CA1253945A - Semiconductor laser - Google Patents
Semiconductor laserInfo
- Publication number
- CA1253945A CA1253945A CA000486983A CA486983A CA1253945A CA 1253945 A CA1253945 A CA 1253945A CA 000486983 A CA000486983 A CA 000486983A CA 486983 A CA486983 A CA 486983A CA 1253945 A CA1253945 A CA 1253945A
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- Canada
- Prior art keywords
- layer
- semiconductor laser
- active layer
- light
- light absorbing
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
- H01S5/22—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure
- H01S5/223—Buried stripe structure
- H01S5/2231—Buried stripe structure with inner confining structure only between the active layer and the upper electrode
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- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Optics & Photonics (AREA)
- Geometry (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Plasma & Fusion (AREA)
- Semiconductor Lasers (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A semiconductor laser is disclosed, which is formed such that on a substrate there are in turn formed a first cladding layer, an active layer, a second cladding layer and a light absorbing layer for limiting a current path and for absorbing a light oscillated out from the active layer. In this case, the light absorbing layer is provided with a removed-away portion of stripe-shape for forming the current path, and in which the width W of the removed-away portion is selected in a range from 1 to 4 µm, the thickness d1 of the active layer is selected so as to satisfy the condition of d1 ? 500 .ANG. and the distance d2 between the active layer and the light absorbing layer is selected in a range from 0.2 to 0.7 µm, respectively.
A semiconductor laser is disclosed, which is formed such that on a substrate there are in turn formed a first cladding layer, an active layer, a second cladding layer and a light absorbing layer for limiting a current path and for absorbing a light oscillated out from the active layer. In this case, the light absorbing layer is provided with a removed-away portion of stripe-shape for forming the current path, and in which the width W of the removed-away portion is selected in a range from 1 to 4 µm, the thickness d1 of the active layer is selected so as to satisfy the condition of d1 ? 500 .ANG. and the distance d2 between the active layer and the light absorbing layer is selected in a range from 0.2 to 0.7 µm, respectively.
Description
lZ539~
Field of the Invention T~e pres~nt invention relates to a semiconductor laser.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematically enlarged cross-sectional view of a prior art semiconductor laser;
Fig. 2 is a graph indicating a noise level relative to a forward current of a semiconductor laser and a power thereof;
Fig. 3 is a schematically enlarged cross-sectional view showing an embodiment of a semiconductor laser according to the present invention;
Fig. 4 is a graph indicating a relation between a threshold value current density and a width of a removed-away portion formed in a light absorbing layer; and Fig. 5 is a graph indicating a distribution of a light coming from an active layer.
DescriPtion of the Prior Art ¦ Generally, prior art semiconductor lasers are roughly ¦ classified into the refractive index-guiding type one and the gain-guiding type one in accordance with its light and carrier confinement mechanism in the longitudinal mode thereof.
As a semiconductor laser of refractive index-guiding type, there is proposed such one as, for example, shown in ~ig. 1. Referring to Fig. 1, this semiconductor laser .
- _ 2 -lZ539~5 is forme~ such that on a GaAs substrate 1 of, for example, N type there are formed a first cladding layer 2 of N
type AQzGal zAs, an active layer 3 of P or N type AQxGal xAs, a se~ond cladding layer 4 of P type AQzGal zAs~ a light absorbing layer 5 of N type AQyGal yAs buried into the second cladding layer 4 and a capping layer 6 of P type with high impurity concentration. The light absorbing layer S has a removed-away portion Sa of stripe-shape having a width W which is provided by removing away the light absorbing layer 5 at its, for example, central por- -tion in the direction perpendicular to the sheet of paper of Fig. 1. The composition of this light absorbing layer S is selected such that its forbidden band width is smaller than that of the active layer 3 and its refractive ir.dex for the light oscillated out from the light emission region of the active layer 3 is higher than that of the light emission region, namely, the active layer 3. That is, in the compositions of the active layer 3 and the light absorbing layer 5, they are selected so as to establish the condition of x > y.
In Fig. l, reference numerals 7 and 8 designate electrodes which are deposited on the capping layer 6 and the substrate l in ohmic-contact therewith, respec-tively.
In this structure of the semiconductor laser, if a predete_mined forward voltage is applied between the electrodes 7 and 8, a light emission is carried out in the active layer 3 of which the carrier and light are confined by the first and sec~nd cladding layers 2 and . In this case, a distance d2 between the active layer 3 and the light absorbing layer 5 is selected to be a distance through which the light oscillated out.f~om the active layer 3 is able to reach the light absorbing layer 5, for example, in a range from 0.3 to 0.4 ~m. If the distance d2 is selected as described above, the light introduced to the light absorbing layer 5 from the active layer 3 is absorbed by the light absorbing layer 5, whereby between the portion below this light absorb-ing layer 5 and the portion corresponding to the removed-away portion 5a of stripe-shape at the center of the light absorbing layer 5 in which the light is hardly absorbed, there is formed a difference of effective refractive index, namely, a difference an of built-in refractive index in the lateral direction. In this case, the width W of the removed-away portion 5a of stripe-shape of the light absorbing layer 5 is gelected in a range from 5 to 8 ~ so that also on the basi of the selection of the distance d2, a difference ansnl - n2 between a refractive index nl of the portion (hereinafter referred to as the central postion) of the active layer 3 opposing to the removed-away portion 5a of the light absor~ing layer 5 and refractive index n2 of the portions of the active layer 3 at both sides of the central portion can ~e presented as, for example, +10 2 to ~10 3.
