Virtual Journal of Laser

A pilot-carrier coherent low-earth-orbit (LEO) satellite to ground (LEO-to-Ground) downlink system using an optical injection phase lock loop (OIPLL) technique is proposed and its feasibility under Doppler frequency shift conditions is demonstrated. A fiber-optic based experimental system is configured and it is demonstrated that a 10 Gbps BPSK transmission system based on the proposed configuration can successfully maintain stable frequency and phase locking status under simulated Doppler frequency shift conditions. It is demonstrated that the stable locking status is maintained over a 10.3 GHz (54
$~$ppm) frequency offset with a maximum rate-of-change of up to 32.4 GHz/s (168 ppm/s), which is ample to meet the requirement for a coherent LEO-to-Ground downlink system. The locking capability of the experimental system for more rapidly changing Doppler frequency shift is investigated.

In this paper, the influence of packaging-induced RF signal degradation on an optoelectronic modulator module is investigated. A directly modulated laser (DML) is modeled and packaged in a butterfly-type package. A distributed 3-D electromagnetic model is built based on this laser module. In the packaging assembly procedure, impedance mismatching and ground discontinuity on microwave transmission will cause unwanted signal decays and resonances. We specify the RF degradation in three regions: 1) the RF connector, 2) the RF substrate, and 3) the mode transition region between the optoelectronic subsystem and the package. The RF transmission characteristics in these regions are extracted and analyzed in detail. The results indicate that by optimizing the packaging design, strong resonances and signal decays can be eliminated or compensated over a wide frequency range. The measured scattering parameters show that the proposed packaging assembly has a resonance-free bandwidth of 31.2 GHz, and the DML module exhibits a wide 3 dB bandwidth of 15.1 GHz.

In the absence of optical isolation, semiconductor lasers (SLs) are susceptible to external perturbations that determine the dynamical properties of the generated signals. Mutually coupled SL systems have been proved to exhibit diverse dynamics with the potential to behave synchronously. In this study, a multinodal star all-optical network topology is investigated in terms of synchronization and robustness; 50 SLs with varied operating frequencies around a central frequency
$omega_{0}$ operate as star nodes and mutually interact with a central SL (hub) through optical injection signals and under specific conditions exhibit a synchronized behavior. Additional nodes that are subsequently connected to the network or nodes that disconnect from the network do not alter the dynamical behavior and the robustness of the system. Especially for the newly connected nodes, appropriate optimization in their operating conditions includes them in the synchronized cluster.

We report an Er-doped, actively Q-switched, fiber laser, generating transform-limited pulses based on single-frequency fiber laser seeded ring cavity. The output pulsewidth can be tuned from hundreds of nanoseconds to several microseconds by changing the repetition rate or the open time of the electrical pulse trigger. This injection-seeded, Q-switched ring cavity fiber laser can be operated over the whole C-band. In addition, a theoretical model is developed to numerically study the pulse characteristics by changing the acousto-optic modulator transmission as well as several cavity parameters, such as the cavity length and loss. The numerical results are in good agreement with the experimental results.

In this paper, we present a new and compact structure for optical amplifiers based on erbium-doped microfiber coil resonators (MCR). The performance of the device is modeled by solving numerically the MCR coupled-mode equations and the rate equations in the steady-state regime. Characteristics of amplifier are analyzed for two-, three-, and four-turn MCRs. The signal gain spectrum is studied in terms of amplifier length, signal wavelength, and signal, and pump power. Simulation results demonstrate that such a device selectively amplifies specific wavelengths due to its resonance nature. It is possible to obtain optical gain as high as 30 dB using a 10 mW pump power and a 0.1
$mu$W signal power. The amplified peaks show very narrow full-width at half-maximum that are very promising for microlasers.

We propose and demonstrate a wavelength-division-multiplexed passive optical network providing both unicast and broadcast services using an offset polarization multiplexing technique. The downstream differential phase-shift keying (DPSK)-formatted unicast and broadcast signals are offset polarization-multiplexed at the central office and demultiplexed and detected at the optical network units without resorting to any polarization tracking. A portion of the offset polarization-multiplexed downstream signals is fed into a polarization-sensitive weak-resonant-cavity Fabry–Perot laser diode for upstream remodulation without any polarization control. Successful transmissions of 10-Gb/s downstream unicast and broadcast DPSK signals as well as 2.5-Gb/s upstream ON–OFF keying signal over a 20-km standard single-mode fiber are experimentally demonstrated. The robustness of the proposed scheme against polarization fluctuation along the link, relative bit delay between the unicast and broadcast signals, frequency deviation of the downstream signals from the delay interferometer (DI), and imperfection of the DI is investigated.

