JP2007133445A - Wavelength converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength converter made of a nonlinear optical crystal that is suitable for obtaining fifth harmonic laser light from laser light of a fundamental wavelength, has excellent thermal stability and high reliability.
The wavelength of laser light having a fundamental wavelength is converted into laser light including a second harmonic by a first nonlinear optical crystal, and after the second harmonic is separated, the wavelength is converted to a fourth harmonic by a second nonlinear optical crystal. A wavelength conversion device for converting and summing this with a laser beam of a fundamental wavelength by a third nonlinear optical crystal to obtain a fifth harmonic,
In at least the second nonlinear optical crystal 1 of the second and third nonlinear optical crystals, the thickness of the laser light in the non-walk-off direction y is a size that does not clip the laser light, and the laser light Is smaller than the width in the walk-off direction x, the width is larger than the spread of the laser beam by the walk-off, and a temperature control element 6 is provided at least on the crystal plane in the non-walk-off direction y. .
[Selection] Figure 1

Description

  The present invention relates to a wavelength conversion device that performs wavelength conversion using a nonlinear optical effect.

  Laser light is generally higher in frequency than radio waves, so it has a large information capacity, and has the same wavelength and the same phase, so it has excellent monochromaticity and directivity, and interference that is not found in ordinary light. In addition, since it can be converged very finely, it has features such as concentrating energy on a small area and realizing high temperature and high pressure locally and instantaneously. It is applied to various fields such as processing technology and medical applications.

  A wavelength conversion element made of a nonlinear optical crystal capable of generating a second harmonic by converting the wavelength of a laser beam, for example, is a laser light source applied to a semiconductor exposure apparatus such as a stepper used for semiconductor microfabrication. And a laser beam generator that can oscillate stable high-power laser beam.

  In recent years, regarding such laser light sources and laser light generators (for example, an exposure light source used in a semiconductor manufacturing apparatus), studies have been made on increasing the output power and shortening the wavelength of laser light.

  Various methods are currently used as means for achieving higher output and shorter wavelength of the laser beam. One of the most promising of these is the short wavelength by nonlinear wavelength conversion of the solid-state laser. There is a method for obtaining laser light in a region, particularly in the ultraviolet region. Solid-state lasers can be provided more efficiently and cheaply than semiconductor lasers, and are considered to be more reliable than gas lasers using KrF, ArF, or the like.

  Here, for example, the fifth harmonic of a neodymium: yttrium / aluminum / garnet laser (hereinafter referred to as Nd: YAG laser) that oscillates a laser beam having a wavelength of 1064 nm, that is, a laser beam having a wavelength of 213 nm has high energy conversion. Since it is efficient, can be miniaturized, and is supplied at a relatively low cost, it is promising as a light source for next-generation exposure apparatuses.

  As a system for obtaining such higher-order harmonics, there is a system disclosed in Patent Document 1 described later. According to this, the fundamental wavelength laser beam is converted into a second harmonic by the first nonlinear crystal, and this is converted into a mixed wavelength laser beam including the third and fourth harmonics by the second nonlinear optical crystal. Further, laser light of each wavelength is selected through an attenuator, a filter, and a beam splitter.

  As a method of actually obtaining the fifth harmonic, first, from the fundamental wave having a wavelength of 1064 nm, the second harmonic having a wavelength of 532 nm is generated by the first SHG (Second Harmonic Generation; hereinafter the same) generation process. In addition, a laser beam having a wavelength of 266 nm (hereinafter sometimes referred to as a fourth harmonic for convenience) is obtained by the second SHG generation process. Then, the fundamental laser beam having a wavelength of 1064 nm and the fourth harmonic wave having a wavelength of 266 nm remaining without being wavelength-converted in the first SHG generation process is subjected to a sum frequency mixing (hereinafter referred to as SFM) process to a fifth harmonic. A method of obtaining laser light having a wavelength of 213 nm to be a wave has been implemented.

  The fifth harmonic generation process will be described with reference to FIG.

  First, a laser beam a having a wavelength of 1064 nm oscillated from an Nd: YAG laser oscillator (not shown) passes through a lithium borate crystal (hereinafter referred to as LBO crystal) 20 which is a nonlinear optical crystal (first SHG). At this time, a laser beam having a wavelength of 532 nm, which is the second harmonic, is generated, but the laser beam b emitted from the LBO crystal 20 is a laser beam including a laser beam having a wavelength of 1064 nm and a laser beam having a wavelength of 532 nm.

  Next, the laser beam b emitted from the LBO crystal 20 enters the separation mirror 24, where the laser beam e with a wavelength of 1064 nm is transmitted, the laser beam c with a wavelength of 532 nm is reflected, and the optical axis is 90 degrees. It is bent and separated.

