Schwartz Electro-Optics (now Q-Peak, Inc.)

Research Division

45 Winthrop Street

Concord, MA 01742-2052

We have demonstrated cw laser operation at wavelengths near 3-mm from monolithic, erbium-doped YSGG, GGG and YAG lasers, by direct upper-state pumping in the 0.97-mm wavelength region with either a Ti:sapphire laser or InGaAs diode lasers. We have obtained slope efficiencies as high as 31% and, with diode-laser pumping, outputs of 0.5 W. In addition, we have demonstrated tunable single-frequency operation from Er:YAG.

Laser transitions of trivalent erbium (Er) at wavelengths near 3 mm operate between the 4I11/2 (upper) and the 4I13/2 (lower) states. The long lifetime of the lower state impedes cw operation, but is mitigated by a rapid thermalization among the lower-state Stark-split levels. Thermalization favors laser action from transitions terminating in the highest-lying Stark levels. In Er:LiYF4 (YLF), cw operation should be possible due solely to thermalization [1]. Upconversion from the lower state, identified as an important factor in early studies of long-pulse, flashlamp-pumped lasers [2-4], also aids in establishing net cw gain for crystals with high Er concentrations.

The first true-cw, 3-mm Er laser [5], operating with the host crystal YLF, was pumped by exciting the 4I9/2 state at a 0.79-mm wavelength with an AlGaAs laser, and produced an output power of ~180 mW, which was later improved to nearly 10 mW [6]. Direct excitation of the upper state by pumping near 0.97 mm leads to higher efficiencies than excitation at 0.79 mm, due to reduced energy losses resulting from other de-excitation processes. Recent experiments with 0.97 mm, Ti:sapphire-pumped Er:GSGG lasers [7] have shown power outputs approaching 130 mW, with slope efficiencies of 36%, greater than the ratio of pump to laser wavelength. The high slope efficiency is attributed in part to repumping of the upper laser state by upconversion.

In the experiments discussed here we extend operation of direct-pumped 3-mm Er lasers to three new crystals, and report on efficient operation with 0.97-mm, InGaAs diode pump lasers at 3-mm output power levels as high as 0.5 W. In addition we demonstrate the first, to our knowledge, single-frequency, tunable output from a cw Er:YAG laser.

As in previous work [1,5-7] the laser configuration we used was a longitudinally pumped monolithic resonator. The laser crystals were 3 mm long and 3 mm in diameter except for the single-frequency monolith, which was 1 mm long and 1.5 mm in diameter. The flat, pump surfaces of the monoliths were coated to be highly transmitting at 0.97 mm and highly reflecting at the laser wavelength. The output surfaces were coated for nominal 99.7% reflection at the laser wavelength and were polished with convex 1- and 4-cm-radii-of-curvature for the 3- and 1-mm lengths, respectively. Materials used for the 3-mm monoliths were 30%-doped Er:YSGG, 30%-doped Er:GGG and 33%-doped Er:YAG. The 1-mm monolith was fabricated from 50%-doped Er:YAG in order to maintain a high level of pump absorption. Unless indicated, all data applies to the 3-mm-long monoliths.

We conducted our initial experiments using a cw Ti:sapphire laser as the pump source, focused to a 40-mm spot (1/e2 radius) on the flat surface of the crystals by a 10-cm-focal-length lens. We obtained power, threshold, slope efficiency and laser wavelength data for each material at 300 K. Our results are presented in Table 1, along with our measured lifetimes for the upper and lower laser states. To our knowledge, this data represents the first cw operation from Er:GGG and the first cw operation in the other hosts via direct pumping of the Er3+ ion. Er:YSGG typically showed the best performance of the three materials, with a slope efficiency of 31% well over threshold.

Table 1. Performance summary of 3-mm erbium lasers at 300 K. Efficiency and power measurements in parentheses indicate results of diode-laser pumping. Other values refer to Ti:sapphire pumping.

We observed laser action from the YSGG and GGG hosts at Ti:sapphire pump wavelengths ranging from 0.91 to 0.99 mm. Figure 1 shows, for the Er:YSGG laser, plots of the ratio of output power to pump power as a function of pump wavelength, for pump powers near threshold (15 mW at 0.97 mm) and far over threshold (400 mW at 0.97 mm). Also shown in the figure is the absorption coefficient for the laser material; laser action occured for absorption coefficients as low as 0.2 cm-1. We find a good agreement between the shape of the laser excitation spectra and the absorption spectra, indicating that pump excited-state absorption is either wavelength-independent or has a minimal effect on laser performance at the excited-state densities encountered in the monoliths.

Fig. 1. Er:GGG laser excitation spectra (solid lines) for two pump levels, along with absorption spectrum of crystal (dashed line).

