Nonlinear Optical Properties of Organic and Polymeric Materials

7 Nonlinear Optical Interactions in Organic Crystal Cored Fibers Β. K. N A Y A R

Downloaded by LOUISIANA STATE UNIV on May 8, 2015 | http://pubs.acs.org Publication Date: September 29, 1983 | doi: 10.1021/bk-1983-0233.ch007

British Telecom Research Laboratories, Martlesham Heath, Ipswich, Suffolk, IP5 7 R E , United Kingdom

A novel technique of producing waveguiding structures by growth of crystals in glass c a p i l l a r i e s is presented. This method of crystal growth is simple and is particularly suited to organic materials. The phase-matching techniques for non-linear interactions in these fibres are discussed and optical second harmonic generation is demonstrated in monomode "benzil crystal cored fibres.

N o n - l i n e a r o p t i c a l i n t e r a c t i o n s occur i n m a t e r i a l s with high o p t i c a l i n t e n s i t i e s and have been used to produce coherent l i g h t over a wide range of frequencies from the f a r i n f r a - r e d t o the u l t r a - v i o l e t . The three wave mixing process i s of p a r t i c u l a r i n t e r e s t as i t can be used f o r o p t i c a l parametric a m p l i f i c a t i o n and o p t i c a l second harmonic generation (SHG) and occurs i n n o n - c e n t r ο s y m m e t r i c m a t e r i a l s . In recent years a number o f organic m a t e r i a l s having second order n o n - l i n e a r i t y of the same order and greater than that of commonly used inorganic m a t e r i a l s have been reported ( j _ - 4 ) · These c r y s t a l s are mainly those having molecules which i n c l u d e benzene r i n g s or conjugated bonds with acceptor o r donor groups. Another advantage i n using these m a t e r i a l s i s t h a t , u n l i k e some inorganic m a t e r i a l s , they e x h i b i t r e s i s t a n c e to o p t i c a l damage at high optical intensities. To use these m a t e r i a l s f o r o p t i c a l mixing i t i s necessary to phase match the i n t e r a c t i n g waves as m a t e r i a l s g e n e r a l l y e x h i b i t chromatic d i s p e r s i o n . For some m a t e r i a l s i t i s p o s s i b l e t o e x p l o i t the m a t e r i a l b i r e f r i n g e n c e f o r phase matching and t h i s method i s known as Angle P h a s e - M a t c h i n g . However, f o r m a t e r i a l s which are i s o t r o p i c or do not possess adequate b i r e f r i n g e n c e , i t i s p o s s i b l e t o c o r r e c t phase mismatch using quasi-phase-matching techniques (_5~6.). These i n p r a c t i c e tend t o be d i f f i c u l t to implement and o f f e r low conversion 1

0097-6156/83/0233-0153$06.00/0 © 1983 American Chemical Society

In Nonlinear Optical Properties of Organic and Polymeric Materials; Williams, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1

154

NONLINEAR OPTICAL PROPERTIES


efficiencies. A l t e r n a t i v e l y , i f they can be used t o f a b r i c a t e a waveguiding s t r u c t u r e , phase matching can be achieved using waveguide d i s p e r s i o n . In t h i s paper, the advantages of n o n - l i n e a r i n t e r a c t i o n s i n waveguiding s t r u c t u r e s are discussed and we r e p o r t on the f a b r i c a t i o n of organic c r y s t a l cored o p t i c a l f i b r e s . These f i b r e s have a c r y s t a l l i n e core and a g l a s s c l a d d i n g . This method o f waveguide f a b r i c a t i o n i s simple and i s p a r t i c u l a r l y s u i t e d t o organic m a t e r i a l s . The o p t i c a l SHG was r e c e n t l y demonstrated (7_) i n b e n z i l c r y s t a l cored f i b r e s and here f u r t h e r r e s u l t s are presented and methods of improving SHG e f f i c i e n c y are d i s c u s s e d . Non-linear Interactions

i n Waveguides

The advent of i n t e g r a t e d o p t i c s has h i g h l i g h t e d advantages of using waveguiding s t r u c t u r e s f o r n o n - l i n e a r i n t e r a c t i o n s and these have been e x p l o i t e d by a number o f researchers f o r o p t i c a l SHG and m i x i n g . These advantages are: 1. Phase matching can be achieved using waveguide dispersion. The waveguide d i s p e r s i o n a r i s e s because the phase v e l o c i t y of a l i g h t wave of a given wavelength i n a.waveguide i s determined by the g u i d i n g r e g i o n dimensions, r e f r a c t i v e index d i f f e r e n c e between the g u i d i n g r e g i o n and the s u b s t r a t e , and the mode o f propagation. In p r a c t i c e the phase matching i s achieved for the i n t e r a c t i n g modes by s u i t a b l y t a i l o r i n g dimensions of the guiding r e g i o n and/or the r e f r a c t i v e index d i f f e r e n c e . 2. The dimensions of the guiding r e g i o n f o r o p t i c a l wavelengths are of the order o f micrometers and as a r e s u l t i t i s p o s s i b l e t o have l a r g e o p t i c a l i n t e n s i t i e s , r e q u i r e d f o r n o n - l i n e a r i n t e r a c t i o n s , with modest o p t i c a l powers. A l s o , as the o p t i c a l energy i s guided i t i s p o s s i b l e t o have long interaction lengths. An approximate comparison of the e f f i c i e n c y of SHG process i n a bulk medium and that i n a waveguiding s t r u c t u r e can be made by c o n s i d e r i n g the product of the i n t e n s i t y of the beam at the fundamental frequency and the i n t e r a c t i o n length. For bulk m a t e r i a l :

