bluuebunny/crossref_metadata_2025 huggingface dataset:
Channel-width tailoring of Index-guided arrays
Authors: Hadley G. Ronald , Buus J.
Published: February 2023

Semiconductor laser arrays are attractive as high-power optical sources. However, a systematic design of arrays capable of near-fundamental-mode operation in high-power cw conditions has not yet been reported. In this work we present such a design based on the results of a self-consistent numerical model.1 A previous study of index-guided arrays operating near threshold 2 has suggested the use of modified outer guide widths to achieve a fundamental lateral mode characterized by near-uniform guide intensities. In the present study we used our numerical model to investigate further the performance of this concept in the high-power regime. Because of the sizable index profile contribution arising from thermal effects, we find that modification of only the outer guides is insufficient to insure a near-uniform fundamental mode envelope. Instead, we propose to increase the width of each guide progressively from the device center so as to compensate approximately for the decreasing index of refraction due to thermal effects.
Nonlinear dynamics of semiconductor laser arrays
Authors: Winful H. G. , Wang S. S.
Published: February 2023

The problem of synchronization of coupled oscillators is fundamental to many areas of science. When synchronization is successful, the several oscillators act as one, with a unique frequency and a well-defined phase relationship between the oscillators. When synchronization fails, the behavior of the oscillators is marvelously complex. Here beat oscillations, quasiperiodic motions, and chaotic dynamics are the norm. Semiconductor laser arrays are an example of a set of coupled nonlinear oscillators. This paper presents a theoretical analysis of the stability of coupled laser arrays. The analysis shows that laser arrays with evanescent coupling are dynamically unstable over large regions of a parameter space spanned by the coupling constant and the pump current. The origin of the instability is a breakdown of phase locking due to an amplitude-phase coupling effect peculiar to semiconductor lasers. In the unstable regime the temporal evolution of the laser output is either quasiperiodic or chaotic. The theoretical results show that the undamped pulsations observed in streak camera measurements of laser arrays are intrinsic to the lasers themselves and are a manifestation of deterministic chaos.
Raman soliton generation and spectral biasing
Authors: Druhl K. J.
Published: February 2023

The exact resonance of pump and Stokes with the Raman transition is essential for successful generation of Raman solitons.1 Even slight detuning (~1 % of the Raman linewidth) leads to rapid soliton decay. Viewed in frequency space, this is due to an asymmetric gain profile for the injected Stokes seed. Through the frequency dependence of gain and Raman dispersion, this leads to the rapid growth of an out-of-phase component of the Stokes beam, which does not participate in the transient reversal of pump depletion. The asymmetric gain profile can be compensated by suitably modifying the total phase shift of the injected seed, resulting in an enhancement of those frequency components that see lower gain. We show that almost complete soliton stability can be achieved this way.
Accelerating reference frame for electromagnetic waves in a rapidly growing plasma: Unruh radiation and the dynamic Casimir effect
Authors: Yablonovitch E.
Published: February 2023

In 1974, Hawking showed1,2 that black holes evaporate by the emission of low-temperature thermal radiation, now named Hawking radiation. Shortly thereafter a closely related effect called Unruh radiation became apparent. According to Unruh3 and Davies,4 observers of the electromagnetic field in an accelerating reference frame should see thermal radiation at a temperature T: where a is the acceleration relative to an inertial frame, c is the speed of light, and ħ and k are Planck's and Boltzmann’s constant, respectively. With an acceleration equivalent to that at the earth’s surface, g = 980 cm/s2, this thermal radiation is at a temperature of only 4 × 10−20 K. Bell has suggested5 that the spin depolarization of electrons accelerating around a storage ring may be interpreted as being caused by such radiation. Boyer6 introduced a physically helpful semiclassical interpretation of Unruh radiation. In his phe­nomenological picture, external zero point electro-magnetic field fluctuations are accelerated up to a nonzero temperature. The observer need not experience the acceleration directly. Davies and Fulling7 argued that the observation of reflected zero point field fluctuations from an accelerating mirror was sufficient to establish the relative accelerating motion.
Suppression of line broadening by fast Rabi flopping effects
Authors: Agarwal G. S. , Singh S.
Published: February 2023

We report a mechanism that leads to the suppression of line broadening effects. We show that a strong competition between the effects of coherent and incoherent pumps can lead to suppression of spontaneous emission or to narrowing of the spectral profiles. Consider the interaction of a system with a broadband incoherent pump, the effects of which can be accounted for by changing the relaxation times T1, T2 in the Bloch equations. For example, for radiative relaxation (rate 2γ) we have where | x | is the Rabi frequency associated with the broadband pump, and Γ is its bandwidth. The spontaneous emission spectrum from such a system in the presence of a coherent drive is the Mollow spectrum with the broadband pump contributing an amount β to the widths of the peaks. We demonstrate that the effects of β can be suppressed if the strength of the coherent pump is such that many Rabi oscillations are possible within the coherence time of the incoherent pump. We show this explicitly by obtaining exact results for spontaneous emission from a model system.
Spectral response matching of photodetectors
Authors: Fang Li , Lian Shentu
Published: February 2023

