JP2000338531A - Wavelength conversion laser device and laser radar device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To instantly switch the oscillation wavelengths by disposing a PPLN (Periodically Poled Lithium Niobate) crystal which generates polarization inversion states with different periods to produce different OPO(Optical Parametric Oscillation) wavelengths along two optical axes in an OPO resonator. SOLUTION: The PPLN crystal 3 having the following property is disposed in the OPO resonator. The PPLN crystal 3 generates polarization inversion states with different periods to produce different OPO wavelengths along the optical axis A and optical axis B, namely, the crystal generates signal light of wavelength λsa and idler light of wavelength λia, or signal light of wavelength λsb and idler light of wavelength λib. When no voltage is applied on an EO element 7, the OPO excitation light is reflected as s-polarized light by a polarizer 8 and propagates along the optical axis A. If a voltage is applied on the EO element 7, the OPO excitation light is transmitted as p-polarized light through the polarizer 8 and propagates along the optical axis B.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength conversion laser device for changing the oscillation wavelength of a laser beam in a very short time, and a laser for applying the wavelength conversion laser device to change the oscillation wavelength according to an object to be measured. The present invention relates to a radar device.

[0002]

2. Description of the Related Art One of the techniques for changing the wavelength of laser light is a technique relating to optical parametric oscillation (hereinafter referred to as OPO). This OPO
Is a phenomenon of generating two wavelengths of the wavelength λs and the wavelength λi from the pumping light of the wavelength λp, and the relationship between the wavelength λs and the wavelength λi is 1 / λp = (1 + λs) + (1 / λi) (1) ). Here, of the light by the general OPO (hereinafter, referred to as OPO light), a short wavelength light is signal light (wavelength λs), and a long wavelength light is idler light (wavelength λ).
Called i).

For example, when an Nd: YAG laser having a wavelength of 1064 nm is used as the excitation light, the wavelength λs = 150
OPO light having a wavelength of 0 nm and a wavelength of λi = 3660 nm can be generated.

[0004] The combination of each wavelength of the signal light and the idler light is determined by an optical crystal (hereinafter referred to as OPO) used for OPO.
The wavelength can be selected with a considerable degree of freedom as long as the above expression (1) is satisfied, depending on the installation angle with respect to the crystal (referred to as crystal), the temperature, the characteristics of the mirror used for the OPO, and the like.

[0005] By utilizing this wavelength selectivity, O
PO is often used as a wavelength tunable light source in a wavelength range where the wavelength tunable laser light source is small. As its uses, for example, there are many applications for physics and chemistry such as spectroscopy, and for remote sensing such as environmental measurements, and also applied to medical uses.

In general, the refractive index of light in a medium varies depending on the wavelength, and this is called wavelength dispersion. In order to realize effective OPO, it is necessary to eliminate this difference in refractive index.

Usually, this difference in refractive index is eliminated by utilizing the difference in the polarization of excitation light, signal light, and idler light, and one method of eliminating this is a method called quasi-phase matching.

This quasi-phase matching means, for example, LiNbO
By creating a domain-inverted grating that periodically inverts the polarization of a ferroelectric crystal exhibiting a secondary nonlinear optical effect as shown in ₃ , the periodic sign inversion structure of the secondary nonlinear optical tensor is realized. This is a method of compensating for the phase mismatch between the pump light and the OPO to achieve the pseudo phase matching.

As an OPO crystal utilizing the quasi phase matching, for example, a periodically poled LiNbO ₃ (Period) is used.
dically Poled Lithium Niobate (hereinafter referred to as PPLN) and RTA (RbTiOAsO ₄ : hereinafter PPRTA)
), KTP (KTiOPO ₄ , hereinafter, PPKT)
P) is well known, and research and development are being actively conducted because of its high conversion efficiency.

In the wavelength tunable laser using the OPO, the following two methods are often used as a wavelength tunable method. One of the methods is to change the angle of the OPO crystal with respect to the oscillation optical axis, and the angle tuning is performed. Another method for changing the temperature of the OPO crystal is called temperature tuning.

[0011]

Some laser radars using a plurality of wavelengths need to instantaneously switch the oscillation wavelength according to the characteristics of an object to be irradiated. In such a laser radar, mechanical angle tuning and temperature tuning are performed as described above to vary the oscillation wavelength, but it is difficult to instantaneously switch the oscillation wavelength. Need to change the wavelength faster than the time required.

Accordingly, an object of the present invention is to provide a wavelength conversion laser device capable of instantaneously switching the oscillation wavelength.

