- Email: [email protected]
Contra-directional coupling between stacked waveguides using grating couplers
15 Decemberr
1997
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications
144
(I 997) I SO- 182
Contra-directional coupling between stacked waveguides using grating couplers Qiudan Xing Department
*, Shogo
Ura, Toshiaki Suhara, Hiroshi Nishihara
of Electronic Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita-shi, Osaka 565. Japan Received 29 May 1997; accepted 23 June 1997
Abstract The contra-directional coupling between two stacked thin film waveguides by grating couplers is studied. In this scheme, a guided mode of the first-story waveguide is coupled by a grating coupler to radiation modes, and the radiation modes are coupled by another grating coupler to a guided mode propagating contra-directionally in the second-story waveguide. A device for demonstration was designed, fabricated and characterized at 800 nm coupling wavelength. A very sharp wavelength selectivity of 0.3 nm FWHM was experimentally confirmed with 1 mm coupling length. 0 1997 Elsevier Science B.V. PACS: 42.82; 42.80.L Kqvwords: Integrated optics; Grating coupler; Thin film waveguide;
Wavelength
1. Introduction Three-dimensional integration of optical components would offer advantages of more compact size and higher performances compared with two-dimensional integration. Waveguides are stacked and functionally coupled for the three-dimensional integration. There have been several reports [l-6] on integrated optic devices which utilized stacked waveguides, such as wavelength filters [3,4], a switching device [5] and a ring resonator [6]. In those devices, two waveguides were stacked closely, and a guided power in one waveguide is transferred to the other waveguide by a directional coupling or a grating assisted coupling through evanescent field overlap. This configuration limits the design flexibility, because it cannot be applied to a structure without evanescent field overlap. In order to overcome this limitation, we have utilized grating couplers for guided mode coupling. A grating coupler [7,8] was used so far for taking out a guided mode into the air (output coupling) or for exciting a guided mode into the
* Corresponding
author. E-mail: [email protected].
0030.4018/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOO30-4018(97)00365-9
filter
waveguide (input coupling). We combined the output coupling by one grating coupler and the input coupling by another grating coupler for the guided mode coupling. This makes it possible to couple two non-neighboring waveguides in three or more stacked waveguides. In this paper, we report on a contra-directional coupling between two stacked waveguides using grating couplers. A schematic view of the proposed configuration is illustrated in Fig. 1. A guided mode in one waveguide is converted by a grating coupler to two radiation modes, and the radiation modes are converted by another grating coupler to a guided mode propagating contra-directionally in another waveguide. A device was designed, fabricated and characterized at a coupling wavelength of 800 nm. A very sharp wavelength selectivity of 0.3 nm FWHM (full width at half maximum) was experimentally confirmed with a coupling length of 1 mm.
2. Operation principle and design consideration The cross-section waves are illustrated
of the structure and the coupled in Fig. 2. The first-story waveguide
Q. Xing et al./
Optics Communications
144 (19971 180-182
Separating layer @d guiding layer
Radiation mode %i(z)
i
Si substrate I/ r Grating couplers Two-story waveguides Fig. 1. Schematic view of contra-directional stacked waveguides with grating couplers.
i
I
I
ii
1st guiding layer Optical buffer layer coupling
between
WC1 and the second-story waveguide WG2 are separated by a separating layer. A guided mode of WGl (the mode index N, ) is output coupled by a grating coupler GC 1 of grating period A, to two radiation modes, namely an air radiation mode and a substrate radiation mode. A radiation angle 0 in the separating layer satisfies the following phase matching condition, n,,sinH
= N, - A/A,,
-N? + A/A:,
(2)
The grating period A2 of the GC2 is determined by Eq. (2) for a specified coupling wavelength A, and the coupling angle 0,. When the wavelength A deviates from A,, the coupling efficiency between the guided modes reduces rapidly because H satisfying Eq. (1) and 0 satisfying Eq. (2) change in opposite way against the A deviation. For a wavelength A shorter than A,, the radiation angle 0 from GCI is larger than 19, as shown by dotted arrows in Fig. 2, while 0 satisfying Eq. (2) is smaller. The coupling equations of the guided modes were deduced by the coupled-mode theory to be expressed by dA(,) d,_ --=
= -cx~A(
dB( z) d:
z) - KI;AB( z)exp(
-KBAA(z)exp(j2dz)-a,B(:),
Fig. 2. Schematic ing couplers.
cross-section
-j2Az),
of stacked waveguides
v(A) ~~~ sinh( K
=
cOsh( KL)
+
[(
aA +
2
KL)
- j A] sinh( KL)
ag)/2
’ (9
K =
/[(
aA +
aYe)/
-
jd]’
-
IKBA17.
