JPH06123907A - Wavelength conversion device - Google Patents

Abstract

PURPOSE:To stabilize operation. CONSTITUTION:The light source 1 of this wavelength conversion device having the light source 1 and a nonlinear optical element 2 consists of a semiconductor laser LD and the oscillated light therefrom is made incident as a fundamental wave on the nonlinear optical element 2 to ride a microfluctuating current on the implantation current of the semiconductor laser LD and to fluctuate its oscillation wavelength, thereby, the wavelength of the input fundamental wave to the nonlinear optical element 2 is fluctuated and the oscillation wavelength of the semiconductor laser LD is withdrawn into the wavelength of the input fundamental wave at which the wavelength conversion efficiency thereof is maximized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength conversion device, and more particularly to a wavelength conversion device having a non-linear optical element for generating a second harmonic wave, a fourth harmonic wave or the like by inputting a fundamental wave to the wavelength converter. Involve

[0002]

2. Description of the Related Art In recent years, the second harmonic generation (SH
Attention has been paid to the laser light of G). In this second harmonic generation, the light of the fundamental wave optical frequency ω is converted into the second harmonic frequency 2ω.
To shorten the wavelength of the laser light.

As a method of generating this short wavelength light, for example, there is a wavelength conversion element that guides a fundamental wave of a semiconductor laser or the like by a non-linear optical element to take out a second harmonic laser light.

When the optical harmonic generation band for the fundamental wavelength of the wavelength conversion element is narrow, if a semiconductor laser or the like is used as the fundamental wave light source for the non-linear optical element, ambient temperature fluctuation or fluctuation of injection current to the semiconductor laser may occur. Due to the oscillation wavelength drift, it is difficult to maintain the agreement between the fundamental wave wavelength input to the nonlinear optical element and the wavelength at which the wavelength conversion efficiency of the nonlinear optical element is maximum.

As a countermeasure for this, a method is generally adopted in which the semiconductor laser and the nonlinear optical element of the fundamental wave light source are kept constant at a predetermined temperature to avoid the wavelength fluctuation, but the wavelength fluctuation is reduced. Is not sufficient, and the wavelength conversion efficiency, and hence the output power of the optical harmonic wave, fluctuates, and this cannot be stably maintained in the maximum state.

[0006]

SUMMARY OF THE INVENTION The present invention enables a wavelength conversion device using a non-linear optical element to stably and stably maintain a maximum wavelength conversion efficiency or a wavelength conversion efficiency close to the maximum wavelength conversion efficiency.

[0007]

According to a first aspect of the present invention, in a wavelength conversion device having a wavelength conversion section including at least a light source and a non-linear optical element, a minute fluctuation signal is applied to the wavelength conversion section. The configuration is such that the input light to the nonlinear optical element is guided to the input fundamental wave wavelength that maximizes the wavelength conversion efficiency of the nonlinear optical element.

In the second aspect of the present invention, as shown in the configuration diagram of one example thereof in FIG. 1, the light source 1 is composed of a semiconductor laser LD, and the oscillation light from this semiconductor laser LD is converted into a nonlinear optical element 2.
Incident as an input fundamental wave.

Then, a minute fluctuation current is applied to the injection current of the semiconductor laser LD to change the oscillation wavelength of the semiconductor laser LD to change the wavelength of the input fundamental wave to the nonlinear optical element 2 and The oscillation wavelength of the semiconductor laser is pulled into the input fundamental wave wavelength that maximizes the wavelength conversion efficiency.

In the third aspect of the present invention, the light from the light source 1 is passed through the external frequency modulator 3 to output light from the external frequency modulator 3 to the non-linear optical element 2 as shown in FIG. It is incident as the input fundamental wave. Then, a minute fluctuation modulation signal is applied to the external frequency modulator 3 to minutely change the wavelength of the input fundamental wave to the nonlinear optical element 2 so that the wavelength conversion efficiency of the nonlinear optical element 2 is maximized. 3 pulls in the output light.

A fourth aspect of the present invention is based on each of the above-mentioned configurations.
Fluctuation of the fundamental wave from the nonlinear optical element 2 or the wavelength-converted output wave due to the minute fluctuation current of the semiconductor laser LD or the minute fluctuation modulation signal of the external frequency modulator 3 and the wavelength fluctuation of the input fundamental wave to the nonlinear optical element 2. The correlation with the fluctuation signal after the optical-electrical conversion of the minute is taken.

The difference between the wavelength of the fundamental wave that maximizes the wavelength conversion efficiency in the nonlinear optical element 2 obtained by this correlation and the wavelength of the fundamental wave that is input to the nonlinear optical element 2 is used as an error signal. Negative feedback is performed in the direction in which the wavelength of the input fundamental wave matches the wavelength of the fundamental wave that maximizes the wavelength conversion efficiency in the element 2, and the oscillation wavelength of the semiconductor laser LD or the output wavelength of the external frequency modulator 3 is set in the nonlinear optical element 2. Match the wavelength with the maximum wavelength conversion efficiency.

