All-fiber System Research Articles

Analytical solution to the problem of polarization drift compensation in an all-fiber QKD system

A method for simultaneous compensation of polarization drift in a fiber-optic quantum channel, and a fully fiber-optic receiver has been successfully developed, implemented, and tested using a commercial QKD system. The method is based on the exact analytical solution we derived, which gives the necessary corrective transformation for polarization. Noise and imperfections in any realistic system require iterative application of the computed corrections. In our case, polarization drift compensation was completed on average in 2-3 iterations. This is an improvement by a factor of 10-100 compared to the common methods based on hill-climbing algorithms.

Quick fabrication method of a thermally expanded core in polarization-maintaining fibers using CO[formula omitted] laser and fiber rotation

Maximizing the transmission of high-peak-power ultrashort pulses through joints between polarization-maintaining optical fibers with different mode field diameters is crucial for improving the efficiency of all-fiber laser systems. Here, we propose a method of fabricating a thermally expanded core by using a CO2 laser as a heating source that does not require a priori splicing of fibers. Unlike standard methods utilizing continuous heating of the fiber, usually on the time scale of minutes, we present the pulsed approach, which can reduce the duration of the whole process to below 30 s. Via fiber rotation, we tailor our method to polarization-maintaining optical fibers. Furthermore, we apply the thermally expanded core method to manufacturing mode field adapters between commercially available polarization-maintaining optical fibers. Exemplary splices reveal an increase in transmission of 30 %, enabling insertion losses lower than 0.4 dB. The presented methods of fabricating a thermally expanded core and mode field adapters show high potential for large-volume production, especially for high-power applications, as the splices can withstand peak powers as high as 50 kW.

Fiber laser system for Rb atomic fountain clock

A compact and robust all-fiber laser system comprising fiber-optical components for a Rb atomic fountain clock is demonstrated. The laser sources were based on the frequency doubling of two seed lasers at a wavelength of 1560 nm, which were locked using digital frequency locking and modulation transfer spectroscopy. During the Sisyphus cooling period, the PZT control voltage of the fiber laser was ramped to detune the laser frequency to 170 MHz, and we get an atomic temperature of 1.9 □K. A series of customized optical fiber splitters, acousto-optic modulators (AOMs), and shutters were integrated into two 2U enclosures as cooling and repumping light modules. The entire laser system was integrated into a 22U cabinet and was characterized via polarization, power, and frequency stability measurements over 100 days. Apply the laser system to the Rb atomic fountain clock, which exhibited a frequency stability of less than 4.5 × 10-16 at the interval of 24 h.

Ultra-broadband mid-infrared supercontinuum generation in square lattice As2S3 chalcogenide photonic crystal fibers

This work presents a numerical model of a photonic crystal fiber made up of chalcogenide glass for highly coherent supercontinuum generation in the mid-infrared spectral region. Numerical simulations based on the finite element method have been performed. An optical dispersion engineering technique has been adopted to minimize the dispersion effect at pump wavelength by alteration of geometrical parameters of designed fiber. We have selected two optimal structures from the simulation results to analyze the nonlinear characteristics and supercontinuum generation. The first fiber, #F1 with a lattice constant of 1.0 μm and a filling factor of 0.3 operates in all-normal dispersion, providing the spectrum SC in the range of 2.4 μm to 8.0 μm with a pump wavelength of 5.0 µm, pulse duration of 90 fs, and peak power of 6 kW. Meanwhile, fiber #F2 has anomalous dispersion regimes. With a peak power of 2 kW, this fiber produces a wide SCG with spectral ranges of 4.4–16 μm. The proposed structures are promising for applications in low-peak power all-fiber optical systems.

Real-time fully digital control scheme for pulse coherent beam combining.

