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OPTICS LETTERS / Vol. 38, No. 20 / October 15, 2013
Tunable optical parametric amplification of a single-frequency quantum cascade laser around 8 μm in ZnGeP2 Quentin Clément, Jean-Michel Melkonian,* Jessica Barrientos-Barria, Jean-Baptiste Dherbecourt, Myriam Raybaut, and Antoine Godard ONERA, the French Aerospace Laboratory, Chemin de la Hunière, 91761 Palaiseau, France *Corresponding author: jean‑[email protected] Received June 6, 2013; revised September 10, 2013; accepted September 11, 2013; posted September 12, 2013 (Doc. ID 191899); published October 7, 2013 We demonstrate optical parametric amplification in ZnGeP2 (ZGP) of the radiation emitted by a single-frequency continuous-wave quantum cascade laser (QCL) in the range 7.8–8.4 μm. The ZGP amplifier is pumped by a singlefrequency parametric source at 2210 nm. For a pump energy of 6 mJ, we report an average gain of 50 over this range and a maximum gain of 111 for 7.5 mJ. An exponential trend is observed when changing the pump energy, with very good agreement with theory. These features are of valuable interest for increasing the standoff detection range of hazardous chemicals and explosives by QCL-based backscattering spectroscopy systems. © 2013 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers; (140.5965) Semiconductor lasers, quantum cascade; (140.3070) Infrared and far-infrared lasers. http://dx.doi.org/10.1364/OL.38.004046
Long-range detection of chemical species by optical spectrometry requires infrared coherent sources emitting nanosecond pulses with high energy, narrow linewidth, and near diffraction limited beams. A spectral range of interest is the 8–12 μm transparency window of the atmosphere in the long-wavelength infrared (LWIR). Indeed, in this so-called “molecular fingerprint” region, many important hazardous chemicals and explosives display non-overlapping rovibrational lines [1], while laser attenuation is low enough to allow for propagation over several kilometers. Over the past 10 years, quantum cascade lasers (QCLs) have attracted a lot of attention for both local and short-range (1 J∕cm2 ), broad transparency range (2– 10 μm), and large available aperture [8]. The crystal is antireflection coated, and we checked that the coatings could withstand a fluence of at least 1 J∕cm2 , the maximum value reached in our experiments. The phase matching properties of this crystal have been checked by preliminary OPO experiments with a resulting idler tunability covering the full 4.6–9.2 μm spectral range with a very good agreement with calculated phase matching curves [9]. The pump source used in these experiments has been described previously [10]. It is a 1064 nm pumped parametric source based on a master-oscillator/poweramplifer (MOPA) scheme that delivers a linearly polarized idler beam at 2210 nm with a pulse duration of 11 ns, a repetition rate of 30 Hz, and a Fourier-transform spectral linewidth (∼100 MHz). The maximum available pulse energy at 2210 nm for our experiments is 12 mJ. The pump is slightly elliptic. Its waist at the location of the ZGP crystal is 1.5 × 0.9 mm (horizontal × vertical) enabling a reasonable match to the QCL beam size, so that additional beam shaping is not necessary. The spatial profiles of the pump and of the QCL beams are displayed in Fig. 2. The QCL emission being vertically polarized, we rotate the beam by 90° with two gold mirrors in order to make it extraordinary polarized as required by type I phase matching in ZGP. After the amplifier, the pump beam is filtered out by a dichroic mirror (highly reflective at 2210 nm and highly transmittive around 8 μm). A long-pass filter with a cutoff wavelength of 7.8 μm is added in order to remove the leaking pump as well as the generated signal around 3 μm. All the flux incoming from the amplified QCL beam is collected by a CaF2 lens and focused onto an uncooled HgCdZnTe detector linked to an oscilloscope. The gain is given by the ratio between the peak voltage of the amplified QCL nanosecond pulse and the cw baseline. Measured voltages are averaged, and each measurement is repeated several times to reduce the measurement uncertainty due to the pump energy pulse-to-pulse fluctuations (which amount to 5% peak to peak). The measurements are corrected from the spectral transmissions of the collection and filtering optics, as measured with a Fourier-transform infrared spectrometer (FTIR). In particular, we take into account the
Fig. 2. Spatial profiles of the QCL and the pump beams at the location of the ZGP crystal. (a) QCL beam, M 2 < 1.2, Gaussian profile with 1.3 mm waist. (b) Pump beam, M 2 < 1.5, elliptic profile with a waist of 1.5 × 0.9 mm (horizontal × vertical).
