Continuously-tunable, bit-rate variable OTDM using broadband SBS slow-light delay line

doi:10.1364/OE.15.008317

« Show journal navigation

Continuously-tunable, bit-rate variable OTDM using broadband SBS slow-light delay line

B. Zhang, L. Zhang, L.-S. Yan, I. Fazal, J.-Y. Yang, and A. E. Willner »View Author Affiliations

Optics Express, Vol. 15, Issue 13, pp. 8317-8322 (2007)
http://dx.doi.org/10.1364/OE.15.008317

View Full Text Article

Acrobat PDF (272 KB)

Journal Search
Article Lookup

Article Tools

Citations

Alert me when this article is cited
Export Citation/Save

Full Text
Article Info
References (13)
Cited By
Figures (7)
Metrics

Abstract

We conceptually compare the advantages of the proposed slow-light-based tunable OTDM to conventional fiber-based fixed OTDM multiplexer. We experimentally demonstrate continuously-controllable OTDM of two 2.5-Gb/s return-to-zero (RZ) signals using broadband SBS-based slow-light as the tunable optical delay line. We show that the time slot of one signal path can be manipulated relative to the other by as much as 75-ps. This continuous slow light tunability dramatically enhances the OTDM system performance which results in a power penalty reduction of 9-dB for the multiplexed data stream. We also demonstrate variable-bit-rate OTDM by dynamically adjusting the tunable slow-light delay according to the input bit-rates. We show efficient two-by-one optical time multiplexing of three different input data streams at 2.5-Gb/s, 2.67-Gb/s and 5-Gb/s.

1. Introduction

During the last few years, slow light has captured much imagination as a potential method for achieving a continuously-tunable optical delay-line. One of the most fundamental and potentially useful applications of an optical delay line is the fine-grain temporal manipulation of one data bit stream relative to the other such that these two can be efficiently time multiplexed with minimal system penalty [1

S. A. Hamilton, B. S. Robinson, T. E. Murphy, S. J. Savage, and E. P. Ippen, “100 Gb/s Optical Time-Division Multiplexed Networks,” J. Lightwave Technol. 20, 2086–2100 (2002). [CrossRef]

]. This bit-level time synchronization to achieve optical-time-division-multiplexing (OTDM) is typically accomplished manually, either by varying a free-space or optical fiber delay-line, choosing from a finite set of discrete optical path lengths, or adjusting the electrical RF cables. These methodologies produce only a fixed set of discrete time delays, whether they be based on lengths of fibers, integrated waveguides, or free-space [1

S. A. Hamilton, B. S. Robinson, T. E. Murphy, S. J. Savage, and E. P. Ippen, “100 Gb/s Optical Time-Division Multiplexed Networks,” J. Lightwave Technol. 20, 2086–2100 (2002). [CrossRef]

]. Another limitation is that once the fixed set of lengths is chosen, the input data-bit-rates to be multiplexed are pre-determined accordingly. However, a truly flexible time synchronizer should feature continuously-controllable delay and dynamically-reconfigurable input bit-rates. A laudable goal would thus be to utilize slow light technique to achieve continuous tunability for efficient time multiplexing of multiple Gbit/s data streams.

To date, several slow-light mechanisms for producing an adjustable optical delay for multi-Gbit/s data signals have been demonstrated, such as stimulated-Brillouin-scattering (SBS) [2–3

M. González Herráez, K. Y. Song, and L. Thévenaz, “Optically controlled slow and fast light in optical fibers using stimulated Brillouin scattering,” Appl. Phys. Lett. 87, 081113 (2005) [CrossRef]

], stimulated-Raman-scattering (SRS) [4–5

J. Sharping, Y. Okawachi, and A. Gaeta, “Wide bandwidth slow light using a Raman fiber amplifier,” Opt. Express 13, 6092–6098 (2005). [CrossRef] [PubMed]

