722 OPTICS LETTERS / Vol. 21, No. 10 / May 15, 1996

2 3 2 Single-mode fiber routing switch

T. A. Birks,* D. O. Culverhouse, S. G. Farwell, and P. St. J. Russell*

Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, UK

Received January 11, 1996

An all-fiber 2 3 2 acousto-optic routing switch is described that has an insertion loss of 0.11 dB, a drive powerof 1.5 mW, and a switching time of 80 ms. The switch has a very simple construction and so has the potentialto be low in cost. 1996 Optical Society of America

Optical fiber switches connect input and output fiberstogether in a controllable way and so perform a verybasic function in telecommunications networks. Thestate of the device is determined by an electricaladdressing signal. As well as being fast, a good switchwill have small values for insertion loss, cross talk, andelectrical drive power and will be of simple constructionto minimize cost. This is particularly important whenswitches with many fiber ports are built up from arraysof 1 3 2 or 2 3 2 elements. Routing switches are usedto reconfigure a network, for example, to establish acommunications link along a certain path or to bypassa segment that has failed or is under maintenance.They can also be used in testing, to insert a successionof sources or test objects into a single measurementsystem.

There are three main technologies in use. Typicaloptomechanical switches are complex and have longswitching times of 20 ms or so, despite the maturity ofthe technology. The insertion loss of 0.5 dB or more,although it is the best available, is still large comparedwith that of other fiber components. More than a wattof electrical power is required for changing state, with astandby power of 50 mW.1 Thermo-optic switches arefaster (2 ms) and have no moving parts, but they sufferfrom greater losses (1.5 dB), worse cross talk, andhigher continuous drive powers (600 mW).2 Electro-optic switches, based on LiNbO3 for example, are fastand require little power; the penalty is a very largeinsertion loss, 5 dB or more.3

There is therefore a need for a low-cost, low-loss,low-power fiber routing switch. To this end, itmakes sense to consider all-f iber components, inwhich the light never leaves the medium of the f iber.Simplicity of construction reduces the cost, and theabsence of breaks and collimation optics minimizesthe loss. We recently developed an acousto-optic(AO) device that is based on one such all-f iber com-ponent, the single-mode fused taper coupler. Theproperties of the device as a 633-nm frequencyshifter and electrically tuned coupler with dissimi-lar ports have been described elsewhere4; here wedescribe the device’s performance as a 1550-nmoptical switch.

The coupler is a special type called a null coupler. Itis made from two fibers with diameters mismatched tothe extent that the resultant coupler does not actuallycouple any light. It is the extreme wavelength-f lattened coupler.5 Like the wavelength-f lattenedcoupler, it can be made by pretapering of one of two

0146-9592/96/100722-03$10.00/0

identical fibers along a short length before both f ibersare fused and elongated together to form the coupler.This gives a device with identical single-mode ports.Light in one input f iber excites just the fundamentalmode in the narrow waist of the coupler. Light in theother input f iber excites just the second mode in thewaist. In both cases the light propagates alongthe waist without further interactions and returns tothe original f iber at the output end of the coupler; seeFig. 1(a).

A f lexural acoustic wave propagating along the cou-pler causes a periodic refractive-index perturbation inthe waist. If a resonance condition is met (the acous-tic wavelength matches the optical beat length be-tween the two modes), light can couple between themodes. Furthermore, if the amplitude of the acous-tic wave is suitably adjusted, complete coupling is pos-sible: light enters one f iber, excites one mode in thewaist, is acousto-optically coupled to the other mode,and emerges from the other fiber at the output; seeFig. 1(b). The acoustic wave is in turn generated ina transducer by an electrical signal. Thus the cou-pler and the transducer together act as an electricallydriven 2 3 2 switch.

The switch has the advantages expected from anall-f iber device. Optically, it is a fused taper cou-pler and so can have the ultralow losses (,0.05 dB)associated with such couplers. The construction issimple, requiring no additional optical components, col-limation, or connections, with the potential for lowcost. Furthermore, the power requirement is very

Fig. 1. (a) Propagation of light through a passive nullcoupler. (b) Acousto-optic switching in the coupler.

1996 Optical Society of America

May 15, 1996 / Vol. 21, No. 10 / OPTICS LETTERS 723

small: because the light waves at the waist of a fusedcoupler f ill the whole of the waist, the acoustic and op-tical waves share the same volume. This means thatall the acoustic energy contributes to the interaction.

The wavelength of operation l is determined by thediameter 2R of the waist, which is easily controlledwhen the coupler is made. A simple analytical expres-sion for the AO resonance condition gives R in terms ofl and the acoustic frequency f , under approximationsthat are valid for the devices that we have constructed6:

R3 ­ Al2yf . (1)

The constant A is equal to 109.4 ms21 for silica f iber.The switch changes state in the time taken for theacoustic wave front to travel along the coupler waist.For a waist of length L, with R chosen to satisfy Eq. (1),this time t is given by

t3 ­ BL3ylf , (2)

where B is equal to 3.144 3 1027 m22 s2 for silica f iber.An AO switch was made as shown in Fig. 2.

