A mechanically tuned broadband Kerr frequency comb (KFC) is demonstrated in a Whispering Gallery Modes (WGM) microresonator. Due to their tiny mode volumes (V) and ultra-high Q factor ~ 108, optical WGM microresonators exhibit low threshold power for nonlinear phenomena (scales as V/Q2). Here we focus on the production and mechanical tuning of a 300 nm wide Kerr Frequency Comb (KFC) in silica stretchable microspheres. The ability to tune the KFC source – and hence to lock it to a narrow atomic line – makes it suitable as the basis for a future miniature atomic clock.
Optical Whispering Gallery Mode (WGM) microresonators are an ideal platform for miniature and high sensitive sensors. They exhibit very small mode volumes and extremely high Q factors of the order of 106 - 109 providing a platform with low nonlinearity thresholds and low detection limits to environmental changes. Here we present and discuss some of the WGM-based applications we have been developing in our lab. Specifically, we present a WGM strain sensor based on a silica stretchable microsphere, with a Q factor of 108 . Tension stress is applied along a microsphere with two fiber tails by pulling on one tail. Consequently, the microsphere's WGMs are shifted and the measurement may serve as a strain sensor. Based on this strain sensor, a WGM magnetic sensor is suggested where the mechanical response of a magnetostrictive material to a varying magnetic field applies the tension stress. In addition, a narrow linewidth laser source for these sensors is described here as well, where a multimode diode laser is locked to a single WGM of the microsphere via self-injection feedback from the microsphere. This optical locking is accompanied with a substantial spectral narrowing of the multimode diode laser.
The importance of average power scaling of fiber lasers (FL) is well known. However, power scaling is strongly limited by factors such as thermal load, and non-linear effects. An alternative path for reaching high powers utilizes the stimulated Raman scattering mechanism, and harnesses its power and brightness enhancement potential to reach high average power, high brightness FL. kW scale Raman FLs have been demonstrated, however they are in core-pumping configurations, meaning that they require an a-priori existing brighter kW laser that acts as their pump modules. There have been only a few publications of Raman FLs where the generated signal has a higher brightness than the pump source at levels of ≥100W, the highest, being at 250W. Here we report a strictly all-fiber clad pumped Raman FL with a CW power output of 800 W with a conversion efficiency of 80%. To the best of our knowledge this is the highest power and highest efficiency Raman FL demonstrated in any configuration allowing brightness enhancement (i.e clad pumped or graded index fiber, excluding step-index core pumped), thus being the first kW-class Raman FL with brightness enhancement. This result was achieved with a specially designed triple-clad fiber (TCF). The core was 25 μm, 0.065 NA, and the inner cladding was 45 μm 0.22NA. The choice of the small inner clad allows obtaining sufficient Raman gain without requiring too long a fiber, as well as being compatible with the waist size of the pump source fiber. In addition this diameter complies with the inner-clad/core ratio which prevents generation of a 2nd Stokes laser beam. Two fiber Bragg gratings at 1120 nm written onto the TCF, were employed as the oscillator’s reflectors. The cavity was pumped by a lower beam-quality source with an M2 of ~8 at 1070 nm. The Raman signal generated in the core, at the first Stokes wavelength of 1120 nm, showed an improved beam-quality in relation to the pump.
We have developed a high power single-mode (SM) monolithic fiber laser at 1018 nm, producing 230 W CW, with an M2 of 1.17 and light to light efficiency of 75%. To the best of our knowledge this is the highest power described in the open literature from a SM fiber laser at this wavelength. Careful simulations were employed which take into account the various wavelength dependent parameters such as the fiber absorption and emission as obtained from the fiber manufacturers, and the cavity mirrors’ reflection, in addition to the fiber geometrical parameters. It was found that the major obstacle for increasing the power at 1018nm is the self-generation of amplified spontaneous emission at wavelengths of 1030-1040nm. If the laser is not designed properly these undesired wavelengths dominate the output spectrum.
Orientation patterned GaAs waveguides for parametric conversion from near to mid-infrared have been fabricated by
MOCVD growth on OPGaAs templates. A monolithic OPO cavity was formed by dielectric facet coating. Parametric
oscillation characteristics were investigated using a pulsed source tunable in the range of 1.98-2.05μm. Type I and II
parametric interactions have been observed, differing in QPM wavelength. OPO threshold power of 7W, using a pulsed
pump, and 5.7W using a CW laser was obtained in a 13mm long waveguide of 39μm period. Overall Parametric peak
power of 0.6W at pulsed pump peak power of 11.6W was generated at signal & idler wavelengths of 3.6μm & 4.5μm
respectively and pump wavelength of 2.015μm. Tuning curves for Type I and type II parametric operation in OPGaAs
WGs have been calculated and verified by the measured signal and idler wavelengths.
