OR WAIT null SECS
New mid-infrared spectroscopic sources, based upon advances in fiber lasers and in nonlinear frequency conversion, are now enabling high-resolution laser spectroscopy in the 2–4 μm wavelength region and beyond. With this in mind, the authors discuss continuous wave (CW) optical parametric oscillators (OPOs) in particular.
Sensors based upon laser absorption spectroscopy enable sensitive and selective detection of gas phase species in a wide range of applications (1). Concentrations in the parts-per-million (ppm) to parts-per-trillion (ppt) ranges (2) can be detected, using relatively compact laser-based devices. Applications include detection of trace moisture in semiconductor process gases, monitoring of atmospheric pollutants with significance in the occurrence of disease, breath analysis for diagnosis of asthma, and measurement of hydrocarbons for combustion diagnostics. A variety of techniques can be used to implement the required detection capability. For fence-line monitoring applications, such as for emission monitoring at chemical plants, long-path absorption measurements can provide concentration integrated over distances of hundreds of meters — in this case a laser beam is directed at a retroreflector and the return signal is measured by a detector adjacent to the source. For in situ sensing, where measurement of the species is performed in proximity to the optical source, prominent absorption techniques are photoacoustic detection, tunable diode laser absorption spectroscopy (TDLAS) using a multipass cell, and cavity ring-down spectroscopy (CRDS).
All of the techniques mentioned here share a common critical requirement for a tunable, single-frequency laser operating in continuous wave (CW) mode. The laser frequency must be tuned to the peak of the absorption feature of the species of interest. Furthermore, its spectral bandwidth must be a small fraction of the width of the absorption feature. These requirements are met most commonly using semiconductor lasers. Using different semiconductor gain media, external cavity diode lasers (ECDLs) using a tunable grating to provide feedback can generate single-frequency output at wavelengths between 630 nm and 2 μm. In addition to ECDLs, single-frequency output also can be obtained, with limited tunability, from monolithic devices such as distributed Bragg reflector (DBR) or distributed feedback (DFB) diodes. These devices exhibit compactness, ruggedness, and high reliability, partly derived from their development for telecommunications applications.
However, for ultrasensitive trace gas detection, it is desirable to operate laser-based sensors at mid-infrared (IR) wavelengths above 2 μm. This is because many gas species have their fundamental vibrational transitions here, in the so-called "molecular fingerprint region." Although gas sensors can be developed successfully using near-infrared sources, the detection sensitivities of such sensors are much lower than those based in the mid-IR, because the absorption line strengths of the fundamental transitions are orders of magnitude higher than those of the corresponding near-IR overtone and vibrational combination transitions (see Figure 1).
Figure 1. Water vapor absorption strength increases from near to mid-infrared (HITRAN data). Vibrational band assignments for the upper state are shown for several features.
Hence, the development of practical mid-IR CW sources is something of a "holy grail" for the sensing community. Much effort and resources have been poured into pushing the spectral coverage of semiconductors into this region. Lead-salt lasers (3) have for years been the only technology available, with wavelength coverage from 3–30 μm. However, their low power output and requirement for cryogenic cooling have largely kept these devices from emerging from the research laboratory. Greater promise of practical mid-IR sources for sensor applications is shown by interband cascade lasers (1) (operating between 2 and 5 μm), and quantum cascade (QC) lasers (4) (above 4 μm). Both types of devices have shown many of the characteristics needed, including high power and narrowband operation. However, to date, although there have been demonstrations of room-temperature CW operation of QC lasers at certain wavelengths, these devices have yet to reach a mature stage of commercial development. As a result, there remains a substantial current need for laser sources that enable basic spectroscopy and sensor development in the mid-IR.
The most promising alternative to semiconductor sources is in nonlinear frequency conversion of near-IR tunable lasers. There are two principal methods of mid-IR generation through the use of nonlinear materials. These are difference frequency generation (DFG) and optical parametric oscillation (see Figure 2). Both techniques allow exchange of energy between three optical frequencies, known as pump, signal, and idler (here vp , vs , and vi ). The interacting waves abide by the energy conservation relation, shown in Figure 2. Difference frequency generation is a process of frequency mixing two single-frequency sources (for example, ECDLs) on a single pass through a nonlinear material to generate tunable narrowband output at a third frequency defined by the difference in energy between the two input frequencies. This technique has been used successfully as the basis of mid-IR sensors in studies of atmospheric chemistry (5). The principal limitation of the technique is that the efficiency of conversion in the CW regime is extremely low, with maximum output power levels measured typically in microwatts (μW) to milliwatts (mW).
Figure 2. Difference frequency generation (DFG) and optical parametric oscillation (OPO) allow frequency conversion from near-IR to mid-IR.
