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Abstract
We demonstrated an eye-safe diamond Raman laser intra-cavity pumped by the 1.3 μm fundamental field for the first time, to the best of our knowledge. The first-Stokes laser at 1634 nm was converted from the 1342 nm fundamental laser, which was produced by an in-band pumped double-end diffusion-bonded a-cut Nd:YVO4 crystal. Under an incident pump power of 21.2 W and an optimal pulse repetition frequency of 25 kHz, the maximum average output power of 2.0 W was obtained with the pulse duration of 5.7 ns and the peak power of 14 kW. The first-Stokes emission was found to be near diffraction limited (M2 ≈ 1.3) and to have a narrow linewidth (∼0.05 nm FWHM; instrument limited).
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nanosecond (ns) pulsed laser sources in the 1.6 μm spectral range attract substantial interest for plenty of applications due to their eye-safe property and high transmittance in the atmosphere, such as range finding, remote sensing, free-space communication, environmental monitoring, etc [1–3]. In addition, owing to the absence of strong absorption in water, these waveband ns pulsed coherent sources are intensively exploited for biological engineering and laser therapy [4,5]. Furthermore, high-peak-power ns pulsed lasers near 1.6 μm can be employed as an efficient pump source for mid-infrared laser generation using parametric frequency down-conversion [6]. In particular, high-resolution measurements over long ranges require pulses with short duration of a few nanoseconds and peak power as high as possible.
To date, various methods have been developed to generate laser emissions at 1.6 µm. Er:Yb-codoped fiber lasers have produced hundreds of watts of continuous-wave (CW) output at 1600 nm [7], but ns pulsed fiber lasers often have limited peak power and large spectral linewidth due to the nonlinear effects, primarily four-wave mixing [8]. Resonantly-pumped Er:YAG lasers can deliver average output power of 75 W for CW operation [9], and pulse energy of 120 mJ for Q-switched operation [10]. However, due to the inherently low gain of Er:YAG crystal near 1.6 μm, few-ns-class pulse duration must be achieved by the cumbersome cavity-dumped Q-switching approach [11]. Besides, power scaling of Er:YAG lasers is a particular challenge due to the problems associated with upconversion, dopant ion clustering, and expensive pump sources [12]. Albeit optical parametric oscillators (OPOs) can generate several watts, few-ns-class eye-safe laser operating at 1.6 μm [13], the thermal issues arising in the nonlinear crystal will cause the onset of poor beam quality and optical damage [14]. Moreover, the requirements of birefringent or quasi-phase-matching make the OPO system sensitive to environment [15].
Over the past few decades, crystalline Raman lasers have been certified to be a highly efficient method for facilitating short pulse duration and high beam quality ns pulsed laser output as its automatic phase matching, Raman beam cleanup, and pulse duration shortening. So far, several Raman active media including BaWO4, SrWO4, YVO4 and Ba(NO3)2 have been successfully employed to achieve the eye-safe radiation at 1.6 μm [16–19]. However, the maximum output power is limited to watt-level owing to the excessive waste heat deposited in the laser gain medium, and the high power laser output with high beam quality will be severely constrained by the poor thermal properties of these Raman media. Compared to other Raman-active materials, the mono-crystalline diamond grown via chemical vapor deposition (CVD) is the most attractive candidate for generating the high power and high beam quality eye-safe Raman lasers as its unrivalled thermal conductivity (2200 W/mK), high Raman gain coefficient (∼17 cm/GW@1 μm), large Raman frequency shift (1332.3 cm-1), low thermal expansion (0.8×10−6 K−1), and high stress fracture limit [20–22]. Currently, plentiful efforts have been focused on cascaded Raman conversion from the mature 1 μm Nd and Yb-doped lasers to the 1.5 μm laser emissions using Diamond [14,23]; however, the “eye-safety” requirements for lidar typically call for wavelengths above 1.5 μm because of the order of magnitude higher maximum permissible exposure limit. Previous diamond Raman laser (DRL) oscillation at 1.63 µm was demonstrated with the external cavity architecture, and the maximum peak power of about 8 kW was attained with the pulse duration of 6 ns and the relatively low repetition rate, resulting from the flashlamp-pumped 1.34 µm Nd:YAP laser [24].
