Linear cavity laser system for ultra-sensitive gas detection via intracavity laser spectroscopy (ILS)

Contaminants are detected optically at concentrations below 1 part-per-million (ppm) and extending to a level approaching 1 part-per-trillion (ppt) by using intracavity laser spectroscopy (ILS) techniques. An optically-pumped solid-state laser (the ILS laser) is employed as a detector. The ILS laser comprises an ion-doped crystal medium contained in a linear laser cavity which may be optically pumped by a diode laser pump laser. A gas sample containing gaseous contaminant species is placed inside the laser cavity and on one side of the ion-doped crystal. The output signal from the ILS laser is detected and analyzed to identify the gaseous species (via its spectral signature). The concentration of the gaseous species can be determined from the spectral signature as well. Advantageously, the linear cavity is relatively small and compact in comparison to other ILS systems. Additionally, the small/compact size of the linear cavity makes the gas detection system constructed therewith amenable to a broad variety of practical applications.

TECHNICAL FIELD 
This invention relates, generally, to the detection of contaminants in 
gases, and more particularly, to the high sensitivity detection of gaseous 
molecules, atoms, radicals, and/or ions by laser techniques generally 
termed intracavity laser spectroscopy (ILS). 
BACKGROUND OF THE INVENTION 
In the preparation of high quality semiconductor material (e.g., silicon 
films) for use in the microelectronics industry, it is well known that 
contaminants must be controlled. Failure to control contaminants, as is 
also well known and appreciated, can result in the loss of significant 
resources as the resultant products are typically not useful for their 
intended purposes. 
Generally, the starting materials in the fabrication of silicon films 
consist essentially of gases, typically denoted either "bulk" (e.g., 
nitrogen or argon) or "specialty" (e.g., hydrogen chloride, hydrogen 
bromide, boron trichloride). The successful operation of a fabrication 
facility designed to prepare semiconductor materials depends directly on 
the purity of the starting gases, as well as the gas handling capabilities 
available to preserve gas purity during the delivery of the gases to the 
process chamber and while material processing is taking place. Suitable 
control of the purity of such starting gases (i.e., monitoring and 
inhibiting high levels of contaminants as may be contained in the gases) 
is essential. 
Under many current techniques, such control is achieved after the fact. 
That is, the silicon films so produced are periodically tested and the 
production line shut down only after such tests reveal the presence of 
high level contaminants. These processes, as will be appreciated by those 
skilled in the art, can lead to the waste of not only starting materials 
but also product which is produced prior to cessation of production. It is 
therefore desirable to monitor and control the contaminants as may be 
contained in such starting gases during production so as unacceptable 
contaminant levels are observed, production can be immediately, or at 
least shortly thereafter, halted. 
Many molecular, atomic, radical, and ionic species are present in the bulk 
and specialty gases used in the preparation (e.g., chemical vapor 
deposition or "CVD") and processing (doping and etching) of semiconductor 
materials that can be viewed as "contaminants." Such contaminants can 
degrade either the quality of the fabricated semiconductor material or the 
efficiency with which the semiconductor material is prepared. These 
contaminant species can interfere with the chemical process directly or 
even cause particles to be formed in the gas delivery lines or process 
chamber which subsequently deposit on the surface of the wafer material 
causing indirect performance defects. 
The first step in controlling and/or eliminating these contaminants is 
their detection in the bulk and specialty gases used as starting 
materials. While this is generally recognized, heretofore practiced 
methods are generally inadequate. This is due, in large part, to the 
situation created by seemingly ever increasing competitive industry 
standards which have developed. Specifically, as the size of 
microelectronic devices has decreased while performance specifications 
have been intensified, the requirements for gas purity (i.e., absence of 
microcontamination) has increased. 
Against this backdrop, it will likely be clear that several measurement 
criteria are important to detector effectiveness: (1) absolute detection 
sensitivity usually stated as parts-per-total number of gas molecules in 
the sample (e.g., parts-per-million or number of contaminant molecules per 
10.sup.+6 background molecules); (2) species selectivity or the capability 
to measure the concentration of one species in the presence of other 
species; (3) rapidity of measurements to obtain a desired signal to noise 
ratio; (4) capability of monitoring contaminants in both non-reactive and 
reactive gases; and (5) linearity and dynamic range of gas concentrations 
that can be measured. 
The current state-of-the-art devices for contaminant detection (e.g., 
water) encompass a variety of measurement techniques. For example, current 
state-of-the-art devices for water vapor detection utilize conductivity 
and electrochemical, direct absorption spectroscopy, and atmospheric 
pressure ionization mass spectroscopy (APIMS) measurement techniques. As 
discussed below, each of these methods fails to adequately address these 
requirements. 
CONDUCTIVITY AND ELECTROCHEMICAL 
Conductivity and electrochemical methods by solid-state devices exist which 
can detect water vapor at the 1 to 100 ppm range. Conductivity and 
electrochemical methods generally require direct physical contact between 
the sample and the device; thus, detection occurs after water molecules 
deposit on the solid-state surface. As a consequence, these devices do not 
perform well, if at all, with utilization of reactive or corrosive gases. 
Indeed, even their performance in non-reactive gases changes and/or 
deteriorates after even short exposures to reactive or corrosive gases. 
The linearity and dynamic range of response are usually limited to about 
one decade. The detection selectivity of these devices with respect to 
different gaseous species also is generally poor since the devices 
themselves will respond to a wide range of species without discrimination. 
Additionally, selectivity is incorporated into the measurements only 
through whatever chemical selectivity, if any, is embodied in the coatings 
used to cover these devices. 
DIRECT ABSORPTION 
Direct absorption spectroscopy generally relates to the passing of light 
through the sample from an external source and measuring the reduction in 
light intensity caused by molecular, atomic, radical, and/or ionic 
absorption in the sample. Detection sensitivity depends directly on the 
subtraction of two large numbers (light intensity from the external source 
before it passes through the sample and its intensity after it exits the 
sample). This limits the detection sensitivity to the extent that direct 
absorption is generally considered a low sensitivity methodology. 
APIMS 
APIMS, initially used in the analysis of impurities in bulk nitrogen and 
argon and ambient air for air pollution studies, is now currently used by 
semiconductor manufacturers to detect trace levels of moisture and oxygen 
in inert bulk gases. With APIMS, the sampled gas is bombarded with 
electrons, or may be flame and photon excited, to produce a variety of 
ions that are then detected directly. Particularly, ionization occurs at 
atmospheric pressure in the presence of a reagent gas in the ionization 
source. APIMS typically exhibits detection sensitivities in the range of 
about 10 parts per trillion (ppt) in non-reactive gases. APIMS cannot even 
be used with reactive gas mixtures. Additional disadvantages of APIMS 
include an average cost between about $150,000 to $250,000, extensive 
purging and calibration procedures, and the need for a knowledgeable 
operator. 
INTRACAVITY LASER SPECTROSCOPY 
In the context of the present invention, laser technology, specifically 
intracavity laser spectroscopy (ILS), is disclosed as being used as a 
detector (sensor) to detect gaseous species (contaminants) at very high 
sensitivity levels. While the methods and apparatus disclosed herein are 
particularly suited for application in fabrication of semiconductor 
components, it should be appreciated that the present invention in its 
broadest form is not so limited. Nevertheless, for convenience of 
reference and description of preferred exemplary embodiments, this 
application will be used as a benchmark. In connection with this 
application, laser technology offers distinct advantages to gaseous 
species (contaminant) detection over known methods and, particularly, to 
water vapor detection. 
In conventional applications of lasers to the detection of gaseous species 
(contaminants), laser produced radiation is used to excite the gas sample 
external to the laser in order to produce a secondary signal (e.g., 
ionization or fluorescence). Alternatively, the intensity of the laser 
after it passes through a gas sample, normalized to its initial intensity, 
can be measured (i.e., absorption). 
Some twenty years ago, another detection methodology, intracavity laser 
spectroscopy, was first explored in which the laser itself is used as a 
detector; see, e.g., G. Atkinson, A. Laufer, M. Kurylo, "Detection of Free 
Radicals by an Intracavity Dye Laser Technique," 59 Journal Of Chemical 
Physics, Jul. 1, 1973. 
Intracavity laser spectroscopy (ILS) combines the advantages of 
conventional absorption spectroscopy with the high detection sensitivity 
normally associated with other laser techniques such as laser-induced 
fluorescence (LIF) and multiphoton ionization (MPI) spectroscopy. ILS is 
based on the intracavity losses associated with absorption in gaseous 
species (e.g., atoms, molecules, radicals, or ions) found within the 
optical resonator cavity of a multimode, homogeneously broadened laser. 
These intracavity absorption losses compete via the normal mode dynamics 
of a multimode laser with the gain generated in the laser medium. 
Traditionally, ILS research has been dominated by the use of dye lasers 
because their multimode properties fulfill the conditions required for 
effective mode competition and their wide tunability provides spectral 
access to many different gaseous species. In particular, measurements at 
visible wavelengths have been conducted using dye lasers having linear 
two-mirror cavities; see, e.g., V. M. Baev, J. Eschner, J. Sierks, A. 
Weiler, and P. E. Toschek, "Regular dynamics of a multimode dye laser", 
Optics Communications, 94 (1992) 436-444; and J. Sierks, V. M. Baev, and 
P. E. Toschek, "Enhancement of the sensitivity of a multimode dye laser to 
intracavity absorption", Optics Communications, 96 (1993) 81-86. 
The liquid dye laser, however, is not compatible with many practical 
applications given its liquid state and the need to maintain physical and 
optical stability. Dye lasers also operate primarily in the visible 
spectral region. The absorption strength of many gaseous species, although 
definitely detectable by ILS, are not as strong in the visible as compared 
to lower energies (e.g., in the near infrared). Higher detection 
sensitivity, therefore, is found when absorption transitions in the 
infrared are utilized. 
Some ILS experiments have been performed with multimode, tunable 
solid-state laser media such as color centers and Ti:Sapphire; see, e.g., 
D. Gilmore, P. Cvijin, G. Atkinson, "Intracavity Absorption Spectroscopy 
With a Titanium: Sapphire Laser," Optics Communications 77 (1990) 385-89. 
ILS has also been successfully used to detect both stable and transient 
species under experimental conditions where the need for high detection 
sensitivity had previously excluded absorption spectroscopy as a method of 
choice. For example, ILS has been utilized to examine gaseous samples in 
environments such as cryogenically cooled chambers, plasma discharges, 
photolytic and pyrolytic decompositions, and supersonic jet expansions. 
