Multigas detection using a sample-grating distributed Bragg reflector diode laser Jie Shao,
Transcription
Multigas detection using a sample-grating distributed Bragg reflector diode laser Jie Shao,
Multigas detection using a sample-grating distributed Bragg reflector diode laser Jie Shao,1,* Yexing Han,1 Jie Guo,1 Liming Wang,1 Ying Han,2 Zhen Zhou,2 and Ruifeng Kan3 1 Institute of Information Optics, Zhejiang Normal University, Jinhua 321004, China 2 Kunshan Hexin Mass Spectrometry Technology Co., Ltd., Kunshan 215311, China 3 Key Laboratory of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China *Corresponding author: [email protected] Received 11 July 2013; revised 1 October 2013; accepted 1 October 2013; posted 2 October 2013 (Doc. ID 193428); published 21 October 2013 A sample-grating distributed Bragg reflector (SG-DBR) laser with 18 preprogrammed channels operating at 1540–1580 nm is characterized and compared for use as a source of tunable diode laser gas absorption spectroscopy. Two gases, CO and CO2 , were targeted in this study by direct absorption spectroscopy and wavelength modulation spectroscopy with second-harmonic detection. In addition, the detectability of sample optical thickness is reported. Potential extensions of this research in the future are assessed using the SG-DBR diode laser as a source for tunable diode laser gas absorption spectroscopy. © 2013 Optical Society of America OCIS codes: (300.6260) Spectroscopy, diode lasers; (130.6010) Sensors. http://dx.doi.org/10.1364/AO.52.007462 1. Introduction Over the past 35 years, tunable diode laser absorption spectroscopy (TDLAS) has matured into a robust and convenient means of measuring a wide variety of gas parameters in difficult real-world environments [1]. The main advantages of the method are (a) the modulating capability of the diode laser, which allows for effective suppression of background emissions, especially using frequency and wavelength modulation; (b) it is a self-calibrating working method; and (c) it is a compact, robust apparatus [1–5]. For diagnostics based on TDLAS, several types of diode laser are commonly used [6]: (a) Fabry–Perot diode lasers (FPDLs; AlGaAs) with a 1 cm−1 tuning range in the kilohertz modulation 1559-128X/13/317462-07$15.00/0 © 2013 Optical Society of America 7462 APPLIED OPTICS / Vol. 52, No. 31 / 1 November 2013 range have been found adequate for scanning. These lasers are inexpensive and have a good output power of 3–20 mW, but they suffer from the “mode hopping” effect. Therefore, the lasers have to be selected for single-mode behavior with a wavemeter before use. Besides, the FPDL has a large threshold current and amplitude modulation coefficient. (b) Vertical cavity surface emitting lasers (VCSELs) have single-mode tuning ranges of up to 30 cm−1 but have a lower maximum power of 1 mW, and the output wavelength has a strong dependence on the injection current, resulting in the need for a highly stable current source [7]. (c) Distributed feedback (DFB) diode lasers offer a similar output to the FPDLs but show no mode hops and therefore do not require selections. They use a Bragg mirror that consists of a periodically repeating change in refractive index whose period lengths determine the wavelengths reflected. Their tuning range (via temperature) is about 27 cm−1 . However, they are much more expensive than FPDLs [8]. (d) External cavity diode lasers (ECDLs) employ an external resonator to tune to the exact wavelength required. The rather slow (100 Hz) mechanical wavelength tuning makes ECDLs unsuitable for environments where large, rapid transmission changes occur. With the rapid development of telecommunication, the widely tunable diode laser (WTDL) sources that rely on sample-grating distributed Bragg reflector (SG-DBR) technology have resulted in a near-IR emission that is tunable over a range of 40– 100 nm [9]. The laser chip is composed of a rear Bragg reflector, a phase section, a gain section, and a front Bragg reflector. The refractive indices of these four sections can be independently adjusted by carrier injection. Depending on these four parameters and the laser chip temperature, mode cartography is performed for subsequent computer-based user access to a specific wavelength [10]. Unlike external cavity technology, the SG-DBR technology permits a fast wavelength switching time (<1 μs) [11]. These sources can offer several opportunities to permit multispecies gas monitoring in physics, chemistry, and biology and can also be applied to the detection of broadband absorbers. We combined a WTDL module (Intune Technologies AltoWave3500) with an absorption spectroscopy technique to demonstrate multispecies detection. The WTDL design used in this study relies on SGDBR technology. Details about the methodology will be presented in the following sections. 