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Adjustable bandwidth dispersionless bandpass FBG optical filter

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Abstract A bandpass optical filter, based on fiber Bragg gratings, is presented in which the bandwidth of a Gaussian spectrum can be continuously adjusted, whilst maintaining near zero group delay slope over the filter bandwidth. The device is also wavelength tunable and the spectral profile is selectable by appropriate grating design. This novel device is employed in a 2R-regenerator, enabling data rate reconfiguration and wavelength conversion, with negligible phase distortion. It will find application wherever a dispersionless reconfigurable bandpass optical filter is required.

©2005 Optical Society of America

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Figures (10)

Fig. 1.
Fig. 1. Setup of the dispersionless, adjustable bandwidth, bandpass optical, filter (ABF). Both FBGs are used in reflection via a four port circulator and probed, such that the spectral profiles are multiplied together but their equal and opposite chirps cancel. The chirp of each FBG is aligned in the same sense to the temperature gradient, such that increasing the gradient leads to a broadening of the spectral bandwidth. A translation stage tensions the FBGs to provide wavelength tuning.
Fig. 2.
Fig. 2. Set-up showing the equipment used to create the linear, adjustable temperature gradient. Also shown are the fiber clamps and the stage used for tensioning the fibers. During operation, there is mineral wool insulation around the connecting rod and on top of the heat source.
Fig. 3.
Fig. 3. Typical spectra of FBG 1 a) (in this case a Gaussian) and FBG 2 b). A Gaussian fit is applied to the Gaussian grating and a Super-Gaussian of order 8 is applied to the flat top.
Fig 4.
Fig 4. The filter spectrum of the ABF, for each of four temperature differences between the hot source and cold sink. A Gaussian fit for each gradient shows that the spectral shape is maintained as the bandwidth increases.
Fig. 5.
Fig. 5. ABF Bandwidth as a function of temperature difference. The bandwidth increases linearly with temperature difference (temperature gradient).
Fig. 6.
Fig. 6. Spectra and group delay for no temperature gradient (left) and maximum gradient (right). The top plots show the transmission of the ABF, fitted with Gaussians. The middle level plots show the measured group delay curves for each individual grating in reflection. The lower level plots shows the combined measured group delay of the ABF, for each of the two temperature difference extremes.
Fig. 7.
Fig. 7. An ultra-short pulse is sent into the bandpass filter resulting in the following output pulses for the case of no gradient (left) and maximum gradient (right). The top level, plots a) and d), shows the hypothetical case of ideal Gaussian spectrum and dispersion exactly zero. In the middle level, plots b) and e), the measured GDR is added to the simulation giving rise to temporal sidelobes. In the lowest level, plots c) and f), the measured transmission spectrum is added, changing the evolution only slightly.
Fig. 8.
Fig. 8. The effect of quadratic dispersion on the output pulses of the ABF, with FWHM=860 pm is shown. In the plot a), the dispersion is 0 ps/nm. In plot b), the dispersion is 5 ps/nm and in plot c), the dispersion has been increased to 10 ps/nm.
Fig. 9.
Fig. 9. The effect of cubic dispersion on the pulses with a FWHM=860 pm is shown. Plot b) shows the case of D3=-15.2 ps/nm2, which approximates the cubic component extracted from the measurement. Note the appearance of sidelobes as also seen in the simulation using the measured GDR (Fig 7e). In the other plots the amount of quadratic dispersion is increased and decreased by factors of two as labeled.
Fig. 10.
Fig. 10. The transfer functions of an optical regenerator based on self-phase modulation and the ABF are shown. The dots show the experimental results and the solid curves the theory. The insets show the autocorrelation traces for the ABF output pulses. In plot a), the bandwidth is adjusted to match the input pulse duration, Δτ=8.3 ps, yielding a typical regenerator transfer function. In plot b), the gradient is increased to broaden the bandwidth to match the smaller pulse duration format, Δτ=4.2 ps.


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