EMCORE Corporation : Patent Application Titled "Terahertz Frequency Domain Spectrometer with Heterodyne Downconversion"

http://www.4-traders.com/EMCORE-CORPORATION-10020198/news/EMCORE-Corporation--Patent-Application-Titled-Terahertz-Frequency-Domain-Spectrometer-with-Heterod-17904837/

By a News Reporter-Staff News Editor at Electronics Newsweekly -- According to news reporting originating from Washington, D.C., by VerticalNews journalists, a patent application by the inventors Logan, Ronald T. (Pasadena, CA); Demers, Joseph R. (North Hollywood, CA), filed on September 20, 2013, was made available online on January 30, 2014.

The assignee for this patent application is Emcore Corporation.

Reporters obtained the following quote from the background information supplied by the inventors: "The invention relates to microwave, millimeter wave and submillimeter wave spectroscopy systems and components and in particular to an apparatus and method for accurately adjusting the frequency of one or both of the optical beams used in a transceiver for terahertz spectroscopy.

"Terahertz devices and systems generally employ electromagnetic energy between 300 GHz and 3 terahertz (3 THz), or wavelengths from 100 to 1000 microns (0.1 to 1.0 millimeters), which is also referred to as the submillimeter or far-infrared region of the electromagnetic spectrum.

"One important application of terahertz systems is THz spectroscopy. Terahertz spectroscopy presents many new instrumentation and measurement applications since certain compounds and objects can be identified and characterized by a frequency-dependent absorption, dispersion, and/or reflection of terahertz signals which pass through or are reflected from the compound or object.

"The generation of terahertz radiation by photomixing is a method of generating quasi-optical signals using an optical-heterodyne converter or photomixer. Typical photomixer devices include low-temperature-grown (LTG) GaAs semiconductor devices, which have been used to generate coherent radiation at frequencies up to 5 THz. The spectroscopy system typically uses two single frequency tunable lasers, such as diode lasers, to generate two optical laser beams which are directed at the surface of the photomixer. By photoconductive mixing of the two beams in the semiconductor material, a terahertz difference frequency between the two optical laser frequencies is generated. In particular, a first laser generates radiation at a first frequency and a second laser generates radiation at a second frequency. The difference frequency, equal to the difference between the first and the second laser frequencies, is swept by the user from microwave through terahertz frequencies by changing the temperature of the lasers, which coarsely changes the frequency of one or both lasers. Other types of tuning mechanisms exist, such as distributed-Bragg-reflector diode lasers with multiple electrodes, grating-loaded external cavities, etc. A terahertz transmitter includes a first photomixer that is optically coupled to the first and the second light source. A first radiative element or antenna is electrically coupled to the first photomixer. In operation, the first antenna radiates a terahertz signal generated by the first photomixer at the difference frequency. A receiver includes a second antenna positioned to receive the signal from the target radiated by the first antenna. The second antenna generates a time varying voltage proportional to the terahertz return signal. A second photomixer is electrically coupled to the second antenna and is optically coupled to the first and the second light source. The second photomixer generates a homodyne downconverted current signal in response to the time varying voltage generated by the second antenna. The downconverted signal is a measurement of the absorption or reflection of the material at each terahertz frequency. This is useful, for example, when used in conjunction with computer processing to identify unknown samples by comparing measured results to a library of reference spectra. This apparatus may also be used to characterize the frequency response characteristics of passive or active components and devices such as waveguides, filters, amplifiers, mixers, diodes, and the like designed to work at terahertz frequencies."

In addition to obtaining background information on this patent application, VerticalNews editors also obtained the inventors' summary information for this patent application: "1. Objects of the Invention

"It is an object of the present invention to provide an improved frequency domain terahertz spectrometer using two continuously tunable semiconductor lasers with the relative frequency of the optical beam applied to respective source and detector photoconductive switches being electronically adjustable.

"It is another object of the present invention to provide a terahertz spectrometer for the identification of a target spectrum with high resolution and detection sensitivity of absorption bands of interest by producing CW radiation in one or more frequency bands, and 'fine tuning' the terahertz radiation in at least some of those bands to identify a spectral signature.

"It is also another object of the present invention to mitigate the interference effect in a frequency domain terahertz spectrometer with finely controllable frequency.

"It is an object of the present invention to provide a method for independently coarsely and finely adjusting the frequency difference between two source lasers forming a composite optical beam used in a frequency domain terahertz spectrometer.

"It is another object of the present invention to provide a method for adjusting the frequency of a laser in a terahertz spectrometer using photoconductive switches to provide more accurate frequency specificity and resolution by 'fine tuning' the terahertz radiation in a frequency band of interest using a reference oscillator.

"It is also another object of the present invention to provide a terahertz spectrometer with adjustable resolution of the order of Hz or 10's of Hz at specific frequency bands or absorption regions of interest.

"It is another object of the present invention to provide a method for adjusting the frequency of a laser in a terahertz spectrometer using photoconductive switches to provide more accurate frequency specificity and resolution by first 'coarsely tuning', and then subsequently 'fine tuning' the terahertz radiation in a frequency band of interest.

