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68 2 Extremely Short-External-Cavity Laser Diode
This section describes the increase in photothermal deflection of an MC
by employinga bimorph and an antireflection structure. This deflection can
be expected to exceed half a wavelength, which is required for a tunable
◦
LD, by the temperature increase of 100 C with an optical absorption rate
above 98%. An antireflection coatingof Au/Si 3 N 4 /Au on the Au and Si 3 N 4
bimorph structure semiconductor MC provides enhanced deflection about 500
times greater than that of the solitary semiconductor MC at off-resonant
frequencies. The designed MC monolithically integrated on InP (λ =1.3 µm)
or GaAs (λ =0.83 µm) will be used for an external-cavity length-changing
type edge-emitting tunable LD.

Design Considerations

An MC and LDs were fabricated on the surface of a GaAs substrate, as shown
in Fig. 2.23. The light emitted from the LD2 onto the side wall of the MC
is partially absorbed, heatingthe MC and producingthe bendingmoment.
When the MC is excited at the resonant frequency by the thermal stress caused
by a modulated laser beam from LD2, the external cavity length between the
MC wall and the LD1 facet varies sufficiently, but insufficiently at off-resonant
frequencies. That variation of the external cavity length causes the wavelength
change of the LD1.
By varying the external cavity length, the laser wavelength can be easily
changed and the wavelength varies every λ/2. Therefore, an MC deflection
of more than λ/2 is necessary for the tunable LD, even at an off-resonant
frequency. Meanwhile, the light absorption should be increased to obtain a
sufficient temperature rise for the large thermal strain. A bimorph structure
and an antireflection coatingare applied to satisfy these requirements. The
followingdescribes the enhanced MC photothermal deflection design for GaAs
and InP materials.

Bimorph Design
Figure 2.45 shows the calculated relationship between photothermal deflection
and thickness for a solitary GaAs MC. The tip deflection d at the free end of
the MC is estimated by (2.28), where α is the thermal expansion coefficient,
δT is the temperature difference between the surfaces of the MC illuminated
by an LD, l and t are the length and thickness of the MC, respectively [2.29].

αl 2
d = δT (2.28)
2t
Tip deflection d depends largely on the absorption coefficient (varies λ/2 due
to the interference between the upper and lower surfaces) of the laser light
and reaches 0.8 nm when illuminated with 1 mW of power.
We designed a metal-dielectric bimorph structure to increase the MC de-
flection by increasingin temperature resultingfrom absorption of light emitted