Abstract
All-optical 3R regenerators and regenerative all-optical wavelength converters are key components in future transparent networks. In a transparent network, transmission distances are not a priori known and therefore regeneration is needed to reduce impairments that accumulate. In addition, wavelength converters are needed to overcome blocking issues at network nodes. Here we review recent progress towards fully regenerative schemes that allow transmission of 40 Gb/s signals over virtually unlimited distances, i.e. we demonstrated all-optical transmission over one million kilometers.

Introduction

Fig. 1 Future networks are likely to consist of “islands of transparency” . In these islands of transparency, the signals are transmitted without intermediate conversion into the electronic domain from source to destination (Fig. 1). The usual optical-to-electrical-to-optical (OEO) converter at network interfaces will be replaced by all-optical devices.

3R all-optical regenerative devices that can perform re-amplification (1R), reshaping (2R), and re-timing (3R) hold promise to replace optical-to-electrical-to-optical (OEO) components in future transparent networks. In particular, 3R regenerators that do not rely on power consuming broadband electronics [1]-[4] may replace OEO regenerator units that are needed to overcome transmission impairments. Additional functionality is needed in networks with optical crossconnects, where optical translator units are used to overcome wavelength blocking. Regenerative all-optical wavelength converters (AOWC) might replace OEO translator units in such crossconnects [5]. As a likely scenario “islands of transparency” [6] will first emerge. In these islands of transparency, the aforementioned devices will enable transmission from source to destination without intermediate conversion into the electronic domain (Fig. 1).

This article reviews techniques that have demonstrated full optical signal regeneration and thus enable transmission over virtually unlimited distances.

3R Regeneration in Soliton Systems
Long-haul experiments based on 40 Gb/s dispersion managed soliton transmission have demonstrated transmission distances of up to 70,000 km at regenerator spacings of 250 km [7]. To achieve these results both reshaping based on soliton transmission and retiming exploiting the so-called synchronous modulation technique [8] has been used. The soliton transmission technique provides a very natural form of pulse shape conservation since a soliton pulse preserves it’s form during the propagation along the fiber. The synchronous modulation technique on the other hand provides retiming by remodulating a signal after a certain transmission distance with the clock derived of it’s own signal. An example of a synchronous intensity modulator is shown in Fig. 2(b). 10 Gb/s soliton transmission based on such techniques have already demonstrated transmission distances of up to one million kilometers in the past [8][9].

3R Regenerators for Pseudo-Linear Transmission Systems

Fig. 2 (a) Scheme of the 3R regenerator comprising fiber 2R regenerators and retiming elements. (b) Synchronous Modulator scheme. (c) Clock recovery and semiconductor optical all-optical wavelength converter for performing retiming and wavelength conversion.

The soliton transmission technique is difficult to apply over commercially deployed fibers. Instead a pseudo-linear transmission techique is used [10]. A 3R regenerator that works in the pseudo-linear transmission regime is shown in Fig. 2. The regenerator consists of three elements [11]. It comprises two fiber based regenerators for signal reshaping and reamplification (2R) and a retiming section (3R).

The fiber based 2R regenerator is shown in more detail in Fig. 2(a). The 2-stage fiber regenerator (left side of Fig. 2(a)) contains a compression stage and a regenerator stage [11]. The compression stage is for suppressing Brillouin scattering. In the regenerator stage self-phase modulation (SPM) of the nonlinear fiber is exploited to broaden the optical spectrum of the input signal. A 1-nm optical filter is placed at the output to select components of the same intensity in the SPM modulated spectrum. Components with less intensity have less broadening and are therefore suppressed by the filter. Components with higher intensity give larger broadening and are brought back to identical intensities determined by the frequency offset of the filter. The 2R stage also provides some small wavelength shift of 1.5 nm. The second single-stage regenerator is therefore used to reset the signal wavelength to its input value. For all-optical regeneration where no wavelength conversion is needed, a simple 40 Gb/s synchronous intensity modulation scheme is used to perform retiming, Fig. 2(b). The 40 Gbit/s signal is intensity modulated synchronously by a sinusoidally driven electroabsorption (EA) modulator. The clock signal is derived from a very simple clock recovery scheme as shown. The signal is detected using a 40 GHz pin photodiode, then filtered using a very high Q (~900) microwave filter and then re-amplified before applying to the reverse biased modulator. The synchronous modulator can be placed either in between the two 2R regenerators or before the first one.

Regenerative Wavelength Conversion
When wavelength conversion is needed, the synchronous modulator is replaced with a 3R semiconductor optical amplifier (SOA) based delay interferometer (DI) wavelength converter [3]. In this device, the input signal P _2R is used for gating the SOA-DI, such that the clock signal P _clk can pass through the device depending on whether the gate is opened or closed. The signal that passes the gate is the clock signal carrying the new wavelength of the clock signal. Wavelength conversion can be performed to any wavelength within approximately 30 nm around the SOA gain maximum. To further improve the signal quality after the wavelength converter we added a saturable absorber (SA) behind the filter at the output of the SOA-DI. The additional 2R regenerator at the output of the scheme improved the signal quality only little. To perform wavelength converter over just a few 400 km loops we did not need the additional fiber regenerator at the output. On the other hand it was needed to provide a perfect 3R regeneration such as needed to demonstrate transmission over thousands of 400 km loops. The clock signal is derived of the input signal. In our case we use the 40-GHz pin photodiode and the high-Q filter of above to detect the signal and to drive an EA modulator which generates the clock signal. However, all-optical clock recovery schemes such as suggested by Sartoris et al. could be used [13] and thus completely eliminate the need for high-speed electronics.

Loop Setup

A simplified loop set up to demonstrate long distance all-optical regeneration is shown in Fig. 3(b). The transmitter is a 40 Gb/s, 33% duty cycle, RZ signal that is obtained by multiplexing four 10 Gbit/s signals with a PRBS of 2³¹ -1 [11]. The receiver consists of a high-Q filter based clock recovery and an OTDM demultiplexer. An electroabsorption modulator demultiplexes the 40 Gbit/s signals down to 10 Gbit/s for error detection. The transmission span consists of four 100 km spans of TrueWaveTM Reduced Slope (TWRS) nonzero dispersion fiber and dispersion compensated fiber. Erbium-doped fiber amplifiers (EDFAs) and backward pumped Raman amplification was used to compensate the 21 dB span losses. A tunable dispersion compensator adjusts the dispersion before the regenerator.

Fig. 3 Loop Setup

The launch power into the spans was between -1 dBm and 3 dBm for the regenerator and the wavelength converter experiments respectively. Input signal wavelengths were 1552.5 nm. The fiber based regenerator and wavelength converter were set such that this wavelength was maintained in the loop. Input-signal and clock signal power into the SOA-DI were 6 and 8 dBm

Fig. 4 Quality of the signal versus transmission distances and (b) Bit-error rate versus received power at one million km.

Results
Fig. 4(a) shows the measured Q value as a function of distance of the fiber-based 3R regenerator and the 3R all-optical wavelength converter [11][12]. Transmission over one million kilometers of fiber is shown for both schemes. The fiber based 3R regenerator shows no sign of a degradation even after one million kilometer of transmission. The wavelength converter shows a first sign of an error floor that we attribute to polarization sensitivity issues of the SOA and the SA that occurred at the end of the experiment.

Conclusions
Full 3R regeneration and wavelength conversion have been achieved exploiting both fiber nonlinear effects and SOA based nonlinearities. This way 40 Gb/s transmission over as much as one million kilometer of fiber has been demonstrated with no or only little signal degradation.

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