The chemical oxygen iodine laser in the presence of a magneticfield. II. Frequency tuning behavior

For pt.I, see ibid., vol.29, no.3, p.933-43 (1993). Results are presented from an experimental/theoretical investigation of the frequency shifting and tuning behavior of a chemical oxygen-iodine laser in the presence of an applied magnetic field. Frequency shifting measurements are presented for both P and S polarization over a range of magnetic field strength from 0 to 3000 G, in a pressure regime (3 torr) where the line shape is Doppler broadened. Theoretical predictions of the location and magnitude of the frequency shifts are shown to be in good agreement with both the present measurements and a previous investigation in the pressure broadened regime     

A novel multilevel coherent optical system: 4-quadrature signaling

A novel multilevel coherent optical system is proposed. It is based on the exploitation of the property that the electromagnetic field propagating in a single-mode optical fiber can be represented by a four-dimensional vector whose components are the phase and quadrature terms of the two polarization components of the electrical field. This allows a wider use of the resources of the electromagnetic field for information transmission in order to obtain a spectrally efficient modulation format with a limited end. The net performance gain with respect to multilevel amplitude and phase modulation (N-APK) and N-PSK increases with an increase in the number of levels N. For instance, for N=32 the gain is 1.6 and 7.7 dB with respect to N-APK and N-PSK systems. The effect of laser phase noise on the system performance is evaluated     

Low-frequency intensity noise in semiconductor lasers

The observed 1/f noise in the light-output power S _p of four different types of heterostructure lasers is explained in terms of spatially uncorrelated gain fluctuations and spontaneous emission fluctuations. Two possible noise sources are suggested: fluctuations in the absorption coefficient and fluctuations in the number of free carriers. Both models are in agreement with the experimental results obtained from index-guided and gain-guided diodes at wavelengths of 1.3 and 0.8 μm. The dependence S_p ∝P^m has been observed with P the average light-output power and m=3/2 under spontaneous emission, a small transition region with m=5/2, m=4 in the superradiation region, and 0⩽m⩽1 in the laser region     

Picosecond pulse generation in a GaAs/GaAlAs single-quantum-welllaser at the first and second subband transition

Picosecond optical pulses are generated in a single-quantum-well laser at the n=2 or n=1 quantized transition by tuning the optical gain spectra via the intracavity losses. The results for the generated pulses are discussed with respect to the influence of differential gain (dg/dN) and nonlinear gain saturation (∈) effects     

Seasat over-land scatterometer data. I. Global overview of theKu-band backscatterer coefficients

Statistics on the backscatter coefficient σ⁰ from the Ku-band Seasat-A Satellite Scatterometer (SASS) collected over the world's land surfaces are presented. This spaceborne scatterometer provided data on σ⁰ between latitude 80° S and 80° N at incidence angles up to 70°. The global statistics of vertical (V) and horizontal (H) polarization backscatter coefficients for 10° bands in latitude are presented for incidence angles between 20° and 70° and compared with the Skylab and ground spectrometer results. Global images of the time-averaged V polarization σ⁰ at a 45° incidence angle and its dependence on the incidence angle are presented and compared to a generalized map of the terrain type. Global images of the differences between the V an H polarization backscatter coefficients are presented and discussed. The most inhomogeneous region, which contains the deserts of North Africa and the Arabian Peninsula, is studied in greater detail and compared with the terrain type     

Dependence of nonlinear gain effect on threshold gain insemiconductor lasers-an optimization for high-speed modulation

The dependence of nonlinear gain parameter and K factor on material gain in semiconductor lasers has been investigated theoretically by calculating (dg/dS)_s=0, derivative of gain g with respect to photon density S. If spectral hole burning is assumed, the square of the line-shape function appears in dg/dS. This implies that the contributions from high-energy electron-hole pairs are reduced, and that (dg/dS)_S=0 takes a finite negative value at transparent point (g=0). The nonlinear gain parameter, therefore, diverges, as the gain approaches zero. The K factor is minimized at a value of material gain, which is estimated to be 4.3 ps^-1 for typical InGaAs/InGaAsP quantum well lasers. The confinement factor should be designed to maintain the gain at the optimum value     

Free-electron laser operation in the intermediate gain region

The modification induced on the well-known antisymmetric gain function of a free-electron laser when the gain coefficient g ₀ ranges up to 10 is studied. Deviations from the linear regime for g₀⩾0.5 are presented, and a simple perturbative analysis which accounts remarkably well for the corrections to the linear gain formula even for g₀=10 is discussed. The effect of inhomogeneous broadening is also discussed     

