JPH01115183A - Laser wavelength stabilization and wavelength stabilized laser apparatus - Google Patents

Abstract

PURPOSE:To stabilize wavelength and reduce variation of laser output by controlling one of two plates of an etalon on the basis of the result of spectroscopic analysis of laser beam and controlling the other plates based on a laser power. CONSTITUTION:A part of laser beam 6 irradiated from a laser oscillator which can vary oscillation wavelength is extracted using a first Fabry-Perot etalon 4 and a second Fabry-Perot etalon 5, and it is then analyzed spectrometrically with a wavelength monitor mechanism 7 to determine oscillation wavelength. The first Fabry-Perot etalon 4 is controlled according to the oscillation wavelength to stabilize the wavelength of laser oscillator. Next, a part of laser beam 6 is extracted, its laser output is measured with a power monitor mechanism 9 to control the second Fabry-Perot etalon 5 and stabilize an output of laser oscillator. Thereby, wavelength can be stabilized and reduction of laser output can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for stabilizing a laser wavelength and a wavelength-stabilized laser device.

[Conventional technology]

Figure 5 shows, for example, magazines [OAN, J, PHYS, VO].
FIG. 2 is a configuration diagram showing a conventional narrow band laser shown in "L63 ('85) 214".

In the figure, (1) is a laser medium, (2) is a total reflection mirror,
(3) is a partial reflecting mirror, (4) is a coarse adjustment etalon, (5) is a fine adjustment etalon, and (6) is a laser beam.

Next, the operation will be explained. In Fig. 5, normally the laser medium (1) is a total reflection mirror (2) and a partial reflection mirror f#, (3
) The light is amplified while reciprocating many times and is extracted as a laser beam (6). By the way, some laser oscillators, such as excimer lasers, semiconductor lasers, dye lasers, and some solid-state lasers, have a wide oscillation wavelength width,
The oscillation wavelength width can be narrowed by inserting a spectroscopic element within the spectral element 6. For example, as in this example, by using a plurality of Fabry-Perot etalons (hereinafter abbreviated as etalons), it is possible to obtain a laser beam close to monochromatic light without limit.

Here, we will particularly describe the case where two etalons, a coarse adjustment etalon (4) and a fine adjustment etalon (5), are inserted into an optical resonator. FIG. 6 is a diagram showing the principle of narrowing the laser emission amplitude, and (a) shows the spectral characteristics of the rough adjustment etalon. The position of the peak of each mountain of this spectral characteristic λm1
is the wavelength to be irradiated by the formula (1). Here, n is the refractive index of the material between the two mirror surfaces that make up the etalon, d is the distance between the mirror surfaces, θ1 is the angle at which light enters the etalon, m
is an integer. Several peaks correspond to differences in m. As is clear from this equation, the peak wavelength of the mountain can be freely changed by changing n+d10. On the other hand, the distance between peaks is called a free spectral range (hereinafter abbreviated as FSR) and is expressed by equation (2). Further, the half-width Δλ1 of each peak is expressed by equation (3). Here, 1 is called finesse and is determined by the performance of the etalon.

On the other hand, FIG. 6(C) shows the spectral characteristics of the gain of the laser medium. If there is no spectroscopic element in the optical resonator, the light is amplified within the range where this gain exists and becomes a laser beam. At that time, set the peak position λml of the rough adjustment etalon to be equal to the wavelength λoK somewhere within the range where the gain exists, and also within 1 m of the wavelength where the gain exists.
If dl etc. are determined so that other peaks other than 1 do not appear,
Due to the presence of the coarse tuning etalon, a state with little loss is achieved only at λ0, and light is amplified and oscillates only around that wavelength.

By the way, if there is only one peak, the FSR
The lowest value of l is fixed, and finesse> depends on the performance of the etalon, and is about 20 at most, so there is a limit to the wavelength width that can be narrowed with only one rough adjustment etalon.

Therefore, another etalon (5) for fine adjustment is used. The spectral characteristics may be as shown in FIG. 6(b), for example. In this case, the peak wavelength λm2 may be set equal to λ0, and the FSR2 may be set such that FSR2≧△λ1.

If you want to make it even narrower, you can use another etalon.

Therefore, by using two etalons, the laser beam that originally had the spectral characteristics as shown in Figure 6 (0) will overlap the peaks of each etalon as shown in Figure 6 (, i). It will oscillate only within a narrow range centered around λ0. In reality, the laser beam passes through the etalon many times during oscillation, resulting in a laser beam.

