JP7000117B2 - Noise reduction device and detection device with it - Google Patents

Description

The present invention relates to a noise reduction device, and more particularly to a noise reduction device used in a detection device such as a guided Raman scattering microscope.

In recent years, a Raman scattering detection device using a nonlinear optical process has been proposed as a device capable of shortening the detection time as compared with a spontaneous Raman scattering detection device. Patent Document 1 describes a guided Raman scattering (SRS) microscope as a nonlinear Raman scattering detection device. According to this SRS microscope, the sample is included in the sample by irradiating the sample with pulsed light of two colors having different optical frequencies (wavelengths), locking-in detecting the change in intensity of the generated pulsed light, and acquiring the SRS signal. Can identify and distinguish between molecules.

Further, in the SRS microscope described in Patent Document 1, the intensity noise of the pulsed light is generated by dividing the optical path of one of the two color pulsed lights into two, giving a time delay to one of them, and then combining the waves. Is reduced.

The noise reduction device of Patent Document 2 reduces noise in a wider frequency band centering on a specific frequency according to the delay time by giving a specific delay and intensity ratio to pulsed light divided into a plurality of parts instead of being divided into two parts. There is.

Patent No. 5623652 WO / 2016/079845

When the SRS signal is measured at high speed, the frequency band of the signal acquired by lock-in detection becomes wide. However, in the configuration described in Patent Document 1, the frequency band in which noise can be reduced is limited, and it is difficult to sufficiently reduce noise over a wide band. In Patent Document 2, a plurality of delayed pulsed lights are combined to solve the problem, but the divided pulsed light is coaxially combined (or guided to a single mode fiber so that it can be used in a microscope). There is a problem that the amount of light is generally lost when (lighting). The loss of the amount of light causes a decrease in the signal, and the SN ratio of the detected signal is decreased (in Patent Document 2, the SN ratio is improved by reducing noise more than the decrease in the signal). For example, induced Raman scattering (and its signal) occurs in proportion to the product of the intensities of two pulsed lights incident on the sample.

An exemplary object of the present invention is to provide a noise reduction device that reduces noise in a wide frequency band with a small amount of light loss.

The noise reducing device as one aspect of the present invention for achieving the above object is a noise reducing device for reducing the noise of the pulsed light of the first cycle, and two pulsed lights of the first cycle are used. The first pulsed light polarized in the first direction and the first direction are described by adding a first delay time between the two demultiplexed pulsed lights and merging them. A first delayed combiner means for outputting a second pulsed light polarized in a direction orthogonal to the first pulsed light and delayed by the first delay time with respect to the first pulsed light, and the first pulse. By splitting the light into two and adding a second delay time between the two split pulsed lights to combine the light, the third pulsed light polarized in the second direction and the above-mentioned A fourth pulsed light polarized in a direction orthogonal to the second direction and delayed by the second delay time with respect to the third pulsed light is output, and two of the second pulsed lights are output. A fifth pulsed light polarized in the second direction and the second pulsed light are combined by imparting the second delay time between the two pulsed lights that have been demultiplexed. A second delayed combine light that is polarized in a direction orthogonal to the direction of the light and outputs a sixth pulsed light that is delayed by the second delay time with respect to the fifth pulsed light . It has a polarization adjusting unit that adjusts the deflection direction of the pulsed light of 1 and the polarization direction of the second pulsed light, and the polarization adjusting unit has the first delay combiner and the second delay. It is characterized by being located between the combine waves . Other aspects of the invention will be demonstrated in embodiments described below.

According to the present invention, it is possible to provide a noise reducing device that reduces noise in a wide frequency band with a small amount of light loss.

It is a schematic diagram explaining the configuration example (the first embodiment) of the noise reduction apparatus. It is a figure which shows the polarization direction of a pulsed light. (A) Time profile of incident pulsed light. (B) Time profile of pulsed light 21 and 22. (C) Time profiles of pulsed light 23, 24, 25 and 26. It is a figure which shows the noise reduction rate (FIG. 3 of Patent Document 2). It is a schematic diagram explaining the structural example (second embodiment) of the noise reduction apparatus. It is a conceptual diagram of the SRS microscope of Example 1. FIG. It is a figure which shows the time profile of the optical pulse train of the SRS microscope of Example 1. FIG. It is a conceptual diagram of the SRS microscope of Example 2. FIG. It is a figure which shows the time profile of the optical pulse train of the SRS microscope of Example 2. FIG. It is a conceptual diagram of the SRS microscope of Example 3. FIG. It is a figure which shows the time profile of the optical pulse train of the SRS microscope of Example 3. FIG.

(First Embodiment)
The noise reduction device 100 of the present embodiment will be described with reference to the accompanying drawings. FIG. 1 is a schematic diagram showing the configuration of the noise reduction device 100. The noise reducing device 100 reduces noise near a specific frequency included in pulsed light emitted by a light source such as a pulse laser by two delayed combined light paths using a polarizing beam splitter. In the present embodiment, a pulse train having a light intensity ratio of 1: 3: 3: 1 is formed at equal time intervals with a small amount of light loss (no loss other than the loss due to the reflectance and transmittance due to the optical element) of the incident pulse light. Generate.

The incident pulse light 20 is divided (demultiplexed) into pulse light 21 (first pulse light) and pulse light 22 (second pulse light) having the same light intensity (pulse energy) by the polarizing beam splitter 1. Since the polarization beam splitter divides the incident light into two orthogonal linear polarizations (P polarization, S polarization), when the incident pulse light 20 is polarized, the polarization direction is not shown at 45 degrees. The polarization direction is rotated by the polarization adjustment unit. For example, using a half-wave plate, the amplitude (or light intensity) of the electric field is adjusted to be equal in the direction of the paper surface of FIG. 1 and the direction perpendicular to the paper surface. Further, a light source (for example, a pulse laser) that generates the incident pulsed light 20 may be adjusted, or a delayed combined light path including a polarizing beam splitter may be rotated around the optical axis.

