JPH01179487A - Distributed feedback-type semiconductor laser - Google Patents

Abstract

PURPOSE:To realize a laser which is small in a leakage current which leaks toward a window region and capable of a high speed modulation by a method wherein a window structure phase shift distributed feedback type semiconductor laser is provided, where a transparent window region formed of a high resistive layer the same as a channel section is formed at a projecting side. CONSTITUTION:A stripe-like light emitting region (mesa section) 130 comprising an active layer 103 and an optical guide layer 102 is formed on a substrate 1, where the light emitting region 130 is sandwiched in between channel sections 140 which are filled with a semiconductor layer. A phase shift section 160 and a diffraction grating 150 are formed near the center of a laser resonator. In the distributed feedback type semiconductor structured as mentioned above, the above channel layers 140 are formed of a high resistive semiconductor which is higher than the laser radiation in energy gap. The parts of the active layer 103 and the optical guide layer 102 on the projecting side are removed, and a window region 120 formed of the high resistive semiconductor layer 106 the same as the above channel sections 140 is formed on the part where the layers 103 and 102 are removed.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a distributed feedback semiconductor laser.

[Conventional technology]

Distributed feedback semiconductor lasers (hereinafter referred to as DFB lasers), which are equipped with a diffraction grating inside the laser cavity, oscillate in a single-axis mode even during high-speed modulation, making them indispensable devices for long-distance, high-capacity optical transmission systems. ing. DFB lasers basically utilize the wavelength selection function of a diffraction grating inside a laser resonator, but there are several types depending on the shape of the diffraction grating and the structure of the laser end face, and each type has its own features. Among these, near the center of the laser resonator,
A so-called phase-shifted DFB laser in which the period of the diffraction grating is shifted has a much better yield of single-axis mode than other structures. However, in this phase-shifted DFB laser, it is necessary to reduce the reflectance of both end faces of the laser as much as possible.

For this purpose, a method of applying an anti-reflection coating using a SiN film or the like on both end faces is generally used, but at present it is extremely difficult to control the reflectance to about 1% or less with good uniformity. One known method for solving this problem is to form a structure called a "window structure" near the end face. For more information about this structure, see IE, EB Journal of Quantum Electronics (K, UTA
KA et al. IEEE.

J, Quantum Electron, VolGE
-20, 1984, PP236-245). A cross-sectional structure of this window structure DFB laser in the cavity axis direction is shown in FIG. As shown in the figure, this laser removes the active layer 203 and light guide layer 202 near the end face, and fills them with InP that is transparent to the laser beam! 205 is formed. This buried layer 205 becomes a window region 220 without a waveguide. In this structure, the active layer 2
The emitted light from 03 is spread by diffraction in the window region 220, and the light reflected by the end face of the window region 220 is reflected in the active layer 20.
The rate of return to 3 (that is, the effective end face reflectance) becomes extremely small. Although it is not clear in FIG. 2, this laser uses a so-called buried structure as a transverse mode control structure, and when growing this buried structure, the buried layer 205 of the window region 220 is grown at the same time. Specifically, this buried layer has a pnp structure of InP.

[Problem that the invention seeks to solve]

The conventional window structure DFB laser has the following problems. One is that since the buried layer 205 of the window region 220 has a pnpl structure, it does not necessarily function as a complete current blocking layer, and does not necessarily function as a complete current blocking layer.
This is the point at which current leaks and the threshold of the laser increases. Second, since the buried layer 205 has a multilayer structure called a pnp structure and is grown by liquid phase epitaxial growth, the yield of manufactured devices is not necessarily high. The third problem is that the pnpi structure increases the stray capacitance of the entire element, making high-speed modulation difficult.

An object of the present invention is to improve these problems and provide a phase-shifted DFB laser that has extremely low leakage current to the window region, has a high manufacturing yield, and is capable of ultra-high-speed modulation.

[Means for solving problems]

The structure of the present invention is a distributed feedback semiconductor laser which has buried layers on both sides of a striped light emitting region including an active layer and a light guide layer, and has a diffraction grating with a phase shift part near the center of the laser cavity. The buried layers on both sides of the light-emitting region are made of high-resistance semiconductor layers with an energy cap larger than the energy of the laser light, and there is a window region transparent to the laser light near at least one emission surface. The active layer and the optical guide layer are removed from the window region, and the high-resistance semiconductor layer, which is the same as the buried layer, is formed in the removed region.

〔Example〕

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 1(a) is a perspective view showing an embodiment of the present invention;
b) is a sectional view in the axial direction of the resonator. This semiconductor laser basically has the same window structure and phase shift DFB as the conventional example.
It's a laser. The vicinity of one end face of the laser becomes a window region 120, and the active layer 103 and light guide layer 102 are removed. The end face of the other active region 110 is a cleavage plane and is coated with an anti-reflection coating. The transverse mode control structure is a so-called buried 13fi. That is, the structure is such that a central mesa portion 130 serving as a laser emission region is sandwiched between channel portions 140 filled with a semiconductor layer. The feature of the present invention is that the channel portion 140 and the window region 1
20 is formed of a high-resistance semiconductor layer 106 that is not transparent to laser light, that is, has a large energy gap. From this slanted active region 110 to the window region 1
Leakage current to 20 and leakage current through channel portion 140 and the like are very small. Further, since the window region 120 and the channel portion 140 are embedded at the same time, the manufacturing process is simple and a high yield can be obtained. Furthermore, since the stray capacitance in the high resistance layer is extremely small, ultrahigh-speed modulation is possible compared to the conventional example having a PNP buried structure.

