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Free space optical propagation affects many application areas, including communication, LIDAR, and directed energy. Optical turbulence causes dynamic changes in the propagated beam that can impact these applications. A primary measure of these effects is the optical scintillation index. In this work, we report on a comparison of experimental measurements of optical scintillation, conducted on a 16 km range across the Chesapeake Bay over a period of three months, to model predictions. Models for turbulence parameters were based on NAVSLaM, a Monin Obhukov similarity theory, and used environmental measurements taken on the range simultaneously with the scintillation measurements. These parameters were then used in two different classes of optical scintillation models: Extended Rytov theory, and wave optic simulation. We show that wave optics simulation matched the data much better than Extended Rytov theory, and that prediction of scintillation using environmental parameters is possible. In addition, we show that optical scintillation over water has different characteristics in stable versus unstable conditions.

We show low-loss propagation in germanium-on-silicon (GOS) waveguides from 6.85 μm to 11.25 μm. The loss is as low as a few dB/cm at a wavelength of 10 μm for both the quasi-TE 00 and quasi-TM 00 modes.

We report a novel spectroscopic technique to characterize photonic waveguides over more than an octave of wavelength. The technique, called mode-crossing spectroscopy, uses a broadband source and a spectrometer that are coupled to the input and output ends of a series of straight dielectric waveguides via single-mode optical fibers. Measurements of the wavelengths at which multiple modes are degenerate in a single waveguide enable the determination of the refractive index of the core, as well as the waveguide width variation. In addition, mode-dependent losses are reported from 700 nm to 1550 nm using waveguides of varying lengths. The core refractive indices are measured within an error of ±0.5%, waveguide width variation to an accuracy of less than 5 nm, and waveguide propagation losses within an error of ±0.5 dB/cm. Ultrabroadband in situ measurements of loss and index over an octave of bandwidth are crucial for the accurate characterization of photonic integrated circuits and devices, especially those based on precision broadband couplers, interferometers, and resonant cavities.

Silicon photonics enables the development of optical components on a chip with the potential for large-scale optical integrated circuits that can be fabricated at the wafer-scale using foundries similar to those used in the electronics industry. Although silicon is a passive optical material with an indirect bandgap, reconfigurable devices have been demonstrated using thermo-optic effects (large phase shifts, but relatively slow with large power consumption) and carrier plasma dispersion effects (high-speed, but small phase shifts). We recently demonstrated a low-power approach for inducing large phase shifts (>2π) using a technique that we call micro-opto-electro-mechanical index perturbation (MOEM-IP). In this initial work we characterized silicon nitride waveguides in which the propagating optical mode’s evanescent field is vertically coupled to silicon nitride microbridges. This interaction leads to an effective index tuning that is a strong function of the waveguide-microbridge separation. We now extend our MOEM-IP approach to different configurations (i.e. in-plane coupling) and material systems (i.e. silicon-oninsulator). Mode perturbation simulations indicate that the MOEM-IP approach is widely applicable to many configurations and material systems enabling large effective index tuning (Δneffective>0.1) requiring microbridge displacements of only a few hundred nanometers. We also examine several device applications that take advantage of MOEM-IP. These include tunable optical filters using high-Q microring cavities and optical phased arrays that enable chip-scale beam steering in two-dimensions using low-power phase shifting enabled by MOEM-IP.

Functionalized silicon nitride waveguides are used to detect Raman scattering from trace concentrations of vapor phase chemicals. Parts-per-billion detection limits are measured using single-mode rib and nanoslot waveguides pumped at 785 nm.

Lasercom, also known as free space optical (FSO) communication, has enjoyed a renewal of interest driven by increasing data rate requirements and the crowding of the RF spectrum, affecting both commercial and military sectors. Military communications must also deal with intentional or unintentional jamming, as well as frequency allocation restrictions, neither of which affects lasercom. The U.S. Naval Research Laboratory (NRL) has been conducting research on lasercom since 1998 with an emphasis on tactical applications. NRL's lasercom research has covered propagation studies in the maritime domain, component development, and systems demonstrations. NRL has developed both conventional lasercom systems and retro-reflecting systems for small platforms. This paper reviews some of the retro-reflecting work, discusses applications of lasercom in the areas of explosive ordnance disposal (EOD) unmanned ground vehicles (UGVs), and unmanned aerial vehicles (UAVs), and describes future directions.

