APPLICATION OF OPTOTHERMAL OPTICALLY BISTABLE DEVICES TO TELECOMMUNICATIONS SWITCHING

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APPLICATION OF OPTOTHERMAL OPTICALLY BISTABLE DEVICES TO TELECOMMUNICATIONS

SWITCHING

G. Buller, C. Paton, S. Smith, A. Walker

To cite this version:

G. Buller, C. Paton, S. Smith, A. Walker. APPLICATION OF OPTOTHERMAL OPTICALLY

BISTABLE DEVICES TO TELECOMMUNICATIONS SWITCHING. Journal de Physique Collo-

ques, 1988, 49 (C2), pp.C2-333-C2-336. �10.1051/jphyscol:1988279�. �jpa-00227696�

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APPLICATION OF OPTOTHERMAL OPTICALLY BISTABLE DEVICES TO TELECOMMUNICATIONS SWITCHING

G.S. BULLER, C.R. PATON, S.D. SMITH and A.C. WALKER Department of Physics, Heriot-Watt University, Riccarton, GB-Edinburgh EH14 4AS, Scotland, Great-Britain

Abstract

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A 1 x 4 spatial switch network, based on bistable nonlinear interference filters and optically controlled route selection, has been demonstrated.

Development of similar devices operating with near-ir diode-laser sources is also described.

1

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INTRODUCTION

In previous work /I/, we demonstrated the use of an optothermal bistable nonlinear

interference filter (NLIF) as an optically controlled 1 x 2 spatial switch - routing video signals between two possible output fibres (see Fig. 1). This utilised the property exhibited by this type of bistable Fabry-Perot resonator, of switching between highly reflecting and highly transmitting states. By using an angled NLIF the change in state can be exploited to achieve data-transparent switching of optical data streams between two output channels with 0.1

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1 THz transmission bandwidth. Our initial experimental

demonstration was performed using 514 nm radiation. To progress this concept further several advances are required: including, operation at near-ir wavelengths using diode-laser sources and the development of extended switching network schemes.

FIBRE. OPTICAL SWITCH- ARGON ION [ MODULATOR

I A

LASER

L

_I

I

DRIVER PULSE

GENERATOR 3ETECTORS

TV CAMERA

Fig. 1.

optical

optically controlled 1 x 2 spatial switch for routing video signals between fibres.

ZnSe/ThF, NLIFs can be operated as BEAT devices /2/ at infrared wavelengths, where there is little absorption within the dielectric materials making up the multilayer structure, by adding, for example, aluminium or silicon absorbing layers.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988279

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C2-334 JOURNAL DE PHYSIQUE

Figure 2 shows the effect of using isolated, fully-absorbing, A1 patches (65 nm thick).

As the area of A1 :is increased the switch power rises, as a result of transverse conduction within the metal layer and the consequent increase in effective area of the device. The ideal arrangement is to match the absorber dimensions to the optimised optical spot size

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¹⁰p in these experiments. Clearly if these devices are to be employed as spatia:l switches thinner, semi-transparent absorbing layers must be used and this further reduces lateral conduction. The use of an independently controllable absorbing layer permits selection of the highest overall transmission through the switch consistent with the maximum acceptable switch power.

I

50 100 1 5 0 XI0 250 300

DIAMETER OF PIXELATED ABSORBER (pm)

Fig. 2. Showing the effect of using isolated absorbing A 1 pixels.

Figure 3 shows an example of simultaneous reflection/transmission characteristics obtained using diode laser illumination (835 nm) with a 90% absorbing A1 layer ('L 10 nm thick) on a ZnSe/ThF, NLIF

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G:L~SS.~(HL).~(HH).~(LH).A~. Switching powers down to 8 mW have been observed and lower operating powers (e.g.

<

1 mW) are expected for small (z 5 p) isolated devices including thin insulating layers. With such pixellated devices, together with carefully matched reflecting stacks forming the high finesse cavity, very much higher contrast is expected ( t 20 dB).

d

⁰ ¹⁰ ²⁰ ⁰ ¹⁰ ²⁰

I-

INCIDENT POWER (mW) INCIDENT POWER (mW)

Fig. 3. Transmission m d reflection characteristics for 90% 'absorbing BEAT using diode laser illumination.

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in advance of the high-bandwidth data (see Fig. 4). Figure 5 shows the simpler 1 x 4 switch currently being studied. It consists of three NLIF bistable elements, each biased close to switch point, either through the input channel (switch X) or by independent beams via holographic focussing lenslets (Y and 2). In these networks the NLIFs have relatively slow responses (z 1 ms) and consequently act as linear components for the high frequency data. However, millisecond control signals can induce switching between their reflecting and transmitting states. These control signals can be in the form of coded header pulses.

By setting up the network such that the later gates require less energy to switch them (e.g. by adjusting the holding beam power), then the routing information can be encoded by varying the energy (peak power or duration) of the pulses in the header sequence. The finite switching time of each gate can be exploited to ensure that the switch pulse energy, corresponding to a particular gate, that is passed on to subsequent (more sensitive) gates, is insufficient to induce further switching. Our demonstration 1 x 4 switch is only a 2-layer network and consequently requires just two control pulse. Figure 6 shows the response of each output to the four alternative control pulse combinations.

Although both contrast and throughput are poor (no attempt has been made to optimise these) the basic concept of an all-optical bistable routing network has been

demonstrated.

OUT

DATA HEADER

&

OUT

OPTICAL HOLDING BEAMS

Fig. 4. A self-routing 1 x 8 data-transparent switch using three optically bistable switches (a.b, and c).

Pig. 5. Demonstration 1 x 4 spatial switch network with optically controlled route selection.

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C2-336 JOURNAL DE PHYSIQUE

5 m s / d l v .

Fig. 6. Inputs and corresponding outputs of 1 x 4 switch as shown i n Figure 5 .

The next step is to fabricate a high contrast, high transmission, spatial switch for operation at diode-laser wavelengths. The various design options to be studied includ the possibility of minimising signal loss by using different wavelengths for the bias/control and the signals.

ACKNOWLEDGEMENTS

Valuable assistance has been provided by Dr. J.G.H. Mathew, Dr. M. Saoudi,

Dr. W. Manookian and Mr. I. Redmond. This work has been partially supported by Unisy Ltd.

REFERENCES

/1/ Paton, C.R., Smith S.D. and Walker, A.C., Proc. OSA Topical Meeting on Photonic Switching (Lake Tahoe 19871, Springer Verlag (1987).

/2/ Walker, A.C., Opt. Commun.,

59

(1986) 145.