JP2013025284A - Short-pulse light generating device and method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable generation of short-pulse light that has a pulse width of 200 femtosecond or less and has a high repetition frequency.SOLUTION: A short-pulse light generating device comprises: a CW light source 1 having an arbitrary center optical frequency; phase modulation means 2 for modulating a phase of continuous laser light from the CW light source 1; an optical gate 5 for increasing peak power by thinning down an optical pulse from the phase modulation means 2; an optical amplifier 6 for amplifying the optical pulse passing through the optical gate 5, and generating broadband light after enlarging an optical spectral range by a non-linear effect in the optical amplifier 6; and dispersion assignment means 7 for compressing the optical pulse amplified by the optical amplifier 6.

Description

  The present invention relates to a short pulse light generating apparatus and method for generating femtosecond short pulse light at a high repetition frequency.

New laser materials and new pulse generation methods have been introduced, and the technology for shortening light pulses has been remarkably advanced. Today, femto (10 ^{-15 s} ) short pulse lasers that cannot be reached with electrical signals are one of the tools that anyone can use today, such as realizing ultra-fine processing not only in the field of basic science but also in industry. Has contributed to humanity in various ways. In recent years, attention has been focused on increasing the pulse repetition frequency in addition to shortening the pulse. The shortening of the light pulse contributes to the improvement of the time resolution of ultra-high-speed phenomenon measurement, and the increase of the pulse repetition frequency contributes to the increase of the signal-to-noise ratio. The short pulse laser is suitable for observing faint ultrafast phenomena. In addition, high-repetition-frequency short-pulse lasers are also expected to be applied to ultra-high-speed communication systems, precision measurement, low-phase noise microwave generation, optical sampling, carrier envelope offset control optical frequency combs, and the like.

Currently, passively mode-locked lasers with resonator devices are used as femtosecond short-pulse laser light sources. A passively mode-locked laser usually has a gain medium for amplifying light and a resonator having a resonator length of L. The resonance frequency of the longitudinal mode of this resonator is an integral multiple of c / 2L (c is the speed of light). The spectrum width of the laser light oscillated by the laser is very narrow, but if the resonance frequency of the resonator is narrower than the spectrum width, the resonator resonates in a plurality of modes (frequencies). At this time, the laser light is intensified at the pulse repetition frequency f _rep = c / (2L) by aligning the phases of the modes (mode synchronization). Thereby, a pulse laser beam having a pulse repetition frequency f _rep is generated. For this reason, the center optical frequency of the passively mode-locked laser depends on the gain medium installed in the resonator.

  For example, when a titanium sapphire crystal is used as the gain medium, the oscillatable wavelength is from the infrared region of 650-1100 nm to the near infrared region, but the wavelength that can oscillate most efficiently is 800 nm, and the laser center light The frequency is limited. In addition, the passive mode-locked laser cannot shorten the resonator length L due to restrictions such as the spatial arrangement of the gain medium and the optical components installed in the resonator, and the pulse repetition frequency determined by the resonator length L is general. The upper limit is about 1 GHz. Furthermore, the adjustment range of the resonator length L for changing the pulse repetition frequency is limited to the transverse mode stability condition of the optical resonator. Therefore, the variable range of the pulse repetition frequency is also small. Furthermore, the mode-locked laser has a problem that it is easily affected by the external environment.

  Recently, a femtosecond short pulse laser light source using a CW (continuous wave) semiconductor laser as a seed light source and using a Mach-Zehnder modulator and a dispersion reducing fiber has been developed (see Non-Patent Documents 1 and 4). In the short pulse laser light sources disclosed in Non-Patent Documents 1 and 4, since no resonator is used, the pulse repetition frequency is not limited. However, with this short pulse laser light source, it is difficult to generate short pulse light of 200 femtoseconds or less due to the limitation of the optical spectrum band determined by the modulation index of the phase modulator.

  Recently, a femtosecond short pulse laser light source has been proposed in which a CW semiconductor laser is used as a seed light source, and an optical pulse train generated by a phase modulator and an intensity modulator is highly repetitively pulsed by an optical gate (Patent Document 1, Non-Patent Document 1). (See Patent Documents 2 and 3). The short pulse laser light sources disclosed in Patent Document 1 and Non-Patent Documents 2 and 3 do not use the nonlinear effect in the optical amplifier, so that the spectral band after optical amplification does not widen, and optical pulse compression after optical amplification Since a single-mode fiber having a normal core diameter of about 10 μm is used as the means, it is difficult to generate pulsed light shorter than 250 femtoseconds due to the influence of a nonlinear process during pulse compression.

