**3. Optical MEMS devices**

Components fabricated using optical MEMS devices are finding an increasing number of applications when optical side of telecommunications is in question. They can be divided in two categories: core and peripheral optical MEMS devices (Table 2.). Core optical MEMS devices incorporate fixed structures (V-groves, gratings, etc.) and moving elements (micro‐ mirrors, attenuators, etc.). Peripheral optical MEMS devices are alignment components and structural components. The key area, when optical MEMS for telecommunications are in question, is related to functional optical devices - devices that involve small moving optical parts necessary for more advanced functionality. They are core optical MEMS with moving elements.


**Table 2.** Optical MEMS for telecommunication applications [6]

One of the simplest functional optical MEMS devices is the variable optical attenuator (VOA) [2, 7]. Typically, a moving micro-structure is designed to either partially block or decouple the lightpath. An example of a blocking VOA is shown in Figure 8. The light from the input fiber is collimated with a lens, partially blocked or attenuated by the MEMS device and recoupled to an output fiber. The MEMS device itself could be actuated horizontally or vertically. Actuators could be electrostatic, thermal or electromagnetic. Such a device could be also used as an on-off switch.

The main goal of optical MEMS is providing a high-performance, low-cost solutions for optical switching and wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM). Depending on the specific application, these devices can be wave‐ length insensitive or wavelength selectable. Wavelength and protocol insensitive device for

**Figure 8.** Schematic of variable optical attenuator (VOA)

The full potential of surface micromachining, bulk micromachining and wafer/chip bonding techniques is still being explored. The key activities are continued development of masks and etches that can yield high aspect ratio structures and the development of deposition techni‐ ques. Special attention is being paid to the development of techniques for creating fully 3D

Components fabricated using optical MEMS devices are finding an increasing number of applications when optical side of telecommunications is in question. They can be divided in two categories: core and peripheral optical MEMS devices (Table 2.). Core optical MEMS devices incorporate fixed structures (V-groves, gratings, etc.) and moving elements (micro‐ mirrors, attenuators, etc.). Peripheral optical MEMS devices are alignment components and structural components. The key area, when optical MEMS for telecommunications are in question, is related to functional optical devices - devices that involve small moving optical parts necessary for more advanced functionality. They are core optical MEMS with moving

**Core optical MEMS Peripheral optical MEMS**

One of the simplest functional optical MEMS devices is the variable optical attenuator (VOA) [2, 7]. Typically, a moving micro-structure is designed to either partially block or decouple the lightpath. An example of a blocking VOA is shown in Figure 8. The light from the input fiber is collimated with a lens, partially blocked or attenuated by the MEMS device and recoupled to an output fiber. The MEMS device itself could be actuated horizontally or vertically. Actuators could be electrostatic, thermal or electromagnetic. Such a device could be also used

The main goal of optical MEMS is providing a high-performance, low-cost solutions for optical switching and wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM). Depending on the specific application, these devices can be wave‐ length insensitive or wavelength selectable. Wavelength and protocol insensitive device for

Alignment components Lenses

Structural components Packaging

Beam steering Fiber-guides

structures.

elements.

**3. Optical MEMS devices**

Fixed Structures V-grooves

Moving Elements Mirrors

as an on-off switch.

Connectors Benches Gratings

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Shutters Filters Attenuators

**Table 2.** Optical MEMS for telecommunication applications [6]

all optical switching is optical cross connect (OXC). It replaced conventional optical-electricaloptical (OEO) switching that required conversion of optical signals to electrical ones, switching of electrical signals and conversion of electrical signals to optical ones. OEO switching solution cannot keep up with rapid data rate increase because expensive transceivers and electrical switch core will have to be replaced. However, all optical switching provide avoidance of conversion stages and core switch is independent of data rate and data protocol, making cross connect ready for data rate upgrades. This solution is also cost effective because the use of expensive power-consuming high-speed electronics, transmitters and receivers is avoided. This complexity reduction significantly improves reliability of the device. A typical MEMS OXC consists of micromirrors made of either polysilicon or crystalline silicon, using siliconon-oxide (SOI), coated with metal for reflectivity. The actuation can be electrostatic, magnetic or combination of the two. Two MEMS approaches for optical switching can be distinguished: 2D (planar) switching and 3D free-space switching [8, 9]. In 2D MEMS the switches are digital because mirror position is bistable (Figure 9.). MEMS micromirrors are arranged in a crossbar configuration and all optical paths lie on a planar (2D) surface (Figure 10.). When a micromirror is activated it moves into the path of the beam and directs the light to one of the outputs. Light can also be passed through the matrix without hitting the micromirror allowing adding or dropping optical channels.

