Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
1999-05-05
2001-12-04
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular beam control device
Tuning
C372S098000, C372S103000
Reexamination Certificate
active
06327280
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to custom optical filters for providing selected wavelength characteristics for use in WDM systems, particularly filters relating to: flat-top WDM mux/demux, flat-top and peaky comb filters or reconfigurable add & drop filters.
BACKGROUND OF THE INVENTION
In the prior art cascaded diffractive elements have been used to increase diffractive efficiency. In addition processing of the beam between diffractive elements is also used. However, processing has been done in the far field. Processing in the far field, where the beam is collimated, and is a Fourier transform of the original beam, makes optical processing of individual or selected wavelengths or channels very complex and difficult, requiring computer processing holographically. Generally the prior art far field processing is used for global processing, to all wavelengths, since local processing of individual wavelength bands is difficult.
An application of optical filtering in the far field of a dispersed beam is disclosed in U.S. Pat. No. 5,805,759 issued to Fujitsu Limited, Sep. 9, 1998. This application discloses an optical equalizer comprising an attenuating filter between two dispersion gratings. Light is processed as a collimated beam. The attenuator introduces variable attenuation primarily to compensate for gain tilt from amplifiers. One hundred percent attenuation can be selectively arranged using metalized blocking strips. Because filtering is performed on the collimated beam, obtaining sufficient wavelength separation is more difficult. It requires high dispersion and long distances between the gratings. The size requirements makes the proposed device impractical for use. Further, working at high dispersion causes greater sensitivity to polarization. Filtering as taught by the Fujitsu application only involves attenuation at varying levels within a dispersed beam. Displacement of the dispersed wavelength bands is not contemplated. The Fujitsu patent does not teach more complex processing achieved by displacing wavelength bands in the focal plane, which can be achieved in a compact device by processing focused wavelength bands in the focal plane.
The present invention has found that optical processing, particularly of selected wavelengths, is more easily accomplished in the focal plane where the wavelengths are more easily separated within a compact space and can be physically processed or moved without complex calculation. In the focal plane position can be correlated to wavelength. This can be used to provide filtering for complex systems. It is often desired to redirect selected wavelengths for add/drop application, or to shift or invert wavelength bands for flat top or other specialized response profiles not possible through simple attenuation.
Simple focal plane optical processing is disclosed in U.S. Pat. No. 5,223,405 issued in 1993 to Hewlett-Packard Company. This reference discloses the use of a slit to permit a selected portion or wavelength band of a dispersed light beam from a diffraction grating to pass. The selected portion of the light beam is then redirected to the diffraction grating to be recombined in order to pass to an output for spectral analysis. The isolation of a selected wavelength band is important for wavelength monitoring and spectral analysis. However, the method and apparatus taught by the Hewlett-Packard reference does not suggest the controlled filtering of multiple discrete wavelengths and varying intensities or multiple inputs which can be achieved by processing the dispersed light in the focal plane.
SUMMARY OF THE INVENTION
The present invention has found that more complex processing in the focal plane can be achieved for custom filtering and multiple simultaneous filtering can be achieved. This processing may include local channel inversion, global inversion of the signal, location shifts of selected wavelengths, and two dimensional multiple simultaneous filtering.
Accordingly, the present invention provides an optical filter comprising:
at least one input light beam comprising multiple wavelengths;
a first dispersive element for dispersing a spectrum of the at least one input light beam;
a processing element in the focal plane of the spectrum of the at least one input light beam for selectively processing a plurality of wavelength bands wherein the intensity and wavelength order vary in a predetermined manner;
dispersive means selected from the first dispersive element and a second dispersive element for providing a second dispersion of the processed wavelength bands for condensing the processed wavelength bands or for redispersing the processed wavelength bands; and,
at least one output for receiving output light from the dispersive means.
A further embodiment in accordance with the present invention provides a tunable laser comprising:
a tunable filter cavity including
an input light beam,
a lens for focusing light from the input source;
a dispersion grating for dispersing an input light beam spectrum; and
a processing element in the focal plane for transmitting a selected wavelength band of the spectrum including a corner cube retro-reflector having an axis parallel to lines of the dispersion grating for redirecting light to the dispersion grating, through the lens to the input light source;
an amplifying medium
a mirror;
a filter input coupled to the laser and aligned with the input light source with reference to the lines of the dispersion grating, for filtering the wavelength of the tunable laser; and
means for separating a filtered output from the filter input light.
Accordingly a method of the present invention comprises a method of providing optical filtering comprising the steps of:
dispersing an input beam of light comprising a plurality of wavelengths;
selecting a plurality of specific wavelength bands at the focal plane for processing to provide predetermined intensities and wavelength order for subsequent dispersion;
dispersing the plurality of selected wavelength bands to recondense or redisperse the selected wavelength bands.
Advantageously, precise and complex filtering can be achieved for numerous applications using a relatively simple filter device. A further advantage is that by processing focused wavelength bands having a separation of tens of microns, a compact, practical device of only a few inches can be constructed.
Additional advantages will be apparent to persons of skill in the art from the detailed description of preferred embodiments together with the following drawings which illustrate preferred embodiments by example only:
REFERENCES:
patent: 5233405 (1993-08-01), Wiuldnauer et al.
patent: 5594744 (1997-01-01), Lefevre et al.
patent: 5808759 (1998-09-01), Fukushima
Delisle Vincent
Ducellier Thomas
JDS Fitel Inc.
Jr. Leon Scott
Teitelbaum Neil
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