JPH04191826A - Light pulse shaping method - Google Patents

Abstract

PURPOSE:To enable the execution of another waveform shaping in the ensuring stage after expected shaping treatment is ended by using optical amplification as means for shaping pulse waveforms and preserving all the original frequency components even after shaping. CONSTITUTION:Pumping light is transmitted so as to irradiate only the corresponding optical amplifier element with a mask 16 in order to selectively excite only the optical amplifier element existing in the position of the frequency component to be amplified. This pumping light and the frequency component of the signal light to be amplified are multiplexed by a multiplexer 17 and are made incident on a fiber 14 which amplifies the signal light. The light past the fiber is reflected by a mirror 8, travels backward in the same optical path and is again amplified by the optical fiber 14; thereafter, the pumping light is removed by a wavelength filter 18 and only the amplified light passes the fiber. This light is diffracted by diffraction gratings 4, 3, is taken out by a half mirror 9 to the outside of the system and finally, the shaping of the light pulse waveforms is completed. All the frequency components are preserved even after the waveform shaping in such a manner and, therefore, the separate waveform shaping is executed in the ensuing stage.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical pulse shaping method for arbitrarily shaping the waveform of an ultrashort optical pulse.

[Conventional technology] A typical conventional optical pulse shaping method will be explained.

What is shown in FIG. 5 is an optical system for a conventional optical pulse shaping method. In the figure, l is a laser light source, 2 is an optical fiber,
3.4 is a pair of diffraction gratings, 5 is a lens, 6 is a slit,
7 is a mask, 8 is a mirror, 9 is a half mirror, 510 is a laser pulse, 11 is a chirped pulse, and 12 is a shaped pulse.

When a laser pulse of 1O is passed through 2 optical fibers,
When the intensity of the optical pulse is strong to a certain extent, the leading and trailing portions of the optical pulse undergo negative and positive frequency modulation, respectively, due to the self-phase modulation effect of the optical fiber. When the wavelength of the optical pulse is shorter than 1.3 μm, the pulse width is expanded due to the positive wavelength dispersion of the optical fiber, and the frequency becomes linearly chirped due to the chromatic dispersion characteristics of the fiber.

In Figure 6, (a) pulse waveform subjected to linear chirping, (b) frequency, <c> frequency spectrum, (
d) It is a short pulse compressed by correcting the time difference. The frequency spectrum is nearly rectangular. By reciprocating this optical pulse between diffraction gratings 3 and 4, it is spatially separated into each frequency component, and further, by utilizing the fact that the optical path difference caused by the difference in frequency has a characteristic opposite to that of linear chirping. , corrects the time difference and compresses it into a short pulse.

Therefore, the spacing between the diffraction gratings must be set to provide an optical path length difference corresponding to the pulse width. The slit 6 is for removing frequency components that cause waveform tailing. The advantage of reciprocating the optical pulses is that not only can the difference in optical path length be doubled, but also that the beam expanded just after point 4 can be focused at the point where the optical pulses are taken out in point 9.

Immediately after the 4th diffraction grating in this optical system, each frequency is spatially separated, so it is possible to perform spatial operations or necessary pulse shaping for each frequency component as described below. . The optical pulse waveform to be generated in advance is Fourier-transformed to obtain a frequency spectrum, and unnecessary frequency components are removed by passing through a mask 7 having a pattern that transmits only desired frequency components, and reflected by a mirror 8. After retracing the same optical path and focusing the spatially wide light pulse on one point, the light pulse is reflected by a half mirror 9 and taken out, thereby finally completing the shaping of the light pulse waveform. What is shown in FIG. 7 is the result of this numerical calculation; the left side is the frequency spectrum, and the right side is the optical pulse waveform.

It can be seen that the pulse waveform can be shaped by removing the central portion from the spectrum of (a) as shown in (b).

[Problems to be Solved by the Invention] However, in this pulse shaping method, unnecessary frequency shaping is blocked and removed by a mask, so that some frequency components are lost. Therefore, it is impossible to perform another waveform shaping at the next stage after the intended processing is completed.

The object of the present invention is to
To solve the drawback that some frequency components are lost in the process of processing, and to provide a means by which another waveform shaping can be performed at the next stage after the intended pulse waveform shaping processing is completed. It is in.

[Means for solving the problem 1 The main feature of the present invention is that optical amplification is used as a means for shaping pulse waveforms, and it differs from conventional technology in that all original frequency components are preserved even after shaping. .

[Operation] According to the optical pulse shaping method of the present invention, all frequency components can be preserved even after waveform shaping without losing some frequency components in the processing process, so another waveform shaping can be performed in the next stage. I can do what I want to do.

[Example] (1) First Example The details will be explained below with reference to the drawings. FIG. 1 is a diagram showing the optical system of the optical pulse shaping method of the present invention, in which 1 is a laser light source, 2 is an optical fiber, 384 is a pair of diffraction gratings, 5 is a lens, 6 is a slit, 8 is a mirror, 9 is a half mirror
1I3-1 is an optical amplification unit using an Er-doped fiber as a gain medium, 10 is a laser pulse, 11 is a chirb pulse, and 12 is a shaped pulse.

Next, the optical amplifier 131 will be explained with reference to FIG. In the figure, 13-1 first amplification.

