What you need to know about the thermal control system for high-speed optical communication modules

The function of the optical module is to realize the mutual conversion and transmission of photoelectric signals. They are mainly used in data center connection scenarios, telecom markets, passive WDM systems, etc.

Currently, the packaging types of optical modules used in China are notably the 100G four-channel small form factor pluggable QSFP28 optical module and the 200G four-channel small form factor plugable double-density optical module QSFP-DD.

With the development of visual communication industry, 400G QSFP-DD optical modules are gradually applied to cloud computing and 5G network.

Increasing the communication rate inevitably involves large power consumption. The heat dissipation environment must be guaranteed to ensure the stable operation of data centers and base stations.

In the past, Microsoft built data centers on the seabed and then Facebook built data centers in the Arctic Circle. Suffice it to say that the heat dissipation of the optical module is significant.

In order to ensure that the high-speed optical communication module can still work stably in this extreme environment, it is necessary to include radical temperature cycle experiments in the module factory test to ensure that the optical module can work under extreme temperatures in general.

There are heat flow meters, a thermal shock test chamber, and a thermoelectric cooler (TEC) to control the temperature of the module.

The temperature control efficiency of the heat flow meter is high, but it is large, expensive, and loud in noise. Due to the limitation of the test fixture, the heat flow meter can only test a single module, not multiple modules in parallel; The temperature control of the thermal shock test chamber is stable, but its volume is significant and its efficiency is low, especially the cooling time is too long.

And due to the structure of the thermal shock test chamber, it is impossible to perform other optical module functional tests while controlling the temperature. In order to facilitate the functional integration test of the optical module, the case size of the temperature control system must be less than 200mm 500mm 500mm and the noise must be lower than 65 decibels.

In summary, neither the heat flow meter nor the thermal shock test chamber is suitable; TEC is chosen as the thermal control system due to its light, compact structure, small footprint and noise-free, which is ideal for optical module integration test.

Temperature control components. At the same time, according to the existing relevant theoretical research: the liquid cooling heat dissipation method has the advantages of ultra-quiet, fast heat dissipation, etc., although the conduction heat dissipation method is small in size and occupies a small area, there is a possibility of natural failure. , chemical building materials and other industries, and the electronics industry has fewer applications; Finally, in combination with the experimental conclusions to optimize the heat dissipation of the hot end, the liquid cooling method was chosen to improve the temperature control efficiency of the TEC.

This paper combines liquid cooling and TEC to develop a thermal control system for high-speed optical communication modules.

The thermoelectric cooler components in the system can be replaced with different test fixtures depending on other package types of the modules under test: It is possible to test a single QSFP-DD optical module or two QSFP28 optical modules in parallel.

Compared with the water cooler, the system heat dissipation system has the advantages of small size and low noise, which will promote the integration test of the optical module function in the later stage.

Hardware composition of the thermal control system

The thermal control system of the high-speed optical communication module mainly includes a thermoelectric device under test (DUT), a DUT holder, a TEC controller, and a TEC heat dissipation system. The TEC heat dissipation system consists of an outer and an inner circulation water path.

The external circulation water path includes a cold plate, a water pump and a TEC cold source; The inner circulation water path consists of a cooling plate, an air outlet, a fan and a water pump.

How the thermal control system works

The basic working principle of the system is that the TEC cooling surface in the thermoelectric more excellent assembly is in direct contact with the module to be measured via a heat sink to realize heat exchange to achieve the temperature control of the module to be measured.

Its heat dissipation surface is in contact with the surface of the water cooling head and exchanges heat through the chilled water cooling liquid to achieve the purpose of cooling; After the temperature of the water cooling liquid rises due to heat absorption, it flows through the flat circulating water channel of the heat dissipation system into the external circulating water pump, and then it is pumped into the TEC cold source to cool down, and finally flows back to the cold plate in the thermoelectric cooling component through the water channel to the continue heat exchange.

The TEC cold source in the heat dissipation system uses liquid cooling to dissipate heat. Its main purpose is to reduce the temperature difference between the two sides of the TEC in the cold source, thereby improving the cooling capacity and efficiency of the heat dissipation system.