JP3149962B2 - Multi-wavelength semiconductor laser device and driving method thereof - 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 device expected as a light source for wavelength-division multiplexed optical communication, wavelength-division multiplexed optical recording, optical operation, and the like, and a driving method thereof. Accordingly, the present invention relates to a multi-wavelength semiconductor laser device capable of simultaneously emitting a plurality of laser beams having a plurality of different wavelengths or a single wavelength, and a driving method thereof.

[0002]

2. Description of the Related Art In recent years, the demand for semiconductor laser devices in the fields of optical communication and optical information processing has been rapidly increasing, and accordingly, demands for device functions have been diversified. In particular, a semiconductor laser device capable of multi-wavelength oscillation is important as a light source for wavelength division multiplex communication or the like.

As a first conventional example of a wavelength tunable semiconductor laser satisfying such requirements, a so-called distributed reflection type (DB) using a grating as a reflector constituting a resonator is known.
R) Semiconductor lasers or distributed feedback (DFB) semiconductor lasers have been proposed in which the wavelength of oscillation is made different by making the pitch of the grating different for each wavelength. 63-329
80). In this case, the structure of the light emitting layer and the like is the same as that of a normal semiconductor laser.

In a second conventional example, the composition and the like of the active layer of a semiconductor laser oscillating each wavelength are changed, that is, the etching and regrowth processes are repeated to form a plurality of active layers having different energy gaps. A device in which semiconductor lasers are arranged to form a multi-wavelength laser device has been proposed.

[0005]

However, the above prior art has the following problems. First, in the first conventional example, since only the active layer having the same simple structure is used and the pitch of the grating is changed, the spectrum of the gain of the active layer with respect to the wavelength is narrow.
It could only change by a percentage.

Further, in the second conventional example, since a plurality of semiconductor lasers are manufactured by repeating the processes of etching and regrowth, the manufacturing method is complicated, and damage is likely to occur during processing, and the yield is poor.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a multi-wavelength semiconductor laser device which solves the above-mentioned problems of the conventional example, has a wide wavelength tunable range, and is easy to manufacture, and a method of driving the same.

[0008]

According to the present invention to achieve the above object, according to the present invention, a plurality of light emitting layers having different band gaps in the ground level and a band gap larger than these light emitting layers provided between the light emitting layers are provided. In a multi-wavelength semiconductor laser device comprising an optical waveguide structure including a barrier layer having a structure and a cladding layer laminated with the structure interposed therebetween, light having a wavelength corresponding to the band gap of each of the plurality of light emitting layers is oscillated. That is, a plurality of resonators having different resonator lengths are arranged side by side in an array in the lateral direction, and the electrodes are configured so that current can be independently injected into the plurality of resonators. It is characterized by.

More specifically, the band gap and the thickness of the barrier layer are such that when carriers are injected into the light emitting layer,
As compared with the case without the barrier layer, the carrier concentration of the light emitting layer having the larger band gap of the ground level is made higher, and the carrier concentration of the light emitting layer having the smaller band gap of the ground level is made lower. It is set, or an intermediate layer whose band gap is smaller than the cladding layer and larger than the light emitting layer is provided between the cladding layer and the light emitting layer, or the barrier layer, the light emitting layer, and the intermediate layer are close to the light emitting layer. At least one of the portion and the portion of the cladding layer adjacent to the light emitting layer
One of them has a p-type or an n-type at least partially due to impurity doping, and the band gap of the barrier layer is larger than the band gap of the intermediate layer near the boundary with the light emitting layer. Big or
When the band gap of the barrier layer is larger than the band gap of the cladding layer, or when a current having an oscillation threshold is injected into the device, the light emitting layer having the smaller band gap of the ground level. , The gain of the emission wavelength (short wavelength) corresponding to the band gap of the ground level in the light emitting layer having the larger band gap of the ground level becomes positive (this is because when oscillating at the short wavelength, The gain at the short wavelength corresponding to the bandgap of the higher-order level in the light emitting layer having the smaller bandgap at the ground level is eaten away, so that the light of the longer wavelength stops oscillating, thereby reducing the oscillation wavelength. This is to make switching completely possible.) Other specific configurations will be apparent from the following description of the embodiments.

In the case of a quantum well, the band gap in the present invention refers to a transition energy from a certain level in the valence band to a certain level in the conduction band, including quantization energy. In addition, the wavelength corresponding to one band gap generally means a certain wavelength range because the wavelength spread occurs in accordance with the energy spread of the injected carriers in the quantum well. Therefore, the resonator length for selectively selecting the wavelength light corresponding to one band gap is not limited to one, and a plurality of resonators having different resonator lengths may be used. In other words, the number of band gaps in the light emitting layer and the number of resonators do not need to be equal, and the gain spectrum formed by current injection into each band gap is sufficiently positive to cover a large wavelength or wavelength region. May be set.

