Stimulated Brillouin scattering (SBS) is the lowest threshold nonlinear effect that limits power scaling in narrow linewidth continuous wave fiber lasers. While several SBS mitigation techniques exist, optical linewidth broadening through external phase modulation has become the predominant method for SBS suppression. Arbitrary waveform generators (AWG) and pseudo-random binary sequence (PRBS) modulation schemes provide enhanced control of lineshape to effectively mitigate SBS but are expensive and difficult to implement. White noise (WNS) phase modulation is simple and easy to implement, but the resulting line shape is non-ideal and has a slow roll-off. Thus, the SBS enhancement obtained experimentally with WNS broadening is reasonably lower than the theoretical value. We attribute this to the increased SBS seeding due to the overlap between the WNS broadened signal and the Brillouin gain spectrum. A modulation scheme that is implemented easily and provides adequate line shape control would be of great advantage from a practical and engineering point of view. In this work, we propose a simple, yet powerful modulation technique to synthesize a line shape to have fast roll-off and improved flatness by incorporating dual sine and noise modulation. An in-house built kW-class, polarization maintaining, multi-stage Ytterbium-doped fiber amplifier is used to quantify the SBS enhancement of the proposed modulation scheme. We experimentally compare the results to that of pure noise broadened modulation at similar RMS linewidths and demonstrate over 2.3x enhancement in SBS limited output power at ~7.3GHz and <1kW SBS unlimited output power at ~10.4GHz in a fully polarization maintained system.
Temporal coherence control is a vital tool to achieve power scaling by overcoming stimulated Brillouin scattering (SBS). Conventional methods through phase modulation, with a sinusoidal drive or white noise source are limited in the achievable linewidth tunability due to the bandwidth of the RF drive and/or power handling of the phase modulator. Linewidth tuning range can be further increased by phase modulation amplification through cascaded four-wave mixing between multiple input lasers. When the input lasers are correlated, linewidth does not change with mixing order. However, in the case of uncorrelated input lasers, the linewidth increases progressively with increase in cascaded order. In this work, continuous linewidth tuning is achieved in two parts. From single frequency to ~7GHz, a single input laser at the required wavelength is line-broadened with phase modulation driven by a filtered white-noise source. Beyond this, two uncorrelated pumps are chosen, similarly broadened, amplified and sent through highly nonlinear fiber which performs cascaded four-wave mixing based phase modulation amplification. A demultiplexer extracts the required cascaded order and the two input lasers are tuned in wavelength appropriately to ensure the output center wavelength is constant. With the two effects together, continuous linewidth tuning at a single C-band wavelength from single frequency to more than 30GHz is achieved. The upper limit can be enhanced with power scaling. These principles can be translated to 1064nm wavelength, relevant to power combining and SBS limited power scaling of Ytterbium lasers using a combination of 1064nm phase modulators and nonlinear PCFs for cascaded four-wave mixing.
High repetition rate pulsed lasers are used for applications such as highspeed optical communications, nonlinear optics and optical sampling. Conventionally, mode locked lasers are used as pulsed sources. However, they suffer from low repetition rate that is not tunable. Electro-optic modulation allows generation of frequency combs with tunable high repetition rate. These combs can be compressed to generate pulses, but the pulse widths are large owing to the limited bandwidth of electro-optic frequency combs. Though, modulators can be cascaded to scale the bandwidth, it is only a linear enhancement and requires additional RF components. Spectral broadening of these combs in nonlinear fibers results in marginally improved bandwidths. The broadening achieved at a given optical power can be enhanced several times by suitably modifying the temporal profile of the comb before spectral broadening with a pulse-shaper. In this work, electro-optic intensity and phase modulators are driven at 25GHz to generate an initial comb. A pulse-shaper adaptively optimises the temporal profile of the electro-optic frequency comb to enhance the spectral broadening in highly nonlinear fiber (HNLF). The optimised comb is power scaled in an Er-Yb co-doped fiber amplifier before HNLF. The adaptively optimised comb is compressed with single mode fiber and is characterized with zero delay implementation of spectral shearing interferometry to obtain the spectral phase and temporal profile of the pulses. The output pulses have 1 dB bandwidth of ~0.6 ps and 3 dB bandwidth of ~ 1 ps with high repetition rate of 25GHz. This adaptive technique is shown to be immune to drifts and changes in modulator drive conditions.
