The adoption of optical technologies by high-volume consumer markets is severely limited by the cost and complexity of
manufacturing complete optical transceiver systems. This is in large part because "boutique" semiconductor fabrication
processes are required for III-V lasers, modulators, and photodetectors; furthermore, precision bonding and painstaking
assembly are needed to integrate or assemble such dissimilar devices and materials together. On the other hand, 200mm
and 300mm silicon process technology has been bringing ever-increasing computing power to the masses by relentless
cost reduction for several decades. Intel's silicon photonics program aims to marry this CMOS infrastructure and recent
developments in MEMS manufacturing with the burgeoning field of microphotonics to make low cost, high-speed
optical links ubiquitous. In this paper, we will provide an overview of several aspects of silicon photonics technology
development in a CMOS fabrication line. First, we will describe fabrication strategies from the MEMS industry for
micromachining silicon to create passive optical devices such as mirrors, waveguides, and facets, as well as alignment
features. Second, we will discuss some of the challenges of fabricating hybrid III-V lasers on silicon, including such
aspects as hybrid integration of InP-based materials with silicon using various bonding methods, etching of InP films,
and contact formation using CMOS-compatible metals.
Avalanche Photodiodes (APDs) are widely used in fiber-optic communications as well as imaging and sensing
applications where high sensitivities are needed. Traditional InP-based APD receivers typically offer a 10 dB
improvement in sensitivity up to 10 Gb/s when compared to standard p-i-n based detector counterparts. As the data rates
increase, however, a limited gain-bandwidth product (~100GHz) results in degraded receiver sensitivity. An increasing
amount of research is now focusing on alternative multiplication materials for APDs to overcome this limitation, and one
of the most promising is silicon. The difficulty in realizing a silicon-based APD device at near infrared wavelengths is
that a compatible absorbing material is difficult to find. Research on germanium-on-silicon p-i-n detectors has shown
acceptable responsivity at wavelengths as long as 1550 nm, and this work extends the approach to the more complicated
APD structure. We are reporting here a germanium-on-silicon Separate Absorption Charge and Multiplication (SACM)
APD which operates at 1310 nm, with a responsivity of 0.55A/W at unity gain with long dark current densities. The
measured gain bandwidth product of this device is much higher than that of a typical III-V APD. Other device
performances, like reliability, sensitivity and thermal stability, will also be discussed in this talk. This basic
demonstration of a new silicon photonic device is an important step towards practical APD devices operating at 40 Gb/s,
as well as for new applications which require low cost, high volume receivers with high sensitivity such as imaging and
We present a monolithic integrated low-threshold Raman silicon laser based on silicon-on-insulator (SOI) rib
waveguide ring cavity with an integrated p-i-n diode. The laser cavity consists of a race-track shaped ring resonator
connected to a straight bus waveguide via a directional coupler which couples both pump and signal light into and
out of the cavity. Reverse biasing the diode with 25V reduces the free carrier lifetime to below 1 ns, and stable,
single-mode, continuous-wave (CW) Raman lasing is achieved with threshold of 20mW, slope efficiency of 28%,
and output power of 50mW. With zero bias voltage, a lasing threshold of 26mW and laser output power >10mW can
be obtained. The laser emission has high spectral purity with a side-mode suppression of >80dB and laser linewidth
of <100 kHz. The laser wavelength can be tuned continuously over 25 GHz. To demonstrate the performance
capability of the laser for gas sensing application, we perform absorption spectroscopy on methane at 1687 nm using
the CW output of the silicon Raman laser. The measured rotationally-resolved direct absorption IR spectrum agrees
well with theoretical prediction. This ring laser architecture allows for on-chip integration with other silicon
photonics components to provide an integrated and scaleable monolithic device. By proper design of the ring cavity
and the directional coupler, it is possible to achieve higher order cascaded Raman lasing in silicon for extending
laser wavelengths from near IR to mid IR regions.
