Pulsed light generation apparatus and pulsed light generation method

Abstract

By using soliton self-frequency shift modulation, two pulsed light beams are reliably formed. [Solution] The pulsed light generation device 100 includes an oscillator 1 that oscillates pulsed light L, a fiber amplifier 2 positioned downstream of the oscillator 1 in the optical path of the pulsed light L and broadening the spectrum of the pulsed light L, a soliton shift fiber 6 positioned downstream of the fiber amplifier 2 in the optical path of the pulsed light L and comprising a polarization-maintaining fiber 60, which modulates the pulsed light L, and a half-wave plate 5 positioned upstream of the soliton shift fiber 6 in the optical path of the pulsed light L, which changes the polarization direction of the pulsed light L in a direction that intersects both the slow axis direction and the fast axis direction. The soliton shift fiber 6 modulates the wavelength of the pulsed light L whose polarization direction has been changed by the half-wave plate 5 using soliton self-frequency shift to form a first pulsed light L1 and a second pulsed light L2.

Description

Technical Field

[0001] The present disclosure relates to a pulsed light generation device and a pulsed light generation method.

Background Art

[0002] There is known a pulsed light generation device including an oscillation unit that oscillates pulsed light and a modulation unit that modulates the wavelength of the pulsed light oscillated by the oscillation unit using soliton self-frequency shift. In such a pulsed light generation device, by increasing the intensity of the pulsed light before modulation by the modulation unit, the pulsed light is split by the modulation, and a plurality of pulsed lights having different wavelengths are formed (a multi-colored soliton is output) (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a pulsed light generation device, it may be desirable to form two pulsed lights having different wavelengths (so-called double solitons) by modulation of the modulation unit. However, in the above technique, if the intensity of the pulsed light before modulation by the modulation unit is increased, there is a risk that three or more pulsed lights will be formed. Therefore, an object of the present disclosure is to provide a pulsed light generation device and a pulsed light generation method capable of reliably forming two pulsed lights by modulation using soliton self-frequency shift.

Means for Solving the Problems

[0005] The pulsed light generation apparatus of the present disclosure is [1] "a pulsed light generation apparatus comprising: an oscillator that oscillates pulsed light; a broadbanding unit disposed downstream of the oscillator in the optical path of the pulsed light and broadening the spectrum of the pulsed light; a modulation unit disposed downstream of the broadbanding unit in the optical path of the pulsed light and comprising a polarization-maintaining fiber and modulating the pulsed light; and a polarization-changing unit disposed upstream of the modulation unit in the optical path of the pulsed light and changing the polarization direction of the pulsed light in a direction intersecting both the slow axis direction and the fast axis direction of the polarization-maintaining fiber, wherein the modulation unit modulates the wavelength of the pulsed light whose polarization direction has been changed by the polarization-changing unit using soliton self-frequency shift, and forms a first pulsed light which is the pulsed light with a polarization direction along the slow axis direction and a second pulsed light which is the pulsed light with a polarization direction along the fast axis direction."

[0006] As a result of diligent research, the inventors have found that by broadening the bandwidth of the pulse light spectrum before modulation using soliton self-frequency shift, it is possible to suppress multi-soliton formation, which occurs when the intensity of the pulse light before modulation is increased. Therefore, in this disclosure, the spectrum of the pulse light is broadened, and the wavelength of the broadened pulse light is modulated using soliton self-frequency shift. This makes it possible to suppress multi-soliton formation when the intensity of the pulse light before modulation is increased.

[0007] Furthermore, as a result of diligent research, the inventors have found that when pulsed light having both a component polarized in the slow axis direction and a component polarized in the fast axis direction is input to a modulation unit including a polarization-maintaining fiber, Raman solitons are generated with each axis as the polarization direction (i.e., pulsed light polarized in the slow axis direction and pulsed light polarized in the fast axis direction are obtained). Therefore, in this disclosure, by changing the polarization direction of the pulsed light input to the modulation unit including the polarization-maintaining fiber to a direction intersecting both the slow axis direction and the fast axis direction, the pulsed light is modulated in the modulation unit using soliton self-frequency shift, resulting in the formation of a first pulsed light polarized in the slow axis direction and a second pulsed light polarized in the fast axis direction. In other words, according to this disclosure, it is possible to reliably form two pulsed lights by modulation using soliton self-frequency shift.

[0008] The pulsed light generation apparatus of the present disclosure may also be [2] "the pulsed light generation apparatus according to [1], comprising a polarizer disposed downstream of the modulation section in the optical path of the pulsed light and adjusting the intensity ratio of the first pulsed light and the second pulsed light." In this case, the polarizer makes it possible to adjust the intensity ratio of the first pulsed light and the second pulsed light.

[0009] The pulsed light generation apparatus of the present disclosure may also be the pulsed light generation apparatus according to [1] or [2], wherein the polarization changing unit is a half-wave plate. In this case, the polarization direction of the pulsed light input to the modulation unit can be changed using the half-wave plate.

[0010] The pulsed light generation apparatus of the present disclosure may also be [4] "a pulsed light generation apparatus according to any one of [1] to [3], which is provided with an optical intensity control unit that is disposed between the oscillator and the modulation unit in the optical path of the pulsed light and controls the intensity of the pulsed light for each pulse." In this case, the wavelengths of the first and second pulsed light formed in the modulation unit can be varied for each pulse by the optical intensity control unit.

