Spectrometry using an optical parametric oscillator

A thermal lens spectrometer utilizes a continuously tunable optical parametric oscillator (OPO) to provide practical, easy-to-build, reliable spectrometer for measuring a wide variety of sample materials over a wide spectral range. A flow cell is placed in the path of the output beam from a tunable optical parametric oscillator, and material to be analyzed is placed in the cell. A laser probe beam is also directed into the cell coaxially with the OPO output beam. The OPO output beam acts as a pump, and the output probe beam from the cell is directed to an intensity sensor. The OPO is tuned over a selected frequency range, under computer control, for example, to produce an output probe beam having an intensity representing the spectral response of the material to be analyzed.

BACKGROUND OF THE INVENTION 
The present invention relates, in general, to tunable laser spectroscopy, 
and more particularly to laser spectroscopy utilizing an optical 
parametric oscillator as a continuously tunable laser source. 
For many spectroscopic and spectrometric applications, such as nonlinear, 
photothermal, and fluorescence spectrometry, widely and continuously 
tunable laser sources are required. Until recently, dye lasers have been 
generally used for tunable laser spectroscopy. However, the tuning range 
of dye lasers tends to be severely limited. Each dye can cover only a few 
hundred angstroms, and the total range that can be covered by laser dyes 
is limited from approximately 400 nm to 1 .mu.m. To extend beyond the 
primary dye laser tuning range, complicated nonlinear optical techniques 
are required. 
Thermal lens spectrometry is known as a highly sensitive method for 
detecting very small quantities of material by absorption of visible, 
ultraviolet or infrared light from a laser source. This method takes 
advantage of the thermal lens effect, which is a thermally induced 
alteration of the index of refraction of an absorbing medium which occurs 
when a laser beam is passed through the medium. In the thermal lens device 
described in U.S. Pat. No. 4,310,762, a converging beam derived from a 
coherent, collimated beam is passed through a reference cell. The 
converging beam is slightly modified by the change in index within the 
cell, due to the thermal lens effect. The modified beam then is passed 
through a sample cell containing the identical medium, with an additional 
medium which is to be identified. The reference cell and the sample cell 
are located at points in the beam path so that any modification in the 
beam caused by a change in the index of refraction in the reference cell 
is canceled by the same medium in the sample cell. Any detectable 
modification in the beam, e.g. expansion or divergence, as it evolves from 
the sample cell is due to the thermal lens effect caused by an additional 
pump beam directed onto the material to be identified. 
U.S. Pat. No. 4,544,274 also uses the thermal lens phenomenon for measuring 
weak optical absorptions when a cell containing sample is inserted into a 
normally continuous-wave laser-pumped dye laser cavity. The pulsewidth of 
the resulting pulsed laser output is related to the sample absorbtivity by 
a simple calibration curve. 
SUMMARY OF THE INVENTION 
Since the invention of the laser, there has been a great deal of interest 
in truly continuously tunable laser sources that cover a wide spectral 
range. Recent developments in the use of nonlinear optical crystals in 
optical parametric oscillators have made it possible to develop 
spectroscopic and spectrometric systems for a wide variety of 
applications. It has now been found that optical parametric oscillator 
(OPO) devices are particularly valuable in thermal lens spectroscopy for 
use in a wide range of environmental and analytical chemistry 
applications. 
Briefly, in accordance with the present invention a thermal lens 
spectrometer utilizes a spectrometry flow cell placed in the path of a 
tunable pump beam such as the signal beam from a solid state, tunable OPO 
utilizing one or more nonlinear crystals. The sample material to be 
analyzed is placed in the cell, and the OPO pump beam is directed through 
the cell. The pump beam is then eliminated by a prism and a bandpass 
filter following the cell. A probe laser beam, from an He-Ne laser, for 
example, is also directed through the cell coaxially with the OPO beam. 
This probe beam is directed through a pin-hole onto a sensor such as a 
photomultiplier, which measures the intensity of the probe light leaving 
the cell as the OPO is tuned through its output tuning range, or a 
selected portion thereof, to obtain the spectrum of the material being 
analyzed as a function of OPO beam wavelength. The change in the intensity 
of the probe beam is due to the wavelength-dependent heating of the sample 
in the flow cell due to the absorption of the pump beam, which is the OPO 
signal beam. 
The solid state spectroscopic system of the invention is compact, and has 
high output power and efficiency as compared to the conventional sources, 
without the need for an amplifier stage, making possible highly sensitive 
measurements, particularly in thermal lens spectrometry (TLS). The high 
sensitivity available in such TLS permits measurements with excellent 
spacial resolution while using only a small sample volume. Further, the 
characteristics of the present device make it suitable for remote sensing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates the cavity configuration of a typical tunable optical 
parametric oscillator (OPO) 10 suitable for use in the present invention. 
