JPH0595165A - Dfb laser diode and its manufacture - Google Patents

Abstract

PURPOSE:To provide a laser diode in which the frequency can vary covering a wide range by forming an active layer in the direction of a resonator in order from the first region to the third region, and making the second region have a line width enlargement factor smaller than those of the first and third regions. CONSTITUTION:In a lambda/4 shift type DFB laser diode, an active layer 13 is made in the direction of a resonator in order of the first region 13a, the second region 13B, and the third region 13C. And the second region 13B is made to correspond to a lambda/4 phase shift point, and the second region 13B has a line width enlargement factor alpha smaIler than those of the first and third regions 13A and 13C. For example, the regions 13A and 13C of the active layer 13 are made so that the lengths L1 and L3 may be 300mum respectively, and the length L2 of the region 13B is set to 600mum. And, while the regions 13A and 13C are made of bulk crystals of nondoped InGaAsP, the region 13B has multilayer quantum well structure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to a semiconductor device, and in particular, it is possible to change the frequency in a wide range.
FB laser diode.

In a high-speed and large-capacity optical communication system, a laser diode is an essential element for generating coherent light. In particular, laser diodes oscillate at a single frequency,
It is required to have a narrow spectrum. Furthermore, in order to transmit information in the form of a modulated light beam, the laser diode is required to be able to change frequency in response to the electrical information signal. This enables FSK modulation. Further, such a wavelength tunable laser diode is useful for constructing a frequency multiplexing communication system.

[0003]

2. Description of the Related Art As a laser diode used in such an optical communication system, Japanese Patent Application No. 1-184144 and 1-2
35929, 1-235930, 1-235928.
And corresponding US patent application S. N. 55
2,116 and European Patent Application No. 90307626.3.
The publication describes a tunable laser diode.

In the DFB laser diode proposed above, a non-uniform intensity distribution of optical radiation is formed in the longitudinal direction of the laser, and the optical intensity is maximized at the longitudinal axis of the laser or substantially in the center of the cavity. In the portion where the light intensity is maximum, strong carrier depletion occurs, and by selectively injecting the modulation current into such a portion, it becomes possible to change the oscillation frequency of the laser in a wide range. In this conventional DFB laser diode, a λ / 4 phase shift point is formed in the approximate center of the diffraction grating used to form the Bragg reflection in the DFB laser in the longitudinal axis direction to intentionally create a non-uniform light intensity distribution. ing. Such depletion of carriers occurring at the central portion in the longitudinal axis direction of the laser diode is referred to as "axial hole burning".

FIG. 12 is a schematic view showing the structure of a DFB laser diode having a λ / 4 phase shift point on a diffraction grating formed on a waveguiding layer. In the conventional DFB laser diode, the inventor of the present invention has found that the frequency change caused by the change of the injection current is not so large as expected from the injection current. In the following, this problem is illustrated in FIG.
Examine in detail with reference to 2-15. For simplicity,
The description will be given for an element having a single electrode in the longitudinal direction.

Referring to FIG. 12, a conventional DFB laser diode has an n-type substrate 1 made of a compound semiconductor material such as InP, and a waveguide layer 2 made of another n-type compound semiconductor material on the substrate 1. Is formed. At the boundary between the substrate 1 and the waveguide layer 2, a diffraction grating 1A which causes Bragg reflection is formed along the longitudinal direction of the laser diode. further,
A .lamda. / 4 phase shift point 1B for concentrating the light radiation at the central portion of the laser diode longitudinal axis is formed substantially at the central portion in the longitudinal axis direction of the diffraction grating 1A. That is, the repeating of the irregularities of the regular diffraction grating is interrupted at the phase shift point 1B, and the phase of the diffraction grating on the right side of the point 1B is shifted by λ / 4 from the left side. However, lambda represents the wavelength of the unevenness of the diffraction grating. An undoped active layer 3 is formed on the waveguide layer 2, and a clad layer 4 made of a p-type compound semiconductor material is further formed on the active layer 3. An electrode 5 is formed on the cladding layer 4, and another electrode 6 is formed on the lower surface of the substrate 1.

