Title: Long-Period Grating Couplers

Long-Period Grating Couplers 

__________________________________________________ 

Kin Seng Chiang 

Department of Electronic Engineering 

City University of Hong Kong, Hong Kong, China 

E-mail: eeksc@cityu.edu.hk 

Acknowledgement: 

Y. Bai, F. Y. M. Chan, Q. Liu, Y. Liu, K. P. Lor, M. N. Ng 

12 June 2007 

NGLC CUHK 1

Outline 

‣ Long-period 

fiber grating (LPFG) 

‣ LPFG couplers 

‣ Long-period waveguide grating (LPWG) 

‣ LPWG couplers 

‣ Conclusion 

12 June 2007 

NGLC CUHK 2

Core mode 

(Effective index N 01 

Long 

ng-Period 

Fiber 

Grating 

(LPFG) 

01 ) 

Band rejection filter 

Λ : pitch (~100 µm) 

Λ 

Cladding mode 

(Effective index N 0m 

0m ) 

Transmission (dB) 

0 

-5 

-10 

-15 

-20 

Resonance wavelength: 

λ o = (N 01 – N 0m )Λ 

Long-period grating 

1120 1220 1320 1420 1520 1620 

Wavelength (nm) 

[A. M. Vengsarkar et al., J. Lightwave Technol., 14 (1996) 58] 

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NGLC CUHK 3



Fiber 


(LPFG) 

UV radiation 

single-mode 

fiber 

cylindrical 

lens 

amplitude 

mask 

broadband 

optical source 

optical spectrum 

analyzer 

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Fiber 


(LPFG) 

Photos taken at the output ends of several fabricated LPFGs 

LP 03 

(Λ = 590 µm) LP 06 

(Λ = 410 µm) LP 08 

(Λ = 295 µm) LP 016 

(Λ = 100 µm) 

[Q. Liu, K. S. Chiang, Y. Liu., J. Lightwave Technol., (2007) accepted] 

12 June 2007 

NGLC CUHK 5

Four-Port LPFG Coupler 

Input 

L 

Λ 

Band-rejection output 

d 

Band-pass output 

L 

z = 0 

z 1 

z 2 z 3 

Broadband Optical Add/Drop Multiplexer (OADM) 

[K. S. Chiang, Y. Liu, M. N. Ng, S. Li, Electron. Lett., 36 (2000) 1408] 

[K. S. Chiang, F. Y. M. Chan, M. N. Ng, J. Lightwave Technol., 22 (2004) 1358] 

[F. Y. M. Chan, K. S. Chiang, J. Lightwave Technol., 24 (2006) 1008] 

12 June 2007 

NGLC CUHK 6

Coupling Between Cladding Modes 

Photo taken at the fiber output ends 

PS1250/1500 Fiber 

External index 

1.377 (589 nm) 

Grating pitch: 

320 µm 

Grating length: 

32 mm 

Interaction length: 

58 nm 

Wavelength: 

1550 nm 

Cladding mode: 

LP 08 

mode 

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LPFG OADM 

-30 

n ex = 1 (air) n ex = 1.440 

-10 

Transmission(dB) 

-40 

-50 

-60 

-70 

-80 

-90 

-100 

-30 -20 -10 0 10 20 30 

λ ο 

−λ (nm) 


-20 

-30 

-40 

-50 

-60 

-30 -20 -10 0 10 20 30 

λ ο 

−λ (nm) 

L = 30 mm, Λ = 400 µm, d = 0 

[K. S. Chiang, Y. Liu, M. N. Ng, S. Li, Electron. Lett., 36 (2000) 1408] 

12 June 2007 

NGLC CUHK 8

η (dB) 

0 

-5 

-10 

-15 

-20 

-25 

LPFG OADM 

Coupling efficiency η at λ 0 

simulation 

experiment 

0 1 2 3 4 5 6 

z 1 

(cm) 

n 1 

= 1.4499 

n 2 

= 1.4443 

n ex 

= 1.333 (water) 

a = 4.05 µm 

ρ = 62.5 µm 

Λ = 205 µm 

L = 30 mm 

d = 0 

C = 12.54 m -1 

κ = 42.8 m -1 

λ 0 = 1545 nm (air) 

