JP2008034882A - Optical amplification element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical amplifying element capable of suppressing a gain fluctuation due to input light intensity without mixing oscillation light in a signal light guiding direction.
An input waveguide 402, a plurality of multimode waveguides 403a to 403c, and an output waveguide 404 are formed on an n-InP substrate 401. The input waveguide 402 and the output waveguide 404 have InGaAsP as a core. The multimode waveguides 403a to 403c are composed of a multimode waveguide composed of a gain medium having InGaAsP as a core, and are highly reflective on both sides of the multimode waveguide 403. The films 409 and 410 are opposed to each other, and the input signal light 411 is propagated in the multimode waveguide 403 while causing laser oscillation in the multimode waveguide 403 in a direction orthogonal to the waveguide direction of the input signal light 411. As a result, the input signal light 411 is amplified.
[Selection] Figure 1

Description

  The present invention relates to an optical amplifying element, and is particularly suitable for application to an optical transmission system using wavelength division multiplexing (WDM).

2. Description of the Related Art Conventionally, as an optical transmission system that transmits a plurality of optical signals having different wavelengths, there is an optical transmission system (WDM system) that uses wavelength multiplexing to transmit a plurality of optical signals having different wavelengths by combining them with one optical fiber. . Further, in the WDM system, not only one-to-one transmission but also networking is rapidly progressing.
In this WDM system, a WDM multiplexing / demultiplexing circuit that merges and branches optical signals according to wavelength, a multiplexing / branching circuit that merges and branches light of all wavelengths at once, and an add / drop that extracts or inserts a specific wavelength An optical element such as a multiplexer (Add-drop multiplexer, ADM) is used, and the signal strength is deteriorated due to an intensity loss caused when an optical signal passes through these optical elements.

For this reason, in a WDM system, an optical amplifying element that amplifies an optical signal transmitted through an optical fiber as light is indispensable.
11A is a cross-sectional view of a conventional optical amplifying element cut along the optical waveguide direction, FIG. 11B is a cross-sectional view taken along the line AA ′ of FIG. As an example of a conventional optical amplifier (Semiconductor Optical Amplifier: SOA), a structure using an n-InP substrate 101 is shown (Non-Patent Document 1).

In FIG. 11, an InGaAsP active layer 102 as a gain medium is formed in a stripe shape on an n-InP substrate 101, and the InGaAsP active layer 102 is buried with a p-InP layer 103 and an n-InP layer 104. .
A p-InP layer 105 is formed on the InGaAsP active layer 102 and the n-InP layer 104, and a p-GaInAs contact layer 106 is formed on the p-InP layer 105. A p-side electrode 107 is formed on the p-GaInAs contact layer 106, and an n-side electrode 108 is formed on the back surface of the n-InP substrate 101.

FIG. 12 is a diagram illustrating saturation characteristics of the optical amplifying element in FIG.
In FIG. 12, when the input light intensity is small, the gain is almost constant even when the input light intensity increases, but when the input light intensity exceeds a certain value, the gain decreases rapidly. Here, in the WDM system, a wavelength multiplexed signal is incident as an optical signal, and the number of wavelength multiplexed signals varies every time it passes through an add drop multiplexer or the like.

It is assumed that an optical signal having a wavelength multiplexing number m is incident on the optical amplifying element. In this case, when the incident light intensity of the optical amplifying element is P ₁ (dBm) in total for m waves, the gain of the optical amplifying element is G ₁ (dBm).
Here, it is assumed that an optical signal is added by the add / drop multiplexer and the wavelength multiplexing number is increased to n. In this case, when the incident light intensity of the optical amplifying element is P ₂ (dBm) in total for n waves, the gain of the optical amplifying element is G ₂ (dB).

  As described above, when the optical amplifying element of FIG. 11 is used in a WDM system, the intensity of incident light varies depending on the number of wavelength multiplexing, and the gain of the optical signal varies. For this reason, in the conventional optical amplifier, as disclosed in Patent Document 1, in order to prevent the gain of the optical signal from fluctuating due to the number of wavelength multiplexing, the gain is set to a certain value by using the oscillation action. Some used a clamping method.

13A is a cross-sectional view of a conventional optical amplifying element cut along the optical waveguide direction, and FIG. 13B is a cross-sectional view taken along the line CC ′ of FIG. 13A.
In FIG. 13, an InGaAsP active layer 202 that is a gain medium is formed in a stripe shape on an n-InP substrate 201, and the InGaAsP active layer 202 is buried with a p-InP layer 203 and an n-InP layer 204. .

  Here, an InGaAsP isolation and confinement (SCH) layer 209 is formed on the lower surface of the InGaAsP active layer 202, and an InGaAsP isolation and confinement (SCH) layer 210 is formed on the upper surface of the InGaAsP active layer 202. A grating is formed on the layer 210. A p-InP layer 205 is formed on the InGaAsP separation confinement layer 210 and the n-InP layer 204, and a p-GaInAs contact layer 206 is formed on the p-InP layer 205. A p-side electrode 207 is formed on the p-GaInAs contact layer 206, and an n-side electrode 208 is formed on the back surface of the n-InP substrate 201.

In the optical amplifying element of FIG. 13, since an optical signal is reflected by the grating formed in the InGaAsP separation confinement layer 210, positive feedback is applied and oscillation can be performed like a DFB laser. However, the coupling coefficient of the grating is smaller than that of a normal DFB laser, and the oscillation threshold is high.
In the laser oscillation state of the optical amplifying element in FIG. 13, the carrier density in the gain medium is clamped to a constant value, but since the oscillation threshold is high, the carrier density is clamped to a value higher than that of a normal DFB laser. .

Therefore, in the DFB type optical amplifying element having the grating of FIG. 6, the carrier density of the gain medium (InGaAsP active layer 202) is constant as long as oscillation occurs, and the gain is proportional to the carrier density of the gain medium. The gain can be clamped to a constant value.
Therefore, in the oscillation state described above, even if the current value injected into the optical amplifying element is increased, the gain of the optical amplifying element can be kept constant only by increasing the light intensity of the oscillation light. When the input signal light intensity increases, the oscillation light intensity decreases and the total light intensity inside the optical amplifying element is kept constant, so that the carrier density of the optical amplifying element may fluctuate. Therefore, the gain of the optical amplifying element can be kept constant.

FIG. 14 is a diagram illustrating saturation characteristics of the optical amplifying element in FIG.
14, in the optical amplifying element of FIG. 13, even when the input light intensity of signal light incident from the outside fluctuates, the gain is maintained at a constant value Go. That is, even when the wavelength multiplexing number of the signal light is changed from m to n and the total input power is changed from P ₁ to P ₂ , the gain is a constant value of Go.

Further, in the optical amplifying element of FIG. 13, the gain decreases only when the intensity of incident light from the outside further increases and the oscillation is suppressed. Conversely, as long as oscillation occurs in the optical amplification element of FIG. 13, the gain can be kept constant regardless of the incident light intensity or the wavelength multiplexing number of the incident signal.
K. Morito et al., Journal of Lightwave Technology, No. 1, p176-181, 2003 fig. 5 JP-A-7-106714

However, when the DFB type optical amplifying element of FIG. 13 is used, since the oscillation light is mixed in the same optical path as the signal light, there is a problem that a wavelength filter for removing the mixed oscillation light is necessary.
Further, since the DFB type optical amplifying element of FIG. 13 has a very strong oscillation light intensity, if the incident signal intensity is small, the oscillation light can be emitted with the same intensity as the signal light even when a normal wavelength filter is used. There was a problem of remaining.
Accordingly, an object of the present invention is to provide an optical amplifying element capable of suppressing gain fluctuation due to input light intensity without mixing oscillation light in the waveguide direction of signal light.

In order to solve the above-described problem, according to the optical amplifying element according to claim 1, at least one of the input waveguide for guiding single-mode light and the input waveguide is optically coupled, and A plurality of multi-mode waveguides configured to guide mode light and include at least a portion of a gain medium, and at least one of the multi-mode waveguides is optically coupled to form single-mode light. By reflecting the spontaneous emission light radiated from the gain medium and the output waveguide that guides the multimode waveguide through the input waveguide in a direction that intersects the propagation direction of the light incident on the multimode waveguide A reflection region that causes oscillation, a high reflection film is formed in the reflection region, an antireflection film is formed on an end surface of the input waveguide and the input waveguide, and the length of the multimode waveguide Each L _i , width is W _i , reflectance of the high reflection film is R _H , and reflectance of the antireflection film is R _AR .
It is characterized by satisfying the relationship.

  As a result, it is possible to generate oscillation light that is guided in a direction different from the waveguide direction of the signal light by the multimode waveguide, and to clamp the gain of the signal light by the oscillation action in the multimode waveguide. As a result, it is possible to increase the overall length of the multi-mode waveguide, thereby increasing the clamp gain. For this reason, it is possible to amplify the input signal light in the gain medium whose gain is clamped by oscillation while preventing the oscillation light from being mixed into the amplified light emitted from the output waveguide, and driving. The current can be reduced. As a result, it is possible to eliminate the need for a wavelength filter or an optical waveguide for separating the signal light and the oscillation light, and it is possible to suppress a gain variation due to the input light intensity while suppressing an increase in element size.

According to another aspect of the present invention, there is provided an optical amplifying element comprising a connection waveguide configured to guide single mode light and connecting the multimode waveguides.
As a result, the multimode waveguides can be coupled to each other with low loss, and the length of the entire multimode waveguide can be increased while ensuring the degree of freedom of arrangement of the multimode waveguides.
According to another aspect of the present invention, multimode waveguides having different widths are directly connected.
According to the optical amplifying element of claim 4, at least one of the input waveguide, the output waveguide, and the connection waveguide is a single core having a material transparent to the wavelength of the signal light. It is characterized by comprising a mode waveguide.
As a result, even when the optical power density is high because the signal light amplified in the multimode waveguide is incident on the connecting waveguide or the output waveguide, the waveform deterioration due to gain saturation is prevented. Can be prevented.

