JPH0773135B2 - Tunable semiconductor laser and transmission wavelength tunable optical filter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength tunable semiconductor laser and a wavelength tunable semiconductor optical filter.

[0002]

2. Description of the Related Art A semiconductor laser capable of controlling an oscillation wavelength and a variable wavelength semiconductor optical filter are indispensable elements for realizing future wavelength-multiplexed optical transmission and frequency-multiplexed optical transmission. These elements are required to have continuous controllability of the oscillation wavelength and the selection wavelength over a wide wavelength range. A wavelength tunable double-waveguide semiconductor laser (hereinafter referred to as TTG LD) is one of conventional examples of a lasing wavelength tunable semiconductor laser.
Can be given. As a report (report example 1), JP-A-2-2
46393 (European Patent Organization EP-8910259)
6.7), Aman et al., (Report example 2) IEEE Pro
ceedings-J, vol. 139, no. 1, p
p. 24-28, 1992, Yamamoto et al. This semiconductor laser has a diffraction grating inside the element, and has a basic structure of a distributed feedback structure that oscillates at a single wavelength by utilizing the wavelength selectivity resulting from the active layer that generates light gain and the change in refractive index. A tuning layer that causes the change and changes in the oscillation wavelength as a result is arranged vertically close to each other. Current can be independently injected into each layer, and laser oscillation can be generated by injecting current into the active layer, and the oscillation wavelength can be controlled by injecting current into the tuning layer or applying an electric field.

Optical filters are required to have no transmission plane dependence in their transmission characteristics. A conventional example of such a semiconductor optical filter is, for example, (Report Example 3) 1990.
The surface emitting laser type wavelength selective filter reported by Kubota et al. This optical filter is a type of light that is vertically incident on a wafer having a double hetero structure having a thick tuning layer. The Fabry-Perot etalon resonator composed of the upper and lower surfaces of the wafer can selectively transmit light of a certain wavelength. In Report Example 3, the transmission wavelength is not tuned, but the transmission wavelength can be controlled by injecting current into the tuning layer and changing the refractive index there.

[0004]

Both the TTG LD and the wavelength selective optical filter inject the current into the tuning layer and utilize the change in the refractive index (mainly due to the plasma effect) generated there to control the oscillation wavelength and the transmission wavelength. To control. The problem at this time is that the optical absorption loss inside the tuning layer increases due to the current injection. This absorption loss is mainly due to the absorption transition between the valence bands of the semiconductor, and has the property of increasing in proportion to the density of the holes by injecting holes simultaneously with the electrons into the tuning layer. is doing.

Therefore, in the case of the TTG LD, injection of the tuning current increases the oscillation threshold current of the laser and lowers the optical output, and eventually the laser oscillation is stopped or the desired optical output is maintained. By not being able to do so, the maximum variable width of the oscillation wavelength is limited. The same problem occurs in a so-called distributed reflection type semiconductor laser (DBR LD) in which an active region and a wavelength control region are arranged in the laser cavity direction and a tuning layer is provided in the wavelength control region.

On the other hand, in the case of the wavelength selective optical filter, the transmittance of light at the transmission wavelength is lowered due to the increase of the optical loss inside the tuning layer, and the extinction ratio between the transmission wavelength and the non-transmission wavelength band is small. There is a problem that it becomes.

An object of the present invention is to introduce a tuning layer having a layer structure in which an increase in absorption loss of light can be suppressed to a small level even by current injection, and on the other hand, a refractive index change which is the same as the conventional one can be obtained. A wide wavelength tunable semiconductor laser, and a semiconductor optical filter having a variable transmission wavelength in a wide range while maintaining a high extinction ratio.

[0008]

A tunable semiconductor laser according to the present invention comprises an active layer on a semiconductor substrate which produces a gain by current injection, a control layer which produces a refractive index change by current injection or voltage application, and the active layer. And a means for independently injecting current into the control layer, the control layer having a multiple quantum well structure having an effective forbidden band width larger than the energy of the wavelength of the oscillated light by 60 meV or more.

