Optical devices used in optical fiber communication can be divided into two categories: optical active devices and optical passive devices. The difference between them is whether photoelectric energy conversion occurs in the process of realizing its own function. If photoelectric energy conversion occurs, it is called active optical device; If there is no photoelectric energy conversion, even if some electrical signals are needed, they are called optical passive devices. There are many kinds of classification methods for optical passive devices. At present, the most commonly used is to classify by function. According to the function of the device on the optical fiber transmission path, it can be divided into optical isolator, optical fiber connector, optical combiner / splitter, optical coupler, optical switch, optical attenuator, optical circulator, filter, etc.
1. Optical fiber connector
There are four main performance indexes to evaluate an optical fiber connector, namely insertion loss, return loss, repeatability and interchangeability.
2. Fiber coupler
Coupler, for short, is a device for shunting or combining, inserting and distributing optical signals.
In terms of power, it is divided into power distributor and optical wavelength (combining / dividing) coupler;
In terms of port form, it is divided into X-shaped (2x2) coupler, Y-shaped (1x2) coupler, star (NxN, n > 2) coupler and tree (1xn, n > 2) coupler;
From the conduction mode, it can be divided into multimode coupler and single-mode coupler.
Manufacturing method of coupler:
Discrete optical element combination type, all fiber type and planar waveguide type. Among them, the all fiber melting taper method has become the main manufacturing method of optical couplers because of its various advantages.
3. Overview of wavelength division multiplexers / Demultiplexers
Optical wavelength division multiplexer is an optical passive device that separates and synthesizes optical wavelengths. The common requirements for WDM and demultiplexer are: enough multiplexing channels, small insertion loss, large crosstalk attenuation and wide passband range. The difference between wavelength division multiplexer and wavelength division multiplexer is that the insertion loss of multiplexer is generally large. The requirements of demultiplexer are as follows: the given working wavelength shall have the lowest insertion loss; Other ports shall have ideal isolation from the optical signal;
Characteristics of WDM / demultiplexer:
1. Central wavelength (or passband)
It is selected by the designer and manufacturer according to the corresponding international standards, national standards or practical application requirements. The dense wavelength division multiplexer ITU-T is specified in the 1550nm region, and 1552.52nm is the standard wavelength. For other wavelengths, the specified interval is 100g (0.8nm), 50g (0.4nm), etc., or an integer multiple thereof is taken as the multiplexing wavelength.
2. Working range of central wavelength
For each working channel, the device must give a range suitable for the spectral width of the light source. This parameter limits the spectral width and central wavelength position of the light source (LED or LD) we choose. It is expressed in 1.0 nm or 10% of the average interval between channels.
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