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2.5 Designs for Related Problems of an ESEC LD 79
The preferred medium reflectivities for LD#2 can be chosen as follows:

R 1 ≥ 0.7, (2.52)
∼
R 2 = 0.01,
h
0.21 ≥ R ≥ 0.14,
3
l
h
0.05 ≥ R − R ≥ 0.02.
3
3
Compared with LD#2, LD#1 has advantages of a large permissible range
medium reflectivity, but has the disadvantage of temperature rise due to low
quantum efficiency.
In summary, the optimum design head consists of an LD facet with a
reflectivity of R 1 = 0.7and R 2 =0.01, and a medium high reflectivity of
∼
h
h
l ∼
0.21 ≥ R ≥ 0.14. The reflectivity difference between the two states R −R =
3 3 3
0.05 and the spacingbetween laser facet and medium is 2 µm. This flying
type optical head is now developingfor the candidate of an ultra-high density
optical near field storage (see Sect. 5.4.2).
Problems
2.1. Calculate (2.27) for Si and show the relationship between the cantilever
resonant frequency f 0 and the length l in the range of 500 µm ≥ l ≥ 0,
thickness t(5 µm ≥ t ≥ 0.5) as a parameter. Here, λ 0 =1.875,E =1.9 ×
2
3
10 12 dyne/cm ,ρ =2.3g/cm ,l is the cantilever length, and t is the thickness.
3
3
2.2. Calculate springconstant K = Et b/4l for Si and show the relationship
between K and the length l in the same conditions described in Problem 2.1.
2.3. Calculate the light output ratio P 2 /P 1 , with medium reflectivity R 3 as a
parameter, versus the medium side laser facet reflectivity R 2 , where P 1 is the
light from PD side and P 2 is from medium side, R 1 =0.7,h =2 µm.
2.4. What are the specific trackingissues that need to be addressed and solved
for the higher disk rotation rate?
2.5. Are there any reasons to use a 1.3-µm wavelength LD?

2.6. Is contamination a serious issue, in practice, for the flyingoptical head?