The spectral response matching of photodetectors under diffused illumination is required in almost all applications, especially in vision research. Compared with the others, the full-filter method has been used widely due to its precision and simplicity. Traditionally, the design is made by an iterative method. With the specular transmittance and reflectance, which are functions of thickness, of the filter under collimated illumination as the input, one can obtain an optimal value of filter thickness so that the experimental data can be matched. This procedure may not converge because of the diffused illumination condition. We propose a more efficient method by introducing the equivalent distance to substitute for the random distance that light traveled in the filters. From this assumption, the equivalent coefficient, which is the ratio of the equivalent and actual distance, is found to be where n is the index of refraction of the filter. The equivalent incident angle of a collimating light beam can also be expressed by c: Therefore, the transmittance and reflectance under diffused illumination can easily be calculated. We measured these parameters for various optical glass. The data conform to the theory very well. The spectral matching algorithm of the detector takes two steps. First, we calculate the initial thickness of the filters according to the transmittance and reflectance, neglecting the multiple reflection between the surfaces of the filters. The theoretical results are then compared with measured data to obtain the coefficient of multiple reflection. The final parameters are to be calculated when this coefficient is taken into consideration.
Energy efficient optical interconnection using dynamic holograms in photorefractive media
Authors: Chiou Arthur E. , Yeh Pochi , Hong John
Published: February 2023

Reconfigurable optical interconnection linking laser arrays and detector arrays plays a key role in optical computing. A generalized crossbar interconnection can be implemented using optical matrix-vector multiplication.1 A spatial light modulator (SLM) can be used to reconfigure the interconnection pattern. For the case of N × N crossbar interconnection, such an architecture has an optical energy efficiency of 1/N. Recently, we proposed a new method of forming dynamic holograms inside a photorefractive crystal so that the hologram is capable of performing reconfigurable optical interconnection with a very high-energy efficiency.2 In this new method, a small fraction of the incident beam is coupled out by using a beam splitter. This small fraction (called a probe beam) is expanded and passes through the SLM encoded with the desired interconnection pattern. The transmitted beam is then recombined with the main beam inside a photorefractive crystal. As a result of nonreciprocal energy coupling, almost all the energy in the main beam is transferred to the probe beam which carries the interconnection pattern. The result is a reconfigurable optical interconnection with broadcasting capability and high-energy efficiency.
Multimode fiber gyro using a mutually pumped conjugator
Authors: McMichael Ian , Beckwith Paul , Yeh Pochi
Published: February 2023

Phase-conjugate fiber-optic gyros (PCFOGs) use phase conjugation to compensate for reciprocal phase changes due to thermal and mechanical effects on the fiber, while at the same time allowing for measurement of the nonreciprocal phase shift produced by rotation. Where the best standard fiber-optic gyros require polarization preserving fibers and couplers to avoid polarization scrambling that is a source of noise and signal fading, the PCFOG can avoid this problem by using polarization preserving phase conjugation. This has the advantage of allowing for use of inexpensive non-polarization preserving and even multimode fibers and components. We previously demonstrated rotation sensing with a PCFOG using multimode fiber and a self-pumped phase-conjugate mirror. However, to form a grating in the self-pumped conjugator, the two inputs to the conjugator (the two counterpropagating outputs from the fiber) must be coherent to within the response time of the conjugator. As a result, this configuration cannot use the preferred technique of fast phase modulation for biasing. We have solved this problem by constructing a PCFOG using a mutually pumped conjugator that does not require coherence between its two inputs.
Four-detector photopolarimeter: precision analysis and low-light-level measurements
Authors: Lopez Ali G. , Azzam Rasheed M. A.
Published: February 2023

A given uncertainty of the output current vector of the four-detector photopolarimeter (FDP) leads to a corresponding uncertainty in determination of the Stokes vector of incident light. Using an FDP instrument matrix measured free from imperfections in the calibration optics, and for a preset level of error for each current, we calculate the maximum possible azimuth error, ellipticity error, and apparent depolarization as a function of the state of polarization of an assumed totally polarized incident beam. Experimental results are also presented that show the effect of reducing the power level of a light beam (from <1 mW to <1 μW, an attenuation range of 3.5 decades) on the precision with which the normalized Stokes parameters (NSPs) are measured. The rms error of each NSP is virtually independent of attenuation over an initial range of 2 decades and rises subsequently. However, it is the digitization (or quantization) error of the analog-to-digital converter that sets the limit on precision in our FDP. The accuracy with which the NSPs are measured is also essentially independent of light level, provided that the adjusted operational–amplifier gains are correctly accounted for.
Fiber-optic robot limb position sensor with holographic encoding
Authors: Knight Edward J. , Anderson Dana Z. , Hermanns Anno , Fienberg Jack
Published: February 2023

