Showing papers on "Dispersion-shifted fiber published in 1983"

Extreme picosecond pulse narrowing by means of soliton effect in single-mode optical fibers

Abstract: We report the narrowing of pulses, initially 7 psec FWHM, to widths as small as 0.26 psec by various lengths, short relative to the soliton period, of single-mode, low-loss optical fiber. Since the ~1.5-μm wavelength lies in the region of negative group-velocity dispersion (∂νg/∂λ 10) soliton number. We show these results to be in at least semiquantitative agreement with prediction based on the nonlinear Schrodinger equation.

Fiber optic amplifier

Abstract: An amplifier for use with fiber optic systems comprises a neodymium YAG crystal placed in series with a signal-carrying optical fiber. The ND:YAG crystal is supplied by the optical fiber with both the signal to be amplified, and pumping illumination. The pumping illumination is coupled onto the optical fiber by a multiplexing coupler which is used to combine the signal to be amplified and illumination from a pumping illumination source onto a single optical fiber. The pumping illumination inverts the neodymium ions within the ND:YAG crystal. The signal to be amplified propagates through this crystal to stimulate emission of coherent light from the neodymium ions, resulting in amplification of the signal. Because this arrangement permits the ND:YAG crystal to be end-pumped with pumping illumination, and because the length of the ND:YAG crystal may be substantially greater that the absorption length for the crystal at the wavelength of the pumping illumination, virtually all of the pumping illumination may be absorbed within the ND:YAG crystal and used for amplification of the signal carried by the optical fiber.

Fiber-optic gyroscopes with broad-band sources

Abstract: The use of broad-band sources, polarized and unpolarized, in fiber gyroscopes with linearly birefringent fibers, is studied theoretically. Polarization-mode coupling in the fiber is modeled using one-mode coupling center. Gyroscope output equations are obtained which are useful in interpreting an origin of fiber noise in the limiting cases of low and high fiber birefringence. Interference effects in the output are shown to be related to mode coupling at particular locations in the fiber. The extent of these locations is governed by a depolarization length for which numerical estimates are given.

Amplification and reshaping of optical solitons in a glass fiber-IV: Use of the stimulated Raman process.

Abstract: By use of the stimulated Raman process, optical solitons can be amplified and reshaped while they propagate through a glass fiber. When an appropriate level is chosen for the pump power (10–100 mW for 10-psec solitons), the solitons can be reshaped adiabatically. The method allows the separation between two repeaters (amplifiers) to be decided by the fiber loss rather than by the fiber dispersion.

The stress-optic effect in optical fibers

Abstract: The importance of the photoelastic effect in controlling polarization in optical fibers is discussed. Measurements of the stress-optic coefficient, its dispersion, and temperature dependence are reported using a fiber measurement method. The results compare closely to data obtained for bulk silica by an extrapolation technique. It is shown that the dispersion of the stress-optic coefficient can have a significant effect on the performance of birefringent fibers and of fiber birefringent devices. Furthermore, the temperature dependence is sufficiently large to be troublesome in fiber sensors.

A single-polarization fiber

Abstract: We report an oPtical fiber which guides only one polarization. This fiber polarizer utilizes high modal birefringence ( 4.7 \times 10^{-4} ) to split the two polarizations of the fundamental mode and an ellipitcal depressed-index cladding to provide a steep wavelength-dependent tunneling loss to the fast axis mode. The stress-induced birefringence is created by an elliptical SiO 2 -B 2 O 2 inner cladding. A 4.7-m length of polarizer fiber exhibits an extinction ratio of 34 dB at a wavelength of 633 nm, with polarizing behavior over a bandwidth of 50 nm. Insertion loss of less than 1 dB has been observed for the guided linear-polarization mode. The polarizing band may be shifted from 620 to 525 nm by bending the fiber to a radius of 0.67 cm. Such fiber polarizers should find a variety of applications in fiber sensors, attenuators, isolators, wavelength filters, and tuners.

