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Motoharu Matsuura, Naoto Kishi, and Tetsuya Miki. Department of Information and Communication Engineering, University of Electro-Communications,.
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Broadband regenerative wavelength conversion and multicasting using triple-stage semiconductor-based wavelength converters Motoharu Matsuura, Naoto Kishi, and Tetsuya Miki Department of Information and Communication Engineering, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan Received December 8, 2006; revised January 30, 2007; accepted February 1, 2007; posted February 5, 2007 (Doc. ID 77924); published April 3, 2007 We demonstrate broadband wavelength conversion with a 320 nm operating wavelength range and channel spacing flexible wavelength-division-multiplexing (WDM) multicasting from a 1550 nm signal using a triplestage cascaded semiconductor-optical-amplifier-based wavelength converter. © 2007 Optical Society of America OCIS codes: 060.2330, 230.1150, 190.2620, 250.5980.

All-optical wavelength conversion is one of the key technologies for future wavelength-divisionmultiplexing (WDM) transmission systems, which include two types: dense WDM (DWDM) and coarse WDM (CWDM). These technologies are both based on the concept of using multiple wavelengths but differ in the spacing of wavelengths and the number of channels. Various transmission experiments have reported the use of more than 1000 channel DWDM systems1 or greater than 300 nm bandwidth CWDM systems2 to date. It is very important to develop alloptical broadband wavelength conversion in such networks, so as to convert an arbitrary wavelength flexibly over such a wide wavelength range. In addition, all-optical multicasting3–6 is an attractive function for conversion of a single data signal into several different wavelengths without limiting the operating wavelength range. However, no broadband multicasting with a wide operating wavelength range over a 1.55 ␮m wavelength window has been reported so far. Previously, we proposed novel broadband wavelength conversion using cascaded semiconductor optical amplifier (SOA)-based wavelength converters with each different gain band.7,8 However, it was very difficult to realize fully flexible wavelength conversion with a type of cascaded scheme. Using this technique, we also demonstrated all-optical multicasting from a 1.55 ␮m to a 1.3 ␮m wavelength window. However, the multicasting range was limited to the 1.3 ␮m wavelength window.9 In this Letter we demonstrate broadband wavelength conversion and WDM multicasting with flexible output wavelength and channel spacing using a triple-stage cascaded SOA-based wavelength converter. This converter also enables us to regenerate the amplitude of the input signal. The experimental setup for broadband wavelength conversion and multicasting is shown in Fig. 1(a). This scheme consists of a dual-stage upconverter (conversion of long to short wavelength) and a singlestage downconverter (conversion of short to long wavelength). Using the dual-stage upconverter, we convert the input into a 1320 nm signal, but also regenerate the amplitude of the input signal by repeat0146-9592/07/091026-3/$15.00

ing inverted wavelength conversion based on crossgain modulation.8,9 After conversion into the 1320 nm signal, we demonstrate downconversion using the single-stage downconverter with a 1.3 ␮m SOA. In the case of conventional wavelength conversion based on cross-phase modulation, conversion into a much longer wavelength is possible because cross-phase modulation in the SOA is effective over wide wavelength range.10,11 In contrast, conversion into a much shorter wavelength outside the gain band of the SOA is very difficult because of the high absorption loss of the SOA.7 Therefore, to realize fully flexible wavelength conversion into a wider wavelength range, we add a single-stage downconverter using a 1.3 ␮m SOA after the conversion into the 1320 nm signal with the dual-stage upconverter. In these wavelength converters, amplifiers SOA1, SOA2, and SOA3, each with a different gain band, are employed. These SOAs have a small-signal gain of over 20 dB at the gain peak wavelengths of 1310, 1400, and 1500 nm, respectively. The 2.5 Gbit/ s nonreturn-to-zero (NRZ) signal at the wavelength of 1550 nm is generated by using an external-cavity laser diode (LD) and LiNbO3

Fig. 1. (a) Experimental setup for broadband wavelength conversion and multicasting. Tx, transmitter, WC, wavelength converter; Rx, receiver. (b) Configuration of cascaded SOA-based wavelength converter. © 2007 Optical Society of America

May 1, 2007 / Vol. 32, No. 9 / OPTICS LETTERS Table 1. Channel Wavelength and Probe Power Injected into SOA1 for Single-Channel and Multicasting Schemes Conversion Scheme Single channel CWDM1 CWDM2 DWDMa

Channel Wavelength (nm)

Input CW Probe Power

1290– 1610 1470, 1510, 1550, 1590 1530, 1550, 1570, 1590 1554.74– 1556.15

7 dBm 4 dBm/ channel 4 dBm/ channel 4 dBm/ channel

a

Eight channels, 25 GHz spacing.

