WO2016006224A1

WO2016006224A1 - Light pulse synchronizer, illumination apparatus, optical microscope, and light pulse synchronization method

Info

Publication number: WO2016006224A1
Application number: PCT/JP2015/003384
Authority: WO
Inventors: Naoki Kohara; Chidane Ouchi
Original assignee: Canon Kabushiki Kaisha
Priority date: 2014-07-11
Filing date: 2015-07-06
Publication date: 2016-01-14
Also published as: JP2016018978A

Abstract

A light pulse synchronizer (50) that synchronizes, with each other, a first light pulse generated at a first repetition frequency and a second light pulse generated at a second repetition frequency includes a modulator (10) configured to change an optical path length of at least one of the first and second light pulses at a modulation frequency, a photodetector (16) configured to receive the first and second light pulses to output a first signal, a timing detector (17) configured to output a second signal based on a signal with the modulation frequency and on the first signal, and an adjuster (19) configured to adjust at least one of the first and second repetition frequencies depending on the second signal.

Description

LIGHT PULSE SYNCHRONIZER, ILLUMINATION APPARATUS, OPTICAL MICROSCOPE, AND LIGHT PULSE SYNCHRONIZATION METHOD

The present invention relates to a light pulse synchronizer, an illumination apparatus, an optical microscope, and a light pulse synchronization method, each of which makes pulse timings of two light pulse trains emitted by two pulse lasers coincide with each other.

Pulse lasers have been utilized in various fields in recent years. As applications of pulse lasers, Raman scattering microscopes utilizing a nonlinear optical process have been studied. For example, CARS (coherent anti-Stokes Raman scattering) microscopes and SRS (stimulated Raman scattering) microscopes are known. These microscopes focus, onto a sample, light pulse trains emitted by two pulse lasers, with pulse timings of the light pulse trains made coincide with each other. That is, it is necessary to synchronize the light pulse trains from the two pulse lasers with each other.

An SRS microscope disclosed in Patent Literature 1 detects, as a pulse timing difference, an output of a photodetector that detects two-photon absorption, and controls repetition frequency of one of the light pulse trains such that the output of the photodetector has a set value. A CARS microscope disclosed in Patent Literature 2 uses two similar photodetectors that respectively detect a pulse timing difference, and utilizes a difference between outputs of the two photodetectors for controlling repetition frequency of one of the light pulse trains. These configurations enable synchronizing the light pulse trains with each other even when light intensities, wavelengths, and/or pulse widths of the light pulse trains change.

[PLT1] International Publication NO. 2010/140614

[PLT2] Japanese Patent No. 4862164

The light pulse train synchronization method disclosed in Patent Literature 1 inevitably requires changing an output circuit of the photodetector and the output set value of the photodetector to synchronize the light pulse trains with each other when the light intensities, the wavelengths and/or the pulse widths of the light pulse trains emitted by pulse lasers change. On the other hand, the CARS microscope disclosed in Patent Literature 2 requires two photodetectors that detect the pulse timing difference, to stably synchronize, with each other, the light pulse trains even when the light intensities, the wavelengths and/or the pulse widths of the light pulse trains emitted by pulse lasers change. The apparatus including the two photodetectors has to be configured such that their sensitivities and wavelength characteristics thereof coincide with each other and such that light intensities and pulse widths input to them are mutually identical. A failure to satisfy this requirement makes it impossible to synchronize the light pulse trains with each other, resulting in a time difference between the pulses of the light pulse trains emitted by the two pulse lasers when the wavelengths change. Furthermore, since the outputs of the photodetectors largely depend on a positional relation between an objective lens and a light receiving surface of each photodetector, it is necessary to make the positional relation between the objective lens and the corresponding photodetector with respect to one of the two lights, and such a positional relation with respect to the other light coincide with each other to synchronize the light pulse trains with each other.

In view of these problems, an object of the present invention is to provide a light pulse synchronizer stably operable even when light intensities, wavelengths, pulse widths of light pulse trains, and/or a positional relation between an objective lens and a photodetector, and the like change.

