[Feng K., Li L., Jiao W.] 632.8-Nm Visible Region (BookZZ.org)

8/17/2019 [Feng K., Li L., Jiao W.] 632.8-Nm Visible Region (BookZZ.org)

1/6

VOI. 40 NO. 7

SCIENCE IN CHINA (Series A)

JUIY 997

632.8 nm visible region waveguide polarizer fabricated

by proton exchange

FENG Kecheng (?3~ %), I Ling (

?&) JIA O Wentao

( )

and WANG Zhaomin (3

)

(Department of Optics Physics, Institute of Optics and Fine Mechanics, Changchun 130022, China)

Received January 10.

997

Abstract Optical waveguide polarizer at 632.

8

nm was fabricated for th e first time using proton exchange

Ti : LiNb0 3. Th e setup measuring the characteristic parameters of the polarizer was given. Th e extinction ratio of the

polarizer was theoretically analyzed and calculated by means of physical optics and dispersion theory for waveguide.

Various factors affecting the device performance are analyzed. Theoretical calculations are in good agreement with the

experimental results.

Keywords:

proton exchange, optics waveguide polarizer, polarization extinction ratio, dispersion theory for

waveguide.

With the development of science and technology in optical communication, optical fiber gyro,

optical fiber sensor and the core of information processing equipment, easily modulated polarizing

optical source are widely employed. The appearance of waveguide polarizer undoubtedly offers us a

new choice. Especially when jointly used with semiconductor lasers, it has the advantages of com-

pact, high output stability, high extinction ratio, good mode selection and being easy to integrate

with other waveguide devices. Great interest has been aroused at home and abroad. However, the

polarizing devices reported are mostly used in infrared or near infrared region[ -31. Reports of

fabrication, measurement and theoretical analyses for 6 3 2 . 8 nm or shorter wavelengths have not

been seen. We have designed and fabricated for the first time the 632.8-nm device with the ex-

tinction ratio larger than

4

dB. We also set up the measuring system to measure the characteris-

tics of the devices. The extinction ratio of the waveguide polarizer is analyzed and calculated using

physical optics and dispersion theory for waveguide. The theoretical calculation is in good

agreement with the experimental results.

Theoretical analyses

1.1 Physical interpretation for waveguide polarizer

The LiNb03 crystal itself is a kind of ferroelectric crystal. The variation of its spontaneous

polarization could greatly affect the index of refraction. Under paraelectric phase, LiC is located

in the oxygen triangle plane of octahedron, N b C 5 s located at the center of oxygen octahedron.

Under ferroelectric phase, Lit and N b + 5have a small shift along C axis. The shift is .in the same

direction as the spontaneous polarization. Therefore the total spontaneous polarization of LiNb03

crystal is contributed by PsLiC and P S N ~ ~ .

Department of Physics, Siping Education College, Siping 136000, China.


2/6

774

SCIENCE IN CHINA (Series

A Vol. 40

Th e infrared measurement shows that for proton exchange LiNb03 waveguide, after the pro-

ton exchange, H + is not exchanged to the Li + position but shifts along

C

axis. Under the action

of internal electric field, it enters the oxygen triangle plane near Lif It is evident tha t for each

replacement of Lif by

H+

there would be a reduction of P sL iC contribution to the total polariza-

tion. On the waveguide layer if the lack concentration of Lif is A N

(

x ) (here, the concentration

refers to atoms per unit volume, x is the depth of the waveguide layer) , the total spontaneous po-

larization is expressed as

AP,

=

P A N ( x ) ,

(1)

where P is the polarization decrease due to the lack of one Li Therefore An,, the variation of

the extraordinary index of refraction (e- light) due to the linear electrooptical effect can be ex-

pressed as

An, =

Y33

n: AN(rc) P / 3 ~ ~ ( ~ 3 31 ) .

(2)

T o examine the correctness of the above expression, we consider a crystal where the distance of Li

to the nearest oxygen plane is 0 . 0 7 1 4 nm and concentration of Li atom is 1. 8 9

x loz8

~ m - ~ .t

room temperature, for =

632 .8 nm, taking Y = 32 .2 X 10- l2 m-V - ,

€33 =

28, €0

=

8.854 X

1 0 - ~ ~ ~ . m - ,, = 2 . 2 0 1 9 .

