Planar lightwave circuit based tunable 3 port filter
View Patent ↗The invention relates to a tunable optical add/drop module (TOADM) monolithically integrated on a single planar lightwave circuit (PLC). The present invention overcomes the shortcomings of the prior art by providing virtual pupils at the interface between the channel waveguides and the slab waveguide on the PLC for focusing each wavelength channel, and additional on-chip lenses on the PLC for transforming the focal plane of the spatially dispersive demultiplexer into a substantially flat plane at the edge of the PLC. On-chip lenses are realized as reflective surfaces within slab waveguiding regions having a surface curvature to provide optical power.
1. A tunable add/drop filter in a planar lightwave circuit (PLC) comprising:
an input port for launching an input optical signal comprising a plurality of wavelength channels into the PLC;
a demultiplexer with an input pupil and an output pupil for spatially separating the input optical signal into the wavelength channels;
a slab waveguide region comprising a high-index core layer, surrounded above and below by lower-index cladding layers for transmitting the separated wavelength channels from the demultiplexer to an add/drop port;
a lens system in the slab waveguide region for focusing the plurality of separated wavelength channels from the output pupil of the demultiplexer onto straight line at an edge of the PLC,
a separate reflector adapted for transverse movement along the edge of the PLC for reflecting the separated wavelength channels;
an add/drop port within a transmissive window within the reflector for receiving one or more of the separated wavelength channels as a drop port, or for inputting a new wavelength channel as an add port, the add/drop port being tunable as a function of transverse motion of the transmissive window along the separated wavelength channels focused at the edge of the PLC;
a multiplexer with an input pupil optically coupled with the reflector and the add/drop port for receiving and multiplexing the one or more wavelength channels reflected by the reflector and any new wavelength channel input at the add port from the lens system in the slab waveguide region, and an output pupil; and
an output port optically coupled to the output pupil of the multiplexer for outputting the multiplexed wavelength channels;
wherein the output pupil of the demultiplexer is positioned with respect to the input pupil of the multiplexer, such that at least one wavelength channel focused on the reflector is optically coupled into the input pupil of the second demultiplexer.
2. A tunable add/drop filter in a planar lightwave circuit (PLC) as defined in claim 1 , wherein the first demultiplexer and the multiplexer comprise arrayed waveguide gratings.
3. A tunable add/drop filter in a planar lightwave circuit (PLC) as defined in claim 2 , wherein the spatially separated wavelength channels of the first demultiplexer and the multiplexer are in reflective communication along a common circle.
4. A tunable add/drop filter in a planar lightwave circuit (PLC) as defined in claim 3 , wherein the common circle comprises a Rowland circle.
5. A tunable add/drop filter in a planar lightwave circuit (PLC) as defined in claim 1 , wherein the lens system in the slab waveguide region comprises a telecentric lens system.
6. A tunable add/drop filter in a planar lightwave circuit (PLC) as defined in claim 5 , wherein chief rays of each reflected wavelength channel are incident on the reflector at equal angles.
7. A tunable add/drop filter in a planar lightwave circuit (PLC) as defined in claim 1 , wherein the lens system comprises at least one reflective surface within a slab waveguiding region, the reflective surface having a curvature to impart optical power.
8. A tunable add/drop filter in a planar lightwave circuit (PLC) as defined in claim 7 , wherein the at least one reflective surface is created through a refractive index gradient causing total internal reflection, and all angles of incidence of the separated wavelength channels on the at least one reflective surface are greater than the critical angle for total internal reflection.
9. A tunable add/drop filter in a planar lightwave circuit (PLC) as defined in claim 8 , wherein the at least one reflective surface is etched to expose an air interface.
10. A tunable add/drop filter in a planar lightwave circuit (PLC) as defined in claim 7 , wherein the at least one reflective surface is etched and metalized.
11. A tunable add/drop filter in a planar lightwave circuit (PLC) as defined in claim 1 , wherein the reflector comprises a separate PLC including an add/drop port waveguide disposed in the transmissive window.
12. A tunable add/drop filter in a planar lightwave circuit (PLC) as defined in claim 11 , wherein the add/drop port waveguide comprises a Mach-Zehnder interferometer.