JP3173582B2 - 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 used in a digital optical transmission system.

[0002]

2. Description of the Related Art Conventionally, a digital optical transmission system has a λ in which the phase of a diffraction grating is shifted by a half period at the center of a laser resonator.
A semiconductor laser having a high single-mode property called a / 4 phase shift distributed feedback semiconductor laser is used. λ / 4
The phase shift structure is a known structure, for example, “1994”
Published by Ohmsha, Japan Society of Applied Physics, Semiconductor Laser 27
Page 2 FIGS. 12 and 12 ".

As shown in the sectional view of FIG. 5, a λ / 4 phase shift distributed feedback semiconductor laser is obtained by shifting the phases of a first diffraction grating 51 and a second diffraction grating 52 by a half cycle.
A phase shift structure 5 is provided at the center of the laser resonator. In this structure, assuming that n _eff is an effective refractive index, the Bragg wavelength λ _B mλ _B = 2 す る n _eff (where m is the order of the diffraction grating and m = 1 when the diffraction grating period Λ is determined from the following equation) Is the period of the first-order diffraction grating.)
There is a feature that the sub-mode suppression ratio can be set high.

However, in this structure, the ratio of the laser light output from the front and the rear changes depending on the bias current.
There is a problem that the front light output cannot be monitored by the rear light output (tracking error). Further, there is a problem that wavelength fluctuation (chirping) at the time of modulation is large and causes a code error in long-distance transmission. These problems are caused by the existence of a λ / 4 phase shift structure in the center of the laser cavity, and the electric field becomes extremely strong in this phase shift portion. As the bias is increased, the internal electric field intensity distribution becomes extremely uneven. This occurs because the change in the refractive index due to the carrier fluctuation greatly differs depending on the position in the resonator.

In addition, the electric field becomes strong near the center of the laser resonator, and it becomes difficult to output light to the outside, so that there is a problem that the current-to-light output conversion efficiency is low.

[0006] As a method for solving this problem, "13th
Proceedings of the 13th IEEE International Conference on Laser Diodes (13th IEEE International
Semiconductor Laser Confe
rence Digest) 1992, 218-219
As shown in the cross-sectional view of FIG. 6A, the diffraction gratings 61 and 62 have a constant period on the resonator axis and a length of a peak portion within one period of the diffraction grating. A method is described in which the ratio of the length of the peak portion to the sum of the lengths of the valley portion and the valley portion (here, defined as the duty ratio) is changed to change the effective feedback amount by the diffraction grating. .

According to this method, the feedback amount near the center of the resonator can be reduced as shown in FIG. 6 (B), which shows the distribution of the coupling coefficient in the axial direction. Therefore, in this structure, the λ
Compared to a / 4 phase shift distributed feedback semiconductor laser, the electric field intensity distribution is flattened in the axial direction of the resonator.

However, in this method, the duty ratio in the vicinity of the center becomes extremely small, and the manufacturing process of the diffraction grating becomes extremely difficult. there were.

[0009]

SUMMARY OF THE INVENTION According to the present invention, the electric field intensity distribution in the axial direction of the laser resonator is made uniform so that the front-to-back output ratio does not change even when the bias current changes. It is an object of the present invention to provide a semiconductor laser whose light output can be easily monitored on a system.

At the same time, by making the electric field intensity distribution in the axial direction of the laser resonator uniform, wavelength fluctuation during modulation is reduced, stable operation is possible even during modulation, and a semiconductor laser having high current-to-light output conversion efficiency. The purpose is to provide.

Another object of the present invention is to provide a semiconductor laser capable of lowering the bit error rate during digital modulation as compared with a conventional semiconductor laser by improving the stability of the laser oscillation mode.

Further, the present invention facilitates a manufacturing process at the time of mass production, reduces variation in characteristics between elements,
It is an object to provide a semiconductor laser having a high yield.

[0013]

According to the present invention, there is provided a distributed feedback semiconductor laser in which optical feedback is provided by a diffraction grating, wherein the diffraction grating is divided into a plurality of grating regions, and a first-order diffraction grating is provided at the center of the laser resonator. The phase is shifted by a half period of the grating, and the grating region is provided on the end face side of the laser resonator.
The order of the diffraction grating in the grating region provided in the center direction is high , and the order of each grating region is 1st to 4th or higher.
The present invention relates to a distributed feedback semiconductor laser characterized by being formed so as to change in four or more steps up to four steps .