According to the present invention having the above-mentioned structure, all the problems of the above-mentioned conventional example can be solved. First, the narrowness of the band of the gain spectrum of the active layer, which is a problem of the first conventional example, is solved by using the active layer (including a plurality of different light emitting layers) of the present invention. It is enlarged. That is, since a plurality of light emitting layers or quantum well layers having a plurality of different energy gaps are used as an active layer, the gain spectrum of the entire active layer becomes the sum of the gain spectra from all the quantum well layers, and the band becomes wide. Oscillation is possible in a wide wavelength range. For example, when AlxGa1-xAs is used for the light emitting layer, the oscillation wavelength can be changed from several tens nm to about one hundred and several tens nm.

The complexity of the fabrication process, which is a problem of the second conventional example, is not the present invention, and resonators having different resonator lengths can be formed at desired oscillation wavelengths using a common active layer formed on the same substrate. Therefore, the multi-wavelength laser device of the present invention can be easily manufactured.

Here, a method of designing the resonator length will be described. The resonator length is L, the reflectivity of the end face is before and after, and R is
Assuming that f, Rb, and the internal loss of light are αi, the threshold gain gth in the oscillation threshold state is expressed by the following equation. gth = αi + 1 / L·ln (1 / RfRb) That is, it can be seen that the shorter L is, the larger gth is.
Since the gain spectrum of the active layer of the semiconductor laser of the present invention changes remarkably with the change in injection current, the oscillation wavelength at that time can be greatly changed by appropriately setting the resonator length L and changing the threshold gain. be able to. Therefore, by appropriately selecting the resonator length L, light having a desired wavelength can be oscillated.

By devising the structure near the active layer as follows, it is possible to further reduce the threshold value and increase the efficiency. That is, a barrier layer having a larger band gap than those light emitting layers is provided between adjacent light emitting layers having different band gaps, and the band gap and the thickness of the barrier layer are adjusted by injecting carriers into the light emitting layer. Occasionally, the carrier concentration of the light emitting layer having the larger band gap is higher than that without the barrier layer, and the carrier concentration of the light emitting layer having the smaller band gap is large enough to make the carrier concentration lower. With such an arrangement, the gain spectrum distribution extends to the short wavelength side under a small carrier injection amount. Therefore, short wavelength light can be oscillated without increasing the current density so much.

As described above, according to the present invention, a plurality of resonators having different lengths are formed in a lateral direction by a simple manufacturing process so that light having a wavelength corresponding to a plurality of band gaps oscillates. And a multi-wavelength semiconductor laser device capable of oscillating a plurality of light beams having different wavelengths by independently injecting current into each other can be realized.

[0016]

The present invention will be described in detail below with reference to embodiments. In order to make the description easy to understand, in the following description, two types of wavelengths are used, so that the number of quantum well light emitting layers is two and the number of juxtaposed resonators is two. Since the same applies to the case of three or more types, it can be easily analogized from the following description.

FIG. 1 is a perspective view of an embodiment of the semiconductor laser device of the present invention. Such a device can be manufactured by an epitaxial film formation that can be controlled on an atomic layer order such as a molecular beam epitaxy (MBE) method or a metal organic chemical vapor deposition (MOCVD) method. Therefore, detailed description is omitted.

In the figure, 1 is an n + -GaAs substrate, and 2 is an n +
A GaAs buffer layer, 3 an n-AlxcGa1-xcAs cladding layer, 4 an optical waveguide structure, and 5 a p-AlxcGa1-
xcAs cladding layer, 6 is a p + -GaAs cap layer, 7
Is an Au / Cr electrode, 8 is an Au-Ge / Au electrode, 9 is S
i3 N4 film.

FIG. 2 schematically shows an energy band diagram of the optical waveguide structure 4 common to two side-by-side resonators. In the figure, 10a and 10b are p-type and n-type Alxc, respectively.
Ga1-xcAs Optical / electron confinement layer (separate
-Confinement, abbreviated as SC layer), 1
1a is an AlxaGa1-xaAs light emitting layer, 11b is an AlxbGa
The 1-xbAs light emitting layer 12 is a p @ + -AlxBGa1-xBas barrier layer. Although the essence of the invention is independent of the SC structure,
Since the light emitting layers 11a and 11b are asymmetric, it is necessary to increase the carrier injection efficiency and clean the light intensity distribution by using S.
The C structure is effective. In this embodiment, the SC structure has a GRIN in which the energy gap, that is, the refractive index gradually changes.
(Graded index) composition, but a step index type (step) that changes stepwise
index)) or a composition that changes linearly.

