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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 12, Iss. 14 — Jul. 12, 2004
  • pp: 3066–3071
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Dual-wavelength optical-pulse source based on diode lasers for high-repetition-rate, narrow-bandwidth terahertz-wave generation

Yuzo Sasaki, Hiroyuki Yokoyama, and Hiromasa Ito  »View Author Affiliations


Optics Express, Vol. 12, Issue 14, pp. 3066-3071 (2004)
http://dx.doi.org/10.1364/OPEX.12.003066


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Abstract

Synchronized dual-wavelength 1-MHz repetition-rate optical pulses were successfully generated by the combination of a gain-switched diode laser and a wavelength-tunable continuous-wave diode laser at 1.5 µm. The timing synchronization of dual-wavelength optical pulses was achieved with four-wave mixing by use of a highly nonlinear optical fiber. This optical pulse source was utilized for terahertz (THz)-wave difference-frequency generation by use of slanted periodically poled LiNbO3 (PPLN). We generated between 1.05 and 2.1 THz by use of the proper grating period of slanted PPLN with a 10-GHz bandwidth.

© 2004 Optical Society of America

1. Introduction

The development of coherent terahertz (THz)-wave sources is of great interest in applications to solid-state physics, molecular analysis, and so on. THz-wave generation with photoconductive switching or optical rectification by use of femtosecond optical pulses has been applied in THz time-domain spectroscopy, taking advantage of its high temporal resolution [1

1. P. R. Smith, D. H. Auston, and M. C. Nuss, “Subpicosecond photoconducting dipole antennas,” IEEE J. Quantum Electron. 24, 255–260 (1988). [CrossRef]

]. Such ultrashort pulses consist of an ultrabroad bandwidth spectral component from dc to the THz region. In contrast, to access the excited states of matter of interest for high-resolution spectroscopy, ultrahigh-speed optoelectronics, and other applications, it is necessary for the THz-wave source to be tunable in frequency and to have a narrow spectral bandwidth.

Difference frequency generation (DFG) is a promising technique for the realization of widely tunable, narrow-bandwidth THz-wave generation. In the early stage of this research, THz oscillation and amplification were expected by utilization of the resonant frequency of a crystal lattice or of the molecule itself [2

2. J. Nishizawa, “History and characteristics of semiconductor lasers,” Denshi Kogaku 13, 17–20 (1963), in Japanese.

], and different ferroelectric materials and semiconductors were explored for efficient and tunable THz-wave generation [3

3. F. Zernike Jr. and P. R. Berman, “Generation of far infrared as a difference frequency,” Phys. Rev. Lett. 15, 999–1001 (1965). [CrossRef]

6

6. K. Suto and J. Nishizawa, “Low-threshold semiconductor Raman laser,” IEEE J. Quantum Electron. QE-19, 1251–1254 (1983). [CrossRef]

]. The recent interest in THz-wave applications has stimulated the development of DFG THz-wave sources [7

7. K. Kawase, M. Mizuno, S. Sohma, H. Takahashi, T. Taniuchi, Y. Urata, S. Wada, H. Tashiro, and H. Ito, “Difference-frequency terahertz-wave generation from 4-dimethylamino-N-methyl-4-stilbazolium-tosylate by use of an electrically tuned Ti:sapphire laser,” Opt. Lett. 24, 1065–1067 (1999). [CrossRef]

11

11. T. Kleine-Ostmann, P. Knobloch, M. Koch, S. Hoffmann, M. Breede, M. Hofmann, G. Hein, K. Pierz, M. Sperling, and K. Donhuijsen, “Continuous-wave THz Imaging,” Electron. Lett. 37, 1461–1463 (2001). [CrossRef]

]. In general, however, nonlinear optical materials have high absorption coefficients in the THz-wave region [9

9. W. Shi, Y. J. Ding, N. Fernelius, and K. Vodopyanov, “Efficient, tunable, and coherent 0.18–5.27-THz source based on GaSe crystal,” Opt. Lett. 27, 1454–1456 (2002). [CrossRef]