We attached a 100-mm core diameter multimode optical fiber along a mechanical arm to sense the position of the arm. The speckle pattern of laser light at the output of the multimode fiber varies as the arm moves. At each of the 128 arm positions we recorded the corresponding speckle pattern as a volume hologram, in the photorefractive crystal Fe:LiNbO3, using an angle encoded reference beam. The encoded angle is unique to the arm’s position. After the recording sequence is completed, any previously recorded position of the arm reconstructs a particular reference wave, thereby indicating the arm’s position. Interpolation allows encoding a virtually continuous range of arm positions. Our robot arm sensor has encoded a maximum travel of ~45° and has an angular sensitivity of ~2 mrad. The number of allowable positions, the sensitivity of the sensor and its noise, are largely determined by the number of fiber modes, the method of attachment, and the volume of the hologram. Although our mechanical arm has only one degree of freedom, the fiber can be used to record an arbitrarily complex motion of an arm having many degrees of freedom.
Quantum noise in light amplifiers
Authors: Collett M. J. , Walls D. F.
Published: February 2023

The principal problem of light amplifiers is to find a means to amplify a coherent amplitude without decreasing the SNR. Consider a signal with amplitude squeezed quantum fluctuations, that is, less noise in the amplitude quadrature than the phase quadrature. The objective is to find a phase sensitive amplifier, which, while amplifying the coherent amplitude, adds quantum noise preferentially to the phase quadrature. We take as a concrete model for the amplifier an intracavity parametric amplifier. We go beyond the usual linear analysis and include the nonlinear regime where pump depletion is important. We find that in the linear region the quantum noise added by the amplification process may be reduced by squeezing the idler mode, while in the nonlinear regime the quantum fluctuations from the pump become more important. These results are of direct application to practical light amplifiers.
Observation of spatial beating at near optical frequencies in the interference of two photons
Authors: Ou Z. Y. , Mandel Leonard
Published: February 2023

An experiment has been performed in which signal and idler photons produced in the process of parametric downconversion are mixed at a beam splitter, and the mixed beam splitter outputs are directed to two photodetectors feeding a coincidence counter. Two optical filters with passbands centered at frequencies ω1, and ω2 are inserted just before the detectors. When the two-photon coincidence rate is measured as a function of the time delay δτ introduced between the two photons, it is found that the rate exhibits a cosine variation with δτ at the difference frequency |ω1− ω2|. The experimental results are consistent with a quantum analysis of the experiment but not with a classical treatment. We have observed a beat note with a period of 37 fs, which is several orders of magnitude shorter than the resolving time of the photodetectors. In principle a beat note should be observable at optical frequencies with this technique.
Paley-Wiener revisited
Authors: Miller Richard T. , Tyler Christopher W.
Published: February 2023

The Paley-Wiener criterion is used to define the universe of functions in the frequency domain for which the impulse responses are causal, i.e., physically realizable in the time domain. By methods similar to those of Paley and Wiener, we prove a kindred result. Specifically, we find a family of functions K
k
[ω, α(ω)] so that given a transfer function amplitude αω, if

∫

−
∞

∞

K
k

d
ω

is finite for some k, there is a .(highly nonunique) transfer phase θ
k
(ω) for which the impulse response I
k
(t) corresponding to α exp(θ
k
), is of the order of exp(kt). A striking corollary is that, if transfer amplitude decays as exp(−|ω|), for any finite γ > 0, then, for any k> 0, there are phase choices for which the corresponding impulse responses are of the order of exp(kt). After normalizing and for large k, these I
k
(t) will for practical purposes vanish on the negative taxis.
Effects of fringe binarization on multiobject joint transform correlation
Authors: Yu Francis T. S. , Cheng Feng , Nagata Toshio , Gregory Don A.
Published: February 2023

In a joint transform correlator (JTC), the reference image and input objects are located at the input plane of a coherent processor. The corresponding joint power spectrum (JPS) is modulated by carrier spatial frequencies determined by the separation between the images. By Fourier transforming the JPS, the cross-correlation functions between the reference image and input objects can be obtained. We discuss the effects of binarization of the JPS on a JTC. It has been shown that binarization of the JPS is applicable to single-input object cases. However, the binarization is in general not applicable to multiobject problems. A computer simulation is used to analyze the effects due to fringe binarization in both noiseless and noisy circumstances. The results show that binarization of the multiobject joint power spectrum generates false and missing correlation peaks.
Noise characterization of femtosecond fiber Raman soliton lasers
Authors: Keller U. , Li K. D. , Rodwell M. J. W. , Bloom D. M.
Published: February 2023

