November 15, 2013 / Vol. 38, No. 22 / OPTICS LETTERS
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Highly efficient and compact cavity oscillator for high-power, optically pumped gas terahertz laser L. J. Geng, Y. C. Qu,* W. J. Zhao, and J. Du National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150080, China *Corresponding author: [email protected] Received July 16, 2013; revised October 8, 2013; accepted October 9, 2013; posted October 9, 2013 (Doc. ID 194025); published November 13, 2013 We demonstrate a highly efficient and compact terahertz cavity oscillator that is based on z-cut crystal quartz used as the dichroic beam splitter, for the first time to the best of our knowledge. With D2 O gas as the active medium, pumped with a multitransverse mode TEACO2 laser, experimental verification was also presented to demonstrate the advantages of this cavity oscillator. With the cavity length of 120 cm, 7.4 mJ pulse energy at pulse repetition frequency of 6 Hz, pulse width of 90 ns, and peak power of 82.2 kW were achieved at a wavelength of 385 μm. Photon conversion efficiency (PCE) of 44% was obtained at the maximum output level from this terahertz cavity oscillator. Furthermore, to our knowledge, this PCE is the highest efficiency ever reported in D2 O gas, 385 μm terahertz cavity laser systems. The beam quality or M2 factor was found to be about 1.77. © 2013 Optical Society of America OCIS codes: (140.3070) Infrared and far-infrared lasers; (140.3410) Laser resonators; (140.3325) Laser coupling; (140.3538) Lasers, pulsed. http://dx.doi.org/10.1364/OL.38.004793
Intense terahertz laser radiation is of special interest in areas such as terahertz imaging [1,2], terahertz digital holography [3,4], terahertz radar [5] and atmosphere remote sensing [6,7]. Among all means of generation, the optically pumped gas terahertz laser (OPGTL) is one of the most powerful ways to generate coherent terahertz radiation, which also requires special cavity design because of the difficulty in separating pump and signal radiance. Many kinds of terahertz cavity configurations have been used to produce coherent terahertz radiation, such as unstable resonator [8,9], oscillator–amplifier [10], metal mesh oscillator [11–15], hole-coupled mirror oscillator [16,17], and intracavity-pumpings [18] configurations. For example, high-power, coherent terahertz radiation could be obtained by means of an unstable resonator and oscillator–amplifier combined with a high-power pump source. Unfortunately, the lengths of these cavities are longer than 4 m, and the photon conversion efficiency (PCE) of these cavities varies from 8.3% to 25.4% [8–10]. A metal mesh oscillator composed of two metal meshes was widely used to generate OPGTL [11–15]. However, with the mesh input coupler, it is hard to get high reflectivity for terahertz radiation and high transmissivity for pump radiation simultaneously. PCE of 15.6% has been obtained in [11]. Moreover, an oscillator with metal mesh mirrors is difficult to manufacture and align. The hole-coupled mirror oscillator has almost full input coupling for the pump radiation. But the energy conversion efficiency (ECE) and beam quality of the laser is restricted by the small apertures of the gain section. To the best of our knowledge, using an intracavity-pumping configuration [18], the highest PCE of 47% at 151.5 μm has been obtained from a NH3 medium. In spite of its complications, the compact form (about 0.8 m length) and high efficiency are the main advantages of this cavity. Besides these methods, by using the etalon effects of antireflection (AR)-coated Ge as the input cavity mirror, an efficient cavity for 151.5 μm terahertz radiation with 0146-9592/13/224793-04$15.00/0
PCE of 35.5% was reported recently [19]. Unfortunately, this cavity could not be used for other terahertz wavelengths. Meanwhile, the thickness accuracy of the input cavity mirror is required to be at a high level. In this Letter, with z-cut crystal quartz used as a dichroic beam splitter (DBS) placed at 45° inside the cavity oscillator, we present a terahertz cavity oscillator for OPGTL. The resonator consists of a terahertz output coupling mirror (M2 ) and a high-reflective mirror (M1 ) with curvatures of 15 m and DBS configuration as shown in Fig. 1. The terahertz radiation is generated from the terahertz Fabry–Perot cavity oscillator formed by two cavity mirrors (M1 and M2 ) with diameters of 38 mm. The input window (IW was a piece of AR-coated ZnSe. The DBS is highly reflective for the pump radiation and highly transparent for the terahertz radiation. As a result, the pump light could almost fully input coupling
