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Silicon Photonics Technology : Ten Years of Research at IIT Madras

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The integrated optoelectronics research group at IIT Madras has been active since 2007 with a determination towards developing a center of excellence for silicon photonics research. The core research theme involves novel designs, CMOS compatible fabrication process optimizations and subsequent experimental demonstrations leading towards cost-effective, energy-eficient and high-speed optoelectronic interconnects for various applications. As of now, various prototype devices like power splitters, ITU channel interleavers, variable optical attenuators, p-i-n phase shifters/modulators, ring resonators, DBR ilters etc., have been demonstrated by exploiting conventional microelectronics technology as well as recently established nano-fabrication facilities at IIT Madras. Their design principle, process development, fabrication and characterizations are described in the present article.
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ISSN : 0971 - 3093
An International Research Journal
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Vol 25, No 7, July, 2016
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Asian Journal of Physics
Vol. 25, No 7 (2016) 923-955
Silicon Photonics Technology:
Ten Years of Research at IIT Madras
B K Das, N Das Gupta, S Chandran, S Kurudi, P Sah, R Nandi, Ramesh K, Sumi R, S Pal,
S Kaushal, S M Sundaram, P Sakthivel, Sidharth R, R Joshi, H Sasikumar, U Karthik,
S Krubhakar, G R Bhatt, J P George, R K Navalakhe, Narendran R, and I Seethalakshmi
Integrated Optoelectronics Labs, Department of Electrical Engineering, IIT Madras, Chennai - 600 036, India
The integrated optoelectronics research group at IIT Madras has been active since 2007 with a determination towards
developing a center of excellence for silicon photonics research. The core research theme involves novel designs, CMOS
compatible fabrication process optimizations and subsequent experimental demonstrations leading towards cost-effective,
               
devices like power splitters, ITU channel interleavers, variable optical attenuators, p-i-n phase shifters/modulators, ring
recently established nano-fabrication facilities at IIT Madras. Their design principle, process development, fabrication
and characterizations are described in the present article.  
Keywords  
1 Introduction
et al in 1984 [1]. The progress in this subject was reviewed and re-emphasized again in 2000 by another
[2]. Further, in 2010, Young and his colleagues presented an overview
on how optical input/output technology was being implemented for highly demanding tera-scale computing
systems [3]     
        
               
Besides passive on-chip optical networking devices, optoelectronic devices like lasers, photodetectors,
solutions. Being an indirect bandgap semiconductor (Eg
light emission. Therefore, hybrid integration of direct bandgap III-V semiconductors is being implemented
[7].                    
photodetectors are being monolithically integrated in silicon-on-insulator (SOI) [8]. The plasma dispersion
art CMOS technology [9]. Here, modulation or switching is based on localized refractive index modulation
by switching free carrier distribution. The carrier concentrations can be controlled by suitable biasing and
effect is the change in imaginary part of the dielectric constant, which eventually causes optical absorption/
attenuation. Nevertheless, this is being exploited to design electronically controlled variable optical attenuators
Corresponding author
e-mail: (B K Das)
924 B K Das, N Das Gupta, S Chandran et al.............
effect in silicon crystal is utilized for cost-effective solutions [11-14].
interactions [15].
Though the interests in silicon photonics research among Indian scientists and engineers were
noticeable here and there, no planned long-term research goals were set until a decade ago. IIT Madras
(India) took the lead to start a silicon photonics research group in 2006. Since then the group at IIT Madras
has been actively engaged in novel design, fabrication and characterizations of silicon photonics devices
for communication systems and sensor applications. By the end of 2011, various prototype devices were
             
Madras [16-19]. With the availability of nanofabrication facilities at the Centre for NEMS and Nanophotonics
(funded by MeitY Govt. of India), the research program was raised to the next level during last couple of
years research outcomes have been presented in international conferences, published in peer reviewed
    [20-33]. The goal of the research group is now set to carry out world class
silicon photonics research encompassing novel device designs, CMOS compatible fabrication process
         
speed optoelectronic interconnects for various applications. In this article, we would like to highlight the
unique development of silicon photonics technology that has been taking place during last ten years at IIT
Madras. We believe this review article will help students, research scholars and perhaps many other research
groups in India and abroad who are aspiring to step into this important area of research.
In section 2, we will discuss details of commercially available SOI substrates and their suitability
as a silicon photonics platform. The basic building blocks like single-mode/multi-mode waveguide design
rule, principle of multimode interference and directional coupler will be also discussed in this section. In
   
      
nm SOI substrates will be discussed in sections-4, 5, and 6, respectively. Finally the conclusion is given in
section 7.
2 Optical waveguides in SOI platform - Design Rules
The success of silicon photonics came with the commercial availability of optical-grade SOI wafers
               