Art example of such an oscillator is described more fully in an article 
"High-efficiency and Narrow-linearwidth Operation of a Two-crystal 
.beta.-BaB.sub.2 O.sub.4 Optical Parametric Oscillator", W. R. Bosenberg 
et al., Appl. Phys. Lett. 55, 1952-1954 (1989). The basic principle of the 
OPO is simple. UV photons (at 355 nm, for example) from a pump beam 12 
produced as the third harmonic of a Nd:YAG pumping laser 14 are directed 
by mirror 16 through two nonlinear Beta Barium Borate (.beta.-BaB.sub.2 
O.sub.4, or BBO) crystals 18 and 20, where they break down by spontaneous 
and stimulated emission into lower-frequency photons called the signal 
(visible) and the idler (infrared) photons. The crystals 18 and 20 are 
included in a Fabry-Perot cavity 22 formed by an output mirror 24 and a 
grating 26, the cavity providing the optical feedback which leads to 
oscillation of the OPO. Mirror 28 in cavity 22 and mirror 16 transmit the 
signal and idler, but reflect the UV pump photons. 
The grating is oriented in the Littrow configuration so as to reduce the 
oscillator linewidth to 0.2-0.6 nm throughout the tuning range of the 
oscillator. Two BBO crystals are used to compensate for the walk-off 
effect because the Poynting vector and the k-vector of the pump wave are 
not in the same direction. The crystals may be grown by using the known 
top-seeded high-temperature solution growth technique. The lengths of the 
crystals are 9 and 8.5 mm and they are cut for type-I phase matching at 
28.6.degree.. 
The two crystals are mounted on separate rotational stages 30 and 32, 
respectively, that are connected to a motor controller 34, which is in 
turn controlled by a standard personal computer 36. This system allows the 
two crystals to be set simultaneously to a phase-matching angle 
corresponding to the desired output wavelength. The grating 26 (Milton 
Roy) has 1800 grooves/mm with a blaze angle of 26.5.degree.. The grating 
is also placed on the same kind of rotational stage 38 and is controlled 
by the computer to set the desired angle. The grooves 40 on the grating 
are positioned normal to the polarization of the OPO signal beam to 
improve the broadband diffraction efficiency and the line-narrowing 
effect. 
The pump source 14 for the OPO is a Q-switched Neodymium-doped Yttrium 
Aluminum Garnet (Nd:YAG) laser system followed by third-harmonic 
generation (THG). The pump beam diameter is reduced to 2 mm by use of a 
telescope; a typical pulse property may be 12 mJ with a 5-6 ns pulse 
duration at 355 nm. The pump beam is steered through the OPO cavity by use 
of mirror 16 and reflected back along the same optical axis by use of 
mirror 28. Both mirrors are 355 nm high reflectors. The incident angles 
are 55.degree. and 0.degree., respectively, for mirrors 16 and 28. The 
output coupler of the OPO 10 is a standard multilayer dielectric-coated 
mirror 24 with 50% reflectivity at 550 nm, which allows signal wave 40 to 
exit the cavity. The resonated OPO signal wave is diffracted by the 
grating, and the first-order diffracted beam is reflected back along the 
cavity axis. This Littrow configuration minimizes loss from the grating. 
The cavity length is 75 mm from the center of the grating to the surface 
of mirror 24. 
The linewidth of the OPO signal wave 40 may be measured by using a 0.5 m 
monochromator 42 equipped with a photodiode. In a test of the equipment, 
wavelength resolution was set to 0.2 nm, which also corresponded to the 
monochromator's wavelength accuracy. The signal from the photodiode in 
monochromator 42 was sent to a boxcar averager 44 (EG&G Model 162) and 
then read into a computer 46. The spectrum was displayed in real time on 
the computer screen. The intensity fluctuation of the OPO output was also 
measured. A hot mirror was used to isolate the signal wave from the idler. 
FIG. 2 is a block diagram of a thermal lens spectrometer 50 based on a 
direct incidence system of the type described by K. Mori et al., 
"Determination of Nitrogen Dioxide by Pulsed Thermal Lens 
Spectrophotometry" Anal. Chem. 55, 1075-1079 (1983) and by S. Kawasaki et 
al. "Thermal Lens Spectrophotometry Using a Tunable Infrared Laser 
Generated by a Stimulated Raman Effect", Anal. Chem. 59, 523-525 (1987). 
