This research represents the first effort to apply vertical cavity surface emitting lasers (VCSELs) to the monitoring of interferometric fiber optic sensors. Modulation of the drive current causes thermal tuning of the laser light frequency. Reflection of this frequency-modulated light from a fiber Fabry-Perot interferometer (FFPI) sensor produces fringe patterns which can be used to measure the optical path difference of the sensor. Spectral characteristics were measured for 850nm VCSELs to determine the combination of dc bias current, modulation current amplitude and modulation frequency for which single mode VCSEL operation and regular fringe patterns are achieved. The response characteristics of FFPI sensors were determined experimentally for square, triangular, saw-tooth waveforms at frequencies from 10kHz to 100kHz. The dependence of VCSEL frequency on the dc bias current was determined from spectral measurements to be ~165GHz/mA. An independent measurement of this quantity based on counting fringes from the FFPI sensor as the laser modulated was in good agreement with this value. The effect of optical feedback into the laser was also studied. By observing the fringe shift as the FFPI sensor was heated, a fractional change in optical length with temperature of 6.95 X 10-6/°C was determined in good agreement with previous measurements on a 1300nm single mode fiber. The performance of 850nm VCSEL/FFPI systems was compared with their counterparts using 1300nm distributed feedback (DFB) lasers. The results of these experiments show that the 850nm VCSEL/FFPI combination gives regular fringe patterns at much lower bias current and modulating current amplitudes than their 1300nm DFB/FFPI counterparts.

Bragg grating reflectors etched in amorphous silicon overlay films have been integrated with Ti:LiNbO3 optical waveguides. With a 12.5 mm long grating segment and an etch depth of ~ 93 nm in a 105 nm-thick silicon film, a narrow (0.05 nm) spectral bandwidth with a record high transmission dip (> 20 dB) was achieved at a wavelength of ~1542 nm for TE polarization on an x-cut, y-propagating substrate. The reflectance in the channel waveguides is found to be strongly dependent on the depth of the etched grating. The 3-dB bandwidth of 0.05 nm obtained for all tested samples is the smallest reported for waveguides in LiNbO3. The effect of the Bragg waveguide loss factor on the transmittance and reflectance spectra is investigated using a model for contra-directional coupling that includes an attenuation coefficient. The Bragg grating spectral characteristics are exploited to fabricate distributed Bragg feedback modulators (DBFM) and Bragg reflector Fabry-Perot modulators (BFPM). The sharp cut-off in transmission and reflection spectra, which is an inherent characteristic of Bragg grating, was tuned by applying voltage via the linear electrooptic effect, to produce intensity modulation. The Bragg grating based modulators consume less electric power compared to polarization intensity modulators (PIMs). The DBFM demonstrates 1/1.6 times the modulating voltage of a PIM with identical waveguide and electrode structure. The BFPM shows 1/3.3 times the modulating voltage of the PIM. No difference in the frequency response is observed among the three modulators. Comparison of the modulation sensitivity in the linear region indicates that the Bragg grating based modulators provide better sensitivity than that of the PIM with identical waveguide and electrode structure. These results indicate the potential advantage of the Bragg grating based modulators for enhanced modulation efficiency over conventional modulators. Further improvements can be expected from the optimization of the electrode design.

This research represents the first effort to apply vertical cavity surface emitting lasers (VCSELs) to the monitoring of interferometric fiber optic sensors. Modulation of the drive current causes thermal tuning of the laser light frequency. Reflection of this frequency-modulated light from a fiber Fabry-Perot interferometer (FFPI) sensor produces fringe patterns which can be used to measure the optical path difference of the sensor. Spectral characteristics were measured for 850nm VCSELs to determine the combination of dc bias current, modulation current amplitude and modulation frequency for which single mode VCSEL operation and regular fringe patterns are achieved. The response characteristics of FFPI sensors were determined experimentally for square, triangular, saw-tooth waveforms at frequencies from 10kHz to 100kHz. The dependence of VCSEL frequency on the dc bias current was determined from spectral measurements to be ~165GHz/mA. An independent measurement of this quantity based on counting fringes from the FFPI sensor as the laser modulated was in good agreement with this value. The effect of optical feedback into the laser was also studied. By observing the fringe shift as the FFPI sensor was heated, a fractional change in optical length with temperature of 6.95 X 10-6/°C was determined in good agreement with previous measurements on a 1300nm single mode fiber. The performance of 850nm VCSEL/FFPI systems was compared with their counterparts using 1300nm distributed feedback (DFB) lasers. The results of these experiments show that the 850nm VCSEL/FFPI combination gives regular fringe patterns at much lower bias current and modulating current amplitudes than their 1300nm DFB/FFPI counterparts.