device layer) were mainly used in large-scale integration of nano-scale electronic devices in a drive to
sustain Moore's law [34]
electronic circuits, the necessity of on-chip high-speed optical interconnects has been increasing. Low-
loss optical waveguide in SOI platform with Si as core material is the key to on-chip high-speed optical
interconnects. In fact, SOI substrate itself could provide 1D waveguide with buried oxide (BOX) as bottom
cladding, intrinsic silicon device layer as core and air/silicon dioxide as top cladding. The 2D waveguide
can be designed with rib and/or with rectangular silicon core and air/SiO2 cladding. In this section, we
discuss the design aspects of three basic waveguide systems, namely, single-mode, multimode and coupled
2.1 Single-mode waveguides
                  
factor, sharp bends (minimum waveguide bending radius) and capability of handling impedance/mode-size
task. In order to achieve compact integrated optical devices with smaller bend radius; core-cladding index
Silicon Photonics Technology: Ten Years of Research at IIT Madras 925
         
the index contrast is high, the waveguide cross-section has to be small enough to make the waveguide single
mode (supporting only the fundamental mode). Further, the smaller cross-section makes the waveguide highly
wavelength dependent (dispersive) as well as lossy. The loss is mainly due to the enhanced interaction of
          
supporting only the fundamental mode is essentially used in optical interconnect applications to reduce
the circuit complexity. Typical cross-sectional view of a SOI waveguide is shown in Fig 1(a); single-mode
guidance for a given operating wavelength band (λ ~ 1550 nm) is achieved by selecting appropriate values
of three parameters namely, waveguide width W, height H, slab thickness h (see Fig 1(a)). Depending
          
dependencies, etc. by suitably scaling these three parameters. Relatively, larger cross-section rib waveguides
WH h  
and polarization independent photonic circuits [20, 35]
possible with larger waveguide structures [36].       
been proposed for large-scale manufacturing of photonics transceiver solutions [37]. Commercially available
         
            
typical design parameters of W ~500 nm, H ~ 250 nm and h < 200 nm (photonic wire waveguide) are used
     [38].
926 B K Das, N Das Gupta, S Chandran et al.............
Fig 1. (a) Schematic cross-sectional view of a SOI optical waveguide; W - width, H - height (device layer
thickness), h - thickness of slab, BOX - buried oxide (SiO2); (b), (c) and (d) are the calculated slab height
cut-off hc         
are highly polarization sensitive with width variation, hc is shown only for TE-like mode.
        Fig 1(a)) is used to evaluate slab height cut-off
hc as a function of W    
      
the extracted results have been shown in Figs 1 (b)-(d). The simulation results were obtained for both TE-
like and TM-like modes. However, we have shown results only for TE-like hybrid mode in case of 250 nm
device layer thickness as TM-like hybrid modes are found leaky for certain slab thicknesses.
Theoretical calculations also reveal that the single-mode waveguides with larger device layer
thickness offer nearly a polarization independent properties (in terms of mode-size and effective index of the
Silicon Photonics Technology: Ten Years of Research at IIT Madras 927
guided mode). Figure 2(a) shows the wavelength dependent structural birefringence comparison of typical
line represents the birefringence of a waveguide with W H h 
represents the birefringence of a waveguide with W    H     h      
line represents the birefringence of a waveguide with W = 500 nm, H = 250 nm and h = 0. It is evident
that the birefringence of larger cross-section waveguides is estimated to be very small (< 10–3). However,
the submicron photonic wire waveguide is found to be highly birefringent (> 0.7). Figure 2 (b) shows the
cross-sections are found to be nearly dispersion free over a wide range operating wavelengths; the photonic
wire waveguides are found to be highly dispersive. This shows that integrated optical components/devices
designed with submicron waveguides are wavelength sensitive.
Fig 2. (a) Birefringence and (b) group index comparison of various waveguide structures in SOI
                  
waveguide (W  H h   W  H  
h  W = 500 nm, H = 250 nm and h = 0), respectively.
928 B K Das, N Das Gupta, S Chandran et al.............
2.2 Multimode Waveguides
Though the single-mode waveguides are mostly preferred in designing integrated optical devices,
multimode waveguides play vital role in some key functions e.g., adiabatically excited fundamental mode
guidance in multimode waveguides offer low propagation loss, power splitter by appropriate interference of
guided modes, low-loss switch matrix in on-chip optical networking, etc. Multimode interference coupler
(MMIC) is found to be very attractive and frequently used to demonstrate many stand-alone integrated
optical devices such as ring resonators [39], Mach-Zehnder interferometers [40], and arrayed waveguide
gratings [41] due to nearly wavelength independent and polarization insensitive operation. MMIC structures
a multi-mode waveguide section and output single-mode waveguide(s). Multi-mode section is designed to
support higher order modes such that the interference of all the excited modes give rise to localized intensity
distribution exactly similar to input intensity distribution and which can be explained well with self-imaging
principle [42].
Fig 3. (a) Schematic top view of a 2×2 MMI coupler with input/output single-mode waveguides,
where Lmmi length of MMI section, Wmmi width of MMI, Wt taper width, Wphwidth
of submicron waveguide and Lt– taper length. (b) Wavelength dependent 3-dB power splitting
  
were carried out for TE-like guided modes.
Silicon Photonics Technology: Ten Years of Research at IIT Madras 929
The localized spots formed in the multimode waveguide region can be tapped through the output
             
