JP2017203939A - Light source device and information acquiring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light source device capable of varying a center wavelength of output pulse light without accompanying a large variation in a spectral line width, and an information acquiring device using the light source device.SOLUTION: The light source device includes: a light source 101 emitting first pulse light; a nonlinear optical medium 103 emitting second pulse light at a wavelength different from that of the first pulse light in response to the incident of the first pulse light; an optical branch 104 for branching the second pulse light; a dispersion medium that imparts predetermined wavelength dispersion to the second pulse light branched by the optical branch, includes a first optical waveguide 105 and a second optical waveguide 106 having opposite signs of wavelength differential values of dispersion at the wavelength of the second pulse light, and has non-zero dispersion value at the wavelength of the second pulse light; an optical multiplexing part 102 for guiding the second pulse light having wavelength dispersion imparted by the dispersion medium and the first pulse light emitting from the light source to the nonlinear optical medium.SELECTED DRAWING: Figure 1

Description

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

  Various kinds of constituent materials of the subject are detected 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. The technology to obtain information is attracting attention.

  In recent years, active research has been conducted to irradiate a subject with two pulse lights having a frequency difference corresponding to the molecular frequency, detect light based on the following scattering generated in the subject, and identify the substance of the subject. Has been done. Examples of such scattering include stimulated Raman scattering (SRS). Examples of such scattering include Coherent Anti-Stokes Raman Scattering (CARS: Coherent Anti-Stokes Raman Scattering).

  The following laser light sources are known as laser light sources that generate two pulse lights having different center wavelengths. That is, an optical parametric resonator (Fiber Optical Parametric Oscillator, hereinafter referred to as FOPO) using four-wave mixing (a kind of optical parametric effect) generated in a nonlinear fiber is known. The FOPO receives the energy of the excitation pulse light incident on the optical fiber, generates pulse light having a wavelength different from that of the excitation pulse light, and oscillates the pulse light in the resonator. That is, when the excitation pulse light is incident on the nonlinear fiber provided in the FOPO, pulse light having a shorter wavelength than the excitation pulse light (signal pulse light, signal light) and pulse light having a longer wavelength than the excitation pulse light ( Idler pulsed light and idler light) are generated simultaneously. At least a part of at least one of the signal pulse light and the idler pulse light is fed back and re-introduced into the resonator in synchronization with the excitation pulse light, thereby at least one of the signal pulse light and the idler pulse light. Oscillate one of them. Then, either or both of the signal pulse light and the idler pulse 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 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 pulse light cannot be changed without a large fluctuation in the spectral line width.

  An object of the present invention is to provide a light source device capable of changing the center wavelength of output pulse light without causing a large fluctuation 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 the first pulsed light and a nonlinear that generates the 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 unit that branches the second pulsed light, and a dispersion medium that gives a predetermined wavelength dispersion to the second pulsed light branched by the optical branching unit, 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 dispersion are opposite to each other at the wavelength of light, and wherein the dispersion value at the wavelength of the second pulsed light is not zero; An optical multiplexing unit that guides the second pulsed light having the chromatic dispersion given by the dispersion medium and the first pulsed light emitted from the light source to the nonlinear optical medium; A light source device is provided.

  According to the present invention, it is possible to provide a light source device capable of changing the center wavelength of output pulse light without causing a large fluctuation in the spectral line width and an information acquisition device using the light source device.

