JPH06252516A - Harmonic generator - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a harmonic generator capable of efficiently and stably generating a harmonic output. [Structure] A fundamental wave 6 emitted from an LD 2 is made incident on a monolithic resonator 5, and its output light is made into a beam splitter 1.
A fundamental wave 6 and a second harmonic wave 7 are separated by passing through 4. The fundamental wave 6 is converted into an electric signal by the photodetector 15, and the temperature of the LD 2, the temperature of the resonator 5, the electric field and the like are controlled based on the electric signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a harmonic generator for converting a fundamental wave emitted from a semiconductor laser into a harmonic using a resonator having a non-linear optical material.

[0002]

2. Description of the Related Art In recent years, various devices have been proposed in which a fundamental wave emitted from a semiconductor laser or the like is incident on a nonlinear optical material to obtain a second harmonic wave or a third harmonic wave. In these devices, a non-linear optical material is arranged in a resonator composed of a plurality of reflecting surfaces, and a fundamental wave is confined in the resonator to be multiplied, whereby harmonics can be efficiently generated. As a resonator, a reflection film is provided on the end surface of the nonlinear optical material, and a monolithic resonator that resonates inside is provided,
There is known a discrete resonator in which a plurality of mirrors are arranged to form a resonator, and a nonlinear optical material is arranged in the resonator. In recent years, the mainstream is shifting from a discrete type to a monolithic type in order to miniaturize the device and improve the variation efficiency to harmonics.

FIG. 3 shows a second harmonic generator using a monolithic resonator as an example of a conventional harmonic generator. This second harmonic generation device 1 includes a semiconductor laser (hereinafter abbreviated as “LD”) 2, a collimating lens 3, a mode matching lens 4, and a monolithic resonator 5 made of a nonlinear optical material such as KNbO ₃ crystal. It is configured. The LD 2 emits the fundamental wave 6 having a wavelength of 860 nm, for example. Two surfaces of the monolithic resonator 5 facing each other in the figure are polished into a spherical shape and provided with a predetermined optical film. That is, the surface on the left side of the drawing forms an incident surface for the fundamental wave 6, and a spherical mirror 8 provided with an optical film that partially transmits the fundamental wave 6.

The surface on the right side of the drawing forms the exit surface of the second harmonic wave 7, and is a spherical mirror provided with an optical film that is highly reflective to the fundamental wave 6 and highly transmissive to the second harmonic wave 7. I am 9. Further, the lower surface of the monolithic resonator 5 in the figure forms a plane mirror 10 that totally reflects both the fundamental wave 6 and the second harmonic wave 7. In the figure, 11a is a Peltier element that controls the temperature of the LD 2, and 11b is a Peltier element that controls the temperature of the monolithic resonator 5.

In the above structure, the fundamental wave 6 having a wavelength of 860 nm emitted from the LD 2 is collimated by the collimator lens 3, passes through the mode matching lens 4, and passes through the monolithic resonator 5 at the point A of the spherical mirror 8. Incident from. At this time, the fundamental wave 6 is incident at a specific angle with respect to the crystal axis a so that the fundamental wave 6 incident on the point A travels parallel to the crystal axis a of the monolithic resonator 5. The incident fundamental wave 6 is reflected in a ring shape at points A, B, and C in the resonator constituted by the two spherical mirrors 8 and 9 and the plane mirror 10 to be multiplied.

In this way, the multiplied fundamental wave 6 propagates in the monolithic resonator 5 in the crystal axis a direction, and when the phase matching condition is satisfied, a part of the fundamental wave 6 has a wavelength 4 of the c-axis linearly polarized light.
It is converted to the second harmonic wave 7 of 30 nm. Then, a part of the fundamental wave 6 is scattered by impurities or the like inside the crystal and becomes return light having a reverse path. When this return light enters the LD 2, the frequency of the LD light is drawn to the resonance frequency of the resonator 5 and locked to the resonance frequency of the resonator 5 with a certain region. This locked frequency range is called a rocking range, and when the amount of light returned from the resonator 5 is 40 dB, the rocking range is about 3.5 GHz.

