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Abstract
Tunable single-longitudinal-mode (SLM) ring Ho:YLF laser with intra-cavity isolator is investigated for 2.05 µm and 2.06 µm band based on single resonator, which is realized dual-band SLM laser conversion by a polarizer. Up to 548 mW SLM power with beam quality factor M2 of 1.1 is achieved at wavelength of 2064.63 nm, and the corresponding slope efficiency is 26.7%. Wavelength tuning ranges from 2063.91 nm to 2065.71 nm and 2050.65nm to 2053.15nm can be demonstrated. The highest SLM power around P12 and R30 CO2 absorption peak of 2064.41 nm and 2050.96 nm are 540 mW and 500 mW, respectively. The power instability within 30 minutes is around 0.14%. As we know, dual-band switched Ho:YLF laser operation at SLM with wavelength tunability is reported for the first time for the potential application of CO2 differential absorption lidar.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Near 2 µm is safe wavelength band for human eyes [1,2] and has transparency window of the atmosphere [3]. 2 µm lasers have been applied in lots of fields, for example, laser lidar systems [4–6], medicine [7], spectroscopy [8], and so on [9,10]. In recent years, single-frequency operation of 2 µm laser has been widely studied. Various approaches have been taken to achieve SLM laser, including nonplanar ring oscillator [11,12], intracavity F-P etalon [13–15] and unidirectional ring laser [16]. Among them, the method of unidirectional ring laser suitable for all kinds of crystal is an extremely effective way to eliminate spatial hole burning and achieve robust SLM operation. Compared with the coaxially pumped linear configuration, such as a distributed feedback laser diode followed by an amplifier, the unidirectional ring laser with longer cavity length is more beneficial for obtaining narrow linewidth and high-power SLM output. Tunable SLM 2.05 µm and 2.06 µm laser covers the absorption peak of CO2 [17,18], which makes it important for the development of CO2 differential absorption lidar [19]. The active remote sensing of atmospheric CO2 R30 and P12 based on differential absorption lidars use respectively 2.05 µm and 2.06 µm laser as the light source with the advantages of independent of external light sources, high spatial resolution, and comprehensive real-time information of CO2 concentration. Compared with R30 absorption peak of 2050.96 nm [20–22], the CO2 absorption at P12 line center of 2064.41 nm [23] is slightly stronger, so that the CO2 differential absorption lidar with 2.05 µm laser source may display a longer detection distance for the same output energy of laser source.
Compared with Tm, Ho-codoped crystals [24,25], Ho:YLF crystal is easier to achieve 2.05 µm or 2.06 µm high-power stable operation output at room temperature. In 2010, W. Koen et al. reported a fiber-laser pumped Ho:YLF oscillator [26]. The highest continuous-wave (CW) power achieved at 2065 nm was 12.4 W with a slope efficiency of 25%. In 2014, J. Kwiatkowski et al. implemented CW Ho:YLF laser with in-band pumped [27]. Up to 11.5 W was acquired at 2050 nm with a slope efficiency of 40.9%. In 2020, Y. P. Wang et al. reported a CW operation ring Ho:YLF laser [28]. As high as 4.14 W power was achieved at 2063 nm with a slope efficiency of 27.3%.
In this paper, the experimental study about the 2.05 µm and 2.06 µm band tunable unidirectional ring Ho:YLF SLM laser with intra-cavity isolator is developed based on single resonator. The dual-band switched can be realized by changing the resonator loss through an intra-cavity polarizer. The SLM power at 2064.63 nm was up to 548 mW with M2 factor of 1.1. The corresponding slope efficiency is 26.7%. The wavelength tunable can be achieved from 2063.91 nm to 2065.71 nm and 2050.65nm to 2053.15 nm by an etalon. At P12 CO2 absorption peak of 2064.41 nm and R30 CO2 absorption peak of 2050.96 nm, the highest SLM power reaches about 540 mW and 500 mW respectively. The power instability measured at 2050.96 nm is around 0.14%. The results show that the proposed laser has potential value in differential absorption lidar for CO2 concentration measurement. To our knowledge, dual-band switched SLM ring Ho:YLF laser with wavelength tuning is the first time performed.
2. Experimental setup
The experimental structure used to demonstrate ring Ho:YLF laser with unidirectional operation is presented in Fig. 1. The self-designed Tm-doped fiber laser pump source has a maximum CW power of 25 W with an M2 factor of ∼1.1. Compared with Tm-doped solid-state laser, the Tm-doped fiber laser has smaller relaxation oscillation, which is beneficial to ensure 2 µm SLM stable output. A coupling system is applied for collimating and focusing the 1.94 µm pump light emited from a Tm-doped fiber laser into the a-cut Ho:YLF medium with 0.25 mm beam radius. The polarization state of the pump light is varied by a quarter-wave plate (QWP) for increasing the absorption of crystal. M1 is a 1.94 µm high reflection (R>99.8%) plane mirror. Using polarizer P1 and P2 for high reflection (R>99.8%) of pump light can ensure 2.05 µm p-wave high transmittance (T>97%) and s-wave high reflectivity (R>99.5%) at 45°. The intracavity loss to result in dual-band switched laser output is affected by changing the angle of P2.
