JP2008258453A - Distributed feedback semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent reduction of power of output light through spatial hole burning (carrier density decrease) caused in a λ/4 phase shift DFB laser. <P>SOLUTION: The λ/4 phase shift DFB laser includes: an active layer 40 for generating stimulated emission light; a diffraction grating layer 30 formed on or under the active layer 40 for attaining a single longitudinal mode; and upper clad layers 41 and 41a and a lower clad layer 21 laminated with the active layer 40 and the diffraction grating layer 30 in between from the upper and lower directions. In the diffraction grating layer 30, a diffraction grating formation region 31, in which an uneven diffraction grating 33 is formed, and a diffraction grating non-formation region 32, in which the diffraction grating 33 is not formed, are formed by turns periodically in a range affected by an evanescent field of the stimulated emission light, which is subjected to wave guide in the active layer 40. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a distributed feedback semiconductor laser (hereinafter simply referred to as a “DFB laser”), and in particular, is disposed adjacent to an active layer for generating stimulated emission light, and has a single longitudinal mode. In the diffraction grating layer for the purpose, a diffraction grating forming region in which concave and convex (groove) diffraction gratings (gratings) are formed and a diffraction grating non-forming region in which no diffraction grating is formed are alternately and periodically formed. The present invention relates to a DFB laser.

The DFB laser achieves a single longitudinal mode operation by controlling the longitudinal mode by utilizing the Bragg reflection of the stimulated emission light generated in the active layer by the uneven diffraction grating.
A conventional DFB laser is described in, for example, the following documents.

JP-A-1-231389 JP-A-8-274406 Japanese Patent Laid-Open No. 11-212038 Hiroo Yonezu “Optical Communication Element Optical One Light-Emitting / Light-Receiving Element” 5th Edition (1984) Engineering Books p. 252-255 Toshiaki Sugawara “Basics of Semiconductor Lasers” 1st edition (1998) Kyoritsu Shuppan p. 123-130 Toshiaki Sugawara “Basics of Semiconductor Lasers” 1st edition (2005) Kyoritsu Shuppan p. 208-217

  Patent Documents 2 and 3 and Non-Patent Document 1 describe general DFB laser technology, and Patent Document 1 and Non-Patent Documents 2 and 3 describe phase-shift DFB laser technology. Has been.

  In general, the Bragg reflection is given by a periodic diffraction grating. In a DFB laser, the diffraction grating is usually formed as a concave / convex (for example, tooth or groove) grating perpendicular to the optical axis of the element in a semiconductor layer (clad layer) adjacent to the active layer of the element. .

  3 (a) and 3 (b) are schematic configuration diagrams showing conventional DFB lasers described in Patent Documents 2 and 3 and Non-Patent Document 1 and the like. FIG. The perspective view which shows the structure of the whole notch, and the figure (b) are the perspective views which show the structure of the diffraction grating layer in the figure (a).

  In the perspective view of FIG. 3A, for example, the horizontal horizontal direction is the X-axis direction, the horizontal vertical direction orthogonal thereto is the Y-axis direction, and is further orthogonal to the X-axis and Y-axis. The vertical direction is the Z-axis direction. As a material for the DFB laser, AlGaAs, InGaAsP, PbSnTe, or the like is used. A conventional example of an InGaAsP DFB laser is shown in FIG.

  This DFB laser has a substantially rectangular n-type InP substrate 1 extending in the horizontal plane of the X axis and the Y axis, rises in the Z axis direction at the approximate center of the substrate 1 in the X axis direction, and stripes in the Y axis direction. A mesa-shaped (convex) n-type InP lower cladding layer 2 extending in a projecting shape is projected. On the lower cladding layer 2, a diffraction grating layer 3 that is a waveguide layer, an InGaAsP active layer 4, and a part of the upper cladding layer 5 a of the p-type InP upper cladding layer 5 are sequentially stacked in the upward direction of the Z axis. ing. In the laminated striped lower cladding layer 2, diffraction grating layer 3, active layer 4, and part of the upper cladding layer 5a, both side surfaces along the Y-axis direction are p-type InP blocking layer 6 and n-type InP. It is sandwiched between current blocking layers made up of the blocking layer 7. The upper clad layer 5 is formed on the block layer 7 and a part of the upper clad layer 5a. A p-type electrode 9 is formed on the upper cladding layer 5 via a p-type InGaAs contact layer 8, and an n-type electrode 10 is also formed on the lower surface of the substrate 1.