], semiconductor-optical-amplifiers (SOA) [6–7

J. Mørk, R. Kjær, M. van der Poel, and K. Yvind, “Slow light in a semiconductor waveguide at gigahertz frequencies,” Opt. Express 13, 8136–8145 (2005). [CrossRef] [PubMed]

], and optical-parametric-amplifiers (OPA) [8–9

D. Dahan and G. Eisenstein, “Tunable all optical delay via slow and fast light propagation in a Raman assisted fiber optical parametric amplifier: a route to all optical buffering,” Opt. Express 13, 6234–6249 (2005). [CrossRef] [PubMed]

]. Two notable recent results demonstrated broadband SBS slow light with 12-GHz [10

Z. Zhu, A. M. C. Dawes, D. J. Gauthier, L. Zhang, and A. E. Willner, “Broadband SBS Slow Light in an Optical Fiber,” J. Lightwave Technol. 25, 201–206 (2007). [CrossRef]

] and 25-GHz [11

K. Y. Song and K. Hotate, “25 GHz bandwidth Brillouin slow light in optical fibers,” Opt. Lett. 32, 217–219 (2007). [CrossRef] [PubMed]

] bandwidth by frequency-broadening the SBS pump spectrum. These achievements are important since SBS can thus be utilized to support multi-Gb/s data streams with minimal signal distortion.

Although many methods have been demonstrated for achieving simple delay, there has been no report in literature of using a slow-light delay to completely control the time multiplexing of two data streams as well as to quantify the reduction of system power penalty due to any timing misalignment. Furthermore, future heterogeneous optical networks, especially with hybrid incoming data-bit-rates, as well as different data modulation formats [12

B. Zhang, L. Yan, I. Fazal, L. Zhang, A. E. Willner, Z. Zhu, and D. J. Gauthier, “Slow light on Gbit/s differential-phase-shift-keying signals,” Opt. Express 15, 1878–1883 (2007). [CrossRef] [PubMed]

], will require variable-bit-rate OTDM multiplexer.

In this paper, we first show conceptually key advantages of using a slow-light based tunable and flexible OTDM multiplexer, compared with conventional fixed set of fiber-lengths based counterparts. We demonstrate experimentally continuously-controllable OTDM of two 2.5-Gb/s 33% return-to-zero (RZ) data channels using broadband SBS-based slow-light as the tunable optical delay line. We show that the time slot of one signal path can be manipulated relative to the time slot of another path by as much as 75-ps at a pump power of 600-mW. This continuous tunability dramatically enhances the dynamic performance of such OTDM system which results in a power penalty reduction of 9-dB at a bit-error-rate (BER) of 10^-9 for the time-multiplexed 5-Gb/s data stream. We also verified conceptually and demonstrated experimentally that variable-bit-rate OTDM multiplexer can be achieved by dynamically adjusting the tunable slow-light delay according to the input bit-rates. We show efficient two-by-one optical time multiplexing of three different input data streams at 2.5-Gb/s, 2.67-Gb/s and 5-Gb/s. All three multiplexed data streams show error-free performance with increasing power penalty due to the limited SBS slow-light bandwidth.

2. Concept of slow-light-based OTDM tunable multiplexer

Conventional fiber-based OTDM multiplexer (MUX) incorporates a fixed set of fiber-lengths which is only suitable for a discrete set of given bit-misalignments of the incoming data streams. As shown in Fig. 1, when two input streams from two different locations pass through a fiber-based fixed OTDM MUX, it is highly likely that they will be misaligned and cause bit-overlapping at the output. By utilizing the continuously-controllable delay feature of slow-light-based OTDM MUX, one could possibly manipulate the relative time misalignment by tweaking the slow-light control knob (e.g., pump power) and align the two streams nicely.

Fig. 1. Due to the fixed set of optical path delays, it is highly likely that conventional OTDM will have misaligned outputs for some certain incoming offsets between bit streams. However, slow-light-based tunable OTDM can always reconfigure its continuous delay according to the incoming offsets and get the output streams well-multiplexed.

