FR2568064A1 - SEMICONDUCTOR LASER FOR LIMITING AND REMOVING NOISE FROM HOPPING AND REDUCTION OF THRESHING CURRENT - Google Patents

Abstract

A. SEMICONDUCTOR LASER. B. LASER CHARACTERIZED IN THAT THE LIGHT ABSORBING LAYER 35 INCLUDES A BAND-SHAPED REMOVED PART 35A TO CONSTITUTE A CURRENT PATH, THE WIDTH W OF THE REMOVED PART 35A BEING BETWEEN 1 AND 4MICRONS, THE THICKNESS D OF THE ACTIVE LAYER 33 MEETS DA 500A CONDITION AND THE DISTANCE BETWEEN ACTIVE LAYER 33 AND LIGHT ABSORBING LAYER 35 IS CHOSEN BETWEEN 0.2 AND 0.7MICRON. C. THE INVENTION CONCERNS SEMICONDUCTOR LASERS.

Description

Semiconductor laser to limit and remove a

  noise of jumps and reduction of the current of. threshold."

The present invention relates to a

semiconductor laser.

  In general, semiconductor lasers

  conductors according to the prior art are schematically

  in refractive index guided lasers and in

  gain-guided lasers, according to the transmission system

  fertility and light passage in longitudinal mode.

  As a semiconductor laser guided refraction index, for example known that

  shown in Figure 1. In Figure 1, the semiconductor laser

  conductor consists of a support 1 GaAs for example of

  type N on which is formed a first coating layer

  2, composed of N-type AlzGCa 1-z As, an active layer 3 of AlxGal xAs of the P or N type, a second P-type AlzGa izAs coating layer 4, an absorbing layer of the N-type AlyGal yAs light 5 embedded in the second coating layer 4 and a

  P-type cover layer 6 having a high concentration

  impurity. The absorbent layer of light

  has a stripped portion 5a in the form of a band of

  W which is achieved by removing the light absorbing layer 5 for example in its central portion in the direction perpendicular to the paper sheet of Figure 1. The composition of this light absorbing layer 5 is chosen so that the width of its forbidden band is less than that of the active layer 3 and its refractive index for the oscillating light, emitted from the photoemissive region of the active layer 3

  is higher than that of the photo-emissive region

  the active layer 3. This means that the composi-

  active layer 3 and that of the absorption layer.

  5 are selected to meet the con-

edition x> y.

  In Figure 1, the numerical references

  7 and 8 designate electrodes deposited on the neck.

  cover 6 and support 1, respectively in

contact with these layers.

In this semicircle laser structure

  when a predetermined direct voltage is applied between the electrodes 7 and 8, the active layer 3 emits light, the carriers and the light of this layer being maintained between the first and the second

  coating layer 2 and 4. The distance d2 between the

  3 and the light absorption layer 5 is chosen to be a distance through which the oscillating light emerging from the active layer 3 can

  reach the light absorption layer 5, that is,

  say for example of the order of 0.3 to 0.4 micron. If the distance d2 is chosen as indicated above, the light sent into the light absorption layer 5 by the active layer 3 is absorbed by this layer 5, the part below the light absorption layer 5. and the portion corresponding to the band-shaped removed portion 5a in the middle of the light absorbing layer 5 in which the light is hardly absorbed, it

  is there an actual difference in refractive index

  i.e., a difference n of the refractive index of interest

  grateful in the lateral direction. The width W of the strip-shaped stripped portion 5a of the layer 5 is chosen between 5 and 8 microns so that at the base of the selection of the distance d2, there is a difference A n = nl-n2 between refractive index n1 of the portion of the active layer 3 (hereinafter referred to as "central portion") facing the removed portion 5a of the absorbent layer 5 and the refractive index n2 of the portions of the active layer 3 of the two sides of the central part to have for example +102 to +10-3. As described above in the active layer 3, the effect of limiting the oscillation of the

  light in the lateral direction is obtained in the

  center section opposite the part removed 5a in the-

  which photo-emissive region is limited.

  Since the refractive index guiding type semiconductor laser is not limited to the structure in which the light absorbing layer 5 is embedded in the second cladding layer 4, operation similar to the above above applies to a semiconductor laser known as CSP (flat-to-channel support) described in

  the publication of the unexamined Japanese patent application

  n ° 143787/1977, we have a support 1 used for ex-

  ple as a light absorbing layer.

  As indicated above, although the refractive index guide type semiconductor laser is made as shown in FIG. 1, the semiconductor laser of this type can be modified.

  into a gain-guided semiconductor laser.

