Measuring Laser Diode Temperature in Optical Transceivers

Optical transceivers carry high-speed data over fiber between servers, switches, and other data center equipment. The core component on the transmit side is the laser diode that generates the optical signal. The thermal behavior of this laser affects its emission, wavelength, optical output, efficiency, and modulation behavior.

The main challenge in measuring the diode temperature is the placement of the sensing element. Sensors that measure ambient or module temperature may not show the local thermal conditions of the laser diode because heat flows through thermal resistances.

TDK Corporation addresses this with its NTCWS Series that combines submillimeter dimensions with Au electrodes and wire-bondable construction to allow the thermistor to be placed closer to the laser diode without occupying significant package area.

How NTCWS measures laser temperature

The TDK NTCWS Series is a negative temperature coefficient (NTC) thermistor with a ceramic sensing element whose resistance changes with temperature. As with any other NTC, its resistance decreases with rising temperatures. The thermistor needs a measurement circuit that can read its resistance, and then convert that value into temperature.

Two specifications are important to understand this operation. TDK offers nominal resistances (B25) of 10 kΩ and 100 kΩ at +25 °C. The B value describes how sharply resistance changes with temperature. TDK specifies B25/85, which means the characteristic is defined by resistance at +25°C and +85°C.

Increasing the value from 10 kΩ to 100 kΩ also raises the impedance of the sensing circuit. A higher resistance reduces both the measurement current and the thermistor self-heating in a given circuit design. However, the higher impedance can also make ADC input leakage, parasitic leakage, and settling behavior more relevant.

For example, the nominal resistance of the 10 kΩ, 3930 K version drops to about 3.60 kΩ at +50°C and 1.10 kΩ at +85°C. The 100 kΩ, 4050 K version measures about 35.3 kΩ at +50°C and 10.27 kΩ at +85°C. The measurement circuitry uses these known resistance-temperature curves to determine the thermistor temperature.

A normal measurement circuit places the NTC thermistor in a voltage divider with a known resistor. When the thermistor resistance changes, the divider voltage also changes. An ADC converter measures that voltage, and the controller calculates the thermistor resistance.

The next step is converting that resistance into temperature. This relationship is nonlinear, so the controller cannot use a simple fixed number of ohms per degree. It can either use TDK's published resistance-temperature data with a lookup table and interpolation, or use a thermistor equation such as the Beta equation over an appropriate temperature range.

The TDK NTCWS devices have ±1% tolerance for R25 and ±1% tolerance for B value. These specifications control different errors. R25 tolerance limits part-to-part resistance variation at +25°C. The B-value tolerance limits the variation in the resistance curve deviation from that reference point.

Simplified assembly for laser temperature sensing

The sensor needs to fit and connect to a very small optoelectronic package for near-laser measurements. The NTCWS devices are roughly 0.31 mm to 0.48 mm square, depending on the variant, and around 0.25 mm thick (Figure 1). This allows the sensing element to be placed inside the laser diode package.

Figure 1: Representation of the NTCWS series device packaging. (Image source: TDK)

TDK also designed the electrodes for package-level assembly. The NTCWS supports Au wire bonding, in which a fine gold wire connects the thermistor electrode to another electrical connection in the package. The NTC ceramic still performs the temperature sensing, but the gold electrode and bond wire provide access to its resistance.

TDK identifies AuSn solder as a supported mounting method. These options give designers more flexibility to integrate the thermistor into the same compact package environment as the laser diode. The exact attachment and bonding processes still have to be qualified for the specific package.

This placement also has an important thermal benefit. A sensor that is close to the laser has a shorter thermal path. Therefore, the measured temperature more accurately shows the laser's local thermal environment. It does not directly measure the laser junction temperature, since some thermal resistance still exists between the laser active region and the sensing element.

The surrounding electronics can then use local measurements for temperature control and monitoring. For example, the system can compare measured temperature with an expected setpoint or operating profile and respond when the thermal behavior deviates from that expectation.

TDK describes close temperature measurements as enabling proactive temperature control and early abnormality detection. The NTCWS does not diagnose a fault, but It provides the system with local temperature information that can be compared with expected limits, rates of change, or operating conditions.

High reliability for harsh environments

A sensor placed inside a laser package needs to maintain a predictable resistance temperature characteristic over time.

When there is a resistance change in the thermistor, it could appear to the control system as a change in laser temperature. TDK pairs the NTCWS electrical specifications with environmental and reliability testing.

The Version 6 datasheet specifies several environmental stress tests. For the 10 kΩ NTCWS3UFfamily, devices are tested for 1,000 hours at +125°C, 1,000 hours at -40°C, and 1,000 hours at +85°C and 85% relative humidity.

The thermal shock testing alternates between -40°C and +125°C for 1,000 cycles. After these tests, the change in resistance must remain within ±1%. The 100 kΩ NTCWS4AF family is tested under the same conditions with a ±2% resistance change limit.

TDK also reports a sulfurization test for the gold electrode thermistor. The device was exposed to hydrogen sulfide at 5 ppm, 40±2°C, and 75±3% relative humidity for 240 hours. They reported that it passed the test.

These results show that the NTCWS resistance characteristic remains under control after defined thermal, humidity, and corrosive-gas stresses.

Conclusion

Measuring laser diode temperature in an optical transceiver is about selecting an accurate sensor that can be placed close enough to the laser to capture its local thermal condition. The TDK NTCWS Series has defined resistance temperature characteristics with R25 and B-value tolerances to provide a predictable electrical response. The submillimeter dimensions, Au electrodes, and support for wire bonding simplify integration into compact optical transceiver assemblies.

To learn more, visit TDK’s NTCWS Series Thermistors.