Peak Power Variation Research Articles

Demodulating Optical Wireless Communication of FBG Sensing with Turbulence-Caused Noise by Stacked Denoising Autoencoders and the Deep Belief Network

Free-space optics communication (FSO) can be used as a transmission medium for fiber optic sensing signals to make fiber optic sensing easier to implement; however, interference with the sensing signals caused by the optical turbulence and scattering of airborne particles in the FSO path is a potential problem. This work aims to deep denoise sensed signals from fiber Bragg grating (FBG) sensors based on FSO link transmission using advanced denoising deep learning techniques, such as stacked denoising autoencoders (SDAE). Furthermore, it will demodulate the sensed wavelength of FBGs by applying the deep belief network (DBN) technique. This is the first time the real FBG sensing experiment has utilized the actual noise interference caused by the environmental turbulence from an FSO link rather than adding noise through numerical processing. Consequently, the spectrum of the FBG sensors is clearly modulated by the noise and the issue with peak power variation. This complicates the determination of the center wavelengths of multiple stacked FBG spectra, requiring the use of machine learning techniques to predict these wavelengths. The results indicate that SDAE is efficient in denoising from the FBG spectrum, and DBN is effective in demodulating the central wavelength of the overlapped FBG spectrum. Thus, it is beneficial to implement an FSO link-based FBG sensing system in adverse weather conditions or atmospheric turbulence.

Double-Brillouin Frequency Spacing Multi-Wavelength Brillouin Erbium Fiber Laser in Double Cavities Configuration

Multi-wavelength fiber laser has great potential for use in dense wavelength-division multiplexing (DWDM) systems, sensors, and microwave photonics. This paper presents a multi-wavelength Brillouin erbium fiber laser (MW-BEFL) with double-frequency Brillouin spacing of approximately 0.176 nm between each adjacent Stokes. The proposed setup utilizes double cavities configuration for odd and even-order Brillouin Stokes to propagate separately in the system. This configuration is flexible as it can be transformed into a single-spacing MW-BEFL by swapping the ports of the circulator; it can also extract the odd-orders Brillouin Stokes by relocating the OSA in the cavity. Up to 25 double-spaced Brillouin Stokes are obtained with a tuning range of 30 nm. The first 8 Stokes have peak power variation within ± 3.7 dB and an average signal-to-noise ratio of 20 dB.

Investigating the effect of laser modulation and input energy on bending mechanism and bending angle in the multi-pass laser forming process through real-time monitoring

Laser forming is a non-contact type process used for bending, spatial forming, and alignment of metallic components through a controlled application of laser energy. The process involves localized rapid heating and cooling, inducing thermal strains exceeding the elastic strain of the material resulting in plastic deformation. The process is broadly governed by the temperature gradient mechanism (TGM), buckling mechanism (BM), and upsetting mechanism (UM). However, in the case of multi-pass laser forming process, heat accumulation and variation in the flow stress because of consecutive thermal cycles and strain hardening due to multiple bending cycles makes the process highly complicated. While operating laser in modulated mode may be a wise choice to handle the heat accumulation, inter-pulse cooling cycles along with variation in peak and average power with change in duty cycle affects the bending mechanism and angle significantly. Therefore, in the present study, a real-time monitoring system was developed to investigate the bending mechanism for a multi-pass laser forming process carried out in laser modulated mode. Also, the effect of average power, peak power, heat accumulation, strain hardening on bending mechanism and bending angle were investigated. Keeping the peak power constant, variation in duty cycle from 100% to 40% resulted in reduced average energy and strain hardening was found to be dominant, decreasing bending angle per pass. On contrast, variation in duty cycle by keeping the average power constant resulted in heat accumulation dominating the bending mechanism.

Stable Triple-Wavelength Random Fiber Laser Based on Fiber Bragg Gratings

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.