Fiber Raman soliton lasers have been extensively used to generate femtosecond pulses in the near-IR region around 1.3 μm. We have investigated the amplitude and phase noise properties of these lasers. Pump pulses from a second harmonic cw mode-locked Nd:YAG laser are launched into 300 m of single-mode optical fiber, generating Raman-shifted pulses that are compressed by the soliton effect. For a single-pass compressor, an amplitude noise floor of ≥50 dB above shot noise at 2 mA of photocurrent was observed, substantially higher than the ≈20-dB excess noise at 1-MHz offset observed in a standard fiber-grating pulse compressor. The amplitude noise is white, which implies that the amplitude fluctuations are uncorrelated from pulse to pulse. The Raman-soliton timing jitter increases to 5.2 ps with increasing pump power (1.6 Wmax) given ≈2-ps jitter from the Nd:YAG pump. Using a synch-pumped ring cavity improved the timing jitter but did not improve the amplitude noise substantially due to the very low Q cavity.
Spatial pattern formation in lasers and cooperative frequency locking
Authors: Lugiato L. A. , Oldano C. , Kaige Wang , Narducci Lorenzo M. , Oppo G.-L. , Pernigo M. A. , Tredicce Jorge R. , Prati F.
Published: February 2023

We discuss aspects of laser dynamics that are connected with transverse effects. We show that a laser capable of operating simultaneously in more than one transverse mode can develop temporal instabilities and pulsations similar to those which are known to occur in much higher gain conditions in the traditional plane-wave models. Over selected ranges of the parameters these lasers can also reach steady-state configurations in which several transverse modes develop synchronized oscillations (cooperative frequency locking). When both the radial and angular degrees of freedom of the cavity field and atomic parameters are taken into account, spontaneous symmetry breaking of the cylindrical symmetry can also be observed.
Experimental evidence of laser instabilities due to transverse effects
Authors: Quel E. J. , Tredicce Jorge R. , Ghazzawi A. , Green C. , Pernigo M. A. , OPPO G.-L. , Narducci Lorenzo M. , Lugiato L. A.
Published: February 2023

Most nonlinear dynamic studies of optical systems in the recent past have focused on temporal behavior of the output radiation.1 However, recent theoretical studies2 have predicted the emergence of spatiotemporal structures and of a new frequency locking effect in a number of passive and active optical systems. Here we present experimental evidence in support of these effects using a CO2 laser with a simple intracavity arrangement that allows the continuous variation of the radius of curvature of the output couplers and, therefore, of the radial mode spacing. When the frequency spacing between consecutive transverse modes is sufficiently small, complex stationary spatial patterns develop in qualitative agreement with the theoretical predictions, and the laser oscillates with radial modes synchronously locked to each other. For larger values of the radial spacing, the frequency locking is lost and the transverse modes tend to operate more independently of one another, as evidenced by the appearance of beating effects in the output signal. The loss of frequency locking also corresponds to a reduction of the average power output. A further increase in the radial mode spacing eventually leads to the standard TEM00 mode configuration.
Homogeneous hole burning in an inverted system
Authors: Chakmakjian Stephen , Koch Karl , Papademetriou Stephano , Stroud C. R.
Published: February 2023

We report the observation of a spectral feature in the gain and absorption profiles of a multilevel system. The gain experienced by the sidebands of a weakly modulated laser field Is measured as a function of modulation frequency. For small modulation frequencies the sidebands experience diminished absorption or gain depending on the time-averaged inversion of the system. The width of the spectral feature depends on spontaneous as well as stimulated population transition rates. This paper Is an extension of single-field modulation experiments In collisionally broadened media.1,2 Two lasers are used in our experiment. One laser is used as an incoherent pump field that provides population for the upper level of the other optical transition. The second laser stimulates this population to the lower or ground level depending on the system configuration. In the experiment we modulate the second laser field. Using alexandrite as an active medium we are able to study both three- and four-level atomic systems.3 The stability of a single-mode laser to the growth of neighboring modes is related to the gain experienced by the sidebands. The competition between modes is strongest when their frequency separation is less than the width of this spectral feature.
Fluctuations of the delay bifurcation in an argon-ion laser
Authors: Bromley D. , Tredicce J. R. , Narducci L. M. , Scharpf W. J.
Published: February 2023

When one of the control parameters of a dynamic system varies slowly through a bifurcation point, the effect of the sweeping can cause a delay in the bifurcation.1 The experimental system studied here is an Ar-ion laser with an AOM mounted inside the cavity to control the cavity losses. The sweeping induces a delay in the switching on of the laser as the system passes through the threshold. Here we present a study of the effect of noise on the delay and how the noise may increase or decrease the effect. The averages and variances were measured for each point of time throughout the sweep along with the statistics of the delay time for different sweep rates. These were compared with the solutions of the Fokker-Planck equation for the single-mode laser near threshold. This effect suggests a way to measure some of the more nebulous parameters of a laser system.
Effect of detection criterion on foveal cone thresholds
Authors: Wesner M. F. , Pokorny Joel , Smith Vivianne C. , Feige D. L.
Published: February 2023

Assuming a binomial sampling of LWS and MWS cones in the fovea and Poisson-distributed quantal stimulus fluctuation, we can estimate three important parameters from threshold detection data: the minimum number of quanta absorbed per cone (C ~ 3–6), the number of cones illuminated by C quanta (N~1–2), and the proportion of LWS cones in the fovea (P
L
).1,2 We combined the method of constant stimuli with a rating response to examine the effect of observer criterion to the parameters C and N. The test stimulus was a 1′ point source of 520-620-nm wavelength, pulsed for 0.5 ms at an eccentricity of 8.5′. A different foveal location was sampled on each trial. Observers responded using one of four rating criteria: (1) definitely present; (2) probably present; (3) possibly present; and (4) definitely not present. Three psychometric functions were constructed corresponding to the cumulation of strict, medium, and loose criteria. The data were fit by the model1,2 and showed that the quantal requirement per cone C decreased systematically as criterion was relaxed.
Flicker sensitivity losses in dark adaptation: spatial factors
Authors: Eisner Alvin
Published: February 2023

