Modern industrial applications increasingly require advanced sensors, high-performance hardware and software. Depending on the field of application, different technologies are specialized and established. Gestural control, movement monitoring and distance measurement, quality control, object identification, product defect detection, safety, human-robot interaction are examples of a series of applications for which various solutions have been implemented.

Vertical-Cavity Surface-Emitting Lasers (VCSEL) technology have gradually become the primary trend in the 3D sensor market adopting the ToF (Time of Flight) measurement, thanks to advantages such as fast scanning, long-distance, high efficiency and excellent resistance to ambient light. VCSEL devices mainly apply to the emission of light sources for the 3D depth detection module, providing solutions with different wavelengths, brightness, size, and beam angle selection.

A ToF type 3D video camera operates by "illuminating" the scene with a modulated light source. The sensor detects the reflected light pulses, converts them into electrical signals and transmits them to the ToF processor, which measures the phase slip between the emitted light and the reflected light, a parameter from which it is able to deduce the distance of the object.

VCSEL has become the reference technology for short-range datacom networks and local networks, thus replacing edge-emitting lasers. Its success is mainly due to the VCSEL lower production costs and to the higher reliability compared to competitors.

VCSEL Technology

Vertical-Cavity Surface-Emitting Lasers (VCSELs) are a relatively new type of laser composed of layers of semiconductor material grown on top of one another on an epitaxial substrate ("Epi"). This growth is typically performed in an epitaxy by the molecular beam (MBE) or through the MOCVD (Metal Organic Chemical Vapor Deposition) technique. The wafer is then processed accordingly to produce individual devices.

The laser beam inside a VCSEL is generated between two layers of distributed Bragg mirrors (distributed Bragg reflector, DBR) placed parallel to the surface of the semiconductor wafer at the base of the diode and separated by one or more quantum wells responsible for the "generation" of the light beam.

Each DBR mirror is made up of an alternation of thin layers (each one has a thickness of a quarter of the laser wavelength) made with materials with high and low refractive index: this configuration ensures a high reflectance index (about 99% against about 30% of standard lasers), and greater electrical efficiency. As a result, light oscillates perpendicularly to the layers and comes out from the top (or bottom) of the device.

VCSEL offers coherent light with direct emission, higher power density, and simple packaging of a similar device. Its structure is easier to assemble if compared to an EEL (edge-emitting laser). The shape of the radius of a VCSEL is a circular point, concerning the elliptical form of FP-EEL (Fabry-Perot Edge Emitting Laser) and DFB. This simple beam structure considerably reduces the complexity and cost for coupling/beam-shaping optics (concerning edge emitters) and increases the efficiency of coupling to the fiber or another medium. This was a strong point for VCSEL technology in the low power markets.

Furthermore, many advantages offered by VCSEL technology can be summarized in the following points as follows:

  • Wavelength stability: the laser wavelength in a VCSEL is really stable since it has been set to a fixed value by the Fabry-Perot cavity. Unlike edge emitters, VCSELs can only operate in a single longitudinal mode.
  • High-temperature operation: VCSEL devices can operate without a cooling system since they can be operated up to 80 °C. As a result, the required cooling system can be small, robust, and portable.
  • Cost: by using simple manufacturing, it becomes much easier to package VCSEL arrays compared to an equivalent edge emitter bar.

Applications

ToF technology can be implemented in different sectors: from automotive to industrial and consumer ones. Applications can be dedicated to gesture control (faces, hands, fingers), game consoles, smart televisions, or portable computing devices.

In the automotive sector, the use of these lasers will increase in LiDAR systems and advanced driver assistance systems for the next generation of autonomous vehicles. Occupant monitoring systems will instead act as an advanced form of in-car camera, allowing the vehicle to understand the positions, dimensions, and activities of the driver and passengers. Based on the measurement of time of flight (ToF) and 3D scanning, these systems could be able to make real-time changes to the vehicle cabin, from disabling airbags for smaller passengers to preconditioning the vehicle's interior in the event of an accident (Figure 1).

VCSEL LiDAR

Figure 1: VCSEL LiDAR in automotive [Source: ams]

LiDAR is an optical detection technology that measures the distance and direction of objects in the vicinity by illuminating them with a laser beam and detecting the reflection of the object. The pulses of light emitted hit objects, reflect and return to the LiDAR system where the receiver detects the return light pulse. Knowing the time allows you to calculate the distance.

VCSEL devices can recognize humans and detect their height, allowing the monitoring of elderly/children. Moreover, for the detection of driver fatigue, VCSEL devices can detect reactions to fatigue, including eyes closed and head bowed, improving the driver’s safety.

Devices

Lextar's VCSEL PV88M series of components is used for 3D depth detection, providing products with various wavelengths, brightness, dimensions and beam angle selection (from 45 ° to 100 °). The series can be applied to check the safety of a vehicle, through the driver's facial recognition. Furthermore, it is able to recognize and detect human height and is suitable for monitoring elderly / children, detection of falls of the elderly (Figure 2).

VCSEL PV88M

Figure 2: VCSEL PV88M [Source: Lextar]

Driver distraction is a significant cause of concern when it comes to road accidents. Voice recognition technology, similar to the one used in computers and smartphones, allows the use of voice commands to control other digital systems. However, it is highly limited in its scope and is still a sign of distraction for the driver.

Osram Opto Semiconductors offers its VCSEL BIDOS® PLPVQ 940A product family, paving the way for new areas of application such as 3D detection. The device acts as a light source, uniformly illuminating the face with infrared light, and as a detector with a camera that is used to capture the user's significant functionality. The image is then compared to the image of the user stored in the system: if the two match, the device will be unlocked.

Ams VCSEL technology includes the epitaxial structure and chip design with the front-end and back-end for signal processing. Ams VCSELs are classified to operate at ambient temperatures up to 150 ° C.

The devices of the Finisar HVS7000 product line are designed to integrate optics and are AECQ and Jedec qualification, and operate in a temperature range from -40 ° C to + 125 ° C. Vertilite offers a wide range of VCSEL products, including CAS8502W. Typical applications include ToF, automotive sensing, and infrared illumination (Figure 3).

VCSEL Finisar

Figure 3: application of a VCSEL in a smartphone [Source: Finisar]

Texas Instruments has developed a system (chipset) that can be integrated into new 3D imaging devices. It includes a ToF type 3D sensor, based on SoftKinetic's DepthSense technology, which supports a modulation frequency above 50 MHz and offers a high signal-to-noise ratio (SNR). The VL53L0X sensor is very advanced and integrates the VCSEL laser driver, the SPAD diode reading and the device management part, including calibration, measurement, and data conversion for the I2C interface. The image below shows the block diagram of the sensor (figure 4).

VL53L0X

Figure 4: block diagram for the VL53L0X [Source: ST Microelectronics]

Conclusion

The light emitted by the VCSEL has the characteristics of monochromaticity, uniform stability, uniform illumination, and high brightness. The VCSELs will become essential components of the Augmented Reality industry and will become leaders in the consumer electronics era. The light source will directly determine the quality of the projection system.