Pulsed light generation device and pulsed light generation method

Abstract

This pulsed light generation device comprises: an oscillation unit that oscillates pulsed light; an amplification unit that enlarges the bandwidth of the pulsed light oscillated by the oscillation unit; and a modulation unit that modulates, using soliton self-frequency shift, the wavelength of the pulsed light obtained by enlarging of the bandwidth of the spectrum by the amplification unit.

Description

Pulsed light generating device and pulsed light generating method

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

[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] In the above-described pulsed light generating device, as described above, when the intensity of the pulsed light before modulation by the modulator is increased, the pulsed light is split by the modulation to form multiple pulsed lights (hereinafter also referred to as "multi-soliton generation"). However, it is very difficult to set all of the multiple pulsed lights to desired wavelengths. Furthermore, since applications in which all of the multiple pulsed lights are simultaneously used in the desired wavelength range are very rare, such multi-soliton generation is often undesirable from a practical standpoint, for example, because it is necessary to remove unnecessary pulsed lights. Therefore, it is desirable to suppress multi-soliton generation when performing wavelength conversion.

[0005] Therefore, an object of the present disclosure is to provide an optical pulse generating device and an optical pulse generating method that can prevent multiple optical pulses from being generated by modulation using soliton self-frequency shift.

[0006] The pulsed light generating device of the present disclosure is [1] "a pulsed light generating device including: an oscillator that oscillates pulsed light; an amplifier that broadens the spectrum of the pulsed light oscillated by the oscillator; and a modulator that modulates the wavelength of the pulsed light, the spectrum of which has been broadened by the amplifier, by utilizing soliton self-frequency shift."

[0007] As a result of extensive research, the inventors have found that multi-soliton generation can be suppressed by broadening the spectrum of pulsed light before modulation using soliton self-frequency shift. Therefore, in the pulsed light generating device of the present disclosure, the spectrum of pulsed light is broadened, and the wavelength of the broadened pulsed light is modulated using soliton self-frequency shift. This makes it possible to suppress multi-soliton generation even when, for example, the intensity of pulsed light before modulation is increased.

[0008] The pulsed light generating device of the present disclosure may be [2] "the pulsed light generating device according to [1]," in which the amplifier unit broadens the spectrum of the pulsed light by similariton amplification." In this case, the amplifier unit can suppress pulse stretching. This makes it possible to effectively achieve modulation using soliton self-frequency shift.

[0009] The pulsed light generating device of the present disclosure may be [3] "the pulsed light generating device according to [1] or [2]," in which the amplifier unit includes a normal dispersion fiber. In this case, the amplifier unit can specifically broaden the spectrum of the pulsed light.

[0010] The pulsed light generating device of the present disclosure may be [4] "the pulsed light generating device according to any one of [1] to [3], wherein the amplifier unit includes a double-clad fiber." In this case, for example, an erbium- and ytterbium-co-doped double-clad fiber can be used for the amplifier unit, thereby effectively increasing the output power of pulsed light before modulation.

[0011] The pulsed light generating device of the present disclosure may be [5] "the pulsed light generating device according to any one of [1] to [4], wherein the amplifier unit sets the spectral width of the pulsed light to 100 nm or more." In this case, it is possible to reliably suppress multi-soliton generation.

[0012] The pulsed light generating device of the present disclosure may be [6] "the pulsed light generating device according to any one of [1] to [5], further comprising a light intensity control unit disposed between the amplifier and the modulator in the optical path of the pulsed light, and configured to control the intensity of the pulsed light for each pulse." In this case, the light intensity control unit can vary the wavelength of the generated pulsed light for each pulse.

[0013] The pulsed light generating device of the present disclosure may be [7] "the pulsed light generating device according to any one of [1] to [6], including a pulse compressor arranged between the amplifier and the modulator in the optical path of the pulsed light, and configured to compress the pulse width of the pulsed light." In this case, modulation utilizing soliton self-frequency shift can be effectively achieved.

[0014] The pulsed light generation method of the present disclosure is [8] "a pulsed light generation method comprising: an oscillation step of oscillating pulsed light; an amplification step of broadening the spectrum of the pulsed light oscillated in the oscillation step; and a modulation step of modulating the wavelength of the pulsed light, the spectrum of which has been broadened in the amplification step, by utilizing soliton self-frequency shift."

