1. Introduction

High power and efficient yellow lasers have attracted increasing attention for important applications in the fields of medical treatment, metrology, remote sensing and visual displays [1]. Based on solid-state laser technology, there are mainly three ways being adopted for realizing yellow lasers. First, Sum-frequency generation of two infrared Nd-based laser lines at 1.06 μm and 1.3 μm has been widely studied [2,3]. By this way, simultaneous dual-wavelength oscillation or both lasers emitting 1.06 and 1.3 μm wavelengths are needed and the systems are complex in order to improve the conversion efficiency. Second, Low output power yellow lasers also have been realized by frequency doubling of weaker transitions in the manifold ⁴F_3/2-⁴I_11/2 in Nd³⁺ doped crystals, such as 1112 nm and 1123 nm lasers for Nd:YAG [4,5]. In recent years, Second harmonic generation (SHG) of Nd-doped solid Raman laser is proved to be the most attractive way to produce an efficient and reliable yellow light source for the applications mentioned above [6]. In 2007, Li et al. reported that an average power of 3.14 W at 590 nm was yielded by frequency doubling of a diode-side-pumped Nd:YAG/BaWO₄ Raman laser, corresponding to an overall conversion efficiency of 3.1% [7]. In 2008, the highest continuous-wave yellow light with output power of 2.51 W at 586.5 nm was generated by intracavity frequency-doubling of a Nd:GdVO₄ self-Raman laser pumped by 880 nm diode laser [8]. More recently, average power of 1.67 W at 559 nm has been generated from sum-frequency mixing of fundamental and first-Stokes wavelengths in Nd:YVO₄ self-Raman laser in [9] and 2.93 W at 590 nm corresponding conversion efficiency of 18.1% has been obtained by Q-switched Nd:YAG/SrWO₄/KTP laser in [10]. Our group has also reported a letter about intracavity doubling of a Q-switched Raman laser based on a YVO₄/Nd:YVO₄ composite crystal [11]. Yellow light with 5.7 W average power was generated at a pump power of 23.5 W, corresponding to an overall diode-yellow conversion efficiency of 24.2% and a slope efficiency of 32%. All the results show that intracavity frequency doubling of self-Raman lasers based on Nd:YVO₄ or Nd:GdVO₄ are efficient ways to obtain yellow light.

Self-Raman laser have the advantage of eliminating the need of a separate Raman crystal and achieving a compact resonator design. On the other hand, the Raman interaction length is limited and thermal loading of the combined laser/Raman process is exacerbated at high average Raman laser powers. It has been proved that the composite of doped and pure crystals can reduce the thermal effects efficiently [12]. Therefore, if a self-Raman crystal and its corresponding pure crystal are compounded this will not only improve the thermal effects, but also increase Raman interaction length for an efficient self-Raman laser operation. In [9], a double-end diffusion-bonded Nd:YVO₄ crystal has been used for sum-frequency mixing in a Nd:YVO₄ self-Raman laser to produce light at 559 nm. Since the Raman gain is increasing with the Raman interaction length, the Raman conversion efficiency will be improved by compounding the self-Raman crystal with a longer pure Raman crystal. In this work, a total length up to 30 mm double-end diffusion-bonded Nd:YVO₄ crystal was adopted for reducing the thermal effects and increasing the interaction length for the stimulated Raman scattering. The focus position of the incident pump light and both the repetition rate and the duty cycle (duty cycle of time the drive outputs no radio frequency energy) of the Q-switch have been optimized. By frequency doubling based on a 15-mm-length, non-critical phase matching (NCPM) (θ = 90°, ϕ = 0°) cut, LBO crystal, the maximum average power of 7.93 W at 588 nm was achieved at a repetition rate of 110 kHz and duty cycle of 5%, while the incident pump power is 26.5 W. The overall diode-yellow conversion efficiency is up to 30% and the slope efficiency is about 43%.

2. Experiments design

The experimental setup of the double-end diffusion-bonded Nd:YVO₄ crystal self-Raman laser with intracavity SHG producing 588 nm light is shown in Fig. 1 . One of the SRS-active vibration modes in YVO₄ crystal is 890 cm⁻¹ [13], the wavelength of the first-Stokes emission is calculated to be around 1176 nm with a fundamental wavelength of 1064 nm. The a-cut 3mm × 3mm × 30mm double-end diffusion-bonded Nd:YVO₄ crystal (fabricated by CASTECH Inc) working as self-Raman crystal (0.3-at.%) is bounded by a 2-mm-long pure YVO₄ end at the pumped facet, and one 18-mm-long pure YVO₄ at the other facet. It was wrapped with indium foil and mounted in a thermoelectric cooled copper block and its surface temperature was kept at about 20 °C during the experiments. The pump source was an 808 nm fibre-coupled laser diode array with a core diameter of 200 μm, a numerical aperture of 0.22 and a maximum power of 27 W. Two plane-convex lenses with the respective focal length of 30 and 60 mm were used to re-image the pump beam with a focus spot diameter of 400 μm into the a-cut Nd:YVO₄ laser crystal. Both coupling lenses were anti-reflection coated at 808 nm and the transmission through the pair lenses was about 96%.

