JPH1078594A - Wavelength converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the wavelength converter which easily makes a phase matching wavelength λpm coincide with a wavelength λres of the fundamental waves being generated. SOLUTION: The light beams generated by an optical amplifier 3 are supplied to a wavelength converting element 1 as pump light beams through an optical fiber 5. The element 1 generates the light beams having the frequency (the 2nd harmonics) which is the double frequency of the pump light beams (the fundamental frequency). An optical coupler 2 outputs the 2nd harmonics components among the light beams from the element 1 and the fundamental frequency components are supplied to a wavelength variable filter 4. The filter 4 limits the band of the supplied fundamental wave components and the components are fed back to the amplifier 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength converter, and more particularly, to a wavelength converter having improved conversion efficiency.

[0002]

2. Description of the Related Art Conventionally, wavelength multiplexing communication for multiplexing and transmitting a plurality of lights having different wavelengths has been studied for the purpose of increasing the line capacity in optical communication. In such wavelength multiplex communication, it is necessary to convert the wavelength of light in order to multiplex a plurality of input signals (optical signals).

As an element for performing such wavelength conversion, for example, “Applied Physics letters, Vol. 63,
P. 1170 (1993) "is indicated LiNbO ₃ optical waveguide element. In addition, as a method for performing highly efficient and stable conversion, for example, “Japan Journal of Applied Physic”
s, Vol. 31, p. 2104 (1992) "(hereinafter,
It is called Document 1. ), Etc., incident wave (fundamental wave)
Harmonic Generation (SH) that generates light (second harmonic) having a half wavelength of the wavelength
G) Elements are known.

In such an SHG element, as shown in FIG. 2, DBR (Distributed Bragg Reflector) gratings X2 and X3 having high reflectivity for the fundamental wave are provided at both ends of the optical waveguide X1. DBR like this
By providing the gratings X2 and X3, the fundamental wave to be emitted from the optical waveguide X1 is reflected, returned to the optical waveguide X1 and resonated, and only the second harmonic generated in the optical waveguide X1 is transmitted. The light is emitted. This makes it possible to generate a second harmonic of the incident light.

[0005] For example, "Journal of Optical Socie"
ty of America B, Vol. 5, p. 267 (198
8) "(hereinafter referred to as Document 2) and the like, an SHG element using a multilayer reflection mirror that reflects a fundamental wave and transmits a second harmonic, instead of the above DBR grating, is known. ing. By using such a multilayer reflective mirror, the fundamental wave can be fed back into the optical resonator (optical waveguide) to amplify the intensity of the fundamental wave, as in the case of using the above-described DBR grating. Thereby, the intensity of the second harmonic, which is proportional to the square of the intensity of the fundamental wave, can be improved, and the wavelength conversion efficiency can be improved.

[0006]

However, the above-mentioned SHG element has the following problems.

(1) The conditions required for the wavelength λres of the fundamental wave that can resonate are very severe. Since there is a requirement for phase matching of the SHG element, the phase matching wavelength λpm
Must be equal to λres. Satisfying such conditions is particularly difficult in practical use.

(2) When light propagates through the SHG element,
Propagation loss occurs. Propagation loss, element length, reflectivity and wavelength conversion efficiency are related to each other, and there is an optimal reflectivity to improve the conversion efficiency, but it is not easy to find this, and the design of the element is complicated. Become.

The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a wavelength conversion device capable of easily matching the phase matching wavelength λpm with the wavelength λres of the oscillating fundamental wave. Aim.

[0010]

SUMMARY OF THE INVENTION A wavelength converter according to the present invention comprises: an optical amplifying means for amplifying incident light; a light amplified by the optical amplifying means; and a wavelength of the incident light. A wavelength conversion unit for converting the light, a light output unit for outputting a part of the light from the wavelength conversion unit, and an optical waveguide made of an optical fiber, and a part of the output light of the wavelength conversion unit is transmitted through the optical waveguide. Light feedback means for entering the light into the amplification means.

