Grating-type optical filter with apodised spectral response

Details Grating-type optical filter with apodised spectral response Grating-type optical filter with apodised spectral response

2001-02-26

2003-04-15

Spyrou, Cassandra (Department: 2872)

Optical waveguides

With optical coupler

Input/output coupler

C385S031000

Reexamination Certificate

active

06549707

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns the field of guided wave optical devices including a grating, that is to say including a part through which an optical wave travels and in which an optical parameter varies in an alternating manner along the path of the light.
2. Description of the Related Art
The invention concerns codirectional couplers and couplers, guided wave Bragg reflectors, fibre type mode converters or Bragg reflectors for VCSEL cavities, for example.
The above devices filter optical waves. They eliminate waves with wavelengths outside a chosen range.
Generally speaking, an optical filter is of good quality when it selects wavelengths to be transmitted and wavelengths to be rejected precisely. In other words, the filter is of good quality if its spectral response takes substantially constant values close to 1 in the range of wavelengths to be transmitted and substantially constant values close to 0 outside of that range.
The spectral responses of current grating filters feature secondary lobes, however.
Various methods have been proposed for producing optical filters in which such secondary lobes are small in amplitude, in other words filters with a high rejection rate. The skilled person knows such methods as apodisation methods.
When the grating device is the site of interference formally described using the principle of coupled modes, apodisation methods have been proposed that consist in modulating an amplitude of variation of the coupling coefficient along the path of the light.
A first such method of varying the amplitude of the coupling coefficient which is routinely used in designing codirectional and contradirectional couplers is implemented by varying an interguide distance. This method is described in B. E. Little, C. Wu, W. P. Huang, “Synthesis of ideal window filter response in grating-assisted couplers”, Optics Lett., vol. 21, pp. 725-727, 1996; B. E. Little, C. Wu, W. P. Huang, “Synthesis of codirectional couplers with ultralow side lobes and minimum bandwidth”, Optics Lett., vol. 22, pp. 1259-1261, 1995; and G. H. Song, “Toward the ideal codirectional Bragg filter with an acousto-optic-filter design”, J. Lightwave Technol., vol. 13, pp. 470-480, 1995. It produces in the order of 30 dB to 40 dB apodisation of the secondary lobes.
The accompanying
FIG. 1
is a top view of an interguide distance variation codirectional coupler of the above kind.
In this device the distance between two guides is caused to oscillate by transverse undulation of one of the two guides which yields an oscillatory distribution of the coupling coefficient along the undulating guide, in accordance with the coupled mode principle. The period of the oscillation determines the initial wavelength or the phase lock length selected by the device.
The undulating guide also has a generally arcuate shape so that the interguide distance is minimum at the centre of the device and maximum at its ends. The average coupling coefficient calculated at each undulation therefore has a maximum in the central part and decreases progressively towards the ends of the guides.
This bell-shaped variation of the average coupling coefficient calculated at each undulation reduces the secondary lobes of the spectral response of the filter.
However, the above method has a major drawback associated with the fact that the average interguide distance must undulate with an amplitude in the range of 2 &mgr;m to 5 &mgr;m, the amplitude of sinusoidal variation being in the order of 1 &mgr;m. Implementing it therefore requires great precision in the variations of the interguide distance, which is difficult to achieve in practice.
Moreover, the coupling coefficient is related to the interguide distance by a non-linear function and this tends to accentuate the negative effects of errors and uncertainties on the interguide distance.
The method is also difficult to implement for vertical couplers and cannot be applied to components in which there is only one guide, for example Bragg reflectors like that shown in the accompanying
FIG. 3
or light mode converters.
A second method, known as the modulation amplitude variation method, consists in varying the coupling coefficient by varying a corrugation profile or varying the refractive index in a fibre.
When the corrugation profile is varied, it is in practice difficult to control corrugation amplitudes so that the modulation of the coupling coefficient is sufficiently precise. The corrugation amplitudes required are generally below 1 &mgr;m.
When the index in the fibre is varied, very complex and very exacting ultraviolet exposure techniques are employed. These methods are described in J. Albert, K. O. Hill, D. C. Jonson, F. Bilodeau, M. J. Rooks, “Moire phase masks for automatic pure apodisation of fibre Bragg gratings”, Electronic Lett., vol. 32 pp. 2260-2261, 1996; P. Kashyap, A. Swanton, D. J. Armes, “Simple technique for apodising chirped and unchirped fibre gratings”, Electronic Lett., vol. 32, pp. 1226-1227, 1996; J. Albert, K. O. Hill, B. Malo, S. Thériault, F. Bilodeau, D. C. Jonson, L. E. Erickson, “Apodisation of the spectral response of fibre Bragg gratings using a phase mask with variable diffraction efficiency”, Electronics Lett., vol. 31, pp. 222-223, 1995; and B. Malo, S. Thériault, D. C. Jonson, F. Bilodeau, J. Albert, K. O. Hill, “Apodised in fibre Bragg grating reflectors photoimprinted using a phase mask”, Electronic Lett., vol. 31, pp. 223-225, 1995 and are specifically addressed to gratings inscribed in the fibres.
A third method, known as the cyclic ration variation method, is described in H. Sakata, “Side lobe suppression in grating-assisted wavelength-selective couplers”, Optics Lett., vol. 17, pp. 463-465, 1992. The cyclic ratio is defined over a period of the grating as the ratio between the length of the part of the period in which the coupling coefficient is positive and the length of the part of the period in which the coupling coefficient is negative. This method varies this ratio along the path of the light.
FIG. 7
shows a grating optical filter of a type known per se in which the cyclic ratio is modified along the length of the filter.
The filter comprises a central guide flanked by portions adapted to modify the value of the coupling coefficient in the parts of the central guide level with these portions.
To be more precise, the portions of the central guide that are flanked on the right have a negative coupling coefficient and the portions of the central guide that are flanked on the left have a positive coupling coefficient.
The central guide can be considered as a succession of sections each made up of a guide part in which the coupling coefficient is negative followed by a guide part in which the coupling coefficient is positive.
In
FIGS. 7
to
9
dashed lines have been drawn between the successive sections constituting the filter and the successive sections have been numbered from 1 to 8.
These sections are all the same length. In other words, the lateral portions are disposed so that each pair consisting of a righthand portion and a lefthand portion has a constant length along the guide.
FIG. 9
shows the distribution of the coupling coefficient along the filter in the direction of increasing section numbers from section
4
to section
8
.
In the graph shown in
FIG. 9
the abscissa axis therefore plots a distance z measured along the filter in the direction of increasing section numbers and the ordinate axis plots the value of the coupling coefficient k at the point of the guide concerned.
A section of the filter therefore consists of a succession of two sub-sections, one in which the coupling coefficient is positive and the other in which it is negative, the absolute amplitudes being substantially equal.
The sections therefore form lobes, each having the same amplitude, which is constant along the filter.
According to the cyclic ratio variation principle that is known per se, the ratio between the length of the negative lobe and the length of the positive lobe within each sect

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