DE10316220A1 - Semiconductor laser amplifier has active layer sandwich between input and output facets and convergent preamplification structure in charge carrier input region - Google Patents

Abstract

A semiconductor laser amplifier comprises input (14) and output (16) facets coupled to an extending semiconductor sandwich of an optically active layer (28) and boundary layers (26,30) with electrical contact regions (12,32) having a narrowing pre-amplification section (20) on the input side and a broadening main output side. An independent claim is also included for a laser system comprising the above.

Description

The The invention relates to a semiconductor laser amplifier and a laser system with at least one semiconductor laser amplifier and a laser.

Semiconductor laser amplifier for optical amplification an optical signal of a laser are known. Such semiconductor laser amplifiers are for example, as edge-emitting semiconductor heterostructures constructed in which the optical signal to be amplified via a End face, the so-called Eingangsfacette, in the semiconductor layer structure is coupled, at least once passes through the opposite end, the so-called output facet, coupled out as amplified optical signal becomes. The amplification process in the Semiconductor laser amplifier is similar to the Working principle of a semiconductor laser, wherein an optically active Layer, for example, a complementary doped semiconductor regions introduced quantum well or Multiquantentopfstruktur, is pumped electrically. The optical Signal gets during its passage through the optically active layer, which is the gain medium represents, over stimulated emission processes of injected into the active layer Reinforced electron-hole pairs.

issues conventional Laser systems with semiconductor laser amplifiers in particular concern unwanted optical feedback of the semiconductor laser amplifier with the signal laser and unsatisfactory optical saturation characteristics in the semiconductor laser amplifier.

task the invention is to propose an improved semiconductor laser amplifier, which improved feedback characteristics in operation with a laser and has improved saturation characteristics. It is another object of the invention to provide a corresponding improved Specify laser system with a laser and an improved semiconductor laser amplifier.

This Objects are achieved by a semiconductor laser amplifier with the in claim 1 specified features and a laser system with the in claim 10 solved characteristics. Preferred embodiments are the subject of the dependent Claims.

• – eine Halbleiterstruktur mit – einer Eingangsfacette, welche zur Einkopplung eines optischen Signals ausgelegt ist; – einer Ausgangsfacette, welche zur Auskopplung eines verstärkten optischen Signals ausgelegt ist; – zumindest einer optisch aktiven Schichtstruktur aus Halbleitermaterial, welche zwischen angrenzenden Begrenzungschichtstrukturen aus Halbleitermaterial angeordnet ist und sich zwischen der Eingangs- und der Ausgangsfacette erstreckt; und
• – zumindest einen ersten und einen zweiten elektrischen Kontaktbereich zur Injektion von elektrischen Ladungsträgern über die Begrenzungsschichtstrukturen in die optisch aktive Schichtstruktur zur Verstärkung des optischen Signals, wobei zumindest einer der Kontaktbereiche einen eingangsfacettenseitigen Vorverstärkungsabschnitt, welcher sich in Richtung von der Eingangsfacette zu der Ausgangsfacette verjüngt, und einen ausgangsfacettenseitigen Hauptverstärkungsabschnitt, welcher sich in Richtung von der Eingangsfacette zu der Ausgangsfacette erweitert, umfaßt.

According to the invention comprises a semiconductor laser amplifier

• A semiconductor structure having an input facet adapted to couple an optical signal; An output facet adapted to extract a amplified optical signal; - At least one optically active layer structure of semiconductor material, which is disposed between adjacent boundary layer structures of semiconductor material and extending between the input and the Ausgangsfacette; and
• At least a first and a second electrical contact region for injecting electrical charge carriers via the boundary layer structures into the optically active layer structure for amplifying the optical signal, wherein at least one of the contact regions has an input facet side preamplification section which tapers in the direction from the input facet to the output facet, and an output facet-side main amplifying section which widens in the direction from the input facet to the output facet.

at the semiconductor laser amplifier according to the invention it is preferably a so-called edge-emitting semiconductor laser amplifier, which preferably a monocrystalline, epitaxially grown semiconductor layer structure includes. On optical signal, which is generally generated by a laser will be over the input facet of the semiconductor structure of the semiconductor laser amplifier at least partially in the optically active layer structure of semiconductor material coupled. The optical signal passes through the active layer structure, in which by "electrical Pumps "electron-hole pairs be introduced so electron-hole recombination via stimulated emission processes for reinforcement of the optical signal. The active layer structure may be, for example, a Quantum well or multi-quantum well structure with optionally suitable vertical wave guide act. The optically active layer structure is preferably between arranged two Begrenzungsschichtstrukuren, which preferably complementary doped to each other, i. one of the boundary layer structures is n-doped during the other boundary layer structure is p-doped. About the corresponding boundary layer structures are electrons or holes introduced into the optically active layer structure to stimulate there Emission processes for reinforcement to effect the optical signal.

The optically active layer structure is by means of the first and second electrically "pumped" electrical contact area to produce a population inversion. Preferably, each of the electrical contact areas is accurate one of the boundary layer structures in electrical and mechanical surface contact connected.

The Invention proposes before, at least one of the contact areas in its lateral form, i.e. in a layer plane parallel to the optically active layer structure, to train in a special way. In particular, at least one the contact areas are designed such that it has a preamplification section exhibits, which is in the propagation direction of the optical signal (Beam direction) tapers in its width. Such a rejuvenation of the Vorverstärkungsabschnitts in the direction from the input facet to the output facet, that yourself also that region of the optically active layer structure, which is electrically pumped and which in a direction perpendicular to the layer plane adjacent to the preamplification section, accordingly rejuvenated. The lateral shape of the contact area in a tapered Vorverstärkungsabschnitt and a flared output facet-side main amplification section is transmitted thus on the optically active layer structure such that in the optically active Schichtstrukur essentially only areas electrically be pumped, which in the normal direction of the layer plane of the optically active layer structure adjacent to the structured contact region are arranged. The area of the optically active layer structure, in which charge carrier be introduced, is thus by structuring the one Contact area in the Vorverstärkungsabschnitt and the main reinforcement section certainly. In the rest Areas of the optical layer structure are not electrical Charge carriers over the Boundary layer structures introduced so that these areas not to reinforcement contribute to the optical signal.

