Noiseless InGaAs avalanche photodiodes (APDs) are a new class of APD
By Ben White, Co-Founder and CEO at Phlux Technology
Infrared systems include laser rangefinders, LiDAR, and Optical Time Domain Reflectometry (OTDR) systems. While each of these instruments does a different job, they have one important thing in common: they all send out pulses of infrared (IR) light and detect any reflections, either from the general environment or from inside an optical fiber. By measuring the time taken from the pulse being sent to its reflection being detected by an IR sensor, it’s possible to determine the distance from the source at which the reflection was created.
The sensors that detect the reflections are photodiodes (PDs). Photons land on them after passing through some kind of lens arrangement and that light is converted into an electrical current. Connect the PD to a trans-impedance amplifier (TIA) and you get a voltage output. Connect this to an analog-to-digital converter (ADC) and you have a digital signal for processing.
Avalanche photodiodes (APDs) are a type of photodiode with a useful characteristic. If you increase the reverse bias voltage on them, they exhibit gain, multiplying the level of current produced. It’s an avalanche effect in which carriers, electrons, and holes, are excited by absorbed photons and strongly accelerated in the strong internal electric field so that they can generate secondary carriers. This makes the diodes so sensitive that some can even detect a single photon.
The more sensitive the diode, the less amplification it needs and the less susceptible it is to electronic noise. The wavelengths to which APDs are most sensitive are determined by the compounds used in their manufacture. Silicon-based APDs show peak sensitivity from around 900nm. By applying a reverse voltage, they can achieve a current multiplication factor of btween 50 and 1000.
For wavelengths from 1000 to 1700nm, APDs based on germanium or indium gallium arsenide (InGaAs) are best. Here, typical current multiplication factors are limited to 10 to 40 due to the higher noise levels they produce, but InGaAs performs better than germanium in this respect.
InGaAs APDs are manufactured in a compound semiconductor process, which makes them more expensive than silicon equivalents. However, silicon APDs have one major drawback. The equipment that uses them operates at around 905nm. Legislation mandates strict limits on the power of lasers that can be used to transmit the IR pulses because, at this wavelength, retinal damage can be caused if the pulses reach a human eye. By contrast, InGaAs APDs, which normally operate from 1300 to 1550nm (the latter being a particularly popular option) are the technology of choice when systems must be “eye safe.” This means that such systems can use more powerful IR lasers and achieve greater distances without endangering eyesight.
The challenge has been to reduce the noise produced in InGaAs APDs and hence improve their sensitivity.
Noiseless InGaAs APDs
After eight years of research at the University of Sheffield, a new class of InGaAs APD technology was recently announced. Through the addition of antinomy alloys, the new APDs demonstrate up to twelve times higher sensitivity than traditional InGaAs components. They can be used with APD gains over 120, have fast overload recovery, and feature ten times lower temperature drift than components without antinomy. What’s more, they have stable high-temperature performance.
Phlux calls these “Noiseless InGaAs APDs” and—while they may not be completely noiseless—”alcohol-free” beer isn’t completely free of alcohol either! They do offer more than an order-of-magnitude reduction in noise and improvement in sensitivity, and so are effectively a new class of APD.
This breakthrough was achieved by designing a new InGaAs APD that incorporates antimony (Sb) alloys into the epitaxial structure. But what does all this mean for end applications?
Boosting performance while reducing system cost and size
Returning to the application examples of high-performance laser rangefinders, LiDAR, and Optical Time Domain Reflectometry (OTDR) systems. All of them that work at 1550nm can achieve up to 50% greater range by simply replacing the existing APD with the new device described. That’s a massive performance boost for simply changing a drop-in replacement diode. In addition, Noiseless InGaAs APDs achieve high gain well before the reverse bias reaches their breakdown voltage, thereby enabling stable operation.
In new designs, there’s the opportunity to make various trade-offs. For example, rather than going for increased range, Noiseless InGaAs APDs could enable 12X greater LiDAR image resolution for a given laser power. Alternatively, up to 30% reduction in system size and weight and up to 40% lower system costs may be achieved by reducing the power of the laser diode that generates the IR pulses and using smaller optical apertures. Lower laser power means simpler thermal management and the new APDs work at up to +85°C without performance degradation, which is a significantly higher temperature than conventional equivalents. In practice, the application, product performance, system size and weight, and cost will determine the optimum trade-off for a particular application.
Single-mode fiber OTDRs typically allow switching between 1310 and 1550nm wavelengths. This is because 1310nm is often best for catching fiber alignment problems and 1550nm for finding bending or cracking issues in the fiber. Comparing reflection and scatter results at both wavelengths helps determine the nature of any faults. Where Noiseless InGaAs APDs are used to replace traditional APDs, the range and accuracy of measurements can be immediately boosted. The temperature stability of the new APD technology is important too because these instruments are most used in the field where environmental conditions vary widely.
Other applications
Of course, the potential applications for the new APD sensors are far wider than those described above. They include, but are not limited to, the following:
- Optical fiber communications
- Imaging
- Laser microscopy
- Raman spectroscopy
- Gas sensing
- Quantum communications
- Free space optical communications
Some of these exciting applications are predicted to show substantial growth in the coming years, not least quantum, free space, and optical fiber communications. In all of them, greater APD sensitivity will create new design opportunities and greater design flexibility. Ultimately, this will result in products with higher performance and more predictable and stable operation.