An Introduction of MEMS Variable Optical Attenuator (VOA)

A MEMS VOA (Micro-Electro-Mechanical Systems Variable Optical Attenuator) is an optical device that utilizes micro-electromechanical systems to control the intensity of optical signals. By adjusting the tilt angle of a micromirror, the transmission path of light within the device is altered, thereby achieving optical attenuation.

A MEMS VOA operates on the principle of a micromirror whose tilt angle can be electrically controlled. Incident light is reflected off the micromirror at an angle determined by the mirror's orientation. By adjusting the mirror's tilt, the coupling efficiency of the reflected light into the fiber can be varied, enabling precise attenuation of the optical signal. In a MEMS-based VOA, a dual-fiber collimator provides input and output ports. The collimated beam is redirected by the MEMS micromirror, establishing an optical path between the ports. The rotation of the micromirror causes beam deflection, leading to optical power attenuation.

In recent years, the manufacturing process of MEMS chips has matured significantly, driving the widespread adoption of MEMS VOAs. MEMS-based products have shown clear advantages in terms of both cost and performance, especially in optical networks. MEMS VOAs can be categorized into reflective and diffractive types, as illustrated in the figure.

Figure (a) depicts the operating principle of a reflective VOA, which employs a silicon-based micromirror. In an unblocking VOA configuration, light enters through a dual-fiber collimator and impinges on the micromirror at an oblique angle. The application of an electric field induces a torsional deformation of the micromirror, resulting in a change in the reflection angle. This variation in reflection angle leads to incomplete coupling of the reflected light into the output fiber, thereby enabling precise attenuation of the optical signal. In the absence of an applied voltage, the micromirror remains planar, facilitating maximum coupling efficiency.

A diffractive VOA, as depicted in Figure (b), leverages dynamic diffraction grating technology. The grating comprises an array of parallel micro-gratings, with each grating's upper surface coated with a 200-300 nm aluminum layer for both electrical conduction and light reflection. The lower surface incorporates a dual-spring structure of Si3N4 and SiO2 to provide mechanical flexibility. The thickness of the underlying etched air gap is tailored to the specific spectral band of the application. Under the influence of an applied voltage, electrostatic forces induce a downward displacement of the spaced gratings, giving rise to a diffraction grating effect, as shown in Figure 5(b). By controlling the applied voltage, the intensity of the first-order diffracted light can be modulated, enabling precise attenuation of the optical signal.
Initially employed in imaging and display systems, this dynamic diffraction grating has demonstrated exceptional performance metrics. These include rapid response times, high precision in attenuation control, a large extinction ratio, and robust resistance to fatigue and wear. Such properties render it an ideal candidate for integration into a wide array of optical communication components, such as optical switch arrays.

Types of MEMS VOA
GLSUN manufactures various types of MEMS VOAs to meet different application requirements:
Single-mode VOA: Suitable for single-mode fiber systems, providing precise control of optical signal intensity under single-mode transmission conditions. It demands a high level of polarization state for the light beam and offers low insertion loss, high stability, and a wide operating wavelength range.
Multimode VOA: Used in multimode fiber networks to achieve dynamic optical power control by adjusting the intensity of multimode light beams. It has a relatively low requirement for the polarization state of the light beam and is suitable for data centers and short-distance communication.
Polarization-maintaining VOA: Specifically designed to maintain the polarization state of the optical signal, suitable for polarization-sensitive systems such as coherent communication and fiber optic gyroscopes.
Mini MEMS VOA: Features small size and high integration, making it ideal for applications with strict space and power consumption requirements.

Applications of MEMS VOA
MEMS VOAs find extensive applications in various fields, including optical communication, fiber optic sensing, and optical measurement. Some key applications include:
Optical power management: MEMS VOAs are used for precise control and equalization of optical power in WDM systems and for protecting optical amplifiers from overpower conditions.
Optical network testing: MEMS VOAs enable dynamic performance testing and calibration in optical networks.
Data center interconnects: MEMS VOAs support adaptive optical power adjustment in high-speed data center interconnects.
Laser output control: MEMS VOAs are used to control the output power of lasers.
Fiber optic sensing: MEMS VOAs are employed for signal modulation and demodulation in fiber optic sensing systems.

With the increasing trend towards higher density integration in optical communication equipment, MEMS VOAs are being designed to be smaller and more power-efficient. To meet the growing demand for dynamic optical networks, the next generation of optical communication systems requires devices with faster response times, which can be addressed by high-speed MEMS VOAs. By integrating with optical switches and splitters, MEMS VOAs can enable the construction of intelligent optical network nodes, offering greater automation and flexibility. Furthermore, future MEMS VOAs will be capable of operating across a wider wavelength range, thereby accommodating the needs of multi-band optical communication networks.

MEMS variable optical attenuators (VOAs) play a crucial role in optical communication due to their high performance, small size, and flexibility. As optical communication technology continues to evolve, MEMS VOAs are advancing towards higher integration, faster response speeds, and greater intelligence, providing strong technical support for the construction of next-generation optical networks.