JPH04306896A - semiconductor laser light source - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser light source that generates light of a single wavelength and is used in optical fiber communications, optical measurement, and the like.

0002

[Prior Art] In fields such as optical fiber communication and optical measurement, compact semiconductor laser light sources that generate light of a single wavelength are used. ) is essential in applied fields. Conventionally, distributed feedback lasers (hereinafter referred to as DFB lasers) and distributed reflection lasers (hereinafter referred to as DBR lasers) with built-in diffraction gratings have been actively studied in order to improve the single wavelength property of semiconductor lasers. ing. For line width reduction, DFB laser and DBR
In lasers, it is important to sharpen the resonant characteristics of the resonator by increasing the optical coupling constant by increasing the resonator length of the semiconductor laser and increasing the depth of the diffraction grating (for example, Electronics Letters, vol.
．． 25, No. 11, pp. 709-710, 1989
, F. KANO et al., “Spectral linewi
dth reduction (580kHz) in
structure-optimized 1.5μ
mButt-jointed distributed
Bragg reflector laser”.：
Electronics Letters, vol. 2
5, pp. 990-992, 1989, Y. Kotak
i etc.,”Tunable narrow-linewi
dth and high-powerλ/4-shi
fted DFB laser”.：Electron
ics Letters, vol. 25, pp. 356
-357, 1989, S. Takano et al., “Sub-
MHz spectral linewidth in
1.5μm separate-confineme
nt-heterostructure (SCH) q
).

0003

[Problem to be solved by the invention] When the coupling constant of the diffraction grating is increased in order to sharpen the resonant characteristics of the resonator in a normal DFB laser, the optical power is significantly concentrated near the center of the laser resonator. . Therefore, more stimulated emission of light occurs in the central portion of the resonator than in other portions. Due to significant stimulated emission in the central portion of the resonator, the carriers present in the laser resonator decrease in the central portion of the resonator, and the gain in the central portion of the resonator decreases. Through this process, the equivalent loss felt by the light inside the laser resonator increases, deteriorating the resonance characteristics. Due to this effect, the number of carriers within the resonator increases in order to maintain the gain of the entire resonator necessary for oscillating the laser, causing a problem of deterioration of the single wavelength property of the laser. Also,
For the same reason, there was also the problem that the line width increased. This problem occurs in the λ/4-shifted DFB laser, which has a structure in which the phase of the diffraction grating is shifted by π/2 inside the resonator, which has good single wavelength property, compared to the conventional uniform grating-type DFB laser. was more prominent. Also,
DBR lasers have a structure that increases the coupling constant of the distributed reflector and sharpens the resonant characteristics of the resonator. It has been difficult to reduce the line width due to the deterioration of resonance characteristics as the loss increases.

[0004] In order to improve the single wavelength property of the generated light, D
A part without a diffraction grating is formed in the central part of the resonator of an FB structure semiconductor laser, and the light propagation constant is changed between the part with and without the diffraction grating, and the length of the part without the diffraction grating in the central part of the resonator is changed. The product of L and propagation constant β1 is (N+1/2
)・π (N is a positive integer) or the propagation constant β2 of the light in the part with the diffraction grating and the propagation constant β1 in the part without it
The structure was manufactured so that the product of the difference Δβ between the two and the length L was π/2 (for example, Electronics Le
tters, vol. 20, No. 10, pp. 391
-393, 1984, F. Koyama et al., “1.5μ
m phase adjusted active d
distributed reflector lase
r for complete dynamic si
However, although the concentration of optical power at the center of the resonator is alleviated in the device with this structure, it is difficult to control the light propagation constant and the length of the part without the diffraction grating in the center. It was difficult to control the length L.

In addition, when sweeping the oscillation wavelength in a DFB structure laser, in a semiconductor laser in which a diffraction grating is formed over the entire length of the resonator, the injection current is spread over the resonator length by dividing the current injection electrode. This was realized by making it non-uniform in the direction (for example, the above-mentioned Y. Ko
taki et al.), there was a problem in that the line width fluctuated because the optical power within the resonator was not flat during the oscillation wavelength sweep.

