Abstract:
An invention is described, which uses terahertz radiation for chemical sensing and gas analysis. Relatively narrow-band THz radiation is generated by impinging an optical pulse train from a short-pulse laser source on a THz emitter. Coherent detection of the resulting THz radiation is accomplished by using a similar optical pulse train to activate the THz sensor. The invention is shown to detect and indicate various concentrations of water vapor in air. Optimal phase biasing conditions give maximum sensitivity to variations in concentration of the species under investigation.

Description:
This is a continuation of application Ser. No. 09/692,322 filed Oct. 20, 2000 now abandoned; the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Systems are known in which ultrashort laser pulses are used to generate and coherently detect terahertz (THz) radiation. Most types of THz systems use single laser pulses to generate broad-band THz radiation. Time-domain, terahertz spectroscopy has been shown to provide a useful analytical tool for measuring properties of molecular vapors, both pure and in mixture with other gases, such as air (see M. van Exter, et al., Optics Letters, Vol 14, p. 1128 (1989)). The use of THz radiation reveals certain features which are not afforded by the use of laser radiation lying in the “usual” range of 200 nm to 10 microns. 
     X. -C. Zhang et al., teach a THz sensing system which employs electro-optic crystals, such as ZnTe, to serve as THz receivers, these having the advantage of higher detection bandwidth (see Q. Wu and(X. -C. Zhang, Appl. Phys. Lett, Vol 67, p. 2523 (1995)). The generation of narrow-band THz radiation can give significant advantages over broadband THz generation in certain spectroscopic applications. Continuous wave narrow-band THz radiation can be generated and detected by photomixing two CW lasers in a THz transceiver, such as a photoconductive antenna, as demonstrated by Verghese et al. (see S. Verghese, et al., Appl. Phys. Lett, Vol. 73, p. 3824 (1998)). Moderately narrow-band THz bursts can be generated by exciting a THz emitter with a train of optical pulses spaced to the desired THz frequency as taught by Siders et al. (see C. W. Siders, et al., Opt. Lett., Vol 24, p. 241 (1999)), and Weling et al. who excites a semiconductor surface (see A. S. Weling, et al., Appl. Phys. Lett, Vol. 64, p. 137 (1994)). Alternatively, Norris demonstrates a method in which a single laser pulse can be used to generate narrow-band THz radiation directly by optical rectification in a periodically poled nonlinear crystal such as PPLN (periodically poled lithium niobate), by exploiting the group-velocity walkoff between the optical pulse and the THz radiation in the crystal (see T. -S. Lee, T. Meade, V. Perlin, H. Winful, T. B. Norris, A. Galvanauskas, “Generation of narrow-band terahertz radiation via optical rectification of femtosecond pulses in periodically poled lithium niobate,” Appl. Phys. Lett., Vol. 76, p. 2505 (2000)). In another method which does not employ ultrafast lasers, T. Heinz et al. teach a THz generation/detection system employing two detuned CW lasers to generate THz radiation at the beat frequency between the two lasers (see A. Nahatha, James T. Yardley, Tony, F. Heinz, “Free-space electro-optic detection of continuous-wave terahertz radiation,” Appl. Phys. Lett., Vol. 75, p. 2524 (1999)). The same two lasers activate the THz receiver, providing narrow-band coherent detection. This system provides very narrow-band THz radiation with ˜MHz linewidths, limited only by the absolute frequency stability of the two CW lasers, and have potential for linewidths less than 1 kHz. 