waveguides. Higher the width of the multi-mode waveguide, more the number of modes supported by the
waveguide and better the resolution of self-images. However, as width of MMIC section increases, the length
at which self-images are formed also drastically increases. The length of MMIC can be greatly reduced
without compromising the image resolution by properly positioning input waveguide(s) and exciting only
certain guided modes in the multi-mode waveguide section. By utilizing paired interference property of
MMIC, one can reduce the length of 2×N coupler for N fold images from 3Lπ/N to Lπ/N, where Lπ is the
   
splitter (see Fig 3(a)    
waveguide has to be positioned at Wmmi
eff /6, where Wmmi
eff is the effective waveguide width of multimode
waveguide section which is always higher than the actual width of the waveguide because of the existence
             
              Wmmi
eff can be
approximated to the physical width of the multi-mode waveguide section (Wmmi). Figure 3(a) shows the
schematic view of typical 2×2 MMIC with input and output ends are interfaced with adiabatically tapered
to reduce the impedance mismatch. The optimum waveguide parameters of the MMIC (for TE polarization)
Table 1. Figure 3(b) shows the wavelength
dependent splitting ratio [r = PCross/(PBar + PCross
The MMIC design using 2 micron device layer SOI is found to be nearly wavelength independent.
Whereas the MMIC with submicron SOI device layer shows a slight wavelength dependent power splitting
characteristics. Polarization dependency of the MMIC is also calculated and it is found that the variation
 L) is 0.6 % of Lmmi   
Table 1. Optimized MMIC parameters (for TE polarization) of micron sized waveguide cross-section
(H  H 
H h Wph Wt  Lt  Wmmi Lmmi
2 1 1.5 3 27 15 350
0.25 0 0.5 1 5 3.6 15
2.3 Coupled waveguides - Directional coupler
Waveguides are said to be coupled when they are exchanging power either in the co-direction
or in the contra-direction propagations. Two parallel waveguides coupled co-directionally via evanescent
        
of a typical DC is shown in Fig 4(a)         
shown in the inset. When the waveguide separation is small enough such that the evanescent tails of the
guided modes of the waveguides overlap each other causing the exchange of power between waveguides.
                 
waveguides. DCs are mainly used as power splitters in integrated optical devices such as Mach-Zehnder
interferometer, ring resonator, etc. The waveguide cross-section design parameters play a major role in
obtaining desired performance of a DC. Figure 4(b) shows the 3-dB length variation from the design value
of a DC as a function of fabrication error for 5 µm, 2 µm and 0.25 µm waveguides. Considering lithographic
limitations in respective technology nodes, the waveguide separations are chosen to be 2 µm, 1 µm and 0.2
µm, respectively.
930 B K Das, N Das Gupta, S Chandran et al.............
Fig 4. Schematic top view of coupled waveguides in a directional coupler (3-dB) power splitter.
Lc     
output regions are shown in the inset, (b) variation of 3-dB length of directional coupler as a
function of fabrication error from designed values of 5 µm waveguide (W = 5 µm, H = 5 µm
and h = 4 µm), 2 µm waveguide (W = 1.5 µm, H = 2 µm and h = 1 µm) and 0.25 µm waveguide
(W = 0.5 µm, H = 0.25 mm and h = 0 µm).
Conventionally, waveguide width is defned using either by photolithography or e-beam lithography,
and ridge height is defned with subsequent reactive ion etching process. The etching can be precisely
controlled. However, the width is highly dependent on the lithography process. If W is reduced/increased
by W because of fabrication error, the separation between the waveguides in a DC is increased/
Silicon Photonics Technology: Ten Years of Research at IIT Madras 931
W. Figure 4(b) shows the variation 3-dB length variation of the DC as function
 W). The designed cross-sections for 5 µm, 2 µm and 0.25 µm waveguides are same
as the values used for birefringence calculation. The variation in 3dB length is defned as:
L3dB [%] =
× 100
3dB is the calculated 3dB length with the respective fabrication error and
3dB is the 3 dB length
for a designed DC. The maximum 3-dB length variation for 5 m waveguide is found to be 1.5%, whereas
that for 2 µm and 250 nm waveguide DCs are 11% and 65%, for the fabrication error of 0.15 µm. This
calculation shows that 250 nm waveguides are highly sensitive to fabrication errors compared to larger
waveguide dimensions such as 2 µm or 5 µm waveguides.
3 Device Fabrication
In general, fabrication parameters for silicon photonics devices are optimized in accordance with
like strip waveguides, 2D photonic crystal, DBR and grating couplers, and (ii) active components like p-i-n
structure for plasma-optic phase-shifters, and microheater for thermo-optic phase-shifters. In this section, we
present fabrication processes standardized for some of these integrated optical components in our labs.
3.1 Waveguide Fabrication
               
