Underwater wireless information transfer is of great interest to the military, industry, and the scientific community, as it plays an important role in tactical surveillance, pollution monitoring, oil control and maintenance, offshore explorations, climate change monitoring, and oceanography research. In order to facilitate all these activities, there is an increase in the number of unmanned vehicles or devices deployed underwater, which require high bandwidth and high capacity for information transfer underwater. Although tremendous progress has been made in the field of acoustic communication underwater, however, it is limited by bandwidth. All this has led to the proliferation of underwater optical wireless communication (UOWC), as it provides higher data rates than the traditional acoustic communication systems with significantly lower power consumption and simpler computational complexities for short-range wireless links. UOWC has many potential applications ranging from deep oceans to coastal waters. However, the biggest challenge for underwater wireless communication originates from the fundamental characteristics of ocean or sea water; addressing these challenges requires a thorough understanding of complex physio-chemical biological systems. In this paper, the main focus is to understand the feasibility and the reliability of high data rate underwater optical links due to various propagation phenomena that impact the performance of the system. This paper provides an exhaustive overview of recent advances in UOWC. Channel characterization, modulation schemes, coding techniques, and various sources of noise which are specific to UOWC are discussed. This paper not only provides exhaustive research in underwater optical communication but also aims to provide the development of new ideas that would help in the growth of future underwater communication. A hybrid approach to an acousto-optic communication system is presented that complements the existing acoustic system, resulting in high data rates, low latency, and an energy-efficient system.

Underwater Optical Wireless Communication

The Keck Observatory began science observations with a laser guide star adaptive optics system, the first such system on an 8-10 m class telescope, in late 2004. This new capability greatly extends the scientific potential of the Keck II Telescope, allowing near-diffraction-limited observations in the near-infrared using natural guide stars as faint as 19th magnitude. This paper describes the conceptual approach and technical implementation followed for this system, including lessons learned, and provides an overview of the early science capabilities.

https://www2.keck.hawaii.edu/optics/aodocs/PWetal2006PASP118_297.pdf

The W. M. Keck Observatory Laser Guide Star Adaptive Optics System: Overview

We present a model for the photoevaporation of circumstellar disks or dense clumps of gas by an external source of ultraviolet radiation. Our model includes the thermal and dynamic effects of 6-13.6 eV far-ultraviolet (FUV) photons and Lyman continuum EUV photons incident upon disks or clumps idealized as spheres of radius rd and enclosed mass M*. For sufficiently large values of rd/M*, the radiation field evaporates the surface gas and dust. Analytical and numerical approximations to the resulting flows are presented; the model depends on rd, M*, the flux of FUV and EUV photons, and the column density of neutral gas heated by FUV photons to high temperatures. Application of this model shows that the circumstellar disks (rd ~ 1014-1015 cm) in the Orion Nebula ("proplyds") are rapidly destroyed by the external UV radiation field. Close (d 1017 cm) to θ1 Ori C, the ionizing EUV photon flux controls the mass-loss rate, and the ionization front (IF) is approximately coincident with the disk surface. Gas evaporated from the cold disk moves subsonically through a relatively thin photodissociation region (PDR) dominated by FUV photons and heated to ~1000 K. As the distance from θ1 Ori C increases, the Lyman continuum flux declines, the PDR thickens, and the IF moves away from the disk surface. At d ~ 3 × 1017 cm, the thickness of the PDR becomes comparable to the disk radius. Between 3 × 1017 cm d 1018 cm, spherical divergence and the resultant pressure gradient in the 103 K PDR forms a mildly supersonic (~3-6 km s-1) but neutral Parker wind. This wind flows outward until it passes through a shock, beyond which gas moves subsonically through a stationary D-type IF. The IF is moved away from the disk surface to a standoff distance rIF 2.5rd. In this regime, the mass-loss rate is determined by the incident FUV photon flux and not the ionizing flux. However, at very large distances, d 1018 cm, the FUV photon flux drops to values that cannot maintain the disk surface temperature at ~103 K. As the PDR temperature drops, the pressure of the FUV-powered flow declines with increasing distance from θ1 Ori C, and again the EUV ionizing photons can penetrate close to the disk surface and dominate the evaporation rate. Radio, Hα, and [O III] observations of externally illuminated young stellar objects in the Trapezium region are used to determine rIF and the projected distances, d⊥, from θ1 Ori C. The observed values of rIF and d⊥ are combined with the theory to estimate the disk sizes, mass-loss rates, surface densities, and disk masses for the ensemble of extended sources in the Trapezium cluster. Observations of rIF, d⊥, and rd in HST 182-413 and a few other sources are used to calibrate parameters of the theory, especially the column of heated PDR gas. The disks have a range in sizes between 14 < log [rd/(cm)] < 15.2, mass-loss rates of -7.7 < log [/(M/yr)]<-6.2, surface densities at disk edge 0.7 < log [Σ(rd)/(g cm-2)] < 2.5 which imply disk surface densities at 1 AU from the central, embedded star of 2.8 < log [Σ0/(g cm-2)] < 3.8 and disk masses of 0.002 < Md/M☉ < 0.07. Σ and Md scale with the adopted ionization time, ti, which we take to be 105 yr. The inferred Σ(rd) for the ensemble of disks suggest that the initial surface density power law of an individual disk, Σ ∝ r-α, is bounded by 1 α 1.5.