By the special design in the lateral plane or layer plane of at least one of the contact areas is thus caused to change the width the optically active layer structure in parallel to the layer plane and the direction perpendicular to the beam direction, in which charge carriers to Generation of a population inversion can be injected in the direction from the input facet to the output facet or in the beam direction first decreases and later increases again.

The optically active layer does not necessarily extend over the whole length the semiconductor structure, i. not necessarily of the input up to the starting facet. Between the preamplification section and the main reinforcement section may be provided an intermediate portion in which the lateral width of the contact area does not change significantly. Preferably only one of the contact areas in the lateral plane with a Vorverstärkungsabschnitt and a main reinforcement section structured. The second contact region is preferably unstructured and extends in the lateral plane, i. in one to the optically active Layer structure parallel layer plane, over the structured contact area out.

The Design of the at least one contact region with a tapered preamplification section and a flared main reinforcing section surprisingly results to particularly favorable Reinforcement properties of the Semiconductor laser amplifier. In particular, the semiconductor laser amplifier according to the invention is essential improved optical saturation properties and significantly improved feedback characteristics on, as later will be described in detail. Hereduch also results in an advantageous Reduction of reinforced spontaneously emitted radiation (ASE radiation) over the induced-emitted Radiation in the optically active layer structure of the semiconductor laser amplifier. at the ASE radiation is an unwanted "interference radiation" or an unwanted amplified optical Signal stimulated-emitiert compared to the desired Radiation of the semiconductor laser amplifier have a low intensity should. How later is described in detail leads the rejuvenating or taped preamplification section to a better optical saturation the optically active layer structure in the main reinforcement section, resulting in spontaneous recombination processes of injected carrier pairs across from stimulated emission processes become less likely and thus the parasitic ASE radiation across from the desired increased induced-emitted Radiation decreases.

Further allows a semiconductor laser amplifier according to the invention due to the particular lateral design of at least one of the contact areas a strong compared to known semiconductor laser amplifiers reduced feedback with the signal laser, which also gives the overall optical properties and the efficiency of a semiconductor laser system with a laser and a semiconductor laser amplifier according to the invention crucial be improved.

According to a preferred embodiment of the invention, a waist section of the one contact region is arranged between the preamplification section and the main reinforcement section. The waist portion is arranged in the direction from the input to the output facets, that is, in the direction of the propagating optical signal, between the preamplification portion and the main amplification portion. The lateral position of the waist portion preferably coincides in operation with a beam waist of the optical signal substantially. In a lateral plane, ie in a layer plane parallel to the optically active layer structure, the optical signal essentially passes through that region of the optically active layer structure ture, which is electrically pumped through the correspondingly structured contact area.

Preferably comprises the at least one contact region comprises an electrically conductive contact layer structure, which adjoins one of the boundary layer structures. The contact layer structure preferably has a metal layer, which in electrical and mechanical surface contact with one of the preferably highly doped boundary layer structures stands.

Preferably The layer planes of the contact layer structure and the optical extend active layer structure substantially parallel to each other. The Contact layer structure, which is a multi-layered structure or a single layer structure, is preferably by planarlithographic preparation techniques defined on the semiconductor structure and thus has a layer or area level which is substantially parallel to the layer plane of the optical active layer structure is. The layer plane of the optically active Layer structure preferably corresponds to the surface level a semiconductor substrate, in or on which the semiconductor structure is defined. The normal direction of the optically active layer structure or of the substrate in this case preferably corresponds to the epitaxial Growth direction of the semiconductor layer structures.

Preferably the normal directions of the input and the output facets run in each case parallel to the layer plane of the optically active layer structure. The input and output facets are, for example around fractured surfaces a monocrystalline semiconductor structure (so-called "cleave" surfaces), wherein the input and output facets in particular run parallel to each other. However, it may also be advantageous to have the input and the To tilt output facet such that their normal directions not parallel to the layer plane of the optically active layer structure run.

The preamplification section and / or the main reinforcement section preferably has a substantially trapezoidal surface configuration. In this case, the surface plane is parallel to the layer plane of the optically active layer structure, ie parallel to the surface plane of the semiconductor substrate. The longer base side of the trapezoidal surface shape of the preamplification section (or the main reinforcement section) faces the input facet (or the output facet), while the shorter base side is directed away from the corresponding facet. Preferably, the surface shape is an isosceles trapezoidal shape. Particularly preferably, both the preamplification section and the main reinforcement section are trapezoidal. Particularly advantageous is a design in which the trapezoidal angles α _V for the preamplification section and α _H for the main reinforcement section, ie the internal angles between the two elongated intersecting trapezoidal sides, are substantially identical. Preferably, the trapezoid angle is between 2 degrees and 15 degrees, more preferably between 3 degrees and 10 degrees.

Preferably applies _Before l / l _main <1, preferably l _Before / l _main <0.8, where l is the length of the _Prior Vorverstärkungsabschnitts and l the length of the _main portion of each of the main reinforcement in the direction from the input to the output facet or in the propagation direction the semiconductor amplifier are simply passing optical signal.

Preferably is at least one area of the electrically conductive Contact layer structure, which of the boundary layer structure facing away, with an electrically conductive heat sink in electrical and mechanical surface contact. Thus Both the electrical and the mechanical contact takes place the contact layer structure over their outer surface, which for example, is soldered to a metal bracket to over the large area contact the Semiconductor device to remove heat loss efficiently.

Preferably has the semiconductor laser amplifier at least one absorber structure for absorbing an undesired one optical signal, wherein the absorber structure at least adjacent to a waist portion of the contact area or adjacent to the entire Contact area is arranged. The absorber structure comprises a material which in the wavelength range of the optical signal has a high absorption coefficient.

