JPH01205488A - Laser device - Google Patents

Abstract

PURPOSE:To stabilize an oscillation wavelength in the case of a narrow band oscillation, by installing a light source projecting a light beam spectrally diffracted only by a wavelength tuning element to the wavelength tuning element, and providing a control mechanism which detects an interference fringe generated and alters a wavelength capable of being transmitted so that it becomes equal to the interference fringe corresponding to the wavelength of oscillation of specified laser beam. CONSTITUTION:At least two of wavelength tuning elements 5, 6, which respectively vary wavelengths capable of being transmitted while spectrally diffracting incident beams and have different transmission wavelength width, are set up into an optical resonator 1 for a laser oscillator. Interference fringes formed by laser beams are detected, and the wavelength capable of being transmitted of the first wavelength tuning element 6 having narrowest transmission wavelength width is changed so that the interference fringes are brought to the reference interference fringes of laser beams having a specified oscillation wavelength while a light source 11 projecting beams spectrally diffracted only by a second wavelength tuning element 5 is fitted to the element 5. Interference fringes, which are spectrally diffracted from beams from the light source 11 and generated, are detected, and the wavelength capable of being transmitted of the wavelength tuning element 5 is altered so that the interference fringes become equal to interference fringes corresponding to the oscillation wavelength of the predetermined laser beams. Accordingly, the centers of the transmission wavelengths of each wavelength tuning element 5, 6 coincide with each other, and the transmission wavelengths are tuned with a fixed narrow band laser oscillation wavelength at all times.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a narrowband laser oscillator, and particularly to stabilization of its oscillation wavelength.

[Conventional technology]

Figure 7 shows, for example, the magazine “OAN, J, PHY8. VOL.
63 ('85).

In the figure, (1) is an optical resonator, and includes a laser medium (2), a total reflection mirror (8), and a partial reflection mirror (4). ). (5) and (6) are wavelength tuning elements for coarse tuning and fine tuning that are provided in the optical resonator to narrow the band of the laser beam, and a typical example is an etalon.

(γ) is a laser beam whose band is narrowed and goes out of the optical resonator (1).

Next, the operation will be explained. In Fig. 7, a laser medium (2) is usually surrounded by an optical resonator consisting of a total reflection mirror (8) and a partial reflection mirror (4), and the light is amplified while traveling back and forth through this optical resonator many times. , is extracted as a laser beam (γ). However, some laser oscillators, such as excimer lasers, semiconductor lasers, dye lasers, and some solid-state lasers, have a wide oscillation wavelength range, and can be oscillated by inserting a wavelength tuning element into the optical resonator. The wavelength width can be narrowed. For example, if a plurality of wavelength tuning elements are used as in this example, a laser beam as close to monochromatic light as possible can be obtained.

In particular, a case will be described in which two wavelength tuning elements, a coarse tuning wavelength tuning element (5) and a fine tuning wavelength tuning element (6), are inserted into an optical resonator. FIG. 8 is a diagram showing the principle of narrowing the laser emission amplitude, and (a) shows the spectral characteristics of the coarse tuning wavelength tuning element (5). The position λ77L1 of each peak of this spectral characteristic becomes the wavelength determined by Equation (1) and Equation 2 (1). Here, n is the refractive index of the material between the two mirror surfaces that make up the wavelength tuning element, d is the distance between the mirror surfaces, θ1 is the angle at which light enters the wavelength tuning element, and m is an integer. be. Several peaks correspond to different m. As is clear from this equation, the peak wavelength of the peak can be freely changed by changing n, d, and θ.

On the other hand, the region between the peaks is called a free light spectral region (hereinafter abbreviated as FSR), and is expressed by equation (2). Further, the half-width Δλ1 of each peak is expressed by equation (3). Here, the value is called finesse and is determined by the performance of the wavelength tuning element.

On the other hand, FIG. 8(C) shows the spectral characteristics of the gain of the laser medium. If there is no wavelength tuning element in the optical resonator, the light is amplified within the range where this gain exists and becomes a laser beam. At that time, a coarse tuning wavelength tuning element (5
) so that the peak position h1 of Due to the presence of the wavelength tuning element, a state with less loss is achieved only at λO, and light is amplified and oscillates only around that wavelength.

By the way, if there is only one peak, the FSR
The minimum value of I is determined, and the finesse 1 is determined by the performance of the wavelength tuning element and is about 20 at most, so there is a limit to the wavelength width that can be narrowed with only one coarse tuning wavelength tuning element.

