156-μs continuously tunable parametric delay line for a 40-gb/s signal

1.56-µs continuously tunable parametric delay line for a 40-Gb/s signal

Evgeny Myslivets1*

, Nikola Alic1, Slaven Moro

1, Bill P. P. Kuo

1, R. M. Jopson

2,

C. J. McKinstrie2, M. Karlsson

3 and Stojan Radic

1

1Department of Electrical and Computer Engineering, University of California San Diego, 9500 Gilman Dr., La Jolla CA 92037, USA

2Bell Laboratories, Alcatel-Lucent, Holmdel, NJ, 3Chalmers University of Technology, Gothenburg, Sweden

*[email protected]

Abstract: Experimental demonstration of an all-fiber, all-optical continuously tunable delay line is reported. The 1.56-µs delay with a record 62,400 time-delay bit-rate product was characterized for a 40-Gbps data channel. The result was enabled by parametric dispersion compensation with cascaded triple-conversion in highly-nonlinear fiber capable of continuous tuning over 39.5 nm.

©2009 Optical Society of America

OCIS codes: (190.4380) Nonlinear optics, four-wave mixing.

References and links

1. R. Ramaswami, and K. N. Sivarajan, “Routing and wavelength assignment in all-optical networks,” IEEE/ACM Trans. Netw. 3(5), 489–500 (1995).

2. S. P. Davis, J. W. Brault, and M. C. Abrams, Fourier Transform Spectrometry, 1st Ed. Academic Press (1983). 3. D. Cotter, R. J. Manning, K. J. Blow, A. D. Ellis, A. E. Kelly, D. Nesset, I. D. Phillips, A. J. Poustie, and D. C.

Rogers, “Nonlinear Optics for High-Speed Digital Information Processing,” Science 286(5444), 1523–1528 (1999).

4. Zh. Jiang, D. E. Leaird, and A. M. Weiner, “Line-by-line pulse shaping control for optical arbitrary waveform generation,” Opt. Express 13(25), 10431–10439 (2005).

5. W. Ng, A. A. Walston, G. L. Tangonan, J. J. Lee, I. L. Newberg, and N. Bernstein, “The 1st demonstration of an optically steered microwave phased-array antenna using true-time delay,” J. Lightwave Technol. 9(9), 1124–1131 (1991).

6. R. S. Tucker, P.-Ch. Ku, and C. J. Chang-Hasnain, “Slow-Light Optical Buffers: Capabilities and Fundamental Limitations,” J. Lightwave Technol. 23(12 Issue 12), 4046–4066 (2005).

7. N. Alic, J. R. Windmiller, J. B. Coles, and S. Radic, “Two-Pump Parametric Optical Delays,” IEEE J. Sel. Top. Quantum Electron. 14(3), 681–690 (2008).

8. E. Myslivets, N. Alic, J. R. Windmiller, R. M. Jopson, and S. Radic, “400-ns Continuously Tunable Delay of 10-Gb/s Intensity Modulated Optical Signal,” IEEE Photon. Technol. Lett. 21(4), 251–253 (2009).

9. N. Alic, E. Myslivets, S. Moro, B. P. P. Kuo, R. M. Jopson, C. J. McKinstrie, and S. Radic, “1.83-µs Wavelength-Transparent All-Optical Delay,” post deadline paper, OFC 2009, PDPA1, San Diego USA (2009).

10. O. F. Yilmaz, S. R. Nuccio, X. Wu, and A. E. Willner, “10-Packet-Depth, 40 Gb/s Optical Buffer with a <0.5ns Reconfigurable Time using.116 ns, Continuously Tunable Conversion/Dispersion Delays,” post deadline paper, OFC 2009, PDPC7, San Diego USA (2009).

11. Y. Dai, X. Chen, Y. Okawachi, A. C. Turner-Foster, M. A. Foster, M. Lipson, A. L. Gaeta, and C. Xu, “1 micros tunable delay using parametric mixing and optical phase conjugation in Si waveguides,” Opt. Express 17(9), 7004–7010 (2009).