Recent progress in high-current photodiodes now makes it possible to efficiently generate over 26 dBm of RF power directly from the output of a photodiode. This paper describes two photodetector designs which demonstrate excellent large- and small-signal behavior. Maximum small-signal compression currents have increased to over 700 mA at 300 MHz. Output RF power amplifier stage efficiencies of over 45% (class AB operation) and 35% (class A) have been achieved from 0.3 to 6 GHz with RF power outputs over 26 dBm. The linearity figure of merit (LFOM) is also shown to be greater than 50, leading to the possibility of implementing very high linearity RF power amplifiers in the future.

Free-space optical communication can allow high-bandwidth data links that are hard to detect, intercept, or jam. This makes them attractive for many applications. However, these links also require very accurate pointing, and their availability is affected by weather. These challenges have limited the deployment of free-space optical systems. The U.S. Naval Research Laboratory has, for the last 15 years, engaged in research into atmospheric propagation and photonic components with a goal of characterizing and overcoming these limitations. In addition several demonstrations of free-space optical links in real-world Navy applications have been conducted. This paper reviews this work and the principles guiding it.

Anisotropic in-plane strain in quantum wells leads to an optical polarization anisotropy that can be exploited for device applications. We have determined that for many anisotropic compressive strain cases, the dependence of the optical anisotropy is linear in the strain anisotropy. This result holds for a variety of well and barrier materials and widths and for various overall strain conditions. Further, the polarization anisotropy per strain anisotropy varies as the reciprocal of the energy separation of the relevant hole sub-bands. Hence, a general result for the polarization anisotropy per strain anisotropy is avialable for cases of compressive anisotropic in-plane strain.

We have developed a high resolution optical technique to measure the electromechanical properties of MEMS microstructures. The technique is applied to microbridges developed for capacitive switching in coplanar radio frequency (RF) waveguides. The thin metal ground plane on the substrate and the bottom of the bridge together form a microcavity for an optical beam. The wavelength of a cavity mode is a sensitive measure of the bridge position relative to the substrate. The technique is applied to the measurement of resonances and damping times of microbridges of varying lengths. It is also used to measure dc changes in bridge height of tenths of nanometers, driven ac displacements of less than a picometer, and bridge displacement noise of hundreds of femtometers per root Hertz. This extreme sensitivity exceeds previously demonstrated optical characterization methods.

We demonstrate an opto-electro-mechanical phase shifter. A suspended microbridge is electrostatically actuated to strongly-interact with a weakly-confined waveguide mode. The resulting effective index tuning substantially exceeds that achievable with thermo-optic and electro-optic approaches.

Optical modulating retro-reflectors enable free-space optical links that have greatly reduced pointing requirements and do not require a laser at one end of the link. However, these types of links can exhibit very high optical scintillation due to the double passage of the beam through the atmosphere. This high scintillation causes fades and surges that can lead to packet errors in the link. It is shown that scintillation can be greatly reduced through a combination of techniques including retro-reflector diversity, aperture averaging and bistatic optical interrogation. Improvements of 20 dB in link performance are demonstrated.

A new method for fabricating microlenses in bulk, glassy Ge 20 Se 80 is demonstrated. Based upon a laser-induced thermal runaway effect, the process has been used to fabricate lenses with diameters between 70 and 180 μm and numerical apertures of about 0.4.

The third-order nonlinearity, /spl chi//sup 3/(/spl omega/,/spl omega/,-/spl omega/), is measured for a mid-infrared intersubband transition in strained InGaAs-AlGaAs multiple quantum wells (MQW's). The high conduction band offset of this system allows an intersubband transition at 3.1 /spl mu/m. The level structure of the quantum well is designed to include a meta-stable trapping level, resulting in a peak saturation intensity of 6 MW/cm/sup 2/ at Brewster's angle, approximately 20 times lower than would be found in a square quantum well with similar linewidth. A near-resonant n/sub 2/ of 8.4/spl times/10/sup -7/ cm/sup 2//W at 3.1 /spl mu/m is calculated. The off-resonant n/sub 2/ is also calculated and shown to be attractive at wavelengths as short as 1.55 /spl mu/m.