The parameter L is the coupling length. The calculated efficiency v(A) is depicted by a solid line in Fig. 3 for the case of crA = 1 mm-‘, (in= 1 mm-‘. IK~~I= 1 mm-’ and L = 1 mm, showing a very sharp wavelength selectivity. The efficiency was calculated to be 25% at A, (A = 0), and the FWHM was 0.2 nm.
3. Device fabrication
and experimental
results
Specifications for device fabrication are listed in Table 1. The WGl and WG2 are single-mode waveguides. The radiation decay factors and the coupling coefficient were
(3)
(4)
where A( z) and B(L) are the amplitudes of the guided modes in WGI and WG2, respectively. The coefficients CY*, ffB and ~~~ represent the radiation decay factors of GCl and GC2, and a coupling coefficient between the two guided modes. The parameter A is a phase mismatching factor defined by
-2 -1.5 -1 -0.5 Wavelength
0
0.5
deviation from
wavelength
d=n
with grat-
The wavelength dependence of the efficiency of the guided mode-guided mode coupling can be written as
(1)
where A and nb2 are the wavelength and the refractive index of the separating layer, respectively. The phase matching condition for input coupling of the radiation modes to a guided mode which propagates contra-directionally in WG2 (the mode index N2) is written as n,zsinH=
181
ALA-1,
1
1.5
2
coupling [nm]
Fig. 3. Wavelength dependence of coupling efficiency. imental result, solid: theoretical result.
Dot: exper-
182
Q. Xing et al. / Optics Cornmunicarions
Table I Specifications
designed
for device fabrication A, =800nm
Coupling wavelength Waveguides WG2 2nd grating layer 2nd guiding layer Separating WGI
layer 1st guiding layer 1st grating layer
Optical buffer layer Substrate Grating couplers Coupling angte Coupling iength GC2 grating period GCl grating period
calculated
144 (19971 180-182
to be cyA= I .04 mm -
/V’?= 1.508 0.7 pm thick 0.80 +m thick 1.90 pm thick N, = 1.517 0.73 pm thick 0.027 pm thick I .77 km thick
TE,, mode index EB resist Corning #7059 SiO TE,’ mode index Coming #7059 Si-N Si-0 Si ’
e,= 6” t= I mm A1 = 0.499 pm .4, = 0.526 Frn
‘, tiB = 1.04 mm _ ’ and
/KBA/ = 1.03 mm - ’ , respectively. A Si substrate was thermally oxidized to grow a SiO, buffer layer. A Si-N layer was deposited by plasma enhanced chemical vapor deposition. An electron-beam (EB) resist was spin coated. The GCl pattern was written by EB scanning and transferred to the Si-N layer by reactive ion etching. A Coming #7059 guiding layer, a SiO, separating buffer layer and a Coming #7059 guiding layer were RF sputtered. Then the EB resist was spin-coated again and the GC2 pattern was produced by EB lithography. A wave from a Ti:AI,O, laser was coupled to be a TE, mode of the WGI and then coupled by the GCI and the GC2 to the guided mode propagating contra-directionally in the WG2. The wavelength was tuned and the wavelength dependence of the coupling efficiency was measured. The coupling wavelength A, was measured to be 795 nm close to the design value of 800 nm. The slight deviation is due to fabrication errors in A,, A,, N, or N?. The measured wavelength dependence of the coupling efficiency is shown by the dots in Fig. 3. The FWHM was 0.3 nm, close to the theoretical prediction of 0.2 nm. The radiation decay factor of grating coupler GCl and GC2 were measured to be cyA= 0.55 mm-’ and (Ye = I .I mm-‘, respectively. The coupling efficiency q(h,) was estimated to be 5%. This efficiency deterioration would be due to the smaller coupling coefficient lk,,\ limited by the radiation decay factor ar, which is smaller than the design value.