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overall transmission of the ZGP crystal, which was carefully measured with the FTIR and at several discrete wavelengths using OPO radiation and compared to published data. The transmission of the ZGP crystal is 63% for the pump at 2210 nm, half of this loss being due to bulk absorption. For the idler in the 7.8–8.4 μm range, the transmission is limited to 81% because of imperfect antireflection coatings, while multiphonon absorption only accounts for 5% of loss. To enable comparison with theory, one has thus to consider the effective pump energy in the crystal, which is lower than the available pump energy. First, we study the evolution of the gain, at a fixed QCL wavelength of 8 μm, when increasing the pump energy from 0.5 to 12 mJ, which corresponds to an effective energy spanning from 0.3 to 7.5 mJ. As shown in Fig. 3, we notice an exponential increase of the gain when increasing the pump energy. Then, we compare our results with theoretical calculations carried out under the undepleted pump approximation. In our case this approximation is valid up to a theoretical gain of 6 · 104 , where pump depletion reaches 5%. Our calculation follows the approach derived by Barnes et al. [11], which we have extended to the case of elliptical beams. Spatial and temporal profiles are assumed to be Gaussian in each direction, an assumption very close to our experimental data. We calculate the spatial overlap of the two beams by integrating along the vertical and the horizontal directions as shown in Eq. (1): ZZ G
2 2x2 2y2 cosh2 ΓLdxdy; exp − w2QCL πw2QCL
(1)
where " Γ
#1 4 ln 2t2 2x2 2y2 2 exp − − 2 − 2 : 3 τ2p wh wv cnp ns ni π 2 τp wh wv
p μ0 8 ln 2ωi ωs d2 E p
(2)
Fig. 3. OPA gain at 8 μm versus effective pump energy for a 15-mm-long ZGP crystal (squares: experimental measurements; line: theoretical calculations without adjustable parameter).
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OPTICS LETTERS / Vol. 38, No. 20 / October 15, 2013
G is the intensity gain experienced by the QCL beam and Γ is the usual parametric gain coefficient derived from the nonlinear wave equations. Data relative to the pump are the energy Ep , the pulse duration τp , the beam waists wh for the horizontal direction and wv for the vertical direction. wQCL is the QCL waist, L the length of the crystal, and d the nonlinear effective coefficient of ZGP, which is almost constant in the range 7.8–8.4 μm. We have taken d 75 pm∕V in our model [8]. This value is coincidentally identical to the value of d14 given in the literature for second harmonic generation at 9.6 μm, because the effect of the phase-matching angle is balanced by Miller’s wavelength scaling rule when pumping at 2 μm [12]. Then, n stands for the optical index and ω for the angular frequency, where p; s; i, are the indices for the pump, signal, and idler, respectively. Finally, c is the speed of light, and μ0 is the magnetic permeability in vacuum. We see in Fig. 3 that the calculated gain G is in good agreement with the measurements without adjusting any parameter. For the highest effective pump energy of 7.5 mJ, we report a gain of 111 whereas we expect 108 from calculations. Starting from 1.1 nJ in the initial QCL beam (peak power of 100 mW), this leads to 122 nJ pulses after amplification (peak power of 11 W). With antireflection coatings transmitting 95%, the effective pump energy would be 11.4 mJ and we predict a theoretical gain of 544. The temporal profile of the amplified QCL radiation is shown in Fig. 4. As expected it is identical to that of the pump. Then, the OPA gain is measured by tuning the QCL wavelength from 7.8 to 8.4 μm. For this experiment we work with an effective pump energy of 6 mJ, which corresponds to a fluence of 0.45 J∕cm2 on the input coating, in order to have a safe margin below the damage threshold. The results are shown in Fig. 5. We report an average gain of 50 in this whole range, with an almost constant gain between 7.9 and 8.2 μm. To complete our theoretical study, we have calculated the global gain of two identical ZGP crystals in a walk-off compensation configuration [13], with imperfect coatings as detailed above. As a signal wave is generated and the
Fig. 5. Gain of the optical parametric amplifier versus QCL wavelength. The type I ZGP crystal was 15-mm-long, and the effective pump energy was 6 mJ.
