IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 8, AUGUST 2003
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Measurement of Multipath Interference in the Coherent Crosstalk Regime Siddharth Ramachandran, Member, IEEE, Jeffrey W. Nicholson, Samir Ghalmi, and Man F. Yan
Abstract—We compare various techniques to measure multipath interference (MPI) in devices exhibiting distributed coherent crosstalk. A notable example of devices in this class is the higher order mode dispersion compensator. These devices are susceptible to deterministic noise as well as intermittent fading, and the measurement technique employed must capture both these manifestations of MPI. We show that, ultimately, slow temporal intensity variations of a continuous-wave distributed-feedback laser yields the best measure of MPI in few-mode fiber devices. Index Terms—Noise measurement, optical crosstalk, optical fiber devices, optical fiber dispersion, optical fiber interference, optical fiber measurements, optical noise, phase noise.
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ULTIPATH interference (MPI), caused by beating between the signal and a weak replica, can severely degrade systems performance. This impairment has been extensively studied in the context of distributed Raman amplification schemes, where the signal interferes with a weak replica resulting from two consecutive Raleigh back-scattering events in the fiber [1]. The signal and replica in this case interfere incoherently, resulting in noise that is deterministic in nature. On the other hand, add–drop nodes formed by pairs of multiplexers and demultiplexers, can also lead to MPI. In this case, the signal and replica interfere coherently at discrete junctures, which manifests in slow as well as fast variations in transmitted power. Recently, devices comprising fibers that support more than one guided mode have gained attention. Notably, dispersion compensation has been demonstrated in few-mode fibers where mode (the so called higher the signal travels in either the order mode dispersion compensators (HOM-DCM) [2], [3]). The existence of signal light in more than one mode can lead to a plurality of distortion effects, such as timing jitter, differential mode delay, and MPI. Of these, the dominant distortion effect in HOM devices is MPI. The interference occurs between copropagating modes, and thus, is coherent in nature, similar to coherent crosstalk in add–drop nodes. However, unlike add–drop nodes, the parasitic modes may be excited in a distributed fashion throughout length of the fiber. Thus, they also share some characteristics with MPI in Raman amplifiers. We evaluate several measurement techniques used to characterize MPI in few-mode fiber devices. As a reference, all measurements are made on an HOM-DCM with known MPI. We will show that the coherent, distributed nature of MPI in few-mode fibers leads to fast fluctuations, manifesting in deterministic noise, as well as slow variations, reminiscent of Manuscript received November 11, 2002; revised April 9, 2003. The authors are with the OFS Laboratories, Murray Hill, NJ 07974 USA (
[email protected]). Digital Object Identifier 10.1109/LPT.2003.814880
fading in mobile communications systems [4]. In addition, the existence of a multitude of interferometers in the HOM fiber calls for careful choice of source as well as detector while measuring MPI. While more than one characterization technique may be needed to evaluate MPI in the most general case, we show that temporal intensity variations of a continuous-wave distributed-feedback (DFB) laser captures most of the relevant information. I. PHASE TO INTENSITY NOISE IN INTERFEROMETERS Consider an interferometer with MPI , comprising a signal in one arm, and a parasitic mode with power with power in the other arm. The oscillatory component of the output intensity is given by (1) where is the relative phase between the two arms. This relative phase is related to several time varying parameters (2) is the effective index difference between the two where modes in which the signal propagates, is the length of fiber, is wavelength, and is the source laser phase noise. Equation (2) indicates that the time scales of phase fluctuations depends on laser phase noise as well as the path-length . A large path-length difference will result difference in large phase (and therefore, intensity) fluctuations even for , , or . The path-length difference small changes in (or equivalently, the relative delay) also determines whether the interference is coherent, or incoherent and depolarized. Typically, index and length would change due to environmental fluctuations such as temperature, and thus, will have long time scales (seconds to minutes), while phase noise of a laser source would fluctuate on a time scale roughly equal to the reciprocal of its bandwidth ( MHz). The HOM-DCM used for evaluations here comprised 2 km of few-mode fiber and a pair of long-period fiber-gratings [5] and that provide the mode conversion between the modes [3]. The relative delay , for 1547-nm light between the mode and the parasitic modes, is roughly 13 ns, for an interferometer comprising 2 km of HOM fiber, while the bandof a DFB laser typically used in communications syswidth , corresponds tems is 2.5 MHz. Thus, the product to an interferometer in which (1) the path-length difference is small, and (2) where the interference is mostly coherent. In addition, a significant component of the MPI in HOM-DCMs arises
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 8, AUGUST 2003
Fig. 1. MPI measurement with ESA. (a) RF power versus frequency. Spectrum shifts in–out of quadrature. Averaging introduces = 2-dB uncertainty in MPI. (b) RIN versus frequency. Measured MPI 48 dB. Measures>70-MHz noise. Slow drift filtered out.
