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62 2 Extremely Short-External-Cavity Laser Diode
(a) (b)
access codes access codes
WA WA
CLK CLK
WB WB

Laser Beam

1.6 mm

unique distance
Fig. 2.37. Discrete block format (DBF)marks with a 1.6-µm track-pitch (a),and
reproduced signals at 900 rpm. LD bias current is 40 mA (b)

Recording
medium
Sampling
RF AMP PLL timing
LD generator

PD
WA
Register + TE Track
A/D D/A error
-
Register signal
WB
Fig. 2.38. Block diagram of track error signal detection. Track error signal is ob-
tained by the signal amplitude difference between the two wobbling signals of the
discrete block format (DBF)

derived from the difference in signal amplitudes between the two wobbling
signals.
Figure 2.38 is a block diagram of the track error signal (TE = WA − WB)
detection system. The reproduced track error signal confirms that the spot
diameter of the lensless but taper-ridged waveguide LD is small enough to
reproduce the DBF servo marks.

Reducing Laser Noise by Optimizing Bias Current
The SNR of the OSL flying head is high because of the large signal amplitude
due to laser switchingand the lack of feedback laser noise due to the external
cavity beingso much shorter (2 µm) than the LD cavity (300 µm). The SNR
is degraded, however, by fluctuation of both high- and low-frequency light
output. The former is caused by laser noise due to mode competition, while
the latter is caused by variation of head-medium spacing(external cavity
length) and of temperature.