What is a Fiber Optic Isolator?
The fiber optic isolator, known as an optical diode, photocoupler, or optocoupler, is a passive device that employs magneto-optic properties to enable unidirectional transmission of light. Its primary function is to prevent undesirable feedback to an optical oscillator, specifically the laser cavity that may harm the laser source or trigger unexpected laser issues like mode hop, amplitude modulation, frequency shift, and more. As a result, the isolator plays an essential role in reducing these effects and is a crucial and valuable device in fiber optic communication systems.
The isolator operates based on the Faraday effect, which relies on the Faraday rotor as its main component. Michael Faraday discovered the Faraday effect in 1842. This effect refers to the phenomenon of rotation of the plane of polarized light when it passes through a material that has been subjected to a magnetic field. The rotation of the polarization plane of light is proportional to the strength of the magnetic field and the distance that the light travels through the material. The direction of the rotation is dependent on the direction of the magnetic field, rather than the direction of light transmission.
Construction of Fiber Optic Isolator
The three main components of the fiber optic isolator are an input polarizer, a Faraday rotator that incorporates a magnet, and an output polarizer. Only linearly polarized light can pass through the input polarizer and enter the Faraday rotator. The Faraday rotator's function involves rotating the incoming light at a specific angle before its arrival at the output polarizer. This enables unobstructed passage of light in the forward direction, while the light in the reverse direction is either absorbed or reflected and cannot pass through the optical isolator. The collaborative function of these three components ensures the smooth transmission of light signals
Working Principle of Fiber Optic Isolator
The optical isolator contains a Faraday rotator, an input polarizer, and an output polarizer. The isolator has two operating modes, namely the forward mode and the backward mode, which are classified based on the direction of light.
During the forward mode, the light is first polarized linearly upon entering the input polarizer. As the light beam approaches the Faraday rotator, the rod of the rotator undergoes a 45° rotation, which causes the light to exit the output polarizer at a 45° angle. Similarly, in the backward mode, the light initially enters the output polarizer at a 45° angle. As it passes through the Faraday rotator, it rotates an additional 45° in the same direction. Then, the 90° polarization light turns vertically towards the input polarizer and cannot leave the isolator, leading to either absorption or reflection of the light beam.
Types of Fiber Optic Isolator
- Polarized Optical Isolator
A polarized optical isolator makes use of the polarization axis to ensure that light travels in a single direction. This device permits light to propagate in a forward direction without any prohibition, while effectively blocking any light from travelling in the opposite direction. Additionally, there exist two types of polarized optical isolators: dependent and independent. The latter is generally more complicated in design and is commonly utilized in EDFA optical amplifiers.
- Composite Optical Isolator
The composite optical isolator is a type of independent polarized optical isolator that is employed in EDFA optical amplifiers, which incorporate several other components such as erbium-doped fiber, wavelength-division multiplexer, and pumping diode laser, among others. Due to the presence of numerous components in the EDFA module, this type of isolator is referred to as a composite optical isolator.
- Magnetic Optical Isolator
The magnetic optical isolator is a type of polarized optical isolator but with a focus on the magnetic component of the Faraday rotator. The Faraday rotator, which is typically a rod made of a magnetic crystal under a strong magnetic field that exhibits the Faraday effect, plays a crucial role in this type of isolator.
Parameters that determine the performance of fiber optical isolators
Fiber optic isolators are designed to work at specific wavelengths. The performance of the isolator can be affected by the wavelength of the light passing through it. The isolator should be optimized for the specific wavelength or range of wavelengths used in the system. Isolators that operate in a narrow range of wavelengths, less than 20 nm, are called narrowband isolators. Their performance is measured by the amount of reverse light they can reduce and the bandwidth in which they can maintain isolation within 3 dB of the peak value.
The insertion loss of the isolator is the amount of power lost when the light passes through the device. A low insertion loss is important for maintaining the overall signal strength in the system. The forward direction of the isolator should have an insertion loss of less than 1 dB, while the reverse direction should have a reduction of at least 35 dB for single-stage isolators and 60 dB for double-stage isolators.
- Polarization mode dispersion (PMD)
Polarization mode dispersion occurs when the polarization of the light passing through the fiber is affected by variations in the fiber. Fiber optic isolators should be designed to minimize the effects of PMD. High birefringent elements are used to build isolators, but they can be susceptible to PMD, which is usually between 50 to 100 fs, particularly for single-stage isolators. Double-stage isolators can be designed to cancel out PMD induced by the first stage.
- Polarization-dependent loss (PDL)
Polarization-dependent loss occurs when the polarization of the light passing through the fiber affects the amount of power lost. This reduces the effectiveness of an optical isolator.
Fiber optic isolators have several applications in optical communication systems, especially in high-speed fiber optic networks. They are used to prevent reflections and feedback from reaching the source, which can cause signal degradation and instability.
Optical isolators are used in various other applications beyond optical communication systems. In industries, they can be used to protect laser diodes and other optical components from back-reflections, while in laboratories they are used in experiments that involve sensitive optical measurements.
They are also used in corporate settings for fiber optic sensing, testing, and measurement applications. Fiber optic isolators are utilized in fiber optic sensing systems for detecting temperature, pressure, strain, and vibration changes.