The strong optical nonlinearity of silicon and tight optical field confinement in silicon waveguides, accompanied by
silicon's unique material properties such as high optical damage threshold and thermal conductivity, enable compact
nonlinear photonic devices to be fabricated in silicon using cost effective CMOS compatible fabrication technology.
By integrating a p-i-n diode into the silicon waveguide, the nonlinear optical loss due to two photon absorption
induced free carrier absorption in silicon waveguides can be dramatically reduced, and efficient nonlinear optical
devices can be realized on silicon chips for high speed optical communications. In this paper, we report recent
development of silicon p-i-n waveguide based nonlinear photonic chips for wavelength conversion and dispersion
compensation applications. Wavelength conversion efficiency of -8.5 dB can be achieved in an 8-cm long p-i-n
silicon waveguide by four-wave mixing in continuous-wave operation, and chromatic dispersion compensation by
mid-span spectral inversion is demonstrated experimentally using silicon spectral inverter at the mid-span of a fiber
optical link, achieving transmission of optical data at 40 Gb/s over 320 km of standard fiber with negligible power
penalty. The unique advantages of using silicon over previously proposed nonlinear optical media for dispersion
compensation are discussed.
Recently, AlGaInAs-silicon evanescent lasers have been demonstrated as a method of integrating active photonic devices on a silicon based platform. This hybrid waveguide architecture consists of III-V quantum wells bonded to silicon waveguides. The self aligned optical mode leads to a bonding process that is manufacturable in high volumes. Here give an overview of a racetrack resonator laser integrated with two photo-detectors on the hybrid AlGaInAs-silicon evanescent device platform. Unlike previous demonstrations of hybrid AlGaInAs-silicon evanescent lasers, we demonstrate an on-chip racetrack resonator laser that does not rely on facet polishing and dicing in order to define the laser cavity. The laser runs continuous-wave (c.w.) at 1590 nm with a threshold of 175 mA, has a maximum total output power of 29 mW and a maximum operating temperature of 60 C. The output of this laser light is directly coupled into a pair of on chip hybrid AlGaInAs-silicon evanescent photodetectors used to measure the laser output.
Photonic integration is one of the important ways to realize low cost and small form factor optical transceivers for future high-speed high capacity I/O applications in computing systems. The photonic integration on silicon platform is particularly attractive because of the CMOS photonics and electronics process compatibility. In this paper, we present design and fabrication of a silicon photonic integrated circuit that is capable of transmitting data at hundreds gigabits per second. In such an integrated chip, 8 high-speed silicon optical modulators with a 1:8 wavelength demultiplexer and an 8:1 wavelength multiplexer are fabricated on a single silicon-on-insulator (SOI) substrate. We review the recent results of individual silicon modulator based on electric-field-induced carrier depletion in a SOI waveguide containing a reverse biased pn junction. We characterize the individual multiplexer/demultiplexer as well as the integrated chip. The basic functionality of the photonic integration is demonstrated.
Recently, low threshold Raman silicon lasers based on ring resonator architecture have been demonstrated. One of the
key elements of the laser cavity is the directional coupler that couples both pump and signal light in and out of the ring
resonator from the bus waveguide. The coupling coefficients are crucial for achieving desired laser performance. In this
paper, we report design, fabrication, and characterization of tunable silicon ring resonators for Raman laser and amplifier
applications. By employing a tunable coupler, the coupling coefficients for both pump and signal wavelength can be
tailored to their optimal values after the fabrication, which significantly increases the processing tolerance and improves
the device performance.
The package integration of optical components with electronic integrated circuits
(ICs) for optical interconnects is a subject of much debate and will, to a large extent,
determine the performance of the optical interconnect system. In this paper we examine
the challenges of incorporating optical interconnects into a computer system; specifically
we cover several ways to integrate the optical components with a central processing
unit (CPU) or chipset.
Critical performance parameters such as the supported distance, power
consumption and the achievable bandwidth are all impacted by the electrical integration
between the IC and the optical components. Additional electrical link issues which also
have a large impact on the performance of the link will be discussed as well; these include
protocol related issues as well as signal integrity concerns, such as the jitter budget.
