JPH04291780A - Semiconductor light emitting apparatus - Google Patents

Abstract

PURPOSE:To provide a semiconductor light emitting apparatus in which reflected light is prevented from returning to a laser and laser oscillation is stabilized. CONSTITUTION:The semiconductor light emitting apparatus comprises a laser section A having an InGaAsP active layer 41, an optical modulator section B having an InGaAsP optical wave guide layer of length L1, and a returning light blocking optical wave guide section C having an InGaAsP optical wave guide layer 43 of length L2. It is constituted to satisfy following relations; T1<=(L1Xn1+2XL2Xn2)/c, T2>=(L1Xn1+2XL2Xn2)/c, where T1, T2 are time intervals within a single modulation period during which the optical modulator section B transmits or intercepts the light respectively.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a semiconductor light emitting device.
In particular, the present invention relates to a semiconductor light emitting device with an optical modulator in which a double heterojunction semiconductor laser and an electroabsorption optical modulator having a double junction are formed on the same semiconductor substrate so as to have a continuous optical waveguide with the same optical axis.

0002

2. Description of the Related Art A conventional semiconductor light emitting device with an optical modulator will be explained with reference to a sectional view shown in FIG. A laser section A and an optical modulator section B are formed on a semiconductor substrate 41. The active layer 43 of the laser section A and the optical waveguide layer 44 of the optical modulator section B, which are double heterojunctioned between the semiconductor substrate 41 and the cladding layer 42, are formed continuously with the same optical axis. ing. Further, a laser section electrode 45 is formed on the cladding layer 42 of the laser section A, and an optical modulator section electrode 46 is formed on the cladding layer 42 of the optical modulator section B. Further, a common electrode 47 is formed on the back surface of the semiconductor substrate 41.

In such a semiconductor light emitting device with an optical modulator, when the light oscillated by the laser part A is output from the end face on the optical modulator part B side, the reflected light from the end face is reflected by the laser part A. returns and makes the laser oscillation unstable. Another problem is that the monochromaticity and narrow line width characteristics of the laser oscillation light are deteriorated. Therefore, in order to prevent reflection at the end face on the side of the optical modulator section B, conventionally, an anti-reflection film 48 was provided on the end face to perform anti-reflection treatment. The most effective method in this case is that the refractive index n of the thin film is equal to the square root of the effective refractive index n1 of the optical waveguide layer 44 of the optical modulator section B,
This method forms a dielectric thin film whose thickness is equal to 1/(4×n) of the laser oscillation wavelength λ. For example, in an InP-based semiconductor light-emitting device with an optical modulator, the refractive index is 1.9.
, a SiN thin film with a thickness of about 200 nm is used.

0004

However, in the conventional method of providing the antireflection film 48 on the end face on the optical modulator section B side, for example, in the case of an InP-based semiconductor light-emitting device with an optical modulator, (1) SiN Since the refractive index and formation rate of the thin film vary depending on the manufacturing method, atmospheric environment, etc., the thickness and refractive index of the SiN thin film cannot be controlled with perfect accuracy. (2) The effective refractive index of the optical waveguide layer 44 of the optical modulator section B also depends on the semiconductor composition and thickness and slightly varies depending on the device. It is difficult to achieve a good reflectance of about 0.1% or less.

[0005] Moreover, in reality, the laser is only 0.1%
Since even less return light would cause instability, a major challenge was to further reduce the influence of reflected light. SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor light emitting device that can prevent reflected light from returning to the laser and stabilize laser oscillation.

[0006]

[Means for Solving the Problems] The above-mentioned problems include a laser section formed on a semiconductor substrate, a laser section formed on the semiconductor substrate in contact with the laser section, and an optical axis that is the same as an active layer of the laser section. an optical waveguide section for preventing return light, the optical waveguide section having an optical waveguide formed on the semiconductor substrate in contact with the optical modulation section and having the same optical axis as the optical waveguide of the optical modulation section; a length L1 of the optical waveguide of the optical modulation section, a length L2 of the optical waveguide of the optical waveguide section for preventing return light, and a refractive index n1 of the optical waveguide of the optical modulation section.
, the refractive index n2 of the optical waveguide of the optical waveguide section for preventing return light;
, the time T for transmitting light during one modulation period of the light modulation section
1. The time T2 during which light is blocked during one modulation period of the light modulation section is determined by the following formula T1 ≦ (L1 × n1 + 2 × L2 × n2 )/c
T2 ≧(L1 ×n1 +2×L2 ×n2)/c
However, c is achieved by a semiconductor light emitting device characterized by a relationship that satisfies the speed of light.