Therefore, another wavelength tuning element (6) for fine tuning is used. The spectral characteristics may be as shown in FIG. 8(b), for example. In this case, the peak wavelength λm2 may be set equal to λ0, and the FSR2 may be set to FSR22Δλ1.

If you want to make it even narrower, you can use one more wavelength tuning element.

In this way, by using two wavelength tuning elements, the laser beam that originally had the spectral characteristics as shown in Figure 8(0) can be changed to Oscillation occurs only in a narrow range centered on λ0 where the peaks overlap. In reality, since the laser beam passes through the wavelength tuning elements many times during oscillation, the linewidth of the laser beam is equal to the wavelength width lb determined by the two wavelength tuning elements.

Now, as described above, it is possible to narrow the wavelength width of a laser beam, but in the conventional example, not much is written about stabilizing the wavelength.

However, as noted in the magazine, short-term stability can be improved by improving the optical resonator or reducing the incident angle θ, but in the long term, thermal problems, especially laser beams, can be improved. A major problem is wavelength shift due to heat generation when passing through a wavelength tuning element. This problem will be explained using FIG. 9.

FIG. 9(a) is an enlarged view of the spectral characteristics of the coarse tuning wavelength tuning element (5), and the solid line represents the spectral characteristics immediately after oscillation. By the way, after oscillation, heat is generated by the laser beam and the wavelength tuning element is deformed. Although this deformation does not degrade the properties of the wavelength tuning element, it does change the gap length of the wavelength tuning element, thereby shifting the wavelength.

There is a relationship expressed by equation (4) between the amount of shift and the change in d due to deformation of the wavelength tuning element.

Δλ=□Δd・・・・・・・・・・・・・・・
(4) Here, the direction of the wavelength shift is determined by the structure of the wavelength tuning element, etc., and if a specific wavelength tuning element is used, the wavelength will be shifted in one direction due to the heat generated by the laser beam. The state of the shift at that time is shown by the dotted line in Fig. 9 (al). On the other hand, a similar wavelength shift has also occurred in the wavelength tuning element (6) for fine tuning. 1 The state is shown in Fig. 9 (b). The amount of wavelength shift of the fine tuning wavelength tuning element becomes smaller by the amount that the interval d2 of the wavelength tuning elements is larger than the interval d1 of the coarse tuning wavelength tuning element.

Now, the problem in this case is that the peak wavelengths λm1 and 2m2 of the spectral characteristics of the two wavelength tuning elements are shifted. At that time, the amount of light transmitted when both are overlapped is reduced compared to the case where λml and 2m2 are equal. The state of laser oscillation at that time is shown in FIG. 9(C). After oscillation for a long time, the laser output undergoes a wavelength shift from λ0 to 2 m2 and the output decreases. Furthermore, when the shift m is large, oscillation in other modes of the wavelength tuning element for fine tuning may also occur.

[Problem to be solved by the invention]

Conventional narrowband laser devices are configured as described above, and a wavelength shift occurs due to thermal deformation of the wavelength tuning element.
The problem is that this method can only be applied to low-power lasers with small thermal deformation because the output power also decreases.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to obtain a laser device whose oscillation wavelength is stabilized when the band is narrowed.

[Means to solve the problem]

A laser device according to the present invention has at least two wavelength tuning elements each having a different transmission wavelength width, each of which can change the wavelength that can be transmitted into an optical resonator of a laser oscillator, and which splits incident light into different wavelengths, and which can be formed from a laser beam. The first wavelength tuning element having the narrowest transmission wavelength width is changed so that the interference fringes are detected and the interference fringes become reference interference fringes for a laser beam having a predetermined oscillation wavelength. A second wavelength tuning element is provided in which a light source that inputs light that is separated by this alone is applied, and a second wavelength tuning element is used to detect interference fringes generated by the separation of the light and to make the interference fringes correspond to the oscillation wavelength of a predetermined laser beam. This changes the wavelength that can be transmitted.

[Effect]

In this invention, the centers of the transmission wavelengths of each wavelength tuning element coincide, and are always tuned to a predetermined narrow band laser oscillation wavelength.

[Embodiments of the invention]

FIG. 1 is a configuration diagram showing a laser device according to an embodiment of the present invention, and (1) to (γ) are similar to those of the conventional example.