We will also discuss the performance of some of the competing electrical
technologies in order to provide a better understanding of the implementation challenge
facing the developers of optical interconnect technology. Rack to rack communications
are quickly moving to optical links, board to board communication is the next step and chip
to chip communication is still further out as the electrical solutions for this topology have a
great deal of headroom.
Recently, AlGaInAs-silicon evanescent lasers have been demonstrated as a method of integrating active
photonic devices on a silicon based platform. This hybrid waveguide architecture consists of III-V quantum wells
bonded to silicon waveguides. The self aligned optical mode leads to a bonding process that is manufacturable in high
volumes. Here give an overview of a racetrack resonator laser integrated with two photo-detectors on the hybrid
AlGaInAs-silicon evanescent device platform. Unlike previous demonstrations of hybrid AlGaInAs-silicon evanescent
lasers, we demonstrate an on-chip racetrack resonator laser that does not rely on facet polishing and dicing in order to
define the laser cavity. The laser runs continuous-wave (c.w.) at 1590 nm with a threshold of 175 mA, has a maximum
total output power of 29 mW and a maximum operating temperature of 60 C. The output of this laser light is directly
coupled into a pair of on chip hybrid AlGaInAs-silicon evanescent photodetectors used to measure the laser output.
We present an electrically pumped silicon evanescent laser that utilizes a silicon waveguide and offset AlGaInAs
quantum wells. The silicon waveguide is fabricated on a Silicon-On-Insulator (SOI) wafer and is bonded with the
AlGaInAs quantum well structure using low temperature O2 plasma-assisted wafer bonding. The optical mode in the
hybrid waveguide is predominantly confined in the passive silicon waveguide and evanescently couples into the III-V
active region providing optical gain via electrical current injection. The device lases continuous wave at 1577 nm with a
threshold of 65 mA at 15 °C. The maximum single-sided fiber-coupled cw output power is 1.8 mW. The maximum operating temperature is 40 °C mainly limited by a high series resistance of the device. Operation up to 60 °C should be achievable by lowering the series resistance and thermal impedance.
High-speed silicon optical modulator is one of key components for integrated silicon photonic chip aiming at Tb/s data
transmission for next generation communication networks as well as future high performance computing applications. In
this paper we review the recent development of the silicon modulator. In particular, we present a high-speed and highly
scalable silicon optical modulator based on the free carrier plasma dispersion effect. The fast refractive index modulation
of the device is due to electric-field-induced carrier depletion in a Silicon-on-Insulator waveguide containing a reverse
biased pn junction. To achieve high-speed performance, a traveling-wave design is employed to allow co-propagation of
electrical and optical signals along the waveguide. We demonstrate high-frequency modulator optical response with 3
dB bandwidth of ~20 GHz and data transmission up to 30 Gb/s. We also highlight the future device optimization for 40
Gb/s and beyond.
We present a chip-scale ring resonator Raman silicon laser and amplifier based on a silicon-on-insulator rib
waveguide with an integrated p-i-n diode structure. The laser cavity consists of a race-track shaped ring resonator
connected to a straight bus waveguide via a directional coupler which couples both pump and signal laser light into
and out of the cavity. The optical propagation loss of the ring resonator is reduced to <0.3 dB/cm on average and the
effective free carrier lifetime in the waveguide can be shortened to <1 ns under reverse biasing, which efficiently
reduces the nonlinear loss due to two-photon absorption induced free carrier absorption. We achieve continuous-wave,
single-mode lasing with threshold of <20 mW and slope efficiency of >23%. Based on the same ring
resonator architecture, we build a compact, chip-scale Raman amplifier that takes advantage of the cavity
enhancement effect to lower the pump power and reduce the device size. We achieve over 3 dB amplification with 3
times less pump power in a 3 cm ring resonator compared to a straight waveguide of the same length. Our
experimental results agree with simulations. The ring resonator based laser and amplifier can be integrated on chip
with other silicon photonics components to provide a monolithic integrated photonic device.