According to one Another aspect of the invention comprises a laser system according to the invention at least a previously described semiconductor laser amplifier according to the invention and at least one laser. Preferably, the laser is designed in this way and re of the semiconductor laser amplifier is arranged that a at least partially generated by the laser optical signal via the input facet coupled into the optically active layer structure of the semiconductor laser amplifier is where a focus of the optical signal in an optical to be active Layer structure parallel layer structure layer or layer plane within the semiconductor laser amplifier.

In the laser system according to the invention, the optical signal to be amplified, ie the incident laser beam, is preferably coupled into the semiconductor laser amplifier in such a way that the optical signal has a (virtual) focus in the layer plane of the optically active layer structure of the semiconductor laser amplifier and not - as conventionally - focused on the input facet. Since the focus of the incoming optical signal in the layer plane does not coincide with the input facet but is preferably spaced apart from the input facet in the optically active layer, the optical power density at the input facet is less than when the incoming optical signal focuses on a smaller area of the input facet becomes. The advantages associated with this will be discussed in detail later.

Preferably, the laser is arranged and positioned relative to the semiconductor laser amplifier such that the focus of the optical signal in the layer structure plane is spaced from the input facet by a distance substantially equal to a length l _before the preamplification section in the direction from the input to output facets. In the layer structure plane, the focus of the incoming optical signal is thus preferably at the same distance from the input facet in the direction of propagation of the optical signal as the preamplification section extends. Accordingly, the beam profile in layer structure plane is adapted to the geometric shape of the preamplification section or the shape of the electrically pumped region of the optically active layer structure.

Preferably the laser is designed and arranged with respect to the semiconductor laser amplifier, that the Focus of the optical signal in the layer structure level substantially is in a region of the optically active layer structure, which in the normal direction of the layer structure plane adjacent to the waist section the contact area is arranged. According to this particularly preferred embodiment a laser system according to the invention the optical input signal is coupled laterally such that in layer structure plane of Focus of the optical signal in a region of the optically active Layer structure lies, which essentially from the waist section the contact area is electrically pumped. The lateral beam path The optical signal can thereby be particularly good for the tapered Vorverstärkungsabschnitt customized whereby a particularly good optical saturation in the main reinforcement section of the semiconductor laser amplifier possible is.

Preferably the laser is designed and arranged with respect to the semiconductor laser amplifier, that this optical signal in the area of the input facet an astigmatic Beam profile, wherein a focus of the optical signal in a plane perpendicular to the layer structure plane substantially lies in an area of the entrance facet. According to this particular preferred embodiment a laser system according to the invention becomes the optical signal or the incoming laser beam in "astigmatic Way "coupled into the semiconductor laser amplifier. While the focus of the optical signal in layer structure plane within of the semiconductor laser amplifier is located, the optical signal is in one of the layer structure level vertical and parallel to the beam direction plane coupled such that in this level of focus of the optical signal substantially with the input facet coincides. The focus of the injected optical signal in the layer structure plane falls thus with the focus perpendicular to the layer structure plane not together, so that it is an astigmatic coupling of the optical signal in the semiconductor laser amplifier is.

at This astigmatic coupling is utilized that, with a suitable choice of optically active layer structure and limiting structures the optical signal essentially within the optically active layer structure in the form guided by waveguide modes becomes. In layer structure level, i. in a lateral direction however, no index or waveguide guidance of the optical signal instead of. In this plane, the optical signal is coupled in such a way that this Radiation field in the optically active layer structure substantially coincides with that part of the optically active layer structure, which is pumped electrically.

Preferably comprises the laser system a coupling optics for coupling the optical Signal in the semiconductor laser amplifier. The coupling optics can comprise at least one cylindrical lens or astigmatic lens, to effect an astigmatic coupling.

According to one another preferred embodiment a laser system according to the invention comprises the coupling optics a slit with a perpendicular to the layer structure level extending slot direction, wherein the slit in the region of a Intermediate focus of the optical signal is arranged to feedback to reduce optical signals from the semiconductor laser amplifier in the laser.

Preferably, at least one optical attenuation or attenuation element is arranged in the beam path between the laser and the semiconductor laser amplifier. The optical signal of the laser passes through the attenuation element once to get to the semiconductor laser amplifier. The signal beam (reflected laser beam) reflected at the input facet of the semiconductor laser amplifier, on the other hand, has to pass through the attenuation element twice. Likewise, the laser reflected ASE radiation must pass through the damping element twice. Thus, the feedback can be attenuation by arranging at least one damping element in the beam path further increase.

The Invention will be more preferred with reference to accompanying drawings embodiments described by way of example. It shows:

1a ) - d ) schematic plan views of conventional versions of semiconductor laser amplifiers with different contact areas;

2a a schematic plan view of a preferred embodiment of a semiconductor laser amplifier according to the invention;

2 B a schematic sectional view of the in 2a illustrated semiconductor laser amplifier along the line AA;

3a a schematic view of the in 2 illustrated semiconductor laser amplifier with cylindrical coupling optics and beam path;

3b a schematic sectional view of the in 3b illustrated semiconductor laser amplifier with coupling optics;

4 a preferred embodiment of a laser system according to the invention with a semiconductor laser amplifier according to 2 and 3 ;

5a a schematic plan view of another preferred embodiment of a semiconductor laser amplifier according to the invention with a rib waveguide and / or absorber structure in the waist portion;

5b a schematic plan view of another preferred embodiment of a laser amplifier according to the invention with curved rib waveguide and / or absorber structure and Vorverstärkungsabschnitt;

5c a schematic plan view of another preferred embodiment of a laser amplifier according to the invention, in which the absorber structure extends along the entire length of the contact region;

6 a diagram showing the composition of the optically active layer structure and the surrounding boundary layer structures as a function of the location in the normal direction of the layer structure level of the optically active layer structure (growth direction of the semiconductor layer structure) of a preferred embodiment of a semiconductor laser amplifier according to the invention;

7a ) to G ) schematic sectional views of different stages of production of a preferred embodiment of a semiconductor laser amplifier according to the invention during the planarlithographic preparation;

8th a light microscope view of a preferred embodiment of a semiconductor laser amplifier according to the invention; and

9 a scanning electron micrograph, perspective view of two semiconductor laser amplifier according to a preferred embodiment of the invention, which are soldered with their respective contact areas on a metallic heat sink and electrically contacted on the back by means of bonding wires.