As described a~ove, in the active layer 3, the confinement effect for the light oscillation in the lateral direction ~2539~5 is produced in the central portion opposing to the removed-away portion 5a, in which the light emission region is restricted.
As the semiconductor laser of refractive index-guiding type, it is not limited to the structure in which the light absorbing layer 5 is buried into the second cladding layer 4 but the similar operation to the above can be carried out for such a semiconductor laser is pos~ible, which is known as a C.S.P (channeled substrate planar) disclosed iIl a publicated document of Japanese patent application unexamined No. 143787/1977 and in which the substrate 1, for example, is used as the light absorbing layer.
As mentioned above, although the semiconductor laser of refractive index-guiding type is constructed as shown in Fig. 1, the semiconductor laser of this structure can be modified into the gain-guiding type semiconductor laser, too. That is, in the ~tructure of Fig. 1, since the light absorbing layer 5 is selected to be a conductive type which is different from that of the second cladding layer 4, there is then an effect that a thyristor struc-ture of P-N-P-N is formed in the portion in which the light absorbing layer ~ exists to thereby limit the current path. As a result, the semiconductor laser of gain-guiding type can be formed by using this light absorbing layer 5 as the current limit region. That is, in a qualitative standpoint, a strong current concen-tration in the stripe portion is produced by reducing ~253945 the width w of the removed-away portion Sa as compared with that of the semiconductor laser of the afore-mentioned refractive index-guiding type or the thickness dl of the active layer 3 is increased or the distance d2 between the active layer 3 and the current limit region (that is, the light absorbing layer 5) is increased so that the effect of absorbing the light from the active layer 3 by the light absorbing layer 5 can be reduced. Thus, a negative refractive index changing amount -Qne provided by the injected carrier becomes dominant as compared with the changing amount ~n of the built-in refractive index so that the difference ~n (where ~n = ~n - ~ne) between the refractive index of the central portion of the active layer 3 and the refractive index of the both side portions lies in a range from -10 2 to -10 3. Thus the semiconductor laser of the gain-guiding type can be made.
However, both of the semiconductor laser of refrac-tive index-guiding type and the semiconductor laser of gain-guiding type have merits and demerits, respectively.
Accordingly, when each of them is used as the writing ar.d/or reading light source of, for example, the video disc, each proposes a practical problem.
More speclfically, in the semiconductor laser of the refractive index-guiding type, since its longitudinal mode is the single mode, when it is used as the writing and/or reading light source in, for example, an optical video disc and so on, there is a defect that a mode hopping noise is caused by a returned light. Fig. 2 is .
lZ539~5 a graph showing measured results of a mode hopping noise caused by a forward current relative to the semiconductor laser. In Fig. 2, a curve 21 indicates measured results of noise produced in the semiconductor laser of refractive index-guiding type and as will be clear therefrom, the mode hopping noise is produced. On the other hand, in the semiconductor laser of refractive index-guiding type, a so-called beam waist position exists near the light end face of the light emission region. There is then an advantage that in practical use, the focal position can be set easily. Further, since a far distant image on the cross-section in the direction parallel to the junction or a so-called far field pattern is symmetrical with each other in the right and left direction, there is then an advantage that similarly in, for example, the practical use, it is easy to obtain a reading and/or writing light of spot-shape having a small distortion. As compared therewith, in the semiconductor laser of gain-guiding type, the beam waist position exists at the inside of about 20 ~m from the light end face of the light emission region, the far field pattern is frequently asymmetrical with each other in the right and left direction, and the astigmatism is large whereby the spot distortion becomes relatively large. As will be clear from a noise characteristic shown by a curve 22 in Fig. 2, the noise level of the semiconductor laser of the gain-guiding type is high as compared with the noise level shown by the curve 21 of the semiconductor laser of refractive ~3,,, ~ - 7 -~2539~5 index-guiding type. ~owever, since the longitudinal mode thereof is the multi-mode, there is then an advan-tage that no mode hopping noise is caused by ~he returned light.
OBJECTS AND SUMM~RY OF THE INVENTION
Accordingly, it is an object of this invention to provide a semiconductor device having a so-called interme-diate characteristic between those of the refractive index-guiding type semiconductor laser and the gain- -guiding type semiconductor laser ~nd which can utilize the advantages of the both and which can complement the defects thereof so that a noise can be reduced.
It is another object of this invention to provide a semiconductor device which is suitable as a writing and/or reading light source in, for example, an optical video disc or digital audio disc.
According to one aspect of the present invention, there is provided a semiconductor laser comprising a substrate on which a first cladding layer, an active layer, a second cladding layer and a light absorbing layer for limiting a current path and for absorbing a light oscillated out from said active layer are se~uentially formed in contact to one another, wherein said light absorbing layer is provided with a removed-away portion of stripe-shape for forming said current path, wherein a width W of said removed-away portion is selected in a range from 1 to 4 ~m, a thickness dl of ~2S39~5 said active layer is selected so as to satisfy the condition of d1 > 500A and a distance d2 between said active layer and said light absorbing layer is selected in a range from 0.2 to 0.7 ~m.
