EXTERNAL GRATING COUPLERS ON PLANAR WAVEGUIDES AS INTEGRATED OPTICAL BISTABLE DEVICES

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EXTERNAL GRATING COUPLERS ON PLANAR WAVEGUIDES AS INTEGRATED OPTICAL

BISTABLE DEVICES

V. Briguet, J. Kramer, W. Lukosz

To cite this version:

V. Briguet, J. Kramer, W. Lukosz. EXTERNAL GRATING COUPLERS ON PLANAR WAVEG-

UIDES AS INTEGRATED OPTICAL BISTABLE DEVICES. Journal de Physique Colloques, 1988,

49 (C2), pp.C2-325-C2-328. �10.1051/jphyscol:1988277�. �jpa-00227694�

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JOURNAL DE PHYSIQUE

Colloque C2, Supplement au n06, Tome 49, juin 1988

EXTERNAL GRATING COUPLERS ON PLANAR WAVEGUIDES AS INTEGRATED OPTICAL BISTABLE DEVICES

V. BRIGUET, J. KRAMER and W. LUKOSZ

Optics Laboratory, Swiss Federal Institute of Technology, CH-8093 Ziirich, Switzerland

R Q S U ~ Q

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On dgcrit le comportement bistable du couplage d'un faisceau laser

~ r + dans des guides d'ondes plans, couplage effect& 2 l'aide d'un rgseau de diffraction dont la distance le separant de la surface du guide d'ondes varie par suite d'une dilatation thermique des deux composantes.

Abstract

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We demonstrate! bistabi+ity coupling argon laser light into a planar waveguide with an external grating coupler which is separated from the waveguide by a small air gap.

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INTRODUCTION

The 'external' grating coupler is a diffraction grating pressed against the planar waveguide in a similar way as a prism coupler. Such couplers have to the best of our knowledge, not been used in integrated optics. The couplers normally used are surface relief gratings on the waveguide. We report on a new type of optical bi- stability (OB) which occurs only with external grating couplers, but not with surface relief grating couplers. The air gap of width d(<A) between the grating and the waveguide plays an essential role in the OB mechanism.

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EXPERIMENTAL RESULTS

The experimental configuration is shown in Fig. 1. The external grating coupler was a commercially available aluminized diffraction grating with 1 / ~ = 2 4 4 2 lines/mm on a glass substrate. The waveguides were SiO -Ti0 films of thickness d -140 nm and refractive index n =I .79 on Pyrex glass substgates of refractive in8ex n =I .47. The diffraction order H=+l of the grating was used to couple He-Ne or argon faser light into the waveguide; either the TEo- or the TM -mode was excited.

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Figure 2 shows results of incoupling experiments at low input power P. We measured both the power PR reflected from the grating coupler (cf. Fig.1) and the 'diffracted' power PD, i.e., the power in the diffraction order 1=-1, as a-function of the angle of incidence a. (Instead of the angle a we use the variable N

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the 'angular detuning', which is defined below). The excitation of a guided modeOis shown by a dip in PR and PD.

Fig. 1

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Schematic of external grating coupler. F, waveguiding film; S, substrate;

P, incident power; a, angle of incidence; P', incoupled power; PR

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reflected power;

PD, power of diffracted beam in order 1=-1; E, glass substrate wlth reflection grating; d, width of air gap.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988277

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C2-326 JOURNAL DE PHYSIQUE

Fig. 2

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Incoupling at low input powers P. Reflected power PR and diffracted power PD versus angular detuning N

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for different gap widths d. h=488 nm. Left: TEo- mode, P=2 mW; right: TM -mod?, P=10 mW; A=488 nm.

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The condition for optimum incoupling is

N n _airsina

+

^IA/A, ₍₁

where N is the effective refractive index of the guided mode, nai the refractive index of air, A the wavelength, A the grating period, and 1=1,2, Ehe diffraction order. The incoupling 'resonance' has a finite angular width, which increases with increasing attenuation of the guided mode. In the experimental situation discussed here the incoupled guided mode is attenuated by interaction of its evanescent field with the metal coating of the grating. The attenuation increases with decreasing air gap width d. Therefore the incoupling curves in Fig. 2 become broader with decreasing d. We a.Lso see in Fig. 2 that the minima of the curves are shifted.

According to Eq. ( I ) , this means that the effective index N depends on d. This effect is caused by the perturbation of the guided mode by the presence of the coupler, in particular by its metal coating. The effective index shifts for the TE

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and TM -modes are of different sign; we find aN(TE )/dd>O and aN(TM )/dd<0. We us? the ang81ar det:uning variable

R

znair(sinao-sina) t8 describe the deeiation of a chosen angle of incidence a from Qts value .a permitting optimum excitation of the guided mode at large gap widths d and low lnput powers P.

