We demonstrate sub-millisecond tuning of a prototype parametric tunable dispersion compensator (P-TDC) based on cascaded polarization-diverse four-wave mixing (FWM) process with a fast tunable and highly wavelength-stable pump light source. The pump light source is developed using a tunable distributed amplification chirped sampled grating distributed reflector laser that is fully wavelength tunable by on-chip heaters with a 3-dB frequency response of 45 kHz, resulting in fast dispersion tuning of less than 50 μs without additional timing jitter. The P-TDC is developed as the first prototype to satisfy essential requirements for practical network uses: stable input-polarization diversity, input-wavelength preservation, and seamless dispersion tunability for entire C-band input wavelengths are simultaneously achieved.
© 2013 Optical Society of America
Since a large-scale IP router consumes large amounts of electricity in present optical networks, optical path switching to cut through IP routers is a promising solution to reduce energy consumption of optical networks. We have proposed dynamic optical path networks (DOPNs) that utilize optical path switching in a dynamic manner to drastically reduce energy consumption . In a practical situation of DOPN, various kinds of signal formats, not only digital coherent signal but also conventional on-off-keying or differential phase-shift-keying signals, coexist depending on bandwidth needs. Hence fast optical dynamic dispersion compensation is a key-enabling technology to achieve effective path switching with a short guard time. Considering the balance between average path connection time of minutes to hours and the switching guard time, the target response time of the dynamic optical dispersion compensator is sub milliseconds for the real use in DOPN.
We have developed parametric tunable dispersion compensator (P-TDC) [2, 3], in which the amount of dispersion is tuned by parametric frequency conversion before propagating through a frequency-dependent dispersive medium. Widely tunable parametric frequency conversion is achieved through four-wave mixing (FWM) process in state-of-the-art highly-nonlinear fibers (HNLFs) . Ultra-fast responses of FWM process enable much faster dispersion tuning compared with the response time in seconds of conventional tunable compensators based on mechanical or macroscopic thermal tunings [5, 6]. The response of the P-TDC practically depends on the wavelength-tuning response of a tunable pump light source for FWM process. We have so far demonstrated dynamic dispersion compensation with a switching guard time of 125 μs by developing a P-TDC using commercially available fast tunable laser diode with carrier injection for the pump source . While the guard time was short enough for the use in DOPN, the compensated signal suffered increment of timing jitter in static conditions since the wavelength fluctuation of the compensated signal due to the wavelength fluctuation of the pump light source was converted to timing jitter by propagating through a dispersive medium. For fast and timing-jitter-free tuning of a P-TDC, a pump light source must satisfy both high wavelength stability and fast wavelength tunability.
In this paper, we report the development of a P-TDC with sub-millisecond tuning responses for DOPN and its successful fast tuning without additional timing jitter. Here the P-TDC is developed as the first black-box prototype that satisfies all the essential practical requirements of polarization diversity, input-wavelength preservation, and seamless dispersion tunability without tuning gap due to FWM guard band for entire C-band input wavelengths. While we have addressed each issue individually so far , the technical solutions are demonstrated simultaneously for the first time in this report. State-of-the-art components are developed to achieve all the practical requirements and timing-jitter-free sub-millisecond tuning: A polarization-maintaining HNLF (PM-HNLF) that has flat zero-dispersion profile in C + L band  is used as a nonlinear medium to achieve stable polarization diversity and seamless tuning for entire C-band input wavelengths, and a tunable distributed amplification chirped sampled grating distributed reflector (TDA-CSG-DR) laser  is used as a pump light source to achieve sub-millisecond dispersion tuning. We employ fast dispersion monitoring based on radio-frequency (RF) power measurement of clock components  and successfully demonstrate timing-jitter-free fast dispersion tunings of less than 50 μs for several tuning combinations.
2. Design of prototype P-TDC
Figure 1 shows the design of the prototype P-TDC. For input-wavelength preservation or in-line operation, P-TDC design with cascaded tunable frequency converters (T-FCs) is employed . The P-TDC consists of a dispersion compensating fiber (DCF) for offset dispersive medium (DCF-1), two T-FCs, and another DCF for frequency-dependent dispersive medium (DCF-2). T-FCs are based on degenerate FWM process with pump-frequency tuning. The pump frequency ωP is common to two T-FCs, which indicates that the first T-FC shifts the input frequency ωS to the intermediate frequency ωI for dispersion tuning and that the second T-FC shifts the intermediate frequency ωI back to the original input frequency ωS. Simultaneous compensation of multi-channel signal essentially requires such input-wavelength preservation since the signal is de-multiplexed by arrayed waveguide gratings with fixed grid after the compensation.
