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WO1999041619A1 - Mesure electro-optique ou magneto-optique d'un rayonnement electromagnetique par impulsion optique comprimee - Google Patents

Mesure electro-optique ou magneto-optique d'un rayonnement electromagnetique par impulsion optique comprimee Download PDF

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Publication number
WO1999041619A1
WO1999041619A1 PCT/US1999/002922 US9902922W WO9941619A1 WO 1999041619 A1 WO1999041619 A1 WO 1999041619A1 US 9902922 W US9902922 W US 9902922W WO 9941619 A1 WO9941619 A1 WO 9941619A1
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free
optical probe
probe signal
space
crystal
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Xi-Cheng Zhang
Zhiping Jiang
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Rensselaer Polytechnic Institute
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Rensselaer Polytechnic Institute
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Priority to US09/617,242 priority Critical patent/US6414473B1/en
Anticipated expiration legal-status Critical
Priority to US10/164,454 priority patent/US6573700B2/en
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R29/00—Arrangements for measuring or indicating electric quantities not covered by groups G01R19/00 - G01R27/00
    • G01R29/08—Measuring electromagnetic field characteristics
    • G01R29/0864—Measuring electromagnetic field characteristics characterised by constructional or functional features
    • G01R29/0878—Sensors; antennas; probes; detectors
    • G01R29/0885—Sensors; antennas; probes; detectors using optical probes, e.g. electro-optical, luminescent, glow discharge, or optical interferometers

Definitions

  • the present invention relates to characterizing free-space electromagnetic radiation using electro- optic or magneto-optic crystal sampling, and more particularly, to electro-optic or magneto-optic measurement of a spatial-temporal distribution of free- space pulsed radiation using a chirped optical pulse.
  • Electro-optic sampling is a powerful technique for the characterization of a repetitive electrical waveform, such as an electrical signal in an integrated circuit (see Kolner et al . , IEEE J. Quantum Electron., Q.E.-22, p. 69 (1986), and Valdmanis et al . , IEEE J. Quantum Electron. , Q.E.-22, p. 79 (1986)), or a terahertz beam in a free-space environment (see United States applications by Zhang et al . , entitled “Electro- Optical Sensing Apparatus and Method for Characterizing Free-Space Electro-Magnetic Radiation," Serial No.
  • Time-domain optical measurements such as the terahertz time-domain spectroscopy in pump/probe geometry of the above-incorporated United States Patent Applications, use a mechanical translation stage to vary the optical path between the pump and the probe pulses.
  • the intensity or polarization of the optical probe beam, which carries information generated by the pump beam, is repetitively recorded for each sequential time delay.
  • this data acquisition for the temporal scanning measurement is a serial acquisition; i.e., the signal is recorded during the probe/pulse sampling through a very small part of the terahertz waveform (roughly the pulse duration of the optical probe beam) .
  • the data acquisition rate in this single channel detection approach is limited to less than 100 Hz for a temporal scan on the order of tens of picoseconds.
  • this relatively low acquisition rate cannot meet the requirement for real- time measurements, such as time-domain terahertz
  • a sensor for characterizing free-space radiation includes one of an electro-optic crystal or a magneto-optic crystal positionable so that the free- space radiation passes therethrough.
  • Means are provided for generating a chirped optical probe signal and for co-propagating the chirped optical probe signal through the crystal with the free-space radiation such that a temporal waveform of the free-space radiation is encoded onto a frequency spectrum of the chirped optical probe signal.
  • the sensor also includes means for decoding a characteristic of the free-space radiation using the chirped optical probe beam with the temporal waveform of the free-space radiation encoded on its frequency spectrum.
  • an imaging system for imaging an object includes means for generating a free-space electromagnetic radiation pulse positionable to pass through the object to be imaged, and one of an electro-optic crystal or a magneto-optic crystal positioned so that the electromagnetic radiation pulse passes through the crystal after passing through the object.
  • the system further includes means for generating a chirped optical probe signal to impinge the crystal simultaneous with the electromagnetic radiation pulse passing therethrough so that a temporal waveform of the radiation is encoded onto a wavelength spectrum of the chirped optical probe signal.