 

Fig. 1 Schematic of the diode-end-pumped intracavity frequency-doubling of acousto-optic Q-switched composite Nd:YVO₄ crystal self- Raman laser

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A film with high transmission at the pump wavelength (HT, T>95%) and high-reflection at both the fundamental and first-Stokes wavelength (HR, R>99.9%) was coated on the incident facet for the pump light (surface of 2-mm-long pure YVO₄) and acted as one of the cavity mirrors. In order to reduce diffraction losses and misalignment sensitivity of cavity, a concave mirror with 300 mm radius of curvature was adopted, which was also HR(R>99.9%) coated at both the fundamental and the first-Stokes wavelength, but HT coated at 588 nm wavelength. Therefore, the fundamental and the Raman laser resonator share the same cavity which leads to a low-loss at the fundamental and the first-Stokes wavelength. A 3mm × 3mm × 15mm LBO crystal with NCPM (θ = 90°, ϕ = 0°) cut was adopted for frequency doubling of the Raman radiation to produce the 588nm yellow light and the temperature was controlled at 41 °C. NCPM permits efficient operation for its maximum effective nonlinear coefficient and acceptance angle, and essentially eliminates the beam walk-off. Both end faces of the LBO crystal were AR coated at 1064, 1176, and 588 nm. In order to prevent absorption of backward-propagating yellow emission in the composite crystal and maximize the yellow output collected through the output coupler, the right facet of the Nd:YVO₄ crystal was HR coated at 588 nm and AR coated at both the fundamental and the first-Stokes wavelength. Between the composite Nd:YVO₄ crystal and LBO crystal, a 30-mm-long acousto-optic Q-switcher (AOS, from Gooch & Housego Co.) was inset to realize Q-switch mode operation, which is antireflection (AR) coated at both 1064 and 1176 nm on both faces and driven at a 40 MHz center frequency with 20 W of radio-frequency power.

3. Experimental results and discussion

First, a resonator with a total length of about 112 mm was designed for yellow light generation. In the experiment, we discovered that the resonator stability was very sensitive to pulse repetition rates and the focus position of pump light in the crystal. Therefore, the focus position of pump light in the crystal has been optimized for the highest output power. The pulse repetition rates of 30, 90, and 110 kHz have been studied, and duty cycle of 10% and 5% for Q-switch operation have been compared. Figure 2 shows average output power at 588 nm versus incident pump power for different repetition rates and duty cycles for Q-switch operation. The Raman threshold reduces with the repetition rate, and the Raman thresholds are 1.5, 2.8, and 4.3 W at the repetition rates of 30, 90 and 110 kHz, respectively. The output power increases with the pump power firstly and reaches a maximum for a given incident pump power and then rapidly decreases at higher pump powers, which is caused by resonator instabilities. Thermal loading of the end-pumped Q-switched Nd-doped laser increases with decreasing repetition rates [14], and the influence of resonator instability by repetition rate is magnified in self-Raman laser. Therefore, its more significant thermal effects lead to a much lower critical pump power of becoming instability and a lower maximum output power. The efficient average output powers at 588 nm were 2.32, 4.95, and 7.32 W at repetition rates of 30, 90, and 110 kHz with incident pump power of 13.2, 19.4, and 24.6 W, respectively. The diode-yellow conversion efficiency is increased from 19.1% (for 11.2 W pump power) at 30 kHz to 29.7% (for 24.6 W pump power) at 110 kHz

 

Fig. 2 Average output power at 588 nm versus incident pump power at repetition rates (f) of 30, 60 and 90 kHz and duty cycle (δ) of 5% and 10%.

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The above output power of 588 nm has been optimized at duty cycle of 5%. Figure 2 also shows that output characteristics of 588 nm laser also are susceptible to the change of duty cycle. The lower Raman threshold of 2.2 W with the Q-switch duty cycle being set at 10% was obtained at repetition rate of 110 kHz, but the output power and conversion efficiency are lower than that of 5% duty cycle. The maximum output power of 6.84 W with the pump power of 25.6 W corresponds to a conversion efficiency of 26.7%. It is worth to notice that the critical pump power for the appearance of instabilities increases with the Q-switch duty cycle.