Generally, in order to obtain a highly efficient and stable second harmonic, it is necessary to match the LD oscillation frequency with the resonance transmission frequency of a resonator made of a non-linear optical material. LD
The oscillation frequency changes depending on the LD current and the LD temperature, and is about -1.5 GHz / m for the LDs used conventionally.
A, there is a dependency of -35 GHz / ° C. Therefore, LD
Is driven by ACC (Auto Current Control) while the temperature is controlled with a precision of about 1/100 ° C. by a Peltier element or the like.

The resonance transmission frequency is the optical resonator length n.
It depends on L (n is a refractive index and L is a resonator length). nL also has a temperature dependence, and in the case of a conventionally used monolithic resonator made of KNbO ₃ , it changes by about 5 GHz / ° C. Therefore, the nonlinear optical material forming the resonator is also temperature-controlled by the Peltier element or the like with an accuracy of about 1/100 ° C. or less.

As described above, in the conventional second harmonic generator, the LD oscillation frequency and the resonance transmission frequency are matched by controlling the temperature of each of the LD and the resonator independently.
I was trying to obtain a highly efficient and stable second harmonic.

[0010]

However, in the above-mentioned second harmonic generator of the related art, the temperature of the resonator made of the LD and the nonlinear optical material is controlled with an accuracy of about 1/100 ° C. Since the place where the temperature is to be controlled and the place where the temperature is actually desired to be controlled cannot be perfectly matched structurally, there is a temperature gradient between them. For this reason, when the ambient temperature changes, the temperature of the place to be controlled does not change, but at the place to be controlled, that is, at the light emitting part of the LD and the resonant part of the nonlinear optical material, the temperature changes and L
The oscillation frequency of the D light and the resonance transmission frequency of the resonator made of the non-linear optical material change. At this time, there is a problem that a relative deviation occurs between the LD oscillation frequency and the resonance transmission frequency, and when the deviation amount exceeds the rocking range, the frequency lock is released and the second harmonic output is stopped. Was there.

Therefore, an object of the present invention is to efficiently and stably generate a harmonic output by adding a control for compensating for the above relative frequency deviation even if it occurs. An object of the present invention is to provide a higher harmonic wave generating device.

[0012]

To achieve the above object, one of the present inventions is to provide an LD, a coupling optical system, a resonator having a nonlinear optical material, and a temperature adjusting means for the LD.
In a harmonic generator comprising the temperature adjusting means for the non-linear optical material, the fundamental wave transmitted through the resonator is detected, and the temperature adjusting means for the LD and / or the non-linearity is detected in accordance with a change in the amount of light. A feedback control device for sending a control signal to the temperature adjusting means of the optical material to change the temperature of the LD and / or the nonlinear optical material is provided.

In the above, the feedback control device includes an oscillator that modulates the current of the LD, a beam splitter that separates a fundamental wave that has passed through the resonator from harmonics, and a fundamental beam splitter that is split by the beam splitter. A photodetector for converting a wave into an electric signal, a lock-in amplifier for phase-detecting the electric signal converted by the photodetector, and the LD so that the error signal obtained by the lock-in amplifier becomes zero. And / or a PI controller that sends a control signal to the temperature adjusting means of the nonlinear optical material.

Another aspect of the present invention is a harmonic generation device comprising an LD, a coupling optical system, a resonator having a nonlinear optical material, and an electric field applying means for applying an electric field to the nonlinear optical material. A feedback control device that detects a fundamental wave that is transmitted through the resonator and sends a control signal to the electric field applying unit according to a change in the amount of light to change the electric field applied to the nonlinear optical material is provided. Characterize.

