A diode-side-pumped actively Q-switched intracavity frequency-doubled Nd:YAG/BaWO4/KTP Raman laser is studied experimentally and theoretically. Rate equations are used to analyze the Q-switched yellow laser by considering the transversal distributions of the intracavity photon density and the inversion population density. An 8.3 W 590 nm laser is obtained with a 125.8 W 808 nm pump power and a 15 kHz pulse repetition frequency. The corresponding optical conversion efficiency from diode laser to yellow laser is 6.57%, much higher than that of the former reported side-pumped yellow laser. The output powers with respect to the incident pump power are in agreement with the theoretical results on the whole.
©2010 Optical Society of America
Stimulated Raman scattering (SRS) is an efficient method to produce new laser lines [1–4]. In recent years, with emergence of excellent Raman crystals, all-solid-state Raman laser has attracted much attention for the advantages of high conversion efficiency, compactness, good mechanical and thermal properties. The yellow-orange lasers in the spectral region from 550 nm to 650 nm have widely applications in metrology, remote sensing, medicine, and so on. But they are hardly obtained by use of intracavity frequency doubling Nd-doped lasers. Frequency-doubled the first Stokes Raman laser is an efficient way to obtain the yellow-orange lasers [5–8].
As a promising Raman medium, BaWO4 crystal has many advantages of high Raman gain for picosecond and nanosecond pulse [9–16], good mechanical and optical properties [17,18] and large physical size. Pavel Cerny et al. did much work to study the characteristics of BaWO4 Raman crystal with picosecond and nanosecond pump source [2,9–12]. In 2005, Y. F. Chen et al. investigated the characteristics of an actively Q-switched Nd:YAG/BaWO4 intracavity Raman laser, generating a 1.56 W 1181 nm Raman laser at an incident pump power of 9.2 W . In 2007, S. T. Li et al. reported an diode-side-pumped intracavity frequency-doubled Nd:YAG/BaWO4 Raman laser, generating a 3.14 W 590 nm laser . In 2009, a 3.36 W CW 1180 nm first order Stokes laser was obtained with a Nd:YVO4/BaWO4 Raman laser . Recently, A. J. Lee et al. reported the generation of 2.9 W CW yellow laser emission from an intracavity Nd:GdVO4/BaWO4/LBO Raman laser .
Rate equations are an important way to describe laser operation theoretically. They are widely used to analyze intracavity frequency-doubled lasers [19,20] and Raman lasers [21–23]. However, there is little report about the theoretical model of the actively Q-switched intracavity frequency-doubled Raman laser , and there is no comparison between the theoretical results and experimental results to our knowledge.
It is possible for end pumped lasers to achieve high conversion efficiency, but high output power is usually impeded by the heat generated in the laser medium. Compared with end pumped yellow laser, the side-pumped laser has lower conversion efficiency, but much higher output power can be obtained. In this paper, we analyze the intracavity frequency-doubled Raman laser based on the rate equations considering the transversal distributions of the intracavity photon density and the inversion population density. By numerically solving these rate equations, in which the thermal effect of the laser medium and Raman medium is taken into account, the theoretical output power is calculated. In the experiment, the characteristics of an efficient diode-side-pumped actively Q-switched intracavity KTP frequency doubled Nd:YAG/BaWO4 Raman laser were studied. A three-mirror linear coupled cavity configuration is used to collect the forward and backward propagation yellow beams. At an incident pump power of 125.8 W and a pulse repetition frequency (PRF) of 15 kHz, an average output power of 8.3 W 590 nm laser was obtained. The corresponding optical-to-optical conversion efficiency from diode laser to yellow laser was 6.57%, much higher than the former reported 3.2% conversion efficiency with side-pumped yellow laser . The average output powers with respect to the incident pump power were in agreement with the experimental results on the whole.
In this paper, we assume the intracavity resonating lasers are in TEM00 mode. The intracavity fundamental photon densities in the laser medium φLL(r,t) and in the Raman medium φLR(r,t), the Raman photon densities in the Raman medium φRR(r,t) and in the frequency-doubling medium φRK(r,t) can be expressed as:
When the Raman laser passes through the frequency-doubling crystal with low conversion efficiency, the output power of the frequency-doubling laser can be written as :
The loss rate of the Raman photon density in the Raman medium caused by second harmonic generation (SHG) satisfies the following relation:Eqs. (4) and (6), the following equation can be obtained:
For intracavity frequency-doubled Raman laser, the yellow light was generated in two directions inside the frequency-doubled crystal. In order to collect the forward and backward yellow beams, a three mirror coupled cavity was employed in the experiment (This will be described in detail later in the third part). When the phase difference between the forward and backward yellow beams is θ, the magnitude parameter of the return pass caused by the coupled cavity is M (0<M<1), the parameter ρ can be expressed :Eqs. (10a) and (10b), we can obtain φLL(0,t) and φRR(0,t). Substituting them into Eqs. (13) and (14), we can calculate the average output power and the pulse energy.
3. Experimental results and discussions
The cavity configuration is shown in Fig. 1 . M1 was a concave mirror with a curvature radius of 3000 mm. It was coated for high reflection (HR) at the range of 1000-1200 nm. M2 was a plane mirror used as the output coupler, coated for HR at 1064 nm (R>99.8%) and 1180 nm (R>99.6%) and partial transmission at 590 nm (T = 90.9%). M3 was coated for HR at 590 nm and HT at 1064 and 1180 nm (T>99.5%). The overall cavity length was 226 mm. A dichroic mirror M4 coated for HT at 590 nm (T>98.5%) and HR at 1064 and 1180 nm (R>99.0%) was used to separate the yellow laser from the residual fundamental laser at 1064 nm and the first Stokes laser at 1180 nm.
