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Extremely high sensitivity gas detection at 2.3 μm using a grazing incidence Tm3+ fibre laser cavity
Sensors and Actuators A 87 (2001) 107±112
Extremely high sensitivity gas detection at 2.3 mm using a grazing incidence Tm3 ®bre laser cavity F.J. McAleaveya, J. O'Gormana, J.F. Doneganb,*, J. Hegartyb, G. MazeÂc a
Optronics Ireland, Trinity College, Dublin 2, Ireland Department of Physics, Trinity College, Dublin 2, Ireland c Le Verre FluoreÂ, Campus Ker Lann, F-35170, Bruz, Brittany, France b
Received 26 July 1999; received in revised form 24 January 2000; accepted 29 February 2000
Abstract Tm3, when doped into ¯uoride ®bre produces a broad emission centred around wavelength, l 2:3 mm in the near infra-red. When placed within a laser cavity and pumped at l 790 nm, tunable laser action is observed. Using a grazing incidence laser cavity arrangement, a wavelength tuning range Dl 130 nm was achieved along with a very narrow emission linewidth Dn 207 MHz. We have used this laser as part of a gas sensing arrangement and have demonstrated an extremely high gas detection sensitivity of better than 50 ppm m using CH4 as a representative hydrocarbon gas. This system has the potential for use as part of a high sensitivity gas detection system for hydrocarbons and also for CO and HF which exhibit optical absorption in the spectral region around l 2:3 mm. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Fibre optical sensors; Gas sensor; Light emission
1. Introduction Thulium in a ¯uorozirconate glass host was ®rst reported [1] as a laser in 1988, subsequently most of the transitions were shown to lase and be wavelength tunable [2]. Of the several tunable transitions we have concentrated on the region around l 2:3 mm in the near infra-red. This transition does not produce laser action in Tm3 doped silica ®bre due to multiphonon emission that limits the lifetime of the excited state coupled with the strong infra-red absorption transitions in the glass host material. However, the ¯uoride ®bre has high transmission out to 5 mm and therefore longer wavelength lasers based on rare-earth ions can be realised. Laser-based absorption spectroscopy has signi®cant advantages for high sensitivity trace gas detection with high species selectivity [3]. Ideally, the laser sources would be in the range of about 5±10 mm where the fundamental and strongest absorption occurs. While research is continuing on developing semiconductor sources for this wavelength range such as quantum cascade lasers [4], these do not presently work at room temperature and consequently deployment of tunable laser based sensors has concentrated to date on gas overtone absorptions in the near infra-red. In this paper, we * Corresponding author. Tel.: 353-1-608-1675; fax: 353-1-671-1759. E-mail address: [email protected] (J.F. Donegan).
report on the development of a narrow linewidth, tunable Tm3 ®bre laser operating at l 2:3 mm for optical gas sensing purposes and investigate grazing incidence gratingloaded laser cavity characteristics in order to explore the highest sensitivity that can be achieved with this laser arrangement. 2. Grazing incidence laser cavity The laser used here is a grazing incidence [5] (sometimes referred to as a Littman/Metcalf) grating-loaded cavity arrangement, as shown in Fig. 1. In this double pass arrangement the diffraction grating functions as both the wavelength-selective element and the output coupler. The highly dispersive grating diffracts only a very narrow spectrum of light back into the ®bre, due to the grazing incidence con®guration and hence the large number of grating lines ®lled. The spectrally narrow feedback forces the multilongitudinal mode ®bre laser to oscillate with a dramatically reduced linewidth, yet with broad tunability over the ®bre laser gain spectrum. A 1.3 m ®bre length was used in this study with a concentration of 2000 ppm of Tm3. The ®bre ends were cleaved and the pump input side was butt-coupled to a high re¯ectivity mirror with index matching gel to minimise
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 0 ) 0 0 4 7 7 - 5
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Fig. 1. Schematic diagram of the grazing-incidence tunable external cavity fibre laser.
etalon effects. A tunable cw Ti: sapphire laser emitting at l 790 nm was used to pump the ¯uoride ®bre using a 10 microscope objective to launch the pump light into the ®bre core. The ®bre has a core diameter of 11 mm and a numerical aperture of 0.15. Using the grazing incidence arrangement, a higher grating dispersion is achieved over other tunable arrangements such as the Littrow mounting arrangement. Full details of the Littrow arrangement for the Tm3 ®bre laser are given in a previous publication [6]. The cavity linewidth may be calculated from the grating equation:
For our experimental geometry, a beam radius in the external cavity of r 1:36 mm at l 2350 nm was calculated for the lens and angles of incidence of (yi) 508 and 858 for the Littrow and grazing incidence arrangements. This yields cavity linewidths of 31.99 and 2.17 GHz for the two cavities. Consequently, a cavity with a grazing incidence arrangement should yield a cavity linewidth more than an order of magnitude smaller than the Littrow arrangement.
