Two major uses of photodetectors in optical communication systems

Photoelectric detection and optical receiver

  At the transmitting end of the optical fiber communication system, the light source is modulated with a low-frequency baseband electrical signal, and then the modulated light signal is transmitted via the optical fiber. Due to factors such as attenuation and dispersion of optical fibers, when a dimmed signal is transmitted to the receiving end, it becomes weak and has waveform distortion. The role of the optical receiver is to detect the weak light signal first, then convert it into an electrical signal, and then restore to the original baseband signal through amplification, shaping, regeneration, and decoding. Therefore, the core component of the optical receiver is a photodetector. In the entire optical fiber communication system, the photoelectric detection device mainly has two purposes: one is for terminal reception of the communication system, and the upper part of the figure is the schematic diagram of terminal receiving of the digital optical fiber communication system; the second is photoelectric conversion for the relay station. After the signal processing continues to transmit, the lower part of the figure below depicts the relay station’s photoelectric conversion process.

FOT Blog: working process diagram of optical communication system

Photoelectric detection principle

  The photodetector is an optoelectronic device made by utilizing the photoelectric effect of a semiconductor. It converts the change of the optical signal into the change of the photocurrent, and reflects the change rule of the information. According to different conversion parameters, the semiconductor photodetector has two basic types: photoconductive type and photodiode type. The conductance of the photoconductive type detector changes with the change of luminous flux. The photodiode is always operated in the reverse bias state. It belongs to the inner photoelectric effect device, and the incident photon does not directly bombard photoelectrons, but merely raises the internal electrons from the lower energy level to the higher energy level. The differential resistance of the photodetector does not change with the luminous flux, and the generated photocurrent is proportional to the luminous flux. Both types of semiconductor photodetection devices have very fast response speeds, but have their own characteristics and different uses. In optical fiber communication systems, the most widely used photodetector is a photodiode because of the small size and long life of such detectors.

Photodetector operating characteristic parameters

  The main function of the photodetector is to convert the optical power signal transmitted from the optical fiber into a current signal, which carries the information of the source. Photoelectric detector basic parameters including the following 6 main features:

  • Photocurrent
    When the incident optical power of the photodetector changes, the photocurrent also changes linearly, thereby converting the optical signal into an electrical signal.
  • Quantum efficiency
    Quantum efficiency, ie, photoelectric conversion efficiency, represents the degree to which the total number of photons received by the photodetector can be converted into the total number of electrons of the photogenerated current.
  • Responsiveness
    TheResponsiveness, also called photoelectric conversion sensitivity, is represented by r, which reflects how much light power is converted into photo-generated current.
  • Cutoff wavelength
    Only when the incident photon energy is greater than the bandgap of the detection device material, photogenerated carriers can be generated, forming a photocurrent. Therefore, for any photoelectric detection device made of any material, there is a minimum frequency or maximum wavelength that can be detected, ie, the upper cutoff wavelength.
  • Dark current
    The dark current represents the reverse current that occurs in the absence of light. It affects the receiver’s signal-to-noise ratio and is an important quality parameter.
  • Response time
    The response time (speed) indicates the ability of the photodetector to respond to the optical signal.

Fiber Optic Communication System Requirements for Photodetection Devices

  The role of the photoelectric detection device is to use the photoelectric effect to convert the optical signal into an electrical signal. The main requirements for photodetection devices are: high sensitivity at the operating wavelength in order to improve the photoelectric conversion efficiency; fast response, good linearity, frequency bandwidth, and the speed of photoelectric conversion is higher than the operating speed of the system, reaching hundreds of Mbit/s From s to thousands of Gbit/s, the bit rate of the communication system can be increased; the additional noise caused by the detection process is small, various measures are taken to reduce the internal noise of the system, and the signal to noise ratio is improved; the cost is low, the reliability is high, and the volume is small Small, long life, the photo-sensitive surface of the detector is matched with the core diameter of the optical fiber to improve the coupling efficiency; the operating voltage is as low as possible and easy to use. In optical fiber communication systems, the photodetector devices that meet the above requirements are the most commonly used PIN photodiodes and avalanche photodiodes (APD).

Photoelectric detection device

  There are many materials that can be used to make photodiode detectors, such as Si, Ge, GaAs, InGaAs, GaAsP, InGaAsP, and the like. According to the photodiode detection PN junction, it can be divided into PN junction type, PIN junction type, Schottky barrier junction type, heterojunction and avalanche photodiode detectors. According to the wavelength response to light to distinguish, photodiode detector can be divided into infrared type, ultraviolet type, blue silicon type. Among them, the photodiode made of Si material, its typical peak response wavelength of 0.94 μm, its series is also more; PIN photodiode and avalanche photodiode APD’s response time is short, so suitable for high-speed transmission applications; Ge The photodiode of the material is also one of the widely used optoelectronic devices. Since its band gap is smaller than Si, it has higher sensitivity in the long wavelength band, but since the Ge material has a relatively large current, the noise is also high. InGaAs photodiodes are one to two orders of magnitude lower, so photodiodes of InGaAs compound materials are widely used. In order to meet the requirements of optoelectronic integrated circuits, integrated optical photodetectors can be fabricated using various waveguide effects.

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