4. Conclusion Contra-directional coupling between two stacked waveguides using grating couplers was studied theoretically and experimentally for three-dimensional integration. A device
was designed and fabricated with 1 mm coupling length. A very sharp wavelength selectivity of 0.3 nm OHM was experimentally confirmed. This wavelength filtering function can be applied to a wavelength-demultiplexing system by using several stacked waveguides. For example, a selective coupling for h, to the second waveguide and another selective coupling for h3 to the third waveguide can be realized simultaneously.
Acknowledgements This research work was financially Grand-in-Aid of Ministry of Education, and Culture
supported Science,
by a Sports
of Japan.
References 111 E.A.J. Marcatili, Bell Syst. Tech. J. 48 (1969) 2071.
Dl S. Asakdwa,
Y. Kokubun. M. Ohyama, T. Baba, Electron. Lett. 29 (1993) 1485. [31 R.C. Alfemess, L.L. Buhl, U. Koren, B.I. Miller, M.G. Young, T.L. Kah, CA. Burrus. G. Raybon. Appl. Phys. Lett. 60 ( 1992) 980. 141 S.-K. Han, R.V. Ramaswamy, R.F. Tavlykaev, Electron. Lett. 31 (1995) 29. [51 W. Sotoyama, S. Tatsuura, K. Motoyoshi, T. Yoshimura, Jpn. J. Appl. Phys. 31 (1992) L1180. 161 S. Suzuki, K. Shuto. Y. Hibino, Photon. Techn. Lett. 4 (1992) 1256. [71 M.L. Dakss. L. Kuhn. P.F. Heidrich, B.A. Scott, Appl. Phys. Lett. 16 (121(1970) 523. [El H. Kogelnik, T.P. Sosnowski, Bell Syst. Tech. J. 49 (1970) 1602.
1997
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications
144
(I 997) I SO- 182
Contra-directional coupling between stacked waveguides using grating couplers Qiudan Xing Department
*, Shogo
Ura, Toshiaki Suhara, Hiroshi Nishihara
of Electronic Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita-shi, Osaka 565. Japan Received 29 May 1997; accepted 23 June 1997
Abstract The contra-directional coupling between two stacked thin film waveguides by grating couplers is studied. In this scheme, a guided mode of the first-story waveguide is coupled by a grating coupler to radiation modes, and the radiation modes are coupled by another grating coupler to a guided mode propagating contra-directionally in the second-story waveguide. A device for demonstration was designed, fabricated and characterized at 800 nm coupling wavelength. A very sharp wavelength selectivity of 0.3 nm FWHM was experimentally confirmed with 1 mm coupling length. 0 1997 Elsevier Science B.V. PACS: 42.82; 42.80.L Kqvwords: Integrated optics; Grating coupler; Thin film waveguide;
Wavelength
1. Introduction Three-dimensional integration of optical components would offer advantages of more compact size and higher performances compared with two-dimensional integration. Waveguides are stacked and functionally coupled for the three-dimensional integration. There have been several reports [l-6] on integrated optic devices which utilized stacked waveguides, such as wavelength filters [3,4], a switching device [5] and a ring resonator [6]. In those devices, two waveguides were stacked closely, and a guided power in one waveguide is transferred to the other waveguide by a directional coupling or a grating assisted coupling through evanescent field overlap. This configuration limits the design flexibility, because it cannot be applied to a structure without evanescent field overlap. In order to overcome this limitation, we have utilized grating couplers for guided mode coupling. A grating coupler [7,8] was used so far for taking out a guided mode into the air (output coupling) or for exciting a guided mode into the
* Corresponding
author. E-mail: [email protected].