Fig. 1. Schematic of the Ho:YLF ring laser with unidirectional operation. F: lens; QWP: quarter-wave plate; M1, M2, M3, M4, M5, M6: mirrors; P1, P2: polarizer; HWP: half-wave plate
The designed ring resonator with a length of 1.33 m consists of two plane-concave mirrors (M2, M3) and two planar mirrors (M4, M5). The beam between M2 and M3 is parallel to the beam between M4 and M5. The angle between the resonator arm M3-M2 and arm M2-M5 is 20°. The angle between the resonator arm M2-M3 and arm M3-M4 is 23°. M2 and M3 have radius of curvature of 400 mm and 300 mm, respectively, with 2.05 µm highly reflective coating (R>99.8%). M4 is also high reflection (R>99.8%) for 2.05 µm. Optimum transmission of output mirror M5 is around 30%. M6 is used for the laser beam splitting and it sends half of the light down one arm to F-P scanning and half down the other arm to wavemeter. The Ho:YLF gain crystal dimension and Ho3+ doped concentration are 4×4×20 mm3 and 0.5 at.%, respectively. Using half-wave plate (HWP) and Faraday rotator, the operation of unidirectional travelling wave in the Ho:YLF ring cavity can be realized based on Faraday effect. Wavelength tuning is performed using a 0.5 mm thick F-P etalon.
According to the Ref. [26], two emission wavelengths of anisotropic Ho:YLF crystal are respectively located at 2.05 µm and 2.06 µm. For π-polarisation, the emission cross sections at 2051 nm and 2064 nm are about 1.5×10−20 cm2 and 1.27×10−20 cm2, respectively. For σ-polarization, the maximum emission cross section at 2064 nm is about 0.78×10−20 cm2 and the emission cross section at 2051 nm is too small to generate oscillating. The free oscillation wavelength switching is typical for the behavior of Ho:YLF laser system with increasing losses (increasing inversion rate β), which can be calculated and analyzed by [29]: ${\sigma _{gain}}(\lambda )= \beta {\sigma _{em}}(\lambda )- ({1 - \beta } ){\sigma _{abs}}(\lambda )$, where$\; {\sigma _{gain}}(\lambda )$, ${\sigma _{em}}(\lambda )$ and ${\sigma _{abs}}(\lambda )$ represent the gain cross section, emission cross section and absorption cross-section, respectively. The relationship between Ho:YLF gain cross section and inversion rate for two bands is shown in Fig. 2.
It can be seen from Fig. 2 that at lower inversion rate (0.22<β<0.39) 2064 nm laser for π-polarisation exhibits higher gain cross section. However, a shorter wavelength 2051 nm laser for π-polarisation emerges higher gain cross section at higher inversion rates (0.39<β<1). The emission of 2064 nm for σ-polarisation needs to ensure that the c-axis of Ho:YLF crystal is horizontal and the inversion rate should meet the condition of 0.22<β<1. In brief, the essence of intra-cavity polarizer angle control is to alter the intracavity loss (inversion rate), so that dual-band switched can be realized. Additionly, at a certain band, wavelength tuning and output power depend on the combination of the transmission peak of F-P etalon and the gain curve of Ho:YLF crystal. By changing the angle of F-P etalon, the transmission peak of the F-P etalon can be tuned to a certain wavelength of the crystal gain curve, so that the unidirectional ring laser can be realized with wavelength tunability, and the SLM output power is gradually decreased as the transmission peak of the etalon shifts away from the gain curve of the crystal.
3. Experimental results and discussion
Lasing characteristics are observed for free-running operation of Ho:YLF laser with no Faraday rotator and HWP. As shown in Fig. 1, the output beam follows the two directions of the red solid line and the red dashed arrow behind the M5 mirror. Figure 3(a) shows the wavelength of 2064.61 nm observed by wavemeter. Since the free-space input wavemeter has the finite spectral resolution (± 0.2 pm) and optical spectrum exists random noises which may be introduced by the laser beam position measurement error and the alignment of laser beam with the internal He-Ne reference laser, the determination of single-longitudinal-mode or multi-longitudinal-mode laser should be achieved by F-P spectrum. The inset F-P spectrum obtained from F-P scanning interferometer shows that the free-running Ho:YLF laser is multi-longitudinal-mode operation. Figure 3(b) shows the pump threshold is about 3.51 W, the highest power achieves 3.26 W, and the slope efficiency is 28.8%. Output laser measured by Glan Thompson prism is horizontal polarization state.
