Abstract: Near field imaging using a THz imaging system is realized by utilizing an interferometric imaging detector array that includes detector elements disposed on a surface curved, physically or artificially, to match substantially the curvature of the wave front for received THz signals. Generally, the near field is an environment wherein the distance to an object of interest is on the order of 10–100 times larger than the physical size of the THz imaging array. Typical distances from the object or target to the imaging array is anticipated to be in the 0.5 m–50 m range. Curvature of the detector array corrects a distortion problem in prior THz imaging systems that utilized planar interferometric imaging arrays based on a planar wave front assumption for received THz signals.

Abstract: Low pressure sensing and imperviousness to corrosion and to the effects of harsh environments are achieved in a pressure sensor that employs a flexible membrane supporting piezoresistive elements. A plurality of piezoresistive elements are aligned substantially collinearly across one surface of the flexible membrane. Innermost piezoresistive elements are disposed in such a way that they experience tension in response to an applied pressure, whereas outermost piezoresistive elements are disposed in such a way that they experience compression in response to the same applied pressure. Contact pads for each end of each piezoresistive element allow the elements to be configured in any number of desirable arrangements. In one exemplary embodiment, four piezoresistive elements are disposed along a main central axis of the membrane. The contacts of the elements are connected to form a Wheatstone bridge.

Abstract: An interferometer having semiconductor optical amplifiers in one or both arms of the interferometer converts information from a first wavelength to another predetermined wavelength. The original data signal (at the first wavelength) is coupled into the interferometer in such a way that it is confined to propagate, either co-directionally or contra-directionally, in only one arm of the interferometer together with the lightwave signal at the predetermined wavelength. This arm includes the semiconductor optical amplifier. In this way, the original data signal is utilized effectively to maximize the interference effect, which is required for wavelength conversion, while minimizing the problems associated with simultaneously saturating amplifiers in the interferometer structure.

Abstract: A menu level indicator facilitates ease of use of a telephone terminal by a user by providing easily understandable information about selectable menu options available in an interactive display on the telephone terminal. The menu level indicator provides a visual indication for permitting the user to directly determine the number of menu options available while at one of multiple selectable menu levels in the menu. The menu level indicator also provides a visual indication for the user as to the relative position of a displayed, selectable menu option with respect to other selectable menu options commonly located at one of the multiple selectable menu levels. As a user maneuvers through the menu levels, such user is able to quickly develop a mental model of the feature menus and easily navigate through these menu selections.

Abstract: Optical system architectures with improved spatial resolution are provided in which the radiation useful for THz spectroscopy and other investigative procedures can be directionally coupled, in a highly efficient manner, into and out of photoconductive structures such, for example, as dipole antennas. An optical system constructed in accordance with an illustrative embodiment of the present invention comprises a source for emitting radiation in a range of frequencies within from 100 GHz to 20 THz, a coupling lens structure for coupling radiation emitted by said source into free space, at least one collimating optical element for collimating received coupled radiation into a beam having a substantially frequency independent diameter and substantially no wavefront curvature, and a detector for detecting the beam collimated by the at least one collimating optical element.

Abstract: A tunable laser whose lasing frequency can be changed by varying the absorbance of absorber layers that are preceding a standing wave of the laser cavity. Changes in the absorbance of the layers are accomplished by varying the strength of the absorber layers through application of voltage to the layers.

Abstract: A cryptographic module, such as a smartcard, is designed to a) store decrypting software programs, and information indicative of predetermined conditions under which an escrow agent is enabled to use the software programs stored on the module to decrypt encrypted data files, and b) records for audit purposes, information indicating every time the software programs are used for decryption.

Abstract: Methods and apparatus are disclosed for using information about a calling party's identity or the importance of a call to control the call waiting signal played to a called party that is busy on another call. Upon determining that a called communication station is busy on another call, information about the identity of the calling party is obtained from the calling station. This information is processed to determine call routing treatment, and particularly to determine whether a call waiting signal is to be played for the call and, if so, what type of call waiting signaling is to be played. The called party can determine information about the identity of the calling party from the call waiting signaling, and thus determine whether to interrupt the call in progress to respond to the call waiting signal.

Abstract: Low optical loss and simplified fabrication are achieved by a nonlinear reflector which incorporates one or more semiconductor quantum wells within a standard semiconductor quarter wave stack reflector. The nonlinear reflector provides an intensity dependent response which permits it to be used for saturable absorption directly in a main oscillating cavity of a laser. Saturation intensity of the nonlinear reflector and thereby related laser modelocking properties can be controlled by disposing the quantum well at a particular position in the reflector structure.

Abstract: Certain material and objects can be characterized by a frequency-dependent absorption, dispersion, and reflection of terahertz transients in signals which pass illuminate the material or object. The present terahertz imaging system analyses that frequency dependence in the time-domain by collecting that transmitted signal propagating through the object and then processing the information contained in those signals for every point or "pixel" on that object. This is a non-invasive imaging technique that is capable of differentiating between different materials, chemical compositions, or environments.

