In open path spectroscopy, tunable diode lasers (TDLs) are designed to focus on single absorption wavelengths specific to a compound of concern in the gaseous form. They are capable of achieving low detection limits and are virtually interferent-free. Open path TDLs are used in atmospheric pollutant studies, fenceline monitoring, process line/tank leak detection, industrial gas-purity applications, and monitoring and control of combustion processes.

Quantitative measurements in direct gas phase laser absorption spectroscopy are often based on Beer’s Law. The Law states that for a constant path length the intensity of the incident light energy traversing an absorbing medium diminishes exponentially with concentration. The mathematical argument is

where I_o(ν) is the intensity of the incident spectrum, α is the optical absorption coefficient of the gas and is a function of the wave number ν, C is the concentration of the gas, and L is the path length. Exhibit 1 presents an illustration of the absorption law.

Gaseous chemicals (especially those with five or fewer atoms) generally have strong absorption fundamentals in the mid- to far- infrared spectrum. The absorption of light by these fundamentals provides a means for detecting and quantifying them down to very low concentrations. The near-infrared also can be used for identifying and quantifying these chemicals. However, absorption in this region is due to overtone or combination bands with strengths that are typically two orders of magnitude weaker than the mid-infrared fundamentals and hence, have much higher detection limits.

A simple tunable diode laser instrument uses a diode to generate light within a narrow frequency range that contains a relatively unique absorption wavelength of the chemical of interest. The laser frequency is either “tuned” by changing the temperature of the diode or by changing the current being fed to it or both so that it matches the spectral absorption line of interest. The degree of absorption at a specific locked on wavelength can be used to calculate a concentration, or it can be calculated using a small wavelength range about the absorption line of interest that is built up in a signal averager and the concentration is calculated from this. Multiple chemicals can be monitored by multiplexing the instrument with more than one diode (usually up to four [Brassington 1995]).

Detection limits are dependent upon the pressure and temperature of the gas and the path length, among other things, with shorter path lengths producing higher detection limits. Instrument performance and technical noise (e.g., optical fringes) will also affect the detection limits that can be achieved. TDLs can be operated over truly open atmospheric paths or can use enclosed cells where the gas to be measured is drawn into a fixed length cell at a specified rate and pressure. The cells used are generally multipass, so the distance the light travels before being measured can be quite large relative to the length of the cell. This feature, which provides near-continuous measurements, can greatly improve the detection limits of the instrument by reducing pressure, controlling temperature, and increasing path length.

There are a number of factors that contribute to the operation of a tunable diode laser. These include materials of construction, type of structure, and tunability. Also, the source can be continuous-wave or pulsed.

Materials of Construction

Commercially available diodes are semiconductors, fabricated from exact combinations of ultra pure materials. The basic materials of construction of these diodes include gallium (Ga), indium (In), arsenic (As), antimony (Sb), phosphorus (P), aluminum (Al), lead (Pb), tin (Sn), selenium (Se), tellurium (Te), and sulfur (S). Usually the TDL is referred to by the constituents of the diode material. For example, a TDL using a diode material made from Indium, Gallium and Arsenic is referred to as an Indium Gallium Arsenide (usually written as InGaAs) laser. Other examples would include Indium Gallium Arsenide Phosphorus (InGaAsP), Aluminum Gallium Indium Phosphorus (AlGaAsP), and Gallium Indium Arsenic Antimony (GaInAsSb) lasers. An exception for this is the TDLs made from Lead compounds are referred to as Lead salt Diode Lasers. Generally TDLs used for measurement in the near infrared can be thermoelectrically cooled or operate at room temperature and pressure. There are some diodes in this class (Curl and Tittel 2002) that can produce frequencies in the near mid-infrared (3-5 µm) and therefore can access a small number of fundamental bands, which greatly improves their detection limits for the chemicals having absorption bands in this range. Note that this does not mean that any given diode can access a small number of fundamental bands but rather that diodes can be fabricated to operate at a specific wavelength within the 3-5 µm frequency range. Non-lead-based diodes are a standard in the communications industry. They are mass produced, easy to obtain, and relatively inexpensive. However, instruments based on Lead salt diode lasers are used to measure fundamentals in the mid-infrared spectrum giving them excellent detection limits. They require cryogenic cooling (usually using liquid nitrogen) and critical and stringent alignment requirements for the collection optics which makes them more difficult to operate.

Structure

As expected, the physical design of a diode laser directly affects it performance. There are a wide variety of diode designs and accessories that are used to improve performance. A simple diode design is the diffused homojunction laser. It is formed by taking a base crystal of a lead salt (for example) semiconductor and diffusing a salt of a different stoichiometry on the top surface (Brassington 1995). This affects the semiconductor band gap and, consequently, the TDL’s laser’s wavelength.

