JP2000244074A - Traveling-wave semiconductor optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To widen a range wherein variation in light-confinement coefficient ratio ΓTE/ΓTM between polarized waves to that in the width of active layer is gentle while a basic mode condition is maintained, in a traveling-wave semiconductor light amplifier. SOLUTION: In a traveling-wave semiconductor light amplifier, a strained bulk active layer 1 comprising bulk crystal into which tensile stress is introduced is provided, optical resonance due to reflection at a gap between a light-incident end surface and a light-emitting end surface is suppressed, signal light having the wave length almost equal to that of a band gap of the strained bulk active layer 1 is made incident from the light-incident end surface, current is injected into the strained bulk active layer 1 to amplify the signal light by induced emission effect, and the amplified signal light is emitted from the light-emitting end surface. In this amplifier, two surfaces vertical to the layer-thickness direction of the strained bulk active layer 1 are sandwiched between single clad layers 2 and 3 respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a traveling-wave type semiconductor optical amplifier, and more particularly, to a wavelength-division multiplexing (WDM) in which the gain difference between polarizations is reduced to make it polarization independent.
Nthth Division Multiplexin
g) The present invention relates to a traveling wave type semiconductor optical amplifier used for a communication system.

[0002]

2. Description of the Related Art In recent years, a wavelength division multiplexing communication system has been developed in which a plurality of signal lights having different wavelengths are multiplexed and transmitted simultaneously through one optical fiber in response to a dramatic increase in communication demand. In this wavelength division multiplexing communication system, since many optical components are used for multiplexing and demultiplexing, an optical signal is attenuated due to loss of each optical component.

[0003] Optical amplifiers are used to compensate for such attenuation. However, since an extremely large number of optical amplifiers are required as compared with the conventional optical fiber system, the optical amplifiers are small in size. It is required that low power consumption operation be possible.

As such a loss compensating optical amplifier, a traveling-wave semiconductor optical amplifier which is small in size and capable of operating with low power consumption is expected. However, such an optical amplifier used in-line has a low gain. Since the polarization dependence is required to be small, a semiconductor optical amplifier (SOA: Sem:
iconic Optical Amplif
The polarization dependence of the gain in (ier) will be described.

[0005] Single transmission internal gain of semiconductor optical amplification for TE polarized light and TM polarized light (hereinafter simply referred to as internal gain)
G ⁱ _{TE, TM} (dB) is, TE light confinement coefficient gamma _TE for polarized light and TM polarized _{light, TM,} material gain for TE and TM polarizations g _{TE, TM} and, by using the active region length L, the following formula It is represented by (1). _{^{G i TE, TM (dB)}} = 10log 10 {exp (Γ TE, TM · g TE, TM · L)} ··· (1)

Accordingly, the gain difference between polarizations ΔG ⁱ _TE-TM (d
B) is derived as the following equation (2). ΔG ⁱ _TE-TM (dB) = G ⁱ _TE (dB) −G ⁱ _TM (dB) = G ⁱ _TE (dB) ｛1−G ⁱ _TM (dB) / G ⁱ _TE (dB)｝ = G ⁱ _TE (DB) {1- (g _TM / g _TE ) / (Γ _TE / Γ _TM )} (2)

In order to suppress the inter-polarization gain difference ΔG ⁱ _TE-TM (dB) to a certain allowable value a (dB) or less, from the above equation (2), ABS (ΔG ⁱ _TE-TM (dB)) ≦ a (dB) Therefore, 1−a (dB) / G ⁱ _TE (dB) ≦ (g _TM / g _TE ) / (Γ _TE / Γ _TM ) ≦ 1 + a (dB) / G ⁱ _TE (dB) • The material gain ratio between polarizations g _TM / g _TE so as to satisfy the condition (3)
It is understood that the optical confinement coefficient ratio Γ _TE / Γ _TM between the polarizations needs to be controlled. In the specification of the present application, the absolute value of x is expressed as ABS (x) for convenience of preparing the specification, and therefore, ABS (ΔG
ⁱ _TE-TM (dB)) represents the absolute value of ΔG ⁱ _{TE -TM} (dB) (the same _applies hereinafter).

Considering this equation (3), even if the allowable value a (dB) of the gain difference ΔG ⁱ _TE-TM (dB) between polarizations is the same,
As the required internal gain G ⁱ _TE (dB) increases, a
Since (dB) / G ⁱ _TE (dB) becomes smaller, (g _TM /
Strict control is required to make g _TE ) / (Γ _TE / Γ _TM ) closer to 1.