During operation, a drive current is injected through the electrodes 5 and 6 to cause stimulated emission in the active layer 3. The light beam formed as a result is guided in the waveguide while feeling the periodic change in the refractive index generated by the diffraction grating. At that time, the light beam is diffracted back and forth by Bragg reflection.

FIG. 13 shows the intensity distribution in the longitudinal axis direction of the light radiation formed in the active layer 3 of the device of FIG. As indicated by the solid line in the figure, the light intensity becomes maximum corresponding to the λ / 4 phase shift point.

Corresponding to the light intensity distribution of FIG. 13, the carrier density also has a distribution as shown in FIG. 14. In this case, a drop or a valley of the carrier density is formed corresponding to the position of the maximum light intensity. .. Further, a distribution of the refractive index in the longitudinal axis direction is generated as shown in FIG. 15 corresponding to the carrier density distribution of FIG.

[0010]

By the way, the present inventor changes the drive current for this purpose when frequency modulation is performed by changing the carrier density to induce the change in refractive index. However, it has been found that the desired displacement of the refractive index distribution is not so large as a whole, and the change of the distribution profile as shown in FIGS.

More specifically, although the carrier density increases at both longitudinal ends of the laser diode when the injection current increases, at the central portion on the longitudinal axis where the λ / 4 phase shift point 1B is formed. Carrier depletion becomes more pronounced. It is considered that this is because the recombination is promoted as a result of the increase in the light intensity.

As described above, as a result of the decrease in carrier density in the substantially central portion in the longitudinal direction of the laser diode, the refractive index averaged in the longitudinal axis direction does not change as expected from the injection current. Only the refractive index profile is shown in Figure 1.
A desired large change in the oscillation frequency cannot be obtained only by the change shown in FIG.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a new and useful DFB laser diode capable of changing the frequency over a wide range and a method for manufacturing the same.

[0014]

The present invention has the following objects.
In the λ / 4 shift type DFB laser diode, the active layer is formed in the first region (13A) in order along the first direction.
A second region (13B) and a third region (13C), the second region corresponding to the λ / 4 phase shift point, and the second region being the first and third regions. A DFB laser diode characterized by having a line width increase coefficient smaller than that of the region, and extending in the first direction and sequentially formed along the first direction. The first area (13A) and the second area (13A
B) and an active layer (1) consisting of a third region (13C)
3 ′) and having a reduced linewidth enhancement factor in the second region, a mask layer (12) is provided on the main surface of the semiconductor layer (12) on which the active layer is formed. ) Is formed and the mask layer is patterned to form a mask pattern (32) consisting of a pair of elongated mask portions (32a, 32b) extending substantially parallel to each other in the first direction, each mask portion having the mask pattern (32). Lateral protrusions (32a1, 32b) protruding laterally corresponding to the second region
1), and the mask portions are arranged symmetrically with respect to an imaginary axis extending in the first direction, the lateral protrusions extending in opposite directions. And forming a multilayer quantum well structure on the main surface of the semiconductor layer using the mask, a quantum well layer (13B1) having a first bandgap and a second band larger than the first bandgap. This is achieved by a manufacturing method characterized by a step of forming by alternately growing a barrier layer (13B2) having a gap.

[0015]

According to the present invention, when the injection current increases, the change in the refractive index in the direction opposite to the first and third regions, which occurs in the second region, and the line width increase coefficient in the second region, It can be suppressed by decreasing it, and thus the change in the refractive index of the entire laser diode can be increased, and as a result, the oscillation frequency of the laser diode can be changed over a wide range. Also, when the active layer is formed to have a multi-layer quantum well structure, the side protrusions (32a1, 32b) are formed.
By forming 1) as a mask, the thickness of the quantum well layer in the second region can be made thicker than that in the first and third regions, so that the light generated in the second region can be formed. The wavelength of the beam can be shifted with respect to the light wavelength at which the diffraction grating causes Bragg reflection, and the line width enhancement factor decreases accordingly.

The principle of the present invention will be described below with reference to FIGS. 16, 17 and 1. Here, FIG. 16 shows the spectrum of the laser diode, and FIG. 17 shows the relationship between the gain G and the carrier density N of the laser diode.