A light pulse synchronizer as one aspect of the present invention is a light pulse synchronizer that synchronizes, with each other, a first light pulse generated at a first repetition frequency and a second light pulse generated at a second repetition frequency and that includes a modulator configured to change an optical path length of at least one of the first and second light pulses at a modulation frequency, a photodetector configured to receive the first and second light pulses to output a first signal, a timing detector configured to output a second signal based on a signal with the modulation frequency and on the first signal, and an adjuster configured to adjust at least one of the first and second repetition frequencies depending on the second signal.

A light pulse synchronization method as another aspect of the present invention is a light pulse synchronization method that synchronizes, with each other, a first light pulse generated at a first repetition frequency and a second light pulse generated at a second repetition frequency and that includes a modulation step of changing an optical path length of at least one of the first and second light pulses at a modulation frequency, a photodetection step of receiving the first and second light pulses to output a first signal, a timing detection step of outputting a second signal based on a signal with the modulation frequency and on the first signal, and an adjustment step of adjusting at least one of the first and second repetition frequencies depending on the second signal.

Further features and aspects of the present invention will become apparent from the following description of exemplary examples with reference to the attached drawings.

The present invention provides a light pulse synchronizer stably operable even when light intensities, wavelengths, pulse widths light pulse trains, and/or a positional relation between an objective lens and a photodetector, and the like change.

FIG. 1 is a conceptual view of a light pulse synchronizer according to an embodiment of the present invention. FIG. 2 illustrates time profiles of light pulse trains received at a photodetector. FIG. 3 illustrates time profiles of an output voltage of the photodetector and a voltage with a modulation frequency. FIG. 4 is a diagram illustrating a relation between an output voltage of a timing detection circuit and a pulse timing difference. FIG. 5 is a conceptual view of a light pulse synchronizer including an optical path length modulator having a configuration different from that illustrated in FIG. 1. FIG. 6 is a conceptual view of an SRS microscope utilizing the light pulse synchronizer.

A detailed description will be given below of a light pulse synchronizer as an optical apparatus that is an embodiment of the present invention, with reference to the attached drawings. In the drawings, identical constituent elements are denoted by the same reference numerals, and a duplicated description thereof will be omitted.

FIG. 1 is a conceptual view of a light pulse synchronizer according to the embodiment of the present invention. A light pulse synchronizer 50 makes a timing of a first pulse generated by a pulse laser (a first light source) 1 at a first repetition frequency and a timing of a second pulse generated by a pulse laser (a second light source) 2 at a second repetition frequency coincide with each other.

A half mirror 3 coaxially combines light pulse trains respectively emitted from the

pulse lasers

1 and 2, and separates them into two directions. One of the separated light pulse trains is utilized by the light pulse synchronizer 50, and the other is utilized by a system, such as a nonlinear optical microscope, which requires synchronized light pulse trains. A dichroic mirror 4 is a multi-layer dielectric film designed to transmit a first light pulse train having a wavelength λ1 and to reflect a second light pulse train having a wavelength λ2 different from the wavelength λ1. Although, in this embodiment, the respectively emitted light pulse trains are coaxially combined by the half mirror 3 and then separated by the dichroic mirror 4 into the light pulse trains with the mutually different two wavelengths, the first and second light pulse trains may be respectively separated by a half mirror before being coaxially combined by the half mirror 3.

The first light pulse train is introduced by a collimator lens 5 to an optical fiber 8. The second light pulse train is reflected by a mirror 6, and then introduced by a collimator lens 7 to an optical fiber 9.

An optical path length modulator (a modulator) 10 increases and decreases a length of the optical fiber 9 at a frequency (a modulation frequency) that can be changed by a user as appropriate, thereby periodically changing an optical path length of the light passing through the optical fiber 9. The optical path length modulator 10 of this embodiment is constituted by a cylindrical-shaped piezoelectric or electrostrictive element and the optical fiber 9 wrapped therearound. The piezoelectric element or the electrostrictive element is shifted in a radial direction of the cylinder in response to a voltage applied to the piezoelectric element or the electrostrictive element that is changed at the modulation frequency. Therefore, the optical fiber 9 is extended and contracted at the modulation frequency, which periodically changes the optical path length of the light passing through the optical fiber 9. As a result, a timing at which the second light pulse train reaches a photodetector 16 changes.

