Cascaded optical modulation system with high linearity

An apparatus for modulating a light beam comprising

• at least two integrated electro-optic cascaded modulator stages, including an input stage and an output stage, the input modulator stage having an optical waveguide input for receiving a light beam and the output stage having at least one optical waveguide output, at least one of said modulator stages being of the balanced-bridge interferometer type with at least one directional coupler section, each stage having separate bias means, wherein at least one directional coupler of said at least one balanced-bridge interferometer type modulator stage is a .DELTA..beta. coupler, said at least one .DELTA..beta. coupler being characterized by a change in a relative phase delay, .DELTA..beta., between two coupler paths thereof, in addition to a change in a coupling coefficient, .epsilon., between said coupler paths, in response to a bias voltage applied to said .DELTA..beta. coupler,
• means for driving the optical waveguide input of the input stage with a continuous wave light source;
• means for producing antiphase modulation of the light beam in said two stages simultaneously;
• and
• means for receiving a portion of the light beam emitted from each optical waveguide output of the output stage and deriving therefrom bias signals applied to said bias means, said at least one directional coupler section, including said at least one .DELTA..beta. coupler, having bias voltages applied thereto which are selected to minimize third and fifth order intermodulation distortions over all possible coupling ratio values.

An optical modulator comprising

• a first electro-optic modulator stage having an optical waveguide input port, N optical waveguide output ports and a first electrical port, said first modulator stage characterized by a first 1.times.N transfer matrix,
• a second electro-optic modulator stage having N optical waveguide input ports, M optical waveguide output ports and a second electrical port, said second modulator stage characterized by a second N.times.M transfer matrix said N input ports of said second modulator stage being butt coupled to said N output ports of said first modulator stage, and
• an electrical splitter having an input connected to receive an input electrical signal and having two electrical output terminals connected to said first and second electrical ports for applying respective first and second modulating voltages thereto, said modulating voltages being voltages derived from said input electrical signal,
• wherein at least one of said modulator stages is a balanced-bridge interferometer (BBI) modulator with at least one directional coupler (DC) section therein, at least one directional coupler of at least one said BBI modulator being a .DELTA..beta. coupler having means for applying a bias voltage thereto, said at least one .DELTA..beta. coupler being characterized by a change in a relative phase delay, .DELTA..beta., between two coupler paths thereof, in addition to a change in a coupling coefficient, .kappa., between said coupler paths, in response to said bias voltage applied to said .DELTA..beta. coupler, the transfer matrix characterizing the BBI modulator stage with at least one .DELTA..beta. coupler being a function of at least the bias voltage applied to the .DELTA..beta. coupler of that modulator stage, said first 1.times.N transfer matrix and said second N.times.M transfer matrix having been selected such that at least one power series coefficient for an overall transfer characteristic between said input port of said first modulator stage and at least one of said M output ports of said second modulator stage is minimized.

A linear electro-optic integrated modulator comprising:

• a first modulating stage of a 1.times.2 interferometer-type electro-optic modulator device, said first stage having an optical waveguide input port, a Y-junction splitter with two waveguide outputs, a first interferometer section including two noninteracting optical waveguides, each optical waveguide having an input port and an output port, said input port, of said two noninteracting optical waveguides being butt coupled to said two waveguide outputs of said Y-junction splitter, said first interferometer section also having modulation and bias means, and a 2.times.2 optical connecting port with two optical input waveguides and two optical output wave-guides, said two optical input waveguides of said connecting port being butt coupled to said output ports of said two noninteracting optical waveguides of said first interferometer section, said two optical output waveguides of said connecting port forming two optical waveguide output ports of said first modulating stage,
• a second modulating stage of a 2.times.M interferometer-type electro-optic modulator device, wherein values for M are selected from the set consisting of the integers 1 and 2, the second stage having a second interferometer section including two noninteracting optical waveguides, each optical waveguide having an input port and an output port, said input ports of said two noninteracting optical waveguides of said second interferometer section forming two optical waveguide input ports of said second modulating stage and being butt coupled to said two optical waveguide output ports of said first modulating stage, said second interferometer section also having modulation means, and a 2.times.M optical multiport with two optical input waveguide ports and M optical output waveguide ports, said two optical input waveguide ports of said multiport being butt coupled to said two optical waveguide output ports of said second interferometer section, said M optical output waveguide ports of said multiport forming M output ports of said second modulating stage,
• means for receiving a modulating electrical signal and deriving therefrom first and second output signals which are linearly related to said modulating electrical signal, said receiving means being connected to provide said output signals, respectively, to the modulation means of the first and second stages;
• and
• means for generating individual bias signals, said bias signals being connected, respectively, to bias means of at least one of said first and second modulating sections, voltage levels of said bias means being mutually adjusted to minimize even-order intermodulation distortions over all possible coupling ratio values, and to minimize third order intermodulation distortion terms in said optical waveguide output over all possible coupling ratio values,
• whereby, when a continuous-wave light source is connected to the modulator optical waveguide input port and an electrical modulating signal is connected to the signal splitting means, the light emitting from the at least one optical waveguide output port is modulated by the modulating signal.