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Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1994. / Vita. Abstract. Includes bibliographical references (leaves 67-68). Also available via the Internet.

Techniques for adapting metamaterials for the improvement of RF energy harvesting and infrared photodetection are demonstrated using experimental and computer simulation methods. Two methods for RF energy harvesting are experimentally demonstrated and supported by computer simulation. In the first method, a metamaterial perfect absorber (MPA) is made into a rectenna capable of harvesting RF energy and delivering power to a load by soldering Schottky diodes onto connected split ring resonator (SRR) structures composing the planar metasurface of the perfect absorber. The metamaterial rectenna is accompanied by a ground plane placed parallel to it, which forms a Fabry-Perot cavity between the metasurface and the ground plane. The Fabry-Perot cavity stores energy in the form of standing waves which is transferred to the SRR structures of the metasurface as AC currents that are rectified by the diodes to create DC power. This type of design enables highly efficient energy harvesting for low input power, creates a large antenna capture area, and uses elements with small electrical size, such that 100 uW of power (enough to operate simple devices) can be captured at ambient intensities ~ 1 - 2 uW/cm2. Two designs using this method are presented, one that operates for linear polarizations at 0.9 GHz and a smaller polarization-independent design that operates around 1.5 GHz. In the second method, the energy stored in the standing waves of an MPA Fabry-Perot cavity is instead harvested by placing a separate energy harvesting antenna within the cavity. The cavity shapes and enhances the incident electric field, and then the separate energy harvesting antenna is designed to be inserted into the cavity so that its shape and/or radiation pattern matches the electric field lines within the cavity and maximally extracts the stored energy. This method allows for great customization of antenna design parameters, such as operating frequency, polarization dependence, and directionality, by swapping out different metasurface and antenna designs. Using this method, the amount of power harvested by a simple dipole rectenna placed within a cavity is improved by a factor of 18 as compared to what it would harvest by itself at an ambient intensity of 35 nW/cm2. Lastly, the addition of plasmonic structures to DWELL (quantum dot-in-a-well) infrared photodetectors is investigated by computer simulation. DWELL photodetectors have the potential to one day replace standard mercury cadmium telluride detectors by being cheaper alternatives with a higher operating temperature. The inclusion of gold plasmonic structure arrays into DWELL detectors enables excitation of surface plasmon polariton modes that increase the responsivity of the detector to incident infrared radiation. The peak responsivity of a DWELL detector is demonstrated to improve by a factor of 8 for a 1 um thick layer of plasmonic structures and by a factor of 15 for a 2 um thick layer. These works are steps forward in making RF energy harvesting practically useful and for improving infrared photodetector performance.

There is an increasing need for tuneable spectrally pure semiconductor laser sources as well as broadband and polarization insensitive semiconductor optical amplifiers based on the InGaASP/InP material system, to be monolithically integrated with other active and passive components in a photonic integrated circuit. This thesis aims to contribute to finding a solution through modelling, experimental characterization and design improvements. In this thesis we have analyzed laterally-coupled distributed feedback (LC-DFB) lasers. These lasers have the gratings etched directly out of the ridge sidewalls thus lowering the cost associated with the re-growth process required if the gratings were otherwise embedded above the active region. The performance characteristics are analyzed for the LC-DFB lasers partitioned into 1-, 2-, and 3-, electrodes with individual bias control at various operating temperatures. The laser exhibits a stable single mode emission at 1560 nm with a current tuning rate of ~14 pm/mA for a tuning of 2.25 nm. The side modes are highly suppressed with a maximum side-mode suppression ratio of 58 dB. The light-current characteristics show a minimum 40 mA threshold current, and power saturation occurring at higher injection currents. The linewidth characteristics show a minimum Lorentzian linewidth of 210 kHz under free-running and further linewidth reduction under feedback operation. The multi-electrode LC-DFB laser devices under appropriate and selective driving conditions exhibit a flat frequency modulation response from 0 to above 300 MHz. The multi-electrode configuration can thus be further exploited for certain requirements. Simulation results and design improvements are also presented. The experimental characterization of semiconductor optical amplifier (SOA) and Fabry-Perot (FP) laser operating in the E-band are also presented. For the SOA, the linear vertical and horizontal states of polarization corresponding to the transverse electric (TE) and transverse magnetic (TM) modes were considered. For various input power and bias, performance characteristics shows a peak gain of 21 dBm at 1360 nm, gain bandwidth of 60 nm and polarization sensitivity of under 3 dB obtained for the entire wavelength range analyzed from 1340 to 1440 nm. The analysis presented in this thesis show good results with room for improvement in future designs.