In the fifth aspect of the present invention, as shown in the block diagram of one example thereof, the light source 1 comprises a semiconductor laser LD, and the oscillation light from this semiconductor laser LD is input to the nonlinear optical element 2 as an input fundamental wave. The nonlinear optical element 2 is made to enter and a minute fluctuation voltage is applied to change the refractive index of the nonlinear optical element 2, and the oscillation wavelength of the semiconductor laser LD is set to the input fundamental wavelength that maximizes the wavelength conversion efficiency of the nonlinear optical element 2. Pull in.

In the sixth aspect of the present invention, as shown in the block diagram of FIG. 4, the output from the wavelength conversion section is obtained as a digital signal, and the light source 1 is composed of a semiconductor laser LD. Oscillating light is input to the nonlinear optical element as the input fundamental wave, a minute fluctuation current is applied to the injection current of the semiconductor laser LD, and the oscillation wavelength of the semiconductor laser LD is changed to change the input fundamental wave wavelength to the nonlinear optical element 2. Then, the oscillation wavelength of the semiconductor laser is pulled to the wavelength of the input fundamental wave that maximizes the wavelength conversion efficiency of the nonlinear optical element 2.

In addition, a digital modulation that maximizes a current that gives a wavelength that maximizes the wavelength conversion efficiency to the injection current of the semiconductor laser LD and minimizes a current that gives a wavelength that has a maximum conversion efficiency different from the maximum conversion efficiency. A current signal is applied to digitally modulate the wavelength-converted light.

The seventh aspect of the present invention, as shown in the configuration diagram of FIG. 5, shows that the output from the wavelength converter is obtained as a digital signal, and the light source 1 is a semiconductor laser LD. The oscillated light is input to the nonlinear optical element 2 as an input fundamental wave.

Then, a minute fluctuating current is applied to the injection current of the semiconductor laser LD to change the oscillation wavelength of the semiconductor laser LD to change the wavelength of the input fundamental wave to the nonlinear optical element 2 and convert the wavelength of the nonlinear optical element 2. The maximum conversion efficiency different from the maximum conversion efficiency is obtained by drawing the oscillation wavelength of the semiconductor laser LD into the input fundamental wave wavelength that maximizes the efficiency and maximizing the voltage that gives the wavelength that maximizes the wavelength conversion efficiency to the nonlinear optical element 2. A wavelength-converted light is digitally modulated by applying a digital modulation voltage signal that minimizes a voltage that gives a wavelength that is efficient.

In the eighth aspect of the present invention, the light source 1 is composed of a semiconductor laser LD, as shown in FIG.
Incident as an input fundamental wave.

Then, a minute fluctuating voltage is applied to this nonlinear optical element to vary the refractive index of this nonlinear optical element, and the oscillation wavelength of the semiconductor laser is set to the input fundamental wave wavelength at which the wavelength conversion efficiency of the nonlinear optical element 2 is maximized. Digital modulation of the wavelength-converted light is performed by drawing in and applying a digital modulation current having a specific current difference to the injection current of the semiconductor laser.

The ninth aspect of the present invention is a wavelength conversion device having a light source 1 and a non-linear optical element 2, as shown in the block diagram of FIG. And the oscillation light from this semiconductor laser LD is
It is incident on the nonlinear optical element 2 as an input fundamental wave.

Then, a minute fluctuating voltage is applied to this nonlinear optical element to vary the refractive index of this nonlinear optical element, and the oscillation wavelength of the semiconductor laser is set to the input fundamental wave wavelength that maximizes the wavelength conversion efficiency of the nonlinear optical element 2. The wavelength-converted light is digitally modulated by pulling in and applying a digital modulation voltage having a specific voltage difference to the nonlinear optical element.

In the tenth aspect of the present invention, in each of the above-mentioned configurations, the non-linear optical element 2 is an optical harmonic generating element. The 11th aspect of the present invention is configured such that, in each of the above-described configurations, the nonlinear optical element 2 is a nonlinear optical element having a quasi phase matching structure.

In the twelfth aspect of the present invention, in each of the above-mentioned configurations, the non-linear optical element 2 is a Cherenkov type non-linear optical element. In the thirteenth aspect of the present invention, in each of the above-described configurations, the nonlinear optical element 2 is LiTa _x Nb _1-x O.
_The configuration is a non-linear optical element of ₃ (x = 0 to 1).

The wavelength conversion in the present invention includes sum frequency mixing, second harmonic generation, fourth harmonic generation and the like.

[0025]

According to the present invention, the semiconductor laser LD is aimed at the wavelength that maximizes the conversion efficiency in the nonlinear optical element 2.
Since the oscillation wavelength of the laser is electrically controlled, the semiconductor laser L
The precision of temperature control with respect to D and the nonlinear optical element 2 is relaxed.

