JPS63166281A - Distributed feedback semiconductor laser - Google Patents

Abstract

PURPOSE:To enable a distributed feedback laser to oscillate in a uniaxial mode up to a high output level, by correcting an equivalent refractive index or diffraction grating cycle of a region having intense optical intensity distribution so as to be smaller than that of a region having weak optical intensity distribution. CONSTITUTION:Diffraction grating 12 having lambda/4 phase shift is formed on an n-type InP substrate 11. Then, an n-type 1.3 mum GaInAsP optical waveguide layer 13, an undoped 1.55 mum active layer 14, a P-type InP clad layer 15 and p<+>type GaInAsP contact layer 16 for example are deposited in that order by means of liquid-phase epitaxy. The width of the optical waveguide stripe including the active layer is varied by means of an appropriate mask, exposure and mesa etching such that the width of a region 17a having a smaller distance between the lambda/4 phase shift position 12a and the cleaved face is smaller than the width of a region 17b having a larger such distance, by about o.15 mum for example. A BH (buried hetero) structure is provided and p electrode 19 and an n electrode 20 are formed. This element is then cleaved such that the lambda/4 phase shift position 12a is deviated from the center of the length of a resonator of 300 mum by 30 mum for example. Si3N4 single-layer films are applied on the opposite end faces.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a distributed feedback semiconductor laser device that can dynamically realize single-longitudinal mode oscillation.

(Prior art) In recent years, dynamic lasers have been used for large-capacity, long-distance, repeaterless optical communication systems using the 1.5 to 1.6 μl wavelength band, which has wavelength dispersion and has the lowest fiber loss region. Single mode oscillation (DSM)
Lonquitudinal Mode 0perat
Distributed feedback semiconductor laser (D F B
; Distributed Feedback L
a5er) has been developed based on Ga1nAsP/InP-based materials.

Unlike the conventional Fabry-Bello type laser with a cleavage plane reflection structure, this distributed J-reduction type semiconductor laser has periodic irregularities that serve as a diffraction grating in the cavity length direction of the optical waveguide. The zero oscillation wavelength of a laser oscillation device that performs optical feedback using the diffraction grating can be fixed to a single axial mode due to the wavelength selectivity of this diffraction grating. Useful in single axis mode operation.

However, if a distributed feedback semiconductor laser does not have a structure that suppresses reflection on both end faces, in principle, two axial modes sandwiching the BraQQ wavelength will oscillate simultaneously. The characteristics of the axial mode varied greatly depending on the phase relationship between the phase of the diffraction grating and the reflecting end face (phase term of reflectance). Since the phase of this end face cannot be controlled in reality, it is extremely difficult to obtain a distributed feedback laser with stable single-axis mode characteristics with good reproducibility, and its production efficiency is extremely low.

In order to solve this problem, a method is proposed in which the phase of the diffraction grating is shifted by π/2 near the center of the leading waveguide in the cavity length direction, and the reflectance of both end faces is kept low (for example, K, Ut
aka, et, at, Electron, L
ett.

1984, 20. DD, 326-327) is known, and is the third such distributed feedback semiconductor laser.
Figure (2) Both end surfaces 2a and 2b of the light-emitting layer/leading wave layer 1, for which the one shown in this figure has been developed, are A R (Ant
1-Reference) A coating is applied to suppress the reflectance to about 1%. In addition, a phase discontinuity portion 4a of the diffraction grating 4 is formed in the central portion 3 of the leading wave layer 1, and the phase shift amount Δθ at this time is π/2=λ/4 (λ: wavelength in the tube).
Make it so that This phase shift of the diffraction grating 4 can be achieved by exposing both a positive resist and a negative resist when forming the diffraction grating using the three-beam interference exposure method (for example, Utaka et al. 1985 Spring Applied Physics Conference Lecture preview,
Lecture No. 29p-ZB-15), or a method of exposure through a phase mask (for example, Shirasaki et al., IEICE 1985 Semiconductor and Materials Division National Conference (Autumn)) Lecture No. 3
11) etc. In principle, the phase-shifted distributed feedback laser fabricated in this way has the lowest axial mode oscillating at arag9 wavelength, and the next axial mode (secondary mode).
The suppression ratio can also be made very large.

