JP2002232083A - Optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical amplifier where the same active layer is used in a wafer plane, having a structure in which increase of the number of electrodes is suppressed as low as possible, to adjust gain against a TE polarization input light and TM polarization input light, while a polarization dependency of gain characteristics is compensated against the polarized input lights which is caused by deviation, etc., of a parameter of an active layer from a design value at manufacturing, for larger tolerance in manufacturing precision and higher yield. SOLUTION: The width of a distortion semiconductor active layer varies at three stages: a region where a TE polarization is mainly amplified, a region for amplifying independent of polarization, and a region for amplifying a TM polarization.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier.
For example, the present invention relates to a technology effective when applied to an optical transmission system such as optical communication, optical switching, and optical information processing.

[0002]

2. Description of the Related Art When considering the construction of an optical transmission system using light such as optical communication, optical switching, and optical information processing, optical loss in an optical fiber or an optical switch becomes a serious problem, and an attenuated optical signal is converted by an optical amplifier. Compensation is essential. Especially semiconductor laser type optical amplifiers (optical amplifiers)
Is very promising because it has the advantages of being compact, highly efficient, and capable of being integrated with semiconductor optical devices such as optical switches.

However, one of the requirements for an optical amplifier used in an optical transmission system is that the optical amplification gain has no polarization dependence. One method is to make the cross-sectional area of the active layer responsible for optical amplification isotropic and equalize the optical confinement coefficients for the TE polarized light and the TM polarized light. However, in this method, the active layer width needs to be on the order of submicron under single mode conditions. However, since the active layer width is based on an etching technique, there is a problem in that it becomes difficult in controllability, yield and other processes. Was.

[0004] For this purpose, an example in which a wide active layer having a large change in polarization dependence of gain characteristics and a large process tolerance and an extensional strain active layer with respect to a shift of the active layer width (reference: JY Emery). , P. Doussiere, L. Goldstein,
F. Pommereau, C. Fortin, R. Ngo, N. Tsherptner, J.
L. Lafragette, P. Aubert, F. Brillouet, G. Laubean
d J. Barrau: "New, Process Tolerant, High Perfor
mance 1.55μm Polarization Insensitive Semiconduct
or Optical Amplifier Based on Low TensileBulk GaIn
AsP "Ecoc'96, vol.3, pp.165-168) was reported and TE
The difference in gain characteristics between the polarization input light and the TM polarization input light is 1
A semiconductor laser type optical amplifier that cuts dB has been reported.

As another example, there has been reported an example in which a mechanism for adjusting the polarization dependence of the gain characteristic is provided to make the polarization independent (see JP-A-8-64904 and JP-A-8-64904). No. 11-17260).

[0006]

Conventionally, as disclosed in the above document, for example, a semiconductor laser type optical amplifier in which the difference in gain characteristics between TE-polarized light and TM-polarized light is less than 1 dB. Has been realized. However, when considering the cascade connection of optical amplifiers (optical amplifying elements) with the increase in the scale of the system, the difference in the gain characteristic is smaller (for example, 0 to 0.1 dB). Devices are desired.

In this case, in the semiconductor laser type optical amplifier of the first document, the amount of strain of the active layer is reduced to 0.0
It must be controlled on the order of 1% or less, which is very difficult in crystal growth technology. Also, JP-A-8-649
In the example of Japanese Patent Application Publication No. 04-204, an active layer for selectively amplifying TE polarized light and an active layer for selectively amplifying TM polarized light are connected in cascade, and the optical gain of each active layer is controlled independently. Although polarization independence is attempted, there is a problem that the number of manufacturing steps increases because a plurality of active layer structures are manufactured.

In the example of Japanese Patent Application Laid-Open No. H11-17260, the sectional structure and the crystal composition are continuously changed in the stripe direction of light by using a selective growth technique, so that the T
A structure in which the gain difference with respect to the E-polarized light and the TM-polarized light is different in the stripe direction is fabricated, and the polarization injection is made independent by changing the current injection region in the stripe direction. If the polarization dependence is to be adjusted, the number of electrodes will increase in proportion to this, and the number of wire bonding steps will increase accordingly, which is problematic in terms of manufacturing cost.

An object of the present invention is to provide a structure in which the same active layer is used in the plane of the wafer and the number of electrodes is minimized.
The gain for the E-polarized light and the TM-polarized light can be adjusted, and the polarization characteristics of the gain characteristics for the respective polarized input light due to deviations of the parameters of the active layer from the design values during fabrication. An object of the present invention is to provide an optical amplifier that can compensate for the problem, that is, has a high manufacturing tolerance and a high yield.

