Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrophotometer
Reexamination Certificate
2001-12-19
2004-06-08
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrophotometer
C356S326000
Reexamination Certificate
active
06747736
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a terahertz wave spectrometer for performing spectroscopic measurements by using a terahertz wave. The terahertz wave is an electromagnetic wave having a frequency of around 1 THz (terahertz).
2. Description of Related Art
An electromagnetic-wave frequency range of around 1 THz (terahertz) is located on the boundary between an optical wave and a radio wave, and is called a terahertz range. More specifically, the terahertz range is defined as a frequency range that includes frequencies of about 100 GHz to 10 THz. The terahertz range can sometimes be defined as a wider frequency range that includes the range of about 100 GHz to 10 THz and further includes neighboring lower- and higher-frequency ranges. For example, the terahertz wave can effectively be used in spectroscopic processes for an infrared region and in imaging processes for the infrared region.
In comparison with other frequency ranges, developments of generators and detectors for this frequency range advance relatively slowly. Many technological and other problems have not yet been solved to apply the generators and detectors in practical uses. It is necessary to develop terahertz wave generators (optical sources) and terahertz wave detectors so that they will be small and easy to handle, in order to realize, in an industrial field, a spectrometer that detects and quantitatively measures the characteristics of a sample by using terahertz wave.
There are recently being developed terahertz wave generators (optical sources) and terahertz wave detectors that employ optical switching devices or electro-optic (EO) crystals. Though it is difficult to generate electromagnetic wave at the terahertz-range frequency by using an electric-circuit oscillator, it is possible to generate and detect electromagnetic wave at the terahertz-range frequency by modulating an electric current, or the like, using a pulse-shaped light.
In order to attain the spectroscopic measurement by using terahertz wave, one method has been proposed to detect terahertz wave and to measure directly the intensity of the temporal waveform of the detected terahertz wave. However, this method provides no limitation or no selection onto the respective frequency components of the terahertz wave. Accordingly, even if the sample presents some special characteristic with respect to a specific frequency range, it is impossible to perform measurement at the specific frequency range only. Thus, only a limited amount of information can be obtained from the spectroscopic process.
A method for performing measurements with frequency selection has been proposed by “Terahertz Electromagnetic Wave: Generation and Applications” by Sakai et al.
Laser Review
, Vol. 26, No.7, pp.515-521 (1998). During the measurement process with frequency selection, a detector performs a sampling measurement. A temporal waveform of the terahertz wave is determined based on the sampling-measurement result. Next, the obtained temporal waveform is subjected to fast Fourier transform (FFT), and the resultant amplitude spectrum is evaluated. In this device, the terahertz wave is scanned only once in a forward direction by a variable optical delay device, thereby sampling the terahertz wave.
Japanese Patent Unexamined Application Publication Nos. 8-320254 and 10-153547 disclose imaging systems that obtain spectroscopic information by using terahertz wave and by using an analog-to-digital converter and a digital signal processor (DSP). According to the methods disclosed by these publications, the DSP retrieves frequency-related information from time domain data by recognizing the characteristic shape of the terahertz wave.
According to the above-described conventional methods, the temporal waveform is first measured, and then frequency-related information, such as an amplitude spectrum, is determined by a computer thereafter. It is therefore impossible to attain a real-time measurement. Additionally, the entire device for attaining those methods has a complicated structure. Especially, the analog-to-digital converter and the DSP are employed to attain the methods disclosed by the publication Nos. 8-320254 and 10-153547. Accordingly, the device, such as a two-dimensional array for performing an imaging operation, becomes complicated and expensive,
SUMMARY OF THE INVENTION
It is an objective of the present invention to solve the above-described problems, and to provide a terahertz wave spectrometer, which can perform spectroscopic measurement in real time and whose device structure is simplified.
In order to overcome the above-described problem, the present invention provides a terahertz wave spectrometer for performing spectroscopic measurement by using terahertz wave, comprising: a predetermined excitation light optical system guiding an excitation light; a terahertz wave generator generating terahertz wave by using the excitation light guided by the predetermined excitation light optical system; a terahertz wave optical system guiding the terahertz wave generated by the terahertz wave generator to a sample for spectroscopic measurement, and further guiding the terahertz wave which has been affected by the sample; a predetermined probe light optical system guiding a probe light that is in synchronization with the excitation light; a terahertz wave detector detecting, using the probe light guided by the predetermined probe light optical system, the terahertz wave that is affected by the sample and that is guided by the terahertz wave optical system, and outputting a detection signal; optical delay vibrating means provided in either one of the excitation light optical system and the probe light optical system, the optical delay vibrating means vibrating, at a predetermined vibration frequency, the length of the optical path of the corresponding one of the excitation light and the probe light, thereby periodically vibrating the irradiation timing of the corresponding one of the excitation light and the probe light onto a corresponding one of the terahertz wave generator and the terahertz wave detector; and spectroscopic processing means performing spectroscopic measurement on the sample based on the detection signal obtained by the terahertz wave detector, the spectroscopic processing means including frequency analyzing means performing frequency analysis on the detection signal that periodically changes in accordance with the vibration frequency, the frequency analyzing means performing the frequency analysis of the detection signal by performing a frequency domain measurement, the frequency-analysis result obtained by the frequency analyzing means indicating frequency-analysis information on the terahertz wave that has been affected by the sample, thereby indicating the spectroscopic information of the sample.
In the terahertz wave spectrometer having the above-described structure, the irradiation timing of the probe light onto the terahertz wave detector is synchronized with respect to the irradiation timing of the excitation light onto the terahertz wave generator, while vibrating or oscillating the difference between the probe light irradiation timing and the excitation light irradiation timing. More specifically, the terahertz wave spectrometer is set up so that the terahertz wave emitted from the terahertz wave generator is transmitted through the predetermined terahertz wave optical system, is affected by the sample upon passing through or reflecting off the sample, for example, and then falls incident on the terahertz wave detector, which in turn detects the terahertz wave by using the probe light. The terahertz wave spectrometer is set up to vibrate or oscillate the irradiation timing of the terahertz wave on the terahertz wave detector and the detection timing of the terahertz wave by the probe light at the terahertz wave detector.
In the case where the optical delay vibrating means is provided in the probe light optical system, the optical delay vibrating means preferably includes a portion constructed to
Font Frank G.
Hamamatsu Photonics K.K.
Lauchman Layla
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