  • the chirped optical probe signal modulated by the free-space radiation is then passed to decoding means for decoding a characteristic of the free-space electromagnetic radiation using the chirped optical probe signal with the temporal waveform of the radiation encoded thereon.
  • the system further includes means for determining a characteristic of the object using the characterization of the free-space electromagnetic radiation pulse after passing through the object.
  • a method for characterizing free-space radiation.
  • the method includes: providing one of an electro-optic crystal or a magneto-optic crystal positionable so that the free- space radiation passes therethrough; generating a chirped optical probe signal and co-propagating the chirped optical probe signal through the crystal with the free-space radiation so that a temporal waveform of the free-space radiation is encoded onto a wavelength spectrum of the chirped optical probe signal; and decoding a characteristic of the free-space radiation using the chirped optical probe signal with the temporal waveform of the free-space radiation encoded on its wavelength spectrum.
  • a measurement technique employing a chirped optical probe beam which allows characterization of a free-space electromagnetic pulse.
  • the measurement technique of this invention provides single-shot measurement ability and ultrafast measuring speed. With these advantages, the technique can be employed with a number of possible applications, including monitoring for transient emitter breakdown, measuring unsynchronized a microwave, spatial-temporal imaging of non-terahertz signals, monitoring various unsynchronized fast phenomenon, such as chemical reactions and explosions, and studying non-linear effects.
  • monitoring for transient emitter breakdown measuring unsynchronized a microwave
  • spatial-temporal imaging of non-terahertz signals monitoring various unsynchronized fast phenomenon, such as chemical reactions and explosions, and studying non-linear effects.
  • Fig. 1 is a schematic of one embodiment of electro-optic measurement of a free-space radiation pulse using a chirped optical probe signal in accordance with the principles of the present invention
  • Fig. 2 graphically depicts spectral distribution of a chirped optical probe pulse with and without a co- propagating terahertz field pulse in accordance with the present invention
  • Fig. 3 depicts a normalized differential spectral distribution ( ⁇ I/I) by adjusting the fixed delay line of Fig. 1 at a step of 1.3 ps;
  • Fig. 4 is a graph of a single-shot spectral waveform of a terahertz pulse measured by a chirped optical probe pulse in accordance with the present invention, wherein the temporal waveform of the terahertz pulse is reconstructed;
  • Fig. 5 is a diagram of one embodiment for spatio- temporal terahertz imaging in accordance with the principles of the present invention
  • Fig. 6 is a graph depicting one dimensional terahertz imaging of a dipole in accordance with the principles of the present invention
  • Fig. 7 is a graph depicting one dimensional
  • Fig. 8 is a graph depicting single-shot one dimensional terahertz imaging of a dipole field without signal averaging, wherein the y-axis corresponds to the spatial position across the dipole emitter;
  • Fig. 9 is a graph depicting one dimensional terahertz imaging of a quadrupole field, wherein the y- axis corresponds to the spatial position across the quadrupole emitter;
  • Fig. 10 is a diagram of dynamic subtraction in accordance with the present invention wherein the signal beam (S) and the reference beam (R) are sent to the spectrometer simultaneously;
  • Fig. 11 is an example of the results of dynamic subtraction in accordance with the present invention wherein (A) comprises images of the CCD spectral traces without a terahertz signal, and (C) images of the CCD spectral traces with a terahertz signal, while (B) depicts the spectral plot without the terahertz signal and (D) is the spectral plot with the terahertz signal;
  • Fig. 12 graphically depicts simulated output results for measurement of a bipolar input terahertz waveform in accordance with the present invention
  • Fig. 13 graphically depicts measured and calculated modulation depth versus optical bias r 0
  • Fig. 14 is a schematic of one embodiment of an electro-optic measurement device for broadband mid- infrared spectroscopy in accordance with the present invention.
