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Abstract
A multimode all-fiber Raman laser enabling cascaded generation of high-quality 1019-nm output beam at direct pumping by highly-multimode (M2>30) 940-nm laser diodes has been demonstrated. The laser is made of a 100/140 graded-index fiber with special in-fiber Bragg gratings which secure sequential generation of the 1st (976 nm) and 2nd (1019 nm) Stokes orders. Comparing different 1019-nm cavity structures shows that the half-open cavity with one FBG and distributed feedback via random Rayleigh backscattering provides excellent quality (M2∼1.3) with higher slope efficiency of pump-to-2nd Stokes conversion than in the conventional 2-FBG cavity. The maximum achieved slope efficiency amounts to about 40% at output powers of up to 12 W limited by the 3rd Stokes generation.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
A beam quality improvement at Raman conversion of a multimode radiation in a multimode graded-index fiber (GIF) known as the Raman beam clean-up effect has been extensively studied in the Raman fiber laser (RFL) scheme based on GIF with multimode laser pumping aiming both the improving RFL performance and the understanding mechanisms of the effect, see e.g. [1,2]. Recently, power scaling capabilities of multimode and multi-clad fiber Raman lasers with brightness enhancement are widely explored demonstrating power level above 1 kW, see [3] for a review, as well as Raman amplifiers based on multimode GIF are introduced reaching output power level of 2 kW [4] with rather good quality (M=2.7-2.8) of output beam at pumping by combined fiber lasers with beam quality M2∼10 in both cases.
Another direction of recent research aims at the development of efficient and reliable RFL directly pumped by high-power multimode LDs [5,6]. Using commercially available multimode GIFs and LDs, efficient Raman conversion of highly-multimode LD radiation (M2=20-30) into a high-quality Stokes beam (M2=2-3) has been demonstrated in an all-fiber scheme comprising laser cavity formed by two in-core fiber Bragg grating (FBGs) and fiber coupling of pump radiation, see [7] for a review. At the same time, with the use of 915-940 nm LDs such RFL are able to generate Stokes radiation at wavelengths 950-976 nm inaccessible for classical lasers based on fibers doped with rare-earth elements, and in particular for Yb-doped fiber laser [8]. The use of multimode GIF with a large core diameter (including standard telecom ones) together with the implementation of special fiber pump combiners for coupling several LDs to the GIF make it possible to significantly increase the maximum output power of lasing. As a result, it was possible to increase the Raman laser power from ∼10 W when pumped by only one LD [9] up to ∼60 W when pumped by three laser diodes [10] at the expense of slight worsening of output beam quality to M2=2.5-3. Optimization of transverse profile of output FBG inscribed by femtosecond point-by-point technique in near-axis part of GIF core allows one to improve beam quality to M2≤2 at 976 nm without significant loss in power and slope efficiency of pump-to-Stokes conversion amounting to about 70% [11]. The achievable brightness enhancement (BE) in such GIF-based Raman lasers could reach ∼70 [11] that is the highest value compared to other types of continuous Raman lasers with output power above 0.1 mW [3], whereas in [12] a high peak power Raman fiber amplifier with BE of 192 and average power of 44 mW is demonstrated. In addition, a half-open cavity with one FBG and distributed feedback via Rayleigh backscattering instead of output FBG turns out to provide even better output beam quality compared to the linear cavity consisting of two FBGs. Thus, M2∼1.6 at ∼30 W was achieved in [13] for the second Stokes at 996 nm in the half-open cavity with random Rayleigh feedback.
Here we report on the development and comparison of LD-pumped cascaded RFL based on GIF with conventional FBG cavity and half-open cavity with random Rayleigh feedback for the 2nd Stokes wave at 1019 nm. Similar to [11], coupling of several 940-nm LDs to the GIF is provided by a special multimode fiber pump combiner and a linear cavity formed by two FBGs secures the 1st Stokes wave (∼976 nm) generation with relatively high beam quality (M2≈1.7-1.8) at low quality of the LD pump beam (M2≈30). It has been shown that the 2nd Stokes waves generated in the random RFL (RRFL) and conventional RFL cavity have near diffraction limited beam quality M2≈1.3-1.4 with slightly better value for RRFL. The slope efficiency of the 2nd Stokes generation is also higher in the RRFL scheme amounting to about 40%, but the output spectrum appears to be sufficiently narrower in the RFL scheme.