It is a block diagram which shows the light source device by 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 typically the dispersion profile in a dispersion medium. It is a graph which shows the dispersion profile in the light source device by 1st Embodiment. It is a graph which shows the dispersion | distribution profile in the light source device by 2nd Embodiment. 5 is a graph showing a phase mismatch Δβ of light propagation constant and an optical parametric gain G in a nonlinear optical medium with β ₂ > 0 and β ₄ > 0. 6 is a graph showing a phase mismatch Δβ of light propagation constant and an optical parametric gain G in a nonlinear optical medium in which β ₂ > 0 and β ₄ <0. 6 is a graph showing a phase mismatch Δβ of light propagation constant and an optical parametric gain G in a nonlinear optical medium with β ₂ <0 and β ₄ > 0. 5 is a graph showing a phase mismatch Δβ of light propagation constant 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. However, the present invention is not limited to the configurations of the following embodiments. In the drawings described below, members denoted by 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 illustrating 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 pulsed light, a nonlinear optical medium 103 that generates signal pulsed light, and a dispersion medium 108 for narrowing the spectral line width of the signal pulsed light. Yes. The light source 101 can change the center wavelength of the excitation pulse light. The nonlinear optical medium 103 and the dispersion medium 108 are disposed in an optical resonator 107 that oscillates signal pulse light. Optical fibers can be preferably used for the nonlinear optical medium 103 and the dispersion medium 108, respectively.

  When the excitation pulse light emitted from the light source 101 is incident on the nonlinear optical medium 103, signal pulse light having a wavelength different from that of the excitation pulse light is generated by 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 excitation pulse light. By utilizing this phenomenon, in the light source device 100 according to the present embodiment, the center wavelength of the output pulse light can be changed in a wide band.

  When the signal pulse light generated in the nonlinear optical medium 103 is incident on the dispersion medium 108, the speed of the light traveling in the dispersion medium 108 differs depending on the wavelength, so that the signal pulse light spreads in time and the chirped pulse light Become. When the signal pulse light having chirp enters the nonlinear optical medium 103 again, the signal pulse light is amplified by receiving the optical parametric effect from the excitation pulse light. At this time, since the signal pulse light spreads much larger in time than the excitation pulse light, only the portion where the signal pulse light and the excitation pulse light overlap in time is amplified. Since the signal pulse light has a chirp, only a limited wavelength component of the signal pulse light is amplified, and the spectral line width of the signal pulse light is narrowed.

  However, since the dispersion value of the dispersion medium 108 generally varies depending on the wavelength, when the center wavelength of the signal pulse light changes, the degree to which the spectral line width of the signal pulse light is narrowed depends on the center wavelength of the signal pulse light. End up. As a result, the spectral line width of the signal pulse light varies 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 whose signs of the wavelength differential value of dispersion are opposite to each other within the wavelength variable range of the signal pulse light. Since the dispersion medium 108 is constituted by the first optical waveguide 105 and the second optical waveguide 106 whose signs of dispersion wavelength differential values are opposite to each other, the wavelength dependency of the dispersion value is reduced as a whole in the dispersion medium 108. . For this reason, according to the present embodiment, it is possible to provide the light source device 100 that can change the center wavelength of the output pulse light without causing a large fluctuation in the spectral line width.

  Before describing the light source device 100 according to the present embodiment in detail, the principle of four-wave mixing, which is the source of optical parametric gain that generates signal pulse light from pump pulse light, will be described.

Four-wave mixing means that when two lights (excitation light) having different frequencies (wavelengths) are incident on a nonlinear optical medium (optical fiber), new light having a wavelength different from any wavelength of the two excitation lights. Is a phenomenon that occurs. At this time, a part of the energy of light incident on the nonlinear optical medium is converted into energy of light newly generated by four-wave mixing. For example, when two lights having frequencies ω ₁ and ω ₂ are 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 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, Two lights of ω _C −Δω are generated symmetrically with respect to the frequency ω _C. In general, light on the high frequency side of two lights generated by degenerate four-wave mixing is called signal light, and 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 the wavelength control and configuration are simpler than in the case where two lights having different frequencies are incident. Such 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, if the propagation constant of excitation light entering the 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 expressed by Equation (1).

Here, [Delta] [beta] phase mismatch of the propagation constants of each of the light in the nonlinear optical medium, gamma is the nonlinear coefficient of the nonlinear optical medium, the P _C is the peak intensity of the excitation light. N ₂ is the nonlinear refractive index of the nonlinear optical medium, A _eff is the effective cross-sectional area of the core of the optical fiber that is the nonlinear optical medium, and c is the speed of light in vacuum.

