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In the frame of ongoing efforts between space agencies to define an on-off-keying-based optical low-Earth-orbit (LEO) direct-to-Earth (DTE) waveform, this paper offers an in-depth analysis of the Variable Data Rate (VDR) technique. VDR, in contrast to the currently adopted Constant Data Rate (CDR) approach, enables the optimization of the average throughput during a LEO pass over the optical ground station (OGS). The analysis addresses both critical link level aspects, such as receiver (time, frame, and amplitude) synchronization, as well as demonstrates the benefits stemming from employing VDR at system level. [ABSTRACT FROM AUTHOR]

A wavelength tunable chaotic external cavity semiconductor laser (ECSL) composed by a distributed-feedback laser diode (DFB-LD) and a broadband chirped fiber Bragg grating (CFBG), is proposed and experimentally demonstrated for generating tunable chaos signals with time delay signature (TDS) suppression. The experimental results show that by using a CFBG with a 3-dB reflection bandwidth of approximately 30 nm and a low dispersion of approximately 1.25 ps/nm, the chaotic ECSL can be tuned over a wide wavelength range, and the generated chaos possesses a low TDS of about 0.04. With the increase of the bias current of the DFB-LD, the chaotic ECSL is tuned towards long-wavelength direction, the effective bandwidth of the chaotic signal is extended, and the TDS of the chaotic signal remains effectively suppressed. To highlight the advantages of the proposed scheme, relevant experiments for the DFB-LD under fiber mirror feedback and fiber Bragg grating feedback are also performed and compared. [ABSTRACT FROM AUTHOR]

An investigation of angled flip-chip integration of a singlemode 850 nm vertical-cavity surface-emitting laser (VCSEL) on a silicon nitride photonic integrated circuit (PIC) is presented. Using numerical FDTD simulations, we consider the conditions under which the VCSEL can be integrated at an angle over a grating coupler with high coupling efficiency and low optical feedback. With both coupling efficiency and feedback decreasing with increasing angle, there is a trade-off. With co-directional coupling, first-order diffraction loss sets in at a critical angle, which further reduces the coupling efficiency. No such critical angle exists for contra-directional coupling. We also experimentally demonstrate angled flip-chip integration of GaAs-based 850 nm single transverse and polarization mode VCSELs over grating couplers on a silicon-nitride PIC. At the output grating coupler, light is either collected by an optical fiber or converted to a photocurrent using a flip-chip integrated GaAs-based photodetector. The latter forms an on-PIC optical link. We measured an insertion loss of 21.9, 17.6 and 20.1 dB with a singlemode fiber, multimode fiber and photodetector over the output grating coupler, respectively. [ABSTRACT FROM AUTHOR]

In this article, we propose an all-fiber self-mixing velocimeter based on a DBR fiber laser with adjustable injection intensity for the first time, and theoretically and experimentally explore the behavior of fiber lasers in detecting non-cooperative target under different injection intensities. When the ultra-strong injection level is attained, the linewidth of the DBR fiber laser will be narrowed to about 10 kHz, thus extending the coherence distance of laser tremendously. Simultaneously, the intensity noise can be also significantly suppressed. The non-cooperative target velocity measurement with 40 km single-channel transmission distance has been successfully achieved. This velocimeter is shown to be of great significance in the field of remote sensing with simple and compact configuration, and adjustable dynamical monitoring range. [ABSTRACT FROM AUTHOR]

Coherent reception becomes an interesting option when data rates in time-division-multiplexed (TDM) passive optical networks (PONs) grow beyond 50 Gbit/s. Controlling the wavelength, i.e., the optical frequency, and the phase of the laser acting as local oscillator (LO) is one of the main technical challenges in the design of coherent TDM PONs. In the optical network units (ONUs), low-cost lasers are required, which come at the expense of wavelength variations and drifts over multiple nanometers due to fabrication imperfections, and temperature variations. This contradicts the requirement of wavelength-stable LOs in coherent receivers. The use of a wavelength locker circuit and a temperature controller is considered as too complex for applications in access networks. In this work, we propose a novel colorless coherent architecture with high resilience to ONU laser wavelength drifts of up to $\pm$ 4 nm ($\pm$ 0.5 THz) for future 100 Gbit/s PON. It allows the use of distributed feedback lasers at the ONU side. This is rendered possible by generating a frequency comb with carefully chosen free spectral range in a quantum-dash mode-locked laser diode at the optical line terminal. In upstream operation, the frequency comb serves as an LO, whereas the same information is modulated onto all comb lines for the case of downstream. As a result, the ONU laser can drift over the entire comb bandwidth without substantial performance penalty. We experimentally demonstrate downstream and upstream operation with an aggregated raw data rate of 96 Gbit/s, respectively. We further introduce advanced digital signal processing (DSP) methods including a coarse frequency offset compensation (CFOC) and a multiple-input multiple-output (MIMO) equalizer to improve the performance of our concept. We show that the receiver sensitivity can be increased by 3 dB for a high-bandwidth receiver when using a 6 × 2 MIMO equalizer scheme. A 4 × 2 MIMO equalizer scheme enables colorless reception even with a limited-bandwidth receiver. [ABSTRACT FROM AUTHOR]

A wavelength-division-multiplexing (WDM)-visible laser light communication (VLLC) and white-lighting ring network with a 150-Gbit/s accumulative transmission rate at central station (CS), a 50-Gbit/s transmission rate at optical node, and a 604-lux white-lighting at CS is successfully demonstrated. Red, green, and blue laser diodes employing two-stage injection locking and optoelectronic feedback techniques are utilized to afford the dual functions of high-speed VLLC links and white-light illumination at reading/writing-level. By deploying optical add-drop multiplexing scheme, this newly-built WDM-VLLC and white-lighting ring network is promising since it allows a point-to-multipoint link and a two-way operation in free-space transmission. Impressively low bit error rate of 10−9, clear PAM4 eye diagrams, and qualified white-lighting for reading and writing are achieved through 7.5 m/9 m/10.5 m/12 m VLLC link with 5 m plastic optical fiber transmission. Such constructed WDM-VLLC and white-lighting ring network shows a potential one for the evolution of high-speed VLLC with white-lighting for reading/writing. [ABSTRACT FROM AUTHOR]