(with positivetone photoresist S1813 or S1805) and subsequent dry etching process (using photoresist as
      
lithography. Figure 5(a) and 5(b) 
e-beam lithography, respectively.
Table 2. Optimized etching parameters for the fabrication of waveguides and some of their measured properties.
Mechanism Etching Chemistry Etch Rate
Typical etch
depth (µm)
Typical surface
roughness (nm)
loss (dB/cm)
5 µm RIE SF6 0.35 1 10  0.5
2 µm ICP-RIE SF6: CHF3:: 15:30 0.95 0.6 20-30  1.3-1.5
250 nm ICP-RIE SF6: CHF3:: 5:18 0.55 0.1 10-20  3.0
The fabricated waveguides are characterized in terms of surface roughness, side-wall angle,
propagation loss, and mode-size, etc. Table 2 summarizes the various etching parameters optimized for the
different dimensions and some typical characterization results. The 5 µm and 2 µm waveguides are cleaved
or polished to prepare the end-facet for optical characterizations. Photonic wire waveguides are characterized
and photonic wire waveguides are shown in Fig 6; they are obtained by imaging the near-feld distribution
        
wire waveguide, end-facet polishing has been carried out though grating coupler is used otherwise. The
estimated mode-size of the photonic wire waveguides appears to be larger than that of theoretical prediction,
932 B K Das, N Das Gupta, S Chandran et al.............
which can be attributed to diffraction limit of the imaging lens and resolution of IR camera used in our
characterization setup. Besides conventional rib waveguides, we have optimized the fabrication process for
2D photonic crystal waveguides in 250-nm SOI substrate. SEM images of typical grating coupler, photonic
crystal waveguide, directional coupler, and cleaved end-facet of a rib waveguide are shown in Figs 7 (a),
(b), (c) and (d), respectively.
Fig 5.  (a) photolithography and (b) e-beam lithography.
Silicon Photonics Technology: Ten Years of Research at IIT Madras 933
Fig 6                  
fabricated with device layer thickness: (a) 5-µm, (b) 2-µm, and (c) 250-nm.
934 B K Das, N Das Gupta, S Chandran et al.............
Silicon Photonics Technology: Ten Years of Research at IIT Madras 935
Fig 7. SEM images of fabricated devices: (a) grating coupler (250-nm SOI), (b) photonic crystal
waveguide (250-nm SOI), (c) directional coupler (250-nm SOI) and (d) cleaved end-facet of
waveguide (2-µm SOI).
3.2 Fabrication of PIN diodes
The p-i-n diode is used as a high-speed active element for reconfgurable silicon photonics devices.
        Figure 8        
diode integration with a rib waveguide in 2 µm SOI. First, the surface layer of rib waveguide is oxidized
   
              
sourced from the liquid POCl3 kept at 0ºC using carrier gases. Oxygen and Nitrogen. The diffusion doping
is carried out for 120 minutes by maintaining the sample temperature at 850ºC; which gives a junction depth
of 0.2 µm. The junction depth for phosphorus increases to ~1.2 µm after all the subsequent high temperature
Fig 8. The fabrication process steps of a p-i-n waveguide phase-shifter.
936 B K Das, N Das Gupta, S Chandran et al.............
processes of oxidation and boron diffusion. Similar steps of oxidation and photolithography are followed
for Boron diffusion (p-type doping). Boron Nitride (BN1100) discs are used as the solid dopant source for
             
              
depth obtained after diffusion is summarized in Table 3.
Table 3. Dopant surface concentrations (extracted from sheet resistance) and estimated junction depths
Type of diffusion Surface concentration (cm-3) Junction depth (µm)
Phosphorous 4 × 1020 1.2
Boron 4.4 × 1020 1.3
Fig 9. Fabrication process steps for an integrated optical microheater based phase-shifter.
3.3 Fabrication of microheater
Fabrication process of integrated optical micro-heaters with single-mode rib waveguide in 2µm SOI
Fig 9. Photolithographically
      
4 Devices with 5 µm SOI
Integrated optical devices with relatively large cross-section waveguides in SOI substrate do not
    [16]. Moreover, one can easily characterize the
splitter, 2×2 directional coupler for 3-dB power splitting, Mach-Zehnder interferometer based ITU channel
interleaver, variable optical attenuator and Mach-Zehnder switch, etc. have been designed and fabricated
using SOI substrates with a device layer thickness of ~ 5 µm. Their design parameters, working principle
   
Silicon Photonics Technology: Ten Years of Research at IIT Madras 937
Fig 10. (a) The confocal microscopic image of an 11th order DBR grating integrated with single-mode
waveguide in a SOI substrate with 5 µm device layer thickness; (b) The transmission characteristics
(TE-pol) of the fabricated DBR with grating period of h = 260 nm,
LDBR = 5.2 mm, waveguide height H = 5 µm, slab height h = 3.8 µm and the waveguide width W
varied from 5.5 µm to 3.5 µm. The spectrum is normalized with the transmission spectrum of straight
waveguide and for a peak output power of 1 mW.
938 B K Das, N Das Gupta, S Chandran et al.............
4.1 DBR Filter
    
    
the huge prospects of CMOS compatible photonic devices in SOI platform, there have been great interest to
a periodicity of         
1550 nm. The fabrication of such sub-micron grating structures requires stringent process control. Therefore,
    
