JP2001133820A - Wavelength converter, quasi phase matching type wave conversion element, and method for using quasi phase matching type wave conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To convert signal wave light of one wavelength into converted wave light of a desired wavelength in a wavelength converter using a 2nd harmonic generation process and a difference-frequency generation process. SOLUTION: This wavelength converter 11 makes signal wave light S made incident from outside and a pump beam P emitted from a laser source 13 join by a 1st optical joining means 17, and then makes them incident to a QPM element 21. This QPM element is configured so as to generate a difference- frequency wave between a signal wave SHG light SSH and the pump beam P while generating the 2nd harmonic wave (signal wave SHG light SSH) of the signal light S. This wavelength converter uses this difference-frequency wave as converted wave light C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a wavelength conversion device,
The present invention relates to a quasi phase matching wavelength conversion element and a method of using the quasi phase matching wavelength conversion element.

[0002]

2. Description of the Related Art Conventionally, wavelength multiplexing has been realized in an optical network system. In the wavelength division multiplexing method, since signal wave lights of different wavelengths are transmitted simultaneously, the communication capacity can be increased. Since wavelengths different from each other are used in this wavelength multiplexing system, an all-optical wavelength conversion device that realizes wavelength conversion between wavelengths is indispensable.

[0003] Document I (FOUR-WAVE MIXING IN A SEMICOND
UCTOR LASER AMPLIFIER, ELECTRONICS LETTERS 7th jan
uary 1988 vol.24 No.1 p.31-32) uses a third-order nonlinear optical effect to generate a four-wave mixing (FWM) process.
A wavelength converter that performs wavelength conversion by this is disclosed.

According to the wavelength conversion device of the document I, a desired converted light can be obtained from the incident signal light. However, the wavelength converter using such a four-wave mixing process has a disadvantage that the wavelength conversion width is very narrow.

Document II (Optical Fiber Communication,
1999, FB6-1 / 39) discloses a wavelength converter using cascade second-order nonlinearity. This wavelength conversion device has a quasi-phase matching type wavelength conversion element (hereinafter sometimes abbreviated as a QPM element). Q of this wavelength converter
When the fundamental wave light and the signal wave light are incident thereon, the PM element generates a second harmonic of the fundamental wave light (referred to as a fundamental wave SHG light) and generates a difference frequency between the fundamental wave SHG light and the signal wave light. Then, the difference frequency light is emitted as converted light. The QPM element of the wavelength conversion device of Document II has a domain inversion region designed to generate a second harmonic of the fundamental light and a difference frequency between the second harmonic and the signal light.

According to the wavelength converter of Document II, by changing the wavelength of the signal wave light, a converted wave light having a wavelength corresponding to the wavelength of the signal wave light can be obtained. In addition, since the width of the wavelength band of the signal wave light at which practical conversion efficiency can be obtained is about 70 nm, the wavelength of the converted wave light is about 7 nm.
It can be changed over about 0 nm. Therefore, wavelength conversion in a wider band is possible than using the four-wave mixing process.

[0007]

However, in the wavelength converter of Document II, the width of the wavelength band of the fundamental light from which practical conversion efficiency can be obtained is very narrow. For example, when considering a conversion efficiency curve of a general QPM element with respect to the wavelength of the fundamental light, the half width of this conversion efficiency curve is only about 0.2 nm on the wavelength axis (for example, when the element length is 4 cm). .

For example, in this case, assuming that the wavelength of the fundamental wave light is λ _P , the wavelength of the signal wave light is λ _S , and the wavelength of the converted wave light is λ _C , from the law of conservation of energy, equation (1) 1 / λ _C = 1 / λ _P −1 / λ _S (1) must be satisfied.

Therefore, the wavelength λ _P of the fundamental light is 0.2
The fact that the wavelength can be changed only with a wavelength width of about nm means that the wavelength λ _P of the fundamental light is a substantially fixed wavelength. Therefore,
As can be understood from equation (1), when the wavelength λ _P of the fundamental light is fixed, one wavelength λ _S of the signal light can be converted into only one wavelength λ _C of the converted light.

Therefore, the wavelength converter of Document II cannot convert a signal wave light of an arbitrary wavelength into a converted wave light of a desired wavelength, and cannot meet the requirements of a wavelength multiplexing system.

The signal wave light incident from the outside is generally pulse light, but the fundamental wave light is light continuous in time. In the wavelength converter of Document II, in order to generate the SHG light of the fundamental wave light, the QPM element needs to have 50 mW to 1 mW.
It is necessary to make fundamental wave light of about 00 mW incident. Therefore, a strong laser beam always enters the inside of the QPM element of the wavelength conversion device, and as a result, optical damage occurs inside the QPM element.

Incidentally, the optical damage means a phenomenon in which the refractive index in the element is partially changed when strong light is incident on the element. Such a refractive index change is known to last for several days. QPM
If a local refractive index change region is formed in the device, the conversion efficiency of the QPM device is significantly reduced.

Therefore, it is possible to wavelength-convert a signal wave light having a specific wavelength into a converted wave light having a desired wavelength.
A wavelength converter capable of wavelength conversion without causing optical damage inside the element has been desired. Similarly, a signal wave light of a specific wavelength can be wavelength-converted into a converted wave light of a desired wavelength,
Preferably, a QPM element capable of wavelength conversion without causing optical damage inside the QPM element and a method of using the QPM element have been desired.

[0014]

SUMMARY OF THE INVENTION Accordingly, a wavelength converter according to the present invention includes a laser light source for emitting a fundamental wave light, and a wavelength converter having a quasi-phase matching type wavelength converter (QPM element). When the fundamental wave light (that is, the pump light) and the signal wave light are made incident on the wavelength converting means, the wavelength converting means converts the wavelength of the signal wave light based on the wavelength of the fundamental wave light. The wave light is emitted. In particular, the wavelength conversion means of the present invention generates the converted wave light as the difference frequency between the signal wave SHG light and the fundamental wave light while generating the signal wave SHG light which is the second harmonic of the signal wave light.