https://iopscience.iop.org/article/10.1086/305658/pdf

Photoevaporation of Disks and Clumps by Nearby Massive Stars: Application to Disk Destruction in the Orion Nebula

I describe a nematic liquid-crystal spatial light modulator that can be used as a high-precision wave-front control device. I present results showing the open-loop correction of wave-front aberrations and demonstrate wave-front shaping by the production and quantification of the first 15 significant Zernike terms.

Wave-front correction and production of Zernike modes with a liquid-crystal spatial light modulator.

▪ Abstract The Orion Nebula (M 42) is one of the best studied objects in the sky. The advent of multi-wavelength investigations and quantitative high resolution imaging has produced a rapid improvement in our knowledge of what is widely considered the prototype H II region and young galactic cluster. Perhaps uniquely among this class of object, we have a good three dimensional picture of the nebula, which is a thin blister of ionized gas on the front of a giant molecular cloud, and the extremely dense associated cluster. The same processes that produce the nebula also render visible the circumstellar material surrounding many of the pre–main sequence low mass stars, while other circumstellar clouds are seen in silhouette against the nebula. The process of photoevaporation of ionized gas not only determines the structure of the nebula that we see, but is also destroying the circumstellar clouds, presenting a fundamental conundrum about why these clouds still exist.

/pdf/the-orion-nebula-and-its-associated-population-4lh8cu6i25.pdf

The Orion Nebula and Its Associated Population

The effect of non-Kolmogorov stratospheric turbulence on star image motion is for the first time experimentally investigated with a ground-based telescope. A new approach permitting isolation of star image motion induced solely by atmospheric turbulence is employed. In this technique Polaris image wander is recorded with the telescope bolted in place to minimize uncontrolled telescope motion. High resolution temporal and spatial statistics of wave-front tilt are obtained. The dependencies of tilt variance, tilt power spectra, and tilt temporal correlation on telescope diameter are investigated for five apertures in the range 0.1-1.5 m. The experimental data show the dependence of tilt variance on telescope diameter does not follow the predictions of the Kolmogorov and von Karman models. The graph of the measured dependence has a “knee” which can be explained only by assuming a non-Kolmogorov stratospheric turbulence effect. The difference between tilt components in different axes indicates anisotropy in stratospheric turbulent inhomogeneities. The slopes of the measured tilt power spectra, approximately -1 in the low frequency range and -8/3 in the high frequency range, do not agree with theoretical predictions. The measured tilt temporal correlation scale is in the range 0.1-1.0 s, and the behavior of the correlation coefficients indicates the effect of large scale inhomogeneities not predicted by the conventional model. Uncontrolled telescope motion is manifested as a “bump” in the tilt power spectra in the range 70-90 Hz, but this makes an insignificant contribution to Polaris jitter variance.

Experimental study of the effect of non-Kolmogorov stratospheric turbulence on star image motion

On September 15, 1996, researchers from Utah State University/Space Dynamics Lab in conjunction with Phillips Lab/Starfire Optical Range and Kjome Research successfully flew and tested a retromodulator laser communication package on a high altitude balloon. This paper addresses the layout and hardware used for the communication link, as well as presenting some preliminary data collected during the 6 hour flight of the balloon. The package was a proof of concept demonstration system for a low-power laser communications systems for small, low Earth orbiting satellites. The ferroelectric liquid crystal based retromodulator design of Utah State provided test patterns for modulation rates up to 20 kilo bits per second. Data was successfully downlinked using a 1200 bps RS232 format and a simplistic receiver. The Starfire Optical Range 1.5-meter telescope located on Kirtland AFB, tracked the balloon, which reached a float altitude of 31 km and collected the modulated light reflected from the payload.

