WO2016125458A1

WO2016125458A1 - Light pulse train synchronizer, light pulse train synchronizing method, illumination apparatus, detection apparatus, and program

Info

Publication number: WO2016125458A1
Application number: PCT/JP2016/000398
Authority: WO
Inventors: Naoki Kohara
Original assignee: Canon Kabushiki Kaisha
Priority date: 2015-02-06
Filing date: 2016-01-27
Publication date: 2016-08-11
Also published as: JP2016145718A

Abstract

A light pulse train synchronizer configured to synchronize a first light pulse train 21 including light pulses produced periodically at a first period and a second light pulse train 22 including light pulses produced periodically at a second period includes a light detector 5, 6 configured to output a first signal corresponding to at least one of a light intensity of the first light pulse train and a light intensity of the second light pulse train, a light detector 10 configured to output a second signal corresponding to a timing difference, and a control circuit 11 configured to output a control signal for adjusting at least one of the first period and the second period so that the second signal is equal to a target value obtained from the first signal.

Description

LIGHT PULSE TRAIN SYNCHRONIZER, LIGHT PULSE TRAIN SYNCHRONIZING METHOD, ILLUMINATION APPARATUS, DETECTION APPARATUS, AND PROGRAM

The present invention relates to a light pulse train synchronizer configured to synchronize timings of pulses of two light pulse trains emitted by two pulse lasers.

In a nonlinear optical microscope using a nonlinear optical process such as a stimulated Raman scattering microscope, light pulse trains emitted from two pulse lasers need to be condensed on a sample with their timings being synchronized (or with a timing difference being kept constant).

PTL 1 discloses a stimulated Raman scattering (SRS) microscope configured to detect, as a pulse timing difference, an output from a light detector that detects two-photon absorption, and to adjust pulse periods so that a detected value is equal to a set value. PTL 2 discloses a coherent anti-Stokes Raman scattering (CARS) microscope configured to adjust the pulse periods based on a difference between outputs from two light detectors that detect the pulse timing difference.

[PTL 1] WO2010/140614

[PTL 2] WO2007/132540

In PTL 1, when the power of light output from the pulse lasers has changed, a timing difference between synchronized pulses changes, or a pulse synchronization cannot be achieved. In PTL 2, the use of the two light detectors allows for synchronization of the light pulse trains when the power of light has changed. However, the two light detectors need to be configured such that their sensitivities are identical and the powers and pulse time widths of light input to the respective light detectors. Otherwise, a light pulse train synchronization would fail, and a timing difference between synchronized pulses would change. Moreover, since outputs from the light detectors largely depend on the arrangement of a condensing objective lens and light receiving surfaces of the light detectors, each light detector needs to have an identical arrangement of the objective lens and the light detector, which is troublesome.

The present invention provides a light pulse train synchronizer, a light pulse train synchronizing method, an illumination apparatus, a detection apparatus, and a program, which are capable of stably operating.

A light pulse train synchronizer as an aspect of the present invention is a light pulse train synchronizer configured to synchronize a first light pulse train including light pulses produced periodically at a first period and a second light pulse train including light pulses produced periodically at a second period, the light pulse train synchronizer including a first detector configured to output a first signal corresponding to at least one of a light intensity of the first light pulse train and a light intensity of the second light pulse train, a second detector configured to output a second signal corresponding to an arrival timing difference between the light pulse included in the first light pulse train and the light pulse included in the second light pulse train, and a controller configured to output a control signal for adjusting at least one of the first period and the second period so that the second signal is equal to a target value obtained from the first signal.

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

The present invention provides a light pulse train synchronizer, a light pulse train synchronizing method, an illumination apparatus, detection apparatus, and a program which are capable of stably operating.

FIG. 1 is a block diagram of a light pulse train synchronizer according to Embodiment 1 of the present invention. FIG. 2 is a graph of a output signal from a light detector 10 illustrated in FIG. 1 according to Embodiment 1. FIG. 3A illustrates a time profile of intensity of a first light pulse train in synchronization illustrated in FIG. 1 according to Embodiment 1. FIG. 3B illustrates a time profile of intensity of a second light pulse train in synchronization illustrated in FIG. 1 according to Embodiment 1. FIG. 4 is a block diagram of a light pulse train synchronizer according to Embodiment 2 of the present invention. FIG. 5 is a graph of an output signal from the light detector 10 illustrated in FIG. 4 according to Embodiment 2. FIG. 6 is a conceptual diagram of an SRS microscope including the light pulse train synchronizer illustrated in FIG. 1 according to Embodiment 3. FIG. 7A illustrates the time profile of the intensities of the first light pulse train in the SRS microscope illustrated in FIG. 6 according to Embodiment 3. FIG. 7B illustrates the time profile of the intensity of the second light pulse train in the SRS microscope illustrated in FIG. 6 according to Embodiment 3.