In addition, according to the optical amplifying element described in claim 5, the waveguide widths of at least two multimode waveguides are different from each other.
This makes it possible to change the injected carrier density for each multimode waveguide. For this reason, it is possible to increase the saturated output power or improve the NF (noise figure) while gaining gain, and it is possible to realize various operation states depending on the use of the optical amplification element.
The optical amplifier according to claim 6 further includes a current injection means for injecting current independently into at least two multimode waveguides.
As a result, the injection current density can be independently controlled for each multimode waveguide, and the optical amplifying element can be efficiently driven even when the waveguide widths are different for each multimode waveguide.

In the optical amplifying device according to claim 7, the input waveguide, the multimode waveguide, and the output waveguide are arranged side by side on the same substrate so that waveguide center axes coincide with each other, and the reflection The region is disposed opposite to both sides of the multimode waveguide.
As a result, the input waveguide, the multimode waveguide, and the output waveguide can be coupled with low loss, and oscillation can be caused in a direction orthogonal to the propagation direction of the signal light. For this reason, it is possible to suppress the gain fluctuation due to the input light intensity while suppressing the increase in the element size, and it is possible to prevent the oscillation light from being mixed into the amplified light emitted from the output waveguide.

Further, according to the optical amplifying element according to claim 8, the input waveguide, the multimode waveguide, and the output waveguide include a core composed of a common gain medium, and the multimode waveguide has the core. The width of the core is wider than the width of the core of the input waveguide and the output waveguide.
As a result, the input waveguide, the multimode waveguide, and the output waveguide can be collectively formed in the same process, and the gain fluctuation due to the input light intensity can be suppressed while suppressing an increase in the manufacturing process. In addition, it is possible to prevent the oscillation light from being mixed into the amplified light emitted from the output waveguide.

According to the optical amplifying element of claim 9, the length of the i-th multi-mode waveguide among the at least two multi-mode waveguides is L _i , the width is W _i , the refractive index is n _eqi , the signal When the optical wavelength is λ, L _i = m _i · n _eqi · W _i ² / λ (where m _i is a positive integer).
As a result, it is possible to increase the length of the multimode waveguide while allowing the self-coupling effect to be exhibited, and to increase the clamp gain. Therefore, it is possible to reduce the size of the multimode waveguide while keeping the injection current density constant, and it is possible to couple the signal light to the fundamental mode of the output waveguide with low loss while reducing the driving current. Can be reduced.

Further, according to the optical amplifying element according to claim 10, a part of the core of the multimode waveguide is configured, and is extended in the direction of the reflection region so as to be wider than the multimode waveguide. A transparent layer transparent to the signal light wavelength is further provided.
Thereby, the core for the oscillation light can be extended to the reflection region, and the loss of the oscillation light in the cavity can be reduced. For this reason, it becomes possible to efficiently feed back the oscillation light in the multimode waveguide, and the requirement for the reflectance of the reflection region can be relaxed.

According to an optical amplification element of an eleventh aspect, the reflection region includes a dielectric multilayer film or a metal film formed on a side wall of the multimode waveguide.
As a result, it is possible to integrally form a reflection region on the substrate on which the input waveguide, the multimode waveguide, and the output waveguide are formed, and to suppress the increase in element size, while guiding the signal light. The oscillation light can be guided in a different direction.

  As described above, according to the present invention, by providing a reflection region that causes oscillation in a direction crossing the propagation direction of the light incident on the multimode waveguide, the amplified light emitted from the output waveguide is provided. It is possible to amplify the input signal light in the gain medium whose gain is clamped by oscillation while preventing the oscillation light from being mixed. For this reason, a wavelength filter or an optical waveguide for separating the signal light and the oscillation light can be dispensed with, and the gain fluctuation due to the input light intensity can be suppressed while suppressing an increase in the element size.

Hereinafter, an optical amplifying element according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a plan view showing a schematic configuration of the optical amplifying element according to the first embodiment of the present invention.
In FIG. 1, on an n-InP substrate 401, an input waveguide 402 that is connected to a multimode waveguide 403a and receives input signal light 411, and a plurality of multimode waveguides 403a to 403c that guide signal light, An output waveguide that connects the multimode waveguides 403a and 403b to each other, a connection waveguide 413b that connects the multimode waveguides 403b and 403c to each other, and a multimode waveguide 403c that outputs the output signal light 412. A waveguide 404 is formed.

Here, the input waveguide 402 and the output waveguide 404 can be constituted by a single mode waveguide made of a gain medium having InGaAsP as a core, and the multimode waveguides 403a to 403c are gain media having InGaAsP as a core. The connection waveguides 413a and 413b can be formed of a single mode waveguide made of a gain medium having InGaAsP as a core. Further, the input waveguide 402, the multimode waveguides 403a to 403c, the connection waveguides 413a and 413b, and the output waveguide 404 can be arranged side by side on the n-InP substrate 401 so that the waveguide center axes coincide with each other. . Further, the length L of each of the multimode waveguides 403a to 403c is set so that the width of each of the multimode waveguides 403 to 403c is W, and the equivalent refractive index in the vertical direction (substrate vertical direction) of each of the multimode waveguides 403a to 403c. n _eq , where λ is the wavelength of the input signal light 411,
L = m · n _eq · W ² / λ (1)
It can be set to satisfy the relationship. However, m is a positive integer. In the following description, the length L of each of the multimode waveguides 403a to 403c that satisfies the relationship of the expression (1) is L _MMI . Here, the length L _MMI of each of the multimode waveguides 403a to 403c indicates the length until light is first condensed in a spot shape in each of the multimode waveguides 403a to 403c. Further, in multimode interference, spots are periodically formed for each length L _MMI , and therefore, the equation (1) indicates that an integral multiple of the length is used as the length of each of the multimode waveguides 403a to 403c. Yes.

  Further, an antireflection film 407 is formed on the end face 405 of the n-InP substrate 401 on the input waveguide 402 side, and an antireflection film 408 is formed on the end face 406 of the n-InP substrate 401 on the output waveguide 404 side. Has been. Further, on both sides of the multimode waveguide 403, highly reflective films 409 and 410 are disposed to face each other. Here, as the highly reflective films 409 and 410, a dielectric multilayer film or a metal film formed on the side wall of the multimode waveguide 403 can be used.

  The input signal light 411 that has entered the input waveguide 402 propagates through the input waveguide 402 and enters the multimode waveguide 403a. When the input signal light 411 enters the multimode waveguide 403a, the input signal light 411 is developed into the eigenmode in the multimode waveguide 403a. That is, the plurality of propagation modes in the multimode waveguide 403a are excited with a power distribution proportional to the overlap integral between the fundamental propagation mode of the input waveguide 402 and the plurality of propagation modes of the multimode waveguide 403a. Each mode excited in the multimode waveguide 403a propagates in the multimode waveguide 403a under a phase condition determined by the respective propagation constant.

The signal light propagated in the multimode waveguide 403a enters the connection waveguide 413a, propagates through the connection waveguide 413a, and then enters the multimode waveguide 403b. Then, the signal light incident on the multimode waveguide 403b propagates through the multimode waveguide 403b while being developed in multimode, and enters the connection waveguide 413b. Then, the signal light incident on the connection waveguide 413b propagates through the connection waveguide 413b, then enters the multimode waveguide 403c, and propagates through the multimode waveguide 403c while being developed in multimode. Then, the signal light that has propagated through the multimode waveguide 403 c enters the output waveguide 404, propagates through the output waveguide 404, and then exits from the end face 406 as output signal light 412.

  Here, when light propagates for a certain distance in each multimode waveguide 403a to 403c, the phase of light in each mode is intensified in each multimode waveguide 403a to 403c, and one or more It may be focused on the spot. This phenomenon is known as a self-imaging effect.

Here, to satisfy the equation (1) relationship, by setting the length L of each multimode waveguide 403a~403c the L _MMI, the signal light propagating in the multimode waveguide 403a~403c The light can be condensed into one spot. As a result, a self-imaging effect can be caused in each of the multimode waveguides 403a to 403c, and the signal light propagated in each of the multimode waveguides 403a to 403c is connected to the connection waveguides 413a and 413b and the output waveguide. Each can be coupled to the fundamental mode of the waveguide 404. For this reason, even when a plurality of propagation modes are excited in the multimode waveguides 403a to 403c, the coupling between the multimode waveguides 403a to 403c, the connection waveguides 413a and 413b, and the output waveguide 404 is performed. Loss can be reduced.

  In addition, since the cores of the input waveguide 402, the multimode waveguides 403a to 403c, the connection waveguides 413a and 413b, and the output waveguide 404 include a gain medium, the input signal light 411 is input to the input waveguide 402 and the multimode waveguide. 403a to 403c, connection waveguides 413a and 413b, and output waveguide 404 are amplified as they propagate through output waveguide 404, and an amplified output signal light 412 can be obtained.

  On the other hand, spontaneous emission light generated in each of the multimode waveguides 403a to 403c is emitted in all directions and reflected by the high reflection films 409 and 410 on both sides of each of the multimode waveguides 403a to 403c, Laser oscillation can be caused in a direction orthogonal to the waveguide direction of the input signal light 411. When laser oscillation occurs in a direction orthogonal to the waveguide direction of the input signal light 411, even when the intensity of the signal light incident on the multimode waveguides 403a to 403c varies, each multimode waveguide 403a. The carrier density of ˜403c can be kept constant, and the gain of the optical amplifying element can be clamped to a constant value.