In this spectrometer, the OPO output signal wave 40 from the OPO 10 is 
used as a pump beam, and is focused into a 1 cm sample flow cell 52 
(Hellma Cells, Inc.) by way of a mirror 54, a lens 56, and a quartz wedge 
58. A beam 60 from an He-Ne laser 62 (Spectra-Physics, Inc. Model 105-1) 
is directed to the surface of wedge 58 and is reflected coaxially with the 
OPO pump beam 40 into the cell 52. The beam 60 is used as the probe beam 
and passes through the cell without focusing, producing a sample output 
probe beam 64 which is directed through a quartz prism 66 to a mirror 67. 
The mirror 67 directs the sample output probe beam 64 through a lens 68 
which expands it to a 10 mm diameter, through a pinhole aperture 70, 
through a polarizer 72 and through a bandpass filter 74 to a 
photomultiplier 76 (Thorn EMI Electron Tubes, Ltd., 9658R). The pinhole 
may have an aperture of 1.2 mm, with the bandpass filter being centered at 
the wavelength of the He-Ne probe; i.e., at 632.8 nm, and with the 
photomultiplier measuring the intensity of the beam center. The intensity 
spectrum may be supplied by way of line 80 to a personal computer for 
display on the computer screen in real time. In a test of the equipment, a 
sample of nitrogen dioxide (supplied by Matheson) was diluted to 0.5% in 
dry air and delivered to the flow cell 52 by way of line 82. The flow rate 
of the sample was 40-50 cc/min and the outlet gas line 84 was then bubbled 
through an NaOH solution (not shown) and discharged. 
To maintain narrow linewidth oscillation over wide range, it is important 
to have synchronized operation and precise alignment between the two 
crystals 18 and 20 and the grating 26. The generated wavelength that 
depends on the crystal angle for two crystals over the entire tuning range 
was measured in the above-noted experimental set-up, and an equation was 
derived for the tuning curve with a fifth-order polynomial fit to these 
data. The error in selecting a given wavelength was confirmed to be within 
the accuracy of the rotational stage. 
FIG. 3 shows a typical OPO output at 620 nm. The spectrum indicated by 
curve 90 was obtained by using the first-order diffracted beam from the 
Littrow configured grating, which was blazed for optimum efficiency around 
this range. The output beam 40 was measured in 0.05 nm steps and 30 points 
were averaged at each step. The full width at half-maximum (FWHM) 
linewidth is estimated to be 0.45 nm from the spectrum. The observed value 
without a grating is close to 1.5-nm FWHM. Thus, one can successfully use 
the grating to reduce the linewidth by a factor of 3 at this wavelength. 
The spectral shape of curve 90, which is almost symmetric, also indicates 
that the angles of two crystals are well synchronized with that of the 
grating. 
Above 620 nm the linewidth increased slightly to 0.5-0.6 nm, which is cause 
by broadening of the phase-matching bandwidth of the crystal near the 
degenerated point. Careful alignment of the crystal angle and grating 
produces narrower oscillation. Precise alignment of the cavity cannot be 
maintained during automatic scanning since the accuracy of the rotational 
stage is limited to 0.0050.degree.. This, however, can be improved with a 
higher resolution grating. A 2400-grooves/mm grating gives a linewidth of 
0.15 nm at 650 nm without sacrificing efficiency significantly. One can 
achieve a narrower linewidth by inserting an etalon over the wide spectral 
range. The Littman configuration can also decrease the linewidth to that 
of the etalon configuration, but its low efficiency still remains a 
problem. The linewidth of the OPO is also affected by the linewidth and 
divergence of the pump beam. However, one can solve these problems by 
using an injection-seeded Nd:YAG laser. A single longitudinal mode can be 
obtained with that pump laser, but the difficulty in stabilizing the 
cavity leads to a limited tuning range. Finally, a proper configuration is 
chosen depending on the application, trading off the advantages of 
simplicity, wide tunability, high efficiency, and narrow linewidth. 
Curve 92 in FIG. 4 shows the output intensity of the signal wave of the OPO 
as a function of wavelength from 450 to 706.5 nm. The OPO was scanned at 
1.5 nm/step and 50 points were averaged at each step. The intensity was 
corrected for the spectral response of the photodiode that was used for 
the measurements. The scanning time was approximately 14 min. This 
spectrum 92 shows the intensity of the signal wave. Since the hot mirror 
used cannot isolate the signal wave from its corresponding idler at 
wavelengths over 680 nm, the intensity above this region includes that of 
the idler wave. A jump in the output intensity around 475 nm is seen from 
etalon effects that are due to internal reflections that are no longer 
lost from the cavity when the surfaces of each crystal rotate into a 
position that is normal to the cavity axis. The conversion efficiency was 
greater than 10% for most of the tuning range and 13% at 650 nm. It was 
12% when a 2400-groove/mm grating was used, and 3% with an additional 
etalon at 650 nm. This spectrum was measured to show that there are no 
spectral gaps over the entire tuning range. Therefore, the spectral 
transmission variation of the hot mirror was not considered, which is the 
main cause of irregular intensity fluctuation. The wavelength dependence 
of the intensity also depends on the transmittance of the output coupler 
24. 
