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: shaojie@zjnu.cn
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
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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
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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.
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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.
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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.
This work was financially supported by the Key
Project of Educational Commission of Zhejiang Province of China (Z201018588), the State Key Laboratory of Fire Science of USTC (HZ2010-KF12),
Qianjiang Talent Project of Science and Technology Department of Zhejiang Province of China
(2011R10065), and the National Natural Science
Fund (61275154).
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