Dual-spectroscopy technique based on quartz crystal tuning fork detector

Journal Pre-proof Dual-spectroscopy technique based on quartz crystal tuning fork detector Linguang Xu, Ningwu Liu, Shen Zhou, Lei Zhang, Jingsong Li

PII:

S0924-4247(19)32156-9

DOI:

https://doi.org/10.1016/j.sna.2020.111873

Reference:

SNA 111873

To appear in:

Sensors and Actuators: A. Physical

Received Date:

24 November 2019

Revised Date:

20 January 2020

Accepted Date:

25 January 2020

Please cite this article as: Xu L, Liu N, Zhou S, Zhang L, Li J, Dual-spectroscopy technique based on quartz crystal tuning fork detector, Sensors and Actuators: A. Physical (2020), doi: https://doi.org/10.1016/j.sna.2020.111873

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Dual-spectroscopy technique based on quartz crystal tuning fork detector

Linguang Xu, Ningwu Liu, Shen Zhou, Lei Zhang and Jingsong Li*

Laser Spectroscopy and Sensing Laboratory, Anhui University, 230601 Hefei, China

Corresponding author: Email address: [email protected]

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Tel./Fax: +86 (0) 551-63861490

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Graphical abstract

Highlights 

Quartz crystal tuning forks (QCTFs) as valuable components have been widely used for developing sensitive sensors.



Broadband tunable ECQCL and QCTF based dual-spectroscopy technique was firstly proposed for gas detection. 1



Comparing to the popular QEPAS technique, a sensitivity enhancement of 4-7 times was obtained for the proposed dual-spectroscopy technique.

Abstract Quartz crystal tuning fork (QCTF) as a sensitive element and valuable component has been widely used for

developing mass and stress sensor, pressure and temperature sensor, optoacoustic transducer and photoelectric detector, due to its high quality factor, high stability and precision, and low power

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consumption. In this paper, QCTF was reported for simultaneous detection of both acoustic signal induced by photoacoustic effect (i.e. photoacoustic spectroscopy) and light signal from molecular absorption process (i.e. direct absorption spectroscopy). A broadband tunable external cavity quantum cascade laser (ECQCL) and two QCTFs with same resonant frequency was demonstrated for dual-spectroscopy technique detection

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of gas concentration. The results indicated that the proposed dual-spectroscopy technique can be efficiently utilized to enhance sensitivity of QCTF based gas sensors. Comparing to the common QEPAS technique, a

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sensitivity enhancement of 4-7 times was obtained for the proposed dual-spectroscopy technique.

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Keywords: ECQCL, QCTF, QEPAS, TDLAS, Dual-spectroscopy

1. Introduction

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Optical gas sensing techniques show significant superiority in terms of sensitivity, selectivity and time resolution. Based on the interaction process between light and matter, it mainly including absorption photoacoustic

spectroscopy

(PAS),

scattering/reflectance

spectroscopy,

interference

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spectroscopy,

spectroscopy and polarization spectroscopy, and so on. Among them, absorption spectroscopy based on Beer-Lambert law and PAS based on PA effect have being universal in gaseous, solid, and liquid phases. Recently, advances in new laser sources and detectors have triggered an increasing application of both spectroscopy techniques in atmospheric monitoring, industrial process control, medical and combustion diagnostics, etc [1-5].