In 1a ) to d ) are schematic top views of typical conventional semiconductor laser structures 100 with differently designed contact areas 102 shown. The viewing direction of the plan views runs along the normal direction of a layer plane of the optically active layer structure, ie in the growth direction of the semiconductor structure. The optical axis, which corresponds substantially to the beam direction of the optical signal to be amplified, is indicated by a dashed-dotted line. In the illustrated semiconductor laser amplifiers 100 it is a so-called "one-pass" semiconductor laser amplifier, which via an input facet 104 coupled optical signal amplified in a single pass through the (not shown) optically active layer structure and as an amplified optical signal on the output facet 106 outputs. The amplification takes place in a known manner by electrical "pumping" of the optically active layer structure, wherein electrical charge carriers from the contact region 102 and a second contact region arranged on the opposite semiconductor structure side is injected into the optically active layer structure via boundary layer structures (not shown). The optically active layer structure thus has electron-hole pairs which recombine via stimulated emission events and so that optical signals can amplify.

The geometry of the electrical contact regions thus determines that layer region of the optically active layer structure which is electrically "pumped" and can contribute to the optical amplification of the optical signal in the semiconductor structure. Only those layer regions of the optically active layer structure which essentially in the normal direction of the layer structure plane, ie in a direction perpendicular to the plane in 1 , below the (structured) electrical contact area 102 lie, contribute to reinforcement of the optical signal, since only these layer areas are electrically pumped. The electrical contact area 102 has for this purpose an electrically conductive contact layer structure (not shown) which is in electrical and mechanical surface contact with the adjacent boundary layer structure. The boundary layer structure, which is typically a highly doped semiconductor structure, adjoins the optically active layer structure.

Conventional semiconductor laser amplifiers often have a strip-shaped contact area 102 constant width, as in 1a ), formed. In 1b ) is another known structure of a semiconductor laser amplifier 100 shown in which the contact area 102 one from the input facet 104 to the starting facet 106 has flared surface shape. In 1c ) is a variant of in 1b ) illustrated known embodiment, in which near the Eingangsfacette 104 a rib waveguide 108 is trained. In 1d ), the input facet side portion of the contact area additionally has a curved shape.

In 2a ) is a schematic plan view of a preferred embodiment of a semiconductor laser amplifier according to the invention 10 shown. The viewing direction of the supervision according to 2a ) corresponds to that of 1 ie perpendicular to a layer structure plane of the semiconductor structure, in particular of the optically active layer structure. The semiconductor laser amplifier 10 has a first contact area 12 on. The contact area 12 preferably extends over the entire semiconductor laser amplifier 10 from an input facet 14 up to an output facet 16 which are formed, for example, as fracture surfaces ("cleave" surfaces) of the monocrystalline semiconductor structure. The entrance facet 14 and the output facet 16 preferably have an antireflection coating to avoid an optical standing wave field within the semiconductor structure and to minimize coupling and decoupling losses. In the semiconductor laser amplifier 10 according to 2 is a "one pass" semiconductor laser amplifier. As already discussed in detail with 1 the optical signal to be amplified is described via the input facet 14 coupled and passes through the (not shown) optically active layer structure substantially along the marked with a dash-dot line optical axis to the output facet 16 to be decoupled.

Unlike all in 1 shown known embodiments of semiconductor laser amplifiers, the in 2a ) illustrated preferred embodiment of the invention a tapered Vorverstärkungsabschnitt 20 and a main reinforcing portion which widens in the beam direction 22 on. The preamplification section 20 is formed substantially trapezoidal, with the longer trapezoidal base of the input facet 14 facing and the shorter trapezoidal base of the input facet 14 turned away. The trapezoid height preferably coincides with the optical axis of the semiconductor laser amplifier 10 together. The trapezoidal side surfaces of the preamplification section 20 are inclined to each other so that in the direction of the Eingangsfacette 14 to the starting facet 16 the preamplification section 20 Tapered, ie narrowed in width. The inside angle between the (extended) sides of the trapezoid is denoted by α _V.

The shorter trapezoidal area of the preamplification section 20 adjoins a waist section 24 which has an approximately constant width or a rounded shape. The waist section 24 goes into the main reinforcement section 22 over, which is formed ausgangsfacettenseitig. The longer base of the substantially trapezoidal main reinforcement section 22 is the starting facet 16 while facing the shorter footprint of the output facet 16 turned away. The length I _{main of} the main reinforcement section 22 is greater than the length I _before the preamplification section 20 , wherein the length is designated in the main radiation direction. The preamplification section 20 and the main reinforcement section 22 preferably have an isosceles trapezoidal shape. The trapezoidal main reinforcement section 22 has a trapezoidal angle α _H , which preferably substantially the trapezoidal angle α _V of Vorverstärkungsabschnitts 20 equivalent.

In 2 B ) is a schematic and simplified sectional view of the embodiment according to 2a ). The section plane runs along the dash-dot line AA of 2a ). The semiconductor structure has a layer structure of substantially parallel semiconductor layers. The contact area 12 adjoins a first boundary layer structure 26 which, on the optically active layer structure 28 is arranged. The optically active layer structure 28 , which may comprise several layers, is in 2 B ) simplified merely as a single line or layer shown. The optically active layer structure 28 is at the second boundary layer structure 30 arranged, which may also have a plurality of partial layers in general. The second contact area 32 is disposed on the opposite side of the semiconductor structure. Electric charge carriers are in operation by the electrical contact areas 12 and 32 over the boundary layer structures 26 and 30 in the optically active layer structure 28 injected.