Further, according to the present invention, since the reference wavelength for controlling the wavelength is always the wavelength at which the conversion efficiency in the nonlinear optical element 2 is maximized, the fluctuation of the optical harmonic output light intensity is suppressed. It can be suppressed and a stable system can always be realized.

Furthermore, according to the present invention, when the wavelength conversion output is a digital signal, the voltage giving the wavelength is maximized so that the conversion efficiency in the nonlinear optical element 2 is maximized, which is different from the maximum conversion efficiency. Since the digital modulation current signal that minimizes the voltage that gives the wavelength with the maximum conversion efficiency is applied, the dynamic characteristic, that is, the output light, is not limited to the static characteristic, that is, the non-digital modulation when the wavelength-converted light such as SHG is always output. Even when the digital modulation is turned on and off, particularly in the off state, the wavelength control can be reliably performed.

That is, according to the present invention, since the reference wavelength for controlling the wavelength is always the wavelength at which the conversion efficiency in the nonlinear optical element 2 is maximized, the fluctuation of the optical harmonic output light intensity is suppressed. It is possible to suppress both static characteristics (when digital modulation is not performed) and dynamic characteristics (when digital modulation is performed), and it is possible to realize a stable system at all times.

[0029]

Before explaining the embodiments of the present invention, in order to facilitate the understanding of the present invention, the principle of matching the oscillation wavelength of the semiconductor laser with the wavelength at which the conversion efficiency of the nonlinear optical element is maximized will be explained. .

Now, consider a case where an optical fundamental wave having an input power Pinω and a wavelength λω is input to a nonlinear optical element (SHG) which generates an optical second harmonic. Regarding the dependence of the optical fundamental wave output power Pout ω of the SHG and the optical fundamental wave input wave λω of the optical second harmonic output power Pout2ω at this time, the SHG of the quasi-phase matching structure is taken as an example. 8 and FIG. 9.

As can be seen from FIGS. 8 and 9, the wavelength dependence of the optical fundamental wave output power and the optical harmonic output power has a peak at a certain wavelength λP, and the maximum conversion efficiency is obtained at this λP. To be

Considering the case where the wavelength of the optical fundamental wave incident on the SHG has a minute fluctuation signal (Δλω), the fundamental wave output power signal (ΔPout from the SHG is accompanied by it.
ω), and the optical second harmonic output power signal (ΔPout2
ω) is as shown in FIGS. 10 and 11.

Therefore, when the inner product of ΔPout ω and ΔPout 2ω is taken with Δλω, the wavelength dependence is linear as shown in FIGS. 12 and 13, respectively, with respect to the characteristics of FIGS. 10 and 11, respectively. A differential curve is obtained.

From this characteristic, λP that gives the maximum conversion efficiency
There is a wavelength region (the region indicated by A in FIGS. 12 and 13) indicating the positive / negative judgment value at the boundary, and when the optical fundamental wave input center wavelength λω exists in this A region, the wavelength λω is relative to λP. Information on whether it exists on the long wavelength side or the short wavelength side,
It is obtained by the signal of FIG. 12 and FIG. 13 whether it is positive or negative.

Therefore, the optical input wavelength λω is changed to λ by this signal.
By performing negative feedback so as to change λω in the direction in which P coincides with P, λω and λP can always coincide with each other.

The basic structure of an example of the present invention will be described with reference to FIG. In FIG. 1, the light source 1 is a semiconductor laser LD, and the semiconductor laser LD is
Frequency f of, for example, several tens of kHz from 00 Hz to 100 MHz
_{It is} modulated by the minute fluctuation current signal Δλω of _ref . The AC component of the minute fluctuation current signal Δλω is 0.00 with respect to the DC component of the injection current supplied to the semiconductor laser LD.
1% to 10%, for example, 0.1% is selected.

In this case, since the oscillation wavelength of the output light of the semiconductor laser LD changes almost linearly with the change of the injection current, the oscillation wavelength of the output light of the semiconductor laser LD is changed by the above-mentioned minute fluctuation current. The oscillation wavelength is modulated by the direct modulation by the minute fluctuating current.

Then, this modulated semiconductor laser LD
The output light ω of is incident on the non-linear optical element 2 and wavelength-converted by this, for example, the second harmonic ω thereof is taken out, but a part of the incident light ω is taken out as it is without wavelength conversion. Be done.

A mirror 5 is arranged downstream of the nonlinear optical element 2. The mirror 5 transmits the wavelength of the second harmonic light 2ω, but the nonlinear optical element 2 having a longer wavelength than this wavelength.
For example, a dichroic mirror that reflects the light having the wavelength of the incident light ω is used.

Then, the light reflected by the mirror 5 is introduced into the photoelectric conversion element 6 such as a photodiode, and this light is detected and taken out as an electric signal. This electric signal becomes a fluctuating current signal ΔPout ω that fluctuates with the fluctuation of the fundamental wave wavelength from the semiconductor laser LD to the nonlinear optical element 2 described in FIG.

