Wavelength dependence of 1/f noise in the light output oflaser diodes: an experimental study

The optical power emitted by a monomode GaAlAs laser is filtered with a monochromator. The 1/f noise in the filtered emission is found to be directly dependent on the noncoherent emission, such as S_pαP_nc^m. Here s_p is the spectral density of the 1/f fluctuations, P_nc is the average noncoherent power, m=3/2 under spontaneous emission, and m=4 in the superradiation and laser regions. Study of the 1/f noise in the optical power in a band centered at the laser wavelength and with variable bandwidth shows three operating regions. (1) LED region (at low currents): the fluctuations with a 1/f spectrum are uncorrelated in wavelength. (2) Superradiation region (at currents close to the threshold): the fluctuations are correlated. (3) Laser region: the 1/f noise apparently is dominated by noncoherent emission within a small optical band around the laser wavelength     

Use of a microwave cavity for sensing dielectric properties ofarbitrarily shaped biological objects

A rectangular waveguide resonator operating in the H₁₀₅ mode at 3.2 GHz is used in determining the change in resonant frequency, ΔF, and the Q factor of the cavity, ΔT , when measured with and without single corn kernels of various shapes and dimensions. By measuring those variables for a kernel oriented in two positions differing by a 90° rotation with respect to the maximum E-field vector, the average values of ΔF and ΔT are found to be independent of shape. The ratio ΔF/ΔT is independent of size and is a function of the material properties (ε'-1)/ε". This function is shown to be related to the material density, moisture content, or other characteristics when all other properties except the one selected remain unchanged     

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     

Multistable fiber-optic Michelson interferometer with electroopticfeedback exhibiting 95 stable states

The multistable behavior of a fiber-optic Michelson interferometer with electrooptic feedback using a piezoelectric phase modulator is investigated. A linear stability analysis of the third-order differential equation used to describe the system reveals the number of stable states increases with the ratio τ/T of the system time constant (τ) and the feedback delay time (T). In laboratory investigations 95 stable states were achieved with τ/T>50     

Limiter-discriminator detection of a coherent optical heterodyneM-ary CPFSK receiver

The performance of a coherent optical M-ary continuous-phase frequency-shift-keying (CPFSK) receiver using limiter-discriminator (L-D) detection is investigated. It is shown that L-D detection of CPFSK optical signals offers the best performance for a large normalized IF beat spectral linewidth, ΔνT. When the modulation index is unity, the receiver is immune to laser phase noise and can produce (M/4) exp (-SNR) symbol error probability, which may be considered as the upper bound if the optimal modulation index is used (SNR is the signal-to-noise ratio per symbol). Optimum modulation indexes are 0.8 and 1 at ΔνT=1% and ΔνT=2%, respectively, for M=4, 8, and 16     

The bandwidth performance of a two-element adaptive array withtapped delay-line processing

The bandwidth performance of a two-element adaptive array with a tapped delay line behind each element is examined. It is shown how the number of taps and the delay between taps affect the bandwidth performance of the array. An array with two weights and one delay behind each element is found to yield optimal performance (equal to that obtained with continuous-wave interference) for any value of intertap delay between zero and T₉₀/B, where T ₉₀ is a quarter-wavelength delay time and B is the fractional signal bandwidth. Delays less that T₉₀ yield optimal performance but result in large array weights. Delays larger than T₉₀/B yield suboptimal signal-to-interference-plus-noise ratio when each element has only two weights. For delays between T₉₀/B and 4T₉₀/B , the performance is suboptimal with only two taps but approaches the optimal if more taps are added to each element. Delays larger than T₉₀/B result in suboptimal performance regardless of the number of taps used     

Effect of conduction-band discontinuity on lasing characteristicsof 1.5 μm InGaAs/In(Ga)AlAs MQW-FP lasers

The characteristic temperature (T₀), relaxation frequency (f_r), differential gain (dg /dn) and nonlinear gain coefficient (ϵ) of 1.5-μm InGaAs/In(Ga)AlAs multiple-quantum-well (MQW) Fabry-Perot (FP) lasers grown by gas source molecular beam epitaxy (GSMBE) are reported. It is found that T₀ is little affected by the difference in the conduction band discontinuity. A maximum T₀ value of 86 K is obtained. The dg/dn and ϵ∈ were calculated from the slope of the f_r versus √ power plot and the damping K-factor. It is demonstrated that dg/dn and ϵ of InGaAs/In(Ga)AlAs MQW lasers increase with an increase in the conduction band discontinuity     