Generally, in a DFB laser diode, coherent light is formed as a result of stimulated emission. In stimulated emission, as is well known, the emission of photons is caused by the interaction of carriers with the quantized electromagnetic field, where the photons are emitted in the mode in which they are already present. In other words, the light beam propagating back and forth in the active layer induces the emission of photons in a phase that is synchronized with the phase of the light beam, whereby an amplification effect of the coherent light is obtained.

By the way, in the laser diode,
There is another light emission mechanism called spontaneous emission. In this mechanism, the carriers interact with the fluctuations of the vacuum field and the photons are directed to modes in which there are few or few other photons. As a result, the light beam generated by spontaneous emission inevitably fluctuates in phase and wavelength,
The spectrum width becomes wider. Such a DFB laser diode has a line width enhancement factor defined by the following equation. α∝ (dn / dN) / (dG / dN) Here, n represents the refractive index, N represents the carrier density, and G represents the gain. Further, dG / dN represents the differential gain of the laser diode. The line width enhancement factor α represents the ratio of spontaneous emission to the total radiation generated in the active layer of the laser diode. In other words, the coefficient α represents the degree of spread of the spectrum of the laser diode.

FIG. 16 shows the spectrum of a typical DFB laser diode for two different linewidth enhancement factors. From this figure, if the line width increase coefficient α is large (α = α1
) It can be seen that the spectrum becomes wider, while when the line width enhancement coefficient α is small (α = α2), the spectrum becomes narrower.
In the conventional DFB laser diode, the line width increase coefficient α is substantially constant in the axial direction, except that the linear width increase coefficient α decreases slightly and gently at the central portion in the axial direction. This is the term dn / dN
Is determined by the composition and structure of the active layer, and the term dG / dN does not change much.

FIG. 17 shows the gain G as a function of carrier density N. As shown in this figure, the gain G monotonically increases with the carrier density N, and the differential gain dG / dN gradually decreases as the carrier density N increases. Since the carriers are significantly depleted in the axial center of the DFB laser diode, the slope dG / dN slightly increases. As a result, it is considered that the above-mentioned coefficient α is slightly decreased. However, this effect is not large, as can be seen from the fact that the slope in FIG. 17 does not change much even if the carrier density N changes.

Under such circumstances, the change in the refractive index with respect to the carrier density is approximately expressed as dn∝αdN. However, the term dG / dN is assumed to be constant. It can be seen from this relational expression that the change in the refractive index in the active layer can be suppressed by decreasing the value of the parameter α.

The present invention suppresses the undesired decrease in the refractive index in the central portion of the active layer as shown in FIG. 15 by changing the value of the line width increasing coefficient α in the axially central portion of the laser diode. ..

FIG. 1 illustrates the principle of the present invention. FIG. 1 shows the axial profile of the line width increase coefficient α. From this figure, the coefficient α is three parts along the axial direction, that is, the part I corresponding to one end of the laser diode and the part III corresponding to the other end. , And the intermediate portion II, the stepwise change. Here, in the first part I and the third part III, the coefficient α is the same, whereas in the second part II, the coefficient α is reduced. By setting the profile of α in this way, it is possible to increase the change in the refractive index of the laser diode as a whole and to increase the change in the frequency accordingly.

[0024]

2 is a vertical sectional view of a DFB laser diode according to the first embodiment of the present invention.

Referring to FIG. 2, the laser diode is n
It is formed on the mold substrate 11. As is well known in the DFB laser diode, a diffraction grating 11a is formed on the upper surface of the substrate 11 to form Bragg reflection in the longitudinal axis direction of the laser. Further, a λ / 4 phase shift point 11A is formed on the diffraction grating 11a so as to correspond to a substantially central portion of the laser diode. The substrate 11 is typically doped to an impurity concentration of 1 × 10 ¹⁸ cm ^-3 .

A waveguide layer 12 having an impurity concentration of 5 × 10 ¹⁷ cm ^-3 is formed on the substrate 11. The waveguiding layer 12 has a substantially flat upper surface, and its thickness varies in the range of 100 nm to 150 nm depending on the peaks and valleys of the diffraction grating 11a.
Further, an active layer 13 having a thickness of 150 nm is formed on the waveguide layer 12 to generate a light beam by stimulated emission. As will be described in detail below, the active layer 13 is composed of three different regions 13A, 13B, 13C having lengths L1, L2, L3 in the longitudinal direction of the laser diode, of which region 13B is λ. / 4 phase shift point 11
Corresponding to A, it is formed in the axial center part.