Standard single-mode fiber, with a diameter of125 mm and an operating wavelength of 1550 nm, wasused. We made the coupler by stretching a pair offibers together in an oxybutane f lame. One fiber hadbeen pretapered to a diameter of 90 mm beforehand.For the chosen acoustic frequency of ,10 MHz, Eq. (1)indicates that the required waist diameter should be,6 mm. For the interaction to be on resonance alongthe whole of the coupler the waist must be uniformin diameter. We achieved uniformity and diametercontrol by using a traveling f lame as the heat source.The final coupler had a waist 25 mm long and shorttaper transitions. The excess loss of the passivecoupler was 0.08 dB, and the maximum splitting ratiowas very small, 1:17,000, indicating that it was a goodnull coupler. Thus, in the OFF state, the switch had across talk of 242 dB and an insertion loss of 0.08 dB.

The acoustic wave was generated by a piezoelectric(PZT) disk driven by a rf electrical supply. The diskwas linked to the fibers by a conical aluminum horn,which concentrates the acoustic wave at the apex. Thedisk was ,0.2 mm thick and ,6 mm in diameter, andthe horn was 5.5 mm in diameter and 3 mm high.The horn was fixed to the f ibers at some distancefrom the coupler. This allowed the taper transitionto concentrate the acoustic energy further and avoidedthe scattering loss that would result from attaching thehorn at the coupler itself, where the light is guided atthe glass–air boundary.

With light of a particular polarization state and awavelength of 1550 nm in one fiber, AO coupling wasobserved at the frequency of 10.6 MHz. Followingadjustment of the acoustic amplitude, more than 99.3%of the light could be coupled, corresponding to a crosstalk of 221.5 dB and an insertion loss of 0.11 dB forthe switch in the ON state. The rf power supplied tothe device was 1.5 mW.

With the switch in the ON state the coupled wavehad a bandpass spectrum, whereas the through wavehad a notch spectrum.4 For f ixed acoustic frequencythe FWHM bandwidth of the main resonance was6 nm. There were also small satellite resonancesspread over a range of 35 nm, which are believed

to be due to slight nonuniformities in the couplerwaist.7 The center wavelength depends on the acous-tic frequency,6 enabling the device to operate as atunable wavelength-selective switch given a suitablybroadband acoustic transducer. The response ispolarization dependent; for the orthogonal polarizationstate less than 1% of the light was coupled.4

We measured the time response of the switch bymodulating the rf drive with a slow square wave. Theresulting curves for the OFF–ON transition are givenin Fig. 3 (similar curves were obtained for the othertransition). The switch changes state in , 50 ms,which compares well with the 68 ms predicted byEq. (2). The time lag between the rf signal and thestart of switching is the time that the acoustic wavetakes to travel from the transducer through the hornand along the f ibers to the coupler. The net effect is adelay of ,80 ms between the electrical signal and thecompletion of optical switching. This switching timeis more than an order of magnitude less than that ofany established switch technology, with the exceptionof electro-optics.

These results were obtained for acoustic frequenciesnear 10 MHz. As a consequence, the switched lightis frequency shifted by 10 MHz. In a succession ofswitching elements the net frequency shift could benoticeable. However, we have obtained similar resultsfor devices driven at ,1 MHz; the net shift for a seriesof these switches is insignificant.

Our all-f iber switch outperforms all other designswith respect to the insertion loss of 0.11 dB. Thedrive power of 1.5 mW and switching time of 80 ms alsocompare favorably with those of the best commerciallyavailable switches. The construction is simple andmonolithic, the optical element being just a parti-cular type of fused coupler; although the waist is

Fig. 2. Construction of the all-f iber switch.

Fig. 3. Time variation of the optical power in each outputof the switch. The rf drive is turned on at t ­ 0.

724 OPTICS LETTERS / Vol. 21, No. 10 / May 15, 1996

very thin, couplers as narrow as 1 mm can surviveshock testing.8 There are no gross moving parts.The coupler has four identical standard single-modefiber ports by default, so there is no need for collimatingoptics and no pigtailing problems. The device shouldtherefore yield a competitive low-cost routing switch.

The Optoelectronics Research Centre is an interdisci-plinary research center funded by the UK Engineeringand Physical Sciences Research Council. T. A. Birksis a Royal Society University Research Fellow.

*Present address, School of Physics, University ofBath, Claverton Down, Bath BA2 7AY, UK.

References

1. Dicon Fiberoptics, Inc., Berkeley, Calif., product in-formation.

2. Photonic Integration Research, Inc., Columbus, Oh.,product information.

3. GEC Advanced Optical Products, Chelmsford, Essex,UK, product information.

4. T. A. Birks, S. G. Farwell, P. St. J. Russell, and C. N.Pannell, Opt. Lett. 19, 1964 (1994); 21, 231 (1996).

5. D. B. Mortimore, Electron. Lett. 21, 742 (1985).6. T. A. Birks, P. St. J. Russell, and C. N. Pannell, IEEE

Photon. Technol. Lett. 6, 725 (1994).7. H. E. Engan, B. Y. Kim, J. N. Blake, and H. J. Shaw, J.

Lightwave Technol. 6, 428 (1988).8. D. R. Campbell and D. W. Stowe, Proc. SPIE 1580, 235

(1991).