High Performance Silicon-on-Sapphire Subwavelength Grating

Coupler for 2.7μm Wavelength

Jingjing Zhang, Junbo Yang, Wenjun Wu, Honghui Jia and Shengli Chang

Center of Material Science, National University of Defense Technology, Changsha 410073, China

Keywords: Beam Splitter, Grating-Assisted Coupler, Coupled Mode Theory, Transfer Matrix Method.

Abstract: Couplers are important parts in integrated optical circuit and high efficiency compact couplers are in great

demand. We report on high-efficiency silicon-on-sapphire (SOS) grating couplers at wavelength of 2.7μm.

A 75% coupling efficiency and 50nm etching depth bandwidth (the tolerance of etching height) from a

standard single-mode fiber to an SOS waveguide is obtained. A basic design principle of the grating coupler

is presented, and some improved structures to enhance the coupling efficiency are proposed and estimated.

The FDTD method is utilized to simulate and design the grating operated under TE polarization. With our

optimization design, the coupling efficiency can be largely increased and beyond 80%. The gratings open

the path to silicon photonic chips for the mid-infrared enabling new nonlinear optical functions as well as

new spectroscopic lab on-a chip approaches.

1 INTRODUCTION

The mid-infrared (MIR) wavelengths, typically

defined to range from 2-20μm (Saleh and Teich,

2007; Richard Soref, 2010; Richard A Soref et al.,

2006), have proven to be useful for a number of

applications. Many astronomy experiments depend

upon the detection of MIR wavelengths (L. Labadie

and O. Wallner, 2009). Chemical bond spectroscopy

benefits from a large range of wavelengths from

visible to past 20μm (H. B. Gray, 1994). Thermal

imaging (such as night vision) depends upon mid-

infrared wavelengths as a source of blackbody

radiation (G. C. Holst and S. W. McHugh, 1992).

Silicon photonic waveguide circuits were

originally conceived to be used for

datacommunication and telecommunication

applications (the wavelengths of 1.3μm and

1.55μm), a myriad of other application domains

have emerged in recent years, including high

performance mid-infrared optical systems operating

in the 2-8μm spectral range

7, 8

. Working at these

longer wavelengths reduces the parasitic nonlinear

absorption in silicon, the two photon absorption,

enormously. Without the two photon absorption we

can fully benefit from the record nonlinear

parameters obtained through the combination of the

high linear index leading to high confinement and

the high nonlinear index of silicon. The mid-IR is

also a region in which second-and third-order

nonlinear optical effects can be exploited to great

advantage. Thus, such systems could find

applications in industrial and environmental

monitoring (Willer et al., 2006), threat detection

(Moore et al., 2004), medical diagnostics (Namjou et

al., 2006) and free-space communication (Capasso et

al., 2002).

The silicon-on-insulator (SOI) and silicon-on-

sapphire (SOS) nanophotonic platform has been

proposed as an excellent system for integrated mid-

infrared (mid-IR) optical devices on account of the

long wavelength transparency of silicon and many

associated CMOS-compatible materials (Soref,

2008). Various methods have been explored for

fabricating passive mid-IR waveguides and

microcavities, with group-IV materials, including

the use of “traditional” SOI rib and channel

waveguides (G. Z. Mashanovich et al., 2001; M. M.

Milošević et al., 2009), silicon-on-sapphire

substrates (T. Baehr-Jones et al., 2010; A. Spott et

al., 2010; F. Li et al., 2011).

Furthermore, numerous Si mid-IR components

have been demonstrated, including ring resonators at

5.5μm 19, modulator (Mackenzie A. Van Camp et

al., 2012), mid-infrared spectrometers (M. Muneeb

et al., 2013), and grating coupler (B. Kuyken et al.,

2011; Nannicha Hattasan et al., 2012; Zhenzhou

Cheng et al., 2012; Ananth Z. Subramanian et al.,

2012). In Ref 22, a grating couplers at wavelength of

2.15μm is demonstrated, which shows a peak

Zhang J., Yang J., Wu W., Jia H. and Chang S..

High Performance Silicon-on-Sapphire Subwavelength Grating Coupler for 2.7µm Wavelength.

DOI: 10.5220/0005333700730077

In Proceedings of the 3rd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2015), pages 73-77

ISBN: 978-989-758-092-5

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

coupling loss of -5.2 dB. In following work

(Nannicha Hattasan et al., 2012), a -3.8 dB coupling

efficiency from a standard single-mode fiber to an

SOI waveguide at 2.1μm is obtained experimentally.

