JP2965023B2 - Multi-wavelength distributed feedback semiconductor laser array and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a semiconductor laser array, and more particularly to a phase shift distributed feedback multi-wavelength semiconductor laser array used in a wavelength division multiplexing optical communication system.

[0002]

2. Description of the Related Art As shown in a sectional view of FIG. 8, a multi-wavelength semiconductor laser array used in a conventional multi-channel optical communication system has a .lambda. A semiconductor laser having a high single-mode property called a / 4 phase shift distributed feedback semiconductor laser is used. The λ / 4 phase shift structure is a known structure, and is described, for example, in “1994, published by Ohmsha, edited by the Japan Society of Applied Physics, semiconductor laser, page 272, FIGS. 12 and 12”. In this structure, the laser oscillation wavelength becomes equal to the Bragg wavelength determined by the period of the diffraction grating, so that the submode suppression ratio can be increased.

Japanese Patent Application Laid-Open No. Sho 62-109388 discloses a multi-wavelength laser array structure in which semiconductor lasers having different diffraction grating periods are arranged in parallel. In this structure, an individual electrode is used for each laser and each is driven at a different oscillation wavelength, so that it can be operated as a multi-wavelength semiconductor laser array.

[0004]

However, in such a semiconductor laser array structure, in order to realize a multi-wavelength laser array, the diffraction grating periods of each semiconductor laser must be formed differently. When the diffraction grating is formed by an electron beam exposure method using an electron beam exposure apparatus, different diffraction grating periods are set for each laser, and diffraction gratings are formed one by one. Therefore, there is a problem that the drawing time is long and the exposure time is too long, which is not suitable for mass production.

Also, as a second problem, the conventional λ / 4
In the phase shift distributed feedback type semiconductor laser, since a λ / 4 phase shift structure exists in the center of the laser resonator, photons concentrate on the phase shift portion, and the internal electric field distribution intensity becomes extremely non-uniform, resulting in carrier fluctuation. Since the change in the refractive index greatly differs depending on the position in the resonator, there is a problem that the wavelength fluctuation during modulation is large.

The present invention has been made in view of such a problem, and a semiconductor laser which exhibits a stable operation even when a semiconductor laser is modulated and which can set each wavelength independently on the same substrate. It is an object of the present invention to provide a multi-wavelength distributed feedback semiconductor laser array having a large number.

Another object of the present invention is to provide a multi-wavelength distributed feedback semiconductor laser array capable of shortening the time required to form a diffraction grating for each laser element, reducing production costs, and mass-producing, and a method of manufacturing the same. And

[0008]

According to the present invention, there is provided a distributed feedback semiconductor laser array in which a plurality of distributed feedback semiconductor laser elements are arranged and arranged on a semiconductor substrate, wherein each of the laser elements has the same diffraction grating period; By providing a phase shift structure of a diffraction grating in which the amount of phase shift is independently set for each laser element at a plurality of locations in the resonator of each laser element, the oscillation wavelength of each laser element can be set independently. The present invention relates to a multi-wavelength distributed feedback semiconductor laser array.

The phase shift structure of each laser element is disposed at a position where the length of the resonator of each laser element is divided at substantially equal intervals, and the phase shift amounts are set so that their absolute values are equal. Is preferred.

The phase shift amount is one of the diffraction grating periods.
It is preferable that the period be / 4 or-／.

Such a multi-wavelength distributed feedback semiconductor laser array comprises a step of applying a first photoresist for forming a diffraction grating on a semiconductor substrate, and a step of applying a first photoresist on the first photoresist. Applying a second photoresist different from the first photoresist, further applying a third photoresist different from the first and second photoresists on the second photoresist, A step of patterning the photoresist into a predetermined shape by lithography, and a step of patterning by lithography the second photoresist in a portion where the surface has appeared after the removal of the third photoresist, thereby forming the second and third photoresists. A step of forming a two-step step, and using the second and third photoresists processed in the step as a phase mask. Exposing the first photoresist to a diffraction grating pattern over the entire surface by light interference exposure, removing the second and third photoresists, and then developing the first photoresist. Forming a first photoresist in a diffraction grating pattern, and etching the substrate using the diffraction grating pattern of the first photoresist as a mask to form a diffraction grating having a predetermined phase shift structure at a predetermined position. And a forming step.