idler wave is amplified in the first crystal, the initial conditions for the calculation of the gain in the second ZGP crystal are different. Note that these initial conditions are also altered by the losses of the coatings between the two crystals. We derive a slightly more complicated formula for the gain of this two-crystal amplifier, which increases from 53 to 6170 at 8 μm, for an effective pump energy of 6 mJ. Cascading ZGP crystals is thus a straightforward way to efficiently enhance the gain. In conclusion, we have demonstrated tunable OPA of a single-frequency EC-QCL in a ZGP-based OPA. An exponential gain has been measured when increasing the pump energy, with a good agreement with theoretical calculations. For 6 mJ of effective pump energy, an average gain of 50 has been achieved with a single crystal in the range 7.8–8.4 μm, only limited by the EC-QCL tunability. For an effective pump energy of 7.5 mJ, we reported a maximum gain of 111 at 8 μm. With improved coatings displaying a reasonable transmission of 95% at the pump wavelength and a slightly longer crystal of 20 mm, we anticipate a gain as high as 104 with a single crystal. Pulse energies higher than 10 μJ, which are not reachable by direct emission from any LWIR semiconductor-based laser source, should thus be expected from this approach. Further energy scaling up to the millijoule range can be achieved through OPA with higher energy lasers by cascading several crystals with larger apertures. This concept has already been validated with an OPO as the master oscillator and could be applied to QCLs as well [6,10]. Furthermore, since we use a single-frequency pump, the spectral properties of the QCL emission are preserved, whatever the value of the gain. Hence, OPA in ZGP is a relevant method for increasing the peak power of LWIR optical sources, paving the way to enhanced detection ranges of chemical species by absorption spectrometry. We acknowledge Direction Générale de l’Armement (DGA) for funding.
Fig. 4. Temporal profile of the amplified QCL radiation. The unamplified QCL power has been normalized to 1 so that the vertical axis is a direct measure of the gain.
References 1. M. E. Webber, M. Pushkarsky, and C. K. N. Patel, J. Appl. Phys. 97, 113101 (2005).
October 15, 2013 / Vol. 38, No. 20 / OPTICS LETTERS 2. E. Holthoff, J. Bender, P. Pellegrino, and A. Fisher, Sensors 10, 1986 (2010). 3. F. Fuchs, S. Hugger, M. Kinzer, R. Aidam, W. Bronner, R. Lösch, Q. Yang, K. Defreif, and F. Schnürer, Opt. Eng. 49, 111127 (2010). 4. C. K. N. Patel, Eur. J. Phys. Special Topics 153, 1 (2008). 5. G. Bloom, A. Grisard, E. Lallier, C. Larat, M. Carras, and X. Marcadet, Opt. Lett. 35, 505 (2010). 6. M. W. Haakestad, G. Arisholm, E. Lippert, S. Nicolas, G. Rustad, and K. Stenersen, Opt. Express 16, 14263 (2008). 7. J.-M. Melkonian, M. Raybaut, A. Godard, J. Petit, and M. Lefebvre, Proc. SPIE 8546, 854607 (2012).
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8. D. N. Nikogosyan, Nonlinear Optical Crystals: A Complete Survey (Springer, 2005). 9. S. Das, G. C. Bhar, S. Gangopadhyay, and C. Ghosh, Appl. Opt. 42, 4335 (2003). 10. M. Raybaut, T. Schmid, A. Godard, A. K. Mohamed, M. Lefebvre, F. Marnas, P. Flamant, A. Bohman, P. Geiser, and P. Kaspersen, Opt. Lett. 34, 2069 (2009). 11. N. P. Barnes, K. E. Murray, M. G. Jani, P. G. Schunemann, and T. M. Pollak, J. Opt. Soc. Am. B 15, 232 (1998). 12. W. J. Alford and A. V. Smith, J. Opt. Soc. Am. B 18, 524 (2001). 13. D. J. Armstrong, W. J. Alford, T. D. Raymond, A. V. Smith, and M. S. Bowers, J. Opt. Soc. Am. B 14, 460 (1997).