The phase mismatch Δβ of the propagation constant of each light in the nonlinear optical medium can be expressed by the following equation (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 a 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.

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

In the phase matching condition of [Delta] [beta] represented by formula (1), and the nonlinear coefficient γ of the nonlinear optical medium, are both positive values to the peak intensity P _C of the excitation light, [Delta] [beta] will take 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. In such a nonlinear optical medium, even if the excitation light is incident, the signal light Neither idler light is generated.

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 Expression (1) is satisfied, that is, the phase matching region is indicated by hatching. As can be seen from FIG. 8A, the range of Δω that satisfies the phase matching condition expressed by the equation (1) exists in a relatively narrow region at a position sufficiently away from the frequency of the excitation light. Accordingly, when excitation light having a specific frequency is incident on the nonlinear optical medium, an optical parametric gain G is obtained in a relatively narrow frequency band as shown in FIG. 8B, and signal light having a narrow frequency band is obtained. And idler light are generated.

FIG. 9 shows the case of β ₂ <0 and β ₄ > 0, and FIG. 10 shows the case of β ₂ <0 and β ₄ <0. As shown in FIGS. 9A and 10A, the range of Δω that satisfies the phase matching condition expressed by the equation (1) is relatively wide. That is, as shown in FIGS. 9 (b) and 10 (b), when pulse light of a specific frequency is incident on the nonlinear optical medium, an 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 nonlinear optical medium satisfying β ₂ > 0 and β ₄ <0 in order to generate pulsed light having a relatively narrow spectral line width using degenerate four-wave mixing. I understand. In such a nonlinear optical medium, the core material and the cladding material are selected so that the refractive index difference between the core and the cladding of the optical fiber becomes an appropriate value, and the shape of the optical fiber is appropriately set. Can be realized.

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 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), (5 ). Further, the frequency width δω and the spectral line width (spectral half width) δλ of the optical parametric gain G are expressed by the following equations (6) and (7), respectively.

Here, 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 of the group velocity dispersion β _{2 at} the zero dispersion wavelength. It is a derivative function. As can be seen from the equation (5), the wavelength of the signal light or idler light can be greatly changed by the coefficient shown in the equation (5) by slightly changing the center wavelength λ _C of the excitation pulse light. As can be seen from the equation (7), if a nonlinear optical medium having a small nonlinear coefficient γ and a large β ₄ is used, signal light or idler light having a narrow spectral line width δλ can be generated. Then, either one or both of signal light and idler light can be used.

  Here, a method for changing the center wavelength of the output pulse light without causing a large fluctuation 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 a dispersion profile of the dispersion medium 108 constituted by the first optical waveguide 105 and the second optical waveguide 106 in which the sign of the wavelength differential value of dispersion is opposite. The dispersion profile of the first optical waveguide 105 is indicated by a dotted line in FIG. 4, and the dispersion profile of the second optical waveguide 106 is indicated 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 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 dispersion in the second optical waveguide 106 is negative. As described above, the wavelength differential values of dispersion in the first optical waveguide 105 and the second optical waveguide 106 are opposite to each other. When the first optical waveguide 105 and the second optical waveguide 106 are connected in series, the dispersion profile of 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 in 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 in the entire dispersion medium 108 is shown by a thick solid line in FIG. As shown in FIG. 4, the dispersion wavelength differential value in the entire dispersion medium 108 is very small. For this reason, even if the center wavelength of the signal pulse light is changed, the wavelength dispersion that the signal pulse light receives from the dispersion medium 108 is substantially constant, and therefore the spectral line width of the signal pulse light is 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 pulse light is not zero. The reason why the sum of the dispersion value in the first optical waveguide 105 and the dispersion value in the second optical waveguide 106 is not zero is to spread the signal pulse light in terms of time. As a result, only the portion where the signal pulse light and the excitation pulse light overlap in time is amplified, and the spectral line width of the signal pulse light is narrowed.

