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Keyless Locks Using VCSEL Sensors

Key Takeaways

  • The VCSEL sensors are monolithic semiconductor laser diodes that emit light perpendicular to the wafer surface.

  • High efficiency, good optical beam quality, reliability, lower power consumption, high speed, and packaging flexibility are some of the advantages VCSEL sensors have over LEDs and EELs.

  • VCSELs can be used in robotics, face recognition, optical mouses, and optical interconnects.

face recognition

Figure 1: VCSEL technology is used in face recognition

The evolution of optical communication has revolutionized signal processing systems. In optic communication, Electro-Magnetic ❲EM❳ waves are used as carrier waves to send data or signals. The efficient sensing of the EM signals is a major requirement to develop reliable optic communication systems for telephone, internet, and cable services. 

Semiconductor light-emitting sources such as Light Emitting Diodes ❲LED❳, Edge Emitting Lasers ❲EEL❳, and Vertical Cavity Surface Emitting Lasers ❲VCSEL❳ are trending sensor devices in use today. VCSEL sensors are often favored over LEDs and EELs, due to VCSELs having high speed, high sensitivity, and high resolution. The VCSEL sensor can be applied in wearable and handheld sensors. 

VCSEL Sensor- Structure, Advantages and Comparison 

With the advent of Artificial Intelligence and the Internet of Things, lock and key pairs will go extinct in the near future. 3-D sensing is a remarkable breakthrough in this array. Generally, humans respect privacy, and as a result, 3-D sensing technology gained popularity in the electronics industry. VCSEL technology is employed for face and object recognition in most consumer electronics.

VCSEL structure

Figure 2:  VCSEL structure (shown above) contains semiconductor layers, active layers, and an inner cavity.

The VCSEL sensors are monolithic semiconductor laser diodes that emit light vertically. It is a variant of Distributed Bragg Reflector ❲DBR❳. The circular beam profile of VCSELs is perpendicular to the wafer surface. The planar structure of the VCSEL and its vertical emission allows testing of the laser in the wafer-level fabrication stage. The fabrication and testing can go hand-in-hand, and this permits the building of higher-order VCSEL sensor assemblies. 

Structure

In the VCSEL structure shown above, there are two types of layers. The electrically conductive semiconductor layer is responsible for the optical feedback and acts as a laser mirror. The amplifying layers are placed in an inner cavity surrounded by electrically conductive layers. The VCSEL emits EM waves of about 850-980nm wavelength when the active layers and semiconductor layers have a thickness of 10nm and 8 µm, respectively. The semiconductor manufacturing equipment can be utilized for the construction of VCSELs, as it is an all-vertical fabrication. Metal-organic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE) are preferred fabrication methods. 

Advantages

Here we will discuss some of the advantages of VCSEL sensors:

  • Small footprint-The area occupied by a VCSEL is small and this increases its use in optical fibers.

  • Lower production cost-The production cost of a VCSEL is less, making it widely used as a cost-effective sensor option. 

  • Low power consumption-The VCSEL sensors require a low threshold and driving current which minimizes power consumption. The design of driving circuits for VCSEL is easy.

  • Modulation rate-The modulation rate of VCSELs reaches 40 Gbit/s at 85℃.

  • Beam quality-The vertical optical beam from VCSEL is directional, coherent, narrow divergence, and symmetric.

  • Temperature dependency-The VCSEL wavelength is less sensitive to temperature variations and there is not much wavelength shift associated with it.

  • High efficiency-The high efficiency of VCSELs make them suitable for battery-powered applications. Concerning the total emitted power, VCSELs are favored over LEDs and EELs.

  • Reliability and long life-The VCSEL is reliable when compared to traditional lasers. The probability of dark line effects and optical damages in VCSELs are low and this increases the shelf life of VCSEL sensors.

  • Arrayability-The 1-D and 2-D arrays of VCSELS can be easily fabricated, and according to the number of arrays, the power density and efficiency are scaled up.

  • Packaging-The flexibility and simplicity in the packaging of VCSEL sensors make it suitable for surface mounting. The flip-chip bonding packaging technology produces compact versions of VCSEL sensors.

Comparison of VCSEL with EEL

VCSEL

EEL

Threshold current less than 1 mA

High threshold current

High optical power efficiency at low power

Medium optical power efficiency

Divergent, circular beam

Astigmatic, elliptical beam

Wafer-level testing possible

Testing possible only after complete fabrication

Simple packaging and mounting is easy

Packaging flexibility is less

Wavelength ranging from 450-2000 nm

Wavelength ranging from 400-1300 nm

Applications of VCSEL Sensors

The compactness, reliability, and low power consumption of VCSELs have increased the use of optical sensors in electronic circuits. The major applications of VCSELs are in automobiles, robotics, surveillance systems, and machine vision in industries. It is the trending technology used for gesture recognition, contact-less sensing, and chip to chip communication. Here we will see some applications of VCSELs. Let's start with the popular application of VCSEL—facial recognition.

Face Recognition

In mobile phones, we personalize the keyless locks with face recognition features. It is the most frequent application of VCSEL sensors. The combo of VCSELs and cameras make face recognition a reality. The VCSEL emits Infra-Red ❲IR ❳ rays and illuminates the face. The camera then captures the face and it is compared with the reference image stored in the device. The device unlocks when the two images match. 

Optical Mouse

A wireless mouse is a typical application of VCSEL sensors. The VCSEL sensor-enabled mouse uses the IR laser diodes instead of LEDs. Low power demand, improved tracking performance, and broad navigability are the advantages gained by pointing devices when LEDs are replaced by VCSEL diodes and sensors.

Optical Interconnects

Interconnect plays an important role in chip-to-chip and internal communication. The physical condition and material of interconnects are factors affecting the signal and power integrity in ICs. The signal loss and power supply variations due to poor condition of interconnects can lead to device misoperation or malfunctioning. The error-free operation of an IC requires interconnects in perfect condition. Traditional copper interconnects are replaced by optical interconnects, and when it comes to short-reach data transmission optical interconnects in servers, data centers, and supercomputers, VCSEL is the first choice. The high reliability, low cost, and energy efficiency of VCSEL sensors affirm their use in optical interconnect applications. 

VCSELs are the epitome of high speed, miniaturized free-space optical systems suitable for communication. The application of VCSELs is not limited to communication, sensing, and tracking. The advantages of  VCSELS are so vast that they can be used anywhere LEDs and EELS were previously used. Next time you use the face recognition feature for unlocking your gadget, remember the VCSEL sensor is working hard behind-the-scenes to make that happen.