In contrast to known semiconductor laser amplifiers, the geometry of at least one of the contact areas 12 . 32 adapted to the radiation field of the optical signal to be amplified. As will be described in detail later, the optical signal to be amplified is preferably coupled into the semiconductor laser amplifier so as to form in the lateral chip plane, ie in the layer structure plane, a beam waist in the interior of the semiconductor structure substantially at the position of the waist portion 24 is adjacent. In a direction perpendicular to the layer structure plane, which with the plane of 2 B ) coincides, the optical signal is coupled in such a way that its beam waist substantially with the Eingangsfacette 14 of the semiconductor laser amplifier 10 coincides.

Such a coupling can, for example, with a cylinder optics 38 be made as in 3a ) and b ) is shown schematically. The beam path of the optical signal is schematically by means of boundary lines 34 represented, wherein the drawing plane of 3a ) and b ) those of 2a ) respectively. b ) corresponds. As in 3a ) is shown, in a lateral plane, ie in layer structure plane, the optical signal 34 not on the input facet 14 focused, but points first inside the semiconductor laser amplifier 10 a beam waist 36 on. The beam waist 36 , which are within the optically active layer structure 28 forms essentially lies in a direction perpendicular to the layer structure plane below the waist section 24 of the structured contact area 12 , As in 3a ) is the lateral beam path 34 the surface design of the contact area 12 adjusted, so that in layer structure plane of the beam path 34 with that region of the optically active layer structure 28 which collapses through the structured contact area 12 electrically "pumped" is.

The coupling of the optical signal 34 in the semiconductor laser amplifier 10 is preferably carried out in an astigmatic manner, which in particular by means of a suitable cylinder optics 38 can be done. As in 3b ) is shown, in a plane perpendicular to the layer structure plane, which is parallel to the plane of the drawing, the optical signal 34 so on the input facet 14 focuses that a maximum intensity component in the optically active layer structure 28 is coupled. In the vertical direction, ie in the normal direction of the layer structure plane, the optical signal in the optically active layer structure 28 preferably guided in the form of waveguide modes, while in the lateral direction no index guide is provided.

In 4 a preferred embodiment of a laser system according to the invention is shown. The laser system comprises a semiconductor laser amplifier according to the invention 10 , preferably a coupling optics 40 as well as a laser 42 , At the in 4 illustrated embodiment is a so-called astigmatic laser or trapezoidal laser 42 which has a different beam divergence and beam waist position in two mutually perpendicular directions. In the in 4 represented plan, which of the drawing plane the 2a ) and 3a ) and which is parallel to the layer structure plane, that of the laser 42 generated optical signal 47 through a first lens 44 bundled, passes through a slit diaphragm 45 to use a second lens 46 such over the input facet 14 of the semiconductor laser amplifier 10 to be coupled, that in the interior of the semiconductor laser amplifier 10 a lateral beam waist 36 in the area of the waist section 24 formed. The slot direction of slot formation 45 is chosen such that it runs parallel to the normal direction of the layer structure plane. Despite anti-reflection coating of the input facet 14 becomes a portion of the optical signal to be injected 47 at the entrance facet 14 reflected. The reflected radiation field 48 is in 4 illustrated by a dashed line, wherein it can be seen that the slit 45 to a significant attenuation of the semiconductor laser amplifier 10 in the signal laser 42 back-reflected signal provides.

In the following, the operation and advantages of the described preferred Auführungsformen inventive semiconductor laser amplifier 10 or laser systems shown. In laser amplifier systems, particularly semiconductor laser amplifiers, optical feedback occurs between the semiconductor laser amplifier 10 and the signal laser 42 on. By this feedback, the beam properties are degraded and - in extreme cases - the coupling can even lead to the destruction of one of the two components. For this reason, in conventional laser systems, a so-called optical isolator is usually indispensable, which between the signal laser 42 and the semiconductor laser amplifier 10 is used. However, such optical isolators represent cost-intensive building components and significantly limit the miniaturization of the laser system.

• 1. Reflexion des Signalstrahls, d.h. des einzukoppelnden optischen Signals 47, an der Eingangsfacette 14 des Halbleiterlaserverstärkers 10 zurück in den Signallaser 42;
• 2. Reflexion der sogenannten ASE-Strahlung (amplified spontaneous emission) aus der Eingangsfacette 14 des Halbleiterlaserverstärkers 10 an der Ausgangsfacette des Signallasers 42 zurück in den Halbleiterlaserverstärker 10; und
• 3. Kopplung der ASE-Strahlung aus der Eingangsfacette 14 des Halbleiterlaserverstärkers 10 in den Signallaser 42.

A distinction is made between the following optical feedback mechanisms:

• 1. Reflection of the signal beam, ie the optical signal to be coupled 47 , at the entrance facet 14 of the semiconductor laser amplifier 10 back to the signal laser 42 ;
• 2. Reflection of the so-called ASE radiation (amplified spontaneous emission) from the input facet 14 of the semiconductor laser amplifier 10 at the output facet of the signal laser 42 back into the semiconductor laser amplifier 10 ; and
• 3. Coupling of the ASE radiation from the input facet 14 of the semiconductor laser amplifier 10 in the signal laser 42 ,

conventional Due to their design, laser systems have a high sensitivity to optical Feedback, because the location of the optical beam waist in the vertical and horizontal Direction with the position of the emitting and reflecting Facet matches. In this area, the wave fronts of the radiation are flat, so that the on the facet surface reflected portion of the radiation with the incident beam path is almost congruent. The reflected radiation thus becomes back to the original one Focus point focused on the emitting facet, what for the under 1st and 2nd cases described equally applies.

In a laser system according to the invention, the reflected radiation field forms 48 in the lateral plane, a focus which lies clearly outside the focal plane of the beam-forming optics. Because the geometry of the reflected radiation field 48 strongly deviates from the incident beam geometry, the reflected radiation field 48 in a simple way by a slit 45 or an aperture can be hidden.