The wavelength tunable semiconductor optical filter of the present invention further comprises a control layer which causes a change in refractive index on the semiconductor substrate by current injection or voltage application, and a Fabry-Perot etalon resonator formed so as to sandwich the control layer. A means for injecting a current into the control layer is provided, and the control layer has an effective forbidden band width of 60 m or more than the wavelength energy of light to be controlled.
It is characterized by having a multiple quantum well structure larger than eV.

[0010]

The principle of the present invention will be described with reference to the drawings. FIG. 4 shows band diagrams of devices having tuning layers of various structures for TTG LDs, and the corresponding field distribution of light and distribution of injected carriers.
(A) is an example of a conventional element using a bulk semiconductor for the tuning layer, (b) is an example of using a multiple quantum well structure (hereinafter referred to as MQW structure) for the tuning layer, and (c) is an MQW structure tuning. It is the example which introduced the tensile strain into the well layer of a layer. In the conventional device, electrons and holes are distributed throughout the layer by injecting current into the tuning layer.
At this time, light absorption occurs due to the transition between valence bands in the region where holes are present, and the problems described in the problems of the conventional technique occur. The refractive index change is caused by the plasma effect in the region where electrons are present, that is, in the entire tuning layer.

Here, in order to reduce the light absorption due to the absorption between valence bands, it is effective to localize the holes, which cause the light absorption, in a narrow region and minimize the overlapping region with the light field. Is. On the other hand, in order to obtain a wide wavelength tunable range, it is necessary to distribute the electrons that cause a change in the refractive index almost uniformly over the entire tuning layer and widen the overlapping area with the optical field.

One of the structures that can realize these is the structure shown in FIG. 4B in which the MQW structure is adopted for the tuning layer. Generally, the density of states of holes in the valence band is about an order of magnitude higher than that of electrons in the conduction band, so as shown in the figure, most of the injected holes are confined in the narrow well layer. On the other hand, electrons can be distributed throughout the tuning layer by overflowing outside the well layer. At this time, it should be noted that the effective forbidden band width of the tuning layer needs to be sufficiently larger than the energy of the oscillation wavelength (60 meV or more).

The energy difference between the oscillation wavelength and the forbidden band width is 60
In the case of meV or less, according to the experiment, induced recombination transition due to laser oscillation light occurs in the tuning layer, and as a result, the carrier density change at the time of current injection is suppressed to a small level,
The refractive index change for wavelength control tends to be suppressed.
Therefore, in order to obtain a wide variable wavelength width, the energy difference needs to be 60 meV or more. In the report example 2, a TTG LD that uses the MQW structure for the tuning layer is shown, but the element of the report example does not satisfy the above conditions, and therefore (the variable wavelength width of about 4 nm is obtained due to the effect of heat generation). However, the variable wavelength width due to a pure carrier density change is as narrow as 1 nm or less.

In the MQW structure tuning layer,
By providing the well layer closer to the side farther than the active layer as shown in FIG. 6B, the light absorption due to the transition between valence bands can be further reduced. That is, the intensity of the light field is strongest in the active layer and becomes weaker as the distance from the active layer increases. Therefore, by providing the well layer on the side farther from the active layer, the overlap between the light field and the hole existing region becomes smaller. .

Further, FIG. 3C shows an example of a structure in which the light absorption can be further reduced by applying tensile strain stress to the well layer of the tuning layer. In the tensile strain MQW structure, band discontinuity in the valence band increases and conversely, conduction band discontinuity decreases. As a result, the wells in the valence band are deeper, while the wells in the conduction band are shallower, holes are more reliably confined in the well layer, and electrons are more evenly distributed throughout the tuning layer. A wider range of wavelength tuning characteristics can be obtained.

The principle of the present invention has been described above in the case of the TTG LD, but the same effect is expected in the current injection type wavelength tunable semiconductor optical filter.

