Biomedical Engineering Reference
In-Depth Information
The device was fabricated with a heteroepitaxial ridge overgrown (HRO)
construction. The structure uses conventional Fabry-Perot cleaved facets,
a cavity length of 250 μm, standard cleaving, and standard ohmic contacts.
The 5% by weight Er doping was added to the In melt during the liquid
phase epitaxial growth of the layer. The Er diffuses into neighboring layers
during the growth process. The HRO laser operated in a single longitudinal
mode, was stable with temperature, had line shifts of less than 1 Å C −1 , was
apparently immune to external reflections, and showed reproducible lasing
frequency in devices from different wafers.
The concept of using rare-earth doping has been used in the past. For exam-
ple, rare-earth ions were incorporated into III-V structures [51] and silicon [52]
by ion implantation and other doping methods. At low temperatures, that is,
77 K, the emissions from the rare-earth samples did not depend on the band-
gap energy of the host semiconductor but obeyed the emission expected of
the particular rare-earth ion. It is also common practice to use rare-earth ions
to produce emissions having a wavelength longer than the band edge wave-
length of the host semiconductor. For example, in light-emitting diodes, rare-
earth ions are used in the “up-converter” [53] where the infrared emission
from a GaAs light-emitting diode is absorbed by phosphor doped with rare-
earth ions such as ytterbium (Yb 3+ ) and erbium (Er 3+ ). The operation is depen-
dent upon the successive absorption of a single photon in the visible region.
Figure 4.24 shows a comparison of the spectra for an Er-doped HRO laser
and a control non-rare-earth-doped HRO laser at different injection levels.
Note that the control HRO laser shows typical multilongitudinal mode opera-
tion including mode hopping that is common with quaternary lasers, while
the Er-doped laser exhibits a clean single longitudinal mode of operation with-
out mode hopping. These initial results are very encouraging, and continued
development will lead to increased commercial availability of these devices.
4.17.11 Solutions to Laser Frequency Instability: Summary
Some of the viable solutions to the problem of laser frequency detailed in the
previous sections are as follows:
• Maintain the laser above threshold during operation. If the laser is
biased substantially above threshold, single-mode operation can be
maintained at a higher level of modulation than when the laser is
biased close to or below the threshold level.
• Operate the laser at an 80% modulation depth maximum. High-
frequency modulation has little effect on the lasing spectrum if the
modulation depth is maintained at or below 80%.
• Operate the laser at an intensity level that ensures stable operation.
• Reduce cavity length or use a laser source that includes an external
cavity.
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