Improved repetition rate mixed isotope CO2 TEA laser D. B. Cohn Citation: Review of Scientific Instruments 85, 094710 (2014); doi: 10.1063/1.4896331 View online: http://dx.doi.org/10.1063/1.4896331 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in CO2 laser-based dispersion interferometer utilizing orientation-patterned gallium arsenide for plasma density measurements Rev. Sci. Instrum. 84, 093502 (2013); 10.1063/1.4819028 Precise measurements of the total concentration of atmospheric C O 2 and C 13 O 2 ∕ C 12 O 2 isotopic ratio using a lead-salt laser diode spectrometer Rev. Sci. Instrum. 79, 043101 (2008); 10.1063/1.2902829 Noncontact infrared laser sensing of magnetoresistance Rev. Sci. Instrum. 79, 023901 (2008); 10.1063/1.2839024 CO 2 laser imaging interferometer on LHD Rev. Sci. Instrum. 72, 1089 (2001); 10.1063/1.1323247 Dual CO 2 laser polarimeter for Faraday rotation measurement in tokamak plasmas Rev. Sci. Instrum. 70, 714 (1999); 10.1063/1.1149470
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 094710 (2014)
Improved repetition rate mixed isotope CO2 TEA laser D. B. Cohna) DBC Technology Corp., 4221 Mesa St, Torrance, California 90505, USA
(Received 17 July 2014; accepted 11 September 2014; published online 29 September 2014) A compact CO2 TEA laser has been developed for remote chemical detection that operates at a repetition rate of 250 Hz. It emits 700 mJ/pulse at 10.6 μm in a multimode beam with the 12 C16 O2 isotope. With mixed 12 C16 O2 plus 13 C16 O2 isotopes it emits multiple lines in both isotope manifolds to improve detection of a broad range of chemicals. In particular, output pulse energies are 110 mJ/pulse at 9.77 μm, 250 mJ/pulse at 10 μm, and 550 mJ/pulse at 11.15 μm, useful for detection of the chemical agents Sarin, Tabun, and VX. Related work shows capability for long term sealed operation with a catalyst and an agile tuner at a wavelength shift rate of 200 Hz. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4896331] I. INTRODUCTION
Active remote detection of chemicals in the atmosphere promises to be a valuable technique for monitoring industrial processes and for early warning of agent dispersal in military scenarios.1 For moderate to long range detection, the CO2 TEA (Transversely Excited Atmospheric) laser has proven to be the transmitter of choice owing to its multi-line, high peak power output in the long wavelength region of the spectrum where many industrial chemicals and chemical agents have significant absorption features. More recently, sensing of chemicals on surfaces with sensors using long wave infrared lasers has been shown to be feasible.2 Figure 1 is a representation of some important chemical agent absorption spectra and the CO2 emission lines for laser gas mixtures with the 12 C16 O2 and 13 C16 O2 isotopes.3 Output with gas mixtures using the normal 12 C16 O2 isotope takes place on roughly 60 lines in four bands over the 9.3–10.7 μm spectral region; however, clefts between the bands can impair chemical sensing, most notably in the 9.8–10.1 μm region. For this reason, the alternative isotope 13 C16 O2 is often considered because it adds bands centered around 10 μm, 10.75 μm, and 11.15 μm, thereby significantly improving detection capability, as in the case of the chemical agents Sarin, Tabun, and VX. The band designations in the figure refer to the 12 C16 O2 V-R manifold. The four bands for the 13 C16 O2 V-R manifold can be clearly distinguished except for the significant overlap region around 9.7 μm. In order to realize the advantage of multiple isotope emission in a practical sensor system it would be desirable to have it available simultaneously from gases mixed within the same laser. The mixed isotope, single laser approach also minimizes transmitter size and weight, an important consideration for many applications. The relatively high cost of 13 C16 O2 suggests that the TEA laser be operated as a sealed unit. In that case, a catalyst is needed to reconstitute CO2 dissociated in the discharge process to achieve acceptable runtimes.4 This is especially true in the case of compact lasers with reduced reservoirs of gas and for operation at high pulse repetition a) Electronic mail: [email protected].
