MITSUBISHI

MITSUBISHI OPTICAL SEMICONDUCTORSMEASURING PROCEDURESIII SIN characteristicLength of external resonator (variable)Effective NA = 0.15FilterPelche (Tc control)BS PBS Detector(MERET MDA438)Fig. 23 S/N ratio characteristic evaluating systemc:;> DC (Signalspectrum analyzer(Noise)The noise of a laser produced in Optical DiskSystems is mainly optical feedback noise.Theoptical feedback noise is generated when the beamfrom a semiconductor laser is reflected from a disksurface or the like and gets in the laser again. Letthe mean optical power by CW drive be Po andfluctuation be 0 Po, then the SI N value is asfollows:PoS/N = 20 log -- [dB]1) PoOptical feedback noise is measured by the set- upas shown in Fig. 23. Optical feedback ratio will bechanged from 0 to 5 % by using filters.For the noise at each optical feedback ratio,measure the worst value to be obtained when thecase temperature has been changed.Make thismeasure by specifying the measuring frequency andthe measuring band width. Figure 24 shows ameasuring example with a measuring frequency of20kHz and a measuring band width of 300Hz.70ML4402f=20kHz, BW=300Hz, T c=25-50'C80r-~----~-------r------4m 90~r·----·~·~---r-------r-----~-0 .-A. ....•Z ' :/'='---_~.~"" --0-- 1 mW;n 100 lII"'=1'f--