This PDF file contains the front matter associated with SPIE Proceedings Volume 10902, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

All-solid-state deep UV lasers (266nm and shorter) have many potential applications including metrology, LIBS, imaging, chem/bio standoff detection, ARPES spectroscopy, and laser surgery. There are few suitable direct laser emission lines at such short wavelengths, making the use of frequency generation by harmonic generation an important tool for UV solid state laser development. The primary limitation of the field is the development of suitable single crystals for nonlinear frequency conversion. The materials must be acentric, have bandgaps substantially wider than the desired conversion line and and be capable of phase matching to these short wavelengths. Several borate crystals have been reported as possible candidates for deep UV nonlinear applications. These include compounds with the formula ABBF (ABe2BO3F2) where A = K, Rb or Cs, and SBBO (Sr2Be2B2O7). Both compounds have very attractive properties for deep UV nonlinear applications including acentric crystal structures, wide band edges, moderate birefringence and reasonable nonlinear coefficients. Both classes are difficult to crystallize and process however. They have two dimensional structures making them somewhat soft, difficult to cut and polish at a critical angle, and making them prone to disorder. The ABBF phases have very attractive optical properties with wide bandgaps and are capable of frequency conversion to 175nm. They are extremely difficult to grow as single crystals however and are extremely soft and difficult to process. To date these are debilitating limitations. The SBBO crystals appear to have nearly as attractive properties with somewhat less of a bandgap than KBBF, but appear to have greater NLO coefficients. They are considerably harder than KBBF but are prone to severe disorder. Recently the application of hydrothermal methods led to the crystal growth of both of these classes of compounds. The hydrothermal growth of KBBF and RBBF led to formation of large single crystals that can be used for new cutting and polishing methods enabling their use in applications such as ARPES. The hydrothermal growth of SBBO greatly reduces the disorder in the crystals enabling a more confident determination of their optical properties. The recent structural redetermination of SBBO and the examination of their optical properties will be discussed.

We developed a high-power, continuous-wave (CW), single-frequency 852nm laser source, for the purpose of fourth harmonic generation at 213nm. Our approach is the doubly resonant sum-frequency mixing (DRSFM) with two fiber sources. An in-house single-frequency master oscillator at 1907nm is amplified by an in-house clad-pumped amplifier to 5W, and a commercial single-frequency master oscillator at 1540nm is amplified by a commercial amplifier to 10W. The two beams are combined via a dichroic mirror to a single beam before incident on a dual-wavelength resonator, consisting of one set of dual-wavelength mirrors. The external resonator is locked to the 1907nm laser frequency, and the frequency of the 1540nm is locked to the resonator, realizing double-resonance. With a periodically-poled stoichiometric lithium tantalate in the resonator, the sum-frequency at 852nm is efficiently generated. All 3 waves are in the same polarization (e-ray), allowing the effective use of Brewster-cut device, eliminating reflection loss for all wavelengths without any antireflection coatings. With 4.6W at 1907nm and 7.7W at 1540nm incident onto the resonator, 5.2W at 852nm was generated, representing the efficiency of greater than 40%. The experimental result indicates our current setup will be more efficient with higher input powers at 1907nm. With both fiber sources at 1540nm and 1907nm being scalable in output power, the output at 852nm is also scalable. By the forth harmonic of 852nm, 0.456 W CW 213nm was generated.

Deep UV Raman spectroscopy is an exquisite tool for chemical sensing and imaging, which provides superior sensitivity and specificity, while allowing Raman measurements to be performed in a bright room with no residual fluorescence background, which can potentially corrupt the useful signal. However, the lack of appropriate laser sources limits the field of applications and penetration of this advanced spectroscopic technique. In this report, I will present our outgoing efforts to develop versatile laser sources in the wavelength range from 200 to 260 nm for fast and reliable deep UV Raman spectroscopy. I will provide with the outline of three different configurations we have developed and will illustrate the power of deep UV Raman spectroscopy with several practical examples.

Thermal effect in high-power CW single-pass 1064 nm SHG for 532 nm generation using MgO:PPLN crystal has been theoretically and experimentally investigated. By careful control of the focusing condition to deal with the well-known thermal issue such as thermal lensing and dephasing in MgO:PPLN crystal, we have successfully generated >7W green output at 532nm using a 25mm long crystal without observing optical damage. This achievement has enhanced the maximum CW green power generated through MgO:PPLN crystal by a factor of 3 as compared to the common optimization under low power condition, and by a factor of 2 as compared to the scheme of single-pass multi-crystal cascading SHG. To our best knowledge, this result gives the highest CW green power via SHG of MgO:PPLN crystal. Furthermore, a systematic study for SHG optimization has shown that >30% SHG conversion efficiency can be achieved from 2W to 7W under output-power dependent optimization.

Most of the nonlinear optical material for ultraviolet generation are borate material such as LBO, BBO, and CLBO. There are the intrinsic disadvantages of those UV devices such as walk-off and hygroscopicity. As a result, the laser system based on those devices always require a special beam shape treatment and an anti-humidity control during storage and in operation. On the other hand, quasi phase matching devices are very attractive since there are no requirement of beam shaping. We focus on periodically poled LaBGeO5 (PP-LBGO) device since it is QPM structure and transparent at UV region. In addition, even it is borate crystal, it does not have hygroscopicity. Therefore, it is easy to treat in the fabrication process and assembling phase. In this paper, fabrication of PP-LBGO for 266nm generation was demonstrated with the first order QPM structure. The periodicity of the first order QPM for 266 nm generation from 532 nm is approximately 2.1 micrometer. The thickness of 0.3 mm LBGO plates are prepared. The electrode pattern was created on the plate. Several different electrode size and poling conditions were investigated. Finally, we successfully fabricated 2.1 micrometer periodically poled structure with 0.3 mm thick device with 10 mm in length. Its aspect ratio of periodic structure was achieved approximately 300. By using fabricated device, the better conversion efficiency for 266 nm generation was confirmed than former 2nd order PP-LBGO device by using pulsed 532 nm laser.

We report on a 3 W Mid-IR supercontinuum extended up to 4.6 μm based on an all-PM thulium doped fiber gainswitched laser seeding an InF₃ fiber. This innovative fiber presents a specific design that increases non-linear effects and shows very weak background losses. Thanks to the versatility of our gain-switched laser, all the pulse parameters have been widely optimized to generate a supercontinuum emission with the highest average power and the largest spectrum.

Kerr-lens mode locked lasers based on polycrystalline Cr:ZnS and Cr:ZnSe have come of age and, arguably, represent the most viable route for generation of ultra-short pulses in the range 2–3 μm. Developed designs of Kerr-lens mode locked oscillators feature high efficiency and provide access to few-cycle MIR pulses with Watt-level power in a very broad range of pulse repetition rates. However, currently available dispersive mirror coatings limit spectral coverage of these oscillators to below one octave hampering their conversion to frequency combs via frequency envelop offset frequency (fceo) control and stabilization. Supercontinuum (SC) generation using photonic waveguides is a promising approach for spectral broadening of pulsed coherent sources at low pulse energies and small footprint. Among many materials promising for this application stoichiomentric Si3N4 (SiN) holds a unique place due to its high nonlinearity, CMOS compatible fabrication process, and spectral coverage over visible-middle-infrared (MIR) range. In the current paper we experimentally demonstrate the generation of a supercontinuum spanning more than 1.5 octaves over 1.2-3.7 um range in a silicon nitride waveguide using sub-40-fs pulses at 2.35 um generated by 75 MHz Cr:ZnS laser. The coupling efficiency was about 16%, which corresponds to 0.56nJ pulse energy and 12.4 kW peak power. We also have observed that threshold for SC generation was about 50 mW of incident power that corresponds to 2.4KW peak power. The demonstrated coherent 1.5 octaves spanning bandwidth is ideal for self-referenced f-2f detection of the fceo. In addition, this represents a promising broadband coherent source for dual comb spectroscopy.

