JPH10206917A - Wavelength converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to reduce the width of the spectra of converted light and to reduce the size of the device itself by limiting the wavelength of the light to be fed back to a semiconductor laser and selectively feeding back only the light of the prescribed wavelength. SOLUTION: This wavelength converter comprises a wavelength conversion element 1 which executes wavelength conversion in accordance with the wavelengths of plural beams of incident light, a semiconductor laser 2 which generates the light being the pumping light of this wavelength conversion element 1 and a wavelength selecting element 4 which is arranged between the wavelength conversion element 1 and the semiconductor laser 2. An optical resonator is formed by the end face 2a of the semiconductor laser 2 and the wavelength selecting element 4 with such constitution. The light generated in the optical waveguide of the semiconductor laser 2 is reflected by the end face 2a or the wavelength selecting element 4, by which laser oscillation is induced. The oscillation wavelength of the laser oscillation is the wavelength selected by the wavelength selecting element 4. The laser light oscillated by the light which is limited in the band in such a manner and is fed back to the semiconductor laser 2 is supplied as the pumping light to the wavelength conversion element 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength converter used in an optical communication system.

[0002]

2. Description of the Related Art Wavelength multiplexing (WDM) 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).

Conventionally, as a wavelength conversion element for converting the wavelength of light, one using a semiconductor optical amplifier, one using four-wave mixing, and one using a semiconductor laser having a saturable absorption region have been proposed. . However, these wavelength conversion elements cannot satisfy the conditions required for an optical communication system such as high efficiency, high speed, wide band, low noise, and polarization independence.

Therefore, the inventors of the present invention have considered using a difference frequency generation (DFG) by quasi-phase matching, which is a kind of the second-order nonlinear optical effect, using an optical waveguide, and have filed an earlier patent application. Japanese Patent Application No. 6-175265) and the following paper [1] CQXu, et.a1. App1.Phys.Lett. 63, 1170 (199)
3) [2] Shoucho Ao, et al. Wavelength division multiplexin in IEICE technical report OCS95-3, P-17 (1995).
g: WDM) It has been proposed to use a wavelength conversion element as a key device of a communication system.

[0005] In the wavelength conversion element used here,
A periodic domain-inverted structure is formed in LiNbO ₃ , and an optical waveguide is formed so that fundamental light propagates through the domain-inverted structure. In these documents, 1.53 μm to 1.5.
An example of realizing light exchange to light of 55 μm is reported.
According to the wavelength conversion element having such a configuration, ultra-high speed, wide band (140 nm or more), low noise (less than quantum noise), exchange between multiple channels is possible, and high conversion efficiency (−30 dB using 4 mW pump light). ) Can be satisfied. This leaves a problem of polarization independence, but shows the effectiveness of incorporating a wavelength conversion technique using DFG into an optical communication system.

Further, other similar proposals are disclosed in the following documents.

[3] SJBYoo, et.a1. App1.Phys.Let
t. 63, 2609 (1996) In this document, a GaAs substrate is used, and other
An aAs substrate is bonded by a direct bonding method of crystals so that the plane directions are opposite to each other, and the bonded GaAs substrate is periodically removed by photolithography to form a pattern whose plane direction is reversed. A periodic domain-inverted structure is formed thereon by metal organic chemical vapor deposition to provide a wavelength conversion element. By forming the crystal in the opposite direction, the direction of polarization also becomes opposite. Therefore, a domain-inverted structure is formed by using two bonded GaAs crystal substrates. With the wavelength conversion element configured in this way, ultra-high speed,
In addition to the characteristics of wideband, low noise, exchange between multiple channels, and high conversion efficiency, polarization independence is realized.

[0008] All of the proposals in the above-mentioned documents [1], [2], and [3] are intended to use a wavelength conversion technique for a switching function in an optical communication system. By utilizing the wavelength conversion technology in this way, it is possible to realize ultra-high speed, wide band, low noise, exchange between multiple channels, and high conversion efficiency which cannot be realized by electronic devices.

By the way, there are a second harmonic generation (SHG), a sum frequency generation (SFG), and a difference frequency generation (DFG) as wavelength conversion techniques utilizing the second-order nonlinear optical effect. As a typical example of an element using an optical waveguide among such techniques, there is one shown in the following literature.

[4] K. Yamamoto, et.a1. App1.Phys.Let
t. 58, 1227 (1991) [5] F. Lauren, et.a1. App1. Phys. Lett. 62, 1872 (19
93) Document [4] reports an example in which phase matching is realized using a Cherenkov radiation scheme using an optical waveguide and wavelength conversion is performed using SHG and SFG. In the method described in this document, the phase instability can always be realized because the instability of the wavelength of the semiconductor laser can be absorbed by the change of the Cherenkov radiation angle, but the radiation direction of the converted light always changes according to the fluctuation of the laser wavelength. Will do. Further, the spread of the oscillation spectrum and the multi-modality of the semiconductor laser are directly reflected as the spread of the converted light spectrum.

In an optical communication system, the spectrum of converted light as well as the fundamental light needs to be sufficiently narrow, so that the method disclosed in this document cannot be used in an optical communication system. Further, the characteristic that the fluctuation of the wavelength of the fundamental wave light appears as a change in the radiation direction of the converted light is extremely inconvenient in application.

In Document [5], wavelength conversion is realized using an optical waveguide as in Document [4], but unlike in Document [4], phase matching is realized by a method called pseudo phase matching. Therefore, the radiation direction of the converted light does not change, but there is a problem that the conversion efficiency itself fluctuates due to the wavelength fluctuation of the fundamental light. Further, similarly to the literature [4], the spectrum width of the fundamental wave is directly reflected on the spectrum spread of the converted light.