Another very important feature of slow-light-based OTDM MUX is its flexibility to be dynamically adaptive to the incoming data-bit-rates so that it could enable a variable-bit-rate OTDM system, whereas conventional fiber-based fixed OTDM could not provide this capability. Shown in Fig. 2 is one typical example of two sets of input bit-rates data streams. By simply control the slow-light knob for producing either 200-ps or 100ps delay, we could effectively multiplex either two 2.5-Gb/s or two 5-Gb/s data streams, respectively. One thing to note is that the incoming bit-rates can in theory be reconfigured to any value, thanks to the continuously-tunable slow-light delay. This nice feature virtually puts no limitations on the delay resolution.

Fig. 2. Concept of slow-light-based OTDM for dynamic reconfiguration of different input data-bit-rates. Incoming bit-rates to be multiplexed can in theory be of any value because of the continuously-controllable delays slow light offer.

3. Experimental setup

The experimental setup is shown in Fig. 3. The tunable laser source (TLS) is modulated with 2¹⁵-1 PRBS data by two Mach-Zehnder modulators (MZM) to generate 2.5-Gb/s return-to-zero (RZ) signals. The second MZM is driven by half-rate clock with 2Vπ swing to generate 33% duty cycle. This 33% RZ signal is then split by a 50:50 fiber coupler. The upper channel is served as a reference and the lower channel is passed through the broadband SBS-based tunable slow-light delay line. Broadband SBS is realized by noise modulating the directly-modulated-laser (DML) at 400 MHz clock [10

Z. Zhu, A. M. C. Dawes, D. J. Gauthier, L. Zhang, and A. E. Willner, “Broadband SBS Slow Light in an Optical Fiber,” J. Lightwave Technol. 25, 201–206 (2007). [CrossRef]

]. The RF amplifier and attenuator are used to control the pump spectral width to be 7-GHz. The broadened SBS pump is then amplified and enters a 2-km Highly Nonlinear Fiber (HNLF), which is served as the slow-light medium. The zero-dispersion wavelength of the HNLF is at 1552 nm, the dispersion slope is 0.045 ps/(nm²km), the effective mode area is 14.5 μm² and the gamma coefficient is 9.1 W^-1km^-1. The 33% RZ signals in the lower channel then counter-propagates with respect to the pump in the HNLF, with its polarization state controlled for maximization of the SBS interaction. One optical delay-line (ODL) is used to emulate the initial offset of the upper and lower arms for demonstration purposes. The amplified and delayed lower channel signal is routed out via a circulator. One attenuator is used to adjust to the SBS gain so as to balance the power with the upper arm. One 7-GHz Fabry-Perot filter is used to suppress the spectral crosstalk from Rayleigh pump backscattering [13

B. Zhang, L.-S. Yan, J.-Y. Yang, I. Fazal, and A. E. Willner, “A single slow-light element for independent delay control and synchronization on multiple Gb/s data channels,” Photon. Technol. Lett. 19, issue. 14, (2007)

]. BER measurement is taken on the multiplexed 5-Gb/s RZ signals.

Fig. 3. Experimental setup of SBS slow-light based tunable OTDM for time multiplexing of two 2.5-Gb/s 33% RZ signals.

4. Experimental results: tunable OTDM of two 2.5-Gb/s data signals

Fig. 4. Bit patterns and spectra comparison when SBS pump is off and turned on to 600mW. This shows efficient OTDM multiplexing after continuously-tunable slow-light of up to 75-ps delay.

