Optical: systems and elements – Light control by opaque element or medium movable in or... – Electro-mechanical
Reexamination Certificate
2001-10-02
2004-03-02
Dunn, Drew (Department: 2872)
Optical: systems and elements
Light control by opaque element or medium movable in or...
Electro-mechanical
C359S234000, C359S236000, C359S578000, C359S889000, C359S891000
Reexamination Certificate
active
06700690
ABSTRACT:
FIELD OF THE INVENTION
This invention relates, generally, to optical filters and more particularly to a variable bandwidth tunable optical filter that is comprised of two transparent optical substrates, upon each of which is deposited a linearly variable multilayer interference filter coating such that the variable bandpass tunable filter can be adjusted to a specific center wavelength by moving the filter pair together linearly through the incident beam and the variable bandwidth of the tunable filter can be adjusted by introducing a relative linear offset between the linearly variable filter pair.
BACKGROUND OF THE INVENTION
Various types of optical filters have been developed in the past, most often for optical devices known generally as spectrometers that are used for measuring and analyzing the spectral or color content of electromagnetic radiation in the frequency range or spectrum of optical wavelengths. These include from ultraviolet, through visible, to near-infrared wavelengths, which include the portion of the electromagnetic spectrum producing photoelectric effects, referred to herein as “light.” Also, various kinds of opto-electronic devices employing optical filters are used for both imaging applications, as by inspecting the spectral reflectance characteristics of a two-dimensional object, and for non-imaging applications.
Spectrometric measurements of light are performed in basically two ways, dispersion-based techniques and filter-based techniques. In the dispersion-based approach, a radiation dispersion device such as a prism or diffraction grating is used to separate the incident polychromatic light into its spectral contents. The spectrally separated light is then projected onto a photodetector to measure the relative intensity in each spectral range. While dispersion-based devices can be effectively used in some applications, they have the disadvantage of being easily knocked out of alignment during use, and thus not suitable for more rigorous applications in the field.
In the filter-based approach to spectral measurement, various types of optical filters are used in conjunction with photodetectors to measure and analyze light. For example, in one approach, a single band-pass filter is placed over a detector to measure a single spectral band of the incident light. In another variation of the filter-based technique, a filter wheel on which several filters are mounted is used in conjunction with a single photodetector or several photodetectors. Alternatively, the discrete filters in the filter wheel can be replaced with a continuous circular variable filter (CVF) which is placed over a detector. Further, the CVF may be placed over several detectors to provide simultaneous spectra in a limited number of bands. These filter-based techniques are limited for practical reasons to use in low resolution spectral measurements of a few bands of light and to non-contiguous bands only.
Other spectrometer devices have been developed that utilize linear variable filters in an attempt to enhance light measuring capabilities. For example, U.S. Pat. No. 5,166,755 to Gat discloses a spectrometer apparatus including a spectrum resolving sensor containing an opto-electronic monolithic array of photosensitive elements which form a photodetector, and a continuous variable optical filter such as a linear variable filter (LVF) that is placed in an overlaying relationship with the photodetector. The LVF and photodetector are mounted in a single housing which serves to support at least the filter and the array in a unitary sensor assembly. The LVF is formed by depositing optical coatings directly onto the photodetector array, or a preformed LVF may be positioned in contact with or slightly above the array.
In U.S. Pat. No. 5,218,473 to Seddon et al., a leakage-corrected linear variable filter is disclosed. This patent describes a conventional linear variable filter system as including an LVF positioned in a spaced apart relationship with a linear detector such as a charge coupled device array. The LVF is paired with a linear detector having comparable dimensions in order to form a detector capable of receiving and discriminating a number of wavelengths of radiation simultaneously.
The use of linear variable filters in spectrometer devices has been limited because of fundamental packaging problems. Depositing an optical coating on the surface of a detector to form an LVF is problematic because of the delicate surface and wiring of the detector array. The placement of a preformed LVF on the surface of the detector array requires the removal of a cover glass that protects the delicate surface of the detector array. Such placement of an LVF during manufacture can damage the surface of the detector array.
Further, the LVF is prone to have diffuse leaks that downgrade its spectral performance and which are unavoidable when the LVF is placed in contact with the detector array surface. The LVF filter works properly only within a limited cone angle of light (numeric aperture). Light outside this limited angle may contain diffuse leaks. The detector array is capable of receiving light within the full hemisphere and will detect the diffuse leaks when placed in contact with or very close to the LVF. In addition, if the LVF is spaced apart from the surface of the detector array, then the LVF will not perform properly since light emitted from one position of the LVF may reach more than one element of the detector array, thereby limiting the spectra resolving power of the LVF.
Accordingly, there is a need for an optical filter that overcomes or avoids the above problems and limitations.
In order to be useful in most applications, an optical filter that is designed to transmit only a given narrow band of wavelengths must sufficiently reject all other wavelengths for which source energy and detector sensitivity both exist. That is, light of all other wavelengths outside this narrow spectral band and within a range set by the limits of the source and the detector must be blocked in order for the filter to operate with the given source and detector. In the case of induced transmittance or Fabry-Perot-type metal dielectric filters, the rejection occurs naturally and such filters can be designed with wide-band blocking without complicating the design of the filter.
All-dielectric bandpass filters can be much more environmentally stable than metal dielectric filters and are preferred in many applications. Their disadvantage is that the bandpass provides natural blocking for only a narrow band of wavelengths above and below the pass band. Additional blocking requires additional stacks of layers, each stack blocking a specific range of wavelengths. Several quarterwave optical thickness (QWOT) stacks generally provide this additional blocking. A quarterwave stack is characterized by its center wavelength in that the stack blocks light by reflection over a wavelength range around its center wavelength. The width of the wavelength range of the stack depends on the stack configuration and the ratio of the indices of refraction of the two coated materials used in the stack. The depth of blocking is controlled by the number of layers in the stack.
It is not uncommon for the all-dielectric filters to have upwards of 200 total layers. Typically, only a relatively few such layers can be formed on a single surface. Thus, these layers must be distributed over several surfaces, for example, over two to four surfaces on one or two substrates, to minimize and balance coating stresses. Otherwise, the use of two substrates with a small air space is acceptable, and in a number of applications it is perfectly acceptable to coat two surfaces of the same substrate.
An exception is found in linear-variable narrow-band filters. These are filters in which the thickness of the coating layers, and hence the wavelength of the pass band, is varied linearly, or at least monotonically, along a dimension of the filter. If such a filter is used in conjunction with an extended detector such as a linear array, it is desirable to mount th
Buchsbaum Philip E.
Lane James D.
Cook, Esq. Dennis L.
Dunn Drew
Fowler White Boggs Banker P.A.
Ocean Optics, Inc.
Pritchett Joshua
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