B Pin plus first

control pulse A Pin plus second control pulse a). b). c).

Figure 4.9 DOS with control signal.

By adjustment of the lateral position of the data signal an unequal split of the data between the output guides is obtained in the absence of a control signal (figure 4.9a). Two control pulses are used for faster switching. The first is used to switch the signal to channel A by causing an index asymmetry and a change in power division between the output guides. A second control pulse causes a spatial mirror image in the index change due to the first control switching the signal back to channel B. The active structure is a multiquantum well GaAs/AlGaAs device grown on a GaAs substrate. The material exhibited an exciton resonance at around 828 nm and experiments were carried out using a centre wavelength of around 875 nm. Experimental results on real devices are shown in figure 4.10 [Kan’an 96],

channel B

ch anne l B

channel A channel A

0 100 200 300 *00 500 °"lo 0 10 20 30 40 50 60 70 60 90

time delay, ps hEB time delay. p» m

a). b).

Figure 4.10

Switching characteristics of DOS. a). One control pulse b). Two control pulses.

By varying the delay of the signal with respect to the control the split between output ports was seen to decay on a time scale of around 225 ps (figure 4.10a) which seems to place a limit on the switching speed of « 4 GHz. Absorption of carriers is the reason for the switching non-linearity. The resolution of the switch is determined by the separation between the two pump pulses. In this experiment 200 fs pulses were used with a separation of 9 ps (figure 4.10b) and an energy per pulse of « 9 pj. The switching window had a rise time of 2 ps which was attributed to cooling of thermalised carriers. (ii) Operating AlGaAs at photon energies at half the bandgap

A new technique which also addresses the problem of the slow relaxation was proposed and demonstrated by Villenewue [Villenewue et al 95], The trade off for this effect is the high power levels required. Directional non-linear couples using this method require powers up to 42W (pulse energy 31 pj). Lower powers were realised in this experiment by minimising the effective area of the waveguide.

(iii) Using semiconductor in a Much Zender interferometer

Placing the semiconductor in a Mach Zender has allowed switching speeds of 90 Gbit/s

to be achieved (section 4.2.4.).

4.4.2. Polymer DOSs using thermo-optics

Commercially available PMMA is used as the waveguide material for Polymer DOSs as in the thermo-optic directional couplers with standard polymer waveguide technology used to fabricate the waveguide onto silicon. The 2x2 switches are based on four equal waveguides in the formation of a symmetric x crossing (see figure 4.11).

E T 2 ’

E T E 2 ’

2

Figure 4.11 Polymer DOS.

Four heating electrodes control the device two on the input guides and two on the output guides. The heated input electrode excites the odd mode whereas the unheated input excites the even mode. The switch operates in the cross or bar state by appropriate heating of the electrodes as in table 4.2.

Heated electrodes [ Sigaal path.

El and E l’ or E2 and E2’ 1 1 to 1’ and 2 to 2’

El and E2’ or E l’ and E2 | 1 to 2’ and 2 to 1’

Table 4.2. Switching configurations of polymer DOS.

In an experiment by Kiel [Kiel et al 96] fixed heating powers of 45 mW were applied to the input side whilst the output heater power was varied. At 45 mW a digital output from the switch was obtained which can be maintained up to a heating power of 100 mW, showing good tolerance to variation in input voltage and temperature. As expected with thermal devices, the response time is again slow (« 1 ms) making them more suitable for

routing of high data rate signals rather than high speed switching. Typical values obtained by Kiel et al [Keil et al 96] when operating at k=1.35pm or 1.55pm gave a crosstalk of <-25 dB with a switching power of >45 mW showing that wavelength independence is evident. Device lengths are around 25 mm.

An alternative method for manufacturing polymer waveguides is in the use of “oversize polymer rib waveguides” as in [Moosburger et al 96]. This structure is based on a guided rib structure initially developed by Marcatili [Marcatili 74]. The guides can be implemented in semiconductor material systems and have a particularly decisive advantage in being single mode in spite of their large size and large index step between core and cladding. To fabricate the devices standard semiconductor technology can be used. Waveguides are formed by etching grooves into a silicon substrate. The polymer waveguide material is the commercially available Cyclotene 3022™ a substance originally designed for electronic applications. It has low intrinsic optical loss when operating around 1.3 pm. The switch is implemented in a typical 1x2 Y waveguide formation (see figure 4.12).

Heater

Pout 1 Pin

Pout 2

Figure 4.12 ‘Y’ junction polymer thermo-optic switch.

With unheated branch arms equal power splitting of the signal is obtained (known as the broadcast function). On heating one arm the light is switched to the opposite path (dn/dt < 0). Insertion loss for the straight through path was ~2.5 dB which compares favourably with the thermo optic switch using PMMA of 6 dB [Kiel et al 96]. The power supplied

to the heaters is relatively high approaching 100 mW when the switching characteristics maximise giving an extinction ratio of around 20 dB over a wide operating power (up to

« 2 0 0 mW ) at an ambient temperature of 25°C. Around 170 mW the extinction ratio

remains at 20 dB even up to 100°C ambient temperature. This is due to the high thermal stability of Cyclotene 3022™ which is useable up to 350°C. The polymer does exhibit wavelength characteristics and at 1.5fim the straight through path has an increased insertion loss of ~4 dB. The extinction ratio was better than 20 dB for any thermal power above ~ 190 mW.

4.5 Acousto Optic Switching

The acoustic optic filter relies on an acoustic controlling signal to change the optical properties of a material. The switch contains an acoustically generated birefringent grating by inputting an acoustic signal into the waveguide of an optical signal. Signals input to the device are flipped orthogonally with respect to TE-TM polarisation and the

signal is frequency shifted by an amount equal to the acoustic frequency. The polarisation conversion is wavelength dependent so the effect can be used for optical filtering using output polarisers [Jackel et al 96].

PBS