12. E. Myslivets, N. Alic, S. Moro, B. P.-P. Kuo, R. M. Jopson, C. J. McKinstrie, A. H. Gnauck, M. Karlsson, and S. Radic, “1.56-µs Continuously Tuneable 40-Gb/s Parametric Delay,” accepted for oral presentation in ECOC 2009, Vienna Austria (2009).

13. S. Radic, R. M. Jopson, C. J. McKinstrie, A. H. Gnauck, S. Chandrasekhar, and J. C. Centanni, “Wavelength division multiplexed transmission over standard single mode fiber using polarization insensitive signal conjugation in highly nonlinear optical fiber,” post deadline paper, OFC 2003, PD12, Atlanta USA (2003).

14. S. Namiki, “Wide-Band and -Range Tunable Dispersion Compensation through Parametric Wavelength Conversion and Dispersive Optical Fibers,” J. Lightwave Technol. 26(1 Issue 1), 28–35 (2008).

15. S. Namiki, and T. Kurosu, “17 ns tunable delay for picosecond pulses through simultaneous and independent control of delay and dispersion using cascaded parametric process,” post deadline paper, ECOC 2008, Th. 3, C.3, Brussels Belgium (2008).

#111225 - $15.00 USD Received 11 May 2009; revised 12 Jun 2009; accepted 20 Jun 2009; published 30 Jun 2009

(C) 2009 OSA 6 July 2009 / Vol. 17, No. 14 / OPTICS EXPRESS 11958

1. Introduction

Continuously tunable delays are key components in a number of applications of high practical interest: all-optical routing [1], optical correlation [2], high-speed gating [3], arbitrary waveform generation [4], beam steering sensing [5], and detection. Optical tunable delay lines can be divided into two main classes based on the operating principle. The first class (commonly called “slow light”) utilizes the rapid refractive index change associated with nearby optical resonances. Although delay values of several pulse durations have been demonstrated with this approach, a fundamental delay-bandwidth issue [6] limits the maximally achievable delay for a given signal bandwidth. In the second class of delay engineering, the common approach is based on signal wavelength translation to a different carrier (or idler) and propagation in a dispersive waveguide. Delay tuning is achieved by changing the wavelength of the created idler resulting in the desired delay in the dispersive waveguide. Highly nonlinear fibers (HNLFs) [7–9], periodically polled lithium niobate structures (PPLN) [10] and Si waveguides [11] have all been used as wavelength converting elements. Conversion in HNLF was used for existing record results as its conversion efficiency and wide operating bandwidth are significantly superior to those of alternative platforms. More importantly, HNLFs can be easily incorporated into fiber-based setups with negligibly small coupling loss, and can be packaged on spools with diameters smaller than 20 mm.

In this paper we demonstrate a two-pump parametric tunable optical delay line based on three cascaded conversions in highly nonlinear fibers. Furthermore, we provide detailed characterization of the demonstrated delay [12]. The performance of this buffering element was measured for a 40-Gbps channel using long (2

31-1) pseudo-random pattern bit-error

testing. The delay configuration provided full compensation for the dispersion and the timing jitter. The measured tuning delay range is the longest achieved value for 40-Gb/s signals reported to date.

2. Principle of operation

The operating principle for delay lines based on wavelength converters and dispersive elements is easily understood [7]: the input signal is mapped to a new carrier wavelength that will propagate with a different group velocity in the dispersive medium, and correspondingly, accumulate a different delay than that of the original signal. It is critically important to note that almost all practical applications [1–5] require that the delayed output signal wavelength be identical to the input wavelength. More importantly, the delay value also must be independent of the operational wavelength. Consequently, an additional wavelength conversion is necessary after the dispersive element.

Owing to the finite bandwidth of the delayed signals, the process of delay accumulation by means of propagation in a dispersive medium inherently imposes a significant amount of group-velocity-dispersion-induced inter-symbol interference on the signal. Therefore, dispersion compensation is necessary. The compensation of dispersion can be facilitated by phase conjugation which, for a properly chosen wavelength conversion process, allows reuse of the dispersive block, in analogy with mid-span phase conjugation [13]. The accumulated delay in the above approach can be calculated by integrating the dispersion profile over the wavelength range between the original and idler [7]. Unfortunately, this technique does not provide for exact compensation of the dispersion across the full tuning interval due to non-vanishing third (and/or higher) order dispersion, since the dispersion compensation and group-velocity-dispersion accumulation do not occur at the same wavelength. Consequently, a non- negligible uncompensated distortion can be a source of impairment at high modulation rates [7].



a)

b)

λI(2)λS

1530 1540 1550 1560 1570 1580 1590 1600 1610 1620

-20

-19

-18

-17

Wavelength, nm

Dis

pe

rsio

n, n

s/n

m

(DPre

+D)

DDPre

1

3 2

Tx DPre

Converter 1no Phase Conj.