We experimentally demonstrate mid-infrared difference-frequency generation in suspended 181 nm thick GaAs waveguides. Generation of the idler at wavelengths between 2800 and 3150 nm is enabled by form-birefringent phase-matching in ultrahigh index-contrast waveguides. Nonlinear mixing has a measured efficiency of 0.4 W⁻¹ in a 1.2 mm long waveguide using a CW signal tunable between 1490 and 1620 nm and a CW pump tunable between 1018 and 1032 nm at powers of a few mW.

We have developed a simulation of a multiple quantum well optical correlator to investigate the effects of saturation on the correlation signal. We present the results of the modeling effort here.

We demonstrate a surface-normal coupled-quantum-well InGaAs-InAlAs electroabsorption modulator that provides optical modulation with a contrast ratio in excess of 1.5 at only 6 V. The device operates at 1.55 /spl mu/m and is based on a novel strain-balanced layer structure. The operating voltage is about two times lower than that of a conventional square quantum-well modulator that achieves a comparable contrast ratio.

We experimentally demonstrate cascaded microcavities for enhanced bandwidth and extinction. The cavities are integrated into waveguides by etching three sets of quarter-wavelength trenches, each set forming a reflector. Experiments compare well with transfer matrix predictions.

We detect trace gases at ppb levels using evanescent-field absorption spectroscopy in microring resonators coated with sorbent polymers. The overtone spectra derive from mid-infrared resonances that provide a signature of analyte toxicity.

Using achromatic gratings techniques we have written dynamic holograms on an optically addressed spatial light modulator with a temporally and spatially incoherent white light source. The resolution and depth-of-field of these holograms are theoretically derived and experimentally verified. We demonstrate writing speeds over an order of magnitude faster than video framing rates.

We demonstrate that micromachining lattice-matched InGaAs quantum wells grown on (001) InP with strained barriers produces precise laterally patterned modifications to the semiconductor band structure. The light-hole and heavy-hole excitonic transitions are mixed and differentially shifted by the micromachining, inducing a surface-normal optical anisotropy characterized by a peak birefringence of Δn=0.028. The measured optical properties agree with calculations based on finite-element models of the strain combined with an eight-band k⋅p model that includes deformation potentials. This technique may find applications in fields such as surface-normal polarization modulators, quasi-phase matching, and optically-acitve piezoelectric materials.

Asymmetric Fabry–Perot multiple quantum well modulators can have much higher extinction ratios than are otherwise possible with surface normal devices. This performance comes at the price of very tight tolerances on the epitaxial growth and high sensitivity to temperature and angle of incidence. We demonstrate that a quantum well modulator with an integrated microelectromechanical mirror can remove these sensitivities.

A new kind of modulating retro-reflector using cat's eye optics and a multiple quantum-well electro-absorption modulator array is described. The device exhibits retro-reflection over a 30/spl deg/ field of view and can support data rates of up to 50 Mb/s using 1-mm pixels. The use of the device in free-space optical communication is discussed.

Anisotropic in-plane strain in quantum wells leads to an optical polarization anisotropy that can be exploited in optoelectronic devices such as modulators. A theoretical model shows that the behavior of the polarization anisotropy with increasing strain anisotropy is radically different for quantum wells under anisotropic tensile and compressive strains of equal magnitude. This strikingly different behavior arises from the different valence-subband mixing that occurs in the cases of anisotropic tensile and compressive strain. Specifically, the mixing of the first heavy- and light-hole subbands that occurs only under anisotropic tensile strain is central to the polarization anisotropy.