Here, the order of the diffraction grating means an integral multiple of the period of the first-order diffraction grating of the diffraction grating. In the present invention, the diffraction grating of the first-order diffraction grating period is provided at both ends of the resonator. Is arranged so that the period of the diffraction grating is an integer multiple of twice, three times or more the period of the first-order diffraction grating near the end face in a longitudinally symmetric manner along the axial direction from the resonator end toward the resonator center. Formed.

Another aspect of the present invention relates to a diffraction grating.
In distributed feedback semiconductor lasers that provide more optical feedback,
Diffraction grating is divided into multiple grating regions,
Phase shifts by half a period of the first diffraction grating at the center of the detector
Then, from the grating region provided on the end face side of the laser resonator,
High order of diffraction grating in the grating area provided in the center direction
Each of which is formed to be
Within the grating region from the end face of the laser cavity to the center.
Of the grating so that the coupling coefficient decreases toward
Distributed feedback type with tee ratio set
The present invention relates to a semiconductor laser. Also in this embodiment, the diffraction grating
Is an integral multiple of the first diffraction grating period of the diffraction grating.
Magnification: In the present invention, the first order is applied to both ends of the resonator.
A diffraction grating with a period of
Diffraction symmetrically along the axial direction toward the center of the shaker
The grating period is twice the period of the first-order diffraction grating near the end face, 3
It is formed so as to be a multiple and an integral multiple.

In the present invention, the duty ratio of the diffraction grating refers to the ratio between the half width of the grating and the length of one period of the grating.

The shape of the grating portion of the diffraction grating is a square wave,
There is no particular limitation as long as the shape functions as a normal diffraction grating, such as a sine wave or a triangular wave.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a conventional λ / 4 phase shift distributed feedback semiconductor laser, as described above, since light feedback becomes too strong near the center, light is concentrated near the center.

In general, the duty ratio of the diffraction grating is 50% (in the case of a square wave, the length of the peak portion and the length of the valley portion of the diffraction grating are equal), and the amount of feedback from the diffraction grating. Is set to be constant on the resonator axis. Also, when the duty ratio is 50%, the coupling coefficient is maximum, and by increasing or decreasing the duty ratio from 50%,
The coupling coefficient can be reduced.

Therefore, it is conceivable to flatten the electric field intensity distribution inside the resonator by changing the period of the diffraction grating. However, if you try to solve the problem that light is concentrated at the center only by changing the period of the diffraction grating,
It is necessary to make the duty ratio near the center approximately 100% or 0%, and the peaks or valleys of the diffraction grating become extremely short, which makes it extremely difficult to manufacture the diffraction grating.

On the other hand, the higher the order of the diffraction grating, the lower the coupling coefficient is, for example, as described in “I Triple E Journal of Quantum Electronics (IEEE).
EJournal of Quantum Electr
onics) vol. QE-12 (1976) pp. 7
37-739 ". Further, an example using a second-order diffraction grating over the same resonator is described in "IEEE Journal of Quantum E.
lactronics) vol. QE-24No. 1,
(1988) pp. 73-82 ". However, the structures described therein cannot make the electric field intensity inside the laser cavity flat.

On the other hand, according to the present invention, since the order of the diffraction grating in the center of the laser resonator is higher, the coupling coefficient becomes smaller in the center, and the feedback of light near the center is performed. The amount can be reduced, and the internal electric field intensity distribution can be flattened.

Although the above-mentioned document describes that the threshold gain difference can be increased by using a higher-order diffraction grating, the same effect can be obtained in the present invention.
By partially using a higher-order diffraction grating, a higher threshold gain difference can be obtained as compared with the conventional example.

Also, in the structure of the present invention, the period of the diffraction grating near the center is several times the period of the diffraction grating near the end face. It is long enough and the fabrication process is easy.

Further, like the conventional λ / 4 phase shift distributed feedback semiconductor laser, the present invention also oscillates at the Bragg wavelength determined by the diffraction period near the end face.

When the main mode of the laser is equal to the Bragg wavelength, the difference between the main mode and the submode is maximized.
In this structure, a threshold gain difference which is an index of mode stability can be as high as that of a λ / 4 phase shift distributed feedback semiconductor laser.

