KR101219444B1 - High Power Laser Device - Google Patents

Abstract

The present invention relates to a high power laser device in which pulse energy is increased by using a resonator made of a combination of a conventional resonator and a multi-reflective mirror.
The present invention includes a first resonator for primarily resonating a pump beam; And a second resonator configured to resonate a laser beam output from the first resonator, the first multi-reflective mirror having a first aperture formed thereon and a second multi-reflective mirror having a second aperture formed thereon.

Description

High Power Laser Device

The present invention relates to a high power laser device and a method thereof. More specifically, the present invention relates to a high power laser device in which pulse energy is increased by using a resonator composed of a combination of a conventional resonator and a multi-reflective mirror.

After the advent of lasers, recent studies in the areas of laser processing, cutting, welding, drilling, trimming, etching, medical treatments such as dental care, spots, tattoo removal, hair removal, LASIK surgery, and research on the interaction of lasers and materials Fields and their applications, such as defense and culture, are becoming very widespread. In the case of industrial and medical lasers, the acceptance of lasers is increasing as more and more attempts are made to use lasers in areas where conventional mechanical and chemical processing and treatment methods are impossible.

Recently, femtosecond lasers have been used in various fields. Femtosecond lasers have the property that light energy is aggregated and emitted for a very short time (10 ^-15 s). As a result, femtosecond lasers exhibit different characteristics from conventional lasers. For example, using femtosecond lasers, the femtosecond laser is irradiated only for a time shorter than the time when heat is transferred to the medium when the laser light is irradiated to the medium, thereby avoiding the heat effect or heat deformation generated in conventional laser processing. In addition, femtosecond lasers can be processed inside without damaging the surface of the medium, and thus are used in fields requiring semiconductor processing (semiconductor, electronic chip, medical, etc.).

However, femtosecond laser alone has a limitation in output in order to be used industrially for increasing yield and processing area. Therefore, it is a limitation in expanding application fields.

A common way to solve this limitation is to increase the output by connecting multiple amplifier stages. However, when the amplification stage is installed, it has the disadvantage of having to bear the cost, and it is a limitation in expanding the application of the industry because a laser specialist for maintenance and repair is required.

D. Herriott et al. Proposed an interferometer using two mirrors (“Off-Axis Paths in Spherical Mirror Interferometers”, Applied Optics, Vol. 3, Issue 4, pp. 523-526, 1964), and SH Cho et al. The interferometer proposed by Herriott et al. Was applied to extend the resonator length due to femtosecond pulse round trip ("Low-repetition-rate high-peak-power Kerr-lens mode-locked TiAl2O3 laser with a multiple-pass cavity", Optics Letters, Vol. 24, Issue 6, pp. 417-419). In order to increase the laser pulse energy, it is required to increase the length of the resonator. For this purpose, S. H. Cho et al. Used a multipass method that preserves complex q-parameters (including state information of the laser beam) using two mirrors. Preserving the complex q-parameters maintains the mode lock condition of the general resonator and also increases the pulse energy of the laser by increasing the length of the resonator.

As a similar example, Korean Patent Publication No. 10-2006-0123311 (published Dec. 1, 2006) adopts a method in which a laser beam is multipathed between two mirrors while preserving complex q-parameters, and a high-order dispersion value in a laser resonator. After shortening to a positive dispersion value to make a short pulse and then compressed by a compressor from the outside to provide a short pulse laser device to generate a femtosecond laser.

However, the laser device using the two mirror structures proposed by Herriott et al. Has a problem that the path of the laser beam is complicated and light alignment is not easy.

In order to solve the above problems, an object of the present invention is to provide a laser device that increases pulse energy without using multiple amplifier stages.

It is also an object of the present invention to provide a laser device that simplifies the path of the laser beam and facilitates light alignment.

The present invention includes a first resonator for primarily resonating a pump beam; And a second resonator configured to resonate a laser beam output from the first resonator, the first multi-reflective mirror having a first aperture formed thereon and a second multi-reflective mirror having a second aperture formed thereon.

In one embodiment, the laser device further comprises a pump laser to generate the pump beam.

The first resonator may include a focusing lens for focusing the pump beam, a gain medium through which the pump beam focused by the focusing lens passes, and a first reflection or retroreflection of the laser beam passing through the gain medium in sequence. It may be configured to include a curved mirror, a first reflective mirror, a second reflective mirror, a second curved mirror, and a third reflective mirror.

The second resonator may further include: a fifth reflection configured to reflect the laser beam passing through the first multi-reflection mirror, the second multi-reflection mirror, and the first resonator to the first multi-reflection mirror through the second aperture; It can be configured to include a mirror.