We have shown previously that the losses of foveal flicker sensitivity following extinction of a bleach can be either precipitous or gradual, depending critically on the spectral characteristics of both the bleach and test.1 These losses, measured with 18-Hz flicker, also depend on the size and retinal location of the stimuli. At an eccentricity of 4.5°, precipitous losses of sensitivity to 20′ diam tests occur earlier than they do at the fovea. As the parafoveal tests are enlarged, the precipitous losses of sensitivity occur later [e.g., at 1 vs 3 min for 20′ vs 1° diam long wavelength tests that follow extinction of a 4.90-log troland (Td) 540-nm bleach]. As the tests are enlarged more (e.g., to 2° following extinction of the 540-nm bleach), the losses become less precipitous, often resembling the gradual losses found at the fovea for certain spectral combinations of bleaches and 20′ diam tests (the same diameter tests we used previously). On the other hand, when stimuli which result in gradual losses foveally (e.g., a 20′ diam 578-nm test following a 580-nm 4.85-log Td bleach) are moved to the parafovea, the losses can become early and precipitous. Making these foveal tests smaller can also result in precipitous losses. The data suggest that (a) precipitous losses are associated with relatively small tests, while delayed or gradual losses are associated with relatively large tests and (b) the relevant spatial dimensions depend on the spectral parameters of the bleach and test as well as on the retinal locus.
Saturation of rod-initiated signals in two-month-old human infants
Authors: Brown Angela M.
Published: February 2023

A model of rod vision in human infants is proposed in which rod-initiated signals saturate after the rods themselves but before most of the extensive spatial pooling of visual signals that occurs in infants. This model is based on an experiment in which psychophysical absolute thresholds were measured on two month olds using 10-ms short-wavelength test flashes that were 6–16° in diameter. Psychometric functions were steep for large stimuli and shallow for small stimuli. According to the model, rod-initiated signals from large stimuli are pooled over a large retinal region, which overcomes the extreme insensitivity of the proximal pathways to some extent, allowing 100% correct performance to be achieved based on rod-initiated signals. For small stimuli, however, the distal rod pathways of the infant are taxed beyond their capacity to respond; the result is saturation of rod-initiated signals at remarkably low test luminances. The psychometric functions are shallow for small stimuli because 100% correct performance is not attained until the test flash has high enough luminance to be detected by means of cone-initiated signals. Increment threshold data were also collected and are consistent with the model.
Measurement of the saturating nonlinearity in the retinal light response
Authors: Liu Lei , Tyler Christopher W.
Published: February 2023

Recordings from human cones exhibit nonlinear response saturation at high intensities, which match the hyperbolic asymptote of the Naka-Rushton equation.1 Psychophysical measurement of this saturation is made difficult by an adaptive gain control mechanism which holds the retinal response at the same operating point on the static nonlinearity. We, therefore, devised a paradigm to clamp the gain control at a fixed adaptation level, allowing direct measurement of the form of the nonlinear response saturation by increment and decrement thresholds. A 7′ test stimulus of 660 nm was viewed in the foveola with a large surround kept at a fixed level of 50,000 trolands (Td). The steady base level of the test stimulus was varied over about a ±1-log unit range but should have had a minimal effect on the local adaptation pool in the region of the test. Two-pulse measures of the response time constant verified constancy of pooled adaptation level over the base intensity range. Up and down test pulses of 65-ms duration were used to probe the static nonlinearity. Up/down asymmetries of up to a factor of 2 occurred at the highest base intensities, as expected from the Naka-Rushton function. A complete specification of the foveolar saturating nonlinearity was constructed from the up/down data.
Can human cones process one million quanta per millisecond?
Authors: Tyler Christopher W. , Hamer Russell D. , Liu Lei
Published: February 2023

Can the photochemical processes involved in light transduction perform linearly under extreme levels of illumination? The space between each outer segment disk appears to act as an independent transduction unit, and there are about 600 disks in human peripheral cones. Thus linearity would be expected up to levels of ~300 quanta absorbed per cone (50% of the independent processing units) within the cone response time (i.e., simultaneously). Beyond this level, increasing numbers of quanta must be processed simultaneously within each interdiscal space. We measured human light response properties at flux levels up to nine million quanta per cone per millisecond [15 million trolands (Td) incident on peripheral cones]. We used conditions in which brief flashes at this intensity were at psychophysical threshold: a 30′ field of 660 nm at 35° eccentricity with an equiluminant white surround. Temporal integration for pulses of different durations was measured in these conditions to determine the response linearity as stimulus duration was reduced. Complete linearity was obtained for durations down to 100 ps, where about one million quanta were absorbed at threshold (more than 1000 quanta absorbed for each outer segment disk).
Rib profile effects on scattering in semiconductor optical waveguides
Authors: Hawkins R. J. , Deri R. J. , Kapon E.
Published: February 2023

