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Why Is Flat Response Required? In the all optical network (AON), the optical signals passed tens of nodes before reaching the destination node, as shown in Fig.1. The ROADM nodes are usually composed of wavelength selective switches (WSS), multiplexers/demultiplexers and optical switches. The wavelength multiplexers/demultiplexers are optical filters, including TFF-based WDM devices, arrayed waveguide gratings (AWG) and optical interleavers.
Why is AWG demanded? As we know, DWDM technology enables transmission of dozens of wavelengths in a single fiber, which expands the capacity of optical fiber communication enormously. The first mux/demux modules for DWDM system are based on thin-file filters (TFFs), as shown in Fig.1 and Fig.2. Both are designed in serial structure. Different wavelengths travel different number of devices in the module and result in different power loss. The loss uniformity degrades with increment of port number. Meanwhile, the maximum loss at the last port is another limitation on the port number. Thus the TFF-based WDM modules are usually limited to be ≤16 channels.
What’s CDC-F ROADM? A ROADM node has a network node interface (NNI) and a user network interface (UNI). The NNI interconnects DWDM signals from/to multiple directions. The DWDM signals are switched between different directions in wavelength granularity. The UNI downloads signals designated to the node and uploads signals from the node in wavelength granularity. In order to realize non-blocking switching and adding/dropping of wavelengths, the new generation of ROADM nodes are required to be colorless, directionless and contentionless (CDC ROADM).
Review of Optical Fiber Communication Based on industrial view, the development of optical fiber communication has experienced four stages and is now in the fifth stage. In 1970, the emergence of optical fiber with low loss and laser diode operating at room temperature initialized optical fiber communication.
However, the wide application of optical fiber communication was in 1990s. USA government released the plan named “National Information Infrastructure (NII)” in 1993. Optical fiber communication technologies were important parts supporting NII and were developed rapidly. The symbolic technology in the period is DWDM, which expands the transmission capacity of optical fiber communication by tens of times. The development of optical fiber communication slowed down in 2001 with the burst of internet bubble.
As we know, optical fiber communication is one of the enabling technologies for Internet and changed the world. The advantage of optical fiber communication is to transmit dozens of wavelengths in a single optical fiber, which is called wavelength-division multiplexing (WDM). The basic devices for WDM transmission are optical filters, which can be realized by fused biconical taper (FBT), thin film filter (TFF), arrayed waveguide grating (AWG) and optical interleaver. TFF and AWG are most commonly used in the WDM system. This paper discusses TFF-based WDM devices.
Thin Film Filter Fabry-Perot interferometer (FPI) is a commonly used interferometer for spectrum filtering. The structure of a FPI is shown in Fig.1, which consists of two glass plates spaced by a spacer with precise thickness. The inner surfaces of the plates are coated for partial reflection and the outer surfaces are usually anti-reflection (AR) coated.
Application of OXC Optical cross connect is a matrix switch usually with N×N ports. The OXC can be used to construct a CDC ROADM (Colorless, Directionless, and Contentionless Reconfigurable Optical Add/Drop Multiplexer), as shown in Fig.1 .
OXC Constructed by 1×N Optical Switches The OXC can be constructed by 1×N optical switches, as shown in Fig.2. It requires 2N 1×N optical switch to construct a N×N OXC. Thus the size and cost of the OXC module increase rapidly with the incensement of port number N. OXC of this structure is usually limited to below 32×32 ports.
Optical switches are widely employed in optical fiber communication systems. The approaches for an optical switch are variable, including mechanical optical switch, thermos-optic switch, acousto-optic switch, electro-optic switch, magneto-optic switch, liquid crystal optical switch, MEMS optical switch, et. al. MEMS optical switches are characterized by compact size, low power consumption and good scalability, which enable their wide applications.
Optical switch is a multiport device. The port configurations include 2×2, 1×N, N×N. Optical switch with N×N ports is usually called OXC (optical cross connect). According to the difference in port configurations, different MEMS chips are employed for realization of the devices. We will discuss optical switches with 2×2 and 1×N ports in this paper.
MEMS technologies are widely employed in optical fiber communication system. The combination of MEMS and optical technologies is usually named MOEMS (Micro-Opto-Electro-Mechanical Systems). The most widely applied MOMES devices include VOA (Variable Optical Attenuator), OS (Optical Switch), TOF (Tunable Optical Filter), DGE (Dynamic Gain Equalizer), WSS (Wavelength Selective Switch) and OXC (Optical Cross Connect).
VOA is widely employed in optical fiber communication system for optical power equalization. Among the variable approaches, MEMS VOAs are characterized by small size, low cost and easy fabrication. There are mainly two types of MEMS VOAs in applications, MEMS shutter and MEMS mirror. The first is usually thermally actuated and the second is usually actuated by electrostatic force.
MEMS is a micro-electro-mechanical system fabricated with IC process. It is usually based on Si-wafer. The mechanical structures are fabricated with processes such as lithography, ion-beam etching, chemical etching and wafer bonding. Meanwhile, electrodes are fabricated along with the mechanical structures for electrical controlling.
MEMS in Life The fist rotary MEMS motor was born in UC Berkley in 1988, as shown in Fig.1 . Then in 1989, the first lateral comb drive emerged in Sandia National Laboratories, where the structures move laterally to the surface .
As we know, physical contact is most important to ensure low IL and high RL for fiber connection. The evolution of endface polishing types experiences physical contact (PC), ultra physical contact (UPC) and angled physical contact (APC), as shown in Fig.1. All the endfaces are spherically polished. The UPC connector has a smaller radius of curvature than the PC connector. For the APC connector, the sphere surface is usually tilted by an angle of 8°. The RL guaranteed by PC, UPC and APC is 40dB, 55dB and 65dB, respectively.