† Corresponding author. E-mail:
Project supported by the National Key R&D Program of China (Grant No. 2016YFA0301401) and the Fund for Shanxi “331 Project” Key Subjects Construction, China (Grant No. 1331KS).
A theoretical model of quasi-three-level laser system is developed, in which both the thermally induced depolarization loss and the effect of energy-transfer upconversion are taken into account. Based on the theoretical investigation of the influences of output transmission and incident pump power on thermally induced depolarization loss, the output performance of 946 nm linearly polarized Nd:YAG laser is experimentally studied. By optimizing the transmission of output coupler, a 946 nm linearly polarized continuous-wave single-transverse-mode laser with an output power of 4.2 W and an optical–optical conversion efficiency of 16.8% is obtained, and the measured beam quality factors are
The realization of 946 nm laser based on Nd:YAG has great potential applications in vapor detection, diffraction absorption radar, and 473 nm blue laser generation by frequency doubling.[1–5] Solid-state blue lasers are attractive due to their numerous applications, such as color display, high-density optical data storage, and underwater communication. In addition, Si photodiode with a quantum efficiency over 99% is available at 946 nm. The use of Nd:YAG laser at 946 nm along with Si photodiode will reduce the detection loss in the generation of squeezed light.[6]
However, it is relatively difficult to obtain an efficient 946 nm Nd:YAG laser, due to the characteristics of the quasi-three-level transition, the need for a high pump deposition density to achieve population inversion, and the extremely small stimulated emission cross section compared with those near 1064 nm and 1320 nm,[7] thereby leading to very strong thermal effects and limiting the power scaling of laser as well.[8] Several methods have been used to reduce the thermal lensing effect and improve the performance of Nd:YAG laser at 946 nm, such as using a composited laser rod,[9] efficient cooling schemes,[10,11] direct pumping and thermally boosted pumping.[12] Another factor that limits the scaling of Nd:YAG laser output is the energy-transfer upconversion (ETU) effect. This ETU effect will reduce the population of the upper laser level, and give rise to an extra heat load in the laser crystal, so the thermal effect of the gain medium becomes more severe and the laser performance is degraded. The influences of the ETU effect on the threshold, output power, and fractional thermal loading of Nd:YAG 946 nm lasers have been investigated theoretically and experimentally.[13–15]
As is well known the realizing of linear polarization fundamental output is very important for efficient frequency doubling. To achieve this, a number of polarizing elements have to be inserted into the laser cavity. Obviously, the presence of thermally induced birefringence will give rise to depolarization loss and lower the conversion efficiency of the laser. Thermally induced birefringence has been analyzed and several methods have been used to compensate for the depolarization loss.[16–18] Hua et al. used a quarter-wave plate to compensate for and reduce the depolarization loss from thermally induced birefringence in an Nd:YAG laser with a polarization output.[16] Wang et al. theoretically described the birefringence characteristics in an Nd:YAG rod by taking the thermal lens effect into account and measured the degree of polarization of the probe beam.[17] Mondal et al. simply used a Glan-Taylor polarizer to reduce the depolarization loss resulting from thermally induced stress birefringence of solid state Nd: YAG laser.[18] However, the study on the influence of the ETU effect on thermally-induced depolarization loss has not been published previously to the best of authors’ knowledge.
In this paper, a diode-end-pumped Nd:YAG laser is built up using a three-mirror folded cavity with a Brewster plate. A theoretical model of a quasi-three-level laser system is developed with consideration of the thermally-induced depolarization loss and the effect of ETU. Based on the theoretical investigation of the influences of output transmission and incident pump power on thermally-induced depolarization loss, the linear polarization output performance of Nd:YAG laser at 946 nm is studied experimentally. By optimizing the transmission of output coupler (OC), a 946 nm linearly polarized continuous-wave (CW) single-transverse-mode (TEM00) solid-state laser is demonstrated.
In a quasi-three-level laser system, an implicit analytical expression for the laser output (Pout) can be derived from the space-dependent rate equation including the effect of ETU[7] and expressed as follows:
The round-trip loss δ inside the cavity can be written as δ = T + δf + δdif + δdep, where δf, δdif, and δdep denote the intrinsic loss, thermally induced diffraction loss,[8] and thermally induced depolarization loss, respectively. The thermal depolarization loss is an additional loss induced by the polarizer existing in the cavity, which can be derived when the laser operates in the fundamental mode and expressed as[19]
If the effect of ETU is not considered, the fractional thermal loading is expressed as ξ0, which under the lasing condition is taken as a quantum defect 1 − λp/λ0, where λp is the pump wavelength. However, if the effect of ETU is taken into account, the fractional thermal loading ξ is no longer a constant, but becomes[15]
For a diode-end-pumped 946 nm Nd:YAG laser with a three-mirror folded cavity configuration and a Brewster plate in the cavity, the thermally induced depolarization loss can be calculated using Eqs. (
Figure
The experimental setup of a diode-end-pumped CW 946 nm Nd:YAG laser is schematically depicted in Fig.
