The wide application of adjustable optical attenuator (VOA) in optical communication
Adjustable optical attenuator (VOA) is widely used in optical communication
The main function of the optical attenuator is to reduce or control the optical signal. For example, in a WDM network system, it is used to reduce the energy gap between channels and maintain a constant signal-to-noise ratio (S / N) for all channels. Therefore, optical amplifiers All DWDM modules are used. There are many types of manufacturing methods for optical attenuators, including changing the loss between a pair of fiber end faces by adjusting the air gap, or applying a filter to affect the incident light. These manufacturing methods can be used for fixed and adjustable optical attenuators ( Variable OpTIcal Attenuator). At present, the price of fixed optical attenuators is relatively low, and they are mainly installed in communication systems to balance light energy, but they are less flexible; relatively, adjustable optical attenuators (VOA) are more advantageous in development and can be active and accurate The signal of balanced light is developed towards the combination of various modules and sub-systems (such as multiplexing / demultiplexing and OADM optical plug-and-drop). The future development is attracting more attention, so this article discusses VOA.
VOA technical demand characteristics How does the increase in the number of optical communication network channels affect optical amplifiers and other system components? For example, EDFA is quite sensitive to the input of light energy, so the number of channels needs to be programmable in the control of light energy, because when the number of channels increases or decreases the light energy entering the EDFA, these input light energy changes It will cause some uncertain effects on EDFA, for example: Gain TIlt of spectrum import. Gain skew is defined as the non-uniform gain when the inversion condition changes. Generally, when designing a broadband amplifier, in order to reduce gain skew and maintain the average inversion energy constant, VOA is used, which can reduce the impact of stimulated Raman scattering. VOA uses its loss characteristics to achieve a variety of spans in EDFA deployment. The way to control the span loss between optical amplifiers is as described above, which is to control the energy of the input light. In many cases, designers need to use VOA to design a network system with a fixed span. For example, when the system equipment has been set up, or when a new optical amplifier is installed, it can be used if the system ages or the system needs to be changed. . VOA can dynamically control the energy of light in a DWDM network. Before multi-channel coupling, it provides energy balance, balance between OADM channels, placement between optical amplifiers, and energy control before entering the receiver.
The ideal VOA of VOA manufacturing technology can accurately control the loss of optical signals. The general characteristics requirements include: low polarization modal dispersion, low polarization related loss, wavelength-independent loss, and low insertion loss. The manufacturing technology and characteristics applied by VOA are shown in Figure 1 below. In addition, as can be seen from Figure 1, among the three technologies, liquid crystal (LC) and microelectromechanical (MEMs) technologies will have lower insertion loss, lower polarization loss and lower dispersion loss, and further VOA technology Refer to Table 1 for comparison.
Figure 1 Various technologies for manufacturing VOA
MEMS technology can be applied to single-channel and multi-channel VOA, where the diffractive MEMS technology is faster and more reliable than stepper motor components, planar waveguides and other MEMS technologies (such as micro-lens or bubble type) , Less complicated and cheaper.
Table 1 VOA technology comparison
VOAs, due to factors such as the complexity of internal materials and components used, can cause attenuation of characteristics such as insertion loss and polarization sensitivity. Through the design of electronically controlled VOA and the use of multi-layer film coupler technology, it provides strong glass mold layer protection of the sintered layer, which is beneficial to the application of VOA. This design provides high reliability, low insertion loss and low polarization Related losses.
VOA application-oriented
a. VOA is used for energy control of the light source channel. A high-order multi-channel light source platform must have good characteristics. This is important for the application of test lines and future EDFA / DWDM interactive connections. In short, a high-order multi-wave The channel light source will integrate the following main functions: polarization control of each channel; refer to the wavelength of the ITU-T specification, providing internal and external modulation rate of 2.5GHz; high output power; independent shutter for each channel; light energy Measurement module; telemetry channel; VOA; powerful user interface and waveguide multiplexer, etc., providing more functions and management.
b. Combining VOA and optical switch to enhance network control using high-speed VOA technology (VOA material reaction rate is close to 1 microsecond), network designers can improve the information flow control between network nodes, and can get high at the demultiplexer Insertion loss.
