K.H. Ahn

Cascadability and functionality of all-optical low-birefringent nonlinear optical loop mirror: experimental demonstration

We verify both experimentally and theoretically the cascadability and the Boolean complete functionality of the low-birefringent nonlinear optical loop mirror (low-bi NOLM) operating with all inputs at the same wavelength. We achieve a low switching energy by using a low-birefringent (Δn∼3.5×10-6), polarization-maintaining fiber to achieve a longer interaction length between two orthogonally polarized pulses. We experimentally demonstrate switching in the cascaded operation of two low-bi NOLMs using picosecond pulses from an erbium-doped fiber laser. This has the potential to have a bit rate of 100 Gb/s. After the two cascaded low-bi NOLMs, the performance is a peak switching contrast of 36:1 and a timing window of 1.7 pulse widths with a switching energy of 9 pJ. In addition, we demonstrate the and and the xor/not operations with the low-bi NOLM showing Boolean completeness. The and operation has a switching contrast of 84.5:1, and the xor/not has a switching contrast of 11.5:1. Finally, we study the gate numerically and find good agreement between experiments and simulations.

Soliton logic gate using low-birefringence fiber in a nonlinear loop mirror

We demonstrate, using numerical simulations, a new all-optical logic gate that combines the mechanisms of soliton dragging and nonlinear-optical loop mirrors (NOLM’s). The design uses specially wrapped low-birefringence polarization-maintaining fiber, which we recently demonstrated in the NOLM configuration. Combining the soliton dragging and NOLM effects increases the timing window of the gate, and using the low-birefringence fiber minimizes the number of cross splices required. For example, we show that by using only one cross splice we can achieve a switching energy of 5.3 pJ and a total timing window of 3.3 pulse widths. We also present a set of design rules for our low-birefringence NOLM.

Synchronization of passively mode-locked erbium-doped fiber lasers and its application to optical communication networks

We synchronized two passively mode-locked erbium-doped fiber lasers using a phase lock loop with a large dynamic range and bandwidth, which is realized by using a novel acoustooptic-modulator-grating scheme. Cross-correlation of the two lasers shows the interlaser jitter is under 2 ps (same as the laser pulse width) for period as long as hours. To prove the quality of phase locking, we apply synchronized lasers in two all-optical network applications, one of which requires the lasers to have the same wavelength and the second requires the lasers to be at different wavelengths. In the single wavelength application, the synchronized lasers drive a cascade of two low-birefringence, polarization maintaining, optical logic gates with switching timing window of 4 and 5 ps, respectively. We obtain nonlinear transmission of /spl sim/50% at a switching energy of 8 pJ and contrast ratio of 16 dB, which are comparable performance as that obtained using a single laser. In the different wavelength application, we use 0.8 ps pulses to switch 2 ps pulses in a two-wavelength nonlinear optical loop mirror demultiplexer with timing window of 5.5 ps. Stable switching is reached at a efficiency as high as 90% at switching energy of 0.8 pJ, and a contrast ratio of 20 dB. Excellent agreement is found between the experimental data and the simulated results, which exclude the timing jitter.

Demonstration and performance analysis for the off-ramp portion of an all-optical access node

Ultrafast processing of packets is demonstrated and the performance analyzed for the off-ramp portion of an all-optical access node. The off-ramp consists of synchronized fiber lasers driving an all-optical header processor that includes nonlinear optical loop mirrors (NOLM), electrooptic router, and demultiplexer in the form of a two-wavelength NOLM. We achieve switching contrasts of 10:1 for the header processor and demultiplexer with switching energies of 10 pJ and 1 pJ, respectively. Also, a proposed measurement technique to obtain eye diagrams is used to analyze the all-optical header processor using the synchronized lasers. Using this technique, we obtain an eye diagram with a Q value of 7.1/spl plusmn/0.36, which corresponds to a worst case BER value of 8.8/spl times/10/sup -12/ for a 95% confidence level. Finally, simulation models are used to verify and compare the experimental results, and we find good agreement. We also use the model to study the various causes for the degradation of the Q value through our system.

