JP4611230B2 - Wavelength converter - Google Patents

Description

  The present invention relates to a wavelength conversion device that performs wavelength conversion using a nonlinear optical effect.

A semiconductor laser that generates light in the wavelength region of 0.5 μm to 0.6 μm has not been realized yet. Therefore, in order to obtain laser light in the wavelength region, second-harmonic generation of light in the infrared region by a nonlinear optical crystal such as KH ₂ PO ₄ , LiNbO ₃ , LiIO ₃ , Ag ₃ AsS ₃ is used. (For example, refer to Patent Document 1).

  The nonlinear optical crystal that generates the second harmonic outputs light (wavelength converted light) in which the frequency of incident light (excitation light) is doubled (wavelength is 1/2). The nonlinear optical crystal has a characteristic that the light intensity of the wavelength-converted light is proportional to the square of the light intensity of the excitation light. In the nonlinear optical crystal having such characteristics, if there is a change in the light intensity of the excitation light, the amount of change in the light intensity of the wavelength converted light is twice the amount of change in the light intensity of the excitation light. Therefore, the wavelength conversion device is required to stabilize the light intensity of the wavelength converted light.

  In order to stabilize the light intensity of the wavelength-converted light to be output, the wavelength converter performs feedback control for converting the light intensity of the wavelength-converted light into an electric signal and feeding it back to the input side of the wavelength converter. FIG. 1 shows a block diagram of the structure of a conventional wavelength converter 101 that performs feedback control. The wavelength conversion device 101 includes a differential amplifier 11, a current drive circuit 12, a semiconductor laser 13, a nonlinear optical crystal 14, an optical branching device 15, a photodiode 16, and a feedback signal circuit 17. The wavelength conversion device 101 in FIG. 1 describes the case where the external electrical signal and feedback control are voltages, but these are not limited to voltages but may be current or power.

  The differential amplifier 11 has two input terminals and one output terminal. The differential amplifier 11 amplifies the voltage difference of the signal input to the input terminal and outputs it from the output terminal.

  The current driving circuit 12 is an electric circuit that supplies a driving current to generate laser light in the semiconductor laser. Although an electric signal from the input side can be directly converted into a drive current, a current source circuit may be incorporated, and the drive current may be supplied by controlling the current source circuit with an electric signal from the input side.

  The semiconductor laser 13 is a semiconductor laser that generates laser light having a wavelength at which the wavelength converted light converted by the nonlinear optical crystal 14 has a desired wavelength. For example, an InGaAs compound laser having a wavelength of 0.98 μm or 1.06 μm and an InP compound laser having a wavelength of 1.3 μm or 1.5 μm can be exemplified.

  The optical splitter 15 is an optical element that splits incident light into two and outputs it in different directions. Examples include prisms and half mirrors.

  The photodiode 16 converts the light intensity of the received light into an electrical signal and outputs it. Since the sensitivity varies depending on the wavelength of the received light, a type with good sensitivity is selected according to the wavelength of the wavelength converted light.

  The feedback signal circuit 17 is an electric circuit that outputs the magnitude of the input current as a feedback signal. When the magnitude of the current is output as a voltage, the feedback signal circuit 17 can be exemplified as a resistance circuit.

The wavelength converter 101 outputs the output light 15a as described below. An external electric signal 10a is input to the current driving circuit 12 as a differential signal 11a via the differential amplifier 11. The current drive circuit 12 converts the voltage value V _COMP of the input difference signal 11a into a drive current 12a and outputs it. Equation 1 shows the relationship between the voltage value V _COMP of the difference signal 11a in the current drive circuit 12 and the current value I _LD of the drive current 12a. G2 is a constant.

The semiconductor laser 13 receiving the drive current 12a outputs excitation light, and the excitation light is wavelength-converted by the nonlinear optical crystal 14 and outputs wavelength-converted light 14a. The light intensity P _PUMP of the pumping light output from the semiconductor laser 13 is expressed by Equation 2 using the threshold current I _th of the semiconductor laser 13 and the conversion efficiency α.