In this example, the short-wavelength (λ2) light-emitting layer 11b is provided on the n-type cladding layer 3 side and the long-wavelength (λ1) light-emitting layer 11a is provided on the p-type cladding layer 5 side. It is difficult for the holes h injected from the side to reach the light emitting layer 11b of the short wavelength (λ2) (compared to the case where the holes h move in the opposite direction). Therefore, the barrier layer 12 is doped with a high concentration of p-type to supply holes in advance. If the holes are sufficiently replenished in advance as described above, the holes are appropriately distributed in both the light emitting layers 11a and 11b when no current flows. In such a case, when discussing laser oscillation, it is sufficient to mainly consider only the electron distribution. The following description of the operation will be made in this case, but other cases (such as replacing p and n in the present embodiment) can be easily analogized. The following can be said about the example in which p and n are exchanged. Ga at room temperature
Considering that the mobility of As holes is 400 cm 2 / V · s, which is smaller than the electron mobility of 8800 cm 2 / V · s, it can be said that non-uniform injection of holes is easier, and therefore p It can be said that the example where n and n are exchanged is more suitable.

Further, the thickness of the barrier layer 12 and the height (depth) of the potential are set to be sufficiently large so that each light emitting layer or quantum well layer 11a is provided at about the threshold current density of an ordinary semiconductor laser. , 11b are unevenly distributed to increase the gain of the well 11b on the high energy gap side (that is, as shown by the oblique line in FIG. 2, the light emitting layer 11b having the larger band gap). The carrier concentration has increased). If the barrier layer 12 is too thin or too low, a uniform carrier distribution similar to that without the barrier layer 12 is obtained. However, in the example of FIG. 1, the well layer 11b having a shorter wavelength (λ2) is used. The barrier layer 12 is set so that the electrons e are distributed at a larger ratio toward the upper side.
However, if the barrier layer 12 is made too thick and / or too high, electrons will not come to the long wavelength (λ1) well layer 11a, so that the setting of the barrier layer 12 requires optimization.

FIG. 3 shows the injection current dependence of the gain spectrum of such an active layer. Curves 1, 2, and 3 are the respective gain spectra when the injection current is gradually increased.
FIG. 3 shows that when the injection current is small, the gain is the largest for the long wavelength light λ1, but when the injection current is large, the gain is largest for the short wavelength light λ2. As described above, since the gain spectrum of the active layer of the semiconductor laser of the present invention is greatly changed by the injection current, the oscillation wavelength can be changed in a large range by appropriately setting the resonator length and controlling the threshold gain. Can be.

As a specific example, GaAs having a thickness of 80 ° is used as the light emitting layer 11a in an AlGaAs semiconductor laser.
Al 0.12 Ga 0.88 A with a thickness of 60 ° as the light emitting layer 11 b
s, Al.sub.0.4 Ga.sub.0.6 having a thickness of 150.degree.
As, thickness 5 as photo-electron confinement layers 10a and 10b
00 ° Al0.3 Ga0.7 As-Al0.5 Ga0.5 As
FIG. 4 shows an experimental result of a change in the oscillation wavelength when the threshold gain is changed by changing the resonator length L in the case of.

FIG. 4 shows that the oscillation wavelength gradually decreases as L decreases. This is because the carrier density peak shifts toward higher energy (short wavelength) as the carrier injection amount increases. Furthermore,
In the case of the active layer of the present invention, a discontinuity between about 830 nm and about 780 nm was observed at L ≒ 180-200 μm. The reason for this is that, as shown in FIG. 4, the increase in the threshold gain with the decrease in L is caused by the peak λ1 → λ2
Because of the discontinuity of

FIGS. 5A and 5B show that L = 300 μm (1 / L = 33 cm −1) and L = 150 in the above embodiment.
FIG. 3 schematically shows a gain spectrum of a μm (1 / L = 67 cm −1) resonator in a laser oscillation threshold state. FIG.
In (a), the threshold gain gth1 at L = 300 μm determines the oscillation conditions. In this case, since the peak of the gain spectrum is at λ1 at the threshold current density, light of λ1 (long wavelength) oscillates.

Similarly, in FIG. 5B, L = 150 μm
The threshold gain gth2 at m determines the oscillation condition. In this case, the peak of the gain spectrum at the threshold current density (increased with the increase of gth2) is at λ2, so that λ2 (short wavelength) Light oscillates.

