Infrared Physics & Technology 136 (2024) 105037
Contents lists available at ScienceDirect
Infrared Physics and Technology
journal homepage: www.elsevier.com/locate/infrared
Microfiber knot resonator augmented quartz-enhanced
photoacoustic spectroscopy
Jiabao Xie a, Haohua Lv a, Junming li a, Chenglong Wang a, Haoyang Lin a, Wenguo Zhu a,
Jieyuan Tang a, Yongchun Zhong a, Xueqing He b, **, Jianhui Yu a, Huadan Zheng a, *
a
b
Key Laboratory of Optoelectronic Information and Sensing Technologies of Guangdong Higher Education Institutes, Jinan University, Guangzhou 510632, China
Department of Physics, Taiyuan Normal University, Jinzhong 030619, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Photoacoustic spectroscopy
Quartz-enhanced photoacoustic spectroscopy
Quartz tuning fork
Gas sensor
In this work, a microfiber knot resonator augmented quartz-enhanced photoacoustic spectroscopy (MKR-QEPAS)
was developed for the analysis of trace gases. In the proof-of-concept experiment, a 1.39 μm laser diode was
utilized for the detection of H2O molecules. Based on the H2O absorption line, the optical fiber-based MKR was
meticulously designed to modulate the transmission spectrum, thereby inducing constructive interference with
the excitation laser wavelength. The MKR was integrated with a clamp-type tuning fork, leading to a pronounced
evanescent resonance enhancement. Consequently, the detection sensitivity of the MKR-QEPAS was improved by
approximately one order of magnitude. The micro ring resonator is posited to hold significant promise for hybrid
integration in QEPAS on-chip sensors.
1. Introduction
Photoacoustic spectroscopy (PAS) gas sensing technology for trace
gas detection has been developed and utilized over recent decades
[1–9]. The principle of PAS dictates that gas molecules are excited from
the ground state to excited states by the modulated laser. Once the
molecules are de-excited, energy absorbed by the molecules is trans­
formed periodically into heat energy through the vibration-translation
(V-T) relaxation, leading to pressure variations, recognized as acoustic
waves [10–12]. Various photoacoustic transducers [13], such as MEMS
microphones [14–16], cantilever beams [17], fiber-optic Fabry-Perot
microphones [18], and quartz tuning forks [19–21], are developing
iteratively and being researched extensively. Quartz-enhanced photo­
acoustic spectroscopy (QEPAS) was introduced as a variant of PAS and
was first investigated in 2002 by Kosterev et al. [22]. Within the QEPAS
system, a quartz tuning fork (QTF) is utilized as a piezoelectric trans­
ducer to transform acoustic waves into electrical signals [23–31]. In
comparison to PAS based on microphones, QEPAS exhibits higher Q
factors of 104-105 and more refined resonance frequency bandwidths of
2–4 Hz [32]. In subsequent years, a pivotal role in trace gas detection
applications has been played by QEPAS, encompassing realms such as
agricultural monitoring [33], gas leak detection [34], industrial process
control [35], and human respiratory analysis [36–38]. A compact 6Urack QEPAS sensor was also crafted [39]. The amplitude of the photo­
acoustic signal can be represented by Eq. (1) [40]:
S∝
α•P•Q
f0
(1)
where α and P are the absorption coefficient of the targeted gas mole­
cules and the light power, respectively. Q and f0 are the quality factor
and the resonance frequency of the QTF, respectively. A proportionality
between the QEPAS signal amplitude and the excitation optical power is
observed.
Since the advent of QEPAS in 2002, a commercial standard QTF
featuring a resonance frequency of 32.768 kHz, typically employed in
timekeeping devices, has been incorporated into a majority of QEPAS
systems. A surge in custom QTFs with enhanced Q and diminished
resonance frequency emerged over the decade, driven by the desire to
leverage the acoustic pressure and curtail background noise induced by
scattered light. Spagnolo et al. delved into a custom QTF with a reduced
resonance frequency down to 4.25 kHz in 2013 [41]. In 2016, a 30.72
kHz QTF was introduced by Ma et al. to augment the signal amplitude
[42]. T-shaped longitudinal cross-sections and grooved prong surfaces
characterized the geometry-enhanced QTFs showcased [43,44]. By
* Corresponding author.
** Corresponding author.
E-mail addresses: hexq_1988@163.com (X. He), zhenghuadan@jnu.edu.cn (H. Zheng).
https://doi.org/10.1016/j.infrared.2023.105037
Received 3 November 2023; Received in revised form 1 December 2023; Accepted 1 December 2023
Available online 2 December 2023
1350-4495/© 2023 Elsevier B.V. All rights reserved.
