JP2001060734A - Ultrashort pulse wide band light wave generating method and device therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a wide band output light spectrum by increasing a non-linear effect in a non-linear medium. SOLUTION: This ultrashort pulse wide band light wave generating device is provided with an ultrashort pulse laser beam generating device 1 for repeatedly outputting plural ultrashort light pulses in the same cycle which are pico-seconds or less, an optical fiber 3 through which an ultrashort light pulse column 1A emitted from the ultrashort pulse laser beam generating device 1 is transmitted, a phase compensating light compressing device 4 for inputting the wide band wavelength of the laser beams transmitted through the optical fiber 3, and for chirp (speed dispersion by wavelength) compensating the wide band laser beams, and a feedback optical path having feedback delay optical paths 7-10 through which compressed lights with shorter light pulses than the ultrashort light pulse column 1A are transmitted. Then, the compressed lights are returned through the feedback optical path to the emitted pulse optical path, and overlapped with the ultrashort light pulse column 1A, and transmitted through the optical fiber 3 and the phase compensating compressing device 4. This processing is repeated at least once so that the wide band and short pulse wavelength can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for generating ultrashort pulse broadband light waves.

[0002]

2. Description of the Related Art Conventionally, techniques in such a field include:
For example, there is one disclosed in the following literature.

[1] A. Baltuska et a
l. , Opt. Lett. 22, 102 (1997). [2] D. Meshulach et al. , Op
t. Commun. 138, 345 (1997). [3] P.I. A. Apanasevich et a
l. , J. et al. Modern Opt. 38, 151 (199
1). The above document [1] discloses an apparatus in which an ultrashort light pulse is passed through an optical fiber once to generate a broadband light wave by a self-phase modulation effect, which is one of the second-order nonlinear effects.

In this technique, after passing through an optical fiber, a dispersion compensating / chirp compensating light such as a diffraction grating pair (reference 1), a prism pair (reference 1), a chirp mirror (reference 1), or a spatial phase modulator (reference 2) is used. Ultra-short light pulses can be generated using a compression device, but in order to obtain a shorter pulse, a wider band of light must be generated. It is necessary to increase the light pulse energy divided by the light pulse time width). However, there is a damage threshold in the optical fiber, and there is a limit in increasing the spire intensity of the input pulse.

[0005] A method in which an optical pulse is passed through a non-linear medium other than an optical fiber (for example, argon gas) and then fed back to the incident side is performed in a broadband light wave generation method such as stimulated Raman scattering or stimulated Brillouin scattering (see above). Reference 3). However, in this case, since the light pulse width used is on the order of nanoseconds to microseconds, the first half of the light pulse is fed back to the non-linear action of the second half. That is, feedback is performed within one pulse, and the nonlinear action is enhanced. This is different from the superposition of the pulse train according to the present invention.

[0006] The wide frequency band light wave generator mentioned here is also a very short light pulse generator. This is because there is a Fourier transform relationship that the product of the frequency band and the light pulse time width is constant.

FIG. 6 is a schematic diagram of a conventional ultrashort pulse broadband lightwave generation system.

In FIG. 1, an ultrashort optical pulse train 101 is transmitted to an optical fiber 102, and an output from the optical fiber 102 is sent to a chirp compensating optical system 103. And a pulse width measurement system 104.

[0009]

However, in the above-mentioned prior art, in order to broaden the optical pulse in the optical fiber, it is necessary to increase the spire intensity of the optical fiber input light, but there is a problem that there is a limit due to optical damage. It is.

Therefore, if the effect of broadening the band can be enhanced even with the same input spire intensity, light waves can be generated in a wider band, and furthermore, shorter light pulses can be generated, greatly expanding the field of industrial application. By superimposing the output light of the optical fiber with the pulse output from the laser in the next cycle and entering the optical fiber again, the nonlinear effect is enhanced even with the same input spire intensity, and the optical pulse of a wider band compared to the prior art is obtained. Can be generated.

