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In this paper we report on the realization of a single-mode Q-switched Nd:YVO4 ring laser at 1342 nm. Unidirectional and single-mode operation of the ring laser is achieved by injection-locking with a continuous wave Nd:YVO4 microchip laser, emitting a single-frequency power of up to 40 mW. The ring laser provides a single-mode power of 13.9 W at 10 kHz pulse repetition frequency with a pulse duration of 18.2 ns and an excellent beam quality (M2 < 1.05). By frequency doubling of the fundamental 1342 nm laser, a power of 8.7 W at 671 nm with a pulse duration of 14.8 ns and a beam propagation factor of M2 < 1.1 is obtained. The 671 nm radiation features a long-term spectral width of 75 MHz.
© 2015 Optical Society of America
1. Introduction
Diode-pumped solid-state lasers operating at the 1.3 μm transition of neodymium are of great interest for several applications such as fiber optical communication, medical treatment and generation of visible and ultraviolet light via nonlinear frequency conversion. The latter allows the development of all solid-state deep-UV light sources at 191.7 nm [1] and 167.75 nm [2] by seventh and eighth harmonic generation, respectively.
Pulsed 1.3 μm laser sources with high peak power are required for highly efficient frequency conversion to the visible and UV spectral range by second-harmonic generation (SHG) and sum frequency generation (SFG). Recently, considerable progress toward higher peak powers of 1342 nm Nd:YVO4 lasers in the picosecond and nanosecond regime have been reported [1–7].
Picosecond pulses with the highest peak power of 7.3 kW from a single oscillator were realized by mode-locking via cascaded second-order nonlinearities [3]. The highest average power of 7.63 W, obtained from a single mode-locked oscillator, was demonstrated by using a semiconductor saturable absorber mirror [4]. With a master oscillator power amplifier system, the average power could be increased to 17 W and the peak power to 8.1 kW [2]. By using a pulse picker and a regenerative amplifier an average power of 11 W and a peak power of 2.57 MW at a pulse repetition frequency (PRF) of 300 kHz was demonstrated [5]. The highest peak power of 4.89 MW was realized with a regenerative amplifier at 1 kHz PRF [6].
Nanosecond pulses with the highest peak power of 110 kW were demonstrated with a cavity dumped oscillator at a PRF of 2 kHz [7]. The highest average power of 15.2 W was realized with an oscillator, actively Q-switched by an acousto-optic modulator [1].
With such picosecond and nanosecond systems, broadband UV and deep-UV radiation can be generated by cascaded frequency conversion. However, many applications in the deep-UV spectral range such as metrology, lithography and fiber Bragg grating (FBG) inscription benefit from a narrow linewidth. The maximum length of a FBG is limited by the bandwidth of the inscribing laser due to self-apodization [8]. Hence, single-longitudinal-mode deep-UV systems are of great interest. However, in recent years, research on single-mode 1.3 μm lasers was focused on continuous wave 1342 nm Nd:YVO4 and Nd:GdVO4 lasers [9, 10], due to their necessity for the manipulation of lithium atoms.
One pulsed single-mode 1.3 μm laser was recently demonstrated by Li et al. with a single-mode Q-switched Nd:GYSGG laser at 1336.6 nm [11]. The laser was pumped with 1 ms long pulses at a PRF of 10 Hz. The system operated in Q-switched mode at 80 Hz during the pump pulse. The pulse duration of the Q-switched pulse was 300 ns and the peak power was 15.3 kW. The pulse energy of the envelope pulse with 10 Hz PRF was 36.7 mJ. The spectral width of the laser system was less than 143 MHz.
In this paper we report on a single-mode, actively Q-switched, 888 nm pumped Nd:YVO4 ring laser at 1342 nm which is very well suitable for cascaded frequency conversion to the visible and UV spectral range. Unidirectional and single-mode operation of the ring laser was achieved by using the injection-locking technique. The cavity was stabilized on the seed laser with the pulse buildup time method [12]. The error signal was generated by modulating the resonator length with a piezo actuator. A continuous wave (cw) single-frequency Nd:YVO4 microchip laser, which was demonstrated by Conroy et al. [13], was used as a compact and reliable seed source. The Q-switched, single-mode ring laser provided an average power of 13.9 W at a PRF of 10 kHz. The pulse duration was 18.2 ns, resulting in a peak power of approximately 72 kW. By second harmonic generation (SHG) a power of 8.7 W at 671 nm was obtained, with the long-term spectral width being 75 MHz.
