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Abstract
A continuous-wave (CW) single-longitudinal-mode (SLM) intracavity Raman laser is demonstrated for the first time, by virtue of the spatial hole-burning free nature of stimulated Raman scattering (SRS) gain. By using a single etalon in the Nd:GdVO4 fundamental laser cavity, the spectral linewidth of the multimode fundamental field is suppressed below the Raman linewidth of Raman crystal BaWO4; hence power in all the longitudinal modes of fundamental field can be extracted by one single Stokes mode. Therefore, the hole-burning free SRS gain exhibits a spectral cleanup effect whereby a stable SLM Stokes field is derived from the multimode fundamental field within a simple standing-wave cavity arrangement. The low-threshold SLM Raman laser delivered 3.42 W SLM Stokes and 1.53 W SLM one-way yellow harmonic at the guide-star wavelength of 589.16 nm. The results here provide a new approach to SLM laser operation with good simplicity and power dynamic range. Further engineering for power scaling and better stability is also discussed.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Single-longitudinal-mode (SLM) lasers are of significant importance in scientific applications like gravitational wave detection, remote sensing and laser cooling [1–3]. SLM operation of a laser may be achieved through a number of means, including a unidirectional ring cavity to eliminate spatial hole burning [4], injection seeding or using frequency-selective elements to suppress undesired modes [5,6], a short-cavity design with large mode spacing combined with narrow-band reflectors [7,8], and some other less- conventional approaches such as twisted-mode techniques, and using saturable absorbers [9,10].
To produce laser wavelengths outside that generated through traditional laser gain media, nonlinear frequency conversion is often used. One approach, stimulated Raman scattering (SRS), can be combined with second harmonic generation (SHG), as an efficient technique to access the yellow spectral region around 590 nm and the 1.5 μm eye-safe region, where narrow-linewidth laser sources are important for applications including laser guide star (LGS), spectroscopy and remote sensing [2,11,12]. Specific requirements for the guide star lasers are considered later in this paper. Prior reports of single-longitudinal-mode Raman lasers are mainly based on the methods applied to inversion lasers. Single-frequency Raman lasers/amplifiers with injection seeding have been demonstrated with fiber, gaseous and crystalline-based systems [12–14]. For the injection-seeded gaseous and crystalline systems, pulsed pumping with high peak power were required due to the relatively low χ(3) nonlinear gain [13,14], while for the fiber systems good power/efficiency have been achieved by virtue of the long interaction length [12]. In 2015, Lee et. al. reported a single-frequency monolithic Nd:YVO4 self-Raman laser which was cryogenically cooled – the Raman gain linewidth at the extremely low temperature was narrowed significantly and the monolithic short cavity provided a large mode spacing [15]; in this case, 1.36 W single-frequency Stokes emission at 1176 nm was obtained under 17.2 W of absorbed diode pump. In 2018, Liu et. al. demonstrated single-frequency operation of an actively Q-switched, intracavity, coupled-cavity Raman laser at 1178.3 nm by inserting two etalons with different thickness into the Stokes cavity [16]. However, the average output power was limited to 41 mW by the insertion losses of the etalons used in the Stokes cavity.
In 2016, researchers at Macquarie University demonstrated a SLM external resonator Raman laser based on the hole burning free nature of the SRS gain [17]. In contrast to inversion laser gain mediums, SRS does not suffer from spatial hole burning, and hence can generate SLM Stokes with a simple standing wave cavity. The laser in [17] was pumped by a 1.06 μm single-mode amplified fiber laser, and delivered up to 4 W SLM Stokes output at 1.24 μm under ~20 W fundamental pump power and eventually became multimode because of thermal loading at higher power. Later, the same group reported a SLM second Stokes Raman laser at 1.5 μm and a SLM Raman laser resonantly pumped by a tunable single-frequency Ti:Sapphire laser has also been demonstrated [2,18]. However, all these approaches are based on the fundamental field itself being single-frequency and the use of the high-threshold extracavity or pump-enhanced schemes, therefore the overall efficiency and the system complexity can be an issue.
