In this report, we summarize our recent achievements in free-space communications in the mid-infrared (MIR) region enabled by directly modulated quantum cascaded laser (QCL) at 4.65 µm (~65 THz). We have experimentally demonstrated a multigigabit free-space transmission link in the lab environment with the QCL operating at room temperature. The QCL chip is mounted on a commercial QCL mount with a water-cooled Peltier element. Multilevel modulation formats at different baud rates are generated and combined with the laser driving current at a custom-made bias-tee to drive and modulate the QCL. A commercial mercury cadmium telluride (MCT, HgCdTe) photovoltaic (PV) MIR detector with a built-in trans-impedance amplifier was used to receive the MIR free-space signal. With the receiver to be the bottleneck of the system bandwidth, the end-to-end 3-dB bandwidth was measured to be around 320 MHz, and the 6-dB bandwidth was around 450 MHz. We have successfully demonstrated up to 6 Gbps free space transmission with multilevel modulation formats, assisted with effective digital equalization techniques despite the limited bandwidth.
KEYWORDS: Quantization, Analog electronics, Radio over Fiber, Data compression, Orthogonal frequency division multiplexing, Modulation, Quadrature amplitude modulation, Transmitters, Receivers, Signal to noise ratio
With the approach of the 5G era, stringent requirements are imposed on the data transport solutions, including both of the supported transmission reach and the capacity. Radio-over-fiber technologies are considered to be promising candidates to cope with both aspects, owing to the low-loss and broad-bandwidth nature of the optical fibers. Meanwhile with such optical transport solutions, signals can be collected from the distributed remote radio sites and processed in a centralized manner. In this report, we target on the digital radio-over-fiber systems, and discuss about several key technologies, focusing on the aspects of coding and transmission, which could potentially enable terabit-scale data transport.
The performance of long-haul high speed coherent optical fiber communication systems is significantly degraded by the laser phase noise and the equalization enhanced phase noise (EEPN). In this paper, the analysis of the one-tap normalized least-mean-square (LMS) carrier phase recovery (CPR) is carried out and the close-form expression is investigated for quadrature phase shift keying (QPSK) coherent optical fiber communication systems, in compensating both laser phase noise and equalization enhanced phase noise. Numerical simulations have also been implemented to verify the theoretical analysis. It is found that the one-tap normalized least-mean-square algorithm gives the same analytical expression for predicting CPR bit-error-rate (BER) floors as the traditional differential carrier phase recovery, when both the laser phase noise and the equalization enhanced phase noise are taken into account.
We implement an SLM to generate laser beams of variable orbital angular momentum, also referred as Laguerre-
Gaussian beams. Input beam polarization takes into account a local birefringence of each pixel of the SLM. We identify
the beam polarization eigenstates allowing generate L-G beams of different order via matching variable birefringence of
every separate pixels. Zero-order beam passing through the SLM can interact with a generated OAM beam to create an
interference pattern. Experimental results demonstrate good agreement with simulations.
For an erbium-doped fiber laser mode-locked by carbon nanotubes, we demonstrate experimentally and theoretically a new type of the vector rogue waves emerging as a result of the chaotic evolution of the trajectories between two orthogonal states of polarization on the Poincare sphere. In terms of fluctuation induced phenomena, by tuning polarization controller for the pump wave and in-cavity polarization controller, we are able to control the Kramers time, i.e. the residence time of the trajectory in vicinity of each orthogonal state of polarization, and so can cause the rare events satisfying rogue wave criteria and having the form of transitions from the state with the long residence time to the state with a short residence time.
KEYWORDS: Digital signal processing, Semiconductor lasers, Phase shift keying, Semiconductors, Receivers, Interference (communication), Signal to noise ratio, Frequency modulation, Transmitters, Optical communications
We discuss about digital signal processing approaches that can enable coherent links based on semiconductor lasers. A state-of-the art analysis on different carrier-phase recovery (CPR) techniques is presented. We show that these techniques are based on the assumption of lorentzian linewidth, which does not hold for monolithically integrated semiconductor lasers. We investigate the impact of such lineshape on both 3 and 20 dB linewidth and experimentally conduct a systematic study for 56-GBaud DP-QPSK and 28-GBaud DP-16QAM systems using a decision directed phase look loop algorithm. We show how carrier induced frequency noise has no impact on linewidth but a significant impact on system performance; which rises the question on whether 3-dB linewidth should be used as performance estimator for semiconductor lasers.