The following explanation will be given by taking as an example the case where there are two wavelength tuning elements, one for coarse tuning and one for fine tuning. (9) is a first interference fringe detector that detects interference fringes formed from a part of the laser beam (γ) guided through a reflecting mirror (8). (10)
is a first wavelength tuning element control mechanism that changes the wavelength that can be transmitted by the fine tuning wavelength tuning element (6) so that this interference fringe becomes a reference interference fringe of a laser beam having a predetermined oscillation wavelength. (old) is a light source that enters the light that is separated by itself into the coarse tuning wavelength tuning element (5), and (13) is a light source in which this light is focused by a condenser lens (1z) and input to the coarse tuning wavelength tuning element (5). () 51 is a second interference fringe detector which detects the interference fringes produced by this light transmitted through the coarse tuning wavelength tuning element (6) and guided through the reflecting mirror I. The interference fringes generated at this time are spectrally separated only by the coarse tuning tuning element (5).(16) adjusts the coarse tuning wavelength so that the interference fringes correspond to the oscillation wavelength of the predetermined laser beam. This is a second wavelength tuning element control mechanism that changes the wavelength that can be transmitted by the tuning element (5). (17) is a selection control mechanism that selects the necessity and priority of coarse tuning or fine tuning control.

Next, the operation will be explained. As in the conventional example, by inserting two wavelength tuning elements (5) and (6) into the optical resonator, a laser beam (γ ) can be obtained. However, as mentioned above, this alone results in unstable wavelength and output, and therefore a control mechanism for the wavelength tuning element as described below is required.

First, the control mechanism of the wavelength tuning element for fine tuning, that is, the wavelength tuning element having the narrowest transmission wavelength width will be explained. Since the transmission wavelength width of this wavelength tuning element is equal to the output wavelength width of the laser beam, this is controlled by changing the wavelength that can be transmitted so that the interference fringes formed by the laser beam become predetermined reference interference fringes. First, an example of detection of interference fringes will be explained with reference to FIG.

In FIG. 2, a portion of the laser beam (γ) is guided to an interference fringe detector (9). The interference fringe detector (9) may have a spectroscopy function using, for example, an etalon, a prism, a grating Fizeau interferometer, etc., but in this embodiment, a case where an etalon and an image sensor are used will be described.

The interference fringe detector (9) consists of an integrator (2) that weakens or diffuses the laser beam, an etaloshi (4), and a lens (2).
It consists of 21. Of the divergent components generated from the isitegrator (20) K, only the component having a specific incident angle θ passes through the etalon (21) and reaches the imaging lens 02.

If the focal length of the lens is f, then light having a component of θ gathers at a focal position at a distance of f·θ from the axis of the lens, forming circular interference fringes. Therefore, the image sensor (2
By observing the position where the light gathers according to 3), θ is determined,
λ can be calculated from the equation for the transmission wavelength of the wavelength tuning element shown above.

By the way, the intensity distribution of light on the image sensor is as shown in FIG. The vertical axis shows the output, and the horizontal axis shows the distance X from the center of the interference fringe. The winter mountains correspond to differences in the order m of the etalon. The interval between winter mountains is called a free spectral range, and the wavelength can be uniquely determined within this range. Moreover, since the free spectral range can be determined by design, such as the distance between the mirror surfaces of the etalon, it is better to design it to be wider than the expected wavelength shift.

In addition, since Fuyuyama has a light intensity distribution corresponding to the wavelength distribution of the laser beam, this is processed and analyzed by the image processing unit (241) to obtain θ.Furthermore, here, the current wavelength λ
is calculated, and the oscillation wavelength is adjusted through the control mechanism 00).

In this way, the interference fringes formed by the laser beam are detected, and the wavelength tuning element having the narrowest transmission wavelength width is In this embodiment, the oscillation wavelength of the laser can be controlled to a predetermined wavelength by changing the angle of the fine tuning wavelength tuning element (6) to change the wavelength that can be transmitted.

Next, the control mechanism of the coarse tuning wavelength tuning element (5) will be explained. In the output of the laser beam, only the transmitted wavelength width component determined by the fine tuning wavelength tuning element appears, and the influence of the coarse tuning wavelength tuning element is hidden. Therefore, it is necessary to control the coarse tuning wavelength tuning element using a light source that spectrally disperses only this element. At this time, if the optical axis of this light source is set to match the laser optical axis, the light from this light source will pass through the fine tuning wavelength tuning element as it is and will be separated by only the coarse tuning wavelength tuning element. In order to do this, it is necessary to have a different wavelength from the laser beam. However, in the case of a light source that enters only the coarse tuning wavelength tuning element from a direction that is angularly shifted from the laser optical axis, for example from an oblique direction, the wavelengths do not necessarily need to be different.

