1. Introduction

All-optical wavelength converters are the key devices for dynamic routing and wavelength reuse in future WDM-based photonic networks. Among various wavelength conversion methods, those utilizing cross-phase modulation (XPM) based on the third order optical nonlinearity in optical fibers are promising because of their ultrafast operation [1] and simple configuration. For example, nonlinear optical loop mirrors (NOLMs) [2, 3] using fiber-based Sagnac interferometers show good performance in various all-optical applications [4] and have the potential of operating at Tbit/s [5, 6].

In conventional NOLMs, however, the converted probe light must be separated from the input signal (pump) light with the aid of optical filters or WDM couplers [7, 8]. Moreover, to achieve wavelength-tunable operation, we have to use externally controlled tunable filters so as to extract the input probe light. Incidentally, we often need relatively large signal (pump) power to induce the sufficient XPM owing to the small nonlinear coefficient in silica fibers [9]. Consequently, it is difficult for fiber-XPM-based devices to convert an input signal to the same or near wavelength and this often limits the flexible use of such converters.

In this letter, we propose for the first time to our knowledge a filter-free wavelength converter using a Sagnac loop followed by a delayed interferometer to improve the wavelength flexibility of the XPM-based wavelength converters. We successfully demonstrate that 10-Gbit/s signals at 1545.5 nm are converted to the signals with arbitrary wavelength using the proposed converter.

2. Principle of filter-free operation

Figure 1 shows the basic configuration of the proposed filter-free scheme using a propagation-diversity Sagnac loop consisting of a fiber loop, a polarization controller, and a 2×2 3-dB coupler. The intensity-modulated signal (pump) light and the CW probe light are launched into the Sagnac loop from the different input ports (1 and 2) of the 3-dB coupler, respectively. Each light is individually split into clockwise- and counterclockwise-components with the same power by the 3-dB coupler. Due to the Sagnac loop configuration, the probe light is all reflected back to its input port 1 because each probe component experiences the same cross-phase modulation induced by each co-propagating signal light component. Since the signal light is also reflected back to its input port 2, both the signal (pump) and probe lights are effectually separated without optical filters regardless of their wavelengths.

The output probe light, after being phase-modulated in the Sagnac loop, is converted into the intensity-modulated light by the self-delayed interferometer when the delay is properly chosen to half of the time slot. It is essential to note that the input signal should be RZ format and its pulse width has to be smaller than the half of the time slot in this method. Moreover it should be pointed out that the self-delayed interferometer requires precise phase tuning according to the probe wavelength in practice. Fortunately, this could be alleviated when adopting fixed frequency spacing allocation such as ITU-T grid wavelength though a stable and proper delayed interferometer is still required for such WDM systems. A delayed interferometer based on the planar lightwave circuits (PLCs) will be a prospective candidate in which the phase delay can be precisely controlled by thermo-optic effect, so that stable operation can be expected as compared with conventional fiber-based delay lines.

 

Fig. 1. Basic configuration of the filter-free wavelength converter using Sagnac loop and self-delayed interferometer.

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3. Experimental results

In order to verify the proposed filter-free operation, we have performed 10-Gbit/s wavelength-conversion experiments. A signal (pump) pulse train, having a width of 20 ps and a repetition rate of 10 GHz, is generated at 1544.5 nm from a distributed feedback laser diode (DFB-LD) and an electro-absorption (EA) modulator. The signal pulse train is intensity-modulated with 2⁵ bit word pattern or 2¹⁵-1 pseudorandom bit sequence (PRBS) at 10 Gbit/s by a LiNbO₃ intensity modulator. The average output power of the modulated signal pulse train is amplified to 16 dBm as a pump by two sets of Er-doped fiber amplifiers (EDFAs). A continuous wave (CW) probe light is generated from an external cavity tunable laser diode and the output power is amplified to 13 dBm by an EDFA. Polarization states of the signal and probe lights are adjusted to the same orientation by polarization controllers (not indicated). The Sagnac loop consists of a 2×2 3-dB fiber coupler, a pigtailed polarization controller (PC), and a 6.25 km conventional dispersion-shifted single-mode fiber (DSF) with 1.54/(W km) non-linear optical coefficient. The zero-dispersion wavelength of the DSF is 1552 nm and the walk-off between the probe (1540-1565 nm) and signal (1544.5 nm) wavelengths is within 20 ps; this is not a critical value for 10-Gbit/s though it must limit the maximum operation speed. The PC in the Sagnac loop controls the polarization state so that the transmitted signal power through the Sagnac loop is minimized. As for the delayed interferometer, we use a polarization-maintaining (birefringent) fiber with a polarization dispersion of 50 ps (equal to half time slot) followed by a polarizer whose principal axis is oriented at 45° with respect to that of the birefringent fiber.

