All-optical polarization-mode dispersion monitor for return-to-zero ...

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Oct 28, 2010 - return-to-zero optical signals at 40 Gbits/s and beyond ... rates in excess of 40 Gbits=s and does not require the use of high-data-rate ...
November 1, 2010 / Vol. 35, No. 21 / OPTICS LETTERS

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All-optical polarization-mode dispersion monitor for return-to-zero optical signals at 40 Gbits/s and beyond Martin Rochette,* Chams Baker, and Raja Ahmad Department of Electrical and Computer Engineering, McGill University, Montreal, Quebec H3A 2A7, Canada *Corresponding author: [email protected] Received August 18, 2010; accepted September 18, 2010; posted October 11, 2010 (Doc. ID 133644); published October 28, 2010 The operation of a polarization-mode dispersion monitor insensitive to chromatic dispersion is demonstrated at 40 Gbits=s. The high-speed processing device is based on the Kerr effect and provides an optical power output as a reading of differential group delay. The monitor is compatible with return-to-zero modulation formats at data rates in excess of 40 Gbits=s and does not require the use of high-data-rate electronics. © 2010 Optical Society of America OCIS codes: 190.4360, 190.7110.

Polarization-mode dispersion (PMD) monitors have become essential components of optical networks at data rates in excess of 40 Gbits=s. At such data rates, the optical signal is subjected to impairments, such as PMD and chromatic dispersion (CD), that continuously and randomly fluctuate under the effect of environmental conditions [1]. To compensate for PMD, a monitor must track the differential group delay (DGD) and the principle states of polarization (PSP) of the incoming signal. The continuous monitoring of these PMD components is then fed to a compensating optical element that dynamically restores the degraded signals. Since PMD and CD are both continuously and independently varying impairments, it is beneficial to use a PMD monitoring design that isolates the PMD degrading effect to the exclusion of all other impairments, such as CD. Electronic PMD monitoring solutions that are compatible with high data rates are technically challenging, since the data rate of optical communication systems stands close to the upper limit imposed by state-of-theart electronics. To cope with this electronics constraint, optical PMD monitoring solutions that are free from highdata-rate electronics have been demonstrated [2–5]. However, none of the PMD monitors presented thus far simultaneously provides CD insensitivity and a DGD reading that can be inferred from a power meter. With this objective in mind, we presented in 2005 a device that fulfills these requirements based solely on four-wave mixing (FWM) but required the use of a secondary probe laser signal [6]. In this Letter, a CD-insensitive PMD monitor based on the Kerr effect is presented. The monitor implementation requires no high-data-rate electronics. A theoretical development is provided and two experimental demonstrations are performed. The first experiment demonstrates the concept and the second experiment shows a realistic monitoring situation for a 40 Gbits=s return-to-zero (RZ) signal. This device differs from the one presented in 2005 [6], as it is based on a combination of self-phase modulation (SPM), cross-phase modulation (XPM), and degenerate (D)-FWM. Although it is theoretically more complex than the FWM-based version, it turns out to be technically much simpler to implement and does not require an additional probe signal to mix with the signal to monitor. 0146-9592/10/213703-03$15.00/0

Figure 1 presents the CD-insensitive PMD monitor. It is based on the nonlinear interaction between the orthogonal polarization components of a PMD-impaired signal. An optical signal at carrier frequency ω0 is modulated in amplitude into an RZ signal. The signal then becomes impaired by PMD and CD before being sent to the PMD monitor. As a result of PMD, the impaired signal is composed of two orthogonal polarization states, p1 and p2, that are time delayed by an amount equal to the DGD. A first bandpass filter (BPF1 ) slices from the impaired signal two narrow bands that are spectrally offset by an amount Δf . In the time domain, the signal after BPF1 then consists of two orthogonally polarized waves modulated with a profile AðtÞ that is sinusoidal with an oscillation frequency Δf . The polarized modulations are also delayed by an amount equal to the DGD and are expressed as pffiffiffiffi P cosðπΔf tÞ; pffiffiffiffi Ap2 ðtÞ ¼ P cos½πΔf ðt − DGDÞ;

