Ammonia sensing system based on wavelength modulation spectroscopy

A sensing system in the near infrared region has been developed for ammonia sensing based on the wavelength modulation spectroscopy (WMS) principle. The WMS is a rather sensitive technique for detecting atomic/molecular species, presenting the advantage that it can be used in the near-infrared region by using the optical telecommunications technology. In this technique, the laser wavelength and intensity were modulated by applying a sine wave signal through the injection current, which allowed the shift of the detection bandwidth to higher frequencies where laser intensity noise was typically lower. Two multi-pass cells based on free space light propagation with 160 cm and 16 cm of optical path length were used, allowing the redundancy operation and technology validation. This system used a diode laser with an emission wavelength at 1512.21 nm, where NH3 has a strong absorption line. The control of the NH3 gas sensing system, as well as acquisition, processing and data presentation was performed.


Introduction
Ammonia (NH 3 ) is a colourless gas composed of nitrogen and hydrogen with a sharp, penetrating odour. Sensitive and continuous monitoring of NH 3 is relevant in several applications such as environment monitoring to quantify NH 3 emissions from coal waste piles in combustion [1], DeNO x processes, which are widely used in power plants and incinerators to reduce NO x emissions [2], or in medicine to analyse breath NH 3 levels as a diagnostic tool [3].
The main optical gas sensor technologies are based on absorption spectroscopy of fundamental bands in the 3 µm-25 µm spectral region, near infrared vibrational overtone, and combination bands from 1 µm-3 µm [4,5]. Various NH 3 analysers and measuring methods have been developed, including the differential optical absorption spectrometer [6], tuneable diode laser absorption spectrometer [7], photo acoustic spectroscopy [8], cavity ring-down spectroscopy [9], Fourier transform infrared spectroscopy [10], and others [11][12][13][14][15]. NH 3 has a rich spectrum in the near infrared region, in the spectral range from 1450 nm to 1560 nm. Recently, several works have been developed which use the absorption lines at 1532 nm and 1512 nm as the operating wavelength for NH 3 sensing [16]. The wavelength modulation spectroscopy (WMS) technique has also been demonstrated in a system using a distributed feedback (DFB) laser diode with an emission wavelength at 1532 nm in conjunction with hollow optical waveguides [17]. Such waveguides were used as long-path sample cells (optical path length of 3 m) which were coiled to reduce the physical extent of the system. A portable diode-laser-based sensor for NH 3 detection, using vibrational overtone absorption spectroscopy at 1532 nm (optical path length of 36 m), was described using a fiber-coupled optical element that made a trace gas sensor rugged and easy to align [16]. The gas sensor was used primarily for NH 3 concentration measurements. An NH 3 sensor based on the combination of resonant photo acoustic spectroscopy and direct absorption spectroscopy techniques with a DFB laser diode operating at 1532 nm, was also described [18]. Another photo acoustic spectroscopy sensor using a laser diode emitting near 1532 nm, combined with an erbium-doped fiber amplifier, was developed for NH 3 trace gas analysis at atmospheric pressure [19]. An instrument based on off-axis integrated cavity output spectroscopy and room-temperature near infrared diode lasers, with an emission wavelength at 1532 nm, was applied for measurement of several gas species [20]. In this case, the combination of high-finesse optical cavities with the simplicity of a direct-absorption-spectroscopy technique results in fast, sensitive, and absolute gas measurement. A compact cavity ring-down spectroscopy was reported for the measurement of atmospheric toxic industrial compounds such as hydrides and hydrazine's derivate of NH 3 [21]. The system used a DFB laser diode with an emission wavelength at 1527 nm, and it was directly modulated to produce the ring-down waveforms.
The current work reports the development of an NH 3 gas sensing system based on the WMS principle. This system uses a diode laser with an emission wavelength at 1512.21 nm, where the absorption coefficient of NH 3 is approximately twice its value at 1532 nm [22]. The proposed sensing system also allows the selection of two multi-pass cells based on free space light propagation with 160 cm and 16 cm of the optical path length.

Wavelength modulation spectroscopy
Absorption spectroscopy is based on a unique signature attenuation of spectral intensity caused by the gas to be monitored. It happens when light radiation interacts with molecular species and it is represented by the Beer-Lambert Law [23,24]: where   t I  and   0 I  are the transmitted and incident light intensities at wavelength λ of the laser diode, respectively, and L is the interaction length or path length that the light has to pass through the gas.
The absorption coefficient     is that by [23,24] where C is the gas concentration, and     is the specific absorptivity of the gas. The WMS is a rather sensitive technique based on absorption spectroscopy for atomic/molecular species detection, presenting the advantage that can be used in the near-infrared region [23,24]. A typical experimental configuration of WMS is composed by a wavelength-tuneable DFB laser diode, a function generator, a gas chamber, a photodetector, and a lock-in amplifier, as illustrated in Fig. 1. This technique consists in applying a sine wave modulation signal, at a frequency of a few kHz, superimposed on the laser bias current, which in turn modulates both the wavelength and the intensity of the laser light. A ramp signal is added to sweep the laser frequency across the gas absorption line. The sinusoidal modulation in the operational kHz range shifts the detection bandwidth to higher frequencies where laser intensity noise is lower [23,24].
From Fig. 1 (see left side wavelength spectrum), the modulated laser light interacts with the absorption line of the target gas and two possible situations may be observed: (1) the modulated frequency is doubled when the laser light is tuned to the center of absorption line and (2) the frequency remains unchanged when the laser is tuned out of the center of the absorption line. Therefore, this interaction between the modulated laser light and absorption line of the target gas generates signals at different harmonics of the applied modulation frequency-see the wavelength dependent responses of the measurement setup at the right side of Fig. 1 [23,24].
Gas concentration is typically detected by the recovery of the second harmonic (2f) of the laser modulation frequency through a synchronous detection, using a lock-in amplifier and reading its amplitude [23,24]. The forms of the first and the second demodulated harmonics are shown in Fig. 1 (see the wavelength dependent responses of the measurement setup at the right side), where the amplitude of the second harmonic (2f) of the laser modulation frequency is proportional to the gas concentration. The first harmonic (f) is proportional to the first derivative of the gas absorption line and equals zero when the wavelength modulated laser light is centered at the NH 3 gas absorption line, therefore providing a rather sensitive mechanism to tune the central laser wavelength to this line [23,24].