By contrast, optical parametric oscillators (OPOs) can provide highly efficient conversion of a near-IR pump laser to longer wavelength output, while retaining the spectral and spatial properties of the pump laser. OPOs long have been regarded as promising tunable coherent sources, since their initial demonstration in the 1960s. However, their development in the commercial arena has been slow, with viable products emerging only in the late 1980s. Even then, OPOs have been available only as pulsed devices, mainly in the nanosecond pulse regime, with output typically extending from ~400 nm to 2500 nm. Although single-frequency, pulsed OPOs are available commercially, these devices are large, expensive systems occupying a substantial fraction of an optical table, and their linewidths are substantially broader than those of CW systems. Hence, beyond applications such as LIDAR, which requires high pulse energy output, they are not well suited to incorporation into practical sensors.
In the continuous wave regime, progress has been slower (6). Although there have been recent attempts to launch CW OPO products, the limited tuning capability of the devices offered has not allowed them to tap successfully into the spectroscopic market. Progress in development of CW devices has been held back by limitations in available pump sources, nonlinear materials, and in the OPO configurations, which could be implemented using them. Ideally, a CW OPO pump source is required to possess essentially all the characteristics that are required of a spectroscopic source in the target OPO output wavelength region — that is, single-frequency spectral output, high beam quality, continuous tunability, and high power. Although various sources can meet some of these demands, until recently, none have been able to provide all of these characteristics. Simultaneously, it is only with the recent development of periodically poled nonlinear materials that crystals now provide enough gain to allow CW oscillation using practical pump lasers. Early demonstrations of CW operation of OPOs were performed with unwieldy sources such as argon ion lasers, and required that the OPO resonated both the signal and idler waves within the OPO cavity. Although demonstration of stable operation and tuning of such devices has been performed (7), the requirement introduces unwelcome complication to the device. However, using periodically poled materials including lithium niobate, operation of singly resonant devices has been enabled (8, 9). These devices can be tuned straightforwardly, either by varying the poling period or temperature of the nonlinear material, or by tuning its pump source. This latter method is particularly attractive, in that it removes the tuning mechanism entirely from the OPO, and takes advantage of well-developed frequency tuning schemes that have been demonstrated in mature near-IR pump sources. Hence, it is with the advances in development of semiconductor lasers and fiber amplifiers, made under the auspices of the telecom industry, that the final leap has been made to CW OPOs fully capable of meeting spectroscopic application needs.
Fiber amplifiers and lasers have made spectacular advances within a period of a few years to the point of being able to generate kilowatt-level CW power in a diffraction-limited beam, while single frequency sources such as DFB semiconductor and fiber lasers provide high spectral quality and tunability, as well as high reliability.
Figure 3. A fiber-pumped CW OPO consists of rack-mounted fiber pump laser and frequency converter head.
Combining these advances, a CW OPO has been developed (Figure 3) pumped by an all-fiber pump source, which demonstrates the characteristics demanded of high-resolution, mid-IR spectroscopy. The pump source for the OPO is a single-frequency fiber laser, which can be tuned by applying piezoelectric strain to the fiber. The power from this source is amplified to a level of several watts by a fiber amplifier. Both sources are configured as packaged, rack-mounted units, similar to those developed for telecom applications. This provides a pump source in which the light is confined entirely within the fiber, without any possibility of misalignment, which can be a problem in typical diode-pumped, solid-state lasers. The OPO is configured as a ring-cavity, singly resonant OPO, with the shorter, signal wavelength resonant. This signal wavelength is held fixed by an intracavity etalon, so that any tuning of the pump source is directly reproduced by the OPO idler wavelength. Hence, rapid mode-hop-free tuning capability with a range of over 50 GHz is available in the spectral output of the mid-IR OPO idler. Similarly, the high spatial and spectral quality of the pump also is transferred to the OPO idler, with diffraction-limited output and linewidth below 1 MHz. Up to 85% of the 1-μm photons are converted to 3-μm photons within the OPO, so power levels of up to 2 W have been measured using only 6 W of pump power (Figure 4).
Figure 4. 2.7-Î¼m OPO power output versus fiber laser power input.
Cavity ring-down spectroscopy (CRDS) is an ideal technology platform for the application of the fiber-pumped OPO. The high sensitivity of the CRDS technique is achieved by coupling laser light into a high finesse Fabry-Perot cavity containing a gas sample (Figure 5). Long effective path-lengths are obtained within the cavity, and absorption is measured by monitoring the time taken for light to leak out of the cavity when the laser is switched off. In the presence of an absorbing species, the "ring-down" time is shortened. The development of the fiber-pumped CW OPO is sponsored by the Optoelectronic Manufacturing group at the National Institute of Standards and Technology (Boulder, CO), as a means to monitor trace impurities in process gases that affect semiconductor manufacture yield. Kris Bertness of NIST has demonstrated a measurement sensitivity of 50 nmol/mol in CRDS measurement of residual moisture in bulk phosphine gas (10). This measurement was made with a near-IR semiconductor laser probing relatively weak combination band transitions of water vapor at 938 nm. The motivation for the development of an OPO operating in the 2.7 μm region has been to allow the same technique to be used while accessing absorption features, which are as much as 100 times stronger (see Figure 1). This potentially can result in a similar increase in detection sensitivity. Such capability can allow rapid, sensitive monitoring of trace water to provide a sensitive measure of real-time variation. This capability should lead ultimately to improvements in semiconductor source gas purity and hence, improved manufacture yield.
