FR2677499A1 - Monomode semiconductor laser with distributed feedback and its method of manufacture - Google Patents

Abstract

In this laser, the optical feedback network (grating) (20) has two alternate phase shifts (22, 24) whose positions are asymmetric with respect to the middle (20M) of this network. The invention applies in particular to the manufacture of lasers intended for fibre-optic telecommunications systems.

Description

Distributed feedback monomodal semiconductor laser and method of manufacturing the same
The present invention relates to a single mode semiconductor laser with distributed feedback of the so-called DFB type. The resonant cavity of such a laser is at least partially formed by an optical feedback network. The product of the pitch H of this network by the effective refractive index of the structure of the laser is usually substantially equal to half the wavelength of the light to be emitted. This network may have one or more localized phase shifts, the usefulness of which will appear later.

 Such a laser is typically intended for a fiber optic telecommunications network. An important characteristic is then the width of the spectral line of the light emitted.

Theoretical articles (CH Henry "Theory of the linewidth of semiconductor lasers", IEEE J. Quantum Electron. Pp 259-264, 1982) show that the line width DNU of a semiconductor laser is, according to certain operating models of this laser, inversely dependent on the light power P it emits
DNU = c / P
c being a constant coefficient.

 The minimum line width reached would then depend on the maximum power (in single mode operation) of the laser.

However, these models assume a homogeneous distribution of free carriers (electrons) and photons in the resonant cavity of the laser.

However, above the resonance threshold, the constituent elements of this cavity (the possible reflectivities on the external faces, the possible phase shifts of the network and essentially the coupling coefficient of this network with light) impose a well-defined profile on the density of photons (these are only stimulated photons). This profile is, especially for lasers with a single phase shift of H / 2 and a high coupling coefficient, far from being homogeneous. This inhomogeneity affects the density of carriers: in fact, the creation of a stimulated photon causes the de-excitation of an electron in the conduction band, and therefore a reduction in the number of free electrons. Consequently, the density of free carriers is lower in regions where the density of photons, that is to say the intensity of light, is high. This phenomenon is commonly called "spatial hole burning". ie "local impoverishment".

Models taking into account this impoverishment (G. Morthier et al "Linewidth of single-mode DFB lasers in the presence of spatial and spectral hole burning", ECOC'89) show that this produces a shift in the line width
DNU = c / P + a
a denoting the offset.

 The achievable line width is then limited by this offset.

This is due to the inhomogeneous profile of photons and carriers.

In addition, this depletion is responsible for the degradation of the modal rejection rate of lasers (H. Soda et al "Stability in single longitudinal Mode operation in GalnAsP / InP phase adjusted DFB lasers"
IEEE J. Quantum Electron. pp 804-814, 1987): it produces an ascent of the secondary modes, and the laser can even become bimode beyond a certain intensity of the supply current of the laser. This depletion thus also limits the maximum single-mode power of the laser.

 One solution to reduce local depletion is to introduce multiple phase shifts in the cavity. Several publications have been made on this subject. There are two phase shifts (two H / 4 phase shifts: J. Whiteaway et al. "The static and dynamic characteristics of single-mode and multiple phase-jump DFB laser structures", 12th IEEE International semiconductor laser conference, 1990, pp 110-111) or 3 (phase shifts of 2H / 5, -2H / 5 and 2H / 5: Ogita et al "Long cavity, multiple phase-shift, distributed feedback laser for linewidth narrowing", Electron. Letters, llth May 1989). These phase shifts modify the profile of photons imposed by the laser cavity. They are generally optimized to create a relatively homogeneous distribution of the light intensity while keeping a good modal selectivity. The phase shifts considered are: 2 phase shifts of the same sign, or three phase shifts, either of the same signs, or of alternating signs distributed symmetrically with respect to the center of the laser cavity.

According to a known interference method, the phase shifts of alternating signs can advantageously be engraved with the aid of a quartz blade, by producing light interference bands on a resin for photolithography. The thickness of this strip is varied over the length thereof and the angles of the light rays are chosen to obtain the desired phase shift. This will be engraved in the region of thickness variation of the blade. This known process is described in an article: "lambda / 4 - Shifted DFB-LD corrugation formed by a novel spatial phase modulating mask. M. Shirasaki, M. Soda, S. Yamakashi and
H. Nakajima, FUJITSU Lab Ltd. IOOC Ecoc '85 ".