l d sin yi sin yd
Fig. 2 displays the light-in/light-out characteristic for the Littman±Metcalf con®guration. Laser oscillation was achieved for a launched pump power of 31.4 mW with a corresponding slope ef®ciency of 19% (56% photon conversion ef®ciency). This slope ef®ciency, which is higher than obtained with the Littrow cavity arrangement [6], would point toward lower intra-cavity losses combined with a higher output coupler value. A tuning curve FWHM of 130 nm at constant pump power was measured and is shown in Fig. 3. The large tunability demonstrates the usefulness of this ®bre laser as a source for trace gas sensing as it emits
(1)
where l is the centre wavelength, d the diffraction groove spacing and yi and yd represent the angles of incidence and diffraction. Differentiating Eq. (1) with respect to the angle of incidence and using Dlsp
@l Dyi @yi
(2)
an expression for the cavity linewidth dependence on y is obtained Dlsp Dyi d cos yi
(3)
3. Laser characteristics
Similarly for a double pass (grazing incidence) Dldp 12 Dyi d cos yi
(4)
where Dlsp and Dldp represent the single pass and double pass configurations, respectively. It is clear from Eq. (4), that as the angle of incidence approaches 908 the linewidth is dramatically reduced. The angular half-width associated with a collimated Gaussian beam is given by: Dyi
l p2r
(5)
Hence equating the Dyi's, the cavity linewidths for Littrow and grazing incidence arrangements are given by DlLittrow
dl cos yi 2rp
DlLittmanÿMetcalf
dl cos yi 4rp
(6) (7)
Fig. 2. Grazing-incidence external cavity tunable fibre laser characteristics.
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109
4. Gas sensing
Fig. 3. Experimental tuning range with a grazing-incidence external cavity arrangement.
over the entire range where hydrocarbon and other industrially important gases such as HF, have absorption bands. The diameter of the plane mirror limited the wavelength tuning range; as the mirror was rotated the effective mirror aperture seen by the light decreased either side of normal incidence. Fig. 4 shows the laser emission linewidth in the grazing incidence con®guration where a 207 MHz linewidth was measured with a scanning Fabry±Perot interferometer with a 30 GHz FSR and a minimum resolvable bandwidth (MRB) of 200 MHz. This linewidth was con®rmed using a scanning Fabry±Perot interferometer with a MRB of 100 MHz. The ®bre laser cavity length of 1.5 m, yields a mode spacing of n 66:67 MHz and consequently the laser oscillates with three modes simultaneously. The emission linewidth is nevertheless still signi®cantly narrower than the gas absorption line at ambient pressure. In the Littrow cavity up to 50 cavity modes (66.67 MHz apart corresponding to 3.2 GHz laser linewidth [6]) can propagate near the peak of the grating re¯ectivity curve whereas only 3±4 modes (207 MHz) propagate under the peak of the grazing incidence re¯ectivity versus wavelength pro®le.
Fig. 4. Grazing-incidence fibre laser linewidth.
Using the grazing incidence arrangement, the narrow linewidth of the laser will lower the minimum detectable gas concentration as compared with the Littrow con®guration [6]. Laser intensity noise plays a signi®cant role in limiting the minimum detectable concentration particularly in the absence of the ability to use harmonic detection techniques using wavelength or frequency modulation. In order to reduce the impact of intensity noise on detection limits an electronic circuit was constructed from reports by McCaul et al. [7] and Haller and Hobbs [8]. This circuit essentially allows the coherent subtraction of common mode noise occurring in two beams one of which is being used in a measurement. This effective subtraction of noise allows near shot-limited detection in ratiometric or differential measurements. In our embodiment of the circuit (shown in Fig. 5) a loop ®lter of 320 Hz bandwidth was inserted between ampli®er A and B to limit the feedback loop bandwidth. The experimental arrangement for gas sensing uses a gas cell with a path length set to 50 m for all experiments described here. Both photodiodes (signal and reference) were connected to the noise cancelling circuit described above. The laser emission wavelength was tuned to the strong methane absorption line centred at l 2373 nm. Changes in gas concentration were again obtained by using computer controlled system to precisely mix selected concentrations in the multi-pass gas cell. Fig. 6 displays data showing methane gas detection over a wide dynamic range and in particular at low concentration levels. The smallest preparable change of concentration was 50 ppm m over a 50 m path length, corresponding to a change of 1 ppm in the gas cell. From Fig. 6 we see that the minimum change in concentration is clearly resolved in the ®gure indicating that the detectivity is signi®cantly better than 50 ppm m. In this low gas concentration regime, small concentration changes in the multi-pass cell requires several minutes for the gas concentration to reach equilibrium and the gas
Fig. 5. Schematic diagram of the principal components of a balanced ratiometric detector.
