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Abstract
A 2052.96 nm single-longitudinal-mode pulsed Ho:YVO4 MOPA system was demonstrated for the first time. The pulsed Ho:YVO4 MOPA system consisted of a unidirectional ring passively Q-switched oscillator and a single-pass amplifier. By inserting an isolator, a half-wave plate and a Cr2+:ZnS plate into the ring Ho:YVO4 oscillator cavity, the single-longitudinal-mode pulsed laser was achieved with an average output power of 1.02 W with pulse width of 910 ns and pulse repetition frequency (PRF) of 67 kHz. Using the residual (non-absorbed) pump power of the oscillator as pump, the single-pass pulsed Ho:YVO4 amplifier obtained an average output power of 1.67 W. The total optical-to-optical efficiency of the pulsed Ho:YVO4 MOPA system was 14.3%. Single-longitudinal-mode pulsed MOPA system based on isolator and Cr2+:ZnS around 2 µm has not been reported yet to the best of our knowledge.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Single-longitudinal-mode pulsed lasers operating around 2 µm which lie in the eye-safe range and atmospheric transmission window have been used for a variety of applications, including coherent Doppler lidars [1] and differential absorption lidars [2]. In 2010, Koch et al. developed a coherent Doppler lidar for wind measurement by using a 2.053 µm, 200 ns Ho:Tm:LuLiF laser as laser source, and performed the performance of the lidar for wind measurement in the atmospheric boundary layer and free troposphere [3]. Generally, the 2 µm Doppler wind lidars need the laser operating with pulse width of hundreds of nanoseconds. Moreover, the wavelength around 2.051 µm is in the absorption peak of CO2 molecule. In 2013, Ishii et al. reported a coherent 2 µm differential absorption and Doppler wind lidar for the measurement of CO2 concentrations with a laser source of 2.051 µm, 150 ns Tm,Ho:YLF laser [4]. In order to realize single-longitudinal-mode pulsed lasers, a common method is injection locking [5,6]. In 2009, Bai et al. reported a 1.25 kHz injection-seeded, single-longitudinal-mode Q-switched Ho:YLF laser [7]. In 2012, Dai et al. built and characterized an injection-seeded, single-longitudinal-mode Q-switched Ho:YAG laser at room temperature [8]. In order to further improve the power and energy of single-longitudinal-mode pulses, MOPA architecture is a promising method [9]. In 2017, Zhang et al. reported a 55.64 mJ single-longitudinal-mode Q-switched Ho:YAG ceramic MOPA with an injection locking seed laser at PRF of 200 Hz [10]. However, MOPA system with an injection locking structure is very complex. To solve this problem, a promising alternative is replacing injection locking seed laser with a Q-switched oscillator.
YVO4 crystal has good mechanical properties similar to YLF crystal and weaker than YAG crystal [11–13]. Holmium-doped YVO4 crystal performs a relatively large emission cross section (2.4×10−20 cm2 around 2.05 µm vs. 1.8×10−20 cm2 around 2.05 µm for Ho:YLF and 1.0×10−20 cm2 around 2.09 µm for Ho:YAG), but a relatively short upper laser level lifetime (∼4 ms vs. ∼7 ms for Ho:YAG and ∼15 ms for Ho:YLF) [14,15]. A laser system based on Ho:YVO4 crystals has the potential to produce outstanding 2 µm laser. Ho:YVO4 lasers have been studied in recent years, including continuous-wave (CW) laser [11], Q-switched laser [16] and CW single-longitudinal-mode laser [17]. However, single-longitudinal-mode pulsed Ho:YVO4 laser hasn’t been reported yet.
In this paper, we present a single-longitudinal-mode pulsed Ho:YVO4 MOPA system in the region of 2052.96 nm. The single-longitudinal-mode pulsed Ho:YVO4 oscillator was designed as a unidirectional ring cavity with an isolator, a half-wave plate and a Cr2+:ZnS plate. By using the residual (non-absorbed) pump power of the oscillator as the pump of the pulsed Ho:YVO4 amplifier, the 2052.96 nm laser power was further improved. As we know, this is the first time to report the single-longitudinal-mode pulsed MOPA system around 2 µm with isolator and Cr2+:ZnS, and this is also the first time to report the Ho-doped vanadate single-longitudinal-mode pulsed laser.
