Optical Modules Structure:
Optical modules usually consist of a transmitter assembly (TOSA, containing a laser LD chip), a receiver assembly (ROSA, containing a photodetector PD chip), a driver circuit, an optoelectronic interface, a heat sink (some models), a housing, a pull ring and so on, and its structure is as shown in the following picture:
The lasers inside the optical module can be divided into vertical cavity surface emitting lasers (VCSEL), Fabry-Perot lasers (FP), distributed feedback lasers (DFB), electro-absorption modulated lasers (EML) and narrowband tunable lasers, etc.; Photodetectors can be classified into PIN junction diodes (PIN) and avalanche photodiodes (APD). Different types of lasers and photodetectors have differences in performance and cost, so we can choose different chip solutions according to their specific specifications.
Optical Modules Design:
The design of an optical module depends on several dimensions: package form, laser type, transmission rate, and price. These include chip-level performance and cost, number of channels (miniaturization, heat dissipation, packaging complexity), and modulation method (chip cost, reliability, design complexity).
Single Channel:
Single-channel is the simplest design approach. This configuration, along with other optical components on the PCB and various electronic chips, constitutes a single-channel optical module. Example:
A 10G optical module with a single 10G chip using NRZ modulation;
A 256 forward optical module with a single 10G chip using the overclocking modulation method;
100G DR1 data center optical module with a single 506 chip using PAM4 modulation.
Multi-channel:
Due to the difficulty of upgrading laser chips, the current single wavelength is up to 50G. But customer demand is rapidly increasing to 400G, 800G or even 1.6T. Therefore, multi-channel design realizes higher transmission rates by combining multiple lasers and receivers. Currently there are two types of multi-channel designs, one is multi-fiber and the other is single-fiber:
Multifiber:
Directly transmit each laser through a separate fiber, the advantage of this method is that the internal structure is simple, and the cost is low. Multiple fibers require the longer the distance, the higher the cost, so the multi-fiber method is mostly used in short- and medium-distance scenarios.
Example: 400G SR8 optical module adopts a standard 16-core multimode MPO connector, and each module contains 8 optical devices and electrical device channels.
Single fiber:
Reducing the number of fibers is necessary for longer transmission distances because of the need to control costs. In this case, a single-fiber design using the principle of wavelength-division multiplexing (CWDM), where four lasers of different wavelengths combine and transmit over a single fiber, and then at the receiving end, a demultiplexer (DeMUX) separates the signal into four different wavelengths for detection. This method requires an LC duplex interface, which reduces the number of fibers required to two.
Example: 400G QSFP-DD ER4 /LR4 / ER4.
MVSLINK (Suzhou Weimei Electronic Technology Co., Ltd. ) is a 12+ years old optical module supplier with a factory, dedicated to providing high-quality optical modules and professional communication solutions for all customers, offering 1 piece OEM service.
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