JP3452131B2 - Distributed feedback semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser, and more particularly to a phase shift distributed feedback (DFB) semiconductor laser having high mode stability and high efficiency used in a digital optical transmission system.

[0002]

2. Description of the Related Art A conventional digital optical transmission system includes a semiconductor laser having a high single-mode characteristic called a λ / 4 phase shift distributed feedback semiconductor laser in which the phase of a diffraction grating is shifted by a half period at the center of a laser resonator. Used. λ /
The four-phase shift DFB structure is a known structure, for example, “1”
994, published by Ohmsha, edited by the Japan Society of Applied Physics, semiconductor laser, page 272, FIGS.

As shown in the sectional view of FIG. 7, a λ / 4 phase shift distributed feedback semiconductor laser is obtained by shifting the phases of a first diffraction grating 43 and a second diffraction grating 44 by a half cycle.
A phase shift structure 48 is provided at the center of the laser resonator.
In this structure, when n ^eff is the effective refractive index, the Bragg wavelength λ ^B determined by the diffraction grating period Λ, that is, λ ^B = 2Λ
Since the oscillation occurs at n ^eff , there is a feature that the submode suppression ratio can be increased.

However, in this structure, since a λ / 4 phase shift structure exists at the center of the laser resonator, the electric field becomes very strong in this phase shift portion, and the internal electric field intensity distribution becomes extremely inadequate as the bias is increased. It becomes uniform, and the optical power output to the outside of the laser resonator becomes small. In addition, low-reflection coating is applied to both end surfaces of the laser resonator, and only about 1% of the light is reflected, so that the same amount of light is output behind the laser as in the front. Therefore, in the laser having this structure, there is a problem that the efficiency of the laser light output with respect to the drive current is low and a high output cannot be obtained.

In order to solve this problem, Japanese Patent Laid-Open Publication No.
No. 00287 discloses that, instead of disposing a λ / 4 phase shift structure at one point of a laser resonator, a diffraction grating periodic modulation equivalent to λ / 4 phase shift is applied to a part of the resonator. Describes a structure that avoids one point concentration in the internal electric field strength distribution. This structure has an advantage that the intensity of light output to the outside is increased by an amount corresponding to the reduction of the electric field concentrated inside.

However, in the structure described in Japanese Patent Application Laid-Open No. 4-100287, although the optical output is improved as compared with a normal λ / 4 phase shift distributed feedback laser, both ends of the laser resonator are low. With a reflective coating, light with almost the same output is emitted from both sides of the laser end face,
The output was only about half that of a semiconductor laser having a high-reflection coating on the rear side.

In the laser disclosed in Japanese Patent Application Laid-Open No. 61-216383, the rear end face of the laser is provided with a cleavage end face or a high reflection coating, the front end face is provided with a low reflection coating, and the λ / Four phase shift structure,
An example is described in which the laser cavity is brought closer to the rear end face at a ratio of 3: 7 to 4: 6 instead of the center of the laser cavity. In this structure, the end face reflectivity on the rear side of the laser is about 30% for the cleavage end face and 90% or more for the high reflection coating. Light is not output much, and the light output from the front is higher than in the conventional example. In addition, it is described that the deterioration of the single mode property is smaller than that of the λ / 4 phase shift DFB laser when the position of the λ / 4 phase shift structure is closer to the rear side than the center of the laser resonator. .

However, in this structure, the conventional λ /
As in the case of the four-phase-shift distributed feedback laser, the electric field intensity becomes extremely high in the λ / 4 phase-shift structure.
Since the / 4 phase shift structure is far away from the laser front end face, the light intensity output from the front of the laser is
Although slightly higher than a / 4 phase shift DFB laser, it is not yet sufficiently high. According to a theoretical study by the present applicant, even if the rear facet reflectance is set to 95%,
The light output ratio from the front to the rear remains at 4: 1.
Also, if an error of several microns occurs at the cleavage position at the stage of cleavage of the laser element, the optical output becomes lower than the design, and the design tolerance for the cleavage error is small. If the cleavage at the time of forming the laser rear end face differs depending on the element, there is a problem that the distribution of characteristics between the laser elements becomes large due to the dispersion of the diffraction grating phase on the high reflection side end face.

Japanese Patent Publication No. 6-65090 (JP-A-60-125882) discloses a structure in which two diffraction gratings having different periods are provided asymmetrically in a resonator. However, the structure described in this publication is intended only for a single mode, and one longitudinal mode having the minimum gain of one diffraction grating and one longitudinal mode having the other minimum gain overlap. In addition, the respective diffraction gratings are set at a constant period satisfying a specific relationship with each other. Therefore, it is difficult to obtain a laser whose output from the output end is sufficiently large only with this structure alone, and there is no description in this publication of the effect of increasing the output.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and has good single-mode stability of a laser beam, and the power of the laser beam from an output end is high. An object of the present invention is to provide a semiconductor laser which is large and has high current-to-light output conversion efficiency.