These and other objects, features and advantages of the semiconductor laser according to the present inven-tion will become apparent from the following detailed description of the preferred embodiment taken in conjuction with the.accompanying drawings! throughout which like reference numerals designate the same elements and parts.
"~
~ ^ .
: - 8a -l~S39~5 DESCRIPTION OF THE PRE~ERRED EMBODIMENT
Now, an embodiment of a semiconductor laser according to the present invention will hereinafter be described with reference to Fig. 3 and the followings. In this embodiment, similarly to the semiconductor laser described in connection with Fig. 1, on a GaAs substrate 31 of a conductive type, for example, N type, there are sequentially formed a first cladding layer 32 of N type made of AQzGal zAs, an active layer 33 of P type or N type made of AQxGal xAs, a second cladding layer 34 made of A~zGal zAs with the conductive type different from that of the first cladding layer 32 or P type, a light absorbing layer 35 made of A~yGal yAs having an effect for limiting a current path and which can absorb the light coming from the active layer 33 and a capping layer 36 of high impurity concen-tration and of conductive type P same as that of the second cladding layer 34.
The light absorbing layer 35 is of a conductive type different from the second cladding layer 34, for example, N type in this embodiment and in order that the light absorbing layer 35 is opposed through the second cladding layer 34 to the active layer 33 with a distance d2, the light absorbing layer 35 is buried into, for example, the second cladding layer 34. Further, the light absorb-ing layer 35 has at its, for example, central portion aremoved-away portion 35a of stripe-shape with a predeter-mined width and which is extended in the direction perpendicular to the sheet of paper of Fig. 3.
The compositions of the first and second cladding layers 32 and 34 are selected to satisfy the condition ...
_ g _ 125~5 of z > x so that the forbidden band widths thereof become larger than that of the active layer 33. Further, the composition of the light absorbing layer 35 is selected such that the forbidden band width thereof becomes smaller than that of the active layer 33 and that the refractive inde~ thereof with respect to the light oscillated from the light emission region of the active layer 33 becomes higher than that of the light emussion region, or the active layer 33. In other words, in the compositions of the active layer 33 and the light absorb-ing layer 35, their compositions are selected so as to satisfy the condition of x > y. Electrodes 37 and 38 are respectively deposited on the capping layer 36 and the substrate 31 in ohmic-contact therewith.
In this structure, the width W of the removed-away portion 35a of the light absorbing layer 35 is selected in a range from 1 to 4 1~m, preferably 2 to 4 ~m. The thickness dl of the active layer 33 is selected so as to satisfy the condition of dl > 500A, preferably 1500A > d1 > 700A. Further, the distance d2 between the active layer 33 and the light absorbing layer 35 is selected in a range from 0.2 to 0.7 ~m, preferably 0.3 to 0.5 ~m.
In the semiconductor laser of this structure, a noise characteristic relative to a forward current I
flowing between the both electrodes 37 and 38 when the forward voltage is applied between the electrodes 37 and 38 becomes as shown by a curve 23 in Fig. 2. In Fig. 2, a curve 24 indicates a relation between the current I and the power P.
~ ..
~;~S~5 As will be clear from the comparison of the curve 23 with the curve 22, the noise level of the semiconductor laser of the pxesent invention is reduced as compared with that of the gain-gulding type semiconductor laser.
That is, if now the semiconductor laser of the present invention is operated by the power of, for example, 5 mW
by which a current Il is flowed, the difference between the curves 22 and 23 becomes about 5 dBm. In other words, when the semiconductor laser is operated at the power of 5 mW, according to this invention, it is possible to reduce the noise by about 5 dBm as compared with the semiconductor laser of gain-guiding type. Furthermore, in the semiconductor laser of the present invention, as will be clear from the curve 23, there is produced no mode hopping noise which is seen just in the curve 21.
More particularly, according to the structure of the present invention, it may be considered that self-excited oscillation is produced by the light and carrier confine-ment which is intermediate between those of the prior art refractive index-guiding type and gain-guiding type ones. In other words it may be considered that the semiconductor laser of the present invention is operated in a range in which the afore-mentioned effective refractive index difference ~n is extremely small positive or negative value as compared with the semiconductor lasers of refractive index-guiding type and gain-guiding type.
Further, according to the semiconductor laser of the present invention, the symmetric property of the far field pattern is improved as compared with the prior art :., . .
~i~5;39'~5 semiconductor laser of gain-guiding type and the diameter of beam spot thereof is made small, thus the threshold current being reduced substantially. It can be understood that based upon these merits the semiconductor laser of this invention is operated as the intermediate semi-conductor laser between the refractive index-guiding type and the gain-guiding type ones.
The afore-described removal of mode hopping noise, low noise, reduction of threshold current and improve-ment of far field pattern can be achieved by particulariz-ing three of the width W of the removed-away portion 35a of the light absorbing layer 35, the distance d2 between the active layer 33 and the light absorbing layer 35 and the thickness d1 of the active layer 33 simultaneously.