The pulsed light 21 and the pulsed light 22 pass through different optical paths and are coaxially combined by the polarizing beam splitter 2. The optical path length between the polarizing beam splitters 1 and 2 is made ΔL larger in the pulsed light 22 than in the pulsed light 21 by utilizing the mirrors 3 and 4. As a result, the pulsed light 22 is delayed by τ (= ΔL / c, c is the speed of light) with respect to the pulsed light 21 after the combined wave.

The combined pulsed light passes through the polarization adjusting unit 5 in order to rotate the polarization direction by 60 degrees. The reason why the polarization direction of the pulsed light is rotated by 60 degrees is to make the ratio of the light intensity split by the polarization beam splitter 6 1: 3 (1: √3 in the absolute value ratio of the electric field) as shown in FIG. be. In FIG. 2, the vertical axis is the direction parallel to the polarization separation surface of the polarizing beam splitter and orthogonal to the optical axis (S polarization direction), and the horizontal axis is the S polarization direction and the direction orthogonal to the optical axis (P polarization direction). ). The pulsed light 21 transmitted through the polarized beam splitter 2 as P-polarized light is divided into pulsed light 23 (third pulsed light) and pulsed light 24 (fourth pulsed light) having a light intensity ratio of 1: 3. ..

The pulsed light 23 and the pulsed light 24 pass through different optical paths and are coaxially combined by the polarizing beam splitter 7. The optical path length between the polarizing beam splitters 6 and 7 is 2ΔL larger in the pulsed light 24 than in the pulsed light 23 by utilizing the mirrors 8 and 9. As a result, the pulsed light 24 is delayed by 2τ (= 2ΔL / c, c is the speed of light) with respect to the pulsed light 23 after the combined wave. That is, the delay time given by the first delayed combined light path including the polarized beam splitters 1 and 2 (first delay time) and the delay given by the second delayed combined light path including the polarized beam splitters 6 and 7. Arrange so that the ratio of time (second delay time) is 1: 2.

Next, the pulse light 22 arriving with a delay of τ with respect to the pulse light 21 will be considered. The polarization direction of the pulsed light 22 is rotated by 60 degrees by the polarization adjusting unit 5 in the same manner as the pulsed light 21. As can be seen from FIG. 2, the pulsed light 22 is orthogonal to the pulsed light 21 even after the polarization direction is rotated. Therefore, the pulsed light 22 reflected by the polarized beam splitter 2 as S polarization is the pulsed light 25 (fifth pulsed light) and the pulsed light 26 (sixth pulsed light) having a light intensity ratio of 3: 1 by the polarized beam splitter 6. It is divided into pulsed light).

The pulsed light 25 and the pulsed light 26 pass through different optical paths and are coaxially combined by the polarizing beam splitter 7. The pulse light 25 passes through the same optical path as the pulse light 23, and the pulse light 26 passes through the same optical path as the pulse light 24. When the pulsed light 21 is divided, the pulsed light 23 having a low light intensity passes through a short optical path, but when the pulsed light 22 is divided, the pulsed light 26 having a low light intensity passes through a long optical path. Further, since the delay time ratio of the first and second delayed combined light paths is set to 1: 2, pulsed light having a light intensity ratio of 1: 3: 3: 1 at equal time intervals is a polarization beam splitter 7. Is ejected from.

This situation will be described with reference to the time profile of the pulsed light of FIG. The incident pulsed light shown in (a) is divided by the first delayed combined light path into two pulsed lights having a time interval τ as shown in (b). Further, the second delayed combined light path divides the light intensity into pulsed light having a light intensity ratio of 1: 3: 3: 1 at a time interval τ.

Since the pulsed light of FIG. 3A is actually repeatedly incident at a frequency determined by the resonator length of the pulse laser or the like, each incident pulse is divided into four as shown in FIG. 3C. ..
Here, noise reduction by a light intensity ratio of 1: 3: 3: 1 will be described. The noise contained in the pulsed light output from the noise reduction device can be expressed as a function of the frequency f as in Eq. (1) (the noise power per unit frequency at the frequency f of the incident light is 1, and each pulsed light has its own. It does not overlap in time, that is, it does not interfere).
| 1/8 + 3e ^(i2πfτ) / 8 + 3e ^(i4πfτ) / 8 + e ^(i6πfτ) / 8 |
= {(1 + cos2πfτ) / 2} ^3/2 equation (1)

A phase component corresponding to the frequency f and the delay time τ is introduced for each of the four pulsed lights having an intensity of 1/8, 3/8, 3/8, and 1/8, and the absolute value of the sum of each term is calculated. By taking it, the noise reduction rate of the noise reduction device is expressed. With m as an integer, the noise is completely canceled at the frequency of f = (2m-1) / (2τ), and the noise is reduced according to the equation (1) at the frequencies in the vicinity thereof. If it is desired to realize a wide range of noise reduction in the vicinity of the frequency fr, the noise reduction device may be set so that m = 1 and τ = 1 / (2 · fr).