Therefore, an object of the present invention is to flatten the optical power distribution within the resonator and reduce the intra-cavity loss due to waveguide coupling loss within the resonator, thereby achieving a single wavelength. An object of the present invention is to provide a semiconductor laser light source that has excellent characteristics and a narrow line width. Another object of the present invention is to make it possible to sweep the oscillation wavelength in a constant narrow linewidth state by nonuniform current injection in the resonator length direction by dividing the current injection amount into a region where the optical power is flat. It is an object of the present invention to provide a semiconductor laser light source that has the following characteristics.

[0007]

[Means for Solving the Problems] In order to achieve the above object, the semiconductor laser light source according to the present invention has an active layer over the entire length of the resonator of the semiconductor laser light source that outputs oscillation light from both ends. The light propagation constant is constant, and the semiconductor laser light source has a diffraction grating only near both ends of the resonator.

[0008]

[Function] In the present invention, by providing a region without a diffraction grating in the center of the resonator and making the light propagation constant constant over the length of the resonator, the central part of the resonator where optical power concentration tends to occur is improved. A structure with a flat optical power distribution can be obtained with good controllability. Further, by providing a diffraction grating at both ends of the resonator, it is possible to improve the resonance characteristics of the resonator. As a result, deterioration of single wavelength property due to optical power concentration inside the resonator during high current injection can be prevented, and a semiconductor laser light source with a narrow linewidth can be realized. Furthermore, by dividing the injected current electrode into a part with a diffraction grating and a part without a diffraction grating in the resonator length direction, and creating a structure that can make the injected current non-uniform in the resonator length direction, a constant narrow linewidth state can be achieved. It is possible to realize a semiconductor laser light source that can sweep the oscillation wavelength while maintaining the oscillation wavelength. Here, the propagation constant of light is determined by the refractive index (equivalent refractive index) that the light perceives in each part of the resonator, and in a part of the resonator, it is expressed as the average value of the equivalent refractive index that varies periodically due to the diffraction grating. As a result, in order to keep the propagation constant constant within the resonator, the thickness of the optical waveguide layer forming the diffraction grating can be adjusted periodically between the thickness of the region without the diffraction grating and the thickness of the region with the diffraction grating. This is achieved by making the varying thickness equal to the average thickness. In addition, in the following examples,
By devising this structure, the propagation constant within the resonator is kept constant.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained in detail below with reference to the drawings. (Embodiment 1) FIG. 1 is a partially cutaway perspective view showing the structure of an embodiment of a semiconductor laser light source according to the present invention. In this embodiment, a buried semiconductor laser structure will be described as an example. In the figure, 1 is a + side electrode, 2 is an n-InP buried layer, 3 is a p-InP buried layer, 4 is an n-InP substrate, 5 is a − electrode, and 6 is a p-InG
7 is an InGaAsP active layer, 8 is an InGaAsP optical waveguide layer, 9 is a p-InP cladding layer, and 10 is a diffraction grating. In addition, region A is the diffraction grating 10
, region B is a region where no diffraction grating 10 is present and only active layer 7 is present, and region C is a region where diffraction grating 10 is present. This semiconductor laser light source has an active layer 7 in the length direction of the resonator, which has the effect of giving gain to the light generated internally and causing oscillation. A diffraction grating 10 is provided in the optical waveguide 8 close to the active layer 7 at both ends of the resonator, and the wavelength of light to be caused to oscillate is selected by the diffraction grating 10.

FIG. 2 is a diagram showing an example of calculation of the optical power distribution inside the resonator of the semiconductor laser light source according to the present invention. In this calculation, the resonator length is 1.5 mm, and the diffraction grating length at both ends is 3 mm.
00 μm and a light source with a coupling constant of 50 cm −1 . In the figure, curve 1 represents the optical power distribution of the present light source, and curve 2 has the same coupling constant as the diffraction grating of the light source according to the present invention, and the resonator length is the same as the active layer with the diffraction grating in the present light source. DFB structure laser (hereinafter referred to as DF
BI) represents the optical power distribution. DFB
It can be seen that the optical power, which was concentrated at the center of the resonator at −I, was flattened with this light source. Furthermore, with this light source,
The threshold gain required for oscillation could be reduced to 2/3 of the value required for DFB-I to oscillate. Furthermore, with the light source according to the present invention, when the injection current to the light source is 300 mA and the output optical power is 10 mW, 50 kHz
The line width of z was obtained.