We demonstrate real-time broadband multi-Gb/s electrical RF synthesizer-free millimeter-wave (MMW) signals generation and wireless transmission at the 5G new radio (NR) frequency band of 47 GHz based on analog radio-over-fiber (A-RoF) fronthaul. This is enabled by a low noise, highly correlated, monolithic C-band semiconductor InAs/InP quantum-dash (QDash) dual-wavelength distributed feedback (DW-DFB) laser. One laser mode is encoded using 4-/6-GBaud multilevel quadrature amplitude modulation (M-QAM) (16-/32-/64-QAM) baseband data signals, the other lasing mode is used as an optical local oscillator for optical-heterodyne remote up-conversion to a MMW carrier of 47.27 GHz. Consequently, optical baseband modulated data signals with data capacity up to 36 Gb/s (6-GBaud × 64-QAM) are transmitted through back-to-back (BtB) and 25-/50-km of standard single mode fiber (SSMF) before the MMW carrier is optically synthesized remotely for free space wireless data transmission and detection over up to 9-m. The end-to-end MMW-over-fiber (MMWoF) wireless link is thoroughly characterized exhibiting promising error-vector-magnitude (EVM) and bit-error-rate (BER) values. The 4-/6-GBaud 16-QAM MMWoF wireless links achieve EVMs down to 6.32%/7.33%, 6.71%/7.78%, and 7.35%/8.91% through BtB, 25-km, and 50-km SSMF, respectively. Similarly, the EVM for 32-QAM and 64-QAM links is observed to be 5.56%/6.56% and 6.05%/6.62%, respectively. Moreover, in each case, the calculated BER is below the forward error correction (FEC) limit of 3.8 × 10−3. The results corroborate the potential and viability of the QDash DW-DFB laser as a simple, efficient and cost-effective alternative to individual laser sources for deployment in broadband photonic MMWoF fronthaul systems of 5G wireless networks. [ABSTRACT FROM AUTHOR]

Laser emissions at 1.6–1.7 μm band have captured considerable attention in recent years. However, limited by the available emission spectrum of typical rare-earth-ion doped gain fibers, it is extremely difficult to realize highly stable laser output in this wavelength region with a simultaneous high spectral purity and high output power. To address these problems, a cost-effective robust random fiber lasing scheme is proposed to generate the high-power and high-spectral-purity laser output at 1.69 μm. The maximum output power is 7.55 W and the spectral contrast between the output lasing and the pump is as high as 24.3 dB. Both the short-time temporal characteristics and the long-time output power fluctuations are measured, which shows excellent stability and robustness of the laser configuration. This work provides an alternative approach to realize high-performance 1.6–1.7 μm band lasing output which is of particular significance for potential applications. [ABSTRACT FROM AUTHOR]

Ultra-stable lasers serve as the backbone for the most advanced optical metrology scientific experiments, enabling the ability to laser interferometry or atomic spectroscopy at the highest levels of precision, thanks to the rapid development of laser wavelength stabilization techniques based on optical or electrical feedback from an external reference such as absorption line or optical cavity. With the increasing interest in making these scientific systems portable and applying it outside of the laboratory, it still remains an open question as to how to realize a laser source that can reach the high wavelength stabilization and still remain sufficiently compact and portable for field use. Here, we describe a wavelength stabilization fiber laser with intracavity reference mechanism, by utilizing the beat note of dual modes inside the laser cavity as the laser wavelength drift reference signal. A laser wavelength drift less than 5 kHz and ADEV 10−13 level at integration time scales from 0.01s to 1000s is achieved, which is to-date the best results for the laser wavelength stabilization without the external frequency reference. The outcome of this work presents a new way to achieve an ultra-stable laser with a simpler, lower cost feature. It offers new insights to promote the highest precision optical metrology scientific application outside of the laboratory, and also gives the possibilities of applying the ultra-stable laser source into a wider range of industrial application. [ABSTRACT FROM AUTHOR]

Integrated polarization stabilizers along with corresponding stabilizing algorithms have been investigated for years to tackle the polarization fluctuation issue in polarization dependent systems such as photonics integrated circuits and coherent optical communication systems. Here, we propose and demonstrate a novel integrated polarization stabilizer which is composed of an in-line polarimeter and two cascaded Mach-Zehnder interferometers-based polarization converter, with a footprint of 2.8 mm × 0.8 mm. Benefitting from the integrated in-line polarimeter and two controlling algorithms, namely feed-forward tracking algorithm and feedback tracking algorithm, the stabilizer can track the incoming light with arbitrary state of polarization in one-time manipulation. The whole process consumes about 30 μs, which is approaching the limitation of the response time of the adopted thermal phase shifters, making the proposed device the fastest one with thermal phase shifters on silicon photonics platform. Meanwhile, the incoming light is endlessly tracked in real time and 99% converted to horizontal linear polarized light. [ABSTRACT FROM AUTHOR]

The paper is focused on a systematic investigation of the circuits for active stabilization of the frequency offset between two semiconductorn ultra-accurate fiber optic time transfer systems. The frequency offset is increased up to 50 GHz, which can be achieved not only with relatively low-noise integrated tunable laser assemblies, but also with ordinary, telecom-grade, distributed feedback lasers. The paper starts by determining the general frequency accuracy and stability, required to keep the uncertainty contribution of the stabilization circuit at a negligible level (assumed here as 1 ps) compared to other contributions of the overall uncertainty of the link calibration. Next, the technical details of the essential building block of the system discussed, which is required to convert the high-frequency offset to lower frequencies to allow convenient frequency measurement, are analyzed. Experimental circuits built with commercially available millimeter wave integrated circuits were tested with the frequency offset complying with telecom dense wavelength division multiplex standards, equal to 12.5 GHz, 25 GHz and 50 GHz. It was found that such stabilization circuits can cause substantial systematic errors, which are related to lasers’ phase noise and the operation with low input optical powers (below −39 dBm in evaluated circuits). These effects were investigated experimentally in detail and countermeasures were proposed. [ABSTRACT FROM AUTHOR]