When excitation 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. 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. L ₂ is the length of the second optical waveguide 106. Further, [Delta] [lambda] _S is the spectral linewidth of the optical signal pulse, [Delta] T _C is the pulse width of the excitation pulse light.

In order to obtain a substantially constant spectral line width within the wavelength variable range of the signal pulse light, it is necessary that the dispersion value of the dispersion medium 108 is substantially constant within the wavelength variable range of the signal pulse light. In other words, the difference between the maximum value of the dispersion value of the dispersion medium 108 within the wavelength variable range of the signal pulse light and the minimum value of the dispersion value of the dispersion medium 108 within the wavelength variable range of the signal pulse light is sufficiently small. It takes a thing. Since the relationship shown in the equation (8) is established, the following equation (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 wavelength variable range of the signal pulse light, and (D ₁ L ₁ + D ₂ L ₂ ) _min is the signal pulse. This is the minimum value of the dispersion value of the dispersion medium 108 within the light wavelength variable range. | (D ₁ L ₁ + D ₂ L ₂ ) _max − (D ₁ L ₁ + D ₂ L ₂ ) _min | is a difference value between them. B is a coefficient, and the smaller the coefficient B, the better the light source device with less fluctuation of the spectral line width.

  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 splitter 104, a first optical waveguide 105, And a second optical waveguide 106.

  The light source 101 emits first pulsed light (excitation pulsed light). The light source 101 can change the center wavelength of the emitted first pulse light. As the light source 101, for example, a laser resonator or the like provided with a wavelength filter can be used. For example, a pulse laser is preferable. Here, as the light source 101, for example, a mode-locked Yb (ytterbium) -doped fiber that can change the center 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 first pulsed light having a pulse width of 10 ps or more can be used as the light source 101 as appropriate. Here, the case where the light source 101 that emits the first pulsed light having a spectral line width of about 0.1 nm is used as an example. However, the spectral line width of the first pulsed light is limited to this. is not. For example, a light source that emits 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 nonlinear optical medium 103, an optical fiber having a high nonlinear coefficient can be suitably used. Examples of the optical fiber having a high nonlinear coefficient include a photonic crystal fiber and a tapered optical fiber. A photonic crystal fiber is a fiber in which a large number of air holes (air holes) are provided in a clad portion of an optical fiber, and the refractive index of the clad can be made extremely lower than that of a core by the air holes. Since the photonic crystal fiber can reduce the effective core diameter (mode field diameter), a large nonlinear effect can be obtained even if the fiber length is about several meters. Moreover, the photonic crystal fiber can obtain arbitrary wavelength dispersion characteristics by adjusting the size and pitch of the holes. A tapered optical fiber is an optical fiber in which an output cladding diameter is made smaller than an input cladding diameter, and can be formed by heating and stretching a normal optical fiber. When the diameter of the clad optical fiber is extremely reduced to, for example, about several μm, a large nonlinear effect can be obtained even when the length is, for example, about several mm. The tapered optical fiber can obtain arbitrary wavelength dispersion characteristics by adjusting the clad diameter and 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, for example, 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 dispersion are opposite to each other, and the total dispersion value in the first optical waveguide 105 and the second optical waveguide 106 is the same. Is not 0 at each wavelength within the wavelength tunable 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 nonlinear optical medium 103, the second pulsed light and the third pulsed light are generated by the optical parametric gain. The center wavelength of the second pulse light is different from the center wavelength of the first pulse light. The center wavelength of the third pulse light is different from both the center wavelength of the first pulse light and the center wavelength of the second pulse light. The wavelength of the second pulse light is, for example, 1200 nm to 1500 nm, and the wavelength of the third pulse light is, for example, 750 nm to 950 nm. The second pulse light and the third pulse light emitted from the nonlinear optical medium 103 are guided to the optical branching device 104. The optical branching device 104 branches and outputs the pulsed light. The second pulsed light and the third pulsed light emitted from the nonlinear optical medium 103 are branched by the optical branching device 104, and are distributed to the dispersion medium 108 including the first optical waveguide 105 and the second optical waveguide 106. Led. 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, the second pulsed light and the third pulsed light are spread over time by the chromatic dispersion in the first optical waveguide 105 and the chromatic dispersion in the second optical waveguide 106 to become pulsed light having a chirp. The second pulse light and the third pulse light that have been given chromatic dispersion 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 that have been given chromatic dispersion by the dispersion medium 108 are broader in time than the first pulsed light and overlapped in time with the first pulsed light. Only the part is amplified. For this reason, only a limited wavelength component is amplified, and the spectral line width is narrowed. Here, a case where the second optical waveguide 106 is located at the subsequent stage of the first optical waveguide 105 will be described as an example, but the present invention is not limited to this, and the subsequent stage of the second optical waveguide 106 is performed. In addition, the first optical waveguide 105 may be located. The optical branching unit (optical branching unit) 104 may include a bandpass filter that cuts light other than a desired wavelength. For example, when the optical splitter 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 Only the other one is guided to the dispersion medium 108.