We develop a set of laser rate equations that accurately describes mechanical amplification in optomechanical oscillators driven by photothermal or radiation pressure forces. In the process we introduce a set of parameters describing gain, stored energy, slope efficiency, and saturation power of the mechanical laser. We identify the three-phonon parametric interactions as a microscopic mechanism enabling self-oscillation. Our theory shows remarkable agreement with our experimental data, demonstrating that optomechanical self-oscillation is essentially a "phonon lasing" process in which an optical pump generates coherent acoustic phonons.

Scintillation is an effect that limits the performance of many optical systems including imagers and free space optical communication links. Scintillation can be especially severe for optical retro-reflectors. This can limit the range of links based on modulating retro-reflectors. In this work we examine the effects of retro-reflector diversity on optical scintillation. This technique uses multiple retro-reflectors, separated by a distance larger than the atmospheric coherence size and illuminated by one interrogating beam, to reduce the scintillation index. We examine the dependence of scintillation index on the number of retro-reflectors, their separation and the degree of coherent interference at the receiver. Data was taken in the field at three different sites: the Naval Research Laboratory's Chesapeake Bay Detachment, where both over land and over water measurements were made, the Fort AP Hill Laser Test Range and China Lake, California. We show that multiple retro-reflectors can reduce the scintillation index on double-pass links. We demonstrate that when retro-reflectors are spaced too closely, coherent interference can increase the scintillation index and greatly expand its frequency content. We also examine the effects of retro-reflector diversity on the margin needed for modulating retroreflector data links.

The U.S. Naval Research Laboratory (NRL) is developing a small size, weight an d power (SWaP) free space lasercomm terminal for small unmanned airborne platforms. The termin al is based on a small gimbal developed by CloudCap Technology. A receiver with a large field of view and with sensitivity sufficient to meet the program range goals is required for this terminal. An InGaAs Avalanche Photodio de (APD) with internal structures engineered to reduce excess noise and k eff in high gain applications was selec ted as the detector. The detector is a 350 micron diameter impact ionization engineered (I 2 E) APD developed by Optogration, Inc. Resu lts of development and characterization of the receiver will be presented. Keyword list: free space lasercomm, unmanned airborne system (UAS), APD, impact ionization engineering (I 2 E) 1. INTRODUCTION NRL is developing a very small SWaP lasercomm terminal desig ned to fit into the T2 gimbal developed by Cloud Cap Technology. The T2 (now called the T300 by Cloud Cap) was chosen because although it is small enough for small UAS applications, it has very good pointing capabilities (0.023 degree pointing resolution) and a very well developed navigation system. The terminal development is being supported by the Office of Naval Research (ONR), Code 30. The terminal has been designed to be compatible with the larger lasercomm terminals being developed under the ONR TALON program under the direction of ONR Code 31. The lasercomm terminal has dual mode capability: the ability to act as one terminal in a direct, high speed lasercomm link for high speed data download to a ground station and the ability to interrogate unattended ground sensors that are equipped with modulating retroreflector devices at lower speeds. Larger terminals with this dual mode capability which have been developed previously by companies such as NovaSol of Honolulu have been referred to as Dual Mode Optical Interrog ators (DMOIs). The small SWaP version discussed in this paper has been dubbed the micro-DMOI ( DMOI). Since the gimbal payload bay is very small (cylindrical with 5.83 inch diameter and 4.84 inch depth), the use of steering mirrors for fine pointing is not practical so the pointing must be done with the gimbal. The receiver aperture is restricted to a diameter of 2.48 inches (63mm) so to meet the distance requirements of the program lasercomm links, the detector needs to have high sensitivity (better than -45 dBm at 100baseT fast Ethernet data rates) and a wide field of view (0.5 degrees minimum). This requires a very large In GaAs detector with high speed and high sensitivity. NRL has recently been working collaboratively with Professor Joe Campbell of the University of Virginia and OptoGration, Inc. (OGI) of Wilmington, MA. on the de velopment and testing of APDs that are fabricated using impact ionization engineeringŽ (I

We demonstrate the detection of water vapor adsorbed into a sorbent polymer layer coated onto highly evanescent SiN rib waveguides by measuring the change in absorption as a function of wavelength, polarization, and analyte concentration.