(Embodiment 2) FIG. 3 is a partially cutaway perspective view showing the structure of another embodiment of the semiconductor laser light source according to the present invention, and the same parts as in the previous figures are given the same reference numerals. In this example, in the semiconductor laser light source of Example 1,
It is characterized in that it is divided into three regions, ie, region A, region B, and region C, by providing an electrical isolation groove 11 in the positive electrode 1 for drive injection current. In this example,
Electrical isolation is achieved by forming isolation grooves 11 and removing the cap layer 6 for facilitating current injection. By changing the amount of current injected into the electrode located above the active layer part without a diffraction grating at the center of the resonator using a separate electrode, the injected carriers inside the resonator can be maintained while keeping the optical power distribution in the center of the resonator flat. When the distribution can be changed and the output power is kept constant when measured,
The line width is narrow, and it is now possible to sweep the oscillation wavelength at a nearly constant value. When the output power was kept constant at 10 mW, an oscillation wavelength of 2 nm could be swept with a line width of 50 kHz.

(Embodiment 3) FIG. 4 is a cross-sectional view showing the structure of still another embodiment of the semiconductor laser light source according to the present invention, and the same parts as in the previous figures are given the same reference numerals. In the figure, the present light source has an active layer 7 over the entire length of the resonator, which has the effect of giving gain to the light generated inside and causing oscillation. A diffraction grating 10 is provided in an optical waveguide 8 close to the active layer 7 at both ends of the resonator, and the wavelength of light to be caused to oscillate is selected by the diffraction grating 10. The phase of the present diffraction grating 10 is shifted by π at a point 13 within the diffraction grating 10.

FIG. 5 is a diagram showing an example of calculation of the optical power distribution inside the resonator of the present light source. In this calculation, the resonator length is 1.
1mm, the length of the diffraction grating at both ends is 300μm, and the coupling constant is 5
0 cm-1, the point 13 that changes the phase is each diffraction grating 10
This was done for the light source located in the center of the area. Conventional D
It can be seen that in a light source that is an FB type semiconductor laser and has a large coupling constant of a diffraction grating, the optical power that was concentrated inside the resonator can be flattened at the center of the resonator in this light source. Furthermore, in this light source, the threshold gain required for oscillation is the same as that for a uniform grating DFB laser (cavity length 600 μm, coupling constant 50 cm-1, hereinafter referred to as a normal DFB laser). It was possible to reduce the value to 2/3. Furthermore, with the light source according to the present invention, when the injection current to the light source is 300 mA and the output optical power is 10 mW, 50 k
A linewidth of Hz was obtained.

(Embodiment 4) FIG. 6 is a sectional view showing the structure of another embodiment of the semiconductor laser light source according to the present invention.
The same parts as in the previous figures are given the same reference numerals. In the figure, a diffraction grating 10 and a diffraction grating 14 are formed near both ends of the resonator of an optical waveguide layer 8 adjacent to an active layer 7 that exists over the entire length of the resonator, and the coupling constant is compared to that of the diffraction grating 10. , the coupling constant of the diffraction grating 14 is large. Further, the phases of diffraction grating 10 and diffraction grating 14 are shifted by π at point 13 within the diffraction grating.

