JP4036291B2 - Optical amplification element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical amplification technique for amplifying input signal light intensity of an arbitrary wavelength in a wavelength division multiplexing optical network.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, as an optical transmission system for transmitting a plurality of optical signals of different wavelengths, an optical transmission system using wavelength multiplexing (WDM system) that transmits the optical signals of different wavelengths coupled to one optical fiber. There is.
Furthermore, the WDM system is rapidly becoming a network in order to enable not only one-to-one transmission but also optical transmission between a plurality of terminals such as one-to-many and many-to-many.
[0003]
In such a WDM system, optical signals are combined / branched according to the wavelength, WDM multiplexing / demultiplexing circuits, a combining / branching circuit that combines and branches all wavelengths, and a specific wavelength is extracted or inserted. The signal strength is deteriorated due to an intensity loss caused by passing through an optical element such as an add-drop multiplexer (ADM). Therefore, the role of an optical amplifying element that amplifies an optical signal transmitted through an optical fiber as light is important.
[0004]
When a conventional semiconductor optical amplifier (Semiconductor optica1 Amplifier, SOA) is used in a WDM system, the following problems occur. That is, in the WDM system, a wavelength multiplexed signal is incident, but the number of wavelength multiplexed signals is considered to change every time it passes through the ADM or the like. Assume that a signal having a wavelength multiplexing number m is incident on the SOA. The incident light intensity is P ₁ (dBm) in total for m waves, and the SOA gain at that time is G ₁ (dB). Here, if a signal is added by ADM and the wavelength multiplexing number is increased to n, then the incident light intensity is P ₂ (dBm) in total for n waves, and the saturated output P _{sat of the} SOA is P _sat <G ₁・ If P ₂ , saturation occurs, and the SOA gain at that time decreases to G ₂ (dB).
In general, the relationship between the power of the gain and the input as described above is a relationship such as the corresponding contents of Patent Document 1 and Non-Patent Document 1 shown below.
[0005]
[Patent Document 1]
US Pat. No. 5,291,328 (FIG. 4)
[Non-Patent Document 1]
Journal of Lightwave Technology Vol.16, No.12 (1998) (page 2228, Fig. 2)
[0006]
[Problems to be solved by the invention]
Thus, when the conventional SOA is used in the WDM system, there is a problem that the gain of the signal varies depending on the number of wavelength multiplexing. Further, in the state where the saturation occurs, there is a problem that the waveform is deteriorated due to a pattern effect in which the signal intensity changes depending on the signal pattern as well as the gain.
[0007]
Accordingly, the present invention has been made paying attention to such an unsolved problem of the conventional technology, and has an object to provide an optical amplifying element that has no gain variation and suppresses the pattern effect. Yes.
[0011]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided an optical amplifying element according to the present invention, comprising: a symmetric Mach-Zehnder interference circuit formed on a substrate, the first arm and the second arm each having amplifying means for amplifying light intensity of input signal light; ,
An input waveguide formed on the input side of the first arm of the symmetric Mach-Zehnder interference circuit;
An oscillation optical waveguide formed on the output side of the first arm and the input side of the second arm of the symmetric Mach-Zehnder interference circuit;
An output waveguide formed on the output side of the second arm of the symmetric Mach-Zehnder interference circuit;
Laser oscillation means having a cavity for generating oscillation formed independently of the amplification means on an oscillation optical waveguide formed on the output side of the first arm,
The oscillation wavelength of the laser oscillation unit, is characterized in that set in the wavelength shorter than the bandgap wavelength of the gain medium of the amplifier means.
[0012]
This makes it possible to keep the gain constant and suppress the pattern effect.
It invention according to claim 2, in the optical amplifier device according to claim 1, wherein said laser oscillating means, Fabry-Perot type laser, DFB (Distributed Feed-back) laser or DBR (Distributed Bragg reflector) is a laser It is characterized by.
[0013]
This makes it possible to keep the gain constant and suppress the pattern effect .
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1 to 5 are diagrams showing an embodiment of an optical amplifying element according to the present invention.
First, a first embodiment of an optical amplifying element according to the present invention will be described with reference to FIG. FIG. 1 is a diagram showing a first embodiment of an optical amplifying element according to the present invention.
[0015]
In the figure, 101 and 102 are semiconductor optical amplifiers (Semiconductor Optica1 Amplifier, SOA), 103 and 104 are multimode interference type 3 dB force coupler (Multi-mode Interference coupler, MMI coupler), 105 is DFB-LD, and 106 and 107 are guides. Waveguide end faces, 108 and 109 are antireflection films, 110 is an input waveguide, 111 and 112 are oscillation optical waveguides, 113 is an output waveguide, and 114 is a substrate. The SOAs 101 and 102 and the MMI couplers 103 and 104 form a symmetric Mach-Zehnder interference circuit 115.