A multiple scattering solution for the effective permittivity of asphere mixture

A recursive algorithm for calculating the exact solution of a random assortment of spheres is described. In this algorithm, the scattering from a single sphere is expressed in a one-sphere T matrix. The scattering from two spheres is expressed in terms of two-sphere T matrices, which are related to the one-sphere T matrix. A recursive algorithm to deduce the (n+1)-sphere T matrix from the n-sphere T matrix is derived. With this recursive algorithm, the multiple scattering from a random assortment of N spheres can be obtained. This results in an N² algorithm rather than the normal N³ algorithm. As an example, the algorithm is used to calculate the low-frequency effective permittivity of a random assortment of 18 dielectric spheres. The effective permittivity deviates from the Maxwell-Garnett result for high contrast and high packing fraction. With a high packing fraction, dielectric enhancement at low frequency is possible     

Error performance of a digitally processed binary ESK frequencyhopping receiver in the presence of large Doppler

Analysis is made of the effects of Doppler on the error rate performance of a low data rate binary FSK frequency hopping receiver, employing a discrete Fourier transform (DFT) technique for baseband detection. Bit detection decision is made by locating the maximum of the DFT outputs which, in the frequency domain, are assumed to be separated by 1/T where T is the bit period. Both the worst case and average error performances are obtained and presented as a function of E_b/N₀ for various values of M where E_b/N₀ is the signal bit energy-to-noise density ratio and M is the degree of freedom associated with the Doppler uncertainty window. The E _b/N₀ degradation as a function of M is also presented     

On linear inductance- and capacitance-time conversions usingNIC-type configurations

The problem of linear inductance- and capacitance-time (L/T, C/T) conversion is approached through the systematic study of four approaches to building astable multivibrators using piecewise linear resistances obtained from one operational amplifier (OA) negative impedance converter (NIC) configuration. A new L/T converter with grounded inductance is found. Formulas for the time period taking into consideration the losses as well as the OA saturation output resistance are derived     

Error probability of M-ary FSK with differential phasedetection in satellite mobile channel

Formulas are derived for the error probability of M-ary frequency shift keying (FSK) with differential phase detection in a satellite mobile channel. The received signal in this channel is composed of a specular signal, a diffuse signal, and white Gaussian noise; hence, the composite signal is fading and has a Rician envelope. The error probability is shown to depend on the following system parameters: (1) the signal-to-noise ratio; (2) the ratio of powers in the specular and diffuse signal components; (3) the normalized frequency deviation; (4) the normalized Doppler frequency; (5) the maximum normalized Doppler frequency; (6) the correlation function of the diffuse component, which depends on the normalized Doppler frequency and the type of the antenna; (7) the number of symbols; and (8) the normalized time delay between the specular and diffuse component (t _d/T) where 1/T is the symbol rate. Except for T_d/T, all normalized parameters are the ratios of the parameter value and symbol rate. The Doppler frequency depends on the velocity of the vehicle and the carrier frequency. The error probability is computed as a function of the various parameters. The bit error probability is plotted as a function of signal-to-noise ratio per bit and other system parameters     

A model for fast switching transients in power systems: the nearzone concept

The author presents a simple time-domain model which makes it possible to predict the order of magnitude of the highest di/ dt values generated by closing switches in electrical power systems. The model is based on traveling-wave analysis. It is demonstrated that two different approaches must be applied, according to whether (a) the closing time, T_s, of the switch is faster than twice the traveling time to the first reflection point or (b) T_s is much slower. Under condition (b) the well-known quasistationary approach di/dt_max=U₀/L can be used, where U₀ is the switched voltage and L is the self-inductance of the line between the stray capacitances located to the left and the right of the switching device. Under condition (a) a new formula must be applied: di/dt _max≈2 U₀/ZT_s, where Z is the line impedance of the line in which the switching device is installed and T_s is the time during which the voltage across the switch collapses from U₀ to zero. Experimental results are given from both fast and slow closing switches     

Convergence and steady-state behavior of a phase-splittingfractionally spaced equalizer

The eigenstructure, the initial convergence, and the steady-state behavior of a phase-splitting fractionally spaced equalizer (PS-FSE) are analyzed. It is shown that the initial convergence rate of a T/3 or, in general, a T/M, PS-FSE employing the least-mean-square (LMS) stochastic gradient adaptive algorithm is half that of a symbol rate equalizer (SRE) or a complex fractionally spaced equalizer (CFSE) with the same time span. It is also shown that the LMS adaptive PS-FSE with symbol rate update converges to a Hilbert transformer followed by a matched filter in cascade with an optimal SRE, and thus forms an optional receiver structure. The LMS PS-FSE is computationally more efficient and introduces less system delay than the CFSE     