2

Quartz crystal tuning fork (QCTF) as a piezoelectric resonator is commonly used for the construction of real time clock circuits in various electronic devices. Due to its small size, high quality factor (Q-factor) and long stability, QCTF is widely used for developed mass sensors microbiology [6,7], stress sensors in scanning probe microscopy [8,9], as well as pressure and temperature sensors in low-temperature physics [10-15]. In 2002, QCTF was firstly reported as an acoustic wave transducer by Kosterev et al. [16], thus the so-called quartz enhanced photoacoustic spectroscopy (QEPAS) was proposed. The standard QEPAS is to focus the excitation laser beam through the gap between the prongs on the QCTF crystal plane. For resolving various drawbacks in real applications, many novel detection strategy have been successively proposed, such as “off-beam” spectrophone for convenient beam arrangement [17,18], micro-resonator configuration and multi-QCTF for signal enhancement [19], as well as custom QCTFs with larger prongs for light sources with

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a limited beam quality (like LED) [20,21]. Two latest review articles nicely summarized the main attributes of QEPAS technique and many representative results [22,23]. Recently, QCTF based light detector was demonstrated for standoff detection of volatile organic compounds (VOCs) by Sun et al [24-26]. Comparing

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with traditional semiconductor detectors based on photoelectric effect, the QCTF detector technology based on piezoelectric and resonant effect may pave a way towards developing a new type of photodetector

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suitable for the whole electromagnetic radiation.

In this paper, a QCTF based detector was firstly demonstrated for simultaneous detection of both acoustic

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signal induced in PAS and light signal in DAS. Base on this process, a dual-spectroscopy technique was thus proposed. To demonstrate the dual-spectroscopy technique for sensitive trace gas detection, a sensor system

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based on a broadband tunable external cavity quantum cascade laser (ECQCL) and two QCTFs with same resonant frequency was integrated for nitrous oxide (N2O) spectroscopy measurement. The results indicated

sensors.

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that the dual-spectroscopy effect can be efficiently utilized to enhance the sensitivity of QCTF based gas

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2. Laser spectroscopy theory 2.1 Absorption spectroscopy

The theory of laser absorption spectroscopy can be described by the Lambert-Beer Law, the full expression of Beer-Lambert Law can be described as： I (  )  I 0 (  ) e x p [  (  )C L   (  )   L   ] A (  )

(1)

where  (  ) is the Rayleigh scattering coefficient,  is the Mie scattering coefficient,  is the absorption 3

coefficient caused by gas density change, and

A ( )

is the transfer function of the measurement system.

Generally for homogeneous gaseous medium, the Lambert-Beer law can be simplified as : I ( )  I 0 ( ) e x p (  (  )C L )

where

I ( )

I 0 ( )

and

(2)

are the intensity of the transmitted and incident light intensity after and before

through a uniform medium of length

L

(cm), respectively.  (  ) is the absorption coefficient at a specific

wavelength which contains information such as the concentration, temperature, and pressure of the target gas. The absorption coefficient can be expressed as: 1

I ( )

ln

I 0 ( )

L

  (    0 )  S (T )  N (T , P )

(3)

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 ( )  

The molecular number density expression is given as follows: N0 

P0

where

T0

(4)

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P

T

N 0  2 .6 8 7 5  1 0

19

(mol/cm-3) is the number density of an ideal gas at

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N ( P ,T ) 

K by :

S (T )  S (T re f )

Q (T ref )

[1  e x p (  h c

v0

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T ref  2 9 6

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The line intensity at the temperature T (K) is related to the intensity

Q (T )

[1  e x p (  h c

kT v0

)]

 hcE 0  exp   k   )]

S (T r e f )

 1 1    T T ref 

T 0  2 7 3 .1 5 K

and

P0  1 atm.

at the reference temperature

     

(5)

where

Q

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k T0

is the partition function of absorbing gas, including the vibrational S (T r e f )

and rotational

Qr

can be directly taken from atmospheric spectroscopy database HITRAN [27].

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partition functions.

Qv

For measuring species concentrations, the absorbance profile was numerically integrated over the contours of individual spectral lines or blended features, to obtain the integrated absorption area 

 

A

:



 (  )d v 

  (

  0 )  S (T )  N ( P , T ) d v  A

(6)



Since the line shape function  ( 

 0 )

satisfies the normalization condition. The integrated absorbance 4

area equals to the product of line intensity and the number density of the absorbing species: 

S  N  A=

 



1

ln

L

I (v )

dv

(7)

I 0 (v )

According to the above description, gas concentration can be directly retrieved by selecting a suitable molecular absorption lineshape (typical Voigt function) and nonlinear least squares fitting algorithm.