[0013]

In order to apply the wavelength conversion technology to an optical switching device that performs wavelength multiplexing, the spectrum width of light must be sufficiently narrow, for example, about 5 MHz or less. However, any of the above-described light conversion elements cannot be used as it is in an optical communication system because the spectrum of the converted light is not sufficiently narrow.

For example, the wavelength conversion element disclosed in the above-mentioned document [3] is generally used in an optical resonator of a Fabry-Perot type laser diode (LD). This point cannot be solved because the spectrum of the oscillating light is generally multimode. Although the half-width of each mode is relatively narrow even in the wavelength conversion element shown in the document [4], the spectrum width is not sufficiently small because the Ti: sapphire laser has multiple modes, so that the optical switching system is not suitable. It cannot be applied as it is. Further, the Ti: sapphire laser device itself is large and it is difficult to apply it to an optical switching system.

It is an object of the present invention to provide a wavelength converter capable of reducing the width of the spectrum of converted light and reducing the size of the device itself.

[0016]

SUMMARY OF THE INVENTION A wavelength converter according to the present invention generates a semiconductor laser for generating a fundamental wave light, and generates and outputs a converted light based on the fundamental wave light from the semiconductor laser and an external signal light. A wavelength conversion unit is provided, and a wavelength selection unit that limits the wavelength of light to be returned to the semiconductor laser and selectively returns only light having a predetermined wavelength.

Further, another wavelength converter according to the present invention comprises:
A semiconductor laser having a high-reflection coating at one end of an optical path and outputting a fundamental wave light generated from an output end face opposite to the high-reflection coating;
PM means and DBR means comprising a periodic refractive index distribution and the like, comprising wavelength conversion means for generating converted light based on fundamental light from a semiconductor laser and signal light from the outside,
The reflectance at the output end face of the semiconductor laser is r, the reflectance at both end faces of the wavelength conversion means is r ′, r ″, and the feedback rate from the DBR means is r _D , r ′, r ″ << r _D , r << r _D , r ′, r ″ << r The reflectance of each end face is set so as to satisfy the following relationship.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment FIG. 1 is a block diagram illustrating a configuration of a wavelength conversion device according to a first embodiment of the present invention. As shown in the figure, the wavelength conversion device includes a wavelength conversion element 1 that performs wavelength conversion based on the wavelengths of a plurality of incident lights,
And a wavelength selecting element 4 disposed between the wavelength converting element 1 and the semiconductor laser 2.

The semiconductor laser 2 is composed of, for example, a Fabry-Perot type semiconductor laser, and has a reflection surface 2a at one end (the side not facing the wavelength selection element 4) of the optical waveguide. The wavelength selection element 4 has a reflection surface on the wavelength conversion element 1 side of the optical path, and the reflection surface and the reflection surface 2a of the semiconductor laser 2 constitute an optical resonator.

The wavelength selecting element 4 is configured so that only light of a specific wavelength from the wavelength selecting element 4 is selectively fed back to the semiconductor laser 2. The half-value width of the spectrum of the light to be fed back is set to a predetermined width, for example, about 5 MHz or less, which is required in optical communication such as a WDM system by appropriately selecting the selection characteristics of the wavelength selection element 4. The laser light which is thus band-limited and oscillated by the light fed back to the semiconductor laser 2 is supplied to the wavelength conversion element 1 as pump light.

In this wavelength conversion device, the signal light incident via the optical fiber is incident on the wavelength conversion element 1 by any of the following methods. In the first method, a bandpass filter (BPF) 3a is inserted between the semiconductor laser 2 and the wavelength selection element 4, and the signal light is made to enter from the lower side (the optical fiber 6a) in FIG. In the second method, a band-pass filter (BPF) 3b is inserted between the wavelength selection element 4 and the wavelength conversion element 1, and the signal light is made to enter from below (the optical fiber 6b) in FIG. Which method the signal light is incident on is selected according to the configuration of the wavelength conversion element 1, the wavelength selection element 4, the BPF, or the like.

The wavelength conversion element 1 includes a semiconductor laser 2
A converted light is generated based on the wavelengths of the pump light and the signal light, and the converted light is output. This wavelength conversion element is configured by an element that performs, for example, DFG (difference frequency generation), SFG (sum frequency generation), and the like.

In the wavelength converter configured as described above,
As described above, an optical resonator is formed by the end face 2a of the semiconductor laser 2 and the wavelength selection element 4. Light generated in the optical waveguide of the semiconductor laser 2 is reflected by the end face 2a or the wavelength selection element 4, and laser oscillation occurs. The oscillation wavelength of the laser oscillation is the wavelength selected by the wavelength selection element 4.

This oscillation is, in principle, the same as that of a Bragg feedback semiconductor laser if the wavelength selection element 4 is a Bragg reflector, and its oscillation wavelength is excellent in monochromaticity (the spectrum of the oscillation wavelength is a single mode). And the half width is narrow) and a stable oscillation state can be obtained.

Here, the spectrum of the feedback light fed back to the semiconductor laser 2 by the wavelength selection element 4 will be examined. The reflectance R of the Bragg diffraction grating constituting the Bragg reflector is given by the following equation.

[0026]

(Equation 1) here,

(Equation 2) It is. Also, κ: coupling coefficient, α: absorption coefficient, L: total length of Bragg reflector, N (λ): refractive index of Bragg region,
Λ _B : period of Bragg reflector, q: Bragg diffraction order.

