Optical Power Fluctuations Research Articles

Digital stabilization of an I/Q modulator in the carrier suppressed single-sideband mode for atom interferometry

We present an all-digital method for stabilizing the phase biases in an electro-optic I/Q modulator for carrier-suppressed single-sideband (CS-SSB) modulation. Using programmable logic on the Red Pitaya STEMlab 125-14 platform, we digitally generate and demodulate an auxiliary radio-frequency tone whose beat with the optical carrier probes the I/Q modulator’s phase imbalances. We implement a multiple-input, multiple-output integral feedback controller that accounts for unavoidable cross-couplings in the phase biases to lock the error signals at exactly zero, where optical power fluctuations have no impact on phase stability. We demonstrate >23dB suppression of the optical carrier relative to the desired sideband at +3.4GHz over a period of 15 h and over temperature variations of 20°C.

Real-time in situ phase sensitivity calibration of interferometric fiber-optic ultrasonic sensors.

Output from a fiber-optic interferometric ultrasonic sensor can be affected by the operating point changes and/or the optical power fluctuations. We report a method for real-time and in situ calibration of the responses to ultrasound-induced phase delays for a low-finesse Fabry-Perot (FP) ultrasonic sensor demodulated using a phase-generated carrier. The carrier is produced from an electro-optic phase modulator, and its zero- and fundamental-carrier frequency components yield two quadrature signals. The same phase modulator is employed to generate controllable laser frequency modulations and calibration phase delay modulations. The response to the known calibration phase delays is used to gauge the sensor output in real time and in situ. The experiment has verified the effectiveness of the calibration method against the signal variations from both operating point changes and optical power variations.

Simultaneous optical power insensitivity and non-volatile wavelength trimming using 2D In4/3P2Se6 integration in silicon photonics

In integrated photonic circuits, microring resonators are essential building blocks but are susceptible to phase errors due to fabrication imperfections and optical power fluctuations. Conventional active phase tuning methods are power-intensive and challenging to integrate into densely packed photonic chips. This study proposes a solution by integrating a thin 2D layer of In4/3P2Se6 (InPSe) onto silicon microring resonators (Si-MRR). This approach mitigates sensitivity to laser power and achieves non-volatile wavelength trimming. Under bias voltage, the device exhibits electro-optic behavior, offering a non-volatile phase trimming rate of −2.62 pm/V to −4.62 pm/V, corresponding to InPSe thicknesses of 45 nm to 120 nm. Low optical losses of 0.0091 to 0.0361 dB/μm were also measured, corresponding to thicknesses of 30 nm to 120 nm. The devices demonstrate stable in-situ resonance wavelength stabilization and bidirectional trimming, ensuring cyclic stability for non-volatile phase control. This advancement enhances the performance of silicon photonics across diverse applications, facilitating high-capacity, high-power operation in compact designs.

Molten fluoride salt-assisted synthesis of titanium carbide (Ti2C) MXene and its application for 2 µm mode-locking in a thulium-doped fiber laser

Titanium carbide (Ti2C), a new two-dimensional material named MXenes, has attracted interest due to its potential applications in numerous fields. Of the many unique characteristics of Ti2C MXene, its nonlinear properties are attractive for optoelectronic applications, specifically for ultrafast laser generation. In this work, a Ti2C MXene was fabricated by etching a MAX phase precursor titanium aluminum carbide (Ti2AlC) using a mixture of lithium fluoride and hydrochloric acid, eliminating the risk of using the harmful hydrofluoric acid. The Ti2C MXene was prepared in solution form and then dropped onto a reduced core diameter of tapered fiber before being used as a saturable absorber (SA). The SA device was inserted into a thulium-doped fiber laser to generate stable mode-locked pulses at a center wavelength of 1951 nm with a pulse width of 1.67 ps. The mode-locked laser was highly stable when tested over time, with peak optical power fluctuations of as little as 0.005 dB measured. The results show that the Ti2C MXene exhibit outstanding performance for ultrafast laser generation.

Wavelength switching technique for phase interrogation of Mach Zehnder interferometer-based optical sensors.