Low-power FLC-based retromodulator communications system

We present H-alpha and I-band images of a approximately 1 min diameter field centered on theta(sup 1) C Ori made with a unique adaptive optics system that uses either starlight or Rayleigh-backscattered laser light to correct for atmospheric wavefront distortion. Approximately one-half of the stars in this region are positionally associated with knots of ionized gas, which are interpreted as photoevaporating envelopes of low-mass stars. The acronyms 'partially ionized globule' (PIGs), external ionized (accretion) disks in the environs of radiation sources (EIDERs), or protoplanetary disks (ProPlyDs) all refer to these same knots. The H-alpha fluxes of the PIGs are proportional to their 2 cm radio continumm flux densities, and for nearly all the ionized knots, the 2 cm brightness temperatures are consistent with theta(sup 1) C Ori as the primary source of ionization. The comet-like morphology of the bright nebulosities is modeled as the result of an equilibrium between photoionization, recombination, and shadowing. The radii of the ionized 'head' of the cometary PIGs grow with distance from theta(sup 1) C Ori; the radii range from approximately less than or equal to 0.05 sec to approximately 0.25 sec. We interpret the size-distance relationship as evidence that the envelopes all have the same density profile and mass-loss rate within a factor of 2. Faint, arcuate wisps are observed 1 sec to 2 sec distance from some of the cometary nebulosities; these are modeled as bow shocks caused by the wind from theta(sup 1) C Ori. The positions of the stars associated with the PIGs in the observational H-R diagram indicate they are pre-main-sequence stars with masses less than approximately 3 solar mass, with approximately 1 solar mass being typical. Their medium I-K color is 2.9.

Photoevaporating stellar envelopes observed with Rayleigh beacon adaptive optics

This paper discusses the laser beam control system requirements for power beaming applications. Power beaming applications include electric and thermal engine propulsion for orbit transfer, station changing, and recharging batteries. Beam control includes satellite acquisition, high accuracy tracking, higher order atmospheric compensation using adaptive optics, and precision point-ahead. Beam control may also include local laser beam clean-up with a low order adaptive optics system. This paper also presents results of tracking and higher-order correction experiments on astronomical objects. The results were obtained with a laser beacon adaptive optics system at Phillips Laboratory's Starfire Optical Range near Albuquerque, NM. At a wavelength of 0.85 micrometers , we have achieved Strehl ratios of approximately 0.50 using laser beacons and approximately 0.65 using natural stars for exposures longer than one minute on objects of approximately 8th magnitude. The resulting point spread function has a full width half maximum (FWHM) of 0.13 arcsec.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Laser beacon adaptive optics for power beaming applications

This paper reports progress in the use of laser beacons for compensating the images of faint objects using adaptive optics. The system describe is located at the USAF Phillips Laboratory's Starfire Optical Range near Albuquerque, New Mexico on a 1.5 m telescope. The adaptive optics system uses a 241 actuator continuous facesheet deformable mirror. A copper vapor laser beacon focused at 10 km range is used to sense higher-order distortions. A natural guide star is used for sensing full aperture tilt. The limiting factor for long exposure performance of the system is tracking--the correction of full aperture tilt. The main goal of the work reported here was to determine the performance of the tracking system under conditions of weak signal but good higher order compensation (similar to conditions encountered in laser beacon adaptive optics). Single axis rms track errors of approximately 20 milliarcsec were achieved for track signal levels equivalent to stars of 13th magnitude. However, we cannot track stars fainter than 9 - 10th magnitude because the laser induces very low level, long lived phosphorescence in the optical train. This appears to be a fundamental limitation for our system as it is presently configured.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Robert Q. Fugate

Papers

Experimental study of the effect of non-Kolmogorov stratospheric turbulence on star image motion

Low-power FLC-based retromodulator communications system

Photoevaporating stellar envelopes observed with Rayleigh beacon adaptive optics

Laser beacon adaptive optics for power beaming applications

Observations of faint objects with laser beacon adaptive optics