  In FIG. 3 (a), the surface of the front side of the substrate 1 in FIG. 3A has an upper semiconductor layer in the same manner as other portions, but here, In order to show a cross section of the upper semiconductor layer, it is drawn like a space.

  In the mesa-shaped diffraction grating layer 3, a plurality of concave and convex diffraction gratings 3a are formed uniformly at regular intervals along the Y-axis direction.

  In the method of manufacturing a DFB laser having such a configuration, for example, in a series of manufacturing processes, each semiconductor layer is epitaxially grown by metal organic chemical vapor deposition (hereinafter referred to as “MOCVD”). .

  In the DFB laser, when a positive voltage is applied to the p-type electrode 9 and a negative voltage is applied to the n-type electrode 10, holes are injected into the p-type electrode 9 and electrons are injected into the n-type electrode 10. Move towards. The moved electrons / holes are recombined in the active layer 4 to generate light having a wavelength corresponding to the energy of the almost forbidden band, and are stimulated and emitted. This stimulated emission light is Bragg-reflected by the diffraction grating layer 3 to cause laser oscillation and output coherent light.

  The oscillation wavelength is determined by the material of the active layer 4. The DFB laser oscillates continuously at room temperature, oscillates in a single mode, has excellent wavelength selectivity, the oscillation wavelength hardly changes even when the current is changed, and the temperature dependence of the oscillation wavelength is the temperature of the refractive index. The dependency is small because it depends on the dependency. Since a part of the waveguide of the DFB laser is a feedback unit, the laser and the waveguide can be connected on a single substrate, and can be used as a light source (laser) in an optical integrated circuit. .

  Among the DFB lasers, there is a laser having a high single longitudinal mode property called a λ / 4 phase-shifted DFB laser in which the phase of the diffraction grating 3 a is shifted by π / 2 mainly at the center of the oscillating diffraction grating layer 3.

  FIG. 4 is a schematic perspective view showing a diffraction grating layer in the conventional λ / 4 phase shift DFB laser described in Non-Patent Document 2 and the like.

  In this diffraction grating layer 3 </ b> A, the phase is shifted by ∧ / 2 (= π / 2) by the diffraction grating 3 b of the λ / 4 phase shift component provided in the approximate center in the Y-axis direction. Such a λ / 4 phase-shifted DFB laser has an advantage that a stable single wavelength oscillation at a Bragg wavelength is possible with a low threshold.

  However, in the conventional λ / 4 phase shift DFB laser as shown in FIG. 4, electric field concentration occurs in the diffraction grating 3b of the λ / 4 phase shift component, and the electric field strength distribution becomes non-uniform as the bias voltage is increased. However, there is a problem that spatial hole burning (spatial density reduction) is caused in a portion where the electric field strength is large, and the power of the output light is reduced.

  The DFB laser according to the present invention includes an active layer for generating stimulated emission light, a diffraction grating layer that is laminated below or on the active layer, and for achieving a single longitudinal mode, the active layer, and the diffraction An upper cladding layer and a lower cladding layer are stacked so as to sandwich the lattice layer from above and below. In the diffraction grating layer, a diffraction grating formation region in which an uneven diffraction grating is formed within a range covered by an evanescent field of the stimulated emission light guided through the active layer. And diffraction grating non-formation regions where the diffraction grating is not formed are alternately and periodically formed.

  According to the DFB laser of the present invention, since the diffraction grating non-formation region where the diffraction grating is not formed is periodically dispersed in the diffraction grating layer for achieving the single longitudinal mode, the λ / 4 phase The electric field intensity distribution is flattened in the shift component, and the output optical power can be increased.

  The DFB laser includes an active layer for generating stimulated emission light, a diffraction grating layer for forming a single longitudinal mode, which is laminated below or on the active layer, and the active layer and the diffraction grating layer. An upper clad layer and a lower clad layer are stacked so as to be sandwiched from above and below. In the diffraction grating layer, a diffraction grating forming region in which an uneven diffraction grating is formed, and the diffraction grating are formed within a range covered by an evanescent field of the stimulated emission light guided through the active layer. Non-diffraction grating non-formation regions are alternately and periodically formed.