OPTICS LETTERS / Vol. 38, No. 20 / October 15, 2013
Tunable optical parametric amplification of a single-frequency quantum cascade laser around 8 μm in ZnGeP2 Quentin Clément, Jean-Michel Melkonian,* Jessica Barrientos-Barria, Jean-Baptiste Dherbecourt, Myriam Raybaut, and Antoine Godard ONERA, the French Aerospace Laboratory, Chemin de la Hunière, 91761 Palaiseau, France *Corresponding author: jean‑[email protected] Received June 6, 2013; revised September 10, 2013; accepted September 11, 2013; posted September 12, 2013 (Doc. ID 191899); published October 7, 2013 We demonstrate optical parametric amplification in ZnGeP2 (ZGP) of the radiation emitted by a single-frequency continuous-wave quantum cascade laser (QCL) in the range 7.8–8.4 μm. The ZGP amplifier is pumped by a singlefrequency parametric source at 2210 nm. For a pump energy of 6 mJ, we report an average gain of 50 over this range and a maximum gain of 111 for 7.5 mJ. An exponential trend is observed when changing the pump energy, with very good agreement with theory. These features are of valuable interest for increasing the standoff detection range of hazardous chemicals and explosives by QCL-based backscattering spectroscopy systems. © 2013 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers; (140.5965) Semiconductor lasers, quantum cascade; (140.3070) Infrared and far-infrared lasers. http://dx.doi.org/10.1364/OL.38.004046
Long-range detection of chemical species by optical spectrometry requires infrared coherent sources emitting nanosecond pulses with high energy, narrow linewidth, and near diffraction limited beams. A spectral range of interest is the 8–12 μm transparency window of the atmosphere in the long-wavelength infrared (LWIR). Indeed, in this so-called “molecular fingerprint” region, many important hazardous chemicals and explosives display non-overlapping rovibrational lines [1], while laser attenuation is low enough to allow for propagation over several kilometers. Over the past 10 years, quantum cascade lasers (QCLs) have attracted a lot of attention for both local and short-range (1 J∕cm2 ), broad transparency range (2– 10 μm), and large available aperture [8]. The crystal is antireflection coated, and we checked that the coatings could withstand a fluence of at least 1 J∕cm2 , the maximum value reached in our experiments. The phase matching properties of this crystal have been checked by preliminary OPO experiments with a resulting idler tunability covering the full 4.6–9.2 μm spectral range with a very good agreement with calculated phase matching curves [9]. The pump source used in these experiments has been described previously [10]. It is a 1064 nm pumped parametric source based on a master-oscillator/poweramplifer (MOPA) scheme that delivers a linearly polarized idler beam at 2210 nm with a pulse duration of 11 ns, a repetition rate of 30 Hz, and a Fourier-transform spectral linewidth (∼100 MHz). The maximum available pulse energy at 2210 nm for our experiments is 12 mJ. The pump is slightly elliptic. Its waist at the location of the ZGP crystal is 1.5 × 0.9 mm (horizontal × vertical) enabling a reasonable match to the QCL beam size, so that additional beam shaping is not necessary. The spatial profiles of the pump and of the QCL beams are displayed in Fig. 2. The QCL emission being vertically polarized, we rotate the beam by 90° with two gold mirrors in order to make it extraordinary polarized as required by type I phase matching in ZGP. After the amplifier, the pump beam is filtered out by a dichroic mirror (highly reflective at 2210 nm and highly transmittive around 8 μm). A long-pass filter with a cutoff wavelength of 7.8 μm is added in order to remove the leaking pump as well as the generated signal around 3 μm. All the flux incoming from the amplified QCL beam is collected by a CaF2 lens and focused onto an uncooled HgCdZnTe detector linked to an oscilloscope. The gain is given by the ratio between the peak voltage of the amplified QCL nanosecond pulse and the cw baseline. Measured voltages are averaged, and each measurement is repeated several times to reduce the measurement uncertainty due to the pump energy pulse-to-pulse fluctuations (which amount to 5% peak to peak). The measurements are corrected from the spectral transmissions of the collection and filtering optics, as measured with a Fourier-transform infrared spectrometer (FTIR). In particular, we take into account the
Fig. 2. Spatial profiles of the QCL and the pump beams at the location of the ZGP crystal. (a) QCL beam, M 2 < 1.2, Gaussian profile with 1.3 mm waist. (b) Pump beam, M 2 < 1.5, elliptic profile with a waist of 1.5 × 0.9 mm (horizontal × vertical).