Figure 5. Cavity ring-down technique measures the rate of leak-out of light trapped in an optical resonator, containing a gas sample.
Further characteristics of the OPO (Figure 6) make it particularly suited to the cavity ring-down technique. First, power levels are very high relative to diode lasers used typically in the technique — power is measured in 100s of milliwatts to watts. Second, the excellent beam quality enabled by the fiber pump source allows much better mode-matching of the laser into an optical cavity. Better mode matching allows more photons to be coupled into the fundamental transverse mode of the ring-down cavity. This improved coupling efficiency results in higher signal throughput and improved absorption signal-to-noise ratio (S/N), ultimately translating into better measurement precision. The detection sensitivity of the CRDS technique also can be enhanced by the narrow linewidth of the source laser. Because the spectral width of the ring-down cavity resonances is extremely narrow, the fraction of incident photons coupled through the cavity is increased as the linewidth of the laser approaches that of the ring-down cavity resonance. As a result, again the S/N of the detected absorption signal is increased. CW OPO linewidths typically are measured in kilohertz, while typical diode laser linewidths are in the megahertz range.
Figure 6. Fiber-pumped CW OPO provides narrow linewidth, wide tunability, and diffraction-limited beam quality.
Third, the broad tuning range permits the selection of strong impurity absorption lines in regions of the lowest interference from the host gas, and increases the likelihood that the same system can be used to monitor multiple impurities. In monitoring moisture in phosphine, for example, the ultimate sensitivity limits are set by the ability to reliably account for the phosphine off-resonant absorption line background. The frequency-stabilized cavity ring-down apparatus used at NIST Boulder was developed originally by colleagues at NIST Gaithersburg (11). Unlike conventional implementations of CRDS, this approach uses the stabilized frequency comb of the ring-down cavity to provide frequency markers, thus enabling low-uncertainty determination of line intensities, and high-resolution measurements of line shapes, as well as measurements of pressure shifting and Doppler-free saturation effects. Project leader Joseph T. Hodges also has worked with the specialty gas industry to perform characterization of the performance of a commercial near-IR-based ring-down system (12) in similar moisture concentration measurements (13). According to Hodges, "tunable single-frequency mid-IR CW laser sources are critical to the realization of high-sensitivity absorption spectroscopy of residual species in gas mixtures. Coupling such spectroscopic-grade light sources with cavity-enhanced and high-spectral resolution absorption methods like CRDS, can enable quantitative gas sensing of trace analytes by discriminating weak analyte absorption from background absorption interferences associated with bulk gases."
Under the NIST development program, initial demonstrations of the OPO in spectroscopic measurement, including the use of cavity-enhanced techniques, have been performed. Collaborative experiments have been performed with Professor Houston Miller of the department of Chemistry at George Washington University (GWU), Washington, DC. The Miller group at GWU has applied cavity enhanced absorption techniques to the detection of trace constituents of air as well as to the development of combustion diagnostics. For applications in this field, the operation of the OPO in the 3–4 μm band is advantageous due to the existence of the fundamental "C–H stretch" vibration in this wavelength region, which allows very sensitive detection of many hydrocarbons and combustion products. In preliminary experiments performed at Aculight (Bothell, WA), the tuning capability of the OPO was illustrated (Figure 7) by generating single-pass spectra of carbon dioxide at 2810 nm (~3559 cm-1 ) and methane at 3167 nm (~3158 cm-1 ). Demonstration of efficient coupling into a ring-down cavity also was performed at the latter wavelength. By scanning the laser frequency over a free spectral range of the ring-down cavity, it was demonstrated that the linewidth of the OPO at 3167 nm was less than 1 MHz (Figure 6). Based upon the sensitivity of previous near-IR ring-down measurements at GWU, we anticipate potential detection sensitivities in the sub-part-per-billion regime using the high absorption strengths at this wavelength. This continuing collaborative work is expected to realize the potential for versatile, ultrasensitive, multi-species trace gas detection for a wide variety of applications.
Figure 7. Single-pass absorption spectra for 60-cm pathlength of CO2 (5%, 50 torr) and CH4 (5%, 30 torr) measured using fiber-pumped OPO. Symbols: experimental data; lines: fits to HITRAN 2004.
It is anticipated that future directions in the development of this technology will include expansion of the wavelength coverage to wavelengths as long as 10 μm using novel nonlinear materials such as optically patterned gallium arsenide (OP-GaAs). OPOs using this material already have demonstrated wavelengths throughout this range using pulsed pumping. Development of more sophisticated electronic controls will provide capabilities for frequency locking, stabilization, and automated tuning, allowing application of the devices to a wider range of spectroscopic techniques. Finally, further investment producing reduction in cost might be the most important factor, enabling the use of OPOs in a wider range of real-world sensor applications beyond initial adoption as a laboratory spectroscopy tool.
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Angus Henderson is principal scientist, and Ryan Stafford is development engineer, at Aculight Corporation (Bothell, WA). J. Houston Miller is with the Department of Chemistry, George Washington University (Washington DC).