2. Experimental setup
The experimental setup, depicted in Fig. 1, consisted of a a Nd:YVO4 microchip seed laser, a high-power Q-switched Nd:YVO4 ring laser and a frequency doubling stage.
Fig. 1 Experimental setup. PD = photodiode, HV = high voltage, HF = high frequency, AOM = acousto-optic modulator, FR = Faraday rotator, TFP = thin-film polarizer. For details see text.
The homemade microchip seed laser was built in a closed housing to avoid instabilities by air turbulence and fluctuations of the microchip temperature. The laser crystal was a 1 at.%-doped Nd:YVO4 crystal with a length of 0.4 mm, which corresponded to a longitudinal mode spacing of approximately 171 GHz (1.03 nm). The aperture of the crystal was 7 × 5 mm2. Both facets featured an anti-reflection coating at 808 nm and 1064 nm. Additionally, the pump facet was highly reflective at 1342 nm and the emission facet featured an output coupling of 8 % at 1342 nm. The microchip was mounted in a oven, allowing tunability of the emission wavelength due to the dependence of the refractive index and the length of the crystal from the temperature. The Nd:YVO4 crystal was pumped by an 808 nm diode laser with an emitter size of 1 × 3 μm2, providing a power of up to 210 mW. The pump beam was imaged one-to-one into the laser crystal by an aspheric lens pair. The microchip laser provided a tunable cw single-frequency power of up to approximately 40 mW, depending on the temperature of the microchip. Before entering the high-power Nd:YVO4 ring laser through the output coupler (mirror R1), a seed power of up to 35 mW was available. To protect the microchip laser from the backward emitted power of the ring laser during unseeded and hence bidirectional operation, two Faraday isolators were implemented in the seed path. A telescope consisting of lenses L2 and L3 served as a beam expander to limit the fluence in the Faraday rotator.
The cavity of the 888 nm pumped ring laser consisted of five mirrors (R1 – R5), with the total optical length of the resonator being 335 mm. Therefore, the longitudinal mode spacing was approx. 895 MHz. The laser crystal was a 0.5 at.%-doped Nd:YVO4 crystal with an aperture of 4 × 4 mm2 (888/1064/1342 nm anti-reflection (AR) coating) and a length of 30 mm. The 888 nm pump beam was focused to a radius of approx. 600 μm inside the crystal. By using a pump wavelength of 888 nm for pumping the Nd:YVO4 crystal, the heat load in the crystal can be reduced by spreading the heat over the length of the crystal due to lower absorption compared to 808 nm or 880 nm pumping [14, 15]. Hence, the thermal lens is reduced allowing higher pump powers and thus achieving higher output powers. By using pump powers in the order of 100 W, there is still strong thermal lensing in the Nd:YVO4 crystal due to the high quantum defect of the 1.3 μm transition [16]. Depending on the pump power and the output power of the laser, the thermal lens was approx. f = 65 mm for lasing at 1342 nm and approx. f = 110 mm for the non-lasing state [16]. The thermal lens was compensated by the convex zero-lens pump mirror (R4) in our experiments, which featured a radius of curvature of 200 mm. The oscillator was actively Q-switched by an acousto-optic modulator (AOM) and operated at a PRF of 10 kHz. The pump mirror R4 was coated for high reflectivity (HR) at 1342 nm and high transmission (HT) at 888 nm just like mirror R3. Mirrors R2 and R5 were coated for HR at 1342 nm and the output coupler R1 had a transmission of 20%. Mirror R2 was mounted on a piezo actuator to adjust the resonator length for injection-locking. The seed laser was focused to a radius of approx. 812 μm inside the laser crystal. About 0.1 % of the power at 1342 nm was transmitted through mirror M1 and detected with an ultrafast germanium photodiode for the stabilization servo loop. Injection-locking was realized by stabilizing the cavity length on the minimization of the Q-switch pulse buildup time of the ring resonator [12]. The error signal was generated by modulating the resonator length with the piezo actuator of mirror R2. Because of the strong thermal lensing in the Nd:YVO4 crystal, 5 % of the output power at 1342 nm was in a diffraction ring around the Gaussian beam profile, with a power of 13.9 W being transmitted through a pinhole.
