Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
2001-10-09
2004-01-27
Evans, F. L (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
C356S328000, C356S333000, C356S334000
Reexamination Certificate
active
06683686
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method and apparatus for performing temporally resolved wavelength measurements and power measurements, of particular application in an optical spectrum analyzer in which measurements performed in the time domain are used to calculate the wavelength and power spectrum of a laser or other light source.
BACKGROUND ART
In applications such as optical telecommunications there is a need for small, portable, rugged and low cost devices that can accurately measure the wavelength and power spectrum of a laser or other light source with improved resolution, increased measurement speed and reduced cost. Parameters of importance in optical spectrum analyzers include wavelength range, wavelength resolution, wavelength accuracy, optical sensitivity, power calibration accuracy and dynamic range.
Existing devices, such as commercially available optical spectrum analyzers, employ spatially resolved methods in which measurements of spatial parameters are used to calculate wavelength spectra of the input light. Generally, input light is firstly collimated and then spectrally dispersed by a diffraction grating. The resulting diffracted light is then typically reflected from several mirrors or diffraction gratings before exiting a slit aperture and is detected by a photodetector. The wavelength range is scanned by adjusting the angle of the diffraction gratings, the position of the photodetector, or both. The spatially resolved data is translated to wavelength data by calibrating against a known reference source. The result is a wavelength spectrum relating the relative intensity of the light source at a measured position or angle to the wavelength of the light source over a given range of values. A feature common to existing systems, therefore, is the performance of measurements in the spatial domain and the collection of data while varying a parameter such as detector position or grating angle. The relative accuracy and resolution of this technique is dependent on the relative positioning accuracy and translational stability of both moving and fixed mechanical components.
U.S. Pat. Nos. 5,233,405 and 5,359,409 disclose similar double-pass scanning monochromator designs for use in an optical spectrum analyzer device, in which an input light beam is spatially dispersed by a diffraction grating and passed through a slit so that a portion of the dispersed light beam can be selected. The monochromator based optical spectrum analyzer includes a motor for rotating the diffraction grating and a shaft angle encoder for sensing grating position. The light that passes through the slit is recombined to produce an output light beam. The input light beam is incident on the diffraction grating during first and second passes. A polarization rotation device rotates the polarization components of the light beam by 90° between the first and second passes so that the output of the monochromator is independent of the polarization of the input light beam. An output optical fiber is translated by a micropositioning assembly in a plane perpendicular to the output light beam during rotation of the diffraction grating to automatically track the output light beam and to provide optical chopping.
U.S. Pat. No. 5,497,231 discloses a monochromator design utilizing a beam-diffracting scanning mirror on a oscillated spring. The spring acts as an electromechanical self-energized oscillation circuit; a sensor detects the deflection of the spring, its output used as a feedback signal for maintaining the spring's oscillation.
U.S. Pat. No. 5,886,785 discloses an optical spectrum analyzer for an incident light beam and a process for analyzing the corresponding spectrum. The spectrum analyzer comprises addressing means, a diffraction grating, a reflecting dihedron, a device for adjusting the rotation of the reflecting dihedron and reception means. A polarization separator divides the incident beam into first and second parallel secondary beams of linearly polarized light along the directions parallel to and perpendicular to the grooves in the grating respectively, and a &lgr;/2 plate placed on the path of the first secondary beam applies a perpendicular polarization direction to this beam. The grating diffracts the secondary beams a first time, the reflecting dihedron exchanges their directions, the grating diffracts them a second time, the &lgr;/2 plate applies a 90° rotation to the polarization state of the second secondary beam and the separator recombines the secondary beams into a single main beam returned to reception means.
U.S. Pat. No. 6,097,487 discloses a device for measuring wavelength, including an interrogation broadband light source and a tunable optical filter. A first portion of the light transmitted through the filter and reflected from, or transmitted through, a fiber Bragg grating of known Bragg wavelength to provide an absolute wavelength reference, and directed to a first detector. A second portion of the light is transmitted through the filter and transmitted through, or reflected from a Fabry-Perot filter with fixed and known free spectral range to create a comb spectrum sampling the interrogation source spectrum to provide an accurate frequency/wavelength scale.
However, there are physical limits to parameters such as grating spacing and slit aperture size. Improvements in resolution and wavelength range require either increased optical path lengths or additional scanning elements (such as secondary diffraction gratings). A greater number of moving parts increases the complexity, instability and cost of the apparatus. Design instability and susceptibility to shock can produce inaccurate measurements in environments that require portability and ruggedness. Consequently improvements in wavelength resolution, scanning speed and scanning range are generally achieved at the expense of increased cost or size, and reduced portability.
Existing optical spectrum analyzers relate the wavelength of light to data measured in the spatial domain by varying and monitoring a parameter such as detector position or grating angle. The relative accuracy and resolution of this technique depends on the relative positioning accuracy and translational stability of moving and fixed mechanical components. The construction of accurate and repeatable mechanical translation and oscillation assemblies leads therefore to a high cost of manufacture.
U.S. Pat. No. 4,732,476 discloses a rapid scan spectrophotometer device that measures the spectral transmission of sample materials that are illuminated by a broadband light source. The spectrophotometer device is a different type of apparatus from an optical spectrum analyzer and has different functions and applications. The disclosed spectrophotometer—in order to measure the relative wavelength parameter—relates that parameter to the temporal difference between detected signals of different wavelengths as they are scanned past a photodetector at a constant speed. Hence, the relative transmission spectra is calculated using, in part, a temporally resolved measurement technique. However, although this spectrophotometer design has advantages in device cost and complexity, its calibration accuracy depends on the incorporation of spatial measurements and the positioning stability of mechanical components.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a new temporally resolved method of measuring at least some properties of a light source.
The present invention provides, therefore, an apparatus for determining the wavelength of a sample source of light, said apparatus having:
a reference light source of known wavelength;
a collimator for collimating light from said sample source and from said reference source;
a dispersing means for receiving and spatially dispersing collimated light from said collimator according to wavelength;
focusing means for focusing dispersed light from said dispersing means; and
a photodetector located in the focal plane of said focusing means and having an aperture for spatially selectively a
Foster Peter Graham
Weigold Adam Mark
Evans F. L
Geisel Kara
Larson & Taylor PLC
Photonica PTY LTD
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