JP 2006-209067 A

I. Morohashi, T. Sakamoto, H. Sotobayashi, T. Kawawanishi, and I. Hosako, “Broadband wavelength-tunable ultrashort pulse source using a Mach-Zehnder modulator and dispersion-flattened dispersion-decreasing fiber”, Opt. Lett., Vol.34, No.15, p.2297-2299, 2009 A.Ishizawa, T.Nishikawa, A.Mizutori, H.Takara, S.Aozasa, A.Mori, H.Nakano, A.Takada, and M.Koga, “Octave-spanning frequency comb generated by 250 fs pulse train emitted from 25 GHz externally phase-modulated laser diode for carrier-envelope-offset-locking ”, Electron.Lett., Vol.46, No.19, p.1343, 2010 T. Kobayashi, H. Yao, K. Amano, Y. Fukushima, A. Morimoto, and T. Sueta, “Optical Pulse Compression Using High-Frequency Electrooptic Phase Modulation”, IEEE J. Quantum Electron., Vol. 24, No .2, p.382-387, 1988 M. Kourogi, B. Widiyatomoko, Y. Takeuchi, and M. Ohtsu, “Limit of Optical-Frequency Comb Generation Due to Material Dispersion”, IEEE J. Quantum Electron., Vol. 31, No. 12, p. 2126, 1995

As described above, among the conventional short-pulse laser light sources based on the passive mode-locked laser having the resonator device, an optical pulse width of 200 femtoseconds or less can be obtained. There is a problem that the variable range of the repetition frequency is limited, and it is difficult to achieve a pulse repetition frequency of 1 GHz or more.
In addition, in a short pulse laser light source that generates a light pulse by frequency-modulating a CW wavelength stabilized light source using a Mach-Zehnder type phase modulator, the pulse repetition frequency can be increased, while the modulation index of the phase modulator is increased. There was a problem that it was difficult to shorten the pulse to 200 femtoseconds or less because of the limitation.
In addition, a pulse repetition frequency can be increased in a short pulse laser light source using a CW semiconductor laser as a seed light source, and an optical pulse train generated by a phase modulator and an intensity modulator using an optical gate. There was a problem that it was difficult to shorten the pulse to less than a second.

  The present invention has been made in order to solve the above-mentioned problems, and has a pulse width of 200 femtoseconds or less for an arbitrary central optical frequency, and further, an order of magnitude compared to a mode-locked laser as a conventional technique. An object of the present invention is to enable generation of short pulse light having a high pulse repetition frequency.

The short pulse light generator of the present invention includes a continuous laser light source having an arbitrary center light frequency, a phase modulation means for phase-modulating the continuous laser light from the continuous laser light source, and thinning out light pulses from the phase modulation means. An optical gate, an optical amplifying unit for amplifying the optical pulse that has passed through the optical gate, and a first dispersion providing unit for compressing the optical pulse amplified by the optical amplifying unit are provided. .
In one configuration example of the short pulse light generator of the present invention, the optical gate is provided in a plurality of stages.
In addition, one configuration example of the short pulse light generation device of the present invention further includes a second dispersion providing unit that compresses the optical pulse from the phase modulation unit between the phase modulation unit and the optical gate. It is characterized by this.
In one configuration example of the short pulse light generator of the present invention, the phase modulation means is provided in a plurality of stages.

In addition, one configuration example of the short-pulse light generating device of the present invention is further provided from the phase modulation unit between the continuous laser light source and the phase modulation unit or between the phase modulation unit and the optical gate. Intensity modulation means for modulating the intensity of the optical pulse is provided.
The short pulse light generator of the present invention further includes a reference frequency generator for supplying a signal for modulating the continuous laser light to the phase modulator, and a signal output from the reference frequency generator. And frequency dividing means for supplying a signal for driving the optical gate to the optical gate.
Further, in the configuration example of the short pulse light generation device of the present invention, reference frequency generation means for supplying a signal for driving the optical gate to the optical gate, and a signal output from the reference frequency generation means are further multiplied. Frequency multiplying means for supplying a signal for optical pulse modulation to the phase modulating means.

  Further, the short pulse light generation method of the present invention includes a phase modulation step for phase-modulating continuous laser light from a continuous laser light source having an arbitrary center optical frequency, and an optical gate for thinning out the optical pulses obtained in this phase modulation step. And an optical amplification step for amplifying the optical pulse obtained in the optical gate step by an optical amplification means, and a pulse compression step for compressing the optical pulse obtained in the optical amplification step. It is.

  According to the present invention, an optical pulse train is generated by using phase modulation means for phase-modulating continuous laser light from a continuous laser light source, and this optical pulse train is thinned out by an optical gate to increase the optical peak power. The femtosecond short pulse laser that has a very simple optical configuration, excellent long-term stability, and is portable in a compact size by expanding the optical spectrum band by the effect and finally compressing the pulse with a dispersion medium. An extremely excellent effect that a light source can be realized is obtained. In the conventional technique, the upper limit of the pulse repetition frequency is about 1 GHz. However, if the present invention is used, a femtosecond short pulse laser light source having a pulse repetition frequency higher by one digit or more than the conventional technique can be realized. .

  In the present invention, by providing a plurality of optical gates, it is possible to generate pulsed light having a shorter pulse width.

  Further, in the present invention, the light transmission loss of the optical gate can be reduced by providing the second dispersion providing unit for compressing the optical pulse from the phase modulating unit between the phase modulating unit and the optical gate. .

