JPS60178685A - Single-axial mode semiconductor laser device - Google Patents

Abstract

PURPOSE:To enable the element of excellent characteristics such as the exposure area of a diffraction grating and the stability of period to be produced with good reproducibility by utilizing the current injection to a wave guide for phase control without any diffraction grating in the middle of two wave guides that form diffraction gratings. CONSTITUTION:A thin film stripe of SiO2 or Si3N4 with a width corresponding to the distance between diffraction gratings 10 and 11 is selectively formed on the surface of an n type InP, and a photo resist film is formed thinly thereon. Further, a pattern of diffraction grating utilizing the two-flux interference of gas laser beam is exposed over the whole surface. Next, only the InP is etched by using the remaining resist as a mask; thereafter, the resist and the dielectric thin film are removed, thus obtaining an InP substrate 1 having the part of formating diffraction grating in stripe form. Then, crystal layers 2-5 are laminated, and further electrodes are formed by evaporation and the like. A forward current is injected from electrodes 7 and 9 of a hetero structural element thus obtained to the p-n junction of the part of forming two diffraction gratings. Thus, a spatial phase difference of pi can be imparted between diffraction gratings of two regions A and B by varying the characteristic of light absorption at the part of the wave guide in a region C according to the amount of carriers injected to this part.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention is a light emitting element used as a light source in an optical communication system using glass fiber, and is a distributed feedback type or The present invention relates to a distributed reflection type semiconductor laser device.

(Prior Art Field) Semiconductor lasers used as light sources for optical communications are required to have stable oscillation wavelengths and oscillation modes even during high-speed modulation. Semiconductor lasers that meet these requirements have a periodic structure in which the thickness of a semiconductor crystal layer periodically changes, and this periodic structure has a light reflection function to obtain a so-called distributed laser oscillation. Feedback type semiconductor lasers and distributed reflection type semiconductor lasers have been known.

Among these, distributed feedback (hereinafter referred to as DFB) lasers have the above-mentioned periodic structure formed in an active layer or a light guide layer formed in contact with the active layer along the direction in which light oscillates.

The manufacturing technology for this type of laser has made great progress in recent years, and devices that operate continuously at room temperature have been obtained, but the yield of devices that oscillate in a single-axis mode is not 100 inches. This is clear from the analysis of Kogelnick et al. (Jo
urnal of Applied Physics+
43 Haku, May issue, 2327 negative - 2335 pages, 197
2), this is because DFB lasers essentially have two oscillation modes with the same threshold gain. However, in actual device structures, the above-mentioned symmetrical spectral structure changes and becomes asymmetric due to the influence of the phase of the diffraction grating at the device end face, the diffraction grating along the waveguide, the crystal composition, the crystal thickness, etc. Therefore, single 1T11 mode oscillation is possible. Regarding such technology, please refer to Mr. Streifer's paper (IEEE Journal of Quan
tum EJectronjcs r QE-118
, April issue, pp. 154-161, 1975) and Mr. Tada's paper ('Giko Communication Society, Optical/Quantum Electronics) rConics Research Group Management Fee, Material No. OQE 77-88 (19
78-01) K is described in detail.

Therefore, in order to obtain a DFB laser that oscillates in a single axis mode, it is necessary to actively incorporate a structure that makes the spectrum asymmetric, and many proposals have been made to avoid this.

Among them, one of the Qs proposed by Mr. Tada et al.
There is an element in which the spatial phase of a diffraction grating installed along the waveguide structure of a DFB laser differs between the left and right sides of the element. As stated on page 107 of the Institute of Electronics and Communication Engineers, Optical and Quantum Electronics Study Group Material/Reference No. OQE 77-84, a DFB laser with this structure has a diffraction grating structure as shown in Figure 1, and in particular, the Phase difference θ of diffraction grating
When takes the value of π, the asymmetry of the spectrum becomes the strongest and single-dll+ mode oscillation can be easily obtained.

Although the DFB laser is thus a useful element, there are significant difficulties when considering the manufacturing process. Conventionally, diffraction gratings for manufacturing DFB lasers and DBR lasers have been formed using two-beam interference of gas laser light. Although this method is extremely excellent for forming a uniform diffraction grating on a large area semiconductor substrate, it is difficult to form a diffraction grating with the structure shown in Figure 1.
Close to Noh.

Among the currently practical technologies, the period is 2000~5ooo
The electron beam exposure method has the TIIJ capability of giving a fine diffraction grating A a spatial phase difference as shown in FIG. 1 with good controllability. However, this method has drawbacks such as the stability of the exposed area of the diffraction grating for two rotations, making it unsuitable for stably producing elements with excellent characteristics.

(Objectives and Features of the Invention) The present invention has been made to overcome the difficulties of I) fabrication of an FB laser having a diffraction grating incorporating a spatial phase difference as described above. The purpose is to provide a semiconductor laser device, and its feature is that the phase side without a diffraction grating is installed between two diffraction grating forming waveguides to provide a spatial phase difference. 1 to waveguide for 1111
5. There is a method that uses ryuin.

