JP2554772B2 - Laser pulse stretcher - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Industrial field of application) The present invention relates to a laser pulse stretcher for expanding the laser pulse width of a dye laser, for example.

(Prior Art) For example, in a high-power dye laser system,
A system consisting of a dye laser oscillator and a dye laser amplifier is used to increase the output. In such a dye laser system, as a means for measuring the increase in output, not only is the laser light output from the dye laser amplifier amplified by the dye laser amplifier, but the pulse width of the laser light is also increased by a laser pulse stretcher. And the dye laser amplifier amplifies the laser light to increase the amplification effect.

Conventionally, it is the third as the above laser pulse stretcher.
The structure shown in FIG. 4 or FIG. 4 was used.
That is, the laser pulse stretcher shown in FIG. 3 has the double-sided reflection half mirror 1 arranged at an angle of 45 degrees with respect to the pulsed laser light L. The laser beam L incident at an angle of 45 degrees to one of the reflecting surface 1a of the double-sided reflecting half mirror 1 is divided into transmitted light L ₁ and the reflected light L _2.

The reflected light L ₂ reflected by the first reflection surface 1a of the double-sided reflection half mirror 1 is sequentially reflected by the reflection surfaces 2a to 5a of the first to fourth reflection mirrors 2 to 5 at an angle of 45 degrees to form a rectangle. After traveling in a loop shape and being delayed for a predetermined time, the beam diameter is corrected by the beam correction lens 6, and then reflected by the other reflection surface 1b of the double-sided reflection half mirror 1. As a result, the reflected light L ₂ is combined with the transmitted light L ₁ that has passed through the double-sided reflection half mirror 1 with a predetermined delay, so that the laser light L ′ with an expanded pulse width can be obtained.

On the other hand, in the laser pulse stretcher having the configuration shown in FIG. 4, the first pulse laser beam L is arranged at an angle of 45 degrees.
Is split by the beam splitter 10 into transmitted light L ₁ and reflected light L ₂ . The transmitted light L ₁ is transmitted through the half mirror 11. The reflected light L ₂ is reflected by the first reflection mirror 12 to form a first through hole formed in the first concave reflection mirror 13.
The light is incident from 14 and the reflection surface 13 of the first concave reflection mirror 13
Multiple reflections are made between a and the reflecting surface 15a of the second concave reflecting mirror 15 arranged so as to face the first concave reflecting mirror 13 and delayed for a predetermined time. Then, the light is emitted from the second through hole 16 formed in the second concave reflection mirror 15, reflected by the second reflection mirror 17, and the beam diameter is corrected by the beam correction lens 18, and then the half mirror 11 is used. Reflect on. As a result, the reflected light L ₂ is combined with the transmitted light L ₁ with a predetermined delay, so that the pulse width of the combined laser light L ′ can be expanded.

However, in the conventional configuration shown in FIG. 3, the reflected light L ₂ is changed by changing the interval between the first to fourth reflecting mirrors 2 to 5.
Must be adjusted, and in the configuration shown in FIG. 4, the delay time must be adjusted by the pair of concave reflecting mirrors 13 and 15. Therefore, according to these conventional configurations, the first to fourth reflection mirrors 2 to 5 are provided.
It often takes a lot of time to adjust the optical system such as the pair of concave reflection mirrors 13 and 15. Also, the reflected light L ₂
Since the beam diameter is different from the transmitted light L ₁ due to the delay, the correction lenses 6 and 18 may have to be used to correct the difference. Furthermore, in order to obtain a predetermined delay time, the distance between the reflection mirrors 2 to 5 must be increased in the configuration shown in FIG. Fourth
In the configuration shown in the figure, since the number of times of multiple reflection between the pair of concave reflecting mirrors 13 and 15 must be increased, it may be very difficult to adjust.

(Problems to be Solved by the Invention) As described above, the conventional laser pulse stretcher is
Not only it takes a lot of time to adjust the delay time by the optical system, but also the overall configuration becomes large.

The present invention has been made based on the above circumstances, and an object thereof is to provide a laser pulse stretcher in which the delay time can be easily adjusted and the overall configuration does not become large. .

[Structure of the Invention] (Means and Actions for Solving the Problem) In order to solve the above problems, the present invention has a first incident end and a first emitting end, and a pulse is applied from the first incident end. It has a first optical fiber into which laser light enters and is emitted from the first emission end, and a second incidence end and a second emission end, and these incidence end and emission end are detachably connected by a connector. The second optical fiber formed into a loop and the first optical fiber
Of the optical fiber and the second optical fiber are optically connected to each other, and the pulsed laser light incident on the first optical fiber is supplied to the first optical fiber and the second optical fiber at a predetermined ratio. And a distribution means for distributing.

According to such a configuration, the first optical fiber to the second optical fiber
The laser light distributed to the optical fiber of the first optical fiber is delayed for a predetermined time by passing through the second optical fiber, and then a part of the laser light is distributed to the first optical fiber so that the laser light is emitted from the first optical fiber. The pulse width of the emitted laser light can be expanded.

