Optical reflection is a significant cause of the performance degradation in fiber lasers and amplifiers. This issue can easily be resolved by the use of optical isolators. An optical isolator is a small device designed to transmit optical signals in one direction. It comes in two versions: polarization maintaining optical isolators and polarization insensitive optical isolators.
While both types of optical isolators block any returning light, the insertion loss in a polarization maintaining optical isolator depends on the input polarization.
Polarization Maintaining (PM) Optical Isolators
Polarization Maintaining (PM) optical isolators are simpler and very compact in design and highly suitable for polarization maintaining fiber applications. They are also used in scenarios where an input free space beam of constant polarization passes across the Faraday optics. In both types of applications, the linearly polarized beam from the source is aligned with the transmission axis of an optical isolator.
While most passive optical components are reciprocal, optical isolators are usually non-reciprocal. Meaning, optical isolators allow an optical beam to pass in the forward direction with minimal losses while preventing it to propagate in the backward direction.
Though different types of optical isolators can be found in the market such as all-fiber isolators, fiber-embedded isolators, fiber Faraday rotator isolators, and waveguide-based isolators, the core a typical commercially available optical isolator consists of a Faraday rotator with 45-degree rotation and a pair of birefringent crystals.
Applications of PM Optical Isolators
PM optical isolators are playing an increasingly important role, especially in modern optical transmission systems and fiber optic systems. They are mainly used in applications that are sensitive to unwanted optical reflections and require polarized light. Even a very low optical reflection can cause a significant increase in laser phase noise, intensity noise, and wavelength stability. Hence, the use of optical isolators in such applications becomes inevitable.
Another crucial application of polarization maintaining optical isolator is distributed-feedback lasers that are widely used in transmission systems. The distributed feedback laser frequency is said to be very sensitive to the reflection coupled back to the laser cavity due to the single-cavity mode. Since the laser gain profile is not flat, the frequency fluctuation also leads to power instability. Hence, it becomes quite essential to achieve isolation from the optical circuitry and its reflection.
In some cases, Fabry-Perot lasers may also require isolation from the system to enhance the power stability. When Fabry-Petro lasers have fewer cavity modes, the need for isolation in the system increases even more.
Besides, PM optical isolators are widely used in telecommunications and other areas such as biotechnology and sensing (such as fiber-optic gyros). Plus, you will also find their extensive usage in other applications such as fiber lasers, fiber amplifiers, and fiber sensors.