The tunable range of this system is actually 450-1675 nm, limited by the 
cavity mirrors that were used. The wavelength accuracy of this system was 
measured for several wavelengths over the tuning range and was confirmed 
to be within half of the linewidth of the oscillation beam at each point. 
The response of the system depends mostly on the rotational rate and the 
minimum step of the rotational stage. The crystals and grating were 
directly mounted on the rotational stages in the experimental set-up; 
therefore, any wavelength could be selected within a few seconds, although 
the scanning time was limited by the repetition of the Nd:YAG laser (10 
Hz) and the desired amount of averaging at each point. 
This compact, solid-state BBO OPO system has several desirable performance 
characteristics for spectroscopic applications. High output power and 
efficiency relative to conventional sources without an amplifier stage 
make sensitive measurements possible, especially for spectrometry such as 
thermal lens spectrum (TLS), which is known as a highly sensitive 
analytical method. For example, it has been reported that TLS is almost 
1000 times more sensitive than conventional absorption spectrometry for 
probing NO.sub.2 diluted in air. Because of this sensitivity, only a small 
sample volume is needed and excellent spatial resolution can be obtained. 
Accordingly, the actual sample volume needed for the thermal lens signal 
is defined only by the beam cross section and the confocal distance of the 
focusing lens. In the experimental set-up described above, the OPO beam 
was tightly focused by lens 56 which led to a short confocal distance (&lt;1 
mm) which allowed the use of a 1-cm flow cell. The thermal lens signal was 
strong enough to obtain a spectrum comparable with that measured with a 
1.2-1.3% sample with a 7.5 cm cell. Furthermore, the BBO OPO TLS system 
makes a fingerprinting assignment (which has been of interest but 
unachievable with conventional dye lasers) feasible because of its broad 
tunability. These characteristics are quite suitable for remote-sensing 
device. 
FIG. 5 shows a thermal lens spectrum 94 of NO.sub.2 from 450 to 590 nm. 
Each peak corresponds well with previously reported spectra. In this 
experiment the idler was not isolated. However, absorption of the idler 
wavelength by the sample was small and so could be neglected. Since the 
intensity of the thermal lens input signal 40 is proportional to the OPO 
power, intensity variations over the tuning range will affect the 
amplitude of the spectral peaks. This is the reason the peaks under 470 nm 
are relatively small. During a scan over such a wide spectral range, 
dispersion of the focusing lens and the beam splitter influences the 
signal intensity. In this experiment, however, the confocal distance was 
estimated to be less than 1 mm. Over the spectral range, the focal point 
and the focal length changed by less than 0.1 mm and a few millimeters, 
respectively. Since the direct incidence method was employed, these 
factors had no significant effect on the spectrum. 
FIG. 6 shows the spectrum 96 of the same sample between 600 and 680 nm, 
reproducing exactly the known spectrum of this molecule. Because NO.sub.2 
shows a complicated spectrum, finer details of the spectrum cannot be 
resolved at this resolution. As the excitation beam is tuned toward the 
wavelength of the He-Ne laser, it can spuriously give a positive signal at 
the photomultiplier tube. However, since the OPO beam is polarized 
linearly and has a fast rise time, it can easily be separated by using a 
polarizer and by changing the gate position of the boxcar. 
Thus, there has been disclosed a computerized BBO OPO system with a narrow 
linewidth oscillation of 0.2-0.6 nm over its visible tuning range. The 
performance of this system is demonstrated by measuring the thermal lens 
spectrum of NO.sub.2, which illustrates that the tunability of the system, 
450-1675 nm, is much broader than that of commercially available dye laser 
systems that require dye changes to achieve a wide spectral region. The 
present system also tunes more quickly and easily over its whole tuning 
range. The OPO system is completely solid state (except for the YAG laser 
pumped by a water-cooled flash lamp) and consists of commercially 
available components, making it a very practical tunable laser system. The 
system succeeded in measuring the entire visible thermal lens spectrum of 
NO.sub.2 in a single scan with adequate resolution to resolve the 
important peaks, thus demonstrating excellent pointing stability and wide 
tunability. 
Although the invention has been described in terms of a particular OPO, as 
pump source, with a particular laser as the probe, it should be understood 
that the system of the invention is not linked to these particular 
devices. Nor is the system limited to the particular sample or wavelength 
described herein, but is limited only by the scope of the following 
claims.