D

DConverter 3

Phase Conj.

Converter 2

Phase Conj.Rx

λI(1)

Dispersion Compensation

λC λL

Fig. 1. Principle of transparent operation with dispersion compensation using three conversions. Acronyms and symbols are transmitter Tx, receiver Rx, dispersions D and DPre; C-band (or blue) pump wavelength λC; L-band (or red) parametric pump wavelength λL; input and output signal wavelength λS; idler or intermediate wavelengths λI

(1) and λI(2).

In order to address this basic limitation, Namiki [14,15] proposed a new, triple-conversion architecture. The main idea in [14,15] is to accumulate delay, not between the original wavelength λS and the newly created idler, but between the original signal and two closely spaced phase conjugated idlers at λI

(1) and λI

(2). Naturally, the approach requires an additional

wavelength conversion as the intermediate λI(2)

idler must now be created from (λI(1)

). The triple-conversion delay principle is illustrated in Fig. 1. In the presence of a non-vanishing wavelength difference between the idlers, a non-zero dispersion slope will result in a net total dispersion because the signal propagates twice through the same dispersive block, once at wavelength λI

(1) and once at wavelength λI

(2). However, full second-order dispersion

compensation can be achieved by introducing a dispersion pre-emphasis, DPre, for the input signal, together with proper choice of wavelengths λI

(1) and λI

(2) [9] (see Fig. 1), satisfying the

relationship:

( )( ) ( ) ( )( )1 2

I Pre S ID D Dλ λ λ+ = (1)

Here, D(λI(1)

) and D(λI(2)

) are the dispersions of the dispersive block at the idler wavelengths. An additional advantage of the approach [14,15] is that the required dispersion is nearly halved relative to the standard scheme. From practical considerations, the value of DPre should be high enough to provide adequate frequency separation between the idlers to facilitate proper filtering.

3. Experimental setup

The setup used in the experiment is shown in Fig. 2. Highly nonlinear fibers (HNLFs) with flattened dispersion profiles were used as nonlinear media for wavelength conversions. The



wavelengths of the pumps were selected to generate idler wavelengths that simultaneously provided (i) the desired delay and (ii) full dispersion compensation. A combination of a phase modulator and an intensity carver was used to generate a 0.8 nm-wide (100 GHz) 40-Gb/s, return-to-zero-differential-phase-shift-keyed (RZ-DPSK) signal at 1562 nm (Tx). The pattern

was a pseudo-random bit sequence of length 231−1. A long pattern length, used as a standard

in OC-768 tests, is necessary to identify any pattern-dependent penalties in the system. The modulated signal mixed with the filtered external noise (ASE) prior to the delay element. The adopted layout enables rigorous performance quantification, allowing assessment of the penalties due to both the signal distortion in the delay element, and the optical noise originating in the delay element.

A 75-km non-zero dispersion shifted fiber was used as dispersion pre-emphasis (DPre), providing a partial third-order dispersion compensation of the dispersive block, and allowing a 5- to 8-nm separation between idler wavelengths over the entire tuning range. After amplification (A2), the signal was shifted to λI

(1) in a two-pump parametric converter (HNLF1),

filtered (F1), and passed through a delay block consisting of a set of dispersion- compensating

fibers (DCFs) with an aggregate dispersion (D) of −19 ns/nm at 1565 nm, as shown by the lower (red) curve in Fig. 1. In order to compensate more than 100 dB of loss in the dispersive block, the DCFs were counter-directionally Raman pumped, resulting in 3.2-dB equalized gain for the entire tuning range. After the dispersive block, the idler at λI