When the SBS pump is off, the ODL is initially set such that the upper and lower arms are offset by 75-ps away from the well-multiplexed condition. The choice of the misalignment is only for proof of concept and can be dependent on various system parameters, such as the slow-light bandwidth, SBS pump power and initial signal pulse-width. We can clearly see from Fig. 4 that when the pump is off, the recorded bit patterns after OTDM have a severe beating region which is due to the bit-overlapping of the two channels. By turning on the SBS pump, the lower channel can be slowed down by up to 75-ps at a pump power of 600-mW so that the delayed bits are right interleaved with respect to the reference upper arm. Multiplexed bit patterns show that the output data stream has doubled the bit-rate to 5-Gb/s with good signal quality. Slight beating still remains in the non-delayed reference bit slots since dispersive slow-light medium broadens the delayed bits whose tails penetrate into the neighboring slot. Spectra of both the original 2.5-Gb/s data streams and the slow-light well-multiplexed 5-Gb/s data streams show that the spectral width remains almost the same. The asymmetry of the spectrum for the multiplexed case is due to the residual Rayleigh pump backscattering even after 7-GHz filtering [13

Fig. 5. Power penalty versus fractional delay and the corresponding eye diagrams. Up to 9-dB power penalty reduction is achieved by using slow-light-based OTDM, which shows its capability for dynamically enhancing the system performance.

Figure 5 quantifies the relative power penalty (with respect to the well-multiplexed case) versus the fractional delay, which is defined as the absolute slow-light delay divided by FWHM of 33% RZ pulses. We show that as the fractional delay increases, the relative power penalty can be reduced gradually, resulting in a maximum power penalty reduction of 9-dB. The eye diagrams corresponding to three pump power levels are shown. The main reason for the improvement is that the beating caused by the bit-overlapping reduces dramatically after efficient slow-light-based multiplexing. Some residual beating in the reference bit-slots is still observed due to the slow-light-induced pulse broadening, as explained in the previous paragraph. Therefore, a crucial requirement for efficient 2:1 OTDM is that the original incoming RZ pulses should ideally occupy less than half the bit slot so that beating region could be minimized after multiplexing. On the other hand, we do not want to have too narrow-width pulses which will waste the slow-light bandwidth.

5. Experimental Results: Variable bit-rate OTDM

We also experimentally demonstrate variable-bit-rate OTDM using SBS-based slow light. Three different input bit-rates of 2.5-Gb/s, 2.67-Gb/s, and 5-Gb/s 33% RZ PRBS data are subsequently passed through the slow-light element. Broadband SBS bandwidth is fixed at 5GHz for different bit-rates comparison. We show in Fig. 6 three eye diagrams after OTDM multiplexing of 2.5-Gb/s to 5-Gb/s, 2.67-Gb/s to 5.34-Gb/s and 5-Gb/s to 10-Gb/s, respectively. We can see from Fig. 7 that the maximum achievable fractional delay after OTDM is reduced with increasing of the input bit-rates. This is because of the limited slow-light bandwidth for higher bit-rate signals. In the meantime, OTDM-induced power penalty also increases with the data bit-rates. This can be attributed to the fact that at higher bit-rate, multiplexed signal not only suffers from slow-light-induced pulse broadening on the delayed channel, but also experiences increased spectral-overlapping crosstalk from Rayleigh pump backscattering [13

]. These limitations can be alleviated if a much wider slow-light BW medium, such as optical parametric amplification (OPA) [8–9

], is used. Future variable-bit-rate N:1 OTDM multiplexing could be enhanced by either cascading multiple 2:1 OTDM MUX or enabling multiple-channel operation in a single slow light element [13

Fig. 6. Variable bit-rate OTDM: Efficient multiplexing of two data streams at three different input bit-rate.

Fig. 7. Power penalty and fractional delay at three different input bit-rates.

6. Conclusion

We experimentally demonstrate continuously-controllable OTDM multiplexing of two 2.5-Gb/s RZ data signals using broadband SBS-based slow light delay line. We show that the time slot of one path can be manipulated relative to the other by as much as 75-ps. This slow-light tunability dramatically enhances the performance of time-multiplexed 5-Gb/s signals, resulting in a power penalty reduction of 9-dB at a BER of 10^-9. We also demonstrate experimentally variable-bit-rate OTDM by adjusting the slow-light delay according to the different input bit-rate. We show error-free 2:1 OTDM of three different input data streams at 2.5-Gb/s, 2.67-Gb/s, and 5-Gb/s. This proof-of-concept demonstration addresses the importance of using slow-light-based tunable delay for OTDM multiplexing functionality.