According to this configuration, the QPM of the wavelength conversion means is
In the element, instead of generating the second harmonic of the fundamental wave light, the second harmonic of the signal wave light is generated, and the difference frequency between the second harmonic (the signal wave SHG light) and the fundamental light is generated. Conversely, this QPM element is configured to generate SHG light of signal wave light. Therefore, although the details will be described later, the wavelength bandwidth of the signal light in which practical conversion efficiency is obtained is narrow, but the wavelength bandwidth of fundamental wave light in which practical conversion efficiency is obtained can be widened. . Therefore, the wavelength of the fundamental light can be changed over a wide wavelength bandwidth. As a result, in this wavelength converter,
It is possible to wavelength-convert a signal wave light having a specific wavelength into a converted wave light having a desired wavelength.

Generally speaking, the fundamental wave light is a CW laser light source (Continuous Wave Laser).
r), which is temporally continuous light (hereinafter sometimes abbreviated as CW light), while the signal wave light is temporally discontinuous pulse light from a pulse laser light source. In the configuration of the present invention, the SHG light of the signal wave light is generated. Therefore, it is necessary to increase the intensity of the signal light, but the intensity of the fundamental light can be reduced. Therefore, since the time average of the intensity of each light incident on the wavelength conversion means is lower than in the related art, it is possible to suppress the occurrence of optical damage inside the QPM element.

Further, the quasi-phase-matched wavelength conversion element (QPM element) of the present invention has a periodic domain inversion region, and quasi-phase-matched wavelength conversion element for emitting signal wave light and fundamental wave light and emitting converted wave light. It is a conversion element, and the following four equations are used: 2π / Λ = 2πn _SH / λ _SH -4πn _S / λ _S , 1 / λ _SH = 2 / λ _S , 2π / Λ = 2πn _SH / λ _SH -2πn _P / λ _P -2πn _C /
λ _C and 1 / λ _C = 1 / λ _SH −1 / λ _P are simultaneously satisfied.

However, the wavelength of the signal wave light is λ _S , the wavelength of the fundamental wave light is λ _P , the wavelength of the converted wave light is λ _C , the wavelength of the second harmonic of the signal wave light is λ _SH, and the domain for light of these wavelengths is The refractive indices of the inversion region are denoted by n _S , n _P ,
Let n _C and n _SH be the domain inversion period of the QPM element.

According to this configuration, in the QPM element,
A second harmonic of the signal wave light can be generated, and a difference frequency between the second harmonic (the signal wave SHG light) and the fundamental wave light can be generated. Therefore, although the details will be described later, the wavelength bandwidth of the signal light in which practical conversion efficiency is obtained is narrow, but the wavelength bandwidth of fundamental wave light in which practical conversion efficiency is obtained can be widened. . Therefore, the wavelength of the fundamental light can be changed over a wide wavelength bandwidth, and as a result,
In this QPM element, signal wave light of a specific wavelength can be wavelength-converted into converted wave light of a desired wavelength.

In this QPM element, the SPM of the signal wave light
HG light is generated. Therefore, it is necessary to increase the intensity of the signal light, but the intensity of the fundamental light can be reduced. Therefore, the time average of the intensity of each light incident on the QPM element becomes lower than in the related art, so that the occurrence of optical damage inside the QPM element can be suppressed.

Further, in the invention of the method of using the quasi phase matching type wavelength conversion element (QPM element) according to the present application, the fundamental wave light and the signal wave light are made incident, and the wavelength of the signal wave light is determined based on the wavelength of the fundamental wave light. In using a QPM element that emits converted and converted wavelength light, a signal wave SHG light, which is a second harmonic of the signal wave light, is generated and converted as a difference frequency between the signal wave SHG light and the fundamental wave light. Wave light is generated.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a wavelength converter and a method of using a QPM element according to the present application will be described below with reference to the drawings. Each figure used in this explanation is
The shapes, sizes, and arrangements of the components are only schematically shown to the extent that these inventions can be understood. In each drawing, the same components are denoted by the same reference numerals, and overlapping description may be omitted. Further, numerical values used in the description of the embodiments are merely examples, and the present invention is not limited to these.

(First Embodiment) FIG. 1 is a diagram schematically showing a configuration of a wavelength converter according to a first embodiment.
Hereinafter, an embodiment of the QPM element and an embodiment of a method of using the QPM element will be described with reference to FIG.

As shown in FIG. 1, the wavelength converter 11
Comprises a laser light source 13 and a wavelength conversion means 15.
In the illustrated example, in particular, the first optical combining means 17 is
5 is provided before the laser light source 13. Further, the wavelength demultiplexing means 19 is provided at a stage subsequent to the wavelength conversion means 15.

The laser light source 13 emits pump light P (pump light wavelength: λ _P ) as fundamental light. This laser light source 13 has a temporally continuous C
It is a CW laser light source that oscillates W light.

Here, the laser light source 13 is, for example, a wavelength-variable laser light source whose output wavelength is variable. Thus, pump light P having an arbitrary wavelength λ _{P in} a certain wavelength band is obtained. As the wavelength variable laser light source, for example, an external resonator type DBR laser light source or the like can be used.

The laser light source 13 can be replaced with a plurality of laser light sources having different output wavelengths, as in the third embodiment described later. In this case, the first optical convergence means 17 may be connected to each of the plurality of laser light sources, so that the single-wavelength pump light P from each of the laser light sources may be combined with the signal wave light S.

The first optical converging means 17 converges the pump light P from the laser light source 13 and the signal wave light S (signal wave light wavelength: λ _S ) incident from the outside to form the pump light P and the signal wave light S. Is emitted, that is, the combined wave light SP is emitted. As the first optical converging means 17, for example, a Y-branch type or directional coupler type optical converging element can be used.

In the first embodiment, the wavelength conversion means 15 comprises only a quasi-phase matching type wavelength conversion element (QPM element) 21. The QPM element 21 of the wavelength conversion means 15
When the pump light P and the signal wave light S, that is, the combined wave light SP are made incident on this element, the wavelength λ _S of the signal wave light S is wavelength-converted based on the wavelength λ _P of the pump light P, and the wavelength-converted converted wave light C (converted light wavelength: λ _C ) is emitted.

As described above, the signal wave SHG, which is the second harmonic of the signal wave light S,
It is characterized in that a converted wave light C is generated as a difference frequency between the signal wave SHG light S _SH and the fundamental wave light P while generating the light S _SH (wavelength of the signal wave SHG light: λ _SH ).

