We report a diamond Raman ring cavity laser resonantly pumped by a tunable Ti:sapphire continuous wave laser. We characterize the laser operation generating first Stokes output and, for the first time, generate second Stokes lasing at a maximum output power of 364 mW with 33.4% slope efficiency at 1101.3 nm. Single longitudinal mode operation is achieved for all first Stokes output powers, but only for lower output powers for second Stokes operation. We discuss possible reasons preventing single longitudinal mode operation.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Current technological developments in quantum physics, biological imaging, precision measurements, etc., demand wavelength flexible lasers with an excellent spectral purity. The wavelength reach of primary laser sources can be expanded using optical parametric oscillators (OPOs), which can provide a widely tunable wavelength shift with high efficiency . Stimulated Raman Scattering (SRS) provides an alternative method for laser wavelength extension; SRS does not require phase matching in contrast to OPOs, but can only provide discrete sets of wavelength shifts and so is most suitable for shifting tunable primary lasers . Diamond is attractive Raman material for this application for its large Raman frequency shift (multiples of 1332 cm−1), high gain coefficient, and outstanding thermal properties .
Diamond Raman lasers (DRL) have been investigated in continuous-wave (cw) and pulsed regimes and at wavelengths ranging from UV to mid infrared (3.8 µm) [4–7] . For cw output, intracavity pumped DRLs produced multi-watt output at Stokes  and intracavity doubled wavelengths and were tunable by over 20 nm when pumped by a tunable semiconductor disk laser . Externally pumped DRLs have been power scaled to hundreds of watts [10,11], albeit mostly in a quasi-cw regime . Single longitudinal mode (SLM) operation of an external cavity DRL has been investigated in [13,14]. A SLM output was observed up to a 1 W of output power, after which the laser became multi-mode.
Using cavity enhancement of the primary laser allows efficient conversion of lower-power primary lasers of a scale that are more common in research labs. As wavelength tuning of Raman lasers is achieved by incorporating a tunable pump laser, Ti:sapphire sources are of particular interest. However, their output power (typically <10 W) is not sufficient for double-pass pumping . We previously have generated watt-level SLM output from a resonantly pumped ring DRL, using a 5 W Ti:sapphire laser . In this paper, we develop this work to include operation at the second Stokes to increase the wavelength extension and discuss the conditions for single longitudinal mode output.
2. The experimental setup
The design of the SLM Raman laser is shown in Fig. 1. The pump laser was a commercial SLM cw Ti:sapphire laser (SolsTiS, M Squared Lasers Ltd.), which was tunable (730-1050 nm) with a linewidth under 50 kHz, M2 factor less than 1.1, and 4.5 W maximum output power at 855 nm. The pump beam was mode matched into a ring resonator by a telescope system and a lens. The ring resonator was designed as a bow-tie cavity with one input/output coupler (IC/OC) mirror (M1) and three HR mirrors (M2, M3, and M4). The details of these resonator mirrors are given in Table 1. The resonator length was controlled by a piezo mounted to M2, and pump enhancement was achieved using the Hansch-Couillaud locking scheme . Dichroic mirror DM1 picked out the Stokes beams returning to the pump laser into a power meter, and DM2 passed only the pump light into the locking system. To promote unidirectional oscillation of the first and second Stokes fields, HR mirrors were used to reinject one of the two output beams back into the cavity as in ; this is discussed below.
The Raman gain medium was a 5 × 2×1 mm3 CVD grown diamond (Element Six Ltd.), installed on a copper holder and placed in the waist (ω0 = 20 µm) between M1 and M2. The diamond was Brewster cut, for propagation along the  direction; the Brewster faces were oriented so p-polarized incident light was aligned with the  direction to access the highest Raman gain .
3. Results and discussion
3.1 First Stokes generation
First, we configured the laser to generate first Stokes unidirectional output as in , using mirror M1 (first Stokes) with reflectivity of 98% for both pump and Stokes wavelengths and utilizing a feedback HR mirror. Note that the Raman gain bandwidth is very much larger than the cavity mode spacing, and so there is no need to control the cavity free spectral range to ensure resonance of all lasing fields which is in contrast to Brillouin lasers that have narrow gain bandwidth .
The power curve is shown in Fig. 2, the inset is the beam profile of the first Stokes output. The first Stokes beam is Gaussian and close to the diffraction limit. Due to using IC/OC with different reflectivity and a different diamond the threshold increased from previous 1.8 W to 2.9 W, however, the slope efficiency also increased from 32% to 45%. Thus, the maximum first Stokes output power generated at 964.9 nm was 0.4 W for 3.8 W pump at 855 nm with the maximum conversion efficiency 10.4% (see Fig. 2). The first Stokes spectrum was a stable SLM at all output powers in accordance with our previous work  (measured by a scanning Fabry-Perot interferometer (FPI) with an FSR of 10 GHz and resolution 67 MHz).
To analyze the behavior of the Stokes output and estimate the cavity loss, we derived a model describing the first and second Stokes threshold and output power. The model combines the enhancement of the pump field in a resonant cavity  with the steady state Raman gain equations , similar to . For the first Stokes, the model predicts similar behavior to ; we will present a full analysis of the behavior and optimization of these cavity-enhanced lasers in a future work.