A fully digital control scheme for non-polarization-maintaining (non-PM) nanosecond pulse coherent beam combining (CBC) is proposed, where digital locking of optical coherence by single-detector electronic-frequency tagging (LOCSET) for active phase control and stochastic parallel gradient descent (SPGD) for active polarization control is proposed. The fully digital control scheme is integrated on a real-time field-programmable gate array (FPGA) empowered hardware platform and then experimentally validated in a four-channel all-fiber non-fully polarization-maintaining nanosecond pulse CBC system. Consequently, the system can be fully locked in 9.5 ms, and the polarization extinction ratio (PER) of the combined beam is 21.5 dB with a CBC efficiency of 95.3%. The fully digital control scheme integrates the advantages of digital LOCSET and multi-channel active polarization control, enhancing the channel scalability and the potential output power of the non-PM pulse CBC system.

Semiconductor optical amplifiers as an optical arbitrary waveform generator for high-energy laser systems.

A simple and straightforward technique is presented as a novel temporally controllable front-end for nanosecond very-high energy laser systems. It is based on an original utilization of a semiconductor optical amplifier (SOA) used as an intensity modulator. The essential characteristics of the component are analyzed in order to evaluate potential limitations. Various parameters of interest for standard operation are displayed, demonstrating its usability and its effectiveness. We demonstrate arbitrary and controllable pulse temporal profiles with duration ranging from 1 nanosecond to 100 nanoseconds and a temporal precision of 1.1 ns. A high extinction ratio is also achieved ensuring a modulation contrast up to 53 dB. The SOA is then integrated into an existing operating system in an ultra-compact, reliable all-fibered system. It is used to seed a 2*200 J laser system, exhibiting excellent performance, and validating its usability under operation conditions without any detrimental effects.

Strong dipole-dipole interactions via enhanced light-matter coupling in composite nanofiber waveguides

We study the interaction of emitters with a composite waveguide formed from two parallel optical nanofibers in regimes of experimental importance for atomic gases or solid-state emitters. Using the exact dyadic Green's function we comprehensively investigate the coupling efficiency and the fiber-induced Lamb shift accounting for variations in emitter positions and fiber configurations. This reveals coupling efficiencies and Purcell factors that are enhanced considerably beyond those using a single fiber waveguide, and robustness in the figures of merit. We finally investigate resonant dipole-dipole interactions and the generation of entanglement between two emitters mediated through the composite waveguide under excitation. We show that the concurrence can be enhanced for two fiber systems, such that entanglement may be present even in cases where it is zero for a single fiber. All-fiber systems are simple in construction and benefit from a wealth of existing telecommunications technologies, while enjoying strong couplings to emitters and offering interesting light-matter functionalities specific to slot waveguides. Published by the American Physical Society 2024

All-fiber GHz few-cycle pulse generation at 2 µm.

We demonstrate an all-fiber GHz mode-locked laser system with few-cycle duration operating at 2 µm. Based on a dispersion-managed mode-locked oscillator, a multi-stage fiber amplifier, and a nonlinear pulse compressor, the laser system can deliver watt-level few-cycle pulses at a fundamental repetition rate of 1.041 GHz. This 2-µm pulsed laser offers outstanding performance metrics, including a pulse duration of 33 fs (corresponding to ∼5 optical cycles) and an average power of 4.17 W. Moreover, the all-fiber laser system exhibits excellent power stability, and the integrated relative intensity noise (RIN) is only 0.052% (10 Hz-1 MHz). It is anticipated that this new, to the best of our knowledge, laser source is promising for frontier applications, including coherent supercontinuum generation, nonlinear frequency conversion, and laser-material interaction.

Modeling and analysis of single-mode widely tunable all-fiber Ho-doped CW master oscillator power amplifier system

We report the design of an all-fiber single-mode polarization maintaining widely tunable Ho-doped master oscillator power amplifier (MOPA) system operating in the wavelength range of 2005-2135 nm based on a pre-amplifier and two power amplification stages. The single clad Ho-doped active fibers in the ring cavity based master oscillator, pre-amplifier, and power amplifiers are pumped using 1950 nm semiconductor laser diode pump sources. The amplification of the MOPA system is investigated over the entire spectral range of 2005-2135 nm. The highest output power achieved by the continuous wave (CW) Ho-doped MOPA system is 43 W at a wavelength of 2028.6 nm using pump power of 20 W with power conversion efficiency (PCE) of 90% for second power amplifier while the total pump power of Ho-doped MOPA system is 45 W. Optical bandpass filters (OBPFs) are employed between different stages of MOPA system to enable amplification over the wider spectral range of 2005-2135 nm by suppressing the amplified spontaneous emission (ASE). Not using OBPFs results in amplification that is limited to the spectral range of 2045-2130 nm. Therefore, a penalty of 45 nm can be avoided at the expense of using the OBPFs between different stages of MOPA system. This flexible MOPA system can be used in multiple applications, especially for space-borne high power transmitters operating in atmospheric transmission windows.