For this reason, it is possible to amplify the input signal light 411 in the gain medium whose gain is clamped by oscillation while preventing the oscillation light from being mixed into the amplified light emitted from the output waveguide 404. As a result, it is possible to eliminate the need for a wavelength filter or an optical waveguide for separating the signal light and the oscillation light, and it is possible to suppress a gain variation due to the input light intensity while suppressing an increase in element size.
Further, by connecting a plurality of multi-mode waveguides 403a to 403c in multiple stages, it is possible to increase the overall length of the multi-mode waveguides 403a to 403c, increase the clamp gain, and reduce the drive current. Can be made.

FIG. 2 is a cross-sectional view illustrating a configuration example of the optical amplifying element cut along line AA ′ in FIG. 1.
In FIG. 2, in the multimode waveguide 403a of FIG. 1, an InGaAsP active layer 502 is formed on an n-InP substrate 501 in a stripe shape. Note that the width of the InGaAsP active layer 502 can be set so that light of a plurality of modes is propagated, and the width of the InGaAsP active layer 502 of the multimode waveguide 403 can be set to 20 μm, for example. Both sides of the InGaAsP active layer 502 are buried with a p-InP current blocking layer 503 and an n-InP current blocking layer 504 that are sequentially stacked on the n-InP substrate 501. Here, by embedding both sides of the InGaAsP active layer 502 with the p-InP current blocking layer 503 and the n-InP current blocking layer 504, a buried heterostructure can be configured.

  A p-InP cladding layer 505 is formed on the InGaAsP active layer 502 and the n-InP current block layer 504. Here, by forming the InGaAsP active layer 502 between the n-InP substrate 501 and the p-InP cladding layer 505, a multimode waveguide 403a made of a gain medium having the InGaAsP active layer 502 as a core is formed. Can do.

A p-GaInAs contact layer 506 is formed on the p-InP cladding layer 505. A p-side electrode 507 is formed on the p-GaInAs contact layer 506, and an n-side electrode 508 is formed on the back surface of the n-InP substrate 501. In addition, the InGaAsP active layer 502 embedded in the p-InP current blocking layer 503 and the n-InP current blocking layer 504 is etched in a mesa shape, and the p-InP current blocking layer 503, the n-InP current blocking layer 504, p The side walls of the InP cladding layer 505 and the p-GaInAs contact layer 506 are exposed. The side walls of the p-InP current blocking layer 503, the n-InP current blocking layer 504, the p-InP cladding layer 505, and the p-GaInAs contact layer 506 are highly reflective along the propagation direction of the input signal light 411. Films 509 and 510 are formed. As the highly reflective films 509 and 510, for example, a dielectric multilayer film such as TiO ₂ or SiO ₂ or a metal film such as Au can be used. The multimode waveguides 403b and 403c in FIG. 1 can have the same configuration.

  On the other hand, the configuration of the input waveguide 402, the connection waveguides 413a and 413b, and the output waveguide 404 cut along the line BB 'is the InGaAsP active layer of the input waveguide 402, the connection waveguides 413a and 413b, and the output waveguide 404. Except for the fact that the width of 502 is different from the width of the InGaAsP active layer 502 of the multimode waveguides 403a to 403c, the same configuration as that of the multimode waveguides 403a to 403c can be adopted. That is, the widths of the InGaAsP active layers 502 of the input waveguide 402, the connection waveguides 413a and 413b, and the output waveguide 404 are set so that single-mode light propagates, and the input waveguide 402 and the connection waveguide 413a. The width of the InGaAsP active layer 502 of 413b and the output waveguide 404 can be set to 0.8 μm, for example.

  In the case where the InGaAsP active layer 502, the p-InP current blocking layer 503, the n-InP current blocking layer 504, the p-InP cladding layer 505, and the p-GaInAs contact layer 506 are formed on the n-InP substrate 501, for example, Epitaxial growth such as MBE (Molecular Beam Epitaxy), MOCVD (Metal Organic Chemical Vapor Deposition), or ALCVD (Atomic Layer Chemical Vapor Deposition) can be used.

  Then, by applying a voltage to the p-side electrode 507, current can be injected into the InGaAsP active layer 502 while confining current in the n-InP current blocking layer 504. When a current is injected into the InGaAsP active layer 502, the InGaAsP active layer 502 can emit light. The light generated in the InGaAsP active layer 502 is reflected by the high reflection films 509 and 510 on both sides of the InGaAsP active layer 502, and laser oscillation is performed in a direction orthogonal to the waveguide direction of the input signal light 411 in FIG. Can be caused.

  That is, since laser oscillation occurs in the laser cavity composed of the highly reflective films 509 and 510 formed in the width direction of the multimode waveguide 403 and the InGaAsP active layer 502, the propagation direction of the oscillation light and the propagation direction of the signal light are determined. Can be orthogonal. For this reason, it is possible to prevent the oscillation light from being mixed into the input waveguide 402 and the output waveguide 404, and the oscillation light and the signal light can be spatially separated. For this reason, a wavelength filter or an optical waveguide for separating the signal light and the oscillation light can be dispensed with, and the gain fluctuation due to the input light intensity can be suppressed while suppressing an increase in the element size.

The multimode waveguides 403a to 403c are multimode waveguides having the InGaAsP active layer 502 as a core and a width W and a length L _MMI for signal light, and a width L _MMI and a length for oscillation light. Since it acts as a multimode waveguide having a length W, both the oscillation light and the signal light can be propagated in the horizontal direction (in-plane direction of the substrate). For this reason, the manufacturing process of a normal semiconductor laser and a semiconductor optical amplifier can be used as it is, and it becomes possible to suppress complication of the manufacturing process of the optical amplifying element and to ensure high reliability. .

  Furthermore, a sufficient space for forming the multimode waveguide 403 can be secured in the width direction of the InGaAsP active layer 502 that is the propagation direction of the oscillation light. For this reason, it is possible to propagate a greater distance than when the oscillation light propagates in the width direction or thickness direction of the single mode waveguide, and a large gain can be obtained. Therefore, oscillation can be caused even if the reflectivity of the high reflection films 409 and 410 is low to some extent, and the demand for the reflectivity of the high reflection films 409 and 410 is relaxed, and the production of the high reflection films 409 and 410 is made. Can be facilitated.

FIG. 3 is a plan view showing the operation principle of the optical amplifying element of the present invention. 3 corresponds to the case where only one multimode waveguide 403a to 403c of the optical amplifying element of FIG. 1 is provided.
That is, in FIG. 3, on an n-InP substrate 601, an input waveguide 602 for inputting the input signal light 611, a multimode waveguide 603 for guiding the input signal light 611, and an output waveguide for outputting the output signal light 612 are provided. A waveguide 604 is formed. Here, the input waveguide 602 and the output waveguide 604 can be composed of a single mode waveguide made of a gain medium having InGaAsP as a core, and the multimode waveguide 603 is made of a gain medium having InGaAsP as a core. It can be composed of a multimode waveguide.

An antireflection film 607 is formed on the end surface 605 of the n-InP substrate 601 on the input waveguide 602 side, and an antireflection film 608 is formed on the end surface 606 of the n-InP substrate 601 on the output waveguide 604 side. Has been. Further, on both sides of the multimode waveguide 603, highly reflective films 609 and 610 are disposed to face each other.
The input signal light 611 that has entered the input waveguide 602 propagates through the input waveguide 602 and enters the multimode waveguide 603. When the input signal light 611 enters the multimode waveguide 603, the input signal light 611 is developed into the eigenmode in the multimode waveguide 603. That is, the plurality of propagation modes in the multimode waveguide 603 are excited with a power distribution proportional to the overlap integral of the fundamental propagation mode of the input waveguide 602 and the plurality of propagation modes of the multimode waveguide 603. Each mode excited in the multimode waveguide 603 propagates in the multimode waveguide 603 under a phase condition determined by the respective propagation constant.

When the light propagates a certain distance, the phases of the light in each mode are intensified with each other in the multimode waveguide 603 and are condensed on one or a plurality of spots. Here, by setting the length L of the multimode waveguide 603 so as to satisfy the relationship of the expression (1), the signal light propagating in the multimode waveguide 603 can be condensed into one spot. it can. Then, the signal light propagated through the multimode waveguide 603 by the length L _MMI enters the output waveguide 604, propagates through the output waveguide 604, and then is emitted from the end face 606 as the output signal light 612.

  As a result, a self-imaging effect can be caused in the multimode waveguide 603, and the signal light propagated in the multimode waveguide 603 can be coupled to the fundamental mode of the output waveguide 604. For this reason, even when a plurality of propagation modes are excited in the multimode waveguide 603, the coupling loss between the multimode waveguide 603 and the output waveguide 604 can be reduced.

For example, when the width W of the multimode waveguide 603 is 20 μm, the equivalent refractive index n _eq of the multimode waveguide 603 is 3.24, the wavelength λ of the input signal light 411 is 1.55 μm, and m = 1, The length L _MMI of the waveguide 603 can be set to 836 μm.
Further, since the cores of the input waveguide 602, the multimode waveguide 603, and the output waveguide 604 include a gain medium, the input signal light 611 propagates through the input waveguide 602, the multimode waveguide 603, and the output waveguide 604. And the amplified output signal light 612 can be obtained.

  On the other hand, the spontaneous emission light generated in the multimode waveguide 603 is emitted in all directions and reflected by the high reflection films 609 and 610 on both sides of the multimode waveguide 603, thereby guiding the input signal light 611. Laser oscillation can be caused in a direction orthogonal to the wave direction. When laser oscillation occurs in a direction orthogonal to the waveguide direction of the input signal light 611, the carrier density of the multimode waveguide 603 is reduced even when the intensity of the signal light incident on the multimode waveguide 603 varies. It can be kept constant, and the gain of the optical amplifying element can be clamped to a constant value.