2.2 Quartz enhanced photoacoustic spectroscopy Photoacoustic spectroscopy (PAS) is a calorimetric spectroscopy technique based on the photoacoustic effect,

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which was founded by Bell in 1880 [28]. In quartz enhanced PAS [29-32], this technique exploits miniaturized quartz tuning forks as optoacoustic transducers to convert an acoustic signal into an electrical signal via quartz piezoelectric properties, instead of microphones used in conventional PAS. The

S 

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photoacoustic signal in QEPAS can be expressed as: Q  P 

(8)

Q

is the quality factor of QCTF,

P

is the optical power,  is the absorption coefficient, and

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where

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f0

f0

is

the resonant frequency of QCTF. The fundamental resonance frequency of QCTF oscillator can be given : 1

k

2

m e ff

 1 .0 1 5

w

E

2  l

2

m e f f  0 .2 4 2 7   l w t

is the effective mass and

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where



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f 

fork quartz crystal vibrating arm, respectively.

(9)

l ,w

, t are length, wide, thickness of the tuning

k  1 / 4 E w (t / l )

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modulus and density of quartz, respectively. The quality factor

3

Q

is the elastic constant, E ,  are the Young's reflects the loss of vibration energy or the

amount of damping that the vibration is subjected to. This value can be calculated according to the following formula:

Q =

f0 f

(10)

where  f is the frequency bandwidth (full width at half maximum) at 5

1/

2

of the maximum signal amplitude,

typically a few Hz. The high sensitivity of QEPAS benefits from the extremely high Q factor (> 104 in a metallic vacuum encapsulation), since a high Q factor corresponds to a long energy accumulation time, and high resonant frequency and narrow resonance frequency band provide the QEPAS selectivity and immunity to environmental acoustic noise. 3. Experimental details The experimental system is shown in Fig. 1. A pulsed external cavity quantum cascade laser (Block Engineering, USA) was used as the excitation light source, which can be tuned between 1130 and 1437 cm-1 (or 6.96–8.85 µm). The ECQCL can be programmed to emit pulses from 50 to 450 ns with pulse repetition rate up to 3 MHz. A collimated and vertically polarized beam of 2×4 mm2 dimension with a < 1 mrad

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divergence is available with a specified single spatial mode of TEM00. For a more detailed description of the ECQCL laser we refer the reader to [25] and [26]. The diverging laser beam is firstly shaped with a diaphragm, and focused into a single-pass gas sample cell. The home-made stainless steel sample cell consists of two CaF2 windows and a QCTF#1 detector. The transmitted laser beam was focused on the

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second QCTF#2 detector. A home-made low noise transimpedance preamplifier circuit with a feedback resistor of 10 MΩ was applied to magnify and transform the piezoelectric current generated by the QCTF

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into a voltage signal. Both QCTF detector signals were digitized by a data acquisition card (NI USB-6259, 1.25 MHz sampling rate) and a data acquisition system based on LabVIEW software. Gas sampling line

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includes some needle valves, an air pump and a digital gauge (Testo 552, Germany).

Air

Lens

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Pressure Meter

N2O

Pump

valve

Lens

QCTF#2

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QCTF#1

CaF2

Diaphragm

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ECQCL

Notebook Comptuer

CaF2

PA cell

Preamplifier

Preamplifier

Data Acquision Card

Fig. 1. Schematic diagram of the ECQCL and QCTF based spectroscopy system

6

1.5mm 0.6mm

2mm y

x1=0 x2=0.15mm x3=0.45mm

3.7mm x

6mm

0.3m m

Half off-beam

Full off-beam

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Side-beam

Fig. 2. Configuration of different QCTF detection scheme

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As shown in Fig. 2, three optical detection schemes were adopted for investigating the QCTF performance, which are defined as on-side, half off-beam and full off-beam, respectively. The dimensions of the QCTF is