FIG. 4 is based on FIG. 3 of Patent Document 2, and is a comparison of noise transmittance spectra in the case of 2, 3, 4, and 5 branches in consideration of the power ratio, and the plot of 4 branches in the figure is τ. The result of the equation (1) at the time of = 12.5 nanoseconds is shown. It shows that noise of 60% or more can be reduced in the frequency range of 40 MHz, centering on 40 MHz in which noise is completely canceled. The light intensity ratio does not have to be exactly 1: 3: 3: 1. For example, when one pulsed light increases by 10% of the light intensity before division to 1: 3.8: 3: 1, the increased noise is not canceled and the minimum value in the noise transmittance shown in FIG. 3 is obtained. Is 0.1 instead of zero. Therefore, even if the ratio of light intensity deviates from 1: 3: 3: 1, there is an effect of noise reduction.

When three or more lights that have passed through different optical paths are coaxially combined using a polarization beam splitter or half mirror, there is generally a loss of light quantity (that is, other than the direction to be output by the polarization beam splitter or half mirror). Light is generated in the direction).

In Patent Document 2, in an optical path divided into two by polarization, the optical path is further divided by polarization to generate pulsed light in which the delay time and the light intensity are adjusted. In the three-division example, first, the incident pulse light is divided into pulse light A and pulse light B. The pulsed light B is divided into pulsed light C and pulsed light D, and combined by a polarized beam splitter via different optical path lengths. The polarization directions of the combined pulse light C and the pulse light D are orthogonal to each other. Noise reduction is realized by combining the pulse light C and the pulse light D with the pulse light A, and at that time, the pulse light C and the pulse light D are output as unidirectional polarization by the polarization beam splitter. .. That is, two pulsed lights in one direction are generated from two pulsed lights in two orthogonal directions. As a result, the pulsed light C and the pulsed light D combine with the pulsed light A and output only half of the incident light amount. This loss of light quantity is not desirable when the output power of a light source such as a pulse laser is limited, or when it is desired to increase the light quantity as much as possible to improve the signal-to-noise ratio of the signal.

In this embodiment, not only noise reduction in a wide frequency band is realized, but also there is no light loss at the time of merging by arranging two-split delay merging means in series (however, the transmittance of the polarizing beam splitter). Excluding loss due to reflectance).

For example, a half-wave plate can be used for the polarization adjusting unit 5 in the present embodiment. The angle of the half-wave plate is set so that the angle in the polarization direction of the pulsed light 21 with respect to the phase-advancing axis or the slow-phase axis is 30 degrees. The angle of the half-wave plate is the light intensity of the pulse light 23 evaluated by blocking the pulse light 22 and the pulse light 24 and the light intensity of the pulse light 24 evaluated by blocking the pulse light 22 and the pulse light 23. May be adjusted so that the ratio of is 1: 3. Further, the output light of the noise reducing device 100 may be evaluated by a spectrum analyzer or the like, and the angle of the half-wave plate may be adjusted so that the noise is reduced most. The polarization adjusting unit 5 only needs to be able to rotate the polarization direction, and a Frenellom wave plate or the like can be used in addition to the half-wave plate.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and modifications can be made within the scope of the gist thereof.

For example, the polarizing beam splitters 1 and 6 can be replaced with a half mirror that splits light in two directions with a light intensity ratio of 1: 1. In this case, since the divided light has the same polarization direction, an element (for example, a half-wave plate) for turning the polarization direction by 90 ° is further introduced in one of the divided optical paths in order to combine without light loss in the polarization beam splitter. do it.

Further, although the first delay time is τ and the second delay time is 2τ, the same noise reduction device can be configured even if the first delay time is 2τ and the second delay time is τ. .. In this case as well, the noise reduction performance does not change because pulsed light having a light intensity ratio of 1: 3: 3: 1 is generated at equal time intervals (only the order of the polarization directions changes).

(Second embodiment)
The noise reduction device 200 of this embodiment will be described with reference to the accompanying drawings. FIG. 5 is a schematic diagram illustrating the configuration of the noise reduction device 200. Similar to the first embodiment, the two delayed combined light paths are used, the light intensity loss is small (no loss other than the loss due to the reflectance and the transmittance due to the optical element), and the light intensity ratio is 1: 3 at equal time intervals. : Generates a 3: 1 pulse train.

Fiber-type polarizing beam splitters 11, 12, 16 and 17 are used to form a delayed combined light path, and the two delayed combined light paths near a specific frequency contained in pulsed light emitted by a light source such as a pulse laser. Reduce noise.

The incident pulse light 20 is divided into pulse light 21 (first pulse light) and pulse light 22 (second pulse light) having the same light intensity (pulse energy) by a fiber type polarizing beam splitter 11, and each of them is polarized. The light is guided to the holding fibers 13 and 14. In order to divide the incident pulse light 20 into equal light intensities, the polarization directions of the incident pulse light 20 are set so as to have the same light intensity on each of the Fast axis and the Slow axis of the polarization holding fiber. The polarization direction may be adjusted by using a polarization controller, or the polarization before guiding the light to the polarization holding fiber by a collimator may be adjusted by a half-wave plate or the like.

The pulsed light 21 and the pulsed light 22 pass through different polarization holding fibers and are guided into one polarization holding fiber by a fiber type polarizing beam splitter 12. The length of the polarization-retaining fiber 14 is increased by ΔL as compared with the polarization-retaining fiber 13. As a result, the pulsed light 22 lags behind the pulsed light 21 by τ (= n · ΔL / c, c is the speed of light, n is the group refractive index of the optical fiber) after the combined wave.