Exemplary embodiments of the present invention will be described below with reference to the accompanied drawings.

Embodiment 1

FIG. 1 is an optical path diagram of a light pulse train synchronizer (hereinafter, simply referred to as a synchronizer) according to Embodiment 1 of the present invention. The synchronizer synchronizes a first light pulse train 21 emitted from a pulse laser (first light source) 1 and a second light pulse train 22 emitted from a pulse laser (second light source different from the first light source) 2. The synchronizer, the pulse laser 1, and the pulse laser 2 are included in an illumination apparatus configured to illuminate a sample.

A light pulse train is a series of light pulses emitted from a pulse laser. In other words, the synchronizer maintains constant an arrival timing difference between a light pulse included in the first light pulse train 21 and a light pulse included in the second light pulse train 22. A wavelength (λ1) of the first light pulse train 21 and a wavelength (λ2) of the second light pulse train 22 are different from each other.

The pulse laser 1 is, for example, a mode-locked laser that can adjust a pulse period (first period) of the first light pulse train 21 by changing its cavity length. The pulse laser 1 adjusts the first period in accordance with a pulse timing shift to accurately set a ratio of the first period and a pulse period (second period) of the second light pulse train 22 to be an integer-to-integer ratio, thereby synchronizing pulse timings. The first light pulse train includes the light pulses produced periodically at the first period, and the second light pulse train includes the light pulses produced periodically at the second period.

A beam splitter 3 reflects part of a light beam emitted from the pulse laser 1 into the right direction, and transmits the rest of the light beam into the bottom direction. A beam splitter 4 reflects part of a light beam emitted from the pulse laser 2 into the bottom direction, and transmits the rest of the light beam into the right direction. The beam splitters 3 and 4 only need to reflect incident light at a certain ratio, and thus may be a glass flat plate. Alternatively, the beam splitters 3 and 4 may be each a flat plate coated to have a predetermined reflectance, or a polarizing beam splitter.

A light detector 5 receives light reflected at the beam splitter 3 to acquire a signal proportional to a power of incident light (the first light pulse train 21). In other words, the light detector 5 serves as a light intensity detector (first detector) configured to output a signal (first signal) corresponding to the light intensity of the first light pulse train 21. A light detector 6 receives light reflected at the beam splitter 4 to acquire a signal proportional to a power of incident light (the second light pulse train 22). In other words, the light detector 6 serves as a light intensity detector (first detector) configured to output a signal (the first signal) corresponding to the light intensity of the second light pulse train 22.

The

light detectors

5 and 6 each include a light receiving element such as a photodiode and an avalanche photodiode, and an electrical circuit configured to convert current produced at the light receiving element into voltage. As described later in Embodiment 2, at least one of the

light detectors

5 and 6 may be provided. In other words, the light intensity detectors only need to detect at least one of the light intensity of the first light pulse train 21 and the light intensity of the second light pulse train 22.

A dichroic mirror 7 transmits the first light pulse train 21 and reflects the second light pulse train 22 to combine the first light pulse train 21 and the second light pulse train 22 on the same axis. The dichroic mirror 7 is a dielectric multi-layer film designed to transmit light of the wavelength λ1 and reflect light of the wavelength λ2.

A beam splitter 8 splits the first light pulse train 21 and the second light pulse train 22 combined through the dichroic mirror 7 into two directions. Part of incident light is reflected to be used in the synchronizer, and the rest of the incident light is transmitted to be used in a system such as a nonlinear optical microscope that needs synchronized light pulse trains.

The beam splitter 8 only needs to reflect incident light at a certain ratio, and thus may be a glass flat plate. Alternatively, the beam splitter 8 may be a flat plate coated to have a predetermined reflectance, or a polarizing beam splitter.