In Ref 24, a TE mode shallow etched apodized

grating with coupling efficiency of 80.6% is

predicted at the wavelength of 2.75μm by finite-

difference time-domain (FDTD) simulation. The

coupling strength of mid-IR gratings can be tuned

from 0.011 to 0.16 by changing the fill factor from

0.05 to 0.42. Authors use apodized structure in the

front section of the grating, and uniform gratings for

the rear section, thus, the design and fabrication is

relatively difficult in practices. In addition, a grating

coupler (using silicon nitride waveguide) at near-

infrared wavelength is presented. An experimental

efficiency of 5.7dB is reported (Ananth Z.

Subramanian et al., 2012). Obviously, grating

coupler at long wavelength is taken more and more

attentions.

In this paper we propose a compact, simple and

uniform structure grating coupler with 75% coupling

efficiency at wavelength of 2.7μm. The rigorous

coupled wave theory is applied to the design and

analysis of this coupler. Finally, the simulation

results are obtained by the finite-difference time

domain method.

2 PRINCIPLE AND DESIGN

Various grating couplers for coupling light between

single mode fiber and submicron sized silicon

waveguides have already been demonstrated at the

1550nm telecom wavelength region (Junbo Yang et

al., 2011; Junbo Yang et al., 2012; Dirk Taillaert,

2005). However the high absorption of silicon

dioxide (for wavelength > 2.6μm) greatly limits their

applications in mid-infrared region. Therefore, we

designed and fabricated the silicon coupler on SOS

substrate. We optimize first the design parameters,

including waveguide width, grating period, fill

factor, grating widths, and grating height. A grating

coupler can be obtained as shown in Fig. 1, which is

composed of subwavelength pillars with uniform

height.

The grating period is T and etching depth h. The

thickness of waveguide and sapphire are H and W,

respectively. A θ=10° tilt from vertical is presumed

to avoid substantial second order Bragg reflection

back into the waveguide when coupling to the

optical fiber. The incident light P

is divided into

four parts throughout the grating: P

, P

and

. P

is the coupling power. P

is the transmission

through the finite grating. P

is the reflection at the

grating. P

is the power that is leaked down. P

can

Figure 1: Schematic of fiber-to-chip grating coupler

structure based on SOS.

be reduced to be very small after optimizing the

structure and incident angle. So, we have:

= P

+ P

(1)

Coupling efficiency η is defined as:











(2)

The underlying sapphire thickness W has a major

influence on the coupling efficiency. Its value is

chosen in such a way that the downward radiated

light which gets reflected at the sapphire/substrate

interface interferes constructively with the direct

upward radiated light. Based on this result the

sapphire thickness W was fixed at 0.88μm which can

reduce leaked to the substrate to improve the grating

couplers directionality.

For an SOS planar waveguide structure, we can

obtain the effective refractive index (ERI) of the TE

mode n

eff

=2.318, we use Rsoft to compute n

eff

=2.318

when the thickness of the waveguide H is 220nm

(The limited thickness of TE

mode) λ =2.7μm as

shown in Fig.2.

Figure 2: The profile of waveguide mode.

Next, according to the phase match condition

between the gratings and the waveguide mode:

T







n



∙sinθ



 mλ

m0， 1， 2⋯

(3)

Where n

clad

=1 (air) and the grating period T is equal

Sapphi

PHOTOPTICS2015-InternationalConferenceonPhotonics,OpticsandLaserTechnology

to 1.2603μm. In addition, the fill factor of grating f

which defined as the ratio of pillar width to grating

period is equal to 0.4. We can control the width of

each pillar to obtain the desired refractive index

distribution. With the calculations given above,

finally, all the parameters required for constructing a

subwavelength grating coupler based on SOS is

listed in Table.1.

Table.1: Design parameters of grating coupler (Unit:

Micrometers).

Parameters λ T h H W f θ

Value

1.260

0.1

0.22 0.88

Simultaneously, the finite-difference time-domain

method, a powerful and accurate method for a finite-

size structure, is chosen to simulate and design this

device. For a 2.7μm wavelength, the coupling

efficiency is about 75% when we consider the TE.

The Ey component of optical filed is given in Fig. 3.

Figure 3: Distribution of optical field.

According to above theoretical analysis, we take

these parameters given in Table 1 as a starting point

to simulate and evaluate the performance of the

grating coupler described above. We will look at the

sensitivity of the grating couplers to variations in the

grating parameters. We introduce errors on the etch

depth, filling factor and grating width (period

tolerance).

In fabrication, considering requirements of

production process, the grating depth is hard to

control because electron beam density which is hard

to control current on the accuracy of the etching

depth. We will evaluate the influence of fabrication

error to the coupling efficiency. The resulting

coupling efficiency as a function of etching depth is

shown in Fig. 4.

