JPH047884A - Semiconductor wavelength control element and semiconductor laser - Google Patents

Abstract

PURPOSE:To increase the change of refractive index of a wavelength control region and enlarge the shift amount of wavelength, by constituting a plurality of guide layers. CONSTITUTION:The following are formed; a plurality of guide layers 4, 7 which are formed on a semiconductor substrate 1, isolated almost in parallel by interposing a barrier layer 5, and optically coupled with each other, a diffraction grating 3 arranged on the guide layer 4, and control electrodes 9, 10 for controlling the carrier concentration of the guide layers 4, 7. In this case, by stacking the guide layers while interposing the barrier layer whose thickness is available for optical coupling, the light confinement more perfect than the case of single layer is realized. Thereby the refraction index can be largely changed in a wavelength control region, and wavelength can be adjusted in a wide range.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a semiconductor wavelength control element having a plurality of guide layers and increasing the light confinement rate in the guide layer, and a semiconductor wavelength control element in which the element is integrated into one. The present invention relates to semiconductor lasers such as DBR lasers.

[Conventional technology]

In optical communications and optical information processing, optical filters are needed to select arbitrary signal light from wavelength multiplexed signals. As a transmission type optical filter element, there is one that uses a semiconductor laser equipped with an optical amplification region and a wavelength control region with a bias current below the oscillation threshold. This structure provides gain in the amplification region and selects the wavelength of transmitted light in the wavelength control region, and the selected wavelength matches the Bragg wavelength determined by the equivalent refractive index of the waveguide and the pitch of the diffraction grating. Here, by injecting carriers into the wavelength control region, it is possible to generate a plasma effect and change the equivalent refractive index, thereby changing the Bragg wavelength, thereby making it possible to tune the selected wavelength.

The Bragg wavelength λg is expressed by λg=2dnar/m from the Bragg equation. m is the order, d is the pinch of the diffraction grating, n
me means equivalent refractive index. Since the wavelength channel width is determined by the amount of change in the refractive index of the wavelength control region, a structure that allows for a large change in the refractive index is desired.

Furthermore, if a region with an active layer is used with a drive current higher than the threshold value, a distributed Bragg reflection type with a light emitting region and an 'IB fii region on the wavelength side can be formed.
ragReflector: Hereinafter referred to as DBR. ) becomes a semiconductor laser. 3-electrode type D shown as a conventional example
The BR laser (see FIG. 6) is capable of wavelength shifting while keeping the phase continuous, and is effective as a coherent light source for the heterodyne system, which is considered to be the mainstream of next-generation optical communications. Its intended use includes a wavelength modulated light source for signal light, and a light source for local light that can be tuned to signal light to obtain a beat signal on the receiver side. A larger amount of wavelength shift is more useful, and a structure with a larger change in refractive index in the wavelength control region is expected.

[Problem to be solved by the invention]

As mentioned above, the selection of the Bragg wavelength depends on the equipolarized refractive index of the waveguide, and the equipolarized refractive index of this waveguide is due to the plasma effect of the guide layer composed of a double hetero head that confines carriers. Strongly affected by refractive index fluctuations. In order to increase the change in the equipolarized refractive index, it is important to effectively incorporate the change in the refractive index of the guide layer, that is, it is sufficient to increase the light confinement rate in the guide layer.

In the conventional structure, since the guide layer is one layer, the guide layer must be made thicker in order to increase the light confinement rate, that is, the light confinement coefficient ξ. However, considering the optical coupling with the light-emitting region, it was not possible to make the guide layer sufficiently thick, so that a high degree of light confinement could not be achieved in the guide layer.

FIG. 7 shows the relationship between the refractive index distribution and the light intensity distribution when this guide layer is made of a single layer.

In FIG. 7(a), the vertical axis represents the thickness direction and the horizontal axis represents the refractive index. In FIG. 7(a), the vertical axis represents the thickness direction and the horizontal axis represents the intensity of light. The shaded part represents the guide layer, and the light intensity distribution in that part is related to the light confinement coefficient ξ. That is, in order to increase the light confinement coefficient ξ, the guide layer must be made thicker.

However, as shown in Figure 3, when the active layer and the guide layer are parallel to each other, there is
In order to obtain high optical coupling in the movement of light in a structure in which the active layer and the guide layer are directly connected, it is important that the propagation constants of the active layer and the guide layer be equal. That is, since the determination of the propagation constant is restricted by the structure of the active layer portion, the thickness of the guide layer is limited, making it impossible to achieve high optical confinement.

[Means to solve the problem]

The object of the present invention is to provide a semiconductor wavelength control element with a high light confinement rate in a guide layer, and a D
The purpose is to use a semiconductor laser such as a BR laser.