2. Experimental setup
The layout of the single-longitudinal-mode passively Q-switched Ho:YVO4 MOPA system is shown in Fig. 1. The pump source was a 1940nm Tm-fiber laser with maximum output power of 11.7 W and beam quality factor M2 of 1.42. The 1940nm pump laser was focused into the Ho:YVO4 crystal in the oscillator with a 1/e2 spot radius of ∼0.11 mm via two 150 mm focal length plane convex lenses. The oscillator was an 8-shaped four-mirror ring cavity. Its cavity length was 1125 mm. Flat concave mirror M1, M2 and M3 had highly reflective coating at 2.05 µm and highly transmissive coating at 1.94 µm with curvature radii of 150 mm, 300 mm and 5000 mm, respectively. Flat mirror M4 was an output coupler (OC) with 30% transmittance at 2.05 µm. In order to reduce the impact of thermal effects, an a-cut Ho:YVO4 crystal with Ho3+-doping concentration of 0.7 at. % was used in the oscillator, which had a cross-section of 3.2 mm × 3.2 mm and a length of 20 mm. An isolator (EOT, makros series 2000-2100 nm) and a half-wave plate were used in the oscillator to make the ring resonator operated at unidirectional operation. The isolator consisted of a faraday rotator and two polarizing beam splitters which were attached to the both ends of the faraday rotator. The faraday rotator surrounded by magnetic field produced a polarization rotation angle of 45° for 2.05 µm laser. By using the faraday rotator and half-wave plate in the oscillator, the polarization states of 2.05 µm laser performed orthogonal for the two opposite directions. Actually, only one direction laser could be produced for the Ho:YVO4 oscillator due to the stronger polarization emission cross-section (2.4×10−20 cm2 at 2.05 µm for π-polarization vs. ∼0.7×10−20 cm2 at 2.05 µm for σ-polarization) of the Ho:YVO4 crystal. The two polarizing beam splitters could further enhance the unidirectional propagation capacity of the oscillator. The unidirectional operation of laser could eliminate the effect of space burning, then the single-longitudinal-mode output could be obtained. A Cr2+:ZnS plate anti-reflective (AR) coated at 2.05 µm was inserted into the cavity as the saturable absorber. The Cr2+:ZnS plate had an initial transmission of 96% at 2.05 µm and was tightly mounted in a water-cooled copper heat sink. Cr2+:ZnS plate has saturable absorption property, which means that the higher 2.05 µm photon density in Cr2+:ZnS plate reduces the intracavity loss caused by Cr2+:ZnS plate. The reduction of the intracavity loss leads to the rapid release of the upper level particles of Ho3+ ions. With the decrease of photon density in Cr2+:ZnS plate, the intracavity loss caused by Cr2+:ZnS plate increases, and then the pulse gap is formed. The radii of TEM00 mode in the Ho:YVO4 crystal and the Cr2+:ZnS plate in the oscillator were designed to be 0.108 mm and 0.463 mm, respectively.
Using the residual (non-absorbed) pump power of the pulsed Ho:YVO4 oscillator as pump, a single-pass pulsed Ho:YVO4 amplifier was designed for further power scaling. An a-cut Ho:YVO4 crystal with Ho3+-doping concentration of 0.6 at. % was used in the amplifier, which had a cross-section of 3.2 mm × 3.2 mm and a length of 30 mm. the Ho:YVO4 crystal used in the amplifier had a relatively larger product of concentration and length than in the oscillator to improve the conversion efficiency of the amplifier. The both end faces of the Ho:YVO4 crystals used in oscillator and amplifier were AR-coated at both 1.94 µm and 2.05 µm. The Ho:YVO4 crystals used in oscillator and amplifier were wrapped with indium foils and mounted in copper heat sinks whose temperature was maintained at 14 °C by thermoelectric cooler. The seed beam was adjusted by two 100 mm focal length plane convex lenses (L2, L3), and the pump beam was adjusted by a 200 mm focal length plane convex lens (L1).
