JP6746371B2 - Light source device and information acquisition device - Google Patents

Description

The present invention relates to a light source device and an information acquisition device.

By irradiating the subject with pulsed light and detecting light reflected or scattered by the subject, light transmitted through the subject, or fluorescence emitted from the subject, various substances related to the constituents of the subject are detected. The technology for obtaining information is drawing attention.

In recent years, a lot of research has been conducted to identify a substance in a subject by irradiating the subject with two pulsed lights having a frequency difference corresponding to the molecular frequency and detecting light based on the following scattering generated in the subject. Has been done. Examples of such scattering include stimulated Raman scattering (SRS: Stimulated Raman Scattering). In addition, examples of such scattering include coherent anti-stokes Raman scattering (CARS).

The following laser light sources are known as laser light sources that generate two pulsed lights having different center wavelengths. That is, an optical parametric resonator (Fiber Optical Parametric Oscillator, hereinafter abbreviated as FOPO) that utilizes four-wave mixing (a type of optical parametric effect) generated in a nonlinear fiber is known. The FOPO receives the energy of the excitation pulse light that has entered the optical fiber, generates pulsed light having a wavelength different from that of the excitation pulsed light, and oscillates the pulsed light in the resonator. That is, when pumping pulse light is made incident on the nonlinear fiber provided in the FOPO, pulsed light having a shorter wavelength than the pumping pulse light (signal pulsed light, signal light) and pulsed light having a longer wavelength than the pumping pulsed light ( Idler pulse light and idler light) are generated at the same time. At least a part of the signal pulsed light and the idler pulsed light is fed back and re-introduced into the resonator in synchronization with the excitation pulsed light, so that at least one of the signal pulsed light and the idler pulsed light. Make either one oscillate. Then, either or both of the signal pulsed light and the idler pulsed light are taken out as an output.

Non-Patent Document 1 discloses a technique for narrowing the spectral line width of signal pulse light by inserting a long optical fiber into the FOPO and increasing the chromatic dispersion for the signal pulse light as a whole resonator. ing.

Thomas Gottschall et al., “Fiber-based optical parametric oscillator for high resolution coherent anti-Stokes Roman scattering (CARS) microscopy”, Optics Express, Vol. 22, No. 18, pp.21921-21928, 8 September 2014.

However, in the conventional technique, the center wavelength of the output pulsed light cannot be changed without causing a large variation in the spectral line width.

It is an object of the present invention to provide a light source device that can change the central wavelength of output pulsed light without causing a large variation in the spectral line width, and an information acquisition device using the light source device.

According to one aspect of the embodiment, a light source that emits a first pulsed light and a nonlinear light source that generates a second pulsed light having a wavelength different from that of the first pulsed light by the incidence of the first pulsed light. An optical medium, an optical branching part for branching the second pulsed light, and a dispersion medium for giving a predetermined wavelength dispersion to the second pulsed light branched by the optical branching part, the second pulse A dispersion medium that includes a first optical waveguide and a second optical waveguide in which the positive and negative of the wavelength differential value of the dispersion at the wavelength of light are opposite to each other, and the dispersion value at the wavelength of the second pulsed light is not 0; possess the the dispersion medium by the chromatic dispersion and the second pulse light given to the first pulse light emitted from the light source, and an optical multiplexer for guiding the nonlinear optical medium, said first difference between the maximum value and the minimum value of the dispersion value of the dispersion medium in the wavelength variable range of 2 pulsed light source device is provided, characterized in der Rukoto below 2.5 ps / nm.

According to the present invention, it is possible to provide a light source device that can change the central wavelength of output pulsed light without causing a large variation in the spectral line width, and an information acquisition device using the light source device.

It is a block diagram showing a light source device by a 1st embodiment. It is a block diagram which shows the light source device by 2nd Embodiment. It is a block diagram which shows the information acquisition apparatus by 3rd Embodiment. It is a graph which shows the dispersion profile in a dispersion medium typically. 6 is a graph showing a dispersion profile in the light source device according to the first embodiment. 9 is a graph showing a dispersion profile in the light source device according to the second embodiment. 7 is a graph showing a phase mismatch Δβ of light propagation constants and an optical parametric gain G in a nonlinear optical medium with β ₂ >0 and β ₄ >0. 7 is a graph showing a phase mismatch Δβ of light propagation constants and an optical parametric gain G in a nonlinear optical medium with β ₂ >0 and β ₄ <0. 7 is a graph showing a phase mismatch Δβ of light propagation constants and an optical parametric gain G in a nonlinear optical medium with β ₂ <0 and β ₄ >0. 7 is a graph showing a phase mismatch Δβ of light propagation constants and an optical parametric gain G in a nonlinear optical medium with β ₂ <0 and β ₄ <0.

Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the configurations and the like of the following embodiments. Further, in the drawings described below, members with the same reference numerals mean the same members or corresponding members.

[First Embodiment]
The light source device according to the first embodiment will be described with reference to the drawings. FIG. 1 is a block diagram showing the light source device according to the present embodiment.

The light source device 100 according to the present embodiment includes a light source 101 that emits excitation pulse light, a nonlinear optical medium 103 that generates signal pulse light, and a dispersion medium 108 that narrows the spectral line width of the signal pulse light. There is. The light source 101 can change the central wavelength of the excitation pulsed light. The nonlinear optical medium 103 and the dispersion medium 108 are arranged in the optical resonator 107 that oscillates the signal pulsed light. Optical fibers can be preferably used for the nonlinear optical medium 103 and the dispersion medium 108, respectively.

When the pump pulse light emitted from the light source 101 is incident on the nonlinear optical medium 103, a signal pulse light having a wavelength different from that of the pump pulse light is generated due to the optical parametric gain in the nonlinear optical medium 103. The center wavelength of the signal pulse light can be greatly changed by slightly changing the center wavelength of the pump pulse light. By utilizing this phenomenon, the light source device 100 according to the present embodiment can change the central wavelength of the output pulsed light in a wide band.

