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We provide an experimental evidence of transient instabilities (TIs) in a terahertz (THz) quantum cascade laser (QCL) under optical feedback, in contrast to the widely accepted claim that THz QCLs are ultrastable against feedback. The TIs appear as periodic oscillations in emitted power or terminal voltage of the laser with an increasing oscillation frequency as feedback increases. The absence of relaxation oscillations and low linewidth enhancement factor in THz QCLs makes them a platform uniquely suitable for exploring external-cavity-related dynamics in semiconductor lasers. This work opens a pathway to a THz sensing and imaging modality based on these TIs, which has much reduced complexity compared to existing approaches using laser feedback interferometry.

In this article, we explore the interplay between the self-pulsations (SPs) and self-mixing (SM) signals generated in terahertz (THz) quantum cascade lasers (QCLs) under optical feedback. We find that optical feedback dynamics in a THz QCL, namely, SPs, modulate the conventional SM interference fringes in a laser feedback interferometry system. The phenomenon of fringe loss in the SM signal — well known in interband diode lasers — was also observed along with pronounced SPs. With an increasing optical feedback strength, SM interference fringes transition from regular fringes at weak feedback (C ≤ 1) to fringes modulated by SPs under moderate feedback (1 C ≤ 4.6), and then [under strong feedback (C > 4.6)] to a SM waveform with reduced number of fringes modulated by SP, until eventually (under even greater feedback) all the fringes are lost and only SPs are left visible. The transition route described above was identified in simulation when the SM fringes are created either by a moving target or a current modulation of the THz QCL. This SM signal transition route was successfully validated experimentally in a pulsed mode THz QCL with SM fringes created by current modulation during the pulse. The effects of SP dynamics in laser feedback interferometric system investigated in this work not only provides a further understanding of nonlinear dynamics in a THz QCL but also helps to understand the SM waveforms generated in a THz QCLs when they are used for various sensing and imaging applications.

The phenomenon of self-pulsation (SP) in terahertz (THz) quantum cascade lasers (QCLs) due to optical feedback was reported recently. In this Letter, we propose a THz imaging modality using the SP phenomenon in a THz QCL. We explore the theoretical oscillation properties of the SP scheme and demonstrate its suitability to perform imaging experimentally. The SP imaging scheme operates in self-detection mode, eliminating the need for an external detector. Moreover, the scheme requires only a fixed current, meaning that one can avoid many of the pitfalls associated with high temperature operation of THz QCLs, including frequency chirp and mode hops caused by sweeping the laser current. This also means that one is free to locate the operating point at the maximum power, to produce the desired beam profile or for highest spectral purity, depending on the application. The SP imaging modality proposed in this work can be translated directly to high operating temperature THz QCLs.

We report measurements of switch-on dynamics, mode competition and frequency selection in a monolithic frequency-tuneable laser using coherent time-domain sampling of the laser emission. We observe hopping between lasing modes on picosecond-timescales and temporal evolution of transient multi-mode emission into steady-state single mode emission.

In this paper, we have designed and proofed a widely tunable microwave photonic filter (MPF) on the basis of self-adaptive optical carrier regeneration method through stimulated Brillouin scattering (SBS) amplification. The MPF system features with a single microwave passband as the result of the processed signal sideband beating with the regenerated optical carrier. Since the Brillouin-laser optical carrier and the SBS amplification are generated from one laser source, the optical carrier regeneration process of the proposed MPF is self-adaptive and carrier-frequency independent. Moreover, by simply varying the wavelength spacing between the optical carrier and the optical filter, the central passband of the MPF can be continuously tuned from 6 to 32 GHz, which is limited by the bandwidth of the modulator and the photodetector. The 3 dB bandwidth and the out-of-band suppression ratio of the MPF can reach 640 MHz and 21 dB, respectively.