An impurity concentration of 5 × 10 ¹⁷ is formed on the active layer 13.
cm ⁻³ clad layer 14 made of p-type InP has a thickness of 15
It is formed to 0 nm. Further, the contact layer 15 made of p-type InGaAsP has a thickness of 0.5 on the cladding layer 14.
It is formed in μm. Ti / Pt on the contact layer 15
The p-side electrode 16 having the / Au structure is formed with a thickness of 1 μm as usual. Further, on the lower surface of the substrate 11, AuG
An n-side electrode 17 made of an e-alloy is formed with a thickness of 1 μm. In addition, Si3 N4 is attached to both ends of the laser diode.
The antireflection film 18 is formed with a thickness of 200 nm.

Next, the structure of the active layer 13, which is the main part of the present invention, will be described in detail.

Referring again to FIG. 2, region 13 of layer 13
A and 13C are made of undoped InGaAsP and are formed so that the lengths L1 and L3 are each 300 .mu.m. On the other hand, the length L2 of the region 13B is 600 μm.
set to m. Overall, the laser diode has a length of about 1200 μm. Furthermore, the active layer region 13
The regions 13B have a multi-layer quantum well (MQW) structure, while the layers 13C and 13C are made of a bulk crystal of InGaAsP.

FIG. 3 shows area 13B, line 9-9 'in FIG.
It is a band structure diagram along the cross section shown in.

Referring to FIG. 3, the region 13B is formed by stacking alternating layers 13B1 and 13B2, of which the layer 13B1 is a quantum well layer made of undoped InGaAs and has a thickness of 90 nm. In contrast, layer 13B2 has 13
It consists of undoped InGaAsP with a thickness of 0 nm and acts as a barrier layer. In Fig. 3, the conduction band is Ec
, And the valence band is represented by Ev. From this band structure diagram, the layer 13B1 has a band gap substantially larger than that of the layer 13B2, and efficiently confine carriers. It can be seen that a quantum well is formed. Furthermore, such a layer 1
The MQW having a structure composed of 3B1 and 13B2 is composed of InGaAsP having a thickness of 265 nm in the vertical direction, and an SCH (separate configuration).
It is sandwiched between the heterostructure) layers 13B3. Typically, in active layer 13, layer 13B1,
The repeating unit consisting of 13B2 is repeated 5 times.

By forming the MQW structure in this manner, discrete quantum levels QL and QL 'are formed in each quantum well due to confinement of carriers in the vertical direction. As a result, carrier recombination is selectively and concentrated between the quantum levels, and the efficiency of stimulated emission is greatly improved. As a result, the differential gain dG / dN in equation (1) increases, which leads to a decrease in the linewidth increase coefficient α.

During operation, an injection current is supplied to the electrodes 16 and 17, and carriers are accumulated in the active layer 13. As a result, carrier recombination occurs efficiently when the injection current exceeds a predetermined threshold value. Generally, in the DFB laser diode, the intensity of the light beam tends to be maximized near the center in the axial direction. However, in the element of this example, this nonuniform light intensity distribution is λ / 4 phase shift points 11
It is further emphasized by providing A. Along with such a light intensity distribution, a carrier density distribution in which strong carrier depletion occurs near the central portion in the axial direction of the element similar to FIG. 14 occurs.

FIG. 4 shows the distribution profile of the refractive index obtained by the present invention. Here, the solid line shows the initial state, and the broken line shows the state when the injection current is increased. For comparison, the refractive index profile obtained by the conventional element of FIG. 12 is shown by a chain line. Even in the case of this embodiment, the carrier distribution as a result of the axial hole burning is shown in FIG.
It becomes substantially the same as that shown in FIG. In the device of this example, an undesired increase in the refractive index near the center of the laser diode is suppressed, as clearly shown in FIG. As a result, the refractive index is displaced downward as a whole as shown in FIG. 4, and the oscillation wavelength is greatly changed accordingly.

