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Abstract
We demonstrated an efficient and tunable single-longitudinal-mode Ho:YLF ring laser based on Faraday effect for application to measure atmospheric carbon dioxide (CO2). Single-longitudinal-mode power at 2051.65 nm achieved 528 mW with the slope efficiency of 39.5% and the M2 factor of 1.07, and the tunable range of about 178 GHz was obtained by inserting a Fabry-Perot (F-P) etalon with the thickness of 0.5 mm. In addition, the maximum single-longitudinal-mode power reached 1.5 W with the injected power of 528 mW at 2051.65 nm by master oscillator power amplifier (MOPA) technique. High efficiency and tunable single-longitudinal-mode based on Faraday effect around 2 μm has not been reported yet to the best of our knowledge.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Single-longitudinal-mode lasers around 2 μm have been widely applied to lots of fields, such as laser lidar [1,2], laser remote sensing [3] and high resolution spectroscopy [4] on account of its advantage of eye-safe [5]. The 2 μm high power single-longitudinal-mode laser previously reported mainly focused on Ho:YAG laser achieved by nonplanar ring oscillator (NPRO) [6,7]. The NPRO technique is only applicable for isotropic crystals but not for anisotropic crystals. The unidirectional ring laser with acousto-optic device is a promising way for 2 μm high power output with anisotropic crystals. In 2017, our group reported single-longitudinal-mode Ho:YLF [8] and Ho:YVO4 [9] unidirectional ring laser based on the acousto-optic effect, and output power were 3.73 W and 941 mW, respectively. The slope efficiency relative to the pump power were 27.1% and 11.1%, respectively. However, there are still some difficulties in achieving tunable single-longitudinal-mode lasing since the way achieved by the unidirectional ring laser with acousto-optic device requires precise adjustment of acousto-optic Q switch and two half-wave plates.
The tunable single-longitudinal-mode output at 2 μm has a wide promising applications in differential absorption lidar for atmospheric H2O and CO2 monitoring [10,11]. The tunable and single-longitudinal-mode lasers have been mainly reported at 1 μm by unidirectional ring resonator based on Faraday effect, since the Faraday material (terbium gallium garnet crystal) used in 1 μm has the high laser-damage threshold and the laser gain medium can be pumped with laser diode. Typically, in 2008, N. Coluccelli et al. reported a single-frequency Tm:LiLuF4 ring laser pumped with a laser diode [12] based on Faraday effect, the maximum power was 120 mW at 1875 nm and the slope efficiency was 13%. In 2013, Wang et al. reported a tunable single-frequency Nd:YVO4/LBO green laser pumped by laser diode with 10.5 W output power [13]. Terbium gallium garnet crystal was used as a unidirectional device. With the development of 1.9 μm fiber laser and magneto-optic material at 2 μm, this technique is a promising way to achieve tunable and high efficiency single-longitudinal-mode output with Ho3+ doped crystal.
In this paper, a tunable single-longitudinal-mode laser with power of 528 mW, tuning range of 178 GHz, slope efficiency of 39.5%, and M2 factor of 1.07 at 2051.65 nm was achieved. In addition, the maximum single-longitudinal-mode power reached 1.5 W with the injected power of 528 mW at 2051.65 nm by MOPA technique and the maximum gain of 7.22 dB was achieved with the injected power of 18 mW. The Ho:YLF crystal is selected as gain medium since the Ho:YLF laser can operate high efficiency and high power at room temperature with a long cavity length compared with the Tm,Ho co-doped crystal. Faraday rotator and a half-wave plate are inserted into four-mirror bow-tie ring resonator for unidirectional operation. Since the laser wavelength ranges from 2050.65 nm to 2053.15 nm which covers the absorption line center of CO2 (2050.967 nm) [14–16], tunable single-longitudinal-mode Ho:YLF laser can be used in differential absorption lidar (DIAL) for the measurement of CO2 concentration. As we know, this is the first time to report the high efficiency and tunable single-longitudinal-mode Ho:YLF ring laser based on Faraday effect.