(4i+1 composition and action of invention) A detailed explanation of the present invention will be given below.

FIG. 2 is a sectional view showing the principle of the present invention, where l is an n-type InP substrate, 2 is an n-type InGaAsP optical guide layer, 3 is a p-type InGaAsP active layer, 4 is a p-type InP cladding layer, and 5 is a A p-type InGaAsP electrode layer, 6 is an n-type ohmic electrode, and 7, 8.9 are p-type ohmic W and poles.

Further, 10 and 11 are diffraction gratings formed on the surface of the InP substrate. In order to realize the configuration of this device, first, an n-type In
Rikamo's S corresponding to the distance between two diffraction gratings on the P surface
Selectively form some thin film stripes such as j02 or 513N4. A photoresist film is formed on this,
A diffraction grating pattern using two-beam interference of gas laser light is exposed on the entire surface from above. After developing the resist, the remaining resist pattern is used as an etching mask to etch only the InP by appropriate means, and then the resist and dielectric thin film are removed. Through these steps, a 1nP substrate having a stripe-shaped diffraction grating forming portion is obtained. On top of this, liquid phase growth, vapor phase growth, and MOCVD methods are applied.

Crystal layer 2 ~ by using an appropriate crystal growth method such as MBE method
5 are laminated, and further electrodes are formed by means such as vapor deposition.

A flow is caused to flow in the positive direction from the electrodes 7 and 9 of the betaro structure element thus obtained to the pn junction of the two diffraction grating forming portions so that the same 1S and flow density are obtained, and the value is gradually increased. Let's consider the case where In the structure of the portion under the electrode 7 and the portion under the electrode 9, the period of the diffraction grating and the thickness of the crystal layer are selected so as to operate as a distributed feedback laser. That is, A is the period of the diffraction grating,
λ is the oscillation wavelength, n+ is the order of the diffraction grating, n is the crystal J*1
If the effective refractive index of the optical waveguide is 4, then A
= (mλ/2n) is required. The injected current is distributed under the electrodes 7 and 9.
/- laser oscillation occurs in two regions A (region below 7) and B (region below 9) when the oscillation threshold of laser is exceeded. At this time, the laser light propagates along the waveguide, so
The light oscillated in region A reaches region 13, and the light oscillated in region B reaches region A, where interaction occurs. At this time, the light oscillated in regions A and B has the same spatial phase as long as it remains in each region, but in region C (where there is no waveguide)
8), the effective refractive index of this region is different from the values in regions A and B, so that a phase change occurs depending on the difference in effective refractive index and the length of region C. Therefore, the second
In the configuration shown in the figure, if the structures of regions A and B are made the same and the same current is injected into regions A and B at the same time, there will be a difference in the spatial phase of the diffraction grating on the left and right sides of the element proposed by Mr. Tada. A distributed feedback type 7-the can be realized. However, as mentioned above, the phase difference θ is the length of area C and areas A and B.
It is determined by the difference in effective refractive index between and C, and θ−π
The only way to achieve this ideal condition is to wait for coincidence. Furthermore, in a so-called unexcited state where no current is injected into region C, the region C
acts as a saturable absorber, causing optical loss, and also causes unfavorable phenomena for single-axis mode oscillation of distributed feedback lasers such as optical twin beams. This difficulty is solved by injecting a positive current from electrode 8 to the pH junction in region C. The carriers injected into the waveguide in region C change the light absorption q゛, unity of this portion according to their desires, and the carriers injected into the waveguide in region A, ! =13, the transparency to the oscillated laser light is increased and the refractive index is also changed. Therefore, the optical loss is reduced compared to the case of non-excitation, and the injection...
The phase difference θ can be adjusted according to the phase difference θ. These facts? If utilized, the flow of carriers injected into the waveguide in region C can be controlled in accordance with the thickness, composition, length, and operating conditions of regions A and B. It is clear that it is possible to provide a spatial phase difference of magnitude π between the diffraction gratings.

Next, we will consider specific numbers. If the wavelength of the light emitted from the currently targeted laser element in the air is λ8, the wavelength in the laser element is λ1, and the effective Jot refractive index of the waveguide is n, then the following relationship holds true.

λ λ −−! 0+ -n On the other hand, the refractive index change coefficient due to carrier injection into a semiconductor crystal is expressed by the following equation (3).

