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Until recently, fiber optics tended to be regarded only as a medium for the transmission of large quantities of data. Work done in our laboratory has exteded the application of fibers into a little-known area, the delivery of power.
In a wavelength division multiplexed (WDM) fiber system, where pulses on different wavelength beams may co-propagate in a single mode fiber, the cross phase modulation (CPM) effects caused by the nonlinearity of the optical fiber are unavoidable.
A DC magnetron sputtering system has been used to actively coat optical fibers with hermetic metal coatings during the fiber draw process. Thin films of Inconel 625 have been deposited on optical fibers and annealed in air at 2000 F. Scanning electron microscopy and Auger electron microscopy have been used to investigate the morphology and composition of the films prior to and following thermal cycling. Issues to be addressed include film adhesion, other coating materials, and a discussion of additional applications for this novel technology.
Described is a fiber-optic temperature sensor that uses a spectrum modulating SiC etalon. The spectral output of this type of sensor may be analyzed to obtain a temperature measurement which is largely independent of the transmission properties of the sensor's fiber-optic link. A highly precise laboratory spectrometer is described in detail, and this instrument is used to study the properties of this type of sensor. Also described are a number of different spectrum analyzers that are more suitable for use in a practical thermometer.
Described is a fiber-optic temperature sensor that uses a spectrum modulating SiC etalon. The spectral output of this type of sensor may be analyzed to obtain a temperature measurement which is largely independent of the transmission properties of the sensor's fiber-optic link. A highly precise laboratory spectrometer is described in detail, and this instrument is used to study the properties of this type of sensor. Also described are a number of different spectrum analyzers that are more suitable for use in a practical thermometer.
A fiber-optic thermometer based on temperature dependent absorption in Nd(3+) doped glass is demonstrated over the 298-573 K range. A broadband detection technique allows the use of the complete spectrum of a pulse modulated light emitting diode. A fiber-optic recirculating loop is employed to construct a reference channel in the time domain by generating a train of pulses from one initial pulse. A theoretical model is developed, and experimental data are shown to compare well with the theory. Possible sources of error and instability are identified, and ways to enhance the performance of the system are proposed.
A numerical model has been developed to calculate the modal phase shift of circular step index profile weakly guiding fibers under axial strain. Whenever an optical fiber is under stress, the optical path length, the index of refraction, and the propagation constants of each mode change. In consequence, the phase of each mode is also modified. A relationship for the modal phase shift is presented. This relation is applied to both single mode and two-mode fibers in order to determine the sensitivity characteristics of strained fibers. It was found that the phase shift is strongly dependent on the core refractive index, n(co). It was also found that it is possible to design fibers which are insensitive to axial strain. Practical applications of strain insensitive fibers are discussed.
A lensless backscatter fiber optic probe is used to measure the size distribution of protein molecules inside an excised, but intact, human eye lens. The fiber optic probe, about 5 mm in diameter, can be positioned arbitrarily close to the anterior surface of the eye; it is a trans-receiver, which delivers a Gaussian laser beam into a small region inside the lens and provides a coherent detection of the laser light scattered by the protein molecules in the backward direction. Protein sizes determined from the fast and slow diffusion coefficients show good correlation with the age of the lens and cataractogenesis.
NASA Langley Research Center is developing instrumentation based upon principles of Optical Frequency-Domain Reflectometry (OFDR) for the provision of large-scale, dense distribution of strain sensors using fiber optics embedded with Bragg gratings. Fiber Optic Bragg Grating technology enables the distribution of thousands of sensors immune to moisture and electromagnetic interference with negligible weight penalty. At Langley, this technology provides a key component for research and development relevant to comprehensive aerospace vehicle structural health monitoring. A prototype system is under development that includes hardware and software necessary for the acquisition of data from an optical network and conversion of the data into strain measurements. This report documents the steps taken to verify the software that implements the algorithm for calculating the fiber strain. Brief descriptions of the strain measurement system and the test article are given. The scope of this report is the verification of software implementations as compared to a reference model. The algorithm will be detailed along with comparison results.