This sub-mode suppression ratio can be estimated by measuring the spectra before and after oscillation, but theoretical analysis shows that instead of this parameter, the threshold gain difference Δα between the main and sub-modes at the oscillation threshold and the resonator It is often discussed using the length product ΔαL, and the two are closely related (for example, Motosugi et al., 1985 IEICE Semiconductor and Materials Division National Conference (Spring) Lecture No. 892).

In addition, in such a semiconductor laser, light is output symmetrically from both end faces, but the light output from the back side has little utility value, and even if this light is used as monitor light, it does not require much. It has been proposed to change the ratio of outputs from the left and right end faces by shifting this phase shift position from the center (for example, Usami et al., 1985 Autumn Institute of Electronics and Communication Engineers, Proceedings of National Conference of Optical Radio Division, Lecture No. 2)
20) An example of this is shown in Figure 4. Figure 4 (a) has a λ/4 phase shift (λ: wavelength in the tube) at the center of the resonator, and the reflection at both end faces is suppressed to 0. This shows a schematic diagram of the structure of a semiconductor laser and its internal light intensity distribution (cavity length L = 300 μm, normalized coupling coefficient L = 1.5).
simulation example).

Figure 4 (b) shows a schematic structural diagram and a simulation example of the light intensity distribution inside the structure when the λ/4 phase shift position ff4a is shifted to the right (about 230 μm) from the center. In case a), the normalized threshold gain increases from about 1.02 to 1.09, and ΔαL increases from 0.7 to 0.57, that is, ΔαL increases to about 2.
It decreases by 0%, and the single-axis modality slightly deteriorates. However, the proportion of the output from the right end surface in the total optical output (Pr/
(Pr+Pe)) increases from 50% to 65%. The external differential quantum efficiency from one side is given by the following equation.

77ex, r -77! 2αL, Pr where η
i is the internal differential quantum efficiency, and αwL is the waveguide loss outside the active layer. Assuming αimL=0.6, 'r)ex
It is possible to improve z by about 30%.

This property increases optical output and is very beneficial from the perspective of long-distance transmission.

Furthermore, instead of this method, it is also possible to make the internal light intensity distribution asymmetric by setting the phase shift position at the center and changing the left and right coupling coefficients L (for example, Numai et al.
1st year (autumn) IEICE National Conference Proceedings t&
Lecture number 219).

FIG. 3(c) shows a schematic diagram of a semiconductor laser having this structure and an example of the light intensity distribution in the cavity direction.

The resonator length is 300 μm, a λ/4 phase shift 4a is provided in the center, and both end faces are made non-reflective. L was set to 2.0 for the normalized coupling coefficient on the left side, and L was set to 1.0 for the right side. In this case, αL is 1 compared to the case in (a) of the same figure.
．． The value increases from 02 to 1,17, and ΔαL decreases to 0.62. However, the output ratio from left and right increases from 50% to 78%. Therefore, η@y, r is 60 from equation (1) above.
% can also be improved.

(Problem to be Solved by the Invention) However, in the distributed feedback semiconductor laser device having the structure described above, a phenomenon called axial hole burning occurs because the optical power is concentrated on the output side end facet, and the output cannot be increased. There was a drawback that ΔaL became smaller due to the change in refractive index.

This is because the carrier density decreases in a region with high optical density, resulting in an increase in the refractive index due to the plasma effect of free electrons, fluctuations in the absorption edge due to injected carriers, etc. In other words, the axial non-uniformity of the light intensity induces the axial non-uniformity of the refractive index, resulting in an equivalent phase shift. Analysis of this phenomenon has already been carried out, and it has been found that 1.3< and L<1.6 are suitable for the normalized coupling coefficient in which ΔαL does not become relatively small even when the optical output increases. (Glda et al., Proceedings of the National Conference of the Optical and Radio Division of the Institute of Electronics and Communication Engineers, Autumn 1986, Lecture number 235)
).

When L is large, ΔαL becomes 0 at relatively low injection levels.
This results in biaxial mode oscillation.

FIG. 4 is a characteristic diagram showing this situation. That is, as the drive current increases, ΔαL monotonically decreases.

