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Abstract
We report a diamond Raman laser that is continuously-tunable across the range from 590 nm to 625 nm producing continuous wave output with up to 8 W. The system is based on an all-fiber and tunable (1020–1072 nm) Yb-doped pump laser with a spectral linewidth of 25 GHz that is Raman-shifted and frequency doubled in a cavity containing diamond and a lithium triborate second harmonic crystal. Despite the broad pump spectrum, single frequency output is obtained across the tuning range 590–615 nm. The results reveal a practical approach to obtain tunable high-power single-frequency laser in a wavelength region not well served by other laser technologies.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Tunable single-frequency lasers are fundamental tools in a plethora of fields including atomic clocks, quantum sensing and computing, gravitational wave detection, isotope separation and remote sensing [1–7]. These applications require single-longitudinal-mode (SLM) lasers to either couple strongly to narrow atomic or molecular transitions or provide the coherence needed for interferometric sensors. The main technologies that currently address these applications are Ti:sapphire lasers [8–11], external-cavity diode lasers (ECDLs) [12–14], distributed feedback (DFB) lasers [15–18], distributed Bragg reflector (DBR) lasers [19,20] and seeded rare-earth dopants fiber lasers [21–24]. Ti:sapphire lasers are a mature technology capable of tunable output in the range 700-1000 nm at powers up to approximately 10 W, whereas ECDLs, DFB and DBR lasers provide access to more near-infrared options that may be amplified to powers up to a few watts using semiconductor amplifiers or much high power for wavelength bands that overlap the spectral ranges of fiber amplifiers. Conversion of these continuous-wave (CW) sources to the visible is usually achieved using external second harmonic generation (SHG) schemes [25–28]. Despite these options, there is a need for high power single frequency sources in the yellow-red spectral range [6,7].
Lasers based on stimulated Raman scattering in conjunction with SHG are interesting sources of single-frequency output in the yellow-red spectral region with excellent potential for scaling to tens-of-watt powers [29–34]. For example, a seeded Raman fiber amplifier combined with external SHG produced SLM yellow emission at 589 nm with a CW power of 75 W [34], an approach that has provided the only commercial product for sodium guide star lasers to date [32]. Using diamond as the Raman medium, 38 W of SLM output at 620 nm [31] and a 22 W SLM output at 589 nm [33] have been demonstrated for a standing-wave Raman laser cavity with intracavity frequency-doubling. In the latter case, the pump laser comprised of a high power (65 W) seeded Yb-doped fiber (YDF) amplifier at the wavelength of 1018 nm to produce diamond Raman shifted output at the 1178 nm sub-harmonic. Based on the highly practical and wide tuning band (1010-1100 nm) of Yb fiber amplifier technology, there is a substantial opportunity to develop similar diamond-Raman systems with widely-tunable Stokes output in the infrared and SHG in the visible range across the yellow-red band.
In this work, a SLM laser tunable across 590 to 615 nm is demonstrated using an intracavity frequency-doubled diamond Raman laser and a Yb-fiber pump laser. This approach has several features that are promising for realizing practical high-power systems. The system is based on a broadband all-fiber pump laser, which due to the broad linewidth is power-scalable without concerns about detrimental nonlinear effects such as stimulated Brillouin scattering (SBS). Stable SLM operation in the standing-wave cavity is achieved via the homogeneous gain properties of the pumped-Raman medium in tandem with the longitudinal-mode competition provided by the intracavity SHG crystal [35]. The large Raman frequency of diamond (1332.3 cm-1) steps the wavelength into the desired band in a single step, in contrast to silica fiber for which two or three cascaded shifts are required. Moreover, the excellent thermal conductivity [36] and broad transparency of diamond crystal enables a potential for scaling to high output powers and greatly extending the wavelength coverage.
