Laser linewidth measurements using digital holographyThornton, Douglas E.; Spencer, Mark F.; Rice, Christopher A.; Perram, Glen P.
doi: 10.1117/12.2529624pmid: N/A
This paper measures the mixing efficiency of a digital holography system at various optical path differences and hologram integrations times. From the measurements, the master oscillator (MO) laser spectral lineshape and linewidth is estimated. The lineshape was Gaussian, which is indicative of laser frequency flicker noise or 1/f noise, and the linewidth decreased by 65% when the integration time was decreased from 100 ms to 0.1 ms. This reduction in the observed MO laser linewidth yields an increased coherence length and time of 280%. A flicker noise model for the linewidth and integration time is used and approximates the measurements to within 17%.
Closed-form expressions for digital-holographic detection in a laboratory settingRadosevich, Cameron J.; Spencer, Mark F.
doi: 10.1117/12.2529645pmid: N/A
Digital-holographic detection provides a distinct way forward to combat the low signal-to-noise ratios (SNRs) associated with tactical applications. With that said, past efforts studied the major digital-holographic recording geometries used with tactical applications and derived closed-form expressions for their SNRs. They also used wave-optics simulations to validate the use of these closed-form expressions and found that for real-world scenarios, where we interfere a comparatively weak-signal beam with a strong-reference beam, the associated “weak-strong” expressions are accurate. For a laboratory setting, however, we often set the signal and reference beams to be nearly equal in power. As such, in this paper we derive closed-form expressions for this specific use case. We also use wave-optics simulations to explore the accuracy of these “equal-power” expressions. Overall, the results show that the equal-power expressions are accurate.
Estimation ambiguities encountered when imaging through turbulencePaxman, R. G.; Carrara, D. A.; Miller, J. J.; Gleichman, K. W.; Rucci, M. A.; Karch, B. K.
doi: 10.1117/12.2533979pmid: N/A
There are many approaches to incoherent imaging through the atmosphere that involve joint estimation of multiple turbulence-induced wavefront-aberration realizations and an object that is common across realizations. These approaches, all of which use short-exposure or “speckle” data, include Multi-Frame Blind Deconvolution (MFBD), Phase-Diverse Speckle (PDS), and Wavelength-Diverse Speckle (WDS). We enumerate fundamental estimation ambiguities that arise within each of these modalities and identify strategies to eliminate some of the ambiguities.
Effects of sinusoidal phase modulation on the signal-to-noise ratio in a digital holography systemMao, Davin; Thornton, Douglas E.; Rice, Christopher A.; Spencer, Mark F.; Perram, Glen P.
doi: 10.1117/12.2528807pmid: N/A
Digital holography (DH) has been demonstrated to be an effective wave-front sensor in low signal-to-noise ratio (SNR) environments due to the use of a strong reference. However, since DH relies on the interference of a signal with the mutually coherent reference, the coherence properties of the master-oscillator (MO) laser can degrade system SNR at long ranges. In this paper, a DH system in the off-axis image plane recording geometry was assembled and used to measure the effects of the MO coherence properties on the SNR. The coherence properties of the MO laser were degraded using sinusoidal phase modulation, which imparted maximum phase shifts of 0.38, 0.55, and 0.73, at modulation frequencies of 20MHz to 100MHz. The measured coherence efficiency losses were closely predicted by the square of the fringe visibility, and deviated from the theoretical predictions by root-mean-squared errors of 0.0397, 0.0373, and 0.1007 for the three depths of modulation, respectively. The empirical data and models presented in this work may be used to assess efficiency losses in a DH system due to coherence effects.
On the degrees of freedom of the phase retrieval problem in phase spaceTestorf, Markus E.
doi: 10.1117/12.2530100pmid: N/A
Phase-space tomography describes the recovery of signals from a tomographic reconstruction of the generalized radiance distribution with measurements being interpreted as projections of phase space. For coherent signals the phase space of the Wigner distribution function is a highly redundant signal representation. For efficient signal recovery based on phase-space information, it is necessary to address these redundancies. To understand the number of independent measurements necessary for the recovery of the WDF, Talbot self-imaging is used to derive a formalism for discrete phase-space tomography and signal recovery.
Laser beam focusing through the scattering medium using bimorph deformable mirror and spatial light modulatorGalaktionov, Ilya; Kudryashov, Alexis; Sheldakova, Julia; Nikitin, Alexander
doi: 10.1117/12.2533985pmid: N/A
Bimorph deformable mirror with the clear aperture of 50 mm and 48 control electrodes and spatial light modulator with resolution of 1920x1080 pixels were used to increase the efficiency of focusing of partially coherent laser radiation, propagated through the 5 mm layer of the scattering suspension of 1 um polystyrene microspheres, diluted in distilled water, with the concentration values ranging from 105 to 106 mm-3. Medium with such parameters can be considered as an equivalent to the mid-dense fog layer with the length ranging from 300–500 meters up to 5 kilometers. Shack- Hartmann sensor was used to measure the distortions of averaged wavefront of laser beam, and CCD camera was used to estimate the intensity distribution of the focal spot in the far-field. Numerical and experimental investigation of the focusing improvement showed that it is possible to increase the peak intensity of the focal spot up to 45–60 %.
