WO2015038217A1

WO2015038217A1 - Fiber optic gas monitoring system

Info

Publication number: WO2015038217A1
Application number: PCT/US2014/044556
Authority: WO
Inventors: Bo DONG; Amiya BEHERA; Bo Liu; Scott ZHANG; Anbo Wang; Jianmin Gong
Original assignee: Virginia Tech Intellectual Properties, Inc.
Priority date: 2013-09-12
Filing date: 2014-06-27
Publication date: 2015-03-19

Abstract

A gas sensing system including one or more gas sensor heads allows the remote detection and monitoring of one or more gases using absorption spectroscopy. A controlled first signal is generated with a laser and transmitted via optical fiber to a gas sensor head. The first signal may interact with one or more gases which are present in a gas chamber of the gas sensor head. The light transmitted through the gas chamber is collected as a second signal and transmitted to data processing devices. The first signal is also transmitted through a reference gas cell (RGC) containing known concentrations of one or more gases to give a third signal. Using the first, second, and third signals, the concentrations of the one or more gases may be determined. A single gas sensor head may be used in detecting and determining the concentration of one or a plurality of different gases.

Description

FIBER OPTIC GAS MONITORING SYSTEM

FIELD OF THE INVENTION The invention generally relates to detection and monitoring of one or more gas concentrations in an ambient environment, and, more specifically, to the remote detection and monitoring of gas concentrations using fiber optics.

BACKGROUND

Coal mines present a variety of safety hazards. Among these is firedamp, a term used to describe one or more flammable gases that may, under certain circumstances, result in explosions if ignited. Other potentially harmful and even lethal gases may also be present, for instance carbon monoxide. Efficient and accurate measurement and monitoring of gas concentrations in environments such as coal mines is important for the reduction and prevention of workplace accidents and deaths resulting from gases and gas explosions.

Some gas sensors and gas sensing systems have been developed in the art for the detection and monitoring of firedamp gases, especially methane.

"An optical-fiber-based gas sensor for remote absorption measurement of low-level CH4 gas in the near-infrared region" Journal of Lightwave Technology 2.3 (1984): 234-237 Institute of Electrical and Electronics Engineers by Chan et al. describes a fully optical gas sensor configured to detect methane gas in air to a sensitivity of about 700ppm. A gas sensor system using such a gas sensor incorporates LEDs or laser diodes.

"Remote Detection of Gases by Diode Laser Spectroscopy" Journal of Modem Optics 35.3 (1988):319-324 by Mohebati et al. describes detecting methane at 2000ppm using a lm optical path length and a 2.5km optical fiber link.

Chinese patent application publication no. CN102841074 A describes measuring methane gas concentration in mines. A constant current source drives a distributed feedback (DFB) laser with an output wavelength near a gas absorption peak for methane. The output wavelength is scanned by changing a temperature of the DFB laser. The light output of the laser is injected into a measured gas chamber. The output light of the measured gas chamber is converted to an electrical signal which is captured by an analog-to-digital (A/D) card. The absorption peak is used for calculating the methane gas concentration. As evidenced by these and other devices and systems in the field, laser spectroscopy is recognized as a worthwhile approach to detecting some gases, especially methane, in environments such as coal mines. However, such environments present a variety of challenges which limit the effectiveness of existing solutions. Dust, humidity, power failures, long signal transmission distances, and confined testing spaces are some factors which limit the applicability, reliability, accuracy, precision, and responsiveness of existing devices and systems for gas detection. There is a persisting need for systems and detectors having compact size, high durability, low maintenance, and high sensitivity, among other desirable features.

SUMMARY

Especially as compared to existing sensors and gas detection systems, exemplary embodiments of the invention offer improved accuracy, reliability, simplicity, convenience, and size compactness. A gas monitoring system is provided in which all electrical components and signals may be provided remote from combustible gases, eliminating the risk of explosions from sparks or electrical discharges. This is accomplished by, for example, the arrangement of entirely optics based sensor heads in an ambient environment in which gas detection is desired and arrangement of an electrical control box at a remote location, the sensor heads and control box being connected and communicatively coupled by one or more fiber optic cables.

Although a number of methods have been researched and developed for gas detection including calorimetric gas detection, photoacoustic spectroscopy, and evanescent wave detection, the method of absorption spectroscopy was selected as an exemplary approach for the present invention owing at least in part to its remarkable reliability and multiple gases detection capability.

Typically, every gas has a unique optical absorption spectrum specific to that gas, and the absorption intensity is directly related to its concentration. For example, methane has a relatively strong absorption peak at 1665nm which does not overlap with the absorption peaks of other gases. By measuring the peak intensity (e.g. by absorption spectroscopy), the concentration of methane diffused into a sensor head may be determined. This may likewise be accomplished for other gases at absorption peaks specific to the respective gases.

In one aspect of the invention, a sensor head is provided with improved expandability and robustness over the prior art. In particular, a gas chamber of the sensor head is substantially vibration and impact insensitive as well as substantially dust proof and water vapor proof, at least for dust and water vapor in coal mines.

In another aspect of the invention, a sensor head is provided that requires minimum maintenance and does not require periodic recalibration.

In yet a further aspect of the invention, a system is provided that detects one or multiple gases using the same one or more gas sensor heads. In exemplary embodiments, multiple gases are detected using the same sensor head, resulting in a simplified overall gas sensing system. This may be accomplished using wavelength division multiplexing (WDM), or, more preferably, wavelength modulation spectroscopy (WMS). In some embodiments, one or more of methane (CH₄), carbon dioxide (CO₂), and carbon monoxide (CO) are detected simultaneously and their respective concentrations determined.

In an exemplary gas sensing system, a distributed feedback (DFB) laser may be used as the light source, where the wavelength is thermally tuned several nanometers around the absorption line of a gas using a thermo-electric cooler (TEC) controller. For example, a DFB laser can be tuned to the absorption line of C0₂, approximately 1572nm. The laser output light is launched into, for example, a segment of silica fiber which may be a single mode optical fiber (SMF). A graded index lens fiber collimator is used to transfer the laser light from the SMF to an interior space of the gas chamber or cell. The gas (e.g. CO₂) modulates the laser light according to its absorption spectrum. After having passed through the gas present in the free interior space of the gas chamber, the laser light is collected by a second collimator and received by another segment of silica fiber which passes the signal to an optical receiver module. At the optical receiver module, the signal is detected and amplified, and the output voltage signal is recorded by an analog-to-digital converter. By using the TEC's intrinsic temperature tuning slope, the laser wavelength can be continuously scanned at a fast speed (e.g. ~lnm/s), with a good repeatability, which will minimize the averaging time, and thus increase the signal-to-noise ratio (SNR).

In order for a gas sensor head to be invulnerable to vibration, impact, and dust, any one or more of the following advantageous features may be provided, without limitation: 1) using single mode lead in fiber and single mode lead out fiber for optimal light collection, efficiency, and alignment insensitivity; 2) a collimated laser beam less than 1mm in size for better dust insensitivity, 3) protective windows for shielding the collimators and providing easy dust removal maintenance; 4) a sealed collimator assembly for longer operation life time; 5) a suspended gas cell for best vibration and impact insensitivity; 6) a multi level (e.g. multi-layer) filter structure for longer filter life and quicker response time; 7) easily changeable filter design for fast maintenance operation; and 8) sealed internal components to be dust and humidity proof.

Further advantages of some exemplary sensor heads according to the invention include the following: 1) Only light is transmitted inside the sensor head and there are no electronic parts, which makes it extremely safe for potentially explosive environments. 2) One sensor head can be used for multiple gases detection and the cross sensitivity is negligible. 3) Because the very low loss of single mode fibers (typically 0.2dB/km), the sensor head is capable of robust long distance, remote operation. 4) Once the sensor head is assembled, no further alignment is needed.

Allowing for the detection and monitoring of multiple gases at the same sensor head involves specific design considerations which are not accounted for in many systems which only detect a single gas, in particular methane. Since the methane absorption coefficient at around 1.66 μιη is 0.4 cnf'atm^"1, for a 10cm gas chamber and methane concentration of 0.004, the absorption is 0.0156, which is detectable without great difficulty, at least as compared to some other gases. For other gases to be monitored by the same sensor head, much higher sensitivity is required. For carbon dioxide, for example, the absorption coefficient at 1.572 μηι is only around 10^"6 cnf'atm^"1. The enormous difference between the absorption coefficients of CH₄ and C0₂ has the effect that methane in a concentration of 2.5ppm absorbs approximately the same amount of optical energy as a volume of gas that is 100% CO?. Smaller sensor heads with smaller gas chambers (e.g. smaller than 10cm in length) and thus shorter optical path lengths are subject to even greater restrictions, since there are fewer gas particles (i.e. gas molecules) to interact with the optical signal. A goal of the minimum detectable absorption is at least 10^"6 cnf'atm^"1 , more preferably at least 10^"7 cnf'atm^"1, and most preferably 10^"8 cnf'atm^"1.

For highly sensitive and accurate determination of gas concentration, the features of the gas sensor head are important as are the data processing hardware, software, and/or firmware that calculates gas concentration using the optical signal from a gas sensor head as an input. In an exemplary embodiment, the optical receiver module includes a photodiode which converts to an electrical current the transmitted laser power passed from the output optical fiber of the sensor head. The electrical current is very weak, therefore a

transimpedance amplifier (TIA) is applied to boost the signal magnitude and convert one or more current signals to one or more voltage signals. In order to prevent saturation and ensure maximum linear amplification of the gas absorption signal, a closed-loop feedback may be used to remove the DC background from the detection by providing a voltage bias. A voltage bias may be applied with a circuit-implemented negative feedback or, alternatively, with computer control. A computer controlled voltage bias affords dual benefits of reduced system complexity and an improved signal-to-noise ratio (SNR).

In an exemplary receiver circuit, features may include 1) a variable gain stage; 2) for the filter circuit, an active third-order low pass Butterworth filter for signal output; and 3) a low pass filter for the bias input signal (e.g. a second-order low pass filter). According to an exemplary embodiment, the variable gain stage employed contains three additional stages (for a total of four), which can be remotely activated by a Digital I/O card. The different gain stages are designed to provide optimal signal amplification for gas concentration (e.g.

methane concentration) from 100% down to lOOppm.

In an exemplary gas sensing system, one or more reference gas cells (RGCs) are provided. In an exemplary embodiment configured to detect and monitor methane (among other possible additional gases), an RGC with a methane concentration of 0.5% was used to estimate the expected absorbance height for a methane concentration of lOOppm. Coal mine regulations in the United States do not permit methane concentrations higher than 1%. One or more RGCs may be used when detecting multiple gases or, more preferably, a single RGC containing known concentrations of multiple gases may be used. The signal from the RGC is advantageously used when determining the gas concentrations at the sensor heads.