2. Experiment
Figure 1 shows the designed Raman fiber laser based on the GIF with direct MM LD pumping. Pump radiation from three fiber pigtailed high-power LDs operating at the wavelength of ∼940 nm is added together by a 3 × 1 multimode fiber pump combiner. Each MM LD was fiber coupled by a 105/125 μm step-index fiber with NA = 0.22 connected to the input ports of the fused pump combiner made of similar multimode step-index fiber with 105-μm core, whereas the output port of the combiner is made of a 100-μm core GIF with numerical aperture NA=0.29. Total power from the combiner output exceeds 200 W while the pump beam quality is worsened to M2>30 (for more details see [11]) A 1-km-long fiber line of 100-μm core GIF with NA=0.29 is spliced to the pump combiner with LDs providing Raman gain. The linear laser cavity for the 1st Stokes wave at wavelength of 976 nm is formed in the GIF by a high-reflection UV-inscribed FBG and a low-reflection fs-inscribed output FBG. In the case of conventional cascaded RFL scheme, a cavity for the 2nd Stokes order is also formed by two FBGs: UV FBG and FS FBG operating at 1019 nm. The reflectivity of input and output FBGs are ∼90% and ∼4%, respectively, for both the 1st and 2nd order cavities. All high-reflective (HR) FBGs are inscribed by CW UV interference pattern formed in core area, while the low-reflective output coupling (OC) FBGs are inscribed point-by-point by femtosecond pulses tightly focused in the center of the GIF core thus providing the predominant reflection of fundamental transverse mode, for more details see [14]. Spectra of 976 and 1019 nm FBG pairs are similar. The reflection spectra of 1019-nm FBGs are shown in the inset of Fig. 1. The spectrum of highly-reflective UV-inscribed FBG is rather broad involving several groups of low-order transverse modes of the GIF, whereas fs-inscribed output FBG has a narrow peak centered at 1019 nm corresponding to the fundamental mode that is by >10 dB higher than multiple higher-order peaks at short-wavelength tail separated by ∼0.5 nm. A half-open cavity with random distributed feedback is implemented by the only UV FBG at 1019 nm (R∼ 90%) placed near the pump input. The output fiber end is cleaved in this case with an angle of >10° in order to eliminate Fresnel reflection. Therefore, the feedback is provided by reflection from the UV FBG at one GIF end and the Rayleigh backscattering distributed along the GIF.
Fig. 1. All-fiber configuration of the cascaded random RFL: LD1, LD2, LD3 – multimode laser diodes; UV FBG – high-reflection fiber Bragg grating inscribed by UV radiation; FS FBG – low-reflection fiber Bragg grating inscribed by femtosecond pulses; L – collimating lens; M1, M2, M3 – selective mirrors; IF – bandpass filter; P1, P2 – power meters; OSA – optical spectrum analyzer. When the FS FBG 1019 nm is added in the scheme, RRFL becomes a conventional RFL at the second Stokes wavelength. Inset: reflection spectra of the HR and OC 1019 nm FBGs.
The laser characteristics are measured in the following way. Selective mirrors M1, M2, M3 are used to separate cascaded Raman generation at 976 and 1019 nm from pump radiation at 940 nm. The residual pump power and Raman signal were measured by a power meter P1 and P2, respectively. The residual radiation passed through mirrors M2 and M3 is used to measure output spectrum and profile of the generated beam by an optical spectrum analyzer (OSA) and Thorlabs M2-measurement system, respectively. A set of bandpass filters IF with different central wavelengths is used to measure quality parameter M2 of the 1st and the 2nd Stokes beams. Stokes powers of different orders were calculated taking into account relative power dependencies measured by the OSA for the individual Stokes spectral lines and the total power measured by power meter P2.
3. Experimental results
First, an experiment was carried out with a conventional two-stage Raman laser. In this case, two pairs of FBGs with coinciding reflection wavelengths, respectively, for the first (976 nm) and second order (1019 nm) Stokes generation wavelengths were installed in the laser scheme. At input pump power of 105 W, the 1st order Stokes generation begins, the power of which increases linearly up to ∼ 11 W with increasing pump power up to 140 W (Fig. 2). After that, second-order Stokes component starts to generate, whereas the first Stokes power starts to slowly decrease. The maximum obtained power of the 2nd Stokes radiation was 8 W. The slope efficiency of pump-to-Stokes conversion amounts to ∼40% (at powers <11 W) and 16% for the 1st and 2nd Stokes, respectively. Figure 3(a) presents the output spectra at different pump powers with high-resolution scan in the inset, which shows that the width of the 2nd Stokes spectral line is rather narrow and its value amounts to ∼0.12 nm at the -3 dB level (0.3 nm at -10 dB level).
Fig. 2. Measured output power of conventional RFL and RRFL at the pump, the first and second Stokes wavelengths versus the input pump power coupled to the GIF. Arrow marks appearance of the 3rd Stokes line in the RRFL spectrum.