[0015] In the pulsed light generation method of the present disclosure, the wavelength of the broadband pulsed light is also modulated by utilizing the soliton self-frequency shift, so that it is possible to suppress multi-soliton generation even when, for example, the output of the pulsed light before modulation is increased.

[0016] According to the present disclosure, it is possible to provide an optical pulse generating device and an optical pulse generating method that can prevent multiple optical pulses from being generated by modulation using soliton self-frequency shift.

[0017] 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 the ultrashort optical pulse output from the filter of Fig. 1. Fig. 5(d) is a graph showing the spectrum of the ultrashort optical pulse output from the filter of Fig. 1. Fig. 6 is a flowchart showing a method for generating optical pulses according to an embodiment. Fig. 7(a) is a graph showing the relationship between the intensity of the ultrashort optical pulse input to a soliton-shifted fiber, the wavelength of the soliton, and the number of solitons. Fig. 7(b) is a graph showing the relationship between the spectral width of the ultrashort optical pulse input to a soliton-shifted fiber, the wavelength of the soliton, and the number of solitons.

[0018] 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.

[0019] 1 , a pulsed light generating apparatus 1 of this embodiment utilizes soliton self-frequency shift (Raman soliton shift) to generate long-wavelength ultrashort pulsed light (pulsed light) L. 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, and a filter 7.

[0020] 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. 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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. Note that the light intensity control section is not particularly limited to the acousto-optic modulator 4, and an electro-optic modulator (EOM), for example, may also be used.

[0025] 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.

[0026] 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 the 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 soliton-shifted fiber 6 can be, for example, a single-mode anomalous dispersion fiber that exhibits anomalous dispersion in the wavelength band of the ultrashort pulsed light L generated by the fiber amplifier 3. In addition, by controlling the acousto-optic modulator 4, solitons with wavelengths different from soliton S1 can be generated. For example, when modulations of intensities M1 and M2 are applied as shown in FIG. 5(c), the wavelength of the soliton S shifts to a wavelength corresponding to the intensities given by M1 and M2 (solitons S1 and S2) as shown in FIG. 5(d). The range and precision of the shift wavelength 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 changed arbitrarily by applying intensity modulation to the pulse train with 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 did not become the soliton S1 or S2).

[0027] The filter 7 filters the ultrashort pulsed light L whose wavelength has been modulated by the soliton-shifted fiber 6. The filter 7 is disposed downstream of the soliton-shifted fiber 6 in the optical path of the ultrashort pulsed light L. In the illustrated example, the filter 7 cuts the non-soliton component S0 of the ultrashort pulsed light L, as shown in FIGS. 5(b) and 5(d). The filter 7 desirably has an OD value of 3 or more. There are no particular limitations on the filter 7, and various filters can be used.

[0028] Next, a pulsed light generating method carried out using the pulsed light generating device 1 will be described with reference to the flowchart of FIG.

[0029] First, an oscillator 2 oscillates an ultrashort pulse light L to generate an ultrashort pulse train with a predetermined period (oscillation step: step S1). A fiber amplifier 3 increases the output power of the ultrashort pulse light L and broadens the spectrum of the ultrashort pulse light L (amplification step: step S2). An acousto-optic modulator 4 controls the intensity of the ultrashort pulse light L for each pulse, depending on, for example, the specifications or conditions required for the pulsed light generating device 1 (step S3).

[0030] Next, the compressor 5 compresses the time width of the ultrashort pulse light L (step S4). The soliton shift fiber 6 modulates the wavelength of the spectrally broadened ultrashort pulse light L using the soliton self-frequency shift to lengthen the wavelength (modulation step: step S5). The filter 7 filters the ultrashort pulse light L with its lengthened wavelength to cut its non-soliton component SO (step S6).

[0031] As a result of extensive research, the inventors have found that, as shown in Fig. 7(a), increasing the intensity of the ultrashort pulsed light L input to the soliton-shifted fiber 6 increases the wavelength of the soliton generated by the soliton self-frequency shift. In this case, they have found that the wavelength tunable range of the ultrashort pulsed light L generated by the pulsed light generating device 1 is expanded. On the other hand, they have found that if the intensity of the ultrashort pulsed light L input to the soliton-shifted fiber 6 is too high, a phenomenon known as multi-soliton generation occurs, resulting in multiple solitons. From a practical standpoint, for example, it is desirable to suppress multi-soliton generation.