Another object of the present invention is to provide a laser radar device capable of instantaneously switching an oscillation wavelength according to an object to be measured.

[0014]

According to the first aspect of the present invention,
An excitation light source that generates the excitation light, an optical parametric oscillation optical system that generates a different wavelength depending on the optical path position of the excitation light generated by the excitation light source, and an optical parametric oscillation of the excitation light generated from the excitation light source A wavelength conversion laser device comprising: an optical path variable optical system that changes a transmission optical path position in the optical system.

According to a second aspect of the present invention, in the wavelength conversion laser device according to the first aspect, the optical parametric oscillation optical system has a nonlinear optical crystal in which polarization inversions having different periods are generated in at least two portions. It is.

According to a third aspect of the present invention, there is provided the wavelength conversion laser device according to the second aspect, wherein the nonlinear optical crystal is Li
One of NbO ₃ , RbTiOAsO ₄ , and KTiOPO ₄ is used.

According to a fourth aspect of the present invention, in the wavelength conversion laser device of the first aspect, the optical path variable optical system polarizes the excitation light generated from the excitation light generation source, and changes the optical path of the excitation light according to the polarization. Change the position.

According to a fifth aspect of the present invention, in the wavelength conversion laser device according to the first aspect, the optical path variable optical system comprises: an electro-optical element for converting the excitation light generated from the excitation light generation source into s-polarized light or p-polarized light; S polarized by this electro-optical element
A polarizer that changes the optical path position of polarized light or p-polarized light.

According to a sixth aspect of the present invention, there is provided an excitation light generation source for generating excitation light, an optical parametric oscillation optical system having different wavelengths generated by an optical path of the excitation light generated by the excitation light generation source, and an excitation light generation optical system. An optical path variable optical system that varies the transmission optical path position of the excitation light generated from the source in the optical parametric oscillation optical system, and receives light from the object of the laser light emitted from the optical parametric oscillation optical system, and responds to the light. Optical path control means for variably controlling the optical path of the excitation light in the optical path variable optical system.

According to a seventh aspect of the present invention, there is provided an excitation light generation source for generating excitation light, and a nonlinear optical crystal in which polarization inversions having different periods are generated in at least two portions, wherein the excitation light generation source generates the excitation light. An optical parametric oscillation optical system in which the generated excitation light has a different wavelength depending on the optical path position in the non-linear optical crystal, an electro-optic element that changes the polarization angle of the excitation light generated from the excitation light source, and polarized light by the electro-optic element A polarizer that guides the nonlinear optical crystal by changing the optical path position of the s-polarized light or the p-polarized light, and receives light from a target object of laser light emitted from the optical parametric oscillation optical system, and responds to the light. An optical path control unit that controls an applied voltage to the electro-optical element to change a polarization angle of s-polarized light or p-polarized light to control an output ratio between s-polarized light and p-polarized light.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a configuration diagram of a laser radar device using the wavelength conversion laser device of the present invention.

The excitation light generating laser device 1 is, for example, an OP
It has a function of generating excitation light of wavelength λp used for O. The excitation light used for this OPO (hereinafter referred to as OPO excitation light) is a linearly polarized laser light, and here the polarization direction is s-polarized light. The s-polarized light is
It has a direction in which polarized light oscillates in a direction perpendicular to the drawing.

The optical parametric oscillation optical system 2 receives the OPO pumping light generated by the pumping light generating laser device 1 and generates the OPO pumping light on each optical path position (on the optical axis A or the optical axis B). OPO light having different wavelengths, that is, a signal light having a wavelength λsa and an idler light having a wavelength λia are generated on the optical axis A, and a signal light having a wavelength λsb and an idler light having a wavelength λib are generated on the optical axis B. ing.

The specific configuration will be described.
An OPO resonator is formed by arranging an OPO incident mirror 4 and an OPO exit mirror 5 so as to sandwich the N crystal 3.

The PPLN crystal 3 is a non-linear optical crystal in which polarization inversion of different periods is generated so that OPO light generates different wavelengths in the optical axis A and the optical axis B. That is, when the wavelength of the OPO pumping light generated by the pumping light generating laser device 1 is λp, the PPLN crystal 3 has an optical axis A at 1 / λp = (1 / λsa) + (1 / λia). (1) In the optical axis B, 1 / λp = (1 / λsb) + (1 / λib) (2) The signal light having the wavelength λsa and the idler light having the wavelength λia, or the signal light having the wavelength λsb and the wavelength λib PPLN between the optical axis A and the optical axis B so as to generate the idler light of
The polarization inversion period of the crystal 3 is set.