U-band multiwavelength Brillouin-Raman random fiber laser with single Brillouin frequency spacing

A single Brillouin frequency interspacing (∼10 GHz) multiwavelength Brillouin-Raman random fiber laser with lasing operation in the U-band is demonstrated through nonlinear interactions of stimulated Raman scattering and stimulated Brillouin scattering as amplification mechanisms. Structurally, dispersion compensation fiber of 11.2 km length was employed as a highly nonlinear gain medium. For a single Brillouin frequency interspacing operation, a quasi-bidirectional double pass distributed Raman amplification was proposed by looping back the forward lasing spectrum together with the residual Raman signal into the laser cavity. The optimized Raman pump power was set to 1020 mW at 1550 nm; whereas Brillouin pump power and wavelength were optimized at 0 dBm and 1640 nm respectively. Under these conditions, 476 channels interspaced by 10 GHz and spectrally flattened within 3 dB peak power variation were generated over ∼40.8 nm spectral bandwidth from 1640.9 to 1681.7 nm, with 16.28 dB average optical signal-to-noise ratio, and −15.25 dBm average Stokes peak power. Meanwhile, total output power of 19.77 dBm was recorded at 20 % optical power conversion efficiency with 0.10 dB power fluctuation during 1 hour of continuous operation at ambient conditions. The proposed U-band Brillouin-Raman random fiber laser has potential applications in optical communication, optical sensing, and microwave photonics.

Accuracy of peak-power compensation in fiber-guided and free-space acoustic-resolution photoacoustic microscopy.

Acoustic resolution photoacoustic microscopy (AR-PAM) has gained much attention in the past two decades due to its high contrast, scalable resolution, and relatively higher imaging depth. Multimode optical fibers (MMF) are extensively used to transfer light to AR-PAM imaging scan-head from the laser source. Typically, peak-power-compensation (PPC) is used to reduce the effect of pulse-to-pulse peak-power variation in generated photoacoustic (PA) signals. In MMF, the output intensity profile fluctuates due to the coherent nature of light and mode exchange caused by variations in the bending of the fibers during scanning. Therefore, using a photodiode (PD) to capture a portion of the total power of pulses as a measure of illuminated light on the sample may not be appropriate for accurate PPC. In this study, we have investigated the accuracy of PPC in fiber-guided and free-space AR-PAM systems. Experiments were conducted in the transparent and highly scattering medium. Based on obtained results for the MMF-based system, to apply PPC to the generated PA signals, tightly focused light confocal with the acoustic focus in a transparent medium must be used. In the clear medium and highly focused illumination, enhancement of about 45% was obtained in the homogeneity of an optically homogeneous sample image. In addition, it is shown that, as an alternative, free-space propagation of the laser pulses results in more accurate PPC in both transparent and highly scattering mediums. In free-space light transmission, enhancement of 25-75% was obtained in the homogeneity of the optically homogeneous sample image.

Performance investigation on novel MPPT controller in solar photovoltaic system

This paper investigates the performance of solar PV system by implementing MPPT algorithm. The IV and PV characteristics of solar photovoltaic system have been analyzed for various parameters of temperature and irradiation. From the characteristics, the peak power variations with respect to physical parameters have been identified using algorithmic approach. To validate the MPPT operation, the P&O algorithm has been implemented and verified the operation under varying environmental conditions. And also, the same has been compared with theoretical approach in this paper.

Nonlinearity Effect on Multiwavelength fiber laser based on Sagnac loop mirror interferometer

This paper demonstrates a multiwavelength fiber laser (MWFL) with erbiumdoped fiber amplifier (EDFA) as gain medium and Sagnac loop mirror interferometer as filter. Stable and flat lasing spectrum was achieved when highly nonlinear fiber (HNLF) was inserted into the laser cavity. The number of lasing lines and extinction ratio (ER) was directly influenced by EDFA gain where at maximum setting, 14 lasing lines with ER of 19 dB were generated. The stable and flat spectrum achieved spanned about 1.6 nm. This is due to the induced intensity dependent loss mechanism in the cavity by adjusting the half-wave plate of the polarization controller in combination with the HNLF. The MWFL output showed peak power variation of about 2.1 dB within 100 minutes of observation time.