Further, since the light intensity of the wavelength-converted light output from the nonlinear optical crystal that generates the second harmonic is proportional to the square of the light intensity of the input excitation light, the light intensity P _LN of the wavelength-converted light 14 a is the semiconductor laser 13. Using the light intensity of the excitation light and the conversion efficiency β, it is expressed by Equation 3.

Therefore, the relationship between the light intensity P _LN of the wavelength converted light 14a and the current value I _LD of the drive current 12a is obtained by substituting Equation 2 into Equation 3 and substituting the constant portion with the wavelength conversion efficiency η of Equation 4. Equation 5 can be derived.

  The wavelength-converted light 14a is branched into two by the optical branching device 15, and one is output to the outside as the output light 15a of the wavelength conversion device 101.

The wavelength conversion device 101 performs feedback control of the output light 15a as follows. One of the optical branching devices 15 that branches the wavelength-converted light 14a at a constant ratio is received by the photodiode 16 as the reference light 15b and is photoelectrically converted as shown in Equation 6. In Equation 6, G5 is a constant, _PPD is the light intensity of the reference light 15b, and _IPD is the current value of the photoelectric conversion current 16a output from the photodiode 16.

Current _{I PD} of the photoelectric conversion current 16a is converted into a feedback signal 17a as in equation 7 in the feedback signal circuit 17. In Equation 7, G6 is a constant, and _VFB is a voltage value of the feedback signal 17a.

The feedback signal 17 a is fed back to the input side of the wavelength conversion device 101 and input to the differential amplifier 11. The differential amplifier 11 amplifies the voltage difference between the externally indicated value V _in of the electric signal 10a and the voltage value V _FB of the feedback signal 17a, and outputs the amplified signal to the current driving circuit 12 as the differential signal 11a. Equation 8 is an expression showing the voltage value V _COMP of the difference signal 11a. G1 is a constant.

Since the feedback signal 17a has a variation information of the light intensity _{P LN} wavelength converted light 14a, the differential signal 11a also have a variation information of the light intensity _{P LN} wavelength converted light 14a. The wavelength conversion device 101 can perform feedback control of the output light 15a by inputting the drive current 12a obtained by converting the differential signal 11a in the current drive circuit 12 to the semiconductor laser 13.
Japanese Patent Laid-Open No. 08-102564

The light intensity P _LN of the wavelength-converted light 14a output from the wavelength conversion device is expressed by _Equation 5, and has a quadratic-curve IL characteristic. FIG. 2 shows IL characteristics of the wavelength conversion device, in which the horizontal axis represents the current value I _LD of the drive current 12a and the vertical axis represents the light intensity P _LN of the wavelength converted light 14a. In FIG. 2, a curve 21 is an IL characteristic of the wavelength conversion device 101, and a broken line is a linear approximation line 22 of the curve 21.

On the other hand, since the current drive circuit 12 has the relationship of Equation 1, the voltage value V _COMP of the difference signal 11a and the current value I _LD of the drive current 12a are in a proportional relationship as shown in FIG.

Here, if the degree minute or 0 threshold current I _th of that negligible semiconductor laser 13 may be directed Equation 9 from equations 1 and 5, the wavelength converted light 14a outputted from the wavelength conversion device The light intensity P _LN is proportional to the square of the voltage value V _COMP of the difference signal 11a. The relationship between the voltage value _{V COMP} of the light intensity _{P LN} and the difference signal 11a of the wavelength conversion light 14a shown in FIG.

On the other hand, in the photodiode 16 described with reference to FIG. 1, the light intensity received and the current output by photoelectric conversion are proportional to each other as shown in Equation 6. That is, from the equations 6 and 7, the wavelength converted light 14a and the feedback signal 17a are in a proportional relationship. Since the feedback signal 17a becomes the differential signal 11a by the differential amplifier 11, the voltage value V _COMP of the differential signal 11a varies in proportion to the variation of the light intensity P _LN of the wavelength converted light 14a. However, since the wavelength converter 101 has the relationship of Equation 9, the wavelength-converted light 14a is corrected in proportion to the square of the fluctuation of the light intensity _PLN by feedback control.

  Therefore, the wavelength conversion device 101 performs feedback control according to the linear approximation line 22 shown by the broken line in FIG. 2 on the light output having the characteristic as the curve 21 in FIG. 2, and the light intensity of the wavelength converted light. There is a problem that it is not possible to make an appropriate correction with respect to fluctuations in the size.

