JPS6173933A - Generation method of optically mixed wave - Google Patents

Abstract

PURPOSE:To simplify constitution and to perform feedback control with less malfunction by amplifying the monitor output of an optically mixed wave generated from two optical paths having a temperature difference to a specific level and using the difference signal as an error signal for feedback control. CONSTITUTION:High frequency light components forming the optically mixed wave by laser light excitation from a laser light source 1 is outputted from two parallel optical paths of nonlinear optical medium 5 having a temperature difference through thermostatic parts 12 and 13 of a thermostatic chambers which are held at temperature a little belt higher than phase matching range temperature. Some of those light components are photodetected and monitored by photodetectors 17 and 20 and an arithmetic unit 18 which is supplied with their outputs amplifies monitor outputs in the state closest to a phase matching state with an amplification factor with which the two outputs become equal to each other. Then, the difference output after the amplification is used as the error signal to perform feedback control over the thermostatic chamber 2. Therefore, a modulator which imposes modulation on phase matching condition is eliminated, the constitution is simplified, and there is not optical higher harmonic variation due to the modulation, so the feedback control is performed with less malfunction and the stable optical mixed wave is generated.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for generating an optical mixed wave by phase matching of laser beams in which the output is stabilized.

(Prior art and its problems) Industries that use lasers have gone from the initial simple processing such as cutting and drilling to making full use of the wavelength dependence of light and the advanced controllability of lasers. It is spreading to photochemistry, semiconductor processes, etc. Along with this, when optical mixed waves such as optical harmonics, sum frequency waves, and parametric optical mixed waves are used as a light source, the light source is required to have a much higher output and stable current than ever before.

In optical mixed wave generation, temperature phase matching means that the optical paths of the excitation laser beam and the optical mixed wave completely overlap, and the conversion efficiency from the excitation laser beam to the optical mixed wave increases with the length of the nonlinear optical medium. It offers advantages such as being able to obtain high-quality optical mixed waves, and is widely used in industrial applications. However, in temperature phase matching, it is generally necessary to stabilize the temperature of the nonlinear optical medium to an accuracy of 0.1°C or less while correcting the self-heating effect in the nonlinear optical medium due to absorption of the excitation laser beam and optical mixed wave. is necessary.

Conventionally, as the most advanced stabilization technology to compensate for these effects and always satisfy the phase matching condition, D
Furthermore, a method has been reported for stabilizing the optical harmonic output by feeding back the phase of the intensity change of the corresponding optical harmonic and the captured sound as an error signal to a control system for phase matching conditions.

A case in which the modulation is performed by temperature is described in Japanese Patent Application No. 48-03110. In addition, a method of performing modulation using the electro-optic effect of a nonlinear optical medium was published in IEJ Journal of Phantoms in 1976.
Electronics (IEI! 13 Journal of
Q of Quantum Electronics) magazine
It is described on page 148 of volume E-12. Figure 5 is a diagram illustrating the operating principle of the device that has passed through the conventional method.
The horizontal axis is temperature and the vertical axis is 2? is the harmonic output.

In the figure, point P is the operating point in the phase matching state where the temperature matches the phase matching temperature T, and points 8 and B are the operating points in the phase matching state where the phase matching condition slightly deviates from the phase matching state, but the second harmonic output The operating point is approximately half of the phase-matched state and at the limit of the allowable temperature range. Amplitude ΔT at points A and B
The time-varying waveforms of temperature modulation and optical harmonic output when temperature modulation is added are shown as a, b, and c. FIG. 6 shows the phase detection output obtained by phase detection of the waveform of c using a as a reference signal, and shows the relationship of changes in harmonic output with respect to minute temperature changes. In order to approach the phase matching state at point A, it is necessary to raise the temperature, and conversely, it is necessary to lower the temperature at point B. This information can be known from the sign of the phase detection output in Figure 6, so the optical harmonic output can be stabilized by feeding back the phase detection output as an error signal to the phase matching condition control system. . , 747 are examples of an apparatus to which a conventional method is applied when modulation is performed by temperature. The operation of 1iVt will be briefly explained.

A laser light source 1 generates excitation laser light, and the excitation laser light is converted into optical harmonics by a nonlinear optical medium 5 installed inside a constant temperature chamber 2. A part of the obtained optical harmonics is reflected by the first beam splitter 6 and received by the photodetector 7. On the other hand, the inside of the constant temperature oven 2 is maintained near the phase matching temperature by the main heater 3 controlled by the main heater 1 source 11.

