Abstract: A method of observing the core region of optical fibers and fiber preforms is disclosed comprising the step of inducing fluorescence in at least one of the index-modifying dopants present in the core being observed by illuminating said fiber/preform with radiation at the peak absorption wavelength for said dopant, and observing the region between the fluorescing edges of said fiber/preform. The core diameter can be determined by measuring the distance between said edges. This technique can be utilized to control the fiber pulling rate during fabrication.

Abstract: The refractive index profile of an optical fiber preform is controlled during manufacture by illuminating the gaseous precursor being fed to the preform substrate tube with ultraviolet radiation, measuring the intensity of either the uv that traverses the gas or the induced fluorescence, comparing this measurement with a reference signal, generating an error signal, in response to this comparison, and controlling composition of the gaseous precursor in response to said error signal. A similar technique can be used to control the thickness and refractive index of each layer deposited upon the substrate tube by forming an x-ray shadowgraph of the layers as they are deposited and comparing the shadowgraph characteristics with appropriate reference signals to form a second set of error signals. The latter are then used to fine tune the fabrication process. The invention can also be employed in conjunction with the vapor axial deposition method of fabricating preforms.

Abstract: By randomly varying the difference in the phase constants and/or the coupling coefficient along the lengths of two or more wavepaths, an equal division power divider is obtained whose operation is not critically dependent upon its dimensions.

Abstract: High speed, integrated optical directional coupler modulators utilize spaces of the order of one micron between waveguides. While one micron delineation of the waveguides can be readily obtained using conformable mask exposure techniques, diffraction effects become significant using thick glass masks required to delineate the electrodes. To overcome this problem, a two-process electrode fabrication procedure is employed. In accordance with the procedure a first pair of electrodes are deposited by a lithographic process. However, this yields a gap between electrodes that is wider than the desired gap. To reduce this distance, the process is repeated with the gap forming region of the mask laterally displaced. The result yields an electrode pattern with the desired spacing and thickness.

Abstract: An optical TE.revreaction.TM mode converter, using highly birefringent materials such as lithium niobate, is wavelength sensitive and, as such, can be used as a wavelength filter. However, to extract the filtered (i.e., the mode converted) signal, a separate, high quality polarization selective element is required. This application discloses means for eliminating the need for a separate polarization sensing element by using mode conversion as a means for spatially separating the filtered signal. The filter comprises a pair of mismatched waveguides (11, 12), and phase matching means (14, 15) for producing selective coupling between TE mode wave energy in one of the guides and TM mode wave energy on the other guide.

Abstract: This application describes a method and apparatus for measuring the transmission characteristics of a class of nonlinear two-ports containing physically inseparable reactive networks, but which can be characterized by an equivalent network comprising input and output linear reactive networks separated by a resistive nonlinear network. In accordance with the method, three signals, f.sub.1, f.sub.2 and f.sub.3, are applied to the test network where signals f.sub.2 and f.sub.3 are variable and differ by a relatively small amount .DELTA.f. So related, the third order intermod frequency, f.sub.p =f.sub.1 -f.sub.2 +f.sub.3, becomes a constant f.sub.p =f.sub.1 +.DELTA.f. It is further shown that the magnitude of the output voltage V.sub.p at frequency f.sub.p is proportional to the square of the magnitude of the input network transfer function H.sub.A (f).

Abstract: Increased mode dispersion in graded-index, multimode optical fibers due to defects in the index profile can be significantly reduced if the average defect is caused to vanish over the length of the fiber. For fibers with non-optimum .alpha.-values, minimum impulse response can be approached provided .alpha. varies along the fiber, such that the average value of .alpha. is equal to .alpha..sub.opt. The effect of fluctuations superimposed upon the index profile are minimized by introducing an odd number of phase reversals in the fluctuations equally spaced along the fiber.

Abstract: Multimode switching is obtained in a four-port device having controllable switching means (22) comprising a thin Fabry-Perot interferometer including, within the cavity, a voltage controllable absorber (35). By the appropriate dimensioning of the cavity and orientation of the incident radiation, multimode operation is obtained.

Abstract: The coupling between a pair of coupled wavepaths (11, 12) is modulated by means of a biasing wave travelling in synchronism with the signal in the coupled wavepaths. In particular, by limiting the coupling to a small interval about the zero crossover point of the biasing signal, subpicosecond pulses can be generated by means of a simple sine wave biasing signal. Such a velocity matched gate can be employed as a pulse generator; a modulator; a multiplexer/demultiplexer; and pulse shape analyzer.

Abstract: Because orthogonally polarized optical waves see unequal electrooptic coefficients and refractive indices for the same applied voltage, it has been difficult to obtain efficient switching of arbitrarily polarized waves. This difficulty is resolved in a reverse .DELTA..beta. directional coupler (10, 11) which employs spatial tapering of the distance between waveguides (D(z)) for efficient through-coupling, and the appropriate choice of effective coupling strengths (s.sub.TE, s.sub.TM) for the two polarizations for efficient cross-coupling.