ILS has been further used to obtain quantitative absorption information 
(e.g., line strengths and collisional broadening coefficients) through the 
analysis of absorption lineshapes. Some of these are described in G. 
Atkinson, "Intracavity Laser Spectroscopy," SPIE Conf., Soc. Opt. Eng. 
1637 (1992) 
Prior art methods of performing ILS, however, while suitable for use in 
laboratory settings are unacceptable for commercial settings. The 
constraints of commercial reality, as briefly noted above, essentially 
dictate that such a detector be conveniently sized, relatively 
inexpensive, and reliable. Laboratory models fail to fully meet these 
requirements. 
A laboratory demonstration of the feasibility of using ILS techniques for 
detecting small quantities of water vapor in a nitrogen atmosphere with a 
Cr.sup.4+ :YAG laser is described in D. Gilmore, P. Cvijin, G. Atkinson, 
"Intracavity Laser Spectroscopy in the 1.38-1.55 .mu.m Spectral Region 
Using a Multimode Cr.sup.4+ :YAG Laser," Optics Communications 103 (1993) 
370-74. The experimental apparatus utilized was satisfactory for 
demonstration of operational characteristics, but undesirable for 
implementation in a commercial application as contemplated by the present 
invention. 
In accordance with various aspects of the present invention, the present 
invention provides a user friendly, i.e., comparatively simple, detection 
system, having the advantages of direct absorption techniques but with 
dramatically increased detection sensitivities, capable of detecting 
gaseous species in reactive and non-reactive samples at a commercially 
viable cost. In this regard, the present invention addresses the long felt 
need for a method and apparatus for the high sensitivity detection of 
contaminants in reactive and non-reactive gas systems in commercial 
settings. 
SUMMARY OF THE INVENTION 
In accordance with various aspects of the present invention, contaminants 
are detected optically at concentrations below 1 part-per-million (ppm) 
and extending to a level approaching 1 part-per-trillion (ppt) by using 
ILS techniques. An optically pumped solid-state laser comprising an 
optical resonator cavity which is a linear cavity formed between two 
mirrors with an ion-doped crystal medium contained therein serves as the 
detector. A gas sample containing gaseous contaminant species, for 
example, water vapor, is placed inside the linear cavity of the ion-doped 
laser (between the two mirrors or reflective surfaces) and on one side of 
the active medium. A variety of ion-doped laser media including 
Tm.sup.3+,Tb.sup.3+ :YLF and Tm.sup.3+ :YAG are described here, but other 
ion-doped crystals having multiple longitudinal and transverse cavity 
modes can be used as well. 
Specifically, a gas detection system for detecting the presence of gaseous 
species in a gas sample is provided. The gas detection system comprises: 
(a) a linear laser cavity formed between a first mirror and a second 
mirror; 
(b) an ion-doped crystal therein having two ends; 
(c) a pumping source located outside the linear laser cavity which has an 
output which optically excites the ion-doped crystal, thereby producing an 
output beam which exits the linear laser cavity; and 
(d) means for containing the gas sample in the laser cavity, the output 
beam of the ion-doped crystal passing through the gas sample prior to 
exiting the linear laser cavity. 
The gas detection system preferably comprises a pumping sources such as a 
semiconductor laser diode, an ion-doped crystal laser (e.g., Cr.sup.4+ 
:YAG), a gas laser, flashlamps, or other suitable forms of optical pumping 
used to provide the optical excitement required to operate the ILS laser, 
a multimode ILS laser operated over the wavelength region in which the 
species of interest absorb, a gas sample placed within the linear laser 
cavity of the ILS laser (either by employing a gas sample cell located 
within the linear laser cavity or by filling the entire intracavity 
optical region with the gas sample), a wavelength dispersive spectrometer 
capable of spectrally resolving the output of the ILS laser, a detector 
capable of measuring the wavelength-resolved intensity of the ILS laser 
output, and an electronic circuit which can read the signal from the 
detector and convert it into electronic signal that can be processed by a 
computer or other digital electronics. The gas detection system may also 
include a modulating device designed to periodically interrupt the 
intensity of the pumping laser beam and the output from the ILS laser. 
A method for detecting the presence of gaseous species in a gas sample is 
also disclosed. The method comprises the steps of: 
(a) directing the output beam of a pumping source to an ion-doped crystal 
contained within a linear laser cavity, thereby producing an output beam 
from the ion-doped crystal which passes through the gas sample which is 
contained in the linear laser cavity prior to exiting the linear laser 
cavity; and 
(b) directing the output beam of the ion-doped crystal after exiting the 
laser cavity to a detector assembly for determining the presence and/or 
concentration of gaseous species in the gas sample.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference is now made in detail to a specific embodiment of the present 
invention, which illustrates the best mode presently contemplated by the 
inventors for practicing the invention. Alternative embodiments are also 
briefly described as applicable. 
As previously briefly noted, the subject matter of the present invention is 
particularly well suited for use in connection with the manufacture of 
semiconductor components, and thus, a preferred exemplary embodiment of 
the present invention will be described in that context. It should be 
recognized, however, that such description is not intended as a limitation 
on the use or applicability of the subject invention, but rather is set 
forth to merely fully describe a preferred exemplary embodiment thereof. 
In this regard, the present invention is particularly suited for detection 
of contaminants. Contaminants as used herein refer to molecular, atomic, 
radical, and ionic species such as may be present in gaseous materials, 
such as in the gaseous materials which are used in the fabrication of 
silicon films, i.e., inlet lines. Alternatively, the term contaminant may 
also refer to the gaseous material itself, such as, for example, when the 
detector of the present invention is used to determine if a line (e.g., 
HCl line) has been sufficiently purged of the gaseous material. 
In accordance with a preferred embodiment of the present invention and with 
momentary reference to FIG. 1A, a gas (contaminant) detector system 10 
suitably comprises a pumping laser system A, an ILS laser and associated 
chamber B, a spectrometer C, and a detector with associated electronics 
(e.g., computer, digital electronics, etc.) D. More particularly, and with 
reference to FIGS. 1B 3, and 4 pumping system A suitably comprises a 
pumping source 100, a beam shaping optics assembly 200 and a beam 
modulation assembly 300; laser and chamber B suitably comprises a chamber 
assembly 400 and an ILS laser 500; spectrometer C suitably comprises a 
spectrometer assembly 600; and detector D suitably comprises a detector 
assembly 700 and a computer system 800. As will be described more fully 
herein, gas detector system 10 advantageously detects gaseous species 
(contaminants) which are suitably contained in a gas sample. In general, 
pumping laser driver system A pumps ILS laser 500, preferably at or near 
(but above) the threshold level such that a laser beam passes through the 
gas sample thereby enabling the spectrum of the gas sample to be obtained. 
This spectrum is suitably detected through use of detector/computer system 
D which, upon manipulation, enables the reliable and accurate 
determination of the presence and concentration at high sensitivity levels 
of gaseous species (contaminants) which may be contained within the gas 
sample. 
With reference to FIGS. 2A-2C, and in order to more fully explain the 
scientific principles utilized in accordance with a preferred embodiment 
of the present invention, the general principles of intracavity laser 
spectroscopy (ILS) are illustratively shown. As is known, in its simplest 
terms, a laser can be described as containing a gain medium, in which 
optical gain is produced, and a resonator, comprised of optical elements 
such as mirrors. Optical losses may appear in both the medium and the 
optical elements comprising the laser cavity (e.g., the resonator). With 
particular reference to FIG. 2A, a laser device in its simplest form can 
be schematically illustrated as including a gain medium 1A around which 
respective mirrors 2A and 3A are placed. Mirrors 2A and 3A are typically 
coated to have high reflectivity surfaces over a broad spectral range. For 
example, the mirror coating on mirror 2A may be totally reflective, while 
the mirror coating on mirror 3A may be partially reflective thereby 
permitting some light to escape from the laser cavity. The spatial region 
between the reflective surfaces of mirrors 2A and 3A in which the gain 
medium is placed defines the laser resonator or cavity, and in the context 
of the present invention relates to the so-called "intracavity region." 
The intensity (I) of the laser output may be determined both by the 
wavelength region over which the gain medium operates (.lambda.) and the 
reflectivity of the resonator elements (e.g., mirrors 2A and 3A). Normally 
this output is broad and without sharp, distinctive spectral features, as 
is shown in the plot of I versus wavelength (.lambda.) provided in FIG. 
2D, which corresponds to the laser of FIG. 2A. 
By selecting different optical elements to form the laser cavity, the 
spectral output of the laser can be altered or "tuned." For example, and 
with particular reference to FIG. 2B, a tuned resonator cavity may include 
a diffraction grating 2B which replaces the highly reflective mirror 2A 
shown in FIG. 2A. As shown, the laser device therefore includes 
diffraction grating 2B, mirror 3B, and a medium 1B positioned 
therebetween. In general, the result in spectral output from this tuned 
laser will be narrowed and appear as wavelengths within the original 
spectral output of the laser defined by the gained medium and the mirrors 
(FIG. 2A). For example, a schematic plot of intensity (I) versus 
wavelength (.lambda.) illustrating a narrowed output is depicted in FIG. 
2E. 
The laser output can also be altered by placing gaseous molecules, atoms, 
radicals, and/or ions in either their ground or excited states inside the 
optical resonator (e.g., cavity). With reference to FIG. 2C, a laser so 
configured may include a highly reflective mirror 2C, a partially 
reflective mirror 3C with a medium 1C, and an intracavity absorber 4 
placed therebetween. In this case, intracavity absorber 4 may comprise 
such gaseous species (e.g., the sample containing contaminants). The 
effect of the intracavity gaseous species on the laser output can be 
observed. For example, a plot of I versus .lambda. for such a device is 
shown in FIG. 2F comprises an absorption spectrum of the gaseous species 
contained within intracavity absorber 4. The distinct absorption features 
illustrated in FIG. 2F arise from the intracavity species losses against 
which the laser gain must compete. 
