Adaptive Cruise Control: Laser Diodes as an Alternative to Millimeter-Wave Radars

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Adaptive cruise control (ACC) is an automotive convenience or safety feature that allows a vehicle's cruise control system to adapt the vehicle's speed to the traffic environment. ACC can be realized by three different technologies: millimeter-wave radar, stereo-metric and light radar or light detection and ranging (LIDAR). Laser diode is a key component of the LIDAR-based ACC. In this article we will investigate the benefits of a LIDAR system while exploring the advances in laser diode technology.

Adaptive cruise control or ACC, like conventional cruise control, maintains the speed set by the driver while automatically adjusting the speed to maintain a pre-set distance between vehicles if the vehicle preceding the ACC-equipped vehicle slows down[1]. The ACC functionality is illustrated in Figure 1. Initially, there is no vehicle in front of the ACC-equipped vehicle and the speed is set at 65 mph. When this vehicle encounters a slow moving vehicle traveling at 55 mph, the ACC system instructs the ACC vehicle to decelerate, maintaining the ACC vehicle speed at 55 mph with a pre-defined distance between vehicles. Once the slower vehicle in front moves away, the ACC vehicle accelerates and reverts to the originally set speed limit of 65 mph. The ACC system allows the vehicle to autonomously slow down and speed up depending on traffic conditions, without driver intervention.

The most prominent technologies of ACC systems are millimeter-wave radar and LIDAR. While it is possible to use a video camera to estimate the speed or distance between vehicles, it is not a reliable technology in varying weather conditions. The radar-based systems operate in the millimeter-wave region of 76 GHz to 77 GHz and can sense distances of up to 150 meters with a resolution of one meter. Shorter frequency radar systems in the 24 GHz range can be used for pre-crash sensing applications that require sensing up to 20 meters. The larger size radar systems provide a challenge to automotive designers when integrating them into vehicles. Also, because of the high costs of manufacturing ACC systems, they are offered as an option only on premium luxury vehicles like the Mercedes S-class.

Alternatively, LIDAR-based ACC systems offer many advantages such as small size, simple assembly, and relatively low cost. Currently, Nissan and Toyota offer mid-size luxury cars equipped with LIDAR-based adaptive cruise control systems. Figure 2 shows the LIDAR sensor manufactured by Omron and Figure 3 is an example of the radar sensor manufactured by Denso.

The laser-based ACC systems measure the distance of vehicles traveling in front of the ACC-equipped vehicle by using triangulation or time-of-flight measurement where the laser pulses are reflected by the preceding vehicle. The sensor uses a high-power laser diode to transmit infrared light pulses with a wavelength in the range of 850 nm to 950 nm. A high-speed PIN photodiode or avalanche photodiode receives the light reflected by the preceding vehicle and determines the distance and the speed relative to the vehicle in front. The time of flight measurement principle is illustrated in Figure 4.

The time-of-flight measurement principle involves sending short duration pulses that are reflected back from the object. The distance to one or more objects can be determined by the delay time of the reflection. The time-of-flight measurement using laser diodes and photodiodes is realized through the following scanning methods: a) beam splitter, b) polygon mirror, c) moving transmission mirror and d) addressable laser array.

The LIDAR-based adaptive cruise control system offers excellent sensing distance. It can detect multiple objects separately, in a wide area, with high precision leading to superior lane recognition. The LIDAR ACC module does not need any special alignment during vehicle assembly since it automatically aligns during operation[3].

The LIDAR module is compact and can be mounted easily inside the vehicle without compromising the design aesthetics. The laser is eye safe and there are no other health concerns. Also, there is no chance of interference with other sensors out on the road.

Laser based ACC's provide ranging capabilities and integration of functions such as vehicle classification, fog sensing, visibility detection to adjust the lighting of the vehicle and headlamp leveling. Additionally, the LIDAR-based ACC system can be used for pre-crash sensing and collision avoidance functionalities to convey warnings to drivers. Ultimately, the laser can be used for vehicle-to-vehicle communications in the intelligent transportation system initiative that many countries are currently developing.