The combined pulsed light passes through the polarization adjusting unit 15 in order to rotate the polarization direction by 60 degrees. The reason why the polarization direction of the pulsed light is rotated by 60 degrees is to make the ratio of the light intensities divided by the fiber-type polarization beam splitter 16 1: 3 (1: √3 in terms of the absolute value of the electric field).
The pulsed light 23 (third pulsed light) and 24 (fourth pulsed light) divided by a light intensity ratio of 1: 3 pass through the polarization-retaining fibers 18 and 19, respectively, and are fiber-type polarized beam splitters. 17 guides the light to one polarization-retaining fiber and outputs the light.

The length of the polarization-retaining fiber 19 is 2ΔL larger than that of the polarization-retaining fiber 18. As a result, the pulsed light 24 lags behind the pulsed light 23 by 2τ (= 2nΔL / c, c is the speed of light, n is the group refractive index of the optical fiber) after the combined wave. That is, the ratio of the delay time due to the first delayed combined light path (first delay time) and the delay time due to the second delayed combined light path (second delay time) is arranged to be 1: 2. The first delayed combine light path is composed of fiber type polarizing beam splitters 11 and 12, and the second delayed combine light path is composed of fiber type polarization beam splitters 16 and 17.

Next, with respect to the pulsed light 22 that arrives with a delay of τ with respect to the pulsed light 21, the pulsed light 25 (fifth pulsed light) and the pulsed light having a light intensity ratio of 3: 1 are the same as in the first embodiment. It is divided into 26 (sixth pulsed light). Finally, pulsed light having a light intensity ratio of 1: 3: 3: 1 is output from the fiber-type polarizing beam splitter 17 at equal time intervals.

The time profile of the pulsed light and the effect of noise reduction are the same as those described in the first embodiment with reference to FIGS. 3 and 4.

For the polarization adjusting unit 5 in the present embodiment, for example, a bulk type or paddle type polarization controller can be used. Further, when the output optical fiber of the fiber type polarizing beam splitter 12 and the input optical fiber of the fiber type polarizing beam splitter 16 are fused, the Fast axis (or Slow axis) of each is shifted by 60 degrees and fused. The fusion point may be the polarization adjusting unit 5. At the fusion point, the polarization direction of the light propagating in the fiber can be changed. The fusion angle of the fiber is such that the ratio of the light intensities of the pulsed light 23 and 24 passed in the state where the fibers are brought close to each other before the fusion is 1: 3 (or the ratio of the light intensities of the pulsed light 25 and 26 is 3: 1). ) Can be fine-tuned. Further, the output light of the noise reducing device 200 may be evaluated by a spectrum analyzer or the like, and the fusion angle may be finely adjusted so that the noise is reduced most.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and modifications can be made within the scope of the gist thereof.

For example, the fiber-type polarizing beam splitters 11 and 16 can be replaced with a branch coupler that splits the input light at a light intensity ratio of 1: 1 and guides each to two polarization-holding fibers. In this case, since the divided light has the same polarization direction, an element for turning the polarization direction by 90 ° may be further introduced in one of the divided fibers in order to combine the waves without light loss. For example, a polarization controller may be further introduced, or a fusion portion in which the Fast axis and the Slow axis are aligned and fused may be provided in the fiber.

Further, although the first delay time is τ and the second delay time is 2τ, the same noise reduction device can be configured even if the first delay time is 2τ and the second delay time is τ. .. In this case as well, the noise reduction performance does not change because pulsed light having a light intensity ratio of 1: 3: 3: 1 is generated at equal time intervals (only the order of the polarization directions changes).

The detection device to which the present invention is applied will be described with reference to FIG. As an example, an SRS (stimulated Raman scattering) microscope is shown. The SRS microscope detects SRS generated by irradiating a sample with two lights having different wavelengths at the same time. SRS is one of the nonlinear optical phenomena and occurs in proportion to the product of the intensities of light of each wavelength. Therefore, in order to efficiently generate SRS, the same spots on the sample are simultaneously irradiated with the light beams of lasers of two wavelengths. When SRS occurs, the intensity of the pulsed light having the shorter wavelength of the two wavelengths is weakened, and the intensity of the pulsed light having the longer wavelength is increased. Further, in order to efficiently generate SRS and to have sufficient Raman spectrum resolution, it is desirable to use a short pulse laser having a pulse time width of 1 to 10 picoseconds. In order to detect SRS with high sensitivity, the cycles of the two pulsed lights are synchronized at a ratio of 1: 2, and a state is created in which the SRS signal is included in each pulsed light in one pulsed light. This SRS signal can be acquired by a lock-in amplifier whose lock-in frequency is the frequency corresponding to the larger pulse period of the two pulsed lights (laser repetition frequency). The noise reduction device of this embodiment reduces noise in a wide frequency region near the lock-in frequency, thereby increasing the sensitivity of acquisition of the SRS signal.

As the pulse laser 501 (first generation means) and the pulse laser 502 (second generation means), an optical pulse train having a pulse period of 1: 2 is used. A solid-state laser (titanium sapphire laser) having a center wavelength of 800 nanometers and a pulse period (first period) of 12.5 nanoseconds is used as the pulse laser 501. For example, Mai Tai of Spectra-Physics is used. As the pulse laser 502, an ytterbium-doped fiber laser having a center wavelength of 1030 nanometers and a pulse period of 25 nanoseconds is used.

Since the SRS signal is generated in a period of 25 nanoseconds, in order to reduce noise in the widest frequency range near the lock-in frequency of 40 MHz, the time interval of the pulsed light divided by the noise reduction device should be 12.5 nanoseconds. good. To create this time interval, the difference in fiber length in the first delayed combiner of the noise reduction device 200 may be 2.5 meters (speed of light 3 x ¹⁰⁸ meters per second, optical fiber). The group refractive index was 1.5). Further, the difference in fiber length in the second delayed combiner means may be 5 meters.