Diffusion and noise in GaAs material and devices

The variation of the diffusion coefficient D(E) versus the electric field strength E is determined at 300 K in n-type GaAs (N_D=3×10^-17 cm^-3 ), using pulsed high-frequency noise measurements. D(E) is found to increase slightly at low field, then to decrease down to one tenth of its ohmic value near the threshold field. Long (⩾4 μm) real n⁺-n-n⁺ Gunn diodes, with an arbitrary doping profile, can be modeled. Comparisons are made, and excellent agreement is found, between experimental and theoretical characteristics of two real diodes, with notch and with gradual doping profiles. The doping profile N_D(x ) is shown to have a considerable influence on the diode behavior, in regard to the electric field profile as well as the noise characteristics. Using the impedance field method, the noise current is modeled and found to by very sensitive in the D(E) variation law, in particular in the range of 2.5-4 kV/cm. The agreement between the experimental noise and the computed noise of real diodes is quite satisfactory when using the D(E) determined     

Generation of short pulses of coherent electromagnetic radiation ina free-electron laser amplifier

Theoretical and experimental studies of the evolution of a frequency-chirped pulse under the influence of both phase and gain dispersion effects induced by the free-electron laser interaction are presented. For the experimental parameters used (electron beam voltage V=150 kV, wiggler periodicity l_w=3.5 cm, gain ~10 dB, input pulse width Δt~200 ns, frequency w ₀/2π=10 GHz, and frequency chirp α/2π~5 MHz/ns), pulses of a few nanoseconds were generated after an interaction length of 2.30 m, in good agreement with theoretical expectations     

Dependence of transient gain on gas-flow velocity and dischargecurrent in the FAF CO2 laser amplifier

Transient behaviors in which the output power amplified through the fast-axial-flow (FAF) CO₂ laser amplifier decreases temporally after initiating the discharge are discussed. The output power becomes stable within a few minutes, i.e. the transient time. These phenomena should strongly depend on input irradiance, plasma length, gas-flow velocity v, and discharge current I _dis. The small signal gain γ₀ is formulated as a function of v and I_dis and the discharge time for a partially-homogeneously-broadened, slightly saturated gain medium at individual vibrational-rotational transitions     

Gain measurements on the KrF(B→X), XeF(B→X), and XeF(C→A) lasertransitions in an XeF(C→A) laser gas mixture

Optimum energy extraction from an electron-beam-pumped XeF(C →A) laser is achieved with a five-component rare gas halide mixture. The characterization and modeling of laser action in such a gas mixture requires a knowledge of small-signal gain and absorption coefficients not only on the blue-green XeF(C→ A) transition, but also in the ultraviolet (UV) region for the competing XeF(B→X) and KrF(B→X ) transitions. The authors report gain measurements on the XeF(C→A) transition and small-signal gain and absorption coefficients at or near both the XeF(B→X ) (351 and 353 nm) and KrF(B→X) (248 nm) transitions. A study of the gain for the UV and visible transitions as a function of Kr and Xe partial pressure is reported, and its impact on the XeF(C→A) kinetics is discussed     

Frequency-locking of 1.5-μm InGaAsP lasers to an atomic kryptonline without dithering the laser frequency

The frequency-locking of a 1.5-μm distributed-feedback (DFB) laser to an atomic Kr 2p₈-3d"₁ transition at 1.5244 μm that does not dither the laser frequency is discussed. Instead, the frequency-discriminant signal was obtained by dithering the Kr line with a small AC magnetic field using the Zeeman effect. The frequency shift of this Kr line was measured to be 1.25 MHz/G, and is linearly proportional to the applied magnetic field. Since the technique avoids dithering of the laser frequency without adding complexity in the servo-loop, it is well suited for developing master oscillators to be used in optical wavelength-division-multiplexed (WDM) networks     

RELEDOP: a full-scale antenna pattern measurement system forhigh-powered HF transmitting antennas

A system is presented that utilizes a towed, 3-axis receiver called RELEDOP. RELEDOP is a receiving elementary dipole with optional polarization that measures the transmit directivity patterns of over-the-horizon backscatter (OTH-B) radars and shortwave broadcast transmitting arrays. A dielectric tow cable and a fiber-optic link to the towing aircraft permit excellent electromagnetic isolation of the RELEDOP sensor from the aircraft. High accuracy in position location (±15 m in x, y, and z) and fast-receiver sampling permit orthogonal polarization component measurements with accuracies of ±0.15° in bearing and ±1 dB in relative amplitude. Calibrated receiving element gains permit measurement of absolute field strength. The field strength data, combined with the position data, permit an assessment of transmitter effective radiated power (ERP); a knowledge of input power permits a computation of absolute gain of the transmitting antenna to ±3 dB or better. The RELEDOP system is described, along with its subsystems; towed sensor, airborne, positioning, ground station, and communications subsystems. Example patterns and flight path tracks illustrate the output graphics     

Optimization of signal sets for partial-response channels. II.Asymptotic coding gain

For Pt. I see ibid., vol.37, no.5, p.1327-141 (1991). For a linear, time-invariant, discrete-time channel with a given transfer function H(f), and information rate R bits/ T, where T is the symbol interval, an optimal signal set of length K is defined to be a set of 2^RK inputs of length K that maximizes the minimum l₂ distance between pairs of outputs. The author studies the minimum distance between outputs, or equivalently, the coding gain of optimal signal sets as K→∞. He shows how to estimate the coding gain, relative to single-step detection, of an optimal signal set length K when K is large     

