Wavelength conversion optical system and pulse light generation device

Abstract

This wavelength conversion optical system includes: a wavelength conversion element in which different light incident positions have different wavelength conversion bands; and a wavelength-specific optical path setting unit for adjusting an optical path for each wavelength band so that light to be converted to the intended wavelength band hits the corresponding incident position in the wavelength conversion element.

Description

Wavelength conversion optical system and pulsed light generating device

[0001] The present disclosure relates to a wavelength conversion optical system and a pulsed light generating device.

[0002] A pulsed light generating device is known that includes an oscillator for generating pulsed light and a modulator for modulating the wavelength of the pulsed light generated by the oscillator using soliton self-frequency shift. In such a pulsed light generating device, the intensity of the pulsed light before modulation by the modulator is increased, thereby splitting the pulsed light into multiple pulsed lights with different wavelengths (outputting multicolored solitons) through the modulation (see, for example, Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2004-527001

[0004] Here, the above-described pulsed light generating device uses ultrashort pulsed light as pulsed light and rapidly changes the wavelength of each pulse of the ultrashort pulsed light. To perform wavelength conversion of such ultrashort pulsed light, a wideband and highly efficient wavelength conversion crystal is required. While using a thick crystal is considered to increase the efficiency of wavelength conversion, this requires a drive system to change the angle of incidence of light onto the crystal for phase matching, which aligns the phases of waves generated at each point in the crystal. However, it is difficult to drive the crystal for each pulse of ultrashort pulsed light (e.g., every several tens of MHz). In contrast, using a thin crystal facilitates phase matching, but increasing the intensity of the fundamental wave incident on the crystal is necessary to increase the efficiency of wavelength conversion. However, increasing the intensity of the fundamental wave introduces various nonlinear effects, which may ultimately reduce the efficiency of wavelength conversion. Thus, in the past, it was difficult to perform wavelength conversion efficiently in a configuration that performs wavelength conversion over a wide band in a short time, such as when performing wavelength conversion of ultrashort pulsed light in which the wavelength is changed pulse by pulse.

[0005] An object of the present disclosure is to provide a wavelength conversion optical system and a pulsed light generating device that perform wavelength conversion with high efficiency in a configuration that performs wavelength conversion over a wide band in a short time.

[0006] (1) A wavelength conversion optical system according to one aspect of the present disclosure includes a wavelength conversion unit that converts light into different wavelength bands depending on the incident position of the light, and a wavelength-specific optical path setting unit that changes the optical path for each wavelength band so that light in the wavelength band to be converted is incident on each incident position of the wavelength conversion unit.

[0007] In a wavelength conversion optical system according to one aspect of the present disclosure, a wavelength-specific optical path setting unit changes an optical path for each wavelength band so that light in a corresponding wavelength band (light in a wavelength band to be converted) is incident on each incident position of a wavelength conversion unit, each of which has a different target wavelength band for conversion. With this configuration, the wavelength-specific optical path setting unit sets an optical path so that each wavelength band is appropriately wavelength-converted at an appropriate incident position of the wavelength conversion element. Therefore, wavelength conversion can be performed appropriately for various (broadband) wavelength bands without using a drive system for moving the wavelength conversion unit. Because wavelength conversion can be performed for a broadband wavelength band without using a drive system, wavelength conversion can be performed appropriately even when the required phase matching width is wide and the movement of the wavelength conversion unit by the drive system cannot keep up with the wavelength conversion, for example, when wavelength conversion of light with a wide wavelength range is performed in an extremely short time, such as wavelength conversion of ultrashort pulse light of ultrabroadband light. In addition, since the wavelength-specific optical path setting unit can set the light to be incident at an appropriate incident position on the wavelength conversion unit for each wavelength band by the wavelength-specific optical path setting unit, the restrictions on the wavelength conversion unit to be used are reduced, and the configuration of the wavelength conversion unit can be appropriately set to perform highly efficient wavelength conversion, thereby enabling highly efficient wavelength conversion. As described above, the wavelength conversion optical system according to one aspect of the present disclosure can perform highly efficient wavelength conversion in a configuration that performs wavelength conversion over a wide band in a short time.

[0008] (2) In the wavelength conversion optical system described in (1) above, the wavelength-by-wavelength optical path setting unit may have a dispersing unit that changes the optical path for each wavelength band by dispersing light for each wavelength band, and a focusing unit that focuses the light from the dispersing unit at an incident position on the wavelength conversion unit. With this configuration, the optical path can be appropriately changed for each wavelength band by the dispersion for each wavelength band by the dispersing unit, and the light can be appropriately focused at each incident position by the focusing unit, thereby making it possible to appropriately perform wavelength conversion over a wide band.

[0009] (3) In the wavelength conversion optical system described in (2) above, the distance from the dispersing section to the focusing section and the distance from the focusing section to the wavelength conversion section may both be set to match the focal length of the focusing section. With this configuration, it is possible to perform wavelength conversion over a wide band with higher accuracy using the dispersing section and the focusing section.

