Optical Isolators - The One-Way Valves of Optical Paths

The Faraday effect is a property of transparent materials whereby, when placed in a magnetic field, the polarization of light traveling along the direction of the magnetic field is rotated as a function of distance. More precisely, the rotation of the polarization direction is proportional to the component of the magnetic field along the direction of light propagation. Moreover, the rotation of the polarization direction induced by the Faraday effect is non-reciprocal.

An optical isolator is a non-reciprocal device that utilizes the Faraday effect to enable unidirectional light transmission. Based on their constituent materials, optical isolators can be categorized into three types: bulk, fiber, and waveguide. Fundamentally, optical isolators consist of a polarizer, an analyzer, and a Faraday rotator. The materials for Faraday rotators are often magnetic crystals such as yttrium iron garnet (YIG) and bismuth iron garnet (BIG).

Working Principle of Optical Isolator
Polarizer A has a polarization direction along the x-axis, while the polarization direction of analyzer B is 45 degrees relative to that of A. The Faraday rotator introduces a 45-degree rotation. For forward-propagating light, after passing through polarizer A, the light is linearly polarized along the x-axis. The Faraday rotator then rotates the polarization counterclockwise by 45 degrees, aligning it with the polarization direction of analyzer B, thus allowing the light to pass through. For backward-propagating light, the light emerging from analyzer B is rotated counterclockwise by 45 degrees again by the Faraday rotator, becoming perpendicular to the polarization direction of polarizer A and is therefore completely blocked.

In optical communication systems, optical isolators are used to eliminate the adverse effects caused by reflected light from discontinuities in the optical path on the light source and the optical system. For example, placing an optical isolator between a light source (LD) and an optical transmission system can eliminate the adverse effects of reflected light on the light source, significantly improving the mode and frequency stability of the light source. In an erbium-doped fiber amplifier (EDFA), an optical isolator can prevent self-oscillation caused by line reflections and effectively reduce the noise of the EDFA. Therefore, optical isolators play a crucial role in fiber optic communications, optical information processing systems, fiber optic sensing, and precision optical measurement systems.

Optical isolators utilize the Faraday effect in magneto-optic crystals. They typically consist of three main components: an optical collimator, a Faraday rotator, and a polarizer. Based on their polarization properties, optical isolators can be categorized into polarization-dependent and polarization-independent types.
The schematic diagrams of these two types are shown in Figure 1 and Figure 2.

For a polarization-dependent optical isolator, when light passes through a Faraday rotator, the polarization direction of the light rotates by an angle Φ = FHL under the influence of a magnetic field, where H is the magnetic field strength, L is the length of the Faraday material, and F is the Verdet constant of the material. As shown in Figure 1, when the input light passes through a vertical polarizer, it becomes vertically polarized light. After passing through the Faraday rotator and rotating by 45 degrees, the polarization direction of the analyzer is 45 degrees relative to the polarization direction of the polarizer, allowing the light to pass through. However, the reflected light, after passing through the analyzer and Faraday rotator, continues to rotate by 45 degrees in the same direction, making its polarization direction perpendicular to the polarization direction of the polarizer, and thus the light cannot pass back. Since only vertically polarized light can pass through the optical isolator, it is called a polarization-dependent optical isolator.

As shown in Figure 2(a), a polarization-independent optical isolator has forward input. When an input light wave containing two orthogonal polarizations is separated by a polarization beam splitter, it becomes vertically polarized light and horizontally polarized light. These two beams pass through the Faraday rotator and rotate 45 degrees in the same direction. Then, they pass through a λ/2 waveplate and rotate 45 degrees. The vertically polarized light becomes horizontally polarized light, and the horizontally polarized light becomes vertically polarized light. Finally, they are combined by the polarization beam splitter and output as a single beam. Figure 2(b) shows the evolution of the polarization state of the reverse input light in the isolator. When the SWP horizontally polarized light is refracted and the vertically polarized light is transmitted, the light cannot be output from the forward input end.

Key Performance Indicators of Optical Isolators
(1)Insertion loss: Insertion loss is a key parameter in optical systems. It represents the amount of optical power that is lost when an optical isolator is inserted into a fiber optic link. This loss is typically expressed in decibels (dB) and is caused by factors such as reflection, absorption, and scattering within the isolator.

(2)Isolation: This refers to the transmission loss in the reverse direction of an optical isolator, also known as reverse isolation. Therefore, the measurement method for isolation is the same as that for insertion loss, except that isolation measurement is conducted in the reverse direction, while insertion loss measurement is conducted in the forward direction.

(3)Polarization-dependent(PDL)：PDL indicates the maximum insertion loss variation when the input polarization changes randomly. The maximum variation in insertion loss at the corresponding output port when the optical signal is input with different polarization states. To measure this, a fiber polarization controller is placed in front of the input port of the isolator to obtain various polarization states, and the maximum and minimum loss values are measured. A fiber polarization controller typically has three movable sections (with fiber wrapped around them) that can be adjusted sequentially. These adjustments cause the fiber to twist, inducing birefringence and resulting in changes in the polarization state. Low PDL is important for system optical-power stability.

In addition to the above indicators, there are also allowable optical power, spot size, damage threshold, wavelength, and wavelength width.

The role of magneto-optic crystals in optical systems cannot be underestimated. For instance, they can be used to eliminate the birefringence effect caused by excessive pump power in laser resonators, isolate the laser from the detrimental effects of reflected light, prevent cross-gain saturation in solid-state laser amplifiers, and ensure unidirectional operation in single-frequency lasers to avoid standing waves.

Optical isolators are extensively used in information transmission fields, including fiber-optic communications and fiber-optic sensing. As a passive optical component that leverages optical information technology, optical isolators find versatile applications in optical systems. By inserting an optical isolator between the seed stage and the amplification stage of a pulsed MOPA fiber amplifier, for example, the operational stability of the amplifier can be substantially improved. Therefore, optical isolators are indispensable in industrial pulsed laser systems, fiber-optic communications, and optical information processing systems.