As described above, 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, and the first optical waveguide 105 and the second optical waveguide 106 are The sum of the dispersion values in is not zero at each wavelength within the wavelength variable 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 compensating fiber (model: DCF38, length: 500 m) manufactured by Thorlab is used. When the first optical waveguide 105 and the second optical waveguide 106 having positive and negative dispersion wavelength differential values are connected in series, a dispersion medium 108 including the first optical waveguide 105 and the second optical waveguide 106 is obtained. The wavelength differential value of the dispersion in the whole becomes very small. The spectral line width Δλ _S of the second pulse 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 and the wavelength in the first optical waveguide 105. FIG. 5B shows the relationship between the dispersion value D ₂ per unit length and the wavelength in the second optical waveguide 106. 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 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 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 wavelength variable range of the second pulsed light from 1200 nm to 1500 nm is about −10 ps / nm. Further, as can be seen from FIG. 5 (c), 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. Pulse width [Delta] T _C of the first pulse light is about 15 ps, the spectral line width [Delta] [lambda] _S of the second pulse light is about 1.5 nm. In this case, the coefficient B in the equation (9) is 0.15 in this case. Within the wavelength variable 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 pulse 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 pulse light is 1.5 ps / nm is described as an example, but the present invention is not limited to this. It is 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 is set. The difference between the maximum value and the minimum value can be 1.5 ps / nm or less.

  Thus, 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. For this reason, according to the present embodiment, the spectral line width of the output pulse light can be made substantially constant even when the center wavelength of the output pulse light is changed.

  In the present embodiment, since the nonlinear optical medium 103 is disposed in the optical resonator 107, the second pulse light and the third pulse light repeatedly pass through the nonlinear optical medium 103. Since the second pulse light and the third pulse light are amplified every time the second pulse light and the third pulse light pass through the nonlinear optical medium 103, the second pulse light and the third pulse 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 splitter 104. In this way, the second pulse light having a substantially constant spectral line width and the third pulse light having a substantially constant spectral line width are extracted outside the optical resonator 107. Note that the first pulse light that has not contributed to the optical parametric oscillation is also output to the outside of the optical resonator 107 via the optical splitter 104. Further, as described above, the optical branching device 104 may include a band-pass filter that cuts light other than a desired wavelength. When a bandpass filter that cuts light other than a predetermined wavelength is provided in the optical splitter 104, only light having a predetermined wavelength is output to the outside of the optical resonator 107.

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

  Note that the spectral line width of the first pulsed light emitted from the light source 101 is preferably 1 nm or less. This is because as the spectral line width of the first pulse light is narrower, four-wave mixing is performed more efficiently, and a sufficient optical parametric gain is 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 in which the sign of the wavelength differential value of dispersion is opposite to each other. Since the dispersion medium 108 is constituted by the first optical waveguide 105 and the second optical waveguide 106 whose signs of dispersion wavelength differential values are opposite to each other, the wavelength dependency of the dispersion value is reduced as a whole in the dispersion medium 108. . For this reason, according to the present embodiment, it is possible to provide the light source device 100 that can change the center wavelength of the output pulse light without causing a large fluctuation in the spectral line width.