The wavelength demultiplexing means 19 converts the converted wave light C emitted from the QPM element 21 into other light transmitted through the QPM element 21 (that is, the signal wave light S, the pump light P, and the signal wave SHG light S _(SH, etc.).
The wavelength demultiplexing means 19 only needs to selectively demultiplex the converted wave light C, and can use a wavelength demultiplexing element in a broad sense, that is, a bandpass filter, a photoacoustic element, or an optical coupler. As shown in the illustrated example, the wavelength demultiplexing means 19 is QPM
Instead of being provided separately from the element 21, the QP
A band-pass filter or the like may be formed directly on the emission end face of the M element 21.

FIG. 2 is a diagram for explaining a wavelength conversion process in the QPM element according to the embodiment. Hereinafter, the wavelength conversion process in the QPM element 21 will be described with reference to FIGS.

As shown in FIG. 2, the QPM element 21 is generally often formed as a waveguide element. In this case, a substrate 23 containing a nonlinear optical crystal, an optical waveguide 25 provided on the substrate 23, and a domain inversion region 2 having a periodically inverted polarization direction along the waveguide 25.
And 7.

As the substrate 23, a LiNbO ₃ substrate, an AlGaAs substrate, a LiTaO ₃ substrate or a KTP
A substrate or the like can be used. Generally LiNbO
_Three substrates are used. When a LiNbO ₃ substrate is used, a domain inversion region 27 is provided on the + c plane of the substrate 23,
Pump light P and signal wave light S are respectively transmitted in the TM direction (FIG. 2).
In the direction perpendicular to the plane of FIG.
1

The QPM element 21 is configured to generate the second harmonic of the signal light S, that is, the signal SHG light S _SH . In order to efficiently generate the second harmonic of the signal wave light S, the QPM element must satisfy the following equations (2) (showing the phase matching condition) and (3) (showing the energy conservation law). It is desirable to design 21 periods Λ. Here, the period of the domain inversion region 27 of the QPM element 21 is set to Λ. Δk indicates the amount of phase mismatch, and n
_S and n _SH indicate the refractive index of the domain inversion region 27 with respect to the light having the wavelength λ _S and the wavelength λ _SH .

ΔkΛ = 2π and Δk = 2πn _SH / λ _SH -4πn _S / λ _S (2) 1 / λ _SH = 2 / λ _S (3) Although the second harmonic is also generated, the second harmonic of the pump light P does not satisfy the phase matching condition and can be ignored. Although it depends on the element length of the QPM element 21, the width of the wavelength band in which the second harmonic can be generated is, for example, 0.2 nm when the element length is 4 cm.
It is as follows.

In theory, the signal wave SHG light S
Field strength E _SH of _SH, as shown in the following equation (4), is proportional to the square of the electric field strength E _S of the signal wave light S.

[0038] ^{_{E SH αE S 2 ··· (4}} ) Further, the QPM element 21 and, at the same time to generate a signal wave SHG light S _SH, difference frequency of the signal wave SHG light S _SH and the pump light P is Is configured to occur. In order to generate the difference frequency between the signal wave SHG light S _SH and the pump light P, in addition to the above expressions (2) and (3), the following expressions (5) and (6) are simultaneously satisfied. QPM
It is desirable to design the period Λ of the element 21. In addition,
n _P and n _C represent the refractive index of the domain inversion region 27 with respect to the light having the wavelength λ _P and the light having the wavelength λ _C.

[0039] ΔkΛ = 2π and _{Δk = 2πn SH / λ SH -2πn} P / λ P -2πn C / λ C ·· · (5) 1 / λ C = 1 / λ SH -1 / λ P ··· ( 6) The electric field intensity E _C of the converted wave light _C is equal to the signal wave SHG light S
Proportional to the product of the field strength E _P of the electric field strength E _SH and the pump light P of _SH, is expressed by the following equation (7) By utilizing the expression (4).

E _C ∝E _SH E _P ∝E _S ² E _P (7) As described above, to simultaneously generate the second harmonic and the difference frequency, equations (2) and (3) are used. , (5) and (6)
It is necessary to design the QPM element 21 so as to satisfy the equations at the same time. However, even if these parameters are slightly deviated, the conversion efficiency may be reduced, but it is possible to obtain a conversion efficiency of no problem in a practical wavelength region.

FIG. 3 is a diagram schematically showing the dependence of the conversion efficiency η on the signal light wavelength λ _S and the pump light wavelength λ _{P in} the QPM element of the embodiment. However,
Here, the QPM element has a wavelength λ _S = 15 of the signal wave light S.
It is designed to satisfy the equations (2) and (3) at 40 nm.

FIG. 3A shows the wavelength λ of the signal wave light S.
Intensity of signal wave SHG light S _SH with respect to _S (horizontal axis) (vertical axis)
Is shown. As shown in FIG. 3A, this conversion efficiency curve has a wavelength λ _S = 1540 nm.
Is the peak wavelength, and has a half width of about 0.2 nm (however, the QPM element length is about 4 cm). That is, in this case, the width of the wavelength band of the signal wave light S at which practical conversion efficiency can be obtained is about 0.2 nm. As is well known, the longer the element length of the QPM element, the narrower the half width. Although the second harmonic of the pump light P is also generated in the QPM element, the second harmonic does not satisfy the phase matching condition of the equation (2), and is rapidly attenuated.

FIG. 3B shows the wavelength λ of the pump light P.
A conversion efficiency curve showing the intensity (vertical axis) of the converted wave light C with respect to _P (horizontal axis) is shown. As shown in FIG.
This conversion efficiency curve has a half-width of about 70 nm with a wavelength λ _P = 1550 nm as a peak wavelength (also QP
The M element length is about 4 cm. ). That is, this QP
In the M element, the converted wave light C having practical intensity can be generated even when the wavelength λ _P of the pump light P is changed to about 70 nm.

Therefore, in this QPM element, the signal wave light S
Wavelength λ_SIs substantially fixed at 1540 nm, but
Wavelength λ of pump light P_PIs about 70 nm centered on 1550 nm
It can be changed in the range of the degree. Therefore,
Wavelength λ of converted wave light C_CIs the equation (3) or (6)
Is required immediately. At this time, the wavelength λ of the converted wave light C _C
To a range of about 70 nm centered on a wavelength of 1530 nm.
It can be any wavelength in the box.