Substituting the above result into eqs. ( 1 ) and ( 2 ) , the calculation gives An,

=

0.11, while

the experimental result is 0 .12 , which are in good agreement. Jolivares

t

a1 41 gave an empiri-

cal formula for the variation of Ano as a function of An,:

Ano = 0.007 0 . 4 An, .

3 )

From eq. (4 ) , Ano= 0.04 can be calculated. It is obvious that extraordinary index of refraction

is evidently increased, while the ordinary index of refraction is decreased. Th is effect forms a spe-

cial waveguide. For ordinary light it has total reflection only on the upper surface, whereas on the

lower surface (the boundary between waveguide and the substract) there is a wave leakage which

attenuates the transmitted light. As a result, the emerging light is polarized.

1 . 2

Calculation of the extinction ratio

Extinction ratio is an important physical parameter of a polarizer. It is defined as

E = 10 og(

I:(t/ I ) ,

(4)

where

I Z

represents the output intensity of the light whose polarization direction is parallel to the

incident plane (i . e. the T M wave or 0-lig ht) I:, represents the output intensity of the light

whose polarization direction is perpendicular to the incident plane ( i . e. TE wave or e-light).

Since the T E wave formed a guided wave in the waveguide when the transmission loss is neglect-

ed,

I ,* I A .

The calculation of extinction ratio is mainly the calculation of the output intensity

of the T M wave. In waveguided optics the dispersion equation for plane waveguide isr5

1 , 2 )

(1 ,3 )

nlkdcos0, 6 2 m x ( m = 0 , 1 , 2 , - a * , 5 )

where 6;;

)

and 6;;

3,

represent the reflection phase difference in ;he two boundaries respective-

ly, n l is the index of refraction of the waveguide layer. For our waveguide polarizer, because of

the wave leakage of T M wave at the boundary between the waveguide and the substrate, the

phase difference takes 6): 2) =

x

while for the upper surface,


3/6

No.

7 632.8 nm VISIBLE REGION WAVEGUIDE POLARIZER

775

where n2 and n3 are the index of refraction of the upper and lower layer respectively, ; is the re-

flection angle at the boundary between the upper and lower layer figure

1 .

Substituting eq. 6 ) into the dispersion equation, we have

m = 0 , 1 , 2 , . . . ) . 7 )

According to eq. 7) different mode order m corresponds to different 0;. For T M mode, the am-

plitude ratio between each reflected wave at the lower surface and the incident wave is

As shown in fig.

1,

if the thickness and length of

the waveguide are d , L respectively, thus trav-

eling length of light along the axis for each back

and forth reflection at the lower boundary is 2s,

and s

= d

tgOi the reflection times are n =

L

2 d . tgOi). After n times reflection, the ratio be-

tween the left energy and the incident energy,

i. e . the total reflectivity for T M mode is

R//

=

r while the output energy is ~ / d = R //

{

Thus, for the

same

TE and T M incident intensi-

Fig.

1 .

The transmission of light in the plane waveguide.

ty, the extinction ratio can be calculated:

E

= 10 l ~ ~ l L ~ / l l ; ;= 10 log l;[,/1[) = 10 ogR//

It can be seen from eq. 9 ) that the smaller the transmission angle is, the bigger the extinction ra-

tio is. Fig.

2

shows the behavior of extinction ratio plotted as a function of transmission angle. In

fact, as 8 ecreases, all the mode would be attenuated rapidly except the fundamental mode.

Therefore the proton exchanged waveguide polarizer can only transmit with fundamental mode.

Fig. 3 shows the calculated and experimental results of extinction ratio of a waveguide polarizer

for =6 32 .8 nm.

The difference between n l , the index of refraction of waveguide layer, and 722, the index of

refraction of the substrate, An is one of the major factors affecting the performance of the polariz-

er . It is the key to improve the performance of the device. Our design parameters are: = 632.8

nm, d =

3

pm, L = 2 000 pm . Fig. 4 shows the relation between An = n l n 2 ) and the ex-

tinction index. It is obvious that the bigger the An , the smaller the extinction ratio. To improve

the performance, it is necessary to improve the exchange condition in order that An is not over

0 . 05 .

