## LightLike Glossary |
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Note: Many of the definitions in this glossary have been copied verbatim from the glossary originally prepared for an earlier version of the software. Some of these definitions still require updating to reflect recent changes to the software.
Absorbing boundaries Absorbing boundaries are a mechanism sometimes used when simulating optical propagation to avoid wraparound. Absorbing boundaries avoid this problem by multiplying the light after each propagation step by a filter ( in the spatial domain, not the spatial frequency domain), which is unity over the central region, then falls smoothly to zero at the edge of the mesh. Absorbing boundaries can themselves produce artifacts, because some light moving toward the edge of the mesh may be reflected back into the inner portion of the mesh, but this is generally less of a problem than wraparound. Absorbing boundaries are often used in combination with spatial filtering, i.e. filtering in the spatial frequency domain. Under some circumstances these techniques can make it possible to use a smaller propagation mesh than would otherwise be needed, reducing simulation execution time. AcsAtmSpec AcsAtmSpec is a C++ class which defines an atmospheric specification, used for a parameter to the LightLike components AtmoPath and GeneralAtmosphere. This specifies the detailed modeling parameters for a particular optical propagation path through the atmosphere: the path length, the altitude at each end, the number and placement of phase screens, the integrated turbulence strength (Cn2) and inner scale for each screen, and so forth. Atmosphere specifications can be created using turbtool, and for certain common cases (e.g. uniformed turbulence, or scaled Clear1) we have also provided a more convenient function call interface. AoTool AoTool is a special purpose graphical interface used for setting up adaptive optics geometries. This includes wavefront sensor subaperture geometries and deformable mirror actuator geometries, influence functions, and slaving relationships. AoTool is implemented using the MatlabTM graphical user interface facility, and comes with a number of Matlab m-files for use in analyzing the properties of candidate geometries and setting up wavefront reconstructor matrices.
birefringence Birefringence is a property associated with anisotropic optical media, such as certain types of crystals or materials under stress, where the index of refraction differs for different polarization components.
One of a number of well-known models for the variation of turbulence strength with altitude, derived from experimental data.
The coherence diameter, sometimes called the “Fried coherence diameter”
Collimated light is light which as it propagates remains compact in the dimensions perpendicular to the direction of propagation. Generally a beam of light will be collimated if and only if it is coherent, the transverse diameter of the beam is very large compared to the wavelength, and the phasefront is very nearly flat.
Cn2 Cn2 is a measure of the strength of turbulence, defined in terms of random variations in the index of refraction. For a detailed description, see Goodman, Statistical Optics.
Coordinate systems In LightLike different coordinate systems can be used in different parts of a system model, so that in setting up each part of the model you can work in whatever coordinate system is most convenient. Thus for setting up a model of a moving you would generally choose to work in a coordinate system moving with the target, while for modeling a moving optical system you would work in a coordinate system moving with it. Then, at the interfaces between coordinate systems, you use TransverseVelocity components to perform the appropriate coordinate transformations in x and y. Displacements in z involve optical propagations, which can be modeled using AtmoPath, GeneralAtmosphere, or VacuumPropagator. Velocity differences in z can generally be neglected.
DMModel DMModel is a C++ class, part of the LightLike class library, which is used to carry all the information necessary to describe a complete adaptive optics geometry (deformable mirror and wavefront sensor), plus the reconstructor matrix used to compute DM actuator commands from wavefront sensor subaperture slopes. FieldSensor FieldSensor is the base class for all non-realistic (field sensing) optical sensor models in the LightLike component library. It in turn is derived from WaveSensor which is the base class optical sensor models, both realistic and nonrealistic. WaveSensor takes care of the logic related to opening and closing the shutter, remeasuring the detected light incident illumination changes, readout lag, and so forth; FieldSensor assumes that the quantity measured is the complex field, defined on a rectangular mesh. Filter Filter is a C++ class, part of the LightLike class library, which is used to specify spatial filters and absorbing boundaries for use in modeling optical propagation.
To propagate a wavefront from one plane to another in LightLike, we make use of the fact that in scalar diffraction theory the field at one plane can be computed from the field at another plane using Fourier transforms in combination with quadratic phase factors, as can be seen by inspecting the form of the Fresnel propagation integral. (see Goodman, Introduction to Fourier Optics) Also, it turns out that by using two Fourier transform rather than just one, one can control the mesh spacing at the propagated plane, which is crucial in wave optics simulation, so wave optics codes generally use a “two-step” Fourier optics propagator.
