A three-dimensional index variation grating in bulk BGG31 glass written using neither hydrogen loading nor germanium doping is demonstrated. This material is useful for fabricating ion-exchanged waveguides, and its photosensitivity to ultraviolet (UV) radiation at 248nm has not been previously explored. Intensity measurements of the Bragg diffracted spots indicated a maximum index variation (Delta n) of similar to 4 x 10(-5)

Geraghty, D. F.

Honkanen, S.

Marshall, W. K.

Peyghambarian, N.

Provenzano, D.

Yariv, A.

English

Caltech Authors - Main

N ~ 2) and those of the odd numbers were set to he g,, = expQa) (n 
= 1, 3, ..., N - 1). The output power of thc transversal filter is 
given from eqn. I by 
1 T = \GI2 = -(1+ e'") x = 1 c o s ( ~ / Z ) / ~  (2) 
IN 
Fig. 2 compares the experimental normalised output power with 
the theoretical output power given by eqn. 2. The circles represent 
the experimental values for a = 0, d3, d2, 2d3 and K. It is con- 
firmed that the collective summation of complex electric fields is 
obtainable by using an MMI combiner. The fibre-to-fibre inser- 
tion loss at zero phase variation was 5.6dB. There are several loss 
origins: they are, a 0.4dB theoretical loss in each MMI splitter and 
combiner, a 0.ldB theoretical loss in the MMI 3dB coupler, a 
0.06dB intersecting waveguide loss for each intersection [6] and a 
0.ldB coupling loss in the dispersion shifted fibre. When we sum 
up these losses, we have a 0.4(dB) x 2 (MMI splitter and com- 
biner) t 0.1 (dB) x 2 (MMI 3dB couplers) + 0.06(dB) x 15 (inter- 
sections) t 0.1 (dB) x 2 (facets) = 2.l(dB) loss without including 
the waveguide propagation loss. The total waveguide length of the 
transversal filter is -27cIn. If we assume a reasonable waveguide 
loss of 0.13dB/cm, the total waveguide propagation loss becomes 
3.5dB. The above loss breakdown sums up to 2.1 + 3.5 = 5.6(dB) 
which agrees with the experimental value. When we exclude the 
3.5dB waveguide loss and 0.2dB fibre coupling losses, the inherent 
loss of the transversal filter is evaluated to be -1.9dB. 
0 ,  I 
MMI combiner. Programmable spectral filters and chromatic dis- 
persion equalisers can be realised by using the presented transver- 
sal filter. Experiments are now in progress and the results will he 
reported in the future. 
Acknowledgments: The authors would like to thank members of 
the Okamoto Research Laboratory and Hyper-Photonic Compo- 
nent Laboratory for their co-operation. 
0 IEE 1999 
Elelcctvonim Letters Online No: I9990940 
nor: 10. 1049/c1:19990940 
22 June 1999 
K. Okamoto, H. Yamada and T. Goh (NTT Photonics Lobomtorim, 
162 Tokai, Naku-gun, l l w a k i ,  319-11, Japan) 
E-mail: okamoto~iba.iccl.nttjp 
References 
JACKSON. K,P., NtiWrON, S.A., MOSLEI.11. B., CUTLER, M.C.C. 
GOODMAN, I.w., and SHAw. H.I.:  'Optical fibcr delay-line signal 
processing', IEEE Trum,  1985, MTT-33, pp. 1933210 
SASAYAMA, K., OKUNO. M, ,  and HABARA, K.: 'Photonic FDM 
multichannel selector using coherent optical transversal filter', 
IEEE J. Lightwave Techno/., 1994, LT-12, pp. 664-669 
TAKIOUCIII. K., JINCUJI.  K., and OHMORI, Y.: 'Variable gTOup-delay 
dispersion equaliser based on a lattice-farm programmable optical 
filter', Elcclron. Lett., 1995, 31, pp. 1240-1241 
TAKIGUCHI, K., IINCUII,  K., OKAMOTO, K., and OHMORI, V.: 'Variable 
group-dclay dispersion equalizer using lattice-form programmable 
optical filtcr on planar lightwave circuit', IEEE J. Se/. Topics 
Qurrrilum Electron., 1996, QE-2, pp. 270-276 
SOI.IIAN0. L.H., and PENNINCS. C.M.: 'Optical multi-mode 
interference devices based an self-imaging: principles and 
applications', IEEE J. Ligblwuvc TccIznoL, 1995, LT-13, pp. 615- 
627 
KOMINATO, T., KLIOH:T, KATOII. K., HIBINO, Y., and YASU, M.: 'Loss 
characteristics of intersecting silica-based waveguides'. 
Optoclectronics Conf. OEC'92, 1992, Paper 1684-1, pp. 138-139 
