OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 22 — Oct. 29, 2007
  • pp: 14901–14906
« Show journal navigation

Generation of two-mode optical signals with broadband frequency tunability and low spurious signal level

Ho-Jin Song, Naofumi Shimizu, and Tadao Nagatsuma  »View Author Affiliations


Optics Express, Vol. 15, Issue 22, pp. 14901-14906 (2007)
http://dx.doi.org/10.1364/OE.15.014901


View Full Text Article

Acrobat PDF (239 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

For continuous millimeter and terahertz-wave applications, a two-mode optical signal generation technique that uses two arrayed waveguide gratings and two optical switch units is presented. In addition to easy and fast operation, this scheme offers broadband frequency tunability and high signal purity with a low spurious mode level. Mode spacing, which corresponds to the frequency of the generated MM/THz-wave signal after photomixing, was successfully swept in the range of 200 ∼ 550 GHz and the optical spurious mode suppression ratio higher than 25 dBc was achieved. In addition, spurious modes characteristics were investigated by using second harmonic generation (SHG) autocorrelation methods for several frequencies.

© 2007 Optical Society of America

1. Introduction

Generation of millimeter and terahertz (MM/THz) waves in the range of 0.1 ∼ 10 THz is attracting interest in various application fields, such as wireless communications, radio astronomy, and imaging systems [1–3

1. A. Hirata, M. Harada, and T. Nagatsuma, “120-GHz wireless link using photonic techniques for generation, modulation and emission of millimeter-wave signals,” IEEE J. Lightwave Technol. 21, 2145–2153 (2003). [CrossRef]

]. However, such a high-frequency signal has a relatively high loss even in waveguide structures, which makes it difficult to generate and handle MM/THz-wave signals with all-electronic configuration. Alternative utilizing attractive advantages of photonic devices such as a low loss and wide bandwidth of photonic components, microwave photonic signal generation techniques with a photomixer have been developed and reported. The microwave photonic techniques include the optical heterodyning of two lasers [4

4. H. Ito, T. Furuta, F. Nakajima, K. Yoshino, and T. Ishibashi, “Photonic generation of continous THz wave using uni-traveling-carrier photodiode,” IEEE J. Lightwave Technol. 23, 4016–4021 (2005). [CrossRef]

], optical injection mode locking [5

5. L. A. Johansson and A. J. Seeds, “Fiber-intergrated heterodyne optical injection phase-lock loop,” Tech. Dig. International Microwave Symp., 1737 (2000).

], microwave photonic frequency up-conversion [6

6. H.-J. Song, J. S. Lee, and J.-I. Song, “Error-free simultaneous all-optical frequency upconvresion of WDM radio-over-fiber signals,” IEEE Photon. Technol. Lett. 17, 1731–1733 (2005). [CrossRef]

], and mode-locked lasers [7

7. K. Sato, I. Kotaka, Y. Kondo, and M. Yamamoto, “Active mode locking at 50 GHz repetition frequency by half-frequency modulation of monolithic semiconductor lasers integrated with electro-absorption modulators,” Appl. Phys. Lett. 69, 2626–2628 (1996). [CrossRef]

]. Optical heterodyning of two lasers offers a very simple configuration, but the phase noise characteristics of the generated signal is poor due to the mismatching of the phases of the two free-running lasers. Mode-locking techniques offer low phase noise signal but make frequency tuning difficult.

Fig. 1. (a). Schematic diagram of the two-mode heterodyning system with an AWG and (b), (c) optical spectra at each node (OFCG: optical frequency comb generator)

Recently, there have been several reports on MM/THz-wave signal generation with an optical comb signal and optical filters [8–11

8. T. Yamamoto, H. Takara, and S. Kawanishi, “270-360 GHz tunable beat signal light generator for photonic local oscillator,” Electron. Lett. 38, 795–797 (2002). [CrossRef]

]. The spectral linewidth and frequency tunability strongly depend on the coherency of the optical comb signal and the frequency tunability of the optical filter, respectively. In addition, the mode spacing of a comb signal should be tunable for continuous frequency sweeping [8