The first light pulse train 21 and the second light pulse train 22 reflected at the beam splitter 8 are condensed on a light receiving surface of a light detector 10 through an objective lens 9. The objective lens 9 preferably has a numerical aperture of 0.5 or larger to have a large two-photon absorption signal detected by the light detector 10.

The light detector 10 serves as a timing difference detector (second detector) configured to output a timing difference signal (second signal) corresponding to an arrival timing difference between the light pulse included in the first light pulse train 21 and the light pulse included in the second light pulse train 22 on the light receiving surface. The light detector 10 includes, for example, a light receiving element such as a photodiode, and an electrical circuit configured to convert current produced at the light receiving element into voltage for outputting. To obtain a two-photon absorption signal, the light receiving element of the light detector 10 has a sensitivity to a wavelength of λ1・λ2/(λ1+λ2) corresponding to a sum of a photon energy (E1∝1/λ1) of the first light pulse train 21 and a photon energy (E2∝1/λ2) of the second light pulse train 22, that is, E1+E2. When λ1 is 1030 nm and λ2 is 800 nm, the light receiving element needs to have a light detection sensitivity at 450 nm.

FIG. 2 is a graph illustrating a relationship between an output signal (vertical axis) from the light detector 10 and a difference (horizontal axis) obtained by subtracting a timing at which the light pulse of the second light pulse train 22 arrived at the light receiving surface from a timing at which the light pulse of the first light pulse train 21 arrived at the light receiving surface of the light detector 10.

The output (timing difference signal) of the light detector 10 includes two-photon absorption signals corresponding to E1+E1 and E2+E2 in addition to a two-photon absorption signal corresponding to E1+E2. The two-photon absorption signals corresponding to E1+E1 and E2+E2 provides an offset to the two-photon absorption signal corresponding to E1+E2. The two-photon absorption signals corresponding to E1+E1 and E2+E2 are produced at the first and second periods, respectively. However, since a frequency range of the output signal (output voltage) of the light detector 10 is set to be small compared to frequencies correspond to these periods, the output signal is a DC component (offset).

The two-photon absorption signal corresponding to E1+E2 is produced when the timing of the light pulse of the first light pulse train 21 and that of the second light pulse train 22 coincide with each other (in other words, when two pulses simultaneously arrive at the light receiving surface of the light detector 10). When peak intensities of the two pulses completely coincide with each other, a maximum signal is obtained. This signal is proportional to a product of the power of the first light pulse train 21 and the power of the second light pulse train 22. Thus, a maximum value of the two-photon absorption signal corresponding to E1+E2 can be expressed as a0・V1・V2 using a proportionality coefficient a0, an output signal V1 of the light detector 5, and an output signal V2 of the light detector 6.

Similarly, the two-photon absorption signal corresponding to E1+E1 can be expressed as a1・V1・V1 using a proportionality coefficient a1 and V1, and the two-photon absorption signal corresponding to E2+E2 can be expressed as a2・V2・V2 using a proportionality coefficient a2 and V2.

The proportionality coefficients a0, a1, and a2 can be easily calculated from data corresponding to FIG. 2 acquired using, for example, an oscilloscope and a data logger, and the output signals of the

light detectors

5 and 6, and are stored in a storage.

The output signal of the light detector 10, the output signal of the light detector 5, and the output signal of the light detector 6 are input to a control circuit 11. The control circuit 11 is a controller configured to output a signal for pulse synchronization to a pulse period adjuster 12 based on the input signals, and includes a micro computer. The control circuit 11 outputs a control signal for adjusting at least one of the first period and the second period so that the output signal (second signal) of the light detector 10 equal to a target value obtained from the first signal, which is expressed by an expression below, to the pulse period adjuster 12. Thus, the control circuit 11 inputs the control signal to at least one of the

pulse lasers

1 and 2.
Target value = a0/2・V1・V2 + a1・V1・V1 + a2・V2・V2 ... (1)

In other words, the control circuit outputs the control signal for adjusting at least one (the first period, in this example) of the first period and the second period so that the timing difference signal is equal to the target value VA. The target value is not limited to VA.

The control circuit 11 includes a storage configured to store information of the target value for the output signal of the light detector 10, and this target value is writable. Thus, the storage is, for example, an EEPROM. The information of the target value includes, for example, Expression (1), the proportionality coefficients a0, a1, and a2, and ε.

