Diode-pumped intracavity yellow–green Raman laser at 560 nm with sum-frequency-generation☆

Optics & Laser Technology 66 (2015) 122–124

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Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec

Diode-pumped intracavity yellow–green Raman laser at 560 nm with sum-frequency-generation☆ Fufang Su a,n, Xingyu Zhang b, Wei tao Wang b, Zhenhua Cong b, Men Shi a, Peigao Han a, Wendi Wu a, Xiuqin Yang a,c, Lili Ma a a

Shandong Provincial Key Laboratory of Laser Polarization and Information Technology, Laser Institute, Qufu Normal University, Shandong 273165, PR China School of Information Science and Engineering, Shandong University, Jinan 250100, PR China c State Key laboratory of Crystal Material, Shandong University, Jinan, 250100, PR China b

art ic l e i nf o

a b s t r a c t

Article history: Received 20 December 2013 Received in revised form 2 July 2014 Accepted 9 August 2014 Available online 19 September 2014

By using Nd:YVO4 as the gain medium and the Raman medium simultaneously, KTP as the sumfrequency crystal, the actively Q-switched operation of the 560 nm Raman laser by sum-frequency generation was realized. The output characteristics versus the incident pump power and the pulse repetition rates were investigated. At a pulse repetition rate of 40 kHz an average power up to 0.9 W was obtained with the incident pump power of 13.7 W, corresponding to a conversion efficiency of 6.6% with respect to the diode laser input power. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Raman lasers Stimulated Raman scattering Sum-frequency-generation

1. Introduction Stimulated Raman scattering (SRS) effect is an efficient means to generate the new laser spectral lines [1–7]. In recent years, with the methods of the second-Harmonic generation or sum-frequency-generation, the new laser sources of visible and UV laser have been obtained [8–10]. Moreover, the lasers around 560 nm have been widely applied in phycoerythrins, immunodetection and fluorescence microscopy etc., especially pulsed laser around 560 nm has applied in medicine, for example for treatment of freckles and wrinkles. In recent years, the sum-frequency generation (SFG) of solid state Raman laser has been shown to be an efficient way to produce the yellow–green lasers [12–16]. Chang et al. reported a Q-switched dual-wavelength emission at 1176 and 559 nm with intracavity Raman and sum-frequency generation based on Nd:YVO4 self-Raman laser with intracavity SFG in BBO crystal [12]. Chen reported a diode-pumped actively Q-switched Nd:YAG laser with KTP as self-sum frequency mixing and selffrequency doubling [13]. Duan et al. presented 2.41 W output power at 559.6 nm based on Nd:YAG/SrWO4 Raman laser with intracavity SFG in KTP crystal [14]. Pask et al. reported 0.77 W

☆ This research is supported by the National Natural Science Foundation of China (No. 11104161 and No. 11104162 and No. 11104160) and Open Project of State Key laboratory of Crystal Material, (Shandong University, KF1103). n Corresponding author. E-mail address: [email protected] (F. Su).

http://dx.doi.org/10.1016/j.optlastec.2014.08.003 0030-3992/& 2014 Elsevier Ltd. All rights reserved.

output power at 559 nm with an intracavity Nd:YAG/KGW Raman laser, with intracavity SFG in BBO crystal [15]. Lee et al. reported 5.3 W cw laser at 559 nm based on Nd:GdVO4 self-Raman laser with intracavity SFG in LBO crystal [16]. The potassium titanyl phosphate (KTP) crystal has a large nonlinear coefficient, excellent thermal stability and high damage threshold, these facts make KTP very favorable for use as second harmonic generators of near-infrared laser light [9–11], sumfrequency generation [12,13], and optical parametric oscillators [17,18]. In this paper, we report a diode-end-pumped actively Q-switched SFG of Nd:YVO4 self-Raman laser at 560 nm. With a c-cut Nd:YVO4 as self-Raman medium, KTP as SFG crystal, a 0.9 W 560 nm laser was obtained at a 13.7 W incident pump power and a 40 kHz pulse repetition frequency(PRF), corresponding to a conversion efficiency of 6.6% with respect to the diode laser input power.

2. Experimental setup The laser configuration of the diode-pumped actively Q-switched SFG of Nd:YVO4 self-Raman laser at 560 nm is depicted in Fig. 1. The pump source is an 808 nm fiber-coupled laser diode with a core diameter of 600 μm, and a maximum output power of 25 W. The cavity was formed by a linear coupled resonator, the input mirror M1 with high-transmission (HT490%) coating at 808 nm and highreflection (HR) coating at 1000–1200 nm. The output coupler M2 has high reflectivity at 1066–1178 nm and high transmission

F. Su et al. / Optics & Laser Technology 66 (2015) 122–124

Laser diode

123

Focusing optics AO Q-switch

560 nm laser

KTP

Nd:YVO4 M1

M3

M2

1178nm laser

Fig. 1. Experimental arrangement of the diode-pumped intracavity yellow–green Raman laser at 560 nm with sum-frequency-generation.