More specifically, the width W is selected in a range from 1 to 4 ~m, preferably 2 to 4 ~m, the thickness d O
is selected so as to satisfy the condition of dl > 500A
o o (preferably 1500A > dl > 700A) and the distance d2 is selected in a range from 0.2 to 0.7 ~m (preferably in a range from 0.3 to 0.5 ~m).
These width W, the thickness d1 and the distance d2 will respectively be described below.
Now, the width W of the removed-away portion 35a of the light absorbing layer 35 will be considered.
Since the current concentration is weakened as the width W is increased as earlier noted, the refractive index change ¦~ne¦ by the injection of the carrier becomes small. If the width W is increased more, the change ~n of the built-in refractive index becomes dominant so that the semiconductor of this invention is opera~ed as 12539~S
the refractive index-guiding type one. In this case, the width W is one of the ~actors which determines the threshold current Ith for the oscillation of the semi-conductor laser. Since the threshold curren~ Ith is given as the product by a threshold current density Jth and the area of the light emission, when the width W is increased, the width of the light emission region, or its area is increased, whereby the threshold current Ith is increased. On the other hand, since the relation between the threshold current density and the width W
i5 shown in Fig. 4, when the width W becomes large, the threshold current density Jth becomes small. Therefore, the threshold current Ith can be suppressed to be small if the width W is in a certain range. Accordingly, in order to reduce the threshold current Ith, the selection of the width W becomes an important factor.
~ he thickness dl of the active layer 33 will be described. If the thickness dl thereof is increased, the area of the light emission area is increased and this leads to the increase of the threshold current Ith as mentioned before. Accordingly, in view of reducing the threshold current Ith, it is preferable that the thickness dl is not so large. Now, the permeation of light from the active layer 33 to the first and second cladding layers 32 and 34 will be considered. As shown in Fig. 5, when the thickness dl is small, the permeation of light presents a steep distribution as shown by a curve 51 in Fig. 5, while when the thickness dl is large, it presents a gentle distribution as shown by a curve 52 in Fig. 5. Accordingly, as the thickness dl is increased, l;~S3945 the light absorbing effect by the light absorbing layer 35 is decreased so that the semiconductor laser of this invention can be operated as the gain-guiding type one, or it becomes possible to avoid the mode hopping noise from being produced.
Further, also the distance d2 between the active layer 33 and the light absorbing layer 35 is one of the factors which determines the effect of the light absorb-ing layer 35 for absorbing the light from the active layer 33. If the distance d2 is small, the light absorbing e~fect is large so that the refractive index-guiding type characteristic becomes dominant in the semiconductor laser. If on the other hand the distance d2 is increased, the light absorbing effect is decreased or lost so that the semiconductor laser of this invention becomes the gain-guiding type one.
As describéd above, by the fact that the width W, the thickness dl and the distance d2 are particularized at the same time, the occurrence of the mode hopping noise can be avoided, the noise level can be reduced as compared with the semiconductor laser of the gain-guiding type, the threshold current Ith is low as compared with the threshold current Ithg of the gain-guiding type one and the far field pattern can be improved.
As set forth above, according to the semiconductor laser of the present invention, the mode hopping noise can be prevented from being produced and as compared with the semiconductor laser of the gain-guiding type, the noise and the threshold current can be reduced and the far field pattern can be improved. Accordingly, l;~S3945 when the semiconductor laser of this invention is used as a writing and/or reading light source of, for example, the video disc and the digital audio disc, there are brought a great deal of advantages that the resolution S and S/N (signal-to-noise) ratio can be improved and that the optical system can be simplified and so on.
The above description is given on a single prererred embodiment of the invention, but it will be apparent that many modifications and variations could be effected by one skilled in the art without departing from the spirits or scope of the novel concepts of the invention, so that the scope of the invention should be d~termined by the appended claim only.
.~
Field of the Invention T~e pres~nt invention relates to a semiconductor laser.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematically enlarged cross-sectional view of a prior art semiconductor laser;
Fig. 2 is a graph indicating a noise level relative to a forward current of a semiconductor laser and a power thereof;
Fig. 3 is a schematically enlarged cross-sectional view showing an embodiment of a semiconductor laser according to the present invention;
Fig. 4 is a graph indicating a relation between a threshold value current density and a width of a removed-away portion formed in a light absorbing layer; and Fig. 5 is a graph indicating a distribution of a light coming from an active layer.
DescriPtion of the Prior Art ¦ Generally, prior art semiconductor lasers are roughly ¦ classified into the refractive index-guiding type one and the gain-guiding type one in accordance with its light and carrier confinement mechanism in the longitudinal mode thereof.
As a semiconductor laser of refractive index-guiding type, there is proposed such one as, for example, shown in ~ig. 1. Referring to Fig. 1, this semiconductor laser .