0034-6748/2014/85(9)/094710/4/$30.00
frequency (PRF) required for the high data rates typical of practical operations in the field.5–7 Willis et al. reported sealed operation with a relatively large Lumonics Model 101 commercial laser.8 It was operated with mixtures of CO2 :N2 :He = 1:1:10.5 where the CO2 component was made up of 50% 13 C16 O2 and 50% 12 C16 O2 . They found that output for mixed isotopes was significantly lower at the band edges than for mixtures with single pure isotopes. Maximum pulse energy at 10 μm was about 600 mJ for pure 13 C16 O2 and mixed 13 C16 O2 plus 12 C16 O2 isotopes, and it was about 1 J at 10.6 μm with 12 C16 O2 . The gas recirculation system was composed of an external flow loop housing a heated platinum-on-alumina catalyst. The laser was capable of a maximum PRF of 2.5 Hz. Fox and Ahl reported operation of a small laser with 20 cm3 gain volume for separate isotope mixtures of CO2 :N2 :He = 1.5:1:10.9 They observed 80 mJ maximum with 12 C16 O2 and 55 mJ at 10 μm with 13 C16 O2 . A 50/50 mixture of the two isotopes was found to significantly reduce gain and the number of emitted lines. The laser was excited by an LC inverter network using a static sparkgap high voltage switch which presumably limited PRF to 1–5 Hz. The use of a catalyst was not indicated. A small laser with 45 cm3 gain volume was described by Zhang et al.10 They report a maximum pulse energy for a CO2 :N2 :He = 1:1:3 mixture of about 350 mJ with 12 C16 O2 and 125 mJ at 10 μm with 13 C16 O2 . They did not report mixed isotope results. Maximum PRF was 10 Hz. The use of a catalyst was not indicated. Our work with isotope mixtures began with the FAL (Frequency Agile Laser) designed originally for operation with 12 16 C O2 .5 That relatively compact laser had a gain length of 28 cm in a sealed vessel with internal catalyst located in the main gas flow loop, and discharge specific energy loading was 240 J/l-atm. Maximum laser pulse rate and wavelength shift rate with the agile grating tuner was 200 Hz. The catalyst was capable of sustaining laser operation for several weeks, limited by small air leaks in the vessel. Multimode output with 12 16 C O2 for the benchmark 10P20 and 9P44 lines was on the order of 200 mJ and 100 mJ, respectively. With mixed isotopes, it was found necessary to use two FAL laser modules
85, 094710-1
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094710-2
D. B. Cohn
FIG. 1. Representative chemical agent absorption spectra and CO2 laser emission lines. Band designations refer to the 12 C16 O2 V-R manifold. The 13 C16 O isotope lines shown dotted. 2
within a single power oscillator to achieve acceptable output in the critical 9P44 and 10 μm regions of the spectrum; but the resulting device was deemed too large for practical applications, and more compact designs were investigated. We wish to report here results of development of a compact, 40 cm3 gain volume TEA laser. It operates with mixed 12 16 C O2 and 13 C16 O2 isotopes and emits multiple transverse mode pulse energies of 110 mJ at the 12 C16 O2 , 9P44 wavelength, 250 mJ at 10 μm, and 545 mJ at 11.1 μm with a power supply limited PRF of 250 Hz. II. APPARATUS
The laser discharge module is composed of two 42 cm long parallel electrodes separated by 1 cm to define a 40 × 1 × 1 cm3 gain volume and it represents a 50% increase in gain length compared to the previous FAL design. Note that the gain length is less than the electrode length because of field shaping curvature at the ends of the electrodes. Main discharge preionization is provided by uv from an auxiliary discharge formed around pyrex tubes with center conductors located adjacent to the grounded electrode on either side and with the conductor attached to the opposing high voltage electrode. The preionization discharge is powered directly by the main discharge circuit. A 6 cm diameter tangential fan rotating at 4000 rpm provides for gas flow across the electrodes and through the internal catalyst module and water cooled heat exchanger. The discharge module is housed in a sealed aluminum vessel with dimensions of 20 cm width, 28 cm height, and 51 cm length. All laser internal structures are made of materials compatible with long term storage with a catalyst. No acrylics or other plastics were used. Beam extraction from the vessel is through ZnSe Brewster windows.