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from mode coupling within the fiber, yielding a plurality of interferometers with even smaller relative delays. The measured MPI of the current HOM-DCM under test, is 39 dB. We now describe four MPI measurements, to understand the most appropriate technique for characterising MPI in copropagating few-mode fiber devices. II. RIN MEASUREMENT USING ELECTRICAL SPECTRUM This is the most extensively employed technique to measure MPI due to Rayleigh scattering in Raman amplifiers [1]. The relative intensity noise (RIN) due to MPI is the Fourier transform of the autocorrelation of (1). For coherent interference when the fields are in quadrature, the RIN is approximately [6] (3) where is the radio frequency (RF) and all other quantities are as defined earlier. Integrating the RIN, measured with an electrical spectrum analyzer (ESA), therefore, yields , the MPI. Fig. 1(a) is a plot of the RF power spectrum, measured with MHz was used, a commercial ESA. A DFB laser and the input power into the ESA was limited to 1 dBm. The spectrum clearly shows the characteristic nulls observed in interferometers in which the two arms are coherent. In addition, changing the input polarization state changes the visibility of the interference spectrum, which indicates that the two arms of the interferometer are not depolarized. Also, note that the interferometer drifts in and out of quadrature on a time scale of several seconds to minutes, changing the RF power (and therefore, measured MPI) by 4 dB. Thus, it is expected that this measurement technique will introduce errors in measured MPI of 2 dB. approximately Fig. 1(b) is a plot of the electrical RIN spectrum (which is twice in decibel scale of the optical RIN) obtained from an averaged RF spectrum of Fig. 1(a). Also shown is the RIN spectrum for the DFB laser alone. The MPI was obtained by integrating the RIN spectrum from 70 MHz to 1 GHz, as this range captured most of the distinguishing features of the HOM. Subtracting this value from the RIN for the DFB laser alone yields , which is related to MPI , by (3). The MPI for the HOM-DCM was deduced to be 48 dB, 9 dB lower than the reference value mentioned earlier.
Fig. 2. Swept wavelength-scanning technique for measuring MPI. (a) Typical scan at 20 nm/s predicts lower (better) MPI than reference. (b) Measured MPI for different scan speeds. Result dependent on scan speed.
The discrepancy is not accounted for by the uncertainty in 2 dB. the measurement technique alone, since that is only This is resolved by considering the time scales of fluctuations in coherent distributed interferometers. As noted earlier, such interferometers can exhibit slow (second to minute timescale) as well as fast (microsecond to picosecond timescale) fluctuations. However, the ESA cannot measure RIN below 100 kHz. Thus, this technique samples only high frequency noise, and may not be suitable to measure noise from devices that exhibit a large frequency spread in their noise spectrum. III. SWEPT WAVELENGTH SCANNING From (1) and (2), the amount of MPI may be indirectly inferred by measuring transmitted intensity as a function of wavelength [7]. Changing the wavelength changes the phase, which enables sampling the [ 1,1] values of the function . Since the signal and replica possess the same degree of polarisability (they are coherent replicas), the MPI is given by (4) is the average power. where is the standard deviation and As noted earlier, the relative delay in the HOM-DCM is 13 ns, which corresponds to an interferometer with a free spectral range of 80 MHz, or 0.64 pm. Since the best wavelength-scanning test sets employ tunable lasers with 3-pm resolution, this technique cannot resolve the cosine transfer function of the interferometer. However, a large number of samples taken over a 1-nm window would enable statistically sampling enough points to yield a good measure of the standard deviation , of (4). A typical scan is shown in Fig. 2(a). The scan speed was set to 20 nm/s. This yielded an MPI of 44 dB, which is 5 dB lower than the reference value. In addition, the measured MPI strongly depends on the scan speed used [Fig. 2(b)]. The finite response time of the detector effectively integrates the response over a range of wavelengths, thus, further reducing the measured MPI as the scan speed is increased. Nevertheless, the MPI deduced at any scan speed is lower than the reference value of 39 dB. The discrepancy may arise from the fact that the HOM fiber represents a multitude of complex interferometers whose coupling ratios change slowly with time. Hence, the wavelength scans at any speed are fast enough to act as a high-frequency-pass filter, thus, filtering out the slow fluctuations.