In the prior art, a system has been adopted in which an ultrashort optical pulse is passed only once through an optical fiber to broaden the band by the self-phase modulation effect (the above-mentioned document 1). No technique has been reported to enhance this effect.

As a method of enhancing the nonlinear effect, a technique of enhancing the nonlinear effect by feeding back the first half of a single pulse to the latter half has been reported by Raman scattering or Brillouin scattering (see the above-mentioned document 2). , A pulse train of light pulses that is short in time is not used. After using a short optical pulse (10 ^-15 seconds, femtosecond) and the first pulse is output from the optical fiber, the time width is compressed by the phase compensator and output from the laser device in the next period. At present, there is no report on increasing the nonlinear effect by superimposing the applied pulses.

An object of the present invention is to provide an ultrashort-pulse broadband lightwave generation method and apparatus capable of eliminating the above-mentioned problems, increasing the nonlinear effect in a nonlinear medium, and broadening the output light spectrum. And

[0014]

According to the present invention, there is provided a method for generating an ultrashort pulse broadband light wave, comprising the steps of: outputting an ultrashort pulse of picoseconds or less in which pulses are repeatedly output at the same period. By passing the emitted pulse light emitted from the laser light generator through a nonlinear medium, the wavelength of the laser light is broadened, and the broadband laser light is chirp-compensated (speed dispersion by wavelength) by a phase compensating light compressor. Compressed light shorter than the emitted pulse light, through a feedback optical path having a feedback delay optical path, returned to the emitted pulse optical path, superimposed on the emitted pulse train, passing through the nonlinear medium and the phase compensation compression device. By repeating one or more times, broadening the wavelength band,
And shortening the pulse.

[2] In the method for generating an ultrashort pulse broadband light wave according to [1], when the compressed light is superimposed on the emitted pulse light, the emitted pulse is a pulse next to the emitted pulse forming the compressed light. There is a feature.

[3] In the ultrashort-pulse broadband lightwave generator, an ultrashort-pulse laser light generator of sub-picosecond or less in which pulses are repeatedly output at the same period, and the light emitted from the ultrashort-pulse laser light generator. By passing the output pulse light through a nonlinear medium and through this nonlinear medium,
A phase compensation light compression device for broadening the wavelength of the laser light and compensating for chirp (velocity dispersion by wavelength) of the broadband laser light; and a compressed light shorter than the output pulse light by the phase compensation light compression device. A feedback optical path having a feedback delay optical path through which the compressed light passes through the feedback optical path, returns to the output pulse optical path, overlaps the output pulse train, and passes through the nonlinear medium and the phase compensation compression device once. By repeating the above, the wavelength is broadened and the pulse is shortened.

[4] The ultrashort pulse broadband lightwave generator according to [3], wherein an optical fiber is used as the nonlinear medium.

[0018]

Embodiments of the present invention will be described below.

First, an ultrashort optical pulse train (mode-locked laser pulse train) output from an ultrashort pulse laser light generator in which a number of pulses are repeatedly output at the same cycle is passed through an optical fiber, and the wavelength is broadened by a self-phase modulation effect. Obtained optical pulse.

The obtained pulse has a broad band and, at the same time, a chirped pulse in which the velocity dispersion due to the wavelength is caused by the refractive index dispersion in the optical fiber and the pulse is elongated in time. Using a dispersion compensating / chirp compensating compression device such as a pair, a chirp mirror, or a spatial phase modulator, an optical pulse equivalent to or shorter in time than the incident light is obtained.

The broadened and shorter pulsed light is superimposed on the next pulse of the mode-locked laser pulse train after passing through a delay optical path. Then, of the two superimposed pulses, the one that is fed back has a spectrum region that is not present in the next pulse, so that not only self-phase modulation but also two pulses having different wavelength bands are generated by the nonlinear medium. Phase modulation (induced phase modulation)
Compared to the prior art case of only one-pulse self-phase modulation,
Generation of a wider band pulse is possible. That is, it is possible to generate a pulse having a wider band than the limit due to input light damage in the related art.

