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The Naval Research Laboratory (NRL) was established in Washington, DC in 1923 and is the corporate laboratory for the U.S. Navy and Marine Corps. Today NRL is a world-class research institution conducting a broad program of research and development (R&D), including many areas of optical science and technology. NRL is conducting cutting-edge R&D programs to explore new scientific areas to enable unprecedented Navy capabilities as well as improving current technologies to increase the effectiveness of Navy and other Department of Defense systems. This paper provides a broad overview of many of NRL’s achievements in optics. Some of the remaining articles in this feature issue will discuss NRL’s most recent research in individual areas, while other articles will present more detailed historical perspectives of NRL’s research concerning particular scientific topics.

Optical-fiber sensors are reviewed for the measurement of temperature, pressure, acceleration, flow of liquids and gases, and the level of a liquid in a container. Optical-fiber sensors are making their first commercial appearance in measuring instruments and controls where such attributes as sensitivity, resistance to hostile environments, and compactness are essential. Through alteration of light by external stimuli, these sensors can now detect virtually any stimulus the other sensors can-from pressure and magnetism to acidity and acceleration-and often more sensitively and accurately, and over a wider dynamic range. Optical-fiber sensors are more rugged and more resistant to corrosion than other sensors, immune to electromagnetic interference, and compatible with optical-fiber telemetry. Several types of fibre sensors that are in commercial production, being field-tested, and in advanced development are discussed.

A Mach‐Zehnder interferometric waveguide modulator is demonstrated which simultaneously modulates both TE and TM modes. Two sets of electrodes are utilized to provide approximate independent control of vertical and horizontal field components across the channel waveguides. In Z‐X–cut LiNbO3 this is shown to provide two polarization‐independent points of operation. With Ti‐diffused waveguides operated at 0.633 μm we obtained 90–95% modulation with applied voltages of 1.3 and 44 V and also with −8 and −36 V. Electrode design and alignment are shown to be important factors in device operation.

A state of the art review is presented on fiber optic systems and integrated optical devices. Advantages of such systems, problem areas, and applications are discussed. (auth)

This paper describes electro‐optically induced tunable deflection in the plane of a waveguide structure by utilizing the liquid‐crystal property of tunable birefringence together with appropriately designed electrodes. The magnitude of tunable deflection achieved in this experiment (>20°) had only been demonstrated in passive structures in the past. Furthermore, by changing the device parameters slightly, electrically controlled total internal reflection or refraction in the plane of the waveguide structure was demonstrated. This allowed operation of the device as a switch where the guided beam was deflected or ``switched'' through a very large angle (>70°) in the plane of the guide.

In a conventional circular cross‐section single‐mode fiber, built‐in and induced birefringences remove the degeneracy and make the fiber a two‐Eigenmode system. We have constructed the first all‐fiber interferometric sensor system and have performed preliminary studies on the effects of birefringence in these interferometers. Theoretical analysis and experimental data supporting this analysis are presented for the 3‐dB coupler and all‐fiber interferometer sensor.

Resolution and efficiency measurements of acousto−optic deflection in optical waveguides are presented and compared to results of previous analysis. Measurements are made on a Ta2O5 film on YX−quartz. Resolution is shown to be limited by coupling, beam divergence, transducer bandwidth, and modal dispersion.

The effect of optical feedback on the spectral characteristics of channel‐substrate planar single‐mode laser diodes operating at room temperature is reported. The impact on the performance of interferometric sensor systems using such sources is discussed. The linewidth for the free‐running laser at 10‐mW output power was determined to be less than 5 MHz at room temperature. Broadening on the order of 40 times the intrinsic linewidth was observed for 0.1% feedback and increased with increasing feedback. The presence of self‐oscillation modes was observed at 0.04% feedback. Satellite modes symmetrically located with respect to the primary mode appeared in the spectrum for feedback greater than 0.04%. These satellite modes are attributed to self‐oscillation in the laser induced by feedback. As the feedback was increased, the satellite mode spectrum began to overlap that of the primary mode, reducing the effective coherence length from 60 m (for the single‐mode linewidth) to less than a few centimeters.

