Ultra Wideband Signals and Systems in Communication Engineering

Resonance conditions for a substrate-superstrate printed antenna geometry which allow for large antenna gain are presented. Asymptotic formulas for gain, beamwidth, and bandwidth are given, and the bandwidth limitation of the method is discussed. The method is extended to produce narrow patterns about the horizon, and directive patterns at two different angles.

Gain enhancement methods for printed circuit antennas

Recent advances in photonic integration have propelled microwave photonic technologies to new heights. The ability to interface hybrid material platforms to enhance light–matter interactions has led to the development of ultra-small and high-bandwidth electro-optic modulators, low-noise frequency synthesizers and chip signal processors with orders-of-magnitude enhanced spectral resolution. On the other hand, the maturity of high-volume semiconductor processing has finally enabled the complete integration of light sources, modulators and detectors in a single microwave photonic processor chip and has ushered the creation of a complex signal processor with multifunctionality and reconfigurability similar to electronic devices. Here, we review these recent advances and discuss the impact of these new frontiers for short- and long-term applications in communications and information processing. We also take a look at the future perspectives at the intersection of integrated microwave photonics and other fields including quantum and neuromorphic photonics. This Review discusses recent advances of microwave photonic technologies and their applications in communications and information processing, as well as their potential implementations in quantum and neuromorphic photonics.

Integrated microwave photonics

A new class of planar optics has emerged using subwavelength gratings with a large refractive index contrast, herein referred to as high-contrast gratings (HCGs). This seemingly simple structure lends itself to extraordinary properties, which can be designed top-down based on intuitive guidelines. The HCG is a single layer of high-index material that can be as thin as 15% of one wavelength. It can be designed to reflect or transmit nearly completely and with specific optical phase over a broad spectral range and/or various incident beam angles. We present a simple theory providing an intuitive phase selection rule to explain the extraordinary features. Our analytical results agree well not only with numerical simulations but also experimental data. The HCG has made easy fabrication of surface-normal optical devices possible, including vertical-cavity surface-emitting lasers (VCSELs), tunable VCSELs, and tunable filters. HCGs can be designed to result in high-quality-factor (Q) resonators with surface-normal output, which is promising for wafer-scale lasers and optical sensors. Spatially chirped HCGs are shown to be excellent focusing reflectors and lenses with very high numerical apertures. This field has seen rapid advances in experimental demonstrations and theoretical results. We provide an overview of the underlying new physics and the latest results of devices.

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High-contrast gratings for integrated optoelectronics

A vertical-cavity surface emitting laser (VCSEL) was invented 30 years ago. A lot of unique features can be expected, such as low-power consumption, wafer-level testing, small packaging capability, and so on. The market of VCSELs has been growing up rapidly in recent years, and they are now key devices in local area networks using multimode optical fibers. Also, long wavelength VCSELs are currently attracting much interest for use in single-mode fiber metropolitan area and wide area network applications. In addition, a VCSEL-based disruptive technology enables various consumer applications such as a laser mouse and laser printers. In this paper, the recent advance of VCSEL photonics will be reviewed, which include the wavelength extension of single-mode VCSELs and their wavelength integration/control. Also, this paper explores the potential and challenges for new functions of VCSELs toward optical signal processing

https://cnst.nctu.edu.tw/mbe08/abstract/6-13.30-14.00-koyama.pdf

Recent Advances of VCSEL Photonics

The invention provides a distributed feedback semiconductor laser which sequentially comprises an N-surface electrode layer, a substrate layer, a buffer layer, a lower waveguide layer, an active layer, an upper waveguide layer, a secondary epitaxial grating layer, an etching self-stop layer, a cladding layer, an ohmic contact layer, a passivation layer and a P-surface electrode layer from bottom to top. The cladding layer and the ohmic contact layer form a waveguide structure, and the waveguide structure is a ridge waveguide structure. An upper grating structure and a lower grating structure in the vertical direction are used for jointly feeding back mode selection. Secondary epitaxial gratings distributed near the active region are efficiently coupled with a light field so that frequencyselection and line width reduction are realized. The single reflection peak line width of the electrode sampling grating is narrow, the electrode sampling grating is fully coupled with a light field in the waveguide for feedback, the working wavelength can be further stabilized, the side-mode rejection ratio is improved, the line width factor is reduced and the laser line width is effectively narrowed.