  • Fig. 15 is a schematic of one embodiment of an electro-optic measurement device in accordance with the principles of the present invention for conversion of a terahertz image into an optical image by converting a two-dimensional field distribution in the sensor crystal into a two-dimensional optical intensity distribution which is recorded by a digital CCD camera;
  • Fig. 16 is a schematic of one embodiment of a free-space magneto-optic sampling device in accordance with the present invention wherein the propagation direction of the probe beam is parallel to the direction of the magnetic component of the terahertz wave ;
  • Fig. 17 is a schematic of one embodiment of an optical device in accordance with the principles of the present invention for chirping a laser pulse continuously by moving a translation stage, wherein the output direction and timing of the laser beam do not change during the optical pulse duration and the chirping rate adjustment;
  • Fig. 18 is a graph of chirped pulse duration
  • Fig. 20 is a schematic of one embodiment of compact optics employed in accordance with the principles of the present invention for dynamic subtraction used in chirped pulse measurement as _ described herein, wherein the angle separation of the Wallaston prism is about 3°.
  • electro-optic With the introduction of a chirped optical probe beam into an electro-optic or magneto-optic sampling apparatus as described in the above-incorporated applications, it is possible to perform free-space electromagnetic pulse measurement.
  • a temporal waveform of a co-propagating terahertz field is linearly encoded onto the frequency spectrum of the optical probe pulse, and then decoded by dispersing the probe beam from a grating to a detector array.
  • electro- optic is intended to encompass either electro-optic measurements or magneto-optic measurements as described in the above-incorporated applications using either an electro-optic crystal or a magneto-optic crystal, respectively.
  • a femtosecond laser beam is split into pump and probe beams 11 & 13, respectively.
  • the geometry is similar to the conventional free-space electro-optic sampling setup described in the above- incorporated applications, except for the use of a grating pair 20 for chirping and stretching the optical probe beam 13, and a grating-lens combination 30 & 32 with a detector array 34 for the measurement of the spectral distribution.
  • the pump beam 11, generated by an ultra short laser 10 is used to generate the terahertz beam 15 from an emitter 12.
  • the terahertz beam is focused onto, for example, electro-optic crystal 14 by a polylens 16.
  • the fixed delay-line 19 is only used for the positioning of the THz pulse, within the duration of the synchronized probe pulse, (acquisition window) and for temporal calibration. Note that although discussed herein with reference to a terahertz pulse, those skilled in the art will understand that the concepts presented are equally applicable to other electromagnetic radiation beams and that a terahertz pulse is only one example.
  • the probe beam is frequency chirped and temporally stretched by grating pair 20 by passing beam 13 through beam splitters 22a & 22b to grating 20 for reflection off mirror 21.
  • the linearly chirped pulse is
  • the chirped probe signal is returned from grating pair 20 to the reflective surface of beam splitter 22b after which the signal is passed through a first polarizer to generate a purely linearly polarized probe beam.
  • This linearly polarized beam is modulated inside the electro-optic crystal, and becomes slightly elliptical due to phase modulation.
  • a second polarizer P is used to convert the phase modulation into an intensity modulation.
  • This second polarizer has a polarization axis that is perpendicular to the polarization axis of the first polarizer.
  • the THz waveform is encoded onto the wavelength spectrum of the probe beam.
  • a spectrometer e.g., comprising grating 30 and lens 32 combination, and a detector array (LDA or CCD) 34 are used to measure the spectral distribution.
  • LDA or CCD detector array
  • the temporal THz signal can be extracted by measuring the difference between the spectral distributions of the probe pulse with and without THz pulse modulation applied via the electro- optic crystal 14.
  • the measured signal is proportional to the THz field under certain conditions.
  • the unchirped probe be is a diffraction limited Gaussian pulse with a central frequency ⁇ 0 and an envelope Gaussian function:
  • the spectral modulation is spatially separated on the CCD array.
  • the measured signal on a CCD pixel with optical frequency ⁇ l r is proportional to the convolution of the spectral function of the spectrometer and the square of the Fourier transform of the chirped pulse:
  • Equation (3) Equation (3) Equation (4) can be written as :
  • Equation (5) can be evaluated by using the method of stationary phase if ⁇ is sufficiently large (see M. Born and E. Wolf Principles of Optics, 6th ed. Pergamon, New York, p. 752 (1980)) .
