Abstract: Several techniques are provided to reduce the spurious reflection generated at the exit surface of a transmission grating. In one embodiment the exit surface of the grating is not parallel with the entrance surface. In another embodiment, the first and second surfaces of the grating are parallel and a second substrate is attached to the second surface of the grating. The second substrate has an entrance surface and an exit surface having at least a wedge angle relative to the entrance surface. In another embodiment, a second substrate is fixedly connected to the second surface of a parallel plate grating, where the second substrate has an entrance surface and an exit surface. The exit surface is curved such that it will be about normal to a beam transmitted over a range of angels of incidence onto the grating region.

Abstract: High resolution fast tunable optical filters are described such that each filter includes a tunable single-peak narrow-bandwidth (SPNB) filter and a tunable etalon in tandem with the tunable SPNB filter, where the bandwidth of the tunable SPNB filter is less than the free spectral range (FSR) of the tunable etalon.

Abstract: Step-phase interferometers are provided for use as optical interleavers/de-interleaver for optical communication. High data rates require a wide band-width to pass the high-speed modulated optical spectrum, and further require a wide stop-band to reject the signal from adjacent channels. The present interferometers provide a steep slope at the transition from the passband to the adjacent stop-band, thereby enlarging the width of both the pass-band and stop-band.

Abstract: An optical spectrum analyzer is implemented with a detector combined with a tunable filter mounted on a stage capable of 360-degree rotation at a constant velocity. Because of the constant rate of angular change, different portions of the input spectrum are detected at each increment of time as a function of filter position, which can be easily measured with an encoder for synchronization purposes. The unidirectional motion of the mirror permits operation at very high speeds with great mechanical reliability. The same improvements may be obtained using a diffraction grating or a prism, in which case the detector or an intervening mirror may be rotated instead of the grating or prism.

Abstract: In an optical etalon with a fixed FSR determined by the cavity length, the time delay is adjusted by an etalon surface coating. The proper cavity length is chosen to achieve a desired FSR, and the coating is independently selected to obtain a desired time delay.

Abstract: A plane-parallel optical window is the spacer of single-cavity filters in the stack used for DWDM applications. Highly reflective quarter-wave stacks are deposited on each side of the optical window and the single-cavity structure so obtained is diced to produce a plurality of filters. Each single-cavity filter so fabricated from the optical window has the same transmission wavelength and is therefore readily stackable for DWDM applications. Alternatively, an optical window with a thickness equal to one half that required for the spacer of a single-cavity filter is coated on a single side. The window is then divided in multiple identical components that can be combined in pairs by placing them in optical contact so as to form individual single-cavity filters with the same transmission-peak wavelength. The transmission peak of the filter can be fine tuned by controlling the temperature of the solid spacer material.

Abstract: A plane-parallel optical window is the spacer of single-cavity filters in the stack used for DWDM applications. Highly reflective quarter-wave stacks are deposited on each side of the optical window and the single-cavity structure so obtained is diced to produce a plurality of filters. Because the single-cavity structure has the same thickness over the entire window area and the quarter-wave-stack deposition process is carried out throughout under the same conditions, each single-cavity filter fabricated from the optical window has the same transmission wavelength and is therefore readily stackable for DWDM applications. Alternatively, an optical window with a thickness equal to one half that required for the spacer of a single-cavity filter is coated on a single side. The window is then divided in multiple identical components that can be combined in pairs by placing them in optical contact so as to form individual single-cavity filters with the same transmission wavelength.

Abstract: A conventional interference tunable filter is combined with a pair of shaping prisms to enlarge the circular input beam in the direction orthogonal to the direction of beam propagation and to the axis of rotation of the tunable filter. The degree of expansion is tailored to minimize the walk-off losses produced by successive reflections in the cavity of the tunable filter. By appropriately sizing the enlargement, the substantially elliptical beam produced by the shaping prisms encompasses sufficient reflected beams after passing through the tunable filter to produce substantially the same filter output that in a conventional filter would require a materially larger input beam. The input beam is preferably first converted to two parallel beams of the same polarization state. Both beams are then expanded by the prisms and processed by the tunable filter.

Abstract: A four-fiber collimator is coupled to the optics of a single interleaver to produce the functionality of two co-packaged interleavers. Two fibers of the collimator are coupled to the core optics of a single interleaver to produce two pairs of output beams. The other two fibers of the collimators are coupled to receive the reflection output beams. The geometry of the optical fibers in the bundle is controlled to produce interleaver outputs with no offset. In another embodiment two fibers of the four-fiber collimator are coupled as inputs to and the other two fibers as outputs from a Fabry-Perot etalon. The geometry of the fibers and the focal length of the collimator are controlled to produce two outputs with peaks offset by a predetermined amount.

Abstract: In a grating based tunable filter, an input beam that includes multiple wavelengths is directed into an input fiber of a circulator. The beam is collimated and then is reflected by a rotationally actuated mirror. The beam is then magnified and propagates onto and through a transmission diffraction grating which causes the different wavelengths of the transmitted beam to deflect into different angles.