By changing the wavelength λ _P of the pump light P, the phase mismatch Δk shown in the equation (5) slightly changes. However, the wavelength λ of the signal wave SHG light S _SH
Fixing the _SH, since by changing the wavelength lambda _C of wavelengths lambda _P and converting wave light C of the pump light P, the change amount of the phase mismatching amount Δk is negligible. That is, the conversion efficiency η is about 70 n
In the range of about m, it can be regarded as constant.

Of course, the wavelength converter 11 of FIG. 1 is not limited to the wavelengths of the signal light S and the converted light C described above, but can be designed to have an arbitrary period .SIGMA. The wavelength λ _S of _S can be converted into the wavelength λ _C of the desired converted wave light C.

However, as described above, in the wavelength converter 11, the QPM element 21 is expressed by the following equations (2), (3),
It is designed to satisfy the equations (5) and (6) at the same time. That is, the periods に お け る in the equations (2) and (5) must be equal. However, if the signal wave light S and the converted wave light C are light having wavelengths close to each other, the amount of phase mismatch Δk in the expressions (2) and (5) can be substantially matched, and therefore, these conditions There can be a period を 満 た す that satisfies the equation.

As described above, according to the wavelength converter 11 of the first embodiment, the QPM element 21 generates the signal wave SHG light S _SH which is the second harmonic of the signal wave light S, The converted wave light C is generated as a difference frequency between the signal wave SHG light S _SH and the pump light P. Therefore, by changing the wavelength lambda _P of the pump light P, and the signal wave light S of one wavelength lambda _S that is substantially fixed, it can be converted into converted wave light C of an arbitrary wavelength.

Further, as described above, in the conventional configuration, in order to generate the SHG light of the pump light, it is necessary to make the pump light having a large intensity of about 50 mW to 100 mW incident on the QPM element. This pump light is CW
This is CW light that is temporally continuous and emitted from the laser light source. Therefore, the conventional wavelength converter has a problem that optical damage occurs inside the QPM element.

However, in the wavelength converter 11 of FIG. 1, in order to generate the SHG light S _SH of the signal wave light S, the signal wave light S, not the pump light P, needs to have a certain intensity or higher. Therefore, the intensity of the signal light S, which is a pulse light, needs to be increased to, for example, about 50 mW to 100 mW.
On the other hand, the intensity of the pump light P, which is the CW light, can be reduced to, for example, about 1 mW. Since the signal wave light S is a pulse light that is discontinuous in time, even if the intensity of the signal wave light S is high, the time average of the intensity is sufficiently smaller than that of the conventional configuration. Therefore, the occurrence of optical damage can be suppressed inside the QPM element 21.

Further, not only when the above-described QPM element is used, but also when a conventionally known QPM element is used, the signal wave SHG light S _SH which is the second harmonic of the signal wave light S is used.
The same effect can be obtained by generating the converted wave light C as the difference frequency between the signal wave SHG light S _SH and the pump light P while generating

Further, in the wavelength converter 11 of FIG. 1, it is preferable that the intensity of the signal wave light S described above is large enough to parametrically amplify the pump light P incident on the wavelength conversion means 15. When the intensity of the signal wave light S incident on the QPM element 21 is increased, parametric amplification occurs, whereby the pump light P incident on the QPM element 21 is increased.
Is converted and amplified. Note that, empirically, if the intensity of the signal wave light S is equal to or higher than the critical value (several hundred mW), the pump light P is considered to be parametrically amplified. When the pump light P is parametrically amplified as described above, the conversion efficiency can be significantly improved.

(Second Embodiment) FIG. 4 is a diagram schematically showing a configuration of a wavelength converter according to a second embodiment.
Hereinafter, a wavelength converter according to a second embodiment as a modification of the wavelength converter according to the first embodiment will be described with reference to FIG.

As shown in FIG. 4, this wavelength converter 41
Comprises a laser light source 13 and a wavelength conversion means 15.
Further, the first optical converging means 17 is provided in front of the wavelength converting means 15, and the wavelength demultiplexing means 19 is
Is provided at the subsequent stage. This is the same as the wavelength conversion device of the first embodiment.

As shown in FIG. 4, the wavelength converter 41
Comprises an optical amplifier 43 for amplifying the amplitude of the signal wave light S. This point is different from the first embodiment.

The optical amplifier 43 is connected to the first optical combining means 17.
And amplifies the signal wave light S in advance before the light wave S is made incident on the first optical converging means 17.

When the signal wave light S is set to a wavelength band of 1.5 μm, the optical amplifier 43 uses, for example, an EDF (Er).
(Doped Fiber) optical amplifier or the like may be used. When the signal wave light S has a wavelength band of 1.3 μm, an NDF (Nd Doped Fiber) optical amplifier or the like can be used.

As shown in the equation (7) of the first embodiment, the electric field strength E _C of the converted light _C is proportional to the square of the electric field strength E _S of the signal light S. Therefore, the amplification efficiency η can be increased by amplifying the amplitude of the signal wave light S by the optical amplifier 43 before making the signal wave light S enter the QPM element. Therefore, according to the wavelength converter 41, the converted wave light C having a higher intensity can be obtained.

As described in the first embodiment, since the signal wave light S is a pulse light, the signal wave light S is converted to the same intensity as the pump light in the conventional configuration (for example, 5 μm).
Even if amplified to 0 mW to 100 mW, optical damage hardly occurs in the QPM element 21.

Preferably, the optical amplifier 43 is connected to A
It is preferable to use a WG coupler. This is because spontaneous emission light (ASE) is emitted from the optical amplifier 43 as is well known, but by using an AWG coupler, the spontaneous emission light can be efficiently demultiplexed. Therefore, the signal wave light S
ASE noise hardly enters the QPM element 21.

(Third Embodiment) FIG. 5 is a diagram schematically showing a configuration of a wavelength converter according to a third embodiment.
Hereinafter, a wavelength converter according to a third embodiment as a modification of the wavelength converter according to the first embodiment will be described with reference to FIG.

As shown in FIG. 5, this wavelength converter 51
Comprises a plurality of laser light sources 531 to 53N and a wavelength conversion means 15. In addition, the first optical converging means 17 is provided before the wavelength converting means 15, and the wavelength demultiplexing means 19 is provided after the wavelength converting means 15.

