Optics: measuring and testing – By shade or color – With color transmitting filter
Reexamination Certificate
2001-07-20
2004-09-21
Nguyen, Thong (Department: 2872)
Optics: measuring and testing
By shade or color
With color transmitting filter
C356S402000, C356S408000
Reexamination Certificate
active
06795188
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to wavemeters for determining the wavelength of an optical beam.
2. Discussion of the Background Art
Wavemeters, i.e. devices for determining the wavelength of an optical beam, are well known in the art. While the main application of wavemeters is to determine the wavelength per se, they can also be applied for tuning a variable (or tunable) laser source. In the latter case, the optical beam can be regarded as being substantially monochromatic, so that the wavemeter fulfils the task of determining the precise wavelength of the variable laser source. In case that the measured wavelength does not coincide with the expected wavelength to be emitted by the variable laser source, adequate correction means might be provided for tuning the variable laser source to the expected or preset wavelength.
A first general principle for determining the optical wavelength makes use of changes in the characteristic properties of some materials in dependency of the wavelength. DE-A-3929845 discloses a dual detector with at least two detectors with different spectral sensitivity and a computer for determining the incident light wavelength from the photocurrent difference or ratio. A similar dual detector is disclosed in DE-A-3030210.
Variable wavelength filters are disclosed e.g. in JP-A-9089674 or GB-A 2288013. In JP-A-9089674, a wavelength detector includes a wavelength independent branching part into which a light beam is input. The branched output is passed to a wavelength-dependent branching part from which a pair of outputs is obtained. The outputs of the wavelength-dependent branching part are passed alternately to a photo-detector through an optical switch. The photo-detector detects the wavelength of the measured light from the ratio of the two outputs. According to GB-A-2288013, an optical fiber-based wavelength meter comprises two serially connected fused optical fiber couplers whose outputs are connected to a photo-detector with wavelength-sensitive gradients of coupling ratios of outputs being of opposite sign and outputs connected to a signal processor.
The above mentioned methods for determining the wavelength based on changes in the characteristic properties dependent on the wavelengths are generally only applicable for determining the wavelength of monochromatic signals and generally exhibit a poor accuracy.
A second possibility for determining the wavelengths applies the principles of diffraction or reflection at optical gratings. In the former case, light is emitted onto a grating, and the angle of the deflected beams indicates the wavelength. Although this solution generally allows determining the wavelength also of polychromatic optical signals, typical drawbacks are poor accuracy and the restriction in wavelengths of typically applicable detectors such as CCD-arrays.
A third possibility for determining the wavelength applies the principle of interference using interferometers such as Fabry-Perot or Fizeau. Those solutions are generally difficult to calibrate and applicable only in a narrow wavelength range.
A further interferometric solution makes use of the interferometric properties of etalons, i.e. plane parallel plates of glass or fused quartz with reflecting surfaces. Due to the interference at the surfaces within the etalon, the light beam transmitted through the etalon exhibits a wavelength dependent transmission characteristics, generally showing a sinus or cosines shape. As disclosed in EP-A-875743, the wavelength resolution can be significantly improved by combining two etalons with different phase dependencies on the wavelength. By providing a phase shift (in particular of n/2) between the two curves, the problem of the ambiguous turn points in the sinus-or cosines-like shapes can be avoided and a tangent relationship on the wavelength can be achieved. Instead of using two separated etalon elements, one birefringent element can be provided in combination with corresponding polarizing elements. Since the resulting transmission-over-wavelength characteristic is not unambiguous due to the periodicity of the curves, main applications are fine-tuning of variable laser sources, whereby the wavelength location is already roughly known and can therefor be assigned to a corresponding period of the curve.
DE-A-4114407 discloses an arrangement for determining the air path wavelength of a light source. Using an already known starting value for the air path wavelength, a current wavelength value is determined based on thermodynamic parameters of the ambient air.
U.S. Pat. No. 4,864,578 (Proffitt) discloses a scannable laser with integral wavemeter. The wavemeter comprises a fine wavelength read-out based on an ambiguous interferometric wavelength determination, and a course wavelength read-out making use of optically active quartz crystals. The amount of polarization rotation is measured as a sample light beam passes along its axis, and this is correlated to the wavelength of the light beam. Both wavelength read-outs are then utilized to determine the wavelength of the incoming light beam.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved wavemeter. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.
A wavemeter according to the invention for determining the wavelength of an incoming optical beam comprises a coarse-measuring unit and a fine-measuring unit. The coarse-measuring unit allows for unambiguously determining the wavelength of the incoming optical beam over a first wavelength range. The fine-measuring unit provides an ambiguous wavelength determination for the incoming optical beam, however with a higher accuracy than the accuracy of the coarse-measuring unit. Although the wavelength determination of the fine-measuring unit is ambiguous within the first wavelength range (e.g. since it provides a periodic wavelength dependency), it is provided to be unambiguous in each of a plurality of unambiguous wavelength ranges, whereby each of the plurality of unambiguous wavelength ranges is smaller than the first wavelength range.
An evaluation unit uses the result of the coarse-measuring unit for identifying one of the plurality of unambiguous wavelength ranges as a second wavelength range that covers the wavelength of the incoming optical beam. The result of the fine-measuring unit is then applied by the evaluation unit for determining the wavelength of the incoming optical beam within that identified second wavelength range.
In other words, the result of the coarse-measuring unit represents a first wavelength value with a first accuracy. However, since the measured value of the fine-measuring unit is generally ambiguous (i.e. plural wavelength values correspond to the measured value of the fine-measuring unit) but only unambiguous within each of the plurality of unambiguous wavelength ranges (i.e. within each of the plurality of unambiguous wavelength ranges, only one wavelength value corresponds to the measured value of the fine-measuring unit), the evaluation unit selects—as the second wavelength range—the one of the plurality of unambiguous wavelength ranges (of the fine-measuring unit) that covers the first wavelength value as provided by the coarse-measuring unit. With the information of the second wavelength range, the evaluation unit can then determine a second wavelength value as the one wavelength value corresponding to the measured value of the fine-measuring unit within the second wavelength range. The second wavelength value then represents the measuring result of the wavemeter for the wavelength of the incoming optical beam.
Thus, the wavemeter according to the invention allows determining the wavelength of the incoming optical beam within the first wavelength range and with the accuracy of the fine-measuring unit.
In case that the fine-measuring unit provides a periodic wavelength dependency, it is clear that in order to provide reasonable measuring results for the wavelength of the inco
Mueller Emmerich
Ruck Clemens
Agilent Technologie,s Inc.
Lavarias Arnel C.
Nguyen Thong
LandOfFree
High-accuracy wavemeter does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High-accuracy wavemeter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High-accuracy wavemeter will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3247167