[0032] Therefore, the inventors have conducted further intensive research and have found that, as shown in FIG. 7( b), the generation of multi-solitons can be suppressed by widening the spectral width of the ultrashort pulse light L input to the soliton-shifted fiber 6, that is, by broadening the spectrum of the ultrashort pulse light L before modulation using the soliton self-frequency shift is performed.

[0033] Therefore, in the pulsed light generating device 1 and the pulsed light generating method, the spectrum of the ultrashort pulsed light L is broadened by the fiber amplifier 3, and the wavelength of the broadened ultrashort pulsed light L is modulated using the soliton self-frequency shift. This increases the intensity of the ultrashort pulsed light L input to the soliton-shifted fiber 6, efficiently lengthening the wavelength of the soliton, and makes it possible to suppress the generation of multi-solitons while widening the wavelength tunable range.

[0034] In the pulsed light generating device 1 and the pulsed light generating method, the fiber amplifier 3 broadens the spectrum of the ultrashort pulsed light L by similariton amplification. In this case, the fiber amplifier 3 can suppress pulse stretching, thereby effectively achieving modulation using soliton self-frequency shift.

[0035] In the pulsed light generating device 1 and the pulsed light generating method, the fiber amplifier 3 includes a normal dispersion fiber. In this case, the fiber amplifier 3 can specifically achieve broadening of the spectrum of the ultrashort pulsed light L.

[0036] In the pulsed light generating device 1 and the pulsed light generating method, the fiber amplifier 3 includes a double-clad fiber. In this case, for example, an erbium- and ytterbium-co-doped double-clad fiber can be used for the fiber amplifier 3, which makes it possible to effectively increase the output power of the ultrashort pulsed light L before modulation.

[0037] In the pulsed light generating device 1 and the pulsed light generating method, the fiber amplifier 3 sets the spectral width of the ultrashort pulsed light L to 100 nm or more. In this case, it is possible to reliably suppress the generation of multi-solitons.

[0038] In the pulsed light generating device 1 and the pulsed light generating method, the intensity of the ultrashort pulsed light L input to the soliton-shifted fiber 6 is modulated at high speed for each pulse by the acousto-optic modulator 4, thereby making it possible to quickly tune the wavelength of the generated ultrashort pulsed light L for each pulse. Although there is a concern that the use of the acousto-optic modulator 4 may significantly reduce the intensity of the ultrashort pulsed light L, this concern can be alleviated by increasing the output power using the fiber amplifier 3.

[0039] The acousto-optic modulator 4 can modulate the intensity of the ultrashort pulsed light L input to the soliton shift fiber 6 to an intensity that does not cause soliton self-frequency shift. In this case, it is possible to selectively thin out a pulse train of a desired repetition frequency in combination with the filter 7, thereby modulating the repetition frequency. In this case, it is possible to modulate the wavelength of the ultrashort pulsed light L at high speed for each pulse, while simultaneously modulating its repetition frequency.

[0040] The pulsed light generating device 1 and the pulsed light generating method use the compressor 5 to compress the pulse time width of the ultrashort pulsed light L input to the soliton shifted fiber 6. In this case, modulation utilizing the soliton self-frequency shift can be effectively achieved.

[0041] The pulsed light generating device 1 and the pulsed light generating method use a fiber laser, which makes them maintenance-free compared to when a titanium sapphire laser is used. The pulsed light generating device 1 and the pulsed light generating method can freely generate ultrashort pulsed light L with a long wavelength by adjusting the intensity (output) of the ultrashort pulsed light L input to the soliton-shifted fiber 6. Furthermore, because no configuration requiring physically moving parts is used when converting the wavelength, wavelength switching can be performed more quickly compared to when a titanium sapphire laser is used, for example.

[0042] Incidentally, in a technique for modulating the wavelength of ultrashort optical pulses L using a modulator by utilizing soliton self-frequency shift, it is also conceivable to perform wavelength conversion by increasing the intensity of the ultrashort optical pulses L before modulation, thereby splitting the ultrashort optical pulses L into multiple optical pulses with different wavelengths (multi-soliton generation). However, in this case, the above-mentioned problems resulting from multi-soliton generation arise. Furthermore, since the modulation speed is, for example, on the order of several hundred kHz, which is much slower than the repetition frequency (on the order of MHz) of typical ultrashort optical pulses L, it is difficult to perform high-speed wavelength modulation, such as pulse-by-pulse wavelength conversion, on ultrashort optical pulses L.