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Fig. 6. Methane gas detection using grazing-incidence fibre laser tuned to gas absorption line at l 2373 nm. Detection over large dynamic range and a minimum detectable concentration of 50 ppm m.
mixing system is operating at twice its minimum change in concentration. Due to the long path length in the signal arm and subsequent time delay (170 ns) between the signal and reference photocurrents, the noise cancelling circuit was seen to operate erratically if large ¯uctuations in the laser intensity noise were observed. Large changes in the laser intensity were due to the open path laser cavity that was susceptible to environmental effects (primarily vibrations) and optical feedback. Optical gas sensing studies have also been carried out at the shorter wavelength of 1.64 mm [3]. Using laser modulation techniques, the detectivity limit in the previous study was 1 ppm m. Lower gas detectivities may be accomplished with the ¯uoride ®bre laser system by employing optical isolators to reduce feedback, and by mounting the laser cavity in an environmental isolation chamber, this will serve to reduce laser intensity noise which is the factor limiting lower gas detection capabilities in this work. Modulation spectroscopy techniques such as wavelength or frequency modulation that reduce the impact of system noise would allow us to further lower detection limits. Wavelength modulation spectroscopy could not be implemented with the current arrangement; nevertheless, replacing the stepper motors with piezo-electric actuators would allow the laser wavelength to be modulated. The use of this laser as a spectroscopic tool was investigated by passing the laser through a gas cell containing a ®xed gas concentration and tuning the laser. A 400 step per revolution stepper motor which was connected to a 100:1 gear box, allowed the rotation stage (one rotation 108) on which the plane mirror was mounted to be rotated in discrete angular steps of 2:4 10ÿ48. Each step corresponded to a wavelength change of 6:56 10ÿ3 nm or 356 MHz.
Another stepper motor was connected to the tilt axis on the plane mirror stage, this was utilised to keep the cavity optically aligned as the mirror was rotated. A typical wavelength scan consisted of moving the motor which rotated the mirror (i.e. changing the wavelength) and then dithering the tilt axis to maximise the laser output power, reading data from the electronic ratio of both the signal and reference arms and looping this sequence of events. Using this apparatus, the laser was scanned in the wavelength range 2309 nm l 2337 nm. The laser was directed through a gas cell containing 50,000 ppm m, the transmission data is plotted in Fig. 7. Overlaid on the graph is data recorded on a
Fig. 7. (a) Measured transmission spectrum of methane (concentration of 50,000 ppm m) using the grazing-incidence external cavity. (b) FTIR measured spectrum.
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111
Results on this system for gas detection will be published elsewhere [10]. Spectroscopy of gases which exhibit absorption features in the spectral region centred at l 2:3 mm, may be studied in detail using this external cavity laser demonstrated here. Due to the large tuning range several gases may be investigated simultaneously, to examine line broadening or collision effects between gases. Such increased sensitivities are suf®cient for applications such as land®ll monitoring, gas leak detection and geochemical path ®nders for mineral exploration. Acknowledgements Fig. 8. A section of measured transmission spectrum of methane (gas linewidth of 4 GHz) using the grazing-incidence external cavity with a Lorentzian lineshape function fitted.
FTIR with a resolution of 4 cmÿ1 (120 GHz) for a concentration of 50% CH4 and 50% air. Clearly, the two spectra have similar shapes, however utilising the narrow linewidth ®bre laser as a spectroscopic probe allows narrow gas absorption features to be observed and examined in detail. A single methane gas absorption feature is plotted in Fig. 8, centred at l 2328:5 nm. A Lorentzian is ®tted to the gas absorption data showing excellent agreement, with a FWHM of 4 GHz. Only one of the hydrocarbon gases was investigated in this work, nevertheless many environmentally and industrially important gases may also be detected and spectrally analysed using this laser system. Of particular interest are carbon monoxide (CO) and hydrogen ¯uoride (HF) which have strong absorption features centred at l 2:3 mm, while their absorption is weak in other spectral regions where conventional laser sources are readily available. Data from the HITRAN database gives line strengths of about 110ÿ21 (cm/molecule) for CH4, CO and HF in the spectral region around 2.3 mm [9]. It should therefore be possible to detect CO and HF with the same high sensitivity demonstrated for CH4 in this study. 5. Conclusions We have demonstrated a tunable Tm3 doped ¯uoride laser using the Littman±Metcalf arrangement and this exhibited a continuous tuning range with a FWHM of 130 nm over the wavelength region (l 2:3 mm) with a narrow emission linewidth (207 MHz). These characteristics give the laser signi®cant potential for laser based hydrocarbon gas sensing. The laser system allows hydrocarbon detection at less than 50 ppm m limited by our ability to prepare low gas concentrations. We have recently implemented a system using a Bragg grating to eliminate the open laser cavity.