Bragg ~ 1550 nm (see Fig 10). The fabrication was carried out using two step
            
~ 3.3 nm (estimated from transmission spectrum) for a grating length of 5.2 mm for TE polarization.
4.2 MMI based 1× 8 Power Splitter
Integrated optical power splitter is an important component in passive/active optical network systems.
Such power splitters are mostly being manufactured in silica-on-silicon platform using planar lightwave
circuit technologies. We have demonstrated multimode interference (MMI) based integrated optical 1×8
power splitter in SOI substrate with large cross-section single-mode input/output waveguides. The device
basically consists of one single mode input waveguide, eight single mode output waveguides and a planar
waveguide (MMI region) in between. The device parameters were optimized by BPM simulation results and
was taken to minimize the wavelength / polarization dependencies, insertion loss, excess loss and throughput
power non uniformity among the output waveguides. The device (dimension ~20 mm × 2 mm) has also been
           
fundamental mode in these waveguides is estimated to be ~ 7.2 µm × 4.6 µm nearly equivalent to that
               
in terms of excess loss, insertion loss, non-uniformity in throughput powers and polarization / wavelength
Silicon Photonics Technology: Ten Years of Research at IIT Madras 939
Fig 11. (a) The scheme 1×8 power splitter designed with 5-µm device layer thickness; (b) Photograph
   ×8 power splitter. FPR - free propagation region, Ri and Ro are
the radii of the Rowland and grating circles, respectively; (c) Wavelength dependent transmission
characteristics (TM-pol.) of a packaged device; (d)
    
(Pin = 5 mW). The devices are found to be nearly polarization independent.
940 B K Das, N Das Gupta, S Chandran et al.............
insensitive. By analyzing the experimental results, we have observed typical insertion loss of 15 dB with
results suggest that such power splitters can be potentially useful in passive/active optical networks operating
4.3 ITU Channel Interleaver
Dense Wavelength Division Multiplexing (DWDM) has recently attracted much interest in-order to
   
the performance at transmitter / receiver ends.
Fig 12. (a) Schematic representation of a 2×2 directional coupler along with fabricated features
(SEM images) of a coupled waveguide section, S-bends with both-sided and one-sided slab, (b)
Wavelength dependent transmission (power splitting ratio) at one of the output port for both TE and
TM polarizations.
Silicon Photonics Technology: Ten Years of Research at IIT Madras 941
Table 4. Final design parameters for 100 GHz ITU channel interleaver fabricated with 5 µm SOI
Waveguide width = 5 µm Waveguide spacing for the directional couplers = 2.5 µm
Rib (slab) height = 1.8 µm (3.2 µm) Length of directional couplers = 1400 µm
Waveguide bending radius = 15000 µm 
Total Device length = 29 mm Operation band = C and L
942 B K Das, N Das Gupta, S Chandran et al.............
Fig 13. (a) Scheme of an unbalanced Mach-Zehnder Interferometer (MZI) used to design the 100
GHz ITU channel interleaver in SOI platform with 5 µm device layer thickness. DC - directional
coupler (for 3-dB power splitting) ; (b) Fiber pigtailed and packaged interleaver; (c) Wavelength
dependent transmission at both the output ports for TE polarization; (d) Experimentally observed
channel positions at bar port and comparison to that ITU channels.
for 100 GHz channel spacing. It is basically an unbalanced Mach-Zehnder Interferometer (MZI) designed
by cascading two identical DCs. The length of DCs is adjusted for 3-dB power splitting for a wide-range of
wavelengths (see Fig 12). The optimized values of critical design parameters have been given in Table 4.
The devices were fabricated with optimized design parameters, and characterized in terms of insertion loss,
to be more than 15 dB with 3 dB channel passband of ~ 40 GHz (see Fig 13 (b)).
4.4 Variable optical attenuator and Mach-Zehnder switch
Design of diffusion doped p-i-n structures with large cross-section (~ 25 µm2) single mode rib
waveguide structures on SOI platform have been studied for the demonstration of variable optical attenuator
       
single-mode p-i-n waveguide section. Since the forward biased p-i-n waveguide offers simultaneous reduction
of refractive index and increment in attenuation because of plasma dispersion effect, we have optimized its
     
these devices were experimentally studied and typical results shown in Figs 14 and 15, respectively. Typical
were found to be slightly deviated from the simulated results, which may be attributed to non-ideal waveguide
of the modulator was found to be 12.4 dB and 12 dB for TE and TM polarizations, respectively. Transient
response of the fabricated MZI switches were obtained by modulating the bias voltage with a rectangular
pulse of duration 500 ns (repetition rate of 1 MHz). The estimated rise-time and fall-time of a MZI switch
were recorded as 61 ns and 159 ns, respectively.
Silicon Photonics Technology: Ten Years of Research at IIT Madras 943
944 B K Das, N Das Gupta, S Chandran et al.............
Fig 14. (a) Schematic representation of cross-sectional p-i-n waveguide phase-shifter/variable
Microscopic image showing top view of a fabricated variable optical attenuator; (c)
characteristics for TE -polarization; (d)
Silicon Photonics Technology: Ten Years of Research at IIT Madras 945
Fig 15. (a) Microscopic image of fabricated MZI switches; (b) I-V characteristics; (c) Modulated
light output as a function of forward current; (d) Switching performance with rectangular
input pulse captured by a digital storage oscilloscope; blue (yellow) color represent modulated
optical output (bias voltage).
5 Devices with 2-µm SOI
        