 The case of 2 phase shifts of opposite signs placed symmetrically with respect to the center of the cavity has not been envisaged, since these phase shifts make the laser dual mode when its faces are treated with anti-reflection. There is then no advantage compared to simple DFB without phase shifts.

 The present invention has in particular the following aims - Simplerly producing a single-mode semiconductor laser with distributed feedback having a single-mode operation and a sufficiently homogeneous distribution of the light intensity over the length of the resonant optical cavity of this laser.

- Reduce the number of phase shifts in the optical feedback network of such a laser while allowing this laser to be easily produced by an interference process using a quartz blade of variable thickness.

- In general, make a laser with single-mode operation and a new arrangement of phase shifts in its optical feedback network without significant local depletion of the density of charge carriers.

 For these purposes, it has in particular a single-mode semiconductor laser with distributed feedback, characterized in that its optical feedback network has two alternating phase shifts whose positions are asymmetrical relative to the middle of this network.

 Using the attached schematic figures, we will describe below, by way of non-limiting example, how the present invention can be implemented.

FIG. 1 represents a view of a laser produced according to the present invention, in section through a longitudinal vertical plane.

Figures 2 and 3 show two details II and III on an enlarged scale of Figure 1 to show the phase shifts of the optical feedback network of this laser.

FIG. 4 represents a view of this laser during a stage of its manufacture, in partial section through a longitudinal vertical plane.

 In accordance with FIG. 1, the laser given as an example comprises a semiconductor wafer 2 defining a longitudinal direction X, a transverse direction Y perpendicular to this longitudinal direction, and horizontal planes parallel to these longitudinal and transverse directions. This plate also defines a vertical direction Z perpendicular to these horizontal planes. Lengths and thicknesses are measured in these longitudinal and vertical directions, respectively.

 The plate 2 carries a lower electrode layer 4 and an upper electrode layer 6, 8 for passing internal electric currents from one to the other of these faces through this plate. The lower layer 4 has a common electrode 4 while the upper layer has a longitudinally successive modulation electrode 6 and an amplification electrode 8.

The electrode 6 can pass two superimposed electric currents, namely a first constant supply current I1 and a variable modulation current Im. The electrode 8 passes a second constant supply current I2. The modulation current Im is controlled to temporarily vary the frequency of the laser and thus carry out differential phase modulation of the known type known as
DPSK.

 The plate 2 comprises an active strip 10 having a length between two ends 10A, 10B. This tape consists of an optically active material and it is vertically interposed between a lower confinement layer 12 of a first type of conductivity and an upper confinement layer 14 of a second type of opposite conductivity.

Each supply current passing through this active ribbon allows this active material to amplify a laser light by recombination of charge carriers that this current injects into this active ribbon from these confinement layers.

This active tape is included in a guide assembly which extends longitudinally while being interposed vertically and transversely between materials having refractive indices lower than that of materials of this assembly. This assembly thus constitutes a light guide for guiding said laser light in the direction
X.

 More particularly, the guide assembly comprises, in addition to the active strip 10 made up of an InGaAs ternary alloy, a passive guide 18 made up of an InAsGaP quaternary alloy and separated from the active strip 10 by an intermediate layer 16. This layer is made up, like the confinement layers 12 and 14, of a binary alloy InP whose refractive index is lower than those of the active strip and the passive guide. Its thickness is small enough to make an optical coupling between this strip and this guide.

 More particularly still, from bottom to top, the lower confinement layer 12, the active strip 10, the intermediate layer 16, the passive guide 18 and the upper confinement layer 14 are encountered.

 Other layers are conventionally formed in the wafer 2 and are not shown. These are in particular lateral layers situated transversely on either side of the guide assembly, and contact layers formed between the confinement layers and the electrodes.

