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We demonstrate a high power high efficiency Raman fiber laser pumped directly by a laser diode module at 976 nm. 80 Watts of CW power were obtained at a wavelength of 1020 nm with an optical-to-optical efficiency of 53%. When working quasi-CW, at a duty cycle of 30%, 85 W of peak power was produced with an efficiency of 60%. A commercial graded-ind...
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Context 1
... experimental laser setup used is shown in figure 1. A laser diode module providing up to 150 W of a polarized free-space collimated output power at 976 nm with a spectral bandwidth of ~3 nm is coupled into a GRIN fiber. ...
Context 2
... beam cross section at the laser diode module out- put window is imaged onto the focusing lens L3 with two 300 mm focal length lenses (L1 and L2). L3 is an aspheric lens ( f = 11 mm) and focuses the pump beam onto the left end (see figure 1) of the GRIN fiber. The Raman GRIN fiber is a commercial MM Thorlabs GIF625, with core/clad dimen- sions of 62.5/125 μm, a core NA 0.275 and a length of 500 m. ...
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Citations
... Conventional mechanisms to suppress higher order modes have been attempted in BE-RFLs in SIF. Bend loss has not shown significant improvement [7], and using mode-selective fiber Bragg gratings (FBGs) has still not achieved diffraction limited beam quality at high power [6,7,11,[13][14][15]. While it was originally posited that this was due to higher order core modes [7], there is increasing evidence that it is due to the Raman conversion of cladding modes [6,10,13]. ...
Cladding-pumped Raman lasers have often been cited for their potential for brightness enhancement, but so far have never achieved high power and beam quality simultaneously. By utilizing a fiber geometry with a larger cladding to core ratio than current high power Raman fiber lasers for brightness enhancement (BE-RFLs), a noise-seeded Raman fiber laser pumped by a 100 ns pulsed laser with record beam quality is achieved with ${{{M}}^2} = {1.3}$ M 2 = 1.3 and 0.4 mJ first Stokes output, with an instantaneous brightness enhancement of 60. The results here support the hypothesis that the limiting factor to high beam quality in BE-RFLs is Raman conversion in the pump cladding, rather than higher order core modes as in most conventional fiber lasers.
... Conventional mechanisms to suppress higher order modes have been attempted in BE-RFLs in SIF. Bend loss has not shown significant improvement [7], and using mode-selective fiber Bragg gratings (FBGs) has still not achieved diffraction-limited beam quality at high power [6,7,11,[13][14][15]. While it was originally posited that this was due to higher order core modes [7], there is increasing evidence that it is due to the Raman conversion of cladding modes [6,10,13]. ...
Cladding-pumped Raman lasers have often been cited for their potential for brightness enhancement, but so far have never achieved high power and beam quality simultaneously. By utilizing a novel fiber geometry with a larger cladding to core ratio than current high power Raman fiber lasers for brightness enhancement (BE-RFLs), a noise-seeded Raman fiber laser pumped by a 100 ns pulsed laser with record beam quality is achieved with M^2 = 1.3 and 0.4 mJ 1st Stokes output, with an instantaneous brightness enhancement of 60. The results here support the hypothesis that the limiting factor to high beam quality in BE-RFLs is Raman conversion in the pump cladding, rather than higher order core modes as in most conventional fiber lasers.
... The power density in the fiber core is sufficient to induce generation of Stokes wave via SRS gain and cavity feedback in a relatively short fiber [4]. In recent time, a new approach to Raman generation has been proposed: the use of multimode fibers with a graded (e.g., parabolic) refractive index profile of the core, which can be directly pumped by high-power multimode laser diodes [2,5,6], including development of an all-fiber scheme [7]. The highest pumpto-Stokes brightness enhancement factor of 73 has been obtained at conversion of highly multimode (M 2~3 4) pump radiation of LD at~940 nm into a Stokes radiation at 976 nm with M 2 < 2 and power > 50 W in a graded-index (GRIN) fiber with 100-µm core [8,9]. ...
We present our recent experimental results on the pulsed regimes of Raman conversion of highly multimode laser diode (LD) pump radiation into the 1st and higher order Stokes radiation in multimode graded-index fibers. Three different linear cavities of Raman fiber laser with the modulation of losses (by acousto-optic modulator, AOM) or gain (by LD current) are explored and compared. An LD with wavelength of 976 nm is used for pumping enabling Raman lasing at wavelength of the 1st (1018 nm) and 2nd (1064 nm) Stokes orders. At ~27.2-kHz repetition rate corresponding to the laser cavity round-trip frequency (i.e., in the mode-locking regime), nanosecond pulses have been observed for both Stokes orders having the highest peak power of ~300 W in the scheme with bulk AOM and the shortest duration of 5–7 ns in the scheme with fiber-pigtailed AOM. At the same time, the beam quality of generated pulses is greatly improved as compared to that for pump diode (M2 > 20) reaching the best value (M2 = 2.05) for the 2nd order Stokes beam in the scheme with the gain modulation and demonstrating also the most stable regime.