The in 4 sketched structure of a laser system according to the invention is based on a combination of a single-mode emitting laser with trapezoid geometry (astigmatic laser) 42 and a semiconductor laser amplifier according to the invention 10 , Due to its contact area design, the semiconductor laser amplifier according to the invention is also referred to as α-semiconductor laser amplifier. By suitable combination of the trapezoidal area length of the signal laser 42 and the distance from laser 42 and the semiconductor laser amplifier 10 can be dispensed with in this case on a cylinder optics. In the vertical direction, ie in a plane parallel to the plane of drawing of 3b ), that is from the signal laser 42 emitted beam collimated. The output facet of the signal laser 42 and thus the beam waist is in the focal plane of the lens 44 , In the lateral direction, ie in a direction parallel to the layer structure plane, is because of the astigmatism of the trapezoidal laser 42 the virtual beam waist continues from the lens 44 removed as the focal plane. Therefore, forms in the beam path to the lens 44 an intermediate focus, at its position - as described - a suitable slit diaphragm 45 can be attached. The slit diaphragm 45 leaves the optical input signal 47 happen while the above-mentioned in 1. and 2. Reflected portions of the radiation are largely hidden. The above-mentioned ASE radiation has a much lower spatial coherence compared to the signal radiation. Therefore, its jet waist diameter is at the position of the slit 54 greater than that of the signal beam. With a sufficiently small diaphragm aperture, therefore, this feedback mechanism can also be suppressed.

Another significant advantage of a laser system according to the invention or of a semiconductor laser amplifier according to the invention 10 is the increased signal gain in the preamplification section 20 , A characteristic of semiconductor laser amplifiers is the gain dependent on the optical power density. The optical power density refers to the optical power related to a surface element flooded by the light field, the gain being the relative optical power increase in relation to the traveled distance. Because of the vertical waveguide, the extent of the light field (of the optical signal) in the vertical direction, ie in the normal direction of the layer structure plane, is approximately constant. In a perpendicular, lateral direction, however, the extent of the radiation field varies since there is no waveguiding of the optical signal here. The convergence of the on the beam waist 36 incoming optical signal can by optical beam shaping by means of lenses in the input-side beam path with respect to the geometry of the contact area 12 be designed. Because the optical input signal is in the range of the input facet 14 Compared to conventional types of laser systems with semiconductor laser amplifiers is wider, the optical power density is significantly lower, resulting in a correspondingly high, unsaturated gain. The high gain and the convergent beam result in the optical power density in the beam waist at the same optical input powers 36 higher than at the corresponding location in conventional designs. As a result, better optical saturation of the main amplification section occurs 22 and there a reduction of the interfering ASE radiation. Same optical output powers on the output facet 16 lower optical input powers are required, or lower coupling efficiencies may be required at the input facet 14 be accepted. A lower necessary coupling efficiency reduces the requirement for the positioning accuracy of the coupling optics 40 , which represents a significant simplification of the system with corresponding cost reduction.

Furthermore, the described embodiment of the laser system according to the invention as well as the described embodiments of the semiconductor laser amplifier according to the invention is characterized by an increased damage threshold for the optical power at the input facet 14 out. The compatibility of optical power at the input facet 14 is determined by the maximum occurring power densities. This applies both to those from the input facet 14 emitted from the Halbleiterlaserver stronger 10 self-generated radiation, as well as for externally injected signal radiation. Due to the advantageously much wider version of the preamplification section 20 in the area of the input facet 14 and the resulting wider extension of the input side radiation field of the optical signal, the optical power densities correspondingly lower. The performance values at which damage to the input facet 14 occurs, are thus significantly increased.

The concept according to the invention of a preamplification section tapering in the beam direction 20 is distinguished from semiconductor laser amplifier designs using a lateral index waveguide to induce tapering of the waveguide to narrow the fundamental mode in an input-side portion of the contact region, thereby achieving greater beam divergence in the divergent trapezoidal region. In the semiconductor laser amplifiers according to the invention 10 preferably no lateral waveguide takes place in the preamplification section 20 , In addition, in the embodiments according to the invention, the optical signal to be amplified is preferably injected "astigmatically", whereby considerably improved feedback attenuations are achieved.

In comparison to conventional semiconductor laser amplifiers with lateral index waveguide (see 1c ) Furthermore, the simpler manufacturability of a semiconductor laser amplifier according to the invention may be mentioned, since the low manufacturing tolerances of the rib waveguide, which often represent a problem in practice, omitted.

In the beam path between laser 42 and semiconductor laser amplifiers 10 can additional (in 4 not shown) optical components, in particular optical attenuation or attenuation elements are arranged. The optical output power of the laser 42 is preferably a multiple of that of the semiconductor laser amplifier 10 required optical power. This opens up the possibility of making a targeted selection of the desired beam properties by introducing the additional optical components and of achieving a further reduction of the optical feedback. For example, by introducing an optical attenuation element, preferably an optical attenuation filter, with an attenuation of, for example, 15 dB in a single beam pass, the feedback attenuation can be increased to 30 dB since the feedback signal must pass through the attenuation element twice. The cost of such additional optical components is very low. When using a birefringent retardation plate (.lambda. / 4-plate), a corresponding reduction of the feedback attenuation is achieved with a substantially lower attenuation of the optical signal in a single pass. Another preferred variant is a wavelength selection in the optical input signal by introducing an optical grating.