Interrogating a remote passive-sensor with high fidelity is one of the biggest challenges in the sensing domain. Recently, random fiber laser (RFL) combined with remote fiber sensors was proposed for optical fiber sensing with high signal-to-noise ratio over ultra-long-distance fiber link. However, only quasi-static sensing has been demonstrated. In this work, a novel dynamic sensing scheme based on backward-pumped random fiber laser is proposed, and its sensing bandwidth is both theoretically and experimentally studied. Particularly, a Raman-gain-modulated power-balance model reveals that the RFL intensity has instantaneous and linear response to remote feedback disturbances, laying the ground for ultra-high-bandwidth sensing; in a proof-of-concept experiment, RFL with 100 km fiber and a far-end fiber Bragg grating is used for dynamic strain sensing, and 65 kHz dynamic sensing is achieved, the sensing bandwidth of which is two orders higher than that determined by the lightwave round-trip time as in the pulse-probing cases. This work paves the way for ultra-high-bandwidth and ultra-long-distance interrogation for optical fiber sensors. [ABSTRACT FROM AUTHOR]

An InP-based mode-locked laser photonic integrated circuit with a repetition rate of 10 GHz is optically synchronized to a SiN microresonator-based dissipative Kerr soliton with a repetition rate of 305 GHz. The synchronization is achieved through regenerative harmonic injection locking assisted with electro-optic division which results in an optical frequency division factor of 18. The repetition rate of the dissipative Kerr soliton is stabilized through electro-optic division and transferred to the mode-locked laser, where we measure a fractional frequency instability in the repetition rate of $10^{-10}$ at 1 s with a $1/\tau$ trend. Furthermore, we also stabilize the repetition rate of the dissipative Kerr soliton using the mode-locked laser’s repetition rate beat as a feedback point. [ABSTRACT FROM AUTHOR]

With on-chip integration enabled, resonant integrated optical gyroscope (RIOG) is a promising choice of rotation rate sensor in the novel micro inertial navigation systems. However, various optical errors, especially the backward-light-related errors including backscattering error and back-reflection error caused by the manufacturing process and heterogeneous materials in the optical elements, limit the performance of RIOG. Here, we propose the frequency spectrum separation method to suppress the backward-light-related errors for improving the detection precision of RIOG. The method can separate the angular velocity signal from the backward-light-related errors in the frequency domain. And, we investigate the optimal strategy of modulation frequencies to obtain maximum spectrum gaps of clockwise and counter-clockwise light for suppressing the backward-light-related errors, and simultaneously maintain maximum demodulation gain and minimum nonlinearity of optical effect for improving detection sensitivity. With the frequency spectrum separation method, the long-term bias stability of RIOG achieves 0.3°/h based on Allan deviation, which is the best reported result to the best of our knowledge. The proposed method of backward-light-related errors suppression can significantly improve the detection precision of RIOG based on a photonic chip, which contributes to the application of RIOG in the novel micro inertial navigation field such as cluster unmanned aerial vehicles and micro-nano satellites. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a novel multi-channel chaos generation scheme, which can simultaneously produce multiple chaotic signals with wideband spectrum and suppressed time-delay signature (TDS). In this scheme, we introduce an external self-phase-modulated feedback (ESPMF) to improve the optical bandwidth of the initial chaos generated by a conventional external-cavity semiconductor laser (ECSL), then optical tunable filters (OTFs) are utilized to simultaneously extract three-channel chaotic outputs. The experimental results show that the proposed scheme has three main advantages. Firstly, it can simultaneously generate multiple chaotic outputs with different central wavelengths and low correlations. Secondly, the bandwidth of original ECSL-based chaos can be improved by several times, and the undesired TDS characteristics in the original chaos can be completely suppressed to an indistinguishable level (lower than 0.02). Thirdly, the bandwidths of these chaotic outputs are flexible and can be adjusted within a wide range of 20 GHz by controlling the filter bandwidths. In addition, we demonstrate an application of the proposed chaos generation scheme in random bit generation (RBG), and multi-channel high-speed random bit sequences with a total generation rate over Tb/s and NIST verified randomness are simultaneously obtained. [ABSTRACT FROM AUTHOR]

A new dither technique is presented to control the bias voltage of an intensity modulator of the Mach-Zehnder type to maintain the quadrature operating point. This method is based on the direct calculation of the operating point deviation. The calculation uses an exact mathematical model of the modulator. The data used for calculating the deviation value are obtained from an analysis based on a fast Fourier transform with a properly set level of vector averaging. Unlike other solutions, the proposed feedback system does not use an approach with a proportional-integral or proportional-integral-derivative controller. Moreover, the presented stabilization method is robust and invariant regarding changes in the input optical power of the modulator and immune to prospective fluctuations of most of the feedback loop parameters. With the proposed approach in its present form, a 3-hour stability of ±0.08° is achieved. This value reflects the limitations of the equipment used. Even better results can be expected using more powerful hardware. [ABSTRACT FROM AUTHOR]

Analog photonic computing has been proposed and tested in recent years as an alternative approach for data recovery in fiber transmission systems. Photonic reservoir computing, performing nonlinear transformations of the transmitted signals and exhibiting internal fading memory, has been found advantageous for this kind of processing. In this work, we show that the effectiveness of the internal fading memory depends significantly on the properties of the signal to be processed. Specifically, we demonstrate two experimental photonic post-processing schemes for a 56 GBaud PAM-4 experimental transmission system, with 100 km uncompensated standard single-mode fiber and direct detection. We show that, for transmission systems with significant chromatic dispersion, the contribution of a photonic reservoir's fading memory to the computational performance is limited. In a comparison between the data recovery performances between a reservoir computing and an extreme learning machine fiber-based configuration, we find that both offer equivalent data recovery. The extreme learning machine approach eliminates the necessity of external recurrent connectivity, which simplifies the system and increases the computation speed. Above 31 dB OSNR, the photonics-based equalization exhibits a lower BER than the respective offline DSP-based KK receiver. [ABSTRACT FROM AUTHOR]

Injecting a spectrally pure oscillator signal into a noisy optoelectronic oscillator (OEO) can result in a drastic reduction of the phase noise; however, a unified explanation of this effect may be obscured by implementation-specific models. We show based on an analytically tractable theory that the power spectral density $S_f(\omega)$ of an ideal injection-locked (IL) oscillator signal is composed of a narrow Lorentzian peak at the center frequency lying on top of a broader roughly Lorentzian pedestal using an Ornstein-Uhlenbeck model obtained from the Adler equation in the weak-noise limit. The injected signal imposes its phase on the IL oscillator providing a restoring force for the phase, leading to this fundamentally different behavior of $S_f(\omega)$ compared with a self-sustaining oscillator. Our treatment is shown to be grounded in the physics-based approach for IL OEOs of A. F. Talla, R. Martinenghi, G. R. Goune Chengui, J. H. Talla Mbé, K. Saleh, A. Coillet, G. Lin, P. Woufo, and Y. K. Chembo, “Analysis of phase-locking in narrow-band optoelectronic oscillators with intermediate frequency,” IEEE J. Quantum Electron., vol. 51, 5000108, 2015. [ABSTRACT FROM AUTHOR]