That is, when the difference in light intensity between the linear approximation line 22 and the curve 21 for the same drive current as in section B of Figure 2 is large, the voltage value V of the instruction value V _in the feedback signal 17a of the electric signal 10a from the outside _Since the difference from _FB becomes large, if the differential amplifier 11 does not have a sufficient gain, the output is saturated, and the fluctuation of the light intensity of the wavelength-converted light cannot be corrected, and the desired light intensity cannot be obtained.

Further, in the section A and the section C of FIG. 2, the slope of the linear approximation line 22 (loop gain) and the slope of the curve 21 (the optical gain that is the light intensity P _LN of the wavelength-converted light 14a with respect to the current value I _LD of the drive current 12a). ) Is different. If feedback control according to the linear approximation line 22 is performed, the optical gain is small with respect to the loop gain in the section A in FIG. 2, so that the correction of the fluctuation of the light intensity of the wavelength converted light is delayed. On the other hand, in section C, since the optical gain is larger than the loop gain, overshoot is likely to occur due to correction of fluctuations in the light intensity of the wavelength converted light.

  Specifically, when a pulse waveform is input as the external electric signal 10a to the wavelength conversion device 101, the pulse shape of the output light 15a may be distorted due to the intensity of the pulse waveform of the external electric signal 10a.

  As a countermeasure for the correction in the section A and the section C in FIG. 2, a means for increasing the time constant of feedback control can be considered, but it has a demerit that high-speed feedback control cannot be performed.

  The present invention has been made in order to solve the above-mentioned problems, and can appropriately correct the fluctuation of the light intensity by feedback control using a feedback signal proportional to the light intensity, so that the wavelength-converted light having a stable light intensity can be obtained. An object of the present invention is to provide a wavelength conversion device capable of obtaining the above.

  In order to achieve the above object, a wavelength conversion device according to the present invention is a current driving circuit that outputs a driving current for driving a semiconductor laser, and a voltage, current, or input signal that inputs the driving current to the current driving circuit. It was assumed to be proportional to the square root of the power value.

  Specifically, the present invention relates to a semiconductor laser that outputs excitation light, a nonlinear optical crystal that converts the wavelength of the excitation light from the semiconductor laser and outputs the wavelength converted light, and the nonlinear optical crystal that outputs the nonlinear optical crystal. An optical branching device for branching a part of the wavelength-converted light, a light intensity detection circuit for detecting the intensity of the wavelength-converted light branched by the optical branching device and outputting it as a feedback signal, and an instruction for an electrical signal from the outside A differential amplifier circuit that amplifies a difference between the value and the value of the feedback signal from the light intensity detection circuit and outputs the difference signal; and the differential signal from the differential amplifier circuit is set to a value of the difference signal And a current drive circuit that converts the drive current proportional to a square root and outputs the drive current to the semiconductor laser.

  The light intensity of the wavelength-converted light from the wavelength converter is proportional to the square of the drive current as described in Equation 5. By outputting a drive current proportional to the square root of the voltage, current or power value of the difference signal input to the current drive circuit by the current drive circuit, the light intensity of the light output from the wavelength converter is the difference. It will be proportional to the signal. That is, the feedback control of the feedback signal proportional to the fluctuation of the light intensity of the wavelength-converted light will correct the wavelength-converted light in proportion to the fluctuation of the light intensity, thereby avoiding the problems described in the prior art. Can do.

  Therefore, the present invention provides a wavelength conversion device that can appropriately correct the fluctuation of light intensity by feedback control using a feedback signal proportional to the light intensity, and obtain wavelength-converted light having a stable light intensity. Can be provided.

  The present invention may further include an offset addition circuit that adds an offset signal to the electrical signal so that a threshold current of the semiconductor laser is added to the drive current output from the current drive circuit.

The offset adding circuit adds an offset signal converted to a current value corresponding to a threshold current of the semiconductor laser via the differential amplifier and the current driving circuit to an external electric signal. By adding the offset signal to the electrical signal, the amount corresponding to the threshold current is added to the drive current, and it is not necessary to consider the semiconductor laser threshold current Ith in _Equation 5. That is, since the accuracy of Equation 9 is increased, accurate feedback control can be performed by inputting the difference signal having the fluctuation information of the light intensity of the wavelength-converted light to the current driving circuit.