[0016]
The operation principle will be described below. Almost the same current is injected into the SOAs 101 and 102 connected to both arms of the symmetric Mach-Zehnder interference circuit 115, and both SOAs have a signal gain of 20 dB. At this time, since the same current is injected into both SOAs as described above, the symmetry of the Mach-Zehnder interference circuit 115 is preserved.
[0017]
Further, due to the nature of the symmetric Mach-Zehnder interference circuit 115, in the Mach-Zehnder interference circuit 115, light incident from the input waveguide 110 is emitted from the output waveguide 113, and light incident from the oscillation optical waveguide 111 is input to the input waveguide 112. It will be in the intersection state where it is emitted to.
The DFB-LD 105 connected to the oscillation optical waveguide 112 is controlled independently of the SOAs 101 and 102, and is set to oscillate regardless of the injection current of the SOAs 101 and 102.
[0018]
Here, the DFB-LD 105 is a distributed feedback (DFB type) semiconductor laser and has a diffractive resonator structure. In other words, a periodic uneven structure (functioning as a diffraction grating) is formed as a guide layer in a place adjacent to the active layer (which may be above or below), and the light returned by the unevenness is oscillated. It is. In this structure, since the refractive index also changes periodically, the reflectivity is increased by Bragg reflection at a wavelength where the phase of the periodically reflected light is in phase, thereby causing laser oscillation.
[0019]
FIG. 2 is a diagram showing the relationship between the oscillation wavelength of the DFB-LD 105 and the gain spectra of the SOAs 101 and 102. In this embodiment, since the oscillation wavelength of the DFB-LD 105 is set to a shorter wavelength side than the band gap wavelength of the gain quality of the SOAs 101 and 102, the peak wavelength λ of the gain spectrum as shown in FIG. _Shorter than _c . Here, when the current and temperature of the DFB-LD 105 and the SOAs 101 and 102 are adjusted, the oscillation wavelength can be set to a wavelength λ _{0 at} which the gain of the SOAs 101 and 102 becomes zero as shown in FIG. At this time, the SOAs 101 and 102 are in a transparent state at the oscillation wavelength λ ₀ of the DFB-LD 105, and the presence of oscillation light does not affect the characteristics of the SOAs 101 and 102.
[0020]
When the intensity of the input signal light 116 increases, the carrier density of the SOAs 101 and 102 decreases, and the gain spectrum becomes like the one-dot chain line in FIG. At this time, at the oscillation wavelength λ ₀ of the DFB-LD 105, the SOAs 101 and 102 have a negative gain, that is, an absorption state. Therefore, the SOAs 101 and 102 absorb the oscillation light of the DFB-LD 105. As a result, the carrier density increases and rapidly recovers to the steady state shown by the solid line in FIG.
[0021]
On the other hand, when the input signal light intensity decreases, the carrier density of the SOAs 101 and 102 increases, and the gain spectrum becomes as shown by the dotted line in FIG. At this time, at the oscillation wavelength λ ₀ of the DFB-LD 105, the SOAs 101 and 102 have a positive gain, that is, are in an amplified state. For this reason, the SOAs 101 and 102 amplify the oscillation light of the DFB-LD 105, and as a result, the carrier density decreases and rapidly recovers to the steady state shown by the solid line in FIG.
[0022]
In this way, even if the input signal intensity varies, the carrier density of the SOAs 101 and 102 does not vary, and the gain is kept constant. Therefore, even if the number of multiplexed wavelengths of the input signal is changed, it is possible to stably operate with minimal fluctuations in gain.
In this embodiment, due to the property of the symmetric Mach-Zehnder interference circuit 115, the oscillation light of the DFB-LD 105 incident from the input waveguide 112 is transmitted through the Mach-Zehnder interference circuit 115 and output from the oscillation optical waveguide 111. The oscillation light is not mixed into the input waveguide 110 and the output waveguide 113 of the signal light, and the signal light and the oscillation light are spatially separated. Therefore, an external filter for removing oscillation light is not necessary.
[0023]
Further, the structure of the SOAs 101 and 102 used in the present embodiment is not particularly limited, and the effects described above can be expected by adopting this configuration for all the SOAs that are normally used. That is, the SOA active layer can be applied to any material such as InGaAsP, GaAs, AlGaAs, InGaAs, and GaInNAs, and the active layer structure is not limited to bulk, MQW, quantum wire, quantum dot, and the SOA waveguide. With respect to the structure, the same effect can be expected even when a pn buried structure, a ridge structure, a semi-insulating buried structure, a high mesa structure, or the like is used regardless of the presence or absence of SCH (separation confinement structure). The substrate is not limited to the n-type substrate, and it goes without saying that the same effect can be obtained with a p-type, a semi-green type, and the like.