[0007]

[Operation] The present invention provides an optical waveguide section for preventing return light having an optical waveguide whose optical axis is the same as that of the optical waveguide of the optical modulation section, in contact with the optical modulation section, and a length L1 of the optical waveguide of the optical modulation section. and its refractive index n1, the length L2 of the optical waveguide of the optical waveguide section for preventing return light, and its refractive index n2, and the time T1 for transmitting light and the time T2 for blocking light in one modulation period of the light modulating section. In this relationship, by controlling to a predetermined value, the reflected light reflected at the end face on the side of the light-preventing optical waveguide section can be blocked by the light modulation section before returning to the laser section.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on illustrative embodiments. FIG. 1(a) is a sectional view showing a cross section perpendicular to the resonator direction of a laser section of a semiconductor light emitting device with an optical modulator according to an embodiment of the present invention, and FIG.
FIG.

[0009] Thickness: 100 μm, impurity concentration: 2×10 18
cm-3 n-type InP substrate 1 with an energy band gap of 1 eV, which has a larger refractive index than the n-type InP substrate 1.
An InGaAsP optical guide layer 2 is formed. This InGaAsP light guide layer 2 and n-type InP substrate 1
A part of the interface has a pitch of 215 nm and a depth of 150 nm.
, a corrugation 3 having a length of 300 μm is formed. The thickness of the InGaAsP light guide layer 2 in the portion where the corrugations 3 are not present is 0.3 μm.

[0010] InGaAs above this corrugation 3
An InGaAsP active layer 41 having a refractive index larger than that of the InGaAsP light guide layer 2, a thickness of 0.2 μm, and an energy band gap of 0.8 eV is formed on the P light guide layer 2. Further, the InGaAsP light guide layer 2 is in contact with the InGaAsP active layer 41, and the length L1 = 20
0μm, thickness 0.4μm, energy band gap 0
．． An InGaAsP optical waveguide layer 42 of 885 eV and a refractive index n1 is formed. Further, on the InGaAsP optical waveguide layer 2 and in contact with the InGaAsP optical waveguide layer 42, InG having a length L2 = 1000 μm, a thickness 0.4 μm, an energy band gap of 0.885 eV, and a refractive index n2 is formed.
An aAsP optical waveguide layer 43 is formed. And In
GaAsP active layer 41, InGaAsP optical waveguide layer 42
, and the InGaAsP optical waveguide layer 43, including the InGaAsP optical waveguide layer 2 below them, have a continuous stripe structure with a width W=1.5 μm and having the same optical axis.

[0011] Furthermore, these striped InGaA
sP active layer 41, InGaAsP optical waveguide layer 42 and I
On the top and side surfaces of the nGaAsP optical waveguide layer 43, a p-type InP cladding layer 5 with an impurity concentration of 5 x 1017 cm-3 is provided.
is formed. In this way, the InGaAsP active layer 4
1. The InGaAsP optical waveguide layer 42, the InGaAsP optical waveguide layer 43, and the InGaAsP optical guide layer 2 have a double heterostructure sandwiched between the p-type InP cladding layer 5 and the n-type InP substrate 1 on the upper and lower sides.

[0012] The n-type InP cladding layer 5
The thickness from the interface of the P substrate 1 to the surface is 2.7 μm, but the thickness is 0 μm from the interface of the n-type InP substrate 1.
．． 7μm, n-type In with impurity concentration 5x1017cm-3
A P current blocking layer 6 is formed. Also, InGa
On the p-type InP cladding layer 5 above the AsP active layer 41, there is a laser part electrode 7 having a three-layer structure of Ti, Pt, and Au.
is formed, and on the p-type InP cladding layer 5 above the InGaAsP optical waveguide layer 42, an optical modulator electrode 8 having a three-layer structure of Ti, Pt, and Au is formed. A common electrode 9 made of only an Au layer is formed on the back surface of the n-type InP substrate 1.