Preferably, the laser beam finally reflected by the second multi-reflective mirror passes through the first aperture and the laser beam passed through the first aperture is transmitted to the output mirror.

Further, preferably, the laser characteristic is changed by adjusting the distance between the output diameter and the first through hole.

The present invention can generate a high power laser beam, in particular a femtosecond laser beam by increasing the pulse energy without using multiple amplification stages. In addition, the present invention has the effect of facilitating light alignment while keeping the path of the laser beam in the resonator simple.

In addition, according to the present invention, the entire length of the resonator can be extended due to repetitive reflection of the laser beam while occupying a small space.

In addition, according to the present invention, there is an effect that can maintain the mode lock even in a situation where the existing complex q-parameters are not preserved.

1 is a view showing the structure of a laser device according to a preferred embodiment of the present invention;
2 is a view showing a multiple reflection mirror in a laser device according to a preferred embodiment of the present invention;
3 shows an equivalent concept wave guide system for analyzing a laser device according to FIG. 1, FIG.
4 is a view showing a pulse train obtained from a long period high power femtosecond laser device according to the present invention,
5 is a view showing the shape of the pulse obtained from the long-period high power femtosecond laser device according to the present invention,
6 is a diagram showing a spectrum obtained from a long period high power femtosecond laser device according to the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals are used to designate the same or similar components throughout the drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear. In addition, the preferred embodiments of the present invention will be described below, but it is needless to say that the technical idea of the present invention is not limited thereto and can be variously modified by those skilled in the art.

1 is a view showing the structure of a laser device according to a preferred embodiment of the present invention, Figure 2 is a view showing a multiple reflection mirror in the laser device according to a preferred embodiment of the present invention.

The laser device 10 according to a preferred embodiment of the present invention includes a pump laser 12 for generating a pump beam 13, a first resonator, and a second resonator.

In one embodiment, the pump laser 12 may be configured as a frequency multiplied Nd: YAG laser or Nd: YVO ₄ laser. The pump beam 13 produced by the pump laser 12 is focused by the focusing lens 14 and transmitted to the gain medium 16.

In a preferred embodiment of the present invention, the first resonator is a general resonator for laser generation, and includes a gain medium 16, first and second curved mirrors CM1 and CM2, and first, second and third reflection mirrors M1, M2, M3) and first and second prisms 18, 20.

The gain medium 16 may consist of titanium: sapphire (Ti: Saphire), and the pump beam 13 excites the gain medium 16. The laser beam passing through the gain medium 16 is reflected by the first curved mirror CM1 and transmitted to the second reflective mirror M2 via the first reflective mirror M1. The laser beam reflected back from the second reflecting mirror M2 is transferred to the first curved mirror CM1 via the first reflecting mirror M1, passes through the gain medium 16, and then the second curved mirror CM2. Is delivered to. The laser beam is reflected back from the second curved mirror CM2 and then transferred to the third reflective mirror M3. In the most general form, an output coupler is provided in a path in which the laser beam reflected from the second curved mirror CM2 is reflected back from the third reflective mirror M3. However, in FIG. 1, the output mirror is not provided in the path reflected by the third reflection mirror M3. This is to transfer the laser beam to the second resonator which will be described later.

The first and second prisms 18, 20 are intended to compensate for the dispersion of the amount by the gain medium 16 and air. When this compensation condition is satisfied, the laser pulse is generated by mode lock by the third order nonlinear optical Kerr effect. Meanwhile, the first and second prisms 18 and 20 may be replaced with chirp mirrors.

The laser pulse may preferably be a femtosecond laser pulse. Specifically, since the laser beam has a strong center portion compared to the edge, when the laser beam penetrates the gain medium 16, the transit time of the center of the laser beam takes longer so that the gain medium 16 acts as a convex lens. do. In this case, the strong part of the laser beam is concentrated more strongly than the weak part. When the aperture is provided to cover the weak edge of the laser beam, only the center portion of the high intensity laser beam is gained by turning the resonator to generate a femtosecond pulse. This approach is called hard aperture mode locking. On the other hand, in the case of replacing the role of the diaphragm with the laser beam of the pump laser 12, this is referred to as soft aperture mode locking.

In a preferred embodiment of the present invention, the second resonator comprises fourth and fifth reflecting mirrors M4 and M5, a first multi-reflective mirror 30 and a second multi-reflective mirror 40. Meanwhile, the first through-hole 32 is formed in the first multi-reflective mirror 30, and the second through-hole 42 is formed in the second multi-reflective mirror 40. The first through holes 32 and the second through holes 42 may be provided by forming holes in the first and second multi-reflective mirrors 40 or forming notches as shown in FIG. 2.