  In the present invention, it is possible to generate pulsed light having a shorter pulse width by providing a plurality of stages of phase modulation means.

  Further, in the present invention, pulse compression is provided by providing an intensity modulation means for intensity-modulating the optical pulse from the phase modulation means between the continuous laser light source and the phase modulation means or between the phase modulation means and the optical gate. Can contribute to pedestal suppression.

  In the present invention, the reference frequency generating means for supplying the signal for modulating the continuous laser light to the phase modulating means, and the optical gate driving signal obtained by dividing the signal output from the reference frequency generating means for the optical gate. By providing the frequency dividing means to be supplied, the phase modulation means and the optical gate can be completely synchronized, and the stability of the intensity of the optical pulse passing through the optical gate can be improved.

  In the present invention, the reference frequency generating means for supplying a signal for driving the optical gate to the optical gate, and the signal for optical pulse modulation obtained by multiplying the signal output from the reference frequency generating means are supplied to the phase modulating means. By providing the frequency multiplication means, the phase modulation means and the optical gate can be completely synchronized, and the stability of the intensity of the optical pulse passing through the optical gate can be improved.

It is a block diagram which shows an example of a structure of the short pulse light generator which concerns on the 1st Embodiment of this invention. It is a figure which shows the optical pulse waveform before the optical gate in the 1st Embodiment of this invention, and the optical pulse waveform after an optical gate. The figure which shows the optical spectrum after passing through a dispersion | distribution provision means in the structure except the optical gate from the short pulse light generation apparatus which concerns on the 1st Embodiment of this invention, and an autocorrelation waveform, and 1st Embodiment of this invention It is a figure which shows the optical spectrum after passing through the dispersion | distribution provision means in the form of, and an autocorrelation waveform. It is a block diagram which shows an example of a structure of the short pulse light generator which concerns on the 2nd Embodiment of this invention. The figure which shows the optical spectrum after passing through a dispersion | distribution provision means in the structure except the optical gate from the short pulse light generation apparatus which concerns on the 2nd Embodiment of this invention, and an autocorrelation waveform, and 2nd Embodiment of this invention It is a figure which shows the optical spectrum after passing through the dispersion | distribution provision means in the form of, and an autocorrelation waveform. It is a block diagram which shows an example of a structure of the short pulse light generator which concerns on the 3rd Embodiment of this invention. It is a block diagram which shows an example of a structure of the short pulse light generator which concerns on the 4th Embodiment of this invention. It is a block diagram which shows an example of a structure of the short pulse light generator which concerns on the 5th Embodiment of this invention. It is a block diagram which shows an example of a structure of the short pulse light generator which concerns on the 6th Embodiment of this invention. It is a block diagram which shows an example of a structure of the short pulse light generator which concerns on the 7th Embodiment of this invention.

[Principle of the Invention]
In order to achieve the above object, the present invention achieves the above-described object by phase modulation step (step S1) for phase-modulating continuous laser light from a continuous laser light source having an arbitrary center optical frequency, and thinning out the optical pulses obtained in the phase modulation step The optical gate step (step S2) for increasing the optical peak power and the optical pulse obtained by the optical gate step are amplified by the optical amplifier, and the broadband light whose optical spectrum band is expanded by the nonlinear effect in the optical amplifier is generated. It is characterized by comprising an optical amplification step (step S3) and a pulse compression step (step S4) for compressing the optical pulse using a dispersion medium capable of suppressing the occurrence of nonlinear effects during pulse compression.

  The present invention is also characterized by a dispersion compensation method. In the conventional pulse compression method, positively dispersed light whose spectral band is expanded by the self-phase modulation effect is compressed by a dispersion shift fiber or a diffraction grating pair having negative dispersion. In contrast, in the present invention, a single mode fiber or glass block having a positive dispersion is obtained by expanding the spectral band of the negative dispersion generated by the phase modulator by a nonlinear effect in an EDFA (Erbium Doped Fiber Amplifier). Compressed.

[First Embodiment]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an example of the configuration of the short pulse light generator according to the first embodiment of the present invention.
This short pulse light generator is a short pulse light source capable of controlling an arbitrary center light frequency and pulse repetition frequency, and phase modulation that performs phase modulation of the CW light source 1 and the CW light from the CW light source 1 with a frequency f _rep. Means 2, a microwave reference frequency generator 3 for generating a signal of repetition frequency f _rep , an optical gate microwave reference frequency generator 4 for generating a signal of repetition frequency f _mod , and light from the phase modulation means 2 An optical gate 5 that increases the peak power by thinning out pulses at regular intervals, an optical amplifier 6 that amplifies the optical pulse that has passed through the optical gate 5, and a dispersion applying means 7 that compresses the optical pulse amplified by the optical amplifier 6 (for example, , Glass block, optical fiber having a large mode field diameter, etc.) and the repetition frequency f _mod output from the optical gate microwave reference frequency generator 4 And an optical gate impulse generator 14 for generating an impulse signal from the above signals.