Also, a configuration in which the optical amplifying means amplifies light (pump light) having a wavelength near the wavelength at which the efficiency of wavelength conversion in the wavelength converting means is maximum, or an optical feedback means capable of controlling the pass band. A wavelength tunable filter is provided, and by adjusting the wavelength tunable filter, the wavelength of light incident on the optical amplifier is limited to a wavelength near a wavelength (phase matching wavelength λ _pm ) at which the wavelength conversion efficiency of the wavelength converter is maximized. It is good also as a structure which performs.

[0012]

FIG. 1 shows the configuration of a wavelength converter according to a first embodiment of the present invention. As shown in the figure, the wavelength conversion device includes a wavelength conversion element 1 for converting the wavelength of incident light, an optical coupler 2 for splitting light from the wavelength conversion element 1, and amplification of incident light. An optical amplifier 3 for performing the operation, a wavelength tunable filter 4 for outputting the light by limiting the band of the incident light, and an optical fiber 5 for connecting the wavelength conversion elements 1 to 4 in a ring shape are provided. This wavelength converter is a fiber ring laser resonator that feeds light emitted from the optical amplifier 3 back to the optical amplifier 3 using the optical fiber 5 as shown in FIG.

The wavelength conversion element 1 includes, as shown in FIG. 3, for example, an optical waveguide 7 formed on a LiNbO ₃ substrate 6;
A QPM-SHG (QuQ) including a domain inversion grating 8 provided on a LiNbO ₃ substrate 6 at regular intervals along an optical waveguide 7 in a direction orthogonal to the waveguide 7.
asiphase Mattinng Second Harmonic Generator: Quasi-phase matching second high-frequency generator)

Interval of domain inversion grating 8 グ
Is about 18 μm, for example, the wavelength is 1.54
For the light of μm (fundamental wave), the following QPM condition is satisfied. That is, half period lambda / 2 of the repetition period lambda, when QPM satisfies the condition expressed by the following equation, an odd multiple of coherence length l _c.

K ₂ -2k ₁ -2π (2m-1) / Λ = 0 where m is a positive integer, and k ₁ and k ₂ are the fundamental wave and the second harmonic of the proton exchange optical waveguide 7, respectively. Wave vectors.

The coherent length l _c is represented by the following equation.

L _c = λ / 4 [N (λ / 2) -N (λ)] where λ is the wavelength of a fundamental wave in a vacuum, and N
(Λ) is the optical refractive index for light of wavelength λ.

In order to satisfy the above QPM condition,
By setting the interval の between the domain reflection gratings 8, the SHG element 1 can have a specific wavelength (for example, 1.
(54 μm) is set so that the efficiency of second harmonic generation is maximized.

In the QPM type SHG device 1 using such LiNbO ₃ as an optical waveguide, TE (tran)
For a svers electric wave, the difference in the refractive index is small and TE light propagating in the optical waveguide is diffused.
ansvers Magnetic) Only light is guided. For this reason, it is desirable to use a polarization maintaining fiber (single mode fiber) for preserving the polarization of the incident light as the optical fiber 5 described above.

The optical coupler 2 has two parallel optical waveguides, and light incident on one of the optical waveguides from the SHG element 1 propagates through this optical waveguide and is supplied to the wavelength tunable filter 4. Part of it is guided into the other optical waveguide,
Output to the outside. The intensity of light guided to the other optical waveguide depends on the length and wavelength of the optical waveguide.

The optical coupler 2 has a fundamental wave component (for example, having a wavelength of 1.54) of the output light from the SHG element 1 as described above.
One-half of the light (μm) is supplied to the wavelength tunable filter 4, and about 90% of the second harmonic component (light having a wavelength of 0.77 μm) is output to the outside.

The optical amplifier 3 is obtained by depositing an antireflection film on both end faces of an optical waveguide of a 1.55 μm band semiconductor laser. A maximum gain of 20 dB can be obtained for incident light of 1.55 μm.

The tunable filter 4 has a multilayer structure, a pass band width of 2.3 nm, and a tunable width of about 3 nm.
0 nm.

Hereinafter, the operation of the wavelength converter configured as described above converts, for example, light (fundamental wave) having a wavelength of 1.54 μm to light (second harmonic) having a wavelength of 0.77 μm. The case will be described.