The influence of external feedback on low‐frequency noise generation (100 Hz–10 kHz) in single longitudinal and transverse mode channel substrate planar laser diodes is reported. An increase in noise by as much as 60 dB was induced by external feedback as small as 0.04%. The induced intensity variations were observed to be sensitive to both phase and amplitude of the externally reflected light.

Piezoelectric leaky wave coupling of a LiNbO3 surface acoustic wave (SAW) to a CdS bulk wave is reported. A tapped SAW delay line is constructed using a CdS bulk‐wave amplifier weakly coupled across an air gap to a yz LiNbO3 SAW delay line. Insertion loss of the tapped delay line with untuned IDT’s is 19 dB. Leaky wave coupling contributes only 1 dB to the insertion loss. Additional delayed pulses 2.5 dB larger than the SAW are easily achieved.

We report the first single‐mode multiterminal star coupler in which fibers are grouped into sets of three fibers. The three fibers in each set were than braided over some length before one or more of the fibers of each set are interchanged with their counterparts in other sets. We utilized the encapsulated etching technique reported earlier to overcome the peculiar geometric problems associated with single‐mode fibers. Excellent output uniformity and very low throughput loss have been achieved in these devices because of our new designs. We also present coupled‐mode theoretical analysis which can be used to describe these new single‐mode star couplers.

The behavior and spectral characteristics of a constricted double heterojunction injection laser in a regime in which the lasing characteristics are dominated by a cavity comprised of two external mirrors is reported. The Fox–Smith configuration is shown to limit the laser’s emission (previously multimode) to a single longitudinal mode with a linewidth of less than 200 kHz. It is also shown that this laser interferometer configuration is effective as an extended, external cavity laser diode sensor.

The amplitude stability of single‐mode channeled‐substrate planar diode lasers has been measured as a function of frequency (100 Hz–1 MHz) and driving current. The relative noise spectrum showed a decrease with increasing frequency and an overall reduction at all frequencies as the output power of the laser was increased. These results are compared with similar measurements of the most stable commercial single‐mode HeNe lasers. The problems of laser amplitude noise in fiber‐optic sensor systems is briefly discussed.

A new acoustoelectric phase shifter is proposed which produces a direct phase shift of a SAW on piezoelectric substrate. Computations of the attenuation and phase lag versus resistivity due to a silicon wafer air‐gap coupled to yz‐LiNbO3 are presented. The use of the variation of the resistivity of the silicon to control the phase of a SAW was demonstrated using the photogeneration of carriers and the field‐effect control of the surface resistivity. A 180° shift was obtained with a +30‐V transverse voltage pulse.

For the first time a surface acoustic waveguide has been made by diffusing titanium into yz‐LiNbO3 to serve as the fast cladding region. The guides are excited by transducers operating at 240 MHz and exhibit low loss. Dispersion characteristics presented illustrate the effects of lateral confinement.

The operation and construction of a new actively controlled SAW power divider is presented. The amount of power delivered to either of two outputs can be controlled with a voltage less than +30 V. The two essential components of this device are a SAW magic tee fabricated on yz LiNbO3 and an acoustoelectric phase shifter which uses silicon air‐gap coupled to the LiNbO3.

Results on the signal intensity and collinear tuning characteristics in CW optical parametric scattering in ADP pumped at 4880 A are presented and discussed. The corresponding idler wave is in a strong absorption region of ADP.

The forward-scattered intensity in the spontaneous parametric process is found to decrease significantly as the idler frequency goes sufficiently far into the 5.7-\ensuremath{\mu} and 1.4-\ensuremath{\mu} infrared absorption bands of LiNb${\mathrm{O}}_{3}$ and N${\mathrm{H}}_{4}$${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$, respectively. These results indicate that, unlike the previously studied cases in GaP and LiNb${\mathrm{O}}_{3}$, the corresponding optical nonlinear susceptibility has essentially no resonance increase and its ionic contribution remains small near these absorption bands. A formula for the forward-scattered intensity is obtained which agrees very well with the observed effects of idler attenuation on the spontaneous parametric scattering in both LiNb${\mathrm{O}}_{3}$ and N${\mathrm{H}}_{4}$${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$.

