Highly sensitive acetylene detection based on multi-pass retro-reflection-cavity-enhanced photoacoustic spectroscopy and a fiber amplified diode laser

In this paper, a multi-pass retro-reflection-cavity-enhanced photoacoustic spectroscopy (PAS) based gas sensor is reported for the first time. The multi-pass retroreflection-cavity consisted of two right-angle prisms and was designed to reflect the laser beam to pass through the photoacoustic (PA) cell four times, which improved the acetylene (C2H2)-PAS sensor signal level significantly. The optical power of a near-infrared distributed feedback (DFB) diode laser emitting a continuous wave (CW) was amplified to 1000 mW with an erbium-doped fiber amplifier. The background noise was reduced with wavelength modulation spectroscopy (WMS) and 2nd harmonic demodulation techniques. The linear optical power and concentration response of such a PAS sensor were investigated, and the experimental results showed excellent characteristics. When the integration the time of the sensor system was set to 1 s, the minimum detection limit (MDL) for C2H2 detection was 8.17 ppb, which corresponds to a normalized noise equivalent absorption coefficient (NNEA) of 1.84 × 10 cmW/√Hz. The long-term stability of such a multi-pass retro-reflectioncavity-enhanced PAS based C2H2 sensor was evaluated by an Allan deviation analysis. It was demonstrated that the multi-pass retro-reflection-cavity-enhanced PAS sensor had an excellent stability. An MDL of 600 ppt was achieved when the integration time was set to ~1000 s. It was verified that the method of multi-pass retro-reflection-cavity-enhanced PAS with an amplified laser source improved the sensor performance significantly. If an appropriate cavity design with increasing reflection times is used, the MDL of such a PASbased sensor can be further improved. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Acetylene (C 2 H 2 ) is a flammable and colorless gas [1] and will explode at certain pressures when it encounters factors such as a shock or an electric spark.C 2 H 2 has several important applications in electrical fault detection in the industrial field and is the key gas to test for low energy arc discharge faults [2].Furthermore, monitoring of C 2 H 2 concentrations can be used to detect gas leakage from electrical transformers and ethylene streams for polyethylene production [3,4].
Photoacoustic spectroscopy (PAS) is a detection technique for trace gases with high detection sensitivity.When the target gas absorbs light, the gas molecules will be excited and relax to the ground state by non-radiative processes which will produce heat energy.This heat energy can lead to local temperature and pressure increases.Therefore, when a target gas absorbs modulated light, an acoustic wave will be generated in the gas sample [5].A sensitive acoustic wave detector can be used to detect this acoustic signal, such as a microphone, a quartz tuning fork or a cantilever [6][7][8].PAS is identified as a suitable technique, which offers several advantages such as nondestructive detection, simplicity, a wide dynamic range and a compact size [9].
The PAS technique has an important advantage that with the increase of optical power of the excitation source the PAS performance can be improved, because the intensity of the PAS signal is directly proportional to the optical power of the excitation source [10].However, the output power of usually used diode lasers significantly limits the PAS based sensor detection sensitivity, because the optical power of diode lasers is only several milliwatts.In order to improve the performance, some PAS based sensors were designed in an intracavity configuration, in which a