In 5a ) to c ) are further preferred embodiments of semiconductor laser amplifier according to the invention in a schematic plan similar to 2a ). By additionally forming, for example, etched depressions 50 in the transition between the tapered and the widening trapezoid zone of the contact area 12 that is essentially in the region of the waist section 24 , a rib waveguide can be defined. The etched depressions are in 5a ) and b ) are represented by black blocks which are lateral on both sides of the waist portion 24 are arranged. As a result, a suppression of higher lateral modes can improve the beam quality of the semiconductor laser amplifier 10 to reach. In addition, the depressions can 50 be filled with a material that absorbs scattered optical radiation. At the in 5b ) embodiment is the contact area 12 in the area of the waist section 24 additionally curved, so as to cause a further improvement of the beam quality and the feedback characteristics. At the in 5c ) illustrated embodiment, the semiconductor laser amplifier 10 a large-scale absorber structure 50 ' which is adjacent to the preamplification section 20 and the main reinforcement section 22 extends.

The following is with reference to 6 to 9 a particularly preferred embodiment of a semiconductor laser amplifier according to the invention 10 described by way of example. In 6 is the material composition of the boundary layer structures 26 and 30 as well as for the optically active layer structure 28 used semiconductor material composition as a function of the position in the normal direction of the layer planes (growth direction of the layers). In particular, the aluminum content x of Al _x Ga _1-x As is shown on the positive ordinate labeled "x". On the negative ordinate, the indium content y of In _y Ga _1-y As is shown. In the 6 The vertical layer structure shown is preferably grown monocrystalline by means of molecular beam epitaxy (MBE) on a (100) -oriented, n-doped gallium arsenide substrate. In 6 the substrate is not shown, but begins only at higher positive abscissa values. The boundary layer structure 30 comprises an n-doped region of AlGaAs, which in 6 with "n-doped region" is designated. The donor is preferably silicon. In the growth direction of the semiconductor layer structure closes at this constant n-do For example, a layer sequence of n-AlGaAs with decreasing aluminum content, an InGaAs quantum well and a layer of p-AlGaAs with increasing aluminum content, which together form the optically active layer structure 28 form. The quantum well is surrounded by a thin bilayer of intrinsic GaAs, and consists of a pressure-strained, about 5 to 12 nm wide InGaAs film. The vertical optical waveguide is generated by the AlGaAs layer sequence with continuously varied aluminum content. The charge carrier injection occurs primarily in the quantum film.

On the basis of schematic sectional views along a cutting direction perpendicular to the layer plane of the semiconductor layer structures, a preferred production method of a preferred semiconductor laser amplifier according to the invention will be described below 10 described. The lateral structuring of the semiconductor laser amplifier 10 is preferably carried out by means of optical contact lithography on the p-doped side of in 6 illustrated semiconductor layer structure. In a first, in 7a ), a contact made of Ti / Pt / Au is applied and patterned by means of conventional lift-off technology. The thus structured p-contact 54 is then used as a mask around the heavily doped boundary layer structure 26 outside the contact area 12 to remove (cf. 7b )). Subsequently, in a second lithography and etching step, recesses are formed just above the quantum film plane, ie the optically active layer structure 28 etched. In 7e ), the intermediate stage is shown after completion of the etching step, wherein the photoresist mask used for the etching 56 still exists. Subsequently, for example, germanium is applied as the absorbing material and by peeling off the photoresist mask 56 Removed in unwanted areas by lift-off technology (see. 7d )). The germanium layer serves as absorber structure 50 ,

Subsequently, preferably an electrically insulating layer 58 for example, applied from Si ₃ N ₄ or SiO ₂ . Within the metallization surface of the p-contact 54 and the area in which the break edges for the separation of the components are later determined, for example, by scribing, the insulating layer 58 removed by reactive ion etching, for example with CF ₄ (cf. 7e )). Below is another Ti / Pt / Au metallization 59 evaporated and again removed by a lift-off process in the scribe area (see. 7f )). The Ti / Pt / Au layer 59 forms with the p-contact 54 the electrically conductive contact layer structure 52 out. In the last step of the lateral structuring, the thickness of the substrate is reduced to about 100 μm and the n-side contact 60 of the second contact area 32 , which consists of a Ge / Au / Ni / Au layer sequence, evaporated (see. 7g )). The electrically pumped part of the optically active layer structure is schematically indicated by reference numerals 61 designated.

An effective suppression of the self-oscillation of the semiconductor amplifier 10 is achieved by reducing facet reflectivity to values around 10 ^-4 or smaller. For this purpose, an antireflective coating is applied, for example by means of ion beam sputtering deposition. A coating with the required low reflectivity over a wide range of wavelengths and angles can only be achieved by multi-layer anti-reflection with at least three layers.

8th shows a light microscope view of a preferred embodiment of a semiconductor laser amplifier according to the invention 10 according to the structuring method described above. The structured contact area 12 with the preamplification section 20 and the main reinforcement section 22 is recognizable. The semiconductor laser amplifier 10 has a total length from the input facet 14 to the starting facet 16 of about 2.5 mm, a width of about 0.5 mm and a height of about 0.1 mm. The embodiment described allows an emission wavelength between 900 nm and 1100 nm, this restriction resulting from the semiconductor material system InGaAs / GaAs / AlGaAs used. Due to different laser-active materials, emission wavelengths can be realized far beyond the stated range. In 8th is in the range of the output facet 16 the overlap of the anti-reflection layer structure visible on the contact side.

In order to achieve a good dissipation of the heat loss, the semiconductor laser amplifier thus produced and isolated 10 with the contact 59 on an electrically conductive heat sink 62 soldered as in the scanning electron microscope perspective view of 9 is shown. At the in 9 illustrated embodiment are two semiconductor laser amplifier 10 parallel on a heat sink 62 applied, the n-contact 60 is electrically contacted by a series of bonding wires.