  The separated laser beam c having a wavelength of 532 nm is further reflected by the total reflection mirror 25 and passes through a β-barium borate crystal (hereinafter referred to as a BBO crystal) 21 which is a nonlinear optical crystal to be a fourth harmonic. A laser beam d having a wavelength of 266 nm, which is a wave, is generated (second SHG). The laser beam d having a wavelength of 266 nm is reflected by the total reflection mirror 26 and the separation mirror 27 and guided to the BBO crystal 22 which is a nonlinear optical crystal capable of sum frequency mixing.

  On the other hand, the laser beam e having a wavelength of 1064 nm transmitted through the separation mirror 24 is transmitted through the separation mirror 27 and guided to the BBO crystal 22.

  In this BBO crystal 22, the laser light e having a wavelength of 1064 nm and the laser light d having a wavelength of 266 nm are sum-frequency mixed to generate a 213 nm ultraviolet laser light that is the fifth harmonic of the fundamental laser light, and this wavelength is 213 nm. Laser beam f is emitted.

Here, as described above, an LBO (LiB ₃ O ₄ ) crystal is widely used as the nonlinear optical crystal in the first SHG. Since this LBO crystal has a high laser damage threshold and low optical loss, it is suitable for use in generating a laser beam (second harmonic) having a wavelength of 532 nm from a fundamental laser beam having a wavelength of 1064 nm. is there.

In addition, as described above, the second SHG and the SFM for generating the fifth harmonic include BBO (β-BaB ₂ O ₄ ) crystal and CLBO (CsLiB ₆ O ₁₀ ) crystal as nonlinear optical crystals. Often used. Note that the LBO crystal used in the first SHG cannot satisfy the phase matching condition due to its optical characteristics, and is inappropriate for generation of laser light having a wavelength of 266 nm.

  However, the CLBO crystal shows good wavelength conversion characteristics when using a laser with a low repetition pulse (for example, about 10 to 20 pulses per second), but for example, every second as used in an exposure apparatus for semiconductor manufacturing. Under high repetitive operation such as several thousand pulses, there is a problem that the harmonic output decreases rapidly.

  Therefore, in an environment where a high repetition pulse laser is required, as the nonlinear optical crystal used for the second SHG (that is, as the nonlinear optical crystal 21 in FIG. 6) and the SFM for generating the fifth harmonic, BBO crystals are used.

JP-T-10-500628 (page 17, line 13 to page 21, line 8, FIGS. 2A and 2B)

  Here, a wavelength conversion element using a BBO crystal (particularly, the nonlinear optical crystal 21 of FIG. 6) is shown in FIG.

  A nonlinear optical crystal 11 shown in FIG. 5A has a laser light incident surface 12 and a laser light emission surface 13 having a lateral length x ′ and a longitudinal length y ′, and a length z ′ (z ′ = 10 in the optical axis direction). A parallelepiped crystal having ˜15 mm). In general, the laser light incident surface 12 and the laser light emitting surface 13 are formed into a square having a lateral length x ′ = vertical length y ′ (x ′ = y ′ = 3 to 5 mm). Although not shown, temperature control elements are arranged on all surfaces other than the entrance surface 12 and the exit surface 13.

  Here, for example, the laser beam 4 having a wavelength of 532 nm enters the nonlinear optical crystal 11 from the laser beam incident surface 12, propagates in the nonlinear optical crystal 11 while receiving a nonlinear optical effect, generating a walk-off, and emitting the laser beam. Wavelength conversion from the surface 13 to laser light 5 with a wavelength of 266 nm is performed.

  That is, the incident laser light 4 is subjected to a nonlinear optical effect in the nonlinear optical crystal 11 and is wavelength-converted while gradually becoming flat laser light while propagating through the length z ′ in the optical axis direction. As shown in FIG. 5 (A), the incident laser beam 4 reaches the emission surface 13 and the incident laser beam 4 is a laser beam 4 ′ having a substantially circular cross section perpendicular to the optical axis (that is, the laser beam incident surface 12). On the laser beam emission surface 13, the laser beam 5 'is flat in the walk-off direction (that is, the x-axis direction in the figure).

  Therefore, as shown in FIG. 5B, heat is generated in the nonlinear optical crystal 11 by absorption of laser light, particularly laser light 5 'flat in the walk-off direction. In particular, in the case of laser light having a short wavelength laser beam or even a wavelength in the ultraviolet region, the heat generation rate is large. The heat generation source (heat source) also has a flat shape similar to that of the laser beam 5 ', and the generated heat is diffused in the direction indicated by the arrow 15 in the figure.

  Here, in general, a temperature control element 14 is provided on a surface other than the laser light incident surface and the laser light emitting surface of the nonlinear optical crystal 11. The temperature control element 14 is configured to relieve and further control the heat, and can remove the instability of the nonlinear optical crystal 11 due to heat.

  However, in spite of such heat control means being arranged in a normal wavelength conversion element, long-term output degradation and periodic output fluctuations may occur, and a highly reliable wavelength It was difficult to say a conversion element.

