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Case Study CS-TC09 Rev. A

Simultaneous Multi-

Gas Detection Using ICL

Photoacoustic Spectroscopy

May, 2024

Page 1

ABSTRACT

Researchers from China have developed and demonstrated a simultaneous, multi-component gas

detection system based on differential Helmholtz resonance spectroscopy with a mid-infrared, tunable,

interband cascade laser (ICL). The first harmonic demodulation method was used under ambient pressure

to detect three light carbon gases: CH

4, C2H4, and C2H6. Using this photoacoustic system, the detection

limits reached 98.8 ppb, 252 ppb, and 33 ppb for methane, ethylene, and ethane, respectively. Multi-

wavelength linear regression was implemented to reduce the effects of cross-interference of the three

gases, maintaining the accuracy of the gas concentration measurements. This new method, using an ICL

with a Helmholtz resonator, provides a simple and fast multiple gas detection technique for atmospheric

research, medicine, the food industry, pollutant monitoring, and the power industry.

PHOTOACOUSTIC SPECTROSCOPY

Photoacoustic spectroscopy (PAS) is a common technique

for trace gas detection, reaching concentration levels as

low as part-per-billion and part-per-trillion levels. Instead of

using scattered light or absorption spectrum from molecules,

PAS uses acoustic detection to measure the effect of

light on a sample. These designs include microphones or

tuning forks in the acoustic detection module or near the

resonator chamber to transduce the acoustic signal into an

electrical signal. Multiple wavelengths of light will produce

a photoacoustic spectrum and can be used to identify the

components in a sample.

PAS, as an indirect detection method, can be advantageous

with high detection sensitivity, good selectivity, fast

detection speed, and long service life.1 These benefits are

critical for reaching part-per-billion detection levels of trace

gases in the atmosphere or lab environment. PAS has

been widely used in atmospheric research, medicine, the

food industry, pollutant monitoring, and the power industry.1

However, most systems are designed to detect only a

single gas, not multiple components simultaneously. A

single device that can measure multiple gases in real-time

can provide safety, environmental, and economic benefits in

research laboratories, field experiments, or even residential

applications.

The light source to generate the photoacoustic effect has

recently improved with the invention of tunable lasers and,

specifically, quantum cascade lasers (QCLs) with narrow

linewidths for higher sensitivity. Along with the laser, the

photoacoustic cell and pressure sensor are critical factors

affecting the detection performance of PAS.1

PROBLEMS AND GOALS

Simple mid-infrared lamps and single-wavelength laser

arrays have achieved multiple gas detection, but they

require mechanical or optical accessories to accomplish

wavelength switching. To achieve multiple component

detection with a single laser, tunable lasers, such as CO

2

gas lasers and QCLs, can be used to cover a wide range

of wavelengths. Wider wavelength ranges provide a better

selection of absorption signal wavelengths of specific

gases. Although QCLs have advanced the PAS capabilities

with extremely narrow linewidths and large tuning ranges,

they require a large threshold current to operate. Interband

cascade lasers (ICLs) allow each injected electron to

be reused at a forward bias to emit multiple photons and

decrease this threshold current and total required electrical

input power.1

A major issue with multiple gas detection when compared

to single gas detection is cross-interference. Some

absorption peaks of different gases may be relatively close

in wavelength, and this may cause the pressure sensor to

decrease in accuracy in detecting concentrations of multiple

gases. The photoacoustic cell and pressure sensor directly

correlate to cross-interference.

Some PAS designs utilize quartz tuning forks and cantilever

beams to detect pressure changes due to sample

absorption of light with high sensitivity. A high-pressure

cell may be required to contain these devices, increasing

frequency noise and cross-interference and leading to

frequent inaccurate measurements. Temperature and high

cost can also deter researchers from using these systems.

A better, more efficient, lower cost, and highly sensitive

design is needed for simultaneous multi-gas detection with

PAS designs.