Optics and Photonics Journal, 2013, 3, 43-46
doi:10.4236/opj.2013.32B010 Published Online June 2013 (http://www.scirp.org/journal/opj)
PAD Spectrometer Based on Wide Tunable Optical
Parametric Oscillator for Noninvasive Medical Diagnostics
D. B. Kolker1,2,3, I. V. Sherstov2,3, A. I. Karapuzikov2, A. A. Karapuzikov2, A. A. Boyko1,2,
M.K.Starikova2, N. Yu. Dukhovnikova1,2, V. N. Loconov3, M. Yu. Shtyrov2,
I. B. Miroshnichenko2, K. G. Zenov2, M. B. Miroshnichenko2
1Novosibirsk State Technical University
2Special Technology LTD
The gas analyzer based on optical parametric oscillators (OPO) and laser photo - acoustic spectroscopy is demonstrated.
The optical parametric oscillators based on fun – out PPLN and bulk crystal AgGaS2 with a two-pass pumping are de-
veloped. Wide tunable OPO is pumped by compact nanosecond Nd: YLF laser. Pulse duration is 5 - 7 ns, maximum
pulse energy is 1.5 mJ at a frequency of 100-2000 Hz. OPO lasing threshold is 10 - 16 mJ/cm2 at the spectral range 2.2 -
4 µm for fun – out PPLN OPO and 12 - 20 mJ/cm2 at the spectral range 4 - 7.5 µm for AgGaS2 OPO. Absorption spec-
tra of gaseous mixtures (CH4, C3H8, C2H6, C2H4 and CO2) and human’s breath were studied.
Keywords: Optical Parametric Oscillator; Nonlinear Optical Crystals; Silver Thiogallate; Fun – out PPLN; Photo -
Acoustic Spectroscopy; Absorption Spectrum
At the present time registration and determination of the
concentrations of various gases in the atmosphere plays
an important role in biology and medical diagnostic. Analy-
sis of the gas composition of atmosphere and human's
breath is one actual directions of research.
The method of laser photo-acoustic spectroscopy (LPAS)
is important for monitoring of chemical compounds in
atmosphere because of its simplicity of practical realize-
tion, safety, cost-effectiveness and extremely high sensi-
tivity (ppb-ppt level) [1,2].
LPAS allows taking measurements in real time. LPAS
detectors do not require expensive high-reflectivity mir-
rors as opposed to cavity ring-down spectroscopy (CRDS)
method. Capabilities of LPAS sensors generally increase
with increasing laser energy as far as acoustic signal are
proportional to optical power absorbed in detector. A
major impact on the field of trace gas detection can be
expected from new extensively tunable solid-state laser
systems working in the mid - IR spectral region.
Early developed systems based on CO2-lasers combined
with resonant photo-acoustic detector (PAD) a described
at . Our approach is determined by possibility of de-
veloping compact sealed-off waveguide CO2 lasers with
specific technical parameters defined by a particular task.
In this respect the recent realization and further im-
provement of optical parametric oscillators (OPOs) and
quantum cascade lasers (QCL) could be an important
breakthrough in the practical application of laser photo-
acoustic spectroscopy (LPAS) in trace gas monitoring
In present time optical parametric oscillation (OPO) is
one of the most effective devices to produce tunable co-
herent radiation in MID-IR.
2. Laser Photo - Acoustic Spectroscopy Gas
2.1. “LaserGasTest” Gas Analyzer
LaserGasTest gas leak detector based on LPAS is pre-
sented at the Figure 1. The leak detector is destined for
measuring extremely small concentration (up to 1 ppb) of
SF6 in high-voltage gas insulated equipment . CO2
laser's lines show a strong overlap with the absorption
band of SF6. So, for development a SF6 gas sensor, it is
enough to use CO2 laser without frequency stabilization
(free-running laser). Practical testing of SF6 LaserGasT-
est shows that sensitivity of 1 ppb of SF6 is enough 
for leak detection. The relative error of measurements of
SF6 concentration is less than 1% .
At the present time, SF6 LaserGasTest (Figure 1) gas
leak detector delivering to China, Japan, Russia and
Copyright © 2013 SciRes. OPJ
D. B. KOLKER ET AL.
South Korea. Nowadays SF6 LaserGasTest is being pre-
pared for entrance to European market.
2.2. Laser Photo - Acoustic Spectroscopy Gas
Analyzer Based on Tunable Optical
Optical parametric oscillator (OPO) possesses broad
wavelength coverage therefore we research new devices
based on OPO for range extension of LPAS sensors. This
technique will allow covering wide spectral range from
2.4 to 8.5 µm. The tuning range of nanosecond OPO
based on PPLN crystal pumped with Nd:YLF laser is 2.4
- 3.9 µm (idler wave). Expansion of the spectral range up
to 8.5 µm is possible by using nonlinear chalcogenide
bulk crystal: LiGaSe2, LiInSe2, AgGaS2, and AgGaSe2
The experimental OPO setup combined with photo -
acoustic detector was developed.