As explained above, it is extremely difficult for conventional distributed feedback semiconductor lasers to maintain single-axis mode oscillation up to high output levels.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a distributed feedback semiconductor laser device capable of single-axis mode oscillation up to a high output level.

[Structure of the Invention] (Means for Solving the Problems) The distributed feedback semiconductor laser device of the present invention includes a diffraction grating waveguide in which a diffraction grating for performing optical feedback is formed in the direction of the resonator, and In a distributed feedback semiconductor laser device having a light intensity distribution in the vertical direction, the equipolarized refractive index or diffraction grating period in the region where the light intensity distribution is strong is equal to the equipolarized refractive index or the diffraction grating period in the m region where the light intensity distribution is weak. This feature is characterized in that it has been corrected so that it is smaller than .

(Function) The present invention maintains a large ΔαL over a wide range of optical output levels,
In order to realize stable single-axis mode operation, the equipolarized refractive index of the region where optical density is concentrated is corrected in advance so as to be small.6 For example, by reducing the width or thickness of the optical waveguide in that region, These include changing the composition to reduce the refractive index itself, and changing the composition of the surrounding crystals on the top and bottom or on the left and right. Equivalently, it also includes reducing the period of the diffraction grating.

(Example) An example in which the present invention is applied to a Ga1nAsP/InP distributed feedback laser will be described below with reference to the drawings.

FIG. 1 is a diagram showing the structure of the laser device of the example, and shows an n-type 1
A diffraction grating 12 with a λ/4 phase shift 12a is formed on the nP substrate 11. Such a laser device is manufactured using a positive/negative two-layer resist method and two-beam interference using a phase mask designed to form a λ/4 phase-shifted diffraction grating 12 on an n-type 1nP substrate 11. It was produced using an exposure method.

After that, the n-type 1.3
．． um band Gal'nAsP optical waveguide scrap 13, undoped 1
．． 55μm band active layer weight: 4, p-type 1nP cladding layer: 1,
A p'''-type Ga1nAsP contact F116 was grown sequentially.Then, using an appropriate mask and exposure method, a mesa
By etching, the properties of the optical waveguide strip including the active layer are changed so that the width of the region 17a, where the distance from the λ/4 shift position fi12a to the cleavage plane becomes shorter, is 0.15 μm narrower than the width of the longer region 17b. I made it.

Also, the darkness 12a with a shift of λ/4 is included in the short region 17a,
A tapered return region 18 was provided at the boundary of the wide region to prevent unnecessary reflection (FIG. 1(b)). InP having a reverse bias junction is grown and buried on both sides of the mesa stripe on the substrate thus prepared.
(buried hetero) structure, n-electrode 19, n-electrode 20
was formed.

Next, this element and the λ/4 phase shift position 12a are separated by 30 μm from the center of the resonator length of 300 μm,
An AR coating 21 made of a single layer of Si3N4 was applied to both end faces.

In Fig. 1(c), the position of λ/4 phase shift remains unchanged,
This is the internal light intensity distribution when the width of the light stripe is not corrected. The stripe shape shown in FIG. 4(b) was basically determined based on the distribution shown in FIG. 2(c). The narrow part also covers the strongest part in the center.

The light intensity distribution in the stripe shape in figure (b) is shown in figure (c).
) and the trend does not seem to change much.

When such a distributed feedback semiconductor laser was operated, it oscillated at a threshold current of 231 A, and as shown in FIG. 2, ΔαL increased as the current was injected, reached its maximum value, and then decreased. Therefore, stable single-longitudinal mode operation was realized up to an optical output level of 20 IIIW or higher. In other words, if the equal polarized refractive index of the optical waveguide section on the side where the optical output is expected to be concentrated is corrected to a small value, the maximum Δα at the threshold value
Oscillation starts at a value of ΔαL smaller than L. After that, the carrier density decreases in the region where light is concentrated, and the refractive index increases relatively due to axial hole burning, so △αL increases.After this, the maximum ΔaL before correction is achieved at a certain current value. However, ΔαL decreases again. Therefore, compared to the case of the conventional device shown in FIG. 5, a current range from the threshold value to the maximum ΔαL is added, and the optical output f! is correspondingly wider. This means that a high ΔaL can be maintained within the range. In this way, in this example, the current injection and increase in optical output act in the direction of increasing △aL, compared to conventional devices that utilize only the region in which increases in current and optical output decrease ΔαL. It's completely new.

