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Reduction of group delay ripple of multi-channel chirped fiber gratings using adiabatic UV correction

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Abstract

We demonstrate reduction of group delay ripple (GDR) from 24 ps to 9 ps peak to peak in a four channel 43 Gb/s dispersion compensating chirped fiber grating by adiabatic UV post processing. The eye opening penalty due to the grating GDR was improved from ~2dB to <1dB for all of the channels over a range of carrier frequencies of 15GHz. Our results demonstrate that at 43 Gb/s, the adiabatic UV correction technique is sufficient to substantially improve multi-channel fiber grating performance. We also discuss three limitations of the correction technique which cause GDR to vary from channel to channel: Noise in the sampling function, cladding mode loss, and varying channel reflectivity. While these limitations are visible in our results they do not reduce the effectiveness of the adiabatic correction for our gratings.

©2004 Optical Society of America

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Figures (8)

Fig. 1.
Fig. 1. Experimental setup used to fabricate the multi-channel gratings and implement the iterative correction method.
Fig. 2.
Fig. 2. Calibration of the adiabatic rescaling parameters to create an accurate correction profile. If the observed differential matches the applied correction profile, then the rescaling parameters A, k, and λ are correct.
Fig. 3.
Fig. 3. The total applied correction profile matches the differential between the GDRs of the initial and final iterations.
Fig. 4.
Fig. 4. Plots of reflection spectrum (left row) and smoothed GDR (right row) of the four channels of the superstructure grating. In each channel plot, black: before correction, green: after correction, red: after anneal. Note that an arbitrary offset was added in x and y to the three reflection and GDR plots (black, green, and red) from each channel to simplify the comparison. The reference level for the reflection spectra in black is 0dB, -1 dB for the green plot, and -2 dB for the red plot.
Fig 5.
Fig 5. 43 Gb/s NRZ eye diagrams at the output of the electrical Bessel filter of the simulation setup without the grating (left plot) and with the grating (right plot). Rectangles of fixed widths (20% of the bit period) are fitted within each eye diagram by adjusting their heights. The reduction in height gives a measure of the EOP.
Fig. 6.
Fig. 6. EOP simulation plots for the four strongest channels before and after correction of the superstructure grating GDR. Zero detuning corresponds to approximate center of the reflection band of Fig. 4. Solid: amplitude, dashed: phase, bold: both. EOP near zero frequency offset improves by 0.5 to 1 dB in all channels.
Fig. 7.
Fig. 7. Amplitude ripple affects the overall EOP of the grating for weak channels after correction. Notice that for the weak channel the EOP due to amplitude only effects already exhibits a small change after the correction has been applied (dark gray plot encircled in red).
Fig. 8.
Fig. 8. Cladding mode loss introduces asymmetry to the multi-channel grating making the channel responses vary. The grating was index matched after the correction process to see the effect of changing the cladding mode loss. The EOPs of two of the channels show effect of cladding mode loss on GDR.

Equations (4)

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Δ n ac ( z ) = Δ n 0 [ sin ( k 1 z + C z 2 + φ 1 ( z ) ) + sin ( k 2 z + C z 2 + φ 2 ( z ) ) ]
= Δ n 0 sin ( k 1 + k 2 2 z + C z 2 + φ 1 ( z ) + φ 2 ( z ) 2 ) cos ( k 1 k 2 2 z + φ 1 ( z ) φ 2 ( z ) 2 )
n ( z ) = A exp { α exp ( β ln ( ( z z 0 ) 2 d 2 ) ) }
R = r exp [ α + α noise ( ω ) + i φ nosie ( ω ) ]
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