[0010]

According to a first aspect of the present invention, there is provided an optical amplifier having a strained semiconductor active layer, wherein the strained semiconductor active layer has first and second electrodes having individual electrodes. , 3 regions, wherein the first region has a width such that the optical amplification of the strained semiconductor active layer is polarization-independent, and the width of the second region is smaller than the width of the first region. The width of the third region is wider than the width of the first region.

According to a second aspect of the present invention, there is provided an optical amplifier according to the second aspect, wherein the active layer according to the first aspect is provided with a light confinement layer.

An optical amplifier according to a third aspect of the present invention that achieves the above object is characterized in that the optical amplifier according to the first and second aspects is integrated with another optical device.

According to a fourth aspect of the present invention, there is provided an optical amplifier for branching a part of output light to an output terminal of the optical amplifier according to the first and second aspects. A polarization beam splitter that divides output light into a TE polarized light component and a TM polarized light component, two light receivers for measuring the light intensities of the TE polarized light component and the TM polarized light component, and light receiving current values of the two light receivers And adjusting a current flowing through the second region and the third region so as to minimize the difference between the two light receiving current values.

According to the above-described means, the optical amplifier of the present invention has a region for selectively amplifying the TM-polarized input light in addition to a region for providing a desired optical gain to the signal light in the same waveguide. Has a region for selectively amplifying the TE-polarized input light, and each region can be independently driven and controlled.
The gain for polarization and TE polarization input light can be adjusted separately.

Therefore, for example, it is possible to compensate for the polarization dependence of the gain characteristic caused by the amount of strain applied to the active layer and the deviation of the active layer width from the design value when actually manufactured,
As a whole, the difference between the gain characteristics of the TM-polarized input light and the gain characteristics of the TE-polarized input light can be reduced to 0.1 dB or less close to zero.

When a new optical component is manufactured by integrating the optical amplifier of the present invention with another optical device, the polarization dependence of the optical device integrated by this optical amplifier is compensated for, so that the entire optical component is compensated. Can be made polarization independent.

Further, an optical coupler for branching a part of the output light to an output terminal of the optical amplifier of the present invention, and a polarization beam splitter for dividing a part of the branched output light into a TE polarization light component and a TM polarization light component And two light receivers for measuring the light intensities of the TE polarization component and the TM polarization component, respectively, and means for comparing the light reception current values of the two light receivers, and minimizing the difference between the two light reception current values. By adjusting the current flowing in the second region and the third region as described above, the optical amplifier of the present invention can be made polarization independent.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is characterized in that the width of a strained semiconductor active layer is changed in three steps, and mainly includes a region for amplifying TE polarization, a region for amplification independent of polarization, and a region for TM polarization. This embodiment will be described below with reference to the drawings.

[Embodiment 1] FIG. 1 shows an embodiment according to claim 1 of the optical amplifier of the present invention. FIG. 1A shows a cross section in a plane along the light propagation direction, and FIG.
1A shows a BB ′ section in a direction orthogonal to FIG. 1A, FIG. 1C shows a CC ′ section in a direction orthogonal to FIG. 1A, and FIG. D- in the direction orthogonal to 1 (a)
The D 'section is shown. This embodiment is 1.5 μm
It is a semiconductor laser type optical amplifying device with a polarization dependence adjusting mechanism for a band, and is formed of an InP-InGaAsP-based compound semiconductor material.

That is, a 1 μm thick n-side electrode 101 made of AuGeNi / Au is formed on the back surface of a substrate 102 made of n-type InP having a thickness of about 100 μm. Further, a buffer layer 103 made of n-InP having a thickness of 0.5 μm is formed on the substrate 102, and a buffer layer 103 having a thickness of, for example, 0.
Active layers 104, 105 and 106 each having a thickness of 2 μm are formed.

The active layer 104 has a width of W2 and a length of L2 along the light propagation direction.
Along the light propagation direction, the width of W1 is equal to the length of L1, and
The active layer 106 has a width of W3 and a width of L3 along the light propagation direction.
The length is formed. Here, the active layers 104 and 10
The strains 5 and 106 are applied so as to be polarization independent at the width W1 of the active layer 105. Further, a 1.5 μm-thick p-type I layer is formed on these active layers 104, 105 and 106.
An over cladding layer 107 made of nP is formed, and a contact layer 108 made of p-type InGaAs with a thickness of 0.4 μm is formed thereon.