[0043] Furthermore, in a technique for modulating the wavelength of ultrashort pulsed light L using a modulation section by utilizing the soliton self-frequency shift, it is conceivable to generate solitons with longer center wavelengths by lengthening the fiber length in the modulation section to extend the interaction length of the soliton self-frequency shift. However, in this case, even if the wavelength is lengthened, the center wavelength remains at, for example, 2.03 μm, and the upper limit of the center wavelength when lengthening the wavelength is small. In addition, even if the wavelength is lengthened, the obtained light becomes multi-soliton, which causes the above-mentioned problems caused by the multi-soliton formation.

[0044] In this embodiment, the ultrashort optical pulse L from the fiber amplifier 3, which broadens the spectrum, is modulated by the soliton shift fiber 6. That is, before the soliton shift fiber 6, which uses soliton self-frequency shift for modulation, broadens the bandwidth of the ultrashort optical pulse L and performs dispersion compensation on the broadened ultrashort optical pulse L, thereby suppressing multi-soliton generation and enhancing the effect of soliton self-frequency shift. This makes it possible to generate solitons whose center wavelengths are tunable up to long wavelengths (e.g., 2.2 μm). Furthermore, suppressing multi-soliton generation enables a one-to-one correspondence between the intensity modulation of the ultrashort optical pulse L and the center wavelength of the soliton. As a result, by rapidly modulating the intensity of the ultrashort optical pulse L using an optical intensity control unit (e.g., AOM or EOM) before the soliton shift fiber 6, it is possible to perform wavelength conversion of the ultrashort optical pulse L for each pulse on the order of MHz to GHz, thereby enabling high-speed wavelength modulation.

[0045] As described above, one aspect of the present disclosure is not limited to the above embodiment.

[0046] In the above embodiment, a fiber amplifier using a double-clad normal dispersion fiber is used as the fiber amplifier 3. However, a fiber amplifier using a single-clad normal dispersion fiber (e.g., erbium-doped) may be used instead. Even in this case, it is possible to at least broaden the spectrum of the ultrashort pulsed light L.

[0047] The respective components in the above-described embodiment and modified examples are not limited to the materials and shapes described above, and various materials and shapes can be applied. Furthermore, the respective components in the above-described embodiment and modified examples can be arbitrarily applied to the respective components in other embodiments or modified examples.

[0048] 1...pulse light generating device, 2...oscillator (oscillating section), 3...fiber amplifier (amplifying section), 4...acousto-optic modulator (light intensity control section), 5...compressor (pulse compression section), 6...soliton shift fiber (modulation section), 7...filter, L, LM1, LM2...ultrashort pulse light (pulse light).

Claims

1. A pulsed light generating device comprising: an oscillator that generates pulsed light; an amplifier that broadens the spectrum of the pulsed light generated by the oscillator; and a modulator that modulates the wavelength of the pulsed light whose spectrum has been broadened by the amplifier, by utilizing a soliton self-frequency shift.

2. The pulsed light generating device according to claim 1, wherein the amplifier broadens the spectrum of the pulsed light by similariton amplification.

3. The pulsed light generating device according to claim 1 or 2, wherein the amplifying section includes a normal dispersion fiber.

4. The pulsed light generating device according to claim 1 or 2, wherein the amplifying section includes a double-clad fiber.

5. The pulsed light generating device according to claim 1 or 2, wherein the amplifier section sets the spectral width of the pulsed light to 100 nm or more.

6. A pulsed light generating device as described in claim 1 or 2, further comprising a light intensity control section disposed between the amplifier section and the modulator section in the optical path of the pulsed light, for controlling the intensity of the pulsed light for each pulse.

7. A pulsed light generating device as described in claim 1 or 2, further comprising a pulse compression section arranged between the amplification section and the modulation section in the optical path of the pulsed light, for compressing the time width of the pulse of the pulsed light.

8. A method for generating pulsed light, comprising: an oscillation step of oscillating pulsed light; an amplification step of broadening the spectrum of the pulsed light oscillated in the oscillation step; and a modulation step of modulating the wavelength of the pulsed light whose spectrum has been broadened in the amplification step by utilizing a soliton self-frequency shift.

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