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Abstract
Frequency doubling of multimode diode-pumped GRIN-fiber Raman laser with improved beam quality (M2=1.9-2.6 depending on configuration) in a simple single-pass scheme with 5-mm PPLN crystal is studied. After scheme optimization and elimination of back reflection and crystal heating effects, an efficient conversion into blue spectral range with output power of about 0.4 W@488 nm and 0.64 W@477 nm has been demonstrated.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Raman fiber lasers (RFLs) are known as reliable sources of high-quality laser radiation at the wavelengths inaccessible for rare-earth doped fiber lasers [1]. As the Stokes frequency shift and gain bandwidth at Raman conversion of pump radiation in silica-based fibers are nearly the same (∼13 THz), RFL generation wavelength may be almost continuously tuned over a whole transmission window in near IR (1-2 µm) via cascaded generation of higher Stokes orders combined with fine tuning within the Raman gain spectrum [2–6]. For high-power RFL operation, singlemode Yb-doped fiber lasers (YDFLs) with output wavelength of ∼1 µm are usually used as pump sources, while RFL cavity is based on nested fiber Bragg grating (FBG) pairs (for each Stokes order) connected to both ends of passive Raman fiber [1–3]. Recently developed FBG-free cavity schemes based on single broadband reflector and random distributed feedback (RDFB) via Rayleigh backscattering along the singlemode fiber, greatly simplify the design of tunable/cascaded RFLs and improve their characteristics [4–6].
Another recent advance in RFL development consists in implementation of its direct pumping by high-power multimode laser diodes (LDs), similarly to rare-earth-doped fiber lasers, e.g. high-power YDFLs [7]. Since cladding pumping of special double-clad passive fiber appeared to be not so efficient as that for active double-clad fibers [8], direct coupling of LD pump radiation into the multimode core of graded-index (GRIN) fiber has been intensively explored starting from works [8,9] with the use of 976-nm and 938-nm LDs, respectively. This approach is based on a possibility of sufficient beam quality improvement at Raman conversion of multimode radiation in GRIN fibers due to the well-known Raman clean-up effect [10]. As a result of these endeavors, output power as high as 154 W at the Stokes wavelength of ∼1020 nm has been reached in the RFL with two high-power 976-nm pump LDs, which radiation was coupled through bulk optics into a 62.5-µm core GRIN fiber placed in a Raman cavity formed by bulk mirrors [11]. The output beam quality parameter M2 grows with increasing power from 4 to 8 while pump radiation has M2∼20. With the cavity formed by special FBGs inscribed in the central part of GRIN fiber core, selection of fundamental transverse mode appears to be possible [12]. The demonstrated 62.5-µm GRIN-fiber Raman laser with FBGs generates nearly diffraction limited beam (M2=1.2) with power of ∼10 W at 954 nm with bulk-optics coupling of a 915-nm pump LD (M2∼20). Increasing pump power by coupling of three LDs through the fiber pump combiner to the GRIN fiber of 85-µm and 100-µm core diameter with in-core FBGs, sufficiently higher powers (49 W and 62 W, respectively [13,14]) have been reached at the expense of slight reduction of the output beam quality to M2=2.2-3 at pump radiation quality M2∼30. The beam quality reduction is primarily defined by beam transformation in pump combiner. In spite of not pure fundamental mode generated, the developed all-fiber scheme of high-power LD-pumped GRIN-fiber Raman laser is quite stable and robust having great potential for generation in new spectral bands (especially ≤1 µm), where application of rare-earth-doped fiber lasers is problematic, see [15] for a review.