     CW heterodyne methods provide superior frequency resolution in the THz measurement system, however suffer the drawbacks of very low efficiency, as well as fringe ambiguity. While these systems can give sub-wavelength resolution, the larger scale TOF (time-of-flight) information is lost. The use of ultrashort pulses to generate and detect THz radiation provides useful TOF information. Furthermore, the use of ultrashort pulses can ultimately result in greater efficiency of THz emission, especially in cases such as that demonstrated by Norris, where the optical pulse is re-used many times in the generation process. As extensions of the THz sensing technology, various systems have been devised which combine THz generation/detection with imaging to give imaging in the THz frequency range. B. Hu et al. teach a THz imaging system which uses single, broad-band THz pulses repetitively while the sample under test is raster scanned through the THz beam (see B. B. Hu and M. C. Nuss, Appl. Phys. Lett, Vol. 20, p. 1716 (1995); see U.S. Pat. No. 5,623,145; and see U.S. Pat. No. 5,710,430). Because this system relies on THz waveform measurement, time delay scanning was required in addition to the raster scanning, both techniques, in turn, requiring the use of a large number of laser shots to complete the measurement even if no signal averaging was used. This type of system generally requires at least several minutes to complete an imaging measurement. Additionally, if spectral information about the image is desired, then complex signal analysis such as Fourier or wavelet transforms are required. A system for performing THz imaging with a single laser shot was first demonstrated by Q. Wu et al. This system works by using an EO field sensor crystal to impart THz image information on an optical beam (an ultrashort laser pulse), and then imaging the optical beam with an optical imaging device such as a CCD camera (see Q. Wu, T. D. Hewitt, and X. -C. Zhang, Appl. Phys. Lett., Vol. 69, 1026 (1996)). 
     SUMMARY OF THE INVENTION 
     The current invention combines advantages of tunable narrow-band THz generation and coherent detection, with the unique properties of ultrashort pulses, those being single-shot capability, TOF information, high pulse intensity, and greater efficiency of THz generation. The current invention provides the additional advantage of using two THz frequencies to give a differential measurement providing greatly enhanced sensitivity and immunity to laser fluctuations. Additionally, the narrowband THz emission can be adapted to single-shot THz imaging systems, which commonly use only a single, broadband THz pulse. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a simplified block diagram of an illustrative narrow-band THz chemical sensing system in accordance with the principles of the present invention. 
     FIG. 2 shows comparisons between THz waveforms and the optical gating waveforms, illustrating the relative phase between them. 
     FIG.  3 ( a ) shows a transmitted THz waveform after propagation through a sample space filled with dry air (upper trace) and air with 50% humidity (lower trace), illustrating the effect of free induction decay when the THz waveform is tuned in resonance with the water vapor absorption. 
     FIG.  3 ( b ) shows a cross-correlation between the transmitted THz waveform and the optical gating pulse, illustrating the effects of phase, and the asymmetry due to free induction decay. 
     FIG. 4 shows the output signal of the THz sensing system when the sample space contains air of varying levels of relative humidity. 
     FIG. 5 shows an illustrative THz DIA (differential absorption) system in which THz pulse bursts of two different frequencies are generated, are propagated through the sample-region, and are then coherently detected. 
     FIG. 6 shows an illustrative example of a single-shot THz imaging system which employs narrowband THz radiation and matched filter detection. 
