GB2163288A - Semiconductor lasers - Google Patents

Abstract

A semiconductor laser comprises a substrate (31) in which there are formed, in turn, a first cladding layer (32), an active layer (33), a second cladding layer (34) and a light absorbing layer (35) for limiting a current path and for absorbing light emitted from the active layer (33). The light absorbing layer (35) is provided with a stripe-shaped removed portion (35a) for forming the current path. The width (W) of the removed portion (35a) is selected to be in a range of from 1 to 4 microns, the thickness (d1) of the active layer (33) is selected to be not less than approximately 500 Angstroms, and the distance (d2) between the active layer (33) and the light absorbing layer (35) is selected to be in a range of from 0.2 to 0.7 microns. <IMAGE>

Description

SPECIFICATION Semiconductor lasers This invention relates to semiconductor lasers.

Generally speaking, previously proposed semiconductor lasers can roughly be classified into lasers of the refractive index-guiding type and lasers of the gain-guiding type in accordance with the light and carrier confinement mechanism in the longitudinal mode thereof.

An example of a previously proposed semiconductor laser of the refractive index-guiding type is shown in Fig. 1 of the accompanying drawings. The semiconductor laser shown in Fig. 1 comprises a GaAs substrate 1 of, for example, Type, having formed thereon a first cladding layer 2 of N-type AIzGa,~zAs, an active layer 3 of P- or N-type AlxGa1-xAst a second cladding layer 4 of P-type AI,Ga, ~,As, a light absorbing layer 5 of N-type AlyGa1~yAs buried in the second cladding layer 4, and a P-type capping layer 6 of high impurity concentration.The light absorbing layer 5 has a stripe-shaped removed portion 5a having a width W which is provided by removing the light absorbing layer 5 at, for example, its central portion in a direction perpendicular to the plane of Fig. 1. The composition of the light absorbing layer 5 is selected to be such that its forbidden bandwidth is smaller than that of the active layer 3 and its refractive index for the light oscillated out or emitted 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, the compositions of the active layer 3 and the light absorbing layer 5 are selected so as to establish the condition that x is greater than y.

The semiconductor laser shown in Fig. 1 includes electrodes 7 and 8 which are deposited on the capping layer 6 and the substrate 1, respectively, in ohmic contact therewith. If a predetermined forward voltage is applied between the electrodes 7 and 8, light emission is carried out in the active layer 3, carriers and light being confined by the first and second cladding layers 2 and 4. A distance d2 between the active layer 3 and the light absorbing layer 5 is selected to be a distance over which the light oscillated out or emitted from the active layer 3 is able to reach the light absorbing layer 5, which distance may be, for example, in a range of from 0.3 to 0.4 microns. if the distance d2 is selected as described above, the light introduced into the light absorbing layer 5 from the active layer 3 is absorbed by the light absorbing layer 5.Accordingly, between the portion below the light absorbing layer 5 and the portion corresponding to the stripe-shaped removed portion 5a at the centre 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.The width W of the stripe-shaped removed portion 5a of the light absorbing layer 5 is selected to lie in a range of from 5 to 8 microns so that, also on the basis of the selection of the distance d2, a difference A n = n1 - n2 between a refractive index n1 of the portion (hereinafter referred to as the central portion) of the active layer 3 opposing the removed portion 5a of the light absorbing layer 5 and a refractive index n2 of the portions of the active layer 3 at opposite sides of the central portion can be, for example, + 10-2 to + 10-. As described above, in the active layer 3, the confinement effect for the light oscillation in the lateral direction is produced in the central portion opposing the removed portion 5a, in which the light emission region is restricted.

Semiconductor lasers of the refractive index-guiding type are not limited to the structure in which the light absorbing layer 5 is buried in the second cladding layer 4. It is possible to carry out a similar operation to that described above for a semiconductor laser which is known as a CSP (channelled substrate planar) laser and is disclosed in Japanese Published Unexamined Patent Application No. 143787/1977 (JP-A-52143787), in which the substrate 1, for example, is used as the light absorbing layer.