Finally, the LIDAR-based adaptive cruise control system is less expensive than radar sensor and has low-power consumption needs, an important criterion for high fuel efficiencies. The laser diode is the key component of the LIDAR sensor and can be manufactured in high volume using standard semiconductor processes. It is relatively inexpensive, contributing to the proliferation of ACC technology across more vehicle platforms.

The LIDAR-based ACC system requires lasers that are capable of sending a high-power light pulse with a short pulse width in the order of 10 ns and meet stringent automotive temperature requirements. The laser diode technology today can boast a lifetime of greater than 10 years at the operating temperature range of -40 °C to +85 °C.

Laser diodes with very high pulse power increase the range of the ACC sensor. The distance range of the ACC sensor depends on many factors: weather conditions, optical design of the system, wavelength of the laser and sensitivity of the detector or the photodiode.

A typical pulsed laser diode has a peak power of 10 W to 25 W. Higher optical powers are achieved through stacking of laser emitters. Multi-emitter or individually addressable laser arrays also offer an alternative to high power laser diodes used in conjunction with rotating prisms.

In LIDAR ACC systems, light pulses of short duration (10 ns to 50 ns) and low pulse repetition frequency (1kHz to 20 kHz) are generated. This results in a duty cycle of 0.001% to 0.1%. The required peak power is in the range of 1 W to 100 W with the average power close to 10 mW in most cases[4].

The laser characteristic (P-I), the diode characteristic (V-I) and the electro-optic efficiency are shown for a 905 nm laser diode with a maximum output power of 25 W at input current of 30 A is shown in Figure 5. The electro-optical efficiency is approximately 20%, depending on the operating point, resulting in a power dissipation of around 40 mW. Due to the low power dissipation, the laser diodes can be suitably packaged in 5 mm radial plastic housings[5]. Such a package is shown in Figure 6.

In order to increase the output power without altering the radiation characteristics of the laser diodes suitable for ranging applications, individual emitters are stacked on top of each other. With a serial connection, the operating voltage increases according to the number of emitters. This is overcome by so-called Nanostack technology patented by OSRAM Opto Semiconductors[6].

In the Nanostack construction, the individual emitters are grown on top of one another by epitaxial growth of the crystal and connected by efficient tunnel junctions. Due to the small distance between the emitters, Nanostack is perceived as an individual emitter in the far field. A schematic of the Nanostack construction is shown in Figure 7.

The laser characteristic of a 905 nm pulse laser diode with triple Nanostack emitter is shown in Figure 8. Peak power of 100 W is reached at 50 A. At 30 A the peak power is 75 W compared to 25 W for the single emitter (see Figure 5).

Laser diodes with built-in driver circuits to address EMI issues are also available for systems that require very high current in the order of 50 A and short switching time of about 10 ns. Smartlaser from OSRAM Opto Semiconductors is such a device that contains built-in field effect transistor and two capacitors connected in parallel[7]. A picture of Smartlaser is shown in Figure 9.

It is essential to keep circuit paths short in order to reduce the parasitic impedance. This has been implemented in an ideal way in the Smartlaser. The basic circuit diagram of the Smartlaser is shown within the dashed lines in Figure 10. The three leads serve to connect the charge voltage (Vc), the trigger voltage (Vtrigger) and ground (GND). The capacitors are charged by the charge voltage. With the application of a trigger signal, the FET switches through, causing the stored energy in the capacitors to discharge through the laser diode, thus creating a short current pulse. In order to keep switching times low, a MOSFET driver should be used at the trigger input. The maximum current, and thus the optical power, can be controlled by the level of the charge voltage Vc. The pulse shape and repetition rate can be controlled via the input trigger signal.

Another interesting laser solution for LIDAR-based ACC systems is the individually addressable multichannel laser array that is shown in Figure 11. The laser array consists of many single emitters that are parallel connected. Many packaging options for such arrays are possible together with integrated optics and electronics.

LIDAR-based ACC systems offer simpler assembly, high reliability and low-cost solution for proliferation across different automotive platforms. Laser diodes in plastic packages are automotive compliant and inexpensive. Laser diodes with integrated driver for short pulses and exact distance measurements have no EMI problems. Customized solutions with optics or multiple emitters offer added functionality to basic ACC systems.