The diffraction grating formation region and the diffraction grating non-formation region,
Interval ∧ is the following formula (1)
∧ = m · λc / 2Neff (1)
Where m is any positive integer
λc: emission wavelength in the diffraction grating layer
Neff: The sum Ltl of the width La of the diffraction grating non-formation region formed discretely with a period satisfying the effective refractive index in the diffraction grating layer is expressed by the following equation (2)
δβ · Ltl≈π / 2 (2)
However, in the diffraction grating layer, the diffraction grating formation region and the diffraction grating non-formation region
The difference in light propagation constant in the region is satisfied.

(Configuration of Example 1)
FIGS. 1A and 1B are schematic configuration diagrams of a DFB laser showing Example 1 of the present invention, and FIG. 1A is a perspective view showing an overall configuration with a part cut away; FIG. 2B is a perspective view showing the configuration of the diffraction grating layer in FIG.

  In the perspective view of FIG. 1A, as in the conventional FIG. 3A, for example, the horizontal horizontal direction is the X-axis direction, the horizontal vertical direction perpendicular thereto is the Y-axis direction, and further A vertical direction perpendicular to the X axis and the Y axis is taken as a Z axis direction. As a material for the DFB laser, AlGaAs, InGaAsP, PbSnTe, or the like is used. A configuration example of an InGaAsP DFB laser is shown in FIG.

  As shown in FIG. 1A, the DFB laser includes, for example, a diffraction grating formation region and a diffraction grating non-formation region using an InGaAsP multiquantum well (hereinafter referred to as “MQW”) as an active layer. This is an InGaAsP-based DFB laser that is configured periodically, and has an approximately rectangular parallel plate-shaped n-type InP substrate 20 extending in the horizontal planes of the X-axis and Y-axis. A mesa-shaped (convex) n-type InP lower cladding layer 21 that protrudes integrally in the direction and extends in a stripe shape in the Y-axis direction is provided. The substrate 20 and the lower cladding layer 21 are collectively referred to as a mesa substrate portion.

  On the lower clad layer 21, a diffraction grating layer 30, which is a striped waveguide layer, is formed in the upward direction of the Z axis. On the upper surface of the diffraction grating layer 30, an InGaAsP active layer 40 for generating stimulated emission light having a uniform thickness is provided over the entire area in the longitudinal direction of the Y axis and with the same width as the upper surface. It has been. The upper surface of the active layer 40 is a mesa clad layer formed of a part of the p-type InP upper clad layer 41 over the entire area of the upper surface in the longitudinal direction of the Y axis and the same width as the upper surface. A clad layer 41a is provided. Thus, the active layer 40 is sandwiched between the lower clad layer 21 and the upper clad layer 41a on the lower side via the diffraction grating layer 30, and the light generated in the active layer 40 is transmitted to the lower clad layer. 21 and the upper clad layer 41a are confined in the active layer, and the lower clad layer 21, the active layer 40, and the upper clad layer 41a form a laminate.

  On the upper surface of the substrate 20 on both sides of the laminate and the diffraction grating layer 30, a current blocking layer is provided. The current block layer is formed by sequentially stacking a p-type InP block layer 42 and an n-type InP block layer 43 from the substrate side. That is, in the laminated lower clad layer 21, diffraction grating layer 30, active layer 40, and part of the upper clad layer 41a, both side surfaces along the Y-axis direction are p-type InP block layers 42 and n. It is sandwiched between current blocking layers made of type InP blocking layer 43.

  The upper clad layer 41 is formed on the block layer 43 and a part of the upper clad layer 41a. A p-type electrode 45 such as gold (Au) is formed on the upper clad layer 41 via a p-type InGaAs contact layer 44 for taking out the electrode, and an n-type electrode such as Au is also formed on the lower surface of the substrate 20. 46 is formed.

  In FIG. 1 (a), as in the conventional case of FIG. 3 (a), the front surface of the substrate 20 having a surface area of about 1/4 is actually the same as the other portions. Although there is a semiconductor layer, here it is drawn like a space in order to show a cross section of the upper semiconductor layer.

  As shown in FIG. 1B, the diffraction grating layer 30 is provided on the upper surface of the lower cladding layer 21 in the longitudinal direction of the Y axis with the same width as the upper surface. The diffraction grating layer 30 includes a diffraction grating forming region 31 for flattening the electric field intensity distribution by making a single longitudinal mode and a diffraction grating non-forming region 32 having a width La, and the diffraction grating forming region 31 is Y It is provided in such a manner that it is periodically dispersed in the longitudinal direction of the shaft (that is, separated by a width La). In each diffraction grating formation region 31, a plurality of concave and convex diffraction gratings 33 are formed uniformly at regular intervals along the Y-axis direction.