So the 0-l igh t can be well extinguished. On the other hand, it can be seen from eq. 9 )

that the length of the waveguide is also an important parameter. According to eq. 9 ) , the

E-L

relation is linear. But L should not be too long; otherwise the insert loss will be increased. The

third factor affecting the performance of the polarizer is its thickness d . The E-d relation is more

complicated. T o obtain the E-d dependence the d-O,, relation must be found in advance. The re-


4/6

776 SCIENCE IN

CHINA

(Series A Vol. 40

Propagation angle/rad L/ pm

Fig. 2 . Extinction ratio of po arizer versus propaga-

tion angle rad)

sults are shown in figure 5.

16

'Fig.

3 .

Length of proton exchange region versus

extinction ratio for

I

6 3 2 . 8 nm.

Difference index of refraction

A n

I

2

30

4

Thickness of waveguide d /pm

Fig. 4 Difference between index of refraction of the Fig. 5 Thickness d of waveguide versus extinction ratio

waveguide region of the substrate n and extinction ratio. E

In brief, the extinction ratio of a waveguide polarizer is proportional to its length. But the

selection of L must take the loss into account. Since the bigger the difference

A n

of index of re-

fraction the smaller the extinction ratio. The proton exchange time and annealing time must be

controlled carefully. The thickness of the waveguide can limit the mode and greatly affect the ex-

tinction ratio as well. In the design of polarizer, it is unwise to consider the extinction ratio as the

only factor, especially when the polarizer is to combine with a semiconductor laser.

Fabrication and measurement

of

the device

2 . 1 Fabrication of the device

The polarizer device employs X-cut and Y-transmission strip waveguide structure as shown

in fig. 6. A combination structure is formed by cutting off a segment waveguide and introducing

a length of proton exchange waveguide region. To assure the fabrication, the width of Ti diffu-

sion is selected as 9 pm; the width of optical waveguide is selected as 10 pm .

Inter rupted s trip Ti film is fabricated on a fine polished and cleaned L iNb03 crystal by high

frequency spatter and peeling off technique. After 9-h diffusion at 050°C in flowing wet Ar gas,

the str ip is cooled to 600°C in oxygen atmosphere. The T i : LiNbOs single mode waveguide is pre-

pared. Then a layer of 0 . 2 pm

A1

film is coated on the surface of the substrate . On the A1 film

the proton exchange waveguide pattern is formed by block titanium diffusion in the interrupted re-


5/6

No.

7

632.8-nm VISIBLE REGION WAVEGUIDE POLARIZER

777

gion. The A1 mask on proton exchange region

is removed by etching. The proton exchange /

technique is a key step in the fabrication. The

device is put in a 30 cm long glass tube in

which benzoxy powder is filled to 10 cm high.

The glass tube is sealed and put into the ex-

change furnace at 210°C for 25 min. Then the

sample is taken out and annealed in air at room

temperature for 2. 5 h. Thus the optical Fig. 6. Structure of Ti d~ffusion roton exchange LiNbOs optical

waveguide polarizer chip is fabricated.

waveguide polarizer. 1 LiNb 03 substrate; 2 Ti diffusion re-

gion; 3 proton exchange region.

d

thickness of exchange re-

The purpose of annealing is to eliminate gion; L length of exchange

region.

the sudden change of index of refraction on the

surface of the device. It can be seen from fig.

7

that the curve of index of refraction is much

smoother than that before annealing. It is approximately linear. Finally the two ends of the de-

vice are cleaned and polished and glued to a 63 2. 8- nm optical fiber. The device is mounted in a

box of 2 . 3 cm x 1 cm.

2 . 2 Measurements of the devices

a

he setup measuring the polarizer extinction ratio is

shown in fig.

8.

Random polarized = 632.8 nm light beam

from a He-Ne laser is changed into circularity polarized by

passing through a

h 4

place polarizer. The energizing light is

2 . 2

changed into linearly polarized light by a Glan prism. The

0 5

emerging light is coupled to the input end of optical fiber by

Dlpm

beam expander and lens of short focal length. The light from

Fig.

