ENABLING LONG RANGE LIDAR WITH OPTICAL FIBERS

In our latest blog post on lidar, we explained its general principle, along with some advantages of using 1550 nm technology and how fiber-based systems can be a strategic choice for the emitting laser. The main advantage of using a 1550 nm light source for a lidar system is that you can generally achieve a longer range of detection.

This time, we will go into more detail on fiber laser sources for lidar applications, with more information on what it enables for autonomous vehicles and other technologies.

Definitions and requirements for autonomous vehicles

The degree of autonomy of a vehicle is defined in the industry over five “Levels” that are defined by how much input is required by the driver.

The first two levels are for driving assistance systems like parking assistance or distance alerts for close vehicles. These levels require the driver’s full attention behind the wheel and are meant to increase safety and convenience but are not for autonomy. Starting at level 3, systems need a much longer range to have good reaction times, especially at very high speeds. This is where long-range1550 nm lidars are advantageous. It is even more important for larger vehicles like trucks that need a longer distance to stop.

Basics of Fiber Amplifier Light Sources for Lidar

In a long range 1550 nm time-of flight (ToF) lidar, rare-earth doped optical fiber will be used in the optical amplifier for the light source. Optical fiber amplifiers can be high powered, are robust and can resist high temperatures. It is a mature technology, its roots coming from the telecommunication industry.

A seed laser (generally a laser diode) will emit a low power at 1.5 µm output with the required pulse characteristics and will go through one or many amplifier stages before being emitted.

There can be a preamplifier stage using single-clad erbium-doped fiber, which will be core-pumped by a single mode pump diode, followed up by an amplifier stage built on a double-clad erbium/ytterbium fiber.

Otherwise, the amplifier can be designed using only double-clad erbium/ytterbium fibers, based on a single stage or a two-stage architecture. For example, the next diagram illustrates a two-stage system.

Coractive supplies both erbium and erbium/ytterbium-doped fibers to act as the amplifier medium in these systems and provides matched passive fibers to make the components required to build the amplifier seen in the diagrams (couplers, pump combiners, isolators).

Our fibers enable the optimization of many critical specifications of the amplifier:

High optical efficiency: to minimize the pump power required and offer great thermal behavior.
Minimized unwanted emissions: high Signal-to-Noise ratio in the 1.5 µm range and low 1 µm ASE (Amplified Spontaneous Emission).
High temperature resistance: Different coatings can enable a temperature resistance of 85°C, 105°C, and up to 125°C.

Other applications and alternative technologies

With 1550 nm lidar systems now achieving a long range of detection (over 500 m), applications other than self-driving cars and trucks are emerging. Ships, mining vehicles and trains are all large equipment in which the vision of the operator is narrow or limited. Lidar systems can add layers of security by sensing where the eyes can’t see.

Stationary lidars may also be used on highways, street intersections, ports, and other types of infrastructure. Uses can vary from pedestrian detection to replacing human-operated tugboats with automated vessels to dock large ships.

Technologies such as wind sensing require polarized light to enable the measurement of velocity at each point. An advantage of optical fibers is that they can be built to maintain polarization and will bring the high power required to achieve good reflection when observing wind and gases. A similar design to what we have presented can emit a polarized beam if built with PM fibers.

Beyond optical fiber amplifiers, semiconductor technologies are also progressing for 1.5 µm laser emission and detection using silicon photonics, as well as photonic integrated circuits (PICs). FMCW lidar (an alternative to Time-of-Flight) is emerging from these semiconductor technologies and can be complementary.

We believe that the future of lidar applications will consist of a variety of technologies that will each find optimal uses. Coractive will be at the forefront of optical fibers for lidar and we are looking forward to the future of mobility.

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