,11

11. T. Kleine-Ostmann, P. Knobloch, M. Koch, S. Hoffmann, M. Breede, M. Hofmann, G. Hein, K. Pierz, M. Sperling, and K. Donhuijsen, “Continuous-wave THz Imaging,” Electron. Lett. 37, 1461–1463 (2001). [CrossRef]

], which prevents efficient THz-wave generation from these materials. Furthermore, it is necessary to satisfy phase-matching conditions for efficient THz-wave generation. One method to overcome this problem, THz-wave generation from near the surface of periodically poled LiNbO3 (PPLN), has been proposed to obtain difference-frequency mixing in a PPLN waveguide [13

13. Y. Avetisyan and K. Kocharyan,, “A new method of terahertz difference frequency generation using periodically poled waveguide,” in Conference on Lasers and Electro-Optics, OSA 1999 Technical Digest (Optical Society of America, Washington, D.C., 1999), p. 380.

]. This surface-emitted method can generate a THz wave perpendicular to the direction of an optical beam by use of a PPLN with an appropriately designed domain structure. The absorption loss is minimized because the THz wave is generated from near the PPLN surface. Moreover, the phase-matching condition can be designed by use of an appropriate grating period of PPLN. As described theoretically, an optical waveguide should enable the realization of efficient interaction of dual-wavelength optical waves, but most proton-exchanged waveguides are made on a +Z or a −Z crystal surface. To avoid this problem, we proposed using bulk slanted PPLN, which can be used to realize a quasi-phase-matched (QPM) interaction in the direction of both launched optical pulses and a perpendicularly generated THz-wave. A detailed explanation of surface-emitted THz-wave generation has been given elsewhere [14

14. Y. Sasaki, A. Yuri, K. Kawase, and H. Ito, “Terahertz-wave surface-emitted difference frequency generation in slant-stripe-type periodically poled LiNbO3 crystal,” Appl. Phys. Lett. 81, 3323–3325 (2002). [CrossRef]

].

For efficient THz-wave conversion, in addition to preparation of an appropriate nonlinear material, the generation of high peak power synchronized dual-wavelength lasers is also important. The dual-wavelength optical pulses must have temporal overlap and a tunable frequency interval. The generation of such dual-wavelength optical pulses with a repetition rate from several hertz to a kilohertz has been reported by use of solid-state lasers or optical parametric oscillation [3

3. F. Zernike Jr. and P. R. Berman, “Generation of far infrared as a difference frequency,” Phys. Rev. Lett. 15, 999–1001 (1965). [CrossRef]

9

9. W. Shi, Y. J. Ding, N. Fernelius, and K. Vodopyanov, “Efficient, tunable, and coherent 0.18–5.27-THz source based on GaSe crystal,” Opt. Lett. 27, 1454–1456 (2002). [CrossRef]

]. We used dual-wavelength optical pulses based on a gain-switched semiconductor laser at 1.5-µm, which makes it easy to achieve high-repetition-rate operation up to the gigahertz order. This feature makes a semiconductor laser a promising candidate for obtaining high-repetition-rate THz-wave generation. Furthermore, 1.5-µm semiconductor lasers have the potential for highly stable operation and a compact setup compared with large-sized solid-state lasers. Kilowatt-level peak power pulses for efficient DFG can be obtained by use of the appropriate repetition rate and subsequent optical amplification. The typical pulse duration of a laser from a gain-switched semiconductor laser is 100 ps; the corresponding bandwidth is several gigahertz.

Here we report on 1-MHz repetition-rate synchronized dual-wavelength optical pulses using a combination of semiconductor lasers and a four-wave mixing (FWM) device. The 1-W average power of dual wavelengths with a 1-MHz repetition rate and a 100-ps duration means that the peak power of the optical pulses reached approximately 10 kW, and, based on such high-repetition-rate laser sources, a 10-GHz narrow-bandwidth THz-wave generation with wide tunability was demonstrated by use of slanted PPLN.

2. Operating principle of dual-wavelength optical pulses for difference-frequency generation

Fig. 1. Schematic of the generation of timing synchronized dual-wavelength optical pulses.