  Thus, in the structure of FIG. 1, since the light absorbing layer 5 is chosen to be of a conductive type different from that of the second coating layer 4, it follows that there is a PNPN thyristor structure in FIG. the part in which the light-absorbing layer 5 exists, which limits the path of the current. As a result, the semiconductor laser

  of the gain-guiding type can be achieved by using

  this light absorbing layer 5 as a current limiting region. This means that qualitatively, there is a high concentration of current in the band-shaped portion, obtained by reducing the width W of the removed portion 5a as compared to a guide-semiconductor semiconductor laser. refraction; the thickness d1 of the active layer 3 can be increased or the distance d2 between the active layer 3 and the current limiting region (i.e., the light absorption layer 5) is increased to reduce the absorption effect of the

  light of the active layer 3 by the absorbent layer 5.

  We thus have a negative variation - 4ne of index of

  refraction by carrier injection which becomes dominant

  compared with the variation An of the integrated refractive index, so that the difference A n (A n = An - An) between the refractive index of the central part e of the active layer 3 and the refractive index of the two lateral parts is between -10 2 and

  -103. There is thus a laser of the type guided by the gain.

  However, both the semiconductor laser

  refractive index guide conductor and laser

  gain-controlled semiconductors have advantages

  respective disadvantages. So when we use

  read each of these lasers as a light source for

  and / or reading, for example for a video disc, each of them is a source of difficulties

in practice.

  More precisely, in a semiconductor laser of the refractive index gain type, as its longitudinal mode is the only mode, when this

  laser is used as a light source for

  tion (or writing) and / or reading, for example for a

  Whether optical or other video, there is the disadvantage that the return of light generates noise jumps. Fig. 2 is a graph showing the results of hopping noise measurements generated by a direct current for the

semiconductor laser.

  In FIG. 2, the curve 21 designates the results of the measurements of the noise generated in the refractive index guided semiconductor laser;

  as it is clear, we have the sound of jumps.

  Furthermore, in the semiconductor laser guided by

  index of refraction, there is a position of shrinking

  beam in the vicinity of the end surface of the photoemissive region. In practice, we have

  the advantage of being able to easily adjust the focal position.

  Moreover, as an image far removed from the trans-

  versale in the direction parallel to the junction or far field diagram is symmetrical in the right direction and the left direction, it has the advantage that for example in practice, it is easy to obtain a point of

  low distortion reading and / or writing light.

  Compared with that in the gain-guided type semiconductor laser, the narrowed portion of the beam is within about 20 microns of the end face of the photoemissive region and the pattern of field is frequently asymmetrical in the right direction and the left direction; astigmatism is important and

  thus the deformation of the point is relatively large.

  As is clear from the characteristic of

  noise represented by curve 22 in Figure 2, the

  noise caliber of the gain-guided semiconductor laser is larger than the noise level represented

  by the curve 21 and corresponding to the semiconductor laser.

  refractive index guide. Since the longitudinal mode is the multiple mode, it is best not to

  generate noise jumps by the light back.

  The present invention aims to

  to create a semiconductor device with a characteristic

  so-called intermediary between that of the semi-

  refractive index-guided conductor and that of the gain-guided semiconductor laser, and that can utilize the advantages of both types of lasers by

  overcoming their disadvantages by reducing noise.

  The invention also aims to create a semiconductor device that can be used as

  light source of inscription and / or reading, for example

  for an optical video disc or a digital audio disc

  America. For this purpose, the invention relates to a semiconductor laser comprising a support on which

  is carried out a first coating layer, a cou-

  active layer, a second coating layer and a light absorption layer to limit the path of the current

  and to absorb the oscillating light coming out of the

  active layer which are successively in contact with each other, the light absorbing layer having a band-shaped portion removed to form the current path, the width W of the removed portion being

  between 1 and 4 microns, the thickness d1 of the neck

  active agent satisfying the condition d1> 500 A and the distance d2 between the active layer and the absorbing layer

  the light being between 0.2 and 0.7 micron.

  The present invention will be described in more detail with the aid of an exemplary semiconductor laser shown in the accompanying drawings, in which: - Figure 1 is a sectional view

  cross-section on a larger scale of a semiconductor laser

according to the prior art.

  FIG. 2 is a graph giving the noise level for a direct current in a laser

  semiconductor; this figure also gives the power

  this. FIG. 3 is a schematic cross-sectional view on an enlarged scale showing an embodiment of a semiconductor laser according to the invention. FIG. 4 is a graph showing the relationship between the threshold current density and the width of the portion removed in the light absorption layer. FIG. 5 is a graph showing the distribution of the light coming from the layer

active.

