532 Chapter 17
a phase reversal between the two components of the refl ection. This causes destructive
interference, and light cannot return to the pickup. It must refl ect at angles which are
outside the aperture of the lens and be lost. Conversely, when light falls on the fl at surface
between bumps, the majority of it is refl ected back to the pickup. The pickup thus sees a
disc apparently having alternately good or poor refl ectivity.
Some discs can be recorded once, but not subsequently erased or rerecorded. These are
known as WORM (write once read mostly) discs. One type of WORM disc uses a thin
metal layer that has holes punched in it on recording by heat from a laser. Others rely on
the heat raising blisters in a thin metallic layer by decomposing the plastic material beneath.
Yet another alternative is a layer of photo-chemical dye that darkens when struck by the
high powered recording beam. Whatever the recording principle, light from the pickup
is refl ected more or less, or absorbed more or less, so that the pickup once more senses a
change in refl ectivity. Certain WORM discs can be read by conventional CD players and
are thus called recordable CDs, whereas others will only work in a particular type of drive.
17.1.3 Magneto-Optical Discs
When a magnetic material is heated above its Curie temperature, it becomes
demagnetized, and on cooling will assume the magnetization of an applied fi eld which
would be too weak to infl uence it normally. This is the principle of magneto-optical
recording used in the Sony MiniDisc. The heat is supplied by a fi nely focused laser; the
fi eld is supplied by a coil that is much larger.
Figure 17.7 assumes that the medium is initially magnetized in one direction only. In
order to record, the coil is energized with the waveform to be recorded. This is too weak
to infl uence the medium in its normal state, but when it is heated by the recording laser
beam the heated area will take on the magnetism from the coil when it cools. Thus a
magnetic recording with very small dimensions can be made.
Readout is obtained using the Kerr effect, which is the rotation of the plane of
polarization of light by a magnetic fi eld. The angle of rotation is very small and needs a
sensitive pickup. The recording can be overwritten by reversing the current in the coil and
running the laser continuously as it passes along the track.
A disadvantage of magneto-optical recording is that all materials having a Curie point
low enough to be useful are highly corrodible by air and need to be kept under an
effectively sealed protective layer.