2. Experimental Setup The experimental setup is schematically illustrated in Fig. 1. The laser was modulated by one or two function generators; one provided a triangular wave (with a voltage ranging from 0 to 1 V for direct absorption (DA) measurements and from 0.2 to 0.8 V for wavelength modulation absorption spectrometry (WMS), and a frequency of some tens of hertz) that provided a “slow” scan of the wavelength across the tunable region of each channel, whereas the other (in fact a part of a lock-in amplifier) provided a sinusoidal function (with an amplitude of up to a couple of hundreds of millivolts and a frequency of some tens of kilohertz) for the WMS measurements. All experiments were done at room temperature, assumed to be 23°C (296 K). The diode laser light was first sent through a fiberbased variable optical attenuator (VOA50-APC, Thorlabs) before it was sent to the sample cell. The output from the attenuator was equipped with a GRIN lens with a working range (or focal region) of several tens of centimeters. The sample cell is a 10 cm long glass cell with either 980 mbars of CO or 1005 mbars of CO2 before the light aimed onto a large-area IR sensitive detector (2033-M, New Focus). The detector signal was sent directly to a 16 bit Data Acquisition card (NI PCI-6251DAQ card, National Instruments) in a computer for data acquisition when DA measurements were performed, or into a lock-in amplifier (SR830 DPS, Stanford) running with a time constant of 100 μs with a 24 dB lowpass filter on the input when WMS measurements were made. In the latter case, the output of the lock-in output was sent to the A/D card. The monolithic integrated WTDL used has a tuning ranges of several tens of nanometers, developed for optical communications. These lasers employ Vernier effect tuning to cover a 1540–1580 nm spectral range, with a side-mode suppression ratio of better than 40 dB and a linewidth is less than 3 MHz. The WTDL design with an INT1190 evaluation card and software evaluation application requires the alignment of reflection peaks from the front and back mirrors with the cavity mode at the desired wavelength. This entails control of four separate currents to achieve complete wavelength coverage over the entire tuning region. As many as 255 user-specified laser channels can be stored in the module memory, and the laser output is coupled to a single-mode polarization maintaining fiber. The laser has been preprogrammed for 18 channels according to Table 1 below. The wavelengths of the various channels have been chosen so that they address a transition in CO, CO2 , or H2 O. The first two species listed were monitored in this research. One selects a channel by sending a command to the module via an RS232 interface. In a given channel, one can modulate the laser wavelength at a frequency of as much as 50 kHz by applying an external voltage to the laser module’s analog modulation input. 3. Assessment of Direct Absorption Spectrum from CO and CO2 Fig. 1. Experimental setup for TDLAS using an SG-DBR diode laser. DA measurements on CO and CO2 have been performed using atmospheric pressure cells. It was first concluded that the 14 channels that target transitions in CO and CO2 indeed include the transitions in the center part of the scan. Some of the results can be seen in Figs. 2 and 3, which show the signals acquired directly from the detector. The fitted baselines are shown in the top panel of each set, the 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS 7463 Table 1. Specification of the 18 Preprogrammed Channels of the In-tune Laser and Summary of the Analytical Signals from Either a Cell with 980 mbars of CO or 1005 mbars of CO2 Measured in DA and in Wavelength Modulation Absorption Spectroscopy SOT (10−3 ) Channel 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 Species Targeted Wavelength (nm) Line Strength (cm1 ∕molecule) Linewidth (γ self ) cm−1 Theory Experiment DA 10−4 WMS 10−4 H2 O H2 O H2 O CO CO CO CO H2 O CO CO CO CO CO2 CO2 CO2 CO2 CO2 CO2 1541.9494 1546.1780 1547.9520 1560.5000 1561.2577 1561.6763 1562.1215 1562.4410 1562.5932 1563.0914 1563.6162 1564.1676 1568.3395 1568.5569 1571.4059 1571.7082 1573.3315 1573.6788 — 3.35 × 10−25 3.16 × 10−25 1.62 × 10−24 3.09 × 10−24 4.13 × 10−24 5.40 × 10−24 3.77 × 10−25 6.91 × 10−24 8.66 × 10−24 1.06 × 10−23 1.27 × 10−23 1.22 × 10−24 1.68 × 10−24 1.51 × 10−23 1.63 × 10−23 1.52 × 10−23 1.34 × 10−23 — — — 0.0491 0.0510 0.0519 0.0527 — 0.0535 0.0542 0.0549 0.0555 0.0720 0.0676 0.0720 0.0729 0.0799 0.0822 — — — 2.6 4.8 6.3 8.1 — 10.2 12.6 15.2 18.1 1.3 2.0 16.5 17.6 15.0 12.9 — — — 1.65 3.34 4.58 5.98 — 7.37 8.93 10.41 11.75 0.88 1.41 7.53 8.39 6.53 6.42 — — — 0.76 0.79 1.0 1.1 — 1.4 1.2 1.5 1.5 0.77 0.73 0.87 1.0 0.86 9.6 — — — 0.40 0.42 0.68 0.74 — 0.62 0.75 0.48 0.54 0.46 0.24 0.19 0.25 0.15 0.20 Fig. 2. (a) DA signals and fitted baselines from the CO cell from channels 04 and 12, (b) the normalized DA signals and fitted signals from the corresponding channels, and (c) the difference (normalized noise) between the normalized signals and the fitted signals. 