100

Semiconductor laser amplifier

102

contact area

104

input facet

106

output facet

108

Ridge waveguide

10

Semiconductor laser amplifier

12

contact area

14

input facet

16

output facet

20

Vorverstärkungsabschnitt

22

Main amplification section

24

waist section

26

Boundary layer structure

28

optical active layer structure

30

Boundary layer structure

32

(Second) contact area

34

ray tracing

36

beam waist

38

cylindrical optics or cylindrical lens

40

coupling optics

42

laser

44

lens or lens system

45

slit

46

lens or lens system

47

incoming Radiation field or optical signal

48

reflected radiation field

50

recesses if necessary filled with absorber material

52

electrically conductive contact layer structure (includes 54 and 59 )

54

p-contact made of Ti / Pt / Au

56

Photoresist mask

58

electrical insulating layer

59

Ti / Pt / Au contact

60

n-contact from Ge / Au / Ni / Au layer sequence

61

electrical pumped part of the optically active layer structure

62

heat sink

Claims (16)

Semiconductor laser amplifier ( 10 ) comprising - a semiconductor structure having - an input facet ( 14 ), which is designed for coupling an optical signal; An output facet ( 16 ) which is designed to extract a amplified optical signal; At least one optically active layer structure ( 28 ) made of semiconductor material, which between adjacent boundary layer structures ( 26 . 30 ) is arranged from semiconductor material and at least partially between the input facet ( 14 ) and the output facet ( 16 ) extends; and - at least a first ( 12 ) and a second ( 32 ) electrical contact area for the injection of electrical charge carriers via the boundary layer structures ( 26 . 30 ) in the optically active layer structure ( 28 ) for amplifying the optical signal, wherein at least one of the contact areas ( 12 ) an input facet side preamplification section ( 20 ), which extends in the direction of the input facet ( 14 ) to the starting facet ( 16 ), and an output facet-side main reinforcing portion (FIG. 22 ), which extends in the direction of the input facet ( 14 ) to the starting facet ( 16 extended). Semiconductor laser amplifier ( 10 ) according to claim 1, wherein between the preamplification section ( 20 ) and the main reinforcement section ( 22 ) a waist section ( 24 ) of the one contact area ( 12 ) is arranged. Semiconductor laser amplifier ( 10 ) according to claim 1 or 2, wherein the at least one contact region ( 12 ) an electrically conductive contact layer structure ( 52 ) attached to one of the boundary layer structures ( 26 ) adjoins. Semiconductor laser amplifier ( 10 ) according to claim 3, wherein the layer planes of the contact layer structure ( 52 ) and the optically active layer structure ( 28 ) are substantially parallel. Semiconductor laser amplifier ( 10 ) according to one of the preceding claims, wherein the normal directions of the input and the output facets ( 14 . 16 ) in each case parallel to the layer plane of the optically active layer structure ( 28 ). Semiconductor laser amplifier ( 10 ) according to one of the preceding claims, wherein the preamplification section ( 20 ) and / or the main reinforcement section ( 22 ) has a substantially trapezoidal surface shape. Semiconductor laser amplifier ( 10 ) According to one of the preceding claims, wherein l _Before / l _main <1, preferably l _Before / l _main <0.8 holds, where l _Before the length of the Vorverstärkungsabschnitts ( 20 ) and l _main the length of the main reinforcement section ( 22 ) in each case in the direction from the input to the output facet ( 14 . 16 ) are. Semiconductor laser amplifier ( 10 ) according to one of the preceding claims and claim 3, wherein at least one surface of the electrically conductive contact layer structure ( 52 ), which of the boundary layer structure ( 26 ) is facing away, with an electrically conductive heat sink ( 62 ) is in electrical and mechanical surface contact. Semiconductor laser amplifier ( 10 ) according to one of the preceding claims with at least one absorber structure ( 50 . 50 ' ) for absorbing an unwanted optical signal, wherein the absorber structure ( 50 . 50 ' ) at least adjacent to a waist portion ( 24 ) of the contact area ( 12 ) is arranged. Laser system with at least one semiconductor laser amplifier ( 10 ) according to one of the preceding claims and at least one laser ( 42 ), whereby the laser ( 42 ) and with respect to the semiconductor laser amplifier ( 10 ) is arranged so that one of the laser ( 42 ) generated optical signal ( 47 ) via the input facet ( 14 ) at least partially into the optically active layer structure ( 28 ) that's why conductor laser amplifier ( 10 ), wherein a focus of the optical signal ( 47 ) in an optically active layer structure ( 28 ) parallel layer structure plane within the semiconductor laser amplifier ( 10 ) lies. Laser system according to claim 10, wherein the laser ( 42 ) and with respect to the semiconductor laser amplifier ( 10 ) is arranged such that the focus of the optical signal ( 47 ) in the layer structure plane of the input facet ( 14 ) is spaced by a distance which is substantially equal to a length l _before the preamplification section (FIG. 20 ) in the direction from the input to the output facet ( 14 . 16 ) corresponds. Laser system according to claim 10 or claim 11 and claim 2, wherein the laser ( 42 ) and with respect to the semiconductor laser amplifier ( 10 ) is arranged such that the focus of the optical signal ( 47 ) in the layer structure plane substantially in a region of the optically active layer structure ( 28 ) which lies in the normal direction of the layer structure plane adjacent to the waist section (FIG. 24 ) of the one contact area ( 12 ) is arranged. Laser system according to one of claims 10 to 12, wherein the laser ( 42 ) and with respect to the semiconductor laser amplifier ( 10 ) is arranged that the optical signal ( 47 ) in the region of the input facet ( 14 ) has an astigmatic beam path, wherein a focus of the optical signal ( 47 ) in a plane perpendicular to the layer structure plane substantially in a region of the input facet ( 14 ) lies. Laser system according to one of claims 10 to 13, wherein the laser system has a coupling-in optical system ( 40 ) for coupling the optical signal ( 47 ) in the semiconductor laser amplifier ( 10 ). Laser system according to claim 14, wherein the coupling-in optics ( 40 ) at least one cylindrical lens ( 38 ). Laser system according to one of Claims 14 or 15, the coupling-in optical system ( 40 ) a slit ( 45 ) with a direction perpendicular to the layer structure plane extending slot direction, wherein the slit ( 45 ) in the region of an intermediate focus of the optical signal ( 47 ) is arranged to provide feedback of optical signals ( 48 ) from the semiconductor laser amplifier ( 10 ) in the laser ( 42 ) to reduce.