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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 3, MARCH 2004
A Novel Dispersion Monitoring Technique Based on Four-Wave Mixing in Optical Fiber Shenping Li and Dmitri V. Kuksenkov
Abstract—We propose a novel dispersion monitoring method based on four-wave mixing (FWM) or parametric amplification effect in optical fiber. The signal pulse stream, which is used as a parametric pump, is mixed with a weak continuous wave light at a different wavelength in a fiber. Due to the FWM effect, a so-called idler light is created at a wavelength symmetric in respect to the pump. The output idler power can be measured and is dependent on the pulsewidth and, therefore, accumulated residual dispersion of the input signal. The concept was successfully demonstrated with 40-Gb/s return-to-zero signal. This approach potentially works for data rates up to terabits per second and can be applied to provide a feedback for automatic dispersion compensation. Index Terms—Nonlinear optics, optical fiber, optical fiber communications.
I. INTRODUCTION
D
YNAMIC chromatic dispersion management has become a critical issue for high bit-rate transmission systems and reconfigurable optical networks, because the accumulated dispersion changes with time due to several time-varying effects. Thus, for management purposes or automated tunable dispersion compensation, it is imperative to implement some form of dispersion monitoring. Previous work on dispersion monitoring resulted in the development of numerous approaches: 1) detecting the intensity modulation resulting from a phase modulation [1]; 2) modulating the frequency of the transmitted data signal and monitoring the clock deviation [2]; 3) inserting in-band subcarriers in the transmitter and monitoring their radio frequency (RF) tones [3], [4]; 4) adding an amplitude-modulated double-sideband subcarrier to the signal and measuring the phase delay between two subcarrier tones [5]; 5) extracting the clock component and measuring its RF power [6], [7]; 6) extracting two single sideband components of the data signal and measuring their phase difference [8]; 7) using spectral filtering of a self-phase modulation (SPM) broadened return-to-zero (RZ) data stream [9]; and 8) employing nonlinear optical detection [10], [11]. In this letter, we propose a new and simple dispersion monitoring method based on four-wave mixing (FWM) in fiber, suitable for the application in automatic dispersion compensation. In previous work, FWM in an optical fiber has been to use as a tool to characterize low power picosecond pulses [12].
II. PRINCIPLE In the proposed method, the input signal pulse stream with an(wavelength ) is used as the FWM pump gular frequency in an optical fiber. A continuous wave (CW) signal with an(wavelength ) is used as the probe signal. gular frequency Through FWM in the fiber, a new pulsed signal (idler) at an(wavelength ) is generated gular frequency at the output of the fiber. Because of the nonlinear relationship between the idler and pump amplitudes, the average idler power depends on the pulsewidth (duty cycle) of the pump. Therefore, the pulsewidth (and the residual dispersion) of the input signal can be determined by measuring the average power of the idler. To better understand the operating principle, let us consider a pulse train with bit period and pulsewidth , assuming for simplicity that pulses are rectangular in shape with a constant peak power . Then the average powers of the pump and the idler are given by, respectively (1) (2) is the peak power of the idler pulse. where Assuming that the total dispersion of the fiber used to produce FWM and resulting pump pulse broadening are negligibly small and neglecting pump depletion, the peak power of the idler pulse can be written as (3) with (4) where and are the peak power of the pump pulse and the power of the CW signal at the input of the fiber, respectively, is the nonlinear coefficient of the fiber, and is the linear wave vector mismatch determined by the type of , and are the respective propagation fiber used, where constants of the CW signal, the idler, and the pump. From (1)–(4), the average output power of the idler is given by (5)
Manuscript received September 11, 2003; revised November 5, 2003. The authors are with Corning Incorporated, Corning, NY 14831 USA (e-mail:
[email protected]). Digital Object Identifier 10.1109/LPT.2004.823751
with
1041-1135/04$20.00 © 2004 IEEE
(6)
LI AND KUKSENKOV: NOVEL DISPERSION MONITORING TECHNIQUE BASED ON FWM IN OPTICAL FIBER
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Fig. 1. Experimental setup. LD: tunable laser diode. VOA: variable optical attenuator. BPF: bandpass filter.