The fiber optic cable connecting gas sensor heads with the control system may be as long as several miles (e.g. one or more miles or one or more kilometers). The control system generally includes laser driver modules and one or more optical receiver modules. The control system may be managed by a fan-less embedded computer, for example PPC-L62T (made by Advantech Corp.) which is equipped with a touch panel and a LCD screen with IP65 protection.

A multiple gas monitoring system (i.e. gas sensing system; gas detection system) is an extension of a single gas sensing system. It is likewise based on absorption spectroscopy and relies on absorption lines at different wavelengths to detect different gases separately. The system is configured to detect and measure absorption lines for each of a plurality of detectable gases, where the absorption lines relied upon are selected so as to have no overlap from one gas to another.

Generally, at least one laser driver module is provided for each gas to be detected, although two laser driver modules per gas may be used for redundancy (where the first is a primary and the second is a backup). No additional optical receiver modules are required for a system configured to detect and monitor a plurality of gases as compared to a system configured to detect and monitor a single gas, although the number of receiver channels might be greater. The number of receiver module channels for the multi-gas monitoring system is '2 + n' where 'n' is the number of sensor heads deployed.

The laser signals from each of the laser drive modules may be combined by optical couplers. The combined laser signal is then divided by one or more couplers to supply optical signal to 1) an input channel of an optical receiver module, 2) a reference gas cell (RGC), and 3) each of one or more sensor heads arranged to allow detection and monitoring of the gases.

In some exemplary embodiments, the combined laser optical signals for a given sensor head is launched into a single mode fiber which runs to that sensor head. After the optical signal has passed through the gas chamber of the gas sensor head, the optical signals transmitted back from the gas sensor head are transmitted to an input channel of the optical receiver module where it is processed to generate a concentration measurement.

Laser driver modules may each include features such as but not limited to temperature sensing, temperature control, a trans-impedance amplifier (TIA), a laser driver (specifically a DFB laser current driver), and a sine wave generator. A/D ports monitor the temperature and driving current of each DFB laser and send the information to the imbedded computer.

Digital I/O ports from the computer control the on and off of the laser driving current and temperature sweeping. Optical receiver module channels collect the light passing through each gas chamber of each sensor head respectively. On the optical receiver module at least three control circuits, including trans-impedance amplifier, auto gain control, and auto bias control, may be installed.

A storage interface (e.g. USB storage interface for thumb drives), an Ethernet port, and wireless network ports may be provided for data transfer and communication to the outer environment. A switch power supply is generally included which may provide, for example, +5V and +/-12V power to the system. The entire system may be sealed in a metal control box with, for example, IP65 protection. These are illustrative examples of a few exemplary embodiments. Modifications such as to the number of ports provided by respective electronic elements will be apparent to those of skill in the art for alternative embodiments having, for example, a variable number of gas sensor heads or a variable number of gases which are detected and monitored in one or more of the gas sensor heads.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic of a gas sensing system for detecting one or more gases; Figure 2 is a simplified block diagram showing measured signals and parameters in relation to system components;

Figure 3A is an isometric view of a sensor head;

Figure 3B is a top view of the sensor head;

Figure 3C is a longitudinal cross sectional view of the sensor head;

Figure 3D is an enlarged cut out view taken from Figure 3C;

Figure 3E is a side view of the sensor head;

Figure 3F is a second longitudinal cross sectional view of the sensor head;

Figure 3G is a top view of the sensor head with the enclosure removed;

Figure 3H is a side view of the sensor head with the enclosure removed;

Figure 4 is a schematic of the arrangement of collimators and protective windows in a sensor head;

Figures 5A and 5B are, respectively, the transmission spectrum and a fast Fourier transform (FFT) thereof for a sensor head with tilted wedged windows as schematically shown in Figure 4;

Figures 6A and 6B are, respectively, the transmission spectrum and a FFT thereof for a sensor head with the protective windows removed;

Figures 7A and 7B are, respectively, signals from a reference gas cell and signals from a sensor head;

Figure 8 is a circuit schematic for an exemplary receiver board;

Figure 9 shows example waveforms measured at channels of an optical receiver module showing sinusoidal variation;

Figure 10 is a simplified block level diagram for simultaneous detection of multiple gases.

DETAILED DESCRIPTION

Referring now to the drawings, and more particularly Figure 1 , a gas sensing system 100 (i.e. gas monitoring system; gas detection system) is shown which includes one or more gas sensor heads 101 , at least one laser driver module 102, and at least one optical receiver module 103. When used for detecting only one gas, a gas sensing system 100 may be referred to as a "single gas sensing system". The at least one laser driver module 102 and the at least one optical receiver module 103 may be housed in the same housing in some configurations, i.e., the two modules do not need to be remote from one another in some fiber optic arrangements; rather, the one or more gas sensor heads 101 may be remote from a housing/enclosure which has both the laser driver module and the optical receiver module. Indeed, in some embodiments, circuitry of the modules 102 and 103 may be integral. The laser driver module 102 is configured to generate and regulate an optical signal (e.g. a laser beam) of controlled wavelength or frequency. The optical signal may be swept over a specific range of wavelengths, i.e. a wavelength spectrum. The sweeping may also be made with regard to frequency and a frequency spectrum, as will be apparent to those of skill in the art. The optical signal wavelength spectrum or frequency spectrum is specifically selected to include one or more maximum absorption peaks of a particular gas such as but not limited to methane (CH₄), carbon dioxide (C0₂), or carbon monoxide (CO).

The optical signal generated by the laser driver module 102 is transmitted to one or more gas sensor heads 101. In many applications the laser driver module 102 and the gas sensor heads 101 will be remote from one another. More specifically, the laser driver module 102 may be arranged in a location isolated or separate from ambient air in which potentially hazardous gases, especially explosive gases, exist in significant concentration or quantity. In contrast, gas sensor heads 101 may be arranged in one or more locations in which hazardous gases may be present and the detection and monitoring of their concentrations or quantities is desired. An exemplary site for installation or use of gas sensor heads 101 is in mine shafts, for example coal mines, or in other confined air spaces.

In exemplary embodiments, gas sensor heads 101 are entirely free and devoid of electronics and electrical signals. In these embodiments, the gas sensor heads 101 are free of electrical charges which could risk discharging and igniting explosive gases (e.g. firedamp). The laser beam or optical signal from the laser driver module 102 is transmitted to the one or more gas sensor heads 101 by one or more optical fibers. Typically, these will be grouped together and protected in one or more fiber optic cables. Because fiber optic cables may be made free of electrically conductive materials or electrically conductive pathways (e.g. free of metal wiring), there is no risk of electrical conduction along the fiber optic cables, and thus the inadvertent deliverance of electrical charge to an environment which may contain explosive gases is completely avoided.

Gas sensor heads 101 are optics-based gas sensors which provide improved performance, durability, and safety as compared to existing gas sensors known in the art. In exemplary embodiments, gas sensor heads 101 are configured to enable detection and monitoring of one or more gas concentrations or quantities in the ambient air of a testing location via absorption spectroscopy. Several scanning methods have been published for purposes of increasing the stability or enhancing the detection limit of gas detection systems. Table 1 lists four example alternatives together with respective advantages and disadvantages. Exemplary embodiments disclosed herein employ wavelength modulation spectroscopy (WMS) with harmonic detection. However, the exemplary embodiments discussed are for illustrative purposes only, and the invention is not necessarily limited to WMS with harmonic detection.

Table 1. Scanning methods comparison

Within a gas sensor head 101 , a chamber is provided in which the optical signal from the laser driver module may interact with gas molecules from the ambient air. A multi-level filtration system reduces or eliminates the introduction of dust (e.g. coal dust) into the chamber while still admitting gas molecules, improving the accuracy of the spectroscopy readings. After the optical signal has passed through the gas chamber and interacted with gas molecules therein, the portion of the beam or optical signal which fully transmits through the chamber is collected by a collimator and passed back into one or more optical fibers. These optical fibers may be referred to as "transmitting" optical fibers to indicate that they transmit optical signals away from a sensor head 101. The above identified optical fibers which deliver optical signals to the sensor head 101 may be referred to as "receiving" optical fibers, since the sensor head 101 is receiving light conveyed by these fibers. The transmitting optical fibers and receiving optical fibers may either or both be part of the same or different fiber optic cables which connect the one or more sensor heads 101 to the one or more laser driver modules 102 and the one or more optical receiver modules 103.

Optical signals from sensor heads 101 are converted to electrical signals by one or more photodetectors. A common and suitable photodetector is a photodiode 104. The electrical signals are then processed by one or more computing devices to determine one or more gas concentrations. One or more computing devices may include, for example, special purpose computers or processors configured to execute particular instructions and perform digital data processing, electrical circuits which perform data processing via specially configured electrical component arrangements, or other devices, hardware, software, or firmware. In some exemplary embodiments, the one or more computing devices are a combination of circuits and integrated computers. Photodetectors may be included in an optical receiver module 103, as is the case with photodiodes 104 in Figure 1, or they may be separate from the optical receiver module 103 and other signal processing/computing devices but communicatively coupled thereto.

One or more optical receiver modules 103 are, similar to the laser driver modules 102, preferably located at one or more locations remote from the gas sensor heads 101 where there are no substantial concentrations of explosive gases. This is desirable because both the laser driver module 102 and the optical receiver module 103 contain electric circuits which have or transmit electrical charges. According to exemplary embodiments, all electrical components, electricity, and electrical charges of laser driver modules 102 and optical receiver modules 103 are separated from the hazardous or potentially hazardous ambient air which the gas sensor heads 101 are configured and arranged to monitor. In the case of mines such as coal mines, it is preferable that any laser driver modules 102 and optical receiver modules 103 are not physically located in the portions and shafts of the mine which may likely contain firedamp. Generally, for a given gas sensing system 100, the one or more laser driver modules 102 and one or more optical receiver modules 103 may be arranged together in the same control box. In some embodiments, the optical receive modules 103 can be co-located with or located in close proximity with the laser driver modules 102, both of which are remote from the gas sensor heads 101. In particular embodiments, an important component in a gas sensing system 100 is one or more reference gas cells (RGCs) 105. A RGC 105 contains a known concentration of at least one gas which the system 100 is configured to detect and monitor. Although multiple RGCs 105 may optionally be used when detecting multiple gases, a single RGC 105 containing known concentrations of multiple gases is sufficient. A RGC 105 is configured to receive a signal from the one or more laser driver modules 102 and provide as an output signal portions of the received signal which are transmitted through the RGC 105. This output signal is then transmitted to an input channel of an optical receiver module 103. In exemplary embodiments, an original signal from the laser driver modules 102 is also transmitted directly to an input channel of an optical receiver module 103 without having passed through a sensor head 101 or a RGC 105.

