SIMULTANEOUS COMPENSATION OF THERMAL EFFECTS IN GGG-BASED CRYOGENIC FARADAY ISOLATOR FOR LASERS WITH HIGH AVERAGE POWER
A. V. Starobor1, D. S. Zheleznov1, O. V. Palashov1.
1Institute of Applied Physics RAS, 46 Uljanov Street, 603950 Nizhny Novgorod, Russia
Intense development of laser technique and average power enhancement of CW and pulse-periodic lasers demands reduction of thermal effects appearing in different optical elements due to laser radiation absorption. One of the devices subject to significant thermal self-action is Faraday isolator (FI) because of strong absorption (~10-3cm-1) of its magneto-optical elements (MOE).
Therefore, the main important FIs characteristic - the isolation ratio - is determined by thermally induced depolarization. There are different ways to decrease the thermally induced depolarization in FI. For example, cooling FI to cryogenic temperatures (T <100K). The magneto-optical figure-of-merit M (it characterizes the medium impact on the FI isolation degree at high average power) [1], increases by cooling, which leads to a reduction of thermal effects. But use of traditional media terbium-gallium garnet (TGG) for cryogenic FI (CFI) is limited due to depolarization, arises because of inhomogeneous distribution of the angle of rotation of polarization plane caused by the temperature dependence of the Verdet constant. Therefore, in paper [2] gadolinium gallium garnet (GGG) was proposed as a MOE for CFI.
In present work CFI with MOE made of GGG with compensation of thermally induced depolarization was theoretically and experimentally investigated. Compensation effect bases on the principle that radiation passing through depolarizing element and quartz rotator partially compensates in another element. We had chosen new scheme of compensation introduced in [3]. The idea was to add a compensator consists of 2 optical elements: quartz rotator and additional optical element (AOE) to 45-degree FI outside magnetic field. Depolarization induced in MOE partially compensates in AOE. Described scheme makes it possible to save the existing magnetic system and MOE configuration, just adding two optical elements. It also allows making AOE of media with thermo-optical properties close to properties of MOE, but differ from the MOE media, as it was done in this paper.
We have theoretically and experimentally demonstrated the possibility of compensation of thermally induced depolarization in CFIs with a GGG-based MOE and two variants of AOE: made of a GGG crystal and FK51. Optimal parameters for the AOE and reciprocal rotator for these media have been calculated in a wide range of laser powers.
With the GGG crystal, compensation by a factor of 13 was achieved at 150K. The depolarization in the design with compensation at 150K is approximately twice as small as with a single GGG at 80K.
Temperature dependences of thermal lenses in GGG and FK51 glass have been measured experimentally. It is found that the thermal lens decreases in GGG by a factor of 20 when the GGG crystal is cooled from 300K to 80K, and a negative thermal lens increases in FK51 glass by a factor of 1.4. The temperature dependence of the thermally induced depolarization has been measured in the FK51 glass. The reduction of the thermally induced depolarization during cooling was by a factor of 1.7. Based on these data, the temperature dependence of the ratio of thermo-optical constants P/Q for GGG and FK51 glass has been measured for the first time. Since at temperatures below 120 K these ratios in both the media differ insignificantly (<20%), the condition of simultaneous compensation of thermal effects can be quite easily satisfied. By using the FK51 glass, we have demonstrated thermal lens compensation by a factor of ≈3.5 with simultaneous compensation of thermally induced depolarization by a factor of 6.3.
Based on the experimental data and theoretical estimates obtained in our study, we may conclude that when GGG crystals with absorption 10-3 cm-1 are used as MOE and AOE, maximum operating power in excess of 20 kW can be achieved. The use of glass with an optimal P/Q ratio will enable simultaneous compensation of thermal lens and thermally induced depolarization by an order of magnitude.
References:
[1] E.A. Khazanov “Faraday isolators for high average power lasers,” Advances in Solid-State Lasers: Development and Applications, INTECH, Croatia (2010).
[2] A. V. Starobor, D. S. Zheleznov, O. V. Palashov, and E.A. Khazanov “Magnetoactive media for cryogenic Faraday isolators,”
JOSA B, Vol. 28, Iss. 6, pp. 1409–1415 (2011)
[3]. I. Snetkov, I. Mukhin, O. Palashov, and E. Khazanov, “Compensation of thermally induced depolarization in Faraday isolators for high average power lasers,” Opt. Express, Vol. 19, pp. 6366-6376 (2011)
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