The constant temperature chamber 2 also includes a modulation heater 4 that modulates the temperature using a modulator 8. Information regarding deviations from the phase matching state caused by environmental temperature or fluctuations in the output of the excitation laser beam can be obtained by extracting a component synchronized with the modulation frequency of the modulator 8 from the output of the photodetector 47 using the phase detector 9. It will be done. The output of the phase detector 9 is fed as an error signal to the main heater power supply 11 via a DC amplifier 10, and the heating power of the main heater 3 is increased or decreased in accordance with the increase or decrease in the error signal. In this way, the temperature of the nonlinear optical medium 5 is reduced by one phase matching temperature. However, in a device to which such a conventional method is applied, it is essential to perform modulation in order to monitor deviation from the phase matching state, so the obtained optical harmonics include an output that matches the modulation frequency of the modulator 8. There is a drawback that fluctuations are included, and this output fluctuation cannot be removed.

Also, due to the modulation, the structure of the lighting system becomes double and standard,
There is a disadvantage that the device is expensive, and another disadvantage is that if the output fluctuation of the excitation laser beam is large, the feedback loop may malfunction, making stable operation impossible.

(Object of the Invention) An object of the present invention is to provide a simple and inexpensive device that does not cause fluctuations in optical harmonics due to modulation in the conventional optical mixed wave generation method for detecting deviation from phase matching. It is an object of the present invention to provide a method for generating an optical mixed wave that is configurable and has little risk of malfunctioning of a feedback loop.

(Structure of the Invention) According to the present invention, there is provided a method for generating an optical mixed wave for obtaining an optical mixed wave from an excitation laser beam using a control means for controlling the phase matching condition of a nonlinear optical medium. The optical path of the excitation laser beam in the medium is made into two parallel optical paths with a slight temperature difference, and the monitored outputs of the optical mixed waves generated from the two optical paths are kept in a phase aligned state. A method for generating an optical mixed wave is obtained, which is characterized in that the two outputs are amplified with amplification degrees that are approximately equal to each other, and the output after the amplification is fed back to the control means as an error signal.

c) Operation of the present invention) The operational feature of the present invention is that the excitation laser beam is passed through two optical paths with a temperature difference, and the difference in the optical mixed wave output between the two optical paths allows the phase matching state to be changed. Since the direction and degree of deviation can be detected, there is no need for a conventionally used modulator, there is no fluctuation in the four waves of the leading height due to this modulation, and the device configuration is simple and inexpensive. It is possible to avoid malfunctions due to fluctuations in

(Example) Next, the present invention will be specifically described with reference to the drawings. FIG. 1 shows an example of gH 1g in which the present invention is applied to second harmonic generation, and the functions and constituent fluorine names are the same as the conventional 14iIl shown in FIG. 7, except for the following points. The temperatures of the constant temperature chamber 2 and the second constant temperature section 13 are set by a temperature control unit 16 to a temperature that matches the allowable temperature range for phase matching. The first constant temperature section A2 is set to a temperature higher than the second constant temperature section 13 by a constant temperature difference that is sufficiently smaller than the allowable temperature range by the temperature control unit [6. 14 is a reflecting mirror, 15 is a second beam splitter that separates the excitation laser beam, and 119 and 21 are generated 2 beam splitters.
Some of the optical harmonics of the book are 17 and 20 first and second, respectively.
third and fourth beam splitters 11 leading to the detector of
8 is the operation unit. At this time, in the state closest to phase matching, the output of the first detection 517 is the second detection D2.
In the following, we will assume a case where the ratio is weakened to 0.01. In the opposite case, the structure will be omitted because it can be easily inferred from the following argument. The arithmetic unit 18 includes a part that amplifies the output of the first detector 17 and a part that amplifies the output of the first detector 17 and the amplified signal and the second detector 2.
It consists of a differential amplifier that amplifies the difference between the output and the zero output. The output of the differential amplifier is input to the temperature control unit 16. FIG. 2 shows the general relationship between the degree of deviation from the phase matching state of the nonlinear optical crystal 5 and the optical harmonic output.

T, , T, is 2 in the first embodiment shown in FIG.
The operating point is shown when the temperatures of the two optical paths are both lower than the phase matching temperature but within the allowable temperature range for phase matching.

FIG. 3 shows the relationship between the temperature of the thermostatic chamber 2 and the output of the arithmetic unit 18. At point P', the temperatures of the two optical paths are T, , T, respectively, as shown in FIG.

This shows the operating point when the motor is running. Arithmetic Uni,
The output of the differential amplifier in step 18 is adjusted to be zero when the phase matching conditions in the two optical paths are most satisfied, so the output of the differential amplifier is adjusted to be constant from the phase matching temperature of step 2. Using the output of this differential amplifier, in which the amount of error can be determined including its sign, the phase matching condition of the two optical paths can always be kept constant by the feedback loop shown in FIG. First detector 17 in calculation unit 18
The amplification degree of the part that amplifies the output of
It can be set in advance as a ratio of the output of the first detector 17 to the output of the second detector 20 in a similar state.

According to the device of the present invention, fluctuations in optical harmonics due to modulation observed in devices applying conventional methods are eliminated, and the nonlinear optical medium 5 can highly stabilize phase matching conditions.
Since it is sufficient for the temperature gradient provided within the nonlinear optical medium 5 to have a value smaller than the half-width of the temperature at which phase matching is possible, the strain applied to the nonlinear optical medium 5 has almost no effect on the reduction in the efficiency of optical harmonic generation.