A one-dimensional array of optical amplification elements using E r F-fiber as a gain medium, 15 is a pump light source, 1
6 is a mask for the pump light source, I7 is a multiplexer/demultiplexer, 18 is a band pass filter that passes only light of the signal wavelength, and 19 is a one-dimensional lens array. Since an Er-doped fiber is used as an amplification element, the wavelength of the laser light source (2) is set to 1.55 μm, which provides the maximum gain, and the pump light source (15) is a laser having a wavelength of 098 μm, which has a high pumping efficiency. Since the process up to the generation of a uniform frequency spectrum distribution is the same as the conventional method, only the subsequent processing will be described.

First, the optical pulse waveform to be generated in advance is Fourier transformed to obtain a frequency spectrum. In order to selectively excite only the optical amplification element located at the position of the frequency component to be amplified,
The pump light is transmitted through a mask No. 16 so as to irradiate only the corresponding optical amplification element.

The frequency components of this pump light and the signal light to be amplified are multiplexed by a 17 waver and input into 14 fibers to amplify the signal light. The light that has passed through the optical fiber is reflected by mirror 8, travels the same optical path backwards, and is amplified again by the optical fiber, after which the pump light is removed by wavelength filter 18 and only the amplified light passes through. The light is diffracted by the diffraction grating 4.3 and taken out of the system by the half mirror 9, thereby completing the shaping of the optical pulse waveform.

The main difference between the conventional method and this embodiment is that the conventional method transmits only desired frequency components and removes unnecessary frequency components using a mask, depending on the frequency spectrum of the optical pulse waveform to be generated. In contrast, the present invention is characterized in that only desired frequency components are selectively amplified. The difference in the configuration of the optical system from the conventional one is that an optical amplification unit is used instead of removing unnecessary frequency components using a mask in step 7. By reciprocating the optical pulse through a gain medium and selectively amplifying only the desired frequency components,
Since the light intensity is made sufficiently large compared to the light intensity of unnecessary frequencies, the influence of unnecessary components on the pulse waveform can be almost ignored. Therefore, selectively amplifying only desired frequency components can be considered equivalent to removing unnecessary frequencies.

(2) Second embodiment What is shown in FIG. 3 is the optical amplification unit shown in FIG.

Optical amplification unit provided in place of G 13-1, G 13-2
It uses a photorefractive crystal as the optical amplification medium. 20 is one of the optical amplification elements using a photorefractive crystal as a gain medium.
In the dimensional array, 21 is a mask for a pump light source, 22 is a pump light, and 23 is a diffraction grating formed by interference of the pump light and signal light. Only the optical amplification method different from the second embodiment will be explained. This optical amplification method uses two-wave mixing. When a weak signal light and a coherent pump light are incident on a crystal, the refractive index corresponding to the interference fringes of both modulates due to the light refraction effect of the crystal. A diffraction grating is formed within the crystal. The pump light is diffracted by this diffraction grating, and its energy is transferred to the signal light, thereby intensifying the signal light. Typical photorefractive crystals include ferroelectric crystals such as BaTiO3 and compound semiconductors such as GaAs, and the sensitivity of the photorefractive effect varies depending on the wavelength used.

(3) Third embodiment What is shown in FIG. 4 is the optical amplification unit shown in FIG.

Optical amplification unit l3-3 provided in place of g13-1
A semiconductor laser is used as an optical amplification medium, and 24 is a one-dimensional array of optical amplification elements each having an anti-reflection coating applied to the end face of the semiconductor laser, and 25 is a current source thereof. Only the parts of the optical amplification method that are different from the first and second embodiments will be explained. In the case of this optical amplification element, a current is injected in the forward direction to maintain an excited state, and a weak signal light is made incident thereon to cause stimulated emission and optical amplification is performed.

Spontaneous emission light that causes noise is removed by 18 narrow band pass filters. In order to avoid absorption of signal light, the wavelength must be smaller (longer) than the semiconductor energy width.

[Effect of the invention E. As explained above, according to the optical pulse shaping method of the present invention, the conventional drawback that some frequency components are lost during the processing process is solved, and even after waveform shaping, all frequency components are This has the advantage of being able to store a different waveform shape in the next stage.

[Brief explanation of the drawing]

FIG. 1 is a diagram explaining the optical system of the optical pulse shaping method of the present invention, FIG. 2 is a schematic configuration diagram of the optical amplification unit 13-1 of the first embodiment, and FIG. 3 is an optical amplification diagram of the second embodiment. Unit 13-
2 is a schematic diagram of the configuration, and FIG. 4 is the optical amplification unit of the third embodiment)
133 is a schematic configuration diagram. FIG. 5 is a diagram explaining the optical system of the conventional optical pulse shaping method. In FIG. 6, (a) is the pulse waveform subjected to linear chirping, (b) is the frequency, (C) is the frequency spectrum, (
d) is a short pulse compressed by correcting the time difference, and Figure 7 shows before (a) and after (b) optical pulse shaping using the conventional method.
FIG. 2 is a diagram showing the results of numerical calculations, in which the left side shows the frequency spectrum and the right side shows the optical pulse waveform. 3.4...Pair of diffraction gratings, 5...Lens, 8. ...Mirror, 9...Half mirror 115...Pump light source, 7,16.21...
...Mask, 14,20°24...Primary array of amplification elements.

Claims (1)

[Claims] A method of shaping the waveform of an optical pulse by spatially separating the optical pulse into each frequency component, selectively amplifying only the optical intensity of a specific frequency component, and spatially focusing the light again.
A pair of diffraction gratings that spatially separate frequency components, a mask that filters only a specific component to be amplified among the spatially separated frequency components, an optical amplification element, a pump light source that excites the optical amplification element, and a mirror. An optical pulse shaping method characterized by using an optical system such as a mirror, a half mirror, or a lens.