Further, according to the present invention, even when the reflectivity of the end face on the rear side of the laser is close to 100%, it is hardly affected by cleavage errors, and the variation in characteristics between elements during mass production is small, so that production can be performed with high yield. It is an object to provide a semiconductor laser.

[0012]

Means for Solving the Problems The first invention of the present application is:
In a distributed feedback semiconductor laser in which optical feedback is performed by a diffraction grating, in a distributed feedback semiconductor laser in which optical feedback is performed by a diffraction grating, a low reflection coating is applied to an output end face of the laser light, and a high reflection is applied to a rear end face on the opposite side. coating is provided, wherein the diffraction grating is less and the output terminal also
A period uniform region comprise provided period has a length exceeding one-half of the total diffraction grating length in the resonator in total a predetermined constant value, and periodic in the periodic uniform area is A periodic fluctuation region having a different fixed period or a modulated period, wherein the periodic fluctuation region is located with respect to the laser resonator center.
And the periodic uniform region provided near the rear side end.
And the periodic fluctuation region are formed so that the phase is continuous.
The present invention relates to a distributed feedback semiconductor laser characterized in that:

A second invention of the present application is directed to a distributed feedback semiconductor laser that performs optical feedback using a diffraction grating.
A low-reflection coating is provided on both end faces of the laser resonator, and the diffraction grating has a first period uniform region, a second period uniform region having a predetermined constant period, and a grating of the period uniform region. period and a periodic variation region having a diffraction grating of different modulated period and the first period uniform region is provided comprising an output terminal, said second periodic uniform territory
The area is provided including the rear side edge, and the first period is uniform.
The sum of the area and the second periodic uniform area is less than two-thirds of the total diffraction grating length in the resonator, and the periodic fluctuation area is asymmetric with respect to the center of the laser resonator. At the rear end, and is in contact with the periodic fluctuation region.
The first periodic uniform region or the second periodic uniform region
Is related to a distributed feedback semiconductor laser characterized in that the one side is formed so as to have a continuous phase .

Further, a third invention of the present application relates to a distributed feedback semiconductor laser that performs optical feedback by a diffraction grating,
A low-reflection coating is provided on both end faces of the laser resonator, wherein the diffraction grating is provided including at least the output end , has a length equal to or less than half of the total diffraction grating length, and has and the periodic low modulation region period is modulated, before
The periodic low-modulation area is provided in contact with the low-period modulation area, and the periodic low-modulation area has a different grating period and a modulated periodic fluctuation area different from the periodic low-modulation area. smaller than the modulation amount in the periodic variation region, as will become asymmetric with respect to the laser resonator central relates distributed feedback semiconductor laser, characterized in that is provided closer to the rear end.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a conventional λ / 4 phase shift distributed feedback semiconductor laser, as described above, since the feedback of light becomes too strong near the center, the electric field intensity distribution of the light is concentrated near the center. In addition, both front and rear end surfaces are coated with low-reflection coating, the end surface reflectivity is suppressed to about 1%, and the same light output is provided not only from the front but also from the rear. The laser output from the device. The laser disclosed in Japanese Patent Application Laid-Open No. 4-100287 has a structure in which substantially equal light outputs can be obtained from both end faces of the laser.

On the other hand, the structure of the first invention of the present application, that is, a first embodiment of the present application in which a low-reflection coating is provided on the output end face and a high-reflection coating is provided on the opposite rear end face. In the structure of the present invention, the periodic fluctuation region in the diffraction grating in the resonator is provided close to the rear end face so as to be asymmetric with respect to the center of the laser resonator.

With such a structure, the period of the diffraction grating is distributed asymmetrically with respect to the center of the laser resonator, and the amount of distributed feedback within the resonator becomes asymmetric, so that the internal electric field intensity distribution is distributed asymmetrically. Thereby, for example, Japanese Patent Laid-Open No.
Compared with the structure described in Japanese Patent No. 0287, an optical output having a larger output from the output end than the rear end can be obtained. Further, in the present invention, the light output from the output end face can be further increased by the high reflection coating on the rear end face. The structure of the present invention is disclosed in
Compared with a structure in which the λ / 4 phase shift position is moved to the rear side as described in Japanese Patent No. 16383, the optical output is increased because the concentration of the internal electric field distribution is small.