Acknowledgments

We gratefully acknowledge the financial support of the DARPA DSO Slow-Light program.

References and links

1.	S. A. Hamilton, B. S. Robinson, T. E. Murphy, S. J. Savage, and E. P. Ippen, “100 Gb/s Optical Time-Division Multiplexed Networks,” J. Lightwave Technol. 20, 2086–2100 (2002). [CrossRef]
2.	M. González Herráez, K. Y. Song, and L. Thévenaz, “Optically controlled slow and fast light in optical fibers using stimulated Brillouin scattering,” Appl. Phys. Lett. 87, 081113 (2005) [CrossRef]
3.	Y. Okawachi, M. S. Bigelow, J. E. Sharping, Z. M. Zhu, A. Schweinsberg, D. J. Gauthier, R. W. Boyd, and A. L. Gaeta, “Tunable all-optical delays via Brillouin slow light in an optical fiber,” Phys. Rev. Lett. 94, 153902 (2005). [CrossRef] [PubMed]
4.	J. Sharping, Y. Okawachi, and A. Gaeta, “Wide bandwidth slow light using a Raman fiber amplifier,” Opt. Express 13, 6092–6098 (2005). [CrossRef] [PubMed]
5.	Y. Okawachi, M. Foster, J. Sharping, A. Gaeta, Q. Xu, and M. Lipson, “All-optical slow-light on a photonic chip,” Opt. Express 14, 2317–2322 (2006). [CrossRef] [PubMed]
6.	J. Mørk, R. Kjær, M. van der Poel, and K. Yvind, “Slow light in a semiconductor waveguide at gigahertz frequencies,” Opt. Express 13, 8136–8145 (2005). [CrossRef] [PubMed]
7.	F. G. Sedgwick, B. Pesala, J. -Y. Lin, W. S. Ko, X. Zhao, and C. J. Chang-Hasnain, “THz-bandwidth tunable slow light in semiconductor optical amplifiers,” Opt. Express 15, 747–753 (2007). [CrossRef] [PubMed]
8.	D. Dahan and G. Eisenstein, “Tunable all optical delay via slow and fast light propagation in a Raman assisted fiber optical parametric amplifier: a route to all optical buffering,” Opt. Express 13, 6234–6249 (2005). [CrossRef] [PubMed]
9.	L. Yi, W. Hu, Y. Su, M. Gao, and L. Leng, “Design and system demonstration of a tunable slow-light delay line based on fiber parametric process,” Photon. Technol. Lett. 18, 2575–2577, (2006) [CrossRef]
10.	Z. Zhu, A. M. C. Dawes, D. J. Gauthier, L. Zhang, and A. E. Willner, “Broadband SBS Slow Light in an Optical Fiber,” J. Lightwave Technol. 25, 201–206 (2007). [CrossRef]
11.	K. Y. Song and K. Hotate, “25 GHz bandwidth Brillouin slow light in optical fibers,” Opt. Lett. 32, 217–219 (2007). [CrossRef] [PubMed]
12.	B. Zhang, L. Yan, I. Fazal, L. Zhang, A. E. Willner, Z. Zhu, and D. J. Gauthier, “Slow light on Gbit/s differential-phase-shift-keying signals,” Opt. Express 15, 1878–1883 (2007). [CrossRef] [PubMed]
13.	B. Zhang, L.-S. Yan, J.-Y. Yang, I. Fazal, and A. E. Willner, “A single slow-light element for independent delay control and synchronization on multiple Gb/s data channels,” Photon. Technol. Lett. 19, issue. 14, (2007)