Low-loss semiconductor waveguide components are essential for integrating guided-wave optical components with other optoelectronic devices. Trade-offs between fabrication-related scattering loss and bend radiation loss will determine the smallest size practical for semiconductor integrated-optic devices. Rib waveguides with tight optical confinement (small mode size, large index differences) are required for compact bends but are more susceptible to fabrication-induced scattering loss from rough surfaces. An understanding of how rib geometry affects scattering would provide a valuable tool to optimize guide performance. Although scattering in 1-D planar guides has been extensively investigated theoretically,1,2 a complete model for 2-D rib guides is not available.
Enhanced self-phase modulation in a regenerative amplifier for subpicosecond pulse compression
Authors: Yan Li , Ling Jun-Da , Ho P.-T. , Lee Chi H. , Burdge Geoffrey L.
Published: February 2023

We have demonstrated a new way to generate subpicosecond pulses with high pulse energy at a high repetition rate. By enhancing the intracavity self-phase modulation in a Nd:phosphate glass regenerative amplifier, we can both amplify the picosecond pulse and broaden the pulse spectrum. The spectrally broadened pulses are then compressed by a pair of gratings to subpicosecond widths. A single Ar-ion laser was used to pump both a neodymium phosphate glass oscillator and a regenerative amplifier.1 The oscillator was actively mode-locked by an acoustooptic modulator to generate 10-ps pulses at 100 MHz with an average power of 50 mW. A seed pulse (~5 pJ) from the oscillator was coupled into the regenerative amplifier which amplified the pulse energy to 16 µJ. Self-phase modulation in the Nd:glass broadened the pulse spectrum to ~15 Å. The pulse was compressed to 1.25 ps by a pair of gratings. To enhance the intracavity self-phase modulation, an unpumped slab of Nd:phosphate glass was inserted into the regenerative amplifier. The pulse spectrum was further broadened to ~33 Å. The pulse was then compressed to 0.55 ps with a pulse energy of 9 µJ and pulse repetition rate of 370 Hz.
Continuous-wave synchronously pumped Q-switch No. 5 dye laser
Authors: Choa F. S. , Liu P. L.
Published: February 2023

Ultrafast tunable pulses between 1300 and 1550 nm are very useful for the study and characterization of devices and materials used in optical communications systems. Recently, picosecond pulses tunable between 1180 and 1530 nm have been generated by using an actively/passively mode-locked pulse train followed by an amplifier to pump dye No. 5.1
Orthonormal trap (and other lens optimization issues)
Authors: Weller Scott W.
Published: February 2023

Optical systems are growing in complexity, and designers of these systems are finding limitations in the well-known optimization methods. In addition, low-order approximations and intuition are playing a decreasing role in lens optimization.
Optimization of merit function evaluation in lens design
Authors: Forbes G. W.
Published: February 2023

The success of global optimization routines in optical design relies heavily on the reliability and efficiency of the methods used for system assessment (i.e., merit function evaluation). In fact, this efficiency is also vital in everyday lens design, since the lion's share of computer time is spent in system assessment. Since the merit function is typically evaluated by tracing a limited set of rays, efficiency can be expressed in terms of the number of rays required to obtain a given accuracy in the measure of overall system performance. For simplicity, the Gaussian pupil is adopted throughout this work. The efficiency of a number of standard system assessment schemes is presented, and some novel highly efficient schemes are described. The key to the new schemes lies in coupling appropriate ray coordinates with the method of Gaussian quadrature. The resulting efficiencies are typically an order of magnitude or more ahead of comparatively simple schemes.
Simulated annealing applied to optical design with discrete lens elements
Authors: Pickrell Mark m. , Hearn Gregory K.
Published: February 2023

We have addressed the problem of designing corrected lens systems using off-the-shelf elements (i.e., lens elements with discreet parameters), as might be found in a catalog. Our approach has been to take an existing algorithm, the simulated annealing algorithm, rederive the underlying statistical mechanics, and apply it to the optical engineering problem.
Least-squares fitting of an aspheric surface by a sphere, conoid, or toroid
Authors: Tatian Berge
Published: February 2023

Spherical fits to an aspheric surface provide a measure of the raw asphericity, give an easily generated starting point for its fabrication, and show the amount of aberration to be expected in a center of curvature test. Conoidal fits have similar uses. We have previously shown that a conoidal fit can be substantially closer than a spherical fit. We extend our methods to include fits to sections without any symmetry, even though the base surface has plane or greater symmetry. The fitting process in the conoidal case takes on many of the characteristics of a lens design optimization problem. Although it is not as complex as the typical lens design, it is difficult because the solution has to be obtained completely automatically in a reliable and reasonably efficient manner for many different kinds of surface. We describe the various techniques used to accomplish this and give examples of some actual fits so obtained.
Generalized field tracing method for analyzing and reducing flare light in optical systems
Authors: Evans Eric D. , Collins Stuart A.
Published: February 2023