FIG. 5 shows the relationship between the change in injection current and the oscillation wavelength. In FIG. 5, the vertical axis represents the oscillation wavelength, and the horizontal axis represents the injection current standardized with respect to the initial state.

Referring to FIG. 5, the broken line a shows the case where the coefficient α is 3.5 over the entire element, while
The alternate long and short dash line b shows the case where the coefficient α is 5.0 over the entire element. Further, the solid line c indicates the coefficient α in the region 13B.
Is set to 3.5. In this case, the areas 13A, 1
The coefficient α is set to 5.0 at 3C.

As is apparent from this figure, the wavelength change with respect to the modulation current is the largest in the device of this embodiment. Further, the element of the present invention gives the sharpest spectrum because the coefficient α is reduced in the region 13B. In the element (curve b) having the coefficient α of 5.0, the range of wavelength change is almost the same as that of this embodiment, but the spectrum of the light beam inevitably broadens because the coefficient α is large.

The laser diode of FIG. 2 can be manufactured in various ways. For example, I corresponding to the active layer 13
nGaAsP is deposited by the MBE method, and only the portion corresponding to the region 13B is removed by, for example, the RIE method.
Expose 2 Further, the regions 13A and 13B of the layer 13
After covering the part corresponding to, with a silicon oxide mask, SC
The H layer 13B3 is deposited on the exposed portion by the MBE method.
Further, on the SCH layer 13B3 thus formed, layers 13B1 and 13B2 are alternately deposited by a predetermined number of repetitions by the MBE method, and another SCH layer 13B3 is further deposited thereon.
To form. After the active layer 13 is formed in this way, the cladding layer 14 and the contact layer 15 are formed by a known method, and the electrodes 16 and 17 are further formed. further,
The antireflection film 18 is formed on both ends in the axial direction to complete the laser diode.

The formation of the waveguide layer 12 and the diffraction grating 11a is clearer than, for example, the above-mentioned conventional technique, and the description thereof will be omitted.

Next, a second embodiment of the present invention will be described together with its manufacturing method. First, the principle of this embodiment is shown in FIG.
A description will be given with reference to (A) and (B).

FIG. 6A shows a mask pattern 22 used to form a semiconductor layer on a specific portion of the substrate 21. On the other hand, FIG. 6B shows the thickness distribution of the semiconductor layer formed on the substrate 21 using the mask pattern 12. The mask pattern 22 is composed of an elongated first mask 22a and a second elongated mask 22b that extend substantially parallel to each other and are separated by a distance W. More specifically, the mask 22a
And a mask 22b define a band-shaped region having a width of W, and the semiconductor layer is grown in this band-shaped region. Further, each of the masks 22a and 22b has protrusions 22a1 and 22b1 extending in opposite directions.

The masks 22a and 22b are made of silicon oxide. When a silicon oxide mask is used, the thickness of the epitaxial layer deposited on the substrate between the mask portions 22a1 and 22b1 is equal to that of the mask 22a and the mask 2
It is known that the thickness becomes larger than the thickness of the epitaxial layer grown in the portion located between the straight portions of 2b. For example, J. Finders et al. , J. Crys
tal Growth, 107, pp. 151-155
See (1991). The cause of this phenomenon has not been completely clarified at this time, but when the group III or group V atoms deposited on the masks 22a and 22b move on the mask surface and reach the exposed portion of the substrate 21, they are deposited. At this time, more atoms are collected on the mask portions 22a1 and 22b1, so that the mask portions 22a1 and 22b1 of FIG.
It is speculated that this is due to the fact that the thickness of the epitaxial layer is selectively increased in the substrate portion located between.

Hereinafter, the manufacturing process of the DFB laser diode according to this embodiment will be described with reference to FIGS. 7 (A), 7 (B), 8 (A),
This will be described with reference to (B), FIGS. 9 (A) and 9 (B).
In the present embodiment, the same reference numerals are given to the portions already described in the previous embodiment, and the description thereof will be omitted.

