Andrew Finch and John H. Flint
Schwartz Electro-Optics Inc., Research Division
Abstract
We report on a diode-pumped, Thulium, Holmium:YLF ring laser which has produced over 2.0 W of single-frequency power with a pump power of 14 W.
Tm,Ho:YLF lasers are proving to be effective sources for a variety of coherent wind sensing applications. The majority of long range systems (>5 km) rely on pulsed i.e. Q-switched sources operating at 10 - 200 Hz repetition rates. However for short range applications single-frequency CW sources are useful and in some cases preferred. This paper details the performance of a breadboard Tm,Ho:YLF ring laser which will eventually be packaged for use in field measurements. To date the system has produced 2.1 W single frequency output which we believe represents the highest single-longitudinal mode power obtained from a Tm,Ho:YLF laser. Previous workers have reported the performance of single-frequency standing wave cavities, but output powers have been limited to 1.2 W [1]. The design used here, as shown in Figure 1, is a modification of a diode pumped Q-switched ring laser previously reported for use in clear-air-turbulence detection [2].
Figure 1. Schematic of the single frequency Tm,Ho:YLF ring laser
The 2-µm laser cavity contains a 10-mm long, 4-mm diameter 6% Tm, 0.5% Ho:YLF crystal mounted in a TE-cooled heat sink inside a purgeable enclosure. The hot sides of the TE-cooler are cooled to 5°C using a Neslab chiller, allowing the YLF crystal block to be cooled to as low as 235 K (-35°C). A single 20-W laser bar is used as the pump source, whose output is coupled to a bifurcated fiber bundle, i.e. the bundle was split into two separate smaller bundles each of which were imaged into either end of the YLF rod through HR/HT flat mirrors. The remainder of the ring resonator is a flat output coupler and a 50-cm curved high reflector. A Brewster-angled acousto-optic Q-switch is inserted in one leg of the cavity.
In previous work the ring laser, when operated in CW mode, was made to lase unidirectionally by employing an external mirror next to the output coupler to retroreflect one output back into the cavity. This acted to suppress that direction of lasing to a very low fluence level and the laser was effectively unidirectional. Its output was not single frequency, however, and typically suffered sporadic amplitude noise. This was due to phase fluctuations of the retroreflected light relative to the cavity field, since the outside mirror was not interferometrically matched to the cavity.
Achieving truly unidirectional (and hence single-frequency) operation requires the insertion of a unidirectional device inside the laser ring cavity. One possibility is to use an optical diode based on the Faraday material YIG. Unfortunately, the available samples of this material apparently had too small a through aperture. Based on recent work performed at the University of Southampton (U.K.) [3,4], an alternative approach is to use an acousto-optical modulator. Operating such a device slightly away from the Bragg angle sets up a differential loss between the two lasing directions in the ring cavity. The device is operated with RF applied just sufficient to suppress the one (back) direction, since the forward direction still sees loss due to the RF applied. Use of these devices may present a larger insertion loss to the cavity compared to an optical diode, however they provide the advantage of having variable control of differential loss. This is useful when optimizing the laser for different operating points and accommodating feedback effects from optics external to the cavity.
In these initial CW experiments, while maintaining constant RF power to the Q-switch, we adjusted it away from its Bragg angle while monitoring the power levels of each lasing direction. Optimum alignment (i.e. maximum differential loss for minimum RF applied -hence minimum insertion/forward loss) was achieved by additional vertical alignment adjustments of the two turning mirrors. Once aligned properly, unidirectional operation was reliably achieved and the output amplitude fluctuations were extremely small (<0.1% rms.). Over 2 W of single frequency output power was achieved (Figure 2), interestingly without any etalons in the cavity. Single frequency operation was verified using two Burleigh scanning interferometers with free spectral ranges of 8 GHz and 150 MHz. We suspect that the AR surface of the crystal and/or the windows around the purged enclosure are acting as etalons. We inserted a 100 µm etalon into the cavity and tuned the laser from 2.0671 to 2.0537 µm. A tuning curve is shown in Figure 3. The tuning was not continuous but separated by ~0.3 nm skips which is indicative of additional etalon elements inside the cavity. A variety of output couplers were tried; the optimum coupling seems to be in the 2.5 - 4% range.
Figure 2. Input/ output data of single frequency ring laser. (Crystal block temperature = 238 K).
Figure 3. Tuning curve of single frequency ring laser (Pump power =14 W, crystal block temperature = 238 K)
These experiments suggest that it may be possible to design a generic ring cavity which can operate as either a multi-milli Joule Q-switched or a multi-Watt, single-frequency CW source without the inclusion or removal of any additional optics.
References
[1] C.P. Hale, S.W. Henderson, and P.J.M. Suni, Proc. on Advanced Solid-State Lasers 15, 407 (1993).
[2] A. Finch, John Flint, CLEO'95, Tech. Digest Series 15, Paper CWH2 232 (1995).
[3] W.A. Clarkson, A.B. Nielson, and D.C. Hanna, Opt. Lett. 17 601 (1992).
[4] M. K. Reed, W. K. Bischel, Opt. Lett. 17 691 (1992).
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