In addition, for the oscillation wavelength of the laser, the phase shift due to the change in the period and the phase shift due to the high reflection coating are equivalently equivalent to about λ / 4 in the entire resonator.
By performing the diffraction grating periodic modulation that gives a phase shift, the laser single mode property can be maintained.

In general, the phase of the diffraction grating at the end face coated with the high reflection coating varies greatly between the elements, but the characteristics are greatly different. Since the diffraction grating has a different period from that of the conventional example, the influence is smaller than that of the conventional example.

In the first invention of the present application, the period uniform region is provided including at least the region on the most output side, and has a total length exceeding one half of the total diffraction grating length in the resonator. . That is, the periodic uniform region may be one continuous region or may be divided into several regions. In the case of a single continuous region, the period uniform region occupies a region exceeding the center from the output end side in all the diffraction gratings in the resonator.

In the case where the periodic uniform region is divided, it is sufficient that the sum thereof exceeds one half of the total diffraction grating length in the resonator. In this case, of the total length
It is preferable that a length exceeding at least one half exists on the output end side from the center of the resonator. By doing so, asymmetry is ensured, and single-mode stability of laser oscillation can be kept high.

The period variation region is a region having a lattice period different from the above-mentioned period uniform region, and may be one continuous region or divided into a plurality of regions. However, one continuous region is more preferable in terms of laser oscillation mode stability. In particular, it is preferably provided in a continuous area including the rearmost end area.

The period of the periodic fluctuation region may be constant or may be modulated as long as the period is different from the period uniform region. In the case where the period is constant, the difference between the period and the lattice period of the region having a uniform period is sufficient. The effect of the present invention can be obtained even if the period is short or long, but the longer is particularly preferable. In addition, when the period is modulated in the period variation region, it is generally preferable that the period varies monotonically, and it is preferable that the difference from the lattice period of the period uniform region gradually increases toward the rear end. . This way,
The influence of variations in the end face phase of the high reflection coating can be further reduced, and the single mode yield can be increased. At this time, it may be changed so as to be shorter than the lattice period of the uniform period region, but it is particularly preferable to change it so as to be longer. When the change is monotonous, for example, a structure in which the change is made linearly in a portion where the change is made can be cited.

Further, the boundary between the periodic fluctuation region and the uniform period region may be either a continuous period or a discontinuous period.

The structure of the second invention of the present application provides a structure in which a low-reflection coating is provided on both end faces of the laser resonator and the period of the diffraction grating is modulated in the period variation region. The first invention differs from the first invention in that the period uniform region is provided including at least the region on the most output side, and has a total length of two thirds or less of the total diffraction grating length in the resonator. Is different. From the viewpoint of the single mode stability of the laser oscillation, it is desirable that the value is about 1/2 or more. However, in order to increase the asymmetry and further increase the output of the laser, the value can be made 1/2 or less. However,
It preferably has a length of one third or more.

When the low reflection coating is provided on both end surfaces as in the second invention, it is essential that the grating in the periodical fluctuation region is modulated, whereby the period of the diffraction grating is reduced. A laser with a larger output from the output end than the rear end because the internal electric field intensity distribution is asymmetrically distributed by asymmetric distribution to the center of the laser cavity and asymmetric distribution feedback in the cavity. Is obtained. If the grating period in the periodic fluctuation region is constant, the internal electric field intensity distribution may not be asymmetric if both ends are low reflection coating, and even if it is, the asymmetry is very slight .

In the second invention, it is preferable that the length of the periodic fluctuation region is one third or more of the total length of the diffraction grating in the resonator, and in order to secure a necessary length of the periodic uniform region. Preferably, it is less than two-thirds.

In the structure according to the second aspect of the present invention, it is generally preferable that the change is monotonic within the period variation region, and it is preferable that the difference from the lattice period of the period uniform region becomes gradually larger toward the rear end. . This makes it possible to further reduce the influence of the end face phase variation as in the case of the first aspect of the invention, and to increase the single mode yield. At this time, the period may be changed so as to be shorter than the lattice period of the period uniform region, but it is particularly preferable to change the period so as to be longer because the effect is extremely remarkable in the second invention. When it changes monotonically, for example,
In a portion having an increase or decrease, a structure that changes linearly can be given. Further, the boundary between the periodic fluctuation region and the periodic uniform region may be either a continuous period or a discontinuous period.

In the structure of the second aspect of the present invention, the laser oscillation wavelength is equivalently given about λ / 4 phase shift in the entire resonator by the phase shift due to the change of the period, thereby improving the laser single mode property. Although it can be maintained, a single mode property can be further improved by providing a phase shift structure at a certain point on the resonator in order to give a clear λ / 4 phase shift to the laser oscillation wavelength.

