A microchip-type laser with multiple scattering from powder as necessary feedback is demonstrated. The laser consists of a transparent ceramic Nd:YAG microchip and a Nd:YAG powder tablet, operates at quasi-continuous-wave regime pumped by a laser diode array.
©2005 Optical Society of America
In recent years there were considerable interests in random lasers [1–6], which usually refer to lasing in disordered media where strong multiple scattering plays a constructive role instead of being only a loss factor as in conventional lasers. Random lasing has been observed in a variety of materials such as laser crystal powder [5–8], dye solution with microparticle scatterers , zinc oxide [2, 9], etc. While many fundamental questions remain, possible applications had been proposed, such as improved phosphor, planar display , and sensor [1, 11] etc.
We apply ideas used in conventional lasers to random laser study [12–14]. It is shown that lasing threshold can be reduced with a reflective mirror closely touching the powder. The presence of the reflective mirror introduces interference between incident 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 . In this paper, we demonstrate a microchip-type laser with multiple scattering from powder as necessary feedback. The laser consists of a transparent ceramic Nd:YAG microchip and a Nd:YAG powder tablet, operates at quasi-continuous-wave (QCW) regime pumped by a laser diode array. The idea may be applied to fabricate interesting light sources.
2. Experimental setup
A Hamamatsu QCW laser diode array (LDA) is used as pump source, which has four bars with center wavelength at ~805nm, and adjustable pulse duration and repetition rate. The fast axe of the laser output is collimated by a cylindrical lens, and then the laser beam is focused by a spherical lens to a spot of about 1mm diameter on the sample. The maximum power at focal point is found to be about 150-W. The sample, as shown in Fig. 1 (left), consists of a 2%-doped 1mm-thick transparent ceramic Nd:YAG microchip and a layer of 4% doped Nd3+:YAG powder tablet with thickness of 3mm. The microchip is coated to be highly reflective at 1064nm (measured to be ~99.5%) at one face. The uncoated face closely touches the powder tablet. The transverse dimension of the sample is 15mm. 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. When measuring emission spectra, a monomode fiber is used to couple some light into an Ando AQ-6317 Optical Spectrum Analyzer.
We choose QCW laser diode as pump source, because wavelength can be tuned to be resonant with Nd3+ absorption and pulse duration can match Nd3+:YAG lifetime well (230µs or shorter dependent on dope concentration, for 4% Nd3+:YAG, about 80µs). Therefore, enough gain can be introduced by relatively low peak pump power and QCW operation.
3. Results and discussion
Before experimenting with the powder, fluorescence from the microchip alone is measured with the setup. Typical emission waveforms are shown in Fig. 1 (right). The pump pulse is rectangular and has duration of 200µs, and rising/drop time of ~3µs. The waveforms are determined by excited state population dynamics in the Nd:YAG microchip. When the rectangular pump pulse is on, fluorescence increases for the cumulation of excited state population. The intensity is saturated after a period of time due to spontaneous emission. After pump pulse is off, the fluorescence decays as usual. By fitting the waveform, lifetime of the emitting level 4F3/2 is found to be ~120µs. The emission signal is weak because the presence of high reflective coating.
Placing the 4% doped Nd:YAG nano-crystalline powder tablet to the back of the microchip, very strong emission is detected. Fig. 2 (left) shows typical emission waveforms with different pump power, from bottom to up corresponding pump power increases homogenously. When pump power is low, emission increases gradually throughout the pump duration. When pump power is higher, emissions show a transition behavior. Emission intensities jump to a high level, and after that increase slowly again. The transition time depends on pump power. Higher the pump power, quicker the transition takes place.
The first phase of homogeneous growth is due to spontaneous emission, because excited state population cumulates when pump is on. At the transition point, the population grows to a critical value, which corresponds to a laser threshold in time dimension. Furthermore, when pump power is a little even higher, there is additional feature of irregular pulsing. Fig. 2 (right) shows a zoom-in view of waveforms shown in left. The repetition rate increases when pump power increases. Duration of the pulses ranges from 0.5µs to 0.2µs, which decreases when pump increases.