Then, an error signal is obtained by correlating the fluctuating current signal ΔPout ω from the photoelectric conversion element 6 and the minute fluctuating current signal Δλω from the local oscillator 4.

In this example, when ΔPout ω and Δλω are input to the multiplication circuit 7 and the signal shown in FIG. 12 is obtained by the inner product of both signals ΔPoutω and Δλω, the output signal of the multiplication circuit 7 is obtained. , Which is introduced into the control circuit 8 so that the injection current to the light source 1, that is, the semiconductor laser LD acts in a direction to match the output wavelength thereof, that is, the optical input wavelength λω to the nonlinear optical element 2 with the above-mentioned λ P. Perform negative feedback operation.

In the example shown in FIG. 1, the semiconductor laser LD of the light source 1 is directly subjected to the minute modulation by the minute fluctuation current. However, as shown in the basic configuration of FIG. For example, the light from the semiconductor laser is introduced into the external frequency modulator 3, and the light whose frequency is modulated by this is incident on the nonlinear optical element 2 as the input fundamental wave. This is a case where a minute fluctuating current from the local oscillator 4 is given as a frequency modulation signal to frequency-modulate the incident light to the nonlinear optical element 2. In this FIG. 2 as well, the same error signal detection and negative feedback as in FIG. 1 are performed, and in FIG. 2, the parts corresponding to those in FIG.

In the examples shown in FIGS. 1 and 2 for the wavelength control, the semiconductor laser LD of the light source 1 is directly subjected to the minute modulation by the minute fluctuation current. As shown in the basic configuration, the nonlinear optical element 2 is applied with a minute fluctuation signal from the local oscillator 3 as an externally applied voltage to cause the refractive index modulation of the nonlinear optical element 2 to obtain the fluctuation of the wavelength shift. Since a signal is obtained, wavelength control is performed in the same manner as when a minute fluctuation signal is applied to the injection current of the semiconductor laser. In FIG. 3, parts corresponding to those in FIGS. 1 and 2 are denoted by the same reference numerals, and redundant description will be omitted.

In each of FIGS. 1 to 3, the error signal is obtained by detecting the output of the fundamental wavelength from the non-linear optical element 2, as shown in FIGS. 14 to 16. Output light whose wavelength is converted by the nonlinear optical element 2,
That is, in the illustrated example, the error signal can be obtained by detecting the light transmitted through the mirror 5. 14 to 16, parts corresponding to those in FIGS. 1 to 3 are denoted by the same reference numerals, and redundant description will be omitted.

FIG. 17 is a block diagram showing an example of a more specific circuit configuration of the device of the present invention described with reference to FIG. 1 or 14. In FIG. 17, 10 is a semiconductor laser LD as the light source 1.
In the driver circuit for driving the semiconductor laser LD, an isolator 11 for preventing light returning to the laser LD is provided in front of the semiconductor laser LD driven by the driver circuit.

The LD driver 10 gives a minute fluctuation current to the injection current of the laser LD in response to a signal of, for example, 40 kHz from the local oscillator 4 to minutely change the oscillation wavelength of the semiconductor laser LD.

The oscillated light from the semiconductor laser LD is made incident on a non-linear optical element 2 of, for example, a QPM (quasi phase matching) type as a fundamental wave, and the non-linear optical element 2 generates a second harmonic. This optical second harmonic is detected by an opto-electric conversion element 6 such as a photodiode, this detection signal is amplified by an amplifier 12, and the phase shifter 13 matches the phase with the signal from the local oscillator 4 to multiply its output by a multiplication circuit. 7
Is introduced together with the signal from the local oscillator 4, and its output is applied to the oscillation output from the local oscillator 4 through the low-pass filter 14 having a cut frequency fc of 1 kHz and the integrator 15 having a cut frequency fc of 1 kHz, for example. In this case, the laser LD is driven by negative feedback, and the operation described in the basic configuration of FIG. 1 is performed.

Here, the quasi-phase matching (QPM) type non-linear optical element 2, for example, SHG, has a substrate 20 whose Z-axis (C-axis) is in the thickness direction of KTP or the like, as an example is shown in FIG. An optical waveguide 21 is formed in a non-linear optical crystal 20 made of, for example, and a polarization inversion structure portion 22 in which polarization is alternately inverted in the thickness direction of the optical waveguide 21 is provided across the optical waveguide 21.

Further, in the external frequency modulator 3 used in the configuration described in FIGS. 2 and 15, the laser light from the semiconductor laser LD is made incident on the nonlinear optical crystal substrate 30 as shown in FIG. An optical waveguide 31 that guides the light is provided, and electrodes 32 to which a modulation voltage is applied are provided on both sides of the optical waveguide 31, for example. A frequency modulator that controls the frequency can be used.