  That is, for example, when an Nd: YAG laser is used as a laser oscillator and a BBO crystal is used in the fourth harmonic generation process, the output is not so rapidly degraded as when a CLBO crystal is used, but the frequency is 7 kHz and the average output When the fourth harmonic wave having a wavelength of 266 nm is generated at about 7 W, when this is carried out for a long time, deterioration of the nonlinear optical crystal accompanied by a drop in the harmonic output is observed. However, these output degradations are due to individual differences.

  Further, in an experiment using a high-power laser light source and trying to generate a fourth harmonic having a wavelength of 266 nm with an output of 2 W or more, for some crystal samples, periodicity that seems to be due to thermal instability The output fluctuated. This was a phenomenon in which the output fluctuated between about 0.5 W and 2 W in a cycle of about 1 minute.

  Such fluctuations in output cause the nonlinear optical crystal itself to absorb the generated short-wavelength laser light, particularly ultraviolet light, leading to a temperature rise of the crystal, and the nonlinear optical crystal has a temperature dependence of the refractive index. This is considered to be because the necessary phase matching condition is not satisfied. Note that the input laser beam used in these experiments is a substantially circular laser beam having a diameter of about 150 μm to 250 μm.

Therefore, the present inventor has said that the thermal instability is
(1) Generation of harmonics (2) Absorption of short-wavelength laser light (particularly ultraviolet laser light) by nonlinear optical crystal (3) (Local) temperature rise of nonlinear optical crystal (4) Phase matching condition is not satisfied It is considered that the harmonic output is lowered (5) The temperature is close to the initial state, that is, it is caused by the cyclic behavior (hereinafter the same).

  In order to solve this, it is needless to say that an ideal solution is to use a nonlinear optical crystal that has no absorption by short-wavelength laser light, particularly ultraviolet laser light. Use is practically impossible.

  In addition, if it is possible to partially relax the periodic behavior, it is considered that the thermal instability can be reduced. However, no effective means has been found at present.

  In other words, at present, typically, an extraordinary ray having a wavelength of 266 nm, which is the fourth harmonic of the Nd: YAG laser, has an absorption of about 2% / cm. Such an increase in temperature is a serious problem because it involves the thermal instability. The same applies to the above-described Patent Document 1.

  The present invention has been made in view of the above-described circumstances, and the object thereof is suitable for obtaining a fifth harmonic laser beam from a fundamental wavelength laser beam, which is excellent in thermal stability, stable and An object of the present invention is to provide a wavelength conversion device made of a highly reliable nonlinear optical crystal.

  As a result of intensive studies to solve the above-mentioned problems, the present inventor, in a wavelength conversion element composed of a nonlinear optical crystal that converts the wavelength of laser light using a nonlinear optical effect, in the non-walk-off direction, It has been found that when the diameter of the nonlinear optical crystal with respect to the diameter of the laser beam is set to a specific value or less, a wavelength converter made of a nonlinear optical crystal having excellent thermal stability, stability and reliability can be obtained.

That is, the present invention includes a first nonlinear optical crystal that converts the wavelength of incident laser light having a fundamental wavelength and emits laser light including a second harmonic; and the emitted laser light from the first nonlinear optical crystal. Separating means for separating the laser light of the fundamental wavelength and the laser light of the second harmonic; and converting the wavelength by making the separated laser light of the second harmonic incident to convert the laser light of the fourth harmonic A second nonlinear optical crystal that emits light; and a sum frequency mixing of the fourth harmonic emission laser light and the fundamental wavelength laser light from the separation mirror to emit a fifth harmonic laser light. A third nonlinear optical crystal; and laser light walks in a plane substantially perpendicular to the optical axis through at least the second nonlinear optical crystal of the second and third nonlinear optical crystals. Configured to propagate while generating off The A wavelength converter,
In at least the second nonlinear optical crystal of the second and third nonlinear optical crystals, the thickness of the laser light in the non-walk-off direction is large enough not to clip the laser light, and the laser light walk The width is smaller than the width in the off direction, the width is larger than the spread of the laser beam due to the walk-off, and a temperature control element is provided at least on the crystal plane in the non-walk-off direction. The wavelength converter (hereinafter referred to as the wavelength converter of the present invention).

  According to the wavelength conversion device of the present invention, the laser beam is transmitted through at least the second nonlinear optical crystal of the second and third nonlinear optical crystals having the laser beam incident surface and the laser beam emission surface. In a wavelength converter that propagates while generating a walk-off in a plane substantially perpendicular to the axis, the thickness of the nonlinear optical crystal in the non-walk-off direction of the laser beam is a size that does not clip the laser beam. Since the width of the nonlinear optical crystal in the walk-off direction is smaller than the width of the nonlinear optical crystal, the heat generated by the laser light propagating in the nonlinear optical crystal can be easily controlled by the temperature control element. By suppressing the local temperature rise of the crystal, the phase matching condition is less likely to fluctuate, the thermal stability is excellent, and the wavelength is stable and reliable. It is possible to construct a changeover device.

