Optics: measuring and testing – By light interference – Spectroscopy
Reexamination Certificate
2001-04-13
2002-10-08
Dunn, Drew A. (Department: 2882)
Optics: measuring and testing
By light interference
Spectroscopy
C356S451000
Reexamination Certificate
active
06462823
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical spectrometers, and more particularly, to wavelength meters based on Michelson Interferometers.
BACKGROUND OF THE INVENTION
An optical wavelength meter is an electronic instrument that measures the wavelength of a light signal input thereto. An optical multi-wavelength meter is one that can simultaneously measure the wavelengths of multiple signals of light such as channels in a wavelength division multiplexing (WDM) system.
Wavelength meters based on Michelson interferometers are well known in the optical arts. Michelson interferometer based wavelength meters operate by measuring an interference signal between two light beams generated from the optical signal being measured. One light beam traverses a fixed path while the other traverses a path that is varied by moving the position of a mirror that is part of that path. The interference signal varies in amplitude as a function of the mirror position. In this type of wavelength mirror, the amplitude of the interference signal is measured as a function of the mirror position at a number of discrete mirror positions. The resulting signal is then subjected to a fast Fourier transform (FFT) to obtain the amplitude of the input optical signal as a function of wavelength. The measurement points are typically defined by a second interference pattern that is generated from a light signal of known wavelength that is reflected from the same mirror.
A common measurement for evaluating a WDM system consists of measuring the noise level centered between channels in a WDM system. The noise measured between channels is a combination of noise generated by the WDM system and noise generated by the instrument. Hence, measurement of the interchannel noise requires that the instrument noise be small compared to the noise in the system under measurement.
One method for minimizing the effects of instrument noise is to take multiple traces and average the results. Each trace requires a FFT. Hence, the multiple traces can impose a severe computational load on the instrument.
In principle, multiple traces can be averaged in the time-domain to obtain an averaged trace that is then subjected to a single FFT. However, to average the time-domain traces, each trace must begin and end at the same point in the mirror's travel. Each of the measurement points of each trace must be taken at the same distance from the zero optical path delay position of the second interference pattern discussed above. More specifically, the measurement points must be taken at the same distance away from the zero optical path delay of the input signal. Ideally, the zero optical path delay position for the input signal and for the second interference pattern are either the same point, or offset by a constant amount over the range of temperatures encountered during the measurement process. If the zero optical path delay position does not move, then the measurements can begin at the same mirror position. However, these positions are normally not equivalent because the distances between the fixed and moving mirrors change due to temperature changes. While the calibration standard used to define the measurement points provides very high accuracy in determining the measurement points once a starting reference fringe is selected, this standard is of little use in measuring the absolute position of the mirror at any time.
Broadly, it is the object of the present invention to provide an improved wavelength meter.
It is a further object of the present invention to provide a wavelength meter that can generate multiple traces that are aligned with respect to one another so that the traces can be averaged.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
SUMMARY OF THE INVENTION
The present invention is a wavelength meter for measuring the wavelength of an input light signal. The meter includes a beam splitter, a fixed mirror, and a moveable mirror, the beam splitter splits the input signal into a first input signal and a second input signal, the first and second input signals being reflected from the fixed and moveable mirrors, respectively, and being recombined by the beam splitter. A measurement detector generates a measurement signal related to the amplitude of the recombined input signal. The meter also includes a reference light source for generating a reference light signal that is split by the beam splitter into first and second reference light signals. The first reference light signal is reflected by the fixed mirror and the second reference light signal is reflected by the moving mirror, the beam splitter recombining the first and second reference light signals to form a combined reference light signal. The amplitude of the combined reference light signal is detected by a reference detector that generates a reference signal related to the amplitude of the combined reference light signal. The wavelength meter also includes a position circuit for generating a start signal indicating that the moveable mirror is at a first position relative to the fixed mirror and that the moveable mirror is moving in a first direction. A controller records a sequence of measurement signal values in response to the start signal and the reference signal. A plurality of such sequences can be averaged together prior to performing a Fourier transform to determine the optical spectrum of the input light signal.
REFERENCES:
patent: 5166749 (1992-11-01), Curbelo et al.
patent: 5523838 (1996-06-01), Nagashima
patent: 5576834 (1996-11-01), Hamada
patent: 5943134 (1999-08-01), Yamaguchi et al.
Braun David M.
Fortenberry Rance M.
Hill Gregory S.
Wheeler Benjamin S.
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