Ap1 ðtÞ ¼

ð1Þ

where P is the amplitude of each polarized wave. Passing through a highly nonlinear medium (HNLM) leads to a nonlinear interaction between the orthogonal polarizations. The nonlinear effects experienced by the PMDimpaired signal include SPM, XPM, and D-FWM [7]. The nonlinear phase shift ϕ turns out to be the same for both polarized field and equals ϕðtÞ ¼ γL½jAp1 ðtÞj2 þ jAp2 ðtÞj2 ;

ð2Þ

where γ is the waveguide nonlinear parameter and L is the nonlinear medium length. The power components between square brackets refer to SPM on one hand and to

Fig. 1. (Color online) PMD monitor and testing setup. PSD, power spectral density. © 2010 Optical Society of America

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XPM and D-FWM from the orthogonally polarized wave on the other hand. The resulting nonlinear fields are expressed as E NLi ðtÞ ¼ expðjω0 tÞApi exp½jϕðtÞ;

ð3Þ

where i takes values i ¼ 1, 2. Delaying one polarization component over the other by using different amounts of DGD affects the XPM and D-FWM, thus enabling the retrieval of DGD from an optical spectrum analysis. The spectrum E NLi ðωÞ of E NLi ðtÞ is calculated by taking the Fourier series transformation of the electric fields in Eq. (3) and using the series identities cos½β cosðωr tÞ ¼ J 0 ðβÞ − sin½β cosðωr tÞ ¼

∞ X n¼1

∞ X n¼1

2J 2n ðβÞ cosð2nωr tÞ;

2J 2n−1 ðβÞ cosðð2n − 1Þωr tÞ;

ð4Þ

where J n are Bessel functions of order n, and ωr and β are constants. This leads to an analytical solution for the power ratio between the spectral component PSD1 at frequency ω0  ð3=2Þ2πΔf generated from nonlinear effects over the spectral component PSD0 at frequency ω0  ð1=2Þ2πΔf from the original signal. Taking the ratio of PSDs leads to   E NLi ω ¼ ω0 þ 32 2πΔf 2  PSD1     ¼ PSD0 E NLi ω ¼ ω0 þ 12 2πΔf ¼

½J 21 ðφSPM =2Þ þ J 22 ðφSPM =2Þcos2 ðπΔf DGDÞ ; ð5Þ J 20 ðφSPM =2Þ þ J 21 ðφSPM =2Þ

where ϕSPM ¼ γPL. Without DGD, this solution takes the form provided in [8], as should be expected. For a constant ϕSPM , PSD0 is also constant and only DGD affects PSD1 . To complete the experimental setup, after the HNLM, a second BPF (BPF2 ) isolates the spectral components of PSD1 . Finally, a power meter indicates the amount of DGD the signal has experienced from a measure of PSD1 . The validation of our design is supported by two experiments. The first experiment is a proof of concept of the PMD monitor without the restriction of bandpass filtering from BPF1 , whereas the second experiment includes a nonideal BPF1 combined with a realistic RZ modulated signal. In the first experiment, the narrow filtering require-

Fig. 2. (Color online) Setup to demonstrate the operation of the PMD monitor in conditions of ideal filtering from BPF1 (see Fig. 1). CW, cw laser source.

Fig. 3. (Color online) Optical spectra of a 40 GHz sinusoidal signal at the output of the HNLM as a function of DGD · Δf .

