JPH0230195B2 - - Google Patents

Description

[Detailed Description of the Invention] [Summary] In a distributed reflection laser, an active waveguide region is embedded in a continuously formed external waveguide region, and alignment between the active waveguide region and the external waveguide region is further improved. This minimizes the reflection of laser light at the coupling part.

[Industrial application field]

The present invention relates to a distributed reflection semiconductor laser.

[Conventional technology]

Currently, it is essential to use dynamic single mode lasers as light sources in the 1.55 μm wavelength band to realize long-distance, high-capacity optical communications.

There are various types of dynamic single mode lasers, such as distributed feedback (DFB) lasers, distributed reflection lasers (DBR) lasers, and composite cavity lasers. Reflection type (DBR) lasers are easy to maintain stable single longitudinal mode operation during high-speed modulation, easy to monolithically integrate with other functional devices,
It is viewed as particularly promising because it has advantages such as the ability to shorten the resonance of the laser resonator.

Next, as distributed reflection lasers, we will provide an overview of the conventionally proposed DBR-ITG laser with an integrated dual waveguide structure and the DBR-BJB laser with a direct coupling structure called the BJB type.

FIG. 2 is a sectional view showing a schematic structure of a DBR-ITG type laser. In the figure, 21 is an active waveguide, 22 is an external waveguide, 23 and 24 are distributed Bragg reflectors, and 25 and 26 are electric field distributions of laser light.

As shown in the figure, for the DBR-ITG type laser,
The active waveguide 21 is formed in parallel on the external waveguide 22 . Therefore, since the coupling between the active waveguide 21 and the external waveguide 22 is in the vertical direction as shown by the arrow, the active waveguide 21
The matching condition between the electric field distribution 25 at the central position of the distribution bragg reflector 24 and the electric field distribution 26 at the position of the distributed bragg reflector 24 depends on the length of the active region indicated by diagonal lines. Since it is difficult to do so, there is a drawback that partial reflection occurs between the active waveguide 21 and the external waveguide 22.

FIG. 3 is a sectional view showing the schematic structure of the DBR-BJB type laser. In the figure, 31 is an active waveguide, 32 and 33 are external waveguides, 34 and 35 are distributed Bragg reflectors, and 36 and 37 are electric field distributions.

This DBR-BJB type laser differs from the DBR-ITG type laser in that it has an active waveguide 31 and an external waveguide 3.
2 and 33 are arranged almost linearly, and it is possible to design the electric field distribution and equivalent refractive index of the active region and the external waveguide region to be equal.

However, due to the actual manufacturing process, this DBR-BJB
In a type laser, the thickness of the external waveguide layer changes at the coupling part, which tends to cause a step, and this step has the disadvantage of causing partial reflection between the active waveguide 31 and the external waveguides 32 and 33. It was hot.

[Problem that the invention seeks to solve]

Conventional distributed reflector (DBR) lasers are difficult to achieve due to the conditions for highly efficient coupling between the active waveguide and the external waveguide, or due to the step difference between these waveguides. There were problems in that reflection occurred in the waveguide coupling portion, multimode laser oscillation was likely to occur, and it was difficult to increase the oscillation efficiency.

[Means for solving problems]

The principle of the present invention is that the active waveguide of a distributed reflection laser is embedded in an external waveguide, and by matching the propagation constant and electric field distribution in both the active waveguide and the external waveguide, This eliminates reflections at the joint.

FIG. 4 is a sectional view showing the structure of a typical example of a distributed reflection laser according to the present invention. In the figure, 4
1 is an active waveguide, 42 is an external waveguide, 43 and 44 are distributed Bragg reflectors, and 45 and 46 are electric field distributions.

As shown in the figure, the front and back of the active waveguide 41 and
Since the external waveguide 42 is formed to surround the upper part, all of the laser light output from the active waveguide 41 is input into the external waveguide 42 . Furthermore, the light reflected by the distributed Bragg reflectors 43 and 44 mainly propagates in the low-loss external waveguide 42,
Some light is emitted to the outside.

In FIG. 4, if the physical dimensions and compositions of the active waveguide 41, external waveguide 42, etc. of the distributed reflection laser are appropriately set, the propagation constants of both waveguide regions can be made approximately equal, and the electric field distribution shown in the figure can be made approximately equal. 45,
46, the electric field distribution in each part of the waveguide can be matched, and the tolerance of the design is high, and reflections between the active waveguide 41 and the external waveguide 42 can be easily reduced to a minimum. It becomes possible to convert into

〔Example〕

The details of the present invention will be explained below with reference to Examples.