Another basic construction is the heterostructure design, which involves alternating layers of semiconductors of differing chemical composition and band gap. The layers do have atomic bonding and therefore are not physically separate. They can be bonded in such a way as to have an abrupt interface or, depending upon the application, the transition can be more gradual with some minor integration of the two crystal structures. Heterostructures are generally fabricated using an epitaxial growth process such as molecular beam epitaxy or metalorganic chemical vapor deposition.

If the layering of the semiconductor is made sufficiently thin, quantum interference effects begin to appear in the motion of the electrons. The placing of a sufficiently thin layer ( to ∼0.1 µm) of a narrower band gap semiconductor between thicker layers, or wider band gap material, creates a quantum well structure. This results in quantization of the valence and conduction-band energy levels. The lasers have predominantly single-mode characteristics and higher efficiency as a result of the quantum effects and can have higher power and higher operating temperatures than lasers obtained from conventional structures (Brassington 1995).

Vertical cavity-surface-emitting diode lasers are constructed in such a fashion that the light emitted by them is perpendicular to the diode surface rather than parallel. While there are a number of construction materials and designs, the basic structure consists of an active p-n layer with a vertical cavity that is grown to fit the desired wavelength. This layer can be sandwiched by other layers, such as those made from oxides to provide both gain and index guiding, but the key features are highly efficient distributed Bragg reflectors (mirrors) placed both above and below the active layer. The laser output from this type of diode has several advantages over other structure types because the beam is circular with low divergence and the emission is single mode with a narrow linewidth.

Tunability

The most widely available commercial diode is based on the Fabry Perot design. In this design, mirrors are placed at either end of the diode facets and provide the means for standing wave formation. The design tends to produce a wide bandwidth. Tuning to a specific wavelength is done by varying the diode temperature and the current going to it. The performance of the basic Fabry Perot laser diode can be improved in various ways.

• The Distributed Bragg Reflector (DBR) design replaces one of the mirrors on one end of the diode with a Bragg reflector. The Bragg reflector provides wavelength-selective feedback and lases on a single longitudinal mode. However, the mode is subject to change with changing temperature and current.
• The distributed feedback (DFB) diode design, replaces one of the mirrors with a diffractive feedback grating. The grating spacing determines the wavelength that is reflected back into the active area. This is one of the most widely used designs.

Another variation of the Fabry Perot design are lasers that use an external cavity with grating and reflecting mirrors. There are two generally used configurations for the external-cavity diode laser grating and mirrors. They are the Littrow and Littman-Metcalf designs.

In the Littrow configuration (Exhibit 2), the grating is aligned so that the first-order diffracted beam goes directly back into the laser. Typical feedback powers are about 10 percent of the output power. The coarse lasing wavelength is determined by the angle of the grating with respect to the laser; wavelength tuning is accomplished by changing this angle. The zeroth order reflected beam is the output (National Institute of Science and Technology webpage).

In the Littman-Metcalf configuration (Exhibit 3), the output beam from the laser is aligned at grazing incidence with the grating. The first-order diffracted beam is then sent to a mirror or retroreflector that reflects the beam back on itself. This reflected beam then hits the grating, and the first-order diffracted beam couples back into the laser. Tuning in this case is achieved by varying the angle of the retroreflector or tuning mirror, which again changes the wavelength for which the laser sees optical feedback. The output is also the zeroth order reflected beam off the grating (National Institute of Science and Technology webpage).

TDL systems usually use photodiodes as detectors. Like the TDLs themselves, these detectors can be constructed from a variety of materials. Lead-salt detectors are generally made from InSb or HgCdTe (MCT) and must be cryogenically cooled. There are some MCT and InAs detectors that operate in photovoltaic or photoconductive mode and use thermoelectric rather than cryogenic cooling. These detectors have a noise level and detectivity that is equal to or slightly worse than their cryogenic counterparts.

Unlike some other laser sources, tunable diodes generally produce a rather divergent laser beam that requires collimating. If an open atmospheric path is being monitored, the optics also will include telescopes to keep the beam as focused as possible. Telescopes generally are not needed in systems using flow-through cells, although these systems can require a number of mirrors to reshape the output beam and match the frequency number of the multipass cell. The more optical elements a system has the more fringes that will be produced. Also multiple optical elements increase the optical susceptibility of the beam to movement and subsequent changing of the optical baseline during the averaging of the absorption signal which will affect detection limits. Fringes can be subtracted if they are stable.