In the example shown in Fig. 1, when the optical axes match, 01)
is the light source. Here, the laser medium (1) is a KrF laser with a center wavelength of 248, while the light source (1)
1) K uses a HeNe laser with a center wavelength of 633M. The light U from the He Ne laser has a line width of 0 and OO
It is stable with 5#I11 or less. Note that when a single mode HeNe laser is used, a linewidth that is even more stable and narrower by an order of magnitude can be obtained. In addition, Ar laser, Na laser beam, etc. can also be used as this light source since their wavelengths are different from the laser oscillation wavelength. The light from such a light source (II) is collected by the lens a, and is incident on the coarse tuning wavelength tuning element (5). At this time, mirror (8), (4
) and the fine-tuning wavelength tuning element (6) are coated with a coating that sufficiently transmits the wavelength of HeNe.

On the other hand, the coarse tuning wavelength tuning element is coated with a coating that has a high reflectance for both the wavelength of the laser beam (γ) and the wavelength of HeNe, and this reflection characteristic is, for example, as shown in Figure 4. It has become. At this time, increase the finesse to prevent the resolution of the coarse tuning wavelength tuning element (5) from decreasing because the wavelength of He Ne is more than twice as long as the wavelength of the KrF laser. Here, the finesse is a quantity representing the resolution of a wavelength tuning element, and is a property that increases as the diameter of a beam passing through the wavelength tuning element becomes smaller or as the reflectance increases.

Now, in such a laser device, when a HeNe laser is incident on the coarse tuning wavelength tuning element (5), only wavelengths having a specific incident angle component are selected.

This beam is reflected by a reflector (1, 4) having a high reflectance at a wavelength of HeNe, for example, and is sent to a second interference fringe detector (1, 4).
5). In the interference fringe detector (I5), as an example of its configuration is shown in FIG. 2, the light guided here is condensed by a lens (25) to produce circular interference fringes. After detecting this interference pattern with the image sensor (26), the image processing unit (
27), the transmission center wavelength λml of the coarse tuning wavelength tuning element can be found. Here, the image sensor is a one-dimensional image sensor or a two-dimensional image sensor placed across one of the interference fringes.
A split photodiode etc. can be used.

In this way, the coarse tuning wavelength tuning element (
The wavelength that can be transmitted can be controlled by changing the angle of the light beam.

Next, a specific example of such a control method will be explained.

FIG. 5 (A+ is a flowchart diagram showing an outline of a method for stabilizing the laser wavelength according to an embodiment of the present invention, in which the position where the spatial light intensity distribution of the laser beam is maximized is determined, and the oscillation wavelength is An example of controlling is shown below.

In step (30), the etalon (21) separates the laser beam, and in step (31), the image sensor (23) measures the one-dimensional light intensity distribution. In step (32), this measurement data is smoothed and subjected to image processing such as removing noise. In step (33), the position X showing the maximum intensity is determined. Next, in step (34), the value χ0 (specified wavelength If they are different, the fine tuning wavelength tuning element (6) is controlled through the mechanism α0) from X>χ0 or X<XOK to set the center of the transmission range of the fine tuning wavelength tuning element. Change the wavelength λm2 (step (35))
, returns to Nutetsububuchi again and repeats this operation until χ-χO is reached. As described above, the wavelength tuning element for fine tuning (
By adjusting 6), the oscillation wavelength of the laser can be kept constant.

On the other hand, for controlling the coarse tuning wavelength tuning element (5), in step (36) the light from the light source (11) is focused, and circular interference fringes are created by only the coarse tuning wavelength tuning element (5). generate. In step (37), -dimensional light intensity distribution is measured. In step (38), this measurement data is smoothed and subjected to image processing such as removing noise. In step (39), the position Y showing the maximum intensity is determined. Next, in step (40), the value Yo (λml- (specified position coordinates corresponding to λm2), and if different, Y>YO or Y (Y
o controls the coarse tuning wavelength tuning element (5) through the control mechanism (16) to change the center wavelength λml of the transmission range of the coarse tuning wavelength tuning element (step (41)), and returns to step (36) again. Repeat this operation until =Yo. In addition, when a two-split photodiode is used as a detector, the diode should be placed at the point YO, and the coarse tuning wavelength tuning element (5) should be controlled through the control mechanism 06) so that the outputs of the two elements are equal. Bye.