The second diode-laser was a 1-W, multimode device (SDL-6462-P1) with a 100-mm stripe width. In order to obtain high pump powers we used two of the devices, oriented for orthogonal polarization and combined into a single beam with a polarizing beamcube. The individual diode laser outputs were collimated with a 4-element lens (f=15 mm, 18-mm aperture). An identical lens focused the combined beams onto the monoliths. Figure 2 shows the input-output curves for the three materials. Power, threshold, and efficiency data for the double-diode pumping scheme are summarized in Table 1.

Fig. 2. Input-output curves for Er-doped monolithic lasers pumped by two-1-W diode lasers.

We used a 0.25-m grating spectrometer to measure the emission spectrum of the Er lasers. Longitudinal modes of the 3-mm-long crystals are spaced approximately 0.7 nm apart, and were resolved by the spectrometer. The 3-mm monoliths generally operated on 3-5 longitudinal modes, with a resultant overall width of ~2.5 nm at FWHM. The 1-mm-long crystal of Er:YAG, when pumped by the Ti:sapphire laser, operated on a single axial mode based on the observed spectrometer output and on the fact that a stable reading of wavelength was evident when the laser output was analyzed by a high-resolution wavemeter (Burleigh Wavemeter Sr.). For the 1-mm crystal we obtained a 40-mW threshold, a 16% slope efficiency and 70 mW of output power for 540 mW of pump.

We placed the 1-mm Er:YAG crystal mount on a thermoelectric cooler to determine the temperature and pump-power tuning rate of the laser wavelength, which we measured with the wavemeter to resolution of 0.01 nm and an absolute accuracy of 0.1 nm. We observed that the monolith switched between the 2.94-mm and 2.83-mm laser transitions, depending on the crystal temperature and pump power. By changing both pump power and crystal temperature we were able to tune the 2.83-mm line over a 1.9-nm range. The 2.83-mm line demonstrated a constant-pump-power, wavelength tuning vs. temperature relation plotted as points in Figure 3. The solid line in the figure is the slope predicted if tuning results entirely from the shift in the cavity mode wavelength with temperature. We calculated the slope, 0.033 nm/C, using a dn/dT and a thermal expansion coefficient of 7.3 and 7.5 x 10^-6, respectively, for the YAG host crystal. The agreement between the data and predicted tuning rate is excellent, and the deviation from a straight line at the extremes of the temperature range may result from a shift with temperature of the transition peak wavelength. The pump-power tuning rate, for a fixed heat-sink temperature of 25 C, was 0.004 nm/mW in the pump-power range 300-500 mW. The two rates combined imply that the monolith temperature, in the volume occupied by the laser mode, rises at a rate of 0.10 C/mW of incident pump power.

Fig. 3. Observed temperature-tuning of single-frequency Er:YAG laser wavelength.

In analyzing our results, we estimated the expected threshold pumping level for the lasers by setting up the rate equations for populations of the upper and lower laser states and finding the steady-state solutions as a function of pump rate. We included terms for upconversion from each state and, in addition, terms accounting for the finite probability that excitation arriving in the ⁴I_9/2 state from upconversion may cross relax and generate two excitations in the lower laser state, rather than excite the upper state after nonradiative relaxation, as discussed by Chou and Jenssen [8]. We measured the upconversion coefficients and the ⁴I_9/2 branching term for YSGG, and employed the upconversion data of Shi et al [9] for Er:YAG. After calculating the cw population densities in the upper and lower states we determined the pumping rate needed to obtain a net gain in the laser crystal, accounting for the thermalized fraction of excitation in the appropriate upper- and lower-state Stark-split levels. We will present details of our calculations in another publication.

We find that pumping rates of 4x10²¹ and 2.5x10²² cm^-3 are predicted to obtain net cw gain in Er:YSGG and Er:YAG at the doping levels we used in our 3-mm monoliths. We calculated the actual threshold pumping rates W_th, at the crystal surface for Ti:sapphire pumping using the formula (for a Gaussian pump)

W_th = a P_th / ( p w₀² E_p ) ,

where a is the pump absorption coefficient, P_th is the threshold pump power, w_o is the pump beam radius, and E_p is the energy of the (965 nm) pump photon. The values for a and P_th for the lasers appear in Table 1, and w_o is 40 mm. We find that W_th is 7.3x10²¹ and 4.7x10²² cm^-3 for Er:YAG and Er:YSGG, which in both cases is slightly less than twice the theoretical values needed to obtain net gain. A more involved threshold calculation requires full accounting for the overlap between the spatial variation in gain in the laser crystal and the laser mode.

As higher-power and higher-brightness, 0.97-mm diode-laser sources become available, we expect that higher levels of 3-mm, cw power output can be generated by Er-doped materials.

6. G. J. Kintz and L. Esterowitz, "Diode pumped CW 2.8 mm Er:LiYF4 laser with high slope efficiency," Paper EL1.6, LEOS '88 Annual Meeting (IEEE/LEOS, Piscataway, NJ, 1988).

If you got here from a search engine, click the icon to go to Recent Technical Papers at Q-Peak