Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
  • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
  • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
  • G01N21/3581—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B3/00—Simple or compound lenses
  • G02B3/02—Simple or compound lenses with non-spherical faces
  • G02B3/04—Simple or compound lenses with non-spherical faces with continuous faces that are rotationally symmetrical but deviate from a true sphere, e.g. so called "aspheric" lenses

Definitions

• the present invention aims a system for capturing point values, to constitute an image with Terahertz radiation. It concerns, in particular, the field of imaging, for example for the control of qualities of manufactured parts.
• the Terahertz domain designates electromagnetic waves whose frequency is between 0.075 THz and 10 THz, in terms of wavelength from 4 mm to 30 ⁇ . These waves are said to be millimeters, they are located between far infrared (or FIR for Far InfraRed), and radar waves (microwave).
• the invention more particularly relates to the range 75-700 GHz. Indeed, in this frequency range, a large part of the non-conductive plastic and composite materials are transparent to the radiation. And the radiation sources are relatively accessible in terms of the ratio between the price of the source and the available optical power.
• ⁇ is the wavelength
• ON is the numerical aperture.
• a diffraction limit of 2.44 mm is the order of one millimeter. This diffraction limit of the order of one millimeter corresponds to the average spatial resolution of a THz imaging system.
• THz beam is invisible to the eye, and, unlike the near infrared, there is no photosensitive map allowing the conversion of radiation to wavelengths visible to the eye. This aspect has an impact on the tuning and alignment of the systems.
• THz radiation generation methods for continuous THz radiation exist and can be used in a device that is the subject of the invention, for example:
• Impatt diode associated with a frequency multiplier to go beyond 0.2
• Oscillator fixed or variable between 0 and 0.02 THz associated with a frequency multiplier
• Quantum Cascade Laser QCL
• the signal generated by the sources usually used in the THz domain has a very pure coherence, the width of the emission line is very fine. This coherent radiation is not conducive to image formation because interference fringes are generated with a very large amplitude on each system allowing it. Typically, these sources have an accuracy of 0.001 Hz with a bandwidth of 1 kHz to -118dBc / Hz.
• the present invention aims to remedy all or part of these disadvantages.
• the present invention aims a device for capturing point values for constituting an image, which comprises:
• an incoherent source of rays whose frequency is between 0.075 THz and 10 THz to illuminate an object
• a radiation sensor originating from the object which comprises a zone sensitive to the radiation coming from the source and which emits an electrical signal representative of the intensity of the rays coming from the source reaching the sensitive zone of the sensor and
• At least one less than one aperture focusing optical system located on the optical path of the rays emitted by the source from the source to the ray sensor passing through the object.
• the imaging device is not subject to the presence of standing waves due to the inconsistency of the source.
• the inventor has determined that this combination of optical and electronic means makes it possible to perform precise point measurements, the relative displacement of these means and the object making it possible to produce linear or matrix images.
• the source used has a bandwidth of several GHz, preferably:
• the source illuminates the object with a broad enough emission spectrum to scan the standing wave in a shorter time than the acquisition time of the sensor.
• At least one optical system has an aspheric optical lens.
• the dispersion (in percentage) of the indices of the materials used on the frequency ranges of the source is less than 1%.
• the dispersion in HDPE high density polyethylene
• HDPE high density polyethylene
• the aspheric lens simplifies optical design, compared to the use of parabolic mirrors.
• At least one optical component of an optical system has an antireflection coating with cone or crater-like microstructures.
• the advantage of this method is that it makes it possible to perform anti-reflective treatments very broadband and insensitive to the orientation of the surface.
• At least one optical system includes an optical lens and wherein the incoherent ray source is configured to illuminate the entirety of the optical lens closest to said source.
• the transmission frequency of the incoherent source is modulated.
• the incoherent source includes a noise source of thermal noise type in a resistor or impedance diode.
• the device comprises a proximity electronics for biasing a nano-transistor comprising the photosensitive zone, by a gate voltage close to its swing voltage where the conventional operation of the transistor is the most nonlinear.
• the rectified signal from the nano-transistor is amplified by forcing asymmetry of the charges in the nano-transistor channel by injecting a current in the transistor channel, between the drain and the source and / or using metallic patterns acting as antennas.
• the rectified signal is a continuous potential difference between the drain and the source of the nano-transistor measured in common or differential mode.
• the proximity electronics comprise a compensation circuit of the offset generated by the injection of the current between the drain and the source, for example by using a subtractor mounting.
• the photosensitive area is a nano-transistor
• the signal generated by the radiation THz is a continuous potential difference between the drain and the source of the nano-transistor measured in common or differential mode.
• the device comprises a proximity electronics for biasing the nano-transistor by a gate voltage close to its swing voltage where the conventional operation of the transistor is the most nonlinear.
• the rectified signal from the nano-transistor is amplified by forcing asymmetry of the charges in the nano-transistor channel by injecting a current in the transistor channel, between the drain and the source and / or using metallic patterns acting as antennas.
• the device comprises at least one low noise and low drift amplifier that amplifies the signal on the dynamics of an analog digital converter.
• the device comprises means for synchronizing the demodulation of the signal with the amplitude modulation signal of the source.
• the device comprises means for synchronizing the digital output of the device with the THz source.
• the sensor and the source are synchronized by a digital clock signal.
• the source is modulated in amplitude by this clock signal which makes it possible to measure and compensate for the signal residue (offset) present at the terminals of the sensor in the absence of THz.
• the compensation is both digital by high frequency subtraction of the signal measured with THz and without THz, but also by adjusting the voltage to subtract from the sensor signal to maintain the overall level within a certain voltage range.
• This architecture guarantees greater sensitivity, improved signal-to-noise ratio.
• the device comprises an encoder of the absolute positions of the object to be imaged, these measured position data being synchronized with the data from the sensor for use over the entire stroke of the displacement of the object. , including in the zones of accelerations, and an exploitation of the displacement back and forth of the object, by the setting of the even and odd lines avoiding a phenomenon of shearing.
• the device according to the invention comprises a splitter plate, the polarization incident on the splitter plate being the polarization in TM mode (acronym for transverse mode for transverse mode) in which the field is parallel to the plane of impact.
• TM mode acronym for transverse mode for transverse mode
• the separating blade is, for example, a blade made of Si-HRFZ whose thickness is calculated to ensure optimal separation. Thanks to these provisions, it is possible to perform measurements in reflectometry. Polarization in TM mode provides better stability of the separation efficiency when the thickness of the separating blade varies in its manufacturing tolerance.
• the device includes means for measuring the intensity of radiation from the object, and / or polarization of the rays from the object, and / or the difference in ray steps from the object.
• the device object of the invention thus allows several types of measurements.
• FIG. 1 represents, schematically and in section, a first embodiment of an incurent terahertz source
• FIG. 2 represents, schematically and in section, a second embodiment of an incurent terahertz source
• FIG. 3 represents, schematically and in section, a first embodiment of an imaging objective implemented in devices that are the subject of the invention
• FIG. 4 represents, schematically and in section, a second embodiment of an imaging objective implemented in devices that are the subject of the invention
• FIG. 5 represents, schematically and in section, a third embodiment of an imaging objective implemented in devices that are the subject of the invention
• FIG. 6 represents, schematically and in section, a fourth embodiment of an imaging objective implemented in devices that are the subject of the invention.
• FIG. 7 represents, schematically and in perspective, a first means of linear illumination of an object, implemented in devices that are the subject of the invention
• FIG. 8 represents a non-uniform illumination curve
• FIG. 9 represents an emission curve of a light source to compensate for the non-uniform illumination shown in FIG. 8,
• FIG. 10 represents an optical system providing substantially the emission curve illustrated in FIG. 9,
• FIG. 11 represents a substantially uniform illumination curve provided by the optical system illustrated in FIG. 10, with the linear illumination means illustrated in FIG. 7,
• FIG. 12 represents, schematically and in perspective, a second linear illumination means having non-symmetrical surfaces
• FIG. 13 represents, schematically and in section, a first particular embodiment of the device which is the subject of the invention.
• FIG. 14 represents, schematically and in section, a second particular embodiment of the device which is the subject of the invention.
• FIG. 15 represents, schematically and in section, a third particular embodiment of the device which is the subject of the invention.
• FIG. 16 represents, schematically, a point sensor
• FIG. 17 represents, schematically, a multi-pixel sensor
• FIG. 18 represents an electronic diagram for a THz microscope. DESCRIPTION OF EMBODIMENTS OF THE INVENTION
• the indices of the materials are constant. within a few percent, or less as discussed above, for HDPE.
• the systems that are the subject of the invention therefore do not have a problem of chromaticity, that is to say that there is no change in the optical behavior of the diopters when the wavelength changes.
• the imaging device that is the subject of the invention uses an incoherent terahertz source (THz) 100.
• THz incoherent terahertz source
• An incoherent THz source 100 is, according to a first example, constituted, as illustrated in FIG.
• a broadband TeraHertz 105 (f ⁇ 200 GHz) transmitter (0.1 GHz ⁇ Ai ⁇ 40 GHz), a filter 1 10 for limiting the emission band,
• An incoherent THz source 100 is, according to a second example, constituted, as illustrated in FIG. 2:
• a transmitter 130 in the low band that is to say in the frequency range from 0.1 GHz to 40 GHz, emitting on a narrow band (f ⁇ 1 GHz),
• the emitter 110 or 130 generates an incoherent signal such that, in a time shorter than the acquisition time of the sensor, the emitter emits a sufficiently broad emission spectrum to scan the standing wave.
• the source used has a bandwidth of several GHz, preferably:
• the transmitter 110 or 130 is a noise source of thermal noise type in a resistance or impedance diode. In embodiments, the transmitter 110 or 130 is a tunable source such as a variable frequency oscillator or JIG.
• the modulator 135 is tuned so that, once multiplied by each frequency multiplier 145, the generated signal has a transmission bandwidth preferably in the 0.1 GHz ⁇ Af ⁇ 10 GHz range. Note that the modulator 135 is flexible over a very narrow band (for example 200 MHz).
• the amplifiers 1 15, 140 or attenuators are used to adjust the input power of the signal before each frequency multiplier 120 or 145.
• the multipliers, amplifiers and attenuators can use structures on PCB (acronym of printed circuit board circuit board) or well guided (at the frequencies considered, we can manipulate these electromagnetic waves confined in metal waveguides).
• the choice of the antenna 125 or 150 makes it possible to parameterize the optical properties of the beam coming out of the last multiplier 120 or 145: polarization, TEM mode (transverse electromagnetic mode for transverse electromagnetic mode), divergence and size of the emission point.
• the source presented in FIGS. 1 and 2 uses an emission spectrum which ensures a scan at least equal to ⁇ / 4, which makes it possible to reduce the sensitivity to standing waves in the image while allowing the choice of the working frequency and the polarization.
• the spatial resolution is related to the size of the smallest spot that can be focused through the system.
• the spatial resolution is therefore proportional to the wavelength ⁇ , and to the numerical aperture ON used in the focusing system.
• the numerical aperture ON is physically limited to a value of 0.5. But in the air with realistic optical indices (of a value of 1, 5 for example), it is very difficult to reach openings smaller than 0.7.
• the divergence of the source is taken into account in order to choose the focal length f of the first lens so that the beam completely fills this first lens.
• the diameter D of the beam is equal to the diameter of the first lens, and the numerical aperture parameter ON is optimized.
• the imaging device that is the subject of the invention uses aspherical lenses (FIGS. 3 to 6 and 13 to 15).
• a spherical lens is a lens whose shape of each surface follows a sphere.
• spherical aberration an aberration related to the curvature of the lens, called "spherical aberration”.
• Marginal rays passing near the edge of the lens
• paraxial rays passing in the center of the lens.
• This effect counterbalances the gain in resolution with the increase of the diameter of the beam, because the focused beam with spherical aberration no longer follows the Airy law mentioned above.
• the introduction of this spherical aberration disrupts the entire propagation of the THz beam by creating rings of light instead of a point of focus ("spot”) well circular: there is a loss of energy in addition to 'a loss of information.
• aspherical lenses For an aspheric lens the shape of the curvature follows a sphere at the center, but out of the center, the curvature is corrected to cancel the spherical aberration. In visible and infrared optics, these lenses are very expensive to manufacture because of their complex shape.
• THz THz
• 3D printing and / or micromachining with a residual roughness between 0.02 and 0.1 mm is carried out.
• the measurement of the source propagation parameters makes it possible to define the illumination at the input of the system: divergence, distribution of the energy
• the dispersion (in percentage) of the indices of the materials used for the lens, on the frequency ranges of the source is less than 1%.
• the dispersion in HDPE is of the order of 0.5% over the frequency range and, more particularly:
• the system is optimized by varying the distances between the optics, the radius of curvature of the surfaces as well as the conicity coefficients (hence the asphericity)
• the system optimization criterion can be defined for example on the minimization of the spot size at several points of the field, or the minimization of the wavefront difference at several points of the field.
• a roughness of 1/100 is sufficient to ensure good transmission efficiency. These roughnesses are compatible with microfraises used on modern lathes and milling machines and 3D wire deposition printing.
• the focus lenses are designed to work in infinite focus mode, ie the incident beam must be collimated (parallel rays) and its diameter must be the same. than the active diameter of the lens.
• the focal point where the beam is the smallest is defined and then the beam diverges again is captured by another lens.
• a first method consists in depositing one or more layers of dielectric material whose optical index is lower than the optical index of the treated lens, for example the optical index of the treatment material is close to the square root of the optical index of the lens.
• the thickness of the layer to be deposited is of the order of a quarter of the average wavelength used. This thickness of the treatment material is relatively large in THz, compared to the infra-red, which poses problems on the deposition technique.
• a second method, preferably used in the imaging device of the invention is to form cone-shaped or crater-shaped microstructures using a micro-machining method, for example "laser drilling” ( laser milling) using a second femto laser.
• laser drilling laser milling
• These microstructures of dimensions smaller than the wavelength create an index gradient apparent on the surface of the diopters which cancels the reflection on the face because there is no longer the index discontinuity that causes the reflection.
• the advantage of this method is that it makes it possible to perform anti-reflective treatments very wide band and less sensitive to the orientation of the surface, unlike the case of layer deposition.
• This type of treatment increases the energy efficiency of multi-lens optical systems by increasing the transmission of each optical diopter. Thanks to antireflection treatments, the energy projected on the sensor is increased by several tens of percent. Antireflection treatments also help eliminate unwanted images and standing waves.
• the THz detector is an RF nano-transistor in GAAS, GAN or INP in a technology less than 0.25 ⁇ which by plasma effect in its channel straightens the THz waves in the 0.1 THz bands. at 2.5 THz; - A proximity electronics 610 for biasing the detector.
• the nano-transistor to be an efficient rectifier must be biased by a gate voltage close to its swing voltage where the conventional operation of the transistor is the most nonlinear.
• the rectified signal is amplified by forcing asymmetry of the charges in the channel by injecting a current in the transistor channel between the drain and the source, which has the effect of increasing the sensitivity of the transistor;
• the rectified signal is a continuous potential difference between the drain and the source of the nano-transistor measured in common or differential mode.
• the proximity electronics 615 has a high impedance so as not to attenuate the amplitude of the signal and to force the injection of the bias current into the transistor and not into the amplification circuit.
• a subtractor assembly compensates for the offset generated by the current injection between the drain and the source.
• One or more low noise and low drift amplifiers amplifies the signal on the dynamics of the Digital Analog Converter ("CAN") 620;
• the analog-to-digital converter has a large dynamic amplitude (> 12 bits) and is fast (> 100 thousand samples per second);
• a central processing unit 625 implementing a signal demodulation algorithm.
• the useful signal is placed at a frequency greater than 100 Hz by modulating the THz source.
• the useful signal at the output of the sensor is derived from the demodulation of the signal received by the electronics.
• the function of the point sensor 600 and its proximity electronics is to straighten the THz wave and to provide a suitable impedance signal for the following electronic stages.
• the subtracter followed by an amplifier, or the differential amplifier makes it possible to amplify the useful signal differentially with respect to the offset signal generated by the injection of the current into the transistor.
• the signal is digitized at a high rate (acquisition frequency> 1 kHz) as close as possible to the analog output.
• Signal demodulation is performed synchronously with the source modulation signal, which provides a maximized signal-to-noise ratio by eliminating all additive noises at a frequency other than the source modulation frequency.
• the digital output of the sensor is synchronized with the absolute positions of the object to be imaged.
• an injection of a current into the channel of the transistor is carried out.
• a current of the order of a few tens of ⁇ is injected into the transistor channel. This has the effect of increasing by several orders of magnitude the electrical response in the presence of THz radiation.
• the injection is done by a current generating component directly connected to the transistor by its drain, the component may be for example a LM334 associated with a resistor, the choice of the value of the resistor allows to set the amount of current injected.
• the useful signal at the terminals of the transistor in the presence of radiation THz being the voltage between its drain and its source, it is necessary to isolate the transistor and the component injecting the current from the rest of the amplification chain by a follower mounting having a very strong impedance.
• the injected current can propagate only in the transistor and not in the amplification chain.
• the resistance of the transistor channel is of the order of kOhm
• the injection of the current into the channel generates a continuous offset across it.
• This offset is present whether there is THz or not.
• This offset can be of the order of several hundred mV which is problematic for the amplification chain. Indeed the amplifiers amplify as much the useful signal generated by the THz as the continuous offset: beyond a certain level the offset causes a saturation of the inputs of the different amplifiers. It is therefore necessary to subtract a DC voltage of the same order of magnitude as the offset voltage to the signal generated by the transistor to allow the amplification chain to remain in its input operating range.
• the amplification is chosen such that the noise generated by it is low, it has a drift as low as possible, and a sufficient bandwidth for the working frequencies.
• an OPA735 amplifier from Texas Instruments has the required performance.
• Two amplification stages are conventionally chosen: the first of fixed gain of 10 or 100 near the sensitive surface, which ensures the transport of a signal several orders of magnitude higher than the level of noise induced by the circuit.
• the second variable gain amplifier (for example between 1 and 10) near the analog-digital converter (ADC). This allows the total to reach a maximum amplification factor of 1000 while being able to adapt the amplification to fill the conversion range of the ADC.
• FIG. 18 shows a transistor 815 receiving a gate voltage 805 and a current injection 810 on its drain, the voltage on the drain being supplied to a follower 820.
• a subtractor 830 subtracts a voltage 825, then two amplifiers 835 and 840 amplify the signal.
• the noise generated by the transistor increases greatly when current is injected. It is no longer interesting to operate in continuous mode (DC) because the noise level is too important, especially the low frequency noise related to the offset voltage. However, by placing the useful signal at a frequency of the order of kHz, a signal-to-noise ratio greater than 60 dB is obtained, compared to approximately 45 dB in continuous mode without injected current. The advantage of current injection is in this signal to noise ratio which is better with the current for a working frequency between 100Hz and 100kHz.
• the procedure is as follows: the source is modulated externally by a programmable electronic component (for example an FPGA) which controls a radiofrequency amplitude modulator.
• the programmable component controls the high and low states of the modulation: on one, the THz is emitted by the source; on the other, no signal is emitted.
• the signal received by the sensor is either the useful signal related to the presence of THz radiation, or the background noise in the absence of THz.
• the programmable component samples the signal from the sensor during the high and low states, it produces two averages: one corresponding to the average signal level on the high state, the other at the average signal level on the state low.
• This architecture ensures greater sensitivity, reduced signal noise and signal synchronization with the optimal object position.
• this multipixel sensor 700 is constituted, as illustrated in FIG. 17:
• Each detector THz is a nano-RF transistor in GAAS, GAN, or INP in a technology less than 0.25 ⁇ which, by plasma effect, in its channel straightens the waves THz in the 0.1 THz bands at 2.5 THz.
• Each nano-transistor is packaged in a micro surface packaging ⁇ 0.5 mm 2 ;
• a nano-transistor for being an efficient rectifier is biased by a gate voltage close to its swing voltage where the conventional operation of the transistor is the most nonlinear.
• the rectified signal is amplified by forcing asymmetry of the charges in the channel by injecting a current between the drain and the source; a proximity electronics 720 for formatting the THz signal.
• the rectified signal is a continuous potential difference between the drain and the source of the nano-transistor measured in common or differential mode.
• the proximity electronics 720 has a high impedance so as not to attenuate the amplitude of the signal and to force the injection of the bias current into the transistor and not into the amplification circuit.
• a subtractor assembly compensates for the offset generated by the current injection between the drain and the source.
• One or more low noise and low drift amplifiers amplifies the signal on the dynamics of the Digital Analog Converter (ADC) 725;
• This digital-to-digital converter 725 has a large dynamic amplitude (> 12 bits) and is fast (> 100 kSamples / s or thousands of samples per second);
• the THz source is modulated in amplitude which makes it possible to perform an averaged differential measurement.
• a high-speed communication system 735 via an Ethernet, USB or cameralink communication protocol (registered trademarks).
• the sensor and the source are synchronized by a digital clock signal.
• the source is modulated in amplitude by this clock signal which makes it possible to measure and compensate for the signal residue (offset) present at the terminals of the sensor in the absence of THz.
• the compensation is both digital by high frequency subtraction of the signal measured with THz and without THz, but also by adjusting the voltage to subtract from the sensor signal to maintain the overall level within a certain voltage range.
• the absolute positions of the object to be imaged are measured by an encoder. These position data are then synchronized with the data from the THz sensor for use over the entire travel path including in the acceleration zones, but also the exploitation of the outward and return displacement by the calibration of the lines. even and odd, avoiding a shear phenomenon. And lastly, a averaging of the data calibrated on the absolute positions and not on the time allows an adaptation at any speed of displacement This architecture guarantees the highest sensitivity, reduced signal noise and synchronization of the signal with the optimal object position.
• the multipixel 700 sensor makes it possible to enlarge the field of view spatially and thus to produce high resolution images with a high refresh rate (line frequency> 1 kHz).
• Each sensor 700 including its proximity electronics is in charge of straightening the THz wave it receives locally and provide a suitable impedance signal for the following electronic stages.
• each subtracter circuit followed by an amplifier, or differential amplifier, amplifies the useful signal.
• the signal is digitized at a high rate (acquisition frequency> 1 kHz) as close as possible to the analog output.
• the signal demodulation is performed synchronously with the source modulation signal on the signals from each pixel, providing a maximized signal-to-noise ratio by eliminating all additive noises at a frequency other than the modulation frequency of the signal. source.
• the multipixel sensor 700 is located in the focal plane of the imaging optical system. Each point sensor is in charge of straightening the THz wave, amplifying the useful signal. The signal is digitized at a high rate (acquisition frequency> 1 kHz) as close as possible to the analog output
• the small surface of the sensors allows a great compactness and thus it results a high spatial resolution.
• the photosensitive components are arranged in line with, for example, a space between two components of the order of 300 ⁇ and an interpixel of 800 ⁇ , each component having a dimension of 500 ⁇ ⁇ 1 mm.
• the individual polarization of each sensor makes it possible to guarantee uniform sensor-to-sensor sensitivity while maintaining high sensitivity.
• Parallel management of all signals ensures a very high acquisition speed.
• Embedded signal processing provides real-time filtering for the best signal-to-noise ratio. Communication via an Ethernet, USB or cameralink communication protocol allows packet management to send high-speed lines to link several muiti pixels sensors without loss of data.
• the starting point for the design of linear sensor imaging systems is the spatial resolution imposed by the sensor.
• the imaging device object of the invention has, in its multipixel embodiments, an inter-pixel of 800 ⁇ , and according to the model, a line of 64, 1 28 or 192 pixels. By choosing the field of view on the side of the object to be imaged, the optimal spatial resolution of the sensor and lens assembly is imposed.
• the optimal spatial resolution of the camera is 0.53 pl / mm (pairs of lines per millimeter) in frequency.
• a higher optical resolution of the system could not be sampled correctly by the camera, a lower optical resolution would lead to having an excessive pixel / mm number so not to benefit from all the available resource.
• This approach also directly indicates the magnification required for the optical system, here 1, 71 x.
• the objective to be designed to answer the problem is a microscope-type objective.
• the microscope objective contrary to the objective of photography or to a telescope (distant telescope), the microscope objective has the following overall characteristics: the scene to be imaged is smaller than the size of the sensor, the pixels of the sensor are of the same order of magnitude as the average details of the scene. The reproduction ratio is much higher than one (a scene of 100 ⁇ x 100 ⁇ will be reproduced on a 10 mm x 10mm sensor: magnification of 100).
• the objective works at a short distance from the object, and its object aperture can reach values such as 0.7.
• microscopic optics makes it possible both to grow the object to be observed, and to separate the details of this object so that it is observable. It is the combination of magnification and resolving power that characterizes such an optical system.
• the scene In the context of microscope objective imaging systems in THz imaging, on the one hand the scene is generally smaller than the size of the detector array, and the detector is on the order of magnitude of the scene details it is therefore necessary to have a magnification of the scene and its details to image it.
• the image of the scene must be formed by ensuring sufficient resolution of the object to be able to observe its details, which generally implies working with large openings, so close to the object and with large diameter optics compared to the stage.
• the pixels of the sensor are large compared to the details of the image to be formed
• the sensor is large:> 150 mm for 192 pixels and
• the resolution to be achieved requires very large openings.
• the consequence of this choice is that the lens is large, of a similar size, or larger than the objects to be imaged.
• the lenses have at least two lens groups:
• 1st group near the object, allows the collection of the light coming from the object with the targeted opening and
• - 2nd group close to the image, allows to form the image of the object with the target image opening.
• the lens actually has three lens groups, the second group being divided into two separate subgroups of a large air space, which is particularly the case when the lens is telecentric. side image.
• Example 1 40 mm object field, magnification 1, 3x, 235 GHz
• the objective 200 has two groups 205 and 210 of a lens each, it is not telecentric side image.
• This 200 lens has been designed with the constraint of an object print larger than 50 mm.
• the MTF (acronym for "modulation transfer function" for modulation transfer function) shows a resolution of 0.4 pl / mm, ie 1.92 mm of resolution relative to the object.
• the spot diagram shows that the system is limited by diffraction on the whole field: the image is not disturbed by the aberrations.
• Example 2 Figure 4: object field 30 mm, magnification 1, 7x, 235 GHz
• the objective 215 has three lenses 220, 225 and 230, the second group being divided into two subgroups of a lens. Telecentric lens on the image side.
• the MTF image shows a resolution of 0.45 pl / mm, ie 1.3 mm of resolution reported to object.
• the spot diagram shows that the system is limited by diffraction on the whole field: the image is not disturbed by the aberrations.
• Goal 235 has a second group divided into two subgroups. Four lenses 240, 245, 250 and 255 are necessary to obtain the magnification of 1 and the telecentricity on the image side.
• the MTF image shows a resolution of 0.40 pl / mm, or 2.5 mm resolution reported object.
• the spot diagram shows that the system is limited by diffraction except at the edge of the field (slight defect).
• the objective 260 of great increase in two groups, with a compensator of field in the first group.
• This objective with three lenses 265, 270 and 275 makes it possible to reach resolutions below the wavelength.
• the image-wise MTF shows a resolution of 0.41 pl / mm, ie 780 ⁇ of resolution referred to object.
• the spot diagram shows that the system is limited by diffraction except at the edge of the field (slight defect).
• the THz imaging lens is used to image an object in the field to an image plane located opposite the lens.
• the condition for operating in the specifications is to have a THz signal input sufficient for the detection sensitivity in the image plane.
• this objective is integrated in a string including a source, a projection system of the illumination of the source, the objective itself, a multi-pixel sensor, and a computerized analysis system (no represent).
• the objective is therefore an element is key in the chain described above because it is he who ensures the optical spatial resolution on the object to be imaged.
• the dimensions of the lens are intrinsically related to its performance: the higher the desired resolution, the larger the lenses, and the length of the lens is important.
• the use of a line of pixels as a sensor requires the use of lighting on a line of the scene in order to focus the energy of the source on the part of the object observed by the linear sensor.
• the source emits radiation on an emission cone determined by its horn.
• the distribution of energy in the emission cone presents a non-uniform and circular energy distribution because it is generally of Gaussian distribution and symmetry of revolution.
• the linearization system of the source has two objectives:
• the illumination of the scene is designed parallel in one direction, and focused in the perpendicular direction.
• the use of a cylindrical lens 290 is a solution to achieve this goal.
• the beam 285 from the source is enlarged to the same diameter as the scene to be illuminated, then with a lens whose curvature is aspherical with a cylindrical geometry (and not with a geometry of revolution) the radiation is focused in a single direction of the space, as shown in Figure 7.
• the linearization 280 by the preceding method has a non-uniform illumination because the distribution of the illumination of the source is not.
• the focusing of the energy distribution of the source in a single direction of space keeps the same inhomogeneous distribution carried along the line of illumination.
• FIG. 8 represents a power distribution 300 at source output 100.
• optical design software it is possible to calculate the optimal revolution geometry distribution to ensure uniform illumination once transformed by the cylindrical lens. Since this distribution is known, the software uses it as a target for optimizing the beam shaping lenses at the output of the source 100. For example, an illumination having a distribution 305, illustrated in FIG. 9, forms a uniform distribution after a cylindrical lens.
• a three-lens system 310, 315, 320, 325 is formed:
• the distribution 305 necessary for the entry of the cylindrical lens 325 is calculated
• optimization is made of a system with two aspherical lenses 315, 320 to form this distribution 305.
• FIG. 11 illustrates the distribution 330 resulting from the implementation of the optical system 310.
• the two beamforming lenses 315 and 320 are replaced by a single lens, having at least one non-symmetrical surface of revolution (sphere, asphere), but representing splines or polynomials.
• a set of two lenses 335 and 340 makes the illumination linear and uniform, as illustrated in FIG. 12:
• the lens 335 is used for shaping the beam to prepare the energy distribution (XY polynomial surface lens, for example)
• the aspherical cylindrical lens 340 achieves the focusing of the beam in only one direction.
• the advantage of this method is that it is simpler to implement in a computation software: a macro of passage of the Gaussian circular illumination of the source to the uniform linear illumination is directly implemented at the level of the optimization without going through an intermediate step of calculating the illumination necessary for the entry of the cylindrical lens 340.
• the computation of the macro complex for a system with symmetry of revolution, breaks this symmetry from the first lens. This brings the advantage of reducing the number of lenses needed.
• the design of the illumination takes into account the source used (wavelength, divergence).
• the illumination system projects enough THz energy onto a plane of space and then uses a collection lens and a sensor to form an image of an object placed in that plane of space.
• the linearization system allows you to choose how to distribute the energy from the source to illuminate the scene most effectively.
• a system benefiting from this type of illumination has the greatest energy efficiency, which makes it possible to use less sensitive sensors, or to make images of very opaque or low-reflective objects.
• the illumination may be a rectangle whose widths and heights may be chosen by the designer.
• the illumination is collimated on the object to be studied which allows on the one hand to have a greater depth in which the illumination is optimal and on the other hand to have the exit pupil of the system. illumination to infinity to avoid having a part of the image of the source that is superimposed with the image of the object.
• This macro is implemented in the design software to optimize two non-symmetrically revolving surface lenses that generate rectangular illumination at the object as defined by the designer.
• a first group realizes the shaping of the beam to reach the desired resolution
• a second group carries out the collection of the stream transmitted through the object to be imaged.
• the spatial resolution is only related to the first optical group located before the object.
• the stream collection part has no impact on the spatial resolution of the formed image.
• the optimization of the second group is based on the principle of energy flow collection: the goal is to concentrate the energy on the sensor as much as possible in order to obtain the largest signal-to-noise ratio.
• Two imaging modes are used depending on the types of objects to be imaged.
• the transmission mode through the object: transmitted measurement of the intensity, and / or polarization, or difference in operation by the sample.
• the reflection mode on the object reflected measurement of the intensity, and / or polarization, or difference in the surface or depth of the sample.
• the proposed punctual imager makes it possible to make images of the object in reflection and in transmission simultaneously. For this purpose, it uses two flow collection groups located upstream and downstream of the object. It can also control the amplitude of the source for calibration to perform absolute measurements.
• the point imaging system can integrate the following elements:
• One or more point or multi-pixel sensors linear
• a computer system that creates the image by synchronizing with the absolute position of the Cartesian table the signal synchronized with the modulation of the source.
• the maximum resolution is obtained for a focusing lens with a very short focal length, of the order of 10 or 20 mm.
• the space available for inserting the sample may be very limited for large samples.
• the proposed imager presents a solution to this problem thanks to a modular system allowing to change resolution according to the dimension of the object to be imaged.
• the beam from the source is first collimated by a first lens.
• the collimated beam propagates with parallel rays as a result of this lens, which makes it possible to place the lens that focuses the radiation at any distance from the collimating lens.
• the plane of focus on the object is chosen such as a short focal lens allowing a high resolution, and thus placed close to the object, or a lens of long focal length allowing a lower resolution but making it possible to measure a larger object. voluminous, the lens being placed close to the source, focus the beam at the same place on the optical axis.
• the collection of flows is done by a group of two lenses placed after the object.
• the collector used has a numerical aperture at least equal to that of the first group, to ensure maximum efficiency.
• the first lens may be identical to that of focusing. It collects the flux coming from the object, and makes the beam with parallel rays (collimated).
• the modularity of resolution described above is preserved thanks to this system by allowing to dispose the second lens at any distance from the first.
• the second lens in front of the detector focuses the THz beam on the photosensitive surface by concentrating the energy to maximize the signal-to-noise ratio.
• the aperture used for this lens is for example 1, 25. This value can be chosen to make a compromise between energy concentration (lower aperture value), and focusing stability: a larger aperture makes it possible to make the focusing on the sensor less dependent on any beam distortions associated with the images. mechanical dilatations or the simple disturbance of the beam by the object.
• the collection of the flow is done by adding a splitter blade and a lens.
• the assembly is placed before the focusing lens on the sample.
• the separating blade makes it possible to let the incident THz beam pass between the source and the object. Once reflected on the object, the THz signal is collected by the focusing lens, then reflected on the splitter plate to be sent at 90 ° to the lens in front of the sensor.
• the lens in front of the sensor is the same as the lens used in transmission mode.
• the splitter blade introduces a loss of signal at each passage by a reflection of a portion of the energy of the source to the go, and by a transmission of a portion of the reflected energy of the object back.
• the THz wave is an electromagnetic wave that can be polarized specifically to study a specific response of materials, such as birefringence.
• a delay component can be used to transform a linear polarization into a circular one, for example:
• the analyzer analyzes the ellipticity, the phase shift of the ellipse and the direction of rotation of the ellipse.
• the techniques used are:
• Modularity offers a very flexible platform for measuring the thickness of samples (objects to be imaged) through print management, the need for adapted spatial resolution, transmission mode imaging for completely transparent materials, reflection for objects with one material opaque, polarization for materials that have a preferential axis of propagation (we speak of birefringence).
• THz source modulation and broadband
• sensors very sensitive, fast
• encoders of the Cartesian table absolute position
• acquisition system synchronization, filtering
• the high-resistivity silicon is very commonly used in the THz domain for the manufacture of optical windows, beam splitter or lens hemispherical collection.
• the image capture system object of the invention uses this material for its semi-reflective blades.
• the very high index of 3.41 makes it possible to obtain on a single material face a separation of the beam in two components reflected and transmitted by 45% and 55% respectively.
• the blade must be carefully designed because the passage through both sides of the blade causes interference of the beam with itself which makes vary significantly the performance of the component.
• the separating blade is the object of antireflection treatment on one side of its material.
• a material with an index close to 1.8 is preferably chosen, and its thickness is adapted to a submultiple of the wavelength.
• a separation efficiency of 50% is obtained by choosing the thickness of the blade: by calculating the components of the incoming wave interference it is possible to determine which thickness leads to an efficiency of 50%.
• the polarization is used in TM mode (acronym of transverse mode for transverse mode) in which the field is parallel to the plane of incidence.
• TM mode acronym of transverse mode for transverse mode
• the inventor has in fact determined that the polarization in TM mode ensures a better stability of the separation efficiency when the thickness of the separating blade varies in its manufacturing tolerance.
• the splitter blade is used on a weakly diverging beam to be most effective. Preferably, it is inserted into a collimated beam.
• the separating blade is crossed twice by the optical beam: once when it comes from the source, a second time when it returns from the object to the detector.
• the splitter blade is used to design an optical system in reflection, where the illumination of the object and the recovery of the flux from the object are on the same side of the object.
• a multipixel imaging device integrates the following elements:
• an optical system consisting of lenses for performing linear illumination
• an optical system consisting of lenses for collecting the illumination after transmission through the object or reflection on the object
• the reflection mode is therefore commonly used. There are several ways to arrange the lighting and the lens to set up this mode.
• the complete optical system has two separate parts 400 and 410, one for linear lighting, the other for the imaging lens.
• the free space makes it possible to insert the object 405 whose images must be captured by the sensor 415.
• the opening of the lighting is adapted to the opening of the lens to not lose energy outside the lens.
• the three right-hand lenses 420 form the illumination which is projected at the level of the image plane of the objective 430. then the objective 430 to be projected onto the object 425. Finally the objective 430 forms the image of the object 425 which is projected at 90 ° on the sensor 415 through a semi-reflective plate 435.
• the lighting is then designed to uniformly illuminate the object 425 through the lens
• the illumination 440 takes into account an additional lens 460 after the cylindrical lens 465: the last lens of the lens 450.
• the illumination is inserted directly at the exit of the objective 450.
• the objective 450 operates thanks to a semi-reflective plate 455 which reflects the light reflected at 90 ° to the other two lenses and the linear sensor 470.
• the advantage of this design is to be more compact, and to introduce fewer losses with fewer lenses crossed by the THz beam between the source and the sensor.
• the illumination system provides a uniform illumination of the scene, the collection system a very high resolution exploited by the extreme compactness of the pixels of the multi-pixel sensor.

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