Contents:
In each case, we compare the experimental results with those provided by the theoretical expressions of the transfer functions obtaining an excellent agreement. Experimental results for tuneable unbalanced Mach Zehnder Interferometers and finite impulse response filters. The second column shows the layout of the implemented structure, while the third and fourth columns show, respectively, the measured modulus and corresponding phase calibrated by the shortest path for the synthesized configuration where the input is in the IN port and the output is the OUT1 port.
Measured curves are displayed for different values of the coupling constants K 1 and K 2 , which are tuned by changing the injection currents to the heater elements of the input and output MZI devices of the UMZI. Changing these values alters the position of the zero in the UMZI transfer function bringing it closer or farther to the unit circle The closer the zero is to the unit circle, the deeper are the notches in the transfer function are and the higher is the phase shift step in the transfer function is ref.
Since the waveguide mesh has a limited number of cells, we were unable to implement a lattice filter by serially cascading UMZI units. However, we could implement a 3-tap transversal filter by the parallel cascade of UMZI units. The results are shown in Fig. The transversal filter is band-pass periodic, as expected, with a FSR of Changing the values of K 1 to K 4 we tuned the positions of the two zeros provided by the structure and, therefore, reconfigure its transfer function. Ring cavities find multiple applications 44 including integrators, differentiators and Hilbert transformers 33 , dispersion compensators 45 , as well as tuneable radiofrequency phase shifters, and true time delay lines The measured results correspond to different values of K 1 and K 2 , which settle the positions of the zero and the pole The IIR filter tunability, which is shown in Fig.
Hence, any MZI can be operated as a constant-amplitude phase shifter. Experimental results for 6-BUL ring resonator infinite impulse response and combined finite impulse response and infinite impulse response filters. By suitably tuning the coupling constants, one can obtain, for instance, filters with special characteristics in the modulus 42 flat passband of the phase shift 44 parabolic.
As in the previous cases, the first column shows the 7-cell hexagonal waveguide mesh configuration where each MZI device is represented by a given color depending on its activation state, the second column shows the layout of the implemented structure and the third and fourth columns show, respectively, the measured modulus and corresponding phase. In the measured results of Fig. When the phase shift is 0, then the resonances of the two cavities are located in the same frequency and the narrowest bandpass is achieved red trace. As a small phase shift is added to one of the cavities, one of the resonances is slightly displaced but there is still a considerable overlapping.
This technique is employed to broaden the response of bandpass filters providing a controlled ripple value The phase shifter flattens the spectral region in between two consecutive notches and provides two slightly parabolic phase shifts of opposed concavity in that region, which correspond to two linear group delay regions of opposed slopes. Within this region, the structure can be employed as a tuneable dispersion compensator or as a true time delay line The TBU marked with an asterisk is an example of how TBUs can be configured in order to extract non-ideal leaking due to optical crosstalk from the circuit.
This structure is employed as a building block for the implementation of special configurations such as maximally flat high-order Butterworth and Chebyshev filters 42 , Experimental results for complex double ring-loaded 6-BUL optical ring resonator filters. These include, among others, switching and broadcasting, mode combiners and splitters, and quantum logic gates. These are relevant examples of signal processing tasks that are needed in different applications and the results are shown in Fig. In addition, any phase relationship between the three output modes can be selected by proper biasing MZIs M21 and M In this case, a backward input swapper where input 1 is routed to output 3, input 2 to output 1 and input 3 to output 2.
The former examples are special cases that illustrate the application of the waveguide mesh as a programmable signal router. Experimental results for multiple input multiple output linear optic transformation devices. As a final example, Fig. We can program the mesh to implement this gate in a very compact layout.
The proposed waveguide mesh photonic processor can implement a wide variety of signal processing functionalities.
In practice, we were limited by the number of available current sources that are required to tune the TBUs, which restricted the number of configurations that we could demonstrate to More complex structures for instance, cascade UMZI lattice filters and more complex quantum logic gates than the ones demonstrated here would be possible, if the fabricated chip were to include more unit cells. This can be achieved if smaller BULs are employed, which is technically feasible.
