We report on the first demonstration of an all-fiber CW Raman laser based on a multimode graded-index fiber directly pumped by multimode fiber-coupled laser diodes. A joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of the graded-index fiber core, results in high-efficiency conversion of a multimode (M2~26) pump at 915 nm into a high-quality (M2~2.6) output beam at 954 nm. About 50 W output power has been obtained with slope efficiency of 67%. The proposed development and integration of key multimode fiber technologies opens the door to new type of LD-pumped high-power high-beam-quality fiber lasers that may operate at almost any wavelength defined by available LDs.
© 2017 Optical Society of America
Rapid development of high-power CW fiber lasers in new millenium resulted in their outstanding performance in terms of beam quality, output power and efficiency enabling wide use in industrial applications. Most of high-power lasers utilize rare-earth (RE) doped double-clad fibers pumped by high-power multimode laser diodes (LDs) into the fiber cladding. The low-quality pump radiation is guided by the multimode inner silica cladding and absorbed in the single-mode RE-doped core which then re-emits the high-quality laser light in the RE emission band, see  for a review. The most powerful Yb-doped fiber laser (YDFL) is able to deliver 10 kW diffraction-limited output beam at ~1.07 μm [2, 3]. This became possible due to the implementation of pump combining and tandem pumping technologies in addition to earlier developed cladding pumping of RE-doped fibers, see [1, 4] for a review.
In order to achieve high pump power, single- or several-emitter diode sources typically pigtailed with a 100-μm multimode fiber are connected by means of fiber pump combiner with common output port before being launched into the RE-doped fiber . There exist also alternative approaches to combine multiple LD radiation, such as GTWave technology based on multi-cladding fibers, see  and citation therein. Although direct diode-pumping is potentially scalable to 10 kW level , all nearly single-mode RE-doped fiber lasers with power ≥3 kW demonstrated to the moment, use a tandem pumping scheme. In such a scheme one or several LD-pumped high-power fiber lasers pump another one . The tandem pumping offers several advantages for power scaling. In particular, it makes possible in-band pumping (close to the emission wavelength) so that the quantum defect heating can be low thus resulting in a reduced thermal load and reduced dimensions of the inner cladding, for more details see [1, 2]. In such a way, 10 kW IPG fiber laser [2,3] was pumped by LD-pumped YDFLs at 1018 nm and emitted at 1070 nm with quantum defect of less than 5%. It is estimated  that several tens kW output is feasible with the tandem pumping, but a further limitation caused by the development of mode instability comes into play .
Laser wavelength flexibility is another important feature of fiber lasers, e.g. they can be tuned by ~100 nm within the spectral band of RE dopants (1-1.15, 1.5-1.65 and 1.9-2.1 μm for Yb, Er and Tm/Ho, accordingly [1–9]). To go beyond the wavelength range of RE elements, stimulated Raman scattering in passive fibers is used, see  for a review. Raman fiber lasers (RFLs) are usually based on single-mode fibers core-pumped by YDFLs or other high-power RE-doped fiber lasers with single transverse mode output that leads to a rather complicated design of RFLs. However, Raman lasers are characterized by a small quantum defect, low background spontaneous emission and absence of photo-darkening effect, which is a problem for RE-doped fiber lasers, especially for YDFLs operating near 1 μm . YDFL-pumped RFLs deliver more than 100 Watts of output power for the first  and higher Stokes orders generated in the nested cavities made of fiber Bragg grating (FBG) pairs (for all intermediate Stokes wavelengths), either in single laser  or in master oscillator - power amplifier (MOPA)  configurations. Much simpler design (free of FBGs) utilizes broadband random distributed feedback via Rayleigh backscattering  providing high-efficiency cascaded generation of high Stokes orders, see  and citation therein. Power scaling of the Raman fiber laser to kW level is shown to be possible for an integrated Ytterbium-Raman fiber amplifier architecture . Injecting a seed laser beam into a 1080-nm Yb-doped fiber MOPA laser results in generation of 1.28 kW power at 1120 nm. Modification of this scheme results in 3.89 kW power even in the absence of seed . Nevertheless, all these schemes are based on a high-power YDFL with some extension necessary for its Raman conversion.