Simultaneous measurement of temperature and strain based on peak power changes and wavelength shift using only one uniform fiber bragg grating

Many efforts have been devoted to simultaneous measurements of strain and temperature by FBG sensors and several improving techniques have been resulted and implemented on the measurement. Most of them are based on two or more FBGs configurations or a single non-uniform FBG implementation. We propose simultaneous measurement of temperature and strain based on peak power changes and Bragg wavelength shifts using only one uniform fiber Bragg grating (FBG). We placed a ramp with the angle of θ, similar to a tilted cantilever beam, on an assumptive structure and stuck a uniform FBG on it. When a uniform strain applied to a structure, the cantilever beam converts it to a non-uniform strain distribution along with itself and consequently the uniform FBG. By creating this non-uniform strain distribution, the peak power of the reflection spectrum of the FBG will be sensitive to strain changes. In addition, the Bragg wavelength shift will be sensitive to both temperature and strain parameters. According to our simulation, temperature sensitivity of 14.15 pm/°C is obtained for FBG sensor without any changes in the peak power. The strain sensitivity of 0.7837 pm/με, and a nonlinear sensitivity according to a quadratic function for peak power variation are also observed.

Brillouin-Raman fiber laser with switchable wavelength spacing based on Brillouin pump distribution

A switchable multi-wavelength Brillouin-Raman fiber laser (MW-BRFL) is presented in this work. The MW-BRFL consists of a full-open linear cavity in which dispersion compensating fiber is employed as the gain medium for Brillouin and Raman amplification. The switching operation for single- and double-wavelength spacing MW-BRFL can be implemented by controlling Brillouin pump (BP) power distribution and direction that enters the laser cavity via two fiber-entry points. This is achievable by optimizing the splitting ratio of an optical coupler and the initial BP power. For the proposed laser setup, a 30/70 optical coupler was found to be the optimum. By setting the initial BP power at −3.0 dBm, 246 laser lines with 20-GHz frequency spacing were observed. The transformation to 10-GHz frequency spacing was attained by increasing the BP power to 11.8 dBm that resulted in 483 laser lines. All the counted laser lines had 3 dB peak power variation and were spread across a 38 nm bandwidth. In a nutshell, the proposed MW-BRFL is promising with respect to its performance, simplicity and flexibility.

Multiwavelength Fiber Laser Covering far L and U Bands in a Dual Cavity Configuration

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

Breach and recurrence of dissipative soliton resonance during period-doubling evolution in a fiber laser

We report on the experimental observation of period-doubling bifurcation of dissipative-soliton-resonance (DSR) pulses in a fiber laser passively mode-locked by using the nonlinear optical loop mirror. Increasing the pump power of the fiber laser, we show that temporally a stable, uniform DSR pulse train could be transformed into a period-doubling state, exhibiting two sets of pulse parameters between the adjacent cavity roundtrip. It is found that DSR pulses in the period-doubling state could maintain the typical feature of DSR pulse: fixed pulse peak power and linear variation in pulse width with respect to the pump power change. The mechanism for achieving period-doubling of DSR pulses is discussed.

Study of low-peak-power highly coherent broadband supercontinuum generation through a dispersion-engineered Si-rich silicon nitride waveguide.

Since the first observation by Alfano and Shapiro in the 1970s [Phys. Rev. Lett.24, 584 (1970)PRLTAO0031-900710.1103/PhysRevLett.24.584], supercontinuum generation study has become an attractive research area in the field of broadband light source design, including its use in various applications associated with nonlinear optics in recent years. In this work, the numerical demonstration of ultrabroadband supercontinuum generation in the mid-infrared (MIR) region via the use of complementary metal-oxide semiconductor compatible Si-rich silicon nitride as the core in a planar waveguide design employing one of two materials, either LiNbO3 or MgF2 glass, as the top and bottom claddings is explored. A rigorous numerical investigation of broadband source design in the MIR using 2 mm long Si-rich silicon nitride waveguides is carried out in terms of waveguide structural parameter variations, input peak power variation, varying unexpected deformation of the waveguide along the core region during fabrication, and spectral coherence analysis. Among the several waveguide models studied, two promising designs are identified for wideband supercontinuum generation up to the MIR using a relatively low input peak power of 50 W. Simulation results reveal that spectral coverage spanning from 0.8 µm to 4.6 µm can be obtained by using a LiNbO3-cladded waveguide, and similar spectral coverage is also predicted for the other design, a MgF2-cladded waveguide. To the best of our knowledge, this is the widest spectral span in the MIR region employing a Si-rich silicon nitride waveguide so far. In dispersion tuning as well as in supercontinuum generation, the effect of possible unexpected waveguide deformation along the transverse directions during fabrication is also studied. No significant amount of spectral change is observed in the proposed model for a maximum of 10° inside/outside variation along the width. On the other hand, even 1° upward/downward variation along the thickness could cause substantial spectral change at the waveguide output. Finally, the obtained output spectra from the proposed waveguides are found to be highly coherent and can be applied in various MIR region applications such as optical coherence tomography, spectroscopic measurement, and frequency metrology.

