The function of the optical transmitter is to convert the digital baseband electrical signal output from the electrical terminal into an optical signal and effectively inject it into the optical fiber line using coupling technology. The electrical-to-optical conversion is achieved by modulating the light source with the digital electrical signal carrying the information. Modulation is divided into two types: direct modulation (internal modulation) and indirect modulation (external modulation). The characteristics of the modulated light source include power, amplitude, frequency, and phase. Currently, direct optical intensity (power) modulation is the most technically mature and widely used in practical fiber optic communication systems.
Basic components of an optical transmitter
Light source
Fiber optic communication transmits optical signals. Therefore, the light source, which is the light-emitting device in a fiber optic communication system, becomes one of the most important components. Its function is to convert the transmitted electrical signal into an optical signal and emit it.
Input interface and line coding
The input interface and line coding circuit together form the input circuit. Its function is to shape the input PCM (Pulse Code Modulation) pulses and convert them into NRZ (Non-Return-to-Zero) code to modulate the light source and external modulation circuit. The basic structure of the input circuit is shown in the figure.
- (1) Equalization and amplification: Compensates for attenuation and distortion caused by cable transmission to ensure correct decoding.
- (2) Code pattern conversion: Converts the HDB3 or CMI code output from the equalizer into NRZ code.
- (3) Multiplexing: The process of transmitting multiple low-speed signals simultaneously over a single large transmission channel.
- (4) Scrambling: If the signal code stream contains long sequences of "0"s or "1"s, it will make clock signal extraction difficult. To avoid this, a scrambling circuit is needed to ensure that "0"s and "1"s appear with equal probability, facilitating clock extraction.
- (5) Clock Extraction: Since both code transformation and scrambling processes require a clock signal, the clock signal is extracted from the PCM signal after the equalization circuit and supplied to other circuits.
- (6) Encoding: As mentioned above, the scrambled code stream should ideally have an equal number of "1"s and "0"s to facilitate clock signal extraction at the receiving end. Furthermore, from a practical perspective, to facilitate continuous error monitoring, inter-channel communication, monitoring, and overcoming DC component fluctuations, the signal code stream after scrambling is encoded in actual fiber optic communication systems to meet the above requirements.
After encoding, the signal is transformed into a line code suitable for transmission over fiber optic lines.
Modulation circuit and control circuit
1) The wavelength of the emitted light should match the low-loss "window" of the optical fiber, meaning the central wavelength should be around 0.85 μm, 1.31 μm, and 1.55 μm. The spectral monochromaticity should be good, i.e., the spectral linewidth should be narrow, to reduce the limitations imposed by fiber dispersion on bandwidth.
2) The electrical-to-optical conversion efficiency should be high, requiring sufficient and stable output optical power at sufficiently low driving current, with good linearity. The directionality of the emitted light beam should be good, i.e., the radiation angle should be small, to improve the coupling efficiency between the light source and the optical fiber.
3) The allowed modulation rate should be high or the response speed should be fast to meet the requirements of high-capacity transmission systems.
4) The device should be able to operate in continuous wave mode at room temperature, requiring good temperature stability, high reliability, and a long lifespan.
5) In addition, the device should be small in size, lightweight, easy to install and use, and inexpensive.
◆Optical transmitter specifications and optical power