An optical circulator is a multi-port (minimum three ports) nonreciprocal passive component.
The function of an optical circulator is similar to that of a microwave circulator—to transmit a lightwave from one port to the next sequential port with a maximum intensity, but at the same time to block any light transmission from one port to the previous port. Optical circulators are based on the nonreciprocal polarization rotation of the Faraday effect.
Starting from the 1990s optical circulators has become one of the indispensable elements in advanced optical communication systems, especially WDM systems. The applications of the optical circulator expanded within the telecommunications industry (together with erbium-doped fiber amplifiers and fiber Bragg gratings), but also expanded into the medical and imaging fields.
Since optical circulators are based on several components, including Faraday rotator, birefringent crystal, waveplate, and beam displacer, we will have to explain these technologies before jumping into the detail of circulator.
1. Faraday Effect
The Faraday effect is a magneto-optic effect discovered by Michael Faraday in 1845. It is a phenomenon in which the polarization plane of an electromagnetic (light) wave is rotated in a material under a magnetic field applied parallel to the propagation direction of the lightwave. A unique feature of the Faraday effect is that the direction of the rotation is independent of the propagation direction of the light, that is, the rotation is nonreciprocal.
The Verdet constant is a measure of the strength of the Faraday effect in a particular material, and a large Verdet constant indicates that the material has a strong Faraday effect. The Verdet constant normally varies with wavelength and temperature. Therefore, an optical circulator is typically only functional within a specific wavelength band and its performance typically varies with temperature. Depending on the operating wavelength range, different Faraday materials are used in the optical circulator.
Rare-earth-doped glasses and garnet crystals are the common Faraday materials used in optical circulators for optical communication applications due to their large Verdet constant at 1310 nm and 1550 nm wavelength windows. Yttrium Iron Garnet and Bismuth-substituted Iron Garnets are the most common materials.
The Verdet constant of the BIG is typically more than 5 times larger the YIG, so a compact device can be made using the BIG crystals. All these materials usually need an external magnet to be functional as a Faraday rotator. Recently, however, a pre-magnetized garnet (also call latching garnet) crystal has been developed that eliminates the use of an external magnet, providing further potential benefit in reducing overall size.
Faraday rotators in optical circulators are mostly used under a saturated magnetic field, and the rotation angle increases almost linearly with the thickness of the rotator in a given wavelength (typically 40 nm) range. The temperature and wavelength dependence of the Faraday rotation angle of the typical BIG crystals at wavelength of 1550 nm is 0.04-0.07 deg/°C and 0.04-0.06 deg/nm, respectively.
Another common material used in the construction of optical circulators is the birefringent crystal. Birefringent crystals used in optical circulators are typically anisotropic uniaxial crystals (having two refractive indices with one optical axis). In an anisotropic medium, the phase velocity of the light depends on the direction of the propagation in the medium and the polarization state of the light. Therefore, depending on the polarization state of the light beam and the relative orientation of the crystal, the polarization of the beam can be changed or the beam can be split into two beams with orthogonal polarization states.
The refractive index ellipsoid for a uniaxial crystal is shown in the above figure. When the direction of the propagation is along the z-axis (optic axis), the intersection of the plane through the origin and normal to the propagation direction So is a circle; therefore, the refractive index is a constant and independent of the polarization of the light. When the direction of the propagation S forms an angle θ with the optic axis, the intersection of the plane through the origin and normal to S becomes an ellipse. In this case, for the light with the polarization direction perpendicular to the plane defined by the optic axis and S, the refractive index, is called the ordinary refractive index no, is given by the radius ro and independent of the angle θ. This light is called ordinary ray and it propagates in the birefringent material as if in an isotropic medium and follows the Snell’s law at the boundary.
On the other hand, for light with the polarization direction along the plane defined by the optic axis and S, the refractive index is determined by the radius re and varies with the angle θ. This light is called the extraordinary ray and the corresponding refractive index is called the extraordinary refractive index ne. In this case ne is a function of θ and can be expressed as
The ne varies from no to ne depending on the direction of propagation. A birefringent crystal with no < ne is called a positive crystal, and one with no > ne is called a negative crystal.
Therefore, the function of a birefringent crystal depends on its optic axis orientation (crystal cutting) and the direction of the propagation of a light. Birefringent crystals commonly used in optical circulators are quartz, rutile, calcite, and YVO4.
HOW OPTICAL CIRCULATOR WORKS
Optical circulators can be divided into two categories.
polarization-dependent optical circulator, which is only functional for a light with a particular polarization state. The polarization-dependent circulators are only used in limited applications such as free-space communications between satellites, and optical sensing.
polarization-independent optical circulator, which is functional independent of the polarization state of a light. It is known that the state of polarization of a light is not maintained and varies during the propagation in a standard optical fiber due to the birefringence caused by the imperfection of the fiber. Therefore, the majority of optical circulators used in fiber optic communication systems are designed for polarization-independent operation.
Optical circulators can be divided into two groups based on their functionality.
Full circulator, in which light passes through all ports in a complete circle (i.e., light from the last port is transmitted back to the first port). In the case of a full three-port circulator, light passes through from port 1 to port 2, port 2 to port 3, and port 3 back to port 1.
Quasi-circulator, in which light passes through all ports sequentially but light from the last port is lost and cannot be transmitted back to the first port. In a quasi-three-port circulator, light passes through from port 1 to port 2 and port 2 to port 3, but any light from port 3 is lost and cannot be propagated back to port 1. In most applications only a quasi-circulator is required.
The operation of optical circulators is based on two main principles.
Polarization splitting and recombining together with nonreciprocal polarization rotation.
Asymmetric field conversion with nonreciprocal phase shift.