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mitters of choice, using a DFB laser for the MPI measurements yields the most relevant MPI values. One drawback of the direct temporal measurement technique is that it requires dwell times of up to 10 min, in order to sample the entire interferometer. This process can be dramatically shortened by a slight modification. Dithering the polarization state into the HOM yields the desired temporal fluctuation (and thus, MPI) within a time scale of several seconds instead of minutes [Fig. 3(c)]. Since the HOM interferometer is essentially a concatenation of several weak crosstalk paths with polarization-dependent coupling efficiencies, modifying the input polarization state eventually yields a state that efficiently couples all the interferometers. Fig. 3. Temporal fluctuation of transmitted intensity. (a) Power meter (20-ms 39 dB. (b) Measurement with fast response) catches slow fading; MPI photodetector (1-s response) yields MPI 49 dB; slow variations filtered out. (c) Dithering polarization state samples all interferometer states faster; yields same MPI as slow measurement shown in (a).
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IV. TRANSMISSION FLUCTUATION MEASUREMENTS Another means to deduce MPI is to measure the intensity fluctuations over time directly with a photodetector. Intensity MHz transmitted fluctuations for DFB laser light through the HOM-DCM were recorded with photodetectors with a variety of response-times. Fig. 3 shows the variation in transmitted intensity for a 20-ms and 1- s response-time detector, respectively. Assuming that the long recording time ensures sampling all states of the interferometer, the MPI is (5) is the peak-to-peak signal fluctuation in decibels. where The detector with the slowest response over the longest scan time yielded the highest MPI, 39 dB [Fig. 3(a)]. With a detector with 1- s response time, the measured MPI was 49 dB [Fig. 3(b)], which is similar to the value obtained from the RIN measurements. This is consistent with the fact that both the RIN measurements as well as the direct intensity measurements with a fast detector, sample only fast variations. Since the worst MPI values are obtained only with long time scale measurements, we conclude that the impact of distributed coherent MPI in HOM fibers is predominantly a slow variation of transmitted power over time. The source bandwidth is a critical parameter to ascertain while measuring MPI by this technique. We found that when the source bandwidth is increased from 2.5 to 100 MHz, the MPI drops by more than 15 dB. Since DFB lasers are the trans-
V. SUMMARY AND CONCLUSION MPI in HOM-DCMs is a combination of discrete as well as distributed coherent crosstalk. This manifests in high frequency deterministic noise, as well as fading. For a 39-dB MPI module, we find that less than 48 dB of the MPI arises from deterministic, high frequency noise, while the rest can be attributed to slow time-scale fading effects. Thus, traditional measurement techniques to deduce system RIN do not adequately characterize the MPI in few-mode fiber devices. The polarization-dithered slow intensity fluctuation technique that we propose and demonstrate, appears to yield the best measure of MPI in these devices. REFERENCES [1] C. R. S. Fludger and R. J. Mears, “Electrical measurements of multipath interference in distributed Raman amplifiers,” J. Lightwave Technol., vol. 19, pp. 536–545, Apr. 2001. [2] A. H. Gnauck, L. D. Garrett, Y. Danziger, U. Levy, and M. Tur, “Dispersion and dispersion-slope compensation of NZDSF over the entire C band using higher order mode fiber,” Electron. Lett., vol. 36, pp. 1946–1947, 2000. [3] S. Ramachandran, B. Mikkelsen, L. C. Cowsar, M. F. Yan, G. Raybon, L. Boivin, M. Fishteyn, W. A. Reed, P. Wisk, D. Brownlow, R. G. Huff, and L. Gruner-Nielsen, “All-fiber, grating-based, higher-order-mode dispersion compensator for broad-band compensation and 1000-km transmission at 40 Gb/s,” IEEE Photon. Technol. Lett., vol. 13, pp. 632–634, June 2001. [4] C. X. Yu, W.-K. Wang, and S. D. Brorson, “System degradation due to multipath coherent crosstalk in WDM network nodes,” J. Lightwave Technol., vol. 16, pp. 1380–1386, Aug. 1998. [5] S. Ramachandran, Z. Wang, and M. F. Yan, “Bandwidth control of longperiod grating-based mode converters in few-mode fibers,” Opt. Lett., vol. 27, p. 698, 2002. [6] J. L. Gimlett and N. K. Cheung, “Effects of phase-to-intensity noise conversion by multiple reflections on gigabit-per-second DFB laser transmission systems,” J. Lightwave Technol., vol. 7, pp. 888–895, June 1989. [7] M. G. Taylor, D. Craig, H. P. Sardesai, A. Khot, and W. Zheng, “Measurement of system penalty due to multipath interference,” in Proc. ECOC 2002, Paper 3.1.7.
