CHIN. PHYS. LETT. Vol. 26, No. 1 (2009) 014212 Observation of Antiphase State in a Self-Q-switched Nd,Cr:YAG Laser * SHI Yu-Xian(施玉显)1** , LI Qi-Nan(李奇楠)1 , DU Shi-Feng(杜仕峰)1 , ZHANG Dong- Xiang(张东香)1 , ZHANG Zhi-Guo(张治国)1 , FENG Bao-Hua(冯宝华)1 , ZHANG Shi-Wen(张世文)2 1 Laboratory of Optical Physics, Chinese Academy of Sciences, Beijing 100190 2 North China Research Institute of Electro-Optics, Beijing 100015 (Received 7 August 2008) We experimentally investigate the antiphase dynamics phenomenon in a self-Q-switched Nd,Cr:YAG laser oper- ating at 946 nm. Due to the effect of spatial hole burning, the Q-switched pulses sequences of one, two and three modes at different pump power are observed. The experimental results show that the pulse sequences display classic antiphase dynamics. PACS: 42. 55. Xi, 42. 60. Gd, 42. 65. Sf The antiphase state was ?rst observed in an N- coupled Josephson junction circuit. In the state, all oscillators have the same period, but each oscillator is shifted by 1/? of a period from its neighbour.[1] The similar states were reported in the output of a multi- mode laser with intracavity doubling crystal,[2?5] sat- urable absorber[6] and gain or loss modulation.[7,8] The antiphase state in multimode solid-state lasers has been proposed as a method for encoded in- formation transmission.[9] In earlier works, the an- tiphase state have been reported in a diode-pumped passively-Q-switched Yb:YAG laser with a saturable absorber,[10?12] and a diode-pumped self-Q-switched Cr,Yb:YAG laser.[13] As a saturable absorber, Cr- doped crystals possess speci?c advantages such as good optical properties, photochemical and thermal stability, high e?ciency high damage threshold and so on. The Nd,Cr:YAG crystal combines the gain medium and the saturable absorber in a single crys- tal and can be used to highly compact systems. Experiments and theoretical analyses about the an- tiphase dynamics phenomenon in the self-Q-switch Nd,Cr:YAG laser operating at 1064 nm, which cor- responds to the four-level transition of Nd3+ ions, have been reported.[14] Compared to the four-level transition, the Nd,Cr:YAG laser operating at 946 nm as quasi-three-level laser has higher pump threshold, lower output power and lower slope e?ciency owing to the reabsorption of the lower laser state. In de- spite of these disadvantages, 946 nm Nd,Cr:YAG laser can be used for intra-cavity doubling blue laser. In this Letter, we report our experimental observation of antiphase state of a laser-diode-pumped multimode self-Q-switched Nd,Cr:YAG 946 nm laser operating as quasi-three levels. Figure 1 shows the experimental setup. The Nd,Cr:YAG crystal with a thickness of 2 mm and an initial transmission of 97.5% is mounted in a water- cooled heat sink whose temperature is maintained at 10? C. The pump facet of the crystal is coated with anti-re?ection for 808 nm pump wavelength and high re?ection for 946 nm laser wavelength, while high tran- sition at 1064 nm and 1320 nm is speci?ed to suppress the oscillation at these transitions. The other side of the crystal is coated for high transition at 946 nm, 1064 nm and 1320 nm. The pump source is a high- brightness ?bre-coupled diode laser with a ?bre core diameter of 400 ?m and numerical aperture of 0.22. A 1:1 coupling system is used to inject the pump laser into the Nd,Cr:YAG crystal. A plane-concave mirror with a radius of curvature of 50 mm and transmission of 5% is used as output coupler. The total cavity length is 37 mm. The laser spectrum is measured by a spectrograph (SpectraPro-500i) with a resolution of 0.05 nm and dispersion of 1.7 nm/mm. In order to study the dynamics of each mode separately and to make sure the detected light be one mode of the laser, we convert the spectrograph to a monochromator and set the central wavelength at the peak position. The intensity waveforms of the total output and a spe- ci?c mode are detected with a photodiode, whose rise time is less than 1ns, and a fast digital oscilloscope (Tektronix 3052B, 500 MHz bandwidth and 5.0 Gs/s). The co-doped Nd,Cr:YAG crystal can act as the gain medium as well as the saturable absorber to produce self-Q-switched pulse sequences. Fig. 1. Experimental setup of a self-Q-switched Nd,Cr:YAG