In particular, in this wavelength converter 51, a plurality of fixed wavelength laser light sources 531 to 53N having different output wavelengths are used instead of the wavelength tunable laser light source of the first embodiment.
Is used. Thus, a plurality of fixed wavelength laser light sources 5
Since 31 to 53N are configured to emit the pump lights P1 to PN, pump lights P1 to PN more stable than the wavelength tunable laser can be obtained.

The first optical converging means 17 is connected to each of the plurality of laser light sources 531 to 53N, so that at least one wavelength of pump light P1 to PN from each of the laser light sources 531 to 53N is combined with the signal wave light S. Merge.

As the first light combining means 17, an element capable of combining a plurality of lights having different wavelengths can be used. However, since the first light combining means 17 can easily combine a large number of pump lights P1 to PN having different wavelengths, an AWG is used. Preferably, a coupler is used. As the wavelength demultiplexing means 19, for example, an AWG coupler can be used.

The wavelength converting means 15 receives the signal light S and the pump lights P1 to PN, converts the wavelength of the signal light S based on the wavelengths of the pump lights P1 to PN, and converts the wavelength-converted converted light C1. ~ CN are emitted. At this time, since the wavelengths of the pump lights P1 to PN are different from each other, the wavelengths of the output converted light C1 to CN are different from each other.

It should be noted that one of the plurality of pump lights P1 to PN having different wavelengths may be selectively made incident on the first optical converging means 17 (for example, only the pump light P1). Pump light (for example, pump lights P1 to P3, etc.)
At the same time. When a plurality of pump lights (for example, pump lights P1 to P3) are simultaneously incident, the signal wave light S
Can be simultaneously converted into converted wave lights of a plurality of wavelengths (for example, converted wave lights C1 to C3, etc.).

That is, according to the wavelength converter 51, the wavelength of one incident signal wave light S can be simultaneously converted into converted wave lights C1 to CN having different wavelengths. This wavelength converter 51 can be used for a system (so-called broadcast optical communication system) for simultaneously wavelength-converting one signal to a plurality of channels and transmitting the signal.

In the wavelength converter 51, as the pump light P1 to PN, the signal light used in the wavelength division multiplexing method defined by the International Standards Organization (meaning a general signal light, the signal light referred to in the present invention) Wave light and converted wave light).

For example, of ITU (International Telecommunication Union), ITU-T (ITU-Te
communication Standardi
Zoning Sector) defines the following wavelength division multiplexing method. In this wavelength division multiplexing method, for example, for a 1.54 μm wavelength band, light having a center wavelength of 1.54 μm and a wavelength separated by 100 GHz (〜0.8 nm) is used as signal light. The wavelength band of this signal light is a wavelength band having a certain width (hereinafter, referred to as a predetermined wavelength band), such as 1.53 μm to 15.6 μm.

That is, in the wavelength converter 51, the plurality of laser light sources 531 to 53N emit pump lights P1 to PN having wavelengths belonging to a predetermined wavelength band. in this way,
When the pump light P1 to PN belonging to the predetermined wavelength band is used, when the signal wave light S belongs to the predetermined wavelength band, as can be understood from the equations (3) and (6) of the first embodiment,
The converted wave light C having a wavelength belonging to the same predetermined wavelength band in a self-aligned manner.
1 to CN can be respectively emitted.

In the wavelength converter 51, the ITU
Since the pump light P1 to PN having the same wavelength band as the wavelength band defined by the international standardization organization such as -T is used, the first optical converging means 17 is an AWG conforming to a generally commercially available international standard. Couplers can be used. Therefore, a special AWG coupler is not required, and the wavelength converter can be easily manufactured.

(Fourth Embodiment) FIG. 6 is a diagram schematically showing a configuration of a wavelength converter according to a fourth embodiment.
Hereinafter, a wavelength converter according to a fourth embodiment as a modification of the wavelength converter according to the second embodiment will be described with reference to FIG.

It is generally known that the conversion efficiency of a QPM element largely changes depending on the polarization direction of incident signal wave light or pump light. Therefore, in the third embodiment, the wavelength conversion means described in the first embodiment is configured as follows, so that the conversion efficiency does not depend on the polarization direction of the signal wave light, that is, does not depend on the polarization. To realize a simplified wavelength converter.

The wavelength conversion device 61 of FIG. 6 includes the laser light source 13 and the wavelength conversion means 62 as in the first embodiment. Further, the first optical converging means 17 is used for the wavelength converting means 6.
2, the wavelength demultiplexing means 19 is provided after the wavelength conversion means 62. Further, the wavelength converter 61 has the signal wave light S as in the second embodiment.
And an optical amplifier 43 for amplifying the amplitude of.

This wavelength conversion means 62
, A polarization separation means 63, and a polarization plane rotation means 65.

The polarization splitting means 63 converts the combined light SP formed by combining the pump light P and the signal wave light S into a first combined wave having two polarization planes (ie, a TM polarization plane and a TE polarization plane) orthogonal to each other. Separation into polarized light SP1 and second combined polarized light SP2.

The polarization plane rotating means 65 rotates the polarization plane of one of the combined polarizations SP1 and SP2 so as to match the polarization plane of the other combined polarization SP2 or SP1. However, in a general LiNbO ₃ substrate or the like, the c-axis is often aligned with a direction perpendicular to the substrate surface. In this case, as in the illustrated example, the polarization plane rotating means 65 is provided on the optical path of the second combined polarization SP2.

Further, for example, a polarization beam splitter can be used as the polarization separation means 63, and a λ / 2 wavelength plate for rotating the polarization direction by 90 degrees can be used as the polarization plane rotation means 65, for example.

Although not shown, preferably, the polarized light separating means 6
Immediately before 3, an optical isolator that transmits light only in the direction toward the polarization splitting means 63 is provided.

As shown in FIG. 6, the wavelength conversion means 62 includes a first light separation means 67a and a second light separation means 6a.
7b and a second optical converging means 69.

The first light separation means 67a is provided with the polarization separation means 6
3 is input to one end face of the QPM element 21 and the first
The light including the converted wave light C incident on the light separating means 67 a is incident on the second optical converging means 69.