A method for determining the phase shift of a Mach Zehnder interferometer (MZI) is presented. It is based on switching the wavelength of continuous wave (CW) laser light illuminating the MZI and measuring the interferometer output amplitudes at DC and switching frequency. The method can measure the MZI phase shift unambiguously over the entire phase shift range of 2π. A practical proof of concept demonstration shows that the method can perform real-time measurement with high repeatability and accuracy limited by the optical frequency drift and power fluctuation of the lasers. The method does not require modifications of the sensor or accessing to the laser electronics and also uses simple detection. It is, therefore, suitable for bio and medical sensing applications.

Development of a 483 nm external cavity diode laser with cat-eye reflector

An external cavity diode laser, equipped with a cat-eye reflector setup, an interference filter, and a polarizing beam splitter, has been demonstrated to produce a laser wavelength of approximately 483 nm. This ECDL is based on the affordably priced commercial laser diode, GH04850B2G. Critical characteristics, such as fluctuations in optical power and output wavelength in response to injected current, were investigated. In anticipation of potential experiments involving rubidium Rydberg atoms, a simulation of the electromagnetically induced transparency spectrum was conducted using this laser setup in conjunction with a 780-nm laser.

Stellar scintillation statistics and the impact of aperture averaging on space-to-ground optical communications.

Statistical probability distributions characterizing received optical power fluctuations, or scintillation, enable performance predictions of space-to-ground optical communication systems. In this paper, we present measurements of stellar scintillation over a wide range of elevation angles and turbulence conditions collected simultaneously with a 5 cm and 40 cm telescope aperture, which allows a comparison between minimal and significant aperture averaging conditions. The measured data is compared to a reasonable set of candidate probability distribution functions (PDFs), including lognormal, which is most often cited in the literature for weak to moderate scintillation. For scintillation indices (SIs) less than about 0.2, the Nakagami-m distribution provides the best representation of the collected data for both apertures and imposes a greater lasercom link penalty than a lognormal distribution, which has been inaccurately implemented as the default probability distribution in the literature. For larger values of the SI, the scintillation is best characterized by a Gamma-Gamma distribution. Additionally, the measured temporal covariance for weak to moderate scintillation conditions is found to be in reasonably good agreement with theoretical predictions.

Multi-wavelength fiber laser via evanescent field confinement with Al2O3 nanolaminate-coated thinned fiber

We present the fabrication and analysis of an Erbium-doped optical fiber laser emitting at multiple wavelengths. This is accomplished via a filter constructed from a fiber taper coated with alumina (Al2O3). The taper is created using the flame brushing induction technique with a combination of Butane (25 %) and Oxygen (75 %). The diameter is reduced to a 20 µm waist and 2 mm burner displacement. The thin film of Al2O3 was deposited via atomic layer deposition (ALD), resulting in a 15 nm layer. The coated fiber taper was then inserted into a ring fiber laser cavity functioning as a selector filter. This fiber laser produced an output comprising six simultaneous wavelengths: 1531.14, 1531.79, 1532.45, 1541.85, 1544.45, and 1562.22 nm. Laser signal stability tests were performed for one hour at 25 °C at the six emission wavelengths, resulting in a maximum optical power fluctuation of 0.15 to 0.33 dB, a wavelength shift variation of 0.02 nm, and a signal-to-noise ratio of 31.79 dB in average. The laser lines emitted have an average spectral width of 0.06 nm. Our results show that the typical performance of the fiber laser configuration based on an optical fiber taper was improved by the addition of the Al2O3 layer and can be further improved. Our findings demonstrate that the standard operation of the fiber laser setup, which utilizes an optical fiber taper, was enhanced through the incorporation of an Al2O3 layer and has the potential for further development.