The external frequency modulator 3 has a structure in which, for example, in the QPM type nonlinear optical element 2 described in FIG. 18, electrodes 32 are provided on both sides of the optical waveguide 21 as shown by a chain line in FIG. As a result, the external frequency modulator 3 can be integrally formed in the nonlinear optical element 2 as SHG, for example.

Next, the principle of performing digital modulation in which the oscillation wavelength of the semiconductor laser is made to coincide with the wavelength at which the conversion efficiency of the nonlinear optical element is maximum and the ON / OFF signal is given while maintaining that state will be described.

As described above with reference to FIGS. 12 and 13, there is a wavelength region A showing a positive / negative judgment value at the boundary of λp which gives the maximum conversion efficiency, and the optical fundamental wave input center wavelength λω
Is present in this A region, whether the information on whether the wavelength λω exists on the long wavelength side or the short wavelength side with respect to λp is positive or negative depending on the signals of FIGS. 12 and 13. Λω and λp can be always matched by negative feedback such that the optical input wavelength λω is changed in the direction of matching λp with this signal.

On the other hand, the dependence of the SHG optical output Pout2ω on the optical fundamental wavelength λω is expressed as Pout2ω ∝ {sin ² (ΔL / 2)} / (ΔL / 2) ² (L: action length), and Δ = {2π / (λω)} · (n2ω-nω) (nω, n2ω: fundamental light, effective refractive index of the second harmonic), which is shown in FIG. 21.

From FIG. 20, there are peaks of .lamda.p giving the maximum conversion efficiency and. ± .n-order .lamda.pn on both wavelength sides.

Therefore, "1", that is, the wavelength in the ON state of the SHG light is set to λp, and "0", that is, the wavelength in the OFF state of the SHG light is set to λpn.

From FIG. 21, for both λp and λpn, the first-order differential curves take positive and negative values with the wavelength as a boundary, and the same information as in each of the above-mentioned examples is obtained. From the above wavelength control principle, ON, O
Since both states of the FF are controllable, wavelength control is performed even during digital modulation.

Next, the basic structure of an example of the present invention during such digital modulation will be described with reference to FIG. 4, parts corresponding to those in FIG. 1 are designated by the same reference numerals, and duplicate description will be omitted. Even in this case, since the oscillation wavelength of the output light of the semiconductor laser LD changes almost linearly with the change of the injection current, the oscillation wavelength of the output light of the semiconductor laser LD slightly changes due to the minute fluctuation current. Its oscillation wavelength is modulated by direct modulation with an electric current.

That is, the light reflected by the mirror 5 provided at the subsequent stage of the non-linear optical element 2 is introduced into the photoelectric conversion element 6 such as a photodiode, and this light is detected and extracted as an electric signal. This electric signal fluctuates with the fluctuation of the fundamental wavelength from the semiconductor laser LD to the nonlinear optical element 2 described in FIG.
t ω.

Then, an error signal is obtained by the correlation between the fluctuating current signal ΔPout ω from the photoelectric conversion element 5 and the minute fluctuating current Δλω from the local oscillator 4.

In this example, ΔPout ω and Δλω are input to the multiplication circuit 7, and the signal explained in FIG. 12 by the inner product of both signals ΔPoutω and Δλω is obtained.
The output signal of the multiplication circuit 7 is introduced into the control circuit 8, whereby the injection current to the light source 1, that is, the semiconductor laser LD, its output wavelength, that is, the optical input wavelength λω to the nonlinear optical element 2 is set to the above-mentioned λp. Negative feedback action is applied to act in the direction of matching with.

After performing this wavelength control, the injection current of the semiconductor laser is digitally modulated by an external modulator. As shown in FIG. 22 this time, the injection current value I ₂ at the oscillation wavelength lambda _1, also OFF state of the semiconductor laser in the injection current value I ₁ of the output of the nonlinear optical element 2 is in the ON state
The oscillation wavelength λ ₂ of the semiconductor laser in FIG. 20 is λ pn (n = 0, ± 1, ± 2, ± 3 described in FIG. 20 above.
...) is set.

Therefore, the wavelength difference between ON and OFF is λp-λ
The injection current to the semiconductor laser that gives pn is
The injection current difference I ₁ -I ₂ in the N and OFF states is set, and digital modulation is performed by the external modulator.

As a result, the same sign of the error signal can be obtained in both ON and OFF states during digital modulation according to the principle described above, and wavelength control can be performed simultaneously with modulation.

In the example shown in FIG. 4, the digital modulation signal is directly applied to the injection current of the semiconductor laser LD of the light source 1. However, as shown in the basic configuration of FIG. By applying a voltage and changing its refractive index, a wavelength shift can be made in the wavelength dependence of the second harmonic conversion efficiency described in FIG. 10 described above, thereby modulating the externally applied voltage. As a result, digital modulation is performed.

The applied voltage in both ON and OFF states is V1
, V2, the voltage difference V1−V2 is the wavelength difference λp−λ when the oscillation wavelength of the semiconductor laser is ON and OFF.
By setting so as to correspond to pn, wavelength control and digital modulation are performed at the same time according to the above principle.