Further, in order to make the gain in the semiconductor optical amplifier completely polarization-independent, ie, ΔG ⁱ _TE-TM (d
In order to make B) = 0, from the above equation (2), (g _TM / g _TE ) / (Γ _TE / Γ _TM ) = 1 Therefore, (g _TM / g _TE ) = (Γ _TE / Γ _TM ) (4)

Next, various approaches for making the gain polarization independent on the basis of whether the material gain g _{TM, TE} of the active layer of the semiconductor optical amplifier is isotropic or anisotropic will be considered. . First, considering the case where the material gain of the active layer is isotropic, the material gain is isotropic, that is, g _TE = g _TM when a non-strained bulk is used for the active layer. In this case, in order to make the gain of the semiconductor optical amplifier polarization independent, the above equation (4) is used.
Based on the above, the optical waveguide may be designed so that the optical confinement coefficients for TE polarized light and TM polarized light are equal, that is, Γ _TE = Γ _TM . For example, in the case of a buried (BH) type optical waveguide structure that has been proven as a semiconductor laser for optical communication, the cross-sectional shape of the active layer is made rectangular so that the optical confinement coefficients Γ _TE and Γ _TM between polarizations can be increased. Can be equal.

Next, considering the case where the material gain g _{TM, TE of} the active layer is anisotropic, when strain is introduced into the active layer, the material gain is anisotropic, that is, g _TE ≠ g _TM Can be Assuming that the material gain for the TM polarized light is k times the material gain for the TE polarized light, that is, g _TM /
Assuming that g _TE = k, when the optical confinement coefficient for TE polarized light is k times the optical confinement coefficient for TM polarized light, that is, when Γ _TE / Γ _TM = k, the gain of the semiconductor optical amplifier becomes polarized. Wave-independent.

Therefore, the above-described BH type optical waveguide structure
In the case of semiconductor lasers for optical communications
Optical confinement between both polarizations
Number ratioΓ_TE/ Γ_TMIntroduce strain into the active layer to compensate for
Material gain ratio g between polarizations_TM/ G _TEControl the gain
Wave-independent.

Several specific methods for realizing such polarization-independent methods have been proposed in response to the above two approaches. Here, each of the proposals will facilitate crystal growth and fabrication processes. Table 1 below considering the nature
As a summary.

[Table 1] In Table 1, the first two are rectangular non-strained bulk active layers, the third is a strained active layer, and the remaining four are strained MQs.
It uses a W (multiple quantum well) active layer.

Among them, the rectangular strain-free bulk active layer is the bulk crystal having no strain, so that crystal growth is the easiest, and the control parameters are the composition and thickness of the active layer. However, to maintain the fundamental mode waveguide condition,
Since a technique for processing an active layer width of about 0.5 μm in units of 0.1 μm or less with high precision is required, the manufacturing process becomes extremely difficult. Mersal
i et. al, Electron. Lett. , Vo
l. 26, pp. 124-125, 1990 (CNET
[1]), S.P. Tsuji et. al, Electr
on. Lett. , Vol. 27, pp. 941-94
3, 1991 (Hitachi [2]); Dous
sieret et. al, OAA'91, pp. 69-
72 (Alcatel [3]); Itoe
t. al, OECC'97, PDP, pp. 2-3 (N
TT [4])）.

[0015] In the case of a rectangular non-strained bulk active layer,
When a selective growth is performed using a SiO ₂ mask or the like as a selective growth mask, the control parameters relating to crystal growth may be added to a factor other than the composition and thickness of the active layer, and crystal growth may be slightly difficult. , S.P. Kitamura
et. al, IEEE Photon. Techno
l. Lett. , Vol. 6, pp. 173-175,
1994 (NEC [5]).

On the other hand, when a strained MQW active layer is used,
The optical waveguide has a high degree of freedom in design, and can be formed even with a typical active layer width of about 1.5 μm in a semiconductor laser for optical communication. Therefore, the manufacturing process becomes easy, but the crystal growth itself becomes difficult.

