A cavity-enhanced photoacoustic (CEPA) sensor was developed based on an
ultra-narrow linewidth whispering-gallery-mode (WGM) diode laser. A
cavity-enhanced photoacoustic module (CEPAM) was designed to match the output
beam from the WGM-diode laser, resulting in an increase in the excitation
light power, which, in turn, significantly enhanced the photoacoustic signal
amplitude. The results show that a signal gain factor of 166 was achieved,
which is in excellent agreement with the power enhancement factor of 175
after considering the power transmissivity. The performance of the sensor was
evaluated in terms of the detection sensitivity and linearity. A 1σ
detection limit of 0.45 ppmV for C2H2 detection was obtained at
atmospheric pressure with a 1 s averaging time.
Introduction
Photoacoustic spectroscopy (PAS) is an important trace gas detection
technique that is widely applied to atmospheric science, breath analysis and
industrial process control (Siciliani et al., 2014; Wojtas et al., 2014; Yin
et al., 2017a). In the PAS technique, modulated excitation light is
selectively absorbed by a target gas and results in the generation of an
acoustic wave by non-radiative energy relaxation processes. One of the unique
PAS advantages is that its sensitivity is proportional to excitation laser
power, and thus the performance of PAS sensors can be improved by increasing
the excitation laser power. When a commercially available telecommunication
diode laser is employed as an excitation light source, a commonly used method
to increase the laser power is to use an erbium-doped fiber amplifier (EDFA)
to boost the output optical power (Chen et al., 2018; He et al., 2018; Peng
et al., 2009). For example, in 2015, Wu et al. (2015) demonstrated a
quartz-enhanced photoacoustic spectroscopy (QEPAS) based H2S sensor
operating at 1582 nm combined with an EDFA (Wu et al., 2015). With a ∼1.4 W optical excitation power and 67 s averaging time, the H2S
detection sensitivity was reduced from several ppmV level down to 142 ppbV
in N2, which is the best value for the H2S QEPAS sensors
reported so far. Yin et al. (2017b) developed a conventional photoacoustic
(CPA) sensor operating at the same wavelength for H2S detection in
SF6 by means of an EDFA and a background-gas-induced high-Q
photoacoustic cell (Yin et al., 2017b). A 1σ detection limit of
109 ppbV was achieved with a 1 s averaging time. An alternative method of
achieving high power is to combine PAS with cavity-enhanced absorption
spectroscopy (CEAS) (Hippler et al., 2010; Kachanov et al., 2013; Wojtas et
al., 2017). In 2015, Patimisco et al. (2015) proposed an intra-cavity QEPAS
(I-QEPAS) sensor for CO2 detection at 4.33 µm (Patimisco
et al., 2015). A power enhancement factor of ∼240 was achieved with an
intracavity power of ∼0.72 W, resulting in a minimum detection limit of
300 pptV at a total gas pressure of 50 mbar with a 20 s integration time.
Cavity-enhanced absorption spectroscopy (CEAS) is based on the use of optical
resonant cavities in order to enhance light interaction with a gas species
inside the cavity (Gherman and Romanini, 2002; He et al.,
2018; Yi et al., 2016). In CEAS setups, a proper locking between the laser
wavelength and the cavity resonance mode must be carried out via two
approaches: (i) the cavity length is controlled by a piezo transducer (PZT)
for the resonance mode to follow laser wavelength; and (ii) the length of the
cavity is fixed, and the laser wavelength is locked to the cavity resonance
mode. When the locking between the laser wavelength and the fundamental
optical mode of the cavity is realized, the power inside the cavity will be
enhanced significantly by a power enhancement factor G, and the value of
G can be calculated according to Eq. (1):
G=Fπ
where F represents the finesse of the optical cavity.
In this manuscript, we developed a cavity-enhanced photoacoustic (CEPA)
sensor system for acetylene (C2H2) detection at 1530.98 nm
based on an ultra-narrow linewidth WGM-diode laser. A Fabry–Perot (F–P)
cavity with a finesse of 550 was designed. The laser wavelength was locked to
the F–P cavity mode by a Pound–Drever–Hall (PDH) locking technique
(Black, 2000; Drewer et al., 1983) since the WGM-diode laser linewidth is far
narrower than that of the cavity mode. A differential photoacoustic cell with
two electret condenser cylindrical microphones was designed to detect the
photoacoustic signal. To enhance the photoacoustic signal, the photoacoustic
cell was inserted between the two cavity mirrors. The performance of the CEPA
sensor was compared with a CPA sensor without cavity and evaluated at
different C2H2 concentration levels.