Here, the PPLN crystal 3 is usually of type
I phase matching is achieved, and the polarization of the OPO light on the optical axis A is orthogonal to the polarization of the OPO excitation light, and the idler light having the wavelength λia becomes p-polarized light. Together with PPLN
In the crystal 3, the polarization of the OPO light is orthogonal to the polarization of the OPO excitation light on the optical axis B, and the idler light having the wavelength λib is p-polarized.

The OPO entrance mirror 4 and the exit mirror 5
On the optical axis A, a coating is applied so that the OPO excitation light is efficiently converted into a signal light having a wavelength λsa and an idler light having a wavelength λia. On the axis B, a coating is applied so that the OPO excitation light is efficiently converted into signal light having the wavelength λsb and idler light having the wavelength λib.

An optical path variable optical system 6 is provided between the pumping light generating laser device 1 and the optical parametric oscillation optical system 2 as an optical parametric oscillation optical system for the OPO pumping light generated from the pumping light generating laser device 1. The function of changing the position of the transmitted light path in 2, ie, the optical axis A or the optical axis B, is provided.

The specific configuration of the optical path variable optical system 6 will be described.
An EO (Electro Optic) element (electro-optical element) 7 and a polarizer 8 are arranged on the optical path of the O excitation light.

The EO element 7 changes the polarization direction of the OPO excitation light from s-polarized light to p-polarized light when a predetermined voltage is applied, and is set so that the polarization of the transmitted light does not change when no voltage is applied. Have been. Here, the p-polarized light is a polarized light having a polarization vibration plane in a direction horizontal to the drawing.
The EO element 7 is composed of KDP (KH ₂ PO ₄ ), LB
O (LiB ₃ O ₅ ) or the like.

The polarizer 8 has a function of transmitting p-polarized light and reflecting s-polarized light. The reflected light path of the polarizer 8 is
A 45 ° mirror 9 is arranged on the transmission optical path of the polarizer 8 so that the traveling direction of the transmitted light coincides with the optical axis A of the PPLN crystal 3. I have.

Therefore, if no voltage is applied to the EO element 7, the OPO excitation light is s-polarized light, is reflected by the polarizer 8 and travels on the optical axis A, and if a voltage is applied to the EO element 7, If so, the OPO excitation light becomes p-polarized light, passes through the polarizer 8, is reflected by the 45 ° mirror 9, and travels on the optical axis B.

On both sides of the optical parametric oscillation optical system 2 on the optical axis B, λ / 2 plates 10 and 11 are disposed, respectively.

On the emission side of the optical parametric oscillation optical system 2, a 45 ° mirror 12 is arranged on the optical axis B.
The polarizer 13 is disposed at a position on the optical axis A where the 45 ° mirror 12 intersects with the reflection optical path.

On the other hand, the optical path control means 14 controls the object 15 of the laser light emitted from the optical parametric oscillation optical system 2.
, And by controlling the voltage applied to the EO element 7 according to the return light C to change the polarization angle of the s-polarized light or the p-polarized light, the output ratio between the s-polarized light and the p-polarized light is changed. It has a control function.

The structure of the optical path control means 14 will be specifically described. When the object 15 is irradiated with the OPO light emitted from the optical parametric oscillation optical system 2, a part of the light becomes reflected light or backscattered light. It returns to the laser radar device as return light C. Thus, in this specification, the return light refers to the reflected light or the backscattered light by the object 15 to be measured.

The light receiving section 16 has a function of receiving the return light C and converting it into an electric signal corresponding to the amount of the received light.

The EO driver control system 17 receives an electric signal from the light receiving section 16 and processes the electric signal to control a signal corresponding to the object 15 to be measured, that is, a control signal indicating a voltage value to the EO element 7. It has a function of sending a signal to the EO driver 18.

The EO driver 18 has a function of receiving a control signal from the EO driver control system 17 and applying a voltage value of 0 V, a predetermined applied voltage value, or an arbitrary voltage value to the EO element 7. are doing.

The EO element 7 changes the polarization direction of the OPO excitation light from the s-polarized light to the p-polarized light when the predetermined voltage is applied as described above.
The polarization is changed so that the polarization of the transmitted light is not changed when no voltage is applied (0 V), and the polarization angle of the linearly polarized light is changed by an angle corresponding to the applied voltage. I have.