An amount of the optical path length change caused by the extension and contraction of the optical fiber 9 is set to approximately equal to a distance by which the light proceeds in a pulse width of the pulse laser 2. When the pulse width is one picosecond, the amount of the optical path length change is approximately 300 μm. In view of a refractive index of a core of the optical fiber 9, a sufficient amount of the increase and decrease in the length of the optical fiber 9 is approximately 200 μm. Using a cylindrical piezoelectric element with a diameter of several tens of millimeters and an optical fiber with a length of approximately 10 m can achieve this 200-μm increase and decrease in the length of the optical fiber at a modulation frequency of approximately 10 kHz.

The first light pulse train is released from the optical fiber 8 again to the space through a collimator lens 11, and then reflected by a mirror 13. The second light pulse train is released from the optical fiber 9 again to the space through a collimator lens 12. A dichroic mirror 14 transmits the first light pulse train, and reflects the second light pulse train. Therefore, the first and second light pulse trains are released to the space, coaxially combined by the dichroic mirror 14, and then collected by an objective lens 15 on a light receiving surface of the photodetector 16. The dichroic mirror 14 may have a configuration identical to that of the dichroic mirror 4. The objective lens 15 with a numerical aperture of 0.5 or more is suitable to allow the photodetector 16 to detect a two-photon-absorption signal having a higher value.

The length of the optical fiber 8 is adjusted such that, in a section from the half mirror 3 to the photodetector 16, an optical path length by which the first light pulse train proceeds and an optical path length by which the second light pulse train proceeds coincide with each other. The light pulse synchronizer of this embodiment makes timings at which the pulses of the first and second light pulse trains reach the photodetector 16 coincide with each other. For this reason, adjusting the optical fiber 8 as described above makes pulse timings of the other pair of the light beams that are separated by the half mirror 3 (and are to be utilized by a nonlinear microscope and the like) coincide with each other.

The photodetector 16 is constituted by, for example, a light receiving element such as a photodiode, and an electrical circuit that converts an electric current generated in the light receiving element into a voltage and outputs the voltage. The light receiving element of the photodetector 16 has a sensitivity to a sum of a photon energy of the first light pulse train (E1, proportional to 1/λ1) and a photon energy of the second light pulse train (E2, proportional to 1/λ2), namely, a wavelength λ1·λ2/(λ1+λ2) corresponding to E1+E2 to acquire the two-photon-absorption signal. When wavelengths of the first light pulse train and the second light pulse train are 800 nm and 1030 nm, respectively, the light receiving element needs to have a photodetection sensitivity of around 450 nm. It is thus suitable that the light receiving element is constituted by a GaAsP photodiode. When the wavelength of one of the first and second light pulse trains is longer than that described above, it is desirable to use a Si photodiode as the light receiving element.

FIG. 2 illustrates time profiles of the light pulse trains received by the photodetector 16. FIG. 2(a) illustrates a time profile of an intensity of the first light pulse train on the light receiving surface of the photodetector 16. FIG. 2(b) illustrates a time profile of an intensity of the second light pulse train on the light receiving surface of the photodetector 16. In this embodiment, a ratio between the first repetition frequency at which the first light pulse train is generated and the second repetition frequency at which the second light pulse train is generated is 2:1. For this reason, as illustrated in FIGS. 2(a) and 2(b), the timing of the pulse contained in the first light pulse train coincides, every other pulse, with the timing of the pulse contained in the second light pulse train. The ratio between the first repetition frequency at which the first light pulse train is generated and the second repetition frequency at which the second light pulse train is generated is not limited to 2:1 as in this embodiment, and any integer ratio may alternatively be adopted. FIG. 2(c) illustrates a time profile of a two-photon-absorption signal (i.e., an electric current generated in the photodiode) corresponding to the sum of the photon energies of the first and second light pulse trains (E1+E2) that is acquired when the first and second light pulse trains are respectively in states illustrated in FIGS. 2(a) and 2(b). FIG. 2(d) illustrates a time profile of the intensity of the second light pulse train that reaches the photodetector 16 after a delay caused by the optical path length modulator 10. FIG. 2(e) illustrates a time profile of a two-photon-absorption signal corresponding to the sum of the photon energies of the first and second light pulse trains (E1+E2) that is acquired when the first and second light pulse trains are respectively in states illustrated in FIGS. 2(a) and 2(d). Since the value of the two-photon-absorption signal is proportional to a product of the intensities of the two pulses, the value of the two-photon-absorption signal that has a difference between the pulse timings of the first and second light pulse trains as illustrated in FIG. 2(e) is lower compared to that of the two-photon-absorption signal that has no difference between the timings as illustrated in FIG. 2(c).

