Patent Document

TECHNICAL FIELD 
   The field of the invention is that of monolithic integrated components having a plurality of sections including a section constituting a Fabry-Perot cavity laser and at least one other section adjacent said laser section. It relates to the production of a mirror between the laser and an adjacent section. 
   PRIOR ART 
   In a device including a laser and a modulator, the light emitted by the laser is coupled into the modulator by means of an optical fiber, for example. The temperatures of the laser and the modulator are usually controlled to guarantee monomode operation of the laser. For long-haul transmission it is important for the emitted wavelength to remain constant. To reinforce monomode operation a semi-reflecting face constituting an exit face of a Fabry-Perot cavity of the laser is treated so that it is reflective at the operating wavelength of the laser. 
   BRIEF DESCRIPTIONS OF THE INVENTION 
   The present invention relates to a laser and an associated component, for example a modulator, when monomode operation of the laser is not essential, for example a laser used for short-haul transmission over distances of the order of 2 km or less. This kind of laser does not require temperature control. However, because the laser is not temperature-controlled, operation is no longer monomode and it is important for the semi-reflecting exit mirror to allow multimode operation. To this end the reflector must have a flat reflectivity response, at least for the wavelengths liable to be emitted by the laser at all operating temperatures at which it is likely to operate. 
   To this end, the invention provides a monolithic integrated component comprising a plurality of sections including a section constituting a laser having a cavity delimited by an external face of the component and by a partially reflecting reflector and another section adjacent said laser section, which component is characterized in that the partially reflecting reflector is disposed between the laser section and said adjacent other section and in that the reflector allows multimode operation of the laser. 
   In one advantageous embodiment the reflector is a Bragg grating allowing multimode operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described next with reference to the accompanying drawings, in which: 
       FIG. 1  is a diagram showing a first embodiment of a component of the invention in section on a plane perpendicular to the plane of the layers and parallel to a direction of propagation of light between two adjacent sections of the component, and 
       FIG. 2  is a diagram showing a preferred embodiment of a component of the invention in section on a plane perpendicular to the plane of the layers and parallel to a direction of propagation of light between two adjacent sections of the component. 
   

   DESCRIPTION OF EMBODIMENTS 
   A monolithic component  1  in accordance with the invention as shown in  FIG. 1  comprises two sections  21 ,  22 , of which a first section  21  is a Fabry-Perot cavity laser and the second section  22  is a subcomponent of the component  1 . The semiconductor laser first section  21  is made up of a stack of layers  2 ,  3 ,  4 ,  5 ,  6  on a substrate and including an active layer  4  of GaInAsP, for example, and electrical and optical confinement layers  3 ,  5 . The stack of layers is terminated at the top and at the bottom by respective surface electric contact layers  6  and  2  that are used to bias the laser section  21 . The laser layer  4  is bordered above and below by the confinement layers  5  and  3 , respectively. 
   A laser cavity  7  is formed in a manner known in the art by cutting the layers, including the active layer  4 , thereby producing mirror faces  8  and  9  that are treated to have the necessary coefficients of reflection, so that the active layer  4  is in a Fabry-Perot cavity  7 . The laser section  21  is integrated on the same substrate as the other section  22 , which forms a subcomponent, for example a modulator, an amplifier or a filter. One of the mirror faces  9  of the cavity  7  is an external face of the component, obtained by cleaving, for example, and the opposite mirror face  8  constitutes an exit face which in practice is obtained by etching at  12  the layers running from one of the surface layers of the component, for example the layer  6 , as far as the active layer  4 , and possibly beyond it. 
   In accordance with the invention, the etched exit face  8  receives a reflective treatment so that its reflectivity response is flat at least for the operating wavelengths liable to be delivered by the laser over the range of temperatures in which the laser is likely to operate. The operation of the laser is therefore multimode and it is not necessary to control the laser temperature using one of the usual control devices. 
   It must nevertheless be pointed out that, in an embodiment of this kind, because of the etching at  12 , two reflective faces  8  and  18  are necessarily created, comprising a required first face  8 , i.e. the face delimiting the Fabry-Perot cavity  7 , and a second face  18  that is inevitably created, i.e. the face of the second section  22  facing the laser cavity  7  on the substrate. Because of this, laser light is not reflected towards the cavity  7  by only one mirror (the mirror  8 ), but by the two mirrors  8  and  18 . It is then necessary to control the reflectivity of the two mirrors  8  and  18  and also that of the optical path between them, for example to obtain an even integer number of half-wavelengths. It is difficult to achieve the required accuracy with existing etching techniques. Also, etching introduces a constant distance between the reflectors  8  and  18  which is reflected in a phase shift that varies as a function of the operating wavelength. Because of this, the adjustment is correct for only one of the operating wavelengths of the cavity  7 , for example the wavelength corresponding to the most probable operating temperature, and is degraded at other operating temperatures. 
   The preferred embodiment shown in  FIG. 2  has the advantages of the first embodiment without the drawbacks thereof just described. 
     FIG. 2  shows a monolithic component  10 . The architecture of the component  10  is analogous to that of the component  1  shown in  FIG. 1 . In  FIG. 2 , items having the same function as items shown in  FIG. 1  are identified by the same reference number, and these components are not described again. The difference between the component  1  shown in  FIG. 1  and the component  10  constituting the preferred embodiment of the invention shown in  FIG. 2  lies in the junction between the two sections  21  and  22 , in that the etching at  12  is no longer present. It is replaced by a reflective Bragg grating  11  produced by a method known in the art after depositing the active layer  4 . The depth to which the lines constituting the grating are etched is such that the bandwidth of the grating allows multimode operation of the laser cavity  7 . The bandwidth can be of the order of ten to a few tens of nanometers, for example 10 to 20 nanometers. Although the bandwidth of a cleaved face can be of the order of a few hundred nanometers, a bandwidth of a few tens of nanometers covering at least the bandwidth of the subcomponent constituting the second section  22  will not generally lead to any penalty. The reflectivity of this kind of array in the operating band can be of the order of 20 to 25%. 
   In the example shown in  FIG. 2 , the second section  22  integrated on the component  10  is a subcomponent known in the art, for example an electro-absorbant electro-optical modulator.

Technology Category: 5