Fig. 1. Experimental setup of cavity oscillator based on z-cut crystal quartz as DBS. M1 , mirror; M2 , crystal quartz (3 mm thick); M3 , off-axis parabolic mirror; IW, input window (ARcoated ZnSe); BS1 and BS2 , ZnSe beam splitters; DBS, dichroic beam splitter (1 mm thick z-cut crystal quartz); BS3 , beam splitter (high-resistivity silicon plate); FPI, Fabry–Perot interferometer; D1 , pyroelectric joulemeter; D2 , HgCdTe detecter; D3 , terahertz energy detecter; D4 , Schottky diode detector. © 2013 Optical Society of America
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into the cavity oscillator through the IW and the DBS. This cavity oscillator guaranteed high input coupling efficiency of the pump radiation, large terahertz gain section, and uniform output coupling over the entire zone of the waveguide tube, promising a high ECE, high power output, and low diverging Gaussian beam. In the experiment, when the D2 O gas molecules are pumped by a TEACO2 laser with a wavelength of 9.26 μm, 385 μm lasing will be generated [20]. The transmittance of 385 μm radiation and the reflectance of 9.26 μm of the z-cut crystal quartz can be seen in Fig. 2. When the incident angle is about 45°, transmittance of 385 μm radiation and reflectance of 9.26 μm are more than 75% and 92%, respectively. A tunable TEACO2 laser, with multitransverse mode output at a pulse repetition frequency (PRF) of 6 Hz, was used as the pump source. The beam cross section and the maximum energy of the TEACO2 laser beam were 22 mm × 20 mm and 1.4 J, respectively, in the present experiment (the pulse duration was approximately 110 ns), which corresponds to a beam intensity of approximately 2.9 MW∕cm2 . The pump energy and the shape of the pump pulse are detected by a Newport 818E-20-50L and HgCdTe detector with bandwidth of 100 MHz, respectively. A pyrex tube (36 mm diameter, 120 cm length) was used as the terahertz waveguide. The gaseous laser medium was D2 O gas with specified isotopic purity of 99.8%. The terahertz energy was fully coupled to the terahertz energy detector (SPJ-A-8-OB, Spectrum Detector) by an off-axis parabolic mirror (M3 ). A Schottky diode detector (VDI, Quasi-Optical Broadband Detector), with a subnanosecond response time at the frequency range from 0.1 to 1 THz radiation, is used to detect the profile of the terahertz pulse. Finally, all electrical signals were recorded by a Tektronix TDS3032C digital oscilloscope with a 300 MHz bandwidth. The output pulse energy of the 385 μm radiation as a function of the D2 O gas pressure is shown in Fig. 3. For different pump energy from 0.57 to 1.4 J (corresponding to intensities from 1.14 to 2.9 MW∕cm2 ), there existed an optimum gas pressure under which a maximum terahertz output is yielded. The corresponding optimum operating
Fig. 2. Transmittance at 385 μm radiation and reflectance at 9.26 μm of 1 mm thick z-cut crystal quartz varied as the incident angle changed from 0° to 50°.
Fig. 3. Terahertz pulse energy as a function of the D2 O gas pressure.
gas pressure increased slightly from 250 to 450 Pa. It can be seen from Fig. 4 that the highest output pulse energy generated by our experiment is about 7.4 mJ, which is pumped by 1.4 J pump energy, and the corresponding PCE is about 44%. PCE was calculated according to the formula by Hodges et al. [21]: η
E THz ; IR E IR · 2λλTHz
(1)
where λIR is the pump light wavelength, λTHz is the terahertz wavelength, E THz is the terahertz energy, and E IR is the pump energy. As shown in Fig. 4, with different D2 O gas pressure, we have measured the 385 μm terahertz pulse energy versus the pump energy varied from 0.57 to 1.4 J. The slope indicates the ECE, which is defined as the ratio of the output energies of the terahertz laser and the pump laser. In Fig. 4(a), ECE increases with increasing gas pressure from 50 to 450 Pa. At the low gas pressure, such as 50 and 150 Pa, with further increasing of the pump energy, the output energy of the terahertz radiation could not increase anymore. Thus, the saturation behavior is observed at the low gas pressure. Figure 4(b) shows that terahertz radiation energy increases with pump energy
Fig. 4. Relationship between terahertz energy and pump energy.