That is, when growing a strained MQW active layer, first, a well layer (well layer), a barrier layer (barrier layer),
In addition, the composition and layer thickness of each optical confinement layer (SCH layer) must be controlled, and the number of control parameters for crystal growth increases to six. Joma e
t. al, Appl. Phys. Lett. vol. 6
2, pp. 121-122, 1993 (OKI
[7]); Ito et. al, IEEE Pho
ton. Technol. Lett. , Vol. 10,
pp. 657-659, 1998 (NTT [8]),
K. Magari et. al, IEEE Photo
n. Technol. Lett. , Vol. 2, pp.
556-558, 1991 (NTT

[9]), A.I. E. FIG.
Kelly et. al, Electron. Let
t. , Vol. 32, pp. 1835-1836, 19
96 (BT [10]); Godefroy
et. al, IEEE Photon. Techno
l. Lett. , Vol. 7, pp. 473-475,
1995 (CNET [14]).

In particular, when the MQW active layer is used and the compression / tensile strain is combined, the growth parameter increases to eight. Mathur et. al, App
l. Phys. Lett. vol. 61 pp. 284
5-2847, 1992 (USC [11]); Du
bovitsky et. al, IEEE Photo
n. Technol. Lett. , Vol. 6, pp.
176-178, 1994 (also USC [1
1]); F. Tiemeier et. al, E
COC '92, pp. 911-914 (Philips
[12]); F. Tiemeier et. a
1, Appl. Phys. Lett. vol. 62, p
p. 826-828, 1993 (also Philip
s [12]) and M.S. A. Newkirk et.
al, IEEE Photon. Technol. Le
tt. , Vol. 4, pp. 406-408, 1993
(ATT [13])｝.

The transition wavelength is determined by the quantum effect due to the well layer,
Since the thickness and composition of the barrier layer change sensitively, for example, the thickness of the well layer needs to be controlled in sub-nm units. Further, if strain is introduced into the crystal, the crystal growth itself becomes difficult.

On the other hand, when a strained bulk active layer is used, the width of the active layer is about 1.0 μm, so that the fabrication process is much easier than that of a rectangular unstrained bulk active layer. There are four control parameters for the crystal growth of the active layer and the optical confinement layer, that is, the composition and the layer thickness. Y. Emery et. a
1, ECOC'96, vol. 3, pp. 165-16
8 (Alcatel [6]) and Y. Emer
y et. al, Electron. Lett. , Vo
l. 33, no. 12, pp. 1083-10884, 1
997 (also Alcatel [6])}.

In the case of the strained bulk active layer, since the quantum effect is not used, the control of the layer thickness is not as strict as in the case of the strained MQW active layer, and although the strain is introduced into the crystal,
The amount of strain may be smaller than that of the strained MQW active layer. Therefore, judging comprehensively from the above, it is considered that the strained bulk active layer has the structure that is most easily manufactured.

Here, referring to FIG. 9, this Alcat
B using strained bulk active layer by el (Alcatorre)
The H-wave traveling-wave semiconductor optical amplifier will be described. FIG. 9 is a schematic perspective view of a traveling-wave type semiconductor optical amplifier. In the first half, the p-type I
The illustration of the nP buried layer 37, the n-type InP current blocking layer 38, the p-type InP cladding layers 36 and 39, the p-type InGaAs contact layer 40, the SiO ₂ film 41, and the p-side electrode 42 is omitted.

In this traveling wave type semiconductor optical amplifier,
Optical confinement of the 100 nm thick InGaAsP layer above and below the 200 nm thick InGaAsP strained bulk active layer 34 (S
CH: Separate Confinement H
(Electrostructure) layers 33 and 35 are provided. In addition, stripe-shaped InGaAs
The optical axis of the P-strain bulk active layer 34 intersects the light input / output end face at an inclination angle of 7 °.

By doing so, the TE polarization and T
It is possible to reduce the light confinement coefficient ratio between M polarized lights,
When the stripe width of the strained bulk active layer is 1.0 μm, the gain difference between polarizations is reduced only by introducing a tensile strain of 0.16%, so that the signal input light 46 incident from one end face is polarized. Amplified output light 4 that is amplified without relying on waves
7 is output. In addition, since antireflection coating films (AR films) 44 and 45 made of a dielectric multilayer film are provided on both end surfaces, resonance of the signal input light 46 is suppressed.

[0025]

[0008] However, as the traveling-wave semiconductor optical amplifier, when the sandwiched SCH structure thick strained bulk active layer in the optical confinement layer is high when the active layer width W _a is wider There is a problem that the next mode becomes easy to start.