photoacoustic (PA) cell was placed inside the laser resonator.Due to the high intracavity optical power, the signal level was increased compared to extracavity measurements.Hess et al. [11] reported an intracavity PAS based sensor for ethylene (C 2 H 4 ) detection based on a He-Ne laser.A signal enhancement of two times was obtained in comparison with the extracavity investigation.Harren et al. [12] demonstrated an intracavity C 2 H 4 -PAS sensor using a 10.6 μm CO 2 waveguide laser.The high intracavity laser power of 100 W improved the sensor sensitivity and a minimum detection limit (MDL) of 6 ppt was achieved.Starovoitov et al. [13] reported an intracavity PAS based sensor for multiple gases detection based on a 13 C 16 O 2 laser with an intracavity laser power of 60 W. The isotopic 13 C 16 O 2 laser offered the feasibility of overcoming the background atmospheric carbon dioxide absorption.The MDL of such sensor for ammonia, arsine and phosphine were 0.3 ppb, 3 ppb and 4 ppb, respectively with a time resolution of 30 s. Bozoki et al. [14] reported an intracavity PAS based sensor using an external-cavity diode laser with emission at 1.13 μm.This resulted in a ten times signal improvement when compared with the condition of placing the PA cell outside the cavity.By using a continuously tunable externalcavity quantum cascade laser emitting at 5.5 μm, Starovoitov et al. [15] demonstrated a similar configuration to that reported in Ref [14].Wang et al. [16] reported a fiber-ring laser intracavity C 2 H 2 -PAS sensor.It made full use of the optical power of ~108 mW in the cavity, which was nearly one order of magnitude higher than the output power.The MDL of such a sensor was 390 ppb with a 2 s integration time.
Optical fiber amplifiers are widely used in optical communications (with three operating wavelength bands, a S band: 1450-1550 nm, a C band: 1520-1570 nm, and a L band: 1565-1610 nm), which can provide significant optical signal amplification.With low noise, fiber compatibility, high gain and polarization independence, there are many advantages of erbium-doped fiber amplifiers (EDFAs) [17,18].An amplification gain of larger than 30 dB can be achieved with an EDFA and an appropriate seed diode laser.Ma et al. [19] reported a C 2 H 2 sensor which was based on quartz-enhanced photoacoustic spectroscopy with an EDFA amplified diode laser.A MDL of 33.2 ppb was achieved with an amplified optical power of 1500 mW.
The intracavity PAS based sensor is associated with optical distortion and additional optical losses in the laser resonator due to the PA cell and the absorber in the cell.Furthermore, the intracavity configuration with a long cavity length suffers from multimode mode laser oscillations.In this paper, a multi-pass retro-reflection-cavity-enhanced PAS based sensor is reported for the first time.A multi-pass retro-reflection-cavity consisting of two right-angle prisms was designed to reflect the laser beam to pass through the PA cell four times, which improved the C 2 H 2 -PAS sensor signal level significantly.Compared with intracavity configuration, the multi-pass retro-reflection-cavity utilizing the total reflection feature of two right-angle prisms has the advantageous of simple design and no influence on the laser resonator.A distributed feedback (DFB) diode laser amplified by an EDFA was employed as the excitation source.In order to reduce background noise, wavelength modulation spectroscopy (WMS) and the 2nd harmonic demodulation techniques were adopted.A sub-ppb level C 2 H 2 detection sensitivity was achieved with the optimized C 2 H 2 -PAS sensor.Further performance improvements of the of the current sensor are discussed in the following section.

