A fiber structure includes first and second optical fibers disposed such that tip portions thereof butt and a sheet-shaped saturable absorber sandwiched between the tip portion of the first optical fiber and the tip portion of the second optical fiber. Each of the tip portions of the first optical fiber and the second optical fiber has a core, a cladding provided around the core, and a ferrule provided around the cladding. The tip portion of the first optical fiber has a protruding shape protruding to a tip side. The saturable absorber has an adhering part at least adhering to the core of the first optical fiber and a non-adhering part present around the adhering part and not adhering to the tip portion of the first optical fiber.

Passively Q-switched lasers and short wave infrared (SWIR) electro-optical systems including such lasers. A passively Q-switched laser may include a gain medium (GM) having a stimulated emission cross section .sub.SE, a saturable absorber (SA) having an absorption cross section (.sub.a) which is less than three times the .sub.SE of the GM, and an optical resonator within which the GM and the SA are positioned, the optical resonator comprising a high reflectivity mirror and an output coupler, wherein at least one of the high reflectivity mirror and the output coupler comprises a curved mirror, directing light within the optical resonator such that an effective cross-section of a laser mode within the SA (A.sub.SA) is smaller than a cross-section of a laser mode within a Rayleigh length of the pump (A.sub.GM).

A fundamental new effect in nonlinear photonic systems is disclosed herein, called n-photon bound states in the continuum, which can be applied to deterministically create large Fock states, as well as very highly intensity-squeezed states of light. The effect is one in which destructive interference gives a certain quantum state of light an infinite lifetime, despite coexisting in frequency with a radiative continuum. For Kerr nonlinear systems, that state is an n-photon (Fock) state of a particular and tunable n. Experimentally-realizable examples are shown which are capable of producing n-photon Fock states, and states with very large intensity squeezing, such as greater than 10 dB. The effect requires only Kerr nonlinearity and linear frequency-dependent (non-Markovian) dissipation, and is, in principle, applicable at any frequency. The theory and concepts are also immediately applicable to nonlinear bosons besides photons, and thus may be implemented in many other disciplines.

A radiation source arrangement including: a radiation source operable to generate source radiation including source energy pulses; and at least one non-linear energy-filter operable to filter the source radiation to obtain filtered radiation including filtered energy pulses. The at least one non-linear energy-filter is operable to mitigate variation in energy in the filtered radiation by reducing the energy level of the source energy pulses which have an energy level corresponding to one of both extremities of an energy distribution of the source energy pulses by a greater amount than the source energy pulses which have an energy level corresponding to a peak of the energy distribution.