Further, in the present invention, the coupling coefficient in the axial direction can be further finely adjusted by not only changing the order of the diffraction grating but also changing the duty ratio of the diffraction grating at the same time. Thereby, theoretically, "I Triple Photonics Technology Letters (IEEE Photonics Technology)
Letters) vo1.2 (1990) pp. 17
0-172 "can be formed, and the electric field strength distribution inside the resonator can be completely flattened.

Further, in the present invention, when the conductivity type of the diffraction grating portion is a current block structure formed by inverting the conductivity type around the diffraction grating portion, the current injected into the active layer can be controlled and the axial direction can be controlled. Any gain distribution can be obtained. Regarding the structure with inverted conductivity type, see the 13th I Triple E International Semiconductor Laser Conference Proceedings (13
the IEEE International Semi
conductor Laser Conference
Digest), 1992, pp. 14-15 ".

In the present invention, as described in "Hart Mortier's doctoral dissertation, Ghent University, 1991, FIG. By distributing both coefficients, the internal electric field intensity distribution can be completely flattened. However, in order to realize the above structure, it is necessary for the coupling coefficient to be a real number. Are set respectively.

As described above, according to the present invention, only by changing the order and the duty ratio of the diffraction grating, the single mode property is high, the electric field intensity distribution inside the resonator is flat, and the wavelength fluctuation during modulation is small. A small laser can be realized.

[0031]

[Embodiment 1] FIG.
FIG. 2 is a structural view (cross-sectional view) of a semiconductor laser 10 having a 0-micron resonator.

In order to form this semiconductor laser, the n-type InP cladding layer 2 is 1 μm thick, the multiple quantum well layer 3 is 0.2 μm thick, and the -Type InGaAsP light guide layer 4 having a thickness of 0.1 μm and having a thickness of p for forming a diffraction grating.
After growing a p-type InGaAsP layer by 0.03 μm, the p-type InGaAsP layer is etched by a known electron beam exposure method and a known lithography to form diffraction gratings 11 to 18 and a phase shift structure 5 on the same plane. .

Further, the p-type InP cladding layer 6 has a thickness of 3
A micron, p-type InGaAsP cap layer 7 having a thickness of 0.
2 μm and p-type InG
a-type electrode 8 on n-type In
An n-type electrode 9 is formed under the P semiconductor substrate 1.

In addition, anti-reflection coating is applied to both end surfaces of the semiconductor laser 10. Here, the diffraction gratings 11 to 1
The etching depth at the time of forming 8 was set to 0.03 μm so that the distributed feedback coupling coefficient κ was about 70 cm ⁻¹ .

The diffraction gratings 11 to 18 are shown in FIG.
As shown in the distribution of the orders in (B), the first diffraction grating 11 has the order of 1, that is, is formed to have a period of 202 nanometers and a length of 20 μm from one end face; Is 2 or 404 nanometers long and 30 microns long and is connected in phase with the first diffraction grating 11; the third diffraction grating 13 has an order of 3 or a period 606 nanometers The fourth diffraction grating 14 is 50 microns long and 50 microns long with a fourth order, i.e., a period of 808 nanometers, which is 50 microns long and connected in phase with the second diffraction grating 12; The third diffraction grating 13 is connected so that the phase is continuous.

Fourth diffraction grating 14 and fifth diffraction grating 15
The phase shift structure 5 whose phase is shifted by a half period of the period of the first diffraction grating 11 is disposed between the fifth diffraction grating 15 and the fifth diffraction grating 15. The sixth diffraction grating 16 is of order 3 or 5 with a period of 606 nanometers.
Formed to a length of 0 microns and connected in phase with the fifth diffraction grating 15; the seventh diffraction grating 17 has an order of 2 or a period of 404 nanometers and is 30 microns long; The eighth diffraction grating 18 is connected to the sixth diffraction grating 16 so as to be continuous in phase; the eighth diffraction grating 18 has an order of 1, that is, a period of 202 nanometers and a length of 20 microns, and the eighth diffraction grating 18 has the same phase as the seventh diffraction grating 17. They are connected so as to be continuous.

By distributing the orders of the diffraction grating in this way, a distribution of the coupling coefficient as shown in FIG. 1C can be obtained.