According to the present invention, the aforementioned problems are solved for the following reasons.

It has been known that a λ / 4 phase shift distributed feedback semiconductor laser oscillates in a single mode at a Bragg wavelength determined by the period of a diffraction grating.
The λ / 4 phase shift structure as shown in FIG. 8 satisfies the laser resonance condition with the Bragg wavelength by shifting the diffraction grating by で period at the center of the resonator. Here, when the shift amount is changed from the half cycle, the laser oscillation wavelength changes so as to slightly deviate from the Bragg wavelength.

However, in the conventional structure, since the phase shift structure exists at only one point at the center of the laser resonator, the photon density concentrates only on the phase shift portion, the internal photon density becomes non-uniform, and oscillation occurs during the modulation operation. The problem that the wavelength is not stable occurs.

Therefore, in the present invention, by distributing and providing the phase shift structure at a plurality of locations in the laser resonator,
The above problem can be solved by preventing the photon density from being concentrated on one point. The number of phase shift structures provided in the laser resonator is two or more, preferably three or more,
It is usually about 10 or less.

It is particularly preferable to distribute the phase shift structure in the laser resonator at substantially equal intervals, since the oscillation thresholds of the lasers can be made substantially equal.

Further, by disposing a plurality of phase shift structures in the resonator and setting different phase shift amounts for the respective semiconductor laser elements constituting the semiconductor laser array,
Even if the diffraction grating period is the same, a plurality of semiconductor laser elements having different oscillation wavelengths can be obtained.

Further, by setting the phase shift amount of each semiconductor laser element independently, it is possible to make the intervals of the oscillation wavelengths obtained from each semiconductor laser element equal.

It is also possible to make the absolute values of the respective phase shift amounts equal and to design the oscillation wavelength of each laser element only by a combination of the positive and negative phase shift amounts having the same absolute value. The number can be reduced.

In the present invention, since the diffraction grating of each semiconductor laser is formed by the interference exposure method, a multi-wavelength semiconductor laser array can be mass-produced.

As described above, according to the present invention, it is possible to obtain a multi-wavelength semiconductor laser array having different oscillation wavelengths arranged at equal intervals regardless of the arrangement of semiconductor lasers having the same diffraction grating period. Due to the shift structure, the electric field distribution inside the resonator is flattened, and wavelength fluctuation during modulation can be reduced.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION

[Embodiment 1] FIG. 1 is a structural view of a six-wavelength semiconductor laser array showing a first embodiment of the present invention.

This semiconductor laser array 1 is shown in FIG.
As shown in (1), semiconductor lasers are arranged in parallel in the laser resonator direction, and the diffraction grating periods of the respective semiconductor lasers are all equal. FIG. 1B shows the first laser 2.
1 is a cross-sectional view of a multi-phase shift distributed feedback semiconductor laser. Other lasers are similarly formed except for the phase shift structure.

This structure is well known for epitaxial growth,
It can be manufactured by a known electron beam exposure method and a known lithography method, and a drive current can be individually injected into each semiconductor laser.

In the laser array device 1 shown in FIG. 1A, three phase shift structures are formed in a diffraction grating of a 300-micron resonator. The three phase shift structures are λ / 8 phase shift or -λ / 8 phase shift,
The phase is shifted by a quarter or -−１ period in terms of the diffraction grating period, that is, a normal λ / 4
This is a half shift amount of the phase shift structure. Further, the phase shift structure is disposed at one position at the center of the resonator and at one position at an intermediate point between the center of the resonator and both ends.

In three phase shift structures, 1/4
Eight combinations are conceivable by combining the period shift or the phase shift of −−１ period.
When the one having the same structure is removed by inverting left and right, there are six combinations.

The first to sixth semiconductor lasers have the same structure as that shown in FIG. 1B for these six combinations.