After a variable attenuator, consisting of a half-wave plate and a thin-film-polarizer, mirrors M2 – M4 and a telescope, consisting of lenses L4 and L5, a power of 12.7 W was available for SHG. The SHG crystal was a non-critically phase-matched bismuth triborate (BiBO) crystal (θ = 0°, ϕ = 0°, Type I) with a length of 15 mm and an aperture of 3 × 3 mm2 (671/1342 nm AR coating). The temperature of the crystal was stabilized around 244 °C to ensure phase-matching. The 1342 nm radiation was focused to a radius of 168 μm inside the SHG crystal. Finally, the 1342 nm and the 671 nm beam were separated by a dichroic mirror M5.
3. Results and discussion
The characteristics of the microchip laser and the fundamental 1342 nm laser in broadband and single-mode operation are presented in Section 3.1. Subsequently, the 671 nm SHG stage is characterized in Section 3.2.
3.1. 1342 nm fundamental laser
The characteristic of the output power of the microchip laser is presented in Fig. 2(a). A power of up to 41.7 mW was obtained for a microchip temperature of 27.4 °C, which corresponded to an efficiency of 21.7 %. The slope efficiency for this configuration was 28.3 %. The output power of the microchip laser decreased for higher temperatures. At a microchip temperature of 68.4 %, the output power dropped to 34.6 mW. This corresponded to an efficiency of 18 %, with the slope efficiency being 24.8 %. The power of the seed laser changed by variation of the microchip temperature because the longitudinal mode spectrum was tuned over the spectral gain profile of the Nd:YVO4 microchip. The emission wavelength of the microchip laser could be tuned by adjusting the temperature and the pump power of the microchip. The emission spectra for different microchip temperatures, measured at a pump power of 138 mW, are shown in Fig. 2(b). The output power of the microchip laser was between 19.3 mW and 21.6 mW, depending on the temperature of the microchip. The measured spectral width was limited by the apparatus function of the optical spectrum analyzer. The emission wavelength could be tuned from 1341.97 nm at a microchip temperature of 27.4 °C to 1342.234 nm at a temperature of 68.4 °C. By measuring the emission spectrum over a broader spectral range, we could also confirm single-mode operation of the microchip laser, since the longitudinal mode spacing was approx. 1 nm. At the full pump power of 210 mW, the microchip laser was tunable from 1342.031 nm to 1342.281 nm, by increasing the microchip temperature from 30.8 °C to 68.4 °C.
Fig. 2 (a) Power characteristic of the Nd:YVO4 microchip laser for a microchip temperature of 27.4 °C and 68.4 °C. (b) Spectra of the microchip laser at a pump power of 138 mW for different temperature of the microchip.
The power characteristic of the ring laser was measured in unidirectional broadband operation. Unidirectional operation was achieved by inserting an additional mirror (HR 1342 nm) between mirror R1 and lens L3. By reflecting the power of the ring laser, which was emitted against the seed direction, back into the cavity, the ring laser was forced to operate unidirectionally. The broadband power characteristic is shown in Fig. 3(a). The power was measured behind mirror M1. The lasing threshold of the ring laser was between 83 W and 83.5 W. A maximum power of 14.6 W at a PRF of 10 kHz was achieved for an absorbed pump power of approx. 98 W. There was a diffraction ring around the Gaussian beam profile due to strong thermal lensing of Nd:YVO4 at 1.3 μm. Approximately 95 % of the output power was in the Gaussian beam and 5% in the diffraction ring which was blocked by the pinhole behind mirror M1.
Fig. 3 (a) Power characteristic of the Q-switched ring laser in broadband operation. (b) Reduction of the Q-switch buildup time (BUT) by injection-seeding. The seed power was 33 mW at 1342.221 nm before entering the high-power ring laser through mirror R1. The resonator length of the ring laser was scanned via the voltage of the piezo actuator.
The reduction of the pulse buildup time in seeded operation is shown in Fig. 3(b). This curve was measured with a seed power of 33 mW at 1342.221 nm. The cavity length was varied by changing the voltage of the piezo actuator from 0 V to 150 V, which corresponded to a piezo travel of 2.1 μm. The reduction of the Q-switch pulse buildup time in seeded operation was approximately 28 ns. This allowed the stabilization of the resonator length with a servo loop by minimizing the buildup time to ensure single-mode operation via injection-locking [12].
