We demonstrate a low relative intensity noise (RIN) (⪅-125dBc/Hz from 1 MHz to 10MHz), Continuous Wave (CW) Supercontinuum (SC) generation in standard telecom fiber by using a low intensity noise 1064nm Ytterbium (Yb) doped fiber amplifier as pump source. The generated SC has ⪆8dB of RIN improvement (over a bandwidth of 300nm from 1400nm to 1700nm) when compared to the RIN of SC generated by a conventional Fiber Bragg Grating (FBG) cavity based Yb fiber pump laser. Integrated Root Mean Square (RMS) RIN of 0.16 % (from 1MHz to 10MHz) was better than the low noise CW supercontinua reported so far.
Cascaded Raman fiber laser (CRFLs) based on random distributed feedback (RDFB) are proven to be simple, wavelength agile and power scalable technology. However, they are limited in terms of spectral purity where a small amount of output power resides in unwanted Stokes orders. This is due to the inherent intensity noise of pump source used for Raman conversion, which gets transferred and amplified to multiple Stokes orders. Since each Stoke’s signal acts as a pump for the next Stoke’s signal, spectral purity keeps reducing with increasing Stokes order. Further, this increases spectral broadening in these systems due to nonlinear effects of self-phase modulation (SPM) and Cross-Phase Modulation (XPM), thereby, limiting the narrow linewidth operation. Broad spectral width in CRFLs is attributed to 1) inherent broad (~40THz) Raman gain spectrum, and to 2) nonlinear spectral broadening through SPM and XPM. By employing narrow bandwidth spectral filters for feedback and low intensity noise sources for the pump, narrow linewidth operation can be achieved. Here, we demonstrate a low-intensity noise (<-101dBc/Hz from 9kHz to 10GHz) CRFL with <= 99% spectral purity over 6 Stokes orders. A low-intensity noise (<-147dBc/Hz from 9kHz to 10GHz), Narrow linewidth (DBR, 5MHz) 1064nm laser as a pump source. It is line-broadened to ~50GHz bandwidth through phase modulation with a noise source and a sinusoidal signal to suppress stimulated Brillouin scattering (SBS). It is then amplified to 73W using amplifier chain and a passive Raman fiber of length 340m is used for Raman conversion. This work paves a way to achieve narrow linewidth CRFLs with very high conversion efficiency.
Cascaded Raman fiber lasers is the only proven technology which enables high power continuous wave (CW) fiber laser sources outside rare-earth emission bandwidths. Among these systems, recently demonstrated cascaded Raman fiber lasers based on Random distributed feedback (RDFB) provide wavelength agility enabling high-power, ultra-broad band wavelength tunable fiber lasers. However, these systems are limited in terms of power scaling due to degrading spectral purity with increasing input pump power which in turn limits the applicability of these systems. This is due to conversion of desired wavelength into next higher order Raman stokes with increase in pump power. In this work, we demonstrate a high power, ultra-high spectral purity, broadly wavelength tunable cascaded Raman fiber laser. This was enabled by culmination of two significant advances over the last year. To terminate the Raman cascade at the required output wavelength, we utilized our recently proposed distributed filtered feedback mechanism. To achieve high spectral purity, we used a recently demonstrated technique of using high power fiber based Amplified Spontaneous emission (ASE) sources as the input pump. A maximum output power of ~33W at 1.5μm and ~27W at 1.4μm was achieved. High spectral purity of <97% at the final wavelength was achieved over a wide-range of output powers.
Measuring the profile of a laser beam is of critical importance, especially for high power laser systems. Although different techniques exist to measure the beam profile, owing to the use of optoelectronic detectors or cameras, they primarily work at lower powers and require tapping and attenuating the beam. In this process, there is potential for the diagnostic system affecting the beam quality. In this work, we propose a simple technique which can measure the beam profile at full power using a thermal imager without the need for additional optical components. The method involves taking a thermal image of the beam while it is incident on an absorptive surface such as a thermopile head which is used to measure optical power. In addition, a second image is taken using a focused incidence on the surface at low powers. The second image which is reused provides the point spread function. We then make use of the linearity of the heat equation which allows the deconvolution of the point spread function from the original image to obtain the actual beam profile. In this work, we utilized the technique to directly analyze the beam profile at full power of a 100 W class fiber laser and analyzed deviations from single-modedness. In addition, we utilized offset splices to few-mode fibers to launch higher order modes at the 100W level and demonstrate their direct characterization of multimode nature of the profile. This technique provides a simple alternative, using instruments present in most laser labs for direct, high power laser beam profiling.
