# Photonic Integration on the Hybrid Silicon Evanescent Device Platform

Authors: Hyundai Park; Alexander W. Fang; Di Liang; Ying-Hao Kuo; Hsu-Hao Chang; Brian R. Koch; Hui-Wen Chen; Matthew N. Sysak; Richard Jones; John E. Bowers
Journal: Advances in Optical Technologies
 (2008)

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## Abstract

This paper reviews the recent progress of hybrid silicon evanescent devices. The hybrid silicon evanescent device structure consists of III-V epitaxial layers transferred to silicon waveguides through a low-temperature wafer bonding process to achieve optical gain, absorption, and modulation efficiently on a silicon photonics platform. The low-temperature wafer bonding process enables fusion of two different material systems without degradation of material quality and is scalable to wafer-level bonding. Lasers, amplifiers, photodetectors, and modulators have been demonstrated with this hybrid structure and integration of these individual components for improved optical functionality is also presented. This approach provides a unique way to build photonic active devices on silicon and should allow application of silicon photonic integrated circuits to optical telecommunication and optical interconnects.


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## Body

## 1. Introduction

Recent research
in silicon
photonics has been driven by the motivation to realize silicon optoelectronic integrated
devices using large scale, low-cost, and highly accurate CMOS technology. Silicon
is transparent at the 1.5μm and 1.3 μm telecommunication wavelengths and has
demonstrated low loss waveguide with losses in the range of 0.2 dB/cm ~ 1 dB/cm.
The large index contrast of silicon waveguides with silicon dioxide cladding
results in highly confined optical modes and reduction of waveguide bend
radii leading to dense photonic integration. This has resulted in advances in
passive devices such as compact filters [1], optical buffers [2], photonic
crystals [3], and wavelength multiplexer/demultiplexers [4, 5].It was only recently that
silicon has been demonstrated as a high-speed modulator. Silicon-based
modulators have been reported using free carrier plasma dispersion in
Mach-Zehnder interferometer structure [6, 7], photonic crystals [8], and ring resonator structures [9]. Strained
silicon has been shown to break the inversion symmetry of silicon allowing
silicon to exhibit linear electro-optic refractive index modulation [10].
Recently, an electroabsorption modulator on silicon has been demonstrated based on
the quantum confined stark effect in strained silicon germanium [11].Light detection is another
major research topic in silicon photonics. Strained germanium and silicon
germanium push the absorption out to 1.55μm wavelength and are attractive since
it is compatible with CMOS processing capabilities [12, 13]. Integration of the
photodetector with the receiver is critical for lower capacitance and higher sensitivity
[14].The indirect bandgap of silicon has been a key hurdle for achieving
optical gain elements. Although Raman lasers and amplifiers [15–17], and
optical gain in nanopatterned silicon have been observed [18], an electrically
pumped silicon waveguide gain element has been an unsolved challenge.An
alternative to fabricating the gain element in silicon is to take prefabricated
lasers and couple them to silicon waveguides.
However, due to the tight alignment tolerances of the optical modes and
the need to align each laser individually, this method has limited
scalability, and it is difficult to envision die attaching more than a few
lasers to each chip without prohibitive expense.
Furthermore, the reflections at the chip interfaces limit the gain and
spectral flatness that can be achieved.Epitaxial
growth of III-V layers on silicon substrates has been investigated as well. The
demonstrations include InGaAs quantum dot lasers [18] and InGaSb quantum well
lasers [19] fabricated on silicon substrate. Those demonstrations have widened
the possibility of building monolithically integrated on-chip laser sources on
the silicon photonics platform. However, an efficient coupling scheme from the III-V
lasers to the silicon waveguide needs to be developed since most of the lasing
mode is in the III-V layers.Recently,
we have demonstrated a hybrid integration platform utilizing III-V epitaxial layers
transferred to silicon to realize many types of photonic active devices through
a single wafer bonding step. The wafer-bonded structure forms a hybrid
waveguide, where its optical mode lies both in silicon and III-V layers. This
structure enables the use of III-V layers for active light manipulation such as
gain, absorption, and electro-optical effect for the amplifiers, lasers,
detectors, and modulators. In this paper, we review the recent
progress on hybrid silicon evanescent devices. In Section2, the device
structure and design issues are introduced. In Section 3, the device
fabrication process is described. Section 4 presents the performance and
characteristics of fabricated silicon evanescent devices. Finally, several
potential future paths of this research are discussed in Section 5.


## 2. Device Platform

Figure1
shows the general structure of hybrid silicon evanescent devices. The hybrid
structure is comprised of a III-V region bonded to a silicon waveguide
fabricated on a silicon-on-insulator wafer. The mesa structure formed on the
III-V region enables the current flow through the multiple quantum well region.
The general structure of III-V layers consists of a p-type contact layer, a p-type
cladding, a p-type separated confinement heterostructure (SCH) layer, an undoped
multiple quantum well layer, n-type contact layer, and n-type super lattices.
Amplifiers and lasers have a wide III-V mesa (12 μm ~ 14 μm) for better heat
conduction and mechanical strength (Figure 1(a)) while a narrow III-V mesa (2 μm
~ 4 μm) is chosen for detectors and modulators for high-speed operation with a
reduced capacitance (Figure 1(b)). The optical mode in this hybrid waveguide
lies both in the silicon waveguide and the multiple quantum well layers. The
confinement factors in III-V and silicon regions of the hybrid waveguide can be
manipulated by changing the silicon waveguide dimensions. The quantum well
confinement factor is a critical design parameter in order to achieve enough
optical gain and absorption while the silicon confinement factor is an
important parameter determining coupling efficiency when the device is
integrated with silicon passive devices. Figure 2 shows three different mode
profiles with three different waveguide widths. In general, the silicon confinement factor increases
as the height or width of the silicon waveguide increases while the quantum well
confinement factor decreases. The epitaxial structures and confinement factors
for each device set will be specified in Section 4.Hybrid silicon evanescent device cross-section structures: (a) a wide III-V
mesa for amplifiers and lasers, (b) a narrow III-V mesa for detectors and modulators.
(a)(b)Mode profiles with different waveguide
widths. Height of the silicon waveguide is fixed at 0.7μm.
(a)(b)(c)


## 3. Fabrication

### 3.1. Plasma Assisted Low-Temperature Wafer Bonding

The transfer of the indium phosphide (InP)-based epitaxial layer structure to the silicon-on-insulator (SOI) substrate is a key step in the fabrication of this
hybrid platform and has direct impact on the device performance, yield, and
reliability. Due to the mismatch between the thermal expansion coefficient of silicon
and indium  phosphide (αSi = 2.6 ×10−6/K,
αInP =  4.8 ×10−6/K), high-temperature (>400°C) annealing steps are not desirable.
Figure 3(a) shows a Nomarski photograph of the top surface of an InP die
transferred to a silicon-on-insulator substrate at 600°C. Crosshatching can be seen for this
high-temperature direct wafer bonding which can lead to degradation of material
quality and scalability issues due to the accumulation of stress over larger
sample sizes. In order to resolve this
issue, low-temperature annealing is used with an oxygen plasma surface
treatment to enable strong bonding [20]. An
annealing temperature of 300°C is chosen to
minimize bonding stress while still being able to convert the weak Hydrogen
bonds formed by the room temperature bonding to strong covalent Si–O–In and
Si–O–P bonds. Figure 3(b) shows a
successful transfer of InP epitaxial layers to SOI with smooth device quality
surface morphology and no interfacial voids.Nomarski microscope images of the transferred III-V surface at bonding temperatures of
(a)600°C and (b) 300°C.
(a)(b)Figure4 is the schematic
process flow of the oxygen plasma assisted low-temperature (300°C) wafer bonding. After rigorous
sample cleaning and close microscopic inspection with 200x magnification, the native
oxide on SOI and InP are removed in standard buffer HF solution (1HF : 7 H2O)
and NH4OH (39%), respectively, resulting in clean, hydrophobic
surfaces. The samples then undergo an oxygen plasma surface treatment to grow
an ultrathin layer of oxide (<5 nm) [21] which leads to
very smooth (rms roughness <0.5 nm) hydrophilic surfaces, which is less
sensitive to the microroughness as compared to hydrophobic bonding. The Si–O–Si bonds
of the oxide (SOI side) are also found to be more strained than conventional
oxides formed in standard RCA-1 cleaning process or other hydrophilic
wet-chemical treatment, and have a higher readiness to break and form new
bonds. O2 energetic ion
bombardment also acts as a final cleaning step to remove hydrocarbons and water-related
species on the sample surface efficiently. The following deionized water dip
further terminates the oxide surface by polar hydroxyl groups OH−, forming bridging bonds between
the mating surfaces to result in spontaneous bonding at room temperature [22]. To strengthen the bond, the bonded sample is
placed in a conventional wafer bonding machine (Suss Bonder SB6E), where the samples
are held together at a pressure of 1.5 MPa and a temperature of 300°C, under vacuum (<4 × 10−4 Torr) from 1 to 12 hours. The 300°C annealing process enhances out diffusion of molecules trapped at the
interface and desorption of chemisorbed surface atoms, such as hydrogen, while
activating the formation of covalent bonds to achieve higher bonding energy [20]. After annealing and cooling, the InP
substrate is selectively removed in a 3HCl : 1H2O solution at room
temperature.Figure 4
Oxygen plasma-assisted
low-temperature wafer bonding process flow.Figure5 shows a 2-inch InP-based
expitaxial  wafer bonded on a SOI sample cleaved from a 6 inch SOI wafer. Smooth III-V morphology with no interfacial
void is achieved in the bonded area. The
two defects on the left-hand side of the figure are due to wafer handling with
tweezers and InP epitaxial surface defects that are 29 μm in diameter. Successful epitaxial transfer on 2 inch wafer
demonstrates the scalability of this oxygen plasma-assisted low-temperature
bonding process, which subsequently paves the way for mass production of the
hybrid devices.Figure 5
Photograph of a 2 inch III-V epitaxial transferred to an SOI sample after300°C anneal for 2 hours and
selective removal of the InP substrate.
InP wafer sidewall is surrounded by CrystalbondTM wax to
protect InP device layer from being etched laterally during the substrate
removal step. The white region in the middle of the wafer is a reflection of the illuminator.


### 3.2. Silicon Waveguide and III-V Back-End Processing

The general procedure of silicon waveguide formation on an SOI wafer and III-V
back-end processing after wafer bonding process is as follows. The silicon
waveguide is formed on the (100) surface of an undoped silicon-on-insulator
(SOI) substrate using Cl2/Ar/HBr-based plasma reactive ion etching.
The thickness of the buried oxide (BOX) is 1 μm for the devices reported in
this paper. The III-V epitaxial layer is then transferred to the patterned
silicon wafer through low-temperature oxygen plasma-assisted wafer bonding,
which was described in Section 3.1. After removal of the InP substrate, mesa
structures on III-V layers are formed by dry-etching the p-type layers using a
CH4/H/Ar-based plasma reactive ion etch. Subsequent wet-etching of the quantum well
layers to the n-type layers is performed using H3PO4/H2O2.
Ni/AuGe/Ni/Au alloy contacts are deposited onto the exposed n-type InP layer.
Pd/Ti/Pd/Au p-contacts are then deposited on the center of the mesas. For
lasers and amplifiers, protons (H+) are implanted on the two sides
of the p-type mesa to create a 4 μm wide current channel and to prevent lateral
current spreading, ensuring a large overlap between the carriers and the
optical mode. Ti/Au probe pads are then deposited on the top of the mesa. Then,
if necessary, the sample is diced into bars and each bar is polished.