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length, width, and thickness equal to 3700 μm, 600 μm, and 300 μm, respectively. To clarify the three detection schemes, the coordinate system of the QCTF was defined as in Fig. 2, and the corresponding laser

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incident positions are labeled with x1, x2 and x3 for three detection schemes, respectively. While, the laser beam height is always fixed at 2 mm from the QCTF top. Two QCTFs with a same resonant frequency was

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specially selected for investigating dual-spectroscopy effect. The resonant profiles of the QCTFs are experimentally measured and the theoretically calculated profiles based-on Lorentz function are also shown

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in the Fig. 3. The fitted optimal resonant frequency for QCTF#1 and QCTF#2 are 32751.1 Hz 32751.0 Hz, respectively. According to equation (10), the Q-factors of 9148 for QCTF#1 and 10802 for QCTF#2 are

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obtained in ambient air, respectively.

7

QCTF#1 Lorentz fit Q=9148

QCTF signal amplitude (V)

2.0

QCTF#2 Lorentz fit Q=10802

1.2 1.0

1.5 0.8 0.6

1.0

0.4 0.5 0.2 0.0 32700

0.0 32720

32740

32760

32780

32800 32700

Frequency (Hz)

32720

32740

32760

32780

32800

Frequency (Hz)

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Fig. 3. The measured QCTF resonant profiles at ambient air and the best Lorentz fit.

4. Results and discussion

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4.1 QCTF based photoacoustic spectroscopy

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In this paper, QCTF as acoustic wave transducer was firstly used to investigate quartz-enhanced photoacoustic spectroscopy (QEPAS). Nitrous Oxide (N2O) was chosen as the analyte to verify the QCTF

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based QEPAS sensor performance. For PA detection scheme, off-axis QEPAS was constructed without employing micro-resonator (mR) for further signal enhancement here, defined as full off-beam detection strategy, as shown in Fig. 2. To verify the linear concentration response of the QEPAS system, a series of

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N2O gas sample with different concentrations were made by dilution of a commercial N2O gas cylinder with a stated purity of >99% in natural abundance with ambient air. The original QEPAS spectra with a single

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scan as a function of N2O concentration is shown in Fig. 4(a), and the signal amplitudes at 1272 cm-1 and 1298 cm-1, respectively, are also plotted in Fig. 4(b). As expected, a good linear dependence of QEPAS

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signal on N2O concentration was obtained, with a regression coefficient R2 of 0.998 (for 1272 cm-1) and 0.999 (for 1298 cm-1), respectively. The calculated R-square values exhibiting an excellent linearity response to N2O concentrations.

8

QCTF signal (V)

0.05

(a)

50.0% 31.13% 17.52% 10.23% 5.38% 3.8% 2.28% air

0.04

0.03

-1

0.06

(b)

2

1272 cm (R = 0.998) -1 2 1298 cm (R = 0.999) Linear fit

0.05

0.04

0.03 0.02 0.02 -4

SD=2.3x10

0.01

0.01

0.00

0.00 1220

1240

1260

1280

1300

1320 -1

1340

1360

0

10

20

30

40

50

Concentration (%)

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Wavenumber (cm )

Fig. 4. (a) Experimentally measured N2O QEPAS spectra under different concentration; (b)

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and the best linear fit at 1272 cm-1 and 1298 cm-1.