- _ 2 -lZ539~5 is forme~ such that on a GaAs substrate 1 of, for example, N type there are formed a first cladding layer 2 of N
type AQzGal zAs, an active layer 3 of P or N type AQxGal xAs, a se~ond cladding layer 4 of P type AQzGal zAs~ a light absorbing layer 5 of N type AQyGal yAs buried into the second cladding layer 4 and a capping layer 6 of P type with high impurity concentration. The light absorbing layer S has a removed-away portion Sa of stripe-shape having a width W which is provided by removing away the light absorbing layer 5 at its, for example, central por- -tion in the direction perpendicular to the sheet of paper of Fig. 1. The composition of this light absorbing layer S is selected such that its forbidden band width is smaller than that of the active layer 3 and its refractive ir.dex for the light oscillated out from the light emission region of the active layer 3 is higher than that of the light emission region, namely, the active layer 3. That is, in the compositions of the active layer 3 and the light absorbing layer 5, they are selected so as to establish the condition of x > y.
In Fig. l, reference numerals 7 and 8 designate electrodes which are deposited on the capping layer 6 and the substrate l in ohmic-contact therewith, respec-tively.
In this structure of the semiconductor laser, if a predete_mined forward voltage is applied between the electrodes 7 and 8, a light emission is carried out in the active layer 3 of which the carrier and light are confined by the first and sec~nd cladding layers 2 and . In this case, a distance d2 between the active layer 3 and the light absorbing layer 5 is selected to be a distance through which the light oscillated out.f~om the active layer 3 is able to reach the light absorbing layer 5, for example, in a range from 0.3 to 0.4 ~m. If the distance d2 is selected as described above, the light introduced to the light absorbing layer 5 from the active layer 3 is absorbed by the light absorbing layer 5, whereby between the portion below this light absorb-ing layer 5 and the portion corresponding to the removed-away portion 5a of stripe-shape at the center of the light absorbing layer 5 in which the light is hardly absorbed, there is formed a difference of effective refractive index, namely, a difference an of built-in refractive index in the lateral direction. In this case, the width W of the removed-away portion 5a of stripe-shape of the light absorbing layer 5 is gelected in a range from 5 to 8 ~ so that also on the basi of the selection of the distance d2, a difference ansnl - n2 between a refractive index nl of the portion (hereinafter referred to as the central postion) of the active layer 3 opposing to the removed-away portion 5a of the light absor~ing layer 5 and refractive index n2 of the portions of the active layer 3 at both sides of the central portion can ~e presented as, for example, +10 2 to ~10 3.
As described a~ove, in the active layer 3, the confinement effect for the light oscillation in the lateral direction ~2539~5 is produced in the central portion opposing to the removed-away portion 5a, in which the light emission region is restricted.
As the semiconductor laser of refractive index-guiding type, it is not limited to the structure in which the light absorbing layer 5 is buried into the second cladding layer 4 but the similar operation to the above can be carried out for such a semiconductor laser is pos~ible, which is known as a C.S.P (channeled substrate planar) disclosed iIl a publicated document of Japanese patent application unexamined No. 143787/1977 and in which the substrate 1, for example, is used as the light absorbing layer.
As mentioned above, although the semiconductor laser of refractive index-guiding type is constructed as shown in Fig. 1, the semiconductor laser of this structure can be modified into the gain-guiding type semiconductor laser, too. That is, in the ~tructure of Fig. 1, since the light absorbing layer 5 is selected to be a conductive type which is different from that of the second cladding layer 4, there is then an effect that a thyristor struc-ture of P-N-P-N is formed in the portion in which the light absorbing layer ~ exists to thereby limit the current path. As a result, the semiconductor laser of gain-guiding type can be formed by using this light absorbing layer 5 as the current limit region. That is, in a qualitative standpoint, a strong current concen-tration in the stripe portion is produced by reducing ~253945 the width w of the removed-away portion Sa as compared with that of the semiconductor laser of the afore-mentioned refractive index-guiding type or the thickness dl of the active layer 3 is increased or the distance d2 between the active layer 3 and the current limit region (that is, the light absorbing layer 5) is increased so that the effect of absorbing the light from the active layer 3 by the light absorbing layer 5 can be reduced. Thus, a negative refractive index changing amount -Qne provided by the injected carrier becomes dominant as compared with the changing amount ~n of the built-in refractive index so that the difference ~n (where ~n = ~n - ~ne) between the refractive index of the central portion of the active layer 3 and the refractive index of the both side portions lies in a range from -10 2 to -10 3. Thus the semiconductor laser of the gain-guiding type can be made.
However, both of the semiconductor laser of refrac-tive index-guiding type and the semiconductor laser of gain-guiding type have merits and demerits, respectively.
Accordingly, when each of them is used as the writing ar.d/or reading light source of, for example, the video disc, each proposes a practical problem.