Abstract: Methods and apparatus are disclosed for providing increased security in a telecommunications network by using quasi-time domain reflectometry techniques to identify those telephone calls which comprise multiple legs. Echo data are collected for the telephone call from a predetermined point in the network to a point where the call originated. The data are processed to generate an indication of whether the telephone call comprises multiple legs, thus identifying those calls most susceptible to unauthorized use. The indication that a telephone call comprises multiple legs is advantageously used together with call attribute information, such as whether the call is placed to an international destination, to determine whether a given multiple-leg call is most likely a valid access to the communication system or most likely fraudulent.

Abstract: A bridge connected to an FDDI network preprocesses messages each comprising source address (SA), destination address (DA) and data, using a data base of SA's maintained in a content addressable memory (CAM). A finite state machine working with the other elements of the FDDI bridge acts as an intelligent controller, to select which messages are to be passed to a buffer memory for further processing and ultimate transmission to a remote network. Messages are only passed to the buffer memory if the DA is different from the SA's stored in the CAM. Circuitry is advantageously provided not only to "teach" the CAM about SA's on the network, but also to monitor information relating to the the last activity in a station associated with a particular SA, so that "stale" SA's can be purged from the CAM.

Abstract: Stimulated Brillouin scattering has been effectively suppressed and the effect of residual amplitude modulation has been mitigated in a dithered optical transmitter by injecting the dithered optical signal into an optical device, such as a highly saturated amplifier, whose small-signal intensity response at the dither frequency dampens the residual amplitude modulation thereby decreasing the eye closure (alternatively, increasing the eye pattern opening). The optical device (e.g., the highly saturated amplifier) exhibits a low frequency cutoff (-3 dB) at a frequency greater than or equal to the inverse of the round trip time for a fiber span with the effective optical transmission length into which the optical signal power is launched. The dither rate is generally in the neighborhood of or below the low cutoff frequency.

Abstract: Method and apparatus of soliton transmission using a non-soliton source are described. The soliton transmission is generally in the form of an optical pulse sequence, Intensity and phase modulation of a continuous wave optical signal generates a pair of optical pulse sequences related to Stokes and anti-Stokes components of the doubly modulated optical signal. By proper alignment of the optical frequency of the cw optical signal with the passband of a filtered transmission line, it is possible to have the filtered transmission line comprising sliding frequency-guiding filters permit only one of the pulse sequences to be selected and thereby form a soliton pulse sequence.

Abstract: An air conditioning control system monitors the temperature in the vicinity of electronic equipment using one or more sensors placed either in the equipment area or at the outlet of an air conditioning duct or vent. The temperature sensors supply signals to a microprocessor that is programmed to adjust the output of a system controller such that the amount of cooling provided by the air conditioning system is controlled. Illustratively, when the air conditioning system is restarted after a power failure, the power applied to one or more air conditioning fan motors is controlled, such as by throttling the motors down to an acceptable speed, to in turn control the cooling effect on the equipment. The amount of cool air produced by the air conditioning system is detected by the sensors, thereby forming a feedback control loop.

Abstract: Telephone calls are directed from a first telecommunications platform to a second telecommunications platform, together with an identifier which uniquely identifies the call and enables telecommunication switches to monitor and maintain control over the call. The unique identifier enables the call to be returned from the second platform to the first platform. Once returned to the first platform, the call can be directed to a third telecommunications platform (and, subsequently, back to the first platform), without requiring the caller to place another call or to re-verify the caller's identity. The call identifier is used either to automatically return the call to the first platform after the occurrence of a predetermined event, or upon detecting predetermined, caller-dialed keystrokes indicating that the caller wishes to return to the interexchange carrier platform.

Abstract: A tunable laser capable of high speed operation is realized by modifying a tunable laser incorporating an N.times.N wavelength grating router to have an output waveguide positioned for capturing second order diffraction optical energy from the router free-space region. The second order diffraction-optical energy is supplied as the laser output at the desired wavelength to which the laser is tuned.

Abstract: Improved PMD performance is achieved in fiber optic cable fabricated in accordance with the principles of the invention by twisting each of a plurality of optical fibers in a controlled manner as the cable is being formed. A controlled amount of twist is imparted to each optical fiber uniformly along its entire length. Uniformity of twist reduces twist-induced strain and promotes optimum PMD reduction.

Abstract: Photocurrent suppression is achieved without deleteriously affecting modulation performance in a surface normal, electro-absorption, quantum well modulator by introducing a sufficient number of non-radiative recombination centers in the quantum well region of the modulator. The presence of the non-radiative recombination centers significantly shortens the lifetime of photogenerated carriers and, thereby, suppresses the photocurrent. Modulation performance characteristics such as contrast ratio are maintained at acceptable levels even though exciton broadening occurs in the quantum wells. The present modulator exhibits a careful balance between defect density in the quantum wells and the acceptable degree of exciton broadening necessary to preserve quantum effects.

Abstract: An electronic synapse circuit is disclosed for multiplying an analog weight signal value by a digital state signal value to achieve a signed product value as a current which is capable of being summed with other such synapse circuit outputs. The circuit employs a storage multiplying digital-to-analog converter which provides storage for the analog weight signal value. Additional circuitry permits programming different analog weight signal values into the circuit, performing four-quadrant multiplication, generating a current summable output, and maintaining the stored analog weight signal value at a substantially constant value independent of the digital state signal values.