4.2 QCTF based direct absorption spectroscopy

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QCTF as optoelectronic detector was tested to investigate direct absorption spectroscopy (DAS). For DAS detection scheme, two QCTFs (defined as #1 for intracavity and #2 for extracavity) with same resonant

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frequency were simultaneously used for recording N2O absorption spectra inside gas cell and outside gas cell, respectively, as shown in the experimental setup, which is defined as on-beam detection strategy. Firstly,

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ambient air was pumped into the gas cell for recording the background signal. Similar to same measurement procedure mentioned above, N2O DAS spectra with different concentrations were successively measured by

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using intracavity QCTF#1 detector and extracavity QCTF#2 detector. Fig. 5 demonstrated the original signals measured from both QCTF detectors with N2O concentration of 17.52%, 3.8% and 1.09%, as well as

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the corresponding background signals. For clear comparison, the results are processed and demonstrated as the normalized signals (i.e. absorption coefficient in unit of cm-1), which shows a good agreement with the simulation based on HITRAN database assuming a Gaussian instrument function with bandwidth of 1.1 cm-1. According to Lamber-Beer law, molecular absorption coefficient (cm-1) is a function of the number density (molecules per unit volume) of the absorbing species (i.e. optical absorption per centimeter). As expected, a good agreement was obtained between intracavity and extracavity (This conclusion will be beneficial to prove the discovery of dual-spectroscopy effect in next section.), except slight difference of noise levels, which mainly due to interference effect induced from gas cell windows. On the other hand, the results prove 9

that the QCTF can be used as a reliable photodetector for DAS.

2.0

QCTF#1

1.8

1.6

1.6

1.0

1.4

1.4

0.8

1.2

1.2

0.6

0.60

QCTF#1

(a)

1.0

17.52% Fit data

0.25 0.20

1.2

(b) 3.8% Fit data

QCTF#1

QCTF#1 0.075 0.050

(d)

0.15 0.00

(e)

0.05

17.52% Fit data

QCTF#2

0.25 0.20

(f)

0.025 0.000

0.00

0.75

3.8% Fit data

QCTF#2

0.100

1.09% Fit data

QCTF#2 0.075

0.15

0.45

0.050 0.10

0.30

(g)

(h)

0.15

0.05

0.00

0.00

17.52% Air background

QCTF#2

0.90

1.0

0.000

3.8% Air background

QCTF#2

0.9

0.5

(j)

0.15 1220

1240

1260

1280

1300

1320 -1

1340

0.6

0.4

(k)

0.3

1360

1220

1240

1260

1280

1300

1320 -1

1340

1360

0.5

1220

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0.30

1.09% Air background

QCTF#2

0.7

0.6

0.45

0.9 0.8

0.7

0.60

(i)

0.025

0.8

0.75

0.00

1.09% Fit data

0.10

0.30

1.05

(c)

1.0 0.100

0.15

0.45

0.60

1.09% Air background

QCTF#1

2.0

1.8

1.4

0.75

QCTF signal (V)

2.2

3.8% Air background

QCTF#1

2.0

1.6

0.4

-1 Absorption coefficients (cm )

2.2

17.52% Air background

1.8

ro of

QCTF signal (V)

2.2

Wavenumber (cm )

Wavenumber (cm )

1240

1260

1280

1300

1320 -1

1340

(l) 1360

Wavenumber (cm )

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Fig. 5. DAS spectra of QCTF#1(a-f) and QCTF#2(g-l) before and after background normalization at

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different sample concentration.

4.3 QCTF based dual-spectroscopy technique

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Based on above discussion, we can see that QCTF can be used as acoustic detector in PAS or light detector in DAS. Here, we focus on investigating simultaneous detection of acoustic signal (excited by PA effect) and

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absorption signal (i.e. light intensity change induced by molecular absorption). For this purpose, the laser beam was adjusted to make part of light focusing on the sidewall of the QCTF#1, the rest of the beam is