  Therefore, the present invention provides a wavelength conversion device that can appropriately correct the fluctuation of light intensity by feedback control using a feedback signal proportional to the light intensity, and obtain wavelength-converted light having a stable light intensity. Can be provided.

  The present invention may further include a threshold current adding circuit that adds a threshold current of the semiconductor laser to the driving current output from the current driving circuit.

  The threshold current adding circuit adds a current corresponding to the threshold current of the semiconductor laser to the drive current. Since the threshold current adding circuit can directly supply the threshold current to the semiconductor laser without going through the non-linear differential amplifier, the wavelength conversion device including the threshold current adding circuit is more accurate. Feedback control can be performed.

  Therefore, the present invention provides a wavelength conversion device that can appropriately correct the fluctuation of light intensity by feedback control using a feedback signal proportional to the light intensity, and obtain wavelength-converted light having a stable light intensity. Can be provided.

  Even if the present invention is a wavelength converter having a relationship in which the light intensity of the output light is proportional to the square of the drive current, an appropriate correction is made for fluctuations in the light intensity by feedback control using a feedback signal proportional to the light intensity. Thus, wavelength-converted light with stable light intensity can be obtained.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiment described below is an example of the configuration of the present invention, and the present invention is not limited to the following embodiment.

(Embodiment 1)
The present embodiment includes a semiconductor laser that outputs excitation light, a nonlinear optical crystal that wavelength-converts the excitation light from the semiconductor laser and outputs the wavelength-converted light, and the wavelength-converted light output by the nonlinear optical crystal. An optical branching device for branching a part, a light intensity detection circuit for detecting the intensity of the wavelength-converted light branched by the optical branching device and outputting it as a feedback signal, an indication value of an electric signal from the outside, and the light A differential amplifier circuit that amplifies a difference from the value of the feedback signal from the intensity detection circuit and outputs the difference signal, and the difference signal from the differential amplifier circuit is proportional to the square root of the value of the difference signal And a current driving circuit that converts the driving current and outputs the driving current to the semiconductor laser.

  A block diagram of the structure of the wavelength converter 105 according to the present invention is shown in FIG. The wavelength converter 105 includes a differential amplifier 11, a current drive circuit 52, a semiconductor laser 13, a nonlinear optical crystal 14, an optical branching device 15, a photodiode 16, and a feedback signal circuit 17. 5, the same reference numerals as those used in FIG. 1 have the same configuration and the same functions and operations. The difference between the wavelength conversion device 105 and the wavelength conversion device 101 in FIG. 1 is that the wavelength conversion device 105 includes a current drive circuit 52 instead of the current drive circuit 12.

The current driving circuit 52 is an electric circuit having a square root circuit that supplies a driving current 52 a that generates laser light to the semiconductor laser 13. The characteristic of the current drive circuit 52 is that the difference signal 11a is converted into the drive current 52a so that the current value I _LD of the drive current 52a is proportional to the square root of the voltage value V _COMP of the input difference signal 11a, as shown in Equation 10. Is. FIG. 6 is a diagram showing the relationship of Equation 10.

An example of the square root circuit is shown in FIG. In FIG. 10, Q1 to Q4 indicate transistors. In the circuit as shown in FIG. 10, when the sum of the base-emitter voltage of Q1 and the base-emitter voltage of Q2 is equal to the sum of the base-emitter voltage of Q3 and the base-emitter voltage of Q4, the collector current of Q1 And the collector current of Q2 and the product of the collector current of Q3 and the collector current of Q4 have the property of being equal. The collector current of Q3 is equal to the input current _{I A,} the collector current of Q4 is equal to the input current _{I B.} Further, the output current _Iout is equal to the collector currents of Q1 and Q2. Therefore, the square root circuit of FIG. 10 has a relationship in which the output current I _out is equal to the square root of the product of the input current I _A and the input current I _B as shown in Equation 11. Accordingly, in the square root circuit of FIG. 10, the output current I _out proportional to the square root of the other current can be obtained by setting one of the input current I _{A and the} input current I _B to a fixed value.