With the external grating coupler and the waveguide described above we observed OB with argon laser light at input powers P>P

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the critical power being P 4 5 mW at A=488 nm. The laser beam was focused to a gpot size of about 80 pn~ on tfie grating.

Some experimental results are shown in Figs. 3-5.

Hysteresis curves of output-power versus input power P at constant angle of incidence.a, i.e., at c:onstant N

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are given in Figs. 3 and 4. By output power we mean either the reflected power

PRO

or the diffracted power PD. Parameters are: 1 .) the polarization of the excited mode (TE or TM ), 2.) the angle of incidence a, or angular detuning and 3.) the initial ga? width do, which is adjusted at low

input power P. ^C)

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In Fig. 5 we present angular hysteresis curves of output powers P and PD versus angle of incidence a

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expressed as angular detuning No

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at consBant input power P.

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(do),

2oo

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Fig. 3

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Diffracted power PD versus input power P for different initial gap widths d

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^where^(d)l<(do)2<(d ) <(d )

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TM mode at h=488 nm. The scan time in which the iRpgt p w e r g2was rampldOua an8 Apyn 8as 62 s. The angular detunings were

a) N ₀= -1 -10 and b) No= - 2 . 5 . 1 0

.

Fig. 4

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a) Reflected power P and b) diffracted power PD versus input power P, at different angular detunings

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The initial gap width do was the same in all experiments. TEo mode at h=488 nm. Scan time 62 s.

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THEORETICAL EXPLANATION

We give a brief qualitative physical explanation of the basic mechanism leading to the observed OB. The OB is of photothermal origin. The incoupled guided mode is attenuated by interaction of its evanescent field with the metal film on the grating. The metal film in the coupling region is heated by the absorbed part P = q P of the incident power P. By heat conduction, both the grating substrate and

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^{sqncg the}

width d ( < A ) of the air gap is very small

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also the waveguide and its substrate get warmer. The temperature increase AT, which is proportional to the absorbed power Pa, induces thermal expansion of the substrates of both the waveguide and the grating, and thus a buckling of their surfaces and a corresponding reduction in the width d

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C2-328 JOURNAL DE PHYSIQUE

Fig. 5

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Reflected power PR and diffracted power P versus angular detuning variablc N at constant input power P. a) P.60 mW and b) P=?S mW for TMo-mode (top) and TEo- m8de (bottom). The angle a is scanned back and forth in 142 s.

of the air gap. In the steady state d is reduced from its initial value do linearly proportional to the adsorbed power Pa, i.e. we have

where R is the thermal expansion coefficient, A the thermal conductivity (assuming that the glass substrates of waveguide and grating have the same thermal

properties), and g a dimension-free geometrical factor, which depends on the laser spot size and the thermal boundary conditions. Since I-, (d) increases with

decreasing d, positive feedback and OB occur for suitagle values of the parameters d and

.

From this explanation it is obvious that the type of OB described in tais ,ap?, cannot occur when surface relief gratings on the waveguide itself are used as grating couplers.

The mechanism of the OB observed with the external grating coupler is essentially the same as that of the prism coupler OB which we demonstrated recently /I-3/.

However, details, as for example, the form of the hysteresis curves (cf. Fig.4b), are different.

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CONCLUSIONS

We have demonstrated that 1.) an external grating can be employed as a grating coupler and 2.) t h e photothermally induced buckling of the surfaces of the grating and the waveguide cause a reduction in the width d of the air gap separating them, and thus OB. Very recently we have demonstrated OB caused by photothermally induced buckling of the mirrors in air-spaced Fabry-Perot etalons with gold or silver mirror coatings on glass or PMMA substrates /4/.

REFERENCES

/I/ W. Lukosz, P. Rirani, and V. Briguet, Opt. Lett.

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(1987) 263.

/2/ P. Pirani, V. Briguet, and W. Lukosz, 14th Congress of the Int. Commission for Optics, ICO-14, ~ugbec, Canada, 24.-28.8.1987, Proc. SPIE, Vol. 813, p.191-192.

/3/ W. Lukosz, P. Pirani, and V. Briguet, in "Photoacoustic and Photothermal

Phenomena", Springer Series in Optical Sciences, Vol. 58, P. Hess and J. Pelzl, Eds., (Springer, Berlin, 1 988 ) p. 466-469.

/4/ W. Lukosz, P. Pisani, and G. Combe (to be published)