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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 13 — Jun. 18, 2012
  • pp: 14109–14116
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Real-time wavelength and bandwidth-independent optical integrator based on modal dispersion

Zhongwei Tan, Chao Wang, Eric D. Diebold, Nick K. Hon, and Bahram Jalali  »View Author Affiliations


Optics Express, Vol. 20, Issue 13, pp. 14109-14116 (2012)
http://dx.doi.org/10.1364/OE.20.014109


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Abstract

High-throughput real-time optical integrators are of great importance for applications that require ultrafast optical information processing, such as real-time phase reconstruction of ultrashort optical pulses. In many of these applications, integration of wide optical bandwidth signals is required. Unfortunately, conventional all-optical integrators based on passive devices are usually sensitive to the wavelength and bandwidth of the optical carrier. Here, we propose and demonstrate a passive all-optical intensity integrator whose operation is independent of the optical signal wavelength and bandwidth. The integrator is implemented based on modal dispersion in a multimode waveguide. By controlling the launch conditions of the input beam, the device produces a rectangular temporal impulse response. Consequently, a temporal intensity integration of an arbitrary optical waveform input is performed within the rectangular time window. The key advantage of this device is that the integration operation can be performed independent of the input signal wavelength and optical carrier bandwidth. This is preferred in many applications where optical signals of different wavelengths are involved. Moreover, thanks to the use of a relatively short length of multimode waveguide, lower system latency is achieved compared to the systems using long dispersive fibers. To illustrate the versatility of the optical integrator, we demonstrate temporal intensity integration of optical waveforms with different wavelengths and optical carrier bandwidths. Finally, we use this device to perform high-throughput, single-shot, real-time optical phase reconstruction of phase-modulated signals at telecommunications bit rates.

© 2012 OSA

1. Introduction

An all-optical integrator offers significant promise for ultrafast optical information processing, optical memory, measurement and computing systems [1

1. M. Ferrera, Y. Park, L. Razzari, B. E. Little, S. T. Chu, R. Morandotti, D. J. Moss, and J. Azaña, “On-chip CMOS-compatible all-optical integrator,” Nat. Commun. 1(3), 29 (2010). [CrossRef] [PubMed]

,2

2. M. H. Asghari and J. Azaña, “Photonic integrator-based optical memory unit,” IEEE Photon. Technol. Lett. 23(4), 209–211 (2011). [CrossRef]

]. As expected for an all-optical technology, a photonic integrator can provide a processing speed which is orders of magnitude faster than its electronic counterpart [1

1. M. Ferrera, Y. Park, L. Razzari, B. E. Little, S. T. Chu, R. Morandotti, D. J. Moss, and J. Azaña, “On-chip CMOS-compatible all-optical integrator,” Nat. Commun. 1(3), 29 (2010). [CrossRef] [PubMed]

]. Recent experimental demonstrations of all-optical temporal signal integration devices include a gain-assisted fiber grating resonant cavity [3

3. R. Slavík, Y. Park, N. Ayotte, S. Doucet, T. J. Ahn, S. LaRochelle, and J. Azaña, “Photonic temporal integrator for all-optical computing,” Opt. Express 16(22), 18202–18214 (2008). [CrossRef] [PubMed]

], a uniform fiber Bragg grating (FBG) [4

4. J. Azaña, “Ultrafast analog all-optical signal processors based on fiber-grating devices,” IEEE Photonics J. 2(3), 359–386 (2010). [CrossRef]

] and a CMOS-compatible photonic chip [1

1. M. Ferrera, Y. Park, L. Razzari, B. E. Little, S. T. Chu, R. Morandotti, D. J. Moss, and J. Azaña, “On-chip CMOS-compatible all-optical integrator,” Nat. Commun. 1(3), 29 (2010). [CrossRef] [PubMed]

]. These all-optical integrators are designed by synthesizing their spectral response as opposed to the time domain response. Two major limitations associated with these devices are that strict wavelength matching between the input optical signal and the spectral response of the integrators is required and the operational optical bandwidth is typically small. A wavelength-independent photonic intensity integrator has been demonstrated recently [5

5. Y. Park and J. Azaña, “Ultrafast photonic intensity integrator,” Opt. Lett. 34(8), 1156–1158 (2009). [CrossRef] [PubMed]