  • Equation (5) gives a self-canceling oscillation, so as to allow the contribution of the integrand to be neglected everywhere except in the vicinity of certain critical points. At the critical point the derivation of the Equation (5) with respect to "t" is zero. In this case it gives:
  • N( ⁇ is proportional to the input THz field under certain approximations .
  • the laser is an amplified Ti: sapphire laser (Coherent Rega 9000) with an average power of 0.9 W and a pulse duration of 200 fs at 250 kHz.
  • the center wavelength of the Ti: sapphire laser is about 820 nm with a spectrum bandwidth of 7 nm.
  • the THz emitter is an 8-mm wide GaAs photoconductor with the bias voltage ranging from 2 kV to 5 kV.
  • the focal lens for the THz beam is a polythelene lens with 5 cm focal length. A 4 mm thick ⁇ 110> ZnTe crystal is used.
  • the optical probe pulse is frequency chirped and time stretched by a grating pair, and the time window can be easily changed by changing the grating distance. This distance is several centimeters corresponding to
  • the dispersion element is a spectrometer (Instrument SA, SPEX 500M) with spectral resolution of 0.05 nm, and dispersion of 1.6 mm/nm.
  • the detector array is a CCD camera (Princeton Instruments, Inc., CCD-1242E) . This CCD camera has 1152x1242 pixels and a full well capacity greater than 500,000 electrons, dynamic range 18 bits, and minimum exposure time 5 ms . The data would be transferred to a computer (not shown) for further processing.
  • Fig. 2 shows the spectral distributions of the chirped probe pulse with and without THz modulation and the differential spectrum distribution ( ⁇ I). This differential distribution reconstructs both the amplitude and phase of the temporal waveform of the THz pulse.
  • the differential spectrum ( ⁇ I) in Fig. 2 shifts horizontally by adjusting the fixed delay line. Moving the fixed delay line is equivalent to placing the terahertz field in a
  • Fig. 3 shows the normalized differential spectrum distribution ( ⁇ I/I) when adjusting the fixed time delay line at a step of 1.3 ps .
  • the offset of the spectrum is shifted for better display.
  • the noise at the edge pixels comes from the spectrum normalization with a small background. These waveforms shift linearly with the fixed time delay step.
  • the total spectral window (1024 pixels) is equivalent to 44 ps, corresponding to 43 fs/pixel.
  • FIG. 4 depicts single-shot measurement of a THz pulse, with a signal- to-noise ratio (SNR) better than 60:1.
  • SNR signal- to-noise ratio
  • the probe beam 99 is focused to a line onto the EO crystal 100 by cylindrical lens 101, the imaging of this line is formed at the entrance plane of the spectrometer 110, therefore one-dimensional spatial and one-dimensional temporal information of the THz field 102 is measured simultaneously.
  • Figs. 6 and 7 show the measured distribution images of THz fields (x position versus time) emitted from dipole and quadrupole emitters, respectively.
  • the measured spatial resolution in the imaging system is better than 1 mm, which is close to diffraction limited resolution in other unchirped THz techniques.
  • Fig. 8 is a plot of a single-shot image from a GaAs photoconductive dipole antenna. This plot contains original data without signal averaging and smoothing. The total time for wavelength division multiplexing and demultiplexing is a few picoseconds. The dipole length is 7 mm, and the bias voltage is 5kV. One-dimensional spatial distribution across the dipole and its temporal THz waveform are obtained
  • the size of the spatio-temporal image is 10 mm by 25 ps .
  • the background light per pixel on the CCD camera is ⁇ 200 counts, whereas that of the modulated probe pulse is ⁇ 50.
  • Typical oscillation features and the symmetric spatial distribution of the far-field pattern from a dipole photoconductive emitter are obtained.
  • Fig. 9 shows a spatio-temporal image of the THz field from a quadrupole antenna.
  • the size of the spatio-temporal image is 10 mm by 40 ps .