We recently reported the highest average power (70 W) from an octave spanning (880nm to >1900nm) CW supercontinuum source module constituted of standard telecom fiber and which can be pumped using an Ytterbium laser source operating at any wavelength. Since many applications demand a spectrally stable and repeatable supercontinuum, we have investigated the spectral stability of this supercontinuum source over an extended period of operation (over 15minutes). The overall change in spectral profile was investigated as a function of time and power cycling of the source. This experiment was carried out at 3 different wavelengths of the Ytterbium fiber laser pumping the supercontinuum and at 3 different output power levels. The RMS value for the spectral change was used as the metric for comparison. It was observed that the changes are small (within 1-dB) over the duration of the continuous run. We attribute this change in spectral profile with time, to the rise in temperature of fiber which reduces the nonlinear coefficient of fiber and can be potentially controlled by better heat sinking the fiber spool. By allowing the fiber to cool down to ambient temperature through power cycling tests, the spectral change was observed to be very small at < 0.4dB. The standard deviation of output power fluctuations measured using a fast photodetector (over several seconds of acquisition, at 1 us time interval) was ~3%. These results show that our supercontinuum source offers excellent spectral and power stability over an extended period of operation.

Silicon photonics, based on silicon-on-insulator waveguides, has been successful in establishing itself as a reliable solution for datacom optical transceivers. However, due to the centrosymmetry of silicon’s crystalline structure and the omnipresence of nonlinear absorptions, conventional silicon photonics has not been as successful for second- and third-order nonlinear integrated-optic applications, respectively. Traditionally, second-order nonlinear integrated optics, X(2) effects, have been pursed in waveguides out of materials such as lithium niobate. However, these devices have their own downsides, such as low optical confinement and bulkiness. A solution is heterogeneously integrating ultrathin layers of such noncentrosymmetric materials on silicon substrates, in order to simultaneously achieve efficient X(2) effects and high optical confinement/compactness, as well as partial or full compatibility with silicon foundry processing. Similarly, researchers have for the most part migrated to heterogeneous integration of materials with high third-order nonlinearity, X(3) effects, but negligible nonlinear loss. Recent efforts typically focus on ultracompactness, reduced loss and dispersion engineering for fairly sophisticated integrated chips and advanced nonlinear functionalities, such as supercontinuum generation and optical comb generation. Progress in second-order nonlinear integrated photonic devices heterogeneously on silicon are reviewed. Demonstrations of second-harmonic generation and spontaneous parametric down-conversion, based on ultracompact lithium niobate photonics on silicon, are presented. Also reviewed are heterogeneous third-order nonlinear devices on silicon, and more interestingly, approaches for integration of second- and third-order devices on the same chip. Example demonstrations will be octave-spanning supercontinuum generation on chalcogenide waveguides on silicon and their seamless integration with lithium niobate devices.

Many biophotonic applications used in ophthalmology, dermatology, or flow cytometry rely on laser sources emitting in the yellow-green spectral range. To enable miniaturized laser sources one has to rely on diode lasers. As no direct emitting diode lasers with excellent beam qualities are readily available in this wavelength range, second harmonic generation (SHG) of novel diode lasers emitting wavelengths of 1120 nm and 1152 nm is one feasible approach. For high SHG conversion efficiencies a high beam quality as well as a high optical output power of the fundamental laser light is needed. Our approach is a hybrid integrated master oscillator power amplifier (MOPA) setup which incorporates an optical micro-isolator to protect the MO against back reflections and thereby keep high mode stability. Using individually designed and machined parts based on the results of optical simulations, a platform to integrate different laser system setups has been realized. Using this platform, we were able to compare different approaches of SHG (using planar waveguide or volume LiNbO₃ crystals) and beam output possibilities (free space or fiber-coupled) to satisfy a wide range of applications. In this work we show novel miniaturized diode laser modules emitting more than 2 W and 1.6 W at wavelengths of 560 nm and 576 nm, respectively, which can be used in a wide range of biophotonic applications.

We have developed the MgO: PPLN ridge waveguide modules for efficient second harmonic generation output and optical parametrical amplification at telecom wavelength. High efficient and high damage resistance of developed MgO: PPLN waveguide modules have enabled efficient optical parametric amplification with >16dB intrinsic gain for the input signal.

UV wavelength laser sources are an important area of research due to their use in atmospheric and atomic sensing; however, diode lasers at these wavelengths often have low power, poor spatial mode quality, and broad optical spectra. An alternative approach to a UV laser source is an IR diode laser with frequency conversion. In this work, a dual element ridge waveguide device is presented for third harmonic generation of UV wavelengths. This design has been successfully implemented to generate and sustain 3mW of UV from 200mW of IR pump in the waveguide.

Compact, widely-tunable, continuous-wave (CW) and ultrashort-pulse laser sources in the visible spectral region are extremely valuable in a wide range of cutting-edge applications such as photomedicine, biophotonics and microscopy. The most promising approach to develop a compact, efficient and widely-tunable visible laser source is second harmonic generation (SHG) in a periodically poled nonlinear crystal containing a waveguide, which not only allows highly efficient frequency conversion even at low pump power levels but also offers an order-of-magnitude increase of wavelength range for efficient SHG by the utilization of multi-mode matching technique. In this respect, semiconductor lasers with their small size, high efficiency, reliability, low-cost and wide spectral range coverage are very promising for the realization of tunable visible laser sources. InAs/GaAs quantum-dot (QD) external-cavity diode lasers (ECDLs), owing to the unique features of QDs, are of special interest for that matter. The use of multi-mode matching technique and SHG in periodically-poled potassium titanyl phosphate (PPKTP) waveguides pumped by tunable QD-ECDLs led to the realization of compact, widely-tunable, visible, CW (567.7 - 629.1 nm and 574 – 647 nm) and picosecond-pulse (600 – 627 nm) laser sources. Furthermore, the ability of QD-ECDLs to generate two tunable longitudinal modes simultaneously allowed the demonstration of dual-wavelength SHG (505.4– 537.7nm wavelength region) from diode-pumped PPKTP waveguides. In addition, compact CW white-light and multicolor laser sources were demonstrated by the use of two ECDLs and a PPKTP waveguide. The demonstrated laser sources represent an important step towards the realization of a compact, room-temperature, tunable laser source in the visible spectral region.

Mirrorless optical parametric oscillators (MOPO) represent a special class of parametric devices based on three-wave nonlinear interaction in which the generated photons counter-propagate. Owing to the phase-matching condition of the counter-propagating waves, MOPOs can sustain oscillation without mirrors and present unique and useful tuning and spectral properties. In this paper, we will review our recent advances in structuring technology to achieve quasi-phase matching periodicities as short as 500 nm in Rb-doped KTiOPO₄, which are necessary to compensate for the large phase mismatch. We will also review the performance of MOPOs both in the ps- and ns- pumping regime. In the latter, our crystals reach single-pass conversion efficiencies exceeding 50%, with mJ-level output energies.