In this work, we demonstrate a SLM intracavity Raman laser utilizing the spectral cleanup property of SRS gain. By using a single etalon in the Nd:GdVO4 fundamental laser cavity, the multimode 1.06 μm fundamental linewidth is suppressed to below 0.1 nm, not SLM but narrower than the Raman linewidth of a BaWO4 crystal, which serves as the Raman gain medium. Multi-watt, stable SLM Stokes is derived from the multimode fundamental through hole burning free Raman gain. Furthermore, the Stokes wavelength could be tuned over 4 nm by tilting the etalon to change the Nd:GdVO4 fundamental laser wavelength. Single-frequency yellow output at the guide-star wavelength of 589.16 nm is obtained by intracavity frequency-doubling the Stokes wave when it was tuned to 1178.32 nm, and the potential for this technology to be used for laser guide-star applications is discussed
2. Spectral behavior of Raman lasers
In contrast to the inversion laser gain medium, the Stokes field is driven from a high-intensity fundamental field, rather than from energy stored in the laser crystal. Thus there can be no spatial hole burning in the Raman process, and this conventional impediment to SLM operation is thus absent. However, despite the lack of spatial hole burning in the Raman process, intracavity Raman lasers rarely operate on a single longitudinal mode, due to effects that are somewhat analogous to spectral hole burning and are detailed as follows. For lasers in the high-dispersion regime, the Raman linewidth defines the width of influence for each Stokes and fundamental mode [19]. Each fundamental mode independently provides gain for a range of Stokes modes centered on that pump mode’s downshifted frequency, with the shape of that gain profile given by the Raman lineshape. This means that the overall gain spectrum for the Stokes field is given by the convolution of the fundamental spectrum with the Raman spectrum. The converse is also true: each Stokes mode independently provides loss for a range of fundamental modes centered on that Stokes mode’s upshifted frequency, and the spectrum of loss presented to the fundamental field is the convolution of the Stokes spectrum with the Raman spectrum.
Theoretically, a single-frequency fundamental is not essential for SLM operation of the SRS-generated Stokes field based on the hole-burning-free nature of SRS gain. Provided the spectral linewidth of the multimode fundamental is below the Raman linewidth, power in all the spectral components (or longitudinal modes) of the fundamental could be extracted by a single Stokes mode efficiently. If the fundamental spectrum is broader than the loss spectrum created by the Raman process, a spectral hole will be depleted in the fundamental spectrum. This is similar in outcome to spectral hole burning in conventional lasers, where a spectral hole in the gain occurs when the laser linewidth convolved with the homogeneous gain linewidth is narrower than the inhomogeneous linewidth of the gain. (To be clear though, in the Raman case the effect is not related to the broadening mechanism of the Raman gain.)
For externally pumped Raman lasers, such spectral hole burning means that a narrowband Raman laser could not extract all of the pump energy from a pump laser with a bandwidth larger than the Raman linewidth, and we would then expect the Raman laser to oscillate with a broader spectrum to access more of the pump spectrum. Intracavity Raman laser are much more complex and dynamic systems, in which the fundamental spectrum is determined by the interplay of the laser gain spectrum and the loss spectrum owing to the Stokes field. If the laser gain linewidth is larger than the Raman linewidth, a narrow Stokes field dominantly depleting the fundamental near line center will encourage the fundamental field to broaden into the wings of the gain profile, with laser gain falling, and decreasing more slowly than the Stokes loss away from linecenter. This feedback effect can result in a severely broadened fundamental spectrum, with the Stokes spectrum also broadening in response [20–22]. The effective Raman gain for these broadened spectra can be substantially less than that for narrow fields and so spectral problems often contribute to the poor slope efficiencies of many intracavity Raman lasers (a worsening spectrum as the laser is pumped harder presents as a lowering of the measured slope efficiency) [23].
We see from this analysis that it is the broadening of the fundamental spectrum is the root cause of the broad Stokes spectrum for intracavity Raman lasers. If we can keep the fundamental spectrum substantially narrower than the Raman linewidth, then a single Stokes mode can in principle, efficiently extract the fundamental power, and in the absence of spatial hole burning, we can expect such SLM operation to be stable against competing Stokes modes. By inserting an etalon into the fundamental cavity to narrow the effective gain profile for the fundamental spectrum, we show that we can prevent the fundamental from broadening sufficiently to allow the Stokes field to operate stably in a single longitudinal mode.