We present an approach for analyzing and reducing image plane flare light in optical systems. The approach uses field calculations as well as standard ray tracing routines to predict the irradiance of stray light fields. A flare analysis routine has been written to trace fields through optical systems and calculate the levels of background flare due to multiple reflections between lens surfaces. We include these flare calculations in the merit function of a lens optimization program and describe an efficient algorithm for calculating a flare figure of merit. Using this approach, we redesigned some simple systems for low flare levels and comparable levels of image aberrations.
Zeeman laser-based active laser interferometer
Authors: Pan Ci-Ling
Published: February 2023

Wavelength tunable laser diodes have recently been used for interferometric fringe stabilization and displacement measurement in the so-called active laser interferometers.1 The basic idea is to compensate for any changes in the interference signal by tuning the frequency of the laser. A frequency- and amplitude-stabilized Zeeman He–Ne laser2 is used instead in this work. Utilizing the extreme narrow linewidth (<1 MHz) of the laser, we have demonstrated that subnanometer resolution in the measurement of the displacement of a piezoelectric transducer is possible. While the single-mode tuning range of the laser is only ~300 MHz, displacement measurements over many wavelengths can be realized by adjusting the optical path length difference of the two arms of the Michelson interferometer. Interferometric fringe stabilization has also been realized with a stabilization factor of more than 2 orders of magnitude.
Influence of intensity noise on the calculated phase from phase-shifting interferometers
Authors: Brophy Chris P.
Published: February 2023

The influence of noise in the measured intensities on the calculated phase from a phase-shifting interferometer is analyzed. A dependence on the particular phase-shifting algorithm used is investigated for the cases of independent intensity noise and intensity noise due to quantization. The method used for the analysis is a straightforward Taylor series expansion of the phase in terms of the multiple intensities necessary for phase-shifting algorithms. When statistically independent intensity noise is present, the variance of the calculated phase improves for algorithms using a larger number of intensity frames: the variance is shown to be independent of the algorithm for the case of error in measured intensities due to quantization. Good agreement is obtained between this method of analysis and Monte Carlo simulations.
Color naming and large color differences
Authors: Boynton Robert M.
Published: February 2023

Color perception is categorical. Although many words for describing colors have been invented and used, there are only eleven basic color terms that are unambiguously employed to describe more than a million discriminably different colors. The 424 samples of the OSA Uniform Color Scales set are intended to sample 3-D subjective color space uniformly. A given distance in this space is, therefore, intended to represent the same number of discriminable steps of color difference, whatever the starting point or direction of measurement. Each basic color term describes a region of subjective color space, with variable exactness rather than a point. Using systematic monolexemic color naming of the OSA samples, we have confirmed the special status of the basic colors and have determined the locations of their centroids and the extents of overlapping regions in standard conditions. The method, which has the dual advantages of requiring no training of subjects or use of comparison stimuli, has been used successfully to examine color constancy, color rendering, color blindness, cultural differences, and color perception in very young children.
New measures of medium color differences
Authors: Wandell Brian A.
Published: February 2023

Brown and MacAdam’s experimental procedure, based on color-matching variability, is a good laboratory model for predicting small color differences of foveated objects. Their analysis played an important role in the development of the CIELUV and CIELAB metrics which specify color tolerances. With the increasing use of color monitors to display information and enhance human performance, the same color metrics have been called on to guide the selection of color differences in visual tasks that require somewhat larger color differences (medium color differences). Since Brown and MacAdam’s seminal work, a number of investigators have investigated medium color difference tasks using a variety of performance criteria (reaction times, probability correct discrimination) and spatial configurations (position uncertainty, retinal location). I review the mathematical representation of metric color differences used by Brown and MacAdam. I describe medium color difference tasks in which performance can be described using Brown and MacAdam’s metric representation, permitting us to compare small and medium color differences. Finally, I describe some tasks requiring medium color differences in which performance is qualitatively different from performance in small color difference judgments.
Color differences required for parallel visual search
Authors: Nagy Allen L. , Sanchez Robert R.
Published: February 2023

In previous experiments we have found that the time required to find a target disk among equalluminance distractors depended on the chromaticity difference between target and distractors. When the chromaticity difference was a small search time increased with the number of distractor items in the display indicating a serial search. When the chromaticity difference was large, the search time was approximately constant regardless of the number of distractors in the display indicating a parallel search. In the experiments to be described here we examined the effects of luminance differences on search times. Search times were measured as a function of luminance difference and number of distractors for targets and distractors of the same chromaticity. When the luminance difference was small, the search time increased with the number of distractors indicating a serial search. When the luminance difference was large, search times were approximately constant suggesting parallel search. Results for targets that differ from distractors in both chromaticity and luminance are also discussed. The chromaticity and luminance differences required for parallel search are many times threshold. The parallel search as a criterion for defining large color differences is discussed.
Optical symbolic logic processor using cascaded LCTVs
Authors: Yu Francis T. S. , Jin Yong , Zhang Chenhua
Published: February 2023