     FIG. 7 shows a schematic design of a tunable quasi-phase-matched nonlinear optical device for generation of frequency-converted pulse sequences from a single input pulse. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates one embodiment of the invention. A single short pulse of optical radiation obtained from an ultrashort pulse laser (USPL)  1  is converted, by a pulse shaper  2 , into a sequence of optical pulses  3 , separated by time intervals which are substantially equal. The repetition frequency of the pulse train is chosen to correspond to the desired Terahertz frequency. In the preferred embodiment, the pulse shaper is a quasi-phase matched nonlinear optical crystal—such as periodically poled lithium niobate (PPLN)—which is constructed with multiple poled zones, so as to produce a train of frequency doubled pulses from a single optical pulse, as taught by Galvanauskas, et al. (see G. Imeshev, A. Galvanauskas, D. Harter, M. A. Arbore, M. Proctor, and M. M. Fejer, “Engineerable femtosecond pulse shaping by second-harmonic generation with Fourier synthetic quasi-phase-matching gratings,” Opt. Lett., Vol. 23, p. 864 (1998)). In general, the pulse shaper need not frequency double the input. The pulse shaper could also be based on diffraction gratings and spatial light modulators, as taught by Weiner (see C. W. Siders, et al, Opt. Lett., Vol. 24, p. 241 (1999)). The optical pulse train is split into two paths by a beam splitter  4 ; one beam  20  impinging on a THz emitter  5 , and the other beam  30  being passed through a controllable optical delay  6  and subsequently impinging on a THz receiver  7 . The THz emitter and receiver are configured with intervening optical elements (not, shown), so that THz radiation from the emitter propagates through a free-space path  8 , and then is incident on the receiver  7  where it is detected. If desired, a gas cell or other sample holder  25  is placed in this free-space path so that the THz radiation passes through it. The receiver is activated and gated by the delayed optical pulse train  30 . If the sum of the optical path lengths of the input (pump) beam path  20  and the free space THz beam path  8  is substantially equal to the path length of the delayed gating beam  30 , then the transmitted THz waveform  27  from emitter  5  and the optical gating waveform  30  will substantially coincide in time upon arrival at the THz sensor  7 , giving the necessary temporal overlap to effect THz detection. The end-result of this measurement (which may be output to a data acquisition device  10 ) is a single value of signal level obtained from the THz detection system, which will depend upon the amplitude of the THz emitter beam  27 , and on the precise timing delay between the THz emitter waveform (as affected by sample  25 ) and the gating optical pulse train  30 . Because no time delay scanning is required, this measurement can be effected using a single laser shot, or with multiple laser shots if signal averaging is desired. 
     Because the transmitted THz waveform  27  and the optical gating waveform pulse train  30  have essentially the same frequency, this effectively provides a form of matched filtering during the detection process. From this nominally pathlength-matched condition, small adjustments can be made in the time delay of the gating pulse train  30 , such that the THz phase between the THz waveform  27  and gating pulse train  30  can be adjusted anywhere in the range of 0 to 360 degrees, as illustrated in FIG.  2 . In this way, the system can be phase-biased to provide maximum sensitivity to changes in either amplitude or phase of the THz waveform  27 . Amplitude sensitivity is maximized by setting the phase to either 0 or 180 degrees. Phase sensitivity is maximized by setting the phase to either 90 or 270 degrees. If the THz beam  27  encounters primarily a change in either index of refraction or path length in the intervening sample medium, the change being smaller than the nominal THz wavelength, then it is advantageous to bias the phase near 90 or 270 degrees. If the THz beam encounters a significant absorption in the sample medium, which would effect primarily a change in amplitude, then it is advantageous to bias the phase near 0 or 180 degrees. Under certain conditions (e.g., resonantly absorptive media) both the amplitude and phase of the THz waveform are changed by propagating through the sample medium, so that the optimal phase condition will deviate from the previously stated values of 0, 90, 180, or 270 degrees. 
     After propagating through a resonantly absorbing medium, the transmitted THz waveform will be followed by radiation due to free induction decay (FID), and the resulting distortion of the original pulse, as has been shown by Grischkowsky when using a single broadband THz pulse (see M. van Exter, et al., Optics Letters, Vol 14, p. 1128 (1989)). A similar effect also occurs when using a narrow-band THz burst, and is illustrated in FIG.  3 ( a ). The optimum phase bias for detection can by found in advance of the real measurement by performing calibration scans of known species at known concentrations, such a scan being shown in FIG.  3 ( b ). Under the optimal phase bias conditions, the resulting output signal level will depend on the concentration of the species in the THz beam paths, a higher concentration giving a higher or lower signal level, depending upon the relative phase. FIG. 4 shows a plot of signal level as a function of vapor concentration (relative humidity) for a THz detection system which was tuned to the water vapor resonance at 1.09 THz. 
     Furthermore, instead of sampling the difference in amplitude of the propagated beam, an amplitude of the free induction decay can be measured. 