  Such an effect brings about a significant performance improvement in the second nonlinear optical crystal that generates the fourth harmonic, but in the third nonlinear optical crystal that generates the fifth harmonic, the final fifth harmonic is achieved. It is also important for maintaining and improving wave performance.

  In particular, with respect to the non-walk-off direction, the thickness of the nonlinear optical crystal with respect to the diameter of the laser beam is preferably 15 times or less. That is, if it exceeds 15 times, it becomes difficult to control the heat generated by the laser beam, and the resulting laser beam becomes thermally unstable. The thickness is more preferably 5 times or less.

  Here, the “walk-off” means a laser beam shift in an anisotropic crystal, and the walk-off direction means a laser beam spreading direction caused by the shift.

  In other words, in a crystal with anisotropy (refractive index anisotropy), the laser beam causes birefringence, and the laser beam having a polarization called an extraordinary ray generated by this birefringence is in the direction of energy flow. Is slightly different from the normal direction of the wave front. For example, in the case of the BBO crystal, the generated short-wavelength laser light becomes an extraordinary ray, and when the laser light having a wavelength of 266 nm is generated, the deviation angle (walk-off angle: hereinafter the same) is about 4.8 degrees. Therefore, the harmonics generated in the vicinity of the laser light incident surface of the nonlinear optical crystal and the harmonics generated in the vicinity of the laser light emitting surface are slightly separated in space.

  As a result, the laser light that has undergone wavelength conversion generates a walk-off in a plane substantially perpendicular to the optical axis, and becomes a laser light having a wide width in the walk-off direction. On the other hand, the laser beam does not spread in the direction perpendicular to the walk-off direction (that is, the non-walk-off direction: the same applies hereinafter). As a result, for example, a laser beam having an aspect ratio of 1: 1 is used. When incident, the generated laser light (harmonic) becomes “flat laser light” both in the crystal and in a state of being emitted from the crystal.

  FIG. 3 conceptually shows this state.

  In FIG. 3, (1) is an input beam pattern. In the figure, the harmonics generated near the entrance show the beam pattern shown in (2) and substantially reflect the input beam pattern shown in (1). The harmonics generated in the beam pattern (2) propagate in the crystal while being shifted to the right (in the x-axis direction in the figure) as shown in FIG. On the other hand, in the case of a BBO crystal, the input beam, that is, the fundamental wave, has an ordinary ray polarization due to the phase matching condition, and no shift (shift) occurs due to walk-off. Therefore, the fundamental wave propagates in the y-axis direction in the figure.

  Here, since the harmonic is generated from the fundamental wave in the whole crystal (propagation direction) direction, for example, the harmonic generated at the position (3) in the figure is on the y-axis. The harmonics generated at the position (2) and propagated to the position (3) in the figure come to a position shifted to the right. For (4) to (6) in the figure, only the harmonic pattern generated at that position and the harmonic generated and propagated in (1) are shown. In the figure, (7), that is, at the position of the exit of the crystal, the beam patterns generated at the positions (2) to (7) are shown.

  Since the laser beam (laser light) actually obtained as an output is a superposition of all the beam patterns generated in the nonlinear optical crystal, it spreads in the walk-off direction as shown in (8) in the figure. It becomes a flat pattern. In the figure, the x-axis and the y-axis indicate the walk-off direction and the wavefront normal direction, respectively.

  As described above, in order to remove the thermal instability of the nonlinear optical crystal, it is necessary to partially relax the above-described periodic behavior. As one of the methods, for example, By shortening the period, the amplitude can be reduced. That is, since this periodic behavior is caused by a time delay, it is important to reduce this time delay (that is, shortening the thermal relaxation time).

  According to the wavelength conversion element used in the wavelength conversion device of the present invention, the heat generated by the laser light propagating in the nonlinear optical crystal can be easily controlled, so that the thermal relaxation time can be shortened. Thus, it is possible to shorten the period of the periodic behavior of the thermal instability, and it is possible to provide a wavelength conversion element with excellent stability.

  Further, as a method of using the wavelength conversion device of the present invention, of the second and third nonlinear optical crystals, at least the second nonlinear optical crystal is moved in the walk-off direction, and the incident position of the laser beam is determined. It is good to change.

  According to this method of use, when using the wavelength conversion device of the present invention, the nonlinear optical crystal is moved in the walk-off direction so that the incident position of the laser beam (that is, the propagation path of the laser beam in the crystal) is changed. For example, even if the crystal partially deteriorates after long-term use, the non-degraded portion (hereinafter referred to as fresh) can be obtained by moving the nonlinear optical crystal (for example, parallel movement). The laser light can be propagated, that is, wavelength-converted, and the life of the wavelength conversion element can be extended without exchanging the nonlinear optical crystal.

  In the wavelength conversion device according to the aspect of the invention, it is desirable that the thickness of the nonlinear optical crystal is not less than twice the diameter of the laser light in the non-walk-off direction of the laser light.

