Optical fiber sensors utilizing Brillouin scattering rely on the principle that the Brillouin frequency shift is a function of the local temperature or strain. Conventional optical fibers, such as standard telecommunications single-mode fibers, have been successfully used in these applications, and most typically in the time domain, such as with BOTDR. Such conventional fibers however are susceptible simultaneously to both temperature and strain, requiring either at least two fibers or specialized cabling to distinguish the effects of a local stress from those of a local change in temperature. Recently, methods utilizing fibers possessing at least two Brillouin frequency shifts, each with different temperature or strain coefficients have been proposed. However, realizing such fibers is challenging, requiring fibers with regions of very different compositions, all of which must have substantial overlap with the optical field, posing significant manufacturing challenges. We present several new specialty optical fibers based on novel and unconventional fabrication techniques with significant potential for use in distributed fiber sensor systems. First, we describe a class of fibers fabricated from materials whose Brillouin frequency shifts are immune to either temperature or strain, with a demonstration of the former using fiber derived from sapphire crystal, and modeling and measurements predicting the latter. The ‘Brillouin-athermal’ fiber enables the measurement of a local strain, independent of the local temperature. Second, we describe and demonstrate a novel group of longitudinally graded (chirped) fibers enabling easily-implemented frequency-domain systems; affording the potential to simplify and reduce the cost of Brillouin-based distributed sensors.
Described herein, for the first time to the best of our knowledge, are results on optical fibers possessing significant compositional gradations along its length due to longitudinal control of the core glass composition. More specifically, MCVD-derived germanosilicate fibers were fabricated that exhibited a gradient of up to about 0.55 weight % GeO2 per meter. These gradients are about 1900 times greater than previously reported fibers possessing longitudinal changes in composition. The refractive index difference is shown to change by about 0.001, representing a numerical aperture change of about 10%, over a fiber length of less than 20 m. The lowest attenuation measured from the present longitudinally-graded fiber (LGF) was 82 dB/km at a wavelength of 1550 nm, though this is shown to result from extrinsic process-induced factors and could be reduced with further optimization. The stimulated Brillouin scattering (SBS) spectrum from the LGF exhibited a 4.4 dB increase in the spectral width, and thus reduction in Brillouin gain, relative to a standard commercial single mode fiber, over a fiber length of only 17 m. The method employed is very straight-forward and provides for a wide variety of longitudinal refractive index and acoustic velocity profiles, as well as core shapes, which could be especially valuable for SBS suppression in high-energy laser systems. Next generation analogs, with longitudinally-graded compositional profiles that are very reasonable to fabricate, are shown computationally to be more effective at suppressing SBS than present alternatives, such as externally-applied temperature or strain gradients.
Recently developed glass-clad crystalline semiconductor core optical fibers potentially offer a series of
advantages over present optical fiber materials including greatly enhanced Raman cross-sections and extended
infrared transparency. Indeed, the low-cost high-through-put fiberization of crystalline materials could permit a
step-jump in performance critical for use in high energy laser, infrared counter-measure, communication, and
sensor systems. Further, the high degree of crystallinity is of considerable scientific value since optical fiber
fabrication is a highly non-equilibrium process and so achieving high degrees of crystallinity is very counter-intuitive
and offers new insight into crystal growth mechanisms. This talk will review progress in glass-clad fibers
possessing cores of highly crystalline silicon and germanium including anomalies in Si and Ge that may benefit
fiber fabrication as well as paths forward to optimization of fiber design and performance.
The properties of glass-clad fibers containing cores of phase pure and highly crystalline
silicon and germanium are reviewed. Although further optimization is required, losses of about 4
dB/m have been achieved at 3 μm and suggest that such semiconductor core fibers could be of
practical value for nonlinear and infrared applications.