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overall transmission of the ZGP crystal, which was carefully measured with the FTIR and at several discrete wavelengths using OPO radiation and compared to published data. The transmission of the ZGP crystal is 63% for the pump at 2210 nm, half of this loss being due to bulk absorption. For the idler in the 7.8–8.4 μm range, the transmission is limited to 81% because of imperfect antireflection coatings, while multiphonon absorption only accounts for 5% of loss. To enable comparison with theory, one has thus to consider the effective pump energy in the crystal, which is lower than the available pump energy. First, we study the evolution of the gain, at a fixed QCL wavelength of 8 μm, when increasing the pump energy from 0.5 to 12 mJ, which corresponds to an effective energy spanning from 0.3 to 7.5 mJ. As shown in Fig. 3, we notice an exponential increase of the gain when increasing the pump energy. Then, we compare our results with theoretical calculations carried out under the undepleted pump approximation. In our case this approximation is valid up to a theoretical gain of 6 · 104 , where pump depletion reaches 5%. Our calculation follows the approach derived by Barnes et al. [11], which we have extended to the case of elliptical beams. Spatial and temporal profiles are assumed to be Gaussian in each direction, an assumption very close to our experimental data. We calculate the spatial overlap of the two beams by integrating along the vertical and the horizontal directions as shown in Eq. (1): ZZ G
2 2x2 2y2 cosh2 ΓLdxdy; exp − w2QCL πw2QCL
(1)
where " Γ
#1 4 ln 2t2 2x2 2y2 2 exp − − 2 − 2 : 3 τ2p wh wv cnp ns ni π 2 τp wh wv
p μ0 8 ln 2ωi ωs d2 E p
(2)
Fig. 3. OPA gain at 8 μm versus effective pump energy for a 15-mm-long ZGP crystal (squares: experimental measurements; line: theoretical calculations without adjustable parameter).
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OPTICS LETTERS / Vol. 38, No. 20 / October 15, 2013
G is the intensity gain experienced by the QCL beam and Γ is the usual parametric gain coefficient derived from the nonlinear wave equations. Data relative to the pump are the energy Ep , the pulse duration τp , the beam waists wh for the horizontal direction and wv for the vertical direction. wQCL is the QCL waist, L the length of the crystal, and d the nonlinear effective coefficient of ZGP, which is almost constant in the range 7.8–8.4 μm. We have taken d 75 pm∕V in our model [8]. This value is coincidentally identical to the value of d14 given in the literature for second harmonic generation at 9.6 μm, because the effect of the phase-matching angle is balanced by Miller’s wavelength scaling rule when pumping at 2 μm [12]. Then, n stands for the optical index and ω for the angular frequency, where p; s; i, are the indices for the pump, signal, and idler, respectively. Finally, c is the speed of light, and μ0 is the magnetic permeability in vacuum. We see in Fig. 3 that the calculated gain G is in good agreement with the measurements without adjusting any parameter. For the highest effective pump energy of 7.5 mJ, we report a gain of 111 whereas we expect 108 from calculations. Starting from 1.1 nJ in the initial QCL beam (peak power of 100 mW), this leads to 122 nJ pulses after amplification (peak power of 11 W). With antireflection coatings transmitting 95%, the effective pump energy would be 11.4 mJ and we predict a theoretical gain of 544. The temporal profile of the amplified QCL radiation is shown in Fig. 4. As expected it is identical to that of the pump. Then, the OPA gain is measured by tuning the QCL wavelength from 7.8 to 8.4 μm. For this experiment we work with an effective pump energy of 6 mJ, which corresponds to a fluence of 0.45 J∕cm2 on the input coating, in order to have a safe margin below the damage threshold. The results are shown in Fig. 5. We report an average gain of 50 in this whole range, with an almost constant gain between 7.9 and 8.2 μm. To complete our theoretical study, we have calculated the global gain of two identical ZGP crystals in a walk-off compensation configuration [13], with imperfect coatings as detailed above. As a signal wave is generated and the
Fig. 5. Gain of the optical parametric amplifier versus QCL wavelength. The type I ZGP crystal was 15-mm-long, and the effective pump energy was 6 mJ.
