In today's era of high-speed information exchange, optical communication constitutes the backbone network of the global digital economy. As the core component for implementing optoelectronic conversion in optical communication systems, the performance of optical modules directly determines the transmission capacity of the network. This article will provide a systematic interpretation of optical modules from the perspectives of technical principles, core components, key indicators, and future evolution.

1、 Introduction: Why choose optical communication?

Starting from Shannon's theorem, the channel capacity of a communication system is directly proportional to its bandwidth and signal-to-noise ratio. Compared with traditional electrical communication, optical communication brings almost infinite bandwidth potential with its extremely high carrier frequency (in the range of 10 ^ 14 Hz). At the same time, the low transmission loss of fiber optic media (up to 0.2 dB/km in the 1550nm window) and extremely strong resistance to electromagnetic interference make it the only feasible solution for long-distance and high-capacity information transmission.

The optical module, located on the network equipment side, is the core hub responsible for signal conversion between the electrical interface and the optical interface.

2、 Technical principles and core architecture of optical modules

The optical module is essentially a complete optoelectronic secondary system. The basic working principle is to convert the electrical signal generated by the device into an optical signal at the transmitting end, transmit it through optical fiber, and then restore the optical signal to an electrical signal at the receiving end.

A standard optical module mainly consists of the following core functional units:

• light source：Usually a semiconductor laser, it is the "heart" of the emitting unit. Its main types include:

• FP laser：Low cost, suitable for short distance and low-speed scenarios.

• DFB Laser ：With extremely narrow linewidth and excellent monochromaticity, dynamic single longitudinal mode output is achieved through built-in grating, making it the mainstream choice for long-distance and high-speed transmission.

• EML laser：Integrating the electroabsorption modulator with DFB laser and achieving signal modulation through electroabsorption effect, it has excellent performance and is a benchmark for high-speed applications (such as 100G and above).

driver IC：Amplify and shape the input electrical signal to provide appropriate modulation current for the laser, ensuring that the laser can accurately change the output light intensity according to the law of the electrical signal.

modulator：Load the electrical signal onto the light wave. The methods include：

• Direct modulation：Modulation is achieved by changing the driving current of the laser. Simple and low-cost, but it can cause chirp effects, limiting transmission distance and speed.

• Indirect modulation：The laser generates continuous light waves, which are modulated by independent devices such as external Mach Zehnder modulators. No chirp, superior performance, suitable for high-speed long-distance transmission.

2. Receiving unit

• photodetector：It is the "eye" of the receiving unit, responsible for converting weak light signals into weak current signals. The mainstream technologies are PIN photodiodes and avalanche photodiodes with higher sensitivity

• Transimpedance amplifier：Converting and amplifying the weak current signal generated by the photodetector into a voltage signal is one of the most critical chips in the entire receiving chain, and its noise and bandwidth performance directly determine the receiving sensitivity.

• Limiting amplifier/clock data recovery：Further shape and amplify the voltage signal output by TIA, and recover the synchronous clock signal from it, ultimately outputting high-quality electrical data stream.

3. Auxiliary and supporting units

• MCU：The "brain" of the optical module is responsible for running management algorithms, storing manufacturer information, real-time monitoring of module working status (such as temperature, bias current, received optical power, emitted optical power, etc.), and communicating with the upper computer through the I2C interface to achieve digital diagnostic monitoring functions.

• Optical components：Including lenses, isolators, etc., used to efficiently couple laser into optical fibers and prevent reflected light from affecting laser operation.

• shell：Provide physical protection, electromagnetic shielding, and standardized electrical interfaces.

3、 Key performance indicators of optical modules

To evaluate the quality of an optical module, attention should be paid to the following core indicators：

• transmission rate：Describe the number of bits transmitted per second by the module, such as 100G, 400G, 800G.

• center wavelength：The common frequency bands are 850nm (multimode), 1310nm (O-band), and 1550nm (C/L band).

• transmission distance：Due to fiber loss and dispersion, ranging from several hundred meters (multimode) to tens or even hundreds of kilometers (single-mode).

• extinction ratio：The ratio of the output optical power of the laser at "1" code and "0" code. The higher the ER, the better the signal quality.

• reception sensitivity：The minimum average received optical power that the receiving unit can recognize under specific error rate conditions. The higher the sensitivity, the stronger the receiving ability of the module.

• power consumption：At high speeds, the power consumption and heat dissipation of modules become key challenges in system design.

4、 The packaging evolution and technological driving of optical modules

The packaging form of optical modules is closely related to their speed, density, and power consumption requirements. Its evolution process clearly reflects the driving force of technological development：

• Towards a higher speed：From 1G/10G SFP/XFP, to 40G/100G QSFP+/CFP/QSFP28, and now to 400G/800G QSFP-DD/OSFP. The speed improvement is achieved through more advanced modulation formats and more channels.

• Towards higher density：QSFP-DD/OSFP and other packages achieve a doubling of port density by increasing the number of channels (from 4 to 8 channels) while maintaining a width similar to SFP+.

• Towards lower power consumption and cost：Silicon photonics technology manufactures optical devices on silicon-based substrates using standard CMOS processes, which is expected to achieve large-scale, low-cost integration of photonic devices and is a key path to reducing the cost and power consumption of high-speed modules in the future.

• Towards deeper integration：Co encapsulated optics (CPO) is considered a breakthrough technology for the next generation. It encapsulates the optical engine and switch chip together in the same slot, greatly reducing the distance between electrical channels, significantly reducing system power consumption and I/O bottlenecks, and is an inevitable choice for 1.6T and higher speeds.

5、 Summary and Prospect

As the cornerstone of optical communication networks, the technological iteration of optical modules is the core driving force for the continuous development of data centers, 5G/6G mobile communication, backbone network transmission, and other fields. In the future, optical modules will continue to evolve in the direction of high speed, low power consumption, high integration, and small size. The mature and large-scale application of silicon optical technology and CPO will lead optical interconnect technology into a new stage of development, providing crucial connectivity capabilities for building future computing infrastructure.