## 3.1. Plasma Assisted Low-Temperature Wafer Bonding

The transfer of the indium phosphide (InP)-based epitaxial layer structure to the silicon-on-insulator (SOI) substrate is a key step in the fabrication of this
hybrid platform and has direct impact on the device performance, yield, and
reliability. Due to the mismatch between the thermal expansion coefficient of silicon
and indium  phosphide (αSi = 2.6 ×10−6/K,
αInP =  4.8 ×10−6/K), high-temperature (>400°C) annealing steps are not desirable.
Figure 3(a) shows a Nomarski photograph of the top surface of an InP die
transferred to a silicon-on-insulator substrate at 600°C. Crosshatching can be seen for this
high-temperature direct wafer bonding which can lead to degradation of material
quality and scalability issues due to the accumulation of stress over larger
sample sizes. In order to resolve this
issue, low-temperature annealing is used with an oxygen plasma surface
treatment to enable strong bonding [20]. An
annealing temperature of 300°C is chosen to
minimize bonding stress while still being able to convert the weak Hydrogen
bonds formed by the room temperature bonding to strong covalent Si–O–In and
Si–O–P bonds. Figure 3(b) shows a
successful transfer of InP epitaxial layers to SOI with smooth device quality
surface morphology and no interfacial voids.Nomarski microscope images of the transferred III-V surface at bonding temperatures of
(a)600°C and (b) 300°C.
(a)(b)Figure4 is the schematic
process flow of the oxygen plasma assisted low-temperature (300°C) wafer bonding. After rigorous
sample cleaning and close microscopic inspection with 200x magnification, the native
oxide on SOI and InP are removed in standard buffer HF solution (1HF : 7 H2O)
and NH4OH (39%), respectively, resulting in clean, hydrophobic
surfaces. The samples then undergo an oxygen plasma surface treatment to grow
an ultrathin layer of oxide (<5 nm) [21] which leads to
very smooth (rms roughness <0.5 nm) hydrophilic surfaces, which is less
sensitive to the microroughness as compared to hydrophobic bonding. The Si–O–Si bonds
of the oxide (SOI side) are also found to be more strained than conventional
oxides formed in standard RCA-1 cleaning process or other hydrophilic
wet-chemical treatment, and have a higher readiness to break and form new
bonds. O2 energetic ion
bombardment also acts as a final cleaning step to remove hydrocarbons and water-related
species on the sample surface efficiently. The following deionized water dip
further terminates the oxide surface by polar hydroxyl groups OH−, forming bridging bonds between
the mating surfaces to result in spontaneous bonding at room temperature [22]. To strengthen the bond, the bonded sample is
placed in a conventional wafer bonding machine (Suss Bonder SB6E), where the samples
are held together at a pressure of 1.5 MPa and a temperature of 300°C, under vacuum (<4 × 10−4 Torr) from 1 to 12 hours. The 300°C annealing process enhances out diffusion of molecules trapped at the
interface and desorption of chemisorbed surface atoms, such as hydrogen, while
activating the formation of covalent bonds to achieve higher bonding energy [20]. After annealing and cooling, the InP
substrate is selectively removed in a 3HCl : 1H2O solution at room
temperature.Figure 4
Oxygen plasma-assisted
low-temperature wafer bonding process flow.Figure5 shows a 2-inch InP-based
expitaxial  wafer bonded on a SOI sample cleaved from a 6 inch SOI wafer. Smooth III-V morphology with no interfacial
void is achieved in the bonded area. The
two defects on the left-hand side of the figure are due to wafer handling with
tweezers and InP epitaxial surface defects that are 29 μm in diameter. Successful epitaxial transfer on 2 inch wafer
demonstrates the scalability of this oxygen plasma-assisted low-temperature
bonding process, which subsequently paves the way for mass production of the
hybrid devices.Figure 5
Photograph of a 2 inch III-V epitaxial transferred to an SOI sample after300°C anneal for 2 hours and
selective removal of the InP substrate.
InP wafer sidewall is surrounded by CrystalbondTM wax to
protect InP device layer from being etched laterally during the substrate
removal step. The white region in the middle of the wafer is a reflection of the illuminator.


## 3.2. Silicon Waveguide and III-V Back-End Processing

The general procedure of silicon waveguide formation on an SOI wafer and III-V
back-end processing after wafer bonding process is as follows. The silicon
waveguide is formed on the (100) surface of an undoped silicon-on-insulator
(SOI) substrate using Cl2/Ar/HBr-based plasma reactive ion etching.
The thickness of the buried oxide (BOX) is 1 μm for the devices reported in
this paper. The III-V epitaxial layer is then transferred to the patterned
silicon wafer through low-temperature oxygen plasma-assisted wafer bonding,
which was described in Section 3.1. After removal of the InP substrate, mesa
structures on III-V layers are formed by dry-etching the p-type layers using a
CH4/H/Ar-based plasma reactive ion etch. Subsequent wet-etching of the quantum well
layers to the n-type layers is performed using H3PO4/H2O2.
Ni/AuGe/Ni/Au alloy contacts are deposited onto the exposed n-type InP layer.
Pd/Ti/Pd/Au p-contacts are then deposited on the center of the mesas. For
lasers and amplifiers, protons (H+) are implanted on the two sides
of the p-type mesa to create a 4 μm wide current channel and to prevent lateral
current spreading, ensuring a large overlap between the carriers and the
optical mode. Ti/Au probe pads are then deposited on the top of the mesa. Then,
if necessary, the sample is diced into bars and each bar is polished.


## 4. Device Results

### 4.1. Silicon Evanescent Lasers

#### 4.1.1. 1550 nm Fabry Perot Lasers

The
first demonstrated device using the silicon evanescent device platform was the Fabry-Perot
(FP) hybrid silicon evanescent laser [23]. The cavity for these lasers was made
by dicing the ends of the hybrid waveguide and polishing them to a mirror
finish. The device presented here has two major changes from the first reported
devices. First, the buried oxide thickness is reduced to 1 μm in order to
reduce the thermal impedance of the device. Second, the III-V mesa was reduced
to 12 μm in order to reduce the device series resistance as shown in Figure
1(a).
The waveguide height, width, rib etch depth, and cavity length were 0.7 μm, 2 μm,
0.5 μm, and 850 μm, respectively. The calculated confinement factors in the
silicon and the quantum well region are 63% and 4%, respectively. The
epitaxial structure of the lasers is specified in Table 1.Table 1
III-V epitaxial layer structure with a 1550 nm
photoluminescence peak.
NameCompositionDoping concentrationThicknessP contact layerP-typeIn0.53Ga0.47As1×1019cm−30.1μmCladdingP-type InP1×1018cm−31.5μmSCHP-typeAi0.131Ga0.34In0.528 As, 1.3Q1×1017cm−30.25μmQuantum wellsAi0.089Ga0.461In0.45As, 1.3Q(9x)undoped10 nmAi0.055Ga0.292In0.653As, 1.7Q(8x)undoped7 nmN layerN-type InP1×1018cm−3110 nmSuper latticeN-typeIn0.85Ga0.15As0.327P0.673, 1.1Q(2x)1×1018cm−37.5 nmN-type InP (2x)1×1018cm−37.5 nmN bonding layerN-type InP1×1018cm−310 nmThe continuous
wave (CW) LI curve for this device is collected on one side with an integrating
sphere as shown in Figure6(a). In order to account for light exiting both
sides of the cavity, the data is multiplied by two. It can be seen that the
maximum laser output power, threshold, and differential efficiency at 15°C are 24 mW, 70 mA, and 16%, respectively. The device shows improvement in output power
and differential efficiency while maintaining a similar threshold when compared
to the first generation device. The maximum operating temperature is 45°C.LI curves for a 1550 nm FP silicon
evanescent laser (a) double-sided continuous wave output power, (b) double-sided
pulsed output power.
(a)(b)Figure6(b)
shows a set of pulsed single-sided output power as a function of applied
current (1 kHz repetition rate, 0.1% duty cycle) for stage temperatures ranging
from 15 to 50°C. The characteristic temperature (T0) and
an above threshold characteristic temperature (T1) [24] are 60 K and
120 K, respectively. The thermal impedance of
the laser is measured using a combination of two experiments. The first set of measurements is used to
establish a baseline for the shift in lasing wavelength as a function of active
region (stage) temperature (dλ/dT) as shown in Figure 7(a). Similar to the characteristic temperature
measurement above, this experiment is performed pulsed to ensure that there is
minimal device heating other than what is provided by the temperature-controlled
stage. The second measurement is
performed CW, and is used to measure the shift in wavelength as a function of
applied electrical power to the laser (dλ/dP) as shown in Figure 7(b). The thermal impedance ZT is then
given by (1) (1)ZT=(dλdT)−1(dλdP).(a) Pulsed measurement
results for the shift in lasing wavelength (single FP mode) as a function of
stage temperature. The inset contains a laser output spectrum at15°C
and 30°C. (b) Results for the shift in lasing wavelength (single FP
mode) as a function of dissipated power.
(a)(b)In both thedλ/dT and dλ/dP experiments, a single
longitudinal mode in the laser spectrum is monitored. Combining the results from
Figures 7(a) and
7(b), the laser thermal impedance is measured to be 42°C/W.The thermal performance
of the hybrid laser depends on several factors.
These include the amount and location of heat that is generated, the
thermal conductivity of the layers surrounding the heat sources, and the
operating temperature of the laser active region. To model the temperature rise as a function
of applied bias, we have employed a two-dimensional finite element modeling
technique. For the hybrid laser cross-section
shown in Figure1(a), there are six major sources of thermal energy. These include resistive heating in the
p-cladding, the n-contact layer, the active region, and the p and n contacts,
along with heat generated by the diode drop associated with the active region. More
detailed information about the values used in the simulation can be found in [25].A two
dimensional temperature profile of the hybrid laser operating at 500 mA is
shown in Figure8(a). The dissipated electrical power and the predicted temperature
rise in the laser active region are plotted as a function of applied current in
Figure 8(b). The contribution of each layer to the total electrical power
dissipation is also shown in the same figure. Combining the simulated
temperature rise, the dissipated electrical power, and the output optical
power, the thermal impedance of the laser is 43.5°C/W, which is
within 5% of our initial experimental results.(a) Two-dimensional temperature profile in
the hybrid laser at a bias current of 500 mA.
(b) Theoretical predictions for the dissipated electrical power in the
various laser sections along with the predicted temperature rise in the active
region as a function of contact current.
(a)(b)To show the effect of
the buried oxide on the thermal impedance,
Figure8(b) also includes a simulation
of the temperature rise in the device when the buried oxide has been
removed. Reducing the thickness of the
BOX layer results in lowering the thermal impedance to 18°C/W. This
illustrates how the high thermal conductivity of silicon should result in the
very low thermal resistances of silicon photonic devices.