On the other hand, the second light splitting means 67b causes the second combined polarized light SP2 whose polarization plane has been rotated to be incident on the other end face of the QPM element 21, and also from the QPM element 21 to the second light separating means 67b. The light including the incident converted wave light C is
The light enters the second light combining means 69. As the first and second light separating means 67a and 67b, a wavelength demultiplexing element such as an AWG coupler, an optical circulator for changing an output terminal according to a traveling direction of light, or the like can be used.

The second light combining means 69 combines the light incident from the first and second light separating means 67a and 67b. As the second optical converging means 69, for example, a Y-branch type or directional coupler type optical converging element or the like can be used similarly to the first optical converging means 17.

As shown in FIG. 6, the wavelength converting means 62
Then, the incident combined light SP is first converted into polarization separation means 6.
3 At this time, the combined light SP is separated by the polarization separation means 63 into a first combined polarization SP1 having a TM polarization plane and a second combination polarization SP2 having a TE polarization plane. Thereafter, the polarization plane of the second combined-wave polarization SP2 is rotated by the polarization plane rotation unit 65 so that the polarization plane coincides with the TM direction.

After the polarization planes coincide with each other in the TM direction as described above, the first combined polarized light SP1 and the second combined polarized light SP2 enter the QPM element 21 from other surfaces opposite to each other.

In the QPM element 21, the first combined polarization SP1 and the second combined polarization SP2 propagate in the same domain inversion region in the opposite direction as the same TM polarization. The conversion efficiencies are equal.

Here, the intensity of the pump light included in the first combined polarization SP1 and the second combined polarization SP2 is set equal. More specifically, the pump light before becoming the combined wave light may be non-polarized light having no specific polarization direction, or the first combined wave polarized light SP1 and the second combined wave polarized light SP
What is necessary is just to make it the polarized light which has a polarization plane of the same angle (namely, 45 degrees) with respect to two polarization planes. For example, in this case, the electric field strength of the pump light incident from both directions of the QPM element 21 can be equalized.

Thereafter, the light emitted from each end face of the QPM element 21 is incident on the second light combining means 69 by the first light separating means 67a and the second light separating means 67b while maintaining its polarization plane. After that, the second optical converging means 69
To join.

Here, assuming that the electric field strength of the signal wave light S included in the combined wave light SP is E _S ,
Electric field strength E _S ^TE and E _S in E direction and TM direction
^TM is expressed by the following equation (8). Here, an angle between the polarization plane of the signal wave light S and the TE direction is set to θ.

[0091] E _S ^TE = E _S cos [theta], also ^{_{E S TM = E S sinθ ···}} (8), as shown in (7), the electric field strength E _C of the converted wave light C is the signal wave light S It is proportional to the square of the electric field strength E _S.
Therefore, from the equations (7) and (8), the electric field strength E _C of the converted wave light C obtained as the sum of the converted wave lights of the TE direction component and the TM direction component is expressed by the following equation (9).

[0092] ^{_{E C α (E S TE)}} 2 + (E S TM) 2 = E S 2 cos 2 θ + E S 2 sin 2 θ = E S 2 ·· · (9) (9) as shown in the formula , The electric field strength E _C of the total converted wave light C emitted from the QPM element 21 does not depend on the angle θ. That is, according to the wavelength conversion device 61, the wavelength of the signal wave light S can be converted into the wavelength of the converted wave light C with a constant conversion efficiency without depending on the polarization state of the signal wave light S.

(Fifth Embodiment) FIG. 7 is a diagram schematically showing a configuration of a wavelength converter according to a fifth embodiment.
Hereinafter, a wavelength converter according to a fifth embodiment as a modification of the wavelength converter according to the second embodiment will be described with reference to FIG.

In the fifth embodiment, the wavelength conversion means described in the first embodiment is configured as follows, so that the conversion efficiency can be improved as in the fourth embodiment. A polarization conversion independent wavelength converter that does not depend on the polarization direction is realized.

The wavelength converter 71 of FIG. 7 includes a laser light source 13 and a wavelength converter 73 as in the first embodiment. Further, the first optical converging means 17 is used for the wavelength converting means 7.
3, and the wavelength demultiplexing means 19 is provided after the wavelength conversion means 73. Further, the wavelength conversion device 71 has the signal light S
And an optical amplifier 43 for amplifying the amplitude of.

The wavelength converting means 73 is provided for the polarization separating means 6.
3 and a second optical converging means 69. This point is the fourth
This is the same as the embodiment.

In particular, the wavelength converting means 73 is composed of two identically structured QPM elements (a first QPM element 75a and a second QPM element 75a).
QPM element 75b).

As in the fourth embodiment, the polarization separating means 63 converts the combined light SP formed by combining the pump light P and the signal light S into two polarization planes orthogonal to each other (ie, a TM polarization plane and a TM polarization plane). The light is separated into a first combined polarization SP1 and a second combined polarization SP2 having a TE polarization plane).

The polarization splitting means 63 shown in FIG. 7 includes the separated first combined wave polarized light SP1 and second combined wave polarized light SP
2 with the first QPM element 75a and the second QPM element 75
b, the light is incident on one different QPM element 75a or 75b.

Generally speaking, the first QPM element 7
Usually, the 5a and the second QPM element 75b are often formed on the same substrate. In this case, since the directions of the c-axis of the first QPM element 75a and the second QPM element 75b match, the same polarization plane rotating means 65 as that of the fourth embodiment is provided on the optical path of one of the QPM elements 75a or 75b. Provide. Here, as in the illustrated example, the polarization plane rotating means 65 is used.
Is provided on the optical path of the second combined-wave polarization SP2 having a TE polarization plane, for example.

The first QPM element 75a converts the wavelength of the signal wave light S contained in the first combined wave polarization SP1 based on the wavelength of the pump light P contained in the first combined wave polarization SP1, and converts the converted wave light C. Emit it. Similarly, the second QPM element 75b outputs the signal wave light S included in the second combined wave polarization SP2.
Of the pump light P included in the second combined polarization SP2.
Is converted based on the wavelength of the light, and the converted wave light C is emitted.

Further, the second optical converging means 69 is connected to the first QPM
The lights incident from the element 75a and the second QPM element 75b are combined.

As shown in FIG. 7, the wavelength converting means 73
Then, the incident combined light SP is first converted into polarization separation means 6.
3 At this time, the combined light SP is separated by the polarization separation means 63 into a first combined polarization SP1 having a TM polarization plane and a second combination polarization SP2 having a TE polarization plane. Thereafter, the polarization plane of the second combined-wave polarization SP2 is rotated by the polarization plane rotation unit 65 so that the polarization plane coincides with the TM direction.