Thus, the absorption spectrum of the intracavity species may appear in the 
spectral output of the laser. In particular, the laser output intensity 
(I) at wavelengths where the stronger intracavity absorption features 
compete effectively against the gain properties of the resonator are more 
reduced. As a result, as illustrated, instead of a relatively smooth 
continuous output, such as shown in FIG. 2D, a structured laser output 
such as shown in FIG. 2F may be observed. The decreases in intensity (I), 
as shown in FIG. 2F, are due to absorption by the gaseous intracavity 
species, i.e., the more intense the absorption features, the larger the 
decrease in the laser output intensity. In accordance with the present 
invention, the absorption spectrum obtained by intracavity laser 
measurements in which an intracavity absorber is employed can be utilized 
for the high sensitivity detection of such gaseous species. It has been 
found that each gaseous species can be uniquely identified by its 
respective absorption spectrum (signature) and thus can be used to 
confidently identify such gaseous species (contaminant). 
The present inventors have found that the appearance of the absorbing 
species (gaseous elements) within the laser resonator before and/or during 
the competition between gain and losses which naturally occur as the laser 
system approaches threshold give rise to enhanced detection sensitivity 
through use of ILS. In view of the fact that the losses associated with 
the intracavity absorber become part of the competition between the gain 
and losses within the laser, even a small absorbance associated either 
with a weak absorption transition and/or an extremely small absorber 
concentration is amplified dramatically during the gain/loss competition. 
As a result, such competition clearly appears in the output of the ILS 
signal (see FIG. 2F). Thus, using these principles, ILS can be utilized to 
detect both weak absorption and/or extremely small absorber 
concentrations. 
ILS detection differs significantly from other spectroscopy methods which 
employ lasers. As described above, the output of a laser used for 
spectroscopy typically excites in a gaseous species, a secondary phenomena 
which is then monitored. Alternatively, output of a laser may be passed 
through a gaseous species and the absorption of selected wavelengths in 
the output of the laser provides means for characterizing the gas. In 
either case, the operation of the laser is separate from and unaffected by 
the gaseous species being measured. 
With ILS detection, however, the operation of the laser is directly 
affected by the gaseous species. In this manner, the ILS laser 500 itself 
acts as a detector. In particular, the output from the ILS laser 500 as it 
exits the laser cavity contains spectroscopic information about the 
gaseous species. This mode of operation is unique to ILS detection and the 
ILS laser 500. 
Accordingly, ILS lasers 500 are distinctly different from conventional 
lasers and possess operational characteristics which are not typical of 
conventional lasers. For example, absorbing species which produce loss are 
intentionally introduced into the laser cavity of ILS lasers 500. These 
absorbing species effect the operation of the ILS laser 500 and alter its 
output. 
Also, unlike lasers employed in conventional applications, ILS lasers 500 
operate at or above but close to threshold (e.g., within 10% of threshold 
power). However, operating near threshold often causes the output of the 
ILS laser 500 to be unstable. Accordingly, additional techniques directed 
to stabilizing the output of the ILS laser 500 may be required. 
In contrast, conventional lasers typically operate well above threshold to 
maximize output. Maximizing output, however, is not the objective of ILS 
lasers 500. Consequently, laser media which are inefficient and/or do not 
produce high output power may be employed for ILS detection when such 
laser media are unfavorable for most other laser applications. The purpose 
of the ILS laser 500 is not to produce light, but to monitor loss within 
the laser cavity. As described above, mode competition inside the laser 
cavity enables such loss within the ILS laser 500 to be detected with 
enhanced sensitivity. 
Since ILS detection possesses increased sensitivity beyond conventional 
optical spectroscopy techniques, interferences from background gases 
having both weak absorption and/or extremely small absorber concentrations 
may be significant, even if such interferences are negligible with 
conventional spectroscopy techniques. 
The detection of gases via ILS can be achieved by using a variety of laser 
systems. (As used herein, the laser system includes both the ILS laser 500 
and the pumping source 100.) These laser systems each share several common 
properties which are required for extremely high detection sensitivity. 
Prior art has identified three such properties. First, the laser systems 
exhibit multimode operation near the energy threshold for lasing. Second, 
the laser systems possess an operational wavelength bandwidth that is 
substantially broad relative to the absorption features of the gaseous 
species or contaminants (i.e., molecules, atoms, radicals, and/or ions) 
being monitored. Third, the laser systems maintain stable intensity and 
wavelength. 
It will be appreciated that a variety of ILS laser systems having different 
physical and optical characteristics meet these above-listed criteria for 
extremely high detection sensitivity. The different physical and optical 
characteristics of the laser systems may also provide distinct advantages 
such as with regard to the experimental conditions (e.g., data acquisition 
times) under which ILS measurements are made. Additionaly, these different 
physical and optical characteristics may inf of the following: (1) the 
gaseous species or contaminant (i.e., molecules, atoms, radicals, and/or 
ions) that can be detected; (2) the respective concentrations of each 
gaseous species that can be determined; and (3) the practical types of 
samples to which detection can be applied. Examples of the latter include 
the total pressure of the sample, the sample size, and the environment 
which the sample is contained (e.g., reactive versus stable environments). 
Against the backdrop of these general principles, in the context of the 
present invention, the present inventors have devised a commercially 
viable contaminant sensor system 10 which provides enhanced detection of 
contaminants in gaseous samples. The contaminant sensor system 10 of the 
present invention possesses each of the abovementioned properties required 
for extremely high sensitivity detection. Additionally, the ILS laser 
system of the present invention is smaller, simpler, and less expensive to 
construct than any ILS laser system disclosed in prior art. 
With reference now to FIGS. 1A, 3, and 4, and in accordance with a 
preferred exemplary embodiment of the present invention, a detection 
system 10 suitably includes pump source 100, an ILS chamber assembly 400 
in which ILS laser 500 is contained. Spectrometer 600 and a 
detector/computer system 700, 800 are suitably optically connected to the 
output from the ILS laser 500 whereat the absorption spectrum is suitably 
manipulated thus enabling the high sensitivity detection of the presence 
and/or concentration of gaseous species (contaminants). 
In order to drive ILS laser 500, system 10 requires a pumping source 100 
which delivers radiation of sufficient power and within a suitable 
wavelength region so as to optically excite the ILS laser at or slightly 
above its threshold. In this regard, it is important that ILS laser 500 
operate such that the gain in the laser medium exceeds the overall optical 
losses, including those associated with the gain medium, mirrors, and 
non-minor intracavity optical elements, as well as the absorption of any 
gaseous species within the optical resonator cavity. Moreover, preferably 
laser 500 operates with multiple longitudinal modes, i.e., over a broad 
wavelength region. Typically, a desirable bandwidth over which laser 
action occurs is between about 2 and 15 nanometers (mm). While ILS laser 
500 can also operate with more than one transverse resonator mode, such is 
not necessary. Suitably, the optical parameters (e.g., average power 
density, peak power density, divergence, and beam diameter) of pumping 
source 100, advantageously match the optical requirements of ILS laser 
500. As will be appreciated, to do so it is necessary to determine how 
many photons can be delivered within a specific volume and at a given 
distance from the pumping source 100 over a particular period of time. In 
general, in accordance with the present invention, such determinations are 
made in accordance with known theoretical and quantitative equations such 
that the pumping source 100 is suitably selected to advantageously match 
the optical characteristics of ILS laser 500. 
Accordingly, a pumping source 100 is selected on the basis of its 
operational wavelength and on its optical parameters in a manner such that 
it can alone be used to excite ILS laser 500. 
It should be appreciated that driver 100 may comprise any suitable optical 
pumping source, either coherent or incoherent, continuous or pulsed, that 
will suitably excite ILS laser 500. As a result, even in accordance with 
the previously recited preferred embodiment, pumping source 100 operates 
in a conventional manner and emits radiation over a desired frequency band 
and having a desired bandwidth. 
In accordance with the present invention, pumping source 100 may comprise a 
semiconductor diode laser, a solid state crystal laser (e.g., Nd:YAG), a 
gas laser, one or more flashlamps, or any other pumping source operating 
at a wavelength .lambda..sub.p which is suitable for pumping ILS laser 
500. Preferably the pumping source 100 comprises a diode laser. 
Use of a diode laser as a pump source 100, however, typically requires use 
of a beam shaping optics assembly 200. Disadvantageously, the output beam 
of a diode laser is highly asymmetric and/or astigmatic as is known in the 
art. Consequently, the volume of the pumping radiation from diode laser 
pump laser 100 that is transferred to the gain medium (i.e., crystal 507) 
does not suitably match the volume that must be optically excited within 
the gain medium of ILS laser 500. As will be described in greater detail 
hereinbelow, beam modification optics, such as beam shaping assembly 200, 
can be utilized to facilitate optical matching between a pump source 100 
comprising a diode laser and the ILS laser 500; that is, to optimize the 
radiation delivered to ILS laser 500 by focusing the required photon 
density into the correct location and volume of the gain medium of the ILS 
laser. Specifically, beam modification system 200 is used to alter the 
pumping radiation of driver 100 to meet the requirements of laser 500. To 
correct the astigmatism, asymmetry, and divergence associated with the 
output beam (beam E) of a diode laser pump laser, normal macroscopic 
optics and/or micro-optics that are placed within several micrometers of 
the semiconductor diode laser may be employed. Examples of beam 
modification optics include diffractive optics, refractive optics, and 
combinations thereof. Specific examples of macroscopic optics which may be 
employed to shape the output beam (beam E) of a diode laser include a beam 
expanding telescope or alternatively, a pair of anamorphic prisms. 
In accordance with the present invention, the gas detection system 10 
comprises an ILS laser 500 having a simplified laser cavity. The laser 
cavity is formed between two mirrors and has a substantially linear 
configuration which does not provide astigmatic compensation. However, the 
linear cavity design employed by the present invention enables a gas 
detector system 10 to be constructed which is substantially smaller, 
lighter, and simpler than prior art ILS systems. Consequently, the gas 
detector system 10 of the present invention is less expensive to construct 
as well as easier to operate than other ILS laser designs. Additionally, 
the gas detection system 10 of the present invention can be constructed to 
be more rugged or mechanically stable as is required by many practical 
applications. 
Referring, in particular, to FIG. 3, a gas detection system 10 of the 
present invention is depicted comprising an ILS laser 500 with a laser 
cavity 902 which is a linear laser cavity. By "linear laser cavity" or 
"linear laser resonator" is meant a laser cavity (or laser resonator) 902 
that is equivalent to a laser cavity formed between only two mirrors. 