The voltage output by the photodetector 16 contains, in addition to a component caused by the two-photon absorption corresponding to E1+E2, a component caused by two-photon absorption by each of the first and second light pulse trains themselves such as E1+E1 and E2+E2, and a component, such as E1 and E2, caused by each original photon energy. However, the components other than E1+E2 are unwanted for the light pulse synchronizer of this embodiment. In particular, a detection of the component caused by each of the original photon energies (i.e., E1 and E2) and having a considerably high value makes it difficult to detect the component corresponding to E1+E2. It is thus desirable to adjust a band gap between a p-n junction of the light receiving element of the photodetector 16 such that the band gap is equal to or more than the photon energies E1 and E2, thereby preventing the components caused by the original photon energies from being detected. When a wavelength corresponding to the photon energy E1 is 800 nm, and a wavelength corresponding to the photon energy E2 is 1030 nm, it is desirable to set the band gap to a value equal to or more than the photon energy E1 (=1.55 electron volts). The GaAsP photodiode satisfies this condition.

Although the components caused by the two-photon absorptions that are unwanted (i.e., E1+E1 and E2+E2) cannot be removed utilizing the band gap, the following method enables acquiring only the component attributable to E1+E2.

The optical path length modulator 10 changes the difference between the timings of the first and second light pulse trains at which the respective pulses reach the light receiving element of the photodetector 16 to alter the component attributable to E1+E2. On the other hand, the components attributable to E1+E1 and E2+E2 are caused by the light pulse trains themselves, and therefore do not alter. For this reason, extracting an amplitude of a modulation frequency component contained in an output voltage of the photodetector 16 enables acquiring only the component attributable to E1+E2.

A timing detection circuit (a timing detector) 17, which is an electrical circuit such as a lock-in amplifier, extracts the amplitude of the modulation frequency component contained in a first voltage (a first signal) that is the output voltage of the photodetector 16, and outputs the amplitude as a second voltage (a second signal). Specifically, the timing detection circuit 17 mixes, with a mixer, the voltage output by the photodetector 16 and a rectangular- or sinusoidal-shaped voltage signal having the modulation frequency, causes the resulting voltage signal to pass through a low-pass filter circuit, and then outputs the same. A cut-off frequency of the low-pass filter is set to approximately 1 kHz, a frequency suitable for feedback control of the frequencies of the light pulse trains.

FIG. 3 illustrates time profiles of the output voltage of the photodetector 16 and the voltage with the modulation frequency utilized by the timing detection circuit 17. FIG. 3 schematically illustrates the time profiles with an assumption that a band frequency of the photodetector 16 is lower than the repetition frequencies of the light pulse trains. That is, pulse-by-pulse time profiles cannot be detected, and therefore the signal with an average value in several to hundreds of pulses is illustrated. The output voltage of the photodetector 16 contains, in addition to the component attributable to the variable value E1+E2, the components attributable to E1+E1 and E2+E2. FIG. 4 illustrates a relation between the pulse timing difference and the output voltage of the timing detection circuit 17.