Further expansion of RFL operation towards shorter wavelengths can be made with nonlinear frequency conversion such as second harmonic generation (SHG). For RFLs based on singlemode fibers, one can convert generation wavelength of near-IR radiation to visible range involving green, yellow and red [16], which is attractive for laser guide star, laser display, or biomedical applications such as bio-imaging. However, in the conventional RFL scheme based on singlemode fiber with nested FBG pairs, singnificant spectral broadening (≥1 nm) in relatively long RFL cavity arises from four-wave mixing processes between multiple longitudinal modes [17] and prevents to efficient SHG. With special measures to reduce the broadening, up to 14W of yellow radiation at 589 nm was generated in a periodically poled lithium tantalate (MgO:sPPLT) pumped by a high-power narrowband linearly-polarized RFL with short cavity [18]. Another approach is based on a narrowband (<3 MHz) oscillator with high-power Raman fiber amplifier and SHG in an external cavity [19]. At last, random DFB cavity is shown to provide reduced spectral broadening as compared to the conventional FBG cavity. This also results in increasing SHG efficiency [20] supported by the contribution of sum-frequency mixing between different frequencies (within phase-matching bandwidth) resulting in 2-times SHG enhancement for multi-frequency radiation as compared to that for single-frequency one.
The developed multimode LD-pumped GRIN-fiber Raman lasers are also attractive for SHG generation since their linewidth is suffciently narrower than that in singlemode fibers, thanks to the larger mode area. So, the linewidth remains below 0.4 nm up to ∼60W output power for 100-µm GRIN-fiber RFL operating at 954 nm [14]. Moreover, different wavelengths available below 1 µm in such RFLs [15] enable generation of blue-green radiation in 470-500 nm range that is hardly possible with other fiber lasers. So, they may be treated as a fiber-based alternative to solid-state lasers with SHG and to Ar/Kr ion lasers still widely used in this spectral region. At that, the obtained beam quality parameter M2∼2 for all-fiber LD-pumped RFL makes this task feasible.
In this paper we study for the first time SHG for the multimode LD-pumped GRIN-fiber Raman lasers operating at 976 nm in different schemes (direct and cascaded 2nd-order Raman conversion with different M2 output values) to find optimal configuration for efficient SHG in this approach, and for high-power RFL operating at 954 nm to explore its wavelength and power scalability.
2. Experimental scheme
The Raman fiber laser consists of 100/140 µm graded-index (GRIN) multimode fiber (L ∼ 1 km) that is pumped by three multimode laser diodes operating either at 938 nm or at 915 nm for one or two-step Raman conversion to 976 nm, respectively (Fig. 1). To combine the pump light of the LDs, we use 3 × 1 fiber pump combiner having 3 input fiber ports made of step-index fiber (105/125µm, NA = 0.15) and an output port made of the same GRIN fiber (100/140µm, NA = 0.29). A pair of FBGs with reflection coefficients 90% and 4% at 976 nm is spliced to the both sides of the GRIN fiber to form the laser cavity for the Stokes wave when pumped by 938-nm LDs. An additional pair of FBGs with reflection coefficients 90% and 4% at 950 nm is inserted (as shown in the inset of Fig. 1) to provide a cascaded generation of the first (950 nm) and the second (976 nm) Stokes order when pumped by 915-nm LDs, see [21] for details. The output fiber end is angle cleaved to prevent back reflection.
Fig. 1. All-fiber configuration of the RFL with direct LD pumping and SHG in PPLN crystal: LD1, LD2, LD3 – multimode laser diodes; UV FBG – fiber Bragg grating inscribed by UV radiation; FS FBG – fiber Bragg grating inscribed by femtosecond (FS) pulses; L1, L3 – collimating lenses; L2 – focusing lens; M1, M2, M3, M3, M4, M5, M6 – dichroic mirrors; P0, P1, P2 – power meters; OSA – optical spectrum analyzer.
An output beam from the laser is collimated by lens L1, the Stokes and the pump waves are separated by a set of dichroic mirrors M1-M3, so that the residual pump and Stokes wave powers are controlled by power meters P0 and P1, respectively. Their spectra are measured by an OSA placed after a dichroic mirror M2. For the second harmonic generation, we use a MgO-doped periodically poled lithium niobate (PPLN) crystal with grating period of 5.1 µm and a length of 4.7 mm. For the focusing, a simple scheme with lens L2 is used, in which the crystal placed in a thermostat is centered in the beam waist. The SHG power is measured by a power meter P2 placed after the dichroic mirror M4 that selects the generated beam at 488 nm from the residual Stokes wave at 976 nm. In the case of cascaded 2nd-order Stokes generation, an additional pair of dichroic mirrors (M5, M6) is inserted in order to filter residual 1st-order Stokes wave at 950 nm.