Figure 2 gives a simplified schematic identifying the separate signals which may be collected at an optical receiver module 103 and how the respective signals are generated. As used herein, a "first signal" 108 refers to the combined signal of all laser driver modules 102, be this for one gas or for multiple gases. The term "first signal" may still be used even if the signal is split (e.g. by one or more couplers 107), since the wavelength and frequency spectrum of the split signal remains the same although the amplitude or intensity may vary. Sensor heads 101 each receive as input the first signal 108 and transmit as output a "second signal" 109. A second signal 109 consists of the portions of the first signal which are transmitted through the gas chamber of the sensor head 101 (i.e. without being absorbed by gas molecules in the gas chamber of the sensor head). A RGC 105 is configured to receive as input the first signal 108 and transmit as output a "third signal" 1 10. A third signal 1 10 consists of the portions of the first signal 108 which are transmitted through a gas chamber of the RGC 105 (i.e. without being absorbed by gas molecules in the gas chamber of the RGC). The first signal 108, the third signal 1 10, and the second signal 109 (optionally a plurality of second signals, if a plurality of sensor heads 101 are used) are transmitted to individual channels of one or more optical receiver modules 103. These channels are indicated as Ch-1 , Ch-2, and Ch-3 in Figure 2. Generally, each receiver channel has one or more photodetectors (e.g. photodiodes 104) configured to convert the respective incoming signals 108, 1 10, and 109 from optical signals to electrical signals. One or more computing devices then determine a concentration of each respective gas of the one or more gases for which the system 100 is configured to detect using at least in part a comparison of the first signal 108, the second signal 109, and the third signal 1 10. The variables identified in Figure 2 will be discussed below in the section having the subheader 'Concentration Estimation Algorithm'. Figure 1 is labeled with the same reference numerals as given in Figure 2 wherever applicable. Figure 1 illustrates an exemplary embodiment having two gas sensor heads 101, and thus there are two respective "second signals", identified as 109' and 109", respectively. In Figure 1 , two laser driver modules 102 are combined in a "module" 1 1 1 for each detectable gas. In this exemplary embodiment, there are three such modules, one each for methane (CFLi), carbon monoxide (CO), and carbon dioxide (C0₂). Alternative embodiments may be configured to detect any combination of these three gases and/or other gases. Each module 1 1 1 generates a signal 106 specific for that module's particular gas. The signal 106 contains at least one peak absorption wavelength specific to the gas of the given module. The respective signals 106 may be combined by a configuration of couplers 107. Each coupler 107 combines two signals into a single signal or splits a single signal into two signals. A series of couplers 107 may be provided to couple three or more signals together into a single signal or split a single signal into three or more signals. Serial coupling and serial splitting are shown in Figure 1. Two 2x2 50:50 optical couplers combine the laser signal from three laser modules for three different gases. Then two 2x2 50:50 optical couplers, and one 2x2 10:90 coupler split the laser power into four channels. However, the assortment and configuration of couplers 107 in Figure 1 is just one illustrative example, and variations or alternatives to the coupler arrangement shown will occur to those of skill in the art. A combination of all signals 106 from all modules 1 1 1 provides the first signal 108 which, as a result of the combination, contains at least one peak absorption wavelength for each gas to be detected

(e.g. CH₄, CO, and C0₂ in this exemplary embodiment). The receiving channels of the optical receiver module 103 are indicated as Chi, Ch2, Ch3, and Ch4 and correspond with the four single mode fibers 1 12 at the right hand side of the optical receiver module 103. In Figure 1 as shown top to bottom, fibers 1 12 carry signals 108, 110, 109', and 109", respectively. The single mode fibers 1 12 which carry second signals (e.g. 109' and 109") from sensor heads 101 may be bundled into one or more fiber optic cables 1 13 together with single mode fibers 1 12' which carry the first signal 108 to the sensor heads 101.

Gas Sensor Head

Figures 3A-3H show an exemplary gas sensor head 101. Many of the support structures which do not require particular optical properties may be machined from aluminum or aluminum alloy. While some exemplary component sizes and materials are described herein, sizes and materials are not limited to those indicated, and those of skill in the art will recognize alternative materials which may be substituted according to the design

considerations of different embodiments.

A sensor head 101 generally includes a shell 201 , a shell end cap 202, one or more coarse filters 203, one or more shell covers 204, and a pipe adapter 205, all of which may be at least partially exposed to the external environment of the sensor head 101. A portion of a (right) collimator presser 206 may also be visible from the exterior of the sensor head 101. The shell 201, shell end cap 202, coarse filters 203, and shell covers 204 collectively contribute to a housing enclosure which shields the sensitive optical elements of the sensor head 101. The use of shell end cap 202 provides greater ease of manufacture and assembly. In alternative embodiments, the shell end cap 202 may be integral with shell 201. Shell covers 204 are fixed to the shell 201 by removable attachments such as screws 207 and

removably/reversibly fix coarse filters 203 to shell 201. The body of shell 201 includes one or more openings for the admission of gas into an interior of the housing enclosure. The coarse filters 203 fully cover the openings, thereby limiting the admission of nongaseous particles (e.g. dust). The coarse filters 203 may be easily and inexpensively replaced with new filters over time, such as part of a regular maintenance program for optimal performance of the sensor head 101. Changing of the coarse filters 203 simply involves the temporary removal of shell covers 204 which fix the coarse filters 203 in place relative the shell 201. Coarse filters 203 may have sponges arranged immediately adjacent to an inner surface thereof. The sponges may be removed and swapped similar to the coarse filters 203.

As indicated above, the body of shell 201 requires at least one opening for the admission of gases (i.e. gas molecules, gas particles). The exemplary gas sensor head 101 shown in Figures 3A-3H has two such openings on opposite sides of the shell 201. With such a configuration, gas may, for example, diffuse or flow into an interior of the sensor head 101 through the first coarse filter 203 and out of the interior of the sensor head through the second coarse filter 203. Similarly, gas may enter and exit an interior of the sensor head 101 through either or both coarse filters 203. The provision of large openings in the shell 201 covered by the coarse filters 203 allows for an increased rate of diffusion and gas flow from the surrounding ambient air into the sensor head 101 , resulting in a faster response time of the sensor head 101 and the gas sensing system 100 in response to changes in the gaseous composition of the ambient air.

The optical elements of the sensor head 101 may be shielded and protected from airborne nongaseous particles or particulates (e.g. dust particulates) by a multi level filtration system (i.e. a dust cover) which includes coarse filter 203 and sponges. At least some embodiments intended for certain operating environments preferably include the multi level filtration system. As an example, coal mine tunnels are very dusty and windy. An exemplary dust cover is easily removed and changed in the field. This helps to secure a reliable and consistent response from the sensor head 101. A multi-level filtration system generally comprises a plurality of filters of different pore sizes. This reduces the risk of clogging any one single filter and reduces the frequency with which filters must be replaced in the course of extended use and maintenance. The coarse filter 203 blocks large size particles and may also include a sponge. The window for the coarse filter is large to increase the response time of the sensor head 101. In an exemplary embodiment, the mesh size of the coarse filter is 80x80 with a pore opening size of 0.152mm. At least one fine filter 208 may be arranged between a coarse filter 203 and the optical elements of the sensor head 101. In an exemplary embodiment, the fine filter 208 has a mesh size of 400^χ400 with a pore opening size of 38.1 μιη. As a result, the fine filter 208 blocks fine particles of size 40μπι and larger. These example filter sizes were selected after study of dust particles sampled from ambient air in a coal mine. Fine filter 208 blocks most of the very fine dust which may get past coarse filter 203 and guarantees that gas may still pass through. Other filter sizes may be employed for various embodiments, particularly for embodiments usable in environments other than coal mines.

Filters of the sensor head 101 such as coarse filter 203 and fine filter 208 may be made of different materials such as plastics or metals, although in an exemplary embodiment both filters 203 and 208 are made of stainless steel. A coarse filter 203 made of stainless steel provides the sensor head 101 with structural rigidity and protection from penetration or puncture by foreign objects or debris. Stainless steel is furthermore corrosion resistant which reduces the risk of rust forming and clogging the filter pores, particularly in the humid air of mining tunnels.

A connector/adapter such as pipe adapter 205 connects a sensor head 101 to a fiber optic cable. A variety of known fiber optic cables may be used in accordance with the invention. Generally, the fiber optic cable includes at least one optical fiber for receiving optical signals and at least one optical fiber for transmitting optical signals. Alternatively, an individual optical fiber may be bidirectional and used for both receiving and transmitting optical signals. This can reduce the size of the sensor head and/or the fiber optic cable. In an exemplary embodiment, one or more single mode fibers are used for receiving and transmitting to minimize signal noise. However, alternative embodiments may use multimodal fibers for reasons such as cost reduction. Multi-mode fibers have the drawback of greater sensitivity to vibration and intermodal interference.

When the sensor head 101 is fully connected or coupled to a fiber optic cable, the receiving signal optical fiber (identified as 'Rx') is communicatively coupled to a first collimator 209 and the transmitting signal optical fiber (identified as 'Tx') is

communicatively coupled to a second collimator 209'. It will be assumed for the purposes of discussion that the receiving optical signals are delivered to the first collimator 209 and the transmitting optical signals are output from the second collimator 209'. However, either collimator 209 or 209' may be used for receiving or transmitting purposes, there being no functional difference in the sensor head 101. In an exemplary embodiment, both collimators 209 and 209' are graded index lens collimators with fiber pigtails which reduce the interference caused by fiber connectors. In contrast, flat end faced FC/PC connectors were tested and observed to introduce high frequency interference spectrum. The use of graded index lens collimators with fiber pigtails substantially reduces the interference. The use of wedged tilted windows also reduces the interference. Note that 'FC is one kind of optical fiber connector especially designed for use in high-vibration environments. In this circumstance, 'PC stands for "physical contact" because the surfaces of these connectors are polished to be very flat so opposing surfaces come into direct contact without any gap.

The collimators 209/209' may be held in place by collimator holders 210. The collimator holders 210 are retained in either end of the chamber 211 by swivel joints 212.

Swivel joints 212 allow for adjustment of the angle of the collimators 209/209'with respect to a center longitudinal axis 213 of the sensor head 101. Each swivel joint 212 is individually adjustable and then may be locked by turning down one or more set screws 214 which frictionally engages the swivel joint 212 to fix its position and orientation relative the surrounding chamber 21 1.