We present a monolithic integrated Raman silicon laser and amplifier based on silicon-on-insulator rib waveguide race-track ring resonator with an integrated p-i-n diode structure. Under reverse biasing, we efficiently reduced the nonlinear loss due to two-photon absorption induced free carrier absorption and achieved continuous-wave net gain and stable, single-mode lasing with output power exceeding 30mW and 10% slope efficiency. The laser emission has high spectral purity with a side mode suppression exceeding 70dB and a laser linewidth of <100 kHz. This ring resonator architecture allows for on-chip integration with other silicon photonics components to provide a highly integrated and scaleable monolithic device. Using the ring resonator architecture, we can build a compact, chip scale Raman amplifier that takes advantage of the resonance effect to increase the effective pump power and reduce the device size. Our simulations suggest that a 3dB net gain can be achieved with 4dB less pump power in a 3cm ring compared to a straight waveguide of the same length.
In recent years there has been a growing interest in using Silicon on Insulator (SOI) as a platform for integrated planar optical circuits, this is mainly due to the high quality yield volume processes demonstrated by the CMOS manufacturing industry and recent MEMS technology progress. In this work we present monolithic integration of Silicon and SiON planar lightwave circuits on a single SOI chip processed in a CMOS fabrication environment. The demonstration of a processing scheme that yields low loss waveguides for both silicon and SiON as well as efficient transition of light between the two materials is the goal of this present work. The patterning of waveguides in both silicon and SiON regions is done in a self aligned process using one lithography mask and two separate dry etch steps each highly selective to one of the two materials. The effect of a high temperature anneal on the IR absorption of SiON related N-H bond was measured using FTIR and waveguide optical loss. Up to 98% reduction in absorption is demonstrated which allows acceptable loss across the C-band. We have achieved low propagation loss, single mode, and rib waveguides for both Silicon and SiON core regions as well as low loss silicon-SiON waveguides junction. The silicon-SiON junction loss has been measured to be 0.9+/-0.1dB, only 0.3dB greater than the theoretical value determined by Fresnel's facet reflection.
With a reverse biased p-i-n structure embedded in a silicon waveguide, we efficiently reduced the nonlinear loss due to two photon absorption induced free carrier absorption and achieved continuous-wave net Raman amplification and lasing in a silicon waveguide on a single chip. The low-loss p-i-n waveguides also enabled efficient wavelength conversion in the 1550 nm band via four-wave mixing in silicon. Here we report the performance characteristics of the silicon based laser, amplifier as well as wavelength converter for different device configurations. With a pump wavelength at 1550 nm, the laser output at 1686 nm is single mode with over 55 dB side mode suppression and has less than 80 MHz linewidth. At 25V reverse bias, the threshold pump power is ~180 mW. The slope efficiency is ~4.3% for a single side output and a total output power of >10 mW can be reached at a pump power of 500 mW. The laser wavelength can be tuned by adjusting the wavelength of the pump laser. A 3 dB on-chip amplification and -8.5 dB wavelength conversion efficiency is achieved in an 8-cm long waveguide at a pump powers of < 640 mW. We demonstrate that a high-speed pseudo-random bit sequence optical data at 10 Gb/s rate can be amplified or converted to a new wavelength channel with clear open eye diagram and no waveform distortion.
We report a novel laser architecture, the silicon evanescent laser (SEL), that utilizes a silicon waveguide and offset AlGaInAs quantum wells. The silicon waveguide is fabricated on a Silicon-On-Insulator (SOI) wafer using a CMOS-compatible process, and is bonded with the AlGaInAs quantum well structure using low temperature O2 plasma-assisted wafer bonding. The optical mode in the SEL is predominantly confined in the passive silicon waveguide and evanescently couples into the III-V active region providing optical gain. This approach combines the advantages of high gain III-V materials and the integration capability of silicon technology. Moreover, the difficulty of coupling an external laser source is overcome as the hybrid waveguide can be self-aligned to silicon-based passive optical devices. The SEL lases continuous wave (CW) at 1568 nm with a threshold of 23 mW. The maximum single-sided fiber-coupled CW output power is 4.5 mW. The SEL characteristics are dependent on the silicon waveguide dimensions resulting in different confinement factors in the III-V gain region.