7 .

Curves of the index of refraction

versus depth before and after annealing.

1.

the output end of optical fiber is coupled to the detector and

Before annealing; 2 after annealing.

measured by a

P-W

power meter. The curve for extinction

ratio versus direction of polarization of the incident light can

be obtained by changing the orientation of the Glan prism.

The parameters of our device are

L = 2mm d =

3

pm. Power of He- 2 3 4

Ne laser is 3

mW

temperature of 1

measurement is 21. 6°C

.

The results

are shown in table 1. Th e advantages

of this measuring method are that the

C Y I

results are apparent. Th e requirement

Fig.

8.

Experimental setup for measurement of extinction ratio.

1

He-Ne

for the laser power is not high

and laser; 2 polarizer; 3

h/4

plate;

4

Glan prism; 5 lens of short focal

the external

effect

is small because the length; 6 single-mode optical fiber; 7 waveguide polarizer; 8 detector;

9

power meter.

emerging light from the polarizer is

directly coupled into the detector to reduce the background light . However this method requires

a strict coaxiality of the measuring devices. Otherwise the measurement accuracy would be badly

affected.


6/6

778 SCIENCE IN CHINA Series A) Vol. 40

Table

1

Measured results of extinction ratio of the waveguide polarizer

Output power

V

Mean value

T E 24.8 nW 16.64 nW

19 .5 1 nW 19.4 2 nW 23. 80 nW

TM 2. 25 pW 1.56 pW

1. 68 pW 1. 34 pW 2. 37 pW

Extinction ratio 40. 29 dB 40.28 dB 40 .4 3 dB 40 .5 0 dB 40 .5 4 dB 40.54 d

Conclusions

Proton exchange waveguide polarizer can be theoretically analyzed with guided-wave theory

and numerical simulative calculation for various factors affecting the extinction ratio to conduct the

optimum design to improve the performance of the device. Waveguide polarizers fabricated by

proton exchange have the advantages of low transmission loss and easy fabrication. By annealing

the light loss can be further decreased and the proton diffusion depth can be increased to meet the

mode matching in the waveguide region. The selection of the design parameters is extremely im-

portant in the fabrication. Those factors such as proton exchange temperature, exchange time,

annealing time and temperature, the length and thickness of waveguide region, the difference of

index of refraction, would greatly affect the transmission efficiency loss and extinction. As for the

measurements, the stability of the light source, the background light and the coaxiality of various

device would directly affect the measurement accuracy. T o improve the device performance and

propose more accurate measuring technique is an important topic in the research. On the other

hand, experiences extracted from the fabrication of proton exchanged waveguide polarizer are in-

structive to the fabrication of other type of polarizers, such as metal cladding optical waveguide

polarizers. In the future, with the increasing applications of the polarizers, there are prospective

developments in this field.

References

Suchoski,

P.G,

Findakly, T.

K .

Leonberger.

F.

J .

Low-loss high extinction polarizers fabricated in LiNb03

by

proton ex-

change, O p t L e t t . 1988, 13 2) 170.

2

Veselka,

J . J .

Bogerro, G. A. Low-loss TM-pass polarizer fabricated by proton exchange for Z-cut Ti: LiNbOs waveguides,

E l e c t i o n . L e t t . 1987, 23 1) 29.

Gao Fubin, Jin Feng. Xing Rubing et a1 1.5

pm

proton exchanged waveguide polarizer for T Eo mode, Op tics Journal in

Chinese), 1995, 15 8):1 102.

4

Jolivares Diaz-Gabrera, M .

A . Direct measurement of ordinary refractive index of proton exchanged LiNb03 waveguides,

O p t . C o rn m a n. 1992, 92 1-3) :40.

5 Xu Senlu, Ling Shide, Opt i cul Waveguides and Appl i ca t ions in Chinese 1 Hangzhou: Zhejiang University Publishing

House, 1990, 46-53.

Documents

[Feng K., Li L., Jiao W.] 632.8-Nm Visible Region (BookZZ.org)