**Figure 9.** Schematic of basic element for 2D optical switches

**Figure 10.** MEMS approach for optical cross connect switching

For switching ultra-high N networks planar switching is being replaced with more robust and cost-effective solution. 3D MEMS is a most promising technology for optical cross connect switches with >1000 input and output ports. In 3D MEMS a connection path is established by tilting two micromirrors independently to direct the light from an input port to selected output port (Figure 11.). This approach requires 2-axis mirror cells that usually consist of a gimbal and a mirror [10]. The gimbal connects to the support structure with a pair of torsional springs and another pair of torsional springs connects the mirror to the gimbal. Second pair of springs is rotated 90° with the respect to the first pair. Each pair of springs san be independently actuated and their combination enables two-directional tilt of the mirror (Figure 12.). A drawback of this approach is that a complex and expensive feedback system is required to maintain the position of the mirrors during external disturbances or drift.

**Figure 11.** 3D MEMS approach for optical cross connect switching

**Figure 12.** Schematic of two-axes single crystal silicon mirror

**Figure 10.** MEMS approach for optical cross connect switching

106 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

**Figure 11.** 3D MEMS approach for optical cross connect switching

For switching ultra-high N networks planar switching is being replaced with more robust and cost-effective solution. 3D MEMS is a most promising technology for optical cross connect switches with >1000 input and output ports. In 3D MEMS a connection path is established by tilting two micromirrors independently to direct the light from an input port to selected output port (Figure 11.). This approach requires 2-axis mirror cells that usually consist of a gimbal and a mirror [10]. The gimbal connects to the support structure with a pair of torsional springs and another pair of torsional springs connects the mirror to the gimbal. Second pair of springs is rotated 90° with the respect to the first pair. Each pair of springs san be independently actuated and their combination enables two-directional tilt of the mirror (Figure 12.). A drawback of this approach is that a complex and expensive feedback system is required to

maintain the position of the mirrors during external disturbances or drift.

The output characteristics of an optical amplifier are not uniform across the laser wavelength spectrum. This is problematic for WDM because each segment of spectrum carries a data channel. For that reason, a dynamic gain equalizer (DGE) is needed to level output spectrum [10]. First, channels are separated by dispersing spectrum through assembly of lens and grating (Figure 13.). Then, they are projected onto the DGE and the output of each channel is tuned independently. The tuning can be performed by either an MEMS micromirror array (Figure 14.) or mechanical anti-reflection switch (MARS) (Figure 15.). MARS uses the strip of dielectric (usually silicon-nitride) membrane and air gap that serve as a spatially variable, tunable multilayer dielectric mirror. When the incident signal is spectrally dispersed along the axis of the device defined by an array of strip electrodes, one obtains a simple and compact tunable spectral shaper.

**Figure 13.** Schematic of WDM DGE using MEMS mirror array, lens and grating

In optical WDM provisioning, a data channel may be dropped or added. MEMS technology provides simple solution in optical add-drop multiplexer (OADM) [10]. The tilting-mirror DGE at larger tilting angles can achieve dynamic channel blocking. The full add-drop function of data channels is obtained by inserting channel blocker between an optical splitter and an optical coupler. Independent control for each cannel is wavelength selectable resulting in flexibility to add/drop any combination of data channels. WDM provisioning using wave‐

**Figure 14.** Tilting micromirrors - schematic of operation

**Figure 15.** Schematic of MARS DGE using continuous membrane and finger electrodes [3]

length-selectable switches (WSS) is being often used. The simplest WSS is a channel blocker, with a single input and output fibre, having the capability to power equalize or completely attenuate the WDM channels. The more capable 1×K WSS has a single input and K output fibres, adding the capability to independently route the individual WDM channels among the K fibres (Figure 16.). WSS with higher K requires a large micromirror tilting angle (>8°) and devices using vertical comb drive or double hinged angle amplification [10]. Gentler anglebias response at large angles can be obtained by using alternative design that uses fringe electrical fields.

There are several other MEMS devices for optical networking applications such as polariza‐ tion-mode dispersion (PMD) compensators, tunable laser, etc [11]. New developments in optical MEMS are based on materials technology and cost-effective processing. Optical MEMS are also benefiting from developments of IC industry such as BSOI technology that provided realization of low stress micromirrors, as well as production of other MEMS devices with reproducible mechanical properties and excellent planarity. Continuing progress results in products with better performances such as large-scale switches, variable attenuators, tunable

**Figure 16.** Schematic of MEMS 1×K WSS

filters, etc. High-voltage drivers and sense electronics are being integrated with highly reliable low-loss optical MEMS devices. Accuracy improvement of IC lithography and reactive ion etching provides necessary precision for optical MEMS production. Since there is a great scope for invention in MEMS device structure, materials and processing, optical MEMS will continue to play an increasingly important role in future of optical networks and ultra-high bandwidth communications.

It should be mentioned that besides functional optical MEMS devices, MEMS technology is also being applied in optoelectronic packaging. Ability to provide accurate passive alignment at low cost is one of the important assets of MEMS technology. MEMS approach provides accurate, low-loss optical connections between different guided wave optical components. Highly reliable connections realized using well characterized materials allow construction of complex interconnections. As an illustration, schematic of optical fibre fixed in a V-shaped groove by the triangular microclip is shown in Figure 17.

**Figure 17.** Schematic of optical fibre fixed in a V-groove by the triangular silicon nitride mechanical microclip