Active Learning-Aided CNN-Based Entropy-Tunable Automatic Modulation Identification for Rate-Flexible Coherent Optical System

Flexible rate and real-time link monitoring are important tasks in the development of software-defined elastic optical networks (EONs). The tunable spectral efficiency characteristic of probabilistic constellation shaping (PCS) naturally provides a possibility to dynamically regulate the rate for future optical communication systems. In this work, we firstly propose an active learning-aided entropy-tunable automatic modulation identification (AL-aided ET-AMI) scheme based on convolution neural network (CNN) model for a PCS-based coherent optical system. An AL-based neural network allows monitoring of the link rate and signal-to-noise ratio (SNR) with tuning entropy or optical power fluctuation. The proposed ALaided ET-AMI scheme is demonstrated over a 350 ∼ 550-Gbps line rate 10-km dual-polarized coherent optical communication system at entropies from 3.5 to 5.5. When the entropy tuning step is 0.1, corresponding to a rate tuning step of 5 Gbps at 50 Gbaud, the recognition accuracy can reach 98% with data aggregation (DA). When the fluctuation of SNR is 1 dB, the recognition rate can reach 87% at an entropy of 4.5 over 400 samples. The verifications show that our proposed AL-aided ET-AMI solution can monitor the rate and SNR performance of PCS-based high-speed rateflexible optical links well. The solution provides a new perspective and tool for future optical systems and network monitoring.

Design and numerical analysis of dual-core photonic crystal fiber-based biosensor

In this study, the coupling and the relative sensitivity characteristics of a Dual-Core photonic crystal fiber (DC-PCF)-based liquid sensor are evaluated by employing the full-vectorial finite element method (FV-FEM). The simulation results demonstrate that coupling length of the proposed sensor varies between 2[Formula: see text]mm and 0.2[Formula: see text]mm length at wide wavelength range (800–1600[Formula: see text]nm) for low index analytes (1.33–1.36). Besides, birefringence of order [Formula: see text] is reported. It is found that analyte with 1.36 index reaches higher sensitivity levels. On the other hand, the coupling length of the proposed sensor can be optimized with the ellipticity ratio of the central core hole. Bending analysis on coupling length and optical power fluctuations is also reported. It is found that the power flow in one core is increasing almost 90% towards the bending direction. The proposed structure with simple design parameters has a great potential in various biomedical applications including DNA detection and can also be employed as fiber-based refractometer in various sensing applications.

Trap Behavior of the Optical Power Fluctuation in AlGaN-Based UV-C LEDs Degradation

The trap behavior of the optical power (OP) fluctuation in the degradation of deep ultraviolet light-emitting diodes (DUV-LEDs) is investigated in this work. The OP fluctuation under current stress is related to the combined action of acceptor activation, migration, and recombination. After stress, the changes observed in the admittance spectroscopy and deep-level transient spectroscopy measurements are due to a reduction in electron traps at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{c}$ </tex-math></inline-formula> –20 meV ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{{1}}$ </tex-math></inline-formula> ) and increases in electron traps at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{c}$ </tex-math></inline-formula> –0.5 eV ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{{2}}$ </tex-math></inline-formula> ), 0.68–0.79 eV ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{{3}}$ </tex-math></inline-formula> ), and 0.95–1.2 eV ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{{4}}$ </tex-math></inline-formula> ). The electron trap <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{{1}}$ </tex-math></inline-formula> , located at the active region interface, has a prominent proportion in the apparent charge distribution profile and recombines with holes to restrict their entry into the quantum wells. The other traps contribute to an increase in the leakage current and nonradiative recombination. Reduction of the interface traps would weaken the hole transport shielding effect, but the hole injection augmentation would be wasted in the nonradiative recombination centers when defects are greatly generated in the active region during the long-term operation. Our study dissects the complex mechanisms in the electrical stress-induced degradation of DUV-LEDs and provides a constructive perspective on fabricating high-reliability AlGaN-based devices.