It shows a maximum coupling efficiency around

75% (grating depth h=0.12μm) and a 1 dB depth

bandwidth of 50 nm (from 0.1μm to 0.15μm). When

the grating height is larger than 0.26μm or lower

than 0.06μm, the mismatch of mode profile between

Figure 4: Coupling efficiency as a function of grating

depth with λ=2.7μm, T=1.2603μm, f=0.4, θ=10º.

grating field and fiber will be large. Consequently, if

the grating is etched too deep or too shallow the

wavelength is obviously shifted to longer or shorter

wavelengths (not shown in Fig.4), and the second

order reflection is also increased. So the coupling

efficiencies are significantly low.

Fig.5 shows the effect of a different filling factor.

It shows a maximum coupling efficiency around

75% with grating depth f=0.4μm.Changing it from

0.4 to 0.35 or 0.45 has only a small effect on the

coupling efficiency curves, in which the coupling

efficiency is larger than 60%.

Figure 5: Coupling efficiency as a function of fill factor

with λ=2.7μm, T=1.2603μm, h=0.12μm, θ=10º.

Fig.6 shows the coupling efficiency as a function

of angle. The θ=10º curve has a maximum for

λ=2.7μm. The case of vertical coupling (θ=0º) is

very interesting from a practical point of view.

However, as above mentioned, in such case, the

second order diffraction is reflecting back into the

waveguide, which results in the coupling efficiency

obviously degenerated.

The effect of random errors on the groove widths

is shown in Fig.7. The coupling efficiency gets its

maximum value when the grating period error is 0

(T=1.2603μm). The errors have a normal (Gaussian)

distribution with half width 40nm (from -20nm to

20nm). Compared to the perfect structure, the

coupling efficiency is obviously reduced to the half

of the maximum.

HighPerformanceSilicon-on-SapphireSubwavelengthGratingCouplerfor2.7μmWavelength

Figure 6: Coupling efficiency as a function of incidence

angle with λ=2.7μm, T=1.2603μm, h=0.12μm, f=0.4.

Figure 7: Influence of period tolerance on coupling

efficiency with λ=2.7μm, h=0.12μm, f=0.4, θ=10º.

In real fabricated structures these errors on the

different parameters are present all at the same time.

In the worst case scenario everything adds up, but it

is also possible that one error cancels the other. We

can conclude that the tolerances to fabrication errors

are very tight, but achievable. The high accuracy

needed is typical for all nanophotonic structures.

Figure 8: Optimized grating coupler using reflector grating

and Bragg reflection layers.

As above discussed, at the coupler grating, part of

the light is reflected P

, part is coupled out P

, part

is transmitted P

and the rest is leaked down to

substrate P

. In order to improve the coupling

efficiency, one way is to add a second grating with

the same etch depth as the coupler grating, which

acts as a reflector behind the coupler grating as

shown in Fig.8. If the structure is properly designed

which the direct reflection at the coupler grating and

the reflection from the reflector grating interfere

destructively, it is possible to reduce the reflection at

the coupler and couple all light out. Simultaneously,

a bottom reflector can further improve the coupling

efficiency to fiber. This bottom reflector can be a

multi-layer dielectric mirror or a metal mirror. We

have chosen for a Bragg reflection layers consisting

of Si/Sapphire pairs. The thickness of each layer

t=λ/4n must be well controlled to obtain a high

reflectivity. n is the refractive index of the layer of

silicon and sapphire. Thanks to the high refractive

index contrast, only a few pairs are needed and the

reflection is broadband. Depend on above optimized

methods, the coupling efficiency beyond 80% can be

realized for mid-infrared wavelength.

4 CONCLUSIONS

In this paper, we proposed a subwavelength grating

coupler with coupling efficiencies exceeding 75% at

a wavelength of 2.7μm with an 50nm depth

bandwidth of 1dB. The coupling efficiency can

reach up to about 80% if the optimized method is

adopted. The gratings open the path to silicon

photonic chips for the mid-infrared enabling new

nonlinear optical functions as well as new

spectroscopic lab on-a chip approaches. Experiments

are being carried out and results will be presented

soon.

ACKNOWLEDGEMENTS

This work was supported by the National Natural

Science Foundation of China (60907003), the

Foundation of NUDT (JC13-02-13), the Hunan

Provincial Natural Science Foundation of China

(13JJ3001), and Program for New Century Excellent

Talents in University (NCET-12-0142). The authors

gratefully acknowledge helpful discussions with

Prof. Zhiping Zhou.

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HighPerformanceSilicon-on-SapphireSubwavelengthGratingCouplerfor2.7μmWavelength