When the first and second light pulse trains synchronize with each other, the output voltage of the photodetector 16 periodically changes with the change in the optical path length of the second light pulse train as indicated by a solid line of FIG. 3. The expression “the first and second light pulse trains synchronize with each other” herein means that the pulse timings of the first and second light pulse trains coincide with each other at a position of the half mirror 3. At the time when the voltage of the modulation frequency indicated by a dashed double-dotted line of FIG. 3 is zero, the pulse timings of the first and second light pulse trains coincide with each other, and therefore the output voltage of the photodetector 16 becomes the largest at these time points. When a value of the voltage with the modulation frequency becomes positive or negative, the pulse timing difference occurs, and thus the output voltage of the photodetector 16 lowers. The output voltage of the timing detection circuit 17 in this case is a time average of a product of the output voltage of the photodetector 16 and the voltage with the modulation frequency respectively indicated by the dashed double-dotted line and the solid line of FIG. 3, and therefore the output voltage has a value of zero as illustrated in FIG. 4.

A description will be given of an output of the timing detection circuit 17 corresponding to when a pulse timing difference between the first and second light pulse trains occurs due to a change in a cavity length of the pulse laser 1 or the pulse laser 2 caused by an external disturbance. This pulse timing difference changes at a frequency lower than the modulation frequency and the low-pass filter cut-off frequency of the timing detection circuit 17.

When the first light pulse train lags behind the second light pulse train, the output voltage of the photodetector 16 becomes the largest in a period of time during which the optical path length modulator 10 delays the second light pulse train (i.e., a period of time during which a value of the signal with the modulation frequency is positive), as indicated by a dotted line of FIG. 3. The output voltage of the timing detection circuit 17 in this case is a time average of a product of the output voltage of the photodetector 16 and the voltage with the modulation frequency respectively indicated by the dotted line and the dashed double-dotted line of FIG. 3, and therefore the value of the output voltage becomes positive as illustrated in FIG. 4. When the first light pulse train lags more, the timing detection circuit 17 outputs a signal with a higher positive value.

When the first light pulse train leads the second light pulse train, the value of the output voltage of the photodetector 16 becomes the largest in a period of time during which the optical path length modulator 10 causes the second light pulse train to lead the first light pulse train (i.e., a period of time during which the value of the signal with the modulation frequency is negative), as indicated by a dashed-dotted line of FIG. 3. The output voltage of the timing detection circuit 17 in this case is a time average of a product of an output voltage of the photodetector 16 and the voltage with the modulation frequency respectively indicated by a dashed-dotted line and the dashed double-dotted line of FIG. 3, and therefore the value of the output voltage becomes negative as illustrated in FIG. 4. When the first light pulse train leads the second light pulse train more, the timing detection circuit 17 outputs a signal having a negative value of a larger absolute value.

As described above, the output voltage of the timing detection circuit 17 reflects the difference between the timings at which the pulses of the first and second light pulse trains reach the half mirror 3.

A feedback circuit 18 outputs a voltage to be applied to a frequency adjuster (an adjuster) 19 included in the pulse laser 2, in order to correct the pulse timing difference corresponding to the output voltage of the timing detection circuit 17.

The frequency adjuster 19 is constituted by a phase modulator or a stage with a mirror, and applies a voltage to the phase modulator or drives the stage to adjust the cavity length of the pulse laser 2. The frequency adjuster 19 may be installed alternatively in the pulse laser 1 to adjust the cavity length of the pulse laser 1.

When the output voltage of the timing detection circuit 17 has a positive value, the first light pulse train lags behind the second light pulse train. Therefore, the frequency adjuster 19 increases the cavity length of the pulse laser 2 to delay the second light pulse train, thereby synchronizing the first and second light pulse trains with each other. For this process, the feedback circuit 18 applies, to the frequency adjuster 19, a voltage that increases the cavity length of the pulse laser 2. Conversely, when the output voltage of the timing detection circuit 17 has a negative value, the feedback circuit 18 outputs a voltage that decreases the cavity length of the pulse laser 2. As described above, the feedback circuit 18 performs feedback control that changes the value of the output voltage of the timing detection circuit 17 to zero to make the pulse timings of the first and second light pulse trains coincide with each other. Alternatively, the feedback circuit 18 may be configured such that the output voltage of the timing detection circuit 17 has a value that is not zero, thereby providing a predetermined timing difference between the pulses of the first and second light pulse trains.

