Optical phase and amplitude measurements of underwater turbulence via self-heterodyne detection.

The creation of underwater optical turbulence is driven by density variations that lead to small changes in the water's refractive index, which induce optical path length differences that affect light propagation. Measuring a laser beam's optical phase after traversing these turbulent variations can provide insight into how the water's turbulence behaves. The sensing technique to measure turbulent fluctuations is a self-heterodyne beatnote enhanced by light's orbital angular momentum (OAM) to obtain simultaneous optical phase and amplitude information. Experimental results of this method are obtained in a water tank that creates a thermally driven flow called Rayleigh-Bénard (RB) convection. The results show time-varying statistics of the beatnote that depend on the incident OAM mode order and the strength of the temperature gradient. Beatnote amplitude and phase power spectral densities are compared to analytic theory to obtain estimates of the turbulent length scales using the Taylor hypothesis that include mean flow speed, turbulent strength, and length scales, and flow dynamics due to intermittency in the RB process.

Synchronous optical intensity and phase measurements to characterize Rayleigh-Bénard convection.

Propagation of a laser beam through the Rayleigh-Bénard (RB) convection is experimentally investigated using synchronous optical wavefront and intensity measurements. Experimental results characterize the turbulence strength and length scales, which are used to inform numerical wave optic simulations employing phase screens. Experimentally found parameters are the refractive index structure constant, mean flow rate, kinetic and thermal dissipation rates, Kolmogorov microscale, outer scale, and shape of the refractive index power spectrum using known models. Synchronization of the wavefront and intensity measurements provide statistics of each metric at the same instance in time, allowing for two methods of comparison with numerical simulations. Numerical simulations prove to be within agreement of experimental and published results. Synchronized measurements provided more insight to develop reliable propagation models. It is determined that the RB test bed is applicable for simulating realistic undersea environments.

Sensing optical phase distortion via beatnote detection of a dual probe beam encoded with orbital angular momentum.

Laser based optical applications such as imaging, ranging, and wireless communications are susceptible to environmental distortions. Inferring the strength of these optical distortions is crucial to obtaining information about the environment in which the system is operating. Our technique of inferring environmental distortion strength leverages the spreading of light's orbital angular momentum (OAM) spectrum combined with heterodyne detection. A laser encoded with OAM can be decomposed into a basis set of helical modes that spreads upon interaction with optical distortions. This mode spreading is quantified using the OAM spectrum that can be measured using mode projection or mode sorting techniques. This new technique, to the best of our knowledge, provides benefits compared to the latter two OAM detection methods such as: low-frequency noise rejection, a simpler optical receiver, lower noise floor, and an inherent optical phase component. Central to the method is the heterodyne detection of the zeroth-order OAM coefficient of a superimposed two-beam, two-frequency, probe. The measured heterodyne signal power is seen to be proportional to the coupling power of each beam's OAM spectra. To test the idea, wave-optic simulations and experiments using spatial light modulators are implemented using a simplified optical turbulence model to represent the environment. The experimental implementation agrees well with simulated and theoretical results.

Spatial and temporal domain filtering for underwater lidar.

Combined spatial and temporal processing techniques are presented to enhance optical ranging in underwater environments. The performance of underwater light detection and ranging (lidar) is often limited by scattering. Previous work has demonstrated that both hybrid lidar-radar, which temporally modulates the amplitude of light, and optical spatial coherence filtering, which spatially modulates the phase of light, have independently reduced the effects of scattering, improving performance. The combined performance of the processing methods is investigated, and experimental results demonstrate that the combined filtering improves the performance of underwater lidar systems beyond what either method provides independently.

Enhanced underwater ranging using an optical vortex.

An optical vortex is used to enhance the ranging accuracy of an underwater pulsed laser ranging system. An experiment is conducted whereby an underwater object is illuminated by a pulsed Gaussian beam, and both the object-reflected and scattered light are passed through a diffractive spiral phase plate prior to being imaged at the receiver. An optical vortex is formed from the spatially coherent non-scattered component of the return, providing an effective way to discriminate the desired objected reflected light from the spatially incoherent scatter. Experimental results show that the optical vortex permits a spatially coherent ballistic target return to be more easily discriminated from spatially incoherent forward scattered light up to eight attenuation lengths. The results suggest new optical sensing techniques for underwater imaging or lidar.

Experimental measurements of the magnitude and phase response of high-frequency modulated light underwater.