3. Experimental results and discussion At first, we measured the output spectra of the Q-switched Raman laser without KTP crystal. The spectrum of output of laser was measured by an optical spectrum analyzer (Yokogawa AQ6315A, 350–1750 nm) with a resolution of 0.05 nm. The output fundamental and first Raman center spectra of c-cut Nd:VYO4 were 1066.7 nm and 1178.7 nm. According to [19], the frequency of SFG was determined by νSL ¼ νS þ νL , the theoretical output spectra were the yellow–green laser at 559.9 nm from sum-frequencygeneration of 1066.7 nm and 1178.7 nm. After confirming the output spectra of the Q-switched Raman laser, the laser cavity was designed for SFM of the fundamental laser and the first Stokes laser. The experimental optical spectrum of the laser output is shown in Fig. 2, the output center wavelength of the laser was at 560.02 nm. From Fig. 2 we can see that the average output power at 560.02 nm comes from the sum-frequency-generation of 1066.7 nm and 1178.7 nm. Because the output coupler has high reflectivity at 1066 nm and 1178 nm, the fundamental laser and the first Stokes laser were weak and hard to detect from the spectral of the output laser. After measuring the output spectra of the Q-switched Raman laser at 560 nm, the yellow–green laser operation at 560 nm at different repetition rates had been investigated. Fig. 3 shows the relation between the output average power of 560 nm yellow– green laser and the incident pump power for the pulse repetition frequencies of 30, 40, and 50 kHz, respectively. From the figure we can see that the threshold of 560 nm laser were 5.1, 5.4 and 5.7 W at the repetition rates of 30, 40, and 50 kHz, respectively. At a pulse repetition rate of 40 kHz an average power up to 0.9 W was obtained with the incident pump power of 13.7 W, corresponding

Intensity(1µW/div)

8

6

4

2

0 559

560

561

Wavelength (0.2nm/div) Fig. 2. The spectrum of the diode-pumped intracavity yellow–green Raman laser at 560 nm with sum-frequency-generation.

Average output power of 560 nm Raman Laser (mW)

(HT490%) at 560 nm. The flat intracavity mirror M3 has high transmission coating at 1066–1178 nm (T499.5%) and high reflection coating at 560 nm (R495%). Because of the high-reflection coating at SFG wavelength on mirror M3, compared to the linear two-mirror-cavity, the advantages of the linear three-mirror-cavity were reduced loss of SFG emission transmitting through the front mirror M1 as well as reduced absorption of SFG emission in the Nd:YVO4 which lowers the thermal load in the Nd:YVO4. The selfRaman laser gain medium was a 0.2-at% Nd3 þ -doped, 15 mm long c-cut Nd:YVO4 crystal. Both sides of the laser crystal are coated for antireflection at 1000–1200 nm. The SFG crystal is a KTP with a dimension of 3  3  8 mm3 (AR coated for 1066 nm & 1178 nm & 560 nm, cut for noncritical phase matching, θ ¼83.51, ϕ ¼01), with KTP crystal 560 nm laser is generated from the SFG of the fundamental laser and the first Stokes laser. The 35-mm-long commercial acousto-optic (AO) Q-switch module (Gooch & Housego Company) was placed between the Nd:YVO4 and the KTP crystal. The overall laser cavity length is 96 mm. All crystals were wrapped with indium foil and mounted in water-cooled blocks in which the temperature was maintained at 20 1C throughout the experiment.

1000

30kHz 40kHz 50kHz

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0

4

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8

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12

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Incident pump power at 808nm(W) Fig. 3. Average output power at 560 nm with respect to the incident pump power for pulse repetition rates of 30, 40 and 50 kHz.

to a conversion efficiency of 6.6% with respect to the diode laser input power and a slope efficiency of 10.6%. According to the output power of the Raman laser, we can calculate the pulse energy of the Raman laser, as we can see from Fig. 4. At the pulse repetition frequency of 30 kHz the obtained maximum pulse energy is 29.5 μJ with the incident pumping power of 13.7 W. A Tektronix digital oscilloscope (TDS5052B 500 MHz 5 Gs/s) with a fast InGaAs p-i-n photodiode is used to record the pulse temporal behavior. Fig. 5 shows the typical time shape for 560 nm laser pulse. It can be seen that the pulse duration of the 560 nm laser was 7.9 ns with a 13.7 W pump power. The corresponding single pulse energy and peak power were 29.5 mJ and 3.7 kW, respectively.

F. Su et al. / Optics & Laser Technology 66 (2015) 122–124

Single pulse energy of 560 nm Raman laser (µJ)

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30

power of 13.7 W. The sum-frequency generation provides easy wavelength-conversion of all solid state Raman lasers using robust crystalline technology.

30kHz 40kHz 50kHz

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References

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0 5

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Incident pump power at 808 nm(W) Fig. 4. Pulse energy at 560 nm with respect to the incident pump power for pulse repetition rates of 30, 40 and 50 kHz. 0.10

Intensity (0.02V/div)

0.08

0.06

560 nm laser

7.9 ns

0.04

0.02

0.00 -40

-20

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Time (20ns/div) Fig. 5. The pulse temporal behavior of the 560 nm SFG Raman laser.

4. Conclusion In conclusion, we have demonstrated a simple and efficient method for generating yellow–green Raman laser at 560 nm by sum-frequency generation. With incident pump power of 13.7 W, the obtained maximum Stokes pulse energy is 29.5 μJ at the pulse repetition rate of 30 kHz. The obtained maximum average power is 0.9 W at the pulse repetition rate of 40 kHz and the incident pump

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