2. Unidirectional operation of Ho:YLF ring laser
High efficient and tunable single-longitudinal-mode Ho:YLF ring laser mainly consisted of a Ho:YLF amplifier and a unidirectional operation of Ho:YLF ring laser which was shown in Fig. 1. The 1.94 μm Tm-doped fiber laser used for pump light was collimated and focused by two lenses and became the beam with spot radius of 0.25 mm. A quarter-wave plate was employed to increase the absorption efficiency of the crystal. The Ho:YLF crystal was 0.5% doped with a dimension of 4 × 4 × 20 mm3, and the end faces was coated with AR films for the wavelengths of both 1.94 and 2.05 μm for 13°C ± 0.02°C by a TEC cooler. P1 and P2 were both polarizer that had high transmittance (T>97%) for 2.05 μm p-wave and high reflectivity (T>99.5%) for 2.05 μm s-wave at 45°. Moreover, such a polarizer had ultra-high reflectivity (R>99.8%) for 1.94 μm pump light. Accordingly, polarizer could be used for pump light injecting and dumping. M1 was a plane mirror coated for high reflectivity at 1.94 μm (R>99.8%). M2 and M3 were both plane concave mirror coated for high reflectivity at 2.05 μm with curvature radius of 400 mm and 300 mm. M4 was a plane mirror coated for high reflectivity at 2.05 μm and M5 was a plane output coupler with a transmittance of 30% at 2.05 μm. A faraday rotator (Optics For Research) and a half-wave plate (AR-coated at 2.05 μm) were inserted into four-mirror bow-tie ring resonator with the length of 1.33 m (FSR = 225MHz) for unidirectional oscillation. The faraday rotator was surrounded by a magnetic field with a polarization rotation angle of 45°. The laser propagation of one direction was horizontal polarization while the other was vertical polarization by combining the faraday rotator and a half-wave plate. Thus only one direction laser which has stronger emission cross section (1.8×10−20cm2 at 2.05 μm, π-polarization) can be produced. The single-longitudinal-mode Ho:YLF operation can be realized without the spatial hole-burning effect. In order to avoid damaging the faraday rotator, pump light was dumped by P2 and the waist diameter of the fundamental mode laser in the faraday rotator remained at 1.7 mm. A Φ10 × 0.5 mm F-P etalon (fused silica) with no coating was employed for frequency tuning.
The output power of the Ho:YLF ring laser linearly increases with the pump power increasing, as shown in Fig. 2(a). Two directions both produce laser without a faraday rotator and a half-wave plate inserted, as shown in Fig. 1. The maximum total power was 2.5 W with the pump power of 12.7 W and the slope efficiency of 39.8%. Output wavelength and F-P spectra are shown in Fig. 2(b) and its inset, respectively. From Fig. 2(b) and its inset, the laser is not single-longitudinal-mode because two directions both produce laser. Wavelength of 2051.66 nm was measured by a wavelength meter (721A, Bristol), and output laser demonstrated multimode by a scanned F-P interferometer with FSR of 1.5 GHz (SA200-18B, THORLABS), as shown in Fig. 2(b) and its inset.
The laser can realize unidirectional operation which shows as the solid line direction behind the M5 in Fig. 1 while a faraday rotator and a half-wave plate inserted into the cavity. As shown in Fig. 3(a), the laser operated at single-longitudinal-mode measured by the scanned F-P interferometer at 2051.65 nm. Output laser was horizontal polarization measured by Glan prism. The laser could maintain single-longitudinal-mode operation without an etalon for the reason that the loss difference between the two directions of propagation was big enough.
Fig. 3 Out wavelength of the single-longitudinal-mode Ho:YLF ring laser and corresponding F-P spectra.
A F-P etalon with the thickness of 0.5 mm was used to tune frequency. The laser was divided into two parts by a plane mirror with 50% transmittance so that output wavelength and F-P spectra could be simultaneously observed. The longitudinal-mode transmission peak was ranging from high frequency to low frequency with the inclined angle of the etalon decreasing. The output frequency of the single-longitudinal-mode laser is determined by the transmission peak of etalon and the maximum value of the gain curve of the crystal. From the Fig. 3, it can be known from the wavelength meter that the wavelength can be tuned from 2050.65 nm to 2053.15 nm (about 178 GHz), and the corresponding F-P spectra shift accordingly. As shown in the inset of Fig. 3, the gap of adjacent two peaks is about 1.5 GHz (the FSR corresponding to voltage of about 11 V).
The wavelength meter data in Fig. 3 do not exhibit the same line width for the different considered frequencies. The random errors may include two respects: with the changing of etalon angle, the output beam has variation in direction so that the error resulting from uncertainties in the laser position measurement; in addition to the position measurement error, the actual laser line width is far less than the resolution of wavelength meter, so the laser line width is broadened due to the finite spectral resolution of the wavelength meter ( ± 0.2 pm). Considering the level of noise, spectral purity is estimated to be 15 dB which is relatively low. It can be further improved by both accurate control of cavity length and laser temperature. In addition, it perhaps also cause by the wavelength meter which is free-space input. The alignment of the laser beam to the internal He-Ne reference laser of wavelength meter will affect the level of noise and signal so that spectral purity will be affected. If a pre-aligned fiber-optic input connector is used, the actual spectral purity may be higher.