[0010] (4) In the wavelength conversion optical system according to any one of (1) to (3), the wavelength conversion unit may include a wavelength conversion element, and the wavelength conversion element may have a fan-shaped structure so that quasi-phase matching of different wavelength bands is achieved at each incident position. In this way, the wavelength conversion unit includes a wavelength conversion element, and the shape of the wavelength conversion element is fan-shaped so that the wavelength bands for which quasi-phase matching is achieved at each incident position are different from each other, thereby making it possible to appropriately perform wavelength conversion for various (broadband) wavelength bands.

[0011] (5) In the wavelength conversion optical system according to any one of (1) to (3), the wavelength conversion element may include a PPLN (periodically poled lithium niobate) having a fan-shaped structure. This structure allows appropriate wavelength conversion for various (broad) wavelength bands.

[0012] (6) A wavelength conversion optical system according to one aspect of the present disclosure includes a first conversion unit arranged at an angle suitable for wavelength conversion of a first wavelength band, and a second conversion unit connected in series to the first conversion unit and arranged at an angle suitable for wavelength conversion of a second wavelength band different from the first wavelength band.

[0013] In a wavelength conversion optical system according to an aspect of the present disclosure, a plurality of conversion units are connected in series, and a first conversion unit performs wavelength conversion of a first wavelength band, and a second conversion unit performs wavelength conversion of a second wavelength band. With this configuration, wavelength conversion of the wavelength bands corresponding to each conversion unit can be performed simply by connecting the conversion units in series. With this configuration, wavelength conversion of a plurality of wavelength bands can be performed appropriately without using a drive system for moving the conversion units. Because wavelength conversion can be performed without using a drive system, wavelength conversion can be performed appropriately even when the required phase matching width is wide and the movement of the conversion units by the drive system is insufficient for wavelength conversion, such as when wavelength conversion of light with a wide wavelength range is performed in an extremely short time, such as wavelength conversion of ultrashort pulsed light of ultrabroadband light. Furthermore, with this configuration, fewer restrictions are placed on the conversion units used, and the configuration of the conversion units can be appropriately set for highly efficient wavelength conversion, thereby enabling highly efficient wavelength conversion. As described above, the wavelength conversion optical system according to an aspect of the present disclosure can perform highly efficient wavelength conversion in a configuration that performs wavelength conversion over a wide band in a short time.

[0014] (7) A pulsed light generating device according to an aspect of the present disclosure includes an oscillator that oscillates ultrashort pulsed light, a wavelength converter that converts the ultrashort pulsed light into different wavelength bands depending on the incident position of the ultrashort pulsed light, and a wavelength-specific optical path setting unit that changes an optical path for each wavelength band so that ultrashort pulsed light of the wavelength band to be converted is incident on each incident position of the wavelength converter. Such a pulsed light generating device can perform wavelength conversion over a wide band in a short time using ultrashort pulsed light, with high efficiency.

[0015] (8) The pulsed light generating device described in (7) above may be a laser device that performs wavelength conversion for each pulse by utilizing soliton self-frequency shift. With this configuration, by utilizing soliton self-frequency shift to perform wavelength conversion for each pulse, it is possible to obtain a laser device that performs wavelength conversion over a wide band in a short time and performs wavelength conversion with high efficiency.

[0016] (9) The pulsed light generating device described in (7) above may be a femtosecond laser device for a two-photon microscope. With this configuration, in the field of two-photon microscopes, wavelength conversion can be performed over a wide band in a short time with high efficiency.

[0017] According to the present disclosure, it is possible to provide a wavelength conversion optical system and a pulsed light generating device that perform wavelength conversion with high efficiency in a configuration that performs wavelength conversion over a wide band in a short time.

[0018] FIG. 1 is a block diagram showing an optical pulse generating apparatus according to an embodiment. FIG. 2(a) is a graph showing the time waveform of an ultrashort optical pulse output from the oscillator of FIG. 1. FIG. 2(b) is a graph showing the spectrum of an ultrashort optical pulse output from the oscillator of FIG. 1. FIG. 2(c) is a graph showing the time waveform of an ultrashort optical pulse output from the fiber amplifier of FIG. 1. FIG. 2(d) is a graph showing the spectrum of an ultrashort optical pulse output from the fiber amplifier of FIG. 1. FIG. 3 is a graph showing a specific example of the spectrum of an ultrashort optical pulse output from the fiber amplifier of FIG. 1. FIG. 4(a) is a graph showing the time waveform of an ultrashort optical pulse output from the acousto-optic modulator of FIG. 1. FIG. 4(b) is a graph showing the spectrum of an ultrashort optical pulse output from the acousto-optic modulator of FIG. 1. FIG. 5(a) is a graph showing the time waveform of an ultrashort optical pulse output from the soliton-shifted fiber of FIG. 1. FIG. 5(b) is a graph showing the spectrum of an ultrashort optical pulse output from the soliton-shifted fiber of FIG. 1. Fig. 5(c) is a graph showing the time waveform of ultrashort pulsed light output from the filter of Fig. 1. Fig. 5(d) is a graph showing the spectrum of ultrashort pulsed light output from the filter of Fig. 1. Fig. 6 is a diagram explaining the generation of second harmonic waves. Fig. 7 is a graph showing the wavelength conversion efficiency in each wavelength band for each crystal length. Fig. 8 is a diagram explaining angular phase matching. Fig. 9 is a diagram explaining an example configuration of a wavelength conversion optical system. Fig. 10 is a diagram explaining an example configuration of a wavelength conversion optical system according to a modified example.