idler wave is amplified in the first crystal, the initial conditions for the calculation of the gain in the second ZGP crystal are different. Note that these initial conditions are also altered by the losses of the coatings between the two crystals. We derive a slightly more complicated formula for the gain of this two-crystal amplifier, which increases from 53 to 6170 at 8 μm, for an effective pump energy of 6 mJ. Cascading ZGP crystals is thus a straightforward way to efficiently enhance the gain. In conclusion, we have demonstrated tunable OPA of a single-frequency EC-QCL in a ZGP-based OPA. An exponential gain has been measured when increasing the pump energy, with a good agreement with theoretical calculations. For 6 mJ of effective pump energy, an average gain of 50 has been achieved with a single crystal in the range 7.8–8.4 μm, only limited by the EC-QCL tunability. For an effective pump energy of 7.5 mJ, we reported a maximum gain of 111 at 8 μm. With improved coatings displaying a reasonable transmission of 95% at the pump wavelength and a slightly longer crystal of 20 mm, we anticipate a gain as high as 104 with a single crystal. Pulse energies higher than 10 μJ, which are not reachable by direct emission from any LWIR semiconductor-based laser source, should thus be expected from this approach. Further energy scaling up to the millijoule range can be achieved through OPA with higher energy lasers by cascading several crystals with larger apertures. This concept has already been validated with an OPO as the master oscillator and could be applied to QCLs as well [6,10]. Furthermore, since we use a single-frequency pump, the spectral properties of the QCL emission are preserved, whatever the value of the gain. Hence, OPA in ZGP is a relevant method for increasing the peak power of LWIR optical sources, paving the way to enhanced detection ranges of chemical species by absorption spectrometry. We acknowledge Direction Générale de l’Armement (DGA) for funding.
Fig. 4. Temporal profile of the amplified QCL radiation. The unamplified QCL power has been normalized to 1 so that the vertical axis is a direct measure of the gain.
References 1. M. E. Webber, M. Pushkarsky, and C. K. N. Patel, J. Appl. Phys. 97, 113101 (2005).
October 15, 2013 / Vol. 38, No. 20 / OPTICS LETTERS 2. E. Holthoff, J. Bender, P. Pellegrino, and A. Fisher, Sensors 10, 1986 (2010). 3. F. Fuchs, S. Hugger, M. Kinzer, R. Aidam, W. Bronner, R. Lösch, Q. Yang, K. Defreif, and F. Schnürer, Opt. Eng. 49, 111127 (2010). 4. C. K. N. Patel, Eur. J. Phys. Special Topics 153, 1 (2008). 5. G. Bloom, A. Grisard, E. Lallier, C. Larat, M. Carras, and X. Marcadet, Opt. Lett. 35, 505 (2010). 6. M. W. Haakestad, G. Arisholm, E. Lippert, S. Nicolas, G. Rustad, and K. Stenersen, Opt. Express 16, 14263 (2008). 7. J.-M. Melkonian, M. Raybaut, A. Godard, J. Petit, and M. Lefebvre, Proc. SPIE 8546, 854607 (2012).
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8. D. N. Nikogosyan, Nonlinear Optical Crystals: A Complete Survey (Springer, 2005). 9. S. Das, G. C. Bhar, S. Gangopadhyay, and C. Ghosh, Appl. Opt. 42, 4335 (2003). 10. M. Raybaut, T. Schmid, A. Godard, A. K. Mohamed, M. Lefebvre, F. Marnas, P. Flamant, A. Bohman, P. Geiser, and P. Kaspersen, Opt. Lett. 34, 2069 (2009). 11. N. P. Barnes, K. E. Murray, M. G. Jani, P. G. Schunemann, and T. M. Pollak, J. Opt. Soc. Am. B 15, 232 (1998). 12. W. J. Alford and A. V. Smith, J. Opt. Soc. Am. B 18, 524 (2001). 13. D. J. Armstrong, W. J. Alford, T. D. Raymond, A. V. Smith, and M. S. Bowers, J. Opt. Soc. Am. B 14, 460 (1997).