Further, a high reflection film 10 consisting of a thin SiN film with a thickness of 10 nm and an Au metal film is formed on the end face on the InGaAsP active layer 41 side, and a high reflection film 10 with a thickness of 10 nm on the end face on the InGaAsP optical waveguide layer 43 side is formed. An antireflection film 11 made of a 200 nm SiN film and having a refractive index of 1.9 is formed. In this way, the laser part electrode 7 has a length of 300 μm.
The laser section A has an InGaAsP active layer 41 and the optical modulator section electrode 8 is made of InG with a length L1 = 200 μm.
An optical modulator part B having an aAsP optical waveguide layer 42 and an InGaAsP optical waveguide layer 4 having a length L2 = 1000 μm.
An optical waveguide section C for preventing return light is constructed.

Note that n1 is the refractive index of the InGaAsP optical waveguide layer 42 and the InGaAsP optical waveguide layer 43,
To be precise, n2 is the effective refractive index of the optical waveguides of the optical modulator section B and the optical waveguide section C for preventing return light, respectively. These effective refractive indices n1 and n2 are both approximately 3 for the laser oscillation wavelength λ = 1.55 μm in this case.
．． It is 6.

Next, FIG. 2 shows the voltage applied to the optical modulator section electrode 8 of the optical modulator section B of the semiconductor light emitting device with an optical modulator shown in FIG. 1 and the waveform of the output light. FIG. 2(a) shows the optical modulator section electrode 8.
FIG. 2B shows the light intensity of the light output from the end face on the side of the optical waveguide section C for preventing return light, corresponding thereto. Note that although there is a time difference between the two that corresponds to the spatial propagation time, it is not considered in FIG. 2 because it is not important for the explanation.

T, T1, and T2 in the figure are the modulation period of the light modulation section B, the time for transmitting light in one modulation period of the light modulation section B, and the time for blocking light in one modulation period of the light modulation section B, respectively. It's time. And the length L1 of the optical waveguide 42 of the optical modulation section B
and its refractive index n1, the length L2 of the optical waveguide 43 of the optical waveguide section C for preventing return light, and its refractive index n2, T1 ≦ (L1 × n1 + 2 × L2 × n2
)/cT2 ≧(L1 ×n1 +2×L2 ×n2
)/c. Here, c is the speed of light in vacuum.

Next, the operation of the semiconductor light emitting device with an optical modulator shown in FIG. 1 will be explained using FIG. 3. FIG. 3 is a diagram for explaining the distribution of light passing through the semiconductor light emitting device with an optical modulator when the semiconductor light emitting device with an optical modulator shown in FIG. 1 is driven in time series as shown in FIG. . Note that here, the boundary between the laser section A and the optical modulator section B, and the boundary between the optical modulator section B and the optical waveguide section C for preventing return light are each indicated by a broken line. Further, 121 is the light existing in the laser section A, 122 is the light existing in the optical waveguide section C for preventing return light, and 123 is the light reflected by the end face on the side of the optical waveguide section C for preventing return light. Light in the optical waveguide section C for light prevention, 124 is the optical modulator section B
and 13 indicate the output light output from the end face on the side of the optical waveguide section C for preventing return light.

FIGS. 3(a) to 3(h) are respectively t of FIG.
It shows where the light exists in the waveguide at times 1 to t6. Also, among these, Figure 3 (c
), (d) are at the time between t2 and t3. It is assumed that the optical modulator section B is in a state of not transmitting light until time t1. At time t1, the optical modulator section B enters a light transmitting state. The time difference up to time t2 is T1. Here, T1 ≦ (L1 × n1 + 2 × L2 × n2 )/c
Therefore, at time t2, the light has already reached the end face on the side of the optical waveguide section C for preventing return light, and is reflected, so that the light 123 comes to a position as shown in FIG. 3(b). Here, the distance ΔL from the tip of the light 123 to the boundary between the optical modulator section B and the optical waveguide section C for preventing return light is zero or larger than zero. This is because (L1 × n1 )/c in the above formula is the time required for light to pass through the optical modulator section B, and (2 × L
2 × n2 )/c is the time required for the light to travel back and forth through the optical waveguide section C for preventing return light, and T1
This is because the relationship is ≦(L1×n1 +2×L2×n2)/c.

At this time, ΔL is as follows: ΔL={(L1 ×n1 +2×L2 ×n2)−T
1 }×c The following conditions are satisfied. Therefore, T1 = (L1 × n1 + 2 × L2 × n2 )/c
In the case of ΔL=0, and T1 < (L1 × n1 + 2 × L2 × n2 )/c
In this case, ΔL>0.