3. Dual-wavelength optical source experiment

Fig. 2. (a) Optical spectrum from a highly nonlinear fiber. The component at 1566 nm was newly generated by means of the FWM process. (b) The optical spectrum after spectral filtering. Only the cw laser light was removed.

Figure 3 shows the second harmonic generation intensity correlation profile for dualwavelength optical pulses. Instead of the entire 100-ps pulse envelope, the beating of the 1.5-THz frequency period in this 12-ps measurement range clearly indicates the timing synchronization between the pump and the FWM optical pulses.

Fig. 3. Second harmonic generation correlation trace for dual-wavelength optical pulses. The beating at 1.5 THz is in a pulse envelope.

4. Terahertz-wave difference-frequency generation experiment

A dual-wavelength optical pulse source was applied to demonstrate its usefulness with a combination of PPLN. Avetisyan et al. proposed and demonstrated surface-emitted THz-wave generation [13

13. Y. Avetisyan and K. Kocharyan,, “A new method of terahertz difference frequency generation using periodically poled waveguide,” in Conference on Lasers and Electro-Optics, OSA 1999 Technical Digest (Optical Society of America, Washington, D.C., 1999), p. 380.

,14

14. Y. Sasaki, A. Yuri, K. Kawase, and H. Ito, “Terahertz-wave surface-emitted difference frequency generation in slant-stripe-type periodically poled LiNbO3 crystal,” Appl. Phys. Lett. 81, 3323–3325 (2002). [CrossRef]

]. We prepared a slanted PPLN 1-mm-thick sample with a grating period of 25–52 µm and a 40-mm interaction length. The amplified optical pulses were launched into a slanted PPLN, and THz-wave generation was successfully detected by use of a 4-K Si bolometer set perpendicular to the slanted PPLN. The average power of THz-wave output exceeded 10 nW, and the estimated peak power of a pulse was greater than 100 µW. The THz wavelength was measured with a scanning Fabry–Perot interferometer consisting of two metal-mesh plates. When the free spectral range (FSR) and the resolution of the interferometer were 33 and 3.3 GHz at FWHM (finesse of 10), respectively, the measured bandwidth of this THz wave was 10 GHz. Figure 4 shows wavelength tuning between 1.05 and 2.1 THz by use of four PPLNs with different grating periods. Although this figure shows the measured wavelength as a function of grating period Λ of PPLNs, each PPLN has a different angle α [14

14. Y. Sasaki, A. Yuri, K. Kawase, and H. Ito, “Terahertz-wave surface-emitted difference frequency generation in slant-stripe-type periodically poled LiNbO3 crystal,” Appl. Phys. Lett. 81, 3323–3325 (2002). [CrossRef]

]. The wavelength of a cw laser was varied from 1563 to 1572 nm whereas that of a DFB laser diode was fixed at 1554 nm. Each measured THz frequency coincided with the frequency interval of the generated optical pulses. These results also indicate that the THz wavelength is determined by the grating period of slanted PPLN.

Fig. 4. THz-wavelength tuning curve of a PPLN DFG. The measured THz wavelength (dots) is in good agreement with the calculated value (solid line).

5. Conclusion

In conclusion, we have reported 1-MHz repetition-rate dual-wavelength optical pulses based on laser diodes and THz-wave generation using slanted PPLN. The combination of a gain-switched DFB laser diode and FWM easily made possible the generation of automatically synchronized optical pulses. We generated THz-wave output from 1.05 to 2.1 THz with a 10-GHz bandwidth by using slanted PPLNs. Although the average THz-wave power was still 10 nW, more efficient generation will achieve a high average output by use of optical waveguide PPLN. With its high average power, this configuration is a promising candidate for a quasi-cw THz-wave source with a megahertz or higher repetition rate and a bandwidth of several gigahertz.

Acknowledgments

The authors are grateful to J. Nishizawa, the former Director and now Director Emeritus and Research Consultant of Photodynamics Research Center, who invited us to investigate terahertz-wave generation from dielectric materials including LiNbO3. The authors also thank C. Takyu and T. Shoji for their excellent technical assistance. The authors are grateful to Sumitomo Electric Industries, Ltd. and NEC Corporation for providing the devices and measurement instruments. This research was supported by a Grant-in-Aid (13GS0002) for Creative Scientific Research from the Japan Society for the Promotion of Science (JSPS). Y. Sasaki also thanks JSPS for the Research Fellowship for Young Scientists.