Time‐resolved spectra from the output of a doubly resonant parametric oscillator (DRO) pumped by a cw laser are presented. Single‐cluster operation of the DRO is observed for low pump powers and a simultaneous multicluster operation at higher pump powers. For the multicluster operation, oscillation by higher‐order modes is also detected.

The purpose of the demodulation scheme in fibre optic interferometers is to transform the optical output of the interferometer into an electrical signal proportional to the amplitude of the relative phase shift. Typical requirements of the detection scheme1,20,4 are: to be able to resolve signals corresponding to 10-6 rad phase shift, to have a linear response and to have a large dynamic range (106 107). Specific requirements such as packaging and low power consumption are also important. In this talk, four types of detection schemes will be discussed, two active homodyne techniques using a) a piezoelectric fibre stretcher) and b) laser tuning2 and two passive homodyne techniques using a) phase modulation3 and b) a (3x3) directional coupler.4 Each of these schemes have achieved prad performance, however, each scheme has specific advantages and disadvantages which will be briefly described below and discussed at the conference.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Over the past several years, a new non -communication optical fiber application hasemerged. This application utilizes optical fibers for sensing. Initial interest centeredaround rate rotation sensing. Since that time, however, acoustic, magnetic, and tempera-ture sensing utilizing optical fibers has evolved into a viable research effort with sig-nificant potential payoff. As an example, laboratory fiber optic acoustic sensors nowrival the best sensitivity obtained with piezoelectric ceramics. These sensors possess aunique geometric versatility previously unavailable. In conjunction with the DefenseAdvanced Research Projects Agency (DARPA), the Navy has begun a Fiber Optic Sensor System(FOSS) program to develop associated technology necessary to realize these sensors.Substantial effort is ongoing at the Naval Research Laboratory (NRL) and other Navylaboratories with considerable contractual effort from universities and industry. Thispaper reviews the status of the FOSS program.IntroductionInitial fiber sensor efforts concentrated on rotation rate sensing.' Acoustic,2magnetic,3 and temperature field sensors utilizing optical fibers are now under intenseinvestigation. Threshold detectabilities of laboratory optical- fiber -acoustic sensorsrival the best piezoelectric devices. For magnetic field sensing the potential fordetecting on the order of 10 -11 to 10-12 G appears possible. That detectability rivalsthe best conventional magnetometers. Optical fiber accelerometers with sensitivities inthe sub pg range also appear feasible.4 Thus fiber optic sensors appear to be extremelycompetitive with conventional devices.Fiber interferometerConsider a typical Mach -Zehnder configuration for an optical fiber sensor (Fig. 1).

As has been recently recognized,(1-4) the polarization properties of integrated optical switches/modulators are of paramount importance in determining the usefulness of these devices in single mode fiber optic data transfer systems. In particular, these switches/modulators must perform efficient, complete switching of light regardless of state of incident polarization. These requirements result from the fact that linearly polarized light coupled into single mode, circular fibers suffers a rapid conversion to other polarization states, with the light emerging being generally elliptically polarized. Therefore any new switch design must be capable of polarization independent operation in order for it to be considered useable. In this paper, we report on the principals of polarization independent operation and describe the fabrication of the first polarization independent switch.

The operation of an all-fiber spectrophone is described. The pressure sensing transducer was constructed by winding 9.2 m of high N.A. fiber into the shape of a 2.54-cm diam hollow cylinder, 4.1 cm in length. A layer of silicon rubber adhesive on the outside surface allowed expansion of the device, while retaining the cylindrical shape and containing the absorbing gas. The fiber ends were then spliced into one arm of an all-fiber Mach-Zehnder interferometer, details of which have been published.1,2

Recent developments in an external cavity laser diode sensor are discussed. The sensor consists of an external reflector mounted such that the mirror to laser facet distance is smaller than the laser cavity. This device has micro-radian sensitivity and a large dynamic range. This sensor has been configured to sense acoustic waves, magnetic and electric fields, current, and temperature and acceleration.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The state-of-the-art of optical fibre sensors is reviewed and the principals of operation are detailed. The characteristics of acoustic, magnetic, gyro, laser diode and other sensors are described. Achievable performance and limitations are discussed and a description of technology used to fabricate the sensor is presented.