FIG. 7 is a diagram showing an example of calculation of the optical power distribution inside the resonator of the light source according to this embodiment. In this calculation, the length of the diffraction grating 10 is 300 μm, and the coupling constant is 50 cm-
1. The length of the diffraction grating 14 is 300 μm, and the coupling constant is 10.
0 cm-1, and the point 13 at which the phase is changed is located at the center of each diffraction grating 10 and diffraction grating 14. It can be seen that a light source with a flat optical power distribution over a wide range in the center of the resonator was achieved. Furthermore, by making the optical powers at both ends of the cavity unequal, the diffraction grating 10 with a small coupling constant
We were able to increase the output light in the direction. In addition, in this light source, the threshold gain required to reach oscillation is
Compared to the oscillation threshold gain of an FB laser, the gain could be reduced to 1/2 or less. With the light source of this structure, a line width of 30 kHz was observed when the injection current was 300 mA and the output optical power was 20 mW.

(Embodiment 5) FIG. 8 is a sectional view showing the structure of another embodiment of the semiconductor laser light source according to the present invention.
The same parts as in the previous figures are given the same reference numerals. In the figure, a diffraction grating 10 is formed near both ends of the resonator of an optical waveguide layer 8 adjacent to an active layer 7 that extends over the entire length of the resonator, and the phase of the diffraction grating 10 is determined within the diffraction grating 10. is shifted by π/2 at point 15. Also,
Antireflection films 16 are formed on both end faces of the resonator.

FIG. 9 shows the calculation results of the optical power distribution within the resonator of the light source according to this embodiment. The length of the diffraction grating 10 is 30
0 μm, the coupling constant was 50 cm −1 , and the point 15 at which the phase was changed was located at the center of each diffraction grating 10 and diffraction grating 14 . We were able to flatten the optical power distribution over a wide range at the center of the cavity. In the light source of this example, the threshold gain required for oscillation could be reduced to ⅓ or less of the oscillation threshold gain of a normal DFB laser. With the light source of this structure, when the injection current is 300 mA and the output optical power is 10 mW, the frequency is 20 kHz.
line width was observed.

(Embodiment 6) FIG. 10 is a sectional view showing the structure of another embodiment of the semiconductor laser light source according to the present invention, and the same parts as in the previous figures are given the same reference numerals. In the figure, a diffraction grating 10 and a diffraction grating 14 are formed near both ends of the resonator of an optical waveguide layer 8 adjacent to an active layer 7 that exists over the entire length of the resonator, and the coupling constant of the diffraction grating 10 is It is smaller than the coupling constant of the diffraction grating 14. Furthermore, each diffraction grating 10, diffraction grating 1
The phase of 4 is shifted by π/2 to the left and right of point 15 in the diffraction grating. Antireflection films 16 are formed on both end faces of the resonator of this light source.

FIG. 11 shows the calculation results of the optical power distribution within the resonator of the light source according to this embodiment. The length of the diffraction grating 10 of the light source is 300 μm and the coupling constant is 50 cm −1 , and the length of the diffraction grating 14 is 300 μm and the coupling constant is 100 cm −1
The point 15 for changing the phase was set at the center of each diffraction grating 10 and diffraction grating 14. We were able to flatten the optical power distribution over a wide range at the center of the cavity. Furthermore, by making the output optical powers from both ends of the resonator unequal, it was possible to increase the output optical power from the diffraction grating 10 side. The threshold gain of the light source in this example is the normal DF
The gain was reduced to 1/4 or less of the oscillation threshold gain of the B laser. With the light source of this structure, a linewidth of 15 kHz was observed when the injection current was 300 mA and the output optical power from the direction of the diffraction grating 10 was 20 mW.

(Embodiment 7) FIG. 12 is a sectional view showing the structure of another embodiment of the semiconductor laser light source according to the present invention, and the same parts as in the previous figures are given the same reference numerals. In the figure, a diffraction grating 10 and a diffraction grating 17 are formed near both ends of the resonator of an optical waveguide layer 8 adjacent to an active layer 7 that extends over the entire length of the resonator, and the phase of the diffraction grating 17 is , are shifted by π from the phase of the diffraction grating 10. Antireflection films 16 are formed on both end faces of this light source.

FIG. 13 shows the calculation results of the optical power distribution within the resonator of the light source according to this embodiment. The length of the diffraction grating 10 and the diffraction grating 17 of the light source was each 300 μm, and the coupling constant was 50 cm −1 . We were able to flatten the optical power distribution over a wide range at the center of the cavity. The threshold gain of the light source of this example is 1 of the oscillation threshold gain of a normal DFB laser.
/3 or less. Injected current of 300 with the light source of this structure
mA and an output optical power of 10 mW, a linewidth of 20 kHz was observed.