For purposes of this discussion, TDL instrumentation can be deployed in four configurations: monostatic, bistatic, passive, and flow-through cell. In the monostatic configuration (Exhibit 4), a TDL source is directed through a collimator, transmitting optics, and the source to be measured, where it is reflected back using a mirror or retroreflector through receiving optics to a detector.

Depending upon the output mode, the detector information is mathematically manipulated to quantitate the absorption. The monostatic configuration can be set up to use a single telescope as the transmitting and receiving optics, or it can use separate telescopes for transmitting and receiving the laser signal. In the bistatic configuration (Exhibit 5), the source and transmitting optics are placed at one end of the path and the detector and receiving optics at the other. One feature that TDLs have in monostatic and bistatic deployments is that the source and detector can be remotely located from the sending and receiving optics via fiber optic cable. This allows for multiple open path units to be set up over varying path lengths using a single source/detector/controller unit.

Unlike the passive configuration of other open path techniques, the one currently in use for TDLs requires topographic backscattering to work (Exhibit 6). The eyesafe laser is directed at a potential emission source from a distance of 20 meters or less and the concentration of the target gas is calculated from light that is scattered back to the detector by surfaces beyond the gas plume.

While not strictly an open path configuration, TDLs are often coupled with flow through cells or ambient air cells. The flow-through cells have the advantage of allowing for the control of pressure and temperature that affect absorption bands by broadening them and increasing the pathlength through multipass mirror arrangements. The net effect is that flow through cells are capable of achieving extremely low detection limits (often in the parts per trillion per volume). The cell need not be static either as some studies have placed them or their intake tubes on the outside of a moving vehicle or aircraft and obtained trace gas measurements in near real time of the ambient air. Three basic designs of these cells are used with TDLs: White, Herriot, and modified Herriot.

In the White configuration (Exhibit 7) light is introduced to mirror C where it is reflected to mirror A and then to mirror B where it exits the cell. By rotating either mirror B or C about an axis perpendicular to the surface of the page the number of passes can be changed. This type of cell can be subject to optical fringing that is similar to the absorption linewidth and is difficult to subtract/suppress. A typical cell of this type would have a 1 meter mirror separation, a volume of 10 liters and a usable path length of 100 meters (Brassington 1995).

The Herriot cell (Exhibit 8) features two spherical mirrors separated by slightly less than their diameter of curvature. Light enters through a hole in one and completes a number of passes before it either exits through the same hole or a hole in the other mirror. The beam bounce path and length is adjusted by changing the distance between the mirrors. Herriot cells can support more than one light source by providing different entrance and exit holes in the mirror.

The modified Herriot uses slightly astigmatic mirrors that greatly increase the number of passes possible and hence the signal strength. Also this arrangement can be constructed so that optical infringement caused by beams falling close to the exit hole is easily suppressed. (Brassington 1995 and Curl and Tittel 2002).

Optical infringement is one of many technical noise issues. Table 1 provides a list of techniques that can be used to enhance signals or reduce noise along with a brief explanation and suggestions where more information can be found.

Table 1. Techniques to Enhance Signals or Reduce Noise in TDL Systems.

Signal Enhancement
Cavity-Enhanced Spectroscopy	Cavity techniques such as cavity ring down spectroscopy and integrated cavity output spectroscopy have significantly higher sensitivity than that obtained in conventional absorption spectroscopy (Curl and Tittel 2002).
Photoacoustic and Photothermal Spectroscopy	These techniques use indirect measurement of pressure waves created when light produces a transient temperature rise in an absorbing medium via non-radiative relaxation processes (Sorokina and Vodopyanov 2003). They are limited to point sources since they require a special absorption cell. For a web-based discussion see Curl and Tittel 2002
Noise Reduction
Balanced Beam and Balanced Ratiometric Detection	These techniques have been developed in order to eliminate technical noise including laser intensity noise to approach the fundamental limit of shot noise. By measuring the laser signal with and without the absorption signal simultaneously, common mode noise can be subtracted and small absorption signals can be recovered (Sorokina 2003). Also see McNesby 1999.
Wavelength and Frequency Modulation Spectroscopy	The ability of a diode laser to change its emission wavelength with injection current permits frequency modulation spectroscopy that allow for the examination of higher harmonics that have high noise immunity (Curl and Tittel 2002).

TDLs have found the most use in detecting small molecule gases. The detection limit of the technique varies widely by instrument and the conditions under which the measurements are made. For the lowest detection limits, the ideal measurement would be with an instrument measuring adsorption in the mid- to far infrared with controlled temperature and pressure and a long pathlength. Detection limits in the parts per trillion have been reported under these conditions (Richter et al., 2002a, and Curl and Tittel 2001). Table 2 gives representative detection limits assuming 10^-5 absorbance, 1 Hz bandwidth, and a 1 m path. The table is taken from the website of Southwest Sciences, Inc.