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directed throughout the cell and finally focused on the QCTF#2, namely half off-beam detection scheme. This optical arrangement ensures us to record both QCTF detector signals simultaneously for each N2O sample, which can eliminate the influence of experimental errors (typically sample concentrations). Fig. 6 present experimentally measured signals (after background normalization) from both QCTF detectors under different N2O concentrations. To verify the linear concentration response, the optical path normalized absorption coefficients at 1272 and 1298 cm-1 are also plotted. The calculated R-square value of a linear fit was equal to approximate 0.99 for both cases, exhibiting an excellent linearity response to the N2O 10

concentrations. As can be seen, the results show that the optical path normalized absorption coefficients from the QCTF#1 inside the cell are always higher than those of the QCTF#2 outside the cell. For clarity, the difference of the absorption coefficients at two peaks (i.e. 1272 and 1298 cm-1) vs. N2O concentrations are plotted in Fig. 7, and the best linear fit is also presented. As can be seen, a good linear response was founded. Linear regression leads to a regression coefficient R2 of 0.98 and 0.99 were obtained for 1272 and 1298 cm-1, respectively. Based on the DAS and QEPAS results mentioned above, we can conclude that the significant discrepancy mainly results from both PA effect and direct absorption process occurred in the gas cell, which was detected by QCTF#1 detector, while the QCTF#2 signal only results from direct absorption process.

1.2 1.0

(a)

1.4 1.2 1.0

0.8

0.8

0.6

0.6

0.4 -3

0.2 0.0 -0.2

1220

1240

1260

1280

1300

-1

1320

1340

1360

Wavenumber (cm ) -1

1.0

lP

1.2

QCTF#1

0.8

0.8

1280

1300

1320

1340

1360

30

35

-1

1272 cm -1 1298 cm Linear fit

QCTF#2

0.6

(c)

0.2 0.0 0

5

10

na

0.6 0.4

1260

-1

1.2

1272 cm -1 1298 cm Linear fit

1.4

1240

-3

SD=1.4x10

Wavenumber (cm )

15

20

25

ur

-1

Absorption cofficients (cm )

1.6

1220

re

0.0 -0.2

1.0

(b)

0.4

SD=1.7x10

0.2

31.13% 17.52% 10.23% 5.38% 3.8% 2.28% 1.43% 1.09% 0.7% 0.36%

QCTF#2

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1.4

1.6

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1.6

1.8 31.13% 17.52% 10.23% 5.38% 3.8% 2.28% 1.43% 1.09% 0.7% 0.36%

QCTF#1

-1

Absorption cofficients (cm )

1.8

30

0.4

(d) 0.2

0.0 35

0

Concentration (%)

5

10

15

20

25

Concentration (%)

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Fig. 6. QCTF detectors spectral signal (a,b) and the normalized absorption coefficients at 1272 and 1298 cm-1 as a function of N2O concentrations (c, d).

11

-1

Absorption cofficients difference(cm )

0.5 -1

1272 cm -1 1298 cm Linear fit

0.4

0.3

0.2

0.1

0.0 0

5

10

15

20

25

30

35

Concentration (%)

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Fig. 7. The difference of absorption coefficients at 1272 cm-1 and 1298 cm-1 between QCTF#1 and QCTF#2 detector.

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Finally, a comparison between the common QEPAS and the proposed dual-spectroscopy technique was experimentally investigated under the same conditions, the results from six samples gas with different

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concentration are summarized in Table 1. For calculating the spectral signal to noise ratio (SNR), the standard deviation (SD) of the non-absorption baseline segments is used to calculate the noise level as

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shown in Fig. 4 and Fig. 6. The ratio between each sample concentration value and the corresponding SNR is defined as the limit of detection (LOD), which is just used for comparison of two detection methods, not

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really represents the ultimate sensitivity of the system. It is worth noting that each spectrum is recorded without any signal averaging in this work. By increasing signal average number, the sensor sensitivity can

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be significantly improved, the optimal value could be determined using Allan variance analysis method [33]. Table 1. Comparison of SNR and LOD achieved using different detection methods.