  Therefore, it is possible to amplify the input signal light 611 in the gain medium whose gain is clamped by oscillation while preventing the oscillation light from being mixed into the amplified light emitted from the output waveguide 604. As a result, it is possible to eliminate the need for a wavelength filter or an optical waveguide for separating the signal light and the oscillation light, and it is possible to suppress a gain variation due to the input light intensity while suppressing an increase in element size.

Here, the configuration of the optical amplifying element cut along the line AA ′ in FIG. 3 can be the same as the configuration in FIG. 2, and the configuration of the optical amplifying element cut along the line BB ′ in FIG. The configuration can be the same as that of the optical amplifying element cut along line BB ′.
For example, it is assumed that the reflectance _RH of the highly reflective films 509 and 510 is 0.9, that is, 90%. In this case, when converted to the decibel display, it becomes 10 × log (R _H ), which corresponds to a reflection loss of 0.46 dB. The spontaneous emission light is emitted in all directions in the multimode waveguide 603, and the light traveling or guided in the width direction of the multimode waveguide 603 is reflected by the high reflection films 509 and 510.

  Here, since the reflection loss is 0.46 dB, if there is a gain of 0.46 dB while this reflected light propagates by the distance W in the width direction of the multimode waveguide 603, the reflection loss and the gain are balanced. As a result, a laser cavity composed of the highly reflective films 509 and 510 and the InGaAsP active layer 502 is formed in the width direction of the multimode waveguide 603, and laser oscillation can be caused in the width direction of the multimode waveguide 603.

  For example, if the width W of the multimode waveguide 603 is 20 μm, laser oscillation can be caused if the gain of the multimode waveguide 603 is only 0.46 dB / 20 μm. When laser oscillation occurs in the multi-mode waveguide 603, the carrier density of the InGaAsP active layer 602 can be kept constant even when the intensity of the signal light incident on the InGaAsP active layer 502 fluctuates. The gain of the element can be clamped to a constant value.

On the other hand, considering the gain of the signal light propagating in the axial direction through the multimode waveguide 403, the length of the multimode waveguide 603 is L _MMI = 836 μm, and the gain of the multimode waveguide 603 is 0.46 dB / 20 μm. It is clamped. For this reason, the gain of the signal light when propagating through the multimode waveguide 603 is
836 μm × (0.46 dB / 20 μm) = 19 dB (2)
It is clamped with.

When the gain of the multimode waveguide 603 is clamped, even if a current is further injected into the multimode waveguide 603, this current is consumed to increase the power of the oscillation light, and the gain of the signal light is increased. There is no contribution. On the other hand, since the antireflection films 607 and 608 are respectively provided on the incident side end surface 605 and the emission side end surface 606, the residual reflectance R _AR is suppressed to 0.1% or less (−30 dB or less). For this reason, even if the gain of the multimode waveguide 603 is only 19 dB, oscillation does not occur in the propagation direction of the input signal light 411, and a traveling wave type optical amplification operation is performed.

FIG. 4 is a diagram showing saturation characteristics of the optical amplifying element of FIG.
In FIG. 4, when a current is injected into the multimode waveguide 603, the gain increases as the current increases. When the current reaches I 1, the gain when propagating by the distance W in the width direction of the multimode waveguide 603 reaches the threshold gain G _lateral , and oscillation occurs in the width direction of the multimode waveguide 603. At this time, the threshold gain G _lateral is
G _lateral = −10 × log (R _H ) (dB) (3)
Given in.

Even when the current is further increased and the current reaches I2 (> I1), oscillation has already occurred in the multimode waveguide 603, so that the carrier density is clamped to a constant value and the gain does not increase. That is, in the conventional optical amplifier, clamp even beyond the current I1, while the gain with increasing current increases monotonically, in the present embodiment, the gain of the multimode waveguide 603 to G ₀ Can be made. At this time, the gain G _{0 of the} signal light is
G ₀ = G _lateral × L _MMI / W (4)
Given in. Here, the gain G ₀ of the signal light is obtained by using the equations (1) and (3):
G ₀ = −10 × log (R _H ) × n _eq × W / λ (5)
It becomes.

In an operating state where the current is I1 or more, that is, in a state where the gain is clamped to G ₀ because oscillation occurs in the width direction of the multimode waveguide 603, the intensity of the input signal light 611 in FIG. Even when the intensity is increased, the total light intensity of the oscillation light and the signal light inside the optical amplifying element is kept constant only by reducing the intensity of the oscillation light. For this reason, the carrier density of the gain medium in the optical amplifying element does not vary, and the gain of the optical amplifying element is kept constant as shown in FIG. As a result, even when the wavelength multiplexing number of the input signal light 611 is changed, gain variation can be suppressed and the wavelength multiplexing optical transmission system can be operated stably.

Here, the optical amplifying element in FIG. 3 uses one multimode waveguide 603, but the optical amplifying element in FIG. 1 has a plurality of multimode waveguides 403a to 403c connected in multiple stages. For example, the operating principle of the optical amplifying element in FIG. 1 is the same as the operating principle of the optical amplifying element in FIG.
Then, in the configuration of FIG. 1, by multistage connection of a plurality of multimode waveguide 403a~403c length is set to L _MMI respectively, while enabling the expression of the self-coupling effect, make clamp gain It becomes possible. For this reason, it is possible to reduce the size of the multimode waveguides 403a to 403c while keeping the injection current density constant, and the signal light can be transmitted to the fundamental modes of the connection waveguides 413a and 413b and the output waveguide 404 with low loss. The drive current can be reduced while enabling the coupling.

For example, it is assumed that the reflectance _RH of the high reflection films 409 and 410 is 0.94, that is, 94%. Then, when the width W = 10 [mu] m of each multimode waveguide 403a to 403c, (5) the formula, the clamp gain of the signal light when the multimode waveguide 403a to 403c propagated only L _MMI is respectively 5.6dB Become. At this time, in each of the multimode waveguides 403a to 403c, the length L _MMI until the light is first condensed in a spot shape is 209 μm according to the equation (1). For this reason, when the multimode waveguides 403a to 403c are cascade-connected in three stages, the clamp gain for one stage of the multimode waveguides 403a to 403c is 5.6 dB. Is 16.8B.

Further, in the embodiment of FIG. 3, the width W and the length L of the multimode waveguide 603 are 20 μm and 836 μm, respectively, whereas in the embodiment of FIG. 1, the width W of the multimode waveguides 403a to 403c. Since the length L is 10 μm and 209 μm, respectively, the area is 1/8 compared.
On the other hand, in the embodiment of FIG. 3, the threshold gain G _lateral is 0.46 dB / 20 μm, and the oscillation threshold gain density per unit length is 0.023 (dB / μm), whereas In the embodiment of FIG. 1, the threshold gain G _lateral is 0.27 dB / 10 μm, and the oscillation threshold gain density per unit length is 0.027 (dB / μm). Therefore, the injection current density is considered to be almost the same. Therefore, even when the multi-mode waveguides 403a to 403c have a three-stage cascade connection configuration, the entire area of the multi-mode waveguides 403a to 403c is 3/8. It can be driven by current.

  The configuration of the waveguide including the gain medium is not particularly limited, and may be applied to all layer structures used in ordinary optical amplifying elements. That is, the shape of the InGaAsP active layer 502 may be MQW (multiple quantum well), quantum wire, quantum dot, etc. in addition to the bulk, and a separate confinement heterostructure (SCH) or A gradient refractive index confinement structure (GRIN-SCH) in which the refractive index is gradually changed may be used. For example, an InGaAsP separation confinement layer or a light guide layer having a band gap wavelength between the gain medium and the InP clad may be provided above or below the gain medium. Further, the material is not limited to the combination of InP and InGaAsP, and other semiconductor materials such as GaAs, AlGaAs, GaInAs, GaInNAs, and AlGaAsP may be used.

As for the waveguide structure, a pn buried structure, a ridge structure, a semi-insulating buried structure, and a high mesa structure may be used. Further, the substrate is not limited to the n-type substrate, and a p-type substrate or a semi-insulating substrate may be used.
In the above-described embodiment, the case where the gain medium is included in the core from the input waveguide 402 to the output waveguide 404 through the multimode waveguide 403 along the propagation path of the signal light has been described. Alternatively, a gain medium may be provided at least at a part of the core or the cladding of the multimode waveguide 403.

  In the above-described embodiment, the case where m = 1 has been described. However, the lengths of the multimode waveguides 403a to 403c may satisfy the equation (1) independently, and are not limited to m = 1. For example, m = 2 may be set for the multimode waveguide 403a, m = 1 for the multimode waveguide 403b, and m = 4 for the multimode waveguide 403c. Any number of multimode waveguides 403a to 403c may be used. Even when the integer m is used, the effect of the present invention can be expected. Furthermore, in the above-described embodiment, the method of connecting the multimode waveguides 403a to 403c in three stages has been described. However, the number of stages is not limited to three, and any number of stages may be used as long as the configuration is n (n is a positive integer). .

FIG. 5 is a cross-sectional view showing a schematic configuration of an optical amplifying element according to the second embodiment of the present invention. 5 corresponds to the components of the optical amplifying element cut along the line AA ′ in FIG. 1, and the configuration of the horizontal waveguide in the embodiment of FIG. 5 is the same as that in FIG. Can be taken.
In FIG. 5, an InGaAsP separation and confinement layer 711 is formed on an n-InP substrate 701. The InGaAsP separation confinement layer 711 can be configured to be transparent with respect to the wavelength of the signal light. An InGaAsP active layer 702 is formed in a stripe shape on the InGaAsP isolation and confinement layer 711. Note that the width of the InGaAsP active layer 702 can be set so that light of a plurality of modes is propagated in the multimode waveguide.

