Got feedback? If laser back reflections are knocking your oscillator out of mode lock and making it impossible to take data, what you need is a Faraday optical isolator. A Faraday isolator acts as an optical diode, allowing propagation of linearly polarized light in one direction, but blocking light in the reverse direction. It consists of a Faraday rotator, two polarizers and a body to house the parts. The Faraday rotator, in turn, consists of magneto-optically active optical material placed inside a permanent magnet (Nd-Fe-B).
The magneto-optical rod is cut from glass (MOS-10), polished to flatness of ë/10, and has parallelism better than 10 arc seconds. It is anti-reflection coated with residual reflection less than 0.2% (each side) in the 765-835 nm range. Polarizers are air-spaced Glan prisms made of calcite. Entrance and exit faces of polarizers are anti-reflection coated with residual reflection of less than 0.3% in the range. Polarizer transmittance is > 94%. This gives a total transmittance of 80% for our isolators. Models are offered with reverse isolation of 38 dB or 60 dB.
Laser light, polarized or not, enters the input polarizer and is linearly polarized, 0°. Linearly polarized light enters the Faraday rotator rod, and the plane of polarization rotates as the light propagates along the axis of the rod. The Faraday rotator is tuned to rotate the plane of polarization by 45°. The light then passes through the output polarizer P2 whose transmission axis is also at 45°. Any back reflected light re-enters the isolator through the output polarizer and becomes polarized at 45°. It then passes through the Faraday rotator which produces another 45° of rotation and is now polarized at 90° or horizontally, and is stopped by the input polarizer, still at 0°. Thus, the laser is isolated from its own reflections which may occur in the application part of the optical set.
Faraday isolators are factory aligned for a central wavelength of 800 nm. Changing the position of the rod allows tuning over a wavelength range from 765-835 nm.
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