Hereinafter, step S1 will be described using an example. Here, the CW light source 1 is a CW semiconductor laser that generates CW light. The center light wavelength of the CW light generated by the CW light source 1 can be arbitrarily selected. Further, a tunable CW light source can be used as the CW light source 1. The phase modulation unit 2 modulates the phase of the CW light from the CW light source 1 according to the signal of the repetition frequency f _rep output from the microwave reference frequency generator 3. Thus, up-chirp and down-chirp can be generated periodically.

  As a pulse light source part composed of the CW light source 1 and the phase modulation means 2, a configuration in which an electro-optic modulator is installed in a Fabry-Perot resonator as disclosed in Non-Patent Document 4 is used, and deep frequency modulation is performed using CW light. , FM sidebands may be generated. In the case of this configuration, the signal output from the microwave reference frequency generator 3 is supplied to the electro-optic modulator. Further, the phase modulation means 2 may be configured by a Mach-Zehnder type phase modulator disclosed in Non-Patent Document 1 and a dispersion reducing fiber. In the case of this configuration, the signal output from the microwave reference frequency generator 3 is supplied to the Mach-Zehnder type phase modulator. In the method of generating an optical pulse train using these phase modulation means 2, generation of an optical pulse train having a maximum pulse repetition frequency of 100 GHz can be realized because the resonator length is not limited.

  Next, step S2 will be described using an example. Since the optical pulse train generated by the phase modulation means 2 has a high pulse repetition frequency and an ASE (Amplified Spontaneous Emission) component superimposed thereon, a wide spectrum due to a nonlinear effect such as a self-phase modulation effect in the optical amplifier. A peak power sufficient to generate band light cannot be obtained. Therefore, in the present embodiment, the optical gate 5 is used to reduce the pulse repetition frequency (duty ratio is reduced), and the optical pulse that has passed through the optical gate 5 is optically amplified to about 1 to 2 W by the optical amplifier 6. As a result, non-linear effects such as a self-phase modulation effect are caused inside the optical amplifier 6 to widen the optical spectrum band and realize generation of a wide spectrum band light necessary for generating a short pulse light having a pulse width of 200 femtoseconds or less. doing.

The optical gate impulse generator 14 generates an impulse signal having a gate frequency (repetition frequency) f _mod in order to drive the optical gate 5. The optical gate 5 operates so as to pass or block the optical pulse from the phase modulation means 2 in accordance with the impulse signal, and passes the optical pulse every time the impulse signal is input. Examples of the optical gate 5 include an intensity modulator, a Fabry-Perot interferometer, and a filter.

In order to generate short pulse light having a pulse width of 200 femtoseconds or less, it is necessary to amplify the energy per pulse to 0.2 nJ or more by the optical amplifier 6. The optical pulse outputted from the phase modulation means 2 and before being inputted to the optical gate 5 has a pulse repetition frequency of fHz (f = f _rep , f _rep is several GHz to several tens GHz, for example) as shown in FIG. And a pulse waveform with a duty ratio of pulse width × f. On the other hand, the optical pulse that has passed through the optical gate 5 has a pulse repetition frequency of (f / n) Hz (f / n = f _rep / n = f _mod , where f _mod is several MHz, for example, as shown in FIG. (1 GHz) and a pulse waveform with a duty ratio of pulse width × f / n (n is a positive integer). By reducing the pulse repetition frequency of the optical pulse in step S2, the effect of expanding the optical spectrum band can be obtained due to the influence of the non-linear effect in the optical amplifier 6 at the subsequent stage.

  Next, step S3 will be described using an example. The optical amplifier 6 amplifies the optical pulse that has passed through the optical gate 5. At this time, the optical spectrum bandwidth is gradually expanded while the laser peak power is increased in the optical amplifier 6. By performing optical amplification of the optical pulse train in this step S3, a wide optical spectrum necessary for shortening the pulse is output due to the nonlinear effect occurring in the optical amplifier 6.

Next, step S4 will be described using an example. The dispersion imparting means 7 made of a dispersion medium compresses the optical pulse amplified by the optical amplifier 6 to a pulse width of 200 femtoseconds or less. By using a medium that can suppress the occurrence of nonlinear effects during pulse compression as the dispersion medium, it is possible to perform pulse compression without causing nonlinear effects when an optical pulse propagates through the dispersion medium. Specifically, any dispersion medium in which the peak power of the propagating laser beam is 10 ¹⁰ W / cm ² or less may be used. Examples of the dispersion medium include a glass block, an optical fiber having a large mode field diameter, and a diffraction grating.

  At this time, the microwave reference frequency generator 3 for optical pulse train generation and the microwave reference frequency generator 4 for optical gate are synchronized. Further, the phase adjustment between the microwave reference frequency generator 3 and the optical gate microwave reference frequency generator 4 is performed by a phase adjuster provided in the microwave reference frequency generators 3 and 4 or a microwave reference frequency generator. This is performed using a phase adjuster provided in the subsequent stage of the third and fourth stages.