Regarding the wavelength control, in the example shown in FIGS. 4 and 5, the semiconductor laser LD of the light source 1 is directly subjected to the minute modulation by the minute fluctuation current.
And, as shown in the basic configuration in FIG. 7, a minute fluctuation signal from the local oscillator 3 is applied to the nonlinear optical element 2 as an externally applied voltage to cause refractive index modulation of the nonlinear optical element 2 to obtain a wavelength shift fluctuation. As a result, an error signal relating to the wavelength is obtained, so that the wavelength is controlled in the same manner as when the minute fluctuation signal is applied to the injection current of the semiconductor laser. 6 and 7, parts corresponding to those in FIGS. 4 and 5 are designated by the same reference numerals, and redundant description will be omitted.

In each of the examples shown in FIGS. 4 to 7, the output of the fundamental wavelength from the nonlinear optical element 2 is detected to obtain the error signal. As described above, the output light whose wavelength is converted by the non-linear optical element 2, that is, the light transmitted through the mirror 5 in the illustrated example can be detected to obtain an error signal. 23-
In FIG. 26, portions corresponding to those in FIGS. 4 to 7 are designated by the same reference numerals and redundant description will be omitted.

FIG. 27 is a block diagram showing an example of a more specific circuit configuration of the device of the present invention described with reference to FIG. 4 or FIG.

In FIG. 27, those parts corresponding to those in FIG. 17 are designated by the same reference numerals, and a duplicate description will be omitted. In this case, the injection current of the semiconductor laser LD is turned on by a digital signal from, for example, the function generator 16 as the external modulator 9.
The difference between the oscillation wavelengths of the semiconductor lasers in the ON / OFF state is applied so as to be λp-λpn as described above, and as described above, wavelength control and digital modulation are performed simultaneously.

FIG. 28 shows the result of actually measuring the harmonic output power and the error signal in such a configuration as described above with reference to FIGS. 11 and 13. FIG. 28
In, solid lines a and b respectively indicate the error signal and the harmonic output power.

In each of these examples of digital modulation, the quasi phase matching (QPM) type non-linear optical element 2, for example, SHG, can be configured as shown in FIG. The external modulation voltage used in the configuration described with reference to FIGS. 7, 24, and 26 is an electrode to which the modulation voltage is applied to, for example, both sides of the optical waveguide 21, as shown in a perspective view of another example in FIG. 32 may be provided, and the wavelength of the fundamental wavelength characteristic of the SHG conversion efficiency may be shifted by changing the refractive index of the guided light by utilizing the electro-optic effect of the electric field on the waveguide 21.

Further, in each of the above-mentioned configurations, the nonlinear optical element 2 may be a Cherenkov radiation type SHG, and the nonlinear optical element is not limited to KTP, but LiT.
a _x Nb _1-x O ₃ (0 ≦ x ≦ 1), etc., the present invention is not limited to the above-mentioned examples, and various modifications can be made without departing from the technical idea of the present invention. Needless to say, it can be changed.

[0074]

According to the configuration of the present invention, the nonlinear optical element 2
Since the oscillation wavelength of the semiconductor laser LD is electrically controlled by targeting the wavelength that maximizes the conversion efficiency in (1), the temperature control accuracy of the semiconductor laser LD and the nonlinear optical element 2 can be relaxed. Further, in performing wavelength control, since the reference wavelength is always the maximum conversion efficiency in the nonlinear optical element, it is possible to suppress fluctuations in, for example, optical harmonic output light intensity due to the nonlinear optical element 2. , Can always realize a stable system.

Further, since this system itself can be constructed only by a simple electric control circuit, its manufacture becomes simple.

Further, in the nonlinear optical element 2 having the quasi phase matching structure QPM, the conversion efficiency can be increased by increasing the length of the domain inversion region, but the narrowing of the wavelength band in which the optical harmonics are generated poses a problem. Since this method can be applied to a non-linear optical element having a very narrow wavelength band, it is possible to stabilize the non-linear optical element having a higher conversion efficiency. Further, it is possible to perform modulation at a minute signal frequency used for wavelength control or at a frequency higher than the frequency band (f) of the electric control system, and to directly modulate the injection current of the semiconductor laser (when the modulation frequency is fm> f ) Enables high-frequency intensity modulation.

Further, according to the configuration of the present invention, by controlling the fundamental wave wavelength in the ON state at the time of digital modulation to a wavelength that maximizes the SHG conversion efficiency, wavelength control is possible even at the time of digital modulation, and stable wavelength is maintained. A system can be realized.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of a device of the present invention.

FIG. 2 is a block diagram of an embodiment of the device of the present invention.

FIG. 3 is a block diagram of an embodiment of the device of the present invention.

FIG. 4 is a configuration diagram of an embodiment of the device of the present invention.

FIG. 5 is a block diagram of an embodiment of the device of the present invention.