When the signal pulsed light generated in the nonlinear optical medium 103 is incident on the dispersive medium 108, the speed of the light propagating in the dispersive medium 108 varies depending on the wavelength, so that the signal pulsed light spreads in time and becomes a pulsed light having a chirp. Become. When the signal pulsed light having the chirp enters the nonlinear optical medium 103 again, the signal pulsed light is amplified by the optical parametric effect from the pumping pulsed light. At this time, since the signal pulsed light spreads largely in time compared with the excitation pulsed light, only the portion where the signal pulsed light and the excitation pulsed light overlap in time is amplified. Since the signal pulsed light has a chirp, only the limited wavelength component of the signal pulsed light is amplified, and the spectral line width of the signal pulsed light is narrowed.

However, since the dispersion value of the dispersion medium 108 generally differs depending on the wavelength, if the center wavelength of the signal pulse light changes, the degree to which the spectral line width of the signal pulse light is narrowed differs depending on the center wavelength of the signal pulse light. End up. As a result, the spectral line width of the signal pulse light differs depending on the center wavelength of the signal pulse light.

Therefore, in the present embodiment, the dispersion medium 108 is configured by the first optical waveguide 105 and the second optical waveguide 106 in which the positive and negative of the wavelength differential value of the dispersion are opposite to each other within the variable wavelength range of the signal pulsed light. Since the dispersion medium 108 is composed of the first optical waveguide 105 and the second optical waveguide 106 whose positive and negative values of the wavelength derivative of dispersion are opposite to each other, the wavelength dependence of the dispersion value is reduced as a whole of the dispersion medium 108. .. Therefore, according to this embodiment, it is possible to provide the light source device 100 capable of changing the central wavelength of the output pulsed light without causing a large variation in the spectral line width.

Before giving a detailed description of the light source device 100 according to the present embodiment, the principle of four-wave mixing, which is the root of optical parametric gain for generating signal pulsed light from pumping pulsed light, will be described.

Four-wave mixing is a new light having a wavelength different from both wavelengths of two pumping lights when two lights (pumping lights) having different frequencies (wavelengths) from each other enter a nonlinear optical medium (optical fiber). Is a phenomenon that occurs. At this time, a part of the energy of the light incident on the nonlinear optical medium is converted into the energy of the light newly generated by the four-wave mixing. For example, when two lights having frequencies ω ₁ and ω ₂ are made incident on the nonlinear optical medium and two lights having frequencies ω ₃ and ω ₄ are newly generated, the respective frequencies are ω ₁ +ω The relationship of ₂ =ω ₃ +ω ₄ is satisfied.

When the frequency of light (excitation light) incident on the nonlinear optical medium is one, that is, when ω ₁ =ω ₂ =ω _C , it is called degenerate four-wave mixing, and the frequencies are ω _C +Δω, respectively. Two lights of ω _C −Δω are generated symmetrically with respect to the frequency ω _C. In general, of the two lights generated by degenerate four-wave mixing, the light on the high frequency side is called signal light, and the light on the low frequency side is called idler light. Hereinafter, the frequency of the signal light is represented by ω _S1 (=ω _C +Δω), and the frequency of the idler light is represented by ω _S2 (=ω _C −Δω).

The degenerate four-wave mixing is widely used as a light source device of an information acquisition device using SRS or CARS because it has a simpler wavelength control and configuration as compared with the case where two lights having different frequencies are incident. The degenerate four-wave mixing will be described below.

ここで、Δβは非線形光学媒質中における各々の光の伝搬定数の位相不整合、γは非線形光学媒質の非線形係数、Ｐ_Ｃは励起光のピーク強度である。また、ｎ_２は非線形光学媒質の非線形屈折率、Ａ_ｅｆｆは非線形光学媒質である光ファイバのコアの有効断面積、ｃは真空中での光の速さである。 In order to efficiently generate degenerate four-wave mixing, assuming that the propagation constant of pumping light incident on a nonlinear optical medium is β _C , the propagation constant of signal light is β _S1 , and the propagation constant of idler light is β _S2 , It is necessary to satisfy the phase matching condition represented by the equation (1).

Here, Δβ is the phase mismatch of the propagation constants of the respective lights in the nonlinear optical medium, γ is the nonlinear coefficient of the nonlinear optical medium, and P _C is the peak intensity of the excitation light. In addition, n ₂ is the nonlinear refractive index of the nonlinear optical medium, A _eff is the effective sectional area of the core of the optical fiber that is the nonlinear optical medium, and c is the speed of light in a vacuum.

The phase mismatch Δβ of the propagation constants of each light in the nonlinear optical medium can be expressed by the following formula (2) using the frequency difference Δω between the signal light and the idler light.
^{_{Δβ = β 2 (Δω) 2}} + β 4 (Δω) 4/12 ··· (2)
Here, β ₂ is the group velocity dispersion of the nonlinear optical medium at the frequency of the excitation light, and β ₄ is the second derivative of the group velocity dispersion β ₂ . The group velocity dispersion β ₂ is the second derivative of the propagation constant β _C of the excitation light.

ここで、Ｌは非線形光学媒質の長さである。 The optical parametric gain G is expressed by the following equation (3).

Here, L is the length of the nonlinear optical medium.

FIGS. 7 to 10 show graphs of equations (2) and (3) for four types in which β ₂ and β ₄ are positive or negative. 7 to 10 are graphs showing the phase mismatch Δβ of the propagation constant of light and the optical parametric gain G in the nonlinear optical medium. FIG. 7 shows the case where β ₂ >0 and β ₄ >0. FIG. 7A is a graph of the equation (2), in which the vertical axis represents the phase mismatch Δβ of the propagation constant of each light in the nonlinear optical medium, and the horizontal axis represents the degenerate four-wave mixing. The frequency difference Δω between the generated signal light and the idler light is shown. FIG. 7B is a graph of the equation (3), in which the vertical axis represents the optical parametric gain G and the horizontal axis represents the frequency difference Δω between the signal light and the idler light. The same applies to the graphs (a) and (b) of FIGS. 8 to 10.

Under the phase matching condition of Δβ expressed by the equation (1), the nonlinear coefficient γ of the nonlinear optical medium and the peak intensity P _C of the pumping light are both positive values, and Δβ is a negative value. .. However, as can be seen from FIG. 7A, when β ₂ >0 and β ₄ >0, there is no region that satisfies the formula (1). That is, as shown in FIG. 7B, the optical parametric gain G expressed by the equation (3) cannot be obtained, and in such a nonlinear optical medium, even if pumping light is incident, the signal light No idler light is produced.