  Both sides of the InGaAsP active layer 702 are buried with a p-InP current blocking layer 703 and an n-InP current blocking layer 704 that are sequentially stacked on the InGaAsP isolation and confinement layer 711. A p-InP cladding layer 705 is formed on the InGaAsP active layer 702 and the n-InP current blocking layer 704.

  A p-GaInAs contact layer 706 is formed on the p-InP cladding layer 705. A p-side electrode 707 is formed on the p-GaInAs contact layer 706, and an n-side electrode 708 is formed on the back surface of the n-InP substrate 701. Further, the InGaAsP active layer 702 buried in the p-InP current blocking layer 703 and the n-InP current blocking layer 704 is etched in a mesa shape, and the InGaAsP isolation confinement layer 711, the p-InP current blocking layer 703, the n-InP The side walls of the current blocking layer 704, the p-InP cladding layer 705, and the p-GaInAs contact layer 706 are exposed. The side walls of the InGaAsP isolation and confinement layer 711, the p-InP current blocking layer 703, the n-InP current blocking layer 704, the p-InP cladding layer 705, and the p-GaInAs contact layer 706 are along the propagation direction of the signal light. Thus, highly reflective films 709 and 710 are formed.

  The configuration of the input waveguide and the output waveguide is the same as that of the multimode waveguide except that the width of the InGaAsP active layer 702 in the input waveguide and the output waveguide is different from the width of the InGaAsP active layer 702 in the multimode waveguide. A configuration similar to that of 703 can be employed. That is, the width of the InGaAsP active layer 702 in the input waveguide and the output waveguide can be set so that single mode light is propagated.

  Then, by applying a voltage to the p-side electrode 707, current can be injected into the InGaAsP active layer 702 while confining current in the n-InP current blocking layer 704. When a current is injected into the InGaAsP active layer 702, the InGaAsP active layer 702 can emit light. The light generated in the InGaAsP active layer 702 is reflected by the high reflection films 709 and 710 on both sides of the InGaAsP active layer 702, causing laser oscillation in a direction orthogonal to the waveguide direction of the signal light. it can.

  Here, by providing the InGaAsP isolation and confinement layer 711 extending in the direction of the high reflection films 709 and 710 in the lower layer of the InGaAsP active layer 702, the core for the oscillation light can be extended to the high reflection films 709 and 710. For this reason, the light generated in the InGaAsP active layer 702 can be guided to the highly reflective films 709 and 710 while being guided by the InGaAsP separation confinement layer 711, and the loss of oscillation light in the cavity can be reduced. It becomes. For this reason, it becomes possible to efficiently feed back the oscillation light in the multimode waveguide, and the requirement for the reflectivity of the highly reflective films 709 and 710 can be relaxed.

FIG. 6 is a plan view showing a schematic configuration of the optical amplifying element according to the third embodiment of the present invention.
In FIG. 6, on an n-InP substrate 901, an input waveguide 902 that is connected to a multimode waveguide 903a and receives input signal light 911, and a plurality of multimode waveguides 903a to 903c that guide signal light, An output waveguide that connects the multimode waveguides 903a and 903b to each other, a connection waveguide 913b that connects the multimode waveguides 903b and 903c to each other, and a multimode waveguide 903c that outputs the output signal light 912. A waveguide 904 is formed.

Here, the input waveguide 902 and the output waveguide 904 can be composed of single mode waveguides having InGaAsP as a core transparent to the wavelength of the input signal light 911, and the multimode waveguides 903a to 903c are A multimode waveguide made of a gain medium having InGaAsP as a core can be used, and the connection waveguides 913a and 913b are made of a single mode waveguide having InGaAsP as a core transparent to the wavelength of the input signal light 911. can do. Further, the input waveguide 902, the multimode waveguides 903a to 903c, the connection waveguides 913a and 913b, and the output waveguide 904 can be arranged side by side on the n-InP substrate 901 so that the waveguide center axes coincide with each other. . Further, the length L of each of the multimode waveguides 903a to 903c can be set so as to satisfy the relationship of the expression (1). The width W of each of the multimode waveguides 903a to 903c can be set so that light of a plurality of modes is propagated, and the widths of the input waveguide 902, the connection waveguides 913a and 913b, and the output waveguide 904 are It can be set so that single mode light is propagated.
Further, an antireflection film 907 is formed on the end surface 905 of the n-InP substrate 901 on the input waveguide 902 side, and an antireflection film 908 is formed on the end surface 906 of the n-InP substrate 901 on the output waveguide 904 side. Has been. Further, on both sides of the multimode waveguide 903, highly reflective films 909 and 910 are arranged to face each other.

FIG. 7 is a cross-sectional view illustrating a configuration example of the optical amplifying element cut along line AA ′ in FIG. 6.
In FIG. 7, an InGaAsP input waveguide core layer 1002, InGaAsP active layers 1003a to 1003c, InGaAsP connection waveguide core layers 1009a to 1009b, and an InGaAsP output waveguide core layer 1004 are formed on an n-InP substrate 1001. . Here, the InGaAsP active layer 1003a is disposed between the InGaAsP input waveguide core layer 1002 and the InGaAsP connection waveguide core layer 1009a, and the InGaAsP active layer 1003b is disposed between the InGaAsP connection waveguide core layers 1009a and 1009b. The InGaAsP active layer 1003c is disposed between the InGaAsP connection waveguide core layer 1009b and the InGaAsP output waveguide core layer 1004.

Note that the widths of the InGaAsP active layers 1003a to 1003c can be set so that light of a plurality of modes is propagated, and the widths of the InGaAsP active layers 1003a to 1003c can be set to 20 μm, for example.
A p-InP cladding layer 1005 is formed on the InGaAsP input waveguide core layer 1002, the InGaAsP active layers 1003a to 1003c, the InGaAsP connection waveguide core layers 1009a to 1009b, and the InGaAsP output waveguide core layer 1004. Here, by forming InGaAsP active layers 1003a to 1003c between the n-InP substrate 1001 and the p-InP clad layer 1005, a multimode waveguide 903a made of a gain medium having InGaAsP active layers 1003a to 1003c as a core is formed. ˜903c can be configured respectively.

  On the p-InP cladding layer 1005, p-GaInAs contact layers 1006a to 1006c are formed. Here, the p-GaInAs contact layers 1006a to 1006c are divided and arranged so as to correspond to the InGaAsP active layers 1003a to 1003c, respectively. Further, p-side electrodes 1007a to 1007c are formed on the p-GaInAs contact layers 1006a to 1006c, respectively, and an n-side electrode 1008 is formed on the back surface of the n-InP substrate 1001.

  Here, by dividing each of the p-GaInAs contact layers 1006a to 1006c and the p-side electrodes 1007a to 1007c, it becomes possible to efficiently inject current into the InGaAsP active layers 1003a to 1003c, and to achieve an InGaAsP input waveguide core. Current can be prevented from being injected into the layer 1002, the InGaAsP connection waveguide core layers 1009a to 1009b, and the InGaAsP output waveguide core layer 1004, and free carrier absorption loss can be reduced.

  In FIG. 6, the input signal light 911 that has entered the input waveguide 902 propagates through the input waveguide 902 and enters the multimode waveguide 903a. When the input signal light 911 enters the multimode waveguide 903a, a plurality of propagation modes in the multimode waveguide 903a are excited and propagate in the multimode waveguide 903a under phase conditions determined by the respective propagation constants. To do. The light propagated in the multimode waveguide 903a enters the connection waveguide 913a, propagates through the connection waveguide 913a, and then enters the multimode waveguide 903b. Then, the signal light that has entered the multimode waveguide 903b propagates through the multimode waveguide 903b while being developed in multimode, and enters the connection waveguide 913b. Then, the signal light incident on the connection waveguide 913b propagates through the connection waveguide 913b, then enters the multimode waveguide 903c, and propagates through the multimode waveguide 903c while being developed in multimode. Then, the signal light propagated through the multimode waveguide 903 c enters the output waveguide 904, propagates through the output waveguide 904, and then is emitted from the end face 906 as the output signal light 912.

In addition, by setting the length L of each of the multimode waveguides 903a to 903c so as to satisfy the relationship of the expression (1), the signal light propagated in each of the multimode waveguides 903a to 903c is connected to the connection waveguide. 913a and 913b can be coupled to the fundamental mode of the output waveguide 904, respectively, and the coupling loss between the multimode waveguides 903a to 903c and the connection waveguides 913a and 913b and the output waveguide 904 can be reduced. it can.
Here, since the core of each of the multimode waveguides 903a to 903c includes a gain medium, the input signal light 911 is amplified as it propagates through each of the multimode waveguides 903a to 903c to obtain an amplified output signal light 912. be able to.

  On the other hand, the spontaneous emission light generated in each of the multimode waveguides 903a to 903c is emitted in all directions and reflected by the high reflection films 909 and 910 on both sides of the multimode waveguides 903a to 903c, thereby inputting the light. Laser oscillation can be caused in a direction orthogonal to the waveguide direction of the signal light 911. When laser oscillation occurs in a direction orthogonal to the waveguide direction of the input signal light 911, even when the intensity of the signal light incident on the multimode waveguides 903a to 903c varies, the multimode waveguides 903a to 903a The carrier density of 903c can be kept constant, and the gain of the optical amplifying element can be clamped to a constant value.

  For this reason, even when the input waveguide 902 and the output waveguide 904 have a butt joint configuration, the gain is clamped by oscillation while preventing the oscillation light from being mixed into the amplified light emitted from the output waveguide 904. The input signal light 911 can be amplified in the gain medium. As a result, it is possible to eliminate the need for a wavelength filter or an optical waveguide for separating the signal light and the oscillation light, and it is possible to suppress a gain variation due to the input light intensity while suppressing an increase in element size.