At the wavelength of 2050.65 nm, frequency stability is observed by F-P interferometer and F-P spectra are recorded in 30 minutes with an interval of 15 seconds. Frequency stability of about 1.12 × 10−7 is achieved by calculating the standard deviation of the longitudinal-mode shift within a same FSR of 1.5 GHz, which may be caused by thermal/mechanical perturbations. In actual DIAL application, the influence of thermal/mechanical perturbations on the laser frequency stability is difficult to be completely eliminated. Passive and active frequency stabilization methods should be adopted simultaneously to improve the frequency stability, such as adding high-precision controlling of the laser pump current, temperature and cavity length. In addition, it is obviously that the laser is quasi-continuous tuning since the FSR of cavity is the minimum tunable gap. Continuous tuning can be achieved by tuning the laser cavity length, such as adding a piezoelectric ceramic transducer upon a cavity mirror or inserting an intracavity phase modulator into the cavity.
The 2051 nm emission band [5,17] is larger than the tuning range of 178 GHz (about 2.49 nm) so that the laser tuning range is close to a free spectral region of the 0.5 mm etalon. Transmission peak of etalon will shift with the variation of the inclined angle of the etalon. When the transmission peak of etalon gradually shifts away from the crystal gain peak, output power will decrease accordingly. The relationship between output power and wavelength for changing the inclined angle of the etalon is shown in Fig. 4. Considering the maximum power density of faraday rotator, the maximum single-longitudinal-mode power of 528 mW is obtained near the wavelength of 2051.7 nm and pump power is limited to 8.1 W. Moreover, minimum single-longitudinal-mode power is 442 mW near the wavelength of 2050.67 nm.
As shown in Fig. 5(a), the output power increases with the pump power increasing at the wavelength of 2051.65 nm. In addition, a maximum power of 528 mW is achieved with slope efficiency of 39.5% for the pump power of 8.1 W. The beam quality was measured by knife-edge method for maximum output power of 528 mW, as shown in Fig. 5(b). The M2 factor is 1.07 calculated by non-linear fitting of beam radius at different positions.
Fig. 5 Lasing properties of the single-longitudinal-mode Ho:YLF ring laser at 2051.65 nm. (a) output power and (b) beam quality.
3. Ho:YLF amplifier
The schematic of a single-longitudinal-mode Ho:YLF MOPA laser is shown in Fig. 6. The dimension of the Ho:YLF crystal at 0.5% doped is 4 × 4 × 50 mm3. Seed laser is focused by a lens with f1 of 200 mm and the beam radius is 0.185 mm in the gain medium. In addition, pump source with the wavelength of 1.94 μm is focused by a lens with f2 of 150 mm and the beam radius is 0.19 mm in the gain medium. P3 and P4 are both polarizer that coating film is the same as P1 and P2, and M6 is a plane mirror coated for high reflectivity at 1.94 μm.
With the injected seed power of 528 mW at 2051.65 nm, the output power increases with the pump power increasing and the maximum power is 1.5 W for the pump power of 19.02 W, as shown in Fig. 7(a) and the overall efficiency (1.5 W output power versus the pump power of two Tm lasers) is 5.53%. The laser is still single-longitudinal-mode operation at 2051.65 nm after the amplified tested by the wavelength meter and scanned F-P interferometer. Figure 7(b) shows the output power and gain versus the injected seed power at a fixed pump power of 19.02 W. From Fig. 7(b), the optical gain decreases and the output power increases with the injected power increasing. The output power of 95 mW and optical gain of 7.22 dB are achieved for an injected power of 18 mW. In addition, when the injected power increases to 528 mW, the maximum power of 1.5 W and gain of 4.53 dB are obtained. The amplified laser wavelength changes along with the injected seed wavelength changing, according to the experiment results.
Fig. 7 (a) Amplified power as a function of the pump power; (b) Output power and gain as a function of the injected seed power.
4. Conclusion
In summary, a high efficiency and tunable single-longitudinal-mode Ho:YLF MOPA laser was experimentally demonstrated. A single-longitudinal-mode power of 528 mW was obtained at 2051.65 nm with slope efficiency of 39.5%. The wavelength tunable range was 178 GHz with M2 factor of 1.07. The maximum single-longitudinal-mode power of 1.5 W is achieved by MOPA technique. Experimental results show that such a method is satisfying to realize high efficiency and tunable single-longitudinal-mode Ho:YLF laser for application of atmospheric CO2 measurement.
Funding
The National Natural Science Foundation of China (Nos. 61308009 and 61405047), the China Postdoctoral Science Foundation Funded Project (Nos. 2016T90287 and 2015M570290), the Fundamental Research Funds for the Central Universities Grant (No. HIT. NSRIF.2015042), and the Heilongjiang Postdoctoral Science Foundation Funded Project (No. LBH-Z14085).
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