The experimental setup consists of pump laser (Q-
switched Nd:YLF) and two OPO: fun - out PPLN OPO
(2.4 - 3.9 µm) and AGS OPO (Figure 2). The photo-
acoustic detector is used for registration of absorption
spectra of gas samples.
Figure 1. Photo of SF6 Laser Gas Test.
2.2.1. Fun - Out PPLN Optical Parametric Oscillator
The monolithic fun - out PPLN OPO cavity consist of
two high-reflectivity mirrors at the signal wave (SDPOPO).
The output mirror is high transparent for the pump and
idler wavelengths but for the signal wavelength this mir-
ror is high reflected. The step motor moves crystal in
relation to pumping beam for the wavelength tuning.
The fun-out PPLN OPO tuning range is presented at
the Figure 3.
Lasing threshold was 10 - 16 mJ/cm2 at the spectral
range 2.2 - 4 µm for fun – out PPLN OPO.
Figure 2. Scheme of experimental setup: Nd: YLF pumping
laser, FI - Faraday isolator, λ/2 - half-wave plate, M1, M5,
M6 - turning mirrors, M4 - turning mirror for fun – out
PPLN OPO, M3, M7 - dichroic mirrors, M8 – beamsplitter,
PAD – photo – acoustic detector, PD – pyrodetector.
Figure 3. Fun out PPLN OPO output as a function of idler
wavelength λidler for pump energy Epump 140 µJ.
Copyright © 2013 SciRes. OPJ
D. B. KOLKER ET AL. 45
2.2.2. AGS Optical Parametric Oscillator
The advantages chalcogenide crystals are following: their
relatively high thermal conductivity, large bandgap and,
as a result, low two-photon absorption and low group
velocities mismatching . Optical parametric oscillator
based on chalcogenide crystals allows covering wide
spectral range from 2 to 11 µm.
The advantage of AgGaS2 (AGS) crystal: high effect-
tive nonlinear coefficient and wide optical transmission
in spectral range 0.5 - 12.0 μm, makes it realistic to gen-
erate infrared parametric radiation in wide spectra range.
The monolithic AGS OPO cavity consists of two high-
reflectivity mirrors at the signal wave. The output mirror
is transparent at the pump and idler wavelengths.
The designed monolithic block allows correcting the
cavity length by changing the distance between two cyl-
indric holders in the flanges. The step motor is using for
the wavelength tuning of OPO.
The energy density of the lasing threshold is JT =
11.59 mJ/cm2 at 4.4 µm (idler wave). (Figures 4 and 5)
3. Laser Photo-acoustic Detector (LPAD)
Unlike the cavity ring-down spectroscopy (CRDS) laser
photo – acoustic detectors (LPAD) do not require the use
of expensive mirrors with very high reflectance. Since
photo-acoustic signal is proportional to the absorbed
optical power in the detector, the limiting parameters
LPAS sensors generally increased with an increase in
energy use of lasers.
Figure 4. Fun-out PPLN OPO.
Figure 5. AGS OPO.
For excitation of photo acoustic spectra we used the
nanosecond mid-IR OPO described in previous part.
More often in PA devices are used sinusoidal modulated
radiation and resonant cells. In the pulsed photo acoustics,
the system is illuminated with a laser pulse rather than
with periodic modulation. In our experiments we used
the photo acoustic resonant cell.
4. Absorption Spectra
Absorption spectra of gaseous mixtures (CH4, C3H8,
C2H6, C2H4 and CO2) and human’s breath were studied
by using tandem OPO-PAD. Absorption spectra of
methane and ethylene are presented on the Figures 6 and
Figure 6. Absorption spectra: absorption spectrum of ex-
perimental gaseous mixture CH4 (red line) and theoretical
absorption spectrum of CH4 (black line) from HITRAN.
Figure 7. Absorption spectra: absorption spectrum of ex-
perimental gaseous mixture C2H4 (red line), absorption
spectrum of C2H4,HITRAN (black line).
Copyright © 2013 SciRes. OPJ
D. B. KOLKER ET AL.
Copyright © 2013 SciRes. OPJ
Mid – IR spectrometer based on AGS OPO and fun - out
PPLN OPO pumped by a 1.053 µm Nd: YLF laser was
demonstrated experimentally. Absorption spectra CH4,
C3H8, C2H6, C2H4 , CO2 and human’s breath were studied
by using tandem OPO-PAD. Probes of real patients
(COPD, tuberculosis, asthma) are studied.
We were demonstrating new perspectives of using
photo-acoustic spectroscopy for noninvasive medical
diagnostics. Compact analytical systems for different
applications can be developed with use of this approach.
This work was performed with support of Ministry of
Education and Science of Russian Federation (contract
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