A tiny NTC chip supports both ends of AI optical interconnect

As the computing density of AI clusters continues to break through physical limits, optical interconnect technology is undergoing a profound architectural transformation. CPO (Co-packaged Optics) integrates the optical engine and ASIC into the same package, while ELSFP "removes" the laser diode from high-temperature environments and places it into independent module of the switch panel. Behind this sophisticated architecture lies a role that is often overlooked but indispensable—the EXSENSE gold termination NTC chip. It is so small it's almost invisible—with dimensions of only 0.21mm × 0.21mm. But within this architecture, gold termination NTC simultaneously serves two completely different roles, safeguarding the temperature security of from the light source to the modulator.

Role One: "Light Source Guardian" in ELSFP

Laser diode is one of the "delicate" components in optical communication system, its output wavelength and power are extremely sensitive to temperature. Every slight change in temperature causes wavelength drift, and in severe cases, it may even cause permanent damage to the laser diode. In traditional pluggable optical transceiver, the laser diode is packed together with the ASIC, bearing the heat dissipated by high-power chip. The design logic of ELSFP is to move the laser diode to the switch panel, in a location with a lower and more stable ambient temperature. But "moving it out" is only the first step, the temperature control of the laser diode itself still needs to be extremely precise.

What does the EXSENSE gold termination NTC chip do here?

Inside the ELSFP module, the EXSENSE gold electrode NTC chip is encapsulated on the heat sink substrate of laser diode and connected to the pin of laser diode via gold wire bonding. Its task is to sense the junction temperature of the laser diode in real time, convert temperature signals into electrical signals, then feed them back to the TEC. The working logic of this closed-loop is:

l NTC detects temperature changes in laser diode → resistance values change

l Signal is amplified and convert via the ADC → Sent to the temperature control chip

l TEC cools or heats based on the error signal→ the laser diode locked at the set temperature

Research shows that using NTC+TEC closed-loop temperature control solution can stabilize the operating temperature of laser diode at 25℃ ±0.02℃, with deviations up to 0.03℃. For DWDM system that require extremely high wavelength accuracy, this level of temperature stability is a prerequisite for reliable signal transmission. For example, an external light source manufacturer explicitly emphasizes "dual-TEC thermal control" in its ELSFP product to maintain wavelength stability and extend lifespan of laser diode. Coincidentally, another external light source manufacturer recently released a 23dBm ELSFP with built-in precision TEC temperature control, completely decoupling the laser diode from the high-temperature environment of the ASIC. The role of the EXSENSE gold termination NTC chip here is as the "light source guardian"—it ensures the external laser diode always operates at the optimal temperature point, outputting stable, pure light signals and sending it to the silicon photonic engine inside the CPO package.

Role Two: "Resonant Point Calibration Engineer" in CPO

After the optical signal enters the CPO package, it goes into the silicon photonic engine. The core modulator in a certain CPO solution is the micro-ring modulator (MRM). The working principle of the MRM depends on an extremely precise physical condition: only when the resonant wavelength of MRM is precisely aligned with the input laser wavelength can light be efficiently coupled and modulated. The problem is that silicon has a very high thermo-optical coefficient. For every 1℃ temperature change, the resonant wavelength drifts by about 70 pm. The CPO package is adjacent to the ASIC, operating at high temperatures with large thermal disturbances. A few Kelvin temperature fluctuations are enough to cause the resonant points to deviate completely from the laser wavelength, causing a sharp decline in modulation efficiency, deterioration of the eye diagram, and communication interruptions. At this time, temperature can be measured by the NTC chip to keep the temperature control system constant. The role of the EXSENSE gold termination NTC chip here is as a "resonant point Calibration Engineer"—it ensures the MRM resonant wavelength is always precisely aligned with the external laser wavelength, allowing the optical signal to be efficiently "written" into the data.

Why must there be two EXSENSE gold termination NTC chips instead of one?

 Some people may ask: Since external laser diode have its own temperature control, and the internal MRM of the CPO also has temperature control, can the same system be used?

The answer is: no. Because they are two independent temperature control objects, they are confronted with two different sets of thermal disturbances. The temperature control goal of an external laser diode is to ensure the laser outputs a set wavelength stably. But it does not know or cannot sense the location of the resonance point of each MRM inside the CPO. There are dozens or even hundreds of MRMs inside the CPO, each with slightly different wavelengths due to manufacturing deviations. External laser diodes cannot "customize" wavelengths for each MRM. More importantly, the transmission link itself introduces instability. Light emitted by external laser diodes, passing through polarization-maintaining fibers, connectors, and coupling structures, when reaching the MRM, changes in temperature between the fiber and connector affect the polarization state and phase of the light. Even if the output of an external laser diode is absolutely stable, the light arriving at the MRM may still "change". Therefore, the NTC of the external laser diode is responsible for "which wavelength emits," while the NTC of the CPO is responsible for "which wavelength MRM catches". The two must work together, but each operates independently.

One EXSENSE gold termination NTC chip with two missions

In the CPO architecture with ELSFP, the role of the EXSENSE gold termination NTC chip goes far beyond just "temperature measurement". In ELSFP, it acts as the "vital sign monitor" of laser diode, ensuring the light source is stable, pure and reliable. In the CPO optical engine, it acts as the "resonant point guardian" of the MRM, ensuring signals are efficiently modulated and transmitted precisely. The two are physically isolated and have different tasks, but their goal is the same: to ensure every beam of light travels on the correct wavelength. For optical transceiver designers and AI network architects, choosing an NTC chip requires considering the differentiated needs of both application scenarios. ELSFP scenarios require extremely excellent long-term stability (lasers diode cannot be frequently replaced), while CPO scenarios demand extreme size and response speed (extreme space compression), and EXSENSE gold termination NTC chips meet the selection requirements of both. Their sizes can be as small as 0.21mm × 0.21mm, supporting miniaturization development and fast response; Accuracy of resistant value/Beta can reach ±0.5% with great consistency, providing more precise temperature references.