By the way, the two wavelength tuning elements (5) and (6) may be controlled at the same time, but for example, if the center wavelength of the fine tuning wavelength tuning element (6) is moved, the laser output may fluctuate. Therefore, if control is performed in a chaotic manner, output fluctuations may be exacerbated. Therefore,
In order to monitor both controls, a selection control mechanism (17) is provided, and the first part (A+, (
H) Allow control selection to be made. In this embodiment, priority is given to control (B) immediately after the start of laser oscillation, and priority is given to control (A) after the operation has stabilized to some extent.

In the above embodiment, an etalon is used as the interference fringe detector, but any element capable of spectroscopy may be used, such as a prism or a Fizeau interferometer.

We also show how to control a wavelength tuning element by image processing the light intensity distribution of interference fringes formed by a laser beam as an interference fringe detector or by splitting light from a light source to determine the wavelength shift. However, even if the light intensity distribution is not subjected to image processing, the same effect can be achieved as long as the interference fringes can be detected, for example by arranging a two-split photodetector and controlling based on the change in light intensity.

As shown in Fig. 6, the light from the light source 01) is incident obliquely so that it enters only the wavelength tuning element for coarse tuning, and the interference fringes that are generated by transmitting or reflecting the light are observed. The coarse tuning wavelength tuning element may be controlled by detection. However, when using reflected light, the intensity distribution of interference fringes is reversed, but by performing signal processing etc., the same effect as in the above embodiment can be achieved.

Furthermore, although the explanation has been made for the case of two wavelength tuning elements, if there are more than two wavelength tuning elements, the one with the narrowest transmission wavelength width is controlled by the interference fringes formed by the laser beam, and the other wavelength tuning elements are spectralized using them. It is only necessary to provide a light source and control using interference fringes generated by the light source.

〔Effect of the invention〕

As explained above, this invention has at least two wavelength tuning elements in the optical resonator of a laser oscillator, detects interference fringes formed by a laser beam, and detects interference fringes formed by a laser beam having a predetermined oscillation wavelength. In order to create stripes, the wavelength that can be transmitted by the wavelength tuning element with the narrowest transmission wavelength width is changed, and a light source is provided that inputs the light that is split only by this wavelength tuning element into the other wavelength tuning elements, and the light is split from this light. We configured a control mechanism that detects the interference fringes generated by the laser beam and changes the wavelength that can be transmitted so that the interference fringes correspond to the oscillation wavelength of a predetermined laser beam, thereby stabilizing the oscillation wavelength when narrowing the band. effective.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing a laser device according to an embodiment of the present invention, Fig. 2 is a diagram showing an example of an interference fringe detector used in an embodiment of the invention, and Fig. 3 is a diagram showing an example of an interference fringe detector used in an embodiment of the invention. A diagram showing an example of a light intensity distribution for explaining the operation of the embodiment, FIG. 4 is a diagram showing the relationship between wavelength and reflectance of a wavelength tuning element used in an embodiment of the present invention, and FIG. A flowchart diagram showing an outline of an example of a stabilization method performed in one embodiment of the present invention, FIG. 6 is a configuration diagram showing a laser device according to another embodiment of the present invention, and FIG. 7 shows a conventional laser device. The configuration diagram, Figure 8 is a diagram to explain that the laser output is narrow banded by multiple wavelength tuning elements, and Figure 9 is a diagram to explain that the laser output is reduced based on the wavelength shift of the wavelength tuning elements. This is a diagram for In the figure, (1) is an optical resonator, (5) is a second wavelength tuning element, (6) is a first wavelength tuning element, (9) is a first interference fringe detector, and (ltO) is a First wavelength tuning element control mechanism, 01) is a light source, 05) is a second interference fringe detector, (
16) is a second wavelength tuning element control mechanism. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Scope of Claims] A laser oscillator having at least two wavelength tuning elements with different transmission wavelength widths, each of which can change the wavelength that can be transmitted into an optical resonator and separates incident light, and a laser extracted from this laser oscillator. a first interference fringe detector that forms and detects a first interference fringe from the beam; and a wavelength tuning element that adjusts the first interference fringe to become a reference interference fringe for a laser beam having a predetermined oscillation wavelength. A first wavelength tuning element control mechanism that changes the wavelength that can be transmitted by the first wavelength tuning element that has the narrowest transmission wavelength width among them, and a second wavelength tuning element other than the first wavelength tuning element. a light source that inputs light to be separated; a second interference fringe detector that detects second interference fringes generated by the separation of the light; and the second interference fringe corresponds to the oscillation wavelength of a predetermined laser beam. and a second wavelength tuning element control mechanism that changes the wavelength that can be transmitted by the second wavelength tuning element so as to produce interference fringes.