Abstract: A graded index, multimode optical waveguide. The waveguide is characterized by a graded profile region, followed by an abrupt drop in index, then a region of constant index. At predetermined intervals, the fiber is modified to have a conventional graded index profile to suppress unwanted modes propagating along the guide.

Abstract: A complementary distortion circuit for reducing the level of IM products in the output of a TWT amplifier. One embodiment of the invention includes a 3 db, 90.degree. coupler to divide the input signal into two components of equal amplitude but differing phase with an IM generator positioned in the path of each component. The IM generator may comprise non-linear phase modulators or non-linear amplitude modulators. The two components are then re-combined in a second, 3 db, 90.degree. coupler. The non-linear phase-modulators may comprise varactors and the non-linear amplitude modulators may comprise varistors. The unit is intended to operate in the microwave region.

Abstract: An optical waveguide in which intermodal coupling is deliberately induced. Perturbations are introduced along the waveguide. The perturbations have a spatial frequency that varies along the guide from f.sub.1 to f.sub.2, where f.sub.2 is .ltoreq. the critical frequency above which coupling to unguided modes occurs and f.sub.1 corresponds to the beat between the n.sup.th and the (n + 1).sup.th modes, the lowest modes of interest propagating in the guide.

Abstract: Digital transmission systems operating over microwave radio generally employ phase modulation rather than amplitude modulation because phase modulation is less sensitive to non-linearities in the transmitter. Coherent demodulation is typically employed in the receiver to ensure maximum immunity to the thermal noise of the radio receiver. This, in turn, requires that the receiver include circuitry to recover an unmodulated carrier from the incoming digital signal for use as a phase reference in the demodulation process. Unfortunately, the phase of the recovered carrier is subject to ambiguities which may result in the demodulated digital signal being transposed and/or inverted, resulting in gross transmission errors. Differential encoding of the digital signal, prior to modulation, overcomes this problem but may create additional problems, such as the inability of the receiver to monitor transmission errors by means of the parity bits which are included in the digital signal to be transmitted.

Abstract: Third order IM distortion in a microwave repeater is measured in situ by applying a small amount of low frequency amplitude-modulation to the payload signal while at the same time adding a test-tone of fixed frequency and amplitude. At the repeator's output, a pair of synchronous detectors, driven in phase quadrature by the test-tone generator, measure the modulation depth of the test-tone. This modulation depth is now a measure of the degree of distortion in the repeater.

Abstract: Microwave repeaters and the like are tested for third-order IM distortion by applying two, swept-frequency test-tones to the repeater. The test-tones are offset by a fixed frequency difference .delta.. A detector circuit, connected to the output of the repeater, measures the amplitude of any signal component of frequency 2.delta. which is a direct measure of the repeater's non-linearity. Because a signal component of frequency 2.delta. can be generated by second-order distortion within the detector itself, the detector includes special circuitry to cancel out any such unwanted term.

Abstract: A circuit for synchronizing a microwave oscillator with a reference source of known frequency. The synchronizing circuit includes a phase-lock loop which compares the output of a free-running, crystal-controlled, voltage-controlled oscillator with the output of a burst oscillator. The output of the burst oscillator has a comb spectrum comprising harmonics of the reference frequency and the phase-lock loop locks onto a selected one of the comb harmonics.

Abstract: A protection switching system for microwave radio employs a frequency-agile-repeater (FAR) which is automatically re-tuned to the frequency of a failed repeater. A shutter associated with the failed repeater reflects the input signal normally employed by the failed repeater into the FAR, while the output of the FAR is reflected by a second shutter into the transmitting antenna.

Abstract: A conventional phase-lock loop circuit is modified for use at microwave frequencies. The modification includes a low-frequency oscillator connected in the phase-lock loop to sweep the voltage-controlled oscillator through the frequency of the incoming signal thereby ensuring that lock-in is achieved. The oscillator is automatically turned off when "lock-in" is achieved, thereby reducing residual phase error.