with 2-µm device layer thickness offers nearly polarization independent, lower propagation loss, and
some devices like MMI based 3-dB power splitter, wavelength independent ITU channel interleaver, large
volume microring resonator and thermo-optic phase shifter which were fabricated in 2-µm SOI platform.
946 B K Das, N Das Gupta, S Chandran et al.............
Fig 16. (a) SEM image showing the junction points of two input/output waveguides and the multimode
waveguide section of a 2 × 2 MMI based 3-dB power splitter, (b) Wavelength dependent transmission
characteristics of a typical MMI based 3-dB power splitter fabricated with 2 µm SOI.
5.1 MMI based 3-dB power splitter
The design principle and an optimized set of parameter values for a typical 2×2 MMI based 3-dB
power splitter with 2 µm SOI have been discussed earlier. The SEM image of such a fabricated device is
shown in Fig 16(a) 
         Fig 16(b)) also exhibit
nearly polarization independent 3-dB power splitting over a broad wavelength of operation (experimental
results shown for C+L bands).
Silicon Photonics Technology: Ten Years of Research at IIT Madras 947
5.2 Compact ITU Channel Interleaver
Following successful demonstration of dispersion-free ITU channel interleaver in 5 µm SOI [20],
we explored a compact design of asymmetric MZI in 2 µm platform (see Fig 17) [24]. In contrast to the
Fig 17. Schematic 3D view of compact interleaver designed with 2-µm SOI
Fig 18. Normalized transmission characteristics (shown for TM polarization) exhibiting a nearly
design of DC based interleaver in 5 µm SOI discussed earlier, two MMI based 3-dB couplers were cascaded
    
output ports alternatively. The entire device footprint is ~ 0.8 mm × 5.2 mm (W × L). The characterization
results represented in Fig 18 show that the device could separate alternate ITU channels into two output ports.
948 B K Das, N Das Gupta, S Chandran et al.............
We also observe a uniform channel extinction of ~ 15 dB with a 3-dB bandwidth of ~ 100 GHz for either
polarization at the output ports over the wavelength range of 1520 nm to 1600 nm. The devices show nearly
dispersion-free response and polarization-independent extinction over a wide wavelength range (C+L optical
5.3 Large-volume Ring Resonator
Ring resonator is most versatile fundamental building block for integrated optics applications.
    
    
              [44].
     
Fig 19. (a) Microscopic top view of ring resonator, (b) Transmission characteristics of ring resonator.
Silicon Photonics Technology: Ten Years of Research at IIT Madras 949
especially to reduce bend-induced loss as well as simultaneously to obtain large free spectral range (FSR).
We designed and fabricated a relatively large volume ring resonator of race track type with MMI coupler
for feeding power into the ring waveguide with 2-µm single-mode waveguides. The device foot print is ~
1.5 × 1.5 mm, with ring radius of 500 µm and MMI coupler length of 1.25 mm. Figure 19 (a) shows the
microscopic view of the fabricated device. Transmission characteristics are shown in Fig 19 (b). The device
shows an extinction of ~10 dB and FSR of 122 pm. The Q-value is estimated to be large ~ 9.6 × 104; which
is a clear evidence of lower round-trip loss even for large volume ring resonator. Such high Q-value ring
resonator device may be useful in sensing applications.
5.4 Thermo-optic Switch
The presence of strong thermo-optic effect in silicon (dn/dT ~2×10-4/K), enables large scale
       [45-46], couplers [47],  [48], delay lines
[49], switches [50],
phase-sensitive devices via thermo-optic effect. However, high thermal sensitivity of resonance devices (dλr /
dT~100 pm/K) leads to thermal crosstalk, especially for densely packed large-scale integrated optical circuits
in SOI platform. It is therefore, important to investigate the temperature distribution and effective control
of thermo-optic phase-shift by a micro-heater integrated with waveguide structures [51]. We fabricated
waveguides integrated with titanium micro-heater placed adjacent to waveguide as discussed in section 3.
The micro-heaters were placed at a 3µm separation from the base of the rib waveguide to avoid optical
losses. There were two sets of micro-heaters with width 2µm and 3µm. The lengths were varied from 50µm
  