In this paper, we demonstrate the first intra-cavity DRL operating at 1.6 µm, to the best of our knowledge. The eye-safe first-Stokes line centered at 1634 nm was driven by an actively Q-switched 1342 nm Nd:YVO4 laser. To alleviate the thermal effect of laser crystal, both the in-band pumping technique and the double-end diffusion-bonded architecture were adopted simultaneously. Under an optimal pulse repetition frequency (PRF) of 25 kHz, the maximum average output power of 2.0 W was achieved with an optical power conversion efficiency of 9.4% and near diffraction limited beam quality (M2 ≈ 1.3). Meantime, owing to the pulse duration shortening effect of stimulated Raman scattering (SRS), the pulse duration was shortened to 5.7 ns, leading to the peak power up to 14 kW. As far as we know, this is the highest average power and the maximum conversion efficiency among the 1.6 μm intra-cavity crystalline Raman lasers [17–19].
2. Experimental setup
The schematic configuration is illustrated in Fig. 1. An 878.4 nm fiber-coupled (core diameter: 200 μm, N. A. = 0.22) laser diode (LD) emitting up to 65 W was employed as the pump source. A pair of plane-convex coupling lenses (1:2 magnification) with anti-reflection (AR) coated at 878.4 nm (R < 0.2%) was utilized to re-image the pump beam with a spot diameter of ∼400 μm into the laser gain medium. The laser gain medium was a 3 × 3 × 30 mm3 double-end diffusion-bonded a-cut Nd:YVO4 composite crystal, which was bounded with two 7 mm long pure YVO4 ends at both facets of 0.4 at.% Nd-doped YVO4 crystal. Both sides of the composite crystal were AR coated at 878.4 nm (R < 0.5%), 1064 nm (R < 0.5%) and 1342 nm (R < 0.4%). Under non-lasing situations, the pump absorption efficiency was measured to be approximately 93%. By combination of the in-band pumping technique and the double-end diffusion-bonded structure, the thermal lensing effect of laser gain medium is expected to be remarkably reduced [25]. A 46 mm long acousto-optic Q-switcher (Gooch & Housego) was AR coated at 1342 nm (R <0.2%) on both facets, and it was driven at the 27.12 MHz ultrasonic frequency with 100 W radio-frequency power. The Raman gain medium was a CVD-grown single-crystal diamond (Type IIIa, Element Six), cut for propagation along the <110> direction with dimensions of 2 × 2 × 6 mm3. Two end faces of the diamond were AR-coated at 1342 nm (R < 0.3%) and 1634 nm (R < 0.5%) to minimize the intra-cavity losses. The <111> crystallographic axis of the diamond was aligned to the fundamental polarization direction for achieving the highest Raman gain [26,27]. During the experiment, Nd:YVO4 and CVD-diamond were wrapped in indium foil and clamped in the brass mounts, cooled with a closed-cycle water chiller at 18°C.
The fundamental resonator was designed to be V-shaped for providing a tight beam waist in the diamond, and it was constituted by an input mirror M1, a folding mirror M2 and an output coupler M4. The fold angle of two arms was set to be 18°. The plane input mirror M1 was coated for high transmission (HT) at 878.4 nm (T > 91%) and 1064 nm (T > 91%), and high reflection (HR) at 1342 nm (R > 99.9%). The HT coating enabled suppression of laser oscillation at 1064 nm. Both the folding mirror M2 and the output coupler M4 were the concave mirrors with radius-of-curvature of 100 mm coated for HR at 1342 nm (R > 99.9%) and partial reflectivity (PR) at 1634 nm (T = 2.2%). The distances between M1 and M2 as well as M2 and M4 were set to be 240 and 76 mm, respectively, resulting in the fundamental resonator geometric length of 316 mm. Based on the method presented in Ref. [28], the thermal lens focal length of the a-cut Nd:YVO4 composite crystal was measured to be about 180 mm under the launched pump power of 21.2 W. According to the ABCD transfer-matrix theory, the spot radii of the 1342 nm TEM00 mode at the laser and Raman media were estimated to be approximately 180 and 80 μm, respectively. The Raman resonator was defined by the flat dichroic mirror M3 and the output coupler M4. The dichroic mirror M3, having HT coated at 1342 nm (T > 98.8%) and HR coated at 1634 nm (R > 99.8%), was placed in the second arm (M2–M4) in order to collect the Stokes light in the backward propagation. The Raman resonator was set to be as short as 14 mm, which gave rise to the calculated Raman laser mode size inside the diamond crystal of about 280 μm under the full Stokes output power, based on the estimated thermal lens focal length of diamond [29,30]. A calibrated optical power meter (Physcience Opto-Electronics, LP-3C) was utilized to detect the output power. The laser spectra were monitored by an optical spectrum analyzer (Yokogawa, AQ6374) with the resolution of 0.05 nm. The pulse temporal behavior was registered by an Agilent digital oscilloscope (DSO90604A, 6 GHz bandwidth) connected to a fast photodiode (DET08CL/M, 5 GHz bandwidth).