[0011] The pulsed light generation apparatus of the present disclosure may be [5] "a pulsed light generation apparatus according to any one of [1] to [4], comprising: a branching unit arranged downstream of the modulation unit in the optical path of the pulsed light and branching the first pulsed light and the second pulsed light; another polarization changing unit arranged downstream of the branching unit in the optical path of the pulsed light and changing at least one of the first polarization direction and the second polarization direction so that the first polarization direction, which is the polarization direction of the first pulsed light, and the second polarization direction, which is the polarization direction of the second pulsed light, coincide; and a sum frequency generation unit arranged downstream of the other polarization changing unit in the optical path of the pulsed light and combining the first pulsed light and the second pulsed light, and emitting sum frequency light by sum frequency generation." In this case, it is possible to obtain sum frequency light from the first and second pulsed light formed in the modulation unit.

[0012] The pulsed light generation apparatus of the present disclosure may also be [6] "a pulsed light generation apparatus according to any one of [1] to [5], which is arranged upstream of the modulation unit in the optical path of the pulsed light and comprises a time width compression unit for compressing the time width of the pulsed light." In this case, modulation using soliton self-frequency shift can be effectively realized in the modulation unit.

[0013] The pulsed light generation method of the present disclosure is [7] "a pulsed light generation method comprising: an oscillation step of oscillating pulsed light; a broadbanding step of broadening the spectrum of the pulsed light oscillated in the oscillation step; a modulation step of modulating the pulsed light after broadening by the broadbanding step using a modulation unit configured to include a polarization-maintaining fiber; and a polarization change step of changing the polarization direction of the pulsed light before modulation by the modulation step to a direction intersecting both the slow axis direction and the fast axis direction of the polarization-maintaining fiber, wherein in the modulation step, the wavelength of the pulsed light whose polarization direction has been changed by the polarization change step is modulated using a soliton self-frequency shift to form a first pulsed light which is the pulsed light with a polarization direction along the slow axis direction and a second pulsed light which is the pulsed light with a polarization direction along the fast axis direction."

[0014] In the pulsed light generation method of this disclosure, since the wavelength of the broadbanded pulsed light is modulated using soliton self-frequency shift, it is possible to suppress the formation of multiple solitons when the intensity of the pulsed light before modulation is increased. Furthermore, by changing the polarization direction of the pulsed light input to the modulation section, which includes a polarization-maintaining fiber, to a direction that intersects both the slow axis direction and the fast axis direction, the pulsed light is modulated in the modulation section using soliton self-frequency shift, resulting in the formation of first and second pulsed light. In other words, according to this disclosure, it is possible to reliably form two pulsed light by modulation using soliton self-frequency shift. [Effects of the Invention]

[0015] This disclosure makes it possible to provide a pulsed light generation device and a pulsed light generation method that can reliably form two pulsed light pulses by modulation utilizing soliton self-frequency shift. [Brief explanation of the drawing]

[0016] [Figure 1] Figure 1 is a block diagram showing a pulsed light generation device according to the first embodiment. [Figure 2] Figure 2 is a graph showing the spectral waveform of the pulsed light output from the fiber amplifier in Figure 1. [Figure 3] Figure 3 is a cross-sectional view showing the polarization-maintaining fibers that make up the soliton-shift fiber in Figure 1. [Figure 4] Figure 4 is a graph showing the spectral waveform of pulsed light output from the soliton-shift fiber shown in Figure 1. [Figure 5] Figure 5(a) is a schematic front view showing the polarizer in Figure 1 in the orientation where the polarization direction is 0°. Figure 5(b) is a graph showing the spectral waveform of pulsed light whose intensity ratio has been adjusted by the polarizer in Figure 5(a). Figure 5(c) is a schematic front view showing the polarizer in Figure 1 in the orientation where the polarization direction is 45°. Figure 5(d) is a graph showing the spectral waveform of pulsed light whose intensity ratio has been adjusted by the polarizer in Figure 5(c). [Figure 6] Fig. 6(a) is a schematic front view showing the state of the polarizer in Fig. 1 where the polarization direction is 90°. Fig. 6(b) is a graph showing the spectral waveform of the pulsed light whose intensity ratio is adjusted by the polarizer in Fig. 6(a). [Figure 7] Fig. 7 is a flowchart showing the pulsed light generation method according to the first embodiment. [Figure 8] Fig. 8 is a block diagram showing the pulsed light generation device according to the second embodiment. [Figure 9] Fig. 9 is a block diagram showing the pulsed light generation device according to the third embodiment. Embodiments for Carrying Out the Invention

[0017] Hereinafter, the embodiments will be described in detail with reference to the drawings. In each figure, the same or corresponding parts are denoted by the same reference numerals, and duplicate explanations are omitted.

[0018] [First Embodiment] As shown in Fig. 1, the pulsed light generation device 100 according to the first embodiment is a device that generates output light LK, which is ultrashort pulsed light, by using soliton self-frequency shift (Raman soliton shift). The pulsed light generation device 100 can be applied as a light source for an optical measurement device or an optical inspection device, for example, a microscope, particularly a two-photon microscope. A two-photon microscope is a type of laser scanning fluorescence microscope that excites a fluorescent dye in a sample by irradiating an ultrashort pulsed laser beam in the near-infrared region onto the sample, and detects a signal (fluorescence) from the sample to perform image analysis. The pulsed light generation device 100 constitutes a wavelength-variable light source whose wavelength is variable for each pulse.