Figure 20 (a) shows the microscopic image of heater. The device was polished using diamond
Figure 20 (b) shows the thermal tuning characteristics
for different lengths of heater and separation from the waveguide edge. Fabry-Perot modulation technique
was used to extract the effective thermo-optic phase-shifts and is presented in Fig 21 (a). Transient response
was also recorded by applying a square wave as shown in Fig 21 (b) 
width of 2 µm integrated with a waveguide of length ~ 20 mm.
950 B K Das, N Das Gupta, S Chandran et al.............
Fig 20. (a) Confocal micrograph of fabricated micro-heaters integrated with single mode rib
waveguides, (b) effective electrical power (Peff
for heater width WH = 2 µm (solid line) and WH = 3 µm (dotted line), and different heater length LH
viz. 50 µm, 100 µm, 200 µm for TE polarized light.
Fig 21. (a) Fabry-Perot response with thermal tuning; (b) Transient response: yellow trace - driving
electrical signal, blue trace - optical output.
Silicon Photonics Technology: Ten Years of Research at IIT Madras 951
6 Devices with 250-nm SOI
 
waveguide modes. The devices are fabricated with input/output grating couplers (see Fig 7(a)) and can be
average propagation loss of ~ 3 dB/cm for TE-like guided modes. Recently, two important integrated optical
Their performance characteristics are discussed in this section.
6.1 Narrow-line-width DBR Filter
The side-wall DBR gratings with a period of 290 nm and lengths up to 750 µm have been integrated
with photonic wire rib waveguides (W = 560 nm, H = 250 nm, and h = 150 nm). SEM image of a single-mode
Fig 22. (a) SEM image of the side wall grating fabricated in SOI photonic wire using e-beam
lithography (b) Transmission characteristics of 500 µm long DBR structure in SOI.
952 B K Das, N Das Gupta, S Chandran et al.............
  W = 50 nm) and the transmission characteristics of a 500
Fig 22 (a) and 22 (b)
fabricated in our labs is extracted to be ~ 2 dB for the guidance of TE-like fundamental mode.
6.2 Microring Resonator
Microring resonators were fabricated with single-mode waveguides (W = 550 nm, H = 250 nm,
and h = 100 -150 nm) supporting only the TE-like guided modes. The ring radius was varied from 10 - 200
µm to study bend-induced waveguide loss, group index of the guided mode, effective free spectral range,
        
coupled, the length of the coupled region (LDC) and gap (s) between them are the deciding parameters for
coupling strength between the bus and ring (for a given set of waveguide parameters). We have optimized the
e-beam lithography process to control the s value ~ 100 nm. SEM image of a fabricated microring resonator
is shown in Fig 23 (a)Fig 23 (b). Typical transmission
characteristics of a microring resonator is shown in Fig 24. The FSR and quality factor of this device is
measured to be ~ 0.89 nm and 141,000, respectively.
Fig 23 (a). SEM image of microring resonator, and (b) zoomed SEM image of directional coupler used in microring
Fig 24. Transmission characteristics of a typical microring resonator fabricated on 250 nm SOI for
TE polarized mode.
Silicon Photonics Technology: Ten Years of Research at IIT Madras 953
7 Conclusions
The objective of this article is to highlight the silicon photonics research activities carried out during
the state-of-art silicon photonics technology and get to know how we have achieved a little success in this
area starting right from the scratch. Our silicon photonics research has been evolved from 5-µm waveguide
technology to submicron or photonic wire waveguide technology. We have also explored the integration of
active elements like p-i-n waveguide phase-shifter and micro-heaters for modulators/switches. Many of our
recent application oriented recent research works include waveguide surface trimming, adiabatic spot-size
and ring resonator based wide-range refractive index sensing, etc. Some of these results have been already
published or will be published in course of time.
We thankfully acknowledge following funding agencies for supporting silicon photonics research at
IIT Madras.
1. DeitY, Govt. of India for the funds to establish Centre for NEMS and Nanophotonics (CNNP) at IIT
2. IRDE/DRDO, Govt. of India for the project “Silicon Nanophotonics - technology development, novel
device design, fabrication and characterization”
3. DST, Govt. of India for the projects, “Development of passive integrated photonic components in SOI
platform” and “Experimental demonstration and modelling of SOI based p-i-n/p-n phase-shifters and
variable optical attenuators with submicron waveguides”.
4. DIT, Govt. of India for the project, “Development of integrated optical single channel add-drop
   
5. RCI, Hyderabad for the project, “SOI integrated optical chip for sensor applications”.
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17. George J P, Dasgupta N, Das B K, Compact integrated optical directional coupler with large cross section silicon
waveguides, Proc SPIE, 7719(2010)77191X-77191X.
18. Bhatt G R, Das B K, Demonstration of ITU channel interleaver in SOI with large cross section single mode
waveguides, Proc SPIE, 8069(2011)806904-806904.
19. Krubhakar I S, Narendran R, Das B K, Design and fabrication of integrated optical 1x8 power splitter in SOI
substrate using large cross-section single-mode waveguides, Proc SPIE, 8173(2011)81730C;doi:1117/12.898475
20. J Lightwave
Tech, 30(2012)140-146.
21. Bhatt G R, Das B K, Improvement of polarization extinction in silicon waveguide devices, Opt Commun,
22. Chandran S, Das B K, Tapering and Size Reduction of Single-mode Silicon Waveguides by maskless RIE", OECC
2012 - OptoElectronics Communication Conference, Bexco, Busan Korea, July 02-06, 2012.
23.   
   