The laser beam reflected from the third reflection mirror M3 via the second curved mirror CM2 is reflected by the fourth reflection mirror M4 and then transferred to the fifth reflection mirror M5. In a preferred embodiment, the laser beam reflected back from the fifth reflection mirror M5 is transmitted to the first multi-reflection mirror 30 through the second through-hole 42 formed in the second multi-reflection mirror 40. . The laser beam is repeatedly reflected between the first multi-reflection mirror 30 and the second multi-reflection mirror 40. In one embodiment, the first multi-reflective mirror 30 may be a reflecting mirror and the second multi-reflective mirror 40 may be a concave mirror. Finally, the laser beam reflected by the second multi-reflective mirror 40 passes through the first through hole 32 of the first multi-reflective mirror 30 and then is transmitted to the output mirror 50.

Referring to FIG. 2, the configuration in which the laser beams in the first and second multi-reflective mirrors 30 and 40 are overlapped and reflected in detail will be described below.

The laser beam reflected by the fifth reflection mirror M5 is transmitted to the first multi-reflection mirror 30 through the second aperture 42 of the second multi-reflection mirror 40, and then the laser beam is first multi-reflection. Reflected repeatedly between the mirror 30 and the second multi-reflective mirror 40. In FIG. 2, the laser beam reflected by the fifth reflective mirror M5 is reflected at point a of the first multi-reflective mirror 30 and then transmitted to a 의 of the second multi-reflective mirror 40, followed by b , b ＇, c, c＇,… An example of passing through the g, g, and g 'points to the output mirror 50 is illustrated.

Compared to the case where the conventional interferometer proposed by Herriott et al. Is used as a resonator (abbreviated as 'Herriott resonator'), in the present invention, the first multi-reflection mirror 30 and the second multi-reflection mirror 40 are respectively formed Due to the first through holes 32 and the second through holes 42, the number of round trip reflections of the laser beam is insufficient. For example, the first multi-reflection mirror 30 is a planar mirror, the second multi-reflection mirror 40 is a concave mirror having a radius of curvature of 4 meters, and the first multi-reflection mirror 30 and the second multi-reflection mirror The laser beam reciprocates between the first multi-reflective mirror 30 and the second multi-reflective mirror 40 in a state where the first through-hole 32 and the second through-hole 42 are not provided in the 40. It is assumed that after (n) has been made eight times, it is delivered to the output mirror 50. In this case, the distance L ₀ between the first multi-reflection mirror 30 and the second multi-reflection mirror 40 is 58.6 cm, and the laser between the first multi-reflection mirror 30 and the second multi-reflection mirror 40 The pattern angle of the beam is 45 degrees.

However, the first multi-reflection mirror 30 and the second multi-reflection mirror 40 is provided with a first through hole 32 and a second through hole 42, the laser beam reflected from the fifth reflection mirror (M5) The laser beam reflected by the second multi-reflective mirror 30 through the second through-hole 42 and finally reflected by the second multi-reflective mirror 40 passes through the first through-hole 32 to the output mirror 50. In the conveyed configuration, the number of round trips n is seven. Thus, complex q-parameters are not preserved. The Herriott resonator acts as a virtual planar mirror when complex q-parameters are preserved. However, if complex q-parameters are not preserved, the Herriott resonator will act as a virtual mirror (or lens) with a large radius of curvature. Nevertheless, since the radius of curvature of the second multi-reflective mirror 40 is large, it is close to the planar mirror, and the change in the general mode locking condition is insignificant within the range in which the pulse mode locking condition (particularly, the femtosecond pulse mode locking condition) is achieved. . Therefore, even though the complex q-parameters are not completely preserved in the present invention, it can be said that there is no influence on the mode locking condition.

In the second resonator according to the present invention will be described to satisfy the substantial mode locking condition as follows.

3 is a view illustrating a wave guide system of an equivalent concept for analyzing the laser device according to FIG. 1.

First, in a typical Herriott resonator consisting of planar mirrors and concave mirrors spaced at distance L ₀ , a single round trip of the laser beam between the planar mirrors and the concave mirrors (ie, the unit cell of the Herriott resonator) is a Ray transfer matrix (1). ray transfer matrix) M _T. (In Equation 1, R ₂ is the radius of curvature of the concave mirror and f is the focal length.)

In the above transfer matrix M _T , the following Equation 2 must be satisfied to satisfy the stable condition.