A direct-modulation OEO (DM-OEO) generating stable 10 GHz and 20 GHz signals is presented. A single loop and a dual loop approach are implemented and compared. We show an output signal of 15 dBm RF power, and a phase noise as low as −135 dBc/Hz at 10 kHz offset from the 10 GHz carrier. The 20 GHz second harmonic exhibits a noise level of −127 dBc/Hz at 10 kHz. A high spur level reduction is also obtained in the dual loop architecture. [ABSTRACT FROM AUTHOR]

Photonics-based techniques spearhead the generation of high-frequency signals in the millimeter-and Terahertz wave, crucial for ultrabroadband mobile wireless link development. Photonic integration is enabling to provide the photonic approach with added advantages of energy-efficiency, flexibility and scalability, in addition to signal quality. We present an optical heterodyne system based on a novel dual laser module containing two InP-Si3N4 hybrid lasers with intracavity wavelength selective optical filters with output optical power per laser of up to 15 dBm (31 mW), wide wavelength tuning of about 60 nm, and narrow optical linewidth below 100 kHz. To the best of our knowledge, we present for the first time the continuous-wave generation of RF frequencies over a wide tuning range from C-band (4 GHz – 8-GHz) to W-band (75 GHz – 110 GHz) achieving record low RF electrical linewidth around 108 kHz and long-term drift < 12 MHz with two free-running lasers. This is the best beat-note linewidth obtained with such an integrated source in a free-runnning regime and with a wide tuning range ever reported. [ABSTRACT FROM AUTHOR]

Wavelength-dependent power excursions in gain-controlled erbium doped fiber amplifiers (EDFA) is a challenging issue in optical networks. We investigate a launch channel power control method using reinforcement learning (RL) to mitigate the power excursions of EDFA systems. A machine learning engine is developed, trained and evaluated with four different policy-gradient RL algorithms that are compared according to two main criteria: achieved power excursion reduction and learning time. Different scenarios are considered with 12-, 24-, 40- active channels at fixed wavelengths and with variable number of active channels (between 12 and 64) assigned randomly at different wavelengths during RL process. We show 62% power excursion reduction in the 40-channel scenario and 28% in the variable scenario, which demonstrates the promising role of online RL approach for controlling power excursion in EDFA systems. [ABSTRACT FROM AUTHOR]

Integrating photonics with CMOS electronics in silicon is essential to enable chip-scale, electronic-photonic systems that will revolutionize classical and quantum communication and computing systems. However, the lack of an on-silicon isolator, capable of blocking unwanted back reflections and ensuring the stable operation of the laser, precluded many previous demonstrations from providing single-chip solutions. For most optical systems employing a laser, magneto-optic isolators have been indispensable, but such isolators are incompatible with silicon. To stabilize on-chip lasers, reflections-cancellation circuits were proposed as a way to reduce the reflections going back to the laser. Yet, a stable laser against time-varying back reflections was never demonstrated. Here we demonstrate a stable quantum well-distributed feedback (QWDFB) laser against slowly time-varying reflections using a reflections-cancellation circuit (RCC) on a foundry-produced, silicon-photonic (SiP) chip. The optical spectrum and the relative intensity noise (RIN) of the laser when the RCC was running is comparable to when an isolator was used. By accurately locking the laser in a stable optical feedback regime, the RCC further enhances the QWDFB laser performance by reducing its linewidth by a factor of 100, down to 3 kHz. Both results are enabled using novel techniques in the design, calibration, tuning, and control of the proposed SiP RCC. The optical insertion loss of the RCC is less than 1.5 dB for reflections smaller than −20 dB and can yield isolation ranges of up to 64 dB. Our device paves the way towards the mass production of fully integrated, low-cost electronic-photonic silicon chips without attaching magneto-optic isolators between the laser and the SiP chip. [ABSTRACT FROM AUTHOR]

A method of generating flat broadband chaos by using a semiconductor laser subject to asymmetric dual-path optical feedback is proposed and demonstrated experimentally. In the asymmetric dual-path optical feedback, one path is connected with an optical band-pass filter (OBPF) to form filtered path, and the other is a traditional optical feedback path named non-filtered path. Through the beating frequency effect between the filtered mode and the non-filtered chaos mode, a chaotic signal with the standard bandwidth of 36.1 GHz and the spectrum flatness of 5.8 dB is generated experimentally. Furthermore, the effects of filter frequency detuning, filter bandwidth and feedback strength on the bandwidth and flatness of chaos are investigated. The results show that the bandwidth enhancement and the flat spectrum can be achieved simultaneously in a wide range of operating parameters. [ABSTRACT FROM AUTHOR]

A bidirectional white-lighting wavelength-division-multiplexing (WDM) visible light communication (VLC)–underwater wireless optical communication (UWOC) converged system with an office-level white light illumination of 514 lux and a symmetric downstream/upstream data rate of 150 Gb/s is demonstrated successfully. Red, green, and blue (RGB) laser diodes with two-stage light injection and optoelectronic feedback schemes are operated to provide bidirectional acceptable office-lighting and high-transmission-rate. White light is created/separated by combining/dispersing RGB lights utilizing two cascaded transmission gratings at the transmitting/receiving side. By employing PAM4 modulation and RGB triple-light WDM technique, the transmission rate is enhanced remarkably to 150 Gb/s for downlink/uplink transmission. Acceptable office-lighting (>500 lux) is attained through 3 m plastic optical fiber transport. Additionally, good bit error rate performance of 10−9 and accepted PAM4 eye diagrams are obtained over 10 m free-space transmission with 5 m ocean underwater link. The bidirectional white-lighting WDM VLC–UWOC converged system demonstrated here is beneficial for the development of white-lighting with high-speed optical wireless communications. [ABSTRACT FROM AUTHOR]

