We experimentally demonstrate an all fiber-based, compact add/drop multiplexer (ADM) of a 160 Gbit/s optical time division multiplexed signal using only 1-m length of our fabricated Bi2O3-based step index type optical fiber with an ultra-high nonlinearity of ~1100 W-1∙km-1 The ADM is based on the cross phase modulation-induced nonlinear polarization rotation principle and simultaneous add/drop operation was easily achieved by use of a polarization beam splitter after the Bi2O3-based nonlinear fiber. Error-free add/drop operation is readily achieved at multiplexed data rates of both 80 Gbit/s and 160 Gbit/s.
©2005 Optical Society of America
The fabrication technology of optical fibers based on bismuth oxide (Bi2O3) material has advanced significantly in recent years, and has resulted in the production of a variety of high-quality Bi2O3-based optical fibers: for example, highly nonlinear Bi2O3 based step-index fiber (Bi-NLF) , rare-earth doped Bi2O3 fiber , and Bi2O3 based holey fiber . In particular, Bi-NLFs, which possess a ultra-high Kerr nonlinearity ~100 times larger than conventional silica-based highly-nonlinear fiber and can be readily fusion-spliced to conventional silica fibers, have attracted huge attention for their practical application to the implementation of a variety of nonlinear optical signal-processing devices within fiber-optic communication systems.
Owing to the extremely-high nonlinear coefficient n2 of the Bi2O3 glass, such a high nonlinearity of γ ~1360 W-1∙km-1 can be readily achieved by use of a conventional step-index fiber structure  without employing any special fiber structure design. This means that only a meter or less in length would be long enough to generate a nonlinear optical phase shift sufficient for obtaining various nonlinear signal-processing functions. The compactness and stability issue of fiber-based devices relative to semiconductor-based devices could thus be substantially improved by use of short length of such ultra-high nonlinearity fiber. In addition to the huge enhancement of Kerr nonlinearity, the Bi-NLF has a unique advantage of a relatively high stimulated Brillouin scattering (SBS) threshold compared to the silica based highly-nonlinear fiber, and this means that any additional SBS suppression scheme is not required for the implementation of a range of optical devices based on four-wave mixing or parametric amplification .
Until now we have performed a series of researches to figure out the possibility and the limitation of the Bi-NLF technology for the implementation of various nonlinear signal-processing devices and showed that it would be possible to realize a range of practical and compact all fiber-based devices using Bi-NLFs: for example, a four-wave mixing-based wavelength converter of a 40-Gbit/s non-return-to-zero (NRZ) signal using a 40-cm Bi-NLF , an optical time-division multiplexed (OTDM) data demultiplexer using just 1-m length of Bi-NLF , and a nonlinear birefringence based wavelength converter of an 80 Gbit/s RZ signal using a 1-m-long Bi-NLF .
Another essential signal processing device in high-speed OTDM systems and the related all-optical networks is the channel add/drop multiplexer (ADM) operating in the time domain . The ADM performs two key functions: One is selectively dropping a base-rate tributary from a multiplexed high bit-rate data stream in the time domain, and the other is adding a new channel into the cleared dropped-bit time slot  at the network nodes. Two complementary switching windows with a high-extinction ratio are thus required for simultaneously realizing good add and drop performance . A variety of OTDM ADMs have been proposed and experimentally demonstrated to date and those are based on optical switches incorporating either semiconductors devices [9, 10] or nonlinear optical fibers [8, 11, 12, 13].
In this paper we further investigate the applicability of our Bi-NLF technology for the implementation of ultra-high speed, nonlinear signal-processing devices. More specifically, we make our efforts to implement an all fiber-based OTDM add/drop multiplexer operable at a data rate of 160 Gbit/s by use of just 1-m length of our fabricated Bi-NLF as a nonlinear interaction medium. Our add/drop multiplexer is based on the nonlinear birefringence principle within nonlinear optical fiber [14, 15]. Both drop and add functions are simultaneously obtained at a single all-fiber-based device. Experimental results show that error-free add/drop operation is readily achieved at a data rate of 160 Gbit/s.
