We will present results for new object spectrum phasor reconstruction methods in speckle imaging. Each phasor
reconstruction algorithm results from minimizing a very naturally defined weighted-least-squares error function. Once
we pick a phasor-based error function, the remaining steps in our algorithms are developed by setting the error function
variation, with respect to each phasor element, to zero. The resulting coupled nonlinear equations for the minimum error
phasor array are then solved iteratively, locating the error function minimum. In these applications, we will specifically
compare and contrast three implementations: 1) Knox-Thompson; 2) bispectrum, using two unit-shift bispectrum planes;
3) bispectrum, using four bispectrum planes. Although we develop and minimize error functions for three specific singleaperture
speckle methods, the approach readily generalizes to other cases.
We will develop and then compare object spectrum phasor reconstruction results for several
speckle imaging approaches. Each phasor reconstruction algorithm results from minimizing a
very naturally defined weighted-least-squares error function. Once we pick a phasor-based error
function, the remaining steps in our algorithms are developed by setting the error function
variation, with respect to each phasor element, to zero. The resulting coupled nonlinear
equations for the minimum error phasor array are then solved iteratively. In the applications, we
will compare and contrast three implementations: 1) Knox-Thompson; 2) Bispectrum, using
only two bispectrum planes; 3) Bispectrum, using four bispectrum planes. In each application
of the three approaches, we first calculate the modulus of the object spectrum using a Wiener-
Helstrom filter to remove the speckle transfer function. The methods then differ only in their
object spectrum phasor reconstructions. In the simulations, we will implement all three methods
on a simple object at low photon-per-frame light levels. Next, we will apply the methods to a
complex extended object.
In this paper, we briefly review optically pumped type-II "W" quantum-well semiconductor lasers that emit in the midinfrared
wavelengths. In addition, we demonstrate on-chip unstable resonator cavity devices that exhibit excellent lateral
We report on optically pumped mid-IR semiconductor lasers that are based on type-II wells. A systematic study of the effect of increasing the In-content in the In<SUB>x</SUB>Ga<SUB>1-x</SUB>Sb hole-well suggests that improved hole confinement results in improved power conversion efficiency at elevated temperatures that is also accompanied by a reduction in threshold power and a reduction in T<SUB>0</SUB>, the characteristics for threshold.
Numerous optical engineering applications lead to two two- dimensional difference equations for the phase of a complex field. We will demonstrate that, in general, the solution for the phase can be decomposed into a regular, single-valued function determined by the divergence of the phase gradient, as well as a multi-valued function determined by the circulation of the phase gradient; this second function has been called the 'hidden phase.' The standard least-squares solution to the two-dimensional difference equations will always miss this hidden phase. We will present a solution method that gives both the regular and hidden parts of the phase. Finally, we will demonstrate the method with several examples from both speckle imaging and shearing interferometry.
Filamentation currently limits the amount of diffraction- limited power that can be obtained from broad-area semiconductor amplifiers. This paper examines the filamentation tendencies of a wide input-aperture tapered amplifier. Experimental measurements of filament gain under varying duty-cycle are offered and compared to theory. A numerical simulation of the device operation, which addresses non-uniform current injection, is also presented.
This paper presents a technique for determining carrier lifetimes which does not require a fast detector or rely on an experimentally complex implementation. The technique is based both on a measurement and a parallel calculation: (1) A Hakki- Paoli measurement of modal gain versus current density, g(J), and (2) A theoretical determination of the modal gain versus carrier sheet density, g(N). Once the gain relationships have been determined, the carrier sheet density, N, can be functionally related to the current density, J, and the lifetime determined. We demonstrate this method on two InGaAs single quantum well lasers. This method may prove particularly useful for carrier lifetime estimations in long-wavelength semiconductor lasers.
Spontaneous emission spectra have ben obtained from semiconductor quantum well lasers of varying epitaxial design. Initial measurements taken normal to the active region through the substrate and a transparent contact exhibited a modulated spectral profile dependent on the collection angle. An image model with the quantum well active region as the source and the p-side metallization as the image plane explains the observed modification and as such, presents an excellent example of a simple cavity quantum electrodynamics (QED) effect in a planar semiconductor laser. The phenomenon is made possible by the proximity of the quantum well active region to the p-side electrical contact of the device. Modification of the spontaneous emission rate and spectra can be substantial and must be accounted for if one hopes to correctly infer modal gain or carrier heating phenomena in a device using this geometry. Alternatively, to avoid the influence of the cavity QED effect, spontaneous emission can be obtained through the side wall of the device. Using this method for collection of spontaneous emission, the effect of quantum well dimensions on carrier heating in single quantum well InGaAs or GaAs active regions was also investigated. Incomplete pinning of the carrier density was observed above threshold in these samples with low duty cycle pumping.However, minimal distortion of the carrier distribution to higher energies was observed at room temperature up to current densities of 1.6 kA cm<SUP>-2</SUP>. Low temperature spontaneous emission spectra revealed gain suppression from carrier heating and possibly spectral hole burning in InGaAs deep and shallow quantum well lasers.