[0024]
In the above embodiment, an example is shown in which an MMI coupler is used as the 3 dB coupler. However, the present invention is not limited to this, and a directional coupler may be used.
Furthermore, in the above-described embodiment, an example in which a DFB-LD is used as a laser transmitter is shown. However, the laser cavity is not limited to the DFB-LD and is formed independently of the semiconductor amplifiers 101 and 102 that are signal light amplifying means. If so, the same effect can be expected. For example, a DBR-LD that forms a cavity by an active region in which an active layer exists, a Bragg reflector that exists at one end of the active region, and a Bragg reflector or reflecting mirror that exists at the opposite end, and the active region and its A similar effect can be expected even when a Fabry-Perot LD that forms a cavity with reflection mirrors at both ends is used. The spectrum of the oscillation light does not necessarily need to be single mode oscillation. Even when an oscillation spectrum laser transmitter having a multi-peak such as a Fabry-Perot LD is used, the short wavelength side of the oscillation spectrum is absorbed and the long wavelength side is amplified across λ ₀ , and the absorption and amplification are balanced in total. Since it is in a steady state at a point, the same effect can be expected.
[0025]
Further, a second embodiment of the optical amplifying element according to the present invention will be described with reference to FIG. FIG. 3 is a diagram showing a second embodiment of the optical amplifying element according to the present invention.
In the figure, 301 and 302 are semiconductor optical amplifiers (SOA), 303 and 304 are multimode interference type 3 dB force couplers (MMI couplers), 305 is a DFB-LD, 306 and 307 are waveguide end faces, and 308 and 309 are antireflection films. , 310 is an input waveguide, 311 and 312 are oscillation optical waveguides, 313 is an output waveguide, 314 is a substrate, and 318 is a loop waveguide. The SOAs 301 and 302 and the MMI couplers 303 and 304 form a symmetric Mach-Zehnder interference circuit 315.
[0026]
The operating principle is exactly the same as in the first embodiment except that the oscillation light having the wavelength λ ₀ output from the DFB-LD 305 enters the symmetric Mach-Zehnder interference circuit 315 from both the oscillation optical waveguides 311 and 312. It is the same.
Therefore, also in the present embodiment, due to the property of the symmetric Mach-Zehnder interference circuit 315, the oscillation light of the DFB-LD 305 incident from the oscillation optical waveguide 312 is transmitted through the Mach-Zehnder interference circuit 315 and output from the oscillation optical waveguide 311. In addition, since the oscillation light of the DFB-LD 305 incident from the oscillation optical waveguide 311 passes through the Mach-Zehnder interference circuit 315 and is output from the oscillation optical waveguide 312, the oscillation light enters the signal light input waveguide 310 and the output waveguide 313. There is no mixing, and signal light and oscillation light are spatially separated. Therefore, an external filter for removing oscillation light is not necessary.
[0027]
It should be noted that the SOA structure in the present embodiment is not particularly restricted as in the first embodiment, and the effects described above are obtained by adopting this configuration for all the SOAs that are normally used. Can be expected. That is, the SOA active layer can be applied to any material such as InGaAsP, GaAs, AlGaAs, InGaAs, and GaInNAs, and the active layer structure is not limited to bulk, MQW, quantum wire, quantum dot, and the SOA waveguide. With respect to the structure, the same effect can be expected even when a pn buried structure, a ridge structure, a semi-insulating buried structure, a high mesa structure, or the like is used regardless of the presence or absence of SCH (separation confinement structure). The substrate is not limited to the n-type substrate, and it goes without saying that the same effect can be obtained with a p-type, a semi-green type, and the like.
[0028]
In addition, the 3 dB coupler is not limited to the MMI coupler, and the same effect can be expected even when a directional coupler is used.
Further, the laser transmitter is not limited to the DFB-LD, and the same effect can be expected if the laser cavity is formed independently of the semiconductor amplifiers 301 and 302 which are signal light amplification means. For example, a DBR-LD that forms a cavity by an active region in which an active layer exists, a Bragg reflector that exists at one end of the active region, and a Bragg reflector or reflecting mirror that exists at the opposite end, and the active region and its A similar effect can be expected even when a Fabry-Perot LD that forms a cavity with reflection mirrors at both ends is used. Further, the spectrum of the oscillation light does not necessarily need to be single mode oscillation.
