DWDM uses a set of optical wave lengths (or channels) like 1,553 nm with channel spacing of 0.8 nm (100 GHz), each wavelength includes information up to 10 Gbps (STM 64). Here more than 100 such channels can be carried and transmitted on a single fiber. Make a trial to squeeze the channels further and to increase data bit rate on each channel.
Let’s experiment that each 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. Each ring-based DWDM optical network deployment 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. Here MEMS plays key role in design and development of such network elements.
MEMS stands for Micro Electro Mechanical Systems which is used to create ultra-miniaturized devices, having dimensions from a few microns to a couple of centimeters across. MEMS are similar to an IC with an ability to integrate moving mechanical parts on the same substrate.
MEMS technology works better in the semiconductor industry. Fabricate these with batch fabrication process which is 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.
While MEMS consists of several 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 make a complete functional system on a chip. MEMS’ is able to include moving mechanical parts on the same substrate.
Because of its small size, it is very easy to use MEMS across places where mechanical devices are virtually impossible to add like 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.
Nowadays, MEMS are exploring an application across domains like telecommunication, bio-sciences, and sensors. But MEMS-based motion, acceleration, and stress sensors are more find 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 also used in nozzles of inkjet printers, and read/write heads of hard disk drives. Currently Automotive industry is using MEMS in ‘fuel injection systems’ and airbag sensors.
The other side design engineers are also using MEMS in their new designs to improve performance of their products. It will minimize the manufacturing cost and time. While combining the multiple functions into MEMS offers more miniaturization, lower component count, and increased reliability.
While, semiconductor industry has developed more in recent times. MEMS development is grown up by this technology. Initially, techniques and materials used for integrated circuit (IC) design and fabrication were adopted for MEMS development and many MEMS-specific fabrication techniques are being developed. Surface micromachining, bulk micromachining, deep reactive ion etching (DRIE), and micro-molding are known as the advanced MEMS fabrication techniques.
With the micromachining method, several layers of polysilicon, typically 1-100 mm thick, are combined to form a three-dimensional structure having metal conductors, mirrors, and insulation layers. A precise etching process eliminates the underlining film (sacrificial layer) leaving an overlaying film referred to as the structural layer capable of mechanical movement.
Surface micromachining is used to produce different types of MEMS devices in commercial volumes. Layers of polysilicon and metal can be seen before and after the etching process.
Bulk micromachining is another way to process the functional components for MEMS. A single silicon crystal is designed to build a high-precision three-dimensional parts like channels, gears, membranes, nozzles, etc. These components are combined 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 basic foundations of a platform t to lead an application-specific approach to MEMS, which is same with the application-specific approach (ASIC),which is more successful in the integrated circuit industry.
It’s a big challenge for telecommunication experts to accommodate ever-expanding array of high bandwidth services in telecommunication networks. Bandwidth demand is increasing in a great way due to expansion of Internet and Internet-enabled services. Arrival of Dense Wavelength Division Multiplexing (DWDM) has solved this technical issue and 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). Nearly 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.
A practical MEMS-based optical switch was invented by scientists at Bell Labs during the year 1999. It works like a seesaw bar having gold plated microscopic mirror at one end. An electrostatic force drags at the other end of the bar down, lifting the mirror which, reflects the light at a right angle. The incoming light then moves from one fiber to the other.
The technology is connecting variety of devices and systems like wavelength add/drop multiplexers, optical provisioning switches, optical cross-connect, and WDM signal equalizers.
Like ring-based SDH/SONET networks, the all-optical DWDM-based networks are started to launch. 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 equal in functions to the SDH/SONET Add Drop Multiplexers (ADM). Here a massive group of chosen 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.
Above discussed two dimensional matrix of Optical switches are used to fabricate OADM to offer very little flexibility. Re-configurable Add Drop Multiplexers (R-OADM) on the other hand allows full flexibility. The channels which are passing through can be accessed, dropped, or new channels can be added. Each channel wavelength 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.
A MEMS device requires an eight-port OADM with 64 individual micro mirrors to control on a MEMS device. It is same as ‘cross bar’ switches used in telephone exchanges.
Various optical switches of this kind have experienced dynamic 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.
Bell Labs technology is used to overcome the limitation of 2D type optical switch. It is popularly known as ‘Free Space 3-D MEMS’ or ‘Light Beam Steering’. This technology uses a series of dual axis micro-mirror as an optical switch. The micro-mirror is added on one of the axis of a set of cross-coupled gimbal rings, via a set of torsion springs. This process enables the mirror to move along two perpendicular axes at any desired angle. Usually the mirror is actuated by electrostatic force applied at four quadrants below the mirror. The complete micro-mirror unit presented with MEMS technology to form a ‘switch fabric’ of 128 or 256 micro-mirrors.
A group of collimated input fibers are 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. With this a set of mirror on the input and output fibers, required light connection can be formed. This process is called ‘light beam steering’.
Let’s switching time of 3D MEMS will switch in less than 10 ms and the micro-mirrors are extremely stable. Optical cross connects offers various unique advantages are across the O-E-O type cross connects. OXC are of high capacity, scalable, truly data bit-rate and data format independent. It forms the optical channels without costly O-E-O conversion. The main advantages of all-optical switching technology are Low footprint and power consumption.
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