  Regarding the non-walk-off direction of the laser beam, if the thickness of the nonlinear optical crystal is made too small with respect to the diameter of the laser beam, the laser beam to be input or output is clipped and an effective output can be expected. It may disappear. Therefore, in the wavelength conversion device of the present invention, it is desirable that the thickness of the nonlinear optical crystal is not less than twice the diameter of the laser beam in the non-walk-off direction of the laser beam. The thickness is more preferably 2 to 5 times, but when the diameter of the laser beam is about 200 to 300 μm, the thickness is as small as about 1 mm in consideration of handling properties and ease of processing. This is because a nonlinear optical crystal can be produced.

  In the wavelength converter of the present invention, the nonlinear optical crystal is a parallelepiped, and a cross section perpendicular to the optical axis in the parallelepiped corresponds to the width in the walk-off direction of the laser light. A rectangular shape having a short side corresponding to the thickness in the non-walk-off direction, and the short side is not more than 15 times the diameter of the laser light in the non-walk-off direction. It is desirable.

  That is, the nonlinear optical crystal is a parallelepiped and the short side is 15 times or less with respect to the diameter of the laser beam in the non-walk-off direction, thereby absorbing laser light (particularly, short wavelength laser light). The heat can be efficiently exhausted or controlled from the heat generation source by suppressing the fluctuation of the phase matching condition due to the temperature change, and at the same time, the damage of the nonlinear optical crystal due to the temperature rise can be suppressed.

  Even in such a case, it is desirable that the short side of the exit surface or the entrance surface is at least three times the diameter of the laser beam from the viewpoint of preventing the clipping.

  In other words, of course, it depends on the wavelength of the laser light and the type and size of the nonlinear optical crystal, but when the exit surface or the entrance surface is rectangular, the crystal processing and handling are also taken into consideration (in the walk-off direction). Side length): (Length of side in non-walk-off direction) = 20: 1 to 2: 1 is desirable. This ratio is further desirably 5: 1 to 3: 1.

  That is, the size of the beam spread by the walk-off is almost proportional to the length of the crystal. For example, when a crystal having a crystal length (z in FIG. 1) of about 10 to 15 mm is used, on the exit side, the walk The size of the beam in the off direction (that is, the beam width) is about 1 mm. Accordingly, a crystal having a width of 2 mm or more (x in FIG. 1) is preferable. In addition, when considering using a fresh spot by parallel movement of the crystal, a wider one is preferable.

  For example, when a BBO crystal is used, in order to produce a crystal whose width (x in FIG. 1) exceeds 10 mm, it is necessary to grow a relatively large crystal, and it can be taken from one grown boule (bulk). Since the amount of the device is limited, for example, assuming that a 0.5 mm thick crystal is used, it seems realistic that the length of the side in the walk-off direction is up to 20 times (that is, 10 mm).

  In the wavelength conversion device of the present invention, at least in the second nonlinear optical crystal, an element for controlling the temperature of the nonlinear optical crystal is provided on a surface other than the laser light incident surface and the laser light emission surface. It is desirable that

  Of course, the temperature control element may be provided only on the long side surface of the nonlinear optical crystal. The temperature control element may be a metal having high thermal conductivity such as copper connected to a Peltier element. Further, it may not be in close contact with the nonlinear optical crystal, and may be provided via, for example, a resin having good thermal conductivity. Further, a temperature control means such as a Peltier element may be provided directly on the nonlinear optical crystal. Furthermore, the temperature control element may be arranged only in the closest part of the optical path of the laser light propagating through the nonlinear optical crystal.

  In the wavelength converter of the present invention, the first nonlinear optical crystal for obtaining the second harmonic laser beam is made of lithium borate to obtain the fourth or fifth harmonic laser beam. The second and third nonlinear optical crystals for the purpose are preferably β-barium borate.

  As described above, the β-barium borate crystal (BBO crystal) is used to generate laser light having a wavelength of 266 nm, which is the fourth harmonic, from laser light having a wavelength of 532 nm, which is the second harmonic of the Nd: YAG laser. It is useful as a nonlinear optical crystal used in the above. That is, the absorption and heat generation of the laser beam by the nonlinear optical crystal are caused by a laser beam having a short wavelength, particularly a laser beam having a wavelength in the ultraviolet region. Therefore, the nonlinear optical crystal is a β-barium borate crystal. When laser light in the ultraviolet region is generated, the effect of the characteristic configuration of the present invention is great.

  According to the present invention, the length (or width) of the nonlinear optical crystal in the walk-off direction with respect to the diameter of the emitted laser light in the walk-off direction on the laser light emitting surface is represented by x in FIG. (Direction) is preferably 3 times or more.

