How to choose a right transmitter by light source parameters?

LED: Light Emitting Diode; LD: Laser Diode; AM: Amplitude Modulation; FM: Frequency Modulation; PM: Phase Modulation; ASK: Amplitude Shift Keying; FSK: Frequency Shift Keying; PSK: Phase Shift Keying

  The complete optical fiber communication system should consist of three main parts: Light emission, optical transmission and optical reception. This blog will introduce the light source, the composition of the optical transmitter, the working process and the main performance indicators.

Optical Transmitter Overview

  In an electric signal communication system, a low-frequency analog or digital baseband signal is transmitted over a long distance using a carrier wave. First, a high-frequency carrier such as a frequency and amplitude stability is selected. The carrier usually uses a continuous sine wave and modulates the carrier with a low-frequency baseband signal. In the analog communication system, the modulation method of the baseband signal to the carrier includes amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM); in the digital communication system, the modulation method of the digital baseband signal to the carrier has amplitude shift keying ( ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK). Similarly, in an optical communication system, a carrier wave is also selected to carry a baseband signal. Here, an optical signal is used as a carrier wave to carry information. As the optical signal of the carrier wave, it should have a very stable wavelength and optical power P (Power), that is, the light emitted from the light source needs to have a stable light intensity I (Intensity) and a stable wavelength λ, using the low frequency simulation to be transmitted or The digital baseband signal modulates a parameter on the optical carrier and then transmits it over long distances through the optical fiber.

  Since the optical signal is selected as carrier information for the carrier, the optical signal can be emitted to the core component of the optical transmitter. In order to ensure the normal operation of the optical fiber communication system, certain technical requirements must be put on the light source. The general requirements for light sources in a optical fiber communication system is listed as following:

The wavelength of light emitted

  In addition to having a highly stable wavelength λ and P (optical power), the light source emits a peak center wavelength λp within several low-loss transmission windows of the fiber. Since the wavelengths of several low-loss windows of the fiber are approximately 0.85 μm, 1.33 μm, and 1.55 μm, the peak value λp emitted by the light source should also be within these several wavelength ranges. Only by satisfying this requirement can the optical signal be reduced in the fiber. The transmission loss within, prolongs the relaying distance of the optical signal transmission.

Power requirement for light source

  The optical power that can be sent out by the selected light source should be large enough to meet the requirements of the communication system for the optical relay distance. Otherwise, the number of relay stations must be increased and the transmission cost must be increased. The light intensity (or power) emitted by the light source should be stable to meet the intensity modulation of the signal to the light source and reduce the intensity modulation noise.

High electro-optic conversion efficiency

  Under the same driving power conditions, increasing the luminous intensity of the light source can increase the relay-free distance of transmission. If the power of the light source can be reduced under the condition of satisfying the output optical power requirement, that is, the luminous efficiency of the light source is increased, the temperature rise of the optical transmitter can be reduced, the reliability of the transmitter can be improved, and the life of the light source can be extended.

Good monochromaticity and directionality

  If the wavelength of the light emitted by the light source is relatively simple, the spectral width Δλ1⁄2 is narrower, which can reduce the transmission dispersion of the optical fiber and reduce the spread of the optical pulse, thereby increasing the information transmission rate. If the directionality of the light source is good, the coupling efficiency of the light source and the optical fiber can be improved. In order to increase the directionality of the light source and reduce the light reflection, the optical transmitter is usually provided with an optical isolator.

Light source noise must be small

  The noise of the communication system includes external interference noise and internal noise of the system. If the intensity and noise of the light source itself is small, the signal to noise ratio of the analog modulation system can be increased, the bit error rate of the digital communication system can be reduced, and the communication environment can be improved and the communication quality can be improved.

Quick response

  The response speed of the light source is faster in order to facilitate high-rate, high-capacity signal transmission. In addition, the light intensity of the light source is better than the linearity of the driving current to ensure that there are enough modulation channels in a dense wavelength division multiplexing (DWDM) system.

Common Light Source

  The most commonly used optical fiber communication systems are semiconductor light sources, including semiconductor light emitting diodes (LED) and semiconductor laser diodes (LD). The reason why semiconductor light sources are mostly used in optical fiber communication systems is that they have the following characteristics: the wavelength of light emitted by semiconductor light sources is suitable for transmission in the low-loss window of optical fibers; the semiconductor light source is small in size, and its light emitting area is in the same order of magnitude as the core diameter of optical fibers. Therefore, there is a higher degree of coupling with the optical fiber, which can increase the coupling efficiency between the optical signal and the optical fiber; the intensity modulation can be easily performed. As long as the baseband signal current is injected into the light source driving circuit, the corresponding modulated optical signal output can be obtained. The purpose of non-coherent modulation; high reliability and long life of semiconductor light source, not only high transmission power, high coupling efficiency with fiber, fast response, but also good coherence of emitted light, so the semiconductor light source is in high-speed, large-capacity digital Optical fiber communication systems are widely used. In recent years, the wavelength tunable laser technology has gradually matured, and it has become a key component in multi-channel wavelength division multiplexing (WDM) optical fiber communication systems and has received widespread attention.

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