OCIS Codes
(060.4230) Fiber optics and optical communications : Multiplexing
(060.4370) Fiber optics and optical communications : Nonlinear optics, fibers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: May 14, 2007
Revised Manuscript: June 12, 2007
Manuscript Accepted: June 12, 2007
Published: June 18, 2007

Citation
B. Zhang, L. Zhang, L.-S. Yan, I. Fazal, J.-Y. Yang, and A. E. Willner, "Continuously-tunable, bit-rate variable OTDM using broadband SBS slow-light delay line," Opt. Express 15, 8317-8322 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-13-8317

Sort: Year | Journal | Reset

References

S. A. Hamilton, B. S. Robinson, T. E. Murphy, S. J. Savage, and E. P. Ippen, "100 Gb/s Optical Time-Division Multiplexed Networks," J. Lightwave Technol. 20,2086-2100 (2002). [CrossRef]
M. González Herráez, K. Y. Song, and L. Thévenaz, " Optically controlled slow and fast light in optical fibers using stimulated Brillouin scattering," Appl. Phys. Lett. 87, 081113 (2005) [CrossRef]
Y. Okawachi, M. S. Bigelow, J. E. Sharping, Z. M. Zhu, A. Schweinsberg, D. J. Gauthier, R. W. Boyd, and A. L. Gaeta, "Tunable all-optical delays via Brillouin slow light in an optical fiber," Phys. Rev. Lett. 94, 153902 (2005). [CrossRef] [PubMed]
J. Sharping, Y. Okawachi, and A. Gaeta, "Wide bandwidth slow light using a Raman fiber amplifier," Opt. Express 13, 6092-6098 (2005). [CrossRef] [PubMed]
Y. Okawachi, M. Foster, J. Sharping, A. Gaeta, Q. Xu, and M. Lipson, "All-optical slow-light on a photonic chip," Opt. Express 14, 2317-2322 (2006). [CrossRef] [PubMed]
J. Mørk, R. Kjær, M. van der Poel, and K. Yvind, "Slow light in a semiconductor waveguide at gigahertz frequencies," Opt. Express 13, 8136-8145 (2005). [CrossRef] [PubMed]
F. G. Sedgwick, B. Pesala, J. -Y. Lin, W. S. Ko, X. Zhao, and C. J. Chang-Hasnain, "THz-bandwidth tunable slow light in semiconductor optical amplifiers," Opt. Express 15, 747-753 (2007). [CrossRef] [PubMed]
D. Dahan and G. Eisenstein, "Tunable all optical delay via slow and fast light propagation in a Raman assisted fiber optical parametric amplifier: a route to all optical buffering," Opt. Express 13, 6234-6249 (2005). [CrossRef] [PubMed]
L. Yi, W. Hu, Y. Su, M. Gao, L. Leng, "Design and system demonstration of a tunable slow-light delay line based on fiber parametric process," Photon. Technol. Lett. 18, 2575-2577, (2006) [CrossRef]
Z. Zhu, A. M. C. Dawes, D. J. Gauthier, L. Zhang, and A. E. Willner, "Broadband SBS Slow Light in an Optical Fiber," J. Lightwave Technol. 25, 201-206 (2007). [CrossRef]
K. Y. Song and K. Hotate, "25 GHz bandwidth Brillouin slow light in optical fibers," Opt. Lett. 32, 217-219 (2007). [CrossRef] [PubMed]
B. Zhang, L. Yan, I. Fazal, L. Zhang, A. E. Willner, Z. Zhu, and D. J. Gauthier, "Slow light on Gbit/s differential-phase-shift-keying signals," Opt. Express 15, 1878-1883 (2007). [CrossRef] [PubMed]
B. Zhang, L.-S. Yan. J.-Y. Yang, I. Fazal and A. E. Willner, "A single slow-light element for independent delay control and synchronization on multiple Gb/s data channels," Photon. Technol. Lett. 19 (2007)

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.

Click here to see a list of articles that cite this paper