Orientation-patterned gallium arsenide (OP-GaAs) and gallium phosphide (OP-GaP) have enabled exciting advances in frequency-combs for spectroscopy in the molecular fingerprint region beyond the transparency limits of quasi-phase matched (QPM) oxides like PPLN and PPKTP. These QPM semiconductors also offer much higher nonlinearities for pumping at 1-m (OP-GaP, d14 ~ 70 pm/V), 1.5m (OP-GaP, d14 ~ 35 pm/V), or 2m (OP-GaAs, d14 = 94 pm/V). In ferroelectric oxides like PPLN, the periodic QPM structure is created by the use of electric-field poling through a photo-lithographically defined insulator to reverse the polarity (and hence the sign of the nonlinear coefficient) of alternating domains. Nonlinear optical semiconductors like GaAs and GaP are not ferroelectric, however, so the QPM grating structure must is created instead by polar-on-nonpolar MBE whereby an inverted GaAs (GaP) epi-layer (grown on a on a thin, lattice-matched non-polar Ge (Si) layer) photolithographically patterned, etched, and regrown to produce thick QPM layers (t ~ 1mm) for in-plane laser pumping. A primary advantage of QPM is that the frequency conversion process can be engineered in various ways through the design and layout of the grating structure to achieve discrete and continuous wavelength tuning by translation across multiple adjacent gratings or fan gratings respectively. Tandem grating periods enable sequential processes such as photon recycling for enhanced efficiency, and chirped grating structures allow spectral tailoring of the frequency conversion process. While previously achieved in ferroelectric oxides, here we demonstrate the same novel structures in QPM semiconductors and discuss the implications for device design.

Using the room-temperature-bonding technique, we have succeeded in fabricating a quasi-phase-matching (QPM) stack of 53 GaAs plates with each thickness of 106 μm and aperture of 5.5 × 5.0 mm for second-harmonic generation of a CO₂ laser at 10.6 μm. An improved process was applied which realized fine alignment of the GaAs plates on the translation stage. The fabricated 5.6 mm long, 53 plate-stacked GaAs-QPM device generated 20 times higher secondharmonic power than the previously fabricated 0.95 mm long QPM stack of nine GaAs plates.

Zinc Selenide exhibits some extremely attractive properties for development as the next generation orientation-patterned semiconductor for mid-infrared frequency conversion. These include a high nonlinear figure of merit (d2/n3 =198), an extremely wide transparency range (0.48-22m), low dispersion (which favors large grating periods and wide spectral acceptance bandwidth), and very low absorption losses. A primary obstacle to producing OP-ZnSe, however, has been the lack of availability of ZnSe single crystals. Extensive efforts to develop melt growth of ZnSe crystals in the late 1990s were abandoned due to severe twinning problems and the replacement of potential ZnSe blue-green diodes with the advent of GaN. The objective of this work, therefore, was to revisit physical vapor transport growth of single crystal ZnSe. Optical-grade polycrystalline ZnSe starting material was vacuum-encapsulated in 13-mm I.D. heavy-walled quartz ampoules, and preliminary unseeded growth was performed using using controlled temperature gradients in horizontal transparent furnaces, which yielded high-optical-quality, cubic centimeter-sized, randomly-oriented single crystals. We further evaluated seeded growth on lattice-matched (100), 4°-offcut GaAs substrates as well as on orientation-patterned GaAs templates. Detailed characterization of ZnSe epitaxial growth quality and quasi-phase matched grating propagation will be reported.

This talk will provide an overview of the latest advances in OPO technology based on novel nonlinear materials such as CdSiP2 (CSP), orientation-patterned GaP (OP-GaP), and MgO:sPPLT, in combination with solid-state and fiber pump laser technology. In the ultrafast time-scales, we have achieved operation of femtosecond OPOs into deep-IR at wavelengths as long as ~8 um by exploiting internal cascaded parametric generation based on MgO:PPLN and CSP using the Kerr-lens-mode-locked (KLM) Ti:sapphire laser as primary pump source, or by external cascaded pumping using a commercial near-IR femtosecond OPO as the intermediate pumping step. More recently, we have also demonstrated successful operation of deep-IR femtosecond OPOs based on CSP at wavelength beyond 8 um by direct pumping with the KLM Ti:sapphire laser, and without the need for intermediate cascading step. We have also recently shown that the development of high-repetition-rate ultrafast OPOs in the picosecond time-scale into the deep-IR is possible by direct pumping using Yb-fiber lasers at 1.064 um. The major improvements in the optical quality of the CSP crystal with low transmission loss over long interaction lengths have made it possible to overcome the low nonlinear gains under low pumping intensities, thus enabling us to develop the first high-repetition-rate picosecond OPO based on CSP and with wavelength tunability out to 6.7 um. In the cw regime, we have demonstrated new a concept for frequency comb generation based on OPOs in doubly-resonant oscillator configuration with dispersive cavities. Using this new concept in a cw OPO based on MgO:sPPLT and pumped at 532 nm in the green, we have generated broadband cw spectra centred at 1064 nm with ~9 THz bandwidth containing >42000 modes. These developments have opened the way for the advancement of OPO sources in new directions and their potential use in a wide range of novel applications including optical microscopy, spectroscopy, and imaging.

Coherent beam combining (CBC) by active phase control could be useful for power scaling fiber-laser-pumped optical frequency converters like optical parametric oscillators (OPOs). We developed an indirect phase control approach based on the phase matching relation intrinsic to efficient nonlinear processes. Previously, we demonstrated coherent combining of second harmonic waves through real time active control of the phases of the fundamental waves, using high bandwidth fibered electro-optic phase modulators. In the case of this 2- wavelength process, it was possible to simultaneously combine both the fundamental and the second harmonic waves. In this paper, we present an experimental demonstration of coherent combining of difference frequency generators emitting an idler wave at 3400 nm. We confirm experimentally the theoretical prediction that through active phase control of the sole 1064 nm pump waves, it’s possible to coherently combine the idler waves efficiently. A residual phase error of 1/28th wave at 3400 nm is achieved, corresponding to an excellent combining efficiency. However, in such a 3-wavelength process, simultaneous combination of the signal and idler waves is not always feasible. This demonstration opens the way to mid-infrared OPO combining. We present the architectures of continuous wave OPOs we are working on.

Chirp of the pulses generated in the 1.4 μm-1.8 μm spectral ranges and in a singly resonant OPO build around a 3 mm long PPLN crystal pumped at 108 MHz by 40 nJ, 1 ps chirped or 350 fs weakly chirped pump pulses centered at 1035 nm is analyzed. Whatever the chirp of the pump pulses, we achieved a pump to signal conversion efficiency < 30%. We demonstrate that by adjusting the cavity length around synchronism condition, chirped or almost chirped free (down to 80 fs) signal pulses are generated. A simple mechanism accounts for this phenomenon.

Since the first demonstration of electric field poling in 1993, the use of quasi-phase matching (QPM) technique has gained wide adoption in a multitude of applications. The QPM field today is dominated mainly by the ferroelectric oxide materials from LiNbO₃ (LN) and KTiOPO₄ (KTP) families, where QPM structures are implemented by the electric field poling technique. While typical QPM devices have a fixed-period, one-dimensional domain grating design, which is the most straightforward to implement, numerous applications require the ability to continuously tune the wavelength over a wider spectral range. For applications where temperature tuning is not desired, a fan-out QPM grating design may be advantageous. The tuning here is performed by transverse translation of the structure in respect to the pump beam, while keeping the crystal temperature constant. While the implementation of fan-out gratings is reasonably well researched in LN, there is a lack of reliable data for KTP isomorphs. Taking into account the high domain growth anisotropy in KTP, an important factor becomes the angle between the domain walls and the b-axis of the crystal. This angle directly affects the quality and dimensions of the QPM device. However, its upper boundary has not been determined to date. In this work we discuss the prospects and limitations of PPKTP devices with fan-out grating designs. We present a fan-out PPRKTP device, where the transverse fan-out rate is 0.5 μm/mm. In an OPO configuration pumped by 532 nm such PPRKTP crystal is able to provide continuously tunable radiation between 0.7 – 2.2 μm.