We have also tested SHG in similar 5-mm long MgO-doped PPLN crystal with grating period of 4.82 µm and resonant wavelength of 954 nm. In this case the 100/140µm GRIN fiber Raman laser with another pair of FBGs (with reflection coefficient of ∼90% and 4% at 954 nm) pumped by 915 nm LDs was used, see [14] for details of the laser.
Note that the beam quality of such GRIN fiber Raman laser (M2=2-3 depending on the output power and the exact wavelength [14]) is greatly improved as compared to that for the LD pump radiation (M2>30) due to the transverse mode selection by FBGs inscribed in the central part of GRIN fiber core (mainly by fs-inscribed output FBG with low reflection). As shown in [21], second-order Stokes generation in GRIN fiber Raman laser leads to further beam quality improvement to M2≤2 (exact value depends on the scheme of cascaded generation). Since the SHG efficiency depends on the beam quality, it is interesting to compare two schemes of GRIN fiber Raman laser operating at 976 nm.
3. Experimental results
3.1. 976-nm GRIN-fiber Raman laser in two schemes
Similar to the 954-nm Raman laser with 915-nm LD pumping [14] the developed 976-nm Raman laser with multimode 938-nm LD pumping (M2∼34 after pump combiner) exhibits slight variation of the output beam quality parameter M2 (from 2.2 to 2.6) when output power increases from several to several tens of Watts. An example of beam quality measurement together with the beam profile in the waist is shown in Fig. 2(a). The −3 dB width of the generated spectra at 976 nm is also almost unchanged in the broad power range (5-35 W) being comparable with the acceptance width of the PPLN crystal, see Fig. 2(b).
Fig. 2. Parameter M2 measurements for the generated 1st Stokes beam at 976 nm of the RFL with 4 W output power (a) and its spectral linewidth compared to the acceptance (phase matching) width of the PPLN crystal (dash line) (b). Inset: intensity profile of the generated beam in the waist (a) and typical spectrum of 1st Stokes lasing (b).
The modification of the scheme to cascaded 976-nm Raman laser with 915-nm LD pumping has resulted in sufficiently better beam quality (M2≤2.0) than that for the 976-nm Raman laser with 938-nm LD pumping, but its power range is limited by the value of 11 W [21]. An example of beam quality measurement together with the beam profile in the waist for the cascaded Raman laser is shown in Fig. 3(a). The measured −3 dB width of the generated spectra at 976 nm is also reduced at comparable power (∼10 W) being within the acceptance width of the PPLN crystal, see Fig. 3(b).
Fig. 3. Parameter M2 measurements for the generated 2st Stokes beam at 976 nm of the RFL with 5.5 W output power (a) and its spectral linewidth compared to the acceptance width of the PPLN crystal (dash line) (b). Inset: intensity profile of the generated beam in the waist (a) and typical spectrum of 2nd Stokes lasing (b).
Therefore, the cascaded Raman generation at 976 nm in the GRIN fiber with 915-nm LD pumping has better beam quality and narrower spectrum than the direct Raman generation at 976 nm with 938-nm LD pumping that makes it attractive for SHG in low-power domain. Note that the cascaded Raman generation based on random distributed feedback via Rayleigh backscattering for the 2nd Stokes order has even better beam quality (M2=1.6) and higher power (up to 27 W [21]), but this regime is very sensitive to any back reflection which is important at SHG in nonlinear crystal.