In the above-described embodiments, the equipolarized refractive index was controlled in the axial direction by changing the width of the optical waveguide (strive), but it is also possible to control it by changing the thickness. The same effect can also be obtained by changing the period of the diffraction grating 12 instead of changing the 0 equivalent refractive index by changing the composition of the guiding waveguide layer or by changing the surrounding buried layer by making it quaternary Ga1nAsPM. be. Further, the present invention can be applied not only to a method in which the λ/4 shift position W 12 a is shifted from the center, but also to a method in which the left and right coupling coefficients are changed with the λ/4 shift position f12a placed at the center. Moreover, it is of course applicable to a mixed method thereof.

As described above, the present invention can be modified and applied in various ways.

[Effects of the Invention] As explained above, according to the distributed feedback semiconductor laser device of the present invention, hole burning in the axial direction occurs by correcting the equipolarized refractive index of the region where the optical density is concentrated to a small value in advance. In this case, it becomes possible to increase the optical output level at which ΔαL decreases, and stable single-axis mode oscillation can be achieved up to a high output level, which greatly contributes to the high output of the distributed feedback laser. Furthermore, the range of L has been relaxed to the normalized coupling coefficient defined by axial hole burning, and stable single-axis mode oscillation has become possible even with a large L. Therefore, the suppression ratio in the 7M mode can be increased, contributing to lowering the threshold value, and the effect is immeasurable.

[Brief explanation of the drawing]

Figure 1 shows the present invention in the 1.55μll band Ga1nAsP/I
Figure 2 shows the current-optical output characteristics and current-ΔαL characteristics in the example. Figure 3 shows the conventional λ/4 shift. Figure 4 is a diagram showing the structure of a distributed feedback laser, and Figure 4 is a diagram showing the relationship between the structure of a conventional device and the axial light intensity distribution.
is an example of a non-reflection structure with a λ/4 phase shift, (b) is an example of a structure in which the phase shift position is shifted by 30 μt from the center, and (c) is an example of a structure in which the phase shift position is located at the center and the left and right sides are L. FIG. 5 is a diagram illustrating an example of the llI mode in which the voltage is changed, and FIG. 5 shows the current-light output characteristics and current-ΔaL characteristics of a conventional device. 11... n -1nP substrate 12... Diffraction grating 12 a... λ/... 1st position shift position 13...
...1.3 μm band n-type Ga1n/1. sP optical waveguide layer 14...1.55μ■AND-1 active layer 15
......n-1nP cladding layer 16---A,
15) trn p + -GalnAsP: 7 contact layer 19...n electrode 20...n electrode

Claims (3)

[Claims] (1) In a distributed feedback semiconductor laser device comprising a diffraction grating waveguide in which a diffraction grating for performing optical feedback is formed in the direction of the cavity, and having a light intensity distribution in the direction of the cavity, a region where the light intensity distribution is strong. A distributed feedback semiconductor laser device characterized in that the equivalent refractive index or the diffraction grating period of the region is corrected to be smaller than the equivalent refractive index or the diffraction grating period of the region where the light intensity distribution is weak. (2) The diffraction grating has a phase discontinuity at a position away from the vicinity of the center in the cavity direction, and the region where the light intensity distribution is strong is the waveguide region where the distance between the phase discontinuity and the output end face is shorter. The distributed feedback semiconductor laser device according to claim 1, wherein the region where the light intensity distribution is weak is a waveguide region having a longer distance between the phase discontinuity portion and the output end face. . (3) The diffraction grating has a phase discontinuity near the center in the cavity direction, and the coupling coefficient of the waveguide on both sides of the phase discontinuity is different, and the waveguide with the small coupling coefficient has a strong light intensity distribution. 2. The distributed feedback semiconductor laser device according to claim 1, wherein the waveguide portion having a large coupling coefficient is a region having a weak light intensity distribution.