High repetition rate frequency combs are predominantly used in optical communications, astronomical spectrographs and microwave photonics. Spectral broadening of electro-optic combs based on cascaded intensity and phase modulators with highly nonlinear fibers (HNLF) provides broadband combs with tunable repetition-rate and center-frequency. Spectral broadening is achieved using nonlinear effects such as self-phase modulation which requires substantial time dependent intensity at the input. To achieve this, the combs are compressed to a pulse using either fiber-based devices or pulse shapers. However, this has resulted in poor quality spectral broadening. Determining the optimal shaping profile of the input electro-optic comb for efficient spectral broadening is not direct due to the complex interplay between multiple parameters such as length, non-linear coefficient and dispersion of the nonlinear media, the initial spectral phases and power of the comb and modulator biasing conditions. This problem has been addressed here using adaptive pulse shaping. We use cascaded electro-optic modulators to generate a comb with 9 lines (within 20dB) around 1550nm at 25GHz repetition-rate. A wave-shaper changes the spectral phase of the comb. Dynamic spectral phase optimization by stochastic perturbations is performed in a closed loop by processing the output spectrum to maximize spectral bandwidth. With an output power of ~210mW, adaptive optimization more than tripled the number of lines to 29 (within 20dB) with a smooth spectral envelope while the unoptimized case causes negligible broadening (11 lines). We anticipate that the demonstrated testbed will enable more advanced methods of machine learning towards optimization and shaping of frequency combs.

Bloembergen and Pershan [1] predicted that along with the transmitted second harmonic beam, a beam at the second harmonic frequency is also reflected from the incident surface of the crystal. Experimental verification of the laws for second harmonic reflection (SHR) in isotropic crystals of GaAs [2] were presented thereafter. SHR from crystalline GaAs under picosecond laser irradiation at 532 nm was demonstrated [3]. To our knowledge, the phenomenon of SHR has not been demonstrated before for anisotropic crystals, nor has it been considered theoretically. Using a 10 picosecond duration laser at 1064 nm at an angle of incidence of 45° upon crystals of CdSiP2 (010), ZnGeP2 (010) and GaAs (110), we observed SHR at 532 nm, which was detected using a silicon detector (Laser Probe RjP-765). The generated SHR depended on the square of the incident irradiance. The dependence of SHR energy on the polarization direction of the incident beam and on the orientation direction of the (001)-axis of the crystal around the sample normal was determined. The detailed theory for Maker fringes in anisotropic media presented earlier [4] was extended to the case of uniaxial crystals of 4 ̅2m symmetry and to isotropic crystals for the determination of the amount of SHR predicted. References [1} N. Bloembergen and P. S. Pershan, “Light Waves at the Boundary of Nonlinear Media”, Phys. Rev. 128, 606 1962. [2] R. K. Chang AND N. Bloembergen, “Experimental Verification of the Laws for the Reflected Intensity of Second-Harmonic Light”, Phys. Rev. 144 (2), 775-780 (1966). [3] A. M. Malvezzi, J. M. Liu, and N. Bloembergen, “Second harmonic generation in reflection from crystalline GaAs under intense picosecond laser irradiation,” Appl. Phys. Lett. 45 (10), 1019–1021 (1984). [4] S. Guha, J. Wei, J.M. Murray, K.T. Zaw, and P.G. Schunemann, “Measurement of d-coefficients of CdSiP2 and ZnGeP2”, To be presented at OSA Advanced Solid State Laser Conference, Boston MA, November 2018.

High symmetry in the nonlinear susceptibility tensor combined with lack of birefringence in zincblende crystals allow for efficient mixing of a wide range of polarization states. Quasi-phasematching (QPM) allows phasematching to be achieved without relying on specific polarization states. Polarization insensitive difference-frequency generation [1] as well as optical parametric oscillation using circularly polarized and depolarized pump sources [2] have been demonstrated in QPM GaAs. Here, we present the six coupled-wave equations that describe chi-2 nonlinear mixing in these materials. We account for the two orthogonal polarization states at each frequency, which leads to six instead of three coupled-wave equations. We calculate the effective nonlinear coefficient by projecting the nonlinear susceptibility tensor onto the field polarization directions. We applied the couple-wave equations to describe optical parametric amplification (OPA) and oscillation (OPO) in non-birefringent crystals and present simulations for QPM zincblende crystals. We show that for both OPA and OPO, there may be back-conversion to the orthogonally polarized pump wave. This back-conversion process is phasematched. Also, because the wave orthogonal to the pump is initially unseeded, this wave can take on the phase to favor back-conversion well before the original pump becomes depleted. This back-conversion process is associated with reduced OPA and OPO gain as well as apparent rotation of the pump polarization angle. [1] S. J. B. Yoo, et al., Appl. Phys. Lett. 68, 2609 (1996). [2] P. S. Kuo, et al., Opt. Lett. 32, 2735 (2007).

Optical parametric oscillators (OPOs) convert the frequency of laser light to almost arbitrary values. Nowadays, they serve as wavelength-agile light sources for spectroscopic applications as well as photon generators for quantum-optical experiments. Conventional OPOs are based on a nonlinear-optical crystal surrounded by a mirror cavity. Regarding the cavity, one often finds the following rule of thumb: the more waves are resonant, the lower is the oscillation threshold and the more difficult is the wavelength tuning. In whispering-gallery-resonator-based OPOs (WGR OPOs), light is guided by total internal reflection in a millimeter-sized spheroidally-shaped nonlinear-optical crystal. Thus, these devices are intrinsically triply resonant. Indeed, they provide microwatt-level oscillation thresholds, i.e., the lowest values of all OPO configurations. However, following the abovementioned rule, their applicability in fields beyond fundamental science might be questionable, because the most striking feature of an OPO, i.e., the wavelength tunability, is hampered. Nevertheless, several experimental studies revealed that the output wavelengths of WGR OPOs could be tuned in well-defined steps over hundreds of nanometers by temperature variation. Combined with strategies for mode-hop free tuning, it is possible, e.g., to tune the output wavelength in a controlled way until it meets MHz-wide resonances. This is sufficient for high-resolution spectroscopy. WGR OPOs are nowadays operated around various center frequencies, covering the visible-to-mid-infrared spectral range. These light sources - despite of their intrinsic triple resonance - might serve as compact and wavelength-agile devices for various applications.

Traditionally, infrared molecular spectroscopy has been performed with frequency-domain measurement techniques. Recent experiments have exploited the outstanding temporal coherence of state-of-the-art femtosecond lasers to overcome long-standing sensitivity and dynamic range limitations of these traditional techniques, with time-domain measurements. Here, we show how state-of-the-art 2-µm femtosecond technology provides (i) Watt-level infrared sources covering the entire molecular fingerprint region, with a spectral brightness exceeding even that of synchrotrons, (ii) background-free, high-sensitivity and high-dynamic range time-domain detection of molecular vibrations via electro-optical sampling with (iii) attosecond temporal accuracy. These advances herald a new regime for time-, frequency- and space-resolved molecular vibrational metrology.

