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Unformatted text preview: Midterm ECE 227A November 4, 2009 Department of Electrical and Computer Engineering University of California, Santa Barbara Name 1. (40 points) A 1550 nm, InP/InGaAsP based epitaxial wafer is grown with an active region consisting
of 38 nm wide strained quantum wells. The conﬁnement factor for the active region
was modeled to be 3%. The material gain can be represented using a logarithmic
relationship between the carrier density and gain with gun; = 1207.29 cm'1 and N“ =
1.22841018 cm'3. For this material, the bimolecular recombination coefficient is B =
031010 ems/s, and the Auger coefficient is C = 310’29 cms/s First, 50 um wide broad area lasers are made in this material, and characterized for
differential efficiency as shown in the table below. a) Determine the injection efficiency and the internal losses for this material The same material is then used to make buried heterostructure FabriPerot lasers,
whose rib width is 3 pm. Additional internal losses due to scattering relative to the
broad area laser case are 2 cm'l. Several BH rib laser bars of different lengths were made, some with both facets cleaved,
and some with one facet HR (r=1) coated after cleaving. For both bar types, it was
determined that their output power is 15 mW at 40 mA bias above threshold. For these
devices, calculate: b) the differential efﬁciencies for both cleaved facets and HRcleaved facet devices
c) laser mirror losses for the cleaved facets and HRcleaved facet devices d) laser cavity length for the cleaved facets and HRcleaved facet devices e) threshold current for the cleaved facets case If the quantum well active region inside the buried rib laser were to be replaced by a simple double heterostructure active region, consisting of a 100 nm thick 1.550 lnGaAsP
(clad by InP), f) what would the confinement factor of this structure be? (25 points)
Using the same quantum well material described in problem #1, we would like to
fabricate 400 um long, 3 pm buried rib lasers suitable for direct modulation up to 10
Gb/s. Based on the optical system design, it was concluded that these lasers need to
have the resonance frequency of 10 GHz at a bias of 20 mA above threshold. To
accomplish this, one facet of these lasers will be HR coated (r=1), and the other will be
coated in order to achieve the desired frequency range. Neglect heating, and assume
that n;=0.87, and <ai> = 15 cm'l. ‘ a) Determine the required differential gain of this new ridge laser b) Determine the required reflectivity of the non HRcoated facet c) Calculate the output power for this laser at 20 mA above threshold (35 points) 3. The 400 um long, 3 pm wide BH lasers made from the material from problem #1 have
100 ohm series resistance, and the total diode voltage of 0.9V in a simple resistordiode
model. The MP substrate is 110 pm thick and 500 um wide, its thermal conductivity is
0.68 W/crnK, and it is bonded to a good heatsink. The threshold current and differential
efficiency characteristic temperatures are 50K and 150K respectively. In pulsed
measurements, these lasers have a threshold current of 20 mA and nd of 0.45 at 25°C. a) Calculate the thermal impedance of the structure b) What is the pulsed threshold current if the heat sink temperature is changed to
45°C? c) Sketch the Ll curve (the more accurate the sketch, the easier part d) will be to solve) d) For the heat sink temperature of ZS’C, find the operating conditions (bias current,
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