More speclfically, in the semiconductor laser of the refractive index-guiding type, since its longitudinal mode is the single mode, when it is used as the writing and/or reading light source in, for example, an optical video disc and so on, there is a defect that a mode hopping noise is caused by a returned light. Fig. 2 is .
lZ539~5 a graph showing measured results of a mode hopping noise caused by a forward current relative to the semiconductor laser. In Fig. 2, a curve 21 indicates measured results of noise produced in the semiconductor laser of refractive index-guiding type and as will be clear therefrom, the mode hopping noise is produced. On the other hand, in the semiconductor laser of refractive index-guiding type, a so-called beam waist position exists near the light end face of the light emission region. There is then an advantage that in practical use, the focal position can be set easily. Further, since a far distant image on the cross-section in the direction parallel to the junction or a so-called far field pattern is symmetrical with each other in the right and left direction, there is then an advantage that similarly in, for example, the practical use, it is easy to obtain a reading and/or writing light of spot-shape having a small distortion. As compared therewith, in the semiconductor laser of gain-guiding type, the beam waist position exists at the inside of about 20 ~m from the light end face of the light emission region, the far field pattern is frequently asymmetrical with each other in the right and left direction, and the astigmatism is large whereby the spot distortion becomes relatively large. As will be clear from a noise characteristic shown by a curve 22 in Fig. 2, the noise level of the semiconductor laser of the gain-guiding type is high as compared with the noise level shown by the curve 21 of the semiconductor laser of refractive ~3,,, ~ - 7 -~2539~5 index-guiding type. ~owever, since the longitudinal mode thereof is the multi-mode, there is then an advan-tage that no mode hopping noise is caused by ~he returned light.
OBJECTS AND SUMM~RY OF THE INVENTION
Accordingly, it is an object of this invention to provide a semiconductor device having a so-called interme-diate characteristic between those of the refractive index-guiding type semiconductor laser and the gain- -guiding type semiconductor laser ~nd which can utilize the advantages of the both and which can complement the defects thereof so that a noise can be reduced.
It is another object of this invention to provide a semiconductor device which is suitable as a writing and/or reading light source in, for example, an optical video disc or digital audio disc.
According to one aspect of the present invention, there is provided a semiconductor laser comprising a substrate on which a first cladding layer, an active layer, a second cladding layer and a light absorbing layer for limiting a current path and for absorbing a light oscillated out from said active layer are se~uentially formed in contact to one another, wherein said light absorbing layer is provided with a removed-away portion of stripe-shape for forming said current path, wherein a width W of said removed-away portion is selected in a range from 1 to 4 ~m, a thickness dl of ~2S39~5 said active layer is selected so as to satisfy the condition of d1 > 500A and a distance d2 between said active layer and said light absorbing layer is selected in a range from 0.2 to 0.7 ~m.
These and other objects, features and advantages of the semiconductor laser according to the present inven-tion will become apparent from the following detailed description of the preferred embodiment taken in conjuction with the.accompanying drawings! throughout which like reference numerals designate the same elements and parts.
"~
~ ^ .
: - 8a -l~S39~5 DESCRIPTION OF THE PRE~ERRED EMBODIMENT
Now, an embodiment of a semiconductor laser according to the present invention will hereinafter be described with reference to Fig. 3 and the followings. In this embodiment, similarly to the semiconductor laser described in connection with Fig. 1, on a GaAs substrate 31 of a conductive type, for example, N type, there are sequentially formed a first cladding layer 32 of N type made of AQzGal zAs, an active layer 33 of P type or N type made of AQxGal xAs, a second cladding layer 34 made of A~zGal zAs with the conductive type different from that of the first cladding layer 32 or P type, a light absorbing layer 35 made of A~yGal yAs having an effect for limiting a current path and which can absorb the light coming from the active layer 33 and a capping layer 36 of high impurity concen-tration and of conductive type P same as that of the second cladding layer 34.
The light absorbing layer 35 is of a conductive type different from the second cladding layer 34, for example, N type in this embodiment and in order that the light absorbing layer 35 is opposed through the second cladding layer 34 to the active layer 33 with a distance d2, the light absorbing layer 35 is buried into, for example, the second cladding layer 34. Further, the light absorb-ing layer 35 has at its, for example, central portion aremoved-away portion 35a of stripe-shape with a predeter-mined width and which is extended in the direction perpendicular to the sheet of paper of Fig. 3.
The compositions of the first and second cladding layers 32 and 34 are selected to satisfy the condition ...
_ g _ 125~5 of z > x so that the forbidden band widths thereof become larger than that of the active layer 33. Further, the composition of the light absorbing layer 35 is selected such that the forbidden band width thereof becomes smaller than that of the active layer 33 and that the refractive inde~ thereof with respect to the light oscillated from the light emission region of the active layer 33 becomes higher than that of the light emussion region, or the active layer 33. In other words, in the compositions of the active layer 33 and the light absorb-ing layer 35, their compositions are selected so as to satisfy the condition of x > y. Electrodes 37 and 38 are respectively deposited on the capping layer 36 and the substrate 31 in ohmic-contact therewith.
In this structure, the width W of the removed-away portion 35a of the light absorbing layer 35 is selected in a range from 1 to 4 1~m, preferably 2 to 4 ~m. The thickness dl of the active layer 33 is selected so as to satisfy the condition of dl > 500A, preferably 1500A > d1 > 700A. Further, the distance d2 between the active layer 33 and the light absorbing layer 35 is selected in a range from 0.2 to 0.7 ~m, preferably 0.3 to 0.5 ~m.
In the semiconductor laser of this structure, a noise characteristic relative to a forward current I
flowing between the both electrodes 37 and 38 when the forward voltage is applied between the electrodes 37 and 38 becomes as shown by a curve 23 in Fig. 2. In Fig. 2, a curve 24 indicates a relation between the current I and the power P.
~ ..