High-sensitivity multi-channel DPSK system with real-time phase lock controller for free-space optical communication

To overcome the power jitters in satellite-to-ground communications caused by atmospheric turbulence, a type of DPSK free-space communication system, assisted by a self-designed real-time phase lock controller, has been established. The system can effectively compensate for power swings in communication links and hence achieve high sensitivity. The wavelength division multiplexing technique is applied to a four-channel DPSK system to provide greater link capacity. With the data rate of a single channel as 2.5 Gbps and unencoded BER as 1 × 10–3, reception sensitivity has been obtained at −53.58 dBm (13.69 photons/bit), −53.59 dBm (13.66 photons/bit), −53.61 dBm (13.59 photons/bit), and −53.63 dBm (13.53 photons/bit) for each independent channel, respectively. The gap between our sensitivity result and the theoretical limit has narrowed to about −3.5 dB. Simultaneously, the DPSK receiver, with our self-designed phase lock controller, has stabilized reception of optical power fluctuations that range from 0 to 40 dB. Additionally, the impact of a four-wave mixing effect on multi-channel system performance has been investigated in detail. Our experimental results present a novel solution for the superior performance of free-space communication links.

Optical flow sensor based on the thermal time-of-flight measurement.

This paper presents a dielectric, all-optical thermal time-of-flight fluid flow velocity sensor. The proposed sensor utilizes a sequence of three short sections of optical fibers, which are positioned in a direction perpendicular to the measured fluid flow. One of these three fiber sections is highly doped with vanadium and acts as an optically controlled heater, while the other two fiber sections contain fiber Bragg gratings (FBG) that act as dynamic temperature sensors. The vanadium-doped fiber is heated periodically by a laser source, while observing temperature variations within the fluid flow downstream by the two fiber sections with inscribed Bragg gratings. The time delay in temperature variations recorded at both FBG sensors correlates directly with the flow rate of the fluid. When the sensor was placed within the glass capillary with inner diameter of 650 µm, it enabled a flow rate measurement range between 1 ml/h and 1200ml/h. The sensor thus provides a broad flow-rate dynamic range and is insensitive to changes in losses in the lead optical fibers or optical heating source power fluctuations. Furthermore, the thermal properties of the measured liquid, for example, the liquid's thermal conductivity and heat capacity, have mostly limited effects on the measurement results, which allows for thermal-principle-based flow velocity measurements in cases of liquids with variable or poorly defined compositions.

Optimized design of the light source for an indoor visible light communication system based on an improved bat algorithm.

Aiming at the problem of uneven distribution of receiving illuminance and optical power on the receiving plane of the visible light communication (VLC) system, this paper proposes a light source optimization method based on an improved bat algorithm (IBA). Taking the rectangular and hybrid layouts with 16 light-emitting diodes (LEDs), as examples, we set the variance of received light power on the receiving plane as the fitness function. By redesigning the speed update method and local search method of the traditional bat algorithm (BA), the IBA is used to optimize the LED half-power angle and LED position that affect the system performance. The simulation results show that, considering the primary reflection of the wall, the method can reduce the received illumination and optical power fluctuation of the receiving plane within a limited number of iterations under different light source layout schemes.

Reflective temperature sensor based on a fiber Bragg grating combined with a fiber loop ringdown technique.

In this paper, a reflective fiber sensing system combined with a fiber loop ringdown technique for temperature detection is proposed. A fiber Bragg grating (FBG) with an initial center wavelength of 1543.33 nm was set as the sensor head. The experimental results showed that the average sensitivity of temperature was 1.003 µs/°C in the temperature range of 30-38°C, and the resolution was 0.007°C, which is 14 times better than the current FBG sensing demodulator. In addition, the system has the advantages of simple structure, low cost, and insensitivity to optical power fluctuation.

Investigation of a multiwavelength thulium-doped fiber laser incorporating a two-mode fiber filter

A multiwavelength, thulium-doped, ring fiber laser was proposed and experimentally demonstrated. The laser was realized exploiting the cooperative effects of a two-mode fiber filter, nonlinear polarization rotation and the four-wave mixing induced by a section of a highly nonlinear fiber with a length of 150 m. By fine-tuning the polarization controllers, stable multiwavelength outputs with 11 lasing lines within the bandwidth of 10 dB were obtained in 60 min. The maximum wavelength shift and optical power fluctuation of the eleven-wavelength operation for the laser are 0.08 nm and 1.86 dB, respectively, and the wavelength interval and the number of lasing lines can be changed by adjusting the length of the two-mode fiber. Furthermore, the effects of the different experimental parameters such as the pump power, output coupling ratio, nonlinear phase shift and the extinction ratio of the filter on the output performance have also been carefully investigated.