Sensor designCharacterization of ultra-narrow linewidth WGM-diode laser
The portable WGM-diode laser (OEwaves, USA, OE4023-153160-PA02-FM100) has an
ultra-narrow linewidth of <200 Hz for 10 µs and good
tunability in the wavelength range of 1529 to 1532 nm with an output power
of 4 mW after stabilizing. The laser frequency can be tuned mode hop-free
over 50 GHz at a tuning rate of 1 GHz s-1 by changing the temperature
of the laser internal resonator and over 1 GHz at a tuning rate of
40 MHz µs-1 by an external voltage to change the length of an
internal piezoelectric crystal. Figure 1 shows the tunable wavelength range
of the WGM-diode laser for seven different wavelength bands when the laser
temperature was fixed and the internal piezoelectric crystal was changed by
an external voltage. Due to the ∼1 GHz limit for each wavelength band,
the tunable range cannot link up end-to-end. The laser beam was collimated by
a fiber-coupled collimator (OZ Optics, Canada, HPUCO-T.3A-1550-P-2AS). The
output beam quality from the collimator was evaluated by a scanning-slit
optical beam profiler (THORLABS, USA, BP209-IR2/M). Figure 2a and b show the
two-dimensional intensity distribution of the laser spot and the
three-dimensional laser beam profile, respectively, at 15.2 cm from the
collimator. The laser beam exhibits an excellent Gaussian fundamental mode
with a spot size of 860 µm.
Tunable wavelength range of the WGM-diode laser for seven
different wavelength bands when the laser temperature was fixed and the
internal piezoelectric crystal was changed by an external voltage.
(a) The two-dimensional intensity distribution of the laser
spot. (b) Three-dimensional laser beam profile.
Design of the cavity-enhanced photoacoustic module
A cavity-enhanced photoacoustic module (CEPAM) consists of a differential
photoacoustic cell, a F–P cavity, and a gas chamber with a gas inlet and
outlet. The differential photoacoustic cell was designed as shown in Fig. 3a,
which resembles the well-known differential Helmholtz resonator
(Starecki and Geras, 2014; Zeninari et al., 1999; Zheng et
al., 2017). It has two identical 90 mm parallel tube-shaped channels with
diameters of 8 mm as two acoustic resonators. Two buffer volumes with
lengths of 10 mm and diameters of 20 mm connect to the two channels at both
ends, thus making the two channels act as acoustic open–open resonators and
create a total optical absorption length of 110 mm. This allows the beam
from the WGM-diode laser to pass through the differential photoacoustic cell
easily. When the laser intensity is modulated at the resonance frequency of
the photoacoustic cell, a standing sound wave generated with the absorption
of a target gas has its maximum acoustic pressure in the middle of the
acoustic resonator. Hence, two selected electret condenser cylindrical
microphones which have the same frequency response sensitivities are
installed on the walls in the middle of each resonator to detect the acoustic
pressure. The gas flow noise and external acoustic disturbances can be
effectively suppressed by using a custom transimpedance differential
preamplifier. The signal coming from the microphone located in the acoustic
resonator not illuminated by the laser beam is subtracted from the one
related to the microphone located in the excited resonator and the resulting
signal is subsequently amplified. Therefore, the performance of the PAS cell
is improved. The resonance frequency of the differential photoacoustic cell
in air was experimentally determined to be f0=1781.0 Hz and the full
width at half maximum (FWHM) of the frequency response curves (resonance
width) was Δf=40 Hz, corresponding to a quality factor Q=f0/Δf=45.
(a) Schematic diagram of the differential photoacoustic
cell. (b) Schematic diagram of the cavity-enhanced photoacoustic
module (CEPAM).
Figure 3b illustrates the schematic of the CEPAM. The F–P cavity
consists of a 25.4 mm plane mirror as the incident mirror and a 25.4 mm
plane–concave mirror with a 1 m radius of curvature as the exit mirror. The
two mirrors were coated with a high reflective coating of 0.995. The cavity
length was 160 mm, which was longer than the 110 mm length of the
differential photoacoustic cell. The designed differential photoacoustic cell
was inserted between the two cavity mirrors as shown in Fig. 3b. A gas
chamber made of polymethyl methacrylate with a gas inlet and outlet was
fabricated and used to provide environmental stabilization of the cavity.
Experimental setup of the CEPA sensor system: f-EOM, fiber-coupled
electro-optic modulator; f-AM, fiber-coupled amplitude modulator; L1,
mode-matching lens; λ/2, half-wave plate; PBS, polarization beam
splitter; λ/4, quarter-wave plate; L2 and L3, focusing lenses; PD,
photodiode detector; SG, function signal generator; LPF, low-pass filter;
PID, proportional–integral–derivative controller; HV, high-voltage
amplifier; LIA, lock-in amplifier; PC, personal computer.