This research was supported in part by Enterprise Ireland under its Strategic Research Programme, grant number ST/ 1997/302. References [1] L. Esterowitz, R. Allen, I. Aggarwal, Pulsed laser emission at 2.3 mm in a Tm3-doped fluorozirconate fibre, Elec. Lett. 24 (1988) 1104±1105. [2] J.Y. Allain, M. Monerie, H. Poignant, Tunable CW lasing around 0.82, 1.48, 1.88 and 2.35 mm in Tm3-doped fluorozirconate fibre, Elec. Lett. 25 (1989) 1660±1662. [3] V. Weldon, P. Phelan, J. Hegarty, Methane and carbon dioxide sensing using a DFB laser diode operating at 164 mm, Elec. Lett. 29 (1993) 560±562. [4] C. Sirtori, J. Faist, F. Capasso, D.L. Sivco, A.L. Hutchinson, A.Y. Cho, Long wavelength infra-red (l 11 mm) quantum cascade lasers, Appl. Phys. Lett. 69 (1996) 2810±2812. [5] M.G. Littman, H.J. Metcalf, Spectrally narrow pulsed dye laser without beam expander, Appl. Opt. 17 (1978) 2224±2227. [6] F.J. McAleavey, J. O'Gorman, J.F. Donegan, B.D. MacCraith, J. Hegarty, G. Maze, Narrow linewidth, tunable Tm3-doped fluoride fibre laser for optical-based hydrocarbon gas sensing, IEEE J. Sel. Topics. Quan. Elec. 3 (1997) 1103±1111. [7] B.W. McCaul, D.E. Doggett, E.K. Thorson, Gas spectroscopy, US Patent Number 5,491,341 (1996). [8] K.L. Haller, P.C.D. Hobbs, Double beam laser absorption spectroscopy : shot noise-limited performance at baseband with a novel electronic noise canceller, Proc. SPIE 1435 (1991) 298±309. [9] L.S. Rothman, R.R. Gamache, A. Goldman, L.R. Brown, R.A. Roth, H.M. Pickett, R.L. Poynter, J.-M. Flaud, C. Camy-Peyret, A. Barbe, N. Husson, C.P. Rinsland, M.A.H. Smith, The HITRAN database: 1986 Edition, Appl. Opt. 26 (1986) 4058±4097. [10] F.J. McAleavey, J. O'Gorman, J.F. Donegan, J. Hegarty, G. MazeÂ, H. Poignant, unpublished.
Biographies F.J. McAleavey received the BSc degree in applied physics in 1992 and the MSc in applied physics in 1995, from Dublin City University. He received the PhD degree in 1998 from the University of Dublin, Trinity College. His doctoral work was concerned with the development of fluoride fibre lasers based on Tm3 ions. These lasers show wide tunability in the near infra red around 2.3 microns and have been used as the optical source for high sensitivity gas detection systems. Dr. McAleavey is a member of the Institute of Physics.
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J. O'Gorman was awarded a PhD in Laser Physics by the University of Dublin, Trinity College, in 1989. Subsequently, he joined Bell Laboratories the R&D arm of AT&T (now Lucent Technologies) where he carried out research on laser diodes and their applications particularly in high speed data communications demonstrating 10 Gbit/s aggregate data rates for interconnect and LAN applications using laser diode arrays. In 1992, he returned to Ireland to join Optronics Ireland as a Senior Researcher. In 1994, he became Centre Manager of the Optronics Ireland R&D laboratories in TCD. His current research interests are the physics and applications of semiconductor light emitters and fibre lasers, particularly novel laser diode and LEDs based on photon mode control, wavelength tunable lasers, fibre lasers and amplifiers and the application of these devices in laser based optical sensors; high speed optical interconnects and ultrashort high power optical switching. J.F. Donegan received the PhD degree from the National University of Ireland, Galway in 1986 for a study of the optical properties of transition metal ion doped oxides. He spent 2 years as a postdoctoral researcher at Lehigh University working on magnetic resonance of defects in semiconductors. On his return to Ireland, he worked for Optronics Ireland on the development of new tunable solid state laser sources. In 1993 he was appointed to the academic staff of the Physics Department in Trinity
College Dublin. His research interests include lasing and many body effects in wide bandgap semiconductors, the optical degradation and recovery of defect damage in semiconductors, the development of tunable fibre lasers for spectroscopic applications. Dr. Donegan is a member of the American Physical Society, the Institute of Physics and the Materials Research Society. J. Hegarty received the PhD degree from the National University of Ireland, Galway in 1976, where his thesis concerned the luminescence of rare-earth doped manganese fluoride. From 1977±1980, he was with the Solid State Spectroscopy group at the University of Wisconsin, Madison. From 1980± 1986, he was a member of the technical staff. AT&T Bell Laboratories, Murray Hill, NJ, where he performed research in optical semiconductors, glasses, optical fibres and magneto-optical waveguides. In 1986 he became Professor of Laser Physics, Trinity College Dublin, where he is head of the semiconductor optoelectronics group. His current research activities include spontaneous emission processes in semiconductor microcavities, semiconductor diode laser physics and applications. Since 1996, he has been Dean of Research in Trinity College Dublin. Dr. Hegarty is a member of the American Physical Society, and a Fellow of the Institute of Physics. G. MazeÂ, Biography not available at time of publication.