HIDA, Y. ,  HIHINO, Y., OKAZAKI. H.,  ilnd OHMORI. Y.: 'IO-m long silica- 
based waveguide with a loss of 1,7dB/m'. IPR'95, Dana Point, 
CA,1995, Paper IThC6 
-60 
1545 1546 ' 1547 1548 i s ls  1550 1551 
A, nm wim 
Fig.  4 Spectral intensity tmnsmirtunces of filter. vlrcn g, has Gaussian 
pl.oJ?Ie 
~ experimental values 
_ _ - -  theoretical values 
To confirm the signal processing capabilities, sinc-type and 
Gaussian-type spectral filter responses have been synthesised. 
Fig. 3 shows the spectral transmittances of the filter when g, = 1 
for all n. Since the functional shape of g,, and the spectral trans- 
mittance G are connected by a Fourier transform relationship, the 
theoretical intensity transmittance is expressed by 
where $ = (2nnC/?4AL = 2nj7FSR and f denotes the optical fre- 
quency. The peak transmittance is -5.6dB. Fig. 4 shows the spec- 
tral transmittances of the filter when g,  has the following 
Gaussian profile: 
(n = 0, I, ..., 15) (4) 1 
The theoretical intensity transmittance also has Gaussian shape as 
shown by the dotted line in Fig. 4. The peak transmittance 
becomes -12dB. An additional loss of 6.4dB is caused by cutting 
out the signal power in each tap arm to form Gaussian tap coeK- 
cients. It is shown from Figs. 3 and 4 that an arbitrary shape of 
the filter characteristics can be realised by the present transversal 
filter. 
Conclusion: The fabrication of a fully integrated coherent optical 
transversal filter has been reported. It is confirmed that the colleo 
tive summation of complex electric fields is obtaiiiable by using an 
Grating formation in BGG3l glass by UV 
exposure 
D. Provenzano, W.K. Marshall, A. Yariv, 
D.F. Geraghty, S. Honkanen and N. Peyghamharian 
A three-dimensional index variation grating in bulk BGG31 glass 
written using neither hydrogen loading nor germanium doping is 
demonstrated. This material is useful for fabricating ion- 
exchanged waveguides, and its photascnsitivity to ultraviolet (LTV) 
radiation at 248nm has not bcen previously explond. Intensity 
measnrcments of the Bragg diffracted spots indicated a milximum 
index Yariation (API) of -4 x 1O-j. 
Introduction: The field of optical communications has recently 
experienced a host of new devices to allow for higher hit rates. 
One area of innovation has been in the research of ion-exchanged 
waveguides [I], and incorporating Bragg gratings into such devices 
could lead to a great range of applications. Permanent index 
changes for gratings written in Ge-Si02 glasses have heen reported 
[2]. Photobleaching has also been performed in a K+-exchanged 
waveguide with the 351nm line from an Ar' laser [3]. However, 
the photosensitivity was reportedly obtained by 7-ray irradiation 
pretreatment. In the waveguide material discussed in this Letter, 
there are no Gedopanls [4] and no pretreatment of any kind, 
including hydrogen loading, thus allowing easier and cheaper fab- 
rication. 
We report the writing of a diffraction grating in a 2mm thick 
glass sample used for making Ag"' ion-exchanged waveguides by 
exposure to radiation from a 248nm KrF excimer laser, without 
germanium doping nor hydrogen loading. BGG31 is a glass of the 
system SiO2iB~O2/A1,O1~a20/F, with 12.5molNa02. The three 
dimensionally written gratings are evidence that neither germa- 
1332 ELECTRONICS LETTERS 5th August 1999 Vol. 35 No. 16 
nium nor hydrogen need be present to produce a reasonable index 
change with 248nm irradiation. The effect of the excimer exposure 
on the transmission of the glass can be seen in Fig. 1. Shown is 
the transmission against wavelength characteristics with and with- 
out exposure (no grating) and compared to a standard Corning 
glass microscope slide. The microscope slide transmission curve 
was unaffected by the excimer lascr. Photobleaching the BGG31 
glass with 248nm radiation changes the absorption spectrum near 
