Silicon photonics is considered an enabling technology for the development of high-performance photon-pair sources for quantum information applications. However, the substantially higher pump intensity and narrow wavelength separation between the photon-pairs and the optical pump impose stringent requirements that lie beyond the capabilities of state-of-the-art silicon spectral filters.
In this invited talk we will present an overview of our recent advances in the field of waveguide Bragg gratings for photonic noise reduction in silicon photon-pair sources, including different strategies harnessing subwavelength and modal engineering to overcome the bandwidth-rejection trade-off in state-of-the-art implementations.
Since the proof of concept of Photonic Crystal Fibers (PCF) by Knight et al., their development over the last two decades has led to progressive enlargement of core sizes while maintaining a transverse single-mode operation enabling power scaling in fiber lasers and amplifiers by pushing further the nonlinear effects and damage thresholds. Numerous fiber designs and laser/amplifier architectures have been investigated in order to make the most of the PCF technology and mainly to mitigate a new deleterious phenomenon responsible of beam quality degradation, the Transverse Mode Instability (TMI), which arose in parallel of the high average powers reached with those fibers. In this context, our research group has developed a PCF, so called Fully-Aperiodic Large-Pitch Fibers (FA-LPF) which proved its relevance with passive as well as active fibers, manufactured with the powder sintering technology known as REPUSIL. In this work, the refractive index of the FA-LPF core is slightly lower than that of the background cladding material (Δn ~ -5x10-5). This depressed-index core feature enables a thermal resilience ensuring an effective single-mode propagation above a certain average power for core size as high as 110µm. Experimental results in amplifier set-up with a 110 µm Yb-doped depressed core FA-LPF led to 110W of amplified signal for 300W of pump with a M² < 1.3. No TMI phenomenon was observed even at maximum pump power despite the average power and the very large mode area involved.
In this presentation, we will introduce the quantum equivalent of white-light interferometry.
However, instead of a classical light source with thermal or poissonian statistics, we use energy-time entangled photon pairs. This provides one with several important advantages, which are highly relevant for high-accuracy measurements of chromatic dispersion:
1.) The interferometer does not have to be balanced, which improves the set-up time, especially when comparing different fibres.
2.) Two data fitting parameters are cancelling out, which reduces systematic errors significantly.
3.) The wavelength at which dispersion is measured is not extracted from the data, but is rather inferred with arbitrary precision using a wavemeter or an atomic reference.
4.) Twice as many interference fringes are observed for the same spectral bandwidth, allowing to measure dispersion in standard telecom fibres down to about 3 cm with a 140 nm bandwidth source.
After introducing the concept and highlighting the quantum advantages, we will demonstrate the performance of the quantum approach by comparing it to the best state-of-the-art approaches. Statistical analysis is performed by 2 times 100 measurements using either technique. In terms of precision, the quantum (classical) approach achieves a 1-sigmal precision of 21 fs/nm/km (51 fs/nm/km). In addition, the classical approach presents a systematic error of 12 fs/nm/km, which is unlikely to occur using the quantum approach, as the related fitting parameters cancel out automatically.
In summary, we believe that combining fundamental and conceptual advantages enabled by quantum optics is a promising approach for the future development of applications requiring precise and accurate measurements.
Bragg filters stand as a key building blocks of the silicon-on-insulator (SOI) photonics platform, allowing the implementation of advanced on-chip signal manipulation. However, achieving narrowband Bragg filters with large rejection levels is often hindered by fabrication constraints and imperfections. Here, we present a new generation of high-performance Bragg filters that exploit subwavelength and corrugation symmetry engineering to overcome bandwidth-rejection trade-off in state-of-the-art implementations. We experimentally show flexible control over the width and depth of the Bragg resonance, unlocking new tools for the implementation of notch filters with arbitrary bandwidth and rejection level. These results pave the way for the implementation of high-performance on-chip wavelength filters with a great potential for nonlinear-based applications, e.g. next generation Si-based photon-pair sources for quantum photonic circuits.