FIG. 3A is a diagram showing an optical spectrum after passing through the dispersion providing means 7 in the configuration in which the optical gate 5 is removed from the short pulse light generator of the present embodiment shown in FIG. 1, and FIG. FIG. 3 is a diagram showing an autocorrelation waveform after passing through the dispersion applying unit 7 in a configuration excluding the optical gate 5, and FIG. 3C is a diagram showing an optical spectrum after passing through the dispersion applying unit 7 in the present embodiment. FIG. 3D is a diagram showing an autocorrelation waveform after passing through the dispersion imparting means 7 in the present embodiment. Here, in the configuration in which the optical gate 5 is removed from the short pulse light generator of the present embodiment, the repetition frequency of the optical pulse train after passing through the dispersion providing means 7 is 25 GHz (that is, f _rep = 25 GHz). On the other hand, the repetition frequency of the optical pulse train after passing through the dispersion providing means 7 in the short pulse light generator of the present embodiment is 1 GHz. According to FIGS. 3C and 3D, it can be seen that the pulse width is also compressed as the optical spectrum band is expanded. In the present embodiment, the pulse width can be compressed to 163 femtoseconds by performing pulse compression using a dispersion medium that does not cause a nonlinear effect in step S4.

  By executing Steps S1 to S4 described above, short pulse light with a pulse width of 200 femtoseconds or less can be generated using the CW light source as a seed light source. As a result, in the present embodiment, it is possible to generate a short pulse light with a pulse width of 200 femtoseconds or less for an arbitrary center optical frequency, and more than one digit compared with the mode-locked laser which is the prior art. A light source having a high pulse repetition frequency and a variable pulse repetition frequency over a wide range can be realized.

[Second Embodiment]
Next, a second embodiment of the present invention will be described. FIG. 4 is a block diagram showing an example of the configuration of the short pulse light generator according to the second embodiment of the present invention.
The short pulse light generator of the present embodiment is a short pulse light source capable of controlling an arbitrary center light frequency and pulse repetition frequency, and phase modulation that performs phase modulation on the CW light source 1 and the CW light from the CW light source 1. a means 2, the repetition frequency f microwave reference frequency generator 3 for generating a signal _rep the repetition frequency f _rep / n (n is a positive integer) microwave reference frequency generator for generating a signal to the optical gate 4 And an optical gate 5 for thinning out the optical pulses from the phase modulation means 2 at regular intervals, an optical amplifier 6 for amplifying the optical pulses that have passed through the optical gate 5, and dispersion application for compressing the optical pulses amplified by the optical amplifier 6 a means 7, the optical gate 10 for thinning the optical pulse that has passed through the optical gate 5 at regular intervals, the repetition frequency _{f rep / n / m (n} , m are positive integers) micro for optical gate for generating a signal A reference frequency generator 13, the repetition frequency f _rep / n optical gate for impulse generator 14 for generating an impulse signal from the signal output from the optical gate for microwave reference frequency generator 4, an optical gate microwave criteria The optical gate impulse generator 15 generates an impulse signal from a signal having a repetition frequency f _rep / n / m output from the frequency generator 13.

  In this embodiment, a new optical gate 10 is added between the optical gate 5 and the optical amplifier 6 of the first embodiment, and the optical gate microwave reference frequency generator is used to drive the optical gate 10. 13 is an example of a multistage optical gate in which an optical gate impulse generator 15 is added. When the extinction ratio of the optical gate is not high, it is effective to use a plurality of optical gates as in this embodiment. For example, in the configuration using one optical gate as in the first embodiment, the ASE component cannot be sufficiently suppressed, and even if the pulse repetition frequency is reduced, pulse light shorter than 163 femtoseconds is generated. The required peak power cannot be obtained (see FIGS. 3C and 3D).

Therefore, in order to obtain sufficient peak power, it is effective to arrange two or more stages of optical gates.
The optical gate impulse generator 14 generates an impulse signal having a gate frequency (repetition frequency) f _rep / n (n is a positive integer) in order to drive the optical gate 5. The optical gate 5 operates to pass or block the optical pulse from the phase modulation means 2 in accordance with the impulse signal having the gate frequency f _rep / n, and allows the optical pulse to pass every time the impulse signal is input. .

The optical gate impulse generator 15 generates an impulse signal having a gate frequency f _rep / n / m (n and m are positive integers) in order to drive the optical gate 10. The optical gate 10 operates to pass or block the optical pulse from the gate 5 in accordance with the impulse signal having the gate frequency f _rep / n / m, and allows the optical pulse to pass every time the impulse signal is input. .
If the ASE component cannot be sufficiently suppressed by the second-stage optical gate 10, the extinction ratio of the first-stage optical gate 5 operating at a frequency (f _rep / n) multiplied by the second-stage gate frequency is _increased . It is also effective to use a high optical gate.