The propagation behavior of high-frequency intensity-modulated signals through turbid water is of significant interest for underwater laser ranging, imaging, and communications. Prior experimental measurements have focused only on the magnitude response of the underwater optical channel to forward-scattered and unscattered modulated light. In this study we include, for the first time to our knowledge, both the magnitude and phase of the underwater optical channel to forward-scattered light. The magnitude and phase response is measured out to 1 GHz, using three different artificial scattering agents in scattering environments in excess of 25 attenuation lengths. The phase response provides additional insight into the behavior of forward-scattered light carrying high-frequency intensity modulation.

Independent component analysis for enhancement of an FMCW optical ranging technique in turbid waters.

Optical detection and ranging in turbid waters are challenged by the effects of absorption and scattering. In particular, backscatter creates a clutter return, which can mask the presence of weak underwater targets. This work explores the use of independent component analysis (ICA), a statistical signal processing approach, to recover weak targets from strong backscatter in turbid waters using a frequency-modulated continuous-wave optical rangefinder. ICA uses statistical differences between target and backscatter returns to suppress the backscatter return. In laboratory test tank experiments, the use of ICA is observed to improve probability of detection at various turbidities and extend target detection range by four optical attenuation lengths.

Propagation of modulated optical beams carrying orbital angular momentum in turbid water.

The attenuation and temporal dispersion of beams with and without orbital angular momentum (OAM) underwater are investigated in a controlled laboratory water tank environment. Both spherical polystyrene beads and a commercial antacid are used to determine the effect of scattering particle size and shape on the results. Varying concentrations of the scattering agents were used to study the propagation of light in both minimally scattered and multiply scattered regimes (over 20 attenuation lengths). To study temporal dispersion, a custom diode seeded fiber amplified laser source is used to modulate beams up to 1 GHz, and diffractive spiral phase plates are used to compare performance over different spatial modes. We observe an increase in received signal with increasing OAM order (|m|=0, 8, and 16) under multiple scattering conditions. Initial experimental results suggest that this variation is dependent on particle shape and size. We do not observe any dependency of OAM order on temporal dispersion.

Digital passband processing of wideband-modulated optical signals for enhanced underwater imaging.

Radar modulation, demodulation, and signal processing techniques have been merged with laser imaging to enhance visibility in murky underwater environments. The modulation provides a way to reject multiple scattered light that would otherwise reduce image contrast and resolution. Recent work has focused on the use of wideband modulation schemes and digital passband processing to resolve range details of an underwater scene. Use of the CLEAN algorithm has also been investigated to extract object features that are obscured by scattered light. Results from controlled laboratory experiments show an improvement in the range resolution and accuracy of underwater imagery relative to data collected with a conventional short pulse system.

Optics research at the U.S. Navy System Commands Laboratories: introduction.

The purpose of this feature issue is to highlight optics research being performed at the U.S. Navy System Commands. Contributed papers cover a range of topics related to the various components of optical systems, including the optical source, the intervening medium, and the optical receiver, and processing techniques that are used to obtain information from the detected signal. While research from only two of the Navy System Commands is represented in this feature issue, it will hopefully enlighten the readers to the variety and high caliber of research being conducted in our Navy's laboratories and pave the way for additional features in the future.

Implementing a Perioperative RN Training Program for Recent Graduates.

In 2010, nurse educators at one health care facility implemented a new program that emphasized placing new graduates in specialty areas, including the hemodialysis unit, the gastroenterology unit, and the OR. Managers in the OR faced staffing challenges because of the difficulty in recruiting and retaining experienced perioperative nurses and the expected retirement of a significant number of staff members. Surgical services managers agreed to participate in the new graduate program and decided to use AORN's Periop 101™ course and a series of monthly simulation training sessions to supplement the program and provide recently graduated nurses with a strong perioperative nursing foundation. In three years, a total of nine newly graduated RNs successfully completed the program. The three-year retention rate was 78%.

Using simulation training to improve perioperative patient safety.

Simulation learning provides medical and nursing personnel with the opportunity to develop and refine their skills without putting patients at risk. Faced with ensuring the competence of a large number of new staff members, the management team at one facility implemented a simulation training program. Surgical team members are able to participate in an ongoing program of simulated scenarios involving surgical drape fires and airway fires, cardiac arrest of patients in the supine position and prone position, respiratory depression in the postanesthesia care unit, and malignant hyperthermia. The simulations help OR staff members identify problems that can happen during real emergencies and help them work as a team to prepare for events that may represent life-threatening situations for patients.

Modulated pulse laser with pseudorandom coding capabilities for underwater ranging, detection, and imaging.

Optical detection, ranging, and imaging of targets in turbid water is complicated by absorption and scattering. It has been shown that using a pulsed laser source with a range-gated receiver or an intensity modulated source with a coherent RF receiver can improve target contrast in turbid water. A blended approach using a modulated-pulse waveform has been previously suggested as a way to further improve target contrast. However only recently has a rugged and reliable laser source been developed that is capable of synthesizing such a waveform so that the effect of the underwater environment on the propagation of a modulated pulse can be studied. In this paper, we outline the motivation for the modulated-pulse (MP) concept, and experimentally evaluate different MP waveforms: single-tone MP and pseudorandom coded MP sequences.