[0019] The pulsed light generation device 100 includes an oscillator (oscillation unit) 1, a fiber amplifier (broadband unit) 2, an acousto-optic modulator (light intensity control unit) 3, a compressor (time width compression unit) 4, a half-wave plate (polarization change unit) 5, a soliton shift fiber (modulation unit) 6, a polarizer 7, a filter 8, a stretcher 9, a fiber amplifier 10, a compressor 11, and a wavelength conversion unit 12.

[0020] Oscillator 1 generates pulsed light L. Various oscillators capable of generating ultrashort pulses L with a time width of picoseconds to femtoseconds can be used as oscillator 1. Oscillator 1 generates a train of ultrashort pulses L that are arranged on the time axis at predetermined time intervals. The pulsed light L generated by oscillator 1 has a repetition rate in the MHz to GHz range.

[0021] The fiber amplifier 2 amplifies the pulsed light L oscillated by oscillator 1, for example, by similariton amplification. The fiber amplifier 2 broadens the bandwidth of the spectrum of the pulsed light L oscillated by oscillator 1 through nonlinear effects. In addition, the fiber amplifier 2 secondarily widens the time width of the pulsed light L. The fiber amplifier 2 is located downstream (after) oscillator 1 in the optical path of the pulsed light L. In this embodiment, the fiber amplifier 2 is located between oscillator 1 and acousto-optic modulator 3 in the optical path of the pulsed light L.

[0022] Fiber amplifier 2 is composed of a fiber amplifier. For the fiber amplifier, for example, an erbium-doped single-clad normal dispersion fiber or an erbium and ytterbium-doped double-clad normal dispersion fiber can be used. A normal dispersion fiber is a fiber in which the dispersion parameter D (ps / nm / km) is negative in the wavelength range in which the fiber is used. There are no particular restrictions on the additives used in the fiber, and various additives can be used. Fiber amplifier 2 amplifies while generating a nonlinear effect using the above normal dispersion fiber, and generates pulsed light L as broadband amplifier light. Figure 2 is a graph showing an example of the spectrum of the broadbanded pulsed light L. In Figure 2, the horizontal axis shows wavelength (nm), and the vertical axis shows intensity (arbitrary unit). In the example in Figure 2, the fiber amplifier 2 broadens the spectral width of the pulsed light L in the 1550 nm wavelength band to 100 nm or more.

[0023] Returning to Figure 1, the acousto-optic modulator 3 controls the intensity of the pulsed light L amplified by the fiber amplifier 2 on a pulse-by-pulse basis. The acousto-optic modulator 3 is a device called an AOM (Acousto Optic Modulator) that modulates the intensity of the pulsed light L on a pulse-by-pulse basis using the power of sound (sound waves). The acousto-optic modulator 3 can modulate the intensity in accordance with the repetition of the pulsed light L. The acousto-optic modulator 3 is positioned between the fiber amplifier 2 and the soliton-shift fiber 6 in the optical path of the pulsed light L. In this embodiment, the acousto-optic modulator 3 is positioned upstream (before) the compressor 4 in the optical path of the pulsed light L, between the fiber amplifier 2 and the soliton-shift fiber 6. The acousto-optic modulator 3 also reduces the repetition rate by thinning out the pulsed light L included in the ultrashort pulse train.

[0024] The compressor 4 compresses the time width of the pulsed light L whose intensity is controlled by the acousto-optic modulator 3. In other words, the compressor 4 applies dispersion in the opposite direction to the dispersion effect that the pulsed light L has experienced up to the previous stage, thereby achieving dispersion compensation that narrows the time width of the broadened pulsed light L. The compressor 4 is located upstream of the soliton shift fiber 6 in the optical path of the pulsed light L. In this embodiment, the compressor 4 is located between the acousto-optic modulator 3 and the soliton shift fiber 6 in the optical path of the pulsed light L. Various compressors can be used as the compressor 4, such as a compressor including an anomalous dispersion fiber, a compressor including a pair of prisms, or a compressor including a pair of grisms. In this embodiment, a diffraction grating type compressor including a pair of diffraction gratings is used as the compressor 4.

[0025] The half-wave plate 5 changes the polarization direction of the pulsed light L. The half-wave plate 5 is a waveplate unit that has the function of creating a half-wavelength (λ / 2) phase difference with respect to the pulsed light L. The half-wave plate 5 is positioned upstream of the soliton shift fiber 6 in the optical path of the pulsed light L. In this embodiment, the half-wave plate 5 is positioned between the compressor 4 and the soliton shift fiber 6 in the optical path of the pulsed light L. Note that instead of the half-wave plate 5, a waveplate unit that can create a half-wavelength phase difference with respect to the pulsed light L may be used, for example, a unit including two quarter-wave plates may be used.

[0026] The soliton shift fiber 6 modulates the wavelength of pulsed light L, whose spectrum has been broadened by the fiber amplifier 2 and whose polarization direction has been changed by the half-wave plate 5, using soliton self-frequency shifting. The soliton shift fiber 6 is positioned downstream of the half-wave plate 5 in the optical path of the pulsed light L. In this embodiment, the soliton shift fiber 6 is positioned between the half-wave plate 5 and the polarizer 7 in the optical path of the pulsed light L. For example, a single-mode anomalous dispersion fiber exhibiting anomalous dispersion in the wavelength band of the pulsed light L can be used as the soliton shift fiber 6.