(Embodiment) An embodiment of the present invention will be described below with reference to FIG. The laser pulse stretcher shown in FIG. 1 comprises a first optical fiber 21 and a second optical fiber 22. The first optical fiber 21 has one end formed at the first incident end 23a and the other end formed at the first emitting end 23b. The pulsed laser light L is incident from the first incident end 23a via the incident side lens 24, and the laser light L'with the expanded pulse width is emitted from the first emitting end 23b as described later. It is emitted through the side lens 25.

The second optical fiber 22 has one end formed at the second entrance end 26a and the other end formed at the second exit end 26b,
The entrance end 26a and the exit end 26b are optically and detachably coupled by a connector 27. Thereby, the second
The optical fiber 22 has a loop shape.

The middle portions of the first optical fiber 21 and the second optical fiber 22 are optically connected by a directional coupler 28 that constitutes a distribution means. That is, this directional coupler 28
Distributes the laser light L incident on the first incident end 23a of the first optical fiber 21 to the emitting end 23b of the first optical fiber 21 and the second optical fiber 22 at a predetermined distribution ratio. It is like this. As a result, a part of the laser light L incident on the first optical fiber 21 is distributed to the second optical fiber 22 based on the distribution ratio set in the directional coupler 28. . The laser light L distributed to the second optical fiber 22 is delayed for a predetermined time by passing through the second optical fiber 22, and then a part of the laser light L is passed through the directional coupler 28 to the first optical fiber 21. It is repeated that the light is distributed to the emitting end 23b, and the remaining part is circulated in the second optical fiber 22 again and further delayed for a predetermined time to be distributed. Therefore, the first of the first optical fiber 21
The laser light L ′ having an expanded pulse width is emitted from the emission end 23b of the laser.

The pulse width of the laser light L incident on the first optical fiber 21 is T, the peak value is 1, and the emitting end of the optical fiber 21 when it enters from the first optical fiber 21 by the directional coupler 28.
If the distribution ratio between 23b and the second optical fiber 22 is (A: B), and the delay time between the pulse laser lights L emitted from the emission end 23b of the first optical fiber 21 is Δt, then the first light fiber
The laser light L that has entered the light source 21 enters the output end 23b of {A /
It is distributed at a ratio of (A + B)} and is emitted to the outside.

In the directional coupler 28, the light is distributed to the second optical fiber 22 at a ratio of {B / (A + B)}. The laser light L distributed to the second optical fiber 22 is
By making one round, the optical signal is delayed by Δt, and is again distributed by the directional coupler 28 to the emission end 23b of the first optical fiber 21 and the second optical fiber 22 at a ratio of (B: A). Therefore, at this time, it is distributed at a ratio of {B ² / (A + B) ² }
Laser light L emitted from the emission end 23b of the optical fiber 21 of
Is overlapped with the laser light L distributed by the ratio {A / (A + B)} on the emitting end 23b side with a delay of Δt.

In addition, to the second optical fiber 22 {A · B / (A + B) ² }
The laser light L divided at the ratio of is delayed by Δt as in the previous time, and is again divided into the first optical fiber 21, the emitting end 23b, and the second optical fiber 22. The laser beams L and Δt are emitted to the first optical fiber 21 and the emission end 23b at a rate of {B ³ / (A + B) ³ } and before that are distributed at a rate of {B ² / (A + B) ² }. Only delayed and overlap.

In this way, the laser beam L split by the directional coupler 28 has peaks of {A / (A + B)} and {B ² /
(A + B) ² }, {B ³ / (A + B) ³ }, ...
The pulse width T is delayed by the delay time Δt and overlaps.

Therefore, the pulsed laser light L as a whole is the sum of the distributed laser light L, and therefore the directional coupler is used.
By appropriately setting the distribution ratio (A: B) of 28 and the delay time Δt, that is, the length of the second optical fiber 22, the pulse width can be changed. In order to change the length of the second optical fiber 2, the connector of the entrance end 26a and the exit end 26b is used.
The connection by 27 is removed, and an optical fiber of a predetermined length, which is provided between both ends thereof with a connector that can be connected to the connector 27 (not shown), may be optically connected and interposed.

Further, the transverse mode of the pulsed laser light L becomes a stable single mode by passing through the single mode first and second optical fibers 21 and 22, and the pattern of the laser light L also diffracts the dye cell during oscillation. It becomes circular from the state it was received.
Therefore, the core diameters of the first and second optical fibers 21 and 22,
By adapting the conditions such as the difference in refractive index between the core and the clad to the wavelength of the dye laser to be used, it is possible to improve the spatial matching with the amplifier (not shown) installed after this laser pulse stretcher. it can.

In the conventional device shown in FIG. 3, for example, 20 ns
When the delay time of ec is desired, the light velocity in the air is about 3 × 10 ⁸ m / s, so after the light is reflected by the double-sided reflection half mirror 1, the first to fourth reflection mirrors 2 to 5 are used. The length of the delay optical path of the laser light L reflected by 6 must be 6 m. Therefore, the device becomes large and it may be difficult to secure the installation place.