References and links

1.

P. R. Smith, D. H. Auston, and M. C. Nuss, “Subpicosecond photoconducting dipole antennas,” IEEE J. Quantum Electron. 24, 255–260 (1988). [CrossRef]

2.

J. Nishizawa, “History and characteristics of semiconductor lasers,” Denshi Kogaku 13, 17–20 (1963), in Japanese.

3.

F. Zernike Jr. and P. R. Berman, “Generation of far infrared as a difference frequency,” Phys. Rev. Lett. 15, 999–1001 (1965). [CrossRef]

4.

T. Yajima and K. Inoue, “Submillimeter-wave generation by difference-frequency mixing of ruby laser lines in ZnTe,” IEEE J. Quantum Electron. QE-5, 140–146 (1969). [CrossRef]

5.

K. H. Yang, J. R. Morris, P. L. Richards, and Y. R. Shen, “Phase-matched far-infrared generation by optical mixing of dye laser beams,” Appl. Phys. Lett. 23, 669–671 (1973). [CrossRef]

6.

K. Suto and J. Nishizawa, “Low-threshold semiconductor Raman laser,” IEEE J. Quantum Electron. QE-19, 1251–1254 (1983). [CrossRef]

7.

K. Kawase, M. Mizuno, S. Sohma, H. Takahashi, T. Taniuchi, Y. Urata, S. Wada, H. Tashiro, and H. Ito, “Difference-frequency terahertz-wave generation from 4-dimethylamino-N-methyl-4-stilbazolium-tosylate by use of an electrically tuned Ti:sapphire laser,” Opt. Lett. 24, 1065–1067 (1999). [CrossRef]

8.

T. Tanabe, K. Suto, J. Nishizawa, K. Saito, and T. Kimura, “Tunable terahertz wave generation in the 3- to 7-THz region from GaP,” Appl. Phys. Lett. 83, 237–239 (2003). [CrossRef]

9.

W. Shi, Y. J. Ding, N. Fernelius, and K. Vodopyanov, “Efficient, tunable, and coherent 0.18–5.27-THz source based on GaSe crystal,” Opt. Lett. 27, 1454–1456 (2002). [CrossRef]

10.

K. Siebert, F. Siebe, M. Thomson, J. Z. Baghbidi, R. Leonhardt, and H. G. Roskos, “Advances in continuous-wave THz generation,” in Terahertz Spectroscopy and Applications II, J. M. Chamberlain, ed., Proc. SPIE3828, 234–243 (1999).

11.

T. Kleine-Ostmann, P. Knobloch, M. Koch, S. Hoffmann, M. Breede, M. Hofmann, G. Hein, K. Pierz, M. Sperling, and K. Donhuijsen, “Continuous-wave THz Imaging,” Electron. Lett. 37, 1461–1463 (2001). [CrossRef]

12.

M. Shall, H. Helm, and S. R. Keiding, “Far infrared properties of electro-optic crystals measured by THz time-domain spectroscopy,” Int. J. Infrared Millim. Waves 20, 595–604 (1999). [CrossRef]

13.

Y. Avetisyan and K. Kocharyan,, “A new method of terahertz difference frequency generation using periodically poled waveguide,” in Conference on Lasers and Electro-Optics, OSA 1999 Technical Digest (Optical Society of America, Washington, D.C., 1999), p. 380.

14.