In its simplest form, a linear laser cavity comprises a laser cavity 902 
formed between a first mirror and a second mirror. It will be appreciated 
that any number of additional mirrors which are planar may be included to 
steer (i.e., alter the path) of a beam which travels from the first mirror 
to the second mirror. The inclusion of these additional mirrors, however, 
does not modify the shape of the beam within the laser cavity 902 
(provided that the distance between the first mirror and the second mirror 
is not changed). Accordingly, the inclusion of additional planar mirrors 
in a laser cavity 902 of a laser does not affect the operation of the 
laser but merely alters the manner in which the laser is physically 
configured. Consequently, a laser cavity 902 formed between a first mirror 
and a second mirror, having additional planar mirrors included therein to 
steer a beam that travels from the first mirror to the second mirror, is 
equivalent to a laser cavity formed solely between the first mirror and 
the second mirror; removing the additional planar mirrors alters neither 
the shape of the beam nor the operation of the laser. The use of such 
additional planar mirrors, however, may be employed to fit a laser cavity 
902 into a package having spatial constraints. 
In accordance with an aspect of the present invention, the ILS laser 500 
comprises an ion-doped crystal 507 which resides within a laser cavity 902 
which is a linear laser cavity. An ILS laser 500 comprising an ion-doped 
crystal 507 within a laser cavity 902 which is a linear cavity is a 
completely novel optical design which has not been disclosed in prior art. 
It will be appreciated that the ILS laser 500 of the present invention 
which is based on a linear cavity 902 requires fewer and simpler optical 
components than prior art designs. 
The ion-doped crystal 507 in the ILS laser 500 may comprise, e.g., 
Tm.sup.3+,Tb.sup.3+ :YLF or Tm.sup.3+ :YAG, and preferably operates at or 
near room temperature. Other suitable ion-doped crystals 507 may also be 
employed in the practice of the present invention. The ion-doped crystals 
507 may comprise, for example, other ion-doped vibronic laser crystals. 
Examples of ion-doped crystals 507 suitably employed in the ILS laser 500 
of the present invention are listed in Table 1. It will be readily 
apparent to those skilled in this art, however, that other ion-doped 
crystals 507 may be employed as is suited to the particular use 
contemplated. Accordingly, it is not intended that the ion-doped crystals 
507 specifically disclosed herein, including those listed in Table 1, are 
to be exhaustive. 
TABLE 1 
______________________________________ 
LIST OF LASER CRYSTALS THAT CAN BE OPTICALLY 
PUMPED BY THE OUTPUT OF A DIODE LASER 
Crystal Operating Wavelength of 
Pump Laser Wavelength, 
(gain medium) 
ILS laser (in .mu.m) 
.lambda..sub.p (in .mu.m) 
______________________________________ 
Cr: Tm: Ho: YAG 
2.10 0.781 
Cr.sup.4+ : YSO 0.980 
Cr.sup.4+ : YAG 
1.38 to 1.53 0.980 
Cr.sup.4+ : YSAG 
1.30 to 1.62 0.980 
Er: GSGG 2.80 0.970 
Er.sup.3+ : YLF 
3.40 to 3.54 at 77K 
0.970 
Er.sup.3+ : YLF 
2.70 to 2.95 0.970 
Er.sup.3+ : Yb.sup.3+ : Glass 
1.532 to 1.534 0.970 
Ho.sup.3+ : YSGG 
2.080 to 2.089, 2.10 
0.780 
Ho.sup.3+ : Tm.sup.3+ : YLF 
2.10 0.781 
Tm.sup.3+ : Ho.sup.3+ : YLF 
2.10 0.780 
Tm.sup.3+ : Ho.sup.3+ : YAG 
2 0.780 
Tm.sup.3+ : Ca Y SOAP 
.about.1.65 to 2.0 
0.780 
Tm.sup.3+ : YLF 
2.295 to 2.424 0.780 
Tm.sup.3+ : Tb.sup.3+ : YLF 
1.449 to 1.455 0.780 
Tm.sup.3+ : Glass 
2.25 to 2.50 0.780 
Tm.sup.3+ : Ca La SOAP 
2 0.780 
Tm.sup.3+ : YOS 
.about.1.7 to 2.1 
0.780 
Tm.sup.3+ : YSGG 
1.85 to 2.14 0.780 
Tm.sup.3+ : YAG 
1.85 to 2.16 0.780 
Tm.sup.3+ : Ho.sup.3+ : YLF 
2.31, 2.08 0.790 
______________________________________ 
Referring now to Table 1, a list of crystals that can be optically pumped 
by a diode laser pump laser 100 is provided. The crystals comprise a host 
material doped with ions. The host materials listed include the following: 
YAG, yttrium aluminum garnet (Y.sub.3 Al.sub.5 O.sub.12); YSO or YOS, 
yttrium orthosilicate (Y.sub.2 SiO.sub.2); YSAG, yttrium scandium aluminum 
garnet (Y.sub.3 Sc.sub.2 Al.sub.5 O.sub.12); GSGG, godalinium scandium 
gallium garnet (Gd.sub.3 Sc.sub.2 Ga.sub.3 O.sub.12); YLF, lithium yttrium 
fluoride (LiYF.sub.2); YSGG, yttrium scandium gallium garnet (Y.sub.3 
Sc.sub.2 Ga.sub.3 O.sub.12); LUAG, lutetium aluminum garnet (Lu.sub.3 
Al.sub.5 O.sub.12); Ca Y SOAP, calcium yttrium oxyapatite silicate (Ca 
Y.sub.4 (Si.sub.2 O.sub.3).sub.4 O); Ca La SOAP, calcium lanthanum 
oxyapatite silicate (Ca La.sub.4 (Si.sub.2 O.sub.3).sub.4 O); and glass. 
The dopant ions include Cr, chromium; Tm, thulium; Ho, holmium; and Er, 
erbium. Accompanying the crystals listed in Table 1 is a wavelength 
corresponding to the pumping radiation and a wavelength or wavelengths 
corresponding to the resultant output from the crystal. 
As depicted in FIG. 3, the ion-doped crystal 507 has one end 904 which has 
a reflective coating deposited thereon. Another end 906 of the ion-doped 
crystal 507 is cut at an angle between about 2.degree. to 3.degree. to 
reduce interference effects. (It is conceivable that the end 906 of the 
ion-doped Crystal 507 is cut at Brewster's angle, however, only in the 
case where the ion-doped crystal is large enough to accommodate such a 
cut). The linear laser cavity 902 as shown in FIG. 3 is formed between a 
first mirror (pump mirror) 908 and a second mirror (output mirror) 910. 
The first mirror 908 comprises the reflective coating deposited on the one 
end 904 of the ion-doped crystal 507. The second mirror 910 comprises a 
curved reflector. The laser cavity 902 is a linear laser cavity as defined 
above, since it is formed between only two mirrors. 
The ion-doped crystal 507 is pumped by pump beam F which is shown in FIG. 3 
as incident on the one end 904 which has a reflective coating deposited 
thereon. Accordingly, the ion-doped crystal 507 is optically pumped. The 
output beam from the ion-doped crystal 507 (beam H) exits the laser medium 
through the other end 906 of the ion-doped crystal which is cut at an 
angle to reduce interference effects as discussed above. The output beam 
from the ion-doped crystal 507 (beam H) extends across the linear laser 
cavity 902 to the second mirror 910. (It will be appreciated that due to 
refraction the beam, i.e., beam H, within the linear laser cavity 902 is 
bent slightly, e.g., between about 2.degree. to 3.degree., at the end 906 
of the ion-doped crystal 507 when the ion-doped crystal is cut at a small 
angle, e.g., between about 2.degree. to 3.degree..) 
It will further be appreciated that longitudinal optical pumping is 
employed to pump the ILS laser 500 depicted in FIG. 3. The terms 
longitudinal optical pumping, longitudinal pumping, and longitudinally 
pumped are used herein in their conventional meaning which is well-known 
in the art. Specifically, the pump beam F, incident on the one end 904, is 
directed along the linear laser cavity 902 in about the same direction as 
the output beam from the ion-doped crystal 507 (beam H) which extends 
across an axis running through the laser cavity from the first mirror 908 
to the second mirror 910 (or in the about the same direction as the beam 
within the ion-doped crystal). It will be appreciated that the ion-doped 
crystal 507 typically has a symmetric axis extending from the one end 904 
to the other end 906. Longitudinal pumping corresponds to pumping in a 
direction parallel to the symmetric axis of the ion-doped crystal 507. 
Analogously, an ion-doped crystal 507 which is pumped by a pump beam 
directed along the linear laser cavity 902 and in approximately the same 
direction as the output beam from the ion-doped crystal 507 (beam H) which 
extends across the laser cavity 902 to the output mirror 910 (or in the 
about the same direction as the beam within the ion-doped crystal) is said 
to be longitudinally pumped. 
Alternatively, transverse optical pumping may be employed to pump the ILS 
laser 500 of the present invention. The terms transverse optical pumping, 
transverse pumping, and transversely pumped, are used herein in their 
conventional meaning which is well-known in the art. In particular, the 
ion-doped crystal 507 may be pumped by a pump beam which is incident on a 
side of the ion-doped crystal such as side 912 shown in FIG. 3. Transverse 
optical pumping corresponds to the case where the pump beam, i.e., the 
output of the pumping source 100, which is incident ion-doped crystal 507, 
is directed perpendicular to the symmetric axis of the ion-doped crystal. 
With transverse optical pumping, the pump beam is directed approximately 
perpendicular to the output beam from the ion-doped crystal 507 (beam H) 
which extends across an axis running through the linear laser cavity 902 
from the first mirror 908 to the second mirror 910. In particular, an 
ion-doped crystal 507 which is pumped by a pump beam incident on a side of 
the ion-doped crystal 507, such as side 912, is said to be transversely 
pumped. Similarly, an ion-doped crystal 507 which is pumped by a pump beam 
which is directed approximately perpendicular to the direction of the 
output beam from the ion-doped crystal 507 (beam H) which extends across 
the linear laser cavity 902 to the output mirror 910, is also said to be 
transversely pumped. 
When the pumping source 100 comprises a semiconductor diode laser or a 
solid state crystal laser (e.g., Nd:YAG), longitudinal pumping may be 
employed. Alternatively, flashlamps or diode lasers can be employed in 
transverse pumping. It will be appreciated that with transverse pumping, a 
plurality of flashlamps configured to pump from more than one side of the 
ion-doped crystal 507, may be used as the pumping source 100. 
