An optical isolator is a passive optical device designed to transmit optical signals in only one direction. It is primarily used to isolate the ports from unwanted optical reflections. The device is characterized by certain features, including:
- Insertion loss
- Degree of isolation
- Return loss
- Operating wavelength
- Optical bandwidth
- Requirements for input polarization
- Maximum optical power
For instance: A 1480nm polarization-maintaining isolator is an optical isolator that operates at a wavelength of 1480nm, while maintaining the state of polarization.
An ideal PM optical isolator is characterized by:
- Low insertion loss
- High degree of isolation
- High extinction ratio
- High return loss
- High precision
- Wide attenuation range
While many optical isolators are used with free-space beams, others are coupled to waveguides, i.e. optical fibers.
What is an optical isolator used for?
1. Semiconductor laser: An optical isolator is mainly used for avoiding unwanted optical reflections, also called feedback. For example, a single-frequency semiconductor laser is highly sensitive to external optical feedback. Even a very low level of optical reflection can result in a substantial increase in laser phase noise, intensity of noise, and wavelength instability. That’s why an optical isolator is needed at the output of each laser diode in applications
2. Fiber Amplifier: Another example where an optical isolator is needed is a fiber amplifier. In such amplifiers, we need unidirectional optical amplification. Bidirectional optical amplification due to optical gain medium can lead to self-oscillation if external optical reflections from connectors and other optical components are strong enough. The use of optical isolators is common among erbium-doped fiber amplifiers (EDFAs) deployed in lightwave systems.
3. Polarizers: Some tunable isolators are designed to adjust the angular orientation of polarizers and optimize the isolation for different wavelengths. In that way, a single device can be used to cover a broad wavelength region.
4. High-Power Applications: Some optical isolators can work at very high optical power levels and thus, are suitable to be used in high-power applications. These high-power optical isolators are used where optical damage may occur due to high peak powers.
5. Fiber Lasers: PM optical isolators are widely used in fiber lasers to avoid unwanted feedback and prevent the possible damage caused by optical reflections while improving amplification.
6. Fiber communication: Laser diodes that are used in fiber communication systems are susceptible to reflected light from the fiber. Hence, PM optical isolators play a great role in fiber communication systems.
The uses and applications of polarization-maintaining isolators are continually increasing as the demand for fiber lasers and fiber communications is increasing in the market.