A storage-efficient method for solving banded Toeplitz systems

A method is presented for solving the banded Toeplitz system Tx=y by decomposing T into its asymptotic upper and lower triangular factors (which are banded and Toeplitz) and a rank-p correction matrix, where p is the bandwidth of T. This way of representing T requires only O(p²) words of storage and allows computation of x in O(2Np) operations. A similar method is presented for the case in which T is bi-infinite and y is zero outside a finite region     

A note on the application of edge-elements for modelingthree-dimensional inhomogeneously-filled cavities

The application of edge-elements for modeling three-dimensional inhomogeneously filled cavities is presented. Explicit representations for the two element matrices [S]_e and [T] _e, are provided in order to facilitate the implementation of the FEM formulation. Also included are the results of a numerical experiment that investigates the rate of convergence of the computations of the dominant resonance frequency of a rectangular cavity when the edge-element formulation is used     

On the three-coefficient window family

The definition of the three-coefficient window family is extended to the form: w(t,b)=0.5(1-b)+0.5 cos(2πt/T)+0.5b cos(4πt/T ), |t|⩽T/2, -0.25⩽b⩽0.25, and its spectral behavior is investigated. It is found that with the parameter b adjusted to different values, this window family can attain a minimum main lobe width of 1.38, a maximum sidelobe attenuation of 64.19 dB, and a highest cutoff rate of 30 dB/oct     

Maximum heat-sink temperature for CW operation of adouble-heterostructure semiconductor injection laser

Assuming that the temperature dependence of the threshold current for pulsed operation is known, an analytical expression for the maximum heat-sink temperature, T_hm, for CW operation of the laser can be derived. The maximum heat-sink temperature is expressed in terms of the characteristic temperature T₀, the room-temperature threshold current for pulsed operation I₀ , the equivalent effective thermal resistance &thetas;, and the equivalent effective series electrical resistance r of the device. It is shown that the values of T_hm can be enhanced by increasing the value of T₀ or by decreasing the values of I₀, &thetas;, and r     

Spectral efficiency of optical FDM systems impaired by phase noise

The required frequency spacings between channels in an optical frequency division multiplexing (FDM) network are considered. The minimum permissible spacings consistent with meeting bit error rate (BER) objectives are derived. The assumed transmission uses on-off keying (OOK), at a data rate 1/T (in bits per second), via external modulation of a laser source having linewidth β (in hertz). The assumed receiver consists of an optical channel selection filter followed by a p-i-n photodiode and a postdetection integrate-and-dump circuit. The analysis estimates the adjacent channel interference (ACI)-induced floor on BER for the middle of three FDM channels, as a function of frequency spacing and linewidth-to-bit rate ratio (βT). For BER=10^-9 and βT ranging from 0.32 to 5.12, the required channel spacing ranges from 5.2 to 27.5 bit rates. The multiplying factors associated with using (wide-deviation) frequency shift keying (FSK), coherent (heterodyne) detection, and infinitely many FDM channels, respectively, are estimated to be 2.0, at most 3.0, and at most 1.37     

Data predistortion techniques using intersymbol interpolation

Two data predistortion techniques are presented that compensate for high-power amplifier (HPA) nonlinearities in digital microwave radio systems by employing quadrature amplitude-modulation (QAM) signal formats. The first one is a T/2-spaced predistortion technique that ensures distortion-free HPA output at two points per symbol interval T. The second is a T/3-spaced predistortion technique which cancels nonlinear distortion at the HPA output at three points per symbol interval. As opposed to conventional data predistortion, which can only compensate for warping of the signal constellation, the new techniques are effective against intersymbol interference. Using the 64- and 256-QAM signal constellations, it is shown that the proposed techniques lead to a very efficient utilization of the available HPA power. It is shown that, of the two techniques, the T/3-spaced data predistortion employs narrower transmit-pulse shaping and achieves higher protection against adjacent-channel interference at the expense of some additional complexity     

Picosecond injection laser: a new technique for ultrafastQ-switching

A modification of a method for self-Q-switching in a laser with an isotypic saturable absorber is suggested and implemented in a multisegmented injection laser. The method consists of the transportation of most parts of the excited absorber population to the amplifier. The carrier transport time must be less than the spontaneous recombination time in the absorber. In a three-section AlGaAs/GaAs double-heterostructure laser with modified Q-switching, optical pulses of 5 ps in duration with a repetition rate as high as 18.5 GHz and peak power above 10 W have been obtained. The latter value is the largest ever reported for a picosecond injection laser. Unique temporal and spectral features exhibited by these lasers have been observed, including the stepped variation of pulse repetition frequency, its dependence on the pump current, large emission spectral width (on the order of kT), spectral chaos and bistability