For example, according to the semi-vector analysis using the finite element method, a strained bulk active layer having a thickness of 200 nm and a composition of 1.55 nm has a thickness of 100 nm with a composition of 1.18 nm.
When the sandwiched Alcatel a light confinement layer in the layer thickness of m, to the waveguide of only the fundamental mode indicates that it must width W _a of the strained bulk active layer below 0.9 .mu.m.

Furthermore, According to the analysis, when the width W _a of the strained bulk active layer is below 0.9 .mu.m, the light confinement coefficient ratio gamma _TE / gamma _TM and polarization dependent gain difference .DELTA.G _TM-TE polarized waves is large changes, in order to suppress the polarization dependent gain difference ratio ΔG ⁱ _TE-TM constant value range in a certain amount of distortion, the width W _a of the strained bulk active layer
Must be controlled within a certain range, and this situation will be described with reference to FIG.

FIG. 10 (a) shows the optical confinement coefficient ratio 偏_TE / Γ _TM between the polarized waves.
It is a diagram showing the active layer width dependency, the active layer width W _a is 0.
9μm drops according rapidly optical confinement factor ratio gamma _TE / gamma _TM polarizations narrows below the active layer width W _a is gamma _TE = gamma _TM at about 0.3 [mu] m.

Referring to FIG. 10B, FIG. 10B shows the relationship between the current value at which the internal gain becomes 25 dB.
Difference ΔG between polarizations ⁱ _TE-TMShows active layer width dependence
FIG. 14 is a diagram showing an active layer width W in which the manufacturing process is relatively easy._a
Is larger, the gain difference between polarizations ΔGⁱ _TE-TMIs
Allowable value, for example, ABS (a (dB)) ≦ 0.5 dB
To do this, as is apparent from the figure, the active layer width W
_aIt is necessary to control within the range of 0.8μm ± 0.1μm.
You. Therefore, even if a strained bulk active layer is used,
If it is left as it is, quite high precision processing technology is required
become.

Therefore, an object of the present invention is to widen a gentle range of a change in the optical confinement coefficient ratio Γ _TE / Γ _TM between polarizations with respect to a change in the active layer width while maintaining the fundamental mode waveguide condition. I do.

[0031]

Here, means for solving the problems in the present invention will be described with reference to FIG. FIG. 1A is a top view, and FIG.
(B) is a schematic sectional view along the optical axis direction. See FIG. 1. (1) The present invention has a strained bulk active layer 1 made of a bulk crystal in which a tensile strain is introduced, and has a light incident end face 5 and a light output end face 6.
The signal light 7 having a wavelength substantially equal to the band gap wavelength of the distorted bulk active layer 1 is incident from the light incident end face 5, and current is injected into the distorted bulk active layer 1. In the traveling wave type semiconductor optical amplifier in which the signal light 7 is amplified by the stimulated emission effect and the amplified signal light 8 is emitted from the light emitting end face 6, two surfaces perpendicular to the thickness direction of the strained bulk active layer 1 are respectively formed. It is characterized by being sandwiched between single cladding layers 2 and 3.

As described above, the optical confinement layer, that is, the SCH layer is not provided on the strained bulk active layer 1 so that the strained bulk active layer 1 comes into direct contact with the clad layer, so that a higher-order mode is hardly generated. It is possible to widen a gentle range of the change of the optical confinement coefficient ratio Γ _TE / Γ _TM between the polarizations with respect to the change of the active layer width W _a while maintaining the waveguide condition, whereby the inter-polarization gain difference ΔG ⁱ _TE it is possible to widen the range of the active layer width W _a to below the allowable value a (dB) in the _-TM, can be relaxed fabrication process conditions. In the specification of the present application, the “single cladding layers 2 and 3” refer to a single cladding layer in which the entire p-side cladding layer and the entire n-side cladding layer acting as cladding layers in the entire configuration are deposited in a single step. It means a single clad layer 2 or 3, even if it is composed of a plurality of layers in the case where the base material is the same, even if the deposition process is a multi-step process, as a matter of course. It is.