12 June 2007 

[K. S. Chiang, F. Y. M. Chan, M. N. Ng, J. Lightwave Technol., 22 (2004) 1358] 

NGLC CUHK 9

Six-Port LPFG Coupler 

Tapping 

fiber 1 

Tapping fiber 2 

L 

Input 

Transmission fiber 

s 

L 

Gratings 

Outputs 

I II III 

z 

z = 0 z = s z = L z = s +L 

Multi-port 

OADM 

Gratings with κL = π/2 

should be used. 

[Y. Liu, K. S. Chiang, IEEE Photon. Technol. Lett., 18 (2006) 229] 

12 June 2007 

NGLC CUHK 10

LPFG OADM 

Coupling at λ 0 

LP 08 mode 

L = 35 mm, Λ = 300 µm 

λ 0 

= 1562.5 nm (in air) 

Contrast: 18 dB 

3-dB bandwidth: 17 nm 

κL: ∼π/2 

Coupling (dB) 

-2 

-5 

-8 

-11 

-14 

-17 

-20 

n =1.420 

Experimental results: 

Tapping fiber 1(●); tapping fiber 2(▲); 

Simulation results: 

d = 0 (yellow); d = 0.2 µm (green); d = 0.5 µm (red) 

0 10 20 30 40 50 60 

Offset Distance (mm) 

The peak coupling efficiencies at s = 50 mm are −3.73 dB (42.4%) and −3.69 

dB (42.8%) for the two tapping fibers, respectively. The maximum total 

power transfer reaches 85.2%. 

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0 

LPFG OADM 

Output spectra from tapping fiber 1 and the transmission fiber 


-10 

-20 

-30 

-40 

-50 

s = 0, 10, 20, 30, 40, 50 mm 

(n = 1.420) 

-60 

1510 1530 1550 1570 1590 1610 


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0 

LPFG OADM 

Output spectra from tapping fiber 2 and the transmission fiber 


-10 

-20 

-30 

-40 

-50 

s = 0, 10, 20, 30, 40, 50 mm 

(n = 1.420) 

-60 

1510 1530 1550 1570 1590 1610 


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LPFG 3 x 3 Coupler 

Equal power outputs at λ 0 regardless which fiber light is launched into 

Three well-aligned 

LPFGs to maintain symmetry – no offset allowed 

Fiber 3 

Fiber 2 

L 

Input 

Outputs 

Fiber 1 

Gratings 

Strong gratings with κL ∼ π should be used. 

[Y. Liu, K. S. Chiang, Q. Liu, Optics Express, 15 (2007) 6494] 

12 June 2007 

NGLC CUHK 14



0 

-5 

-10 

-15 

-20 

-25 

-30 

-35 

λ 0 

Fiber 2: red 

Fiber 3: yellow 

Fiber 1: white 

n = 1.448 

1480 1500 1520 1540 1560 1580 


12 June 2007 

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0 

-5 

-10 

-15 

-20 

-25 

-30 

-35 

λ 0 



Fiber 2: red 

n = 1.448 

1480 1500 1520 1540 1560 1580 


12 June 2007 

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0 

-5 

-10 

-15 

-20 

-25 

-30 

-35 

λ 0 


Fiber 2: red 


n = 1.448 

1480 1500 1520 1540 1560 1580 


12 June 2007 

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0 


Power Ratio (dB) 

-3 

-6 

-9 

-12 

-15 

-18 

Simulation (d = 0 µm) 

Simulation (d = 0.5 µm) 

1.40 1.41 1.42 1.43 1.44 1.45 

Refractive Index 

Using three 32-mm 

mm-long over-coupled gratings and a suitable surrounding refractive index, 

we achieved a total coupling efficiency of ~72%, which is equivalent to a total loss of ~1.4 dB 

or a loss of ~0.46 dB/port. The non-uniformity in the splitting ratios obtained was ~1%. 