[Second Embodiment]
A light source device according to a second embodiment will be described with reference to FIG. FIG. 2 is a block diagram illustrating 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, a first optical waveguide 205, and the like. And a second optical waveguide 206. The first optical waveguide 205 and the second optical waveguide 206 constitute a dispersion medium 208. The nonlinear optical medium 203 and the dispersion medium 208 are arranged in an optical resonator 207 that oscillates signal pulse light.

  In the present 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 (model: NL-PM-750, length: 110 m) manufactured by NKT Photonics 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 whose signs of the dispersion wavelength differential values are opposite to each other are connected in series, the dispersion wavelength differential value in the entire dispersion medium 208 becomes very small. ing. The spectral line width of the second pulse light is, for example, about 1.5 nm. In the present embodiment, similarly to the first embodiment, when the first pulsed light is incident on the nonlinear optical medium 203, the second pulsed light (signal pulsed light) and the third pulsed light are caused by the optical parametric gain. Light (idler pulsed light) is generated.

FIG. 6 is a graph showing a dispersion profile in the light source device according to the present embodiment. FIG. 6A shows the dispersion value D ₁ per unit length of the first optical waveguide 205, and FIG. 6B shows the dispersion value D 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. It has become. As can be seen from FIG. 6A, the wavelength differential value of 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 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 the second pulsed light from 1200 nm to 1500 nm is 2.5 ps / nm. ing. Pulse width [Delta] T _C of the first pulse light is about 15 ps, the spectral line width [Delta] [lambda] _S of the second pulse light is about 1.5 nm. In this case, the coefficient B in the equation (9) is 0.25 in this case. 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 pulse light is 2.5 ps / nm is described as an example, but the present invention is not limited to this. It is 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 is set. The difference between the maximum value and the minimum value can be 2.5 ps / nm or less.

  As described above, also in this embodiment, the dispersion medium 208 is configured by the first optical waveguide 205 and the second optical waveguide 206 in which the positive and negative of the wavelength differential value of dispersion are opposite to each other. Since the dispersion medium 208 is constituted by the first optical waveguide 205 and the second optical waveguide 206 whose signs of dispersion wavelength differential values are opposite to each other, also in this embodiment, the wavelength dependence of the dispersion value is the dispersion medium. 208 as a whole is reduced. Therefore, also in the present embodiment, it is possible to provide the light source device 200 that can change the center wavelength of the output pulsed light without causing a large fluctuation in the spectral line width.

[Third Embodiment]
An information acquisition apparatus according to the third embodiment will be described with reference to FIG. FIG. 3 is a block diagram illustrating the information acquisition apparatus according to the present embodiment. The information acquisition apparatus 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 pulse lights, the light reflected by the subject 315, the light transmitted through the subject 315, and the light emitted from the subject 315. At least one of the above is detected. And the information regarding the said subject 315 is acquired based on the detected light.

  SRS imaging is a technique for acquiring molecular vibration imaging using 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 pulse lights having different wavelengths, that is, the Stokes light is intensity-modulated, and the two pulse lights are irradiated to the subject in synchronization. Stimulated Raman scattering occurs when the difference frequency between the two pulse lights matches the molecular frequency of the molecules constituting the subject, and the intensity-modulated Stokes light is amplified. At this time, in accordance with the intensity modulation of the Stokes light, the intensity-modulated pulse light, that is, the pump light is also intensity-modulated, and the intensity modulation due to stimulated Raman scattering of the pump light emitted from the subject is detected. As a result, molecular vibration imaging of the subject can be performed. In addition, by changing the center wavelength of the pulsed light and changing the difference frequency between the two pulsed lights, it can be made to match the molecular frequency of various molecules, and is specific to the molecular group constituting the analyte. A signal can be obtained.

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

  Pump light from the bandpass filter 309 and Stokes light from the optical modulator 308 are incident on the optical multiplexer 310. The optical multiplexer 310 multiplexes a plurality of pulse lights having 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 Stokes light combined by the optical multiplexer 310 are installed 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 object 315 is irradiated.

