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Distributed Bragg Reflector semiconductor lasers are ideally suited for quantum technology applications due to their high efficiency, small footprint, and tunability in combination with the frequency stability provided by an integrated frequency selective element. Here we present the design, fabrication, and electro-optical characterization of DBR lasers optimized for strontium-based quantum technology applications at 689.449 nm. The devices achieved an optical output power of up to 88 mW, four times more than previously reported, as well as a spectral linewidth of 0.4 MHz. [ABSTRACT FROM AUTHOR]

A DBR-laser-based SOA-assisted extended-reach EML (AXEL) was investigated in order to improve the optical feedback tolerance. The performance was tested in relative intensity noise measurement and fiber transmission experiment. A superior optical feedback tolerance was obtained in comparison with a conventional AXEL. The excess RIN induced by optical feedback is suppressed at 3-dB higher feedback power for the DBR-AXEL. 40 km fiber transmission of 10-Gbit/s signal with a feedback factor of −15 dB was achieved. The power budget was 41.3 dB. The feedback-induced power penalty was 0.52 dB. [ABSTRACT FROM AUTHOR]

We for the first time present an all-passive cascaded optical frequency transfer technique over a fiber link, in which the phase noise of the each fiber link introduced by environmental perturbation and the laser repeater station are simultaneously compensated by using passive phase noise cancellation. The laser repeater station consisting of a cavity-stabilized laser is employed as a regenerative amplifier with a 45 dB optical gain. We demonstrate a cascaded optical frequency transfer with two 100 km spooled fiber links and one laser repeater station, illustrating an instability of $4.8\times 10^{-15}$ at the integration of 1 s and $4.9\times 10^{-19}$ at 10,000 s improved by a factor of 2 compared to the single-span 200 km fiber link. Dividing the whole fiber link into sub-sections could significantly improve the bandwidth of the phase noise compensation. Additionally, all-passive phase noise cancellation has the advantage of avoiding the servo bumps in the transferred light relative to the conventional active technique. The proposed technique provides a promising solution for high-performance and robust long-distance optical frequency transfer. [ABSTRACT FROM AUTHOR]

A semi-analytical dimensionless formula is presented for predicting the period-one frequency of an optically-injected laser over a wide range of injection powers and positive detunings. The result shows that for any given injected optical power, there exists an optimal detuning at which the period-one frequency is maximized. The formula is applied to a tabletop optical injection system with a commercially-available quantum-well distributed feedback laser with previously unknown design parameters. By extrapolating the linearity of the period-one frequencies at zero-detuning and weak-injection regimes, the physical control parameters of the laser are approximated to the analytical formula for wide ranges of injection powers and positive detunings. The accuracy of the model is experimentally verified, achieving period-one frequencies as high as 84.7 GHz. [ABSTRACT FROM AUTHOR]

Optical neural networks offer radically new avenues for ultrafast, energy-efficient hardware for machine learning and artificial intelligence. Reservoir Computing (RC), given its high performance and cheap training has attracted considerable attention for photonic neural network implementations, principally based on semiconductor lasers (SLs). Among SLs, Vertical Cavity Surface Emitting Lasers (VCSELs) possess unique attributes, e.g. high speed, low power, rich dynamics, reduced cost, ease to integrate in array architectures, making them valuable candidates for future photonic neural networks. This work provides a comprehensive analysis of a telecom-wavelength GHz-rate VCSEL RC system, revealing the impact of key system parameters on its performance across different processing tasks. [ABSTRACT FROM AUTHOR]

Practical needs related to infrastructure management are driving optical network operators to include Raman amplification in order to improve the performance of long fiber spans. Compared to standard erbium-doped fiber amplifier (EDFA) management, Raman amplifiers require a greater degree of control and monitoring due to their distributed nature. Inevitably, this update leads to a key consideration; the introduction of additional telemetry devices with respect to standard EDFA photodiodes, resulting in an increase in required investments. In this work, we present an embedded controller architecture in combination with an ad-hoc probing procedure to manage Raman amplification within disaggregated optical networks, using only standard integrated equipment, allowing an efficient implementation without the introduction of optical channel monitors (OCMs). This proposal is validated using a fully representative experimental campaign, testing both the probing procedure on a single fiber span and the operation of a Raman amplifier using the extracted information. [ABSTRACT FROM AUTHOR]