The pulse period adjuster 12 includes a phase modulator or a stage to which a mirror is attached, and adjusts the cavity length of the pulse laser 1 by voltage application to the phase modulator or driving of the stage. The adjustment of the cavity length changes the first period, thereby adjusting a pulse timing. The pulse period adjuster 12 may be installed not in the pulse laser 1 but in the pulse laser 2 to adjust the cavity length of the pulse laser 2. The present invention only requires a pulse period adjuster configured to adjust at least one of the first period and the second period needs.

The control circuit 11 controls the first period so that a timing difference illustrated by a dashed and single-dotted line in FIG. 2 is obtained. FIGs. 3A and 3B illustrate relationships between the intensities (vertical axis) of the first light pulse train 21 and the second light pulse train 22, respectively, in pulse synchronization on the light receiving surface of the light detector 10 and time t (horizontal axis). When the pulse synchronization is achieved, a timing difference between an intensity peak of a particular light pulse 21A of the first light pulse train 21 and an intensity peak of a particular light pulse 22A of the second light pulse train 22 coincides with a timing difference TD in the pulse synchronization illustrated in FIG. 2. When the pulse synchronization is continuously achieved, a light pulse 22B of the light pulse train 22 arrives at a light-receiving portion of the light detector 10 with the timing difference illustrated in FIG. 2 to a light pulse 21B adjacent to the light pulse 21A.

In FIG. 3, the pulse periods of the first light pulse train 21 and the second light pulse train 22 have a ratio of 2:1, which is, however, an example, and the present invention is applicable when the pulse periods have a ratio of m:n. Here, m and n are arbitrary natural numbers.

When the output signal of the light detector 10 is higher than the target value VA, the light pulse 21A and the light pulse 22A are close to each other as compared to a pulse synchronization state illustrated in FIG. 3. Thus, the control circuit 11 outputs a signal for reducing the first period so that the first light pulse train 21 advances (for reducing the cavity length) to the pulse period adjuster 12. In contrast, when the output signal of the light detector 10 is lower than the target value VA, the control circuit 11 outputs a signal for increasing the first period so that the first light pulse train 21 retards relative to the second light pulse train 22 (for increasing the cavity length) to the pulse period adjuster 12. A frequency range and a voltage amplitude of the signal output to the pulse period adjuster 12 are set such that a stable control is performed near a region illustrated by a dashed and single-dotted line in FIG. 2.

The above-described control maintains a constant timing difference between the light pulse of the first light pulse train 21 and that of the second light pulse train 22 on the light receiving surface of the light detector 10.

Next follows description of a behavior of the synchronizer when an output power of the pulse laser 1 or the pulse laser 2 has changed. In a conventional synchronizer, the target value VA is a fixed value. In FIG. 2, a dashed line illustrates a relationship between the output signal of the light detector 10 and a pulse timing difference when the output power is decreased. Not only the two-photon absorption signal corresponding to E1+E2 decreases, but also the two-photon absorption signals corresponding to E1+E1 and E2+E2 which provide an offset voltage decrease. Consequently, the output signal of the light detector 10 does not reach the target value VA set before the output power decrease, and thus the pulse synchronization cannot be performed. For a small change amount of the output power, the pulse synchronization can be performed, but the pulse timing difference changes with the change of the output power. Thus, in the conventional synchronizer, when the power of light input to the synchronizer has changed, the pulse synchronization cannot be continued or becomes unstable (the pulse timing difference is not constant).

The control circuit 11 changes (adjusts) the target value VA to a target value VB illustrated by a dashed and double-dotted line in FIG. 2 based on detection results of the

light detectors

5 and 6. This change of the target value allows the pulse synchronization to continue and the pulse timing difference to be kept constant even when the power of light input to the synchronizer changes.

The control circuit 11 calculates the target value based on information (in this embodiment, Expression (1), and the proportionality coefficients a0, a1, and a2) of the target value stored in the storage when at least one of the detection results of the

light detectors

5 and 6 has changed (calculating step). The calculating step is constantly performed in this embodiment, but may be performed every time the detection results of the

light detectors

5 and 6 change or may be periodically performed. Accordingly, the target value is updated as needed. The target values VA and VB are both expressed by Expression (1). Since the timing difference TD is maintained simultaneously, Expression (1) also has a function to maintain a certain timing difference.