Y. Sasaki, A. Yuri, K. Kawase, and H. Ito, “Terahertz-wave surface-emitted difference frequency generation in slant-stripe-type periodically poled LiNbO3 crystal,” Appl. Phys. Lett. 81, 3323–3325 (2002). [CrossRef]

OCIS Codes
(190.2620) Nonlinear optics : Harmonic generation and mixing
(190.4410) Nonlinear optics : Nonlinear optics, parametric processes
(260.3090) Physical optics : Infrared, far

ToC Category:
Research Papers

History
Original Manuscript: May 10, 2004
Revised Manuscript: June 16, 2004
Published: July 12, 2004

Citation
Yuzo Sasaki, Hiroyuki Yokoyama, and Hiromasa Ito, "Dual-wavelength optical-pulse source based on diode lasers for high-repetition-rate, narrow-bandwidth terahertz-wave generation," Opt. Express 12, 3066-3071 (2004)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-14-3066


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References

  1. P. R. Smith, D. H. Auston, and M. C. Nuss, ???Subpicosecond photoconducting dipole antennas,??? IEEE J. Quantum Electron. 24, 255???260 (1988). [CrossRef]
  2. J. Nishizawa, ???History and characteristics of semiconductor lasers,??? Denshi Kogaku 13, 17???20 (1963), in Japanese.
  3. F. Zernike, Jr., and P. R. Berman, ???Generation of far infrared as a difference frequency,??? Phys. Rev. Lett. 15, 999???1001 (1965). [CrossRef]
  4. T. Yajima and K. Inoue, ???Submillimeter-wave generation by difference-frequency mixing of ruby laser lines in ZnTe,??? IEEE J. Quantum Electron. QE-5, 140???146 (1969). [CrossRef]
  5. K. H. Yang, J. R. Morris, P. L. Richards, and Y. R. Shen, ???Phase-matched far-infrared generation by optical mixing of dye laser beams,??? Appl. Phys. Lett. 23, 669???671 (1973). [CrossRef]
  6. K. Suto and J. Nishizawa, ???Low-threshold semiconductor Raman laser,??? IEEE J. Quantum Electron. QE- 19, 1251???1254 (1983). [CrossRef]
  7. K. Kawase, M.Mizuno, S. Sohma, H. Takahashi, T. Taniuchi, Y. Urata, S. Wada, H. Tashiro, and H. Ito, ???Difference-frequency terahertz-wave generation from 4-dimethylamino-N-methyl-4-stilbazolium-tosylate by use of an electrically tuned Ti:sapphire laser,??? Opt. Lett. 24, 1065???1067 (1999). [CrossRef]
  8. T. Tanabe, K. Suto, J. Nishizawa, K. Saito, and T. Kimura, ???Tunable terahertz wave generation in the 3- to 7-THz region from GaP,??? Appl. Phys. Lett. 83, 237???239 (2003). [CrossRef]
  9. W. Shi, Y. J. Ding, N. Fernelius, and K. Vodopyanov, ???Efficient, tunable, and coherent 0.18???5.27-THz source based on GaSe crystal,??? Opt. Lett. 27, 1454???1456 (2002). [CrossRef]
  10. K. Siebert, F. Siebe, M. Thomson, J. Z. Baghbidi, R. Leonhardt, and H. G. Roskos, ???Advances in continuous-wave THz generation," in Terahertz Spectroscopy and Applications II, J. M. Chamberlain, ed., Proc. SPIE 3828, 234???243 (1999).
  11. T. Kleine-Ostmann, P. Knobloch, M. Koch, S. Hoffmann, M. Breede, M. Hofmann, G. Hein, K. Pierz, M. Sperling, and K. Donhuijsen, ???Continuous-wave THz Imaging,??? Electron. Lett. 37, 1461???1463 (2001). [CrossRef]
  12. M. Shall, H. Helm, and S. R. Keiding, ???Far infrared properties of electro-optic crystals measured by THz time-domain spectroscopy,??? Int. J. Infrared Millim. Waves 20, 595???604 (1999). [CrossRef]
  13. Y. Avetisyan and K. Kocharyan,, ???A new method of terahertz difference frequency generation using periodically poled waveguide,??? in Conference on Lasers and Electro-Optics, OSA 1999 Technical Digest (Optical Society of America, Washington, D.C., 1999), p. 380.
  14. Y. Sasaki, A. Yuri, K. Kawase, and H. Ito, ???Terahertz-wave surface-emitted difference frequency generation in slant-stripe-type periodically poled LiNbO3 crystal,??? Appl. Phys. Lett. 81, 3323???3325 (2002). [CrossRef]

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