In this paper, we investigate the phase noise elimination employing an optical pilot carrier in the high speed coherent
transmission system considering the equalization enhanced phase noise (EEPN). The numerical simulations are
performed in a 28-Gsymbol/s quadrature phase shift keying (QPSK) coherent system with a polarization multiplexed
pilot carrier. The carrier phase estimation is implemented by the one-tap normalized least mean square (NLMS) filter and
the differential phase detection, respectively. Simulation results demonstrate that the application of the optical pilot
carrier is very effective for the intrinsic laser phase noise cancellation, while is less efficient for the EEPN mitigation.
We report measurements of beat and birefringence correlation lengths, and the PMD value on a long single mode fiber. The proposed technique is based on the analysis of maximum and minimum polarization dependent gain in a fiber Raman amplifier. Results are consistent with a standard technique based on differential group delay measurements.
A solid-state dye laser with a microcavity whose size is comparable to the lasing wavelength is modeled by means of
the finite element method. The position of the pumping source affects the lasing mode spectrum. In comparison with a
single point source at the edge of the cavity, a random distribution of excitation sources in the central gain part of the
microcavity leads to depletion of odd longitudinal modes and to higher output efficiency of the laser radiation in even
modes. The simulation results are explained by simple physical considerations.
Wavelength tunability of a microcavity solid-state dye laser is modeled by means of the finite element method. We
investigate the application of two phenomena, thermoelastic expansion of the microcavity material and thermo-induced
change of refractive index, in order to tune microcavity mode frequencies by the variation of the effective optical path.
An optimized size of the laser microcavity is defined depending on the operation wavelength bandwidth and the glass
temperature of the gain material.
We report a new stochastic model for studying the polarization-dependent gain and gain fluctuations in Raman fiber
amplifiers with randomly varying birefringence as a function of the pump state of polarization and the polarization
mode dispersion parameter. Based on the presented model and our previous experimental data for polarization dependent
gain we develop a new experimental technique for birefringence correlation length measurement.
The apodization of diffractive optical elements can be realized by a local change of their diffraction efficiency. In the case of lithographic elements with step-like structure of the period, the variable diffraction efficiency can be achieved by a gradual transformation of the 2m step kinoform into its conjugate counterpart across the apodization region. In the present contribution we show experimental results confirming this idea, which until now was verified only by simulations. The apodized quaternary grating with locally varying diffraction efficiency was obtained on a SLM device as a programmable diffractive optical element by changing gradually the period's profile. Knowledge of the phase heights of the SLM's pixels is required for successful implementation of the apodization function. It was determined from Fresnel images of the binary phase gratings with different phase step height programmed on the SLM. The Fresnel images become then binary and their visibility depends on the phase height of the grating in a known way, what makes possible to calibrate the SLM.
With uniform partially coherent illumination of a sufficiently small coherence width, linear holographic axicons produce an extended axial line image of a desired length and a nearly uniform intensity. This result is demonstrated by numerical diffractive simulations based on generalized axicon phase functions and the space-frequency theory of optical coherence. The conclusions are also verified through analytical calculations employing the asymptotic method of stationary phase.
The potentials of laser-induced microwave photoconductivity (LMP) as a diagnostic method for semiconductor surface studies are considered in this paper. The principals and practical realization aspects including a block-diagram and technical characteristics of the LMP apparatus, and some experimental data on electron-recombination properties of Si and GaAs surfaces are presented. The data include investigation of non equilibrium carrier relaxation channels and rates, influence of various defects (traps, adhesion centers, dislocations etc.), and spatial variations of these relaxation characteristics in surface layers modified with different procedures (ion implantation, laser annealing, mechanical treatment). Among the remote action of surface modification and interaction of locally modified zones. These effects are shown to be very important in the physics and technology of semiconductor surface modification. It is concluded that the LMP method is very informative diagnostic tool for testing and studying of semiconductor surfaces and device structures used in microelectronics, power electronics and optoelectronics.
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