A photonic approach to generating an optoelectronic oscillator (OEO) based on an integrated mutually coupled (IMC) distributed feedback (DFB) laser is reported. Compared with the traditional construction of an OEO, the IMC-DFB has functions of the laser source, intensity modulator, and microwave photonic filters (MPFs), which could simplify the OEO scheme. An optical feedback loop (O-Loop) and an optoelectronic feedback loop (OE-Loop) are used simultaneously in the proposed scheme. The optical loop was used to reduce the optical linewidth of the IMC-DFB to improve the quality of the laser source in the OE loop. The IMC laser consisted of two distributed feedback (DFB) laser sections with a semiconductor optical amplifier (SOA) section in between. The optical linewidth of the IMC-DFB is reduced from 5.86 MHz to 3.94 kHz due to the feedback of the optical loop. By tuning the current of the SOA and DFB sections, tunable microwaves ranging from 24.9 to 46.5 GHz with single-sideband (SSB) phase noise below −110 dBc/Hz at a 10 kHz offset from the carrier were realized. [ABSTRACT FROM AUTHOR]

A versatile arbitrary bias control technology for Mach-Zehnder modulators (MZMs) based on a linear-frequency modulated (LFM) signal is proposed. An LFM signal is applied to the direct current (DC) port of an MZM together with a DC voltage. The output microwave signal is measured and cross-correlated with the fundamental and frequency-doubled components of the LFM signal. Their ratio serves as the feedback signal to control the bias point. An experiment is carried out. Compared to the conventional single-frequency pilot-based bias control technique, the peak power of the LFM component in the output electrical signal decreases by more than 27 dB, and the error vector magnitude (EVM) of the output microwave signal evaluated by 1000 symbols is reduced from 13.632% to 5.929%. [ABSTRACT FROM AUTHOR]

Optoelectronic feedback on a laser diode is demonstrated to generate two distinct modes of periodic pulse-train formation depending on the injection current J of the laser leading to microwave combs in two distinct regimes. For J close to the threshold current Jth, the pulse repetition rate ƒrep is the inverse of the loop delay. This behavior is attributed to feedback-induced gain-switching and leads to comb spacings in the tens of MHz range. In contrast, for J ≅ 2Jth, the repetition rate ƒrep is observed to be related to the relaxation-oscillation frequency ƒRO. The potential to generate pulse trains and associated microwave combs may find use in metrology, optical communications, optical sampling, and spectroscopy. [ABSTRACT FROM AUTHOR]

In multi-chip integrated photonic micro-systems, it is important to protect the laser source from reflection because the semiconductor laser is sensitive to the back reflection induced by the roughness of the waveguide sidewall or other on-chip optical structures. In order to maintain the stable operation of the light source, an additional micro-ring resonator coupled with the laser source is design before the optical circuit which enables the laser diode to enter a self-injection locking state and improves the reflection tolerance. It is experimentally demonstrated in the hybrid system that by tuning the phase shifter and the micro-ring resonate wavelength, the distributed feedback (DFB) laser enters a self-injection locking state, resulting significant improvement of the critical reflection ratio from −18.69 dB to −4.31 dB. The method can isolate the external reflection of laser diode without using heterogeneous integration, which provides a new idea for on-chip source isolation. The parameters design of the micro-ring and the coupling method with the laser are also introduced in the letter. [ABSTRACT FROM AUTHOR]

This paper proposes a novel approach and apparatus along with its calibration method to use the digital predistortion (DPD) technique in radio-over fiber (RoF) cloud RAN (C-RAN) transmitters that encompass physically spaced central baseband unit (C-BBU) and remote radio head (RRH). The proposed approach requires a calibrated observation path (COP) connecting the RRH to the BBU to mitigate any hardware impairments and distortion using a broadband post-distortion technique. This feedback (FB) link provides on-demand tracking of the change in the downlink (DL) optoelectronic component’s behavior and sends a copy of the radio frequency (RF) output signal to the C-BBU for DPD identification and processing. The proposed technique is experimentally validated using 40 MHz, and 100 MHz LTE-A as DL data signals, while the post-compensation model is built by a 200 MHz LTE-A signal. [ABSTRACT FROM AUTHOR]

Two new dynamic-state switching phenomena are experimentally observed in a vertical-cavity surface-emitting laser with polarization-preserved external-cavity optical feedback. One is switching between a steady state and a quasi-periodic state, and the other is switching between two different steady states. Both switching phenomena occur in the same polarization and the switching period is equal to the round-trip time in the external feedback cavity. The evolution of the two switching phenomena is experimentally investigated in detail. This work not only enriches the understanding of laser nonlinear dynamics but also provides an all-optical alternative for generation of special signals for possible applications. [ABSTRACT FROM AUTHOR]