  In the above-described embodiment, the connection waveguides 913a and 913b and the output waveguide 904 are configured by single mode waveguides having a core made of a material transparent to the wavelength of the input signal light 911. For this reason, even when the signal light amplified by the multimode waveguides 903a to 903c is incident on the connection waveguides 913a and 913b and the output waveguide 904 in a condensed state, and the optical power density is increased. Waveform deterioration due to gain saturation can be prevented. Further, the input waveguide 902, the connection waveguides 913 a and 913 b, and the output waveguide 904 are formed from a single mode waveguide whose core is made of a material transparent to the wavelength of the input signal light 911, whereby the input waveguide 902. Therefore, it is not necessary to inject current into the connection waveguides 913a and 913b and the output waveguide 904, and the drive current can be reduced.

  In the above-described embodiment, the method of configuring all of the input waveguide 902, the connection waveguides 913a and 913b, and the output waveguide 904 using a material that is transparent with respect to the wavelength of the input signal light 911 has been described. The input waveguide 902, the connection waveguides 913a and 913b, and the output waveguide 904 are not necessarily configured using a material that is transparent to the wavelength of the input signal light 911. The input waveguide 902 and the connection waveguide 913a are not necessarily required. 913b and the output waveguide 904 may be made of a material transparent to the wavelength of the input signal light 911. For example, when it is desired to reduce the current, it is effective to configure the input waveguide 902, the connection waveguides 913a and 913b, and the output waveguide 904 using a material that is transparent with respect to the wavelength of the input signal light 911. However, when NF is more important, the input waveguide 902 is preferably made of a material having a gain with respect to the wavelength of the input signal light 911.

  Moreover, although the case where the gain medium is included in the core for all of the multimode waveguides 903a to 903c has been described, the gain medium may be provided at least in part of the core or the cladding of the multimode waveguides 903a to 903c. Also in the embodiment of FIG. 6, various modifications can be made to the configuration of the waveguide, the composition and structure of the core layer or gain medium, the presence or absence of the SCH structure, and the like as in the embodiment of FIG.

FIG. 8 is a plan view showing a schematic configuration of the optical amplifying element according to the fourth embodiment of the present invention.
In FIG. 8, on an n-InP substrate 1101, an input waveguide 1102 that is connected to a multimode waveguide 1103a and receives input signal light 1111 and a plurality of multimode waveguides 1103a to 1103c that guide signal light, An output waveguide that connects the multimode waveguides 1103a and 1103b to each other, a connection waveguide 1113b that connects the multimode waveguides 1103b and 1103c to each other, and a multimode waveguide 1103c that outputs the output signal light 1112. A waveguide 1104 is formed.

  Here, the input waveguide 1102 and the output waveguide 1104 can be constituted by a single mode waveguide made of a gain medium having InGaAsP as a core, and the multimode waveguides 1103a to 1103c are gain media having InGaAsP as a core. The connection waveguides 1113a and 1113b can be formed of a single mode waveguide made of a gain medium having InGaAsP as a core.

Further, the input waveguide 1102, the multimode waveguides 1103a to 1103c, the connection waveguides 1113a and 1113b, and the output waveguide 1104 can be arranged side by side on the n-InP substrate 1101 so that the waveguide center axes coincide with each other. . Further, the widths W ₁ , W ₂ , and W ₃ of the multimode waveguides 1103a to 1103c can be set to be different from each other so that light of a plurality of modes is propagated. Further, the widths of the input waveguide 1102, the connection waveguides 1113a and 1113b, and the output waveguide 1104 can be set so that single-mode light is propagated. In addition, the lengths L ₁ , L ₂ , and L ₃ of the multimode waveguides 1103a to 1103c can be set so as to satisfy the relationship of the expression (1).

  Further, an antireflection film 1107 is formed on the end surface 1105 of the n-InP substrate 1101 on the input waveguide 1102 side, and an antireflection film 1108 is formed on the end surface 1106 of the n-InP substrate 1101 on the output waveguide 1104 side. Has been. Furthermore, highly reflective films 1109 and 1110 are arranged opposite to each side of the multimode waveguides 1103a to 1103c.

FIG. 9 is a cross-sectional view illustrating a configuration example of the optical amplifying element cut along line AA ′ in FIG. 8.
In FIG. 9, an InGaAsP active layer 1202 is formed on an n-InP substrate 1201. Here, the width of the InGaAsP active layer 1202 is set so that light of a plurality of modes is propagated in the multimode waveguides 1103a to 1103c, and is set to be different for each of the multimode waveguides 1103a to 1103c. The input waveguide 1102, the connection waveguides 1113a and 1113b, and the output waveguide 1104 can be set so that single mode light is propagated.

  A p-InP cladding layer 1205 is formed on the InGaAsP active layer 1202. Here, by forming an InGaAsP active layer 1202 between the n-InP substrate 1201 and the p-InP clad layer 1205, the multimode waveguides 1103a to 1103c made of a gain medium having the InGaAsP active layer 1202 as a core are formed. Can be configured.

Then, p-GaInAs contact layers 1206 a to 1206 c are formed on the p-InP cladding layer 1205. Here, the p-GaInAs contact layers 1206a to 1206c are divided and arranged so as to correspond to the multimode waveguides 1103a to 1103c, respectively. In addition, p-side electrodes 1207 a to 1207 c are formed on the p-GaInAs contact layers 1206 a to 1206 c, respectively, and an n-side electrode 1208 is formed on the back surface of the n-InP substrate 1201.
Here, by dividing the p-GaInAs contact layers 1206a to 1206c and the p-side electrodes 1207a to 1207c, the injection current density can be controlled independently for each of the multimode waveguides 1103a to 1103c.

  In FIG. 8, the input signal light 1111 that has entered the input waveguide 1102 propagates through the input waveguide 1102 and enters the multimode waveguide 1103a. When the input signal light 1111 enters the multimode waveguide 1103a, a plurality of propagation modes in the multimode waveguide 1103a are excited and propagate in the multimode waveguide 1103a under phase conditions determined by the respective propagation constants. To do. Then, the light propagated in the multimode waveguide 1103a enters the connection waveguide 1113a, propagates through the connection waveguide 1113a, and then enters the multimode waveguide 1103b. Then, the signal light incident on the multimode waveguide 1103b propagates through the multimode waveguide 1103b while being developed in multimode, and enters the connection waveguide 1113b. Then, the signal light incident on the connection waveguide 1113b propagates through the connection waveguide 1113b, then enters the multimode waveguide 1103c, and propagates through the multimode waveguide 1103c while being developed in multimode. The signal light that has propagated through the multimode waveguide 1103 c enters the output waveguide 1104, propagates through the output waveguide 1104, and is then emitted from the end face 1106 as output signal light 1112.

The lengths L ₁ , L ₂ , and L ₃ of the multimode waveguides 1103a to 1103c are set so as to satisfy the relationship of the expression (1), thereby propagating through the multimode waveguides 1103a to 1103c. Can be coupled to the fundamental modes of the connection waveguides 1113a and 1113b and the output waveguide 1104, respectively, and between the multimode waveguides 1103a to 1103c and the connection waveguides 1113a and 1113b and the output waveguide 1104. The coupling loss can be reduced.
Here, since the core of each of the multimode waveguides 1103a to 1103c includes a gain medium, the input signal light 1111 is amplified as it propagates through each of the multimode waveguides 1103a to 1103c, and an amplified output signal light 1112 is obtained. be able to.

  On the other hand, the spontaneous emission light generated in each of the multimode waveguides 1103a to 1103c is emitted in all directions and reflected by the high reflection films 1109 and 1110 on both sides of the multimode waveguides 1103a to 1103c. Laser oscillation can be caused in a direction orthogonal to the waveguide direction of the signal light 1111. When laser oscillation occurs in a direction orthogonal to the waveguide direction of the input signal light 1111, each of the multimode waveguides 1103 a to 1103 a is changed even when the intensity of the signal light incident on the multimode waveguides 1103 a to 1103 c varies. The carrier density of 1103c can be kept constant, and the gain of the optical amplifying element can be clamped to a constant value.

For this reason, even when the widths W ₁ , W ₂ , and W _{3 of the} multimode waveguides 1103 a to 1103 c are set to be different from each other, the oscillation light is prevented from being mixed into the amplified light emitted from the output waveguide 1104. However, the input signal light 1111 can be amplified in the gain medium whose gain is clamped by oscillation. As a result, it is possible to eliminate the need for a wavelength filter or an optical waveguide for separating the signal light and the oscillation light, and it is possible to suppress a gain variation due to the input light intensity while suppressing an increase in element size.

In addition, by setting the widths W ₁ , W ₂ , and W ₃ of the multimode waveguides 1103 a to 1103 c to be different from each other, it is possible to change the injected carrier density for each of the multimode waveguides 1103 a to 1103 c. For this reason, it is possible to increase the saturated output power or improve NF while gaining gain, and various operation states can be realized according to the use of the optical amplifying element.

Now, the reflectance _{R H} of the high reflection film 1109 is assumed to be 0.94 (94%). At this time, the oscillation threshold gain of the multimode waveguides 1103a to 1103c is 0.27 dB according to the equation (3). Now, assuming that the widths W ₁ , W ₂ , and W _{3 of the} multimode waveguides 1103 a to 1103 c are 8 μm, 10 μm, and 12 μm, respectively, the signal light when propagating through the multimode waveguides 1103 a to 1103 c according to the equation (5) The clamp gains are 4.5 dB, 5.6 dB, and 6.7 dB, respectively. Also, the lengths L _{1 to} L ₃ of the multimode waveguides 1103a to 1103c at this time are expressed as L _MMI as L _i , n _eq as n _eqi , W as W _i , and m as m _{i in} equation (1). Further, when m _i = 1, they are 134 μm, 209 μm, and 301 μm, respectively. Therefore, as shown in the present embodiment, when the multimode regions having the waveguide widths W ₁ , W ₂ , and W ₃ are connected in three stages vertically by the single mode connection waveguides 1113a and 1113b, the clamp gain is obtained in the entire three stages. Is 16.8 dB. This is the same value as the configuration of the first embodiment shown in FIG.