Rev. Sci. Instrum. 85, 094710 (2014)
Pulsed discharge power is provided by an inverting capacitive discharge circuit switched through a hydrogen thyratron.5 Effort was made to minimize circuit inductance in the interest of achieving arc-free high pressure operation. An array of 12, 2700 nF ceramic capacitors is attached between parallel plates and connected to the high voltage electrode by 10 insulated feedthroughs. A series of 10 cylindrical electrode spacers located on either side of the grounded electrode provide for a symmetric current return with low transverse gas flow impedance. The thyratron is mounted in a closely fitting coaxial cylindrical housing with end mounted cooling fan. High voltage capacitor charging is achieved through a constant current 2 kJ/s capacitor charging supply. With the tangential fan rotating at 4000 rpm, maximum discharge PRF is 200 Hz, limited by the high voltage power supply capacitor charging capability. At a charge voltage of 20 kV maximum PRF is 250 Hz. Arc-free operation is obtained at 25 kV charge voltage and 800 Torr fill pressure giving a specific discharge energy loading of 240 J/l-atm. A gas mixing manifold system provided for testing various mixture ratios. The ratio CO2 :N2 :He = 1:1:3 was found to be optimum in terms of maximizing output energy with long term discharge stability. In the case of mixtures of isotopes, the CO2 content was divided equally between 12 C16 O2 and 13 16 C O2 . For all of the tests reported here, fill pressure was 800 Torr. The optical resonator was designed to optimize extraction efficiency, defined as the ratio of optical mode to gain cross section, while resisting misalignment by component movement due to thermal effects. The ZnSe output coupler has a 10 m radius of curvature and 87% reflectivity at 10 μm. The 10 m radius allows for filling of the gain volume combined with ease of alignment and long term resonator stability. At the other end of the 78 cm long resonator is a 150 line/mm grating of ruled gold on invar blazed for 10 μm. This combination of optical parameters gives a multiple transverse mode across the 1 × 1 cm output beam at the coupler.
III. OUTPUT CHARACTERISTICS
Initial benchmark testing was conducted with the normal 12 C16 O2 isotope. Pulse energy measurements were made using a Gentec Joulemeter Model QE25LP-S-MB with calibration that was revalidated by the company. Line assignments were made with a Macken Instruments spectrum analyzer, Model 16A with resolution of 0.003 μm. The laser was operated at a 1 Hz PRF. Output on representative lines is shown in Table I. Efficiency refers to the ratio of output pulse energy to discharge energy stored in the capacitors. Multimode output on strong lines at the center of the bands exceeds 0.5 J. Useful output is obtained on the 9P44-52 lines at the long wavelength edge of the 9P branch, where as shown in Figure 1, emission is favored for detection of some important chemical agents. These results represent a significant improvement over those with the previous FAL owing to the increase in gain length. The increased gain is also responsible for appearance of the useful 9P46-52 lines that were not emitted with FAL.
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094710-3
D. B. Cohn
Rev. Sci. Instrum. 85, 094710 (2014)
TABLE I. Output energy for selected lines, 12 C16 O2 isotope.