  With respect to the walk-off direction, the nonlinear optical crystal is moved in the walk-off direction (parallel movement) when the crystal is deteriorated by long-term use by making the length three times or more the diameter of the laser beam. ), The laser beam can be propagated in the part where the crystallinity has not deteriorated (fresh spot), and the life of the wavelength conversion element can be extended without exchanging the nonlinear optical crystal (ie, simple calculation, A lifetime of at least three times that of a typical wavelength conversion element). The upper limit of the length is not particularly limited, but when a nonlinear optical crystal having a length of 10 mm (z direction) is used, a laser beam having a width (diameter) of about 1 mm is generated on the emission side. The upper limit of the length is preferably 10 times the diameter of the laser beam.

  Next, preferred embodiments of the present invention will be described.

  FIG. 1 shows a nonlinear optical crystal that can be used, for example, when generating the fourth harmonic (wavelength 266 nm) from the second harmonic (wavelength 532 nm) of an Nd: YAG laser that oscillates laser light with a wavelength of 1064 nm. In FIG. 1, the walk-off direction is the x-axis direction in the figure, and the non-walk-off direction is the y-axis direction in the figure.

  A nonlinear optical crystal 11 shown in FIG. 1 (A) has a laser beam incident surface 2 and a laser beam exit surface 3 having a lateral length x and a longitudinal length y, and is a parallelepiped nonlinear optical system having a length z in the optical axis direction. It is a crystal. Although not shown, temperature control elements are arranged on all surfaces other than the entrance surface 2 and the exit surface 3.

  Here, for example, the laser beam 4 having a wavelength of 532 nm enters the nonlinear optical crystal 1 from the laser beam incident surface 2, propagates in the nonlinear optical crystal 1 while receiving a nonlinear optical effect, generating a walk-off, and emitting the laser beam. Wavelength conversion from the surface 3 to laser light having a wavelength of 266 nm is performed.

  That is, the incident laser beam 4 is subjected to nonlinear optical effects in the nonlinear optical crystal 1 and is wavelength-converted while being gradually flattened while propagating through the length z in the optical axis direction. As shown in FIG. 1A, if the incident laser beam 4 is a laser beam 4 ′ having a substantially circular cross section perpendicular to its optical axis, the laser beam exit surface 3 has a walk-off direction. In other words, the laser beam 5 ′ is flat in the x-axis direction in the drawing.

  Therefore, as shown in FIG. 2, when the incident laser beam 4 having the diameter A in the walk-off direction is incident on the nonlinear optical crystal 1 (of course, the laser beam 4 is not walked off at this point), the walk The width of the laser light having the off angle θ increases in the walk-off direction as it travels through the nonlinear optical crystal 1, and the width of the laser light 5 in the walk-off direction when it is emitted from the laser light emission surface 3. Is the width B.

  That is, as shown in FIG. 4A, (1) when laser light 4 ′ having a diameter t in the non-walk-off direction and a diameter A in the walk-off direction is incident on the laser light incident surface 2 of the nonlinear optical crystal 1, As the laser light propagates through the nonlinear optical crystal 1, (2) the laser light has a width C in the walk-off direction on the surface perpendicular to the optical axis, and (3) on the exit surface 3 The laser light having a width B in the walk-off direction is emitted from the nonlinear optical crystal 1.

  Thus, based on the nonlinear optical effect, the laser light propagating in the nonlinear optical crystal having anisotropy gradually expands in the walk-off direction as A → C → B in the figure. However, the diameter t is substantially constant in the non-walk-off direction. According to the present invention, in FIG. 4A, the length y of the nonlinear optical crystal 1 in the non-walk-off direction is not more than 15 times the diameter t of the laser light 4 ′, and the relationship is y ≦ 15t holds.

  For example, when the nonlinear optical crystal is a BBO crystal (length z = 10 mm) and a laser beam having a wavelength of 532 nm (circular laser beam diameter φ = about 200 μm) is incident, the walk-off angle is about 4.8 °. Therefore, while the diameter A is about 0.2 mm (200 μm), the width B is about 1 mm, and the laser light in the walk-off direction is expanded about five times.

  Accordingly, as shown in FIG. 1B, heat is generated by absorption of the laser light 5 ′ flat in the walk-off direction by the nonlinear optical crystal 1, and this heat also has the same shape (heat source 5 ′). Divergence in the direction indicated by the middle arrow 7.

  At this time, since the temperature control element 6 is provided on the side surface of the nonlinear optical crystal 1, the heat is controlled by the temperature control element 6, and the instability of the nonlinear optical crystal 1 due to heat, that is, nonlinear optics. It is designed to remove the (local) temperature rise of the crystal.

  As described above, according to the present embodiment, in the non-walk-off direction, the distance between the heat source and the temperature control element is relatively close, and the local temperature rise of the nonlinear optical crystal 1 is suppressed, thereby achieving phase matching. It is possible to construct a wavelength conversion element that is less likely to change conditions, has excellent thermal stability, and is stable and highly reliable. Further, since the heat relaxation time can be shortened, the period of the periodic behavior described above can be shortened, and the stability of the wavelength conversion element can be improved.