To cancel the noise at the lock-in frequency, the time interval of the pulsed light may be (12.5 + 25 · m) nanoseconds (m is an integer greater than or equal to 0). However, as m is larger, the frequency range in which noise can be reduced becomes narrower, and the length of the optical fiber of the noise reducing device must be lengthened, so that there is no merit.

FIG. 7A shows an optical pulse train 31 generated by the pulse laser 501. Each pulse has a spread of several picoseconds, but is represented as a line with a length proportional to the light intensity in the figure. When the optical pulse train 31 is incident on the noise reduction device 200, an optical pulse train 33 having a time profile as shown in FIG. 7B is generated. The delay time in the noise reducing device 200 is slightly deviated from 12.5 nanoseconds so as not to overlap with the connected pulsed light in time and cause interference. The amount of deviation from 12.5 nanoseconds may be 10 to several tens of picoseconds as long as it is equal to or larger than the pulse width (exaggerated in FIG. 7). With this degree of deviation, the effect on the noise reduction rate is small.

FIG. 7 (c) shows the optical pulse train 32 generated by the pulse laser 502. It is assumed that the wavelength (λ1) of the optical pulse train 33 is smaller than the wavelength (λ2) of the light pulse train 32. When the pulses of the optical pulse train 33 and the light pulse train 32 are focused at the same point on the sample at the same timing as shown in FIGS. 7 (b) and 7 (c), they are transmitted through the sample by SRS. The intensity of the pulsed light changes. The light pulse train 33 having a small wavelength has a small light intensity when SRS occurs. FIG. 7D shows the light intensity of the light pulse train 33 changed by SRS. It shows a negative value because the light intensity decreased due to SRS. The amount of change is exaggerated and illustrated, and since it is actually a minute amount of about ^10-4 of the light intensity of the original pulse, lock-in detection is performed.

The amount of change in light intensity detected by lock-in corresponds to the SRS signal, and the information of the molecule contained in the point where the light beam is focused is reflected. For example, when the resonance frequency of the molecular vibration included in the above point and the difference (c / λ1-c / λ2) between the optical frequencies of the two lasers match, the SRS signal becomes large. c is the speed of light. By changing λ2 with the wavelength selection filter 503, the SRS signal can be acquired while changing (c / λ1-c / λ2). That is, the Raman spectrum can be acquired. It is possible to estimate what kind of molecules are contained in the sample from the Raman spectrum. The SRS microscope can acquire a spectrum equivalent to that of a microscope using spontaneous Raman. Since the scattering efficiency of SRS is much higher than the scattering efficiency of spontaneous Raman scattering, the SRS microscope can acquire the Raman spectrum in a shorter time than the microscope using spontaneous Raman scattering.

The wavelength selection filter uses a spectroscopic element such as a diffraction grating and has a width of about 0.3 nanometer from the wavelength range of the light generated by the pulse laser 502 (about 40 nanometers centered on 1030 nanometers). Select and cut out an arbitrary wavelength region. The pulsed light output by the wavelength selection filter changes in time width corresponding to 0.3 nanometer width, but the time profile (repetition frequency) is maintained even after passing through the wavelength selection filter.

The optical pulse train 33 that has passed through the wavelength selection filter 503 is changed in the traveling direction by the mirror 504, and then undulated coaxially with the light pulse train 32 by the dichroic mirror 505. The optical pulse train 32 is emitted as parallel light by the collimator lens 506 in order to combine waves.

The SRS microscope is configured as a laser scanning microscope. The optical pulse trains 32 and 33 are coaxially incident on the beam scanner 507, deflected by the beam scanner 507, and emitted. The beam scanner 507 is composed of a galvano scanner and a resonant scanner, and changes the direction of the optical axis in two orthogonal directions. For simplification of the figure, the two mirrors in the beam scanner 507 are represented by one mirror in FIG. If a resonant scanner (scan frequency 8 kHz) and a galvano scanner (scan frequency 15 Hz) are used, 500 lines of images can be acquired at 30 frames per second.

The light beam deflected by the beam scanner 507 is incident on the objective lens 508 through a relay lens (not shown). The objective lens 508 irradiates the same spot on the sample 509 with the optical pulse trains 32 and 33. The objective lens 508 is preferably an objective lens having a large numerical aperture (NA) from the viewpoint of spatial resolution for detecting an SRS signal and a signal-to-noise ratio.

The sample 509 is sandwiched by a cover glass having a thickness of several tens to 200 micrometers (not shown). Due to the deflection of the light beam by the beam scanner 507, the light spot focused on the sample 509 is two-dimensionally scanned, and the SRS signal is two-dimensionally imaged. Since the SRS signal is generated only in the focused light spot, a three-dimensional image can be obtained by moving the sample 105 in the optical axis direction by a stage (not shown).

The condenser lens 510 is an objective lens or a condenser lens having an NA equal to or higher than the NA of the objective lens 508 so as to receive all the light transmitted through the sample 509 and subjected to intensity modulation by SRS. The light beam emitted from the condenser lens 510 passes through the filter 511 and a relay lens (not shown), and then irradiates the light receiving surface of the photodiode (detector) 512. The filter 511 is composed of a dielectric multilayer film, blocks light having a wavelength of λ2, and transmits light having a wavelength λ1. The photodiode 512 is irradiated with an intensity-modulated optical pulse train 32 by SRS. As the photodiode 512, a silicon photodiode having a sensitivity of 800 nanometers and having a cutoff frequency of 40 MHz or more is used.

