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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 24 — Nov. 21, 2011
  • pp: 24102–24108
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Large tunable fractional delay of slow light pulse and its application to fast optical correlator

Norihiro Ishikura, Toshihiko Baba, Eichi Kuramochi, and Masaya Notomi  »View Author Affiliations


Optics Express, Vol. 19, Issue 24, pp. 24102-24108 (2011)
http://dx.doi.org/10.1364/OE.19.024102


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Abstract

The slow light device based on photonic crystal coupled waveguide was fabricated, and a tunable delay of 72 ps was obtained for 2-ps-wide slow light pulses by local heating, which corresponds to a tunable fractional delay of 36. This value was further enhanced to 110 by compressing the output pulses through self-phase modulation and dispersion compensation in external fibers. We applied this device to optical correlator as a delay scanner, where the fractional delay determines the resolution of the delay scanning. Using this correlator, we successfully observed sub-picosecond pulses at a scan frequency up to 2 kHz, which is 100 times faster than that of mechanical scanners in conventional correlators.

© 2011 OSA

1. Introduction

2. Tunable delay

Of course, this pulse width is still limited by the bandwidth and the remnant dispersion. As a simple method that breaks this limit, we compressed the output pulse. In this experiment, we amplified the peak power of output pulse from −6 to 37 dBm by an EDFA and expanded its spectrum through the self-phase modulation in highly nonlinear fiber (HNLF). We used a 22 m HNLF with a dispersion of −1.4 ps/nm/km (−0.030 ps/nm). Then we successively connected 10 m long single-mode fiber (SMF) with a dispersion of 16 ps/nm/km (0.17 ps/nm) to compensate for the total dispersion including the device, other fibers and optics. Consequently, the pulse width was reduced to an average of 0.52 ps. This changes the delay tuning, as shown in Fig. 2(b) and (c). Fixed and tunable fractional delays increased to 290 and 110, respectively. As mentioned above, this tunable value can be regarded as the resolution of the delay scanning in the optical correlator. The correlation is obtained from the convolution integral of two pulses. In this study, we used the cross-correlation method where one pulse goes through the device under test (DUT) and the other is the reference (REF). Therefore, if the REF is narrower, we can measure narrower DUT pulses.

3. Optical correlator

Now we propose an optical correlator using the PCCW and pulse compressor as a delay scanner of the REF pulse, as shown in Fig. 3
Fig. 3 Cross-correlator using locally heated PCCW and pulse compressor.
. Here, P is modulated by a pulsed current from the signal generator (SG) to the laser so that the delay is scanned like a saw-tooth wave. The interval between currents is set to be sufficiently longer than the thermal relaxation time in the PCCW. The heating position x and the amplification in the EDFA cannot be changed dynamically with P. With such limited control parameters, the delay changed nonlinearly and the intensity and dispersion fluctuated nonuniformly. To avoid these problems, we limited the scan range to 22 ps, transformed the pulsed current appropriately, and the cross-correlation trace was calibrated by post-processing of the signal. This scan range is still sufficient for measuring ps to sub-ps pulses. After passing the PCCW, the REF pulse was compressed as mentioned above. Meanwhile, the branched pulse passed through the DUT. Here, fibers with bandwidth- and wavelength-tunable band-pass filter (BPF) were considered as DUT. The correlation waveform was observed through second-harmonic generation (SHG) using photomultiplier (PMT) connected with oscilloscope (OSC).