2. Experimental setup and results
Figure 1 shows the experimental setup for our 160-Gbit/s OTDM channel add/drop multiplexer using only 1-m length of the fabricated Bi-NLF. 1.6-ps Gaussian pulses were first generated from a 10-GHz mode-locked erbium glass laser operating at a wavelength of 1555 nm and subsequently modulated to provide a 27-1 pseudorandom bit sequence (PRBS) at 10 Gbit/s using a high-speed LiNbO3 modulator. These pulses were then amplified to a ~15 dBm average power with an erbium-doped fiber amplifier (EDFA) and multiplexed up to an aggregate bit rate of 160 Gbit/s using a commercially available four-stage passive multiplexer (MUX), which was designed to provide a PRBS at the output port for only a 27-1 PRBS input signal. As a control pulse source we used a 10-GHz mode-locked erbium fiber ring laser (EFRL) operating at a wavelength of 1545 nm and synchronized to the 10-GHz base rate of data pulses. These control pulses with a 3-ps temporal width were amplified up to ~29 dBm after a 3-nm bandpass filter. A tunable optical delay line was used to adjust the arrival time of the control pulses relative to the 1545-nm data pulses. The 160-Gbit/s data pulses and the control pulses were combined together by a 50:50 coupler and launched into a 1-m-long Bi-NLF. The peak power of the control pulse within the Bi-NLF was ~7 W considering the 50:50 coupler and the 2.5 dB splicing losses. Polarization controllers (PCs) were included on both the control and the 160-Gbit/s data launching paths into the Bi-NLF so that they were properly polarized at the fiber input to yield maximum nonlinear birefringence. A 3-nm bandpass filter was employed at the output of the Bi-NLF to filter out the residual control beam components. After the bandpass filter a polarization beam splitter was used to selectively obtain a demultiplexed channel (drop-channel) by the synchronized control pulse at one output port (drop port) while the rest of the multiplexed channels with a cleared drop-channel time slot (through-channels) were extracted from the other port (add port). A new base-rate data channel, which was split from the 10-Gbit/s base-rate data pulses using a 90:10 coupler, was added to the empty time slot of the through-channels with a proper time-delay by use of a 50:50 coupler. Both a polarization controller and an optical attenuator were also used to adjust the polarization and the optical power of the added pulses to the same status of the through-channels.
We then constructed a 160-to-10-Gbit/s data-demultiplexing switch to assess the add/drop performance in terms of bit error rate (BER). The 160-to-10-Gbit/s demultiplexing switch was based on a nonlinear optical loop mirror (NOLM) incorporating a 150-m long highly nonlinear dispersion-shifted fiber (HNL-DSF), which has a zero dispersion wavelength (λ0) at 1602 nm. The demultiplexed signal was then fed into a receiver module composed of an optical preamplifier, a 1-nm bandpass filter, and a photodetector to measure BER performance.
The propagation loss of our fabricated Bi-NLF was 0.8 dB/m at 1550 nm. The mode-field diameter and the NA were 1.97 μm and 0.64, respectively. The input and output splicing losses were measured to be 2.5 dB and 2.6 dB, each. Special care was taken of controlling arc strength and time due to the material discrepancy between Bi2O3 and Silica, and a high NA silica fiber (NA ~0.35) was employed between them to achieve good intermediate mode matching. We define the splicing loss as the overall loss from the Bi-NLF to the standard silica single mode fiber (SMF) including the loss between the high NA fiber and the SMF. Measured group velocity dispersion (GVD) was -260 ps/nm-km at a wavelength of 1550 nm. The fiber nonlinearity was measured to be ~1100 W-1∙km-1 at 1550 nm, which is ~100 times larger than that of the conventional, silica-based HNL-DSF. Further details of our Bi-NLF are fully described in Ref. .
Our OTDM add/drop multiplexer is based on cross phase modulation (XPM)-induced nonlinear polarization rotation [14, 15]. XPM between the control and the multiplexed data beams results in nonlinear birefringence for the data beam where these beams overlap temporally within the fiber. Only the nonlinearly polarization-rotated channel (drop-channel) by the synchronized control pulse passes through one output port of the polarization beam splitter and is dropped whilst the rest of the channels with a cleared drop-channel time slot (through-channels) are extracted from the other port of the beam splitter. A new channel (add-channel) is then added to the empty time slot of the reflected through-channels using a fiber coupler.
