What are MEMS Optical Switches?

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- GoPhotonics

Apr 13, 2023

Optical switches are devices that can selectively switch or route an optical signal from one fiber to another, without converting the signal into an electrical signal. They can be based on different principles such as electro-optic, thermo-optic, acousto-optic, and micro-electromechanical systems (MEMS) technology. 

A Micro-Electro-Mechanical System, or MEMS, refers to a micro-device or system that incorporates micro-machines, micro-actuators, signal processing, and control circuits. They are important components in optical communication systems that enable the routing and switching of optical signals. The creation of micro-mechanical structures involves various techniques such as photolithography, ion beam etching, chemical etching, wafer bonding, and more.

Figure 1: Schematic of a MEMS optical switch

Electronic technology is used to drive MEMS devices through mechanisms such as electrostatic attraction, electromagnetic force, electrostriction, and thermocouple. Electrostatic attraction involves the use of electrical charges to attract or repel the MEMS structure, while electromagnetic force utilizes a magnetic field to generate motion. Electrostriction, on the other hand, depends on the deformation of a material due to an applied electric field, and thermocouple involves the conversion of temperature gradients into electrical signals to generate mechanical motion. Among these mechanisms, electrostatic attraction is most commonly used technology in MEMS devices due to its simple preparation, ease of control, and low power consumption. The schematic of a MEMS optical switch is shown in figure 1.

To create a MEMS optical switch, a number of small mirrors are etched onto a silicon crystal. A micromirror is used to reflect light beams in a MEMS optical switch. By utilizing either electrostatic or electromagnetic force, the microarray can be rotated to alter the direction of the input light, effectively turning the light path on and off.

Structure of MEMS Optical Switch

Figure 2: Structure of a MEMS optical switch

The MEMS optical switches have two basic types of port configurations: Matrix type configuration and Fan-out type configuration. In the matrix-type or symmetric configuration, there are multiple input and output ports, arranged in a matrix format. 2 x 2, and N x N are examples of this type of configuration. When the optical switch has N x N ports, it's commonly referred to as an optical cross-connect (OXC). In the case of a fan-out type or asymmetric configuration, there is a single input port and multiple output ports arranged in a fan-out pattern. Some examples of this type are 1 x 1, 1 x 2, 1 x 3, 1 x 4, etc.

The structure of a MEMS optical switch is shown in figure 2. 

Figure 3: Structure of a 1 x N MEMS optical switch

The structure of a MEMS-based 1 x N optical switch, consisting of a MEMS torsion mirror, a collimating lens, and a multi-fiber pigtail. The MEMS mirror is usually assembled on a TO (Transistor Outline) base and then the collimating lens is attached to the sub-assembly through the TO cap. Hence, the sub-assembly is actively aligned with the multi-fiber pigtail. The structure of a 1x N MEMS optical switch is shown in figure 3.

Principle of MEMS Based Optical Switches

The working principle of an optical switch depends on the specific technology used in the switch, which can be based on different principles such as electro-optic, thermo-optic, acousto-optic, and micro-electromechanical systems (MEMS) technology. When the optical signal enters the switch, it is directed to the switching element, which then redirects the signal to the desired output fiber. The switching element is controlled by an external signal, such as an electrical or acoustic signal, which determines the routing of the optical signal.

Figure 4: Schematic of MEMS based optical switch

In the case of MEMS optical switches, it operates based on micro-electro-mechanical systems that employ optical micro-mirrors or arrays of them to manipulate the direction of light beams and alter optical paths. They work on a very simple principle by using tiny mirrors that can be moved by electricity or magnetism to control the direction of light beams. By changing the angle of these mirrors, the switch can route light to different places, turning the light on or off as needed. The schematic of a MEMS based optical switch is shown in figure 4.

When an input optical signal is incident on the micro-mirror, the angle of the mirror is changed, and the light is reflected to a different output terminal. This movement of the mirror is controlled by applying a voltage or magnetic field to the electrostatic actuator integrated into the mirror structure.

Working of MEMS Optical Switches

Figure 5: Working of MEMS optical switch

The working of MEMS optical switch is shown in figure 5. The light from the input fiber is collimated onto the first rotating mirror. Then, it is directed onto the second mirror at varying angles determined by the first mirror's positioning. This leads to different light configurations, each travelling in a unique direction. At each output channel, the light is collimated back into a fiber. The MEMS microcontroller controls the mirror's orientation by mechanically tilting it to the user-defined switch position.

Types of MEMS Optical Switches

In terms of spatial structure, MEMS-based optical switches can be categorized into two types: 2D switches and 3D switches.

2D switches have mirrors that rotate only in one plane, while 3D switches have mirrors that can rotate in multiple planes, providing more flexibility in routing the optical signal.

Figure 6: Structure of 2D MEMS optical switch

The 2D MEMS optical switch incorporates a rotating mirror on the silicon substrate by utilizing surface micromechanical manufacturing techniques. By controlling the micro mirror's rotation, the collimated light can be directed to the designated output terminal. When the micro mirror is horizontal, the light beam can pass through it. And, when the micro mirror rotates perpendicular to the silicon substrate, it reflects the incident light beam, enabling the light to pass through the corresponding output port of the micro mirror.

The add and drop ports are usually located on the edge of the switch, away from the main grid of input and output ports in a 2D MEMS optical switch. They are used to add or drop individual channels from a multi-channel optical signal. An add port is used to add a new channel for an existing optical signal and a drop port, on the other hand, is used to drop a specific channel from a multi-channel optical signal.

Figure 7: Structure of 3D MEMS optical switch

In the case of 3D MEMS optical switch, the micro mirror can rotate along two axes, allowing for different angles to alter the output of the optical path. The multiple micro-mirrors of this 3D switches are arranged in a grid pattern, and each mirror can be rotated in two directions, providing more degrees of freedom for routing the optical signal. These arrays typically appear in pairs, with the input light reaching the first array mirror, reflecting onto the second array mirror, and then reflecting again to the output port.

Advantages of MEMS Optical Switches

  • Low insertion loss
  • Low crosstalk
  • Low polarization sensitivity
  • High extinction ratio
  • High switching speed
  • High reliability
  • Low power consumption
  • Compact size
  • High integration

Disadvantages of MEMS Optical Switches     

  • High cost
  • Limited scalability
  • Sensitivity to environmental factors
  • Limited wavelength range

Applications of MEMS Optical Switches

MEMS optical switches are used in optical cross-connects to enable the routing of optical signals between different input and output fibers. 

These devices can add and drop individual wavelengths of light from a multi-wavelength optical signal. They can be used to switch individual wavelengths of light to different output fibers.

MEMS optical switches can be used to switch optical packets between different input and output fibers in an optical packet-switching network. They can also be used in optical test and measurement equipment to switch between different optical channels for testing and analysis.

These optical switches are also used as optical sensors to detect the presence or absence of light in a particular fiber or channel.