We used coupled-mode theory in an Yb-doped multimode fiber amplifier to compute the effects of gain saturation, nonlinear index, and fiber curvature on the evolution of the field. A positive nonlinear index results in power transfer to lower-order modes, usually the fundamental LP10 mode, and for negative nonlinear index the reverse is predicted. The nonlinear interaction between modes breaks the core’s cylindrical symmetry, resulting in recombination of degenerate LP mode pairs into super-modes: consisting of an expected in- and anti-phase pair, but also a quadrature of super-mode that reflects an increase of “information” capacity associated with nonlinearity. Convergence to all three super-modes was observed in our simulations, but the last more often. We also present observed evidence of mode phasing in experiments with two fiber amplifiers.
The collapse of the Soviet Union opened many areas of laser technology to the West. E-beam- pumped semiconductor lasers (EBSL) were pursued for 25 years in several Soviet Institutes. Thin single crystal screens of II-VI alloys (ZnxCd1-xSe, CdSxSe1-x) were incorporated in laser CRTs to produce scanned visible laser beams at average powers greater than 10 W. Resolutions of 2500 lines were demonstrated. MDA-W is conducting a program for ARPA/ESTO to assess EBSL technology for high brightness, high resolution RGB laser projection application. Transfer of II-VI crystal growth and screen processing technology is underway, and initial results will be reported. Various techniques (cathodoluminescence, one- and two-photon laser pumping, etc.) have been used to assess material quality and screen processing damage. High voltage (75 kV) video electronics were procured in the U.S. to operate test EBSL tubes. Laser performance was documented as a function of screen temperature, beam voltage and current. The beam divergence, spectrum, efficiency and other characteristics of the laser output are being measured. An evaluation of the effect of laser operating conditions upon the degradation rate is being carried out by a design-of-experiments method. An initial assessment of the projected image quality will be performed.
The collapse of the Soviet Union opened many areas of laser technology to the West. E-beam-pumped semiconductor lasers were pursued for 25 years in several Soviet institutes. Thin single crystal screens of II-VI alloys (ZnCdxSe1-x, CdSxSe1-x) were incorporated in laser CRTs to produce scanned visible laser beams at average powers greater than 10 W. Resolutions of 2500 lines were demonstrated by the Russians. MDA-W is performing a contract for ARPA to assess e-beam- pumped semiconductor laser technology for high brightness, high resolution RGB laser projection application. Transfer of II-VI crystal growth and screen processing technology is underway, and initial results will be reported. Various techniques (cathodoluminescence, one- and two- photon laser pumping, etc.) were used to assess material quality and screen processing damage. High voltage (75 kV) video electronics were designed and procured in the U.S. to operate test laser tubes obtained from Russian sources. The primary military applications of interest to MDA are flight simulator visuals, command center large screen projection and possibly cockpit display. Laser radar, designation, and communication are secondary applications. Some of the commercial applications included electronic cinema, teleconference display, university auditorium display, laser microscopy, and optical computing. Applications and plans for commercialization will be discussed.
We have designed a monolithic 8 X 8 optical crossbar switch that can provide a lossless link for a fiber optic databus or communication system. The device is based upon a semiconductor active (with gain) waveguide structure that can be used as a building block at a monolithic device or multi-device level to configure a high bandwidth space division switch matrix. We have fabricated and characterized a first generation of devices and currently are involved in characterizing the second and designing the third. We will discuss our concepts for applying our device concepts to higher levels of network integration for military and commercial applications.
A semiconductor laser phased-array transmitter is described that operates by coherently combining the output of many semiconductor laser amplifiers to form a near-diffraction-limited beam in the far field with sufficient power for most free-space optical communications applications. Because the outputs of the amplifiers are coherently combined, modulation formats requiring coherent detection can be used with this type of transmitter. A one-dimensional phased array of semiconductor optical amplifiers is used to amplify a 200 Mbps PSK signal which is then detected using a self-homodyne technique. The communications performance when the phased array is used is not degraded compared to the performance when the array is not used. In addition, the array introduces no additional phase noise to cause performance degradation.
A semiconductor laser phased-array transmitter is described that operates by coherently combining the output of many semiconductor laser amplifiers to form a near-diffraction-limited beam in the far field with sufficient power for most free space optical communications applications. Because the outputs are coherently combined, modulation formats requiring coherent detection can be used with this transmitter. Results are presented on the operation of the phased array and on preliminary communications experiments conducted with this transmitter.