O Now, as described above, the wavelength width of a laser beam can be narrowed, but in the conventional example, not much is written about stabilizing the wavelength.

However, as described in the magazine, short-term stability can be improved by improving the vanguard equipment and reducing the angle of incidence θ, but in the long term there are thermal problems, especially when the laser beam A major problem is the wavelength shift caused by the heat generated during transmission.

This problem will be explained using FIG. 7.

FIG. 7(a) shows an enlarged view of the spectral characteristics of the rough adjustment etalon.The solid line represents the spectral characteristics immediately after oscillation. By the way, after oscillation, heat is generated by the laser beam and the etalon is deformed. Although this deformation does not degrade the characteristics of the etalon, it does change the etalon gap length.
As a result, the wavelength is shifted. There is a relationship expressed by equation (4) between the amount of shift and the change in d due to the deformation of the etalon.

Here, the direction of the wavelength shift depends on the structure of the etalon, etc. If a specific etalon 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 Figure t-47 (IIL).

On the other hand, the fine tuning etalon also undergoes a similar wavelength shift. The situation is as shown in FIG. 7(b).

The wavelength shift of the fine tuning etalon is made smaller by the amount 11Ad2 between the etalons which is larger than dl of the coarse tuning etalon.

Now, the problem in this case is that the peak wavelengths λml and 7m2 of the spectral characteristics of the two etalons are shifted.

At that time, the amount of light transmitted when both are overlapped is reduced compared to the case where λml=λm2. The state of laser oscillation at that time is shown in Fig. 7 (C) h After long-time oscillation, the laser output is λ0
As the wavelength shifts from 7m2 to 7m2, the output decreases.

When the amount of shift is large, oscillations in other modes of the fine adjustment etalon may also occur.

[Problem that the invention seeks to solve]

Conventional narrowband laser devices are configured as described above, and do not have a means to correct the wavelength shift due to thermal problems in the etalon, but instead prevent the output reduction when using two etalons. Since the method does not have any means, there is a problem that it can only be applied to low-power lasers with small thermal deformation.

This invention was made to solve the above-mentioned problems, and it is possible to stabilize the wavelength when the lightning area is narrowed, and also to suppress the decrease in output.

[Means for solving problems]

The method for stabilizing the laser wavelength according to the present invention spectrally spectra a part of the laser beam whose wavelength has been selected by two etalons,
By controlling a single etalon based on the analysis and results of the spectroscopic laser beam, the wavelength of the laser beam is stabilized, and changes in the laser output from a part of the laser beam are measured, and the changes in the output By analyzing this and controlling the other etalon, the decrease in laser output is also suppressed.

Further, a wavelength stabilized laser device according to another aspect of the present invention selects the wavelength by including two etalons, a fine tuning etalon and a coarse tuning etalon, and also controls the wavelength of the laser beam extracted from the laser oscillator. The device is equipped with a servo mechanism that guides the oscillator to a wavelength monitor mechanism, measures the oscillation wavelength, and drives the υ fine tuning etalon according to the measured wavelength to change the wavelength.

Furthermore, apart from the wavelength monitor mechanism, it is equipped with a power meter for measuring changes in laser output, and a power monitor mm consisting of a part for recording output changes, and a coarse adjustment etalon is set based on the signal from the power monitor mechanism. It is equipped with a servo mechanism for control.

[Effect]

The laser wavelength stabilization method and wavelength-stabilized laser according to the present invention can fix the wavelength to an arbitrary wavelength by measuring the wavelength shift, and can suppress the decrease in output that occurs when the wavelength shifts.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

In FIGS. 1 and 2, (1) to (5) are similar to the conventional example. (6) is a laser beam, (7) is a wavelength monitor mechanism, (8) is a control mechanism, (9) is a power monitor mechanism, αG, α1) is a servo mechanism for controlling the etalon, (2) is an integrator, The α peak is a Fabry-Perot etalon, (14) is an imaging lens, and (to) is an imaging element for observing interference fringes generated by the Fabry-Perot etalon 03, for example, a -dimensional image sensor.

αQ is an image processing unit for analyzing interference fringes.

Next, the operation will be explained. As in the conventional example, by inserting two etalons (4) and (5) into the optical resonator, a laser beam (6) with a narrow rooting wavelength width and an arbitrary wavelength λ0 within the range where gain exists is generated. can be obtained. However, this is not enough - as mentioned above, the wavelength and output are unstable, so an etalon control mechanism as described below is required.