J. Xie et al.
Infrared Physics and Technology 136 (2024) 105037
2020, pilot-line manufactured 28 kHz QTFs were unveiled by Zheng
et al. [1]. In recent developments, a clamp-type QTF with an aperture
diameter approaching ~1 mm and Q factors surpassing 104 was inno­
vatively crafted, rooted in commercially available standard QTFs,
marking a cost-efficient and uncomplicated avenue for procuring
custom QTFs [45].
For the purposes of eliminating the precise focusing optics from the
optical open path and achieving extended and distributed sensing,
evanescent-wave microfiber QEPAS sensors were conceived [46]. The
utilization of the evanescent wave, as opposed to the free space laser, is
championed for photoacoustic excitation. Capitalizing on the merits of
optical fibers, a 3 km fiber with myriad tapered detection units in an
evanescent-wave microfiber QEPAS system was exhibited [47]. None­
theless, constraints in optical power within the evanescent field and
limited engagement between the tapered fiber and the QTF culminated
in the evanescent-wave microfiber QEPAS sensor exhibiting diminished
signal amplitude. The potency of microfibers in forging optical reso­
nance, given their compact stature and minimal loss, is recognized.
When a microfiber is configured into a closed circuit, a circular cavity
based on the microfiber, denoted as a ring resonator, is engendered via
evanescent coupling at the overlap [48–50].
In this letter, a QEPAS sensor rooted in a fiber resonator, specifically
the microfiber knot resonator augmented quartz-enhanced photo­
acoustic spectroscopy (MKR-QEPAS), is showcased for trace gas anal­
ysis. This avant-garde QEPAS spectrophone introduces three distinct
advantages: (1) a diminutive size relative to free-space optical cavity
resonators; (2) an elongated excitation optical trajectory; (3) amplified
excitation optical prowess. Such compactness permits the MKR to be
situated within the aperture of the clamp-type QTF, sustained by the
microfiber’s friction at the conjunction. The MKR exhibited better
robustness than traditional fiber ring resonators, which reduced the
susceptibility to airflow disturbance and enhanced the compatibility for
the clamp-type QTFs. An extended excitation path fosters heightened
and robust interactions between molecules and the QTF. An augmen­
tation in excitation power, achieved by aligning the constructive inter­
ference condition with the excitation wavelength, profoundly elevates
the photoacoustic effect. In juxtaposition with conventional evanescentwave microfiber QEPAS, a tenfold enhancement in detection sensitivity
is observed.
factor of clamp-type QTF were measured to be 35765.6 Hz and 10360,
respectively. The diameter d, necessary for fiber to generate an
evanescent wave, can be at the sub-wavelength scale by controlling the
location of the flame and fiber stretching speed, which results in
microfibers. In this work, a microfiber with a diameter d of 1.29 μm was
produced by a tapered fiber puller from a standard single-mode 9/125
µm fiber, as illustrated in Fig. 1(b). The microfiber was positioned
through the circular aperture of the custom clamp-type QTF, forming the
traditional microfiber QEPAS. According to the waveguide in fiber, a
shallow evanescent wave region emerged in the second medium. As the
microfiber’s diameter d was reduced below the sub-wavelength scale,
both the transmission length and penetration depth of this evanescent
wave increased. Consequently, as the microfiber diameter d decreases,
the evanescent field’s power, crucial for efficient absorption and pho­
toacoustic excitation outside the microfiber, is nonlinearly enhanced.
This enhancement significantly augmented the coupling efficiency of the
microfiber knot resonator [46,50]. A micro-knot ring was then formed
based on the same diameter d of 1.29 μm, as shown in Fig. 1(c). Ad­
justments to the diameter D of this micro-knot ring were made to cali­
brate the transmission spectrum of the MKR. Under the guidance of an
optical microscope, the MKR was positioned into the aperture of the
clamp-type QTF. The alignment ensured the MKR and the QTF aperture
were coaxially integrated, resulting in an effective excitation path
spanning from 0.3 mm to 1.865 mm.
3. Theory analysis of the microfiber knot resonator
In Fig. 2(a), the field distribution of the employed MKR is illustrated.
This distribution was analyzed using COMSOL Multiphysics® finite
element analysis software. The MKR can be likened to a directional ring
coupler. The governing principle of the MKR can be elucidated through
optical waveguide theory. When light is introduced from the micro­
fiber’s entrance, it is observed that it bifurcates at the knot. A fraction of
this light continues directly through the microfiber’s exit. Conversely,
another portion is coupled into the preordained ring via evanescentwave propagation. After completing a circumferential journey within
the ring, this light once again divides at the knot. Owing to this mech­
anism, constructive interference can be manifested within the ring
resonator. Under such conditions, it is discerned that the optical power
accumulated within the MKR exceeds that of the input.