In Expression (1), the proportionality coefficient in the first term is set to be a0/2 as a value for which the pulse synchronization is most stable. However, the pulse synchronization is possible even when the coefficient is set to another value in a range of 0 to a0. When the proportionality coefficient a0/2 is replaced with the proportionality coefficient a3 (=a0・ε), Expression (1) is rewritten as described below. The value ε takes a value in a range of 0 to 1, and is 1/2 or its vicinity when the pulse synchronization is most stable.
Target value = a3・V1・V2 + a1・V1・V1 + a2・V2・V2 ... (1')

A timing difference between the light pulse of the first light pulse train 21 and that of the second light pulse train 22 at the light receiving surface of the light detector 10 can be adjusted by adjusting the proportionality coefficient in the first term in a range of 0 to a0. This adjustment leads to adjust a timing difference between light pulses transmitted through the beam splitter 8 and used in the nonlinear optical microscope. The timing difference can be adjusted also by differentiating an optical path length from the beam splitter 8 to the light receiving surface of the light detector 10 between the first light pulse train 21 and the second light pulse train 22. For example, a glass block is inserted between the beam splitter 8 and the objective lens 9, and a distance passing through the glass block can be changed to adjust the pulse timing difference. Alternatively, a dichroic mirror may be disposed between the beam splitter 8 and the objective lens 9 so that the first light pulse train 21 and the second light pulse train 22 pass through different optical paths and then are combined again. This allows change of the length of one of the optical paths so as to adjust the pulse timing difference.

Another aspect of the present invention is a light pulse train synchronizing method executed by the control circuit 11 to synchronize light pulses produced periodically at the first period and light pulses produced periodically at the second period. This light pulse train synchronizing method includes the step of outputting a signal for adjusting at least one of the first period and the second period so that the timing difference signal is equal to the target value, to the pulse period adjuster. The light pulse train synchronizing method is executed by a computer and includes the calculating step and the outputting step. The calculating step calculates a target value from the first signal corresponding to at least one of the light intensity of the first light pulse train 21 and the light intensity of the second light pulse train 22. The second signal corresponds to the timing difference between the light pulse included in the first light pulse train 21 and the light pulse included in the second the light pulse train 22. The outputting step outputs a control signal for adjusting at least one of the first period and the second period so that the second signal is equal to the target value, to the pulse period adjuster 12. In this embodiment, a change of the second signal illustrated in FIG. 2 from the state represented by a solid train to the state represented by a dashed train does not need to be determined, and a pulse period adjustment is performed by setting again the target value based on a change of the first signal, thereby achieving a reduced calculation load of the control circuit 11. Another aspect of the present invention is a program that causes the computer to execute the light pulse train synchronizing method. This program may be stored in, for example, a non-transitory computer-readable media.

Embodiment 2

FIG. 4 is a block diagram of a synchronizer according to Embodiment 2 of the present invention. This embodiment does not include the beam splitter 4 and the light detector 6 illustrated in FIG. 2, but includes, as the pulse laser 2, a laser configured to output a stable power of light. In this case, since the light pulse train 22 has a constant power, a0・V2 in the two-photon absorption signal a0・V1・V2 illustrated in FIG. 2 can be regarded as a constant b0. Similarly, the two-photon absorption signal a2・V2・V2 can be regarded as a constant b1. Thus, in Embodiment 2, the output signal of the light detector 10 is as illustrated in FIG. 5.

The proportionality coefficients b0, a1, and b1 can be calculated from data corresponding to FIG. 5 acquired using, for example, an oscilloscope and a data logger, the output signals of the light detector 5 and the light detector 10 both when the light pulse train 21 is blocked and not blocked. The proportionality coefficients b0, a1, and b1 may be stored in the storage. In Embodiment 2, the target values VA and VB of the output signal of the light detector 10 are set as in Expression (2), thereby achieving the pulse synchronization, similarly to Embodiment 1.
Target value = b0/2・V1 + a1・V1・V1 + b1 ... (2)

Similarly to Embodiment 1, when the proportionality coefficient b0/2 is replaced with the proportionality coefficient b2 (=b0・ε), Expression (2) is rewritten as described below. The value ε takes a value in a range of 0 to 1, and is 1/2 or its vicinity when the pulse synchronization is most stable.
Target value = b2・V1 + a1・V1・V1 + b1 ... (2')