We have developed a compact 4 × 80-Gbps transmitter-receiver optical subassembly (TROSA) module using a chip-to-chip optical butt-coupling method. For a Tx submodule, four directly modulated laser (DML) chips precisely bonded on a silicon DML-carrier are simultaneously butt-coupled to a vertically polished silica-based arrayed waveguide grating (AWG) chip, revealing good optical characteristics insensitive to external optical feedback. A Rx submodule is realized by passive alignment between a 45°-polished AWG and four surface-illuminated (SI)-PD chips, and subsequently integrated with the Tx submodule into a single package case to form the compact TROSA module whose size is only 25 mm × 15 mm × 6.4 mm and compliant to fit into the QSFP-DD/OSFP transceiver. The fabricated TROSA module shows clear Tx optical eye patterns at 80-Gbps PAM4 signals for all channels, achieving the Rx sensitivities within −7.1∼−6 dBm at a bit error rate (BER) of 2.4 × 10−4. [ABSTRACT FROM AUTHOR]

We investigate the feasibility of pulse amplitude modulation-N (PAM-N) signal transmission over a photonics-based cost-effective THz wireless link. A direct detection-based THz wireless link was established by using commercialized optical and electrical components designed for the WR 3.4 frequency band (220–330 GHz). At the THz signal transmitter, a uni-traveling carrier photodiode (UTC-PD) was used as a photomixer. The maximum output power of the UTC-PD was ∼47 μW when it was supplied with +17 dBm of optical power. At the THz signal receiver, the Schottky barrier diode was used as a diode detector; an offline digital signal processor (DSP) with a decision feedback equalizer was also used to mitigate the inter-symbol interference penalty. In the back-to-back configuration (i.e., antenna of Tx and Rx are placed at a distance of ∼1 cm), the driving condition of the optical modulator was optimized to obtain the best transmission performances of the PAM-N signal. A carrier frequency of 300 GHz is carefully selected considering the bit error rate (BER) dependency on the carrier frequency. After the optimization procedure, the BER performances were analyzed by varying the PAM order, baud rate, and wireless transmission distance. Finally, a 90 Gb/s PAM-8 signal is successfully transmitted over a wireless distance of 1.4 m, in which the measured BER is below the 20% soft-decision forward error correction threshold (2.0 × 10–2). [ABSTRACT FROM AUTHOR]

Coherent optical communication is considered as an indispensable solution to the ever-increasing demand for higher data rates. To reduce the cost and form factor of coherent transceivers, full integration of photonic devices including lasers, modulators, amplifiers, photodetectors, and other components is necessary. However, as fabricating optical isolators on chip remains extremely challenging, optical feedback, which can degrade the coherence of semiconductor lasers, becomes the main obstacle, thwarting large-scale photonic integration. An appealing solution to such a problem is to use semiconductor lasers with intrinsic insensitivity to optical feedback as the integrated light sources. The heterogenous Si/III-V lasers, with their built-in high-Q resonators, are expected to possess a robustness to optical feedback which exceeds by several orders of magnitude compared to commercial III-V distributed feedback (DFB) lasers, which will be validated here. We present data showing that the heterogeneous Si/III-V lasers can preserve their phase coherence under much larger optical feedback and therefore function without severe degradation in isolator-free coherent optical communication systems. [ABSTRACT FROM AUTHOR]

Single section quantum dash (QDash) mode locked lasers (MLL) can provide a flat and broadband optical frequency comb with low energy consumption, operational simplicity and large-scale low-cost production possibilities. MLL longitudinal modes can be employed as single carriers with a regular spectral spacing in a dense wavelength division multiplexing (DWDM) link, making them promising components for next generation DWDM transceivers. However, individual modes of a MLL suffer from relatively high phase noise while high capacity coherent transmission requires carriers with low phase noise. Optical feedback, which is a well-known method to reduce the linewidth of a single mode laser, can be used to stabilize comb lasers. This article reports on the investigation of phase noise properties of comb lines delivered from a single section QDash MLL under optical feedback and shows that each MLL longitudinal mode optical linewidth can be drastically narrowed. It enables coherent transmission with extended link budget over 50 km at Tbit/s line rates. [ABSTRACT FROM AUTHOR]

With external optical feedback (EOF), a laser diode (LD) can operate at different dynamic states. In this work, an LD with EOF is set at period-one (P1) oscillation state to generate microwave photonic (MWP) signal for sensing. Firstly, the P1 state boundary of the LD is determined and then the influence of the LD controllable parameters on the boundary is studied by solving the well-known Lang–Kobayashi equations. A set of parameters selection rule for designing an LD based MWP sensing system is obtained. In addition, a measurement algorithm for recovering the displacement from an MWP sensing signal is developed. By making full use of the sensing information carried in both amplitude and frequency of the MWP signal, displacement sensing with high resolution and large range can be achieved. Both simulations and experiments are conducted to verify the proposed method and show it is capable of realizing wide measurable range, high measurement sensitivity, and high resolution sensing. [ABSTRACT FROM AUTHOR]

This paper reports a reconfigurable architecture to generate and maintain the linearity of frequency-swept semiconductor lasers for both current rising (up-sweep) and falling (down-sweep) edges, respectively. This method combines both passive and active controls to achieve a high frequency-sweep linearity using an integrated digital architecture. Both the passive and active control utilize the same feedback—the interferometric signal of the laser's output—which has a frequency proportional to laser frequency-sweep velocity. This architecture is highly reconfigurable, allowing key parameters to be changed, including sweep velocity, tuning range, and sweep duration, making it a highly versatile and flexible approach suitable for a range of semiconductor lasers and a variety of applications. This architecture was implemented on an FPGA to validate the concept and evaluate its performance. By precisely tracking the interferometric feedback signal, it was found that this approach allowed a semiconductor laser to generate and maintain a phase error of less than $\frac{\pi }{2}$ within the regions of interest at different sweep velocities for both up-sweep and down-sweep cycles. Given that the delay length of the interferometer is 226 ns, the instantaneous optical frequency error is within 0.12% at 1552 nm. [ABSTRACT FROM AUTHOR]