Here, the amount of phase shift is set so as to be equivalent to about λ / 4 phase shift in total with the phase shift caused by the diffraction grating period modulation.

FIG. 6 shows the threshold gain difference (indicator of single mode property) with respect to the amount of phase shift at one point of the present structure. The improvement of the single-mode property of the structure of the present invention by separating the phase shift into the periodic modulation of the diffraction grating and the phase shift at one point in this manner is, as apparent from FIG. 1/8 cycle to 1 /
This is effective for less than two cycles.

Here, the point where the one-point shift structure is provided is as follows.
Although it may be provided at the center of the resonator as in the conventional example, it is preferably provided at the boundary between the period uniform region and the period variation region. Further, in the first invention, it is possible to approach the rear end face of the laser, and in the second invention, it is possible to approach the output end face.

As described above, also in the second invention of the present application, the electric field intensity distribution inside the resonator can be controlled asymmetrically with respect to the center without the electric field being concentrated at the center.
The laser light output can be increased. Further, the single mode yield can be made equal to that of a conventional λ / 4 phase shift distributed feedback laser.

The structure of the third invention of the present application is the same as that of the second invention.
In the structure of the present invention, the period uniform region is defined as a low period modulation region in which the period is modulated in the region (a region where the period is modulated, but the modulation amount is smaller than the period fluctuation region, and the period fluctuation region is To avoid confusion).) However, the length of the region of the periodic low modulation region has a total length equal to or less than half of the total diffraction grating length in the resonator. Then, the modulation amount in the low period modulation region is set smaller than the modulation amount in the period fluctuation region. Specifically, when comparing the averages of the modulation amounts in the entire region, it is preferable that the average value in the low periodic modulation region is smaller than the average value in the periodic fluctuation region.

That is, in the second invention, when the phase shift structure approaches the output end face, as described above, the efficiency is superior, but with respect to the laser single mode, the phase shift structure is located at the center. It tends to be worse than the one that is located. Therefore, in the third invention, in order to prevent this deterioration, the period can be modulated even in the period uniform region on the output side, a phase shift can be given, and the laser single mode stability can be improved.

In a preferred form of the third invention, the periodic fluctuation region is monotonically changed so that the grating period gradually increases or decreases toward the rear end, and the periodic low modulation region Within, the grating period monotonically changes so as to gradually increase or decrease toward the rear end. In a more preferred form, the periodic variation region has a lattice period that monotonically changes so as to gradually increase toward the rear end, and even in the low period modulation region, the lattice period is on the rear side. It changes monotonically so that it becomes longer gradually toward the edge. The most preferable form is a form in which the grating period increases linearly toward the rear end, and the gradient (absolute value) of the grating period in the low-period modulation region is smaller than the period variation region.

In the structures of the second and third inventions of the present application, a flat region having a constant grating period may be provided at the rear end of the laser resonator. By providing such a flat region, a laser with small variations in characteristics can be obtained even if an error occurs in the cleavage position of the laser end face. In the second invention of the present application, when the grating period in the flat region is equal to the grating period in the uniform period region, the flat region is a part of the uniform period region.

The length of the flat region is preferably long in order to reduce the influence of the cleavage error, for example, preferably 5 μm or more.
In the case of the third invention) and the length and position of the periodic low modulation region (in the case of the third invention).

[0039]

The present invention will be described in more detail with reference to the following examples. In the following description, it is assumed that the laser oscillation wavelength is a 1.3-micron wavelength band for optical communication. It can be applied to band lasers.

Embodiment 1 FIG. 1A shows a semiconductor laser 1 having a resonator width of 300 μm according to Embodiment 1 of the present invention.
FIG. In order to form this semiconductor laser, first, a first diffraction grating 3 as a periodic fluctuation region and a second diffraction grating as a periodic uniform region are formed on an N-type InP semiconductor substrate 2 by a known electron beam exposure method and a known lithography.
Is formed. The etching depth at the time of forming the first diffraction grating 3 and the second diffraction grating 4 is set to 0.02 μm so that the distributed feedback coupling coefficient κ is 40 to 50 cm ⁻¹ .

The N-type InGaAsP light guide layer 5 is 0.1 μm thick, the multiple quantum well active layer 6 is 0.2 μm thick,
P-type InGa cladding layer 7 having a thickness of 3 μm
AsP cap layers 8 are each formed to a thickness of 0.2 microns. Next, P-type In is formed by a well-known electrode forming method.
A P-type electrode 9 is formed on the GaAsP cap layer 8, and an N-type electrode 10 is formed below the N-type InP semiconductor substrate 2. A high-reflection coating 11 is applied to both end surfaces of the semiconductor laser 1 on the end surface on the first diffraction grating 3 side, and a low-reflection coating 12 is applied on an end surface on the second diffraction grating 4 side.