[0027] The soliton shift fiber 6 outputs a soliton with a wavelength corresponding to the intensity of the input pulsed light L when pulsed light L is input. For example, the soliton shift fiber 6 outputs a soliton having a wavelength in the wavelength range of 1800 nm to 2200 nm. The soliton is pulsed light L whose wavelength is modulated by soliton self-frequency shift. The wavelength of the soliton can be controlled by the intensity of the pulsed light L input to the soliton shift fiber 6. When the intensity of the pulsed light L input to the soliton shift fiber 6 is increased, the wavelength of the soliton shifts to the longer wavelength side, while when the intensity of the pulsed light L input to the soliton shift fiber 6 shifts the wavelength of the soliton to the shorter wavelength side. However, if the intensity of the pulsed light L input to the soliton shift fiber 6 becomes excessive, multi-solitonization may occur, and multiple solitons may be generated for a single pulsed light L.

[0028] In contrast, in this embodiment, the spectral width of the pulsed light L input to the soliton shift fiber 6 is pre-broadened by the fiber amplifier 2. This makes it possible to form solitons while suppressing the multi-solitonization that occurs when the intensity of the pulsed light L input to the soliton shift fiber 6 is increased. Furthermore, in this embodiment, by controlling the intensity of the pulsed light L for each pulse using the acousto-optic modulator 3, it becomes possible to control the wavelength of the solitons generated in the soliton shift fiber 6 for each pulse.

[0029] As shown in Figure 3, the soliton shift fiber 6 is a polarization-maintaining fiber 60 configured to maintain a specific polarization state. The polarization-maintaining fiber 60 is an optical fiber in which the polarization plane maintenance characteristics of the transmitted light are enhanced by making the refractive index different between the mutually orthogonal slow axis and fast axis. In the illustrated example, the polarization-maintaining fiber 60 is composed of a stress-applied polarization-maintaining fiber using the photoelastic effect. The polarization-maintaining fiber 60 may also be composed of a structural polarization-maintaining fiber having a non-axisymmetric core shape.

[0030] The polarization-maintaining fiber 60 has a core 60a, a cladding 60b, and a pair of stress-applying materials 60c. The core 60a is located in the center of the cross-section of the polarization-maintaining fiber 60. The refractive index of the core 60a is higher than that of the cladding 60b. The cladding 60b surrounds the core 60a. The pair of stress-applying materials 60c are arranged within the cladding 60b so as to be located on both sides of the core 60a on the slow axis X1. In the polarization-maintaining fiber 60, the core 60a is given birefringence by applying tensile stress to it by utilizing the fact that the thermal shrinkage rate of the stress-applying material 60c is greater than that of the cladding 60b. Due to this difference in refractive index, when light propagates through the polarization-maintaining fiber 60, the component propagating along the fast axis X2 propagates faster than the component propagating along the slow axis X1.

[0031] Returning to Figure 1, the polarizer 7 is an element that allows only pulsed light L with a specific polarization direction to pass through. The polarizer 7 includes a rotation mechanism that allows its orientation to be rotated (i.e., it can rotate around a rotation axis along the optical axis of the pulsed light L). By changing the orientation of the polarizer 7 using the rotation mechanism, the polarization direction of the pulsed light L that passes through is changed, and the intensity ratio of the first pulsed light L1 and the second pulsed light L2, as described later, is adjusted (details will be described later).

[0032] Filter 8 removes the non-soliton components (components that did not become solitons) from the optical components generated by the soliton shift fiber 6, excluding the solitons. Filter 8 is located downstream of the soliton shift fiber 6 in the optical path of the pulsed light L. In this embodiment, filter 8 is located between the polarizer 7 and the stretcher 9 in the optical path of the pulsed light L. There are no particular restrictions on filter 8 as long as it can remove or attenuate the non-soliton components, but it is preferable to use one with an optical density (OD value) of 3 or more. For example, a wavelength-separated filter may be used as filter 8. Filter 8 may be implemented as a standalone unit or as part of a wavelength-division multiplexing coupler. The non-soliton components removed by filter 8 may be reused.

[0033] The stretcher 9 extends the time width of the pulsed light L (soliton) from which the non-soliton component has been removed by the filter 8, thereby reducing the peak power of the pulsed light L. This suppresses the occurrence of undesirable nonlinear optical effects during amplification by the subsequent fiber amplifier 10. The stretcher 9 is positioned downstream of the soliton-shift fiber 6 in the optical path of the pulsed light L. In this embodiment, the stretcher 9 is positioned between the filter 8 and the fiber amplifier 10 in the optical path of the pulsed light L. The stretcher 9 may extend the time width of the pulsed light L by transmitting the pulsed light L through at least one of the fiber and / or the medium to impart dispersion. Alternatively, the stretcher 9 may extend the time width of the pulsed light L by imparting dispersion to the pulsed light L in a spatial system using a diffraction grating pair or a prism pair, etc. The dispersion may be either normal dispersion or anomalous dispersion.

[0034] The fiber amplifier 10 amplifies the pulsed light (soliton) modulated by the soliton-shift fiber 6. The fiber amplifier 10 is located downstream of the soliton-shift fiber 6 in the optical path of the pulsed light L. In this embodiment, the fiber amplifier 10 is located between the stretcher 9 and the compressor 11 in the optical path of the pulsed light L. The fiber amplifier 10 includes a fiber amplifier. The fiber amplifier of the fiber amplifier 10 is a normally dispersed fiber, for example, a thulium-doped fiber. The laser medium added to the fiber of the fiber amplifier 10 is not particularly limited and may be a rare earth element such as ytterbium, erbium, neodymium, or Bi. The fiber amplifier 10 has a gain band corresponding to, for example, 1800 nm to less than 2200 nm.