Referring to FIG. 7A, a silicon oxide layer 31 having a thickness of about 0.1 nm is formed on the waveguiding layer 12, and then patterned to form a mask pattern as shown in FIG. 7B. 32 is formed. Here, the mask pattern 32 is composed of a pair of masks 32a and 32b extending substantially in parallel, and these are arranged symmetrically to each other. Side projections 32a1 and 32b1 are provided on the masks 32a and 32b, respectively.
Are formed, which extend in opposite directions to each other. At that time, a band-shaped region having a width W is defined on the upper main surface of the waveguide layer 12 corresponding to a portion located between the mask 32a and the mask 32b. The main surface is exposed. The strip region is defined by a pair of straight side edges. In the illustrated example, the width W is set to 2 μm. Further, the masks 32a and 32b are both provided with the protrusions 32a1 and 32a1.
The portion where 32b1 is not formed has a width A, and in the illustrated embodiment, A is set to about 4 μm. On the other hand, each of the masks 32a and 32b has a width B of about 10 μm in the protruding portions 32a1 and 32a2. The protrusions 32a1 and 32b1 correspond to the regions 13B defined in FIG. 2, while the remaining portions of the masks 32a and 32b correspond to the regions 13A and 13B. See FIG. 7 (B).

While using the mask 32, the active layer 1 of FIG.
The active layer 13 'corresponding to 3 is a quantum well layer 13a' made of InGaAsP and a barrier layer 1 made of InGaAs.
It is formed by alternately depositing 3b 'by the MOVPE method. In that case, the barrier layer 13b 'is deposited on the waveguide layer 12 first, and then the quantum well layer 13a' is deposited.
See FIG. 8 (A). In this step, TMI, TMG, arsine and phosphine are introduced into the reaction vessel as a raw material for the layer 13a ′, and TM as a raw material for the layer 13b ′.
I, TMG and arsine are introduced.

When growing the layer 13a ', TMI, T
MG, arsine and phosphine 0.5SC each
CM, 0.5SCCM, 0.5SCCM and 0.5S
Introduced at CCM flow rate. On the other hand, the layer 13b 'is TM
Flow rate of 0.5 SCC for I, TMG and arsine
It is done by introducing with M, 0.5 SCCM and 0.5 SCCM. By this growth process, the layers 13a 'and 13b' are formed in the regions 13A and 13C to have a thickness of 5 nm and 8 nm, respectively. On the other hand, in the region 13B, the layers 13a 'and 13b' are formed to have a thickness of 7 nm and 11 nm, respectively.

Repeated and alternating deposition of layers 13a 'and 13b' results in a structure in which the thickness of active layer 13 'is increased in region 13B as shown in FIG. 8B. For example, layers 13a 'and 13b' are deposited in repeated 5 cycles, in which case the active layer has a thickness of region 13
57 nm in A and 13C, and 79 nm in the region 13B.

After the structure of FIG. 8B is formed, the silicon oxide masks 33a and 33b are removed by a wet etching process using, for example, HF, and the cladding layer 14 is further formed on the structure thus formed by the MOVPE method. To be done. Further, the contact layer 15 is formed on the clad layer 14, and the electrodes 16 and 17 are formed on the upper surface of the contact layer 15 and the lower surface of the substrate 11, respectively.
The B laser diode is completed. See FIG. 9 (A).

When the electrode 16 is formed, first, an insulating layer such as silicon oxide is formed on the upper surface of the contact layer 15,
This is patterned to form a first insulating layer portion 16a and a second insulating layer portion 16b extending substantially parallel to each other in the longitudinal axis direction of the laser, and between the insulating layer portion 16a and the insulating layer portion 16b. The exposed upper surface of the contact layer 15 may be exposed in a strip shape, and the electrode 16 may be in contact with this exposed portion. See FIG. 9 (B). By doing so, the current injected into the laser diode can be narrowed down to the side of the laser.

In the laser diode according to the present invention, as a result of the increase in the thickness of the quantum well layer 13a ', the region 1
The wavelength of the optical radiation formed by the recombination of carriers at 3B increases. That is, the wavelength of the optical radiation formed in the region 13B of the laser diode is
It deviates from the wavelength of the optical radiation formed at 13C. On the other hand, the diffraction grating 11a is tuned so as to cause Bragg reflection with respect to the light radiation formed in the regions 13A and 13C. It is well known that such a wavelength shift increases the differential gain dG / dN,
Along with this, the line width increase factor decreases. For example, S.
Ogita et al. , Electronics
Letters, 23, pp. 393-394 (198
See 7). Therefore, the DFB laser according to the present embodiment also has the distribution of the line width increasing coefficient similar to that shown in FIG.

