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64 2 Extremely Short-External-Cavity Laser Diode
(a) Non-coated (b) AR-coated
(R=32%) (R=0.01%)
Light output (a.u.) Light output (a.u.)

0
0 2 4 6 8 0 2 4 6 8
External cavity length (mm) External cavity length (mm)
Fig. 2.40. Light output of a composite cavity LD as a function of external-cavity-
length L ex. Light output varies by a half-wavelength period as L ex varies (a).Vari-
ation can be suppressed by an antireflection coating on the LD facet facing the
recording medium (b)

Figure 2.40 shows a typical suppression of light output variation induced
by reducingfacet reflectivity (medium reflectivity R 3 =0.30). The variation
usingan uncoated LD (left side) with a facet reflectivity of R 2 =0.32 is
extremely high due to strong interference between internal and feedback light,
both of which have almost the same intensity. Usingan antireflection-coated
LD (right figure), on the other hand, with extremely low reflectivity of R 2 =
1 × 10 −4 weakens interference and greatly reduces variation of light output.
Figure 2.41 shows the relationship between facet reflectivity R 2 and the
variation ratio F, defined as the ratio of the amplitude change. The variation
ratio decreases as R 2 decreases. At a facet reflectivity R 2 =1 × 10 −4 ,the
variation ratio was reduced to the extremely small value of 1.6 compared to
9.2 for an uncoated LD.

Increasing Thermal Write Power
Laser power density on the medium is increased not only by the taper-ridged
waveguide laser, but also by the highly thermally conductive slider and the re-
duction of LD–PD attachment error. Light output is higher for the slider with
the higher thermal conductivity (AlN) than the lower thermal conductivity
(sapphire). Not only the thermal property but also the electrical properties,
precise fabrication, and head/medium reliability must be considered when
choosinga slider material. We used an AlN slider for the followingexperi-
ments.
As the head-medium spacingdecreases the light output increases due to
increased light feedback. The spacing h, set at 2 µm to keep the beam diameter
bellow 1 µm, is the sum of the slider flyingheight h 0 , LD–PD attachment error
h 1 (facet-to-slider surface error), and the protective-layer thickness h 2 . Since
h 0 =0.9 µmand h 2 =0.24 µm in this experiment, h 1 must be < 0.9 µm. We