Here, as typical values of LiNbO ₃ , κ = 5.0 × 10 ⁻⁴ μm ⁻¹ and α = 0.5 dB / c
Assuming that m, L = 4 mm and q = 17, the half width of the Bragg reflection spectrum is approximately 0.1 nm.

On the other hand, the longitudinal mode interval of the semiconductor laser 2 is
The semiconductor laser 2 has a cavity length of 350 μm and a wavelength of 0.78.
In the case of μm, it is approximately 0.3 nm. Therefore, if the length L of the Bragg reflection region is set to be longer than the above value (= 4 mm), this wavelength converter can be used for an optical communication system. .

In the actual design, the half-width of the Bragg reflection spectrum is calculated by using the equation (1), and the half-width is calculated so as to be equal to or less than the longitudinal mode interval of the semiconductor laser used as the pump light. What is necessary is just to design a DBR region (Bragg reflector).

When both the pump light from the semiconductor laser 2 and the signal light from the optical fiber 3 oscillated by the feedback light whose wavelength is limited as described above enter the wavelength conversion element 1, based on the wavelengths of these lights, Converted light is generated. For this wavelength conversion element 1, it is convenient to use, for example, a quasi phase matching (QPM) element having a periodically poled structure. The period Λ of the periodic domain inversion structure is set so as to satisfy the following quasi phase matching condition. k3−k2−k1 = 2π / Λ (2) Here, k3, k2, and k1 are the wave numbers of the pump light, the signal light, and the converted light, respectively.

For example, when the above-mentioned DFG element is used as the wavelength conversion element 1, light having a frequency of a difference between the incident pump light and the signal light is generated as converted light, and when the SFG element is used, Light having a frequency equal to the sum of the pump light and the signal light is generated.

When the wavelengths of the pump light, the signal light, and the converted light are λ3, λ2, and λ1, respectively, when a DFG element is used, the relationship between these wavelengths is as follows from the law of conservation of energy. 1 / λ1 = 1 / λ3-1 / λ2 (3) Similarly, when an SFG element is used, 1 / λ1 = 1 / λ3 + 1 / λ2 (4)

By using the above-described wavelength converter in an optical communication system and supplying signal light modulated according to transmission data or the like as signal light, the wavelength of the signal light is converted to perform wavelength switching. Can be realized.

By the way, in order to obtain converted light having both stable intensity and wavelength, it is necessary that the wavelength of the pump light be stable. In the wavelength converter described above, the wavelength of the light fed back to the semiconductor laser 2 is limited (selected) by the wavelength selection element 4, so that the oscillation wavelength of the semiconductor laser 2 becomes the wavelength selected by the wavelength selection element 4. ,
The instability of the oscillation wavelength of the Fabry-Perot type semiconductor laser can be reduced, and the wavelength of the pump light generated in the semiconductor laser 2 can be stabilized.

The phase matching condition of the wavelength conversion element 1 is as follows.
Since it is determined by the wavelengths of the pump light and the signal light, it is required that the wavelengths of the pump light and the signal light be stable in order to stably satisfy the phase matching condition.

In optical communication, signal light is generally generated by a distributed feedback laser (DFB laser) or a Bragg feedback laser (DBR laser), so that the wavelength is inherently stabilized.

However, generally, Fabry-Perot type semiconductor lasers are often used to generate pump light, and the wavelength stability is low. The pump light does not use the Fabry-Perot type semiconductor laser, and the above-mentioned DBR whose wavelength is stable
It is sufficient to use a laser or a DBR laser. However, if the wavelengths of the signal light and the pump light are both determined, the phase matching condition of the wavelength conversion element is also determined, and the degree of freedom in configuring the wavelength conversion device is determined. Is technically difficult because of the complete disappearance. For this reason, at the stage of configuring the wavelength converter, it is preferable to secure a margin for adjustment of the wavelength of the pump light.
Perot type semiconductor lasers are often used.

During operation, it is required that the wavelength of the pump light is also stable as described above. In the above-described wavelength converter, as described above, the light fed back to the semiconductor laser 2 by the wavelength selection element 4 is limited. Therefore, even if a Fabry-Perot type semiconductor laser is used as the semiconductor laser 2, the pump light can be reduced. The wavelength can be stabilized. Therefore, the phase matching condition of the wavelength conversion element 1 determined by the wavelengths of the signal light and the pump light is stably satisfied, so that both the intensity and the wavelength of the converted light can be stabilized.

Second Embodiment In the above-described first embodiment, the wavelength selection element 4 is disposed between the wavelength conversion element 1 and the semiconductor laser 2. 2, the wavelength conversion element 1 can be disposed between the semiconductor laser 2 and the wavelength selection element 4, as in the wavelength conversion device according to the second embodiment shown in FIG.

In this wavelength conversion device, a reflection surface is formed on the surface of the wavelength selection element 4 farther from the wavelength conversion element 1. The reflection surface and the reflection surface 2a of the semiconductor laser 2 constitute an optical resonator.

In this wavelength conversion device, the signal light incident via the optical fiber is incident on the wavelength conversion element 1 by any of the following methods.

In the first method, a band pass filter (BPF) 13a is provided between the semiconductor laser 2 and the wavelength conversion element 1.
Is inserted, and the signal light is made to enter from below (the optical fiber 16a) in FIG. In the second method, a band-pass filter (BPF) 1 is provided between the wavelength conversion element 1 and the wavelength selection element 4.
3b is inserted, and signal light is made to enter from the lower side of FIG. 2 (optical fiber 16b). In the third method, the wavelength selection element 4
The signal light is made incident from the right (optical fiber 16c) of FIG. Which method the signal light is incident on is selected according to the configuration of the wavelength conversion element 1, the wavelength selection element 4, the BPF, or the like.