[0019] Hereinafter, the embodiments will be described in detail with reference to the drawings. In each drawing, the same or corresponding parts are designated by the same reference numerals, and duplicated explanations will be omitted.

[0020] 1 , a pulsed light generating apparatus 1 of this embodiment generates long-wavelength ultrashort pulsed light (pulsed light) L by utilizing soliton self-frequency shift (Raman soliton shift). For example, the pulsed light generating apparatus 1 is a femtosecond laser apparatus for a two-photon microscope. The pulsed light generating apparatus 1 includes an oscillator 2, a fiber amplifier 3, an acousto-optic modulator 4, a compressor 5, a soliton-shifted fiber 6, a stretcher fiber 7, a fiber amplifier 8, a compressor 9, and a wavelength conversion optical system 10.

[0021] The oscillator 2 constitutes an oscillation unit that oscillates ultrashort pulsed light L. As shown in FIG. 2( a), the oscillator 2 generates an ultrashort pulse train with a predetermined period F1. The wavelength band of the ultrashort pulsed light L oscillated from the oscillator 2 may be a band including 1550 nm, for example. As shown in FIG. 2( b), the oscillator 2 here oscillates ultrashort pulsed light L having a spectrum with a first spectral width H1 and a first intensity K1. The oscillator 2 is not particularly limited, and various oscillators can be used.

[0022] The fiber amplifier 3 constitutes an amplifying section that broadens the spectrum of the ultrashort pulsed light L oscillated by the oscillator 2. The fiber amplifier 3 broadens the spectrum of the ultrashort pulsed light L by similariton amplification and also increases the output power of the ultrashort pulsed light L. The fiber amplifier 3 is disposed between the oscillator 2 and the soliton-shifted fiber 6 in the optical path of the ultrashort pulsed light L.

[0023] The fiber amplifier 3 includes a fiber amplifier. The fiber amplifier of the fiber amplifier 3 is a normal dispersion fiber, which is a double-clad fiber co-doped with erbium and ytterbium. That is, the fiber amplifier 3 performs amplification while causing a nonlinear effect by the normal dispersion double-clad fiber so as not to stretch, and obtains ultrashort pulsed light L as broadband amplified light. The normal dispersion fiber is a fiber in which the dispersion parameter D (ps / nm / km) is negative. There are no particular restrictions on the dopant used in the fiber amplifier 3, and various dopant types may be used.

[0024] 2(c) and 2(d), the fiber amplifier 3 broadens the spectral width of the ultrashort pulsed light L to a second spectral width H2 that is wider than the first spectral width H1. The fiber amplifier 3 increases the intensity of the ultrashort pulsed light L to a second intensity K2 that is higher than the first intensity K1. Specifically, as shown in Fig. 3, the fiber amplifier 3 sets the spectral width of the ultrashort pulsed light L to 100 nm or more. In Fig. 3, the horizontal axis represents the wavelength of the ultrashort pulsed light L, and the vertical axis represents the intensity of the ultrashort pulsed light L relative to a predetermined intensity.

[0025] The acousto-optic modulator 4 constitutes a light intensity control unit that controls the intensity of the ultrashort pulsed light L for each pulse. The acousto-optic modulator 4 is a device that modulates the ultrashort pulsed light L by utilizing the force of acoustics (sound waves) and is called an AOM (Acousto Optic Modulator). In this embodiment, the acousto-optic modulator 4 is disposed between the fiber amplifier 3 and the soliton-shifted fiber 6 in the optical path of the ultrashort pulsed light L. Note that the acousto-optic modulator 4 may be disposed at any position between the oscillator 2 and the soliton-shifted fiber 6. As shown in FIGS. 4( a) and 4(b), the acousto-optic modulator 4 controls the intensity of the ultrashort pulsed light L so that it changes for each pulse. For example, when intensity modulations M1 and M2 are applied as shown in FIG. 4(a), ultrashort pulsed light LM1 and LM2 corresponding to the intensities given by M1 and M2 are generated as shown in FIG. 4(b). The range and precision of intensity modulation of the ultrashort pulsed light L (LM1, LM2) depend on the performance of the acousto-optic modulator 4. The intensity of each pulse light in the pulse train of the ultrashort pulsed light L can be modulated arbitrarily by the acousto-optic modulator 4.

