NGN - Micro Electro Mechanical Systems
DWDM uses a set of optical wave lengths (or channels) around 1,553 nm with channel spacing of 0.8 nm (100 GHz), each wavelength can carry information up to 10 Gbps (STM 64). More than 100 such channels can be combined and transmitted on a single fiber. Efforts are on to squeeze the channels further and to increase data bit rate on each channel.
Experimentally, transmission of 80 channels, each carrying 40 Gbps (equivalent to 3.2 Tbits/sec) on a single fiber has been tested successfully over a length of 300 km. Deployment of point-to-point and ring-based DWDM optical network requires a newer type of network elements that can manipulate signals on the run without a costly O-E-O conversion. Optical amplifiers, filters, optical add drop multiplexers, de-multiplexers, and optical cross connect are some of the essential network elements. MEMS plays an important role in design and development of such network elements.
MEMS is an acronym for Micro Electro Mechanical Systems. It is used to create ultra-miniaturized devices, having dimensions from a few microns to a couple of centimeters across. These are quite similar to an IC, but with an ability to integrate moving mechanical parts on the same substrate.
MEMS technology has its roots in the semiconductor industry. These are fabricated using batch fabrication process similar to a VLSI. A typical MEMS is an integrated microsystem on a chip that can incorporate moving mechanical parts in addition to electrical, optical, fluidic, chemical, and biomedical elements.
Functionally, MEMS includes a variety of transudation mechanisms to convert signals from one form of energy to another.
Many different types of micro-sensors and micro-actuators can be integrated with, signal processing, optical subsystems, and micro-computing to form a complete functional system on a chip. MEMS’ characteristic ability is to include moving mechanical parts on the same substrate.
Due to small size, it is possible to use MEMS at places where mechanical devices are virtually impossible to put; such as, inside a blood vessel of a human body. Switching and response time of MEMS devices is also less than conventional machines and they consume lesser power.
Application of MEMS
Today, MEMS are finding application in every sphere. Telecommunication, bio-sciences, and sensors are the major beneficiaries. MEMS-based motion, acceleration, and stress sensors are being deployed massively in aircraft and spacecraft to increase safety and reliability. Pico satellites (weighing about 250 gm) are developed as inspection, communication, and surveillance devices. These use MEMS-based systems as payload as well as for their orbital control. MEMS are used in nozzles of inkjet printers, and read/write heads of hard disk drives. Automotive industry is using MEMS in ‘fuel injection systems’ and airbag sensors.
Design engineers are putting MEMS in their new designs to improve performance of their products. It reduces manufacturing cost and time. Integration of multiple functions into MEMS provides higher degree of miniaturization, lower component count, and increased reliability.
Design and Fabrication Techniques
In the last few decades, the semiconductor industry has grown to its maturity. MEMS development is benefited largely by this technology. Initially, techniques and materials used for integrated circuit (IC) design and fabrication were borrowed directly for MEMS development, but now many MEMS-specific fabrication techniques are being developed. Surface micromachining, bulk micromachining, deep reactive ion etching (DRIE), and micro-molding are some of the advanced MEMS fabrication techniques.
Using the micromachining method, various layers of polysilicon, typically 1-100 mm thick, are deposited to form a three-dimensional structure having metal conductors, mirrors, and insulation layers. A precise etching process selectively removes an underlining film (sacrificial layer) leaving an overlaying film referred to as the structural layer capable of mechanical movement.
Surface micromachining is used to manufacture a variety of MEMS devices in commercial volumes. Layers of polysilicon and metal can be seen before and after the etching process.
Bulk micromachining is another widely used process to form functional components for MEMS. A single silicon crystal is patterned and shaped to form high-precision three-dimensional parts like channels, gears, membranes, nozzles, etc. These components are integrated with other parts and subsystems to produce completely functional MEMS.
Some standardized building blocks for MEMS processing and MEMS components are multi-user MEMS processes (MUMPs). These are the foundations of a platform that is leading to an application-specific approach to MEMS, very similar to the application-specific approach (ASIC), that has been so successful in the integrated circuit industry.
All Optical DWDM Networks and MEMS
Today’s telecommunication experts are facing unprecedented challenge to accommodate ever-expanding array of high bandwidth services in telecommunication networks. Bandwidth demand is exponentially increasing due to expansion of Internet and Internet-enabled services. Arrival of Dense Wavelength Division Multiplexing (DWDM) has resolved this technological scarcity and altogether changed the economics of core optical network.