Narrow-linewidth distributed feedback semiconductor laser and preparation method thereof

We propose a microwave photonic temperature interrogation system with high resolution and sensitivity based on temperature-to-time mapping with a simple structure. In this work, a bidirectional chirped optical signal is constructed, and the temperature information is mapped to the time interval change of the output pulses by filtering the bidirectional chirped optical signal using the transmission spectrum of a Fabry-Perot filter. Only a low-speed oscilloscope is required to extract the time interval of the output pulses in the proposed scheme, and complex data processing as well as synchronizing processes are avoided. In a proof-of-concept experiment, a high temperature interrogation sensitivity of 18.24 µs/°C and a high resolution of 0.035°C are realized, demonstrating good potential for practical applications.

Simple method for microwave photonic temperature interrogation with high resolution and sensitivity.

A single-passband microwave photonic filter with 3 dB bandwidth of 650 kHz, center frequency drift of ±50kHz over spectral range of 2–18 GHz is experimentally demonstrated based on a Brillouin dynamic grating. To the best of our knowledge, this is the highest spectral resolution single-passband MPF reported to date, with high stability, wide tunability and a simple structure.

A single-passband microwave photonic filter with sub-MHz bandwidth

We propose and experimentally demonstrate an on-chip microwave phase shifter based on Coupling-modulated Microring Resonators (CMR). CMR is a structure that connected the transmission port and upload port of the normal ring resonator. In this case, the transmission spectrum can be tuned by properly adjusting the phase shift accumulation in the connected waveguide of the CMR. A deep notch of the transmission spectrum with 16 dB extinction ratio and 3-dB bandwidth of 17 GHz is chosen to achieve the phase shifter. And a phase shift ranging from 15 to 199 degree at 15 GHz with a radio-frequency power variation more than 7 dB is realized. After applying 1V voltage on the CMR, the extinction ratio changed to 5 dB. A quasi-linear phase shift range from 35 to 177 degree at 15 GHz with a radio-frequency power variation less than 2.5 dB is obtained. In addition, the characteristic of the phase shift at 20 GHz and 25 GHz is also tested and investigated. Moreover, this experiment indicates that the CMR has potential to solve the conflict between the large range of phase shift and small radio-frequency power variation in the ring resonator based phase shifter.

Microwave photonics phase shifter with small radio-frequency power variation based on Coupling-modulated Microring Resonators

We propose an optical frequency comb (OFC) generator for ultra-flat and multiple frequency-spaced OFC. By appropriately set the amplitude and phase of radio frequency (RF) signals fed to the parallel phase modulators, an ultraflat OFC with multiple frequency spacing can be generated. Since the frequency spacing of the generated OFC can be multiplied, it can be much larger than the typical modulation bandwidth of electro-optic modulators. Previously reported ultra-flat and multiple frequency-spaced OFC generators suffer from complex and invalid configuration since they employed either more than one-stage modulators or single-stage modulators attached with other components such as optical switches, which suffer from light leakage and limited extinction ratio in practice. Compared with previously reported method, Our scheme has much simpler configuration since only single-stage parallel modulators are used. The operating conditions for generation of multiple frequency spacing and spectrum flattening are clarified. The concept is verified in numerical analysis for generation of N×25GHz-spaced OFC. Performance analysis of the proposed scheme is also investigated.

Ninghua Zhu

Papers

Narrow-linewidth distributed feedback semiconductor laser and preparation method thereof

Simple method for microwave photonic temperature interrogation with high resolution and sensitivity.

A single-passband microwave photonic filter with sub-MHz bandwidth

Microwave photonics phase shifter with small radio-frequency power variation based on Coupling-modulated Microring Resonators

Optical frequency comb generator based on interference among parallel phase modulators