High-tolerance broadband-optical switch in planar lightwave circuits
Broadband optical switches based on adiabatic couplers having a pair of asymmetric waveguides with variable curvature sections include in a 2×2 configuration based on a Mach-Zehnder interferometer two such adiabatic couplers, and in a 1×2 or 2×1 configuration an adiabatic coupler and an Y-splitter. Each adiabatic coupler includes two waveguide branches of different but constant widths having curved sections with varying radii, separated over a coupling length by a changing spacing therebetween and blending in an asymmetric intersection area, and two symmetric branches. In the 2×2 switch, the two adiabatic couplers face each other with their respective symmetric branches, and are connected by the two identical arms along a main propagation axis in a mirror image. The utilization of the variable curvature adiabatic couplers in silica MZI switches on a silicon substrate provides switches with an exceptional broadband range (1.2-1.7 μm), very high extinction ratios (>35 dB), low fabrication sensitivity and polarization independent operation. The switches are significantly smaller than known broadband switches, have significantly smaller excess loss, faster switching time and low power consumption.
1. A 2×0.2 optical switch having a main propagation axis, comprising:
a) a first 3 dB adiabatic coupler having
i) a first pair of constant width, asymmetric waveguide branches, each said branch extending between a proximal end and a distal end, each said branch having a curved section with a variable curvature, said branches separated over a coupling length by a changing spacing therebetween and blending in a symmetric intersection area at said distal end, and
ii) two symmetric branches connected to said intersection area at said distal end,
b) a second 3 dB adiabatic coupler having
iii) a first pair of constant width, asymmetric waveguide branches, each said branch extending between a proximal end and a distal end, each said branch having a curved section with a variable curvature, said branches separated over a coupling length by a changing spacing therebetween and blending in a symmetric intersection area at said distal end, and
iv) two symmetric branches connected to said intersection area at said distal end, wherein said first and second adiabatic couplers facing each other along the main optical propagation axis in a mirror image,
c) two identical arms connecting said first and second pairs of symmetric branches optically to each other along the main propagation axis, and
d) at least one active element coupled to at least one of said identical arms for dynamically changing an optical property of said at least one arm, whereby the implementation of the switch in a planar lightwave circuit provides a switch that is practically polarization independent, and has a low loss and a very high extinction ratio over an exceptionally broad band range.
2. The switch of claim 1 , wherein said asymmetry of said waveguide branches includes different waveguide widths.
3. The switch of claim 1 , wherein said variable curvature is defined by a finite number of different radii.
4. The switch of claim 1 , wherein said variable curvature is defined by a plurality of smoothly and monotonically varying radii.
5. The switch of claim 1 , wherein said curved waveguide sections are implemented as a series of discrete circular bends with different monotonically decreasing or increasing radii values.
6. The switch of claim 1 , wherein said first and second adiabatic couplers are identical.
7. The switch of claim 1 , wherein each said adiabatic coupler includes a pair of symmetric input/output bends, each said bend connected to a respective said curved section of a branch at said proximal branch end and configured to match the respective width of said branch.
8. The switch of claim 7 , wherein at least one of said couplers further includes a mediating waveguide located between said intersection area and said symmetric branches.
9. The switch of claim 7 , wherein said connection of each said bend to a respective curved section is mediated by an adiabatic taper.
10. The switch of claim 1 , wherein said couplers and said identical arms are built of silica on a silicon substrate, and wherein said optical property of said arm includes an index of refraction of said arm.
11. The switch of claim 10 , wherein said extinction ratio is selected from the group consisting of an extinction ratio of at least 30 in the C+L bands, and an extinction ratio of at least 32 in the 10.3 μm wavelength window.
12. The switch of claim 1 , wherein said at least one active element coupled to at least one of said identical arms includes an active element on each of said identical arms, whereby a combined use of said two active elements can actively compensate for any asymmetry effect in said arms.