Line
Output (mJ)
Efficiency (%)
9R20 9P20 9P44 9P46 9P48 9P50 9P52 10R20 10P20
513 562 343 276 212 119 18 644 700
5.1 5.6 3.4 2.7 2.1 1.2 0.2 6.4 6.9
Output energy versus wavelength was obtained for mixed C16 O2 and 13 C16 O2 isotopes. The results are shown in Figure 2. The resolution of the spectrometer and its readout did not have enough precision to differentiate between many of the lines from the two isotopes in the overlap regions; therefore, only representative lines were obtained in order to determine the form of the emission bands. Not all available lines are shown. Bars at the bottom of the graph indicate regions of spectral overlap for the two isotopes as represented in Figure 1. The band designations refer to the 12 C16 O2 V-R manifold. In general, output on the lines clearly attributable to 12 16 C O2 is reduced by about half compared to the benchmark case of 12 C16 O2 only mixtures. In the overlap regions, there are energy outliers at 9.62 μm and 10.65 μm that appeared in repeated testing which may be due to the addition of several lines that could not be resolved with the laser cavity grating. Willis et al. mention the close coincidence by 0.09 cm−1 of two lines around 10.65 μm. Lines at positions 9P46-52 which had been observed with the single 12 C16 O2 isotope are lost due to the reduced gain with mixed isotopes. Energy output is equivalent on the 12 C16 O2 9R and 13 C16 O2 lines around 10 μm. The highest output is on the 13 C16 O2 lines centered at 10.8 μm and 11.15 μm. Measurement of the two-dimensional far field beam mode profile was made at the focus of a 1 m focal length lens with a Spiricon Pyrocam III infrared camera. The pro12
FIG. 3. Transverse mode profile at 10R20, stable TEM11 .
file for the 10P20 line is shown in Figure 3. It is a four-lobe TEM11 pattern that was found to be stable shot-to-shot. The depth of modulation determined from the superimposed intensity curves is about 25%. Divergence, taken as the ratio of beam diameter at the 1/e2 intensity points to lens focal length, is 3.5 mrad vertical and 4.6 mrad horizontal. In prior measurements with two FAL modules in series within a 143 cm resonator, single mode output with 1.56 mrad divergence was obtained with no loss in output energy compared to output scaled from a single module. No attempt was made to achieve single mode output with the present laser by extending the resonator length. Waveform measurements were made with a TE-cooled, PV HgCdTe detector placed at the exit of a 5 cm diameter gold plated integrating sphere. Typical traces are shown in Figure 4 for (a) the line at 10 μm attributable to the 13 C16 O2 isotope and (b) the 10R20 line attributable to 12 C16 O2 . The monotonically rising line in each trace is the pulse integral. Bandwidth of the detection system was limited to 20 MHz to eliminate mode beating modulation and improve observation of the pulse envelope. The signal amplitudes for each line were adjusted individually to maximize their display on the oscilloscope. Based on the half energy level of the integral, the full-width-half maximum for the 10 μm and 10R20 pulses is 180 ns and 150 ns, respectively. IV. DISCUSSION
FIG. 2. Output energy versus wavelength for mixed isotopes.
Development of the original FAL laser involved extensive sealed long term testing with a catalyst type formulated by Phillips Petroleum, now Chevron-Phillips. Testing of this type in comparison to others then available is reported by Lewis et al. The laser reported here was designed with an internal catalyst module and wire preheater through which the entire high speed gas flow passes before flowing through the heat exchanger. The internal module was adopted as opposed to an external flow loop in order to minimize size and complexity of the system. In experiments with a laser system mockup, the catalyst volume and activity level were validated for an oxygen level anticipated at a PRF of 200 Hz. The heat exchanger set at a water cooling temperature of 18 ◦ C was found to
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094710-4
D. B. Cohn
Rev. Sci. Instrum. 85, 094710 (2014)
FIG. 4. Intensity versus time waveforms at wavelengths of (a) 10 μm for 13 C16 O2 and (b) 10R20 for 12 C16 O2 .
maintain the gas between the electrodes at a temperature of 25 ◦ C for a catalyst gas temperature of 50 ◦ C. The low inductance discharge module design of the present laser was used previously in a separate device to achieve arc-free operation up to 1500 Torr; however, it was necessary to reduce the electrode separation (and gain volume) to accommodate the power supply voltage limit. Energy extraction experiments showed no advantage for operation above about 800 Torr; therefore, 800 Torr was taken as the maximum pressure in the laser reported here. The laser is scalable in output energy with increase in the electrode separation, charge voltage, and a modest increase in overall size. ACKNOWLEDGMENTS
This work is based on laser development and field test efforts under several U.S. Army contracts administered by the
Edgewood Chemical Biological Center. The author is grateful for the many helpful discussions with their technical team. 1 M.