When the electrical signal from the input side is a current, the current is left as it is. When the electrical signal is power or voltage, the electrical signal is converted into a current, and then the current is input to the input current I _A or the square root circuit. input as the input current _{I B.} The current drive circuit 52 outputs an output current _Iout proportional to the square root of the current value as a drive current. Further, the current drive circuit 52 may include a current source circuit, and output the drive current by controlling the current source circuit with an output current _Iout .

Here, when the threshold current Ith of _Formula 5 is 0, _Formula 12 can be derived from Formula 5 and Formula 10, and the light intensity P _LN of the wavelength converted light 14a output from the wavelength converter is a difference signal. The relationship is proportional to the voltage value V _{COMP of} 11a. The relationship between the voltage value _{V COMP} of the light intensity _{P LN} and the difference signal 11a of the wavelength conversion light 14a shown in FIG.

Wavelength converter 105 outputs the output light 15a as explained in the wavelength converter 101 of Figure 1 to the outside, and inputs a feedback signal 17a which is proportional to the light intensity P _LN wavelength converted light 14a to the differential amplifier 11 .

As described above, the voltage value V _COMP of the difference signal 11a fluctuates in proportion to the fluctuation of the light intensity P _LN of the wavelength converted light 14a. However, since the current drive circuit 52 has the relationship of several tens, the wavelength converter 105 The feedback signal 17a proportional to the fluctuation of the light intensity _PLN of the wavelength converted light 14a enables accurate feedback control.

  Therefore, since the wavelength converter 105 can avoid the trouble described in the conventional wavelength converter 101 and can obtain a stable light intensity, even if the electric signal 10a from the outside has a pulse waveform, It becomes difficult to produce distortion in the pulse shape.

(Embodiment 2)
In this embodiment, an offset addition circuit that adds an offset signal to the electrical signal so that a threshold current of the semiconductor laser is added to the drive current output from the current drive circuit is described in the first embodiment. It is further provided in the wavelength conversion device described in 1.

  A block diagram of the structure of the wavelength converter 108 according to the present invention is shown in FIG. In FIG. 8, the same reference numerals as those used in FIG. 5 have the same configuration and the same functions and operations. The difference between the wavelength converter 108 and the wavelength converter 105 of FIG. 5 is that the wavelength converter 108 further includes an offset addition circuit 80.

Offset adding circuit 80, so that the current value I _th of the threshold current of the semiconductor laser 13 to the drive current 52a output from the current driving circuit 52 is added, to the instruction value V _in of the electrical signals 10a from the outside The offset signal 80a is added. For example, the offset addition circuit 80 adds the voltage value V _off of the offset signal 80a as shown in Equation 13 to the instruction value V _in of the electric signal 10a from the outside. That is, the current value of the drive current 52a from the current driving circuit 52 when the voltage value _{V off} is indicated value _{V in} of the electrical signals 10a from the voltage value _{V FB} and external feedback signal 17a of the offset signal 80a is 0 _{I LD} There is a value such that the threshold current I _th.

Wavelength converter 108 includes an offset adder circuit 80, to constantly input a threshold current to the semiconductor laser 13 by adding the voltage value _{V off} of the offset signal 80a to the command value _{V in} of the electrical signals 10a from the outside be able to.

Therefore, it is not necessary to consider the threshold current value Ith in _Equation 5, and the wavelength converter 108 can perform accurate feedback control with a feedback signal proportional to the light intensity, and can obtain a stable light intensity. .

(Embodiment 3)
The present embodiment further includes a threshold current adding circuit for adding the threshold current of the semiconductor laser to the driving current output from the current driving circuit in the wavelength conversion device described in the first embodiment. Yes.

  A block diagram of the structure of the wavelength converter 109 according to the present invention is shown in FIG. 9, the same reference numerals as those used in FIG. 5 have the same configuration and the same functions and operations. The difference between the wavelength converter 109 and the wavelength converter 105 of FIG. 5 is that the wavelength converter 109 further includes a threshold current adding circuit 90.

The threshold current adding circuit 90 adds an offset current I _{off corresponding} to the threshold current value I _th of the semiconductor laser 13 to the driving current 52 a output from the current driving circuit 52.