3. Results and discussion
3.1 Oscillator
The output characteristics of the single-longitudinal-mode Ho:YVO4 oscillator versus 1940nm pump power are given in Fig. 2. When the Cr2+:ZnS plate was removed from the oscillator cavity, the Ho:YVO4 oscillator operated in CW mode. The Ho:YVO4 oscillator obtained a maximum CW output power of 1.10 W at the incident pump power of 11.7 W, corresponding to an optical-to-optical efficiency of 9.4% and a slope efficiency of 13.2%. When the Cr2+:ZnS plate was inserted into the oscillator, the Ho:YVO4 oscillator operated in Q-switched mode. The single-longitudinal-mode Q-switched Ho:YVO4 oscillator performed a maximum average output power of 1.02 W under the incident pump power of 11.7 W, corresponding to an optical-to-optical efficiency of 8.7% and a slope efficiency of 11.6%. It was estimated that about 45.2% of the pump power was absorbed by the Ho:YVO4 crystal at the maximum pump power, corresponding to residual (non-absorbed) pump power of 6.52 W.
The output longitudinal mode of the CW Ho:YVO4 oscillator was monitored by a Fabry-Perot scanning interferometer with a free spectral range (FSR) of 1.5 GHz, and displayed by a oscilloscope(Tektronix, MSO 3034), as shown in Fig. 3. The blue line is the PZT’s driving voltage of Fabry–Perot scanning interferometer. In a period of voltage change, the voltage increases first and then decreases, corresponding cavity length of Fabry-Perot scanning interferometer decreases first and then increases. Thus, two sets of the peaks were measured in one period of voltage change. The two peaks of each set of peaks were separated by the 1.5 GHz free spectral range (FSR), and no other messy peaks was measured, which meant that the CW Ho:YVO4 oscillator had only one longitudinal mode.
The temporal profiles of the pulsed Ho:YVO4 oscillator were measured by a InGaAs detector (12.8 GHz) combined with a digital phosphor oscilloscope (1 GHz). The InGaAs detector and the digital phosphor oscilloscope were sufficient for the characterization of the oscillator which provided a typical mode spacing of ∼267 MHz. A portion of a pulse at the peak and its fourier transformation for the pulsed Ho:YVO4 oscillator at the pump power of 11.7 W are shown in Fig. 4(a). The position of ∼267 MHz in the fourier transformation spectrum had no peak, which indicated that the pulsed Ho:YVO4 oscillator operated in single-longitudinal-mode. Compared with Fig. 4(a), Fig. 4(b) was obtained by tapping on one of the resonator mirror mounts to disturb the laser cavity. The position of ∼267 MHz in this fourier transformation spectrum had an obvious peak, which indicated that the Ho:YVO4 oscillator operated in a multi-longitudinal-mode at that moment.
Fig. 4. Temporal profiles and FFT curves of (a) the single-longitudinal-mode pulse and (b) the multi-longitudinal-mode pulse for the pulsed Ho:YVO4 oscillator.
The spectra output of the Ho:YVO4 oscillator were measured at the pump power of 11.7 W, as shown in Fig. 5. The spectra were recorded by the laser spectrum analyzer (Bristol 721). As shown in Fig. 5(a), the central wavelength of the output laser of the CW Ho:YVO4 oscillator was measured to be 2052.93 nm. The central wavelength of the output laser of the pulsed Ho:YVO4 oscillator is shown in Fig. 5(b). The laser was emitting at 2052.96 nm. The slight difference of the central wavelengths between CW and pulsed Ho:YVO4 oscillator was mainly due to the loss of the Cr2+:ZnS plate.