Abstract: Continuously variable phase shift is obtained by means of two variable phase shifters (20,21) located in two separate, switchable parallel wavepaths (18, 19). The first phase shifter (20) is variable over, at least, the range between zero and 180 degrees. The second phase shifter (21) is variable over, at least, the range between 180 degrees and 360 degrees. Switching means (22, 23) are provided for switching the signal to one or the other phase shifter. A control circuit (16) simultaneously causing the phase shift through the two phase shifters to vary in the opposite sense. When maximum phase shift is reached, the control circuit switches the signal from one phase shifter to the other, thereby providing continuously variable phase shift through the network.

Abstract: The separate steps of forming an optical fiber preform and then drawing the fiber are combined into an essentially simultaneous process wherein a "stub" preform (18) is formed by means of the so-called "vapor-phase axial-deposition" method while the fiber (17) is simultaneously drawn from the end of the stub that is opposite to the end upon which the particulate matter (14, 15) is being deposited.

Abstract: A transmitter of information to be communicated in secure form is optically coupled to a composite optical fiber at one end, the fiber being connected to a receiver of the secure information at the other end. The composite optical fiber has cores including a central core for carrying the secure information, one or more guard signal cores, a cladding matrix surrounding all the cores, and a metallic or other nontransmissive film surrounding the central core but not the guard cores. A guard signal optical transmitter section is coupled to the guard cores for jamming any leakage of secure information through the film, and a guard signal receiver section is provided at the other end of the fiber for monitoring purposes. The optical transmitter apparatus employs pulse code modulation (PCM) and sends related security bits in the guard and secure signals.

Abstract: A synchronous digital transmission system (12), operating over a prescribed range of bit rates, is interfaced with subscribers (10, 11) whose data sources and receivers have different bit rates by means of transmitter and receiver interface networks (13, 14). The transmitter interface network (13) comprises means (20, 21, 23, 24) for phase-locking an integral subharmonic f.sub.o /N of a variable frequency oscillator (22) to an input clock signal, f.sub.1, associated with an input data stream, and means (25, 27) for encoding the data stream for operation at a bit rate corresponding to the fundamental frequency, f.sub.o, of the local oscillator (22) and including one code violation at a prescribed rate, f.sub.1 /M, related to the input clock frequency f.sub.1. The receiver interface network (14) includes means (30, 31, 32) for decoding the received signal and for recovering the input data stream and clock signal.

Abstract: This application describes a nondestructive method for determining the refractive index profile of optical fibers and fiber preforms by measuring the density distribution of the light that is focused by the fiber/preform core acting as the lens. The fiber/preform is immersed in an index matching medium and illuminated by means of a collimated light beam directed at right angles to the fiber/preform axis. The density distribution of the light focused by the fiber/preform core is then measured along a direction normal to the fiber/preform axis at a distance L from the axis that is less than the focal length of the fiber/preform. The index profile is determined from these measurements by two numerical integrations.

Abstract: Orthogonally polarized waves are more effectively decoupled in a waveguide that is fabricated in a manner so as to deliberately enhance stress-induced birefringence. This characteristic is accomplished by introducing a geometrical and material asymmetry in the preform from which the optical fiber is drawn. Three methods of preparing the preform are disclosed. Optical waveguides capable of transmitting power with only one direction of polarization are desirable for use with integrated optical devices which are polarization sensitive.

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 sensitive, nondestructive method of viewing the internal structure of multilayered optical fiber preforms is described. The method comprises the steps of transversely illuminating the preform, and intercepting the light that traverses the preform on a viewing screen. To obtain a complete picture, the preform is rotated 360 degrees about its axis. The technique provides a detailed view of the core size and eccentricity, and of the structure of the multilayered core.

Abstract: In a duplex optical communication system employing, at each location, the same electrooptic device alternately as a source and as a detector of optical wave energy, and using a single optical fiber to connect pairs of locations, interference produced by reverse Rayleigh scattering along the fiber is minimized by means of R-C equalizers. The modulation signal applied to the electrooptic device (11) during its transmitting period is simultaneously applied to the equalizer (20). During the receiving period, the signal stored in the equalizer is coupled to the local receiver (18), along with the signal from the electrooptic device, wherein it combines destructively with the reverse Rayleigh scattered signal produced along the fiber by the original transmission.

Abstract: An address verification system is described for use in a communications system comprising a transmission path (14) having sequence of remote stations (1, 2, ... n) distributed therealong. After accessing the desired station, a loop-back (32) is established at the addressed station to a second transmission path (15) having a second sequence of stations (1', 2' ... n'). Verification is obtained by transmitting a series of p+1 signal bursts, where p is the total number of stations in the loop-back path. By designing each station so that it deletes one of the signal bursts, a single burst will be received at a verification detector (23) when and if the proper station is addressed. It is an advantage of the invention that it can be used with all types of communication systems. It is a further advantage that it permits the use of identical repeaters at all the remote stations.