Figure 4(a)
Fig. 4 Cross-correlation waveforms and their width Δτobs for different f. (a) Waveforms with scan range of 22 ps (red) and 5 ps (purple). (b) Δτobs measured for different scan ranges. Values are normalized by that at f = 100 Hz.
shows correlation waveforms for different scan frequencies f and scan ranges. Here, the DUT pulse width ΔτDUT and REF pulse width ΔτREF were fixed at 0.2 and 0.5 ps, respectively. Assuming these pulses to be Gaussian, the width of the correlation waveform, Δτcor, is given by the relation Δτcor2 = ΔτDUT2 + ΔτREF2. Therefore, ΔτREF is dominant in this observation. For a scan range of 22 ps, the waveform was maintained at f < 1 kHz, while it trailed at higher f due to the slow thermal response. When the scan range was limited to 5 ps, the tail was not so evident up to f = 2 kHz. Figure 4(b) shows the normalized Δτcor for different f and scan ranges. The waveform broadened at higher frequencies, but the broadening was suppressed by narrowing the scan range. If 10% broadening in Δτcor is acceptable, the highest f reaches 2 and ~3 kHz with scan ranges of 5 and 3 ps, respectively.

To estimate the thermal response time of the delay scan, we modulated P with a step-like current and switched the delay between the REF (0.5 ps) and DUT (0.2 ps) pulses, from completely separated to just overlapped. In this case, the cross-correlation waveform is given by the convolution integral of the ideal waveform and thermal response. The thermal response is derived by deconvoluting the ideal waveform for two Gaussian pulses of 0.2 and 0.5 ps widths from the correlation waveform. Figure 5
Fig. 5 Measured thermal response (blue) and exponential fit (black) estimated using the step-like heating.
shows the normalized separation between two pulses, which corresponds to the delay scan of the REF pulse. The obtained response curve fits well to the exponential function with a 1/e time constant of 19 μs. Taking the Fourier transform of this exponential function, 50 and 90% frequency bandwidths are derived to be 8.3 and 2.8 kHz, respectively. The 90% bandwidth is in good agreement with the result of Fig. 4(b) with the shortest scan range. Within this bandwidth, the error in the delay scan for Fig. 4(a) can be smaller than 10%. The response time is determined by the thermal equilibrium between the PCCW area and outer area. As aforementioned, the heating laser spot was elliptical with a 230 μm width across the PCW. Therefore, the outer area having a larger thermal capacity was heated simultaneously, and this might delay the equilibrium. If the laser spot is narrowed so that only the PCCW area is irradiated, the response time will be shortened and the frequency bandwidth will be enhanced.

Figure 6
Fig. 6 Cross-correlation waveforms at f = 1 kHz for different widths of DUT pulse measured by using the proposed correlator.
shows the waveforms for different ΔτDUT, which were controlled by changing the bandwidth of the BPF. Here, the small nonlinearity of Δt and intensity fluctuation in the PCCW were calibrated by the waveform at ΔτDUT = 6.0 ps, which was measured separately using a commercial auto-correlator. As ΔτDUT decreased from 6.0 ps, the waveform narrowed and converged at ΔτDUT < ΔτREF = 0.5 ps. It shows that the proposed system functions as a correlator.

4. Conclusion

We improved the tunable slow light characteristics obtained by locally heating the photonic crystal coupled waveguide. A tunable fractional delay of 36 was obtained for picosecond pulses, which is the highest record for on-chip slow light. By compressing output pulses externally, this value was further enhanced to 110. We considered this as a tunable resolution and applied to the delay scanning in the optical correlator. We successfully observed the sub-picosecond pulse with a scan frequency up to 2 kHz, which is at least 100 times faster than mechanical scanners in conventional correlators. It is limited by the thermal response time of 19 μs, and will be improved by limiting the heating area. In this study, we employed the external laser heating, but it can be replaced by integrated heaters. In addition to the delay, heaters can be incorporated to tune the target wavelength in the range of several 10 nm. Such a compact and flexible tunable delay will allow one-chip correlation systems.

Acknowledgment

This work was partly supported by The FIRST Program of the Japan Society for the Promotion of Science.

References and links

1.

R. S. Tucker, P. C. Ku, and C. J. Chang-Hasnain, “Slow-light optical buffers: capabilities and fundamental limitations,” J. Lightwave Technol. 23(12), 4046–4066 (2005). [CrossRef]

2.

T. Baba, “Slow light in photonic crystals,” Nat. Photonics 2(8), 465–473 (2008). [CrossRef]

3.

