Nonlinear optical device and nonlinear optical system

Abstract

A nonlinear optical device (102) comprises: an emitting unit (106) for generating pump light, Stokes light, first probe light, and second probe light delayed from the first probe light, and emitting the light at a sample; and a detector (108) for detecting the intensity of scattered light from the sample. By emitting pump light and Stokes light at the sample, resonance and non-resonant background (NRB) are generated, and when the first probe light is emitted at the sample, the intensity of a rising edge part of a non-resonant background (NRB) signal is detected without detecting a resonance signal, and when the second probe light is emitted at the sample, the intensity of a signal in which a falling edge part of the NRB signal and the resonance signal are mixed is detected. The optical path lengths of the first probe light and the second probe light are set such that the intensity of the rising edge part of the NRB signal is equal to the intensity of the falling edge part.

Description

Nonlinear optical devices and systems

[0001] The present invention relates to nonlinear optical devices and systems.

[0002] In nonlinear optical processes such as coherent anti-Stokes Raman scattering (CARS), molecular vibrations in a sample are excited by irradiating it with two laser beams (pump beam and Stokes beam) of different angular frequencies, and scattered light from the sample is detected by irradiating it with a probe beam. Resonance occurs when the difference in angular frequency between the pump beam and the Stokes beam matches the angular frequency of a vibrational mode of the target molecule, while non-resonant background occurs due to the presence of components other than the target molecule (e.g., water). Recently, various methods have been proposed to remove the non-resonant background signal.

[0003] The first method for removing the non-resonant background signal involves separately measuring a sample consisting only of background other than the target molecule (hereinafter referred to as the BG sample) and an actual sample in a nonlinear optical system. When the BG sample is irradiated with pump light, Stokes light, and probe light, a non-resonant background signal is detected. On the other hand, when the pump light, Stokes light, and probe light are irradiated on an actual sample, a signal containing a mixture of a strong non-resonant background signal and a weak resonant signal is detected. The non-resonant background signal can be removed by performing spectral analysis using the measurement data of the BG sample and the measurement data of the actual sample.

[0004] The second method for eliminating the non-resonant background signal is to delay the probe light relative to the pump light and Stokes light. The non-resonant background signal is obtained by elastic scattering, so it appears immediately upon simultaneous irradiation with the pump light, Stokes light, and probe light. On the other hand, in the resonance process, the resonant signal appears later than the non-resonant background signal due to the interaction between the photon and the electron of the target molecule. The second method utilizes the delay of the resonant signal to irradiate the probe light at a timing when the non-resonant background signal decays and the resonant signal becomes dominant. This allows measurement data with a high ratio of resonant signal to non-resonant background signal to be obtained (see, for example, Non-Patent Documents 1 and 2).

[0005] Gombojav O. Ariunbold, Supriya Nagpal, Bryan Semon, “Quantitative time-resolved buildup in coherent anti-Stokes Raman scattering,” arXiv:1912.12575v1 [physics.optics] 29 Dec 2019Romedi Selm, Martin Winterhalder, Andreas Zumbusch, Guenther Krauss, Tobias Hanke, Alexander Sell, and Alfred Leitenstorfer, “Ultrabroadband background-free coherent anti-Stokes Raman scattering microscopy based on a compact Er:fiber laser system,” Optics Letters Vol. 35, No. 19, pp. 3282-3284 (2010)

[0006] However, the first method described above requires sample switching in order to measure the BG sample and the actual sample separately in the nonlinear optical system. This may affect the optical components of the nonlinear optical system and also poses the problem of poor operability. The second method described above only obtains measurement data with a high proportion of resonant signals, but does not reliably remove non-resonant background signals. In particular, the second method is not effective for samples in which the contribution of background is overwhelmingly large, such as solutions with a much higher solvent content than solute. Therefore, when using the second method, it is better to measure the BG sample and the actual sample separately, as with the first method, and sample switching is required.

[0007] The present invention has been made in view of the above-mentioned problems, and has as its object to provide a nonlinear optical device and system capable of removing nonresonant background signals without replacing the sample.