Experimental setup of the sensor system
The experimental setup of the CEPA sensor system is depicted in Fig. 4. A
mode-matching lens (L1), a half-wave plate (λ/2), a polarization
beam splitter (PBS) and a quarter-wave plate (λ/4) were placed in
front of the cavity. Two focusing lenses (L2 and L3) were used to focus the
transmitted and reflected lights from the cavity onto the photodiode
detectors (PD1 and PD2) (THORLABS, USA, PDA10CF-EC), respectively. In order
to achieve the mode matching, a ramp signal was generated by the first
function signal generator (SG1) (Agilent, USA, Model 33500B) to scan the
laser wavelength. The transmitted signal was detected by the PD1 and recorded
by an oscilloscope (Tektronix, USA, DPO 2024). The second function signal
generator (SG2) (Tektronix, USA, AFG 3102) generated two sine-wave signals
with the same frequency (15 MHz) and an adjustable phase difference. One of
them was applied to a fiber-coupled electro-optic modulator (f-EOM) (Keyang
photonics, China, KY-PM-1550-10-PP-FA) to modulate the laser wavelength.
Another was directed to the mixer (Mini-Circuits, USA, ZLW-1) and mixed with
the reflected signal that was reflected by the PBS to the PD2. A low-pass
filter with an upper cutoff frequency of 1 MHz (Stanford Research Systems,
USA, Model SR560) was used after the output of the mixer to acquire a
low-frequency signal as an error signal. The error signal was directed to a
proportional–integral–derivative (PID) controller (Stanford Research
Systems, USA, Model SIM960), which can provide a control signal to adjust the
laser wavelength locking it to the cavity mode.
To sweep over a full cavity free-spectral range (FSR) of ∼0.9 GHz, a 10 Hz ramp signal was applied to the WGM-diode laser, as shown in
Fig. 5a. The transmitted signal and the error signal were recorded using
the oscilloscope as shown in Fig. 5b and c, respectively. Based on the
FSR and the linewidth of the cavity (Δν) from Fig. 5b, the
finesse F (FSR/Δν) is 550. Therefore, a power enhancement factor G of
175 is obtained according to Eq. (1), which means that the detection
sensitivity of PAS can be improved by 175 times.
(a) Laser scan signal. (b) Transmitted signal from
the cavity. (c) Error signal from the low-pass filter following the
mixer.
Comparison between the CEPA and CPA signal amplitudes from a 500 ppmV C2H2/N2 gas mixture.
(a) CEPA signal at the different C2H2
concentration levels. (b) Linearity of the CEPA sensor system.
To verify the improved performance of the CEPA sensor system based on the
WGM-diode laser, a C2H2 absorption line located at 1530.98 nm
with an intensity of 4.00×10-21 cm molecule-1 was selected
as a target line. The wavelength of the WGM-diode laser was tuned to the
target line by means of a wavelength meter (HighFinesse, Germany, WS-6). The
laser wavelength was locked to the cavity mode. In order to generate a
photoacoustic signal, a fiber-coupled amplitude modulator (f-AM) (Photline
Technologies, France, MX-LN-10) with a DC bias generated by a high-voltage
amplifier (Piezomechanik GmbH, Germany, SVR 200-3) and a square wave of
50 % duty cycle generated by the SG3 (Tektronix, USA, AFG 3102), was
employed to modulate the laser intensity before the laser beam entered the
collimator. The f-AM can provide a DC extinction ratio of 20 dB, which can
meet the requirement of the intensity modulation. The frequency of the square
wave was 1781.0 Hz, corresponding to the resonance frequency of the
differential photoacoustic cell. The loss of the f-EOM and the f-AM
results in a final incident power of 0.7 mW in front of the F–P cavity.
The photoacoustic signal from the differential preamplifier was fed into a
lock-in amplifier (LIA) (Stanford Research Systems, USA, Model SR830), which
demodulated the signal in the 1-f mode. The reference signal for the LIA
was from the TTL signal output of SG3. A 12 dB/oct filter slope and a 1 s
time constant were set for the LIA, corresponding to a detection bandwidth of
Δf=0.25 Hz. The demodulated signal from the LIA was recorded by a
personal computer and the data were processed with a LabVIEW software
program. A certified 500 ppmV C2H2 gas cylinder was used. The
different concentrations of C2H2/N2 gas mixtures were
produced by a gas dilution system (Environics Inc., USA, Model EN4040).
Results and discussion
The CEPAM was first filled with 500 ppmV C2H2. The measurements
were carried out at atmospheric pressure and room temperature. The signal
amplitudes from the CEPA sensor system are shown in Fig. 6. As a comparison,
the signal amplitudes from a CPA sensor without the F–P cavity are also
shown in Fig. 6. The CEPA sensor effectively enhanced the signal amplitude
from 44.3 to 7366.8 µV, corresponding to a signal gain factor of
166. Considering the ratio of the powers of incident and transmitted lights
is ∼95 % for the F–P cavity, the power enhancement factor
verified by PAS is 175, which is in excellent agreement with the anticipated
value.