DWDM uses a set of optical wavelengths (or channels) around 1553 nm with channel spacing of 0.8 nm (100 GHz), each wavelength can carry information up to 10 Gbps (STM 64). More than 100 such channels can be combined and transmitted on a single fiber. Efforts are on to squeeze the channels further and to increase data bit rate on each channel.
Experimentally, transmission of 80 channels, each carrying 40 Gbits/sec (equivalent to 3.2 Tbits/sec) on a single fiber has been tested successfully over a length of 300 km. Deployment of point-to-point and ring-based DWDM optical network requires a newer type of network elements that can manipulate signals on the run without a costly O-E-O conversion. Optical amplifiers, filters, optical add drop multiplexers, de-multiplexers and optical cross connect are some of the essential network elements. MEMS plays an important role in design and development of such network elements. We will discuss Optical Add Drop Mux (OADM) and Optical Cross Connect (OXC) in detail.
Breakthrough in Optical Switching
A practical MEMS-based optical switch was demonstrated by scientists at Bell Labs during the year 1999. It functions like a seesaw bar having gold plated microscopic mirror at one end. An electrostatic force pulls the other end of the bar down, lifting the mirror which, reflects the light at a right angle. The incoming light thus moves from one fiber to the other.
The technological success is in fact a building block of variety of devices and systems, such as wavelength add/drop multiplexers, optical provisioning switches, optical cross-connect, and WDM signal equalizers.
Optical Add Drop Multiplexer
Similar to the ring-based SDH/SONET networks, the all-optical DWDM-based networks are beginning to take off. The superiority of ring-based network over mesh network has already been established by SDH network designers. In all-optical ring, bandwidths (ls) can be reserved for protection purpose. Optical Add Drop Multiplexers (OADM) are functionally similar to the SDH/SONET Add Drop Multiplexers (ADM). A group of selected wavelengths (ls) can be added or dropped from a multi wavelength light signal. OADM eliminates costly O-E-O (optical to electrical and back) conversion.
A two dimensional matrix of Optical switches as described above is used to fabricate such OADM offer very little flexibility. Re-configurable Add Drop Multiplexers (R-OADM) on the other hand allows full flexibility. Any of the channel passing through can be accessed, dropped, or new channels can be added. Wavelength of a specific channel can be changed to avoid blocking. Optical switches or OADM of this kind are known as 2D or N2 switches because the number of switching elements required are equal to the square of the number of ports, and because the light remains in a plane of two dimensions only.
An eight-port OADM requires 64 individual micro mirrors with their control on a MEMS device. It is quite similar to ‘cross bar’ switches used in telephone exchanges.
Optical switches of this kind have undergone stringent mechanical and optical tests. Average insertion loss is less than 1.4 db with excellent repeatability of ± 0.25 db over 1 million cycles. 2D/N2 type OADM having configuration larger than 32 × 32 (1024 switching mirrors) become practically unmanageable and uneconomical. Multiple layers of smaller switch fabrics are used to create larger configurations.
Optical Cross Connect
TThe limitation of 2D type optical switch has been overcome by a yet innovative optical switching technology by Bell Labs. It is popularly known as ‘Free Space 3-D MEMS’ or ‘Light Beam Steering’. It uses a series of dual axis micro-mirror as an optical switch. The micro-mirror is mounted on one of the axis of a set of cross-coupled gimbal rings, via a set of torsion springs. This arrangement allows the mirror to move along two perpendicular axes at any desired angle. The mirror is actuated by electrostatic force applied at four quadrants below the mirror. The complete micro-mirror unit is replicated using MEMS technology to form a ‘switch fabric’ of 128 or 256 micro-mirrors.
An array of collimated input fibers is aligned to a set of mirrors that can re-direct the light by tilting the mirror in X and Y-axis to second set of mirrors aligned to collimated output fibers. By precisely aiming a set of mirror on the input and output fibers, a desired light connection can be made. This process is called ‘light beam steering’.
Switching time of 3D MEMS switch is less than 10 ms and the micro-mirrors are extremely stable. Optical cross connects based on this technology offer various unique advantages over the O-E-O type cross connects. OXC are of high capacity, scalable, truly data bit-rate and data format independent. It intelligently routes the optical channels without costly O-E-O conversion. Low footprint and power consumption are additional advantages of the all-optical switching technology.