  Here, when considering the gain per unit length in the multimode regions having different widths, 0.27 dB / 8 μm = 0.03375 B / μm in the region of 8 μm width, and 0 in the region of 10 μm width. .27 dB / 10 μm = 0.027 dB / μm and the width of 12 μm is 0.27 dB / 12 μm = 0.0225 dB / μm, and the narrow width region has a higher gain per unit length. You can see that Since the gain per unit length of the semiconductor medium is proportional to the carrier density at the time of non-saturation, the narrower the width of the multimode region, the higher the operation state. Therefore, as a matter of course, the current injected into the p-side electrodes 1207a to 1207c is controlled so that the injection current density is higher in a narrower region in a plurality of multimode regions.

In general, it is known that NF improves as the injected carrier density increases. Therefore, as in this embodiment, by setting the waveguide width on the input side in order so that the output side becomes wider like W ₁ <W ₂ <W ₃ , the NF becomes better toward the input side. It becomes possible to operate the optical amplifier. The NF of the entire element is almost determined by the NF on the input side. This is because the spontaneous emission light is superimposed on the input signal light as noise added near the input end, whereas the spontaneous emission light superimposed on the input signal light and the incident end (except the vicinity of the input end) ( This is because the change in the ratio between the signal light and the noise becomes small because both (noise) are amplified. Therefore, the NF of the entire device can be improved by narrowing the waveguide width W ₁ so that the NF is improved in the vicinity of the input end as in this embodiment.

  On the other hand, the increase in saturation output is due to the following reason. That is, as described in the first embodiment, the optical amplifying element of the present invention is in a state where the total light intensity or photon density of the signal light and the oscillation light inside the active layer having gain is constant. Operate. Gain saturation occurs when the signal light intensity increases and the oscillation light intensity becomes zero. In this embodiment, since a multimode waveguide is used, in order to increase the saturation output, a structure that lowers the intensity distribution of signal light or the photon density distribution inside the active layer may be used. . Now, considering the intensity of signal light, the light intensity or photon density can be reduced by increasing the mode cross-sectional area of the waveguide mode of the signal light. In the case of the present embodiment, since the signal light propagates through the multi-mode waveguide, the power of the signal light is distributed to more waveguide modes by widening the waveguide width, and spreads over the entire waveguide width. In addition, the light intensity or photon density is reduced. In the case of a single mode waveguide, there is an upper limit on the waveguide width due to the single mode condition. However, since the multimode waveguide is used in this embodiment, there is no restriction on the waveguide width.

  Moreover, since the width W of the multimode waveguide and the length L of the multimode waveguide satisfy the relationship of the expression (1), the signal light propagating in the multimode waveguide is condensed by the self-imaging effect. All are output as single mode. Therefore, the saturation output when using a multimode waveguide is the signal light intensity (saturation output density) when the intensity of the oscillation light becomes zero and the intensity of the signal light propagating in the multimode waveguide ( The saturation output is proportional to the product of the saturation output density and the mode cross-sectional area, or the waveguide width (more precisely, the waveguide cross-sectional area). The input signal light is amplified without being saturated in the multimode waveguide until the saturation power density is reached, and is condensed and output by the self-imaging effect.

Therefore, simply increasing the waveguide width increases (saturated output density × waveguide width), and it is possible to improve the saturated output. For example, if the waveguide width is doubled, the average light intensity (photon density) in the waveguide is halved. Thus, by widening the waveguide width, a margin of double is produced before the signal light intensity reaches the saturation power density. Conversely, since the waveguide width is doubled, the total power output when the signal light intensity reaches the saturation power density is also doubled. This corresponds to the saturation output of the element being doubled.
In this way, by setting the width of the multimode waveguide as W ₁ <W ₂ <W ₃ , the NF is improved while obtaining the same gain as the configuration of the first embodiment shown in FIG. The effect of increasing the saturation output power is expected.

In the above-described embodiment, the method has been described in which the widths W ₁ , W ₂ , and W _{3 of the} multimode waveguides 1103a to 1103c are gradually increased from the input side to the output side. For example, the widths W ₁ , W ₂ , and W ₃ of the multimode waveguides 1103 a to 1103 c are set to W ₁ <W ₂ > W, without being limited to a method of gradually increasing from the input side to the output side. _It may be set to be ₃ . In this case, not only the output-side multimode waveguide 1103c but also the input-side multimode waveguide 1103a can be operated in a high injection state with a high carrier density, and an application in which signal light is incident from both directions. It becomes a structure suitable for. As described above, one of the merits of this configuration is that the width relationship of the multimode area can be arbitrarily determined according to the purpose. On the contrary, it can be said that one of the features of this embodiment is that various operation states can be easily realized simply by changing the width of the multi-mode region.

  In the embodiment of FIG. 8 as well, the configuration of the waveguide, the composition and structure of the core layer or gain medium, the presence or absence of the SCH structure, and the like can be variously modified as in the embodiment of FIG. In the embodiment of FIG. 8, the input waveguide 1102 and the output waveguide 1104 have also been described with respect to a method of configuring a single mode waveguide made of a gain medium having InGaAsP as a core. The waveguide 1102 and the output waveguide 1104 may be configured using a medium that is transparent with respect to the wavelength of the input signal light 1111. The connection waveguides 1113a and 1113b may also be configured using a medium transparent to the wavelength of the input signal light 1111.

  In the above-described embodiment, the case where m = 1 has been described. However, the lengths of the multimode waveguides 1103a to 1103c may satisfy the equation (1) independently and are limited to m = 1. The effect of the present invention can also be expected when an arbitrary integer m is used for each of the multimode waveguides 1103a to 1103c. Furthermore, in the above-described embodiment, the method of connecting the multimode waveguides 1103a to 1103c in three stages has been described. However, the number of stages is not limited to three, and any number of stages may be used as long as the configuration is n (n is a positive integer). .

FIG. 10 is a plan view showing a schematic configuration of the optical amplifying element according to the fifth embodiment of the present invention.
In FIG. 10, on an n-InP substrate 1301, an input waveguide 1302 for inputting input signal light 1311 connected to the multimode waveguide 1303a, and a plurality of multimode waveguides 1303a to 1303c for guiding the signal light, An output waveguide 1304 that is connected to the multimode waveguide 1303c and outputs the output signal light 1312 is formed.

  Here, the multimode waveguide 1303a is directly connected to the multimode waveguide 1303b, and the multimode waveguide 1303b is directly connected to the multimode waveguide 1303c. Further, the input waveguide 1302 and the output waveguide 1304 can be constituted by a single mode waveguide made of a gain medium having InGaAsP as a core, and the multimode waveguides 1303a to 1303c are made of a gain medium having InGaAsP as a core. The multimode waveguide can be configured as follows.

Further, the input waveguide 1302, the multimode waveguides 1303a to 1303c, and the output waveguide 1304 can be arranged side by side on the n-InP substrate 1301 so that the waveguide center axes coincide with each other. Further, the widths W ₁ , W ₂ , and W ₃ of the multimode waveguides 1303a to 1303c can be set to be different from each other so that light of a plurality of modes is propagated. Further, the widths of the input waveguide 1302 and the output waveguide 1304 can be set so that single-mode light is propagated. Further, the lengths L ₁ , L ₂ , and L ₃ of the multimode waveguides 1303a to 1303c can be set so as to satisfy the relationship of the expression (1).

  Further, an antireflection film 1307 is formed on the end surface 1305 of the n-InP substrate 1301 on the input waveguide 1302 side, and an antireflection film 1308 is formed on the end surface 1306 of the n-InP substrate 1301 on the output waveguide 1304 side. Has been. Further, highly reflective films 1309 and 1310 are arranged opposite to each side of the multimode waveguides 1303a to 1303c.

  The input signal light 1311 that has entered the input waveguide 1302 propagates through the input waveguide 1302 and enters the multimode waveguide 1303a. When the input signal light 1311 enters the multimode waveguide 1303a, a plurality of propagation modes in the multimode waveguide 1303a are excited and propagate in the multimode waveguide 1303a under phase conditions determined by the respective propagation constants. To do. The light propagated in the multimode waveguide 1303a enters the multimode waveguide 1303b. Then, the signal light incident on the multimode waveguide 1303b propagates through the multimode waveguide 1303b while being developed in multimode, and enters the multimode waveguide 1303c. The signal light incident on the multimode waveguide 1303c propagates through the multimode waveguide 1303c while being developed in multimode. The signal light that has propagated through the multimode waveguide 1303 c enters the output waveguide 1304, propagates through the output waveguide 1304, and is then emitted from the end face 1306 as the output signal light 1312.

The lengths L ₁ , L ₂ , and L ₃ of the multimode waveguides 1303a to 1303c are set so as to satisfy the relationship of the expression (1), and propagated in the multimode waveguides 1303a to 1303c. Signal light can be coupled to the fundamental mode of the output waveguide 1304, and the coupling loss between the multimode waveguides 1303a to 1303c and the output waveguide 1304 can be reduced.
Here, since the cores of the multimode waveguides 1303a to 1303c include a gain medium, the input signal light 1311 is amplified as it propagates through the multimode waveguides 1303a to 1303c to obtain an amplified output signal light 1312. be able to.