First, when a current is injected into the optical amplifier 3, an electric field generated by carriers due to this current causes
Electrons in the optical amplifier 3 are excited. When light enters from outside the optical amplifier 3 in such a state, recombination of electrons excited by the light occurs, and at this time, light is emitted (stimulated emission).

By the way, the carriers excited by the electric field as described above cause recombination within a short time even if no external light is incident, and light is emitted. When this light enters the optical amplifier 3 again via the SHG element 1, the optical coupler 2, and the wavelength tunable filter 4, stimulated radiation is caused by this light. Thereby, the so-called ASE (Am
plified Spontaneous Emission (amplified spontaneous emission)
Is generated.

Here, in this wavelength converter, since the wavelength tunable filter 4 that passes only light of a predetermined wavelength (the wavelength of the fundamental wave of the SHG element 1) is used, a ring-shaped resonance Round the vessel, and again, the optical amplifier 3
Is only light within the pass band of the tunable filter 4. This light is amplified by the optical amplifier 3 and again enters the optical amplifier 3 via the SHG element 1, the optical coupler 2, and the wavelength tunable filter 4.

The output light from the SHG element 1
Light corresponding to the second harmonic component is extracted outside by the optical coupler 2.

When the feedback of light as described above is repeated, laser oscillation eventually occurs. Since the wavelength of the laser light generated by the laser oscillation depends on the wavelength of the light fed back to the optical amplifier 3, the wavelength can be freely set by setting the pass band of the wavelength tunable filter 4.

Therefore, if the pass band of the tunable filter 4 is set to the QPM wavelength (λ _pm ) of the SHG element 1,
Laser oscillation can be performed at the QPM wavelength of the HG element 1. As a result, the intensity of the QPM wavelength component incident on the SHG element 1 is increased, and the intensity of the QPM wavelength component is increased by 2%.
By increasing the generation intensity of the second harmonic component generated at an intensity corresponding to the power, the efficiency of wavelength conversion can be improved.

As described above, in the wavelength converter according to the first embodiment, the oscillation wavelength can be freely set by controlling the pass band of the wavelength tunable filter. The QPM wavelength λpm of the conversion element (SHG element) can be easily matched,
Conversion efficiency can be improved.

In this embodiment, since the wavelength conversion element (SHG element) itself is inside the optical resonator, light (fundamental wave) generated in the optical resonator and transmitted through the reflection surface is converted into the wavelength conversion element. , The intensity of the fundamental wave can be improved. Therefore, the intensity of the fundamental wave is 2
The conversion efficiency can be improved by improving the intensity of the second harmonic proportional to the power.

Further, in this wavelength converter, compared with the case where a conventional SHG element using a reflection mirror is used,
Since no reflecting mirror is used, it is possible to reduce a design burden such as setting an optimum value of the reflectance of the mirror.

FIG. 4 shows a configuration of a wavelength converter according to a second embodiment of the present invention. As shown in the figure,
The wavelength conversion device includes a wavelength conversion element 11, an optical circulator 12, a fiber grating 13, a wavelength tunable filter 15, an optical amplifier 16,
And an optical fiber 17 for connecting the optical amplifier 16 in a ring shape.

As the wavelength conversion element 11, Q using the same LiNbO ₃ as an optical waveguide as in the first embodiment described above.
When a PM type wavelength conversion element (SHG element) is used, it is preferable to use the above-mentioned polarization maintaining fiber as the fiber 17 because the SHG element guides only TM light as described above.

The optical circulator 12 outputs incident light to a path corresponding to the incident direction. For example, light incident from the wavelength conversion element 11 is supplied to the fiber grating 13, and light incident from the fiber grating 13 is output to the outside.

The fiber grating 13 totally reflects only light of a specific wavelength (for example, 0.77 μm).

The tunable filter 15 and the optical amplifier 16 are
The wavelength tunable filter 15 has the same configuration as that of the first embodiment described above. The wavelength tunable filter 15 transmits only the fundamental wave component of the incident light and supplies the light to the optical amplifier 1. Amplify and output light.

