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Abstract
10 Gb/s non-return-to-zero (NRZ) on-off keyed (OOK) optical data packets are synchronized and time-multiplexed using a 26-ns tunable all-optical delay line. The delay element is based on wavelength conversion in periodically poled lithium niobate (PPLN) waveguides, inter-channel chromatic dispersion in dispersion compensating fiber (DCF) and intra-channel dispersion compensation with a chirped fiber Bragg grating (FBG). Delay reconfiguration time is measured to be less than 300 ps.
©2007 Optical Society of America
1. Introduction
Optical packet switching holds the promise of highly efficient use of the available bandwidth in an optical network. As with electronic packet switching, a key enabling technology is the ability to controllably delay/buffer a data packet so as to synchronize packets and rapidly resolve output port contention. Desirable characteristics of this delay include large tuning range, continuous tunability, and rapid reconfiguration [1]. Previously published results for optical delays used in packet switching have typically been chosen from a finite set of discrete optical path lengths. This methodology produces only a fixed set of delays, whether they are in fiber [2, 3], waveguides [4], or free-space [5]. However, a laudable goal for a truly flexible and efficient packet switch would be to generate any arbitrary delay value. Recently, tunable delays have been shown using wavelength conversion via self-phase modulation [7] or four wave mixing [8, 9] coupled with dispersive elements. In this method, a data stream was converted to a slower-propagating wavelength, followed by conversion back to the original wavelength. Note that proper dispersion compensation was necessary. The result is that a data bit arrives at the output, delayed in time relative to a data bit that had always resided at the original, faster-propagating wavelength. Bit-level delays were shown [10], but little was reported relating to using these delays for time manipulation and synchronization of full data packets. In this paper, we experimentally demonstrate synchronizing and multiplexing of optical data packets using a 26-ns tunable optical delay based on wavelength conversion and inter-channel chromatic dispersion. We achieved tunability for 10-Gbit/s non-return-to-zero (NRZ) on-off keyed (OOK) data packets, multiplexed two packet streams, and measured a 10-9 bit error rate (BER). The delay reconfiguration time from one delay value to another was measured to be <300 ps.
2. Concept of conversion/dispersion based delay module for packet synchronization
Shown in Fig. 1 is the packet-synchronizing scheme. Two packet streams on two different wavelengths are routed such that the packets requiring delay (packet-3 on λ1) pass through the delay module.
Fig. 1. Packet 3 (P3) passes through the delay module which consists of periodically-poled lithium-niobate (PPLN) λ-converters, a dispersion compensating fiber (DCF) and a chirped fiber Bragg grating (FBG).
After a packet is delayed for synchronization, it is then multiplexed with the non-delayed stream (λ2) using a 3-dB optical coupler. As shown in Fig. 2, the delay module uses wavelength-dependent chromatic dispersion generated by a dispersive element, such as dispersion compensating fiber (DCF). Using sum frequency generation (SFG) followed by difference frequency generation (DFG) in PPLN-1 [11], we convert λ1 to the desired wavelength λc. The optical delay equals the total wavelength shift (λ1-λc) multiplied by the total dispersion (ps/nm) of the DCF [9, 10]. We use a second PPLN to convert the delayed signal back to the original wavelength, thereby preserving the original wavelength at the output. Since the DCF also causes data-degrading intra-channel dispersion, we use a chirped fiber Bragg grating (FBG) centered at λ1 and with the opposite dispersion of the DCF in order to perform output dispersion compensation [12].
Fig. 2. In the two scenarios, λ1 is converted to different λc’s, resulting in different group velocities due to inter-channel dispersion. The undesired intra-channel dispersion is compensated by a FBG. (PPLN : periodically poled lithium niobate waveguide)
3. Two stage wavelength conversion in PPLN
The wavelength conversion process is shown in Fig. 3. SFG in Fig. 3(a) is produced by using two laser pumps that are spectrally equidistant from the quasi-phase-matched (QPM) PPLN wavelength. In PPLN-1, the two pumps are the input signal at λ1 and a local pump at λpump-1. These two pumps mix via the first χ 2 process of SFG to generate λp/2={(λ1+λpump-1)/2}/2. This λp/2 mixes with another input “dummy” wavelength λdummy-1 to produce a converted output at λc=2λp-λdummy-1 via the second χ 2 process of DFG. By tuning λdummy-1, we can tune our converted signal λc to almost any desired value within the PPLN bandwidth.