The intensity modulation by SRS is 40 MHz (cycle 25 nanoseconds) for the repetition frequency of 80 MHz (pulse period 12.5 nanoseconds) of the optical pulse train 32. The current-voltage conversion circuit 513 is a filter circuit and an amplifier circuit for amplifying a weak 40 MHz component while attenuating a large 80 MHz component contained in the current signal generated by the photodiode 512 and outputting it as a voltage.

The synchronous detection circuit (for example, a lock-in amplifier) 514 is an electronic circuit using a mixer. The synchronous detection circuit 514 extracts a component synchronized with the repetition frequency of 40 MHz of the pulse laser 502 included in the voltage signal output by the current-voltage conversion circuit 513, and outputs the component as a voltage. The output voltage of the synchronous detection circuit 514 indicates how much SRS occurred at the condensing point in the sample 509.

The computer 515 uses the control signal of the beam scanner 507 to create a two-dimensional image of the output signal (SRS signal) of the synchronous detection circuit 514 and display it. The computer 515 can also display a three-dimensional image of the SRS signal acquired by moving the sample 509 in the optical axis direction on a stage (not shown). The computer 515 can also display the Raman spectrum from the SRS signal acquired by changing the wavelength of at least one of the two pulse lasers.

Since the noise in the vicinity of 40 MHz included in the optical pulse train 32 is reduced in a wide frequency band by the noise reduction device 200, high sensitivity can be maintained even if a signal acquisition of 10 megasampling or more per second is performed. That is, it can have the performance limited by the shot noise generated in principle. Further, since the noise reducing device 200 has a small loss of light intensity, the sample 509 can be irradiated with a higher light intensity than the conventional one. As a result, the SRS signal is improved and the detection device can be made highly sensitive.

The detection device to which the present invention is applied will be described with reference to FIG. As in Example 1, an SRS microscope will be taken as an example. The description of the part overlapping with the first embodiment will be omitted.

In the first embodiment, the pulsed light generated by the two pulse lasers has a pulse period of 1: 2 synchronized by a pulse synchronization device (not shown). In this embodiment, one pulsed laser constitutes an SRS microscope. That is, a configuration example that does not use the pulse synchronization device is shown.

The optical pulse train generated by the pulse laser 501 is split into two by a half mirror 521, and one of them is input to the noise reduction device 200 as an optical pulse train 31 as in the first embodiment. On the other hand, the wavelength is changed by the wavelength conversion means 522 (second generation means). As a wavelength changing means, for example, an optical parametric oscillator (OPO) can be used. By changing the wavelength by OPO, a Raman spectrum can be obtained in the same manner as in Example 1.

9 (a) is a time profile of the optical pulse train 31, and FIG. 9 (c) is an optical pulse train 34 whose wavelength is changed by OPO. Since the repetition frequency of both the optical pulse trains 31 and 34 is 80 MHz, SRS also occurs at 80 MHz. Similar to the first embodiment, even if the SRS signal is detected by the lock-in detection, the intensity of the optical pulse train itself is evaluated, and the SRS cannot be detected with high sensitivity.

Therefore, the lock-in detection of the SRS signal can be performed by reducing the 80 MHz component of the intensity of the optical pulse train by the noise reducing device of this embodiment.

In order to reduce the noise of 80 MHz, the time interval of the pulsed light divided by the noise reducing device 200 may be 6.25 nanoseconds. To create this time interval, the difference in fiber length in the first delay combiner of the noise reduction device 200 may be 1.25 meters (speed of light 3 x ¹⁰⁸ meters per second, optical fiber). The group refractive index was 1.5). Further, the difference in fiber length in the second delayed combiner means may be 2.5 meters.

This operation corresponds to doubling (160 MHz) the repetition frequency of the optical pulse train of 80 MHz, but it is possible to suppress the variation in intensity as compared with simply dividing the optical pulse train into two and doubling the frequency. This effect is the same as the noise reduction rate shown in FIG. 4, and it is possible to suppress the variation in strength more in the 4-branch than in the 2-branch.

The time interval of the pulsed light divided by the noise reducing device 200 may be (6.25 + 12.5 · m) nanoseconds (m is an integer of 0 or more). However, as m is larger, the frequency range in which noise can be reduced becomes narrower, and the length of the optical fiber of the noise reducing device must be lengthened, so that there is no merit.

FIG. 9B shows an optical pulse train 35 generated by the noise reduction device 200. FIG. 9D shows the light intensity of the light pulse train 35 changed by SRS. The change in light intensity due to SRS can be detected by a lock-in amplifier having a repetition frequency of 80 MHz of pulsed light generated by the pulse laser 501 as a lock-in frequency. The detection method is the same as that of the first embodiment except for the lock-in frequency.

The detection device to which the present invention is applied will be described with reference to FIG. Similar to Examples 1 and 2, an SRS microscope will be taken as an example. The description of the part overlapping with the first embodiment will be omitted.

In the first embodiment, the repetition frequency generated by the pulse laser 502 is set as the lock-in frequency, and the lock-in detection of the SRS signal is performed. In the first embodiment, the repetition frequency generated by the pulse laser 501 is set as the lock-in frequency, and the lock-in detection of the SRS signal is performed.

In this embodiment, an example is shown in which the generation of SRS is modulated by modulating the pulsed light at an arbitrary frequency using an external modulator, and the lock-in detection of the SRS signal is detected.

