Device for managing pulses in pump-probe spectroscopic analysis

Description

[0001] The present invention relates to a device for managing light pulses for measuring the reaction of a sample exposed to a first light pulse called a "pump" pulse, the measurement being carried out by analyzing a light signal emitted by the sample subjected to a second light pulse, called a "probe" pulse, offset relative to the "pump" pulse by a determined time interval.

[0002] The field of interest is that of pump-probe spectroscopy and, in general, any application which requires the use of two ultrashort laser pulses separated by a known and variable time delay allowing the scanning of the total time interval to be investigated.

[0003] In recent years, studies on the dynamics of biological systems have been the subject of much work. It has been shown, in particular, that such dynamics can present, in certain cases, a continuous evolution over several temporal orders ranging from picoseconds to seconds.

[0004] Pump-probe spectroscopy is a technique that allows the temporal dynamics of a system to be measured during a reaction: a pump pulse triggers a reaction, then a time-shifted probe pulse measures the induced changes. It is then necessary to be able to shift one laser pulse relative to another throughout the time regime studied, this shift having to be accurate to the nearest picosecond.

[0005] Application US 2008 / 186486 discloses a laser flash photolysis device for measuring the reaction of a sample exposed to a first “pump” light pulse, the measurement being carried out by analyzing a signal emitted by the sample subjected to a second “probe” light pulse offset by a time interval regulated by a delay generator.

[0006] US 5,258,612 A describes a device for a pump-probe experiment using the accumulation of a charge of a capacitor in order to measure the time interval between a first "pump" light pulse and a second "probe" light pulse.

[0007] Currently, there are three ways to generate two separate pulses of a known and variable delay.

[0008] The first method involves using a single laser system with the beam split into two arms. One arm is delayed relative to the other by a mechanical delay line. This system is limited by the length of the delay line in the picosecond-nanosecond range. In addition, it requires special attention to maintain sufficient pointing stability during mechanical scanning.

[0009] The second method is to use two unsynchronized laser oscillators. The delay is chosen by selecting, for amplification, the pair of pulses with the delay closest to the desired delay. The selection is generally carried out by opening and closing a Pockels cell around the desired pulse. Because of the asynchronism between the two laser oscillators and the resulting time jitter, the actual delay between the two pulses thus selected is known with an accuracy that cannot fall below the oscillation period of the laser cavities, typically of the order of ten nanoseconds. On the other hand, this system allows arbitrarily long delays to be achieved.

[0010] The third method involves using two synchronized laser oscillators whose delay is varied by varying their relative time phase. Combined with pre-amplification selection as described in the second method, this system overcomes the limitations of the previous solutions by allowing long delays to be achieved while maintaining a precision of the order of a picosecond. However, it requires a costly and complex implementation that cannot be achieved retrospectively on standard commercial laser oscillators, unless this functionality has been planned by the manufacturer during the laser design. This system is therefore not accessible to most potential users.Similarly, it is possible to use laser oscillators of different frequencies by exploiting the principle of stroboscopy applied to the femtosecond domain, the temporal coincidence between the two pulse trains being measured by an optoelectronic device. However, in the proposed implementations, this approach requires, to work well, that the frequencies of the two oscillators are close and known with a very high precision which requires an active control of the laser cavity lengths. This is generally not the case if it has not been planned before the acquisition of the systems.

[0011] These different methods have been described in various publications, the main ones being: Bredenbeck, Helbing and Hamm., Rev. Sci. Instrum. Vol. 75 p. 4462. This 2004 article describes a synchronous electron scanning solution proposed by the Institute of Physical Chemistry in Zurich. Long delays are given as multiples of the oscillator repetition period through the selection by a Pockells cell of the pulse to be amplified. Short delays are generated by introducing a known phase between the two laser oscillators. With this method, it is possible to achieve an accuracy of 2 ps over an interval extending up to 50 µs.

[0012] Takagi and Adachi (RSI Vol. 70, p. 2218) introduced the asynchronous optical scanning method in 1999. The scanning is achieved by introducing a known repetition rate difference between the two oscillators. If the introduced difference is negligible compared to the repetition frequency f 1 and f 2 of the two oscillators, it has been shown that this results in a magnification of the time axis f 1 / f 2 and consequently an increment in the spectral resolution. In Takagi's work, a time magnification of 760,000 times was measured.