T. F. Krauss, “Why do we need slow light?” Nat. Photonics 2(8), 448–450 (2008). [CrossRef]

4.

Y. A. Vlasov, M. O’Boyle, H. F. Hamann, and S. J. McNab, “Active control of slow light on a chip with photonic crystal waveguides,” Nature 438(7064), 65–69 (2005). [CrossRef] [PubMed]

5.

F. N. Xia, L. Sekaric, and Y. Vlasov, “Ultracompact optical buffers on a silicon chip,” Nat. Photonics 1(1), 65–71 (2007). [CrossRef]

6.

M. Notomi, E. Kuramochi, and T. Tanabe, “Large-scale arrays of ultrahigh-Q coupled nanocavities,” Nat. Photonics 2(12), 741–747 (2008). [CrossRef]

7.

A. Melloni, A. Canciamilla, C. Ferrari, F. Morichetti, L. O'Faolain, T. F. Krauss, R. De La Rue, A. Samarelli, and M. Sorel, “Tunable Delay Lines in Silicon Photonics: Coupled Resonators and Photonic Crystals, a Comparison,” IEEE Photonics J. 2(2), 181–194 (2010). [CrossRef]

8.

J. Cardenas, M. A. Foster, N. Sherwood-Droz, C. B. Poitras, H. L. R. Lira, B. B. Zhang, A. L. Gaeta, J. B. Khurgin, P. Morton, and M. Lipson, “Wide-bandwidth continuously tunable optical delay line using silicon microring resonators,” Opt. Express 18(25), 26525–26534 (2010). [CrossRef] [PubMed]

9.

F. Shinobu, N. Ishikura, Y. Arita, T. Tamanuki, and T. Baba, “Continuously tunable slow-light device consisting of heater-controlled silicon microring array,” Opt. Express 19(14), 13557–13564 (2011). [CrossRef] [PubMed]

10.

D. Mori and T. Baba, “Wideband and low dispersion slow light by chirped photonic crystal coupled waveguide,” Opt. Express 13(23), 9398–9408 (2005). [CrossRef] [PubMed]

11.

T. Kawasaki, D. Mori, and T. Baba, “Experimental observation of slow light in photonic crystal coupled waveguides,” Opt. Express 15(16), 10274–10281 (2007). [CrossRef] [PubMed]

12.

T. Baba, T. Kawaaski, H. Sasaki, J. Adachi, and D. Mori, “Large delay-bandwidth product and tuning of slow light pulse in photonic crystal coupled waveguide,” Opt. Express 16(12), 9245–9253 (2008). [CrossRef] [PubMed]

13.

J. Adachi, N. Ishikura, H. Sasaki, and T. Baba, “Wide Range Tuning of Slow Light Pulse in SOI Photonic Crystal Coupled Waveguide via Folded Chirping,” IEEE J. Sel. Top. Quantum Electron. 16(1), 192–199 (2010). [CrossRef]

14.

G. J. Tearney, B. E. Bouma, and J. G. Fujimoto, “High-speed phase- and group-delay scanning with a grating-based phase control delay line,” Opt. Lett. 22(23), 1811–1813 (1997). [CrossRef] [PubMed]

15.

E. Margallo-Balbás, M. Geljon, G. Pandraud, and P. J. French, “Miniature 10 kHz thermo-optic delay line in silicon,” Opt. Lett. 35(23), 4027–4029 (2010). [CrossRef] [PubMed]

OCIS Codes
(070.4550) Fourier optics and signal processing : Correlators
(230.3120) Optical devices : Integrated optics devices
(130.5296) Integrated optics : Photonic crystal waveguides

ToC Category:
Integrated Optics

History
Original Manuscript: September 16, 2011
Revised Manuscript: October 26, 2011
Manuscript Accepted: October 28, 2011
Published: November 10, 2011