Semiconductor Laser Amplifier for Single Mode Optical Fiber Communications

Abstract: The recent advent of very low loss single mode optical fibers (< 0.2 dB/km at the 1.55μπι wavelength) opens the possibility for very long distance, high information bandwidth (^1 Gbit/s) communication systems, as transoceanic cable links or trunk networks. With such fibers, especially when chromatic dispersion is negligible, the range is only limited by attenuation, so there would be no need for complete signal regeneration (pulse shaping, timing, etc...). In these conditions, direct light amplifiers would be very attractive as \"on-line\" amplifiers as an alternative to sophisticated optoelectronic repeaters; they could also be used in a receiver as low noise preamplifiers, in combination with a pinphotodiode, especially at the 1.3 ... 1.55 μη wavelength, where avalanche photodiodes (APD) are more \"noisy\" than silicon APDs. It is our purpose to discuss the possibility for semiconductor laser amplifiers to be used in a single mode fiber communication system. We shall first derive the power budget improvement due to the insertion of a laser amplifier in a digital optical communication system. A review of semiconductor laser amplifiers (SCLA) characteristics of the traveling-wave type (TW), Fabry-Perot type (FP) and injection locked laser type (ILL) will then be presented, with emphasis on recent experimental results. Finally, possible applications of these devices in single mode fiber communication systems will be discussed. 2 Noise characteristics of a laser amplifier in an optical communication system

Observation of pulse restoration at the soliton period in optical fibers.

Abstract: Restoration in both shape and spectrum of a (train of) 6.4-psec optical pulses has been observed at the soliton period in a single-mode fiber. The source was an F2+ color-center laser at 1.55 μm, and the fiber was 1.3 km long, which was one soliton period for this pulse width and wavelength. As predicted by the nonlinear Schrodinger equation, pulse restoration occurs despite initial spectral broadening from self-phase modulation and temporal compression as a result of negative group-velocity dispersion acting on the chirped pulse.

Wavelength multiplexing in single-mode fiber couplers.

Abstract: Theoretical and experimental studies of wavelength-division multiplexing in a single-mode fiber optic coupler fabricated by mechanical polishing are reported. The variable spacing geometry of the device allows fine tuning of the center wavelength of operation. Wavelength selectivities ranging from 200 to 35 nm have been experimentally demonstrated, with cross talk ranging from 50 to 10 dB. Selectivity control is simply achieved by proper choice of the interaction length of the coupler. The dependence of the multiplexer behavior on all relevant parameters is investigated and found to satisfy predicted results.

Optical pulse reshaping based on the nonlinear birefringence of single-mode optical fibers

Abstract: We report the observation of strong reshaping of 3-psec dye-laser pulses by nonlinear birefringence during passage through a 150-cm-long, single-mode optical fiber and a crossed polarizer. For lower-intensity input pulses to the fiber, the transmitted pulses were observed to be proportional to the cube of the input pulses. With increased intensity, more-complicated pulse shapes were obtained. Our experimental results agree well with the predicted intensity dependence of the reshaping action.

How modal noise in multimode fibers depends on source spectrum and fiber dispersion

Abstract: The speckle contrast for multimode fibers, and thus the modal noise, is essentially given by the impulse response of the fiber and the power spectrum of the source. Theoretical and experimental results show that laser sources with a large spectrum of narrow longitudinal modes may cause high speckle contrast and important modal noise over more than 1-km length in graded-index multimode fibers.

Measurement of fiber birefringence by wavelength scanning: effect of dispersion

Abstract: Scanning the wavelength in high-birefringence fibers inherently measures the group delay difference, and not the phase delay difference, between the two polarization modes. Dispersion of the birefringence will result in a 10–20% error in fiber beat lengths interpolated from such measurements, even if only stress birefringence is present. Geometrical dispersion can introduce even larger errors in some fibers.

