JPS6248204B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical pulse shaper that can obtain optical pulses of arbitrary shapes, particularly optical pulses used in laser fusion.

Laser light used for fuel irradiation in laser fusion requires a light pulse called an inverse square distribution waveform, which rises in several nanoseconds while having a rapidly increasing slope (differential coefficient). However, a high-speed optical pulse with such a shape cannot be obtained from a normal laser oscillator, and a conventional low-speed (compared to the rise of the desired optical pulse) modulator can
Of course, since this cannot be generated, it is necessary to generate a new pulse shaper using a pulse shaper suitable for this purpose.

Conventionally, optical pulse shapers for laser fusion extract one pulse from a pulse train obtained from a mode-locked laser oscillator, divide it into an appropriate number of pulses using a large number of semi-transparent mirrors, and divide them into The method used was to give different relative delay times and then re-synthesize them. However, since this type of optical pulse shaper extracts one pulse from a pulse train, most of the energy obtained from the laser oscillator is wasted, and the energy is not used effectively. In addition, since a large number of reflecting mirrors are used,
It is extremely difficult to adjust the optical axis, is susceptible to fluctuations in environmental conditions such as temperature and atmospheric pressure, and is difficult to maintain in practice.In addition, it has various drawbacks such as only being able to obtain the same pulse waveform and lacking in versatility. It was hot.

In other conventional pulse shapers, a single optical pulse extracted from the output optical pulse train of a mode-locked laser oscillator is incident on an etalon plate, and a pulse cutter cuts out a desired envelope from the resulting optical pulse train. I was getting a light pulse train. but,
Also in this optical pulse shaper, one optical pulse is extracted from the optical pulse train, so the laser oscillation output energy is not effectively utilized.

An object of the present invention is to make effective use of optical energy by using a plurality of optical pulses of a mode-locked optical pulse train obtained from a laser oscillator, and to perform optical adjustment and maintenance by using fewer reflecting mirrors. It is an object of the present invention to provide an optical pulse shaper which is particularly suitable for use in laser fusion, and which facilitates the processing and allows the envelope waveform of a pulse train to be appropriately selected.

According to the present invention, in an optical pulse shaper that obtains a pulse train with a desired envelope from a plurality of optical pulses of a mode-locked optical pulse train having equal time intervals, an input semi-transparent mirror and at least two total reflection mirrors are provided. It is composed of a ring-shaped optical circuit whose one-round time is variable around the time interval of the mode-locked optical pulse train, and an optical pulse extractor placed in the ring-shaped optical circuit, and further The optical pulse extractor is characterized in that it is formed of a polarization direction rotation element driven by a control electric signal and a polarizer whose transmittance or reflectance differs depending on the polarization direction. A pulse shaper is obtained.

Further, according to the present invention, there is also obtained a pulse shaper in which one of the total reflection mirrors forming the ring-shaped optical circuit also serves as a polarizer of the light pulse extractor.

Furthermore, according to the present invention, an optical pulse shaper can be obtained in which an optical intensity modulator is added for obtaining a desired envelope optical pulse train from a mode-locked optical pulse train.

Now, before describing the present invention, in order to facilitate comparison with the prior art, a conventional example of a pulse shaper will first be described with reference to the configuration diagram of FIG. In the figure, a single optical pulse 2 is extracted by an optical pulse extractor 10 from an optical pulse train 1 obtained from a mode-locked laser oscillator. A single optical pulse 2 is divided into an appropriate number of optical pulses (four optical pulses in the figure) by an optical pulse combiner 11, given different relative delay times, and combined as an output optical pulse train 3. . Regarding the principle of this optical pulse shaper, please refer to "Applied Optics" published in the United States in December 1976.
(Applied Optics) Magazine, Volume 15, Issue 12, Page 3054~
It is described in detail on page 3061, so it will be omitted here. In the above conventional example, only one optical pulse is used from the optical pulse train 1 obtained from the mode-locked laser oscillator, and the other optical pulses are discarded, so energy is not used effectively. It turns out there isn't. In addition, as the pulse synthesizer 11, 8
Although we used an example of a configuration using a single reflector, the actual configuration is made up of about 20 reflectors, so as mentioned above, it is difficult to adjust the optical axis, and the temperature It is susceptible to fluctuations in atmospheric pressure environmental conditions, and furthermore, only optical pulse trains with the same envelope shape can be obtained.

Another conventional example is the optical pulse shaper shown in the block diagram of FIG. Similar to FIG. 1, this is a single optical pulse 2 extracted from the optical pulse train 1 by the optical pulse extractor 10.
When the light is incident on the etalon 12, an optical pulse train 4 having a repetition time determined by the spacing between the etalons is obtained on the output side. A desired optical pulse train is extracted from the optical pulse train 4 using an optical pulse cutter 13 and obtained as an output optical pulse train 5. Regarding this optical pulse shaper, please refer to the document “UCRL-
51908" (published 1, 7, 1976), detailed explanation will be omitted in the main text. In this example as well, only one optical pulse of the optical pulse train obtained from the mode-locked laser oscillator is used, so energy is not used effectively.