The wavelength converter configured as described above operates in the same manner as the wavelength converter according to the first embodiment described above, and similarly to the first embodiment, the wavelength selector 4 applies the wavelength selection element 4 to the semiconductor laser 2. Since the wavelength of the light to be fed back is limited, the wavelength of the pump light can be stabilized, and both the intensity and the wavelength of the converted light can be stabilized.

The method of injecting the signal light into the wavelength converter in the first and second embodiments is selected depending on the actual design of the optical communication system.

Third Embodiment FIG. 3 shows a configuration of a wavelength converter according to a third embodiment of the present invention. As shown in the figure, the wavelength converter includes a semiconductor laser 22 for generating pump light, a band-pass filter 23 for transmitting pump light from the semiconductor laser 22, a quasi-phase matching part (QPM part) 24c, A wavelength conversion element 24 integrally formed with a Bragg reflection portion (DBR portion) 24d and a signal light from an optical fiber 26 are propagated to the wavelength conversion element 24, and light from the wavelength conversion element 24 is transmitted to the optical fiber 26. And an optical isolator 25 that does not propagate.

On one end face of the semiconductor laser 22 (an end face opposite to the end face facing the wavelength conversion element 24), a high reflection coating portion (preferably 100% reflection) 22a is formed.

The reflectance of the output end face 22b of the semiconductor laser 22 opposite to the reflection face 22a is r, the reflectance of the end faces 24a and 24b of the wavelength conversion element is r ′, r ″, and DB.
Assuming that the feedback rate from the R portion 24c is r _D , surface treatment such as coating is performed so as to satisfy the following relationship: r ′, r ″ << r _D r << r _D (5) r ′, r ″ <r The reflectivity of each end face 22b, 24a, 24b is set by selection, and the configuration is such that oscillation light (pump light) is output from the low reflection side 22b of the semiconductor laser 22.

The pump light output from the low reflection side 22 b of the semiconductor laser 22 enters the wavelength conversion element 24 via the band pass filter 23. The input side of the pump light of the wavelength conversion element 24 has a QP having a periodically poled structure.
An M portion 24c is formed, and a distribution feedback structure portion (DBR) formed of a periodic refractive index distribution is formed on the opposite side. In such a configuration, as shown in FIG.
Pump light (wave number k
p) and the signal light (wave number vector ks) are phase-matched. Therefore, it is necessary to maintain the intensity of the pump light by sufficiently increasing the reflectivity of the DBR portion that reflects the pump light.

In this wavelength converter, the signal light (for example, 1.55 μm) from the optical fiber 26 is transmitted through the optical isolator 25 from the side opposite to the pump light (for example, 0.78 μm) from the semiconductor laser 22. Wavelength conversion element 2
4. This corresponds to the third method in the second embodiment shown in FIG.

In the wavelength converter configured as described above,
DBR portion 24d of wavelength conversion element 24 and semiconductor laser 2
An optical resonator is constituted by the second high reflection coating portion 22a. Accordingly, the wavelength of the light reflected by the DBR portion 24d is limited, and the oscillation wavelength of the laser is limited, so that the laser oscillates in the single wavelength mode. That is, D of the wavelength conversion element 24
Since the wavelength of the light reflected by the BR portion 24d is determined by the period of the periodic structure of the refractive index of the DBR portion 24d,
The semiconductor laser 22 oscillates at a wavelength determined by the period of the periodic structure of the refractive index (referred to as Bragg condition).

Pump light from the semiconductor laser 22 (specifically, pump light reflected by the DBR portion 24d)
When the signal light from the optical fiber 26 enters the wavelength conversion element 24, light having a wavelength based on the wavelengths of these lights (for example, light having a frequency corresponding to the wavelength difference between these lights) is generated in the QPM portion 24c. . The converted light generated in this manner is selectively reflected with respect to the wavelength in the band-pass filter 23 and is extracted to the outside. As a result, the signal light (wavelength 1.55 μm) and the converted light (wavelength 1.57 μm) are
Outputs downward in the middle.

Here, the oscillation operation of the semiconductor laser 22 will be described in more detail. Assuming that the entire wavelength conversion device shown in FIG. 3 is a laser oscillator, it can be considered that a plurality of resonator structures are configured with the active region of the semiconductor laser 22 interposed therebetween.

That is, a resonator constituted by the high reflection coating portion 22a and the end face 22b of the semiconductor laser 22 (a first resonator, which is a resonator when only the semiconductor laser 22 is viewed as a Fabry-Perot type laser) High reflection coating portion 22a of the semiconductor laser 22
(Second resonator) composed of the semiconductor laser and the end face 24a of the wavelength conversion element 24, a resonator composed of the high reflection coating portion 22a of the semiconductor laser and the DBR portion 24d (third resonator), and the semiconductor laser. High reflection coating part 2
A resonator (fourth resonator) constituted by 2a and end face 24b is conceivable. There are other portions that can be regarded as resonance structures, but these contribute less to laser oscillation than the above-described first to fourth resonators and can be ignored.

In this embodiment, the semiconductor laser 2
2 is a semiconductor laser having a normal cavity length of 350 μm, the oscillation mode interval of the first resonator is naturally equal to the mode interval of the semiconductor laser 22 having a normal cavity length of 350 μm, which is about 0.3 nm. The interval between the oscillation modes of the above-mentioned resonator is an extremely narrow interval of about several hundredths or less. On the other hand, the oscillation mode of the third resonator corresponds to a wavelength corresponding to a high reflection condition determined by the refractive index period of the DBR portion 24d, and exists at intervals of about 100 nm.