As shown in FIG. 1A, the six combinations of the phase shift structures are such that the first laser 2 has a period of 1/4,-／, and 1/4 with respect to the diffraction grating period. Phase shift, the second laser 3 is 1/4,-1/4, -1
/ 4 period phase shift, and the third laser 4
4, −−１, and − ／ phase shifts.
The laser 5 has a phase shift of １ ／, ４, 周期 cycle, the fifth laser 6 has a phase shift of ４, ４, １ ／ cycle, and the sixth laser 6 has Laser 7 is-／, 1
/ 4, -−１ phase shift.

The six semiconductor lasers 2 to 7 are:
Both ends are anti-reflective coated.

FIG. 2 is a diagram showing the distribution of the oscillation wavelength of the six-wavelength semiconductor laser array device 1 according to the first embodiment. That is, despite the fact that the diffraction grating periods of the respective semiconductor lasers are all the same, different oscillation wavelengths are obtained due to the different phase shift portions. Although the wavelength intervals of the semiconductor lasers are not uniform, the average is 0.56 nm.

In this embodiment, a semiconductor laser array having six wavelengths is realized with three phase shift structures. However, the number of phase shift structures is not limited to this, and may be increased. . Also, the number of lasers with different wavelengths can be changed.

In this embodiment, the distributed feedback coupling coefficient κL is set to 3. However, the present invention is not limited to this. When the coupling coefficient κL is set large, the range of the oscillation wavelength obtained in the semiconductor laser array can be widened.

[Second Embodiment] Next, a second embodiment of the present invention will be described in detail with reference to the drawings.

The laser 16 whose sectional view is shown in FIG.
As in the first embodiment of the present invention, three phase shift structures are formed on a diffraction grating in a 300-micron resonator. As shown in FIG. 3A, in the semiconductor laser array 14 according to the second embodiment of the present invention, the three phase shift structures each correspond to a diffraction grating period.
The first laser 15 has a phase shift of 1/4,-／, and 1/4 period, and the second laser 16 has a phase shift of 1/4, -1 /
The fourth laser 18 has a phase shift of 4, -1 / 5.15 cycles, the third laser 17 has a phase shift of -1/4,-1/4,-1 / 3.615 cycles, and the fourth laser 18 has 1/4, 1 /
The fifth laser 19 has a phase shift of-／, ４, 1 / 5.15 cycles, and the sixth laser 20 has a phase shift of １ ／, 1 / 3,615 cycles. 4, 1/4,
This is a phase shift of − cycle. Further, the second aspect of the present invention
The six semiconductor lasers 15 to 20 according to the embodiment are all coated with anti-reflection coating on both end surfaces.

FIG. 4 is a diagram showing the distribution of the oscillation wavelength of the six-wavelength semiconductor laser array device 14 according to the second embodiment of the present invention. That is, despite the fact that the diffraction grating periods of the respective semiconductor lasers are all the same, different oscillation wavelengths are obtained due to the different phase shift portions.
Also, the wavelength spacing of each semiconductor laser is equal,
0.56 nanometer.

In this embodiment, a semiconductor laser array having six wavelengths is realized with three phase shift structures. However, the number of phase shift structures is not limited to this, and may be increased. . Also, the number of lasers with different wavelengths can be changed.

In this embodiment, the distributed feedback coupling coefficient κL is set to 3. However, the present invention is not limited to this. When the coupling coefficient κL is set large, the range of the oscillation wavelength obtained in the semiconductor laser array can be widened.

Third Embodiment Next, a third embodiment of the present invention will be described in detail with reference to the drawings.

The semiconductor laser array 22 shown in FIG. 5 has four phase shift structures formed at three locations on a diffraction grating in a 300-micron resonator, similarly to the first and second embodiments of the present invention. It is an array of semiconductor lasers. As shown in FIG. 5, the three phase shift structures are such that the first laser 23 is が, − ／, 1 /
The phase shift is four periods, and the second laser 24
4, − レ ー ザ, and −1 / 3.02 phase shifts, and the third laser 25 emits −−１, １ ／, 1 / 3.0
Four cycles of phase shift, and the fourth laser 26
The phase shift is 4, 1/4,-1/4 cycle.

Next, this manufacturing method will be described with reference to FIG.