From the above, the two resonator lengths L1 and L2 of the embodiment of the present invention shown in FIG. 1 are determined. In the structure of the active layer, L1 ≧ 200 μm and 180 μm ≧ L2
The degree is appropriate. In the embodiment of FIG. 1, L1 ≒ 300 μm
m, L2 ≒ 170 μm, 840 nm, 780, respectively
A light having a wavelength of nm is emitted.

Here, a method of manufacturing the two resonators shown in FIG. 1 will be described. After fabricating semiconductor laser structures arranged in an array in the horizontal direction, applying processes such as photoresist coating, patterning, etc., dry etching such as reactive ion beam etching is performed so that two resonators are formed vertically. The groove 15 and the like are etched to produce a structure in which two semiconductor lasers having different resonator lengths are arranged in parallel with a common emission surface. That is, in this embodiment, in order to confine current and light in a stripe-shaped region, a ridge-type waveguide of a parallel resonator is formed by a method such as etching with a reactive ion beam. Film 9 is plasma C
After forming the film by the VD method, only the upper part of the ridge is removed by etching, and the electrode 7 is deposited. After that, in order to separate the element and produce the resonator of the etching facet,
A dry etching process such as reactive ion beam etching is performed, and after lapping, an electrode 8 is deposited on the back surface of the substrate 1. Thereby, semiconductor lasers having two different resonator lengths are formed in an array in parallel. As a structure for confining light and current, any method used in a normal semiconductor laser may be used.

The parallel semiconductor laser arrays manufactured as described above can oscillate at different wavelengths and are monolithically arranged on the same substrate, so that the difficulty of optical arrangement can be eliminated. As described above, according to the present invention, a multi-wavelength semiconductor laser device that can be independently modulated can be realized.

FIG. 6 shows an example of the multi-wavelength semiconductor laser device according to the present invention shown in FIG. 1 in which multiplexers of respective wavelengths are provided at the emission sections of the lasers LD1 and LD2. The addition of the multiplexer enables light of different wavelengths to be emitted from one emission end. FIG. 6A shows a Y-shaped multiplexer, and FIG. 6B shows a total reflection slit type multiplexer, but other forms may be used.

The current injection into each resonator may be performed with a single electrode, or a plurality of electrodes may be provided so that currents can be injected at different current densities in the resonance direction.

In the above embodiment, for convenience of explanation, the case where Alx Ga1-x As is used as the semiconductor has been described. However, it is apparent that any semiconductor material capable of forming a heterostructure may be used. In addition, it is apparent that the number and types of light emitting layers and the number of resonators are not limited to two as described above, but may be three or more. For the number and types of the light emitting layers, refer to the above-mentioned JP-A-63-211786.

Further, the laser device of the present invention can be used as a high-efficiency optical amplifier operating in a wide wavelength range. That is, the semiconductor laser device of the present invention is injected with a current slightly smaller than the threshold current value at which laser oscillation occurs, and a wavelength near the wavelength of light oscillating through one end face of the device from an external light source. Is incident, and light having the same wavelength as the incident light is extracted from the other end face.

Since the laser device according to the present invention has a gain over a wider wavelength range than the conventional device, it can be used as a high-efficiency optical amplifier that operates over a wide wavelength range and easily oscillates short-wavelength light.

Further, the laser device of the present invention can be used as a highly efficient optical wavelength converter operating in a wide wavelength range. That is, a current slightly smaller than a threshold current value for laser oscillation is injected into the semiconductor laser device of the present invention, and light having a photon energy larger than the band gap of the light emitting layer is incident on the device from an external light source. . Then, carriers are generated, so that light having a wavelength different from the incident light is emitted from the light emitting layer of the element, and emitted from the end face. The emitted light has a set wavelength of λ1 (close to) if the biased current is close to the threshold current value of light of wavelength λ1, and λ2 (close to) if it is close to the threshold current value of light of λ2.
Set wavelength. When this element is used, it can be used as a high-efficiency optical wavelength converter that operates in a wider wavelength range than the conventional element and easily oscillates short-wavelength light.

[0036]

As described above, in the multi-wavelength semiconductor laser device of the present invention, a plurality of laser devices can be monolithically formed in an array in the lateral direction on the same substrate by an easy manufacturing process. Different light can be emitted.

Also, by appropriately setting the structure near the active layer, particularly the band gap and the thickness of the barrier layer, it is possible to shorten the resonator length and increase the threshold gain without increasing the threshold gain.
Since short-wavelength light can be oscillated, laser efficiency is high and continuous oscillation at room temperature can be easily achieved. Due to the above effects, the present laser device is suitable as a light source for a wavelength multiplexing system such as wavelength multiplexing optical communication and wavelength multiplexing optical recording.

