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Unformatted text preview: Final
ECE 227A December 7, 2005 Department of Electrical and Computer Engineering Universin of California, Santa Barbara (40 pts) 1. Two different types of DFB lasers are fabricated from the same 1.55 pm wavelength wafer that contains a uniform grating and 2 pm wide BH lateral waveguides. The ﬁrst are
stande AR—coated DFB lasers with a 500 nm length. The threshold current on these is
measured to be 30 mA and the differential efﬁciency, considering both ends, is found to be
0.5. As expected two degenerate axial modes are observed. The second type of laser is formed by cleaving out 250 um length devices from this same wafer, and then applying AR coatings to only one end. In this case the spectra of some
devices show marginal MSRs, while others show relatively good MSRs. The devices with
the best MSRs are measured, and these provide threshold currents of 30 mA and differential
efﬁciencies, considering both ends, of 0.6. From other measurements is also determined that the threshold modal gain of these shorter lasers is twice that of the longer lasers. 3 a What is m. the internal efﬁciency?
i 6 b. What is <a,>, the incremental modal loss? a c. What is K, the grating coupling coefﬁcient? 9 d. Assuming a gain characteristic of the form, I'g = Fgo 1n(J/J,,), what are Fgo and J,? 8 e. What MSR would be expected in the better 250 um devices, if operated at twice
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with gain. phase and grating sections measuring 400, 100, and 200 microns in length,
respectively. The modal loss in the gain section is known to be 15 cm“, and it is found to be 5
cm" in the passive phase and minor secn'ons. The internal efﬁciency is assumed to be 0.75, and the DBR power reﬂection and transmission are found to be 0.2 and 0.7, respectively. The other
mirror is a cleaved facet. é (a) What is the axial mode spacing?
7 (b) What are the differential efﬁciencies out of each end? Q (c) Ifwe wish to move the DBR Bragg wavelength by 5 run, by how much does the
effective index of the minor need to be shifted? Md) By how much would an axial mode shit! for the index shiﬂ of (c)? ,lc/mm
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