Table 2. Detection Limits of Lead Salt Diode Lasers (Mid-Infrared) and Near Infrared Diode Lasers.

Chemical (Symbol)	Lead Salt Mid Infrared in ppb	InGaAsP Near Infrared in ppb
Acetylene (C2H2)	3.5	80
Ammonia (NH3)	0.80	800
Carbon Dioxide (CO2)	0.13	3000
Carbon Monoxide (CO)	0.75	30000
Formaldehyde (H2CO)	8.4	50000
Hydrogen Bromide (HBr)	7.2	600
Hydrogen Chloride (HCl)	0.83	150
Hydrogen Cyanide (HCN)	12	290
Hydrogen Fluoride (HF)	-	10
Hydrogen Iodide (HI)	-	2100
Hydrogen Sulfide (H2S)	-	20000
Methane (CH4)	1.7	600
Nitric Oxide (NO)	5.8	60000
Nitrogen Dioxide (NO2)	3.0	340
Nitrous Oxide (N2O)	0.44	1000
Oxygen (O2)	-	78000
Ozone (O3)	11	-
Phosphine (PH3)	6.2	1000
Sulfur Dioxide (SO2)	14	-
Water (H2O)	2.0	60

• Ammonia and hydrogen sulfide monitoring in agricultural applications
• Trace gas detection in process streams and open atmosphere
• Fenceline monitoring for specific gases
• Traffic pollution monitoring (NO, CO₂ and CO)
• CO monitoring in front of electro filtering pollution control equipment
• Combustion gas monitoring (boilers, incinerators)
• Stack monitoring for aluminum, glass, pulp and paper, smelting, and steel production facilities
• Process tank and line monitoring for fugitive emissions
• Quality control monitoring for specialty gases
• Natural gas line monitoring for impurities
• Breath analyzing

This section, which is not meant to be all inclusive, offers examples of successful TDL implementation for open-path monitoring.

Air Pollution from Motor Vehicles

TDLs have been deployed in several different configurations to measure exhaust gases, such as CO, CO₂, and NO, along roadways. In one configuration, the laser source is directed parallel to the road to a retroreflector 100 meters away, and an average of the target gases is obtained. This type of measurement provides an idea of the areal pollution at any given time. In another configuration, the laser source is placed at the approximate height of a vehicle muffler and directed across the roadway to a retroreflector. As cars pass through the beam, a measurement of their individual pollutant plumes is made. This configuration can be coupled with a camera that photographs the vehicle’s licence plate number, so vehicles having emissions above a prescribed threshold can be contacted and requested to have the problem repaired.

Quality Control in Pipeline Transmissions

El Paso Natural Gas has installed a distributed feedback diode laser tuned to 1.37µm to detect water in a high methane background. The laser, which is set up in a monostatic configuration across the pipeline, provides near real time information on the water content of user ready gas being transmitted through the pipeline. Detecting high water content in the gas before it is delivered is important to avoid poor performance at the point of use and icing in the pipeline that could curtail flow.

Environmental Fence-Line Monitoring at a Refinery

Along with an open path FT-IR system, a TDL fence-line monitoring system has been in operation at the TOSCO refinery in Rodeo, California, since 1997. The system consists of two monostatic configurations deployed along the north and south fence lines of the plant. The one-way optical path of the north fence line is 930 m long and the south path is 955 m. Both systems are set to monitor every five minutes and to sound an alarm if concentrations of some 26 target compounds exceed pre-set concentration levels. Each month, a report is developed that evaluates system performance and summarizes the chemicals detected, their concentrations, and the system detection limit for them. The TDL system monitors for H₂S and NH₃. The concentration data in ppm are transferred to refinery computers. No spectral data are preserved. (See Pawloski and Iverson, 1998, and the U.S. EPA webpage.)

Hydrogen Fluoride Alkylation Unit Monitoring

Terra Air Services has installed near infrared TDLs at two refineries in Louisiana to monitor potential releases from hydrogen fluoride (HF) alkylation units. Both units are set up in a monostatic configuration. One is placed next to the alkylation unit and has a one-way path of 100 m and the other is in a fenceline deployment with about a 200 m-path line.