  DESCRIPTION OF THE PREFERENTIAL EMBODIMENT:

  An embodiment of a semiconductor laser according to the present invention will be described

  hereinafter with the aid of FIG. 3 and the following figures.

  In this embodiment, analogously to the semiconductor laser described in connection with FIG. 1, there is a 31 GaAs carrier with conductivity, for example N type, on which layer a first coating layer 32 is successively produced. AlzGal zAs type N, an active layer 33 in AlxGal xAs of type P or N type, a second layer of coating 34 in AlzGal zAs type

  of conductivity different from that of the first

  coating 32 or type P, a light absorbing layer 35 of AlyGa yAs having the effect of limiting the path of the current and which can absorb the light

  from the active layer 33 as well as a recovery layer

  36 with a high concentration of impurity and with a conduction type P, identical to that of the second layer of

coating 34.

  The light absorbing layer 35 is of different conductivity from that of the second coating layer 34, for example N-type in this case.

  embodiment so that the absorbing layer

  35 be opposed through the second

  coating layer 34 to the active layer 33, a

  At d2, the light-absorbing layer 35 is embedded, for example, in the second coating layer 34. In addition, the light-absorbing layer 35 has, for example, a stripped-off central portion 35a having a predetermined width, and extending in the direction

  perpendicular to the paper sheet of Figure 3.

  The composition of the first cladding layer 32 and the second cladding layer 34 is chosen to satisfy the condition z> x so that the width of the forbidden band of each of the

  layers is greater than that of active layer 33.

  In addition, the composition of the light absorbing layer is chosen so that the width of its forbidden band is less than that of the active layer 33 and its refractive index with respect to the oscillating light coming from the light emitting region of the active layer 33 is greater than that of the photoemissive region or layer 33. In other words, in the composition of the active layer 33 and the absorbent layer la. light 35,

  the compositions are chosen to satisfy the conditions

  tion x> y. The electrodes 37 and 38 are deposited respectively

  on the coating layer 36 and on the support 31

  by having an ohmic contact with these layers.

  In this structure, the width W of the removed portion 35a of the absorption layer of

  light 35 is between 1 and 4 microns, preferably

  this between 2 and 4 microns. The thickness d1 of the active layer 33 is chosen so as to satisfy the condition

O O O

  dl> 500 A and preferably 1500 A> d1> 700 A. Moreover, the distance d2 between the active layer 33 and the layer

  light absorption 35 is selected in the range of

  between 0.2 and 0.7 micron, preferably between 0.3

and 0.5 micron.

  In the semiconductor laser having this structure, there is a noise characteristic for a direct current I passing between the two electrodes 37 and 38 when a direct voltage is applied between the electrodes 37 and 38 as shown in the curve 23 at the

  Figure 2. In Figure 2, curve 24 represents the relationship between

  between current I and power P.

  As is clear from the comparison

  because of the curve 23 and the curve 22, the noise level of the semiconductor laser according to the invention is

  decreased compared to that of the semiconductor laser.

  Gain-guided motor. If the semiconductor laser according to the invention works for example with a power

  of 5 mW with a current II, the diffe-

  this between the curves 22 and 23 is of the order of 5 dB.

  In other words, when the semiconductor laser operates at a power of 5 mW according to the invention, it is possible to reduce the noise by about 5 dB compared to the semiconductor laser guided by the

  gain. Moreover in the semiconductor laser according to the in-

  vention, as is clear from curve 23,

  there is no noise of jumps, as for curve 21.

  In more detail, according to the structure of the

  According to the invention, it is possible to estimate that a self-excited oscillation is maintained by keeping the light and the carriers confined, and which corresponds to an intermediate situation between the refractive index guided laser and the gain guided laser, according to the invention. the prior art. In other words, it is believed that the semiconductor laser according to the invention operates in a range in which the difference of the effective refractive index λ n, mentioned above, is extremely low in the positive or negative direction. negative in comparison with refractive index guided semiconductor lasers and

guidance by gain.

  Moreover in the semiconductor laser

  according to the invention, the symmetry characteristic of the far-field pattern is improved by comparison with a prior art semiconductor laser with gain-guidance and the diameter of the beam point is very small.

  low, which significantly reduces the threshold current.

  It is clear that, depending on such characteristics, the semiconductor laser of the invention operates as a semiconductor laser intermediate between a refractive index guided laser and a laser guided laser.

