optical isolators (original) (raw)

Author: the photonics expert (RP)

Definition: devices which transmit light in one direction but not in the opposite direction

Alternative term: optical diode

Categories: general optics, photonic devices

• optical elements
  • polarization optics
    * polarizers
    * waveplates
    * Berek compensators
    * Brewster plates
    * depolarizers
    * polarization scramblers
    * Faraday rotators
    * optical isolators
    * Faraday isolators
    * Faraday circulators
    * Faraday isolators
    * Faraday mirrors
    * Babinet–Soleil compensators
    * fiber polarization controllers
    * (more topics)

Related: Faraday rotatorsFaraday isolatorsFaraday circulators

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DOI: 10.61835/bcn Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What are Optical Isolators?

Optical isolators (also called optical diodes) 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

Many isolators are used with free-space beams, while others are coupled to waveguides, e.g. optical fibers.

A variant is an optical circulator (Faraday 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 to the last port back to port 1.

Types of Optical Isolators

Faraday 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 can 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 [1]. 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 [4, 5]. Here, the rotation of the acoustic field causes a direction-dependent shift of resonance frequencies. If those optical resonances are positioned such that the ring is resonant for one propagation direction, resonant 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.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is an optical isolator?

An optical isolator, also called an optical diode, is a device that transmits light in one direction but blocks it in the opposite direction. It exhibits low propagation loss for forward-propagating light and much higher loss for backward-propagating light.

What is the most common type of optical isolator?

The vast majority of optical isolators are Faraday isolators. They operate based on the Faraday effect, where a magnetic field rotates the polarization direction of the light passing through a special optical material.

Why are conventional optical isolators difficult to use in photonic integrated circuits?

Conventional Faraday isolators are hard to integrate into photonic integrated circuits because they require a strong magnetic field, which is difficult to generate and confine on a compact chip.

Are there optical isolators that work without magnetic fields?

Yes, some isolators can be made with waveplates instead of Faraday rotators. For photonic integrated circuits, novel types are being developed which use other principles, for example based on acoustic effects in waveguide resonators.

Bibliography

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