Although the semiconductor laser of Fig. 1 is constructed as a semiconductor laser of the refractive index-guiding type, a semiconductor laser of the structure shown in Fig. 1 can be modified to form a semiconductor laser of the gain-guiding type. That is, in the structure of Fig. 1, the fact that the light absorbing layer 5 is selected to be of a conductivity type which is different from that of the second cladding layer 4 leads to an effect that a P-N P-N thyristor structure is formed in the portion in which the light absorbing layer 5 exists to thereby limit the current path. Accordingly, a semiconductor laser of the gain-guiding type can be formed by using this light absorbing layer 5 as a current limit region.That is, from a qualitative standpoint, a strong current concentration in the stripe-shaped portion is produced if the width W of the removed portion 5a is reduced as compared with that of the semiconductor laser of the above-described 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 absorption of the light from the active layer 3 by the light absorbing layer 5 can be reduced.Thus, a negative change in refractive index amounting to - Ane provided by the injected carriers becomes dominant as compared with the amount of change An of the built-in refractive index so that the difference An (where An = An - Ane) between the refractive index of the central portion of the active layer 3 and the refractive index of both side portions lies in a range of from - 10-2 to - 10-3. Thus, a semiconductor laser of the gain-guiding type can be made.

Both semiconductor lasers of the refractive index-guiding type and semiconductor lasers of the gain-guiding type have respective merits and demerits. Accordingly, when either of them is used as a writing and/or reading light source of, for example, a video disc, each gives rise to a practical problem.

More specifically, in the case of a semiconductor laser of the refractive index-guiding type, since its longitudinal mode is a single mode, when it is used as a writing and/or reading light source in, for example, an optical video disc and so on, the defect arises that mode hopping noise is caused by returned light. Fig. 2 is a graph showing measured results of mode hopping noise against forward current flowing through the semiconductor laser. In Fig. 2, a curve 21 indicates measured results of noise produced in a semiconductor laser of the refractive index-guiding type, and, as will be clear therefrom, mode hopping noise is produced. On the other hand, in a semiconductor laser of the refractive index-guiding type, a 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 in the right and left directions, there is then an advantage that similarly in, for example, practical use, it is easy to obtain reading and/or writing light of spot-shape having small distortion. In contrast, in a semiconductor laser of the gainguiding type, the beam waist position exists inside of (about 20 microns from the light end face of) the light emission region, the far field pattern is frequency asymmetrical in the right and left directions, and 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 a semiconductor laser of the gainguiding type is high as compared with the noise level shown by the curve 21 of a semiconductor laser of the refractive indexguiding type. However, since the longitudinal mode thereof is multi-mode, there is an advantage that no mode hopping noise is caused by returned light.

According to the present invention there is provided a semiconductor laser comprising a substrate on which a first cladding layer, an active layer, a second cladding layar and a light absorbing layer for limiting a current path and for absorbing light emitted from the active layer are formed sequentially in contact with one another, wherein the light absorbing layer is provided with a stripe-shaped removed portion for forming said current path, a width of the removed portion is selected to be in a range of from 1 to 4 microns, a thickness of the active layer is selected to be not less than about 500 Angstroms, and a distance between the active layer and the light absorbing layer is selected to be in a range of from 0.2 to 0.7 microns.

A preferred embodiment of this invention described hereinbelow seeks to provide a semiconductor laser having a characteristic intermediate or between those of semiconductor lasers of the refractive index-guiding type and the main gain-guiding type and which can utilise the advantages of both and can complement the defects thereof so that noise can be reduced.

The preferred semiconductor laser is suitable (for example) as a writing and/or reading light source in, for example, an optical video disc or digital audio disc. However, lasers embodying the invention have more general applicability.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which like references designate like items throughout, and in which: Figure 1 is a schematically enlarged crosssectional view of a previously proposed semiconductor laser; Figure 2 is a graph illustrating noise levels and power relative to forward current of semiconductor lasers; Figure 3 is a schematically enlarged crosssectional view of a semiconductor laser embodying the present invention; Figure 4 is a graph illustrating a relationship between a threshold value current density and a width of a removed portion formed in a light absorbing layer; and Figure 5 is a graph illustrating the distribution of a light coming from an active layer.

