Conventional laser resonators yield multimodal output, especially at high powers and short cavity lengths. Since highorder modes exhibit large divergence, it is desirable to suppress them to improve laser quality. Traditionally, such modal discriminations can be achieved by simple apertures that provide absorptive loss for large diameter modes, while allowing the lower orders, such as the fundamental Gaussian, to pass through. However, modal discrimination may not be sufficient for short-cavity lasers, resulting in multimodal operation as well as power loss and overheating in the absorptive part of the aperture.
In research to improve laser mode control with minimal energy loss, systematic experiments have been executed using phase-only elements. These were composed of an intra-cavity step function and a diffractive out-coupler made of a computer-generated hologram. The platform was a 15-cm long solid-state laser that employs a neodymium-doped yttrium orthovanadate crystal rod, producing 1064 nm multimodal laser output. The intra-cavity phase elements (PEs) were shown to be highly effective in obtaining beams with reduced M-squared values and increased output powers, yielding improved values of radiance. The utilization of more sophisticated diffractive elements is promising for more difficult laser systems.
Optical limiters transmit low intensity input light while blocking input light with the intensity exceeding certain limiting threshold. Conventional passive limiters utilize nonlinear optical materials, which are transparent at low light intensity and turn absorptive at high intensity. Strong nonlinear absorption, though, can result in over- heating and destruction of the limiter. Another problem is that the limiting threshold provided by the available optical material with nonlinear absorption is too high for many applications. To address the above problems, the nonlinear material can be incorporated in a photonic structure with engineered dispersion. At low intensity, the photonic structure can display resonant transmission via localized mode(s), while at high intensity the resonant transmission can disappear, and the entire stack can become highly re ective (not absorptive) within a broad frequency range. In the proposed design, the transition from the resonant transmission at low intensity to nearly total re ectivity at high intensity does not rely on nonlinear absorption; instead, it requires only a modest change in the refractive index of the nonlinear material. The latter implies a dramatic increase in the dynamic range of the limiter. The main idea is to eliminate the high-intensity resonant transmission by decoupling the localized (resonant) modes from the input light, rather than suppressing those modes using nonlinear absorption. Similar approach can be used for light modulation and switching.
The size reduction of a laser cavity is highly desirable during the process of designing a laser system, as it allows reducing the weight and size of the laser system as a whole, as well as increasing the robustness of the system’s operation under severe environmental conditions. This cavity length reduction should be achieved without sacrificing the output laser beam quality, especially in the far field region. One approach to reducing the laser cavity length is based on the selective generation of single higher order transverse radiation modes. We show that a single transverse mode generation is essential for producing high radiance, high beam quality far field distributions with short length laser cavities. In the past, the selection of a single high order transverse mode was performed by employing amplitude masks or localized, non-uniform pumping of the gain medium. Both approaches resulted in significant cavity losses, and an associated increase in the laser oscillation threshold, as well as a reduction in laser efficiency. In this work, we provide details of a “lossless” intra-cavity mode formation technique employing circular-shaped diffractive phase structures. The radial size of the diffractive structures can be optimized for the selection of specific transverse higher order laser cavity modes. We also define a lossless external-cavity transformation of the selected output transverse higher order laser cavity modes with diffractive phase plates that results in the formation of far field distributions containing high intensity on-axis peaks. Spatial characteristics of the transformed output laser cavity modes were analyzed, including encircled beam powers, as well as encircled power M2 functions. Results of this work can be applied to reducing cavity lengths of various laser systems.
Developing a customized micro-mirror array (MMA) is costly and time consuming. Characterization experiments can be nearly as labor-intensive as fabrication. It is therefore desirable to have a computational simulation as a cost-effective and efficient means to conduct exploratory investigations such that appropriate parameters and optical characteristics can be obtained prior to any physical tests. In this article, we present our simulation of a novel MMA and preliminary diffraction analysis. Using geometric mapping, we created a three-dimensional visualization of the 5 × 5 MMA based on userspecified poses, which provides an interactive virtual view of the MMA geometry. A number of parameters are used to allow customizable simulation of various micro-mirror poses within the hardware limits. In addition, the far-field diffraction is simulated based on the given mirror geometry. This simulation package provides a tool for studying optical properties of MMAs, and it enables computational means to evaluate the performance of MMA as a beam steering element of Infrared Countermeasure (IRCM) systems.
