Evaluation of White Light Sources For an Absolute Fiber Optic Sensor Readout System

McConaghy, C F

Lawrence Livermore National Laboratory

关键词: Light Sources;    Wavelengths;    71 Classical And Quantum Mechanics, General Physics;    Titanium Oxides;    Readout Systems;   

DOI  :  10.2172/15009728
RP-ID  :  UCRL-TR-200273
RP-ID  :  W-7405-ENG-48
RP-ID  :  15009728

美国|英语

来源: UNT Digital Library

【 摘 要 】

This report summarizes work done in pursuit of an absolute readout system for Fabry-Perot optics sensors such as those built both by FISO and LLNL. The use of white light results in a short coherence length reducing the ambiguity of the Fabry-Perot gap measurement which is required to readout the sensor. The light source coherence length is the critical parameter in determining the ability to build a relative or an absolute system. Optical sources such as lasers and LEDs are rather narrow in optical spectral bandwidth and have long coherence length. Thus, when used in interferometric sensor measurements, one fringe looks much like another and it is difficult to make an absolute measurement. In contrast, white light sources are much broader in spectral bandwidth and have very short coherence lengths making interferometry possible only over the coherence length, which can be 1 or 2 microns. The small number of fringes in the interferogram make it easier to calculate the centroid and to unambiguously determine the sensor gap. However, unlike LEDs and Lasers, white light sources have very low optical power when coupled into optical fibers. Although, the overall light output of a white light source can be hundreds of milliwatts to watts, it is difficult to couple more than microwatts into a 50-micron core optical fiber. In addition, white light sources have a large amount of optical power in spectrum that is not necessarily useful in terms of sensor measurements. The reflectivity of a quarter wave of Titanium Oxide is depicted in Figure 2. This coating of Titanium Oxide is used in the fabrication of the sensor. This figure shows that any light emitted at wavelengths shorter than 600 nm is not too useful for the readout system. A white light LED spectrum is depicted in Figure 3 and shows much of the spectrum below 600 nm. In addition Silicon photodiodes are usually used in the readout system limiting the longest wavelength to about 1100 nm. Tungsten filament sources may have much of their optical power at wavelengths longer than 1100 nm, which is outside the wavelength range of interest. An incandescent spectrum from a tungsten filament is depicted in Figure 4. None of this is to say that other types of readout systems couldn't be built with IR detectors and broadband coatings for the sensors. However, without reengineering the sensors, the wavelength restrictions must be tolerated.