After inputting all measured properties of the laser into the model (e.g., input/output coupling, waist sizes, gain coefficient of 14.4 cm/GW) the passive loss value which best matches the data (as shown in Fig. 2) was 1.7%, a surprisingly high value. Diamond absorption and scattering, is estimated at 0.1% , and unwanted combined transmission of HR mirrors is estimated at 0.3%. Residual birefringence in diamond will change the polarization orientation of the Stokes field , which may contribute loss from the Brewster faces of the diamond.
3.2 Second Stokes generation
For the second Stokes generation the laser was setup according to Fig. 1. To force the laser to cascade to the second Stokes field, all mirrors were designed to be highly reflecting at the first Stokes wavelength, with the IC/OC mirror M1 (second Stokes) now HR at the first Stokes and designed to couple the pump wavelength in and the second Stokes wavelength out. The mirror design (Fig. 1 inset) allows some degree of tunability while still having appropriate reflectivity values for each of the three fields. These mirror properties, combined with additional factors of reducing pump power and slightly-decreasing Raman gain at longer wavelengths, resulted in the highest second Stokes output power being measured at a wavelength of 1101.3 nm, with a corresponding pump wavelength of 851.5 nm. Second Stokes lasing could be observed between 1100 nm and 1115 nm. A broader tuning range should be possible with optimized mirror parameters.
We characterized the laser at this optimum second Stokes wavelength of 1101.3 nm. Shown in Fig. 3, the first and second Stokes reached threshold at 0.9 W and 2.8 W of pump power respectively. The second Stokes output power increased linearly with a slope of 33.4%, up to 364 mW and reached conversion efficiency of 9.5% at the maximum pump power of 3.86 W. Inset is the beam profile of the second Stokes beam, which shows a Gaussian shape close to the diffraction limit.
Assuming the parasitic loss of all the resonated fields is identical, the loss parameter value of 1.12% in our model gives good agreement with the experimental results as shown in Fig. 3(a). The difference compared to the first Stokes laser may largely reflect the different optimisations of the systems, particularly finding a better path in the diamond crystal.
The second Stokes spectra at different output power levels, as monitored by the FPI, are shown in Figs. 3(b)–3(d). The FPI frequency-axis was calibrated using the SLM pump source. The second Stokes output was stable SLM (see Fig. 4) with a resolution limited FWHM of 67 MHz up to about 140 mW of output power. As the power increased above 140 mW, the second Stokes field started to mode-hop and/or oscillate at several modes; above 162 mW, the second Stokes was in a highly multi-mode regime with a range of wavelengths comparable to the free spectral range of the FPI. Interestingly, even at the maximum pump power with multimode second Stokes output, the first Stokes remained stable SLM with only an occasional mode-hop.
The directions of oscillation of the intracavity fields were important in determining the stability of single mode operation. While the cavity enhanced pump field can only travel right to left through the diamond in Fig. 1, the closely-equal forward and backward Raman gain in principle allow the first and second Stokes field to oscillate in either direction in the ring cavity. As in , for the first Stokes laser, reinjection of the left-to-right first Stokes field into the cavity lead to predominantly unidirectional operation, which was necessary to get stable SLM output. For the second Stokes laser, we reinjected the 0.1% first Stokes leakage through the mirror M3, and reinjected the right-to-left second stokes output beam. As a result, the pump and the first Stokes field propagated predominantly in the same direction while the second Stokes field propagated in the opposite direction. We found that choosing counter-propagating directions for the first Stokes and the second Stokes field increased the maximum SLM output power of the second Stokes. Allowing bidirectional first Stokes operation by blocking its reinjection mirror caused the second Stokes output to be multimode for all output powers. We also investigated the behavior of co-propagating fields by changing the feedback direction of the first Stokes field, resulting in similar multimode operation. We speculate that counter-propagating the interacting fields suppresses the transfer of noise from one field to the next. This can be understood in the time domain as local intensity/phase variations do not stay aligned when fields are counter-propagating as they would for co-propagating fields; in the frequency domain, the same physics is understood by considering that four-wave mixing interactions are not phase matched for counter-propagating fields .
For the present design, the first Stokes reinjection was relatively weak due to low transmission of M3 (which was nevertheless the most transmissive mirror at that wavelength, with all mirrors designed to be HR); we measured the power ratio in the two directions to be approximately 3:1. We expect that second Stokes SLM operation will be stable at higher powers by improving the first Stokes unidirectionality - this could be achieved by deliberately designing a larger first Stokes transmission for one of the cavity mirrors.
In conclusion, we have demonstrated a second Stokes CW DRL in a triply resonant ring cavity configuration. The maximum output power reached 364.4 mW with 33.4% slop efficiency. A second Stokes model was used to characterize the loss and slope efficiency of the second Stokes system. The highest stable SLM power achieved was 140 mW. The second Stokes SLM power should be increased by improving the unidirectional operation of the first Stokes. With our current mirrors, the input wavelength of 851.5 nm was converted to 1101.3 nm. With properly designed mirrors the SLM second Stokes laser can in principle extend the wavelength coverage of the Ti:sapphire pump laser from 845-930 nm to 1090-1236 nm.
Australian Research Council (LP110200545).
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