Silicon Raman Amplifiers, Lasers, and Their Applications
Bahram Jalalia, Ozdal Boyrazb, Varun Raghunathana, Dimitri Dimitropoulosa and Prakash Koonatha
aUCLA Department of Electrical Engineering, Los Angeles, CA 90095-1594
bUniversity of California, Irvine, Department of Electrical Engineering, Irvine, CA 92697
Silicon Photonics is emerging as an attractive technology in order to realize low cost, high density integrated optical
circuits. Realizing active functionalities in Silicon waveguiding structures is being pursued rigorously. In particular, the
Stimulated Raman scattering process has attracted considerably attention for achieving on-chip light generation,
amplification and wavelength conversion. This paper reviews some of the recent efforts in using the Raman nonlinear
process to realize amplifiers, and lasers. First the prospects of Raman process in realizing high gain amplifiers are
discussed theoretically. Following this experimental results on amplification with gains as high as 20dB are presented.
Some of the recent results in realizing pulsed and CW lasers with reverse-biased carrier sweep out are presented. The
paper is concluded by highlighting some of the applications of the Raman process in Silicon in realizing mid-IR sources
and also the use of SiGe as a flexible Raman medium are discussed.
From telecommunications to remote sensing Photonics is penetrating into different areas in a tremendous speed.
Nowadays, photonic devices are essential building blocks of fiber optic networks that form the backbone of the internet.
In addition, photonics is playing an increasing role in biomedical applications ranging from laser surgery to photo
dynamic therapy. However, ultimately this fast growth is limited by the cost of the photonic devices. Silicon is the
ultimate manufacturing technology and represents an unprecedented convergence of technological sophistication and
economy of scale. Ideally, photonic devices should also be manufactured in silicon to fuel the growth of this technology.
Being able to tap into silicon’s vast manufacturing base will reduce the cost of photonic devices, which in turn will
accelerate penetration of photonics into mass markets .
In addition to its low cost, its compatibility with silicon IC manufacturing, and separately with silicon MEMS
technology have stimulated a significant amount of research and generated significant attention to silicon photonics field.
In 2004, the U.S. Defense Advanced Research Projects Agency (DARPA) launched the Electronic and Photonic
Integrated Circuit (EPIC) Program, the first of its kind, dedicated to silicon photonics. The four-year program is funding
research and development at universities and industry with the ultimate goal of producing production capable active and
passive silicon optoelectronic circuits. More recently, the US Air Force has launched multidisciplinary University
Research Initiative (MURI) program dedicated to this topic. Additionally, the first International Conference on Group IV
Photonics was also launched in 2004. The same year also witnessed the demonstration of the first silicon laser . The
rapid pace of progress is continuing, and the first quarter of 2005 saw a demonstration of the Raman laser with direct
electrical modulation capability , a report of the first continuous-wave (CW) silicon Raman laser , and a lossless
electro-optical modulator based on the Raman amplification .
While a myriad of high performance passive devices were demonstrated in the 1990s , the creation of active
devices proved to be much more difficult. Unfavorable physical properties, such as the near-absence of Pockel’s effect
caused by the symmetric crystal structure, and the lack of efficient optical transitions due to the indirect band structure,
were the culprits. High order nonlinear effects, third order nonlinear effects in particular, are the only viable option for
designing active silicon devices. Compared to other integrated optics platforms, a distinguishing property of silicon is the
tight optical confinement made possible by the large index mismatch between the silicon and SiO2. The index
difference, ∆n =2, between the cladding layer and the waveguiding layer allows localizing the optical beam in a very
small area and hence boosting the effective nonlinearity several orders of magnitude to make active devices possible in
Active and Passive Optical Components for WDM Communications V, edited by Achyut K. Dutta, Yasutake Ohishi,
Niloy K. Dutta, Jesper Moerk, Proc. of SPIE Vol. 6014, 601402, (2005) · 0277-786X/05/$15 · doi: 10.1117/12.630907
Proc. of SPIE Vol. 6014 601402-1
silicon. Raman scattering was proposed and demonstrated in 2002 as a mean to bypass these limitations and to create
optical amplifiers and lasers in silicon . The approach was motivated by the fact that the stimulated Raman gain
coefficient in silicon is 103 – 104 times larger than that in fiber. The modal area in a silicon waveguide is roughly 100
times smaller than in fiber, resulting in a proportional increase in optical intensity. The combination makes it possible to
realize chip-scale Raman devices that normally require kilometers of fiber to operate. The initial demonstrations of
spontaneous Raman emission from silicon waveguides in 2002 was followed by the first demonstration of stimulated
Raman scattering  and parametric Raman wavelength conversion [9-11]. Other merits of the Raman effect include the
fact that it occurs in pure silicon and hence, does not require rare earth dopants, and that the spectrum is tunable through
the pump laser wavelength.