The light pulse synchronizer of this embodiment is capable of determining, based on a sign of the value of the output voltage of the timing detection circuit 17, whether the first light pulse train lags behind or leads the second light pulse train. This capability enables synchronizing the light pulse trains with each other even when the light intensities and/or the pulse widths of the light pulses change because only the absolute value of the output voltage of the timing detection circuit 17 changes and the sign does not change. Similarly, this capability enables synchronizing the light pulse trains with each other even when the wavelengths of the light pulses change within a range in which the photodetector 16 has a sensitivity. In addition, since this embodiment utilizes only one photodetector, it is not necessary to make a positional relation between one objective lens and one photodetector, and a positional relation between the other objective lens and the other photodetector coincide with each other as described in Patent Literature 2.

Moreover, although the wavelengths of the lights emitted by the two pulse lasers are mutually different in this embodiment, utilizing polarizing beam splitters instead of the dichroic mirrors enables easily synchronizing, with each other, light pulse trains with mutually identical wavelengths emitted by the two pulse lasers.

Further, although this embodiment changes the optical path length of the optical fiber 9 by the optical path length modulator 10, alternatively changing an optical path length of the optical fiber 8 can also provide the above-described effect. Furthermore, although the single optical path length modulator is provided in this embodiment, multiple optical path length modulators may alternatively be provided. For example, an optical path length modulator identical to the optical path length modulator 10 is used to extend and contract the optical fiber 8. In this case, a phase of the extension and contraction of the optical fiber 8 is inverted with respect to a phase of the extension and contraction of the optical fiber 9. This phase inversion doubles an amount of the optical path length modulation performed by each of the two optical path length modulators, which enables stably synchronizing light pulse trains with wider pulse widths with each other.

Although the optical path length modulator 10 of this embodiment extends and contracts the cylindrical piezoelectric element in the radial direction of the cylinder to extend and contract the optical fiber 9 wrapped therearound, the optical fiber may be linearly extended and contracted. As illustrated in FIG. 5, an optical path length modulator 30 as an alternative optical path length modulator utilizing neither optical fibers nor collimator lenses may be constituted by an electrically-operated stage (a driver) 31, and mirrors (reflectors) 32 to 35.

The photodetector 16 may be replaced with other photodetector having a sensitivity to a wavelength corresponding to the photon energy sum E1+E2 and having a wavelength band equal to or higher than the modulation frequency.

Next, with reference to FIG. 6, a description will be given of an SRS (stimulated Raman scattering) microscope utilizing the light pulse synchronizer of this embodiment. SRS microscopes simultaneously focus, by an optical system, two lights with mutually different wavelengths onto a sample, and detect, by a detection system, an SRS generated by the focusing. The SRS is one of nonlinear optical phenomena that occurs in proportion to a product of intensities of lights having wavelengths. In order to efficiently generate the SRS, SRS microscopes synchronize, with each other, light pulse trains such that light beams emitted by two lasers with mutually different wavelengths are collected on an identical point and such that two light pulses with mutually different wavelengths are simultaneously collected. The generation of the SRS decreases an intensity of one of the two light pulses that has a shorter wavelength, and increases an intensity of the other that has a longer wavelength. In order to efficiently generate the SRS, it is desirable to utilize pulse lasers that emit short light pulses each having a pulse width of 1 to 10 ps.

This embodiment utilizes, as the pulse laser 1, a solid laser (a titanium sapphire laser) with a center wavelength (λ1) of 800 nm and a repetition frequency of 80 MHz. This embodiment also utilizes, as the pulse laser 2, an ytterbium-doped fiber laser with a center wavelength (λ2) of 1030 nm and a repetition frequency of 40 MHz.