Absorption line selection
Diode lasers emitting in the optical communication band are mature and inexpensive.Therefore, such diode lasers are a good choice to use for near-infrared gas detection.Several molecules, such as water vapor (H 2 O) and C 2 H 2 have a strong absorption in this spectral region.According to the HITRAN 2016 database [20], the C 2 H 2 absorption lines in the 1.5 μm at a standard atmospheric pressure and temperature of 296 K is shown in Fig. 1.It could be seen from the simulation in Fig. 1(a) that a line at the wavelength of 1530.37 nm (6534.37 cm −1 ) is one of the strongest C 2 H 2 absorption lines which is also free from spectral interference of H 2 O.This 6534.37 cm −1 C 2 H 2 absorption line was measured using a tunable diode laser absorption spectroscopy (TDLAS) method to show the spectral contour.A Herriot multi-pass gas cell with an optical length of 10 m and a C 2 H 2 :N 2 gas mixture with a concentration of 510 ppm were used in this investigation.From the measured results shown in Fig. 1(b), an absorption linewidth of 0.17 cm −1 was found.

EDFA amplified diode laser characterization
The excitation source of the sensor system was a DFB fiber-coupled diode laser which emits a continuous wave (CW) at 1.53 μm.The wavelength of the CW-DFB diode laser can be tuned to cover the 1530.37 nm C 2 H 2 absorption line by changing the laser injection current and the temperature of the thermoelectric controller (TEC).When the temperature of the TEC was 22 °C and the laser was tuned at the same wavelength of the C 2 H 2 absorption line, the output power was ~17 mW.A wavelength meter with a resolution of 0.2 pm was used to measure the emission spectrum of the diode laser.The measured result is shown in Fig. 2(a).It can be seen that the diode laser emission spectrum has a signal-to-noise ratio (SNR) of ~25 dB and a linewidth of 0.14 cm −1 .An EDFA was used to amplify the optical power emitted from the diode laser.To increase the pumping efficiency, an erbium (Er 3+ )-ytterbium (Yb 3+ ) co-doped fiber was used in the EDFA.The EDFA has two stages, a preamplifier and a power amplifier, and the output power of the EDFA can be varied.It is necessary to minimize the amplified spontaneous emission (ASE) of EDFA so that the noise level of amplified laser source can be reduced.This was achieved with two narrow-band filters whose center wavelength was 1530.33 nm and the transmission bandwidth of the filters was only 1 nm.The spectrum of amplified excitation source is shown in Fig. 2(b).As can be seen from Fig. 2, when the input power of seed laser was 17 mW, the output power was 1000 mW with a SNR of >25 dB for the amplified diode laser.The increased SNR for the amplified diode laser was mainly due to the use of two narrow-band filters.

Multi-pass retro-reflection-cavity-enhanced PAS sensor configuration
A schematic of the multi-pass retro-reflection-cavity-enhanced PAS based C 2 H 2 detection system is depicted in Fig. 3. Two narrow-band filters and an opto-isolator were adopted inside the EDFA in order to avoid back reflections on the DFB laser and EDFA.A fiber collimator (FC) was used to collimate the laser beam.The focal length of FC was 18.67 mm.The laser beam passed through the resonant PA cell which had a central cylinder tube acting as the acoustic resonator.The PA cell operated in the first longitudinal resonant mode.The performance of the PA cell depends on the Q factor and cell constant.The Q factor for the first longitudinal resonant mode increases with increasing radius of the PA cell.However, the cell constant decreases when the radius of the PA cell increases.Furthermore, when the radius of the PA cell is too small the laser beam become difficult to couple into the PA cell for the multi-pass retro-reflection-cavity configuration.For the length consideration, the cell constant increases with the length of the PA cell but the resonant frequency decreases with length.Based on the above analyses, the radius and the length of acoustic resonator were chosen to be 5 mm and 100 mm, respectively.The two buffers, whose radii were 25 mm and 50 mm in length, were fixed to both sides of the cylindrically resonator.The two buffers can reduce the noise from the gas flow as well as the interference signal generated by the window absorption.Two calcium fluoride (CaF 2 ) windows were attached on both ends of the PA cell.A condenser microphone with detection sensitivity of 50 mV/Pa was used to detect the acoustic signal in the PA cell.The multi-pass retro-reflection-cavity consisted of two rightangle prisms which were placed at both sides of the resonant PA cell.The retro-reflectioncavity was used to reflect the laser beam within the resonant PA cell with a 4-times passage to enhance the optical absorption.The side length of the two right-angle prims were 14 mm and 20 mm, respectively.The 510 ppmv (parts in 10 6 by volume) C 2 H 2 was diluted with pure nitrogen (N 2 ).Two mass flow controllers were used to control the flow rate.WMS and the second harmonic demodulation techniques were employed to improve the detection sensitivity.A dual channel function generator provided a sawtooth wave and a high-level voltage signal.The sawtooth wave was used for scanning the wavelength of the laser source by changing the injection current.The high-level voltage signal was used to trigger the lockin amplifier to demodulate the second harmonic.The lock-in amplifier provided a sine wave for modulating the laser wavelength and the frequency of sine wave was equal to half of the resonant frequency of the PA cell.The integration time of the sensor system was 1 s.The detective bandwidth of lock-in amplifier was 78 mHz.