(Embodiment 8) FIG. 14 is a sectional view showing the structure of another embodiment of the semiconductor laser light source according to the present invention, and the same parts as in the previous figures are given the same reference numerals. In the figure, a diffraction grating 10 and a diffraction grating 18 are formed near both ends of the resonator of an optical waveguide layer 8 adjacent to an active layer 7 that extends over the entire length of the resonator, and the phase of the diffraction grating 18 is The phase of the diffraction grating 10 is shifted by π. Furthermore, the coupling constant of the diffraction grating 18 is
It is larger than the coupling constant of . Antireflection films 16 are formed on both end faces of the resonator of this light source.

FIG. 15 shows the calculation results of the optical power distribution within the resonator of the light source according to this embodiment. The length of the diffraction grating 10 of the light source is 300 μm and the coupling constant is 50 cm −1 , and the length of the diffraction grating 18 is 300 μm and the coupling constant is 100 cm −1
And so. We were able to flatten the optical power over a wide range in the center of the resonator. Furthermore, by making the optical power at both end faces of the resonator unequal, the output optical power from the diffraction grating 10 side could be increased. The threshold gain of the light source of this example is 1/4 of the oscillation threshold gain of a normal DFB laser.
Reduced to below. Injected current of 300mA with this structure light source
, a line width of 15 kHz was observed when the output optical power was 20 mW.

(Embodiment 9) FIG. 16 is a sectional view showing the structure of another embodiment of the semiconductor laser light source according to the present invention, and the same parts as those in the previous figures are given the same reference numerals. This example is characterized in that the semiconductor laser light sources of Examples 1 to 8 described above are separated into three regions by providing an electrical isolation groove 19 in the drive injection current electrode 1. In this embodiment, a portion of the cap layer 2 and the + electrode 1 to facilitate current injection are
Electrical isolation is achieved by forming and removing isolation grooves 19. This separation groove 19 is formed so as to be located at the boundary between the region where the diffraction grating is present and the region where there is no diffraction grating. The diffraction grating 10 and the diffraction grating 14 have any of the structures described in the explanations of the first to eighth embodiments. By changing the amount of current injected into the electrode located above the active layer part without a diffraction grating at the center of the resonator using the divided electrodes, carriers are injected inside the resonator while keeping the optical power distribution in the center of the resonator flat. The distribution can be changed, and when measured while keeping the output power constant, the linewidth is narrow and the oscillation wavelength can be swept at a nearly constant value. As an example, in a light source having a structure in which the + electrode 1 of the semiconductor laser light source explained in Example 3 is divided, when the output power is constant at 10 mW, 2
We were able to sweep the oscillation wavelength of nm.

In the embodiments of the present invention described above, a buried structure has been described, but it goes without saying that the same effect can be obtained by using other structures, such as a ridge structure. Further, the shape and structure of the active layer are not particularly limited, but it goes without saying that the effects of the present invention are effective regardless of whether the active layer has a bulk structure or a multiple quantum well structure.

[0026]

[Effects of the Invention] As explained above, according to the present invention,
By providing a region without a diffraction grating in the center of the resonator and making the light propagation constant constant over the length of the resonator, the optical power distribution is flattened in the center of the resonator where optical power concentration tends to occur. A structure with good controllability can be obtained. Further, by providing a diffraction grating at both ends of the resonator, the resonance characteristics of the resonator can be improved. Furthermore, by flattening the optical power distribution within the resonator and reducing the threshold gain required for oscillation, it is possible to realize a semiconductor laser light source with a narrow linewidth without deterioration of single wavelength property during high current injection. In addition, by dividing the current injection electrode into a part with a diffraction grating and a part without a diffraction grating, and changing the amount of current injected depending on the position of the divided electrode, oscillation can be achieved while keeping the line width at a constant narrow value. Extremely excellent effects such as the realization of a semiconductor laser light source that can sweep the wavelength can be obtained.

