Photonics, Volume 10, Issue 8 (August 2023) – 95 articles

In order to capture true-color information of distant targets under extremely low light, a large-format low-light imaging system is designed based on an RGB filter wheel. By decomposing the system indicators, this study proposes a method for acquiring low-light true-color images using a large-aperture, low-distortion optical lens combined with an RGB filter wheel capable of multi-line sequential exposure. The optical field segmentation is achieved using a four-panel optical reflective prism, and the images from four high-sensitivity SCOMS detectors are stitched together to form a composite image. The working principle of the system is explained, and the low-light imaging capability is thoroughly evaluated. The dimensions and rotation speed of the filter wheel are then calculated in detail, ensuring accurate synchronization of the filter wheel’s speed and exposure time. The calculation method for the parameters of the four-panel reflective prism structure is investigated, mathematical expressions for the geometric parameters of the prism assembly are provided, and a prism assembly suitable for four-way spectral separation is designed. Based on the research and design results, a large-swath-width, low-light true-color imaging system is developed that is suitable for an environmental illuminance of 0.01 lux. The system achieves a ground pixel resolution of 0.5 m (at an altitude of 5 km) and an effective image resolution of 4 K × 4 K, and is capable of accurately reproducing target color information. Laboratory and field flight tests verified that the large-swath-width images obtained by the imaging system are clear, with high contrast and resolution. After image fusion and spectral registration, the color images exhibit full saturation and high fidelity, meeting the requirements of low-light true-color imaging under airborne conditions. The design methodology of this low-light imaging system can serve as a reference for the development of airborne low-light imaging equipment. Full article

The short-wavelength mid-infrared spectral range of the 2 μm waveband has the advantages of low transmission loss and broad gain bandwidth, making it a promising candidate for the next optical fiber communication window. It is thus highly desired to develop high-performance silicon photonic components in this waveband. Here, an efficient dual-layer grating coupler was designed on a 220 nm thick silicon-on-insulator based on raised polysilicon to address the low directionality issue. For the fiber tilted at an angle of 10°, the grating coupler’s simulated coupling efficiency reaches 80.3% (−0.95 dB) at a wavelength of 2002 nm. The 1 dB bandwidth is 66 nm. The structure is completely compatible with the standard silicon photonic fabrication process, making it suitable for large volume fabrication. Full article

All-fiber, polarization maintaining, narrow-bandwidth, Yb-doped fiber lasers with randomly distributed feedback operated near 976 nm were realized for the first time. It was shown that the laser operated in a single, longitudinal mode regime during intervals of a few seconds. At other times, the laser generated a few longitudinal modes, but its bandwidth was always below the resolution of the optical spectrum analyzer (0.02 nm). The linewidth of each single longitudinal mode of the laser was estimated to be below 20 kHz. The reasons for this observed laser behavior were discussed and methods for achieving stable, continuous wave operation in the single-longitudinal-mode regime were proposed. Full article

A novel channel-modulated static birefringent Fourier transform imaging spectropolarimeter (CSBFTIS) is introduced, which is based on a double Wollaston prism (DWP). With an adjustable air gap (AG), the spectral resolution can be adjusted by changing the AG. The CSBFTIS combines the channel-modulated imaging spectropolarimeter and the slit-free static birefringent Fourier transform imaging spectrometer technology with adjustable spectral resolution. The device is compact and robust, with a wide spectral range and a large luminous flux. Compared with various previous spectropolarimeters, it can greatly reduce the size of the spectral image data to adapt to different application requirements. A prototype is built, and simulation and experiments are carried out, and the results prove the effectiveness of the method. Full article

The resonant external field-assisted Breit–Wheeler process (Oleinik resonances) for strong electromagnetic fields with intensities that are less than the critical Schwinger field that has been theoretically studied. The resonant kinematics were studied in detail. The case of high-energy initial gamma quanta and emerging ultrarelativistic electron–positron pairs was studied. The resonant differential cross section was obtained. The generation of narrow beams of ultrarelativistic positrons (for Channel A) and electrons (for Channel B) was predicted with a probability that significantly exceeded the corresponding nonresonant process. Full article

We present a comparative theoretical study of silicon crystals irradiated with X-ray free-electron laser pulses, using hard X-ray photons of various energies. Simulations are performed with our in-house hybrid code XTANT based on Monte Carlo, Tight Binding and Molecular Dynamics simulation techniques. By comparing the strength of the coherently scattered signal and the corresponding electronic radiation damage for three X-ray photon energies available at the SACLA free-electron laser facility, we conclude that it would be beneficial to use higher photon energies for “diffraction-before-destruction” experiments. Full article