~;~S~5 As will be clear from the comparison of the curve 23 with the curve 22, the noise level of the semiconductor laser of the pxesent invention is reduced as compared with that of the gain-gulding type semiconductor laser.
That is, if now the semiconductor laser of the present invention is operated by the power of, for example, 5 mW
by which a current Il is flowed, the difference between the curves 22 and 23 becomes about 5 dBm. In other words, when the semiconductor laser is operated at the power of 5 mW, according to this invention, it is possible to reduce the noise by about 5 dBm as compared with the semiconductor laser of gain-guiding type. Furthermore, in the semiconductor laser of the present invention, as will be clear from the curve 23, there is produced no mode hopping noise which is seen just in the curve 21.
More particularly, according to the structure of the present invention, it may be considered that self-excited oscillation is produced by the light and carrier confine-ment which is intermediate between those of the prior art refractive index-guiding type and gain-guiding type ones. In other words it may be considered that the semiconductor laser of the present invention is operated in a range in which the afore-mentioned effective refractive index difference ~n is extremely small positive or negative value as compared with the semiconductor lasers of refractive index-guiding type and gain-guiding type.
Further, according to the semiconductor laser of the present invention, the symmetric property of the far field pattern is improved as compared with the prior art :., . .
~i~5;39'~5 semiconductor laser of gain-guiding type and the diameter of beam spot thereof is made small, thus the threshold current being reduced substantially. It can be understood that based upon these merits the semiconductor laser of this invention is operated as the intermediate semi-conductor laser between the refractive index-guiding type and the gain-guiding type ones.
The afore-described removal of mode hopping noise, low noise, reduction of threshold current and improve-ment of far field pattern can be achieved by particulariz-ing three of the width W of the removed-away portion 35a of the light absorbing layer 35, the distance d2 between the active layer 33 and the light absorbing layer 35 and the thickness d1 of the active layer 33 simultaneously.
More specifically, the width W is selected in a range from 1 to 4 ~m, preferably 2 to 4 ~m, the thickness d O
is selected so as to satisfy the condition of dl > 500A
o o (preferably 1500A > dl > 700A) and the distance d2 is selected in a range from 0.2 to 0.7 ~m (preferably in a range from 0.3 to 0.5 ~m).
These width W, the thickness d1 and the distance d2 will respectively be described below.
Now, the width W of the removed-away portion 35a of the light absorbing layer 35 will be considered.
Since the current concentration is weakened as the width W is increased as earlier noted, the refractive index change ¦~ne¦ by the injection of the carrier becomes small. If the width W is increased more, the change ~n of the built-in refractive index becomes dominant so that the semiconductor of this invention is opera~ed as 12539~S
the refractive index-guiding type one. In this case, the width W is one of the ~actors which determines the threshold current Ith for the oscillation of the semi-conductor laser. Since the threshold curren~ Ith is given as the product by a threshold current density Jth and the area of the light emission, when the width W is increased, the width of the light emission region, or its area is increased, whereby the threshold current Ith is increased. On the other hand, since the relation between the threshold current density and the width W
i5 shown in Fig. 4, when the width W becomes large, the threshold current density Jth becomes small. Therefore, the threshold current Ith can be suppressed to be small if the width W is in a certain range. Accordingly, in order to reduce the threshold current Ith, the selection of the width W becomes an important factor.
~ he thickness dl of the active layer 33 will be described. If the thickness dl thereof is increased, the area of the light emission area is increased and this leads to the increase of the threshold current Ith as mentioned before. Accordingly, in view of reducing the threshold current Ith, it is preferable that the thickness dl is not so large. Now, the permeation of light from the active layer 33 to the first and second cladding layers 32 and 34 will be considered. As shown in Fig. 5, when the thickness dl is small, the permeation of light presents a steep distribution as shown by a curve 51 in Fig. 5, while when the thickness dl is large, it presents a gentle distribution as shown by a curve 52 in Fig. 5. Accordingly, as the thickness dl is increased, l;~S3945 the light absorbing effect by the light absorbing layer 35 is decreased so that the semiconductor laser of this invention can be operated as the gain-guiding type one, or it becomes possible to avoid the mode hopping noise from being produced.
Further, also the distance d2 between the active layer 33 and the light absorbing layer 35 is one of the factors which determines the effect of the light absorb-ing layer 35 for absorbing the light from the active layer 33. If the distance d2 is small, the light absorbing e~fect is large so that the refractive index-guiding type characteristic becomes dominant in the semiconductor laser. If on the other hand the distance d2 is increased, the light absorbing effect is decreased or lost so that the semiconductor laser of this invention becomes the gain-guiding type one.
As describéd above, by the fact that the width W, the thickness dl and the distance d2 are particularized at the same time, the occurrence of the mode hopping noise can be avoided, the noise level can be reduced as compared with the semiconductor laser of the gain-guiding type, the threshold current Ith is low as compared with the threshold current Ithg of the gain-guiding type one and the far field pattern can be improved.
As set forth above, according to the semiconductor laser of the present invention, the mode hopping noise can be prevented from being produced and as compared with the semiconductor laser of the gain-guiding type, the noise and the threshold current can be reduced and the far field pattern can be improved. Accordingly, l;~S3945 when the semiconductor laser of this invention is used as a writing and/or reading light source of, for example, the video disc and the digital audio disc, there are brought a great deal of advantages that the resolution S and S/N (signal-to-noise) ratio can be improved and that the optical system can be simplified and so on.