Similar to the second embodiment, there is one pulse laser, and one of the pulse lasers split into two by the half mirror 521 changes the wavelength through the wavelength conversion means 522 (third generation means). The optical pulse train 34 output by the wavelength conversion means 522 modulates either the optical path length, the light intensity, or the polarization direction of the pulsed light contained in the light pulse train 34 by the modulation means 523. Since the SRS signal decreases when the timing of the two pulsed lights in the sample deviates, the SRS signal changes in synchronization with the modulation of the optical path length of the pulsed light. Since SRS is generated in proportion to the product of the intensities of two pulsed lights, when the light intensity of one of the pulsed lights is modulated, the SRS signal changes in synchronization. Since the SRS signal becomes the largest when the polarizations of the two pulsed lights are matched, the SRS signal changes in synchronization with the modulation of the polarization direction.

That is, the SRS signal can be detected by the lock-in detection in which the frequency for modulating the pulsed light is the lock-in frequency. The modulation frequency is arbitrary, but the higher the frequency, the less susceptible it is to light source noise such as 1 / f noise, and the signal band that can be acquired by the lock-in amplifier can be widened. The modulation means 523 can be configured by using an electro-optical element such as a Pockels cell. In order to acquire the SRS signal by this lock-in detection with high sensitivity, the noise reduction device of this embodiment can be applied.

As an example, the modulation means 523 modulates the light intensity at a frequency that is one-fourth of the repetition frequency of the pulsed light generated by the pulse laser 501. The time profile of the pulsed light output by the modulation means 523 is shown in FIG. 11 (c).

As shown in FIG. 11D, since the SRS signal is modulated at 20 MHz and generated, the noise in the vicinity of 20 MHz included in the pulsed light to be detected is reduced by the noise reducing device of this embodiment, so that the signal detection is enhanced. Sensitivity can be increased.

In order to reduce the noise of 20 MHz, the time interval of the pulsed light divided by the noise reducing device 200 may be 25 nanoseconds. To create this time interval, the difference in fiber length in the first delay combiner of the noise reduction device 200 may be 5 meters (speed of light 3 × ¹⁰⁸ meters per second, group refraction of optical fiber). The rate was 1.5). Further, the difference in fiber length in the second delayed combiner means may be 10 meters.

FIG. 11A is a time profile of the optical pulse train 31 input to the noise reduction device 200, and FIG. 11B is a time profile of the optical pulse train 37 output by the noise reduction device 200. The change in light intensity due to SRS can be detected by a lock-in amplifier having a modulation frequency of 20 MHz for pulsed light by the modulation means 523 as a lock-in frequency. The detection method is the same as that of the first embodiment except for the lock-in frequency.

When the pulsed light is modulated by N times the pulse period of the pulsed light generated by the pulse laser (N is an integer of 1 or more), the time interval of the divided pulse generated by the noise reduction device should be N / 2 of the pulse period. Noise near the lock-in frequency can be reduced. That is, the two delay times may be N / 2 times the pulse period and N times the pulse period.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these examples, and various modifications and modifications can be made within the scope of the gist thereof.

For example, it can also be used in an optical microscope for detecting light generated by a nonlinear optical phenomenon such as CARS (coherent anti-Stokes Raman scattering) and multiphoton excitation (particularly two-photon excitation) fluorescence. It can also be used to detect light modulated by a nonlinear optical phenomenon such as multiphoton absorption (particularly two-photon absorption).

Further, in the above embodiment, the polarization direction is rotated by 60 degrees by the polarization adjusting unit (5, 15). However, the polarization adjusting unit may be omitted by rotating the two delayed combined light paths relative to each other around the optical axis.

It should be noted that "the ratio of the intensity of the first pulse light to the intensity of the second pulse light is 1: 1", "the ratio of the intensity of the third pulse light to the intensity of the fourth pulse light is 1: 3", and "the fifth pulse". The expression "the ratio of the intensity of light to the intensity of the sixth pulsed light is 3: 1" includes the case where the intensity ratio is not strictly satisfied. Specifically, since the effect of the present invention can be obtained even when at least one of the third to sixth pulsed lights has a 10% increase or a 10% decrease in the light intensity before division, these can be obtained. The expression also includes this case.

Further, the expression "the ratio of the first delay time to the second delay time is 1: 2 or 2: 1" also includes the case where the time ratio is not strictly satisfied. Specifically, the present invention also satisfies | τ ₁ − τ _2/2 | <τ _1/6 when the ratio of the first delay time τ ₁ and the second delay time τ ₂ is 1: 2. The effect of can be obtained. Further, the effect of the present invention is also obtained when | τ ₁ / 2-τ ₂ | <τ _2/6 is satisfied when the ratio of the first delay time τ ₁ and the second delay time τ ₂ is 2: 1. Obtainable. Therefore, the expression includes these cases as well.

The technical elements described herein or in the drawings exhibit their technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the techniques exemplified in the present specification or the drawings achieve a plurality of purposes at the same time, and achieving one of the purposes itself has technical usefulness.

1, 2, 6, 7 Polarized beam splitter 3, 4, 8, 9 Mirror 5 Polarized adjusting unit 20 Incident pulsed light 21 First pulsed light 22 Second pulsed light 23 Third pulsed light 24 Fourth pulsed light 25 5th pulse light 26 6th pulse light

Claims (25)