We take advantage of the dual emission properties of up-conversion fluorescence (UCF) and second harmonic generation (SHG) in Er³⁺-doped perovskite Na_0.95Er_0.05Nb_0.9Ti_0.1O₃ to fully temporally characterize the ultrashort laser pulse that excites Er³⁺-ion fluorescence. The chirped pulses from a broadband Ti:Sa oscillator are temporally characterized using the dispersion scan (d-scan) technique by using the SHG signal in the host perovskite at the same point where UCF is being produced by the same pulse. The pulse durations obtained range from ~45 fs to ~8 fs and positive and negative spectral phases are unambiguously identified. The temporal characterization is compared using a standard non-linear crystal and perfect agreement is obtained. These results show that it is possible to temporally characterize in-situ ultrashort laser pulses while they are inducing a UCF process, as long as the host generates second-harmonic signal. Full article

Simultaneous phase-shifting interferometry (SPSI) can simultaneously obtain multiple phase-shifted interferograms and can realize the dynamic wavefront measurement with the use of a phase-shifting algorithm. From the respect of a beam-splitting technique and phase shift achievement of the phase-shifting units, research progress on spatial phase shifting approaches for SPSI systems are classified and summarized, and the key problem affecting SPSI technology is discussed. To ensure the measurement accuracy, it is necessary to perform accurate position registration for multi-channel phase-shifted interferograms before the implementation of a phase-shifting algorithm, and so, the methods of position registration for multi-channel interferograms are also reviewed. This review is expected to prompt research on related fields of phase-shifting interferometry. Full article

Calibration of the imaging environment is an important step in computational imaging research, as it provides an assessment of the imaging capabilities of an imaging system. Visibility is an important quantity reflecting the transparency of the atmosphere. Currently, transmissometers and optical scatterometers are the primary methods for visibility measurement. Transmissometers measure visibility along a single direction between the transmitter and receiver but encounter challenges in achieving optical alignment under long baseline conditions. Optical scatterometers measure the visibility within a localized area since they collect only a small volume of air. Hence, both transmissometers and optical scatterometers have limitations in accurately representing the visibility distribution of an inhomogeneous atmosphere. In this work, a multi-channel visibility distribution measurement via the optical imaging method is proposed and validated in a standard fog chamber. By calibrating the attenuation of infrared LED arrays, the visibility distribution over the entire field of view can be calculated based on the atmospheric visibility model. Due to the large angle of divergence of the LED, the need for optical alignment is eliminated. In further discussion, the key factors affecting the accuracy of visibility measurement are analyzed, and the results show that increasing the measurement baseline, increasing the dynamic range of the detector, and eliminating background light can effectively improve the accuracy of visibility measurement. Full article

This article proposes an improved wavelet threshold denoising for laser self-mixing interference signals. The improved wavelet threshold function exhibits smoothness and continuity near the threshold. By replacing hard or soft wavelet threshold with the improved wavelet threshold, it can eliminate the generation of fake self-mixing interference peaks due to local oscillation induced by hard wavelet threshold, as well as the loss of self-mixing interference peaks due to over-smoothness induced by the soft wavelet threshold. Compared with hard and soft wavelet threshold denoising, theoretical simulations and experimental results demonstrate that the displacement of vibrations are well reconstructed based on the improved wavelet threshold denoising. Full article

The work reported in the paper addresses the thermal diffusivity (TD) tuning of the electronic sensor material polyvinylidene fluoride (PVDF). The thermal properties of electronic material were found to influence the device characteristics significantly, demanding novel techniques for TD tuning. The TD value of the carbon sources—hydroxyethyl cellulose (HC), lignin (LG), and camphor soot (CS) and their composites—were measured by the sensitive nondestructive evaluation technique—photothermal beam deflection. When the HC and LG enhanced the TD of PVDF by 237.5% and 27.5%, respectively, CS was found to lower it by 11.25%. The spectroscopic analysis revealed the variation of hydroxyl groups in the samples and suggested its prominence in deciding the TD value. The Fourier transform infrared analysis and beam deflection measurements exhibited a positive correlation between hydroxyl groups and TD, except for the composite PVDF combined with soot. In this case, the amorphous carbon in soot reduced PVDF’s TD due to the heat trap mechanism of carbon allotropes. The induced variation of TD of PVDF via carbon source integration is attributed to the closure of pores in PVDF, revealed through the optical microscopic images, thereby suggesting a methodology for enhancing or reducing TD of PVDF. Full article