Understanding femtosecond pulse propagation in biological media has become increasingly relevant with the widespread use of femtosecond laser systems for imaging and diagnostic applications. Intense, femtosecond pulses are prone to undergo nonlinear effects in media. The ability to accurately simulate the nonlinear processes femtosecond pulses undergo in biological media is critical for designing new diagnostic techniques and determining maximum permissible exposure (MPE) limits for laser safety standards such as ANSI Z136.1. The combination of strong absorption, broad bandwidth, and dispersive effects makes standard nonlinear simulation methods based on the slowly varying envelope approximation unsuitable for the study of near-infrared (near-IR) pulses in water and biological tissues. Building off an existing linear ultrafast pulse propagation model we present preliminary work simulating supercontinuum broadening in water without using an envelope approximation. Using a one-dimensional simulation of self-phase modulation, we explain the infrared continuum broadening observed in a previous experiment in water using 35 fs near-IR pulses, but fail to explain the visible continuum, suggesting that the continuum is further broadened by self-focusing. We then extend the model to simulate the propagation of near-IR pulses in the human eye at the 100 fs ANSI MPE limit for pulse durations from 10 fs to 1 ps. Using this simulation, we explore the implications of supercontinuum generation on the ANSI MPE limits.

We investigate the nonlinear optical properties of ZnSe and ZnS using ultrashort (pulse duration approximately 200 fs) midwave infrared laser pulses between 3 and 4 μm. Multiple harmonic generation in both materials was observed, as well as significant spectral modification of the fundamental pulse. Simulations using a nonlinear polarization model enhanced with ionization compared favorably with experimental data. Random quasi phase matching in the materials is the likely generator of the observed harmonics.

We demonstrate the development of broadband, infrared frequency combs tunable from 3 to 27 microns. The source is based on using a robust, few-cycle Er:fiber comb (10 fs pulse duration) to drive intra-pulse difference frequency generation within a quasi-phase-matched nonlinear medium (e.g. periodically poled lithium niobite or orientation patterned gallium phosphide). Since the down-converted light has a longer optical period, the electric field of this longer wave light can be directly sampled by the few-cycle Er:fiber pulse via electro-optic sampling (EOS), directly yielding spectroscopic information on the infrared light. Further, by implementing EOS in a dual frequency comb configuration, we can increase the spectroscopic acquisition speed to a rate of 50 Hz. This dual-comb EOS configuration enables a measurement bandwidth spanning 370 – 3300 cm^-1 with a resolution down to the 100 MHz (0.003 cm^-1) spacing of the infrared comb. Due to the brightness of this comb source and the broad acquisition bandwidth, we can perform high resolution and high sensitivity spectroscopy on chemically and biologically relevant compounds spanning the molecular fingerprint region, with an outlook towards fast acquisition, infrared frequency comb microscopy.

ZnGeP2 is one of the most useful nonlinear optical crystals for generation of high power and high energy infrared radiation in the 2 to 8 m spectral range because of its high nonlinear coefficient along with high thermal conductivity. Growth technology has advanced over the years and samples with improved optical quality arising from fewer defects, impurities and free carrier concentration are now available. But there have not been many direct measurements of the refractive indices of ZnGeP2, especially recently. One of the first papers describing nonlinear optical frequency conversion in ZnGeP2 also provided the values of temperature dependent refractive indices [1]. The temperature dependent birefringence in ZnGeP2 was measured by Fischer et al [2] and the room temperature indices were measured by Zelmon [3] with samples available in the 1990s and early 2000s. Temperature dependent Sellmeier coefficients were obtained by Ghosh [4] based on the data from 1971 – but there have been no new temperature dependent measurements since the work by Fischer in 1997 on the birefringence. We have measured the ordinary and extraordinary indices of ZnGeP2 for the wavelength range of 0.9 to 12 m, over a temperature range of 90 K to 475 using high resolution FTIR spectra of a recently grown sample (ZGP 391Q) of thickness 150 m with the c-axis lying on the sample face. The spectra were taken with light polarized along and perpendicular to the c-axis. The temperature dependent Sellmeier coefficients are obtained, expressed in the form given by Kato [5]. The values of the coefficients at 296 K vary a little from those in [5]. The index values obtained from the current Sellmeier equations were used to determine the dependence of the signal and idler wavelengths as a function of the crystal cut angle (theta) for various values of pump wavelengths, for birefringent phase matching of Types I and II. The results obtained show good match with experimental values. REFERENCES 1. G. D. Boyd, E. Buehler, and F. G. Storz, “Linear and nonlinear optical properties of ZnGeP2 and CdSe,” Appl. Phys. Lett. 18, 301–304, 1971. 2. D. W. Fischer, M. C. Ohmer, P. G. Schunemann, and T. M. Pollak, “Direct measurement of ZnGeP2 birefringence from 0.66 to 12.2 m using polarized light interference,” J. Appl. Phys. 77, 5942–5945, 1995, D. W. Fischer and M. C. Ohmer, “Temperature dependence of ZnGeP2 birefringence using polarized light interference,” J. Appl. Phys. 81, 425–431, 1997. 3. D. E. Zelmon, E.A. Hanning and P.G. Schunemann, “Refractive-index measurements and Sellmeier coefficients for zinc germanium phosphide from 2 to 9 m with implications for phase matching in optical frequency-conversion devices”, J. Opt. Soc. Am. B, 1307-1310, 18(9), 2001. 4. G. Ghosh, “Sellmeier coefficients for the birefringence and refractive indices of ZnGeP2 nonlinear crystal at different temperatures”, Appl. Opt., 37(7), 1205-1212, 1998. 5. K. Kato, ‘‘Second harmonic and sum frequency generation in ZnGeP2,’’ Appl. Opt. 36, 2506–2530, 1997.

Laser sources operating near a wavelength of four microns are important for a broad range of applications that require power scaling beyond the state-of-the-art. The highest power demonstrated in the spectral region from a solid-state laser source is based upon nonlinear optical (NLO) conversion using the NLO crystal ZnGeP2 (ZGP). High-power operation in ZGP is known to be limited by thermal lensing. By comparing the figure of merit for thermal lensing in ZGP with other NLO crystal candidates, CdSiP2 (CSP) particularly offers significant advantages. However as was the case with ZGP during its early development, the physics of observed crystal defects, and their relevance to power scaling, was not at first sufficiently understood to improve the crystal’s characteristics as a NLO wavelength conversion element. During the past decade, significant progress has been made (1) with the first reported growth of a large CSP crystals, (2) in understanding the crystal’s characteristics and its native defects, (3) in improving growth and processing techniques for producing large, low-loss crystals, and (4) in demonstrating CSP’s potential for generating high-power mid-infrared laser light. The paper will summarize this progress.

