A nonlinear wave mixing system with grating assisted phase matching is provided. The system includes a pump laser and a nonlinear waveguide. The pump laser is used to generate pump light at a select wavelength. The nonlinear waveguide is configured to generate produced light from the pump light that is directed into the nonlinear waveguide. The nonlinear waveguide includes at least one backward grating that is configured to diffract the produced light in a backward direction relative to a direction the produced light travels in the nonlinear waveguide to reach the backward grating. The backward grating having a grating momentum that generates counter-propagating phase matching in the produced light.

A hollow-core anti-resonant-reflecting fibre (HC-AF) includes a hollow-core region, an inner cladding region, and an outer cladding region. The hollow-core region axially extends along the HC-AF. The inner cladding region includes a plurality of anti-resonant elements (AREs) and surrounds the hollow-core region. The outer cladding region surrounds the inner cladding region. The hollow-core region and the plurality of AREs are configured to provide phase matching of higher order hollow-core modes and ARE modes in a broadband wavelength range.

A method for extending and enhancing bright coherent high-order harmonic generation into the VUV-EUV-X-ray regions of the spectrum involves a way of accomplishing phase matching or effective phase matching of extreme upconversion of laser light at high conversion efficiency, approaching 10.sup.3 in some spectral regions, and at significantly higher photon energies in a waveguide geometry, in a self-guiding geometry, a gas cell, or a loosely focusing geometry, containing nonlinear medium. The extension and enhancement of the coherent VUV, EUV, X-ray emission to high photon energies relies on using VUV-UV-VIS lasers of shorter wavelength. This leads to enhancement of macroscopic phase matching parameters due to stronger contribution of linear and nonlinear dispersion of both atoms and ions, combined with a strong microscopic single-atom yield.

A pump beam (12) is subjected to pulse front tilting, and then guided through an imaging optics (30) and then coupled into the nonlinear optical medium through an entry surface of the nonlinear optical medium. THz radiation is generated in the optical medium by nonlinear optical processes, in particular by optical rectification, via the pump beam. The pulse front tilt of the pump beam required to satisfy the velocity matching condition of v.sub.p,cs cos()=v.sub.THz,f is induced as a sum of a plurality of pulse front tilts, where each pulse front tilt is induced separately as a partial pulse front tilt of the pump beam in subsequent steps. The last step of pulse front tilting of said pump beam (12) is performed by coupling the pump beam (12) into the nonlinear optical medium through a stair-step structure (40) formed in the entry surface of the nonlinear optical medium.

A nonlinear fiber interferometer is disclosed suitable for fiber sensor and other applications. A first nonlinear fiber section amplifies probe and conjugate sidebands of a pump through four-wave mixing. A second section introduces a phase shift to be measured, for example from a sensor. A third nonlinear fiber section amplifies with phase-sensitive gain to increase signal-to-noise ratio. Based on phase-sensitive output power of probe and/or conjugate components, the phase shift can be measured. Superior performance can be obtained by balancing gain between the (first and third) nonlinear sections. Non-fiber, for example photonic integrated circuit, embodiments are disclosed. Differential sensing, alternative detection schemes, sensing applications, associated methods, and other variations are disclosed.

An optically nonlinear crystal is arranged for frequency-doubling an input pulse. The crystal has parallel facets each coated with a reflective coating. The crystal is arranged with respect to the input pulse such that the input pulse makes a plurality of forward and reverse passes between the coatings. A frequency-doubled pulse is generated on the forward passes. The input pulse and the frequency-doubled pulse propagate with different group velocities in the crystal such that temporal separation the pulses occurs. The crystal and reflective coatings are configured such that the temporal separation does not exceed a predetermined value.

An exemplary system can be provided which can include, for example, a plurality of source arrangements configured to provide a plurality of electro-magnetic radiations to at least one of at least one sample or at least one reference structure, a first arrangement configured to receive a first radiation(s) from the reference structure(s), a second arrangement configured to receive a second radiation(s) from the sample(s), where a portion(s) of the second radiation(s) can be in an invisible spectrum, a third arrangement configured to combine the first radiation(s) and the second radiation(s) into a third radiation(s), and a fourth arrangement configured to convert the third radiation(s) into a further radiation in a visible spectrum based on the at least one portion.

A hollow-core fiber (100) of non-bandgap type comprises a hollow core region (10) axially extending along the hollow-core fiber (100) and having a smallest transverse core dimension (D), wherein the core region (10) is adapted for guiding a transverse fundamental core mode and transverse higher order core modes, and an inner cladding region (20) comprising an arrangement of anti-resonant elements (AREs) (21, 21A, 21B) surrounding the core region (10) along the hollow-core fiber (100), each having a smallest transverse ARE dimension (d.sub.i) and being adapted for guiding transverse ARE modes, wherein the core region (10) and the AREs (21, 21A, 21B) are configured to provide phase matching of the higher order core modes and the ARE modes and the ARE dimension (d.sub.i) and the core dimension (D) are selected such that a ratio of the ARE and core dimensions (d.sub.i/D) is approximated to a quotient of zeros of Bessel functions of first kind (u.sub.lm,ARE/u.sub.lm,core), multiplied with a fitting factor in a range of 0.9 to 1.5, with m being the m-th zero of the Bessel function of first kind of order 1, said zeros of the Bessel functions describing the LP.sub.lm ARE modes and LP.sub.lm higher order core modes, respectively. Furthermore, an optical device (200) including the hollow-core fiber (100) and a method of manufacturing the hollow-core fiber are described.

A method and system for utilizing the laser defocusing effect for phase matching of high order harmonic generation in a tight focusing geometry are provided. The most suitable focusing geometry, especially those five parameters including: (1) the aperture size of the adjustable iris, (2) the focus position with respect to the gas cell, (3) the backing pressure of the gas cell, (4) the focal length and (5) the length of the gas cell, for achieving phase matching of the target harmonic order and maximal yield of the target harmonic order are disclosed.

The present application relates to generating terahertz radiation, wherein a pump pulse is subjected to pulse front tilting, the thus obtained pump pulse having tilted pulse front is coupled into a nonlinear optical medium and THz pulse is generated by the optical medium in nonlinear optical processes, particularly by means of optical rectification by the pump pulse. The application also relates to a terahertz radiation source (100), comprising a pump source (10) for emitting a pump pulse and a nonlinear optical medium for generating THz pulse. The pump source (10) and the nonlinear optical medium define together a light path, said light path is arranged to guide the pump pulse from the pump source (10) to the nonlinear optical medium. A first optical element (20) having angular-dispersion-inducing property and imaging optics (30) are disposed in the light path after each other in the propagation direction of the pump pulse.