Optical fiber gyro

Abstract: An optical fiber gyro system provides a pair of separated optical paths containing partial common portions. A phase difference Δθ is detected using a plurality of detectors which yield sin and cos functions in Δθ. Functions are formed as linear couplings of these functions, and the phase difference Δθ is obtained using the function of best sensitivity for the given range of Δθ.

Depolarization in a single-mode optical fiber

Abstract: The reduction of the degree of polarization of broad-band light due to propagation in ordinary single-mode fiber is examined theoretically and experimentally. Previous work is extended to account for polarization-mode coupling along the fiber by developing a model for one discrete mode-coupling center and extending it qualitatively to include multiple centers. The existence of nonzero degree of polarization in long lengths of fiber is shown to be due to mode coupling at particular positions along the fiber and the degree of polarization is shown to be related to the degree of coherence associated with the mode-coupling site. The experimental results generally support the model developed.

Third-harmonic and three-wave sum-frequency light generation in an elliptical-core optical fiber

Abstract: Third-harmonic generation and three-wave sum-frequency light generation in a short-length elliptical-core optical fiber pumped by pulses from a Q-switched and mode-locked Nd:YAG laser, operating at 1.064 μm, have been observed in the 355–385-nm spectral range. In particular, the third harmonics of the pump (λ0 = 1.064 μm, λ0/3 = 354.7 nm) and of the first Raman Stokes (λ1 = 1.116 μm λ0/3 = 372.0 nm) lines have been obtained with 5-kW peak power of the laser pulses.

Sensitive all-single-mode-fiber resonant ring interferometer

Abstract: A new type of fiber interferometer is presented which utilizes a high-finesse all-single-mode-fiber ring resonator. The measured sensitivity to periodic phase shifts induced in the fiber ring is 1.0 \times 10^{-6} rad/ \sqrt{Hz} or better over the 100-Hz-10-kHz frequency range with maximum sensitivity of 1.9 \times 10^{-7} rad/ \sqrt{Hz} at 10 kHz. This sensitivity, while high, is not shot-noise limited and appears to be determined by the spectral bandwidth of the single-frequency laser source.

Comparison of efficiency and feedback characteristics of techniques for coupling semiconductor lasers to single-mode fiber

Abstract: The coupling of CSP lasers to single-mode fibers with different coupling structures made on the fiber face is investigated. In this case easy to make coupling arrangements such as tapers and microlenses, result in a high launching efficiency (∼2-dB loss), in contrast to launching from gain-guided lasers with strong astigmatism and a broader far-field pattern. Index-guiding lasers exhibit, however, a higher sensitivity to optical feedback. Laser output power and wavelength are changed due to reflections from the fiber tip. Critical distances exist which lead to a highly unstable laser spectrum. A comparison of the influence of various fiber faces on laser power and wavelength stability is presented. It is concluded that a tapered fiber end with a large working distance reduces the influence on the laser's performance.

Temperature dependence of stress birefringence in an elliptically clad fiber.

Abstract: The stress birefringence in a fiber with a three-component silica-glass elliptical cladding is found to be a strongly nonlinear function of temperature. The observed birefringence is a factor of 1.6 greater than that predicted from linear approximations of this dependence and estimates of the fiber material’s properties. The fiber’s birefringence can be predicted from a linear extrapolation of the low-temperature data.

Stabilized fiber optic sensor

Abstract: A fiber optic sensor comprises a length of optical fiber, forming a loop, and a fiber optic directional coupler for optically closing the loop. The loop and coupler form a resonant cavity for light circulating therethrough. A PZT cylinder, about which the fiber loop is wrapped, is utilized to control the total round trip phase delay of the circulating light, and thus, control the intensity of the optical output signal. The phase delay is adjusted to a point where the optical output signal is at maximum sensitivity to changes in phase. When the fiber loop is exposed to, e.g., acoustic waves, the loop length changes correspondingly, thereby causing the phase delay, and thus, the optical output signal to vary. By detecting variations in output signal intensity, the frequency and intensity of the acoustic waves may be determined. The sensor also includes a feedback system for stabilizing the fiber loop against low frequency thermal drift.