  On the other hand, the spontaneous emission light generated in each of the multimode waveguides 1303a to 1303c is emitted in all directions, and is reflected by the high reflection films 1309 and 1310 on both sides of the multimode waveguides 1303a to 1303c. Laser oscillation can be caused in a direction orthogonal to the waveguide direction of the signal light 1311. When laser oscillation occurs in a direction orthogonal to the waveguide direction of the input signal light 1311, the multimode waveguides 1303 a to 1303 a to 1303 a to 1303 c are changed even when the intensity of the signal light incident on the multimode waveguides 1303 a to 1303 c varies. The carrier density of 1303c can be kept constant, and the gain of the optical amplifying element can be clamped to a constant value.

For this reason, even when the widths W ₁ , W ₂ , and W _{3 of the} multimode waveguides 1303a to 1303c are set to be different from each other, the oscillation light is prevented from being mixed into the amplified light emitted from the output waveguide 1304. However, the input signal light 1311 can be amplified in the gain medium whose gain is clamped by oscillation. As a result, it is possible to eliminate the need for a wavelength filter or an optical waveguide for separating the signal light and the oscillation light, and it is possible to suppress a gain variation due to the input light intensity while suppressing an increase in element size.

In addition, by setting the widths W ₁ , W ₂ , and W ₃ of the multimode waveguides 1303a to 1303c to be different from each other, it is possible to change the injected carrier density for each of the multimode waveguides 1303a to 1303c. For this reason, it is possible to increase the saturated output power or improve NF while gaining gain, and various operation states can be realized according to the use of the optical amplifying element.

  In the embodiment of FIG. 8 as well, the configuration of the waveguide, the composition and structure of the core layer or gain medium, the presence or absence of the SCH structure, and the like can be variously modified as in the embodiment of FIG. In the embodiment of FIG. 10, the input waveguide 1302 and the output waveguide 1304 have also been described with respect to the method of configuring a single mode waveguide made of a gain medium having InGaAsP as a core. The waveguide 1302 and the output waveguide 1304 may be configured using a medium that is transparent with respect to the wavelength of the input signal light 1311.

  In the above-described embodiment, the case where m = 1 has been described. However, the lengths of the multimode waveguides 1303a to 1303c may satisfy the equation (1) independently and are limited to m = 1. The effect of the present invention can also be expected when an arbitrary integer m is used for each of the multimode waveguides 1303a to 1303c. Further, in the above-described embodiment, the method of connecting the multi-mode waveguides 1303a to 1303c in three stages has been described. .

Next, the range of structural parameters that can be expected to have the effects of the above-described embodiment will be described.
In the optical amplifying element described above, traveling wave type amplification is performed on the input signal light, and laser oscillation occurs in the width direction of the multimode waveguide. For this reason, the relationship between the reflectivity R _H of the highly reflective film, the width W _{i of} the i-th multimode waveguide, the length L _{i of} the i-th multimode waveguide, and the reflectivity R _AR of the antireflection film is This will be explained as follows. When the clamped gain per unit length is G _{clamp — i} (dB), the threshold gain G _lateral for _causing oscillation in the width direction of the multimode waveguide is
G _lateral = G _{clamp — i} × W _i = −10 × log (R _H ) (dB) (6)
On the other hand, in order to prevent oscillation in the propagation direction of the signal light, it is necessary that the reflection loss in the cavity due to reflection by the antireflection film is larger than the signal gain G _signal . Since the reflection loss in the cavity due to reflection by the antireflection film is expressed by 10 × log (R _AR ), in order to prevent oscillation in the propagation direction of the signal light,

It is necessary to satisfy this condition. Here, n is the number of multimode waveguides. When the G _clamp of the formula (7) is deleted using the formula (6) and the formula (7),

It becomes. If the relationship between the structural parameters satisfies the relationship of equation (8), the operation of the above-described optical amplifying element can be expected.

  The above-described optical amplifying element can be applied to applications such as optical transmission processing systems using light such as optical communication, optical exchange, and optical information processing, and in particular, prevents fluctuations in gain of an optical signal due to the number of wavelength multiplexing. This makes it possible to suppress an increase in the size of the wavelength division multiplexing optical transmission system.

1 is a plan view showing a schematic configuration of an optical amplifying element according to a first embodiment of the present invention. It is sectional drawing which shows the structural example of the optical amplification element cut | disconnected by the AA 'line | wire of FIG. It is a top view which shows the principle of operation of the optical amplification element of this invention. It is a figure which shows the saturation characteristic of the optical amplification element of FIG. It is sectional drawing which shows schematic structure of the optical amplification element which concerns on 2nd Embodiment of this invention. It is a top view which shows schematic structure of the optical amplification element which concerns on 3rd Embodiment of this invention. It is sectional drawing which shows the structural example of the optical amplification element cut | disconnected by the AA 'line of FIG. It is a top view which shows schematic structure of the optical amplification element which concerns on 4th Embodiment of this invention. It is sectional drawing which shows the structural example of the optical amplification element cut | disconnected by the AA 'line of FIG. It is a top view which shows schematic structure of the optical amplification element which concerns on 5th Embodiment of this invention. 11A is a cross-sectional view of a conventional optical amplifying element cut along the optical waveguide direction, and FIG. 11B is a cross-sectional view taken along line AA ′ of FIG. 11A. It is a figure which shows the saturation characteristic of the optical amplification element of FIG. 13A is a cross-sectional view of a conventional optical amplifying element cut along the optical waveguide direction, and FIG. 13B is a cross-sectional view taken along the line CC ′ of FIG. 13A. It is a figure which shows the saturation characteristic of the optical amplification element of FIG.

Explanation of symbols

401, 501, 601, 701, 901, 1001, 1101, 1201, 1301 n-InP substrate 402, 602, 802, 902, 1102, 1302 Input waveguides 403a to 403c, 603, 903a to 903c, 1103a to 1103c, 1303a to 1303c Multimode waveguide 404, 604, 804, 904, 1104, 1304 Output waveguide 405, 406, 605, 606, 905, 906, 1105, 1106, 1305, 1306 End face 407, 408, 607, 608, 907, 908 1107, 1108, 1307, 1308 Antireflection film 409, 410, 509, 510, 609, 610709, 710, 909, 910, 1109, 1110, 1309, 1310 High reflection film 411, 611, 911, 11 11, 1311 Input signal light 412, 612, 912, 1112, 1312 Output signal light 502, 702, 1002, 1003a to 1003c, 1202 InGaAsP active layer 503, 703 p-InP current blocking layer 504, 704 n-InP current blocking layer 505, 705, 1005, 1205 p-InP cladding layer 506, 706, 1006a to 1006c, 1206a to 1206c p-GaInAs cap layer 507, 707, 1007a to 1007c, 1207a to 1207c p-side electrode 508, 708, 1008, 1208 n Side electrode 711 InGaAsP separation confinement layer 413a, 413b, 913a, 913b, 1113a, 1113b Connection waveguide 1002 InGaAsP input waveguide core layer 1009a-1009b InGaAsP connection waveguide core layer 1004 InGaAsP output waveguide core layer

Claims (11)

An input waveguide for guiding single mode light;
A plurality of multimode waveguides configured to at least one be optically coupled to the input waveguide to guide a plurality of modes of light and to include a gain medium at least in part;
An output waveguide in which at least one of the multimode waveguides is optically coupled to guide single mode light;
Reflecting the spontaneous emission light radiated from the gain medium so as to cause the reflection region to oscillate in a direction crossing the propagation direction of the light incident on the multimode waveguide via the input waveguide; In the multimode waveguide, the gain region of the light incident on the multimode waveguide and the gain region of the oscillated light are matched to each other,
A highly reflective film is formed in the reflective region,
An antireflection film is formed on the input waveguide and the end face of the input waveguide,
When the length of the multimode waveguide is L _i , the width is W _i , the reflectance of the high reflection film is R _H , and the reflectance of the antireflection film is R _AR ,
An optical amplification element characterized by satisfying the relationship:   The optical amplifying element according to claim 1, further comprising a connection waveguide configured to guide single mode light and connecting the multimode waveguides.   2. The optical amplifying element according to claim 1, wherein multimode waveguides having different widths are directly connected.   At least one of the input waveguide, the output waveguide, and the connection waveguide is composed of a single mode waveguide having a core made of a material transparent to the wavelength of signal light. Item 4. The optical amplifying device according to any one of Items 1 to 3.   5. The optical amplifying element according to claim 1, wherein the waveguide widths of at least two multimode waveguides are different from each other.   6. The optical amplifying element according to claim 1, further comprising current injection means for injecting current independently into at least two multimode waveguides.   The input waveguide, the multimode waveguide, and the output waveguide are arranged side by side on the same substrate so that the central axes of the waveguides coincide with each other, and the reflection regions are arranged opposite to both sides of the multimode waveguide. The optical amplifying element according to claim 1, wherein the optical amplifying element is provided.   The input waveguide, the multimode waveguide, and the output waveguide each include a core made of a common gain medium, and the width of the core of the multimode waveguide is that of the input waveguide and the output waveguide. 8. The optical amplifying element according to claim 7, wherein the optical amplifying element is wider than the width of the core. Of at least two multimode waveguides, if the length of the i-th multimode waveguide is L _i , the width is W _i , the refractive index is n _eqi , and the signal light wavelength is λ,
L _i = m _i · n _eqi · Wi ² / λ (where m _i is a positive integer)
The optical amplifying element according to claim 1, wherein   It further comprises a transparent layer that constitutes a part of the core of the multimode waveguide, is extended in the direction of the reflection region so as to be wider than the multimode waveguide, and is transparent to the signal light wavelength. The optical amplifying element according to claim 1, wherein:   11. The optical amplifying element according to claim 1, wherein the reflective region includes a dielectric multilayer film or a metal film formed on a side wall of the multimode waveguide.