2. Experimental setup
Figure 1 shows a schematic of the system including the pump laser and diamond Raman laser designs. The pump laser comprises of a home-made linearly-polarized fiber laser with tunable wavelength from 1020 nm to 1072 nm, based on a ring-cavity tunable oscillator and two stages of fiber amplifier. The tuning element in the ring resonator consisted of an etalon-based filter (Agiltron, Inc) with a tuning range from 1010 nm to 1090 nm, a 3 dB bandwidth of 1 nm and a tuning resolution of > 0.1 nm. The ring laser also included approximately 2.5 m-long polarization-maintaining large-mode-area ytterbium-doped gain fiber, a polarization dependent isolator ensuring unidirectional laser propagation and linearly polarization, a 50:50 output coupler at 1030${\pm} $20 nm and a 976 nm laser diode pumping combiner. The seed laser longitudinal mode spacing was 19.5 MHz. Active fibers with high core/cladding ratio were used in the ring resonator (15/130 µm; PLMA-YDF-15/130-VIII, Nufern) and pre-amplifier (20/130 µm; PLMA-YDF-20/130-VIII, Nufern) and main-amplifier (20/130 µm; PLMA-YDF-20/130-VIII, Nufern) to suppress amplified spontaneous emission [37,38]. By optimizing the coil radii of the LMA fibers, the pump provided an output beam with beam quality of M2=1.24. After collimation and feedback-isolation, the pump beam was mode matched into the standing-wave diamond Raman and SHG resonator with a plano-convex f = 50 mm lens, shown in the dashed box at the bottom of the Fig. 1.
Fig. 1. Top: Schematic of the fiber laser with tunable wavelength from 1020 nm to 1072 nm. Bottom right: the diamond Raman laser with intracavity SHG. PLMA-YDF: polarization maintaining large mode area ytterbium-doped fiber, LD: laser diode. Bottom left: Photographs of diamond Raman laser running at the yellow (top) and red (bottom) ends of the spectrum.
The diamond laser resonator comprised of two plano-concave mirrors - an input mirror with 50 mm radius of curvature and an output coupler with 100 mm radius of curvature. The input mirror coatings were highly transmitting at the pump wavelengths (>98.5% at 1020-1080 nm), highly reflective at the Stokes wavelengths (>99.9% at 1180-1260 nm) and partially transmitting at the SHG wavelengths (30%-80% at 590-625 nm). The output coupler coatings were highly reflective at the pump and Stokes wavelengths (>99.9% at 1020-1260 nm) and highly transmitting at the SHG wavelengths (>99.5% at 590-630 nm). The Raman medium is a CVD-grown single-crystal diamond (Type IIa, from Element Six Ltd) with dimensions of 8 (L) ×4 (W) ×1.2 (H) mm3 and cut-plane for beam propagation along the <110> axis. The SHG crystal was LiB3O5 (LBO) cut at θ = 90°, ϕ = 0° with dimensions of 10 (L) × 4 (W) × 4 (H) mm3 and placed on a TEC temperature-controlled mount to match the phase-matching range of 10-40°C. The diamond crystal was placed at the waist of the near-concentric resonator and approximately 4 mm away from the LBO. The pump beam waist radius in the diamond was calculated as 27 µm and the Stokes beam waist radii in the diamond and LBO were calculated as 32 and 90 µm, respectively. The direction of pump polarization, diamond <111> axis and LBO slow axis were co-aligned to provide the highest Raman gain and the correct polarization for SHG phase matching. The surfaces of the diamond crystal and the LBO crystal were anti-reflection coated at the pump (R<1.5%, diamond; R<0.1%, LBO) and Stokes (R<0.2%, diamond; R<0.1%, LBO) wavelengths. The reflection loss of the LBO crystal at SHG wavelengths was less than 1%.
3. Experimental results and discussion
Figure 2 shows spectra for the pump, Stokes and SHG with normalized intensity as a function of the ring laser filter setting. The pump and Stokes spectra were measured using a spectrometer (MS9710C, Anritsu) with a resolution of 0.06 nm whereas the SHG was monitored by a second spectrometer (USB2000, Ocean Optics) with a resolution of 1.42 nm. In order to increase the output power and simultaneously prevent the amplified spontaneous emission at peak emission band of YDF, the operating wavelength of the pump was only tuned from 1020 nm to 1072 nm. After phonon frequency transmitting of diamond lattice, the produced Stokes wavelength covered a range of 1180 - 1250 nm. Then the intracavity SHG offered an output wavelength from 590 - 625 nm. The continuous tuning was achieved within a 5 min period by varying the pump wavelength and LBO temperature simultaneously. Two photographs of the diamond Raman laser were taken when the output was tuned from yellow to red, as shown in Fig. 1.