First, as shown in FIG. 6A, a high-resolution first photoresist 27 for forming a diffraction grating is applied to a thickness of 0.1 μm on a semiconductor substrate 13, and the temperature is reduced from 120 ° C. to 1 μm.
Bake at 50 ° C.

Next, as shown in FIG. 6 (2), a second photoresist 28 is applied on the first photoresist 27 and baked at 90 ° C.

Next, as shown in FIG. 6 (3), a third photoresist 29 is further formed on the second photoresist 28.
And baked at 90 ° C. The thicknesses of the second photoresist 28 and the third photoresist 29 are determined using Snell's law depending on the amount of phase shift to be formed.

Next, as shown in FIG. 6D, exposure is performed by a mercury lamp using a photomask, and only the third photoresist 29 is patterned. Here, since the third photoresist 29 has higher sensitivity than the second photoresist 28, the second photoresist 28 is not patterned, and only the third photoresist 29 is patterned.

Next, as shown in FIG. 6 (5), the second photoresist 28, the surface of which has been removed by removing the third photoresist 29, is exposed by a mercury lamp using a photomask. According to the sensitivity of photoresist,
Exposure is performed for a longer time than the step (4). This allows the second
Is patterned. As a result, as shown in FIG. 6 (5), a two-step stepped shape is formed by the second photoresist 28 and the third photoresist 29.

Next, for example, as shown in FIG.
Phases of the second photoresist 28 and the third photoresist 29 are determined by the well-known interference exposure method described in “Semiconductor Lasers and Integrated Circuits,” edited by Yasuharu Suematsu, ed. Using the mask as a mask, the entire surface of the first photoresist 27 is exposed to a diffraction grating pattern.

At this time, the wavelength of light used for exposure is not absorbed by the second photoresist 28 and the third photoresist 29. A phase shift structure can be formed in a stepped portion between the second photoresist 28 and the third photoresist 29. The phase shift amount is
It can be controlled by the thickness of each photoresist. The sign of the phase shift is determined by the orientation of the photoresist edge with respect to the direction of exposure. When light for interference exposure is irradiated from the left side as shown in FIG. 6 (6), the sign at the right end (a) of each photoresist is-.
And a + is formed at the left end (b) of the photoresist.

Next, as shown in FIG. 6 (7), the second photoresist 28 and the third photoresist 29 are removed,
The first photoresist 27 is developed to form a diffraction grating pattern.

As described above, the diffraction grating pattern 30 can be formed on the semiconductor substrate 13 by etching the substrate using the first photoresist 27 as a mask, as shown in FIG. Then, for example, -1
The semiconductor laser array can be obtained by dividing the phase shift between the /3.02 cycle and the 1 / 3.02 cycle by cleavage or the like.

The multi-wavelength semiconductor laser array device 22 can be manufactured by using the well-known epitaxial growth and the well-known electrode pattern forming method on the semiconductor substrate provided with the diffraction grating pattern as described above.

The four semiconductor lasers 23 to 26 are all provided with anti-reflection coating on both end surfaces.

FIG. 7 is a diagram showing the distribution of the oscillation wavelength of the four-wavelength semiconductor laser array according to this embodiment. That is, despite the fact that the diffraction grating periods of the respective semiconductor lasers are all the same, different oscillation wavelengths are obtained due to the different phase shift portions. Further, the wavelength intervals of the respective semiconductor lasers are uniform, that is, 0.94 nm.

Further, in this embodiment, a semiconductor laser array having four wavelengths with three phase shift structures has been described. However, it is to be noted that the number of phase shift structures and the number of lasers having different wavelengths are not limited thereto. Although the distributed feedback coupling coefficient κL is set to 3, the same applies to the case where 2 to 4 is usually desirable.

[0054]

According to the present invention, a multi-wavelength distributed feedback semiconductor having a large number of semiconductor lasers on a single substrate, which can operate stably at the wavelength even when the semiconductor laser is modulated, and which can set each wavelength independently. A laser array can be provided.

Further, according to the present invention, it is possible to provide a multi-wavelength distributed feedback semiconductor laser array capable of shortening the time for forming the diffraction grating of each laser element, reducing the production cost and mass-producing, and a method of manufacturing the same. Can be.