Citation
Norihiro Ishikura, Toshihiko Baba, Eichi Kuramochi, and Masaya Notomi, "Large tunable fractional delay of slow light pulse and its application to fast optical correlator," Opt. Express 19, 24102-24108 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-24-24102


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References

  1. R. S. Tucker, P. C. Ku, and C. J. Chang-Hasnain, “Slow-light optical buffers: capabilities and fundamental limitations,” J. Lightwave Technol.23(12), 4046–4066 (2005). [CrossRef]
  2. T. Baba, “Slow light in photonic crystals,” Nat. Photonics2(8), 465–473 (2008). [CrossRef]
  3. T. F. Krauss, “Why do we need slow light?” Nat. Photonics2(8), 448–450 (2008). [CrossRef]
  4. Y. A. Vlasov, M. O’Boyle, H. F. Hamann, and S. J. McNab, “Active control of slow light on a chip with photonic crystal waveguides,” Nature438(7064), 65–69 (2005). [CrossRef] [PubMed]
  5. F. N. Xia, L. Sekaric, and Y. Vlasov, “Ultracompact optical buffers on a silicon chip,” Nat. Photonics1(1), 65–71 (2007). [CrossRef]
  6. M. Notomi, E. Kuramochi, and T. Tanabe, “Large-scale arrays of ultrahigh-Q coupled nanocavities,” Nat. Photonics2(12), 741–747 (2008). [CrossRef]
  7. A. Melloni, A. Canciamilla, C. Ferrari, F. Morichetti, L. O'Faolain, T. F. Krauss, R. De La Rue, A. Samarelli, and M. Sorel, “Tunable Delay Lines in Silicon Photonics: Coupled Resonators and Photonic Crystals, a Comparison,” IEEE Photonics J.2(2), 181–194 (2010). [CrossRef]
  8. J. Cardenas, M. A. Foster, N. Sherwood-Droz, C. B. Poitras, H. L. R. Lira, B. B. Zhang, A. L. Gaeta, J. B. Khurgin, P. Morton, and M. Lipson, “Wide-bandwidth continuously tunable optical delay line using silicon microring resonators,” Opt. Express18(25), 26525–26534 (2010). [CrossRef] [PubMed]
  9. F. Shinobu, N. Ishikura, Y. Arita, T. Tamanuki, and T. Baba, “Continuously tunable slow-light device consisting of heater-controlled silicon microring array,” Opt. Express19(14), 13557–13564 (2011). [CrossRef] [PubMed]
  10. D. Mori and T. Baba, “Wideband and low dispersion slow light by chirped photonic crystal coupled waveguide,” Opt. Express13(23), 9398–9408 (2005). [CrossRef] [PubMed]
  11. T. Kawasaki, D. Mori, and T. Baba, “Experimental observation of slow light in photonic crystal coupled waveguides,” Opt. Express15(16), 10274–10281 (2007). [CrossRef] [PubMed]
  12. T. Baba, T. Kawaaski, H. Sasaki, J. Adachi, and D. Mori, “Large delay-bandwidth product and tuning of slow light pulse in photonic crystal coupled waveguide,” Opt. Express16(12), 9245–9253 (2008). [CrossRef] [PubMed]
  13. J. Adachi, N. Ishikura, H. Sasaki, and T. Baba, “Wide Range Tuning of Slow Light Pulse in SOI Photonic Crystal Coupled Waveguide via Folded Chirping,” IEEE J. Sel. Top. Quantum Electron.16(1), 192–199 (2010). [CrossRef]
  14. G. J. Tearney, B. E. Bouma, and J. G. Fujimoto, “High-speed phase- and group-delay scanning with a grating-based phase control delay line,” Opt. Lett.22(23), 1811–1813 (1997). [CrossRef] [PubMed]
  15. E. Margallo-Balbás, M. Geljon, G. Pandraud, and P. J. French, “Miniature 10 kHz thermo-optic delay line in silicon,” Opt. Lett.35(23), 4027–4029 (2010). [CrossRef] [PubMed]

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