I

ω

L=(P/TTV

2

ο

)(TTW

2

ο

/λ)=Ρ/λ

where, gaussian beam d i f f r a c t i o n has been assumed and W i s beam r a d i u s at the focus. Whereas, f o r a three dimensional guide: q

2

I L=(P/X )L=(PA)(LA) ω where, i t has been assumed that the guide dimensions are comparable t o the wavelength o f the fundamental wave.


the

7.

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Organic Crystal Cored Fibers

155

The advantages described above o f u s i n g -waveguiding s t r u c t u r e s can c a n c e l out i f the f i e l d overlap i n t e g r a l of the i n t e r a c t i n g modes i s s m a l l . This a r i s e s since the SHG efficiencyis proportional to i t .

dA


where, Ε Ε^ A

ω

i s the e l e c t r i c f i e l d d i s t r i b u t i o n of the fundamental mode. i s the e l e c t r i c f i e l d d i s t r i b u t i o n o f the SH mode. i s the waveguide c r o s s - s e c t i o n a l a r e a .

The overlap i n t e g r a l places c o n s t r a i n t s on mode types i n t o which SH can be coupled and i s maximum when both the fundamental and the SH propagate as fundamental waveguide modes. In p r a c t i c e , phase matching i s g e n e r a l l y achieved f o r SH as a higher order mode due t o the waveguide d i s p e r s i o n c h a r a c t e r i s t i c and t h i s leads t o low conversion e f f i c i e n c i e s . I t has been shown by Ito et a l (Q) that i t i s p o s s i b l e i n some cases t o maximise the overlap by using a m u l t i l a y e r guiding r e g i o n . The waveguiding s t r u c t u r e s used f o r SHG have been planar with the f o l l o w i n g s t r u c t u r e s : 1 . a l i n e a r guide on a n o n - l i n e a r substrate ( 9 - 1 2 ) ; 2 . a n o n - l i n e a r guide on a l i n e a r substrate ( 1 3 - 1 6 ) ; 3 . a n o n - l i n e a r guide on a n o n - l i n e a r substrate ( 1 J - 2 0 ) . R e c e n t l y , s t r i p e waveguides have been f a b r i c a t e d by T i i n d i f f u s i o n i n t o a LiNbC^ substrate f o r SHG ( 2 1 - 2 2 ) . It has not been p o s s i b l e , up t o now, t o r e a l i s e an e f f i c i e n t n o n - l i n e a r o p t i c a l device using these s t r u c t u r e s due t o a number of f a c t o r s in a d d i t i o n t o the c o n s t r a i n t s placed by the overlap i n t e g r a l . The most s t r i n g e n t requirement that has been d i f f i c u l t t o achieve i s the need t o maintain uniform guide dimensions t o w i t h i n a few percent over the waveguide l e n g t h i n order to maintain phase matching. The other problems that have been serious are the s c a t t e r i n g l o s s e s at the g u i d e - s u b s t r a t e i n t e r f a c e and the o p t i c a l damage as a r e s u l t o f high o p t i c a l i n t e n s i t i e s . It should be p o s s i b l e with c a r e f u l design o f waveguide s t r u c t u r e s and improved f a b r i c a t i o n techniques t o obtain g r e a t e r SHG e f f i c i e n c i e s than those obtained with buJk media. Organic c r y s t a l cored f i b r e s As an a l t e r n a t i v e t o planar waveguiding s t r u c t u r e s we report here the f a b r i c a t i o n of c r y s t a l cored f i b r e s i n which i t i s p o s s i b l e t o maintain uniform guide dimensions over long lengths. These f i b r e s with organic c r y s t a l core m a t e r i a l having l a r g e second order n o n - l i n e a r i t y could be used f o r m i n i a t u r i z a t i o n of v i s i b l e l a s e r sources and r e a l i z a t i o n of parametric a m p l i f i e r s for o p t i c a l communications.