We design cascaded waveguide microcavities. Using realistic design parameters based on previously demonstrated single-stage devices, we model filters with >60dB extinction, 100GHz FSR and 1GHz bandwidth. Initial experimental results are presented.

We demonstrate V π L values in the optical L-band in suspended quantum well waveguides between 109 and 199 mV-cm, which result from the strong out-of-plane index contrast that tightly confines the mode to the quantum well core.

We report absolute measurements of thermal-mechanical noise in microelectromechanical systems. The devices are studied with an optical microcavity technique that has a resolution on the order of tens of femtometers per root hertz. The measured noise spectrum agrees with the calculated noise level to within 25%, a discrepancy most likely due to uncertainty in the effective dynamic mass of the vibrating bridge. These measurements demonstrate that thermal-mechanical noise can be the dominant noise source in actuated microelectromechanical devices.

The NRL Transportable Atmospheric Testing Suite (TATS) system was used to measure time resolved scintillation over a variety of different ranges at China Lake in December 2010. In this paper an analysis of scintillation effects on retro-reflector links is presented. Scintillation index, power spectral density and probability distribution functions are deduced from the measured irradiance fluctuations. Effects of aperture averaging using multiple retro-reflectors is examined as well as the contribution to the irradiance fluctuations due to enhanced backscatter.

Optical scintillation is an effect that limits the performance of many optical systems including imagers and free space optical communication links. The Naval Research Laboratory is undertaking a series of measurement campaigns of optical scintillation in a variety of environments. In December of 2010 measurements were made over a one week period in the desert at China Lake, CA. The NRL TATS system was used to measure time resolved scintillation over a variety of different ranges and terrains. This data has been used to determine fade rate and duration as a function of weather and link margin. Temporal correlation of fades has also been calculated. This data allows simulation of a variety of communication protocols and the effects of those protocols on link throughput. In this paper we present a comparison of different protocols for both direct and retroreflector links.

The U.S. Naval Research Laboratory (NRL) is characterizing InGaAs avalanche photodiodes (APDs) with internal structures engineered to reduce dark counts and ionization coefficient ratio (keff). Recently, much progress has been made in the use of APDs in linear mode for photon counting applications.1 However, the best results in linear mode single photon counting in InGaAs devices have been obtained by cooling the devices well below 200 K to reduce dark current. The single photon counting capability is due to the high gain available in the tail of the APD gain distribution, and this high gain tail is enhanced by reducing the ionization ratio (keff) and dark noise.2 Since recent promising results in linear mode APD photon counting have involved engineering the APDs to reduce keff, it is likely that these devices will also perform much better than standard APDs in free space lasercomm applications at temperatures which can easily be reached by thermoelectric coolers, or even uncooled, due to the keff reduction. NRL has obtained several InGaAs APDs of both the standard design and of a new design using impact ionization engineering from OptoGration, Inc. of Wilmington, MA. Some results of characterization of these APDs will be presented.

We report on the free space optical transmission of FM audio/video signals using a 6.3mm diameter InGaAs modulating retro-reflector.

As military sensors and systems become more sophisticated, tactical situations will require reliable, high data rate communications. The current RF communication systems are increasingly competing for the limited amount of RF spectrum and bandwidth. One possible wa y to augment the current RF communication systems is by the use of free space lasercomm in tactical networks for links in which di rect line of sight is possible. Free space lasercomm has been demonstrated over horizontal distances greater than 10 nautical miles and at data rates greater than 1 gigabit/sec. Lasercomm links do not require any RF frequency allocation, nor do they have an RF signature. They are inherently low probability of intercept and detection and they are very difficult to jam due to the very narrow divergence of the communication beams and the ve ry narrow acceptance angle of the receivers. 1-6 The U.S. Naval Research Laboratory has demonstrated the use of free space lasercomm in tactical networks at Trident Spectre 2009 and Empire Challenge 2010. This paper will discuss these lasercomm demonstrations and present packet error rate te st data captured at both. Keyword list: free space lasercomm, tactical network, modulating retroreflector, Trident Spectre, Empire Challenge

We experimentally demonstrate and explain phonon “lasing” (“PhASER”) in a photo-pumped micromechanical cavity that exhibits all the characteristic of a standard laser near threshold and is described by the standard laser equations.

