Birefringence in single-mode optical fiber due to elliptical core deformation and stress anisotropy

A formula for the difference between propagation constants of two polarization modes caused by elliptical core deformation and stress anisotropy is given for single-mode optical fibers. Birefringence in circular and elliptical core fibers subject to an externally applied uniaxial pressure are measured. Birefringence caused by elliptical core deformation and stress anisotropy are experimentally separated by measuring their wavelength dependences. A value of3.36 times 10^{-10}cm²/gram weight (gw) (=3.43 times 10^{-12}m²/N) is obtained for the stress-optical constant of silica fibers. Residual stress corresponding to external pressure of 240 gw/cm is observed for a fiber with 0.35 core ellipticity.     

Optimum design for high-power and high-efficiency GaAs Hi-Lo IMPATT diodes

Optimum structure parameters of GaAs hi-lo IMPATT diodes to give a high-efficiency and a high-output power are experimentally determined in the frequency range from 7 to 18 GHz. The highest efficiency is obtained at any frequency when a diode has a punch-through factor of about 0.6 and is depleted with an electric field strength of 10 kV/cm at the end of the lo region under operation. The optimum lo-region carrier concentration to achieve the highest efficiency and the upper limit of a junction area to obtain a high-output power without decreasing efficiency are found to be simply related to frequency by the following equations:(N_{L})_{opt} = (4 times 10^{14}) cdot (f/7)^{3}cm^-3and(S_{j})_{max} = (20 times 10^{-4}) cdot (f/10)^{-1.9})cm²), respectively, wherefis in gigahertz.     

Measurements of anomalous gain coefficients in transversely excited CO2lasers

We have made a detailed study of the gain coefficients of more than 150 transitions in the [00^{0}1-10^{0}0]_{I,II}and [00^{0}2-10^{0}1]_{I,II}bands of CO2. Measurements were made in a transversely excited discharge at pressures of 80 and 425 torr. Many lines were found to have anomalously high gain coefficients, but all the anomalies are accurately predicted by a model which takes into account the effect of overlapping transitions. Our results indicate that the majority of transitions in an atmospheric pressure discharge have anamalous gain coefficients under typical discharge conditions. This fact has not been generally recognized, and the neglect of overlapping transitions can lead to errors in determining rotational temperatures.     

Second-harmonic generation in KB5O8· 4H2O from 217.1 to 315.0 nm

Second-harmonic generation (SHG) has been observed in KB5O8ċ 4H2O (KB5) between 217.1 and 315.0 nm by angle tuning in a single crystal using a single cut. A conversion efficiency of 9.2 percent was observed for type I noncritical phase matching at 217.1 nm for a peak power of 15 kW at 434.2 nm. The nonlinear coefficients d31and d32are estimated to be approximately1.1 times 10^{-10}ESU (4.0 times 10^{-25}m/V) and0.08 times 10^{-10}ESU (0.29 times 10^{-25}m/V), respectively.     

Antenna impedance measurements and sheath effects in an RF generated plasma

A device using an RF discharge quasi-homogeneous plasma to measure antenna impedance is described. The plasma diagnosis is done by using a new probe made of a transparent resonant cavity. The electron densities (2 times 10^{8}to6 times 10^{9}particles/cm³) and collision frequencies (6 times 10^{7}to1.5 times 10^{8}per second) are controlled by the discharge power. The density distribution measurements indicate a quasi-homogeneous region of 20 by 20 by 14 cm. Impedance measurement results are given for a a thick unipole between 100 and 750 MHz. The values ofX = (omega_{p}/omega)^{2}range from 0 to 44. They agree fairly well with Deschamps model theory applied to scaled frequency measurements as well as to a quasistatic simple analytical formula. Sheath effects enhanced by a dc bias are observed especially at low frequencies and the sheath thickness increase is calculated and found consistent with Pavkovich's parabolic potential assumption. The possibility of electronic tuning by a suitable bias is suggested.     