The pump source is a fiber-coupled diode laser with a central wavelength of 808 nm a maximum output power of 30 W and a core diameter of 400 μm. The pump is coupled into the Nd:YAG crystal with a spot radius of 200 μm via two lenses each with a focal length of 30 mm. To overcome the thermal effect caused by the deformation of the Nd:YAG crystal face, a composite Nd:YAG rod is used. The composite Nd:YAG rod has a diameter of 3 mm and a length of 11 mm, which consists of a 5-mm-long 1 at.% Nd-doped part in the middle with two 3-mm-long undoped end caps. Both end faces of the Nd:YAG rod are anti-reflection (AR) coated at 946 nm (R946 nm < 0.25) and high-transmission (HT) coated at 808 nm (T808 nm > 90%). The Nd:YAG rod is tightly wrapped with an indium foil for reliable heat transfer and mounted in a copper block, which is temperature controlled by a temperature controller with an accuracy of ±0.01 °C (Model YG-4S, Yuguang). To reduce the reabsorption loss, the temperature of Nd:YAG rod is controlled at 12 °C.
A three-mirror folded cavity with a plane mirror (M1) and two plano-concave mirrors (M2, M3) is designed and built. The Brewster plate is inserted into the cavity as a polarizer to realize linear polarization output. To suppress the 1.06 μm emission, the input coupler (M1) was coated for HT at both 808 nm and 1.06 μm (T808 nm, 1.06 μm > 95%) and high reflection (HR) at 946 nm (R946 nm > 99.5%). The radii of both plano-concave mirrors are 100 mm. M2 is coated for HR at 946 nm (R946 nm > 99.5%). The output coupler (M3) is coated to be of partial transmission at 946 nm that will be optimized experimentally to mitigate the effect of ETU and improve the laser performance. Considering that the measured thermal focal length of the laser crystal is 35 mm at a pump power of 25 W,[20] the cavity length between M1 and M2, and that between M2 and M3 are designed to be 120 mm and170 mm, respectively, for the following three purposes. The first purpose is that laser beam radius is 140 μm and the optimum pump-mode matching can be realized.[8] The second purpose is that the cavity stability condition of A + D approaches to 0 at a pump power of 25 W, the resonator is thermally insensitive and the influence of the thermal lensing effect can be weakened. The third purpose is that there is a beam waist with a radius of 80 mm between M2 and M3, and efficient frequency doubling can be achieved when a nonlinear crystal is sandwiched between mirrors M2 and M3.
In order to study the influence of thermally induced depolarization loss on the polarization output performance of CW 946 nm Nd:YAG laser, several OCs with transmissions of 0.6%, 3%, 6%, 7.5%, and 10% are used in the experiment. The plots of 946 nm output power versus the transmission of OC at a pump power of 25 W are shown in Fig.
Using an OC with an optimum transmission of 6%, the curves of the output power of 946 nm laser versus incident pump power with and without Brewster plate in the cavity are measured as shown in Fig.
The beam quality factors of the linearly polarized 946 nm laser at an output power of 4.2 W are measured using a laser beam analyzer (Model M2-200, SPIRICON) as shown in Fig.
A diode-end-pumped Nd:YAG laser is built up using a three-mirror folded cavity with a Brewster plate in the cavity. A theoretical model of quasi-three-level laser system with consideration of the thermally induced depolarization loss and the effect of ETU is developed, and the influences of output transmission and incident pump power on thermal depolarization loss are theoretically analyzed. In order to improve the linear polarization output performance of Nd:YAG laser at 946 nm, the dependence of the output power at 946 nm on output transmission is studied experimentally and theoretically. The theoretical predictions are in good agreement with experimental results. Using an OC with an optimum transmission of 6%, a CW TEM00 linear polarization output power of 4.2 W at 946 nm is obtained with an optical–optical conversion efficiency of 16.8%. The measured beam quality factors of 946 nm laser are
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