Case Study CS-LDTC16 Rev. A

Page 3

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Figure 3. Schematic diagram of QTF optimization.

Insert: real picture of different QTFs. QTF1: commercial

QTF with high f

0. QFT2: structurally optimized QTF with

low frequency and round head. QFT3: PDMS modified

low-frequency QTF with P-Q-G sandwich structure.

PDMS polydimethylsiloxane, a polymer material with low

Φ and high β.1

during vibration. The PDMS, with low effective thermal

conductivity and high thermal expansion coefficient,

effectively increases the local temperature and stress of

vibration which is a vast improvement from the commercial

QTF.1 With the polymer-coated QTF, f

0 was reduced by 70%

from ~32.8 kHz to ~ 9.5 kHz.

To verify that both the optimized MPC and QTF can be

used in LITES, an absorption line of CO was selected at

4587.64 nm as a target line for the experiment (

Figure 4).

A distributed feedback (DFB) quantum cascade laser (QCL)

with a center wavelength of 4.69 µm was used as the

Figure 4. Schematic diagram of CO-LITES sensor

based on intelligent algorithm optimized MPC with

double helix pattern and PDMS modified round-head QTF

with low

f

0. F-lens focusing lens, TIA transimpedance

amplifier, A1 and A2 two apertures used to determine the

incident angle of the laser beam.1

excitation source and temperature controlled at 35ºC. With

an input current of 301 mA, the output power of the laser

reached 145 mW and passed through apertures before

reaching the MPC with the double helix spot pattern.

Inside the MPC the laser is reflected 259 times before

exiting and being focused on the three QTFs with different

characteristics to test the sensor performances.

Wavelength modulation spectroscopy (WMS) was used to

suppress the background noise with ramp and sine waves

set to the resonant frequencies of the respective QTFs. The

noise of each QTF could be found using the relationship

between the second harmonic (2f) signal amplitude and

the current modulation depth and is used to determine the

total signal-to-noise ratio (SNR). With the determined SNR

values for each QTF, the minimum detection limit (MDL)

can be calculated: MDL = C/SNR, where C is the gas

concentration.

RESULTS

To fully test each QTF and the entire LITES system,

researchers first determined the noise for each QTF by

experimenting on 1 ppm CO in the optimized MPC. The

noise values were found to be 957 nV, 834 nV, and 852 nV

for QTF1, QTF2, and QTF3, respectively. Based on the 2f

signals, and the resulting noise levels, the SNRs for the

three QTFs were calculated to be 4106.58, 15215.83, and

43485.92, respectively. QTF2 and QTF3, the optimized

QTFs, achieved great improvement compared to the

commercial QTF1. The structurally optimized QTF2 had an

SNR over 3.71 times better, and the PDMS-modified QTF3

was 10.59 times higher than the commercial-grade QTF1.

The MDL was calculated to be 23 ppt for the CO-LITES

sensor based on QTF3. The relationship between CO

concentration and the 2f signal can be seen in

Figure 5.

Different concentrations of CO were tested, and the signals

of the CO-LITES sensor were directly proportional to the

respective concentrations. As shown in that figure, the

high R2 value of 0.99 for the linear fitting indicates the CO-

LITES had an excellent linear response to the ultra-low CO

concentration.1 By adjusting the average integration time

to 500 seconds, the MDL of the CO-LITES sensor was

improved to 920.7 ppq.

For this novel CO-LITES sensor, the mid-infrared QCL

was utilized with an AFSA optimized three-mirror MPC with

a double helix to significantly improve the absorption of

CO in a relatively compact design. By optimizing the QTF

by PDMS, researchers were able to achieve the highest

detection sensitivity reported in LITES sensors.1 Other

real-world applications tested include CO concentration

on the campus of Harbin Institute of Technology and CO

concentration of human breath.