439 MHz, 94 fs, low-threshold mode-locked all fiber ring laser

Pulsed fiber lasers with high repetition rates play a crucial role in applications such as high-efficiency processing and bio-optical imaging. However, many ring fiber mode-locked lasers primarily focus on shortening the cavity length to achieve high repetition rate pulse output. Attention is often directed towards the gain fiber and cavity structure, with less consideration given to the challenge of increasing mode-locking threshold power. In this study, we present a high-repetition-rate annular all-fiber laser system with dispersion management. Through strategic adjustment of the intracavity dispersion away from the near-zero dispersion region and careful control of the intracavity gain strength, we have successfully lowered the mode-locking threshold power. This approach can generate pulses with a repetition rate of 439.71 MHz and a mode-locking threshold power of approximately 600 mW. Following pulse compression, the achieved pulse width is 94.8 fs. Its time-bandwidth product is 0.368. To our knowledge, this represents the highest fundamental repetition rate currently attained using a ring all-fiber resonator, accompanied by the lowest mode-locking threshold. Our work introduces a ring fiber laser system characterized by a low mode-locking threshold and a high repetition rate, offering a valuable method for achieving higher fundamental repetition rate pulses.

High-Power GHz Burst-Mode All-Fiber Laser System with Sub 300 fs Pulse Duration

An all-fiber low-repetition-rate SESAM mode-locked fiber oscillator combined with a dispersion-managed active fiber loop produces a flexible GHz burst-mode laser source. The high-power output is then produced by amplifying the GHz burst-mode laser source using an all-fiber chirped-pulse amplification system. Then, the laser is compressed using a grating pair compressor; a maximum amplified power of 97 W is obtained. This results in a compressed high power of 82.07 W with a power stability RMS of 0.09% and beam quality better than 1.2. Accurate dispersion control allows for the production of a high-quality pulse duration of 265 fs.

Photonic quasi-crystal fiber electro-optical modulator

The integration of graphene with optical fiber is considered to be a new interdisciplinary research hotspot for functional fiber. In this paper, an electro-optical modulator based on a six-fold Stampfli-type photonic quasi-crystal fiber (PQF) is theoretically proposed with a sandwiched graphene/hexagonal boron nitride/graphene (Gr/hBN/Gr) film covering all the hole walls. This design exhibits a strong light-graphene interaction with an excellent modulation depth of ∼64 dB mm−1 at 1550 nm by applying an external bias voltage (below 30 V) on both graphene layers. As the Fermi level of the graphene changes with voltage, the fiber shows ‘On’ and ‘Off’ states, serving well as a light-switch. For the modulator performance, the dependence of modulation depth on multiple factors is studied in terms of the layer numbers of graphene and hBN films, the incident wavelength, and the structure parameters. Interestingly, an attenuation peak occurs due to the epsilon-near-zero effect in graphene and shows a linear relationship between the wavelength and the Fermi level. This design provides a guidance for the integration of PQF and graphene, and holds great promise for future all-fiber systems.

