OPTICAL ISOLATOR

Model 5AFI38 - 800

MANUAL

Specifications

Aperture 5 mm dia

Wavelength range of turnability 765 - 835 nm

Reverse isolation > 38dB

(Extinction ratio better than 1:3000)

Material of magneto-optical rod MOS-10

Transmittance of MOS-10 rod > 98%

Polarizer transmittance > 94%

Rod holder can be screw out from the magnet-holder to clean the glass.

Warning: Avoid kicking and dropping rotator - magnets can reduce the field strength. The magnet-holder should not be detached.

Introduction

An optical isolator is a device which transmits the linearly polarized light in one direction, and stops the most of light (with any polarization) in 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 the permanent magnet (Nd-Fe-B).

Magneto-optical rod is cut from glass (MOS-10), polished to flatness l/10 and parallelism better than 10 arc seconds. It is anti-reflection coated with residual reflection less than 0.2% (each side) in the range 765-835 nm.

Polarizers are air-spaced Glan prisms made of calcite. Entrance and exit faces of polarizes are anti-reflection coated with residual reflection less than 0.3% in the range 765-835 nm. Polarizer transmittance > 94%.

Operation of an optical Isolator

Laser light, polarized or not, enters the input polarizer P₁ and is linearly polarized, say in the vertical plane (0°). Linearly polarized light enters the Faraday rotator rod, the plane of polarization rotates as the light propagates along the axis of the rod. The Faraday rotator has to be 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°. So, the most of light intensity emerges from the output of the Isolator. Any backward 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.

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Wavelength tuning

Verdet constant of the MOS-10 rod is wavelength dependent. Thus, the tuning of the Isolator for the definite wavelength is required. Wavelength tuning is achieved by rotating a threaded housing which contains the MOS-10 rod, thereby moving the rod into or out of the magnet, more or less of the rod is exposed to the magnetic field, thus controlling the amount of rotation. The complete procedure of adjustment of the Isolator is described below.

Optical Isolator is factory aligned at central wavelength 800nm. For another central wavelength please re-align isolator.

Adjustment of an optical Isolator

Adjustment of the Isolator can be made in three steps with the help of a light beam at working wavelength.

1. Adjustment of input polarizer P₁:

MOS-10

1.1. Remove the output polarizer (free the screw and put out);

1.2. Free the screw of input polarizer and tune it to maximal light transmittance;

1.3. Fix the input polarizer by screw.

2. Adjustment of glass (wavelength tuning):

MOS-10

2.1. Put a beam splitter before the input polarizer to direct a back-reflected beam aside;

2.2. Put a mirror at the output of glass to direct transmitted beam back through the MOS-10;

2.3. Tune the glass holder to minimal intensity of light reflected by a beam splitter.

3. Adjustment of output polarizer P₂:

MOS-10

3.1. Install the output polarizer and tune it to minimal intensity of light reflected by a beam splitter;

3.2. Fix the output polarizer by screw.

The isolator can be mounted on rods or by using the assembly surfaces so that the laser polarization can be oriented horizontal or vertical.

The direction of light beam is indicated by the arrow.