We report an efficient room-temperature operation of a resonantly pumped laser at . The maximum continuous wave (CW) output power of with slope efficiency of 56% was achieved with resonant pumping by an Er-fiber laser at . With pumping by a commercial laser diode bar stack, a quasi-CW (QCW) output of and maximum slope efficiency of versus absorbed pump power were obtained. This is believed to be the first resonantly (in-band) pumped, room-temperature laser.
©2012 Optical Society of America
Due to its good mechanical, thermo-optical, and spectroscopic properties, doped is considered to be a promising laser material for efficient operation in the eye-safe wavelength domain [1,2]. Recently, a CW output from a room-temperature (RT) laser was demonstrated by Sulc et al . This laser, pumped into the manifold of the ion at and emitting from the manifold at , has demonstrated a slope efficiency of versus the absorbed pump. It has been shown that much higher efficiency can be demonstrated via resonant pumping directly into the manifold. Such pumping allows for a low quantum defect () operation  and offers much better power scaling potential. This pump-lase scheme based on transitions operates particularly well at cryogenic temperatures, and lately we reported 84% slope efficiency for a cryo-cooled resonantly (in-band) pumped laser . Also, the resonantly pumped RT laser (which is a very close analog of an one) demonstrated slope efficiency of 57.9% . Similar high efficiency can be expected for an RT laser as well, but, to the best of our knowledge, laser efficiency of resonantly pumped at RT has not been explored so far.
This paper presents the RT spectroscopic characterization of in the 1450 to range ( transitions) and reports the performance of what is believed to be the first RT in-band pumped laser.
For spectroscopic characterization, we used the 0.5%-doped crystal grown by the Czochralski technique. Absorption spectra between the and the multiplets were taken using a Cary 6000i spectrophotometer operating in the fixed-slit-width mode with resolution. Figure 1 shows polarization-resolved ground-state absorption spectra of . There are two strong -polarized absorption lines: at and around . For polarization, there are three distinctive transitions with approximately equal strengths, spread between 1519 and .
Emission spectra of were obtained by illuminating the 0.5%-doped sample with a diode laser. Luminescence was collected with the Optical Spectrum Analyzer (Yokogawa, model AQ6370C). A polarization beam splitter was inserted into collecting optics to separate - and -polarized signals. The emission cross sections of the transitions, shown in Fig. 2, were calculated via the standard Fuchtbauer–Landenburg method , using the lifetime of the manifold of .
It can be seen that in the -polarized emission spectrum of there are three emission maxima where lasing can be expected: at (cross section ), at (), and at (). The relevant energy level scheme of the and the manifolds can be found in  and in . In the -polarized spectrum, there is one transition at suitable for laser operation with the peak cross section of .
Laser experiments were carried out with the antireflection-coated, long, thick 0.7% sample. The crystallographic axis of the crystal was normal to the axis of the laser cavity, thus enabling longitudinal pumping in any chosen polarization. The crystal was bonded between copper plates water-cooled to . A simplified experimental laser setup is shown in Fig. 3.
Two different sources were used for pumping into major absorption lines of : a CW Er-fiber laser with narrowband output ( FWHM) and a commercial, spectrally narrowed ( FWHM), fast- and slow-axis collimated laser diode bar stack (QPC Lasers). The Er-fiber laser output wavelength was tuned to the absorption band with large cross section and relatively small cross section.
The unpolarized pump beam was focused into the crystal by the spherical lens.
The diameter of the almost cylindrical pumped region inside the crystal was approximately ( level). The short laser cavity () consisting of a concave output coupler () and a flat dichroic mirror ( at , at ) resulted in the mode diameter of for a good mode matching.
Figure 4 shows the CW output power of the laser pumped by a Er-fiber laser. The best efficiency was achieved with the 5% outcoupling. The maximum obtained output power was , and the maximum slope efficiency . The fraction of the absorbed pump, carefully derived from the measurements of the transmitted pump while the laser was operational, was calculated at 41% to 44%. This relatively low value is due to the weak absorption of the -polarized component of the unpolarized pump. No noticeable heating of the crystal was observed. The output of the laser was -polarized and centered at with the bandwidth varying from 1.5 to with the absorbed pump.
The second pump source, the InGaAsP/InP laser diode bar stack, was operated in a QCW regime with a 5% duty cycle and . The incident pump beam was polarized, and its wavelength was tuned to the absorption band (cross section ) by varying the coolant temperature of the stack. A cylindrical telescope was used to provide a nearly equal divergence of the pump beam in the vertical and horizontal directions (see Fig. 3). A polarizing cube and a half-wave plate placed in front of the diode stack served as a variable pump attenuator. The collimated pump was focused into the crystal by a spherical lens (). The diameter of the pump spot was in the center of the crystal and at the crystal ends. We used a long laser cavity and concave output couplers with a curvature of . Their reflections varied from 80% to 95%. This cavity configuration provided a fairly good pump-cavity mode spatial overlap inside the crystal.
Figure 5 shows the QCW output power of the laser versus the absorbed pump for 10% outcoupling. The maximum QCW output power of and the maximum slope efficiency of were obtained. The observed nonlinearity of the input-output plot is caused by a stronger radial variation of the excitation relative to the case of Er-fiber pumping. Thus, at low pump levels, only low-order modes are involved in lasing. With the gradual increase of pump power, the number of involved modes going into lasing increases, which improves overall efficiency of the laser.
The fraction of the absorbed pump power in this case was calculated as when the laser operated just above the threshold. The fraction reduced to approximately 0.4 at the maximum of the absorbed pump (). This relatively low utilization of the incident pump power is due to a small cross section of the absorption transition. As in the previous case, the -polarized output spectrum was centered at . Its full bandwidth reached at the maximum absorbed pump power.
We presented the results of a RT spectroscopic characterization of single crystal in the spectral range pertinent to the resonantly (in-band) pumped laser operation. We reported for the first time, to the best of our knowledge, the RT performance of the resonantly pumped laser. The maximum CW output power of at with slope efficiency of 56% was achieved with Er-fiber laser pumping at . With QCW pumping by a commercial laser diode bar stack at , a QCW output power of at and maximum slope efficiency of versus the absorbed pump power were demonstrated.
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