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Broadband cavity spectrometer apparatus and method for determining the path length of an optical structure

Broadband cavity spectrometer apparatus and method for determining the path length of an optical structure description/claims

The present invention relates to apparatus and methods for measuring the optical path length in optical cavities.

An optical cavity may be any region bounded by two or more reflective interfaces that are aligned to provide multiple reflections of light waves. Optical cavities have been monitored or measured using a single wavelength illumination source such as a helium/neon (HeNe) laser. A change in the cavity size is detected by observing the change in reflected or transmitted intensity at the single wavelength. Monitoring a single wavelength reflected intensity requires a much larger signal-to-noise ratio (S/N) than a broadband technique. In certain applications, the cavity is filled with a fluid that is designed to have an index of refraction as close as possible to that of the bounding surfaces, making the reflectivity very small. In such cases, the S/N will be small and thus may not be sufficient to use a single wavelength technique. In a single wavelength system, there is not a one-to-one correspondence between the measured intensity to the optical path length; in other words, a measured intensity may correspond to any number of optical path lengths. Therefore, a single-wavelength system cannot determine the absolute value of the optical path length, it can only detect changes. The change in optical path length as measured by a single-wavelength system is sufficient for some servo applications where the path length is to be held constant; however, this allows the possibility of mode-hopping where the servo unintentionally and undesirably locks onto a different spectral mode.

Therefore, there is a need for an apparatus and method for measuring or monitoring an optical cavity path length with an output that provides the absolute value of the optical path length, has better S/N tolerance, is free of mode-hopping limitations, and offers near real time operation.

A broadband light source with a sufficiently long coherence length is impinged upon an optical cavity. In one embodiment, the broadband laser light reflects from the first and second surfaces of the optical cavity generating multiple reflected light beams. Generally, the two most intense beams will be those that are only reflected once: one from the first surface and one from the second. These two beams are sufficient to produce the necessary interference signal. Therefore, ignoring the weaker reflected beams will not change the functionality of the invention and for simplicity the discussion is limited to the first two beams.

In another embodiment, the modulated light beam transmitted through the optical cavity is used to produce the necessary interference signal. In general, when light is incident on a reflective surface, it is partially transmitted and partially reflected. The transmitted light beam is primarily comprised of the following two beams: that which is transmitted through both the first and second surfaces of the optical cavity; and that which is transmitted through the first surface, reflected from the second surface, reflected again from the first, and finally transmitted through the second surface. These two beams are sufficient to produce the necessary interference signal. Therefore, ignoring the weaker transmitted beams will not change the functionality of the invention and for simplicity the discussion is limited to these two beams.

The two broadband reflected (or transmitted) light beams are phase shifted from one another by an amount proportional to the optical path length (OPL) of the optical cavity. They interfere with each other and produce a modulated light beam that has a spectrum that looks approximately like the broadband laser's spectrum multiplied by a cosine with a frequency (4*π*n*d/λ), where n is the index of refraction of the medium within the cavity, d is the physical separation between the cavity surfaces, and λ is the wavelength of the light. The combined modulated light is coupled to a spectrometer which outputs a spectrum that is a measure of the intensity of the light as a function of wavelength, over a range determined by the spectrometer specifications. The spectrum is changed to a function of wavenumber, where for our purposes the wavenumber is the reciprocal of the wavelength. The spectrum is Fourier transformed, resulting in the Fourier amplitude as a function of a variable that is the reciprocal of the wavenumber. The Fourier transform contains at least one peak that is located at the independent variable coordinate 2*n*d, where n*d is the OPL.

The Fourier transform of the spectrum will contain a DC peak and at least one other peak that is related to the OPL. The certainty that the Fourier transform of the spectrum of the light reflected from or transmitted through the optical cavity has a peak located at a coordinate equal to twice the OPL is a key feature of the present invention. By locating and tracking the position of this peak, one is able to measure and track the OPL directly. Due to the nature of the spectrum of the light reflected from the optical cavity, this OPL peak will be the most prominent one aside from the DC peak (whose location is known). This feature adds to the ease of tracking the OPL with the Fourier transform. If efficient Fourier algorithms are used with sufficiently fast computer technology, the OPL may be tracked in real-time, for example, during a manufacturing process.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings in which:

FIG. 1 is a diagram illustrating an apparatus for practicing embodiments of the present invention;

FIG. 2A is a diagram of the broad band input spectrum and the spectrum of the reflected light from the optical cavity;

FIG. 2B is a diagram illustrating which wavelengths of incident light have constructive interference for a given cavity and which have destructive interference;

FIG. 2C is a diagram illustrating a graph of a Fourier transform of a reflected spectrum from an optical cavity according to embodiments of the present invention; and

FIG. 3 is a flow diagram of method steps according to embodiments of the present invention.

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