[0013] In 2004 (Keimann, Goble, and Holzwarth, “Time domain mid-infrared frequency-comb spectrometer,” Opt. Lett. 29, p. 1542 (2004)), this advantage was improved to a time magnification of 45,500,000 times, corresponding to 13 cm-1 of spectral resolution. This work was carried out with femtosecond lasers, emphasizing the discrete nature of pulse spectra, which paved the way for “Frequency Comb Spectroscopy,” which exploits the beating phenomenon between each frequency line constituting the spectrum of the two shifted pulses. The advantages over conventional Fourier transform spectroscopy experiments are in the measurement sensitivity, spectral width, and resolution achieved with measurement times of a few tens of microseconds.

[0014] An optoelectronic synchronization method has also been proposed in patent application WO / 2007 / 045773 but the time resolution obtained is only interesting when the frequencies of the two oscillators are close.

[0015] It would therefore be advantageous to obtain a laser pulse management device allowing a delay between two pulses to be obtained which can vary from the picosecond to the second with picosecond precision and this using standard femtosecond lasers.

[0016] To solve one or more of the drawbacks cited above, the devices according to claims 1, 2 and 3 are the subject of the present invention.

[0017] These devices thus advantageously allow to rely on a free operation of the two oscillators, operation not requiring any control and feedback system of the optical cavities. In addition, the time precision is not directly proportional to the frequency difference between the two oscillators and can thus be easily less than the picosecond for the typical frequencies used in standard lasers. Thus, most of the time, it is simple to implement the device on an experiment using two pre-existing amplified laser chains, just by sending a small fraction of the oscillator beams on the device.

[0018] Particular features or embodiments, usable alone or in combination, are: The device further comprises two pulse selectors positioned respectively on each of the two beams, each selector being adapted to transmit to a sample a particular pulse of the beam on which it is positioned, the transmitted pulse being selected by the computer so that the time interval between the selected pulse of the first beam and the selected pulse of the second beam is a predetermined value; the pulse selectors are light amplifiers; the computer is adapted to select a series of pairs of pulses, each pair being composed of a pulse of the first beam and a pulse of the second beam, such that all the time intervals of the series of pairs of pulses allow sampling of a predetermined measurement interval.

[0019] The invention will be better understood by reading the following description, given solely by way of example, and with reference to the appended figures in which: there figure 1 is a schematic view of a pump-probe spectroscopy system comprising a device according to a first embodiment of the invention; the figure 2 is a timing diagram of the pulses of two lasers having different repetition frequencies according to the principle of asynchronous scanning and showing the coincidences; figure 3 is a schematic view of an electronic assembly of an acquisition card of the device of the figure 1 ; there figure 4A is a timing diagram of the electronics of the figure 3 showing a coincidence signal in a phase where the two lasers are not in coincidence; figure 4B is a timing diagram of the electronics of the figure 3 showing a coincidence signal in a phase where the two lasers are in coincidence; figure 5 is a flowchart of the operation of the device of the figure 1 ; and the figure 6 is a schematic view of a pump-probe spectroscopy system comprising a device according to a second embodiment of the invention.

[0020] In reference to the figure 1 , a device operating on the principle of heterodyne sampling comprises, in a conventional manner, a pulsed “pump” laser source 1 and a pulsed “probe” laser source 3 emitting respectively a “pump” beam and a “probe” beam. The two laser sources are typically femtosecond lasers therefore emitting pulses of the order of a hundred femtoseconds. The “pump” and “probe” pulse durations are equal or not. Similarly, the central wavelengths of the “pump” and “probe” beams are equal or not depending on the measurement to be carried out.

[0021] Each beam passes through an optical amplifier 5, 7 before being combined together by a combiner 9. For example, the combiner 9 comprises a mirror and a semi-transparent plate.

[0022] The beams thus combined are then directed towards an experimental device 11 in which a sample to be measured is placed.

[0023] The response of the sample is received by a photo-detector 13 then transmitted to an acquisition system 15.