In the present embodiment, the active layer 13 'is formed on the mask 3
It is advantageous in that it can be grown without interruption in the middle except the step of forming 2. That is, the step of etching away the active layer 13 corresponding to the region 13B as in the first embodiment can be omitted. As a result, the active layer 1
3'can be formed without causing substantial defects, and the efficiency of the laser diode is improved.

FIG. 10 shows a third embodiment of the present invention. This embodiment is an example in which the present invention is applied to a DFB laser diode having a three-electrode structure, that is, a laser diode having split electrodes 15A, 15B and 15C instead of the single electrode 16 in the previous embodiment. The other parts have the same structure as the element of the second embodiment, and the description thereof will be omitted. Also in the case of this embodiment, the distribution of the line width increase coefficient similar to that in FIG. 1 can be obtained.

FIG. 11 shows the refractive index distribution of the laser diode in the longitudinal axis direction. Also in this case, the area 13B
The change in the refractive index in the opposite direction can be suppressed, and the laser diode shows a large change in the oscillation frequency according to the modulation of the injection current.

In the above description, the structure of the laser diode of the present invention is not limited to that described in the embodiments. For example, the waveguiding layer 12 may be replaced by the active layers 13, 1
The diffraction grating 11a may be formed on the upper surface of the waveguide layer 12 by forming the upper portion of 3 '. Further, one or more buffer layers may be formed in the substrate 11 to improve the crystal quality of the substrate 11. Such a modification is self-evident and will not be described.

Further, in the distribution of the line width increase coefficient shown in FIG. 1, the active layer is formed by a single MQW structure, and the distribution shown in FIG.
It can also be realized by increasing the total thickness in the central portion 13B as in the structure shown in FIG. Area 1 in this case
The number of quantum well layers included in the MQW structure in 3B is larger than that in the region 13A or 13B. Also in this structure, the differential gain dG / dN increases in the region 13B. The configuration of this modified example is apparent from FIG. 8B, and further description is omitted. Alternatively, the active layer may be formed of a single MQW and the quantum well structure may be erased by ion-implanting impurities such as Zn at the portions corresponding to the regions 13A and 13B.

Furthermore, the present invention is not limited to the embodiments described above, and various modifications and changes can be made within the scope of the present invention.

[0057]

According to the present invention, by changing the distribution of the line width increasing coefficient in the longitudinal axis direction in the DFB laser diode so as to decrease in the portion where the light intensity becomes maximum,
It is possible to suppress a change in the refractive index in the opposite direction that occurs in a region where the light intensity is large when the injection current is modulated, and thus a large change in the refractive index and a large change in the oscillation wavelength can be obtained for the laser diode as a whole.

[Brief description of drawings]

FIG. 1 is a diagram showing the principle of the present invention.

FIG. 2 is a sectional view showing a structure of a DFB laser diode according to a first embodiment of the present invention.

FIG. 3 is a band structure diagram showing a main part of the laser diode of FIG.

FIG. 4 is a diagram showing a refractive index distribution generated in the laser diode of FIG.

5 is a diagram showing a relationship between injection current and oscillation wavelength change in various DFB laser diodes including the laser diode of FIG.

6 (A) and (B) are D according to the second embodiment of the present invention.
It is a figure which shows the principle of the manufacturing process of a FB laser diode.

7A and 7B are DFs according to a second embodiment of the present invention.
It is a figure (1) which shows the manufacturing process of a B laser diode.

8A and 8B are DFs according to the second embodiment of the present invention.
It is a figure (2) which shows the manufacturing process of a B laser diode.

9A and 9B are DFs according to a second embodiment of the present invention.
It is a figure (3) showing a manufacturing process of a B laser diode.

FIG. 10 is a sectional view showing a structure of a DFB laser diode according to a third embodiment of the present invention.

11 is a view showing a refractive index distribution generated in the active layer of the laser diode of FIG.

FIG. 12 is a vertical cross-sectional view showing a conventional DFB laser diode.

13 is a diagram showing a light intensity distribution in an active layer of the laser diode of FIG.

