Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
2000-09-25
2003-06-24
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
C356S331000
Reexamination Certificate
active
06583873
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to devices incorporating dispersive volume transmission gratings, and more particularly to Raman and/or fluorescence spectrometers that have continuous tuning of the excitation wavelength.
2. Discussion of Related Art
Holographic volume transmission gratings are used in a variety of devices and are proven to have high efficiency over a wide spectral range. (For example, see, Tedesco et al, U.S. Pat. No. 5,011,284, the entire contents of which are incorporated herein by reference.) Unlike the situation with widely used reflection surface-relief gratings, the directions of the incident and efficiently diffracted beams are symmetrical with respect to the periodicity of the grating with volume transmission gratings. Wavelength tuning in spectrometers with reflection gratings can be performed by rotating the grating in a Littrow-like configuration. In George et al. U.S. Pat. No. 4,752,130, the entire contents of which are incorporated herein by reference, spectrometers, monochrometers and other devices are described that use volume transmission gratings. However, the devices described in George et al require individually moving at least one grating and at least one mirror with complex devices to match the diffracted and/or incident beam directions, determined by Bragg's conditions. The resultant optical layouts described in George et al are too cumbersome to have significant practical applications. Instead, commercially available spectrometers (e.g., HOLOPROBE made by KAISER OPTICAL SYSTEMS, INC.) utilize “snap-in” gratings. Other conventional devices have stacks of gratings for changing the spectral range of coverage. Such devices are not practical for many applications, including scientific research applications, in which flexible changes of both the excitation wavelength and the spectral range are essential.
For example, flexible changes of both the excitation wavelength and the spectral range are important for spectrometers used for analyzing secondary radiation emitted by a sample under primary excitation by a laser or other source of radiation when it is necessary to distinguish between Raman and fluorescence signals. The Raman signal has essentially the same wavenumber shift with respect to the excitation frequencies, while the luminescence one preserves the positions of the bands on the wavelength scale. Thus, measurements with two or more different excitations permit one to sort out the Raman and fluorescence signals. Moreover, in many cases, the Raman intensities depend critically on the excitation wavelength (resonance Raman), and provide information about electronic and other properties of the sample. Similar information can be obtained from the fluorescence excitation spectra, by measuring the intensity of a particular band as a function of the excitation wavelength. Also, wavelength tuning in a wide spectral range is necessary when measuring the optical properties of substances to investigate their electronic properties.
Therefore, there is a need for improved wavelength tunable devices, such as improved wavelength tunable spectrometers and spectrographs. The conventional devices used for measurements of emission, absorption or reflection spectra in a wide spectral range are surface relief single-grating spectrographs with a CCD array detector. Although adequate for some applications, this kind of conventional device is large and bulky when high spectral resolution is required, e.g., 0.1 nm or higher, because of the necessity to increase the focal length of the spectrograph, and also normally has a decreased throughput resulting from such physical limitations.
Some conventional spectrographs that use reflection gratings have a very important feature that allows the user to change the spectral coverage (and spectral resolution, concomitantly) rapidly without any realignment, thus preserving the calibration. This is realized by having two or more gratings on the same rotation turret, driven by a computer. However, current spectrographs with volume gratings use a different principle: they have either snap-in gratings, or they have a stack of gratings dispersing the spectra on different strips of the CCD shifted in a direction perpendicular to the spectral direction. In the first case, recalibration is needed after each change of the grating, while in the second case, throughput loss results. Neither method is flexible enough for many applications, including scientific measurements, because a change of the central wavelength would require a different grating.
Prism-based selecting elements are widely used in practice for laser intra-cavity wavelength selectors and laser monochrometers. Prism-based selecting elements have high transmission and can be wavelength-tuned, but because of very limited dispersion, prism filters are inadequate for many applications. For example, in the case of Raman spectroscopy, low-frequency laser plasma lines (below 100-200 cm
−1
) leak through the system and appear in the Raman spectra as spikes, which can mask the useful Raman signal. Surface-relief grating monochrometers serve adequately in some cases, but they have several problems. Aberrations originating from their off-axis spherical collimating mirror optics cause significant problems. Thus, using surface-relief gratings in the part of the system delivering the laser beam (laser filters and beam splitters) would deteriorate the quality of the laser spot on the sample and, consequently, the spatial resolution of the device. The throughput of grating monochrometers is polarization and wavelength-dependent and normally does not exceed 50%. Grating monochrometers are also bulky, and in the case of the double-subtractive monochrometer, which is used as the laser-rejecting stage for conventional Raman spectrometers, require accurate and time-consuming alignment. The same is true for the use of a grating monochrometer as a laser beam-splitter. Consequently, although the use of surface relief grating optics is adequate for some laser and nonlaser spectroscopic applications, they are not adequate for devices that require a rapid change of excitation and/or spectral range, they are bulky and complicated, and they are not sufficiently efficient for many applications.
In the case of Raman/fluorescence/excitation spectrometers, it is extremely important to filter out the excitation radiation so that none of the unwanted radiation (e.g., plasma lines of the ion laser tube) is present as spurious bands in the measured spectra. Another important feature of these devices is to provide a way to inject the excitation radiation into the optical system and then to reject the excitation radiation before the spectrograph stage (i.e., analysis of the spectrum). Different types of filters are currently in use for cleaning the laser radiation. Simple color glass filters are adequate for non-demanding applications such as for observing fluorescence spectra or high-frequency Raman spectra. In this case, a neutral beam splitter can be used for injecting the laser radiation and a color glass rejection filter for removing the laser radiation from the signal. Since color glass filters have a very broad edge between the transmission and absorption spectral range, the use of this configuration is very limited. Interference filters and beam splitters offer more of an abrupt edge for both edge and notch type dichroic filters.
There are two types of interference filters: multilayer thin dielectric films with different refractive indexes deposited between two highly reflective layers (i.e., the Fabry-Perot principle) and, alternatively, low/high refractive index periodic structures produced by laser interference in photosensitive materials (i.e., the “holographic” technique). In the latter case, because the refractive index can be changed smoothly with respect to the coordinates of the medium, much sharper changes in the transmission/reflection coefficient can be achieved with much better rejection close to the excitation wavelength. Di
Goncharov Alexander F.
Struzhkin Viktor V.
Smith Zandra V.
Stock, Jr. Gordon J
The Carnegie Institution of Washington
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