One of interesting possibilities to simplify the RFL design is its direct pumping by cheap and reliable high-power multimode LDs, similar to the design of LD-pumped RE-doped fiber lasers [1–9]. To continue the analogy, it is possible to employ cladding pumping, but it requires development of special double-clad passive fibers [18, 19]. In this way, Raman lasing at 1120 nm with output power of 100 W and beam quality parameter M2~1.6 was demonstrated with a multimode YDFL as a pump source , whereas at direct diode pumping only 6 W power was obtained . From technical and economical points of view, it seems to be more attractive to use standard graded-index (GRIN) multimode fibers. They currently have very high quality and low cost due to their wide use in local telecommunication networks. In this case, one can directly couple the multimode radiation of high-power LDs into the multimode GRIN fiber core. In addition, using commercially available multimode LDs with operating wavelengths of 915-940 nm, it is possible to obtain high-power Raman lasing in wavelength range of 950-980 nm, which is problematic for YDFLs. At the same time, the RFL beam quality may be high enough due to the beam clean-up effect at Raman conversion of CW radiation in GRIN fibers . Recently, short-pulse propagation in GRIN fibers also attracts much attention owing to quite interesting spatio-temporal nonlinear dynamics including Kerr self-cleaning of multimode beam [21, 22].
Developing the concept of GRIN fiber lasers, the first CW LD-pumped RFL operating below 1 μm has been demonstrated lately . The output beam generated at 980 nm exhibited much better quality than the LD pump beam at ~940 nm. Shorter wavelength generation at 954 nm was obtained in the 2.5-km GRIN fiber with LD pumping at 915 nm . A possibility to scale output power of the LD-pumped RFLs up to 20 W , 80 W  and 154 W  was demonstrated using short (1.5, 0.5 and 0.2 km, respectively) GRIN fibers and higher pump powers. However, these RFLs were pumped by 976-nm LDs providing Raman output at ~1020 nm that is not as interesting, because much higher power at this wavelength is available from Yb-doped fiber lasers. On the other hand, in spite of significant improvement of the beam quality at Raman conversion, generation remains to be far from singlemode one: M2 = 5 at 20 W for the typical multimode LD (M2 = 22)  and M2 = 2.9 at 40 W for the brighter pump LD (M2 = 12) . It was demonstrated that the beam quality becomes much better (M2~1.3 at 10 W power level) with the cavity mirrors made of fiber Bragg gratings inscribed by femtosecond (fs) pulses in the central part of GRIN fiber core, i.e. FBGs with special 3D structure . However, all these works used bulk optics for coupling the pump beam into the GRIN fiber core.
In this paper, we report on the proof-of-principle demonstration of GRIN-fiber Raman laser efficiently converting high-power multimode LD pump radiation into the high-quality output beam at the wavelength of ≤1μm in a simple and reliable all-fiber configuration integratimg three key multimode fiber technologies: fiber pump combining/coupling, GRIN fiber core pumping, and in-core fiber cavity formation by fs-inscribed FBGs having special 3D structure.
2. Experimental setup
Experimental setup is shown in Fig. 1. Output radiation of three high-power multimode LDs at wavelength of 915 nm is combined by a 3x1 multimode fiber pump combiner. The spliced combiner has 3 input ports with multimode (MM) 110-μm core step-index fiber coupled to LDs and output port made of MM GRIN fiber with 100-μm core. The output port is fusion spliced to the RFL cavity based on GRIN fiber with 85-μm or 62.5-μm core. The laser cavity was formed by highly-reflective (FBG1) and output (FBG2) gratings inscribed in the core of the same graded-index fiber. It appears that highly-reflective FBG1 has better power characteristics at CW UV inscription by interferometric technique, whereas output FBG2 provides better transverse mode selection being inscribed in the central part of GRIN fiber by point-by-point technique using fs pulses (for more details see ).