Water-cooled stacked-actuator deformable mirror for atmospheric applicationsToporovskiy, Vladimir; Kudryashov, Alexis; Samarkin, Vadim; Sheldakova, Julia; Rukosuev, Alexey
doi: 10.1117/12.2533984pmid: N/A
High-power laser systems are widely used for technological purposes or laser fusion experiments, but also to deliver the energy to some remote places here on the Earth as well as in space. Probably the only instrument to improve the phase structure of the laser beam is to use methods and elements of adaptive optics. The heart of any adaptive optical system is of course the deformable mirror. That is why the research and development of such optical elements continues for many years already. Here we propose a new stacked actuator mirror to be used to correct for not just the turbulent phase fluctuations, but also it should work with high CW power laser beams. Therefore, our mirror is a water-cooled one. Another important feature of this device is that each actuator could be easily substituted in case of failure. The mirror is 120 mm in diameter and has 121 control elements. Main characteristics of a mirror are presented in this work.
Measuring the Fried parameter of transmissive phase screens using digital-holographic detectionHorst, Samuel; Radosevich, Cameron J.; Pellizzari, Casey J.; Spencer, Mark F.
doi: 10.1117/12.2529680pmid: N/A
This paper makes use of digital-holographic detection in the off-axis image plane recording geometry to determine the Fried parameter of transmissive phase screens. Digital-holographic detection, in practice, provides us with an estimate of the complex-optical field (i.e., both the amplitude and wrapped phase); thus, we can use this estimate for determining the Fried parameter of transmissive phase screens, especially when the resulting aberrations follow Kolmogorov statistics. As such, this paper uses two experimental setups and Lexitek phase plates, which make use of Kolmogorov statistics to create aberrations with a prescribed Fried parameter. In both experimental setups, we place the Lexitek phase plates under test near the single-receiver lens of our digital-holographic system and assume isoplanatic conditions. In the first experimental setup, we uniformly illuminate a chrome-on-glass bar chart backed by Labsphere Spectralon®. We then use digital-holographic detection and an image-sharpening algorithm to indirectly measure the aberrations and determine the Fried parameter. In the second experimental setup, we send a collimated beam through the Lexitek phase plates. We then use digital-holographic detection to directly measure the aberrations and determine the Fried parameter. The results show that the first experimental setup overestimates the prescribed Fried parameter by 20%-60%, whereas the second experimental setup produces less variability with estimates of ±20% of the prescribed Fried parameter.
Spatially resolved indirect imaging of objects beyond the line of sightRangarajan, Prasanna; Willomitzer, Florian; Cossairt, Oliver; Christensen, Marc P.
doi: 10.1117/12.2529001pmid: N/A
The nascent field of indirect imaging is concerned with the recovery of information pertaining to objects that are beyond the line-of-sight (LoS) and hidden from view. Current approaches to indirect imaging are either limited in their ability to recover spatially resolved imagery (resolution of few centimeters at 1-meter standoff) or impose severe restrictions on the imaging geometry. The present work examines two approaches that recover spatial detail on hidden objects by exploiting spatial and spectral correlation in the light scattered by the objects. Experiments have demonstrated the ability to discern sub-millimeter spatial detail, on centimeter sized objects positioned 1-meter behind a wall.
Hyper-Laplacian priors combined with rotating pupils for image restoration in sparse aperture systemsZhang, Luting; Zhao, Yuejin; Dong, Liquan; Liu, Ming; Du, Haoyuan
doi: 10.1117/12.2527381pmid: N/A
A hyper-Laplacian can model the heavy-tailed distribution of gradients in natural scenes well, which have proven effective priors for deconvolution and denoising. However, because of missing point spread function (PSF) information in the two-dimensional spatial domain of optical sparse aperture (OSA) systems, a hyper-Laplacian prior of single exposure cannot recover the missing information of images. The main focus of this paper is on combining hyper-Laplacian priors with a pupil and its rotated pupils to compensate PSF information and improve the image quality in OSA systems. A scheme of rotating the pupil that has double apertures is analyzed. The cost function relative to multiple degraded images and PSFs obtained by rotating the pupil is established. The alternating minimization algorithm consisting of two phases is implemented to acquire restored images. In one phase, the non-convex part of the problem is solved. In the other phase, the fast Fourier transforms (FFTs) are used to solve a quadratic equation in the frequency domain. Using the peak signal-to noise ratio (PSNR), a quantitative analysis is provided. Simulation results show that hyper-Laplacian priors combined with rotating pupils can restore images better than a hyper-Laplacian prior of single exposure in an OSA system. Taking spoke-square image as the test image, the PSNR is 28.34 dB with two rotations and 23.52 dB without rotation. Moreover, the numbers of rotating the pupil that lead to different changes of the image quality are demonstrated.