With continued reference to FIG. 3, in some applications, it may be 
necessary that the incoming beam be appropriately focused into the laser 
medium (e.g., ion-doped crystal or glass) 507 within ILS laser 500. In 
accordance with a particularly preferred aspect of the present invention, 
a focusing lens (not shown) suitably comprising an optical focusing lens 
with an AR coating centered about a wavelength .lambda..sub.p is employed. 
ILS laser 500, in the simplest case, comprises an optical resonator cavity 
902 defined by the entire optical path length between respective mirrors 
908 (first mirror) and 910 (second mirror). In those cases where system 10 
is used to detect gases (contaminants) within a sample which does not 
chemically react with the components of the laser itself(e.g., gain medium 
or crystal 507, mirrors, mechanical mounting, and the like), the resonator 
cavity can be defined by the region between mirrors 908 and 910. In such a 
case, the gas sample region (i.e., the region where the gas sample 
resides) comprises the region between mirrors 908 and 910 (excluding the 
laser crystal 507). 
However, for samples which do chemically react with one or more of the 
laser components (e.g., a corrosive or reactive gas), it is desirable to 
separate the gas sample region from such components. In accordance with a 
preferred embodiment of the present invention, a separate sample system 
400A may be advantageously utilized to isolate the sample from the laser 
components. 
In accordance with this preferred aspect of the present invention, sample 
system 400A preferably comprises a gas sample cell body 406 suitably 
maintained within a gas sample cell holder (not shown). Respective cell 
windows 404 and 405 are suitably mounted on the distal ends of gas sample 
cell body 406 and provide optical access to the sample within the cell 
body. Windows 404 and 405 also suitably seal cell body 406. An inlet 
conduit 408 and an outlet conduit 409 are operatively connected to gas 
cell body 406. 
Couplings 408 and 409 are advantageously employed to ensure efficient and 
effective passage of a gas sample into and out of gas (contaminant) sample 
cell system 400A. Accordingly, the gas detector system 10 of the present 
invention can continuously monitor a flowing gas at variable pressures 
including high pressure. In particular, the use of the gas sample cell 
body 406 advantageously enables the operation of the ILS laser 500 when 
measuring gases having a pressure which is different (i.e., higher or 
lower) than atmospheric pressure or the pressure for which the ILS laser 
was designed to laser. Without such a gas sample cell body 406, lasing 
would be difficult to achieve when monitoring a gas sample having a 
different pressure from the pressure at which the ILS laser 500 was 
aligned. Thus, the gas sample cell body 406 allows stable operation of the 
ILS laser 500 for a gas sample having a pressure in excess of atmospheric 
pressure or the pressure which the ILS laser was design to lase. 
Alternatively, the gas sample may have a pressure less than atmospheric 
pressure or the pressure which the ILS laser 500 was design to lase (e.g., 
when a vacuum exists in the gas sample cell body 406). Additionally, the 
gas sample cell body 406 enables stable operation of the ILS laser 500 for 
a gas sample having a pressure which fluctuates. 
Suitably, cell body 406 comprises a stainless steel or aluminum body having 
dimensions suitably in the range of 10 to 90 millimeters (mm). Preferably, 
the body 406 has an opening therein which is symmetrically in the center 
of gas sample cell body 406. Preferably, the diameter of the opening in 
cell body 406 is suitably selected to be significantly larger than the 
diameter of the incoming beam such that optical alignment of gas sample 
system 400A may be easily obtained. 
The thickness of windows 404, 405 is suitably selected to avoid 
interferometric effects which may interfere with the quality of the ILS 
absorption spectrum obtained through operation of the gas detection system 
10. In accordance with this aspect, the material used in forming windows 
404, 405 is optimally chosen to minimize absorption losses in the region 
over which ILS laser 500 operates. Windows 404, 405 may be formed from an 
optically compatible material, such as Infrasil.TM. available from 
Research Electro Optics of Boulder, Colo. Windows 404, 405 are suitably 
oriented at Brewster's angle and have antireflection coatings so as to 
further minimize reflective losses from the window surfaces. 
As so configured, gas sample cell 406 suitably permits beam H to pass 
through the gaseous sample to be analyzed. Couplers 408, 409, are suitably 
selected to provide easy adjustment such as may be required to realign 
and/or align windows 404, 405 within ILS laser 500 without significantly 
altering the threshold pumping conditions. The resonator cavity 902, in 
the case where system 400A is employed, is suitably defined by the 
physical length between mirrors 908 and 910 (including the laser crystal 
507 and including the region between windows 404, 405 as well as windows 
404 and 405 themselves that comprise the sample system 400A). 
In the event that system 400A is present within chamber 400, it is 
necessary that any gases (contaminants) within chamber 400 that are to be 
detected are suitably removed or eliminated such that the absorption 
spectrum of the sample obtained through use of the gas detection system 10 
is accurate as to the amount or presence of those gases (contaminants) 
within the gas sample contained within the sample system. In accordance 
with a preferred aspect of the present invention, chamber 400 
advantageously evidences a sealed container which can be either purged of 
gas(es) (contaminant(s)) to be detected, or evacuated to remove gas(es) 
(contaminant(s)) to be detected, or in which the level of gas(es) 
(contaminant(s)) can otherwise be reduced below the level to be detected 
in the sample system 400A. Continuous removal of the contaminants can be 
achieve, for example, by gettering, as described more fully below. 
More particularly, reducing gases (contaminants) in chamber 400 (excluding 
sample system 400A) to an acceptable level may suitably comprise purging 
or evacuating the chamber such that the level of gases (contaminants) is 
below that to be detected in the gas sample within system 400A. It will be 
appreciated that the loss contributed by the gases in the chamber 400 will 
be comparable to loss contributed by the gases in the gas sample cell body 
406 when the ratio between (1) the concentration of gases in the chamber 
and (2) the concentration of gases in the gas sample cell body is equal to 
the ratio between (1) the length of the cavity (i.e., between mirror 908 
and mirror 910) and (2) the length that the ILS laser beam traverses in 
the cell body. 
In such cases where the contaminant comprises water vapor, it is necessary 
that water levels in chamber 400 be reduced below those which are 
contained within the sample. In accordance with the present invention, 
detection levels of up to 10 parts per trillion (ppt) are obtainable. 
While any now known or hereafter devised method for removing contaminants 
(e.g., water) from chamber 400 (excluding the sample system 400A) can be 
practiced within the context of the present invention, preferably, the 
chamber is appropriately sealed and inert gases, such as nitrogen are 
pumped therein. In some instances, it may be necessary to further evacuate 
the chamber 400 so as to create a vacuum which removes substantially all 
contaminants contained therein. Also, it may be useful to heat the chamber 
400 while evacuating. Application of such heating or "baking" will enable 
a higher level of vacuum to be achieved if the chamber 400 is subsequently 
cooled while continually being evacuated. In accordance with yet a further 
aspect of the present invention, a getter (not shown) may be 
advantageously employed with chamber 400 to provide even further 
elimination of water within the chamber. As will be appreciated by those 
skilled in the art, a getter (e.g., a molecular sponge) having the 
capacity for continuously absorbing water may be utilized to reduce the 
level of water (contaminants) below the water concentration that is to be 
detected in the gas sample cell 404-406 (e.g., 10 ppt). 
The sample is suitably communicated to system 400A by connecting a gas line 
to connectors 408, 409 and feeding the gas into the sample system (for 
example, when the sample comprises a corrosive gas). 
However, in such cases where the sample does not chemically react with the 
laser components, the gas sample region may nominally be defined by the 
physical region between mirrors 908 and 910 (excluding laser crystal 507). 
A sample is suitably communicated into the chamber 400 itself (for 
example, when the sample comprises a non-corrosive gas). 
As briefly mentioned above, ILS sensor 500 suitably optically detects 
gaseous species (contaminants, e.g., water vapor) contained in a sample 
placed within chamber 400. In accordance with the present invention, ILS 
laser 500 suitably comprises a crystal 507 mounted in a crystal holder 
(not shown). Crystal 507 is suitably mounted in the crystal holder such 
that the crystal also is optimally placed with reference to the incoming 
beam. As previously briefly mentioned, the incoming beam is suitably 
shaped through use of beam shaping assembly 200 such that incoming beam F 
suitably matches the mode volume of the ILS gain medium (e.g., crystal 
507). 
It will be appreciated, however, that the distance between any reflective 
surfaces (e.g., mirrors and windows) within the ILS laser 500 must not be 
such that any interference occurs inside the ILS laser. Interference 
patterns are produced if the distance between the reflective surfaces 
equals an integer of number wavelengths comparable to the wavelength at 
which the ILS laser crystal 507 operates. 
ILS laser crystal 507 preferably operates in a wavelength region suitable 
for detection of the contaminants contained within the gas sample (e.g., 
water vapor) over which a signature absorption spectrum can be obtained. 
As previously mentioned, laser crystal 507 generally exhibits the 
properties of a multimode laser system. It will be appreciated that the 
mode spacing of output of the laser crystal 507 is required to be small 
enough to accurately represent the absorption features of the gas sample. 
Light produced by laser crystal 507 preferably has a mode spacing of less 
than about 1 gigahertz (GHz), thus ensuring accurate spectral replication 
of absorption bands. A particularly preferred laser medium comprises a 
crystal 507 cut at a small angle (e.g., 2.degree. to 3.degree.) to 
minimize interference effects, as described above. 
Laser crystals currently available, while improving in efficiency, have 
considerable losses associated with them. The losses translate to heat. In 
accordance with the present invention crystal 507 suitably is mounted in a 
manner allowing for the effective removal of the heat thus generated in 
operation. It should be appreciated, however, that as the efficiency of 
laser crystals continue to improve as new crystals are developed, the need 
or requirements on heat removing devices will be reduced and likely, at 
some point, the losses will be small enough that the need to remove the 
heat may be eliminated all together. However, using crystals presently 
available, ILS laser system 500 preferably further comprises a heat sink 
system (not shown). 
In accordance with a preferred aspect of the present invention, optical 
excitation of the ion-doped crystal 507 is provided by pumping source 100 
which comprises a semiconductor diode laser 914 as shown in FIG. 3. The 
semiconductor diode laser 914 is powered by an electrical power supply 916 
and cooled by thermoelectric cooler 918. The semiconductor laser diode 914 
and the thermoelectric cooler 918 are mounted in a heatsink 920 provided 
to dissipate heat generated by the semiconductor diode laser. 