     THz DIA (Differential Absorption) System 
     Another embodiment of the invention is shown in FIG. 5, where two nonlinear optical pulse shapers  52  and  53  (which convert optical radiation obtained from USPL  1  via a beam splitter  57 ) are used together simultaneously with a single THz emitter  55 , and a single THz detector  56 . The pulse shaper could be similar to those described by Weling et al. and by Siders et al. The first pulse shaper  52  is tuned so that the first THz waveform coincides with an absorption resonance of the species under study, while the second pulse shaper  53  is adjusted so that the second THz waveform does not coincide with this absorption resonance (the output beam paths of the first pulse shaper and the second pulse shaper are combined at  58 ). There is a substantial time delay (&gt;1 ns) between the two THz waveforms. Separate detection of the two THz frequencies is obtained by time-gating the data acquisition properly, requiring that the time delay between the THz waveforms be on the order of 1 ns or greater (according to state of the art acquisition systems). Using two THz frequencies simultaneously gives much greater immunity to laser noise, and therefore much greater sensitivity. This is similar in principle to the well known technique of differential absorption lidar (DIAL); however, according to the invention it is performed in the THz frequency region, which is not covered by lasers. 
     In yet another embodiment of the invention, two separate THz emitters, and two separate THz detectors could be used, one for each THz frequency. In this type of system, one would still have to insure that the two THz beams pass through the same sample region  59 ; however, the ns order delay between the THz pulse trains could be dispensed with. This system has the advantage that phase variations from thickness variations of the sample material would have smaller effects on the error in measurement. The non-absorbed signal can be used to correct the phase. 
     Narrow-band THz Imaging 
     The above-described systems use a single THz detector for the (or each) THz beam, which has the effect of spatially integrating over the profile of the THz beam. This effectively discards any spatial information imparted to the THz beam by local variations in the sample under test. However, this narrow-band THz technique can be combined with single-shot THz imaging, as illustrated in FIG.  6 . The invention employs an ultrashort pulse laser emitting a single optical pulse, which is passed through pulse shaper  2  to obtain the pulse train. This signal is split; one portion used to activate the emitter  65 , while the other portion passes through an electro-optic sensor crystal  68 , via, for example, a beam expander  67 . Both the THz emitter input pulse and the expanded optical pulse pass through the EO crystal  68  simultaneously (an imaging lens  71 , arranged between object  70  and EO crystal  68 , may be deployed along the path of the THz emitter pulse), whereby any spatial information from the THz beam is imparted to the expanded optical beam by modification of the polarization properties of the expanded optical beam across its spatial profile. The resultant optical beam is then sent through polarizing optics (not shown), and finally to an imaging device such as a CCD camera  69 , which then records the resulting imposed intensity variations. In the current state of the art, this measurement is done with a single broadband THz pulse, which gives limited spectral information. However, by using trains of ultrashort pulses for generation and detection, as described in this invention, it is possible to obtain single-shot THz imaging at a specific THz frequency, thus giving the capability of hyper-spectral imaging at particular THz wavelengths. This provides an enhancement in functionality over the single-shot, broadband THz imaging shown, for example, by Wu et al. (see Q. Wu and X. -C. Zhang, Appl. Phys. Lett, Vol 67, p. 2523 (1995)). Additionally, the spectral image information is obtained without the need for complex signal processing such as Fourier or wavelet transforms. 
     FIG. 7 shows a schematic design of a tunable quasi-phase-matched nonlinear optical device for generation of frequency-converted pulse sequences from a single input pulse. The tunability is designed into the structure by a lateral variation in the spacing between the poled regions of the device, and the tunability of the pulse repetition frequency is effected by laterally positioning the input pulse to propagate along one of the several poled regions. By changing the position of the frequency doubled sections it is also possible to achieve a variation of the phase delay. 
     For example, referring to FIG. 7, the region with “tilted” poled region affects the phase delay. Furthermore, the poled regions may be “tilted” the same amount such that the difference between all “tilted” poled regions is constant. 
     It is understood that the invention described here is not limited to the specific embodiments described above, but that many variations in the invention can be implemented, but which would still fall within the spirit of the invention.