Wavelength measurement adjustment

Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer

Reexamination Certificate

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Reexamination Certificate

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06798522

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to determining the wavelength of an optical beam.
2. Discussion of the Background Art
Determining the wavelength of optical signals is common object in optical applications. A description of the most common principles for determining the wavelength is given in the pending European Patent Application 00117607.2-2217 and the teaching thereof is incorporated herein by reference.
It is clear that the accuracy of such wavemeters directly affects the accuracy of the entire measurement. Typical sources for inaccuracies are electrical, and/or mechanical, and/or environmental variations (temperature, air pressure, and gravity).
In order to improve accuracy, tunable laser sources or wavemeters are usually calibrated in certain intervals e.g. at a factory site with devices having known wavelength characteristics.
Although calibration usually improves the accuracy of the measurement, it is clear that the sources of inaccuracy mentioned above still persist after the calibration and will still adversely affect the wavelength accuracy. Another disadvantage, of course, results from the additional effort that has to be spent for the calibration process.
SUMMARY OF THE INVENTION
It is an object of the present invention to further improve accuracy for wavelength measurements. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.
According to the invention, a wavelength-determining unit for determining the wavelengths of a plurality of successive optical signals comprises a wavemeter unit, an absolute-measuring unit having unambiguous wavelength properties at known absolute wavelength values, and an evaluation unit. The wavemeter unit determines (e.g. successive) wavelength values for the optical signals. The absolute-measuring unit determines such of the known absolute wavelength values covered by the optical signals. Both devices—the wavemeter and the absolute measuring unit—receive the same optical signals and run substantially simultaneously.
The evaluation unit receives the determined wavelength values from the wavemeter unit and the covered known absolute wavelength values from the absolute-measuring unit. The evaluation unit compares the determined wavelength values from the wavemeter with the covered known absolute wavelength values, and corrects the determined wavelength values based on the covered known absolute wavelength values.
Thus, the invention provides a correction or adjustment of the measuring results that is suitable to provide an online correction. Such simultaneous calibration of wavelength values provides strong improvements in comparison to single factory site calibrations, which cannot cover individual conditions of the setup and environment during measurement. This is in particular useful when the measurement setup, in particular the wavemeter unit, is susceptible for variations, e.g. by thermal or mechanical influences, which can affect the measuring conditions and/or accuracy.
In a preferred embodiment, the wavemeter unit has a wavelength characteristic known in principle or derived from former measurements. In that case, the evaluation unit adjusts the known wavelength characteristic based on the covered known absolute wavelength values, and corrects the determined wavelength values accordingly.
The correction of the determined wavelength values or the wavelength characteristics of the wavemeter is preferably accomplished by correlating the covered known absolute wavelength values with determined wavelength values or with the wavelength characteristics of the wavemeter unit, e.g. by comparing the covered known absolute wavelength value with the wavelength values determined by the wavemeter unit for the same optical signal. The evaluation unit can then determine one or more offset and/or corrections values for correcting the determined wavelength values or for calibrating the wavelength characteristics of the wavemeter unit.
The absolute-measuring unit makes use of unambiguous wavelength properties like absolutely known transmission features as provided e.g. by gas absorption cells. In such gas absorption cells, the incoming light is passed through a gas cell acting as an optical filter having known absorption lines of the gas as absolutely known transmission features. Such filters are described e.g. in U.S. Pat. No. 5,780,843 for controlling high accuracy tunable laser sources.
A preferred embodiment of the wavemeter unit makes use of the interferometric principle, such as the Fizeau, Michelson or Fabry-Perot interferometer or uses e.g. a combination of different etalons (which can be also realized based on polarization effects) as disclosed in detail in the aforementioned EP-A-875743. Those interferometric units generally provide a periodic dependency over the wavelength, but exhibit a higher resolution than the units employing wavelengths dependent material properties.
For providing the wavelength correction of the invention, the optical signals are swept over a wavelength range wherein the absolute-measuring unit has at least one of the known absolute wavelength characteristics. By analyzing the measured transmitted power of the absolute-measuring unit together with the wavelength-results derived from the wavemeter unit, a relation between the absolutely known transmission features and the derived wavelength-results can be established. This can result for example in one or more correction values (offset, polynomial coefficients) relating to an e.g. factory based calibration of the wavemeter unit. Because this online calibration reflects the instantaneous measurement conditions it is more accurate than a timely and geographically separated factory based calibration could ever be.
In another embodiment, a separate wavelength source is employed providing optical signals out of the sweep band. The interference path difference in an interferometer of the wavemeter can thus be measured or controlled. This, however, is not applicable for dispersion drift and a very frequency stable source is needed.
The invention can be partly or entirely embodied by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit. In particular, software programs might be applied by the evaluation unit and for controlling a wavelength sweep of a light source.


REFERENCES:
patent: 5450207 (1995-09-01), Fomenkov
patent: 5978391 (1999-11-01), Das et al.
patent: 2002/0131045 (2002-09-01), Anderson
patent: 2002/0163646 (2002-11-01), Anderson
patent: 1041373 (2000-04-01), None
patent: 2280261 (1995-01-01), None
patent: WO 98/36252 (1998-08-01), None

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