Intracavity frequency doubling of the Stokes field will not affect this interplay between the fundamental and Stokes fields, and so we still expect SLM Stokes operation to be possible. Indeed, provided the second-order phase-matching bandwidth is much larger than the Stokes mode spacing, sum-frequency mixing (SFM) between Stokes modes will actively stabilize SLM operation, as already observed in lasers and OPOs [24,25].
3. Experimental arrangements
The design of the SLM Raman laser is shown in Fig. 1. The pump source was a fiber-coupled LD (nLight element e06) emitting at 878.6 nm with a narrow linewidth of ~0.3 nm (FWHM); these spectral characteristics were independent of pump power. The core diameter and the NA of the fiber were 200 µm and 0.22, respectively. A 1:2 multi-lens coupler refocused the pump light into a a-cut Nd:GdVO4 laser crystal which has an anti-reflective (AR) coating (R<0.1%) at the pump and laser wavelengths on both facets. The 0.3-at.%-doped, 10 mm long Nd:GdVO4 crystal was measured to absorb ~75% of the incident pump under non-lasing conditions. A 16 mm long a-cut BaWO4 crystal which was AR coated for both 1.06 μm fundamental and 1.18 μm Stokes was used as Raman crystal. The BaWO4 Raman peak at 925 cm−1 has a FWHM linewidth of 48 GHz [26]. The fundamental polarization was parallel to the crystal’s a-axis to avoid a secondary Raman peak at 332 cm−1 [27]. Both the laser and Raman crystals were wrapped in indium foil and mounted in water-cooled copper holders maintained at 20°C. The Nd:GdVO4 fundamental laser cavity was defined by a flat mirror M1 and two concave mirrors M2 and M3, each having radii of curvature (ROC) of 100 mm. The distance between the Nd:GdVO4 crystal and the pump mirror M1was ~4 mm. M1 and M2 were AR coated at 880 nm (R<0.2%) and for high reflectivity (HR) at the fundamental (R>99.97%) and Stokes (R>99.99%) wavelengths. In the case where the laser was designed to deliver maximum Stokes output power, mirror M3 served as the output coupler and was coated for HR at the fundamental and 0.45% transmission at the Stokes wavelengths. When the laser was designed to generate yellow second-harmonic output, M3 was coated for HR at both the fundamental and the Stokes and had high transmission (T>98.8%) for the yellow. The Stokes cavity comprised mirror M3 and a flat dichroic mirror M4, which was coated for HR at the Stokes (R>99.9%) and highly transmissive (T>99.5%) at fundamental. The Stokes cavity length was set as short as possible (18 mm for Stokes output and 29 mm for yellow output), to widen the longitudinal mode spacing of the Stokes field, for the purpose of better SLM stability. The crystal used for frequency doubling was a LBO crystal cut at θpm = 90°, φ pm = 0° (dimensions 4 × 4 × 8.5 mm3, with both surfaces AR coated for 1063 – 1180 nm). It was mounted in a copper holder heated to ~41°C, to achieve non-critical phase matching (NCPM).
Fig. 1 Schematic of the single-frequency intracavity Raman laser. Fundamental cavity: M1-M2-M3; Stokes cavity: M3-M4. M1-M2: ~104 mm; M2-M3: ~70 mm; M3-M4: ~18 mm for Stokes output without LBO in the cavity and ~29 mm for yellow output with LBO in.
Thermal lens focal lengths were estimated based on data in [28]. Considering the estimated ( + )80 mm thermal lens focal length in the Nd:GdVO4 and the astigmatic negative thermal lens focal lengths in BaWO4 of (-)50 mm in the c-axis direction and (-)300 mm in the a-axis direction based on the measured Stokes output power of around 3 W, the 174 mm long fundamental cavity resulted in elliptical TEM00 mode spots of around 130 × 200 μm (radii) in the laser crystal and near-circular mode spots of around 130 μm in the Raman crystal, respectively, under the maximum pump of 19-20 W used in the experiment. The Stokes beam spot in the BaWO4 crystal was estimated to be a 100 × 180 μm (radii) elliptical mode.