W. P. Petryk, Appl. Spectrosc. Rev. 42, 287 (2007). K. Goyal, M. Spencer, M. Kelly, J. Costa, M. DiLiberto, E. Meyer, and T. Jeys, Proc. SPIE 8018, 80180N-1 (2011). 3 S. W. Sharpe, T. J. Johnson, P. M. Chu, J. Kleimeyer and B. Rowland, Proc. SPIE 5085, 19 (2003). 4 P. Lewis, J. Fortino, P. Gozewski, F. Faria-e-Maia, A. Doucette, and S. Brown, Proc. SPIE 2702, 385 (1996). 5 D. B. Cohn, J. A. Fox, and C. R. Swim, Proc. SPIE 2118, 72 (1994). 6 D. B. Cohn, W. S. Griffin, L. F. Klaras, E. J. Griffin, H. C. Marciniak, J. A. Fox, and C. R. Swim, Proc. SPIR 4036, 210 (2000). 7 R. E. Warren, R. G. Vanderbeek, and J. L. Ahl, Proc. SPIE 7665, 766504 (2010). 8 C. Willis, P. A. Hackett, and J. M. Parsons, Rev. Sci. Instrum. 50, 1141 (1979). 9 J. Fox and J. L. Ahl, J. Appl. Phys. 61, 2403 (1987). 10 L. L. Zhang, Y. C. Qu, W. J. Zhao, D. M. Reng, and X. Y. Hu, Laser Phys. 18, 1021 (2008). 2 A.
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 094710 (2014)
Improved repetition rate mixed isotope CO2 TEA laser D. B. Cohna) DBC Technology Corp., 4221 Mesa St, Torrance, California 90505, USA
(Received 17 July 2014; accepted 11 September 2014; published online 29 September 2014) A compact CO2 TEA laser has been developed for remote chemical detection that operates at a repetition rate of 250 Hz. It emits 700 mJ/pulse at 10.6 μm in a multimode beam with the 12 C16 O2 isotope. With mixed 12 C16 O2 plus 13 C16 O2 isotopes it emits multiple lines in both isotope manifolds to improve detection of a broad range of chemicals. In particular, output pulse energies are 110 mJ/pulse at 9.77 μm, 250 mJ/pulse at 10 μm, and 550 mJ/pulse at 11.15 μm, useful for detection of the chemical agents Sarin, Tabun, and VX. Related work shows capability for long term sealed operation with a catalyst and an agile tuner at a wavelength shift rate of 200 Hz. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4896331] I. INTRODUCTION
Active remote detection of chemicals in the atmosphere promises to be a valuable technique for monitoring industrial processes and for early warning of agent dispersal in military scenarios.1 For moderate to long range detection, the CO2 TEA (Transversely Excited Atmospheric) laser has proven to be the transmitter of choice owing to its multi-line, high peak power output in the long wavelength region of the spectrum where many industrial chemicals and chemical agents have significant absorption features. More recently, sensing of chemicals on surfaces with sensors using long wave infrared lasers has been shown to be feasible.2 Figure 1 is a representation of some important chemical agent absorption spectra and the CO2 emission lines for laser gas mixtures with the 12 C16 O2 and 13 C16 O2 isotopes.3 Output with gas mixtures using the normal 12 C16 O2 isotope takes place on roughly 60 lines in four bands over the 9.3–10.7 μm spectral region; however, clefts between the bands can impair chemical sensing, most notably in the 9.8–10.1 μm region. For this reason, the alternative isotope 13 C16 O2 is often considered because it adds bands centered around 10 μm, 10.75 μm, and 11.15 μm, thereby significantly improving detection capability, as in the case of the chemical agents Sarin, Tabun, and VX. The band designations in the figure refer to the 12 C16 O2 V-R manifold. The four bands for the 13 C16 O2 V-R manifold can be clearly distinguished except for the significant overlap region around 9.7 μm. In order to realize the advantage of multiple isotope emission in a practical sensor system it would be desirable to have it available simultaneously from gases mixed within the same laser. The mixed isotope, single laser approach also minimizes transmitter size and weight, an important consideration for many applications. The relatively high cost of 13 C16 O2 suggests that the TEA laser be operated as a sealed unit. In that case, a catalyst is needed to reconstitute CO2 dissociated in the discharge process to achieve acceptable runtimes.4 This is especially true in the case of compact lasers with reduced reservoirs of gas and for operation at high pulse repetition a) Electronic mail: [email protected].