As described above, the output beam (beam E) of the semiconductor diode 
laser 914 is highly asymmetric and/or astigmatic. To correct the asymmetry 
and/or astigmatism associated with the output beam (beam E) of the 
semiconductor diode laser 914, beam shaping assembly 200 is employed. The 
beam shaping assembly 200 enables the output beam (beam E) of the 
semiconductor diode laser 914 to be optically matched to the mode volume 
of the ILS gain medium (i.e., the ion-doped crystal 507) contained within 
the ILS laser 500. FIG. 3 shows the beam shaping assembly 200 comprising 
macroscopic optics which include a pair of anamorphic prisms 922 and a 
pair of lenses 924. Alternatively, a beam expanding telescope or 
micro-optics that are placed within several micrometers of the 
semiconductor diode laser 914 may be employed. 
As will be appreciated by those skilled in the art, the quality of the 
quantitative information obtainable through use of the gas detection 
system 10 depends, at least in part, on stable operation of ILS laser 500. 
In the context of the present invention, the stability of ILS laser 500 
depends directly on how reproducibly the ILS laser reaches threshold. 
Desirably, pumping source 100 suitably pumps ILS laser 500 continuously 
near threshold where its greatest sensitivity may be obtained. However, 
not all drivers are capable of reliably operating in a continuous fashion. 
In addition, operating continuously tends to require substantial effort to 
maintain amplitude and wavelength stability of the ILS laser 500 which may 
have an adverse impact on cost and thereby produce an adverse impact on 
the commercial viability of the gas detection system 10. 
As an alternative to operating ILS laser 500 in a continuous mode (cw), and 
in accordance with a preferred embodiment of the present invention, the 
ILS laser is operated in a "pulsed mode" or a "chopped mode". As used 
herein, the terms "pulsed mode" and "chopped mode" refer to processes for 
reproducibly exposing ILS laser 500 (i.e., ion-doped crystal 507) to 
pumping radiation such that the ILS laser will be switched on and off. 
Chopping corresponds to causing the pump radiation to alternate between 
zero intensity and a fixed intensity value at a fixed frequency and over a 
fixed (often symmetric) duty cycle. In contrast, pulsing corresponds to 
causing the pump radiation to alternate between zero intensity and a 
non-zero intensity (which is not necessarily fixed) over a duty cycle 
which may be varied and which is typically asymmetric. (Alternatively, the 
pump radiation can be modulated such that the intensity of the pump beam 
does not reach zero intensity but fluctuates alternately between at least 
two intensity levels which brings the ILS laser 500 alternately above and 
below threshold.) 
Through operation in the chopped mode or the pulsed mode, stable operation 
of ILS laser 500 consistent with the quantitative spectral and 
concentration measurements may be obtained in a commercially viable 
manner. Preferably, the ILS laser 500 is operated in the pulsed mode or 
the chopped mode or is otherwise modulated. Operation in the continuous 
mode, however, can be utilized for certain circumstances. 
As described above, the pulsed mode or the chopped mode have been shown to 
provide advantages with respect to stability and detection sensitivity. 
Such intensity modulation (e.g., interruption) can be achieved utilizing, 
among other things, a mechanically operated chopper, an acousto-optic 
modulator, a shutter, and the like. 
As shown in FIG. 4, a mechanical or electro-optic (e.g., acousto-optic) 
modulator 926 can be inserted between the pumping laser beam F and the 
ion-doped crystal 507. The mechanical or electro-optic modulator 925 is 
powered and controlled by a modulator driver 928. 
Alternatively, the output intensity of pumping source 100 may be modulated 
instead of secondarily chopping the output beam (beam E). For example, in 
the case where the pumping source 100 comprises a semiconductor diode 
laser 914, the electrical power from the power supply 916 to the 
semiconductor diode laser can be pulsed or modulated. Alternating voltages 
to the semiconductor diode laser 914 are provided which thereby cause the 
output of the semiconductor diode laser to fluctuate between high and low 
intensity levels. The high and low intensity levels of the output of the 
pumping source 100 are such that the ion-doped crystal 507 is optically 
excited just above and below the threshold required for lasing. The ILS 
laser 500 is consequently turned on and off. 
While any now known or hereafter devised manner of producing the chopped 
mode or the pulsed mode can be utilized in accordance with the present 
invention, advantageously such modes are obtained through use of 
modulation assembly 300. Desirably, modulating device 300 does not steer 
the pumping beam and is synchronized to modulate the intensity of the ILS 
laser output beam exiting chamber 400. 
In accordance with this aspect of the present invention, beam E is 
periodically prevented from reaching ILS laser 500 by the modulation 
assembly 300 which periodically blocks and transmits the pumping laser 
beam E. It should be appreciated that the modulation assembly 300 may 
comprise a variety devices, e.g., mechanical or electro-optical, which 
periodically blocks or modulates the pumping laser beam. As previously 
mentioned, in accordance with the present invention, the intensity of the 
pumping radiation emanating from pumping source 100 must only fall below 
that required to make ILS laser 500 reach threshold and therefore, is not 
required to reach a zero value. It will be further appreciated, however, 
that the total optical pumping energy (i.e., the integrated intensity) 
delivered by the pumping source 100 to the ILS laser 500, during each 
period of modulation, must remain constant. 
With either the pulsed mode or chopped mode, the output of ILS laser 500 
which contains the absorption information may be periodically sampled. 
Advantageously, the output beam E from the pumping source 100 is 
modulated, while modulation device 304 suitably modulates the output beam 
of ILS laser 500 that exits chamber 400, thereby periodically sampling the 
output of the ILS laser. Modulation assembly 300 alternatively blocks 
pumping beam E from reaching ILS laser 500 gain medium (e.g., crystal 
507), while modulator 304 alternatively blocks ILS laser beam exiting 
chamber 400 from reaching both spectrometer 600 and detector 700. ILS 
laser 500 output exiting chamber 400 is suitably directed to modulator 
304. 
FIG. 4 shows that the output of the ILS laser 500 (beam G) having passed 
through the gaseous species to be monitored is directed to a spectrometer 
assembly 600. Prior to reaching the spectrometer assembly 600, however, 
the output of the ILS laser 500 (beam G) passes through modulation device 
304. 
In accordance with various aspects of the present invention, modulator 304 
comprises an acousto-optic modulator. It should be appreciated, however, 
that other available devices, for example, another mechanically operated 
chopper or even a shutter may be suitably employed for this purpose. As 
discussed above, to extract quantitative information from the ILS laser 
500 exiting beam, modulator 304 periodically samples the output of ILS 
laser 500 which contains the absorption data of contaminants (e.g., 
gaseous species) contained in the particular sample. 
Advantageously, pumping laser beam F is modulated, while modulation device 
304 suitably modulates the output beam of ILS laser 500 that exits the 
laser cavity 902, thereby periodically sampling the output of the ILS 
laser. Modulator 926 alternatively blocks pumping beam F from reaching ILS 
laser 500 gain medium (e.g., crystal 507), while modulator 304 
alternatively blocks the beam exiting the laser cavity 902 (beam G). 
It will be appreciated that instead of employing modulator 304, detector 
assembly 700 may be alternately switched on and off to periodically sample 
the output of ILS laser 500. 
While the specific form of modulation is variable, use of modulation 
enables generation of a reproducible, effective optical path length within 
ILS laser 500. Stated another way, by varying the generation time 
(t.sub.g), i.e., the time period over which intracavity mode competition 
within ILS laser 500 is permitted to occur, the effective absorption path 
length within the intracavity resonator can be controlled and selected to 
achieve optimum quantitative application of the ILS gas detector 10. 
Modulation of the output of the semiconductor diode laser 914 is 
synchronized with modulation device 304 such that quantitative information 
from ILS laser 500 can be extracted in a time-resolved manner. Pump beam F 
is effectively delivered to ILS laser 500 (i.e., ion-doped crystal 507) 
intermittently by passing pump beam F through modulation assembly 300. 
Delivering radiation intermittently alternatively brings ILS laser 500 
near threshold and below threshold. After the generation time, t.sub.g, 
elapses with ILS laser 500 at or slightly above its threshold, ILS laser 
500 output is deflected by modulator 304 to the entrance of spectrometer 
assembly 600 and detector assembly 700 for detection. However, ILS laser 
500 output beam G is deflected to spectrometer 600 and detector 700 for 
only a short time interval determined by the synchronization of modulation 
assembly 300 and modulation device 304. The synchronization of modulation 
assembly 300 and modulation device 304 ensures that radiation from ILS 
laser 500 is sampled over a well-defined time interval (t.sub.g). The time 
interval between when the output of the semiconductor diode laser 914 is 
not interrupted by the modulation assembly 300 and when modulator 304 
opens is determined by t.sub.g. 
Synchronization of modulation assembly 300 and modulation device 304 may be 
achieved by several conventional methods such as, for example, through 
electronic control by a digital circuit (not shown) operated by computer 
802 operatively connected to detector 10. Typically, synchronization of 
modulation assembly 300 and modulation device 304 will be suitable to 
generate generation times (t.sub.g) on the order of less than about 300 to 
500 microseconds (.mu.sec), more preferably on the order of less than 
about 10 to 100 .mu.sec, and optimally on the order of less than about 1 
.mu.sec. Such synchronization results in the modulation assembly 300 
allowing the output of the pumping source 100 to pass uninterrupted when 
modulator 304 is closed. 
The generation time, t.sub.g, can be varied without the use of modulator 
304 by pulsing the output of the pumping source 100 shown as semiconductor 
diode laser 914 in FIG. 4. As described above, pulsing corresponds to 
causing the pump radiation to alternate between zero intensity and a 
non-zero intensity value (which is not necessarily fixed) over a duty 
cycle which may be varied thereby bringing the ILS laser 500 (i.e., 
ion-doped crystal 507) alternately below and above (or at) threshold. 
Accordingly, the ILS laser 500 is turned off and on. The duration over 
which the ILS laser 500 lases may be varied by changing the duty cycle of 
the pump beam; in particular, the duration over which the pumping source 
100 (e.g., semiconductor diode laser 914) pumps the ILS laser to about 
threshold. Accordingly, the generation time (t.sub.g), i.e., the time 
period over which intracavity mode competition within ILS laser 500 is 
permitted to occur, is varied. In this case, the detector assembly 700 
remains continuously activated and the output beam of the ILS laser 500 
which exits laser cavity 902 is allowed to continuously reach the 
spectrometer assembly 600 and detector assembly. 