Reduction of modal noise due to fiber/photodetector misalignment in SSMF links based on 850 nm VCSELs through a low-cost polymer coupling structure.

A low-cost polymer-based structure is proposed to improve the coupling between a fiber end section and photodetector active surface in optical links based on standard single-mode fiber (SSMF), which employs vertical cavity surface emitting lasers operating at 850 nm, i.e., below the SSMF cutoff wavelength. Considering receivers as small-area detectors, which are generally necessary to guarantee high-speed operation but at the same time are particularly subject to power fluctuations due to modal noise (whose impact is in turn enhanced in the presence of fiber-to-photodetector misalignment), significant achievements are demonstrated by employing the presented structure. Indeed, in the presence of a misalignment of $ \pm 4 $±4 to $ \pm 6\;{\unicode{x00B5}{\rm m}} $±6µm, which is nowadays typically achievable, the relative optical power fluctuations due to modal noise reduce in the presented case more than four times (2.5% from more than 10%) with respect to the case of butt-coupling, which implies an increase of the same factor in the output signal-to-noise ratio at the receiver end.

Systematic calibration error requirements for gravitational-wave detectors via the Cramér–Rao bound

Gravitational-wave (GW) laser interferometers such as Advanced LIGO (The LIGO Scientific Collaboration 2015 Class. Quantum Grav. 32 074001) transduce spacetime strain into optical power fluctuation. Converting this optical power fluctuation back into an estimated spacetime strain requires a calibration process that accounts for both the interferometer’s optomechanical response and the feedback control loop used to control the interferometer test masses. Systematic errors in the calibration parameters lead to systematic errors in the GW strain estimate, and hence to systematic errors in the astrophysical parameter estimates in a particular GW signal. In this work we examine this effect for a GW signal similar to GW150914, both for a low-power detector operation similar to the first and second Advanced LIGO observing runs and for a higher-power operation with detuned signal extraction. We set requirements on the accuracy of the calibration such that the astrophysical parameter estimation is limited by errors introduced by random detector noise, rather than calibration systematics. We also examine the impact of systematic calibration errors on the possible detection of a massive graviton.

Hybrid Fiber Fabry–Perot Interferometer With Improved Refractometric Response

The reflectance of a hybrid fiber Fabry–Perot interferometer modulated by the Fresnel reflection at the fiber tip was used to assess the refractive index (RI) of liquid samples. Notwithstanding the closeness of RI values between samples and fiber core, well-defined interference spectra in Fourier domain were obtained after a robust signal processing algorithm. Then, the analysis of these spectra allowed us to derive a mathematical expression to obtain the RI value of the fiber tip surrounding media. Next, to assess the fiber refractometer measuring capability a series of liquid samples, typically used for RI calibration, with values ranging from 1.3924 to 1.5882 were measured. A standard deviation of 7 × 10−4 between the measured and actual RI values was estimated. Finally, the refractometer was used to monitor the RI of a sample, with a high thermo-optic coefficient, while it was heated. The accurate and compact all-fiber refractometer proposed here exhibited immunity to optical power fluctuations and one of the largest dynamic ranges reported. This set of combined features make it very attractive for real-time label-free biosensing applications.

Raman Distributed Temperature Sensor with Optical Dynamic Difference Compensation and Visual Localization Technology for Tunnel Fire Detection.

The field of tunnel fire detection requires a Raman distributed temperature sensor (RDTS) with high-accuracy and visual localization. A novel temperature demodulation method to improve the temperature measurement accuracy of the RDTS systems is presented. This method is based on the optical dynamic difference compensation algorithm, which can eliminate the optical power fluctuation. In addition, the visual localization technology is presented by using the longitudinal lining model (LLM) of a three-dimensional (3D) temperature display, which enhances the engineering application of RDTS in tunnel fire detection. Experimental results indicate that the temperature measurement accuracy is optimized from 7.0 °C to 1.9 °C at the sensing distance of 18.27 km by using the presented method. We provide a solution for temperature field monitoring as well as fire visual localization of the tunnel through RDTS systems.