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic diagram of an ultrashort pulse broadband lightwave generation system showing an embodiment of the present invention, and FIGS. 2 to 5 are diagrams showing examples of a chirp compensator.

As shown in FIG. 1, this ultrashort-pulse broadband light wave generating system comprises an ultrashort-pulse laser light generator 1.
Mode-locked laser (that is, a laser that generates an ultrashort optical pulse train 1A), an optical fiber 3, and a phase compensation compression device (chirp compensation optical system: chirp compensation device)
4. Delay adjusting optical paths 7, 8, 9, 10 and partial reflecting mirror 2. Reference numerals 5, 6, and 11 are reflecting mirrors, and reference numeral 12 is a spectrum and pulse width measurement system.

Also, in FIGS.
26 is a diffraction grating pair, 22 is a lens, 23 and 28 are spatial phase modulators, 27 is a plane mirror, 29 is a spherical mirror, and 31 is a prism pair.

Then, the ultrashort optical pulse train 1A is passed through the optical fiber 3 to obtain a broadband optical pulse by the self-phase modulation effect. The obtained pulse is a chirped pulse whose pulse broadens temporally due to dispersion in the optical fiber 3 at the same time as having a wide band.

This is compensated for by using the phase compensation compression device 4 to make an optical pulse equal to or shorter in time than the incident light. This broadened and shortened light is converted into a delay adjustment light path 7 for the next pulse of the ultrashort light pulse train 1A.
After passing through the optical fiber 3, the partial reflection mirror 2 overlaps the optical fiber 3.

With this configuration, of the two superposed pulses, the one that feeds back has a wider band spectrum than the pulse before passing through the optical fiber. Has no spectral range. For this reason, not only self-phase modulation but also induced phase modulation having a modulation effect is caused, so that a wider band pulse is generated than in the case of only one-pulse self-phase modulation of the related art (see FIG. 6). Is possible. That is, it is possible to generate a pulse having a wider band than the limit due to input light damage in the related art.

In this apparatus, since feedback is performed while also performing chirp compensation at all times, light that has passed through the partial reflecting mirror 2 from the side of the delay adjusting optical paths 7 to 10 in FIG. In comparison, the pulse width is shorter than that of the conventional method because the spectrum is wider than that of the conventional method.

In a conventional method, an optical fiber having a core diameter of 3 μm and a length of 4 mm is applied to a spire having an intensity of 100 kW and a pulse time width of 10 μm.
When a pulse of 0 fs is passed once, the wavelength band becomes 720-92.
The band is broadened to 0 nm, and the bandwidth Δλ becomes 200 nm.

According to the present invention, a pulse of 100 kW, 100 fs, a pulse of 100 kW,
When a pulse of 0 fs is superimposed and incident simultaneously, the wavelength band is broadened to 570 to 1200 nm, and the bandwidth Δλ is 6
30 nm. When this pulse is time-compressed, 8.4
An extremely short optical pulse of fs can be generated.

When a 200 kW, 100 fs pulse is once passed through an optical fiber by the conventional method (see FIG. 6), the wavelength band is 680 to 980 nm, and the bandwidth Δλ is 300 nm.
Therefore, the band is not widened by the ultrashort pulse broadband light wave generation method of the present invention.

The specific examples of the chirp compensator 4 described in FIG. 1 include a diffraction grating pair 21, 25, 26 as shown in FIGS. 2 to 4, and a prism pair 3 as shown in FIG.
1, or as shown in FIG. 2 or FIG.
28 and the like. Similar results are obtained with any of these. Although not shown, compensation can also be made by reflecting once or multiple times with a chirp mirror.

The present invention is suitable for the following applications.

