Page 89 -

78 2 Extremely Short-External-Cavity Laser Diode

h
R
3
P W
I
R 3
Light output P , W Write
,
P
E
Erase
P E

I E I W
Bias current
Fig. 2.58. Write-erase performance for a phase change medium. The write-erase
power margin, P W − P E and P W − P E, for a phase change medium is shown

with our analysis, consideringthe followingdesign quantities [2.33]:
LD efficiency : 0.2 ≥ R 1 × R eff ≥ 0.05, (2.46)
2
Light output ratio : P 2 /P 1 ≥ 8, (2.47)
Medium write sensitivity (absorption) : A ≥ 0.75. (2.48)
Write–erase power margin for P W =30 mW and P E = 15 mW for the phase
change medium shown in Fig. 2.58
P W − P ≥ 10 mW and P − P E ≥ 10 mW, (2.49)

E W
Permissible read power : P R ≤ 1.5mW, (2.50)
l
h
which is 1/10 of the erasingpower P E . Both R 1 and R − R are restricted
3 3
to some appropriate values examined later.
Prefeared Reflectivity Design
Reflectivity design was performed for two kinds of LD; the wavelength of
LD#1 is 1.3 µm and LD#2 is 0.83 µm (LD#2 has a higher quantum efficiency
than LD#1). Appropriate choices are made for LDs and the phase change
medium from the criteria of (2.46)–(2.50).
As the effective reflectivity R eff (medium reflectivity) decreases, the light
2
output ratio (P 2 /P 1 ) increases, but the write-erase power margin (P W − P
E
and P − P E ) decreases. The preferred medium reflectivities for LD#1 can
W
be chosen as follows:
R 1 ≥ 0.7, (2.51)
∼
R 2 = 0.01,
h
0.21 ≥ R ≥ 0.14,
3
h
l
0.10 ≥ R − R ≥ 0.02.
3
3