1. Introduction

Tunable SLM lasers in the eye-safe band have been widely used in Coherent Wind Measurement Lidars and Differential Absorption Lidars [1–3]. There are kinds of way to achieve SLM operation, such as the micro-chip laser [4], the laser with intracavity etalons [5], the twisted mode cavity [6], nonplanar ring oscillator (NPRO) [7] and planar ring resonator which based on the Faraday effect or an acousto-optic effect. Unidirectional operation with planar ring resonator has the advantage of stability and flexible choice of cavity type and applicable for both isotropic crystals and anisotropic crystals. In 1991, Clarkson et al. demonstrated a unidirectional SLM Nd:YLF laser [8]. Up to 340 mW SLM output was achieved based on an acousto-optic modulator and the optical-to-optical efficiency was 34%. In 2004, Shen et al. demonstrated a SLM Ho:YAG ring laser based on acousto-optic modulator [9], and 3.7 W SLM power was obtained with the optical-to-optical efficiency of 41%. In 2008, Kim et al. demonstrated a 1645 nm unidirectional ring Er:YAG laser based on the acousto-optic effect [10], and 4.7 W SLM output power was realized with an optical-to-optical efficiency of 33%. In 2017, Wu et al. reported a unidirectional Ho:YLF ring laser based on the Faraday effect [11], and 528 mW SLM output at 2051 nm was achieved with an optical-to-optical efficiency of 6.5%. In 2020, Dai et al. demonstrated a SLM Ho:GdTaO₄ unidirectional ring laser based on the Faraday effect, and 392 mW SLM power was achieved at 2068 nm with an optical-to-optical efficiency of 10% [12]. From these results we can see that the SLM power obtained by using the acousto-optic effect is generally higher than that obtained by using the Faraday effect due to the low insertion loss and high damage threshold of AOM.

In the past few years, many groups have carried out in-depth works on SLM laser around 1.6 µm for its eye-safe and high atmospheric transmission characteristics. Er:YAG lasers operated at 1645 nm and 1617 nm are widely used. CH₄ absorption lines exist in the 1645 nm region, so SLM running at 1645 nm can be used in Differential Absorption Lidar. In comparison, the radiation at 1617 nm is more suitable for Wind Lidar since it is far away from atmospheric absorption lines [13]. However, due to the strong quasi-three-level character that 1617 nm laser operation needs more population inversion. In 2007, Stoneman et al. demonstrated a 1617 nm Er:YAG laser which used an etalon to enforce 1617 nm operation. And average power of 6.5 W at 1 kHz was obtained [14]. In 2008, Kim et al. reported an Er:YAG laser at 1617 nm with a multimode continuous-wave output power of 31 W [15]. In 2012, Zhu et al. demonstrated a SLM Er:YAG laser operating at 1.6 µm [16]. Two etalons were used to achieve single frequency and wavelength selection. The SLM output powers were 792 mW and 640 mW at 1645 nm and 1617 nm, respectively.

In this work, a tunable unidirectional SLM Er:YAG ring laser at 1.6 µm region is demonstrated. An AOM is employed to realize the unidirectional operation. The highest SLM output power is 10.4 W and 8.7 W at the wavelength of 1645.22 nm and 1617.33 nm, and the wavelength can be tuned from 1616.77 nm to 1617.51 nm and 1644.51 nm to 1646.12 nm. The line width is measured to be 125 kHz at 1617 nm and 131 kHz at 1645 nm. In addition, by rotating the BRF, we also obtain 9.1 W stable simultaneous dual-wavelength single-frequency operation of 1617 nm and 1645 nm. To the best of our knowledge, the SLM power at 1617 nm is an order of magnitude higher than that of the previous tunable SLM laser and BRF is used for the first time to achieve wavelength tuning and linewidth narrowing for 1.6 µm laser.

