Laser metrology systems are a key component of stellar interferometers, used to monitor path lengths and dimensions internal to the instrument. Most interferometers use 'relative' metrology, in which the integer number of wavelengths along the path is unknown, and the measurement of length is ambiguous. Changes in the path length can be measured relative to an initial calibration point, but interruption of the metrology beam at any time requires a re-calibration of the system.
The MSTAR sensor (Modulation Sideband Technology for Absolute Ranging) is a new system for measuring absolute distance, capable of resolving the integer cycle ambiguity of standard interferometers, and making it possible to measure distance with sub-nanometer accuracy. We describe the design of the system, show results for target distances up to 1 meter, and demonstrate how the system can be scaled to kilometer-scale distances. In recent experiments, we have used white light interferometry to augment the 'truth' measurements and validate the zero-point of the system.
MSTAR is a general-purpose tool for conveniently measuring length with much greater accuracy than was previously possible, and has a wide range of possible applications.
Absolute metrology measures the actual distance between two optical fiducials. A number of methods have been employed, including pulsed time-of-flight, intensity-modulated optical beam, and two-color interferometry. The rms accuracy is currently limited to ~5 microns. Resolving the integer number of wavelengths requires a 1-sigma range accuracy of ~0.1 microns. Closing this gap has a large pay-off: the range (length measurement) accuracy can be increased substantially using the unambiguous optical phase.
The MSTAR sensor (Modulation Sideband Technology for Absolute Ranging) is a new system for measuring absolute distance, capable of resolving the integer cycle ambiguity of standard interferometers, and making it possible to measure distance with sub-nanometer accuracy. In this paper, we present recent experiments that use dispersed white light interferometry to independently validate the zero-point of the system. We also describe progress towards reducing the size of optics, and stabilizing the laser wavelength for operation over larger target ranges.
MSTAR is a general-purpose tool for conveniently measuring length with much greater accuracy than was previously possible, and has a wide range of possible applications.
The MSTAR sensor (Modulation Sideband Technology for Absolute Ranging) is a new system for measuring absolute distance, capable of resolving the integer cycle ambiguity of standard intrferometers, and making it possible to measure distance with sub-nanometer accuracy. The sensor uses a single laser in conjugation with fast phase modulators and low-frequency detectors. We describe the design of the system - the principle of operation, the metrology source, beam-launching optics, and signal processing - and show results for target distance up to 1 meter. We then demonstrate how the system can be scaled to kilometer-scale distances.
Fiber Bragg grating written into the core by a sideways exposure to an ultra violet-laser interference pattern have shown great promise for use as practical strain sensors in large structures. One way to sense the strain of the grating is by using active interrogation whereby the fiber Bragg grating is used as the optical feedback element of a laser cavity, and the lasing wavelength is monitored as the system output. Compared to passive broadband techniques, the fiber Bragg laser sensor provides much stronger optical signals, thus leading to a much improved signal-to- noise ratio. In order to optimize the power output from this sensor, one wishes to model the output from the fiber laser in terms of the Er-doped fiber parameters, the pump characteristics, the cavity mirror reflectivities and losses in the cavity. In this paper we solve the rate and propagation equations for a Fabry Perot cavity to obtain explicit closed form equations for the output power, threshold pump power, as well as for the optimum length. Experiments where we used an electron cyclotron resonance plasma enhanced chemical vapor deposition apparatus to deposit dielectric thin films on one fiber end point in order to change the reflectivity of a cavity mirror, while monitoring the reflectivity in situ, verify the validity of the model.
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