A commercial diode-side-pumped Nd:YAG module (Beijing GK Laser Technology Co. Ltd., GKPM-50B) was used as the fundamental laser source. The doping concentration of Nd:YAG crystal was 1-at.%, and the length was 60 mm. The Raman active medium is an a-cut BaWO4 crystal with a length of 46.6 mm. The frequency doubling medium is a KTP crystal with dimensions of 3 × 3 × 6 mm3, cut at an angle of θ = 68.7°, ϕ = 0° for type II phase matching condition. Both sides of the Nd:YAG, BaWO4 and KTP crystals are anti-reflection (AR) coated at 1064 and 1180 nm (R<0.2%). The entrance face of the Nd:YAG is also AR coated at 808 nm. A 35-mm-long acousto-optic (AO) Q-switch (Gooch & Housego Company) is placed between the Nd:YAG crystal and the BaWO4 crystal. Both faces were HT coated at 1064 nm and 1180 nm (R<0.2%). All the crystals are wrapped with indium foil and mounted in water-cooled copper blocks. The water temperature is maintained at 20°C.
The average output power is measured by a power meter (Molectron PM10) connected to Molectron EPM2000. The pulse temporal behavior is recorded by a Tektronix digital phosphor oscilloscope (TDS 5052B, 5 G Samples/s, 500 MHz bandwidth) with a fast p-i-n photodiode.
Figure 2 shows the average output power of the 590 nm laser with respect to the incident 808 nm pump power at the PRFs of 10, 15 and 20 kHz, respectively. The solid symbols are the experimental results, the lines are theoretical results. It can be seen that the average output power strongly depended on the repetition frequency. For Q-switched laser, more population inversions are accumulated at lower repetition frequency. Then high pulse energy and peak power can be obtained. The maximum pulse energy at 10, 15 and 20 kHz is 0.67, 0.55 and 0.37 mJ respectively. The SRS belongs to the third order nonlinear effect, high peak power and pulse energy usually leads to high conversion efficiency. So with the same pump power, the 590 nm laser power at 10 kHz was higher than the powers at 15 and 20 kHz. And the laser at 10 kHz also had higher slope efficiency. In the experiment, the slope efficiency was 8.87%, 8.08% and 7.33% for the 10 kHz, 15 kHz and 20 kHz respectively. However high pulse energy and peak power may damage the coatings of the crystals. In order to avoid the damage of the crystal coatings, the pump powers at 10, 15 and 20 kHz were within 93.5, 126 and 137 W respectively. The maximum output power of 8.3 W was obtained at 125.8 W pump power and 15 kHz PRF, the corresponding optical conversion efficiency from diode laser to yellow laser is 6.57%, much higher than that of the former reported side-pumped yellow laser .
Table 1 shows the parameters for the calculation. Only the 1064 nm fundamental laser, 1180 nm first Stokes laser and 590 nm yellow laser were obtained in the experiment, so we ignored the high order Stokes and anti-Stokes laser generation during the theoretical calculation. The thermal effect in the laser medium and Raman medium became more and more serious with increasing pump power, it impacted the stabilization of the resonator and impeded the increasing of the 590 nm laser power. So it must be taken into account. The thermal focal length of the crystal could be calculated with the theoretical formula :1]. With the pump power increasing from 45 W to 131 W, the thermal focal length in the BaWO4 crystal changed from −2205 mm to −245 mm at 15 kHz. Taking the thermal effect in the Nd:YAG and BaWO4 crystals into account, we could obtain the Raman photon densities in the Raman medium φRR(r,t) by numerically solving Eqs. (10a) and (10b). Substituting φRR(r,t) into (13-14), the output power and pulse energy could be calculated. As shown in Fig. 2, the theoretical results are in agreement with the experimental results on the whole. Some discrepancies can also be observed, the reasons may be as follows: First, nonlinear conversion strongly depends on the laser power intensity. The thermal effect can impact the beam cross section and power intensity. Maybe the thermal focal length was not accurate. Second, we approximate that the 1064 nm, 1180nm and 590 nm lasers are in TEM00 mode. In fact, there are some other high-order modes in the experiment. Third, some parameters used in the calculation may be not accurate.
Figure 3 shows the pulse width of the 590 nm laser with respect to the incident 808 nm pump power at the PRFs of 10, 15 and 20 kHz, respectively. Figure 4 shows the pulses temporal behavior of the 1180 nm and 590 nm lasers with a 125.8 W pump power and a 15 kHz PRF. The pulse duration of the 590 nm laser was 20 ns, the corresponding peak power was 27.6 kW.
In summary, the rate equations have been used to analyze the Q-switched intracavity frequency doubled Raman laser. The characteristics of a diode-side-pumped actively Q-switched intracavity KTP frequency doubled Nd:YAG/BaWO4 Raman laser have been investigated. At an incident pump power of 125.8 W and a pulse repetition frequency of 15 kHz, an 8.3 W yellow laser was obtained. The corresponding optical conversion efficiency from diode laser to yellow laser was 6.57%. The average output powers with respect to the incident pump power are in agreement with the experimental results on the whole.
This research is supported by the National Natural Science Foundation of China (60778012, 50721002) and the Grant for State Key Program of China (2004CB619002).
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