The half mirror 3 coaxially combines the light beams respectively emitted from the

pulse lasers

1 and 2, and separates them into two directions. One of the separated light beams enters the light pulse synchronizer, and the other light beam enters an SRS microscope 100. The

pulse lasers

1 and 2, and the light pulse synchronizer 50 of this embodiment together constitute an illumination apparatus.

The light beams from the

pulse lasers

1 and 2 that coaxially enter a beam scanner 101 are deflected by the beam scanner 101, and then exit therefrom. The beam scanner 101 is constituted by a galvanometer scanner and a resonant scanner, and changes an optical axis tilt in two directions orthogonal to each other. For simple illustration, two mirrors in the beam scanner 101 are illustrated in FIG. 6 as one mirror representative thereof. Utilizing the resonant scanner (with a scan frequency of 8 kHz) and the galvanometer scanner (with a scan frequency of 15 Hz) enables acquiring thirty frames of a 500-line image per second.

After deflected by the beam scanner 101, the light beams enter an objective lens 104 through

lenses

102 and 103. Arranging the

lenses

102 and 103 such that the beam scanner 101 and an entrance pupil of the objective lens 104 are conjugate to each other enables collecting the light beams on a sample 105 without a light amount change due to light shielding even after the deflection of the light beams by the beam scanner 101. Focal lengths of the

lenses

102 and 103 are selected such that a size of the entrance pupil of the objective lens 104 and a size of each of the entering light beams are identical to each other. This selection minimizes a size of a light spot formed by the light beams collected by the objective lens 104, which improves a spatial resolution at which an SRS signal is detected. An increase in an intensity of the light spot increases a value of the SRS signal, resulting in a corresponding improvement in an SN ratio at which the SRS signal is detected. In light of the spatial resolution and the SN ratio at which the SRS signal is detected, it is desirable that the objective lens 104 be an objective lens having a large numerical aperture (NA).

The sample 105 is sandwiched using a cover glass (not illustrated) with a thickness of several tens to 200 μm. The light spot formed by the light beams collected on the sample 105 through the deflection of the light beams by the beam scanner 101 is two-dimensionally scanned to convert the SRS signal into a two-dimensional image. Since the SRS signal is locally present in the light spot formed by the collected light beams, moving the sample 105 in an optical axis direction by a stage (not illustrated) enables providing a three-dimensional image as well.

An objective lens 106 is constituted by an objective lens with an NA equivalent to or higher than that of the objective lens 104 to be capable of receiving all of the lights transmitted through the sample 105 and subjected to intensity modulation by the SRS. After exiting from the objective lens 106, the light beams are transmitted through a filter 107 and a lens 108, and then introduced to a light receiving surface of a photodiode 109. The filter 107, which is constituted by a multi-layer dielectric film, cuts off the light with a wavelength λ2, and transmits the light with a wavelength λ1. A light pulse train emitted from the pulse laser 1 and subjected to the intensity modulation by the SRS on a pulse-by-pulse basis is introduced to the photodiode 109. As the photodiode 109, a silicon photodiode is utilized that has a sensitivity to light pulse trains with a wavelength of 800 nm and that has a cut-off frequency of 40 MHz or higher.

An electric current/voltage conversion circuit 110 is an electrical circuit to output, as a voltage, an electric current signal generated at the photodiode 109.

A synchronization circuit 111, which is constituted by a mixer circuit or a lock-in amplifier, extracts an amplitude of a component with a frequency of 40 MHz from the voltage signal output by the electric current/voltage conversion circuit 110, and outputs the amplitude as a voltage. The output voltage of the synchronization circuit 111 indicates a level of the SRS occurring on a condensing point of the sample 105.

A calculator 112 converts the output signal of the synchronization circuit 111 into the two-dimensional image by utilizing a control signal of the beam scanner 101, and displays the two-dimensional image. The calculator 112 is capable also of converting an SRS signal acquired by the movement of the sample 105 in the optical axis direction by the stage (not illustrated) into a three-dimensional image for a display. The calculator 112 is further capable of displaying a Raman spectra based on an SRS signal acquired by changing at least one of the wavelengths of the two pulse lasers.