To achieve ± 100-ps2 seamless tuning range around 0 ps2, the dispersion characteristics of DCF-1 and DCF-2 were newly chosen and carefully adjusted in this prototype development: DCF-1 had the net second-order dispersion (SOD) of 434.7 ps2 and the net third-order dispersion (TOD) of −2.08 ps3 at 1550 nm, and DCF-2 had the net SOD of 183.9 ps2 and the net TOD of 6.78 ps3 at 1550 nm. Figure 2 shows the group velocity dispersion (GVD) tuning characteristics of the P-TDC for various input wavelengths of 1530.33, 1540.16, 1550.12, and 1560.20 nm. In practice, T-FC based on degenerate FWM process suffers wavelength-tuning gap around its input signal wavelength because pump wavelength is set between the signal and idler wavelengths. The wavelength-tuning gap causes GVD-tuning gap in the P-TDC. To avoid this, the intermediate wavelengths are limited to longer wavelengths than the input ones. The tuning characteristics show that the conversion-wavelength range from 1556 to 1586 nm corresponds to the GVD tuning range of ± 100 ps2 for 30-nm input wavelength range in C-band, which indicates that 50-nm wavelength conversion range to longer wavelengths from the input ones is required to make this design into real system.
3. Development of prototype P-TDC
3.1 PM-HNLF for wideband frequency tuning via degenerate FWM
The conversion range of the T-FC based on FWM process with pump-wavelength tuning is limited by phase mismatching mainly due to residual dispersion of a nonlinear medium. The P-TDC employs a state-of-the-art PM-HNLF that has nearly zero dispersion in C + L band  to achieve wider tuning range. The polarization-maintaining property is effective for long-term stable polarization-diversity operation without active feedback. The PM-HNLF has an ultra-low dispersion slope of 0.003 ps/nm2/km at 1550 nm and two zero dispersion wavelengths of 1539 and 1610 nm. The loss and nonlinear coefficient are 1.74 dB/km and 21 W−1km−1, respectively. Here we focus on wavelength conversion to longer wavelength for C-band input wavelengths and experimentally investigate degenerate FWM process in the PM-HNLF with a length of 50 m. The pump power is set at 23 dBm, which is below the stimulated Brillouin scattering threshold of 25 dBm. The states of polarization of signal and pump are aligned to obtain maximum conversion efficiency. Figure 3 shows the conversion efficiencies to longer wavelengths for input wavelengths from 1530 to 1565 nm. The 3-dB conversion range to longer wavelengths is more than 50-nm for all the input wavelengths, enabling the P-TDC with a seamless GVD tuning range of ± 100 ps2 for entire C-band input wavelengths.
3.2 Fast and wavelength-stable pump light source using TDA-CSG-DR laser
In the GVD tuning process of a P-TDC, wavelength fluctuation of a pump light source is converted to the wavelength fluctuation of a signal through FWM process, resulting in timing jitter after frequency-time conversion by propagating through a frequency-dependent dispersive medium. This phenomenon is specific and critical issue for fast dispersion tuning of a P-TDC. In order to achieve sub-millisecond dispersion tuning without additional penalty due to timing jitter for a 40-Gbaud signal, not only fast wavelength tunability of less than one millisecond but also high wavelength stability corresponding to timing jitter of less than 1 ps are required for a pump light source. The required wavelength stability is related to the SOD of the dispersive medium, and 1-ps timing jitter corresponds to wavelength stability of approximately 300 MHz in the proposed P-TDC configuration. These considerations indicate that so-called linewidth of a laser is negligible, however, wavelength instability caused by the noise of a fast laser driver must be well taken care since such an electric circuit has trade-off between the amount of noise and response time in principle.
We develop a fast tunable pump light source consisting of TDA-CSG-DR laser with a narrow linewidth of less than 300 kHz and fast laser driver with digital-analog hybrid circuits. TDA-CSG-DR laser has a single stripe structure based on Vernier effect and is fully wavelength tunable by thermo-optic effects of on-chip heaters with a 3-dB frequency response of 45 kHz [10, 13]. The fully heater-based tuning enables a response within sub milliseconds and surely contributes to high wavelength stability because the heaters are insensitive to high-frequency noise of over a few hundreds of kHz from the laser driver circuit. Considering the practical situation in which a clock recovery circuit at a receiver works effectively for slow timing variation within its loop bandwidth of approximately 10 MHz, the pump light source developed with TDA-CSG-DR laser can achieve timing-jitter-free operation in the prototype P-TDC. Figure 4 shows the distribution of wavelength-switching time in the fast pump light source. We investigate it for more than 1000-channel combinations: the source and destination channels are all of 96 ITU-T 50-GHz-grid wavelengths and randomly selected 11 wavelengths in C-band, respectively. All the measured switching time is less than 800 μs. The switching response is found to be highly related to heater power differences between source and destination channels . We estimate from the relation and the calibration current table of the laser that the switching time is less than 1 ms for all the channel combinations, which indicates that the P-TDC has sub-millisecond response. We also recently demonstrated that the laser was finely tunable with 6.25-GHz grid , corresponding to tuning resolution of less than 1 ps2 in the proposed P-TDC configuration.