[0024] In the embodiment of the light pulse management device of the figure 1 , a sampling blade 17, 19 is installed on the path of each beam at the output of each laser source 1, 3 so as to capture on photo-detectors 21, 23 a trace of each pulse. Those skilled in the art understand that the transparency of the blades is chosen so as to only convey to the photo-detectors 21, 23 the power necessary for their operation.

[0025] The photo-detectors 21, 23 are connected to a computer 25.

[0026] The computer 25 includes an interface for controlling the optical amplifiers 5, 7 and is connected to the acquisition system 15.

[0027] The operation of the device is as follows.

[0028] Generally, two oscillators designed in free operation produce two combs of pulses 31, 33 of different and highly stable frequencies. The relative delay, or relative time phase, between the two combs gradually scrolls between the moment when a coincidence occurs and the next coincidence according to the principle of asynchronous scanning, figure 2 .

[0029] Ω 1 and Ω 2 being the respective frequencies of the lasers, it is possible to define a time phase for each of the lasers: Φ 1 = Ω 1 (t-τ 1 ) and Φ 2 = Ω 2 (t-τ 2 ) (τ 1 and τ 2 defining the time shift of the two pulse trains and t being the time coordinate). These laws being known, it is then possible to accurately predict the relative phase ΔΦ = Φ 2 - Φ 1 , and therefore the delay between all the pulses produced by the two oscillators.

[0030] To measure the phase laws of the two oscillators, the calculator detects the coincidence pulses while accurately counting the number of pulses produced by each of the two lasers between the coincidences. This law of evolution of the time interval between two pulses is therefore determined by interpolation or extrapolation. This makes it possible to determine in particular Ω 2 / Ω 1 with very high precision.

[0031] Indeed, let Δt be the time interval during which we can assume that the oscillators remain perfectly stable, i.e. that there is no measurable jitter phenomenon. With current oscillators, Δt is typically of the order of 1 ms. T is the period of the pulse train and is equal to about 10 ns for 100 MHz oscillators and δτ the time precision of the coincidence measurement.

[0032] The number of pulses produced during the period Δt is N = Δt / T, which therefore corresponds to a number of coincidences of the order of n = N δτ / T. Taking into account the stability of the oscillators, the precision on the measurement of the parameters τ 1 and τ 2 is therefore of the order of δτ / n The ultimate accuracy of the device is therefore linked to the method used to detect coincidences.

[0033] In the embodiment of the figure 3 , coincidence detection is completely electronic.

[0034] The photo-detectors 21, 23 are fast photodiodes whose electrical signal is sent to an electronic card of the computer 25.

[0035] The electrical signals are sent to two D flip-flops 41, 43. One of the signals is used as a common clock, the other signal is distributed successively to the input of the first flip-flop then of the second flip-flop with a slight delay, τ' for a first delay line 45 and τ" for a second delay line 47. At each new clock pulse, a coincidence is signified by a difference in state between the outputs of the flip-flops.

[0036] THE figures 4A And 4B show the timing diagram of signals 51, 53 at the output of delay lines 45, 47 and of signal 55 at the output of the photodetector and serving as a clock for the two D flip-flops 41, 43. Signal 57 shows the signal at the output of the exclusive OR combination of the two outputs of the D flip-flops. figure 4A shows the signals when the two lasers are not in coincidence. Thus, signal 57 remains at 0 and the figure 4B shows the signals during a coincidence, signal 57 then switching from 0 to 1.

[0037] The inventors have built a prototype according to this scheme which allows a precision of the order of δτ ≈ 30 ps. Two optical fibers coupled to two Si PIN photodiodes were connected in differential pairs to the coincidence detector and the computer implemented in a Spartan family FPGA from the manufacturer Xilinx. With the above numerical values, this gives a rate of 300 coincidences per millisecond. Laser frequencies sufficiently stable with respect to this rate allow these coincidences to be averaged to obtain a final accuracy slightly lower than 2 ps. This value can be further improved by using a specialized circuit (ASIC) or by increasing the coincidence rate by multiplying the number of electronic channels with different and known delays.

[0038] It should be noted as a variant that it is also possible to use known devices of the digital time converter type (in English “time to digital converter”).

