Get to Know All About Optical Isolator

An optical isolator is a device that is designed in such a way that it allows the optical signal to travel in the forward direction along with blocking reflections that would travel in the backward direction. In many applications in optical systems, optical isolators are critically important. For example, to external optical feedback, a single-frequency semiconductor laser is very susceptive. To cause a significant increase in laser phase noise, intensity noise, and wavelength instability even a very low level of optical reflection from an external optical circuit, on the order of − 50 dB, is sufficient. Thus, at the output of each laser diode, an optical isolator is needed in applications needing low optical noise and optical frequency that is stable single.

In sophisticated lightwave systems, an optical isolator is used where one cannot allow light reflection that might perturb the laser oscillator. However, to maintain system performance one can introduce an optical isolator, but at times cost prevents this.

On a Faraday rotator sandwiched between two polarizers, a traditional optical isolator is based. In this particular configuration, the optical signal that comes from the left side then passes through the first polarizer whose optical switch is in the vertical direction, which matches the polarization orientation of the input optical signal. Then, in a clockwise direction, the polarization of the optical signal is rotated by a Faraday rotator by 45 degrees.

Concerning the first polarizer, the optical axis of the second polarizer is oriented 45 degrees, which then allows the optical signal to pass through with little attenuation. The reflected optical signal has to pass through the Faraday rotator from right to left only when there is a reflection from the optical circuit at the right side. As the Faraday rotator is a nonreciprocal device so in the same direction as the input signal, the polarization state of the reflected optical signal will rotate for an additional 45 degrees, thus becoming horizontal, which is perpendicular to the optical amplifier of the first polarizer. In this way, the first polarizer effectively blocks the reflected optical signal and the unidirectional transmission of the optical isolator is assured.