Long range, combined UV-IR LIDAR system was built and tested. The system was developed to
operate as a multi-wavelength DIAL in the IR (8-11 μm), dual exciting wavelengths LIF LIDAR in the
UV, and aerosol map and track at 1.5 μm. The IR transmitter is a continuous tunable solid-state
Tandem Optical Parametric Oscillator (OPO) [1]. The first OPO stage generates the 1.5 μm beam and
the second OPO stage pumped by the first, generates the IR band. In the UV the transmitter generates
and transmits either the 266 nm or the 355 nm wavelengths sequentially. All the outgoing laser beams
are prealigned to ensure geometric overlap over the measured paths. Energy references are measured
for each beam on every pulse. The receiver is based on a single reflective telescope with coatings
optimized for both the UV and the IR. The optical signal is routed between the different detection
packages by means of a computerized optical scanner mirror. The receiver-transmitter layout is
based on periscope geometry and is equipped with a large θ-φ scanner. Computer control enables fast
switching between the different measurements and wavelengths, data acquisition and spatial scan as
well.
The system was built inside a mobile trailer and was field tested to descriminate aerosol types in a
complex enviroment [2].
Optical parametric generator (OPG) is a very attractive optical down-conversion configuration since it is a single pass
process and no cavity mirror's alignment is required. Thus the system configuration is much more simple and robust.
Traditionally, OPG processes were demonstrated using a pump source with a pulse length of the order of picoseconds or
less. This is because GW/cm2 order of magnitude pump irradiance was required to excite an OPG process, and such
irradiance in nanosecond long pulses commonly damages the non-linear crystal. The introduction of periodically poled
crystals with high non-linear coefficients has significantly lowered the threshold for parametric processes. This progress
in non-linear crystals enables exciting OPG processes at less than 100MW/cm2 irradiance, using nanoseconds long
pulses from Q-switched lasers. We present an OPG with a threshold of less than 10 MW/cm2 using an 80 mm long
Periodically Poled Lithium Niobate (PPLN) non-linear crystal. High signal conversion efficiency and high power were
obtained at 25 nanosecond pulse length, 10 kHz repetition rate pumping without damaging the crystal. Theoretical
approaches for explaining this OPG regime are discussed.
We report experimental realization of a new type of optical parametric oscillator, in which oscillation is achieved by
polarization rotation in a linear retarder, followed by nonlinear mixing of the two polarized beams. The mixing is
performed by a type II degenerate parametric down conversion in a PPKTP crystal, pumped at 1064nm by a pulsed
Nd:YAG laser. A single, linearly polarized beam is generated at the degenerate wavelength. The output spectrum has a
narrow linewidth (below the instrumentation bandwidth of 1nm), and is highly stable at degeneracy with respect to
variations in the crystal temperature. The frequency locking is explained in terms of balanced roundtrip phase-matching
condition.
Techniques are presented to model optical nonlinear frequency conversion of highly distorted beams with M-squared values as high as 30. Random superpositions of Gaussian-Hermite modes are used to create the field distributions of the incident beams. Split-step Fourier transform techniques are used for the calculation of nonlinear conversion.
We present a theoretical and experimental investigation of the efficiency, beam quality and signal bandwidth of a confocal, unstable OPO resonator. Reduction by more than 20 times of the divergence as well as bandwidth narrowing by factor of ~5 of the signal beam, in comparison to the plane-parallel resonator, are obtained. Resonator configurations have been found where the beam quality of the signal exceeds the beam quality of the pump.
A compact IR transmitter for the 8-12 micrometers atmospheric window is presented. The transmitter consists of two optical parametric oscillators (OPOs) in series, pumped by a 1.064 micrometers Nd:YAG laser. The first conversion stage is a double-pass non-critically phase-matched KTP OPO. A singly resonant configuration is used - the signal at 1.574 micrometers is resonated and coupled out with a 73 percent reflectivity output mirror. The first OPO's signal serves as a pump for a double-pass type I phase-matched AgGaSe2 OPO. This second OPO resonates the signal and couples out the idler at 8-11 micrometers . We eliminate high oscillating intensities inside the cavity by means of a low feedback. The low feedback causes a high threshold level, but have a minor influence on the total efficiency. Pumped by 6.5mJ at 1.574 micrometers , the AgGaSe2 OPO produced up to 0.5mJ at 8.5 micrometers , with beam quality of M2 equals 4-5 and spectral width of 4-5cm-1. Small physical dimensions, simplicity, and fairly good stability, makes this tandem OPO system usable for remote sensing applications. The described system is currently used for laboratory aerosol backscatter measurements.
We present a simple analytical solution for singly resonant OPO. The present analysis permits calculating the depletion efficiency of the OPO even when the resonated signal suffers from strong intracavity losses. To the best of our knowledge, this is the first model, which yields an analytical formula in the depleted signal case. The model can be a useful tool to design cavity mirror reflectivity for a given pump intensity and intracavity losses.
Intracavity second harmonic generation of a passively Q- switched cw Nd:YAG laser is studied. The saturable absorber used was a Cr4+:YAG crystal, and the second harmonic generator was a flux grown KTP crystal. The results were compared to a case of acousto-optic Q-switched case. To allow for a unidirectional second harmonic output beam, a folded laser cavity was constructed. However, due to a partially transmitting mirror in the cavity, two output counter propagating second harmonic beams, were generated. Unexpectedly, 180 degrees out of phase periodic power variations of the two output beams were observed, as a function of the distance from the nonlinear crystal to the mirror.
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