0034-6748/2014/85(9)/094710/4/$30.00
frequency (PRF) required for the high data rates typical of practical operations in the field.5–7 Willis et al. reported sealed operation with a relatively large Lumonics Model 101 commercial laser.8 It was operated with mixtures of CO2 :N2 :He = 1:1:10.5 where the CO2 component was made up of 50% 13 C16 O2 and 50% 12 C16 O2 . They found that output for mixed isotopes was significantly lower at the band edges than for mixtures with single pure isotopes. Maximum pulse energy at 10 μm was about 600 mJ for pure 13 C16 O2 and mixed 13 C16 O2 plus 12 C16 O2 isotopes, and it was about 1 J at 10.6 μm with 12 C16 O2 . The gas recirculation system was composed of an external flow loop housing a heated platinum-on-alumina catalyst. The laser was capable of a maximum PRF of 2.5 Hz. Fox and Ahl reported operation of a small laser with 20 cm3 gain volume for separate isotope mixtures of CO2 :N2 :He = 1.5:1:10.9 They observed 80 mJ maximum with 12 C16 O2 and 55 mJ at 10 μm with 13 C16 O2 . A 50/50 mixture of the two isotopes was found to significantly reduce gain and the number of emitted lines. The laser was excited by an LC inverter network using a static sparkgap high voltage switch which presumably limited PRF to 1–5 Hz. The use of a catalyst was not indicated. A small laser with 45 cm3 gain volume was described by Zhang et al.10 They report a maximum pulse energy for a CO2 :N2 :He = 1:1:3 mixture of about 350 mJ with 12 C16 O2 and 125 mJ at 10 μm with 13 C16 O2 . They did not report mixed isotope results. Maximum PRF was 10 Hz. The use of a catalyst was not indicated. Our work with isotope mixtures began with the FAL (Frequency Agile Laser) designed originally for operation with 12 16 C O2 .5 That relatively compact laser had a gain length of 28 cm in a sealed vessel with internal catalyst located in the main gas flow loop, and discharge specific energy loading was 240 J/l-atm. Maximum laser pulse rate and wavelength shift rate with the agile grating tuner was 200 Hz. The catalyst was capable of sustaining laser operation for several weeks, limited by small air leaks in the vessel. Multimode output with 12 16 C O2 for the benchmark 10P20 and 9P44 lines was on the order of 200 mJ and 100 mJ, respectively. With mixed isotopes, it was found necessary to use two FAL laser modules
85, 094710-1
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094710-2
D. B. Cohn
FIG. 1. Representative chemical agent absorption spectra and CO2 laser emission lines. Band designations refer to the 12 C16 O2 V-R manifold. The 13 C16 O isotope lines shown dotted. 2
within a single power oscillator to achieve acceptable output in the critical 9P44 and 10 μm regions of the spectrum; but the resulting device was deemed too large for practical applications, and more compact designs were investigated. We wish to report here results of development of a compact, 40 cm3 gain volume TEA laser. It operates with mixed 12 16 C O2 and 13 C16 O2 isotopes and emits multiple transverse mode pulse energies of 110 mJ at the 12 C16 O2 , 9P44 wavelength, 250 mJ at 10 μm, and 545 mJ at 11.1 μm with a power supply limited PRF of 250 Hz. II. APPARATUS
The laser discharge module is composed of two 42 cm long parallel electrodes separated by 1 cm to define a 40 × 1 × 1 cm3 gain volume and it represents a 50% increase in gain length compared to the previous FAL design. Note that the gain length is less than the electrode length because of field shaping curvature at the ends of the electrodes. Main discharge preionization is provided by uv from an auxiliary discharge formed around pyrex tubes with center conductors located adjacent to the grounded electrode on either side and with the conductor attached to the opposing high voltage electrode. The preionization discharge is powered directly by the main discharge circuit. A 6 cm diameter tangential fan rotating at 4000 rpm provides for gas flow across the electrodes and through the internal catalyst module and water cooled heat exchanger. The discharge module is housed in a sealed aluminum vessel with dimensions of 20 cm width, 28 cm height, and 51 cm length. All laser internal structures are made of materials compatible with long term storage with a catalyst. No acrylics or other plastics were used. Beam extraction from the vessel is through ZnSe Brewster windows.