(1) Element Structure FIG. 1 is a sectional view showing the element structure of one embodiment of a distributed reflection (DBR) laser having an active waveguide buried structure according to the present invention.

In the figure, 1 is an n-InP substrate, 2 is λg=
1.55μm GaInAsP active waveguide layer, 3 is λg=
1.35μm GaInAsP protective layer, 4 λg = 1.35μm
5 is a p-InP layer, 6 is a GaInAsP outer waveguide layer.
is a p-GaInAsP cap layer, 7 is a p-InP layer,
8 is an n-InP layer, 9 is a p-GaInAsP layer, 10 is a primary grating forming a distributed Bragg reflector, 11 is a Zn diffusion region, 12 is a metal electrode,
13 indicates a SiO ₂ insulating layer. Note that λg is the bandgap wavelength.

(2) Manufacturing method Next, a manufacturing method for realizing the element structure shown in FIG. 1 will be described.

FIG. 5 shows an example of the manufacturing method.

First, GaInAsP with a bandgap wavelength λg (hereinafter omitted) of 1.55 μm was grown on an n-InP substrate 1 using the LPE growth method, as shown in FIG. 5a.
Active waveguide layer 2 and GaInAsP with 1.35 μm composition
A wafer 51 is manufactured by sequentially growing the protective layer 3.

Next, using a stripe mask stretched in the (011) direction, as shown in FIG.
4 is removed by selective etching onto the n-InP substrate, and further a primary grating 55 constituting a distributed blur reflector is formed on the n-InP substrate.
form 56.

Next, a second growth is performed, and as shown in FIG.
A GaInAsP cap layer 6 is sequentially grown over the entire surface.

After this, further burying (BH) processing is performed to selectively etch the mesa in the reverse direction in the lateral direction as shown in FIG.
A current confinement layer consisting of an InP layer 7, an n-InP layer 8, and a p-GaInAsP layer 9 is formed (see FIG. 1).

In this way, the outer waveguide layer 4 surrounding the top and front and back of the active waveguide layer 2 can be easily formed in one step.

In addition, this method has the advantage that the active waveguide region can be obtained with high precision because the etching for forming the active waveguide region is performed at the stage when the active waveguide layer and the protective layer are grown on the substrate.

FIG. 7 shows an example of another manufacturing method. In this example, the outer waveguide layer 4 is formed in two steps.

First, as shown in FIG. 7a, an n-InP substrate 1
A GaInAsP active waveguide layer 2, a GaInAsP protective layer 3, and a GaInAsP outer waveguide layer 4 are sequentially grown thereon.

Next, as shown in FIG. 7B, the active waveguide region 71 is left and the other regions 72 and 73 are etched and removed to the top of the n-InP substrate, and primary gratings 74 and 75 are formed.

Next, as shown in FIG. 7c, a second growth is performed to form a GaInAsP outer waveguide layer 4',
Layer 5' etc. are formed.

In this way, a more interconnected outer waveguide layer 4, 4' is obtained.

(3) Matching of propagation constant and electric field distribution FIG. 8 is an enlarged view of a part of the element structure of the embodiment shown in FIG.

In the figure, ns: refractive index of n-InP substrate 1 and p-InP layer 5 nact: refractive index of active waveguide layer 2 ncov: refractive index of protective layer 3 next: refractive index of outer waveguide layer 4 tact: active Thickness of waveguide layer 2 tcov: Thickness of protective layer 3 text: Thickness of external waveguide layer 4.

In order to match the propagation constant and electric field distribution between the external waveguide region 81 and the active waveguide region 82, the refractive index next of the external waveguide layer 4 is set to the refractive index nact of the active waveguide layer 2. Refractive index of layer 3
It is sufficient to set it to a value intermediate between nact and ncov so that a certain weighted average of nact and ncov can be equal to next.

Furthermore, the refractive index ns of the n-InP substrate 1 and the p-InP layer 5 must be smaller than both next and nact because it is necessary to confine the laser light within the waveguide.

Therefore, one condition is that the relationship nact>next>ncov≧ns holds between the refractive indices of the materials constituting each layer. By appropriately controlling the material composition and thicknesses tact, tcov, and text of each layer under these conditions, it becomes possible to match the propagation constant and electric field distribution.