Optical feedback has an impact on the tunability of lasers. We created a model of a tunable distributed Bragg reflector (DBR) laser describing the effect of optical feedback from a constant reflector distance on the wavelength tuning. Theoretical and experimental results are in good agreement. A further discussion of the model sheds light on design rules to reduce the effect of optical feedback on the tuning behavior. We introduced a new parameter called mode loss difference (MLD) as a metric for the feedback tolerance of the tuning behavior. A large MLD indicates higher tolerance of the laser to cavity length variations. [ABSTRACT FROM AUTHOR]

We propose graph representations for reconfigurable photonic mesh circuits. Waveguide mesh circuits are abstracted into a graph to highlight the connectivity and topology. We model the optical ports as graph nodes. Performance metrics for each connection are incorporated into the edge attributes in categories such as propagation loss, crosstalk penalty, power consumption, phase accumulation, and so on. We use three types of graph representations for tunable couplers to model the flow of light and create a circuit graph representation to an example hexagonal mesh. The representation should respect the physics of waveguide circuits (e.g. directional flow of light). Of the three types, the directed graph with eight artificial nodes performs best for solving light distributions with feedback paths. This graph representation is demonstrated in four distribution cases: a single pair input-output, multi-pair inputs and outputs without collisions, a single input to multiple outputs (distribution), and multiple distributions without collisions. The programming tolerance against malfunctioning tunable elements is also demonstrated. With this circuit representation, we can reduce all these distribution cases to different graph problems and leverage a wealth of existing algorithms developed in graph theory to program the photonic mesh. Thus we provide a systematical strategy to design and program complex reconfigurable photonic circuits, especially in photonic meshes with feedback paths. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a Software-Defined Network (SDN) enabled Hybrid Polling Algorithm for packet Contention Resolution (HPACR) in optical Data Center Network (DCN) based on Fast Optical Switches (FOS). Fast Semiconductor Optical Amplifier (SOA)-based buffer-less FOS with nanoseconds reconfiguration time are employed for interconnecting Top-Of-the Racks (TORs) switches with distributed flow control. Under the realistic 10 Gb/s server traffic, the network performance of the DCN employing the proposed HPACR algorithm is experimentally assessed. Results show that the HPACR increases 13.6% throughput at the high load of 1.0 and decreases the 75% contention ratio for the load >0.5 with respect to the traditional packet contention resolution methods. Based on the experimental parameters, a simulation platform is built to numerically evaluate the HPACR performance of DCN with 64 TORs connecting 1280 servers. The results show that HPACR improves the system performance by increasing the throughput by 5% compared with existing algorithms, reducing the TOR-to-TOR latency by 18.5%, and decreasing the packet loss by 14.1% at the load of 1. [ABSTRACT FROM AUTHOR]

This article presents a detailed study of injection-locking characteristics of a single-loop optoelectronic oscillator (OEO) under strong radio frequency (RF) signal injection. We provide formulas for the steady-state phase-angle after locking, and the locking time for a higher level of a locking signal, considering only the phase perturbation. We also provide a compact expression for the main spectrum components of the pulled OEO in terms of the beat frequency, which is useful to evaluate and quantify the injection-pulling effects in the OEO. The experimental results provide partial support to the conclusion of the analysis. The analytical and experimental results are presented in design curves, which can be used to determine the required properties of an injection-locked OEO for a specific performance of the oscillator. [ABSTRACT FROM AUTHOR]

Probabilistically shaped orthogonal frequency-division multiplexing (PS-OFDM) signal is proposed and experimentally demonstrated in a low-cost intensity-modulation and direct-detection (IM-DD) system for optical access networks. The concatenation of constant composition distribution matcher (CCDM) and low-density parity-check (LDPC) code sequentially implements probabilistic amplitude shaping (PAS) and channel coding. Thanks to the precoding scheme, all data subcarriers can be treated equally by one optimized probabilistic distribution. Experimental results demonstrate that constant amplitude zero autocorrelation sequence (CAZAC) precoding can offer optimal performance compared to orthogonal circulant transform (OCT) precoding and discrete Fourier transform spread (DFT-spread) in terms of increasing generalized mutual information (GMI) and reducing PAPR. Moreover, probabilistically-shaped 64-QAM OFDM signal can provide shaping gains of 1.04/0.94/0.64 dB at 4.0/3.6/3.0 bits/QAM symbol compared to traditional 64-QAM OFDM signal. Meanwhile, compared to traditional 16-QAM OFDM, it can offer shaping gains of 1.64/0.64 dB at 3.6/3.0 bits/QAM symbol. In addition, net data rate ranging from 32.66 to 43.55-Gb/s over 20-km single mode fiber (SMF) can be achieved by only adjusting the probabilistic distribution. [ABSTRACT FROM AUTHOR]

We propose a wideband chaos generation scheme that shows significant bandwidth enhancement and excellent time delay signature (TDS) suppression, by introducing self-phase modulation in the feedback loop of external-cavity semiconductor laser (ECSL) and passing the ECSL-generated chaos through a dispersive component. Proof-of-concept experiment and thorough complementary numerical simulations are carried out to study the efficient bandwidth and TDS characteristics of the generated chaos. The results demonstrate that with the joint contributions of self-phase modulation and dispersion, the efficient bandwidth of chaos can be enhanced by several times and the spectrum gets much flatter. Simultaneously, the TDS can be completely suppressed, in a wide operation parameter range. The proposed wideband chaos generation scheme shows great potential for the applications of fast random bit generation, high-resolution chaotic radar, and high-speed chaos communication. [ABSTRACT FROM AUTHOR]

Semiconductor mode-locked lasers can be used in a variety of applications ranging from multi-carrier sources for wavelength-division-multiplexed communication systems to time-base references for metrology. Their packaging in compact chip- or module-level systems remains, however, burdened by their strong sensitivity to back-reflections, quickly destroying the coherence of the mode-locking. Here, we investigate the stabilization of mode-locked lasers directly edge coupled to a silicon photonic integrated circuit, with the objective of moving isolators downstream to the output of the photonic circuit. A 2.77-kHz 3-dB RF linewidth, substantially improved compared to the 15.01 kHz of the free running laser, is obtained in the best case. Even in the presence of detrimental reflections from the photonic circuit, substantial linewidth reductions from 20 to 8.82 kHz, from 572 to 14.8 kHz, and from 1.5 MHz to 40 kHz are realized. [ABSTRACT FROM AUTHOR]