Rev. Sci. Instrum. 85, 094710 (2014)
Pulsed discharge power is provided by an inverting capacitive discharge circuit switched through a hydrogen thyratron.5 Effort was made to minimize circuit inductance in the interest of achieving arc-free high pressure operation. An array of 12, 2700 nF ceramic capacitors is attached between parallel plates and connected to the high voltage electrode by 10 insulated feedthroughs. A series of 10 cylindrical electrode spacers located on either side of the grounded electrode provide for a symmetric current return with low transverse gas flow impedance. The thyratron is mounted in a closely fitting coaxial cylindrical housing with end mounted cooling fan. High voltage capacitor charging is achieved through a constant current 2 kJ/s capacitor charging supply. With the tangential fan rotating at 4000 rpm, maximum discharge PRF is 200 Hz, limited by the high voltage power supply capacitor charging capability. At a charge voltage of 20 kV maximum PRF is 250 Hz. Arc-free operation is obtained at 25 kV charge voltage and 800 Torr fill pressure giving a specific discharge energy loading of 240 J/l-atm. A gas mixing manifold system provided for testing various mixture ratios. The ratio CO2 :N2 :He = 1:1:3 was found to be optimum in terms of maximizing output energy with long term discharge stability. In the case of mixtures of isotopes, the CO2 content was divided equally between 12 C16 O2 and 13 16 C O2 . For all of the tests reported here, fill pressure was 800 Torr. The optical resonator was designed to optimize extraction efficiency, defined as the ratio of optical mode to gain cross section, while resisting misalignment by component movement due to thermal effects. The ZnSe output coupler has a 10 m radius of curvature and 87% reflectivity at 10 μm. The 10 m radius allows for filling of the gain volume combined with ease of alignment and long term resonator stability. At the other end of the 78 cm long resonator is a 150 line/mm grating of ruled gold on invar blazed for 10 μm. This combination of optical parameters gives a multiple transverse mode across the 1 × 1 cm output beam at the coupler.
III. OUTPUT CHARACTERISTICS
Initial benchmark testing was conducted with the normal 12 C16 O2 isotope. Pulse energy measurements were made using a Gentec Joulemeter Model QE25LP-S-MB with calibration that was revalidated by the company. Line assignments were made with a Macken Instruments spectrum analyzer, Model 16A with resolution of 0.003 μm. The laser was operated at a 1 Hz PRF. Output on representative lines is shown in Table I. Efficiency refers to the ratio of output pulse energy to discharge energy stored in the capacitors. Multimode output on strong lines at the center of the bands exceeds 0.5 J. Useful output is obtained on the 9P44-52 lines at the long wavelength edge of the 9P branch, where as shown in Figure 1, emission is favored for detection of some important chemical agents. These results represent a significant improvement over those with the previous FAL owing to the increase in gain length. The increased gain is also responsible for appearance of the useful 9P46-52 lines that were not emitted with FAL.
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094710-3
D. B. Cohn
Rev. Sci. Instrum. 85, 094710 (2014)
TABLE I. Output energy for selected lines, 12 C16 O2 isotope.