3. Experimental results and discussions
Preliminary testing of the actively Q-switched Nd:YVO4 laser was implemented by replacing the Raman output coupler with a plane mirror having a PR coating at 1342 nm (T ≈ 5%). For ease of comparison, the PRF was chosen to be 25 kHz due to its optimized Raman conversion efficiency in the following investigations. As displayed in Fig. 2, this oscillator reached the threshold at 0.8 W of incident pump power and delivered the maximum average power of 4.0 W under the launched pump power of 22.4 W, corresponding to an optical power conversion efficiency of 17.9%. With the increasing of incident pump power, the resonator became unstable, and the roll-over effect of average output power was observed. This phenomenon can be attributed to the thermal lensing effect of laser crystal. Further efforts to improve the average output power will focus on reducing the thermal lens strength in the Nd:YVO4 crystal and optimizing the fundamental resonator structure. Under the full output power, the pulse duration was determined to be ∼60 ns, and the central wavelength was measured to be 1341.7 nm with a narrow full-width at half-maximum (FWHM) of 0.11 nm (18.3 GHz). By using the scanning-knife-edge method, the beam quality factor M2 of the 1342 nm laser was measured to be 1.6 under the maximum launched pump power of 22.4 W.
Fig. 2. Average output power with respect to incident pump power for the actively Q-switched Nd:YVO4 laser operating at 1342 nm.
Afterward, the output performances of the intra-cavity DRL were evaluated at the PRF of 25 kHz. The laser emission spectrum was monitored under the incident pump power of 21.2 W, as depicted in Fig. 3. As we can see, the central wavelength of the DRL was located at 1633.7 nm associated with the Raman shift of 1332.3 cm−1, and no other Stokes line was observed over the whole pump power range. Spectral width of the DRL was determined to be approximately 0.05 nm (5.6 GHz) FWHM (resolution limited), which was slightly smaller than that of the fundamental laser. It can be explained by the fact that the fundamental spectrum was totally covered by the Raman linewidth of Diamond (45 GHz), thus all the longitudinal modes of the fundamental field can be extracted only by a few Stokes modes, due to the hole-burning free SRS gain [31, 32]. As a result, the effective Raman gain coefficient from fundamental field to first-Stokes field can exceed 65% of its steady state value g0, which is beneficial for efficient Raman conversion [33].
Figure 4(a) shows the power transfer of the DRL at 1634 nm with respect to the incident pump power. The threshold of the DRL occurred for an incident pump power of 1.5 W, which was about twice as high as that of the fundamental laser. The maximum average output power of 2.0 W was achieved under the incident pump power of 21.2 W, corresponding to an optical-to-optical conversion efficiency of 9.4% and a slope efficiency of 10.2%. Based on the actively Q-switched 1342 nm Nd:YVO4 laser under the same pumping condition, the conversion efficiency from the fundamental field to the first-Stokes field was speculated to be as high as ∼51%, which is 32 times larger than that of the external cavity DRL at 1.6 μm [24]. To our knowledge, the average power and conversion efficiency reported here are also the highest values among the 1.6 μm intra-cavity crystalline Raman lasers [17–19]. Although the diamond length (6 mm) employed here was far less than other Raman crystal lengths (usually equal to or greater than 30 mm), the higher Raman conversion efficiency obtained by diamond could be mainly attributed to three aspects: (i) the 2-4 times higher Raman gain coefficient of diamond compared to other Raman media, such as BaWO4, SrWO4, and YVO4, (ii) the narrower fundamental and Stokes spectra resulting in higher effective Raman gain, and (iii) the small spot radius of fundamental laser on Raman crystal as well as good mode overlap between the pump beam and the fundamental beam, which was enabled by the elaborated V-shaped resonator configuration. In addition, further enhancement of the Raman conversion efficiency might be anticipated by increasing the diamond length, reducing the intra-cavity losses, and optimizing output coupling at the first-Stokes wavelength. During laser characterizations, the average output power at 1634 nm was very stable, typically varying by <3.2% over one hour. Similar to the fundamental laser operation, the cavity became unstable for the incident pump power exceeding 21.2 W, and an obvious decline in average output power was detected. In comparison with the fundamental laser, the beam quality of the DRL has been improved due to the so-called ‘Raman beam cleanup’ effect, and its beam propagation factor M2 was measured to be 1.3 at the highest output power. The good beam quality indicated that thermal effects in the highly thermally conductive Raman material can be negligible at such power levels. In all cases, there were no evidences of diamond damage.