8. T. Yamamoto, H. Takara, and S. Kawanishi, “270-360 GHz tunable beat signal light generator for photonic local oscillator,” Electron. Lett. 38, 795–797 (2002). [CrossRef]

, 9

9. A. Hirata, H. Togo, N. Shimizu, H. Takahashi, K. Okamoto, and T. Nagatsuma, “Low phase noise photonic millimeter-wave generation using an AWG integrated with a 3-dB combiner,” IEICE Trans. Electron. E88-C, 1458–1464 (2005). [CrossRef]

]. A supercontinuum scheme [8

8. T. Yamamoto, H. Takara, and S. Kawanishi, “270-360 GHz tunable beat signal light generator for photonic local oscillator,” Electron. Lett. 38, 795–797 (2002). [CrossRef]

,10

10. T. Kuri, T. Nakasyotani, H. Toda, and K.-I. Kitayama, “Characterizations of supercontinuum light source for WDM millimeter-wave-band radio-on-fiber systems,” IEEE Photon. Technol. Lett. 17, 1274–1276 (2005). [CrossRef]

] and the amplified fiber loop external resonator [11

11. S. Fukushima, C.F.C. Silva, Y. Muramoto, and A. J. Seed, “Using an optical frequency comb generator, optically injection locked lasers, and a unitraveling-carrier photodiode,” IEEE J. Lightwave Technol. 21, 3043–3051 (2003). [CrossRef]

] have been used to generate optical comb signal with high coherency and mode spacing tunability. In addition, for filtering optical modes from a comb signal, a fiber Bragg grating (FBG) filter [8

8. T. Yamamoto, H. Takara, and S. Kawanishi, “270-360 GHz tunable beat signal light generator for photonic local oscillator,” Electron. Lett. 38, 795–797 (2002). [CrossRef]

], arrayed waveguide grating (AWG) [9

9. A. Hirata, H. Togo, N. Shimizu, H. Takahashi, K. Okamoto, and T. Nagatsuma, “Low phase noise photonic millimeter-wave generation using an AWG integrated with a 3-dB combiner,” IEICE Trans. Electron. E88-C, 1458–1464 (2005). [CrossRef]

,10

10. T. Kuri, T. Nakasyotani, H. Toda, and K.-I. Kitayama, “Characterizations of supercontinuum light source for WDM millimeter-wave-band radio-on-fiber systems,” IEEE Photon. Technol. Lett. 17, 1274–1276 (2005). [CrossRef]

], and sampled grating distributed Bragg reflector (SG-DBR) laser have been used [11

11. S. Fukushima, C.F.C. Silva, Y. Muramoto, and A. J. Seed, “Using an optical frequency comb generator, optically injection locked lasers, and a unitraveling-carrier photodiode,” IEEE J. Lightwave Technol. 21, 3043–3051 (2003). [CrossRef]

]. The FBG and AWG simplified the operation but limited the tunability, and the SG-DBR lasers offered tunability over a wide range but the condition for an injection lock should be satisfied.

In this paper, two-mode optical signal for MM/THz-wave was presented. It was generated with a supercontinuum-based, high-coherency optical comb signal generator, two optical switches (OSWs) and two identical AWGs. Commercially available OSWs and AWGs for communication applications were used, which makes it easy to control electrically and accurately. The mode spacing of the two-mode signal was swept in the range of 200 ∼ 550 GHz and tuning resolution was as good as that of the reference RF sources (< 10 Hz). In addition, fast tuning (< 10 ms) limited by the optical switch and reference RF sources was achieved. The optical spurious mode suppression ratio (OSMSR) was higher than 25 dBc over the whole frequency range, and assuming a uniform frequency characteristic of a photomixer, high-purity generated signal is expected with a spurious suppression ratio higher than 50 dBc.

