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A Distributed Bragg Reflector Silicon Evanescent Laser
Alexander W. Fanga, Brian R. Kocha,b, Richard Jonesb, Erica Lively a,
Di Liang a, Ying-Hao Kuo a, and John E. Bowersa
a
University of California Santa Barbara, ECE Department, Santa Barbara, CA 93106, USA
b
Intel Corporation, 2200 Mission College Blvd, SC-12-326, Santa Clara, CA 95054, USA
Email: [email protected]
Abstract
We report a distributed Bragg reflector silicon evanescent laser operating continuous wave at
1596nm. The lasing threshold and maximum output power are 65mA and 11mW, respectively and
shows open eye-diagrams under direct modulation at 2.5 Gb/s.
I. Introduction
Recently, hybrid integration has received a lot of attention as a
method to create electrically pumped laser sources on silicon.
This new interest is due to developments in the transfer of thin
crystalline III-V films to silicon [1,2] leading to the
demonstration of micro-disk, and Fabry-Perot III-V membrane
lasers coupled to silicon waveguides [3,4] and Fabry-Perot,
and racetrack hybrid silicon evanescent lasers [5], These
devices allow for the use of electrically pumped III-V gain
regions while enabling scalable manufacturing through
alignment free bonding that is absent in conventional gold
bump bond die attach of III-V active devices. Hybrid silicon
evanescent lasers (SEL) utilize both III-V regions and silicon
waveguide regions within the device, allowing for processing
in the silicon region to define the cavity along with lasing
properties. Earlier this year, we demonstrated a distributed
feedback (DFB) SEL that utilized surface corrugated gratings
beneath the bonded III-V region to realize an electrically
pumped single wavelength laser on silicon [6]. Here, we
demonstrate a distributed Bragg reflector (DBR) SEL where
passive gratings are placed on both sides of the active region
in order to form a wavelength selective cavity.
Figure 1: (top panel) DBR-SEL side-view topographical
structure, (center panel) DBR-SEL top-view topographical
structure, (bottom panel) Microscope image of DBR-SEL
II. Device Structure
The DBR-SEL is fabricated on the silicon evanescent
waveguide platform as described in reference [6]. The silicon
waveguide has a width, height, and rib etch depth of 2 μm, 0.7
μm, and 0.5 μm, respectively. This results in silicon and
quantum well confinement factors of 66 % and 4.4 %.
A 440 micron long silicon evanescent gain region and two 80
micron long tapers are placed inside the cavity. The tapers
provide an adiabatic transition between the passive silicon
regions and the silicon evanescent waveguide regions by
varying the width of the upper III-V layers along the length of
the taper, reducing the reflection and allowing for low loss
coupling between these two regions.
The device topography consists of a two passive Bragg
reflector mirrors placed 600 microns apart to form an optical
cavity. The back and front mirror lengths are 300 microns and
100 microns. The surface corrugated gratings are formed
during silicon processing prior to wafer bonding through ebeam lithography. They have an etch depth and duty cycle of
25 nm and 75 %, respectively, with an upper cladding of SU-8
leading to a grating κ of 80 cm-1. The power reflectivity of the
gratings can be calculated using the following expression [7]:
ܴ ൌ ‫݄݊ܽݐ‬ଶ ሺߢ‫ܮ‬ሻ
resulting in power reflectivities of 97% and 44% for the back
and front mirrors, respectively.
978-1-4244-1768-1/08/$25.00©2008 IEEE
III. Experimental Results
The laser output power is measured with an integrating sphere
at the front mirror of the laser. The front mirror L-I is shown
in Figure 2. The device has a lasing threshold of 65 mA, a
maximum device output power, of 11 mW, and a differential
efficiency of 15%. The laser operates up to 45 °C. The kinks
in the LI are from mode hopping and will be discussed later.
The device has lasing turn-on voltage of 2.4 V and series
resistance of 11.5 ohm.
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the current sweet. In intermediate regimes where a dip occurs
in the L-I, the lower adjacent mode has not moved into the
spectral gain-reflectivity peak and other adjacent modes lase.
Figure 2: DBR-SEL L-I curve for various temperatures
measured out of the front mirror.
The lasing spectrum is shown in Figure 3 with a lasing
peak at 1597.5 nm when driven at 200 mA. The device has a
free spectral range (FSR) of 0.47 nm, which corresponds to a
group index of 3.86 based on the sum of the physical cavity
length and mirror penetration depths of 61 and 42 microns.