350nm and appears to add a new absorption hand near 520nm, 
with no noticeable differences above 650n111. 
I U ,  
om0 L 300 400 A, nm 500 800 m 
Fig. 1 Tranamirriun spectrum j b r  exposed and unaxpo.red sa~nples of 
BGG31 glms 
~ microscope slide 
__ BGG31 glass unexposed - BGGll glass exposed 
For the grating inscription, the BGG31 glass sample was placed 
directly under a phase mask with a pitch of 1049.28nm (IBSEN 
Micro Structures A I S )  and exposed with the excimer laser at 
248nm (Lambda Physik Compex 110). For hest results, the glass 
was placed in direct contact with the phase mask and exposed for 
IOmin with 0.84J/cm2 per pulse at 50Hz over an area of 2cm x 
2mm. The beam was collimated in the long dimension (perpendic- 
ular to the phase mask rulings) and slightly focused in the short 
dimension. Other trials were carried out with gaps of 125 and 
2 5 0 ~  between the phase mask and the sample, but in each case 
the diffraction spots from thc Ar" laser probe were reduced in 
intensity. This is consistent with reports [5, 61 that it is not the 
temporal coherence that limits the Cringe visibility with masklglass 
(or fibre) separation, but the spatial coherence. 
Thc diffraction orders from the phase mask obey the equation 
where m is the diffracted order, h = M n m ,  and A,,,,,Tk is the mask 
period = 1049.28nm. Each pair of beams produces an index varia- 
tion grating in the material with a unique angle $. Fig. 2 shows an 
example of a grating produced with the -2, + I  orders. The angle 4 
for a given grating is determined as the bisector of the angle 
between the propagation directions of any two orders, denoted mI 
and m,. The f l  orders produce gratings with $ = 0, as will the 52 
orders. c1 is the Bragg angle, measured as the angle of the incident 
(or reflected) ray with the tangent to the grating lines. 
Characterisation of the writtcn gratings was accomplished by 
probing these gratings with the 476nm line from an Ar+ laser 
operating at -5OmW. Evidence for a volumc grating was that a 
very precise incident angle e,$ was needed to observe Bragg dif- 
fraction (Fig. 2). If only surface gratings had been produced by 
the excimer laser, then Bragg diffraction would have occurred at a 
continuum of angles. The Bragg condition for these photowritten 
gratings is 
cos4 sinal= r n X  (2) 1 
where the teim in brackets is the actual period of the desired grat- 
ing, derived from the intensity distribution of any two diffracted 
orders mi and m2. m is the diffraction order from the probe, and 
here his 476nm. e, and e,,,, can be obtained from a in eqn. 2 and 
account for the background material index of 1.4. 
Fig. 2 Bragg diffraction porn one of induced volume grating.v 
Probing was performed with 47hnm line from Ar" laser 
Parallel lines show induced index-variation volume hologram 
-2 
I I I I 
~ 
ELECTRONICS LETTERS 5th August 1999 Vol. 35 No. 16 1333 
2 4 6 8 10 
10 
An (XI d5) 
Fig. 3 Computed index change (An) Jhr grating written from t l  phase 
mask orders 
(i) expectcd peak reflectivity for grating length lmm 
(ii) expected peak rcflectivily far grating length 0.5mm 
Horizontal line is measured reflectivity 
Using the intensity data for the observed diffraction spots, it 
was possible to estimate An for the strongest grating written in the 
glass (ml = 1 and m2 = -1). It was necessary to know the penetra- 
tion depth of the grating, for that determines the interaction 
length, and hence the peak reflectivity, for the grating. According 
to both Othonos and Dyer [5, 61, KIF excimer lasers have the spa- 
tial coherence to provide high fringe contrast out to only a few 
hundred micrometres past the phase mask. We performed expo- 
sures with mask/glass separations of 125 and 2 5 0 ~ ~  each with 
increasingly faint gratings produced, so at best, it was interpolated 
that the penetration depth is between 500pni and Imm. However, 