Demodulation of interferometric sensors using a fiber-optic passive quadrature demodulator

Abstract: The passive quadrature demodulator (PQD) eliminates the phase stretcher and feedback electronics frequently used in fiber interferometric sensors by passively extracting the desired signal using two distinct interferometers which differ in phase by \pi/2 . A fusion technique is described to fabricate a fiber PQD which is sufficiently stable with respect to temperature, polarization, and wavelength to maintain the sensitivity of interferometric sensors constant to 0.25 dB.

An all single-mode fiber Michelson interferometer sensor

Abstract: A Michelson interferometer made from a single-mode fiber directional coupler with silver mirrors deposited on fiber ends is presented. This interferometer is simple to fabricate and has a theoretical visibility of unity, irrespective of the splitting ratio of the coupler. A theoretical analysis of its performance is described. Locking to quadrature was achieved by tuning the frequency of the laser source at 1.52μm. Results are presented for the interferometer operating over one kilometer of single-mode fiber, in a passive remote sensing configuration.

Measuring chromatic dispersion of fibers

Abstract: The chromatic dispersion of an optical fiber such as a 10 to 50 km length of single mode fiber is measured by launching into the fiber the outputs of at least three commonly modulated semiconductor lasers having output wavelengths close to a region of minimum chromatic dispersion of the fiber. At a remote end of the fiber the relative phases of the signals from the three lasers are determined and a chromatic dispersion profile of the fiber is derived from the measured relative phases and the output wavelenghts of the lasers by assuming a parabolic relationship between signal propagation times and wavelength. The profile can be determined by defferentiating a parabolic function fitted to the three date points derived using said three lasers or by directly generating a linear relationship between chromatic dispersion and wavelength on the basis of two values of chromatic dispersion derived from these three data points.

Progress in monomode transmission techniques in the United Kingdom

Abstract: The key results obtained from experimental systems studies over monomode fiber and cable links are given for both direct detection and coherent optical systems. Direct detection techniques have been studied at 1300 nm using conventional lasers and at 1500 nm with narrow line transmitters of the injection-locked and external cavity types. In the case of coherent optical systems, developments of the same transmitters have been used in conjunction with amplitude and phase shift key (PSK) modulation techniques to obtain improved performance. Increases in receiver sensitivity of up to 14 and 17 dB have been obtained using, respectively, heterodyne and homodyne detection techniques. These results approach theoretical predictions and offer the possibility of much greater transmission capacity and extended regenerator spacings for monomode systems.

Single mode fiber optic single sideband modulator and method of frequency

Abstract: A fiber optic frequency shifter comprising two waveguides having different indices of refraction. In some embodiments the waveguides are two modes of propagation in one fiber. Plural distributed coupling ridges, or electrodes mounted adjacent piezoelectric materials, are independently driven to apply sinusoidally varying forces to the fiber. In some embodiments, the phase relationship of the driving signals for the electrodes or ridges is such that a travelling acoustic wave is launched in the fiber. In other embodiments, regions of stress in the fiber are created by an acoustic wave coupled into the fiber from a transducer coupled to an acoustic medium surrounding the fiber. The input carrier light is shifted in frequency by the frequency of the acoustic wave.