A semiconductor laser embodying the present invention will now be described with reference to Fig. 3. In similar manner to the semiconductor laser described with reference to Fig. 1, the semiconductor laser of Fig. 3 comprises a GaAs substrate 31 of a desired conductivity type (for example N-type) on which there are formed sequentially a first cladding layer 32 of N-type Al,Ga1 2As, an active layer 33 of P-type or N-type Alx Ga,~xAs, a second cladding layer 34 of AIzGa 1,As having a conductivity type (for example P-type) different from that of the first cladding layer 32, a light absorbing layer 35 of AlyGa- 1~yAs effective to limit a current path and which can absorb the light coming from the active layer 33, and a capping layer 36 of high impurity concentration and of a conductivity type (for example P-type) which is the same as that of the second cladding layer 34.

The light absorbing layer 35 is of a conduc tively type different from that of the second cladding layer 34 (for example N-type) and, in order that the light absorbing layer 35 opposes, through the second cladding layer 34, the active layer 33 with a distance d2, the light absorbing layer 35 is buried in, for example, the second cladding layer 34.

Furthe, the light absorbing layer 35 has, for example, at a central portion thereof, a stripeshaped removed portion 35a of a predetermined width which extends in the direction perpendicular to the plane of Fig. 3.

The compositions of the first and second cladding layers 32 and 34 are selected to satisfy the condition that z is greater than x so that the forbidden bandwidths thereof become larger than that of the active layer 33. Further, the compositions of the light absorbing layer 35 is selected such that the forbidden bandwidth thereof becomes smaller than that of the active layer 33 and such that the refractive index thereof with respect to the light oscillated out or emitted from the light emission region of the active layer 33 becomes higher than that of the light emission region, or the active layer 33. In other words, the compositions of the active layer 33 and the light absorbing layer 35 are selected so as to satisfy the condition that x is greater than y. Electrodes 37 and 38 are deposited on the capping layer 36 and the substrate 31, respectively, in ohmic contact therewith.

In this case, the width W of the removed portion 35a of the light absorbing layer 35 is selected to lie in a range of from 1 to 4 microns, preferably 2 to 4 microns. The thickness dl of the active layer 33 is selected so as to satisfy the condition that d1500 Ang- stroms, that is so that it is not less than about 500 Angstroms. Preferably, dl is between 700 and 1 500 Angstroms. Further, the distance d2 between the active layer 33 and the light absorbing layer 35 is selected to be in a range of from 0.2 to 0.7 microns, preferably 0.3 t 0.5 microns.

In a semiconductor laser of this structure, a noise characteristic relative to a forward current I flowing between the electrodes 37 and 38- when a forward voltage is applied between the electrodes 37 and 38 is as shown by a curve 23 in Fig. 2. In Fig. 2, a curve 24 indicates a relationship between the current I and the power P.

As will be seen from comparing the curve 23 with the curve 22, the noise level of the semiconductor laser of Fig. 3 is reduced as compared with that of the semiconductor laser of the gain-guiding type. That is, if the semiconductor laser of Fig. 3 is operated at a power level of, for example, 5 mW at which a current Ii flows through it, the difference between the curves 22 and 23 is about 5 dBm. In other words, when the semiconductor laser of Fig. 3 is operated at a power level of 5 mW, it is possible to reduce the noise by about 5 dBm as compared with the semiconductor laer of the gain-guiding type. Furthermore, in the semiconductor laser of Fig. 3, as will be seen from the curve 23, no mode hopping noise (which can be seen only in the curve 21) is produced.More particularly, according to the semiconductor laser structure described above with reference to Fig. 3, selfexcited oscillation is produced by light and carrier confinement which is intermediate between those of previously proposed semiconductor lasers of the refractive index-guiding type and the gain-guiding type. In other words, the semiconductor laser of Fig. 3 is operated in a range in which the afore-mentioned effective refractive index difference An is of an extremely small positive or negative value as compared with semiconductor lasers of the refractive index-guiding type and the gain-guiding type.

Further, the symmetry of the far field pattern of the semiconductor laser of Fig. 3 is improved as compared with the previously proposed semiconductor laser of the gainguilding type and the diameter of the beam spot thereof is made small, whereby the threshold current is reduced substantially. It can be understood that, based upon these merits, the semiconductor laser of Fig. 3 is operaed as an type of semiconductor laser which is intermediate or between semiconductor lasers of the refractive index-guiding type and the gain-guiding type.