We propose a method for generating three-dimensional light fields with given intensity and phase distributions using purely phase transmission functions. The method is based on a generalization of the well-known approach to the design of diffractive optical elements that focus an incident laser beam into an array of light spots in space. To calculate purely phase transmission functions, we use amplitude encoding, which made it possible to implement the designed elements using a single spatial light modulator. The generation of light beams in the form of rings, spirals, Lissajous figures, and multi-petal “rose” distributions uniformly elongated along the optical axis in the required segment is demonstrated. It is also possible to control the three-dimensional structure of the intensity and phase of the shaped light fields along the propagation axis. The experimentally generated intensity distributions are in good agreement with the numerically obtained results and show high potential for the application of the proposed method in laser manipulation with nano- and microparticles, as well as in laser material processing. Full article

Stable and reliable optical power converting devices are obtained using vertical multi-junction laser power converters. They are based on the GaAs and the InP material systems and are used for power-over-fiber or power-beaming applications. This study demonstrates that, in addition to providing the overall best conversion efficiencies with output voltages ideal for various applications, these semiconductor photovoltaic devices are very tolerant to beam non-uniformity, partial illumination, or beam displacement variations. Examples are given with two tight beams, each covering as little as ~7% of the cell area. An optical input power of 10 W was converted with still an efficiency of Eff ~59.4%. For an input power of 20 W, the illuminated area was set to ~22% without significantly affecting the conversion efficiency of Eff ~60%. Remarkably, for a beam diameter at ~65% of the chip length (i.e., covering ~35% of the chip area), a converted power of 29.5 W was obtained using a 12-junction GaAs device with a conversion efficiency of 61%. For a 10 junction InP-based device, an efficiency of Eff = 51.1% was obtained at an output voltage reaching as high as V_oc = 5.954 V for an average optical intensity of 69 W/cm² and an illumination area of ~57%. Full article

The interaction of high-contrast high-intensity laser radiation with solids allows us to create hot or warm plasma of solid or even over-solid density, such as in the case of inertial fusion particularly. The multicharged ions contained in it can no longer be considered isolated. As a result, this leads to a decrease in the ionization potentials and to the disappearance of a number of bound ionic states. To describe the ionization potential depression, two major approaches are now used predominantly, where the key parameter is either average interelectronic or interionic distance. Since neither of the approaches can be substantiated purely theoretically, their applicability can only be established by comparison with experimental results. In recent experiments with X-ray free-electron lasers, it was concluded that the ionization potential depression rather depends on the interelectronic distance. However, when measuring ionization potentials, it was assumed that the main role in ionization processes is played by the direct photoionization of the ion ground state. In the present paper, we show that stepwise photoionization processes should play a significant role in dense plasma, disrupting a straight correspondence between the threshold in direct photoionization by X-ray laser photons and the actual ionization potential of multicharged ions. It means that the measurement results mentioned above are not correct, and the main conclusion about the importance of the interelectronic distance for depression of the ionization potential is not correct. Full article

The analysis of greenhouse gas emission characteristics on urban roads is of great significance for understanding the sources and sinks of urban greenhouse gases and their changing patterns. Based on tunable diode laser derivative absorption spectroscopy technology, which features high resolution, high sensitivity, and fast response, a vehicle-mounted system capable of simultaneously detecting CO₂ and CH₄ has been developed. The system has a response time of 0.38 s, with detection sensitivities of 5 ppb for CH₄ and 0.2 ppm for CO₂, power consumption of approximately 4.8 W, a weight of less than 3 kg, and dimensions of 255 mm × 275 mm × 85 mm. Using this system, monitoring campaigns were conducted on the same road in Beijing, running north–south, during different time periods in April and June 2023. The results show that there is little correlation between changes in CO₂ and CH₄ concentrations on the road, and these gas concentrations exhibit different influencing factors and spatiotemporal characteristics. The CO₂ concentration on the road is primarily related to the degree of traffic congestion and does not exhibit significant seasonal variations. The average CO₂ concentration measured on the road is much higher than the global average CO₂ concentration during the same period. On the other hand, the CH₄ concentration on the road is not strongly correlated with traffic congestion but is closely related to the leakage of methane from specific emission wells or covers. The CH₄ concentration is higher in the morning, gradually decreases as the sun rises, and then increases again after sunset. The CH₄ concentration measured at night in June is significantly lower than that in April, reflecting some seasonal variation. The CH₄ concentration on the Beijing urban road is slightly higher than the global average CH₄ concentration during the same period. The vehicle-mounted experiments verified the feasibility of using this self-developed system for vehicle-mounted detection of greenhouse gas concentrations on urban roads. The research results can provide data for analyzing the spatial pattern of regional carbon sources and sinks. Full article