Highly efficient few-mode spatial beam self-cleaning at 1.5µm.

We experimentally demonstrate that spatial beam self-cleaning can be highly efficient when obtained with a few-mode excitation in graded-index multimode optical fibers. By using 160 ps long, highly chirped (6 nm bandwidth at -3dB) optical pulses at 1562 nm, we demonstrate a one-decade reduction of the power threshold for spatial beam self-cleaning, with respect to previous experiments using pulses with laser wavelengths at 1030-1064 nm. Self-cleaned beams remain spatio-temporally stable for more than a decade of their peak power variation. The impact of input pulse temporal duration is also studied.

Classification of weld penetration condition through synchrosqueezed-wavelet analysis of sound signal acquired from pulse mode laser welding process

Abstract Monitoring weld condition using acoustic method is quite challenging due to some factors, hence the importance to further explore the use of signal analysis method not only to diminish the effect of noise, but more importantly, to obtain a distinct correlation with weld condition. The main goal of this work is to determine the significance of the feature extracted from synchrosqueezed-wavelet analysis in classifying sound signals that derived from varied weld penetration conditions. In achieving the aim, sound signal was acquired during pulse mode laser welding process with variation in peak power and pulse width that produced half penetration and full penetration weld joints. The trends of time domain, frequency domain, and wavelet analysis features of the acquired sound from half and full penetration welds were compared prior to the support vector machine (SVM) classification analysis. The comparison between all the features displayed a clear distinction in signals between half and full penetration welds for the case of features extracted from synchrosqueezed-wavelet analysis. In SVM binary classification analysis, the use of same feature as input recorded 96.94 % of average classification accuracy, which appeared to be the highest. Additionally, comparison with band power, exhibited 27.7 % improvement in classification precision. From these findings, it is concluded that the use of features extracted from synchrosqueezed-wavelet analysis as input for classification of sound signals from various penetration conditions is indeed significant. This work contributes to an alternative way in dealing with random sound signals in view of developing an efficient weld penetration monitoring system in future.

Two-Port Network Theory-Based Design Method for Broadband Class J Doherty Amplifiers

This paper proposes a new methodology for designing wideband Class J Doherty power amplifiers (DPAs). The paper begins by presenting a network analysis that leads to a set of design equations to govern the synthesis of the combiner network parameters that satisfy Class J load requirements. The combiner network produces a complex-to-complex Doherty load modulation capable of avoiding potential transistor voltage clipping due to a mismatch between the fundamental and harmonic impedances. Consequently, this method improves the high power AM-AM characteristic and reduces the DPA's peak power variation versus frequency. A wideband Class J DPA was designed as a proof-of-concept demonstrator to operate from 2.7 GHz to 4.3 GHz. Under continuous wave stimuli, over the entire band, the measured AM-AM distortion in the Doherty region of the fabricated DPA was found to be lower than 1.2 dB with a relatively constant output power of 38.9 ± 0.3 dBm at saturation. Moreover, good drain efficiencies of about 42% and 54% were recorded at 6-dB output back-off and saturation powers, respectively. In addition, the linearizability of the fabricated DPA was confirmed under both intra- and inter-band carrier-aggregated signal stimuli. In fact, measurements using an 80 MHz inter-band carrier-aggregated signal revealed that the proposed DPA could deliver an adjacent channel leakage ratio of better than -48 dBc after digital pre-distortion with a good average drain efficiency of 45% - 49% over the entire band of interest.

