Sat. Oct 1st, 2022
Image of a roller coaster race track.
enlarge Kind of like this, except for photons.

Sometimes it seems like you can’t go a week without hearing new lidar news. This is troubling, as lidar has long been the desperation application for lasers. If you’d made a new pulsed laser and couldn’t figure out what it might be good for, you’d come up with something that needed a remote measurement and claimed your laser was useful for that application. At best, you would give the laser to an atmospheric scientist and have him measure the density of aerosols in the upper atmosphere.

Okay, that might be a little cynical, but for a very long time, lidar research was an unexplored backwater harbor on the sea of ​​laser physics. But new research shows how much this has changed: a new device that has wide-ranging applications and is likely to have a huge impact on optical communications. Still, the device is marketed as great for lidar, which it very well could be.

Lidar is difficult

It wasn’t that lidar was unappealing to engineers in the past. It’s more that lidar was impractical. Lasers were large, the pulse duration was too long, the optics for collecting the signals were large, the electronics were simple… but large. Lidar instruments were delicate. The idea of ​​fitting an expensive and fragile device into the grille of a nitrous oxide-injected Honda Civic, powered by a hormonal boy racer, probably didn’t excite many engineers.

Then powerful lasers became smaller, cheaper and less delicate. The electronics required for optical communication can be easily adapted to lidar. Suddenly lidar started to look good.

For lidar to work effectively, you must efficiently combine a laser beam reflected from an object with a reference beam. The internal routing and external optics form one system, and misalignment between the transmit and receive beam results in miscalculation of direction. The simplest solution is to use the same optics to send and receive signals. But this is expensive and requires fast micromechanical or electro-optical devices. It is much better to have a passive optical circuit that simply redirects the light depending on its function. To do that, we need a non-reciprocal optical device.

A typical non-reciprocal device is called a circulation pump (see box). These are bulky devices where the light is usually passed through a relatively strong magnetic field and a special material that is almost certainly viewed obscurely by engineers responsible for manufacturing machinery. In a circulation pump, light entering port 1 will exit port 2, while light entering port 2 will exit port 3, and light entering port 3 will exit port 1 (assuming there are only three ports).

However, for lidar, what you may want is more complicated. A powerful pulse of light has to travel from gate 1 to gate 2, where it goes to the outside world. The signal coming back enters port 2 and exits port 3. A weaker reference signal enters port 1 and exits port 3 to mix with the return signal. Under these conditions, the outgoing laser pulse and the reflected laser pulse are automatically matched. The reference and the received pulses are also automatically aligned.

Make silicon refuse to answer

Creating this kind of device is exactly what the researchers have achieved, albeit in rather artificial conditions. But they’ve done it using standard silicon-integrated optical fabrication techniques, meaning if it works as a unit as expected, it can be manufactured very cheaply.

The researchers relied on silicon’s nonlinearity to ensure that the device’s behavior would change depending on the brightness of the light in the device. Essentially, if light is bright enough, it can change the properties of the material it moves (this is what happens when you set termites on fire with a magnifying glass). However, in the small gap between a material that is not changed by light and it catches fire, the change can be helpful. Optical engineers spend a lot of time throwing themselves and their expensive gear on this hole, with only the occasional survivor emerging without their hair on fire.

The device is basically a pair of waveguides that make up four ports, only three of which are in use. Ports 1 and 2 are directly connected while port 3 is in a separate waveguide. The researchers created an oval racetrack resonator between the two straight waveguides. In addition, there is a small defect in the waveguide placed between ports one and two. This defect acts like a mirror in low light. However, if the intensity is high enough, the optical properties of the defect are changed and the light pulse can be sent.

Light traffic ahead

A strong pulse of light is sent to port 1, where a small portion is transferred to the ring through a process called evanescent coupling. The light in the ring spins, transitions to the next waveguide and exits port 3. The light from port 1 exiting port 3 is the reference pulse. But most of the strong light pulse continues outside the ring, encounters the defect and modifies it, preventing it from reflecting the pulse. Therefore, the rest of the pulse is sent out of port 2 and out to the outside world.

"Artists" impression of the resonator.  The laser pulse (in pink) enters through port 1 (brown).  A small section jumps to the race track and then to the next waveguide where it exits gate 3.  The rest passes through the defect (light blue) and exits port 2. The lidar return signal (dark blue) enters port 2, bounces off the defect and exits port 3. Half of the lidar return signal is lost through port 4.
enlarge “Artists” impression of the resonator. The laser pulse (in pink) enters through port 1 (brown). A small section jumps to the race track and then to the next waveguide where it exits gate 3. The rest passes through the defect (light blue) and exits port 2. The lidar return signal (dark blue) enters port 2, bounces off the defect and exits port 3. Half of the lidar return signal is lost through port 4.

Chris Lee

However, light entering through port 2 (light that the lidar would have received from the outside world) travels in the opposite direction. The light from port 2 still enters the circuit, but cannot go out at port 3 because it is going the wrong way. Under normal circumstances, it would exit port 4. This is where the defect plays its part. But when the light intensity is low, the defect is still reflective. It reflects the pulse back in the direction it came from. On the way back, the light is coupled in the ring and can go out of port 3, just like the reference signal from port 1.

Note, however, that the transmission of light between the waveguide and the ring is reciprocal, meaning that about half of the light coming in from port 2 has to leave port 4 and half from port 3. This sounds bad, but is it actually not. These types of devices often have high losses no matter how they are built, so I don’t see a 50 percent loss as a big problem. Given that auto-lidar operates in the 100m range, where the signals are relatively strong, it is likely to be able to absorb the additional losses.

The researchers have not demonstrated this with a true lidar unit. Instead, all the light pulses traveled through optical fibers with varying delays. The signals they ended up with seem rather strong for a lidar return signal. Nevertheless, this is all made with standard processes, so connecting to a real system will not be a difficult task.

Nature Photonics, 2020, DOI: 10.1038/s41566-020-0606-0 (About DOIs)

By akfire1

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