FIG. 8 shows a case where β ₂ >0 and β ₄ <0. In FIG. 8A, the range in which the phase matching condition of Δβ represented by the equation (1) is satisfied, that is, the phase matching region is shown by hatching. As can be seen from FIG. 8A, the range of Δω satisfying the phase matching condition represented by the equation (1) exists in a relatively narrow region at a position sufficiently distant from the frequency of the excitation light. Therefore, when pumping light of a specific frequency is incident on the nonlinear optical medium, as shown in FIG. 8B, the optical parametric gain G is obtained in a relatively narrow frequency band, and the signal light having a narrow frequency band is obtained. And idler light are generated.

FIG. 9 shows the case where β ₂ <0 and β ₄ >0, and FIG. 10 shows the case where β ₂ <0 and β ₄ <0. As shown in FIGS. 9A and 10A, the range of Δω satisfying the phase matching condition represented by the equation (1) is relatively wide. That is, as shown in FIGS. 9B and 10B, when pulsed light having a specific frequency is incident on the nonlinear optical medium, the optical parametric gain G is obtained in a relatively wide frequency band. , Signal light and idler light are generated over a wide frequency band.

From the above, it is preferable to use a non-linear optical medium that satisfies β ₂ >0 and β ₄ <0 in order to generate pulsed light with a relatively narrow spectral line width using degenerate four-wave mixing. I understand. In such a non-linear optical medium, the core material and the clad material are selected and the shape of the optical fiber is appropriately set so that the refractive index difference between the core and the clad of the optical fiber has an appropriate value. Can be achieved by.

ここで、ω_０は非線形光学媒質の零分散周波数、λ_Ｃは励起パルス光の中心波長、λ_０は非線形光学媒質の零分散波長、β_３は零分散波長における群速度分散β_２の１次導関数である。式（５）から分かるように、励起パルス光の中心波長λ_Ｃをわずかに変化させることによって、信号光又はアイドラー光の波長を式（５）に示す係数の分だけ大きく変化させることができる。また、式（７）から分かるように、非線形係数γが小さく、β_４の大きな非線形光学媒質を用いれば、スペクトル線幅δλの狭い信号光又はアイドラー光を生成することが可能である。そして、信号光とアイドラー光とのうちのいずれか一方又はこれらの両方を利用することができる。 The frequency shift amount (frequency difference between the signal light and the idler light) Δω and the wavelength shift amount Δλ of the optical parametric gain G generated by the degenerate four-wave mixing with respect to the pump pulse light are expressed by the following equations (4) and (5), respectively. ). The frequency width δω and the spectrum line width (spectrum half-width) δλ of the optical parametric gain G are expressed by the following equations (6) and (7), respectively.

Where ω ₀ is the zero-dispersion frequency of the nonlinear optical medium, λ _C is the center wavelength of the pump pulse light, λ ₀ is the zero-dispersion wavelength of the nonlinear optical medium, and β ₃ is the first-order group velocity dispersion β _{2 at} the zero-dispersion wavelength. Is the derivative. As can be seen from the formula (5), the wavelength of the signal light or the idler light can be largely changed by the coefficient shown in the formula (5) by slightly changing the central wavelength λ _C of the pump pulse light. Further, as can be seen from the equation (7), it is possible to generate signal light or idler light having a narrow spectral line width δλ by using a nonlinear optical medium having a small nonlinear coefficient γ and a large β ₄ . Then, either one or both of the signal light and the idler light can be used.

Here, a method of changing the central wavelength of the output pulsed light without causing a large variation in the spectral line width will be described below. FIG. 4 is a graph schematically showing a dispersion profile in a dispersion medium. FIG. 4 shows the dispersion profile of the dispersion medium 108 constituted by the first optical waveguide 105 and the second optical waveguide 106 in which the positive and negative of the wavelength differential value of the dispersion are opposite. The dispersion profile of the first optical waveguide 105 is shown by a dotted line in FIG. 4, and the dispersion profile of the second optical waveguide 106 is shown by a broken line in FIG. As shown in FIG. 4, the dispersion value of the first optical waveguide 105 increases as the wavelength increases. The wavelength differential value of the dispersion in the first optical waveguide 105 is positive. On the other hand, the dispersion value of the second optical waveguide 106 decreases as the wavelength increases. The wavelength differential value of the dispersion in the second optical waveguide 106 is negative. As described above, the wavelength differential values of the dispersion in the first optical waveguide 105 and the second optical waveguide 106 are opposite in positive and negative. When the first optical waveguide 105 and the second optical waveguide 106 are connected in series, the dispersion profile in the entire dispersion medium 108 composed of the first optical waveguide 105 and the second optical waveguide 106 is as follows. Become. That is, the dispersion profile of the entire dispersion medium 108 is represented by the sum of the dispersion profile of the first optical waveguide 105 and the dispersion profile of the second optical waveguide 106. Therefore, the dispersion profile of the entire dispersion medium 108 is as shown by the thick solid line in FIG. As shown in FIG. 4, the wavelength differential value of the dispersion in the entire dispersion medium 108 is very small. Therefore, even if the central wavelength of the signal pulsed light is changed, the chromatic dispersion received by the signal pulsed light from the dispersion medium 108 becomes substantially constant, and therefore the spectral line width of the signal pulsed light becomes substantially constant. As shown in FIG. 4, the sum of the dispersion value in the first optical waveguide 105 and the dispersion value in the second optical waveguide 106 is a non-zero value. That is, the dispersion value of the dispersion medium 108 at the wavelength of the signal pulsed light is not 0. The sum of the dispersion value in the first optical waveguide 105 and the dispersion value in the second optical waveguide 106 is not 0 because the signal pulsed light is spread in time. As a result, only the portion where the signal pulsed light and the excitation pulsed light temporally overlap each other is amplified, and the spectral line width of the signal pulsed light is narrowed.

When pumping pulsed light having a certain pulse width is incident on the dispersion medium 108 having a certain dispersion characteristic, signal pulsed light having a certain spectral line width is obtained, and therefore the following equation (8) is established.
(D ₁ L ₁ +D ₂ L ₂ )∝ΔT _C /Δλ _S (8)
Here, D ₁ is a dispersion value per unit length in the first optical waveguide 105, D ₂ is a dispersion value per unit length in the second optical waveguide 106, and L ₁ is a length of the first optical waveguide 105. Now, L ₂ is the length of the second optical waveguide 106. Further, Δλ _S is the spectral line width of the signal pulse light, and ΔT _C is the pulse width of the pump pulse light.