Since the output power of 588nm light was still hindered by the resonator stability, we tried to shorten the total length of resonator. The laser still operated in the stable region with the pump power close to the maximum while the total length of resonator was shortened to about 108mm. The litter shortening of cavity length also increased the critical pump power for the appearance of instabilities. At a repetition rate of 110 kHz and duty cycle of 5%, the average power of 588 nm light is up to 7.93 W while the incident pump power is 26.5 W, corresponding to the overall diode-yellow conversion efficiency of 30% and the slope efficiency of 43%. The temporal pulse profile of 588nm light was recorded by a PIN photodiode and displayed by a Model TDS3052B (500 MHz) dual-line oscilloscope. At a repetition rate of 110 kHz, the pulse width is about 18 ns as shown in Fig. 4 and the pulse energy is up to 72 μJ with the incident pump power of 26.5 W. The yellow light spot recorded by CCD camera is displayed in Fig. 5 . The power stability of the yellow light was investigated by a Model LPM-100 power meter and the fluctuation was about 3.5% at the maximum output power of 7 W in half an hour. This fluctuation may attribute to the variation of temperature in double-end diffusion-bonded Nd:YVO₄ crystal and LBO crystal temperature. It can be seen from Fig. 2 and Fig. 3 , the output power dropped at pump power near the Raman thresholds which didn’t appear in our previous work when the YVO₄/Nd:YVO₄ composite crystal was adopted [11]. The adoption of double-end diffusion-bonded Nd:YVO₄ crystal has resulted in the output power and conversion efficiency remarkable increasing comparing with the our previous results using 20 mm length YVO₄/Nd:YVO₄ composite crystal. Figure 6 displayed the measured spectrum of yellow light, only the wavelength of 588 nm appears in the measured wavelength range from 500 nm to 650 nm.

 

Fig. 4 The temporal pulse profile of yellow light at the output power of 7.9 W

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Fig. 5 The yellow light spot

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Fig. 3 Average output power at 588 nm versus incident pump power with the total length of resonator shortened to 108 mm

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Fig. 6 The measured spectrum of yellow light

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When the output coupler was replaced by the mirror with the transmission optimized for 1064 nm output, the output power increased linearly with the pump power increased to 26.5W and the resonator didn’t become instable while the pulse repetition rates changed. With an input pump power of 26.5 W, average output powers of 9.8 to 13.7 W at 1064 nm were obtained at pulse repetition rates between 30 and 110 kHz. The experiment results show that the actively Q-switch self-Raman laser owns more serious and complicated thermal effects than normal actively Q-switched Nd-doped fundamental lasers for its additional thermal lens effect resulting from SRS progress. Since the thermal loading of SRS process is proportional to the average power density of the first Stokes power laser in the crystal, thermal lensing of the self-Raman laser is exacerbated at high average Raman power [15]. Therefore, the Raman laser performance is always hindered by resonator instability and the Raman gain coefficient decreases with the increase of the temperature in the crystal. The resonator stability caused by thermal lens effect in our experiment was sensitive to the focus position of pump light in the crystal, pulse repetition rates and duty cycle of Q-switch. The advanced studies on thermal effects need to be further investigated.

4. Conclusion

In summary, we have demonstrated a high power and efficient 588 nm light generated by intracavity frequency doubling of an acousto-optic Q-switched self-frequency Raman laser. A 30-mm-length double-end diffusion-bonded Nd:YVO₄ crystal was utilized for efficient self-Raman laser operation by reducing the thermal effects and increasing the interaction length for the stimulated Raman scattering. A 15-mm-length LBO with non-critical phase matching (θ = 90°, ϕ = 0°) cut was adopted for efficient second-harmonic generation. The focus position of incident pump light and both the repetition rate and duty cycle of the Q-switch operation have been optimized. At a repetition rate of 110 kHz and duty cycle of 5%, the average power of 588 nm light is up to 7.93 W while the incident pump power is 26.5 W, corresponding to a overall diode-yellow conversion efficiency of 30% and the slope efficiency of 43%. The adoption of double-end diffusion-bonded Nd:YVO₄ crystal results in an remarkably increased output power and conversion efficiency compared to the our previous results using 20 mm length YVO₄/Nd:YVO₄ composite crystal. At a repetition rate of 110 kHz, the pulse energy is up to 72 μJ with the incident pump power of 26.5 W.