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.
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.
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.
In this work, we demonstrate an architecture to perform Raman-based power combining and simultaneous wavelength conversion of two independently controlled high-power Ytterbium doped fiber lasers operating at different wavelengths into a single laser line at the 1.5-micron band. Specifically, we have been able to achieve an in-band output power of ∼99W with a conversion of ∼64% of the quantum limited efficiency. This power combining is illustrated for different cases of the input wavelengths of the Ytterbium fiber laser. In each case, we have been able to demonstrate a power combining of >87 W in the final 1.5-micron band, with more than 85% of the fraction of the power residing in the final desired band.
We have demonstrated a ~34 W continuous wave supercontinuum using the standard telecom fiber (SMF 28e). The supercontinuum spans over a bandwidth of ~1000 nm (>1 octave) from 880nm to 1900 nm with a substantial power spectral density of >1mW/nm from 880-1350 nm and ~50-100mW/nm in 1350-1900 nm. The distributed feedback Raman laser architecture was used for pumping the supercontinuum which ensured high efficiency Raman conversions and helped in achieving a very high efficiency of ~44% for supercontinuum generation. Using this architecture, Yb laser operating at any wavelength can be used for generating the supercontinuum and this was demonstrated by using two different Yb lasers operating at 1117nm and 1085 nm to pump the supercontinuum.
Cascaded Raman lasers enable high powers at various wavelength bands inaccessible with conventional fiber lasers. However, the input and output wavelengths are fixed by the multitude of fiber gratings in the system providing feedback. In this work, we demonstrate a high power, tunable, grating-free cascaded Raman fiber laser with an output power of >30W and a continuous tuning range from 1440nm to 1520nm. This corresponds to the entire in-band pumping region of Erbium doped gain media. Our system is enabled by three novel aspects – A grating free feedback mechanism for Raman lasers, a filter fiber to terminate the Raman cascade at the required wavelength band and a tunable high-power Ytterbium doped fiber laser as input. In this work, the primary system is a novel, cascaded Raman conversion module which is completely color blind to the input pump source and does wavelength band conversion at high efficiency. In addition, the conversion module also provides high spectral purity of greater than 85% at the required wavelength by terminating the cascade using high distributed losses provided by specialty Raman filter fibers. Using a high-power Ytterbium doped fiber laser continuously tuned from 1060nm to 1100nm and Raman filter fiber with distributed loss beyond 1520nm, we achieve a continuously tunable 1440nm to 1520nm laser corresponding to 5th or 6th Raman Stokes shift of the input. To the best of our knowledge, the reported powers at these wavelengths have been the highest for tunable Raman fiber lasers and is currently only limited by the input power.
In this work, we report and analyse the surprising observation of a rainbow of visible colors, spanning 390nm to 620nm, in silica-based, Near Infrared, continuous-wave, cascaded Raman fiber lasers. The cascaded Raman laser is pumped at 1117nm at around 200W and at full power we obtain ∼100 W at 1480nm. With increasing pump power at 1117nm, the fiber constituting the Raman laser glows in various hues along its length. From spectroscopic analysis of the emitted visible light, it was identified to be harmonic and sum-frequency components of various locally propagating wavelength components. In addition to third harmonic components, surprisingly, even 2nd harmonic components were observed. Despite being a continuous-wave laser, we expect the phase-matching occurring between the core-propagating NIR light with the cladding-propagating visible wavelengths and the intensity fluctuations characteristic of Raman lasers to have played a major role in generation of visible light. In addition, this surprising generation of visible light provides us a powerful non-contact method to deduce the spectrum of light propagating in the fiber. Using static images of the fiber captured by a standard visible camera such as a DSLR, we demonstrate novel, image-processing based techniques to deduce the wavelength component propagating in the fiber at any given spatial location. This provides a powerful diagnostic tool for both length and power resolved spectral analysis in Raman fiber lasers. This helps accurate prediction of the optimal length of fiber required for complete and efficient conversion to a given Stokes wavelength.
We demonstrate a simple to implement, drive scheme for standard laser diode modules (without wavelength locking) used for pumping rare-earth doped lasers and amplifiers. This scheme enables an “always-resonant” mode of operation. The deleterious effect accompanying power/current tuning - drifts of emission wavelength of the diodes from the peak absorption band of the gain medium is completely avoided. In this work, we demonstrate the drive mechanism and its performance in a fiber amplifier. We anticipate this scheme to have significant impact in enabling a cost-effective solution which achieves an optimal balance of efficiency, nonlinearity and reliability in laser systems.
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