The pulse traces in single-mode and broadband operation are shown in Figs. 4(a) and 4(b). In single-mode operation, a pulse duration of 18.2 ns full width at half maximum (FWHM) was achieved, with the pulse energy fluctuations being σ < 0.6 %. The pulse trace in broadband operation featured mode-beating peaks which are typical for a multi-longitudinal mode Q-switched laser. Fast Fourier transformation (FFT) of the measured pulse traces was a reliable method to ensure optimum alignment of the seed laser. The FFTs of the single-mode and the broadband pulse are presented in Figs. 4(c) and 4(d). The FFT of the singe-mode pulse was smooth without any peaks, whereas there were several peaks in the FFT of the broadband pulse. The frequency spacing of those peaks corresponded to the longitudinal mode spacing of the ring cavity. There was a continuous transformation between those two states (perfect injection-locking and bidirectional, unseeded operation) depending on the quality of the alignment of the seed laser. Deviations from perfect alignment were visible as ripples in the single-mode pulse trace, indicating that there was an additional longitudinal mode oscillating. In the FFT, even small deviations from ideal alignment were visible, which could not yet be observed in the pulse trace by eye. Hence, using the FFT function of an oscilloscope ensured correct alignment of the seed laser.
Fig. 4 (a) Q-switched pulse of the ring laser in single-mode operation. (b) Q-switch pulse of the ring laser in broadband operation, with mode-beating peaks visible. (c) Fast Fourier transformation (FFT) of the single-mode pulse. (d) FFT of the broadband pulse. The FFT of the pulse is a reliable method to optimize the alignment of the seed laser.
The influence of the power of the seed laser on the power of the high-power ring laser is shown in Fig. 5(a). The power in forward direction, emitted in seed direction, is represented by black squares. The residual power, emitted against the seed direction is depicted by red dots. The power in forward direction was measured behind the pinhole and the residual power was determined with the reflected beam of the TFP behind the first Faraday rotator. Below a seed power of approx. 4 mW incident on mirror R1, injection-locking was not possible and the ring laser was therefore completely unseeded and in bidirectional operation. For seed powers of more than 4 mW but below 12 mW, the ring laser became partially seeded but not single-mode. The FFT of the pulse trace confirmed that at least one additional longitudinal mode was oscillating and hence mode-beating was visible. The residual power against the seed direction decreased from 500 mW at 4 mW seed power to 12 mW at 12 mW seed power. Above a seed power of 13 mW, the ring laser was completely seeded and in single-mode operation. The FFT of the pulse trace was smooth and did not show any mode-beating effects. The residual power decreased from approx. 11 mW at a seed power of 13 mW to 0 mW for seed powers above 24 mW. The ring laser provided an average power of 13.9 W at a pulse repetition frequency of 10 kHz resulting in a peak power of approximately 72 kW. We used the full seed power of 33 mW for our further experiments to ensure the widest tunability of the ring laser in seeded operation. At full power, the seed laser featured a wavelength of 1342.221 nm.
Fig. 5 Dependence of the power in forward direction (black squares) and the residual power emitted against the seed direction (red dots) from (a) the injected seed power and (b) the seed wavelength. The gray dashed line represents the broadband emission wavelength in free running operation.
The power of the ring laser with respect to tuning of the seed wavelength is shown in Fig. 5(b). Over the whole tuning range from 1342.031 nm to 1342.281 nm, injection-locking was successful and single-mode operation was achieved. The laser provided a power between 13.7 W and 13.9 W above a seed wavelength of 1342.111 nm, with the residual power being below 5 mW. The two-state behavior around the gain maximum, marked by the dashed line (free running wavelength), was caused by different positions of the piezo actuator. The dip of the output power at a seed wavelength of approx. 1342.1 nm could be attributed to absorption in water vapor. Below a seed wavelength of 1342.088 nm, the output power of the ring laser decreased to a power of 13.2 W at 1342.031 nm due to lower gain. In principle, the seed wavelength is a probe for the spectral gain profile of the Nd:YVO4 crystal. Below a seed wavelength of 1342.11 nm, the residual power emitted against the seed direction increased to a power of 28 mW at 1342.031 nm. The spectra of the ring laser in broadband operation and in single-mode operation are presented in Fig. 6(a). The spectral width in broadband operation was 0.175 nm (FWHM). The measured spectral width in single-mode operation was limited by the apparatus function of the optical spectrum analyzer. For our further experiments, we used a seed wavelength of 1342.221 nm which was the centroid wavelength in broadband operation to ensured optimum stability of injection-locking and maximum power.
Fig. 6 (a) Spectra of the ring laser in broadband operation (black) and injection-locked operation (color). The single-mode spectra were measured for different temperatures of the microchip and therefore different seed wavelengths. (b) M2 measurement and beam profile of the injection-locked ring laser.
