All-optical regeneration

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All-optical signal regeneration with

wavelength multicasting at 610 Gb/s using asingle electroabsorption modulator

K. K. Chow and C. Shu

Department of Electronic Engineering and Center for Advanced Research in Photonics

The Chinese University of Hong Kong

Shatin, N.T., Hong Kong

[email protected]

Abstract: All-optical signal regeneration with wavelength multicasting hasbeen demonstrated using cross-absorption modulation in a singleelectroabsorption modulator for the first time. We show that the input signalwavelength can be simultaneously converted to 6 different wavelengths at10 Gb/s with signal regeneration. The output extinction ratio, the linewidth,and the pulse shape show a significant improvement. A negative powerpenalty of 2 dB is obtained at 10

-9bit-error-rate level.

2004 Optical Society of AmericaOCIS codes: (070.4560) Optical data processing; (060.2360) Fiber optics links and subsystems

References and links

1. T. H. Wood, J. Z. Pastalan, C. A. Burrus, JR., B. C. Johnson, B. I. Miller, J. L. Demiguel, U. Koren, M. G.

Young, Electric field screening by photogenerated holes in multiple quantum wells: A new mechanism forabsorption saturation, Appl. Phys. Lett. 57, 1081-1083 (1990).

2. N. Edagawa, M. Suzuki, S. Yamanoto, Novel wavelength converter using an electroabsorption modulator,IEICE Trans. Electron. E81-C, 1251-2157 (1998).

3. S. Hojfeldt, S. Bischoff, and J. Mork, All-optical wavelength conversion and signal regeneration using anelectroabsorption modulator, IEEE J. Lightwave Technol. 18, 1121-1127 (2000).

4. T. Otani, T. Miyazaki, S. Yamamoto, 40-Gb/s optical 3R regenerator using electroabsorption modulators foroptical networks, IEEE J. Lightwave Technol. 20, 195-200 (2002).

5. A. D. Ellis, J. K. Lucek, D. Pitcher, D. G. Moodie, D. Cotter, Full 1010 Gbit/s OTDM data generation anddemultiplexing using electroabsorption modulators, Electron. Lett. 34, 1766-1767 (1998).

6. E. S. Awad, P. S. Cho, C. Richardson, N. Moulton, J. Goldhar, Optical 3R regeneration using a single EAM forall-optical timing extraction with simultaneous reshaping and wavelength conversion, IEEE Photon. Technol.Lett. 14, 1378-1380 (2002).

7. K. K. Chow and C. Shu, All-optical wavelength conversion with multicasting at 610 Gbit/s usingelectroabsorption modulator, Electron. Lett. 39, 1395-1397 (2003).

8. L. Xu, N. Chi, K. Yvind, L. J. Christiansen, L. K. Oxenlwe, J. Mrk, P. Jeppesen, and J. Hanberg, 740 Gb/sbase-rate RZ all-optical broadcasting utilizing an electroabsorption modulator, Opt. Express 12, 717- 723(2004), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-3-416

9. S. Gao, X. Jia, X. Hu, and D. Li, Wavelength requirements and routing for multicasting connections inlightpath and light-tree models of WDM networks with limited drops, IEE Proc. Commun. 148, 363-367(2001).

10. R. K. Pankaj, Wavelength requirements for multicasting in all-optical networks, IEEE/ACM Trans.Networking 7, 414-424 (1999).

11. R. Libeskind-Hadas and R. Melhem, Multicast routing and wavelength assignment in multihop opticalnetworks, IEEE/ACM Trans. Networking 10, 621-629 (2002).

1. Introduction

All-optical regeneration of degraded signals is desirable for future large-scale opticalnetworks. Cross-absorption modulation (XAM) in an eletroabsorption modulator (EAM) [1] is

(C) 2004 OSA 28 June 2004 / Vol. 12, No. 13 / OPTICS EXPRESS 3050

#4596 - $15.00 US Received 10 June 2004; revised 23 June 2004; accepted 24 June 2004
mailto:[email protected]:[email protected]


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a promising technique for all-optical signal regeneration and wavelength conversion owing toits high-speed performance, low-chirp characteristics, and capability to maintain the samelogical polarity. Much research has been focused on the investigation of the EAM-basedwavelength converter, time-division demultiplexer, and its application for all-optical signalregeneration [2-6]. Recently, wavelength multicasting has been successfully demonstratedbased on XAM in an EAM [7, 8]. Many advanced WDM networks require the support ofmulticasting, where a number of applications such as video distribution and teleconferencing

require a multicast connection to be established [9-11]. In this paper, we further combine thecapability of signal regeneration with wavelength multicasting of the setup. With the XAMeffect of the EAM, a degraded input signal is successfully multicasted to 6 differentwavelength channels with significant improvement in the extinction ratio (ER), the linewidthand the pulse shape. A complete 10 Gb/s bit-error-rate (BER) test is also performed and anegative power penalty of 2 dB is obtained at 10

-9BER level for all output channels.