Fig. 2. (a). Schematic diagram of the system with two AWGs and (b) optical and electrical spectra before and after photomixing

2. Tunable optical filtering with AWGs and OSWs

Figure 1(a) shows a schematic diagram of the two-mode heterodyning system with an AWG as an optical filter, and Figs. 1(b) and 1(c) show optical spectra at each node. As shown in Fig. 1(b), when desired optical modes are placed within the passband of an AWG channel, undesired modes are cut out clearly (node B) and a beat signal with low spurious signals is generated by a photomixer (node C). However, as can be seen in Fig. 1(c), when the mode spacing of the comb signal is tuned, and desired optical modes are placed at the edge of the passband or out of the passband of the AWG channel, the insertion loss for the desired mode is increased and undesired neighboring modes are not suppressed well. After photomixing, these modes produce high spurious signal level, which degrade signal purity. Because of this problem, the tunable frequency range for high-purity signal is limited to approximately the passband bandwidth of the AWG channels.

This problem can be solved by introducing one more identical AWG, as shown in Fig. 2(a), whose center wavelength is shifted by as much as half of the channel spacing of the AWGs. As shown in Fig. 2(b), even if the wanted mode is placed around the edge of the channel passband of AWG1, it can be filtered out completely and undesired neighboring modes can be suppressed well by selecting the channel of the other AWG (AWG2) with the OSWs. Therefore, the combination of two AWGs and two OSWs works as a tunable optical filter with a discrete tuning step. The frequency tuning-step is equal to half of the AWG channel spacing, and the passband bandwidth and stopband attenuation are determined by those of the AWGs. In addition, the maximum tuning span is determined by the number of channels of the AWGs and OSWs. Hence, with an optical comb signal and commercially available AWGs and OSWs, it is possible to generate the two-mode signal having a mode spacing of over 1 THz with low-level spurious signals and easy to tune or control the mode spacing electrically.

Fig. 3. Experiment setup (LD: laser diode. PM: optical phase modulator. DDF: dispersion decreasing fiber. AWG: arrayed waveguide grating. OSW: optical switch. and PSS: polarization state stabilizer)

3. Experiments and results

Figure 3 shows the experimental setup for generating two-mode signal with two AWGs and two OSWs. An optical comb signal was generated by using the supercontinuum technique with a distributed feedback laser diode (LD), electro-optic phase modulator, Er-doped fiber amplifier, and dispersion-decreasing fiber (DDF). The wavelength of the LD and the length of the DDF were 1549.2 nm and approximately 2 km, respectively. The mode spacing of the generated comb signal (f comb) was defined by a reference RF synthesizer and tuned in the range of 22 ∼ 28 GHz. The generated optical comb signal was divided by two and input to the two identical AWGs, which have 32 channels and 25-GHz channel spacing. By controlling the operation temperature of the AWGs, center wavelengths were shifted in order to set the channels of the AWGs in an interleaved configuration. With respect to the center channel, higher and lower wavelength channels of the AWGs were connected to two Mach-Zehnder-interferometer-based OSWs (OSW A and B), symmetrically. At the output of the OSWs, the polarization state stabilizers (PSSs) were utilized to match the polarization states of two selected modes, and then merged with a polarization-maintaining fiber-coupler. Finally, the mode spacing of two-mode signal was tuned by changing f comb and controlling the OSWs through a computer, and tuning speed was approximately a few msec, limited by the OSWs and RF synthesizer.

Fig. 4. Measured OSMSR with one AWG (dotted line) and two AWGs (solid line)

In order to estimate the spurious characteristics after photomixing for one-AWG and two-AWG configurations, the optical powers of desired and undesired modes at the coupler output were measured in the mode-spacing range of 200 ∼ 550 GHz with the step of 200 MHz. Then OSMSR, defined as the power ratio between the desired and undesired modes was obtained. As shown in Fig. 4, when one AWG was used, the OSMSR reached approximately 2 ∼ 4 dBc, periodically. On the contrary, with two AWGs, it stayed higher than 25 dBc for the whole mode spacing range. Assuming a uniform frequency characteristic of a photomixer, it is expected that a spurious suppression ratio of a generated MM/THz signal would be higher than 50 dB because of the square-law detection of photomixers.

Fig. 5. Measured optical spectra of two-mode signal having a mode-spacing of 525 GHz with (a) one and (b) two AWGs

Figures 5 show measured optical spectra of two-mode signal having a mode-spacing of 525 GHz (f comb = 23.86 GHz) with one and two AWGs, respectively. With one AWG, desired and undesired optical modes are observed at the edge of the passband of the AWG channel. These undesired modes may contribute to the generation of high spurious signals at approximately 548.86 and 572.73 GHz after photomixing. However, as shown in Fig. 5(b), the desired optical modes were filtered out clearly with two AWGs, resulting in a high-purity MM/THz-wave signal after photomixing.