7464 Detectability of SOT APPLIED OPTICS / Vol. 52, No. 31 / 1 November 2013 Fig. 3. (a) DA signals and fitted baselines from the CO2 cell from channels 13 and 16, respectively, (b) the normalized DA signals and fitted signals from the corresponding channels, and (c) the difference (normalized noise) between the normalized signals and the fitted signals. normalized signals and fitted signals are shown in the middle panel, and the normalized noises are shown in the bottom panel from the channels that address a CO or CO2, respectively. The following nomenclature has been used for the fits and units given in Table 1. The absorption of light by the analyte has been assumed to follow Beer’s law, which has been written as Iυ I 0 e−αυ I 0 e−SχυnL ; (1) where αυ represents the relative absorbance, the sample optical absorption (dimensionless), S is the line strength (in cm2 cm−1 ∕molecule), υ is the detuning frequency (cm−1 ), χυ is an area normalized lineshape function (in 1∕cm−1 ), n is the density of absorbers (in molecules∕cm3 ), and L is the interaction length (in cm). I and I 0 are the transmitted and incident power, respectively. It can clearly be seen from Figs. 2 and 3 that the absorption signals reside on a nonlinear background. There is obvious nonlinear power-versus-modulation voltage dependence from the scan. So the incident power (I 0 ) is obtained by fitting a fourth-order polynomial expression describing absorption as Iυ a bυ cυ2 dυ3 eυ4 e−αυ : where δυ is the HWHM width in unit of cm−1. We will denote the measured SOT on resonance by α0, whereas the corresponding estimated entity was calculated by Eq. (3). Table 1 gives the relative absorption on resonance (α0 ) of each transition, obtained by calculating Eq. (4) and fitting an expression of Eq. (2). The signal-to-noise ratio (SNR) is defined as the ratio of the experimental α0 divided by the standard deviation (σ), which terms are calculated from the normalized residual. From the results, the CO detection limitation is about 1290 ppm·m from channel 12, and the CO2 detection limitation is about 1150 ppm·m from channel 15 but not the largest line strength of channel 16, which has larger nonlinear background from power-versusmodulation voltage. As can be seen from Table 1, the relative absorption for the strongest transitions is around 10−2 and for the smallest transitions is about 10−3 . All the measured absorptions show a good degree of agreement with the magnitude of line strength according to Table 1. It is worth noting that all measured α0 values are in general slightly smaller than calculated from HITRAN, but they are still linear with the calculated α0 , as shown in Fig. 4. (2) It is in most cases straightforward to identify the analytical absorption signal from the fitted Eq. (2) despite these background signals, although accurate quantifications are more difficult to perform. The fitted incident power (I 0 ) is shown (called baseline) as a solid line in the top panel and the relative absorbance [αυ] as a solid line in the middle panel, respectively. To assess the quality of the fit, the residual is shown in the bottom panel. Statistical analysis showed the magnitude of the 1 × 10−4 . is between 7.3 × 10−5 and 1.5 × 10−4 level. The results showed that the sensitivity was often limited by DA techniques but not for the SG-DBR diode laser [12]. Detectability for small values of α0 is often a technical limitation of absorption spectrometry. α0 represents the sample optical thickness (SOT) on resonance (when υ υ0 ). For small absorbance, αυ ≪ 1, α0 normally is a small entity that also takes the role of the relative absorbance on resonance, ΔI∕I 0 (when υ υ0 ), according to Beer’s law α0 Snlχjυυ0 : (3) In this study, all experiments were done at about 1 atm, and the χυ mentioned above is the area normalized Lorentz function. So, the SOT can be written as α0 Snl 1 ; πδυ (4) Fig. 4. Comparison of measured α0 and calculated α0 from (a) CO and (b) CO2 . 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS 7465 The reason is that most normalized absorption signals do not have a pure Lorentzian form and the fitted linewidths are larger than the HITRAN given, also from the etalon effects caused by the laser, the fiber, or the cell. 4. Assessment of Wavelength Modulation Absorption Spectrum from CO and CO2 WMS, which is a sensitivity improvement method that has been widely used in TDLAS, is the most common technique for reducing the 1∕f type of noise in absorption measurements. In short, the wavelength of the laser is sinusoidally modulated at a frequency of f , which often is in the tens of kilohertz range. For a weak absorbance, the detected signal is fed to a lock-in amplifier in order to extract a certain nth harmonic of the detected signal at a detection frequency nf, where n used to be 2. Some of the WMS signals detected at 2f are shown in Figs. 5 and 6 using the same sample cells that target transitions in CO and CO2 , which indeed include the transitions in the center part of the scan. Fig. 5. (a) 2f WMS signals and fitted data from the CO cell from channels 04 and 12 that address a transition in CO and (b) the residual from the raw signals and fitted signals. 