In the system, gain is provided by the homogeneous ceramic microchip and powder tablet as well. The positive feedback is provided by the highly reflective coating at one side and backscattering from the powder tablet at the other side. Because the experiments are carried out in the same condition, the relative intensity is meaningful. We see in the case of powder-microchip hybrid sample emission is about 200 times stronger than that in the microchip-alone case. The emission is also much stronger than that in one-mirror case , which results from better amplification in the homogeneous microchip, because the homogeneous region is better pumped than powder.
The repetition rate versus pump power is investigated. The repetition rates are calculated by counting the number of pulses in an interval and repeat it in different interval and that of different record of waveforms, as well, and after that averaged. Since the repetition rate varies even within one waveform (the pulses are denser at the end of pump period), we count the rate at the end of pump period for each pump level. The results are shown in Fig. 3. The behavior may remind us relaxation oscillation as in conventional lasers , in which case the repetition rate has a square root relation with pump power. However, from the detail of the pulse train we think it is rather different from relaxation oscillations. At low pump power just above pulsing threshold, pulses are very sparse), the distance between pulse to pulse is much larger compared to pulse duration and irregular. But the pulse amplitude doesn’t decrease accordingly. We observe very high pulses time to time. The emission is not likely just modulated but is a kind of bursting behavior. Most probably, the observed pulsing is due to some abrupt change in modes resulting from enhanced nonlinearity in powder. However, we have no solid interpretation on this pulsing phenomenon at present.
We measure the relative emission intensities by integrating over the emission waveforms. The result is shown in Fig. 3. There is a threshold point around peak pump power of about 60W, above which emission intensity increases more rapidly and linearly with respect to pump power. This is one important proof of lasing. The transition is much steeper than that observed in one-mirror random laser. This means less spontaneous emitted photons are involved in the laser dynamics. The β factor is smaller in current case.
The spectra of the emissions are investigated. Fig.4 shows the averaged emission spectra taken at different pump levels, 156, 139, 106, 85, and 48 watts, respectively. One can see the emission is red-shifted with increasing pump power, which is due to thermal effect . The linewidth is measured to be about 0.03nm, which is limited by the instrument spectral resolution. Compared to spontaneous emission linewidth of ~0.5nm, this spectral narrowing is another proof of lasing. Unlike that in one-mirror random laser [5, 13], no linewidth broadening at higher pump is observed.
Compared to the random laser with one mirror, this microchip-type random laser contains a homogeneous part, which allow efficient pumping and provide most of gain. This is confirmed by much stronger emission. The powder is pumped by the residual pump laser. Moreover, the lasing threshold is comparable in spite of the 1mm distance between reflective coating and powder, which is expected to cause extra leakage of photon. The extra loss is compensated by better pump efficiency. We cannot observe any difference in emission dynamics between this microchip-type and one-mirror random lasers.
In the demonstration, microchip and powder of Nd:YAG is used, which is a common laser materials. However, beside the physical interests of the system, one possible application is to fabricate interesting light sources with materials that cannot form in good optical quality. The homogeneous part of the structure allows better absorption of pump so as to provide more gain, but it is not necessary to be in laser quality. The powder need not be doped, because the key role of it is providing feedback. The idea can be extend to a scheme of powder-homogeneous material-powder sandwich, where the powder provides feedback while the homogeneous part provides most of gain. This may be useful for the situations where reflective coating by periodic structure is not possible. Thermal problem will be a major practical issue when applying random laser to real world light source, because thermal conduction is very low too in optically scattering media like powder. From a viewpoint of engineering, a homogeneous part is welcome for introducing gain and removing heat. Possible field of application includes semiconductor light sources and planar display, etc.
We have demonstrated a microchip-type laser with multiple scattering from powder as necessary feedback. The laser consists of a transparent ceramic Nd:YAG microchip and a Nd:YAG powder tablet, operates at quasi-continuous-wave regime pumped by a laser diode array. The idea has potential application in fabricating interesting light sources.
This work is supported by the 21st Century COE program of Ministry of Education, Science and Culture of Japan.
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