In the conventional apparatus shown in FIG. 4, the pair of concave reflecting mirrors 13 and 15 causes multiple reflection to obtain a delay time, so that the apparatus itself can be miniaturized. However, the distance between the pair of concave reflecting mirrors 13 and 15 is 0.1 m, and the other optical path lengths are
When the length is 0.1 m, the laser light L is used as a pair of concave reflection mirrors 1.
After reflecting 58 times between 3 and 15, the second concave reflecting mirror 1
It is possible to obtain a delay time of 20 nsec by emitting light from the second through hole 16 of No. 5. Therefore, it takes a lot of time and effort to adjust the laser light L to be reflected 58 times between the pair of concave reflection mirrors 13 and 15 and then taken out.

On the other hand, in the configuration of the present invention, in order to obtain a delay time of 20 nsec, the propagation speed of scattered light in the second optical fiber 22 serving as a delay circuit is about 2.1 × 10 when the material of the optical fiber is quartz. ^Since it is ⁸ m / s, the total length of the second optical fiber 22 may be 4.2 m. Since the optical fiber can be wound up, it does not take up much space even if it is long. In addition, when changing the delay time, it is sufficient to change the length of the second optical fiber 22 and does not require the troublesome adjustment of the optical system as in the conventional case. Further, the laser light L that is delayed and the laser light L that is not delayed are
Since it passes through the second optical fibers 21 and 22, there is no difference between the beam diameters of the two. Therefore, a correction lens for adjusting the beam diameter as in the prior art is not necessary.

Further, by satisfying the relationship of the following expression (1), it is possible to set the laser light of a specific wavelength to the single mode.

Here, n is the refractive index of the core, Δ is the refractive index difference between the core and the clad, a is the radius of the core, and λ is the wavelength of the laser light.

Therefore, when λ = 600 nm, n = 1.5, and Δ = 0.3%, the condition for the single mode is a <4 μm. As described above, by setting the laser light L to the single mode, the intensity distribution of the emitted laser light L can be set to the Gaussian mode, so that the matching of the space with the amplifier installed in the subsequent stage can be improved.

In the above-mentioned embodiment, the first optical fiber 21 and the second optical fiber 21
The optical fiber 22 and the middle part thereof are optically connected directly by the directional coupler 28, and two short optical fibers 31, 32 which are optically connected in advance are built in the directional coupler 28. Then, as shown in FIG. 2, these ends are connected to the directional coupler 28.
Connectors 31a, 31b, 32a, 32b on the sides of the
While being connected to one end of the first and second optical fibers 21, 22
Are each divided into two. Then, the one end 21a and the other end 21b of the first optical fiber 21 and the one end 22a and the other end 22b of the second optical fiber 22 which are respectively divided into two at the other ends of the corresponding pair of connectors. May be connected to each other. With such a configuration, the first and second optical fibers 21 and 22 can be attached to and detached from the directional coupler 28, so that the lengths of these optical fibers can be easily adjusted. In the case of this embodiment, the second optical fiber 22 is not only detachably connected to the entrance end 26a and the exit end 26b, but also detachable to the directional coupler 28. Therefore, the second optical fiber 22 is divided into two.
The length of the second optical fiber 22 can be changed by exchanging the one end 22a or the other end 22b with a different length.

[Advantages of the Invention] As described above, according to the present invention, the pulse width of the laser beam can be expanded by using the optical fiber. Therefore, not only the device can be downsized, but also the laser having a predetermined pulse width can be used. Adjustments to obtain light can be easily made.
Further, since the incident end and the emission end of the second optical fiber which is connected to the distribution means in the middle and has a loop shape are detachably connected by the connector, the distance between the incidence end and the emission end is large. By connecting an optical fiber of a predetermined length to
You can change the length of the loop. This has the advantage that the pulse width of the laser light output can be changed according to the change in the length of the loop.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a block diagram showing another embodiment of the present invention, FIGS. 3 and 4
Each of the figures is an explanatory diagram showing a conventional configuration. 21 …… First optical fiber, 22 …… Second optical fiber, 28
... Directional coupler (distribution means).

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Claims (2)

(57) [Claims] 1. A first optical fiber having a first entrance end and a first exit end, the pulsed laser light entering from the first entrance end and exiting from the first exit end; A second input end and a second output end, and the input end and the output end are detachably connected by a connector to form a loop.
Of the optical fiber and the first optical fiber and the middle part of the second optical fiber are optically connected to each other, and the pulsed laser light incident on the first optical fiber is supplied to the first optical fiber and the second optical fiber. A laser pulse stretcher comprising: an optical fiber; and a distribution means for distributing the optical fiber at a predetermined ratio. 2. The laser pulse stretcher according to claim 1, wherein the second optical fiber is detachably connected to the distributing means.