[0008] A nonlinear optical device according to one aspect of the present invention includes an irradiation unit that generates at least broadband light having a predetermined bandwidth, first probe light, and second probe light delayed from the first probe light as incident light onto a sample to be measured, and irradiates the sample with the incident light, and a detector that detects the intensity of scattered light from the sample. When the irradiation unit irradiates the sample with at least the broadband light, resonance due to molecular vibrations of the sample and non-resonant background are generated, when the irradiation unit irradiates the sample with the first probe light, the detector does not detect the intensity of a resonance signal due to molecular vibrations, but detects the intensity of a rising portion of a non-resonant background signal generated due to the presence of the non-resonant background as a first scattering signal, when the irradiation unit irradiates the sample with the second probe light, the detector detects the intensity of a second scattering signal in which a falling portion of the non-resonant background signal and a resonance signal are mixed, and the optical path lengths of the first and second probe lights in the irradiation unit are set so that the intensities of the rising and falling portions of the non-resonant background signal are equal.

[0009] A nonlinear optical system according to one aspect of the present invention includes the above-described nonlinear optical device and a computer connected to a detector of the nonlinear optical device and performing a spectral analysis of light scattered from a sample.

[0010] According to the present invention, the optical path lengths of the first and second probe lights are set so that the intensity of the rising edge of the non-resonant background signal as the first scattering signal and the intensity of the falling edge of the non-resonant background signal as the second scattering signal are equal, thereby irradiating the same sample with the first and second probe lights, thereby making it possible to remove the non-resonant background signal based on the first and second scattering signals without replacing the sample.

[0011] Fig. 1 is a configuration diagram illustrating an example of a nonlinear optical system according to an embodiment of the present invention. Fig. 2 is a schematic diagram illustrating incident light on a sample and scattered light from the sample. Fig. 3 is a schematic diagram illustrating a temporal change in the intensity of incident light on a sample and a temporal change in the intensity of scattered light from the sample. Fig. 4 is a schematic diagram illustrating a mechanism for varying the optical path length of a first probe light and a second probe light. Fig. 5 is a graph showing Raman spectra of acetaminophen at different concentrations.

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings, in which the same or similar components are designated by the same reference numerals throughout the drawings.

[0013] 1 shows an example of the configuration of a nonlinear optical system 100 according to this embodiment. The nonlinear optical system 100 includes a nonlinear optical device 102 and a computer 104. The nonlinear optical device 102 includes an irradiation unit 106 that generates incident light and irradiates the incident light onto a sample S, and a detector 108 that detects the intensity of scattered light from the sample S.

[0014] The irradiation unit 106 generates, as incident light, pump light and Stokes light having different angular frequencies, a first probe light, and a second probe light delayed from the first probe light. The irradiation unit 106 includes a light source 110, an optical parametric amplifier (OPA) 112, a photonic crystal fiber (PCF) 114, a delay line 120, a dichroic mirror 130, a beam combiner 132, and an objective lens 140.

[0015] The light source 110 outputs a laser beam having a predetermined wavelength. The OPA 112 converts the wavelength of the laser beam from the light source 110 to output three laser beams (a first laser beam 151, a second laser beam 152, and a third laser beam 153).

[0016] The first laser light 151 and the second laser light 152 have the same frequency, the first laser light 151 is used as pump light, and the second laser light 152 is used as first probe light and second probe light.

[0017] The PCF 114 generates a wide bandwidth supercontinuum light (hereinafter referred to as SC light) from the third laser light 153. The wavelength bandwidth of the SC light can be set arbitrarily, but is preferably 700 nm to 2000 nm, and more preferably 900 nm to 1200 nm. The SC light is used as a Stokes light.

[0018] The delay line 120 includes a beam splitter 122 and mirrors 124, 126, and 128. The beam splitter 122 splits the second laser light 152 into a first probe light and a second probe light. The first probe light is incident on a dichroic mirror 130. The second probe light has a longer optical path length than the first probe light, and is incident on the dichroic mirror 130 via mirrors 124, 126, and 128. The design of the optical path lengths of the first and second probe lights will be described later.

[0019] The dichroic mirror 130 transmits the Stokes light from the PCF 114, reflects the first probe light and the second probe light, and causes the Stokes light, the first probe light, and the second probe light to be incident on the beam combiner 132. The beam combiner 132 combines the pump light, which is the first laser light 151, with the incident light from the dichroic mirror 130. The combined light from the beam combiner 132 is focused on the sample S by the objective lens 140.

[0020] The scattered light from the sample S is collimated by the objective lens 142 and is incident on the optical filter 144. The optical filter 144 allows only the anti-Stokes light from the incident light from the objective lens 142 to pass through.