The side mode suppression ratio is 50 dB.
Figure 4: L-I curve and spectrum versus current at a stage
temperature of 18 °C.
We measured the modulation characteristics of the
device by using a bias-T to drive the laser simultaneously
with a DC current and an RF signal while measuring the
electro-optic (EO) response on a photodetector. Figure 5
shows the overall photodetected EO response of the laser
with all connected components under small signal
modulation (-10 dBm). S11 measurements indicate that the
electrical contact geometry is not limiting the performance
of the device, so reflected power has not been factored out
of these curves. In addition, a 2 pF device capacitance was
extracted from the S11 measurement, resulting in an RC
limited bandwidth of 7 GHz. As expected, when the DC
bias current on the laser increases the resonance frequency
increases. Under higher modulation powers the resonance
peak is significantly damped. The 3 dB electrical bandwidth
at 105 mA is ~2.5 GHz.
Figure 3: Optical spectrum of the DBR-SEL driven at 200 mA
Figure 4 shows the lasing spectrum as a function of drive
current along with the corresponding L-I curve. It can be seen
that as the device heats with larger current injection, the lasing
mode moves to longer wavelengths due to the thermo-optic
effect in the cavity. When the mode moves far enough from
the reflection peak of the mirrors and the gain can no longer
support this lasing mode, a longitudinal mode hop to a lower
mode occurs. It can be seen from the L-I that the transition
between these two adjacent modes is not continuous through
Figure 5- Photodetected frequency response of the DFBSEL for 3 different bias currents with a stage temperature of
18 °C
We directly modulated the laser biased at 105 mA DC
current with a 2.5 Gb/s, 231-1 PRBS electrical signal having
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insulator waveguide circuit” Optics Express, Vol. 15, Issue 11, pp.
6744-6749
[4] G. Roelkens et al., “Laser emission and photodetection in an
InP/InGaAsP layer integrated on and coupled to a silicon-oninsulator waveguide circuit,” Opt. Express 14, 8154-8159 (2006).
[5] A. W. Fang, et al., “Electrically pumped hybrid
AlGaInAs-silicon evanescent laser,” Opt. Express 14, 9203-9210
(2006).
[6] A. W. Fang, et al., “A distributed feedback silicon evanescent
laser,” Opt. Express 16, 4413-4419 (2008).
[7] Michael Bass, Handbook of Optics IV, McGraw-Hill (2001),
page 9.4
20 mW of RF power. The resulting eye diagram is shown
in Figure 6. The extinction ratio is 8.7 dB. Although the
modulation bandwidth increases at higher DC currents, the
extinction ratio decreases unless larger RF modulation
powers are used. For example an open eye diagram at 4
Gb/s can be obtained, but it has an ER closer to 6 dB.
Improving the laser design to decrease the threshold current
and increase the differential gain is expected to significantly
improve the modulation bandwidth in future devices.
Figure 6- Eye diagram of a 2.5 Gb/s directly modulated
DBR-SEL.
IV. Conclusions
We report a distributed Bragg reflector laser on the
silicon evanescent platform operating in the 1596 nm
regime with a side mode suppression ratio of 50 dB. The
laser operates continuous wave with a lasing threshold of 65
mA and maximum output power of 11 mW at 15 C. The
laser showed open eye diagrams with extinction ratios of
8.7 dB and 6 dB for data rates of 2.5 Gb/s and 4 Gb/s,
respectively. This demonstration paves the way for the use
of directly modulated lasers on silicon integrated with
silicon interleavers or arrayed waveguide gratings in order
to create low cost, silicon wavelength division multiplexed
transmitters.
V. Acknowledgements
The authors would like to thank J. Shah, M. Haney, D.
Blumenthal, L. Coldren, M. Paniccia, H. Park, and H.-W.
Chen for insightful discussions. This work was supported
by a grant from Intel Corp. and from DARPA/MTO DODN
program and ARL under award number W911NF-05-10175 and W911NF-04-9-0001.
VI. References
[1] D. Pasquariello, et al., “Plasma-Assisted InP-to-Si Low
Temperature Wafer Bonding,” IEEE J. Sel. Topics Quantum
Electron. 8, 118, (2002).
[2] Q. Tong et al., “Low temperature InP/Si wafer bonding,” Appl.
Phys. Lett. 84, 732, (2004).
[3] J. Van Campenhout, et al., “Electrically pumped InP-based
microdisk lasers integrated with a nanophotonic silicon-on
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