  • the quadrupole has three parallel electrodes separated by 3 mm. The center electrode is biased and the two adjacent electrodes are grounded.
  • the field pattern from two back-to-back dipoles shows opposite polarity depending on the spatial position (y axis) .
  • Temporal oscillation from each dipole can be resolved individually.
  • the layered structure in the y-axis direction is due to the optical inhomogeneity of the sensor crystal.
  • a defect point in the ZnTe crystal causes an offset in the field strength of the temporal waveform (E axis in the figure) .
  • a high-quality ZnTe crystal with good spatial homogeneity will provide better spatial resolution.
  • Fig. 10 shows the setup. Before the second polarizer 200, a beam splitter 201 is used to pick up part of the beam, this beam 202 is used as a real-time reference (R) and sent to the spectrometer 203 simultaneously with the signal beam (S) 204.
  • R real-time reference
  • S signal beam
  • Fig. 20 is an example of the input coupling of a spectrometer showing how to create two beams from one input beam with a lateral displacement at the entrance plane.
  • a Wallaston prism is an ideal component to split one beam into two beams.
  • the input beam is the polarized probe beam which is modulated by, for example, a terahertz electric field via Pockels effect inside the electro-optic crystal.
  • the Wallaston prism is preferably set so that the polarization of one output beam (e.g., PI) is perpendicular to that of the input beam, while the polarization of another beam (e.g., P2) is parallel to that of the input beam.
  • beam PI is the signal beam, and beam P2 can be taken as the reference beam.
  • the intensity of beam P2 will need to be decreased to the same level as beam PI.
  • an attenuator cannot be used becaus-e the two beams are not separated spatially. (They are separated at the entrance plane, but the separation is small and it is impractical to use an attenuator there.) Instead, a polarizer can be used. The polarization of this
  • Fig. 11 depicts single-shot experimental results.
  • the left panel is the image_s of the CCD spectral traces of the signal (S) and reference (R) beam, the right panel is the plots of these spectra.
  • S signal
  • R reference
  • the signal and the reference spectra have good overlap, indicating that the reference is good.
  • the THz pulse is on, its modulation on the signal spectrum is obviously visible in the CCD image picture.
  • the measured signal is proportional to the THz field.
  • the chirp rate is limited by the laser pulse bandwidth or the pulse duration. This limited chirp rate constrains the temporal resolution.
  • V2 ⁇ T the interval between the maximum and minimum.
  • the spectral function of spectrometer can be approximated by a Guassian function:
  • Equation (7) Equation (7)
  • Equation (12) the measured normalized differential intensity function N(t) in Equation (12) is similar to the bipolar THz field in Equation (10), except that the characteristic time increases by a factor of
  • the larger the chirp rate the better the temporal resolution.
  • the temporal resolution T mln is defined when the input pulse is so narrow that the broadening factor is equal to 2.
  • the temporal resolution is equal to the square root of the product of the original probe beam duration and the chirped pulse duration.
  • the physics can be understood in the following way: since the THz pulse within the duration of the synchronized probe pulse window (acquisition window) only modulates a portion of the probe pulse spectrum, the limited frequency bandwidth in the modulated spectrum cannot support the required temporal resolution. If the pulse duration of the chirped probe beam T c , is comparable to the duration of the THz waveform, then the temporal resolution will be
  • Tmin JT0T T Hz .
  • Comp c ared with the samp rling ⁇ ? > method by J. varying the optical path, the temporal resolution decreases by a factor of -/ ⁇ ⁇ Hz / ⁇ o ⁇ For example,
  • the estimated limit of the temporal resolution is 1 ps.
  • the simulated distortion of the THz waveform with several different chirp rates is shown in Fig. 12.
  • ⁇ s 0 and focus on the chirp rate dependence of temporal resolution.
  • the x axis is dimensionless time t as defined in Equation (13), and the y axis is the relative signal.
  • T c or T 0 must be reduced.