0030.4018/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOO30-4018(97)00365-9
filter
waveguide (input coupling). We combined the output coupling by one grating coupler and the input coupling by another grating coupler for the guided mode coupling. This makes it possible to couple two non-neighboring waveguides in three or more stacked waveguides. In this paper, we report on a contra-directional coupling between two stacked waveguides using grating couplers. A schematic view of the proposed configuration is illustrated in Fig. 1. A guided mode in one waveguide is converted by a grating coupler to two radiation modes, and the radiation modes are converted by another grating coupler to a guided mode propagating contra-directionally in another waveguide. A device was designed, fabricated and characterized at a coupling wavelength of 800 nm. A very sharp wavelength selectivity of 0.3 nm FWHM (full width at half maximum) was experimentally confirmed with a coupling length of 1 mm.
2. Operation principle and design consideration The cross-section waves are illustrated
of the structure and the coupled in Fig. 2. The first-story waveguide
Q. Xing et al./
Optics Communications
144 (19971 180-182
Separating layer @d guiding layer
Radiation mode %i(z)
i
Si substrate I/ r Grating couplers Two-story waveguides Fig. 1. Schematic view of contra-directional stacked waveguides with grating couplers.
i
I
I
ii
1st guiding layer Optical buffer layer coupling
between
WC1 and the second-story waveguide WG2 are separated by a separating layer. A guided mode of WGl (the mode index N, ) is output coupled by a grating coupler GC 1 of grating period A, to two radiation modes, namely an air radiation mode and a substrate radiation mode. A radiation angle 0 in the separating layer satisfies the following phase matching condition, n,,sinH
= N, - A/A,,
-N? + A/A:,
(2)
The grating period A2 of the GC2 is determined by Eq. (2) for a specified coupling wavelength A, and the coupling angle 0,. When the wavelength A deviates from A,, the coupling efficiency between the guided modes reduces rapidly because H satisfying Eq. (1) and 0 satisfying Eq. (2) change in opposite way against the A deviation. For a wavelength A shorter than A,, the radiation angle 0 from GCI is larger than 19, as shown by dotted arrows in Fig. 2, while 0 satisfying Eq. (2) is smaller. The coupling equations of the guided modes were deduced by the coupled-mode theory to be expressed by dA(,) d,_ --=
= -cx~A(
dB( z) d:
z) - KI;AB( z)exp(
-KBAA(z)exp(j2dz)-a,B(:),
Fig. 2. Schematic ing couplers.
cross-section
-j2Az),
of stacked waveguides
v(A) ~~~ sinh( K
=
cOsh( KL)
+
[(
aA +
2
KL)
- j A] sinh( KL)
ag)/2
’ (9
K =
/[(
aA +
aYe)/
-
jd]’
-
IKBA17.
The parameter L is the coupling length. The calculated efficiency v(A) is depicted by a solid line in Fig. 3 for the case of crA = 1 mm-‘, (in= 1 mm-‘. IK~~I= 1 mm-’ and L = 1 mm, showing a very sharp wavelength selectivity. The efficiency was calculated to be 25% at A, (A = 0), and the FWHM was 0.2 nm.
3. Device fabrication
and experimental
results
Specifications for device fabrication are listed in Table 1. The WGl and WG2 are single-mode waveguides. The radiation decay factors and the coupling coefficient were
(3)
(4)
where A( z) and B(L) are the amplitudes of the guided modes in WGI and WG2, respectively. The coefficients CY*, ffB and ~~~ represent the radiation decay factors of GCl and GC2, and a coupling coefficient between the two guided modes. The parameter A is a phase mismatching factor defined by
-2 -1.5 -1 -0.5 Wavelength
0
0.5
deviation from
wavelength
d=n
with grat-
The wavelength dependence of the efficiency of the guided mode-guided mode coupling can be written as
(1)
where A and nb2 are the wavelength and the refractive index of the separating layer, respectively. The phase matching condition for input coupling of the radiation modes to a guided mode which propagates contra-directionally in WG2 (the mode index N2) is written as n,zsinH=
181
ALA-1,
1
1.5
2
coupling [nm]
Fig. 3. Wavelength dependence of coupling efficiency. imental result, solid: theoretical result.