Fig. 3. Output characteristic of free-running Ho:YLF laser. (a) Optical spectrum and F-P spectrum. (b) Output power
With a Faraday rotator and HWP added into the cavity, the Ho:YLF laser output is only in the direction of the red solid line in Fig. 1. F-P spectrum as shown in Fig. 4, two extreme peaks repeatedly appear between the lowest voltage and the highest voltage. The gap of adjacent two peaks is about 1.5 GHz free spectral region (FSR) of F-P scanning interferometer, which indicates that the Ho:YLF laser is operating in SLM.
Figure 5(a) shows the center wavelength of 2064.63 nm for SLM Ho:YLF laser is obtained. Figure 5(b) shows that SLM power up to 548 mW can be achieved with 5.28 W pump power, and the slope efficiency is 26.7%.
The beam radius measured by the 90/10 knife-edge technique is fitted with Gaussian beam propagation equation to obtain the beam quality factor M2 of 1.1 in Fig. 6. As shown in Fig. 7, the wavelength tuning can be realized with the help of an etalon and the range of SLM laser tuned from 2063.91 nm to 2065.71 nm.
Fig. 7. SLM F-P spectrum and optical spectrum. (a) The detective wavelength is 2063.91 nm. (b) The detective wavelength is 2065.71 nm
We previously reported a 2.05 µm SLM tunable laser based on Ho:YLF ring laser [30]. In this work, based on matching of intracavity loss on free oscillation laser, we achieve dual-band (2.05 µm and 2.06 µm) switched unidirectional ring Ho:YLF laser with wavelength tunability. The 2.05 µm SLM laser output and wavelength tuning can be realized by changing the angle of P2 and etalon, respectively. Figure 8 shows the relationship between tuning wavelength and output power in dual-band. For 2.06 µm band, the output power at 2063.91 nm and 2065.71 nm is respectively 453 mW and 338 mW. For 2.05 µm band, the tunable wavelength range is 2050.65nm to 2053.15nm, corresponding to the power of 442 mW and 460 mW, respectively. The corresponding tuning ranges of 2.06 µm and 2.05 µm bands are respectively 127 GHz and 178 GHz, while the laser is quasi-continuous tuning and the limitation of the FSR of cavity results in a minimum tunable gap of 225 MHz. At the commonly reported P12 CO2 absorption wavelength of 2064.41 nm and R30 CO2 absorption wavelength of 2050.96 nm, the highest SLM power achieves about 540 mW and 500 mW, respectively.
Figure 9(a) shows the threshold pump power of 2050.96 nm SLM laser is about 6.96 W with the slope efficiency of 38.4%. The SLM Ho:YLF laser is lasing on the π-polarisation, and the higher threshold and slope efficiency of 2.05 µm laser than 2.06 µm laser is result from that the 2.05 µm laser requires more energy to realize higher inversion rate and achieves higher utilization of pump light. Such a trend of experiment results is in good agreement with the simulation calculations (see Fig. 2). Figure 9(b) shows the corresponding F-P spectra and measured wavelength. In order to acquire the frequency stability at 2050.96 nm, F-P spectra are measured and recorded by F-P scanning interferometer and digital oscilloscope at 15 second intervals within 30 minutes. By calculating the standard deviation of the longitudinal-mode shift in the same 1.5 GHz FSR, the frequency stability of about 1.15×10−7 is obtained. The power stability within 30 minutes of SLM laser is given in Fig. 10. Power standard deviation is calculated to be 7.14 mW, corresponding to a power instability of around 0.14%.
Fig. 9. Output characteristic of SLM Ho:YLF laser at 2050.96 nm. (a) Output power. (b) Optical spectrum and F-P spectrum
4. Conclusion
In conclusion, a 2.05 µm and 2.06 µm dual-band switched Ho:YLF SLM laser is demonstrated by changing the angle of an intra-cavity polarizer. The SLM power is the most to be 548 mW at 2064.63 nm, and the corresponding slope efficiency achieves 26.7% with M2 factor of 1.1. The wavelength tuning ranges of 2063.91 nm to 2065.71 nm and 2050.65nm to 2053.15nm can be obtained with the help of an etalon. The highest power of SLM laser are 540 mW and 500 mW near the P12 and R30 CO2 absorption peaks of 2064.41 nm and 2050.96 nm, respectively. The power instability within 30 minutes of 0.14% is acquired. The results conclude that the proposed laser has great application potential in CO2 differential absorption lidar.
Funding
National Natural Science Foundation of China (61975084, 62005125); Natural Science Foundation of the Jiangsu Higher Education Institutions of China (19KJB140011); Startup Foundation for Introducing Talent of NUIST (2241131801051, 2241131801052); High-level Innovation and Entrepreneurship Talents Introduction Program of Jiangsu Province of China.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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