3.2. SHG results and discussion
In connection with focusing optimization for maximum SHG a dimensionless focusing parameter ξ is often used. The parameter can be related to the nonlinear PPLN crystal length l and to the beam waist diameter as follows [23]:
where ne is the effective refractive index of the crystal (ne≈2.2 for a lithium niobate [22]). It is known, that the focusing parameter ξ should be close to 2.84 [23]. Taking into account non-ideal beam quality (M2∼2.2), we estimate the beam waist diameter for the wavelength λ=976 nm as $2\sqrt {{M^2}} {w_0}$=33 µm. For the first experiment we have chosen lens L2 with the focal length of 30 mm providing beam waist diameter close to this value. The focal length of collimating lens L3 was 60 mm. Then we compared SHG to 488 nm for two schemes of 976-nm GRIN-fiber Raman laser.The first experiment with 938-nm LD pumping have shown that a doubling of the RFL radiation frequency in the crystal reveals feedback, which manifests itself in an increase of laser power and a change in its spectrum. The spectrum broadened and parasitic lasing at higher-order transverse modes appeared. To reduce the feedback, a bulk polarizing Faraday isolator is added to the scheme before the focusing lens L2. Nevertheless, a weak feedback still remains as a third-order Stokes line appears at ∼42 W power at 976 nm, which was absent in the laser generation without the doubling scheme. The measured beam waist diameter after lens L2 with f = 30 mm amounts to 34 µm, in agreement with the estimate. Measurements of phase matching by varying the crystal temperature have shown that the crystal is heated by visible radiation with increasing SHG power at 488 nm, and the optimum temperature shifts from the initial 86.5°C to 84°C (Fig. 4(a)). As a result, the SHG power saturates at pump powers >12W (circles in Fig. 5). Replacing L2 with a lens of longer focal length f = 60 mm increases the waist diameter to ∼60 µm. This reduces the thermal load on the crystal, and the drift of optimal temperature with increasing power is not as significant as that for L2 with f = 30 mm (Fig. 4(b)). The suppression of the temperature drift results in elimination of SHG power saturation at high powers and the power dependence becomes nearly quadratic (stars in Fig. 5). Thus, the output power at 488 nm approaches ∼400 mW. At the same time it seems that spot beams values of 34 and 60 µm available in our experiments lye on the both sides of SHG efficiency dependence away from the optimal value, so that the difference between SHG powers isn’t so large in this two cases. The quadratic nonlinearity coefficient of SHG obtained from the fitting is about 1.8·10−3 1/W. It is almost two times lower than coefficient (3.8·10−3 1/W) measured in the case of SHG with cascaded Raman laser at 976 nm and f = 30 mm for L2 (squares in Fig. 5). Such difference in SHG efficiency is explained by a better beam quality and narrower linewidth for cascaded Raman lasing. However, maximum power at 976 nm is limited in the 2nd-order RFL scheme, so absolute power values at 488 nm are lower in this case. Note that only one polarization component of randomly polarized Raman laser is used for SHG in PPLN.
Fig. 4. Temperature tuning curves for PSHG in the PPLN crystal for waist diameter 2w0=34 µm (a) and 2w0 = 60 µm (b) at different input pump powers Pin at 976 nm, shown in legends.
SHG experiments for 954-nm Raman laser (spectral FWHM ∼0.3 nm) with 915-nm LD pumping (its beam quality is shown in Fig. 6(a)) and similar PPLN crystal (l = 5 mm) have been also performed in the initial focusing scheme with L2 of focal length f = 30 mm providing beam waist of ∼34 µm. The optimum crystal temperature for phase matching is near 94°C, the obtained coefficient of quadratic nonlinearity is ≈5.4·10−3 1/W. The maximum power of the second harmonic at 477 nm is about 640 mW with a 954-nm pump power of ∼11.5 Watts (Fig. 6(b)). Further increase of the pump power results in the SHG power saturation, similar to the 488 nm SHG scheme with the same L2 (f = 30 mm). Note that this effect can be eliminated with the use of lens of longer focal length. Nevertheless, the achieved SHG power and efficiency at 477 nm are about 2 times higher than that at 488 nm with the similar design of RFL (single-step Raman conversion of LD pump radiation). The difference is explained by slightly longer crystal and better RFL beam quality (M2 = 2.2 at power 12.6 W, see Fig. 6(a)) that is defined by using of better LDs (NA = 0.15 instead of NA = 0.22) which provides better quality of the pump radiation (M2∼30 after pump combiner). Quality of the crystals is also slightly different.
Fig. 6. (a) Parameter M2 measurements for the generated 1st Stokes beam at 954 nm of the RFL with 12.6 W output power. (b) Measured SHG power at 477 nm as a function of 954 nm polarized pump power (squares) and its quadratic fitting (dash line). Inset: intensity profile of the generated beam in the waist.