12 June 2007 NGLC CUHK 18



Waveguide Grating 

(LPWG) 

Phase grating 

Cladding 

Corrugation 


Cladding 

Core 

Core 

‣ More flexible designs 

‣ Wider range of materials available 

‣ Easier realization of active devices 

[V. Rastogi, K. S. Chiang, Appl. Opt., 41 (2002) 6351] 

[Q. Liu , K. S. Chiang, V. Rastogi, J. Lightwave Technol., 21 (2003) 3399] 

12 June 2007 

NGLC CUHK 19

Fabrication of LPWG 

Photolithography and reactive ion etching (RIE) 

[K. S. Chiang, K. P. Lor, C. K. Chow, H. P. Chan, V. Rastogi, Y. M. Chu, PTL, 15 (2003) 1094] 

[K. S. Chiang, C. K. Chow, H. P. Chan, Q. Liu, K. P. Lor, EL, 40 (2004) 422] 

Double ion-exchange process 

[M. Christophe, H. Bertrand, C. Laurent, O. Jacquin, G. Cyril, Opt. Commun., 233 (2004) 97] 

Laser/UV writing 

[K. P. Lor, Q. Liu, K. S. Chiang, PTL, 17 (2005) 594] 

Nano-imprint lithography 

[A. Perentos, G. Kostovski, A. Mitchell, PTL, 17 (2005) 1458] 

Thermo-optically optically induced gratings 

[M. S. Kwon, S. Y. Shin, PTL, 17 (2005) 145] 

[K. S. Chiang, C. K. Chow, Q. Liu, H. P. Chan, K. P. Lor, PTL, 18 (2006) 1109] 

12 June 2007 

NGLC CUHK 20

LPWG 

Devices 

Band rejection filter 

Band-pass filter 

Epoxy 

BCB 

Epoxy 

BCB 

[PTL, 15 (2003) 1094; EL, 40 (2004) 422; APL, 86 (2005) 

241115; Opt. Exp. 13 (2005) 1150; OL, 31 (2006) 2716] 

PDL compensator/Variable attenuator 

Chromium Electrode 

(Metal Grating) 

Epoxy 3505 

[Appl. Opt., 45 (2006) 2755] 

Add/drop multiplexer 

BCB 

12 June 2007 

Epoxy 3507 

Si 

[PTL, 18 (2006) 1109] 

y 

Temperature Controller 

(Heat Pump) 

z 

x 

[Opt. Exp., 14 (2006) 12644] 

NGLC CUHK 21

LPWG Coupler 

LPWG1 

LPWG2 

Epoxy 3505 

BCB 

BCB 

SiO 2 

Si 

y 

z 

x 

Coupling between the guided modes of individual cores through coupling to the 

cladding mode of the composite structure by pure grating effects 

Applications: Long-range coupler, power distributor, add/drop multiplexer 

[Y. Bai, K. S. Chiang, J. Lightwave Technol., 23 (2005) 4363] 

12 June 2007 

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LPWG Coupler 

LPWG1 

LPWG2 

1.8 µm 

3 3 µm 

BCB 

Epoxy 3505 

BCB 

7 7 µm 

8 µm 

8 µm 

9 µm 11 µm 

SiO 2 

/Si 

SiO 2 

Grating pitch Λ: : 107 µm; Corrugation depth: ∼100 nm; Grating length : 11 mm 

[Y. Bai, Q. Liu, K. P. Lor, K. S. Chiang, Optics Express, 14 (2006) 12644] 

12 June 2007 

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LPWG Coupler 


0 

-2 

-4 

-6 

-8 

-10 

-12 

-14 

1545 

17.8 o C 

21.8 o C 

25.2 o C 

29.2 o C 

LPWG1 in, LPWG1 out 


1560 1575 1590 1605 1620 


Coupling efficiency 

0.40 

0.35 

0.30 

0.25 

0.20 

0.15 

0.10 

0.05 

0.00 



21.8 o C 

25.2 o C 

17.8 o C 

29.2 o C 

1545 1560 1575 1590 1605 1620 


The two channels show different characteristics, which is due to the 

fact that the fabricated waveguide structure is slightly asymmetrical. 