In addition, it is essential to make the chip operation robust against departures of the TBUs from their designed values. Recent works 54 , 55 have reported both theoretical and practical solutions to overcome this limitation in CMOS-compatible silicon photonics platforms. Hybrid integration with III—V materials would be necessary in order to incorporate the optical sources and the modulator to the proposed optical core.
Future work on electronic integration is required in order to integrate the current sources. Based on our results, further increasing the number of TBUs to be integrated would require two different metal layer levels to enable on-chip electrical routing. Another important issue is related to how the input signal is directed into a specific input port of the mesh network and how a signal is directed from a port of the mesh to a photodetector.
These operations require some degree of optical interconnection that can be provided by the mesh itself provided its size is large enough. In contrast to application-specific devices, multipurpose photonic processors enable a wide variety of applications on the same chip, providing flexible and fast adaptive design topologies and circuit parameters. Fabricated in a CMOS compatible technology, multi-task processors enable high-production volume reducing the price per chip. In summary, we have designed, fabricated and demonstrated an integrated reconfigurable photonic signal processor. The chip is based on a tuneable hexagonal silicon waveguide mesh configuration where the hexagon sides are composed of two waveguides, which can be variably coupled or switched by means of a programmable TBU implemented by means of a Mach-Zehnder interferometer.
These devices have applications in a wide variety of fields including communications, biophotonics, sensing, multiprocessor interconnections, switching, and quantum information. Hence, this work represents an important step towards the realization of the new paradigm of multipurpose reconfigurable photonic processors. The device was fabricated at the Southampton Nanofabrication Centre at the University of Southampton. Another e-beam lithography and nm silicon dry etching step was performed to produce the optical waveguides. Photolithography was then performed to define isolation trench openings, followed by a deep dry etching process to etch through the top cladding, silicon overlayer, and buried oxide layer.
These trenches provided thermal isolation to adjacent devices and improved the efficiency of the heaters. A subsequent photolithography and dry etching step realized electrodes used to provide localized heating to tune the devices.
The resist was then stripped and the wafers diced into individual dies. These dies were then mounted onto PCBs and a wire bonding process was used to provide electrical connections both within the die and between the die and the PCB. We performed a static characterization of the test cell in four different dies to extract information regarding the main optical properties of the integrated waveguides. Measurements included: differential path length to characterize propagation losses, cascaded bends structures to characterize bend losses and two different cascaded and coupled MMI structures to characterize MMI insertion losses and bandwidth.
For the outer perimeter TBUs, the process consisted in injecting optical power into one of the ports of the TBU and sweeping the electrical current bias applied to one of the two heaters. This process was carried out for the 76 thermal tuners present on each of the two characterized PCBs. Altogether with resistance and output optical power, we obtained as a result the normalized coupling constant calibration curves of each TBU. Electronic supplementary material. Supplementary Information accompanies this paper at doi Change history: A correction to this article has been published and is linked from the HTML version of this paper.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. National Center for Biotechnology Information , U. Nat Commun. Published online Sep Thomson , 2 Ali Z.
David J. Ali Z. Goran Z. Author information Article notes Copyright and License information Disclaimer. Corresponding author. Received Apr 13; Accepted Jul This article has been corrected. See Nat Commun. This article has been cited by other articles in PMC.
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The grating-based couplers are, however, polarization- and wavelength-dependent and show relatively low coupling efficiencies when compared to the end-fire couplers. Integrated photonics can realize LIDAR within a footprint the size of a postage stamp, scan without moving parts, and be produced in high volume at low cost. So far we have described the basic approaches to SDM as independent, however the most recent research is looking to combine multiple approaches to achieve much higher levels of spatial channel count. Transmission schemes employing channel linearization and diagonalization based on: a DBP and b inverse and direct NFT. The problem with laser integration on silicon is that silicon has an indirect bandgap and hence is a very inefficient light emitter.
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