(1) was amplified (A5),

then converted (and phase-conjugated) to λI(2)

in a single-pumped parametric converter

containing a 175-m HNLF (HNLF2) characterized by a nonlinearity coefficient, γ, of 15 W−1

km−1

, and a zero-dispersion wavelength, λ0, of 1562 nm. The signal subsequently passed through the previously used dispersive line, was amplified (A6) and converted back to the

original wavelength λS in a third 100-m dispersion flattened HNLF (HNLF3) with γ = 12 W−1

km

–1. The output optical signal-to-noise ratio (OSNR) after each of the two passes through the

dispersive block exceeded 30 dB (within a 0.1-nm bandwidth). The two-stage thin film tunable filters (F1-3) were used after every converter stage to separate the created signals from the parametric pumps. One-stage tunable filters, not shown in Fig. 2, were placed after every amplifier to suppress the spontaneous noise. The variable optical attenuators (VOA1-2) were used to limit power launched into DCF blocks to eliminate distortions induced by fiber nonlinear effects. The polarization states of the signals were optimized by polarization controllers PC2-6 to provide maximum conversion efficiency.

BoostersλλλλC

λλλλL

λλλλS

λλλλL

λλλλL & λλλλC

Raman

DλλλλI(1)& λλλλI

(2)

Red pump

Blue pump

Converters

HNLF1

HNLF2

HNLF3

5 x DCF

Rx

λλλλI(1)

λλλλI(2)

λλλλS

Tx

PM

A1

A2

A3

A4

A7

A5

PC2PC1

A6

PC3

PC4

PC5

PC6

VOA1

VOA2

DPre

F1

F2

F3

ASE

Fig. 2. The experimental setup. Acronyms: ASE – amplified spontaneous emission (noise); PC – polarization controller; A – amplifier; F – filter; Tx – transmitter; Rx – receiver; HNLF – highly nonlinear fiber; VOA – variable optical attenuator; DCF – dispersion compensating fiber.

At the output, the pre-amplified (A7) 40-Gb/s delayed signal was detected in a balanced DPSK receiver (Rx), electronically de-multiplexed into four 10-Gb/s streams and the bit-



error-ratio (BER) was measured in order to quantify the performance of the constructed delay element. The performance penalty was quantified by varying the OSNR at the transmitter side (see Fig. 2).

Two of the HNLFs were sufficiently short so the pump phase dithering suppressing the stimulated Brillouin scattering (SBS) was required only in the second one-pump converter. It is important to note that only two pumps (PλC = 2 W, PλL = 3 W) distributed among the converting stages were used. This configuration guaranteed identical input and output signal wavelengths [9], thus fulfilling a fundamental requirement for an optical delay block. In particular, the red pump λL was utilized at every conversion stage, while the blue pump λC was used for the first and the last wide-band conversions. In the experiment, only the red pump (L-band) was phase modulated using a single 50-MHz harmonic, while the blue (C-band) pump was left unmodulated. The pump carrier modulation is transferred to a carrier modulation of the created idlers causing significant phase modulation to amplitude modulation (PM-AM) transfer, as well as deterministic jitter after propagation through the highly dispersive line. The particular combination of four wave mixing processes used presented an opportunity for a previously proposed [9] phase dithering cancellation approach.

By properly matching the phases of the modulated pumps and the generated idlers, the carrier modulation can be perfectly compensated after passing all three converters, as illustrated in Fig. 3. The dotted horizontal arrows indicate the direction of the pump carrier shift, whereas the shaded areas schematically depict the corresponding broadened carrier bandwidth, which is a consequence of the pump phase modulation.

Converter 1 Converter 3Converter 2

Signal Carrier Modulation is compensated

Pump Carrier Modulation

λ

λλλλI(2)→→→→ λλλλSλλλλS→→→→ λλλλI

(1) λλλλI(1)→→→→ λλλλI

(2)

Fig. 3. A principle of the carrier modulation compensation.