Here, N is the number of injected carriers, e is the charge element, and n.

is the refractive index when there are no injected carriers, ω is the frequency of oscillation light, ε0 is the permittivity in vacuum, J is the effective mass of electrons, and mζ is the effective mass of holes. Considering the $/l structure of a DFB laser with an oscillation wavelength of around 1.55 μm made of InP/InGaAsP based material and calculating 81 using equation (3), I
The amount of change in refractive index caused by a change in the number of injected carriers in X 10I8crn-3 is about 0.2%. This change in refractive index, &1, is 1 for the value of λ1 from the relationship in equation (2).
It can be seen that a change of about 0λ is caused. Since λ1 is approximately 4800X, the required value of the waveguide length in the C region is given by the following equation in order to obtain the phase π, that is, the - change in optical wavelength H2400X by carrier injection of I x 1018 crf3.

-X 4800A = 115.2 pm (a)0 Since 2400X corresponds to 2π of the phase of the first-order diffraction grating, it is sufficient for our purpose. In addition, Ink/InGa
Considering the carrier lifetime of AsP system 4JN, the %I current value required to obtain an injection carrier density of l x 10"z-3 is in the range of 10 to 100 mA, and these values are extremely practical. be.

〔Example〕

Based on the above printing concept, I
We fabricated an nP/InGaAsP buried distributed feedback laser. The liquid phase growth method was used for crystal growth, and the detailed structure (Ll) and the structure of the crystal layer were already clarified by the inventor of Book X!+2 in the paper (E]ecLon
icsLetters, Volume 18, Pages 27-29,
[182]. The cross-sectional view through the active 4/1 layer is exactly the same as FIG. 2, and the length of the diffraction grating forming portion in regions A and 13 is 200 μm, and the length of the 3F flat waveguide layer in region C is also 200 μm. 1m.

, the thickness of the active layer 3 is 01 μm, and the thickness of the light guide layer 2 is 1 μm.
1.1μm11. The width of the active layer 3 and light guide layer 20 is 15μ
It was set as n1. The same 1J1 lattice is second-order and has a period of 4805A.
,Deep”,! +5 (IA.

When a forward direct current was simultaneously applied to electrodes 7 and 9 of this device mounted on a heat sink at the same current density, laser oscillation occurred when the sum of both reached 78 mA. Figure 3 shows the optical output vs. current characteristics at that time, but there was a so-called kink, and when we measured the spectrum, it oscillated in a single-longitudinal mode at 95 mA from the threshold. , when the current was injected higher than that, oscillation occurred in two longitudinal modes.

Next, when we measured the same characteristics as shown in Figure 3 by passing a constant current through electrode 8 in advance, we found that the characteristics of electrode 8 changed depending on the value of the positive direction injection current. . Figure 4 shows that the positive injection current from electrode 8 is 3.
With the characteristics at 7 mA, the oscillation threshold at which the sum of the currents from electrodes 7 and 9 breaks is 69 mA. The optical output-current characteristic was smooth, and when the spectrum was measured, single-longitudinal mode oscillation was observed from the threshold value to 312 mA. At this time, a light output of 23 mW was obtained.

(Effects of the Invention) As explained above, according to the present invention, it is possible to obtain a high-yield distributed feedback semiconductor laser that oscillates in the 91-longitudinal mode using conventional manufacturing techniques, and it is possible to obtain a high-yield distributed feedback semiconductor laser that oscillates in the 91-longitudinal mode. This is useful for the realization of the 8 system.

Note that the structure of the present invention can be used for other alloys such as GaAs/GaAAAs, and also for InP/InGaAsP systems.
1:3 μmη and 12 μm ranges other than 5 pm are also useful.

Furthermore, depending on the device configuration, the diffraction grating can be formed on a light guide layer that has been epitaxially grown in advance. Round 41r
The order of the lattice is 2nd order and 1st order.

It is also clear that tertiary 1t can be used.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing the structure of the diffraction grating r of a conventionally proposed device.
7, and FIGS. 3 and 4 are optical output-box and flux diagrams of an embodiment of the device shown in FIG. 2. I-n-type 1nl) substrate, 2-n-type 1nGaAsP
Optical guard layer, p-type InGaAsP active layer, 4-
p-type InP cladding layer, 5- p-type InGaAsP electrode layer, 6... n-type ohmic electrode, 7.8.9...
P-type ohmic electrode, 10, II...diffraction grating. Patent Applicant Nippon Telegraph and Telephone Public Corporation Agent Tsuneo Shiramizu and one other person 3 Figure t L (DCs mA) Figure 4 'E' t (DC, mA)

Claims (1)

[Claims] A periodic structure in which the film thickness changes periodically along the oscillation direction of a laser beam is formed in a part of a semiconductor crystal consisting of a plurality of single crystal thin films including at least an active layer formed on a semiconductor substrate. In a semiconductor laser device having a distributed feedback function, there is one optical waveguide region without a periodic structure sandwiched between two regions with a periodic structure, and these three 91 regions correspond to each other. 1b independent of each other,
By adjusting the current injected from the three electrodes, single-mode oscillation is facilitated, and the length of the oscillation is changed to make it 1(I). 114. A Qi-1 monochrome semiconductor laser device having the following structure.