[0035] The compressor 11 compresses the time width of the pulsed light L amplified by the fiber amplifier 10. The compressor 11 applies dispersion in the opposite direction to the dispersion effect on the pulsed light L up to the preceding stage, thereby achieving dispersion compensation that narrows the time width of the broadened pulsed light L. The compressor 11 is located downstream of the fiber amplifier 10 in the optical path of the pulsed light L. In this embodiment, the compressor 11 is located between the fiber amplifier 10 and the wavelength conversion unit 12 in the optical path of the pulsed light L. Various compressors can be used as the compressor 11, such as a compressor including an anomalous dispersion fiber, a compressor including a pair of prisms, or a compressor including a pair of grisms. In this embodiment, a diffraction grating type compressor including a pair of diffraction gratings is used as the compressor 11.

[0036] The wavelength conversion unit 12 performs wavelength conversion on each pulse of the pulsed light L whose time width has been compressed by the compressor 11. The wavelength conversion unit 12 may, for example, convert the wavelength of pulsed light L between 1800 nm and less than 2200 nm to between 900 nm and less than 1100 nm, which is usable for two-photon microscopy. The wavelength conversion unit 12 may also perform the above 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 with half the wavelength (second harmonic). The wavelength conversion unit 12 is located downstream of the compressor 11 in the optical path of the pulsed light L. The wavelength conversion unit 12 outputs the pulsed light L after wavelength conversion as output light LK.

[0037] In this embodiment, the half-wave plate 5 changes the polarization direction of the pulsed light L to a direction that intersects both the slow axis X1 direction and the fast axis X2 direction of the polarization-maintaining fiber 60 constituting the soliton shift fiber 6. In this case, pulsed light L having both a component whose polarization direction is the slow axis X1 direction and a component whose polarization direction is the fast axis X2 direction is input to the polarization-maintaining fiber 60.

[0038] As a result, in the polarization-maintaining fiber 60, Raman solitons are generated with the slow axis X1 and the fast axis X2 as their respective polarization directions. In other words, as shown in Figure 4, a first pulse light L1 with the slow axis X1 as its polarization direction and a second pulse light L2 with the fast axis X2 as its polarization direction are generated, and these are shifted to the longer wavelength side with a constant difference in their central wavelengths. The first pulse light L1 and the second pulse light L2 are solitons whose polarization directions are orthogonal to each other (polarization orthogonal). Hereafter, the first pulse light L1 and the second pulse light L2 will also be referred to as "double solitons".

[0039] Furthermore, in this embodiment, the polarizer 7 changes the polarization direction of the pulsed light L that is passed through by changing the orientation of the polarizer 7 using a rotation mechanism, thereby adjusting the intensity ratio of the first pulsed light L1 and the second pulsed light L2. For example, as shown in Figure 5(a), the polarizer 7 can be rotated so that the polarization direction A0 of the pulsed light L that is passed through is 0° with respect to the reference direction (up and down direction in the figure). Also, for example, as shown in Figure 5(c), the polarizer 7 can be rotated so that the polarization direction A0 of the pulsed light L that is passed through is tilted 45° with respect to the reference direction. Also, for example, as shown in Figure 6(a), the polarizer 7 can be rotated so that the polarization direction A0 of the pulsed light L that is passed through is tilted 90° with respect to the reference direction.

[0040] When the polarizer 7 is rotated in a direction where the polarization direction A0 is 0°, as shown in Figure 5(b), the polarization directions of the first pulse light L1 and the second pulse light L2 become orthogonal to each other. As a result, the first pulse light L1 is completely blocked, while the second pulse light L2 passes through completely. Therefore, in this case, the first pulse light L1 disappears, and the intensity ratio of the first pulse light L1 to the second pulse light L2 becomes 0:1.

[0041] Furthermore, when the polarizer 7 is rotated so that the polarization direction A0 is 45°, as shown in Figure 5(d), the polarization directions of the first pulse light L1 and the second pulse light L2 become orthogonal to each other. As a result, a portion of the first pulse light L1 and the second pulse light L2 is blocked while the rest passes through. Therefore, in this case, the first pulse light L1 disappears, and the intensity ratio of the first pulse light L1 to the second pulse light L2 becomes 1:1. The intensity ratio corresponds to the ratio of the heights of the first pulse light L1 and the second pulse light L2 in Figure 5(d).

[0042] Furthermore, when the polarizer 7 is rotated so that the polarization direction A0 is 90°, as shown in Figure 6(b), the polarization directions of the first pulse light L1 and the second pulse light L2 become orthogonal to each other. As a result, the second pulse light L2 is completely blocked, while the first pulse light L1 passes through completely. Therefore, in this case, the second pulse light L2 disappears, and the intensity ratio becomes first pulse light L1:second pulse light L2 = 1:0.

[0043] Next, the pulsed light generation method performed using the pulsed light generation device 100 will be explained with reference to the flowchart in Figure 7.

[0044] As shown in Figure 7, the pulse light generation method according to this embodiment comprises an oscillation step S01, a broadband step S02, a light intensity control step S03, a time width compression step S04, a polarization change step S05, a modulation step S06, an intensity ratio adjustment step S07, a time width extension step S08, an amplification step S09, a time width compression step S10, and a wavelength conversion step S11.

