Optical switches are key components in optical switching, possessing one or more selectable transmission ports that can convert or perform logical operations on optical signals in optical transmission lines. They have widespread applications in fiber optic network systems.
Optical switches can be divided into two main categories: mechanical and non-mechanical. Mechanical optical switches rely on the movement of optical fibers or optical components to change the optical path; non-mechanical optical switches rely on electro-optic, acousto-optic, or thermo-optic effects to change the refractive index of the waveguide, thereby altering the optical path. The structure and working principles of these two types of optical switches are described below.
Mechanical optical switch
New types of mechanical optical switches include micro-electromechanical system (MEMS) optical switches and metal thin-film optical switches.
Microelectromechanical Systems (MEMS) optical switches are fabricated on a semiconductor substrate material, creating an array of micro-mirrors capable of minute movement and rotation. These micro-mirrors are very small, approximately 140 μm x 150 μm, and under the influence of a driving force, they switch the input optical signal to different output fibers. The driving force applied to the micro-mirrors is generated using thermal, magnetic, or electrostatic effects. The structure of a MEMS optical switch is shown in the figure.
When the micro-mirror is in orientation 1, the input light is output through output waveguide 1; when the micro-mirror is in orientation 2, the input light is output through output waveguide 2. The rotation of the micro-mirror is controlled by a voltage (100-200V). This device features a small size, a high extinction ratio (the ratio of output optical power in the on-state to the output optical power in the off-state), insensitivity to polarization, low cost, moderate switching speed, and insertion loss less than 1 dB. The structure of a metal thin-film optical switch is shown in Figure 3-40. In this type of optical switch, the waveguide core layer is below the bottom cladding, and a metal thin film is above it, with air between the metal thin film and the waveguide. A voltage applied between the metal thin film and the substrate generates an electrostatic force on the metal thin film. Under this force, the metal thin film moves downward and contacts the waveguide, changing the refractive index of the waveguide and thus altering the phase shift of the optical signal passing through the waveguide. In Figure 3-40c, without voltage, the gold thin film is lifted, and the phase shift in both arms is the same, so the optical signal is output from port 2; with voltage applied, the metal thin film contacts the waveguide, causing a π phase shift in that arm, and the optical signal is output from port 1.
Non-mechanical optical switch
Non-mechanical optical switches include types such as liquid crystal optical switches, electro-optic effect optical switches, thermo-optic effect optical switches, and semiconductor optical amplifier switches.
A liquid crystal optical switch is fabricated by creating polarized light beam branching waveguides on a semiconductor material. A groove is etched at a specific angle at the intersection of the waveguides, and liquid crystal is injected into the groove. A heater is placed beneath the groove. When the groove is not heated, the light beam passes straight through; when heated, bubbles are generated within the liquid crystal, and due to total internal reflection, the light changes direction and is output into the desired waveguide.
Electro-optic and thermo-optic effects utilize the phenomenon that the refractive index of certain materials changes with voltage and temperature, thereby enabling the creation of optical switching devices.
Semiconductor optical amplifier (SOA) optical switches achieve switching functionality by changing the bias voltage of the semiconductor optical amplifier.
The main parameters of optical switches include wavelength range, insertion loss, optical return loss, crosstalk, optical input power, polarization-dependent loss, repeatability, switching speed, and lifetime.
Optical filter
Optical filters are wavelength-selective devices that have important applications in fiber optic communication systems, such as filtering noise in optical amplifiers as discussed in the previous section. Especially in WDM fiber optic networks, where each receiver must select the required channel, filters become an indispensable component. Filters are divided into two main categories: fixed filters and tunable filters. The former allows a signal light of a specific wavelength to pass through, while the latter can dynamically select wavelengths within a certain optical bandwidth. The functions and classification of optical filters are shown in the figure.
The transmission characteristics of a practical optical filter are shown in the figure. The main parameters of a fixed-wavelength optical filter are the center wavelength λ2 and the bandwidth Δλ. In addition to these, there are also parameters such as insertion loss and isolation.
Fiber optic grating
Fiber Bragg gratings utilize defects introduced during fiber manufacturing, using ultraviolet light irradiation to create a periodic variation in the refractive index distribution of the fiber core. The filtering effect of a fiber Bragg grating is shown in the figure; wavelengths satisfying the Bragg grating condition are totally reflected, while other wavelengths pass through, making it an all-fiber notch filter.
There are two methods for fabricating fiber Bragg gratings:
(1) Interference method: The interference method utilizes the principle of two-beam interference. A beam of ultraviolet light is split into two parallel beams, creating an interference field outside the optical fiber. By adjusting the lengths of the two interference arms, the period of the resulting interference fringes can be made to meet the requirements for fabricating the fiber Bragg grating.
(2) Phase mask method: The phase mask method uses a pre-fabricated mask. When ultraviolet light passes through the phase mask, interference occurs, creating an interference field on the cylindrical surface of the optical fiber, thus writing the grating into the fiber.