Equations (5) and (6) clearly show that the average output power of the idler is a function of the pulsewidth of the is positive, the value input signal (pump) pulse. When always decreases with the increasing of pulsewidth . For negative , the same type of dependence satisfies or is observed when ( is positive integer). From (5) and (6), it can be seen that for the above two conditions, changes inversely the average output power of the idler with the pump pulsewidth . What this means is that if the operational parameters and/or the fiber specifications are chosen properly, the pulsewidth of the input signal (pump) can be unambiguously determined from the average power of the idler. Since the pulsewidth of the input signal in turn depends on the amount of the accumulated residual dispersion, the idler power can be used for dispersion (or chirp) monitoring. III. EXPERIMENTAL SETUP AND RESULTS To demonstrate the concept, we built the experimental setup, as shown in Fig. 1. A 1-km-long dispersion-shifted fiber (DSF) was used as the nonlinear medium. The zero-dispersion wavelength of the fiber is 1533 nm. The dispersion slope, attenuation (at 1550 nm), and mode field diameter of the fiber are 0.046 ps/nm /km, 0.58 dB/km, and 4.4 m, respectively. A 33% duty cycle 40-Gb/s RZ pseudorandom binary sequence nm was used as the input. signal at a wavelength Variable (in steps of 10 ps/nm) amount of dispersion was imposed on the signal by using premeasured lengths of standard single-mode and dispersion-compensating fibers. After the preset fiber span the signal was boosted by an erbium-doped fiber amplifier (EDFA). Then an optical filter with 1-nm full-width at half-maximum spectral width was used to filter the amplified spontaneous emission noise from the amplified signal. A variable optical attenuator was used to control the power of the signal. The amplified signal (pump) and a CW and 6-dBm power) from probe (with wavelength a tunable laser diode were mixed by a 3-dB optical coupler and launched into the DSF. Finally, two tunable optical filters (with spectral bandwidth 0.6 and 1.0 nm, respectively) were used to , and its power was select the idler FWM measured by an optical power meter. In the first experiment, the wavelength of the CW probe nm, producing a wavelength offset was set at nm. The experimental results are illustrated in Fig. 2. In Fig. 2, curves (a) and (b) represent the change of the with the residual dispersion normalized average power of of the pump signal for two (average) pump power levels,
Fig. 2. Normalized average idler power versus residual dispersion for (a) the : nm and the average wavelength offset of the CW probe signal of mW. (b) power of the input signal of P : nm and P mW. (c) mW. : nm and P
1 =16
= 14 = 21
1 = 25 1 =25
= 21
and mW, respectively. For each measurement point, the polarization state of the CW signal was adjusted to maximize the idler power. Since the input RZ signal is carved by a dual-drive Mach–Zehnder modulator and, therefore, has negligible chirp, the -axis values in Fig. 2 (as well as Fig. 4) directly represent the amount of dispersion imposed on a signal by the preset fiber span. As expected, the power of the idler is maximized when the residual dispersion is zero and decreases monotonically moving in either direction from the zero dispersion point until the pulses become significantly overlapped in time. The asymmetry of the curves about the zero dispersion point is caused by the SPM effect because SPM produces additional negative dispersion, and to some extent, changes the wave-vector mismatch and, therefore, the idler gain. This asymmetry could be reduced if a fiber with negative dispersion was used. In the second experiment, CW probe wavelength was tuned nm, nm away from the pump signal waveto length. The average power of the pump was set at 21 mW. The measured result is shown in Fig. 2(c). Again, the polarization state of the CW signal was adjusted to maximize the idler power for each measurement point. It is evident that the device sensitivity can be increased by increasing the wavelength detuning between the pump signal and the CW probe. The measured [curve (b)] and calculated [curve (a)] results on the change of the idler power with the pulsewidth of the pump (for the pump power of 21 mW) are illustrated in Fig. 3. All parameters of the calculation are the same as in the experiment. We believe that the difference in shape between the two curves is caused by the square pulse assumption in the model. Since the FWM efficiency is strongly polarization-dependent, field application of the proposed dispersion monitor would require either automatic polarization control or scrambling the polarization state of the pump signal and/or CW probe. The second approach is obviously more economical and was experimentally investigated. All experimental conditions were the same as in the first two experiments. Fig. 4 shows the change of the normalized average idler power with the residual dispersion of the pump signal for the case when polarization state of the CW signal is scrambled. Similar results were achieved when scrambling the polarization state of the pump signal.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 3, MARCH 2004
Fig. 3. Normalized average idler power versus the pulsewidth of the pump for mW. (a) Simulation result. (b) Experimental result. the pump power of P
= 21
response time, it can potentially be used to monitor the dispersion of signals with data rates up to terabits per second. In summary, we have proposed a novel dispersion-monitoring scheme based on FWM in fiber. The method is successfully demonstrated using 1-km long DSF with 40-Gb/s 33% RZ signal. It is shown that the measurement sensitivity can be adjusted by changing the fiber length and the wavelength detuning between the signal and the CW probe. The suggested technique is suitable for the application to dispersion monitoring for automatic compensation. In addition, it is potentially bit-rate independent and expected to work for data rates of up to terabits per second. REFERENCES
Fig. 4. Normalized average idler power versus residual dispersion when the polarization state of the probe signal is scrambled. (a) The wavelength offset of : nm and the average power of the input signal the CW probe signal of mW. (b) mW. : nm and P of P
= 14
1 =25 1 =25
= 21
Because the fiber dispersion has a finite slope, the dispersion-related pump pulse broadening increases and the FWM efficiency decreases with the increase of the wavelength de, limiting the operational bandwidth of the device. tuning This problem can be solved by using a specially designed fiber with reduced dispersion slope, for example, similar to the one reported in [13]. Since the nonlinear fiber with very low birefringence can be achieved with present fiber manufacturing techniques (for example, the degree of the model birefringence of the nonlinear fiber used in our experiments is about 5 10 , which is similar to that of standard single-mode fibers) and the length of the nonlinear fiber used in this device is relatively short, the birefringence of the fiber should have a very small influence on the measurement accuracy. For application in automatic dispersion compensation, the ability to deduce the dispersion sign is important. The scheme presented here could be easily extended to include the differential measurement similar to that reported in [6] and [11]. The obvious limitation of the proposed method is that it only works for RZ (pulse) modulated signals. It can be expected, however, that this will become less important as the systems progress to higher data rates, where RZ is the modulation format of choice. Since the technique is based on fiber nonlinearity with femtosecond
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