Collision effect on the saturation character of vibration-rotation transitions for 00°1-10°0 band of CO2

Experimental studies of collision effects on the saturation characteristics of vibration-rotation transitions for00deg1-10deg0 band of CO2is described. Saturation was studied in a passive absorption cell inside the laser resonator. The saturation value could be altered by varying the cell temperature and the pressure of CO2. Vibration-rotation transitions, at pressures greater than or equal to 1 mmHg, were found to be saturated homogeneously, in spite of the fact that the Lorenz width was much less than the Doppler width. This is explained by the high number of collisions during the lifetime in a vibrational state. In this case the spectrum of a single molecule corresponds to that of a Doppler profile. Cross sections for the destruction of levels of00deg1-10deg0by added gases have been obtained, which att = 800degK appeared to be σCO2-He =6 times 10^{-19};sigmaCO2-Ne =2.8 times 10^{-18}; sigmaCO2-CO2=6.6 times 10^{-18}; sigmaCO2-N2=1.2 times 10^{-17}cm²The introduction of sufficiently large absorption caused self-sustained radiation pulsation. When the field influenced the saturating system for only a short period of time, with the interaction time being commensurate with the period of time between collisions, the line was saturated nonhomogeneously. This was expressed by the fact that with the scanning of the laser frequency, a peak power output was observed, corresponding to Lamb's hole, in the center of a saturation line.     

Planar flexible fiber-optic acoustic sensors

The acoustic sensitivity of optical fibers is considered analytically. High sensitivity is obtained with low bulk modulus, very thick polymeric coatings. The fiber coatings play an important role in determining the fiber acoustic sensitivity. The very thick coating limit is realized by embedding optical fibers in an appropriate elastomer (polyurethane). A planar flexible fiber-optic interferometric acoustic sensor is developed by wrapping optimized single-mode fibers in a planar spiral form, and then embedding the fiber in a thin polyurethane layer. The acoustic sensitivity of the sensor is found to be high, and frequency (0.2-2.5 kHz) and static pressure independent. The sensor is found to be compatible with water, and is compared favorably to that of a planar polyvinylidene fluoride sensor of similar geometry     

When does the modular distance induce a metric in the binary case? (Corresp.)

The modular distance induces a metric if and only if the nonadjacent form of the modulusMhas one of the following forms:1) 2^{n}+2^{n-2} pm 2^{i}, wheren-igeq 4; 2) 2^{n} - 2^{j} pm 2^{i}, where2 leq n -j leq 5andj-igeq 2; 3) 2^{n} pm 2^{j}, wheren -j geq 2; 4) 2^{n}.     

Silicon field-effect transistors as radiation detectors for the Sub-THz range

The nonresonance response of silicon metal-oxide-semiconductor field-effect transistors (Si-MOSFETs) with a long channel (1–20 μm) to radiation in the frequency range 43–135 GHz is studied. The transistors are fabricated by the standard CMOS technology with 1-μm design rules. The volt-watt sensitivity and the noise equivalent power (NEP) for such detectors are estimated with the calculated effective area of the detecting element taken into account. It is shown that such transistors can operate at room temperature as broadband direct detectors of sub-THz radiation. In the 4–5 mm range of wavelengths, the volt-watt sensitivity can be as high as tens of kV/W and the NEP can amount to ${{10^{ - 11} - 10^{ - 12} W} \mathord{\left/ {\vphantom {{10^{ - 11} - 10^{ - 12} W} {\sqrt {Hz} }}} \right. \kern-\nulldelimiterspace} {\sqrt {Hz} }}${{10^{ - 11} - 10^{ - 12} W} \mathord{\left/ {\vphantom {{10^{ - 11} - 10^{ - 12} W} {\sqrt {Hz} }}} \right. \kern-\nulldelimiterspace} {\sqrt {Hz} }}. The parameters of detectors under study can be improved by the optimization of planar antennas.     

Optical absorption and fluorescence studies in high pressure cesium-xenon mixtures

The pressure and temperature dependent absorption and fluorescence spectra of the cesium-xenon (CsXe) molecule have been examined. In contrast to previous investigations of the alkali-rare gas molecules, cesium atomic states that have weakly allowed optical transitions have been studied and have been shown to form excimer levels that are attractive for application as potential dissociation lasers. In particular, the (Cs[7^{2}S]Xe)* excimer appears promising as a source of high-energy laser radiation due to 1) its large dissociation energy (0.132 eV), 2) its stimulated emission cross section ofsimeq10^{-17}cm², and 3) its small population threshold inversion densities (simeq10^{13}cm^-3).     

The liquid-phase epitaxial growth of low net donor concentration (5 × 1014-5 × 1015/cm3) GaSb for detector applications in the 1.3 - 1.6 µm region

We report on the liquid-phase epitaxial (LPE) growth of low doped n-GaSb for long wavelength photodiodes. Very low net donor concentration epilayers (5 times 10^{14}-5 times 10^{15}/cm³) were grown both from undoped Ga rich solutions in the300-375degC range and by compensation using Ga-rich solutions atsim500degC with intentionally added Te. The preparation of Au-GaSb Schottky diodes forC-Vprofiling is also discussed.     