This research investigates and demonstrates a unique configuration of a multiwavelength fiber ring laser covering far L and U bands by employing a booster optical amplifier (BOA), a stabilizer beam, and a Mach-Zehnder interferometer (MZI) in a dual cavity arrangement. By reducing the loss in the central cavity from 10 to 0 dB, the number of generated lasing wavelengths can be increased from 15 to 30, respectively. The observed side mode suppression ratio (SMSR) for most of these lasing wavelengths is larger than 60 dB. The generated lasing lines are relatively stable where peak power and wavelength variations of within ~1 dB and ~0.08 nm are observed. [ABSTRACT FROM AUTHOR]

In this letter, we report the dynamics and performance of a 1.3- $\mu \text{m}$ InAs/GaAs directly modulated quantum-dot laser (QDL) under strong feedback. At a direct modulation speed of 10 Gbps, error-free back-to-back operation of the QDL under a feedback level of −12 dB was realized. Feedback-resistant fiber transmission over 20 km under a maximum optical feedback level of −9 dB was also demonstrated with a power penalty of less than 1 dB at a bit error ratio (BER) of 10−3. The performance of the QDL was compared with directly modulated quantum-well laser (QWL) with similar parameters, showing a clear advantage in feedback resistance of QDL. [ABSTRACT FROM AUTHOR]

We evaluate the effects of the external optical feedback on the on-off keying (OOK) and four-level pulse amplitude modulation (PAM4) signals in the single-mode fiber (SMF) link implemented by using a commercial 10G-class 1.55- $\mu \text{m}$ vertical-cavity surface-emitting laser (VCSEL). The results show that the PAM4 signal is much more sensitive to the external optical feedback than the OOK signal. For example, in the back-to-back condition, an error floor is observed at the bit-error rate (BER) of $\sim 8\times 10 ^{\mathbf {-3}}$ for the 10-Gbaud PAM4 signal when we set the optical feedback ratio to be −14 dB. In comparison, the 10-Gbaud OOK signal has a power penalty of only 0.4 dB (@BER $= 5\times 10 ^{-5}$) under the same conditions. The results also show that the PAM4 signal becomes more sensitive to the optical feedback after the SMF transmission due to the shallowed BER curve by the fiber dispersion. In addition, we confirm that the effects of the optical feedback are independent of the baud rate. From these results, we conclude that the error-free transmission of the high-speed PAM4 signal cannot be achieved if there is even one bad fiber connector having >2.5% reflection in the VCSEL-based SMF link. [ABSTRACT FROM AUTHOR]

Practical deployment of elastic optical networks requires key enabling technologies supporting the miniaturization of flexible optical transponders for provisioning large-bandwidth services at low-cost per users. A silicon photonics (SiP) reconfigurable optical frequency comb (OFC) demultiplexer based on passive optical filtering exhibiting a minimum spurious tone rejection (STR) of nearly 30dB at 12.5GHz tone spacing is here reported. Low power penalty due to inter-channel crosstalk (IXT) originated by finite STR is observed. Improved filter designs for reduced IXT and/or operation at 6.25GHz comb spacing are experimentally and numerically verified. By monolithic integration with SiP OFC sources and I-Q modulators the scheme has potentials for realizing ultra-compact multiple-flow optical transmitters with adaptive bandwidth and channel spacing. [ABSTRACT FROM AUTHOR]

we report a tunable self-injection locked violet laser diode external cavity system exhibiting a continuous wavelength tunability of 5.15 nm (400.28 - 405.43 nm) with mean side-mode-suppression-ratio (SMSR) and linewidth of ~23 dB and ~190 pm, respectively. The effects of injection current and temperature indicate a robust system besides being cost-effective and straightforward. Moreover, a successful indoor on-off keying transmission at two different locked modes on a 0.4 m free space channel showed ~10 times improvement in the bit-error-rate (BER) with value ${\sim }8\times 10^{-4}$ at 2 Gb/s, and better performance on 0.8 m channel length at 1.75 Gb/s compared to the free-running laser case. Our work is a potential step towards the realization of future high data capacity narrow-wavelength-spaced multiplexed optical wireless communication system wherein continuously tunable laser sources are expected to play a crucial role as transmitters. [ABSTRACT FROM AUTHOR]

