Posted in | Laser

Mid-Infrared Light can Now be Used to Detect Gases

Vertical-cavity surface-emitting lasers (VCSELs) are a class of small, semiconductor-based lasers with the ability to emit optical beams from their top surface. VCSELs are predominantly used for gas sensing applications.

According to the molecular structure, each type of gas has the ability to absorb a specific set of energies. These bands of absorption lines are identical to fingerprints that allow the gases to be detected in a highly sensitive unambiguous manner by using an appropriate tunable laser, such as a tunable VCSEL.

Various significant gases - such as carbon dioxide, methane, and nitrogen dioxide - can be detected using mid-infrared (mid-IR) light with wavelengths in the range of 3 to 4 µm. However, application-grade VCSELs haven’t yet been developed to suit this wavelength range.

Consequently, the expanding requirement of affordable, portable, and compact gas sensors has resulted in the pressing demand for semiconductor sources of mid-IR light that are highly energy-efficient.

In order to satisfy such unmet demands, a research team at the Walter Schottky Institute of the Technical University of Munich (TUM), Germany, has worked to develop a technique for expanding the wavelength coverage of VCSELs in this significant area. The outcomes of the research have been published this week in the journal Applied Physics Letters, published by AIP Publishing.

Conventional VCSELs lack in their performance in the comparatively long wavelengths of the mid-IR range. This is partly because of the side effects of heating which have a disproportionate impact on the IR wavelengths.

The “buried tunnel junction” structure of VCSELs, in which a material barrier is lodged between the standard n-type and p-type materials of the semiconductor, reduces the side effects. Such a structure leads to the resistance-like behavior of the device and enables the optical properties to be tuned in the desired range.

The buried tunnel junction VCSEL concept has already yielded high-performance VCSELs within the entire 1.3- to 3-micron wavelength range. And so-called type-II ‘W’ quantum well active regions have been used successfully to make conventional edge-emitting semiconductor lasers with excellent performance within the 3- to 6-micron wavelength range.

Ganpath K. Veerabathran, Doctoral Student, Walter Schottky Institute

The researchers combined the tunnel junction VCSEL structure with the traditional edge-emitting laser designs in which the beam is emitted in a direction parallel to the bottom surface, in this wavelength band, to form a buried tunnel junction VCSEL including a single-stage, type-II material active region to expand the wavelength coverage of electrically pumped VCSELs.

The progress achieved by the research team is specifically remarkable as it is the first ever demonstration of electrically pumped, tunable, single-mode VCSELs with the ability to emit continuous wave up to 4 µm.

It marks a significant step from state-of-the-art devices emitting at three microns in a continuous wave, and up to 3.4 microns in pulsed mode, respectively. Further, our demonstration at four microns paves the way for application-grade VCSELs within the entire 3- to 4-micron wavelength range, because the performance of these VCSELs generally improves at shorter wavelengths.

Ganpath K. Veerabathran, Doctoral Student, Walter Schottky Institute

Of critical note is the fact that though gas-sensing systems operating within mid-IR wavelength range and using other types of lasers are available in the market, when compared to VCSELs they consume more power, are expensive, and are chiefly employed in industrial applications to detect trace gases in the area of safety and monitoring.

The 4-micron VCSEL demonstrates that low-power, battery-operated, portable and inexpensive sensing systems are within reach. Once sensing systems become more affordable, there’s great potential for deployment by industries, such as the auto industry for emission monitoring and control, and these systems may even find uses within our homes.

Ganpath K. Veerabathran, Doctoral Student, Walter Schottky Institute

In the near future, the focus of the research team will be on enhancements “in terms of the maximum operation temperature and optical output power of the VCSELs,” stated Veerabathran. “In the future, it may be possible to extend this concept to make VCSELs emit further into the mid-infrared region beyond 4 microns. This would be beneficial because the absorption strength of gases typically becomes orders of magnitude stronger, even for relatively small wavelength increases.”

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