In the present embodiment, the gate frequency of the first-stage optical gate 5 is 1 GHz (= f _rep / 25 GHz), and the gate frequency of the second-stage optical gate 10 is also 1 GHz (= f _rep / _25/1 GHz). .
As in the first embodiment, the optical pulse that has passed through the optical gate 10 is optically amplified to about 1 to 2 W by the optical amplifier 6, so that a nonlinear effect such as a self-phase modulation effect is generated inside the optical amplifier 6. Thus, the optical spectrum band is expanded, and the optical pulse amplified by the optical amplifier 6 is pulse-compressed by the dispersion applying means 7 to a pulse width of 200 femtoseconds or less. As in the first embodiment, the microwave reference frequency generator 3 for generating the optical pulse train and the microwave reference frequency generators 4 and 13 for the optical gate are synchronized.

FIG. 5A is a diagram showing an optical spectrum after passing through the dispersion imparting means 7 in the configuration in which the optical gates 5 and 10 are removed from the short pulse light generator of the present embodiment shown in FIG. FIG. 5B is a diagram showing an autocorrelation waveform after passing through the dispersion imparting means 7 in the configuration excluding the optical gates 5 and 10, and FIG. 5C is a diagram showing light after passing through the dispersion imparting means 7 in the present embodiment. FIG. 5D is a diagram showing a spectrum, and FIG. 5D is a diagram showing an autocorrelation waveform after passing through the dispersion imparting means 7 in the present embodiment. Here, the repetition frequency of the optical pulse train after passing through the dispersion providing means 7 in the configuration in which the optical gates 5 and 10 are removed from the short pulse light generator of the present embodiment is 25 GHz (that is, f _rep = 25 GHz). . On the other hand, the repetition frequency of the optical pulse train after passing through the dispersion providing means 7 in the short pulse light generator of the present embodiment is 1 GHz.

  By using a two-stage optical gate as in the present embodiment, the pulse peak intensity is increased and the optical spectrum band is expanded in the optical amplifier 6. According to FIGS. 5C and 5D, it can be seen that the pulse width is also compressed as the optical spectrum band is expanded. In this embodiment, the pulse width can be compressed to 90 femtoseconds.

When the optical gate frequency is further reduced, the peak power is further increased, and pulse light with a shorter pulse width can be generated. For example, the gate frequency of the first-stage optical gate 5 is 1 GHz (= f _rep / 25 GHz), and the gate frequency of the second-stage optical gate 10 is 250 MHz (= f _rep / 25/4 GHz). At this time, when the optical pulse that has passed through the optical gate 10 is amplified by the optical amplifier 6 and the optical pulse amplified by the optical amplifier 6 is pulse-compressed by the dispersion applying means 7, the pulse width is compressed to 70 femtoseconds.
As described above, in this embodiment, it is possible to generate pulsed light having a shorter pulse width than that in the first embodiment.

[Third Embodiment]
Next, a third embodiment of the present invention will be described. FIG. 6 is a block diagram showing an example of the configuration of a short pulse light generator according to the third embodiment of the present invention. The same components as those in FIG. 1 are denoted by the same reference numerals.
The short pulse light generator of the present embodiment is a short pulse light source capable of controlling an arbitrary center light frequency and pulse repetition frequency, and includes a CW light source 1, a phase modulation means 2, and a microwave reference frequency generator 3. The optical pulse from the optical gate microwave reference frequency generator 4, optical gate 5, optical amplifier 6, dispersion applying means 7, optical gate impulse generator 14, and phase modulation means 2. It is comprised from the dispersion | distribution provision means 11. FIG.

The present embodiment is an example in which a dispersion providing unit 11 is inserted between the phase modulation unit 2 and the optical gate 5 of the first embodiment. When the pulse width of the optical pulse having the repetition frequency f _rep output from the phase modulation means 2 is larger than the optical gate time width (time width when the optical gate 5 is turned on), light transmission loss occurs. Therefore, as in the present embodiment, the dispersion providing means 11 is inserted in front of the optical gate 5, and the pulse width of the optical pulse from the phase modulation means 2 is set to be equal to or smaller than the optical gate time width. Transmission loss can be reduced. As the dispersion imparting means 11, an optical fiber, fiber grating, a planar lightwave circuit, or the like can be used.

[Fourth Embodiment]
Next, a fourth embodiment of the present invention will be described. FIG. 7 is a block diagram showing an example of the configuration of a short pulse light generator according to the fourth embodiment of the present invention. The same components as those in FIG. 1 are denoted by the same reference numerals.
The short pulse light generator of this embodiment is a short pulse light source capable of controlling an arbitrary center light frequency and pulse repetition frequency, and includes a CW light source 1, a plurality of phase modulation means 2, and a microwave reference frequency generator. Output from the reference 3, the microwave reference frequency generator 4 for the optical gate, the optical gate 5, the optical amplifier 6, the dispersion applying means 7, the impulse generator 14 for the optical gate, and the microwave reference frequency generator 3. The phase adjusters 16 and 17 adjust the phase of the signal having the repetition frequency f _rep .