The resonance condition pertaining to the optical path length for the
microfiber knot resonator is encapsulated by Eq. (2) [51]:
2. Structure of the microfiber knot resonator QEPAS
The schematic diagram of the MKR-QEPAS spectrophone is pre­
sented in Fig. 1(a). A clamp-type QTF with an aperture of Ø = ~0.97 mm
was fabricated using the methodology detailed in [16]. The aperture was
positioned at a distance △X of ~0.7 mm from the QTF opening, which is
the optimum for laser excitation. The resonance frequency f0 and Q
1
4
π • D • neff = (m + ) • λm
(2)
where neff is the effective refractive index of the microfiber, D is the
diameter of the knot ring, and λm is the wavelength corresponding to the
order m. When resonance conditions are met within the MKR, the
transmission T is dictated by Eq. (3) [52]:
⃒ ⃒2
⃒E2 ⃒
(α − |t|)2
T 2 = ⃒⃒ ⃒⃒ =
(3)
E1
(1 − α|t|)2
where E2 and E1 are the output and input intensity of the light, respec­
tively. α represents the portion of the intensity remaining after one
round-trip. t represents the transmission coefficient past the coupling
knot. The length of the cavity π⋅D resonating at the wavelength of
7181.156 cm− 1 was obtained by solving Eq. (2) and simulation. The
length of the cavity determined the phase of the evanescent wave,
leading to varied transmission spectrums and resonance effects under
different phase conditions. With a ring diameter D being 0.593 mm, both
a free spectral range (FSR) approximating ~0.74 nm and a quality factor
(Q) nearing ~51,575 are ascertained. As depicted in Fig. 2(b), the
resonance of the MKR is observed at a wavenumber of 7181.156 cm− 1,
coinciding precisely with the desired H2O molecules’ absorption line.
Fig. 1. Schematic diagram of MKR-QEPAS spectrophone; (b) a tapered mi­
crofiber with the diameter of 1.29 µm; (c) a microfiber knot with a ring
diameter of 593.62 µm.
2
J. Xie et al.
Infrared Physics and Technology 136 (2024) 105037
Fig. 2. (a) The field distribution of the microfiber knot resonator at the laser wavenumber of 7181.156 cm− 1. (b) Transmission spectrum of microfiber knot resonator
under the laser wavenumber from 7174 to 7189 cm− 1.
4. Experimental setup
optimized. Fig. 4 elucidates that the QEPAS 2f signal amplitude grows
with modulation depth, but plateaus past 5.41 cm− 1. Consequently,
5.41 cm− 1 was identified as the optimal modulation depth.
The experimental setup of the MKR-QEPAS gas sensing system is
depicted in Fig. 3. As a proof-of-concept, the H2O molecules in the
ambient air were selected as the target. A 10-mW distributed feedback
semiconductor laser (EP1392-5-DM-B01-FA), whose wavelength centers
at 1.39 μm, was employed as the excitation light source. The emission
wavelength of the laser could be adjusted by changing the diode tem­
peratures and injection currents. A ramp wave with a period of 600 s and
an amplitude of 120 mV was added to the laser driver to tune the laser
wavelength from 7180.134 cm− 1 to 7182.176 cm− 1 across the absorp­
tion line of H2O. The laser beam was coupled into the MKR made by
single-mode fiber. Wavelength modulation with the second harmonic
(2f) technique was employed as the detection method [53]. A sinusoidal
wave with a frequency of f = f0/2 generated by a function generator
(Tektronix-AFG-3102) was sent to the laser driver to modulate the
wavelength and provide a reference signal for the lock-in amplifier
(OE2031 DSP). A time constant of 1 s and a slope filter of 12 dB/octave
were set, corresponding to a detection bandwidth of 0.125 Hz.
5. Optimization of the modulation depth
Utilizing wavelength modulation, the modulation depth has been
found to significantly influence the 2f QEPAS signal amplitude, espe­
cially in the context of varying absorption line profiles. For performance
optimization in MKR-QEPAS, modulation depth was experimentally
Fig. 4. Optimization of the laser modulation depth.
Fig. 3. Schematic diagram of the MKR-QEPAS experimental setup.
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J. Xie et al.