The reflectance of the output end face 22b of the semiconductor laser is defined as r, with the reflectance of the high reflection coating portion 22a of the semiconductor laser 22 being substantially 100%.
4 are represented by r ′, r ″, DB
Assuming that the feedback rate from the R portion 24d is r _D , if the coating is made so as to satisfy the relationship of r ′, r ″ << r _D r << r _D (5) r ′, r ″ <r ,
The oscillation mode by the second resonator is rarely selected, and the mode determined by the first and third resonators is selected. Specifically, the mode closest to the wavelength of the feedback light from the DBR portion 24d of the third resonator among the modes at 0.3 nm intervals of the first resonator is selected.

Of the return light from the DBR portion 24d, the only one that can be the wavelength of the oscillation light is that the mode interval is about 100 nm and the gain width of the active region (of the laser diode 22) is about 100 nm. Limited to wavelength. In addition, since the half width of the spectrum of the feedback light from the DBR portion 24d is about 0.1 nm as described above, one mode of the first resonator having a wider mode interval can be selected without any problem. The spectral half width of the selected mode is equal to the half width of a normal semiconductor laser having a cavity length of 350 μm, which is generally on the order of several MHz. This half-value width does not change even in a semiconductor laser oscillating in a normal Fabry-Perot mode without the above-mentioned external feedback, and the half-value width of each mode is still several MHz. This value is unique to the Fabry-Perot type laser unless a special operation such as adding a special gain fluctuation or the like to the active region of the semiconductor laser is applied.

On the other hand, since the mode interval between the second and fourth resonators is about several hundredth of the mode interval between the first resonators, the mode interval is smaller than the spectral half width of the feedback light from the DBR portion 24d. Since the half-width of the feedback light from the DBR portion 24d becomes so narrow that a plurality of oscillation modes are included, the selection of the oscillation mode by the feedback light from the DBR portion 24d cannot be performed stably. Become. For this reason, in this wavelength conversion device, the reflectances r ′ and r ″ of the end faces 24 a and 24 b of the wavelength conversion element are set so that the modes by the second and fourth resonators do not become dominant. 22b reflectance r and DBR portion 24
It is configured to be sufficiently smaller than the feedback ratio r _D from d.

In this wavelength converter, since the wavelength conversion element 24 is formed integrally with the QPM portion 24c and the DBR portion 24d, the quasi-phase matching condition can be satisfied with sufficient accuracy, and The wavelength can be determined. For this reason, the operation when configuring the system can be simplified, and aging of the configured system can be reduced.

In the wavelength converter shown in FIG.
The direction in which the converted light is extracted (band-pass filter 2 in FIG. 3)
3) (downward direction).
If it is inconvenient to mix pump light in the design of the system, the problem is solved by adopting the configuration shown in FIG.

In order to sufficiently increase the feedback efficiency of the pump light, it is necessary to sufficiently increase the reflectance of the DBR portion 24d. In this wavelength converter, the return light from the DBR portion 24d reciprocates. Since there is a wavelength conversion part (QPM part 24c) on the way, this QPM part 24c
The attenuation of the pump light must be as small as possible. However, if the conversion efficiency of the converted light is increased,
Since the attenuation of the pump light in the QPM portion 24c increases, especially when a high-efficiency wavelength converter is configured,
It is necessary to set the feedback efficiency of the pump light in consideration of the overall efficiency.

As described above, in the wavelength converter according to the third embodiment, the semiconductor laser 2 that generates the pump light is used.
2 is band-limited by the DBR portion 24d, the oscillation wavelength of the semiconductor laser 22 is extremely stable, as in the first and second embodiments. The half width of the spectrum is extremely narrow. For this reason, the wavelength of the converted light generated in the wavelength conversion element 24 can be stabilized, and the half width of the spectrum of the converted light can be made extremely narrow. Use of such a wavelength conversion device in a WDM system can contribute to improvement in system performance.

Fourth Embodiment FIG. 4 shows the configuration of a wavelength converter according to a fourth embodiment of the present invention. As in the third embodiment shown in FIG. 3 described above, this wavelength conversion device includes a wavelength conversion element 34 in which a semiconductor laser 32, a bandpass filter 33, a QPM portion 34c and a DBR portion 34d are integrally formed. And an optical isolator 35 for preventing light from the bandpass filter 33 from propagating to the optical fiber 36.

A high-reflection coating (preferably 100% reflection) 32a is formed on one end face of the semiconductor laser 32 (the end face opposite to the end face facing the wavelength conversion element 34).

The semiconductor laser 32 and the wavelength conversion element 3
4 is an output end face 32b of the semiconductor laser 32.
Where r is the reflectance of the wavelength conversion elements, r ′ and r ″ are the reflectances of the end faces 34 a and 34 b of the wavelength conversion element, and r _D is the feedback rate from the DBR portion 34 d, r ′, r ″ << r _D r << r _D (5) The reflectance is adjusted by coating or the like so as to satisfy the relationship of r ′, r ″ <r, and the low reflection side 32 b of the semiconductor laser 32.
Is configured to output oscillation light (pump light).

The pump light from the semiconductor laser 32 enters the wavelength conversion element 34 through the band pass filter 33. Contrary to the above-described third embodiment, the wavelength conversion element 34 has a distributed feedback structure (DBR) portion 34d having a periodic refractive index distribution formed on the incident side of the pump light. A quasi phase matching (QPM) portion 34c made of a periodically poled structure is formed on the side.

