A coupled resonator was developed for high efficient room-temperature single-frequency laser operating near 2 μm optical spectral region. 721 mW stable single-longitudinal-mode oscillation at 1991 nm was obtained when the absorbed pumping power was 2.4 W. The optical-to-optical efficiency was 30%, and the slope efficiency was 46%. 5 nm of frequency tuning range was obtained with stable output power. The beam propagation factors M2 were 1.43 and 1.42 in x and y directions, respectively.
©2010 Optical Society of America
Solid-state lasers operating at 2 μm spectral region have growing numbers of applications in spectroscopy, medicine, remote sensing, and fundamental research. Due to many absorption lines in the atmosphere around 2 μm spectral region, 2 μm lasers are widely applied in atmospheric monitoring [1,2]. 2 μm lasers are preferable to pump solid-state lasers, such as optical parametric oscillators (OPOs) and optical parametric amplifiers (OPAs) operating in the mid-infrared region because of the high quantum efficiency [3,4]. Owing to the strong absorption by liquid water, 2 μm lasers are ideal light sources for biomedical applications and eye-safe lidar .
In applications such as coherent wind mapping, or DIAL, single frequency oscillation is needed. There are several techniques to obtain single frequency lasers, e.g. microchip [6,7], non-planar ring oscillators (NPRO) [8,9], twisted-mode  and intra-cavity etalons [11,12]. As for microchip laser, it was difficult to obtain high power single frequency output due to very short active medium. In 2009 Z. Lin et al. reported a diode-pumped single-frequency microchip Cr,Tm:Ho: AG laser with 32 mW single frequency output power at 10°C . NPRO had high single frequency output power, but fabrication of monolithic non-planar resonator and coating was costly. Besides, only crystals with magneto-optical effect were suitable for NPRO. Due to the monolithic nature, frequency tuning in large range and Q switched operation from a NPRO remain big challenges. As for twisted-mode technology, the active medium needed to be isotropic along resonator axis. In addition, intra-cavity components caused instability and efficiency reduction. In 2009 Y. Zhang et al. reported a diode pumped single-frequency Tm:YAG laser using twisted-mode technique . The maximum single-frequency output power was 514 mW when the pump power was 7.72 W with the efficiency of the single-frequency operation of 6.6%. Another common method to realize single-frequency oscillation was to use intra-cavity etalons as mode selectors, but the intra-cavity etalons induced losses and the efficiency of the laser was reduced. In 2009, a single frequency Tm:LuAG laser was reported. The maximum single frequency laser power was 148mW when the pump power was 3.43 W, and the efficiency was 4.3% .
A coupled cavity was another technique to achieve single frequency oscillations. Coupled cavity had been invented decades ago, but has not been applied in solid-state lasers until recent years. In this letter, we will report a coupled cavity single-longitudinal-mode laser setup. 721 mW stable single-longitudinal-mode oscillation at 1991 nm was obtained with the slope efficiency of 46% when the absorbed pump power was 2.4 W. By changing the relative positions of the two mirrors in the passive cavity, 5 nm frequency tuning was realized. To our knowledge this was the first time that coupled cavity technique was applied to get single frequency oscillation in 2 micron spectral region. The single frequency efficiency we obtained was also the highest among all the results we mentioned above.
2. Coupled-cavity design
The consequences of coupling two or more cavities had been observed and utilized in many systems [13,14]. Referring to the right part of Fig. 1 , the present design consisted of two coupled resonators. One resonator was formed by a plane input mirror and a concave output mirror with the curvature radius of 50 mm. The reflectivity of the concave mirror was 92%. A Tm:YAP crystal was put in this cavity, which was the active cavity. The second resonator, which was the passive cavity, was formed by a 0.1 mm thick uncoated quartz etalon and the concave mirror. The total length of the cavity was 25 mm, while the distance between the etalon and the output mirror was 4 mm. The passive cavity also served as a resonant output coupler with variable reflectivity. For two mirrors with reflectivity of R1 and R2, the effective reflectivity profile of the coupled mirror set was given in :Fig. 2(a) . The bandwidth of the passive cavity was 22.8 GHz (FWHM). There were five longitudinal modes within the bandwidth. The adjacent free-running laser modes had a frequency difference of 5 GHz. By adjusting the two cavities carefully, we made one laser mode match with the peak of the effective reflectivity of the passive cavity, so that this laser mode had the highest gain whereas the other modes suffered high losses and were suppressed. This oscillating mode was the common resonant mode of the active and passive cavities, and we called it the mode of the coupled cavity. Therefore, the frequency separation between adjacent modes (FSBAM) of the coupled cavity was the lowest common multiple of the FSBAM of the active cavity and that of the passive cavity. By properly designing and adjusting the relative lengths of the active and passive cavity, we made only one coupled cavity mode was in the thin etalon’s transmission bandwidth and the gain of this mode was above the threshold. So only this mode oscillated. Therefore the coupled cavity conditions were met, and stable single frequency oscillation was obtained.
