Provided here are: a mesa strip which has an n-type cladding layer, an active layer and a p-type cladding layer that are stacked sequentially on a surface of an n-type substrate; Fe-doped semi-insulating layers which are embedded along both sides of the mesa stripe, each up to a height higher than the mesa stripe; n-type blocking layers which are stacked on respective surfaces of the Fe-doped semi-insulating layers located on the both sides of the mesa stripe, and which are spaced apart from each other with an interval that is a space corresponding to a central portion of the active layer and is thus narrower than the active layer; p-type cladding layers which are formed on back surfaces of respective mesa-stripe-side end portions of the n-type blocking layers; and a p-type cladding layer which buries a top of the mesa stripe, the p-type cladding layers and the n-type blocking layers.

An optical semiconductor device outputting a predetermined wavelength of laser light includes a quantum well active layer positioned between a p-type cladding layer and an n-type cladding layer in thickness direction. The optical semiconductor device includes a separate confinement heterostructure layer positioned between the quantum well active layer and the n-type cladding layer. The optical semiconductor device further includes an electric-field-distribution-control layer positioned between the separate confinement heterostructure layer and the n-type cladding layer and configured by at least two semiconductor layers having band gap energy greater than band gap energy of a barrier layer constituting the quantum well active layer. The quantum well active layer is doped with 0.3 to 1×10.sup.18/cm.sup.3 of n-type impurity.

An optical semiconductor device outputting a predetermined wavelength of laser light includes a quantum well active layer positioned between a p-type cladding layer and an n-type cladding layer in thickness direction. The optical semiconductor device includes a separate confinement heterostructure layer positioned between the quantum well active layer and the n-type cladding layer. The optical semiconductor device further includes an electric-field-distribution-control layer positioned between the separate confinement heterostructure layer and the n-type cladding layer and configured by at least two semiconductor layers having band gap energy greater than band gap energy of a barrier layer constituting the quantum well active layer. The quantum well active layer is doped with 0.3 to 1×10.sup.18/cm.sup.3 of n-type impurity.

A photonic crystal surface-emitting laser includes a substrate, an n-type cladding layer, an active layer, an index matching layer and a photonic crystal structure. The n-type cladding layer is disposed over the substrate. The active layer is disposed over the n-type cladding layer. The index matching layer is disposed over the n-type cladding layer and is arranged around the active layer. The index matching layer is electrically insulating, and an effective refractive index of the index matching layer is substantially identical to an effective refractive index of the active layer. The photonic crystal structure is disposed over the active layer and the index matching layer.

An optical filter includes a first loop mirror, a second loop mirror, a first waveguide optically coupled to the first loop mirror and the second loop mirror, and a first access waveguide. The first loop mirror includes a first loop waveguide and a first multiplexer/demultiplexer. The second loop mirror includes a second loop waveguide and a second multiplexer/demultiplexer. The first loop waveguide is optically coupled to the first multiplexer/demultiplexer. The second loop waveguide is optically coupled to the second multiplexer/demultiplexer. The first waveguide is optically coupled to the first multiplexer/demultiplexer and the second multiplexer/demultiplexer. The first access waveguide is optically coupled to the first waveguide.

A semiconductor device includes an electrode which is arranged on an organic material with an insulation film interposed therebetween and which does not easily peel away from the organic material along with the insulation film. An insulation film in a region including pad portions of a phase shift electrode and a modulation electrode has openings at the centers of the pad portions of the phase shift electrode and the modulation electrode, the edge portions of which are formed on the phase shift electrode and the modulation electrode. In this way, the adjoining edges of the phase shift electrode and modulation electrode and the insulation film are all covered by the insulation film so as not to be exposed to the atmosphere. By covering the cracks that occur in the insulation film in the production process with the insulation film made of SiO.sub.2, SiN.sub.X, SiON.sub.X or the like, an organic solvent such as acetone or ethanol used in the process can be prevented from seeping in between the insulation film and the organic material through the cracks in the insulation film.

An active layer of a quantum cascade laser includes an active layer includes a plurality of emission regions and a plurality of injection regions. Each emission region includes an injection barrier layer, and an light-emitting quantum well layer that has at least two well layers, and that emits infrared light by undergoing an intersubband transition. Each injection region includes an extraction barrier layer, and a relaxation quantum well layer that creates an energy level for relaxing the energy of carriers from the each emission region. One of adjacent two well layers in the light-emitting quantum well layer of the each emission region on the side of the extraction barrier layer is deeper than a second well layer on the side of the injection barrier layer. The each emission region and the injection region are alternately stacked.

An optical device for a quantum random number generator comprising: a source of phase randomised pulses of light, the source of phase randomised pulses of light further comprising a plurality of gain-switched lasers, each gain-switched laser having an output, and each gain-switched laser being configured to emit a stream of pulses such that the phase of each pulse in the stream of pulses is randomised, and an optical pulse combiner, the optical pulse combiner being configured to receive streams of pulses from the output of each gain-switched laser, combine the streams of pulses with one another into a combined stream of pulses and direct the combined stream of pulses into at least one output of the optical pulse combiner, the at least one output of the optical pulse combiner being the output of the source of phase randomised pulses of light; wherein the source of phase randomised pulses of light is configured such that the streams of pulses of light emitted by the plurality of gain-switched lasers are temporally offset relative to one another, a phase measurement element, the phase measurement element being configured to receive the combined stream of pulses from the output of the source of phase randomised pulses of light; and an optical detector, the optical detector being optically coupled to the phase measurement element.

A semiconductor optical device may include a semiconductor substrate; a mesa stripe structure that extends in a stripe shape in a first direction on the semiconductor substrate and includes a contact layer on a top layer; an adjacent layer on the semiconductor substrate and adjacent to the mesa stripe structure in a second direction orthogonal to the first direction; a passivation film that covers at least a part of the adjacent layer; a resin layer on the passivation film; an electrode that is electrically connected to the contact layer and extends continuously from the contact layer to the resin layer; and an inorganic insulating film that extends continuously from the resin layer to the passivation film under the electrode, is spaced apart from the mesa stripe structure, and is completely interposed between the electrode and the resin layer.

A light emitting device includes a first active layer on a substrate, a current spreading length, and a plurality of mesa regions on the first active layer. At least a first portion of the first active layer can comprise a first electrical polarity. Each mesa region can include, at least a second portion of the first active layer, a light emitting region on the second portion of the first active layer with a dimension parallel to the substrate smaller than twice the current spreading length, and a second active layer on the light emitting region. The light emitting region can be configured to emit light with a target wavelength from 200 nm to 300 nm. At least a portion of the second active layer can comprise a second electrical polarity.