Therefore, the ratio of the OPO excitation light traveling to the optical axis A and the optical axis B is changed by changing the voltage applied to the EO element 7 by the EO driver control system 17, and the wavelength λia is accordingly changed. The output ratio between the idler light and the idler light having the wavelength λib also changes.

Next, the operation of the laser radar device configured as described above will be described.

The excitation light generating laser device 1 generates, for example, OPO excitation light having a wavelength λp and linearly polarized light having a polarization direction of s-polarized light. This OPO excitation light is
The light enters the variable optical path system 6.

The EO element 7 of the optical path variable optical system 6 changes the polarization direction of the incident OPO excitation light from a s-polarized light to a p-polarized light when a predetermined voltage is applied. The excitation light is sent to the polarizer 8 without changing its polarization.

The polarizer 8 is provided with the OPO from the EO element 7.
If the excitation light is p-polarized, the OPO excitation light is transmitted,
If it is s-polarized light, it reflects.

Therefore, if no voltage is applied to the EO element 7, the OPO excitation light becomes s-polarized light, is reflected by the polarizer 8, travels on the optical axis A, and if no voltage is applied to the EO element 7. , Becomes p-polarized light, passes through the polarizer 8, and
The light is reflected by the 5 ° mirror 9, passes through the λ / 2 plate 10, and travels on the optical axis B.

The OPO pumping light traveling on the optical axis A is optically resonated by the OPO resonator composed of the PPLN crystal 3, the OPO incidence mirror 4 and the emission mirror 5, and the signal light of wavelength λsa and the wavelength of λia Idler light is generated.

Here, assuming that the idler light having the wavelength λia is used among the signal light having the wavelength λsa and the idler light having the wavelength λia, the PPLN crystal 3 usually has a type
This is eI phase matching, in which the polarization of the OPO light is orthogonal to the polarization of the OPO excitation light on the optical axis A, and the idler light having the wavelength λia becomes p-polarized light and passes through the polarizer 13 to be emitted.

On the other hand, the excitation light traveling on the optical axis B is λ / 2
The polarization is rotated by 90 ° by the plate 10 to become s-polarized light,
PPLN crystal 3, OPO entrance mirror 4 and exit mirror 5
Optically resonates with an OPO resonator composed of
Signal light and idler light of wavelength λib are generated. Here, if it is assumed that the idler light having the wavelength λib is used among the signal light having the wavelength λsb and the idler light having the wavelength λib, the PPLN crystal 3 is usually of type I phase matching, and the OPO on the optical axis A is Similarly, idler light having a wavelength λib of p-polarized light is generated and emitted from the OPO resonator. The emitted idler light of wavelength λib is p-polarized light,
When transmitted through the λ / 2 plate 11, the light becomes s-polarized light,
After being reflected by the mirror 12, the light is reflected by the polarizer 13 and emitted.

The idler light having the wavelength λia or the idler light having the wavelength λib emitted as described above is applied to the object 15, and a part of the idler light becomes reflected light or backscattered light and returns to the laser radar device as return light C. come.

The light receiving section 16 receives the return light C, converts the return light C into an electric signal corresponding to the amount of the received light, and sends the electric signal to the EO driver control system 17.

The EO driver control system 17 includes the light receiving unit 1
6, the electrical signal from the laser radar device is processed according to the measurement object 15 and the measurement contents, and as a result, the control signal corresponding to the object 15, that is, the voltage to the EO element 7 A control signal indicating the value is sent to the EO driver 18.

The EO driver 18 receives a control signal from the EO driver control system 17 and applies a voltage value of 0 V, a predetermined applied voltage value, or an arbitrary voltage value to the EO element 7.

Therefore, the EO element 7 changes the polarization direction of the OPO excitation light from the s-polarized light to the p-polarized light when the predetermined voltage is applied as described above, so that the voltage is not applied (0 V).
If so, the polarization of the transmitted light is kept unchanged.

The EO element 7 changes the polarization angle of the linearly polarized light by an angle corresponding to the applied voltage value. Thus, the ratio of the OPO excitation light traveling to the optical axis A and the optical axis B is changed by changing the voltage applied to the EO element 7, and the idler light having the wavelength λia and the idler light having the wavelength λib are accordingly changed. The output ratio also changes.