For the Raman laser characterization, its output radiation was collimated by lens L1 and separated by selective mirror M1 into the residual pump at 915 nm and the laser beam at 954 nm. Their powers were measured by power meters P1 and P2, respectively. Optical spectra were measured by optical spectrum analyzer (OSA) Yokogawa AQ6370. Beam quality parameter M2 was measured with a conventional scheme consisting of a focusing lens and a beam master head (Coherent BM-7) installed on a translation stage.
As a first step, experiments with 62.5-μm GRIN Raman fiber of optimal length (1.1-km)  were repeated in the new all-fiber configuration. In this case, the pump power from the output port of the pump combiner can be only partially coupled to the Raman fiber. For better coupling, a short 85-μm GRIN fiber piece was spliced to the combiner’s output port. The total losses for coupling of pump radiation to the 62.5-μm GRIN fiber including those at pump combiner and splices are measured to be 75%. Such high losses limit the available pump power. To reduce the coupling losses, similar experiments were conducted for GRIN Raman fiber of 85-μm core diameter with in-core FBGs. Their reflection spectra are shown in the inset of Fig. 1. In this case, the fiber was directly spliced to the pump combiner output port, so the losses decreased to 50%.
Figure 2(a) shows the measured residual pump and generated Stokes wave powers in the 1.1-km-long 62.5-μm GRIN fiber as a function of launched pump power in comparison with the data from  for pump configuration based on a single LD with bulk-optics coupling. One can see that the laser generation threshold increases from ~40 to 50 W in the new all-fiber configuration. Note that the values of transmitted pump power below the generation threshold are the same in both configurations thus confirming correctness of the comparison. The slope efficiency and the 2nd Stokes (996 nm) threshold increase from 38 to 47% and from 10 to 16 W of the generated power, respectively. The 2nd threshold increase is reasoned by the elimination of Fresnel reflection at the input fiber facet after the modification from free-space to all-fiber pump coupling. As a result, the maximum power at 954 nm increases to about 16 W at 82 W launched pump power, whereas the generation linewidth (at −3 dB level) is left at the level of 0.4 nm [Fig. 2(b)]. The beam quality worsens only slightly from M2≈1.3 for pump configuration with bulk optics. The worsening manifests in the appearance of additional peak at −10 dB level with wavelength of 953.5 nm corresponding to the nearest transverse mode [Fig. 2(b)]. At the same time, after the transition to all-fiber configuration, the RFL operation becomes much more stable.
The combined pump power of three 915-nm LDs is sufficient to observe Raman lasing at 954 nm in 85-μm GRIN fiber, for which the threshold is ~85W [Fig. 3(a)]. Moreover, 2nd Stokes threshold is also reached. It grows from ~135W to ~180W of coupled pump power at reducing fiber length from 1.95 to 1.5 km. At that, only slight increase of the 1st Stokes threshold (to ~90 W) is observed with the fiber shortening. As a result, maximum power at 954 nm grows from 21 to 49 W, while the residual pump power varies from 20 to 25 W only.
A comparison of the Raman lasing thresholds in the all-fiber configuration based on 85-μm GRIN fiber of optimum length (1.5 km in Fig. 3(a)) and 62.5-μm GRIN fiber [Fig. 2(a)] shows that the threshold grows by almost two times in proportion with the core area increase. However, the slope efficiency of pump-to-Stokes conversion is higher for the 85-μm GRIN fiber (67%) in spite of sufficiently stronger integral pump attenuation (compare slopes before the lasing threshold). It may be reasoned by a lower quality of the generated beam in this case. To check this, we measured the output beam profile and its radius as a function of distance along the beam waist [Fig. 3(b)]. Fitting of this dependence gives M2 = 2.6 for both axes at 32 W output power that is almost two times of the quality factor for the 62.5-μm fiber laser (M2~1.3), while the beam profile appears to be close to Gaussian. The measured M2 value is only slightly (<10%) varying with power in 10-40 W range, but sufficiently grows with the fiber lengthening (M2≈3.1 for 1.95 km). We have also measured parameter М2 for the transmitted pump radiation, which amounts to М2≈26. It is significantly higher than the value measured in 62.5-μm GRIN fiber (М2≈21) without pump combiner .