In order to evaluate the performance of the CEPA sensor system in terms of
minimum detection limit and linearity, pure N2 and five different
concentration levels of the C2H2/N2 gas mixtures varying from
100 to 500 ppmV were fed into the CEPAM. The sensor system was operated at
atmospheric pressure and at room temperature. Sixty data points of the CEPA
signal were recorded continuously with a 1 s averaging time at each
concentration level as shown in Fig. 7a. With pure N2, the 1σ noise level was found to be 6.6 µV. The scatter of consecutive
measurements at a certain concentration level did not depend on the
concentration and was in agreement with pure N2. For a 500 ppmV
C2H2/N2 gas mixture, a signal amplitude of
7366.8 µV was observed, and hence a signal-to-noise ratio (SNR) of
1110 can be achieved which corresponds to a minimum detection limit (1σ) of 0.45 ppmV. The plot in Fig. 7b is a representation of the same
measurements after 60 sensor readings of each concentration step are
averaged. This plot confirms the linearity of the sensor response to a
concentration with a R2 value of >0.9993.
In 2017, Ma et al. (2017) used QEPAS technique to
detect C2H2 and obtain a detection sensitivity of 33.2 ppb (Ma
et al., 2017), which is 13 times better than this CEPA sensor. The possible
reasons are the following. (1) The CEPA sensor employed the acetylene
absorption line located at 6531.76 cm-1, which was different from the
Yufei Ma line at 6534.37 cm-1. The line strength of 6531.76 cm-1
is ∼3 times lower than that of 6534.37 cm-1. (2) The cavity mode
was not moved to the top of the absorption line. In this way, the
C2H2 spectral wing was detected, which resulted in a sensitivity
loss of ∼3 times. (3) The effective optical power in the optical cavity
was 116 mW. But Ma et al. (2017) used an EDFA to boost the laser power to
1500 mW. In fact, the signal enhancement achieved from the cavity is more
important in this research. A comparative detection limit can be expected
after selecting the same absorption line and setting the cavity mode to the
top of the absorption line.
The CEPA technique is very different from the CPA technique. It is true that
the use of an optical cavity makes the sensor system more complicated.
However, the CEPA technique has the potential to further improve the detect
limit if a higher finesse cavity is employed since the detection sensitivity
is proportional to the excitation optical power.
Conclusions
A CEPA sensor system based on a WGM-diode laser was demonstrated. The
WGM-diode laser has an ultra-narrow linewidth and was used as the excitation
source. A cavity-enhanced photoacoustic module was designed to enhance the
laser power density inside the optical cavity and the photoacoustic cell,
resulting in a signal gain factor of 166, when the laser wavelength was
locked on the fundamental optical mode of the F–P cavity by a
PDH-locking technique. The combination of the cavity-enhanced absorption
spectroscopy and photoacoustic spectroscopy can improve the excitation light
power effectively, leading to a significant gain of the photoacoustic signal
amplitude. The use of the fiber-coupled elements and the WGM-diode laser have
the potential to develop a compact CEPA sensor system with a higher detection
sensitivity with respect to the CPA sensor system. A further improvement of
the CEPA sensor system can be achieved by using a higher finesse cavity or a
higher power laser source.
The supplement related to this article is available online at: https://doi.org/10.5194/amt-12-1905-2019-supplement.
Data availability
Data used for this study can be found in the Supplement.
Author contributions
YP and LD contributed the central idea, analyzed the data,
and wrote the initial draft of the paper. HW developed the idea of the study.
The remaining authors contributed to refining the ideas, carrying out
additional analyses and finalizing this paper.
Competing interests
The authors declare that they have no conflict of
interest.
Special issue statement
This article is part of the special issue “Advances in
cavity-based techniques for measurements of atmospheric aerosol and trace
gases”. It is not associated with a conference.
Acknowledgements
Lei Dong acknowledges support by the National Key R&D Program of China
(2017YFA0304203), the National Natural Science Foundation of China (NSFC)
(61622503, 61575113, 61805132, 11434007), the Changjiang Scholars and
Innovative Research Team in University of Ministry of Education of China
(IRT_17R70), the 111 project (D18001), the Outstanding Innovative Teams of
Higher Learning Institutions of Shanxi, the Foundation for Selected Young
Scientists Studying Abroad, Sanjin Scholar (2017QNSJXZ-04) and Shanxi
“1331KSC”. Frank K. Tittel acknowledges support by the US National Science
Foundation (NSF) ERC MIRTHE award and the Robert Welch Foundation (grant no.
C0568).
Review statement
This paper was edited by Weidong Chen and reviewed by two
anonymous referees.
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