In the following, the operation of the wavelength converter configured as described above generates light (second harmonic) having a wavelength of 0.77 μm from light (fundamental wave) having a wavelength of 1.54 μm, for example. The case will be described.

First, similarly to the above-described first embodiment, a current is injected into the optical amplifier 16 and an ASE (Amplified Sponta
neous Emission (amplified spontaneous emission).
This light is transmitted through a wavelength conversion element 11, an optical circulator 12,
It is supplied to the wavelength tunable filter 15 via the fiber grating 13, and
Only light of a specific wavelength (fundamental wave) is transmitted and fed back to the optical amplifier 16.

If the optical amplification factor of the optical amplifier 16 is larger than the optical loss inside the wavelength converter (in the above-mentioned loop),
The fundamental wave component resonates and laser oscillation occurs. Since the wavelength of the laser-oscillated light is determined by the pass band of the wavelength tunable filter 15 as in the first embodiment, it can be easily controlled by changing the pass band of the wavelength tunable filter 15. The maximum output power of the optical amplifier is determined by the saturation output power of the optical amplifier 16.

In the wavelength conversion element 11, a second harmonic is generated by such a fundamental wave. This second harmonic component enters the fiber grating 13 via the optical circulator 12 together with the fundamental wave component.

In such a fiber grating 13,
The incident fundamental wave is reflected by the grating surface
Is the fundamental wave (wavelength λ) reflected in the direction opposite to the incident direction.
ω Is 1.54 μm) is reflected by the adjacent grating.
Optical path difference is odd multiple of half the wavelength,
The intensity of the reflected light in this direction becomes zero. Therefore, the fundamental wave is light
Optical amplifier 1 passing through the fiber grating as it is
6 is fed back.

However, half of the wavelength for the fundamental wave
Is equivalent to the wavelength of the _second harmonic (light having a wavelength λ ₂ ω of 0.77 μm), and this second harmonic is reflected by the fiber grating 13 in the incident direction and enters the optical circulator 12. I do. The second harmonic incident on the optical circulator 12 in such a direction is output to the outside as described above.

In the first embodiment, the converted light (that is, the second harmonic) is extracted outside by the optical coupler 2 shown in FIG. However, the optical coupler 2
Since the wavelength separation property is not perfect, a part of the fundamental wave may be output to the outside, and the intensity of the fundamental wave in the resonator is slightly reduced. In addition, since the output light contains a fundamental wave component, it is necessary to remove the fundamental wave component when only the second harmonic component is required.

On the other hand, in the second embodiment, only the fundamental wave is transmitted through the fiber grating 13 in FIG. 4 and fed back to the optical amplifier 16, the second harmonic is reflected, and the output is output through the optical circulator 12. can do. The wavelength separation by the fiber grating 13 is superior to the optical coupler 2. Therefore,
The wavelength converter according to the second embodiment includes the first
In addition to the same effects as the wavelength conversion device according to the embodiment,
The intensity of the fundamental wave component to be fed back can be improved, and the fundamental wave component in the output light can be reduced.

FIG. 5 shows the configuration of a wavelength converter according to a third embodiment to which the present invention is applied.

As shown in the figure, the wavelength conversion device comprises a wavelength conversion element 21 and optical circulators 22, 24.
, Fiber gratings 23 and 25, a wavelength variable filter 26, an optical coupler 27, an optical amplifier 28, an optical fiber 30 for connecting the wavelength conversion elements 11 to the optical amplifier 28 in a ring shape, and a signal light source 29. I have.

In the above-described first and second embodiments, the QPM-type which generates the second harmonic of the input light is used as the wavelength conversion element.
Although the case where the SHG element is used has been described, in the third embodiment, a QPM-DFG (Quasiphase Mattinng-type) using LiNbO ₃ as an optical waveguide as a wavelength conversion element.
A case in which a Dfference Frequency Generation (quasi phase matching difference frequency generation) element is used will be described.