In a typical Herriott resonator, the transfer matrix (M _T ⁿ ) when the laser beam reciprocates n times between the plane mirror and the concave mirror can be expressed as Equation 3, where n = 1 in Equation 3 and the result is expressed as In contrast to 1, the result of equation (4) is derived.

In Equations 3 and 4, θ means an angle at which the laser beam is reflected every time. On the other hand, as a condition for complex q-parameters to be preserved, the transfer matrix M _T ⁿ should be ± I (I is the identity matrix). Thus, equation (5) is derived.

In Equation 5, n is the number of round trips, and m is the number of semicircular arcs completed by the reflected laser beam.

With the above results, for example, and R2 = 4m, n = 8, and that in the case of m = 2, θ = 45 ° , that is L ₀ = 58.6 cm.

The above is an analysis of a general Herriott resonator. In the case of the second resonator of the laser device according to the present invention, the first and second apertures 32 and 42 of each of the first and second multi-reflective mirrors 30 and 40 are provided. The effect does not preserve complex q-parameters perfectly. This disadvantage can be overcome by adjusting the position of the output mirror 50.

1 and 3, z ₂ represents a virtual lens according to the presence of the second through hole 42, L _δ is a distance between the first multi-reflective mirror 30 and the output mirror 50, and L _e is Indicates the position of the virtual lens. On the other hand, z ₁ represents the position of the output mirror assuming that only the first resonator according to the present invention is used. However, in reality, as the second resonator is additionally provided, the output diameter is not provided at z ₁ .

Then, the relationship between them can be expressed as in Equation 6. (In Equation 6, f ₂ is the focal length of the second multi-reflective mirror 40)

Applying equation (6), L ₂ = 400 cm, L ₀ = L _e = 58.6 cm, L _δ is determined to be 13.3 cm.

Table 1 shows L _δ and L _e And f _e (focal length of the virtual lens).

L _δ (cm) L _e (cm) f _e (cm) 1.5 80 -141 15 56 -128 25 40 -121 35 27 -113

4 shows a pulse train obtained from a long period high power femtosecond laser device according to the present invention. With a repetition rate of 13.15 MHz (MHz), the time interval between pulses is 76 ns. Continuous aperture was converted into pulses by soft aperture mode locking, and pulse energy of about 172 nJ was obtained at 9.5W pumping, and the higher order dispersion value was about -600 fs2.

5 is a shape of a pulse obtained from a long period high power femtosecond laser device according to the present invention. When measured by an interference autocorrelator method, it is shown that the pulse is generated at 26 femtoseconds when the half-width width is 40 femtoseconds and the shape of the pulse emitted from the laser is assumed as the sech function.

6 is a spectrum obtained from a long period high power femtosecond laser device according to the present invention. The full width at half maximum is 39 nm.

It will be apparent to those skilled in the art that various modifications, substitutions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. will be. Accordingly, the embodiments disclosed in the present invention and the accompanying drawings are not intended to limit the technical spirit of the present invention but to describe the present invention, and the scope of the technical idea of the present invention is not limited by the embodiments and the accompanying drawings. . The protection scope of the present invention should be interpreted by the following claims, and all technical ideas within the equivalent scope should be interpreted as being included in the scope of the present invention.

10 laser device 12 pump laser
14: focusing lens 16: gain medium
18, 20: 1st, 2nd prism CM1, CM2: 1st, 2nd curved mirror
M1, M2, M3, M4, M5: 1st, 2nd, 3rd, 4th, 5th reflective mirror
30: first multi-reflective mirror 32: first through-hole
40: second multiple reflection mirror 42: second aperture
50: output diameter

Claims (6)

A first resonator that primarily resonates the pump beam; And
A fifth reflection reflecting a first multi-reflection mirror having a first aperture, a second multi-reflection mirror having a second aperture, and a laser beam transmitted from the first resonator to the first multi-reflection mirror through the second aperture A second resonator including a mirror to resonate the laser beam output from the first resonator;
Laser device comprising a. The method of claim 1,
And a pump laser to generate the pump beam. The method of claim 1,
The first resonator,
A focusing lens for focusing the pump beam, a gain medium through which the pump beam focused by the focusing lens passes, a first curved mirror reflecting or retroreflecting a laser beam passing through the gain medium, a first reflecting mirror, And a second reflective mirror, a second curved mirror, and a third reflective mirror. delete The method according to any one of claims 1 to 3,
And the laser beam finally reflected from the second multi-reflective mirror passes through the first aperture and the laser beam passed through the first aperture is transmitted to the output mirror. The method of claim 5, wherein
The laser device, characterized in that for changing the laser characteristics by adjusting the distance between the output diameter and the first aperture.

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