Photonic generation of a low-phase-noise microwave signal based on period-one (P1) dynamics of an optically injected semiconductor laser (OISL) subject to subharmonic modulation from an optoelectronic oscillator (OEO) is firstly proposed and experimentally demonstrated. Under suitable injection parameters, an OISL can be driven into period-one (P1) dynamics, which can be regarded as a photonic microwave with a relatively large linewidth. By further introducing an OEO with a frequency of 1/2 P1 frequency to directly modulate the OISL, a low-phase-noise photonic microwave with P1 frequency can be obtained. For an OISL operating at P1 oscillation with a frequency of 16.96 GHz, after imposing a 1/2 subharmonic modulation of 8.48 GHz on the OISL by an OEO, the photonic microwave linewidth is reduced from 5.23 MHz to less than 1 kHz, and the single sideband (SSB) phase noise of the final photonic microwave is −102.5 dBc/Hz@10 kHz, which is close to the SSB phase noise of the OEO signal (−104.3 dBc/Hz@10 kHz). [ABSTRACT FROM AUTHOR]

We propose and demonstrate an improved Talbot laser including a regenerative RF feedback loop. This architecture greatly enhances both amplitude and noise performances of the pulsed laser, while preserving the broadband tunability of its repetition rate. The phase noise power spectral density of the associated RF signal is found to be independent of the repetition rate. For a 4.9 GHz repetition rate, the timing jitter is as low as 54 fs (integrated between 10 kHz and 100 MHz offset frequency). This architecture paves the way towards tunable high performances coupled opto-electronic microwave oscillators. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate a real-time polarization tracking and equalization method based on the state of polarization detection from the adaptive finite impulse response (FIR) filter coefficients in a polarization division multiplexed (PDM) coherent optical communication system. By calculating the variation of the polarization within a time interval, the polarization rotation speed can be evaluated. In a field-programmable gate array (FPGA), a feedback structure for polarization rotation speed detection and FIR coefficients update can be built up. In this feedback structure, the update coefficient of the filter will be adjusted to an optimal value with the change of detection result. The experimental results show that the establishment of this feedback structure improves the polarization tracking ability in the real-time coherent optical receiver. [ABSTRACT FROM AUTHOR]

A wavelength-tunable chaotic optical signal generation and on-chip O/E conversion scheme based on an integrated laser-photodiode chip subject to filtered optical feedback is proposed and experimentally demonstrated. The integrated chip consists of a Fabry–Perot (FP) gain section and a photodiode, which was fabricated on an n-InP substrate with the same active material. By applying a filtered feedback signal to the FP gain section, wavelength-tunable chaotic carrier ranging from 1576 to 1587 nm was achieved. On-chip microwave chaotic signal generation with a standard bandwidth over 5.5 GHz was obtained using the integrated photonic chip. [ABSTRACT FROM AUTHOR]

A compact, micro integrated, diode laser-based dual-wavelength master oscillator power amplifier at 785 nm is presented. Laser emission from a Y-branch distributed Bragg reflector laser is coupled into a tilted ridge waveguide amplifier on a $5\times 25$ mm2 micro optical bench. At 25°C and 20 mW pump power, an optical output power of 0.5 W is obtained. The corresponding emission wavelengths are 784.62 and 785.24 nm, resulting in a spectral distance of 10 cm $^{-1}$. The measured spectral bandwidths and side mode suppression ratios are 0.02 nm (0.3 cm $^{-1}$) and 30 dB, respectively. At both wavelengths beam propagation parameters of 1.3 ($\text{M}{^{2}}_{4\boldsymbol \sigma }$) are obtained. This allows for efficient dual-wavelength single-mode fiber coupling. In addition, the device does not show any effects from optical feedback. It neither shows a lateral spatial tilt of the far field intensity distributions for the two wavelengths, previously reported for single Y-branch diode lasers. Based on these parameters, the device is suitable for demanding applications such as shifted excitation Raman difference spectroscopy or confocal Raman microscopy. [ABSTRACT FROM AUTHOR]

A tunable all-optical microwave oscillator, which has no electrical microwave devices as well as photoelectric conversion and electro-optic modulation, is proposed and experimentally demonstrated. In this scheme, a distributed-feedback laser diode is utilized to be a light source and create a seed microwave signal from the period-one (P1) oscillation by a self-delayed optical feedback injection. A semiconductor optical amplifier-based all-optical feedback loop is introduced to perform optical–optical feedback modulation and mode selection. Meanwhile, a short fiber loop is coupled to further suppress the residual side modes. In the experiment, single-mode microwave photonic signals with good quality are obtained, whose frequencies can be tuned from 6.10 to 15.59 GHz by adjusting the optical feedback ratio. The measured single-sideband phase noise is −89 dBc/Hz at 10-kHz offset from a carrier of 10.98 GHz. [ABSTRACT FROM AUTHOR]