Diploma thesis deals with the construction of a moder semiconductor optical amplifier SOA. In the first chapter optical comunication and the use of optical amplifiers is described. In the next chapter some types of optical fibers, their chemical composition and properties are described in detail. The third chapter overviews optical amplifiers, their use today, describes their advantages and disadvantages and their proper use in optical comunications. The fourth chapter describes the solution of the objective, the right layout of separate blocks and connections used in measurements. Fifth chapter describes used components, describes their basic parameters and proper use in circuit. The last chapter evaluates outputs of measurements and their impact on the objective.

This thesis focuses on the theory, fabrication, and characterization of 2-port surface acoustic wave (SAW) resonators, as well as the application of their Fabry-Pérot resonance modes for strain sensing. The thesis includes three articles. In the first article, a fabrication method for high frequency SAW devices using traditional UV photolithography equipment is developed. It is well known that SAW sensors become more sensitive at higher frequencies but realizing high frequency devices requires small features which challenge existing photolithography methods. The proposed process is a modified version of a previously reported tri-layer lift-off photolithography process intended for Si or SiO2 substrates which allows for compatibility with materials that are piezoelectric and pyroelectric, often used as the substrate in SAW devices. The process uses a lithographic tri-layer consisting of layers of lift-off resist (LOR) on the bottom, back anti-reflection coating (BARC) in the middle, and photoresist (PR) on top, improving resolution by a factor of two over traditional lift-off photolithography techniques. We demonstrate the fabrication of a SAW device with an interdigital transducer (IDT) pitch of 4 μm (minimum feature size of 1 μm) on 128o Y-X cut lithium niobate, whose operating frequency is measured as 994.5 MHz. The 2-Port SAW devices that are used in subsequent chapters are fabricated using this process. The second article proposes a method of analyzing acoustic Fabry-Pérot spectra, by analogy with optical cavities, to determine key SAW parameters. In our experiment, 2-port SAW resonators, consisting of two interdigital transducers (IDTs) laterally separated by a free surface cavity length, are used to generate SAWs on 128o Y-X lithium niobate that are trapped between the two IDTs which also act as Bragg reflectors. Fabry-Pérot cavity peaks can be observed through the electrical S11 (reflection) spectrum measured on one IDT, hence a 2-Port resonator is equivalent to an acoustic Fabry-Pérot cavity/resonator. Measurements of the free spectral range and linewidths are then fitted to linear models to obtain the free surface velocity and attenuation of SAW waves, as well as the reflection of interdigital transducers (IDTs), all of which are crucial design parameters. Our method of analyzing Fabry-Pérot spectra provides a convenient method for determining key characteristics of SAW waves and cavities. In the third article, a surface acoustic wave (SAW) strain sensor based on measuring acoustic Fabry-Pérot resonance peaks from a 2-port SAW resonator is demonstrated. A theoretical analysis is proposed to estimate the frequency sensitivity to strain of IDT and cavity resonances and to predict strain distributions in both the cavity and IDT regions of a 2-port SAW resonator bonded to a tapered cantilever beam. The frequency stability of cavity resonance peaks for fabricated 2-port SAW resonators of different cavity length are measured and analyzed to determine the cavity length which exhibits maximum frequency stability. A cross-correlation analysis technique is then introduced to improve the detection of the frequency shift of SAW resonances and enable multimode frequency shift detection. The measured frequency sensitivity to strain of the cavity resonances of a resonator 10 mm in length (operating frequency = 97.7 MHz) was found to be -103.2 ± 0.2 Hz/με while demonstrating excellent linearity (R2 = 0.9999). By considering a minimum signal to noise ratio (SNR) of 3 dB, the device exhibits a minimum strain resolution of only 234 nε.