  • the smallest T 0 is determined by the measurable time window, which should be larger than the THz duration. Therefore, a shorter original probe pulse (or equivalently broader spectrum) is more desirable. In principle, a wider bandwidth can support a shorter pulse duration. A white-continuum probe pulse with a higher chirp rate should provide better temporal resolution.
  • the THz signal is extracted by subtracting the probe spectra with and without THz modulation.
  • a large modulation depth is essential, that is why two crossed polarizers (zero optical bias) are used instead of the balance detection geometry (Linear optical bias) as in
  • I 0 is the input light intensity
  • ⁇ the contribution by the scattering
  • F 0 the optical bias induced by the residue birefringence of the ZnTe crystal plus the intrinsic birefringence of the compensator
  • r the electric field induced
  • Equation (18) is slightly different than the common notation (see Amnon Yariv "Opto-Electronics” 4th ed., Oxford University Press, p. 328 (1991)) .
  • ⁇ we add ⁇ to include the scattering contribution, and the optical phase terms r 0 , r are twice their counterparts in the above-reference Yariv publication.
  • modulation depth we define the modulation depth as:
  • Equation (24) the group velocity mismatch (GVM) is not considered, this is a good approximation because for a ZnTe crystal the influence of GVM is not significant.
  • GVM group velocity mismatch
  • the modulation depth should be bigger than the laser fluctuation which is on the order of 1%.
  • -29- pulse measurement technique provides some unique features; including a single shot ability, and ultrafast measuring speed. With these advantages, the technique of the present invention can be used in the field where the conventional sampling techniques can not. Some possible applications follow.
  • a biased THz emitter If a biased THz emitter is working under high bias voltage, a strong laser pulse could lead to damage to the emitter. This is a single event, and conventional sampling techniques are obviously not suitable to identifying this breakdown.
  • the chirped pulse technique is an ideal tool, and spatial as well as temporal information can be obtained simultaneously.
  • a linear CCD detector For a single point measurement, a linear CCD detector can be used.
  • the frame rate of the linear CCD can go to several tens of kHz.
  • the obtainable number of waveforms is at most 100. Therefore, the chirped pulse technique can be used in fast changing phenomena, such as chemical reactions and explosions.
  • the electro-optic measurement using a chirped optical pulse can also be used to generate and detect broadband mid-infrared terahertz pulses. Further, real-time, two-dimensional terahertz wave imaging is possible. Finally, free-space magneto-optic sampling is also possible as initially discussed herein. Each of these aspects is discussed below.
  • Several zincblende crystals including GaAs, ZnTe, CdTe, InP, and GaP) with differing thickness were used as the emitters and sensors.
  • LiTa0 3 and BBO as emitter materials. From LiTa0 3 we obtained a bandwidth extending to 43 THz. Three hundred and fifty milliwatts of the laser power was focused on the emitter by a gold-coated off-axis parabolic mirror with a 5-cm effective focal length.
  • the broadband THz radiation generated from the emitter by optical rectification was collimated and then focused on the sensor by a pair of f/0.6 off-axis parabolic mirrors.
  • the laser probe beam collinearly travels with the THz beam profile. If the emitter crystal is transparent to the optical excitation beam, such as ZnTe or GaP, a silicon wafer is placed after the emitter crystal to block the optical beam and transmit only the THz beam.
  • the electro-optic modulation induced by the ultrafast Pockels effect can be detected using a pair of balanced photodiodes.
  • the temporal waveform of the mid-infrared transient is sampled.
  • THz imaging By illuminating an electro-optic crystal with a THz beam and an optical readout beam, then detecting the optical beam with a linear diode array or CCD camera, time-resolved 1-D or 2-D images, respectively, of pulsed far-infrared radiation can be achieved.
  • This system is capable of noninvasively imaging moving objects, turbulent flows, or explosions.
  • Fig. 15 schematically illustrates the experimental arrangement for free-space electro-optic THz imaging.
  • Silicon lenses are used to focus the THz radiation on a ⁇ 110> oriented ZnTe crystal.
  • An optical readout beam with a diameter larger than that of the THz beam probes the electric field distribution within the crystal via the Pockels effect.