Infrared Physics and Technology 136 (2024) 105037
6. Result and comparison
The 2f signals for conventional microfiber QEPAS and MKR-QEPAS
are illustrated in Fig. 5. Here, the laser’s temperature was maintained
at 32 ◦ C, and its wavenumber spanned from 7180.134 cm− 1 to
7182.176 cm− 1. With reference to the HITRAN database, the target of
the H2O absorption line was at 7181.156 cm− 1, boasting an intensity of
1.492 × 10− 20 cm/molecule. The detailed comparison of these config­
urations is tabulated in Table 1. The MKR-QEPAS registered an ampli­
tude of 2.09 × 10− 4 V for the 2f signal, markedly surpassing the 7.39 ×
10− 6 V observed in the microfiber QEPAS setup. Noise analysis yielded
1σ standard deviations of 1.77 µV and 0.56 µV for MKR-QEPAS and
microfiber QEPAS respectively. Consequently, the signal-to-noise ratio
(SNR) for MKR-QEPAS was markedly superior, at 118, compared to the
microfiber QEPAS’s 13.2. Overall, the SNR was elevated by a factor of
ten in the MKR-QEPAS relative to the conventional microfiber QEPAS.
Considering the total output power of the laser, a normalized noise
equivalent absorption (NNEA) coefficient of 2.97 × 10− 6 cm− 1٠ W/Hz1/
2
was obtained for the MKR-QEPAS.
7. Conclusion
Fig. 5. 2f signal comparison of microfiber QEPAS and MKR-QEPAS.
An augmented QEPAS technique leveraging a fiber resonator, termed
as MKR-QEPAS was showcased for trace gas detection. A cost-effective
single-mode fiber-based MKR was cohesively integrated with a clamptype QTF. Comprehensive theoretical and experimental analyses were
performed on the field distribution and transmission spectrum of the
MKR, in alignment with the deployed laser wavelength. Coaxial
coupling of the MKR with the clamp-type QTF’s aperture resulted in a
pronounced evanescent field boost. This led to a detection sensitivity
increment of approximately an order of magnitude relative to traditional
microfiber-based QEPAS. A NNEA coefficient of 2.97 × 10− 6 cm− 1٠ W/
Hz1/2 was obtained for H2O detection. Alternatively, when the resonant
wavelength aligned with the absorption lines of gas molecules, the
system enabled simultaneous detection of either the different absorption
lines within single-component gas molecules or multiple absorption
lines in multi-component gas molecules. Given the inherent advantages
of optical fibers, MKR-QEPAS harbors the immense potential for ultrasensitive, expansive, and distributed gas sensing networks. Moreover,
the micro ring resonator based on a light guide on a silicon chip, used to
integrate filtering, coupling, and modulating optoelectronic devices also
paves the way for innovative hybrid integration within a quartz tuning
fork to achieve highly sensitive trace gas detection.
Table 1
Performance Comparison of microfiber QEPAS and MKR-QEPAS.
Configurations
Microfiber QEPAS
MKR-QEPAS
Signal (V)
− 6
7.39 × 10
2.09 × 10− 4
1σ Noise (µV)
SNR
0.56
1.77
13.20
118.08
Data availability
Data will be made available on request.
Acknowledgements
This work is supported by the National Key Research and Develop­
ment Program of China (2021YFB2800801), National Natural Science
Foundation of China (62375111, 62005105, 12174156, 12174155,
62105125, 62075088, 62175137), Natural Science Foundation of
Guangdong Province (2020B1515020024, 2019A1515011380), the
Science and Technology Projects of Guangzhou (202102020445), KeyArea Research and Development Program of Guangdong Province
(2019B010138004), Special Project in Key Fields of the Higher Educa­
tion Institutions of Guangdong Province (2020ZDZX3022), Open foun­
dation of CEPREI (NO. 19D09), Foundation for Distinguished Young
Talents in Higher Education of Guangdong (2018KQNCX009), the
Fundamental Research Funds for the Central Universities (21619402,
11618413), State Key Laboratory of Applied Optics (SKLAO-201914),
Special Funds for the Cultivation of Guangdong College Students’ Sci­
entific and Technological Innovation (No. pdjh2023a0052), the Applied
Basic Research Program of Shanxi Province (202203021222250).
CRediT authorship contribution statement
Jiabao Xie: Conceptualization, Data curation, Formal analysis,
Investigation, Methodology, Software, Validation, Visualization,
Writing – original draft. Haohua Lv: Supervision. Junming li: Super­
vision. Chenglong Wang: Software. Haoyang Lin: Supervision. Wen­
guo Zhu: Funding acquisition, Project administration, Resources,
Supervision. Jieyuan Tang: Funding acquisition, Project administra­
tion, Resources, Supervision. Yongchun Zhong: Project administration,
Resources, Supervision, Validation. Xueqing He: Funding acquisition,
Project administration, Resources, Supervision. Jianhui Yu: Funding
acquisition, Project administration, Resources. Huadan Zheng:
Conceptualization, Funding acquisition, Methodology, Project admin­
istration, Resources, Supervision, Validation, Writing – review &
editing.
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