[0026] The compressor 5 constitutes a pulse compression section that compresses the time width of the pulses of the ultrashort pulsed light L. In this embodiment, the compressor 5 is disposed between the acousto-optic modulator 4 and the soliton-shifted fiber 6 in the optical path of the ultrashort pulsed light L. The compressor 5 may be disposed at any position between the fiber amplifier 3 and the soliton-shifted fiber 6. Even if the ultrashort pulsed light L is stretched (for example, by several picoseconds) by the fiber amplifier 3, the compressor 5 compresses the time width of the ultrashort pulsed light L and outputs ultrashort pulsed light L with a time width spread of a certain amount or less (less than 1 picosecond). The compressor 5 is not particularly limited, and various compressors can be used.

[0027] The soliton-shifted fiber 6 constitutes a modulation section that utilizes soliton self-frequency shift to modulate the wavelength of the ultrashort pulsed light L, the output of which has been increased while broadening its spectrum in the fiber amplifier 3. The soliton-shifted fiber 6 is disposed downstream of the fiber amplifier 3 in the optical path of the ultrashort pulsed light L. As shown in FIGS. 5( a) and 5(b), the soliton-shifted fiber 6 lengthens the wavelength of the ultrashort pulsed light L to generate soliton S1. The wavelength band of the lengthened ultrashort pulsed light L may be, for example, a band including 1800 nm to 2200 nm. The soliton-shifted fiber 6 may be, for example, a single-mode anomalous dispersion fiber that exhibits anomalous dispersion in the wavelength band of the ultrashort pulsed light L generated in the fiber amplifier 3. In addition, by controlling the acousto-optic modulator 4, solitons with wavelengths different from the soliton S1 can be generated. For example, when the wavelength of the soliton S is modulated by intensities M1 and M2 as shown in FIG. 5C, the soliton S shifts to a wavelength corresponding to the intensities M1 and M2 (solitons S1 and S2) as shown in FIG. 5D. The range and precision of the wavelength shift of the soliton S depend on the performance of the acousto-optic modulator 4. The shift wavelength of each soliton S in the soliton train generated from the pulse train of the ultrashort optical pulse L can be arbitrarily changed by applying intensity modulation to the pulse train using the acousto-optic modulator 4. In the illustrated example, the ultrashort optical pulse L modulated by the soliton self-frequency shift contains a non-soliton component S0 (a component that does not become soliton S1 or S2). A filter (not shown) that cuts the non-soliton component S0 of the ultrashort optical pulse L may be provided downstream of the soliton shift fiber 6. Such a filter may have an OD value of 3 or greater.

[0028] The stretcher fiber 7 is a stretcher that expands the time width of the ultrashort pulsed light L. The wavelength band of the ultrashort pulsed light L whose time width is expanded by the stretcher fiber 7 is, for example, 1800 nm to 2200 nm. The stretcher fiber 7 is configured, for example, by combining a first fiber (not shown) that expands the time width of the ultrashort pulsed light L with a first characteristic and a second fiber (not shown) that expands the time width of the ultrashort pulsed light L with a second characteristic different from that of the first fiber. The first fiber and the second fiber may be configured to expand the time width of the ultrashort pulsed light L by generating a difference in the optical path length of each wavelength due to a difference in the refractive index of each wavelength when the ultrashort pulsed light L passes through. The first fiber expands the time width of the ultrashort pulsed light L including, for example, a wavelength band of 1800 nm to 2200 nm, output from the soliton shift fiber 6, with the first characteristic, and outputs the expanded time width to the second fiber. The first fiber may be, for example, a normal dispersion fiber. The second fiber is connected to the first fiber, and widens the time width of the ultrashort pulsed light L input from the first fiber using the second characteristic, and outputs the widened time width to the fiber amplifier 8 .

[0029] The fiber amplifier 8 amplifies (increases the output power of) the pulsed light whose time width has been expanded by the stretcher fiber 7. The fiber amplifier 8 includes a fiber amplifier. The fiber amplifier of the fiber amplifier 8 is a normal dispersion fiber, for example, a thulium-doped fiber. The laser medium doped in the fiber of the fiber amplifier 8 is not particularly limited and may be a rare earth element such as ytterbium, erbium, or neodymium, or may be Bi or the like. The wavelength band of the ultrashort pulsed light L amplified by the fiber amplifier 8 is, for example, 1800 nm to 2200 nm. The fiber amplifier 8 capable of reliably amplifying the pulsed light in a wide wavelength band includes, for example, a first fiber amplifier (not shown) having a high gain G1 (not shown) on the first wavelength side, which is the short wavelength side of the ultrashort pulsed light L, and a second fiber amplifier (not shown) having a high gain G2 (not shown) on the second wavelength side, which is the long wavelength side, and is configured by combining a filter between these amplifiers to attenuate amplified light containing noise generated by ASE and soliton self-frequency shift. As an example, the short wavelength (first wavelength) is a wavelength that is equal to or greater than 1800 nm and less than 2000 nm, and the long wavelength (second wavelength) is a wavelength that is equal to or greater than 2000 nm and less than 2200 nm. For example, light on the second wavelength side of the ultrashort pulsed light L that includes a wavelength band of 1800 nm to 2200 nm generates amplified light due to ASE and noise caused by soliton self-frequency shift during amplification in the first fiber amplifier described above, which not only prevents sufficient amplification in the second fiber amplifier described above but also causes parasitic oscillation. However, these effects can be suppressed by a filter provided between the fiber amplifiers, so the light is reliably amplified and output to the compressor 9.

