Two Stage Faraday Isolators

Tunable Diode Laser Isolators DLI

Overview

Diode lasers are extremely sensitive to reflected radia-tion. Standard Faraday isolators achieve a 30dB isolation, which is not sufficient to suppress undesirable feedback. The DLI was developed for the special requirements of diode lasers. At the heart of this development is the use of two coupled isolator stages.

This arrangement combines the exit polarizer of the first stage with the entry polarizer of the second stage to form one central polarizer. The isolator can be coarse tuned by altering the effective magnetic field in the two stage isolator. A precise wavelength adjustment is possible by rotating the central polarizer with a micro­meter set screw. The output polarization and the beam position will not change with these adjustments. This allows the influence of the smallest feedback effects on the laser to be quantitatively examined.

Operation

This isolator is easily integrated into existing equipment. It can be adjusted to match any wavelength without changing the laser polarization or displacing the laser beam.

The blocked radiation is deflected out of the isolator at 90° with respect to the beam axis. It is not absorbed by the interior of the isolator, but is available at the side surfaces of the polarizer and the exit window.

All optical surfaces are antireflection coated and the surfaces normal to the beam axis are tilted. The polari-zers and the lateral exit window are mounted to allow easy cleaning of the external optical surfaces. This guarantees that the isolation is not reduced by residual reflections and scattering from the isolator.

Adjusting the isolator to the desired wavelength is carried out with the micrometer on top of the isolator and extending or retracting the cylindrical part (4) as shown in the drawing on the next page. When the part is com­pletely pushed into the isolator casing, the isolator can be tuned from 810 to 890nm with very low variation in insertion loss. The cylinder can be extended 20mm by turning the two screws: item 6 on the drawing. In this position the isolator can be adjusted from 745nm to 815nm. If somewhat higher insertion losses are accep­table, the isolator can be adjusted over a con­siderably larger range. An individually calibrated adjustment curve is provided with each isolator.

The isolator can be mounted by using the assembly surfaces (7) so that the laser polarization is oriented either horizontally or vertically. The isolator can also be moun­ted on rods and with the help of clamping devices through a detachable baseplate.

The polarizers (1,5) are protected by removable security rings (8). The rings can be oriented to block (protected position) or provide access to the diverted radiation.

Applications

These isolators are typically used to improve the power and frequency stability of diode lasers used in spectroscopy, interferometry and precision control and alignment appli­cations. They are also useful for injection locking and with Ti: Sapphire lasers to achieve stable modelocking.

• especially high isolation
• wide tuning range
• two coupled isolator stages with two crystals
• choose between two wavelength ranges
• no beam displacement
• input polarization = output polarization
• TGG crystal (high Verdet constant)
• rare earth magnet (high remanence)
• easily integrated into existing equipment
• antireflection coatings on all optical surfaces
• individually calibrated adjustment curve supplied with each isolator
• mounting: four M4 threads on a 40x40 mm square pattern on bottom of the casting; removable 50x60 mm base plate; or mounting of the cylindrical isolator part (Ø 48 x 20 mm)