[0029]
Further, a third embodiment of the optical amplifying element according to the present invention will be described with reference to FIG. FIG. 4 is a diagram showing a third embodiment of the optical amplifying element according to the present invention.
In the figure, 401 and 402 are semiconductor optical amplifiers (SOA), 403, 404 and 419 are multimode interference type 3 dB force couplers (MMI couplers), 405 is a DFB-LD, 406 and 407 are waveguide end faces, and 408 and 409 are reflections. The prevention film, 410 is an input waveguide, 411, 412, and 418 are oscillation optical waveguides, 413 is an output waveguide, and 414 is a substrate. The SOAs 401 and 402 and the MMI couplers 403 and 404 form a symmetric Mach-Zehnder interference circuit 415.
[0030]
The principle of operation is that the oscillation light having the wavelength λ ₀ output from the DFB-LD 415 is branched into two by the MMI 3 dB coupler 419 and enters the symmetric Mach-Zehnder interference circuit 415 from both the oscillation optical waveguides 411 and 412. This is exactly the same as the first embodiment shown in FIG. 1 and the second embodiment shown in FIG.
Therefore, also in this embodiment, due to the property of the symmetric Mach-Zehnder interference circuit 415, the oscillation light of the DFB-LD 405 incident from the oscillation optical waveguide 412 passes through the Mach-Zehnder interference circuit 415 and is output from the oscillation optical waveguide 411. Further, since the oscillation light of the DFB-LD 405 incident from the oscillation optical waveguide 411 is transmitted through the Mach-Zehnder interference circuit 415 and output from the oscillation optical waveguide 412, the oscillation light enters the signal light input waveguide 410 and the output waveguide 413. There is no mixing, and signal light and oscillation light are spatially separated. Therefore, an external filter for removing oscillation light is not necessary.
[0031]
Note that the SOA structure in the present embodiment is not particularly limited as in the first and second embodiments, and this configuration is adopted for all the SOAs that are normally used. Therefore, the effect as described above can be expected. That is, the SOA active layer can be applied to any material such as InGaAsP, GaAs, AlGaAs, InGaAs, and GaInNAs, and the active layer structure is not limited to bulk, MQW, quantum wire, quantum dot, and the SOA waveguide. With respect to the structure, the same effect can be expected even when a pn buried structure, a ridge structure, a semi-insulating buried structure, a high mesa structure, or the like is used regardless of the presence or absence of SCH (separation confinement structure). The substrate is not limited to the n-type substrate, and it goes without saying that the same effect can be obtained with a p-type, a semi-green type, and the like.
[0032]
In addition, the 3 dB coupler is not limited to the MMI coupler, and the same effect can be expected even when a directional coupler is used.
Further, the laser transmitter is not limited to the DFB-LD, and the same effect can be expected if the laser cavity is formed independently of the semiconductor amplifiers 401 and 402 which are signal light amplification means. For example, a DBR-LD that forms a cavity by an active region in which an active layer exists, a Bragg reflector that exists at one end of the active region, and a Bragg reflector or reflecting mirror that exists at the opposite end, and the active region and its A similar effect can be expected even when a Fabry-Perot LD that forms a cavity with reflection mirrors at both ends is used. Further, the spectrum of the oscillation light does not necessarily need to be single mode oscillation.
[0033]
Further, a fourth embodiment of the optical amplifying element according to the present invention will be described with reference to FIG. FIG. 5 is a diagram showing a fourth embodiment of the optical amplifying element according to the present invention.
The fourth embodiment shown in FIG. 5 is obtained by replacing the MMI 3 dB coupler 419 in the third embodiment shown in FIG. 4 with a multimode interference type (MMI) 1: 2 coupler 519.
[0034]
In the figure, 501 and 502 are semiconductor optical amplifiers (SOA), 503 and 504 are multimode interference type 3 dB force plastics (MMI couplers), 505 is a DFB-LD, 506 and 507 are waveguide end faces, and 508 and 509 are antireflection films. , 510 are input waveguides, 511, 512, and 518 are oscillation optical waveguides, 513 is an output waveguide, 514 is a substrate, and 519 is a multimode interference (MMI) 1: 2 coupler. The SOAs 501 and 502 and the MMI 3 dB couplers 503 and 504 form a symmetric Mach-Zehnder interference circuit 515.
[0035]
The principle of operation is exactly the same as that of the third embodiment shown in FIG. 4 except that the oscillation light having the wavelength λ ₀ output from the DFB-LD 515 is branched into two by the multimode interference type 1: 2 coupler 519. It is the same.