#### 4.1.2. 1310 nm Fabry Perot Lasers

1310 nm hybrid
silicon lasers are also important for many data and telecommunication
applications [26].
The epitaxial layer structure used here (Table 2) contains an electron blocking layer between the quantum wells and the SCH
layer for better injection efficiency [27]. The layer is designed to provide a
high conduction band offset between the barrier and the SCH layer to prevent
electrons from leaking out of the quantum well region while a valence band
offset is kept low not to alter the hole flow into the quantum wells.Table 2
III-V epitaxial layer structure with a 1303 nm
photoluminescence peak.
NameCompositionDoping ConcentrationThicknessP contact layerP-typeIn0.53Ga0.47As1×1019cm−30.1μmCladdingP-type InP1×1018cm−31.5μmSCHP-typeAi0.4055Ga0.064In0.5305 As, 0.9Q1×1017cm−30.25μmElectron
blocking layerAi0.4764Ga0.0189In0.5047Asundoped10 nmQuantum wellsAi0.055Ga0.292In0.653As, 1.0Q(9x)undoped10 nmAi0.178Ga0.1234In0.0986As, 1.5Q(8x)undoped7 nmN layerN-type InP1×1018cm−3110 nmSuper latticeN-typeIn0.85Ga0.15As0.327P0.673, 1.1Q(2x)1×1018cm−37.5 nmN-type InP (2x)1×1018cm−37.5 nmN bonding layerN-type InP1×1018cm−310 nmLasers with fundamental transverse mode or
with second-order transverse mode can be designed and fabricated.
Figure9(a) shows the simulated quantum well
confinement factor as a function of silicon waveguide height while keeping the
waveguide width and slab thickness at 2.5 μm and
0.2 μm, respectively. Quantum well confinement represents the mode overlap to the 4 μm wide
quantum well at the center, where electrons and holes are injected. In the tall
silicon height regime (right hand part in Figure 9(a)), a higher quantum well
confinement factor for the second transverse mode exists. In the short silicon
height regime (left-hand part in Figure 9(a)), the fundamental mode has a higher
quantum well confinement factor. Figure 9(b)
shows the fundamental mode and second transverse mode with different waveguide
heights illustrating the fundamental mode increasingly lies more in the III-V
region as the silicon height decreases. The second transverse mode also
undergoes an increase in III-V confinement factor but splits into two lateral
lobes at lower silicon heights. This splitting reduces the modal overlap with
the 4 μm wide
excited quantum well region at the center.The confinement factor in III-V region simulation for different silicon
waveguide height (a) confinement factor calculations for the fundamental mode
and second transverse mode as a function of waveguide height (b) fundamental
and second transverse modes for waveguide heights of 0.7, 0.475, and 0.4μm. The image aspect ratio (Height:Width) is
4 : 1.
(a)(b)Two different silicon  waveguide heights of
0.4μm and
0.7 μm have
been  chosen to study the lasing mode selection depending on different quantum
well confinement factors. The width and slab height of the silicon waveguide is
2.5 μm and
0.2 μm,
respectively. The device length is ~850 μm.
Figures 10(a) and 10(b) show the simulated mode profiles with the largest
quantum well confinement factor for waveguide heights of 0.4 μm and
0.7 μm,
respectively. The measured lasing mode profiles in
Figures 10(c) and 10(d) agree
with the simulation results, indicating the quantum well confinement factor
primarily determines the lasing mode and can be engineered by changing the silicon
waveguide dimensions.The simulated and measured lasing optical mode profiles (a) simulated optical
mode for a waveguide height of 0.4μm, (b) simulated optical mode for a
waveguide height of 0.7 μm, (c) measured optical mode for a
waveguide height of 0.4 μm, and (d) measured optical mode for
a waveguide height of 0.7 μm.
(a)(b)(c)(d)Figure11 shows the measured single-sided
fiber-coupled CW output power as a function of the injected current at different
temperatures for two different lasers (0.4 μm and
0.7 μm
waveguide heights). The threshold current for both devices at 15°C is 30 mA. The output power at 100 mA is 2.9 mW and 5 mW and its corresponding
differential quantum efficiency is 2.5% and 8% for  the fundamental mode and
second transverse mode lasing devices, respectively. The estimated double-sided
total output power is shown in the secondary y-axis taking account for the
output power from the both facets and a coupling loss of 5 dB between the
device and the lensed fiber. The double sided total output power at 100 mA is
estimated  to be 18 mW and 31 mW, and its total differential quantum efficiency
is 15% and 50% for the fundamental mode and second transverse mode lasing
devices, respectively. The device lasing with second transverse mode (0.7 μm
waveguide height) operates up to 105°C and has better performance than the device lasing
with a fundamental mode primarily due to the higher quantum well confinement
factor (10% versus 7%).
Moreover, the overall performance of 1.3 μm lasers is superior to 1.5 μm
lasers previously described because the carrier blocking layer is incorporated
and because of the reduced intravalence band absorption and Auger scattering.LI curves for 1310 nm FP silicon
evanescent lasers (a) fundamental mode lasing device, (b) second transverse
mode lasing device. The secondaryy-axis represents the estimated total laser
output power.
(a)(b)


#### 4.1.3. 1550 nm Integrated Racetrack Laser and Photodetectors

Figure12(a) shows the layout of an
integrated hybrid silicon evanescent racetrack laser and two photodetectors operating
at 1550 nm [28]. The same epitaxial structure described in
Table 1 is used both
for the laser and the detector. This laser does not rely on facet dicing or
polishing and can be tested on-chip with simple probing of the laser and
photodetectors.(a) The layout of the racetrack resonator and the photodetectors. (b) A top view SEM micrograph of
two racetrack resonator lasers. The racetrack resonator lasers on the top and
bottom have radii of 200 and 100μm, respectively. (c) The LI curve for a laser
with a bend radius, R, of 200 μm, and directional
coupler coupling length, Linteraction, of 400 μm for various
temperatures. (d) The spectrum for a laser with R
= 100 μm and Linteraction =  400 μm.
(a)(b)(c)(d)The waveguide height, width, and rib etch
depth were 0.69μm, 1.5 μm, and 0.5 μm, respectively. The scanning electron
microscope (SEM) image of the fabricated devices is shown in
Figure 12(b). It
consists of a racetrack ring resonator with a straight waveguide length of 700 μm
and ring radii of 200 and 100 μm. A directional coupler is formed on the bottom
arm by placing a bus waveguide 0.5 micron away from the racetrack. Since
clockwise and counterclockwise propagating modes of ring lasers are only weakly
coupled, two 440 μm long photodetectors are used to collect the laser power;
the clockwise being collected at the left detector, and the counterclockwise
being collected at the right detector. These photodetectors have the same
waveguide structure as the hybrid laser, and the only difference being that
they are reverse biased to collect photogenerated carriers.To estimate the
laser output power from the measured photocurrent, the responsivity of the
detector is first measured by dicing and polishing a discrete detector in the
same chip and launching laser light into the detector through a lensed fiber.
The fiber coupled responsivity was measured to be 0.25 A/W at 1580 nm. Taking
into consideration the ~30% reflection off the waveguide facet and an estimated
5.25 +/− .25 dB coupling loss, the photodetector responsivity is estimated to
be in the range of 1.25–1.11 A/W. This corresponds to an internal quantum
efficiency of around 92%. A responsivity of 1.25 A/W is used for laser power
estimation such that the laser power values are on the conservative side. These
measurement results are also consistent with the measured responsivity from
stand alone photodetectors [29].The total laser
output power collected at both detectors as a function of current and temperature is shown in
Figure12(c) for a laser with a ring radius of 200 μm and a
coupler interaction length of 400 μm. The laser has a total output power of 29 mW with a maximum lasing temperature of
60°C. The differential efficiency is 17% and the laser threshold is
175 mA at 15°C. The laser
spectrum is shown in Figure 12(d) with its lasing peak in the range of 1592.5 nm.Since the two modes
of propagation are not coupled and are degenerate, mode competition is typical
in such lasers with a ring cavity structure.
Figure13(a) shows the
photocurrent measured separately from the left (PD1) and the right (PD2)
detectors as a function of the laser drive current for a laser with a ring
radius of 100 μm and a coupler interaction length of 100 μm. There are two
distinct regions of operation above threshold: (1) unidirectional bistable (low-bias
current), and (2) alternating oscillation (high-bias current). In the unidirectional bistable region, the
laser is lasing in either one direction or the other and the laser output
switches from one direction to the other direction as the laser diode current
increase. At higher bias (>360 mA),
the laser enter the alternating oscillation region, the output power exhibit
oscillatory behavior. The unidirectional bistability of the racetrack laser can
be alleviated if one of the detectors is forward biased to inject light into
the cavity (Figure 13(b)). The forward-biased
photodetector acts as an ASE source, which increases the photon density of the
clockwise mode, leading to greater simulated emission and mode selection. This
shows that the lasing direction can be controlled by forward biasing one of the
photodiodes; that is, lasing either clockwise or counterclockwise.(a) L-I curves of CW and CCW direction, (b) L-I curve of CW
direction when the PD2 is forward bias with 50 mA.
(a)(b)


#### 4.1.4. 1550 nm Mode Locked Lasers

Silicon hybrid lasers can be mode
locked at a variety of frequencies from 10 GHz to 40 GHz [30], for potential
applications in optical pulse generation, OTDM, WDM, and regenerative
all-optical clock recovery [31].An FP ML-SEL is shown in Figure14(a)
and its test setup is shown in Figure 14(b). The 39.4 GHz FP ML-SEL had a total
cavity length of 1060 μm and a saturable absorber (SA) length of 70 μm. For all
devices presented here, separate gain and SA sections were electrically
isolated using proton implantation. Passive
mode locking was achieved for a range of gain currents between 195 mA and 245 mA with similar output characteristics. For
a gain current of 206 mA and a saturable absorber reverse bias voltage
of 0.4 V, the pulse has a sech2 shape with 4.2 picoseconds full
width at half maximum (FWHM) pulsewidth, extinction ratio (ER) over 18 dB between
the peak and null, peak power of 4.5 dBm in fiber, and FWHM optical spectral
width of 0.9 nm. The time bandwidth product is 0.4, close to the transform
limited value of 0.32 for sech2 pulses, indicating minimal chirp. The laser was capable of stable mode locking
for a range of gain currents between 195 mA and 245 mA with similar output
characteristics.(a) Scanning electron micrograph of one end
of an FP ML-SEL. (b) Schematic of the general experimental setup. For some
measurements, more components were required as detailed in the references.
EDFA = erbium doped fiber amplifier, SHG = second harmonic generation autocorrelator,
OSA = optical spectrum analyzer, RFSA = 41 GHz radio frequency spectrum analyzer,
PD = 40 GHz photodetector.
(a)(b)By applying an RF signal to the
saturable absorber section, hybrid mode locking occurs and the jitter of the
pulses can be considerably reduced [32]
without changing the pulsewidth or spectral width. To achieve subharmonic
hybrid mode locking, a 20 GHz RF source with 17 dBm of RF input power is used.
For these conditions, the absolute jitter of this laser is 1 picosecond and the
locking range was 5 MHz. For all measurements in this paper, the jitter was
evaluated by integrating two times the single-sideband noise from 1 kHz to 100 MHz offset from the carrier frequency.Increasing the cavity length to 4.16 mm resulted in 10 GHz mode locking. The saturable absorber was 80μm long and the
cavity was divided into 4 equal length gain sections that can be biased
differently depending on the application. The bias conditions used for pulse
generation were as follows: Gain 1 and Gain 2 (adjacent to the saturable
absorber) were biased together at 531 mA, Gain 3 was biased at 140 mA, Gain 4
at 149 mA, and the SA was biased with −2.3 V. The FWHM pulsewidth was 3.9 picoseconds
with a sech2 shape, the ER is over 18 dB, the peak power in fiber
was 11.9 dBm, and the FWHM spectral width was 3.8 nm, containing 45 modes
evenly spaced at 10.16 GHz. The time
bandwidth product is 1.7, indicating significant chirping. Chirp can be reduced
in future designs by incorporating passive silicon waveguide sections in the
laser cavity with a shorter gain section [33]. This is fairly easy to do on
this platform and is a potential advantage of the platform for lower repetition
rate mode-locked lasers. Changing the SA bias between −0.5 and −2 V changes the
output pulsewidth between 9 and 4 picoseconds, while the ER and output power do
not change significantly. Figure 15(b) shows results for passive and hybrid
mode locking with different RF powers and RF injection frequencies. Also shown
is the residual jitter, which is the jitter of the hybrid mode locked laser
compared to that of the RF drive source. The minimum residual jitter is 199 femtoseconds
with 15 dBm RF drive at 10.16 GHz. At
this RF power, the locking range was 260 MHz.(a) Wavelength spectrum and autocorrelation trace
of the 40 GHz FP ML-SEL output. The fit to the optical spectrum is the Fourier
transform of the autocorrelation fit. (b) Jitter measurements for the 10 GHz FP
ML-SEL versus the RF input power.
(a)(b)Increasing the combined gain section
bias currents to a total of 1 A generates over 100 optical modes with power
levels within 10 dB of the peak mode power. If the modes have high enough
quality, the single ML-SEL could be combined with an AWG and used as a multiple
wavelength source for wavelength division multiplexing applications [34].
Across 100 modes, the linewidth was below 500 MHz and the OSNR was near 15 dB.
The linewidths of these modes are too large for some applications, but for
short reach applications the signal quality is sufficient. The mode
quality can be improved by injecting a stable CW laser into the hybrid mode-locked
laser, inducing optical injection locking [34]. In this case, the
spectral width narrows to 30 modes within 10 dB, but the linewidth of each of
these modes is reduced to that of the injected signal (<100 kHz). Also the
OSNR  is improved to over 25 dB for the majority of the modes. It is expected that better stabilization of
the hybrid mode-locked laser through packaging would allow for a wider spectral
width under optical injection locking, allowing for more high-quality modes to
be generated.A 30.4 GHz racetrack ML-SEL [31] is
shown in Figure 16(a). A racetrack mode-locked laser has the advantage that its
cavity is defined by lithography. Since the repetition rate is determined by
the cavity length, this allows for the repetition rate to be precisely
determined and repeated across different devices. Racetrack lasers can also be integrated monolithically
with other components. This laser has a cavity length of 2.6 mm, an absorber
length of 50 μm, and 2 separate gain sections. The gain sections were biased
together at 410 mA and the absorber was biased with  −0.66 V. This resulted in
Gaussian-shaped output pulses with 7.1 picoseconds FWHM pulsewidth, 0.5 nm FWHM
spectral width, and approximately 10 dB ER. The time bandwidth product was 0.43. The peak power was 6.8 dBm onchip, determined
from onchip photocurrent measurements.Figure 16
(a) SEM image of the 30.4 GHz racetrack ML-SEL.
(b) Eye diagrams of the input data sent to the device and the regenerated,
all-optically recovered clock.With a 30.4 GHz, 13 dBm RF signal
applied to the SA section, this laser had 364 femtoseconds of absolute jitter
and 50 MHz locking range. The laser
could also be synchronized to 30 Gbps optical input signals with average powers
below 0 dBm in the input waveguide meaning that it can perform all-optical
clock recovery. To test this application, we intentionally degraded a 30.4 Gbps
return to zero 231-1 pseudorandom bit stream by adding timing jitter
and reducing the extinction ratio, and we used the racetrack ML-SEL to recover
the clock all-optically [31]. The input ER was 3.8 dB and the input jitter
was 14 picoseconds. The recovered clock
had an ER of 10.4 dB and jitter of 1.7 picoseconds. The input data and output clock eye diagrams
are shown in Figure 16(b). The device’s extreme regenerative capabilities and
potential for integration with other components indicate that it could be used
as part of a silicon integrated optical 3R regenerator.