FIG. 6 is a block diagram of an embodiment of the device of the present invention.

FIG. 7 is a block diagram of an embodiment of the device of the present invention.

FIG. 8 is a diagram showing the wavelength dependence of the fundamental wave output power of the fundamental wave light derived from the nonlinear optical element.

FIG. 9 is a diagram showing the fundamental wavelength dependency of the second harmonic output power derived from the nonlinear optical element.

FIG. 10 is an explanatory diagram of minute fluctuations of the fundamental wave wavelength to the nonlinear optical element and fluctuations of the fundamental wave output due to the minute fluctuations.

FIG. 11 is an explanatory diagram of minute fluctuations of the fundamental wave wavelength to the nonlinear optical element and fluctuations of the second harmonic output due to the minute fluctuations.

FIG. 12 is a diagram showing the fundamental wave wavelength dependence of the inner product of the fundamental wave wavelength minute fluctuation signal Δλω and the fundamental wave output power signal ΔPout ω.

FIG. 13 is a diagram showing the fundamental wavelength dependence of the inner product of the fundamental wavelength minute fluctuation signal Δλω and the optical harmonic output power signal ΔPout ₂ ω.

FIG. 14 is a configuration diagram of an embodiment of the present invention.

FIG. 15 is a configuration diagram of an embodiment of the present invention.

FIG. 16 is a configuration diagram of an embodiment of the present invention.

FIG. 17 is a block diagram of an embodiment of the present invention.

FIG. 18 is a schematic enlarged perspective view of an example of a non-linear optical element.

FIG. 19 is an enlarged schematic perspective view of another example of the nonlinear optical element.

FIG. 20 is a diagram showing the fundamental wavelength dependence of the second harmonic output power from the nonlinear optical element.

FIG. 21 is a diagram showing a first-order differential curve obtained from the fundamental wave wavelength dependency of the second harmonic output power.

FIG. 22 is a diagram of a signal of digital modulation.

FIG. 23 is a configuration diagram of an embodiment of the present invention.

FIG. 24 is a configuration diagram of an embodiment of the present invention.

FIG. 25 is a configuration diagram of an embodiment of the present invention.

FIG. 26 is a configuration diagram of an embodiment of the present invention.

FIG. 27 is a block diagram of an embodiment of the present invention.

FIG. 28 is a diagram showing measured values of an error signal and optical harmonic output power.

FIG. 29 is a schematic enlarged perspective view of another example of the nonlinear optical element.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source 2 Nonlinear optical element 3 External frequency modulator 4 Local oscillator 5 Mirror 6 Optical-electrical conversion element 7 Multiplier circuit 8 Control circuit 9 External modulator 10 LD driver 11 Isolator 12 Amplifier 13 Phase shifter 14 Low-pass filter 15 Integrator 16 Function generator 20 Substrate 21 Optical waveguide 22 Polarization inversion structure 32 Electrode

Claims (13)