Results and discussion
The resonant frequency was an important parameter for the PA cell.Therefore, it was initially measured experimentally.The detected signal will be a maximum when the resonant frequency of the PA cell was obtained [21].Therefore, the signal at different modulation frequencies was measured with a 510 ppm C 2 H 2 :N 2 mixture.Figure 4 shows the experimental frequency response of the PA cell.The data were fitted by a Lorentz line shape.As can be seen in Fig. 4, the resonant frequency (ƒ 0 ) of the PA cell was 1580 Hz.This value corresponds to the first longitudinal resonant mode of the resonator.The Q factor was estimated from the ratio of ƒ 0 /G 0.7 , where G 0.7 is the width of resonance contour at a level of Max/√2 = 0.7Max (Max is the maximum of the resonance contour).The G 0.7 was 62.6 Hz.Hence, the Q factor of the PA cell was 25.2.The wavelength modulation depth for the PAS sensor system should be optimized to obtain the highest 2nd harmonic component (2f) [22].The 2f signal level was investigated when the sensor system was without an EDFA and multi-pass retro-reflection-cavity.The dependence of the PAS signal amplitude as a function of the laser wavelength modulation depth is shown in Fig. 5.It can be seen that with an increase of the modulation depth, the PAS signal amplitude increased at first, but when the modulation depth was larger than 0.18 cm −1 the PAS signal started to decline.Therefore, the value of 0.18 cm −1 was found to be the optimum modulation depth.
The maximum output power of the diode laser amplified with an EDFA was 1000 mW.When the modulation depth was 0.18 cm −1 , the 2f signal for the C 2 H 2 -PAS sensor was investigated with the maximum available laser power.The results are shown in Fig. 6.For comparison, the investigation was also performed under the same conditions when the excitation source was the seed laser whose output power was 17 mW.The results from Fig. 6 showed that the 2f signal amplitude of the amplified diode laser achieved an obvious improvement of 60-fold when compared to one of the seed lasers.After the multi-pass retroreflection-cavity was assembled for the C 2 H 2 -PAS sensor system, the 2f signal amplitude became 20.92 mV, which had a 3.65 times improvement compared to the signal value of 5.73 mV for the system without this cavity enhancement.The 3.65 times improvement was somewhat lower than the designed 4 times, which was mainly the result of an optical power loss from the retro-reflection-cavity and the PA cell windows.The performance of multi-pass retro-reflection-cavity-enhanced PAS sensor was further investigated when the optical power levels ranged from 300 mW to 1000 mW.The 2f signal of the multi-pass retro-reflection-cavity-enhanced PAS sensor as a function of laser optical power is shown in Fig. 7(a) and Fig. 7(b).The results from Fig. 7 shows that with an increase of optical power, the signal amplitude of the multi-pass retro-reflection-cavity-enhanced PAS sensor can be improved.To verify the concentration response of the multi-pass retro-reflection-cavity-enhanced PAS based C 2 H 2 sensor system, a 510 ppm C 2 H 2 :N 2 gas mixture was diluted with dry N 2 .The PAS signal was measured with the maximum optical power of 1000 mW.The measured multi-pass retro-reflection-cavity-enhanced PAS signal peak amplitude as a function of C 2 H 2 concentrations is shown in Fig. 8. From Fig. 8, we can see that the reported sensor has a large dynamic range of nearly five orders of magnitude.In order to determine the background signal, an ultra-high purity dry N 2 was filled into the resonant PA cell.The amplitude of the background signal was determined to be 0.62 μV and the 1σ background noise was 0.34 μV.The noise sources in the laser PAS based sensor were the surroundings, the microphone, the electronic circuits, the absorption of the laser beam by the cell wall and the absorption of the laser beam by the windows on the two ends of the PA cell.Based on the measured signal level shown in Fig. 8, the 1σ MDL of the multi-pass retro-reflection-cavity-enhanced PAS based C 2 H 2 sensor was 8.17 ppb when the time constant of lock-in amplifier was 1 s.The corresponding normalized noise equivalent absorption coefficient (NNEA) was 1.84 × 10 −8 cm −1 W/√Hz.The long-term stability of such a multi-pass retro-reflection-cavity-enhanced PAS based C 2 H 2 sensor system was evaluated with an Allan deviation analysis.The resonant PA cell was flushed with pure N 2 at a constant flow rate and the measurements lasted for more than two hours.The measured results are depicted in Fig. 9.It was observed that when the integration time was 100 s, a MDL of 2 ppb was obtained.The Allan deviation followed a 1/√t dependence for time sequences with a maximum of 1000 s, and a minimum MDL of 600 ppt was achieved with an integration time of ~1000 s.It was demonstrated that the multi-pass retro-reflection-cavity-enhanced structure had an excellent sensor system stability.