After the polarization planes coincide with each other in the TM direction in this way, the first combined polarization SP1 and the second combined polarization SP2 enter one different QPM element 75a or 75b.

In each of the QPM elements 75a and 75b,
The first combined wave polarization SP1 and the second combined wave polarization SP2 are
Since the same TM-polarized light propagates in the same domain-inverted region, the conversion efficiencies for the combined polarizations SP1 and SP2 are equal.

Further, similarly to the fourth embodiment, the intensity of the pump light included in the first combined polarization SP1 and the intensity of the pump light included in the second combined polarization SP2 are made equal. Specifically, the pump light before becoming the combined light is converted to light in a non-polarized state having no specific polarization direction or to the polarization plane of the first combined wave polarized light SP1 and the second combined wave polarized light SP2. What is necessary is just to make it the polarized light which has a polarization plane of an equal angle (namely, 45 degrees). For example, in this case, the first QPM element 75a
In addition, the electric field intensity of the pump light incident on the second QPM element 75b can be made equal.

Thereafter, the first QPM element 75a and the second
The light emitted from the end face of the QPM element 75b enters the second light combining means 69 while maintaining its polarization plane and is combined.

In the wavelength converter 71 of the fifth embodiment, the equations (8) and (9) described in the fourth embodiment are satisfied. Therefore, the first QPM element 75a and the second
The electric field strength of the converted wave light represented by the sum of the converted wave lights emitted from the QPM element 75b does not depend on the angle of the plane of polarization of the signal wave light. That is, according to the wavelength converter 71,
The wavelength of the signal light S can be converted into the wavelength of the converted light C with a constant conversion efficiency without depending on the polarization state of the signal light S.

Further, in the fifth embodiment, it is necessary to prepare QPM elements having the same performance, but it is easier to integrate as compared with the fourth embodiment.

[0110]

As is clear from the above description, the wavelength converter of the present invention generates the signal wave SHG light, which is the second harmonic of the signal wave light, while generating the difference between the signal wave SHG light and the fundamental wave light. The converted wave light is generated as the frequency. Therefore, the wavelength bandwidth of the signal light in which practical conversion efficiency can be obtained is narrow, but the wavelength bandwidth of fundamental wave light in which practical conversion efficiency can be obtained can be widened. Therefore, unlike the conventional configuration, the wavelength of the fundamental light can be changed over a wide wavelength bandwidth. Therefore, according to the wavelength converter of the present invention, it is possible to wavelength-convert a signal wave light having a specific wavelength into a converted wave light having a desired wavelength.

Also, generally speaking, the signal wave light is pulse light, and the fundamental wave light is CW light. In the case of the conventional configuration, it was necessary to make the CPM light of high intensity enter the QPM element. On the other hand, in the wavelength converter of the present invention, QP
It is sufficient that the CW light having a very small intensity is incident on the M element, and it is necessary to impose a large intensity pulse light on the QPM element. Becomes smaller. Therefore, the occurrence of optical damage inside the QPM element can be suppressed.

The quasi phase matching type wavelength conversion element (QPM element) of the present invention has the above-mentioned four equations, namely, 2π / Λ = 2πn _SH / λ _SH -4πn _S / λ _S , 1 / λ _SH = 2 / λ _S , 2π / Λ = 2πn _SH / λ _SH -2πn _P / λ _P -2πn _C /
λ _C and 1 / λ _C = 1 / λ _SH −1 / λ _P are simultaneously satisfied. Therefore, this QPM element can generate the second harmonic of the signal wave light and also generate the difference frequency between the second harmonic (the signal wave SHG light) and the fundamental wave light. Therefore, the wavelength bandwidth of the signal wave light for which practical conversion efficiency can be obtained is narrowed,
Conversely, the wavelength bandwidth of the fundamental light from which practical conversion efficiency can be obtained can be widened. Therefore, the wavelength of the fundamental wave light can be changed over a wide wavelength bandwidth. As a result, the QPM element can convert the signal wave light of a specific wavelength into the converted wave light of a desired wavelength.

In this QPM element, the SPM of the signal wave light
HG light is generated. Therefore, it is necessary to increase the intensity of the signal light, but the intensity of the fundamental light can be reduced. Therefore, the time average of the intensity of each light incident on the QPM element becomes lower than in the related art, so that the occurrence of optical damage inside the QPM element can be suppressed.

Further, in the invention of the method of using the QPM element,
While generating the signal wave SHG light, which is the second harmonic of the signal wave light, the converted wave light is generated as a difference frequency between the signal wave SHG light and the fundamental wave light. Therefore, the wavelength bandwidth of the signal light in which practical conversion efficiency can be obtained is narrow, but the wavelength bandwidth of fundamental wave light in which practical conversion efficiency can be obtained can be widened. Therefore, the wavelength of the fundamental light can be changed over a wide wavelength bandwidth. Therefore, according to the method of using the QPM element, it is possible to wavelength-convert a signal wave light having a specific wavelength into a converted wave light having a desired wavelength.

Further, in the invention of the method of using the QPM element, it is sufficient that CW light of very small intensity is made incident on the QPM element, and it is necessary to make pulse light of high intensity incident on this QPM element. The time average of the intensity of these incident lights becomes smaller. Therefore, the occurrence of optical damage inside the QPM element can be suppressed.

[Brief description of the drawings]

FIG. 1 is a diagram schematically illustrating a configuration of a wavelength conversion device according to a first embodiment.

FIG. 2 is a diagram for explaining a wavelength conversion process in the QPM element according to the embodiment;

FIG. 3 is a diagram schematically illustrating the dependence of the conversion efficiency η on the signal light wavelength λ _S and the pump light wavelength λ _{P in} the QPM element of the embodiment.

FIG. 4 is a diagram schematically illustrating a configuration of a wavelength conversion device according to a second embodiment.

FIG. 5 is a diagram schematically illustrating a configuration of a wavelength conversion device according to a third embodiment.

FIG. 6 is a diagram schematically illustrating a configuration of a wavelength conversion device according to a fourth embodiment.