2. Experimental setup

The setup of tunable unidirectional SLM Er:YAG laser is shown in Fig. 1. An Er:YAG ceramic rod with a dimension of Φ4 mm × 50 mm and a doping concentration of 0.25 at.% is utilized as the gain medium, with two sides are polished and antireflection coated within a range of 1400 nm to 1700 nm. The low doping level of the Er:YAG rod can reduce energy transfer up-conversion (ETU) effect [17]. The Er:YAG ceramic rod is attached to a thermoelectric cooler (TEC) whose temperature is maintained at 18 °C. A 1532 nm fiber laser with linear polarization is employed as the pump source and the maximum output power is 30 W. The line width of the fiber is ∼0.2 nm, and its beam quality M² is less than 1.1. Five mirrors form a closed ring cavity and the total length is 1160 mm, where M1 and M3 are 45° dichroic flat mirrors coated for highly transmitting at 1532 nm and highly reflecting at 1600 nm to 1700 nm (R>99.9%). M5 is a highly reflected dichroic flat mirror working at angle of 0°. M2 is a highly reflective mirror with a curvature of 300 mm. M4 is the output coupler with the radius curvature of 300 mm. The radius of the pump beam is focused to 280 µm in the middle of the Er:YAG rod by a lens f1 with a focal length of 200 mm. A fused silica AOM (MQH068, Gooch Inc) with both faces antireflection coated from 1400 nm to 1700 nm is placed in the ring cavity to provide the loss difference required for the unidirectional operation of the backward propagating beam, and the full RF power of the AOM is 50 W with the frequency shift of 68 MHz. A 2.5 mm thick quartz birefringent filter is used to tune the wavelength and to narrow the line width.

 

Fig. 1. The experimental setup of the SLM Er:YAG ring laser. AOM: acousto-optic modulator; BRF: birefringent filter; BC: beam combiner.

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3. Experimental results and discussion

There are two intense transitions between the upper manifold of ⁴I_13/2 and the lower manifold of ⁴I_15/2 for the Er³⁺ ion, and the corresponding output wavelengths are 1617 nm and 1645 nm respectively. The emission cross-section at 1617 nm is larger than that at 1645 nm, but it is more difficult to achieve 1617 nm operation at the room temperature, due to higher threshold population inversion needed for 1617 nm [18]. Therefore, the Er:YAG laser always oscillates at 1645 nm without mode selection. However, the wavelength of 1617 nm is also attractive because its spectrum is located in the atmospheric window [19].

In this work, we inserted a 2.5 mm thick quartz birefringent filter at the Brewster angle in the ring cavity for the wavelength selection and linewidth narrowing. We simulate the transmittance when rotating the BRF around its optical axis. Figure 2 shows the calculated transmission and the gain spectrum of the Er:YAG. The blue line shows the Er:YAG emission spectrum and the red line, green line and purple line are the transmittance of the BRF at different wavelengths when rotating the BRF. It can be seen from the calculation results that wavelength selection can be achieved by using only one piece of BRF with a thickness of 2.5 mm.

 

Fig. 2. Er:YAG emission spectrum and simulated transmittance of BRF at different wavelength.

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Then we investigate the free oscillation (without AOM and BRF) of the ring laser. Figure 3(a) shows the variation of the output power versus the pump power at different output couplers. The maximum output powers are 12.55 W, 11.34 W, 9.84 W, corresponding to the transmittance of 5%, 10%, 20%, respectively. Figure 3(b) shows the wavelength and F-P scanning results under free oscillation at the transmittance of 5%, from which the output beam operates in multimode and the central wavelength is at 1645 nm. The multi-longitudinal-mode of the laser is attributed to the spatial hole burning effect under bi-directions operation.

 

Fig. 3. Output characteristic of free oscillation Er:YAG ring laser. (a) Output power (b) Output spectrum and F-P longitudinal spectrum.

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To get SLM operation, an AOM is inserted into the cavity. When the AOM is placed slightly away from the Bragg angle, the forward-propagating beams and the counter-propagating beams will have different diffraction angles, which will result in different losses, and this can enforce unidirectional operation and eliminate the spatial-hole-burning effect to achieve the SLM operation. The transmission of the output coupler is chosen as 5%. Figure 4(a) shows the results measured by using the spectrometer and the F-P scanning interferometer. It shows that the output laser operates in SLM with the central wavelength of 1645.22 nm. A 2.5 mm-thick quartz birefringent filter is inserted into the cavity at the Brewster angle for wavelength tuning. Figure 4(d) shows the relationship between the SLM output power and the pump power of the Er:YAG ring laser operating at different wavelength. Up to 10.4 W SLM output at 1645 nm is obtained with a slope efficiency of 45%. By rotating the BRF, up to 9.1 W simultaneous dual-wavelength operation is achieved. Figure 4(b) shows the patterns measured by the spectrometer and the F-P scanning interferometer, which clearly shows that the laser operates at simultaneous dual-wavelength and the relative intensity of the two peaks are stable. The pattern of the F-P scanning interferometer shows that both modes operate in single longitudinal mode. Such a high power simultaneous dual-wavelength laser can be applied to dual-frequency Differential Absorption Lidar and terahertz generation [20]. SLM output at 1617.33 nm is also realized, with the maximum power of 8.7 W, and a slope efficiency of 40%, as shown in Fig. 4(c).