Evolution of the generated spectra with increasing power at ≈954 nm in 1.5-km fiber is shown in Fig. 4(a). The shape of generated spectrum is close to the hyperbolic secant with exponential wings. Its width grows as square root of the power (both for −3 dB and −10 dB levels, corresponding best-fit curves are shown in Fig. 4(b)), just like the spectrum of conventional Raman lasers based on long singlemode fibers , owing to an interplay of self-phase modulation and dispersion. The only sign of higher-order transverse mode impurities consists in visible asymmetry of the line that appears already at 4 W and doesn’t change significantly with increasing power. This fact is in agreement with the weak dependence of the quality factor M2 on the laser power. Note that the 3-dB linewidth at maximum power remains to be smaller than the distance between the neighboring groups of transverse modes: ~0.45 nm for λ = 954 nm, d = 85 μm, NA = 0.28, n1 = 1.45. This estimate agrees with the mode peaks in the FBG spectra in Fig. 1.
In addition to narrow linewidth, the developed all-fiber configuration based on conventional multimode GRIN passive fibers greatly improves the beam brightness at the Raman conversion. Given that the brightness is B = P/(M2λ)2, we estimate the enhancement factor as BRFL/BLD~20. It is sufficiently higher than the enhancement achieved with bulk optics in [25, 26] despite of higher output power and optical efficiency in that case. So, the main factor is the better M2 ratio provided by mode-selection properties of FBGs implemented in our case. Moreover, there is enough room for further improvement of the RFL power and efficiency thus increasing the brightness enhancement factor.
The proposed approach based on development and integration of three key technologies – LD pump coupling through the multimode fiber pump combiner, core pumping of GRIN fiber and in-core cavity made of 3D FBGs - opens the door to new type of LD-pumped high-power high-beam-quality CW fiber lasers which may operate at almost any wavelength defined by available LDs. The proof-of-principle experiment performed here has already demonstrated output power level of 50 W at 954 nm (which is not covered by RE-doped fiber lasers) with high slope efficiency (67%) in simple and reliable all-fiber configuration. The configuration based on conventional multimode GRIN passive fiber pumped by three commercial multimode LDs at 915 nm. The obtained output beam quality (M2~2.6) is not far from that for single-mode RE-doped fiber lasers and considerably better than that for other LD-pumped Raman fiber lasers of the same power level [19, 25]. The quality improvement is provided jointly by the Raman beam clean-up effect and mode-selection properties of the narrow-band output FBG inscribed by fs pulses in the central part of the GRIN fiber. At the same time, the beam quality is almost independent on output power that indicates a formation of a stable group of coupled low-index modes. As a result, the shape and power broadening of generated spectrum is quite similar to those in singlemode fiber Raman lasers.
Note that this is only the first step demonstrating basic principles of the new type of CW high-power fiber laser. The next steps towards a better pump coupling at higher powers and a better fundamental mode selection will result in further improvements in optical efficiency, output power and brightness of such source. This makes it very attractive for applications, such as efficient/bright source for pumping solid-state/fiber lasers, second harmonic generation, laser displays, bio-medical imaging. Interesting fundamental challenges also appear, e. g. investigation of mode instability in a graded-index fiber. It may behave differently from that in step-index fibers, thus breaking the existing limits of high power fiber lasers.
Russian Science Foundation (14-22-00118).
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