2. THEORY AND EXPERIMENTS
Classical electrodynamics provides an intuitive macroscopic description of the Raman scattering process. In the
spontaneous scattering, thermal vibrations of lattice at frequency ωv (15.6 THz in silicon) produce a sinusoidal
modulation of the susceptibility. The incident pump field induces an electric polarization that is given by the product of
the susceptibility and the incident field. The beating of the incident field oscillation (ωp) with oscillation of the
susceptibility (ωv) produces induced polarizations at the sum frequency, ωp + ωv (anti-Stokes), and at the difference
frequency ωp - ωv (Stokes). Quantum statistics dictates the ratio of Stokes power to anti-Stokes power and its function of
the ratios of the Bose occupancy factors. In the stimulated amplification case, the interaction of the pump and Stokes
waves produces a driving force for atomic vibrations and enhances the transfer of power from the pump to the Stokes
Microscopically, the direct coupling of light with atomic vibrations is very weak. This is generally true in
semiconductors owing to the large atomic mass that appears (squared) in the denominator of the cross section. Hence,
electrons must mediate the Raman scattering process in semiconductors. The scattering proceeds in three steps. First,
the incident photon excites the semiconductor into an intermediate step by creating an electron-hole pair. Next, the pair
is scattered into another state by emitting a phonon via the electron-phonon interaction Hamiltonian. In step three, the
electron-hole pair in the intermediate step recombines radiatively with emission of a scattered photon. While electrons
mediate the process, they remain unchanged after the process. Transitions involving electrons are virtual and hence, do
not have to conserve energy although momentum must be conserved.
g = 20cm/GW
β β = 0.5cm/GW
α αP = α αS = 1dB/cm
Length = 1cm
Figure 1 shows the achievable cw Raman gain silicon waveguide for different pump intensities. The net gain is the
difference between the Raman gain, which increases linearly with pump intensity, and the Two-Photon absorption (TPA)
induced free carrier loss which increases quadratically with intensity [12,13]. Carrier lifetime determines the steady state
density of free carriers generated by TPA and hence, the free carrier loss that is induced by the pump. Fortunately,
Pump Intensity (Wcm-2)
Net Gain (dB)
τ τeff = 1nsec
τ τeff = 10nsec
τ τeff = 40nsec
τ τeff = 100nsec
Fig. 1. Impact of carrier lifetime on achievable C.W. Raman gain. Gain increases with intensity while loss rises
as intensity squared and dominates when lifetime is long.
Proc. of SPIE Vol. 6014 601402-2
carrier lifetime in SOI waveguide is determined by (i) recombination at the buried oxide interface, (ii) recombination at
the waveguide surface, which should be small for a low loss waveguide with smooth side walls, and (iii), in the case of
rib waveguides by lateral diffusion of carriers out of the waveguide core . Fortunately, carrier lifetimes of few
nanoseconds or less can be achieved in submicron waveguides [15-18]. Additionally, active carrier removal using a pn
junction can lower the lifetime [13,14,19,20], albeit at the expense of on-chip electrical power dissipation. The
dependence of the net gain on pump intensity suggests that tapered structures that maintain optimum pump intensity
along the waveguide length can be used to obtain maximum amplification .
Free carrier buildup can be avoided by using pulsed pumping with a pulse repetition period that is longer than the
carrier lifetime. Figure 2 shows the measure amplification when 30ps, 20 MHz pump pulses are used. The input signal
beam experiences a pump on-off amplification of 20dB , clearly demonstrating the potential of the Raman approach
in rendering silicon as an optically-active medium. Using active removal of carriers, a CW gain of 3dB has also been
Figure 3 shows the transfer function of the first silicon Raman laser . The laser used a 1.7cm long silicon
waveguide gain medium in an external cavity configuration and was pumped by 30ps, 20 MHz pump pulses. It had a
0 0.3 0.6 0.9 1.2
Pump Intensity (GW/cm2)Pump Intensity (GW/cm2)
10 1020 20 3030 40 40
0 0.3 0.6 0.9 1.2
Fig. 2. On-off optical gain as a function of peak pump pulse power. Maximum gain of 20dB is obtained.
Fig. 3. Measured laser output power with respect to peak pump power. Lasing threshold is measured to be at 9 W peak power
level and the slope efficiency is calculated to be ~ 14%.