In order to obtain a substantially constant spectral line width within the variable wavelength range of the signal pulsed light, the dispersion value of the dispersion medium 108 needs to be substantially constant within the variable wavelength range of the signal pulsed light. In other words, the difference between the maximum value of the dispersion value of the dispersion medium 108 within the variable wavelength range of the signal pulsed light and the minimum value of the dispersion value of the dispersion medium 108 within the variable wavelength range of the signal pulsed light is sufficiently small. Requires that. Since the relationship shown in the expression (8) is established, the following expression (9) is established.
_{| (D 1 L 1 + D} 2 L 2) max - (D 1 L 1 + D 2 L 2) min | = B × (ΔT C / Δλ S) ··· (9)
Here, (D ₁ L ₁ +D ₂ L ₂ ) _max is the maximum value of the dispersion value of the dispersion medium 108 within the variable wavelength range of the signal pulsed light, and (D ₁ L ₁ +D ₂ L ₂ ) _min is the signal pulse It is the minimum value of the dispersion value of the dispersion medium 108 within the variable wavelength range of light. _{| (D 1 L 1 + D} 2 L 2) max - (D 1 L 1 + D 2 L 2) min | It is these difference values. B is a coefficient, and the smaller the coefficient B, the better the light source device in which the fluctuation of the spectral line width is small.

Next, the light source device 100 according to the present embodiment will be described in more detail. As shown in FIG. 1, the light source device 100 according to the present embodiment includes a light source 101 that emits pulsed light, an optical multiplexer 102, a nonlinear optical medium 103, an optical branching device 104, and a first optical waveguide 105. And a second optical waveguide 106.

The light source 101 emits a first pulsed light (excitation pulsed light). The light source 101 can change the central wavelength of the emitted first pulsed light. As the light source 101, for example, a laser resonator provided with a wavelength filter or the like can be used, and for example, a pulse laser is suitable. Here, as the light source 101, for example, a mode-locked Yb (ytterbium)-doped fiber that can change the central wavelength from 1020 nm to 1060 nm, has a spectral line width of about 0.1 nm and a pulse width of about 15 ps. Use a laser. Here, the case where the light source 101 that emits the first pulsed light having a pulse width of about 15 ps is used as an example, but the pulse width of the first pulsed light is not limited to this. For example, a light source that emits a first pulsed light with a pulse width of 10 ps or more can be used as the light source 101 as appropriate. Further, here, the case where the light source 101 which emits the first pulsed light having a spectral line width of about 0.1 nm is used as an example, but the spectral line width of the first pulsed light is not limited to this. is not. For example, a light source that emits the first pulsed light having a spectral line width of 1 nm or less can be used as the light source 101 as appropriate.

As the non-linear optical medium 103, an optical fiber having a high non-linear coefficient can be preferably used. Examples of the optical fiber having a high nonlinear coefficient include a photonic crystal fiber and a tapered optical fiber. The photonic crystal fiber is a fiber in which a large number of holes (air holes) are provided in the clad portion of the optical fiber, and the holes can make the refractive index of the clad extremely lower than that of the core. Since the photonic crystal fiber can reduce the effective core diameter (mode field diameter), a large nonlinear effect can be obtained even when the fiber length is about several meters. Further, the photonic crystal fiber can obtain an arbitrary wavelength dispersion characteristic by adjusting the size and pitch of the holes. The tapered optical fiber is an optical fiber in which the output clad diameter is smaller than the input clad diameter, and can be formed by heating and stretching a normal optical fiber. If the cladding diameter of the tapered optical fiber is extremely small, for example, about several μm, a large nonlinear effect can be obtained even if the length is about several mm. The tapered optical fiber can obtain an arbitrary wavelength dispersion characteristic by adjusting the cladding diameter and the length. Here, as the nonlinear optical medium 103, for example, a photonic crystal fiber (LMA-PM-5) manufactured by NKT Photonics is used. Such a photonic crystal fiber has a zero dispersion wavelength of 1050 nm. The length of the photonic crystal fiber is, eg, about 50 cm.

In the first optical waveguide 105 and the second optical waveguide 106, the positive and negative of the wavelength differential value of the dispersion are opposite to each other, and the total of the dispersion values in the first optical waveguide 105 and the second optical waveguide 106. Is not 0 at each wavelength within the variable wavelength range.

The first pulsed light emitted from the light source 101 is guided to the nonlinear optical medium 103 via the optical multiplexer (optical multiplexer) 102. When the first pulsed light enters the non-linear optical medium 103, the second pulsed light and the third pulsed light are generated by the optical parametric gain. The central wavelength of the second pulsed light is different from the central wavelength of the first pulsed light. The center wavelength of the third pulsed light is different from both the center wavelength of the first pulsed light and the center wavelength of the second pulsed light. The wavelength of the second pulsed light is, for example, 1200 nm to 1500 nm, and the wavelength of the third pulsed light is, for example, 750 nm to 950 nm. The second pulsed light and the third pulsed light emitted from the nonlinear optical medium 103 are guided to the optical branching device 104. The optical splitter 104 splits and outputs the pulsed light. The second pulsed light and the third pulsed light emitted from the non-linear optical medium 103 are branched by the optical branching device 104 to the dispersion medium 108 including the first optical waveguide 105 and the second optical waveguide 106. Be guided. When the second pulsed light and the third pulsed light are incident on the first optical waveguide 105 and the second optical waveguide 106, the following occurs. That is, due to the wavelength dispersion in the first optical waveguide 105 and the wavelength dispersion in the second optical waveguide 106, the second pulsed light and the third pulsed light temporally spread and become chirped pulsed light. The second pulsed light and the third pulsed light to which the chromatic dispersion has been given by the dispersion medium 108 are guided again to the nonlinear optical medium 103 via the optical multiplexer 102. The second pulsed light and the third pulsed light to which the chromatic dispersion has been given by the dispersion medium 108 spread widely in time compared to the first pulsed light, and overlapped in time with the first pulsed light. Only the part is amplified. Therefore, only a limited wavelength component is amplified and the spectral line width is narrowed. Here, the case where the second optical waveguide 106 is located after the first optical waveguide 105 will be described as an example, but the present invention is not limited to this, and the latter stage of the second optical waveguide 106 is not provided. The first optical waveguide 105 may be located at. Further, the optical branching device (optical branching unit) 104 may include a bandpass filter that cuts light other than a desired wavelength. For example, when the optical branching device 104 includes a bandpass filter that cuts one of the second pulse light and the third pulse light, the second pulse light and the third pulse light are combined. Only the other of them will be guided to the dispersion medium 108.