This letter reports on a 1.3- $\mu \text{m}$ reflection insensitive transmission with a quantum dot laser directly grown on silicon in the presence of strong optical feedback. These results show a penalty-free transmission at 10 GHz under external modulation with −7.4-dB optical feedback. The feedback insensitivity results from the low linewidth enhancement factor, the high damping, the absence of off-resonance emission states, and the shorter carrier lifetime. This letter paves the way for future on chip high-speed integrated circuits operating without optical isolators. [ABSTRACT FROM AUTHOR]

We present a simple and straightforward method to determine the phase shift between the amplitude modulation (AM) and the frequency modulation (FM) of a laser diode. The approach is based on the measurement of AM and FM signals produced by the dynamical response of a laser diode exposed to weak optical feedback. The AM signal is measured directly on a photodiode detecting the emitted power, and the FM signal is demodulated to intensity modulation through the edge filtering techniques using: 1) a volumetric Bragg grating or 2) a Mach–Zehnder interferometer as filters prior to the signal detection. In this letter, we show experimentally that the phase shift determination unveils the linewidth enhancement factor of semiconductor lasers as predicted by the Lang and Kobayashi theory. [ABSTRACT FROM AUTHOR]

The phase of an interferogram carries the desired information for various applications such as 3-D profilometry. It can be obtained with a wavelength tuning interferometer by acquiring a series of interferograms while changing the laser wavelength. The phase achieved by the interferometer, however, has severe error due to the laser output power change, which is produced inherently from changing the laser wavelength. To eliminate such phase error, we propose a real-time closed-loop feedback control system to keep the laser output power stable during the wavelength tuning. The control system integrates a photodetector, a data acquisition card, and an electro-optic amplitude modulator with our software to achieve the purpose. The experimental result of the system can steadily output a desired intensity value and the measurement speed is up to 600 KHz. Both its control principles as well as its experimental validation are depicted in this letter. [ABSTRACT FROM AUTHOR]

A simple frequency-tunable optoelectronic oscillator based on a distributed-feedback (DFB) laser under optical feedback at period-one (P1) oscillation is proposed and experimentally demonstrated. The directly modulated DFB laser functions as an active tunable microwave photonic filter and a high-efficiency modulator due to the high-efficiency response at the P1 oscillation frequency, eliminating the needs of the narrowband electrical bandpass filter and high-gain RF amplifier. By tuning the optical feedback ratio, tunable microwaves ranging from 8.6 to 15.2 GHz with single sideband phase noise below −110.88 dBc/Hz at a 10-kHz offset from the carrier were realized. [ABSTRACT FROM AUTHOR]

The stabilization of a relatively simple microwave oscillator tunable across the full X-band based on a laser subjected to optical feedback is achieved. Specifically, a resonance effect based on locking the two inherent dynamic frequencies of the system, as well as, optoelectronic feedback are utilized to achieve a sub-ps jitter and typical phase noise in the range of −107 dBc/Hz at 10 kHz offset from an oscillation frequency that can be tuned from 5.5 to 12.1 GHz. Further, the microwave signal is extracted from the laser-diode injection terminals and eliminates the need for multiple lasers, radio-frequency filters, and external RF sources. This architecture, therefore, realizes a compact and low-cost tunable microwave photonic oscillator. [ABSTRACT FROM AUTHOR]

We demonstrate the gigabit-per-second transmission over a step-index plastic optical fiber (SI-POF) of 10-m length with a pulse-amplitude modulation (PAM). A multilayer perceptron-based equalizer is used to mitigate an intersymbol interference and non-linearity in the system. Using this equalizer with 32-PAM, a data rate of 10 Gb/s is achieved over the 10-m SI-POF at a bit error rate of 10−2, which is below the 20% forward error correction limit. [ABSTRACT FROM AUTHOR]