The present embodiment is an example in which the phase modulation means 2 of the first embodiment is configured in multiple stages. The operation of each phase modulation means 2 is the same as in the first embodiment. The phase adjuster 16 adjusts the phase of the signal of the repetition frequency f _rep output from the microwave reference frequency generator 3 so that the second phase modulation means 2 is synchronized with the first phase modulation means 2. Adjust the timing. Similarly, the phase adjuster 17 adjusts the phase of the signal of the repetition frequency f _rep output from the microwave reference frequency generator 3, so that the final phase modulation means 2 is synchronized with the initial phase modulation means 2. Adjust the timing so that

Thus, in the present embodiment, the modulation index can be made larger than that of the short pulse laser light source disclosed in Non-Patent Document 2 by using the multi-phase synchronized phase modulation means 2 synchronized with each other. As compared with the first embodiment and the short pulse laser light source disclosed in Non-Patent Document 2, pulse light having a shorter pulse width can be generated. Since the configuration using the multistage synchronous phase modulation means 2 contributes to the expansion of the optical spectrum bandwidth in the optical amplifier 6, it is an effective means for generating short pulse light.
Needless to say, this embodiment may be combined with the second embodiment.

[Fifth Embodiment]
Next, a fifth embodiment of the present invention will be described. FIG. 8 is a block diagram showing an example of the configuration of a short pulse light generator according to the fifth embodiment of the present invention. The same components as those in FIG. 1 are denoted by the same reference numerals.
The short pulse light generator of the present embodiment is a short pulse light source capable of controlling an arbitrary center light frequency and pulse repetition frequency, and includes a CW light source 1, a phase modulation means 2, and a microwave reference frequency generator 3. And an optical gate microwave reference frequency generator 4, an optical gate 5, an optical amplifier 6, a dispersion applying unit 7, an optical gate impulse generator 14, and an intensity modulation of an optical pulse from the phase modulation unit 2. And intensity modulating means 12 to be configured.

  The present embodiment is an example in which the intensity modulation means 12 is inserted between the phase modulation means 2 and the optical gate 5 of the first embodiment. As described above, in the present embodiment, by arranging the intensity modulation means 12, the non-linear chirp portion generated in the phase modulation means 2 can be removed, which can contribute to pedestal suppression when pulse compression is performed.

[Sixth Embodiment]
Next, a sixth embodiment of the present invention will be described. FIG. 9 is a block diagram showing an example of the configuration of the short pulse light generator according to the sixth embodiment of the present invention. The same components as those in FIG. 1 are denoted by the same reference numerals.
The short pulse light generator of the present embodiment includes a CW light source 1, a phase modulation means 2, a microwave reference frequency generator 3, an optical gate 5, an optical amplifier 6, a dispersion providing means 7, an optical gate. Impulse generator 14, frequency divider 9 that _divides the signal of repetition frequency f _rep output from microwave reference frequency generator 3 into 1 / n, and repetition frequency output from frequency divider 9 and a phase adjuster 18 for adjusting the phase of the signal f _rep / n.

In this embodiment, the two microwave reference frequency generators 3 and 4 used in the first to fifth embodiments can be simplified to one, and the microwave reference frequency generator for optical gates is used. Instead of 4, a frequency divider 9 and a phase adjuster 18 are used.
The phase adjuster 18 adjusts the timing so that the optical gate 5 is synchronized with the phase modulation means 2 by adjusting the phase of the signal of the repetition frequency f _rep / n output from the frequency divider 9.

  In this embodiment, since the common microwave reference frequency generator is used for the phase modulation means 2 and the optical gate 5, the phase modulation means 2 and the optical gate 5 can be completely synchronized, and pass through the optical gate 5. The effect that the stability of the intensity of the light pulse to be improved can be expected.

When a multi-stage optical gate is used as in the second embodiment, in addition to the configuration shown in FIG. 9, the microwave reference frequency generator is used instead of the optical gate microwave reference frequency generator 13. A frequency divider that _divides the signal of the repetition frequency f _rep output from the frequency divider 3 into 1 / n / m, and the phase of the signal of the repetition frequency f _rep / n / m output from the frequency divider. By adjusting, a phase adjuster that adjusts timing so that the optical gate 10 is synchronized with the phase modulation means 2 may be used.

[Seventh Embodiment]
Next, a seventh embodiment of the present invention will be described. FIG. 10 is a block diagram showing an example of the configuration of the short pulse light generator according to the seventh embodiment of the present invention. The same components as those in FIG. 1 are denoted by the same reference numerals.
The short pulse light generator of the present embodiment includes a CW light source 1, a phase modulation unit 2, an optical gate microwave reference frequency generator 4, an optical gate 5, an optical amplifier 6, and a dispersion applying unit 7. , An optical gate impulse generator 14, a frequency multiplier 8 that multiplies the frequency of the signal of the repetition frequency f _rep / n output from the optical gate microwave reference frequency generator 4, and an output from the frequency multiplier 8. And a phase adjuster 19 for adjusting the phase of the signal having the repetition frequency f _rep .