[0021] The detector 108 detects the intensity of the anti-Stokes light that has passed through the optical filter 144 and converts the intensity of the anti-Stokes light into an electrical signal. As the detector 108, a photodetector such as a photomultiplier tube (PMT) can be used, but is not particularly limited to this.

[0022] The computer 104 is a computer such as a personal computer, and is connected to the detector 108. The computer 104 performs A / D conversion of the electrical signal from the detector 108, performs spectrum analysis of the anti-Stokes light, and displays the analysis results on a monitor.

[0023] The nonlinear optical device 102 is not limited to the configuration shown in FIG. 1 . For example, instead of generating the pump light, the Stokes light, the first probe light, and the second probe light from a single light source 110, these lights may be generated from two or more light sources. Also, instead of the beam splitter 122, a switch may be used to switch between the optical path of the first probe light and the optical path of the second probe light. Furthermore, instead of generating the first probe light and the second probe light from a single laser light (second laser light 152), the first probe light and the second probe light may be generated separately.

[0024] Next, the nonlinear optical process performed in the nonlinear optical system 100 and the design of the optical path lengths of the first and second probe beams will be described with reference to FIGS.

[0025] First, the pump beam and the Stokes beam are irradiated onto the sample S (step 202 in FIG. 2 , graph A in FIG. 3 ). When the first probe beam is irradiated onto the sample S, the detector 108 detects the intensity of the rising portion of the non-resonant background signal (hereinafter referred to as the NRB signal) as the first scattering signal (step 204 in FIG. 2 , graph B in FIG. 3 ).

[0026] When the angular frequency difference between the pump light and the Stokes light matches the angular frequency of the vibration mode of the target molecule in the sample S, molecular vibration is excited, but a resonance signal (hereinafter referred to as an R signal) has not yet appeared at the time of step 204. The intensity of the rising portion of the NRB signal detected in step 204 is denoted as n1.

[0027] 2, the R signal begins to appear after the NRB signal, and the NRB signal reaches a peak intensity and then begins to attenuate (graph C in FIG. 3). At the time of step 206, the second probe light has not yet reached the sample S.

[0028] When the second probe light is irradiated onto the sample S, the detector 108 detects the intensity of the second scattered signal, which is a mixture of the falling edge of the NRB signal and the R signal (step 208 in FIG. 2, graph D in FIG. 3). The intensity of the falling edge of the NRB signal included in the second scattered signal is denoted as n2.

[0029] The optical path lengths of the first and second probe beams are designed as follows. Before manufacturing the nonlinear optical device 102, a test is performed in which the BG sample and the actual sample are intermittently irradiated with the probe beam while varying the optical path lengths of the first and second probe beams. Then, based on the detection results from the detector 108, the positions of the optical components are adjusted to satisfy the following conditions (i) to (iii). This makes it possible to manufacture a nonlinear optical device 102 in which the irradiation timings of the first and second probe beams are fixed to optimal values. (i) When the first probe beam is irradiated, the R signal does not yet appear, and the intensity n1 of the rising portion of the NRB signal is detected. (ii) When the second probe beam is irradiated, the intensity of a scattered signal containing a mixture of the falling portion of the NRB signal and the R signal is detected. (iii) The intensity n1 of the rising portion of the detected NRB signal is equal to the intensity n2 of the falling portion of the detected NRB signal.

[0030] The computer 104 obtains the intensity of the R signal by subtracting (dividing) the intensity of the first scattered signal (=the rising portion of the NRB signal) from the intensity of the second scattered signal (=the R signal+the falling portion of the NRB signal).

[0031] 2 and 3 show an example in which the first probe light is irradiated after the pump light and the Stokes light are irradiated, but as described above, the optimum value of the irradiation timing of the first probe light is determined based on tests performed before the manufacture of the nonlinear optical device 102. Therefore, the irradiation timing of the first probe light differs depending on the manufactured nonlinear optical device 102.

[0032] The optical path lengths of the first probe light and the second probe light may be calibrated after manufacturing the nonlinear optical device 102. In this case, as shown in Fig. 4, the irradiation unit 106 includes an optical path length varying mechanism 410 that varies the optical path length of the first probe light and an optical path length varying mechanism 420 that varies the optical path length of the second probe light.