According to

Embodiments

1 and 2, the timing difference between the first light pulse train 21 and the second light pulse train 22 is kept constant. In addition embodiment 2 has an effect of stabilizing the pulse synchronization when the light receiving element of the light detector 10 has no sensitivity to two-photon absorption of E2+E2, that is, no sensitivity to half of the wavelength λ2 due to a characteristic such as bandgap. In this case, since the two-photon absorption signal corresponding to E2+E2 (a2・V2・V2) is fixed to zero, the output signal V2 of the light detector 6 does not need to be used to deal with a change of the two-photon absorption signal corresponding to E2+E2. Thus, the pulse synchronization can be stably performed without the light detector 6. However, since a change of the two-photon absorption signal corresponding to E1+E2 (a0・V1・V2) cannot be dealt with, the pulse timing difference between the first light pulse train 21 and the second light pulse train 22 changes when the power (V2) of the second light pulse train 22 has changed.

The configuration disclosed in PTL 2 includes two detectors configured to detect a two-photon absorption signal as the pulse timing difference, and synchronizes light pulse trains through a control based on a difference between signals from the two detectors. The two-photon absorption signal depends on the power and pulse width of emitted light, and alignment (of an objective lens and the light receiving surfaces of the detectors), and these parameters need to be identical between the two detectors in PTL 2, which is troublesome.

In a simple configuration according to

Embodiments

1 and 2, only the single light detector 10 is provided as a light detector that detects a two-photon absorption signal sensitive to the alignment. Use of the light detectors 5 and 6 (or the light detector 5 only) allows the light pulse trains to be stably synchronized even when the power of light has changed.

Embodiment 3

FIG. 6 is a block diagram of a microscope system (detection apparatus) according to Embodiment 3 of the present invention. The microscope system includes an SRS microscope 100 and a light pulse train synchronizer 200. The SRS microscope 100 combines two light pulse trains of different wavelengths emitted from two

pulse lasers

1 and 2, and irradiates a sample 105 simultaneously with the two light pulse trains in a condensed manner, thereby detecting stimulated Raman scattering (SRS) light produced through the irradiation. Thus, the SRS microscope 100 is a nonlinear optical microscope that irradiates the sample with the two light pulse trains of different wavelengths emitted from the two pulse lasers and observes the sample through a nonlinear optical process. The synchronizer 200 synchronizes light pulse trains emitted from the two

pulse lasers

1 and 2.

The SRS is a nonlinear optical phenomenon, and its signal is produced proportionally to a product of intensities of light of different wavelengths. An efficient production of the SRS is achieved by condensing light beams of two wavelengths from lasers at an identical position, and synchronizing light pulse trains of two wavelengths so that the light pulse trains are simultaneously condensed. The SRS weakens the intensity of one of the light pulse trains of two wavelengths, which has a shorter wavelength, and strengthens the intensity of the light pulse train of a longer wavelength. A short pulse laser of a pulse time width of 1 to 10 picoseconds is desirably used to efficiently produce the SRS.

The

pulse lasers

1 and 2 produce light pulse trains whose pulse periods are 2:1. FIG. 7A illustrates the first light pulse train 21 produced by the pulse laser 1, and FIG. 7B illustrates the second light pulse train 22 produced by the pulse laser 2. In FIGs. 7A and 7B, a horizontal axis represents time (t), and a vertical axis represents a light intensity. The wavelength (λ1) of the first light pulse train 21 is larger than the wavelength (λ2) of the second light pulse train 22.

The pulse laser 1 is an ytterbium-doped fiber laser of a central wavelength of 1030 nm and a pulse period of 25 nanoseconds. The pulse laser 2 is a solid-state laser (titanium-sapphire laser) of a central wavelength of 800 nm and a pulse period of 12.5 nanoseconds, which is, for example, Mai Tai of Spectra-Physics.

When the first light pulse train 21 and the second light pulse train 22 are condensed at an identical position on the sample while their timings coincide with each other as illustrated in FIGs. 7A and 7B, the light intensities of the light pulse trains transmitted through the sample change due to the SRS. In FIG. 7B, the intensities of

light pulses

41, 43, and 45 become small, and the intensities of

light pulses

42 and 44 do not change. Since the intensities of adjacent pulses have a minute difference therebetween, the difference is detected by synchronized detection.