For example, according to the semi-vector analysis using the finite element method, the strained bulk active layer 1 made of InGaAsP having a wavelength composition of 1.55 nm and a thickness of 200 nm is formed by InP.
This shows that the width of the strained bulk active layer 1 may be 1.2 μm or less in order to make only the fundamental mode waveguide when directly sandwiched by the cladding layers. In this case, the light confinement coefficient ratio between the polarizations 波_TE / _{Γ TM} region having an almost constant spreads.

In order to make the gain between polarizations ΔG ⁱ _TE-TM at 0.5 dB or less at a current value at which the internal gain is 25 dB, the active layer width W _a must be within a range of 0.8 μm ± 0.4 μm. may be controlled, tolerance of the active layer width W _a is a conventional 4
Double.

(2) Further, according to the present invention, in the above (1), the layer thickness of the strained bulk active layer 1 is 175 to 250 nm, and the introduced strain amount is -0.13 to -0.23%. It is characterized by being.

As described above, the layer thickness of the strained bulk active layer 1 is 17
5 to 250 nm, and the introduced strain amount is -0.1
If 3 to -0.23%, do not provide SCH layer
2 to 6, since the effect of
And the layer thickness d of the strained bulk active layer 1 _aFrom 100 nm to 300
Optical confinement coefficient ratio between polarizations when changed in the range of nm
_TE/ Γ_TMOf active layer width and gain difference ΔG between polarizationsⁱ
_TE-TMOf the active layer width will be described.

[0037] FIGS. 2 (a) see FIG. 2 (a), the layer thickness d _a of the strained bulk active layer is 100nm
FIG. 9 is a diagram showing the dependence of the optical confinement coefficient ratio 偏_TE / Γ _TM between the polarizations on the active layer width in the case of FIG.
Although the region where the optical confinement coefficient ratio Γ _TE / Γ _TM between polarizations is almost constant is wider than that in the case where the CH layer is provided, the SCH
Even when no layer is provided, it is almost constant over a considerably wide range.

[0038] FIG. 2 (b) when the layer thickness d _a of the reference view. 2 (b) is strained bulk active layer is 100 nm, the activity of the polarization dependent gain difference ΔG ⁱ _TE-TM at a current value that internal gain is 25dB is a diagram showing a layer width dependency, the case without the SCH layer, the active layer width W _a to the less than 0.5dB polarization dependent gain difference ΔG ⁱ _TE-TM as compared with the case of providing the SCH layer Although the range is slightly expanded, S
Even when the CH layer is not provided, the value is 0.5 dB or less in a considerably wide range of 1.0 μm ± 0.4 μm.

FIG. 3A shows the thickness d of the strained bulk active layer._aIs 150 nm
Optical confinement coefficient ratio between polarizations in the case of Γ_TE/ Γ_TMActive layer
FIG. 5 is a diagram showing width dependence, in which a SCH layer is also provided.
When there is no optical closing between the polarizations compared to the case where the SCH layer is provided
Constriction coefficient ratio Γ _TE/ Γ_TMBut the area that is almost constant is expanding
However, even when the SCH layer is not provided,
It is constant.

[0040] see FIG. 3 (b) FIG. 3 (b) when the layer thickness d _a of the strained bulk active layer is 150 nm, the polarization dependent gain difference ΔG ⁱ _TE-TM activity at a current value that internal gain is 25dB FIG. 5 is a diagram showing the layer width dependency. In this case, too, when the SCH layer is not provided, the inter-polarization gain difference ΔG ⁱ _TE-TM is 0.5
Although the scope of the active layer width W _a to a dB or less has spread slightly, 1.0 .mu.m ± even if not provided SCH layer
It is 0.5 dB or less in a very wide range of 0.3 μm.

[0041] FIGS. 4 (a) see FIG. 4 (a), the layer thickness d _a of the strained bulk active layer is 200nm
FIG. 4 is a diagram showing the dependence of the optical confinement coefficient ratio Γ _TE / Γ _TM between polarizations on the active layer width in the case of (1). In this case, when the SCH layer is not provided, the polarization the spread of light confinement coefficient ratio gamma _TE / gamma _TM region having an almost constant becomes remarkable, quite narrow region having an almost constant light confinement coefficient ratio gamma _TE / gamma _TM polarizations in the case without the SCH layer Has become.