FIG. 9 shows the active waveguide layer 2 in FIG.
bandgap wavelength λg = 1.60 μm, tact =
Assuming 0.1μm, tcov=0.1μm, ncov=3.16,
Next and λg of outer waveguide layer 4 are next = 3.30
(λg = 1.15μm), 3.35 (λg = 1.25μm), 3.40 (λ
g
= 1.35 μm), text
When changing in the range of approximately 0.1 to 0.6 μm,
This is an example of experimental data showing a change in the coupling efficiency Cout between the active waveguide region 81 and the external waveguide region 82.

As shown, over 90% and 100% over a wide range
A very high coupling efficiency has been obtained.

(4) Device characteristics FIG. 10 is an example of data on oscillation characteristics obtained using the device structure of the example shown in FIG.
As shown, threshold current Ith = 180 in pulse operation
mA is obtained and single mode operation is confirmed at 1.14 times Ith. However, the element length is 100 μm for the active waveguide region, and 170 μm and 30 μm for the distributed bragged reflector regions on both sides, respectively.
It was hot.

(5) Other embodiments The element structure of the embodiment shown in FIG.
Although a primary grating is formed on the upper surface of the InP substrate, the present invention also applies to structures in which this grating is formed on the upper facing surface of the external waveguide as in the conventional example shown in FIGS. 2 and 3. can be applied as well.

FIG. 11 shows one such embodiment. In the figure, 111 is a substrate, 1
12 is an active waveguide layer, 113 is a protective layer, 114
is an outer waveguide layer, 115 and 116 are first order gratings forming a distributed Bragg reflector.

In this example, grating 115,1
16 is formed after growing the outer waveguide layer 114.

Further, laser light generated by a distributed reflection laser may have various polarization states. In such a case, by arranging a metal film on the side surface of the external waveguide to form a configuration filter, only the laser light with polarization in the TE configuration parallel to the metal film is allowed to pass, and the polarization at right angles to this is allowed to pass.
It is possible to block TM polarized laser light. In other words, the planes of polarization can be aligned.

FIG. 12 shows one such embodiment. In the figure, 121 is a substrate, 12
2 is an active waveguide layer, 123 is a protective layer, 124 is an external waveguide layer, 125 and 126 are gratings, and 127 and 128 are metal films such as Au forming a shape filter.

As yet another embodiment, by making the entire laser element into a buried structure, alignment in the cleavage plane can be improved and reflections can be reduced.

Note that the present invention is not limited to the embodiments described above, and many modified configurations that are obvious in the technical field are also included within the scope of the present invention.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to achieve extremely good coupling between the active waveguide and the external waveguide in a distributed reflection laser, and the occurrence of reflection, scattering, etc. can be almost eliminated. A distributed reflection laser that performs efficient and stable single-mode laser oscillation can be realized.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing the element structure of one embodiment of the present invention, FIG. 2 is a sectional view showing the structure of a DBR-ITG type laser, and FIG. 3 is a sectional view showing the structure of a DBR-BJB type laser. FIG. 4 is a cross-sectional view of the distributed reflection laser according to the present invention, FIG. 5 is an explanatory diagram showing the manufacturing method of the example device shown in FIG. 1, and FIG. 6 is an external view to explain the state during the embedding process. 7 is an explanatory diagram showing another manufacturing method of the example element shown in FIG. 1, FIG. 8 is an explanatory diagram of matching of propagation constant and electric field distribution according to the present invention, and FIG. 9 is an explanatory diagram of coupling efficiency characteristics. 10 is a graph showing the oscillation characteristics of the device according to the example, FIG. 11 is a cross-sectional view of the device according to another example of the present invention, and FIG. 12 is a graph showing the device according to still another example of the present invention. FIG. In Figure 1, 1 is an n-InP substrate, 2 is
GaInAsP active waveguide layer, 3 is GaInAsP protective layer,
4 indicates a GaInAsP external waveguide layer, and 10 indicates a grating.

Claims (1)

[Claims] 1 An active waveguide layer 2 formed in a predetermined area on the substrate 1, a protective layer 3 laminated on the active waveguide layer 2, and a layer in contact with the front and rear and upper surfaces of the active waveguide layer 2 and the protective layer 3. The active waveguide layer 4 has an outer waveguide layer 4 formed to surround the active waveguide layer 4 and a distributed Bragg reflector provided along a predetermined portion of the outer waveguide layer 4, and by controlling the refractive index. A distributed reflection laser characterized in that the propagation constant and electric field distribution are matched between the layer 2 and the external waveguide layer 4.