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Two algorithms of data processing are proposed to shorten the unmeasurable dead-zone close to the zero-position of measurement, i.e., the minimum working distance of a dispersive interferometer using a femtosecond laser, which is a critical issue in millimeter-order short-range absolute distance measurement. After demonstrating the limitation of the conventional data processing algorithm, the principles of the proposed algorithms, namely the spectral fringe algorithm and the combined algorithm that combines the spectral fringe algorithm with the excess fraction method, are presented, together with simulation results for demonstrating the possibility of the proposed algorithms for shortening the dead-zone with high accuracy. An experimental setup of a dispersive interferometer is also constructed for implementing the proposed data processing algorithms over spectral interference signals. Experimental results demonstrate that the dead-zone using the proposed algorithms can be as small as half of that of the conventional algorithm while measurement accuracy can be further improved using the combined algorithm.

A coherent terahertz (THz) link at 200 GHz , with a variable data rate up to 11 Gbit/s, featuring a very high sensitivity at the receiver, is investigated. The system uses a quasi-optic unitravelling carrier photodiode (UTC-PD) emitter and an electronic receiver. The coherent link relies on an optical frequency comb generator at the emission to produce an optical beat note with 200 GHz separation, phase-locked with the receiver. Bit error ratio testing has been carried out using an indoor link configuration, and error-free operation is obtained up to 10 Gbit/s with a received power <2 µW.

A simple and energy-efficient photonic system to generate continuously tunable, low phase noise, sub-THz waves based on COTS components is presented. The optical scheme is based on the use of a commercial vertical cavity surface emitting laser under gain switching modulation that provides a very flat optical frequency comb generator (OFCG) with 23 modes in a 20 dB bandwidth. The laser only needs 15 dBm continuous wave radiofrequency input power and 9 mA of bias current to provide this OFCG. Two optical injection locking stages filter and amplify the two desired modes that are detected in a photodiode to produce the desired sub-THz signal at the frequency difference of these two selected modes. As an example, demonstrated is the generation of a very stable 88.2 GHz tone with lower linewidth than 10 Hz using a reference of 4.2 GHz to generate the OFCG.

The rapid development of optical frequency combs from their table-top origins towards chip-scale platforms has opened up exciting possibilities for comb functionalities outside laboratories. Enhanced nonlinear processes in microresonators have emerged as a mainstream comb-generating mechanism with compelling advantages in size, weight, and power consumption. The established understanding of gain and loss in nonlinear microresonators, along with recently developed ultralow-loss nonlinear photonic circuitry, has boosted the optical energy conversion efficiency of microresonator frequency comb (microcomb) devices from below a few percent to above 50%. This review summarizes the latest advances in novel photonic devices and pumping strategies that contribute to these milestones of microcomb efficiency. The resulting benefits for high-performance integration of comb applications are also discussed before summarizing the remaining challenges.

Network theory has played a dominant role in understanding the structure of complex systems and their dynamics. Recently, quantum complex networks, i.e. collections of quantum systems arranged in a non-regular topology, have been theoretically explored leading to significant progress in a multitude of diverse contexts including, e.g., quantum transport, open quantum systems, quantum communication, extreme violation of local realism, and quantum gravity theories. Despite important progress in several quantum platforms, the implementation of complex networks with arbitrary topology in quantum experiments is still a demanding task, especially if we require both a significant size of the network and the capability of generating arbitrary topology-from regular to any kind of non-trivial structure-in a single setup. Here we propose an all optical and reconfigurable implementation of quantum complex networks. The experimental proposal is based on optical frequency combs, parametric processes, pulse shaping and multimode measurements allowing the arbitrary control of the number of the nodes (optical modes) and topology of the links (interactions between the modes) within the network. Moreover, we also show how to simulate quantum dynamics within the network combined with the ability to address its individual nodes. To demonstrate the versatility of these features, we discuss the implementation of two recently proposed probing techniques for quantum complex networks and structured environments.

Optical frequency combs, a revolutionary light source characterized by discrete and equally spaced frequencies, are usually regarded as a cornerstone for advanced frequency metrology, precision spectroscopy, high-speed communication, distance ranging, molecule detection, and many others. Due to the rapid development of micro/nanofabrication technology, breakthroughs in the quality factor of microresonators enable ultrahigh energy buildup inside cavities, which gives birth to microcavity-based frequency combs. In particular, the full coherent spectrum of the soliton microcomb (SMC) provides a route to low-noise ultrashort pulses with a repetition rate over two orders of magnitude higher than that of traditional mode-locking approaches. This enables lower power consumption and cost for a wide range of applications. This review summarizes recent achievements in SMCs, including the basic theory and physical model, as well as experimental techniques for single-soliton generation and various extraordinary soliton states (soliton crystals, Stokes solitons, breathers, molecules, cavity solitons, and dark solitons), with a perspective on their potential applications and remaining challenges.