  • the 2-D THz field distribution in the sensor crystal is converted into a 2-D optical intensity distribution after the readout beam passes through a crossed polarizer.
  • a linear diode array or a digital CCD camera then records the optical image.
  • Magnetic field detection methods vary widely. Hall effect magnetonomers and optical Kerr effect spectroscopies are quite common.
  • the most sensitive device though, is the superconducting quantum interference device, or SQUID, with a magnetic field resolution of 10 ⁇ 14 Tesla. This far surpasses the detection capabilities of any other magnetic sensing device.
  • SQUID technology is the 2 MHz detection bandwidth, and cost.
  • Photoconductive switches are the most efficient and cost effective method, thus far, for investigating a transient magnetic field.
  • the free-space magneto-optic sampling technique discussed here is an extension of the photoconductive switch and the well-established electro-optic sampling technique. This magnetic technique offers the potential coherent measurement of transient magnetic fields, with a potential bandwidth spanning into the terahertz (THz) frequency range.
  • THz terahertz
  • This experiment includes a regeneratively- amplified Ti : sapphire laser (Coherent RegA), producing 4 ⁇ J pulses at a 250 kHz repetition rate, 200 fs pulse duration, and 800 nm wavelength.
  • Fig. 16 illustrates the pump/probe arrangement. With the optical beam split by a 95/5 beam-splitter, the two beams are recombined at the magneto-optic (MO) sensor. The stronger, time-delayed beam serves as the excitation beam for the THz field generation from a biased GaAs emitter. The weaker probe beam explores the induced Faraday rotation in the sensor crystal, produced by the
  • the -34- collinear magnetic component of the THz radiation With the introduction of the magnetic component of the THz wave, the index of refraction in the magneto-optic sensor is modulated via the Faraday effect.
  • the measured temporal waveform is a time-dependent, intensity-modulated, index birefringence response by the MO sensor material.
  • a pair of balanced photodiodes records this waveform through a lock-in amplifier connected to a computer.
  • This free-space technique provides magnetic transient field detection extending to hundreds of gigahertz.
  • the potential to obtain THz bandwidth response by proper geometry considerations is expected. Larger magnetic fields than the current 10 "5 Tesla will improve the signal-to-noise ratio, while thinner sensors will provide the subpicosecond temporal response .
  • the probe beam is spectrally chirped and temporally stretched by a pair of gratings. It is highly desired that the chirping rate and the chirped pulse duration be adjustable, but the direction and timing of the laser pulse remain unchanged. We have designed an apparatus to satisfy such requirements. This device improves the performance of the above-described chirped pulse method by simplifying the optical alignment procedure and keeping the same timing during the measurement.
  • the direction of the output beam is fixed while adjusting the chirping rate
  • Fig. 17 depicts one example of the setup. To realize the above performances, it is required that:
  • G2 and Ml be on the same translation stage (dashed box) ; and the moving axis of the stage must be parallel with glg2, m ⁇ m2 ;
  • Mirror M2 is perpendicular to m ⁇ m2 .
  • g2g2' m ⁇ mV , therefore the beam path or the timing is unchanged.
  • the laser pulse After passing through the device, the laser pulse is stretched, and the pulse duration is roughly the measurable time window. It is important to design the device with a proper pulse duration. The following is an analysis of one practical device.
  • ⁇ is the central wavelength
  • ⁇ the spectral width
  • c is the speed of light in vacuum
  • d the groove density of the grating
  • y is the incident angle
  • b the slant distance between gratings.
  • Fig. 19 is the simulated chirped pulses in time domain with 2 slant separations of the gratings, assuming Gaussian temporal pulse shape.
  • the pulse duration is proportional to the slant distance b (see Equation 27).
  • -38- have 15 to 30 ps Full Width at Half Maximum (FWHM) .
  • the centers of the pulses are at the same time position as required by the design.

