PAD Spectrometer Based on Wide Tunable Optical Parametric Oscillator for Noninvasive Medical Diagnostics

doi:10.4236/opj.2013.32B010

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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

3OKTAVA LTD

Email: [email protected]

Received 2013

ABSTRACT

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

1. Introduction

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 [3]. 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

[4].

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

Analyzer

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 [5]. 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 [5]

for leak detection. The relative error of measurements of

SF6 concentration is less than 1% [6].

At the present time, SF6 LaserGasTest (Figure 1) gas

leak detector delivering to China, Japan, Russia and

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

Parametric Oscillator

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

[7-10].

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.

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 [8]. 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).

D. B. KOLKER ET AL.

5. Conclusions

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.

6. Acknowledgements

This work was performed with support of Ministry of

Education and Science of Russian Federation (contract

16.522.11.2001).

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