Large aperture phased array antennas operating at millimeter wave frequencies are designed for space-based communications and imaging platforms. Array elements are comprised of active T/R modules which are linked to the central processing unit through high-speed fiber-optic networks. The system architecture satisfying system requirements at millimeter wave frequency is T/R level data mixing where data and frequency reference signals are distributed independently before mixing at the T/R modules. This paper demonstrates design procedures of a low loss high-speed fiber-optic link used for transmission of data signals over 600-900 MHz bandwidth inside satellite. The fiber-optic link is characterized for transmission of analog and digital data. A dynamic range of 79 dB/MHz was measured for analog data over the bandwidth. On the other hand, for bursted SMSK satellite traffic at 220 Mbps rates, BER of 2 x 10 to the -7th was measured for E(b)/N(o) of 14.3 dB.
This paper reports on the development of a photoelastic fiber optic strain gage sensitive to transverse strain. The sensing element is made from an epoxy resin which is stress frozen to passively achieve the quadrature condition. Light, emitted from an LED operating at 820 nm, is transmitted to and from the sensing element via multimode fibers and the signal is detected using a dual channel operational photodiode/amplifier. This unique combination of optics and electronics produces a fiber optic sensor having a high signal to noise ratio which is lead-in/out insensitive. Results show that strains on the order of 1 microstrain can be measured over an 800 microstrain range and that dummy gages can be used for temperature compensation.
Fiber Optic Bragg Grating taps are fabricated and characterized at various wavelengths using a modified Talbot interferometric technique. Gratings are fabricated by tilting the photosensitive fiber to angles up to 45 degrees w.r.t. the writing angle. Diffraction characteristics of the tilted grating is monitored in first and second orders.
This paper describes an electrically passive fiber-optic interferometer which uses dual frequency-modulated laser diodes. Experimental results show that this type of interferometer can attain a displacement range of 100 micron with subnanometer resolution. This technique can serve as the basis for a number of high-precision fiber-optic sensors.
Issues regarding the use of optical fiber thermometers to control heater settings in a microgravity vacuum furnace are addressed. It is desirable to use these probes in environments such as the International Space Station, because they can be operated without re-calibration for extended periods. However, the analysis presented in this paper shows that temperature readings obtained using optical fiber thermometers are corrupted due to emissions from the fiber when extended portions of the probe are exposed to elevated temperatures.
Issues regarding the use of optical fiber thermometers to control heater settings in a microgravity vacuum furnace are addressed. It is desirable to use these probes in environments such as the International Space Station, because they can be operated without re-calibration for extended periods. However, the analysis presented in this paper shows that temperature readings obtained using optical fiber thermometers can be corrupted by emissions from the fiber when extended portions of the probe are exposed to elevated temperatures.
A method of polishing a roof-shaped, or bevelled, tip having a predetermined angle on the end of an optical fiber is disclosed. The method comprises placing an optical fiber in contact with an abrasive tape at an angle corresponding to the desired predetermined angle. The fiber is moved towards the tape beyond the point of contact so as to cause a bend in the fiber. Upon moving the abrasive tape, the compliance of the fiber, by virtue of the bend, provides a pressure of the fiber against the tape to enable polishing to occur. As polishing continues the pressure decreases until enough material is removed so as to eliminate the bend and impart the predetermined angle to the fiber tip. The fiber may be rotated to polish the other side in a similar manner, if so desired.
A modification to a commercial Fourier Transform (FT) Raman spectrometer is presented for the elimination of thermal backgrounds in the FT Raman spectra. The modification involves the use of a mechanical optical chopper to modulate the continuous wave laser, remote collection of the signal via fiber optics, and connection of a dual-phase digital-signal-processor (DSP) lock-in amplifier between the detector and the spectrometer's collection electronics to demodulate and filter the optical signals. The resulting Modulated Fourier Transform Raman Fiber-Optic Spectrometer is capable of completely eliminating thermal backgrounds at temperatures exceeding 300 C.
Through the use of fiber optics, a series of very narrow slits have been constructed and placed at the exit plane of a spectrograph. Plasma radiation which is dispersed by the spectrograph is incident on the slits and is transmitted to separate phototubes via the quartz fibers. With this technique, time resolved measurements of the spectral shape of the hydrogen H beta line have been made and used to determine the electron density of a transient plasma. Data obtained in a shock tube indicated that the thermodynamic conditions behind the reflected shock in a mixture of 20 percent H 80 percent He correspond to theoretically predicted conditions.