As described above, in one embodiment,
The optical axis A and the optical axis B have different OPO wavelengths, that is, wavelength λ.
sa signal light and idler light of wavelength λia or wavelength λsb
PPLN crystal 3 in which polarization inversions of different periods are generated so as to generate signal light and idler light of wavelength λib
Is disposed in the OPO resonator, and if no voltage is applied to the EO element 7, the OPO excitation light is reflected by the polarizer 8 as s-polarized light to travel on the optical axis A, and a voltage is applied to the EO element 7. When applied, the OPO excitation light is converted into p-polarized light to form a polarizer 8.
Is transmitted and travels on the optical axis B, so that the wavelength control can be instantaneously switched on the order of ns, corresponding to several ns of the voltage rise time of a general electro-optical system. For example, in a certain type of laser radar using a plurality of wavelengths (such as a type that measures a distance by reflected light or a type that measures a gas distribution in the atmosphere by reflected light), oscillation occurs according to the characteristics of the object 15. Some wavelengths need to be switched in a very short time, and can be used as a light source of such a laser radar for wavelength switching.

Further, since the polarization angle of the linearly polarized light is changed by an angle corresponding to the voltage applied to the EO element 7, the ratio of the OPO excitation light traveling to the optical axis A and the optical axis B can be changed. The output ratio between the idler light having the wavelength λia and the idler light having the wavelength λib can be changed in a very short time.

Also, the entire laser radar device can be made compact.

[0060]

As described above, according to the present invention, it is possible to provide a wavelength conversion laser device capable of instantaneously switching the oscillation wavelength.

Further, according to the present invention, it is possible to provide a laser radar device capable of instantaneously switching an oscillation wavelength according to an object to be measured.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of a laser radar device using a wavelength conversion laser device according to the present invention.

[Explanation of symbols]

 1: laser device for generating pump light, 2: optical parametric oscillation optical system, 3: PPLN crystal, 4: OPO incidence mirror, 5: OPO emission mirror, 6: variable optical path system, 7: EO element (electro-optical element) 8, 13: polarizer, 9, 12: 45 ° mirror, 10, 11: λ / 2 plate, 14: optical path control means, 15: object, 16: light receiving unit, 17: EO driver control system, 18: EO driver.

Claims (7)

[Claims] An excitation light generating source for generating excitation light; an optical parametric oscillation optical system having a different wavelength depending on an optical path position of the excitation light generated by the excitation light generation source; A variable optical path system for changing a position of a transmitted light path of the excitation light in the optical parametric oscillation optical system. 2. The wavelength conversion laser device according to claim 1, wherein the optical parametric oscillation optical system has a nonlinear optical crystal in which polarization inversions having different periods are generated in at least two portions. 3. The method according to claim 1, wherein the non-linear optical crystal is LiNb.
O_Three, RbTiOAsO ₄, KTiOPO₄Out of
3. The wavelength according to claim 2, wherein said wavelength is used.
Conversion laser device. 4. The optical path variable optical system polarizes the excitation light generated from the excitation light generation source, and changes an optical path position of the excitation light according to the polarization. Item 2. A wavelength conversion laser device according to item 1. 5. The variable optical path optical system, comprising: an electro-optical element for converting the excitation light generated from the excitation light source into s-polarized light or p-polarized light; and an s-polarized light or a p-polarized light polarized by the electro-optical element. The wavelength conversion laser device according to claim 1, further comprising: a polarizer that changes an optical path position. 6. An excitation light source for generating excitation light, an optical parametric oscillation optical system having different wavelengths generated by an optical path of the excitation light generated by the excitation light source, and an excitation light generated from the excitation light generation source. An optical path variable optical system that varies a transmission optical path position of the excitation light in the optical parametric oscillation optical system, and receives light from an object of laser light emitted from the optical parametric oscillation optical system, and responds to the light. A laser path control unit that variably controls an optical path of the excitation light in the optical path variable optical system. 7. An excitation light generating source for generating excitation light, and a nonlinear optical crystal in which polarization inversions having different periods are generated in at least two portions, wherein the excitation light is generated by the excitation light generating source. An optical parametric oscillation optical system having a different wavelength generated depending on the optical path position in the nonlinear optical crystal; an electro-optic element for changing a polarization angle of the excitation light generated from the excitation light source; a polarizer that changes the optical path position of the s-polarized light or the p-polarized light and guides the nonlinear optical crystal to the nonlinear optical crystal; and receives light from an object of laser light emitted from the optical parametric oscillation optical system, and responds to the light. Controlling the voltage applied to the electro-optical element to adjust the s-polarized light or the p-polarized light.
A laser radar device comprising: an optical path control unit that controls an output ratio between the s-polarized light and the p-polarized light by changing a polarization angle of polarized light.