The active layer 10 is formed on the contact layer 108.
Electrodes 109, 110, and 111 for injecting current into regions 4, 105, and 106 are formed. The electrodes 109, 110 and 111 are made of AuZnNi / Au having a thickness of 1 μm.
It is composed of The optical signal input end face on the side where the active layer 104 is formed and the optical signal output end face on the side where the active layer 106 is formed are provided with a non-reflection coated face 112,
113 are formed.

As shown in FIGS. 1B, 1C, and 1D, the buffer layer 103, the active layers 104, 105, and 1
06, over cladding layer 107 and contact layer 108
Is formed with an insulating layer 114 for embedding. Now, here, the difference (ΔG _TE-TM ) of the gain characteristics with respect to each polarization input light is as follows: G _TE is the gain characteristic with respect to the TE polarization input light, Γ _TE and Γ _TM are the optical confinement coefficients for each polarization light, If g _TE and g _TM are material gains for the respective polarized lights, they can be expressed by the following equation (1). ΔG _TE-TM (dB) = G _TE (1−Γ _TM g _TM / Γ _TE g _TE ) (1) Ratio g _{TM between} material gains g _TE and g _TM for each polarized light.
/ G _TE can be changed depending on the amount of strain applied to the active layer. In the case of extensional strain, the material gain ratio g _TM /
g _TE is 1 or more, and in the case of compressive strain, it is 1 or less.

Therefore, the light confinement coefficient ratio Γ _TE / Γ _TM in the region of the active layer 105 and the material gain ratio g for polarized light
_When the strain amount of all the active layers is designed so that _TM / g _TE becomes equal (polarization independent condition), in the region of the active layer 104 that is narrower than the active layer 105, the light confinement coefficient ratio Γ _TE / Γ _TM becomes The light confinement coefficient ratio Γ _TE / Γ _{TM of the} active layer 105 is smaller than the above, and the TM-polarized input light is more selectively amplified by the above equation. In the area of the active layer 106 wider than the active layer 105, the light confinement coefficient is the ratio gamma _TE / gamma _TM is larger than the light confinement coefficient ratio gamma _TE / gamma _TM of the active layer 105, the it can be more selectively amplify TE polarized input light by formula.

That is, according to this embodiment, the active layer 1
In step 05, a desired optical gain is given to the signal light, and the active layer 104,
106 makes it possible to adjust the difference in gain characteristics between polarizations in the entire device. Therefore, for example, the difference in the gain characteristic between the polarizations is reduced by 0.1% by compensating the in-plane distribution of the amount of strain in the active layer and the shift caused by the process of the active layer width.
Since it can be set to a value close to 0, which is less than or equal to dB, tolerance of accuracy in crystal growth technology and process technology is widened, which leads to improvement in yield, and the semiconductor laser type optical amplifying device of the present invention can be used for other purposes. When a new optical component is manufactured by integrating with an optical device, even if the integrated optical device itself and coupling loss have some polarization dependence, it is possible to compensate for it.

[Embodiment 2] FIG. 2 shows an optical amplifier according to a second embodiment of the present invention. FIG. 2A shows a cross section in a plane along the light propagation direction, and FIG.
2A shows a BB ′ cross section in a direction orthogonal to FIG. 2A, FIG. 2C shows a CC ′ cross section in a direction orthogonal to FIG. 2A, and FIG. D- in the direction orthogonal to 2 (a)
The D 'section is shown. This embodiment has a configuration in which a light confinement layer is provided in the active layer in the first embodiment.

This embodiment is a semiconductor laser type optical amplifying device having a polarization dependency adjusting mechanism for 1.5 μm band, and is made of InP-InGaAsP-based compound semiconductor material. That is, as shown in FIG. 2, a 1 μm thick n-side electrode 201 made of AuGeNi / An is formed on the back surface of a substrate 202 made of n-type InP having a thickness of about 100 μm.
Are formed.