Photonic engineering of the terahertz emission from a quantum cascade laser (QCL) is fundamental for the exploitation of this unique source in a myriad of applications where it can be implemented, such as spectroscopy, imaging, and sensing. Active control of the frequency, power, polarization, and beam profile has been achieved through a variety of approaches. In particular, the active control of the emitted frequency, which is difficult to determine a priori, has been achieved through the integration of a photonic structure and/or by using external cavity arrangements. In this work, an external cavity arrangement, which implements a metamaterial/graphene optoelectronic mirror as an external feedback element, is proposed and demonstrated. The reflectivity and dispersion properties of the external active mirror were tuned via electrostatically gating graphene. It was possible to electronically reproduce the mode-switch occurring in a QCL emitting ∼2.8 THz by mechanically changing the external cavity length formed by an Au mirror. The external cavity arrangement was investigated and described in the framework of the self-mixing theory. These results open a way for the all-electronic engineering of the QCL emission by the use of a fast reconfigurable external mirror. This approach can uniquely address both power and frequency control, with ∼100 MHz reconfiguration speeds, using an integrated external element. Furthermore, the metamaterial/graphene mirror's strong dispersive properties might be implemented for the active mode locking of THz QCLs. Finally, this approach offers a unique opportunity to study the laser dynamics and mode competition in THz QCLs in the self-mixing feedback regime.

The typical modal characteristics arising during laser feedback interferometry (LFI) in multi-mode terahertz (THz) quantum cascade lasers (QCLs) are investigated in this work. To this end, a set of multi-mode reduced rate equations with gain saturation for a general Fabry-Pérot multi-mode THz QCL under optical feedback is developed. Depending on gain bandwidth of the laser and optical feedback level, three different operating regimes are identified, namely a single-mode regime, a multi-mode regime, and a tuneable-mode regime. When the laser operates in the single-mode and multi-mode regimes, the self-mixing signal amplitude (peak to peak value of the self-mixing fringes) is proportional to the feedback coupling rate at each mode frequency. However, this rule no longer holds when the laser enters into the tuneable-mode regime, in which the feedback level becomes sufficiently strong (the boundary value of the feedback level depends on the gain bandwidth). The mapping of the identified feedback regimes of the multi-mode THz QCL in the space of the gain bandwidth and feedback level is investigated. In addition, the dependence of the aforementioned mapping of these three regimes on the linewidth enhancement factor of the laser is also explored, which provides a systematic picture of the potential of LFI in multi-mode THz QCLs for spectroscopic sensing applications.

A wideband and high resolution fiber delay line system based on a self-fabricated distributed-feedback (DFB) laser is demonstrated in this article, where the delay time drift due to temperature variance is compensated using the extensibility of the standard single-mode fiber (SMF). The DFB laser with the amplitude bandwidth of 18 GHz and the in-band phase variance within 60° is applied as the optical source to provide wideband microwave signal delay. The temperature-dependence delay variance is measured up to 42.8 ps/km/°C in SMF delay line. A compensation capability of 600 ps is reached with a fiber stretcher acted on a motor-driven 300 m SMF. The proposed system not only provides delay drift compensation for the environmental-sensitive systems, but also a feasible solution to realize continuously tuning of delay for stable systems. The tunable range of the microwave delay could be further extended using longer stretched fiber. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1219–1222, 2015

A quartz-enhanced photoacoustic spectroscopy-based central wavelength stabilized distributed feedback diode laser is demonstrated in this letter. A quartz tuning fork is used as a sharp transducer to generate an error signal for wavelength tuning and stabilization without photodetector, which contributes to low cost, compact size, and freedom from ambient acoustic noises. The central wavelength of the diode laser obtains quickly fine-tuning at various modulation depths because it varies in time with the driving current. The wavelength stability of the diode laser is thus enhanced effectively. The wavelength stabilized diode laser is suited for absorption spectroscopy-based gas sensors.