This paper, for the first time, experimentally presents direct comparisons of two 25 GHz spaced wavelength division multiplexing (WDM) systems using conventional dual-polarization (DP) 16-ary quadrature amplitude modulation and duobinary-shaped DP-quadrature phase shift keying (QPSK) modulation. Both systems operate at the same bit rate per channel of 112 Gbit/s, yielding a spectral efficiency of 4.1 bit/s/Hz. The comparisons are conducted for three different cases, i.e., the back-to-back sensitivity, the nonlinear tolerance over a 640-km standard single-mode fiber link, and the phase-noise tolerance (by means of simulations). The results show that the duobinary-shaped DP-QPSK system not only provides a 3.4 dB superior back-to-back sensitivity, but also exhibits a 3 dB higher tolerance against nonlinear impairments after 640 km transmission with three WDM channels. In addition, the numerical results indicate that both investigated systems provide similar tolerances to the laser phase noise given that the block length used in the carrier phase estimation is optimized.

We propose and experimentally demonstrate a new colorless wavelength-division-multiplexed passive optical network (WDM-PON) architecture with the Rayleigh backscattering (RB) interferometric beat noise mitigation by using cross-remodulation architecture. The proposed WDM-PON has a simply configuration by combining two WDM-PONs at two wavelength bands to support twice the number of users. We experiment different
$m$-quadrature amplitude modulation (QAM) (
$m = 16, 32$ and 64) orthogonal frequency division multiplexing (OFDM) downstream signal and the remodulated on-off keying (OOK) upstream signal by using the 2.5 GHz directly modulated laser (DML) and 1.2 GHz reflective semiconductor optical amplifier (RSOA) respectively. Hence, the total data rate achieved for the downstream signals are 10 Gb/s, 12.5 Gb/s, and 15 Gb/s respectively for different
$m$ -QAM. For the upstream signal, we over-drive the RSOA and 2.5 Gb/s OOK upstream traffic can be achieved.

A novel, simple, and short cavity design of single longitudinal mode (SLM) tunable erbium-doped fiber ring laser using a graphene-based saturable absorber is proposed and demonstrated as a tunable signal source. The SLM output is then mixed with another output signal from a tunable laser source (TLS) to generate tunable radio frequency (RF) signals. The tunable SLM fiber ring laser uses a short length of 1 m highly doped erbium-doped fiber as the gain medium. Graphene is used as a saturable absorber to generate the SLM operation, as opposed to the commonly used unpumped erbium-doped fiber. The tuning range of the fiber ring laser is determined by a tunable fiber Bragg grating, which can be tuned from 1547.88 to 1559.88 nm. A continuous wavelength spacing tuning range of 0.020–0.050 nm is obtained between the output of the SLM fiber ring laser and the TLS which is then mixed in a 6 GHz bandwidth optical-to-electrical convertor. This generates a corresponding RF signal of between 2.4 and 5.9 GHz with a low variation in output power. The current RF signal generation is limited by the frequency bandwidth of the optical-to-electrical convertor.

Yb
$_{2}$O
$_{3}$doped yttrium-rich alumino-silicate nano-particles based D- and P- (pentagonal) shaped optical fibers with core diameter
$sim$ 30–35
$mu$ m are fabricated using the conventional MCVD process and solution doping technique. Parameters of different stages of the fiber preforms fabrication are optimized to get uniform distributions of Al, Y, F, and Yb ions in the core region, ensured by the data of an EPMA analysis. In the presence of small amounts of fluorine, the size of nano-particles is maintained within 5–10 nm; the EDX data reveal that the nano-particles are rich in yttrium-alumino-silicate phase and are dispersed uniformly across the preforms core. The critical parameters of the processes involved at the fibers fabrication along with the nano-structuring and spectroscopic features are highlighted.