Cryogenics covers a wide range of applications including health, food, industry and physics. For instance, quantum computing is a great challenge due to the huge needs of cooling power at very low temperatures. Cryogenics is also closely linked to space activities, either with launcher propellants or infrared earth observation for climate monitoring. In this particular case, the infrared imaging sensors are cooled down to cryogenic temperatures to improve the signal-to-noise ratio, and hence, the image resolution. 

The link between temperature and the operating wavelength of photonic components is well-known. Semiconductor components, like laser diodes, have been investigated, leading to the characterization of the temperature dependence of physical parameters including band gap energy, carrier mobility, effective masses, and internal losses. However, no experimental results on commercial semiconductor optical amplifiers (SOAs) at cryogenic temperatures were available before our first publication in 2022. Since then, we have continued our exploration of such components in the cryogenic regime. In this current paper we present the outcomes of our recent investigations on an SOA working in the  [293 - 55K] temperature range, and new application perspectives for cryogenics.

The results were obtained on a dedicated test bench built around a cryocooler and its cryostat. A commercial-type cryocooler, providing about 1 W at 55 K, is used for cooling a 64P37E-type SOA from Aeon, which is located on a custom supporting plate at the cold tip. The optical and electrical signals are provided via feed-throughs. The SOA is driven by a DC current source in the range of 0.1 to 100mA, and both the current and the voltage are monitored. The optical setup is a pump-probe configuration, where a weak data optical signal is modulated using a 50GHz bandwidth modulator, and the optical pump is used to replace the electrical signal, as it will be presented further on. The optical signals can be measured either with an optical spectrum analyser or a 20 GHz bandwidth vector network analyser. The latter controls the modulator and receives the data through a photodiode and an electrical amplifier.

We first characterized the static SOA parameters over the full temperature range. It should be noted that the following results are corrected for the coupling losses, which are temperature-dependent. While the SOA operates at room temperature with a gain of 33.7dB and a current of 420mA, a gain of 41.0dB is achieved with only 50mA when cooled at 70K. The Noise Figure drops from 5.2dB at room temperature, down to 3.4dB at 70K, which is close to the theoretical limit of 3dB. Dynamic performances are also dramatically improved at cryogenic temperatures : the modulation efficiency is enhanced by +15.8dB, while the cross-gain modulation bandwidth is increased by 55%. Finally, at 60K we were also able to operate the SOA without electrical power, providing only optical signals and observing a gain of 23dB.
SOA are foreseen as a future replacement for traditional Erbium-Doped Fiber Amplifiers (EDFA), thanks to their small footprint and low unit cost. The main usages are in optical telecommunication networks, such as national fiber backbones and satellite transceivers. Based on our results, we performed simulations of an inter-satellite optical transmission with cryocooled SOA, for which we assumed a data rate of 10 Gbit/s. The results show a gain of +6.8dB on the link budget. This margin can be used to decrease 4-fold the optical transmitted power, thus reducing the power needed from the satellite platform. As SOAs and laser diodes rely on the same physical principles, we have extrapolated our results to the concept of an EDFA with cryocooled pump laser diodes. Our calculations show that the wall-plug efficiency of such a device is 30% higher, cryocooler included.

Thanks to our results, we believe that cryogenics can be combined with photonics to create a new disruptive use case: cryophotonics. A broad range of applications could be impacted, such as optical signal processing, all-optical switching, quantum computing…

Keywords
photonics|optical communications|laser|cryophotonics