A fused silica etalon with thickness of 100 μm and coating for R = 30% at 1063 nm was used to control the fundamental linewidth. The free spectral range (FSR) and the full width at T = 98% of the etalon are 3.85 nm and 0.22 nm at the fundamental wavelength λf of 1.06 μm. The etalon was placed into the fundamental cavity between the Nd:GdVO4 crystal and mirror M2, which was close to a beam waist, and where the TEM00 resonator mode was around 130-140 μm. The etalon inserted some loss into the fundamental resonator, but no loss was introduced to the Stokes cavity. The laser spectrum was recorded by a Spectrum Analyzer (Bristol 771A) which measured wavelengths to an accuracy of 0.2 pm and displayed the laser spectrum to a resolution of 2 GHz. The longitudinal mode spacing of the 18 mm Stokes cavity was ~4.5 GHz, therefore it is straightforward to observe the number of longitudinal modes using this spectrum analyzer.
4. Experimental results
4.1 Single-longitudinal-mode Stokes output
We first consider the characteristics of the free-running intracavity Raman laser, ie the laser without an etalon in the cavity. The fundamental field exhibited enormous spectral broadening due to SRS. Figure 2(a) shows the fundamental spectrum under the maximum pump power of 18.7 W. Compared with the case of SRS off [22], the fundamental spectral linewidth was broadened from ~0.2 nm to over 0.7 nm (186 GHz), much broader than the 48 GHz linewidth of the 925 cm−1 Raman transition in BaWO4. The fundamental spectrum consisted of several peaks separated by 0.12-0.13 nm which are attributed to the weak etalon effects between the component facets. The SRS threshold was 1.3 W incident LD power and the maximum Stokes power reached 1.72 W under pump power of 18.7 W, with corresponding optical efficiency being 9.2%, as shown in Fig. 3. The Stokes spectrum was also consisted of several separated peaks. The Raman gain profile, which is determined by the convolution of measured fundamental spectrum and the Lorentzian Raman lineshape [21,25], is also shown in Fig. 2(a).
Fig. 2 Measured Stokes and fundamental spectra and the calculated Raman gain profile based on the fundamental spectra for the cases of (a) free-running and (b) with the etalon in the cavity, under incident diode pump power of 18.7 W. Inset: zoomed Raman gain profile based on the narrowed fundamental spectrum and the Stokes cavity mode spacing.
Fig. 3 Power transfers for two Raman laser: the free-running laser, and the SLM (λs = 1178.3 nm with λf = 1062.5 nm) Stokes laser which incorporated the 100 μm etalon. The lines are a guide to the eye.
With the etalon inserted into the fundamental laser cavity, the fundamental laser was forced to operate at the etalon transmission peak wavelength and the spectral linewidth was narrowed significantly to ~0.05 nm (~13 GHz). Figure 2(b) gives the fundamental spectrum with etalon under incident pump power of 18.7 W. For the 100 μm fused silica etalon used in the experiment, the fundamental wavelength was ~1062.5 nm with the etalon tilted at an angle of ~0.5° to the longitudinal axis of the resonator. Considering the ~0.76 GHz mode spacing of the fundamental cavity, the fundamental laser is estimated to contain over 15 longitudinal modes. From the Raman gain profile with narrowed fundamental spectrum in Fig. 2(b), we know that the gain difference between the central Stokes longitudinal mode and the neighboring modes is only 2% - 3% (assuming that the central Stokes mode was just at the gain peak). However, since the narrowed fundamental spectrum was totally covered by the 48 GHz Raman linewidth, a SLM Stokes field, derived from the multimode fundamental field, in the standing-wave Stokes cavity was observed as anticipated, by virtue of the hole-burning free nature of the SRS gain.