Direct measurement of the modulation transfer function (MTF) of focal plane arrays (FPAs) using random laser speckle approaches for the visible/near-infrared wavelength band has been well documented over the last 20 years. These methods have not transitioned to the midwave infrared (MWIR) primarily because other techniques have been sufficient and MWIR laser sources with sufficient output power have been unavailable. However, as the detector pitch decreases, MTF measurements become more difficult due to diffraction, while potential MTF degradation due to lateral carrier diffusion crosstalk makes accurate MTF characterization critical for sensor system design. Here, a random laser speckle FPA MTF measurement approach is adapted for use in the MWIR that utilizes a quantum cascade laser coupled with an integrating sphere to generate the appropriate in-band random speckle. Specific challenges associated with the technique are addressed including the validity of the Fresnel diffraction assumptions describing the propagation of the random speckle field from the integrating sphere to the FPA. Improved methods for estimating the power spectral density (PSD) of the measured speckle that reduce data requirements are presented. The statistics and uniformity of the laser speckle are presented along with PSD measurements and estimated MTFs of a MWIR FPA.
Micro-Electro-Mechanical Systems (MEMS) Micro-Mirror Arrays (MMAs) are widely used in advanced laser
beam steering systems and as adaptive optical elements. The new generation of MEMS MMAs are fabricated
by bulk micromachining of a single Silicon-On-Insulator wafer. Optical characterization of MEMS MMAs can
be done by direct detection of the reflected beams or by using more advanced wavefront measuring techniques,
such as a phase-shifting interferometer or Shack-Hartmann wavefront sensor. In the case of an interferometer,
the geometry of the tested MMA can be calculated after performing the phase unwrapping procedure, which
can be quite complex. In the latter case of the Shack-Hartmann wavefront sensor, careful selection of a highquality
array of microlenses is required in order to match the capabilities of the wavefront sensor to the measured
wavefront produced by the MMA. The presented digital Shack-Hartmann technique is a modified approach for
wavefront characterization based on digital processing of the interferometer data. The optical wavefront from
the tested MMA is mixed with the reference wavefront. Then the recorded interference intensity image is Fourier
transformed producing digitally synthesized images of the optical beams in the far field. Therefore, the digital
version of the Shack-Hartmann wavefront sensor does not require the use of an array of microlenses and is
primarily limited by the detector array geometry. One can digitally generate any configuration of subapertures
corresponding to various geometries of microlenses. However, this new technique does require coherent optical
mixing of the two wavefronts in order to produce the interference pattern.
Coherent high resolution imaging of distant targets is a challenging problem whose complexity involves understanding
of diffractive properties of the optical imaging system, speckle properties of target, statistical properties
of the atmospheric refractive index variations, coherence properties of the light source and noise limtations of the
detector array to mention a few major issues. In a coherent laser radar imaging system, the target is illuminated
with a laser beam, and the scattered light wave from the target light wave is mixed with a local oscillator wave
from the same source. The system is referred to as spatial heterodyne imaging system. The sparse aperture
imaging approach is based on reducing the total irradiance collection area using smaller sub-apertures, while
the resulting resolution of the sparse aperture imaging array is equivalent to an aperture larger than the subapertures.
The Ladar and Optical Communications Institute (LOCI) currently has a prototype of the high
resolution sparse aperture imaging testbed with an indoor and outdoor range capability. In collaboration with
industry partners, academic institutions, and the government this testbed is becoming an innovative research
tool in the field of high resolution coherent ladar imaging.
We present a formula connecting the change in the output of a multiple-channel waveguide to a micron-scale alteration in the physical properties of a segment of the waveguide. The analysis is motivated by the need to "tune" the properties of photonic devices after fabrication, necessitated by the present impossibility of fabrication to within the severe tolerances required to produce desired output characteristics. The results point to a method by which tuning can be carried out in an optimal manner. Example device application is illustrated by use of a simulation program.
We desire to simulate in the laboratory the speckle fields that would be produced by laser light scattered off of rough
surfaces. We are investigating the ability of liquid crystal spatial light modulators to simulate under electronic control
the statistical properties of speckle fields. Characteristics of the liquid crystal spatial light modulator, including temporal
response, spatial correlation, and discreteness in phase modulation will affect the ability to accurately simulate arbitrary
speckle fields. In the work reported here we describe that status of characterization of two types of liquid crystal
modulators. The method for accomplishing this characterization is described.
Liquid Crystal Optical Phased Arrays (LCOPA) capable of steering optical beams over large angles require very
large number of individually addressable electrodes that can be reduced by grouping the electrodes into periodic
pattern to modulate phase profiles with consequent stepwise phase corrections made by an additional LCOPA.
Such phase ramp-corrector configuration allows for reductions in the total number of the addressed electrodes and
results in lower costs of development and manufacturing of LCOPA devices. Characterization of the device made
by Teledyne Scientific for an experimental RF/EO antenna has been accomplished. Issues concerning optical
beam steering efficiency, incident angle dependency and transparent electrodes alignment were investigated.