VCSEL Array

What is a VCSEL array?

A VCSEL array is a monolithic (linear or 2D) array of vertical-cavity surface-emitting lasers. Each VCSEL outputs a circular beam and can be directly modulated at high speed. This makes these devices ideal for both high-speed short-reach datacom and optical sensing.

A VCSEL array, which stands for Vertical-Cavity Surface-Emitting Laser array, is a technology used in several areas of photonics – mostly within sensing and communications applications and often in high volumes. It consists of multiple VCSELs (Vertical-Cavity Surface-Emitting Lasers) arranged in a one- or two-dimensional grid or pattern on a semiconductor substrate or chip. 

VCSELs are a type of semiconductor diode laser that emits light vertically from the top surface of the chip, as opposed to edge-emitting lasers that emit light from the lateral surface – see Figure 1. Laser operation is enabled by careful shaping of the electrode to avoid blocking/absorption of the laser light.

The VCSEL has two advantages over edge-emitting devices that make it a better choice in several applications.

 

Figure 1

Figure 1. A VCSEL produces a symmetric round beam that is much easier to manipulate and utilize than the elliptical beam from an edge-emitting device.

 

Optical properties. Edge emitters produce an elliptical beam which is also highly divergent and often astigmatic. Therefore their integration often requires more complex beam-shaping optics. In a VCSEL however, the output beam is symmetric, circular, and considerably less divergent. This makes the VCSEL output much easier to focus to a spot or couple into an optical fiber, and reduces the cost and complexity of systems.

Electronic properties. All diode lasers can be directly modulated by switching their drive current. But while edge emitters are widely used in communications applications, the fastest transmission often requires the additional integration of an external (e.g., Mach-Zender) modulator. In contrast, the short cavity and certain other aspects of VCSEL architecture mean they can be optimized for much faster direct modulation than typical edge emitters – see Figure 2. Again, this reduces the overall complexity and cost.

 

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Figure 2. The short cavity of a VCSEL contributes to its extremely fast on/off modulation speeds that benefit datacom applications.

 

VCSEL arrays consist of multiple individual VCSELs arranged in rows and columns. Such an architecture provides two advantages – higher power and multi-channel operation. While the optical power of edge emitters can be scaled by increasing the cavity length, this is not possible in VSCEL, where power is increased by increasing the number of emitters. This is very useful in sensing applications where significant optical output power is required, such as for time-of-flight or structured-light cameras.

The individual emitters in some VCSEL arrays can be independently operated.  This allows considerable flexibility in shaping and steering the emitted light. It also provides a datacom source for multi-channel use, but with the small size, efficiency, and packaging simplicity only found with a monolithic chip.

In terms of their use, VCSEL arrays are commonly found in a number of high-speed data communication applications, such as optical interconnects in data centers and high-speed networks. They can transmit data at very high speeds due to their ability to modulate light rapidly. Their modest per-channel power means that they are better for short-reach – up to 100s of meters – applications rather than long-haul systems. This combination means that VCEL arrays are particularly well suited for optical interconnects within the data hyperscale centers now needed to support the dramatic growth in the use of artificial intelligence (AI) and machine learning (ML).

 

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Figure 3. VCELs and VCSEL arrays are widely used in autonomous vehicles for sensing applications based on Lidar.

 

As noted above, VCSELs and VCSEL arrays are also well-suited for a number of optical sensing and imaging applications. These include facial recognition systems for mobile phones, PCs, or automatic door locks. A major high-volume application is in LiDAR (Light Detection and Ranging) systems for ADAS (Advanced Driver Assistance Systems) - see figure 3. Their ability to emit structured and coherent light patterns is useful for depth perception and mapping, for lane-tracking, traffic proximity sensing, and automated parking.

VCSEL arrays also find applications in various industrial and consumer electronics. Some standout examples include laser printers, optical mice, and gesture recognition systems.

In the areas of biomedical and healthcare: VCSEL arrays are already well-established for used medical devices for applications like blood oxygen sensing. Their compact size and advantageous beam characteristics make them very easy to integrate in wearable devices where space is a major constraint. 

Sensing and Measurement: VCSEL arrays can also be found in a host of other applications in industrial/commercial sensing and measurement. These include process-control applications based on spectroscopy and/or gas sensing, as well as certain types of environmental monitoring.

To summarize, VCSEL arrays offer advantages such as precise control, scalability, and ease of integration into various systems, making them a crucial component in numerous optical and photonic technologies. Their continued development has led to improved performance and expanded applications in a wide range of fields.

 

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