Sensing system design
The NH 3 gas sensing system proposed in this experiment was based on the WMS principle (Section 2) and it was composed by two free-space light-propagation multi-pass cells. Such cells, with distinct optical path lengths, namely, 160 cm and 16 cm, (see Fig. 2) were selected through an optical switch. Fig. 2 Schematic diagram of sensing system for NH 3 gas sensing using WMS.
A reference gas cell in order to fine tune the wavelength emission of the DFB laser diode was also used, with an optical path length of 5.5 cm and a pressure of 0.263 atm at room temperature.
The control of the NH 3 gas sensing system presented in Fig. 2, as well as acquisition, processing, and data presentation was performed using a LabVIEW ® application.
The DFB laser diode provided 7 mW at 1512.21 nm with a specified line-width < 2 MHz. The fiber was spliced to a 99/1(%) directional coupler with the 99% power arm sent to the optical switch. The 1% power arm was used as the reference beam passing through the reference NH 3 gas cell.  The laser wavelength and intensity was modulated by applying a sine wave signal with a frequency of 5 kHz. The initial injection DC current (laser bias) and temperature were set in the laser controller to 61.3 mA and 21.58 ºC, respectively, in order to tune the emission wavelength DFB laser diode to the desired wavelength of the NH 3 absorption line (1512.21 nm).
To lock the laser wavelength to the NH 3 gas absorption central line, the experimental setup had a feedback-loop that consisted in part of the emitted light passing through the reference gas cell. This means that, on Channel B of the lock-in amplifier (Fig. 2), the amplitude of the first harmonic must equal zero, thus tuning the laser wavelength to the targeted gas.
The amplitude of the second harmonic (channel A of lock-in amplifier), which was proportional to the gas concentration, was normalised by the average laser intensity read by the photodetector, therefore providing compensation for undesirable optical power fluctuations.

Ammonia sensing
Real-time concentration measurement was performed by introducing NH 3 gas inside the multi-pass cells, with 16 cm and 160 cm of optical path length, and sealed at the room temperature and atmospheric pressure (1 atm). The different NH 3 gas concentrations were achieved by mixing a calibrated NH 3 sample in an N 2 environment. Figure 4 illustrates the obtained results. The proposed sensing system shows linear responses in the ammonia concentration range from 0 to 10%. The limit of detection (LOD) was 0.12% and 0.06%, for the multi-pass cells with 16 cm and 160 cm, respectively. The limit of detection was defined according to [25]: where b  and b  are the mean and the standard deviations of the blank measures (0% of concentration), respectively, and D k is a factor chosen according to the confidence level desired ( D k was chosen to equal 3 that corresponds to a confidence level of about 90%). The sensitivity of the proposed system S S is the slope of the calibration curve, where C is the NH 3 concentration and WMS S is the normalized signal measured by the sensing system that is proportional to gas concentration. This signal ( WMS S ) was obtained through division of the lock-in amplifier signal by the average value of the photodetector signal. In this case, the sensitivity obtained was 0.73/% and 1.8/%, for the multi-pass cells with 16 cm and 160 cm, respectively. The interaction of light with gas in the optical path length is higher for the multi-pass cell with 160 cm, thus causing an increase in sensitivity.
The resolution of the system was also measured for each multi-pass cell. The results are depicted in Fig. 5. The change in the signal ( WMS S ) associated with a step change of 3% was measured by the system. Based on the step changes and rms fluctuations, resolutions of 0.014% and 0.028% were achieved for the multi-pass cells with 16 cm and 160 cm, respectively. The shorter cell presented better resolution because the number of total internal reflections (one internal reflection) was inferior to the one with 160 cm (ten internal reflections), therefore increasing the optical loss with the associated degradation of the signal-to-noise ratio. For the system resolution to benefit from the larger length interaction of light with gas in the case of the 160 cm multipass cell, an increased level of optical power would be required, which could be achieved considering optical amplification.
This sensing system also shows a fast response and recovery time below 1 s. Table 1 summarizes the system parameters for ammonia sensing.

Conclusions
In this work, an ammonia gas sensing system in the near infrared region has been developed based on the WMS principle. The sensing system was composed by two multi-pass cells sensing heads based on free space light propagation with 16 cm and 160 cm of optical path length, permitting redundancy operation and technology validation. The DFB laser diode used in the sensing system worked at the wavelength of 1512.21 nm, where the NH 3 gas has a unique absorption response and a stronger absorption line when compared to the 1532 nm region, which has been used in most systems reported in the literature, to detect ammonia in the infrared region. The experimental results showed that the best sensitivity (1.80/%) as well as the best limit of detection (0.06%), were achieved with the multi-pass cell with 160 cm of optical path length. However, the cell with 16 cm presented better resolution (0.014%) when compared with the one with 160 cm. This sensing system also showed a fast response and recovery time below 1 s allowing its use in harsh environments were concentrations between 15% and 28% by volume may be found.