A curve (A) in FIG. 4 shows an electric field intensity distribution inside the semiconductor laser 10 according to the first embodiment of the present invention. The ratio between the minimum value and the maximum value of the internal electric field strength is 0.95, and the conventional λ shown in a curve (D) in FIG.
It can be seen that the flatness of the internal electric field intensity distribution is greater than 0.33 of the / 4 phase shift distributed feedback laser.
In addition, the threshold gain difference indicating the mode running property of the laser is about 0.78 in the structure of the first embodiment of the present invention.
was gotten. This is higher than 0.77 of the conventional λ / 4 phase shift distributed feedback laser.

In this embodiment, the laser oscillates at a wavelength of 1.3 μm, which is the Bragg wavelength determined by the period of the first diffraction grating 11.

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

In this embodiment, when L is the laser cavity length, the normalized coupling coefficient κ × L is set to about 2 (the distributed feedback coupling coefficient κ in the diffraction gratings 11 to 18 is about 70 cm ^−1). 2 to 4 are desirable. As the normalized coupling coefficient is set to a large value, it is necessary to reduce the amount of light distribution feedback near the center of the resonator.By increasing the number of orders on the resonator axis, the diffraction grating near the center of the resonator is required. Increasing the order can further reduce the coupling coefficient. As the normalized coupling coefficient increases, the laser oscillation mode becomes more stable, and the first diffraction grating 1
Even if a laser device in which the lengths of the first and eighth diffraction gratings 18 are different from the design can be obtained, the influence can be reduced.

[Embodiment 2] FIG. 2A is a structural view (cross-sectional view) of a semiconductor laser 20 having a 300-micron resonator according to a second embodiment.

In this semiconductor laser, as in the first embodiment, each layer is laminated on an n-type InGaAsP semiconductor substrate 1 by a known epitaxial growth and a known electrode forming method, and a known electron beam exposure method and a known lithography method are used. To form the diffraction gratings 21 to 28 and the phase shift structure 5 on the same plane. The periods of the diffraction gratings 21 to 28 are set similarly to the first embodiment of the present invention, and the distribution of the order of the diffraction grating is the same as that in FIG.

In addition, both end surfaces of the semiconductor laser 20 are provided with an anti-reflection coating. Diffraction gratings 21 to 28
Is formed at 0.03 μm so that the distributed feedback coupling coefficient κ is about 70 cm ⁻¹ .

As shown in FIG. 2B, the distribution of the duty ratio of the diffraction gratings 21 to 28 is set so that the duty ratio of each of the diffraction gratings 21 to 28 decreases toward the center. By doing so, FIG.
As shown in (C), the distribution of the coupling coefficient in the entire resonator can be smoothly changed, and the internal electric field distribution can be completely flattened.

A curve (B) in FIG. 4 shows an electric field intensity distribution inside the semiconductor laser 20 according to the second embodiment of the present invention. The ratio between the minimum value and the maximum value of the internal electric field strength is about 1.0, and the conventional λ shown in the curve (D) in FIG.
It can be seen that the flatness of the internal electric field intensity distribution is greater than 0.33 of the / 4 phase shift distributed feedback laser.
Further, about the threshold gain difference indicating the mode stability of the laser, about 0.8 was obtained in the structure of the second embodiment. This is higher than 0.77 of the conventional λ / 4 phase shift distributed feedback laser.

In this embodiment, the laser oscillation wavelength is 1.3 μm, which is the Bragg wavelength determined by the average period of all the diffraction gratings.

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

In this embodiment, when L is the length of the laser resonator, the normalized coupling coefficient κ × L is set to about 2 (the distributed feedback coupling coefficient κ in the diffraction gratings 21 to 28 is about 70 cm ^−1). 2 to 4 are desirable. As the normalized coupling coefficient is set to a large value, it is necessary to reduce the amount of light distribution feedback near the center of the resonator.By increasing the number of orders on the resonator axis, the diffraction grating near the center of the resonator is required. Increasing the order can further reduce the coupling coefficient. As the normalized coupling coefficient increases, the laser oscillation mode becomes more stable, and the first diffraction grating 2
Even if a laser device in which the length of the first and eighth diffraction gratings 28 is different from the design can be obtained, the effect can be reduced.