Also, in this wavelength converter, unlike the third embodiment, the phase matching is achieved for the pump light (wave number vector kp) and signal light (wave number vector ks) propagating rightward in FIG. Taken.

On the other hand, the signal light (here, wavelength 1.55 μm)
m) enters the band-pass filter 33 through the optical fiber 36 and the optical isolator 35 in a direction perpendicular to the pump light (here, the wavelength of 0.78 μm), and is selectively reflected by the band-pass filter 33 so as to have a wavelength. The light is incident on the conversion element 34. This corresponds to the first method of the first embodiment.

In the wavelength converter configured as described above,
An optical resonator is formed by the high reflection coating portion 32a of the semiconductor laser 32 and the DBR portion 34d of the wavelength converter 34. The semiconductor laser 32 is formed by the DBR portion 34d.
Is limited to a wavelength determined by the period (Bragg condition) of the periodic distribution of the refractive index of the DBR portion 34d. As a result, the semiconductor laser 32 oscillates at a limited wavelength (single wavelength mode).

The pump light passing through the DBR portion 34 d and the wavelength conversion element 34
Is incident on the QPM portion 34c, light (converted light) having a frequency equal to the difference between the pump light and the signal light is generated in the QPM portion 34d. This converted light is output from the right end of the wavelength conversion element 34 in FIG.

Since the wavelength converter limits the wavelength of the light fed back to the semiconductor laser 32 as in the third embodiment, the oscillation wavelength of the pump light generated by the semiconductor laser 32 is stabilized. And the half width of the spectrum of the pump light can be made extremely narrow. Therefore, the wavelength of the converted light generated in the wavelength conversion element can be stabilized, and the half-value width of the spectrum of the converted light can be extremely narrowed.

In this wavelength converter, the above-described third
As in the embodiment of FIG.
Since it is formed integrally with the DBR portion 24c, the quasi-phase matching condition can be satisfied with sufficient accuracy, and the oscillation wavelength can be determined. For this reason, the operation when configuring the system can be simplified, and aging of the configured system can be reduced.

In this wavelength converter, pump light is mixed in the output of the converted light, so it is necessary to design in consideration of this point. However, the feedback light for oscillation of the semiconductor laser 32 reciprocates. Wavelength conversion part (QPM part 3)
Since there is no 4c), there is no need to consider the attenuation of the pump light in the QPM portion 34c, and it is possible to easily design such as increasing the intensity of the pump light.

Fifth Embodiment As shown in FIG. 5, the configuration of a wavelength converter according to a fifth embodiment of the present invention is shown. This wavelength conversion device has a DBR portion 46a near the output end of an optical fiber 46 instead of the DBR portion 24d and the isolator 25 of the wavelength conversion element 24 according to the third embodiment shown in FIG. .

This wavelength converter comprises a semiconductor laser 42 and a band-pass filter 43, as in the third embodiment.
And a wavelength conversion element 44 on which a QPM portion 44c is formed.

On one end face of the semiconductor laser 42 (the end face opposite to the end face facing the wavelength conversion element 34), a high reflection coating portion (preferably 100% reflection) 32a is formed.

The semiconductor laser 42 and the wavelength conversion element 4
4 is an output end face 42b of the semiconductor laser 42.
, The reflectance of the wavelength conversion element end faces 44 a and 44 b is r ′, r ″, and the DBR portion 46 of the optical fiber 46.
Assuming that the feedback rate from a is r _D , the reflectance is adjusted by a coating or the like so as to satisfy a relationship of r ′, r ″ << r _D r << r _D (5) r ′, r ″ <r And the low reflection side 42b of the semiconductor laser 42
Is configured to output oscillation light (pump light).

The pump light from the semiconductor laser 42 enters the wavelength conversion element 44 through the band pass filter 43. As described above, only the QPM portion 44c is formed in the wavelength conversion element 44, and the DBR portion 46 is formed near the output end of the optical fiber 46.

In this wavelength converter, as in the third embodiment described above, phase matching is performed on the pump light (wave number vector kp) and the signal light (wave number vector ks), both of which propagate to the left in FIG. Is taken. Therefore, it is the same as the third embodiment described above in that the reflectivity of the DBR portion that reflects the pump light needs to be sufficiently large.

On the other hand, the signal light (here, the wavelength 1.55 μm)
m) is guided to an optical fiber 46, and this optical fiber 46
The laser beam passes through the DBR portion 46a near the emission end of the laser beam and enters the wavelength conversion element 44 from the side opposite to the pump light (here, the wavelength is 0.78 μm) from the semiconductor laser 42. This corresponds to the third method of the second embodiment.

In the wavelength converter configured as described above,
An optical resonator is formed by the high reflection coating portion 42a of the semiconductor laser 42 and the DBR portion 46a near the emission end of the optical fiber 46. The wavelength of the light fed back to the semiconductor laser 42 by the DBR portion 46a is limited to a wavelength determined by the structure of the DBR portion 46a. As a result, the semiconductor laser 42 oscillates at a limited wavelength (single wavelength mode).

When the pump light reflected by the DBR portion 46a and the signal light incident on the wavelength conversion element 44 via the DBR portion 46a enter the QPM portion 44c, the Q
Light (converted light) having a frequency equal to the difference between the pump light and the signal light is generated in the PM portion 44c. This converted light is output from the left end of the wavelength conversion element 44 in FIG. 5, reflected by the bandpass filter 43, and converted into signal light (wavelength 1.55 μm) and converted light (wavelength 1.57 μm) in the downward direction in FIG. Is output to

In this wavelength converter, as in the above embodiments, the light fed back to the semiconductor laser 42 is wavelength-limited by the DBR portion 46a, so that the oscillation wavelength of the pump light is extremely stable. Further, the half width of the spectrum of the pump light is extremely narrow. Therefore, the wavelength conversion element 4
4, the wavelength of the converted light generated can be stabilized, and the half width of the spectrum of the converted light can be extremely narrowed. By using such a wavelength converter in a WDM system, it is possible to contribute to the improvement of system performance.