Sesame (Sesamum indicum), an important oil crop, is widely grown in tropical and subtropical regions. It provides part of the daily edible oil allowance for almost half of the world's population. A limited number of co-dominant markers has been developed and applied in sesame genetic diversity and germplasm identity studies. Here we report for the first time a whole genome survey used to develop simple sequence repeat (SSR) markers and to detect the genetic diversity of sesame germplasm. From the initial assembled sesame genome, 23,438 SSRs (≥5 repeats) were identified. The most common repeat motif was dinucleotide with a frequency of 84.24%, followed by 13.53% trinucleotide, 1.65% tetranucleotide, 0.3% pentanucleotide and 0.28% hexanucleotide motifs. From 1500 designed and synthesised primer pairs, 218 polymorphic SSRs were developed and used to screen 31 sesame accessions that from 12 countries. STRUCTURE and phylogenetic analyses indicated that all sesame accessions could be divided into two groups: one mainly from China and another from other countries. Cluster analysis classified Chinese major sesame varieties into three groups. These novel SSR markers are a useful tool for genetic linkage map construction, genetic diversity detection, and marker-assisted selective sesame breeding.

We report on the high detection sensitivity of a laser feedback interferometry scheme based on a terahertz frequency quantum cascade laser (QCL). We show that variations on the laser voltage induced by optical feedback to the laser can be resolved with the reinjection of powers as low as ∼-125 dB of the emitted power. Our measurements demonstrate a noise equivalent power of ∼1.4 pW/√Hz, although, after accounting for the reinjection losses, we estimate that this corresponds to only ∼1 fW/√Hz being coupled to the QCL active region.

A dual-loop nonlinear optoelectronic oscillator with an inbuilt ultra-wideband (UWB) band pass filter is proposed to generate chaotic-UWB signals for the first time. To avoid the spectrum-overlay-induced interference between UWB signals and the other existing narrow band signals, a microwave photonics filter is built up in the loops to filter out the corresponding frequency components. In the experiment, the chaotic-UWB signals with a frequency notch response located at 7.5 GHz, 5 GHz, or simultaneous 7.5 and 5 GHz are generated as the time delay difference between the two loops is set at 133, 200, or 400 ps, respectively. The peak sidelobe levels of the autocorrelation traces are as low as -33.7 dB at least, which implies that the generated UWB signals could be applied in random UWB radar directly.

The characteristics of the typical nonlinear states in an optically injected laser system with single- and dual-beam injection are analyzed and compared numerically. Both single- and dual-beam optical injection systems can generate periodic oscillations and chaos states but with different characteristics. By observing the evolution process of periodic oscillations, it is found that dual-beam injection induced periodic oscillations have multiple time series patterns including continuous periodic signals with single peak (CPS) and continuous periodic signals with subharmonic (CPSSH), while for single-beam injection with either of the dual-beam injection parameters generated periodic oscillations, the extremum numbers of the time series equal to the frequency period numbers of the optical spectrum. In addition, the great advantages of bandwidth enhancement effect for chaos oscillations are obtained by using dual-beam injection. Furthermore, extensive numerical simulations reveal that dual-beam injection system undergoes period-doubling routes to chaos, which is identical to that in single-beam injection system. Finally, the characteristics of newly clustered NDFWM dynamics in dual-beam injection system are analyzed.

We propose a downlink broadcast transmission scheme of radio-over-fiber (RoF) system based on period-one (P1) dynamic of dual-beam optically injected semiconductor laser with tunable microwave subcarrier frequency. The transmission performance of 2.5 Gb/s data in a 60 GHz RoF system is demonstrated through numerical simulations. It is found that the proposed transmission scheme can easily generate a single-sideband (SSB) optical modulation by adjusting the injection strength level of two master lasers (ML), which is favorable to reduce the fading effect due to chromatic dispersion. Furthermore, influences of the pulse amplitudes and duty cycles of the downlink data on the transmission properties are investigated. It is observed that the oscillation of the relative power at the base station induced by fiber dispersion does not vary apparently with the modulation parameters. Once the SSB spectrum is generated by dual-beam optical injection, the downlink transmission performance of the proposed RoF system keeps good stability and reliability.