modulator (pseudo-random binary sequence 231 − 1). As preamplifiers for the input data signals, an erbium-doped fiber amplifier, a thulium-doped fiber amplifier (TDFA), and a praseodymium-doped fiber amplifier (PDFA) are employed in each wavelength region of the input signal. The probe and data signal powers injected into SOA2 are 8.0 and 5.0 dBm, while the powers injected into SOA3 are 8.0 and 3.0 dBm, respectively. The channel wavelength and probe power injected into SOA1 are shown in Table 1. Figure 1(b) shows the configuration of the cascaded SOA-based wavelength converter in the dual-stage upconverter. To adjust the state of polarization (SOP) of the input probe beams, polarization controllers (PCs) at the output of each LD and a polarizer (Pol.) at the input of the converter are employed. In this experiment, the inverted wavelength conversions are performed in each wavelength converter so as to utilize the amplitude regeneration effect of the cascaded upconversion.9 In the single-stage downconverter, we adjust the SOP of the probe beam after wavelength demultiplexing as shown in Fig. 1, since the optimum SOPs of the multicasting outputs differ depending on wavelength. In this converter, noninverted wavelength conversion based on nonlinear polarization rotation is performed.11 As a demultiplexer (DEMUX), a tunable optical bandpass filter (BPF) is employed for the single-channel and CWDM schemes, whereas an arrayed-waveguide grating with 25 GHz channel spacing is employed for the DWDM scheme. To estimate the amplitude regeneration effect of our proposed converter, we also employ an amplitude spontaneous emission (ASE) noise source, consisting of a variable optical attenuator (VOA) and dual-stage erbium-doped fiber amplifier, at the input of the dualstage converter. To investigate the broadband conversion performance of our proposed scheme, we measure the converted signal quality with a changing input probe power for single-channel operation. Figure 2 shows the relative power penalty as a function of the continuous-wave (CW) probe power injected into SOA1 at three probe wavelengths. In the case of 1550 and 1610 nm outputs, the power penalties become larger as the input probe powers are decreased. This means that a higher input probe power is required for better performance, if the wavelength of the probe light is located outside the gain bandwidth of SOA1. In contrast, there is an optimum input power around 5 dBm for the case of 1290 nm output, because the performance strongly depends on the input power

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balance between the input data and probe signals. Thus, in the following experiments, the input probe power is set to 7 dBm for single-channel operation, while the input power for multicasting is set to 4 dBm/ channel, since input powers over 13 dBm might cause damage to SOA1. Figure 3 shows the error-free (bit-error rate BER = 10−9) received power of the converted signals as a function of the optical signal-to-noise ratio (OSNR) of the input signal. The insets show the output eye patterns of the back-to-back and converted signals when the input OSNR is set to 19 dB with 0.1 nm resolution. Compared with the back-to-back signal, we obtain much better OSNR tolerances of the converted signals for the single-channel and multicasting schemes. These results indicate that the amplitude of the degraded input signal is well regenerated (owing to the amplitude regeneration effect) by repeating the inverted wavelength conversion.9 On the other hand, slow rise and fall times of the converted waveform are observed. These are due to the slow carrier recovery time of the employed SOAs. Since the proposed scheme has no obstacle to the operating bit rate except for the slow recovery time, higher-bit-rate

Fig. 2. Relative power penalty as a function of probe power injected into SOA1.

Fig. 3. Measured input OSNR tolerances of the back-toback and converted signals. Insets, eye patterns of output signals when the input OSNR is 19 dB/ 0.1 nm.

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Fig. 4. Measured BER characteristics for the cases of (a) single-channel wavelength conversion, (b) CWDM1 and CWDM2 multicasting, and (c) DWDM multicasting with 25 GHz channel spacing.

Fig. 5. Output spectra of eight-channel CW probes using (a) SOA1 and (b) a 1.55 ␮m SOA for DWDM multicasting.

operation will be possible using SOAs with a faster recovery time. The BER characteristics for the single-channel and various multicasting schemes are shown in Fig. 4. For all the output wavelengths, error-free operations with low power penalties are achieved. It should be noted that the received power differences for each measured output wavelength are due mainly to the wavelength dependence of the employed powermeter sensitivity. In addition, the limit of the operating wavelength above 1610 nm is due not to the performance of the converter, but to our equipment. Thus it will be possible to realize a wider operation with better equipment. Compared with single-channel outputs, multicasting outputs have larger power penalties at the same wavelength 共1550 nm兲 because of the power penalty dependence of the injected probe power, as shown in Fig. 2. Using our proposed scheme, we also obtain good conversion performances for the DWDM multicasting scheme. In the case of conventional DWDM multicasting in a 1.55 ␮m window, the channel spacing should be broader than 100 GHz so as to avoid nonlinear effects induced by a 1.55 ␮m SOA, such as four-wave mixing.4–6 On the other hand, the proposed scheme enables us to realize 25 GHz spacing DWDM multicasting with good conversion performance, since SOA1 (1.3 ␮m SOA) can suppress strong nonlinear effects compared with a 1.55 ␮m SOA. Figure 5(a) shows the output spectra of the eight-channel CW probe lights at SOA1 when the total probe power of 13 dBm is injected into SOA1. Compared with the 1.55 ␮m SOA as shown in Fig. 5(b), no remarkable spectral components induced by four-wave mixing are observed. We also demonstrate DWDM multicasting with 50 GHz and

100 GHz channel spacing and obtain good performances, similar to 25 GHz DWDM multicasting. These findings indicate that there is no channel spacing dependence of the conversion performance using our proposed scheme. In conclusion, we demonstrated broadband wavelength conversion and channel spacing flexible multicasting using a triple-stage cascaded wavelength converter. The proposed scheme also enables us to achieve amplitude regeneration of the input signal. This technique will be useful for broadband wavelength conversion and DWDM–CWDM multicasting with amplitude regeneration for future ultrawideband photonic networks. M. Matsuura’s @ice.uec.ac.jp. References

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