"It is another object of the present invention to provide a method for adjusting the frequency of a laser in a terahertz spectrometer using photoconductive switches to provide more accurate frequency specificity and resolution by 'fine tuning' the terahertz radiation in a frequency band of interest.

"It is still another object of the present invention to provide a self-contained, field portable terahertz spectrometer system in a highly compact configuration capable of identifying or imaging an object utilizing a laser with an electronically adjustable or controllable frequency.

"It is still another object of the present invention to provide a field portable terahertz spectrometer system with separately packaged source and detector heads which may be manually positioned by the operator.

"Some implementations may achieve fewer than all of the foregoing objects.

"2. Features of the Invention

"Briefly, and in general terms, the present disclosure provides an apparatus for analyzing, identifying or imaging a target, including a first housing including first and second lasers having tunable frequencies and coupled to first and second optical fibers respectively; a second housing including (i) a first photoconductive switch activated by an optical beam from the first optical fiber for producing electromagnetic radiation in a range of frequencies greater than 100 GHz, and (ii) a radiator for directing said radiation to a target; and a third housing including (i) a receiver for acquiring electromagnetic radiation in a range of frequencies greater than 100 GHz from the target, and (ii) a second photoconductive switch activated by an optical beam from the second optical fiber and coupled to the electromagnetic radiation from the receiver and functioning to generate an electrical signal representative of some characteristic of the target.

"In another aspect, the disclosure provides an apparatus for analyzing, identifying or imaging an object, including a source of CW signals in a range of frequencies greater than 100 GHz directed to said object; and a detector for acquiring spectral information reflected from or transmitted through said object and performing a heterodyne downconversion for generating an electrical signal representative of some characteristics of the object.

"In another aspect, the disclosure provides a method for analyzing, identifying or imaging an object, including generating CW signals in a range of frequencies lying above 100 GHz and directing them to said object; and acquiring spectral information reflected from or transmitted through said object and performing a heterodyne downconversion for generating an electrical signal representative of some characteristics of the object.

"In another aspect, the disclosure provides a method for analyzing, identifying or imaging a target by providing first and second lasers having first and second output beams respectively having different frequencies; frequency shifting the first output beam to produce a third beam; generating a CW radiative beam using a first photoconductive switch in the range of frequencies greater than 100 GHz from the first and third beams; causing the CW radiative beam to be substantially simultaneously focused on or through the target; combining the first beam and the second beam into a composite fourth beam; acquiring a spectral information signal from said target using a second photoconductive switch activated by said composite fourth beam; and generating an electrical signal representative of a characteristic of said target using said spectral information signal and said composite fourth beam.

"In another aspect, the disclosure provides a method comprising providing first and second lasers having tunable frequencies for producing a first optical beam and a second optical beam respectively with different frequencies; frequency shifting or modulating the first optical beam to produce a finely adjustable frequency shifted third optical beam; producing a composite fourth beam from the second and the third optical beams; producing a composite fifth beam from the first and the second optical beams; coupling the fourth optical beam to a first photoconductive switch for producing a CW radiative beam in a range of frequencies greater than 100 GHz; directing the CW radiative beam to be focused on or through a target; and detecting the radiative beam reflected from or transmitted through the target by a second photoconductive switch coupled to the composite fifth optical beam; and generating an electrical signal representative of some characteristic of the target.

"In another aspect, the disclosure provides a method for terahertz spectroscopy including sweeping a source of CW radiative beams over a range of frequencies greater than 100 GHz, including a first photoconductive switch activated by a first composite optical laser beam; directing the radiative beam to be focused on a target; and acquiring spectral information from the target by a second photoconductive switch coupled to a second composite optical beam; and finely adjusting the frequency difference between the first composite beam and the second composite optical beam by a frequency modulator in the path of one of the first and second optical beams used to generate the first composite optical beam, for generating additional electrical signals representative of some characteristic of the target in a selected frequency band.

"In another aspect, the disclosure provides a method for terahertz spectroscopy including sweeping a source of CW radiative beams in one or more predetermined frequency bands lying in a range of frequencies greater than 100 GHz; acquiring spectral information from the target; and processing the spectral information to determine the presence of a specific spectral signature to identify a compound of interest.

"In another aspect, the disclosure provides a method for identifying a compound in a target using terahertz spectroscopy by storing a spectral signature of the predetermined compound; sweeping a CW radiative beam over at least one frequency band in the range of frequencies greater than 100 GHz; directing the radiative beam to a target; acquiring spectral information from the target; and determining whether the spectral signature of the predetermined compound is present in the acquired spectral information by tuning the frequency of the CW radiative beam in frequency increments of less than 100 MHz in the frequency region of the spectral signature.

"Some implementations or embodiments may incorporate or implement fewer of the aspects or features noted in the foregoing summaries.

"Additional objects, advantages, and novel features of the present invention will become apparent to those skilled in the art from this disclosure, including the following detailed description as well as by practice of the invention. While the invention is described below with reference to preferred embodiments, it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional applications modifications and embodiments in other fields, which are within the scope of the invention as disclosed and claimed herein and with respect to which the invention could be of utility