Terahertz (THz) quantum cascade lasers (QCLs) are compact sources of radiation in the 1–5 THz range with significant potential for applications in sensing and imaging. Laser feedback interferometry (LFI) with THz QCLs is a technique utilizing the sensitivity of the QCL to the radiation reflected back into the laser cavity from an external target. We will discuss modelling techniques and explore the applications of LFI in biological tissue imaging and will show that the confocal nature of the QCL in LFI systems, with their innate capacity for depth sectioning, makes them suitable for skin diagnostics with the well-known advantages of more conventional confocal microscopes. A demonstration of discrimination of neoplasia from healthy tissue using a THz, LFI-based system in the context of melanoma is presented using a transgenic mouse model. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

In this paper, major photonic microwave generation and modulation methods are described. Their advantages and limitations are identified and compared through the analysis of the key characteristics relevant to practical photonic microwave applications. The focus of this paper is on the photonic microwave based on the nonlinear dynamics of optically injected semiconductor lasers. Both single-beam optically injected semiconductor lasers and dual-beam optically injected semiconductor lasers are considered. In particular, the three dynamic states of stable injection locking, period-one oscillation, and period-two oscillation are considered and demonstrated for photonic microwave generation, modulation, and conversion applications. Several prospects for future research and development in this area are discussed.

We present a frequency-tunable RF carrier generation and broadband data upconversion technique for radio-over-fiber (RoF) systems. A dual-parallel Mach-Zehnder modulator is used to realize single-mode modulation (SMM). On the other hand, frequency-tunable RF carrier generation with single-sideband (SSB) modulation is performed using stimulated Brillouin scattering (SBS) with three types of configuration. The optical carrier-to-sideband ratio of the SMM-SSB modulated signal can be adjusted to achieve the best received sensitivity performance of the RoF system by simply modifying the pump power in the SBS process. Finally, the transmission performance of the RoF downlink system is examined. The power penalty is less than 1 dB at the bit-error rate of 10-9 after 25-km single-mode fiber transmission.

Transmission properties of data amplitude modulation (AM) and frequency modulation (FM) in radio-over-fiber (RoF) system are studied numerically. The influences of fiber dispersion and nonlinearity on different microwave modulation schemes, including double side band (DSB), single side band (SSB) and optical carrier suppression (OCS), are investigated and compared. The power penalties at the base station (BS) and the eye opening penalties of the recovered data at the end users are both calculated and analyzed. Numerical simulation results reveal that the power penalty of FM can be drastically decreased due to the larger modulation depth it can achieve than that of AM. The local spectrum broadening around subcarrier microwave frequency of AM due to fiber nonlinearity can also be eliminated with FM. It is demonstrated for the first time that the eye openings of the FM recovered data can be controlled by its modulation depths and the coding formats. Negative voltage encoding format was used to further decrease the RF frequency thus increase the fluctuation period considering their inverse relationship.

The frequency-pushing effect in single-mode diode lasers subject to single-beam injection and dual-beam injection is studied experimentally. The physical process of the frequency-pushing effect has been analyzed in detail using a medium-gain model in a single-beam injection system. A ?pushed to lock? phenomenon has been observed in dual-beam injection system and has also been analyzed using the medium-gain model. This model can also be used to explain the maximum of conversion efficiency in an injection-locked frequency convertor. First, we produce a conventional mapping of nonlinear dynamic regions, including stable locking, period doubling, chaos, undamped relaxation oscillations, and bistability, as functions of the detuning frequency and injection ratio in single-beam injection system. Then, a pushed stable locking range is observed when two masters are injected simultaneously. The direction of pushing is towards negative detuning frequencies and low injection ratios, which allows a larger detuning frequency and lower injection ratio in choosing the lasers used as pump source in injection-locked frequency convertor.

In this work, optical wavelet de-noising with several different types of wavelets such as db4, coif4 and dmey wavelet was applied at the end of the 40 Gbit/s multi-span intensity-modulated fiber communication systems. The results of numerical simulations carried out in different fiber links demonstrated that the optical wavelet de-noising method could remove the random amplitude fluctuation induced by the interaction of EDFA’s ASE noise and optical fiber’s dispersion and nonlinearity. The SNR and BER curves of the optical bit sequence without and with optical wavelet de-noising in dispersion compensation fiber link were plotted to show the effectiveness of the wavelet de-noising in the fiber-optic communication systems and wavelet de-noising with demy wavelet can achieve better result than with other type wavelet.