[0020] This will be explained using specific numerical values. Now, the period T of the modulation signal shown in Fig. 2 is T = 55 p seconds (18 GHz
, RZ modulation), and T1 and T2 are each T1 =
Assume that 26 p seconds and T2 = 29 p seconds. Time t=t
2, the light travels through the optical waveguide C for preventing light return from {26p
sec×3×1014 (μm/sec)}/3.6=2167μ
It should have advanced by m.

Therefore, ΔL in FIG. 3(b) is ΔL=
It is 33 μm. Next, at time t2, the optical modulator section B begins to absorb light, and does not transmit light until time t3. As time t3 approaches, FIGS. 3(c) and 3(d)
), and at time t3, no light exists in the optical modulator section B and the optical waveguide section C for preventing return light. Here, 123 and 124 indicated by dotted lines are areas where no light exists.

That is, the last part of the light that has passed through the optical modulator section B up to time t3 travels through the optical waveguide at {29 p seconds×3×1014 (μm/second)}/3.6=2
It advances by 417 μm. The distance that the light that was in the part of the optical modulator section B closest to the laser section A at the instant of time t2 passes through before being reflected by the end face on the side of the optical waveguide section C for preventing return light and returning is (200 μm + 1000 μm). )×2=2400 μm, all the reflected light is absorbed by the optical modulator section B, and no light remains in the optical waveguide. Therefore, next, the optical modulator section B
Even if it becomes a transmitting state, the reflected light does not return to the laser section A. Thereafter, in the same manner, at times t4, t5, and t6, the values change as shown in FIGS. 3(e), (f), (g), and (h).

As described above, according to this embodiment, the laser section A
Even if the oscillated light is reflected by the end face on the side of the optical waveguide section C for preventing return light, the reflected light will not return to the laser section A. Therefore, under any conditions, the oscillation is not destabilized due to the return light to the laser section A, so the laser maintains stable oscillation, and the monochromaticity and narrow linewidth characteristics of the oscillated light are maintained. The above effects are new effects added to the effects of the antireflection treatment of the end face.

[0024] Note that reflections outside of this semiconductor light emitting device with an optical modulator, such as reflections that occur when optically splicing with an optical fiber, can be dealt with with an external optical isolator that utilizes the Faraday effect or the like. . Next, a semiconductor light emitting device according to another embodiment of the present invention will be described using a cross-sectional view shown in FIG.

On the same semiconductor substrate 21, a laser section A1 is provided.
In addition, a large number of optical elements 22 and 23 are formed with the same optical axis. That is, the optical waveguide layer 25 which is double heterojunctioned between the semiconductor substrate 21 and the cladding layer 24 is
They are formed continuously with the same optical axis. A common electrode 26 is formed on the back surface of the semiconductor substrate 21. Further, between the laser section A1 having the laser section electrode 28 and the other optical element 22 having the optical element section electrode 29, there is provided an optical modulator section B1 having an optical modulator section electrode 30, and this optical modulator section B1 having the optical modulator section electrode 30. An optical waveguide section C1 for preventing return light is formed adjacent to B. In addition, in the same manner, the optical element 22 and the optical element part electrode 3
Between the other optical elements 23 having the optical modulator section electrode 32
An optical modulator section B2 having an optical waveguide section B2 and an optical waveguide section C2 for preventing return light adjacent to the optical modulator section B2 are formed.

As in the case of the above embodiment, the length L1 and the refractive index n1 of the optical waveguides 25 of the optical modulation sections B1 and B2, and the length of the optical waveguide 43 of the optical waveguide sections C1 and C2 for preventing return light. L2 and its refractive index n2, light modulation section B
1. By controlling the modulation period T of B2, the time T1 during which the light modulator B transmits light, and the time T2 during which the light modulator B blocks light, the oscillated light from the laser section A1 is transmitted to the adjacent optical element 22.
It is possible to prevent the light from the optical element 22 from being reflected by the adjacent optical element 23 and returning to the laser section A, and it is also possible to prevent the light from the optical element 22 from being reflected by the adjacent optical element 23 and returning to the optical element 22.

As described above, according to this embodiment, the laser section A
1. Light modulation sections B1 and B provided between optical elements 22 and 23
2 and the return light prevention optical waveguide sections C1 and C2 function as an optical isolator and can prevent return light between the laser section and the optical element or between the optical elements, so that the operation of the laser and the optical element is reduced. can be kept stable. This can be effectively used in cases where a large number of optical elements are formed on the same semiconductor substrate to configure an optical logic circuit.