Proposal of novel structure for wide wavelength tuning in distributed Bragg reflector laser diode with single grating mirror

We report a novel structure that is capable of wide wavelength tuning in the distributed Bragg reflector laser diode (DBR-LD) with a single grating mirror. This device's DBR section has two tuning elements, plasma, and heater tunings, which are implemented simultaneously on the top of a single waveguide by using an in-between dielectric layer. For the proposed structure, a three-dimensional thermal simulation was conducted. The results showed that the temperature profile within the waveguide is highly affected by the position of heater metal and thermal conductivity of the p-cladding layer. As a result, it is important to use a uniform temperature region in the DBR section for a wide tuning range and stable single-mode operation. For a 550-μm long DBR-LD with a 250-μm long DBR section, a tuning range of 26 nm (i.e., 7 nm for plasma tuning and 19 nm for heater tuning); an SMSR of more than 45 dB; and a peak power variation of less than ± 2.5 dB were obtained. From the comparisons of two DBR-LDs with only one tuning element, we confirmed that using the dielectric layer is a very effective way of achieving a wide tuning under the independent tuning operation.

Selectable and stable C-band multi-wavelength ring cavity erbium-doped fiber laser employing Sagnac loop and tunable filter

A tunable C-band multi-wavelength ring cavity erbium-doped fiber laser based on a Sagnac loop and tunable filter was developed and experimentally demonstrated. In this fiber laser, a Sagnac loop fabricated using a 3 × 3 coupler and polarization-maintaining fiber is employed as the comb filter. A 4-m-long C-band and 0.8-m-long L-band erbium-doped fiber serve as the gain medium, and a tunable filter connected to the 3 × 3 coupler is used for laser wavelength adjustment. In the experiment described herein, switchable single-wavelength lasing was realized from 1528.6 nm to 1562.5 nm in 4.9-nm intervals, and the peak power variation was less than 1.12 dB during 10 min of scanning at 26 °C. In addition, switchable double- and triple-wavelength lasing were realized, for which the tunable wavelength scope was 28.6 nm and 19.9 nm, and the power fluctuation was less than 1.14 dB and 1.39 dB, respectively.

Multi-channel optical fiber refractometer based on tree topology structure.

A multi-channel optical fiber refractometer based on S fiber taper (SFT) cascaded with fiber Bragg grating (FBG) is proposed and experimentally demonstrated. The compact SFT acts as an in-line Mach-Zehnder interferometer (MZI) to be used as a band-pass filter (BPF) to tailor the FBG reflection, whose transmission spectrum is sensitive to the surrounding refractive index (SRI). Thus, the peak power of the FBG reflection is modulated by the SRI. The three-channel SRI monitor can be achieved by discriminating the different center wavelength and peak power variation of the FBG reflections. Experimental results show that the RI sensitivities of Channels 1, 2, and 3 are 345.54, 349.57, and 463.60dB/RIU, respectively. Moreover, the temperature cross-sensitivity can be solved by interrogating the center wavelength of the FBG reflection.

Dual-point reflective refractometer based on parallel no-core fiber/FBG structure

Abstract A novel dual-point reflective fiber-optic refractometer based on multimode interference (MMI) effect and fiber Bragg grating (FBG) reflection is proposed and experimentally demonstrated, which adopts parallel structure. Each point of the refractometer consists of a single mode-no core-single mode fiber (SNS) structure cascaded with a FBG. Assisted by the reflection of FBG, refractive index (RI) measurement can be achieved by monitoring the peak power variation of the reflected FBG spectrum. By selecting different length of the no core fiber and center wavelength of the FBG, independent dual-point refractometer is easily realized. Experiment results show that the refractometer has a nonlinear relationship between the surrounding refractive index (SRI) and the peak power of the reflected FBG spectrum in the RI range of 1.3330–1.4086. Linear relationship can be approximately obtained by dividing the measuring range into 1.3330–1.3611 and 1.3764–1.4086. In the RI range of 1.3764–1.4086, the two sensing points have higher RI sensitivities of 319.34 dB/RIU and 211.84 dB/RIU, respectively.