[0042] refer to FIG. 4 (b) FIG. 4 (b) when the layer thickness d _a of the strained bulk active layer is 200 nm, the polarization dependent gain difference ΔG ⁱ _TE-TM activity at a current value that internal gain is 25dB FIG. 6 is a diagram illustrating the layer width dependency. In this case, when the SCH layer is not provided, the gain difference between polarizations ΔG ⁱ _{TE-TM is set} to 0.
Although the range of the active layer width W _a to the 5dB below has spread, 0.8 [mu] m ± the case without the SCH layer 0.
It is as narrow as 1 μm.

[0043] FIGS. 5 (a) see FIG. 5 (a), the layer thickness d _a of the strained bulk active layer is 250nm
FIG. 4 is a diagram showing the dependence of the optical confinement coefficient ratio Γ _TE / Γ _TM between polarizations on the active layer width in the case of (1). In this case, when the SCH layer is not provided, the polarization the spread of light confinement coefficient ratio gamma _TE / gamma _TM region having an almost constant becomes remarkable, quite narrow region having an almost constant light confinement coefficient ratio gamma _TE / gamma _TM polarizations in the case without the SCH layer Has become.

[0044] see FIG. 5 (b) FIG. 5 (b) when the layer thickness d _a of the strained bulk active layer is 250 nm, the polarization dependent gain difference ΔG ⁱ _TE-TM activity at a current value that internal gain is 25dB FIG. 6 is a diagram illustrating the layer width dependency. In this case, when the SCH layer is not provided, the gain difference between polarizations ΔG ⁱ _{TE-TM is set} to 0.
Although the scope of the active layer width W _a of the order to 5dB below has spread somewhat, since the absolute value is small as 0.7 [mu] m ± 0.2 [mu] m, the upper limit the effect of not providing the SCH layer is obtained . When the SCH layer is not provided, the width is considerably narrowed to 0.7 μm ± 0.1 μm.

[0045] FIGS. 6 (a) see FIG. 6 (a), the layer thickness d _a of the strained bulk active layer is 300nm
FIG. 4 is a diagram showing the dependence of the optical confinement coefficient ratio Γ _TE / Γ _TM between polarizations on the active layer width in the case of (1). In this case, when the SCH layer is not provided, the polarization Although the light confinement coefficient ratio Γ _TE / Γ _TM has an almost constant expansion, the absolute expansion is narrow.

[0046] FIG. 6 (b) see FIG. 6 (b) when the layer thickness d _a of the strained bulk active layer is 300 nm, the polarization dependent gain difference ΔG ⁱ _TE-TM activity at a current value that internal gain is 25dB FIG. 6 is a diagram illustrating the layer width dependency. In this case, when the SCH layer is not provided, the gain difference between polarizations ΔG ⁱ _{TE-TM is set} to 0.
Although the scope of the active layer width W _a of the order to 5dB below has spread slightly, the absolute value thereof is 0.75 .mu.m ± 0.15 [mu] m
Therefore, the practical effect of not providing the SCH layer cannot be expected. When the SCH layer is not provided, the width is considerably narrowed to 0.7 μm ± 0.1 μm.

Judging comprehensively from the above, the strain bulk activity
Layer thickness d of layer 1_aIs 100 nm or 150 nm
In the case of 175 nm or less, the active layer
Width W _aIs sufficiently large, while the strained bulk active layer
Layer thickness d_aIs greater than 250 nm, such as 300 nm, the SCH
Active layer width W even without layer_aThe tolerance of the
Layer thickness d of luk active layer 1_a175 to 250 nm
Is preferable, and in that case, the distortion amount is −0.13.
~ -0.23% is preferred.

[0048]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Here, an embodiment of the present invention will be described with reference to FIGS. 7 and 8. First, referring to FIG. 7, a traveling-wave type semiconductor according to an embodiment of the present invention will be described. A schematic configuration of the optical amplifier will be described. FIG. 7 is a schematic perspective view of a traveling-wave semiconductor optical amplifier according to the present invention. In the first half, p represents a state of an active layer.
-Type InP buried layer 15, n-type InP current block layer 16,
p-type InP cladding layers 14 and 17, p-type InGaAs contact layer 18, SiO ₂ film 19, and p-side electrode 20
Are not shown.

First, the MOCV is placed on the n-type InP substrate 11.
Using Method D (metal organic chemical vapor deposition), for example, an n-type InP buffer layer 12 having a thickness of 180 nm, a tensile strain amount of −0.13 to −0.23%, and a thickness of 175-
An InGaAsP strained bulk active layer 13 having a wavelength composition of 1.55 μm of 250 nm and a p-type InP clad layer 14 are sequentially deposited.

