Broadband picometer-scale resolution on-chip spectrometer with reconfigurable photonics

Miniaturization of optical spectrometers is important to enable spectroscopic analysis to play a role in in situ,or even in vitro and in vivo characterization systems.However,scaled-down spectrometers generally exhibit a strong trade-off between spectral resolution and operating bandwidth,and are often engineered to identify signature spectral peaks only for specific applications.In this paper,we propose and demonstrate a novel global sampling strategy with distributed filters for generating ultra-broadband pseudo-random spectral responses.The geometry of all-pass ring filters is tailored to ensure small self-and cross-correlation for effective information acquisition across the whole spectrum,which dramatically reduces the requirement on sampling channels.We employ the power of reconfigurable photonics in spectrum shaping by embedding the engineered distributed filters.Using a moderate mesh of MZls,we create 256 diverse spectral responses on a single chip and demonstrate a resolution of 20 pm for single spectral lines and 30 pm for dual spectral lines over a broad bandwidth of 115 nm,to the best of our knowledge achieving a new record of bandwidth-to-resolution ratio.Rigorous simulations reveal that this design will readily be able to achieve single-picometer-scale resolution.We further show that the reconfigurable photonics provides an extra degree of programmability,enabling user-defined features on resolution,computation complexity,and relative error.The use of SiN integration platform enables the spectrometer to exhibit excellent thermal stability of±2.0℃,effectively tackling the challenge of temperature variations at picometer-scale resolutions.

Advances in cost-effective integrated spectrometers

The proliferation of Internet-of-Things has promoted a wide variety of emerging applications that require compact,lightweight,and low-cost optical spectrometers.While substantial progresses have been made in the miniaturization of spectrometers,most of them are with a major focus on the technical side but tend to feature a lower technology readiness level for manufacturability.More importantly,in spite of the advancement in miniaturized spectrometers,their performance and the metrics of real-life applications have seldomly been connected but are highly important.This review paper shows the market trend for chip-scale spectrometers and analyzes the key metrics that are required to adopt miniaturized spectrometers in real-life applications.Recent progress addressing the challenges of miniaturization of spectrometers is summarized,paying a special attention to the CMOS-compatible fabrication platform that shows a clear pathway to massive production.Insights for ways forward are also presented.

Quantum dot mode-locked frequency comb with ultra-stable 25.5 GHz spacing between 20℃ and 120℃

Semiconductor mode-locked lasers(MLLs)are promising frequency comb sources for dense wavelength-divisionmultiplexing(DWDM)data communications.Practical data communication requires a frequency-stable comb source in a temperature-varying environment and a minimum tone spacing of 25 GHz to support high-speed DWDM transmissions.To the best of our knowledge,however,to date,there have been no demonstrations of comb sources that simultaneously offer a high repetition rate and stable mode spacing over an ultrawide temperature range.Here,we report a frequency comb source based on a quantum dot(QD)MLL that generates a frequency comb with stable mode spacing over an ultrabroad temperature range of 20–120℃.The two-section passively mode-locked In As QD MLL comb source produces an ultra-stable fundamental repetition rate of 25.5 GHz(corresponding to a 25.5 GHz spacing between adjacent tones in the frequency domain)with a variation of 0.07 GHz in the tone spacing over the tested temperature range.By keeping the saturable absorber reversely biased at-2 V,stable mode-locking over the whole temperature range can be achieved by tuning the current of the gain section only,providing easy control of the device.At an elevated temperature of 100℃,the device shows a 6 d B comb bandwidth of 4.81 nm and 31 tones with>36 d B optical signal-to-noise ratio.The corresponding relative intensity noise,averaged between 0.5 GHz and 10 GHz,is-146 d Bc∕Hz.Our results show the viability of the In As QD MLLs as ultra-stable,uncooled frequency comb sources for low-cost,large-bandwidth,and low-energy-consumption optical data communications.

Iterative photonic processor for fast complexvalued matrix inversion

An N×N iterative photonic processor is proposed for the first time, we believe, for fast computation of complexvalued matrix inversion, a fundamental but computationally expensive linear algebra operation. Compared to traditional digital electronic processing, optical signal processing has a few unparalleled features that could enable higher representational efficiency and faster computing speed. The proposed processor is based on photonic integration platforms–the inclusion of Ⅲ-V gain blocks offers net neutral loss in the phase-sensitive loops. This is essential for the Richardson iteration method that is adopted in this paper for complex linear systems. Wavelength multiplexing can be used to significantly improve the processing efficiency, allowing the computation of multiple columns of the inverse matrix using a single processor core. Performances of the key building blocks are modeled and simulated, followed by a system-level analysis, which serves as a guideline for designing an N×N Richardson iteration processor. An inversion accuracy of>98%can be predicted for a 64×64 photonic processor with a>80times faster inversion rate than electronic processors. Including the power consumed by both active components and electronic circuits, the power efficiency of the proposed processor is estimated to be over an order of magnitude more energy-efficient than electronic processors. The proposed iterative photonic integrated processor provides a promising solution for future optical signal processing systems.

Bridging the gap between resonance and adiabaticity:a compact and highly tolerant vertical coupling structure

We present a compact,highly tolerant vertical coupling structure,which can be a generic design that bridges the gap between conventional resonant couplers and adiabatic couplers for heterogeneously integrated devices.We show insights on relaxing the coupler alignment tolerance and provide a detailed design methodology.By the use of a multisegmented inverse taper structure,our design allows a certain proportion of the odd supermode to be excited during the coupling process,which simultaneously facilitates high tolerance and compactness.With a total length of 87μm,our coupler is almost threefold shorter than the state-of-the-art alignment-tolerant adiabatic couplers and outperforms them by demonstrating a more than 94%coupling efficiency(for<0.3 d B coupling loss)with±1μm misalignment tolerance,which,to our best knowledge,is a new record for III-V-on-silicon vertical couplers.Furthermore,our design has high tolerance to fabrication-induced structural deformation and ultrabroad bandwidth.These features make it particularly suitable for building densely integrated III-V-onsilicon photonic circuits with commercially available microtransfer printing assembly tools.The proposed design can be widely adopted in various integration platforms.