Dual-spectroscopy (QCTF#1)

SNR

LOD (ppm)

SNR

LOD (ppm)

SNR ratio

31.13

156

1995

768

405

4.92

17.52

100

1752

438

400

4.38

10.23

54

1894

269

380

4.98

5.38

32

1681

152

353

4.75

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QEPAS (QCTF#1)

Sample

Concentration (%)

12

20

1900

106

358

5.30

2.28

12

1900

78

292

6.50

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3.80

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Note: SNR- signal to noise ratio, LOD - limit of detection calculated from the ratio between

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sample concentration and SNR.

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As can see from Table 1, the SNR decreases with N2O sample concentration decreases. Fox example, the experimental signals with same N2O concentrations recorded in QEPAS and the proposed dual spectroscopy

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method are shown in Fig. 8 for clear comparison. Taking gas sample with the lowest concentration (N2O=0.36%) as an example, the absorption profile of QEPAS spectrum is almost indistinguishable, except the strongest absorption section near 1300 cm-1. Based on this experimental data, the LOD for QEPAS

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detection method is estimated to be about 2000 ppm (1σ). For a similar procedure, the LOD for the proposed dual spectroscopy detection method is ~ 327 ppm (1σ). Therefore, the LOD ratio of 6.1 is obtained for gas

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sample of N2O=0.36% between two different detection methods. Since each spectrum is recorded with a single scan acquisition without any signal averaging, the calculated LOD ratio factor has slight discrepancy for each sample due to noise fluctuation. Based on the theoretical and experimental data analysis discussed above, the sensitivity enhancement factor of 4-7 times is finally concluded in this study.

13

N2O = 2.28%

(a)

0.15

Dual spectroscopy signal (cm-1)

0.002

0.001

0.000 1200

1220

1240

1260

1280

1300

1320

1340

0.10

0.05

0.00

-0.05 1200

1360

1220

1240

Wavenumber (cm-1)

1280

1300

1320

1340

1360

0.04

(c)

N2O = 0.36%

Dual spectroscopy signal (cm-1)

0.03

0.0012

QEPAS signal (V)

1260

Wavenumber (cm-1)

0.0014

0.0010

0.0008

0.0006 1200

(b)

N2O = 2.28%

1220

1240

1260

1280

1300

1320

1340

1360

(d)

N2O = 0.36%

0.02

0.01

0.00

-0.01

-0.02 1200

1220

1240

Wavenumber (cm-1)

1260

1280

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QEPAS signal (V)

0.003

1300

1320

1340

1360

Wavenumber (cm-1)

Fig. 8. Comparison of N2O spectral signals recorded in both detection methods: (a) and (b) are QEPAS signal and dual spectroscopy signal at N2O=2.28%; (c) and (d) are QEPAS signal and dual spectroscopy

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signal at N2O=0.36%, respectively.

Although the absolute sensitivity is outside the scope of this work, two main factors limiting the LOD is

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analyzed. Firstly, the LOD is limited by the system noise, especially the preamplifier circuit used for QCTF. Electrical properties of the quartz crystal require suitable amplifier topology. That is very crucial for

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obtaining of high SNR and high sensitivity. According to reference [34], a new electronic amplifier, well matched to the quartz crystal characteristics is being designed. Second, the LOD significantly depends on

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laser power. In order to evaluate the linear response of the QCTF detector, a near-infrared DFB diode laser at 1550 nm with higher output power is used to replace the ECQCL. The QCTF signals as a function of laser power ranging from 0 to 13 mW is shown in Fig. 9. Therefore, we believe that the sensitivity can be

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improved, if a high power laser source is available [32]. For example, a fiber coupled near-infrared DFB

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diode lasers can be easily combined with an erbium-doped fiber amplifier to achieve W-level power.

14

1.6

Equation

y = a + b*x

Weight

No Weighting 0.00488

Residual Sum of Squares

1.4

0.9982

Adj. R-Square

Value

QCTF signal (V)

1.2

Standard Error

D1

Intercept

0.04463

0.00865

D1

Slope

0.10959

0.00124

1.0 0.8 0.6 0.4 0.2

Exp. data Linear fit

0.0 0

2

4

6

8

10

12

14

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DFB laser power (mW)

Fig. 9. The relationship between QCTF signal amplitude and incident laser power (The pre-amplifier gain

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factor is about ten).