[0030] The compressor 9 compresses the time width of the ultrashort pulsed light L amplified by the fiber amplifier 8. The compressor 9 may have a diffraction grating pair (not shown). The diffraction grating pair may be configured to compress the time width by shifting the phase of each wavelength by causing a difference in the optical path length of each wavelength. The wavelength band of the ultrashort pulsed light L1 whose time width is compressed by the diffraction grating pair is, for example, 1800 nm to 2200 nm.

[0031] The wavelength conversion optical system 10 is configured to perform wavelength conversion on a pulse-by-pulse basis for the ultrashort pulse light L output from the compressor 9. The wavelength conversion optical system 10 may perform wavelength conversion on a pulse-by-pulse basis to a wavelength band of 900 nm to 1100 nm, for example. The wavelength conversion optical system 10 may convert light of 1800 nm to 2200 nm, for example, to 900 to 1100 nm, which is usable in two-photon microscopes. The wavelength conversion optical system 10 may perform the wavelength conversion by second harmonic generation, a phenomenon included in nonlinear effects. Second harmonic generation is a phenomenon in which the original wavelength (fundamental wave) is converted into light (double wave) with half the wavelength.

[0032] The conversion of a fundamental wave to a second harmonic wave in second harmonic generation will now be described. FIG. 6 is a diagram illustrating the generation of a second harmonic wave. As shown in FIG. 6, when high-intensity ultrashort pulsed light, which is the fundamental wave, enters a crystal (nonlinear optical medium) that is a wavelength conversion element, polarization oscillation generates a second harmonic wave, which is a harmonic component of electron oscillation. In detail, the harmonic wave (second harmonic wave) is generated at each point in the crystal through which the ultrashort pulsed light passes. The final second harmonic wave is the sum of these second harmonic waves generated at each point.

[0033] While the example shown in Figure 6 shows an example in which the phases of the doubled waves at each point are aligned, this does not necessarily mean that they are aligned. Because reactions occur sequentially, such as the crystal responding to the fundamental wave and then generating a doubled wave, differences in the crystal's response are reflected in the phase of the doubled wave, resulting in misalignment of the phases of the doubled waves at each point. If the doubled waves at each point are added together when their phases are not aligned, the doubled waves will cancel each other out or overlap, resulting in an insufficient magnitude of the added doubled wave. For this reason, it is important to determine the crystal conditions so that the phases of the doubled waves generated at each point are aligned as closely as possible (i.e., phase matching). A phase-matched state is one in which the refractive index of the fundamental wave and the refractive index perceived by the harmonic wave are equal.

[0034] FIG. 7 is a graph showing the wavelength conversion efficiency in each wavelength band for a nonlinear optical crystal, such as single-crystal BBO (Beta-BaB2O4), for each crystal length. In FIG. 7, the horizontal axis represents the wavelength band, and the vertical axis represents the normalized wavelength conversion efficiency in second-harmonic generation (SHG). The solid line in FIG. 7 represents the wavelength conversion efficiency in each wavelength band for a relatively thick crystal with a crystal length of 1 mm, while the dashed line represents the wavelength conversion efficiency in each wavelength band for a relatively thin crystal with a crystal length of 0.2 mm. Note that the maximum wavelength conversion efficiency is higher for thicker crystals, but because the wavelength conversion efficiency in FIG. 7 is normalized, the upper limit of the wavelength conversion efficiency is 1 for all crystals. As shown in FIG. 7, the longer (thicker) the crystal, the more stringent the conditions for superimposing the doubled waves at each point, resulting in a narrower wavelength band in which wavelength conversion can be performed with high efficiency. Therefore, from the perspective of widening the wavelength band in which phase matching is achieved, it is considered to use a relatively thin crystal.

[0035] However, because the intensity of the second harmonic wave is proportional to the square of the crystal length, it is difficult to increase the wavelength conversion efficiency with relatively thin crystals. Because the intensity of the second harmonic wave also depends on the intensity of the fundamental wave, it is possible to ensure the intensity of the second harmonic wave by increasing the intensity of the fundamental wave. However, amplifying the fundamental wave to compensate for the effect of the square of the crystal length described above is not cost-effective. It is also possible to spatially increase the intensity of the fundamental wave by focusing the light onto a very small area of ​​the crystal. However, focusing the fundamental wave to a small area also generates nonlinear effects from third order onward, which inhibit second-harmonic generation and ultimately reduce wavelength conversion efficiency. Furthermore, high intensity may damage the crystal itself. For these reasons, it is difficult to sufficiently increase the wavelength conversion efficiency with the relatively thin crystals described above.

