Lidar Laser Diode Selection for Compact Imaging Systems

Although lidar tends to be discussed in the context of self-driving cars, there are many applications for lidar systems outside of the automotive industry. New UAVs that require environmental awareness can include a miniature lidar system alongside their other sensors, providing images of the surrounding environment that contain a significant amount of information.

Many precision measurements in scientific studies, turbulence measurements in aircraft, and other routine atmospheric measurements also use lidar systems. As an example, lidar is frequently used to gather remote measurements of atmospheric constituents, such as aerosol particles, water vapor, ice crystals, or trace gases. Scattering lidar techniques can be used to measure the altitude of cloud bases or track plumes of pollution. No matter which type of measurement you intend to gather, you’ll need to choose the right lidar laser diode for your application.

The Lidar Signal Chain

Whether you are designing a lidar imaging system for autonomous vehicles, advanced UAVs, or other advanced imaging systems, a pulsed laser diode will be a fundamental part of the signal chain alongside photodiodes or PIN arrays. Similar to a radar signal chain, you will need to carefully design the transmit and receive sides to scan the surrounding environment and create a depth map. These systems combine a number of standard and advanced components for driving a lidar laser diode and detecting low level laser signals reflected from distant targets.

There are a number of different lidar techniques for use in different applications. UAVs and autonomous vehicles that require imaging and depth measurements in a single dataset can use a scanning lidar system to generate point-by-point maps of the surrounding environment. In this application, laser pulses are scanned across the surrounding environment, and the reflected light is gathered with an infrared photodetector (currently 905 nm is used, but this may later move to 1500 nm). Distance can be calculated from a time-of-flight measurement, which can easily be performed with time-to-digital converter ICs. When used alongside a radar system, the two sets of sensors can simultaneously be used for target tracking (i.e., distance and velocity measurements) and imaging.

These imaging applications require a pulsed laser diode to gather point-by-point distance measurements, yielding a distance map of the surrounding environment. Atmospheric measurements based on differential absorption use two or more pulsed laser diodes, depending on the atmospheric constituent or other quantity (temperature or turbulence) being measured.

Lidar laser diode altitude map from a UAV

Pulsed Lidar Laser Diode Selection Criteria

Here we want to focus on the pulsed laser diode on the transmit side. Note that continuous wave (CW) laser diodes can be operated as pulsed diodes, although datasheets for these diodes do not normally quote the laser pulse length you can expect when driving a CW diode with voltage/current pulses. If you did want to drive a CW laser diode as a pulsed diode, you should measure the pulse length using an autocorrelation measurement, which is difficult without sensitive optical equipment.

The following points should be considered when selecting a pulsed laser diode for use in any lidar imaging system:

• Power output. This will determine two aspects of your system. The first is the particular safety requirements for your new product as pulsed lasers can cause eye damage at high power output. This will also determine the useful range for your system. Higher power outputs will provide longer range measurements.
• Wavelength. Different applications tend to use different wavelengths. As was mentioned above, some applications use more than one wavelength, which must be accounted for when selecting a laser diode.
• Duty cycle. Pulsed laser diodes are normally driven with high voltage, low duty cycle pulses. Although the instantaneous voltage can be quite high, the average power consumption is rather low thanks to the low duty cycle. The manufacturer will recommend a particular duty cycle that should be used to reach a specific pulse length.
• Minimum pulse length and maximum repetition rate. Datasheets for laser products normally quote repetition rates as a maximum as this is related to the recovery time of the laser medium. The minimum pulse length will determine the required duty cycle you must use to drive the laser diode.

Osram Opto SPLPL90

The SPLPL90 lidar laser diode from Osram Opto is designed for use in automotive systems, but it is also suitable for other ground-based systems with shorter range. This 2-pin laser diode provides 100 ns pulses at 915 nm with 25 W peak output power. Because this laser diode operates at 915 nm, there may be safety concerns if the system is used in an urban area at ground level.

The SPLPL90 lidar laser diode. From the SPLPL90 datasheet.

Excelitas Technology DPGEW1S09H

The DPGEW1S09H is one of a series of laser diodes that emit at 905 nm. This series of diodes (part numbers DPGEW1S09H to DPGEW3S09H) emit 30 ns pulses with peak power output reaching 45 W. The smaller pulse time can provide slightly higher resolution compared to the other lidar laser diodes presented here. See page 38 of the datasheet for more options from Excelitas Technology.

Hamamatsu L11854-336-05

The L11854-336-05 laser diode from Hamamatsu provides high power output (100 W) for long-range lidar systems operating at 905 nm. These diodes are designed to operate at 0.1% duty cycle and produce 100 ns pulses, requiring a specialized laser driver to produce pulses. The longer range makes this lidar laser diode a better choice for UAV or other remote sensing applications.

Photograph of the L11854-336-05 pulsed laser diode from Hamamatsu. From the L11854-336-05 datasheet.

Advanced lidar systems need the right lidar laser diode for building 3D maps of their surroundings, and Octopart^® is here to give you access to a huge range of lidar laser diodes. Try using our Part Selector guide to determine the best option for your next product.

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