Note that the SOA structure in the present embodiment is not particularly limited as in the first, second, and third embodiments, and is not limited to all the commonly used ones. By taking this configuration for the SOA, the effects described above can be expected. That is, the SOA active layer can be applied to any material such as InGaAsP, GaAs, AlGaAs, InGaAs, and GaInNAs, and the active layer structure is not limited to bulk, MQW, quantum wire, quantum dot, and the SOA waveguide. With respect to the structure, the same effect can be expected even when a pn buried structure, a ridge structure, a semi-insulating buried structure, a high mesa structure, or the like is used regardless of the presence or absence of SCH (separation confinement structure). The substrate is not limited to the n-type substrate, and it goes without saying that the same effect can be obtained with a p-type, a semi-green type, and the like.
[0036]
In addition, the 3 dB coupler is not limited to the MMI coupler, and the same effect can be expected even when a directional coupler is used. Furthermore, the 1 × 2 coupler is not limited to the MMI coupler, and the same effect can be expected even if Y branch is used.
Further, the laser transmitter is not limited to the DFB-LD, and the same effect can be expected if the laser cavity is formed independently of the semiconductor amplifiers 301 and 302 which are signal light amplification means. For example, a DBR-LD that forms a cavity by an active region in which an active layer exists, a Bragg reflector that exists at one end of the active region, and a Bragg reflector or reflecting mirror that exists at the opposite end, and the active region and its A similar effect can be expected even when a Fabry-Perot LD that forms a cavity with reflection mirrors at both ends is used. Further, the spectrum of the oscillation light does not necessarily need to be single mode oscillation.
[0037]
【The invention's effect】
As described above, according to the optical amplifying element of the present invention, it is possible to keep the gain constant even when the input signal intensity varies, and to suppress the pattern effect even in the saturated state.
[Brief description of the drawings]
FIG. 1 is a diagram showing a first embodiment of the present invention.
FIG. 2 is a diagram showing a relationship between an SOA gain spectrum and an oscillation wavelength.
FIG. 3 is a diagram showing a second embodiment of the present invention.
FIG. 4 is a diagram showing a third embodiment of the present invention.
FIG. 5 is a diagram showing a fourth embodiment of the present invention.
[Explanation of symbols]
101, 102 SOA
103, 104 MMI3dB coupler 105 DFB-LD
106, 107 End faces 108, 109 Antireflection film 110 Input waveguide 111, 112 Oscillation optical waveguide 113 Output waveguide 114 Substrate 115 Symmetric Mach-Zehnder interference circuit 116 Input signal light 117 Output signal light 301, 302 SOA
303, 304 MMI3dB coupler 305 DFB-LD
306, 307 End face 308, 309 Antireflection film 310 Input waveguide 311, 312 Oscillation optical waveguide 313 Output waveguide 314 Substrate 315 Symmetric Mach-Zehnder interference circuit 316 Input signal light 317 Output signal light 318 Loop waveguide 401, 402 SOA
403, 404 MMI3dB coupler 405 DFB-LD
406, 407 End face 408, 409 Antireflection film 410 Input waveguide 411, 412 Oscillation optical waveguide 413 Output waveguide 414 Substrate 415 Symmetric Mach-Zehnder interference circuit 416 Input signal light 417 Output signal light 418 Oscillation optical waveguide 419 MMI3dB coupler 501, 502 SOA
503, 504 MMI3dB coupler 505 DFB-LD
506, 507 End faces 508, 509 Antireflection film 510 Input waveguide 511, 512 Oscillation optical waveguide 513 Output waveguide 514 Substrate 515 Symmetric Mach-Zehnder interference circuit 516 Input signal light 517 Output signal light 518 Oscillation optical waveguide 519 MMI 1: 2 coupler

Claims (2)

A symmetrical Mach-Zehnder interference circuit formed on the substrate, each having an amplification means for amplifying the light intensity of the input signal light in the first arm and the second arm;
An input waveguide formed on the input side of the first arm of the symmetric Mach-Zehnder interference circuit;
An oscillation optical waveguide formed on the output side of the first arm and the input side of the second arm of the symmetric Mach-Zehnder interference circuit;
An output waveguide formed on the output side of the second arm of the symmetric Mach-Zehnder interference circuit;
Laser oscillation means having a cavity for generating oscillation formed independently of the amplification means on an oscillation optical waveguide formed on the output side of the first arm,
Optical amplifier, characterized in that the oscillation wavelength of the laser oscillation unit, and set the wavelength shorter than the bandgap wavelength of the gain medium of the amplifier means.   2. The optical amplifying element according to claim 1, wherein the laser oscillation means is a Fabry-Perot laser, a DFB (Distributed Feed-back) laser, or a DBR (Distributed Bragg reflector) laser.

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