14 is a diagram showing a carrier density distribution in the active layer of the laser diode of FIG.

15 is a diagram showing a refractive index distribution in the active layer of the laser diode of FIG.

FIG. 16 is a diagram showing a relationship between a spectrum of a laser diode and a line width increase coefficient.

FIG. 17 is a diagram showing a relationship between a gain of a laser diode and a carrier density.

[Explanation of symbols]

 1,11 Substrate 1A, 11a Diffraction grating 1B, 11A λ / 4 phase shift 2,12 Waveguide layer 3,13,13 'Active layer 4,14 Clad layer 15 Contact layer 5,6,16,17 Electrode 18 Antireflection film

Claims (10)

[Claims] 1. In a λ / 4 shift type DFB laser diode, an active layer is formed in a first region (13A) in order along a cavity direction.
), The second area (13B) and the third area (13C)
), The second region is formed corresponding to a λ / 4 phase shift point, and the second region has a line width increase coefficient smaller than that of the first and third regions. A DFB laser diode characterized by the following. 2. The first and third regions (1) of the active layer
3A, 13C) is made of a bulk crystal of a semiconductor material, while the second region (13C) of the active layer is a quantum well layer (13B1) made of a first semiconductor material having a first band gap. , A barrier layer (13B2) made of a second semiconductor material having a second bandgap substantially larger than the first bandgap, and the quantum well layers are discrete. Quantum level (QL,
DFB laser diode according to claim 1, characterized in that it has a thickness such that QL ') is formed. 3. The first semiconductor material is InGaAsP
3. The DFB laser diode according to claim 2, wherein the first semiconductor material is InGaAs. 4. The active layer (13 ′) comprises a quantum well layer (13B1) made of a first semiconductor material having a first bandgap and a second band substantially larger than the first bandgap. A barrier layer (13B2) made of a second semiconductor material having a gap, the quantum well layer has a thickness that forms a discrete quantum level, and the quantum well layer and the barrier layer alternate. To form the active layer, and the quantum well layer is formed on the second region (1
3B), the first and third regions (13A, 1)
DFB laser diode according to claim 1, characterized in that it has a greater thickness than in (3C). 5. The quantum well layer (13B1) made of a first semiconductor material having a first bandgap and the second band substantially larger than the first bandgap. A barrier layer (13B2) made of a second semiconductor material having a gap, the quantum well layer has a thickness that forms a discrete quantum level, and the quantum well layer and the barrier layer alternate. To form the active layer, and the active layer (13 ') is formed on the second region (1
3B), the first and third regions (13A, 1)
DFB laser diode according to claim 1, characterized in that it has a thickness greater than 3C). 6. A second region (1) of the active layer (13 ′).
DFB laser diode according to claim 5, characterized in that 3B) comprises more quantum well layers than the first and third regions (13A, 13C). 7. A first region (13A) extending in a first direction and sequentially formed along the first direction, a second region (13B) and a third region (13C). In the method for manufacturing a DFB laser diode having an active layer (13 ′) made of a) and a reduced line width enhancement factor in the second region, the main part of the semiconductor layer (12) on which the active layer is formed is formed. Forming a mask layer (12) on the surface, patterning the mask layer, and a pair of elongated mask portions (32) extending substantially parallel to each other in the first direction.
a, 32b), so that each mask portion has lateral protrusions (32a1, 32b1) that project laterally corresponding to the second region,
Further, each mask portion is arranged symmetrically with respect to an imaginary axis extending in the first direction, in which case the side protrusions are formed so as to extend in mutually opposite directions. A multi-layer quantum well structure on the main surface of the semiconductor layer, a quantum well layer (13B1) having a first bandgap and a barrier layer (13B2) having a second bandgap larger than the first bandgap. Manufacturing method characterized by forming by alternately growing and 8. The method according to claim 7, wherein the step of alternately depositing the quantum well layer (13B1) and the barrier layer (13B2) is performed by using an organic material. 9. The method according to claim 8, wherein the step of alternately depositing the quantum well layers (13B1) and the barrier layers (13B2) is performed by a MOVPE method. 10. The method of claim 7, wherein depositing the mask layer comprises depositing a silicon oxide layer as a mask layer.