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increasing when gas pressure is higher than the value of optimum pressure. In this case, the slope of the terahertz energy decreased obviously with the increase of the gas pressure. The pulse profiles of the terahertz laser and the corresponding pump laser were measured by Schottky diode detector and HgCdTe detector. The temporal pulse shapes of the pump laser oscillation (upper) and the pulse shape of the terahertz laser oscillation (lower) are shown in Fig. 5. About 40 ns delay between the start of terahertz radiation pulse and the start of pump pulse was observed. Both the pulses of the TEACO2 laser and the terahertz laser consist of a main pulse and are followed by a tail pulse. For the main pulse, the full width at half-maximum (FWHM) for the pulse waveform of the TEACO2 laser and the terahertz laser are 110 and 90 ns, respectively. Considering the maximum terahertz pulse energy is 7.4 mJ, the peak power of the maximum terahertz pulse output is about 82.2 kW. The wavelength of the terahertz laser was measured by the scanning Fabry–Perot interferometer (FPI) consisting of two Ge plates. As shown in Fig. 6, the solid curve is an Airy function fitted to the experimental data. Although the finesse (∼4.5) of the FPI is not high enough for directly resolving the actual linewidth of the terahertz wave, the characteristic period of the transmission curve clearly shows a terahertz wavelength of 385 μm. A polyethylene polarizer (1200 grooves per mm) was used to measure the polarization component of the terahertz oscillator output. The oscillator output contained both polarization components with a typical ratio of 2.5:1, perpendicular to parallel. To determine the beam quality factor M2 , we routed the terahertz laser radiation though a 100 mm focal length lens and measured the 1∕e2 beam radius along
Fig. 5.
Pulse profiles of pump laser and the terahertz laser.
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Fig. 6. Measured terahertz-wave transmission energy (filled dots) versus the FPI gap. A terahertz wavelength of 385 μm can be determined from periodicity of the fitting curve.
the propagation direction at the maximum output energy level by using a 90∕10 knife-edge technique (shown in Fig. 7). By fitting the Gaussian beam standard propagation equations to these data, the fit yields M2 1.77. The inset of Fig. 7 is the measured beam intensity distribution by using the knife-edge technique. In conclusion, with z-cut crystal quartz used as a DBS, we present a highly efficient, compact terahertz cavity oscillator for high-power OPGTL. When D2 O gas is pumped by a TEACO2 laser with wavelength of 9.26 μm, an intense terahertz radiation at a wavelength of 385 μm can be generated. With cavity oscillator length of 120 cm, the terahertz cavity oscillator produced pulse energy of 7.4 mJ under the incident pump energy of 1.4 J, corresponding to a PCE of 44%. This PCE, to the best of our knowledge, is the highest efficiency ever demonstrated in D2 O gas, 385 μm terahertz cavity laser systems. Pulse width, PRF, and wavelength of the coherent terahertz output were 90 ns, 6 Hz, and 385 μm, respectively. The maximum peak power and average power were about 82.2 kW and 44.4 mW. A beam quality of M2 1.77 was found.
Fig. 7. M2 measurement of the D2 O gas OPGTL laser at wavelength of 385 μm. Inset: typical Gaussian spatial distribution of terahertz transverse beam along horizontal diameter.