Fig. 2. Tunable spectra of the pump, first-order Stokes and SHG. Middle: The pump spectra with a tunable range from 1020 nm to 1072 nm; Right: the first-order Stokes with tunable spectra from 1180 nm to 1250 nm; Left: the SHG spectra with an output from 590 nm to 625 nm. The pump and Stokes were measured at about 10 nm intervals, and the SHG 5nm. Data for figure available in Dataset 1, Ref. [49].
As reported previously in Ref. [39], a pump laser spectrum of width less than spontaneous Raman linewidth is critical for SLM Stokes operation and efficient gain extraction. In this case, since the filter etalon has a Gaussian transmission profile and the insertion loss increases successively from the central wavelength, the number of oscillating longitudinal modes in the ring-cavity is determined by the pump intensity. At the pump power of 1 W, the ring oscillator delivered an output power of within 150 - 200 mW with a spectral full width at half maximum (FWHM) that varied up to 25 GHz. The conversion efficiency at each wavelength in the Yb emission band differs under the same pump power and gain fiber length, resulting the variety of output power. After the amplifiers and isolator, the pump laser power and spectral linewidth varied with wavelength as in Fig. 3(a). As the final isolator was designed and coated specifically at 1020 nm, the power suffered extra losses at longer wavelengths. As a result, the injected pump power was over 50 W in the range from 1020 nm to 1050 nm and decreased to 44.5 W at 1070 nm. The linewidth FWHM in the range 1020 - 1027 nm was less than 18 GHz (measurement resolution limited) increasing to 25 GHz at 1070 nm.
Fig. 3. (a) The injected pump power and spectral linewidth as a function of wavelength. (b) The SHG output power and LBO temperature versus output wavelength. Blue open square: the calculated phase-matching temperature at 620 nm and 625 nm. Data for figure available in Dataset 1, Ref. [49].
SLM operation was obtained across the range 590-615 nm. At 590 nm, the threshold of the Stokes resonator was 19 W and at an injection pump power of 51 W and an LBO phase-matching temperature of 39°C, the output power was 8.0 W with a slope efficiency of 25%. The longitudinal mode structure was investigated by using a scanning F-P interferometer (SA200-5B, ThorLabs) with a free spectrum range (FSR) of 1.5 GHz and a measurement resolution of 8 MHz. In the range from 590 nm to 615 nm, the measured SHG linewidth was instrument limited. Two scanning transmission peaks spaced by the FP FSR and a zoomed SLM spectral structure (inset) are shown in Fig. 4 for the example of λ = 590 nm.
Fig. 4. The SLM characteristics of 590 nm output measured using a scanning F-P interferometer. Inset: Zoom-in of the peak showing a FWHM bandwidth of 8 MHz. Data for figure available in Dataset 1, Ref. [49].
Tuning was achieved by varying the seed laser wavelength and simultaneously decreasing the LBO temperature. For noncritical phase-matching across the range 590 nm to 615 nm, the temperature was decreased from 39°C to 12°C (Fig. 3(b) - blue curve). The black curve in Fig. 3(b) shows the SHG output power at longer wavelengths decreases to 6.6 W at 615 nm, to 4.0 W at 620 nm and 1.2 W at 625 nm. The large decrease in power for λ > 620 nm is attributed the combination of lower pump power and the use of LBO temperatures that were intentionally elevated above optimal phase matching temperature to prevent the condensation of water vapor on the crystal surfaces. The LBO temperature was kept above 10°C, whereas the calculated phase-matching temperature using SNLO [40] at 620 nm and 625 nm was 8.5°C and 5.3°C, respectively (refer blue open squares in Fig. 3(b)). At 625 nm, the conversion efficiency decreased to 2.7%. Note that the SHG spectrum when using non-optimal temperatures leads to oscillations on multi-longitudinal modes due to the weakening of SHG mode self-suppression [31]. One solution is to substitute the LBO with a cut angle suitable for higher temperature phase matching or satisfying type II phase matching. Note that in the Stokes spectrum, some satellite side-peaks were evident in the wings of some Stokes peaks, such as the peaks measured at 1200 nm and 1250 nm. The intervals between peaks are approximately 8 GHz (0.04 nm) and fall within the instrument resolution of about 12 GHz (0.06 nm). As there was no cavity length locking control, it is likely that the side peaks are a result of mode-hops captured during the integration time of the sweeping spectrometer.