In an exemplary embodiment, the diameter of the collimated beam is less than 1 mm. The beam length is defined as the distance between the opposing end surfaces of the two collimators 209/209' . The working distance (i.e. absorption distance) is the distance through which the beam can interact with gas molecules. As will be discussed in greater detail below, a protective window 215 is arranged adjacent each collimator 209/209 'between the pair of collimators. The working distance is therefore shorter than the beam length by a small amount. In a sample embodiment, the working distance is 2.5 inches (63.5 mm). This short working distance is advantageous in that it allows for a smaller, more compact sensor head 101. However, it also increases the dust protection requirement. In an exemplary embodiment, a sensor head is provided with an IP56 rating according to the IP Code, International Protection Marking, IEC standard 60529. The first number '5' means the sensor head has a solid particle protection level 5, or "dust protected". This indicates that ingress of dust may not be entirely prevented, but it does not enter in sufficient quantity to interfere with the satisfactory operation of the equipment. The second number '6' is a water proofing level. The sensor head has a protection level 6, indicating that a powerful water jet has little or no harmful effects. The water proof level may be increased easily if needed in alternative embodiments. A long term dust test was performed with a sample exemplary embodiment, and the results showed that the sensor head 101 meets the dust requirements for successful long term operation in a coal mine. The response time is not significantly affected after a prolonged period of dust accumulation.

Commercially available fiber collimators may be used in some embodiments. For some embodiments, spacers may be included in the connectors to optimize the coupling efficiency if the collimator and fiber connectors are independent. In an exemplary embodiment, collimators 209 and 209' are small size graded index collimators with fiber imbedded, resulting in almost zero loss between a collimator 209/209'and a fiber connector 218. Fiber connectors 218 connect a sensor head 101 with a fiber optic cable. An exemplary fiber optic cable is a single mode fiber such as SMF28 from the Corning Company. This is a relatively expensive optical fiber with a silica core, cladding, and a polymeric coating. A typical diameter of a single mode fiber core is 8.2 um, which has the effect that even micro- scale misalignment can have a significant effect on sensor head loss.

Collimators in different exemplary embodiments may have different working distances. The working distance of a collimator is the distance over which a beam emitted by the collimator will stay substantially of constant diameter. At a distance above a collimator's working distance, the beam will gradually diverge. A sensor head 101 may have a working distance which is greater than the collimator 209 working distance as long as the receiving collimator 209' has a diameter greater than the beam diameter at the point at which the beam meets receiving collimator 209' . In an exemplary embodiment, a sensor head 101 may have working distance of, for example, 2.5 inches yet use collimators having a working distance of, for example, 25mm. According to some embodiments, a light beam is provided having a beam diameter which does not exceed 2mm. In some embodiments, a light beam is provided having a beam diameter between 450 μπι and 1.6 mm.

Inside the chamber 21 1 and between the two collimators 209 and 209 'there are generally window holders 216, windows 215, and an orientation tube 217. Each of the window holders 216 holds a window 215 which functions to shield and protect the respective collimator 209 or 209'from direct exposure to ambient air which enters an interior of the sensor head 101. In an exemplary embodiment, windows 215 are an extremely hard material with exceptional scratch resistance such as sapphire crystal. This improves the durability of the sensor head, allowing cleaning of an inside of the chamber and the windows 215 without introducing defects to the window surfaces which could affect their transmission of an optical signal. Clamping rings 221 may be pressed onto the outer edges of the windows 215 to make them more robust.

The orientation tube 217 is arranged to substantially fill the chamber 21 1 between each of the window holders 216 and serves to control the distance between the two window holders 216. This ensures tight tolerances on the working distance of the sensor head. An exemplary assembly for the sensor head may be summarized as follows. First, the orientation tube 217 is inserted into the chamber 21 1. Second, the two window holders 216 (holding the windows 215) are pushed in, one from each end of the sensor head until they reach the orientation tube 217. Then, the swivel joint 212 is similarly pushed in from each end. As long as each window holder 216 contacts the orientation tube 217, the working distance is fixed.

Chamber 21 1 and orientation tube 217 each include one or more openings for permitting the passage of gas molecules which pass through the one or more openings in shell 201 and coarse filter 203. In the exemplary embodiment of Figures 3A-3H, two such openings are provided on opposite sides of the sensor head. In an exemplary embodiment, such openings in the chamber 21 1 are each covered by a fine filter 208 discussed above. Chamber covers 222 may be removably fixed to the chamber 21 1 to secure the peripheral edges of fine filter 208 to the chamber 21 1 such that fine filter 208 substantially covers the entirety of the opening. A gasket 223 may be arranged between the chamber 21 1 and the fine filter 208 / chamber cover 222. Similar to the shell cover 204 with respect to the coarse filter 203, a chamber cover 222 may be temporarily removed for cleaning or replacement of the fine filter 208. In the case of the sensor head 101 shown in the Figures, two openings are provided in the chamber 21 1 and orientation tube 217 to allow gas to enter and exit (e.g. by diffusion) the free space through which the laser beam passes. Therefore, two fine filters 208 and two chamber covers 222 are provided. Alternative embodiments may have just one opening with one fine filter 208 and chamber cover 222 or more than two openings, fine filters, and chamber covers.

As previously discussed, direct scanning laser spectroscopy utilizes the optical and atomic phenomenon whereby different gases selectively absorb light energy of particular wavelengths. By passing a light beam through a gas and detecting and evaluating the changes in the light after interaction with the gas particles (i.e., gas molecules) as compared to the light prior to interaction with the gas particles, both the identity of the gas particles and their respective concentrations may be ascertained.

Gas sensor head 101 allows interaction of an optical signal and gas particles in a regulated space remote from electrical processing elements. In an exemplary embodiment, a light signal (i.e. electromagnetic radiation or a collection of photons characterizable according to a wavelength, a frequency, and/or an intensity) passes through sensor head 101 by sequentially being passed through the following elements: a first fiber connector 218, a receiving internal optical fiber 219 (preferably single mode), a first collimator 209, a first window 215, an internal cavity of chamber 21 1 and/or orientation tube 217, a second window 215, a second collimator 209', a transmitting internal optical fiber 220 (preferably single mode), and a second fiber connector 218. The adjectives "receiving" and "transmitting" are used for descriptive purposes as to whether light which is passing through an optical element is traveling toward or away from the internal cavity of chamber 21 1. "Receiving" is used to indicate the transmission of light which has not entered the internal cavity. "Transmitting" is used to indicate the transmission of light which has exited the internal cavity. In other embodiments and variations, one or more additional elements may be included anywhere in this sequence of elements, and some of the listed elements may be omitted in these or other embodiments. Collectively, the elements directly involved in the optical pathway are configured to provide robust and reproducible laser spectroscopy. More specifically, provided a particular gas or gas composition in the chamber 21 1 and a particular incoming/receiving light signal at Rx, the sensor head 101 outputs a consistent and high signal-to-noise ratio (SNR) light signal at Tx.

In the exemplary embodiment of Figures 3A-3H, elastomeric O-rings 224 are arranged between the gas chamber 21 1 and the collimator pressers 206 to maintain a soft contact therebetween. This arrangement advantageously reduces transfer of vibrations or mechanical shock which could otherwise be passed from the shell 201 to the collimator pressers 206 to the gas chamber 21 1. Besides providing shock absorbance, O-rings 224, 224', and 224" also help seal the gas chamber 21 1 from ingress of moisture or dust from a direction other than through the course and fine filters. The internal optics assembly is also covered at the end by chamber end cap 225. In alternative embodiments, the gas chamber 21 1 may be fully suspended within the shell 201 by rubber supports. This provides very good vibration and shock resistance but at a cost of greater bulk/size. In an exemplary embodiment, the openings in shell 201 which permit gas molecules to enter and exit an interior of the sensor head 101 are at least 0.1 inch² (square inches),

9 9 9 9 9

preferably larger, e.g. 0.2 inch^", 0.3 inch^", 0.4 inch^", 0.5 inch^", or 0.6 inch^". The openings may be even larger. Of sample embodiments which were manufactured and tested, a first

9

sample embodiment had an opening size of 0.1256 inch^" and a second sample embodiment

9

had an opening size of 0.6inch^". Significant improvement in sensor head 101 response time was observed with the second sample embodiment as compared to the first. It is noted that the second sample embodiment is that which is depicted in Figures 3A-3H.

It is also desirable to minimize the diffusion distance, i.e. the distance from the coarse filter 203 to the center of the chamber 21 1. In the first sample embodiment, the diffusion distance was 2.3 ". In the second sample embodiment, the diffusion distance was reduced to 0.25". In alternative embodiments, the diffusion distance may be reduced to a distance less than 0.25". A short diffusion distance increases the response time of the sensor head.

In an exemplary embodiment, the volume of the internal space in gas chamber 21 1 is also preferably minimized in order to increase the response time. In the first sample embodiment, the internal volume of chamber 211 was 4.26" ^χ 3.1 " ^x 16". In the second sample embodiment, the internal volume of chamber 21 1 was 1.625" x 1.625" ^x 4.82". In this embodiment, the distance between the two windows 215, and thus the actual distance in which the laser light entangles itself (i.e. interacts with) the environmental air, is 2.5 inches.

A smaller cavity length to the inside of chamber 21 1 together with short collimators

209/209' (as previously discussed) further allows for a short overall length to a sensor head 101. In the first sample embodiment, the length of the sensor head 101 was 15 inches. In alternative embodiments, the length of the sensor head may be as small as, for example 14", 13", 12", 1 1 ", 10", 9", 8", 7", or smaller than 7". In the second sample embodiment, the length of the sensor head 101 was reduced to 7.8". The overall diameter of the sensor head 101 may similarly be minimized. The first sample embodiment had a diameter of 4.26". In alternative embodiments, the diameter of a sensor head may be, for example, 4", 3", 2.5", smaller than 2.5", 2.25", 2", or even smaller than 2". The second sample embodiment had a diameter of 2.25". A reduction in the total number of components may reduce the manufacturing and production complexity and help contribute to an overall lighter sensor head.

In the sensor head 101 , the minimum diameter of the laser beam that travels in the ambient air within the chamber 21 1 may be, for example, 450μπι. The laser power needed to ignite methane air is above 1W. In an exemplary system 100, the maximum optical power generated by a laser of the laser driver module 102 is less than lOmW. Thus, the laser power delivered to the sensor head is more than 100 lower than the ignition power limit.

Prototype gas sensor heads were manufactured and tested in the course of developing preferred design features and criteria. In some embodiments (not shown in the Figures), interference fringes with high frequency and low frequency were observed in the

transmission spectra. In order to analyze the sources of these interference fringes, detailed transmission spectra of the prototype sensor heads were acquired with a component test system (CTS). In the Fourier transform of the spectrum, peaks corresponding to optical path differences of 120μηι, 18mm, and 30mm were distinguished. These values were a very good match with, respectively, the thickness of the protective windows, the distance between the collimator surface and the protective window, and the distance between the two windows. It was thus determined that a solution to minimize these sources of interference was to reduce the reflections generated by the protective windows 215.