5. Conclusion

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In this paper, QCTF based detectors were detailedly investigated as acoustic and light signal detectors for spectroscopic applications. Simultaneous detection of both acoustic signal induced by photoacoustic effect

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and absorption signal from molecular absorption process was reported, and a sensitive dual-spectroscopy technique based on this process was thus proposed. To demonstrate the QCTF based dual-spectroscopy

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technique for sensitive trace gas detection, a sensor system based on a broadband tunable ECQCL and two QCTFs with same resonant frequency was integrated for N2O spectroscopy measurement. The results indicated that the dual-spectroscopy effect can be efficiently utilized to enhance the sensitivity of QCTF

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based gas sensors. Comparing to the popular QEPAS technique, a sensitivity enhancement of 4-7 times was obtained for the dual-spectroscopy technique. We believe that the sensitivity enhancement effect will be

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more significant, if a high power laser source is available [32]. For example, fiber coupled near-infrared distributed feedback (DFB) diode lasers can be easily combined with an erbium-doped fiber amplifier to achieve W-level power [20,31].

Declaration of Competing Interest 15

The authors declare that there is no conflict of interest.

Author Statement Quartz crystal tuning fork (QCTF) has been a versatile component for developing various transducers in many fields, such as mass sensors for biochemical and microbiological applications, stress sensors for measuring liquid density and high-resolution atomic force microscopy, as well as pressure and temperature sensors in low-temperature physics. In this paper, a dual-spectroscopy technique is proposed for sensitive gas detection. To explore the

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capabilities of this technique, a broadband tunable external cavity quantum cascade laser (ECQCL) and two quartz crystal tuning fork (QCTFs) with same resonant frequency is successfully demonstrated for N2O spectroscopy measurements. The results indicated that a sensitivity enhancement of 4-7 times was obtained for the proposed dual-spectroscopy technique, which can be further improved by employing a

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high power laser sources.

Acknowledgements

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The authors gratefully acknowledge the financial support from the National Program on Key Research and Development Project (2016YFC0302202), the National Natural Science Foundation of China (61905001,

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41875158, 61675005, 61705002).

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Author Biograhpy

Jingsong Li received his PhD in 2008 at Hefei Institute of Physical Science (HIPS), Chinese Academy of

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Sciences (CAS). He has job experiences as a researcher at Reims University (France), Max Planck Institute for Chemistry (Germany), and Swiss Federal Laboratories for Materials Science and Technology (Switzerland)

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for several years. In 2013, He joined Department of Physics and Materials Science, Anhui University (AHU), working on development and implementation of mid-infrared quantum cascade lasers and sensitive

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spectroscopy techniques for atmospheric chemistry, soil ecosystems and environmental applications, etc. Currently, Dr. Li is the director of Laser Spectroscopy and Sensing Lab. He is a member of Chinese Society

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for Optical Engineering (CSOE), SPIE, IEEE, OSA and EGU.

Linguang Xu is currently a PhD student at Anhui University, China. He works on high sensitive laser

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spectroscopy techniques for trace gas sensing and advanced digital signal processing algorithm. Ningwu Liu is currently a PhD student at Anhui University, China. His main areas of interest are atmospheric

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trace gas and VOCs sensing using laser spectroscopy. Shen Zhou received his PhD degree at Chengdu institute of optoelectronics, Chinese academy of sciences, China in 2017. His research interests focus on high sensitivity laser spectroscopy techniques for trace gases sensing. Lei Zhang received his PhD degree at Zhejiang University, China in 2016. His research interests focus on high precision photoelectric detection technology and advanced signal processing algorithms. 20

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