[0017]

Embodiments Embodiment 1 of the present invention will be described in detail below with reference to the drawings. FIG. 1A is a diagram showing the structure of a wavelength tunable semiconductor laser having a wavelength of 1.55 μm, which is a first embodiment of the present invention. 240 nm period on the surface, depth 3
P-type InP substrate 1 on which a 0 nm diffraction grating 2 is formed
There is a ridge having a width of about 2 μm in which a tuning layer 3, an n-type InP spacer layer 4, an active layer 5, and a p-type InP clad layer 6 are laminated in this order from the bottom. An n-type InP contact layer 7 is laminated in contact with the side surface of the ridge, and a p-type InP buried layer 8 and ap ⁺ -type InGaAs cap layer 9 are sequentially laminated on the ridge. The thickness of each layer is 0.1 μm for the spacer layer 4, 0.5 μm for the cladding layer 6, 1 μm for the contact layer 7, 2 μm for the buried layer 8 and 0.3 μm for the cap layer 9. The electrode 1 is provided on the buried layer 8.
0, the electrode 11 on the contact layer, and the substrate 1
An electrode 12 is provided on the lower surface of the.

The layer structure of the tuning layer 3 and the active layer 5 is shown in FIG. 1 (b) using a schematic diagram of the energy band structure of a semiconductor. The active layer 5 has a total thickness of 130 nm and is made of In
GaAsP (wavelength composition 1.15 μm) barrier layer 120
And five InGaAs well layers 110 having a thickness of 10 nm. The well layers are barrier layers 12 each having a thickness of 10 nm.
Separated by 0. Tuning layer 3 has a total thickness of 3
00 nm, InGaAsP (wavelength composition 1.30 μ
m) Barrier layer 140 and three layers of 5 nm thick InGaA
sP (wavelength composition 1.45 μm) well layer 130.
The well layers 130 are barrier layers 140 each having a thickness of 10 nm.
And is located at approximately the center of the tuning layer 5. The effective forbidden band width of the tuning layer 3 is 0.8
75 eV (corresponding light wavelength 1.417 nm),
Energy of laser oscillation wavelength 1.55 μm (0.8e
It is 75 meV larger than V).

When a device having a device length of 600 μm was manufactured, laser oscillation was exhibited at an oscillation threshold current of 25 mA, and a maximum optical output of 15 mW was obtained. The variable wavelength width was 10 nm for a tuning current of 150 mA. FIG. 5 shows the relationship between the measured wavelength variation and the optical output. It can be seen that the decrease in the optical output at the time of wavelength tuning is suppressed as compared with the conventional case where the bulk semiconductor is used for the tuning layer, and as a result, the variable wavelength width is improved from 5 nm of the conventional element to 10 nm.

(Embodiment 2) FIG. 2A is a diagram showing the structure of a wavelength tunable semiconductor laser having a wavelength of 1.55 μm according to a second embodiment of the present invention. On the p-type InP substrate 1 on the surface of which a diffraction grating 2 having a period of 240 nm and a depth of 30 nm is formed, n-type InGaAsP (wavelength composition 1.3
μm) Guide layer 21, n-type InP buffer layer 22, active layer 5, n-type InP spacer layer 4, tuning layer 3, p
The type InP clad layer 6 has a ridge having a width of about 2 μm, which is sequentially stacked. An n-type InP contact layer 7 is contacted with the side surface of the ridge, and a p-type InP contact layer 7 is formed on the ridge.
The buried layer 8 and the p ⁺ type InGaAs gap layer 9 are sequentially stacked. The thickness of each layer is 0.1μ for the guide layer 21.
m, the buffer layer 22 is 0.1 μm, and the spacer layer 4 is 0.
1 μm, clad layer 6 is 0.5 μm, contact layer 7 is 1 μm, buried layer 8 is 2 μm, cap layer 9 is 0.3 μm.
μm. The electrode 10 is on the buried layer 8, the electrode 11 is on the contact layer, and the electrode 1 is on the substrate 1.
Two are provided.

