A Ho:YAG rod amplifier is demonstrated for vortex beams with orbital angular momentum. Simulations show nice correlation with experimental data and potentially high gain. This could be used for complex modes and spatial division multiplexing.
This work investigates the dynamic and nonlinear properties of quantum dot (QD) lasers directly grown on silicon with a view to isolator-free applications. Among them, the chirp parameter, also named the αH factor, is featured through a thermally insensitive method analyzing the residual side-mode dynamics under optical injection locking. The αH at threshold is found as low as 0.32. Then, the nonlinear gain is investigated from the gain compression factor viewpoint. The latter is found higher for epitaxial QD lasers on silicon than that in heterogeneously integrated quantum well (QW) devices on silicon. Despite that, the power dependence of the αH does not lead to a large increase of the chirp coefficient above the laser’s threshold at higher bias. This effect is confirmed from an analytical model and attributed to the strong lasing emission of the ground-state transition, which transforms into a critical feedback level as high as −6.5 dB, which is ∼19 dB higher than a comparable QW laser. Finally, the intensity noise analysis confirms that QD lasers are overdamped oscillators with damping frequencies as large as 33 GHz. Altogether, these features contribute to fundamentally enhancing the reflection insensitivity of the epitaxial QD lasers. This last feature is unveiled by the 10 Gbit/s error-free high-speed transmission experiments. Overall, we believe that this work is of paramount importance for future isolator-free photonics technologies and cost-efficient high-speed transmission systems.
The precise extraction of magnetic tunnel junction parameters at device level is important for understanding the weak point and its root cause in the stack design, which allows for future developments. The related variability is also vital for a reliable memory technology. Current test methods, however, are limited either to the material level or low efficiency. In this work, a device-in-series structure is proposed that directly monitors the statistical properties of the devices. This allows for a massively parallel measurement and, in this way, permits an accurate, high-efficiency testing with the device-to-device variability embedded intrinsically. Based on this method, we studied the temperature dependence of spin-transfer torque magnetoresistive random access memory’s retention from 12 to 300 K, using a statistical domain wall switching model. The synthetic antiferromagnetic layers are more immune to the temperature change, compared with the free layer. The magnetoresistance is found to be a convex function of the temperature below 100 K, which contrasts the single-device measurements. The results show that as the temperature decreases, the domain wall shrinks and the zero-field energy barrier still increases.
We investigate the linewidth enhancement factor of 1.3 μm quantum dot lasers epitaxially grown on silicon. Both the calculation and experiment show small (<;1) linewidth enhancement factor at gain peak due to high dot uniformity. By varying the modulation p doping level, the linewidth enhancement factor at threshold can be continuously changed across zero from negative to positive.
Recent years have witnessed the proliferation of Low-power Wide Area Networks (LPWANs) in the unlicensed band for various Internet-of-Things (IoT) applications. Due to the ultra-low transmission power and long transmission duration, LPWAN devices inevitably suffer from high power Cross Technology Interference (CTI), such as interference from Wi-Fi, coexisting in the same spectrum. To alleviate this issue, this paper introduces the Partial Symbol Recovery (PSR) scheme for improving the CTI resilience of LPWAN. We verify our idea on LoRa, a widely adopted LPWAN technique, as a proof of concept. At the PHY layer, although CTI has much higher power, its duration is relatively shorter compared with LoRa symbols, leaving part of a LoRa symbol uncorrupted. Moreover, due to its high redundancy, LoRa chips within a symbol are highly correlated. This opens the possibility of detecting a LoRa symbol with only part of the chips. By examining the unique frequency patterns in LoRa symbols with time-frequency analysis, our design effectively detects the clean LoRa chips that are free of CTI. This enables PSR to only rely on clean LoRa chips for successfully recovering from communication failures. We evaluate our PSR design with real-world testbeds, including SX1280 LoRa chips and USRP B210, under Wi-Fi interference in various scenarios. Extensive experiments demonstrate that our design offers reliable packet recovery performance, successfully boosting the LoRa packet reception ratio from 45.2% to 82.2% with a performance gain of 1.8 times.
This paper proposes a new reliability evaluation method for servo systems based on accelerated degradation test. The weaknesses of systems or components are found via Failure Mode and Effect Analysis (FMEA), by which the status parameters reflecting the system degradation could be determined. Then, an accelerated degradation test (ADT) platform design is introduced, where testing, loading, measuring equipment and control method are given in detail. Based on this platform, the ADT is implemented for a permanent magnet synchronous motor (PMSM) servo system. With the degradation data of the status parameters, the Arrhenius model is introduced to verify the MTBF of the system and evaluate its reliability.
The linewidth enhancement factor (α H ) is an important parameter for semiconductor lasers. In this paper, we investigate, both theoretically and experimentally, the key parameters that affect α H of InAs/GaAs quantum dot lasers. Both dot uniformity and doping density are found to be critical in achieving small α H in quantum dot lasers. The prospects for quantum dot lasers in isolator-free and narrow linewidth applications are also discussed.
Abstract Photonic integrated circuits (PICs) have enabled numerous high performance, energy efficient, and compact technologies for optical communications, sensing, and metrology. One of the biggest challenges in scaling PICs comes from the parasitic reflections that feed light back into the laser source. These reflections increase noise and may cause laser destabilization. To avoid parasitic reflections, expensive and bulky optical isolators have been placed between the laser and the rest of the PIC leading to large increases in device footprint for on-chip integration schemes and significant increases in packaging complexity and cost for lasers co-packaged with passive PICs. This review article reports new findings on epitaxial quantum dot lasers on silicon and studies both theoretically and experimentally the connection between the material properties and the ultra-low reflection sensitivity that is achieved. Our results show that such quantum dot lasers on silicon exhibit much lower linewidth enhancement factors than any quantum well lasers. Together with the large damping factor, we show that the quantum dot gain medium is fundamentally dependent on dot uniformity, but through careful optimization, even epitaxial lasers on silicon can operate without an optical isolator, which is of paramount importance for the future high-speed silicon photonic systems.
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