Basic optical logical array processing has been reported recently. Thus a programmable optical symbolic logic processor can be constructed with three cascaded LCTVs.1 This symbolic logic processor can perform not only sixteen basic logic operations but also two-level sum-of-product logic functions. Based on the shadow-casting method,2 two input patterns and the operation mask are generated by a microcomputer. The combinations of these three patterns provide the basic sixteen logic gates. This symbolic logic processor can also perform any two-level sum-of-product logic function. To perform a logic function, two input patterns are written onto two LCTVs, respectively, in n × n arrays. Each column on LCTV1 represents one logic variable, while each row on LCTV2 represents one logic variable. The operation mask written onto LCTV3 is determined according to the specific logic function. Thus any two-level sum-of-product logic function can be performed in parallel, and it can be changed by switching the operation mask stored in the microcomputer.
Single SLM optical pattern recognition
Authors: Javidi Bahram , Odeh Souheil F. , Kuo Chungjung , Horner Joseph L. , Gregory Don A.
Published: February 2023

We describe a joint Fourier transform image correlator that employs thresholding at both the input plane and the Fourier plane. This suggests using a single binary spatial light modulator (SLM) to read in sequentially the binarized input signal and the binarized Fourier transform interference intensity. The performance of the single SLM joint Fourier transform correlator (JTC) is compared with that of the classical JTC in the areas of correlation peak intensity, peak-to-sidelobe ratio, signal-to-noise ratio, and correlation width. We know that the single SLM JTC outperforms the classical JTC in all such areas. Using a single binary SLM results in a significant reduction in cost, size, and complexity of the system.
Effect of ion-ion interactions on the lifetimes of the 2900-nm laser levels of Er3+
Authors: Shi W. Q. , Machan J. , Birnbaum M. , Bass M. , Kokta M.
Published: February 2023

The effects of ion-ion interactions in heavily doped Er:YAG, Er,Nd:YAG, and Er,Nd;YALO on the lifetimes of the 4/11/2 and 4/13/2 levels of Er3+ were studied. In Er:YAG, the 4/13/2 lifetime first increases with increasing concentration and then decreases. The increase can be attributed to radiation trapping and the decrease to concentration quenching. In Nd(1 %), Er(15%):YAG, the 4/11/2 level lifetime is halved, while the 4/13/2 level lifetime suffers an approximately sixty-fold reduction, but τ13/2 > τ11/2. However, study of a YALO crystal (1% Nd, 15% Er) disclosed a τ11/2/τ13/2 ratio of ≈ 1.7, indicating the feasibility of four-level laser operation. Ion-ion interaction coefficients were obtained from an analysis using the rate equation model. Laser operation at 2900 nm with Nd,Er:YAG, and Nd,Er:YALO is presented.
Kinetics of gradual degradation in semiconductor lasers
Authors: Khaital Yu. L. , Salzman Joseph , Beserman R.
Published: February 2023

Gradual degradation in semiconductor lasers is caused by the creation and migration of point defects which require jumping of atoms from their initial position to disordered ones (with activation energy E
a
of a few electron volts). The injection of a nonequilibrium electron-hole plasma may increase the probability of structural changes and reduce their activation energy by δE ≃ F
c
− F
v
through nonradiative recombination1 (F
c
and F
v
are the quasi-Fermi levels). Such enhancement is possible only if the recombination is synchronized with the atom jump event, and the released energy δE is localized around this atom. In the present model, such synchronization is inherent: the fluctuating atom forms a transient point defect which behaves as a nonradiative recombination center of lifetime δ
τ
≃ 10−13 − 10−12s. Free carriers are trapped at these centers leading to synchronous release of localized energy δE. This model is based on the kinetic many-body theory of short-lived large energy fluctuations of atoms.2,3 The laser degradation rate K is given by K = K
o
exp[−(E
a
− δE)/kT] with K
o
and δE expressed in terms of the laser parameters (carrier concentration, energy gap, defect concentration, etc.) Calculated laser lifetimes are in good agreement with experimental data.
All-optical demultiplexing by frequency upconversion
Authors: Lin Hong , Smith John Stephen
Published: February 2023

We propose the use of frequency upconversion for time division demultiplexing. Suppose we receive an N-channel signal at frequency f1 and make a local short-pulse laser at frequency f2 have the same repetition rate as one of the channels. Mixing them properly in a nonlinear crystal, we can upconvert only one channel to frequency f3 = f1 + f2. We can demultiplex N channels by N such units. The frequency upconversion is a very fast (femtosecond domain or 10E5 GHz) and a low noise process. We show that nearly 80% conversion efficiency of 1.3-0.58 μm can be expected for 50-W peak power pumping at 1.06 μm in a 3-mm long barium sodium niobate crystal.1 The use of waveguide structure or high nonlinear organic materials may relax the pumping power requirement substantially.