### 4.2. Silicon Evanescent Amplifiers and Photodetectors

Optical amplifiers are key components in
realizing high levels of photonic integration as they compensate for optical
losses from individual photonic elements.
The hybrid silicon evanescent amplifier structure is similar to the
offset quantum well structure which has typical quantum well confinement factor
in the range of 2% to 4% and it is suitable for preamplifiers [35]. In this
section, the integration of an amplifier and a detector for improved receiver
sensitivity is discussed [36].Figure17(a) shows a device structure of the
integrated device with an amplifier and a detector. At the transition between
the passive silicon waveguide and the hybrid waveguide of the amplifier, the
width of the III-V mesa is tapered from 0 μm
to 4 μm over a length of 70 μm to increase the coupling
efficiency and to minimize reflection. The width from 4 μm to 14 μm is tapered more abruptly over 5 μm since III-V mesas wider than
4 μm do not laterally affect the
optical mode. A 7° tilted abrupt junction is used between the passive
silicon waveguide and the detector hybrid waveguide. The details of the
epitaxial structure of the III-V layers are summarized in
Table 1. The III-V
mesa width of the amplifier is 14 μm.
The III-V mesa width of the detector is 3 μm
at the p cladding layer and 2 μm
at the p SCH and the quantum well layers to reduce the capacitance of the
device. The detector p and n pads are designed to be 100 μm apart from center to center to
use a standard GSG RF probe for high-speed testing. The SEM image of the eight fabricated devices
is shown in Figure 17(b), and the close view of the III-V amplifier taper and
the III-V detector mesa are shown in Figures 17(c)
and 17(d) respectively. The
total length of the amplifier and the detector is 1.24 mm and 100 μm, respectively.(a) Top view of the integrated
device. (b) SEM image of eight fabricated devices. (c) Close view of the III-V
taper of the amplifier. (d) Close view of the narrow III-V mesa of the
detector.
(a)The
internal quantum efficiency is measured to be around 50% at a reverse bias
voltage of 2 V at 1550 nm as shown in
Figure18(a). The amplifier gain is also
measured by taking photocurrent from the detector and the maximum gain is 9.5 dB
at 300 mA as shown in Figure 18(b). The reflection from the III-V taper is
estimated from the ripples at the ASE spectrum and it is less than 5×10−4.
The taper loss is estimated to be in the range of 0.6 dB to 1.2 dB by measuring
the photocurrent from the reverse-biased amplifier [36].
Figure 19 shows the
saturation characteristics of the integrated device. The overall responsivity is
5.7 A/W and the device is saturated by 0.5 dB at an output photocurrent of 25 mA. The device bandwidth is measured to be 3 GHz from the time domain impulse
measurements even though the estimated RC limited bandwidth is 7.5 GHz. The
current bandwidth is limited by carrier trapping in the quantum wells. Higher bandwidth can be achieved by
incorporating a thinner SCH layer and a bulk absorbing region or new quantum wells with small valence
band offset. The bit error rate (BER) was measured with 2.5 Gbps NRZ 231-1
pseudorandom bit sequence (PRBS) with different amplifier gains and the result
is shown in Figure 20. The purple data points are baseline measurements without
the amplification by launching the signal to input 2. The BER data at an
amplifier current of 100 mA shows worse receiver sensitivity than the receiver
sensitivity without amplification (baseline) because the amplifier is
below transparency. Once the amplifier is driven beyond transparency, the
power penalty becomes negative as shown in the three BER curves on the left
side. At the maximum gain of 9.5 dB, the  power penalty is
−8.5 dB compared to
the baseline and the receiver sensitivity at a BER of 10−9 is −17.5 dBm. The 1 dB difference between the gain and the measured power penalty is due
to the ASE noise. Better sensitivities would be achievable with a good
transimpedance amplifier and this device can be integrated with silicon passive
wavelength demultiplexers for high-speed WDM receivers [37].(a) Internal quantum efficiency
of the detector. (b) Gain of the amplifier.
(a)(b)Figure 19
Saturation
characteristics of the integrated optically preamplified receiver.Figure 20
Optically preamplified receiver bit error rate curves measured
with 231-1 NRZ 2.5 Gbps transmissions with different amplifier gains.


### 4.3. Silicon Evanescent Electroabsorption Modulators

In this
section, we review hybrid silicon evanescent electroabsorption modulators
(EAMs). The modulator structure described here can be integrated with lasers,
amplifiers, and photodetectors using quantum well intermixing [38] enabling
integrated high-speed transmitters. The cross-sectional structure is shown in
Figure 1(b) and the III-V epitaxial
structure with photoluminescence at 1478 nm is summarized in
Table 3. AlGaInAs is
chosen as the multiple quantum well material because typically it has a large conduction band offset which
provides a stronger carrier confinement and produces strong
quantum confined Stark effect with higher extinction ratio [39, 40]. The silicon waveguide was fabricated with a height
of 0.5 μm and a slab thickness of 0.3 μm. The
silicon waveguide has a width of 1.5 μm for passive segments and is tapered to
0.8 μm in the hybrid modulator region for a larger quantum well confinement
factor. The width of the III-V mesa is 4 μm at the top InP cladding layer and 2 μm at the SCH and quantum
well layers to reduce the capacitance of the device [40, 41]. The overall layout of the device is the same as
the detector describe in the previous section except for the width of the III-V
mesa is tapered from 0 to 2 μm
over a length of 60 μm to increase the coupling efficiency and to minimize
reflection as in the amplifiers. The hybrid EAM has a total
length around 220 μm with 100 μm absorber and two 60 μm long tapers. The
photograph of
the fabricated device is shown in Figure 21.Table 3
III-V epitaxial layer structure with a 1478 nm photoluminescence
peak.
NameCompositionDoping concentrationThicknessP contact layerP-typeIn0.53Ga0.47As1×1019cm−30.1μmCladdingP-type InP1×1018cm−31.5μmSCHP-typeAi0.160Ga0.320In0.520As, 1.3Q1×1017cm−30.15μmQuantum wellsAi0200Ga0.330In0.470As, 1.19Q(11x)Undoped7 nmAi0.080Ga0.330In0.590As, 1.55Q(10x)Undoped11 nmSCHAi0.160Ga0.320In0.520As, 1.3QUndoped0.1μmN layerN-type InP1×1018cm−3110 nmSuper latticeN-typeIn0.85Ga0.15As0.327P0.673, 1.1Q(2x)1×1018cm−37.5 nmN-type InP (2x)1×1018cm−37.5 nmN bonding layerN-type InP1×1018cm−310 nmFigure 21
Photograph of the fabricated EAM modulator.Figure22 shows the relative extinction at wavelength
of 1550 nm under various reverse
biases. More than 10 dB extinction can be achieved with a reverse bias voltage
of 5 V. The device has a series resistance around 30 Ω and capacitance of 0.1 pF measured from the impedance measurements resulting in a RC limited bandwidth
of ~20 GHz. It matches with the measured small signal modulation response as
shown in Figure 22.Figure 22
EAM extinction versus bias.Figure 23
Measured small signal response (solid). Calculated response using an RC model (dashed line).To investigate the
performance of large signal modulation, the   modulator is driven with a 231-1
pseudorandom bit sequence (PRBS). The device is biased at −3 V with a
peak-to-peak drive voltage of 3.2 V. The modulated light is collected with a
lensed fiber and amplified with an EDFA. Figure 24(a) shows eye diagrams
measured at nonreturn-to-zero (NRZ) 10 Gbps. The 10 Gbps signal has an
extinction ratio of 6.3 dB, which is slightly lower than the DC extinction due
to additional microwave
voltage drop at cladding and ohmic contacts. The eye is
clearly open with
quality factor (Q factor) close to 12. The rise and fall times of the signal are
about 27 picoseconds, which is, as expected, faster than the driving signal as
shown in Figure 24(b).(a) Measured 10G NRZ eye diagrams; (b) pulse train of drive
signal (dashed) and optical modulated signal (solid). The drive signal has rise/fall time around 35 picosesonds. The modulated signal shows rising/falling times of about 27 picseconds.
(a)(b)