We measure and theoretically describe the polarization anisotropy of the facet reflectivity in rectangular, subwavelength waveguides. This effect is increasingly impacting the performance of integrated optical devices as the lateral size scale shrinks.

We demonstrate a new add-drop filter architecture combining an asymmetric Y-branch mode conversion waveguide and a high index contrast shifted grating mode-conversion cavity. Measurements show add-drop functionality in a linear Fabry-Perot filter.

We report on the free space optical transmission of AM composite video signals using a 10 mm diameter InGaAs modulating retro-reflector.

We demonstrate enhanced electro-optic phase shifts in suspended InGaAs/InGaAsP quantum well waveguides compared to attached waveguides. The enhancement stems from an improved overlap between the optical mode and the multiple quantum well layers in thin waveguides when the semiconductor material beneath the waveguide is selectively etched. The measured voltage length product is 0.41 V-cm and the measured propagation loss is 2.3 +/- 0.7 dB/cm for the TE mode in the optical L-band.

Free space optical (FSO) communication has enjoyed a renewal of interest in the past decade driven by increasing data rate requirements and decreasing amounts of radio frequency spectrum. These needs exist in both the commercial and military sectors. However military communications requirements differ in other ways. At the U.S. Naval Research Laboratory (NRL) we have been conducting research on FSO communications for over ten years with an emphasis on tactical applications. NRL's FSO research has covered propagation studies in the maritime domain, new component development, and systems demonstrations. In addition NRL has developed both conventional, direct, laser communications systems and retro-reflecting systems. In this paper we review some of this work and discuss possible future applications of FSO communications.

Using Raman scattering measurements in suspended semiconductor waveguides coupled with finite-element analysis, we elucidate the properties of the guided-mode phonon-polariton, which plays a critical role in many current terahertz generation approaches.

We review properties of free-space optical links for analog transmission including RF performance for various photoreceiver configurations and experimental results at 1 GHz over a 32 km link.

Beam fanning is examined as a noise source in monochromatic and achromatic two-wave mixing. We show that fanning acts mainly as a parasitic loss for low beam ratios, whereas it contributes a background noise floor for high beam ratios. In the first regime, fanning loss is much smaller in achromatic two-wave mixing than in the monochromatic case.

We show that two-beam coupling in BaTiO3, using a laser diode as the source, can be passively stabilized against mode hops and multi-longitudinal mode operation using achromatic grating techniques.

We report micro-electro-mechanical systems (MEMS) chemical sensors with improved on-chip optical interrogation method. A silicon-on-insulator (SOI) MEMS resonator is coupled to a waveguide Fabry-Perot microcavity enabling high resolution displacement and mechanical resonance measurement. By coating the MEMS resonator with a chemo-selective polymer, exposure to chemical analytes results in beam bending (static mode) as well as mass loading and a mechanical resonant frequency shift (dynamic mode). Proof-of-concept relative humidity sensing is demonstrated. Integrated waveguide power splitters/combiners are investigated for interconnecting many sensors on a chip.

Free-space optical (FSO) links typically carry digital data over short to moderate distances (100m - 50 km). From a military perspective, FSO links have the advantages of low probability of interception, inherent anti-jam capability and a reasonable degree of covertness. In certain applications it is desirable to minimize size, weight and power of the transmitter and this pushes the designer to eliminate power-hungry digitizers and to transmit raw analog, instead of digital, information. However, the transmission of analog signals presents significant technical challenges due to strong fluctuations in received optical power resulting from atmospheric turbulence. In this case the standard RF link properties of gain, noise factor, and linearity must be described entirely in statistical terms. In this paper we present preliminary experimental results from a short (500m), single-tone, 5 MHz analog FSO link and compare the data to theoretical predictions. Theory suggests, and our preliminary experimental results confirm, that the statistical properties of all the RF performance metrics of a FSO analog link are determined entirely by the statistical properties of the received optical power. We show that the distribution of values of RF link parameters can be obtained directly from the distribution of received optical power, without the need for modeling the received power with a continuous function.