Systems and techniques for reducing group delay ripple in a dispersion compensating grating

A method for reducing group delay ripple in a fiber grating, comprising:

• (a) mounting the grating into a trimming setup that includes a trimming device and a scanning assembly for causing a region of the grating to be scanned along its length by the trimming device output, the grating having a chemical composition that is sensitive to the trimming device output, such that exposure to the trimming device output causes a change in the effective refractive index of the scanned region, the amount of change varying as a function of scanning velocity;
• (b) selecting a region of the grating to be trimmed;
• and
• (c) programming a scanning velocity profile into the scanning assembly that is calculated to modify the effective refractive index of the selected grating region along its length so as to smooth out group delay ripple in the selected grating region,
• wherein step (c) includes
• (c1) with respect to a selected region of the grating, obtaining data with respect to the relationship between wavelength and group delay;
• (c2) performing a linear fit of the data obtained in (c1), and subtracting the data from the fit to obtain the relationship between wavelength deviation and group delay;
• (c3) converting the relationship obtained in (c2) into a relationship between effective refractive index deviation and group delay;
• (c4) converting the relationship obtained in (c3) into a relationship between effective refractive index deviation and distance along the length of the selected grating region;
• and
• (c5) using the relationship obtained in (c4) and a calibration curve to calculate the scanning velocity profile.

A trimming setup for reducing group delay ripple in a fiber grating, comprising:

• a mounting assembly for receiving a grating to be trimmed;
• a trimming device;
• and
• a scanning assembly for causing a region of the grating to be scanned along its length by the trimming device output, the grating having a chemical composition that is sensitive to the trimming device output, such that exposure to the trimming device output causes a change in the effective refractive index of the scanned region, the amount of change varying as a function of scanning velocity;
• and
• programmable controller means for causing the trimming device to be scanned along the length of the grating region according to a scanning velocity profile constructed by
• in the selected grating region, obtaining group delay data for a range of wavelengths, wherein group delay ripple corresponds to deviations of the group delay data with respect to a linear fit of the relationship between the group delay data and wavelength, and wherein the range of wavelengths corresponds to a range of respective distances along the length of the selected grating region;
• converting deviations in group delay data at wavelengths within the range of wavelengths into corresponding deviations in effective refractive index at respective distances along the length of the selected grating region;
• and
• constructing the scanning velocity profile using the deviations in effective refractive index in conjunction with a calibration curve indicating the relationship between scanning velocity and changes in effective refractive index caused by the trimming device,
• such that the scanning of the trimming device along the selected grating region modifies the effective refractive index of the selected grating region along its length so as to smooth out group delay ripple in the selected grating region.