As described above, the first optical waveguide 105 and the second optical waveguide 106 are opposite in the positive and negative of the wavelength differential value of the dispersion, and the first optical waveguide 105 and the second optical waveguide 106 are the same. The sum of the dispersion values in is not 0 at each wavelength within the variable wavelength range. Here, for example, the following first optical waveguide 105 and second optical waveguide 106 are used. As the first optical waveguide 105, for example, a single mode fiber (model: HI-1060, length: 2000 m) manufactured by Corning is used. As the second optical waveguide 106, for example, a dispersion compensation fiber manufactured by Thorlab (model: DCF38, length: 500 m) is used. When the first optical waveguide 105 and the second optical waveguide 106 whose wavelength differential values of dispersion are opposite in polarity are connected in series, the dispersion medium 108 including the first optical waveguide 105 and the second optical waveguide 106 is connected. The wavelength differential value of the total dispersion is very small. The spectral line width Δλ _S of the second pulsed light is, for example, about 1.5 nm.

FIG. 5 is a graph showing a dispersion profile in the light source device according to the present embodiment. FIG. 5A shows the relationship between the dispersion value D ₁ per unit length in the first optical waveguide 105 and the wavelength. FIG. 5B shows the relationship between the dispersion value D ₂ per unit length in the second optical waveguide 106 and the wavelength. FIG. 5C shows the dispersion value D ₁ L ₁ +D ₂ L _{2 in} the entire dispersion medium 108, that is, the total dispersion value. As can be seen from FIG. 5A, the wavelength differential value of the dispersion in the first optical waveguide 105 is positive. On the other hand, as can be seen from FIG. 5B, the wavelength differential value of the dispersion in the second optical waveguide 106 is positive. As can be seen from FIG. 5C, the dispersion value of the dispersion medium 108 in the variable wavelength range of the second pulsed light from 1200 nm to 1500 nm is about −10 ps/nm. As can be seen from FIG. 5C, the difference between the minimum value and the maximum value of the dispersion value of the dispersion medium 108 in the wavelength variable range of the second pulsed light from 1200 nm to 1500 nm is 1.5 ps/nm. ing. The pulse width ΔT _C of the first pulsed light is about 15 ps, and the spectral line width Δλ _S of the second pulsed light is about 1.5 nm. The coefficient B in the above equation (9) is 0.15 in this case. Within the variable wavelength range of the second pulsed light, the absolute value of the wavelength differential value of the dispersion medium 108 is 0.013 ps/nm ² or less. The spectral line width of the second pulsed light is narrowed to about 1.5 nm, for example. Here, the case where the difference between the maximum value and the minimum value of the dispersion value of the dispersion medium 108 in the wavelength variable range of the second pulsed light is 1.5 ps/nm has been described as an example, but the present invention is not limited to this. Not something. By appropriately setting the configuration and material of the waveguide used as the first optical waveguide 105 and the waveguide used as the second optical waveguide 106, the dispersion value of the dispersion medium 108 in the wavelength variable range of the second pulsed light The difference between the maximum value and the minimum value can be 1.5 ps/nm or less.

As described above, according to the present embodiment, the pulsed light incident on the dispersion medium 108 receives substantially constant wavelength dispersion from the dispersion medium 108 even when the center wavelength changes. Therefore, according to the present embodiment, it is possible to make the spectral line width of the output pulse light substantially constant even when the center wavelength of the output pulse light is changed.

In this embodiment, since the nonlinear optical medium 103 is arranged inside the optical resonator 107, the second pulsed light and the third pulsed light repeatedly pass through the nonlinear optical medium 103. Each time the second pulsed light and the third pulsed light pass through the nonlinear optical medium 103, the second pulsed light and the third pulsed light are amplified, so that the second pulsed light and the third pulsed light are amplified. The intensity of light can be increased. The second pulsed light and the third pulsed light oscillated in the optical resonator 107 are output to the outside of the optical resonator 107 via the optical branching device 104. In this way, the second pulsed light having a substantially constant spectral line width and the third pulsed light having a substantially constant spectral line width are extracted to the outside of the optical resonator 107. The first pulsed light that has not contributed to the optical parametric oscillation is also output to the outside of the optical resonator 107 via the optical branching device 104. In addition, as described above, the optical branching device 104 may include a bandpass filter that cuts light other than the desired wavelength. When the optical branching device 104 is provided with a bandpass filter that cuts off light other than the predetermined wavelength, only the light having the predetermined wavelength is output to the outside of the optical resonator 107.

In order to form a good optical resonator 107, the pulse rate of the first pulsed light is set to an integral multiple of the free spectral interval (FSR: Free Spectral Range) of the optical resonator 107 at the wavelength of the second pulsed light. Preferably. By satisfying such a relationship, the second pulsed light or the third pulsed light can be efficiently oscillated in the optical resonator 107, and the second pulsed light or the third pulsed light having a high peak intensity can be obtained. Can be output. When the central wavelength of the first pulsed light is changed from 1020 nm to 1060 nm, the central wavelength of the second pulsed light is changed from 1200 nm to 1500 nm, and the central wavelength of the third pulsed light is changed from 750 nm to 950 nm, for example. To do.

The spectral line width of the first pulsed light emitted from the light source 101 is preferably 1 nm or less. This is because the narrower the spectral line width of the first pulsed light is, the more efficiently four-wave mixing is performed, and the optical parametric gain is sufficiently secured.