Infrared Fourier transform spectroscopy is a technique ideally suited for the analysis of broad and complex spectra and has, therefore, been widely utilized in molecular IR spectroscopy and astrophysics.1 In the technique, spectral information of the source is obtained by computing the Fourier transform of the interferogram generated by scanning a mirror of a Michelson interferometer which is illuminated with radiation from the source.

Fiber optic sensors appear to offer significant performance advantages when compared to conventional acoustic, magnetic, and rotation sensors. This paper will review the state of the art in this newly developing technology area.

The development of fiber optic interferometric sensor systems has created new demands for single mode semiconductor diode lasers with low output noise power. Recently, several single longitudinal and transverse mode GaAlAs laser diodes have become available. Several experimenters have reported the deleterious effects of optical feedback resulting from fiber optic coupling on the laser's mode structure and high frequency noise spectrum.1-3 Our interest for interferometric sensors is in the low frequency noise, up to a few kilohertz. We report here the first such low frequency noise measurements in fiber coupled and uncoupled single mode diode lasers. Additionally, we also show that the use of index matching fluid can be used to greatly reduce optical feedback and its associated low frequency noise, upon coupling into an optical fiber.

Single mode guided light data transfer systems offer greatly expanded capabilities over multimode optical fiber systems. An overview of the status and capabilities of present single mode devices will be given with emphasis on sources, switches and couplers. Discussion will center on single mode coupling problems because low loss rugged couplers and splicers represent the most significant technological barrier to be overcome. A single mode fiber optic switching terminal presently under development at NRL will be described.© (1977) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A Fabry-Perot interferometric sensor utilizing a single-mode optical fiber has been demonstrated, exhibiting a finesse of 18 and a sensitivity to phase modulation of 7.9 \times 10^{-7} rad/ \sqrt{Hz} at 1 kHz.

In the single-mode optical communication systems, active/passive couplers, mixers and multiplexers constitute key components. Between those components and also between terminals and those components, we need optical transmission lines of substantial lengths. Environmental changes in temperature and mechanical stress and pressure along the transmission lines cause almost constant fluctuation in the phase of any particular mode at any particular point, especially at the input ports of those optical components. When the phase fluctuation is coupled with polarization rotation in the conventional round single-mode fibers, and with interferometric couplings to be found in couplers, mixers and multiplexers, noise and mode instability occur. In this presentation, we will report theoretical and experimental studies on these problems.

Significant device advantages in the areas of microwave spectrum analysis and high speed A/D conversion appears to be possible through the use of microoptics. The attractive features of microoptics for these applications include a compact, rugged means to package optical singal processors, and an approach to achieve increased performance over existing technologies. Emphasis in this paper will center on the spectrum analyzer, will include comparisons of acoustooptics technqiues with conventional approaches for signal sorting, and will describe the state of the technology and its proposed uses.

An optical fiber acoustical sensor for detecting sound waves in a fluid mum. An optical fiber coil through which a light beam is transmitted is placed in a fluid medium. A sound wave propagating through the fluid medium and incident on the optical fiber coil changes the index of refraction and the length of the optical fiber at the area of incidence. These changes cause a shift in the transmitted light which is detectable to denote the presence of the sound wave.