Fig. 4. (a) Average output power, (b) pulse duration, and peak power of the intra-cavity DRL with respect to the incident pump power.
The pulse duration of the DRL as a function of incident pump power was recorded at the PRF of 25 kHz. As visualized in Fig. 4(b), the pulse duration declined monotonously with the incident pump power, and go down to 5.7 ns for the maximum incident pump power of 21.2 W. Moreover, we simultaneously recorded the oscilloscope trace of the 1342 nm fundamental laser through the leaking beam from M2, as depicted in the inset of Fig. 5(b). It can be seen that the pulse duration of the DRL is much narrower than that of fundamental beam (∼25 ns pulse duration) due to the pulse duration shortening effect of SRS process [34]. As a result, the maximum peak power at 25 kHz PRF was up to approximately 14 kW. In contrast to the external cavity DRL at 1.63 μm [24], the peak power has been improved by about twice. Furthermore, the pulse train and single pulse profile were monitored under the full output power, as plotted in Fig. 5. Clearly, it can be seen from Fig. 5(a) that the pulse train of the intra-cavity DRL was very stable, and the peak-to-peak intensity fluctuations at 1634 nm were measured to be better than ±2.4%. We tentatively speculated that this fluctuation mainly resulted from the thermal and mechanical instabilities of current laser system. As illustrated in Fig. 5(b), a smooth single pulse was observed on the oscilloscope without any sub-pulse.
Fig. 5. Pulse train and temporal pulse profile of the intra-cavity DRL under the optimal PRF of 25 kHz and the incident pump power of 21.2 W; Inset: temporal pulse profile of the 1342 nm fundamental laser.
Under the fixed incident pump power of 21.2 W, the output characteristics of the DRL were further investigated by extending the PRF from 10 to 50 kHz with an increment of 5 kHz. It can be seen from Fig. 6(a) that the average output power climbed to the highest level firstly with boosting the PRF from 10 to 25 kHz, and then began to descend for larger PRF. We suspect that the superior output power at 25 kHz can be attributable to the balance between the intra-cavity fundamental average power and the intra-cavity fundamental peak power or Raman conversion efficiency. Interestingly, based on the numerical model of actively Q-switched intra-cavity Raman lasers [35], we concluded that the optimal PRF is mainly related to the upper-laser-level lifetime τf of laser crystal, and it can be estimated to about 2/τf. This conclusion has been experimentally verified in this work (25 kHz for ∼90 μs lifetime of Nd:YVO4) and our previous publications (4 kHz for ∼500 μs lifetime of Nd:YLF) [32,36]. Accordingly, with the PRF increasing from 10 to 50 kHz, the pulse energy of the DRL decreased smoothly from 154.4 to 16.0 μJ. Furthermore, as plotted in Fig. 6(b), the pulse duration of the DRL increased roughly from ∼6 to 11 ns, which lead to the peak power dropping from 25.3 to 1.4 kW. Because of the non-standardized robust deposition of AR coatings on diamond [37], which is a key limiting factor for high power and high energy Raman laser output, hence the lower repetition rate was not attempted to protect the dielectric coating of diamond.
Fig. 6. (a) Average output power and pulse energy, (b) pulse duration and peak power as a function of PRF for the intra-cavity DRL under the launched pump power of 21.2 W.
4. Conclusion
To conclude, we reported the first realization of intra-cavity DRL at 1.6 µm. Driven by the in-band pumped actively Q-switched 1342 nm Nd:YVO4 composite laser, the highest average output power up to 2.0 W was achieved with an optical to optical conversion efficiency of 9.4% under the optimal PRF of 25 kHz. Accordingly, the pulse duration was determined to be 5.7 ns with the peak power of 14 kW. Meanwhile, the highest peak power up to 25.3 kW was obtained with a PRF of 10 kHz. To the best of our knowledge, this is the highest average power and the maximum conversion efficiency achieved at 1.6 µm from the crystalline Raman intra-cavity setup. Transversal fundamental mode output was realized with near diffraction limited beam quality of M2 ≈ 1.3 over the entire pump power range. Future efforts are dedicated to scale average output power and expand pulse operation mode, which are well suited to address the demand in diversified applications.
Funding
National Natural Science Foundation of China (62175093, 62175091, 61935010, 51872307, 51972149); Research and Development Program in Key Areas of Guangdong Province (2020B090922006); Guangzhou Science and Technology Project (201904010294).
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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