A difference frequency generation optical transmitter and sum frequency generation optical receiver operating in the mid-infrared wavelength range for use in free space optical satellite communications are described. By using mid-infrared light, the transmitter/receiver can mitigate atmospheric scintillation, scattering, and other non-ideal optical effects in the communication channel. This is achieved through the use of nonlinear optical crystals designed for difference frequency generation in the case of the transmitter and sum frequency generation for the receiver. High-speed modulated communication signals can thus be frequency converted to the mid-infrared wavelength range by a relatively low cost, compact and high-power optical communication system.

An apparatus (1) is proposed for providing coupling between at least a first input signal with a first signal frequency, and a second input signal with a second, different signal frequency. The apparatus comprises: a first input port (3); a second input port (5); a first output port (9); a second output port (11); a first waveguide (13); a second waveguide (15), the second waveguide (15) being made of or comprising non-linear material such that a first electromagnetic field generated by a first-waveguide signal in the first waveguide (13) and a second electromagnetic field generated by a second-waveguide signal in the second waveguide (15) are arranged to overlap in the non-linear material; a periodic structure (31, 33); and a phase-matching arrangement (37).

A method, apparatus, and system for non-linear optical process. A first light of a first wavelength is routed in a first loop in a main nonlinear optical waveguide. The first loop has a first length for the first light of the first wavelength. A second light of a second wavelength is routed in a second loop that includes portions of the main nonlinear optical waveguide and a first extension optical waveguide. The second loop has a second length for the second light of the second wavelength. A third light of a third wavelength is routed in a third loop that include portions of the main nonlinear optical waveguide and a second extension optical waveguide. The third loop has a third length for the third light of the third wavelength.

An optical waveguide structure comprises a first coupler and a second coupler that, in combination, direct a first-wavelength light to travel through a nonlinear-optical waveguide, the two couplers and an extension waveguide but not a secondary waveguide, a first resonator loop is defined for which the first-wavelength light is resonant. The two couplers, in combination, also direct a second-wavelength light to travel through the nonlinear-optical waveguide, the two couplers and the secondary waveguide but not the extension waveguide, wherein a different second resonator loop is defined for which the second-wavelength light is resonant.

A system for resonantly enhanced frequency conversion includes a nonlinear crystal for frequency converting a pump laser beam, and mirrors forming a ring resonator for the pump laser beam such that a closed propagation path of the pump laser beam, inside the ring resonator, passes through the nonlinear crystal. The mirrors include an adaptive mirror, a curved-mirror pair positioned in a first segment of the propagation path spanning between the adaptive mirror and the nonlinear crystal, and an input coupler for coupling the pump laser beam into the ring resonator. The curved-mirror pair forms an imaging system having conjugate planes at the adaptive mirror and the nonlinear crystal. The input coupler is positioned in a second segment of the propagation path that spans between the adaptive mirror and the nonlinear crystal and does not include deflection by the curved-mirror pair.

An apparatus and method for wavelength conversion of a signal, for example, a dual-polarization signal, is disclosed. The apparatus implements a single-loop counter-propagating wavelength conversion scheme which provides both up-conversion and down-conversion of the signal within the same loop. Nonlinear wavelength conversion devices in the loop provide both up-conversion and down-conversion of the polarization components of the signal within the loop depending on whether the polarization component travels through the nonlinear conversion device in a clockwise or a counter-clockwise direction. The wavelength-converted signal is available to be extracted from the wavelength-conversion loop. An all-optical wavelength-division multiplexing transponder based on the wavelength-conversion scheme is also disclosed.

An optical waveguide structure comprises a nonlinear optical waveguide comprising a nonlinear optical material having a second order nonlinear coefficient that changes with a direction of light propagation. A first portion of the nonlinear optical waveguide in which a light propagating through the first portion is affected by a positive value of a second order nonlinear coefficient. A second portion of the nonlinear optical waveguide in which the light propagating through the first portion is affected by a negative value of a second order nonlinear coefficient, wherein a set of dimensions in the nonlinear optical waveguide in the first portion and the second portion is selected to cause the light to have a phase walk-off that is an odd multiple of 180 degrees.

An optical element includes Strontium tetraborate SrB.sub.4O.sub.7 (SBO) crystal plates that are cooperatively configured to create a periodic structure for quasi-phase-matching (QPM) is used in the final frequency converting stage of a laser assembly to generate laser output light having a wavelength in the range of 125 nm to 183 nm. One or more fundamental light beams having fundamental wavelengths between 1 and 1.1 μm are doubled and/or summed using multiple intermediate frequency conversion stages to generate one or more intermediate light beam frequencies (e.g., second through eighth harmonics, or sums thereof), and then the final frequency converting stage utilizes the optical element to either double a single intermediate light beam frequency or to sum two intermediate light beam frequencies to generate the desired laser output light at high power and photon energy levels. A method and inspection system incorporating the laser assembly is also described.

A technique to generate terahertz radiation is disclosed, where a pump beam (12) is coupled into an optical element (50) made of a medium with non-linear optical properties having plane-parallel front and rear boundary surfaces (51, 52), wherein the pump beam (12) is split into a set of partial pump beams (121) by reflection and/or diffraction on a periodic relief structure (53) of said optical element (50). The partial pump beams travel along a direction at an angle γ that satisfies the velocity matching condition of v.sub.p,cs cos(γ)=v.sub.THz,f within the given medium, where v.sub.p,cs is the group velocity of the pump beam, v.sub.THz,f is the phase velocity of the terahertz radiation and the speed a planar envelope (212) travels toward the front boundary surface (51) of the optical element (50), and angle γ is the angle formed by the pulse front envelope and the phase front of the pump beam.

A pulsed third-harmonic laser system includes a pulsed laser, an extra-cavity nonlinear crystal, and an intracavity nonlinear crystal. The pulsed laser generates fundamental laser pulses and couples out a portion of each fundamental laser pulse out of the laser resonator to undergo second-harmonic-generation in the extra-cavity nonlinear crystal. Resulting second-harmonic laser pulses are directed back into the laser resonator and mixes with the fundamental laser pulses in the intracavity nonlinear crystal to generate third-harmonic laser pulses. The pulsed third-harmonic laser system thus maintains a non-zero output coupling efficiency regardless of the efficiency of the second-harmonic-generation stage, while the third-harmonic-generation stage benefits from the intracavity power of the fundamental laser pulses.