The obtained SHG at 477 nm has no analogues in the class of fiber-based lasers. As for SHG at 488 nm, in paper [24] the SHG power of 83 mW at 489 nm was obtained with 980-nm Yb-doped fiber laser and PPLN crystal. In similar experiments [25,26] the demonstrated SHG power did not exceed 20 mW. High power CW laser systems operating in blue spectral range are usually based on the frequency doubling of single-frequency radiation in an external cavity with BiBO or LBO nonlinear crystals. For example, Burkley et. al [27] used Yb-doped fiber amplifier to generate 6.3 W at a wavelength of 972.5 nm and LBO and CLBO crystals in external cavities to obtain 2.4 W and 0.53 W at wavelengths of 486 nm and 243 nm, respectively. Recently the first neodymium-doped fiber laser with frequency doubling in an internal enhancement cavity integrated within the fiber laser cavity and generating about 7.5 W of CW power near 450 nm was demonstrated by Leconte et. al [28]. However, these schemes are much more complicated than the one used in our paper with single-pass PPLN crystal and simple multi-frequency pump source based on the LD-pumped GRIN fiber.
4. Conclusion
We have demonstrated that recently developed LD-pumped GRIN-fiber Raman laser with improved beam quality (M2=1.9-2.6 depending on configuration) at wavelengths 950-980 nm in simple and robust all-fiber configuration with mode-selective in-fiber FBGs [13–15,21], may be efficiently converted into blue spectral range in a simple single-pass scheme with PPLN crystal of ∼5 mm length and ∼0.3 nm acceptance width. It has been revealed that the parasitic feedback from the crystal influences the RFL beam quality, but it is easily eliminated by means of polarizing isolator. Another limiting effect is the heating of the crystal by blue radiation. It results in saturation of the SHG power from the multimode pump radiation at the level of 0.3-0.6 W depending on the crystal and pump beam quality (at ∼10 W linearly polarized power level). It has been shown that this limiting effect can be eliminated by increasing the beam waist diameter to ∼60 µm. Comparing obtained output parameters for 488-nm (∼0.4 W) and 477-nm (∼0.6 W) SHG schemes with single-step conversion of multimode LD radiation in RFL and similar PPLN crystals, one can see that higher SHG power (and efficiency) is observed for 477 nm wavelength. This mainly defined by the better beam quality of the Stokes radiation at 954 nm due to better LDs used for pumping (NA = 0.15 instead of NA = 0.22). It should be noted that optimization of the mode-selective FBGs may reduce this difference, as our recent studies show that the 1st Stokes beam quality of 2-2.2 is possible with NA = 0.22 LDs at power level of up to 50 W after the FBGs optimization [29]. Another way to improve the beam quality of a GRIN-fiber RFL, which has been tested here, is to employ a cascaded 2nd-order Stokes generation resulting in M2=1.9-2 for 976-nm radiation with the use of two FBG pairs. However, the power in this case is limited by ∼10 W. Higher power (∼20W) and better beam quality (M2=1.6) are possible in random lasing scheme (with one FBG) at the 2nd stage [21], but this scheme is much more sensitive to the parasitic feedback from the crystal. This problem may be solved in the future. To increase the SHG efficiency, it is also important to generate polarized radiation in the RFL. We believe that with the discussed measures several Watts of blue power is feasible at the existing power level of the LD-pumped RFLs.
Other fiber lasers operating at ∼980 nm based on special rare-earth-doped fibers are rather complicated and do not experience long-term stability because of photo-darkening and other limiting effects [7]. The best result is recently obtained by Li et. al [30] who report about a monolithic laser based on a photonic bandgap Yb-doped fiber, which has high beam quality of M2∼1.25 and demonstrates good stability over 60 hours at output power of 75 W at 978 nm. However, shorter wavelengths are hardly possible for these lasers, whereas the developed robust and simple fiber laser platform based on LD-pumped GRIN fiber enables high-power generation at different wavelengths below 978 nm, which may be efficiently converted to blue radiation in 477-488 nm (and potentially broader) range with Watt power level that is very attractive for applications.
Funding
Russian Foundation for Basic Research (19-52-53021, 18-52-7822).
Disclosures
The authors declare no conflicts of interest.
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