As described above, according to the present embodiment, the dispersion medium 108 is configured by the first optical waveguide 105 and the second optical waveguide 106 whose positive and negative wavelength differential values of dispersion are opposite to each other. Since the dispersion medium 108 is composed of the first optical waveguide 105 and the second optical waveguide 106 whose positive and negative values of the wavelength derivative of dispersion are opposite to each other, the wavelength dependence of the dispersion value is reduced as a whole of the dispersion medium 108. .. Therefore, according to this embodiment, it is possible to provide the light source device 100 capable of changing the central wavelength of the output pulsed light without causing a large variation in the spectral line width.

[Second Embodiment]
The light source device according to the second embodiment will be described with reference to FIG. FIG. 2 is a block diagram showing the light source device according to the present embodiment. The light source device 200 according to the present embodiment includes a light source 201 that emits first pulsed light (excitation pulsed light), an optical multiplexer 202, a nonlinear optical medium 203, an optical branching device 204, and a first optical waveguide 205. , And a second optical waveguide 206. A dispersion medium 208 is constituted by the first optical waveguide 205 and the second optical waveguide 206. The nonlinear optical medium 203 and the dispersion medium 208 are arranged in an optical resonator 207 that oscillates signal pulsed light.

In this embodiment, for example, the following first optical waveguide 205 and second optical waveguide 206 are used. As the first optical waveguide 205, for example, a single mode fiber (model: HI-1060, length: 1000 m) manufactured by Corning is used. As the second optical waveguide 206, for example, a photonic crystal fiber manufactured by NKT Photonics (model: NL-PM-750, length: 110 m) is used. The positive and negative of the wavelength differential value of the dispersion of the first optical waveguide 205 and the second optical waveguide 206 are opposite to each other. Since the first optical waveguide 205 and the second optical waveguide 206 in which the positive and negative of the wavelength differential value of the dispersion are opposite to each other are connected in series, the wavelength differential value of the dispersion in the entire dispersion medium 208 becomes very small. ing. The spectral line width of the second pulsed light is, for example, about 1.5 nm. Note that, also in the present embodiment, as in the first embodiment, when the first pulsed light enters the nonlinear optical medium 203, the second pulsed light (signal pulsed light) and the third pulsed light are generated by the optical parametric gain. Light (idler pulse light) is generated.

FIG. 6 is a graph showing a dispersion profile in the light source device according to the present embodiment. 6A shows a dispersion value D ₁ per unit length of the first optical waveguide 205, and FIG. 6B shows a dispersion value D ₁ per unit length of the second optical waveguide 206. ₂ is shown. FIG. 6C shows the dispersion value D ₁ L ₁ +D ₂ L _{2 in} the entire dispersion medium 208, that is, the total dispersion value. As can be seen from FIG. 6C, the wavelength differential value of the dispersion value in the entire dispersion medium 208 including the first optical waveguide 205 and the second optical waveguide 206, that is, the slope of the total dispersion value is very small. Has become. As can be seen from FIG. 6A, the wavelength differential value of the dispersion in the first optical waveguide 205 is positive. On the other hand, as can be seen from FIG. 6B, the wavelength differential value of the dispersion in the second optical waveguide 206 is positive. As can be seen from FIG. 6C, the difference between the minimum value and the maximum value of the dispersion value of the dispersion medium 208 in the wavelength variable range of 1200 nm to 1500 nm of the second pulsed light is 2.5 ps/nm. ing. The pulse width ΔT _C of the first pulsed light is about 15 ps, and the spectral line width Δλ _S of the second pulsed light is about 1.5 nm. The coefficient B in the above equation (9) is 0.25 in this case. Note that here, the case where the difference between the maximum value and the minimum value of the dispersion value of the dispersion medium 208 in the wavelength variable range of the second pulsed light is 2.5 ps/nm has been described as an example, but the present invention is not limited to this. Not something. By appropriately setting the configuration and materials of the waveguide used as the first optical waveguide 205 and the waveguide used as the second optical waveguide 206, the dispersion value of the dispersion medium 208 in the wavelength variable range of the second pulsed light The difference between the maximum value and the minimum value can be 2.5 ps/nm or less.

As described above, also in the present embodiment, the dispersion medium 208 is constituted by the first optical waveguide 205 and the second optical waveguide 206 whose positive and negative wavelength differential values of dispersion are opposite to each other. Since the dispersion medium 208 is composed of the first optical waveguide 205 and the second optical waveguide 206 whose positive and negative of the wavelength differential value of the dispersion are opposite to each other, the wavelength dependence of the dispersion value is also the dispersion medium in this embodiment. 208 reduced overall. Therefore, also in this embodiment, it is possible to provide the light source device 200 capable of changing the central wavelength of the output pulsed light without causing a large variation in the spectral line width.

[Third Embodiment]
An information acquisition device according to the third embodiment will be described with reference to FIG. FIG. 3 is a block diagram showing the information acquisition device according to the present embodiment. The information acquisition device 300 according to the present embodiment is, for example, a microscope that performs SRS imaging, that is, an SRS microscope. The light source device 100 according to the first embodiment or the light source device 200 according to the second embodiment described above can be used as the light source device of the SRS microscope according to the present embodiment.

The information acquisition apparatus 300 according to the present embodiment irradiates the subject 315 with two pulsed lights, the light reflected by the subject 315, the light transmitted through the subject 315, and the light emitted by the subject 315. Detecting at least one of the Then, the information regarding the subject 315 is acquired based on the detected light.

SRS imaging is a method for acquiring molecular vibration imaging by utilizing a phenomenon called stimulated Raman scattering in which Stokes light is amplified by the interaction between pump light and Stokes light incident on a substance. Specifically, one of the two pulsed lights having different wavelengths, that is, the Stokes light is intensity-modulated, and the two pulsed lights are synchronized and applied to the subject. Stimulated Raman scattering occurs when the difference frequency between the two pulsed lights coincides with the molecular frequency of the molecules forming the subject, and the intensity-modulated Stokes light is amplified. At this time, according to the intensity modulation of the Stokes light, the pulse light that has not been intensity-modulated, that is, the pump light is also intensity-modulated, and the intensity-modulated component of the pump light emitted from the subject due to stimulated Raman scattering is detected. This enables molecular vibration imaging of the subject. Also, by changing the central wavelength of the pulsed light and changing the difference frequency between the two pulsed lights, it is possible to match the molecular frequencies of various molecules, which is unique to the group of molecules constituting the subject. You can get a signal.