First, the control mechanism of the fine adjustment etalon will be explained.

In FIG. 2, a portion of the laser beam (6) is guided to a wavelength monitoring mechanism (7). The wavelength monitor mechanism (power is measured by the power of the
It is sufficient to have a spectroscopy function using an etalon, a prism, a grating Fizeau interferometer, etc. as shown in ctronics QE-14-('78) 17J, but in this example, as shown in Fig. 2, A case in which an etalon and an image sensor are used will be explained below.

The wavelength monitoring mechanism (7) consists of an integrator (6) that weakens or diffuses the laser beam, an etalon (to), and lenses α and ◇. Among the firing components generated by the integrator @, only the component having a specific incident angle θ passes through the etalon and reaches the zero station of the imaging lens. When the focal length of the lens is f, light having a component of θ gathers at a distance of fθ from the axis of the lens at the focal point, forming circular interference fringes. Therefore, by observing the position where the light gathers on the image sensor (C), θ can be determined, and λ can be calculated from the formula for the transmission wavelength of the etalon shown earlier.O By the way, the light on the image sensor The intensity distribution of is shown in Figure 3. 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. Furthermore, since the free spectral range can be determined depending on the role of the FP, it is designed to be wider than the expected wavelength shift.

Also, is this because winter mountains have a light intensity distribution that corresponds to the wavelength distribution of the laser beam? An image processing unit αQ is required to process and output θ2. Furthermore, the current wavelength λ is calculated here, and the wavelength of the oscillator is adjusted through the servo mechanism 00.

FIG. 4(A) is a flowchart 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 is performed. An example of wavelength control is shown below.

The laser beam is separated into spectra by the etalon at the step α power, and the light intensity distribution in the sea dimension is measured by the image sensor QG at the step Q frame. In step (11, this measurement data is smoothed and subjected to image processing such as removing noise, and in step ('', the position x showing the maximum intensity is determined, and then in step Cp
Compare the value KO (specified wavelength (specified position coordinate corresponding to C) obtained with The center wavelength λm2 of the area is changed (of the step), and the process returns to step αη again and repeats this operation until !::XQ is reached.

By adjusting the fine tuning etalon as described above, the oscillation wavelength of the laser can be kept constant.

Next, the control mechanism of the rough adjustment etalon (4) will be explained. In FIG. 1, a portion of the laser beam (6) is guided to a power monitor mechanism (9). The power monitor mechanism (9) consists of a part that measures the laser output and a part that records the obtained laser output.
) in either direction, determine whether the laser power increases or decreases, and then determine how to adjust the crude etalon (4).

According to this determination, the center wavelength λm2 of the coarse adjustment etalon (4) is adjusted by the servo mechanism αη. A flowchart of this adjustment is shown in FIG. When laser oscillation starts, the phenomenon shown in FIG. 7 occurs, and the laser output changes. Therefore, the output p is measured in step (a), the result is recorded, and it is compared with the previous measurement result po in step (a). When the outputs are different, the coarse adjustment etalon (4) is adjusted using the servo mechanism (2) depending on whether p>po or pupa. This operation is continued until the rough adjustment etalon (4) reaches thermal equilibrium and the laser output becomes constant.

By the way, the two etalons (4) and (5) may be controlled at the same time, but for example, the fine adjustment etalon (5) may be controlled simultaneously.
) The laser output may fluctuate due to excessive movement of the center wavelength, and chaotic control may even exacerbate the fluctuations in the output. Therefore, in order to monitor both controls, a control mechanism (8) is provided, and 70-
The first part of the chart (4) (B) allows selection of control. In this example, immediately after the start of laser oscillation, (B)
Priority is given to (g) control after the operation has stabilized to some extent.

In the above embodiment, the wavelength monitor mechanism (7) and the power monitor mechanism (9) were provided separately, but the image processing section of the wavelength monitor mechanism was originally configured to obtain a light intensity distribution as shown in FIG. If the device is operated for a long time without any control, a wavelength shift and output change will occur, as shown by the broken line. Therefore, if we measure the intensity change of the peak that appears in the image processing section, we can obtain
This has the same effect as providing a power monitor mechanism.

In addition, although an etalon was used as the wavelength monitor and mechanism in the above embodiment, any spectroscopic element such as a Fizeau interferometer, grating, or prism may be used. It has the same effect as the example.