Narrow linewidth fiber lasers find widespread applications in beam combining, frequency conversion and remote detection. Power scaling of these lasers is mainly limited by Stimulated Brillouin scattering (SBS). Currently, SBS is mitigated through linewidth broadening and/or fibers with enhanced mode area. The latter suffers from problems of beam degradation and modal instability making line broadening the primary technique for SBS suppression. Line broadening can be achieved with phase modulation of lasers using white noise, pseudo-random bit streams or arbitrary waveform generators. The simplest implementation is with white noise source with the latter two requiring greater resources. We recently demonstrated a 10GHz linewidth 0.5kW polarization maintaining fiber laser, where it was observed that the SBS threshold did not directly scale with linewidth. This effect was identified as arising from the slow roll-off of the spectrum in white-noise modulated spectra which seeds the SBS process. The seeding is due to the reflections from the fiber end facet at these broadened linewidths where the spectrum has appreciable power at the Stokes wavelength. This is anticipated to be fundamental limiter for power scaling of narrow linewidth fiber lasers. In this work we overcome these drawbacks through a simple phase modulation scheme that incorporates noise waveforms together with sinusoidal modulation. This enables the spectrum to have sharp roll-off with flatter central region resulting in substantial reduction in seeding of SBS from end facet. With this simple architecture, we demonstrate scaling of SBS limited power by more than 1.5 times over pure noise modulation.
High power polarised narrow linewidth sources are of immense importance in coherent beam combining (increased path length accuracy), spectral beam combining (reduced angular spread), non-linear frequency conversion (increased parametric gain) and remote detection. Power scaling of narrow linewidth sources is primarily limited by Stimulated Brillouin Scattering which can be overcome by spectral broadening before amplification to higher powers. Various applications have different requirements of spectral purity and power which can be met if the source line width is tunable. We demonstrate a narrow line width polarisation maintaining (PM) laser with continuous linewidth tuning from ~2.88 GHz to ~9.88 GHz with over 20 W of output power with a polarisation extinction ratio (PER) greater than 20 dB. We achieve continuous linewidth tuning through pure phase modulation of a 1064 nm DBR laser with a white noise source whose bandwidth and power are tuned with a low pass filter bank and variable attenuation of the drive voltage to the modulator. This dual control mechanism enables gapless tuning of the linewidth of the source. A cascade of optimized PM Yb-doped fiber preamplifier and power amplifier is used to scale the output power to over 20W. Combined with the wavelength tuning of the DBR seed, the tunable spectral width makes it versatile for use in a wide range of applications.
We report the surprising observation of yellow to red visible light flashes in the splice point connecting the seed stage to the power amplifier in a high power, narrow-linewidth, polarisation-maintaining Ytterbium doped fiber laser. The multistage laser delivers upto 500W of power with a tuneable linewidth between 2.88 GHz to 9.88 GHz at 1064 nm. For different linewidths, the visible flashes were observed at different power levels of the laser. We observed a strong correlation between these flashes to the appearance of backward pulses with the onset of Stimulated Brillouin Scattering (SBS). We identify the cause for the flashes to be a two part phenomena. Beyond a threshold level, SBS results in the formation of high peak power pulses. These pulses undergo cascaded Raman scattering into higher order stoke wavelengths. These higher order pulses are unaffected by the isolator separating the amplifier stages and moves back into the seed stage with lower effective area, higher NA fibers. We recently demonstrated that 2nd and 3rd harmonic generation can occur in high NA, low effective area fibers assisted by Cherenkov-type phase matching between core light in the NIR and cladding light in the visible. Through processing of the images of the flash acquired with high resolution, we identified the wavelengths to be a mixture of the second harmonic components of the 2nd and 3rd order Raman Stokes of the 1064 nm laser wavelength (1175nm/588nm and 1240nm/620nm). We anticipate the use of these flashes as a potential monitor for the onset of SBS.
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.
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.
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.
Continuous-wave(CW) supercontinuum sources find applications in various domains such as imaging, spectroscopy, test and measurement. They are generated by pumping an optical fiber with a CW laser in the anomalous-dispersion region close to its zero-dispersion wavelength. Modulation instability(MI) sidebands are created, and further broadened and equalized by additional nonlinear processes generating the supercontinuum. This necessitates high optical powers and at lower powers, only MI sidebands can be seen without the formation of the supercontinuum. Obtaining a supercontinuum at low, easily manageable optical powers is attractive for many applications, but current techniques cannot achieve this. In this work, we propose a new mechanism for low power supercontinuum generation utilizing the modified MI gain spectrum for a line-broadened, decorrelated pump. A novel two-stage generation mechanism is demonstrated, where the first stage constituting standard telecom fiber slightly broadens the input pump linewidth. However, this process in the presence of dispersion, acts to de-correlate the different spectral components of the pump signal. When this is sent through highly nonlinear fiber near its zero-dispersion wavelength, the shape of the MI gain spectrum is modified, and this process naturally results in the generation of a broadband, equalized supercontinuum source at much lower powers than possible using conventional single stage spectral broadening. Here, we demonstrate a ~0.5W supercontinuum source pumped using a ~4W Erbium-Ytterbium co-doped fiber laser with a bandwidth spanning from 1300nm to 2000nm. We also demonstrate an interesting behaviour of this technique of relative insensitivity to the pump wavelength vis-a-vis zero-dispersion wavelength of the fiber.