In order to characterize our 1-m Bi-NLF-based ADM, we first estimated switching windows of both the drop and the add ports by sliding the control pulse across the data pulse in the time domain and measuring the output optical power. The full width at half maximum (FWHM) of the measured windows was 3.4 ps and 6.5 ps for the drop and the add ports, respectively, as shown in Fig. 2. High on-off extinction ratios of ~20 dB and ~16 dB were achieved for the drop and the add ports, each. The switching window property difference could be attributed to the induced nonlinear phase shift of less than p, which was caused by the walk-off effect and the insufficient control pulse peak power. Further optimization of the GVD property of the BI-NLF and the peak power of the control pulses should allow for narrower and symmetric switching windows. However, these switching window characteristics are still good enough for our 160-Gbit/s ADM experiment.
We tested the add/drop multiplexer at multiplexed data rates of both 80 Gbit/s and 160 Gbit/s. Fig. 3(a) shows measured eye diagrams for the input OTDM channels, the drop-channel, the through-channels, the newly reconstructed OTDM channels after the ADM, and the demultiplexed add-channel, which were measured using a fast p-i-n diode and sampling oscilloscope of a combined ~70 GHz bandwidth. High quality of simultaneous add-drop operation is clearly evident from the eye diagrams for both data rates of 80 Gbit/s and 160 Gbit/s.
To quantify the ADM performance from a system perspective, BER measurements at a base rate of 10 Gbit/s were performed for the drop-channel, all of the demultiplexed through-channels, and the demultiplexed add-channel for the multiplexed data of both 80 Gbit/s and 160 Gbit/s. In case of the 80 Gbit/s multiplexed data, high performance and error-free add/drop operation was readily achieved as shown in Fig. 4. The drop-channel was observed to have a power penalty of ~0.7 dB at BER=10-9 compared to the back-to-back curve whilst power penalties for all of the 7 demultiplexed through-channels were less than 0.6 dB. The demultiplexed add-channel was found to exhibit a ~1.6-dB penalty relative to the back-to-back curve, which is ~1 dB higher than that of the 7 demultiplexed through-channels.
The ADM also shows error-free operation at a multiplexed data rate of 160 Gbit/s as shown in Fig. 5 in spite of small increase of overall power penalties. Compared to the back-to-back curve, a power penalty of ~1.3 dB was observed for the drop-channel and power penalties for all of the 15 demultiplexed through-channels were less than 1.8 dB. The demultiplexed add-channel had a ~3.3-dB power penalty relative to the back-to-back curve. The 1.5-dB power penalty increase of the demultiplexed add-channel relative to the demultiplexed through-channels is believed to be mainly due to insufficient drop-channel suppression. Note that the switching window extinction of the add port was ~4 dB less than that of the drop port. We also noticed that the power penalties of the demultiplexed through-channels (channel number 8 and 10 in Fig. 5(b)) next to the demultiplexed add-channel were slightly higher than that of the drop-channel. This can be attributed to the wide temporal width (~6.5 ps FWHM) of the drop port switching window, as shown in Fig. 2. This explanation can be reconfirmed through the use of the eye diagrams in Fig. 3(b) where the through-channel eyes next to the cleared drop-channel time slot exhibit a small amount of reduction of their amplitudes.
We have experimentally demonstrated the use of only 1-m length of our fabricated Bi2O3-based step index type optical fiber with an ultra-high nonlinearity of ~1100 W-1∙km-1 at 1550 nm for the implementation of all fiber-based 160-Gbit/s OTDM add/drop multiplexer. The ADM was based on the Kerr shutter principle using nonlinear birefringence and simultaneous add/drop operation was easily achieved by use of a polarization beam splitter after the Bi-NLF. The experimental results showed that error-free add/drop operation was readily achieved at multiplexed OTDM data rates of both 80 Gbit/s and 160 Gbit/s. In fact, the use of 1-m length of our Bi-NLF in this ADM configuration does not provide better performance than the use of a few hundred meter length of highly nonlinear silica fiber at present ; however, we believe that comparable or better performance should be possible in the near future considering the progress of our design and fabrication technology of Bi-NLF. The Bi-NLF has been investigated as a strong candidate for practical implementation of a range of nonlinear optical signal processing devices, and further applications are certain to be found in the near future.
References and links
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