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This work was partially supported by the Fundamental Research Funds for the Central Universities (Grant No. HIT.NSRIF.2014043) and the Funds for National Key Laboratory of Science and Technology on Tunable Laser. References 1. P. Dean, Y. L. Lim, A. Valavanis, R. Kliese, M. Nikolić, S. P. Khanna, M. Lachab, D. Indjin, Z. Ikonić, P. Harrison, A. D. Rakić, E. H. Linfield, and A. G. Davies, Opt. Lett. 36, 2587 (2011). 2. A. W. M. Lee, Q. Qin, S. Kumar, B. S. Williams, Q. Hu, and J. L. Reno, Appl. Phys. Lett. 89, 141125 (2006). 3. S. H. Ding, Q. Li, Y. D. Li, and Q. Wang, Opt. Lett. 36, 1993 (2011). 4. K. Xue, Q. Li, Y. D. Li, and Q. Wang, Opt. Lett. 37, 3228 (2012). 5. A. A. Danylov, T. M. Goyette, J. Waldman, M. J. Coulombe, A. J. Gatesman, R. H. Giles, X. Qian, N. Chandrayan, S. Vangala, K. Termkoa, W. D. Goodhue, and W. E. Nixon, Opt. Express 18, 16264 (2010). 6. J. S. Melinger, Y. Yang, M. Mandehgar, and D. Grischkowsky, Opt. Express 20, 6788 (2012). 7. E. R. Brown, D. L. Woolard, A. C. Samuels, T. Globus, and B. Gelmont, Microwave Symposium Digest, 2002 IEEE MTT-S International, (IEEE, 2002), Vol. 3, p. 1591. 8. R. Behn, I. Kjelberg, P. D. Morgan, T. Okada, and M. R. Siegrist, J. Appl. Phys. 54, 2995 (1983).
9. R. Behn, M. Dupertuis, I. Kjelberg, P. A. Krug, S. A. Salito, and M. R. Siegrist, IEEE J. Quantum Electron. 21, 1278 (1985). 10. K. Sasaki, N. Takada, O. Takahashi, M. Nagatsu, T. Tsukishima, T. Okada, S. Okajima, Y. Tsunawaki, S. Sudo, K. N. Sato, K. Kondo, H. Arimoto, and K.-L. Sato, J. Appl. Phys. 77, 1378 (1995). 11. D. R. Cohn, T. Fuse, K. J. Button, B. Lax, and Z. Drozdowicz, Appl. Phys. Lett. 27, 280 (1975). 12. M. S. Durschlag and T. A. DeTemple, Appl. Opt. 20, 1245 (1981). 13. D. A. Weitz, W. J. Skocpol, and M. Tinkham, Opt. Lett. 3, 13 (1978). 14. J. Qin, X. Zheng, X. Luo, X. Huang, and Y. Lin, IEEE J. Quantum Electron. 34, 32 (1998). 15. Z. Drozdowicz, R. J. Temkin, and B. Lax, IEEE J. Quantum Electron. 15, 865 (1979). 16. R. E. M. de Bekker, L. M. Claessen, and P. Wyder, J. Appl. Phys. 68, 3729 (1990). 17. E. R. Mueller, T. E. Wilson, J. Waldman, J. T. Kennedy, and R. A. Hart, Appl. Phys. Lett. 64, 3383 (1994). 18. H. Hirose and S. Kon, IEEE J. Quantum Electron. 22, 1600 (1986). 19. L. Miao, D. Zuo, Z. Jiu, and Z. Cheng, Opt. Commun. 283, 3171 (2010). 20. F. Keilmann, R. L. Sheffield, J. R. R. Leite, M. S. Feld, and A. Javan, Appl. Phys. Lett. 26, 19 (1975). 21. D. T. Hodges, F. B. Foote, and R. D. Reel, Appl. Phys. Lett. 29, 662 (1976).