We demonstrate the stable operation of a narrow linewidth semiconductor laser source based on a coherent optical negative feedback system. The linewidth was reduced to 3 kHz from 13.5 MHz. The power spectral density of frequency modulation (FM) noise was reduced by 35 dB, while the relative intensity noise was low enough for a light source for optical communications (less than −140 dB/Hz). Additionally, the system showed stable operation more than an hour. Our results indicate that the instability of a laser system is caused by fluctuations in the feedback loop length due to the mechanical vibrations, which can be suppressed by the robust integration of the system. [ABSTRACT FROM AUTHOR]

This letter presents a self-alignment system using a continuous and a proportional-to-position-error correction algorithm. The system is designed to address uninterrupted point-to-point high-speed line-of-sight optical wireless links. The alignment correction employs amplitude detection from a mm2scale CMOS-compatible 4-PD array connected to a 4-channel receiver and followed by digital-processing and feedback toward the receiver lens position. The proposed self-alignment method is validated with a receiver position varying over about 5-cm range and a receiver angle covering around ±10 degree with roughly ×4 field-of-view widening due to the automatic alignment. A 50-Mb/s ON-OFF-keying transmission link is maintained across 4 meters distance using a 650-nm-red light-emitting-diode source with a bit-error-rate below the forward-error-correction limit of 3.8 × 10-3. [ABSTRACT FROM PUBLISHER]

Mode characteristics and spectral linewidths are investigated for single-mode and dual-mode AlGaInAs/InP square microlasers. For a single-mode square microlaser with a side length of 22 μm, connected with a 2-μm-wide waveguide, a linewidth of 20 MHz is obtained by means of a non-zero delayed self-heterodyne method. Dual-mode square microlaser linewidths are also analyzed using an optical heterodyne, and a strong mode correlation is experimentally demonstrated. Furthermore, we successfully realize the narrow-linewidth microlaser and microwave signal generation through a dual-loop optical feedback technique. [ABSTRACT FROM PUBLISHER]

A linearly chirped microwave waveform (LCMW) generator based on a monolithically integrated dual-mode amplified feedback laser (AFL) is demonstrated. The proposed LCMW generator is very simple, only consisting of an AFL, an arbitrary waveform generator, and a photodetector. By applying a sweeping signal to the amplifier section of the AFL, the beating frequency of the AFL’s output can be changed in a chirped manner. LCMW is experimentally demonstrated using a heterodyne-beating technique. The measured microwave waveform has a pulse duration of 1~\mu \texts , a bandwidth of 3.3 GHz (ranging from 31.5 to 34.8 GHz), corresponding to a time-bandwidth product of 3.3 \times 10^3 and a compression ratio of 2.6 \times 10^3 . [ABSTRACT FROM PUBLISHER]

A simple design of a hybrid integrated short-external-cavity chaotic semiconductor laser module is presented and fabricated. A distributed feedback laser chip is directed to the transflective mirror through a collimating micro-lens, and a part of the light is reflected by the mirror into the laser chip to induce chaos. Transmitted through the mirror, the generated chaotic light is coupled into a pigtail fiber as output by a focusing micro-lens. It should be pointed out that the part between the transflective mirror and the emitting surface of the chip works as a short external straight feedback cavity. All the components above together with chip submount, heat sink, and thermoelectric cooler are encapsulated by a commercial 14-pin butterfly package. In our experimental fabrication, the intensity reflectivity of the transflective mirror is optimized as 5% and the external-cavity length is 2 mm. Experimental tests show that chaotic light with a bandwidth larger than 4.5 GHz is readily obtained from the chaotic laser module by adjusting laser bias current. [ABSTRACT FROM PUBLISHER]

A least squares-based channel estimation algorithm is proposed for optical orthogonal frequency division multiplexing-based multiuser multiple input single output visible light communications. The pilot sequence and pilot tones are optimized to derive the minimum-mean-square error of the channel estimation. Each user estimates the channel independently, and sends the estimates of the channels to the transmitter. Based on the estimated channels, zero-forcing precoding is applied at the transmitter to achieve the spatial multiplexing gain. The simulated performances of channel estimation and data detection are provided. [ABSTRACT FROM PUBLISHER]

Laser chaos is promising for constructing high range-resolution and anti-jamming radars, whereas the range resolution and anti-jamming capability are unfortunately limited by laser relaxation oscillation and external-cavity resonance, respectively. Here, we theoretically propose using a broadband white chaos generated by optical heterodyne of two chaotic external-cavity semiconductor lasers as source signal to construct a radar system with enhanced range resolution and anti-jamming capability. The white chaos not only has a white-noise-like wide spectrum without relaxation oscillation, but also has no signature of external-cavity resonance. Benefitting from these merits, the proposed white-chaos radar shows a marvelous unambiguous detection performance with subcentimeter range resolution. Moreover, the elimination of external-cavity resonance enables the randomness improvement of source signal, and resultantly, the anti-jamming capability is enhanced as well. [ABSTRACT FROM PUBLISHER]