The etalon will clearly introduce insertion losses into the fundamental cavity, leading to higher losses which increased the SRS threshold from 1.3 W to 2.2 W. However, since the narrowed fundamental and Stokes fields both contributed to the effective Raman gain, the maximum Stokes power under the fixed maximum pump was improved significantly to 3.42 W. We did not further increase the pump power to avoid the risk of crystal damage. With the measured fundamental and Stokes spectra, effective Raman gain coefficient geff can be calculated by using the Eq. (3).31) in [19]. For the free-running case, the broadened spectra result in a geff only ~22% of the steady state Raman gain coefficient g0 at maximum pump power. With the narrowed fundamental and resultant SLM Stokes, the geff increased to ~70% of g0 at maximum pump power, and thus enhanced the Stokes power significantly.
The longitudinal mode behavior of the single-frequency Raman laser was further characterized by using a scanning Fabry–Pérot interferometer (FPI) with a FSR of 1.5 GHz and a stated resolution of 7.5 MHz. Figure 4 gives the scanning FPI trace at the maximum output power of 3.42 W. The SLM operation was rather stable without need for active cavity stabilization, showing no evidence of mode-hop or multi-mode operation over several minutes of monitoring, at maximum power. The linewidth was 30-35 MHz and did not show obvious dependence on the pump power. The signal to noise ratio in Fig. 4 was measured to be over 40, and the SLM operation of the Stokes field, which was derived from the multimode fundamental was still very stable under the maximum pump which is 8.5 times of SRS threshold. We coin the term “spectral cleanup” to describe this behavior. The SRS conversion exhibits spectral cleanup which not only extends the spectral coverage of laser output, but also provides a simple and cost effective method of realizing compact single-frequency laser which can operate efficiently over a large dynamic range. This is in contrast with the SLM extracavity diamond Raman laser reported in [17], which became multimode when the pump power exceeded 1.8 times of the SRS threshold.
Fig. 4 FPI trace of the Raman laser operating at 3.42 W SLM Stokes output (FPI: Thorlabs SA200-8B with a FSR of 1.5GHz and a resolution of 7.5 MHz).
Wavelength tuning of the Raman laser was achieved by tilting the etalon to change the fundamental wavelength. Figure 5(a) shows the tuning curve recorded under the incident pump power of 16.5 W. The filled/unfilled symbols correspond to the nth and (n + 1)th orders, and it can be seen that stable Stokes was not obtained for tilt angles between 2.8° and 3.1° due to unstable dual-wavelength operation of fundamental. The fundamental tuning range was 1061.9-1065.6 nm, which was limited by the FSR of the 100 μm fused silica etalon used, and was far wider than the ~1 nm FWHM emission bandwidth of the Nd-vanadate. A corresponding Stokes tuning range of 1177.57-1182.12 nm was achieved. The intracavity Raman laser was capable of operating efficiently even when tuned well into the wings of the laser emission band. The Stokes output remained stably SLM over the whole tuning range, and the polarization of both the fundamental and the Stokes did not change. It can be seen that the Stokes power at 1179.1 nm, which corresponded to the 1063.1 nm emission peak of Nd:GdVO4, was ~20% lower than that at 1178.3 nm with the λf of 1062.5 nm. This was due to the walk-off losses introduced by tilting the etalon, as required for wavelength tuning. Walk-off losses for the fundamental were calculated according to the method in [29], and are shown in Fig. 5(b). The etalon need to be tilted at ~6.4° to tune the λf to 1063.1 nm, which brought a theoretical walk-off loss of ~0.7% to the fundamental laser. In contrast, the tilt angle and corresponding walk-off losses were quite tiny when operating at the λf of 1062.5 nm.
Fig. 5 (a) Stokes wavelength tuning curve of SLM Raman laser under incident diode pump power of 16.5 W, and (b) the calculated walk-off loss for the fundamental field induced by the tilted etalon as a function of external tilt angle wavelength. Walk-off losses are shown for the etalon angles for which SRS occurred, and the corresponding Stokes wavelengths are also shown.
4.2 SLM yellow output at the guide star wavelength (589.16 nm)
Yellow output was achieved by inserting the LBO crystal into the Stokes cavity and replacing M3 with a mirror coated for HR at fundamental and Stokes and AR at yellow. The Stokes cavity was then extended to 29 mm in order to accommodate the LBO crystal, with the corresponding longitudinal mode separation being ~3 GHz (0.014 nm at 1178.32 nm). The spectral content of the yellow output was characterized by using a scanning FPI (Thorlabs SA200-5B) which has a FSR of 1.5 GHz and a stated resolution of 7.5 MHz in the yellow spectral region.