[0045] In the oscillation step S01, the oscillator 1 generates pulsed light L. In the broadbanding step S02, the fiber amplifier 2 broadens the spectrum of the pulsed light L generated in the oscillation step S01. In the light intensity control step S03, the acousto-optic modulator 3 controls the intensity of the pulsed light L whose spectrum was broadened in the broadbanding step S02, pulse by pulse. In the time width compression step S04, the compressor 4 compresses the time width of the pulsed light L whose intensity was controlled in the light intensity control step S03.

[0046] In the polarization change step S05, the polarization direction of the pulsed light L before modulation in the modulation step S06 is changed to a direction that intersects both the slow axis X1 direction and the fast axis X2 direction of the polarization-maintaining fiber 60. In the modulation step S06, the wavelength of the pulsed light L whose polarization direction was changed in the polarization change step S05 is modulated by the soliton shift fiber 6 using soliton self-frequency shift to form a double soliton.

[0047] In the intensity ratio adjustment step S07, the polarizer 7 adjusts the intensity ratio of the double soliton formed in the modulation step S06 (the intensity ratio of the first pulse light L1 and the second pulse light L2). After modulating the intensity ratio of the double soliton, the non-soliton component generated in the modulation step S06 is removed by the filter 8. In the time width extension step S08, the stretcher 9 extends the time width of the double soliton whose intensity ratio was adjusted in the intensity ratio adjustment step S07.

[0048] In amplification step S09, the fiber amplifier 10 amplifies the double soliton whose time width was expanded in time width expansion step S08. In time width compression step S10, the compressor 11 compresses the time width of the double soliton amplified in amplification step S09. In wavelength conversion step S11, the wavelength conversion unit 12 performs wavelength conversion on the double soliton whose time width was compressed in time width compression step S10, and outputs it as output optical light LK.

[0049] As described above, the pulsed light generation device 100 and pulsed light generation method make it possible to suppress the formation of multi-solitons that may occur when the intensity of the pulsed light L before modulation is increased by modulating the wavelength of the broadband pulsed light L using soliton self-frequency shift. Then, by changing the polarization direction of the pulsed light L input to the soliton shift fiber 6 of the polarization-maintaining fiber 60 to a direction that intersects both the slow axis X1 direction and the fast axis X2 direction, the pulsed light L is modulated using soliton self-frequency shift, resulting in the formation of a double soliton (first pulsed light L1 and second pulsed light L2). In other words, the pulsed light generation device 100 and pulsed light generation method make it possible to reliably form a double soliton by modulation using soliton self-frequency shift.

[0050] The pulsed light generator 100 is equipped with a polarizer 7 downstream of the soliton shift fiber 6 in the optical path of the pulsed light L. In this case, the polarizer 7 makes it possible to adjust the intensity ratio between the first pulsed light L1 and the second pulsed light L2.

[0051] The pulsed light generator 100 is equipped with a half-wave plate 5 as a polarization changing unit. In this way, the pulsed light generator 100 can use the half-wave plate 5 to change the polarization direction of the pulsed light L input to the soliton shift fiber 6.

[0052] The pulsed light generator 100 includes an acousto-optic modulator 3 between the oscillator 1 and the soliton-shift fiber 6 in the optical path of the pulsed light L. In this case, the wavelength of the double soliton formed in the soliton-shift fiber 6 can be varied for each pulse by the acousto-optic modulator 3.

[0053] The pulsed light generator 100 is equipped with a compressor 4 upstream of the soliton-shift fiber 6 in the optical path of the pulsed light L. In this case, modulation using soliton self-frequency shift can be effectively realized in the soliton-shift fiber 6. Furthermore, in this case, it is possible to further suppress the formation of multi-solitons when the intensity of the pulsed light L before modulation is increased.

[0054] In this embodiment, while preventing the formation of multiple solitons that occur due to the high intensity of the pulsed light L, double solitons with orthogonal polarization directions can be reliably formed by modulation using soliton self-frequency shift. In addition, the formed double solitons can be wavelength-converted by the wavelength conversion unit 12 to output the second harmonic of the double soliton, and furthermore, the wavelength of each second harmonic can be varied for each pulse. That is, it is possible to generate light waves with two variable wavelengths for each pulse.

[0055] [Second Embodiment] Next, a second embodiment will be described. In describing this embodiment, the differences from the first embodiment will be explained, and redundant explanations will be omitted.

[0056] As shown in Figure 8, the pulse light generation device 200 according to the second embodiment differs from the first embodiment in that, instead of the acousto-optic modulator 3, stretcher 9, fiber amplifier 10, compressor 11, and wavelength conversion unit 12 (see Figure 1), it is equipped with a branching unit 22, stretcher 23, fiber amplifier 24, half-wave plate (other polarization change unit) 25, stretcher 26, fiber amplifier 27, multiplexing unit 28, compressor 29, and wavelength conversion unit 30.

[0057] The branching section 22 splits the first pulsed light L1 and the second pulsed light L2, which have had non-soliton components removed by the filter 8, into two separate streams. The branching section 22 is located downstream of the filter 8 in the optical path of the pulsed light L. Examples of branching sections 22 include wavelength division filters such as dichroic filters, wavelength division multiplexing couplers, and spatial division multiplexing optical systems or optical elements.