3.3 Prototype configuration
Figure 5 shows the configuration of the P-TDC. The signal to be compensated propagates through DCF-1 and is launched into the first T-FC after amplification by an erbium doped fiber amplifier (EDFA). The first T-FC shifts the signal wavelength to an intermediate wavelength through degenerate FWM process. For input-polarization diversity, the T-FC employs an optical diversity loop  consisting of a coupler, an optical circulator, a polarization beam splitter/combiner, and the 50-m long PM-HNLF. While such a polarization-diversity loop was achieved with all-fiber implementation in previous reports [16, 17], here we newly developed compact polarization-diversity modules using free-space micro-optics for long-term stability. The pump power imbalance of the two output ports to the PM-HNLF is suppressed to less than 0.5 dB in C + L-band, and the temporal variation is negligibly small in laboratory environment. The fast tunable pump light source using TDA-CSG-DR laser generates pump light. The pump light is amplified by a polarization-maintaining EDFA and is divided into two by a polarization-maintaining optical coupler to be commonly used for the first and the second T-FCs. Then the signal with an intermediate wavelength propagates through DCF-2 and is launched into the second T-FC after amplification. The second T-FC has the same configuration as the first one and operates to convert the intermediate wavelength back into the original signal wavelength. All the components except for the fast tunable pump source are put in a 19-inch rack-mountable box, and the active components of EDFAs and band-pass filters (BPFs) are controlled through a computer.
4. Experimental demonstration of sub-millisecond tuning of P-TDC
Figure 6 shows the experimental setup to demonstrate fast dispersion tuning of the P-TDC. The transmitter generates 43-Gb/s NRZ-OOK signal with PRBS 231-1 at 1549.72 nm, and the prototype P-TDC controls the amount of the GVD of the signal. Dispersion monitoring technology based on clock tone monitoring  is employed to measure GVD tuning responses. The dispersion monitor consists of a photo detector, a 43-GHz narrow pass-band filter, a 43-GHz narrow-band amplifier, an RF detector, and an oscilloscope (OSC). Since the 43-GHz clock power is related to the residual GVD of the signal, the direct current voltage from the RF detector corresponds to the amount of GVD generated from the P-TDC. The monitoring response is limited by the RF detector, and the output bandwidth is approximately 1 MHz. For the measurement of signal quality after the P-TDC, a typical receiver setup consisting of a pre-amplifier, photo-detectors, and a clock recovery with a loop bandwidth of 16 MHz is used. We measure bit-error-rate (BER) characteristics and eye-diagrams of the signal.
Table 1 shows the relation between pump wavelength and the amount of GVD generated from the P-TDC for the signal wavelength of 1549.72 nm. The P-TDC is set at 0 ps2 when the pump wavelength is 1561.83 nm: the amounts of GVD of DCF-1 and DCF-2 are perfectly cancelled. We measured tuning responses from 0 ps2 to several GVD settings by changing the pump wavelength from 1561.83 nm to the values shown in Table 1. Figure 7 shows the spectra of degenerate FWM process in the first and the second T-FCs. The first T-FC successfully shifted the signal wavelength to each intermediate wavelength for GVD tuning, and the second T-FC also successfully converted the intermediate wavelength back into the original signal wavelength. The conversion efficiencies were approximately −20 dB for the pump power of 22 dBm.
Figure 8 shows the tuning responses measured by the OSC and eye diagrams at the output of the P-TDC. The black line indicates the switching trigger for the laser heaters to change the wavelength, and the dashed lines indicate target monitoring voltages corresponding to each GVD setting. The target voltages were measured in static conditions before the response measurements. The measurement results show that the tuning responses of less than 50 μs are achieved in these GVD settings. In principle, the tuning response of the P-TDC is the sum of the wavelength-switching time of the pump light source and the transmission delay of DCF-2. The switching times of the measured pump-wavelength combinations are 10 to 29 μs as shown in Table 1, and the delay of DCF-2 is approximately 5 μs, which is consistent with the results measured by using the dispersion monitor. Thus we experimentally demonstrated sub-millisecond tuning responses of the P-TDC.
Then we set the P-TDC at 0 ps2 and measured BER characteristics to evaluate static performances. Figure 9 shows BER characteristics and eye diagrams at OSNR of 30 dB. The OSNR penalty from the back-to-back measurement at a BER of 1x10−9 is less than 0.4 dB. The slight OSNR penalty is caused by in-band amplified spontaneous emission (ASE) noise of the signal because optical filters are used in loss and amplification process of the P-TDC. The root-mean-square timing jitters measured from the eye diagrams are 1.0 ps in both cases, which indicates that the wavelength stability of the pump light source is high enough for successful P-TDC operation.
We demonstrated less-than 50-μs GVD tunings of the P-TDC using TDA-CSG-DR laser for several GVD settings at the input wavelength of 1549.72 nm. Additional timing jitter induced by the wavelength fluctuation of the pump light source was not observed, and the ONSR penalty induced by in-band ASE noise from the P-TDC was approximately 0.4 dB. The P-TDC has ± 100-ps2 seamless GVD tunability for C-band input wavelengths and sub-millisecond tuning responses. This prototype satisfies all the essential practical requirements for dynamic dispersion management in DOPN.
This work was supported in part by Special Coordination Funds for Promoting Science and Technology of MEXT, Japan.
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