Grating coupler with high efficiency
An optical coupling device includes a first waveguide layer, including a first semiconductor material, which is disposed over a dielectric substrate layer. A dielectric intermediate layer overlies the first waveguide layer. A second waveguide layer, which includes a different, second semiconductor material, is disposed over the dielectric intermediate layer and is patterned to define a waveguide. A first grating in the second waveguide layer diffracts light of a given wavelength from the waveguide into a specified diffraction order at a given coupling angle, whereby a first fraction of the light propagates out of the device while a second fraction of the light is diffracted into the intermediate dielectric layer in a conjugate diffraction order. A second grating in the first waveguide layer diffracts the second fraction of the light into a second diffraction order, propagating out of the device at the given coupling angle.
1 . An optical coupling device, comprising:
a dielectric substrate layer;
a first waveguide layer, comprising a first semiconductor material, which is disposed over the dielectric substrate layer;
a dielectric intermediate layer overlying the first waveguide layer;
a second waveguide layer, which comprises a second semiconductor material, different from the first semiconductor material, which is disposed over the dielectric intermediate layer and is patterned to define a waveguide;
a first grating, which is formed in the second waveguide layer and is configured to diffract input light of a given wavelength from the waveguide into a specified diffraction order at a given coupling angle, whereby a first fraction of the input light propagates out of the optical coupling device into a transmitted beam at the given coupling angle while a second fraction of the input light is diffracted by the first grating into the dielectric intermediate layer in a conjugate diffraction order; and
a second grating, which is formed in the first waveguide layer and is configured to diffract the second fraction of the input light into a second diffraction order, propagating out of the device at the given coupling angle, whereby the second grating acts as a reflector for the conjugate diffraction order to enhance coupling efficiency of the input light into the transmitted beam.
2 . The optical coupling device according to claim 1 , wherein the first grating is configured to diffract the input light of a first polarization into the specified diffraction order, and wherein the second grating is further configured to diffract incoming light of the given wavelength that is incident on the device at the given coupling angle with a second polarization, orthogonal to the first polarization, into the first waveguide layer.
3 . The optical coupling device according to claim 2 , wherein the first polarization is a TE polarization propagating in the waveguide in the second waveguide layer, and wherein the first waveguide layer comprises a further waveguide, and the second polarization is a TM polarization, which propagates in the further waveguide.
4 . An optical transceiver, comprising:
the optical coupling device according to claim 2 ;
an optical transmitter, which is coupled to transmit the input light of the first polarization into the waveguide in the second waveguide layer; and
an optical receiver, which is coupled to receive the incoming light of the second polarization from a further waveguide in first waveguide layer.
5 . The optical transceiver according to claim 4 , wherein the input light comprises coherent radiation, and wherein the transceiver comprises a mixer, which is coupled to mix a part of the transmitted light with the incoming light that is received through the further waveguide and to output the mixed light to the optical receiver.
6 . The optical transceiver according to claim 5 , wherein the first grating is further configured to diffract the incoming light of the first polarization that is incident on the device at the given angle into the waveguide in the second waveguide layer, and wherein the transceiver comprises a further mixer, which is coupled to mix a further part of the transmitted light with the incident light of the first polarization that is received through the waveguide in the second waveguide layer for output to a detector.
7 . An apparatus for optical sensing, comprising: an optical sensing substrate; and an array of optical transceivers according to claim 4 , disposed on the optical sensing substrate and configured to transmit the input light and receive the incoming light via respective optical coupling devices.
8 . The apparatus according to claim 7 , wherein the optical coupling devices have respective coupling angles that vary across the array.
9 . The optical coupling device according to claim 1 , wherein the first waveguide layer comprises silicon, and the second waveguide layer comprises silicon nitride.
10 . A method for optical coupling, comprising:
depositing a first waveguide layer, comprising a first semiconductor material, over a dielectric substrate layer;
depositing a dielectric intermediate layer over the first waveguide layer;
depositing a second waveguide layer, which comprises a second semiconductor material, different from the first semiconductor material, over the dielectric intermediate layer;
patterning the second waveguide layer to define a waveguide;
forming in the second waveguide layer a first grating configured to diffract input light of a given wavelength from the waveguide into a specified diffraction order at a given coupling angle, whereby a first fraction of the input light propagates into a transmitted beam at the given coupling angle while a second fraction of the input light is diffracted by the first grating into the intermediate dielectric layer in a conjugate diffraction order; and
forming in the first waveguide layer a second grating, which is configured to diffract the second fraction of the input light into a second diffraction order, propagating at the given coupling angle, whereby the second grating acts as a reflector for the conjugate diffraction order to enhance coupling efficiency of the input light into the transmitted beam.
11 . The method according to claim 10 , wherein the first grating is configured to diffract the input light of a first polarization into the specified diffraction order, and wherein the second grating is further configured to diffract incoming light of the given wavelength that is incident on the second grating at the given coupling angle with a second polarization, orthogonal to the first polarization, into the first waveguide layer.
12 . The method according to claim 11 , wherein the first polarization is a TE polarization propagating in the waveguide in the second waveguide layer, and wherein the method comprises patterning the first waveguide layer to define a further waveguide, wherein the second polarization is a TM polarization, which propagates in the further waveguide.
13 . The method according to claim 11 , and comprising:
coupling an optical transmitter to transmit the input light of the first polarization into the waveguide in the second waveguide layer; and
coupling an optical receiver to receive the incoming light of the second polarization from a further waveguide in first waveguide layer.
14 . The method according to claim 13 , wherein the input light comprises coherent radiation, and the method comprises mixing a part of the transmitted light with the incoming light that is received through the further waveguide and outputting the mixed light to the optical receiver.
15 . The method according to claim 14 , wherein the first grating is further configured to diffract the incoming light of the first polarization that is incident at the given angle into the waveguide in the second waveguide layer, and wherein the method comprises mixing a further part of the transmitted light with the incident light of the first polarization that is received through the waveguide in the second waveguide layer for output to a detector.
16 . The method according to claim 10 , wherein the first waveguide layer comprises silicon, and the second waveguide layer comprises silicon nitride.