At this time, the periodic distribution of the first diffraction grating 3 (periodically variable region) and the second diffraction grating 4 (periodically uniform region) is, as shown in FIG. The period is 202.2 nanometers and 100 microns long, and the second diffraction grating 4 has a period of 202.0 nanometers and 20 microns.
It is 0 microns long. Here, the first diffraction grating 3 and the second diffraction grating 4 were formed such that the phase was continuous.

FIG. 4 shows the electric field intensity distribution in the resonator having this structure. From this figure, it can be seen that the electric field intensity is higher at the output end (at the position of 300 μm in the axial direction) than at the rear end (at the position of 0 μm in the axial direction) in the resonator. Also, it can be seen that the concentration of the electric field in the resonator is reduced.

In fact, in the semiconductor laser 1, the light output from the end face on the second diffraction grating 4 side was about six times as powerful as the end face on the first diffraction grating 3 side. Also, regarding the threshold gain difference indicating the mode stability of the laser,
In the semiconductor laser having this structure, 0.6 is obtained, and a practical level of 0.5 or more is achieved.

FIG. 5 is a graph comparing the efficiency of the present embodiment with that of Comparative Example 3 described later with respect to the laser cleavage position error. As shown in FIG. 5, even if an error occurs in the cleavage position of the laser end face between the laser elements, variation in characteristics is reduced.

In the present embodiment, the length of the laser resonator is set to 300 μm, but is not limited to this.

[Embodiment 2] FIG. 2A is a structural view of a semiconductor laser 21 of Embodiment 2. This structure is the same as that of the first embodiment except for the diffraction grating, and can be formed in the same manner as the first embodiment.

At this time, the periodic distributions of the first diffraction grating 23 (periodically variable region) and the second diffraction grating 24 (periodically uniform region) are as shown in FIG.
Has a period of 202.4 nanometers at one end face, and changes to 202.0 nanometers as the distance from the end face increases, and is 100 microns from the end face. The second diffraction grating 24 has a period of 20
With a 2.0 nanometer period, it is 200 microns long, and the total cavity length is 300 microns. The phase between the first diffraction grating 23 and the second diffraction grating 24 was formed to be continuous.

FIG. 4 shows the electric field intensity distribution in the resonator having this structure. From this figure, it can be seen that the electric field intensity is higher at the output end (at the position of 300 μm in the axial direction) than at the rear end (at the position of 0 μm in the axial direction) in the resonator. Also, it can be seen that the concentration of the electric field in the resonator is reduced.

In fact, in this semiconductor laser 21, the light output from the end face on the second diffraction grating 24 side was about five times as powerful as the end face on the first diffraction grating 23 side. Further, as for the threshold gain difference indicating the mode stability of the laser, 0.65 is obtained in the semiconductor laser having this structure,
A practical level of 0.5 or more has been achieved.

In this embodiment, the length of the laser resonator is set to 300 μm. However, the present invention is not limited to this. It is assumed that the period of the diffraction grating period modulation changes linearly. The same effect can be obtained.

In the present embodiment, the first diffraction grating 23 has a different period, but the first diffraction grating 23 has a wavelength near the laser end face.
At a length of about 0 microns, the period may be constant. By doing so, even if the laser cleavage position is shifted by about several microns, the diffraction grating period on the laser end face can be set to the design value.

Third Embodiment FIG. 3A is a structural view of a semiconductor laser 31 according to a third embodiment. In this structure, the diffraction gratings are composed of a first diffraction grating 33 (periodically uniform region), a second diffraction grating 34 (periodically variable region), and a third diffraction grating 35.
(Periodically uniform region), and can be formed in the same manner as in the first embodiment. The etching depth at the time of forming the first diffraction grating 33, the second diffraction grating 34, and the third diffraction grating 35 is 0.03 μm so that the distributed feedback coupling coefficient κ is about 70 cm ^−1. I do. Further, a first low reflection coating 36 and a second
Is applied.

At this time, the period distribution of the first diffraction grating 33, the second diffraction grating 34, and the third diffraction grating 35 is as shown in FIG.
2.0 nanometers, 10 microns long from one end. The period of the second diffraction grating 34 is 202.2 nanometers at a portion that is in contact with the first diffraction grating 33, and becomes 20 μm as the distance from the first diffraction grating 33 increases.
It changes to be 2.0 nanometers and is 140 microns long. The period of the third diffraction grating 35 is 20
2.0 nanometers and 150 microns long.