[0036] Therefore, in order to achieve broadband phase matching, which is a problem with thick crystals, while using a relatively thick crystal, a configuration can be considered in which a drive system is provided to change the angle of incidence of light on the crystal. Figure 8 is a diagram explaining angular phase matching. In the configuration shown in Figure 8, a birefringent crystal 500 is prepared, and a drive system (not shown) adjusts the angle of incidence of light on the crystal 500 to adjust the refractive index, thereby achieving broadband phase matching.

[0037] However, in a configuration having a driving system such as that shown in FIG. 8, it is impossible to drive the driving system for each pulse of ultrashort pulsed light (for example, every several tens of MHz), and it is difficult to use this configuration to perform wavelength conversion of ultrashort pulsed light.

[0038] As described above, in the past, it was difficult to perform wavelength conversion with high efficiency in a configuration that performs wavelength conversion over a wide band in a short time, such as when performing wavelength conversion of ultrashort pulsed light in which the wavelength is changed for each pulse.

[0039] To address these issues, the wavelength conversion optical system 10 according to this embodiment performs highly efficient wavelength conversion using a relatively thick crystal, and also performs broadband wavelength conversion of ultrashort pulsed light by dispersing light for each wavelength band and changing the optical path for each wavelength band without using a drive system. Below, a detailed configuration example of the wavelength conversion optical system 10 will be described with reference to FIG.

[0040] Fig. 9 is a diagram illustrating an example of the configuration of the wavelength conversion optical system 10. As shown in Fig. 9, the wavelength conversion optical system 10 includes a diffraction grating (dispersion unit) 101, a lens (light-collecting unit) 102, a wavelength conversion element (wavelength conversion unit) 103, a lens 104, and a diffraction grating 105. The diffraction grating 101 and the lens 102 constitute a wavelength-specific optical path setting unit that changes the optical path for each wavelength band so that light in a wavelength band to be converted at each incident position (details will be described later) is incident on each incident position of the wavelength conversion element 103.

[0041] The diffraction grating 101 disperses the ultrashort pulsed light L output from the compressor 9 into each wavelength band (spectroscopes the ultrashort pulsed light L), thereby changing the optical path for each wavelength band. Specifically, the diffraction grating 101 disperses the ultrashort pulsed light L into each wavelength band so that light of a wavelength band to be converted at each incident position is incident on each incident position of the wavelength conversion element 103.

[0042] The lens 102 is a lens that focuses the light from the diffraction grating 101 onto the incident position of the wavelength conversion element 103. The distance from the diffraction grating 101 to the lens 102 and the distance from the lens 102 to the wavelength conversion element 103 are both set to match the focal length f of the lens 102, and may be set to, for example, the focal length f of the lens 102.

[0043] The wavelength conversion element 103 is a crystal that converts different wavelength bands depending on the incident position of light. The wavelength conversion element 103 has a fan-shaped structure so that quasi-phase matching of different wavelength bands can be achieved at each incident position. More specifically, the wavelength conversion element 103 may be a PPLN (Periodically Poled Lithium Niobate) with a fan-shaped structure. That is, the wavelength conversion element 103 may be a so-called fan-out PPLN. The crystal length (thickness) of the wavelength conversion element 103 may be, for example, about 1 mm to 20 mm.

[0044] Quasi-phase matching is a technique for achieving pseudo-phase matching by preventing the generation of the doubled wave to be canceled out of the doubled waves that are added together, or by inverting the phase of the doubled wave to be canceled. Here, we will explain an example of quasi-phase matching in which the phase of the doubled wave to be canceled out is inverted (the polarity of the crystal is inverted).

[0045] The wavelength conversion element 103 has a fan-shaped structure, which makes it possible to achieve quasi-phase matching of any wavelength band at each incident position of the crystal. That is, the period of the part where the double waves cancel each other varies depending on the wavelength band, and by using a crystal whose period is inverted according to the fan-shaped structure, the wavelength bands at which quasi-phase matching is achieved at each incident position of the crystal can be made different from each other. In such a wavelength conversion element 103, it is possible to specify in advance which wavelength band quasi-phase matching is achieved at each incident position, and by setting the above-mentioned diffraction grating 101 and lens 102 so that light of the wavelength band to be converted is incident at each incident position, it is possible to appropriately perform wavelength conversion for each of the light of various wavelength bands.

[0046] The lens 104 is a lens that focuses the light wavelength-converted by the wavelength conversion element 103 onto the diffraction grating 101. The distance from the wavelength conversion element 103 to the lens 104 and the distance from the lens 104 to the diffraction grating 105 are both set to match the focal length f of the lenses 102 and 104, for example, the focal length f of the lenses 102 and 104.

[0047] The diffraction grating 105 outputs the light that has been dispersed for each wavelength band by the diffraction grating 101 and then passed through the lens 102, the wavelength conversion element 103, and the lens 104 as wavelength-converted ultrashort pulsed light L. The number of grooves in the diffraction grating 105 may be 2N, which is twice the number N of grooves in the diffraction grating 101, in order to restore the dispersion of the light after wavelength conversion.