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Abstract

On décrit un procédé pour caractériser des impulsions d'énergie électromagnétique (15) au moyen d'un faisceau sonde optique comprimé. Un cristal électro-optique ou magnéto-optique (14) est placé de telle sorte que le rayonnement en espace libre et le signal de sonde optique comprimé se propagent ensemble, de préférence, dans une direction commune co-linéaire, par le biais du cristal dans lequel une forme d'onde temporelle du rayonnement en espace libre subit un codage linéaire sur un spectre de la longueur d'onde du signal de sonde optique comprimé. La forme d'onde temporelle du rayonnement en espace libre est ensuite recomposée au moyen, par exemple, d'une soustraction dynamique de la distribution spectrale du signal de sonde optique comprimé non modulé de la distribution spectrale du signal de sonde optique comprimé modulé par le rayonnement en espace libre.
PCT/US1999/002922 1996-05-31 1999-02-10 Mesure electro-optique ou magneto-optique d'un rayonnement electromagnetique par impulsion optique comprimee Ceased WO1999041619A1 (fr)

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Application Number Priority Date Filing Date Title
US09/617,242 US6414473B1 (en) 1996-05-31 2000-07-14 Electro-optic/magneto-optic measurement of electromagnetic radiation using chirped optical pulse
US10/164,454 US6573700B2 (en) 1996-05-31 2002-06-06 Method of characterizing free-space radiation using a chirped optical pulse

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US7443498P 1998-02-11 1998-02-11
US60/074,434 1998-02-11

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WO2001094955A3 (fr) * 2000-06-09 2002-04-18 Univ Michigan Imageur de champs electromagnetiques a balayage pourvu d'un systeme a base de fibre optique pour la cartographie des champs electro-optiques
US6906506B1 (en) 2001-06-08 2005-06-14 The Regents Of The University Of Michigan Method and apparatus for simultaneous measurement of electric field and temperature using an electrooptic semiconductor probe
US10761126B2 (en) 2015-06-29 2020-09-01 Osaka University Electro-optic probe, electromagnetic wave measuring apparatus, and electromagnetic wave measuring method
CN114527477A (zh) * 2020-10-30 2022-05-24 大众汽车股份公司 带有提高的作用范围的激光雷达传感器系统
CN115219420A (zh) * 2022-06-09 2022-10-21 云南大学 新型飞秒时域微纳空间分辨多功能磁光仪
CN116057929A (zh) * 2020-09-14 2023-05-02 大众汽车股份公司 借助于可见辐射脉冲到周围环境中的视觉对象投射
CN118432724A (zh) * 2024-06-04 2024-08-02 北京红山信息科技研究院有限公司 一种太赫兹-自由空间光收发机一体化方法

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Cited By (9)

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WO2001094955A3 (fr) * 2000-06-09 2002-04-18 Univ Michigan Imageur de champs electromagnetiques a balayage pourvu d'un systeme a base de fibre optique pour la cartographie des champs electro-optiques
US6677769B2 (en) 2000-06-09 2004-01-13 The Regents Of The University Of Michigan Scanning electromagnetic-field imager with optical-fiber-based electro-optic field-mapping system
US6906506B1 (en) 2001-06-08 2005-06-14 The Regents Of The University Of Michigan Method and apparatus for simultaneous measurement of electric field and temperature using an electrooptic semiconductor probe
US10761126B2 (en) 2015-06-29 2020-09-01 Osaka University Electro-optic probe, electromagnetic wave measuring apparatus, and electromagnetic wave measuring method
CN116057929A (zh) * 2020-09-14 2023-05-02 大众汽车股份公司 借助于可见辐射脉冲到周围环境中的视觉对象投射
CN114527477A (zh) * 2020-10-30 2022-05-24 大众汽车股份公司 带有提高的作用范围的激光雷达传感器系统
CN114527477B (zh) * 2020-10-30 2025-09-26 大众汽车股份公司 带有提高的作用范围的激光雷达传感器系统
CN115219420A (zh) * 2022-06-09 2022-10-21 云南大学 新型飞秒时域微纳空间分辨多功能磁光仪
CN118432724A (zh) * 2024-06-04 2024-08-02 北京红山信息科技研究院有限公司 一种太赫兹-自由空间光收发机一体化方法

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