[0039] This makes it possible to exploit pulse pairs even when there is no coincidence and therefore to increase the number n of acquisitions contributing to the measurement and thus to improve the precision of the latter.

[0040] So, figure 5 , having carried out, step 51, a precise measurement of the phase laws of the two oscillators, the computer is then capable of determining, step 53, the value of the time shift between any pulse of the “pump” signal and any subsequent pulse of the “probe” signal. And therefore, for a determined shift value, it is capable of selecting, step 55, a pair of “pump” - “probe” pulses having this time shift value.

[0041] The computer 25 then controls, step 57, the amplifiers 5, 7 to only allow the corresponding pulses to pass through and amplify them. In parallel, it informs, step 59, the acquisition system 15 of the delay between the “pump” pulse and the “probe” pulse.

[0042] As indicated in the preamble, in "pump"-"probe" spectroscopy, we seek to measure the reaction of a sample over a varied time range separating the "pump" pulse from the "probe" pulse in order to study the dynamics of the sample.

[0043] Thanks to the device described, it is sufficient to previously configure the range of delays to be studied, for example from 1ns to 1ms, and the number of samples, for example with a step of 1 ps and the computer is then able to select the series of pulses covering the requested range. It will be noted that the order of the measurements can be arbitrary and depend only on the sequence of the pulses so as to minimize the measurement time. Also, the computer 25 is connected to the acquisition system 15 so as to provide the latter with the information on the delay between pulses corresponding to the measurement in progress. A simple sorting process on the duration of the delays then makes it possible to order the measurements. An interesting variant consists of not using the selectors 5 and 7 which makes it possible to exploit all the pairs of pulses produced by the two oscillators 1 and 3, the delays being determined a posteriori by the computer.In this case, the time dynamics is limited to the period of the oscillators typically of the order of 10ns.

[0044] In a second embodiment, figure 6 , the photo-detectors 21, 23 and the electronic acquisition card are replaced by an optical device.

[0045] This comprises an interferometer 61, possibly fibered, on which the two laser beams are aligned. The coincidences corresponding to the temporal superposition of two pulses are detected by the optical linear interferences generated. The interference signal is detected with a photodiode 63 and sent to an acquisition card of the computer 25 which compares it to a threshold value. Alternatively, it is possible to use a photodiode on each of the two outputs of the interferometer to carry out differential detection.

[0046] Optical coincidence detection as described allows for a precision δτ of the order of the pulse duration, typically 100fs. With the numerical values ​​above, this gives a coincidence rate n / Δt of the order of one coincidence per millisecond. It is possible to increase the coincidence rate at the expense of time precision by placing a filter reducing the spectral band before the photodetector. Such a filter is also necessary when the spectra of the two oscillators are not identical. Finally, in the case where the spectral overlap between the two oscillators is zero, coincidences can be detected using a non-linear optical process (two-photon absorption, frequency summation, etc.).

[0047] The invention has been illustrated and described in detail in the drawings and the foregoing description. The foregoing description should be considered illustrative and exemplary and not as limiting the invention to this description alone.

[0048] In the claims, the word "comprising" does not exclude other elements and the indefinite article "a / an" does not exclude a plurality.

Claims

1. Device for managing light pulses for measuring the reaction of a sample exposed to a first light pulse called the "pump" pulse, the measurement being performed by analysis of a signal emitted by the sample subjected to a second light pulse, called the "probe" pulse, shifted with respect to the "pump" pulse by a determined time interval, comprising: • two pulsed laser sources (1, 3) of ultra-short pulses; • two optical detectors (21, 23) suited to detecting the pulses of two light beams emitted by the two pulsed laser sources (1, 3) of ultra-short pulses respectively, the first beam being called the "pump" beam and the second beam being called the "probe" beam, each source emitting pulses with respective repetition frequencies that are different and arbitrary in the direction of said sample; the detectors being connected to • a computer (25) suited to determining the time interval between two pulses coming from the first and the second beam, respectively, and constituting the "pump" and "probe" pulses for measuring the reaction of the sample; and • an acquisition system (15) for analysing the measurement of the reaction of the sample, having as input parameter the time interval determined by the computer (25) for the measurement involving the two pulses; • said computer (25) comprising a connector to the acquisition system (15), characterized in that the repetition frequencies of the sources are stable over a determined period (Δt) and in that the computer (25) uses an algorithm making use of the stability of the repetition frequencies to determine said time interval on the basis of measurement of the phase laws of the sources, the computer (25) being suited to detecting time coincidences between a pulse of the first beam and a pulse of the second beam, and to counting the number of pulses produced by each beam between two coincidences in order to determine the law of evolution for the time interval between two pulses by interpolation or extrapolation.