  On the other hand, in the case of the nonlinear optical crystal shown in FIG. 5, especially when a BBO crystal is used as the nonlinear optical crystal, the size of the BBO crystal is usually about 10 mm to 15 mm in length z ′ in the optical axis direction. The direction perpendicular to the optical axis (x ′ and y ′, where x ′ = y ′) is generally about 3 mm to 5 mm from the viewpoint of handling properties.

  That is, the size of the incident surface 12 and the exit surface 13 in the nonlinear optical crystal shown in FIG. 5 is approximately the same as the incident laser beam (the diameter of about 150 μm to 250 μm when the fourth harmonic is generated in the Nd; YAG laser described above). The diameter is about 15 to 30 times the diameter of the circular laser beam.

  Therefore, heat generated by absorption of laser light having harmonic components, particularly laser light in the ultraviolet region, passes through the crystal until it is conducted to the mount serving as a heat sink, that is, the temperature control element 14. It must be conducted over a distance several times larger than the size of this, resulting in a time delay, that is, a longer thermal relaxation time.

  That is, for example, the thermal conductivity of a metal used as a crystal mount whose temperature is controlled by a Peltier element is about 400 W / m · K in the case of copper, for example, and 1 to 1.5 W / m in a BBO crystal. Considering that K is about 1 / hundredth, the effect of processing the nonlinear optical crystal 1 small in the non-walk-off direction helps shorten the time until the generated heat is alleviated. It is clear.

  This shortening of the thermal relaxation time is useful for fine adjustment when the crystal output angle is finely adjusted to satisfy the phase matching condition and the harmonic output is increased. That is, the manufacturing time of a laser light source (or wavelength conversion device) using a wavelength conversion element made of a nonlinear optical crystal is shortened.

  This is because when the harmonic output fluctuates by changing the angle of the crystal or the like, the time until the harmonic output converges to a constant value due to a change in thermal conditions is shortened.

  Further, when observing deterioration (long-term deterioration) of the nonlinear optical crystal accompanied by a decrease in harmonic output, for example, when the area of the cross section perpendicular to the optical axis of the nonlinear optical crystal is changed to 1/20 of the conventional case, The rate of deterioration was reduced (improved) to about 1/8. That is, it is considered that the above-mentioned long-term deterioration can be suppressed by processing the size of the nonlinear optical crystal to be particularly small in the non-walk-off direction.

  Here, a description will be given of reducing the size of the nonlinear optical crystal, particularly in the non-walk-off direction.

  This can be said for a generally used nonlinear optical crystal. In particular, in a BBO crystal, there is a walk-off angle due to the nonlinear optical effect, and the generated harmonics are substantially perpendicular to the walk-off direction ( That is, in the non-walk-off direction), the beam parameters are the same as those at the time of input, and in the walk-off direction, the laser beam moves away while generating a walk-off. The optical axis of the nonlinear optical crystal is in this plane, and therefore the phase matching angle is also defined in this plane.

  In addition, when laser light having a wavelength of 266 nm is generated in the BBO crystal, the walk-off angle is 4.8 °. Therefore, for example, the laser light generated by a crystal having a length of 10 mm is flat with a width of about 1 mm. Has an output pattern. Therefore, the nonlinear optical crystal cannot be reduced more than a certain amount in the walk-off direction.

  Therefore, by making it smaller in the non-walk-off direction and making it larger than the walk-off width of the harmonic laser beam in the walk-off direction, temperature control of the nonlinear optical crystal is facilitated, that is, thermal instability is suppressed. At the same time, the long-term deterioration described above is suppressed, and further, by shortening the period of the periodic behavior described above, it is possible to realize a wavelength conversion element capable of generating laser light with a stable and highly reliable optical output. Become.

  Further, by enlarging the nonlinear optical crystal in the x-axis direction in FIG. 1, that is, in the walk-off direction, when the crystal is deteriorated, the laser beam hits the undegraded portion by the parallel movement of the crystal. Thus, the lifetime of the wavelength conversion element can be extended without exchanging crystals.

  For example, as shown in FIG. 4B, the crystallinity deteriorates if the crystal is partially deteriorated (particularly, the optical path 5 ′ of the laser beam having the width B2 in the walk-off direction) by using for a long time. By moving the nonlinear optical crystal 1 in the walk-off direction while avoiding the portion (for example, parallel movement), the laser light can propagate through the portion (fresh spot) 9 where the crystallinity has not deteriorated, that is, the wavelength can be converted. In addition, the life of the wavelength conversion element can be extended without exchanging the nonlinear optical crystal.

  As mentioned above, although this invention was described about preferable embodiment, this invention is not limited to embodiment mentioned above.