When the pulse timings of the first and second light pulse trains coincide with each other as illustrated in FIGS. 2(a) and 2(b), and the light beams are collected on the identical point on the sample 105, the light intensities of the light pulses transmitted through the sample 105 change. Specifically, in FIG. 2(a), light intensities of

pulses

1, 3 and 5 decrease, and those of

pulses

2 and 4 do not change. A difference between the detected light intensities corresponds to the SRS signal, and reflects information on molecules contained at the point on which the light beams are collected. For example, when a resonance frequency of molecular vibration at the point on which the light beams are collected, and a difference between light frequencies of the two lasers (c/λ1-c/λ2) match with each other, the SRS signal has a large value. Symbol c represents the speed of light. Acquiring the SRS signal while changing the difference between the light frequencies of the two lasers (c/λ1-c/λ2) enables acquiring the Raman spectra. Types of the molecules contained in the sample can be estimated from the Raman spectra. For this reason, the SRS microscope is capable of acquiring Raman spectra equivalent to those acquired by microscopes utilizing spontaneous Raman scattering. A scattering efficiency of the SRS is significantly higher than that of the spontaneous Raman scattering, which allows the SRS microscope to acquire the Raman spectra in a period of time shorter than that required by the microscopes utilizing the spontaneous Raman scattering. The SRS microscope of this embodiment is capable of stably synchronizing, by the light pulse synchronizer, the light pulse trains with each other even when at least one of the wavelengths of the two lasers are changed to acquire the Raman spectra.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2014-143216, filed July 11, 2014, which is hereby incorporated by reference herein in its entirety.

10 optical path length modulator (modulator)
16 photodetector
17 timing detection circuit (timing detector)
19 frequency adjuster (adjuster)

Claims

A light pulse synchronizer that synchronizes, with each other, a first light pulse generated at a first repetition frequency and a second light pulse generated at a second repetition frequency, the light pulse synchronizer comprising:
a modulator configured to change an optical path length of at least one of the first and second light pulses at a modulation frequency;
a photodetector configured to receive the first and second light pulses to output a first signal;
a timing detector configured to output a second signal based on a signal with the modulation frequency and on the first signal; and
an adjuster configured to adjust at least one of the first and second repetition frequencies depending on the second signal.
The light pulse synchronizer according to claim 1, wherein the first signal is a signal corresponding to a difference between timings at which the first and second light pulses are received.
The light pulse synchronizer according to claim 1 or 2, further comprising an optical fiber through which the first and second light pulses pass.
The light pulse synchronizer according to claim 3, wherein the modulator includes one of a piezoelectric element and an electrostrictive element, each configured to extend and contract the optical fiber.
The light pulse synchronizer according to any one of claims 1 to 4, further comprising a reflector configured to reflect the first and second light pulses.
The light pulse synchronizer according to claim 5, wherein the modulator includes a driver configured to move the reflector.
The light pulse synchronizer according to any one of claims 1 to 6,
wherein the photodetector includes a photodiode configured to receive the first and second light pulses, and
wherein the photodiode is configured to detect an electric current generated by two-photon absorption.
An illumination apparatus comprising:
a first light source configured to emit the first light pulse;
a second light source configured to emit the second light pulse; and
the light pulse synchronizer according to any one of claims 1 to 7.
An optical microscope comprising:
the illumination apparatus according to claim 8, and
an optical system configured to focus the first and second light pulses onto a sample.
The optical microscope according to claim 9, further comprising a detection system configured to detect light whose intensity is modulated by Raman scattering generated by focusing the first and second light pulses onto the sample.
A light pulse synchronization method that synchronizes, with each other, a first light pulse generated at a first repetition frequency and a second light pulse generated at a second repetition frequency, the method comprising:
a modulation step of changing an optical path length of at least one of the first and second light pulses at a modulation frequency;
a photodetection step of receiving the first and second light pulses to output a first signal;
a timing detection step of outputting a second signal based on a signal with the modulation frequency and on the first signal; and
an adjustment step of adjusting at least one of the first and second repetition frequencies depending on the second signal.