Optical isolators are devices which transmit light in one direction but not in the opposite direction. More precisely, they exhibit a relatively low propagation loss in one direction, but a much higher propagation loss in the other direction.
Essential characteristics of optical isolators are the following:
- the insertion loss
- the degree of isolation, i.e., the power loss for light in the unwanted direction (usually expressed in decibels)
- the return loss (relevant e.g. in optical fiber communications)
- the operation wavelength and the optical bandwidth in which sufficiently high isolation is achieved
- requirements on the input polarization
- the maximum optical power (peak or average power) which can be transmitted
A variant is an optical circulator, having at least three optical ports. Light injected into port 1 will exit at port 2, while input at port 2 will be sent to port 3, and input at port 3 to port 1.
Types of Optical Isolators
The vast majority of optical isolators are Faraday isolators, based on the Faraday effect, i.e., a rotation of the polarization direction caused by a magnetic field. This is a kind of magnetically induced optical activity.
Essential characteristics of Faraday isolators are:
- Many Faraday isolators transmit only light with a suitable direction of linear polarization, but there are also polarization-independent versions.
- The insertion loss can be rather small, and a high degree of optical isolation (e.g. well above 30 dB) can be achieved.
- Faraday isolators can be made for a wide range of optical wavelengths. Although each device works well only within a limited bandwidth, that bandwidth can be substantial – sufficient for a wide range of applications.
- Such devices can be made for operation with very high optical powers by using a sufficiently large beam area. Power limitations arise due to thermal effects (particularly thermal lensing and depolarization) associated with parasitic absorption.
- Relatively compact low-power isolators can also be made. However, it is hard to develop ultra-compact devices for use in photonic integrated circuits. Here, the requirement of a strong magnetic field is a major problem.
Isolators Based on Waveplates
One realize a type of optical isolator using a waveplate instead of a Faraday rotator. While the basic purpose of optical isolation can be fulfilled with that concept, it comes with some limitations, such as the sensitivity to polarization changes in reflected light . For some applications, this approach is superior to the one based on a Faraday isolator, for example because the setup can be substantially more compact.
Optical Isolators Based on Acoustic Effects
As mentioned above, conventional types of optical isolators based on the Faraday effect are hard to implement in the context of photonic integrated circuits. Therefore, alternative methods are under development, where non-reciprocal light propagation is obtained based on completely different physical mechanisms, not involving magnetic fields. In particular, it has been demonstrated that one can utilize rotating sound waves in a ring-shaped waveguide resonator, coupled to a straight waveguide [2, 3]. Here, the rotation of the acoustic field causes a direction-dependent shift of resonance frequencies. If those optical residences are positioned such that the ring is resonant for one propagation direction, resident coupling of the ring to the straight waveguide can cause substantial propagation losses. For the opposite propagation direction, the induced loss can be much lower.
This operation principle allows one to realize rather compact isolators for use on photonic chips. The main limitations are that the usable optical bandwidth is small and that a substantial electric power is required to drive piezo actuators creating the sound wave. However, that power may be substantially reduced with further optimization.
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|||R. Paschotta, Spotlight article on Poor Man's Isolator|
|||H. Tian et al., “Magnetic-free silicon nitride integrated optical isolator”, Nature Photonics 15, 828 (2021), doi:10.1038/s41566-021-00882-z|
|||D. B. Sohn, O. E: Örsel and G. Bahl, “Electrically driven optical isolation through phonon-mediated photonic Autler-Townes splitting”, Nature Photonics 15, 822 (2021), doi:10.1038/s41566-021-00884-x|