In this wavelength converter, the signal light and the converted light are selectively reflected and output using the band-pass filter 43, so that in principle the pump light is completely generated in the direction in which the converted light is output. Do not mix. Therefore, if it is inconvenient to mix pump light in the design of the system,
The problem is solved by adopting the configuration shown in FIG. However,
In order to make the return efficiency of the pump light large enough, DB
The point that the reflectance of the R portion 46a needs to be sufficiently large is the same as in the third embodiment.

In this wavelength converter, the third
Similarly to the embodiment, since the QPM portion 44c is sandwiched between the return light from the DBR portion 46a and the reciprocating light,
In order to make the return efficiency of the pump light large enough,
As in the embodiment, the reflectivity of the DBR portion 46a must be sufficiently large to minimize the attenuation of the pump light in the QPM portion 44c. However, as in the third embodiment, when the conversion efficiency of the converted light is increased, the attenuation of the pump light in the QPM portion 44c is increased. It is necessary to set the return efficiency of the pump light in consideration of the efficiency.

In this wavelength converter, the QPM portion 44c and the DBR portion 46a are not integrated, so that
The quasi-phase matching condition and the determination of the oscillation wavelength cannot be satisfied simultaneously while maintaining sufficient accuracy. By the way, QP
Since the manufacturing process differs between the M part and the DBR part,
When these are integrated, it is difficult to form both parts simultaneously under optimal conditions. In this wavelength converter, Q
Since the PM portion 46a and the DBR portion 44c are not integrated, they can be separately created, and it is easy to create them under optimal conditions.

The wavelength converters of the above-described third to fifth embodiments require some care when designing an actual device, but it is only necessary to select an optimum configuration for the actual device.

As described above, in the wavelength converter of each of the above embodiments, the half width of the spectrum of the converted light is WD
The spectrum width of light required in an optical communication system such as the M system (for example, 5 MHz) can be reduced to about or less, specifically, about 5 MHz or less. Also,
Since a semiconductor laser can be used, the size of a device using such a wavelength converter can be reduced.

The wavelength converter as described above is, for example, a WD
It is used as a wavelength switching element for switching between different wavelengths in an M (wavelength multiplexing) switching system. FIG. 6 shows a configuration example of such a WDM switching system.

This WDM switching system converts the wavelength of the selected light into an optical splitter 50 for splitting the input light, a wavelength selector 51 for selecting light of a specific wavelength from the split light. A wavelength conversion element (the above-described wavelength conversion device is used) 52, a wavelength selection element 53 for limiting the wavelength of the output light of the wavelength conversion element to a specific wavelength, and a multiplex for synthesizing the output light from the wavelength selection element 53. And a vessel 54. Further, this system includes a control unit (not shown) for controlling each of the wavelength selection element 51, the wavelength conversion element 52, the wavelength selection element 53, and the like according to the use state of the light of each wavelength.

This control unit detects, for example, when the channel formed by the light of wavelength λ1 in the output transmission line is congested, and detects the congestion of the channel input with the wavelength λ1. The control of each wavelength selecting element 51, the wavelength converting element 52, the wavelength selecting element 53, and the like is performed so as to be converted into the wavelength (for example, the wavelength λ2).

Based on such control, first, the light of wavelength λ1 is separated and selected by the optical branching element 50 and the wavelength selecting element 51, and is guided to the wavelength converting element 52. Next, the light of wavelength λ1 is converted into light of wavelength λ2 by the wavelength conversion element 52, and only the light of wavelength λ2 is output to the multiplexer 54 by the wavelength selection element 53. And transmits it to the output transmission line.

By performing such wavelength exchange, even if a channel of a specific wavelength of the output transmission line is in use, if another wavelength channel is free, wavelength conversion is performed and communication can be continued. it can. As a result, smooth communication can be performed without taking measures such as temporary standby, and communication efficiency can be improved.

As a method of stabilizing the wavelength of the light to be converted (corresponding to the pump light from the semiconductor laser in each of the above embodiments), for example, Japanese Patent Laid-Open No.
And JP-A-5-112297 and JP-A-5-66440. In these methods, in a device for generating the second harmonic of the oscillation light (converted light) of the semiconductor laser, a wavelength fixing method (Bragg locking) for limiting the wavelength of the light that returns to the semiconductor laser by the Bragg reflection is used. Have been.

However, as in the case of performing difference frequency generation or the like in each of the above-described embodiments, the Bragg-locking method is used in a wavelength converter that requires two or more types of fundamental light (in this case, corresponding to pump light and signal light). Has not been adopted in the past.

Further, the Bragg rocking method used in the inventions described in each of the above publications is intended only to stabilize the oscillation wavelength of the semiconductor laser, and as in each of the above embodiments. This is not the purpose of narrowing the half-width of the spectrum of the oscillation light to ensure the purity of the wavelength.

Therefore, in the inventions described in each of the above-mentioned publications, problems such as optimization of the length of the Bragg structure for narrowing the half width of the spectrum of the oscillating light have not been studied. It is impossible to complete the above embodiments as merely extensions of the invention. Completing a series of technologies by focusing on the fact that difference frequency generation can be incorporated into optical switching technology by focusing on the spectral narrowing of converted light, requires a major change in thinking, and is easily completed I can't.