[0033] Known optical path length varying means can be used as the optical path length varying mechanisms 410 and 420. For example, as shown in FIG. 4, the optical path length varying mechanism 420 can change the optical path length of the second probe light by using a stage to move the positions of the mirrors 124 and 126 of the delay line 120-1 in the directions of the double-headed arrows. The optical path length varying mechanism 410 for the first probe light can also have a structure similar to that of the delay line 120-1. Alternatively, any optical path length varying means using various optical components such as a lens, a polygon mirror, or a resonant scanner can be used. The optical path lengths of the first and second probe lights can be changed using the optical path length varying mechanisms 410 and 420, and tests similar to those performed before the nonlinear optical device 102 was manufactured can be performed, and the optical path lengths of the first and second probe lights can be designed to satisfy the above conditions (i) to (iii).

[0034] 5 shows Raman spectra of acetaminophen at different concentrations as an example of measurements using the nonlinear optical system 100 of this embodiment. It can be clearly seen from FIG. 5 that the spectral values ​​increase as the concentration of acetaminophen increases.

[0035] As described above, the nonlinear optical device 102 and the nonlinear optical system 100 of this embodiment detect the intensities of the first scattering signal (= the rising portion of the NRB signal) and the second scattering signal (= the falling portion of the R signal + NRB signal) by irradiating the same sample S with the first probe light and the second probe light, respectively. The optical path lengths (irradiation timing) of the first probe light and the second probe light are set so that the intensity n1 of the rising portion of the NRB signal and the intensity n2 of the falling portion of the NRB signal are equal. This allows the intensity of the R signal to be determined by subtracting the intensity of the first scattering signal from the intensity of the second scattering signal. Therefore, measurement data of the R signal can be obtained by subtracting the NRB signal without replacing the sample as in the conventional method. This improves operability and enables high-throughput measurements.

[0036] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the spirit of the present invention. Other embodiments and modifications made by those skilled in the art are also included in the present invention.

[0037] For example, the pump light shown in the above-described embodiment is not necessarily required. At least the first scattering signal and the second scattering signal can be obtained by irradiating the sample S with broadband SC light, first probe light, and second probe light. In this case, a certain angular frequency component of the SC light becomes the pump light, and another angular frequency component of the SC light becomes the Stokes light. When the angular frequency difference between the pump light and the Stokes light matches the angular frequency of a vibration mode of a target molecule in the sample S, molecular vibration can be excited.

[0038] 100 Nonlinear optical system 102 Nonlinear optical device 104 Computer 106 Irradiation unit 108 Detector 110 Light source 112 OPA 114 PCF 120, 120-1 Delay line 122 Beam splitter 130 Dichroic mirror 132 Beam combiner 140, 142 Objective lens 144 Optical filter 410, 420 Optical path length variable mechanism

Claims

1. A nonlinear optical device comprising: an irradiation unit that generates at least broadband light having a predetermined bandwidth, first probe light, and second probe light delayed from the first probe light as incident light on a sample to be measured, and irradiates the sample with the incident light; and a detector that detects the intensity of scattered light from the sample, wherein when the irradiation unit irradiates the sample with at least the broadband light, resonance due to molecular vibration of the sample and non-resonant background occur; when the irradiation unit irradiates the sample with the first probe light, the detector does not detect the intensity of a resonance signal due to the molecular vibration, but detects the intensity of a rising portion of a non-resonant background signal generated by the presence of the non-resonant background as a first scattered signal; when the irradiation unit irradiates the sample with the second probe light, the detector detects the intensity of a second scattered signal that is a mixture of the falling portion of the non-resonant background signal and the resonance signal; and the optical path lengths of the first probe light and the second probe light in the irradiation unit are set so that the intensities of the rising and falling portions of the non-resonant background signal are equal.

2. The nonlinear optical device according to claim 1, wherein the irradiating unit generates supercontinuum light as the broadband light.

3. The nonlinear optical device according to claim 1, wherein the irradiation unit further generates pump light, and when the irradiation unit irradiates the sample with the pump light and the broadband light as Stokes light, the resonance and the nonresonant background occur.

4. The nonlinear optical device according to claim 1, wherein the optical path lengths of the first probe light and the second probe light in the irradiation unit are fixed.

5. The nonlinear optical device according to claim 1, wherein the irradiation unit includes an optical path length varying mechanism for changing the optical path lengths of the first probe light and the second probe light.

6. A nonlinear optical system comprising: a nonlinear optical device according to any one of claims 1 to 5; and a computer connected to the detector of said nonlinear optical device, for performing a spectral analysis of the scattered light from said sample.

7. The nonlinear optical system of claim 6, wherein the computer determines the intensity of the resonance signal by subtracting the intensity of the first scattered signal from the intensity of the second scattered signal.

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