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optical isolator[′äp·tə·kəl ′ī·sə‚lād·ər]
A device that is interposed between two systems to prevent one of them from having undesired effects on the other, while transmitting desired signals between the systems by optical means. Optical isolators are used for both electrical systems and optical systems such as lasers.
An optical isolator for electrical systems is a very small four-terminal electronic circuit element that includes in an integral package a light emitter, a light detector, and, in some devices, solid-state electronic circuits. The emitting and detecting devices are so positioned that the majority of the emission from the emitter is optically coupled to the light-sensitive area of the detector. The device is also known as an optoisolator, optical-coupled isolator, and optocoupler. The device is housed in an integral opaque package so that the only optical emission impinging on the detector is that produced by the emitter. This configuration of components can perform as a solid-state electronic transformer or relay, since an electronic input signal causes an electronic output signal without any electrical connection between the input and the output terminals.
Optical isolators are used in electrical systems to protect humans or machines when high-voltage or high-power equipment is being controlled. In addition, optical isolators are used in electronic circuit design in situations where two circuits have large voltage differences between them and yet it is necessary to transfer small electrical signals between them without changing the basic voltage level of either.
The need for optical isolation has broadened considerably since the advent of lasers. It is often necessary to prevent light from reentering the laser, irrespective of any electrical consideration. One example is a small laser followed by high-power laser amplifiers. If the powerful amplified light reenters the small (master oscillator) laser, it can destroy it. Another example is a frequency-stabilized laser, whose oscillation frequency is perturbed by reentering (injected signal) light.
A polarizer-plus-quarterwave-plate isolator prevents laser light from reentering the laser when the light is scattered back by specular reflectors. This device cannot ensure isolation if there is diffuse reflection or if polarization-altering (birefringent) optics are encountered. Another limitation of this isolator is that the transmitted light is circularly polarized.
In contrast to the quarter-wave polarizer isolator, the Faraday isolator can provide truly one-way transmission irrespective of polarization changes from the exit side if an exit polarizer (which passes light that has undergone the Faraday rotation after passing though the entrance polarizer) is used in addition to the entrance polarizer. For example, it isolates against diffuse reflections and any light source on the exit side.
The isolation properties of an acoustooptic deflector are based on the fact that light deflected by it is shifted in frequency by an amount equal to the acoustic frequency. The reflected beam, passing through the deflector a second time, is again shifted in frequency by the same amount and in the same sense if the deflector is operated in the Bragg mode. Hence, the reflected light that is returned to the laser is shifted in frequency by an amount 2f, where f is the frequency of the acoustic wave. Provided the frequency of the light returned to the laser is not close to any resonant frequency of the laser cavity, it will not perturb the laser and will simply be reflected from the output mirror.