Case Study CS-LDTC16 Rev. A

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Figure 1. a) Structure model of three-mirror MPC. d: distance between mirror and origin. α: angle between mirrors; θ: the

angle between the incident laser and the z axis; φ: the angle between the projection of the incident ray in the x-y plane and

the x axis. h

1(x0, y0): coordinates of laser entry hole. b) Theoretical model of artificial fish swarm algorithm (AFSA) design

algorithm. Position: P=[d

1, d2, d3, x0, y0, θ, φ], seven-dimensional position information for fish swarm.

1

METHOD

Researchers from the Harbin Institute of Technology,

China have developed an ultra-sensitive light-induced

thermoelastic spectroscopy (LITES) system based on an

intelligent algorithm-optimized multi-pass cell (MPC) and

low-frequency quartz tuning fork (QTF) for carbon monoxide

(CO) detection. To increase absorption through longer

optical path length (OPL) and to design an optimized MPC

to better balance OPL with volume, researchers employed

an artificial fish swarm algorithm (AFSA) (

Figure 1). The

AFSA was used to optimize the geometric structure of the

MPC with a three-mirror model with a double helix structure

by determining the path the laser light takes through the

gas sample and mirror reflections. It is difficult to greatly

extend the OPL without creating an extensive volume and

a large system to contain it. By iteratively adjusting the

parameters of the mirror placement, angles, and spacing,

the AFSA maximized path length while keeping the cell

compact and minimizing optical losses. The OPL/V ratio

of the novel MPC using the AFSA was 25.8 m/165.8 mL

which results in a highly dense pattern of spot

distribution on the mirrors in a compact design. The

simulated and measured distribution of the double helix

pattern can be seen in

Figure 2. This can increase

gas-light interaction time leading to high sensitivity

without increasing the physical size of the system.

To improve the standard commercial-grade QTF with high

resonant frequency (f

0), researchers needed to reduce

the f

0 to increase the energy accumulation time which

would contribute to improving the detection sensitivity

of the sensor.1 The parameters that affect the f

0 include

the thickness and length of the fork fingers, elasticity,

and density of the QTF. Researchers customized a

QTF by increasing the length from 3.9 mm to 9.1 mm,

decreasing the width from 0.36 mm to 0.25 mm, adding

round heads to enhance stress during vibration, and

using gold electrodes to improve oxidation and corrosion

resistance.1 To further improve the design, the QTF was

coated with polydimethylsiloxane (PDMS) (

Figure 3)

to reduce heat diffusion and increase the stress

Figure 2. Double helix pattern of three-mirror MPC. Simulated (a) and Measured (b) distribution of double helix pattern.1