laser: LD, laser diode; BS, beam splitter; PD, photodiodes; MC, monochromator; DO, digital oscilloscope. *Supported by the National Natural Science Foundation of China under Grant No 60438020, and the National Basic Research Programme of China under Grant No 2007CB613205. **Email: yxshi@aphy.iphy.ac.cn c 2009 Chinese Physical Society and IOP Publishing Ltd 014212-1 CHIN. PHYS. LETT. Vol. 26, No. 1 (2009) 014212 The Nd,Cr:YAG crystal act as the gain medium and the saturable absorber and produce self-Q- switched pulse sequences. Although there are hun- dreds of longitudinal modes within the gain curve, the system selects only a few of them to lasing due to the spatial hole burning (SHB) e?ect. In our experiment, the self-Q-switch pulse threshold is 1.58 W and the minimum pulse width of 36 ns is obtained. The rep- etition rate of the pulse train increases with increas- ing pump power. With the pump power increasing, the number of lasing modes increases. There is only one mode at pump power of 1.58–1.86 W, and there are two modes at pump power of 1.96–3.86 W. The third mode appears when the pump power increases to 4.02 W. The central wavelength of each mode changes with the pump power increasing. The space between each mode is almost a constant, which is about 0.1 nm. These modes compete for the gain and make use of the inversion particles in di?erent longitudinal dimen- sions of the crystal and go to oscillate. These las- ing modes generate self-organized dynamics such as antiphase dynamics. We identify this phenomenon by measuring contribution of each mode to the total output laser frequency and select the incident pump power of 1.78 W, 3.06 W, 3.86 W and 4.02 W as shown in Figs. 2–5. At a pump power of 1.78 W, there is only one mode at 946.40 nm and the Q-switched pulse train is quite stable with a repetition rate of 2.6 kHz as shown in Fig. 2. Fig. 2. (a) Spectrum of the laser at incident pump power of 1.78 W, (b) oscilloscope trace of a train of Q-switched pulses at the pump power of 1.78 W. When the pump power is 3.06 W, the spectrum shows two modes whose wavelengths are 946.34 nm and 946.44 nm, the repetition of the two modes both are 5.7 kHz and the total repetition rate is 11.4 kHz as shown in Fig. 3. Compared the repetition of the two single modes, each mode is shifted from each other by a 1/2 of a period. That is to say, the output Q- switched pulses display an antiphase state in time due to the SHB. When the pump power increases to 3.86 W, we can found that there are two modes with wavelength of 946.38 nm and 946.50 nm as shown in Fig. 4. The ?rst mode contains four groups of pulses and the second mode contains two groups. We can see that the six groups of pulses are arranged at antiphase state. The total repetition is 16 kHz. Fig. 3. (a) Spectrum of the laser at incident pump power of 3.06 W. Oscilloscope trace of a train of Q-switched pulses at pump power of 3.06 W at different wavelengths: (b) the total, (c) 946.34 nm, (d) 946.44 nm. Fig. 4. (a) Spectrum of the laser at incident pump power of 3.86 W. Oscilloscope trace of a train of Q-switched pulses at pump power of 3.86 W at different wavelengths: (b) the total, (c) 946.38 nm, (d) 946.50 nm. At the pump power of 4.02 W, we can see from Fig. 5(a) that there are three modes at 946.29 nm, 946.39 nm and 946.52 nm, respectively. The repeti- tion of the ?rst mode is 4 kHz, the second is 9 kHz and the third is 4 kHz, the total repetition is 17 kHz. Each mode shifts from the neighbour by 1/3 of a pe- riod and the three groups of pulses are arranged at the classic antiphase state as shown in Fig. 5. 014212-2 CHIN. PHYS. LETT. Vol. 26, No. 1 (2009) 014212 Fig. 5. (a) Spectrum of the laser at incident pump power of 4.02 W. Oscilloscope trace of a train of Q-switched pulses of at pump power of 4.02 W at different wave- lengths: (b)the total, (c) 946.29 nm, (d) 946.39 nm, (e) 946.52 nm. In conclusion, we have investigated experimen- tally the antiphase dynamics in a self-Q-switched Nd,Cr:YAG laser, and we have measured the spectrum of the laser and the repetition of each laser mode at di?erent pump power. The two or three modes display classical antiphase dynamics. 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