Although the diamond Raman and SHG resonator was adapted from Ref. [33], the maximum optical-to-optical conversion efficiency was about 15.7%, compared to 24.5% in the former case. The lower conversion efficiency here is mainly attributed to a higher threshold as a result of a broader pump linewidth and the lower available pump power. For a CW external cavity Raman laser, the effective Raman gain coefficient, ${g_{eff}}$ is given by ${g_{eff}=g_0\Delta {\omega _R}/(\Delta {\omega _R} + \Delta {\omega _P} + \Delta {\omega _S})}$, where ${g_0}$ is the monochromatic Raman gain coefficient, $\Delta {\omega _R}$ is the spontaneous Raman linewidth pumped by a SLM source, $\Delta {\omega _P}$ and $\Delta {\omega _S}$ are the pump and Stokes linewidths, respectively [41]. Therefore, the calculated effective Raman gain coefficient for a pump linewidth of 20 GHz is approximately 16% lower than the 8 GHz of [33], given that $\Delta {\omega _R}$ is 45 GHz [42] and that $\Delta {\omega _S}$ is small in this case of SLM operation. In order to further investigate the impact of pump linewidth, a second pump laser at 1025.3 nm with linewidth of 125.5 GHz (approximately three times of $\Delta {\omega _R}$) was employed. The diamond Raman resonator was analyzed without the LBO crystal to eliminate the impact of SHG, and the cavity length was shortened from 157 mm to 154 mm for tightening the waist of Stokes spatial mode. Since the effective Raman gain coefficient varies inversely with the pump linewidth, the threshold of the diamond laser increases with the broadening of the pump linewidth. The laser threshold for the broad bandwidth laser was 31.7 W, 2.8 times higher than that for the ring-cavity fiber laser (threshold of 11.4 W). The initial effective Raman gain coefficient was calculated as 0.72${g_0}$ and 0.264${g_0}$ for the narrow and broad linewidth pump, respectively, corresponding to a disparity of 2.7 times which agreed well with the threshold departure.
In the present experiments, the output coupling at Stokes and the LBO length was specified to optimally extract the SHG with a low leakage of power at the Stokes (< 0.5 W). It is also feasible to optimize the scheme to produce high power output at the Stokes by increasing the Stokes output coupling and shortening the length of SHG crystal. For example, in the present cavity structure, when the output coupling at Stokes is increased to 1.2% and the LBO length shortens to 0.5 cm, an output power of 9 W at Stokes is expected at the pump power of 50 W. These results are calculated by adapting the model in Ref. [43].
The results indicate that diamond Raman frequency-doubling oscillator pumped by tunable YDF multimode laser comprises a practical route for high power SLM lasers tuning in visible band. Notably, an all-fiber pump with simple standing-wave diamond resonator keeps a compact and potentially low-cost and robust laser system. The fiber Raman amplifier and external SHG scheme is a strong competitor to generate CW SLM lasers due to its high output power and robust system [32]. However, a broadly tunable SLM Raman fiber source has not been reported, to the best of our knowledge. Besides the SLM widely tunable seed laser (e.g. diode laser) is difficult to obtain and highly expensive, SBS represents a major parasitic effect and barrier for power scaling of narrow-linewidth fiber Raman amplifiers. Although SBS has also been observed in diamond lasers [44–47], it can be readily suppressed by optimizing the cavity length. Diamond Raman resonators are typically hundreds of times shorter than Raman fiber lengths so that SBS can be suppressed by virtue of adjusting a cavity length for which the Brillouin wavelengths fall between resonances [47]. The high thermal conductivity of diamond indicates that higher power systems are likely without inducing thermal lensing problems in the diamond [48].
4. Conclusions
In summary, a continuously tunable SLM visible laser generated in an external diamond Raman frequency-doubling resonator is demonstrated. The pump was an all-fiber YDF laser with wavelength tunability from 1020 nm to 1072 nm. By tuning the pump wavelength, a SHG output from 590 nm to 625 nm was obtained with a maximum CW power of 8 W with single frequency operation in the range 590–615 nm. This approach is a practical route for single-frequency outputs in visible and infrared band and has the potential for power scaling to tens-of-watts.
Funding
National Natural Science Foundation of China (62005073); Australian Research Council (LP160101039); Natural Science Foundation of Hebei Province (F2020202026); Air Force Office of Scientific Research (FA2386-18-1-4117).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available in Dataset 1, Ref. [49].
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