], where the integration operation is based on the superposition of an infinite set of continuously delayed replicas of the input signal. The replicas are generated by intensity modulating a rectangular-like incoherent broadband optical spectrum with the input drive signal, and the time delay is introduced by first-order chromatic waveguide dispersion. While the system in [5

5. Y. Park and J. Azaña, “Ultrafast photonic intensity integrator,” Opt. Lett. 34(8), 1156–1158 (2009). [CrossRef] [PubMed]

] enables wavelength-independent integration operation, it still suffers from a few limitations. First, since an incoherent broadband light source is required, the integrated signal exhibits a low signal-to-noise ratio (SNR), and single-shot operation is not possible. Second, the system cannot integrate large-bandwidth signals, such as ultrashort optical waveforms, due to the use of an electro-optical modulator. Third, km-length dispersive fibers are required to produce a sufficient integration time windows, due to the relatively small chromatic waveguide dispersion coefficient associated with single mode fibers. This long length results in both high loss and high system latency, and may cause signal distortion due to the non-negligible higher-order chromatic dispersion and polarization mode dispersion [6

6. M. H. Asghari, Y. Park, and J. Azaña, “Photonic intensity integrator with combined high processing speed and long operation time window,” in CLEO:2011—Laser Applications to Photonic Applications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper CThI2.

].

2. Basic concept

ε(θ,z)=n=1anJ0(bnθθc)exp(Dbn2zθc).
(1)

Δτ=τ2τ1n1csinθcn1c
(3)

3. All-optical integrator experiment

To characterize the optical intensity integrator, we use a 500-fs optical pulse to measure the impulse response. The pulse has a center wavelength of 1545 nm and a 3dB bandwidth of 20 nm. The output signal from the MMF is detected by a 17-GHz bandwidth photodiode and recorded by a 50 gigasample/second real-time oscilloscope. As shown in Fig. 1, the output waveform, or equivalently, the impulse response of the system, depends on the beam launch conditions, such as offset, tilt angle and width of the input beam [12

12. R. Ryf, S. Randel, A. H. Gnauck, C. Bolle, R. Essiambre, P. Winzer, D. W. Peckham, A. McCurdy, and R. Lingle, “Space-division multiplexing over 10 km of three-mode fiber using coherent 6 × 6 MIMO processing,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper PDPB10.

].

To demonstrate the wavelength-independent operation of the proposed integrator, we choose the input optical waveforms with different carrier wavelengths, which are generated by intensity modulating continuous-wave (CW) optical carriers with three wavelengths of 1535, 1540 and 1545 nm. The generated optical waveforms have a pulse width of 80 ps and repetition rate of 780 MHz. The optical pulses are transmitted through the MMF-based optical integrator. The input pulses (red curves) and the integration results (blue curves) of the three cases are shown in Figs. 2(a)
Fig. 2 Experimental demonstration of the wavelength and bandwidth independent operation of the MMF-based optical integrator. A tunable laser is modulated as the pulse source. The optical pulses are transmitted through the MMF based optical integrator. The red curves are the input temporal waveforms and the blue curves are the output. The wavelength of the laser is tuned to 1535nm, 1540nm and 1545nm, respectively, as shown in (a), (b) and (c). Modulated ASE is also used as a wideband source, and the waveform of the integrator’s input and output is shown in (d).
, 2(b) and 2(c). As expected, the integrator performance remains invariant as a function of carrier wavelength. Broadband operation of the integrator is also verified by using a 30 nm broadband amplified spontaneous emission (ASE) source as the optical carrier. A similar integration result is obtained, as shown in Fig. 2(d).