## 4.1. Silicon Evanescent Lasers

### 4.1.1. 1550 nm Fabry Perot Lasers

The
first demonstrated device using the silicon evanescent device platform was the Fabry-Perot
(FP) hybrid silicon evanescent laser [23]. The cavity for these lasers was made
by dicing the ends of the hybrid waveguide and polishing them to a mirror
finish. The device presented here has two major changes from the first reported
devices. First, the buried oxide thickness is reduced to 1 μm in order to
reduce the thermal impedance of the device. Second, the III-V mesa was reduced
to 12 μm in order to reduce the device series resistance as shown in Figure
1(a).
The waveguide height, width, rib etch depth, and cavity length were 0.7 μm, 2 μm,
0.5 μm, and 850 μm, respectively. The calculated confinement factors in the
silicon and the quantum well region are 63% and 4%, respectively. The
epitaxial structure of the lasers is specified in Table 1.Table 1
III-V epitaxial layer structure with a 1550 nm
photoluminescence peak.
NameCompositionDoping concentrationThicknessP contact layerP-typeIn0.53Ga0.47As1×1019cm−30.1μmCladdingP-type InP1×1018cm−31.5μmSCHP-typeAi0.131Ga0.34In0.528 As, 1.3Q1×1017cm−30.25μmQuantum wellsAi0.089Ga0.461In0.45As, 1.3Q(9x)undoped10 nmAi0.055Ga0.292In0.653As, 1.7Q(8x)undoped7 nmN layerN-type InP1×1018cm−3110 nmSuper latticeN-typeIn0.85Ga0.15As0.327P0.673, 1.1Q(2x)1×1018cm−37.5 nmN-type InP (2x)1×1018cm−37.5 nmN bonding layerN-type InP1×1018cm−310 nmThe continuous
wave (CW) LI curve for this device is collected on one side with an integrating
sphere as shown in Figure6(a). In order to account for light exiting both
sides of the cavity, the data is multiplied by two. It can be seen that the
maximum laser output power, threshold, and differential efficiency at 15°C are 24 mW, 70 mA, and 16%, respectively. The device shows improvement in output power
and differential efficiency while maintaining a similar threshold when compared
to the first generation device. The maximum operating temperature is 45°C.LI curves for a 1550 nm FP silicon
evanescent laser (a) double-sided continuous wave output power, (b) double-sided
pulsed output power.
(a)(b)Figure6(b)
shows a set of pulsed single-sided output power as a function of applied
current (1 kHz repetition rate, 0.1% duty cycle) for stage temperatures ranging
from 15 to 50°C. The characteristic temperature (T0) and
an above threshold characteristic temperature (T1) [24] are 60 K and
120 K, respectively. The thermal impedance of
the laser is measured using a combination of two experiments. The first set of measurements is used to
establish a baseline for the shift in lasing wavelength as a function of active
region (stage) temperature (dλ/dT) as shown in Figure 7(a). Similar to the characteristic temperature
measurement above, this experiment is performed pulsed to ensure that there is
minimal device heating other than what is provided by the temperature-controlled
stage. The second measurement is
performed CW, and is used to measure the shift in wavelength as a function of
applied electrical power to the laser (dλ/dP) as shown in Figure 7(b). The thermal impedance ZT is then
given by (1) (1)ZT=(dλdT)−1(dλdP).(a) Pulsed measurement
results for the shift in lasing wavelength (single FP mode) as a function of
stage temperature. The inset contains a laser output spectrum at15°C
and 30°C. (b) Results for the shift in lasing wavelength (single FP
mode) as a function of dissipated power.
(a)(b)In both thedλ/dT and dλ/dP experiments, a single
longitudinal mode in the laser spectrum is monitored. Combining the results from
Figures 7(a) and
7(b), the laser thermal impedance is measured to be 42°C/W.The thermal performance
of the hybrid laser depends on several factors.
These include the amount and location of heat that is generated, the
thermal conductivity of the layers surrounding the heat sources, and the
operating temperature of the laser active region. To model the temperature rise as a function
of applied bias, we have employed a two-dimensional finite element modeling
technique. For the hybrid laser cross-section
shown in Figure1(a), there are six major sources of thermal energy. These include resistive heating in the
p-cladding, the n-contact layer, the active region, and the p and n contacts,
along with heat generated by the diode drop associated with the active region. More
detailed information about the values used in the simulation can be found in [25].A two
dimensional temperature profile of the hybrid laser operating at 500 mA is
shown in Figure8(a). The dissipated electrical power and the predicted temperature
rise in the laser active region are plotted as a function of applied current in
Figure 8(b). The contribution of each layer to the total electrical power
dissipation is also shown in the same figure. Combining the simulated
temperature rise, the dissipated electrical power, and the output optical
power, the thermal impedance of the laser is 43.5°C/W, which is
within 5% of our initial experimental results.(a) Two-dimensional temperature profile in
the hybrid laser at a bias current of 500 mA.
(b) Theoretical predictions for the dissipated electrical power in the
various laser sections along with the predicted temperature rise in the active
region as a function of contact current.
(a)(b)To show the effect of
the buried oxide on the thermal impedance,
Figure8(b) also includes a simulation
of the temperature rise in the device when the buried oxide has been
removed. Reducing the thickness of the
BOX layer results in lowering the thermal impedance to 18°C/W. This
illustrates how the high thermal conductivity of silicon should result in the
very low thermal resistances of silicon photonic devices.


### 4.1.2. 1310 nm Fabry Perot Lasers

1310 nm hybrid
silicon lasers are also important for many data and telecommunication
applications [26].
The epitaxial layer structure used here (Table 2) contains an electron blocking layer between the quantum wells and the SCH
layer for better injection efficiency [27]. The layer is designed to provide a
high conduction band offset between the barrier and the SCH layer to prevent
electrons from leaking out of the quantum well region while a valence band
offset is kept low not to alter the hole flow into the quantum wells.Table 2
III-V epitaxial layer structure with a 1303 nm
photoluminescence peak.
NameCompositionDoping ConcentrationThicknessP contact layerP-typeIn0.53Ga0.47As1×1019cm−30.1μmCladdingP-type InP1×1018cm−31.5μmSCHP-typeAi0.4055Ga0.064In0.5305 As, 0.9Q1×1017cm−30.25μmElectron
blocking layerAi0.4764Ga0.0189In0.5047Asundoped10 nmQuantum wellsAi0.055Ga0.292In0.653As, 1.0Q(9x)undoped10 nmAi0.178Ga0.1234In0.0986As, 1.5Q(8x)undoped7 nmN layerN-type InP1×1018cm−3110 nmSuper latticeN-typeIn0.85Ga0.15As0.327P0.673, 1.1Q(2x)1×1018cm−37.5 nmN-type InP (2x)1×1018cm−37.5 nmN bonding layerN-type InP1×1018cm−310 nmLasers with fundamental transverse mode or
with second-order transverse mode can be designed and fabricated.
Figure9(a) shows the simulated quantum well
confinement factor as a function of silicon waveguide height while keeping the
waveguide width and slab thickness at 2.5 μm and
0.2 μm, respectively. Quantum well confinement represents the mode overlap to the 4 μm wide
quantum well at the center, where electrons and holes are injected. In the tall
silicon height regime (right hand part in Figure 9(a)), a higher quantum well
confinement factor for the second transverse mode exists. In the short silicon
height regime (left-hand part in Figure 9(a)), the fundamental mode has a higher
quantum well confinement factor. Figure 9(b)
shows the fundamental mode and second transverse mode with different waveguide
heights illustrating the fundamental mode increasingly lies more in the III-V
region as the silicon height decreases. The second transverse mode also
undergoes an increase in III-V confinement factor but splits into two lateral
lobes at lower silicon heights. This splitting reduces the modal overlap with
the 4 μm wide
excited quantum well region at the center.The confinement factor in III-V region simulation for different silicon
waveguide height (a) confinement factor calculations for the fundamental mode
and second transverse mode as a function of waveguide height (b) fundamental
and second transverse modes for waveguide heights of 0.7, 0.475, and 0.4μm. The image aspect ratio (Height:Width) is
4 : 1.
(a)(b)Two different silicon  waveguide heights of
0.4μm and
0.7 μm have
been  chosen to study the lasing mode selection depending on different quantum
well confinement factors. The width and slab height of the silicon waveguide is
2.5 μm and
0.2 μm,
respectively. The device length is ~850 μm.
Figures 10(a) and 10(b) show the simulated mode profiles with the largest
quantum well confinement factor for waveguide heights of 0.4 μm and
0.7 μm,
respectively. The measured lasing mode profiles in
Figures 10(c) and 10(d) agree
with the simulation results, indicating the quantum well confinement factor
primarily determines the lasing mode and can be engineered by changing the silicon
waveguide dimensions.The simulated and measured lasing optical mode profiles (a) simulated optical
mode for a waveguide height of 0.4μm, (b) simulated optical mode for a
waveguide height of 0.7 μm, (c) measured optical mode for a
waveguide height of 0.4 μm, and (d) measured optical mode for
a waveguide height of 0.7 μm.
(a)(b)(c)(d)Figure11 shows the measured single-sided
fiber-coupled CW output power as a function of the injected current at different
temperatures for two different lasers (0.4 μm and
0.7 μm
waveguide heights). The threshold current for both devices at 15°C is 30 mA. The output power at 100 mA is 2.9 mW and 5 mW and its corresponding
differential quantum efficiency is 2.5% and 8% for  the fundamental mode and
second transverse mode lasing devices, respectively. The estimated double-sided
total output power is shown in the secondary y-axis taking account for the
output power from the both facets and a coupling loss of 5 dB between the
device and the lensed fiber. The double sided total output power at 100 mA is
estimated  to be 18 mW and 31 mW, and its total differential quantum efficiency
is 15% and 50% for the fundamental mode and second transverse mode lasing
devices, respectively. The device lasing with second transverse mode (0.7 μm
waveguide height) operates up to 105°C and has better performance than the device lasing
with a fundamental mode primarily due to the higher quantum well confinement
factor (10% versus 7%).
Moreover, the overall performance of 1.3 μm lasers is superior to 1.5 μm
lasers previously described because the carrier blocking layer is incorporated
and because of the reduced intravalence band absorption and Auger scattering.LI curves for 1310 nm FP silicon
evanescent lasers (a) fundamental mode lasing device, (b) second transverse
mode lasing device. The secondaryy-axis represents the estimated total laser
output power.
(a)(b)


### 4.1.3. 1550 nm Integrated Racetrack Laser and Photodetectors

Figure12(a) shows the layout of an
integrated hybrid silicon evanescent racetrack laser and two photodetectors operating
at 1550 nm [28]. The same epitaxial structure described in
Table 1 is used both
for the laser and the detector. This laser does not rely on facet dicing or
polishing and can be tested on-chip with simple probing of the laser and
photodetectors.(a) The layout of the racetrack resonator and the photodetectors. (b) A top view SEM micrograph of
two racetrack resonator lasers. The racetrack resonator lasers on the top and
bottom have radii of 200 and 100μm, respectively. (c) The LI curve for a laser
with a bend radius, R, of 200 μm, and directional
coupler coupling length, Linteraction, of 400 μm for various
temperatures. (d) The spectrum for a laser with R
= 100 μm and Linteraction =  400 μm.
(a)(b)(c)(d)The waveguide height, width, and rib etch
depth were 0.69μm, 1.5 μm, and 0.5 μm, respectively. The scanning electron
microscope (SEM) image of the fabricated devices is shown in
Figure 12(b). It
consists of a racetrack ring resonator with a straight waveguide length of 700 μm
and ring radii of 200 and 100 μm. A directional coupler is formed on the bottom
arm by placing a bus waveguide 0.5 micron away from the racetrack. Since
clockwise and counterclockwise propagating modes of ring lasers are only weakly
coupled, two 440 μm long photodetectors are used to collect the laser power;
the clockwise being collected at the left detector, and the counterclockwise
being collected at the right detector. These photodetectors have the same
waveguide structure as the hybrid laser, and the only difference being that
they are reverse biased to collect photogenerated carriers.To estimate the
laser output power from the measured photocurrent, the responsivity of the
detector is first measured by dicing and polishing a discrete detector in the
same chip and launching laser light into the detector through a lensed fiber.
The fiber coupled responsivity was measured to be 0.25 A/W at 1580 nm. Taking
into consideration the ~30% reflection off the waveguide facet and an estimated
5.25 +/− .25 dB coupling loss, the photodetector responsivity is estimated to
be in the range of 1.25–1.11 A/W. This corresponds to an internal quantum
efficiency of around 92%. A responsivity of 1.25 A/W is used for laser power
estimation such that the laser power values are on the conservative side. These
measurement results are also consistent with the measured responsivity from
stand alone photodetectors [29].The total laser
output power collected at both detectors as a function of current and temperature is shown in
Figure12(c) for a laser with a ring radius of 200 μm and a
coupler interaction length of 400 μm. The laser has a total output power of 29 mW with a maximum lasing temperature of
60°C. The differential efficiency is 17% and the laser threshold is
175 mA at 15°C. The laser
spectrum is shown in Figure 12(d) with its lasing peak in the range of 1592.5 nm.Since the two modes
of propagation are not coupled and are degenerate, mode competition is typical
in such lasers with a ring cavity structure.
Figure13(a) shows the
photocurrent measured separately from the left (PD1) and the right (PD2)
detectors as a function of the laser drive current for a laser with a ring
radius of 100 μm and a coupler interaction length of 100 μm. There are two
distinct regions of operation above threshold: (1) unidirectional bistable (low-bias
current), and (2) alternating oscillation (high-bias current). In the unidirectional bistable region, the
laser is lasing in either one direction or the other and the laser output
switches from one direction to the other direction as the laser diode current
increase. At higher bias (>360 mA),
the laser enter the alternating oscillation region, the output power exhibit
oscillatory behavior. The unidirectional bistability of the racetrack laser can
be alleviated if one of the detectors is forward biased to inject light into
the cavity (Figure 13(b)). The forward-biased
photodetector acts as an ASE source, which increases the photon density of the
clockwise mode, leading to greater simulated emission and mode selection. This
shows that the lasing direction can be controlled by forward biasing one of the
photodiodes; that is, lasing either clockwise or counterclockwise.(a) L-I curves of CW and CCW direction, (b) L-I curve of CW
direction when the PD2 is forward bias with 50 mA.
(a)(b)