We review the history, development, design principles, experimental operating characteristics, and specialized architectures of interband cascade lasers for the mid-wave infrared spectral region. We discuss the present understanding of the mechanisms limiting the ICL performance and provide a perspective on the potential for future improvements. Such device properties as the threshold current and power densities, continuous-wave output power, and wall-plug efficiency are compared with those of the quantum cascade laser. Newer device classes such as ICL frequency combs, interband cascade vertical-cavity surface-emitting lasers, interband cascade LEDs, interband cascade detectors, and integrated ICLs are reviewed for the first time.

Optical interferometry has emerged as a cornerstone technology for high-precision length measurement, offering unparalleled accuracy in various scientific and industrial applications. This review provides a comprehensive overview of the latest advancements in optical interferometry, with a focus on grating and laser interferometries. For grating interferometry, systems configurations ranging from single-degree- to multi-degree-of-freedom are introduced. For laser interferometry, different measurement methods are presented and compared according to their respective characteristics, including homodyne, heterodyne, white light interferometry, etc. With the rise of the optical frequency comb, its unique spectral properties have greatly expanded the length measurement capabilities of laser interferometry, achieving an unprecedented leap in both measurement range and accuracy. With regard to discussion on enhancement of measurement precision, special attention is given to periodic nonlinear errors and phase demodulation methods. This review offers insights into current challenges and potential future directions for improving interferometric measurement systems, and also emphasizes the role of innovative technologies in advancing precision metrology technology.

Optical frequency combs emerge as a promising technology that enables highly sensitive, near-real-time spectroscopy with a high resolution. The currently available comb generators are mostly based on bulky and high-cost femtosecond lasers for dense comb generation (line spacing in the range of 100 MHz to 1 GHz). However, their integrated and low-cost counterparts, which are integrated semiconductor mode-locked lasers, are limited by their large comb spacing, small number of lines and broad optical linewidth. In this study, we report a demonstration of a III-V-on-Si comb laser that can function as a compact, low-cost frequency comb generator after frequency stabilization. The use of low-loss passive silicon waveguides enables the integration of a long laser cavity, which enables the laser to be locked in the passive mode at a record-low 1 GHz repetition rate. The 12-nm 10-dB output optical spectrum and the notably small optical mode spacing results in a dense optical comb that consists of over 1400 equally spaced optical lines. The sub-kHz 10-dB radio frequency linewidth and the narrow longitudinal mode linewidth (<400 kHz) indicate notably stable mode-locking. Such integrated dense comb lasers are very promising, for example, for high-resolution and real-time spectroscopy applications. Optical-frequency combs: long cavity leads to dense comb An integrated mode-locked laser that can generate a dense optical comb with 1400 narrow lines is promising for high-resolution spectroscopy. An emerging technology, optical frequency combs enable highly sensitive, near-real-time spectroscopy with high resolution, but they use bulky and costly femtosecond lasers as sources. Now, Zhechao Wang and co-workers from Ghent University in Belgium and Eindhoven University of Technology in The Netherlands have demonstrated a compact, low-cost alternative laser for generating optical frequency combs. They use a long, low-loss silicon spiral waveguide to realize a long laser cavity. This allows the laser operate with a record-low repetition rate of 1 gigahertz, which in turn enables a large number of narrow lines to be generated within a 12-nm wide comb. This laser has the potential to be used in cost-sensitive applications such as mobile spectroscopic analysis.

In this work, we demonstrate a tunable optical frequency comb (OFC) source based on a cascaded frequency modulator (FM) and two Mach–Zehnder modulators (MZMs). The setup includes one FM and two MZMs, and a sinusoidal RF signal source that directly drive all these modulators. A Flat OFC source with a high number of comb lines, and tunable frequency spacing and center wavelength is analytically modelled and simulated. The results reveal that 51 comb lines with a frequency spacing of 25 GHz are generated when only FM is used. Thirteen of these lines have power variations of 1 dB. Next, by cascading FM with two MZMs, 127 comb lines are obtained. In addition, 101 of these lines have power variations of 1 dB. An optical frequency comb, with tunable frequency spacing ranging from 10 to 40 GHz is successfully generated. Moreover, the center wavelength of the generated OFC can be tuned from 1310 to 1610 nm.