Our results demonstrate how discrete solitons can be used to effectively route and switch light signals at junctions of evanescently coupled waveguide arrays. Non-planar network topologies are supported by such routing and switching elements.
Note that the waveguide arrays investigated in this survey are merely weakly coupled due to their relatively large separation (32µm), which has been chosen for experimental convenience. To allow for circuits combining several elements on the same length, as depicted in , one needs to reduce the waveguide separation, and thereby increase the coupling, which is well possible by one order of magnitude for directly written waveguides in fused silica29
We would like to emphasise the close correspondence of the numerical results and the experimental data shown in and . The only major difference between the two is the stronger localisation of the blocker beam in the simulations, which were carried out assuming continuous wave (cw) illumination (see Methods). On the other hand, the experiments are subjected to the spatio-temporal dynamics inherent to a pulsed excitation, whereby the low-power slopes of the pulse do not contribute to localisation30
. Therefore, the lower switching contrast in the experiment can be attributed to the temporal dynamics of the gate pulse propagation. To improve the performance of the junctions beyond these proof-of-principle values (~10dB for the T-junction), one can use longer gate pulses, and thereby an increased interaction window. Then, switching contrasts of ~15dB, as obtained from the simulations, can be expected. A further enhancement is possible by shifting the blocker pulse between the first waveguides of the two output branches instead of merely chopping its amplitude.
It is also insightful to have a look at the reflection loss for the signal at the junctions. Comparing to the data found in simple bends (see Supplementary Data
& Discussion, Fig. S1), one finds that considerably more light is reflected at the junctions ( and , central column). This arises from an additional topological defect caused by the branching of the array31
. However, this defect is overcome when the blocker beam is present, as the lower branch effectively vanishes for the signal. Hence, the remaining branches are equivalent to a bent array and exhibit a similarly weak reflection.
However, when it comes to building larger networks out of cascades of such elementary junctions, even small reflection losses are crucial. For instance, at the gated T-junction about 6% of the power radiates back into the input port, whereas 17% is lost in the ungated case. Such reflection losses can be mitigated by a detuning of the pivotal waveguide, as it has been numerically32
shown for bent arrays.
The propagation loss in the laser-written waveguides is in the order of 0.4dB/cm, amounting to a global loss of 60% over the sample length. However, for a networking device one would use stronger coupling29
, meaning that more junctions can be accommodated on a given propagation length. Also note, that losses as low as 0.05dB/cm have been reported for fs-laser written waveguides after thermal treatment34
It may be of interest to compare the presented laser-written waveguide array junctions with similar structures constituted by photonic crystal waveguides. For example, integrated Mach-Zehnder interferometers have been realised, with a thermal35
control of the light propagation. Such structures can be much more miniaturised; due to their enormous refractive index contrast, the light is normally confined to the µm-range, whereas the transverse dimension of our splitter is ~100µm. Interestingly, the reflection loss at a Y-splitter is predicted to be similar to the loss occurring at our devices, if the splitters are engineered accordingly37
. Even though propagation losses are much higher for photonic crystal waveguides (~8dB/cm38
), the much smaller propagation lengths (some 10…100µm per device instead of 1…10cm) more than compensate for it.
On the other hand, these remarkable properties come usually at the expense of a narrow bandwidth, whereas devices relying on directional coupling in laser written waveguides have been demonstrated to operate for spectra spanning more than an octave39
. Moreover, all photonic crystal waveguides reported so far were entirely planar. Networks constructed out of them will thereby suffer from the aforementioned disadvantageous scaling properties11
. Finally, the nonlinear response time is not limited in fused silica24
, at the cost of requiring relatively large energies for the gate pulse.
Therefore, photonic crystal devices will be superior when it is desired to realise highly miniaturised, low-loss, energy-efficient networks with a moderate number of elements for signals at a predetermined spectral range and where speed is not absolutely critical. For broadband applications as well as for very large networks, where the better scaling of 3D topologies becomes relevant12
, or when the speed of operation is more important than energy-efficiency, the waveguide array devices presented in this manuscript may provide a very useful alternative.
Notably, such routing and switching schemes in waveguide arrays may allow for a variety of further all-optical devices for communication and information processing. In two different examples are given. The left column shows an optically gatable wavelength splitter. The coupling strength between adjacent guides, and hence, the transverse velocity of signals, increases with wavelength29
. We envision a Y-junction where the high-power pulse can swap sides via curved waveguide sections. In such a 3D-device the signal may be split into its spectral components depending on the input position of the blocker pulse. A further possibility is the realisation of a (classical) all-optical controlled-NOT (CNOT) gate (right column of ). The high-power pulse controls the gate, by blocking the signal and preserving the target bit if sent into the centre of the structure () or flipping the bit if propagating in the decoupled waveguide (). Alternatively, a nonlinear CNOT gate could be constructed where the target bit is set by the control beam's power.
Proposition of other integrated all-optical devices.
In conclusion, we experimentally studied 2D all-optical routing and switching schemes in fs laser-written waveguide arrays. The investigated structures can be combined to complex non-planar photonic networks in which discrete solitons direct light signals along specific paths. The presented approach suggests a new concept of building purely optical devices which may be used for a rapid routing and processing of data in form of optical pulses. The instantaneous nature of the Kerr nonlinearity sets no fundamental limits to the achievable speed of operation.