The second harmonic generation (SHG) autocorrelation traces of two-mode signals having several mode-spacing frequencies are shown in Fig. 6. Since the SHG autocorrelation traces represent the instantaneous intensity of optical signals with respect to time, it is possible to estimate waveforms of MM/THz signals after photomixing. As shown in Fig. 6(a), at 375, 425, 475, and 525 GHz, where the desired modes were placed at the edge of AWG channels in the one-AWG configuration, the autocorrelation traces look like the waveform of an amplitude-modulated signal. This envelope variation must be due to high spurious signal level and implies that the level of spurious signal would be high after photomixing. However, as can be seen in Fig. 6(b), the autocorrelation traces with two AWGs show an almost constant envelop for all frequencies, implying the single frequency MM/THz signal generation with low spurious signal level after photomixing.

Fig. 6. Measured SHG autocorrelation traces for several mode-spacing frequencies with (a) one and (b) two AWGs

4. Conclusion

For a photonic MM/THz-wave signal generation, a two-mode optical signal generation technique that uses commercially available AWGs and OSWs was described and experimentally demonstrated. This scheme offers not only easy and fast operation but also broadband frequency tunability and a low spurious signal level. The mode spacing of two-mode signal, which corresponds to the frequency of generated signal after photomixing, was swept in the range of 200 ∼ 550 GHz with an OSMSR higher than 25 dBc. The maximum tuning span can be easily extended to over 1 THz with a high OSMSR.

Acknowledgments

The authors thank T. Kimura for his assistance, and Drs. A. Hirata, N. Kukutsu and Y. Kado for their encouragement and discussions.

This work was supported in part by the National Institute of Communications Technology, Japan.

References and links

1.

A. Hirata, M. Harada, and T. Nagatsuma, “120-GHz wireless link using photonic techniques for generation, modulation and emission of millimeter-wave signals,” IEEE J. Lightwave Technol. 21, 2145–2153 (2003). [CrossRef]

2.

J. W. Waters, “Submillimeter-wavelength heterodyne spectroscopy and remote sensing of the upper atmosphere,” Proceeding of the IEEE 80, 1679–1701 (1992). [CrossRef]

3.

D. M. Mittleman, R. H. Jacobsen, and M.C. Nuss, “T-ray imaging,” J. Sel. Top. Quantum Electron. 2, 679–692 (1996). [CrossRef]

4.

H. Ito, T. Furuta, F. Nakajima, K. Yoshino, and T. Ishibashi, “Photonic generation of continous THz wave using uni-traveling-carrier photodiode,” IEEE J. Lightwave Technol. 23, 4016–4021 (2005). [CrossRef]

5.

L. A. Johansson and A. J. Seeds, “Fiber-intergrated heterodyne optical injection phase-lock loop,” Tech. Dig. International Microwave Symp., 1737 (2000).

6.

H.-J. Song, J. S. Lee, and J.-I. Song, “Error-free simultaneous all-optical frequency upconvresion of WDM radio-over-fiber signals,” IEEE Photon. Technol. Lett. 17, 1731–1733 (2005). [CrossRef]

7.

K. Sato, I. Kotaka, Y. Kondo, and M. Yamamoto, “Active mode locking at 50 GHz repetition frequency by half-frequency modulation of monolithic semiconductor lasers integrated with electro-absorption modulators,” Appl. Phys. Lett. 69, 2626–2628 (1996). [CrossRef]

8.

T. Yamamoto, H. Takara, and S. Kawanishi, “270-360 GHz tunable beat signal light generator for photonic local oscillator,” Electron. Lett. 38, 795–797 (2002). [CrossRef]

9.

A. Hirata, H. Togo, N. Shimizu, H. Takahashi, K. Okamoto, and T. Nagatsuma, “Low phase noise photonic millimeter-wave generation using an AWG integrated with a 3-dB combiner,” IEICE Trans. Electron. E88-C, 1458–1464 (2005). [CrossRef]

10.