In order to minimize the interference between the two surfaces of a protective window 215, two titled wedge angle windows may be used. This is schematically illustrated in Figure 4. Different window thicknesses may be used for different embodiments. Generally, it is desirable to minimize window thickness, since thicker windows require greater a length of chamber 21 1 and, as a result, the total length of the sensor head. Window thickness may be at least as small as 120μιη to ΠΟμπα. However, at this thickness the windows may be fragile with a greater propensity for breaking (e.g. in the scenario the sensor head experiences a sharp impact). A window thickness of at least 1mm was determined to be a suitable thickness for minimal size yet robust strength.

Referring to Figure 4, when the laser beam 301 propagates from the transmitting collimator 209 to the receiving collimator 209' (in direction 302 as shown in Figure 4), the beam must pass through each of the two windows 215. More specifically, the beam must pass through each of the two faces for each window 215 for a total of four surfaces, labeled S 1 , S2, S3, and S4 in Figure 4. Consider an example where the beam/light runs into each surface and about 4% of the beam's light energy is reflected due to the air/glass refractive index difference (this example assumes the glass is silica; for sapphire crystal about 7% of the light energy is reflected). The remaining 96% of the light energy passes through the surface. The 4% reflected light from S2 goes backward (i.e. in the direction 303). This 4% backward light runs into S I where, for the same reason as before, 96% of incident light passes and 4% reflects. Now this 4%*4% light goes forward (i.e. in the direction 302). Similarly S3 and S4 will also have a 4%*4% double reflected light going forward. If S I , S2, S3 and S4 are all parallel to each other, this weak "multi-reflected light" (referred to herein as interference) will eventually be received by receiving collimator 209'. To reduce the interference, windows 215 may be wedged and/or rotated (e.g. one window rotated with respect to the other). This or another arrangement of the windows which has the effect of all the window faces (i.e. S I , S2, S3, and S4) being non-parallel to one another is preferred in exemplary embodiments. If surfaces SI , S2, S3, and S4 are not parallel, the multi-reflected light will not go forward in exactly the same path as the primary laser beam 301 due to refraction according to Snell's law. As a result, interference is advantageously reduced. Although the angle of reflected light may only be changed by a very small amount (e.g. whatever the angle of the wedge), once the "multi-reflected light" propagates the gas chamber's working distance 304 (for example, 2.5 inches), it will no longer be received by the receiving collimator 209' .

Different angles may be used for each of the wedge shapes of windows 215. In an exemplary embodiment, one side of each window 215 was polished to form a one degree wedge angle. The interference between a window 215 and other optical components was removed by additionally tilting each of the windows 215 10 degrees from a transverse plane 305 of the gas chamber. Different angles larger or smaller than 10 degrees may also be used. Because each window holder may be installed at a very small angle due to small variations allowed by design tolerances, it is advantageous to deliberately tilt the windows an angle greater than 1 degree.

A visual comparison of Figures 5A-5B with 6A-6B demonstrates that windows 215 introduce insignificant interference to the optical signal. Figure 5A shows a detected transmission spectrum and Figure 5B shows the corresponding fast Fourier transform (FFT) of a sensor head with tilted wedge windows 215 such as are described in the preceding paragraphs and schematically shown in Figure 4. A transmission spectrum and FFT thereof of an identical sensor head but with the windows 215 removed is shown in Figures 6A-6B. The close consistency of Figure 5 A and 6A and the close consistency of Figures 5B and 6B supports the conclusion that the titled/angled wedge windows 215 cause minimal interference on optical signals transmitted therethrough.

In the trial test which provided the data plotted in Figure 5A, the amplitude of the interference was measured as 0.03dB. In comparison, a design having windows which were not wedged and angled had an interference of 1.2dB. Additionally, the standard deviation of the measured methane concentration at zero concentration methane was 8ppm for the sensor head with wedged/tilted windows 215 as compared to 50ppm for the sensor head with windows which were not wedged/tilted. A performance comparison test was made between the exemplary sensor head 101 as illustrated in Figures 3A-3H and a sample commercial reference gas cell 105. In both cases, the light signal transmitted through the respective gas chambers did not include peak absorption wavelengths for any gases therein. Thus, in the absence of interference from the physical apparatus itself, the probing laser spectrum should be identical to the transmitted signal. (Referring back to Figure 2, this is to say that signals 109 and 1 10 should be identical to signal 108 in a system without interference and without any signal absorption by gases in the sensor head 101 chamber or the RGC 105 chamber). Figure 7A shows the transmitted signal of the commercial reference gas cell together with the probing laser spectrum, and Figure 7B shows the transmitted signal of the exemplary sensor head according to the present invention together with the probing laser spectrum. In each of Figures 7A and 7B, only one sinusoidal curve is apparent because the transmitted signals and the respective probing laser spectrums were, at the resolutions shown, substantially identical and therefore superimposed on one another. This result is indicative that the integrity and quality of the gas sensor head 101 is equally as good as the commercial reference gas cell 105.

An exemplary sensor head was tested at different concentrations with a gas flow cell made by Wavelength Electronics (Bozeman, MT). The results are shown in Table 2. For zero concentration of methane, the standard deviation of the measurements was about 8ppm and for 10% of methane, the relative measurement standard deviation was 0.8%.

Table 2. Test results with the commercial gas flow cell.

The Lhoist North America Kimballton underground limestone mine is located near the town of Pembroke in Giles County, Virginia. An early prototype sensor head was placed in a humid and dusty area of this limestone mine to test the durability of the sensor head. The conditions underground are typically 55°F and 99% humidity. The sensor was placed approximately 20 feet up the belt line from the primary crusher on the 10^th level (-1000 feet underground). No obvious loss increase was observed in the 17 day environmental test. Sample exemplary embodiments of the sensor head were manufactured and subjected to a further dust test or a vibration test. The initial pre-test loss of the first sensor head was 4.5 dB, and the initial pre-test loss of the second sensor head was 7.0 dB.

After a continuous 23 day dust test, loss of the first sensor head stayed at 4.5 dB. Small variation was observed which may be attributed to disturbance of the fiber. Even with a heavy dust covering on the shell, the chamber inside the shell was dust free to the naked eye.

Sensitivity to vibration was tested by placing the second sensor head on an air compressor which would cause vibration. The frequency of the vibration was estimated to be 200 Hz and the amplitude was about 1 mm. Each time the air compressor ran for about 10 minutes. Two cycles of 10 minutes were conducted, and the loss of the second sensor head remained at 7.0 dB which indicated that the sensor head was robust.

A method for quantifying response time of an exemplary prototype embodiment of the gas sensor head was conducted according to the Methane Monitor Performance Test (ASTP 2229 by Mine Safety and Health, Administration Approval & Certification Center (MSHA)) and the work done by CD. Taylor, J.E. Chilton & A.L. Martikainen with National Institute for Occupational Safety and Health. Based on input from Kevin Dolinar with MSHA, the response time of both increasing concentration and decreasing concentration were specifically measured.

The test sensor head was arranged in a test box with two fans used to blow the air inside the testing box in which the sensor head was positioned. When the mixed gas entered the box, the readout of the sensor changed from zero to a certain value. Because the box was sealed, the reading would be stable after a certain time. Here acetylene was the gas used to test response time of the sensor head when the gas diffuses into the head, and a Component Test System (CTS) was the instrument used to measure the wavelength spectrum of the sensor head. CTS is in some ways similar to an optical spectrum analyzer (OSA). The sensing spectrum range is limited from 1520nm to 1570nm, but the resolution and sampling frequency are much higher than OSA. Here, the CTS instrument sweeps the optical wavelength from 1520 to 1570 nm at a frequency of 5Hz, each sweep scanning 200,000 wavelength points, measuring the transmitted optical signal through the sensor head at each point. The increasing concentration test resulted in a response time of about 5 seconds. The decreasing concentration test resulted in a response time of about 23 seconds. The response time is defined as the time required for a concentration increase from 0 to 90% maximum or the concentration decrease from maximum to 10%. These differences in definition contribute to the differing response times for the increasing concentration test versus the decreasing concentration test.

In an exemplary embodiment, a gas sensing system 100 is designed to operate without periodic calibration. However, a calibration cover may be provided for testing the performance of the gas sensor head. An exemplary calibration cover includes a tube which is slightly larger than the sensor head and two gaskets to seal the two end of the tube once the tube is slid over the sensor. There is a small opening on the side of the tube which is configured (e.g. in size and shape) to connect to a testing gas cylinder. The calibration cover limits the access of ambient air to the sensor head, permitting gases to reach the sensor head only if passed through the small opening in the side of the calibration cover tube. In this way, a controlled known concentration of a gas may be delivered to the gas sensor head from the testing gas cylinder.

In a prototype sensor head 101 arranged according to Figures 3A-3H, a sensitivity test was conducted with methane and demonstrated a sensitivity at least as low as 200 ppm. A multiple gas sensing system 100 using wavelength modulation spectroscopy (WMS) should be sufficiently sensitive and able to detect gas levels at least as low as 100 ppm, 75 ppm, 50ppm, 25 ppm, or even 10 ppm.

Laser Driver Module

A laser driver module 102 generally comprises a laser driver 1 14, a light source such as laser 1 15, and a thermoelectric cooler (TEC) driver 1 16. The laser driver module 102 may be embodied as a single circuit board or may include a plurality of circuit elements which are interconnected but not mounted on the same circuit board (e.g. a plurality of circuit boards are used). In an exemplary embodiment, the laser 1 15 is a distributed feedback (DFB) laser with a center wavelength several nanometers around a principle absorption line of a gas that is to be detected and the concentration of which is to be monitored.

The laser 1 15, e.g. a distributed feedback (DFB) laser, is controlled and driven by the laser driver 1 14 together with the TEC driver 1 16. The TEC driver 1 16 is primarily responsible for performing wavelength scanning by modulating and adjusting the laser chip temperature of the laser 1 15. A bench top laser diode driver and TEC controller (such as the ITC510 made by Thorlabs) may be used to sweep the laser wavelength in a lab setting.

However, an on-line detection system should use driver and controller modules suited for installation in the conditions where the sensor will be installed for industry purposes, e.g. a coal mine. Different printed circuit boards (PCBs) and integrated circuits (ICs) are available in the market and may be used for this purpose. As examples, two exemplary PCB modules are the PLD 200 and HTC1500 (both by Wavelength Electronics). These two modules perform very stably.

In an exemplary embodiment, the laser 1 15 may be modulated in a sinusoidal fashion. The TEC driver 1 16 may be an imbedded TEC driver. The TEC driver 116 is interfaced with a thermo-electric cooler (TEC) and a temperature sensor inside a case of the laser 1 15.

