Semiconductor lasers are usually used in devices for optical data storage. For low laser currents, a laser diode emits incoherent light. It is only when a threshold current is reached that the laser diode emits coherent light. On account of fluctuations of the current, changes in temperature or an optical feedback into the laser diode, discontinuous jumps between different wavelengths of the emitted light occur during operation. This effect is referred to as mode hopping.
The mode hops are effected in an irregular sequence, the laser diode jumping back and forth rapidly between different wavelengths. During the mode hops, the output intensity of the laser diode fluctuates within a certain scope, which leads to an increased intensity noise. In applications for optical data storage, the mode hopping caused by the optical feedback by the light beam that is focused onto the storage medium and reflected from the latter constitutes a considerable noise factor.
Radio-frequency modulators are used for improving the signal-to-noise ratio. The oscillator generates a low-noise AC current that is added to the DC current for operation of the laser diode. In this case, the DC current is chosen such that it lies just below the threshold current for laser activity, so that the laser diode is switched on and off by the AC current. This switching on and off forces the laser diode into fast mode hopping. The low-frequency component of the laser noise as a result of the mode hopping is thus shifted to high sidebands around the frequency of the radio-frequency modulator. Since the frequency ranges of such oscillators lie between 300 MHz and 500 MHz, it is possible to filter the noise as a result of mode hopping from the read and servo channel.
Since the introduction of the compact disc (CD), a large number of new formats have been established for optical storage media with higher capacities and higher read and write speeds. By way of example, the data rate of the digital versatile disc (DVD) is 11.08 Mbps. At the present time, the drive speed that is theoretically achievable mechanically is 10,000 revolutions per minute, which approximately corresponds to a 20×-DVD drive. This results in a data rate of 221.6 Mbps. A 32×-DVD drive would have a data rate of 354 Mbps. It is expected that future storage media will yield even higher data rates.
Nowadays, powerful signal processing methods are used in the reproduction path of drives for optical storage media in order to recover the data stream. A high-speed analog-to-digital converter samples the input signal of a photodetector of the optical scanner in order to feed it to the further signal processing for example by an FIR filter (finite impulse response) and a sampling rate converter for clock recovery. If the sampling rate moves into the frequency range of the radio-frequency modulator, it can happen that the laser diode is precisely switched off when the sampling is performed since, as described above, the laser diode is switched on and off by the radio-frequency modulator. An erroneous sample is determined in this case.
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