  In a minute region located at the center of the focal point of the objective lens 314, stimulated Raman scattering based on the molecular vibration of the molecule of the subject 315 occurs, thereby causing a change in intensity between the pump light and the Stokes light. Stimulated Raman scattering does not occur at a location outside the minute region located at the center of the condensing point, so that no intensity change occurs between the pump light and the Stokes light at that location. Note that the larger the numerical aperture (NA) of the objective lens 314, the smaller the size of the spot of light irradiated on the subject 315, and the smaller the size of the micro area where stimulated Raman scattering occurs. .

  Pump light that is intensity-modulated by stimulated Raman scattering generated in a minute region located in 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. Is done. The SRS signal detected by the light receiving element 319 is acquired as an image signal by the information acquisition unit 320. That is, the information acquisition unit 320 acquires the light received by the light receiving element 319 as an electrical signal.

  In general, since the Raman scattering cross section σ of a molecule is small, the intensity change of pump light due to stimulated Raman scattering is also weak. For this reason, when detecting the SRS signal from the intensity change of the pump light, the SRS signal may be buried in a noise component or the like. In this 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 converted from the Stokes light from the light modulator 308. Detection is performed in synchronization with the modulation frequency. In this embodiment, molecular vibration imaging of the subject 315 is acquired. Since the synchronously detected signal is amplified, the SRS signal can be detected with high sensitivity.

  The synchronization detector 321 acquires a signal in synchronization with the modulation of light received by the light receiving element 319. As the synchronization detector 321, a lock-in amplifier, an FFT analyzer, or the like can be used. The FFT analyzer can detect an SRS signal at a higher speed than the lock-in amplifier. In addition, in FIG. 3, although the case where the synchronous detector 321 and the control means 322 are separate is shown as an example, 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. Are listed.

  When the X scan mirror 312 is driven, the condensing point scans the inside of the subject 315 in the X direction, and when the Y scan mirror 313 is driven, the condensing point is Y inside the subject 315 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 scan the focal point two-dimensionally on the subject 315. The X direction is, for example, the left-right direction in FIG. 3, and the Y direction is, for example, the depth direction in FIG.

  Further, each time two-dimensional scanning is completed, the subject 315 is moved by moving the stage 316 to move the focal point by a predetermined distance in the optical axis direction and repeating the same two-dimensional scanning as described above. It is also possible to obtain a three-dimensional image.

  In addition, after the completion of one two-dimensional scan or three-dimensional scan, the difference frequency between the pump light and the Stokes light is changed by changing the center wavelength of the first pulsed light emitted from the light source 301. May be. Thereby, the difference frequency between the pump light and the Stokes light may be matched with the molecular frequencies of various molecules included in the subject 315. As a result, a two-dimensional or three-dimensional molecular vibration image can be obtained.

  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 peak intensity of the pulsed light increases as the pulse width of the pulsed light output from the light source devices 100 and 200 becomes narrower, and the presence or absence of a nonlinear effect occurring in the subject 315 can be detected with high accuracy. The pulse rate of the pulsed light emitted from the light source 301 is preferably 1 MHz or more and 1 GHz or less. The pulse rates of the second pulse light and the third pulse 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 measurement speed is preferably 1 MHz or more because of the restriction of the measurement speed that is practically required for the information acquisition device 300, and is preferably 1 GHz or less because of the thermal destruction that occurs in the subject 315.

  In addition, since the information acquisition device 300 is suitably used for observation of living tissue, each pulsed light emitted from the light source devices 100 and 200 has a wavelength that is easy to transmit, with little reflection, absorption, and scattering by the living body. 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, a mode-locked Yb (ytterbium) doped fiber laser can be suitably used as the light source 301.

  Thus, according to this 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 apparatus 300 that can acquire information with uniform resolution.