Therefore, if the multi-wavelength distributed feedback semiconductor laser array of the present invention is used, large-capacity transmission is possible in optical communication, so that it is possible to cope with an increase in subscribers and communication services. Thus, a low-cost optical communication system for subscribers can be realized.

[Brief description of the drawings]

FIG. 1A is a plan view of a six-wavelength semiconductor laser array according to a first embodiment. FIG. 3B is a sectional structural view of a first semiconductor laser portion.

FIG. 2 is a diagram illustrating a distribution of oscillation wavelengths of the six-wavelength semiconductor laser array according to the first embodiment.

FIG. 3A is a plan view of a six-wavelength semiconductor laser array according to a second embodiment. FIG. 3B is a sectional structural view of a second semiconductor laser portion.

FIG. 4 is a diagram illustrating a distribution of oscillation wavelengths of a six-wavelength semiconductor laser array according to a second embodiment.

FIG. 5 is a plan view of a four-wavelength semiconductor laser array according to a third embodiment.

FIG. 6 is a diagram illustrating a manufacturing method according to a third embodiment.

FIG. 7 is a diagram showing a distribution of oscillation wavelengths of a four-wavelength semiconductor laser array according to a third embodiment.

FIG. 8 is a structural diagram of a conventional λ / 4 phase shift distributed feedback semiconductor laser.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Multi-wavelength semiconductor laser array 2 1st laser 3 2nd laser 4 3rd laser 5 4th laser 6 5th laser 7 6th laser 8 Cap layer 9 2nd cladding layer 10 Active layer 11th 1 clad layer 12 diffraction grating 13 semiconductor substrate 14 multi-wavelength semiconductor laser array 15 first laser 16 second laser 17 third laser 18 fourth laser 19 fifth laser 20 sixth laser 21 diffraction grating 22 Multi-wavelength semiconductor laser array 23 First laser 24 Second laser 25 Third laser 26 Fourth laser 27 First photoresist 28 Second photoresist 29 Third photoresist 30 Diffraction grating 31 Diffraction grating

Claims (4)

(57) [Claims] 1. A distributed feedback semiconductor laser array in which a plurality of distributed feedback semiconductor laser elements are arranged and arranged on a semiconductor substrate, wherein each of the laser elements has the same diffraction grating period, and a cavity of each of the laser elements is provided. A multi-wavelength distribution feedback type characterized in that the oscillation wavelength of each laser element is set independently by providing a diffraction grating phase shift structure in which the amount of phase shift is set independently for each laser element at a plurality of locations. Semiconductor laser array. 2. A phase shift structure of each of the laser elements,
2. The multi-wavelength distribution feedback according to claim 1, wherein the resonators of the respective laser elements are arranged at positions where the lengths thereof are divided at substantially equal intervals, and the phase shift amounts are set so that their absolute values are equal to each other. Semiconductor laser array. 3. The method according to claim 1, wherein the amount of phase shift is one of the diffraction grating periods.
3. The multi-wavelength distributed feedback semiconductor laser array according to claim 2, wherein the cycle is / 4 or-／. 4. A step of applying a first photoresist for forming a diffraction grating on a semiconductor substrate, and applying a second photoresist different from the first photoresist on the first photoresist. A step of applying a third photoresist different from the first and second photoresists on the second photoresist, and a step of patterning the third photoresist into a predetermined shape by lithography. Forming a two-step step by the second and third photoresists by patterning the second photoresist in a portion where the surface has appeared by removing the third photoresist by lithography. Using the second and third photoresists processed in the above process as a phase mask, and applying an optical interference exposure method to the first photoresist. Exposing the entire surface to a diffraction grating pattern, removing the second and third photoresists, and then developing the first photoresist to form a first photoresist in a diffraction grating pattern shape. A multi-wavelength distributed feedback semiconductor laser comprising: a step of etching a substrate using the diffraction grating pattern of the first photoresist as a mask to form a diffraction grating having a predetermined phase shift structure at a predetermined position. Array manufacturing method.