the optical power p opt is negligibly small there the other part of the curve for laser diode currents exceeding the threshold current i th has considerably more slope an operating point i m p m is shown the range of the characteristic line between the dip and this operating point can be well approximated by a straight line section above the operating point the characteristic line becomes distinctly flatter this overproportional flattening occurs especially when the laser diode temperature rises or when the laser diode ages with a rising temperature or with an ageing laser diode the slope of the overall characteristic line above the threshold current i th decreases furthermore the threshold current i th increases below the diagram of the characteristic line is shown a variation with time of the laser diode current i laser by way of example the laser diode current i laser consists of a bias current i 0 a modulation current i mod and a pilot current i pilot the bias current i 0 is purely a dc current it provides that the laser diode current i laser is always larger than the threshold current i th the modulation current i mod is in this case modulated with a high frequency binary data signal whose frequency lies in the range above 140 mhz the third component of the laser diode current.