A buffer layer 203 made of n-InP having a thickness of 0.5 μm is formed on the substrate 202, and a 0.2 μm-thick bulk layer having a composition having a bandgap wavelength of 1.58 μm is formed thereon. Active layers 207, 208, 209
And a band gap wavelength of 1.2μ so as to sandwich it above and below
Light confinement layers 204, 205, and 206 made of InGaAsP having a composition of 0.1 m and a thickness of 0.1 m are formed. Also, the bulk active layer 207 and the light confinement layer 204
Is 200 μm in width of 0.4 μm along the light propagation direction.
, And the bulk active layer 208 and the optical confinement layer 205 have a width of 0.55 μm along the light propagation direction.
The bulk active layer 209 and the light confinement layer 206 are formed with a width of 1.0 μm and a length of 200 μm along the light propagation direction.

Here, the bulk active layers 207, 208, 2
09 is strained so that the width of the bulk active layer 208 is 0.55 μm and is polarization independent. Further, a p-type I layer having a thickness of 1.5 μm is formed on the bulk active layers 207, 208, 209 and the optical confinement layers 204, 205, 206.
An over cladding layer 210 made of nP is formed, and a contact layer 211 made of p-type InGaAs with a thickness of 0.4 μm is formed thereon. Also, contact layer 2
Electrodes 212, 213, and 214 for injecting current into the regions of the active layers 207, 208, and 209 are formed on the substrate 11.

The electrodes 212, 213 and 214 are made of AuZnNi / Au having a thickness of 1 μm. The non-reflection coated surfaces 215 and 216 are formed on the optical signal input end face on the side where the bulk active layer 207 is formed and on the optical signal output end face on the side where the bulk active layer 209 is formed. As shown in FIGS. 2B, 2C, and 2D, the buffer layer 203, the bulk active layers 207, 208,
209, light confinement layers 204, 205, 206, over cladding layer 210 and contact layer 211
A buried insulating layer 217 is formed.

FIG. 3 shows the optical confinement coefficient ratio Γ _TE / Γ _TM for each polarized light in the above structure. As shown in FIG. 3, in the region of the bulk active layer 207, the ratio is 1.07.
5. In the region of the bulk active layer 209, the ratio is 1.13.
5, in the region of the bulk active layer 209, the ratio is 1.2.

That is, when the tensile strain is designed in all the active layers so that the material gain ratio with respect to the polarized light is 1.135 (polarization independent condition) at the optical confinement coefficient ratio of 1.135 in the region of the bulk active layer 208. In the region of the bulk active layer 207, the light confinement coefficient ratio is smaller than 1.135. Therefore, the TM-polarized light is more effectively amplified by the above equation, and conversely, the light confinement coefficient ratio is 1 in the bulk active layer 209. .135
Since it is larger, the TE-polarized input light can be more effectively amplified by the above equation. Therefore, the third embodiment has the same effects as those of the first embodiment. In particular, according to the configuration having the light confinement layer, the change in the light confinement coefficient ratio with respect to the width of the active layer is gradual. There is a feature.

[Embodiment 3] FIG. 4 shows an embodiment of an optical amplifier according to claim 4 of the present invention. This embodiment is an application example of the optical amplifier of the present invention. That is, an optical coupler 402 for branching a part of the output light is connected to the output terminal of the optical amplifier 401, and one of the branched output lights is provided with T
A polarization beam splitter 403 for separating the E-polarized light component and the TM-polarized light component, and two photodetectors 404 for measuring the light intensity of the TE-polarized light component and the TM-polarized light component at the subsequent stage;
405 and a comparator 406 for comparing the light reception current values of the two light receivers.

Therefore, if the current flowing in the second region and the third region is adjusted by the comparator 406 so as to minimize the difference between the two received light current values, the semiconductor laser type with the polarization dependence adjusting mechanism can be obtained. The optical amplification element can be made polarization independent. As described above, the invention made by the inventor has been specifically described based on the embodiment. However, the present invention is not limited to the embodiment, and various modifications may be made without departing from the gist of the invention. Of course, it is possible.

For example, in the first and second embodiments, the width of the active layer on the input end face side is narrowest and the width of the active layer on the output end face side is widest in the signal light propagation direction. Although the case has been described, a similar effect can be obtained regardless of the order of the first, second, and third regions. In the first and second embodiments, the case where the width of the active layer changes stepwise has been described. However, the width may be changed continuously along the signal light propagation direction.

In the second embodiment, the case of a bulk active layer has been particularly described. However, a similar effect can be obtained by using a strained multiple quantum well active layer.
Of course, the thickness of the active layer or the optical confinement layer may be changed, or only the upper and lower sides of the optical confinement layer may be used. Further, in the first and second embodiments, the semiconductor laser type optical amplifying device using the n-type InP substrate has been described. However, the same applies to the case where a p-type InP substrate or another compound semiconductor substrate is used. The effect can be obtained.