Then, after depositing an SiO ₂ film on the entire surface, the major axis is inclined by, for example, 7 ° with respect to the plane to be the cleavage plane and the width is set to 0.6 to This is patterned into a stripe shape of 1.4 μm, and this stripe-shaped SiO ₂ mask (not shown)
Is performed by reactive ion etching (RIE) using C ₂ H ₆ + H ₂ + O ₂ until the n-type InP buffer layer 12 is reached, and the active layer width W _a
Forms a stripe-shaped mesa of 0.6 to 1.4 μm.

Next, using the SiO ₂ mask as a selective growth mask, p-type InP
The buried layer 15 and the n-type InP current block layer 16 are selectively grown. Next, after removing the SiO ₂ mask, a p-type InP cladding layer 17 and a p-type InGaAs contact layer 18 are sequentially deposited on the entire surface.

Next, an SiO ₂ film 19 is deposited on the entire surface, an opening is formed so as to project on the stripe-shaped mesa, and then a p-side electrode 20 is formed, and an n-type I
On the back surface of the nP substrate 11, an n-side electrode 21 is formed.

Next, after cleaving along the cleavage plane, a dielectric multilayer film is deposited on the cleavage plane to form an anti-reflection coating film 22,
By setting to 23, the basic configuration of the traveling wave semiconductor amplifier is completed. The combination of the p-type InP clad layer 14 and the p-type InP clad layer 17 is a “single clad layer” in the present application,
When the buffer layer 12 is thin, the combination of the n-type InP buffer layer 12 and the n-type InP substrate 11 becomes a “single cladding layer” in the present application.

In this traveling wave type semiconductor optical amplifier,
Since the non-reflection coating films 22 and 23 are provided on the cleavage plane, that is, the light incident end face and the light output end face, light resonance due to reflection between the light incident end face and the light output end face is suppressed. The signal input light 24 near 55 μm is converted to InGaAs.
The light is amplified by the stimulated emission effect in the P-strained bulk active layer 14 and is emitted as amplified output light 25 from the light emitting end face.

Next, the operation and effect of the embodiment of the present invention will be described with reference to FIG. 8 See FIG. 8, the layer thickness d _a of the InGaAsP strained bulk active layer 13
Is 200 nm, the absolute value AB of the inter-polarization gain difference ΔG ⁱ _{TE-TM at} a current value at which the internal gain is 25 dB
This figure shows the measured values of the active layer width dependence of S (ΔG ⁱ _TE-TM ) for different tensile strain amounts. As the strain amount increases, the absolute value ABS (ΔG ⁱ _{TE− TM)} is reduced, tensile strain is smallest active layer width dependency in the case of -0.21%, the active layer width W _a is 0.6 to 1.4
Absolute ABS of gain difference between polarizations over a wide range of μm
(ΔG ⁱ _TE-TM ) is 0.5 to 0.7 dB.

The distortion amount was 0% (no distortion) and -0.11.
% Of the cases, polarization dependent gain difference .DELTA.G ⁱ by the active layer width W _a
Although _TE-TM is different about 1 dB, the actual of the maximum and minimum gain measurement with a pseudo-random polarized light also affected, such as aging of the ripples and the optical coupling due to residual reflection of the end face, polarization gain difference .DELTA.G ⁱ _TE It is considered that the active layer width dependence of _-TM is small.

In the optical waveguide mode analysis, the active layer width W
Although _a got predicted that there is a higher-order mode at 1.4 [mu] m, in the range of this evaluation, the particular problem was observed on the properties, the active layer width W _a, 1.4 [mu] m
Up to acceptable.

As described above, by adjusting the amount of strain introduced into the tensile strain active layer, the active layer width W _a in the manufacturing process is adjusted.
It has been confirmed that a gain-polarization-independent semiconductor optical amplifier having a large tolerance to the fluctuation of the optical signal can be realized.

Although the embodiments of the present invention have been described above, the present invention is not limited to the configurations and conditions described in the embodiments, and various modifications are possible. For example, in the above embodiment, the active layer of the semiconductor optical amplifier has a 1.55 μm wavelength composition so that the attenuation in the optical long-distance communication fiber is small, but other compositions such as a 1.3 μm wavelength composition are used. But it's good.