Gain.

The suppression of the noise of jumps

  described above, low noise, reduced

  threshold and the improvement of the elec-

  can be obtained by defining in particular

  the three parameters namely the width W of the removed portion 35a of the light absorption layer, the distance d2 between the active layer 33 and the light absorption layer 35 and the thickness d1 of the layer

  active 33, and this simultaneously. More

  cise, the width W is chosen in a range of between 1 micron and 4 microns and preferably between 2 and

  4 microns; the thickness d1 is chosen so as to satisfy

  o do with condition d> 500 A (preferably

0 0

  1500 A> d1> 700 A) and the distance d2 is chosen in the range between 0.2 and 0.7 micron (preferably

  in the range of 0.3 to 0.5 micron).

The width W, the thickness d1 and the

  distance d2 will respectively be explained below.

  The width W of the removed portion 35a of the light absorbing layer 35 will first be examined. As the current concentration decreases when the

  width W increases as indicated above, the variation

  refractive index I | by injection of

  become weak. If the width W continues to increase, the variation En of the integrated refractive index dominates * 11 and the semiconductor according to the invention functions as

  a refractive index guide semiconductor.

  In this case, the width W is one of the coefficients that determines the threshold current Ith to oscillate the semiconductor laser. Since the threshold current Ith is the product of the threshold current density Jth and the light emission surface, if the width W increases, the width of the photoemissive region or its

  surface increases and so the Ith threshold current increases.

  Moreover, as the relation between the density of cou-

  As shown in FIG. 4, as the width W increases, the threshold current density 3th becomes small. Therefore, the Ith threshold current can be made negligible, if the width

  W reaches a certain range. In order to reduce the

  threshold Ith, the choice of width W becomes a

important element.

  The thickness d1 of the active layer 33 will be examined below. If the thickness d1 increases, the

  the area of the photo-emissive zone increases, which is

  due to an increase in the Ith threshold current as already indicated. Thus in terms of the reduction of the threshold current Ith, it is preferable that the thickness d1 is not too important. The light transparency for the active layer 33 to the first and second coating layers 32 and 34 will be discussed hereinafter. As shown in FIG. 5, when the thickness d1 is small, the

  light transparency has an accelerated distribution

  killed as shown by curve 51 in the figure

  5 whereas if the thickness d1 is important, it is

  by a soft distribution as shown in curve 52 in Figure 5. Thus, when the thickness d1 increases, the light absorption effect by the absorbent layer

  the light 35 decreases, so that the semiconductor laser

  According to the invention, it is possible for the driver to operate with gain control or else to avoid the emission of jumping noise. In addition, the distance d2 between the active layer 33 and the light absorption layer 35 is one of the elements that determines the absorption action.

  by the layer 35 to absorb the layer

  active layer 33. If the distance d is small,

  the light absorption effect is important and the character

  The refractive index guidance property dominates in the semiconductor laser. If, on the other hand, the distance d2 increases, the light absorption effect decreases or disappears and the semiconductor laser becomes a laser

guided by the gain.

  As indicated above, because

  the width W, the thickness d1 and the distance d2 are chosen

  in a particular way, at the same time, the

  noise and the noise level is reduced by comparison

  reason with a gain-guided semiconductor laser; the threshold current Ith is weak compared to the threshold current Ithg of a gain-guided laser

  and the far-field scheme is improved.

  As indicated above, in the case of the semiconductor laser of the invention, the emission of jumping noise can be avoided and compared with a gain-guided semiconductor laser, the noise and threshold current and improve the

  distant field diagram. As a result, the semiconductor laser

  conductor of the invention is used as a light source

  for the registration and / or reading of, for example, a

  video or a digital audio disc and has a large number of advantages, such as improved resolution and S / N ratio (signal-to-noise ratio).

  which simplifies the optical system.

CA TI ON R E V E N

  Semiconductor laser having a carrier (31) carrying a first coating layer

  (32), an active layer (33), a second

  (34) and a light absorbing layer (35) for limiting a current path and for absorbing light

  emitted from the active layer (33), these layers being made

  characterized in that the light absorbing layer (35) has a band-shaped removed portion (35a) to form a current path, the width (W) of the removed portion (35a) being between 1 and 4 microns, the thickness (dl) of the active layer (33) satisfies the o condition (d1 at 500 A) and the distance (d2) between the active layer (33) and the light absorbing layer (35)

  is chosen between 0.2 and 0.7 micron.