This paper addresses the issue of reduced image quality due to annular turning marks formed by single-point diamond turning (SPDT) during the processing of metal-based mirrors and infrared lenses. An ideal single-point diamond turning marks diffraction action model to quantitatively analyze the impact of turning marks diffraction on imaging quality degradation is proposed. Based on this model, a fast estimation algorithm for the optical modulation transfer function of the system under turning marks diffraction (TMTF) is proposed. The results show that the TMTF algorithm achieves high computational accuracy, with a relative error of only 3% in diffraction efficiency, while being hundreds of times faster than rigorous coupled wave analysis (RCWA). This method is significant for reducing manufacturing costs and improving production efficiency, as it avoids the problem of being unable to compute large-size optical systems due to computational resource and time constraints. Full article

Narrow-linewidth lasers have a high spectral purity, long coherent length, and low phase noise, so they have important applications in atomic clocks, precision measurement, and quantum computing. We inject a transmitted laser from a narrow-linewidth (∼15 kHz) flat-concave Fabry–Perot (F-P) cavity made from ultra-low expansion (ULE) optical glass into an 852 nm distributed Bragg reflector-type laser diode (DBR-LD), of which the comprehensive linewidth is 1.67 MHz for the free running case. With an increase in the self-injection power, the laser linewidth gradually narrows, and the injection locking current range gradually increases. The narrowest linewidth measured by the delayed frequency-shifted self-heterodyne (DFSSH) method is about 365 Hz, which is about $1/4500$ of the linewidth for the free running case. Moreover, to characterize the laser phase noise, we use a detuned F-P cavity to measure the conversion signal from the laser phase noise to the intensity noise for both the free running case and the self-injection lock case. The laser phase noise for the self-injection lock case is significantly suppressed in the analysis frequency range of 0.1–10 MHz compared to the free running case. In particular, the phase noise is suppressed by more than 30 dB at an analysis frequency of 100 kHz. Full article

In this work, we have proposed and optimized a segmented cladding fiber (SCF) with a large mode area (LMA) consisting of a uniform core and a double cladding. The outer cladding of the SCF consisted of a periodic alternation of high- and low-refractive-index segments, while the inner cladding consisted of a resonant layer of rods surrounded by high-refractive-index rings. The three geometrical parameters chosen as design variables were the (a) doped rod refractive index differences, (b) refractive index differences of rings, and (c) ring thickness. Using the Box–Behnken approach, we selected thirteen different design cases and modeled them numerically using the finite element method (FEM). In order to optimize the features of the proposed fiber, such as the effective mode area (EMA), we applied response surface methodology (RSM). The EMA of the optimal SCF was significantly improved and markedly enlarged to about 706 µm² at a wavelength of 1.550 µm. Different properties of the optimized double-cladding octo-wing SCF (DC-OW-SCF), such as confinement losses in the core mode and the first higher-order mode, were studied. The DC-OW-SCF offers the advantages of feasibility in fabrication using the powder-in-tube (PIT) method and provides the possibility of utilization in compact amplifier devices and high-power fiber lasers. Full article

A 1083 nm laser, corresponding to a characteristic spectral line of ³He 2³S₁-2³P, is the core light source for spin-exchange optical pumping-free technology, and thus has important developmental significance. In this paper, precise wavelength 1083.34 nm semiconductor lasers with 285 mW output power, −144.73 dBc/Hz RIN noise and 30.9952 kHz linewidth have been successfully achieved via reasonable chips design, high-quality epitaxial growth process and ultra-low reflectivity coating fabrication. All the results show the highest output power and ultra-narrow linewidth of the single-frequency 1083 nm DFB semiconductor laser achieved in this paper, which can fully satisfy the requirement of quantum magnetometers. Full article

In this work, we experimentally analyze the periodic oscillations that take place in staircase APDs with separate absorption and multiplication regions when operating under continuous laser light. These oscillations increase in frequency when the APD gain increases. We have verified that they are not affected by the parameters (gain and bandwidth) of the transimpedance amplifier, and thus originate inside the APD. The phenomenon is analyzed systematically by considering devices with different thicknesses of the absorption region. Possible physical interpretations related to the flux of holes generated by impact ionization are provided. Full article