As a parametric amplified noise, nonlinear phase noise limits the channel capacity in multispan fiber-optics transmissions. In this paper, the reduction of nonlinear phase noise by use of Wiener filtering is studied. Extracting signal from time-domain concepts, parametric amplified noise covering signal spectrum in frequency domain can be suppressed considerably. Timing jitter and ghost pulses due to intrachannel nonlinearities can also be compensated. Nonparametric Wiener filtering is used at the end of six configurations of dispersion compensation (DC) transmission lines: pre- and postlumped DC, distributed DC without optical phase conjugation (OPC), dispersion inversion (DI) about midlink OPC with, respectively, one-for-multispan, single type span and alternate type span on each side of OPC. It is found that single-DI and alternate-DI schemes have higher input power capacity than one-for-multi-DI scheme. Further reductions in the nonlinear phase noise accumulation for all these DC maps are obtained with Wiener filtering, and higher absolute dispersion can decrease signal fluctuation due to nonlinear phase noise, as well.

Considerable interest exists for sensing and imaging technologies in the terahertz (THz) spectral range, in particular for the interrogation of materials of an organic or biological nature. Development in THz quantum cascade lasers is seeing higher operating temperatures and peak output powers in pulsed mode, accentuating their place as the preferred source of coherent THz frequency radiation. Technological development of interferometric sensing schemes continues to take advantage of practical improvements in THz quantum cascade lasers. In this Summary, we give a brief overview of some recent developments in this regard.

A new design scheme for a distributed algorithm for routing and wavelength assignment (RWA) is developed in this paper, and the communication rules between the nodes to exchange signaling packets are discussed. The Adaptive-Alternate-Routing-Least-Load (AARLL) algorithm is implemented in the distributed scheme for the first time. Under dynamic traffic circumstances, the influence of the race condition, which does not exist in centralized scheme, on the network performance is analyzed and the analyzed results show that the race condition has a major impact on network performance only under light traffic load, while under medium and heavy traffic load the impact is very small. To analyze the performance loss caused by adopting the distributed algorithm for RWA, the capacity loss factor (CLF) is introduced and the calculated results show that CLF does not exceed 6% under medium traffic load.

We develop a novel method to measure the frequency response of the optoelectronic devices. In this method, an electro-absorption modulator is utilized not only as a modulator but also as a photodetector with the identical frequency response. Based on this property, the frequency responses of the optoelectronic devices can be obtained and the results show good consistency with the measurement results by traditional methods. Instead of using broadband modulators or photodetectors, the frequency response of high bandwidth optoelectronic devices can be measured by using the low bandwidth optoelectronic devices.

A high bandwidth optical transceiver module for an analog optical link in Ku band is developed using a directly modulated distributed-feedback laser module. A bandwidth of 18 GHz can be achieved with in-band flatness less than ±3 dB. The gain can reach about -1 dB. The transceiver module performs with high linearity, with an input 1-dB compression point of 2.9 dBm and an input third-order intermodulation point of 9.8 dBm at 18 GHz. A spurious free dynamic range of about 101 dB·Hz2/3 can be achieved.

The T-matrix method, or the T-matrix formulation of scattering, is a framework for mathematically describing the scattering properties of an object as a linear relationship between expansion coefficients of the incident and scattering fields in a basis of vector spherical wave functions (VSWFs). A variety of methods can be used to calculate the T-matrix. We explore the applicability of the extended boundary condition method (EBCM) and point matching (PM) method to calculate the T-matrix for scattering by cylinders in optical tweezers and hence the optical force acting on them. We compare both methods with the discrete-dipole approximation (DDA) to measure their accuracy for different sizes and aspect ratios (ARs) for Rayleigh and wavelength-size cylinders. We determine range of sizes and ARs giving errors below 1% and 10%. These results can help researchers choose the most efficient method to calculate the T-matrix for nonspherical particles with acceptable accuracy.