After eight years of development, optical fiber sensors are beginning to emerge as competitive devices for performing sensing tasks such as required for aircraft engine and flight controls, for shipboard machinery and damage controls, for medical probing and for industrial process controls. Fiber sensors have proved to be accurate and capable of operation in harsh environments contaminated with high EMI, explosives, or corrosive gases. Because of these attributes, fiber sensors offer unique opportunities to solve existing sensor problems encountered in many applications. Additionally, the compatibility of optical sensors with optical telemetry makes possible the development of all optical, multi-element sensor systems capable of supporting large numbers of high bandwidth sensor elements while at the same time eliminating the requirement for transmitting electrical power remote from the monitoring site. This combination of compatible fiber sensors and fiber telemetry represents an intrinsic advantage over conventional electrical technology.After eight years of development, optical fiber sensors are beginning to emerge as competitive devices for performing sensing tasks such as required for aircraft engine and flight controls, for shipboard machinery and damage controls, for medical probing and for industrial process controls. Fiber sensors have proved to be accurate and capable of operation in harsh environments contaminated with high EMI, explosives, or corrosive gases. Because of these attributes, fiber sensors offer unique opportunities to solve existing sensor problems encountered in many applications. Additionally, the compatibility of optical sensors with optical telemetry makes possible the development of all optical, multi-element sensor systems capable of supporting large numbers of high bandwidth sensor elements while at the same time eliminating the requirement for transmitting electrical power remote from the monitoring site. This combination of compatible fiber sensors and fiber telemetry represents an intrinsic advantage over ...

The spectral and temporal output of a CW pumped, doubly resonant parametric oscillator (DRO) are reported. The DRO consists of a Ba 2 - NaNb 5 O 15 crystal in a hemispherical cavity with mirrors coated for degenerate operation; the pump radiation is 5145A from a continuous argon ion laser. Pump feedback is eliminated by the use of Faraday rotator, and cavity length changes are minimized by coupling together the components of the DRO. The output of the oscillator is analyzed with a 1.0 meter spectrometer (.5A resolution) and a streak camera with an S-1 response and 20 nsec temporal resolution. The experimental results show that for pump powers just above threshold, the DRO operates on a single cluster of modes for the duration of one scan. Mode jumping occurs on most scans and large frequency sweeps can also be observed. At higher pump powers (2-3 times above threshold), another cluster of signal modes starts to oscillate, sometimes simultaneously with the first cluster. This second cluster occurs at the next coincidence of signal-idler pairs for which ω p - ω so - ω io = 0. For all of these pump powers, oscillation in higher transverse order modes is observed; these can be eliminated by use of an aperture within the DRO cavity. Sources of these frequency instabilities and their interpretation will be discussed.

Measurements have been made of the phase noise of a single‐mode diode laser in an unbalanced Michelson interferometer, as a function of optical path difference. The noise increased linearly with increasing optical path difference. The origin of the phase noise is discussed.

A single-mode all-fibre Fourier transform spectrometer is described. A resolution of ˜1 cm−1 (˜0.7 A) is demonstrated at 820 nm. The device can be configured as a remote gas sensor.

We present an approximate analysis of the model fields and dispersion relations of anisotropic dielectric waveguides of rectangular cross section. We initially assume that the dielectric tensor is diagonal in both the guiding and substrate regions of the channel, with a step discontinuity at their interface. Off-diagonal elements in the dielectric tensor are then accomodated by means of coupled mode theory; continuous transverse variation of the dielectric tensor is taken into account by adapting a heuristic technique due to Miller. We have found that the anisotropy may act to decouple orthogonally polarized components of the field, resulting in almost-linearly-polarized modes for many cases of practical interest. We have also considered the coupling of energy between two such channel waveguides running parallel to one another.

Two types of fibre-optic current sensor have been developed and successfully tested in the laboratory: magnetostrictive jacketing materials and a heating effect, to produce a phase shift in the optical fibre. A ruggedised all-fibre interferometer powered by a diode laser was used to detect the phase changes induced by the electrical current.

A sensitivity of less than 1 μg for a fibre optic accelerometer is demonstrated. The accelerometer measures the strain in the fibre when a mass is accelerated. An all-fibre Mach-Zehnder interferometer was used to detect the change in optical path length produced by the strain in the fibre.