Peak Pump Power (W)Peak Pump Power (W) Peak Pump Power (W)Peak Pump Power (W) Peak Pump Power (W)Peak Pump Power (W)
10 1010 101010 151515 1515 1520 2020 2020 20
0.50.50.5 0.5 0.5 0.5
18.104.22.168.5 1.5 1.5
2.52.5 22.214.171.124 2.5
Peak Output Power (W)
Peak Output Power (W)
1550nm pump1550nm pump 1550nm pump
Peak Output Power (W)Peak Output Power (W) Peak Output Power (W)Peak Output Power (W)
Pump: 1540 nm
Stokes: 1675 nm
Proc. of SPIE Vol. 6014 601402-3
Cou*d pump pow.. (nlW
slope efficiency of 14% and produced output pulses with more than 2.5 watts of peak power. These experiments reported
in 2004 also produced the first observation of anti-Stokes generation in any type of Raman laser . The anti-Stokes
beam is generated by parametric Raman conversion within the laser cavity [9-11]. The anti-Stokes power is much lower
than the Stokes due to the lack of phase matching and the anti-Stokes signal not being resonant in the cavity. With a
phase matched and doubly-resonant waveguide design, these lasers will be able to produce significant output in two
wavelength bands, simultaneously. In particular, if pumped at 1430nm, the laser will produce radiation at 1550nm and
1320nm, the two most important communication bands. Another unique feature of silicon Raman laser, that was
recently demonstrated, is the electrical modulation using an integrated p-n junction diode . Although the laser is
optically pumped, from a modulation and switching perspective, it is a diode laser.
Figure 4 shows the transfer function of the first CW silicon Raman laser . Continuous lasing was achieved by
active removal of free carriers, using a reverse-biased p-n junction which spans the length of the waveguide. The laser
used a 4.8cm long s-shaped waveguide with HR coated facets. It produces 9 mW of output power when 600mW of
pump power is present in the waveguide. Under these conditions, the laser draws approximately 50mA at 25V of reverse
Several open and exciting areas of research lie ahead. Size reduction is clearly a priority if such devices are to be
economical. From this point of view, compact disk or ring type resonators or photonic bandstructure cavities would be
desired [15,24,17]. In addition, from a power dissipation point of view, reducing the intrinsic lifetime is preferred over
active removal of carriers, unless the latter can be achieved at the one volt level or below. Another limitation of the
present state-of-the-art is the rigid vibrational spectrum in silicon. The Stokes shift of 15.6 THz (optical phonon
frequency) restricts the operating wavelength of the amplifier or the laser, for a given pump wavelength. Further, the
100GHz intrinsic Raman linewidth limits the number of WDM channels that can be amplified with a single pump.
Silicon-germanium (SiGe) alloys offer a path to spectrum engineering in silicon Raman devices. Raman amplification
and lasing in SiGe-on-oxide waveguides have recently been demonstrated . Preliminary results show that it is
possible to engineer the spectral shift in these structures, Fig. 5. Optimized structures that consider strain, phonon, and
waveguide engineering represent an exciting and rich topic of future research.
In terms of applications, amplification will be the most likely impact of recent breakthroughs in Raman based silicon
photonics. The lack of amplification has prevented the extent of photonic integration to one or two devices per chip.
Raman amplification can make possible highly integrated subsystems on-a-chip. With respect to the laser, cascaded
cavity Raman lasers that emit in the mid-IR represent a promising path forward. These lasers would extend the
Fig. 4. Threshold characteristics of CW silicon Raman laser demonstrated in a 5 cm silicon waveguide
and with using reverse biased p-i-n diode for carrier sweep out .
Proc. of SPIE Vol. 6014 601402-4
wavelength range of III-V lasers to mid-IR where important applications such as laser medicine, bio-chemical sensing,
and free space optical communication await the emergence of a practical and low cost laser. Figure 6 is the transmission
spectrum of silicon in the mid-IR which shows that the low loss window extending from 1.1 to 7µm does exist. The
well-known absorption of the SiO2 at mid-IR is not a major concern as the material can be mostly removed by
undercutting of the silicon waveguide. A fortuitous consequence of mid-IR operation is that TPA should vanish for
wavelengths beyond approximately 2.3µm.
This work was supported by DARPA. The authors thank Dr. Jag Shah of DARPA/MTO for his support.
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1673.2 1673.6 1674 1674.4
Wavelength (nm)Wavelength (nm)Wavelength (nm)Wavelength (nm)
1673.41673.41673.4 1673.41673.41673.41673.41673.41673.61673.61673.61673.61673.6 1673.61673.61673.61673.8 1673.81673.8 1673.81673.8 1673.81673.81673.816741674 16741674 16741674167416741674.21674.21674.21674.2 1674.21674.21674.21674.21674.41674.41674.4 1674.41674.4 1674.41674.41674.41674.61674.61674.61674.61674.61674.61674.61674.6
37GHz 37GHz37GHz 37GHz37GHz37GHz37GHz37GHz
Wavelength (nm)Wavelength (nm)Wavelength (nm) Wavelength (nm)
Power (dBm)Power (dBm)
1673.2 1673.6 1674 1674.4
Power (dBm)Power (dBm)Power (dBm)Power (dBm)
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Wavelength (µ µm)Wavelength (µ µm)
Fig. 6. Transmission characteristics of silicon.
Proc. of SPIE Vol. 6014 601402-5
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