An electric-field sensitive fiber with coaxial electrodes for optical phase modulation

It was shown that an electric-field sensitive optical fiber utilizing a radially poled piezoactive copolymer jacket is suitable for application as an all-fiber optical phase modulator. The jacket used was a copolymer of vinylidene fluoride and trifluoroethylene (P(VDF73/TrFE27)), 63-μm thickness, and was made radially piezoactive by corona discharge poling. The optical response of induced phase shift due to applied dc voltages was measured to be between0.80 times 10^{-5}and1.24 times 10^{-5}rad/(V/m) per meter of fiber. The performance of the jacketed fibers with coaxial electrodes as an optical phase modulator was tested over a wide frequency of range, from 20 Hz to 20 KHz.     

Nearly noncritically phase-matched UV generation at 3547 Å in NaCHO2

Sodium formate (NaCHO2) has been found to be very nearly noncritically phase matchable for type-2 frequency mixing of the Nd:YAG laser fundamental and second-harmonic radiation at room temperature. The nonlinear optical constant was determined to be d32(NaCHO2) =(1.25 pm 0.25) times 10^{-9}ESU.     

Performances of a frequency offset locked He-Xe laser system at 3.51 µm

A highly stabilized frequency offset locked He-Xe laser system was constructed for high resolution laser spectroscopy of H2CO [5_{1,5}(upsilon = 0) rightarrow 6_{0,6}(upsilon_{5}= 1)] at 3.51μm. It is composed of three He-Xe lasers. The first laser is H2CO-stabilized and is used as a frequency reference in the system. The second laser is frequency offset locked to the first laser by using the beat frequency between these lasers, and is used as a local oscillator. The third laser is frequency offset locked to the second laser, and is used to observe the H2CO spectrum by slowly varying the beat frequency between these lasers. The frequency stability of the first laser, measured against a similarly stabilized and synchronously modulated laser, was1.0times10^{-14}attau = 100s, where τ represents the integration time. The frequency traceability of the second laser to the first laser was expressed as8.0times10^{-13} cdot tau^{-1}for 10 msleq tau leq 100s. It was found that this value of the traceability was independent of the frequency modulation of the first and second lasers. The frequency traceability of the third laser to the second laser was nearly equal to that of the second laser described previously. The variable range of the frequency of the third laser was 19 MHz. In this range, the frequency traceability of the third laser to the second laser was independent of the beat frequency between these two lasers. From these results, it was concluded that this system can be used for the observation of the H2CO spectrum.     

Multichannel coherent optical communications systems

Balanced coherent receivers perform substantially better than single-detector receivers in multichannel optical fiber FDM communications systems since the balanced approach eliminates the direct-detection and signal-cross-signal interference. The permissible channel spacingDdepends on the intermediate frequency fIF, on the bit rate Rb, and on the modulation/demodulation format. In particular,Dincreases by 2 Hz for every 1-Hz increase of the fIF. The signal-to-interference ratio SIR, as defined in the text, provides a simple measure of the amount of the interference generated by undesired channels. The criterion SIR = 30 dB is selected in this paper and leads to the following minimum channel spacings: for heterodyne systems,3.8R_{b}for FSK,9.5R_{b}for ASK, and12.4R_{b}for PSK; for homodyne systems,7.5R_{b}for ASK and10.5R_{b}for PSK. Simultaneous transmission of several channels generates an excess shot noise studied here for the first time. If the local oscillator power is 40 dB above the received signal power and 2000 channels are transmitted without optical prefiltering, the excess shot noise power penalty is less than 1 dB.     

Conversion of near infrared to visible light by optical mixing

Collinear 3.3913- and 0.6328-micron laser beams were mixed in LiNbO3to produce the sum frequency corresponding to 0.5333 microns. The laser beams were generated simultaneously in a single He-Ne laser tube. With incident powers of 4.43 and 2.75 mW at 0.63 and 3.39 microns, respectively,3.65 times 10^{-7}mW were generated at 0.53 microns. The magnitude of this conversion is in satisfactory agreement with theory. Some of the characteristics of this system when viewed as an infrared detector are also given.     