[0058] The stretcher 23 extends the time width of the first pulsed light L1 that was branched at the branching section 22, thereby reducing the peak power of the first pulsed light L1. This suppresses the occurrence of undesirable nonlinear optical effects during amplification by the subsequent fiber amplifier 24. The stretcher 23 is positioned in the optical path of the first pulsed light L1. The other configurations of the stretcher 23 are the same as those of the stretcher 9 (see Figure 1).

[0059] The fiber amplifier 24 amplifies the first pulse light L1, whose time width has been stretched by the stretcher 23. The fiber amplifier 24 is located downstream of the stretcher 23 in the optical path of the first pulse light L1. The other configurations of the fiber amplifier 24 are the same as those of the fiber amplifier 10 (see Figure 1).

[0060] The half-wave plate 25 changes the first polarization direction of the first pulsed light L1 so that its first polarization direction matches the second polarization direction of the second pulsed light L2. The half-wave plate 25 is a waveplate unit that has the function of creating a half-wavelength (λ / 2) phase difference with respect to the first pulsed light L1. The half-wave plate 25 is located downstream of the fiber amplifier 24 in the optical path of the first pulsed light L1. Alternatively, a waveplate unit that can create a half-wavelength phase difference with respect to the pulsed light L may be used instead of the half-wave plate 25, for example, a unit containing two quarter-wave plates may be used.

[0061] The stretcher 26 extends the time width of the second pulsed light L2 that was branched at the branching section 22, thereby reducing the peak power of the second pulsed light L2. This suppresses the occurrence of undesirable nonlinear optical effects during amplification by the subsequent fiber amplifier 27. The stretcher 26 is positioned in the optical path of the second pulsed light L2. The other configurations of the stretcher 23 are the same as those of the stretcher 9 (see Figure 1).

[0062] The fiber amplifier 27 amplifies the second pulse of light L2, whose time width has been stretched by the stretcher 26. The fiber amplifier 27 is located downstream of the stretcher 26 in the optical path of the second pulse of light L2. The other configurations of the fiber amplifier 27 are the same as those of the fiber amplifier 10 (see Figure 1).

[0063] The multiplexing unit 28 combines the first pulsed light L1, whose polarization direction has been changed by the half-wave plate 25, and the second pulsed light L2, which has been amplified by the fiber amplifier 27, on the same optical axis. The multiplexing unit 28 is composed of, for example, a mirror and a dichroic mirror. The multiplexing unit 28 is located downstream of the half-wave plate 25 in the optical path of the pulsed light L.

[0064] The compressor 29 compresses the time width of the first pulse light L1 and the second pulse light L2 combined in the multiplexing unit 28. The compressor 29 applies dispersion in the opposite direction to the dispersion effect that the first pulse light L1 and the second pulse light L2 experienced up to the previous stage, thereby achieving dispersion compensation that narrows the expanded time width of the first pulse light L1 and the second pulse light L2. The compressor 29 is located downstream of the multiplexing unit 28 in the optical path of the pulse light L. Various types of compressors can be used as the compressor 29, such as a compressor including an anomalous dispersion fiber, a compressor including a prism pair, or a compressor including a grism pair.

[0065] The wavelength conversion unit 30 emits sum-frequency light as output light LK by sum-frequency generation using the first pulse light L1 and second pulse light L2 combined in the wave-combining unit 28. In the wavelength conversion unit 30, the first pulse light L1 and second pulse light L2 combined in the wave-combining unit 28 are incident on the wavelength conversion crystal, and sum-frequency light is emitted from the wavelength conversion crystal by sum-frequency generation. The wavelength conversion unit 30 is located downstream of the compressor 29 in the optical path of the pulse light L. In this embodiment, the optical path lengths of the first pulse light L1 and the second pulse light L2 are configured to be the same, so that, for example, the arrival timing of the first pulse light L1 and the second pulse light L2 at the wavelength conversion crystal of the wavelength conversion unit 30 is synchronized. The wave-combining unit 28 and the wavelength conversion unit 30 constitute the sum-frequency generation unit.

[0066] As described above, in the pulsed light generation apparatus 200 and pulsed light generation method according to this embodiment, it is possible to reliably form a double soliton by modulation using soliton self-frequency shift.

[0067] Furthermore, the pulsed light generator 200 includes a branching section 22 located downstream of the soliton shift fiber 6 in the optical path of the pulsed light L, a half-wave plate 25 located in the optical path of the first pulsed light L1, and a multiplexing section 28 and a wavelength conversion section 30 located downstream of the half-wave plate 25 in the optical path of the pulsed light L. In this case, sum-frequency light can be obtained from the double soliton formed by the soliton shift fiber 6 by sum-frequency generation.

[0068] In this embodiment, while preventing the formation of multiple solitons that occur due to the high intensity of the pulsed light L, double solitons with orthogonal polarization directions can be reliably formed by modulation using soliton self-frequency shift. In addition, sum-frequency light can be output by sum-frequency generation using the formed double solitons, that is, it is possible to generate light waves in which each pulse has three wavelengths.

[0069] [Third Embodiment] Next, a third embodiment will be described. In describing this embodiment, the differences from the second embodiment described above will be explained, and redundant explanations will be omitted.

[0070] As shown in Figure 9, the pulse light generation device 300 according to the third embodiment differs from the second embodiment in that it further includes another stretcher 31. The other stretcher 31 further extends the time width of the second pulse light L2, whose time width has been extended by the stretcher 26. The other stretcher 31 is positioned between the stretcher 26 and the fiber amplifier 27 in the optical path of the pulse light L. This ensures that the pulses of the first pulse light L1 and the pulses of the second pulse light L2 are reliably superimposed in time by the wavelength conversion unit 30.