Figure 2 shows the output spectra from port 1 of the Sagnac loop, together with the input spectra, measured with the resolution bandwidth of 0.01 nm when the probe light wavelength is set to (a) 1542 nm and (b) 1547 nm, respectively. You can see the input probe light is reflected back to port 1 and its single line spectrum is broadened due to the cross-phase modulation (XPM) by the intensity-modulated input pump signal at 1544.5 nm through the Sagnac loop. Probe light loss through the Sagnac loop, measured by an optical power meter without signal light, is as low as <1 dB. This is because the in-phase interference condition holds at port 1 between clockwise and counter-clockwise probe light components. On the other hand, the output pump signal is well suppressed at port 1 because the out-of-phase interference condition holds between two signal light components. The leaked signal power into port 1, measured by the optical power meter without probe light, is less than -17 dBm, thus, the unwanted signal power is suppressed by more than 33 dB including the loss of DSF. As a result, more than 26 dB ratio of the probe light to pump signal light power is obtained both for 1542 and 1547 nm probe wavelengths. We have also confirmed the similar results in the range of 1540-1560 nm probe wavelengths.

 

Fig. 2. Output spectra from the Sagnac loop when the probe light wavelength is (a) 1542 and (b) 1547 nm.

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The wavelength dependence of the probe loss through the Sagnac loop is less than 0.7 dB in the above range. The residual wavelength dependence is attributed to the dependence of the 3-dB coupler or the polarization controller used in the Sagnac loop. The third small spectra are newly generated at the symmetrical position with respect to the signal as shown in Fig. 2. This is due to an idler wave generation by the four-wave mixing (FWM) in the fiber loop. Although this is undesirable for wavelength conversion, it has been confirmed that the idler wave power can be smaller than the residual signal power provided that the wavelength separation between signal and probe is larger than 1 nm.

Figure 3 shows the wavelength-converted waveforms after the delayed interferometer when using a 32-bit word pattern. Figure 3(a) is the input pump signal (1544.5 nm) waveform for reference, and Figs. 3(b) and 3(c) are the converted waveforms when the probe wavelength is set to 1546 nm and 1545 nm, respectively, corresponding to 1.5 nm and 0.5 nm up-conversion. We can see the good converted waveforms without waveform degradation as well as pattern effects in these cases.

 

Fig. 3. Waveforms of the wavelength-converted signal. (a) input pump signal, (b) and (c) wavelength-converted signals at wavelengths of 1546nm and 1545m, respectively, (d) output waveform without probe light, (e) output waveform when the probe wavelength is the same as the signal wavelength (1544.5 nm). Δλ is a wavelength separation between signal and probe lights.

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To estimate the pump signal crosstalk, that is, the signal leakage into port 1, we measured an output waveform without probe light under the same pump signal level as for the above cases. As is shown in Fig. 3(d), no distinct bit pattern can be observed. This indicates that the proposed method can sufficiently suppress undesirable pump signal without any conventional optical filters. Thus we can conclude that the modulated pulse patterns observed in Figs. 3(b) and 3(c) result from only the phase-modulated probe light.

Figure 3(e) shows the extreme case in which the probe wavelength is set to 1544.5 nm, just the same as the signal wavelength. In this case, very large fluctuation in pulse amplitude have been observed for every mark bit although its instantaneous amplitude increases up to twice of other cases. This may be attributed to the interference effects between co-propagating signal and probe light components. This is because the fluctuations are observed only when both the signal and probe lights having almost the same wavelength are launched into the NOLM.

Finally, we measured the bit error rate (BER) of the proposed wavelength converter using 2¹⁵-1 PRBS signal at 10 Gbit/s. In this measurement, we use an automatic-gain controlled (AGC) EDFA with 1550nm-1560nm bandwidth as an optical preamplifier and a 50kHz-15GHz bandwidth RF amplifier in the receiver circuit. Figure 4 shows the measured BER curves versus the received optical power incident to the AGC-EDFA, as a function of probe light wavelengths (1550nm-1560nm). For comparison, eye-diagrams measured by a Uni-Traveling-Carrier Photodiode (UTC-PD) without bandwidth-limited RF amplifiers are also shown in the inset. We can see good eye-diagrams and confirm the stable operation below 10^-9 BER at the received power of more than -25 dBm. These results indicate that the proposed wavelength converter has small wavelength dependence, the power penalty of which is less than 1 dB at BER of 10^-9 in the wavelength rage of 10 nm. This wavelength range could be expanded when we use the appropriate optical amplifiers having wider bandwidth.

 

Fig. 4. The measured eye-diagrams and BER curves as a function of the probe wavelength.

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4. Conclusion

We have proposed a filter-free wavelength converter based on XPM in silica fibers by using a Sagnac loop together with a self-delayed interferometer. Without any optical filters, we have successfully demonstrated flexible wavelength-conversion operation at 10-Gbit/s. This technique can be scaled for higher bit rates (more than 100 Gbit/s) by using a shorter length of highly nonlinear fiber [9] because this is based on the ultrafast optical nonlinearity in optical fibers and the walk-off between signal and probe light is the main limiting factor in such kind of the converter. Some of polarization insensitive techniques proposed in the conventional NOLMs [8] are also applicable to our filter-free scheme.