The first diffraction grating 33 and the second diffraction grating 34
Are formed so that the phase of the diffraction grating is continuous. Further, the second diffraction grating 34 and the third diffraction grating 3
5, the phase is 1 / of the period of the third diffraction grating 35.
A structure (phase shift structure 38) that shifts by four periods (λ / 8 wavelength) was adopted.

FIG. 4 shows the electric field intensity distribution in the resonator having this structure. From this figure, it can be seen that the electric field intensity is higher at the output end (at the position of 300 μm in the axial direction) than at the rear end (at the position of 0 μm in the axial direction) in the resonator. Also, it can be seen that the concentration of the electric field in the resonator is reduced.

In fact, in the semiconductor laser 31, the light output from the end face on the third diffraction grating 35 side was about 2.5 times as powerful as the end face on the first diffraction grating 33 side. The threshold gain difference indicating the mode stability of the laser is 0.7 in the semiconductor laser having this structure, and has reached a practical level of 0.5 or more.

In this embodiment, the length of the laser resonator is set to 300 μm. However, the present invention is not limited to this. It is assumed that the diffraction grating period modulation changes its period linearly. The same effect can be obtained.

[Embodiment 4] In the structure of this embodiment, in Embodiment 3, the period of the second diffraction grating 34 is continuous with the period of the third diffraction grating 35 at the boundary. In other words, the period of the second diffraction grating 34 at this time is such that the portion in contact with the first diffraction grating 33 is 202.3 nanometers, and the distance from the first diffraction grating 33 becomes 22.3 nm.
It changes to be 02.0 nanometers and is 140 microns long. Further, between the second diffraction grating 34 and the third diffraction grating 35, a structure in which the phase is shifted by １ ／ cycle (λ / 16 wavelength) of the cycle of the third diffraction grating 35 (phase shift). Structure 38).

The electric field strength distribution in the resonator having this structure is as follows.
4 shows the same distribution as the electric field intensity distribution of Example 3 in FIG.
In the resonator, the output end (at the position of 300 μm in the axial direction) had a higher electric field strength than the rear end (the position of 0 μm in the axial direction), and the concentration of the electric field in the resonator was reduced.

[Embodiment 5] In the structure of this embodiment, in Embodiment 4, the period of the first diffraction grating and the second diffraction grating is continuous at the boundary. That is, as shown in FIG.
The period of the diffraction grating 83 (flat region) is 202.3 nanometers and 10 microns, and the period of the second diffraction grating 84 (periodic fluctuation region) is 202. 3 nm, and changes to 202.0 nm as the distance from the first diffraction grating 83 increases, and is 140 microns long. Third
Has a period of 202.0 nanometers and a length of 150 microns.

Further, between the second diffraction grating 84 and the third diffraction grating 85, a structure in which the phase is shifted by ８ period (λ / 16 wavelength) of the period of the third diffraction grating 85. (Phase shift structure 88).

The electric field strength distribution in the resonator having this structure is
Example 3 The distribution was the same as the electric field intensity distribution.

[Embodiment 6] In the structure of this embodiment, in Embodiment 5, the length of the second diffraction grating 84 is set to 190 μm, the length of the third diffraction grating 85 is set to 100 μm, and The period of the diffraction grating No. 3 was also changed. FIG.
As shown in FIG. 7, the period of the third diffraction grating 95 at this time is such that the portion in contact with the second diffraction grating 94 is 202.0.
Nanometers, and changes away from the second diffraction grating 94 to 201.9 nanometers, and is 100 microns long. That is, the third grating (low-period modulation area) is formed to have a smaller gradient of periodic fluctuation than the second grating (periodic fluctuation area).

Further, between the second diffraction grating 94 and the third diffraction grating 95, a structure in which the phase is shifted by 周期 period (λ / 8 wavelength) of the period of the third diffraction grating 95. (Phase shift structure 98).

The electric field intensity distribution in the resonator having this structure is as follows.
4 shows the same distribution as the electric field intensity distribution of Example 3 in FIG.
In the resonator, the output end (at the position of 300 μm in the axial direction) had a higher electric field strength than the rear end (the position of 0 μm in the axial direction), and the concentration of the electric field in the resonator was reduced.

