Optical Connections Magazine Spring 2024

PAUL MOMTAHAN COMPOUND SEMICONDUCTORS

optical communications. Gallium arsenide also provides VCSEL arrays for 3D sensing in devices such as smartphones, smartwatches, and VR headsets, and in automotive applications such as in- cabin sensing and LiDAR. Additional gallium arsenide applications include gas sensing, biomedical sensing, and computer mice. Indium phosphide PICs also have a wide range of emerging and potential applications. These include defence, automotive LiDAR, 3D sensing for wearables and cell phones, solar cells, and medical sensing. Indium phosphide PICs could also have a role in quantum computing and in neuromorphic computing for artificial intelligence and machine learning. For example, Eindhoven University of Technology demonstrated an indium phosphide photonic neuromorphic PIC in 2020. Compound semiconductors even have an evolving role to play in electronic applications. Indium phosphide and

Furthermore, as silicon photonics modulators are unlikely to be efficient in enabling symbol rates much beyond 120 to 140 Gbaud, future generations of coherent optical engines at 1.6 Tbps per wavelength and beyond will be dependent on compound semiconductor modulators, with indium phosphide and thin-film lithium niobate (LiNbO3) the leading candidates. In addition, indium phosphide can detect DWDM light, so can also be used for the photodetectors, though germanium can also provide this function in silicon photonics. Indium phosphide uniquely enables all these photonic functions to be integrated into a single photonic integrated circuit (PIC) in contrast to the photonics for early coherent optical engines that comprised of hundreds of discrete components connected with coupling optics, which resulted in bulky and expensive devices with a less than optimal mean time between failure (MTBF).

analogue converter (DAC) and analogue- to-digital converter (ADC) and additional functions such as forward error correction (FEC), framing, multiplexing, encryption, and performance monitoring. The digital ASIC, often referred to as simply the DSP, is made of silicon leveraging the same state-of-the- art CMOS process node technologies (ie 7nm, 5nm, 3nm) used in premium smart phones and the latest high-end computing, graphics and artificial intelligence chips. Next, an analogue ASIC integrates multiple drivers and transimpedance amplifiers (TIAs). The drivers take the low voltages from the DAC and convert them to the higher voltages required by the modulator. In the receive direction, the TIAs take the currents from the photodetectors and convert them to the voltages required by the ADC. The analogue ASIC is typically made from silicon germanium (SiGe). And finally, we have the photonics. This includes the laser that generates the light with the desired frequency/wavelength. Then we have the modulator that takes the light from the laser, and by changing the phase and amplitude, encodes the data. It does this by using an electric field to change the refractive index of the material the light is passing through. The coherent modulator itself includes four individual Mach-Zehnder modulators (MZMs) in addition to splitters, combiners, phase shifters, a polarisation rotator, and a polarisation beam combiner. Then on the receive side we have the photodetectors and passive photonics including a polarisation beam splitter and 90° hybrid. Some coherent optical engines may also include an optical amplifier to boost the power of the signal and a tuneable optical filter (TOF) to minimise any out-of-band noise resulting from this amplification. Only the compound semiconductor indium phosphide can provide laser and optical amplification functions at DWDM frequencies, so some amount of indium phosphide will always be required in all coherent optical engines. Silicon cannot perform these functions as it is an indirect bandgap semiconductor, meaning that excited electrons produce heat, not light. Indium phosphide also has an inherently superior modulation effect compared to silicon. This enables more bandwidth (GHz), higher symbol rates (Gbaud), and higher date rates (Gbps), thus reducing cost and power consumption. This performance advantage is especially important for the highest- performance embedded segment of the coherent optical engine market. Indium phosphide modulators also require less voltage and length for a given phase change compared to silicon, resulting in lower power consumption and more compact devices, which can especially be an advantage for coherent pluggables.

Figure 2- Compound Semiconductor Applications

gallium arsenide are already used today for some niche applications such as high-radio-frequency electronics for mobile communications. However, as we reach the limits on shrinking silicon chip geometry, the industry is also looking at compound semiconductors, with their higher electron mobility and superior temperature stability, as potential successors. Silicon carbide (SiC) and gallium nitride (GaN) are the primary compound semiconductor R&D candidates for succeeding silicon as the mainstream semiconductor material. So, to summarise, in addition to silicon, domestic production of compound semiconductors including indium phosphide should also be considered a strategic imperative. Furthermore, indium phosphide PIC fabrication is a scarcer resource, and domestic production of key supply inputs such as indium phosphide wafers also needs to be considered. Infinera applauds the passage of the CHIPS and Science Act and the equivalent European legislation, which will help compound semiconductor manufacturers such as Infinera to expand domestic production of indium phosphide PICs and boost US and European countries’ global competitiveness.

As is the case with conventional electronics, with advanced fabrication and integration capabilities in production, manufacturing one PIC is far more cost- effective than manufacturing individual optical components and then integrating and packaging them. PICs also have a dramatic impact on footprint, enabling the miniaturisation of optical devices. Power consumption is also reduced, while performance can be improved due to minimised optical coupling losses when connecting optical functions with waveguides inside the PIC, as opposed to coupling optics between discrete components. And equipment failures are reduced, as these coupling optics are eliminated as a source of failure. These benefits have enabled indium phosphide PICs to play a key role in evolving coherent optical engines, with potential future applications inside the data centre including coherent lite and linear drive optics. Another compound semiconductor, gallium arsenide (GaAs), provides the vertical-cavity surface-emitting lasers (VCSELs) and photodetectors for short- reach optics over multi-mode fibre at 850 nm. Compound semiconductors also have important applications beyond

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ISSUE 36 | Q1 2024

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