... Meanwhile, the output powers of OPOs and non-diamond Raman lasers are yet to approach even hundreds of watts; restricted by thermal lensing in the gain materials. The only other technology competitive enough to pursue such power gains is the fiber-based Raman lasers [98,[101][102][103][104][166][167][168][169][170][171][172][173], as evident from the figure. ...
... Meanwhile, the output powers of OPOs and non-diamond Raman lasers are yet to approach even hundreds of watts; restricted by thermal lensing in the gain materials. The only other technology competitive enough to pursue such power gains is the fiber-based Raman lasers [98,[101][102][103][104][166][167][168][169][170][171][172][173], as evident from the figure. As already discussed, power enhancement also brings in an additional requirement of good beam quality. ...
Optical parametric oscillators (OPOs) and Raman lasers are two nonlinear-based laser technologies that extend the spectral range of conventional inversion lasers. Power and brightness scaling of lasers are significant for many applications in industry, medicine, and defense. Considerable advances have been made to enhance the power and brightness of inversion lasers. However, research around the power scaling of nonlinear lasers is lacking. This paper reviews the development and progress of output power of continuous-wave (CW) crystalline OPOs and Raman lasers. We further evaluate the power scalability of these two laser technologies by analyzing the cavity architectures and gain materials used in these lasers. This paper also discusses why diamond Raman lasers (DRLs) show tremendous potential as a single laser source for generating exceedingly high output powers and high brightness.
... Recently, a direct LD pumping in RFL scheme was successfully implemented in telecom multimode graded-index fibers (GIF) [7][8][9]; see also [10,11] for a review of the main results obtained. It appears possible to efficiently convert highly multimode LD radiation into a Stokes beam with improved beam quality and brightness, thanks to the Raman beam cleanup effect arising from the specific transverse structure of SRS gain in graded-index fibers [12,13]. ...
We review our recent experimental results on the cascaded Raman conversion of highly multimode laser diode (LD) pump radiation into the first- and higher-order Stokes radiation in multimode graded-index fibers. A linear cavity composed of fiber Bragg gratings (FBGs) inscribed in the fiber core is formed to provide feedback for the first Stokes order, whereas, for the second order, both a linear cavity consisting of two FBGs and a half-open cavity with one FBG and random distributed feedback (RDFB) via Rayleigh backscattering along the fiber are explored. LDs with different wavelengths (915 and 940 nm) are used for pumping enabling Raman lasing at different wavelengths of the first (950, 954 and 976 nm), second (976, 996 and 1019 nm) and third (1065 nm) Stokes orders. Output power and efficiency, spectral line shapes and widths, beam quality and shapes are compared for different configurations. It is shown that the RDFB cavity provides higher slope efficiency of the second Stokes generation (up to 70% as that for the first Stokes wave) with output power up to ~30 W, limited by the third Stokes generation. The best beam quality parameter of the second Stokes beam is close to the diffraction limit (M2~1.3) in both linear and half-open cavities, whereas the line is narrower (<0.2 nm) and more stable in the case of the linear cavity with two FBGs. However, an optimization of the FBG reflection spectrum used in the half-open cavity allows this linewidth value to be approached. The measured beam profiles show the dip formation in the output pump beam profile, whereas the first and second Stokes beams are Gaussian-shaped and almost unchanged with increasing power. A qualitative explanation of such behavior in connection with the power evolution for the transmitted pump and generated first, second and third Stokes beams is given. The potential for wavelength tuning of the cascaded Raman lasers based on LD-pumped multimode fibers is discussed.
... The realization of Laser Diode (LD)-pumping brought about the rapid power ramping in GRIN Raman lasers. Nevertheless, the complex coupling and reflection of laser in the free-space configuration resulted in limitation of either the output efficiency or the beam quality [30][31][32][33]. The all-fiber configuration is accomplished by the manufacturing of fiberized combiner and fiber Bragg gratings (FBGs) based on GRIN fiber [34]. ...
Raman fiber lasers (RFLs) are currently promising and versatile light sources for a variety of applications. So far, operations of high power and brightness-enhanced RFLs have absorbed enormous interests along with rapid progress. Nevertheless, the stable Raman lasing at high power levels remains challenged by the thermal effects. In an effort to realize more effective thermal management in high power RFLs, here we demonstrate, for the first time, an all-fiberized RFA employing metal-coated passive fiber enabling high power lasing. By employing aluminum to the cladding of graded-index (GRIN) passive fiber, the thermal abstraction of the laser devices is more sufficient to support low-temperature operation. The maximum output power reaches 3.083 kW at 1130 nm with a conversion efficiency of 78.7%. To the best of our knowledge, this is the first Raman laser generation based on metal-coated passive fiber. Meanwhile, it is also the highest power attained in the fields of all kinds of Raman lasers based on merely nonlinear gain.