The excitation pulsed light (first pulsed light) emitted from the light source 301 is split into two by the optical splitter 302, and one of the split lights is modulated by the optical modulator 308 to be Stokes light for the information acquisition device 300. To use. Then, the other light split by the optical splitter 302 is incident on the nonlinear optical medium 304 via the optical splitter 303, and the second pulsed light (signal pulsed light) and the third pulsed light (idler pulsed light) are input. And generate. As the non-linear optical medium 304, an optical fiber having a high non-linear coefficient can be preferably used. Examples of the optical fiber having a high nonlinear coefficient include a photonic crystal fiber and a tapered optical fiber. The second pulsed light and the third pulsed light are branched by the optical branching device 305, incident on the dispersion medium 323 including the first optical waveguide 306 and the second optical waveguide 307, and passed through the optical branching device 303. And enters the nonlinear optical medium 304 again. In this way, the second pulsed light and the third pulsed light circulate in the optical resonator 324 to oscillate, and the second pulsed light and the third pulsed light with a narrowed spectral line width are obtained. The second pulsed light and the third pulsed light oscillated in the optical resonator 324 are output via the optical branching device 305 and enter the bandpass filter 309. The bandpass filter 309 allows, for example, the second pulsed light to pass therethrough and blocks the third pulsed light. Therefore, only the second pulsed light enters the optical multiplexer 310 via the bandpass filter 309. The second pulsed light that enters the optical multiplexer 310 via the bandpass filter 309 is used as pump light for the information acquisition device 300.

The pump light from the bandpass filter 309 and the Stokes light from the optical modulator 308 enter the optical multiplexer 310. The optical multiplexer 310 multiplexes a plurality of pulsed lights having mutually different center wavelengths. As the optical multiplexer 310, an optical coupler, a diffraction grating, a prism or the like can be used. The pump light and the Stokes light multiplexed by the optical multiplexer 310 are set on the stage 316 via the beam expander 311, the X scan mirror 312, the Y scan mirror 313, and the objective lens 314. The subject 315 is irradiated.

In a minute region located at the center of the condensing point of the objective lens 314, stimulated Raman scattering based on the molecular vibration of molecules of the subject 315 occurs, which causes an intensity change between the pump light and the Stokes light. Since stimulated Raman scattering does not occur at a portion outside the minute region located at the center of the condensing point, there is no intensity change between the pump light and Stokes light at that portion. The larger the numerical aperture (NA: Numerical Aperture) of the objective lens 314, the smaller the size of the spot of the light with which the subject 315 is irradiated, and accordingly the size of the minute region in which stimulated Raman scattering occurs. ..

The pump light whose intensity is modulated by stimulated Raman scattering generated in a minute area located at the center of the condensing point passes through the condensing lens 317 and the bandpass filter 318, enters the light receiving element 319, and is detected as an SRS signal. To be done. The SRS signal detected by the light receiving element 319 is acquired by the information acquisition unit 320 as an image signal. That is, the information acquisition unit 320 acquires the light received by the light receiving element 319 as an electric signal.

In general, the Raman scattering cross section σ of a molecule is small, so that the intensity change of pump light due to stimulated Raman scattering is also weak. Therefore, when detecting the SRS signal from the intensity change of the pump light, the SRS signal may be buried in the noise component or the like. In the present embodiment, the information acquisition unit 320 including the synchronization detector 321 and the control unit 322 is used, and the intensity modulation of the pump light received by the light receiving element 319 and converted into an electric signal is performed by the Stokes light from the optical modulator 308. It is detected in synchronization with the modulation frequency of. Then, in the present embodiment, molecular vibration imaging of the subject 315 is acquired. Since the signal detected synchronously is amplified, the SRS signal can be detected with high sensitivity.

The synchronization detector 321 acquires a signal in synchronization with the modulation of the light received by the light receiving element 319. A lock-in amplifier, an FFT analyzer, or the like can be used as the synchronization detector 321, but the FFT analyzer can detect the SRS signal at a higher speed than the lock-in amplifier. Note that, in FIG. 3, the case where the synchronization detector 321 and the control unit 322 are separate bodies is shown as an example, but the information acquisition unit 320 in which these are integrated may be used. As the information acquisition unit 320 in which the synchronization detector 321 and the control unit 322 are integrated, for example, the control unit 322 is configured by a computer including a CPU, and an application having a synchronization detection function is built in the control unit 322. Some of them are listed.

When the X scan mirror 312 is driven, the light condensing point scans the inside of the subject 315 in the X direction, and when the Y scan mirror 313 is driven, the light condensing point moves inside the subject 315 in the Y direction perpendicular to the X direction. Scan in the direction. Therefore, a two-dimensional image can be acquired by driving the X scan mirror 312 and the Y scan mirror 313 to two-dimensionally scan the subject 315 for a focal point. The X direction is, for example, the horizontal direction in FIG. 3, and the Y direction is, for example, the depth direction in FIG.

Further, every time the two-dimensional scanning is completed, the condensing point is moved by a predetermined distance in the optical axis direction by moving the stage 316, and the two-dimensional scanning similar to the above is repeated, whereby the object 315 is It is also possible to obtain a three-dimensional image of.

Further, after one two-dimensional scan or three-dimensional scan is completed, the central frequency of the first pulsed light emitted from the light source 301 is changed to change the difference frequency between the pump light and the Stokes light. May be. Thus, the difference frequency between the pump light and the Stokes light may be made to match the molecular frequency of various molecules contained in the subject 315. This makes it possible to obtain a two-dimensional or three-dimensional molecular vibration image.

The pulse width of the pulsed light emitted from the light source devices 100 and 200 used in the information acquisition device 300 according to the present embodiment is preferably 1 ns or less, and more preferably 100 ps or less. This is because the narrower the pulse width of the pulsed light output from the light source devices 100 and 200, the greater the peak intensity of the pulsed light, and the presence or absence of the non-linear effect occurring in the subject 315 can be accurately detected. The pulse rate of the pulsed light emitted from the light source 301 is preferably 1 MHz or higher and 1 GHz or lower. Further, the pulse rates of the second pulsed light and the third pulsed light emitted from the light source devices 100 and 200 are also preferably 1 MHz or more and 1 GHz or less. This is because the frequency is preferably 1 MHz or higher due to the restriction of the measurement speed that is practically required for the information acquisition device 300, and is preferably 1 GHz or lower due to the restriction of thermal destruction that occurs in the subject 315.