Abstract: A DPSK demodulator is implemented in Sagnac interferometer configuration with a delay line element introduced in one or both of the optical paths of the transmitted and reflected beams. Because the two reflected and transmitted beams travel on the same optical path (though in opposite directions), the Sagnac interferometer provides all the advantages of a common-path interferometer, is thermally and mechanically stable, and the phase requirements are greatly reduced. In its simplest form, the Sagnac DPSK demodulator. In the preferred embodiment, the beam-splitting surface and one of the mirrors are combined into a rhomb beam-splitter structure and the other two mirrors into a right-angle prism. In a DQPSK embodiment, the input beam is split by an upfront beam splitter into two parallel beams that are then directed toward the rhomb-beam-splitter/right-angle-prism combination of the invention.

Abstract: A switchable free-spectral-range mode selector is used to change the free spectral range of a free-space delay-line interferometer. The mode selector consists of a rotatable device with at least one transparent plate selected to produce the desired change in the free spectral range of the delay-line interferometer. The device may be rotated in and out of the free-space optical path of on of the interferometer arms. If used as a DPSK demodulator, the device enables operation at multiple predetermined free spectral ranges. In the preferred embodiment, the demodulator includes a 50/50 beam-splitter cube combined with two cavities. The mode selector consists of a plurality of different transparent slabs attached to a rotatable shaft so that any one of the slabs or none may be inserted in the appropriate optical path to obtain the desired FSR mode of operation.

Abstract: Rhomb beam splitters are used to implement various optical demodulators, in particular PSK and DPSK demodulators. The parallel beam-splitting and reflective surfaces of the rhomb structure produce reflection and transmission beams that exit the beam splitter parallel to one another and with a relative phase shift determined by the distance between the surfaces. These features afford the advantages of low cost, compactness, easily tunable, and temperature-insensitive optical structures for constructing a variety of optical demodulators.

Abstract: Rhomb beam splitters are used to implement various optical demodulators, in particular PSK and DPSK demodulators. The parallel beam-splitting and reflective surfaces of the rhomb structure produce reflection and transmission beams that exit the beam splitter parallel to one another and with a relative phase shift determined by the distance between the surfaces. These features afford the advantages of low cost, compactness, easily tunable, and temperature-insensitive optical structures for constructing a variety of optical demodulators.

Abstract: The invention provides a wedge pair suitable for use in interferometers. The wedge pair produces a phase shift between beams of light propagating in the interferometer. The invention includes a wedge pair and a mechanism for translating a first wedge of the pair with respect to a second wedge of the pair, where the first wedge has the same wedge angle and material of the second wedge and where the vertex of the first wedge and the vertex of the second wedge are pointed in opposite directions.

Abstract: An absorptive coating is in contact with an optical beam splitter such that a signal beam and a reference beam incident on the beam splitter from opposite sides are can be combined to produce two interference beams having a phase difference of about ? 2 .

Abstract: An optical spectrum analyzer is implemented with a detector combined with a tunable filter mounted on a stage capable of 360-degree rotation at a constant velocity. Because of the constant rate of angular change, different portions of the input spectrum are detected at each increment of time as a function of filter position, which can be easily measured with an encoder for synchronization purposes. The unidirectional motion of the mirror permits operation at very high speeds with great mechanical reliability. The same improvements may be obtained using a diffraction grating or a prism, in which case the detector or an intervening mirror may be rotated instead of the grating or prism.

Abstract: A rhomb beam splitter, rather than a conventional cube, is used in combination with a mirror reflecting both the reflected and transmitted beams to obtain a delay-line interferometer demodulator with a substantially common path. This significantly reduces the sensitivity of the device to environmental changes and also greatly simplifies the manufacture process. A polarization-insensitive coating or a wave plate, a thermal phase tuner with a micro-heater, and two compensator plates are used in the paths of the beams to balance the optical path lengths and to compensate for polarization and environmental effects.

Abstract: A switchable free-spectral-range mode selector is used to change the free spectral range of a free-space delay-line interferometer. The mode selector consists of a rotatable device with at least one transparent plate selected to produce the desired change in the free spectral range of the delay-line interferometer. The device may be rotated in and out of the free-space optical path of on of the interferometer arms. If used as a DPSK demodulator, the device enables operation at multiple predetermined free spectral ranges. In the preferred embodiment, the demodulator includes a 50/50 beam-splitter cube combined with two cavities. The mode selector consists of a plurality of different transparent slabs attached to a rotatable shaft so that any one of the slabs or none may be inserted in the appropriate optical path to obtain the desired FSR mode of operation.

Abstract: An interferometer includes a means for splitting, at a splitting location, an input light beam into a first beam and a second beam; and means for recombining, at a recombination location, the first beam and the second beam. The interferometer is designed such that the first beam will travel a first optical path length (OPL) from the splitting location to the recombination location, and the second beam will travel a second OPL from the splitting location to the recombination location and such that when the input light beam has been modulated at a data rate comprising a time interval, then the difference in optical path lengths between the first OPL and the second OPL is about equal to the time interval multiplied by the speed of light.