The active layer is made of InGaP-based material,
Similar effects can be obtained by using other compound semiconductor materials such as nGaAlAs, InAlAs, and AlGaAs. In the first and second embodiments, each region has a continuous structure. However, in order to make the voltage application effect effective, an isolation groove or an insulating region is provided between the electrodes of each region. Needless to say, it is possible to provide and electrically insulate between the electrodes of each inclined plane.

[0038]

As described above, the optical amplifier of the present invention can be used in the TE amplifier together with the region contributing to the optical amplification of the entire signal light.
Since it has a region in which the gain for the polarized light input and the gain for the TM polarized light can be separately adjusted, it is possible to compensate for the amount of distortion of the active layer and the deviation of the active layer width from the design values during fabrication. As a result, a semiconductor laser type optical amplifying device with a polarization dependent adjusting mechanism capable of widening the tolerance of accuracy in crystal growth technology and process technology is provided.

[Brief description of the drawings]

FIG. 1A is a cross-sectional view of an optical amplifier according to a first embodiment of the present invention in a plane along a light propagation direction, and FIG. 1B is a second region in FIG. 1A. B- orthogonal to
FIG. 1C is a cross-sectional view taken along a line CC ′ orthogonal to the first region in FIG. 1A, and FIG. 1D is a cross-sectional view taken along a line DD ′ orthogonal to the third region in FIG. FIG. 1 (e) is a cross-sectional view, and FIG. 1 (e) is a graph showing a change in the width of the active layer in the length direction of FIG. 1 (a).

FIG. 2A is a cross-sectional view of the optical amplifier according to a second embodiment of the present invention in a plane along a light propagation direction, and FIG. 2B is a second region in FIG. B- orthogonal to
FIG. 2C is a cross-sectional view taken along the line CC ′ orthogonal to the first region of FIG. 1A, and FIG. 2D is a cross-sectional view taken along the line DD ′ orthogonal to the third region of FIG. FIG. 2E is a graph showing a change in the width of the active layer and the light confinement layer in the length direction of FIG. 1A.

FIG. 3 is a graph showing a change in an optical confinement coefficient when the widths of a bulk active layer and an optical confinement layer are changed in the optical amplifier according to the second embodiment of the present invention.

FIG. 4 is an explanatory diagram of an optical amplifier according to a third embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 101 n-side electrode 102 substrate 103 buffer layer 104, 105, 106 bulk active layer 107 over cladding layer 108 contact layer 109, 110, 111 electrode 112, 113 non-reflective coating surface 114 burying insulating layer 201 n-side electrode 202 substrate 203 buffer Layers 204, 205, 206 Optical confinement layer 207, 208, 209 Bulk active layer 210 Over cladding layer 211 Contact layer 212, 213, 214 Electrode 215, 216 Non-reflective coating surface 217 Embedding insulating layer 401 Optical amplifier 402 Optical coupler 403 Polarization Wave beam splitter 404, 405 Receiver 406 Comparator

 ────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yasuo Shibata F-term (reference) in Nippon Telegraph and Telephone Corporation 2-3-1 Otemachi, Chiyoda-ku, Tokyo 5F073 AA07 BA01 BA04 CA12 CB02 EA22

Claims (4)

[Claims] 1. A semiconductor device comprising: a strained semiconductor active layer, the strained semiconductor active layer having first, second, and third regions each having an individual electrode, wherein the first region is provided for optical amplification of the strained semiconductor active layer. Is a width that is polarization independent, the width of the second region is smaller than the width of the first region, and the width of the third region is wider than the width of the first region. Optical amplifier. 2. The optical amplifier according to claim 1, wherein an optical confinement layer is provided on the active layer. 3. An optical amplifier, wherein the optical amplifier according to claim 1 and 2 is integrated with another optical device. 4. An optical coupler for branching a part of output light to an output terminal of the optical amplifier according to claim 1 and a polarization for separating the branched output light into a TE polarized light component and a TM polarized light component. A wave beam splitter, two light receivers for measuring the light intensities of the TE polarization component and the TM polarization component, respectively, and means for comparing the light reception current values of the two light receivers, and a difference between the two light reception current values. An optical amplifier characterized in that currents flowing through the second and third regions are adjusted so as to minimize.