The six requirements that must be met as a light source device for optical fiber communication

What are the six requirements of optical fiber communication for light source devices?
1. emission light of moderate wavelength
The wavelength of the light wave emitted by the light source device must fall near 0.85 μm, 1.31 μm and 1.55 μm where the optical fiber exhibits low attenuation.
Good 2. temperature characteristics
The output characteristics of the light source device, such as the light emission wavelength and the size of the emitted light power, generally change with the temperature change, especially its performance is easy to deteriorate at higher temperatures. In the early and mid-term of optical fiber communication, it is often necessary to add a refrigerator and automatic temperature control circuit to the semiconductor laser, and some of the current excellent performance of the laser can not need any temperature protection measures.
Long 3. working life
Optical fiber communication requires its light source device to work continuously for a long time, so the longer the working life of the light source device, the better. The end of the life of the light source device is not a complete failure as we imagine, but its luminous power is reduced to half of its initial value or its threshold current is increased to more than twice its initial value. At present, semiconductor lasers with a working life of nearly one million hours (about 100 years) have been put into commercial use.
4. small size light weight
The light source device is to be installed in an optical transmitter or an optical repeater, and in order to miniaturize these devices, the light source device must be small in size and light in weight.
5. luminescence spectrum narrow
The spectral line width of the light emitted from the light source device should be as narrow as possible. Because if the spectral line is too wide, it will increase the dispersion of the fiber, reducing the transmission capacity and transmission distance of the fiber (when the dispersion is limited). For example, for a long-distance, large-capacity optical fiber communication system, the spectral line width of the light source should be less than 2nm.
6. emission power is large enough
The light source device must be able to work continuously at room temperature, and its incoming fiber power is large enough, at least several hundred microwatts, of course, more than one milliwatt (odBm) is better. What we emphasize here is the optical power of the fiber and not the pure luminous power. Because only the optical power after entering the fiber has practical significance,
Due to the extremely small geometric size of the optical fiber (the core diameter of the single-mode optical fiber is less than 10 microns), the light source device is required to have a high coupling efficiency with the optical fiber.
At present, the light source devices often used in optical fiber communication can be divided into two categories, namely light emitting diode (LED) and laser diode (LD). Of course, the LD may include a heterojunction laser diode, a distributed feedback type laser diode, a multi-quantum well type laser diode, and the like (in terms of structure). The so-called optical fiber communication is to use low attenuation optical fiber to transmit light waves carrying communication information, but not all light can be used for optical fiber communication. It is hoped that the laser with good phase and character is used as the carrier of communication information, because it has the advantages of good frequency consistency, constant phase difference and strong directivity.
Since Mayman invented the world's first laser (ruby) in 1960, laser technology has developed by leaps and bounds and has been widely used in various fields, especially in the industrial field. However, as of 1970, none of the lasers produced in the world could be used for optical fiber communication because most lasers are bulky and cannot work in continuous wave. Communication is required to work continuously for a long time. Only in 1970, when Bell Laboratories in the United States developed the world's first GaAsAl laser that could work in continuous wave at room temperature, did it find a suitable light source for optical fiber communication.