Embodiment 3 FIG. 3A is a structural view (cross-sectional view) of a semiconductor laser 30 having a 300-micron resonator according to a third embodiment.

In this semiconductor laser, as in the first embodiment of the present invention, each layer is laminated on an n-type InGaAsP semiconductor substrate 1 by a known epitaxial growth and a well-known electrode forming method. The diffraction gratings 31 to 36 and the phase shift structure 5 are formed on the same plane by well-known lithography. The diffraction gratings 31 to 36 are set so that the order increases from the end face toward the center, similarly to the first embodiment of the present invention.

However, in this embodiment, the diffraction gratings 31 to 3 are used.
6 is formed by growing an n-type InGaAsP layer and then etching it. That is, the conductivity type of the diffraction grating is the same as that of the p-type InGaAsP
The conductivity type is opposite to the light guide layer 5 and the p-type InP clad layer 6.

The carrier density of the n-type InGaAsP layer forming the diffraction gratings 31 to 36 is 2 × 10 ¹⁷ cm.
^-3 , and the p-type I
The carrier density of the nP cladding layer 6 is 5 × 10 ¹⁷ cm ⁻³
It is about. By changing the carrier concentration in this way, it is preferable to make adjustments so that the diffraction gratings of different conductivity types and the surrounding light absorption coefficients are equal.

The semiconductor laser 30 has an anti-reflection coating on both end faces. Diffraction gratings 31-36
Is formed at 0.03 μm so that the distributed feedback coupling coefficient κ is about 70 cm ⁻¹ .

As shown in FIG. 3B, the distribution of the coupling coefficients of the diffraction gratings 31 to 36 is set so that the coupling coefficient decreases toward the center. Also, the diffraction gratings 31 to 3
6 blocks the current injected into the active layer because the electrical conductivity type is inverted with respect to the surroundings. Since the duty ratio of the diffraction grating varies depending on the position, the gain of the active layer as shown in FIG. A distribution can be obtained.

By doing so, the coupling coefficient and the active layer gain can be changed, and the internal electric field distribution can be completely flattened.

A curve (C) in FIG. 4 shows an electric field intensity distribution inside the semiconductor laser 30 according to the third embodiment of the present invention. The ratio between the minimum value and the maximum value of the internal electric field strength is about 1.0, and the conventional λ shown in the curve (D) in FIG.
It can be seen that the flatness of the internal electric field intensity distribution is greater than 0.33 of the / 4 phase shift distributed feedback laser.
Further, about the threshold gain difference indicating the laser single mode property, about 0.8 was obtained in the third embodiment of the present invention. This is higher than 0.77 of the conventional λ / 4 phase shift distributed feedback laser. In this embodiment, the laser oscillation wavelength is 1.3 μm, which is the Bragg wavelength determined by the average period for all the diffraction gratings.

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

In this embodiment, the length L of the diffraction grating is 300 microns, and the normalized coupling coefficient κ × L is set to about 2 (the distributed feedback coupling coefficient κ in the diffraction gratings 31 to 36). Was set to about 70 cm ^-1 ), but this is preferably 2 to 4. As the normalized coupling coefficient is set large, it is necessary to increase the gain of the active layer gain distribution near the center of the resonator. As a result, the laser oscillation mode is further stabilized, and even if a laser device in which the lengths of the first diffraction grating 31 and the sixth diffraction grating 36 are different from the design at the time of laser cleavage may be obtained, the effect is reduced. can do.

[0060]

According to the present invention, the electric field intensity distribution in the axial direction of the laser resonator is made uniform, so that the front-to-back output ratio does not change even when the bias current changes, and the Thus, a semiconductor laser whose light output can be easily monitored can be provided. At the same time, by making the electric field intensity distribution in the axial direction of the laser resonator uniform, wavelength fluctuations during modulation are reduced, and stable operation is possible even during modulation.
A semiconductor laser having high current-to-light output conversion efficiency can be provided.

Further, according to the present invention, it is possible to provide a semiconductor laser capable of lowering the bit error rate during digital modulation as compared with a conventional semiconductor laser by increasing the stability of the laser oscillation mode.

Further, according to the present invention, it is possible to provide a semiconductor laser having a high yield by facilitating a manufacturing process in mass production, reducing variations in characteristics between elements.