Second-order nonlinear interactions offer many properties advantageous to ultrafast laser sources. We are developing these devices to enable lasers and frequency combs in new regimes of operation. A key concept for this work has been nonlinearity management intracavity. For high repetition rate frequency combs, their high power per comb line is attractive for high-speed and high-sensitivity molecular spectroscopy. We have introduced a new type of nonlinear device to address the long-standing Q-switching damage problem which had limited the repetition rate achievable from SESAM-modelocked femtosecond solid-state lasers. We use adiabatic excitation of cascaded quadratic nonlinearities, implemented via two-dimensionally quasi-phase-matching gratings, to achieve a large self-defocusing nonlinearity intracavity with low loss. This offers energy-efficient soliton formation for stable modelocking, and also intensity clamping via self-defocusing (preventing SESAM damage prior to stable modelocking). We have developed a repetition-rate stabilized >10 GHz 100-fs modelocked laser with this approach. Combined with highly nonlinear waveguides, supercontinuum generation, f-2f interferometry, and mid-infrared generation are within reach. For high power lasers, thin-disk laser (TDL) oscillators offer a compelling route towards high-energy pulses for industrial or scientific applications, since they bypass the complexity of conventional laser-amplifier systems. However, the highest power TDL oscillators have been operated in vacuum chambers because the nonlinearity of air destabilizes the pulse formation. We have explored intracavity nonlinearities to address such challenges. We demonstrate nonlinear-mirror modelocking, and nonlinear-cancellation via cascaded quadratic nonlinearities, recently obtaining >200 W from a SESAM-modelocked TDL operated in ambient air. Scaling towards kW average powers will be discussed.

Plasma based particle accelerators driven by either lasers or particle beams are most probably the future of the particle accelerators technology. In a laser driven plasma based particle accelerators a stable synchronization of the electron bunch and of the plasma wake field in the range of less than 2 fs is necessary in order to optimize the acceleration. For this purpose we are developing a new shot to shot feedback system with a time resolution of less than 1 fs. As a first step, stable THz pulses are generated by optical rectification of a fraction of the plasma generating high energy laser pulses in a nonlinear lithium niobate crystal. It is planed that the generated THz pulses will energy modulate the electron bunches shot to shot before the plasma to achieve the time resolution. In this contribution we systematically investigate the influence of the optical properties as well as the theoretical description of the THz generation on the conversion efficiency of the generation of short THz pulses. We compare different approximations for the modeling of the generation dynamics (full second order calculation or first order slope varying approximation SVA) and of the dielectric function (linear approximation of the dispersion relation, influence of the free carries generated by the pump adsorption and their saturation, decreasing of the pump intensity) in order to investigate the importance of a detailed description of the optical properties.

We develop a modification of the Z-scan technique that simultaneously measures the nonlinear refraction and absorption while using a single detector. This modification utilizes a quadrant cell detector to measure both the change in absorption and spot size due to the optical nonlinearity without the use of a partially closed aperture or second detector. This improves the ease of alignment, requires half the detectors and utilizes all of the available beam power. This modification is especially useful at IR wavelengths, where alignment can be difficult and detector responsivities are low limiting available signal to noise.

We demonstrate tunable narrowband THz generation by optical rectification of a femtosecond pulse in Orientation Patterned Gallium Phosphide. Center frequencies of 0.9 - 3.8 THz with average power up to 15 μW were achieved using a 1.064 µm fiber laser for the pump laser. Biomolecular characterization for an early application of this system is also shown in this work by anisotropic spectroscopic signature detection of molecular crystals in the THz region.

Terahertz (THz) is an innovative form of electromagnetic radiation providing unique spectroscopy capabilities in critical fields, ranging from biology to material science and security. The limited availability of high-resolution imaging devices, however, constitutes a major limitation in this field. In this work, we tackle this challenge by proposing an innovative type of time-space nonlinear Ghost-Imaging (GI) methodology that conceptually outperforms established single-pixel imaging protocols. Our methodology combines nonlinear pattern generation with time-resolved single-pixel measurements, as enabled by the state-of-the-art Time-Domain Spectroscopy (TDS) technique. This approach is potentially applicable to any wave-domain in which the field is a measurable quantity. The full knowledge of the temporal evolution of the transmitted field enables devising a new form of full-wave reconstruction process. This gives access not only to the morphological features of the sample with deeply subwavelength resolution but also to its local spectrum (hyperspectral imaging). As a target application, we consider hyperspectral THz imaging of a disordered inhomogeneous sample.

The generation of a wide-range THz wave is investigated from a photoconductive antenna excited using a chaotic oscillation multimode semiconductor laser with optical delayed feedback by an external mirror. The stable THz wave is obtained from the multimode-laser diode excited photoconductive antenna by using a laser chaos. For a high sensitive detection, a metal V-grooved waveguide (MVG) is also used. About 10 times high amplitude signal is obtained using laser chaos. The signal is also increased about 1.6 time using MVG compared to Si lens. As the MVG gap is narrower from 200 to 20[μｍ], the detected signal is increased about twice.

We report a second-Stokes diamond Raman laser in eye-safe wavelength capable of high power and large-scale-factor brightness enhancement. Using a quasi-continuous 1.06 μm pump of power 823 W (0.85% duty cycle) and M2 up to 6.4, a maximum output power of 302 W was obtained with an M2 = 1.1 providing an overall brightness enhancement factor of 6.0. The output power is the highest single-mode power reported for Er-doped and Raman fiber lasers (~300 W). The measurements are in good agreement with model calculations, which we use to optimize and predict performance over wider range of power and input beam quality. The results highlight a novel pathway to high brightness eye-safe lasers based on relatively incoherent 1.0–1.1 μm pumps. The concept may be adapted and extended to other wavelength regions by using other pumps or via higher-order (3+) cascading. For example, to generate high brightness red output near 0.62 – 0.67 m by using second harmonic pumps near 0.53 μm. A large number of wavelength options are conceivable as a result of the wide transparency of diamond (0.23–3.8 µm, and > 6 µm).

Cascaded Raman fiber lasers are agile and scalable offering high optical powers at various wavelength bands inaccessible with rare-earth doped fiber lasers. Although several architectures for building cascaded Raman lasers exist, only the use of cascaded Raman resonators (CRRs) provide a high degree of power-independent wavelength conversion. A cascaded Raman resonator comprises of nested cavities built with two sets of high reflectivity fiber Bragg gratings at fixed Stokes wavelengths and thus can be used only for a fixed input wavelength; thereby restricting its use to a specific Ytterbium-doped fiber laser. The need for fabricating separate grating sets for each input wavelength compromises the simplicity and cost-effectiveness of this technique. Here, we demonstrate through experiment and simulations that the simple inclusion of a distributed broadband reflector at the first-order Stokes component along with the grating sets makes the CRR module very flexible to the input wavelengths, with remarkable improvement in efficiency over a widerange of inputs. In our experiment, a 17W Ytterbium-doped fiber laser tunable from 1055nm to 1080nm is used to pump a CRR module designed for an input wavelength of 1117nm and output wavelength of 1480nm. In conventional operation, for a non-resonant pump input into the CRR, nearly all the output was still unconverted pump. However, with the addition of the broadband distributed feedback reflector for the first-order Stokes component we achieved the 6thorder Stokes at 1480nm over the entire tuning range with a significant improvement in conversion ranging from ~33% to 86% of output at 1480nm.

The correct identification of the third-order nonlinear optical effect of stimulated Raman scattering (SRS) led in the last years to a versatile method to generate new laser wavelengths resulting from a photon-phonon-interaction. There is the possibility to down- (Stokes) or up-shifting (anti-Stokes) of the pump laser frequency. The size of the frequency shift depends on the Raman-active material and the excitability of their SRS-promoting vibration-modes. Prominent Raman crystals include BaNO3 and other nitrates, KGW and other tungstates, YVO4 and other vanadates as well as diamond. Recently, we observed SRS in the laser crystal LuAlO3 with one SRS-active phonon mode and the natural crystal Spodumene (α-LiAlSi2O6), which has three corresponding SRS-active vibration modes. Selective amplification of one particular spectral line generated through SRS is possible by placing the Raman crystal into a frequency-selective optical resonator, whose optical feedback is selective for only one Stokes- or anti-Stokes component. Raman lasers can be used in many applications, e.g. differential absorption LIDAR systems (DIAL, Light Detection and Ranging) to detect trace gases like carbon dioxide (CO2), ozone (O3) or water vapor (H2O). Various pumping schemes and resonator designs have been investigated focusing on good conversion efficiency, high spatial beam quality and high pulse energy of the output beam. The DIAL technique requires laser sources with high average output power combined with an excellent beam quality (M2 < 2). One possible solution can be found in an effect called beam-cleanup, which takes place by using Raman lasers and amplifiers.