Fig. 3. Lasing spectra in a broad range and 2nd Stokes line in high resolution in the inset (a); beam quality of the 1st order Stokes wave (10W) (b) and 2nd order Stokes (7W) (c) in the 2-cascaded RFL. Insets: intensity profile of the generated beam in the waist.
Beam quality parameters were also measured for the 1st and 2nd Stokes waves. Figure 3(b) shows the measured profile of the 1st-order Stokes beam divergence, from which the parameter M2 was calculated by standard procedure [15] amounting to 1.72-1.74, which turns out to be close to that obtained in a single-stage laser described in [11]. The quality of the second Stokes beam (Fig. 3(c)) further improves to 1.33-1.40.
To study the Raman laser with a random distributed feedback, the output low-reflective fs-FBG (1019 nm) was removed from the laser, thus a half-open cavity was formed. In the RRFL scheme, the lasing threshold of the 2nd Stokes raises to 156 W, while the power of the 1st Stokes wave at the given pump power reaches 25 W (Fig. 2). Note that at the 1st Stokes threshold a linear growth of the residual pump power stops as well as at the 2nd Stokes threshold the growth of 1st Stokes power also stops due to the depletion via Raman conversion. At higher powers, the slope efficiency of the 1st Stokes becomes higher amounting to about 67% that is close to data of [11]. The dynamics of the residual pump power also changes: while in a conventional two-stage laser the residual pump power increases monotonically as the 1st and 2nd Stokes thresholds are close, in random RFL there is a clear decline induced by pump-to-1st-Stokes conversion, whereas the growth resumes with the appearance of the 2nd Stokes wave that is reasoned by the necessity to compensate increasing nonlinear losses of the 1st Stokes wave (due to its conversion to the 2nd Stokes wave), just like in the case of RFL based on singlemode fiber [16]. The maximum power of the 2nd Stokes wave reaches 12 W limited by the starting 3rd order Stokes generation which leads to the decrease of the 2nd Stokes power in the last point. The slope efficiency of the 2nd Stokes in this laser is about 37%. The slope efficiency as well as the maximum power values here are slightly lower than those for the 2nd Stokes wave at 996 nm [13] where no 3rd order Stokes wave was observed up to maximum power reached at ∼200 W pump power. Note that the absolute pump-to-Stokes conversion efficiencies here are 16.3% and 6.5% for the 1st and 2nd Stokes waves, respectively.
Output spectra for RRFL scheme at different pump powers are presented in Fig. 4(a). The high-resolution spectrum of the 2nd Stokes line in the inset shows that its width at the -3 dB level turns out to be 0.8 nm (1.9 nm at -10 dB level). It is sufficiently larger than that in conventional RFL scheme with output fs FBG which reduces the linewidth according to the narrow reflection spectrum of the output FBG, whereas in the RRFL scheme the linewidth is nearly equal to the linewidth of the highly-reflective UV FBG (see inset in Fig. 1).
Fig. 4. Lasing spectra in a broad range and 2nd Stokes line in high resolution in the inset (a) beam quality of the 1st order Stokes wave (10W) (b) and 2nd order Stokes (7W) (c) in the RRFL. Inset: intensity profile of the generation in the waist.
Figures 4(b) and 4(c) show the measurements of the beam quality parameter for the 1st and 2nd Stokes waves in the RRFL with Rayleigh distributed feedback. It can be seen that M2 parameters of both the 1st Stokes (M2=1.75-1.77) and the 2nd Stokes (M2=1.31-1.37) turn out to be close to the measured values for a conventional two-stage RFL, in contrast to [13], where a sufficient improvement in the quality of the second Stokes beam was demonstrated when the scheme was changed from a classical two-stage to a half-open cavity with Rayleigh feedback. We believe that this discrepancy in the results may indicate an existence of a certain limiting value of the parameter M2 in this type of GIF, so that below this value there is no noticeable improvement in the beam quality of the RRFL in comparison with the conventional two-stage RFL. In addition, it is possible that a slightly worse slope efficiency of pump conversion to the 2nd Stokes wave is associated with the better beam quality here in comparison with the results of work [13]. Nevertheless, both [13] and present results demonstrate higher 2nd Stokes slope efficiency in RRFL in comparison with RFL scheme in spite of narrower RFL spectrum observed here. So, we can conclude that nonlinear losses are not so important as linear losses on OC FBG and probably not optimal reflection coefficient of OC FBG in the RFL as compared with low integral reflection in the RRFL that is shown to be close to optimal in singlemode fiber [16], so there is a room for increasing the slope efficiency of multimode RFL by means of optimizing the 2nd Stokes OC FBG reflectivity. At the same time, the maximum power value in our case is limited by the relatively low threshold of the 3rd order Stokes generation, which reason may be different than that in [13].