[0015]

According to one aspect of the present invention, the fundamental wave (resonant transmitted light) transmitted through the resonator is controlled by the feedback control device.
Is detected and a control signal is sent to the temperature adjusting means of the LD and / or the temperature adjusting means of the non-linear optical material in accordance with the change in the light amount to change the temperature of the LD and / or the non-linear optical material. Etc., and a relative deviation occurs between the LD oscillation frequency and the resonance transmission frequency, the deviation is corrected by the feedback control, and the harmonic output can always be generated efficiently and stably. it can.

According to another aspect of the present invention, the fundamental wave (resonance transmitted light) transmitted through the resonator is detected by the feedback control device, and a control signal is sent to the electric field applying means according to the change in the light quantity. The refractive index of the nonlinear optical material is changed by the so-called EO effect that changes the electric field applied to the nonlinear optical material, and the relative deviation between the LD oscillation frequency and the resonance transmission frequency due to the change of the ambient temperature is corrected. Therefore, the harmonic output can always be generated efficiently and stably.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. Note that the substantially same parts as those of the apparatus of FIG. 3 described as the conventional example are designated by the same reference numerals, and the description thereof will be omitted.

FIG. 1 is a schematic configuration diagram of a second harmonic generation device 101 which is an embodiment of the present invention. In the figure, the parts indicated by reference numerals 2 to 11 are the same as in the conventional example of FIG. Reference numerals 12a and 12b are temperature regulators for adjusting the temperatures of the LD 2 and the monolithic resonator 5, respectively, and 13 is an LD drive power source. A beam splitter 14 separates the fundamental wave 6 and the second harmonic wave 7 emitted from the monolithic resonator 5, and in this embodiment, the fundamental wave 6 is reflected by 99% or more and the second harmonic wave 90%. Permeate more. Reference numeral 15 is a photodetector, which is an APD in this embodiment. Reference numeral 16 is an oscillator for modulating the LD current, 17 is a lock-in amplifier for phase detection, and 18 is a PI controller for feeding back the phase detection signal to the LD current.

In the above structure, the fundamental wave 6 emitted from the LD 2 enters the monolithic resonator 5 through the collimating lens 3 and the mode matching lens 4, and becomes a ring type at points A, B and C in the resonator 5. When reflected, multiplied, and propagated through the monolithic resonator 5 in the crystal axis a direction, a part thereof is converted into the second harmonic wave 7 and output from the point B of the spherical mirror 9 on the exit side. .

At this time, since the reflectance of the spherical mirror 9 with respect to the fundamental wave 6 is not 100%, a part of the fundamental wave 6 is generated.
Is also emitted together with the second harmonic wave 7. Further, a part of the fundamental wave 6 is scattered by impurities or the like inside the crystal forming the monolithic resonator 5, and becomes return light having a reverse path. When this return light enters the LD 2, the frequency of the LD light is drawn to the resonance frequency of the resonator 5 and locked to the resonance frequency of the resonator 5 with a certain region. The above operation is the same as that of the conventional device shown in FIG.

The second harmonic generation device 101 differs from the conventional device in that feedback control as described below is performed. That is, the oscillator 16 outputs a sin wave having a frequency of 1 KHz and an amplitude of 0.1 mV,
The LD current of the D drive power supply 13 is modulated. Accordingly L
The D oscillation frequency slightly changes within the rocking range.
As a result, the intensity (resonance transmission power) of the fundamental wave 6 transmitted from the monolithic resonator 5 also slightly changes. The fundamental wave 6 is separated from the second harmonic wave 7 by the beam splitter 14, converted into an electric signal by the photodetector 15, and phase-detected by the lock-in amplifier 17, whereby a minute change signal of the resonance transmission power, that is, an error. You can get a signal.