The work reports for the first time on fibre-based Raman conversion with relatively large Stokes shift pumped by double-scale laser pulses having various degree of coherence. It was discovered that the degree of coherence of the pump pulses affects significantly the amount of the wavelength shift, intensity, and spectral width of frequency-downconverted radiation. At lower coherence within double-scale pulses, the magnitude of intra-pulse femtosecond field oscillation grows, leading to stronger nonlinear pulse interaction with the optical medium. This discovery suggests new approaches to nonlinear transformation of partially coherent laser pulses, typical of many mode-locked generation regimes of fibre lasers.

Power scaling of narrow-linewidth, continuous-wave, fiber lasers with near-diffraction-limited beam quality is primarily limited by stimulated Brillouin scattering (SBS). Among several SBS mitigation techniques, line broadening by phasemodulation has been widely used. Recently, enhanced SBS seeding (threshold reduction) due to spectral overlap between the backscattered, line-broadened signal and the SBS gain spectrum has been reported. Backscattering of the signal is composed of the Rayleigh component and reflections from the end termination. However, in high power amplifiers with small lengths of optical fiber used, the Rayleigh component of the backscatter is anticipated to be small. Here, we report conclusive experimental evidence that even very small reflections from the output facet are enough to substantially reduce the SBS threshold due to spectral overlap. We demonstrate this in a 500W, white noise phasemodulated, narrow-linewidth, polarization-maintaining power amplifier operating at 1064nm. Two commonly used fiber terminations are utilized. In the first case, the amplifier is terminated by a high-power laser cable with an end-cap and anti-reflection coating and in the second case, by an angle cleaved passive delivery fiber. Back-reflections from the angle cleaved facet (<80) providing ~70dB isolation (ideal case) was enough to enhance SBS. We analyzed the threshold differences between the two cases as a function of linewidth from 4.91GHz to ~10GHz. At smaller linewidths, the difference was negligible while at larger linewidths, there was a substantial difference in thresholds (<20%). This linewidth dependent difference in thresholds was accurately simulated by the backward seeding of SBS by the linebroadened signal, thus conclusively proving this effect.

Stimulated Brillouin scattering (SBS) based distributed optical fiber sensors have been deployed in a myriad of potential applications. Recently, the characteristics of SBS in few-mode-fibers (FMFs) have been investigated for designing optical sensors of high selectivity. For example, monitoring SBS of the individual modes in a two-mode fiber (TMF) allows simultaneous sensing of temperature and strain. In optical communications, on contrary, SBS degrades the signal-to-noise ratio (SNR) and limits the channel capacity. We here experimentally measure the threshold power required to stimulate Brillouin scattering in an FMF when using different mode-pair combinations as pump and probe signals. In particular, we use mode-division-multiplexing (MDM) to launch different linearly-polarized (LP) modes into the both ends of a TMF. For each mode-pair, we gradually raise the pump power until observing the transition from spontaneous to stimulated Brillouin scattering. The results presented here are considerably important for designing efficient FMF-based optical communications/sensing system.

A 10 mm long PPLN crystal pumped by 125 nJ, 250 fs pulses centered at 1035 nm yielded by Yb3+ femtosecond fiber oscillator generates femtosecond signal and idler pulses tunable in the 1.35 μm - 1.65 μm and 2.6 μm - 4.2 μm spectral ranges. A numerical model accounting for both second- and third-order nonlinear processes well agree with the recorded signal conversion efficiency (up to 42%), the spectral and temporal profile of the generated pulses. Pulse to pulse stability is drastically improved injecting this compact and versatile device with a continuum generated in a photonic fiber. Further improvements are discussed.

In multiple scattering media at high excitation intensities, compulsory light scattering (SRS) of organic laser dyes arises simultaneously with their chaotic generation. Together, they create a coupled effect of SRS-RL, in which RL radiation plays a stimulating role for SRS. This radiation stimulates the forced oscillation of the dye molecules at frequencies whose Stokes lines fall within the range of the RL spectrum. As a result, on all these lines there is a SRS. The radiation spectrum of dyes becomes quasilinear, in which, on the background of a continuous component due to RL, there are Stokes lines of SRS corresponding to the segment of the spectrum of Stokes lines of combinational scattering within the range of the RL spectrum. At present, only the influence of RL on the onset of SRS has been investigated, and the reverse effect has not been studied in practice. At the same time, for the understanding of the regularities of the WRC-RL process, the effect is very important, since it can be used practically in Raman scattering spectroscopy. Therefore, the purpose of this work is to study this effect. To this end, the radiation of dyes injected into the vesicular polymer film alone and in the mixture was investigated.

This work reports the experimental demonstration of supercontinuum (SC) generation with broad spectral output of 950 nm maximum or a 3.7 dB variation within the range from 1300 to 1600 nm and demonstrate how some fiber properties enhance nonlinearities and shows different SC output spectrum. Broad spectrum was achieved throughout a subnanosecond microchip pulsed laser operating at 1064 nm. Three different conventional fibers were employed: highnumerical aperture fiber (HNAF), dispersion shifted fiber (DSF) and single mode fiber (SMF). SMF was wrapped in different diameters to induce fiber losses. Several combinations of these three fibers and diameters resulted in high spectral width as well as high flatness in the telecom band.

Stimulated polariton scattering (SPS) is one of the most important non-collinear phase-matching nonlinear process. In SPS, the angle between the THz and pump waves is as large as 65°, so it is necessary to study the spatial intensity distributions of the pump, Stokes and THz waves effected by the non-collinear phase-matching. In this work, we take the injection-seeded terahertz wave parametric generator (is-TPG) as an example to obtain the numerical solutions of the coupled-wave equations. The simulation results are focused on the spatial-temporal intensity distributions and the energy input-output characteristics. It is notable that the spatial intensity distributions of the pump and Stokes beams along the THz-wave propagation direction are not uniform as the inputs. The pump wave is depleted along the THz-wave propagation direction while the Stokes wave is increasing at the same time.

Organic semiconductor dyes play a very important role in different criteria and fields of science, technology, and engineering for different applications, especially the absorption of visible light for solar cell devices, photo sensors, fluorescent, photoconductive devices. Basic fuchsine belongs to the triphenylmethane family. It is used on a large scale as a coloring agent for the staining of biological tissues, leather, and textile materials. The Z-scan technique is based on the principles of spatial distortion of the beam and offers simplicity, as well as a very high sensitivity for measuring both the nonlinear refractive index as well as the non-linear absorption coefficient. For the present work we propose the study of 6 star-type dyes derived from basic fushcine with azo groups added, by the Z-scan technique. A 1 × 10^-4 M dilution of each dye in methanol was prepared. Subsequently, the linear characterization was made with the help of the UV-vis technique and finally, the analysis was carried out in the Z-scan equipment. The dyes have a λmax between 500-570 nm, and with n₂ = -1.7 × 10^-9 to 6.63 × 10^-9 and β = 1.1 × 10^-3 to 6.3 × 10^-4. These results allow us to propose these dyes as good options for their incorporation into solar cells as well as optical absorbers.