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

[Brief description of the drawings]

FIG. 1 is a diagram showing a semiconductor laser device according to a first embodiment of the present invention. (A) Structure diagram (cross-sectional view) (B) Distribution of diffraction grating order (C) Distribution of coupling coefficient

FIG. 2 is a diagram showing a semiconductor laser device according to a second embodiment of the present invention. (A) Structural diagram (cross-sectional view) (B) Distribution of diffraction grating duty ratio (C) Distribution of coupling coefficient

FIG. 3 is a diagram showing a semiconductor laser device according to a third embodiment of the present invention. (A) Structure diagram (cross-sectional view) (B) Distribution of coupling coefficient (C) Distribution of active layer gain

FIG. 4 is a diagram showing an electric field intensity distribution inside a semiconductor laser device. Curve (A): First embodiment Curve (B): Second embodiment Curve (C): Third embodiment Curve (D): Conventional example

FIG. 5 is a first conventional example of a λ / 4 phase shift type DFB.
It is a structural diagram of a laser.

FIG. 6A is a structural view of a semiconductor laser device according to a second conventional example, and FIG. 6B is a distribution of the duty ratio of the diffraction grating (B).
FIG. 4 is a diagram showing a distribution (C) of the coupling coefficient.

[Explanation of symbols]

 Reference Signs List 1 n-type InP semiconductor substrate 2 n-type InP cladding layer 3 multiple quantum well layer 4 p-type InGaAsP light guide layer 5 phase shift structure 6 p-type InP cladding layer 7 p-type InGaAsP cap layer 8 p-type electrode 9 n-type electrode 10 semiconductor Laser 11 First diffraction grating 12 Second diffraction grating 13 Third diffraction grating 14 Fourth diffraction grating 15 Fifth diffraction grating 16 Sixth diffraction grating 17 Seventh diffraction grating 18 Eighth diffraction grating 20 Semiconductor laser 21 first diffraction grating 22 second diffraction grating 23 third diffraction grating 24 fourth diffraction grating 25 fifth diffraction grating 26 sixth diffraction grating 27 seventh diffraction grating 28 eighth diffraction grating Reference Signs List 30 semiconductor laser 31 first diffraction grating 32 second diffraction grating 33 third diffraction grating 34 fourth diffraction grating 35 fifth diffraction grating 36 sixth diffraction grating

Claims (6)

(57) [Claims] In a distributed feedback semiconductor laser that performs optical feedback by means of a diffraction grating, the diffraction grating is divided into a plurality of grating regions, and the phase is shifted by a half period of the primary diffraction grating at the center of the laser resonator. The grating region is shifted so that the order of the diffraction grating of the grating region provided in the center direction is higher than that of the grating region provided on the end face side of the laser resonator , and the order of each grating region is 1st to 4th order.
A distributed feedback semiconductor laser characterized by being formed so as to change in four or more stages . 2. A distributed feedback type in which optical feedback is performed by a diffraction grating.
In a semiconductor laser, this diffraction grating is divided into a plurality of grating regions,
At the center of the shaker, the phase shifts by half the period of the primary diffraction grating.
From the grating region provided on the end face side of the laser resonator.
The order of the diffraction grating in the grating region provided in the center direction is
Is formed so as to be higher, further, within each grating region divided into a plurality, as the coupling coefficient toward the center from the end surface of the laser resonator is reduced, the duty ratio of the diffraction grating is set the distribution feedback type semiconductor laser you, characterized in that. 3. The duty ratio of the diffraction grating is 50.
% Or less, and is set so as to decrease from the end face of the laser resonator toward the center in each of the plurality of divided grating regions.
The distributed feedback semiconductor laser as described in the above. 4. The diffraction grating has a current block structure in which the conductivity type of the grating portion is inverted from the conductivity type around the grating portion, and the carrier density is different between the grating portion and the periphery thereof. Item 4. The distributed feedback semiconductor laser according to any one of Items 2 and 3 . 5. The distributed feedback semiconductor laser according to claim 4, wherein said diffraction grating has a carrier density set so that a light absorption coefficient is equal between said grating portion and its periphery. 6. The distributed feedback semiconductor laser according to claim 4, wherein the conductivity type of the diffraction grating is n-type, and the conductivity type around the diffraction grating is p-type.