3. Results and discussions

As explained in the previous two sections, the signal delay is varied by tuning the wavelengths of both pump lasers. Specifically, the wavelength of the L-band pump, λL, defines the delay value (blue curve in Fig. 4a) while that of the C-band pump, λC, provides the precise dispersion compensation (green curve in Fig. 4b). The pump positions were determined using the measured DCF dispersion profiles (Fig. 1). In the experiment, the L-band laser was tuned from 1568.5 nm to 1608 nm, addressing the full tunable delay range of 1.56 µs (see Fig. 1). Full dispersion compensation of the 40-Gb/s signal requires precise pump positioning (< 0.1nm) owing to its spectral width.



0 0.5 1 1.5 2

Time, us

1.56 us

λλλλL

= 1608 nm

λλλλL

= 1568.5 nm

λλλλL

= 1608 nm

λλλλL

= 1568.5 nm

0 0.5 1 1.5 2

Time, us

1.56 us

λλλλL

= 1608 nm

λλλλL

= 1568.5 nm

λλλλL

= 1608 nm

λλλλL

= 1568.5 nm

1570 1580 1590 1600 16100

0.5

1

1.5

2

Red Pump Wavelength λL, nm

Dela

y,

µs

1570 1580 1590 1600 16101556

1557

1558

1559

1560

Blu

e P

um

p W

ave

len

gth

λC,

nm

a) b)

1570 1580 1590 1600 16100

0.5

1

1.5

2

Red Pump Wavelength λL, nm

Dela

y,

µs

1570 1580 1590 1600 16101556

1557

1558

1559

1560

Blu

e P

um

p W

ave

len

gth

λC,

nm

a) b)

Fig. 4. a) Measured delay and the blue pump wavelength as a function of the red pump wavelength. b) The waveforms corresponding to extreme red pump positions.

The performance is shown in Fig. 5. The measured performance penalty at a 10−9

BER varied between 6 and 7 dB across the full tuning range, (see Fig. 5). The sizeable penalty is attributed to concatenated filtering and the delay-element noise figure. The signal is filtered fourteen times before exiting the setup, resulting in significant equivalent bandwidth limiting. In order to verify the effect of filtering, an additional measurement was performed on the 40-Gb/s RZ-DPSK back-to-back signal by adding a 0.4-nm filter, corresponding approximately to the equivalent bandwidth of the concatenated filtering inside the delay element. The performance curve is shown in the blue dashed line in Fig. 5. As can be seen, the penalty from bandwidth restriction alone is 3 dB, implying an additional 3- to 4-dB penalty due to the

Fig. 5. Measured delay line performance of the 40-Gb/s RZ-DPSK (return-to-zero differential-phase-shift-keyed) channel.

delay-element noise figure of the Raman-amplified dispersive block and the three wavelength conversions. The result is in perfect accord with the previously measured performance of a



10-Gb/s signal [9] which was unaffected by filtering and exhibited only a 3-dB penalty due to the excess noise build-up in the dispersive block.

Thus, it can be concluded that ultimate limit to the all-optical delay generation based on the adopted approach is ultimately limited by noise accumulation in the dispersive block, and can be extended by adopting dispersive elements with significantly lower loss per unit dispersion. Although, the method used significantly relaxes the delay limitations due to the finite third (and higher) order dispersion, the effect of the third order dispersion becomes more pronounced for signal bandwidths wider than 40 GHz. However, the last obstacle can be significantly diminished by proper dispersion engineering of the dispersion pre-emphasis element(s) [15].

4. Conclusion

A record delay-bitrate product of 62,400 is demonstrated for 40-Gb/s RZ-DPSK signal using a parametric delay element employing parametric dispersion compensation in a self-conjugating architecture based on a cascade of one degenerate and two non-degenerate four-wave mixing processes. The result is the first reported all-optical delay of a binary 40-Gb/s signal surpassing 1 µs, and is the record delay-bandwidth product demonstrated to date. The 40-Gb/s RZ-DPSK performance characterization validates the delay implementation in high-speed amplitude and phase modulated formats.

Acknowledgment

This material is based on research sponsored in part by the Air Force Research Laboratory (AFRL) and the Defense Advanced Research Projects Agency (DARPA) under agreement FA8650-08-1-7819 Parametric Optical Processes and Systems. The authors gratefully acknowledge Sumitomo Electric R&D for providing dispersive fibers used in the experiment.



156-μs continuously tunable parametric delay line for a 40-gb/s signal

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