Extremely high sensitivity gas detection at 2.3 mm using a grazing incidence Tm3 ®bre laser cavity F.J. McAleaveya, J. O'Gormana, J.F. Doneganb,*, J. Hegartyb, G. MazeÂc a
Optronics Ireland, Trinity College, Dublin 2, Ireland Department of Physics, Trinity College, Dublin 2, Ireland c Le Verre FluoreÂ, Campus Ker Lann, F-35170, Bruz, Brittany, France b
Received 26 July 1999; received in revised form 24 January 2000; accepted 29 February 2000
Abstract Tm3, when doped into ¯uoride ®bre produces a broad emission centred around wavelength, l 2:3 mm in the near infra-red. When placed within a laser cavity and pumped at l 790 nm, tunable laser action is observed. Using a grazing incidence laser cavity arrangement, a wavelength tuning range Dl 130 nm was achieved along with a very narrow emission linewidth Dn 207 MHz. We have used this laser as part of a gas sensing arrangement and have demonstrated an extremely high gas detection sensitivity of better than 50 ppm m using CH4 as a representative hydrocarbon gas. This system has the potential for use as part of a high sensitivity gas detection system for hydrocarbons and also for CO and HF which exhibit optical absorption in the spectral region around l 2:3 mm. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Fibre optical sensors; Gas sensor; Light emission
1. Introduction Thulium in a ¯uorozirconate glass host was ®rst reported [1] as a laser in 1988, subsequently most of the transitions were shown to lase and be wavelength tunable [2]. Of the several tunable transitions we have concentrated on the region around l 2:3 mm in the near infra-red. This transition does not produce laser action in Tm3 doped silica ®bre due to multiphonon emission that limits the lifetime of the excited state coupled with the strong infra-red absorption transitions in the glass host material. However, the ¯uoride ®bre has high transmission out to 5 mm and therefore longer wavelength lasers based on rare-earth ions can be realised. Laser-based absorption spectroscopy has signi®cant advantages for high sensitivity trace gas detection with high species selectivity [3]. Ideally, the laser sources would be in the range of about 5±10 mm where the fundamental and strongest absorption occurs. While research is continuing on developing semiconductor sources for this wavelength range such as quantum cascade lasers [4], these do not presently work at room temperature and consequently deployment of tunable laser based sensors has concentrated to date on gas overtone absorptions in the near infra-red. In this paper, we * Corresponding author. Tel.: 353-1-608-1675; fax: 353-1-671-1759. E-mail address: [email protected] (J.F. Donegan).
report on the development of a narrow linewidth, tunable Tm3 ®bre laser operating at l 2:3 mm for optical gas sensing purposes and investigate grazing incidence gratingloaded laser cavity characteristics in order to explore the highest sensitivity that can be achieved with this laser arrangement. 2. Grazing incidence laser cavity The laser used here is a grazing incidence [5] (sometimes referred to as a Littman/Metcalf) grating-loaded cavity arrangement, as shown in Fig. 1. In this double pass arrangement the diffraction grating functions as both the wavelength-selective element and the output coupler. The highly dispersive grating diffracts only a very narrow spectrum of light back into the ®bre, due to the grazing incidence con®guration and hence the large number of grating lines ®lled. The spectrally narrow feedback forces the multilongitudinal mode ®bre laser to oscillate with a dramatically reduced linewidth, yet with broad tunability over the ®bre laser gain spectrum. A 1.3 m ®bre length was used in this study with a concentration of 2000 ppm of Tm3. The ®bre ends were cleaved and the pump input side was butt-coupled to a high re¯ectivity mirror with index matching gel to minimise
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 0 ) 0 0 4 7 7 - 5
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Fig. 1. Schematic diagram of the grazing-incidence tunable external cavity fibre laser.
etalon effects. A tunable cw Ti: sapphire laser emitting at l 790 nm was used to pump the ¯uoride ®bre using a 10 microscope objective to launch the pump light into the ®bre core. The ®bre has a core diameter of 11 mm and a numerical aperture of 0.15. Using the grazing incidence arrangement, a higher grating dispersion is achieved over other tunable arrangements such as the Littrow mounting arrangement. Full details of the Littrow arrangement for the Tm3 ®bre laser are given in a previous publication [6]. The cavity linewidth may be calculated from the grating equation:
For our experimental geometry, a beam radius in the external cavity of r 1:36 mm at l 2350 nm was calculated for the lens and angles of incidence of (yi) 508 and 858 for the Littrow and grazing incidence arrangements. This yields cavity linewidths of 31.99 and 2.17 GHz for the two cavities. Consequently, a cavity with a grazing incidence arrangement should yield a cavity linewidth more than an order of magnitude smaller than the Littrow arrangement.