(Example of production method of Example 1)
FIGS. 2A to 2G are schematic manufacturing process diagrams illustrating an example of a method of manufacturing the InGaAsP DFB laser in the first embodiment.

  The InGaAsP DFB laser according to the first embodiment is manufactured by, for example, steps 1 to 7 such as the following (a) to (g).

Step 1 in FIG.
A silicon oxide film (SiO 2 film) (not shown) is formed on the surface of the n-type InP substrate 20 by, for example, a chemical vapor deposition (hereinafter referred to as “CVD”) method. A resist pattern (not shown) is formed by lithography. Using the resist pattern, the SiO2 film is periodically patterned in a stripe shape. After the resist pattern is removed, dry etching is performed using, for example, reactive ion etching (hereinafter referred to as “RIE”) using the patterned SiO 2 film 50 as a mask, and the diffraction grating non-formation region 32 having a width La is formed. Form.

Step 2 in FIG.
For example, the diffraction grating formation region 31 is formed using a resist (not shown) as a mask by photolithography using a two-beam interference exposure method and wet etching.

Step 3 in FIGS. 2-1 (c) and 2-2 (d)
The SiO 2 film 50 is removed, and the n-type InGaAsP lower light guide layer 34 is epitaxially grown using, for example, the MOCVD method. Thereby, the diffraction grating layer 30 including the diffraction grating formation region 31, the diffraction grating non-formation region 32, and the lower light guide layer 34 is formed. Further, for example, by using the MOCVD method, the active layer 40 including the 10-cycle multiple quantum well structure and the InGaAsP upper optical guide layer and the p-type InP upper cladding layer 41 are continuously epitaxially grown.

Step 4 in FIG. 2-2 (e)
The surface of the upper clad layer 41 is patterned by, for example, a CVD method using a Si02 film (not shown) in a stripe pattern, and after removing the resist pattern used for patterning the SiO2 film, the patterned SiO2 film 52 is masked. Then, the p-type InP upper cladding layer 41, the InGaAsP active layer 40, the diffraction grating layer 30, and the n-type InP substrate 20 are mesa-etched. This mesa etching is performed by, for example, a dry etching method using a RIE method and a wet etching method.

Step 5 in FIG. 2-2 (f)
Using the SiO2 film 52 as a mask, the p-type InP block layer 42 and the n-type InP block layer 43 are successively embedded by, for example, MOCVD, and epitaxial crystal growth is performed.

Step 6 in FIG. 2-3 (g)
After removing the SiO 2 film 52, the p-type InP upper clad layer 41 and the p-type InGaAs contact layer 44 are formed on the exposed mesa-like upper clad layer 41a and the n-type InP block layer 43 by, for example, MOCVD. Epitaxial crystal growth is performed continuously. At this time, the p-type InP upper clad layer 41a on the mesa that is immediately below the SiO2 film 52 and the p-type In upper clad layer 41 newly grown by MOCVD are made of the same material. It becomes a new p-type InP upper cladding layer.

  After that, if the p-type electrode 45 is formed on the p-type InGaAs contact layer 44, the lower surface of the n-type InP substrate 20 is polished, and the n-type electrode 46 is formed below the polished surface, the DFB shown in FIG. The laser is complete.

(Operation of Example 1)
In the DFB laser, when a positive voltage is applied to the p-type electrode 45 and a negative voltage is applied to the n-type electrode 46, holes are injected into the p-type electrode 45 and electrons are injected into the n-type electrode 46, as in the conventional case. These move toward the active layer 40. The moved electrons / holes are recombined in the active layer 40 to generate light having a wavelength corresponding to the energy of the almost forbidden band, and are stimulated and emitted. This stimulated emission light is Bragg-reflected by the diffraction grating layer 30, and laser oscillation is generated to output coherent light.

Next, the characteristic operation of the first embodiment will be described with reference to FIG.
The propagation constant β of light in the medium is generally expressed by the following equation (3) by the effective refractive index Neff of the medium and the wavelength λc of light in vacuum.
β = 2πNeff / λc (3)

In the DFB laser according to the first embodiment, since the effective refractive index is slightly different between the diffraction grating forming region 31 and the diffraction grating non-forming region 32 in the diffraction grating layer 30, the propagation constants are also slightly different. If the difference between the propagation constants is δβ and the sum of the widths La of all the diffraction grating non-formation regions 32 per DFB laser is Ltl, this product δβ · Ltl is the phase shift amount with respect to the light wave. Therefore, the following equation (2)
δβ · Ltl≈π / 2 (2)
By forming the width La so as to satisfy the above, a diffraction grating equivalent to the diffraction grating of the λ / 4 phase shift DFB laser can be formed in the DFB laser manufactured according to the first embodiment.