2. Device for managing light pulses for measuring the reaction of a sample exposed to a first light pulse called the "pump" pulse, the measurement being performed by analysis of a signal emitted by the sample subjected to a second light pulse, called the "probe" pulse, shifted with respect to the "pump" pulse by a determined time interval, comprising: • two pulsed laser sources (1, 3) of ultra-short pulses; • two optical detectors (21, 23) suited to detecting the pulses of two light beams emitted by the two pulsed laser sources (1, 3) of ultra-short pulses respectively, the first beam being called the "pump" beam and the second beam being called the "probe" beam, each source emitting pulses with respective repetition frequencies that are different and arbitrary in the direction of said sample; the detectors being connected to • a computer (25) suited to determining the time interval between two pulses coming from the first and the second beam, respectively, and constituting the "pump" and "probe" pulses for measuring the reaction of the sample; and • an acquisition system (15) for analysing the measurement of the reaction of the sample, having as input parameter the time interval determined by the computer (25) for the measurement involving the two pulses; • said computer (25) comprising a connector to the acquisition system (15), characterized in that the repetition frequencies of the sources are stable over a determined period (Δt) and in that the computer (25) uses an algorithm making use of the stability of the repetition frequencies to determine said time interval on the basis of measurement of the phase laws of the sources, the optical detectors (21, 23) being combined in the form of an interferometer (61), the interference signal of which is detected by at least one photodiode (63) in such a way that the temporal superposition of two pulses triggers a coincidence signal for the computer (25).

3. Device for managing light pulses for measuring the reaction of a sample exposed to a first light pulse called the "pump" pulse, the measurement being performed by analysis of a signal emitted by the sample subjected to a second light pulse, called the "probe" pulse, shifted with respect to the "pump" pulse by a determined time interval, comprising: • two pulsed laser sources (1, 3) of ultra-short pulses; • two optical detectors (21, 23) suited to detecting the pulses of two light beams emitted by the two pulsed laser sources (1, 3) of ultra-short pulses respectively, the first beam being called the "pump" beam and the second beam being called the "probe" beam, each source emitting pulses with respective repetition frequencies that are different and arbitrary in the direction of said sample, the optical detectors each having one photodiode suited to transforming each pulse into an electrical signal acting as input signal for an electronic "time-to-digital converter" (TDC) circuit; the detectors being connected to • a computer (25) suited to determining the time interval between two pulses coming from the first and the second beam, respectively, and constituting the "pump" and "probe" pulses for measuring the reaction of the sample; and • an acquisition system (15) for analysing the measurement of the reaction of the sample, having as input parameter the time interval determined by the computer (25) for the measurement involving the two pulses; • said computer (25) comprising a connector to the acquisition system (15), characterized in that the repetition frequencies of the sources are stable over a determined period (Δt) and in that the computer (25) uses an algorithm making use of the stability of the repetition frequencies to determine said time interval on the basis of measurement of the phase laws of the sources.

4. Device according to Claim 1, 2 or 3, characterized in that it furthermore comprises two pulse selectors (5, 7) positioned on each of the two beams, respectively, each selector being suited to transmitting toward a sample a particular pulse of the beam on which it is positioned, the transmitted pulse being selected by the computer (25) in such a way that the time interval between the selected pulse of the first beam and the selected pulse of the second beam has a predetermined value.

5. Device according to Claim 4, characterized in that the pulse selectors (5, 7) are light amplifiers.

6. Device according to any one of the preceding claims, characterized in that the computer (25) is suited to selecting a series of pulse pairs, each pair being composed of a pulse of the first beam and of a pulse of the second beam, in such a way that all of the time intervals for the series of pulse pairs allow sampling of a predetermined measurement interval.