Mixing and modulating methods and structures using nonlinear optical amplifiers

DETAILED DESCRIPTION The present invention is directed to mixing and modulating methods and structures which can generate intermodulation products of radio-frequency signals on an optical carrier without the penalty of an optical conversion loss and without the need for radio-frequency mixers, electro-optic modulators and expensive polarization-maintaining optical fibers.
These goals are realized with an intermodulation generating process that amplifies an optical carrier signal through an optical amplifier which has a nonlinear transfer function between an optical input port and an optical output port.
The optical amplifier also has a bias port and electronic signals are coupled into either the bias port or the optical input port.
The nonlinear transfer function generates intermodulation products of the electronic signals and simultaneously modulates them onto an amplified optical carrier signal at the optical output port.
The modulated and amplified optical carrier signal can be transported to a remote site in low-loss optical fibers and the intermodulation products detected by a photodetector.
Subsequent filtering can be used to produce a desired product, e.
g.
, a sum or a difference product.
Thus, mixing and modulating methods of the invention generate intermodulation products in optical carriers without the need for radio-frequency mixers and electro-optic modulators.
The use of a semiconductor optical amplifier (SOA) as the nonlinear optical amplifier has the additional advantage that its fabrication techniques (e.
g.
, photolithography) facilitate its integration with other system structures on semiconductor chips.
Preferably, polarization-insensitive SOA's are used to avoid the cost of polarization-maintaining optical fibers.
SOA's accept electronic signals over a wide range of radio frequencies (e.
g.
, microwave and millimeter-wave) and provide optical gain (typically>10 dB).
The ability to couple electronic signals into either the bias port or the optical input port lends nonlinear optical amplifiers a versatility which can be advantageously used in optoelectronic systems, i