### 4.1.4. 1550 nm Mode Locked Lasers

Silicon hybrid lasers can be mode
locked at a variety of frequencies from 10 GHz to 40 GHz [30], for potential
applications in optical pulse generation, OTDM, WDM, and regenerative
all-optical clock recovery [31].An FP ML-SEL is shown in Figure14(a)
and its test setup is shown in Figure 14(b). The 39.4 GHz FP ML-SEL had a total
cavity length of 1060 μm and a saturable absorber (SA) length of 70 μm. For all
devices presented here, separate gain and SA sections were electrically
isolated using proton implantation. Passive
mode locking was achieved for a range of gain currents between 195 mA and 245 mA with similar output characteristics. For
a gain current of 206 mA and a saturable absorber reverse bias voltage
of 0.4 V, the pulse has a sech2 shape with 4.2 picoseconds full
width at half maximum (FWHM) pulsewidth, extinction ratio (ER) over 18 dB between
the peak and null, peak power of 4.5 dBm in fiber, and FWHM optical spectral
width of 0.9 nm. The time bandwidth product is 0.4, close to the transform
limited value of 0.32 for sech2 pulses, indicating minimal chirp. The laser was capable of stable mode locking
for a range of gain currents between 195 mA and 245 mA with similar output
characteristics.(a) Scanning electron micrograph of one end
of an FP ML-SEL. (b) Schematic of the general experimental setup. For some
measurements, more components were required as detailed in the references.
EDFA = erbium doped fiber amplifier, SHG = second harmonic generation autocorrelator,
OSA = optical spectrum analyzer, RFSA = 41 GHz radio frequency spectrum analyzer,
PD = 40 GHz photodetector.
(a)(b)By applying an RF signal to the
saturable absorber section, hybrid mode locking occurs and the jitter of the
pulses can be considerably reduced [32]
without changing the pulsewidth or spectral width. To achieve subharmonic
hybrid mode locking, a 20 GHz RF source with 17 dBm of RF input power is used.
For these conditions, the absolute jitter of this laser is 1 picosecond and the
locking range was 5 MHz. For all measurements in this paper, the jitter was
evaluated by integrating two times the single-sideband noise from 1 kHz to 100 MHz offset from the carrier frequency.Increasing the cavity length to 4.16 mm resulted in 10 GHz mode locking. The saturable absorber was 80μm long and the
cavity was divided into 4 equal length gain sections that can be biased
differently depending on the application. The bias conditions used for pulse
generation were as follows: Gain 1 and Gain 2 (adjacent to the saturable
absorber) were biased together at 531 mA, Gain 3 was biased at 140 mA, Gain 4
at 149 mA, and the SA was biased with −2.3 V. The FWHM pulsewidth was 3.9 picoseconds
with a sech2 shape, the ER is over 18 dB, the peak power in fiber
was 11.9 dBm, and the FWHM spectral width was 3.8 nm, containing 45 modes
evenly spaced at 10.16 GHz. The time
bandwidth product is 1.7, indicating significant chirping. Chirp can be reduced
in future designs by incorporating passive silicon waveguide sections in the
laser cavity with a shorter gain section [33]. This is fairly easy to do on
this platform and is a potential advantage of the platform for lower repetition
rate mode-locked lasers. Changing the SA bias between −0.5 and −2 V changes the
output pulsewidth between 9 and 4 picoseconds, while the ER and output power do
not change significantly. Figure 15(b) shows results for passive and hybrid
mode locking with different RF powers and RF injection frequencies. Also shown
is the residual jitter, which is the jitter of the hybrid mode locked laser
compared to that of the RF drive source. The minimum residual jitter is 199 femtoseconds
with 15 dBm RF drive at 10.16 GHz. At
this RF power, the locking range was 260 MHz.(a) Wavelength spectrum and autocorrelation trace
of the 40 GHz FP ML-SEL output. The fit to the optical spectrum is the Fourier
transform of the autocorrelation fit. (b) Jitter measurements for the 10 GHz FP
ML-SEL versus the RF input power.
(a)(b)Increasing the combined gain section
bias currents to a total of 1 A generates over 100 optical modes with power
levels within 10 dB of the peak mode power. If the modes have high enough
quality, the single ML-SEL could be combined with an AWG and used as a multiple
wavelength source for wavelength division multiplexing applications [34].
Across 100 modes, the linewidth was below 500 MHz and the OSNR was near 15 dB.
The linewidths of these modes are too large for some applications, but for
short reach applications the signal quality is sufficient. The mode
quality can be improved by injecting a stable CW laser into the hybrid mode-locked
laser, inducing optical injection locking [34]. In this case, the
spectral width narrows to 30 modes within 10 dB, but the linewidth of each of
these modes is reduced to that of the injected signal (<100 kHz). Also the
OSNR  is improved to over 25 dB for the majority of the modes. It is expected that better stabilization of
the hybrid mode-locked laser through packaging would allow for a wider spectral
width under optical injection locking, allowing for more high-quality modes to
be generated.A 30.4 GHz racetrack ML-SEL [31] is
shown in Figure 16(a). A racetrack mode-locked laser has the advantage that its
cavity is defined by lithography. Since the repetition rate is determined by
the cavity length, this allows for the repetition rate to be precisely
determined and repeated across different devices. Racetrack lasers can also be integrated monolithically
with other components. This laser has a cavity length of 2.6 mm, an absorber
length of 50 μm, and 2 separate gain sections. The gain sections were biased
together at 410 mA and the absorber was biased with  −0.66 V. This resulted in
Gaussian-shaped output pulses with 7.1 picoseconds FWHM pulsewidth, 0.5 nm FWHM
spectral width, and approximately 10 dB ER. The time bandwidth product was 0.43. The peak power was 6.8 dBm onchip, determined
from onchip photocurrent measurements.Figure 16
(a) SEM image of the 30.4 GHz racetrack ML-SEL.
(b) Eye diagrams of the input data sent to the device and the regenerated,
all-optically recovered clock.With a 30.4 GHz, 13 dBm RF signal
applied to the SA section, this laser had 364 femtoseconds of absolute jitter
and 50 MHz locking range. The laser
could also be synchronized to 30 Gbps optical input signals with average powers
below 0 dBm in the input waveguide meaning that it can perform all-optical
clock recovery. To test this application, we intentionally degraded a 30.4 Gbps
return to zero 231-1 pseudorandom bit stream by adding timing jitter
and reducing the extinction ratio, and we used the racetrack ML-SEL to recover
the clock all-optically [31]. The input ER was 3.8 dB and the input jitter
was 14 picoseconds. The recovered clock
had an ER of 10.4 dB and jitter of 1.7 picoseconds. The input data and output clock eye diagrams
are shown in Figure 16(b). The device’s extreme regenerative capabilities and
potential for integration with other components indicate that it could be used
as part of a silicon integrated optical 3R regenerator.


## 4.1.1. 1550 nm Fabry Perot Lasers

The
first demonstrated device using the silicon evanescent device platform was the Fabry-Perot
(FP) hybrid silicon evanescent laser [23]. The cavity for these lasers was made
by dicing the ends of the hybrid waveguide and polishing them to a mirror
finish. The device presented here has two major changes from the first reported
devices. First, the buried oxide thickness is reduced to 1 μm in order to
reduce the thermal impedance of the device. Second, the III-V mesa was reduced
to 12 μm in order to reduce the device series resistance as shown in Figure
1(a).
The waveguide height, width, rib etch depth, and cavity length were 0.7 μm, 2 μm,
0.5 μm, and 850 μm, respectively. The calculated confinement factors in the
silicon and the quantum well region are 63% and 4%, respectively. The
epitaxial structure of the lasers is specified in Table 1.Table 1
III-V epitaxial layer structure with a 1550 nm
photoluminescence peak.
NameCompositionDoping concentrationThicknessP contact layerP-typeIn0.53Ga0.47As1×1019cm−30.1μmCladdingP-type InP1×1018cm−31.5μmSCHP-typeAi0.131Ga0.34In0.528 As, 1.3Q1×1017cm−30.25μmQuantum wellsAi0.089Ga0.461In0.45As, 1.3Q(9x)undoped10 nmAi0.055Ga0.292In0.653As, 1.7Q(8x)undoped7 nmN layerN-type InP1×1018cm−3110 nmSuper latticeN-typeIn0.85Ga0.15As0.327P0.673, 1.1Q(2x)1×1018cm−37.5 nmN-type InP (2x)1×1018cm−37.5 nmN bonding layerN-type InP1×1018cm−310 nmThe continuous
wave (CW) LI curve for this device is collected on one side with an integrating
sphere as shown in Figure6(a). In order to account for light exiting both
sides of the cavity, the data is multiplied by two. It can be seen that the
maximum laser output power, threshold, and differential efficiency at 15°C are 24 mW, 70 mA, and 16%, respectively. The device shows improvement in output power
and differential efficiency while maintaining a similar threshold when compared
to the first generation device. The maximum operating temperature is 45°C.LI curves for a 1550 nm FP silicon
evanescent laser (a) double-sided continuous wave output power, (b) double-sided
pulsed output power.
(a)(b)Figure6(b)
shows a set of pulsed single-sided output power as a function of applied
current (1 kHz repetition rate, 0.1% duty cycle) for stage temperatures ranging
from 15 to 50°C. The characteristic temperature (T0) and
an above threshold characteristic temperature (T1) [24] are 60 K and
120 K, respectively. The thermal impedance of
the laser is measured using a combination of two experiments. The first set of measurements is used to
establish a baseline for the shift in lasing wavelength as a function of active
region (stage) temperature (dλ/dT) as shown in Figure 7(a). Similar to the characteristic temperature
measurement above, this experiment is performed pulsed to ensure that there is
minimal device heating other than what is provided by the temperature-controlled
stage. The second measurement is
performed CW, and is used to measure the shift in wavelength as a function of
applied electrical power to the laser (dλ/dP) as shown in Figure 7(b). The thermal impedance ZT is then
given by (1) (1)ZT=(dλdT)−1(dλdP).(a) Pulsed measurement
results for the shift in lasing wavelength (single FP mode) as a function of
stage temperature. The inset contains a laser output spectrum at15°C
and 30°C. (b) Results for the shift in lasing wavelength (single FP
mode) as a function of dissipated power.
(a)(b)In both thedλ/dT and dλ/dP experiments, a single
longitudinal mode in the laser spectrum is monitored. Combining the results from
Figures 7(a) and
7(b), the laser thermal impedance is measured to be 42°C/W.The thermal performance
of the hybrid laser depends on several factors.
These include the amount and location of heat that is generated, the
thermal conductivity of the layers surrounding the heat sources, and the
operating temperature of the laser active region. To model the temperature rise as a function
of applied bias, we have employed a two-dimensional finite element modeling
technique. For the hybrid laser cross-section
shown in Figure1(a), there are six major sources of thermal energy. These include resistive heating in the
p-cladding, the n-contact layer, the active region, and the p and n contacts,
along with heat generated by the diode drop associated with the active region. More
detailed information about the values used in the simulation can be found in [25].A two
dimensional temperature profile of the hybrid laser operating at 500 mA is
shown in Figure8(a). The dissipated electrical power and the predicted temperature
rise in the laser active region are plotted as a function of applied current in
Figure 8(b). The contribution of each layer to the total electrical power
dissipation is also shown in the same figure. Combining the simulated
temperature rise, the dissipated electrical power, and the output optical
power, the thermal impedance of the laser is 43.5°C/W, which is
within 5% of our initial experimental results.(a) Two-dimensional temperature profile in
the hybrid laser at a bias current of 500 mA.
(b) Theoretical predictions for the dissipated electrical power in the
various laser sections along with the predicted temperature rise in the active
region as a function of contact current.
(a)(b)To show the effect of
the buried oxide on the thermal impedance,
Figure8(b) also includes a simulation
of the temperature rise in the device when the buried oxide has been
removed. Reducing the thickness of the
BOX layer results in lowering the thermal impedance to 18°C/W. This
illustrates how the high thermal conductivity of silicon should result in the
very low thermal resistances of silicon photonic devices.


