Advancements in VCSEL Technology with Spectroscopy

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Vertical Cavity Surface Emitting Lasers (VCSELs) are pivotal to a lot of technological advancements recently, and have several advantages over Edge Emitting Lasers (EELs) and Light Emitting Diodes (LEDs). New applications for VCSELs are found every day.

Optical spectrometers with a high resolution, high speed triggering response as well as short integration time specifications are required to test VCSELs by manufacturers of these devices and device integrators. The world’s most trusted instrument supplier to support applications with VCSEL technology is Avantes.

Introduction to VCSEL Structure

A semiconductor-based light source, Vertical Cavity Surface Emitting Lasers are grown in mass production using standard thin film deposition techniques. These techniques include using metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) to deposit films on Gallium Arsenide (GaAs) wafers. A coherent beam of light is emitted from the surface of the VCSELs.

The structure is composed of two extremely reflective distributed Bragg reflector (DBRs) mirrors which are parallel to the water surface. It is made by alternating layers of low and high refractive indices which are capable of yielding intensity reflectivities. Typically, the DBRs are doped in order to form the diode junction and they are used to deliver a carrier signal in order to stimulate emissions in the active region.

Optical feedback is provided by the active laser medium through which the carrier signal passes. The light between the reflective matrices is amplified by the active laser medium inciting laser propagation, which is possible at a current where the round-trip gain exceeds the round-trip loss.

The VCSEL laser has a low threshold current for laser propagation as a result of the vertically oriented gain region of the VCSEL design being shorter than required for other semiconductor lasers. The DBR which has the lowest refractivity is subsequently out-coupled for coherent light emission.

The VCSEL Advantage

The VCSEL’s vertical design gives it numerous advantages over edge emitters. Before an edge emitting laser can be tested, the deposition processes must be complete and the elements must be die cut from the wafer. The manufacturing time and materials will have been wasted if thin films are used or there are defects in the wafer.

By contrast, VCSELs can be mass produced using ordinary semiconductor thin film deposition methods in a process which is able to be tested at numerous stages of production. This includes wafer testing which enables thousands of VCSELs to be processed simultaneously on a single three-inch wafer. This increases the efficiency of production and reduces costs.

The VCSEL design also enables the connection of multiple elements into two-dimensional, which is a benefit as it increases power output and the larger output aperture allows for better coupling efficiency with optical fibers as it produces a lower divergence angle of the output beam.

VCSELs consume less power than other laser-light emitting devices as a result of the placement of distributed Bragg reflector (DBRs) which lower the threshold current in order to achieve laser propagation. Even so, they are capable of high-power output. Characteristic of VCSELs, the wavelength tenability is achieved by adjusting the thickness of the reflector layers in the active region with the help of microelectromechanical systems.

Current Applications for VCSEL Technology

VCSELs were introduced 40 years ago, and since then they have been used in numerous applications across hundreds of markets and industries, both commercial and industrial. Today, VCSELs can be found everywhere. Signal processing is one of the key functionalities of these devices, which may take the form of sensing or communications.

The signal processing power of VCSELs – which emit in the 1310 nm and 1550 nm bands – is vital to fiber optic communication, as it delivers pulses of light forming an electromagnetic carrier wave which can be modulated in order to carry signals for internet, cable, and telephony. (Larson)

The most familiar and widely used of VCSELs capabilities is the laser mouse on your computer (VCSEL Wiki). Other such common VCSEL applications include miniature atomic clocks, laser printers, collision avoidance systems in equipped vehicles, and facial recognition in mobile devices (VCSEL Wiki).

The Future of VCSELs

At the University of Notre Dame’s Department of Electrical Engineering, research undertaken by Kitsmiller, Drummer, Johnson, et al. investigated the use of frequency domain diffuse optical spectroscopy (fd-DOS) employing near-infrared tunable VCSELs in order to develop a miniaturized system which can perform high-resolution deep tissue scans in non-invasive biomedical imaging.

There have been technological constraints on advances in noninvasive monitoring, particularly wearable sensors, due to the commercial availability of miniature light sources which are capable of producing coherent near-infrared light in the first biological diagnostic window between 650 nm and 1350 nm.

Accuracy, sensitivity, and spatial resolution can be increased by the addition of spectral content, however, the size and complexity of the system can be added to with the addition of laser or LED components. This competes with the objective of developing handheld and wearable sensors and monitoring devices.

Avantes Spectrometers in VCSEL Manufacturing and Testing

Numerous VCSEL manufacturers and integrators use Avantes AvaSpec instruments in order to test the performance of these devices. Three commonly used spectrometers in VCSEL characterization are the AvaSpec-ULS4096CL-EVO, the AvaSpec-ULS3648-USB2, and the new Mini4096CL. A lot of customers have found that these instruments constitute a viable substitute for more expensive optical spectrum analyzers (OSA) for many of their common VSEL tests.

Many VCSELs produced nowadays emit in the near infrared. High resolution gratings (1200 or 1800 grooves/ mm) blazed in the NIR are featured in common configurations of these instruments. 5- or 10-micron slits which provide the highest resolution possible can be used courtesy of the inherent power of the devices being measured. This can, however, result in certain levels of signal attenuation. Customers have cited some of the following important advantages of these instruments: easy software integration, appropriate sensitivity enabling high speed tach times for testing, and high resolution and pixel count.

A range of typical VCSEL parameters are measured by the Avantes spectrometers used in this application. These parameters include centroid, spectral peak wavelength, side mode suppression ratio, RMS spectral bandwidth, and Full Width Half Maximum (FWHM). The AvaSpec range of instruments is ideally suited to handle this source type, because certain VCSELs are operated in pulsed mode. Both the AS7010 USB3/Ethernet and the AS5216 USB2 electronics boards are capable of controlling the window of integration for the pulsed signal by sending or receiving TTL external triggers.

Avantes’ range of optical sampling accessories – which includes integrating spheres and cosine correctors along with their fiber optic patchcords – provide for the means of collecting light emission from the VCSEL at the device interface, regardless of the size or power of the devices.

Conclusion

Regarding VCSEL testing and characterization, affordable alternative to OSAs are spectrometers provided by Avantes. The AvaSpec range of spectrometers offers high-speed, high sensitivity, and high resolution characteristics which make them perfect for this application. Integration with production software is made simple by Avantes’ easy to implement DLL library.

Resources

This information has been sourced, reviewed and adapted from materials provided by Avantes BV.

For more information on this source, please visit Avantes BV

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