l d sin yi sin yd
Fig. 2 displays the light-in/light-out characteristic for the Littman±Metcalf con®guration. Laser oscillation was achieved for a launched pump power of 31.4 mW with a corresponding slope ef®ciency of 19% (56% photon conversion ef®ciency). This slope ef®ciency, which is higher than obtained with the Littrow cavity arrangement [6], would point toward lower intra-cavity losses combined with a higher output coupler value. A tuning curve FWHM of 130 nm at constant pump power was measured and is shown in Fig. 3. The large tunability demonstrates the usefulness of this ®bre laser as a source for trace gas sensing as it emits
(1)
where l is the centre wavelength, d the diffraction groove spacing and yi and yd represent the angles of incidence and diffraction. Differentiating Eq. (1) with respect to the angle of incidence and using Dlsp
@l Dyi @yi
(2)
an expression for the cavity linewidth dependence on y is obtained Dlsp Dyi d cos yi
(3)
3. Laser characteristics
Similarly for a double pass (grazing incidence) Dldp 12 Dyi d cos yi
(4)
where Dlsp and Dldp represent the single pass and double pass configurations, respectively. It is clear from Eq. (4), that as the angle of incidence approaches 908 the linewidth is dramatically reduced. The angular half-width associated with a collimated Gaussian beam is given by: Dyi
l p2r
(5)
Hence equating the Dyi's, the cavity linewidths for Littrow and grazing incidence arrangements are given by DlLittrow
dl cos yi 2rp
DlLittmanÿMetcalf
dl cos yi 4rp
(6) (7)
Fig. 2. Grazing-incidence external cavity tunable fibre laser characteristics.
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4. Gas sensing
Fig. 3. Experimental tuning range with a grazing-incidence external cavity arrangement.
over the entire range where hydrocarbon and other industrially important gases such as HF, have absorption bands. The diameter of the plane mirror limited the wavelength tuning range; as the mirror was rotated the effective mirror aperture seen by the light decreased either side of normal incidence. Fig. 4 shows the laser emission linewidth in the grazing incidence con®guration where a 207 MHz linewidth was measured with a scanning Fabry±Perot interferometer with a 30 GHz FSR and a minimum resolvable bandwidth (MRB) of 200 MHz. This linewidth was con®rmed using a scanning Fabry±Perot interferometer with a MRB of 100 MHz. The ®bre laser cavity length of 1.5 m, yields a mode spacing of n 66:67 MHz and consequently the laser oscillates with three modes simultaneously. The emission linewidth is nevertheless still signi®cantly narrower than the gas absorption line at ambient pressure. In the Littrow cavity up to 50 cavity modes (66.67 MHz apart corresponding to 3.2 GHz laser linewidth [6]) can propagate near the peak of the grating re¯ectivity curve whereas only 3±4 modes (207 MHz) propagate under the peak of the grazing incidence re¯ectivity versus wavelength pro®le.
Fig. 4. Grazing-incidence fibre laser linewidth.
Using the grazing incidence arrangement, the narrow linewidth of the laser will lower the minimum detectable gas concentration as compared with the Littrow con®guration [6]. Laser intensity noise plays a signi®cant role in limiting the minimum detectable concentration particularly in the absence of the ability to use harmonic detection techniques using wavelength or frequency modulation. In order to reduce the impact of intensity noise on detection limits an electronic circuit was constructed from reports by McCaul et al. [7] and Haller and Hobbs [8]. This circuit essentially allows the coherent subtraction of common mode noise occurring in two beams one of which is being used in a measurement. This effective subtraction of noise allows near shot-limited detection in ratiometric or differential measurements. In our embodiment of the circuit (shown in Fig. 5) a loop ®lter of 320 Hz bandwidth was inserted between ampli®er A and B to limit the feedback loop bandwidth. The experimental arrangement for gas sensing uses a gas cell with a path length set to 50 m for all experiments described here. Both photodiodes (signal and reference) were connected to the noise cancelling circuit described above. The laser emission wavelength was tuned to the strong methane absorption line centred at l 2373 nm. Changes in gas concentration were again obtained by using computer controlled system to precisely mix selected concentrations in the multi-pass gas cell. Fig. 6 displays data showing methane gas detection over a wide dynamic range and in particular at low concentration levels. The smallest preparable change of concentration was 50 ppm m over a 50 m path length, corresponding to a change of 1 ppm in the gas cell. From Fig. 6 we see that the minimum change in concentration is clearly resolved in the ®gure indicating that the detectivity is signi®cantly better than 50 ppm m. In this low gas concentration regime, small concentration changes in the multi-pass cell requires several minutes for the gas concentration to reach equilibrium and the gas
Fig. 5. Schematic diagram of the principal components of a balanced ratiometric detector.