Fig. 3. a). λ1 (input signal) and λpump-1 constitute the pumps for PPLN-1. By tuning λdummy-1, the output λc can be tuned.
At PPLN-2 in Fig. 3(b), λc and λpump-2 are both equidistant from the QPM of PPLN-2 and constitute the two pumps. By tuning λdummy-2, we can ensure that the output wavelength is equal to λ1 again, thus preserving the original signal wavelength. Although PPLN devices can have a bandwidth of more than 70 nm [11], our bandwidth is limited to ~25 nm since we use erbium-doped fiber amplifiers (EDFAs) to amplify the signals.
We note that the “continuous” delay range has small “gaps”, such that the converted wavelength cannot be spectrally located at the QPM or the local pump. However, since the QPM is temperature tunable [11], we can avoid this problem by slightly tuning the QPM and the local pump wavelengths such that the gaps can be removed. Furthermore, another inaccessible λ is the input signal λ1, and this corresponds to zero delay.
4. Experimental setup and results
The experimental setup is shown in Fig. 4. The 196-bit, NRZ-OOK 10-Gbit/s data packets are 19.6-ns long, and the packet guard time of empty space is 8 bits (800 ps). Packets are generated electronically and drive Mach-Zehnder intensity modulators. We manually programmed the pulse pattern generator (PPG) in order to generate the stream of 196-bit data packets, and the BERs were measured by programming the PPG and error detector accordingly. The 19.74-km DCF has a total dispersion of -1742 ps/nm at 1550 nm, loss of 10.7 dB, and dispersion slope of -0.22 ps/nm2/km. The chirped FBG has a positive dispersion of +2020 ps/nm, a 0.456-nm bandwidth at 1546.4 nm, and peak reflectivity of 89%. PPLN-1 and PPLN-2 have QPM wavelengths of 1550.1 and 1554.7 nm, respectively, at 91.5 °C. Laser diode (LD) λpump-1 and LD λpump-2 are fixed at 1553.8 and 1562.8 nm, respectively. LD λdummy-1 and LD λdummy-2 are tuned according to the desired converted wavelength, λc. Polarization controllers (PCs) are inserted in the input path of each PPLN since the converters are polarization dependent [13]. The input powers are: (i) for PPLN-1, λ1 and λpump-1 are each 14 dBm and λdummy-1 is 10 dBm, and (ii) for PPLN-2, λpump-2 and λdummy-2 are each 12 dBm and λc is 5 dBm. The converted wavelength is -13 dBm for PPLN-1 and -19 dBm for PPLN-2. The filters at the output of PPLN-1 and PPLN-2 have 3-dB bandwidths of 0.8 and 1.2 nm, respectively. The receiver is a 10-Gbit/s p-i-n device.
Fig. 4. Experimental setup: LD (laser diode), Mod (modulator), PPG (pulse pattern generator), PPLN (periodically-poled lithium-niobate), DCF (dispersion compensating fiber), FBG (fiber Bragg grating), PC (polarization controller), Circ (circulator), EDFA (erbium doped fiber amplifier) and Rx (receiver). Note that ovals are simple passive couplers.
The packets on λ1 (1546.4 nm) are routed through the delay module. Figure 5 shows three example delay scenarios of 0, 10 and 26.4 ns, corresponding, respectively, to λc at 1556.7, 1551.92 and 1542.5 nm. The inset eye diagram is for the packet stream that has been delayed by 26.4 ns.
