Compensation of thermally induced modal distortions in Faraday isolators

Abstract

Two methods of compensation of thermal lensing in high-power terbium gallium garnet (TGG) Faraday isolators have been investigated in detail: compensation by means of an ordinary negative lens and compensation using FK51 Schott glass possessing a negative. Key thermooptic constants for TGG crystals and FK51 glass were measured. We find that the contribution of the photoelastic effect to the total thermal lens cannot be neglected for either TGG or FK51. We define a figure of merit for compensating glass and show that for FK51, an ordinary negative lens with an optimal focus is more efficient, but requires physical repositioning of the lens for different laser powers. In contrast, the use of FK51 as a compensating element is passive and works at any laser power, but is less effective than simple telescopic compensation. The efficiency of adaptive compensation can be considerably enhanced by using a compensating glass with figure of merit more than 50, a crystal with natural birefringence or gel.

High-vacuum-compatible high-power Faraday isolators for gravitational-wave interferometers

“…The use of thermal birefringence compensation [8] and thermal lens compensation techniques [9,10] allows for significantly greater laser powers over conventional FI designs. Selection of low-absorption optical elements [terbium gallium garnet (TGG) magneto-optical crystals and polarizers] minimizes the amount of absorbed power and subsequent thermal gradients that lead to thermal lensing and depolarization.…”

“…Thermally induced depolarization in TGG depends on the crystal orientation with respect to the incident radiation polarization [23]. Figure 7 shows the results of measurements of thermally induced depolarization for [001] TGG crystals such as those used in Virgo, oriented to produce the maximum thermally induced depolarization.…”

“…Depolarization in two TGGs is equal to quadruple depolarization in a single TGG, in exact accordance with the theory. The solid lines correspond to a theoretical calculation for thermally induced depolarization [23]. Figure 7 shows that while both crystals have rather good optical quality, in order to limit the backpropagating light to less than 10 −3 (30 dB) required in gravitational wave detector operation [24] at the level of its operating laser power [2,5,6], compensation of the thermally induced depolarization using an intermediate quartz rotator is needed for the VPFI design [8].…”

Injection-locked single-frequency laser with an output power of 220 W

“…This resonator has got an asymmetric structure for selective higher order mode discrimination and features a piezo-actuated mirror for active length control. Both systems are optically decoupled by a Faraday isolator specifically designed for high power levels [8]. To injection-lock the high-power oscillator following the Pound-Drever-Hall scheme [9], an EOM was placed between NPRO and power amplifier.…”

“…8, a birefringence and thermal lens compensated Faraday isolator was integrated to shield the 35 W amplifier from the backward propagating light of the ring oscillator [8]. Furthermore, two safety shutters and diagnostic photo-diodes at different positions for an online characterization of the system performance have been included.…”