  Moreover, according to this embodiment, since the light source devices 100 and 200 can be reduced in size and cost compared to the conventional SRS microscope device, the information acquisition device 300 as a whole can be reduced in size and cost. can do.

[Modified Embodiment]
As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.
For example, in the first embodiment, a case where a single mode fiber is used as the first optical waveguide 105 and a dispersion compensation fiber is used as the second optical waveguide 106 has been described as an example. However, 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.
In the second embodiment, a case 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 has been described as an example. However, 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.
Moreover, in 3rd Embodiment, although the case where the information acquisition apparatus 300 was an SRS microscope was demonstrated to the example, it is not limited to this. For example, the information acquisition device 300 may be an information acquisition device that acquires various types of spectral information, such as a CARS microscope, a fluorescence microscope, and an endoscope. The light source device 100 according to the first embodiment and the light source device 200 according to the second embodiment can be used as appropriate for such an information acquisition device.
In the above embodiment, the case where the number of the optical waveguides 105, 106, 205, 206 included in the dispersion media 108, 208 is two has been described as an example. However, 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 realizes 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 This process can be realized. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

DESCRIPTION OF SYMBOLS 101 ... Light source 103 ... Nonlinear optical medium 105 ... 1st optical waveguide 106 ... 2nd optical waveguide 107 ... Optical resonator 108 ... Dispersion medium

Claims (16)

A light source that emits first pulsed light;
A nonlinear optical medium 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 branching unit for branching the second pulsed light;
A dispersion medium that imparts a predetermined wavelength dispersion to the second pulsed light branched by the light branching unit, wherein the first and second of the wavelength differential values of dispersion are opposite to each other at the wavelength of the second pulsed light A dispersion medium having a dispersion value other than 0 at the wavelength of the second pulsed light,
An optical multiplexing unit that guides the second pulsed light given the chromatic dispersion by the dispersion medium and the first pulsed light emitted from the light source to the nonlinear optical medium;
A light source device comprising: 2. The light source device according to claim 1, wherein the nonlinear optical medium further generates third pulse light having a wavelength different from any of the first pulse light and the second pulse light. 3. The light source device according to claim 1, wherein a difference between a maximum value and a minimum value of the dispersion value of the dispersion medium in a wavelength variable range of the second pulse light is 2.5 ps / nm or less. . 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 1.5 ps / nm or less. Light source device. 5. The light source according to claim 1, wherein an absolute value of a wavelength differential value of the dispersion medium is 0.013 ps / nm ² or less at a wavelength of the second pulsed light. apparatus.   6. The light source device according to claim 1, wherein at least one of the first optical waveguide and the second optical waveguide includes a dispersion compensating fiber.   6. The light source device according to claim 1, wherein at least one of the first optical waveguide and the second optical waveguide includes a photonic crystal fiber.   The light source device according to claim 1, wherein a center wavelength of the first pulsed light is variable. The nonlinear optical medium and the dispersion medium are arranged in an optical resonator that oscillates the second pulsed light;
9. The pulse rate of the first pulse light is an integral multiple of a free spectral interval of the optical resonator at a center wavelength of the second pulse light. The light source device described.   The light source device according to claim 1, wherein the nonlinear optical medium includes a photonic crystal fiber.   The light source device according to any one of claims 1 to 10, wherein a spectral line width of the first pulsed light is 1 nm or less.   The light source device according to claim 1, wherein a pulse width of the first pulsed light is 10 ps or more. The light source device according to any one of claims 1 to 12,
When two pulse lights having different center wavelengths are emitted from the light source device and the subject is irradiated with the two pulse lights, 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 other of the two pulsed lights is the modulated first pulsed light. The information acquisition apparatus according to claim 13.   The information acquisition apparatus according to claim 13 or 14, wherein the pulse rates of the two pulse lights are both 1 MHz or more and 1 GHz or less. An information acquisition unit for acquiring the light received by the light receiving element as an electrical signal;
The information acquisition device according to claim 13, wherein the information acquisition unit includes a synchronization detector that acquires a signal in synchronization with modulation of light received by the light receiving element. .

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