  For example, as described above, the shape of the nonlinear optical crystal described above may be 15 times or less of the thickness of the nonlinear optical crystal with respect to the diameter of the laser beam in the non-walk-off direction. For example, the laser light incident surface and the laser light emission surface may not be parallel to each other. Further, the shape is not limited to a parallelepiped, and may be any shape such as a tapered flat shape, a cylindrical shape, and a truncated cone shape as long as it satisfies the characteristic configuration of the present invention. . The present invention may also be applied to a wavelength conversion element in which incident light is reflected in a crystal and emitted after internal resonance. In this case, the laser light emitting surface and the laser light incident surface are the same surface. It may be.

  In a nonlinear optical crystal, it is desirable to provide a temperature control element on a surface parallel to the optical axis of the laser beam, but it is not necessary to provide it on all surfaces (that is, all around the optical axis). It may be a part. In particular, the temperature control element may be provided only in the non-walk-off direction.

  Furthermore, the shape of the laser light incident on the nonlinear optical crystal may not be circular on a plane perpendicular to the optical axis of the laser light, but may be elliptical or flat laser light.

  The nonlinear optical crystal may be configured such that the laser beam propagates only in a specific direction, or may be configured such that the laser beam reciprocates in the crystal. Further, the laser beam may be propagated through the crystal along a plurality of optical axes.

  The wavelength conversion device of the present invention can be used for a laser wavelength conversion device in the field of optoelectronics such as an optical disk device and a laser printer, for example, as well as a wavelength conversion device for a semiconductor exposure apparatus (stepper).

Further, as the nonlinear optical crystal, in addition to BBO (β-BaB ₂ O ₄ ), other nonlinear optical crystals that generate the walk-off as described above can be used, and shapes suitable for the respective crystals can be used. Can be made.

  The wavelength conversion apparatus of the present invention has high energy conversion efficiency and is useful for obtaining a fifth harmonic that is promising as a light source for a next-generation exposure apparatus with high stability and reliability.

It is a principal part schematic perspective view (A) of the wavelength conversion element used for the wavelength converter of this invention, and the front view (B) of the laser beam emission side of the same wavelength conversion element. FIG. 2 is a schematic plan view of the wavelength conversion element. It is a conceptual diagram explaining the principle of walk-off generation | occurrence | production. Schematic diagram (A) showing the occurrence of walk-off of laser light from the laser light incident surface to the laser light emission surface in the nonlinear optical crystal according to the present invention, and the laser light emission surface when moving the nonlinear optical crystal during its use It is a schematic diagram (B). It is a principal part schematic perspective view (A) of a general wavelength conversion element, and a front view (B) of the laser beam emission side of the wavelength conversion element. It is a schematic diagram of a general wavelength converter.

Explanation of symbols

1, 11, 20, 21, 22 ... nonlinear optical crystal, 2, 12 ... laser light incident surface,
3, 13 ... Laser light exit surface, 4, 4 '... incident laser light, 5, 5', 9 ... emitted laser light,
6, 14 ... temperature control element, 7, 15 ... heat flow, 8 ... propagating laser beam,
24, 27 ... separation mirror, 25, 26 ... reflection mirror

Claims (5)

A first nonlinear optical crystal that converts the wavelength of incident laser light having a substrate wavelength and emits laser light including a second harmonic; and laser light having the fundamental wavelength that is emitted from the first nonlinear optical crystal. Separating means for separating the second harmonic laser beam and a second harmonic laser beam incident thereon, wavelength-converted to emit a fourth harmonic laser beam; A third nonlinear optical crystal that emits a fifth harmonic laser beam by summing the non-linear optical crystal; the fourth harmonic emission laser beam and the fundamental wavelength laser beam from the separating means; And the laser light propagates through at least the second nonlinear optical crystal of the second and third nonlinear optical crystals while generating a walk-off in a plane substantially perpendicular to the optical axis. A wavelength converter configured to Te,
In at least the second nonlinear optical crystal of the second and third nonlinear optical crystals, the thickness of the laser light in the non-walk-off direction is large enough not to clip the laser light, and the laser light walk The width is smaller than the width in the off direction, the width is larger than the spread of the laser beam due to the walk-off, and a temperature control element is provided at least on the crystal plane in the non-walk-off direction. Wavelength converter.   At least the second nonlinear optical crystal is a parallelepiped, and a cross section of the parallel hexahedron perpendicular to the optical axis has a long side corresponding to the width in the walk-off direction, and the non-walk-off The wavelength conversion device according to claim 1, wherein the wavelength conversion device has a quadrangle having a short side corresponding to the thickness in the direction.   2. The wavelength conversion device according to claim 1, wherein an element for controlling the temperature of the nonlinear optical crystal is provided on a surface other than the laser light incident surface and the laser light emission surface in at least the second nonlinear optical crystal.   The wavelength conversion device according to claim 1, wherein the first nonlinear optical crystal is made of lithium borate, and the second and third nonlinear optical crystals are made of β-barium borate.   2. The wavelength conversion according to claim 1, wherein at least the second nonlinear optical crystal of the second and third nonlinear optical crystals is moved in the walk-off direction to change the incident position of laser light. apparatus.

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