FIG. 7 is a diagram schematically illustrating a configuration of a wavelength conversion device according to a fifth embodiment.

[Explanation of symbols]

 11, 41, 51, 61, 71: wavelength conversion device 13, 531 to 53N: laser light source 15, 62, 73: wavelength conversion device 17: first optical converging device 19: wavelength demultiplexing device 21, 75a, 75b: QPM Element 43: Optical amplifier 63: Polarization separating means 65: Polarization plane rotating means 67a: First light separating means 67b: Second light separating means 69: Second light combining means

Claims (20)

[Claims] 1. A laser light source for emitting a fundamental wave light, and wavelength conversion means having a quasi-phase matching type wavelength conversion element, wherein the wavelength conversion means, when the fundamental wave light and the signal wave light are made incident thereon, In a wavelength conversion device that converts the wavelength of the signal wave light based on the wavelength of the fundamental wave light and emits the converted wave light, the wavelength conversion unit may include a signal wave that is a second harmonic of the signal wave light. A wavelength converter that generates the converted wave light as a difference frequency between the signal wave SHG light and the fundamental wave light while generating SHG light. 2. The wavelength conversion device according to claim 1, further comprising an optical amplifier that amplifies the amplitude of the signal wave light before performing the wavelength conversion. 3. The wavelength conversion device according to claim 1, further comprising a first optical converging means provided before the wavelength converting means, for converging the fundamental wave light and the signal wave light to emit a merged wave light. A wavelength conversion device characterized in that: 4. The wavelength conversion device according to claim 1, wherein the laser light source is a laser light source that oscillates temporally continuous light. 5. The wavelength conversion device according to claim 1, wherein the laser light source is a wavelength-variable laser light source whose output wavelength is variable. 6. The wavelength conversion device according to claim 3, wherein the laser light sources are a plurality of laser light sources having different output wavelengths, and the first optical convergence unit is connected to each of the plurality of laser light sources. Thus, a wavelength conversion device, wherein a fundamental wave light of at least one wavelength from the laser light source is combined with the signal wave light. 7. The wavelength conversion device according to claim 3, wherein said first optical convergence means is an AWG coupler. 8. The wavelength conversion device according to claim 1, wherein light belonging to a wavelength band of signal light used in a wavelength division multiplexing method defined by an international standardization organization is used as the fundamental wave light. Wavelength converter. 9. The wavelength conversion device according to claim 1, wherein the intensity of the signal wave light is set to a magnitude such that the fundamental wave light incident on the wavelength conversion means is parametrically amplified. Conversion device. 10. The wavelength conversion device according to claim 3, wherein the wavelength conversion means converts the combined wave light into a first combined wave polarization having a first polarization plane among two polarization planes orthogonal to each other. Polarization splitting means for splitting into second combined polarized light having two polarization planes,
A polarization plane rotating means for rotating the first polarization plane or the second polarization plane so that the directions of the polarization planes coincide with each other; and the first and second polarization planes in which the first and second polarization planes coincide with each other. 2. The wavelength conversion device according to claim 1, wherein the two combined polarization waves are incident from different end faces of the quasi phase matching type wavelength conversion element facing each other. 11. The wavelength conversion device according to claim 10, wherein said wavelength conversion means includes an optical isolator provided before said polarization separation means. 12. The wavelength converter according to claim 10, wherein the fundamental light is a non-polarized light having no specific polarization direction, or a polarization plane of the first and second combined polarizations. Characterized in that the polarized light has a polarization plane at an angle of 45 degrees to each of the wavelengths. 13. The wavelength conversion device according to claim 10, wherein the wavelength conversion means includes a second light converging means for converging each light emitted from the quasi-phase matching type wavelength conversion element. Characteristic wavelength converter. 14. The wavelength conversion device according to claim 3, wherein the wavelength converting means converts the combined light into a first combined polarization having a first polarization plane among two polarization planes orthogonal to each other. Polarization splitting means for splitting into second combined polarized light having two polarization planes,
And two quasi-phase-matching wavelength conversion elements having the same structure, and the first and second combined-wave polarizations are respectively incident on one different quasi-phase-matching wavelength conversion element. Wavelength converter. 15. The wavelength conversion device according to claim 14, wherein the wavelength conversion means includes an optical isolator provided in a stage preceding the polarization separation means. 16. The wavelength conversion device according to claim 14, wherein the fundamental light is a non-polarized light having no specific polarization direction, or a polarization plane of the first and second combined polarizations. Characterized in that the polarized light has a polarization plane at an angle of 45 degrees to each of the wavelengths. 17. The wavelength conversion device according to claim 14, wherein the wavelength conversion unit includes a second optical converging unit that converges each light emitted from each of the quasi phase matching type wavelength conversion elements. A wavelength converter characterized by the above-mentioned. 18. A quasi-phase-matched wavelength conversion element having a periodic domain inversion region and emitting a converted wave light upon incidence of a signal wave light and a fundamental wave light, wherein the wavelength of the signal wave light is λ _S , Let the wavelength of the fundamental light be λ _P ,
The wavelength of the converted wave light is λ _C , the wavelength of the second harmonic of the signal wave light is λ _SH , and the refractive indices of the domain inversion regions for light of these wavelengths are n _S , n _P , n _C and When n _{SH is set and} the domain inversion period of the domain inversion region is set as Λ, 2π / ｎ = 2πn _SH / λ _SH -4πn _S / λ _S , 1 / λ _SH = 2 / λ _S , 2π / Λ = 2πn _{SH / λ SH -2πn P / λ} P -2πn C /
A quasi-phase-matched wavelength conversion element that simultaneously satisfies λ _C and 1 / λ _C = 1 / λ _SH −1 / λ _P. 19. Injecting fundamental wave light and signal wave light,
In using a quasi-phase-matched wavelength conversion element that converts the wavelength of the signal wave light based on the wavelength of the fundamental wave light and emits the converted wave light, a signal that is a second harmonic of the signal wave light A method of using a quasi-phase matched wavelength conversion element, wherein the converted wave light is generated as a difference frequency between the signal wave SHG light and the fundamental wave light while generating a wave SHG light. 20. The method of using a quasi phase matching type wavelength conversion element according to claim 19, wherein a converted wave light of a desired wavelength is obtained by changing a wavelength of the fundamental wave light. How to use type wavelength conversion element.