By tuning the fundamental wavelength to 1062.51 nm, single-frequency Stokes at 1178.32 nm was resonated and single-frequency yellow output at the guide-star wavelength of 589.16 nm was obtained. The etalon tilt angle was approximately 0.5 degrees. As shown in Fig. 6, the (SRS) threshold for yellow output was 1.8 W and the maximum one-way stable single-frequency yellow power at 589.16 nm was 1.53 W for an incident pump power of 16.5 W. To the best of our knowledge, this is the first solid-state Raman laser providing CW single-frequency yellow output at the guide-star wavelength. Further increasing the pump power was found to reduce the stability of SLM operation, e.g. bursts of multimode operation frequently alternated with seconds of SLM operation. Multimode yellow powers up to 2.07 W were obtained under incident pump power of 19.7 W, considerably larger than the 0.82 W yellow output centered at ~589.6 nm (linewidth >0.2 nm) that could be achieved for the free-running laser. This again highlights the benefit of using an etalon to control spectral broadening and thereby yield higher effective Raman gain. Considering the backwards propagating yellow output was not collected, it should be possible to further increase the yellow power by a factor approaching two by using a suitable intracavity yellow reflector in the Stokes cavity [30].
Fig. 6 Power transfer of the one-way yellow output at the guide-star wavelength. Inset: SLM yellow spectrum under 16.5 W diode pump power.
It is interesting to note that that the Stokes output described in section 3.1 oscillated on a single-longitudinal mode over the full pump range, whereas the laser described here tended towards multi-mode operation at pump powers above 16.5 W, even though the intracavity SFM between Stokes longitudinal modes would tend to stabilize the SLM yellow operation [24,25]. Primarily this is associated with the smaller Stokes mode separation for the 29 mm long cavity used for yellow generation, compared to the 18 mm long Stokes cavity, which did not require an LBO crystal. This means, there is a lesser gain difference between the central and adjacent Stokes modes. The effect of Stokes cavity length was investigated, and it was found that SLM operation could be achieved for Stokes cavities as long as 110 mm, thus demonstrating the underlying principle behind SLM operation in spatial hole-burning free Raman lasers. However, the longer Stokes cavity was observed to result in the SLM operation of the laser being less stable, and this is consistent with more-closely spaced longitudinal modes being less resilient to the vibrations and thermal fluctuations that inevitably occur in a water-cooled laser, which has not been engineered for SLM operation. The optical bench on which the laser was built was not floating, and tapping the bench caused modes to hop. At higher powers, temperature fluctuations in the laser, Raman and SHG crystals, and the etalon are more likely to occur. Heating the LBO to 41°C in yellow output experiment would also cause air turbulence and worsen the SLM stability. The effects of heating could be clearly seen in the laser warm-up phase, where mode hopping and wavelength drift were observed over a period of several seconds prior to the laser reaching steady state. Fig. 7(a) shows the wavelength stability of the 1.53 W single-frequency yellow output recorded over a period of 300 seconds. It can be seen that the yellow output occasionally exhibited a wavelength hop of ~7 pm (~6 GHz), which corresponded to the mode hop of the Stokes wave with the mode spacing of ~3 GHz. The damped oscillations evident in Fig. 7(a) are characteristic of mechanical disturbance and are also apparent in fig.4 of [2]. The linewidth of the single-frequency yellow output was measured to be 13-20 MHz by using the scanning FPI, as shown in Fig. 7(b), which was narrower than that of the Stokes output.
Fig. 7 (a) Wavelength stability and (b) FPI trace of the 1.53 W SLM yellow output under 16.5 W pump.