Further, in the above embodiment, In
Although described as a GaAsP / InP optical amplifier,
Other compound semiconductors such as GaAs / AlGaAs may be used as long as the combination of the active layer and the cladding layer can introduce a predetermined amount of tensile strain into the active layer.

In the above embodiment, the n-type InP buffer layer is used as the n-side cladding layer. However, the n-type InP cladding layer is not provided, and the n-type InP buffer layer is formed on the n-type InP substrate.
The GaAsP strained active layer may be directly grown, and the n-type InP substrate may be used as the n-side cladding layer.

In the above-described embodiment, the current constriction is performed by providing the current block layer on the side of the stripe-shaped mesa. However, the current confinement is not indispensable. It is a good thing.

[0063]

According to the present invention, the cladding layer is provided on two surfaces perpendicular to the layer thickness direction of the striped tensile-strained bulk active layer without the SCH layer interposed therebetween. , The gentle range of the change in the optical confinement coefficient ratio Γ _TE / Γ _TM between the polarizations with respect to the change in the active layer width can be increased, whereby the gain difference ΔG ⁱ _TE between the predetermined values can be increased. it is possible to widen the allowable range of the active layer width W _a to the _-TM, since tolerance to kick the fabrication process is increased, to achieve a high performance polarization-dependence of the traveling-wave semiconductor optical amplifier This greatly contributes to the practical application of the wavelength division multiplexing optical communication system.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a basic configuration of the present invention.

FIG. 2 is a characteristic diagram when _a layer thickness da of _a strained bulk active layer is 100 nm.

FIG. 3 is a characteristic diagram when _a layer thickness da of _a strained bulk active layer is 150 nm.

[Figure 4] a strained bulk active layer thickness d _a is a characteristic diagram in the case of 200 nm.

5 is a characteristic diagram when the layer thickness d _a of the strained bulk active layer is 250 nm.

[6] of the strained bulk active layer thickness d _a is a characteristic diagram in the case of 300 nm.

FIG. 7 is a schematic perspective view of a traveling-wave semiconductor optical amplifier according to an embodiment of the present invention.

FIG. 8 is an explanatory diagram of an operation and effect in the embodiment of the present invention.

FIG. 9 is a schematic perspective view of a conventional traveling wave type semiconductor optical amplifier.

FIG. 10 is an explanatory diagram of a problem in a conventional traveling-wave semiconductor optical amplifier.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 strained bulk active layer 2 single clad layer 3 single clad layer 4 buried layer 5 light incident end face 6 light output end face 7 signal light 8 amplified signal light 11 n-type InP substrate 12 n-type InP buffer layer 13 InGaAsP Strained bulk active layer 14 p-type InP cladding layer 15 p-type InP buried layer 16 n-type InP current blocking layer 17 p-type InP cladding layer 18 p-type InGaAs contact layer 19 SiO ₂ film 20 p-side electrode 21 n-side electrode 22 None Reflection coating film 23 Non-reflection coating film 24 Signal input light 25 Amplified output light 31 n-type InP substrate 32 n-type InP buffer layer 33 InGaAsP light confinement layer 34 InGaAsP strain bulk active layer 35 InGaAsP light confinement layer 36 p-type InP cladding layer 37 p-type InP buried layer 38 n-type InP current blocking layer 39 p InP cladding layer 40 p-type InGaAs contact layer 41 SiO ₂ layer 42 p-side electrode 43 n-side electrode 44 antireflection coating film 45 non-reflective coating film 46 signal input light 47 amplified output light

Claims (2)

[Claims] 1. A strained bulk active layer comprising a bulk crystal in which a tensile strain has been introduced, wherein resonance of light due to reflection between a light incident end face and a light emitting end face is suppressed, and a band of the strained bulk active layer is suppressed. A signal light having a wavelength substantially equal to the gap wavelength is incident from the light incident end face, current is injected into the strained bulk active layer to amplify the signal light by a stimulated emission effect, and the amplified signal light is emitted from the light emitting end face. A traveling wave type semiconductor optical amplifying device, wherein two surfaces perpendicular to the thickness direction of the strained bulk active layer are sandwiched by a single cladding layer. . 2. The strained bulk active layer having a thickness of 175 to 2
50 nm, and the introduced strain amount is -0.13.
2. The traveling-wave semiconductor optical amplifier according to claim 1, wherein the value is .about.-0.23%.