Silicon photonics is considered an enabling technology for next generation datacom applications, providing ultra-compact and high-bandwidth transceivers that are cost-effectively fabricated at the existing CMOS facilities. Among photonic devices developed in silicon, Bragg gratings are routinely used for the realization of key functionalities including wavelength filtering, dispersion engineering and sensing. However, the realization of Bragg filters that simultaneously provides narrowband operation and high rejection remains a challenge in the Si platform. Indeed, the small core size of Si wires, together with the high index contrast between the silicon and the oxide cladding results in a strong interaction of the optical mode with the Bragg structure. Several approaches have been proposed to implement narrowband Bragg filters in Si wires including ultra-small corrugations (a few nanometres), periodic claddings, sub-wavelength engineering or inter-mode coupling. Nevertheless, these filters typically have comparatively weak light rejection performance due to fabrication errors limiting the accurate control of the grating geometry over few millimeter-long waveguide structures.
In this work, we present a novel waveguide Bragg grating geometry that leverages the large index contrast between Si and air in membrane waveguides to overcome these limitations, yielding both narrow bandwidth and high rejection ratio. We use a novel waveguide corrugation geometry that radiates out the higher order modes, allowing effective single-mode operation for micrometric fully etched membrane waveguides. The high mode confinement of these waveguides results in weak interaction with the sidewall corrugation, thus narrowband operation is achieved. On the other hand, the high rejection ratio is achieved by combining reflection and radiation effects within the Bragg resonance. Based on this concept, we designed and experimentally demonstrated notch filters in single-etch suspended Si waveguides with cross-sections as large as 0.5 µm (height) by 1.1 µm (width). We show a narrow bandwidth of 4 nm for a 500 nm wide corrugation, with a high rejection ratio exceeding 50 dB for a filter length of only 700 µm
The silicon-on-insulator (SOI) platform allows for a miniaturization of optical elements at the micron size. It is now a mature technology, with high quality material and well-known fabrication processes. Another advantage stems in its compatibility with the CMOS facilities. The SOI platform is used already for numerus applications such as datacom, sensing or manipulation of quantum objects. Bragg filters are often used for on-chip the rejection of pump lasers. They can also be used for sensing purposes. By periodically modulating a standard waveguide width, it is possible to realize a 1-D photonic crystal with a forbidden wavelength band. In principle, the bandwidth and central wavelength of this bandgap can be tailored just by a proper design of the introduced corrugation. However, the very large index contrast of Si-wires makes the realization of narrowband rejection filters a technological challenge, requiring multiple etching steps or corrugation widths of a few tens of nanometers. Sub-wavelength nanostructuration of Si waveguides has shown to allow narrowband operation with a single-etch process, but reported rejection levels remained limited.
Here we present an innovative differential corrugation approach that allows the realization of narrowband rejection optical filters with relaxed fabrication constraints. By sub-wavelength engineering of the waveguide geometry we experimentally demonstrated simultaneous high rejection of 50dB and narrowband operations less than 3nm.
We propose new subwavelength designs based on subwavelength structures having two subperiods in each Bragg period. We also investigate the equivalent asymmetric structure to reduce the index contrast in the periods and further reduce the bandwidth without adding any new fabrication constraints. We have fabricated the sub-wavelength engineered filters in standard SOI wafer with a 220 nm thick Si guiding layer and bottom oxide layer of 2 µm. We have used electron beam lithography with 5 nm step-size and have patterned the structurse by dry etching with an inductively coupled plasma etcher. Finally, we have covered the devices with PMMA to provide symmetric cladding.
We report results showing that subwavelength Bragg filter geometries allow a drastic reduction of the operating optical bandwidth to the 0.6nm-2.5nm range if compared with regular Bragg filters (20 nm) while retaining still a strong rejection level of around 40dB. Similarly, each asymmetric version was observed to be bandwidth narrower.
To sum up, this paper is an investigation of advanced SOI waveguide Bragg mirrors. We report that the use of subwavelength corrugations and a judicious of waveguide Bragg asymmetry allow to push the extinction ratio/operating bandwidth beyond its traditional limit.
We report an original all-optical synchronization scheme suitable for a quantum relay based experiments at telecom wavelengths. After discussing the entangled photon sources’ performances, we validate our scheme by performing a two-photon interference at the relay station.
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