By adding the current value I _off of the offset signal 90a to the current value I _LD of the drive current 52a, it is not necessary to consider the threshold current value I _th of the semiconductor laser in Equation 5, and the wavelength conversion device 109 has the wavelength An effect similar to the effect described for the conversion device 108 can be obtained. Further, since the threshold current adding circuit 90 can be supplied directly to the threshold current of the current value I _th in the semiconductor laser 13 without passing through the nonlinear differential amplifier 11, the wavelength conversion apparatus 109 even more accurate Feedback control can be performed.

  In addition, although the wavelength converter 105 of FIG. 5, the wavelength converter 108 of FIG. 8, and the wavelength converter 109 of FIG. 9 have described the case where the electric signal and feedback control from the outside are voltage, these are limited to voltage. In some cases, it may be current or power.

  The wavelength converter of the present invention can be used as a semiconductor laser for communication, optical recording, and measurement.

It is a block diagram which shows the structure of the conventional wavelength converter. It is a graph of the IL characteristic of a wavelength converter. It is a graph of the VI characteristic of the current drive circuit used for the conventional wavelength converter. It is the graph which showed the relationship between the light intensity of the wavelength conversion light of a conventional wavelength converter, and a difference signal. It is a block diagram which shows the structure of the wavelength converter which concerns on one Embodiment of this invention. It is a graph of the VI characteristic of the current drive circuit used for the wavelength converter which concerns on embodiment of this invention. It is the graph which showed the relationship between the optical intensity of the wavelength conversion light of the wavelength converter which concerns on embodiment of this invention, and a difference signal. It is a block diagram which shows the structure of the wavelength converter which concerns on other embodiment of this invention. It is a block diagram which shows the structure of the wavelength converter which concerns on other embodiment of this invention. It is an example of the square root circuit which the current drive circuit 52 has.

Explanation of symbols

101, 105, 108, 109 Wavelength converter 10a External electric signal 11 Differential amplifier 11a Differential signal 12, 52 Current drive circuit 12a, 52a Drive current 13 Semiconductor laser 14 Nonlinear optical crystal 14a Wavelength converted light 15 Optical branching device 15a Output light 15b Reference light 16 Photodiode 16a Photoelectric conversion current 17 Feedback signal circuit 17a Feedback signal 21 Curve 22 Linear approximation line 80 Offset addition circuit 80a Offset signal 90 Threshold current addition circuit 90a Offset signal I _LD drive current 12a or drive current 52a of the current value _{P LN} wavelength converted light 14a light intensity _{I PD} photoelectric conversion current 16a of the current value _P light intensity of the _PD reference light 15b _{V FB} command value V of the electrical signal 10a from the voltage value _{V in} an external feedback signal 17a voltage value a of the _COMP differential signal 11a, B, C segment _th semiconductor value of the threshold current of the laser 13 _{V off} offset signal 80a Q4 transistor from the voltage value _{I off} offset current 90a of a current value Q1 of

Claims (3)

A semiconductor laser that outputs excitation light; and
A nonlinear optical crystal that wavelength-converts the excitation light from the semiconductor laser and outputs a second harmonic as wavelength-converted light;
An optical branching device for branching a part of the wavelength-converted light output by the nonlinear optical crystal;
A light intensity detection circuit that detects the intensity of the wavelength-converted light branched by the optical splitter and outputs a feedback signal proportional to the intensity of the wavelength-converted light ;
A differential amplifying circuit that amplifies a difference between an instruction value of an external electric signal and a value of the feedback signal from the light intensity detection circuit, and outputs the difference signal;
A current drive circuit that converts the differential signal from the differential amplifier circuit into the drive current proportional to the square root of the value of the differential signal and outputs the drive current to the semiconductor laser;
A wavelength conversion device comprising:   2. The offset adding circuit for adding an offset signal to the electrical signal so that a threshold current of the semiconductor laser is added to the driving current output from the current driving circuit. The wavelength converter described in 1.   2. The wavelength conversion device according to claim 1, further comprising a threshold current adding circuit that adds a threshold current of the semiconductor laser to the driving current output from the current driving circuit.

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