Line
Output (mJ)
Efficiency (%)
9R20 9P20 9P44 9P46 9P48 9P50 9P52 10R20 10P20
513 562 343 276 212 119 18 644 700
5.1 5.6 3.4 2.7 2.1 1.2 0.2 6.4 6.9
Output energy versus wavelength was obtained for mixed C16 O2 and 13 C16 O2 isotopes. The results are shown in Figure 2. The resolution of the spectrometer and its readout did not have enough precision to differentiate between many of the lines from the two isotopes in the overlap regions; therefore, only representative lines were obtained in order to determine the form of the emission bands. Not all available lines are shown. Bars at the bottom of the graph indicate regions of spectral overlap for the two isotopes as represented in Figure 1. The band designations refer to the 12 C16 O2 V-R manifold. In general, output on the lines clearly attributable to 12 16 C O2 is reduced by about half compared to the benchmark case of 12 C16 O2 only mixtures. In the overlap regions, there are energy outliers at 9.62 μm and 10.65 μm that appeared in repeated testing which may be due to the addition of several lines that could not be resolved with the laser cavity grating. Willis et al. mention the close coincidence by 0.09 cm−1 of two lines around 10.65 μm. Lines at positions 9P46-52 which had been observed with the single 12 C16 O2 isotope are lost due to the reduced gain with mixed isotopes. Energy output is equivalent on the 12 C16 O2 9R and 13 C16 O2 lines around 10 μm. The highest output is on the 13 C16 O2 lines centered at 10.8 μm and 11.15 μm. Measurement of the two-dimensional far field beam mode profile was made at the focus of a 1 m focal length lens with a Spiricon Pyrocam III infrared camera. The pro12
FIG. 3. Transverse mode profile at 10R20, stable TEM11 .
file for the 10P20 line is shown in Figure 3. It is a four-lobe TEM11 pattern that was found to be stable shot-to-shot. The depth of modulation determined from the superimposed intensity curves is about 25%. Divergence, taken as the ratio of beam diameter at the 1/e2 intensity points to lens focal length, is 3.5 mrad vertical and 4.6 mrad horizontal. In prior measurements with two FAL modules in series within a 143 cm resonator, single mode output with 1.56 mrad divergence was obtained with no loss in output energy compared to output scaled from a single module. No attempt was made to achieve single mode output with the present laser by extending the resonator length. Waveform measurements were made with a TE-cooled, PV HgCdTe detector placed at the exit of a 5 cm diameter gold plated integrating sphere. Typical traces are shown in Figure 4 for (a) the line at 10 μm attributable to the 13 C16 O2 isotope and (b) the 10R20 line attributable to 12 C16 O2 . The monotonically rising line in each trace is the pulse integral. Bandwidth of the detection system was limited to 20 MHz to eliminate mode beating modulation and improve observation of the pulse envelope. The signal amplitudes for each line were adjusted individually to maximize their display on the oscilloscope. Based on the half energy level of the integral, the full-width-half maximum for the 10 μm and 10R20 pulses is 180 ns and 150 ns, respectively. IV. DISCUSSION
FIG. 2. Output energy versus wavelength for mixed isotopes.
Development of the original FAL laser involved extensive sealed long term testing with a catalyst type formulated by Phillips Petroleum, now Chevron-Phillips. Testing of this type in comparison to others then available is reported by Lewis et al. The laser reported here was designed with an internal catalyst module and wire preheater through which the entire high speed gas flow passes before flowing through the heat exchanger. The internal module was adopted as opposed to an external flow loop in order to minimize size and complexity of the system. In experiments with a laser system mockup, the catalyst volume and activity level were validated for an oxygen level anticipated at a PRF of 200 Hz. The heat exchanger set at a water cooling temperature of 18 ◦ C was found to
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094710-4
D. B. Cohn
Rev. Sci. Instrum. 85, 094710 (2014)
FIG. 4. Intensity versus time waveforms at wavelengths of (a) 10 μm for 13 C16 O2 and (b) 10R20 for 12 C16 O2 .
maintain the gas between the electrodes at a temperature of 25 ◦ C for a catalyst gas temperature of 50 ◦ C. The low inductance discharge module design of the present laser was used previously in a separate device to achieve arc-free operation up to 1500 Torr; however, it was necessary to reduce the electrode separation (and gain volume) to accommodate the power supply voltage limit. Energy extraction experiments showed no advantage for operation above about 800 Torr; therefore, 800 Torr was taken as the maximum pressure in the laser reported here. The laser is scalable in output energy with increase in the electrode separation, charge voltage, and a modest increase in overall size. ACKNOWLEDGMENTS
This work is based on laser development and field test efforts under several U.S. Army contracts administered by the
Edgewood Chemical Biological Center. The author is grateful for the many helpful discussions with their technical team. 1 M.
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