A photonic frequency down-converter based on a frequency-doubling optoelectronic oscillator (OEO) using two cascaded electro-absorption modulators (EAMs) is proposed and experimentally demonstrated. The use of the two cascaded EAMs allows the OEO feedback loop used to generate the local oscillator (LO) signal and the down-conversion section to be structurally isolated, so that the proposed photonic frequency down-converter has high RF to LO isolation. The phase noise of the frequency-doubled 20-GHz LO signal and that of the down-converted 3-GHz IF signal were measured to be −97.80 and −95.32 dBc/Hz at an offset frequency of 10 kHz, respectively. The spurious free dynamic range measured using a two-tone test was 87.02 dB $\cdot $ Hz2/3. The bit error rate performance was also investigated with a 20-GHz RF signal modulated by 1.25 Gb/s data. [ABSTRACT FROM PUBLISHER]

The synthesis of wavelength-tunable broadband chaotic signals is experimentally demonstrated by using a weak-resonant-cavity Fabry–Perot laser diode (WRC-FPLD) subject to filtered feedback. To perform the broadband tunability from 1544 to 1556 nm, a tunable optical filter is used for providing filtered feedback to drive the laser into chaos. With adjusting the central wavelength of the filter, about 20 longitudinal modes of the WRC-FPLD can individually realize the chaotic output within the tunable range of the filter. Moreover, by suitably selecting the feedback power, the chaos signal with a flat power spectrum and broad bandwidth up to ~30.0 GHz can be achieved. [ABSTRACT FROM PUBLISHER]

We demonstrate how phase-locking a whispering gallery mode-stabilized semiconductor laser to a high-stability Erbium-based distributed feedback fiber laser makes an outstanding optical reference for photonic microwave generation using an optical frequency comb. Using only commercially available compact lasers, it allows generation of a 12-GHz microwave signal with absolute levels of phase noise below −47 dBc/Hz at 1 Hz and −170 dBc/Hz at 50 kHz from the carrier. [ABSTRACT FROM PUBLISHER]

The first Sagnac loop coherent phase modulated RF photonic link employing an ACP-OPLL linear phase demodulator was presented. This structure demonstrated stable signal transmission over a 1-km long coherent RF photonic link. Except for optical loss of the long optical fiber, no penalties in the noise and linearity performance were observed. [ABSTRACT FROM PUBLISHER]

This letter demonstrates that a commercial travelling-wave Mach–Zehnder modulator is a versatile device. Next to being a modulator, it is also able to function as an electro-optical temporal integrator with an integration window twice as long as its propagation delay. Using the similar principle and being driven by a periodic RF frequency, the modulator is able to perform as an optical time-gate isolator that blocks any reverse-travelling lightwave, but simultaneously allows forward-travelling periodically-pulsed optical signals to pass. In the experiment, sub-nanosecond forward-travelling time gates and reverse-travelling optical power extinction >20 dB are successfully demonstrated. [ABSTRACT FROM PUBLISHER]

A tunable three-dimensional silicon nitride racetrack resonator was experimentally demonstrated, with a microring resonator on a bottom layer and a feedback cross-coupled waveguide on a top layer. At a 1559.3-nm wavelength, an extinction ratio of more than 14 dB and free spectral range of 2.7 nm can be realized with a quality factor of about 1.88\times 10^4 . The filter performance, such as the resonance wavelength and extinction ratio, can be thermo-optically tuned based on the electrical control of a heater above the feedback waveguide. The presented device has a potential to be applied as a tunable modulator/switch as well as a highly sensitive sensor. [ABSTRACT FROM PUBLISHER]

A novel optical feedback technique using self-injection locked and phase locked loop (SILPLL) is employed for phase noise reduction of free-running microwave oscillators. Phase noise reduction of 27 dB at 10-kHz offset has been demonstrated by applying this technique for a 10-GHz state of art dielectric resonator oscillator achieving −137 dBc/Hz at 10-kHz offset. A phase noise prediction of this feedback technique is also presented, which very closely corroborates with experimental results. Analysis of SILPLL using the modeling has shown that further phase noise reductions could be achieved, by reducing the flicker phase noise of the long delay lines and microwave amplifiers. Phase noise of −147 dBc/Hz at 10-kHz offset is predicted when the relative intensity noise (RIN) is reduced to the RIN of −170 dB/Hz for an optical power leading to a photocurrent of 5 mA and the \textb\mathrm {\mathbf {-1}} (flicker noise) levels of −125 dBc/Hz for RF amplifiers. [ABSTRACT FROM PUBLISHER]

