VCSELs for ToF Applications - How VCSELs can be best put to use

05/11/2022 Know-How

VCSELs (vertical-cavity surface-emitting lasers) are superior to LEDs when it comes to short switching times and a narrow optical spectrum—making them the best choice for time-of-flight (ToF) applications. To make the best use of the abilities of VCSEL chips, however, the packages need to be optimized for them.

A brief look at the ToF principle can help better judge the benefits of VCSELs. ToF uses the speed of light to measure distances or—with a ToF camera—to capture three-dimensional images. There are two methods for this: the direct and the indirect ToF method.

Direct ToF measures the time between the sending of an optical pulse and the arrival of the reflected light pulse.

Indirect ToF sends bursts comprising multiple pulses with a high modulation frequency (measured in MHz) and measures the phase shift between the transmitted signal and the received, reflected light signal. Figure 1 shows how this method employed by ToF cameras is used to capture 3D images.


Rising Modulation Frequency Requires Low-Inductance Design

The first important aspect of a VCSEL is the modulation frequency. The higher it is, the better the depth resolution and short-range detection of the ToF camera. Currently, modulation frequencies of up to 100MHz are used, which corresponds to a period length of 10ns. With a duty cycle of between 30 and 50%, this means that the VCSEL has a turn-on time of just 3ns to 5ns. In other words, the rise time and fall time of the light source must be much shorter than the turn-on time—typically under 1ns for a modulation frequency of 100MHz. VCSEL chips offer rise and fall times of well under 1ns, making them far superior to LEDs, which usually only achieve around 10ns.

However, a special low-inductance design is required for the package to prevent deterioration in the superior switching behavior of the VCSEL chip. The substrate design here is critical.


Field of View: Making a Circle a Square

The second key aspect is the light output curve. The field of view (FoV) or lighting range is expressed as a vertical and horizontal angle, usually measured at full width at half maximum (FWHM). This describes the illuminated area—which is almost rectangular—to illuminate the field of view of the camera as optimally as possible. However, a naked VCSEL chip has a ring-shaped beam pattern with a beam divergence (i.e. the further away you get from the source, the wider the beam becomes) of around 15° to 25°. A diffuser lens reshapes the beam into its rectangular form (Figure 3).

But the form is not the only important factor—the light distribution within the FoV is also key. Figure 4 shows the horizontal and vertical beam diagram for a VCSEL from the manufacturer Lextar. The maximum beam intensity here is not in the middle (at 0°), but at the outer edges. This bathtub-shaped pattern ensures that the area is evenly lit. This is shown in Figure 5, where the light distribution is visible on a flat surface. The light intensity is distributed evenly over a large angle—meaning that the FoV of the ToF camera is also evenly illuminated. VCSELs also have an advantage over LEDs here, the latter having a circular light pattern with the maximum value at the 0° axis.


Eye Safety

If the laser beams from a VCSEL strike the eye, they can cause serious damage. This is especially dangerous in applications such as driver monitoring or person counting if the VCSEL lens has been damaged or detached as a result of excessive mechanical stress, as this can change the beam pattern, which may intensify the radiation significantly.

To prevent this, Lextar has integrated a monitoring diode in the VCSEL package alongside the VCSEL chip (Figure 6). It uses part of the radiated light, which is reflected back to the photodiode through the lens. A damaged or detached lens would change or lose the light reflection to the photodiode and thus change the photocurrent. If an anomalous photocurrent is detected, the VCSEL can be disabled immediately, thus preventing harmful laser radiation emissions.


Heat Management for Large Ranges

For a ToF camera to achieve large ranges, VCSELs are operated with high currents measuring several amperes. This requires effective heat management design in the package substrate to enable efficient heat dissipation. Ceramic packages and silver-rich die-bonding pastes are used for this purpose.

VCSEL chips with a good package provide a light source with superb properties in relation to radiation intensity, modulation frequency and optical properties, allowing more powerful ToF systems compared to LEDs.

For a variety of applications, Lextar offers VCSELs with two typical ToF operating wavelengths of 850 and 940nm. 940nm is invisible to the human eye, while 850nm VCSELs have a visible red dot on the VCSEL—especially apparent in the dark. At 850nm, the camera sensitivity is 50 to 100% higher than at 940nm. This means that the signal-to-noise ratio (SNR) is better at 850nm than at 940nm. In terms of sunlight rejection, however, 940nm models are often the better choice, as the intensity of sun radiation is much lower in this range.


Optimized Driver Circuit Improves the System as a Whole

A complete lighting unit requires not only the VCSEL but also the driver, which should be selected in consideration of the following technical parameters:

  • Short delivery time: The most important components of a driver circuit include a field-effect transistor (FET) gate, the gate driver, and passive components. These are usually immediately available, enabling a prototype for a lighting unit to be developed within a short time.

  • Design flexibility: A trigger circuit with discrete components offers more flexibility and can be adapted to a variety of design requirements (e.g. peak currents, pulse forms, adjustable rise times).

  • Power: GaN-FET switches generally have a lower resistance (RDSon) compared to corresponding silicon-based products. This enables the driver to handle higher current spikes, which can increase the efficiency of the overall lighting unit.

  • Scalable driver parameters: Ideally, the driver electronics will support modulation frequencies of up to 100MHz or even more.

IC-Haus, for example, offers the iC-HG 6-channel laser driver, which enables spikeless switching of VCSELs with properly defined currents at CW (continuous wave) frequencies of up to 200MHz. The channels can be paralleled for full 3A CW operation and a total of 9A of pulsed current. TTL or LVDS inputs enable the VCSEL to be activated or deactivated with ease or different current levels to be selected; these currents are defined by the voltages at the control inputs. The integrated thermal shutdown mechanism protects the iC-HG from damage caused by overheating. The iC-HG30 even allows for a frequency of 250MHz and 6A CW operation/30A pulse current. It is also clocked for AEC-Q100 qualification.


Numerous Potential Applications

A ToF camera in the car can help to enhance comfort and driving safety, for example by monitoring the driver, passenger, and objects in the passenger seats. They can be used to detect if the driver is distracted or tired before there is an accident. Interior space monitoring also enables head and body positions to be detected, for example to optimize airbag control. It can also be used for gesture control. Outside the vehicle, a ToF application can support assisted and autonomous driving.

Melexis has developed a demo kit for infrared lights in ToF applications, specially for monitoring vehicle interiors. The demo board has powerful onboard processing capabilities for hand gesture recognition and high image resolution for object classification. This makes it suitable not only for gesture recognition and driver monitoring, but also skeletal tracking, person and obstacle detection, and traffic monitoring.

The demo board is equipped with a Lextar PV85Q series VCSEL, which offers high efficiency and a narrow spectral bandwidth (1.8nm). Its various optical power options allow for the detection of multiple objects, 3D depth assistance, and presence detection. This VCSEL includes a photodiode to ensure eye safety. The fully integrated MLX75027 VGA (640 × 480) optical ToF image sensor is delivered with DepthSense pixels measuring 10µm × 10µm.

But VCSELs are not limited to ToF applications. 2D vision systems and other new applications such as optical data communication can also benefit from the higher modulation rate and intensity of the VCSELs.


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Figure 1: A ToF camera uses the indirect ToF method.