Next, an embodiment of the optical pulse shaper according to the present invention will be described with reference to the configuration diagram of FIG. In this embodiment, a ring-shaped optical circuit 30a is formed by an input semi-transparent mirror 31 and two total reflection mirrors 32 and 33.
and a light pulse extractor 34 placed therein. Optical pulse extractor 34
, a polarization direction rotating element 341 such as a Pockel cell driven by a control electric signal, and a polarizer 34 whose reflectance or transmittance varies depending on the polarization direction.
It consists of 2. As shown in FIG. 3a, when the polarization direction rotation element 341 is not driven, the light passes through the polarizer 342 and does not exit the ring-shaped optical circuit 30a. However, as shown in FIG. 3b, when the polarization direction rotation element 341 is driven and the polarization direction of the light is rotated by 90 degrees, the output light 22 is reflected by the polarizer 342 and goes out of the ring-shaped optical circuit 30a. taken out. Note that the one-turn time t ₀ of the ring-shaped optical circuit 30a can be varied by, for example, moving the position of the reflecting mirror 32 and realigning the optical circuit.

Regarding the operation of this optical pulse shaper, please refer to the operation waveform diagrams in FIGS. 4a and 4b for the case where a finite number of linearly polarized optical pulse trains 20 such as those obtained from a Q-switch mode-locked laser oscillator are incident. I will explain. As shown in FIG. 5A, it is assumed that the incident light pulse train 20 has n=7 component pulses 201 to 207 with equal time intervals _t1 . First, 2t ₀ > t ₁
Consider the case >t ₀ . First, the first component pulse 201 is applied to the semi-transparent mirror 31 of the ring-shaped optical circuit 30a.
The pulse 211 shown in FIG. to go into. Part 2
After time (t ₁ −t ₀ ) from the time of the second incident, the second component pulse 202 of the incident optical pulse example 20 enters the semi-transparent mirror 31, so the optical pulse train 212 circulates around the ring-shaped optical circuit 30a. . Here, if N is the rank of any component pulse in the incident pulse train 20, then after (N-1)× _t0 time after the component pulse 201 first enters, the component pulse is composed of N component pulses. , the optical pulse train whose interval is (t ₁ −t ₀ ) enters the loop of the ring type optical circuit 30a. At this time, when the polarization direction rotation element 341 is driven, the polarization of the light passing through this element 341 is rotated by 90 degrees, reflected by the polarizer 342, and is reflected by the ring-shaped optical circuit 30.
It is taken out of a. In this way, the output optical pulse train 22 of the pulse shaper obtained approximately (N-1)t ₁ hour after the incident optical pulse train 20 starts to enter the ring-shaped optical circuit 30a has N component pulses. and the time interval of its component pulses is (t ₁
−t ₀ ), and the total time width is (N−1)×(t ₁ −t ₀ )
becomes. Further, the light intensity P _{N (put)} of the Nth component pulse of the output pulse train 22 is expressed by the following equation.

P _N(put) = P _N(io) T _n1 R _p × (R _n1 R _n2 R _n3 T _s ) ^nN (However, nN) Here, P _N(io) is the Nth component of the incident optical pulse train 20 The light intensity of the pulse, T _n1 is the incident semi-transparent mirror 3
1 transmittance, R _p is the reflectance of the polarizer 342, R _n
1 , R _n2 and R _n3 are the semi-transparent mirror 31 and the reflecting mirror 3, respectively.
2 and 33, and T _s is the transmittance of the optical pulse extractor 34 which is not driven.

Next, the case of t ₁ <t ₀ will be explained with reference to the operation waveform diagrams in FIGS. 5a and 5b. Initially, as in the case of FIG. 4, the first component pulse 201 of the incident optical pulse train 20 shown in FIG. However, the second component pulse 202 of the incident optical pulse train 20 has already entered the incident semi-transparent mirror 31 before that time (t ₀ − t ₁ ), so as shown in FIG. The optical pulse train 212 whose order is temporally reversed is the ring-type optical circuit 30.
Go around a. Below, component pulse 20
After (N-1)×t ₁ hour after the input of the light beam 1, an optical pulse train composed of N component pulses enters the ring-shaped optical circuit 30a. At this time, if the light extractor 34 is operated in the same manner as in the case of FIG.
taken outside. In this way, the output light pulse train 22 of the pulse shaper obtained approximately (N-1)t ₁ hour after the incident light pulse train 20 starts to enter has N component pulses, and the time width is (N
−1)×(t ₀ −t ₁ ). In addition, the output light pulse train 2
The order of the second component pulse is the reverse of the order of the component pulses of the incident optical pulse train 20, and the light intensity of the Nth component pulse is P _N(put) = P _{(o-N+1)(io )} ×T _n1 R _p ×(R _n1 R _n2 R _n3 T _s ) ^(N-1) . Here, P _(o-N+1)(io) is the light intensity of the (n-N+1)th component pulse of the incident optical pulse train. As mentioned above, since the one-round time t ₀ of the ring-shaped optical circuit in the example of FIGS. 4 and 5 is variable, the time width (n-1)×|t of the resulting output light pulse train 22 ₀ −t ₁ | is variable.