The two packet streams at λ1 and λ2 are initially offset by more than a packet length. By changing λc from 1556.76 to 1542.5 nm, we introduce a delay of 26.4 ns that results in aligning the λ1 packet into the vacant slot between packets 1 and 2 on λ2 (1552 nm). Figure 7 shows the synchronized packets that are multiplexed together using a 3-dB optical coupler. The inset eye diagram of Fig. 7 shows that the delayed data packet is slightly more noisy than the non-delayed packet. We believe that this is due to the added ASE arising from the non-optimally-filtered high-power EDFAs that were needed to overcome the PPLNs conversion efficiency of -25 dB. Note that any distortion can be minimized by employing PPLNs with higher conversion efficiency, low noise EDFAs and matched dispersion compensation. Note that the maximum residual dispersion between the DCF and FBG is around 310 ps/nm when the converted wavelength is 1542.5 nm.
Fig. 7. Packet streams λ2 (non-delayed) and λ1 (delayed by 26.4 ns) synchronized and multiplexed. MUX=multiplexer. Our multiplexer is a simple 3-dB passive coupler.
We measured the BER of a manually-programmed bit sequence for the back-to-back (non-delayed) λ1 packet stream, delayed λ1 packet stream, and multiplexed λ1+λ2 packet stream. Figure 8 shows a ~2.5-dB power penalty at a BER=10-9 for the multiplexed signal. This penalty is due to the non-idealities of the delay module. Since higher bit rates are much more sensitive to residual chromatic dispersion, finer control on dispersion compensation would be required.
Fig. 8. Measured bit error rate (BER) for back-to-back, delayed single packet stream and multiplexed data stream. Power penalty of 2.5 dB is observed.
5. Reconfiguration time of optical delay
The delay can be tuned by either changing the input wavelength or the converted wavelength. For our demonstration to dynamically reconfigure the optical delay, we keep λc constant and change λ1 between the two values of 1548.5 and 1551.76 nm. We use this approach for this section because we want to remove from our measurement the additional frequency-dependent time-of-flight of different wavelengths inside the DCF. The modified approach and experimental setup is shown in Figs. 9 and 10.
Fig. 9. (a) Output spectra of the two PPLNs when the switch is in the OFF position. (b) Output spectra when the 2x2 switch is turned ON.
Fig. 10. Experimental setup for measuring the reconfiguration time of the delay scheme. The inset shows that the reconfiguration time is 276 ps, which is the time when one delayed signal vanishes (blue-colored) and the new delayed signal (pink-colored) appears. Ovals are passive couplers.
Two input lasers (1548.5 and 1551.76 nm) that are equidistant from the QPM of PPLN-1 are input to a multi-GHz 2×2 prism-based electro-optic switch. The output port-1 of the switch is connected to a 3-dB coupler. The continuous-wave signal on one output of the coupler is fed to PPLN-1 as λpump-1, while the other coupler output becomes λdummy-2 of PPLN-2. Output from port-2 of the switch is routed through the modulator and input to PPLN-1 as λ1. We keep λdummy-1 and λpump-2 fixed at 1553.8 and 1562.8 nm, respectively. The final output from PPLN-2 is at two different wavelengths, 1560.2 and 1557.5 nm, depending on whether the case is the “switch-off” or “switch-on” position (see Fig. 9). We split the output of PPLN-2, put optical filters with center wavelengths located at the two possible final output values, and observed both of the outputs on a dual-channel optical oscilloscope. The path lengths for both of these outputs are kept equal. As shown in the inset of Fig. 10, the observed elapsed reconfiguration time is ~276 ps.
We note that the reconfiguration time in our experiment includes the inherent delay of the switch. Furthermore, we chose the values of the switched lasers at 1548.5 nm and 1551.76 nm due to the particular QPM settings of the available PPLNs.
Acknowledgments
We gratefully acknowledge Professor A.L. Gaeta for insightful discussions. This work was supported by the DARPA DSO Slow-Light Program, the DARPA UPR Opto-electronic Materials Research Center, and the National Science Foundation.
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