By stacking a guide layer on one guide layer with a barrier layer interposed therebetween to a thickness that allows optical coupling, that is, by increasing the number of guide layers, it becomes possible to confine light more than in the case of one layer.

In addition, when applied to a semiconductor laser, in order to put a semiconductor wavelength control element with multiple guide layers into practical use, the propagation constant of light and the intensity distribution of light in the active layer and the guide layer should be equal, and the central axis should be the same. This is a combined configuration.

[Effect]

In the device of the present invention configured in this way, high optical confinement is possible in the guide layer region, as shown in FIG.

FIG. 4 shows the relationship between the refractive index distribution and the light intensity distribution when the guide layer has two layers.

In FIG. 4(a), the vertical axis indicates the thickness direction and the horizontal axis indicates the refractive index, and in FIG. 4(b), the vertical axis indicates the thickness direction and the horizontal axis indicates the intensity of light. Each shaded area represents a guide layer, and the light intensity distribution in that area is the light confinement coefficient ξ
related to. That is, it can be seen that a large light confinement coefficient can be obtained by forming the guide layer into two layers.

Furthermore, for practical use in semiconductor lasers, we have invented a structure (see FIG. 5) that allows high optical coupling with the light emitting region. Fifth
In the structure shown in the figure, the light propagation constant and light intensity distribution of the active layer 6 and the second guide layer 7 are equal, and their central axes are aligned. Therefore, the mode of light caused by the active layer 6 and first guide layer 4 in the light emitting region and the mode of light caused by the second guide layer 7 and first guide layer 4 in the wavelength control region are both light modes. The intensity distribution and propagation constant become equal, and a 100% optical coupling coefficient Co at the boundary is obtained.

Therefore, there is no propagation loss of light in the emission N region and the wavelength control I region, and the light confinement rate in the guide layer is high, allowing a large refractive index change in the wavelength side fIl region.

Furthermore, in an optical waveguide such as a two-layer guide layer, not only the fundamental (zero-order) mode but also the first-order mode (see FIG. 5) can exist with respect to the vertical transverse mode.

However, since the propagation constants of the 0th-order mode and the 1st-order mode light are different, they are Bragg-reflected by diffraction gratings with different pitches. In other words, depending on the pitch of the diffraction grating, it is possible to select light that propagates in the 0th-order mode or the 1st-order mode.

〔Example〕

The structure of the semiconductor wavelength control element of the present invention is shown in FIG. In addition, a wavelength variable D in which a semiconductor wavelength control element is integrated
The structure of the BR laser is shown in FIG.

Here, the steps for manufacturing the wavelength tunable DBR laser shown in FIG. 2 will be described using FIG. 3.

Hereinafter, the plurality of guide layers will be referred to as a first guide layer 4 and a second guide layer 7, respectively.

i) A Sn-doped n-type 1nP buffer 7 layer 2 with a carrier concentration of 5×10"cm-' is grown on a Sn-doped n-type 1nP substrate 1 with a carrier concentration of 1X10"cm-'. Thereafter, the D B RH region A diffraction grating 3 is formed by etching the portion where the diffraction grating 3 is formed using an etching mask for forming a diffraction grating formed by a resist interference exposure method.

ii) After removing the mask, a first layer of ■ nGaAsP with a bandgap wavelength of 1.3 μm and a carrier concentration lXl0''cm-Sn-doped n-type layer lattice-matched with TnP is placed on top of the mask.
Grow the guide N4 and further increase the carrier concentration lXl0”
A Sn-doped n-type 1nP barrier layer 5 of cm- is grown. Next, lattice matching with InP is performed to achieve a band-gearth wavelength of 1.
An active layer 6 of 55 μm (7) InC; aAsP is grown, and a Zn-doped p-type 1nP cladding layer 8 with a carrier concentration of 7×10”cm is further grown. 1.3μ on top of the active layer 6 of the band
Although an m-band InGaAsP anti-meltback layer is provided, it is omitted in the figure.

1ii) The portion that will become the light emitting region is masked with the SiNx film 14, and the other portions of the cladding layer 8 and active layer 6 are removed by selective etching.

iv) The portion removed by etching has the same composition as the first guide layer 4 and has a carrier concentration I x 10''c.
Grow a second guide layer 7 of m-Zn doped p-type 1 nGaAs P, then with a carrier concentration of 7 x 10 l7c
An m''3 Zn-doped p-type 1nP cap layer 15 is grown. However, the design is such that the conditions for optical coupling between the active layer 6 and the second guide layer 7 are satisfied.

(2) After removing all the masks, the cladding layer 8 is grown thicker again.

