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60 2 Extremely Short-External-Cavity Laser Diode
(a) (b)
R (HRC) R (ARC) h R l R h
1 2 3
3
R
R h 3
P P 3
1 2 Light output
PD LD
R l Data signal
3
eff l h l
R 2 th l th
Bias current
Fig. 2.35. LD used in an OSL flying head forms a complex cavity with the recording
medium acting as an external mirror. (a) R 1 and R 2 are the power reflectivities of
l
h
the LD facets, and R 3 and R 3 are those of the two states for the external recording
eff
medium. R 2 is the effective reflectivity coupled with the external recording medium.
(b) Light output versus bias current in the two medium reflectivity states. There
h
l
l
h
are two threshold currents: I th for R 3 and I th for R 3 . Data bits are read as the laser
l
l
h
h
switches from R 3 to R 3 , biased between I th and I th ,bythePD on theoppositeside
of the medium
The last SiN layer increases adhesion to the glass substrate. The reflectivity,
transmitivity, and absorptivity of the as-deposited GeSbTe medium were
0.24, 0.22, and 0.54.
Basic Read/Write Operation
The LD used in an optically switched laser (OSL) head forms a complex cavity
with the recordingmedium actingas an external mirror. In Fig. 2.35a, R 1 and
l
h
R 2 are the power reflectivities of the LD facets, and R and R are those of the
3
3
two states for the external recordingmedium. R eff is the effective reflectivity
2
coupled with the external recording medium. Figure 2.35b shows the light
output versus bias current in the two medium reflectivity states. There are
h
two threshold currents: I h for the high reflectivity R and I l for the low
th 3 th
l
reflectivity R . Data bits are read as the light output difference between the
3
two states.
Data bits are written, in this experiment, as the laser-induced phase change
l
h
shifts the medium reflectivity from R (as depo. state) to R (crystal state).
3 3
During the write operation, the light output changes from P w to P at a
w
fixed write current of I w according to the reflectivity change. The light output
difference P − P R (nearly equals to P − P w ) is less than 1 mW, which does

R w
not destroy the information even if the medium stops. If the light output
were 20 mW, the variation ratio in the write process would be only 5%. We,
thereby achieve stable write operation. To increase the signal-to-noise ratio
(SNR) and light output from the medium side laser facet, the medium side
LD facet is coated with an antireflection film of (SiO) (Si 3 N 4 ) 1−x [2.14].
x
Data Signal
Data signals are obtained by the light output difference due to the medium
reflectivity. Varyingthe medium reflectivity R 3 as a parameter, Figs. 2.36a