[Brief description of the drawings]

FIG. 1 is a perspective view of a parallel type two-wavelength semiconductor laser device according to an embodiment of the present invention.

FIG. 2 is an energy band diagram near an active optical waveguide layer according to an example.

3 is a diagram showing a change in gain spectrum depending on an injection current when carriers are injected into the active layer of FIG. 2;

FIG. 4 is a diagram illustrating the dependence of the oscillation wavelength on the resonator length in a threshold state of a semiconductor laser device embodying the present invention.

FIG. 5A shows a threshold gain and a gain spectrum when light having a wavelength λ1 is oscillated, and FIG. 5B shows a wavelength λ.
2 shows a threshold gain and a gain spectrum when the light of No. 2 oscillates.

FIG. 6 is a diagram showing a semiconductor laser device having a multiplexed waveguide section according to an embodiment of the present invention.

[Explanation of symbols]

1 n + -GaAs substrate, 2 n + -GaAs buffer layer, 3 n-AlxcGa1-xcAs
Cladding layer, 4 optical waveguide structure, 5 p-AlxcGa1-xcAs
Cladding layer, 6p + -GaAs cap layer, 7 Au / Cr electrode, 8 Au-Ge / Au electrode, 9 Si3 N4 film 10a, 10b Photo-electron confinement layer 11a, 11b Light-emitting quantum well layer 12 Barrier layer 15 Groove

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H01S 5/00-5/50

Claims (10)

(57) [Claims] 1. An optical waveguide structure including a plurality of light emitting layers having different ground level band gaps and a barrier layer provided between the light emitting layers and having a larger band gap than the light emitting layers, and the light guide. In a semiconductor laser device having a cladding layer laminated with a waveguide structure interposed therebetween, a plurality of cavity lengths having different lengths are provided so as to oscillate light having a wavelength corresponding to the band gap of each of the plurality of light emitting layers. A multi-wavelength semiconductor laser device comprising a plurality of resonators arranged side by side in an array in a lateral direction so that current can be independently injected into the plurality of resonators. 2. The bandgap and thickness of the barrier layer are such that when carriers are injected into the light-emitting layer, the carrier concentration of the light-emitting layer having a larger bandgap in the ground level than when no barrier layer is provided. 2. The multi-wavelength semiconductor laser device according to claim 1, wherein the carrier concentration of the light emitting layer having a smaller ground level band gap is set lower. 3. The multi-wavelength semiconductor laser device according to claim 1, wherein an intermediate layer having a band gap smaller than that of the cladding layer and larger than that of the light emitting layer is provided between the cladding layer and the light emitting layer. 4. At least one of the barrier layer, the light emitting layer, a portion of the intermediate layer adjacent to the light emitting layer, and a portion of the cladding layer adjacent to the light emitting layer is at least partially doped with p-type or n-type. 4. The multi-wavelength semiconductor laser device according to claim 1, comprising: 5. The multi-wavelength semiconductor laser device according to claim 3, wherein a band gap of the barrier layer is larger than a band gap of the intermediate layer near a boundary with the light emitting layer. 6. The multi-wavelength semiconductor laser device according to claim 1, wherein a band gap of said barrier layer is larger than a band gap of said cladding layer. 7. When a current having an oscillation threshold value is injected into the device, a light-emitting layer having a smaller ground gap has a lower ground level than a light-emitting layer having a larger band gap. 3. The multi-wavelength semiconductor laser device according to claim 1, wherein the gain of the emission wavelength corresponding to the band gap is positive. 8. A wavelength corresponding to at least one of the band gaps in the plurality of light emitting layers by injecting current into the multi-wavelength semiconductor laser device according to claim 1 or 2 in a forward direction and controlling the amount of current. 3. The method for driving a multi-wavelength semiconductor laser device according to claim 1, wherein the laser light is oscillated. 9. A multi-wavelength semiconductor laser device according to claim 1, wherein a current slightly smaller than a threshold current value for laser oscillation is injected, and laser oscillation is performed from an external light source through one end face of the device. 3. The multi-wavelength semiconductor laser device according to claim 1, wherein light having a wavelength near the wavelength of the light is incident, and light having the same wavelength as the incident light is extracted from the other end face. Method. 10. A multi-wavelength semiconductor laser device according to claim 1, wherein a current slightly smaller than a threshold current value for laser oscillation is injected, and laser oscillation is performed from an external light source through one end face of the device. 3. The multi-wavelength semiconductor laser device according to claim 1, wherein light having a wavelength near the wavelength of the light is incident, and light having a wavelength different from the incident light is emitted from an end face of the device. Method.