Optical Fiber Delay Lines Research Articles

Fiber optic delay line memory

A model for digital synchronous delay line memory (DSDLM) is given, yielding limitations on the maximum number of bits that can be reliably stored for a given change in delay medium temperature and amount of medium dispersion. More than 22 million bits can be stored when single-mode optical fiber is used as the medium, but only if operated at the wavelength of minimum dispersion and by limiting thermal fluctuations to within 0.002 degrees C. A DSDLM is being constructed using such fiber, along with lithium niobate directional couplers as the switching elements. Signal regeneration errors, switch crosstalk, and polarization losses are negligible for the implementation. With a modulation frequency of 100 MHz, a single-line 2000-bit memory can be reliably operated without thermal compensation given a temperature fluctuation of <80 degrees C.

‘Synchronous’ CDMA demonstration for fibre-optic networks with optical processing

‘Synchronous’ code-division multiple-access (S/CDMA) for fibre-optic networks with optical processing is investigated. A two-user experiment demonstrating the feasibility of such a network is performed at 10 Mbit/s (500 Mchip/s) using modified prime sequence codes and optical processing. Fibre-optic delay-lines are used for code sequence generation and correlation without using fast electronic components. An environment in which S/CDMA would be particularly suitable is discussed.

Optical steering of dual band microwave phased array antenna using semiconductor laser switching

The first microwave phased array antenna steered by optical delay lines is presented. Delay times specified by the antenna steering angle were implemented by switching bias currents of high speed lasers pigtailed to the fibre optic delay lines. No beam squint was observed in the radiation pattern of the antenna as its frequency was switched from the L- to the X-band. >

Wide-band fiber optic signal processor

Operation of a 16-tap fiber-optic delay line configured as a bandpass filter with a fundamental frequency of 2.0 GHz over the frequency range from 0 to 12.5 GHz is discussed. Equal tap weights and spacings were used to facilitate comparison of experimental and predicted results. More than 10/sup 11/ analog multiplications per second are performed in this relatively simple signal processing system. Other processors for performing functions such as matched filtering and correlation can be implemented by appropriate choice of the mirror reflectances and fiber lengths.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Prototype Binary Fiber Optic Delay Line

The operation and features of an electronically switched, programmable binary fiber optic delay line are discussed. The design, component selection, fabrication, testing, and evaluation of a prototype are presented. The prototype comprises seven delay stages, has a maximum delay of 5µs and a resolution of 39 ns, operates over the 0.5 to 1 GHz band with a dynamic range of 36 dB, and has a power consumption of 19 W.

Phase induced intensity noise in concatenated fiber-optic delay lines

A fiber-optic differential-delay-line structure converts the phase noise of its short coherence source to intensity noise at its output. If this structure is followed by a similar system, intensity noise is generated as well as filtered by each of the two systems. The power spectrum of the noise at the output of concatenated systems built of Mach-Zehnder and the recirculating delay lines is theoretically studied. The spectral structure of phase-induced intensity noise at the output of these concatenated systems is analyzed, taking into account polarization effects and emphasizing the physical interpretations of the results. An experimental verification is presented for the case of two recirculating delay lines in series.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Switched optical delay-line signal processors

In conjunction with broad-band switching matrices, optical fiber delay lines might provide versatile controllable signal processing devices. Methods of composing single and multiple variable delays and both recirculating and nonrecirculating dynamically configurable lattice filters are described. Over the response bandwidths of semiconductor lasers and detectors, optoelectronic matrices are particularly suitable for this application.

Optoelectronic time division multiplexing

The concept of using optoelectronic (photoconductive) switches as the sampling element in time division multiplexing is introduced in the context of VLSI off-chip data transmission. A 4:1 multiplexer was fabricated in Cr : GaAs, activated by a GaAs laser via optical fibre delay lines and operated at 2.5 Gbit/s.

Fiber optic transversal filters for RF direction finding

Abstract A wide-band electro-optic direction-finding (DF) processor employing a network of fiber optic delay lines is described. This DF filter offers a potential instantaneous bandwidth of 20 GHz and allows for multiple, simultaneous beam-angular responses with peaks that are independent of frequency. Preliminary results of an eight-laser feasibility test model DF device with two transversal filters show good agreement between predicted and measured angular responses. This experimental matched-delay filter operates in the 200–1,000 MHz frequency range and can simultaneously monitor two angles of arrival. The laboratory results obtained in this investigation suggest that the fiber optics matched-delay filter can be useful for wide-band direction finding.

An experimental photonic time-slot interchanger using optical fibers as reentrant delay-line memories

The system interchanges data from time slots in its input to time slots in its output. Carried by single-mode optical fibers and switched by lithium-niobate directional couplers, the data remains in photonic form from input to output. The data format is a series of frames, each with a start-bit and N time slots with B bits each. An electronic controller reads the arbitrary one-to-one assignment of N input time slots to N output time slots (time-slot permutation) and orchestrates photonic directional couplers to steer the data among input, output, and N recirculating fiber-optic delay lines. While a prototype with N = 3 , B = 8 , and a pedestrian 90-MHz bit rate is a 3 × 3 switch, where each channel carries 28.8 Mbit/s, the basic design is not limited to these numbers.