Hill bump. The Hill bump is a feature of the power spectrum of fluctuations in the refractive index of air due to turbulence. Based on work by the Russian mathematician Kolmogorov, the power spectral density curves is expected to consist of three distinct regions: At low frequencies and large scales we expect to see features related to the processes with give rise to the turbulence in the first place; little is known about the actual shape of the spectrum in this regime. At intermediate frequencies and scales the spectrum goes over to a straight power law, reflecting the self-similarity of the energy cascade as larger turbules contribute energy to smaller turbules; the a slope is –11/3. At the highest frequencies and smallest scales viscosity becomes a factor, breaking the self-similarity, and the downward slope of the power spectrum increases rapidly. However just before it does so, there is a small region where the power actually goes up appreciably above the –11/3 line; this is the Hill bump.
One of a number of well-known models for the variation of turbulence strength with altitude, derived from experimental data. Incoming (describing a Light input or output ) Many LightLike optical components can act upon light incident upon them from either of two opposite directions, and typically these components have two or more Light inputs and two or more Light outputs. To help the user keep track of which inputs are related to which outputs, we have adopted a convention of referring to one direction as incoming and the other as outgoing, which by convention refer to light propagating in the direction of decreasing and increasing z, respectively. In general there is nothing to prevent one from connecting a component the opposite way, but for components which act upon the phase of the light one must take the sign reversal into account.
inner scale The inner scale is a feature of the power spectrum of fluctuations in the refractive index of air due to turbulence. Based on work by the Russian mathematician Kolmogorov, the power spectral density curves is expected to consist of three distinct regions: At low frequencies and large scales we expect to see features related to the processes with give rise to the turbulence in the first place; little is known about the actual shape of the spectrum in this regime. At intermediate frequencies and scales the spectrum goes over to a straight power law, reflecting the self-similarity of the energy cascade as larger turbules contribute energy to smaller turbules; the a slope is –11/3. At the highest frequencies and smallest scales viscosity becomes a factor, breaking the self-similarity, and the downward slope of the power spectrum increases rapidly. The scale size marking the transition between these two parts of the spectrum is called the “inner scale”. integrationMethod IntensitySensor IntensitySensor is the base class for all realistic (intensity sensing) optical sensor models in the LightLike component library. It in turn is derived from WaveSensor which is the base class optical sensor models, both realistic and nonrealistic. WaveSensor takes care of the logic related to opening and closing the shutter, remeasuring the detected light incident illumination changes, readout lag, and so forth; IntensitySensor assumes that the quantity measured is intensity, defined on a rectangular mesh, and integrates it for each exposure interval. Kolmogorov turbulence model
Light is a C++ class, part of the LightLike library, used to model optical interfaces, such as the interfaces between different optical components. Most subsystems in the LightLike component library have one or more inputs and/or outrputs of type Light. Light sources, e.g. lasers, generally have a single Light input, while optical sensors, e.g. cameras generally have a single Light input. Two-way optical components, e.g. lens and mirrors, typically have two Light inputs and two Light outputs, one of each for each propagation direction.
Markov approximation A mathematical approximation underlying the standard wave optics approach for modeling propagation through the turbulent atmosphere. The import in this context is that the turbulence realizations for different phase screens may be generated independently, because the correlation length for phase perturbations is much much smaller than the distances between phase screens.
outer scale The outer scale is a feature of the power spectrum of fluctuations in the refractive index of air due to turbulence. Based on work by the Russian mathematician Kolmogorov, the power spectral density curves is expected to consist of three distinct regions: At low frequencies and large scales we expect to see features related to the processes with give rise to the turbulence in the first place; little is known about the actual shape of the spectrum in this regime. At intermediate frequencies and scales the spectrum goes over to a straight power law, reflecting the self-similarity of the energy cascade as larger turbules contribute energy to smaller turbules; the a slope is –11/3. The scale size marking the transition between these two parts of the spectrum is called the “outer scale”. Outgoing (describing a Light input or output ) Many LightLike optical components can act upon light incident upon them from either of two opposite directions, and typically these components have two or more Ligjht inputs and two or more Light outputs. To help the user keep track of which inputs are related to which outputs, we have adopted a convention of referring to one direction as incoming and the other as outgoing, which by convention refer to light propagating in the direction of decreasing and increasing z, respectively. In general there is nothing to prevent one from connecting a component the opposite way, but for components which act upon the phase of the light one must take the sign reversal into account. Phase screens Phase screens are surfaces perpendicular to an optical propagation path defining an optical path difference as a function of position in the x-y plane. Phase screens are used in combination with vacuum propagations to model propagation through the turbulent atmosphere. Phase screens are typically generated randomly, but made to conform to a specified power spectrum characteristic of a particular turbulence model, such as the classic Kolmogorov model.