Development of optical information processing technology of 100 terabits or more Frequency multiplexing large-capacity high-speed optical communication light source High-frequency clock pulse light source of electronic computer Light source for evaluation of ultra-high-speed nonlinear optical devices, elements and materials Selective reaction control at the molecular level in the medical field and development of new materials Molecular reaction control devices by multiple selective delayed optical resonance excitation, birth of new molecules, light-induced proliferation, multidimensional photomolecular diagnosis, selective light Molecular therapy Intelligent optical analyzer It is expected to be applied to such a wide range of technical fields.

It should be noted that the present invention is not limited to the above-described embodiment, and various modifications are possible based on the spirit of the present invention, and these are not excluded from the scope of the present invention.

[0037]

As described above, according to the present invention, the following effects can be obtained.

(A) After chirp-compensating the output light of the optical fiber through an ultrashort optical pulse train through an optical fiber, an appropriate delay is applied, and the optical fiber is superimposed on the next pulse of the ultrashort optical pulse train. , The output light spectrum can be broadened as compared with the conventional system.

(B) As a secondary effect, it is possible to generate a shorter pulse in a longer time than in the conventional method.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an ultrashort pulse broadband lightwave generation system showing an embodiment of the present invention.

FIG. 2 is a schematic diagram of a chirp compensator (part 1) shown in FIG.

FIG. 3 is a schematic diagram of a chirp compensator (part 2) shown in FIG. 1;

FIG. 4 is a schematic diagram of a chirp compensator (part 3) shown in FIG. 1;

FIG. 5 is a schematic diagram of a chirp compensator (part 4) shown in FIG. 1;

FIG. 6 is a schematic diagram of a conventional ultrashort pulse broadband lightwave generation system.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 ultrashort optical pulse laser beam generator 1A ultrashort optical pulse train 2 partial reflector 3 optical fiber 4 phase compensation compressor 5,6,11 reflector 7,8,9,10 delay adjustment optical path 12 spectrum and pulse width measurement system 21, 25, 26 Diffraction grating pair 22 Lens 23, 28 Spatial phase modulator 27 Planar mirror 29 Spherical mirror 31 Prism pair

Claims (4)

[Claims] 1. A method in which a pulse light emitted from an ultrashort pulse laser light generation device of picoseconds or less, in which pulses are repeatedly output at the same cycle, is passed through a nonlinear medium.
Broadening the wavelength of the laser light, a compressed light shorter than the emission pulse light by chirp compensation of the broadband laser light by a phase compensation light compression device,
Returning to the output pulse optical path through a feedback optical path having a feedback delay optical path, superimposing on the output pulse train, and passing through the nonlinear medium and the phase compensation compressor.
An ultrashort-pulse broadband light wave generation method, characterized in that a wavelength is broadened and a pulse is shortened by repeating at least twice. 2. The ultrashort pulse broadband light wave generating method according to claim 1, wherein when the compressed light is superimposed on the emitted pulse light, the emitted pulse is a pulse next to the emitted pulse forming the compressed light. An ultrashort-pulse broadband lightwave generation method, characterized in that: 3. An apparatus for generating an ultrashort pulse laser beam of picoseconds or less in which pulses are repeatedly output at the same cycle, and (b) an output pulse light emitted from the ultrashort pulse laser light generator. (C) a phase compensating light compressor for broadening the wavelength of the laser light by passing through the nonlinear medium and chirp-compensating the broadband laser light; and (d) a phase compensating light compressing device. A feedback optical path having a feedback delay optical path for passing compressed light shorter than the emission pulse light, and (e) returning the compressed light to the emission pulse optical path through the feedback light path and superimposing the emission light on the emission pulse train. Repeating the passage through the non-linear medium and the phase compensation compression device at least once, thereby broadening the wavelength band and shortening the pulse. That ultra-short pulse broadband light wave generating device. 4. The ultrashort pulse broadband lightwave generator according to claim 3, wherein an optical fiber is used as said nonlinear medium.