An integrated electro-optic modulator comprising:

• an input Y-junction having an optical input port for receiving light and having a pair of optical outputs;
• a first interferometer section having a pair of waveguides forming optical paths connected to receive light from the Y-junction optical outputs, said interferometer section having a pair of optical outputs, said interferometer section having modulating electrodes disposed thereon for receiving an electrical modulating signal such that light traversing at least one optical waveguide is modulated in response to said electrical modulating signal, said interferometer section also having a bias means for receiving an electrical bias signal;
• a first directional coupler section having a pair of optical waveguides, said directional coupler having a predetermined coupling ratio, the coupler optical waveguides being connected to receive the optical outputs of the first interferometer section, and having a pair of optical outputs, said coupler section having a bias means for receiving an electrical bias signal;
• a second interferometer section of like structure to the first interferometer section and having its waveguides connected to receive the optical outputs of the first directional coupler;
• a second directional coupler of like structure to the first directional coupler and having its optical paths connected to receive the optical outputs of the second interferometer section, said coupler having a pair of optical outputs, said outputs comprising a pair of modulator optical output ports;
• means for receiving an electrical modulating signal and deriving therefrom a pair of signals for connection, respectively to the modulating electrodes of at least one of the first and second interferometer sections;
• and
• means for biasing first and second interfero meter sections and at least one directional coupler to voltage levels adjusted to minimize intermodulation distortion terms in the optical outputs and to maximize the linear modulation efficiency, wherein at least one of said directional couplers is a .DELTA..beta. coupler characterized by a change in a relative phase delay, .DELTA..beta., between two coupler paths thereof, in addition to a change in a coupling coefficient, .kappa., between said coupler paths, in response to a bias voltage applied to said .DELTA..beta. coupler, said biasing means adjusting the voltage level applied to said .DELTA..beta. coupler to minimize at least a third order intermodulation distortion term and adjusting the voltage level applied to at least one of said interferometer sections to minimize even order intermodulation distortion terms for the particular bias voltage level applied to said .DELTA..beta. coupler.

An optimal modulator comprising

• a first electro-optical modulator stage having an optical waveguide input port, two optical waveguide output ports and a first electrical port, said first modulator stage characterized by a first 1.times.2 transfer matrix,
• a first branch of a second electro-optic modulator stage having two optical waveguide input ports, M optical waveguide output ports and a second electrical port, said first branch characterized by a second K.times.M transfer matrix one of said input ports of said first branch being butt coupled to a first one of said two output ports of said first modulator stage,
• a second branch of said second electro-optic modulator stage of like structure to said first branch having K optical waveguide input ports, N optical waveguide output ports and a third electrical port, said branch characterized by a third K.times.N transfer matrix one of said input ports of said second branch being butt coupled to a second one of said two output ports of said first modulator stage, and
• an electrical splitter forming a linear electrical network having an input connected to receive an input electrical signal and having three electrical output terminals connected to said first, second and third electrical ports for applying respective first, second and third modulating voltages thereto, said modulating voltages being voltages derived from said input electrical signal and selected to minimize intermodulation distortions of at least one order.