4. Application 1: Integration of wavelength-division multiplexing (WDM) waveform

The wavelength- and bandwidth-independent nature of this integrator is usually desirable for performing the integration of a wavelength-division multiplexing (WDM) waveform. As an example of this application, we apply the optical intensity integrator to an input optical signal consisting of three successive optical pulses with different center wavelengths. The three pulses, each having a pulse width of 30 ps and temporal spacing of 130 ps, are generated by first shaping the broadband spectrum of a femtosecond laser using a three-channel optical bandpass filter (1540.5, 1543.7 and 1546.9 nm), and then temporally stretching the filtered pulses using a 500 meter long dispersive fiber. The optical power spectrum and the temporal intensity profile of the generated optical signal are shown in Figs. 3(a)
Fig. 3 Demonstration of optical integration of a WDM signal. (a) Spectrum of the input signal. The three peaks are centered at 1540.5, 1543.7 and 1546.9 nm. (b) Input signal waveform, with each pulse in time corresponding to a different center wavelength. (c) Integral of the WDM signal. The red curve is the simulation result and the blue curve is the experimental result.
and 3(b), respectively. Figure 3(c) shows the integration of this signal using the integrator. We use the data in Fig. 3(b) to perform a numerical integration for comparison. The results of this calculation are shown in Fig. 3(c) (red curve). The obtained experimental results (blue curve) show excellent agreement with the theoretical results over the entire integration time window.

5. Application 2: Real-time high-throughput optical phase reconstruction

To demonstrate the utility of the optical integrator for ultrafast optical data processing performed in real-time, we applied it to perform high-throughput optical phase reconstruction of phase-modulated signals at telecommunications bit rates. Optical phase reconstruction is critical to the performance of coherent optical communications systems and fiber-optic distributed sensing systems [13

13. S. Liang, C. Zhang, W. Lin, L. Li, C. Li, X. Feng, and B. Lin, “Fiber-optic intrinsic distributed acoustic emission sensor for large structure health monitoring,” Opt. Lett. 34(12), 1858–1860 (2009). [CrossRef] [PubMed]

,14

14. A. Pasquazi, M. Peccianti, Y. Park, B. E. Little, S. T. Chu, R. Morandotti, J. Azaña, and D. J. Moss, “Sub-picosecond phase-sensitive optical pulse characterization on a chip,” Nat. Photonics 5(10), 618–623 (2011). [CrossRef]

]. The temporal phase profile of an optical signal is typically monitored using a frequency-discriminator-based phase differentiator followed by an optical integrator. Due to the lack of wavelength-insensitive optical integrators, only the derivative of the phase (frequency chirp) can be characterized in real-time [15

15. F. Li, Y. Park, and J. Azaña, “Single-shot real-time frequency chirp characterization of telecommunication optical signals based on balanced temporal optical differentiation,” Opt. Lett. 34(18), 2742–2744 (2009). [CrossRef] [PubMed]

]. While the complete phase information can be obtained by performing the integration digitally, it is very challenging to implement ultrafast high-throughput real-time integration in the digital domain due to the limited bandwidth of the digitizer and the digital signal processor. Here, we demonstrate complete optical phase reconstruction in real-time by combining a phase differentiator with our optical intensity integrator.

Figure 4
Fig. 4 Demonstration of real-time high-throughput optical phase reconstruction of phase-modulated signals. A CW laser (λ = 1550.02 nm) is phase modulated and transmitted through an edge filter, which serves as a differentiator. The beam is launched into the optical integrator and the phase is reconstructed. (a) The electrical drive signal. (b) Spectrum of the laser and spectral response of the edge filter. (c) Waveform of the output from the edge filter. (d) Waveform of the output from the optical integrator representing the reconstructed phase.
shows the experimental setup of the phase reconstruction system. An optical carrier from a CW laser source (λ = 1550.2 nm) is phase modulated by electrical pulses using a phase modulator. The drive electrical signal, as shown in Fig. 4(a), has a repetition rate of 7.8 GHz and pulse width of 80 ps. The phase modulated light is transmitted through an edge filter, which has a linear amplitude response. Figure 4(b) shows the spectrum of the laser (red curve) and the spectral response of the edge filter (green curve). The edge filter serves as a frequency discriminator, which converts the differential of the phase (instantaneous frequency) into the temporal intensity profile of the optical signal with a DC offset [15

15. F. Li, Y. Park, and J. Azaña, “Single-shot real-time frequency chirp characterization of telecommunication optical signals based on balanced temporal optical differentiation,” Opt. Lett. 34(18), 2742–2744 (2009). [CrossRef] [PubMed]

]. Figure 4(c) shows the measured temporal waveform at the output of the edge filter. This signal is then launched into the optical integrator. Figure 4(d) shows the output signal. The DC component of the integrated signal is blocked and a low-noise microwave amplifier with 20-dB gain amplifies the signal. The integration time window is reduced to 200 ps by offsetting the incident beam to increase the signal-to-noise ratio. The phase of the original optical signal is reconstructed in real-time using the edge filter and the developed optical integrator.