The rocking range usually has a shape as shown in FIG. 2 (A), and this error signal is shown in FIG. 2 (B).
As shown in, the rocking range (LR) is differentiated. The resonance transmission power is most stable at the center (CP) of the rocking range, that is, when the error (differential) signal becomes zero. Therefore, a control signal is sent to the temperature controller 12a of the LD2 through the PI controller 18 so that the differential signal becomes 0, and the temperature of the LD2 is controlled through the Peltier element 11a, resulting in the LD frequency and the resonant transmission frequency. It is possible to correct the deviation of and obtain a stable second harmonic output. At this time, the PI controller 18
Functions to perform control according to the magnitude of this deviation and the change over time.

In the above embodiment, the PI controller 18 sends a control signal to the temperature controller 15b of the monolithic resonator 5 to control the temperature of the monolithic resonator 5 via the Peltier element 11b. The deviation between the frequency and the resonant transmission frequency may be corrected.

Further, an electric field applying device (not shown) is provided in the monolithic resonator 5, and a control signal is sent from the PI controller 18 to the electric field applying device to cause the monolithic resonator 5 to move.
It is also possible to control the electric field (EO effect) related to the above to correct the deviation between the LD frequency and the resonant transmission frequency.

[0025]

As described above, according to the present invention,
Conventionally, the LD frequency and the resonance transmission frequency, which have been independently controlled, are controlled by the feedback control mechanism so as to match with each other. Therefore, there is an effect that a stable harmonic wave output can be obtained even with a change in ambient temperature. Be brought. Further, since the feedback is applied to the part where the tolerance for stability is the strictest, such as the temperature of the LD or the resonator, or the electric field applied to the resonator, it is possible to optimize the entire harmonic generator.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a second harmonic generation device that is an embodiment of the present invention.

FIG. 2 is a chart showing a relationship between an LD oscillation frequency and a resonant transmission power and a relationship between an LD oscillation frequency and an error signal in a rocking range.

FIG. 3 is a schematic configuration diagram showing an example of a conventional second harmonic generation device.

[Explanation of symbols]

 2: LD 3: Collimating lens 4: Mode matching lens 5: Monolithic resonator 6: Fundamental wave 7: Second harmonic wave 11a: Peltier element 11b: Peltier element 12a: Temperature controller 12b: Temperature controller 13: LD drive Power supply 14: Beam splitter 15: Photodetector 15a: First photodetector 15b: Second photodetector 16: Oscillator 17: Lock-in amplifier 18: PI controller 101: Second harmonic generation device

Claims (3)

[Claims] 1. A harmonic generator comprising a semiconductor laser, a coupling optical system, a resonator having a nonlinear optical material, a temperature adjusting means for the semiconductor laser, and a temperature adjusting means for the nonlinear optical material, The fundamental wave transmitted through the resonator is detected, and a control signal is sent to the temperature adjusting means of the semiconductor laser and / or the temperature adjusting means of the non-linear optical material according to the change of the light quantity, and the semiconductor laser and / or Alternatively, a higher harmonic wave generating device is provided with a feedback control device for changing the temperature of the nonlinear optical material. 2. The feedback control device includes an oscillator that modulates the current of the semiconductor laser, a beam splitter that separates a fundamental wave that passes through the resonator from a harmonic, and a fundamental beam splitter that is split by the beam splitter. A photodetector for converting a wave into an electric signal, a lock-in amplifier for phase-detecting the electric signal converted by the photodetector, and an error signal obtained by the lock-in amplifier are set to 0,
The harmonic generator according to claim 1, comprising a PI controller that sends a control signal to the temperature adjusting means of the semiconductor laser and / or the temperature adjusting means of the nonlinear optical material. 3. A harmonic generation device comprising a semiconductor laser, a coupling optical system, a resonator having a nonlinear optical material, and an electric field applying means for applying an electric field to the nonlinear optical material. Detects the coming fundamental wave, sends a control signal to the electric field applying means according to the change in the light quantity,
A harmonic generation device, comprising a feedback control device for changing an electric field applied to the nonlinear optical material.