Regulation of the laser temperature may be based on a Proportional Integral (P-I) feedback process, whereby the TEC driver 116 reads the temperature from the temperature sensor and proceeds to set the temperature to a set value using the TEC. To change the temperature in the form of a sine wave, the appropriate signal may be generated in the D/A channel and fed to the laser driver. The proportional and integral parameters of the TEC driver 1 16 may be controlled by a potentiometer and capacitor, respectively.

The laser driver 114 is connected to the laser 115 and is usable to control the driving current in the laser 1 15, more specifically the laser diode (LD) of the laser 1 15, and thus regulate the optical power generated. This includes switching the laser 115 on or off. This may be controlled by the embedded computer 121 through the digital I/O card. A laser driver module 102 interfaces with an analog-to-digital (A/D), digital-to-analog (D/A), and digital I/O cards. These may be separate or combined into a common interfacing module 1 17, as exemplified in Figure 1.

As an example, an exemplary laser driver module 102 may be configured for permitting methane detection with the sensor head 101 and optical receiver module 103. The laser driver module 102 is used to sweep the peak wavelength of the laser 1 15 near/across the absorption line of methane. This kind of sweep may be achieved by changing the temperature of the laser 1 15 in a periodic fashion (e.g. a sine wave / sinusoidal wave) (e.g. using a sine wave generator 1 18) with the help of the thermo-electric cooler (TEC) driver 1 16. For detecting the methane absorption at 1650.874 nm, the temperature of the laser 1 15 may be varied from, for example, 18°C to 24°C periodically at a frequency of 200 mHz. Temperature may be monitored with a temperature sensor 1 19.

In an exemplary embodiment of a gas sensing system 100, at least one laser driver module 102 but preferably at least two modules 102 are included, where the first may serve as a primary and the second may serve as a backup. Features of a laser driver module 102 may include: remote laser enabling using a digital I/O card connected to the laser driver, laser temperature, and driving current monitoring using an A/D card; protection circuitry for TEC and laser driver current modulation; improved structural support for heat sinks and fans; and on board laser fan power connectors. Trim potentiometers may be included for controlling the protection limits of the TEC or laser driver current modulation. These circuit elements will trigger if the input signal ever exceeds the limits of the safe operating conditions of the laser. The protection limits may be set according to the specific device parameters of the TEC and laser used for a given embodiment.

The scanning rate of the spectrum sweep of the laser driver module 102 may vary according to component limitations of different embodiments. In one sample prototype embodiment, a scanning rate of 2 scans per 5 seconds was achieved. The rate was limited by the thermo-electric cooler (TEC) inside the laser which caused the laser case to heat up when a higher scanning signal was applied. The TEC driver set-point input cannot be too high (e.g. greater than 3.7 V) or too low (e.g. less than 0.5 V) at any point in time because it will make the laser too cold or too hot. Hence diode circuits may be used to clamp the voltage to appropriate level in case the D/A channel gets disconnected and the signal sent to the TEC driver becomes a floating voltage. As a similar safety feature, a diode circuit may be included for the laser current modulation, that is to say the signal sent to the laser driver. An external heat-sink and/or fan may be used to help reduce the risk of the laser overheating and thus increase the maximum scanning rate. The prototype was modified to include an external heat- sink and fan, with the result of an improved maximum scanning rate of 2 scans/sec.

While interfacing the D/A card and Digital I/O card channels to the circuit, care must be taken to avoid a floating voltage at these nodes. This possibility may be removed by using pull-up and pull-down resistors at those nodes.

The TEC driver 1 16, laser driver 1 14, and the laser 1 15 may each have decoupling capacitors (e.g. 10 uF and 0.1 uF) so that any noise caused by the supply voltage or other circuit components is shunted through the capacitor. Furthermore, ferrite beads may used to connect the digital ground to the analog ground in order to suppress high frequency noise.

Optical Receiver Module

In an exemplary embodiment, the optical receiver module 103 includes one or more photodetectors such as photodiodes 104 which convert the transmitted laser power (an optical signal) passed from, for example, a transmitting optical fiber of a sensor head 101 to an electrical current (an electrical signal).

An optical receiver module 103 may be embodied in a single circuit board (e.g. a printed circuit board, or PCB) or may include a plurality of circuit elements which are interconnected but not mounted on the same circuit board (e.g. a plurality of circuit boards and circuit elements may be used). Generally, an optical receiver module 103 may be interchangeably referred to herein as a receiver circuit or a receiver board.

An optical receiver module 103 has a plurality of input channels. In an exemplary embodiment, the optical receiver module 103 comprises at least 3 substantially identical input channels, one channel each for receiving optical signals 109/1097109" from one or more sensor heads 101 , an optical signal 1 10 from a reference gas cell (RGC) 105, and a direct signal 108 from the DFB laser 1 15. An additional input channel may be provided for each additional sensor head in excess of 1. As a result, in an exemplary gas sensing system 100, there are a total of '2 + n' receiver channels, where 'n' is the number of sensor heads 101 connected to the system. In some embodiments, multiple optical receiver modules 103 may be stacked together to increase the number of receiver channels. For example, stacking two three-input receiver circuits together allows for six channel receiving capabilities.

An optical receiver module 103 outputs data indicating a concentration of one or more gases detected at the one or more gas sensor heads 101. In an exemplary embodiment, this data is supplied to a processor or embedded computer 121 , typically through one or more interfacing modules such as D/A card 122 or interfacing module 1 17. Data containing gas concentration information may be shared with or provided on, for example, a user interface device 123, such as an LCD screen and touch panel, stored on a computer readable medium such as a permanent or removable storage device 124 (e.g. a hard drive), or transferred to one or more servers which may be part of a computer network which includes a plurality of computers any of which may have access to the concentration information. A communication module 126 may be provided for transferring the data and include one or more hardwired or wireless connection devices, optionally including but not limited to local area network (LAN) connections and Wi-Fi. The concentration information may be input to a specialized computer program embodied in a set of instruction stored on a non-transitory computer readable medium which, when executed by one or more processors of one or more computers, causes such processors to perform one or more additional calculations, comparisons, or other data processing steps in which the gas concentration is used as an input variable.

The signals 108, 110, and 109 associated with each of three input channels of an optical receiver module 103 may be respectively called 1) the laser channel signal 108, 2) the RGC channel signal 1 10, and 3) the sensor channel signal 109. These terms may be used herein to refer to each signal in the form of an optical signal or in the form of an electrical signal (e.g. a current signal generated by a photo-diode exposed to an optical signal, or a voltage signal generated from such a current signal). Generally, an exemplary optical receiver module 103 receives signals at all input channels as optical signals and provides for their conversion to electrical signals. In alternative embodiments, however, this conversion may be performed by an intermediary device or another module, and the optical receiver module 103 may be configured to receive the converted electrical signal instead of the original optical signal.

The laser channel signal 108 is the optical signal generated by one or more laser driver modules 102 without modification by any gas. The laser channel signal 108 may also be some other signal which contains substantially the same information as the original optical signal generated by the laser driver module(s). The RGC channel signal 1 10 corresponds to the optical signal output of a RGC 105. Notably, this signal was at one point in time substantially identical to the original optical signal 108 generated by the laser driver module 102 but then modified by the selective absorbance/transmittance of the gas in the RGC. Similarly, the sensor channel signal 109 corresponds to the optical signal output of a gas sensor head 101. This signal was also at one point in time substantially identical to the original optical signal 108 generated by the laser driver module but then modified by the selective absorbance/transmittance of one or more gases present in the ambient air in the chamber of the gas sensor head 101.

Figure 8 shows a circuit schematic for an exemplary optical receiver module 103. An exemplary optical receiver module 103 includes at least three major parts: 1) a trans- impedance amplifier (TIA) 801 , 2) a filter circuit 802, and 3) a DC removal circuit 803.

A trans-impedance amplifier (TIA) 801 converts a raw signal from a photo-diode (PD) 104 to an electrical signal of appropriate magnitude. The output of the photo-diode 104 (and thus the input of the TIA 801) is generally a current signal, while the output of the TIA 801 is generally a voltage signal. The current signal from the photo-diode 104 is ordinarily very weak.

An optical receiver module 103 may have a plurality of gain stages 804 from which a suitable gain may be selected and then applied to the input current signal. This is the case for the exemplary optical receiver module 103 shown in Figure 8, where the TIA 801 , DC removal circuit 803, and gain stages 804 collectively contribute to a variable gain TIA 805. In an exemplary embodiment, the variable gain TIA 805 has 4 different gain stages for different intensities of received optical power. Alternative embodiments may have fewer than 4 or more than 4 gain stages. Generally, the gain stages 804 are switched using MOSFETs, relay switches (e.g. Double Pole, Double Throw (DPDT) relay switches), and the Digital I/O card. As an example, Table 3 shows values of feedback resistor (R_F) and feedback capacitor (C_F) for 4 different gain stages. The capacitor values are chosen such that the TIA circuit has the maximum possible frequency bandwidth without making the feedback system unstable. Table 3. Values of resistors and capacitors in the optical receiver module TIA

In some embodiments, an uncompensated operational amplifier (op-amp) (e.g. OPA657 made by Texas Instruments) may be used in the variable gain TIA. However, this can produce instability as the gain is increased. In order to eliminate this problem, a frequency compensated op-amp (e.g. OPA227) may be used instead. Operational amplifiers such as OPA2209 by Texas Instruments may also used in the receiver board such as is shown in Figure 8.

In order to prevent saturation and ensure maximum linear amplification of the gas absorption signal (the electrical signal beginning at the photo-diode 104), a closed- loop feedback may be used to remove the DC background from the detection by providing a voltage bias. A voltage bias may be applied with a circuit-implemented negative feedback or, alternatively, with computer control. In the variable gain TIA 805 of Figure 8, a DC removal circuit 803 is be used to remove the DC component in the electrical signal. An appropriate bias signal is generated from the D/A card 122 and fed back to the TIA 801 through a voltage follower. In an exemplary embodiment, bias application and gain stage selection are automatic.

Input and output labels for Figure 8 are explained in Table 4.

Table 4. Input and output labels for Figure 8 circuit schematic.

Label Name Port type Description

Input DC signal from D/A card to change

BIAS_Chl

the DC bias level in the TIA circuit Output Actual DC level sensed from the TIA

DC Chl circuit. This is fed to the panel PC through the A/D card

Output Output signal from the TIA after the

DC has been removed. This is treated

AC_Chl as the AC signal. This signal is also sampled and fed to the computer through the A/D card.

Input Three Digital I/O pins to change the gain of the TIA, e.g. when DIA_Chl

DIA Chl , is high (+5V), the Double Pole, DIB Chl , Double Throw (DPDT) relay next to DIC ChI that pin is turned ON, and the

effective resistance values on the TIA circuit changes.