6. Conclusion

Acknowledgments

This work was partially supported by National Science Foundation Center for Integrated Access Networks (CIAN) and the National Natural Science Foundation of China 61177012. C. Wang is supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). The authors are grateful to members of Photonics Laboratory at University of California, Los Angeles, for valuable discussions.

References and links

1.

M. Ferrera, Y. Park, L. Razzari, B. E. Little, S. T. Chu, R. Morandotti, D. J. Moss, and J. Azaña, “On-chip CMOS-compatible all-optical integrator,” Nat. Commun. 1(3), 29 (2010). [CrossRef] [PubMed]

2.

M. H. Asghari and J. Azaña, “Photonic integrator-based optical memory unit,” IEEE Photon. Technol. Lett. 23(4), 209–211 (2011). [CrossRef]

3.

R. Slavík, Y. Park, N. Ayotte, S. Doucet, T. J. Ahn, S. LaRochelle, and J. Azaña, “Photonic temporal integrator for all-optical computing,” Opt. Express 16(22), 18202–18214 (2008). [CrossRef] [PubMed]

4.

J. Azaña, “Ultrafast analog all-optical signal processors based on fiber-grating devices,” IEEE Photonics J. 2(3), 359–386 (2010). [CrossRef]

5.

Y. Park and J. Azaña, “Ultrafast photonic intensity integrator,” Opt. Lett. 34(8), 1156–1158 (2009). [CrossRef] [PubMed]

6.

M. H. Asghari, Y. Park, and J. Azaña, “Photonic intensity integrator with combined high processing speed and long operation time window,” in CLEO:2011—Laser Applications to Photonic Applications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper CThI2.

7.

E. D. Diebold, N. K. Hon, Z. Tan, J. Chou, T. Sienicki, C. Wang, and B. Jalali, “Giant tunable optical dispersion using chromo-modal excitation of a multimode waveguide,” Opt. Express 19(24), 23809–23817 (2011). [CrossRef] [PubMed]

8.

A. Shah, C. J. Hsu, A. Tarighat, A. H. Sayed, and B. Jalali, “Coherent optical MIMO (COMIMO),” J. Lightwave Technol. 23(8), 2410–2419 (2005). [CrossRef]

9.

H. R. Stuart, “Dispersive multiplexing in multimode optical fiber,” Science 289(5477), 281–283 (2000). [CrossRef] [PubMed]

10.

S. Murshid, B. Grossman, and P. Narakorn, “Spatial domain multiplexing: A new dimension in fiber optic multiplexing,” Opt. Laser Technol. 40(8), 1030–1036 (2008). [CrossRef]

11.

U. Levy, H. Kobrinsky, and A. Friesem, “Angular multiplexing for multichannel communication in a single fiber,” IEEE J. Quantum Electron. 17(11), 2215–2224 (1981). [CrossRef]

12.

R. Ryf, S. Randel, A. H. Gnauck, C. Bolle, R. Essiambre, P. Winzer, D. W. Peckham, A. McCurdy, and R. Lingle, “Space-division multiplexing over 10 km of three-mode fiber using coherent 6 × 6 MIMO processing,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper PDPB10.

13.

S. Liang, C. Zhang, W. Lin, L. Li, C. Li, X. Feng, and B. Lin, “Fiber-optic intrinsic distributed acoustic emission sensor for large structure health monitoring,” Opt. Lett. 34(12), 1858–1860 (2009). [CrossRef] [PubMed]

14.

A. Pasquazi, M. Peccianti, Y. Park, B. E. Little, S. T. Chu, R. Morandotti, J. Azaña, and D. J. Moss, “Sub-picosecond phase-sensitive optical pulse characterization on a chip,” Nat. Photonics 5(10), 618–623 (2011). [CrossRef]

15.