[0071] As described above, in the pulsed light generation apparatus 300 and pulsed light generation method according to this embodiment, it is possible to reliably form a double soliton by modulation utilizing soliton self-frequency shift.

[0072] [Differentiation] The embodiments described above are not limited to the above-described embodiments.

[0073] In the above embodiment, an acousto-optic modulator 3 was used as the light intensity control unit, but the embodiment is not limited to this, and for example, an electro-optic modulator (EOM) may be used as the light intensity control unit. In the above embodiment, multiple acousto-optic modulators 3 may be arranged.

[0074] In the second and third embodiments described above, the polarizer 7 may be positioned at any location between the soliton shift fiber 6 and the branching portion 22 in the optical path of the pulsed light L. In the second and third embodiments described above, instead of the half-wave plate 25, another half-wave plate having the function of creating a half-wavelength phase difference with respect to the second pulsed light L2 may be placed in the optical path of the second pulsed light L2. In the third embodiment described above, instead of or in addition to the other stretcher 31, a stretcher may be provided that is placed in the optical path of the first pulsed light L1 to extend the time width of the first pulsed light L1.

[0075] In the above embodiment, a fiber amplifier made of a normally dispersed, double-clad fiber was used as the fiber amplifier 2. However, a fiber amplifier made of a normally dispersed, single-clad fiber (e.g., erbium-doped) may be used instead. In this case as well, it will still be possible to broaden the spectrum of the pulsed light L.

[0076] The components in the above embodiments and modifications are not limited to the materials and shapes described above, and various materials and shapes can be applied. Furthermore, the components in the above embodiments and modifications can be arbitrarily applied to the components in other embodiments or modifications. [Explanation of Symbols]

[0077] 1...Oscillator (oscillating unit), 2...Fiber amplifier (broadbanding unit), 3...Acousto-optic modulator (light intensity control unit), 4...Compressor (time width compression unit), 5...Half wave plate (polarization change unit), 6...Soliton shift fiber (modulation unit), 7...Polarizer, 22...Branching unit, 25...Half wave plate (other polarization change unit), 28...Multiplier unit (sum frequency generation unit), 30...Wavelength conversion unit (sum frequency generation unit), 100, 200, 300...Pulsed light generator, L...Pulsed light, L1...First pulsed light, L2...Second pulsed light, LK...Output light.

Claims

1. An oscillator that emits pulsed light, A broadbanding unit is positioned downstream of the oscillator in the optical path of the pulsed light, and broadens the spectrum of the pulsed light. A modulation unit is located downstream of the broadband section in the optical path of the pulsed light, and is configured to include a polarization-maintaining fiber, which modulates the pulsed light. The optical path of the pulsed light includes a polarization changing unit, which is positioned upstream of the modulation unit and changes the polarization direction of the pulsed light in a direction that intersects both the slow axis direction and the fast axis direction of the polarization-maintaining fiber, The modulation unit is A pulse light generation device that modulates the wavelength of the pulse light, whose polarization direction has been changed by the polarization changing unit, using soliton self-frequency shifting to form a first pulse light, which is the pulse light with a polarization direction along the slow axis, and a second pulse light, which is the pulse light with a polarization direction along the fast axis.

2. The pulsed light generation apparatus according to claim 1, further comprising a polarizer positioned downstream of the modulation section in the optical path of the pulsed light, which adjusts the intensity ratio of the first pulsed light and the second pulsed light.

3. The pulse light generation apparatus according to claim 1 or 2, wherein the polarization changing section is a half-wave plate.

4. The pulsed light generation apparatus according to claim 1 or 2, further comprising an optical intensity control unit disposed between the oscillator and the modulation unit in the optical path of the pulsed light, which controls the intensity of the pulsed light for each pulse.

5. A branching section is located downstream of the modulation section in the optical path of the pulsed light, and branches the first pulsed light and the second pulsed light, A polarization changing unit is provided, which is located downstream of the branching portion in the optical path of the pulsed light, and which changes at least one of the first polarization direction and the second polarization direction so that the first polarization direction, which is the polarization direction of the first pulsed light, and the second polarization direction, which is the polarization direction of the second pulsed light, coincide. A pulse light generation apparatus according to claim 1 or 2, comprising: a sum frequency generation unit disposed downstream of the other polarization change unit in the optical path of the pulse light, which combines the first pulse light and the second pulse light and emits sum frequency light by sum frequency generation.

6. The pulse light generation apparatus according to claim 1 or 2, further comprising a time width compression unit arranged upstream of the modulation unit in the optical path of the pulse light, for compressing the time width of the pulse light.

7. An oscillation step that generates pulsed light, A broadbanding step which broadens the spectrum of the pulsed light oscillated in the oscillation step, A modulation step in which the pulsed light after broadening in the broadening step is modulated using a modulation unit configured to include a polarization-maintaining fiber, The system includes a polarization change step which changes the polarization direction of the pulsed light before modulation by the modulation step to a direction that intersects both the slow axis direction and the fast axis direction of the polarization-holding fiber, In the modulation step, A method for generating pulsed light, comprising modulating the wavelength of the pulsed light whose polarization direction has been changed by the polarization change step using soliton self-frequency shift to form a first pulsed light which is the pulsed light with a polarization direction along the slow axis direction and a second pulsed light which is the pulsed light with a polarization direction along the fast axis direction.

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