Parametric stability of self-modulation oscillations in a ring system formed by a laser and a fiber-optic delay line

An analysis is made of the influence of variation of the effective delay time of light in a waveguide and also of the parameters of the waveguide and other components of the system on the frequency of self-modulation oscillations in a ring system formed by a laser, a fiber-optic delay line, and a photodetector. It is shown that such a system can be used as a sensitive fiberoptic detector and also in determination of the dispersion properties of the waveguide. The results are given of an experimental investigation.

Ultrafast All-Optical Synchronous Multiple Access Fiber Networks

Two synchronous multiple access schemes, TDMA and CDMA, are proposed for fiber optic networks using optical signal processing. Network synchronization is achieved by using a central modelocked laser which also serves as the source for each station. The data are converted into a high-bandwidth optical signal using electrooptic modulators. The accessing schemes use optical fiber delay lines. The feasibility of these schemes is discussed.

TDMA fibre-optic network with optical processing

A fibre-optic network with time-division multiple access (TDMA) is implemented using optical processing. The network is capable of accommodating 50 stations transmitting at 10Mbit/s. Synchronisation is achieved using a central optical source with 2ns pulses. Integrated electro-optic modulators and optical fibre delay lines are used to multiplex the stations.

Fast position-sensitive shadowmeter

A simple optical detector of position versus time providing submillimeter spatial and nanosecond time resolution is described. The spatial beam profile of a nanosecond-pulse-width nitrogen laser is sampled simultaneously when the UV light strikes multiple turns of a spiral fiber-optic delay line. Fluorescent light coupled into the fiber converts the profile samples into serial bursts of light. These bursts are detected by avalanche photodiodes; electrical pulses are recorded on fast oscilloscopes. An interposed absorbing or refracting material casts a ‘‘shadow’’ which is measured by comparison with a reference recording. Applications are suggested, and two long-time-base recording methods are described.

Microwave-frequency response of an optical-fibre delay-line filter

The frequency response of a single-mode fibre recirculating delay-line filter has been measured from 0 to 18 GHz using a broadband measurement system that includes an inteferometric waveguide modulator and a high-speed photodetector. An optical-fibre notch filter having a fundamental passband at 740 MHz exhibited 23 overtones that were uniform to within ±1.5 dB over the entire measurement range.

Gigahertz analog repeater for fiber optic delay lines

A gigahertz analog fiber optic repeater is used to extend the achievable delay time for radar delay line applications. The repeater consists of a silicon avalanche photodiode (APD), a wide-band amplifier, and a GaAlAs laser diode transmitter. This repeater has an optical gain of 14.5 dB, a 42 dB electrical dynamic range, and a noise figure of approximately 6.5 dB. The frequency response is flat within ±2 dB over the frequency range from 10 MHz to 1.3 GHz. The nanosecond pulse fidelity is such that the subtraction between input and output pulses is 20 dB below the pulse amplitude.

Time delay multiplexing of optical spectra with a fiber optic array

Fiber optic delay lines, a grating monochromator, a single photomultiplier, and time-correlated photon counting techniques are combined to make a 30-channel absorption spectrometer. The transit time delay in a series of optical fibers of different lengths permits the sequential arrival at the detector of the various wavelength components of a light pulse. While of restricted practicality because of the required low counting rate, the instrument suggests the feasibility of single pulse spectral measurements with a fiber optic multiplex spectrometer.

Noncoherent radar moving target indicator using fiber optic delay lines

A prime candidate for detection of low flying targets over the sea surface is high resolution radar with a noncoherent delay line canceller. Fiber optic delay lines are capable of providing the required large timebandwidth (TB) product. Optimum information capacities in the spectral region around 1.3 {\mu} m are estimated theoretically to correspond to timebandwidth products of 10^8 and 10^6 for single-mode (based on 1-nm source spectral width) and graded-index multimode fibers, respectively. Pulse-to-pulse subtraction with 17-dB cancellation has been experimentally demonstrated using a 940- m graded-index multimode fiber as delay line. Higher cancellation ratios are anticipated from the use of wider bandwidth fibers.

Encoding-decoding optical fibre network

Two multicoupled multimode fibres can act either as an encoder or as its matched-filter decoder, depending on which terminals are used. Such a network which may find applications in communications and microwave systems has been demonstrated using fused T couplers, fibre connectors, and multimode optical-fibre delay lines.

Fibre-optic delay-line devices for r.f. signal processing

A bandpass filter and a discriminator for r.f. signals have been constructed by using fibre-optic delay lines, and their frequency-response characteristics have been investigated. Both devices had a fundamental frequency of 193 MHz, and the filter had a 3 dB passband of 10 MHz.