F. Li, Y. Park, and J. Azaña, “Single-shot real-time frequency chirp characterization of telecommunication optical signals based on balanced temporal optical differentiation,” Opt. Lett. 34(18), 2742–2744 (2009). [CrossRef] [PubMed]

OCIS Codes
(070.6020) Fourier optics and signal processing : Continuous optical signal processing
(200.4740) Optics in computing : Optical processing
(060.5625) Fiber optics and optical communications : Radio frequency photonics
(320.7085) Ultrafast optics : Ultrafast information processing

ToC Category:
Ultrafast Optics

History
Original Manuscript: April 16, 2012
Revised Manuscript: May 29, 2012
Manuscript Accepted: May 31, 2012
Published: June 11, 2012

Citation
Zhongwei Tan, Chao Wang, Eric D. Diebold, Nick K. Hon, and Bahram Jalali, "Real-time wavelength and bandwidth-independent optical integrator based on modal dispersion," Opt. Express 20, 14109-14116 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-13-14109


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References

  1. M. Ferrera, Y. Park, L. Razzari, B. E. Little, S. T. Chu, R. Morandotti, D. J. Moss, and J. Azaña, “On-chip CMOS-compatible all-optical integrator,” Nat. Commun.1(3), 29 (2010). [CrossRef] [PubMed]
  2. M. H. Asghari and J. Azaña, “Photonic integrator-based optical memory unit,” IEEE Photon. Technol. Lett.23(4), 209–211 (2011). [CrossRef]
  3. R. Slavík, Y. Park, N. Ayotte, S. Doucet, T. J. Ahn, S. LaRochelle, and J. Azaña, “Photonic temporal integrator for all-optical computing,” Opt. Express16(22), 18202–18214 (2008). [CrossRef] [PubMed]
  4. J. Azaña, “Ultrafast analog all-optical signal processors based on fiber-grating devices,” IEEE Photonics J.2(3), 359–386 (2010). [CrossRef]
  5. Y. Park and J. Azaña, “Ultrafast photonic intensity integrator,” Opt. Lett.34(8), 1156–1158 (2009). [CrossRef] [PubMed]
  6. M. H. Asghari, Y. Park, and J. Azaña, “Photonic intensity integrator with combined high processing speed and long operation time window,” in CLEO:2011—Laser Applications to Photonic Applications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper CThI2.
  7. E. D. Diebold, N. K. Hon, Z. Tan, J. Chou, T. Sienicki, C. Wang, and B. Jalali, “Giant tunable optical dispersion using chromo-modal excitation of a multimode waveguide,” Opt. Express19(24), 23809–23817 (2011). [CrossRef] [PubMed]
  8. A. Shah, C. J. Hsu, A. Tarighat, A. H. Sayed, and B. Jalali, “Coherent optical MIMO (COMIMO),” J. Lightwave Technol.23(8), 2410–2419 (2005). [CrossRef]
  9. H. R. Stuart, “Dispersive multiplexing in multimode optical fiber,” Science289(5477), 281–283 (2000). [CrossRef] [PubMed]
  10. S. Murshid, B. Grossman, and P. Narakorn, “Spatial domain multiplexing: A new dimension in fiber optic multiplexing,” Opt. Laser Technol.40(8), 1030–1036 (2008). [CrossRef]
  11. U. Levy, H. Kobrinsky, and A. Friesem, “Angular multiplexing for multichannel communication in a single fiber,” IEEE J. Quantum Electron.17(11), 2215–2224 (1981). [CrossRef]
  12. R. Ryf, S. Randel, A. H. Gnauck, C. Bolle, R. Essiambre, P. Winzer, D. W. Peckham, A. McCurdy, and R. Lingle, “Space-division multiplexing over 10 km of three-mode fiber using coherent 6 × 6 MIMO processing,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper PDPB10.
  13. S. Liang, C. Zhang, W. Lin, L. Li, C. Li, X. Feng, and B. Lin, “Fiber-optic intrinsic distributed acoustic emission sensor for large structure health monitoring,” Opt. Lett.34(12), 1858–1860 (2009). [CrossRef] [PubMed]
  14. A. Pasquazi, M. Peccianti, Y. Park, B. E. Little, S. T. Chu, R. Morandotti, J. Azaña, and D. J. Moss, “Sub-picosecond phase-sensitive optical pulse characterization on a chip,” Nat. Photonics5(10), 618–623 (2011). [CrossRef]
  15. F. Li, Y. Park, and J. Azaña, “Single-shot real-time frequency chirp characterization of telecommunication optical signals based on balanced temporal optical differentiation,” Opt. Lett.34(18), 2742–2744 (2009). [CrossRef] [PubMed]

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