We calculated the transmission of the 0.1 mm uncoated etalon. The transmission bandwidth of the uncoated etalon was 500 GHz (FWHM) as shown in Fig. 2(b). Since this bandwidth was much larger than the one of the passive cavity, it was not difficult to match their resonant frequencies in order to have stable oscillations.
Furthermore, we also calculated the effective reflectivity of the coupled mirror set with respect to the distance d in Eq. (1) at a fixed wavelength. The effective reflectivity changed one cycle with d changing less than one micron. This means that d needed to be adjusted very precisely in order to make a laser mode match with the peak of the effective reflectivity of the passive cavity.
3. Experimental setup
The schematic diagram of the single-frequency Tm:YAP laser is shown in Fig. 1. The pump source was a fiber-coupled diode laser with maximum output power of 2.8 W. The wavelength was tuned to 795 nm which was the absorption peak of thulium in YAP. The diameter of the fiber core was 100 µm, and the numerical aperture of the fiber was 0.22. A 5% thulium-doped Tm:YAP crystal was used as the active material. Three peaks in the fluorescent spectra of the Tm:YAP crystal were observed when it was pumped at 795 nm . The overall bandwidth was approximately 200 nm. The Tm:YAP crystal was grown along b-axis with dimensions of 3 mm × 3 mm in cross section and 5 mm in length. Both sides of the crystal were anti-reflecting coated at 795 nm and 1990 nm. The Tm:YAP crystal was wrapped in an indium foil and was held between copper holders. The temperature of the laser crystal was controlled at 293 K.
4. Experimental results
The laser output was monitored by a FP interferometer with FSR of 3.75 GHz. When there was no mode selector in the cavity, five to seven modes were visible in one FSR and the oscillations were chaotic as shown in Fig. 3(a) . When we inserted the thin etalon and adjusted the active and passive cavities carefully and the coupled cavity conditions mentioned in section 2 were met, stable single-longitudinal-mode oscillation was realized as shown in Fig. 3(b). Two FSRs were displayed in Fig. 3(b), and the slightly different distances was due to the nonlinearity of the PZT displacement with respect to the driving voltage.
Figure 4 shows the multimode and the single longitudinal mode laser output power with respect to the absorbed pump power. The maximum multimode output power reached 782 mW when the absorbed pump power was 2.4 W. The slope efficiency was 50% and the optical-to-optical efficiency was 33%. The maximum single frequency output power was 721 mW, with the slope efficiency and the optical-to-optical efficiency of 46% and 30%. The efficiency of single frequency cavity was degraded by 8% compared to the free running laser. This might due to the fact that the effective reflectivity of the passive cavity was 5.6%, which was not the optimum value of the output coupler of this laser setup. We also used two etalons as mode selectors in the same setup, but only 344 mW single frequency output was obtained with same maximum pump power . From the coupled cavity setup, higher single frequency output power is possible with proper design of the effective reflectivity of the passive cavity.
Frequency tuning was realized by adjusting the distance d between the etalon and the concave mirror. We changed d, and adjusted the cavity. When stable single frequency oscillation was shown by the FP interferometer, we measured the output power and the wavelength. Single frequency oscillation was obtained at several wavelengths from 1987.2 nm to 1991.0 nm as shown in Fig. 5 . The maximum fluctuation of the output power was 5.7%. The wavelengths we measured in Fig. 5 were arbitrary because there was no active control in our setup. Smooth frequency tuning was feasible if an active control loop was added.
We measured the dependency of the output power with respect to the temperature of the laser crystal as shown in Fig. 6 , and the output power dropped by 9.2% when the temperature was increased from 12°C to 28°C. The frequency tuning coefficient of the laser with temperature was given by :
The M2 factors of the output beam at 721 mW single frequency output power was measured. The M2 factors were 1.43 and 1.42 for x direction and y direction, respectively.
In summary, a diode-pumped, room-temperature, high efficient, single-longitudinal-mode Tm:YAP laser had been developed with a coupled cavity configuration. 721 mW single-frequency output power was obtained at 1991 nm, and the slope efficiency was 46%. The output wavelength was tuned over 5 nm. The measured beam propagation factors were 1.4 in both directions. It is anticipated that the efficiency and wavelength tuning range can be improved further by optimizing the parameters of the coupled cavity, i.e. the distance between the two mirrors and the reflectivity. The compact setup with high efficiency can be used as seeding signal for fiber amplifier or injection seeding system for power scaling. High power single-frequency lasers at 2 μm optical region have potential applications on eye-safe coherent lidar and optical remote sensing.
The authors acknowledge many helps from Dr. MingWei Gao and Dr. YunShan Zhang from Beijing Institute of Technology. This work is supported by the National Science Foundation of China (60778011).
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