Conclusions
In conclusion, a multi-pass retro-reflection-cavity-enhanced PAS based sensor with a high detection sensitivity was demonstrated for the first time.The excitation source was a near infrared, CW-DFB, pigtailed telecommunication diode laser which emits at 1.53 μm.The optical power of the excitation source was amplified to 1000 mW with an EDFA.In order to reduce the background noise and simplify the data processing, the WMS and a 2nd harmonic detection technique were used.A retro-reflection-cavity consisting of two right-angle prisms was designed to reflect the laser beam to pass through the PA cell four times.Experimental results showed that the retro-reflection-cavity had a 3.65 times signal improvement compared to the one without this cavity enhancement.This difference was mainly the result of an optical power loss from the retro-reflection-cavity and the PA cell windows.The linear optical power and concentration response of such a PAS sensor were investigated and the experimental results showed excellent characteristics.Finally, with an integration time of 1 s, a MDL of 8.17 ppb for C 2 H 2 detection was achieved.The calculated NNEA was 1.84 × 10 −8 cm −1 W/√Hz.The long-term stability of the reported multi-pass retro-reflection-cavityenhanced PAS based C 2 H 2 sensor was evaluated by an Allan deviation analysis.When the integration time was 1000 s, the MDL could be improved to 600 ppt.The integration time of 1000 s followed a 1/√t dependence demonstrated that such retro-reflection-cavity-enhanced PAS sensor had a great stability.The design of such retro-reflection-cavity-enhanced PAS method using an amplified diode laser improved sensor system performance significantly which was verified by the sub-ppb detection sensitivity.If an appropriate cavity design with increasing reflection times is used, the MDL of such a PAS-based sensor could be further improved.Furthermore, the performance of the reported sensor system could be improved if the output power of the EDFA is increased.

Fig. 1 .
Fig. 1.Absorption lines: (a) 510 ppm C 2 H 2 and 2% H 2 O in the 1.53 μm spectral region based on the HITRAN 2016 database; (b) Spectral contour for the 6534.37 cm −1 C 2 H 2 absorption line measured by the TDLAS method.

Fig. 2 .
Fig. 2. Diode laser emission spectrum: (a) Seed diode laser with output power of 17 mW; (b) EDFA amplified diode laser with output power of 1000 mW.

Fig. 3 .
Fig. 3. Schematic configuration of the multi-pass retro-reflection-cavity-enhanced PAS sensor system with an EDFA amplified diode laser.

Fig. 5 .
Fig. 5. 2f signal amplitudes as a function of the modulation depth.

Fig. 6 .
Fig. 6. 2f signal for C 2 H 2 -PAS sensor consisting of a seed laser, an EDFA amplified diode laser and a multi-pass retro-reflection-cavity-enhanced configuration, respectively.

Fig. 7 .
Fig. 7. Multi-pass retro-reflection-cavity-enhanced PAS signal with a modulation depth of 0.18 cm −1 .(a) 2f signals at different optical power levels.(b) Signal peak values as a function of optical power.

Fig. 8 .
Fig.8.The concentration response of the multi-pass retro-reflection-cavity-enhanced PAS signal with a modulation depth of 0.18 cm −1 and an optical power of 1000 mW.