Calculated coupling efficiency at λ 0 : ~18% 

12 June 2007 

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LPWG Coupler 

Resonance wavelength (nm) 

1620 

1610 

1600 

1590 

1580 

1570 

1560 

Band-rejection (LPWG1) 


Band-pass (LPWG2) 


4.7 nm/ o C 

16 18 20 22 24 26 28 30 

Temperature ( o C) 

The operating wavelength of the coupler can be tuned over the (C+L) 

+L)-band 

with a temperature control of only ~20 °C. 

12 June 2007 

NGLC CUHK 25

LPWG Array 

LPWG1 

LPWG2 

LPWG10 

Epoxy 3505 

BCB BCB BCB 

SiO 2 

Si 

y 

z 

x 

Applications: Multi-port add/drop multiplexer, long-range coupler, power distributor 

[K. S. Chiang, K. P. Lor, Q. Liu, C. K. Chow, Y. M. Chu, H. P. Chan, Japanese 

Journal of Applied Physics, 43 (2004) 5690] 

[Y. Bai, K. S. Chiang, J. Lightwave Technol., 24 (2006) 3856] 

12 June 2007 

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LPWG Array 

10-grating array with a core separation of 80 µ m 

Grating pitch Λ: : 83 µm; Corrugation depth: ∼80 nm; Grating length : 10 mm 

BCB core: 2 µm m thick 5.5 µm m wide; Nominal cladding thickness: ~4.7 µm 

[Y. Bai, Q. Liu, K. P. Lor, K. S. Chiang, Optics Express, 14 (2006) 12644] 

12 June 2007 

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LPWG Array 


0 

-3 

-6 

-9 

-12 

-15 

-18 

35.9°C 

Ch10 

Ch8 

Ch1 

Ch2 

Ch9 

-21 

1500 1520 1540 1560 1580 1600 1620 


Cladding thickness (µm) 

4.8 

4.7 

4.6 

4.5 

4.4 

4.3 

4 

5 6 

7 

3 

8 

2 

1 

9 

10 

0 100 200 300 400 500 600 700 800 900 

x (µm) 

A good matching in the resonance wavelength is found between LPWG1 and 

LPWG9 and, to a less extent, between LPWG2 and LPWG8. The other gratings 

do not show obvious resonance dips in the (C+L)-band. 

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LPWG Array 


0.12 

0.10 

0.08 

0.06 

0.04 

0.02 

9 

2 

3 

LPWG1 input 

35.9°C 

(a) 

8 

0.00 

1500 1520 1540 1560 1580 1600 1620 


12 June 2007 

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LPWG Array 


0.12 

0.10 

0.08 

0.06 

0.04 

0.02 

10 

3 

1 

LPWG9 input 

35.9°C 

8 

2 

5 

(b) 

0.00 

1500 1520 1540 1560 1580 1600 1620 


The coupling between LPWG1 and LPWG9 reaches ~11%. The array allows effective 

power transfer between two waveguides that are separated by as much m 

as 640 µm 

using pure grating effects. 

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Resonance wavelength (nm) 

1620 

1600 

1580 

1560 

1540 

1520 

LPWG Array 



-3.8 nm/ o C 

20 25 30 35 40 45 

Temperature ( o C) 

The resonance wavelength can be tuned over the (C+L)-band with a 

temperature control of only ~25 °C. 

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Conclusion 

‣ A wide variety of coupling devices can be realized 

with LPGs in fibers and waveguides (OADMs( 

OADMs, , multi- 

port couplers, widely tunable couplers, etc.). 

‣ Narrow-band couplers could be formed by using 

high index contrast waveguides (e.g., silicon optics). 

‣ Packaging issues (especially for fiber devices). 

12 June 2007 

NGLC CUHK 32

Thank You! 

_____________ 

Q & A 

12 June 2007 

NGLC CUHK 33

Title: Long-Period Grating Couplers

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