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Highly efficient and compact cavity oscillator for high-power, optically pumped gas terahertz laser L. J. Geng, Y. C. Qu,* W. J. Zhao, and J. Du National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150080, China *Corresponding author: [email protected] Received July 16, 2013; revised October 8, 2013; accepted October 9, 2013; posted October 9, 2013 (Doc. ID 194025); published November 13, 2013 We demonstrate a highly efficient and compact terahertz cavity oscillator that is based on z-cut crystal quartz used as the dichroic beam splitter, for the first time to the best of our knowledge. With D2 O gas as the active medium, pumped with a multitransverse mode TEACO2 laser, experimental verification was also presented to demonstrate the advantages of this cavity oscillator. With the cavity length of 120 cm, 7.4 mJ pulse energy at pulse repetition frequency of 6 Hz, pulse width of 90 ns, and peak power of 82.2 kW were achieved at a wavelength of 385 μm. Photon conversion efficiency (PCE) of 44% was obtained at the maximum output level from this terahertz cavity oscillator. Furthermore, to our knowledge, this PCE is the highest efficiency ever reported in D2 O gas, 385 μm terahertz cavity laser systems. The beam quality or M2 factor was found to be about 1.77. © 2013 Optical Society of America OCIS codes: (140.3070) Infrared and far-infrared lasers; (140.3410) Laser resonators; (140.3325) Laser coupling; (140.3538) Lasers, pulsed. http://dx.doi.org/10.1364/OL.38.004793
Intense terahertz laser radiation is of special interest in areas such as terahertz imaging [1,2], terahertz digital holography [3,4], terahertz radar [5] and atmosphere remote sensing [6,7]. Among all means of generation, the optically pumped gas terahertz laser (OPGTL) is one of the most powerful ways to generate coherent terahertz radiation, which also requires special cavity design because of the difficulty in separating pump and signal radiance. Many kinds of terahertz cavity configurations have been used to produce coherent terahertz radiation, such as unstable resonator [8,9], oscillator–amplifier [10], metal mesh oscillator [11–15], hole-coupled mirror oscillator [16,17], and intracavity-pumpings [18] configurations. For example, high-power, coherent terahertz radiation could be obtained by means of an unstable resonator and oscillator–amplifier combined with a high-power pump source. Unfortunately, the lengths of these cavities are longer than 4 m, and the photon conversion efficiency (PCE) of these cavities varies from 8.3% to 25.4% [8–10]. A metal mesh oscillator composed of two metal meshes was widely used to generate OPGTL [11–15]. However, with the mesh input coupler, it is hard to get high reflectivity for terahertz radiation and high transmissivity for pump radiation simultaneously. PCE of 15.6% has been obtained in [11]. Moreover, an oscillator with metal mesh mirrors is difficult to manufacture and align. The hole-coupled mirror oscillator has almost full input coupling for the pump radiation. But the energy conversion efficiency (ECE) and beam quality of the laser is restricted by the small apertures of the gain section. To the best of our knowledge, using an intracavity-pumping configuration [18], the highest PCE of 47% at 151.5 μm has been obtained from a NH3 medium. In spite of its complications, the compact form (about 0.8 m length) and high efficiency are the main advantages of this cavity. Besides these methods, by using the etalon effects of antireflection (AR)-coated Ge as the input cavity mirror, an efficient cavity for 151.5 μm terahertz radiation with 0146-9592/13/224793-04$15.00/0
PCE of 35.5% was reported recently [19]. Unfortunately, this cavity could not be used for other terahertz wavelengths. Meanwhile, the thickness accuracy of the input cavity mirror is required to be at a high level. In this Letter, with z-cut crystal quartz used as a dichroic beam splitter (DBS) placed at 45° inside the cavity oscillator, we present a terahertz cavity oscillator for OPGTL. The resonator consists of a terahertz output coupling mirror (M2 ) and a high-reflective mirror (M1 ) with curvatures of 15 m and DBS configuration as shown in Fig. 1. The terahertz radiation is generated from the terahertz Fabry–Perot cavity oscillator formed by two cavity mirrors (M1 and M2 ) with diameters of 38 mm. The input window (IW was a piece of AR-coated ZnSe. The DBS is highly reflective for the pump radiation and highly transparent for the terahertz radiation. As a result, the pump light could almost fully input coupling
Fig. 1. Experimental setup of cavity oscillator based on z-cut crystal quartz as DBS. M1 , mirror; M2 , crystal quartz (3 mm thick); M3 , off-axis parabolic mirror; IW, input window (ARcoated ZnSe); BS1 and BS2 , ZnSe beam splitters; DBS, dichroic beam splitter (1 mm thick z-cut crystal quartz); BS3 , beam splitter (high-resistivity silicon plate); FPI, Fabry–Perot interferometer; D1 , pyroelectric joulemeter; D2 , HgCdTe detecter; D3 , terahertz energy detecter; D4 , Schottky diode detector. © 2013 Optical Society of America