A unique advantage of some exemplary embodiments is the dynamic range of gas concentrations which can be detected and accurately quantified. For example, a dynamic range of 100pm to 100% methane is possible for an individual system embodiment. The variable gain trans-impedance amplifier (TIA) 805 may be required to convert optical power into an electrical signal over a variation range of, for example, 20dB. Hence the circuit needs a different value of bias voltage (VBIAS) to avoid saturating the op-amp output of the TIA 801. Programming within the optical receiver module performs this process automatically by looking at the output from the TIA 801 and determining an appropriate VBIAS-

As a circuit protection feature, a diode may be placed in series with the power supply (typically +12 V and -12 V) to the receiver circuit. This protects the receiver circuit from any kind of damage in case a reverse voltage is applied. As yet further circuit features which may be used, each op-amp may be arranged with decoupling capacitors (e.g. 10 uF and 0.1 uF) so that noise caused by the supply voltage or other circuit components is shunted through the capacitor. While performing DC removal, op-amp buffer (or voltage follower) may be used to avoid impedance loading. Ferrite beads may be used to connect the digital ground to the analog ground in order to suppress high frequency noise. Figure 9 shows a sample output of a TIA or variable gain TIA 805. Absorption peaks are readily apparent, as is the sinusoidal variation resulting from the modulation of the temperature of the laser by a sinusoidal function. A filter circuit 802 follows the TIA 801 and removes noise from the electrical signal output of the TIA 801 before it is collected using the A/D card of interfacing module 1 17. In the exemplary embodiment shown in Figure 8, the filter circuit 802 is a 3^rd order low-pass filter.

Other Gas Sensing System Components

According to some exemplary embodiments, a gas sensing system 100 may include one or more user interfaces for receiving input from a user and/or generating or displaying output for viewing or interaction by a user (e.g. a human user). According to the gas sensing system 100 schematically depicted in Figure 1 , the imbedded computer 121 is

communicatively coupled to an output device such as user interface device 123 with a display. A gas concentration level (e.g. a methane concentration level) and other relevant parameters may be stored on or in association with the imbedded computer 121 and displayed with the display 123.

The display 123 or other output device connected to the system 100 by

communication module 126 may include an interface such as a graphical user interface (GUI) which may provide or be otherwise involved with functionality including but not limited to viewing or plotting current data (i.e. real time or quasi-real time data), viewing or checking past recorded data, executing statistical processes such as curve fitting, and performing additional data processing. The GUI and associated computer programming instructions may be created using a variety of known programming software, for example Visual C# or C++.

User interfaces are usable for viewing data and setting system parameters. In exemplary embodiments, there may be at least four different main tabs in the interface. The first is a 'user' tab which permits a user to monitor concentration readings at the one or more sensor heads. Space may be provided for multiple concentration plots, one each for separate sensor head readings, or the readings may be superimposed on a single graph. A 'history' tab permits access and lookup capabilities for data collected in the past and stored on the computer, a network, or a computer readable medium such as a hard drive. The past data may be plotted in various formats. A 'developer' tab displays the real-time signals from various channels and provides easy-control of important parameters. One or more of the tabs as well as the entire user interface may have one or more security features such as password protection. This is particularly advantageous for the developer tab. A fourth tab is laser control. Here, parameters for the one or more lasers / laser driver modules may be individually set. These parameters may include but are not limited to maximum and minimum voltage values, a sampling rate, a sampling period, a signal amplitude, and a signal mean or average.

One feature of an exemplary user interface, may be a notification system for alerting one or more users when a gas concentration exceeds an established threshold. A user may be provided the option to set a concentration limit above which a safety alarm is triggered.

One or more laser driver modules 102 and one or more optical receiver modules 103 may be arranged together in one or more control boxes. In an exemplary embodiment having just one such control box, all of one or more sensor heads 101 are connected to the control box by one or more optical fibers, preferably single mode optical fibers which may be grouped and protected in one or more fiber optic cables 1 13. A control box is generally connected to a power supply (e.g. 1 10V AC power) and connected to a network via, for example, an Ethernet cable. This allows a user to interact with and optionally change settings at the control box while physically located at a location other than the location of the control box.

One advantage of a control box is to provide a protective housing for laser driver modules, optical receiver modules, and other electronic components of the gas sensing system. In some embodiments, the control box may be an explosion proof enclosure. In addition to laser driver modules 102 and optical receiver modules 103, a control box may include any one or more of the following: interfacing module 1 17 (which may be, for example, a Multifunction Data Acquisition (DAQ) Module such as USB-6216 by National Instruments), measurement computing D/A card 122 for signal biasing (which may be, for example, USB 3101FS by Measurement Computing Corp.), a USB hub (e.g. a USB 7-port hub), a low noise DC power supply 125, a surge protector strip, embedded computer 121, user interface device 123, communication module 126, storage device 124, and other optical components (couplers 107, isolators, and reference gas cell 105). A multifunction DAQ module like USB-6216 includes three different modules, i.e. A D card, D/A card, and Digital I/O card.

One or more cooling devices such as fans may be included to circulate air throughout the control box, thereby reducing the risk of components overheating. Around 95% of the heat inside a control box may be generated by the DC power supply unit. As discussed previously, the laser driver module also generally generates heat and benefits from heat dissipation. Therefore, small cooling devices such as CPU fans may be arranged on top of the laser for cooling. The laser, TEC, and laser driving cooling elements may also be arranged in

the direct path of air flow from the control box fan.

Concentration Estimation Algorithm

In order to determine the concentration of a gas, e.g. methane, from one or more

electrical signals of an optical receiver module, Beer-Lambert's law may be employed. Beer- Lambert's law relates the transmittance T to the gas concentration C . This is given in the

following equation.

Where l^_ut and /_;„ are the optical signal intensity before and after entering the gas

chamber, I is the light path length, and C is the concentration of the gas.

Beer Lambert's law can be simplified in the following way when the gas

concentration is quite low.

i.e. l_Q^_t ¾ - aCll; foraCi 1

Figure 2 shows a simplified block diagram of an exemplary system with the measured

signals V_t,V_z, V_s and other parameters. The three different channels of the sensor system are

depicted as CH— 1.CH - 2, and CH - 3₅ respectively.

The following is a list of definitions for parameters in Figure 2:

/ a : M eon I aser i n t n si ty

&l_t: Amplitude of sinusoidal variation in laser intensity

ω: Radian frequency of sinusoidal variation in laser intensity

t-.time

a: absorption coefficient for the gas and it is a function of time

R_t R-,R~ Ratios by which the laser output intensity is divided between three channels.

G_itG₂, G_s:Trans - impedance gains of photo receivers 1,2, and 3 respectively.

L~,L_S: Optical loss inside the Reference Gas Cell and the Gas Sensor Head respectively.

C₃, C_a : Cor entr ulion of y s inside the Reference Gus Cell nd ike Gus Sensor fieud resOeciii ely.

I ~,!₃'.OpLiLul pul lenyLh for Reference Gus Cell and Gus Sensor Mtud respectively.

DC_s,DC_a: Added DC bias values to the Reference Gas Cell and Gas Sensor Head signals respectively. Vi.Vs.VazOutput voltage signals from Photo receivers 1, 2, and 3 respectively.

L_2l, L-zi'.O ticc loss ratio for channel 2, and 3 w.r. t. channel 1.

V~,V₂- Scaled voltage signals from channel 2 and 3 respectively.

Offset₂-_L,Cffset_2i:Off set required to match the DC level of \ , V, with that of V_t. V~^' _,V -.Shifted voltage signals from V~,V. respectively.

peak_RGC, peak_seyisor'. Extracted absorption peaks from RGC and Sensor Head respectively. A_RSC,A_sens:,₇-:Absorbance for RGC and Sensor Head respectively.

C ^' _rctioz Concentration ratio between RGC and Sensor Head.

Typical measured waveform patterns for , , V-, and _aon the oscilloscope are shown in Figure 9. The labeling in Figure 9 corresponds with Figure 2, the three signals

corresponding with the three respective optical receiver channels.

An exemplary concentration estimation algoritlim is described below. The analytical expressions for various calculated signals are written in brackets adjacent to simplified descriptive expressions. It should be noted that optical and electrical noises are ignored while writing these expressions.

Time-domain expressions for l ,V₂,anc V- are,

\ = -RiG o - R ₁G₁Al₀siv.(iut)

V₃ = DC. -R L.G.;_Be- ^c*^:* - R ₂L.G_zM₀sin(ot)e-^a(-^{t c} i

V₃ - DC₃ -R₃L₃G₃i_ae-" ^r>^!> -

Step-1: Scaling

L_2i = Slope of V.vs.l^graph

R_iG₁

L_3l = Slope uf V- ι i. \ y: uph -fl₁G₁AI₀sin(iut)e^'""e,:t)c!^I2 - Λ_ιί?₁Δ1₀5ίη(ωί)β^"'^ϊ(ί;ΐί:»

Step-2: Shifting

Off set _2l = Y— intercept of V-vs.l^grapk DC,

R₂L_iG_i

Off set _3l = Y— intercept of V^vs.l^graph | DC,

R_SL_;G₃

= V~ - Off vL_li [=

- R₁G_iAI₀sln(ojL ti-^,1'(r,r>-^,z] = V₃ - Offsets [= -R.G ^-^^'^ - R^Mosin^e-^^i] Step-3: Normalization for absorption peak extraction Step-4: Absorbance calculation

A RGC log(x>eak_RGC) [= (t)C₂ l ₂]

Step-5: Concentration calculation ratio = Slo

C3 = C <- -2— C ^ ra .tio

Hence the unknown concentration C, is calculated from the known parameters C-, and l _s .

Wavelength modulation spectroscopy (WMS) is an exemplary technique for a gas sensing system 100 configured to detect a plurality of gases, where the same optical fiber carries the information for a plurality of gas absorptions.

In the case of WMS, a high frequency (several 100 Hz to kHz) sinusoidal current modulation is fed to the laser driver. At the same time, laser temperature scanning is accomplished by a lower frequency ramp signal for obtaining a linear wavelength scanning. This differs from single gas sensing systems such as for detecting methane alone, where a sine wave may be employed instead of a ramp signal. Thus there are two processes simultaneously changing the laser emission wavelength: 1) Ramp wave fed to the TEC driver 1 16, and 2) Sine wave fed to the Laser driver 1 14. As a result, the optical signal emitted from the laser performs a wavelength modulated scanning of the gas absorption curve.

Gas absorption coefficient a is a nonlinear function (typically described by a Lorentz or Voigt profile) of optical frequency v . Since the optical wavelength or frequency i is being scanned linearly in the time domain by a ramp wave to the TEC driver 1 16, a is now a nonlinear function in time. Therefore the sinusoidal wavelength modulation to the laser driving current generates several harmonic signals at the multiples of this sine modulation frequency f .