Experimental cascaded operation of low-birefringence nonlinear-optical loop mirrors

We experimentally demonstrate the operation of a low-birefringence (low-bi) nonlinear-optical loop mirror (NOLM) that has the advantages of low switching energy, tolerance to timing jitter, and cascadability. Because cascading two all-optical logic gates is an important step toward high-speed optical signal processing and header processing in time-division-multiplexed networks, we also demonstrate the cascaded operation of two low-bi NOLMs. Using a passively mode-locked fiber laser that produces 450-fs pulses at a wavelength of 1.55 microm, we achieve a 10.7:1 switching contrast ratio and a 2.7-pulse-widths-wide timing window after the cascaded gates. The results agree well with theoretical predictions and confirm the advantages of the long interaction length associated with orthogonally polarized pulses in low-bi (Deltan ~ 3.0 x 10(-6)) polarization-maintaining fiber.

System performance measurements for an all-optical header processor using 100-Gb/s packets

We experimentally measure the eye diagram of an all-optical header processor using a crosscorrelator to achieve picosecond resolution. By varying 100-Gb/s header packets, we measure an eye diagram with a Q-value of 7.1 at 12-pJ packet pulse energy for the all-optical header processor consisting of all-optical logic gates and synchronized fiber lasers. From the Q-value, we also statistically calculate the potential bit-error-rate performance of 7.0/spl times/10/sup -13/. By measuring the change in Q-values as a function of input power, we find that input power fluctuations degrade the performance of the header processor by reducing the switching contrast of the logic gates in the header processor.

Experimental demonstration of a low-latency fiber soliton logic gate

We experimentally demonstrate switching in a 50 m-long soliton logic gate with a switching energy of 40 pJ using 490 fs pulses at 1.553 /spl mu/m from an erbium-doped fiber laser. A full characterization of this gate shows a peak contrast ratio of 4.2:1, a timing window of 1.1 pulse width and cascadable operation. To our knowledge, the gate length of 50 m is at least six times shorter than other designs with comparable switching energies. The low-latency of this gate is possible due to a low-birefringent polarization-maintaining fiber that possesses a high polarization extinction ratio of up to 20 dB with a low birefringence of 2.6/spl times/10/sup -6/. The low birefringence leads to a longer walk-off length between two orthogonally polarized pulses, where the walk-off length for 490 fs pulses is 56 m. We also study this gate numerically and find good agreement between the simulations and experiments.

Self-synchronization of 100-Gbit/s TDM packets using a semiconductor laser amplifier and an intensity discriminator

Using a semiconductor laser amplifier followed by an intensity discriminator, we demonstrate packet clock extraction from a 100-Gbit/s eight-bit packet where the first pulse is 20 dB higher than the remaining bits. This extracted pulse can be used to process high-speed packets with low timing jitter within each packet frame. Previous demonstrations of self-synchronization have involved marker pulses at different wavelengths, polarization, intensity, or bit period. Our scheme allows all pulses in the packet to be identical, which simplifies packet generation and propagation in high-speed TDM systems.

High-speed header processing using synchronized fiber lasers driving all-optic logic gates

We show all-optic logic gates driven by synchronized erbium-doped fiber lasers (EDFLs) to perform 100 Gb/s header processing in TDM packetized networks or optical add/drop multiplexers. We experimentally demonstrate AND and XOR logic functions in low-birefringent nonlinear optical loop mirrors (Low-Bi-NOLMs) with switching energy as low as 9 pJ and peak switching contrast of 8:1. We show that the Low-Bi-NOLMs satisfy decision-making capabilities in a header processor such as cascadability and Boolean-complete logic functions. Our novel AOM-grating scheme of synchronization provides 30 times the length and 10 times the bandwidth as compared to the scheme using piezo-electric transducers (PZTs). We have achieved synchronization of two passively mode-locked EDFLs with a timing jitter of approximately 1.3 times the pulse width. We are now integrating the synchronized EDFLs and all-optic logic gates, which are key components for packet addressing for 100 Gb/s applications.