The thermal conductivity of the nonlinear optical medium 5 is usually smaller than that of metals, etc., and it is easy to create a temperature gradient, so the structure of the constant temperature chamber 2 is simple, and another advantage of the present invention is that the device can be manufactured at low cost. . By taking advantage of the symmetry of the circuits from the calculation unit to G18 and arranging the light detectors in a symmetrical manner, it is possible to suppress the effects of detector drift and external noise, resulting in even higher stability. A high optical harmonic output can be obtained by the present invention. Figure 4 shows a fl to which the present invention is applied.
It is a figure which shows the Example of c2. In the figure, 22 is a differential amplifier, n is a folding mechanism, and the names and functions of other parts are the same as in the first embodiment shown in FIG.

In this case, both optical harmonics generated in the first and second optical paths are input to the second detector 20. The differential amplifier 22 is used to remove optical harmonic components generated in the first optical path. Note that the intensifier 24 is used to correct the effect that the optical harmonics generated in the first optical path are attenuated by the folding mirror and the second optical path. This configuration has the advantage that the optical harmonics obtained can be obtained with a single beam, and that the number of optical parts is reduced compared to the configuration shown in FIG. 1, making it easier to adjust the apparatus.

1 and 2 to which the present invention is applied as shown in FIGS. 1 and 4.
In addition to the embodiment described above, an integration circuit is provided between the first and second detectors 17 and 20 and the calculation unit 18 in FIGS. 1 and 4 as a configuration that takes advantage of the effects of the present invention and achieves even higher stability. can be placed.

By adding an integrating circuit, high frequency noise in the detection system can be reduced.
Even higher output stabilization can be achieved.

In the embodiments shown in FIGS. 1 and 4, a part of the excitation laser beam overlaps the optical path of the output beam, but if the excitation laser beam is not needed, a prism or a By installing a filter, only the optical harmonics can be separated and used.

In the above embodiments to which the present invention is applied, optical harmonic generation has been described, but it goes without saying that other parameters)
The present invention can also be applied to other phase-matchable optical mixed wave generation such as IJ book mixing and sum frequency generation.

(Effects of the Invention) According to the present invention, it is possible to eliminate output fluctuations caused by modulation of a modulator that modulates the phase matching condition and the resulting optical mixed wave, which could not be avoided with conventional methods, and fluctuations in pump laser light. Since it is possible to avoid malfunctions caused by this, an optical mixed wave generator having higher stability and reliability than the conventional one can be obtained. Furthermore, since no modulator is required, the device configuration can be made simple and inexpensive.

[Brief explanation of the drawing]

5 and 6 are block diagrams of an apparatus for explaining the operating principle of the conventional method, and FIGS. 1 and 4 are diagrams showing the first and second embodiments to which the present invention is applied, respectively. Figures 2 and 5
The figure shows the relationship between the temperature of the nonlinear medium and the optical harmonic output, a
, b, and c show time-varying waveforms when temperature modulation is applied to the nonlinear medium. Figure $6 shows the relationship between the temperature of the nonlinear medium and the phase detection output. Figure 3 shows the relationship between the temperature of the constant temperature oven and the output of the arithmetic unit. In the figure, 1 is a laser light source, and 2 is a constant temperature device. 3 is a main heater, 4 is a modulation heater, 5 is a nonlinear optical medium, 6 is a first beam beam filter, 7 is a photodetector, 8 is a modulator, 9 is a phase detector, and 10 is a DC amplification! , tt is the main heater power supply, 12 is the first constant temperature section, 13 is the constant temperature part of 1a2, 14 is the reflector, 15 is the second beam splitter, 116 is the temperature control unit, 17 is the first detection unit. 18 is an arithmetic unit, 19 is a third beam splitter, 120 is a second detector, 21 is a fourth beam stimulator, 22 is a differential excitation amplifier 119ia, 23 is a folding mirror, 2
4 is an amplifier. l: Laser) death 2: Kanon dies Ia: blasphemy knit 20: fj42-liquid product 4 mouth 22: differential 3111 width device 23: jar return L/* 24: platform sz

Claims (1)

[Claims] In an optical mixed wave generation method for obtaining an optical mixed wave from an excitation laser beam using a control means that controls phase matching conditions of a nonlinear optical medium, the optical path of the excitation laser beam in the nonlinear optical medium is changed by changing the optical path of the excitation laser beam with a slight temperature difference. Two parallel optical paths are provided, and the outputs of the optical mixed waves generated from the two optical paths are each amplified with an amplification degree that makes the two outputs equal in a state closest to the phase matching state. . A method for generating an optical mixed wave, characterized in that the difference between the amplified outputs is fed back to the control means as an error signal.