A 28-GHz millimeter-wave (MMW) carrier optically heterodyned from an orthogonally polarized tri-color (3λ-⊥-SCM) transmitter, which exhibits single-carrier modulation (SCM) to suppress the dispersion induced fading distortion after long-reach transmission, is demonstrated for building up the long-reach MMW-over-fiber with a distance of 75 km. The SCM-based encoding is obtained by seeding the tri-color master into slave colorless laser diode, which only supports TE-mode feedback with a polarized suppression ratio over 40 dB. After resuming back to the SCM tri-color optical carrier with orthogonal polarization (3λ-⊥-SCM), its delivered data can suppress the intensity noise induced by beating between adjacent modes such that the single-to-noise (SNR) can be improved by 2 dB. Comparing with the traditional parallel polarized multi-carrier modulation (MCM) based tri-color optical carrier (3λ-//-MCM), the orthogonally polarized tri-color optical carrier with SCM (3λ-⊥-SCM) can deliver 64 quadrature amplitude modulation orthogonal frequency-division multiplexing (QAM OFDM) data with its maximal encoding a bandwidth of 8.3 GHz (50 Gbit/s), which provides the error vector magnitude (EVM) of 8.8%, the SNR of 21.2 dB and the bit error rate of 3.5 × 10–3 after passing through a 75-km-long single-mode fiber. To perform the MMW wireless access network (WAN) link after 75-km fiber wired transmission, the heterodyned 28-GHz MMW carrier is obtained from the orthogonally polarized SCM tri-color optical carrier (3λ-⊥-SCM) with its peak power improved by 29 dB (from –50 to –31 dBm) and CNR enhanced by 17 dB (from 32 to 49 dB), as compared to those of the parallel polarized MCM tri-color case (3λ-//-MCM). After wireless transmission for 10-m with horn-antenna pair in free space, the maximal encoding bandwidth of the 16-QAM OFDM delivered by the orthogonally polarized SCM tri-color optical carrier (3λ-⊥-SCM) is 4.5 GHz (with 18 Gbit/s capacity). [ABSTRACT FROM AUTHOR]

We report a fiber optofluidic laser based on a commercial air-clad optical fiber (ACF). The ACF with air holes surrounding the silica core supports pseudo whispering gallery mode, which can provide optical feedback for lasing. 1 mm Rhodamine 6G in quinoline (n = 1.63) is withdrawn into the ACF using the capillary action. Due to the high refractive index difference between gain medium and silica, about 83% of light intensity is confined within the liquid gain medium, leading to a low threshold of about 1.1 μJ/mm2. The lasing mechanism is investigated by both numerical simulation and experimental demonstration. The laser emission was observed with various refractive index in the range between 1.52 and 1.63. The proposed fiber optofluidic laser is promising for biochemical sensing. [ABSTRACT FROM AUTHOR]

We present the design and numerical study of a nonlinear equalizer for optical communications based on silicon photonics and reservoir computing. The proposed equalizer leverages the optical information processing capabilities of integrated photonic reservoirs to combat distortions both in metro links of a few hundred kilometers and in high-speed short-reach intensity-modulation-direct-detection links. We show nonlinear compensation in unrepeated metro links of up to 200 km that outperform electrical feedforward equalizers based equalizers, and ultimately any linear compensation device. For a high-speed short-reach 40-Gb/s link based on a distributed feedback laser and an electroabsorptive modulator, and considering a hard decision forward error correction limit of $0.2 \times 10^{-2}$ , we can increase the reach by almost 10 km. Our equalizer is compact (only 16 nodes) and operates in the optical domain without the need for complex electronic DSP, meaning its performance is not bandwidth constrained. The approach is, therefore, a viable candidate even for equalization techniques far beyond 100G optical communication links. [ABSTRACT FROM AUTHOR]

In the latest radio interferometer, a system equipped with a large number of receivers from low frequency to high frequency ($\sim$ THz) was realized. In addition, an ultra-wide bandwidth receiver and a super-terahertz receiver are being developed as next-generation receivers. In this situation, a new local oscillator capable of handling next-generation receivers is necessary. Due to the very wide bandwidth requirement, an optical local oscillator, which can correspond to a system that conventionally requires a high-frequency phase-locked oscillator with a different amplifier for each receiver, is expected. An optical phase-locked oscillator can be realized at a remote station that is synchronized with a transmitted dual-wavelength reference signal from a reference station. The high-frequency reference signal transmission is required because it is necessary to suppress the multiplication order to reduce individual multiplied noise at the remote station. In this work, we focus on an external modulator-scheme optical phase-locked oscillator that performs a high-frequency phase comparison between photonic microwave signals at a remote station. The system makes fiber-coupled stations a coherent system. It is possible for the optical phase-locked oscillator to handle a very wide bandwidth receiver, a set of receivers with many bands, and a multifeed receiver with one device. [ABSTRACT FROM AUTHOR]

The alignment and fixation of multiple single-mode optical fibers to photonic integrated circuits is currently a challenging, expensive, and time-consuming task. In this paper, we present a concept for a sub-micrometer accurate multi-fiber array, where fibers are actively aligned with respect to each other and fixated to a flat carrier using ultraviolet-curable adhesive. Adhesives are prone to shrinkage, which can disturb the fiber alignment. As a result, especially, the fixation process forms the bottleneck in reaching the required alignment and not the alignment process itself. Simulations are performed to investigate the sensitivity of process variables on the adhesive bond geometry, which is important for the shrinkage amplitude. Furthermore, an experimental setup has been designed and fabricated to measure the shrinkage-induced fiber displacement for three selected types of adhesives. The results show a controllable adhesive shrinkage, where fibers can be aligned with a position reproducibility of $\pm {\text{40}}$ nm, which is more than sufficient for the most critical fiber alignment applications. With this concept, an important step can be made in enabling sub-micrometer accurate photonic interconnects in a cost-effective way, which is suitable for automated production. [ABSTRACT FROM AUTHOR]

In this paper, we present a monolithically integrated coherent receiver with on-chip grating couplers, 90° hybrid, photodiodes and transimpedance amplifiers. A transimpedance gain of 7.7 kΩ was achieved by the amplifiers. An opto-electrical 3 dB bandwidth of 34 GHz for in-phase and quadrature channel was measured. A real-time data transmission of 64 GBd-QPSK (128 Gb/s) for a single polarization was performed. [ABSTRACT FROM AUTHOR]