A noise reduction device that reduces the noise of pulsed light in the first cycle.
By splitting the pulsed light of the first period into two and adding a first delay time between the two split pulsed lights to combine the light, the light is polarized in the first direction. A first pulsed light that outputs a pulsed light and a second pulsed light that is polarized in a direction orthogonal to the first direction and delayed by the first delay time with respect to the first pulsed light. Delayed wave means and
A third pulsed light polarized in the second direction is obtained by splitting the first pulsed light into two and adding a second delay time between the two split pulsed lights to combine the first pulsed light. The pulsed light and the fourth pulsed light polarized in the direction orthogonal to the second direction and delayed by the second delay time with respect to the third pulsed light are output, and the second pulsed light is output. The fifth pulsed light polarized in the second direction is obtained by splitting the pulsed light into two and adding the second delay time between the two splitted pulsed lights to combine the light. And a sixth pulsed light that is polarized in a direction orthogonal to the second direction and delayed by the second delay time with respect to the fifth pulsed light, and outputs a second delayed combined wave. Means and
It has a polarization adjusting unit that adjusts the deflection direction of the first pulsed light and the polarization direction of the second pulsed light.
The polarization adjusting unit is a noise reducing device characterized in that it is located between the first delayed combining unit and the second delayed combining unit. The polarization adjusting unit branches the first pulsed light into the third pulsed light and the fourth pulsed light at a predetermined intensity ratio in the second delayed combining unit, and the second pulsed light is divided into the third pulsed light and the fourth pulsed light. A claim that rotates the deflection direction of the first pulsed light and the polarization direction of the second pulsed light so that the light is branched into the sixth pulsed light and the fifth pulsed light at a predetermined intensity ratio. The noise reducing device according to 1. The noise reducing device according to claim 2, wherein the predetermined intensity ratio is substantially 1: 3. The noise reducing device according to claim 1 or 2, wherein the polarization adjusting unit rotates the deflection direction of the first pulse light and the polarization direction of the second pulse light at a common rotation angle. The noise reduction device according to claim 4, wherein the common rotation angle is about 30 degrees or about 60 degrees. The noise reducing device according to any one of claims 1 to 5, wherein the polarization adjusting unit includes a half-wave plate or a fusion unit in which the ends of a pair of optical fibers are fused to each other. The noise reducing device according to any one of claims 1 to 6, wherein the first delayed wave splitting means includes a first demultiplexing polarization beam splitter and a first wave split beam splitter. The noise reducing device according to claim 7, wherein the first combined wave polarization beam splitter combines the first pulsed light and the second pulsed light, each of which is linearly polarized light. The noise reducing device according to any one of claims 1 to 8, wherein the light intensity ratio of the first pulsed light to the second pulsed light is substantially 1: 1. The noise reducing device according to any one of claims 1 to 9, wherein the second delayed wave combine means has a second demultiplexing polarization beam splitter and a second demultiplexing polarization beam splitter. The second combined wave polarized beam splitter combines the third pulsed light, the fourth pulsed light, the fifth pulsed light, and the sixth pulsed light, each of which is linearly polarized. Item 10. The noise reduction device according to Item 10. The noise reducing device according to any one of claims 1 to 11, wherein the first delayed wave combination means imparts the first delay time by a difference in length of a polarization holding fiber. The noise reducing device according to any one of claims 1 to 12, wherein the second delay wave combining means imparts the second delay time by a difference in the length of the polarization holding fiber. A polarization holding fiber is provided between the first delayed merging means and the second delayed merging means, and the polarization holding fiber is a pair of optical fibers so as to change the polarization direction of the pulsed light. Has a fused portion in which the ends of the
The noise reducing device according to any one of claims 1 to 5, wherein the polarization adjusting unit is a fusion unit. One of claims 1 to 14, wherein the shorter delay time of the first delay time and the second delay time is N / 2 times the first cycle (N is an integer of 1 or more). The noise reduction device described in the section. The noise reduction device according to claim 15, wherein the longer delay time of the first delay time and the second delay time is N times the first cycle (N is an integer of 1 or more). The noise reducing device according to any one of claims 1 to 16, wherein the first cycle corresponds to a repeating cycle of pulsed light from a light source. The noise reducing device according to any one of claims 1 to 17, wherein the ratio of the delay time between the first pulsed light and the second pulsed light is 1: 2 or 2: 1. The noise reduction device according to any one of claims 1 to 18.
With the first generation means for generating pulsed light in the first period,
An irradiation means for irradiating the sample with the pulsed light via the noise reducing device, and an irradiation means.
A detection device comprising a detection means for detecting light from the sample. It has a second generation means of generating pulsed light in the second period.
The irradiation means irradiates the sample with the pulsed light generated by the first generation means and the pulsed light generated by the second generation means via the noise reducing device.
The detection device according to claim 19, wherein the detection means detects light modulated in the second cycle. The detection device according to claim 20, wherein the second cycle is N times the first cycle (N is an integer of 1 or more). The detection device according to claim 20 or 21, wherein the detection means has a lock-in amplifier having a frequency corresponding to the second cycle as a lock-in frequency. A third generation means for generating pulsed light in the first cycle,
It has a modulation means that modulates at least one of the optical path length, the light intensity, and the polarization direction of the pulsed light generated by the third generation means by N times the first period.
The irradiation means irradiates the sample with pulsed light generated by the first generation means via the noise reducing device and pulsed light generated by the third generation means via the modulation means, and the detection is performed. The detection device according to claim 19 , wherein the means is to detect light modulated by N times the first period. 23. The detection device according to claim 23, wherein the detection means includes a lock-in amplifier having a lock-in frequency having a frequency corresponding to N times the first cycle. The modulated light is intensity-modulated by two-photon excitation in the sample, light generated by multiphoton excitation in the sample, light generated by nonlinear Raman scattering in the sample, and two-photon absorption in the sample. The pulsed light generated by the first generation means, the pulsed light generated by the first generation means whose intensity was modulated by multiphoton absorption in the sample, or the first intensity-modulated intensity by induced Raman scattering in the sample. The detection device according to any one of claims 19 to 24, wherein the generation means is generated pulsed light.

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