Comparative Examples 1-3 For comparison with the examples of the present invention, a structure in which a λ / 4 phase shift structure is provided at the center (Comparative Example 1) is described in JP-A-4-100287. Instead of arranging the λ / 4 phase shift structure at one point of the laser resonator as described above, the diffraction grating period modulation such that the λ / 4 phase shift is equivalently applied to a part of the resonator,
A structure that avoids one point concentration in the internal electric field intensity distribution (Comparative Example 2), a high reflection coating is applied to the rear end face of the laser and the front end face as described in JP-A-61-216383. With a low reflection coating on
FIG. 4 shows the results of a study on a structure in which the λ / 4 phase shift structure was brought closer to the rear end face at a ratio of 3: 7 (Comparative Example 3).

From FIG. 4, it can be seen that the concentration of the electric field at the center of the resonator is large in Comparative Example 1, and the electric field concentration at the center of the resonator is reduced to some extent in Comparative Example 2, but the electric field strength at the output end is not sufficient. Furthermore, in Comparative Example 3, it can be seen that the electric field concentration in the λ / 4 phase shift structure is large, and the electric field intensity at the output end is not yet sufficient.

[0069]

According to the present invention, the power of the laser beam from the output end can be increased by making the electric field intensity distribution asymmetric while relaxing the electric field concentration at one point in the laser resonator. Therefore, a semiconductor laser having high current-to-light output conversion efficiency can be provided.

Further, according to the present invention, even when the reflectivity of the end face on the rear side of the laser is close to 100%, it is hardly affected by the cleavage error, the characteristic variation between the elements during mass production is small, and the production can be performed with high yield. The semiconductor laser which can be provided can be provided.

Further, the laser of the present invention has good stability of single mode, and has a lower bit error rate at the time of digital modulation than a conventional semiconductor laser.

Therefore, by using the laser of the present invention, large-capacity transmission is possible in optical communication, and the number of subscribers and the communication service can be increased. Furthermore, a low-cost optical communication system for subscribers can be realized.

[Brief description of the drawings]

FIG. 1A is a sectional structural view of a semiconductor laser device according to a first embodiment, and FIG. 1B is a diagram showing the distribution of the diffraction grating period.

FIGS. 2A and 2B are a cross-sectional structural view of a semiconductor laser device according to a second embodiment and a diagram illustrating a distribution of a diffraction grating period.

3A is a cross-sectional structural view of a semiconductor laser device according to a third embodiment, and FIG. 3B is a diagram illustrating a distribution of a diffraction grating period.

FIG. 4 is an internal electric field intensity distribution of an example of the present invention and a comparative example.

FIG. 5 is a diagram showing a change in efficiency due to an error in a cleavage position in Example 1 and Comparative Example 3.

FIG. 6 is a diagram showing the phase shift amount dependency at one point of the single mode property of the third embodiment.

FIG. 7 is a structural diagram of a conventional λ / 4 phase shift type DFB laser.

8A is a sectional structural view of a semiconductor laser device according to a fifth embodiment, and FIG. 8B is a diagram showing a distribution of a diffraction grating period.

9A is a sectional structural view of a semiconductor laser device according to a sixth embodiment, and FIG. 9B is a diagram illustrating a distribution of a diffraction grating period.

[Explanation of symbols]

1 Semiconductor laser 2 N-type InP semiconductor substrate 3 First diffraction grating (periodic fluctuation region) 4 Second diffraction grating (periodic uniform region) 5 N-type InGaAsP light guide layer 6 Multiple quantum well active layer 7 P-type InP cladding layer 8 P-type InGaAsP cap layer 9 P-type electrode 10 N-type electrode 11 High reflection coating 12 Low reflection coating 21 Semiconductor laser 23 1st diffraction grating (periodic fluctuation area) 24 Second diffraction grating (periodic uniform region) 31 Semiconductor Laser 33. 1st diffraction grating (period uniform area) 34 Second diffraction grating (periodic fluctuation region) 35 Third diffraction grating (periodic uniform region) 36 First low reflection coating 37 Second low reflection coating 38 Phase shift structure 81 Semiconductor Laser 83 First diffraction grating (flat area) 84 Second diffraction grating (periodic fluctuation region) 85 Third diffraction grating (periodic uniform region) 88 Phase shift structure 91 Semiconductor Laser 94 Second diffraction grating (periodic fluctuation region) 95 Third diffraction grating (periodic low modulation area) 98 Phase shift structure

────────────────────────────────────────────────── ─── Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 JICST file (JOIS)

Claims (18)