 An optical feedback network 20 is formed by the guide assembly. It extends longitudinally between two ends 20A, 20B which coincide longitudinally with the two ends lOA, lOB of the active strip 10. It is formed by a network surface which separates two materials 16, 18 with different refractive indices and which s extends longitudinally by raising and lowering periodically according to a network pitch H measured longitudinally. This pitch is adapted to a wavelength of said laser light in order to reflect this light longitudinally in a distributed manner over the length of this grating and thus to cause at least one resonance mode to appear for this light, each such mode of resonance being associated with a wavelength of this light close to the wavelength to which this network pitch is adapted. This network has the effect that the guide assembly constitutes, for the laser light, a resonant cavity , the longitudinally extreme faces of the plate 2 carrying anti-reflective coatings.

 According to a known arrangement indicated above, it presents a longitudinal succession of network phase shifts, the positions and values of which are chosen to select a single said resonance mode and the associated wavelength and to regularize the longitudinal distribution of the intensity of the light with this wavelength in the active ribbon. These phase shifts are alternated, that is to say that their absolute values are equal and that they have an alternately positive and negative algebraic sign.

 In known lasers, such phase shifts, alternated or not, are distributed symmetrically over the length of the resonant cavity. In addition, when they are alternated, they are in odd number so that the symmetry of the network is complete, that is to say relates to the positions, the values and the signs of the phase shifts, a phase shift being located in the middle of the cavity. On the contrary, according to the present invention, said succession of phase shifts consists of two phase shifts 22, 24, the positions of these phase shifts being asymmetrical relative to the ends 20A, 20B of the feedback network 20.

 More particularly, these phase shifts are located on the same side of a medium 20M of the feedback network 20. More particularly still, they have the same absolute value equal to 25%, to within 10%, of the network pitch H and are located, starting from the same end 20B of the feedback network 20, at two distances respectively equal to 13% and 46%, more or less 4%, of the length of this network. They are illustrated in FIGS. 2 and 3, it being understood that the actual shape of the engraving is significantly different from the very schematic shape shown.

 The choice of these values and positions of the phase shifts has the effect, in the case where the end faces of the plate 2 are not reflective and where the feedback network extends continuously from one end to the other of the active strip, to ensure a single mode operation of the laser and a sufficiently homogeneous distribution of the light intensity over the length of this strip.

 Other asymmetrical arrangements would however be preferable in other cases, the asymmetry making it possible to usefully use two alternating phase shifts, provided that the optimum values and positions are sought in each case.

 A method of manufacturing such a laser comprises the following operations - operations of depositing semiconductor layers to form the lower confinement layer 12, the guide assembly 10, 16, 18 and the upper confinement layer 14, - a network etching operation which is longitudinally selective and which is carried out between two said operations of depositing two semiconductor layers 16, 18 of different refractive indices to form the network surface 20, - a delimitation etching operation which is transversely selective for delimiting said guide assembly transversely, - operations for depositing lateral layers to complete the semiconductor wafer 2 on either side of this guide assembly, - and operations for forming the electrodes 4, 6, 8.

 In accordance with FIG. 4, the network etching operation includes the following operations: Depositing a layer of photosensitive resin 30 on a semiconductor layer which constitutes a network base layer and which is for example the intermediate layer 16.

- Installation on this layer of resin of a phase shift blade 31.

This blade is transparent. It extends longitudinally, alternately having first and second thicknesses El and
E2 separated by transition zones 32 and 34 covering the future network phase shifts. This blade is for example made of silica.

The variation of its thickness is obtained by ionic machining. It has anti-reflective coatings 31A and 31B.

- Insolation of this resin layer through this phase shift blade by two light beams B1 and B2 from the same source of sunshine 36. These two beams have asymmetrical directions relative to the surface to be exposed. They show transverse interference lines thanks to which this resin has, vis-à-vis a selective washing medium, a selective sensitivity according to these lines. This source of exposure is for example constituted by a laser 36. The beam which it emits is divided by a semitransparent plate 38 and returned by mirrors 40, 42 and 44 to form the beams B1 and B2.

- Washing of this resin using this selective washing medium so that residual bands of this resin resist this medium and constitute a network mask reproducing these lines.

- Attack of this network base layer through this network mask.

- And washing of these residual strips of resin.

