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Faraday isolator MT-5/800-AD
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  • Aperture 5 mm dia
  • Wavelength 800 nm
  • Reverse isolation > 38 dB
  • Material of magneto-optical rod MOS-10
  • Transmittance of magneto-optical rod > 99%
  • Transmittance of polarizer > 94% and 99%
  • Transmittance of quartz rotator > 99%
  • Transmittance of Isolator > 92%

rod-holder can be screwed out from the magnet-holder to clean the rod.

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

Fig. 1

 1-body fixing isolator, 2-input polarizer, 3-stop screw of polarizer, 4- stop screw of polarizer-holder, 5-stop screw of isolator, 6-quartz rotator fixing screw.

Optical Broadband Isolator

An optical isolator is a device that 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, quartz rotator 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 Magneto-Optical Glass(MOS-10), polished to flatness l /10 and parallelism better than 1 arc minute. It is anti-reflection coated with residual reflection less than 0.2% (each side) in the range 700-900 nm.

Quartz rotator is a crystal quartz plate with optical axis oriented along the direction of the optical beam. The thickness of the plate is 4,1 mm . The plate turns the polarization of the radiation at 45° by wavelength l =800 nm. It also has anti-reflection coating with residual reflection less than 0.2% (each side) in the range 700-900 nm. Polarizers are air-spaced Glan prism made of natural calcite and air-spaced Roshon prism made of natural calcite and quartz glass. Entrance and exit faces of polarizers are anti-reflection coated with residual reflection less than 0.3% in the range 700-900 nm.

Operation of an optical broadband Isolator

Laser light, polarized or not, enters the input polarizer P 1 and is linearly polarized, say in the horizontal 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 quartz rotator and the net rotation is 0 ° . The output polarizer P 2 is oriented at 0 ° . 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 0 ° . It then passes through the QC (quartz compensator) and Faraday rotator, where QC rotates polarization by 45 ° in one direction, while the Faraday element produces 45 ° in same direction, the summary rotation becomes 90 ° and the backward radiation is stopped by the input polarizer. Thus, the laser is isolated from beam reflections that may occur in the application part of the optical set. Moreover, the wavelength dependence of Faraday rotation for backward beam is substantially compensated by that of rotation in QC.

Optical Isolator is factory aligned at central wavelength 800nm. For this wavelength the Isolator exhibits the widest band of operation. To re-aline the Isolator, please, use this wavelength.

 Wavelength tuning

Verdet constant of the MOS 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 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.

Adjustment of an optical broadband Isolator

The Isolator can be adjusted for any input light polarization. The output light polarization is rotated 90 ° in respect to input. Adjustment of the Isolator can be made in free steps with the help of a light beam at working wavelength and light intensity indicator (e.g. photo-diode + meter).

1. Adjustment of MOS rod (wavelength tuning):

M 1 beam splitter mirror; M 2 back-reflecting beam mirror (reflection ~90%); D 1 , D 2 photo-detectors.

1.1 Turn the isolator 180° so that the polarizer P 2 is the input polarizer. Loosen the screws 4 (fig. 1) and by rotating adjust the polarizer to maximum signal at photo-detector D 1 . Fix the screws 4 (fig. 1).
1.2. Put a beam splitter M 1 before the polarizer P 2 to direct a back-reflected beam using mirror M 2 aside.
1.3. Free the screw-nut 2 (fig. 2) and tune the crystal holder to minimal intensity of light reflected by a beam splitter at photo-detector D 2. Fix the screw-nut 2 (fig. 2).

 2. Adjustment of input polarizer P 1 :

•  Turn the isolator 180° so that the polarizer P 1 is the input polarizer. Assemble the scheme as shown in fig. 4.
•  Loosen the screws 4 (fig. 1) and by rotating adjust the polarizer to maximum signal at photo-detector D 1 .
•  Fix the screws 4 (fig. 1).

3. Adjustment of output polarizer P 2 :

3.1. Put the quartz plate into the P 2 polarizer holder and fix it with screw 6, fig. 1.
3.2 Assemble the scheme as shown in fig. 5.
3.3. Install the output polarizer and tune it to minimal intensity of light reflected by a beam splitter at photo-detector D 2 and to maximum intensity at photo- detector D 1 .
3.4. Fix the output polarizer by the screws 4 (fig.1).

Remarks:

  1. If polarization plane of your installation does not coincide with isolator polarization plane (for example the isolator is adjusted for horizontally polarized light, but your installation radiation is vertically polarized), then you can loosen stop screw of isolator-5 (fig. 1.) and turn the isolator inside the frame until it does coincide.
  2. The arrow on the frame of the isolator shows the direction of input radiation.
  3. This modification of isolator provides a side output of radiation from input polarizer. Use protective covers to avoid eye damage .
  4. Avoid ingress of extraordinary ray from Roshon prism to your photo-detectors in process of adjustment (fig. 6). When optical isolator is aligned extraordinary ray is absent.

Fig.6.Propagations of beams in isolator with Glan Roshon polarizers. When optical isolator is aligned extraordinary ray is absent.