Chromatic dispersion controlling is crucial in designing practical optical communication systems, and nonlinear systems. Ultra-flattened chromatic dispersion has been numerically performed in silica photonic crystal fibers (PCFs) whose chromatic dispersion variation can be as small as ±0.5 ps/km/nm. However, keeping arrays of air holes at precise sizes and shapes is highly required to realize the targeted dispersion. Consequently, this requires much effort in controlling air pressure during fiber fabrication and is considered as a disadvantage of PCFs. In this report, we propose a novel fiber structure for flexible controlling of chromatic dispersion. The fiber structure is obtained by adding six solid rods around the core of a step-index fiber. Ultra-flattened and close-to-zero chromatic dispersion can be realized by using this fiber structure. The variation of chromatic dispersion from 2.5 to 3.7 μm is as small as 0 ± 0.2 ps/km/nm. Using a laser pumping at 2 μm into a 5-cm-long fiber, highly coherent supercontinuum (1.2 – 3.3 μm at -20 dB level) is experimentally generated.

The mid-infrared (mid-IR) spectral region is very important topic of research because the molecular fingerprint of most of the molecules find in this region. Therefore, the mid-IR supercontinuum has been of great interest for the application of spectroscopic chemical sensing, metrology, and hyper-spectral imaging. Presently available mid-IR light sources such as optical parametric oscillators, quantum cascade lasers, thermal emitters and synchrotron radiation are not suitable for such mid-IR applications where we require broadband, spatial coherence, portable and high brightness of laser sources. Supercontinuum generation using the optical fibers has been one of the prominent approaches to obtain broadband mid- IR light sources. In this work, we have numerically investigated an all-normal dispersion engineered tapered tellurite step-index fiber structure for the generation of coherent supercontinuum spectrum in the mid-IR region. Supercontinuum spectrum spanning 1.04 – 4.34 μm is obtained by 200 fs laser pulse pumping of coupled peak power of 44 kW at 2.0 μm. Broadband and coherent mid-IR supercontinuum light is generated in a 4 cm long tapered step-index tellurite fiber. Coherent mid-IR supercontinuum spectrum reported in this work is expected to have potential applications for a variety of important applications in various fields including imaging, early cancer detection, sensing, and precision spectroscopy.

Pulsed diode laser sources in the yellow-green spectral range are highly demanded for applications like label-free imaging or stimulated emission depletion (STED) microscopy. However, direct emitting diode lasers are not yet providing sufficient emission characteristics and lifetime behavior. In this work pulsed laser sources that make use of reliable near infrared diode lasers in the wavelength range from 1122 nm to 1178 nm and single-pass second harmonic generation are presented. A ridge waveguide laser with integrated distributed Bragg-reflector (DBR-RWL) is operated in gain-switched pulsed operation. The laser pulses are further amplified by a subsequent tapered amplifier (TPA) in a master oscillator power amplifier (MOPA) configuration. Here, the TPA is in cw-operation. The amplified laser pulses are then coupled into a periodically poled lithium niobate ridge waveguide crystal. The whole setup fits into a butterfly housing with a footprint of only 47 mm × 76 mm which also supports a polarization maintaining fiber output. The concept has been realized at emission wavelengths of 561 nm and 589 nm, respectively. At 561 nm laser pulses with a full width at half maximum (FWHM) of 100 ps and peak power of more than 2.5W could be demonstrated, which is a nearly tenfold increase compared to previous work without TPA. The pulses at 589 nm have similar temporal characteristics but slightly lower optical output power of more than 1W pulse peak power.

A periodically poled MgO – doped LiNbO₃ (MgO:LN) non-degenerate photon pair source is utilized for spontaneous parametric down-conversion of 532 nm photons into time-energy entangled pairs of 794 and 1614 nm photons. The entangled photons are separated using previously detailed sorting optics, such that each wavelength is independently directed through one of two modified Mach-Zehnder interferometers – also known as a Franson interferometer – after which they are fiber-optically guided to high-efficiency photon detectors. Output from the detectors is sent to a high resolution time tagger, where coincidences between the entangled photons are recorded. By varying the length of the long path in one Mach-Zehnder interferometer, it is possible to observe high visibility sinusoidal fringes in the measured coincidence rates (while no variation is seen in single photon detection rates). These fringes – due to interference between the photon probability amplitudes – are indicative of a violation of the Bell inequality, and confirm inconsistencies with local hidden variable theory for the correlations of the time-energy entangled photon pairs.

We present our efforts to power-scale the output from a single crystal, linear cavity Zinc Germanium Phosphide (ZnGeP₂) optical parametric oscillator (OPO). Our initial doubly-resonant OPO produced a maximum total output power of 1.1 W while double-pass pumped with a 7 W 150 ns pulsed Ho:YLF laser. Next, we developed a pump source with higher power (38 W) and shorter pulse lengths (65 ns). This resulted in the demonstration of a maximum 5 W output from the OPO. However, the higher pump power could no longer pass through an optical isolator (limited damage threshold) and single pass pumping had to be used to prevent feedback to the pump laser. In addition, a longer folded OPO cavity had to be used to rid the cavity of the transmitted pump light using available optics. Overall, this resulted in a slightly less efficient optical to optical conversion (14 % vs. 15 %). To improve efficiency the OPO cavity was subsequently optimised in terms of pump and cavity mode sizes and output coupler mirror reflectivity (50 % vs. 80 % at 3 to 5 μm). The output coupler was also specified for high transmission of the pump allowing a short cavity length of only 20 mm with a ZnGeP2 crystal length of 15 mm. Operating near degeneracy, a total output power of 14 W was achieved when single-pass pumped with 48.8 W (conversion efficiency of 29 %).

This paper presents several broad-spectrum sources generated by pumping a 532nm laser microchip, with 600ps pulses and various combinations of conventional thin-core optical fibers. Fibers with a core of 2.1μm, 2.5μm and 3.5μm were used; with lengths of 38m, 23.3m and 19.7m respectively. We present the results derived from experimenting with each type of fiber separately, as well as making combinations among them, obtaining broad spectrum light sources (supercontinuous light) with a spectral width of approximately 300nm and powers greater than -20dBm.

We focused on frequency conversion of simple and compact CW PM-Yb-doped fiber laser based on FBGs with wavelength adjustable function. By using the temperature dependency of FBG’s center wavelength, it is possible to adjust the oscillation wavelength by controlling both FBG temperatures. We tested 6 μm core based FBGs with at the center wavelength of 1040 nm, 1064 nm and 1090 nm. We found that it is possible to tune approximately 500 pm by controlling FBG temperatures between 15 and 85 degree-C. The tunability of wavelength was is around 6.0~8.5 pm/degree-C. It was possible to achieve 10 W of output for each wavelength. In addition, we applied this configuration to 10 μm core fiber at center wavelength of 1064 nm. It was obtained wavelength adjustability of 8.3 pm/degree-C with output of over 35 W. Its linewidth is narrower than 50 pm and suitable for frequency conversion. The tuning range was similar to the 6 μm core fiber case and slight difference and variations might be related to wavelength, refractive index distribution, thermal expansion coefficient, and fixation condition of FBGs. By using over 35 W fiber laser, it is possible to realize around 10 W of green-light SHG laser combined with high conversion efficiency QPM devices such as PP-Mg: SLTs with wavelength tunable function.