So, we intend to find a reason of the relatively low 3rd order Stokes threshold, that is only ∼10 W in our scheme. As long as the mirror M2 transmission spectrum does not allow us to register the third-order Stokes at a wavelength of 1065 nm, we placed the OSA instead of power meter P2 and obtained RRFL lasing spectra in a broad wavelength range that is shown in Fig. 5.
Fig. 5. RRFL lasing spectra in broad wavelength interval at 180 W pump power (top) and corresponding pump LD spectrum (bottom).
At a pump power of 180 W the generation of the 3rd order Stokes begins and at 194 W it becomes strong enough to sufficiently deplete the 2nd order Stokes beam. To further identify the reason, we also measured an emission spectrum of pump MM LD in a broad spectral range. We noticed that it has low-power spectral noise component near the 1065 nm which is precisely the 3rd order SRS gain maximum. We have estimated the noise power as 10-20 mW at 180 W of coupled pump power that is enough to seed a stimulated Raman scattering at 1065 nm. Suppressing this component may raise the threshold for generating the third Stokes and thus increase the maximum achievable power of the second Stokes wave.
4. Conclusion
In this work, we demonstrated for the first time the 2nd-order Raman lasing at 1019 nm in the all-fiber scheme of directly LD-pumped (at 940 nm) multimode graded-index fiber Raman laser with FBG cavity formed for the intermediate 1st -order Stokes wave (976 nm). Two different cavity schemes for the 2nd-order Stokes generation were investigated and it was revealed that the generated beam quality of conventional RFL and random RFL is excellent in both cases and almost independent of power, M2=1.3-1.4, that is close to the diffraction limit. Though random Rayleigh feedback gives slightly better value, it is close to that one obtained with optimized output FBG, that differs from earlier reported results with M2=1.6-2 for the 2nd – order RFL based on GIF [13]. This may indicate that below a certain limiting value of the generated beam quality (M2<1.4) in a given 100-um MM graded-index fiber, the use of random Rayleigh feedback does not give a significant improvement of the output beam quality. To the best of our knowledge, the obtained parameter M2∼1.3 is the best result of generated beam quality in a 100-um highly multimode GIF. The slope efficiency of the 2nd order Stokes generation is >2 times higher in case of RRFL (40%) compared to the conventional RFL (16%) which agrees with [13]. The maximum power reached is ∼12 W and is limited by the 3rd order Stokes generation at 1065 nm. The relatively low threshold of the 3rd order Stokes is explained by the presence of 1065 nm noise component in the MM pump laser diode emission. Its elimination will definitely increase the power at 1019 nm by several times, similar to 996 nm RRFL [13]. In contrast to the 1st Stokes generation at 1019 nm directly from multimode 976-nm LD pump providing moderate beam quality of the output beam (M2 is about 5 with bulk optics cavity [5], and M2 is better but still limited by the value of ∼2 if special FBGs [11] or random Rayleigh feedback [17] to be implemented at this wavelength), the sequential conversion of 940-nm LD pump (M2∼30) to the 1st Stokes at 976 nm (M2≈1.8) and then to the 2nd Stokes at 1019 nm (M2≈1.3) results in nearly diffraction limited beam quality at the expense of slight reduction of pump-to-output Stokes efficiency. Excellent beam quality and good slope efficiency demonstrated for the 1019-nm RRFL together with further power scaling capabilities by increasing coupled pump power combined with shortening of the GIF, similar to [17], is a good opportunity to develop hundreds watts all-fiber source at ∼1018 nm of new type that is in great demand for tandem-pumping schemes of multi-kW Yb-doped fiber lasers because of limitations of Yb-doped fiber lasers at this wavelength such as photo-darkening and amplified spontaneous emission at longer wavelengths [18].
In conclusion, we offer excellent beam quality laser source of new type based on cascaded Raman lasing in a simple configuration of LD-pumped multimode graded-index fiber with sequential improvement of beam quality at conversion of highly multimode pump to the 1st and 2nd order Stokes waves. Relatively high quality of the generated beam at both Stokes orders offers their efficient frequency doubling that will transfer the generated spectrum at 976-1019 nm into blue-green spectral range (488-510 nm) [19] thus enabling implementation of this source in bio-imaging and display technologies. Emphasizing that the best output parameters of the 2nd Stokes wave have been obtained in the half-open cavity with random Rayleigh feedback which may be further explored for efficient generation of multiple Stokes orders, similar to random RFL based on singlemode fiber [20].
Funding
Russian Science Foundation (21-72-30024).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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