Digital calibration method to enable depth-resolved all-fiber polarization sensitive optical coherence tomography with an arbitrary input polarization state

We present a fully integrated depth-resolved all fiber-based polarization sensitive optical coherence tomography (PSOCT). In contrast to conventional fiber-based PSOCT systems, which require additional modules to generate two or more input polarization states, or a pre-adjustment procedure to generate a circularly polarized light, the proposed all-fiber PSOCT system can provide depth-resolved birefringent imaging using an arbitrary single input polarization state. Utilizing the discrete differential geometry (DDG)-based polarization state tracing (PST) method, combined with several geometric rotations and transformations in the Stokes space, two problems induced by the optical fibers can be mitigated: 1) The change in the polarization state introduced by the optical fibers can be effectively compensated using a calibration target at the distal end of the probe, and the computations of the local axis orientation and local phase retardation can be achieved with a single arbitrary input polarization state, eliminating the need for a pre-defined input polarization state, allowing a flexible system design and user-friendly experimental procedure; 2) The polarization mode dispersion (PMD) induced by the optical fibers can be compensated digitally without the requirement of additional input polarization states, providing an accurate PSOCT imaging result. To demonstrate the performance of the proposed method, the depth resolved PSOCT results of a plastic phantom and in vivo skin imaging are obtained using the proposed all-fiber PSOCT system.

Robust low-noise 795-nm single-frequency fiber laser with continuous frequency tuning

This study presents a highly efficient and low-noise all-fiber single-frequency (SF) 795-nm laser system realized using a simple and robust single-pass frequency-doubling configuration. The foundation of the system is a 1590-nm erbium-doped fiber amplifier operating in a master oscillator power amplifier configuration seeded by a distributed feedback semiconductor laser at 1590.52 nm. The main amplifier exhibits an exceptional amplification efficiency of 30.5 %. By utilizing a periodically poled lithium niobate waveguide, the 795-nm laser is frequency-doubled with an output power reaching 2.3 W, corresponding to an optical efficiency of 44 % of the fundamental laser. Crucial to its application in precision measurements and quantum technologies, the SF 795-nm laser exhibits a relative intensity noise close to −150 dBc/Hz in the frequency range of 0.1 to 10 MHz. Additionally, a continuous frequency-tuning range of 11.7 GHz is achieved by modulating the current with a 2.5-mA 1-Hz triangular wave. Notably, this is the first report on an all-fiber SF 795-nm laser system.

Circular lattice benzene-core PCFs with flat near-zero dispersion for low-power broad-spectrum supercontinuum generation

A circular photonic crystal fiber infiltrated with benzene with different air-hole diameters is proposed as a new supercontinuum light source. Optical properties related to dispersion, effective mode area, nonlinear coefficient, and attenuation of the fundamental mode are investigated numerically. Two optimized structures are selected and verified against supercontinuum generation (SCG) in detail. The first structure (#F1) possesses all-normal dispersion, while the second (#F2) has a zero-dispersion wavelength. The possibility of coherent, octave-spanning SCG is proved by a 40 fs pulse, 1.064 μm wavelength, and 0.45 kW of power in-coupled into the core of #F1. Otherwise, injecting a 90 fs duration, 1.5 μm wavelength, and 0.555 kW peak power pump pulse into #F2 generates a broad SC spanning 0.76–4.23 μm. With the advantages of flat near-zero dispersion, high nonlinearity, low attenuation, and low input power used for SCG, the proposed fibers may lead to new low-cost all-fiber optical systems.

High-sensitivity fiber optic graphene resonant accelerometer.

This study proposes a high-sensitivity resonant graphene accelerometer based on a pressure-induced sensing mechanism. The accelerometer design encompasses an optical fiber and a vacuum-sealed graphene resonator affixed to a silicon sensitive film, incorporating a proof mass. This indirect sensing mechanism effectively mitigates the vibration mode aliasing of graphene and the proof mass while ensuring a minimal energy loss in the operating resonator. The mechanical vibration of graphene is excited and detected through an all-fiber optical system. Notably, the proposed sensor demonstrates a sensitivity of 34.3 kHz/g within the range of 0-3.5 g, which is eight times higher than comparable accelerometers utilizing a proof mass on a graphene membrane. This work exhibits a novel, to the best of our knowledge, approach to an acceleration measurement using 2D resonators, exhibiting distinct advantages in terms of compact size and heightened sensitivity.

Versatile design for temporal shape control of high-power nanosecond pulsed fiber laser amplifier.