[Claims] 1. A wavelength conversion device comprising a wavelength conversion section including at least a light source and a nonlinear optical element, wherein the wavelength conversion efficiency of the nonlinear optical element is maximized by giving a minute fluctuation signal to the wavelength conversion section. The wavelength conversion device is characterized in that the input light to the nonlinear optical element is drawn into the input fundamental wave wavelength. 2. The wavelength conversion device according to claim 1, wherein the light source is a semiconductor laser, and oscillated light from the semiconductor laser is incident on the nonlinear optical element as an input fundamental wave to inject the semiconductor laser. A minute fluctuation current is applied to the current to vary the oscillation wavelength of the semiconductor laser to vary the wavelength of the input fundamental wave to the nonlinear optical element, so that the wavelength conversion efficiency of the nonlinear optical element becomes maximum. A wavelength conversion device, wherein the oscillation wavelength of the semiconductor laser is pulled in. 3. The wavelength conversion device according to claim 1, wherein the light from the light source is incident on the nonlinear optical element through the external frequency modulator with the output light of the external frequency modulator as an input fundamental wave. The minute fluctuation modulation signal is given to the external frequency modulator to minutely change the wavelength of the input fundamental wave to the nonlinear optical element, and the output of the external frequency modulator is set to the wavelength at which the wavelength conversion efficiency of the nonlinear optical element is maximized. A wavelength conversion device that draws in light. 4. A minute fluctuation current of a semiconductor laser or a minute fluctuation modulation signal of an external frequency modulator, and a fundamental wave or wavelength-converted output wave from the nonlinear optical element due to wavelength fluctuation of an input fundamental wave to the nonlinear optical element. Of the fluctuation component of the optical-electrical conversion of the fluctuation component of the, the wavelength conversion efficiency in the nonlinear optical element obtained by the correlation and the fundamental wave wavelength that maximizes the wavelength of the input fundamental wave to the nonlinear optical element The difference is used as an error signal, and the error signal is negatively fed back in the direction in which the wavelength of the input fundamental wave coincides with the fundamental wave wavelength at which the wavelength conversion efficiency in the nonlinear optical element is maximized, and the oscillation wavelength of the semiconductor laser or The output wavelength of the external frequency modulator is matched with the wavelength that maximizes the wavelength conversion efficiency of the nonlinear optical element. Wavelength converter. 5. The wavelength conversion device according to claim 1, wherein the light source is a semiconductor laser, and oscillated light from the semiconductor laser is made incident on the nonlinear optical element as an input fundamental wave to cause the nonlinear optical element to enter the nonlinear optical element. A wavelength conversion characterized in that a minute fluctuation voltage is applied to change the refractive index of the nonlinear optical element, and the oscillation wavelength of the semiconductor laser is pulled to the input fundamental wavelength that maximizes the wavelength conversion efficiency of the nonlinear optical element. apparatus. 6. The wavelength converter according to claim 1, wherein the output from the wavelength converter is obtained as a digital signal, the light source is a semiconductor laser, and the oscillation light from the semiconductor laser is an input fundamental wave. As a result, the injection current of the semiconductor laser is applied with a minute fluctuating current, and the oscillation wavelength of the semiconductor laser is changed to change the input fundamental wave wavelength to the nonlinear optical element. The wavelength of the wavelength conversion efficiency is maximized by pulling the oscillation wavelength of the semiconductor laser into the wavelength of the input fundamental wave, and the injection current of the semiconductor laser is maximized by giving a current that gives the wavelength with the maximum wavelength conversion efficiency. Is to perform digital modulation of wavelength-converted light by applying a digital modulation current signal that minimizes the current that gives a wavelength with a different maximum conversion efficiency. Wavelength converter according to claim. 7. The wavelength conversion device according to claim 1, wherein the output from the wavelength conversion unit is obtained as a digital signal, the light source is a semiconductor laser, and oscillation light from the semiconductor laser is an input fundamental wave. As a result, the injected current of the semiconductor laser is applied with a minute fluctuation current to change the oscillation wavelength of the semiconductor laser to change the input fundamental wave wavelength to the nonlinear optical element. Different from the maximum conversion efficiency by pulling the oscillation wavelength of the semiconductor laser to the input fundamental wave wavelength that maximizes the wavelength conversion efficiency, and maximizing the voltage that gives the wavelength that maximizes the wavelength conversion efficiency to the nonlinear optical element. A characteristic is that the wavelength-converted light is digitally modulated by applying a digital modulation voltage signal that minimizes the voltage that gives the wavelength that provides the maximum conversion efficiency. That wavelength conversion device. 8. The wavelength converter according to claim 1, wherein the output from the wavelength converter is obtained as a digital signal, the light source is a semiconductor laser, and the oscillation light from the semiconductor laser is an input fundamental wave. As the incident on the non-linear optical element, a minute voltage modulation signal is applied to the non-linear optical element to change the refractive index of the non-linear optical element, and the wavelength conversion efficiency of the non-linear optical element becomes maximum at the input fundamental wave wavelength. The current that draws the oscillation wavelength of the semiconductor laser and maximizes the current that gives the maximum wavelength conversion efficiency to the injection current of the semiconductor laser, and minimizes the current that gives the wavelength that has a maximum conversion efficiency different from the maximum conversion efficiency. A wavelength conversion device characterized in that a digitally modulated current signal is applied to perform digital modulation of wavelength-converted light. 9. The wavelength conversion device according to claim 1, wherein the output from the wavelength conversion unit is obtained as a digital signal, the light source is a semiconductor laser, and the oscillation light from the semiconductor laser is an input fundamental wave. As the incident on the non-linear optical element, a minute voltage modulation signal is applied to the non-linear optical element to change the refractive index of the non-linear optical element, and the wavelength conversion efficiency of the non-linear optical element becomes maximum at the input fundamental wave wavelength. A wavelength conversion device which draws the oscillation wavelength of the semiconductor laser and applies a digital modulation voltage having a specific voltage difference to the nonlinear optical element to perform digital modulation of wavelength-converted light. 10. The non-linear optical element is an optical higher harmonic wave generating element, as set forth in claim 1, 2, 3, 4, 5,
The wavelength conversion device according to 6, 7, 8 or 9. 11. The wavelength conversion according to claim 1, 2, 3, 4, 5, 6, 7, 8 or 9, wherein the nonlinear optical element is a nonlinear optical element having a quasi phase matching structure. apparatus. 12. The non-linear optical element according to claim 1, wherein the non-linear optical element is a Cherenkov type non-linear optical element.
The wavelength conversion device according to 3, 4, 5, 6, 7, 8 or 9. 13. The nonlinear optical element is LiTa _x Nb _1-x.
The wavelength conversion device according to claim 1, 2, 3, 4, 5, 6, 7, 8 or 9, wherein the wavelength conversion device is an O ₃ nonlinear optical element.