[0048] Next, the effects of the wavelength conversion optical system 10 and the pulsed light generating device 1 according to this embodiment will be described.

[0049] The wavelength conversion optical system 10 according to this embodiment includes a wavelength conversion element 103 that converts different wavelength bands depending on the incident position of light, and a wavelength-specific optical path setting unit that changes the optical path for each wavelength band so that light of the wavelength band to be converted is incident on each incident position of the wavelength conversion element 103.

[0050] In the wavelength conversion optical system 10 according to the present embodiment, the wavelength-specific optical path setting unit changes the optical path for each wavelength band so that light in the corresponding wavelength band (light in the wavelength band to be converted) is incident on each incident position of the wavelength conversion element 103, which has a different wavelength band to be converted at each incident position. With this configuration, the wavelength-specific optical path setting unit sets the optical path so that each wavelength band is appropriately wavelength-converted at an appropriate incident position on the wavelength conversion element 103, thereby enabling appropriate wavelength conversion of various (broadband) wavelength bands without using a drive system for moving the wavelength conversion element 103. Because wavelength conversion can be performed on a broadband wavelength band without using a drive system, appropriate wavelength conversion can be performed even when the required phase matching width is wide and the movement of the wavelength conversion unit 103 by the drive system cannot keep up with the wavelength conversion, for example, when wavelength conversion of light with a wide wavelength width is performed in an extremely short time, such as wavelength conversion of ultrashort pulse light of ultrabroadband light. Furthermore, the configuration in which the wavelength-by-wavelength optical path setting unit causes light to be incident on an appropriate incident position of the wavelength conversion element 103 for each wavelength band reduces the restrictions placed on the wavelength conversion element 103 to be used, and the configuration (e.g., shape, etc.) of the wavelength conversion element 103 for performing highly efficient wavelength conversion, such as the thickness of the wavelength conversion element 103, can be appropriately set, thereby enabling highly efficient wavelength conversion. As described above, the wavelength conversion optical system 10 according to this embodiment can perform highly efficient wavelength conversion in a configuration in which wavelength conversion is performed over a wide band in a short time.

[0051] The wavelength-by-wavelength optical path setting unit may include a diffraction grating 101 that changes the optical path for each wavelength band by dispersing light for each wavelength band, and a lens 102 that focuses the light from the diffraction grating 101 at an incident position of the wavelength conversion element 103. With this configuration, the optical path can be appropriately changed for each wavelength band by the dispersion for each wavelength band by the diffraction grating 101, and the light can be appropriately focused at each incident position by the lens 102, thereby making it possible to appropriately perform wavelength conversion over a wide band.

[0052] The distance from the diffraction grating 101 to the lens 102 and the distance from the lens 102 to the wavelength conversion element 103 may both be set in accordance with the focal length of the lens 102. With this configuration, the diffraction grating 101 and the lens 102 can be used to perform wavelength conversion over a wide band with higher precision.

[0053] The wavelength conversion element 103 may be formed into a fan-shaped structure so that the quasi-phase matching of different wavelength bands can be achieved at each incident position. In this way, by making the shape of the wavelength conversion element 103 fan-shaped and making the wavelength bands that achieve quasi-phase matching at each incident position different from each other, it is possible to appropriately perform wavelength conversion for various (wideband) wavelength bands.

[0054] The wavelength conversion element 103 may be a PPLN (Periodically Poled Lithium Niobate) with a sector structure, which can appropriately perform wavelength conversion for various (broad) wavelength bands.

[0055] The pulsed light generating device 1 according to this embodiment includes an oscillator 2 that oscillates ultrashort pulsed light L, a wavelength conversion element 103 that converts the ultrashort pulsed light L to different wavelength bands depending on the incident position of the ultrashort pulsed light L, and a wavelength-specific optical path setting unit that changes the optical path for each wavelength band so that the ultrashort pulsed light L of the wavelength band to be converted is incident on each incident position of the wavelength conversion element 103. The pulsed light generating device 1 described above can perform wavelength conversion with high efficiency in a configuration that performs wavelength conversion over a wide band in a short time using ultrashort pulsed light.

[0056] The pulsed light generating device 1 may be a laser device that performs wavelength conversion for each pulse by utilizing soliton self-frequency shift. With this configuration, by utilizing soliton self-frequency shift to perform wavelength conversion for each pulse, it is possible to obtain a laser device that performs wavelength conversion over a wide band in a short time and with high efficiency.

[0057] The pulsed light generating device 1 may be a femtosecond laser device for a two-photon microscope. With such a configuration, in the field of two-photon microscopes, wavelength conversion can be performed over a wide band in a short time with high efficiency.

[0058] In this embodiment, as described above, the pulsed light generating device 1 is used in a two-photon microscope. In this case, multicolor imaging is possible in which fluorescence observation is performed while switching the wavelength for each pulse.

[0059] As described above, one aspect of the present disclosure is not limited to the above embodiment.