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into the cavity oscillator through the IW and the DBS. This cavity oscillator guaranteed high input coupling efficiency of the pump radiation, large terahertz gain section, and uniform output coupling over the entire zone of the waveguide tube, promising a high ECE, high power output, and low diverging Gaussian beam. In the experiment, when the D2 O gas molecules are pumped by a TEACO2 laser with a wavelength of 9.26 μm, 385 μm lasing will be generated [20]. The transmittance of 385 μm radiation and the reflectance of 9.26 μm of the z-cut crystal quartz can be seen in Fig. 2. When the incident angle is about 45°, transmittance of 385 μm radiation and reflectance of 9.26 μm are more than 75% and 92%, respectively. A tunable TEACO2 laser, with multitransverse mode output at a pulse repetition frequency (PRF) of 6 Hz, was used as the pump source. The beam cross section and the maximum energy of the TEACO2 laser beam were 22 mm × 20 mm and 1.4 J, respectively, in the present experiment (the pulse duration was approximately 110 ns), which corresponds to a beam intensity of approximately 2.9 MW∕cm2 . The pump energy and the shape of the pump pulse are detected by a Newport 818E-20-50L and HgCdTe detector with bandwidth of 100 MHz, respectively. A pyrex tube (36 mm diameter, 120 cm length) was used as the terahertz waveguide. The gaseous laser medium was D2 O gas with specified isotopic purity of 99.8%. The terahertz energy was fully coupled to the terahertz energy detector (SPJ-A-8-OB, Spectrum Detector) by an off-axis parabolic mirror (M3 ). A Schottky diode detector (VDI, Quasi-Optical Broadband Detector), with a subnanosecond response time at the frequency range from 0.1 to 1 THz radiation, is used to detect the profile of the terahertz pulse. Finally, all electrical signals were recorded by a Tektronix TDS3032C digital oscilloscope with a 300 MHz bandwidth. The output pulse energy of the 385 μm radiation as a function of the D2 O gas pressure is shown in Fig. 3. For different pump energy from 0.57 to 1.4 J (corresponding to intensities from 1.14 to 2.9 MW∕cm2 ), there existed an optimum gas pressure under which a maximum terahertz output is yielded. The corresponding optimum operating
Fig. 2. Transmittance at 385 μm radiation and reflectance at 9.26 μm of 1 mm thick z-cut crystal quartz varied as the incident angle changed from 0° to 50°.
Fig. 3. Terahertz pulse energy as a function of the D2 O gas pressure.
gas pressure increased slightly from 250 to 450 Pa. It can be seen from Fig. 4 that the highest output pulse energy generated by our experiment is about 7.4 mJ, which is pumped by 1.4 J pump energy, and the corresponding PCE is about 44%. PCE was calculated according to the formula by Hodges et al. [21]: η
E THz ; IR E IR · 2λλTHz
(1)
where λIR is the pump light wavelength, λTHz is the terahertz wavelength, E THz is the terahertz energy, and E IR is the pump energy. As shown in Fig. 4, with different D2 O gas pressure, we have measured the 385 μm terahertz pulse energy versus the pump energy varied from 0.57 to 1.4 J. The slope indicates the ECE, which is defined as the ratio of the output energies of the terahertz laser and the pump laser. In Fig. 4(a), ECE increases with increasing gas pressure from 50 to 450 Pa. At the low gas pressure, such as 50 and 150 Pa, with further increasing of the pump energy, the output energy of the terahertz radiation could not increase anymore. Thus, the saturation behavior is observed at the low gas pressure. Figure 4(b) shows that terahertz radiation energy increases with pump energy
Fig. 4. Relationship between terahertz energy and pump energy.
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increasing when gas pressure is higher than the value of optimum pressure. In this case, the slope of the terahertz energy decreased obviously with the increase of the gas pressure. The pulse profiles of the terahertz laser and the corresponding pump laser were measured by Schottky diode detector and HgCdTe detector. The temporal pulse shapes of the pump laser oscillation (upper) and the pulse shape of the terahertz laser oscillation (lower) are shown in Fig. 5. About 40 ns delay between the start of terahertz radiation pulse and the start of pump pulse was observed. Both the pulses of the TEACO2 laser and the terahertz laser consist of a main pulse and are followed by a tail pulse. For the main pulse, the full width at half-maximum (FWHM) for the pulse waveform of the TEACO2 laser and the terahertz laser are 110 and 90 ns, respectively. Considering the maximum terahertz pulse energy is 7.4 mJ, the peak power of the maximum terahertz pulse output is about 82.2 kW. The wavelength of the terahertz laser was measured by the scanning Fabry–Perot interferometer (FPI) consisting of two Ge plates. As shown in Fig. 6, the solid curve is an Airy function fitted to the experimental data. Although the finesse (∼4.5) of the FPI is not high enough for directly resolving the actual linewidth of the terahertz wave, the characteristic period of the transmission curve clearly shows a terahertz wavelength of 385 μm. A polyethylene polarizer (1200 grooves per mm) was used to measure the polarization component of the terahertz oscillator output. The oscillator output contained both polarization components with a typical ratio of 2.5:1, perpendicular to parallel. To determine the beam quality factor M2 , we routed the terahertz laser radiation though a 100 mm focal length lens and measured the 1∕e2 beam radius along
Fig. 5.