Optical properties of a fluorescence molecule can be drastically changed by surface plasmons excited in neighboring metallic nanostructures. Here we investigated the fluorescence enhancement behavior of fluorescein isothiocyanate (FITC) labeled polystyrene nanoparticles coupled to silver island films (SIFs) via a 15 nm polymethyl methacrylate separation layer theoretically and experimentally. Up to 24-fold fluorescence enhancement was experimentally achieved when the annealing time of the 25 nm Ag films was 50 min, which is in good agreement with the theoretical simulation result based on the finite-difference time-domain method. Furthermore, significant fluorescence spectral distortion on SIFs was also observed compared with samples on glass slides, which is sufficiently related to the scattering properties of SIFs and the lifetimes of FITC.

A compact and highly linear quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for the measurement of water vapor concentration in the air is demonstrated. A cost-effective quartz tuning fork (QTF) is used as the sharp transducer to convert light energy into an electrical signal based on the piezoelectric effect, thereby removing the need for a photodetector. The short optical path featured by the proposed sensing system leads to a decreased size. Furthermore, a pair of microresonators is applied in the absorbance detection module (ADM) for QTF signal enhancement. Compared with the system without microresonators, the detected QTF signal is increased to approximately 7-fold. Using this optimized QEPAS sensor with the proper modulation frequency and depth, we measure the water vapor concentration in the air at atmospheric pressure and room temperature. The experimental result shows that the sensor has a high sensitivity of 1.058 parts-per-million.

We experimentally demonstrate a novel tunable true-time delay line with separate carrier tuning using dual-parallel Mach-Zehnder modulator and stimulated Brillouin scattering-induced slow light. The phase of the optical carrier can be continuously and precisely controlled by simply adjusting the dc bias of the dual-parallel Mach-Zehnder modulator. In addition, both the slow light and single-sideband modulation can be simultaneously achieved in the stimulated Brillouin scattering process with three types of configuration. Finally, the true-time delay technique is clearly verified by a two-tap incoherent microwave photonic filter as the free spectral range of the filter is changed.

First-order nonlinear perturbations of several fiber spans may be summed up linearly to yield the total nonlinear response of a group of spans, if nonlinear effects are neglected beyond the first order. Such linear additivity of first-order nonlinear perturbations about the nonlinear coefficients enables nonlinear compensations between one span with stronger nonlinearity and a group of spans with weaker nonlinearity, provided that there is a scaled translation symmetry between the strongly and the weakly nonlinear spans, and the dispersion of each span is properly managed. Excellent nonlinear compensations are achieved both in systems with an optical phase conjugator in the middle and in systems without.

Single-mode frequency-tuneable semiconductor lasers based on monolithic integration of multiple cavity sections are important components, widely used in optical communications, photonic integrated circuits and other optical technologies. To date, investigations of the ultrafast switching processes in such lasers, essential to reduce frequency cross-talk, have been restricted to the observation of intensity switching over nanosecond-timescales. Here, we report coherent measurements of the ultrafast switch-on dynamics, mode competition and frequency selection in a monolithic frequency-tuneable laser using coherent time-domain sampling of the laser emission. This approach allows us to observe hopping between lasing modes on picosecond-timescales and the temporal evolution of transient multi-mode emission into steady-state single mode emission. The underlying physics is explained through a full multi-mode, temperature-dependent carrier and photon transport model. Our results show that the fundamental limit on the timescales of frequency-switching between competing modes varies with the underlying Vernier alignment of the laser cavity.

Remarkable progress in terahertz (THz) technology over the past decade has been driven by the potential applications of THz waves in areas such as biomedical imaging, long-range screening, and organic materials identification [1]. This growth is in no small measure related to the success of the quantum cascade laser (QCL) which has established itself as one of the most promising radiation sources at terahertz frequencies [2]. The appeal of these novel semiconductor lasers stems from their compact size, broad spectral coverage (∼ 1–5 THz), and high output powers [3]. The ability of THz QCLs to generate coherent emission with quantum noise-limited linewidths, make them particularly suited to the development of interferometric THz sensing and imaging systems.