This research proposed a novel pulse-shaping design for directly shaping distorted pulses after the amplification. Based on the principle of the design we made a pulse shaper. With this pulse shaper, we successfully manipulate the pulse's leading edge and width to achieve an 'M'-shaped waveform in an amplification system. Comparative experiments were conducted within this system to compare the output with and without the integration of the pulse shaper. The results show a significant suppression of the nonlinear effect upon adding the pulse shaper. This flexible and effective pulse shaper can be easily integrated into a high-power all-fiber system, supplying the capability to realize the desired output waveform and enhance the spectral quality.

Optimizing optical pulse breakup for efficient supercontinuum generation in an all-fiber system

Ultra-broadband supercontinuum (SC) is generated by an all-fiber system with well-defined pulses (WDPs) as a seed laser. Through properly adjusting the lengths of the fiber segments in the system, sub-pulses with high peak powers are generated through the process of optical pulse breakup. Then, the broken optical pulses are launched into a hybrid nonlinear, which consists of a highly nonlinear optical fiber and a photonic crystal fiber, generating a SC of 1.8 W covering a spectral range from 554 nm to 2.17 µm. In this study, it is demonstrated through experimental observation that optical pulse breakup is an essential process for WDPs before the pulses are launched into a nonlinear optical fiber to generate SC. However, pulse breakup must not happen before the amplification of the WDPs; otherwise, the amplification efficiency is decreased, leading to a subsequent low efficiency of SC generation. The proper breakup of the WDPs after they are efficiently amplified is accomplished by sending these pulses through an optimum length of regular optical fiber, which depends on the peak power of the amplified WDPs. An optical fiber of an insufficient length leads to insufficient pulse breakup, which does not significantly increase the peak power of the pulses, resulting in little enhancement of SC generation. An excessively long fiber leads to excessive pulse breakup, which stretches the pulsewidth and splits the pulse energy, resulting in decreasing the efficiency of SC generation.

High-power, highly efficient, all-fiber wavelength-tunable polarization-maintaining Tm-doped fiber MOPA system

A 2 μm all-fiber wavelength-tunable polarization-maintaining Tm-doped fiber master oscillator power amplifier (MOPA) system is presented. We demonstrate an all-fiber ring cavity Tm-doped fiber master-oscillator (MO) with a tuning range of 60 nm (1959.9–2020.1 nm). The maximum output power of 82.4 mW is obtained at 1991 nm with the launched pump power of 16 W at 793 nm, corresponding to a slope efficiency of 10.8 % concerning launched pump power. Finally, through the first stage amplification, the power is successfully amplified to 38.92 W, the slope efficiency is about 31.4 %, and the polarization extinction ratio (PER) reaches 21.4 dB. Then a theoretical simulation is carried out to calculate the influence of different fiber lengths on the slope efficiency and output power of the amplifier stage when the pump power is increased to 420 W, which provides theoretical guidance for the subsequent experiments. Through the simulation, it is found that the output power will be greatly improved by choosing the appropriate fiber length and increasing the pump power.

Hundreds of picosecond pulses amplifier based on Yb-doped tapered fiber for the generation of 100 W average output power

Hundreds of picosecond pulses amplifier based on Yb-doped tapered fiber can serve as the primary power amplifier stage for an all-fiber and polarization-maintained picosecond pulse laser system. Here, we report a high-power narrow-linewidth amplifier stage by using the Yb-doped tapered fiber. In the regime, we have demonstrated amplification of 201.4 ps pulses centered at 1031.4 nm with a spectral width of 0.54 nm to 105 W average power at a repetition rate of 29.3 MHz. The picosecond pulses had a high beam quality with a measured M2 value of 1.36 ∼ 1.51, and the optical-to-optical conversion efficiency reached 56 %. To the best of our knowledge, this is the first demonstration of a high-power amplifier based on Yb-doped tapered fiber, capable of amplifying hundreds of picosecond pulses beyond the hectowatt-level average power, while maintaining high beam quality. This achievement highlights the immense potential of Yb-doped tapered fiber in the field of picosecond pulse power amplification. At last, we have refined the model for the transmission and amplification of picosecond pulses in Yb-doped tapered optical fibers, thereby enhancing simulation accuracy by 8 % and providing valuable references for the in-depth research of tapered fiber pulse amplifiers.