Introduction

Principle of Operation

Faraday isolators are optical components which allow light to travel in only one direction. In principle, the function of an optical isolator is analogous to that of a diode.

Faraday isolators are composed of three elements:

• Entrance Polarizer
• Faraday Rotator
• Exit Polarizer

Thin film polarizers are commonly used as entrance and exit polarizers and typically are in the form of a special polarizing beam splitter cube (see chapter C). These pola­ri­­zers have an extremely high extinction ratio and are designed for use with high power lasers. The polarizer entrance and exit faces are coated with an antireflection coating for the specified wavelength range. The key element of the Faraday isolator is the Faraday rotator. The rotator consists of a strong permanent mag­net which contains a crystal with a high Verdet constant.

Light of any polarization entering the entrance polarizer exits as linearly polarized light: in this example vertical or 0°. Since laser light is usually linearly polarized, one can match the orientation of the entrance polarizer and the laser by simply rotating the isolator. Light passes through the Faraday rotator. The rotator is made of a Terbium Gallium Garnet (TGG) crystal which is placed in a strong homogeneous magnetic field. The crystal length and the magnetic field strength are adjusted so the light polarization is rotated by 45° on exiting the crystal. In this example a counter clockwise rotation when viewed in the north/south direction of the magnetic field (+45°). The exit polarizer is also oriented at +45°, so that the maximum beam intensity is transmitted.

If light of any polarization, but with a reversed direction of propagation, meets the exit polarizer, it leaves at +45°, passes through the Faraday rotator and is again rotated by +45°. The non-reciprocal nature of the Faraday effect results in the direction of rotation once again being counter clockwise as viewed in the north/south direction of the magnetic field. Upon leaving the Faraday rotator, the polarization has gone through two +45° rotations resulting in a total rotation of +90°. In this polarization direction it is deflected laterally by the entrance polarizer.

Multistage Faraday Isolators

The maximum isolation of the Faraday isolator is limited by inhomogenieties of the TGG crystal and the magnetic field. It is, however, possible to square the extinction ratio by placing two isolators in series and arranging the polarity of the two magnets to be opposite to each other. This way the polarization direction of the transmitted light remains unchanged in the transmission direction and the effect of both magnetic fields is enhanced. This arrangement also leads to a more compact isolator. The strength of this effect depends on the distance between the two magnets and is used to tune the isolator to diffe­rent wavelengths. The adjustment is necessary be­cause the rotational angle of the TGG crystal is wave­-length dependent.

Literature:

1. Bergmann, Schäfer

    Lehrbuch der Experimentalphysik, Band 3, Optik de Gruyter, Berlin 1993

2. R. Wynands et. al.

    A compact tunable 60-dB Faraday optical isolator for the near infrared

    Rev. Sci. Instrum. 63, 5586-5590 (1992)

3. M. Lenzner et. al.

    Sub-20-fs, kilohertz-repetition-rate

    Ti:sapphire amplifier, Optics Letters, Vol. 20, No. 12, 1397-1399