The semiconductor wafer produced in the above manner is mesa-processed and buried to form a semiconductor laser structure.
1. By forming the p-type electrode 9 of the phase control region 12 and the DBR region 13 separately, and forming the n-type electrode 10 on the back side of the substrate to connect them, the wavelength tunable DBR laser shown in FIG. 2 can be obtained. Created.

In this embodiment, an n-type substrate semiconductor laser is used, but it is obvious that the present invention can be applied to a semiconductor laser using a p-type substrate, and the active layer 6 may have a different bandgap wavelength or a multilayer structure.

Further, although the first guide layer 4 and the second guide layer 7 each cause optical coupling, the light confinement rate of each region of the first guide layer 4 and the second guide layer 7 is high ( If so, there is no problem with multiple configurations with different compositions, however,
If the bandgap wavelength of each guide layer is longer than the wavelength generated in the light emitting region, absorption loss will increase, so it must be shorter than the light emission wavelength. Further, the barrier layer 5 separating the first guide layer 4 and the second guide layer 7 needs to have a lower refractive index and a larger band gap than the guide layer in consideration of light confinement and carrier confinement.

Furthermore, if the compositions of the first guide layer 4 and the second guide layer 7 are different, it is possible to omit the barrier layer 5.Furthermore, the diffraction grating may be provided on any guide layer.
Naturally, if it is provided on a plurality of guide layers, it must be constructed so that Bragg reflection occurs effectively.

The above description can also be applied to semiconductor wavelength control elements and wavelength tunable DBR lasers using semiconductor crystals other than InP.

〔Effect of the invention〕

Since the structure includes a plurality of guide layers, it is possible to increase the light confinement coefficient ξ without increasing the thickness of the guide layer.

As a result, it has become possible to improve the amount of change in refractive index.

Specifically, it is as follows.

In the conventional technology, the cladding layer has a refractive index of 3.17 and the guide layer (single layer) has a refractive index of 3.17 for a wavelength of 1.55 μm.
．． The equivalent refractive index when the 387 thickness is 0.2 μm is 3.
It becomes 210. At this time, if the refractive index of the guide layer changes by 11% due to the plasma effect, the amount of change in the equivalent refractive index △n is -0,
0011, and the amount of change Δλ in the Bragg wavelength is -5 nm. Next, in the present invention, the refractive index is 3.17, the thickness is 0.
When two guide layers with a thickness of 0° and 2 μm are used with a barrier layer of 1 μm in between, the amount of change in the equivalent refractive index Δn is −0,0.
018, and the amount of change Δλ in Bragg wavelength is -8.6 layers m
becomes. In this way, a wavelength change approximately 70% larger than in the case of one layer is possible.

In addition, in the configuration of the present invention, since the guide layer is made of two layers,
The reduction in optical coupling at the boundaries of each region, which occurs when the guide layer is made thicker, does not occur.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the structure of the present invention, FIG. 2 is a diagram showing the structure of the present invention, FIG. 3 is a diagram showing the procedure for manufacturing the structure of the present invention, and FIG.
The figure is a diagram for explaining the relationship between the refractive index distribution and the light intensity distribution when the guide layer has two layers, and FIG. 5 is a diagram for explaining the present invention in detail. FIG. 6 is a diagram showing the configuration of the prior art, and FIG. 7 is a diagram illustrating the relationship between the refractive index distribution and the light intensity distribution when the guide layer is made of a single layer in the prior art. 1 ・ 3 ・ 5 ・ 7 ・ 9 ・ 12... Buffer layer, 4... First guide layer 6... Active layer, De layer, 8 Clad layer, - Electrode, 11... Luminescent head stock 13 ...DBR region, 14 - Cap layer.・Substrate, 2. ・Diffraction grating, ・Barrier layer, ・Second guy electrode, 10. ・Phase control region, ・SiNx film, 15. Engraving of drawing Figure 1 Patent applicant Anritsu Corporation Agent Patent attorney Ryuta Koike Figure 2 Figure 4 Figure 3 Figure 5 Figure 2'〒0 Soil liquid length control 11 Bale transmission and removal'L 朕βO゛βC'βa''βD≠βｑ Figure 6 Existence 9 Yes procedure (hosei sho (method) % formula % Name of invention Semiconductor wavelength control device and semiconductor laser correction person

Claims (2)

[Claims] (1) A plurality of guide layers formed on a semiconductor substrate, extending substantially parallel and separated with a barrier layer interposed therebetween, and optically coupled to each other, and a diffraction device provided in at least one of the guide layers. A semiconductor wavelength control element comprising a grating and a control electrode for controlling carrier concentration in the guide layer. (2) A semiconductor laser in which the semiconductor wavelength control element is integrally combined.