In the present embodiment, the two microwave reference frequency generators 3 and 4 used in the first to fifth embodiments can be simplified to one, and instead of the microwave reference frequency generator 3. In addition, a frequency multiplier 8 and a phase adjuster 19 are used.
The phase adjuster 19 adjusts the timing so that the phase modulation means 2 is synchronized with the optical gate 5 by adjusting the phase of the signal of the repetition frequency f _rep output from the frequency multiplier 8.

  In this embodiment, since the common microwave reference frequency generator is used for the phase modulation means 2 and the optical gate 5, the phase modulation means 2 and the optical gate 5 can be completely synchronized, and pass through the optical gate 5. The effect that the stability of the intensity of the light pulse to be improved can be expected.

In place of the frequency multiplier 8, a comb generator and an RF band pass filter may be used.
When a multistage optical gate is used as in the second embodiment, in addition to the configuration shown in FIG. 10, an optical gate microwave is used instead of the optical gate microwave reference frequency generator 13. A frequency divider that _divides the signal of the repetition frequency f _rep / n output from the reference frequency generator 4 into 1 / m, and a signal of the repetition frequency f _rep / n / m output from this frequency divider. It is sufficient to use a phase adjuster that adjusts the timing so that the optical gate 10 is synchronized with the phase modulation means 2 by adjusting the phase.

  As described above, the short-pulse light generators of the first to seventh embodiments have arbitrary center optical frequency and pulse repetition frequency, and it is also possible to continuously vary the center optical frequency and pulse repetition frequency. . In the first to seventh embodiments, the generation of short pulse light having a pulse repetition frequency higher than that of the prior art by one digit or more can be realized with a very simplified optical configuration using a CW light source as a seed light source. It contributes more than ever to the development of ultra-precision machining using ultra-short pulse light, ultra-high-speed optical communication, spectroscopy of ultra-high-speed phenomena, cell manipulation and DNA observation, and the present invention is superior to the prior art. I understand that.

  As mentioned above, although the example of the short pulse light generator of this invention was explained in full detail with reference to drawings, the concrete structure is not restricted to the 1st-7th embodiment, The 1st-7th Embodiments may be combined as appropriate, and changes in design and the like within the scope of the present invention are also included in the present invention.

  The present invention can be applied to a technique for generating a short pulse light of femtosecond order at a high repetition frequency of GHz order.

  DESCRIPTION OF SYMBOLS 1 ... CW light source, 2 ... Phase modulation means, 3 ... Microwave reference frequency generator, 4, 13 ... Microwave reference frequency generator for optical gates, 5, 10 ... Optical gate, 6 ... Optical amplifier, 7, 11 ... Dispersion imparting means, 8... Frequency multiplier, 9... Frequency divider, 12... Intensity modulating means, 14 and 15 .. impulse generator for optical gate, 16, 17, 18, 19.

Claims (8)

A continuous laser source with an arbitrary center optical frequency;
Phase modulation means for phase modulating the continuous laser light from the continuous laser light source;
An optical gate for thinning out the optical pulse from the phase modulation means;
An optical amplifying means for amplifying the optical pulse that has passed through the optical gate;
A short pulse light generator comprising: first dispersion imparting means for compressing the light pulse amplified by the light amplifying means. In the short pulse light generator of Claim 1,
The optical pulse generation device according to claim 1, wherein the optical gate is provided in a plurality of stages. In the short pulse light generator of Claim 1 or 2,
The short pulse light generating device further comprising a second dispersion providing unit for compressing the optical pulse from the phase modulating unit between the phase modulating unit and the optical gate. In the short pulse light generator according to any one of claims 1 to 3,
A short-pulse light generating apparatus, wherein the phase modulation means is provided in a plurality of stages. The short pulse light generator according to any one of claims 1 to 4,
And an intensity modulation means for intensity-modulating an optical pulse from the phase modulation means between the continuous laser light source and the phase modulation means or between the phase modulation means and the optical gate. Short pulse light generator. In the short pulse light generation device according to any one of claims 1 to 5,
Further, a reference frequency generating means for supplying a signal for modulating the continuous laser light to the phase modulating means,
A short pulse light generator comprising: frequency dividing means for supplying an optical gate driving signal obtained by dividing the signal output from the reference frequency generating means to the optical gate. In the short pulse light generation device according to any one of claims 1 to 5,
Further, a reference frequency generating means for supplying a signal for driving the optical gate to the optical gate,
A short pulse light generator comprising: frequency multiplying means for supplying a signal for optical pulse modulation obtained by multiplying the signal output from the reference frequency generating means to the phase modulating means. A phase modulation step for phase-modulating continuous laser light from a continuous laser light source having an arbitrary center optical frequency;
An optical gate step for thinning out the optical pulses obtained in this phase modulation step;
An optical amplification step of amplifying the optical pulse obtained in this optical gate step by an optical amplification means;
And a pulse compression step of compressing the optical pulse obtained in the optical amplification step.

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