The afore-described removal of mode hopping noise, low noise, reduction of threshold current and improvement of the far field pattern can be achieved by selecting appropriate values for all three of the following parameters: the width W of the removed 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 dl of the active layer 33. More specifically, the width W is selected to be in a range of from 1 to 4 microns (preferably 2 to 4 microns), the thickness dl is selected so that it is not less than approximately 500 Angstroms (preferably between 700 and 1 500 Angstroms), and the distance d2 is selected to be in a range of from 0.2 to 0.7 microns (preferably in a range of from 0.3 to 0.5 microns).

These three parameters, namely the width W, the thickness dl and the distance d2 will now be discussed.

The width W of the removed portion 35a of the light absorbing layer 35 will first be considered. Since the current concentration is weakened as the width W is increased, as noted earlier, the refractive index change lanes caused by the injection of carriers becomes small. If the width W is increased more, the change An of the built-in refractive index becomes dominant so that the semiconductor laser of Fig. 3 is operated as a refractive index-guiding type laser. In this case, the width W is one of the factors which determines the threshold current (Ith) for oscillation of the semiconductor laser.Since the threshold current Ith is equal to the product of a threshold current density Jth and the area of light emission, increasing the width W increases the width (and therefore the area) of the light emission region whereby the threshold current kh is increased. On the other hand, since the relationship between the threshold current density and the width W is as shown in Fig. 4, then when the width W becomes large the threshold current density Jth becomes small. Therefore, the threshold current Ith can be suppressed so as 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.

The thickness dl of the active layer 33 will now be considered. If the thickness dl is increased, the area of the light emission region is increased and, for the reason mentioned above, this leads to an increase of the threshold current Ith. Accordingly, with a view of reducing the threshold current Ith, it is preferable that the thickness dl not be so large. The permeation of light from the active layer 33 to the first and second cladding layers 32 and 34 will now 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. 3, while when the thickness dl is large, it presents a relatively gentle distribution as shown by a curve 52 in Fig. 5.Accordingly, as the thickness dl is increased, the effect of light absorption by the light absorbing layer 35 is decreased so that the semiconductor laser of Fig. 3 can be operated as a semiconductor laser of the gain-guiding type, or it becomes possible to avoid mode hopping noise being produced.

Further, the distance d2 between the ative layer 33 and the light absorbing layer 35 also is one of the factors which determines the effect of the light absorbing layer 35 in absorbing light from the active layer 33. If the distance d2 is small, the light absorbing effect 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 Fig. 3 becomes a gainguiding type of laser.

As described above, by virtue of the fact that the width W, the thickness dl and the distance d2 are all three selected to have appropriate values, the semiconductor laser of Fig. 3 is superior in performance to the previously proposed gain-guiding type of laser described above in that the occurrence of 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 semiconductor laser of the gain-guiding type, and the far field pattern can be improved. Accordingly, use of the semiconductor laser of Fig. 3 as a writing and/or reading light source of, for example, a video disc and a digital audio disc, leads to the advantages that the resolution and signal to noise ratio can be improved and that the optical system can be simplified and so on.

Claims (5)

1. A semiconductor laser comprising a subsrate 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 light emitted from the active layer are formed sequentially in contact with one another, wherein the light absorbing layer is provided with a stripe-shaped removed portion for forming said current path, a width of the removed portion is selected to be in a range of from 1 to 4 microns, a thickness of the active layer is selected to be not less than about 500 Angstroms, and a distance between the active layer and the light absorbing layer is selected to be in a range of from 0.2 to 0.7 microns.

2. A semiconductor laser according to claim 1, wherein the width of the removed portion is selected to be in a rage of from 2 to 4 microns.

3. A semiconductor laser according to claim 1 or claim 2, wherein the thickness of the active layer is selected to be in a range of from 700 to 1 500 Angstroms.

4. A semiconductor laser according to claim 1, claim 2 or claim 3, wherein the distance between the active layer and the light absorbing layer is selected to be in a range of from 0.3 to 0.5 microns.

5. A semiconductor laser substantially as herein described with reference to Fig. 3 of the accompanying drawings.