  Here, in the diffraction grating layer 30 in which the diffraction grating formation region 31 and the diffraction grating non-formation region 32 are combined, the interval between the periodically separated diffraction gratings 33 is continuous between the diffraction gratings in which the phase of the diffracted light is discrete. The diffraction grating 33 is formed so that the property is maintained.

(Effect of Example 1)
According to the first embodiment, by periodically separating the diffraction grating non-formation regions 32 in the diffraction grating layer 30, the electric field distribution inside the resonating diffraction grating layer is flattened, so that the formula (2) is satisfied. In addition, by forming the width La of the diffraction grating non-formation region 32, a λ / 4 phase shift equivalent diffraction grating can be formed. In such a DFB laser in which the diffraction grating 33 is formed discretely, the diffraction grating formation region 31 and the diffraction grating non-formation region 32 are combined in the same diffraction grating layer 30, so that A structure in which electric field concentration hardly occurs can be configured.

(Modification)
The present invention is not limited to the first embodiment, and various usage forms and modifications are possible. For example, the following forms (A) to (D) are available as usage forms and modifications.

  (A) In FIG. 1A, even if the diffraction grating layer 30 is provided on the upper surface of the active layer 40, substantially the same function and effect can be obtained.

  (B) In the first embodiment, the DFB laser in which the diffraction grating formation region 31 and the diffraction grating non-formation region 32 are periodically formed using InGaAsP MQW for the active layer 40 has been described, but using an InGaAsP bulk, The DFB laser of Example 1 can also be configured.

  (C) In the first embodiment, the InGaAsP DFB laser using InP as the substrate 20 and InGaAsP as the active layer 40 has been described. However, the present invention uses AlGaAs, PbSnTe, etc. as other DFB laser materials. It can be applied to all DFB lasers.

  (D) The DFB laser of the present invention can be changed to structures, shapes, materials, manufacturing methods, etc. other than those shown in the drawings.

It is a schematic block diagram of the DFB laser which shows Example 1 of this invention. FIG. 5 is a schematic manufacturing process diagram showing an example of a method for manufacturing an InGaAsP-based DFB laser in Example 1. FIG. 5 is a schematic manufacturing process diagram showing an example of a method for manufacturing an InGaAsP-based DFB laser in Example 1. FIG. 5 is a schematic manufacturing process diagram showing an example of a method for manufacturing an InGaAsP-based DFB laser in Example 1. It is a schematic block diagram which shows the conventional DFB laser. It is a schematic perspective view which shows the diffraction grating layer in the conventional (lambda) / 4 phase shift DFB laser.

Explanation of symbols

20 n-type InP substrate 21 n-type InP lower clad layer 30 diffraction grating layer 31 diffraction grating formation region 32 diffraction grating non-formation region 33 diffraction grating 40 active layer 41, 41a p-type InP upper clad layer 42 p-type InP block layer 43 n Type InP block layer 44 p type InGaAs contact layer

Claims (2)

An active layer for generating stimulated emission light;
A diffraction grating layer that is laminated under or on the active layer to achieve a single longitudinal mode;
An upper clad layer and a lower clad layer laminated so as to sandwich the active layer and the diffraction grating layer from above and below,
In the diffraction grating layer, a diffraction grating forming region in which an uneven diffraction grating is formed within a range covered by an evanescent field of the stimulated emission light guided through the active layer, and a diffraction in which the diffraction grating is not formed A distributed feedback semiconductor laser characterized in that the lattice non-formation regions are alternately and periodically formed. The diffraction grating formation region and the diffraction grating non-formation region,
Interval ∧ is the following formula (1)
∧ = m · λc / 2Neff (1)
Where m is any positive integer
λc: emission wavelength in the diffraction grating layer
Neff; the sum Ltl of the widths La of the diffraction grating non-formation regions formed discretely with a period satisfying the effective refractive index in the diffraction grating layer is expressed by the following equation (2)
δβ · Ltl≈π / 2 (2)
However, in the diffraction grating layer, the diffraction grating formation region and the diffraction grating non-formation region
2. The distributed feedback semiconductor laser according to claim 1, wherein a difference in propagation constant of light in the region is satisfied.

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