1. Introduction

Carbon dioxide (CO₂) and methane (CH₄) have been recognized as the most important anthropogenic greenhouse gases by the International Panel of Climate Change (IPCC) [1]. Among those gases, CH₄ has an estimated global warming potential per molecule 25 times greater than CO₂ over a 100 years horizon and 72 times greater over a 20 years horizon [2]. Despite of its comparatively low atmospheric abundance, it is the second most significant anthropogenic greenhouse gas contributing to the global climate warming [2]. Precisely quantifying the global sources and sinks of CH₄ is mandatory for predicting climate change, and towards developing strategies against greenhouse gas abatement [2–5]. Therefore, the accurate measurement of the global atmospheric CH₄ concentration becomes a major mission of current climate prediction. Space-based integrated path differential absorption (IDPA) lidar has potential to fulfill this task, which requires a laser source emitting at around 1645 nm wavelength that coincides with appropriate absorption-line of the CH₄ having high pulse energy and stabilized operation as well as narrow-linewidth spectral properties [2,6,7]. Normally, those lasers can be indirectly generated by optical parametric oscillator/amplifier systems [2] or stimulated Raman scattering effects [8]. However, methods using such lasers show the disadvantages of complexity and sensitivity to environmental perturbations, so developing pulsed and frequency stabilized narrow-linewidth laser sources emitting at 1645 nm with simple-compact structure are highly encouraged. An attractively alternative method is through resonantly pumped Erbium-doped YAG crystals [9,10] or ceramics [11] to generate that particular wavelength directly. In this approach, resonant pumping facilitates lasing with small quantum defect, simplifies thermal management, and enables power scaling. Combined with available 1470 or 1532 nm laser diodes, the whole laser system becomes more compact and efficient.

Appropriate Q-switcher and wavelength selector are important components to obtain high energy and narrow linewidth Q-switched Er:YAG laser. Passive Q-switcher, such as Transition Metal (Cr) doped II-VI semiconductors (ZnSe and ZnS) [12–14], graphene [15,16], topological insulators [17], have been used to modulate the Er:YAG laser. However, the stability and power scaling ability are limited. Another method to modulate the Er:YAG laser is to incorporate an acoustic-optic (AO) or electro-optic (EO) modulator into the laser resonator. In recent years, actively Q-switched Er:YAG [4,5,10,18] or Er:YLuAG [19] laser had been demonstrated at 1645 nm. Compared with the passive Q-switching method, the laser performance can be enhanced greatly, which facilitates its practical applications. Another issue we want to mention is the laser linewidth. The narrow linewidth operation can be obtained by intra-cavity wavelength selectors, such as etalons, dispersive prisms and gratings [20–22]. Among these wavelength selectors, the etalon and dispersive prism incorporated into the cavity show the disadvantages of being lossy, bulky, and complex. In addition, the selected spectrum is usually not sufficiently narrow. The volume Bragg grating (VBG), fabricated from holographically exposed photo-thermal-refractive (PTR) glass, possesses excellent properties, such as optical, thermal, and mechanical stabilities, making it a perfect wavelength selector to obtain a wavelength-locked laser with narrow linewidth operation [23–29]. Recently, Bragg grating or VBG has been used as a wavelength selector in Er:YAG laser by Moskalev et al. [21] and Zhang et al. [22]. Moreover, the VBG can act as a wavelength selector and a pump input mirror simultaneously to obtain continuous-wave wavelength-locked and narrow linewidth laser output [30]. However, the Q-switched narrow-linewidth Er:YAG laser utilizing a VBG as a wavelength selector and a pump input mirror simultaneously, have not yet been reported.

In this paper, we report on a wavelength-locked Q-switched Er:YAG laser resonantly pumped by a laser diode. By using VBG as a wavelength selector and a pump input mirror simultaneously, the Er:YAG laser is very compact and stable. When the incident pump power is 21.6 W, it emitted 185 ns pulses with 1.36 mJ pulse energy and near diffraction-limited beam quality (M² = 1.08) at 1 kHz pulse repetition frequency. The central wavelength of the Er:YAG laser was stably locked at 1645.3 nm with the spectral 3-dB linewidth of less than 0.08 nm, which coincides to one of the CH₄ strong absorption-lines.

2. Experimental setup

The schematic diagram of the Er:YAG laser in our experiments is shown in Fig. 1. The pump source is a 35 W fiber-coupled laser diode operated at 1532 nm with the fiber core diameter of 200 μm and numerical aperture of 0.22. The pump beam was collimated (by Lens 1, f = 40 mm) and focused (by Lens2, f = 125 mm) with a spot radius of about 320 μm on a 40 mm-long 0.25 at. % doped Er:YAG crystal rod. Both end facets of the crystal rod were antireflection-coated for the 1400- to 1700-nm wavelength regime. The rod was wrapped by indium foil and mounted in a copper heat sink to maintain the temperature around 15 °C using a water cooler. A compact plane-concave resonator (~270 mm) was employed for the Q-switched Er:YAG laser. The VBG (OptiGrate Corp.), 15 mm thick with a clear aperture of 8 × 6 mm², has high diffraction efficiency (>99.5%) at the wavelength of 1645 nm (at 22°C in air) with a spectral width (FWHM) of 0.37 nm, was used as a wavelength selector and a pump input mirror simultaneously. Both end facets of the VBG were broadband antireflection-coated to reduce the loss of incident pump light and laser cavity. To ensure efficient heat removal, the VBG was wrapped by indium foil and mounted in a copper heat sink. The output coupler with a curvature radius of 500 mm had different transmittances of 13 and 21% at the lasing wavelength of 1645 nm and a high reflectivity (>99.8%) at the pump wavelength of 1532 nm. The RTP Pockels cell (quarter-wave voltage: 0.85 kV@1645 nm, contrast ratio: >1000:1@600 nm) with a quarter-wave plate and a Brewster-angled undoped YAG polarizer (reflection-coated for the s-polarized lights) constituted the voltage ON type EO Q-switcher. According to the ABCD matrix theory, the TEM₀₀ mode radius at the middle of the Er:YAG and RTP Pockels cell was calculated to be about 360 μm and 450 μm, respectively. An optical spectrum analyzer (Ando, AQ-6317B) with a resolution of 0.05 nm was used to measured the output spectra.

