Optical hybrid in free-space: advanced demodulation solution for quantum to space applications

Exail has an extensive experience in developing innovative technologies enabling ever higher transmission rate for fiber-optic telecommunications, in particular devices that can demodulate optical signals with very complex modulation. Developed for the telecom industry, Exail COH 90° Optical Hybrid now finds relevant applications in quantum communication (QKD) and space communication (optical communications in space).

Exail demodulation solution: a ground-breaking approach based on telecom technologies

Telecommunications have historically been based on the coding of information in binary mode (series of 0 and 1). With the use of optic fibers, the binary coding was applied through the amplitude modulation of laser light, driver of the information. This “on-off keying” modulation implies a “bit time” rate relative to the frequency of modulation per second. A bit time of 1 nanosecond means a transmission rate of 1 Gbit/s. A modulation solution is needed for the emission laser, and a demodulation solution is needed at the receiver side to convert the amplitude modulation in an electrical signal with data usable by a computer.

When the need for transmission rate higher than Gbit/s came, new up-to-date modulation and demodulation technologies had to be developed. Exail has been able to offer various products for such complex demodulation, based on free-space design and a long-term expertise in telecom technology. A series of products (MICS series or “Mux/Demux”) is dedicated to the multiplexing and demultiplexing, meaning the capability to transfer in a unique optic fiber many laser beams of different wavelengths (i.e 40 laser beams at 40 different wavelengths – 1528 to 1560 nm, the C-band – in a typical telecom fiber). Another series of products (MINT series or “DPSK demodulator) uses another property of light for the demodulation, the phase of the electro-magnetic wave. The modulation is easier and faster with the phase (0 or π) than with the amplitude. The optical DPSK (Differential Phase Shift Keying) demodulation is made possible with the MINT product, and it applies to transmission rate around 10 times higher (10 Gbit/s).

Exail COH 90° optical hybrids solution

Then came the generation of “QAM” technologies, a modulation technique using both phase and amplitude of the electromagnetic wave. On top of the previous advantages, this solution allows the use of intermediate values (between 0 and 1, between 0 and π) for the modulation. It gives much more possibilities to code the information. The QAM16 and QAM64 technology can for example deal with 4 to 6 bits in a single bit time.

The demodulation of the signal is complex and made possible by Exail COH 90° optical hybrids solution. The phase, the amplitude and even the polarization are extracted by performing four 90° phase stepped interferences between the signal and a reference, a local oscillator at a specific frequency. If the signal processing is not mastered by Exail, its core competencies rely in the design and the assembly of a free-space optical solution adapted to any kind of modulation. The COH 90° optical hybrids, the most advanced Exail demodulation solution, is very flexible. It can be applied to simple modulation, and then to more complex modulation without any update.

From telecom to space applications

If the telecom market was the first addressed by these modulation solutions, a large range of new applications find it useful today. Being able to demodulate the phase of a signal, in particular, is very powerful for example in metrology, for instance as an optic sensors measuring a tiny physical parameter that interacts with the phase of the electro-magnetic wave. In the medical field as well, it has some application: the Optical Coherent Tomography (OCT) uses it to image under-skin tissues by the demodulation of reflected signal.

Exail COH 90° optical hybrids is also qualified for space applications, such as laser communications or satellites constellation. They are manufactured following an in-house process of ultra-accurate assembling of micro-optics. The full process, from optical active alignment to gluing and assembling process, is based on technologies that have proved their abilities to respect constraints of space deployment. Due to the increasing demand for space products, Exail acquired in 2022 a new hermetic seam-sealing system to carry out mass production of its components, sealed in a controlled atmosphere, at atmospheric pressure.

Many other devices exist in the Telecom wavelength band, the “C-band” (i.e silicon chip), but Exail free-space solution is the only one able to address both the C-band and the L-band (wavelength from 1570 to 1630 nm). It is also better in the C-band than the silicon solution, thanks to the better performance of the free-space solution optics for the demodulation of polarization (reaching 25 to 30 dB of extinction rate), as well as better balanced uniformities (0,2 dB) and 90° phase shift accuracy (phase shift can be tunable as an option). Today, perspectives are even promising for Exail demodulation solution towards all Telecom bands with a single device working from O- to L-band (from 1260 nm to 1620 nm).