[0060] For example, the configuration of the wavelength conversion optical system is not limited to the above embodiment, and may be, for example, the configuration shown in FIG.

[0061] Fig. 10 is a diagram illustrating an example of the configuration of a wavelength conversion optical system according to a modified example. As shown in Fig. 10, the wavelength conversion optical system according to the modified example includes a lens 201, a BBO (Beta-BaB2O4) 202 (first conversion unit), a lens 203, a lens 204, a BBO 205 (second conversion unit), and a lens 206. The lens 201 is a lens that focuses light onto the BBO 202. The lenses 203 and 204 are lenses that focus light from the BBO 202 onto the BBO 205. The lens 206 is a lens that focuses and emits light from the BBO 205.

[0062] The BBO crystals 202 and 205 are nonlinear optical crystals that perform wavelength conversion. The BBO crystal 202 is arranged at an angle (phase matching angle) suitable for wavelength conversion of a first wavelength band. The BBO crystal 205 is connected in series to the BBO crystal 202 and arranged at an angle (phase matching angle) suitable for wavelength conversion of a second wavelength band different from the first wavelength band.

[0063] This configuration allows wavelength conversion of the wavelength bands corresponding to the BBOs 202 and 205, respectively, using a simple configuration in which the BBOs 202 and 205 are connected in series. This configuration allows appropriate wavelength conversion of multiple wavelength bands without using a drive system to move the BBOs 202 and 205. Because wavelength conversion can be performed without using a drive system, it is possible to appropriately perform wavelength conversion even when the required phase matching width is wide and the movement of the conversion unit by the drive system is insufficient for wavelength conversion, such as when wavelength conversion of light with a wide wavelength range is performed in an extremely short time, such as wavelength conversion of ultrashort pulse light of ultrabroadband light. Furthermore, this configuration reduces restrictions on the conversion unit used, allowing the configuration (shape, etc.) of the BBOs 202 and 205, such as the thickness of the BBOs 202 and 205, to be appropriately set for highly efficient wavelength conversion, thereby enabling highly efficient wavelength conversion. As described above, the wavelength conversion optical system according to this modified example allows highly efficient wavelength conversion in a configuration that performs wavelength conversion over a wide band in a short time.

[0064] It is possible to connect the BBOs 202 and 205 in parallel rather than in series, but in this case, some wavelength bands will become unusable when the wavelength bands are divided. That is, the light in the wavelength band between the first wavelength band and the second wavelength band will become unusable. For this reason, from the perspective of using a wide wavelength band, it is preferable that the BBOs 202 and 205 are connected in series. In the above example, two types of BBOs were used, but the number of BBOs may be increased depending on the number of wavelength bands to be converted.

[0065] 1...pulse light generating device, 2...oscillator (oscillating section), 10...wavelength conversion optical system, 101...diffraction grating, 102...lens, 103...wavelength conversion element, 202...BBO (first conversion element section), 205...BBO (second conversion element section).

Claims

1. A wavelength conversion optical system comprising: a wavelength conversion unit that converts different wavelength bands depending on the incident position of light; and a wavelength-specific optical path setting unit that changes the optical path for each wavelength band so that light of the wavelength band to be converted is incident on each incident position of the wavelength conversion unit.

2. The wavelength conversion optical system according to claim 1, wherein the wavelength-by-wavelength optical path setting section has: a dispersion section that changes the optical path for each wavelength band by dispersing light for each wavelength band; and a focusing section that focuses light from the dispersion section onto an incident position of the wavelength conversion section.

3. The wavelength conversion optical system according to claim 2, wherein the distance from the dispersing section to the focusing section and the distance from the focusing section to the wavelength conversion section are both set in accordance with the focal length of the focusing section.

4. The wavelength conversion optical system according to claim 1, wherein the wavelength conversion section comprises a wavelength conversion element, and the wavelength conversion element has a sector structure so as to achieve quasi-phase matching of wavelength bands different from each other at each incident position.

5. The wavelength conversion optical system according to claim 1, wherein the wavelength conversion section includes a PPLN (periodically poled lithium niobate) having a fan-shaped structure.

6. A wavelength conversion optical system comprising: a first conversion unit arranged at an angle suitable for wavelength conversion of a first wavelength band; and a second conversion unit connected in series to the first conversion unit and arranged at an angle suitable for wavelength conversion of a second wavelength band different from the first wavelength band.

7. A pulsed light generating device comprising: an oscillator for oscillating an ultrashort pulsed light; a wavelength converter for converting the wavelength band to different wavelengths depending on the incident position of the ultrashort pulsed light; and a wavelength-specific optical path setting unit for changing the optical path for each wavelength band so that ultrashort pulsed light of the wavelength band to be converted is incident on each incident position of the wavelength converter.

8. The pulsed light generating device according to claim 7, which is a laser device that converts the wavelength of each pulse by utilizing soliton self-frequency shift.

9. The pulsed light generating device according to claim 7, which is a femtosecond laser device for a two-photon microscope.

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