New refractive index profiles of dispersion-flattened highly nonlinear fibers for future all-optical signal processing in wdm optical networks

In this paper, we demonstrate new dissimilar refractive index profiles for highly nonlinear ultra-flattened dispersion fibers with noteworthy effective area \((A_\mathrm{eff})\) for future optical signal processing. The newly proposed fibers named from Type 1 to Type 5 have a flattened dispersion over S, C, L and U bands. Predominantly, few-mode HNL-UFF fiber of Type 3 yields dispersion-flattened characteristics over a range of 250 nm of optical communication spectrum with a mere 0.2 ps/nm km variation in dispersion and a dispersion slope of \(0.0057\hbox { ps}/\hbox {nm}^{2}\) km due to the contribution of higher-order modes to the dispersion characteristics of the fiber. Moreover, it has a moderate nonlinear coefficient of \(8.03\hbox { W}^{-1}\,\hbox {km}^{-1}\). By modifying the refractive index profile of Type 3 fiber, Type 4 and Type 5 fibers are obtained in order to ensure single-mode operation, while the zero flattened dispersion characteristics of the fiber are compromised. Among the newly proposed fibers, Type 4 fiber offers a very low ITU-T cutoff wavelength of \(1.33~\upmu \hbox {m}\), whereas in the case of Type 5 fiber it is \(1.38~\upmu \hbox {m}\). Moreover, Type 4 and Type 5 fibers have good nonlinear coefficients of \(12.26\hbox { W}^{-1}\,\hbox {km}^{-1}\) and \(11.45\hbox { W}^{-1}\,\hbox {km}^{-1}\), respectively. By virtue of the proposed optimized index profile, an insensitive behavior toward bending is displayed by Type 3, Type 4 and Type 5 fibers. In addition, Type 4 fiber provides a better splice loss of 0.25 dB.     

The radar cross section of ogives, double-backed cones, double-rounded cones, and cone spheres

Experimental measurements have been made of the nose-on backscattering radar cross section over a range of diameters from 0.05 to 1.0 wavelengths of a series of ogives with total included nose angles of40deg,75deg, and120deg; of cone spheres with nose angles of15deg,30deg,40deg60degand120deg; of double-rounded cones with nose angles of15deg,40deg, and75deg; and of double-backed cones with nose angles of40deg. Further, a series of ogives of approximately the same diameter and with nose angles of25deg,40deg,60deg,75deg,95deg,120deg,160degand180deghave also been measured. All the present results have been obtained using two monostatic phase-locked CW balanced-bridge model measurement radars operating at frequencies of 9 Gc and 35 Gc. For targets with maximum dimensions less than two inches, cross sections of less than10^{-7}square meters at 9Gc and10^{-6}square meters at 35 Gc can be measured with an accuracy of pm0.5 db. The experimental data is sufficiently comprehensive to provide qualitative explanations of the scattering mechanism in the Rayleigh and resonance regions, and to make rigorous tests of the various theoretical predictions. An extension of the physical optics theory of Adachi was found to predict accurately the echo area of the ogive, the double-rounded cone, and double-backed cone in the resonance region. The Rayleigh theory was found to be generally accurate for all the models considered. The impulse analysis of Kennaugh and Moffatt was found to be accurate for the cone sphere.     

High-modulus low-linear-expansion-coefficient loose-jacket optical fibers

Optical fibers loosely jacketed with high-modulus low-linear-expansion-coefficient polymers have been proposed, and the mechanical, thermal, and optical properties have been investigated. The loose-jacket tube is made of highly oriented polyoxymethylene (POM), which is produced by tensile-drawing the isotropic POM tube with dielectric heating. The oriented POM exhibits Young's moduli 20-40 GPa and linear expansion coefficients of the order of10^{-5}-10^{-6}degC^-1. Owing to the low linear expansion coefficients of the material, the oriented POM loose-jacket optical fiber has no low-temperature excess losses, which is due to fiber bends caused by thermal contraction of the loose tube, in the-60-80degC temperature range.     

Multisideband Detection Radio-Over-Fiber Link With Phase Modulation and Fiber Demodulation for Vector Signal Transmission

We propose and demonstrate a multisideband detection (MSD) radio-over-fiber (RoF) link with optical phase modulation (PM) and fiber demodulation to transmit vector signals. In the scheme, the optical PM signal is demodulated by the fiber dispersion. The fading effect can be almost entirely overcome thanks to the MSD. In case of 50-Msymbols/s vector signal with frequencies of subcarrier ranging from 2 to 12 GHz over 25-, 50-, and 100-km single-mode fibers, respectively, the RoF-link's error vector magnitude of better than 4.5% and spurious-free dynamic range of more than 97 ${hbox {dB}}cdot hbox{Hz}^{2/3}$ are obtained experimentally.