The layer structure of the tuning layer 3 and the active layer 5 is shown in FIG. 2 (b) using a schematic diagram of the semiconductor energy band structure. The active layer 5 has a total thickness of 130 nm and is made of In
GaAsP (wavelength composition 1.15 μm) barrier layer 120
And five InGaAs well layers 110 having a thickness of 10 nm. The well layers are barrier layers 12 each having a thickness of 10 nm.
Separated by 0. The tuning layer 3 has a total thickness of 300 nm and is made of InGaAsP (wavelength composition 1.30).
μm) barrier layer 141 and three layers of 10 nm thick InG
aAsP (wavelength composition 1.45 μm) well layer 131, and the well layers 131 are barrier layers 14 each having a thickness of 5 nm.
Separated by 1. Further, tensile strain corresponding to lattice mismatch of about 1% is introduced into the well layer 131 during crystal growth. The well layer 131 is provided in the tuning layer 5 so as to be closer to the side farther from the active layer 3. The difference between the effective forbidden band width of the tuning layer 3 and the energy of the oscillation light is 75 meV as in the first embodiment.

When a device having a device length of 600 μm was manufactured, laser oscillation was exhibited at an oscillation threshold current of 27 mA, and a maximum optical output of 15 mW was obtained. The variable wavelength width was 13 nm for a tuning current of 150 mA. FIG. 5 shows the relationship between the measured wavelength variation and the optical output. It can be seen that the tendency of the reduction of the public output and the variable wavelength width at the time of wavelength tuning are further improved as compared with the device shown in the first embodiment.

In the first and second embodiments, the active layer 3 has MQW.
Although the structure is adopted, the active layer 3 may be made of an InGaAsP bulk semiconductor. The diffraction grating 2 is a so-called phase shift type diffraction grating in which unevenness is inverted in the center of the element, and if coating films for suppressing light reflection are formed on both end surfaces of the element, stability of single wavelength oscillation characteristics is improved. An excellent semiconductor laser can be obtained. The semiconductor laser wafers shown in Examples 1 and 2 can be manufactured by metal organic chemical vapor deposition (MOVPE method). The ridge portion including the tuning layer 3 and the active layer 5 can be formed with excellent uniformity by using the selective growth technique by MOVPE method.

The present invention also provides the TTG LD shown here.
Not only is it effective for a distributed Bragg reflector semiconductor laser (DBR LD) in which a tuning layer and an active layer are arranged in the cavity axis direction.

(Embodiment 3) FIG. 3A is a structural diagram of a wavelength tunable semiconductor optical filter for light having a wavelength of 1.55 μm, which is a third embodiment of the present invention. n-type InP substrate 3
N-type InGaAsP (wavelength composition 1.15 μm) on top of 1
Etching stop layer 32, n-type InP buffer layer 33
Are laminated, and the tuning layer 34 and the p-type InP clad layer 35 are partially formed only in the light incident region on the layer. On the side surface of the tuning layer 34, p-type I
It has a current block structure composed of nP 36 and n-type InP 37, and a p-type InP buried layer 38 is laminated on the entire upper surfaces of the cladding layer 35 and the current block layer 37. Furthermore, p ⁺ -type InGa is formed on a portion other than the light incident region on the above.
An As gap layer 39 and an electrode 42 are formed thereon. The semiconductor substrate 31 is removed by etching from below in the light incident region, and the etching stop layer 32 is exposed at that portion. Highly reflective films 40 and 41 made of a dielectric multilayer film having a reflectance of 95% are formed on the surfaces of the exposed etching stop layer 32 and the buried layer 38, and an electrode 43 is formed on the lower surface of the semiconductor substrate 31. ing. The thickness of each layer is 100 μm for the semiconductor substrate 31, 0.2 μm for the etching stop layer 32, 0.2 μm for the buffer layer 33, 0.3 μm for the cladding layer 35, and both the p-type and n-type block layers 36, 37. 0.5 μm, the buried layer 38 is 0.5 μm, and the cap layer 39 is 0.3 μm. The diameter of the light incident area is about 100 μm.