Investigation of the effect of scattering agent and scattering albedo on modulated light propagation in water.

A recent paper described experiments completed to study the effect of scattering on the propagation of modulated light in laboratory tank water [Appl. Opt.48, 2607 (2009)APOPAI0003-693510.1364/AO.48.002607]. Those measurements were limited to a specific scattering agent (Maalox antacid) with a fixed scattering albedo (0.95). The purpose of this paper is to study the effects of different scattering agents and scattering albedos on modulated light propagation in water. The results show that the scattering albedo affects the number of attenuation lengths that the modulated optical signal propagates without distortion, while the type of scattering agent affects the degree to which the modulation is distorted with increasing attenuation length.

Effect of scattering albedo on attenuation and polarization of light underwater.

Recent work on underwater laser communication links uses polarization discrimination to improve system performance [Appl. Opt.48, 328 (2009)] [in Proceedings of IEEE Oceans 2009 (IEEE, 2009), pp. 1-4]. In the laboratory, Maalox antacid is commonly used as a scattering agent. While its scattering function closely mimics that of natural seawaters, its scattering albedo can be much higher, as Maalox particles tend to be less absorbing. We present a series of experiments where Nigrosin dye is added to Maalox in order to more accurately recreate real-world absorption and scattering properties. We consider the effect that scattering albedo has on received power and the degree of depolarization of forward-scattered light in the context of underwater laser communication links.

Propagation of modulated light in water: implications for imaging and communications systems.

Until recently, little has been done to study the effect of higher modulation frequencies (>100 MHz) or short (<2 ns) pulse durations on forward-scattered light in ocean water. This forward-scattered light limits image resolution and may ultimately limit the bandwidth of a point-to-point optical communications link. The purpose of this work is to study the propagation of modulated light fields at frequencies up to 1 GHz. Results from laboratory tank experiments and their impact on future underwater optical imaging and communications systems are discussed.

Backscatter suppression for underwater modulating retroreflector links using polarization discrimination.

Free space optical links underwater have the potential to enable short range (<100 m) high-bandwidth (megabits per second) data links that have a low probability of detection and interception. The use of a retroreflecting free space optical link in water has the added advantage of allowing much of the weight and power burden of the link to remain at one end. While modulating retroreflectors have been successfully implemented in above-water links, the underwater environment introduces new challenges. The focus of this paper is to address these challenges and to investigate techniques for minimizing their effect on the link performance.

Demodulation techniques for the amplitude modulated laser imager.

A new technique has been found that uses in-phase and quadrature phase (I/Q) demodulation to optimize the images produced with an amplitude-modulated laser imaging system. An I/Q demodulator was used to collect the I/Q components of the received modulation envelope. It was discovered that by adjusting the local oscillator phase and the modulation frequency, the backscatter and target signals can be analyzed separately via the I/Q components. This new approach enhances image contrast beyond what was achieved with a previous design that processed only the composite magnitude information.

Testing psychodynamic psychotherapy skills among psychiatric residents: the psychodynamic psychotherapy competency test.

OBJECTIVE: Training in psychodynamic psychotherapy remains a core requirement of psychiatric residency training programs, yet no standard measures of competency exist to document residents' knowledge and skills in this area. To address this issue, the authors developed a written test of applied knowledge of psychodynamic psychotherapy technique and theory, the Psychodynamic Psychotherapy Competency Test. Their goal in this article was to evaluate the validity of this test. METHOD: The test was given to a group of 36 psychoanalytic experts and 206 residents in their second, third, and fourth psychiatric postgraduate years from 10 programs located in different parts of the United States. Program directors provided information on the number of hours of psychodynamic didactic teaching, supervision, and resident-conducted psychodynamic psychotherapy and rated the psychodynamic psychotherapy skills of residents in their fourth postgraduate year on the basis of cumulative supervisor reports. RESULTS: There were significant differences in test performance between residents and faculty experts and between residents in their second and fourth postgraduate years: more advanced residents and experts had progressively better scores. The mean scores of fourth-year residents in different programs differed significantly, but the scores of second-year residents did not. Higher test scores were positively associated with both number of hours of resident-conducted psychotherapy and number of hours of supervision. Among fourth-year residents, test scores correlated significantly with program director evaluations. CONCLUSIONS: This initial study supports the validity of the Psychodynamic Psychotherapy Competency Test as well as the feasibility of testing psychotherapy skills in a standardized fashion.