## 4.1.2. 1310 nm Fabry Perot Lasers

1310 nm hybrid
silicon lasers are also important for many data and telecommunication
applications [26].
The epitaxial layer structure used here (Table 2) contains an electron blocking layer between the quantum wells and the SCH
layer for better injection efficiency [27]. The layer is designed to provide a
high conduction band offset between the barrier and the SCH layer to prevent
electrons from leaking out of the quantum well region while a valence band
offset is kept low not to alter the hole flow into the quantum wells.Table 2
III-V epitaxial layer structure with a 1303 nm
photoluminescence peak.
NameCompositionDoping ConcentrationThicknessP contact layerP-typeIn0.53Ga0.47As1×1019cm−30.1μmCladdingP-type InP1×1018cm−31.5μmSCHP-typeAi0.4055Ga0.064In0.5305 As, 0.9Q1×1017cm−30.25μmElectron
blocking layerAi0.4764Ga0.0189In0.5047Asundoped10 nmQuantum wellsAi0.055Ga0.292In0.653As, 1.0Q(9x)undoped10 nmAi0.178Ga0.1234In0.0986As, 1.5Q(8x)undoped7 nmN layerN-type InP1×1018cm−3110 nmSuper latticeN-typeIn0.85Ga0.15As0.327P0.673, 1.1Q(2x)1×1018cm−37.5 nmN-type InP (2x)1×1018cm−37.5 nmN bonding layerN-type InP1×1018cm−310 nmLasers with fundamental transverse mode or
with second-order transverse mode can be designed and fabricated.
Figure9(a) shows the simulated quantum well
confinement factor as a function of silicon waveguide height while keeping the
waveguide width and slab thickness at 2.5 μm and
0.2 μm, respectively. Quantum well confinement represents the mode overlap to the 4 μm wide
quantum well at the center, where electrons and holes are injected. In the tall
silicon height regime (right hand part in Figure 9(a)), a higher quantum well
confinement factor for the second transverse mode exists. In the short silicon
height regime (left-hand part in Figure 9(a)), the fundamental mode has a higher
quantum well confinement factor. Figure 9(b)
shows the fundamental mode and second transverse mode with different waveguide
heights illustrating the fundamental mode increasingly lies more in the III-V
region as the silicon height decreases. The second transverse mode also
undergoes an increase in III-V confinement factor but splits into two lateral
lobes at lower silicon heights. This splitting reduces the modal overlap with
the 4 μm wide
excited quantum well region at the center.The confinement factor in III-V region simulation for different silicon
waveguide height (a) confinement factor calculations for the fundamental mode
and second transverse mode as a function of waveguide height (b) fundamental
and second transverse modes for waveguide heights of 0.7, 0.475, and 0.4μm. The image aspect ratio (Height:Width) is
4 : 1.
(a)(b)Two different silicon  waveguide heights of
0.4μm and
0.7 μm have
been  chosen to study the lasing mode selection depending on different quantum
well confinement factors. The width and slab height of the silicon waveguide is
2.5 μm and
0.2 μm,
respectively. The device length is ~850 μm.
Figures 10(a) and 10(b) show the simulated mode profiles with the largest
quantum well confinement factor for waveguide heights of 0.4 μm and
0.7 μm,
respectively. The measured lasing mode profiles in
Figures 10(c) and 10(d) agree
with the simulation results, indicating the quantum well confinement factor
primarily determines the lasing mode and can be engineered by changing the silicon
waveguide dimensions.The simulated and measured lasing optical mode profiles (a) simulated optical
mode for a waveguide height of 0.4μm, (b) simulated optical mode for a
waveguide height of 0.7 μm, (c) measured optical mode for a
waveguide height of 0.4 μm, and (d) measured optical mode for
a waveguide height of 0.7 μm.
(a)(b)(c)(d)Figure11 shows the measured single-sided
fiber-coupled CW output power as a function of the injected current at different
temperatures for two different lasers (0.4 μm and
0.7 μm
waveguide heights). The threshold current for both devices at 15°C is 30 mA. The output power at 100 mA is 2.9 mW and 5 mW and its corresponding
differential quantum efficiency is 2.5% and 8% for  the fundamental mode and
second transverse mode lasing devices, respectively. The estimated double-sided
total output power is shown in the secondary y-axis taking account for the
output power from the both facets and a coupling loss of 5 dB between the
device and the lensed fiber. The double sided total output power at 100 mA is
estimated  to be 18 mW and 31 mW, and its total differential quantum efficiency
is 15% and 50% for the fundamental mode and second transverse mode lasing
devices, respectively. The device lasing with second transverse mode (0.7 μm
waveguide height) operates up to 105°C and has better performance than the device lasing
with a fundamental mode primarily due to the higher quantum well confinement
factor (10% versus 7%).
Moreover, the overall performance of 1.3 μm lasers is superior to 1.5 μm
lasers previously described because the carrier blocking layer is incorporated
and because of the reduced intravalence band absorption and Auger scattering.LI curves for 1310 nm FP silicon
evanescent lasers (a) fundamental mode lasing device, (b) second transverse
mode lasing device. The secondaryy-axis represents the estimated total laser
output power.
(a)(b)


## 4.1.3. 1550 nm Integrated Racetrack Laser and Photodetectors

Figure12(a) shows the layout of an
integrated hybrid silicon evanescent racetrack laser and two photodetectors operating
at 1550 nm [28]. The same epitaxial structure described in
Table 1 is used both
for the laser and the detector. This laser does not rely on facet dicing or
polishing and can be tested on-chip with simple probing of the laser and
photodetectors.(a) The layout of the racetrack resonator and the photodetectors. (b) A top view SEM micrograph of
two racetrack resonator lasers. The racetrack resonator lasers on the top and
bottom have radii of 200 and 100μm, respectively. (c) The LI curve for a laser
with a bend radius, R, of 200 μm, and directional
coupler coupling length, Linteraction, of 400 μm for various
temperatures. (d) The spectrum for a laser with R
= 100 μm and Linteraction =  400 μm.
(a)(b)(c)(d)The waveguide height, width, and rib etch
depth were 0.69μm, 1.5 μm, and 0.5 μm, respectively. The scanning electron
microscope (SEM) image of the fabricated devices is shown in
Figure 12(b). It
consists of a racetrack ring resonator with a straight waveguide length of 700 μm
and ring radii of 200 and 100 μm. A directional coupler is formed on the bottom
arm by placing a bus waveguide 0.5 micron away from the racetrack. Since
clockwise and counterclockwise propagating modes of ring lasers are only weakly
coupled, two 440 μm long photodetectors are used to collect the laser power;
the clockwise being collected at the left detector, and the counterclockwise
being collected at the right detector. These photodetectors have the same
waveguide structure as the hybrid laser, and the only difference being that
they are reverse biased to collect photogenerated carriers.To estimate the
laser output power from the measured photocurrent, the responsivity of the
detector is first measured by dicing and polishing a discrete detector in the
same chip and launching laser light into the detector through a lensed fiber.
The fiber coupled responsivity was measured to be 0.25 A/W at 1580 nm. Taking
into consideration the ~30% reflection off the waveguide facet and an estimated
5.25 +/− .25 dB coupling loss, the photodetector responsivity is estimated to
be in the range of 1.25–1.11 A/W. This corresponds to an internal quantum
efficiency of around 92%. A responsivity of 1.25 A/W is used for laser power
estimation such that the laser power values are on the conservative side. These
measurement results are also consistent with the measured responsivity from
stand alone photodetectors [29].The total laser
output power collected at both detectors as a function of current and temperature is shown in
Figure12(c) for a laser with a ring radius of 200 μm and a
coupler interaction length of 400 μm. The laser has a total output power of 29 mW with a maximum lasing temperature of
60°C. The differential efficiency is 17% and the laser threshold is
175 mA at 15°C. The laser
spectrum is shown in Figure 12(d) with its lasing peak in the range of 1592.5 nm.Since the two modes
of propagation are not coupled and are degenerate, mode competition is typical
in such lasers with a ring cavity structure.
Figure13(a) shows the
photocurrent measured separately from the left (PD1) and the right (PD2)
detectors as a function of the laser drive current for a laser with a ring
radius of 100 μm and a coupler interaction length of 100 μm. There are two
distinct regions of operation above threshold: (1) unidirectional bistable (low-bias
current), and (2) alternating oscillation (high-bias current). In the unidirectional bistable region, the
laser is lasing in either one direction or the other and the laser output
switches from one direction to the other direction as the laser diode current
increase. At higher bias (>360 mA),
the laser enter the alternating oscillation region, the output power exhibit
oscillatory behavior. The unidirectional bistability of the racetrack laser can
be alleviated if one of the detectors is forward biased to inject light into
the cavity (Figure 13(b)). The forward-biased
photodetector acts as an ASE source, which increases the photon density of the
clockwise mode, leading to greater simulated emission and mode selection. This
shows that the lasing direction can be controlled by forward biasing one of the
photodiodes; that is, lasing either clockwise or counterclockwise.(a) L-I curves of CW and CCW direction, (b) L-I curve of CW
direction when the PD2 is forward bias with 50 mA.
(a)(b)


## 4.1.4. 1550 nm Mode Locked Lasers

Silicon hybrid lasers can be mode
locked at a variety of frequencies from 10 GHz to 40 GHz [30], for potential
applications in optical pulse generation, OTDM, WDM, and regenerative
all-optical clock recovery [31].An FP ML-SEL is shown in Figure14(a)
and its test setup is shown in Figure 14(b). The 39.4 GHz FP ML-SEL had a total
cavity length of 1060 μm and a saturable absorber (SA) length of 70 μm. For all
devices presented here, separate gain and SA sections were electrically
isolated using proton implantation. Passive
mode locking was achieved for a range of gain currents between 195 mA and 245 mA with similar output characteristics. For
a gain current of 206 mA and a saturable absorber reverse bias voltage
of 0.4 V, the pulse has a sech2 shape with 4.2 picoseconds full
width at half maximum (FWHM) pulsewidth, extinction ratio (ER) over 18 dB between
the peak and null, peak power of 4.5 dBm in fiber, and FWHM optical spectral
width of 0.9 nm. The time bandwidth product is 0.4, close to the transform
limited value of 0.32 for sech2 pulses, indicating minimal chirp. The laser was capable of stable mode locking
for a range of gain currents between 195 mA and 245 mA with similar output
characteristics.(a) Scanning electron micrograph of one end
of an FP ML-SEL. (b) Schematic of the general experimental setup. For some
measurements, more components were required as detailed in the references.
EDFA = erbium doped fiber amplifier, SHG = second harmonic generation autocorrelator,
OSA = optical spectrum analyzer, RFSA = 41 GHz radio frequency spectrum analyzer,
PD = 40 GHz photodetector.
(a)(b)By applying an RF signal to the
saturable absorber section, hybrid mode locking occurs and the jitter of the
pulses can be considerably reduced [32]
without changing the pulsewidth or spectral width. To achieve subharmonic
hybrid mode locking, a 20 GHz RF source with 17 dBm of RF input power is used.
For these conditions, the absolute jitter of this laser is 1 picosecond and the
locking range was 5 MHz. For all measurements in this paper, the jitter was
evaluated by integrating two times the single-sideband noise from 1 kHz to 100 MHz offset from the carrier frequency.Increasing the cavity length to 4.16 mm resulted in 10 GHz mode locking. The saturable absorber was 80μm long and the
cavity was divided into 4 equal length gain sections that can be biased
differently depending on the application. The bias conditions used for pulse
generation were as follows: Gain 1 and Gain 2 (adjacent to the saturable
absorber) were biased together at 531 mA, Gain 3 was biased at 140 mA, Gain 4
at 149 mA, and the SA was biased with −2.3 V. The FWHM pulsewidth was 3.9 picoseconds
with a sech2 shape, the ER is over 18 dB, the peak power in fiber
was 11.9 dBm, and the FWHM spectral width was 3.8 nm, containing 45 modes
evenly spaced at 10.16 GHz. The time
bandwidth product is 1.7, indicating significant chirping. Chirp can be reduced
in future designs by incorporating passive silicon waveguide sections in the
laser cavity with a shorter gain section [33]. This is fairly easy to do on
this platform and is a potential advantage of the platform for lower repetition
rate mode-locked lasers. Changing the SA bias between −0.5 and −2 V changes the
output pulsewidth between 9 and 4 picoseconds, while the ER and output power do
not change significantly. Figure 15(b) shows results for passive and hybrid
mode locking with different RF powers and RF injection frequencies. Also shown
is the residual jitter, which is the jitter of the hybrid mode locked laser
compared to that of the RF drive source. The minimum residual jitter is 199 femtoseconds
with 15 dBm RF drive at 10.16 GHz. At
this RF power, the locking range was 260 MHz.(a) Wavelength spectrum and autocorrelation trace
of the 40 GHz FP ML-SEL output. The fit to the optical spectrum is the Fourier
transform of the autocorrelation fit. (b) Jitter measurements for the 10 GHz FP
ML-SEL versus the RF input power.
(a)(b)Increasing the combined gain section
bias currents to a total of 1 A generates over 100 optical modes with power
levels within 10 dB of the peak mode power. If the modes have high enough
quality, the single ML-SEL could be combined with an AWG and used as a multiple
wavelength source for wavelength division multiplexing applications [34].
Across 100 modes, the linewidth was below 500 MHz and the OSNR was near 15 dB.
The linewidths of these modes are too large for some applications, but for
short reach applications the signal quality is sufficient. The mode
quality can be improved by injecting a stable CW laser into the hybrid mode-locked
laser, inducing optical injection locking [34]. In this case, the
spectral width narrows to 30 modes within 10 dB, but the linewidth of each of
these modes is reduced to that of the injected signal (<100 kHz). Also the
OSNR  is improved to over 25 dB for the majority of the modes. It is expected that better stabilization of
the hybrid mode-locked laser through packaging would allow for a wider spectral
width under optical injection locking, allowing for more high-quality modes to
be generated.A 30.4 GHz racetrack ML-SEL [31] is
shown in Figure 16(a). A racetrack mode-locked laser has the advantage that its
cavity is defined by lithography. Since the repetition rate is determined by
the cavity length, this allows for the repetition rate to be precisely
determined and repeated across different devices. Racetrack lasers can also be integrated monolithically
with other components. This laser has a cavity length of 2.6 mm, an absorber
length of 50 μm, and 2 separate gain sections. The gain sections were biased
together at 410 mA and the absorber was biased with  −0.66 V. This resulted in
Gaussian-shaped output pulses with 7.1 picoseconds FWHM pulsewidth, 0.5 nm FWHM
spectral width, and approximately 10 dB ER. The time bandwidth product was 0.43. The peak power was 6.8 dBm onchip, determined
from onchip photocurrent measurements.Figure 16
(a) SEM image of the 30.4 GHz racetrack ML-SEL.
(b) Eye diagrams of the input data sent to the device and the regenerated,
all-optically recovered clock.With a 30.4 GHz, 13 dBm RF signal
applied to the SA section, this laser had 364 femtoseconds of absolute jitter
and 50 MHz locking range. The laser
could also be synchronized to 30 Gbps optical input signals with average powers
below 0 dBm in the input waveguide meaning that it can perform all-optical
clock recovery. To test this application, we intentionally degraded a 30.4 Gbps
return to zero 231-1 pseudorandom bit stream by adding timing jitter
and reducing the extinction ratio, and we used the racetrack ML-SEL to recover
the clock all-optically [31]. The input ER was 3.8 dB and the input jitter
was 14 picoseconds. The recovered clock
had an ER of 10.4 dB and jitter of 1.7 picoseconds. The input data and output clock eye diagrams
are shown in Figure 16(b). The device’s extreme regenerative capabilities and
potential for integration with other components indicate that it could be used
as part of a silicon integrated optical 3R regenerator.