Suppressing the impact of atmospheric turbulence on laser beam propagation is a bottleneck problem in the application of free space optical communications, with the primary difficulty being the lack of a quantitative description of the effect of turbulence on a laser beam. In this paper, we propose a quantitative description of the effect of turbulence and express a quantitative analysis of the effect of atmospheric turbulence on Bessel–Gaussian beam based on the extended Huygens–Fresnel principle. The results of numerical analysis confirm the validity of the theoretical description. This study also shows that the influence of turbulence is stronger when the annular shape of the Bessel–Gaussian beam is more obvious. The method also provides theoretical guidance for reverse engineering the amplitude and phase distribution of the initial laser beam, thereby effectively reducing the impact of atmospheric turbulence on laser beam propagation. Full article

A quasi photonic crystal (QPC) dual-band absorber for laser stealth is designed and numerically studied. It consists of a defective two-dimensional photonic crystal on a thick Ni film. The defective photonic crystal is a continuous Ge layer with air holes, but some of the holes are periodically removed. Under a normal incidence that is perpendicular to the plane of the defects, the absorptivity can achieve 92.8% at the 1.064 μm wavelength and 93.2% at the 1.55 μm wavelength. Within large incident angles (<45 degrees), the dual-band absorptivity is still >80%. Additionally, the emissivity is as low as 5.8%~20.0% and 2.8%~5.8% in the 3–5 μm and 8–14 μm atmospheric windows. It is found that the introduced defects can couple the incidence into the structure and lead to spectral peaks (electromagnetic resonances) even without the bottom Ni film. With the help of the high-loss Ni film, the resonances are transformed into magnetic or/and electric modes of different orders. As a result, the QPC generates four absorption peaks. They are superimposed in pairs, resulting in enhanced absorption of the two laser wavelengths. Full article

Photon counting detector arrays are commonly used for deep space optical communication receivers operating on the principle of intensity modulation/direct detection (IM/DD). In scenarios where beam parameters can vary at the receiver due to scattering, it is important to estimate beam parameters in order to minimize the probability of error. The use of array of detectors increases the sensitivity of the receiver as compared to single photo-detector of the same size. In this paper, we present the derivation of a maximum likelihood estimator (ML) for peak optical intensity, providing both numerical and closed form expressions for the estimator. Performance of both forms of ML estimator are compared using the mean squared error (MSE) criterion and Cramer–Rao Lower Bound (CRLB) is also derived to assess the proposed estimator’s efficiency. This research contributed to the advancement of estimation techniques and has practical implications for optimizing deep space optical communication systems. Full article

A polarization analysis method based on a multi-parameter model is proposed to address the polarization effect analysis and calibration requirements of a dual-coded snapshot spectral polarization imaging system. A full-link polarization effect model for a spectral polarization imaging system is established that includes a digital micromirror array (DMD), prism grating prism (PGP), micro-polarizer array detector (MPA), and multi-film. The influence of parameters such as the refractive index, incident angle, grating refractive index, constant, prism refractive index, vertex angle, multi-layer film complex refractive index, and film thickness on the optical transmittance of the system are analyzed. Using a dynamic data exchange mechanism to perform full-link, full-FOV, and full-pupil ray tracing on the optical system, the polarization effect distribution of the system under different degrees of polarization (DOP) and wavelengths is obtained. A calibration experiment for the controllable incident wavelength and DOP using narrowband filters and glass stacks is established. The experimental results show that in the 420 nm, 532 nm, and 635 nm wavelength bands, the MSEs of the calibrated values are 1.3924 × 10⁻⁴, 1.6852 × 10⁻⁴, and 1.6735 × 10⁻⁴, respectively. It is proven that the calibration method based on a multi-parameter model is feasible. Finally, the spectral polarization image at 532 nm is calibrated. The contrast ratio of metallic aluminum is calibrated from 7.13 to 15.33. This study provides a theoretical basis for the analysis and calibration of polarization effects in a dual-coded snapshot spectral polarization imaging system. Full article