by separating the mask and glass, mechanical vibrations as well as 
fibre heating could have been the largest factor in reducing the 
grating strength. Shown in Fig. 3 are the theoretical peak reflec- 
tivity curves for a grating with two different depths. The left curve 
assumes a penetration depth of lmm, and the right curve assumes 
a penetration depth of 0.5inm. Peak reflectivity for a uniform 
grating is given by [7] 
(3) 
Note that even if the penetration depth is 0.5mm, the effective 
grating length (L) that the argon beam interacts with whcn inci- 
dent at 27" is 0.17mm. For lmm penetration, L = 0.34mm. The 
horizontal line is the measured reflectivity for the first order of the 
strongest grating. The An is expected to lie on this line, 
Conclusion: We have demonstrated volume gratings written in 
BGG31 glass with an excimer laser at 2118nm. The glass has nei- 
ther germanium nor hydrogen to enhance the photosensitivity. 
The index change inferred was (3.9 f 1.3) x lW5, a reasonable 
value for this composition of glass. Tests of tlie transmission prop- 
erties of these gratings with light coupled into the channel 
waveguides havc been performed [XI, and better than 90% trans- 
mission dips at a Bwgg wavelength of 1.531" have been obtained. 
Aclnowledgmentr. We thank Inlegrierte Optik GmbH, Waghitusel- 
Kirrlach, Germany, for providing the BGG31 samples. This work 
is supported by DARPA and the Ofice or Naval Research 
(ONR). 
(E) IFF 1999 3 June 1999 
D. Provenzano, W.K. Marshall and A. Yariv (Califiirnin Institute ,If 
Technology, Mail Stop 128-95. Pasadena, CA 91125, USA) 
E-mail: danpra~its.caltech.ed~ 
D.F. Geraghty (NP Photonic Technologie.7, Tucson, Arizona, USA) 
S. Honkancn and N. Peyghambarian (Optical Scimces Cemw, 
University of Arizona, USA) 
References 
LI, M.J., HONKANEN, s., WANC, w.J., LEONELLI, R.,  ALBERT, I., and 
NAJAFI, S.I.: 'Potassium and silver ion-exchanged dual-core glass 
waveguides with gratings', Appl, Phys. Lert.. 1991, 58, (23), pp. 
2607-2609 
NISHII, I.: 'Permanent index changes in Ge-SiO, glasses by excimcr 
laser irradiation', Mal. Sei, Eng., 1998, B54, (1/2), pp. 1-10 
ROMAN, J.E., and WINICK,  K.A.: 'Photowritten gratings in ion- 
exchanged glass waveguidcs', Opt. Lett.,  1993, 18, ( IO),  pp. 808- 
810 
optical properties of Gc-doped silica glass during exposure lo a 
KrF excimer laser', J ,  Non-C~yr. Solids, 1997, 222, pp. 266-271 
OTHONOS, A., and LBO, x.: 'Novel and improved methods of writing 
Bragg gratings with phase masks', IEEE Photonics Technol. Left.,  
1995, 7, (IO), pp. 1183-1185 
DYER, P.P., FARLEY, R.J., and GIBOL, R.: 'Analysis of grating 
formation with excimer laser irradiated phase masks', Opt. 
Cummun., 1995, 115, (314), pp. 327-334 
OTHONOS, A.: 'Fiber Brags gratings', Rev. Sci. Iwstium., 1997, 68, 
( IZ) ,  pp. 4309-4341 
YARIV. A., and PEYOHAMBARIAN. N.: 'Gratings photowritten in ion- 
exchanged glass channel waveguides', Electron. Lett., 1999, 35, (7), 
pp. 585-587 
MIYAKB, Y., NISHIKAWA, *I., and WATANAHE, E.: 'Changes in the 
OBRAGHTY. D.F., PROVENZANO, D., HONKANEN, S. ,  MARSHALL, W.K., 
Multi/demultiplexer consisting of one parent 
10GHz-spaced AWG and eight subsidiary 
100GHz-spaced AWGs 
K. Takada  a n d  K. Okamoto 
A 320-chaunel IOGHz-spaccd multiidemultiplexer is reported 
which is realised by connecting eight IOOGHz-spaccd AWGs in 
parallel to one lOGI3z-spaced AWG. The subsidiary AWGs RI? 
conventional IOOGHz-spaced AWGs and only anc phase- 
compensated 1OGHz-spaced AWG is needed. The configuration 
is more promising far developing an ultra-dense multi1 
demultiplexer than that reported previously in which sixteen 
10GHz-spaced AWGs  we^ used. 
Introduction: Large-scale multiidemultiplexers with narrow chan- 
nel spacing are attractive for use in increasing the capacity of 
wavelength division multiidemultiplexing (WDM) systems. We 