A compact external cavity diode laser module with 1.4-W narrowband emission at 445 nm is presented. A commercially available broad-area GaN-based laser diode is used as gain medium and a volume Bragg grating is integrated for wavelength stabilization. The laser module is realized on a conduction cooled package with a footprint of 25 mm $\times $ 25 mm. The spectral width is smaller than 50 pm over the whole operating range. A suppression of the amplified spontaneous emission with more than 50 dB is measured. The laser module is suitable for subsequent nonlinear frequency conversion into the deep ultraviolet spectral range. [ABSTRACT FROM PUBLISHER]

A training-based channel estimation algorithm is proposed for the faster-than-Nyquist polarization division multiplexed m-ary quadrature amplitude modulation ( $m = \,\, 4$ , 16, 64) systems with Tomlinson-Harashima precoding (THP). This is robust to the convergence failure phenomenon suffered by the existing algorithm, yet remaining format-transparent. Simulation results show that the proposed algorithm requires a reduced optical signal-to-noise ratio to achieve a certain bit error rate in the presence of first-order polarization mode dispersion and phase noise introduced by the laser linewidth. [ABSTRACT FROM PUBLISHER]

We present a highly linear quantum well (QW) optical phase modulator that employs a mixed QW structure. This modulator demonstrated an excellent phase modulation linearity. Its third-order intermodulation intercept point in the optical phase excursion per unit length was measured to be greater than $8\pi $ /mm. The measured optical loss is $\sim 1$ dB/mm. The new phase modulator design should enable a more compact optical phase-locked loop photonic integrated circuit with a wider bandwidth. [ABSTRACT FROM PUBLISHER]

The influence of feedback level and shifted frequency on laser spectrum and power spectrum is investigated experimentally and theoretically. Due to optical frequency-shifted feedback, the spectrum of Nd:YVO4 microchip laser is expanded into one with multi equal-spaced sidebands around its initial central frequency. The detail of expanded spectrum depends on both the shifted frequency and relaxation oscillation frequency. Optical feedback level decides the possible maximum spectrum range. Nevertheless, the actual spectrum range depends on both the frequency shift and the feedback level. The power spectrum turns from simple to complex with the increase of feedback level when the shifted frequency is a constant. A rate-equation model with optical frequency-shifted feedback is built to interpret the experimental phenomena. The theoretical results accord with the experiments very well. This letter can help choose appropriate parameters for feedback interferometry systems and promote the accomplishment of optical frequency-shifted feedback diagram, which could be a guide for feedback interferometry applications. [ABSTRACT FROM PUBLISHER]

A photonic integrated circuit is demonstrated that couples three Fabry–Pérot (FP) lasers together via an integrated star coupler. The FP lasers rely on a common cleaved facet and individual multimode interference reflectors (MIRs) for lasing operation. The MIRs are monolithically integrated with each FP laser using a single growth step, which allows for epitaxial regrowth-free processing. By suitable independent tuning of each Fabry–Pérot, a single mode operation and a mode selection can be attained from the coupled cavity system. The mode selectivity and side-mode suppression ratios of $\sim 40$ dB are demonstrated. [ABSTRACT FROM PUBLISHER]

We demonstrate a novel method that makes an efficient use of laser nonlinear dynamics when subject to optical self-injection for subwavelength displacement sensing purposes. The proposed methodology combines two different phenomena taking place inside the laser cavity: optical self-injection, which results in optical feedback interference, and laser continuous wave frequency modulation, giving rise to a wavelength sweeping effect in the laser’s emission. We present a combination of these phenomena to measure vibration amplitudes below $\lambda /2$ with the resolutions of a few nanometers, bandwidth dependent upon the distance of external target, amplitude, and frequency of current modulation. The basic theoretical details and a mathematical model are presented for the developed measurement principle. Experimental results with the system working as a vibrometer to measure a target vibration of amplitude {\lambda }/{5} (137.5 nm) with a mean peak-to-peak error of 2.4 nm just by pointing the laser diode onto the target and applying some signal processing are also demonstrated. [ABSTRACT FROM PUBLISHER]