7466 APPLIED OPTICS / Vol. 52, No. 31 / 1 November 2013 The modulation amplitude used was 125 mV for the channels addressing the CO cell that maximized the net 2f -WMS CO signal (in agreement with the fact that the modulation amplitude should be 2.2 times the HWHM of the absorption profile), whereas it was 200 mV for the channels addressing the CO2 cell that used the amplitude that maximized the net 2f -WMS CO2 signal, which was slightly larger because of the larger width of the absorption profile of CO2. Figure 5 shows the measured 2f -WMS signal from the lock-in amplifier (referred to as the raw data) with the dashed lines and the fitted 2f -WMS signal with the solid lines in the upper panel (of each part) from the channels that target a CO transition, whereas the solid lines in the lower panel illustrate the residual signal from the difference between the measured signal and the fitted signal. Figure 6 illustrates the corresponding signals from the glass cell containing CO2 from the channels targeting this species. Fig. 6. (a) 2f WMS signals and fitted data from the CO2 cell from channels 13 and 16 that address a transition in CO2 and (b) the residual from the raw signals and fitted signals. All 2f -WMS signals were fitted according to χ 1 υ; υ¯ a χ 3 υ; υ¯ a ; S aυ χ 2 υ − υ¯ a b 2 (5) where aυ a0 a0 υ a00 υ2 , a0 is a factor including both the sensitivity of the system and the concentration of the sample, and a0 , a00 , and b represent the associated intensity modulation of the laser, a0 and a00 at slow modulations and b for kilohertz modulations. υ is the detuning frequency, and χ i υ − υ¯ a is the various Fourier coefficients for a wavelength modulated normalized Lorentzian line shape function. As can be seen from the Figs. 5 and 6, the channels that target a transition in CO and CO2 gave rise to a clear WMS signal. The relative magnitudes of the analytical signals (S) are obtained from the fitted curve and are reasonable, although they do not perfectly agree with the line strengths given in Table 1. Meanwhile, the standard deviations (σ) are calculated from the residuals, which obviously looked like a sinusoidal structure. This kind of structure seriously reduces the detection limitation. The detectability of SOT related by WMS could be analyzed by the ratio of the relative magnitudes of the analytical signals (S) to the standard deviation of the residual (σ), which is shown in Table 1. Also, the CO detection limitation can be calculated from channels 11 and 12 and is about 420 ppm·m, and the CO2 detection limitation is about 230 ppm·m from the channel 17. From the results, the detected limitations using the WMS technique are only a few times better than those using a DA method. The main reason is that the modulation voltage is nonlinear with the output of the SG-DBR laser, which is the so-called residual amplitude modulation (RAM). From Figs. 2 and 3, the SG-DBR diode laser’s output power has a largely nonlinear modulation voltage dependence, and the nonlinearity is different in every channel. As has been discussed above as well as in the literature, a nonzero output-versusmodulation voltage dependence (i.e., RAM) creates not only a nonconstant level in DA spectrometry, from which a weak absorption signal is to be measured, but also background signals in WMS that significantly affect the detectability and prevent sensitive detection of analytical species. The other reason is that the windows of the sample cells cause an obvious etalon effect, which also affects the detectability in WMS. Anyway, it is still enough for application in an industry where accuracy is a few percent. 5. Conclusion In this study, CO and CO2 were measured by a preprogrammed WTDL. The last analyses indicate that the detectability of the SOT is about 1 × 10−4 using DA and 1 × 10−5 using 2f WMS. These good fits between DA and 2f WMS show the sensitivity and accuracy of the sensor and confirm the potential utility for application in gas sensing. The conclusion of the investigation of DA and WMS signals is that the accuracy of the assessment of concentration is significantly limited by the nonlinear background signals. In conclusion, we have demonstrated that the SG-DBR diode laser could be used for measurement of concentration combined with DA spectroscopy and WMS techniques, which presents exciting opportunities for applications in absorption-based multigassensing regimes. Such wide wavelength tuning is not possible with conventional single-frequency DFB or FPDL or VCSEL lasers, for their use is usually limited to the detection of one gas. It’s obvious that replacing all these single-frequency lasers with such a widely tunable laser device will reduce the complexity and cost of the simultaneous multigas detection system. 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