[Brief explanation of drawings]

FIG. 1 is a partially cutaway perspective view showing the configuration of an embodiment of a semiconductor laser light source according to the present invention.

FIG. 2 is a diagram illustrating an example of calculation of the optical power distribution within the resonator of the semiconductor laser light source according to the embodiment of FIG. 1;

FIG. 3 is a partially cutaway perspective view showing the structure of another embodiment of the semiconductor laser light source according to the present invention.

FIG. 4 is a cross-sectional view of a main part showing a configuration according to still another embodiment of the semiconductor laser light source according to the present invention.

FIG. 5 is a diagram illustrating an example of calculation of the optical power distribution within the resonator of the semiconductor laser light source according to the embodiment of FIG. 4;

FIG. 6 is a sectional view of a main part showing the structure of another embodiment of the semiconductor laser light source according to the present invention.

7 is a diagram illustrating an example of calculation of the optical power distribution within the resonator of the semiconductor laser light source according to the embodiment of FIG. 6; FIG.

FIG. 8 is a sectional view of a main part showing the structure of another embodiment of the semiconductor laser light source according to the present invention.

9 is a diagram illustrating an example of calculation of the optical power distribution within the resonator of the semiconductor laser light source according to the embodiment of FIG. 8; FIG.

FIG. 10 is a sectional view of a main part showing the structure of another embodiment of the semiconductor laser light source according to the present invention.

11 is a diagram illustrating an example of calculation of the optical power distribution within the resonator of the semiconductor laser light source according to the embodiment of FIG. 10; FIG.

FIG. 12 is a sectional view of a main part showing the structure of another embodiment of the semiconductor laser light source according to the present invention.

13 is a diagram illustrating an example of calculation of the optical power distribution within the resonator of the semiconductor laser light source according to the embodiment of FIG. 12; FIG.

FIG. 14 is a sectional view of a main part showing the structure of another embodiment of the semiconductor laser light source according to the present invention.

15 is a diagram illustrating an example of calculation of the optical power distribution within the resonator of the semiconductor laser light source according to the embodiment of FIG. 14; FIG.

FIG. 16 is a sectional view of a main part showing the structure of another embodiment of the semiconductor laser light source according to the present invention.

[Explanation of symbols]

1 + side electrode 2 n-InP buried layer 3 p-InP buried layer 4 n-InP substrate 5 - electrode 6 p-InGaAsP cap layer 7 I
nGaAsP active layer 8 InGaAsP optical waveguide layer 9 p-InP cladding layer 10 Diffraction grating 11 Electrical isolation groove 12 Diffraction grating 13 Point 14 Diffraction grating 15 Point 16 Antireflection film 17 Diffraction grating 18 Diffraction grating 19 Electrical isolation groove

Claims (8)

[Claims] 1. A semiconductor laser light source that outputs oscillation light from both ends, which has an active layer over the entire length of the resonator of the semiconductor laser light source, has a constant light propagation constant, and has an active layer near both ends of the resonator of the semiconductor laser light source. A semiconductor laser light source characterized by having only a diffraction grating. 2. The semiconductor laser light source according to claim 1, wherein the phase of the diffraction grating changes by π within the diffraction grating. 3. The semiconductor laser light source according to claim 1, wherein the phase of the diffraction grating changes by π/2 within the diffraction grating. 4. The semiconductor laser light source according to claim 1, wherein the phases of the diffraction gratings at both ends of the resonator differ by π. 5. The semiconductor laser light source according to claim 2, wherein the coupling constants of the diffraction gratings at both ends of the resonator are different. 6. The semiconductor laser light source according to claim 3, wherein the coupling constants of the diffraction gratings at both ends of the resonator are different. 7. The semiconductor laser light source according to claim 4, wherein the coupling constants of the diffraction gratings at both ends of the resonator are different. 8. The semiconductor laser light source according to claim 1, wherein the drive current injection electrode is divided into a portion without a diffraction grating and a portion with a diffraction grating.