As shown in FIG. 3, the QPM-DFG element includes a LiNbO ₃ substrate 6 and a LiNbO ₃
A proton exchange optical waveguide 7 formed on a substrate 6;
A domain inversion grating 8 is provided on the NbO ₃ substrate 6 at regular intervals along the proton exchange optical waveguide 7 in a direction orthogonal to the proton exchange optical waveguide 7. The repetition period の of the domain inversion grating 8 is 18 μm. The repetition period Λ is set so as to satisfy the following QPM condition.

K ₃ -k ₂ -k ₁ = 2π / Λ Here, k ₃ , k ₂ , and k ₁ are the wave numbers of the pump light, the signal light, and the converted light, respectively.

When a QPM wavelength conversion element using LiNbO ₃ as a waveguide is used as the wavelength conversion element as in the first embodiment, the optical fiber 30 is desirably a single mode polarization preserving type.

The amplification characteristics of the optical amplifier 28 are optimized for light in the 0.77 μm band, and the pass band of the wavelength tunable filter 26 is set to 0.77 μm. The wavelength of the signal light generated by the signal light source 29 is 1.53 μm.
m.

The grating constants of the fiber gratings 23 and 25 are set so as to reflect converted light (light having a wavelength of 1.55 μm) and signal light (light having a wavelength of 1.53 μm).

The optical circulators 22 and 24 output the incident light to an output destination corresponding to the incident direction. Thus, the optical circulators 22 and 24 output the converted light and the signal light reflected by the fiber gratings 23 and 25, respectively, to the outside.

Hereinafter, for example, the wavelength of the pump light is 0.77 μm.
The operation when converted light having a wavelength of 1.55 μm, which is light having a frequency difference between the frequencies of m and signal light of 1.53 μm, will be described.

First, a current is injected into the optical amplifier 28 to oscillate a laser. The oscillation wavelength at this time is about 0.77 μm, which is different from the first and second embodiments.

Next, the pass band is controlled by the wavelength tunable filter 26 to adjust the laser oscillation wavelength.
It is set to oscillate at a wavelength within a predetermined error range from the wavelength (0.77 μm).

Further, a signal light (1.53 μm) is generated and supplied to the wavelength conversion element 21 via the optical coupler 7.

As described above, the pump light (having a wavelength of 0.77 μm)
m) and signal light (light having a wavelength of 1.53 μm) are incident, the wavelength conversion element 21 generates light having a difference frequency between these lights, that is, converted light having a wavelength of 1.55 μm.

The component of the converted light, the pump light, and the signal light that has passed through the wavelength conversion element 21 enters the fiber grating 23 via the optical circulator 22, and only the converted light is totally reflected by the fiber grating 23. The light enters the optical circulator 22 again and is output to the outside.
The remaining pump light and signal light that have passed through the fiber grating 23 enter the fiber grating 25 via the optical circulator 24, and only the signal light is totally reflected by the fiber grating 25, enters the optical circulator 24 again, and enters the outside. Is output.

Further, the wavelength of the pump light transmitted through the fiber grating 25 is limited by the wavelength variable filter 26 and fed back to the optical amplifier 28.

As described above, according to the third embodiment, as in the first and second embodiments, since the wavelength conversion element is inside the resonator, the intensity of the pump light (fundamental wave) is increased. , And the conversion efficiency can be improved because the QPM wavelength matches the resonance wavelength of the laser.

In the third embodiment, since the wavelength conversion element that generates light having a difference frequency between two incident lights is used, light having a difference frequency between pump light and signal light can be generated. Since two sets of fiber gratings and an optical circulator are used, the signal light and the converted light are separately supplied to the outside while the pump light (fundamental wave) necessary for laser oscillation is fed back to the optical amplifier 28. Can be output to

In each of the embodiments described above, the multilayer film type wavelength conversion filter is used. However, the same effect can be obtained by using an AO (Acoustic Optical) filter for limiting the wavelength of light using an acousto-optic effect. can get.

In each of the above embodiments, a QPM element using LiNbO ₃ as an optical waveguide is used as a wavelength conversion element. However, another type of wavelength conversion element, for example, a four-wave mixing type that mixes four lights is used. The present invention can also be applied to a wavelength conversion element of a sum frequency generation type or the like that generates light having a sum frequency of a plurality of input lights. Further, in each of the above-described embodiments, the optical coupler, the fiber grating, and the optical circulator are used in order to output the wavelength-converted light to the outside.