(57) [Claims] 1. A distributed feedback semiconductor laser that performs optical feedback by means of a diffraction grating, wherein a low reflection coating is provided on an output end face of the laser beam, and a high reflection coating is provided on a rear end face opposite to the laser light. , a period uniform area having a length period provided include an output terminal also less than one-half of the total diffraction grating length in the resonator in total a predetermined constant value, the <br / > and a periodic variation regions with different constant period or modulated cycles and cycle uniform area of periodic, the period fluctuation region is to the laser cavity center, the
The periodic uniform region and the peripheral region are provided near the rear end.
A distributed feedback semiconductor laser characterized in that the phase change region is formed so as to have a continuous phase . Wherein said periodic variation region in the laser cavity
2. The distributed feedback semiconductor laser according to claim 1, wherein the distributed feedback semiconductor laser is provided in a region between the center and the rear end . 3. The distributed feedback semiconductor laser according to claim 1, wherein a grating period in the periodic fluctuation region is larger than a lattice period of the uniform period region and is constant. 4. The periodic variation region is characterized in that the lattice period therein is formed such that the difference from the lattice period of the periodic uniform region gradually increases toward the rear end. The distributed feedback semiconductor laser according to claim 1. 5. The periodic fluctuation region is formed such that a lattice period therein gradually becomes longer toward a rear end than a lattice period of the periodic uniform region. The distributed feedback semiconductor laser as described in the above. 6. A distributed feedback semiconductor laser that performs optical feedback by means of a diffraction grating, wherein a low reflection coating is provided on both end faces of the laser resonator, and wherein the diffraction grating has a first period having a predetermined constant value. And a second periodic uniform region, and a periodic variable region having a diffraction grating having a modulated period different from the grating period of the periodic uniform region. The first periodic uniform region has an output provided including an end, said second periodic uniform region, et provided include rear end
Between the first periodic uniform region and the second periodic uniform region.
The sum is less than two-thirds of the total diffraction grating length in the resonator, and the periodic fluctuation region is provided near the rear end so as to be asymmetric with respect to the center of the laser resonator ; The first periodic uniform region in contact with the periodic variation region or
Is that one side of the second periodic uniform region has a continuous phase.
A distributed feedback semiconductor laser characterized by being formed as described above . Wherein said periodic variation region in the laser cavity
7. The distributed feedback semiconductor laser according to claim 6, which is provided in a region between the center and the rear end . 8. The periodic variation region, wherein a lattice period therein is formed such that a difference from a lattice period of the uniform period region gradually increases toward a rear end. Item 8. A distributed feedback semiconductor laser according to item 6 or 7. 9. The periodic fluctuation region is formed such that a lattice period therein gradually becomes longer toward a rear end than a lattice period of the periodic uniform region. Distributed feedback semiconductor laser. 10. A shift structure in which the phase of a diffraction grating is shifted from １ ／ period to less than 周期 period at a point on a resonator axis.
10. The distributed feedback semiconductor laser according to any one of items 9. 11. The periodic fluctuation region according to claim 1, wherein the length of the region is not less than ４ and less than の of the total diffraction grating length in the resonator. 2. The distributed feedback semiconductor laser according to 1. 12. The periodic fluctuation region according to claim 6, wherein the length of the region is not less than one third and less than two thirds of the total diffraction grating length in the resonator. 2. The distributed feedback semiconductor laser according to 1. 13. A distributed feedback semiconductor laser that performs optical feedback by means of a diffraction grating, wherein a low-reflection coating is provided on both end faces of a laser resonator, and wherein the diffraction grating is provided including at least an output end. A low-period modulation area having a length equal to or less than half the diffraction grating length and having a period modulated in the area; and a low-period modulation area that is provided in contact with the low-period modulation area and is different from the low-period modulation area A periodic fluctuation region that is modulated differently from the grating period; the modulation amount in the low periodic modulation region is smaller than the modulation amount in the periodic fluctuation region; and the periodic fluctuation region is located with respect to the center of the laser resonator. A distributed-feedback semiconductor laser, which is provided near a rear end so as to be asymmetric. 14. The distribution according to claim 13, wherein the periodic fluctuation region is formed such that a lattice period therein becomes gradually longer or shorter toward a rear end. Feedback semiconductor laser. 15. The distributed feedback semiconductor laser according to claim 14, wherein said periodic fluctuation region is formed such that a lattice period therein gradually becomes longer toward a rear end. 16. A method according to claim, characterized in that the phase of the diffraction grating has a shift structure to be 1/2 cycle less shifted from 1/8 cycle in one point on the resonator axis 13
16. The distributed feedback semiconductor laser according to any one of items 15 to 15. 17. The distribution according to claim 13, wherein the length of the periodic low-modulation region is equal to or more than ４ of the total diffraction grating length in the resonator. Feedback semiconductor laser. 18. The periodic fluctuation region according to claim 13, wherein the length of the region is at least one-third or more and less than two-thirds of the total diffraction grating length in the resonator. 2. The distributed feedback semiconductor laser according to 1.