2. Experiment

The experimental setup on wavelength multicasting with signal regeneration is shown infigure 1. The input signal (S) is a 2

31-1 bits pseudorandom return-to-zero (RZ) signal at 10

Gb/s, which is prepared by gain-switching a distributed-feedback (DFB) laser diode at 10GHz followed by external modulation. In order to demonstrate the signal regenerationproperties of our setup, an input signal with an ER of around 5 dB is prepared. The signal thenpropagates in 3-km standard single-mode fiber (SMF) so that the pulse width is broadened.Next, the signal is amplified by an erbium-doped fiber amplifier (EDFA) and is launched into

a commercial 1.55-m EAM through an optical circulator. The EAM module consists of anInGaAsP waveguide buried with Fe-doped InP. At the opposite port, 6 cw inputs generatedfrom a WDM laser source are combined by a WDM multiplexer (MUX) and are launched intothe EAM. The separation of each channel is 200 GHz. In our setup, the EAM is reverse-biasedat -3.0 V. Nonlinear optical transmission in the EAM is realized by applying the input opticalsignal to produce a large number of photo-generated charge carriers in the highly absorptive

waveguide. During the high-level of the NRZ signal, due to drift and diffusion the photo-generated holes and electrons are separated from each other. Therefore, the charge neutralityis lifted and gives rise to a screening of the applied electric field. The reduction of the electricfield and bandfilling by the photo-generated carriers significantly decrease the absorption and

Fig. 1. Experimental setup on signal regeneration with wavelength multicastingusing cross-absorption modulation in an electroabsoption modulator. EA MOD:electroabsorption modulator; PS: polarization scrambler; EDFA: erbium-doped

fiber amplifier; MUX: multiplexer; DEMUX: demultiplexer; BER test set: bit-error-rate test set.

RZ Inputsignal

EAMOD

Isolator

DEMUXDEMUX

WDMso

urce

Output signal

DC bias

MUXMUX

EDFA

ReceiverBER

test set

3-km SMF

AB

C




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create a transmission window [1]. This XAM introduced by the input signal will thusmodulate the counter-propagating cw light beams [3, 7]. Due to the nonlinear characteristicsof the transmission window, output signal regeneration is possible [5]. The converted signal ateach wavelength is obtained through a WDM demultiplexer (DEMUX) connected to theoptical circulator.

3. Results and discussion

Figure 2 shows the spectrum obtained from port B of the optical circulator without the

DEMUX. The input signal is at s=1547.7 nm, and the 6 output channels vary from 1=1550.5

nm to 6=1558.5 nm with a 200 GHz spacing. The input power of s is 13 dBm and the cwinput power of each channel is 7 dBm. Figure 3(a) shows the spectrum of the input signal withlinear vertical scale measured by an optical spectrum analyzer with 0.01 nm resolution. Sincethe input RZ signal is prepared by gain-switching a DFB laser diode followed by externalmodulation, it is observed that the 3-dB linewidth is broadened to 60 GHz. Figure 3(b) showsthe output spectrum of channel 1 with linear vertical scale and it is found that the 3-dBlinewidth is greatly reduced to 2 GHz.

The power penalty of the wavelength converter has been studied by performing BERmeasurements at 10 Gb/s. In the experiment, the cw inputs of all the 6 channels are alwaysON and BER measurements on individual channels are performed. Figure 4 plots the outputBER against the received optical power of each channel. The insets show the eye-diagrams ofthe input signal (upper) at port A and output signals (lower) at port C. Since the input signalhas propagated in the 3-km SMF, the pulse width is broadened to 60 ps. The lower traceshows the output eye-diagram of channel 1 at 15 dBm received power. We can find that the

pulse width is reduced to 25 ps. The improvement is due to the nonlinear optical transmissioncharacteristic of the EAM under reverse bias, which allows reshaping of the degraded signal[5]. The ER of the output signal also improves from 5 dB to 12.6 dB. The low opticaltransmission at small pump power allows noise suppression on zeros [5] so that ERimprovement is obtained. Note that similar results are also obtained for the other 5 channels.

1535 1540 1545 1550 1555 1560 1565 1570

Wavelength (nm)

Intensity(10d

B/div)

1 65432

s

Fig. 2. Optical spectrum showing all the 6 output channels.




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By comparing the BER performance of the input and output signals, a negative power penaltyof 2 dB at 10

-9BER level is obtained for all the channels.

Since the EAM is inherently polarization independent, it is worth noting that the setup

can accept arbitrary input signal polarization without affecting the output signal quality. Inorder to study the polarization characteristics of the converted signal, a polarization analyzeris used to monitor the output state of polarization (SOP). The SOP of the input signal is fullyrandomized by a polarization scrambler and the degree of polarization (DOP) is less than 5%.Figure 5(a) depicts the Poin are sphere of the input signal. It shows that the SOP israndomized over the whole sphere. Figure 5(b) depicts the corresponding Poin are sphere ofthe channel 1 output accumulating for 30 minutes, showing that the SOP is confined to a small

Wavelength (0.1 nm/div)

Intensity

(a.u.)

(a)

Wavelength (0.1 nm/div)

Inte

nsity(a.u.)

(b)

Fig. 3. Optical spectra showing (a) the input signal and (b) the channel 1 output.

-10

-9

-8

-7

-6

-5

-25 -20 -15 -10 -5 0

Received power (dBm)

log(BER)

channel 1

channel 2

channel 3

channel 4

channel 5

channel 6

back to back

Time

Signal(a.

u.)

Fig. 4. Plot of the bit-error-rate against the received power in a 10 Gb/s BERmeasurement. The inset shows the eye diagrams of the input signal (upper) and

the channel 1 output (lower).




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spot on the sphere even though the input polarization keeps changing randomly. The DOP ofthe converted signal is measured to be over 98%. Since many devices used incommunications, such as modulators and amplifiers, are polarization sensitive, the stablepolarization output of the EAM-based wavelength converter is very desirable for opticalnetwork applications.

4. Conclusion

In conclusion, all-optical signal regeneration with wavelength multicasting up to 610 Gb/shas been demonstrated using cross-absorption modulation in an electroabsorption modulator.The results show a significant improvement in the output signal quality and a negative powerpenalty of 2 dB is obtained.

Acknowledgments

The work described in this paper was supported by the Research Grants Council of theHKSAR, China (Project No. CUHK 4196/03E).

Fig. 5. Poin are spheres of (a) the input signal with random polarization and (b)

the output signal with a stabilized state of polarization.

(a) (b)



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