The beam quality of the single-mode ring laser at full power is shown in Fig. 6(b). The laser provided a Gaussian shaped beam profile with an ellipticity of 0.96. The laser featured an excellent beam quality with a beam propagation factor of M2 = 1.04 in x-direction and M2 = 1.01 in y-direction.
3.2. 671 nm SHG stage
A power of up to 12.7 W incident on the BiBO crystal was available for SHG. The power characteristic of the frequency doubling stage at a pulse repetition frequency of 10 kHz is presented in Fig. 7(a). A power of up to 8.7 W at 671 nm was achieved, which corresponded to a conversion efficiency of 68.5 % at a fundamental power of 12.7 W. The pulse trace of the second harmonic at full power is shown in Fig. 7(b). The pulse duration was 14.8 ns (FWHM) and the pulse energy fluctuations were σ < 0.8 %. Hence, there was only a minor increase of the fluctuations compared to the fundamental pulses. The second harmonic provided a peak power of approximately 55 kW. The beam quality of the 671 nm beam is presented in Fig. 7(c). The beam profile was Gaussian shaped with an ellipticity of 0.98. The second harmonic featured an excellent beam quality with a beam propagation factor of M2 = 1.06 in x-direction and M2 = 1.05 in y-direction.
Fig. 7 (a) Power characteristic of the SHG conversion stage. (b) Pulse trace of the second harmonic pulse at a power of 8.7 W at 10 kHz PRF. (c) M2 measurement and beam profile of the second harmonic at a power of 8.7 W.
The spectral width of the 671 nm beam was characterized by using a scanning confocal Fabry-Perot interferometer (FPI) with a free spectral range (FSR) of 300 MHz and an oscilloscope (2 GHz bandwidth). The short-term spectral width was determined by measuring the spectrum with a single FPI sweep with a duration of approximately 11 ms. The quantitative result of this measurement is shown in Fig. 8(a). The peaks in the spectrum were not longitudinal modes but laser pulses at a pulse repetition frequency of 10 kHz, which were sampled during the FPI measurement. A full FPI sweep consisted of approx. 110 pulses. By considering the free spectral range of 300 MHz, the short-term spectral width could be determined to be 45 MHz (FWHM). A screenshot of the oscilloscope for the short-term measurement is presented in Fig. 8(b), showing the spectrum and the piezo ramp of the FPI. To determine the long-term spectral width of the second harmonic, a FPI measurement with active display persistence of the oscilloscope was taken. With this method, the impact of the frequency jitter due to modulation of the resonator length for the stabilization of the ring laser and the influence of vibrations and drifts can be quantified. The screenshot of the oscilloscope with persistence for a 5 min period is presented in Fig. 8(c). The peak-to-peak spectral width was determined to 75 MHz (FWHM), which was a factor of 1.67 broader than the short-term width.
Fig. 8 Measurement of the spectral width of the second harmonic with a scanning confocal Fabry-Perot interferometer (FPI) with a free spectral range of 300 MHz. (a) FPI spectrum measured with a single sweep of the FPI. (b) Screenshot of the oscilloscope during a single sweep of the FPI. (c) Screeshot of the oscilloscope with persistence on for a 5 min period.
4. Conclusion
In conclusion, we have presented a high-power, Q-switched, single-mode Nd:YVO4 ring laser operating at 1342 nm. Single-mode operation was achieved with the injection-locking technique. The seed laser was a single-frequency continuous wave Nd:YVO4 microchip laser with a power of up to 40 mW, depending on the emission wavelength. Due to strong thermal lensing in the laser crystal, there was a diffraction ring around the Gaussian beam profile which was blocked by a pinhole. The fundamental single-mode laser provided a pulse duration of 18.2 ns and an average power of 13.9 W at a pulse repetition frequency of 10 kHz, which corresponded to a peak power of approximately 72 kW. Fast Fourier transformation of the pulse trace proved to be a reliable method to optimize the alignment of the seed laser to ensure single-mode operation. The wavelength of the injection-locked 1342 nm laser could be tuned from 1342.031 nm to 1342.281 nm. The beam profile was Gaussian shaped and featured an excellent beam quality ( and ). By second harmonic generation in a BiBO crystal, a power of 8.7 W at 671 nm was obtained. The second harmonic beam provided a Gaussian beam profile with a diffraction limited beam quality ( and ). The long-term spectral width of the 671 nm radiation was 75 MHz. To the best of our knowledge, our setup provided the highest average power as well as peak power of a Q-switched, single-mode neodymium laser operating at the 1.3 μm transition.
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