T. Kuri, T. Nakasyotani, H. Toda, and K.-I. Kitayama, “Characterizations of supercontinuum light source for WDM millimeter-wave-band radio-on-fiber systems,” IEEE Photon. Technol. Lett. 17, 1274–1276 (2005). [CrossRef]

11.

S. Fukushima, C.F.C. Silva, Y. Muramoto, and A. J. Seed, “Using an optical frequency comb generator, optically injection locked lasers, and a unitraveling-carrier photodiode,” IEEE J. Lightwave Technol. 21, 3043–3051 (2003). [CrossRef]

OCIS Codes
(040.2840) Detectors : Heterodyne
(190.7070) Nonlinear optics : Two-wave mixing
(060.5625) Fiber optics and optical communications : Radio frequency photonics

ToC Category:
Nonlinear Optics

History
Original Manuscript: August 13, 2007
Revised Manuscript: September 24, 2007
Manuscript Accepted: September 25, 2007
Published: October 26, 2007

Citation
Ho-Jin Song, Naofumi Shimizu, and Tadao Nagatsuma, "Generation of two-mode optical signals with broadband frequency tunability and low spurious signal level," Opt. Express 15, 14901-14906 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-22-14901


Sort:  Year  |  Journal  |  Reset  

References

  1. A. Hirata, M. Harada, and T. Nagatsuma, "120-GHz wireless link using photonic techniques for generation, modulation and emission of millimeter-wave signals," IEEE J. Lightwave Technol. 21, 2145-2153 (2003). [CrossRef]
  2. J. W. Waters, "Submillimeter-wavelength heterodyne spectroscopy and remote sensing of the upper atmosphere," Proceeding of the IEEE 80, 1679-1701 (1992). [CrossRef]
  3. D. M. Mittleman, R. H. Jacobsen, and M. C. Nuss, "T-ray imaging," J. Sel. Top Quantum Electron. 2, 679-692 (1996). [CrossRef]
  4. H. Ito, T. Furuta, F. Nakajima, K. Yoshino, and T. Ishibashi, "Photonic generation of continous THz wave using uni-traveling-carrier photodiode," IEEE J. Lightwave Technol. 23, 4016-4021 (2005). [CrossRef]
  5. L. A. Johansson and A. J. Seeds, "Fiber-intergrated heterodyne optical injection phase-lock loop," Tech. Dig. International Microwave Symp., 1737 (2000).
  6. H.-J. Song, J. S. Lee, and J.-I. Song, "Error-free simultaneous all-optical frequency upconvresion of WDM radio-over-fiber signals," IEEE Photon. Technol. Lett. 17, 1731-1733 (2005). [CrossRef]
  7. K. Sato, I. Kotaka, Y. Kondo, and M. Yamamoto, "Active mode locking at 50 GHz repetition frequency by half-frequency modulation of monolithic semiconductor lasers integrated with electro-absorption modulators," Appl. Phys. Lett. 69, 2626-2628 (1996). [CrossRef]
  8. T. Yamamoto, H. Takara, and S. Kawanishi, "270-360 GHz tunable beat signal light generator for photonic local oscillator," Electron. Lett. 38, 795-797 (2002). [CrossRef]
  9. A. Hirata, H. Togo, N. Shimizu, H. Takahashi, K. Okamoto, and T. Nagatsuma, "Low phase noise photonic millimeter-wave generation using an AWG integrated with a 3-dB combiner," IEICE Trans. Electron.E 88-C, 1458-1464 (2005). [CrossRef]
  10. T. Kuri, T. Nakasyotani, H. Toda, and K.-I. Kitayama, "Characterizations of supercontinuum light source for WDM millimeter-wave-band radio-on-fiber systems," IEEE Photon. Technol. Lett. 17,1274-1276 (2005). [CrossRef]
  11. S. Fukushima, C.F.C. Silva, Y. Muramoto, and A. J. Seed, "Using an optical frequency comb generator, optically injection locked lasers, and a unitraveling-carrier photodiode," IEEE J. Lightwave Technol. 21, 3043-3051 (2003). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited