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WO2025137196A1 - Systèmes et procédés de détermination d'un indice de réfraction d'un matériau - Google Patents

Systèmes et procédés de détermination d'un indice de réfraction d'un matériau Download PDF

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Publication number
WO2025137196A1
WO2025137196A1 PCT/US2024/060908 US2024060908W WO2025137196A1 WO 2025137196 A1 WO2025137196 A1 WO 2025137196A1 US 2024060908 W US2024060908 W US 2024060908W WO 2025137196 A1 WO2025137196 A1 WO 2025137196A1
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Prior art keywords
clock signal
digital
optical
reference clock
signal
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Roger William RUSACK
Rohith SARADHY
Erich FRAHM
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University of Minnesota Twin Cities
University of Minnesota System
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University of Minnesota Twin Cities
University of Minnesota System
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    • 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
    • G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
    • G01N21/4133—Refractometers, e.g. differential

Definitions

  • a material’s IoR indicates the degree of reflection of an incident beam of light off of the material’s surface (Fresnel coefficient), the change in angle of the trajectory of light at the material’s surface (Snell’s Law), and the value of the critical angle for light at the surface of the material.
  • Various methods for measuring IoR are used in industry. One method makes use of the physical principles of Snell’s Law. A beam of light is redirected through a medium and the deviation in the position is detected with a position-sensitive photodetector. Another approach entails splitting a beam of mono-chromatic light, directing one beam to pass though the material under review and the other to pass around the material.
  • the DDMTD incorporates modern RF integrated circuits to achieve a high level of precision, capable of measuring a change in the optical path length of a signal to a precision of better than 1e-13 second (100 femtoseconds (fs)), in some examples of at least 1e-14 second (10 fs) by taking repeated measurements over a period of 0.1 seconds. In some examples, a measurement precision on the order of 1e-14 seconds (10 fs) with an acquisition rate of 1 Hz is provided.
  • FIG. 6 is a functional block diagram of portions of an IoR measuring system in accordance with principles of the present disclosure
  • FIG. 7 is a functional block diagram of portions of an IoR measuring system in accordance with principles of the present disclosure
  • FIG. 8 is a functional block diagram of portions of an IoR measuring system in accordance with principles of the present disclosure
  • FIG. 9 is a functional block diagram of portions of an IoR measuring system in accordance with principles of the present disclosure
  • FIGS.10-17 are graphs reporting results of testing described in the Examples section.
  • the present disclosure relates to systems and methods for determining or measuring an index of refraction (IoR) of a material, such as an optically transparent material.
  • IoR index of refraction
  • the system 20 includes a digital dual mixer time difference (DDMTD) circuit 30, a clock signal generator 32, a transmitter module 34, a detector module 36, and an interface assembly 38. Details on the various components are provided below.
  • DDMTD digital dual mixer time difference
  • the clock signal generator 32 operates to generate an electric clock signal that is provided to the DDMTD circuit 30 and the transmitter module 34.
  • the transmitter module 34 converts the electric clock signal to an optical clock signal.
  • the optical signal is transmitted through the material under review and is received or detected by the detector module 36 as a recovered optical signal.
  • the material under review is arranged relative to transmitter and detector components of the modules 34, 36 by the interface assembly 38.
  • the detector 2023-353 / U639.176.111 module 36 converts the recovered optical signal to an electrical signal that is delivered to the DDMTD circuit 30.
  • the DDMTD circuit 30 operates to generate reference and test signals from which the IoR of the material under review can be determined, for example by operation of an analyzer 40.
  • the system 20 can optionally include, or be connected to, a memory 42 and/or user interface components such as a user input component 46 (e.g., keyboard) and a display 48 for conveying the determined IoR and/or other information.
  • the systems and methods of the present disclosure can determine or measure the IoR of a material in real time by measuring to a very high precision the velocity of light in the medium.
  • the DDMTD circuit 30 is capable of detecting phase differences in digital clocks to the level of a few tens of femtoseconds (fs); as light in vacuum travels 30 ⁇ m in 100 fs, measurements to that precision are available.
  • the DDMTD circuit 30 is generally configured to generate and measure the time difference between two digital clock signals (in some non-limiting examples, two out- of-phase digital clock signals) with very fine resolution (sub-picosecond) using a relatively low speed circuit.
  • two digital clock signals in some non-limiting examples, two out- of-phase digital clock signals
  • sub-picosecond very fine resolution
  • a digital reference clock signal (u ref ) is an input to the first flip-flop FF ref
  • a digital test clock signal (u test ) is an input to the second flip-flop FF test .
  • the input digital clock signals are each fanned out to the corresponding flip-flop with a fan-out chip (Fan Out).
  • the digital reference clock signal uref is also delivered to the PLL, for example via another fan-out chip Fan Out.
  • the input digital clock signals are heterodyned at the corresponding flip-flop FFref, FFtest by mixing with an offset frequency clock (CLK PLL) generated by the “helper” PLL that is phase-locked to the reference clock signal u ref .
  • CLK PLL offset frequency clock
  • the heterodyned outputs of the two flip-flops are beat clocks designated as BEAT CLK A (the heterodyned reference clock signal) and BEAT CLK B (the heterodyned test clock signal).
  • BEAT CLK A the heterodyned reference clock signal
  • BEAT CLK B the heterodyned test clock signal
  • the BEAT CLK A, BEAT CLK B, and CLK PLL are delivered to a sampling circuit or a controller (e.g., the analyzer 40 (FIG.1)) to count the number of CLK PLL cycles between the transitions of BEAT CLK A and BEAT CLK B.
  • a controller e.g., the analyzer 40 (FIG.1)
  • Other techniques for measuring or determining the phase difference between the beat clocks are also acceptable.
  • a tunable parameter of the DDMTD circuit 30 is an offset factor (Noffset) that affects the sampling rate and the precision which are inversely related.
  • the frequency of the offset clock, v ddmtd can be selected using Equation 1 as: ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ Eq.1 ⁇ ⁇ 1 where the offset number of input cycles required for a full phase cycle of the beat clock, whose frequency (vbeat) is given by Equation 2 as: 1 ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ Eq.2
  • the time with a minimum sensitivity (t min ) that is given by Equation 3 as: 1 ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇
  • the time difference between uref and utest ( ⁇ t) can be measured by Equation 4 as: ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ 2023-353
  • Equation 5 As the time difference between the input clocks is determined from both the positive and negative edges of the beat clocks, the highest frequency that it can be measured is provided by Equation 5 as: 2 ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ 2 ⁇ 1 Eq.5 [21]
  • the setup and hold times may be violated and the output of the flip-flop becomes metastable. This instability can last for several clock cycles of the input clock.
  • This metastability can be removed through various methods. One such method is to estimate the exact time of the transition using the average time of the first and last transitions. The length of the metastability can be determined by the setup and hold times of the flip-flops used.
  • the flip-flops FF ref , FF test are high-performance D-type flip- flop chips, such as a differential D flip-flop with a maximum input clock frequency or clocking rate of at least 10 GHz, optionally on the order of 12 GHz, and setup and hold times optionally on the order of 15 picoseconds (ps).
  • the flip- flops FF ref , FF test can be, or can be akin to, a flip-flop component available under the trade designation NB7V52M from ON Semiconductor Corp.
  • the clock multiplier PLL can be a PLL-based jitter attenuator/clock multiplier integrated circuit, optionally with an RMS jitter on the order of 90 fs.
  • the clock multiplier PLL can be, or can be akin to, a jitter attenuating clock multiplier component available under the trade designation Si5344 from Silicon Labs.
  • the fan-out chips Fan Out can 2023-353 / U639.176.111 be a high performance equalizer receiver (signal enhancer) package, optionally with a random jitter + deterministic jitter on the order of (0.2 + 3.0) ps.
  • the fan-out chips Fan Out can be, or can be akin to, a fan out buffer, equalizer receiver component available under the trade designation NBVQ1006M from ON Semiconductor Corp.
  • the DDMTD circuit 30 is capable of measuring the beat frequency with higher precision as compared to conventional components with setup and hold times on the order of 800 ps.
  • the clock signal generator 32 can assume various forms appropriate for generating an oscillating clock signal (oscillating between a high and low state), and can be a precision clock generating system.
  • the clock signal generator 30 incorporates any crystal or crystal resonator- based design, for example a crystal oscillator (XO).
  • the clock signal generator 32 can be an oven-controlled crystal oscillator (OCXO) or similar construction with high stability (low jitter) and accuracy.
  • the transmitter module 34 can have various constructions capable of converting an electric clock signal to an optical clock signal, and transmitting or emitting the optical 2023-353 / U639.176.111 signal.
  • the transmitter module 34 includes driver or converter electronics and a corresponding optical emitter, for example a laser driver and laser device (e.g., laser diode, solid state laser, etc.).
  • Other optical emitter formats are also acceptable, for example light emitting diode or similar transducer.
  • the transmitter module 34 can include a high-bandwidth optical emitter, although other configurations/emitted radiation wavelengths are also acceptable. More generally, the transmitter module 34 can be selected or configured in accordance with the material to be tested, and in particular to emit radiation (i.e., the optical signal) at wavelengths to which the material under test is otherwise optically transparent (e.g., an emitter configured to emit radiation at wavelengths longer than 1.1 ⁇ m for measuring the IoR of a silicon wafer). [25]
  • the detector module 36 can have various constructions of receiving or detecting an optical signal and converting the recovered optical signal to an electrical signal.
  • the detector module 34 includes a detector device and corresponding converter circuitry.
  • a format of the detector device corresponds with a format of the emitter device of the transmitter module (i.e., the detector device is formatted to detect or sense radiation at the wavelengths being emitted by the emitter device), and can include an optical transducer or sensor such as a photodetector or photosensor.
  • the interface assembly 38 can assume various forms for arranging the emitter and detector components of the modules 34, 36 relative to the material under test, and vice- versa.
  • the interface assembly 38 can include or provide a fixture-type device that optically aligns the emitter and detector components along an optical path, and establishes a holding region that facilitates placement of the material under test across the optical path.
  • the interface assembly 38 can optionally include one or more optical components that facilitate or enhance optical coupling between the emitter and the detector (e.g., lens, mirror, etc.).
  • the analyzer 40 can assume a wide of forms capable of determining or indicating the IoR of the material under review.
  • the analyzer 40 can be or include a 2023-353 / U639.176.111 conventional electronic instrumentation with signal processing (e.g., off-the-shelf digital/analog circuits, time-to-digital converters (with filters), field programmable gate arrays (FPGA), etc.).
  • the analyzer 40 can be or include a processor or microprocessor, embedded microprocessor, embedded controller, digital signal processor (DSP), etc., configured to execute program code stored as software in the memory 42.
  • the program code when executed by the processor, causes the processor to implement the IoR determination functions described herein.
  • the processor can reside in any suitable computing equipment, such as a personal computer, laptop, mobile electronic device, server or cloud-based computational platform.
  • the processor can further cooperate with the memory 42 to store data.
  • Some of the computer implemented methods of the present disclosure operate one or more algorithms or equations described herein.
  • the analyzer 40 can be formatted to address metastability of the outputs of the DDMITD flip-flops FFref, FFtest (FIG. 2).
  • the analyzer 40 can include or be associated with circuitry components that operate to estimate a mid-point of the instability.
  • a sampling circuit or module 60 can be provided with, or electronically connected to, the analyzer 40 that includes an FPGA with digital filter data acquisition (DAQ) arrangement.
  • DAQ digital filter data acquisition
  • the electronic filter(s) provided with the sampling circuit 60 can be designed to precisely determine the time difference between the phase changes of the flip-flops of the DDMTD circuit 30.
  • the sampling circuit or module 60 can optionally include one or more of low-pass filters (that can function to find the estimates of the true transition of the output clocks), a phase-locked loop (PLL) carrier board (that can function to attenuate the jitter in the two output clocks), a time-difference circuit (TDC), etc.
  • PLL phase-locked loop
  • TDC time-difference circuit
  • precise measurements can be objected with the TDC 2023-353 / U639.176.111 alone, and enhanced results can be obtained with the TDC and the PLL working in tandem.
  • sampling circuity or module 70 useful with the systems and methods of the present disclosure is provided in FIG.4.
  • two clocks ck_in0 and ck_in1 are converted from optical to electrical signals with small form-factor pluggable (SFP) optical transceivers 80.
  • the offset clock is generated by a phased-locked loop carrier board (PLL) 82.
  • PLL phased-locked loop carrier board
  • the two outputs of the DDMTD circuit 30 are passed through to low pass filters (LPF) 84, 86, respectively; their outputs (beat clocks 0 and 1) are cleaned and stabilized by two PLLs 90, 92, whose output is sent to a time difference circuit (TDC ) 94 for enhanced measurements.
  • LPF low pass filters
  • TDC time difference circuit
  • Other circuity components or module configurations are also acceptable.
  • FIG. 5 illustrates, in simplified form, portions of one example of an IoR measuring system of the present disclosure arranged to measure or determine the IoR of a material under test 100.
  • the IoR measuring system of FIG. 5 can have any of configurations described above with respect to the system 20 (FIG. 1), and includes the DDMTD circuit 30, the clock signal generator 32, the transmitter module 34, the detector module 36, and the interface assembly 38 (referenced generally).
  • the transmitter module 34 includes a laser driver 120 and a laser 122
  • the detector module 36 includes a photodetector 130 and a photodetector converter circuit 132.
  • the interface assembly 38 establishes optical alignment between the laser 122 and the photodetector 130 such that a beam 134 emitted by the laser 122 is received at the photodetector 130.
  • the material under test 100 is along the optical path 134.
  • the interface assembly 38 further includes an upstream lens 140 and a downstream lens 142.
  • the lenses 140, 142 are arranged in the optical path and are configured to collimate and focus the beams of optical signals emitted upstream and downstream of the material under test 100.
  • the system can further include a controller or processor programmed to perform a measuring operation as described below.
  • the clock signal generator 32 generates an electrical source clock signal that is distributed to both the laser driver 120 and the DDMTD circuit 30 (as the reference input signal (uref) to the DDMTD circuit 30).
  • the laser driver 120 generates optical signals based off of the clock signal; the optical signals are than transmitted through the material under test 100 by the laser 122. After passing through the material under test 100, the optical signals are received or detected by the photodetector 130; the photodetector converter circuit 132 converts the received optical clock signal to an electrical signal that is distributed to the DDMTD circuit 30 (as the digital test clock signal (utest) to the DDMTD circuit 30).
  • the optical clock signal When traveling through the material under test 100, the optical clock signal will experience a delay that depends on both the refractive index of the material under test 100 and the path length.
  • the difference in phase between the heterodyned source clock and recovered or test clock can be used to measure the velocity of light in the material under test 100, for example by counting the number of CLK PLL cycles between the transitions of BEAT CLK A and BEAT CLK B.
  • the phase difference between the heterodyned clocks can be calculated to a designated sensitivity.
  • refractive index of a medium is defined as the ratio of velocity of light in vacuum to the velocity of light in that particular medium. This is a well-known principle that constraints the length, refractive index and the delay of the light pulses in a medium that can be used to estimate one given the other two.
  • refractive index n of the material under test 100 that has a path length of L that experiences a delay of Td for an optical signal of a given wavelength ⁇ can be determined by Equation 6 as follows: 2023-353 / U639.176.111 ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ Eq.6 where c is the velocity of between the refractive index with the medium and without the medium present in the cavity.
  • a conversion factor ⁇ t of the DDMTD circuit 30 for a given DDMTD offset parameter Noffset can be described by Equation 4 above.
  • the conversion factor ⁇ t can be the delay between the clocks uref and utest measured from the group delay or phase difference between the heterodyned clocks ( ⁇ t beat ).
  • ⁇ t beat can only be measured in steps of PLL clock period which leads to a theoretical sensitivity of tmin given by Equation 7 below where ⁇ ref is the time period of the reference clock ⁇ ref.
  • jitter of the clock e.g., on the order of a ps
  • the DDMTD circuit 30 can operate with a repetition rate of approximately 1 kHz.
  • the value of IoR can be repeatedly measured at that frequency with a precision determined by the thickness of the material under test. For example, for a material with an index of refraction of 1 ⁇ 4 and a thickness of 5 mm, the IoR can be measure to a precision of 0.1% every 0.1 seconds with some systems and methods of the present disclosure.
  • precisions on the order of 1e-14 seconds (10 fs) at a sampling rate on the order of 1 Hz are provided, for example by employing one or more of the sampling circuitry schemes implicated by FIG. 3.
  • the intrinsic precision can depend on the mechanical stability of the system, signal integrity of the clock through both the 2023-353 / U639.176.111 electrical and optical paths, and the stability of the clock.
  • the offset-factor Noffset can be adjusted, leading to an increase in the precision at the same sampling rate by extending the data collection over longer intervals to average out high frequency noise.
  • precision of measurements provided by the systems and methods of the present disclosure can be determined as: ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ where l is the length speed of light; ⁇ n is the difference in refractive index to the reference material; and ⁇ t is the difference in time with respect to the reference material.
  • some systems and methods of the present disclosure can be characterized as a linear system that is easy to calibrate to achieve very good accuracy. The effects of mechanical instability drop as ⁇ n approaches small values. Precision improves as l increases. While thermal effects affect both ⁇ l and ⁇ ⁇ t , this can be mitigated by increasing l and implementing temperature stabilization techniques.
  • ⁇ t can be reduced by one or more of increasing the sampling rate and reducing uncorrelated clock jitter.
  • some systems and methods of the present disclosure provide for the optimization of two major variables: sampling rate to improve ⁇ t precision and the length l of the medium being evaluated.
  • the systems and methods of the present disclosure can be used for absolute IoR measurements (subject, in some examples, to rigidity and cross-calibration) and/or relative IoR measurements in a wide variety of end-use applications.
  • 2023-353 / U639.176.111 systems and methods of the present disclosure can be applied for direct measurement of real-time variations in the index of refraction of a material as would be beneficial in pharmaceutical, petroleum, food/beverage, and other manufacturing process.
  • the systems and methods of the present disclosure are useful for IoR measurements of gases, liquids, and all materials that are transparent to the wavelength of the emitted optical clock signal.
  • the interface assembly 38 generally reflected by FIG. 5 is but one arrangement envisioned by the present disclosure.
  • the interface assemblies of the present disclosure can incorporate or implement various other optical components, can effect different arrangements of the emitter and detector components relative to a material under test, can incorporate additional emitters/detectors, etc.
  • FIG.6 illustrates, in simplified form, another interface assembly 150 useful with the IoR measuring systems and methods of the present disclosure.
  • the interface assembly 150 maintains a laser (or other emitter device) 152 in optical alignment with a photodetector (or other detector device) 154, and establishes a holding region or cavity 156 (referenced generally) for placement of a material under test 158 relative to the optical path.
  • the interface assembly 150 includes first and second concave mirrors 160, 162.
  • the first mirror 160 is optically between the laser/emitter 152 and the material under test 158, and is configured to transmit in one direction while reflecting in the other through a window aligned with the optical beam.
  • the second mirror 162 is optically between the material under test 158 and the detector/photodetector 154 and is configured to transmit only a fraction of the incident optical beam, characterized by a transmission coefficient T of the mirror 162 and reflects based on a reflection coefficient R of the mirror 162.
  • the delay caused by the material under test 158 is 2023-353 / U639.176.111 multiplied by the number of reflections that occur in the cavity 156.
  • the number of reflections considered by the delay calculation can be adjusted by adjusting, for example, the voltage threshold of the photo-diode driver circuit.
  • the refractive index of, for example, a gaseous medium can readily be characterized provided the values of R, T, and a length L of the cavity 156 are chosen correctly. The precision could improve by a factor that depends on the number of reflections, and this parameter is readily tunable. Additional features can be included with the arrangement of FIG.6 to provide for high mechanical stability (as mechanical noise may also be amplified by the arrangement).
  • FIG.6 Another interface assembly 200 useful with the IoR measuring systems and methods of the present disclosure is shown in simplified form in FIG.7.
  • the interface assembly 200 is akin to the arrangement of FIG.5, arranging a material under test 202 along an optical path from a laser (or other emitter) 210 to a detector 212.
  • a reference laser 220 and reference detector 222 are provided.
  • the lasers 210, 220 and the detectors 212, 222 are identical, as is the path length/cavity between the corresponding components.
  • any optical components e.g., lenses
  • the refractive index of the material under test 202 is measured (including operation of the laser 210 to emit an optical clock signal that is sensed by the detector 212 as described above) simultaneously with that of the reference cavity/no material (including operation of the reference laser 220 to emit the identical optical clock signal that is sensed by the reference detector 222).
  • the data or information from the reference arrangement can be used to cancel out any correlated fluctuations (e.g., arising due to environmental effects). Comparing the relative phases of the two recovered signals can be used to measure the absolute IoR value.
  • FIG. 8 Another interface assembly 250 useful with the IoR measuring systems and methods of the present disclosure is shown in simplified form in FIG. 8.
  • the arrangement includes a first emitter 260 (e.g., laser), a first detector (e.g., photodiode) 262, a second emitter 270 and a second detector 272.
  • the interface assembly 250 optically aligns the first emitter 260 with the first detector 262 as represented by a first beam 280, and the second emitter 270 with the second detector 272 as represented by a second beam 282.
  • the emitters 260, 270 are maintained relative to one other, and relative to a material under test 290, such the first and second beams 280, 282 pass through the material under test 290 and collectively define an intersection angle ⁇ .
  • the arrangement of FIG.8 can serve to reduce mechanical instability.
  • the effects of mechanical instability can be rectified by correlating the delayed signal from both of the detectors 262, 272.
  • Equation 10 below accounts for the delayed signal from the first detector 262 (denoted by “+” in the equation) and from the second detector 272 (denoted by “–“ in the equation) and is uniquely solvable for n( ⁇ ) for rotation of the material test by a small, unknown angle ⁇ .
  • FIG. 9 Another interface systems and methods of the present disclosure is shown in simplified form in FIG. 9.
  • the arrangement of FIG. 9 includes an emitter (e.g., laser) 310 and detector (e.g., photodiode) 312 as described above, and a mirror 314.
  • the interface assembly 300 maintains the emitter 310 relative to the detector 312 and the mirror 314 such that the mirror 314 reflects an emitted beam 316 to the detector 312, and provides a holding region within which a material under test is placed along the beam path.
  • the configuration of FIG.9 locates the emitter 310 and the detector 312 on the same side of a material under test 320, facilitating same side measurements.
  • the fan-out chips Fan Out were each a buffer, equalizer receiver fan-out NBVQ1006M from ON Semiconductor Corp.
  • the transmission module included a laser driver and a laser; the receiver module included a photodetector (photodiode) and conversion circuitry.
  • the laser transmitter and the photodiode receiver were provided as part of an SFP transceiver available under the trade designation FTLF1318P3BTL from Finisar Corp. Lens elements were placed in the optical path to both collimate and focus the beams of optical signals.
  • Off-the- shelf components were used to complete the various connections, including a fixed fiber-to-fiber coupler (FBC-1310-APC from Thorlabs) and optical fibers.
  • the emitter, detector and other optical components were selected for experiments with optical light at a wavelength of 1310 nm for convenience/use of off-the-shelf components.
  • Outputs from the DDMTD circuit e.g., BEAT CLK A, BEAT CLK B, CLK PLL
  • Tests were performed on a transparent cube of unknown refractive index with dimensions of 39.5 mm x 39.5 mm x 39.5 mm.
  • Data 2023-353 / U639.176.111 reported below were collected in bursts over a period of 30 – 35 minutes, with and without the test material inserted into the optical path through multiple acquisitions, acquiring approximately 200 milliseconds (ms) of data each time. After each burst, the data were analyzed, recorded and the measurement resumed. The total acquisition time for the data was approximately 10 seconds.
  • FIG. 11 shows the expected precision as a function of the sampling rate with the data that were collected, showing stability ⁇ ⁇ n( ⁇ ) as a function of the sampling rate. As a point of reference, lower ⁇ n( ⁇ ) implies better precision. The number of samples considered for the standard deviation is shown next to each data point in the plot of FIG.11. [45] FIG.
  • FIG. 12 shows the effect on the signal phase when introducing the transparent cube into the optical path of the clock signal, and in particular the effect of the delay ⁇ t between u ref and u test measured with the test material in the optical path.
  • the delay was averaged over 0.1 second and was acquired at a rate of 1.6 kHz.
  • the data was not acquired continuously, but approximately 30 – 35 minutes of data with and without the test material in the optical path was taken, out of which approximately 5 seconds worth of data was collected (both with and without the test material).
  • the plots of FIGS.13A and 13B show the stability of the measurements without the material inserted into the optical path at a sampling rate of approximately 10 Hz.
  • FIGS. 14A and 14B show the stability of the measurements with the material inserted into the optical path at a sampling rate of approximately 10 Hz.
  • FIG. 14A reports the delay ⁇ t between u ref and u test as a function of time of acquisition;
  • FIG. 14B provides a histogram of measurements from which it was concluded that the stability ⁇ t of the measurements with the test material in the optical path to be 120 fs.
  • the refractive index of the test material was calculated to be 1.5719 ⁇ 0.0001 averaging the full data sample. This represents a precision of 100 ppm or one part in 10 4 at the sampling rate of approximately 0.2 Hz. At a sampling rate of 10 Hz, the precision becomes approximately 700 ppm for each measurement, increasing at 1.6 kHz acquisition rate to 5400 ppm.
  • Another IoR measuring system was prepared in accordance with the topology of FIG. 3 and included the DDMDT circuit as described in the Examples above.
  • the offset clock generated by a PPL was shared between the DDMTD board and a low-pass filter board.
  • the two outputs of the DDMTD board were passed through to the low-pass filters; their outputs (beat clocks 1 and 2) were sent to a TDC board.
  • the clock generator board had an inbuilt delay chip capable of shifting the skew of the clock in steps of approximately 5 ps. After optimizing various factors such as the topology, PLL, and TDC configurations, a precision of approximately 10 fs at a sampling rate of approximately 1 Hz for the noise floor was achieved. For example, FIG.15 reports a single shot measurement, collecting 399 data-points in a time-window of approximately 1.3 seconds.
  • ⁇ of the distribution was 92.33 ⁇ 0.01 ps with a ⁇ of 210 fs.
  • FIG.16 reports the repeated measurements showing the fluctuations in the single- shot measurements.
  • the standard deviation ( ⁇ ) fluctuations in the mean is 11.32 fs.
  • the clock was routed through an optical cavity using two SFPs where were known to introduce 2023-353 / U639.176.111 jitter. The results of this test are reported in FIG. 17 that otherwise provide a single shot measurement of clock phase through the optical cavity averaged over an acquisition time window of 9.6 seconds to retrieve back the precision.
  • ⁇ of the distribution is 1518.910 ⁇ 0.014 ps with a ⁇ of 870 fs.
  • the systems and methods of the present disclosure provide a marked improvement over previous designs. Index of refraction of a medium or material (solid, liquid or gas) can be measured in real time at exceedingly high levels of precision. Unlike refractometers, the systems and methods of the present disclosure do not rely on a measurement of total internal reflection at a surface; instead, the refractive index across the material is measured (and which may not be the same as at the surface). Thus, surface deposits on the material or object under evaluation (e.g., unexpected deposits on a glass window) will not substantively affect measurements obtained by the systems and methods of the present disclosure. Further, the systems and methods of the present disclosure entail static measurement without any moving parts, rendering the systems simple to build, maintain, and calibrate.

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  • General Health & Medical Sciences (AREA)
  • General Physics & Mathematics (AREA)
  • Immunology (AREA)
  • Pathology (AREA)
  • Investigating Or Analysing Materials By Optical Means (AREA)

Abstract

L'invention concerne des systèmes et des procédés de mesure ou de détermination de l'indice de réfraction d'un matériau (100) à l'aide d'un signal d'horloge numérique propre (faible oscillation). Un signal électrique, l'horloge source, est généré et converti en un signal optique. Le signal optique est transmis à travers le matériau testé (100) puis détecté avec un capteur optique (130, 132) qui convertit le signal en un signal électronique (le signal récupéré). La différence de phase entre l'horloge source et l'horloge récupérée, telle que générée par un circuit numérique de mesure de différence de temps à double mélangeur DDMTD (30), est utilisée pour mesurer la vitesse de la lumière dans le matériau testé (100). Dans certains modes de réalisation, le DDMTD (30) incorpore des circuits intégrés RF de modem pour obtenir un niveau de précision élevé, capables de mesurer un changement de la longueur de trajet optique d'un signal à une précision de 100 femtosecondes, éventuellement à une précision de 10 femtosecondes.
PCT/US2024/060908 2023-12-19 2024-12-19 Systèmes et procédés de détermination d'un indice de réfraction d'un matériau Pending WO2025137196A1 (fr)

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4289403A (en) * 1977-03-04 1981-09-15 Isco, Inc. Optical phase modulation instruments
US4325635A (en) * 1980-10-07 1982-04-20 The United States Of America As Represented By The Secretary Of The Army Heterodyne indicial refractometer
JPH04121642A (ja) * 1990-09-12 1992-04-22 Brother Ind Ltd 光集積型ヘテロダイン干渉屈折率分布測定装置
JPH1062115A (ja) * 1996-08-23 1998-03-06 Nippon Soken Inc レーザ干渉計
US20040135992A1 (en) * 2002-11-26 2004-07-15 Munro James F. Apparatus for high accuracy distance and velocity measurement and methods thereof

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4289403A (en) * 1977-03-04 1981-09-15 Isco, Inc. Optical phase modulation instruments
US4325635A (en) * 1980-10-07 1982-04-20 The United States Of America As Represented By The Secretary Of The Army Heterodyne indicial refractometer
JPH04121642A (ja) * 1990-09-12 1992-04-22 Brother Ind Ltd 光集積型ヘテロダイン干渉屈折率分布測定装置
JPH1062115A (ja) * 1996-08-23 1998-03-06 Nippon Soken Inc レーザ干渉計
US20040135992A1 (en) * 2002-11-26 2004-07-15 Munro James F. Apparatus for high accuracy distance and velocity measurement and methods thereof

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
ROHITH SARADHY ET AL: "A sub-picosecond digital clock monitoring system", JOURNAL OF INSTRUMENTATION, vol. 18, no. 1, 19 January 2023 (2023-01-19), XP020442375, DOI: 10.1088/1748-0221/18/01/T01003 *

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