The detected difference between the intensities corresponds to an SRS signal upon which information of a molecule included in a position where light beams are condensed is reflected. For example, when the resonant frequency of vibration of the molecule included in the position coincides with a difference (c/λ2-c/λ1) between light frequencies of two lasers, the SRS signal becomes large. Here, c represents the speed of light. The SRS signal is acquired while the difference (c/λ2-c/λ1) between the light frequencies of the two lasers is changed, thereby acquiring a Raman spectrum. The wavelength of at least one of the two lasers is changed to obtain the Raman spectrum. The Raman spectrum can be used to estimate a molecule contained in the sample. The SRS microscope can acquire a spectrum equivalent to a microscope utilizing spontaneous Raman scattering. Since the scattering efficiency of the SRS is extremely larger than the scattering efficiency of the spontaneous Raman scattering, the SRS microscope can acquire the Raman spectrum in a shorter time than a time needed by the microscope utilizing the spontaneous Raman scattering.

The light beams emitted from the

pulse lasers

1 and 2 enters into the synchronizer 200, and the first light pulse train 21 and the second light pulse train 22 are synchronized so that their timings coincide with each other on the sample observed through the SRS microscope. The light pulse train 21 and the light pulse train 22 combined on the same axis through the dichroic mirror 7 transmit through the beam splitter 8 and enter into the SRS microscope 100.

The SRS microscope 100 has the configuration of a laser scanning microscope. The light beams from the two pulse lasers are fed to a beam scanner 101 on the same axis, and are deflected by the beam scanner 101. The beam scanner 101 includes a galvanoscanner and a resonant scanner, and changes the direction of an optical axis in two directions orthogonal to each other. To simplify the drawing, these two mirrors in the beam scanner 101 are illustrated as one mirror in FIG. 6. The use of the resonant scanner (scanning frequency of 8 kHz) and the galvanoscanner (scanning frequency of 15 Hz) allows for acquisition of 30 frames of an image of 500 lines per second.

The light beams deflected through the beam scanner 101 are fed to 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 have a conjugate relation allows the light beams to condense on the sample 105 while the light quantities of the light beams deflected through the beam scanner 101 are not changed due to light-shielding. The magnification of an optical system provided by the

lenses

102 and 103 is selected such that the sizes of the entering light beams are equal to the size of the entrance pupil of the objective lens 104. This can minimize the size of a light spot condensed through the objective lens 104 and improve the spatial resolution of detection of the SRS signal. A higher intensity of the light spot results in a larger SRS signal, thereby improving the S/N ratio of detection of the SRS signal. The objective lens 104 desirably has a large numerical aperture (NA) in terms of the spatial resolution of detection of the SRS signal and the S/N ratio.

The sample 105 is placed between a cover glass and a glass slide. The beam scanner 101 provides two-dimensional scanning of the light spot condensed on the sample 105 to obtain the SRS signal as a two-dimensional image. The SRS signal is produced only on a condensed light spot, and thus a three-dimensional image can be obtained by moving the sample 105 on an unillustrated stage in the direction of the optical axis.

A lens 106 has an NA equivalent to or higher than the NA of the objective lens 104 so as to receive all light transmitted through the sample 105, which has an intensity modulation by the SRS. A light beam emitted from the lens 106 is transmitted through a filter 107 and a lens 108, and incident on a light receiving surface of a photodiode 109. The filter 107 includes a dielectric multi-layer film that blocks light of the wavelength λ1 and transmits light of the wavelength λ2. The photodiode 109 receives and detects light that is emitted from the pulse laser 2 and has intensity modulation by the SRS per pulse. The photodiode 109 is a silicon photodiode having a sensitivity to light of 800 nm and a cutoff frequency of 40 MHz or larger.

The frequency of the intensity modulation by the SRS is 40 MHz (with a period of 25 nanoseconds) for the repetition frequency of 80 MHz (with a pulse period of 12.5 nanoseconds) of the light pulse train 22 from the pulse laser 2. A current voltage conversion circuit 110 is an electrical circuit to output a current signal produced by the photodiode 109 as voltage.

A synchronized detection circuit 111 extracts the amplitude of a 40 MHz component of a voltage signal output from the current voltage conversion circuit 110, and outputs this amplitude as voltage. The synchronized detection circuit 111 is a mixer circuit or a lock-in amplifier. An output signal from the synchronized detection circuit 111 indicates the degree of the SRS at a light-condensed point on the sample 105.