Optical gain in GaAs/GaAlAs graded-index separate-confinementsingle-quantum-well heterostructures

Optical model gain in both the TE and TM polarizations of graded-index separate-confinement single-quantum-well heterostructure lasers measured at various levels of injection current on samples with different quantum-well widths is discussed. Lasers with wide quantum wells (⩾120 Å) have emission and gain spectra which exhibit two peaks, caused by the n=1 and n=2 subband transitions. With ordinary cavity parameters, the saturation gain of the n=1 subband transitions is lower than the cavity loss of the laser, and the lasers always lase at the n=2 transitions. Reducing the quantum-well width increases the saturation gain of the n=1 transitions enough to allow lasing from them, even in cases of higher cavity loss. Further, for a fixed cavity loss, reduction of the quantum-well width decreases the threshold current density for n =1 lasing transitions, while that for n=2 lasing increases. The superlinear increase of the material gain with the decrease of the well width reduces the minimum cavity length for n =1 subband lasing. Narrower quantum wells with higher mirror reflectivity allow shorter cavity lengths while retaining n=1 lasing, resulting in low threshold current     

Kinetic processes in electron beam-excited XeF (C40aA) laser media

Fundamental processes affecting the operation and performance of electron beam-excited XeF(C→A) laser media have been analyzed and modeled. Emphasis has been placed on conditions typical of high current density (~250A cm^-2), short pulse (~10 ns FWHM) e-beam excitation of high pressure (~6 atm) multicomponent mixtures comprised of Ar-Kr-Xe-NF₃-F₂ . Computation of the temporal evolution of excited and ionized species for such circumstances has permitted identification of the factors controlling XeF(C) formation and loss, and has resulted in the identification of the primary transient species that absorb radiation in the blue-green spectral region. The data so obtained serve to explain measured XeF(C→A) properties, particularly net gain, under conditions for which the C→ A laser energy density and efficiency values are comparable to those of the UV XeF(B→X) laser     

Electromagnetic fields induced in a person due to devices radiatingin the 10 Hz to 100 kHz range

Using dipole and image theory with prolate spheroid models, a qualitative evaluation is made of the electric, E, and magnetic, H¯, fields induced at selected points inside a person due to external electric, E¯₀, and magnetic, H¯₀, fields within the 10 Hz to 100 kHz range. Amplitude measurements of the E¯₀ and H¯₀ vectors are made at selected points in the region with the person absent. The evaluation is valid for any field radiating device if the amplitude and phase angles of the external fields are measured in accordance with the stated protocol     

Electromagnetic scattering from an infinite elliptic metalliccylinder coated by a circular dielectric one

The scattering from an infinite elliptic metallic cylinder coated by a circular dielectric one is considered. The electromagnetic field is expressed in terms of both circular and elliptical cylindrical wave functions, which are connected with one another by well-known expansion formulas. In the special case of small h=ka/2 (a being the interfocal distance of the elliptic conductor and k the wavenumber of the dielectric coating), exact, closed-form expressions of the form S (h)=S(0)[1+g "h²+O(h⁴)] are obtained for the scattered field and the various scattering cross sections of the problem. Both polarizations are considered for normal incidence. Graphical results for various values of the parameters are given     

Field theoretical treatment of E-plane waveguide junctionswith anisotropic medium

E-plane waveguide junctions containing an anisotropic medium are analyzed. The analysis is based on the equivalence principle and on cavity field expansions. Using the equivalence principle, magnetic surface currents are introduced at the imaginary boundaries chosen between the central region of the junction and the waveguides. The electric displacement D in the junction is expressed in terms of a solenoidal set and an irrotational set. Matching the tangential magnetic field at the imaginary boundaries leads to a matrix equation, the unknown of which are the amplitudes of the scattered waveguide modes. Using this method, the performance of E-plane waveguide junctions with full-height and partial-height ferrite post is analyzed. The influence of the completeness terms G_oq on the numerical results of an empty E-plane Y-junction is shown. The numerical results are compared with previously published experimental and theoretical results     

Intensity noise in long-wavelength superluminescent sources

Noise in broadband 1.3-μm superluminescent diodes (SLDs) is investigated experimentally, using a balanced detector arrangement to determine the excess noise factor as a function of photodetector current. Measurements were made in both the low-frequency 1/f, regime (<500 kHz) and the high-frequency quantum noise spectral region. The data at higher frequencies are in agreement with predictions of the quantum amplifier model, with values of the spontaneous emission coupling factors ranging from 1.2 to 1.9. It is also found that noise for one polarization of the light is uncorrelated with the noise for the orthogonal polarization over the 0-1 MHz frequency range. This implies that the 1/f, noise is not related to carrier density (gain) fluctuations in the active region of the device. An integrated optic chip design to compensate for the excess intensity noise in fiber gyroscopes is proposed