In the above embodiments, the DFG is mainly used as the wavelength conversion element. However, the SFG may be used to obtain converted light. In this case, it is necessary to set the oscillation wavelength of the semiconductor laser as the pump light to, for example, around 1.55 μm, and to change the transmission characteristics and the like of the band-pass filter for SFG in accordance with this.

In addition to DFG and SFG, optical parametric oscillation / amplification, which is a second-order nonlinear optical effect, can also be used as a wavelength conversion element to obtain converted light. Furthermore, it is also possible to use a third-order or higher-order nonlinear optical effect, and in particular, wavelength conversion by four-wave mixing can be used. When these different optical effects are used, it is needless to say that it is necessary to change the characteristics of the bandpass filter, the oscillatable wavelength band of the semiconductor laser, and the like.

[0099]

According to the wavelength conversion apparatus of the present invention, the spectral width of the oscillation wavelength of the semiconductor laser can be reduced by limiting the wavelength of the light fed back to the semiconductor laser by the wavelength selection means. In this case, the spectral width of the converted light generated in the above can be narrowed.

Further, another wavelength conversion means according to the present invention comprises:
By limiting the wavelength of the light fed back to the semiconductor laser by the distributed feedback (DBR) means, it is possible to narrow the spectrum width of the oscillation wavelength of the semiconductor laser and narrow the spectrum width of the converted light generated in the wavelength conversion means. it can.

[Brief description of the drawings]

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

FIG. 2 is a diagram illustrating a configuration of a wavelength conversion device according to a second embodiment of the present invention.

FIG. 3 is a diagram illustrating a configuration of a wavelength conversion device according to a third embodiment of the present invention.

FIG. 4 is a diagram illustrating a configuration of a wavelength conversion device according to a fourth embodiment of the present invention.

FIG. 5 is a diagram illustrating a configuration of a wavelength conversion device according to a fifth embodiment of the present invention.

FIG. 6 is a block diagram illustrating a configuration example of a main part of the WDM switching system.

[Explanation of symbols]

1, 24, 24, 34, 44 wavelength conversion element, 2, 2
2, 32, 42 semiconductor lasers, 3a, 3b, 13a to
13c, 23, 33, 43 band pass filter, 24
c, 34c, 44c QPM part, 24d, 34d, 4
6a DBR part

Claims (15)

[Claims] A semiconductor laser for generating a fundamental wave light; a wavelength converting means for generating and outputting converted light based on the fundamental wave light from the semiconductor laser and an external signal light; and a wavelength of light to be fed back to the semiconductor laser. And a wavelength selecting means for selectively feeding back only light of a predetermined wavelength. 2. The wavelength converter according to claim 1, wherein said wavelength selecting means is disposed between said semiconductor laser and said wavelength converting means. 3. The wavelength converter according to claim 1, wherein said wavelength selecting means is disposed between said semiconductor laser and said wavelength converting means. 4. The wavelength according to claim 1, further comprising signal light incidence means for causing the signal light supplied from a direction different from that of the fundamental wave light from the semiconductor laser to enter the wavelength conversion means. Conversion device. 5. The wavelength conversion device according to claim 1, further comprising a conversion light output unit that outputs the converted light from the wavelength conversion unit in a direction different from a direction of the fundamental wave light from the semiconductor laser. . 6. The semiconductor laser according to claim 1, wherein the half-width of the spectrum of the light fed back by said wavelength selecting means is not more than a predetermined half-width, and said semiconductor laser oscillates at the wavelength of the light fed back by said wavelength selecting means. The wavelength converter according to any one of claims 1 to 3. 7. A semiconductor laser having a high-reflection coating at one end of an optical path and outputting a fundamental wave light generated from an output end face opposite to the high-reflection coating, and quasi-phase matching means (hereinafter referred to as QPM means). And distributed feedback means (hereinafter referred to as DBR means) for limiting the wavelength of light fed back to the semiconductor laser, and generates converted light based on fundamental wave light from the semiconductor laser and signal light from outside. The reflectance of the output end face of the semiconductor laser is r, the reflectance of both end faces of the wavelength conversion means is r ′, r ″, and the feedback rate from the DBR means is r _D , r ', R "<< r _D , r << r _D , r', r"<< r The reflectance of each end face is set so as to satisfy the following relationship: 8. The wavelength conversion device according to claim 7, wherein said QPM means is arranged on an input side of the wavelength conversion means for inputting a fundamental wave light from said semiconductor laser. 9. The wavelength converter according to claim 7, wherein said QPM means is arranged on an output side of said converted light of said wavelength converting means. 10. The wavelength according to claim 7, further comprising signal light incidence means for causing the signal light supplied from a direction different from that of the fundamental wave light from the semiconductor laser to enter the wavelength conversion means. Conversion device. 11. The wavelength conversion device according to claim 7, further comprising a conversion light output unit that outputs the converted light from the wavelength conversion unit in a direction different from the fundamental wave light from the semiconductor laser. . 12. The wavelength conversion means according to claim 8, wherein said QPM means and DBR means are formed integrally. 13. The wavelength conversion device according to claim 8, wherein said DBR means is formed near an output end of an optical fiber for propagating signal light to be incident on a wavelength conversion element. 14. The signal light incidence means comprises a bandpass filter for selectively reflecting the signal light, and comprises an optical isolator for preventing return light output from the wavelength conversion means via the bandpass filter. The wavelength converter according to claim 10, wherein: 15. The wavelength conversion device according to claim 11, wherein said converted light output means comprises a band-pass filter for selectively reflecting said converted light.