A simple and powerful method using continuous wave supercontinuum lasers is demonstrated to perform spectrally resolved, broadband frequency response characterization of photodetectors in the NIR Band. In contrast to existing techniques, this method allows for a simple system to achieve the goal, requiring just a standard continuous wave(CW) high-power fiber laser source and an RF spectrum analyzer. From our recent work, we summarize methods to easily convert any high-power fiber laser into a CW supercontinuum. These sources in the time domain exhibit interesting properties all the way down to the femtosecond time scale. This enables measurement of broadband frequency response of photodetectors while the wide optical spectrum of the supercontinuum can be spectrally filtered to obtain this information in a spectrally resolved fashion. The method involves looking at the RF spectrum of the output of a photodetector under test when incident with the supercontinuum. By using prior knowledge of the RF spectrum of the source, the frequency response can be calculated. We utilize two techniques for calibration of the source spectrum, one using a prior measurement and the other relying on a fitted model. Here, we characterize multiple photodetectors from 150MHz bandwidth to >20GHz bandwidth at multiple bands in the NIR region. We utilize a supercontinuum source spanning over 700nm bandwidth from 1300nm to 2000nm. For spectrally resolved measurement, we utilize multiple wavelength bands such as around 1400nm and 1600nm. Interesting behavior was observed in the frequency response of the photodetectors when comparing broadband spectral excitation versus narrower band excitation.
Demand for bandwidth in optical communications necessitates the development of scalable transceivers that cater to these needs. For this, in DWDM systems with/without Superchannels, the optical source needs to provide a large number of optical carriers. The conventional method of utilizing separate lasers makes the system bulky and inefficient. A multi-wavelength source which spans the entire C-band with sufficient power is needed to replace individual lasers. In addition, multi-wavelength sources at high repetition rates are necessary in various applications such as spectroscopy, astronomical spectrograph calibration, microwave photonics and arbitrary waveform generation. Here, we demonstrate a novel technique for equalized, multi-wavelength source generation which generates over 160 lines at 25GHz repetition rate, spanning the entire C-band with total power >700mW. A 25GHz Comb with 16 lines is generated around 1550nm starting with two individual lasers using a system of directly driven, cascaded intensity and phase modulators. This is then amplified to >1W using an optimized, Erbium-Ytterbium co-doped fiber amplifier. Subsequently, they are passed through Highly NonLinear Fiber at its zero-dispersion wavelength. Through cascaded Four Wave Mixing, a ten-fold increase in the number of lines is demonstrated. A bandwidth of 4.32 THz (174 lines, SNR>15 dB), covering the entire C-band is generated. Enhanced spectral broadening is enabled by two key aspects - Dual laser input provides the optimal temporal profile for spectral broadening while the comb generation prior to amplification enables greater power scaling by suppression of Brillouin scattering. The multi-wavelength source is extremely agile with tunable center frequency and repetition rate.
DWDM with/without super-channel based photonic networks require the use of optical carriers with equalized amplitudes and frequency stabilization of adjacent carriers to realize reliable high bandwidth optical communication systems with high spectral efficiency and long reach. Cascading of electro-optic (EO) modulators is a versatile method for generating tuneable, high repetition rate frequency combs which can be used as sources for the carriers. However, the number of lines produced with this technique is limited by the number of phase modulators. Nonlinear spectral broadening is an attractive option for bandwidth scaling; however, bandwidth scaling of single carrier combs through four wave mixing suffers from unequal comb lines or power limitations due to Brillouin scattering. A simpler technique to increase the number of comb lines would involve using multicarrier excitations for comb generation which would result in a proportional increase in the comb lines. Further, dual-carrier excitation enables an excellent temporal profile for nonlinear spectral broadening. However, since the two carriers have uncorrelated drifts, the resultant frequency combs would be unsuitable for most applications. This issue can be overcome by frequency offset locking the two lasers. Here, we demonstrate frequency offset locking (MHz accuracy) of two diode lasers spaced by 100GHz by using an optical phase locked loop which locks one laser to a RF harmonic of the other. This allows for the generation of frequency comb lines locked to each other even post nonlinear broadening. Using this technique, we demonstrate a 25GHz frequency comb with >90 lines (2THz) in the C-band.
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