Since the gas absorbance '.4 ' (= aCl) is a function of concentration C , the resulting harmonics are also functions of gas concentration. For a sinusoidal modulation of laser current at a frequency , the first harmonic signal is generated at / , the second harmonic signal is generated at 2/ , and so on. Although the 2f (or second harmonic) signal is weak compared to the 1/ (or first harmonic) signal, it has the advantage of having an absolute zero background. In exemplary embodiments, this harmonic signal is detected by a Lock-in amplifier (LIA) after the optical receiver module 103 converts the received optical signal to an electrical signal. The LIA can be a bench-top instrument (e.g. SR810 from Stanford Research Sys.), a field programmable gate array (FPGA) module, or a software

implementation programmed in, for example, C++ or C#.

For a gas sensing system 100 such as shown in Figure 1 , a software implemented lock-in amplifer (LIA) may be used, with the program instructions for the LIA being stored on a storage device 124 and executed by the computer 121. The electrical voltage signal from the optical receiver module 103 is first sampled by the A/D card of interfacing module 1 17 and sent to the computer 121 . Then the computer 13 1 , upon execution of the LIA software, separates the harmonic signals from the sampled signal data of the optical receiver module 103. By using different modulation frequencies / for different gases, the respective harmonic signals can be separated in the computer 121 with the software Lock-in Amplifiers. Thus the concentrations of all of a plurality gases can be measured simultaneously.

A simplified block diagram is shown in Figure 10 to explain this principle. In this scheme, both CO and CO2 gas inside the gas chamber are being detected at the same time. (However, this example is equally applicable to any combination of one or more of CO, CO2, CH₄, and other gases.) DFB Laser 1001 is centered at a wavelength of 1566 nm, a peak absorption wavelength of CO. It is temperature modulated by a ramp of frequency /0 and current modulated by a sine wave of frequency fl . Temperature modulation is controlled by a TEC driver 1 16 , and current modulation is controlled by a laser driver 1 14. Similarly, DFB Laser 1002 is centered at a wavelength of 1572 nm, a peak absorption wavelength of CO2. It is also being temperature modulated by a ramp of f 0 , but the current modulation is done by a sine wave at frequency [2 . f^'2 is selected such that it is not a multiple of fl and the ramp frequency /0 « /l, 2 . The optical signals from the two lasers 1001 and 1002 are combined by a coupler 107 and then sent to the gas sensor head 101 which, in this example, contains CO and CO2. The transmitted optical signal is collected by an optical receiver module 103 and sent to the Lock-in Amplifiers 1003 and 1004 for harmonic detection, from which the individual gas signals are obtained.

This method for simultaneous detection and monitoring of multiple gases, be it two, three, or more than three different gases, minimizes cost by reducing the use of multiple optical fibers in a fiber cable and multiple optical receiver modules when several gases need to be monitored.

For the exemplary system shown in Figure 1 , the particular wavelengths at which absorptions (or conversely, transmittances) may be detected and measured are listed in Table 5. Table 5. Example peak absorption wavelengths for different gases

Although some features of the invention have been identified and discussed herein with reference to individual exemplary embodiments, it should be recognized that except where specifically stated otherwise, features described with relation to one exemplary embodiment are not limited thereto and are also applicable to and usable with other embodiments. While exemplary embodiments of the present invention have been disclosed herein, one skilled in the art will recognize that various changes and modifications may be made without departing from the scope of the invention as defined by the following claims.

Claims

CLAIMS Having thus described our invention, what we claim as new and desire to secure by Letters Patent is:

1. A gas sensing system, comprising:

one or more laser driver modules which generate a first signal having a wavelength spectrum which includes for one or more gases at least one peak absorption wavelength specific for each respective gas of said one or more gases;

one or more gas sensor heads configured to receive said first signal, each gas sensor head of said one or more gas sensor heads providing as a second signal portions of said first signal which are transmitted through an interior space of a gas chamber configured to admit one or more gases from ambient air;

at least one reference gas cell configured to receive said first signal and provide as a third signal portions of said first signal which are transmitted through said at least one reference gas cell, said at least one reference gas cell containing a known concentration of said each respective gas of said one or more gases;

one or more photodetectors configured to convert said first signal, each said second signal, and said third signal from optical signals to electrical signals; and

one or more computing devices which determine a concentration of said each respective gas of said one or more gases using at least in part a comparison of said first signal, said second signal, and said third signal which have been converted to electrical signals,

wherein said one or more laser driver modules, at least one reference gas cell, one or more photodetectors, and one or more computing devices are configured for isolation from ambient air in which hazardous gases potentially exist in significant concentration or quantity and said one or more gas sensor heads are configured for arrangement in one or more locations in which hazardous gases potentially exist in ambient air, and

wherein said one or more laser driver modules and said one or more photodetectors are communicatively coupled to said one or more gas sensor heads by a connection which includes a fiber optic cable.

2. The gas sensing system of claim 1 , wherein said one or more photodetectors and said one or more computing devices are part of an optical receiver module.

3. The gas sensing system of claim 1 , wherein at least one gas sensor head of said one or more gas sensor heads comprises

said gas chamber;

a first collimator for transferring said first signal from an optical fiber to said gas chamber as a light beam traveling in said interior space of said gas chamber;

a second collimator for collecting as said second signal portions of said light beam which are transmitted tlirough said interior space and transferring said second signal to an optical fiber;

a pair of protective windows between said first collimator and said second collimator which separate said first collimator and said second collimator from said interior space of said gas chamber, wherein said pair of protective windows has at least one of

four window faces through which said light beam passes which are all non parallel to one another, and

wedged window shapes with at least one window rotated with respect to the other; and

a housing enclosure protecting said gas chamber, said first collimator, said second collimator, and said pair of protective windows, said housing enclosure including one or more filters configured to admit passage of said one or more gases and filter passage of airborne particulates.

4. The gas sensing system of claim 3, wherein said first collimator and said second collimator are graded index lens collimators with fiber pigtails.

5. The gas sensing system of claim 3, wherein said light beam has an optical path length of 2.5 inches or less in said interior space of said gas chamber.

6. The gas sensing system of claim 1 , wherein at least one gas sensor head of said one or more gas sensor heads comprises

said gas chamber;

a first collimator for transferring said first signal from an optical fiber to said gas chamber as a light beam traveling in said interior space of said gas chamber; a second collimator for collecting as said second signal portions of said light beam which are transmitted through said interior space and transferring said second signal to an optical fiber;

a pair of protective windows between said first collimator and said second collimator which separate said first collimator and said second collimator from said interior space of said gas chamber, wherein each protective window has a wedge shape with respect to a common longitudinal axis of the gas chamber and are rotated with respect to each other about said common longitudinal axis so as to present non-parallel surfaces opposing one another; and

7. The gas sensing system of claim 1 , wherein said wavelength spectrum of the first signal includes a plurality of peak absorption wavelengths each specific to one of a plurality of gases and wherein said one or more computing devices determine a concentration of each gas of said plurality of gases using at least in part a comparison of said first signal, said second signal, and said third signal.

8. The gas sensing system of claim 7, wherein said plurality of gases includes one or more of methane, carbon monoxide, and carbon dioxide.

9. The gas sensing system of claim 1 , wherein said fiber optic cable comprises single mode fibers for transmitting one or more of said first signal and said second signal.

10. The gas sensing system of claim 1 , wherein the gas sensing system detects gases with concentrations as low as 200ppm.

1 1. The gas sensing system of claim 10, wherein the gas sensing system detects gases with concentrations as low as 1 OOppm.

12. The gas sensing system of claim 1 , wherein the gas sensing system provides a dynamic range of gas detection from as low as 200ppm to as high as 100%.

13. The gas sensing system of claim 12, wherein the gas sensing system provides a dynamic range of gas detection from as low as lOOppm to as high as 100%.

14. A method for detecting and determining the concentration of one or more gases, comprising the steps of:

generating with one or more laser driver modules a first signal having a wavelength spectrum which includes for one or more gases at least one peak absorption wavelength specific for each respective gas of said one or more gases;

with one or more gas sensor heads, transmitting said first signal through an interior space of a gas chamber of each gas sensor head, said gas chamber being configured to admit one or more gases from ambient air;

for said each gas sensor head, providing as a second signal portions of said first signal which are transmitted through said interior space of said gas chamber;

transmitting said first signal through at least one reference gas cell containing a known concentration of said each respective gas of said one or more gases;

providing as a third signal portions of said first signal which are transmitted through said at least one reference gas cell;

converting with one or more photodetectors said first signal, each said second signal, and said third signal from an optical signal to an electrical signal; and

determining with one or more computing devices a concentration of said each respective gas of said one or more gases at each location of said one or more gas sensor heads using at least in part a comparison of said first signal, said second signal corresponding to each respective sensor head, and said third signal which have been converted to electrical signals.

15. The method of claim 14, wherein said first transferring step passes said light beam through an optical path length of 2.5 inches or less in said interior space of said gas chamber.

16. The method of claim 14, wherein in said generating step, said wavelength spectrum of said first optical signal includes a plurality of peak absorption wavelengths each specific to one of a plurality of gases and wherein said determining step determines a concentration of each gas of said plurality of gases.

17. The method of claim 16, wherein said plurality of gases includes one or more of methane, carbon monoxide, and carbon dioxide.

18. A gas sensor head, comprising:

a gas chamber configured to admit one or more gases from ambient air into an interior space of said gas chamber;

a first collimator for transferring a first signal from an optical fiber to said gas chamber as a light beam traveling in said interior space of said gas chamber;

a second collimator for collecting as a second signal portions of said light beam which are transmitted through said interior space and transferring said second optical signal to an optical fiber;

19. The gas sensor head of claim 18, wherein said gas sensor head is configured to communicatively couple with electronic devices by a connection consisting of a fiber optic cable.

20. The gas sensor head of claim 19, wherein said gas sensor head is configured to communicatively couple with electronic devices over a distance of one or more kilometers.

21. The gas sensor head of claim 18, wherein said first collimator and said second collimator are graded index lens collimators with fiber pigtails.

22. The gas sensor head of claim 18, wherein said light beam has an optical path length of 2.5 inches or less in said interior space of said gas chamber.

23. The gas sensor head of claim 18, wherein said gas sensor head is configured to detect a plurality of gases.

24. The gas sensor head of claim 23, wherein said plurality of gases includes one or more of methane, carbon monoxide, and carbon dioxide.

25. The gas sensor head of claim 18, wherein said first and second collimators either receive from or transmit to single mode optical fibers.

26. The gas sensor head of claim 18, wherein each protective window has a wedge shape with respect to a common longitudinal axis of the gas chamber and are rotated with respect to each other about said common longitudinal axis so as to present non-parallel surfaces opposing one another.