The optical pulse shaper shown in the above embodiment can be used for a wider range of purposes than conventional ones by inverting the shape of the incident envelope optical pulse or varying the time width. can be done. In addition, since the ring type optical circuit 30a uses only three reflecting mirrors, optical axis adjustment is easy.
And it is easy to maintain.

FIG. 6 shows the configuration of another embodiment according to the present invention. In the figure, the polarizer 343 of the optical pulse extractor 34' is a ring-shaped optical circuit 30'a.
It is formed so as to double as a single total reflection mirror that forms the mirror, which simplifies the structure.

Furthermore, FIG. 7 is a configuration diagram showing an example of application in which a light intensity modulator 40 is added to the input side of the optical pulse shaper shown in FIG. 3 or FIG. An optical pulse train 1 obtained from a mode-locked laser oscillator is applied to an optical intensity modulator 40, and an optical pulse train 41 with an appropriately shaped envelope is obtained from its output side. Note that the response speed of the optical intensity modulator 40 used for this can be approximately Nt ₁ , which is significantly slower than the envelope width of the desired output pulse train, and is therefore sufficiently available in the prior art. When the optical pulse train 41 generated by the optical intensity modulator 40 enters the ring-shaped optical circuit 30a (or 30'a) including the optical pulse extractor 34 (or 34'), Output pulse train 42 is obtained by operations similar to those described. Moreover, in this optical pulse shaper, by changing the modulation form of the optical intensity modulator 40, the shape of the envelope of the resulting output optical pulse can be arbitrarily changed.

As is clear from the above explanation, according to the present invention, since shaping is performed using a plurality of incident light pulses, the energy Not only can the usage efficiency of the pulse train be greatly increased, but it is also possible to change the time width and invert the shape of the envelope of the pulse train obtained as an output, and furthermore, by combining it with a light intensity modulator, the envelope The shape of can be changed arbitrarily. Further, according to the present invention, the polarizer of the optical pulse extractor can also serve as a total reflection mirror forming a ring-shaped optical circuit, so that the optical pulse is small and highly versatile, requiring fewer reflecting mirrors. A shaper is obtained.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a conventional optical pulse shaper using a large number of reflecting mirrors, Fig. 2 is a block diagram of a conventional pulse shaper using an etalon and a pulse cutter, and Figs. 4b is a diagram showing the configuration of an embodiment according to the present invention in the non-operating and operating states of the polarization direction rotation element, respectively; FIGS. 4a and 4b
are the operating waveform diagrams of the incident pulse train and the ring type optical circuit operating under the condition of 2t ₀ > t ₁ > t ₀ in the embodiment shown in FIG. 3, and FIGS. 5 a and b.
are operation waveform diagrams of the incident pulse train and the ring type optical circuit operating under the condition of t ₁ <t ₀ in the embodiment of FIG. 3, and FIG. 6 shows another embodiment according to the present invention. The configuration diagram, FIG. 7, is a configuration diagram showing an example of application when an optical intensity modulator is added to the input side of the optical pulse shaper shown in FIGS. 3 and 6.
In the figure, 30 and 30' are optical pulse shapers;
0a, 30'a are ring type optical circuits, 31 is an entrance semi-transparent mirror, 32, 33 are total reflection mirrors, 34, 34' are optical pulse extractors, 40 is a light intensity modulator, 341 is a polarization direction rotation element, 342 , 343 are polarizers.

Claims (1)

[Claims] 1. In an optical pulse shaper that obtains an optical pulse train with a desired envelope from a plurality of optical pulses of a mode-locked optical pulse train having equal time intervals, an input semi-transparent mirror and at least two total reflection mirrors are used. Consists of a mirror,
It is composed of a ring-shaped optical circuit whose one-round time is variable around the time interval of the mode-locked optical pulse train, and an optical pulse extractor placed in the ring-shaped optical circuit, An optical pulse shaper characterized in that the extractor is formed of a polarization direction rotation element driven by a control electric signal and a polarizer whose transmittance or reflectance differs depending on the polarization direction. . 2 Among the total reflection mirrors forming the ring-type optical circuit,
The optical pulse shaper according to claim 1, wherein one of the sheets also serves as a polarizer of the optical pulse extractor. 3. Claims 1 and 2, characterized in that an optical intensity modulator for obtaining an optical pulse train with a desired envelope from a mode-locked optical pulse train is added at the front stage of the ring type optical circuit. Optical pulse shaper.