Dot: exper-
182
Q. Xing et al. / Optics Cornmunicarions
Table I Specifications
designed
for device fabrication A, =800nm
Coupling wavelength Waveguides WG2 2nd grating layer 2nd guiding layer Separating WGI
layer 1st guiding layer 1st grating layer
Optical buffer layer Substrate Grating couplers Coupling angte Coupling iength GC2 grating period GCl grating period
calculated
144 (19971 180-182
to be cyA= I .04 mm -
/V’?= 1.508 0.7 pm thick 0.80 +m thick 1.90 pm thick N, = 1.517 0.73 pm thick 0.027 pm thick I .77 km thick
TE,, mode index EB resist Corning #7059 SiO TE,’ mode index Coming #7059 Si-N Si-0 Si ’
e,= 6” t= I mm A1 = 0.499 pm .4, = 0.526 Frn
‘, tiB = 1.04 mm _ ’ and
/KBA/ = 1.03 mm - ’ , respectively. A Si substrate was thermally oxidized to grow a SiO, buffer layer. A Si-N layer was deposited by plasma enhanced chemical vapor deposition. An electron-beam (EB) resist was spin coated. The GCl pattern was written by EB scanning and transferred to the Si-N layer by reactive ion etching. A Coming #7059 guiding layer, a SiO, separating buffer layer and a Coming #7059 guiding layer were RF sputtered. Then the EB resist was spin-coated again and the GC2 pattern was produced by EB lithography. A wave from a Ti:AI,O, laser was coupled to be a TE, mode of the WGI and then coupled by the GCI and the GC2 to the guided mode propagating contra-directionally in the WG2. The wavelength was tuned and the wavelength dependence of the coupling efficiency was measured. The coupling wavelength A, was measured to be 795 nm close to the design value of 800 nm. The slight deviation is due to fabrication errors in A,, A,, N, or N?. The measured wavelength dependence of the coupling efficiency is shown by the dots in Fig. 3. The FWHM was 0.3 nm, close to the theoretical prediction of 0.2 nm. The radiation decay factor of grating coupler GCl and GC2 were measured to be cyA= 0.55 mm-’ and (Ye = I .I mm-‘, respectively. The coupling efficiency q(h,) was estimated to be 5%. This efficiency deterioration would be due to the smaller coupling coefficient lk,,\ limited by the radiation decay factor ar, which is smaller than the design value.
4. Conclusion Contra-directional coupling between two stacked waveguides using grating couplers was studied theoretically and experimentally for three-dimensional integration. A device
was designed and fabricated with 1 mm coupling length. A very sharp wavelength selectivity of 0.3 nm OHM was experimentally confirmed. This wavelength filtering function can be applied to a wavelength-demultiplexing system by using several stacked waveguides. For example, a selective coupling for h, to the second waveguide and another selective coupling for h3 to the third waveguide can be realized simultaneously.
Acknowledgements This research work was financially Grand-in-Aid of Ministry of Education, and Culture
supported Science,
by a Sports
of Japan.
References 111 E.A.J. Marcatili, Bell Syst. Tech. J. 48 (1969) 2071.
Dl S. Asakdwa,
Y. Kokubun. M. Ohyama, T. Baba, Electron. Lett. 29 (1993) 1485. [31 R.C. Alfemess, L.L. Buhl, U. Koren, B.I. Miller, M.G. Young, T.L. Kah, CA. Burrus. G. Raybon. Appl. Phys. Lett. 60 ( 1992) 980. 141 S.-K. Han, R.V. Ramaswamy, R.F. Tavlykaev, Electron. Lett. 31 (1995) 29. [51 W. Sotoyama, S. Tatsuura, K. Motoyoshi, T. Yoshimura, Jpn. J. Appl. Phys. 31 (1992) L1180. 161 S. Suzuki, K. Shuto. Y. Hibino, Photon. Techn. Lett. 4 (1992) 1256. [71 M.L. Dakss. L. Kuhn. P.F. Heidrich, B.A. Scott, Appl. Phys. Lett. 16 (121(1970) 523. [El H. Kogelnik, T.P. Sosnowski, Bell Syst. Tech. J. 49 (1970) 1602.