sensitive surface of pin diode 2 that receives the light by a principal face of the semiconductive crystal 50 micrometers for the optical fiber 60 to 200 micrometers for the diode however since the sensitive surface of the diode in the case of very high frequency signals is as small as possible in order to have only a small capacity it is necessary in this case to position fiber 4 with respect to diode 2 with a precision of u00b1 10 micrometers if the optical head is an emitting head provided with an electroluminescent diode called a del diode an adjustment precision of u00b1 5 micrometers is necessary in order to optimize the alignment of a diode having a diameter of 50 micrometers with a fiber of the same diameter furthermore if the optical head is an emitting head provided with a laser diode which is the case of fig1 the diode emits the light through a band about 4 micrometers wide and 0 2 micrometer thick this band being apparent on a lateral face of the semiconductor crystal an alignment precision of u00b1 1 micrometer is thus necessary it can thus be seen that it is necessary to obtain a positioning of the different components of an optical head to within more or less 1 micrometer accuracy which from the mechanical point of view is already somewhat delicate but.