Narrow-line-width pulsed solid-state lasers in the 2 μm eye-safe spectral region are usually used as the lidar emitter in coherent Doppler lidars for wind field measurement or in differential absorption lidars (DIALs) for monitoring the concentrations of gases such as CO₂ and H₂O^[1^–4^]. A lidar configuration requires an emitter with high energy to increase the detection range, a long pulse width to improve the spectrum resolution and a narrow line width to enhance the spectral purity^[5^]. A critical technique for obtaining a narrow-line-width pulsed laser is the injection of a single-frequency continuous-wave (CW) laser into a Q-switched slave laser^[6^,7^]. In addition, there are two ways to obtain a long pulse width: one is to increase the cavity length of the Q-switched slave laser; and the other is to ensure that the slave laser operates at a lower energy. Increasing the cavity length, however, leads to difficulties with the cavity design, a complex structure and a large laser volume. Compared with a longer resonator, it is an effective approach for realizing a pulsed laser with narrow line width, long pulse width and high energy, by making the Q-switched slave laser operate at a lower energy and then increasing the output energy through an amplifier system.

A Ho-doped single-frequency solid-state laser is a significant means for obtaining a 2 μm narrow-line-width laser that benefits from an insensitivity to temperature, low quantum defects and low upconversion losses^[8^,9^]. Anisotropic holmium-doped yttrium lithium fluoride (Ho:YLF) crystals have the capability to generate high energy lasers due to the combination of a high emission cross-section and a long upper-level lifetime (14 ms) for Ho³⁺ transition^[10^,11^]. More importantly, the output spectrum of a Ho:YLF laser can be tuned over many CO₂ characteristic absorption lines^[12^–14^]. The most commonly used CO₂ absorption line around the 2050 nm emission peak of Ho:YLF crystal is 2050.967 nm. In 2009, Bai et al. proposed a single-frequency injection-seeded Q-switched Ho:YLF laser for CO₂ measurement^[15^]. With a pulse repetition frequency (PRF) of 1.25 kHz, the output energy and pulse width of a single-frequency Ho:YLF laser were 5.5 mJ and 50 ns, respectively. The online wavelength was locked to the R30 CO₂ absorption line at 2050.967 nm, and the offline wavelength was 2051.023 nm. In 2014, Gibert et al. reported a single-frequency two-wavelength pulsed Ho:YLF laser for CO₂ DIAL applications^[16^]. At a PRF of 2 kHz, a maximum single-frequency output energy of 13.5 mJ was obtained, with a pulse width of 42 ns. The online wavelength was locked to the R30 CO₂ absorption line at 2050.967 nm. Near the 2064 nm emission peak of Ho:YLF crystal (π- or σ-polarization), however, the P12 CO₂ absorption line at 2064.414 nm was another choice because it has higher absorption intensity, and there is no absorption of other trace gases. Therefore, the single-frequency pulsed laser around 2064 nm based on Ho:YLF crystal is an extremely promising emitter for DIAL systems.

In this paper we propose a single-frequency single-pass Ho:YLF amplifier around the P12 CO₂ absorption line. The output characteristics of the Ho:YLF amplifier under free-running and single-frequency operation were compared for the first time. An output energy of 24.2 mJ with a pulse width of 250 ns at a PRF of 100 Hz was obtained. The energy stability during 30 min was improved from 3.8% to 1% when the single-frequency pulsed Ho:YLF laser was amplified. The pulse build-up time of the Ho:YLF amplifier under single-frequency operation was 0.37 μs shorter than that of free-running operation. The line width of the single-frequency pulsed Ho:YLF amplifier measured by a heterodyne technique was 2.81 MHz.

The single-pass amplifier with three adjacent Ho:YLF crystals was end-pumped by an ellipse-polarization Tm-doped fiber laser with a central wavelength of 1940 nm. At a total pump power of 33.9 W, the powers of p-polarized and s-polarized light were 17.6 W and 16.3 W, respectively. The pump beam was focused by the spherical positive lens f3 with a focal length of 200 mm. The waist radii of the pump beam in the front face of the first Ho:YLF crystal were 0.34 mm in the horizontal position and 0.44 mm in the vertical position. The single-frequency pulsed Ho:YLF seed laser beam was focused by the spherical positive lens f2 with a focal length of 240 mm, and the waist radius was 0.48 mm in the center of the first Ho:YLF crystal. M6 was a coated plane mirror, with high reflectivity from 1.94 μm to 2.06 μm. P2 and P3 were 45° polarizers with high reflectivity at 1.9 μm, high transmission for 2.06 μm p-polarized light and high reflectivity for 2.06 μm s-polarized light. Three a-cut Ho:YLF crystals with the dimensions 5 mm × 4 mm × 60 mm, 5 mm × 4 mm × 30 mm and 4 mm × 5 mm × 40 mm were doped with 0.5 at.% Ho³⁺, wrapped with indium foil, and mounted on a copper heat sink. Both end faces of the three crystals were coated with 1.9 μm and 2.1 μm antireflection (AR) coating. The temperature of the three Ho:YLF crystals was controlled at 18 ± 0.1°C by circulating water. The first two Ho:YLF crystals were oriented with their c-axes horizontal and the last Ho:YLF crystal was oriented with its c-axis vertical, so that the p-polarized pump laser through the first two crystals and the s-polarized pump laser through the last crystal had a low loss due to the polarization absorption characteristics of the Ho:YLF crystal. Under a pump power of 33.9 W, the absorption efficiencies of p-polarized and s-polarized pump light through the first Ho:YLF crystal were 53% and 27%, respectively. To further increase the absorption efficiency of p-polarized light, the second Ho:YLF crystal was used. The corresponding absorption efficiencies of p-polarized and s-polarized pump light were 70.1% and 41.7%, respectively. In order to increase the absorption efficiency of s-polarized light, the last Ho:YLF crystal was employed. The corresponding absorption efficiencies of p-polarized and s-polarized pump light were 75.7% and 72.8%, respectively, resulting in a total absorption efficiency of 74.3%.

The temporal pulse shapes of the Ho:YLF amplifier were measured with a high-speed InGaAs detector (1 GHz bandwidth) connected to a digital oscilloscope (1 GHz bandwidth) at a PRF of 100 Hz; and the fast Fourier transform (FFT) curves of the temporal shapes were calculated, as shown in Figure 6. The emergence of mode beating under free-running operation as shown in Figure 6(a) was due to the multi-longitudinal-mode operation of the ring laser. The free spectrum range (FSR) of 116 MHz was obtained from the FFT curve, which was in correspondence to the 2.53 m ring laser. Compared with Figure 6(a), the temporal pulse shape of the Ho:YLF amplifier at 24.2 mJ in Figure 6(b) was relatively smooth, and there were no mode beating spikes from the FFT curve due to the single-frequency operation of the ring laser.

We compared a single-pass Ho:YLF amplifier under free-running and single-frequency operation. The output energy of 24.2 mJ with a pulse width of 250 ns at a PRF of 100 Hz was obtained. The output spectrum of the single-frequency pulsed Ho:YLF amplifier was around the P12 CO₂ absorption line. The energy stability during 30 min was improved from 3.8% to 1% after the single-frequency pulsed Ho:YLF laser was amplified. The pulse build-up time of the Ho:YLF amplifier under single-frequency operation was 0.37 μs shorter than that under free-running operation. Compared with free-running operation, the absence of mode beating of the temporal pulse shape was due to the single-frequency operation of the Ho:YLF amplifier. The line width of the single-frequency pulsed Ho:YLF amplifier measured by a heterodyne technique was 2.81 MHz.