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Fig. 6. Methane gas detection using grazing-incidence fibre laser tuned to gas absorption line at l 2373 nm. Detection over large dynamic range and a minimum detectable concentration of 50 ppm m.
mixing system is operating at twice its minimum change in concentration. Due to the long path length in the signal arm and subsequent time delay (170 ns) between the signal and reference photocurrents, the noise cancelling circuit was seen to operate erratically if large ¯uctuations in the laser intensity noise were observed. Large changes in the laser intensity were due to the open path laser cavity that was susceptible to environmental effects (primarily vibrations) and optical feedback. Optical gas sensing studies have also been carried out at the shorter wavelength of 1.64 mm [3]. Using laser modulation techniques, the detectivity limit in the previous study was 1 ppm m. Lower gas detectivities may be accomplished with the ¯uoride ®bre laser system by employing optical isolators to reduce feedback, and by mounting the laser cavity in an environmental isolation chamber, this will serve to reduce laser intensity noise which is the factor limiting lower gas detection capabilities in this work. Modulation spectroscopy techniques such as wavelength or frequency modulation that reduce the impact of system noise would allow us to further lower detection limits. Wavelength modulation spectroscopy could not be implemented with the current arrangement; nevertheless, replacing the stepper motors with piezo-electric actuators would allow the laser wavelength to be modulated. The use of this laser as a spectroscopic tool was investigated by passing the laser through a gas cell containing a ®xed gas concentration and tuning the laser. A 400 step per revolution stepper motor which was connected to a 100:1 gear box, allowed the rotation stage (one rotation 108) on which the plane mirror was mounted to be rotated in discrete angular steps of 2:4 10ÿ48. Each step corresponded to a wavelength change of 6:56 10ÿ3 nm or 356 MHz.
Another stepper motor was connected to the tilt axis on the plane mirror stage, this was utilised to keep the cavity optically aligned as the mirror was rotated. A typical wavelength scan consisted of moving the motor which rotated the mirror (i.e. changing the wavelength) and then dithering the tilt axis to maximise the laser output power, reading data from the electronic ratio of both the signal and reference arms and looping this sequence of events. Using this apparatus, the laser was scanned in the wavelength range 2309 nm l 2337 nm. The laser was directed through a gas cell containing 50,000 ppm m, the transmission data is plotted in Fig. 7. Overlaid on the graph is data recorded on a
Fig. 7. (a) Measured transmission spectrum of methane (concentration of 50,000 ppm m) using the grazing-incidence external cavity. (b) FTIR measured spectrum.
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Results on this system for gas detection will be published elsewhere [10]. Spectroscopy of gases which exhibit absorption features in the spectral region centred at l 2:3 mm, may be studied in detail using this external cavity laser demonstrated here. Due to the large tuning range several gases may be investigated simultaneously, to examine line broadening or collision effects between gases. Such increased sensitivities are suf®cient for applications such as land®ll monitoring, gas leak detection and geochemical path ®nders for mineral exploration. Acknowledgements Fig. 8. A section of measured transmission spectrum of methane (gas linewidth of 4 GHz) using the grazing-incidence external cavity with a Lorentzian lineshape function fitted.
FTIR with a resolution of 4 cmÿ1 (120 GHz) for a concentration of 50% CH4 and 50% air. Clearly, the two spectra have similar shapes, however utilising the narrow linewidth ®bre laser as a spectroscopic probe allows narrow gas absorption features to be observed and examined in detail. A single methane gas absorption feature is plotted in Fig. 8, centred at l 2328:5 nm. A Lorentzian is ®tted to the gas absorption data showing excellent agreement, with a FWHM of 4 GHz. Only one of the hydrocarbon gases was investigated in this work, nevertheless many environmentally and industrially important gases may also be detected and spectrally analysed using this laser system. Of particular interest are carbon monoxide (CO) and hydrogen ¯uoride (HF) which have strong absorption features centred at l 2:3 mm, while their absorption is weak in other spectral regions where conventional laser sources are readily available. Data from the HITRAN database gives line strengths of about 110ÿ21 (cm/molecule) for CH4, CO and HF in the spectral region around 2.3 mm [9]. It should therefore be possible to detect CO and HF with the same high sensitivity demonstrated for CH4 in this study. 5. Conclusions We have demonstrated a tunable Tm3 doped ¯uoride laser using the Littman±Metcalf arrangement and this exhibited a continuous tuning range with a FWHM of 130 nm over the wavelength region (l 2:3 mm) with a narrow emission linewidth (207 MHz). These characteristics give the laser signi®cant potential for laser based hydrocarbon gas sensing. The laser system allows hydrocarbon detection at less than 50 ppm m limited by our ability to prepare low gas concentrations. We have recently implemented a system using a Bragg grating to eliminate the open laser cavity.
