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In-band OSNR and chromatic dispersion monitoring using a fibre optical parametric amplifier

T. T. Ng, J. L. Blows, M. Rochette, J. A. Bolger, I. Littler, and B. J. Eggleton

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Abstract

This paper presents an all-optical, in-band optical signal-to-noise ratio (OSNR) and chromatic dispersion monitor. We demonstrate monitoring over the 1 nm bandwidth of our signal, which is a 10 GHz pulse train of 8.8 ps pulses. The monitor output power, as measured on a slow detector, has a 1.9 dB dynamic range when the signal OSNR is varied by 20 dB, and a 1.6 dB dynamic range when ±150 ps/nm of chromatic dispersion is applied. Cascaded four-wave mixing occurring in the optical parametric amplifier provides the nonlinear power transfer function responsible for the monitoring. An analysis using the signals’ probability density functions show that the nonlinear power transfer function provides preferential gain to clean undispersed pulses when compared to noisy and/or dispersed pulses. Our analysis includes a consideration of the applicability of the device to high duty cycle systems, and simulations on monitoring of a 40 Gb/s pulse train with a 50% duty cycle.

©2005 Optical Society of America

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Supplementary Material (1)

Media 1: AVI (1785 KB)     

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Figures (8)
Fig. 1.
Fig. 1. (top) Pulses reach higher gains than noise of the same average power. The pulses receive more average gain and a larger average output. (middle) Schematic illustration of gain as a function of instantaneous input power for a nonlinear power transfer function. High instantaneous input powers receive high gains. (bottom) Output signals have different average powers because of the different gain received.

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Fig. 2.
Fig. 2. Frame 1 of the animation. (left) The plot of average output power against OSNR. (right) The pdf of the input (dashed line), a quadratic PTF (dotted line) and the product of the pdf with the PTF (solid line). The shaded area indicates the average output power. [Media 1]

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Fig. 3.
Fig. 3. (left) The pdf of the undispersed pulse train (dashed line), a quadratic power transfer function (dotted line) and the product of the pdf with the power transfer function (solid line). The shaded area indicates the average output power. (right) The pulse train is dispersed resulting in pulse broadening and compression of its pdf to lower powers.

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Fig. 4.
Fig. 4. Output of the optical parametric amplifier. Only the signal and pump waves are launched, but many waves are generated.

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Fig. 5.
Fig. 5. The experimental setup. TDE: Tunable dispersive element, BPF: Bandpass filter, VA: Variable attenuator, C: Coupler, PS: Polarisation scrambler, DSF: Dispersion shifted fibre, ATT: Attenuator.

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Fig. 6.
Fig. 6. (a) The experimental nonlinear power transfer function. (b) The gain spectrum of the signal.

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Fig. 7.
Fig. 7. (a) Dispersion monitoring curves for a clean pulse train and two pulse trains of OSNR 15.8 dB and 9.4 dB. The points are experimental data and the thick lines are from numerical simulation with uncertainty indicated by the thin lines. (b) The autocorrelation measured pulse widths at ‘A’. (c) OSNR monitoring curves for an undispersed pulse train and one with -78 ps/nm dispersion. The boxed area is the OSNR region of interest for high speed systems and is shown magnified in (d).

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Fig. 8.
Fig. 8. (a) The pdf of a 40 Gb/s signal is compared to the pdf of ASE. The quadratic PTF amplifies the upper part of the ASE giving it more gain than the signal (b) OSNR monitoring curve simulated for a 50% duty cycle signal using a quadratic PTF.

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Equations (2)

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P ( p a , p b ) = ∫ p a p b pdf in ( p ) d p
〈 p out 〉 = ∫ 0 ∞ PTF ( p ) . pdf in ( p ) d p .

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