Flue Gas Emissions Monitoring

The monitoring of flue gas emissions can be used for waste incinerator process control and determining if pollutant emission standards are being met. Bjorøy et al. (1998) report on the installation of a system to provide in situ monitoring of O₂, HCl, HF, CO, and dust in the stack of a 27 MW circulating fluidized bed combined boiler and incinerator at a paper mill. The boiler produces steam for use in plant production and may vary from 20 to 100% capacity. The incinerator is designed to burn municipal waste, plastic, wood, paper, waste oil, and coal. The burning of such widely varying materials puts a high demand on process control and flue gas cleaning.

The installed system consists of near infrared TDLs using a second harmonic detection system. Ports are installed opposite each other on the stack, and transmitting and receiving units are placed in them using standard flanges that are purged with dry air to keep the optical windows clean. The electronics unit, which includes the diodes and photoamplifier are connected by cable and housed in a separate building that can be up to 80 m from the transmitting unit. The burning efficiency of the incinerator is monitored by CO and O₂ measurements, and the HF and HCl readings give an indication of the scrubber performance. While the diode measuring HF requires very low detection limits (0.05 mg/Nm³ were achieved at this site) the CO detector needed a wide dynamic range to accommodate concentration fluctuations that varied from 50 to over 9000 mg/Nm³.

Volcanic Gas Monitoring

Volcanic gas was sampled at the summit crater of the Masaya volcano in Nicaragua using a DFG type TDL (Richter et al. 2001). The device was set up in the back compartment of an air-conditioned vehicle. Outside air was pumped through a Teflon tube into a multipass cell at reduced pressure. Continuous measurements of CO₂, SO₂, H^35,37Cl, H₂O, and CH₄ were achieved with field sensitivities of 3 ppm, 3 ppm, 15 ppb, 670 ppm, 32 ppb respectively.

HF Monitoring at a Phosphogypsum Storage Area

A phosphate fertilizer plant was planning on expanding its operations. As a part of obtaining a permit for the expansion, the state required the plant to determine how much HF and other fluoride species they would be contributing to the ambient air. A TDL was used to measure HF emissions from the existing storage area, and these measurements were incorporated into a model to predict what additional emissions an increased capacity would generate. The TDL study, which was combined with an open path FTIR instrument for other fluoride species, was successful in obtaining the plant expansion permit.

Atmospheric Gases

Measurements of stratospheric gases have been made by TDL over distances up to a kilometer using a retroreflector that is lowered from the gondola of a balloon. The studies were specifically directed at partitioning of active nitrogen in the stratosphere. While NO₂, NO, and HNO₃ were the primary target compounds, HCL, CH₄, O₃, N₂O, CO, and CO₂ were also measured. Since there is little broadening of the spectral lines due to atmospheric pressure at the altitude where the measurements were made, very good detection limits were achieved.

This section provides summaries of research applications and suggests some potential uses of TDLs in the environmental and industrial area.

Hand Held TDLAS Gas Detector for Measuring Industrial Process Releases

Frish et al. (2000) report on the development of a hand held detector for measuring releases of gases in industrial settings. The device, which is the size of a large flashlight, employs sending and receiving telescopes and a photodetector. The laser source and receiver electronics are configured in a simple wavelength modulation spectroscopy mode. An ILX Model LDX 3620 current source powers the fiber-coupled, telecommunications-grade, distributed-feedback diode laser. The laser assemblage contains a thermoelectric cooler for temperature regulation and an optical isolator to prevent feedback of reflected light. The instrument operates in a passive open path mode and uses plant structures up to 20 m away to reflect light back to the receiving unit. Table 3 provides a list of gases with detection limits that can be sensed with the instrument.

Table 3. Detection Limits (ppm/m) for Hand Held TDLAS Gas Detector

HF	0.2	HCN	1.0
H2S	20.0	CO	40.0
NH3	5.0	CO2	40.0
CH4	1.0	NO	30.0
HCl	0.15	NO2	0.2
H2O	1.0	O2	50.0

Measurement of Combustion Products in Jet Engine Exhaust

Research and development activities are being undertaken (Allen et al., 1998; Allen et al., 2000; Allen et al., 2001; Upschulte et al., 1998a; Upschulte et al., 2000) to determine the feasibility and usefulness of equipping jet engines with real-time measurement systems that can be used in their design to maximize the power to fuel expended ratio and to control the fuel and air mix of airborne aircraft to maximize fuel efficiency. The challenges presented in conducting real-time measurement of gases (CO, CO₂, NO, NO₂, H₂O, O₂, and CH₄) that indicate the efficiency of a jet engine especially in flight include instrument stability, optical access, potentially high temperatures and pressures, flow distortions and non-uniform flow properties. Tunable diode lasers in the near infrared have been shown to have the potential to meet these challenges.

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Physical Sciences, Inc.
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Sharp Microelectronics of the Americas
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E-mail: science@swsciences.com

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