As described above, however, the total optical pumping energy or integrated 
intensity delivered by the pumping source 100 to the ILS laser 500 during 
each period of modulation must remain constant, even though the duration 
over which the ILS laser outputs light changes. To maintain a constant 
total optical pumping energy, the intensity level of the pump beam is 
adjusted with each different period of modulation over which t.sub.g is 
varied. Accordingly, both the intensity of the pump beam, and duration 
over which the pumping source, e.g., semiconductor diode laser 914, pumps 
the ILS laser 500 to threshold, are changed to provide different 
generation times. 
Pulsing the output of the pumping source 100 can be achieved by externally 
controlling the transmission of the pump beam with a "pulser". 
Alternatively, the output intensity of the semiconductor diode laser 914 
(i.e., pumping source 100) may be modulated by varying the electrical 
power supplied to the diode laser pump laser. As described above, the 
electrical power supplied to the semiconductor diode laser 914 can be 
modulated to alternately obtain voltages just above and below that 
required to cause the ion-doped crystal 507 to lase. 
Accordingly, the gas detection system 10 of the present invention may 
include any of the following configurations each of which enables the 
generation time to be varied: 
(1) The output of the pumping source 100 may be chopped with an external 
chopper (e.g., modulation assembly 300) and the detector 700 may be 
continuously activated with transmission of the output from the ILS laser 
500 to the detector being controlled by a pulser (e.g., modulator 304) to 
enable periodically sampling; 
(2) The output of the pumping source 100 may be chopped with an external 
chopper (e.g., modulation assembly 300) and the detector 700 may be pulsed 
on and off to enable periodically sampling of the output from the ILS 
laser 500; 
(3) The output of the pumping source 100 may be pulsed with an external 
pulser (e.g., modulation assembly 300) and the detector 700 may be 
continuously activated with the duration of the interaction between the 
output of the ILS laser 500 and the gaseous species being controlled by 
the duration of the pulses from the pumping source which cause the ILS 
laser to lase; 
(4) In the case where the pumping source 100 comprises, e.g., a 
semiconductor diode laser 914, the diode laser pump laser may be pulsed by 
varying the electrical power supplied to the semiconductor diode laser and 
the detector 700 may be continuously activated with the duration of the 
interaction between the output of the ILS laser 500 and the gaseous 
species being controlled by the duration of the pulses from the 
semiconductor diode laser which cause the ILS laser to lase; 
(5) In the case where the pumping source 100 comprises, e.g., a 
semiconductor diode laser 914, the diode laser pump laser may be chopped 
by varying the electrical power supplied to the semiconductor diode laser 
and the detector 700 may be continuously activated with the transmission 
of the output from the ILS laser 500 to the detector being controlled by a 
pulser (e.g., modulator 304) to enable periodically sampling; and 
(6) In the case where the pumping source 100 comprises, e.g., a 
semiconductor diode laser 914, the diode laser pump laser may be chopped 
by varying the electrical power supplied to the semiconductor diode laser 
and the detector 700 may be pulsed on and off to enable periodically 
sampling of the output from the ILS laser 500. 
In accordance with the present invention, output beam G from ILS laser 500 
after passing through sample system 400A is directed to spectrometer 
assembly 600. Such direction can be obtained, such as shown in FIG. 4, 
through use of a folding mirrors 930 and 932. Mirrors 930 and 932 
preferably comprises plane mirrors containing a coating for high 
reflectivity in the desired spectral region of operation of the ILS laser 
500. 
With continued reference to FIG. 4, spectrometer 600 comprises dispersive 
gratings designed to spectrally resolve a coherent beam, in particular, 
the absorption spectrum of the contaminant in the sample to be detected. 
Suitably, the spectral dispersion of the spectrometer 600 is sufficiently 
large to dearly resolve the absorption features of such contaminant, thus 
enabling the identification of the "signature" of each contaminant and the 
quantitative determination of the concentration of the contaminant. While 
any now known or hereafter devised spectrometer may be utilized in 
accordance with the present invention, preferably spectrometer 600 
comprises two diffraction grating 607 and 609 operating in conjunction 
with an optical beam expanding assembly comprising lenses 603 and 605 and 
a focusing lens 611. Lens 603 preferably comprises a negative lens and 
lens 605 preferably comprises a collimating lens; each preferably having 
an AR coating centered about the absorption spectrum of the contaminant in 
the sample to be detected. Lens 611 also preferably having an AR coating 
centered about the absorption spectrum of the contaminant, focuses the 
output of the spectrometer onto multichannel array detector 701. 
The spectral region over which the ILS laser 500 operates is produced by 
spectrometer 600 and is displaced spatially across a plane where the 
multichannel array detector 701 located. A computer 802 for operating and 
reading information from the multichannel detector 701 is operatively 
connected thereto. As a result, the entire spectrally dispersed absorption 
spectrum of the particular contaminant sought to be identified through use 
of the gas detection system 10 can be obtained. The positions and relative 
intensities of the specific absorption features of the contaminant can be 
utilized to uniquely identify the detected gas (contaminant) as well as 
quantitatively determine the amount of the gas (contaminant) so detected. 
The detector 701 may comprise, for example, an InGaAs multichannel (256 
pixel, 100 .mu.m spacing) array detector. The light detected by the 
multichannel detector 701 is preferably transduced into electronic signals 
at each detector element (pixel) with signals thereafter transferred to an 
analog-to-digital (A/D) converter (not shown). Once the data is so 
converted, it is sent to a computer 802 which may be suitably programmed 
to convert the electronic signals into spectral information, i.e., 
spectral signatures identifying a particular gas (contaminant) and 
concentration of gases (contaminants). 
Alternatively, the output of the ILS laser 500 (beam G) having passed 
through the gaseous species to be monitored can be directed to a 
spectrometer assembly 600 having at least one dispersive optical element 
(e.g., diffraction gratings 607 and 609) therein which can be scanned with 
respect to wavelength. The output of the spectrometer assembly 600 can 
then be directed to a single channel detector. The spectral signature of 
the gaseous species in the laser cavity 902 is obtained by scanning the 
dispersive optical element while the light transmitted through the 
spectrometer assembly 600 passes through an appropriate aperture (e.g., 
slit) placed in from of the single channel detector. The intensity of the 
light transmitted through the spectrometer assembly 600, i.e., the output 
of the spectrometer assembly, is recorded as the dispersive optical 
element is scanned. 
The concentration of the gaseous species can be determined from the 
intensity of the absorption feature(s) found in the spectral signature. It 
will be appreciated that the absorption feature(s) found in the spectral 
signature must be calibrated. Since intracavity laser spectroscopy offers 
increased sensitivity beyond prior art methods, weak transitions 
previously not measured may become measurable for the first time with the 
gas detection system 10 of the present invention. In such cases, these 
weak transitions can be used to identify the spectral signature and 
certify the presence of the gaseous species. Such weak transitions can 
also be calibrated by the gas detection system 10 thereby enabling the 
concentration of the gaseous species to be determined by the intensity of 
the absorption feature(s) corresponding to these weak transitions. 
In accordance with an aspect of the present invention, absorption data for 
water vapor recorded by the gas detection system 10 of the present 
invention is presented in FIG. 5. The spectral signature for water was 
obtained using an ILS laser 500 comprising an ion-doped crystal 507 made 
of Tm.sup.+3, Tb.sup.+3 :YLF which was optically excited with a 
semiconductor diode laser 914. The gas detection system 10 employed to 
obtain the absorption data was similar to that shown schematically in FIG. 
4 except that a modulator 926 was not employed. Rather, the electrical 
power to the semiconductor diode laser 914 was modulated instead. FIG. 5 
shows a plot corresponding to the spectral signature of water for the 
spectral region between 1450 to 1455 nanometers in wavelength. Water 
absorption lines at 1452.5 and 1452.1 nanometers are indicated by arrows 
934 and 936, respectively. 
It will be appreciated that the output from the ILS laser 500 (beam G) can 
alternatively be transmitted via an optical fiber link to a remote site 
for spectral analysis. In particular, beam G can be coupled into an 
optical fiber or an optical fiber bundle. The output of the ILS laser 500, 
after having passed through the gaseous species, is thereby carried to the 
spectrometer assembly 600 which is located at the remote site. Under the 
proper conditions, it has been demonstrated that such optical fiber 
transmission does not distort the spectral data. 
Table 2 summarizes a variety of configurations of the gas detection system 
10 of the present invention. Each configuration corresponds to a separate 
embodiment of the present invention. The design parameters which can be 
varied that are listed in Table 2 include the following: 
(1) The modulation may comprise a chopper, a pulser, or modulator external 
to the pumping source or modulation of the electric power to e.g., a 
semiconductor diode laser 914, which serves as the pumping source; 
(2) The gas sample may be confined to a separate sample system 400A or may 
be confined to the chamber (or housing) 400; 
(3) The output from the ILS laser 500 can be transmitted directly to the 
spectrometer assembly 600 or can be coupled into an optical fiber and 
carried to the spectrometer assembly; and 
(4) The spectral signature of the gaseous species can be obtained by using 
a fixed wavelength spectrometer and a multichannel detector 701 or by 
using a scanned wavelength spectrometer and a single channel detector. 
TABLE 2 
__________________________________________________________________________ 
SUMMARY OF VARIOUS CONFIGURATIONS OF THE GAS 
DETECTION SYSTEM OF THE PRESENT INVENTION 
Modulation Sample Cell 
Optical Connection 
Spectrometer/Detector 
__________________________________________________________________________ 
None (1) Intracavity cell with 
(1) Direct or use mirrors 
(1) Fixed wavelength spectrometer and 
sealed windows 
to direct beam 
multichannel detector 
OR OR OR 
(2) Housing only 
(2) Fiber-optic coupler 
(2) Scanned wavelength spectrometer 
and 
single channel detector 
Chopper, pulser, or 
(1)) Intracavity cell with 
(1) Direct or use mirrors 
(1) Fixed wavelength spectrometer and 
modulator (located be- 
sealed windows 
to direct beam 
multichannel detector 
tween pumping source and 
OR OR OR 
ILS laser) (2) Housing only 
(2) Fiber-optic coupler 
(2) Scanned wavelength spectrometer 
and 
single channel detector 
Electronic modulation of 
(1) Intracavity cell with 
(1) Direct or use mirrors 
(1) Fixed wavelength spectrometer and 
diode laser power above 
sealed windows 
to direct beam 
multichannel detector 
and below threshold 
OR OR OR 
(2) Housing only 
(2) Fiber-optic coupler 
(2) Scanned wavelength spectrometer 
and 
single channel detector 
__________________________________________________________________________ 
Additionally, a modulator 304 may control the transmission of the output of 
the ILS laser 500 to the spectrometer assembly 600 and the detector 
assembly 700 or the detector may be switched on and off rather than 
employing modulator 304. Alternatively, the detector assembly 700 may be 
continuously activated without the use of modulator 304 by pulsing the 
output of the pumping source 100, e.g., semiconductor diode laser 914. 
