Beam combiner
A beam combiner is disclosed that comprises a planar lightwave circuit that is based on undoped silicon nitride-based surface waveguides, wherein the planar lightwave circuit comprises a plurality of input ports, a mixing region, and an output port, and wherein the mixing region comprises a plurality of directional couplers that are arranged in a tree structure. Embodiments of the present invention are capable of combining a plurality of light signals characterized by disparate wavelengths on irregular spacings with low loss. Further, the present invention enables high-volume, low cost production of beam combiners capable of combining three or more light signals into a single composite output beam.
1. A beam combiner that includes:
a planar lightwave circuit comprising;
a plurality of input ports;
a mixing region, the mixing region comprising a plurality of directional couplers that are arranged in a first arrangement comprising a tree structure; and
a first output port that is optically coupled with the plurality of input ports via the mixing region; and
a first polarization filter that is optically coupled with a first input port of the plurality of input ports, the first polarization filter receiving a first light signal comprising a first polarization mode having a first magnitude and a second polarization mode having a second magnitude, wherein the first polarization filter provides the first light signal to the first input port such that the first polarization mode has a third magnitude that is substantially equal to the first magnitude and the second polarization mode has a fourth magnitude that is less than the second magnitude.
2. The beam combiner of claim 1 wherein the first input port comprises a first waveguide portion and a second waveguide portion, the first waveguide portion being characterized by a refractive-index contrast that is lower than the refractive-index contrast of the second waveguide portion.
3. The beam combiner of claim 2 wherein the first input port comprises a first surface waveguide that includes a first tapered region having a first length, wherein the cross-sectional area of the first surface waveguide changes monotonically along the first length.
4. The beam combiner of claim 1 wherein the planar lightwave circuit comprises a first surface waveguide having an inner core comprising silicon dioxide, an outer core comprising silicon nitride, and an upper cladding comprising silicon dioxide.
5. The beam combiner of claim 1 wherein the planar lightwave circuit comprises a first surface waveguide having a core comprising silicon nitride.
6. The beam combiner of claim 1 wherein the planar lightwave circuit comprises a first surface waveguide comprising:
a first layer comprising silicon nitride; and
a second layer comprising silicon dioxide, the second layer disposed on and in contact with the first layer, and the second layer being a TEOS-based layer.
7. The beam combiner of claim 1 wherein the planar lightwave circuit comprises an attenuator comprising a first directional coupler, a second directional coupler, and a phase adjuster, wherein the first directional coupler and the second directional coupler are optically coupled in a series arrangement, and wherein the phase adjuster controls attenuation of light propagating through the attenuator based on a first control signal.
8. The beam combiner of claim 1 wherein a second input port of the plurality thereof is operative for receiving a light signal from a source, and wherein the second input port includes a mode-matching region that comprises:
a first end facet having a first mode-field diameter that substantially matches the mode-field diameter of the source; and
a first interface having a second mode-field diameter that substantially matches the mode-field diameter of a waveguide of the planar lightwave circuit, wherein the first mode-field diameter and second mode-field diameter are different.
9. The beam combiner of claim 8 wherein the source of the first light signal is a laser diode.
10. The beam combiner of claim 1 wherein the first input port is substantially wavelength tunable over a wavelength range.
11. The beam combiner of claim 1 wherein the planar lightwave circuit comprises a first waveguide portion and a second waveguide portion, the first waveguide portion and second waveguide portion being optically coupled, wherein the first waveguide portion is characterized by a first refractive-index contrast, and wherein the second waveguide portion is characterized by a second refractive-index contrast that is higher than the first refractive-index contrast.
12. The beam combiner of claim 1 wherein the plurality of input ports comprises a first directional coupler that is dimensioned and arranged to (1) enable each of a first light signal and a second light signal in a first waveguide to substantially completely couple from the first waveguide into a second waveguide and (2) substantially disable optical coupling of each of a third light signal and a fourth light signal in the second waveguide from the second waveguide into the first waveguide.
13. The beam combiner of claim 8 wherein the source of the first light signal is an optical fiber.