demonstrated a 320-channel 10GHz-spaced multiidemultiplexer 
consisting of one parent 100GHz-spaced arrayed-waveguide grat- 
ing (AWG) and 16 subsidiary 10GHz-spaced AWGs [l] thus 
showing the potential of parallel coiinection for increasing the 
number of narrow channels. In practice, however, it is rather diffi- 
cult to use so many 10GHz-spaced AWGs because a sophisticated 
phase compensation technique [2] is needed to reduce the crosstalk 
beiween the AWGs. We report that the same 320-channel multi/ 
demultiplexer can be constmcted by connecting eight conventional 
100GHmpaced AWGs in parallel with only one 10GHz-spaced 
AWG. This configuration greatly reduces the required number of 
AWGs. 
Conjiguration: The configuration of our multi/demultiplexer is 
shown in Fig. 1. It consists of a parent 10GHmpaced AWG and 
eight identical 100GHmpaced 32x32 AWGs, AWGk (k = 1, 2, ..., 
8). The ports on the left and right sides of each AWG w e  denoted 
as the input and output ports, respectively, and they are numbered 
1 ,2, ... from top to bottom. Input port 8 of the parent AWG was 
used as the multildemultiplexer input port, and every output port 
was connected to either the input or output port of AWGk as fol- 
lows Output port 2k-1 was connected to input port 16 of AWGk 
through optical fibres. Output ports 2k (k = 1, ..., 8) were con- 
nected to output ports 20, 20, 17, 19, 16, 16, 13, and 18 of AWGk 
(k  = 1, ..., X), respectively, through optical fibres. 
s 
16 ports 
m 
Fig. 1 Sclzematic dianrrrm of 320-channel mulfi/demulriplexer 
rn 
Fig. 2 Optical frequency relation berween prisshandr of parent A WG 
ond subsidiary A WG with input ports 8 a d  16 for light incidence 
Gaussian passbands obtained at different diffraction orders 
from output port j (= 1, 2, ..., 16) of the parent AWG, input port 8 
of which is used for light incidence, appear repeatedly at FSR,, = 
160GHz intervals and the group is named n,. Changing the ont- 
put port shifts the group by an integer multiple of Avp = IOGHz, 
as shown in the upper part of Fig. 2. Flat-top passbands F, (m = 
'1, 2, ..., N,) obtained from all N, = 32 output ports of AWGk, 
input port 16 of which is used for light incidence, have a hand- 
width of B y ,  = 8OGHz and are arranged at AV$ = IOOGHz inter- 
vals, as shown in the lower part of the Figure. Each passband is 
used to block unwanted passbands in n,. 
Principle of operation: The passbands obtained from N, output 
ports of AWGk in the multildemultiplexer are given by mnltiply- 
ing n2x.l with F,,, (m = 1, 2, ..., YJ. Under the condition that the 
bandwidth SW, < FSR,, one Gaussian passband at most in I&,, 
is allowed to pass through every Fm. The number of unblocked 
and available passbands from AWGk is N, x AvJFSR, = 20 and 
thus a group of 160 Gaussian passbands is obtained from all the 
output ports of AWGk (k  = I ,  2, ..., 8). Since the cyclic condition 
of N, x AV$ = FSRs is approximately satisfied for AWGk, the free 
spectral range of which is FSR,, almost the same passbands as Fm 
(m = 1, 2, ..., N,) are obtained from its N, input ports when an 
output port other than 16 is used for light incidence. Therefore, 
another group of 160 passbands is obtained from all input ports of 
AWGk (k = 1, 2, ..., 8). The bandwidth and cyclic conditions 
ensure that the two groups are combined to give 320 Gaussian 
passbands arranged at AV,, intervals. 
There is at least one output port m in AWGk for every value of 
k, such as output port 20 of AWGl as shown in Fig. 2, the pass- 
1334 ELECTRONICS LETTERS 5th August 1999 Vol. 35 No. 16 


Grating formation in BGG31 glass by UV exposure

Potassium and silver ion-exchanged dual-core glass waveguides with gratings',

http://authors.library.caltech.edu/746/1/PROel99.pdf

Grating formation in BGG31 glass by UV exposure

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