Point source modeling
reference wave In wave optics simulation optical wavefronts are represented using a complex mesh, where the complex amplitude corresponds to the field amplitude, and the complex phase angle corresponds to the phase of the wavefront with respect to some “reference wave”. Depending on the particular modeling problem, one might choose to use a planar or spherical reference wave, converging or diverging, depending on the nature of the light sources and sensors to be modeled and the size of the regions of interest at either end of the propagation path. You can use different reference waves for different light sources, as described in (ADD LINK TO USER’S GUIDE).
speedOfLight speedOfLight is a constant, equal to the speed of light in vacuum (c = 2.997925x108 m/s), defined in the file “PhysicalConstants.h”, which is used by the LightLike library, so the symbol speedOfLight is always available for use in setting expressions. It is most often useful for synchronizing event timing between one end of the optical propagation path and the other, as described in (ADD LINK TO USER’S GUIDE). It can also be useful in computing the lead ahead angles (e.g. v/c) for applications involving active illumination imaging and/or beam projection. Spatial filtering SpeckleModel
Speckle realizations Speckle realizations are used to model the light from an incoherent source. First, the total intensity incident on the reflector is computed, then multiplied by the specified reflectance map to obtain the reflected intensity. For each speckle realization, we create a field with that intensity pattern and a random phase pattern. Two different approaches for generating the random phase pattern are presently supported. In the first approach the phase is uniformly distributed on [-p:p] and delta-correlated. This results in putting significant energy into the highest spatial frequencies (highest angles) represented on the propgation mesh; depending on the mesh spacing, mesh size, propagation distance, and wavelength this can lead to wraparound when we propagate, where energy leaves one side of the mesh, then reappears on the other. To avoid this, we can use spatial filtering and/or absorbing boundaries, or we can turn to the second speckle approach. In the second approach we generate a delta-correlated phase pattern as before, and combine it with the reflected intensity to create a field, also as before. However we then perform a vacuum propagation back to the target plane, then multiply the resulting field by a (spatial domain) filter which is set to unity over a region somewhat larger than the receiving aperture, then rolls off smoothly to zero. Finally, we do a second vacuum propagation, back to the source plane, and that gives us the speckle field. The net result is similar to spatial filtering, except that the effective spatial filter varies gradually as one moves across the source mesh, at each point preserving just that part of the radiation which after propagation will wind up near the receiver aperture. For large incoherent sources this can sometimes reduce the size of the propagation mesh required significantly, reducing simulation execution time.
The Strehl ratio is one of the most commonly used performance metrics for both imaging and beam projection systems. It is defined as the ratio of the on-axis intensity obtained for a given system under given conditions to the on-axis intensity which would have been obtained with a diffraction limited system with the same aperture and total power at the same range. A rated quantity, sometimes called “peak Strehl” is often used for systems operating under conditions where the maximum of the intensity is not necessarily expected to occur on-axis. Peak Strehl is defined as the ratio of maximum of the long-term average intensity to the on-axis intensity for the corresponding diffraction limited system. Wave
wave optics Wave optics, or wave optics simulation, is a well-established paradigm for the numerical modeling of optical effects, including diffraction effects. Wave optics is based upon scalar diffraction theory, as described in Goodman, Fourier Optics.
Wavefront reconstruction
WaveReceiverDescription WaveReceiverDescription is a C++ class, part of the LightLike library, used to carry all them information about an optical receiver, e.g. a sensor, necessary for the correct modeling the propagation of light from an arbitrary light source to that receiver. This includes, for example, the minimum and maximum wavelength the receiver is sensitive to, and the physical location of the receiver. WaveSensor LightLike class library
Wraparound Wraparound is a common problem encountered when simulating optical propagation, a consequence of the finite extent of the propagation mesh: any light which in the course of propagation goes past the edge of the mesh on one side immediately reappears on the other. This is a simulation artifact, not representative of the behavior of the physical system being modeled, so it is a source of error.
X (coordinate axis)
Y (coordinate axis)
Z (coordinate axis) The z axis of all coordinate systems used in LightLike is defined to lie along the nominal propagation direction. By convention z=0 corresponds to the aperture of the main optical system, while the object being imaged or illuminated lies at some z>0. For purposes of labeling the Light input and outputs of two-way optical systems, the direction of increasing and decreasing z is referred to as outgoing and incoming respectively. |