The PRF and pulse width of the pulsed Ho:YVO4 oscillator at different pump power are illustrated in Fig. 6. When pump power increased from 5.21 W to 11.7 W, the PRF of the oscillator increased from 30 kHz to 67 kHz, and the pulse width of the oscillator decreased from 1626 ns to 910 ns. Increasing the pump power can increase the pump rate, which reduces the time for Cr2+:ZnS plate to reach the number of fluorescence photons required for low loss, thus leading to the increasing of repetition rate. The PRF drift of the pulsed Ho:YVO4 oscillator at the maximum average output power was measured to be ± 1.3% in 30 min. Moreover, large fluctuation of the repetition rate was obtained at higher average laser power density (>392 W/cm2) in Cr2+:ZnS plate, which might because that the Cr2+:ZnS plate was not fully recovered from saturation at high average laser power density.
3.2 Amplifier
To make full use of the pump power, a single-pass pulsed Ho:YVO4 amplifier was employed. The output characteristics of the single-longitudinal-mode passively Q-switched Ho:YVO4 amplifier versus initial 1940 nm pump power are depicted in Fig. 7. The solid lines shown in Fig. 7 are the theoretical output calculations of the amplifier based on the amplifier model given in [18]. With the spot radius of about 0.12 mm, the pulsed Ho:YVO4 amplifier performed a maximum average output power of 1.67 W, which was 1.64 times of the oscillator average output power. It meant that the optical-to-optical efficiency was up to 14.3% by using the pulsed Ho:YVO4 MOPA system. In addition, the average output powers of the pulsed Ho:YVO4 amplifier were lower when the spot radii were 0.15 mm and 0.18 mm, and the maximum average output powers of the pulsed Ho:YVO4 amplifier were 1.39 W and 1.15 W, respectively. The experimental results are consistent with the theoretical calculations. With the increase of the sizes of the pump laser spot and the seed laser spot, the number of Ho3+ ions involved in the amplification process increases. This increases the reabsorption of 2.05 µm laser, therefore the pump threshold (seed power equal to output power) of the Ho:YVO4 amplifier will increase. Under the limited pump power, with the increase of the sizes of the pump laser spot and the seed laser spot, the output power of the amplifier decreases. The experimental results and theoretical calculations showed that increasing the amplifier spot radius would result in a strong absorption of 2.05 µm laser in the Ho:YVO4 crystal. Due to the limitation of space distance between mirror mounts, the performance of the pulsed Ho:YVO4 amplifier with spot size of less than 0.12 mm was not performed, but the theoretical maximum average output power was calculated to be 1.7 W when the amplifier spot radius was about 0.105 mm, which was similar with the experimental results shown in this paper.
The beam quality factors of the pulsed Ho:YVO4 amplifier were measured at the maximum output power of 1.67 W. The beam radii of the output laser along the beam propagation direction were measured by 90/10 knife-edge method. As shown in Fig. 8, the beam quality factors M2 were calculated to be 1.063 and 1.062 in the x and y directions, respectively.
4. Conclusion
We have built a 2052.96 nm single-longitudinal-mode passively Q-switched Ho:YVO4 MOPA system and studied its output performances. The CW Ho:YVO4 oscillator with isolator and half-wave plate achieved an output power of 1.1 W. And the pulsed Ho:YVO4 oscillator with isolator, half-wave plate and Cr2+:ZnS plate achieved an average output power of 1.02 W with pulse width of 910 ns and PRF of 67 kHz. Further, using the residual (non-absorbed) pump power of the pulsed Ho:YVO4 oscillator as pump, the single-pass pulsed Ho:YVO4 amplifier obtained an average output power of 1.67 W. The total optical-to-optical efficiency of the pulsed Ho:YVO4 MOPA system was 14.3%. The results demonstrate that the MOPA system with a passively Q-switched unidirectional ring oscillator and a single-pass amplifier is an effective way to produce 2 µm single-longitudinal-mode pulsed laser. Future work will be concentrated on the improvement of the conversion efficiency of the Ho:YVO4 MOPA system.
Funding
National Natural Science Foundation of China (51572053).
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