Abstract: In accordance with the disclosed invention, a first group of optical fiber waveguides is spliced to a second, similar group of optical fiber waveguides using a unique foldable holder. The holder illustratively comprises a relatively thin wafer formed of a bendable material, such as a plastic or metal, and includes first and second sections disposed about a centrally extended fold line. Each section of the holder includes a plurality of grooves laterally spaced in a parallel relation symmetrically about the fold line for receiving and aligning the individual fibers in each fiber group to be spliced. To achieve the desired splice, the fibers in a first group are placed and then fixed in the grooves of the first section of the holder, while the fibers in a second group are placed and then fixed in the grooves of the second section of the holder.

Abstract: An optical cable which is relatively immune to the effects of bending and/or tension comprises a helical optical fiber loosely confined within a protective sheath. Apparatus is disclosed for fabricating the helical fiber. For example, as the fiber is drawn from a heated preform the fiber is subjected to periodic, pulsating jets of an inert gas, such as nitrogen, which physically deform the fiber into the desired helical configuration.

Abstract: Defective repeaters in a multihop, digital microwave transmission system are located by the use of a special test pattern which is generated at the transmitting terminal. At each repeater station, the two outputs of the phase demodulator are compared to determine if the test pattern has been properly received from the upstream repeaters. The special test pattern, S, has the characteristic that S = S(2N); that is to say, the test pattern equals the complement of itself delayed by 2N symbols. This special characteristic makes it unnecessary to employ a differential decoder in the test apparatus at each repeater station.

Abstract: Intelligible crosstalk in microwave radio systems employing single-sideband modulation is substantially eliminated by phase-modulating the AM carrier-wave prior to its modulation by the baseband signal. The phase-modulation may comprise a periodic signal, such as a single sinusoidal tone or pseudo-random noise, or it may comprise an aperiodic signal such as a truly random noise. In this latter event, the coding signal is transmitted to the receiving location to ensure proper demodulation of the encoded signal.

Abstract: A feedbackward, distortion-compensating circuit suitable for pre- and post-distortion compensation of non-linear amplifying devices. The circuit taps-off a portion of the output signal of the compensating circuit, cubes it, and combines it with the input signal to that circuit. Appropriate adjustments to the phase and amplitude of the cubed signal ensure that the output signal of the non-linear amplifying device is free from all significant distortion.

Abstract: Apparatus for interconnecting a time-division multiplex carrier system, such as T1, with a frequency-division multiplex carrier system, such as LMX-3. The interconnexion is effected in an all digital manner by the use of digital filters, thus avoiding the necessity of down-converting either system to voiceband.

Abstract: An adaptive equalizer to compensate for the effects of frequency-selective fading on a microwave radio system. The equalizer includes two Bode-type bump networks and an AGC amplifier whose slope and gain are respectively controlled by pilot tones added to the message load. Importantly, the frequency characteristics of the two Bode networks are non-overlapping, thus ensuring stability.

Abstract: In a digital transmission system of the type which employs cross-polarized radio channels, the failure of a transmitter associated with a particular channel can cause an undesired signal cross-over into the receiver of the failed channel. According to the instant invention, each channel in the system is uniquely encoded by a combination of bitstream inversion and bitstream delay at the head end of the system. Thus, if a signal crossover occurs an excessive number of parity violations are detected in the failed channel and a switch to a protection channel is requested.

Abstract: A process for manufacturing preforms from which clad optical fibers having longitudinal, eccentric, azimuthal index inhomogeneities may be drawn. In one embodiment, the known CVD process for making preforms is modified by the use of asymmetric heating to produce circumferentially alternating deposits of doped and undoped glass which act as the fiber core when the preform is subsequently collapsed and drawn into a fiber.

Abstract: A color coding filter is combined with a single area-imaging charge-coupled-device (CCD) to form a color television camera. Three-phase CCD of frame transfer type provides a 2:1 interlace scanning format, and the color filter is designed for compatibility with this format. The filter is formed from a plurality of different color transmissivity areas arranged in a specific grid-like pattern to provide the distinct filter characteristics required for each of the two interlaced fields. With this filtering, the CCD generates a continuous luminance and two line-sequential chrominance (color difference) signals.

Abstract: Alternate lines of a 526 line video signal are time-stretched and transmitted over a transmission channel of limited bandwidth. At the receiver, the lines are compressed back to their original duration and displayed in an interlaced manner on a conventional receiver by blanking alternate lines in each field. In another embodiment of the invention, adjacent transmitted lines of the video signal are averaged at the receiver and the resultant signals are substituted for the non-transmitted lines of the video signal.