Claims (5)

1 / Single-mode semiconductor laser with distributed feedback, characterized in that its optical feedback network (20) has two alternating phase shifts (22, 24) whose positions are asymmetrical relative to the medium (20M) of this network. 2 / laser according to claim 1,  this laser comprising a semiconductor wafer (2) defining a longitudinal direction (X), a transverse direction (Y) perpendicular to this longitudinal direction, and horizontal planes parallel to these longitudinal and transverse directions, this wafer also defining a vertical direction (Z ) perpendicular to these horizontal planes, lengths and thicknesses being measured in these longitudinal and vertical directions, respectively,  this plate carrying a lower electrode layer (4) and an upper electrode layer (6, 8) for passing internal electric currents from one of these layers to the other through this plate,  this plate comprising an active ribbon (10) having a length between two ends (1or, lOB), this ribbon being made of an optically active material and being vertically interposed between a lower confinement layer (12) of a first type of conductivity and an upper confinement layer (14) of a second type of conductivity opposite to the first so that a said internal electric current constituting a supply current and passing through this active tape allows this active material to amplify a laser light by recombination of charge carriers that this current injects into this active ribbon from these confinement layers,  this plate comprising a guide assembly (10, 16, 18) which includes this active ribbon and which extends longitudinally, being interposed vertically and transversely between materials having refractive indices lower than that of materials of this assembly, so constituting a light guide to guide said laser light in said longitudinal direction,  this guide assembly constituting an optical feedback network (20) which extends longitudinally between two ends (20A, 20B) substantially coinciding, in the longitudinal direction, with the two ends (10A, lOB) of the active strip (10), this network being formed by a network surface which separates two materials (16, 18) of different refractive indices and which extends longitudinally by raising and lowering periodically according to a network pitch (H), this pitch being measured longitudinally and being adapted to a wavelength of said laser light in order to reflect this light longitudinally in a distributed manner over the length of this grating and thus to cause at least one resonance mode to appear for this light, each such mode of resonance being associated with a wavelength of this light close to the wavelength to which this grid pitch is adapted,  this feedback network having a longitudinal succession of network phase shifts, the positions and values of which are chosen to select a single said resonance mode and the associated wavelength and to regularize the longitudinal distribution of the intensity of the light having this wavelength in said active strip, these phase shifts being alternated,  this laser being characterized by the fact that said succession of phase shifts is constituted by two phase shifts (22, 24), the positions of these phase shifts being asymmetrical relative to the ends (20A, 20B) of the feedback network (20). 3 / Laser according to claim 2, characterized in that the two said network phase shifts (22, 24) are located on the same side of a medium (20M) of the feedback network (20). 4 / A laser according to claim 3, characterized in that the two said network phase shifts (22, 24) have an absolute value equal to 25%, to within approximately 10%, of said network pitch H and are located, from the same end (20B) of the feedback network (20), at two distances respectively equal to 13% and 46%, more or less 4%, of the length of this network. 5 / A method of manufacturing a laser according to claim 2, this method comprising - operations of depositing semiconductor layers to form said lower confinement layer (12), said guide assembly (10, 16, 18) and said layer upper confinement (14), - a network etching operation which is longitudinally selective and which is carried out between two said operations of depositing two semiconductor layers (16, 18) of different refractive indices to form said network surface ( 20), - a delimitation engraving operation which is transversely selective in order to delimit said guide assembly transversely, - operations for depositing lateral layers to complete said semiconductor wafer (2) on either side of this guide assembly, - and operations for forming said electrodes (4, 6, 8),  said network etching operation comprising the following operations - depositing a layer of photosensitive resin (30) on a said semiconductor layer which is a network base layer 16, - placing a blade on this resin layer transparent phase shift (31) extending longitudinally and alternately having a first (El) and a second (E2) thicknesses separated by transition zones (32, 34) covering the future so-called network phase shifts, - exposure of this layer of resin through this phase shift blade by two light beams (B1, B2) coming from the same insolation source (36) to form transverse interference lines thanks to which this resin presents, with respect to a medium of selective washing, a selective sensitivity according to these lines, - washing of this resin using this selective washing medium so that residual bands of this resin resist this medium and const itute a network mask reproducing these lines, - attack of this network base layer through this network mask, - and washing of these residual strips of resin.