In addition, since the information acquisition device 300 is preferably used for observing living tissue, each pulsed light emitted from the light source devices 100 and 200 has a wavelength that is small in reflection, absorption, and scattering by the living body and is easily transmitted. It is preferable. Therefore, the center wavelength of each pulsed light emitted from the light source devices 100 and 200 is preferably 300 nm or more and 1500 nm or less, and more preferably 700 nm or more and 1300 nm or less. For example, as the light source 301, a mode-locked Yb (ytterbium)-doped fiber laser can be preferably used.

As described above, according to the present embodiment, the light source devices 100 and 200 according to the first embodiment or the second embodiment are used as the light source of the information acquisition device 300. In the light source devices 100 and 200 according to the first embodiment or the second embodiment, the spectral line width of the output signal pulse light is substantially constant regardless of the center wavelength. Therefore, according to the present embodiment, it is possible to provide the information acquisition device 300 that can acquire information with uniform resolution.

Further, according to the present embodiment, the light source devices 100 and 200 can be downsized and the cost can be reduced as compared with the conventional SRS microscope device. Therefore, the entire information acquisition device 300 can be downsized and the cost can be reduced. can do.

[Modified Embodiment]
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 changes can be made within the scope of the gist thereof.
For example, in the first embodiment, a case has been described as an example where a single mode fiber is used as the first optical waveguide 105 and a dispersion compensating fiber is used as the second optical waveguide 106, but the present invention is not limited to this. For example, a dispersion compensating fiber may be used in both the first optical waveguide 105 and the second optical waveguide 106.
Further, in the second embodiment, a case has been described as an example where a single mode fiber is used as the first optical waveguide 205 and a photonic crystal fiber is used as the second optical waveguide 206, but the present invention is not limited to this. .. For example, a photonic crystal fiber may be used in both the first optical waveguide 205 and the second optical waveguide 206.
Further, in the third embodiment, a case has been described as an example where the information acquisition device 300 is an SRS microscope, but the present invention is not limited to this. For example, the information acquisition device 300 may be an information acquisition device such as a CARS microscope, a fluorescence microscope, and an endoscope that acquires various types of spectral information. The light source device 100 according to the first embodiment and the light source device 200 according to the second embodiment can be appropriately used for such an information acquisition device.
Further, in the above embodiment, the case where the number of the optical waveguides 105, 106, 205 and 206 included in the dispersion media 108 and 208 is two has been described as an example, but the present invention is not limited to this. For example, the dispersion media 108 and 208 may be configured by connecting three or more optical waveguides in series.

The present invention supplies a program that implements one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus read and execute the program. It can also be realized by the processing. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

101... Light source 103... Non-linear optical medium 105... First optical waveguide 106... Second optical waveguide 107... Optical resonator 108... Dispersion medium

Claims (15)

A light source that emits a first pulsed light,
A non-linear optical medium for generating a second pulsed light having a wavelength different from that of the first pulsed light by the incidence of the first pulsed light;
An optical branching unit for branching the second pulsed light;
A dispersion medium that gives a predetermined wavelength dispersion to the second pulsed light branched by the optical branching unit, wherein the positive and negative of the wavelength differential value of the dispersion are opposite to each other at the wavelength of the second pulsed light. A dispersion medium having a dispersion value at the wavelength of the second pulsed light which is not 0,
An optical multiplexer that guides the second pulsed light, to which the chromatic dispersion is given by the dispersion medium, and the first pulsed light emitted from the light source to the nonlinear optical medium.
Have a,
A light source device , wherein a difference between a maximum value and a minimum value of a dispersion value of the dispersion medium in a wavelength variable range of the second pulsed light is 2.5 ps/nm or less . The light source device according to claim 1, wherein the nonlinear optical medium further generates a third pulsed light having a wavelength different from both the first pulsed light and the second pulsed light. According to claim 1, wherein the difference between the maximum value and the minimum value of the dispersion value of the dispersion medium in the wavelength variable range of the second pulse light is less 1.5 ps / nm Light source device. Wherein the wavelength of the second pulse light, the absolute value of the wavelength derivative of the dispersion medium, a light source according to any one of claims 1 to 3, characterized in that 0.013ps / nm ² or less apparatus. Wherein the first at least one of the optical waveguide and the second optical waveguide of the light source apparatus according to any one of claims 1 to 4, characterized in that it comprises a dispersion compensating fiber. Wherein the first at least one of the optical waveguide and the second optical waveguide of the light source apparatus according to any one of claims 1 to 4, characterized in that it comprises a photonic crystal fiber. The light source device according to any one of claims 1 to 6, characterized in that the central wavelength of the first pulse light is variable. The nonlinear optical medium and the dispersion medium are arranged in an optical resonator that oscillates the second pulsed light,
8. The pulse rate of the first pulsed light is an integral multiple of the free spectrum interval of the optical resonator at the central wavelength of the second pulsed light, according to any one of claims 1 to 7. The light source device described. It said nonlinear optical medium, a light source device according to any one of claims 1 to 8, characterized in that it comprises a photonic crystal fiber. The light source device according to any one of claims 1 to 9 , wherein the spectral line width of the first pulsed light is 1 nm or less. The light source device according to any one of claims 1 to 10, characterized in that the pulse width of the first pulse light is more than 10 ps. A light source device according to any one of claims 1 to 11 ,
Two pulsed lights having different central wavelengths are emitted from the light source device, and when the two pulsed lights are irradiated to the subject, the light reflected by the subject, the light transmitted through the subject, And a light-receiving element that receives at least one of the light emitted from the subject. One of the two pulsed lights is the second pulsed light,
The information acquisition device according to claim 12 , wherein the other of the two pulsed lights is the modulated first pulsed light. The information acquisition apparatus according to claim 12 or 13 , wherein the pulse rates of the two pulsed lights are both 1 MHz or more and 1 GHz or less. Further comprising an information acquisition unit for acquiring the light received by the light receiving element as an electric signal,
The information acquisition unit, the information acquiring apparatus according to any one of claims 12 to 14, characterized in that it comprises a synchronization detector for acquiring a signal in synchronism with the modulation of light received by said light receiving element ..

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