156

NONLINEAR OPTICAL PROPERTIES

Organic c r y s t a l cored f i b r e s have been f a b r i c a t e d by growth i n g l a s s c a p i l l a r i e s of a s i n g l e c r y s t a l from melt u s i n g a modified v e r t i c a l Bridgeman technique. This method of c r y s t a l growth i s only a p p l i c a b l e t o m a t e r i a l s which are chemically s t a b l e on m e l t i n g and have a melting point l e s s than the g l a s s transformation temperature. In a d d i t i o n , f o r o p t i c a l guidance i n these f i b r e s i t i s necessary to s e l e c t a c l a d d i n g g l a s s from commercially a v a i l a b l e g l a s s e s ( 2 3 ) having a r e f r a c t i v e index l e s s than the c o r e . I n i t i a l studies on the growth o f organic c r y s t a l s i n g l a s s c a p i l l a r i e s were c a r r i e d out by a number of researchers ( 2 ^ - 2 7 ) t o gain understanding o f c r y s t a l growth mechanism i n small bore c a p i l l a r i e s and t o i d e n t i f y u s e f u l m a t e r i a l s f o r f a b r i c a t i o n of f i b r e o p t i c components. The growth of m - n i t r o a n i l i n e , m-dinitrobenzene, 2 - b r o m o - U - n i t r o a n i l i n e , m-dihydroxybenzene, acetamide, b e n z i l and f o r m y l - n i t r o p h e n y l h y d r a z i n e , has been c a r r i e d out i n 1 5 y m t o 8 0 y m bore diameter g l a s s c a p i l l a r i e s . The best r e s u l t s f o r v o i d f r e e s i n g l e c r y s t a l growth were obtained f o r m-dinitrobenzene, acetamide and b e n z i l . With these m a t e r i a l s , v o i d f r e e s i n g l e c r y s t a l cored f i b r e s o f lengths upto 5cms have been obtained. R e c e n t l y , s i m i l a r lengths of v o i d free m-dinitrobeneze and b e n z i l c r y s t a l s have been grown i n g l a s s c a p i l l a r i e s with bore diameters i n the 2ym to 10μπι range f o r s i n g l e mode operation ( 7 , 2 8 ) . For guided wave n o n - l i n e a r i n t e r a c t i o n s i n these f i b r e s i t i s a l s o necessary that the m a t e r i a l be non-centrosymmetric, o p t i c a l l y transparent i n the wavelength region of i n t e r e s t and should have a non-zero SH tensor c o e f f i c i e n t which i s a m u l t i p l i c a t i v e f a c t o r with one of the transverse e l e c t r i c f i e l d components. The l a t t e r requirement i s necessary as the f i e l d s i n a weakly g u i d i n g f i b r e are very n e a r l y transverse ( 2 9 ) . In the e a r l i e r s t u d i e s on growth of m-dinitrobenzene and m - n i t r o a n i l i n e i t was found that these c r y s t a l s grow i n g l a s s c a p i l l a r i e s with t h e i r c r y s t a l c - a x i s along the f i b r e a x i s . As these m a t e r i a l s have t h e i r l a r g e s t SH tensor c o e f f i c i e n t s along t h e i r c - a x i s and because the f i e l d s i n the f i b r e are t r a n s v e r s e they cannot be used f o r three wave mixing i n t h i s c o n f i g u r a t i o n . F o r , SHG i t i s d e s i r a b l e to f a b r i c a t e f i b r e s which allow s i n g l e mode operation at the fundamental wavelength so that a l l the launched power i s a v a i l a b l e f o r conversion. The frequencies for which s i n g l e mode operation i s p o s s i b l e i n a step-index u n i a x i a l c r y s t a l cored f i b r e with c r y s t a l axis along the f i b r e a x i s are given by (30) ^ Ω

^

—

2.1+05C

UT

Λ

η,/η Ν

1

: Ζ

η

> Ζ

n 1

1

a(N.A.)

^

2.1+05c

—

a(N.A.)

^

ζ — 1


7.

Organic Crystal Cored Fibers

NAYAR

157

•where a c N.A. n^ n η z

is is is is is is

the the the the the the

core r a d i u s . v e l o c i t y of l i g h t i n vacuum. 2 1/2 numerical aperature and i s given by (n^ - η ) c r y s t a l r e f r a c t i v e index t r a n s v e r s e t o the f i b r e a x i s . c r y s t a l r e f r a c t i v e index along the f i b r e a x i s . c l a d d i n g g l a s s r e f r a c t i v e index.


F a b r i c a t i o n of b e n z i l c r y s t a l cored f i b r e s In t h i s i n i t i a l e x p l o r a t o r y study, b e n z i l , a p o s i t i v e u n i a x i a l c r y s t a l of t r i g o n a l c l a s s and point group 3 2 vas used to demonstrate phase matched second harmonic generation i n c r y s t a l cored f i b r e s . B e n z i l s m e l t i n g p o i n t i s 95°C and good q u a l i t y c r y s t a l s can be grown -with ease from i t s m e l t . The b e n z i l s t r a n s m i s s i o n spectrum, shown i n F i g u r e 1 , was determined for a 5.U9mm t h i c k c r y s t a l grown from melt u s i n g a Perkin-Elmer spectrophotometer. This shows that b e n z i l cannot be used f o r e f f i c i e n t frequency doubling of 0 . 9 y m wavelength r a d i a t i o n . B e n z i l has two non-zero SH tensor c o e f f i c i e n t s , namely d ^ and d ^ of which i s of the l a r g e r magnitude (3_0 · magnitude or d^ ^ c o e f f i c i e n t and the SH coherence l e n g t h have been measured by Jerphagnon (32) to be ( 11 .5+1 . 5 )xd^ of

Nonlinear Optical Properties of Organic and Polymeric Materials

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