Focused femtosecond beam filamentation after amplitude masks has been studied experimentally and numerically. We deduced conditions (energy per hole, diameter and geometrical composition of holes, focal length) providing for the formation of the regularized bundle of filaments or single on-axis filament at the given pulse duration and beam diameter. We showed that a light channel with small diameter (∼200 μm) and overcritical peak power may be formed well before both the focal distance and the Marburger length, and this channel collapses due to self-focusing and forms the filament. The start position of such a filament can be predicted based on the linear propagation equation, while a more sophisticated non-linear approach that takes into account the Kerr nonlinearity, plasma effects, etc., helps to describe the temporal structure of a filament, its frequency, and its angular spectrum. Full article

We have experimentally implemented a photonics-aided large-capacity fiber-mm-wave wireless communication system employing a simple dual-polarized single-input single-output (SISO) wireless based on polarization multiplexing at the W-band. To compare the performance of different algorithms, 18G-baud, and 35G-baud 16-level quadrature-amplitude-modulation (16QAM), probabilistically shaped 16QAM (PS-16QAM), 64QAM and PS-64QAM signal using different carrier phase recovery (CPR) algorithms are transmitted in the system. Moreover, we compare the Viterbi–Viterbi (VV), improved new algorithm based on VV (NVV), blind phase search (BPS), and two-stage BPS algorithms’ computational complexity to better compare different algorithms. Using the experiment result, we can demonstrate that the BPS algorithm is about half a magnitude better than the NVV algorithm for PS-QAM signals, while the NVV algorithm has the lowest computational complexity. Additionally, we also achieve error-free wireless transmission at a net data rate of 324.1 Gb/s with the bit error ratio (BER) below the forward-error correction (FEC) threshold of 1 × 10⁻² assuming soft-decision forward-error correction (SD-FEC) when using the BPS algorithm. Full article

Quantum entanglement in macroscopic systems plays an important role in quantum information processing. Here, we show that the steady-state entanglement between the two cavity modes in the macroscopic Laguerre–Gaussian (L–G) cavity optorotating system can be enhanced by placing a degenerate optical parametric amplifier (OPA) inside the cavity. The two L–G cavity modes are coupled to the same rotating mirror and are respectively driven at the red and blue mechanical sidebands. We use the logarithmic negativity to quantify the steady-state entanglement between the two cavity modes. We study the influences of the nonlinear gain and phase of the OPA, the temperature of the environment, and the angular momentums of the two cavity modes on the entanglement between the two cavity modes. In the cryogenic environment temperatures, when the angular momentums of the two cavity modes are identical, the enhancement of the entanglement between the two cavity modes by the OPA is the most significant. Full article

DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) has been introduced to visible light networking framework for connected vehicles (LiNEV) systems as a modulation and multiplexing scheme. This is to overcome the light-emitting diode (LED) bandwidth limitation, as well as to reduce the inter-symbol interference caused by the multipath road fading. Due to the implementation of the inverse fast Fourier transform, DC-OFDM suffers from its large peak-to-average power ratio (PAPR), which degrades the performance in LiNEV systems, as the LEDs used in the vehicles’ headlights have a limited optical power-current linear range. To tackle this issue, discrete Fourier transform spread-optical pulse amplitude modulation (DFTS-OPAM) has been proposed as an alternative modulation scheme for LiNEV systems instead of DCO-OFDM. In this paper, we investigate the system performance of both schemes considering the light-emitting diode linear dynamic range and LED 3 dB modulation bandwidth limitations. The simulation results indicate that DCO-OFDM has a 9 dB higher PAPR value compared with DFTS-OPAM. Additionally, it is demonstrated that DCO-OFDM requires an LED with a linear range that is twice the one required by DFTS-OPAM for the same high quadrature amplitude modulation (QAM) order. Furthermore, the findings illustrate that when the signal bandwidth of both schemes significantly exceeds the LED modulation bandwidth, DCO-OFDM outperforms DFTS-OPAM, as it requires a lower signal-to-noise ratio at a high QAM order. Full article

We demonstrate a generation of three lasing wavelengths with the assistance of Rayleigh backscattering as the stabilizer of peak power variations. The proposed laser consists of a combination of the semiconductor optical amplifier (SOA) and erbium-doped fiber amplifier (EDFA) as the amplifying media. Three fiber Bragg gratings are employed as the selective wavelength selectors at 1544, 1547 and 1550 nm. At 110 mA SOA current and 18 dBm EDFA output power, a flattened output spectrum with 0.9 dB peak power variation is attained. In terms of stability, the maximum peak power fluctuation for the individual laser is 0.24 dB within 120 minutes observation period. Without the Rayleigh backscattering effect, the peak power flatness is severely degraded. This shows that the weakly distributed photons can be utilized as peak power stabilizers in fiber laser systems. Full article