... Raman conversion in multimode graded-index (GRIN) fibers is accompanied by sufficient improvement of the output beam quality in comparison with that for pump radiation which is known as Raman beam clean-up effect [1]. This effect offers opportunities to create amplifiers and lasers of new type based on directly 9xx-nm diode pumped or tandem pumped passive GRIN fibers with fiber Bragg gratings (FBGs), which may operate at almost any wavelength in transmission window (0,95-2 micron) of silica fibers [2][3][4][5][6][7][8]. ...
Raman conversion in multimode GRIN fibers is accompanied by sufficient improvement of the output beam quality in comparison with that for pump radiation, which offers opportunities to create amplifiers and lasers of new type based on directly 9xx-nm diode pumped passive GRIN fibers, which may operate at almost any wavelength in transmission window (0.95-2 micron) of silica fibers. Here we review experimental results and physical mechanisms leading to the beam quality improvement (beam cleaning) in multimode Raman lasers and amplifiers, including effects of Raman gain, fiber Bragg gratings and Rayleigh backscattering feedback in GRIN fibers.
... In [9], using 976 nm pump diodes, a 20 W Raman laser was demonstrated, which generates radiation at a wavelength of ~1020 nm with a beam quality of M 2 ~ 5. In [10,11] close beam qualities (M 2 ~ 4-5) but significantly higher powers of 80 and 154 W were obtained at the same wavelength. The relatively poor quality of the output beam in [9][10][11] is a consequence of the use of bulk mirrors in the cavity. ...
... In [10,11] close beam qualities (M 2 ~ 4-5) but significantly higher powers of 80 and 154 W were obtained at the same wavelength. The relatively poor quality of the output beam in [9][10][11] is a consequence of the use of bulk mirrors in the cavity. Using special FBGs as resonator mirrors, the beam quality close to the diffraction limit (M 2 ~ 1.2) was obtained for 10 W of output power at 954 nm with a free space coupling of pump LD radiation at 915 nm [12]. ...
... At the same time, the enhancement factor for brightness defined as P/(M 2 λ) 2 at the conversion of the diode pump to the Raman laser radiation amounts to about 68. Such a beam quality and brightness enhancement factor are the best results for single-stage Raman lasers with a highly multimode GRIN fiber with diode pumping [8][9][10][11][12][13][14], despite the worse beam quality of the pump diodes used. This result was achieved by improving the quality of the FBGs, providing better discrimination of higher transverse modes. ...
... It should be noted that with the increasing of pump power, another peak at 1070 nm appeared besides the main peak at 1064 nm. It could be ascribed to that the Raman gain spectrum of Yb doped fiber had two peaks whose shift frequencies were 13 and 14.7 THz [28], [29]. Owing to the high output power, we measured the beam quality with β F L factor [30]. ...
... Higher powers were produced by Glick et al. [36,37] where up to 154 W with efficiency of 65% at 1020 nm were demonstrated, with diode pumps and bulk mirrors as shown in Fig. 3. ...
... Zlobina) et al. [41] increased the output power to 62 W and the efficiency to~30% with a BE of 25, by double passing the pump, using specially designed FBGs that reflected the Stokes and the pump. Double passing the pump to increase the efficiency has been demonstrated before [25,36,37,42] with bulk mirrors. However in [41] it is accomplished for the first time in an all-fiber configuration. ...
... As the pump power is increased, the SRS lasing threshold is also reached in more peripheral regions of the fiber core, so those areas also begin to participate in Raman laser conversion and the effective Raman beam diameter in the GRIN core grows larger, resulting in poorer beam quality. This causes an increase in M 2 (and consequently a lower BE) for higher pump powers in GRIN-core Raman fiber lasers, as has been seen in many studies [31,36,37]. The increasing M 2 at high powers is generally more pronounced in configurations which incorporate bulk mirrors since they tend to support all the fiber modes. ...
A large variety of high power, high brightness, rare earth doped fiber lasers have been demonstrated throughout recent years. Several obstacles, such as modal instabilities, heat removal and photo-darkening, have been identified as the limiters and have complicated the further power increase. Raman fiber sources have inherent advantages in regard to these challenges and therefore may offer a high power high brightness alternative. Up to over kW power Raman fiber laser and amplifier sources have recently been demonstrated with multimode graded index fiber (1002 W) as well as with multi-clad clad-pumped fiber (1200 W) in brightness enhancing configurations. In these schemes, the output Stokes-shifted-signal obtained higher brightness than the input pump source, due to the higher Raman gain associated with the lower order modes. A review of the recent results obtained with various configurations is presented, followed by discussion on the challenges and the paths towards further increase in power and brightness towards achieving near diffraction limited multi-kW Raman fiber sources.