Further, in each of the above embodiments, the loop structure is constituted by the optical fiber, but the same effect as described above can be obtained even when the loop structure is constituted by the reflection mirror.

In addition, modifications can be made as appropriate within the scope of the technical concept of the present invention.

[0069]

According to the wavelength conversion device of the present invention, the wavelength conversion means is provided inside the loop-shaped optical resonator comprising the optical amplification means and the optical feedback means. The intensity of light incident on the wavelength converting means can be improved as compared with the case where the wavelength converting means is provided. Therefore, the efficiency of wavelength conversion can be improved by increasing the intensity of wavelength-converted light whose intensity increases in accordance with the intensity of incident light.

Further, the optical amplification means amplifies light (pump light) having a wavelength near the wavelength at which the wavelength conversion efficiency of the wavelength conversion means is maximum, or the optical feedback means can control the pass band. A configuration in which a wavelength tunable filter is provided, and by adjusting the wavelength tunable filter, the wavelength of light incident on the optical amplification unit is limited to a wavelength near a wavelength (phase matching wavelength) at which the wavelength conversion efficiency of the wavelength conversion unit is maximized. Accordingly, it is possible to easily limit the oscillation wavelength of the loop-shaped optical resonator including the optical amplifying unit and the optical feedback unit to a wavelength suitable for the wavelength conversion element.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a wavelength conversion device according to a first embodiment to which the present invention has been applied.

FIG. 2 is a side sectional view showing a structure of a conventional resonance type wavelength converter.

FIG. 3 is a diagram illustrating a configuration of a wavelength conversion element included in the wavelength conversion device according to the first embodiment of the present invention.

FIG. 4 is a block diagram illustrating a configuration of a wavelength conversion device according to a second embodiment to which the present invention has been applied.

FIG. 5 is a block diagram illustrating a configuration of a wavelength conversion device according to a third embodiment to which the present invention has been applied.

[Explanation of symbols]

1, 11, 21 wavelength conversion element, 2, 27 optical coupler,
3, 16, 28 optical amplifier, 4, 15, 26 wavelength tunable filter, 5, 17, 30 optical fiber, 12, 22,
24 optical circulator, 13, 23, 25 fiber grating, 29 signal light source

Claims (7)

[Claims] An optical amplifier for amplifying the incident light; a wavelength converter for receiving the light amplified by the optical amplifier, and converting a wavelength of the incident light; An optical output unit that outputs a part of the light, an optical feedback unit that has an optical waveguide made of an optical fiber, and makes a part of the output light of the wavelength conversion unit incident on the optical amplification unit via the optical waveguide. A wavelength conversion device comprising: 2. The wavelength conversion device according to claim 1, wherein said optical amplification means amplifies light having a wavelength near a wavelength at which the wavelength conversion efficiency of said wavelength conversion means is maximized. 3. The optical feedback means includes a wavelength tunable filter capable of controlling a pass band, and adjusting the wavelength tunable filter converts a wavelength of light incident on the optical amplifying means into a wavelength converted by the wavelength converting means. 2. The wavelength converter according to claim 1, wherein the wavelength is limited to a wavelength near the wavelength at which the efficiency of the wavelength conversion becomes maximum. 4. The wavelength converter according to claim 1, wherein said light output means outputs light whose wavelength has been converted by the wavelength conversion means. 5. An optical fiber constituting the optical waveguide,
5. The wavelength conversion device according to claim 1, comprising a polarization maintaining fiber that retains the polarization of the incident light. 6. The apparatus according to claim 1, wherein said converted light output means comprises an optical coupler for outputting, to the outside, light whose wavelength has been converted by said wavelength converting means, of the light from said wavelength converting means. Wavelength converter. 7. The traveling light changing means for changing the traveling direction of the light whose wavelength has been converted by the wavelength converting means, out of the light from the wavelength converting means, and the traveling direction by the reflecting means. 2. The wavelength conversion device according to claim 1, further comprising: an optical circulator that outputs only the light whose wavelength has been changed to the outside.