FIG. 3B shows the layer structure of the tuning layer 34 using a schematic diagram of the energy band structure of a semiconductor. The total thickness of the tuning layer 34 is 1 μm, in which InGaAsP (wavelength composition 1.3 μm) is included.
Ten barrier layers 132 and 10 InGaAsP (wavelength composition 1.45 μm) well layers 142 having a thickness of 5 nm are included. The well layers 142 are barrier layers 1 each having a thickness of 90 nm.
They are separated by 32, and a tensile strain stress of lattice mismatch of about 1% is introduced similarly to the device of Example 2. Such a device can be manufactured by MOVPE crystal growth method or gas source MBE crystal growth method.
This element is a high reflection mirror 4 formed on the upper and lower surfaces of the wafer.
A Fabry-Perot etalon resonator is constituted by 0 and 41 in the direction perpendicular to the wafer. Resonator length is about 2 μm
As a result, the free spectral range of the resonance mode is as wide as several 10 nm.

When a current was injected into this device and its light transmission characteristics were measured, the transmission wavelength changed to a short wavelength side of 12 nm with respect to a tuning current of 200 mA. The transmission wavelength width is 0.5 nm, the extinction ratio between the transmission wavelength and the non-transmission wavelength is 15 dB at maximum, and the maximum tuning current (20
An extinction ratio of about 10 dB was obtained even when 0 mA was injected. Although the present embodiment has described the characteristics of this element as a transmission type optical filter, this element can also operate as a reflection type optical filter.

The wavelengths of 1.
Although the element in the 55 μm band has been described, the wavelength may be in another wavelength band, for example the 1.3 μm band. In that case, if the layer structure of the tuning layer or the active layer is optimized accordingly, good wavelength tuning characteristics can be obtained. InGaAs may be used for the well layer instead of InGaAsP. Also, the semiconductor material system is InGaAs shown here.
It is not necessary to use P / InP system, for example, InAlAs
The / InGaAs / InP system may be adopted.

[0029]

The tunable semiconductor laser according to the present invention comprises:
It is possible to control the oscillation wavelength with a wide wavelength width of about 10 nm or more while maintaining oscillation at a single wavelength.
M), which is a promising light source for frequency division multiplexing (FDM) optical transmission systems. Conventionally, since the variable wavelength width of the light source is narrow, for example, the local oscillation side of the coherent FDM system has covered a desired wavelength band with a plurality of variable wavelength lasers, but if the semiconductor laser of the present invention is used, about 100 channels are used. Wavelength band (about 10 nm) used by FDM system
There is an advantage that one element can cover the above. Further, since the oscillation wavelength of the laser can be arbitrarily set in a wide range also on the transmission side, there is an advantage that the work of selecting an element matching the transmission wavelength becomes easier than in the past.

The wavelength tunable semiconductor optical filter according to the present invention has a wide variable wavelength width and is therefore promising as a channel selection element for an optical WDM transmission system. Since it is a plane-incidence type, the transmission characteristics have no polarization plane dependence, and therefore there is an advantage that the system does not require a polarization plane controller. In addition, a single element cannot have a variable wavelength width exceeding the free spectral range (10 to several tens of nanometers) at maximum, but two or more elements with slightly different free spectral ranges are vertically stacked to overlap resonance modes. By using, it is expected that the limit value of the variable wavelength range is theoretically much wider than the free spectral range, and that the extinction ratio between the transmission wavelength and the non-transmission wavelength can be made very large.

[Brief description of drawings]

FIG. 1 is a diagram showing a structure of a wavelength tunable semiconductor laser which is a first embodiment of the present invention. (A) is a cross-sectional structure of the device, and (b) is a schematic band diagram showing the layer structure of the active layer and the tuning layer.

FIG. 2 is a diagram showing a structure of a wavelength tunable semiconductor laser which is a second embodiment of the present invention. FIG. 6A is a band diagram showing the layer structure of the active layer and the tuning layer, and FIG.

FIG. 3 is a diagram showing a structure of a transmission wavelength tunable semiconductor optical filter that is a third embodiment of the present invention. (A) is a cross-sectional structure of the device, and (b) is a schematic band diagram showing the layer structure of the tuning layer.

FIG. 4 is a band diagram of a tuning layer illustrating the principle of the present invention.

FIG. 5 is a diagram showing a change in optical output during wavelength tuning of the wavelength tunable semiconductor laser of the present invention.