First, a phase shift diffraction grating 150 with a period of 236 nm is formed on a p-InP substrate 101. Next, by liquid layer epitaxial growth, p-
InGaAsP light guide layer (λg=1.3μm) 102
．． InGaAsP active layer (λg=1.55 μm) 103
．． n-InP cladding ffJ n-InP cladding layer 104. n-InGaAsP cap layer (λg=1
．． 3μm>105 are grown sequentially. Next, using the SiO2 film as an etching mask, the window region 120 and channel portion 140 from the cap layer 105 to the light guide layer 102 are removed by selective etching. Channel section 140
The width of each mesa portion is 20 μm, and the width of each mesa portion is 1.5 μm. Although it is not clear from FIG. 1, the window region 120 becomes a groove (but in a direction perpendicular to the groove of the channel portion) like the channel portion 140 due to this etching. The width of this groove is 50 μm, and the window area 1 of the actual device
The length of 20 is approximately 1/2 of this length. At this time, the diffraction grating 1
The phase shift portion 160 of 50 is positioned approximately at the center of the active region 110 or slightly toward the end surface of the active region 110. Next, using the above-mentioned Si02M as a mask, the channel portion 140 and the window region 120 are simultaneously filled with an Fe-doped InP layer (high resistance layer) by hydride vapor phase epitaxy. At this time, by controlling the growth time and the like, it is possible to embed the element so that the surface of the element becomes substantially flat. Next, the 5i02 film is removed. FIG. 1 shows this state. Next, the 5i02 film is again formed on the entire surface except for the central mesa portion 130, and then electrodes are formed on both sides of the device. Finally, the device is cut out by cleavage, and an anti-reflection coating is applied to the end face of the active region 110 using a SiN film.

In this element, the effective reflectance on the window region 120 side is so small that it can be ignored, so the anti-reflection coating on the active region 110 side will not have a large effect on the laser characteristics even if the reflectance is not controlled too strictly. , so a high single-axis mode yield can be obtained. The characteristics of the actually manufactured device are as follows. The oscillation wavelength is 1.55μm, and the average threshold is 1
The yield of devices operating in single-axis mode up to 20 mW or more at 2 mA and an average differential quantum efficiency of 0.2 W/A was 95% or more. In addition, a high cutoff frequency of 14 GHz was obtained for the small signal response characteristic. Also 1 0
Modulation of G b/s was possible.

In the above embodiments, p-InP was used as the substrate, but n-InP was used as the substrate.
There is no big difference even if InP is used. It is also possible to form window regions on both end faces. The growth method is
Although a combination of liquid-phase epitaxial growth and hydride vapor-phase epitaxy was used, it is also possible to grow double heterostructures and high-resistance layers using other methods such as metal-organic vapor-phase epitaxy. In addition, if the laser wavelength is other than the 1.55 μm band, 1.
Similar effects can be obtained for elements in the 3 μm band and elements made of other materials such as AIGaAs.

〔Effect of the invention〕

As described above, according to the present invention, leakage current to areas other than the active layer is extremely small, manufacturing yield is high, and
A phase-shifted I/DFB laser capable of high-speed modulation can be realized. In fact, for a 1.55 μm band element, the average threshold value is 12 m.
A. A yield of 95% in single-axis mode was obtained. Mana 10
High-speed modulation of Gb/s was possible.

[Brief explanation of the drawing]

FIG. 1(a) is a perspective view showing one embodiment, FIG. 1(b) is a sectional view in the axial direction of the resonator, and FIG. 2 is a sectional view showing a conventional example. In the figure, 110°210 is the active region, 1
20, 220 are window areas, 101 is a substrate, 102.202
is a light guide layer, 103.203 is an active layer, 104 is a cladding layer, 105 is a cap layer, 205 is a buried layer, 1
06 is a high resistance layer, 130 is a mesa portion, 140 is a channel portion, 150 is a diffraction grating, and 160 is a phase shift portion.

Claims (1)

[Claims] A distributed feedback semiconductor laser comprising buried layers on both sides of a striped light emitting region including at least an active layer and a light guide layer, and a diffraction grating having a phase shift portion near the center of the laser cavity, The buried layers on both sides of the light-emitting region are made of high-resistance semiconductor layers with an energy cap (forbidden width) larger than the energy of the laser beam, and a layer transparent to the laser beam is formed near at least one emission surface. A window region is formed, the active layer and the light guide layer are removed from the window region, and the high-resistance semiconductor layer, which is the same as the buried layer, is formed in the removed region. Distributed feedback semiconductor laser.