THz wireless signals have become of interest for future broadband wireless communication. In a scenario where the wireless signals are distributed to many small remote antenna units, this will require systems which allow flexible frequency tuning of the generated THz carrier. In this paper, we demonstrate experimentally the implementation of two tuning methods using an optical frequency comb generator for coherent optical frequency tuning in THz wireless-over-fiber systems. The first method is based on using a photonic integrated circuit optical phase lock loop (OPLL) subsystem implemented as a high quality optical filter for single comb line selection and optical amplification. The OPLL generates an optical carrier, which is frequency and phase stabilized in reference to one of the optical comb lines with a frequency offset precisely selectable between 4 and 12 GHz. The second method is based on optical single sideband suppressed carrier (SSB-SC) modulation from the filtered comb line using an optical IQ modulator. With this technique, it is possible to suppress the other unwanted optical tones by more than 40 dB. This generated optical carrier is then heterodyned with another filtered optical comb line to generate a tuneable and stable THz carrier. The full system implementations for both methods are demonstrated by transmitting THz wireless signal over fiber with 20 Gb/s data in QPSK modulation. The system performance and the quality of the generated THz carrier are evaluated for both methods at different tuned THz carrier frequencies. The demonstrated methods confirm that a high quality tuneable THz carrier can easily be implemented in systems where dynamic frequency allocation is required. [ABSTRACT FROM AUTHOR]

A self-oscillating optical frequency comb generator (SOFCG) is demonstrated by applying the optoelectronic loop feedback to an optical frequency comb generator (OFCG) based on a dual-drive Mach–Zehnder modulator. The resulting SOFCG provides 23 comb lines with a frequency spacing of 11.84 GHz, corresponding to the oscillation frequency defined by the optoelectronic loop. The corresponding OFCG is also implemented by replacing the optoelectronic feedback loop with a microwave synthesizer at 11.84 GHz. A 94.8-GHz millimeter (mm)-wave signal is then generated by selecting two tones and heterodyning in a high-speed photodetector for both the SOFCG and the OFCG. The SOFCG system offers superior single sideband phase noise performance compared to the OFCG approach. Using the SOFCG developed here, it is possible to generate mm-wave signals up to 260 GHz, and this method is applicable to multiband radio-over-fiber (RoF) links. An RoF link is implemented with the SOFCG at 94.8 GHz. An LTE Advanced OFDM FDD 64-QAM signal of 20-MHz bandwidth is transmitted over a 1.3-m wireless distance. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a compact optical-microwave phase detector taking advantage of polarization modulation. The proposed polarization modulator-based phase detector (PolM-PD) shows 460-attosecond residual timing jitter integrated from 1 Hz to 100 kHz when synchronizing an 8-GHz dielectric resonator oscillator to a free-running 250-MHz mode-locked Er-fiber laser. The absolute single-sideband phase noise of the locked 8-GHz DRO is –138 dBc/Hz (–165 dBc/Hz) at an offset frequency of 10 kHz (10 MHz). The proposed PolM-PD has a compact structure implemented by all commercially available components. Because no fiber loop and delay line are used, the proposed PolM-PD can be potentially integrated on a chip. [ABSTRACT FROM AUTHOR]

An electrooptic chaos source is proposed based on phase-modulation-to-intensity-modulation conversion theory and an analog–digital hybrid time-delay feedback loop. The analog part takes the digital binary sequences from shift registers as input and converts them into an analog noise-like signal, from which new bits are determined. The dynamical characteristic of the generated waveform is studied in detail by means of spectrum analysis, periodicity analysis, correlation performance, and complexity analysis. Moreover, the evolution between chaos and stochastic noise is observed and measured by adopting the complexity–entropy curves when a random perturbation is introduced. Such a perturbation can help with recovering chaotic dynamics from degradation. Based on this chaotic source, a secure and robust synchronization scheme that can exactly reproduce the signal is put forward. The effect of parameters’ mismatch and noise level on the synchronization quality is also investigated. The scheme has the potential to be applied in various applications when a robust random source is needed. [ABSTRACT FROM AUTHOR]

The effect of optical feedback on the stability and linewidth of a photonic microwave generated in an optically injected single-mode vertical-cavity surface-emitting laser has been investigated in both cases of single feedback and double feedback. The stability of the photonic microwave is quantified by measuring the microwave frequency range using the long sweep time of the radio frequency spectrum analyzer. The effect of the feedback phase on the side peaks in the single feedback has also been studied. The experimental results show that both single feedback and double feedback can reduce the linewidth of the photonic microwave and improve its stabilization. A narrow linewidth photonic microwave spectrum with suppressed side peaks has been achieved in a single feedback configuration at the optimal feedback phase condition. The simulated results using the spin-flip model confirm that side peaks suppression in the single optical feedback configuration is sensitive to the feedback phase. For nonoptimal feedback phase conditions, single optical feedback can generate many side peaks which are related to external cavity. These side peaks can be suppressed by introducing the second feedback. [ABSTRACT FROM AUTHOR]

In this paper, a simple scheme for linear frequency-modulated (LFM) waveform generation based on a frequency-sweeping optoelectronic oscillator is proposed and demonstrated. The OEO is built up with an optically injected semiconductor laser and the oscillation frequency can be tuned by adjusting the optical injection strength. By applying an injection strength controller in the OEO for rapid frequency sweeping, an LFM microwave waveform can be generated. When the sweep period of the output frequency matches with the round-trip time of the OEO cavity, signal quality of the generated LFM waveform can be significantly enhanced by the high Q optoelectronic oscillation. In the experiment, an LFM signal with a bandwidth as large as 7 GHz, a chirp rate reaching 0.18 GHz/ns, and a time-bandwidth product (TBWP) up to 2804.2 is generated. The corresponding electrical spectrum is a frequency comb with a contrast as high as 47 dB. Based on this system, an improved scheme for extending the frequency and bandwidth of the generated LFM signal is proposed by employing a polarization modulator to implement microwave photonic frequency multiplication. With this method, an LFM waveform with a TBWP as large as 13839.1 (bandwidth 15.6 GHz; temporal period 887.12 ns) is obtained. [ABSTRACT FROM AUTHOR]