Passive Optical Applications
Two examples of using Isomet's crystals passively are outlined below.
Quartz optical rotators based on optical activity have been used extensively in a
wide variety of optical systems because they are insensitive to rotation and only
require alignment normal to the incident beam. However, due to small rotatory
power at 1550 nm (~1.4°/mm), it is impractical to use quartz rotators in
polarization-based passive fiber-optic telecommunications components and other
near-IR optical systems. We have recently discovered TeO2 can be grown in both
dextro-rotatory and laevo-rotatory orientations. This allows both clockwise and
counter-clockwise polarization rotation to be achieved with crystals of reasonable
length. Our new TeO2optical rotators exhibit the following characteristics:
• Wide transparency: 0.33-5.0 µm
• High rotatory power in the near-IR (~11.5°/mm@1550 nm)
• Easy alignment: rotator is aligned normal to the incident beam (no rotational alignment required)
• Very low temperature sensitivity
• Low cost
• On either side of the design wavelength, the polarization remains purely linear (unlike half-wave plates where the polarization becomes elliptical)
• For birefringent filters used in telecommunications, all filtering crystals can be fabricated with the same orientation, and multiple TeO2 rotators can be used to set the polarization rotation for each crystal. Thus, mechanical rotation or multiple crystal orientations are eliminated.
• Low loss: visible/near-IR anti-reflection coatings with R<0.15% per surface can be easily applied. The material itself has negligible absorption from 400-4000 nm.
Thermal Compensation Technology
We have recently developed the capability to design and produce passive telecom components based on birefringent materials. One of the most critical requirements for de-interleavers and demultiplexers (and other passive filtering components) is frequency stability over the required 0 to 70°C operating range. Our athermal filter designs utilize a dual-crystal approach where a second crystal is used to compensate the temperature drift of a first wavelength-filtering crystal.