## 4.2. Silicon Evanescent Amplifiers and Photodetectors

Optical amplifiers are key components in
realizing high levels of photonic integration as they compensate for optical
losses from individual photonic elements.
The hybrid silicon evanescent amplifier structure is similar to the
offset quantum well structure which has typical quantum well confinement factor
in the range of 2% to 4% and it is suitable for preamplifiers [35]. In this
section, the integration of an amplifier and a detector for improved receiver
sensitivity is discussed [36].Figure17(a) shows a device structure of the
integrated device with an amplifier and a detector. At the transition between
the passive silicon waveguide and the hybrid waveguide of the amplifier, the
width of the III-V mesa is tapered from 0 μm
to 4 μm over a length of 70 μm to increase the coupling
efficiency and to minimize reflection. The width from 4 μm to 14 μm is tapered more abruptly over 5 μm since III-V mesas wider than
4 μm do not laterally affect the
optical mode. A 7° tilted abrupt junction is used between the passive
silicon waveguide and the detector hybrid waveguide. The details of the
epitaxial structure of the III-V layers are summarized in
Table 1. The III-V
mesa width of the amplifier is 14 μm.
The III-V mesa width of the detector is 3 μm
at the p cladding layer and 2 μm
at the p SCH and the quantum well layers to reduce the capacitance of the
device. The detector p and n pads are designed to be 100 μm apart from center to center to
use a standard GSG RF probe for high-speed testing. The SEM image of the eight fabricated devices
is shown in Figure 17(b), and the close view of the III-V amplifier taper and
the III-V detector mesa are shown in Figures 17(c)
and 17(d) respectively. The
total length of the amplifier and the detector is 1.24 mm and 100 μm, respectively.(a) Top view of the integrated
device. (b) SEM image of eight fabricated devices. (c) Close view of the III-V
taper of the amplifier. (d) Close view of the narrow III-V mesa of the
detector.
(a)The
internal quantum efficiency is measured to be around 50% at a reverse bias
voltage of 2 V at 1550 nm as shown in
Figure18(a). The amplifier gain is also
measured by taking photocurrent from the detector and the maximum gain is 9.5 dB
at 300 mA as shown in Figure 18(b). The reflection from the III-V taper is
estimated from the ripples at the ASE spectrum and it is less than 5×10−4.
The taper loss is estimated to be in the range of 0.6 dB to 1.2 dB by measuring
the photocurrent from the reverse-biased amplifier [36].
Figure 19 shows the
saturation characteristics of the integrated device. The overall responsivity is
5.7 A/W and the device is saturated by 0.5 dB at an output photocurrent of 25 mA. The device bandwidth is measured to be 3 GHz from the time domain impulse
measurements even though the estimated RC limited bandwidth is 7.5 GHz. The
current bandwidth is limited by carrier trapping in the quantum wells. Higher bandwidth can be achieved by
incorporating a thinner SCH layer and a bulk absorbing region or new quantum wells with small valence
band offset. The bit error rate (BER) was measured with 2.5 Gbps NRZ 231-1
pseudorandom bit sequence (PRBS) with different amplifier gains and the result
is shown in Figure 20. The purple data points are baseline measurements without
the amplification by launching the signal to input 2. The BER data at an
amplifier current of 100 mA shows worse receiver sensitivity than the receiver
sensitivity without amplification (baseline) because the amplifier is
below transparency. Once the amplifier is driven beyond transparency, the
power penalty becomes negative as shown in the three BER curves on the left
side. At the maximum gain of 9.5 dB, the  power penalty is
−8.5 dB compared to
the baseline and the receiver sensitivity at a BER of 10−9 is −17.5 dBm. The 1 dB difference between the gain and the measured power penalty is due
to the ASE noise. Better sensitivities would be achievable with a good
transimpedance amplifier and this device can be integrated with silicon passive
wavelength demultiplexers for high-speed WDM receivers [37].(a) Internal quantum efficiency
of the detector. (b) Gain of the amplifier.
(a)(b)Figure 19
Saturation
characteristics of the integrated optically preamplified receiver.Figure 20
Optically preamplified receiver bit error rate curves measured
with 231-1 NRZ 2.5 Gbps transmissions with different amplifier gains.


## 4.3. Silicon Evanescent Electroabsorption Modulators

In this
section, we review hybrid silicon evanescent electroabsorption modulators
(EAMs). The modulator structure described here can be integrated with lasers,
amplifiers, and photodetectors using quantum well intermixing [38] enabling
integrated high-speed transmitters. The cross-sectional structure is shown in
Figure 1(b) and the III-V epitaxial
structure with photoluminescence at 1478 nm is summarized in
Table 3. AlGaInAs is
chosen as the multiple quantum well material because typically it has a large conduction band offset which
provides a stronger carrier confinement and produces strong
quantum confined Stark effect with higher extinction ratio [39, 40]. The silicon waveguide was fabricated with a height
of 0.5 μm and a slab thickness of 0.3 μm. The
silicon waveguide has a width of 1.5 μm for passive segments and is tapered to
0.8 μm in the hybrid modulator region for a larger quantum well confinement
factor. The width of the III-V mesa is 4 μm at the top InP cladding layer and 2 μm at the SCH and quantum
well layers to reduce the capacitance of the device [40, 41]. The overall layout of the device is the same as
the detector describe in the previous section except for the width of the III-V
mesa is tapered from 0 to 2 μm
over a length of 60 μm to increase the coupling efficiency and to minimize
reflection as in the amplifiers. The hybrid EAM has a total
length around 220 μm with 100 μm absorber and two 60 μm long tapers. The
photograph of
the fabricated device is shown in Figure 21.Table 3
III-V epitaxial layer structure with a 1478 nm photoluminescence
peak.
NameCompositionDoping concentrationThicknessP contact layerP-typeIn0.53Ga0.47As1×1019cm−30.1μmCladdingP-type InP1×1018cm−31.5μmSCHP-typeAi0.160Ga0.320In0.520As, 1.3Q1×1017cm−30.15μmQuantum wellsAi0200Ga0.330In0.470As, 1.19Q(11x)Undoped7 nmAi0.080Ga0.330In0.590As, 1.55Q(10x)Undoped11 nmSCHAi0.160Ga0.320In0.520As, 1.3QUndoped0.1μmN layerN-type InP1×1018cm−3110 nmSuper latticeN-typeIn0.85Ga0.15As0.327P0.673, 1.1Q(2x)1×1018cm−37.5 nmN-type InP (2x)1×1018cm−37.5 nmN bonding layerN-type InP1×1018cm−310 nmFigure 21
Photograph of the fabricated EAM modulator.Figure22 shows the relative extinction at wavelength
of 1550 nm under various reverse
biases. More than 10 dB extinction can be achieved with a reverse bias voltage
of 5 V. The device has a series resistance around 30 Ω and capacitance of 0.1 pF measured from the impedance measurements resulting in a RC limited bandwidth
of ~20 GHz. It matches with the measured small signal modulation response as
shown in Figure 22.Figure 22
EAM extinction versus bias.Figure 23
Measured small signal response (solid). Calculated response using an RC model (dashed line).To investigate the
performance of large signal modulation, the   modulator is driven with a 231-1
pseudorandom bit sequence (PRBS). The device is biased at −3 V with a
peak-to-peak drive voltage of 3.2 V. The modulated light is collected with a
lensed fiber and amplified with an EDFA. Figure 24(a) shows eye diagrams
measured at nonreturn-to-zero (NRZ) 10 Gbps. The 10 Gbps signal has an
extinction ratio of 6.3 dB, which is slightly lower than the DC extinction due
to additional microwave
voltage drop at cladding and ohmic contacts. The eye is
clearly open with
quality factor (Q factor) close to 12. The rise and fall times of the signal are
about 27 picoseconds, which is, as expected, faster than the driving signal as
shown in Figure 24(b).(a) Measured 10G NRZ eye diagrams; (b) pulse train of drive
signal (dashed) and optical modulated signal (solid). The drive signal has rise/fall time around 35 picosesonds. The modulated signal shows rising/falling times of about 27 picseconds.
(a)(b)


## 5. Conclusion

Recent
progress of hybrid silicon evanescent devices has been reviewed in this paper.
Discrete lasers, amplifiers, photodetectors, and electroabsorption modulators
have been demonstrated. Racetrack  lasers
integrated with photodetectors, mode locked lasers, and photodetectors with
preamplifiers have also been presented as examples of photonic integration with
this hybrid device structure. These demonstrations show the potential for
realizing active functionality on the silicon photonics platform. One of the important
paths of this research is improving performance of individual devices, that is,
device efficiency and thermal performance, in conjunction with studies on
device reliability. Another path is the development of bonding of III-V
materials to large size silicon wafers (>6 inch). The bonding process can
be wafer scale or can be used for die attach. The optimum size of III-V material to
use depends on the density of active devices required for the
silicon wafer [42]. The hybrid silicon evanescent device platform provides a
unique way to build photonic active devices on silicon, and those studies will
expedite the applications of silicon photonic integrated circuits in optical
telecommunications and optical interconnects.



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*Source: 682978-2008-06-09.xml*