Why Are Thermoelectric Coolers Used in Fiber Optic Communication Equipment?

A dense wavelength division multiplexing system carries 80 channels on a single fiber. Each channel is spaced 50 GHz apart, which corresponds to about 0.4 nanometers. If the laser diode in one channel drifts by more than 0.1 nanometers because of a temperature change, the channel overlaps with its neighbor, and the bit error rate rises. The Thermoelectric Coolers inside the optical module are the component that prevents this drift. They hold the laser at a constant temperature, typically within 0.1 degrees Celsius, regardless of the ambient temperature outside the module. This guide explains why thermoelectric coolers are used in fiber optic communication equipment and how to specify them correctly.

1. Why Does a Laser Diode in Fiber Optic Equipment Need Temperature Stability at All?

The emission wavelength of a laser diode depends on the bandgap energy of the semiconductor material, and the bandgap energy depends on temperature. For a typical distributed feedback laser used in optical communications, the wavelength drift is approximately 0.1 nanometers per degree Celsius. The output power also changes with temperature because the threshold current increases as the temperature rises. In a dense wavelength division multiplexing system, the channel spacing is 50 or 100 GHz, which corresponds to 0.4 or 0.8 nanometers. A temperature change of 4 degrees Celsius would shift the wavelength by 0.4 nanometers, which is enough to cause crosstalk with the adjacent channel. The Thermoelectric Coolers maintain the laser temperature within 0.1 degrees Celsius, which keeps the wavelength drift below 0.01 nanometers. In our factory, we manufacture Thermoelectric Coolers for optical modules with a temperature stability of plus or minus 0.05 degrees Celsius. Fuzhou X-Meritan Technology Co., Ltd. has been manufacturing thermoelectric coolers for over 10 years.

The temperature stability is also important for the laser lifetime. A laser that operates at a higher temperature degrades faster because the defect propagation in the semiconductor material is thermally activated. Reducing the laser temperature by 10 degrees Celsius can increase the lifetime by a factor of two. The thermoelectric cooler allows the laser to operate at a constant, optimized temperature even when the ambient temperature is high.

2. How Do Thermoelectric Coolers Hold Laser Temperature Within a Fraction of a Degree?

A Thermoelectric Coolers is a solid-state heat pump that uses the Peltier effect. When a direct current flows through a junction of two dissimilar semiconductors, one side of the junction absorbs heat and the other side releases heat. Reversing the current direction reverses the heat flow. The cooler is mounted between the laser diode and the module housing. A thermistor mounted on the laser provides the temperature feedback. A control circuit compares the measured temperature with the setpoint and adjusts the current to the cooler. The table below shows the operating mode and power consumption at different ambient temperatures for a typical optical module.

The bidirectional capability of the cooler is important because the ambient temperature can be below the laser setpoint in cold environments. In that case, the cooler operates in heating mode to bring the laser up to the setpoint. In our factory, we test the heating and cooling capacity of every Thermoelectric Coolers at multiple ambient temperatures. The test data is provided with each shipment so that the module designer can verify the performance.

3. What Advantages Do Thermoelectric Coolers Offer Over Compressor or Fan-Based Cooling in Optical Modules?

Optical modules have specific requirements that favor thermoelectric cooling over other methods. The module is small, typically a few centimeters wide. It must operate for 10 to 20 years without maintenance. It must not generate vibration that could disturb the optical alignment. It must not emit electromagnetic interference that could affect the signal. The table below compares thermoelectric coolers with fan-based and compressor-based cooling for optical module applications.

4. Why Does the Reliability of a Thermoelectric Cooler Determine the Service Life of the Entire Optical Module?

The Thermoelectric Coolers is one of the few active components inside the optical module. If it fails, the laser temperature rises, the wavelength drifts, and the module stops meeting the specification. The failure of the cooler is therefore a failure of the entire module. There are three primary failure modes. The first is solder fatigue at the junctions between the semiconductor pellets and the ceramic substrates. The fatigue is caused by thermal cycling, which is common in outdoor equipment. The second is the degradation of the bismuth telluride material, which reduces the cooling capacity over time. The third is the loss of hermetic sealing, which allows moisture to enter and corrode the junctions. The table below shows the reliability test conditions that we apply to our coolers.

In our factory, we perform these tests on a sample basis for each production lot. The test data is used to calculate the mean time to failure and to verify that the cooler meets the module manufacturer's reliability requirement. X-Meritan  provides the reliability test report with each shipment.

5. How Should Optical Module Designers Specify and Verify a Thermoelectric Cooler?

The specification of a Thermoelectric Coolers for an optical module must include six parameters. The first is the maximum cooling capacity, which is the heat that the cooler can pump at the maximum current and zero temperature difference. The second is the maximum temperature difference, which is the difference between the hot side and the cold side at zero heat load. The third is the rated current and voltage. The fourth is the dimensions and the mounting pattern. The fifth is the lead wire configuration. The sixth is the reliability grade. The verification at receipt should include a cooling capacity test, a resistance test, and a visual inspection. In our factory, we test every cooler before shipment and provide a test certificate with the serial number. Fuzhou X-Meritan Technology Co., Ltd. also provides application support to help the module designer select the correct cooler for the thermal load.

We recommend that the designer calculate the total heat load on the laser, which includes the electrical power dissipated by the laser, the heat absorbed from the surrounding air, and the heat conducted through the mounting screws. The cooler must be sized with a margin of at least 30 percent above the calculated heat load. A cooler that is undersized will not maintain the setpoint at high ambient temperature, and the module will fail the wavelength stability specification.

Summary for Optical Module Designers

Thermoelectric Coolers are used in fiber optic communication equipment because they maintain the laser diode at a constant temperature, which stabilizes the emission wavelength and extends the laser lifetime. They are solid-state devices with no moving parts, which makes them suitable for the compact, maintenance-free, and vibration-sensitive environment of an optical module. The reliability of the cooler determines the service life of the entire module, so the cooler must be specified with the correct cooling capacity, temperature difference, and reliability grade. The verification at receipt should include a cooling capacity test and a resistance test. Fuzhou X-Meritan Technology Co., Ltd. has been manufacturing Thermoelectric Coolers for over 10 years and provides full specification and reliability support for optical module designers.