reflective mirrors by quasi-continuous-wave laser diode pumping. Microchip type random laser and random laser master oscillator ceramic power amplifier were ...
CTuE4-3 Random laser study with nano-crystalline Nd3+:YAG powder Yan Feng, Jianren Lu, Shenghong Huang, Mitsuru Musha, Ken-ichi Ueda
University of Electro-Communications, Japan
Abstract
Fig.1 shows a schematic diagram of experimental setup. A Hamamatsu QCW laser diode array is used as pump source, which has four bars with center wavelength at ~805nm, and adjustable pulse duration and repetition rate. The laser beam is focused to a spot of about 1mm diameter on the sample. The maximum power at focal point is found to be about 150-W. In the case of one-mirror experiments, the sample consists of a powder tablet with dimension of φ16×3 mm3 and a mirror. The mirror is highly reflective at 1064nm (measured to be ~99.5%) and well transmissive at 805nm (~ 90%). The coated faces are at powder side. In the case of microchip and MOPA experiments, the sample consists of the powder tablet and a 2%-doped 1mm-thick transparent ceramic Nd:YAG microchip The microchip is coated to be highly reflective at 1064nm (measured to be ~99.5%) at one face. In microchip experiments the uncoated face closely touches the powder tablet; in the MOPA experiments the coated face touches the powder.
We have demonstrated lasing in Nd3+:YAG nano-crystalline powder with the help of one reflective mirrors by quasi-continuous-wave laser diode pumping. Microchip type random laser and random laser master oscillator ceramic power amplifier were also investigated. Random lasers 1, which usually refer to lasing in disordered media where strong multiple scattering plays a constructive role instead of being a loss factor as in conventional lasers, have received considerable interests in recent years. Random lasing has been observed in a variety of materials such as laser crystal powder, dye solution with microparticle scatterers, zinc oxide, etc. While many fundamental questions remain, possible applications had been proposed, such as improved phosphor, planar display, and sensor etc. We had proposed using reflective mirror for reducing the threshold 2. In a one-mirror structure, better overlap between pumped region and lasing modes can be guaranteed. It was also shown that in such a half-close system eigenmode structure is different. That is because the mirror introduces interference between incident light and reflected light. This effect is expected to collaborate with coherent backscattering, which is an interference effect between counter-propagation waves resulting an enhancement in exact backscattering direction.
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Fig.1 a schematic diagram of experimental setup.
We have demonstrated lasing in Nd3+:YAG nano-crystalline powder with the help of one reflective mirrors by quasi-continuous-wave laser diode pumping 3, 4. Interestingly, irregular pulsing was observed above a pulsing threshold, whose repetition rate increases when excitation power increases. After that, we have pushed the idea further, we demonstrate a microchip-type laser with multiple scattering from powder as necessary feedback 5, and a hybrid master oscillator power amplifier (MOPA) with a random laser as a master oscillator and ceramic microchip as a power amplifier.
The powder is provided by Konoshima Chemical Co., Ltd. SEM image shows the powder particles have average diameter of about 250nm. The dielectric volume fraction was found to be about 50% by comparing the mass density of the power tablet and bulk Nd3+:YAG. The emission from the sample, which leaks through the highly reflective coating, is collected by the same spherical lens, which focused the pump light, separated from the pump light by a dichromatic mirror and an interference filter at 1064nm (bandwidth 10nm), and then focused to detectors. When measuring temporal behavior, an InGaAs photodiode is used.
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When measuring emission spectra, a monomode fiber is used to couple some light into an Ando AQ-6317 Optical Spectrum Analyzer.
The repetition rate increases when pump power increases. Typical duration of the pulses decreases from 0.5µs to 0.2µs when pump increases to highest available power.
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Fig.3 emission spectra from one-mirror experiments
Fig. 3 shows the averaged emission spectra taken in one-mirror experiments at different pump levels, 148, 132, 115, 95, 85, and 80 watts, respectively. For comparison, emission spectrum from a powder sample without mirrors and that from a crystalline sample at room temperature are also shown.
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In summary we will present our investigation on random laser with nano-crystalline Nd:YAG powder with novel configurations. Emission intensity [arb. units]
References
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Fig.2 Waveforms from one-mirror experiments
Fig.2 top) shows typical emission pulses with different pump power in one-mirror experiments, from bottom to up corresponding pump powers are 48, 53, 68, 81, 106, 156 W, respectively. The pump pulse is rectangular and has a duration of 200µs, and rising/drop time of ~3µs. Fig. 2 bottom) shows a zoom-in view of waveforms.
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H. Cao, Waves Random Media 13, R1 (2003). Y. Feng and K.-i. Ueda, Phys. Rev. A 68, 025803 (2003). Y. Feng, J.-F. Bisson, J. Lu, et al., Appl. Phys. Lett. 84, 1040 (2004). Y. Feng, J. Lu, S. Huang, et al., in SPIE, edited by R. M. D. L. Rue, P. Viktorovitch, C. M. S. Torres and M. Midrio (Bellingham, WA, 2004), Vol. 5450, p. 388. Y. Feng, S. Huang, G. Qin, et al., Opt. Express 13, 121 (2005).