FIG. 6 shows an embodiment of the ILS laser 500 of the present invention 
that includes means 938 for periodically switching the detector 701 on and 
off, as well as means 940 for varying electrical power supplied to the 
diode laser 914. FIG. 6 also shows how output beam G can be carried to the 
spectrometer assembly 600 using a fiber optic link 942. 
Advantageously, diode laser pumping can be employed to provide optical 
excitation for a variety of different types of ion-doped crystals 507, 
each having different compositions; see, e.g., Table 1. Consequently, the 
gas detection system 10 of the present invention can be used to detect a 
broad variety of gaseous species (i.e., molecules, atoms, radicals, and/or 
ions) having absorption features at widely varying wavelengths. 
FIG. 7 lists a limited number of ion-doped crystals 507 which are presently 
available that can be optically excited using a semiconductor diode laser 
914 and their respective tuning ranges which reside in the wavelength 
range between about 1000 to 3000 nanometers. Ion-doped crystals 507 listed 
which lase in continuous (CW) mode at room temperature include the 
following: Yb.sup.3+ :YAG, Cr:Forsterite, Cr.sup.4+ :YAG, Tm:Tb:YLF, 
Er:Yb:Glass, Tm.sup.3+ :YAG/YSGG, Tm.sup.3+ :YLF, and Er.sup.3+ :YLF. 
Additionally, an ion-doped crystal 507 comprising CO.sub.2 :MgF.sub.2 can 
operate in continuous mode when cryogenically cooled while an ion-doped 
crystal comprising Cr.sup.2+ :ZnSe can operate at room temperature in 
pulsed mode. (It will be appreciated that the potential tuning range of 
Cr.sup.2+ :ZnS/ZnSe/ZnTe is shown in FIG. 7.) 
FIG. 7 additionally shows the near infrared spectral absorption of a some 
gaseous species. (It will be appreciated that the ranges in wavelength of 
the spectral absorption for H.sub.2 O.sub.2, CO, SO.sub.2, CH.sub.4, and 
NO are calculated overtones.) Accordingly, FIG. 7 indicates some examples 
of the gaseous species that can be probed using an ILS laser 500 
comprising an ion-doped crystal 507 which is optically pumped with a 
semiconductor laser diode 914. 
In accordance with a preferred aspect of the present invention, the 
semiconductor diode laser 914 which may be employed as the source of 
optical pumping is operated electrically. Consequently, the diode laser 
pumping source 100 is relatively small and compact in comparison to other 
sources of optical pumping. Additionally, given the low optical pumping 
energies required for diode laser pumping, the thermal management of the 
ILS laser 500 is less difficult than for gas detection systems 10 shown in 
prior art. Also, the cost is reduced and the operation is simplified in 
contrast to many gas detection systems 10 shown in prior art. 
Advantageously, the linear laser cavity 902 of the present invention 
includes fewer optical elements than designs based on laser cavities 
defined by three mirrors. Accordingly, the complexity of the external 
cavity is reduced thereby increasing mechanical stability (i.e., 
ruggedness), as well as lowering the cost of the gas detection system 10. 
The small/compact size of the gas detection system 10 based on the linear 
laser cavity 902 is amenable to a broad variety of practical applications. 
Specifically, the compactness of the gas detection system 10 comprising an 
ion-doped crystal 507 within a linear laser cavity 902 which has a 
semiconductor diode laser 914 as a pumping source 100 can be directed to a 
completely distinct set of applications in gas detection. In particular, 
the gas detection system 10 which utilizes a diode laser pump laser 100 
for optical excitation of the ILS laser 500 is expected to find 
application in semiconductor manufacturing, process control, environmental 
monitoring, air quality and safety certification, health and safety 
certification, nuclear energy production, and medical diagnostics. 
Thus, there has been disclosed an apparatus for detecting the presence and 
concentration of contaminants in a gas utilizing detector system 10. In 
accordance with a preferred embodiment of the present invention, a method 
for high sensitivity detection is also disclosed herein. The method 
suitably comprises reducing gases (contaminants) in sample chamber 400 to 
an acceptable level, placing a sample of gas to be detected in sample 
system 400A, pumping ILS laser 500 at or near threshold, periodically 
sampling the optical output from the ILS laser, preferably via modulation 
assembly 300 and modulator 304, measuring the absorption spectrum of the 
gases (contaminants) within the sample with spectrometer assembly 600 and 
detector assembly 700, and analyzing the absorption spectrum to identify 
the gaseous species (contaminants) and determine its concentration within 
the sample utilizing computer/software system 800. 
More particularly, reducing gases (contaminants) in chamber 400 (excluding 
sample system 400A) to an acceptable level may suitably comprise purging 
or evacuating the chamber such that the level of gases (contaminants) is 
below that to be detected in the gas sample within the sample system. As 
discussed previously, other mechanisms for reducing the level of gases 
(contaminants) may be utilized provided they can reduce the level to an 
acceptable level. Desirably, chamber 400 is effectively sealed prior to 
delivery to a user in a relatively tamper-proof manner. 
A sample is suitably communicated to system 400A by connecting a gas line 
to connectors 408, 409 and feeding the gas into the sample system (for 
example, when the sample comprises a corrosive gas), or into the chamber 
400 itself (for example, when the sample comprises a non-corrosive gas). 
Pumping ILS laser 500 at or near threshold, more particularly, comprises 
selecting the correct pumping source 100 power, focusing conditions at 
laser crystal 507 utilizing optional beam modification optics 200, and 
modulation conditions utilizing modulator system 300. The method for 
detecting gaseous species in accordance with the present invention further 
comprises driving ILS laser 500 at or slightly above threshold. In 
accordance with the present invention, driver 100 suitably pumps ILS laser 
500. Where necessary, pumping beam E is suitably shaped by beam shaping 
assembly 200 to meet the optical requirements of ILS laser 500. Further, 
where gas detection system 10 is operated in a pulsed or chopped mode, as 
described above, modulation assembly, and in particular, modulator 926 
periodically interrupts pump beam F thereby preventing beam F from 
reaching ILS laser 500. Beam F output from modulator 926 and beam shaping 
assembly 200 is suitably directed to ILS laser 500. 
In accordance with this method, as beam F enters chamber 400 through window 
(not shown) disposed in the wall of the chamber, beam F is suitably 
directed to ILS laser 500. Beam F suitably pumps crystal 507 at or near 
threshold, and the output beam is suitably directed to the gas sample 
within system 400A. The exiting beam, containing the absorption data from 
the gas (contaminant) sample, then exits gas chamber 400 through another 
window (not shown) suitably disposed in a wall of the chamber. 
ILS laser 500 may be operated in a pulsed mode or a chopped mode using 
modulator 304 which is suitably synchronized to modulator 926, and which 
periodically samples the output beam from the ILS laser and passes the 
sampled output thus obtained to spectrometer assembly 600 and detector 
assembly 700. Alternatively, the electrical power supplied to the pumping 
source 100 (for example, when a diode laser pump laser is employed) may be 
modulated and synchronized with modulator 304. Suitably, mirrors 930 and 
932 direct sampled output beam G from ILS laser 500 to spectrometer 
assembly 600 and detector assembly 700. Alternatively, instead of using 
modulator 304, detector assembly 700 may be switched on and off to sample 
the output from ILS laser 500. 
The method for detecting gaseous species in accordance with the present 
invention further comprises analyzing output beam G from the ILS laser 
500. Preferably, spectrometer assembly 600 spectrally resolves and 
detector assembly 700 suitably analyzes beam G from ILS laser 500. 
Spectrometer assembly 600 suitably spectrally disperses beam G from ILS 
laser 500 through beam expanding assembly optics, i.e., lenses 603 and 
605, diffraction gratings 607, 609 and focusing lens 611. 
Spectrally-resolved ILS absorption data exiting spectrometer assembly 600 
is suitably displaced spatially to be detected by multichannel detector 
701. 
It will be appreciated that the gas detection system 10 can be utilized to 
obtain absorption spectra for contaminants, such as water vapor, in 
corrosive (e.g., HCl) or non-corrosive (e.g., N.sub.2) over a variety of 
wavelength regions. 
Given the relationship between intensity and concentration, once a 
characteristic signature of the contaminant gas, e.g., water vapor, is 
obtained, the concentration of the contaminant contained within the sample 
can be readily obtained. In accordance with the present invention, 
computer 802 can be suitably programmed to interpret the data and provide 
an output indicative of the presence and/or concentration of the 
contaminant contained within the sample. 
In accordance with the apparatus and method of the present invention, the 
output signal (beam G) from the ILS laser 500 is detected and analyzed to 
identify the gaseous species (via its spectral signature) and to determine 
its concentration. Those skilled in the art will appreciate that the 
detection levels available through practice of the present invention 
generally exceed those which are obtainable through use of conventional 
devices. Moreover, gas detection system 10 can be used in-line and obtain 
ready, near real-time measurement of the presence and amount of the 
contaminant contained in a specific sample, thus addressing the many 
disadvantages associated with the use of such conventional devices. In 
particular, the method of the present invention provides rapid, in situ 
detection of gaseous species within gas samples at detection levels which 
are not available in prior art. 
It should be understood that the foregoing description relates to preferred 
exemplary embodiments of the invention, and that the invention is not 
limited to the specific forms shown herein. Various modifications may be 
made in the design and arrangement of the elements set forth herein 
without departing from the scope of the invention as expressed in the 
appended claims. Moreover, the application of gas detection system 10 as 
well as the location of the ILS gas detector, e.g., in a semiconductor 
fabrication assembly, can vary as may be desired. For example, the 
specific placement of the various elements within the ILS chamber 400 and 
gas detector system 10 itself may be modified so long as their 
configuration and placement suitably enables optical excitation of ILS 
laser 500 in a readily reproducible manner. These and other modifications 
in the design, arrangement, and application of the present invention as 
now known or hereafter devised by those skilled in the art are 
contemplated by the amended claims.