This research was supported in part by Enterprise Ireland under its Strategic Research Programme, grant number ST/ 1997/302. References [1] L. Esterowitz, R. Allen, I. Aggarwal, Pulsed laser emission at 2.3 mm in a Tm3-doped fluorozirconate fibre, Elec. Lett. 24 (1988) 1104±1105. [2] J.Y. Allain, M. Monerie, H. Poignant, Tunable CW lasing around 0.82, 1.48, 1.88 and 2.35 mm in Tm3-doped fluorozirconate fibre, Elec. Lett. 25 (1989) 1660±1662. [3] V. Weldon, P. Phelan, J. Hegarty, Methane and carbon dioxide sensing using a DFB laser diode operating at 164 mm, Elec. Lett. 29 (1993) 560±562. [4] C. Sirtori, J. Faist, F. Capasso, D.L. Sivco, A.L. Hutchinson, A.Y. Cho, Long wavelength infra-red (l 11 mm) quantum cascade lasers, Appl. Phys. Lett. 69 (1996) 2810±2812. [5] M.G. Littman, H.J. Metcalf, Spectrally narrow pulsed dye laser without beam expander, Appl. Opt. 17 (1978) 2224±2227. [6] F.J. McAleavey, J. O'Gorman, J.F. Donegan, B.D. MacCraith, J. Hegarty, G. Maze, Narrow linewidth, tunable Tm3-doped fluoride fibre laser for optical-based hydrocarbon gas sensing, IEEE J. Sel. Topics. Quan. Elec. 3 (1997) 1103±1111. [7] B.W. McCaul, D.E. Doggett, E.K. Thorson, Gas spectroscopy, US Patent Number 5,491,341 (1996). [8] K.L. Haller, P.C.D. Hobbs, Double beam laser absorption spectroscopy : shot noise-limited performance at baseband with a novel electronic noise canceller, Proc. SPIE 1435 (1991) 298±309. [9] L.S. Rothman, R.R. Gamache, A. Goldman, L.R. Brown, R.A. Roth, H.M. Pickett, R.L. Poynter, J.-M. Flaud, C. Camy-Peyret, A. Barbe, N. Husson, C.P. Rinsland, M.A.H. Smith, The HITRAN database: 1986 Edition, Appl. Opt. 26 (1986) 4058±4097. [10] F.J. McAleavey, J. O'Gorman, J.F. Donegan, J. Hegarty, G. MazeÂ, H. Poignant, unpublished.
Biographies F.J. McAleavey received the BSc degree in applied physics in 1992 and the MSc in applied physics in 1995, from Dublin City University. He received the PhD degree in 1998 from the University of Dublin, Trinity College. His doctoral work was concerned with the development of fluoride fibre lasers based on Tm3 ions. These lasers show wide tunability in the near infra red around 2.3 microns and have been used as the optical source for high sensitivity gas detection systems. Dr. McAleavey is a member of the Institute of Physics.
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J. O'Gorman was awarded a PhD in Laser Physics by the University of Dublin, Trinity College, in 1989. Subsequently, he joined Bell Laboratories the R&D arm of AT&T (now Lucent Technologies) where he carried out research on laser diodes and their applications particularly in high speed data communications demonstrating 10 Gbit/s aggregate data rates for interconnect and LAN applications using laser diode arrays. In 1992, he returned to Ireland to join Optronics Ireland as a Senior Researcher. In 1994, he became Centre Manager of the Optronics Ireland R&D laboratories in TCD. His current research interests are the physics and applications of semiconductor light emitters and fibre lasers, particularly novel laser diode and LEDs based on photon mode control, wavelength tunable lasers, fibre lasers and amplifiers and the application of these devices in laser based optical sensors; high speed optical interconnects and ultrashort high power optical switching. J.F. Donegan received the PhD degree from the National University of Ireland, Galway in 1986 for a study of the optical properties of transition metal ion doped oxides. He spent 2 years as a postdoctoral researcher at Lehigh University working on magnetic resonance of defects in semiconductors. On his return to Ireland, he worked for Optronics Ireland on the development of new tunable solid state laser sources. In 1993 he was appointed to the academic staff of the Physics Department in Trinity
College Dublin. His research interests include lasing and many body effects in wide bandgap semiconductors, the optical degradation and recovery of defect damage in semiconductors, the development of tunable fibre lasers for spectroscopic applications. Dr. Donegan is a member of the American Physical Society, the Institute of Physics and the Materials Research Society. J. Hegarty received the PhD degree from the National University of Ireland, Galway in 1976, where his thesis concerned the luminescence of rare-earth doped manganese fluoride. From 1977±1980, he was with the Solid State Spectroscopy group at the University of Wisconsin, Madison. From 1980± 1986, he was a member of the technical staff. AT&T Bell Laboratories, Murray Hill, NJ, where he performed research in optical semiconductors, glasses, optical fibres and magneto-optical waveguides. In 1986 he became Professor of Laser Physics, Trinity College Dublin, where he is head of the semiconductor optoelectronics group. His current research activities include spontaneous emission processes in semiconductor microcavities, semiconductor diode laser physics and applications. Since 1996, he has been Dean of Research in Trinity College Dublin. Dr. Hegarty is a member of the American Physical Society, and a Fellow of the Institute of Physics. G. MazeÂ, Biography not available at time of publication.