Tailorable ITO thin films for tunable microwave photonic applications.

Tunability is a fundamental prerequisite for functional devices and forms the backbone of reconfigurable microwave photonic (MWP) signal processors. In this paper, we explore the use of indium tin oxide (ITO) thin films, notable for their combination of optical transparency and electrical conductivity, to provide tunability for integrated MWP devices. We study the impacts of post-thermal annealing on the structural, electrical, and optical properties of ITO films. The annealed ITO microheater maintains a low total insertion loss of just 0.1 dB while facilitating the tunability of the microring across the entire free spectral range (FSR) using less than half the voltage required by its non-annealed counterpart. Furthermore, the post-annealed ITO film exhibits a 30% improvement in response time, enhancing its performance as an active voltage-controlled microheater. Leveraging this advantage, we employed the post-annealed device to demonstrate continuous tunable radio frequency (RF) phase shifts from 0-330° across a frequency range spanning 15 GHz to 40 GHz with only 5.58 mW of power. The flexibility in modifying the ITO thin film properties effectively bridges the gap between achieving low-loss and high-speed thermo-optic based microheaters.

On-chip simultaneous measurement of humidity and temperature using cascaded photonic crystal microring resonators with error correction.

We present the design, fabrication, and characterization of cascaded silicon-on-insulator photonic crystal microring resonators (PhCMRRs) for dual-parameter sensing based on a multiple resonances multiple modes (MRMM) technique. Benefitting from the slow-light effect, the engineered PhCMRRs exhibit unique optical field distributions with different sensitivities via the excitation of dielectric and air modes. The multiple resonances of two distinct modes offer new possibilities for enriching the sensing receptors with additional information about environmental changes while preserving all essential properties of traditional microring resonator based sensors. As a proof of concept, we demonstrate the feasibility of extracting humidity and temperature responses simultaneously with a single spectrum measurement by employing polymethyl methacrylate as the hydrophilic coating, obtaining a relative humidity (RH) sensitivity of 3.36 pm/%RH, 5.57 pm/%RH and a temperature sensitivity of 85.9 pm/°C, 67.1 pm/°C for selected dielectric mode and air mode, respectively. Moreover, the MRMM enriched data further forges the capability to perform mutual cancellation of the measurement error, which improves the sensing performance reflected by the coefficient of determination (R2-value), calculated as 0.97 and 0.99 for RH and temperature sensing results, respectively.

Integrated microwave photonic phase shifter with full tunable phase shifting range (> 360°) and RF power equalization.

We report a novel microwave photonic phase and amplitude control structure based on a single microring resonator with a tunable Mach Zehnder interferometer reflective loop, which enables the realization of a continuously tunable microwave photonic phase shifter with enhanced phase tuning range while simultaneously compensating for the RF power variations. The complimentary tuning of the phase and amplitude presents a simplistic approach to resolve the inherent trade-off between maintaining a full RF phase shift while eliminating large RF power variations. Detailed simulations have been carried out to analyze the performance of the new structure as a microwave photonic phase shifter, where the reflective nature of the proposed configuration shows an effective doubling of the phase range while the amplitude compensation module provides a parallel control to potentially reduce the RF amplitude variations to virtually zero. The phase range enhancement, which is first verified experimentally with a passive only chip, demonstrates the capability to achieve a continuously tunable RF phase shift of 0-510° with an RF amplitude variation of 9 dB. Meanwhile, the amplitude compensation scheme is incorporated onto an active chip with a continuously tunable RF phase shift of 0-150°, where the RF power variations is shown to be reduced by 5 dB while maintaining a constant RF phase shift.

Double-pass microwave photonic sensing system based on low-coherence interferometry.

In this Letter, we propose and experimentally demonstrate, to the best of our knowledge, a novel high-performance microwave photonic sensing system employing a reflective double-pass spectrum-slicing sensing scheme, based on low-coherence interferometry in combination with a dispersive medium. The setup is implemented by configuring a double-pass spectrum slicing sensing scheme, which significantly increases the output power level of a low-coherence optical source by approximately 12 dB to compensate for the optical loss of the system. Moreover, since the light passes through the same optical path twice, the conversion efficiency between the applied optical path difference and the dependent radiofrequency (RF) resonance shift is doubled compared to the conventional approaches. It is also possible to realize a very high resolution thanks to the broad bandwidth of the semiconductor optical amplifier (SOA) spectrum. In addition, this SOA-based scheme enables the future realization of a fully integrated sensing system. As an application example, a highly sensitive displacement sensor was investigated, and the experimental results presented a highly linear relationship between the applied OPDs and the RF frequency shifts. The proposed sensing system successfully achieved a high conversion slope of 5.56 GHz/mm and a nearly constant resolution of approximately 124 µm using a Gaussian power density spectrum.

Silicon-on-insulator microring resonator sensor based on an amplitude comparison sensing function.

A novel, highly sensitive integrated sensor based on a silicon-on-insulator microring resonator is proposed and experimentally demonstrated. To achieve a fast-response and cost-effective sensing system, the new structure establishes a linear amplitude comparison sensing function (ACSF) by monitoring the optical powers from both the through port and drop port of an add-drop microring resonator simultaneously, where the contrast of the two ports eliminates the effect of unexpected power fluctuation of the input laser on sensor performance. A highly enhanced linear relationship between the resonant wavelength shift and the ACSF value is achieved with an R-squared value over 0.99. A proof-of-concept experiment for temperature sensing demonstrates an almost constant ACSF with only ±0.9% discrepancy, while the laser power is varied between 0 dBm and -7 dBm.

Distributed optical signal processing for microwave photonics subsystems.

We propose and experimentally demonstrate a novel and practical microwave photonic system that is capable of executing cascaded signal processing functions comprising a microwave photonic bandpass filter and a phase shifter, while providing separate and independent control for each function. The experimental results demonstrate a single bandpass microwave photonic filter with a 3-dB bandwidth of 15 MHz and an out-of-band ratio of over 40 dB, together with a simultaneous RF phase tuning control of 0-215° with less than ± 3 dB filter shape variance.