[0028]

As described above, according to the present invention, the laser section is formed on a semiconductor substrate, and the optical axis is the same as the active layer of the laser section, which is formed on the semiconductor substrate in contact with the laser section. A light modulation section having an optical waveguide, and an optical waveguide section for preventing return light, which is formed on a semiconductor substrate in contact with the light modulation section and has an optical waveguide having the same optical axis as the optical waveguide of the light modulation section. , the length L1 of the optical waveguide of the optical modulation section and its refractive index n1, the length L2 of the optical waveguide of the optical waveguide section for preventing return light
The light oscillated by the laser section is Even if the reflected light is reflected by the end face on the side of the optical waveguide section for preventing return light, the reflected light can be blocked by the optical modulation section before returning to the laser section, thereby preventing the reflected light from returning to the laser section. Can be done.

[0029] This prevents the oscillation from becoming unstable due to the return light to the laser section and maintains stable oscillation of the laser.
Monochromaticity, narrow line width characteristics, etc. of laser oscillation light can be improved.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a semiconductor light emitting device with an optical modulator according to an embodiment of the present invention.

FIG. 2 is a diagram for explaining a method of driving the semiconductor light emitting device with an optical modulator shown in FIG. 1;

3 is a diagram for explaining the distribution of light passing through the semiconductor light emitting device with an optical modulator when the semiconductor light emitting device with an optical modulator shown in FIG. 1 is driven in time series as shown in FIG. 2; FIG. .

FIG. 4 is a sectional view showing a semiconductor light emitting device according to another embodiment of the present invention.

FIG. 5 is a sectional view showing a conventional semiconductor light emitting device with an optical modulator.

[Explanation of symbols]

1... n-type InP substrate 2... InGaAsP optical guide layer 3... corrugation 41... InGaAsP active layer 42... InGaAsP optical waveguide layer 43... InGaAsP optical waveguide layer 5... p-type InP cladding layer 6... n-type InP current blocking layer 7... laser Part electrode 8...Light modulator part electrode 9...Common electrode 10...High reflection film 11...Anti-reflection film 121...Light in the active layer of the laser part 122...Light in the optical waveguide of the light modulator part 123...Prevention of return light Light 12 in the optical waveguide of the optical waveguide section
4... End face return light into the optical waveguide of the optical waveguide section for preventing return light 13... Output light 21... Semiconductor substrates 22, 23... Optical element 24... Clad layer 25... Optical waveguide layer 26... Common electrode 28... Laser part electrode 29, 31... Optical element section electrodes 30, 32... Optical modulator section electrode 41... Semiconductor substrate 42... Cladding layer 43... Active layer 44... Optical waveguide layer 45... Laser section electrode 46... Optical modulator section electrode 47... Common electrode 48...Anti-reflection film A, A1...Laser section B, B1, B2...Light modulator section C, C1, C2...Optical waveguide section for preventing return light L1...Length of the optical waveguide of the light modulation section L2...Prevention of return light Length n1 of the optical waveguide of the optical waveguide section for use
...Refractive index n2 of the optical waveguide of the optical modulation section ...Refractive index T of the optical waveguide of the optical waveguide section for preventing return light
...Modulation period T1 of the light modulation section ...Time T for light to pass through one modulation period of the light modulation section
2... Time c for blocking light during one modulation cycle of the light modulating section...
speed of light

Claims (1)

[Claims] 1. A laser section formed on a semiconductor substrate; a laser section formed on the semiconductor substrate in contact with the laser section;
an optical modulation section having an optical waveguide that has the same optical axis as the active layer of the laser section; and an optical modulation section that is formed on the semiconductor substrate in contact with the optical modulation section, and that has an optical axis that is the same as the optical waveguide of the optical modulation section. a length L1 of the optical waveguide of the optical modulation section, a length L2 of the optical waveguide of the optical waveguide section for preventing return light, and a length L2 of the optical waveguide of the optical modulation section; refractive index n1 of the optical waveguide, refractive index n2 of the optical waveguide of the optical waveguide section for preventing return light, time T1 for transmitting light within one modulation period of the optical modulation section, and time T1 during one modulation period of the optical modulation section. The time T2 for blocking light is determined by the following formula T1 ≦ (L1 × n1 + 2 × L2 × n2 )/c
T2 ≧(L1 ×n1 +2×L2 ×n2)/c
A semiconductor light emitting device characterized in that c has a relationship satisfying the speed of light.