With a reverse biased p-i-n structure embedded in a silicon waveguide, we efficiently reduced the nonlinear loss due to two photon absorption induced free carrier absorption and achieved continuous-wave net gain and lasing in a silicon waveguide cavity on a single chip. We report here the laser characterization for different cavity lengths from 1.6 to 8 cm. With a pump wavelength at 1550 nm, the laser output at 1686 nm is single mode with over 55 dB side mode suppression and has less than 80 MHz linewidth. The lasing threshold depends on the p-i-n reverse bias voltage. With 25V bias, the threshold pump power is ~180 mW. The slope efficiency is ~4.3% for a single side output and a total output power of >10 mW can be reached at a pump power of 500 mW. The laser wavelength can be tuned by adjusting the wavelength of the pump laser. In addition to the laser line at Stokes wavelength, a narrow linewidth anti-Stokes line at 1434.3 nm is also generated in the laser cavity through parametric conversion process.
Due to the mature silicon fabrication technology and vast existing infrastructures, silicon photonics has a chance to offer low cost solutions to telecommunications and data communications. It could also enable a chip-scale platform for monolithic integration of optics and microelectronics circuits for applications of optical interconnects for which high data streams are required in a very small footprint. Two key building blocks needed for any silicon based optoelectronics are silicon based light source and high-speed optical modulator. This paper gives an overview of recent results for a fast (>1GHz) silicon modulator and a silicon Raman laser. We present optical characterization of a high speed metal-oxide-semiconductor (MOS) capacitor-based silicon optical modulator. We show that a Mach-Zehnder interferometer (MZI) structure with a custom-designed driver circuit results in the realization of a silicon modulator transmitting data at 2.5 Gb/s with an extinction ratio of up to 2.8 dB. In addition we show that by reducing the waveguide dimensions one can improve the phase efficiency. In addition, as single crystal silicon possesses higher (four orders of magnitude) Raman gain coefficient as compared to silica, it is possible to achieve sizeable gain in chip-scale silicon waveguide for optical amplification and lasing. With a 4.8 cm long waveguide containing a reverse biased p-i-n diode, we demonstrate lasing operation using a pulsed pump laser. We achieve ~10% slope efficiency. We in addition model a continuous-wave silicon Raman laser and show that higher conversion efficiency and lower threshold power can be realized with optimised cavity device design.
Recently, we have realised a polarisation independent optical racetrack resonator whose resonant dips for TE and TM align to better than 1pm. The devices had a Free Spectral Range (FSR) of only several hundred picometres. This in large part was to the relatively large bend radius (~ 400μm) designed and fabricated with initial focus on producing low bend loss devices. Modelling of the bend loss of the same dimension devices shows that the bend radius can be reduced significantly (down to ~25μm) to produce race track ring resonator with an FSR that is approximately 400% larger than that of those previously fabricated, whilst retaining polarisation independence. This paper will focus on the proposed enhancement of these devices as well as the impetus for their investigation.
Silicon photonics, especially that based on silicon-on-insulator (SOI), has recently attracted a great deal of attention. The mature industrial infrastructure of CMOS fabrication offers an opportunity for low cost silicon based opto-electronic solutions for applications ranging from telecommunications to chip-to-chip interconnects. The high volume and high performance manufacturing disciplines are advantageous to electro-optics application development and fabrication. However, many technical hurdles still need to be addressed. This paper will give an overview of these opportunities as well as discuss some practical issues and challenges concerning processing silicon photonic devices in a high volume CMOS manufacturing environment.