ments on BPF1 were avoided by replacing the RZ signal and BPF1 with a pair of cw lasers. Figure 2 shows the setup used for the first experiment. The input signal is a sinusoidal power profile made from the superimposition of two copolarized cw signals. The lasers are offset in frequency by Δf ¼ 40 GHz (0:32 nm) and placed on either side of a central wavelength of 1531:5 nm. The power of each laser is 22:0 dBm at the HNLM input. The setup also includes a CD emulator (Teraxion, model TDCMX), a PMD emulator (General Photonics, model PDLE-101), an HNLM with a waveguide nonlinear parameter γ ¼ 10:5 W−1 km−1 , length L ¼ 1:0 km and chromatic dispersion parameter DHNLM ¼ −0:71 ps=nm-km, and BPF2 with a FWHM bandwidth of 0:8 nm and tunable central wavelength within the C-band. Increasing amounts of PMD and CD were added to the signal during the experiment. Figure 3 shows the spectra measured at the HNLM output and as a function of the ratio DGDΔf . Sidelobes representing PSD1 are observed at frequency ω0  ð3=2Þ2πΔf as a result of the nonlinear interaction between orthogonal polarization components. The magnitude of PSD1 depends on the delay between orthogonal polarization components. The maximum of PSD1 is observed when DGDΔf ¼ 0, whereas a minimum occurs when the DGD represents one-half of the sinusoid period, or DGDΔf ¼ 0:5. Figure 4 shows the resulting power P 1 after filtering PSD1 in the wavelength range of 1532:1–1532:9 nm. DGD up to 76 ps and CD up to 40 ps=nm were added to the input signal. We observe an excellent match between experimental results and Eq. (5). As expected from theory, the nonlinearly generated spectral components are independent of CD because both polarized sinusoidal

Fig. 4. (Color online) Average power measured between spectral components 1532.1 to 1532:9 nm as a function of the product DGDΔf and for different values of chromatic dispersion.

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Fig. 5. (Color online) Experimental spectrum of the RZ signal modulated at 40 Gbits=s, before and after filtering with a 50 GHz (0:4 nm) BPF1.

components Ap1 and Ap2 experience a CD-induced group delay of the same magnitude and, thus, no relative delay occurs between them. A small discrepancy between theory and experiment occurs at DGDΔf ¼ 0:5 due to a background of spontaneous emission noise superimposed to the input RZ signal, as observed from the spectrum of Fig. 3. This first experiment clearly shows that the DGD can be precisely measured from nonlinear interactions between orthogonal components of an incident signal. The second experiment consists in testing the PMD monitor with a realistic RZ signal format and nonideal BPF1 , as depicted in Fig. 1. The data signal consists of a 231 − 1 pseudorandom bit sequence of 12:5 ps RZ pulses at a data rate of 40 Gbits=s and a wavelength of 1554:5 nm. Figure 5 shows the experimental spectrum of the RZ signal, clearly showing prominent peaks separated by 40 GHz. This signal is sent to BPF1 , a Gaussian BPF with a 50 GHz FWHM that isolates and equalizes two of the natural peaks of the RZ signal. The filter is slightly offset from the center wavelength of the signal in order to get this result. Take note that, although the best filtering solution would completely isolate any two spectral components from the original signal and extinguish all other spectral components, we instead take advantage of the natural peaks by equalizing them. This case is thus nonideal but simple to implement. After propagation through the HNLM, new spectral components are being generated on either side of the filtered spectrum. Figure 6 shows the resulting power P 1 after filtering PSD1 in the wavelength range of 1554:85–1555:65 nm, as a function of DGDΔf . This measurement was performed at HNLM input power levels of 18.7, 21.6, and 24:5 dBm and at CD levels of 0, 20, and 40 ps=nm. The output power contrast under DGD variation is 12:0 dB, independently of the input signal power. We believe that this power contrast could be increased by using a BPF2 with a narrower bandpass, as well as eliminating undesirable spectral components left unfil-

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Fig. 6. (Color online) Optical power as a function of DGD at three different power levels at the input of the HNLM. BPF2 is centered at 1555:25 nm.

tered by BPF1 . Finally, even CD has a negligible impact on the monitor response, arising from the quasi-sinusoidal profile of the temporal signal after filtering with BPF1 . In conclusion, a simple and efficient all-optical PMD monitor based on the Kerr effect was successfully demonstrated. Results show CD-independent monitoring of PMD with an output response contrast of 12 dB between DGD values included between zero and one-half of a bit period. Any RZ modulation format with amplitude or phase encoding can be monitored by this device, because the nonlinear phase shift central to the operation of this monitor depends solely on the power profile of the pulses being used. While this monitor avoids the requirement of high-data-rate electronic components, it is also expected to operate at data rates well beyond 40 Gbits=s because of the short (