A computer 112 uses a control signal of the beam scanner 101 to produce a two-dimensional image of the output signal (SRS signal) from the synchronized detection circuit 111, and displays the image. The computer 112 can display a three-dimensional image based on the SRS signal acquired by moving the sample 105 on the unillustrated stage in the direction of the optical axis. In addition, the computer 112 can display the Raman spectrum based on the SRS signal acquired by changing the wavelength of at least one of the two pulse lasers.

Other Embodiments

Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a 'non-transitory computer-readable storage medium') to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)^TM), a flash memory device, a memory card, and the like.

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.

5, 6 ... light detector (first detector)
10 ... light detector (the second detector)
11 ... the control circuit (controller)
21 ... the first light pulse train
22 ... the second light pulse train
200 ... light pulse train synchronizer
VA, VB ... the target value

Claims

A light pulse train synchronizer configured to synchronize a first light pulse train including light pulses produced periodically at a first period and a second light pulse train including light pulses produced periodically at a second period, the light pulse train synchronizer comprising:
a first detector configured to output a first signal corresponding to at least one of a light intensity of the first light pulse train and a light intensity of the second light pulse train;
a second detector configured to output a second signal corresponding to an arrival timing difference between the light pulse included in the first light pulse train and the light pulse included in the second light pulse train; and
a controller configured to output a control signal for adjusting at least one of the first period and the second period so that the second signal is equal to a target value obtained from the first signal.
The light pulse train synchronizer according to claim 1, wherein the target value obtained from the first signal is expressed as a3・V1・V2 + a1・V1・V1 + a2・V2・V2 where V1 represents an output signal of the first signal corresponding to the light intensity of the first light pulse train, V2 represents an output signal of the first signal corresponding to the light intensity of the second light pulse train, and a1, a2, and a3 represent proportionality coefficients.
The light pulse train synchronizer according to claim 2, wherein the proportionality coefficient a3 is expressed as a0・ε where a0 represents a proportionality coefficient and ε is a value in a range of 0 to 1.
The light pulse train synchronizer according to claim 3, wherein ε is 1/2.
The light pulse train synchronizer according to claim 1,
wherein the light intensity of the second light pulse train is constant, and
wherein the target value obtained from the first signal is expressed as b2・V1 + a1・V1・V1 + b1 where V1 represents an output signal of the first signal corresponding to the light intensity of the first light pulse train, V2 represents an output signal of the first signal corresponding to the light intensity of the second light pulse train, a1, b1, and b2 represent proportionality coefficients.
The light pulse train synchronizer according to claim 5, wherein the proportionality coefficient b2 is expressed as b0・ε where b0 represents a proportionality coefficient and ε is a value in a range of 0 to 1.
The light pulse train synchronizer according to claim 6, wherein ε is 1/2.
The light pulse train synchronizer according to any one of claims 1 to 7, wherein the controller inputs the control signal to at least one of a first light source configured to produce the first pulse train and a second light source configured to produce the second pulse train.
The light pulse train synchronizer according to any one of claims 1 to 8, wherein the second detector includes a photodiode and is configured to convert current produced through two-photon absorption into voltage.
The light pulse train synchronizer according to claim 9, further comprising an objective lens that condenses the first light pulse train and the second light pulse train on a light receiving surface of the photodiode.
An illumination apparatus comprising:
the pulse train synchronizer according to any one of claims 1 to 10;
a first light source configured to produce the first pulse train; and
a second light source configured to produce the second pulse train.
A detection apparatus comprising:
the illumination apparatus according to claim 11; and
a light receiver configured to detect intensity-modulated light of the first and second light pulse trains emitted on a sample.
The detection apparatus according to claim 12, wherein the light receiver detects intensity-modulated light through stimulated Raman scattering caused by irradiation of the first and second light pulse trains on the sample.
The light pulse train synchronizing method of synchronizing a first light pulse train including light pulses produced periodically at a first period and a second light pulse train including light pulses produced periodically at a second period, the method comprising the steps of:
calculating a target value from a first signal corresponding to at least one of a light intensity of the first light pulse train and a light intensity of the second light pulse train; and
outputting a control signal for adjusting at least one of the first period and the second period so that a second signal corresponding to an arrival timing difference between the light pulse included in the first light pulse train and the light pulse included in the second light pulse train is equal to the target value calculated at the calculating step.
A program configured to cause a computer to execute the light pulse train synchronizing method according to claim 14.