Filament formation is currently a limiting factor in the development of high power, spatially coherent semiconductor amplifiers. An experimental and theoretical investigation has been conducted to examine the filamentation tendencies of tapered amplifier structures. Experimental measurements of the far-field intensity distribution of a tapered amplifier which has been intentionally `seeded' to filament are compared to a perturbative solution of the paraxial wave equation. This model is used to address several design issues which can be optimized to suppress filamentation. The effect of non-uniform carrier injection due to carrier- induced bandgap changes is also investigated numerically.
Semiconductor lasers present enormous potential for free-space laser communications. Recently, a new class of devices based on a master oscillator power amplifier configuration has emerged as the leading contender. The paper illustrates the monolithically integrated flared amplifier (MFA-MOPA) device. It incorporates an index-guided, single-lateral-mode- distributed Bragg reflector (DBR) master oscillator section that diffracts a relatively low- power, narrow spectral bandwidth signal into a tapered amplifier section. The tapered amplifier section then amplifies the output to a one-watt or greater level while the divergence precludes the formation of filaments, maintaining good beam quality during the amplification process. The final output facet of the amplifier section is anti-reflection coated so that feedback into the DBR master oscillator is minimized, while outcoupling the amplified power.
A variety of broad-area semiconductor laser devices are currently providing high-power with near diffraction-limited performance. These new devices share an important design element: the suppression of filamentation tendencies. Specific designs include traveling-wave amplifiers and reflective-wave amplifiers, tapered amplifiers, unstable resonator oscillators and most recently, curved or angled grating DBR/DFB oscillators. These latter designs offer the advantages of filament suppression and angular spectrum filtering, while providing strong spectral filtering as well. We review the motivation and design principles for several of the new broad-area DBR/DFB devices and describe the algorithms that have been developed in order to provide accurate numerical simulations and performance predictions.
The phenomenon known as laser induced medium perturbation (LIMP) or mode-medium interaction (MMI) is responsible for a loss of nearly 50% of the output energy from a focussed far-field spot in some pulsed CO2 lasers. A simple theory was previously developed to explain this result in terms of the formation of a phase grating in the gain medium due to enhanced vibrational-translational (VT) relaxation heating in regions where the intracavity flux is high. Attempts to employ this simple theory to model recent experimental results and predict and avoid the onset of this loss mechanism in the design of long pulse CO2 lasers demonstrated that it was inadequate for several reasons. In the present work, these shortcomings are examined and a new model is presented which incorporates wave optics and hydrodynamic relaxation effects. The results of calculations with this new model (time dependent Strehl ratio and pressure dependence) are in good agreement with experimental data for both electron-beam sustained and UV-preionized CO2 discharge lasers. This model is capable of determining laser design points which essentially eliminate the far-field energy loss.
This paper reviews and discusses the performance of unstable resonators concentrating on the design, analysis, and test of oscillators used on electric discharge and electron-beam pumped laser devices. The work covers over 15 years of device development including pulsed and continuous wave CO2, CO and excimer lasers, both subsonic and supersonic flow. The emphasis is on the review of the energy extraction and beam quality experimental results from a wide range of devices. The effect of flow characteristics on laser performance is discussed. The interaction between the optical mode formation and the laser medium, called the 'Mode-Medium Interaction', is presented in terms of both theoretical and experimental understanding. The device design constraints imposed by optical material limitations is also discussed.
An output beam with a nearly ideal focal spot can be obtained by combining an unstable ring resonator with 90 deg beam rotation with a lasing medium which has gain that would be too low for the effective use of a more conventional unstable resonator. As an example, results are presented of a simulation of an oxygen-iodine transfer laser with moderate gain. The simulation included a fairly detailed representation of the laser kinetics, including the effect of transverse gas flow. Results of the simulation include laser mode profiles and far-field patterns.
Several on-the-chip designs of diffraction-limited, broad-area semiconductor lasers are described. In all cases, the devices achieve single lateral-mode operation as unstable resonators with magnifications greater than, or approximately equal to, two. In the first class of devices, the unstable resonator is realized by creating a diverging mirror at the outcoupling facet. In the second class, a diverging quadratic index profile is created under the active region so that the divergence is distributed uniformly between the front and back facets. The modeling shows that optimum device designs exist for both types of devices. For these optimum designs, stable diffraction-limited operation is predicted for up to six times threshold.