Cascaded grating resonator filters with external input-output couplers
First Claim
1. A surface wave device for providing a narrow bandpass characteristic about a center frequency f0 to an input signal, said device comprising a filter having a series of cascaded grating resonators for providing the transmission characteristic proximate to said center frequency f0, and at least one track-changing element for coupling said input signal to said filter, characterized in that said track-changing element includes a four-port directional coupler having a first port positioned to receive said input signal and a fourth port for coupling energy into said filter, at least one periodic chirped and tapered grating having grating elements positioned so as to selectively reflect signal energy from second and third ports of said directional coupler.
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Abstract
Acoustical and optical surface wave filters are described wherein the filter characteristic near resonance is provided by cascaded grating resonators. The out-of-band suppression for these resonators is provided by a track changing element that includes a four-port directional coupler having one port connected to receive input energy and a fourth port connected to couple energy into the cascaded grating resonators. Tapered and chirped reflection gratings are positioned so as to selectively reflect energy present at the second and third ports of the directional coupler. In a surface acoustic wave embodiment of the invention, the directional coupler is provided by a multistrip coupler. In an optical embodiment, the directional coupler function is provided by waveguiding structures that are brought sufficiently close to induce 3 dB coupling.
47 Citations
11 Claims
- 1. A surface wave device for providing a narrow bandpass characteristic about a center frequency f0 to an input signal, said device comprising a filter having a series of cascaded grating resonators for providing the transmission characteristic proximate to said center frequency f0, and at least one track-changing element for coupling said input signal to said filter, characterized in that said track-changing element includes a four-port directional coupler having a first port positioned to receive said input signal and a fourth port for coupling energy into said filter, at least one periodic chirped and tapered grating having grating elements positioned so as to selectively reflect signal energy from second and third ports of said directional coupler.
- 7. A surface acoustic wave device having elements positioned on the surface of a substrate to process acoustic wave energy propagating in said substrate comprising at least one multistrip coupling element having four ports, a first one of which is positioned to receive an input acoustic signal and present output signals with approximately equal power at second and third ports, first and second tapered and chirped gratings positioned on said substrate so as to selectively reflect the acoustic signals presented by said second and third ports of said multistrip coupler, and a cascaded grating resonator filter positioned on said substrate so as to receive at its input an acoustic signal present at the fourth port of said multistrip coupler.
- 10. An optical surface wave device having elements deposited and formed on the surface of a substrate processing an input optical signal comprising at least two thin film waveguides deposited in the surface of said substrate and shaped so as to be proximate to each other in a predetermined region of said substrate thereby establishing 3 dB coupling between said two waveguides, a cascaded grating resonator filter positioned on the surface of said substrate over one of said two waveguides on one side of said 3 dB coupler region, and a tapered and chirped grating having its elements formed on the surface of said substrate over said two waveguides on the opposite side of said 3 dB coupler region.
Specification