24. Karthik U, Das B K, Polarization-independent and dispersion-free integrated optical MZI in SOI substrate for
    
25. Sakthivel P, Dasgupta N, Das B K, Simulation and experimental studies of diffusion doped p-i-n structures for
    
26. Joshi R, B K, Gupta N D, Design of 2D Photonic crystals for integrated optical slow light applications, IWPSD-
2013, Noida, India, Dec 2013.
27. Chandran S, Kaushal S, Das B K, Monolithic integration of micron to submicron waveguides with 2D mode-size
     
28. 
 
29. 
12th International Conference on Fibre Optics and Photonics, Kharagpur, India , 13-16 December 2014 (Paper-
30. 
Conference on Fibre Optics and Photonics, Kharagpur, India , 13-16 December 2014 (Paper- 4B.3).
31. Sidharth R, Das B K, Semi-analytical model of arrayed waveguide grating in SOI using Gaussian beam
approximation, Appl Opt, 54(2015)2158-2163.
32. Chandran S, Das B K , Surface trimming of silicon photonics devices using controlled reactive ion etching chemistry,
Photonics and Nanostructures – Fundamentals and Applications, 15(2015)32-40.
Silicon Photonics Technology: Ten Years of Research at IIT Madras 955
33. Chandran S, Sundaram S M, Das B K, Method and apparatus for modifying dimensions of a waveguide, Patent
Filed – 3799/CHE/2015.
34. Celler G K, Cristoloveanu Sorin, Frontiers of silicon-on-insulator, J Appl Phys, 93(2003)4955-4978.
35. Hsu S.-H, Tseng S.-C, You H.-Z, Birefringence characterization on soi waveguide using optical low coherence
interferometry, in 7th IEEE International Conference on Group IV Photonics, 2010.
36. 
Photonics Technology: Ten Years of Research at IIT Madras 955 adiabatic evolution, Opt Express, 18(2010)15790-
37. Nature Photonics, 5(2011)268-
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IEEE Photonics Technology Letters, 22(2010)1485-1487.
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[Received: 30.7.2016]
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Full-text available
2D photonic crystal waveguides have been designed in both GaAs/AlGaAs and SOI platforms for slow-light applications in planar lightwave circuits. The design parameters have been optimized using commercial FDTD tool. The extracted group velocity of a single-mode SOI photonic crystal waveguide has been shown to be 3–10 times slower than that of a photonic wire waveguide operating in third generation optical communication window (λ ~ 1,550 nm). This reduced group velocity has been exploited to design a plasma dispersion based compact SOI modulator with Lπ ~70 μm.
Full-text available
The arrayed waveguide grating structure can be used as an important component in high-speed CMOS optical interconnects in silicon-on-insulator (SOI) platform. However, the performance of such device is found to be extremely sensitive to the fabrication-related errors in defining the critical features. In the absence of an appropriate analytical model, one needs to rely on numerical computation to analyze the device characteristics and fabrication tolerances. Because compact design of such a device structure has foot-print ∼mm2 and the smallest features can be as small as ∼500 nm×220 nm (waveguide cross section), it demands a huge computational budget to optimize the design parameters. A semi-analytical model using Gaussian beam approximation of guided mode profiles has been developed to analyze the output spectrum of arrayed waveguide grating and to estimate the phase errors due to waveguide inhomogeneities. This model has been validated with existing numerical methods and published experimental results. It has been observed that a probabilistic waveguide width variations of ΔW∼5 nm can cause a cross-talk degradation of about 40 dB (25 dB) for a device (operating at λ∼1550 nm) fabricated on SOI substrate with 220 nm (2 μm) device layer thickness.
Data transport across short electrical wires is limited by both bandwidth and power density, which creates a performance bottleneck for semiconductor microchips in modern computer systems - from mobile phones to large-scale data centres. These limitations can be overcome by using optical communications based on chip-scale electronic-photonic systems enabled by silicon-based nanophotonic devices. However, combining electronics and photonics on the same chip has proved challenging, owing to microchip manufacturing conflicts between electronics and photonics. Consequently, current electronic-photonic chips are limited to niche manufacturing processes and include only a few optical devices alongside simple circuits. Here we report an electronic-photonic system on a single chip integrating over 70 million transistors and 850 photonic components that work together to provide logic, memory, and interconnect functions. This system is a realization of a microprocessor that uses on-chip photonic devices to directly communicate with other chips using light. To integrate electronics and photonics at the scale of a microprocessor chip, we adopt a 'zero-change' approach to the integration of photonics. Instead of developing a custom process to enable the fabrication of photonics, which would complicate or eliminate the possibility of integration with state-of-the-art transistors at large scale and at high yield, we design optical devices using a standard microelectronics foundry process that is used for modern microprocessors. This demonstration could represent the beginning of an era of chip-scale electronic-photonic systems with the potential to transform computing system architectures, enabling more powerful computers, from network infrastructure to data centres and supercomputers.