For a laser to be useful as an artificial sodium guide star, it must meet certain requirements in relation to power, polarization and spectral properties. Its wavelength must match the Sodium D2a resonance at 589.16 nm, and ideally it would have some tunability around this wavelength so it can be frequency-locked when required or tuned off the resonance in order to estimate the Rayleigh background. Its linewidth needs to be sufficiently narrow (≤50 MHz is typical) compared to the sodium resonance to enable efficient excitation of the sodium ions in the mesosphere. Linear (or circular) polarization is preferred as it has been shown to be more effective in terms of fluorescence yield, and high beam quality is required to define a sufficiently bright artificial star. CW or quasi-cw lasers are suitable, with macro-micropulse operation said to beneficial to avoid saturation of the transition [31]. Finally, the laser power needs to be high enough to deliver a sufficiently-high photon return; this can vary between 100 and 1000 photons cm−2s−1 depending on the application. Higher returns are required for visible astronomy, than for near-infrared astronomy [32,33]. Details about sodium guide star requirements can be found in several papers [31–36].
While lasers with powers as low as 2.4 W have been used on-sky effectively, for near-infrared astronomy [6], the current gold standard is the Toptica laser [37], which has output powers of 20 W and is based on Yb fiber amplifier technology, which is present on many new state-of-the-art telescopes. There is however a trade-off between cost and photon returns, and there continues to be interest in developing new laser guide star sources. Alternative lasers proposed include dye lasers, sum frequency lasers based on combining two solid laser sources eg Nd:YAG at 1064 nm and Nd:YAG at 1319 nm, and very recently frequency-doubled optically-pumped semiconductor lasers [38]. This latter development, which has already demonstrated powers of 12 W at 589 nm, could prove to be a disruptive technology owing to it’s relative simplicity and low pump power requirements.
The laser reported in this paper meets all the guide-star requirements detailed above, with the exception of output power. Like the semiconductor guide-star laser in [38], it has the strongly positive attributes of relative simplicity and high overall efficiency. The achievement of single longitudinal mode operation of a Raman laser, by virtue of the spatial hole burning free gain therein, and without the requirement for a single longitudinal mode pump laser, represents a breakthrough in crystalline Raman lasers. The yellow laser output power reported here is 1.53 W, however we anticipate it will be straightforward to engineer a laser with continuous-wave output power of around 5 W, and with quasi-cw pumping, we are hopeful that powers up to 10 W will be achievable. As shown in this paper, spectral control of the fundamental leads to enhanced output powers, compared to free-running lasers, and we have previously reported free-running cw yellow output powers of 4.3 W at 586 nm [39] and quasi-cw (50% duty cycle) yellow output powers of 6.5 W at 586.5 nm [40]. With further engineering, we are cautiously optimistic that yellow Raman lasers will emerge as an alternative and more-affordable technology for laser guide-stars application that will be useful for near-infrared astronomy, and possibly visible-astronomy platforms.
5. Conclusion
We have demonstrated a practical and efficient, CW, single-frequency intracavity Raman laser based on the spectral cleanup effects in the spatial hole-burning free SRS gain media. By suppressing the spectral linewidth of the Nd:GdVO4 fundamental laser to below the Raman linewidth of the Raman crystal, BaWO4 by means of an intracavity etalon, multiwatt stable single-frequency Stokes output and its second harmonic were achieved. The linewidth-narrowed Raman laser also led to enhanced Raman gain, and the SLM Stokes and yellow powers were considerably higher than those of the free-running case. Furthermore, the wavelength of single-frequency Stokes wave could be significantly tuned over several nanometers by tilting the etalon to change the fundamental wavelength, which largely extended the spectral coverage of solid-state Raman lasers and CW single-frequency yellow output could be tuned to the guide-star wavelength of 589.16 nm.
Our findings are highly significant in the context of crystalline Raman lases and approaches to SLM laser operation more generally. Further engineering is predicted to result in stable narrow-wavelength sources, and while it is likely that active cavity stabilization will be required for a practical or commercial device, such methods are becoming increasingly mainstream.
Funding
Shandong Province Key R&D Program (2017CXCC0808); Key laboratory of Opto-electronic Information Technology, Tianjin University, Ministry of Education, China.
Acknowledgments
The authors acknowledge Prof. Jiyang Wang and Prof. Huaijin Zhang at Shandong University for providing the BaWO4 crystal. Quan Sheng would like to acknowledge the support from China Scholarship Council (CSC).
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