 

Fig. 4. SLM characteristics of unidirectional Er:YAG ring laser. (a) Output spectrum and F-P longitudinal spectrum at 1645 nm. (b) Spectrum of simultaneous dual-wavelength operation of 1645 nm, 1617 nm. (c) Output spectrum and F-P longitudinal spectrum at 1617 nm. (d) SLM output power versus the pump power.

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Wavelength tuning around 1617 nm and 1645 nm are also investigated. By finely adjusting the BRF, tuning range of 1616.77 nm to 1617.51 nm and 1644.51 nm to 1646.12 nm are achieved. When the laser operates at dual-wavelength, the relative output power of two wavelengths can be adjusted by rotating the birefringent filter. The SLM output power versus the tuning wavelength is shown in Fig. 5. The wavelength tuning range covers the CH₄ absorption lines which could be used in CH₄ Differential Absorption Lidar.

 

Fig. 5. Wavelength tuning range and corresponding output power under SLM operation.

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Figure 6 shows the fluctuations of the wavelength and power for the SLM laser operating at 1645 nm and 1617 nm in 30 minutes. The standard deviation of 1645 nm output power fluctuation within 30 minutes is 0.03 W, corresponding to a power instability of ∼ 0.29%. The standard deviation of the wavelength is 0.005 nm. For 1617 nm, the two standard deviations are 0.025 W (power instability of ∼0.28%) and 0.003 nm within 30 minutes, respectively.

 

Fig. 6. The fluctuations of the wavelength and power for the SLM Er:YAG laser at 1645 nm (a) and 1617 nm (b).

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The line width of the unidirectional single-longitudinal-mode Er:YAG ring laser is measured by the time-delayed self-heterodyne method, as shown in Fig. 1. The SLM laser is divided into two beams through the second AOM placed at the Bragg angle, and the frequency of the first-order diffraction beam is shifted by 68 MHz. The zero-order beam is coupled into the delay fiber with a length of 20 km. The two beams are combined by a beam combiner (BC) and the beat signal is detected by an InGaAs detector and analyzed with a spectrum analyzer (N9020A MXA, Agilent Inc). Figure 7(a) shows the optical RF spectrum of heterodyne signal at 1617.33 nm after Lorentz fitting when the birefringence filter is inserted. The 3 dB line width is measured to be 125 kHz, and the line width without birefringence filter is measured to be 280 kHz. Figure 7(b) shows the line width with BRF at 1645.22 nm, the 3 dB line width is measured to be 131 kHz and the line width is 303 kHz without BRF.

 

Fig. 7. 3 dB line width of the SLM Er:YAG laser. (a) with BRF inserted into the ring cavity of 1617.33 nm. (b) with BRF inserted into the ring cavity of 1645.22 nm.

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As shown in Fig. 8. The beam diameters of the SLM Er:YAG laser at 1617.33 nm under output power of 8.7 W are measured at different positions. The fitted M² factors in x and y directions are 1.21 and 1.32, respectively.

 

Fig. 8. M² factor of the unidirectional ring Er:YAG laser at 1617.33 nm.

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4. Summary

In summary, a 1.6 µm tunable unidirectional SLM Er:YAG ring laser is demonstrated. By inserting an AOM into the cavity to force unidirectional operation, up to 8.7 W and 10.4 W SLM power are obtained at 1617.33 nm and 1645.22 nm. A birefringent filter is utilized to achieve wavelength switching and tuning. By finely adjusting the BRF inserted in the cavity, the wavelength in a range of 1616.77 nm to 1617.51 nm and 1644.51 nm to 1646.12 nm can be tuned. Besides, up to 9.1 W simultaneous dual wavelength operation of 1617 nm and 1645 nm is achieved. The line width of the SLM Er:YAG laser is measured by the delayed self-heterodyne method. And the linewidth is measured to be 125 kHz at 1617 nm and 131 kHz at 1645 nm. The measured M² factors are 1.21 and 1.32 in the x and y directions, respectively. For all we know, such a high power stable simultaneous dual-wavelength laser at 1.6 µm has not been reported and 8.7 W is the highest SLM power at 1617 nm. This high efficiency SLM laser running around 1.6 µm could be used as the light sources of the Wind Measurement Lidar or Differential Absorption Lidar systems.