Polarization-maintaining fiber system and method of manufacturing the same

Abstract: In an optical fiber system comprising first and second polarization-maintaining fibers, a first orthogonal coordinate system of the first polarization-maintaining fiber is azimuthally rotated relative to a second orthogonal coordinate system of the second polarization-maintaining fiber by the use of an optical coupler by a preselected angle between 0° and 90°, both exclusive. Preferably, the optical coupler is shorter than a beat length of each fiber and the preselected angle is equal to 45°. The optical fiber system is applicable to an optical sensor for sensing a variable physical parameter. In the sensor, a light beam is incident onto one end of the first polarization-maintaining fiber so that a plane of polarization of the light beam is matched to one of those planes of the first polarization-maintaining fiber which are determined by the first orthogonal coordinate system. A third polarization-maintaining fiber is coupled through an additional optical coupler to the second polarization-maintaining fiber. The optical fiber system can be manufactured either by twisting and locally fusing a single polarization-maintaining fiber or by splicing together two polariation-maintaining fibers.

Inverse-square wavelength dependence of attenuation in infrared polycrystalline fibers.

Abstract: The total-attenuation coefficient αT in polycrystalline thallium bromoiodide fibers varies as λ−2, in sharp contrast to the Rayleigh λ−4 dependence in silica fibers. We developed a model showing that this λ−2 attenuation results from the combination of bulk scattering from large-scale, optically thin imperfections and surface scattering and absorption. The model also explains our scattering sphere result that the attenuation coefficient that is due to scattering, αS, at 10.6 μm increases linearly with fiber length, in contrast to the situation in glass fibers in which αS is constant along the fiber.

Review of Fundamentals of Optical Fiber Systems

Abstract: This paper is a brief review of the principles of optical fiber systems to serve as an introduction to the specific application and technology papers in this issue. Fibers, transmitters, receivers, point-to-point transmission systems, multipoint buses, and sensor systems are discussed. Subsystems are reviewed in terms of thier input/ output characteristics.

Thermal characteristics of optical pulse transit time delay and fiber strain in a single-mode optical fiber cable.

Abstract: Thermal characteristics of optical pulse transit time delay and fiber strain in a single-mode optical fiber cable are investigated theoretically and experimentally Measurements of the transit time delay shift are made by a spatial interference technique using a 15-in long fiber, six-fiber unit, and cable Experimental results for a jacketed fiber whose fiber axis is well centered in nylon coating are in good agreement with those predicted from the theory A jacketed fiber whose fiber axis is positioned eccentrically from the jacket center exhibits a small change in fiber strain at low temperature due to fiber buckling compared with that for the well-centered jacketed fiber The loss increase for the off-centered jacketed fiber is explained by the buckling model Furthermore, thermal characteristics of the unit-type cable examined here are found to coincide with those for the constituent six-fiber unit

Fiber Optical Intensity Sensors

Abstract: Fiber optical intensity sensors are made of optical fibers connected to a miniaturized optical sensor which is based on either a mechanical light valve, a light scattering process, a photoemission or photoabsorption process, or the transmission property of the optical fiber itself. Use of the optical fiber to transmit light to and from the optical sensor makes it possible to construct an electrically passive sensor with remotely located electronic readout capability. Major advantages of this type of sensors are electromagnetic interference immunity, electrical passiveness and safety, miniature size, and ruggedness, In this paper, the large variety of fiber optical intensity sensors reported to date will be classified by their sensing mechanism. Examples will be given for each category of sensors with reported performances.

Inline optical fiber attentuator

Abstract: An inline single-mode fiber attenuator (10) is disclosed which may be formed by a tandem combination of a birefringent polarization-preserving fiber (12) and a single polarization fiber (14). The birefringent fiber functions as a variable wave plate and the single polarization fiber functions as a fiber polarizer. By continuously changing the local birefringence of the birefringent fiber with, for example, tension, pressure, or temperature, the phase difference between the two polarization components of light traveling through the birefringent fiber is continuously modified. The difference in phase causes suppression of one of the polarization components as it enters the fiber polarizer and, therefore, the output of the fiber polarizer, the sum of the two polarizations, is attenuated. The attenuator may be tuned by changing the local birefringence of the birefringent fiber. An inline optical fiber bandpass filter may be formed by cascading a plurality of appropriately arranged inline fiber attenuators formed in accordance with the present invention.