Pulse profiles of pump laser and the terahertz laser.
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Fig. 6. Measured terahertz-wave transmission energy (filled dots) versus the FPI gap. A terahertz wavelength of 385 μm can be determined from periodicity of the fitting curve.
the propagation direction at the maximum output energy level by using a 90∕10 knife-edge technique (shown in Fig. 7). By fitting the Gaussian beam standard propagation equations to these data, the fit yields M2 1.77. The inset of Fig. 7 is the measured beam intensity distribution by using the knife-edge technique. In conclusion, with z-cut crystal quartz used as a DBS, we present a highly efficient, compact terahertz cavity oscillator for high-power OPGTL. When D2 O gas is pumped by a TEACO2 laser with wavelength of 9.26 μm, an intense terahertz radiation at a wavelength of 385 μm can be generated. With cavity oscillator length of 120 cm, the terahertz cavity oscillator produced pulse energy of 7.4 mJ under the incident pump energy of 1.4 J, corresponding to a PCE of 44%. This PCE, to the best of our knowledge, is the highest efficiency ever demonstrated in D2 O gas, 385 μm terahertz cavity laser systems. Pulse width, PRF, and wavelength of the coherent terahertz output were 90 ns, 6 Hz, and 385 μm, respectively. The maximum peak power and average power were about 82.2 kW and 44.4 mW. A beam quality of M2 1.77 was found.
Fig. 7. M2 measurement of the D2 O gas OPGTL laser at wavelength of 385 μm. Inset: typical Gaussian spatial distribution of terahertz transverse beam along horizontal diameter.
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OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
This work was partially supported by the Fundamental Research Funds for the Central Universities (Grant No. HIT.NSRIF.2014043) and the Funds for National Key Laboratory of Science and Technology on Tunable Laser. References 1. P. Dean, Y. L. Lim, A. Valavanis, R. Kliese, M. Nikolić, S. P. Khanna, M. Lachab, D. Indjin, Z. Ikonić, P. Harrison, A. D. Rakić, E. H. Linfield, and A. G. Davies, Opt. Lett. 36, 2587 (2011). 2. A. W. M. Lee, Q. Qin, S. Kumar, B. S. Williams, Q. Hu, and J. L. Reno, Appl. Phys. Lett. 89, 141125 (2006). 3. S. H. Ding, Q. Li, Y. D. Li, and Q. Wang, Opt. Lett. 36, 1993 (2011). 4. K. Xue, Q. Li, Y. D. Li, and Q. Wang, Opt. Lett. 37, 3228 (2012). 5. A. A. Danylov, T. M. Goyette, J. Waldman, M. J. Coulombe, A. J. Gatesman, R. H. Giles, X. Qian, N. Chandrayan, S. Vangala, K. Termkoa, W. D. Goodhue, and W. E. Nixon, Opt. Express 18, 16264 (2010). 6. J. S. Melinger, Y. Yang, M. Mandehgar, and D. Grischkowsky, Opt. Express 20, 6788 (2012). 7. E. R. Brown, D. L. Woolard, A. C. Samuels, T. Globus, and B. Gelmont, Microwave Symposium Digest, 2002 IEEE MTT-S International, (IEEE, 2002), Vol. 3, p. 1591. 8. R. Behn, I. Kjelberg, P. D. Morgan, T. Okada, and M. R. Siegrist, J. Appl. Phys. 54, 2995 (1983).
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