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1. Optimizing the sensitivity of high repetition rate broadband transient optical spectroscopy with modified shot-to-shot detection

   https://doi.org/10.1063/5.0143355  

   Hall, Siedah J.; Budden, Peter J.; Zats, Anne; Sfeir, Matthew Y. (April 2023, Review of Scientific Instruments)

   A major limitation of transient optical spectroscopy is that relatively high laser fluences are required to enable broadband, multichannel detection with acceptable signal-to-noise levels. Under typical experimental conditions, many condensed phase and nanoscale materials exhibit fluence-dependent dynamics, including higher order effects such as carrier–carrier annihilation. With the proliferation of commercial laser systems, offering both high repetition rates and high pulse energies, have come new opportunities for high sensitivity pump-probe measurements at low pump fluences. However, experimental considerations needed to fully leverage the statistical advantage of these laser systems have not been fully described. Here, we demonstrate a high repetition rate, broadband transient spectrometer capable of multichannel shot-to-shot detection at 90 kHz. Importantly, we find that several high-speed cameras exhibit a time-domain fixed pattern noise resulting from interleaved analog-to-digital converters, which is particularly detrimental to the conventional “ON/OFF” modulation scheme used in pump-probe spectroscopy. Using a modified modulation and data processing scheme, we achieve a noise level of 10−5 in 4 s for differential transmission, an order of magnitude lower than for commercial 1 kHz transient spectrometers for the same acquisition time. We leverage the high sensitivity of this system to measure the differential transmission of monolayer graphene at low pump fluence. We show that signals on the order of 10−6 OD can be measured, enabling a new data acquisition regime for low-dimensional materials. 

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2. A 28GHz 41%-PAE linear CMOS power amplifier using a transformer-based AM-PM distortion-correction technique for 5G phased arrays

   Ali, S. N.; Agarwal, P.; Baylon, J.; Gopal, S.; Renaud, L.; Heo, D. (January 2018, 2018 IEEE International Solid - State Circuits Conference - (ISSCC))

   To fulfill the insatiable demand for high data-rates, the millimeter-wave (mmW) 5G communication standard will extensively use high-order complex-modulation schemes (e.g., QAM) with high peak-to-average power ratios (PAPRs) and large RF bandwidths. High-efficiency integrated CMOS power amplifiers (PA) are highly desirable for portable devices for improved battery life, reduced form factor, and low cost. To meet simultaneous requirements for high efficiency and reasonable linearity, PAs intended for use with complex modulation are often operated in Class-AB mode [1,2]. For small input amplitude in Class-AB, the device is turned-on and has an input capacitance (Cgs) of ~(2/3)WLCox. As the input amplitude becomes large, the device turns-off for part of the RF cycle, thus reducing its effective input capacitance. This input capacitance-modulation effect creates an input-amplitude-dependent phase shift in Class-AB mode resulting in an amplitude-modulation to phase-modulation (AM-PM) distortion [2]. Consequently, it degrades linearity metrics (e.g., error vector magnitude (EVM), adjacent channel power ratio (ACPR)) in complex-modulation systems. External linearization techniques (e.g., digital pre-distortion) are often used in transmitters to meet linearity requirements, but they are complex in nature and expensive to implement. Apart from these, few works at low-GHz frequencies are reported to improve the PA's intrinsic linearity using a varactor-or PMOS-based AM-PM correction methods [1,2]. These works reduce the design overhead of external linearization systems; however, the inclusion of additional capacitive element to correct AM-PM degrades gain and efficiency, which is not optimal for mmW frequencies 

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3. Synthetic FM triplet for AM-free precision laser stabilization and spectroscopy

   https://doi.org/10.1364/OPTICA.507655  

   Kedar, Dhruv; Yao, Zhibin; Ryger, Ivan; Hall, John_L; Ye, Jun (January 2024, Optica)

   The Pound–Drever–Hall (PDH) cavity-locking scheme has found prevalent uses in precision optical interferometry and laser frequency stabilization. A form of frequency modulation spectroscopy, PDH enjoys superior signal-to-noise recovery, large acquisition dynamic range, wide servo bandwidth, and robust rejection of spurious effects. However, residual amplitude modulation at the signal frequency, while significantly suppressed, still presents an important concern for further advancing the state-of-the-art performances. Here we present a simplified and improved scheme for PDH using an acousto-optic modulator to generate digital phase reference sidebands instead of the traditionally used electro-optic modulator approach. We demonstrate four key advantages: (1) the carrier and two modulation tones are individually synthesized and easily reconfigured, (2) robust and orthogonal control of the modulated optical field is applied directly to the amplitude and phase quadratures, (3) modulation synthesis, demodulation, and feedback are implemented in a self-contained and easily reproducible electronic unit, and (4) superior active and passive control of residual amplitude modulation is achieved, especially when the carrier power is vanishingly low. These distinct merits stimulate new ideas on how we optimally enact PDH for a wide range of applications. 

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4. Programming nonreciprocity and harmonic beam steering via a digitally space-time-coded metamaterial antenna

   https://doi.org/10.1038/s41598-023-34195-8  

   Vosoughitabar, Shaghayegh; Wu, Chung-Tse Michael (December 2023, Scientific Reports)

   Abstract Recent advancement in digital coding metasurfaces incorporating spatial and temporal modulation has enabled simultaneous control of electromagnetic (EM) waves in both space and frequency domains by manipulating incident EM waves in a transmissive or reflective fashion, resulting in time-reversal asymmetry. Here we show in theory and experiment that a digitally space-time-coded metamaterial (MTM) antenna with spatiotemporal modulation at its unit cell level can be regarded as a radiating counterpart of such digital metasurface, which will enable nonreciprocal EM wave transmission and reception via surface-to-leaky-wave transformation and harmonic frequency generation. Operating in the fast wave (radiation) region, the space-time-coded MTM antenna is tailored in a way such that the propagation constant of each programmable unit cell embedded with varactor diodes can toggle between positive and negative phases, which is done through providing digital sequences by using a field-programmable gate array (FPGA). Owing to the time-varying coding sequence, harmonic frequencies are generated with different main beam directions. Furthermore, the space time modulation of the digitally coded MTM antenna allows for nonreciprocal transmission and reception of EM waves by breaking the time-reversal symmetry, which may enable many applications, such as simultaneous transmitting and receiving, unidirectional transmission, radar sensing, and multiple-input and multiple-output (MIMO) beamformer. 

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5. Power Efficient Multi-Carrier Baseband Processing for 5G and 6G Wireless

   https://doi.org/10.1109/IEEECONF51394.2020.9443544  

   Skrimponis, Panagiotis; Hadi Mirfarshbafan, Seyed; Studer, Christoph; Rangan, Sundeep (November 2020, 54th Asilomar Conference on Signals, Systems, and Computers)

   Power consumption is one of the significant challenges in millimeter wave (mmWave) systems due to the need to support wide bandwidths and large numbers of antennas. This paper explores energy efficient implementations of the baseband trans-receiver components for a multi-carrier 3GPP New Radio (NR) system. The analysis considers key components including channel selection filters, digital beamforming and FFT engines for the OFDM processing. A methodology is presented for optimizing bit widths in various components, which is critical in low power designs. Fully digital and analog beamforming architectures are also compared. Preliminary power estimates are provided using a TSMC 28 nm process for a 400 MHz system at 28 GHz similar to 5G systems today and a hypothetical 1.6 GHz system at 140 GHz for potential 6G deployment. 

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