Because of their compact size, ring resonators can be a cost effective solution for many Dense Wavelength Division Multiplexing (DWDM) components, as well as many low cost applications such as part of optical sensor circuits, or low cost optical signal processing. Modulators, filters, add-drop multiplexers, and switches are all components that can be realised with a ring resonator. Their potentially large Free Spectral Range (FSR), finesse, and quality factor, together with the potential for low cost fabrication, make them a viable alternative to many current DWDM devices. However, for such devices to be commercially viable, they need to be insensitive to the polarisation state of the input signal. The results obtained herein show that a single input/output optical racetrack resonator has been fabricated so that the minima in the resonance spectra align to better than 1pm. The rings also exhibit relatively low loss with measured Q-factors of approximately 90,000 and finesse values of 12.
In this paper the optical characterization of a novel, metal-oxide-silicon (MOS) capacitor-based, high speed, silicon optical modulator is presented. By using a capacitor based rather than the conventional p-i-n junction based architecture to modulate the free carrier density inside the waveguide, we show the realization of a fast, 2.5-GHz, optical modulator.
In an effort to determine low-cost alternatives for components currently used in DWDM, optical ring resonators are currently being investigated. The well-known microfabrication techniques of silicon, coupled with the low propagation loss of single crystal silicon, make SOI an attractive material. Laterally coupled racetrack resonators utilising rib waveguides have been fabricated and preliminary results are discussed. An extinction ratio of 15.9 dB and a finesse of 11 have been measured.
There is a trend in photonic circuits to move to smaller device dimensions for improved cost efficiency and device performance. However, the trend also comes at some cost to performance, notably in the polarisation dependence of the circuits, the difficulty in coupling to the circuits, and in some cases, in increased device complexity. This paper discusses a range of Silicon-on-Insulator (SOI) based optical devices, and the advantages and disadvantages in moving to smaller waveguide dimensions. In particular optical phase modulators based upon the plasma dispersion effect and ring resonators are considered, together with a device for coupling to small waveguides, the so-called Dual Grating Assisted Directional Coupler (DGADC). The advantages of moving to small dimensions are considered, and some preliminary experimental results are given. In particular, progress of the DGADC is evaluated in the light of promising experimental results.
We present design, fabrication, and testing of a high-speed all-silicon optical phase modulator in silicon-on-insulator (SOI). The optical modulator is based on a novel silicon waveguide phase shifter containing a metal-oxide-semiconductor (MOS) capacitor. We show that, under the accumulation condition, the drive voltage induced charge density change in the silicon waveguide having a MOS capacitor can be used to modulate the phase of the optical mode due to the free-carrier plasma dispersion effect. We experimentally determined the phase modulation efficiency of the individual phase shifter and compared measurements with simulations. A good agreement between theory and experiment was obtained for various phase shifter lengths. We also characterized both the low- and high-frequency performance of the integrated Mach-Zehnder interferometer (MZI) modulator. For a MZI device containing two identical phase shifters of 10 mm, we obtained a DC extinction ratio above 16 dB. For a MZI modulator containing a single-phase shifter of 2.5 mm in one of the two arms, the frequency dependence of the optical response was obtained by a small signal measurement. A 3-dB bandwidth exceeding 1 GHz was demonstrated. This modulation frequency is two orders of magnitude higher than has been demonstrated in any silicon modulators based on current injection in SOI.
In silicon based photonic circuits, optical modulation is usually performed via the plasma dispersion effect or via the thermo-optic effect, both of which are relatively slow processes. Until relatively recently, the majority of the work in Silicon-on-Insulator (SOI) was based upon waveguides with cross sectional dimensions of several microns. This limits the speed of devices based on the plasma dispersion effect due to the finite transit time of charge carriers, and on the thermo-optic effect due to the volume of the silicon device. Consequently moving to smaller dimensions will increase device speed, as well as providing other advantages of closer packing density, smaller bend radius, and cost effective fabrication. As a result, the trend in recent years has been a move to smaller waveguides, of the order of 1 micron in cross sectional dimensions. In this paper we discuss both the design of small waveguide modulators (of the order of ~1 micron) together with a presentation of preliminary experimental results. In particular two approaches to modulation are discussed, based on injection of free carriers via a p-i-n device, and via thermal modulation of a ring resonator.