In addition, in the above embodiment, a method was shown in which the optical intensity distribution of the separated laser beam is image-processed as a wavelength monitoring mechanism to determine the wavelength shift and the fine adjustment etalon is fully driven. Needless to say, a method that allows monitoring would have the same effect. As a method that does not perform image processing on the light intensity distribution, for example, an optical sensor is placed at X=IQ in Figure 3 to serve as a wavelength monitoring mechanism, and the fine adjustment etalon is changed back and forth from the optimum state to determine the current X=IQ.
There is also a method of controlling the fine adjustment etalon by setting the direction of the optimum state of the fine adjustment etalon to the child side based on the change in the light intensity at .

〔Effect of the invention〕

As described above, according to the present invention, in a laser oscillator using two etalons, one is controlled based on the spectral results of the laser beam, and the other is controlled based on the laser power. The effect is that it is possible to stabilize the laser output and to reduce fluctuations in the laser output.

Further, according to another invention of the present invention, a laser oscillator that can select the laser oscillation wavelength by disposing two etalons for coarse tuning and fine tuning in an optical resonator, A servo mechanism that separates a part of the laser beam, determines the oscillation wavelength, and controls the fine tuning etalon based on the above results.A servo mechanism that measures the output of the laser beam and controls the coarse tuning etalon based on the direction of the output change. Since we set up
This not only keeps the laser's oscillation wavelength stable, but also has the effect of stabilizing its output.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing a wavelength stabilized laser according to an embodiment of the present invention, Fig. 2 is a block diagram showing a wavelength monitoring mechanism in the present invention, and Fig. 3 is an interference fringe on an image sensor of the wavelength monitor section. FIG. 4 is a flowchart showing an outline of a laser wavelength stabilization method according to an embodiment of the present invention, FIG. 5 is a +14 diagram showing a conventional narrow band laser, and FIG. The figure is an explanatory diagram for explaining a method of determining wavelength using two etalons, and FIG. 7 is an explanatory diagram for explaining that an output cost is generated due to a difference in wavelength shift between two etalons. In the figure, (1) d laser medium, (4) coarse tuning etalon, (5) fine tuning etalon, (7) wavelength monitor mechanism,
(8) is the control mechanism, (9) is the power monitor mechanism, αco
, 1L1) is a servo mechanism. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (6)

[Claims] (1) Using a first Fabry-Perot etalon and a second Fabry-Perot etalon, a part of the laser beam emitted from a laser oscillator with a variable oscillation wavelength is taken out and separated by a wavelength monitor mechanism to determine the oscillation wavelength. a step of determining the wavelength of the laser oscillator by controlling the first Fabry-Perot etalon according to the oscillation wavelength; a step of extracting a part of the laser beam and measuring the laser output with a power monitor mechanism; A method for stabilizing a laser wavelength, in which the output of the laser oscillator is stabilized by controlling the second Fabry-Perot etalon and another one. (2) The laser wavelength according to claim 1, characterized in that the oscillation wavelength is controlled by determining the position where the intensity is maximum in the spatial light intensity distribution of the separated laser beam. Stabilization method. (3) The wavelength monitoring mechanism uses a third Fabry-Perot etalon and measures the light intensity distribution of ring-shaped interference fringes that appear when the laser beam passes through the device. The method for stabilizing the laser wavelength according to item 1 or 2. (4) Stability of the laser wavelength according to any one of claims 1 to 3, wherein the power monitor mechanism measures the maximum intensity of the spatial light intensity distribution appearing in the wavelength monitor mechanism. method. (5) Detect the light intensity at a point near the peak of the spatial light intensity distribution of the separated laser beam, and predict the optimum state of the etalon from the change in light intensity when the etalon is moved before and after the optimum state. , Claim 1 in which the oscillation wavelength is controlled by controlling the etalon.
Method for stabilizing the laser wavelength as described in section. (6) For selecting the laser oscillation wavelength within the optical resonator,
A laser oscillator with a variable wavelength that includes a Fabry-Perot etalon for fine tuning and a Fabry-Perot etalon for coarse tuning, a wavelength monitor mechanism that monitors the wavelength of a laser beam extracted from this laser oscillator, and a wavelength monitor mechanism that monitors the wavelength of a laser beam extracted from the laser oscillator; a servo mechanism for controlling the fine adjustment Fabry-Perot etalon based on the signal; a power monitor mechanism for measuring the output of the laser beam and analyzing changes in the laser output;
A wavelength stabilized laser device comprising a servo mechanism for controlling the coarse tuning Fabry-Perot etalon based on a signal from the power monitor mechanism.