 

Fig. 1 Experimental setup of the Q-switched narrow-linewidth Er:YAG laser. L1, Lens 1. L2, Lens 2. P, polarizer. QWP, quarter-wave plate. Q-S, Q-switcher.

Download Full Size | PDF

3. Experimental results and discussions

For comparison, lasing characteristics of the resonantly diode-pumped Q-switched Er:YAG laser in the free-running operation mode (i.e., without VBG wavelength selectors to lock the operating wavelength) by using a plane dichroic mirror (with high transmittance (>94%) at the pump wavelength of 1532 nm and high reflectivity (>99.8%) at the lasing wavelength of 1645 nm) as the pump input mirror were also investigated.

The output spectra of the Q-switched Er:YAG laser under the free-running and wavelength-locked operation regimes at the incident pump power of 21.6 W with different transmittances of 13 and 21% of the output coupler are depicted in Fig. 2. In the free-running operation mode, when a 13% transmission of output coupler was used, the central wavelength of the Er:YAG laser was determined to be 1645.1 nm with a spectral 3-dB linewidth of about 0.43 nm. When the output coupler was replaced with that with a transmittance of 21%, the central wavelength of the Er:YAG laser was shifted to 1645.2 nm, corresponding to a spectral 3-dB linewidth of 0.32 nm. Then, the VBG acting simultaneously as a wavelength selector and a pump input mirror was used to substitute for the plane dichroic mirror, the Er:YAG laser operating wavelength was locked at 1645.3 nm which is within the strong CH₄ absorption-line with stable and sharp spectrum, with either 13% or 21% transmission of the output coupler. In the wavelength-locked operation mode, the laser using 21% transmission of the output coupler displayed a higher spectrum narrowing performance, with a measured 3-dB linewidth of 0.08 nm (limited by the measurement resolution of the optical spectrum analyzer and the actual spectrum linewidth may be less than 0.08 nm), which is narrower than the reported results obtained by Wang et al. (0.16 nm) [20] and Wang et al. (0.1 nm) [4] by using a single etalon as an intra-cavity wavelength selector in resonantly diode-pumped Er:YAG lasers. Although using two or more etalon can obtain narrower linewidth or even single longitudinal mode output, the scheme will result in a complicated laser system. We would like to mention that, in the experiment, the spectrum showed a large jitter in free-running operation mode, while in the wavelength-locked operation mode, almost no wavelength shift was observed, which demonstrates the excellent stabilizing property of the VBG. Furthermore, the operating wavelength can be finely tuned by adjusting the temperature of VBG [31,32].

 

Fig. 2 Output spectrum characteristics of the resonantly pumped Q-switched Er:YAG laser under the free-running and wavelength-locked operation regimes with different transmittances of 13 and 21% of the output coupler at the incident pump power of 21.6 W. OC, output coupler.

Download Full Size | PDF

In view of the narrowest spectrum was obtained in the case of using the output coupler with 21% transmission, we used this output coupler to obtain the Q-switched Er:YAG laser and studied its output pulse characteristics. The Q-switched pulse width and energy of that Er:YAG laser with respect to incident pump power at 1 kHz pulse repetition frequency are shown in Fig. 3. As the incident pump power varied from 16.9 W to 21.6 W, the pulse width (FWHM) decreased from 418 ns to 185 ns, while the pulse energy increased from 0.05 mJ to 1.36 mJ, respectively. The typical pulse trace is shown in the inset of Fig. 3. Obviously, it is more stable than the result reported in [15].

 

Fig. 3 Q-switched pulse width and pulse energy as function of incident pump power. Inset: Typical pulse trace at the repetition rate of 1 kHz.

Download Full Size | PDF

The output beam quality of the Q-switched wavelength-locked Er:YAG laser at pulse energy of 1.36 mJ was measured by the knife-edge scanning method. M² parameter was fitted to be 1.08, as illustrated in Fig. 4.

 

Fig. 4 Measured beam radii of the Q-switched wavelength-locked Er:YAG laser at different positions along propagation axis.

Download Full Size | PDF

4. Conclusions

In conclusion, we have demonstrated a stable wavelength-locked Q-switched narrow-linewidth Er:YAG laser with compact cavity structure by using a VBG as a wavelength selector and a pump input mirror simultaneously. When the incident pump power reached 21.6 W, it emitted 185 ns pulses with 1.36 mJ pulse energy and near diffraction-limited beam quality (M² = 1.08) at 1 kHz pulse repetition frequency. The central wavelength of the Er:YAG laser was stably locked at 1645.3 nm with a spectral 3-dB linewidth of less than 0.08 nm, which coincides to the strong absorption-line of CH₄. This spectral linewidth is narrower than the reported results obtained by using a single etalon as an intra-cavity wavelength selector. The work may provide an inroad for developing more stable and miniaturized IDPA lidar transmitter.