[Explanation of symbols]

1 p-type InP semiconductor substrate 2 diffraction grating 3 tuning layer 4 n-type InP spacer 5 active layer 6 p-type InP clad layer 7 n-type InP contact layer 8 p-type InP buried layer 9 p ⁺ InGaAs cap layer 10, 11, 12 electrode 21 p-type InGaAsP guide layer 22 p-type InP buffer layer 31 n-type InP substrate 32 n-type InGaAsP etching stop layer 33 n-type InP buffer layer 34 tuning layer 35 p-type InP clad layer 36 p-type InP block layer 37 n-type InP block layer 38 p-type InP buried layer 39 p ⁺ InGaAs cap layer 40 and 41 highly reflective films 42 and 43 electrodes 110 InGaAs well layers 120 InGaAsP barrier layers 130, 131, 132 InGaAsP well layers 140, 141 and 142 InGa sP barrier layer

Claims (10)

[Claims] 1. An active layer which produces a gain by current injection on a semiconductor substrate, a control layer which produces a refractive index change by current injection or voltage application, and means for independently injecting current into the active layer and the control layer. And a tunable semiconductor laser having a multi-quantum well structure in which the effective forbidden band width of the control layer is larger than the energy of the wavelength of oscillation light by 60 meV or more. 2. The control layer and the active layer are arranged so as to form a stripe shape parallel to each other with a spacer layer interposed therebetween, and a diffraction grating is formed adjacent to the control layer or the active layer. The active layer is vertically sandwiched by a lower semiconductor substrate and an upper buried layer and laterally sandwiched by a contact layer, and has electrodes separated from each other below the semiconductor substrate and above the buried layer and the contact layer. The wavelength tunable semiconductor laser according to claim 1, wherein 3. The semiconductor substrate and the buried layer are p-type InP,
Spacer layer and contact layer are n-type InP, active layer is In
GaAs / InGaAsP multiple quantum well structure, control layer is InGaAs / InGaAsP or InGaAsP /
3. The wavelength tunable semiconductor laser according to claim 2, wherein the wavelength tunable semiconductor laser has an InGaAsP multiple quantum well structure. 4. The control layer has a multiple quantum well structure including a well layer in which a lattice constant is smaller than that of a semiconductor substrate so that a tensile stress is applied in a growth plane. 3. The wavelength tunable semiconductor laser described in 3. 5. The wavelength tunable semiconductor laser according to claim 2, wherein the well layer in the control layer is provided so that its center is located closer to the side farther from the active layer. 6. A control layer which causes a refractive index change on a semiconductor substrate by current injection or voltage application, a Fabry-Perot etalon resonator formed so as to sandwich the control layer, and means for injecting current into the control layer. The tunable optical filter, wherein the control layer comprises a multi-quantum well structure having an effective forbidden band width of 60 meV or more larger than the energy of the wavelength of light to be controlled. 7. A surface-type filter that allows light to enter from above and below the device, wherein a control layer is sandwiched between a semiconductor substrate below and a buried layer above, and a control layer above the buried layer and below the semiconductor substrate. The surface has electrodes except for the light passage area,
7. The wavelength tunable optical filter according to claim 6, wherein the Fabry-Perot etalon resonator is formed between the surface of the buried layer in the light passage region and the lower surface of the semiconductor substrate. 8. The Fabry-Perot etalon resonator is formed between a high reflection film provided on a buried layer in a light passage region and a high reflection film provided below a semiconductor substrate, and the light passage region is formed. 8. The wavelength tunable optical filter according to claim 7, further comprising a current blocking layer between the buried layer and the semiconductor substrate except the above. 9. The tunable wavelength according to claim 7, wherein the semiconductor substrate has n-type InP, the buried layer has p-type InP, and the control layer has an InGaAs / InGaAsP or InGaAsP / InGaAsP multiple quantum well structure. Optical filter. 10. The control layer has a multi-quantum well structure including a well layer in which a lattice constant is smaller than that of a semiconductor substrate and a tensile stress is applied in a growth plane to the control layer. The tunable optical filter described in 8 or 9.