The above description is given on a single prererred embodiment of the invention, but it will be apparent that many modifications and variations could be effected by one skilled in the art without departing from the spirits or scope of the novel concepts of the invention, so that the scope of the invention should be d~termined by the appended claim only.
.~
Claims
WE CLAIM AS OUR INVENTION
A semiconductor laser comprising a substrate on which a first cladding layer, an active layer, a second cladding layer and a light absorbing layer for limiting a current path and for absorbing a light oscillated out from said active layer are sequentially formed in contact to one another, wherein said light absorbing layer is provided with a removed-away portion of stripe-shape for forming said current path, a width W of said removed-away portion is selected in a range from 1 to 4 µm, a thickness d1 of said active layer is selected so as to satisfy the condition of d1 ? 500.ANG. and a distance d2 between said active layer and said light absorbing layer is selected in a range from 0.2 to 0.7 µm.
A semiconductor laser comprising a substrate on which a first cladding layer, an active layer, a second cladding layer and a light absorbing layer for limiting a current path and for absorbing a light oscillated out from said active layer are sequentially formed in contact to one another, wherein said light absorbing layer is provided with a removed-away portion of stripe-shape for forming said current path, a width W of said removed-away portion is selected in a range from 1 to 4 µm, a thickness d1 of said active layer is selected so as to satisfy the condition of d1 ? 500.ANG. and a distance d2 between said active layer and said light absorbing layer is selected in a range from 0.2 to 0.7 µm.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP149239/84 | 1984-07-18 | ||
| JP59149239A JPH0685455B2 (en) | 1984-07-18 | 1984-07-18 | Semiconductor laser |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1253945A true CA1253945A (en) | 1989-05-09 |
Family
ID=15470921
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000486983A Expired CA1253945A (en) | 1984-07-18 | 1985-07-17 | Semiconductor laser |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPH0685455B2 (en) |
| KR (2) | KR860001502A (en) |
| CA (1) | CA1253945A (en) |
| DE (1) | DE3525703A1 (en) |
| FR (1) | FR2568064A1 (en) |
| GB (1) | GB2163288A (en) |
| NL (1) | NL8502080A (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5161167A (en) * | 1990-06-21 | 1992-11-03 | Mitsubishi Denki Kabushiki Kaisha | Semiconductor laser producing visible light |
Family Cites Families (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS52143787A (en) * | 1976-05-26 | 1977-11-30 | Hitachi Ltd | Semiconductor laser |
| DE3065856D1 (en) * | 1979-02-13 | 1984-01-19 | Fujitsu Ltd | A semiconductor light emitting device |
| JPS5640292A (en) * | 1979-09-11 | 1981-04-16 | Fujitsu Ltd | Semiconductor laser |
| US4329189A (en) * | 1980-02-04 | 1982-05-11 | Northern Telecom Limited | Channelled substrate double heterostructure lasers |
| US4323859A (en) * | 1980-02-04 | 1982-04-06 | Northern Telecom Limited | Chanelled substrate double heterostructure lasers |
| JPS5736882A (en) * | 1980-08-15 | 1982-02-27 | Nec Corp | Stripe type double hetero junction laser element |
| JPS57170584A (en) * | 1981-04-15 | 1982-10-20 | Hitachi Ltd | Semiconductor laser device |
| GB2105099B (en) * | 1981-07-02 | 1985-06-12 | Standard Telephones Cables Ltd | Injection laser |
| JPS5967677A (en) * | 1982-07-01 | 1984-04-17 | Semiconductor Res Found | Photo integrated circuit |
| GB2139422B (en) * | 1983-03-24 | 1987-06-03 | Hitachi Ltd | Semiconductor laser and method of fabricating the same |
-
1984
- 1984-07-18 JP JP59149239A patent/JPH0685455B2/en not_active Expired - Lifetime
-
1985
- 1985-06-19 KR KR1019850004333A patent/KR860001502A/en not_active IP Right Cessation
- 1985-06-19 KR KR1019850004333A patent/KR0128711B1/en active
- 1985-07-17 CA CA000486983A patent/CA1253945A/en not_active Expired
- 1985-07-18 NL NL8502080A patent/NL8502080A/en not_active Application Discontinuation
- 1985-07-18 FR FR8511015A patent/FR2568064A1/en active Granted
- 1985-07-18 DE DE19853525703 patent/DE3525703A1/en not_active Ceased
- 1985-07-18 GB GB08518182A patent/GB2163288A/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| JPS6127694A (en) | 1986-02-07 |
| JPH0685455B2 (en) | 1994-10-26 |
| GB8518182D0 (en) | 1985-08-21 |
| DE3525703A1 (en) | 1986-02-20 |
| FR2568064B1 (en) | 1994-04-22 |
| GB2163288A (en) | 1986-02-19 |
| KR860001502A (en) | 1986-02-26 |
| NL8502080A (en) | 1986-02-17 |
| KR0128711B1 (en) | 1998-04-07 |
| FR2568064A1 (en) | 1986-01-24 |
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