c. VOA modularization and packaging miniaturization development
A high-end multi-channel light source platform must provide good characteristics and provide the function of managing components. Among them, the application of VOA should pay special attention to the development of modularization and packaging miniaturization. Take the combination of VOA and optical plug extractor to enhance network control as an example. Because of the use of high-speed VOA technology, the VOA material response speed is close to 1 microsecond, allowing network designers to improve information flow control between network nodes today. The demand is for high-priced equipment, and high insertion loss can be obtained at the demultiplexer. VOA still has some applications for modular assembly, the following are listed:
EDFA + VOA = gain flattened optical amplifier
CADM (Configurable Add / Drop Module) + VOA = flat gain configurable optical plug extractor
ASE light source + VOA = gain controller
MUX / DEMUX + VOA = multi-channel equalizer
OA's future trend toward miniaturization and reduction of system complexity is the development trend of future components. For buyers, those functions must be accurately integrated together, but it is certain that integration is imperative. In addition, how to smoothly control the amount of attenuation, and to meet the low voltage and current requirements of the system control panel, and to meet the standards set by Telcordia on VOA, such as Telcordia GR-910 requirements for optical attenuators, etc., are also to be resolved Subject. In addition, VOA integrated pump module (pump module) can obtain excellent optical functional characteristics, including laser frequency, temperature stability and accurate energy sensitivity, compared with the traditional single fiber pump module, not only better characteristics, Basically, it can also save costs. Therefore, the ideal EDFA pump laser source integrated application design includes: optical switch connection (opTIcal crossconnects) loss compensation; energy control and amplification on the DWDM network; optical preamplifier arrays for high speed detector arrays; And pumping of waveguide amplifier arrays (waveguide amplifier arrays) or optical fiber laser transmission arrays (lasertransmitter arrays), etc., of course, there are places where VOA can be used.
ElectroniCast divides the optical attenuator market into the following parts: flat backplane: fixed (including bulkhead, connector buildout) optical cable assembly: fixed (including patch cords / leads, in-line) adjustable: manual (flat backplane / Stand alone), electronic control According to the company's survey, the global sales of optical attenuators were about US $ 301 million in 2000, and the compound growth rate was about 33.9% by 2005, and the market reached US $ 1.2 billion, of which the telecommunications market accounted for The proportion is the largest and the compound annual growth rate is the highest, mainly due to the development of Fibre Optic Subscriber Loop (FITL) and the need to upgrade long-distance DWDM networks. Among them, the adjustable optical attenuator (electronic control mode) includes magneto-optic, acousto-optic and electro-optic. This part mainly controls the attenuation range by changing the voltage, this part accounts for about 70%; in addition, in the manual adjustable optical attenuation Attenuators account for about 12%. Overall, in 2000, adjustable optical attenuators accounted for 82% of the global optical attenuator sales of US $ 301 million. The distribution of various optical attenuators is shown in Table 2.
Vendors invest in general VOA array applications including: gain equalization of EDFA in DWDM systems, control of path related signals, and control of dynamic saturation at the receiver. The foreign investment manufacturers and their development status are as follows:
SDL company integrates VOA and DWDM technology. At present, the company has been acquired by JDSU. SDL claims to be the first to introduce the integration of 40-channel DWDM and 40-channel VOA as a single module component, and uses a planar optical waveguide circuit (Planar Lightguide Circuit ; PLC) Integration of AWG multiplexer and thermo-optical VOA. Gemfire invested USD 63 million to introduce VOA arrays and multi-port laser pump arrays. Gemfire is an integrated optical communications company with a reported capital of more than USD 85 million, including Cisco Systems, Corning, Finisar , Intel Capital, TriQuint Semiconductor and venture capital Hook joint investment, previously invested in Gemfire include Eastman Kodak, LG Electronics, Microvision and Ricoh Silicon Valley. Gemfire's multi-port VOA array can provide gain equalization and dynamic saturation optical signal control. VOA array components are integrated on the optical integrated circuit (IC) platform, using a unique polymer waveguide to provide low insertion loss, small size and less energy Loss, to achieve the purpose of electronically controlling the amount of attenuation. Gemfire's pump laser source array is built on the company's own planar light integrated circuit platform, just like its electronic integrated circuit predecessor. Compared with a single pump laser source device, each output can be used at a lower cost Pass more functions, and all functions can be done in a small package. Gemfire optical integrated circuit 980nm multi-port number pump laser source array, from small embedded optical modules to provide eight independently controllable fiber output. As for the initial investment fields of Chinese manufacturers, most of them are based on mechanically modulating optical attenuators. In consideration of the development status of foreign manufacturers, domestic manufacturers should be more active in different technical methods.
For example: electronically controlled optical attenuator (EVOA), etc., to rank among the international technical standards.
Jilin Province Wanhe light Co.,Ltd , https://www.wanhelight.com