Flexible Wavelength Research Articles

High-precision 1.5 µm band tunable single-frequency fluoro-sulfo-phosphate fiber laser based on a self-injection locking scheme

In this paper, a high-precision 1.5 µm band tunable single-frequency Er3+/Yb3+ co-doped fluoro-sulfo-phosphate (EYFSP) fiber laser based on a self-injection locking scheme is experimentally demonstrated. A self-developed 4 cm long EYFSP fiber with a high gain coefficient (3.2–4.7 dB/cm) at the C-band serves as the gain medium for laser output. The single longitudinal mode (SLM) operation is realized via a 15 cm long commercial unpumped gain fiber and a self-injection locking scheme, while flexible wavelength tuning is controlled by a fiber Fabry–Perot tunable filter (FFP-TF). The laser wavelength is tuned from 1528.08 to 1555.55 nm, with a tuning precision of up to 10–18 pm and a tuning speed of 30 nm/s. Throughout the entire tuning range of 27.47 nm, the measured laser linewidth is less than 1.3 kHz, the relative intensity noise remains below −140dB/Hz at frequencies above 3 MHz, and the power fluctuation is as low as 1% over a measurement period of 2 h. These results demonstrate that the high-precision 1.5 µm band tunable single-frequency EYFSP fiber laser is a promising candidate for future coherent optical communication systems and optical fiber sensing.

Harmonic‐Assisted Super‐Resolution Rotational Measurement

AbstractEnhancing rotational measurement resolution and broadening the detectable spectral range are two critical and unresolved matters within the realm of motion perception. The rotational Doppler effect (RDE) is combined with the harmonic generation process to create a rotational measurement scheme that offers flexible detection wavelength conversion, exponential improvement of measurement resolution, and real‐time display of detection results. In the experiments, a cascaded second harmonic generation process is employed to attain a fourfold enhancement in rotational resolution and demonstrate how low‐cost silicon‐based detectors can be used for real‐time detection of infrared objects. This scheme employs a Gaussian beam within the nonlinear process to achieve high conversion efficiency, thereby enabling potential for subsequent cascade amplification. Additionally, it is fully compatible with existing RDE schemes, allowing for co‐amplification of rotational resolution at both the front‐end and back‐end. This research could offer a more precise and cost‐effective method for remote sensing detection.

High‐Performance Flexible PbS Nanofilm Wavelength Sensor with Detection Region Ranging from DUV to NIR

AbstractIn this study, on the fabrication of a flexible wavelength sensor is reported, which is achieved by growing a layer of lead sulfide (PbS) nanofilm on both sides of a polyethylene terephthalate substrate using the chemical bath deposition method, followed by the deposition of two parallel Au interdigital electrodes. Experimental result shows that the photocurrent ratio of the two photodetectors monotonically decreases with increasing wavelength in the range from 265 nm (UV) to 2000 nm (NIR), indicating that the incident light wavelength can be distinguished by the photocurrent ratio. Notably, the as‐constructed wavelength sensor exhibits superior performance compared to most previously reported filter‐less designs, achieving an average absolute error of 11.5 nm and an average relative error of 1.1%. It is also found that the sensor exhibits excellent mechanical flexibility and environmental stability. Furthermore, by introducing the back‐end circuit, real‐time detection of the wavelength of monochromatic light and the peak wavelength of LED light are achieved, with detection errors not exceeding 2.8% and 2.6%, respectively. It is believed that the flexible PbS nanofilm wavelength sensor prepared in this study has potential application in future portable and flexible optoelectronic devices.

DSP-based inter-channel interference monitoring in flexible wavelength-routed networks

The efficiency of optical networks employing flexible wavelength division multiplexing (WDM) can be increased by maximizing the throughput of each individual channel, provided that the position of its neighboring channel is known with sufficient accuracy in order to avoid inter-channel interference. In this paper, we propose a digital signal processing (DSP) algorithm, leveraging the use of an artificial neural network (ANN), to estimate the neighboring channels’ distance by processing raw digital samples from a standard coherent receiver. We present an efficient dataset design approach, based on Latin hypercube sampling (LHS), in order to effectively optimize and validate the algorithm under different assumptions on the optical WDM channels. We investigate the accuracy of the ANN-based DSP scheme through simulation analysis, highlighting its potential in relation to the characteristics of the optical network. Finally, we validate our approach in an experimental setup using standard commercial coherent transceivers. The experimental results show that the distance from the neighboring WDM channels can be estimated with a root mean square error of less than 1.5 GHz for a channel under test with a symbol rate of 52 GBaud.

Spectral Manipulations of Random Fiber Lasers: Principles, Characteristics, and Applications

AbstractRandom fiber lasers (RFLs), a new variety of random lasers, have become a vigorous research spot over the past decades. RFLs possess superiorities in efficiency, structural flexibility, directionality, cost, etc. Particularly, RFLs are discovered to be good platforms for the construction of various forms of high‐performance lasers. Herein, recent progress in the spectral manipulation of RFLs and the applications of spectral‐tailored RFLs are reviewed. Both theoretical and experimental investigations of the unique spectral properties of RFLs up to now are presented. The superior wavelength flexibility of RFLs which can achieve any desired lasing wavelength in the 1–2.1 µm band is highlighted. Via spectral manipulation, RFLs have shown the capability of high‐spectral‐purity, narrow‐bandwidth, and multi‐wavelength output. Furthermore, the RFLs featuring unique spectral properties can lead to various promising applications, which are concisely summarized, including optical fiber communication, optical fiber sensing, imaging, supercontinuum generation, nonlinear‐frequency conversion, mid‐infrared lasing pump sources, and laser‐driven inertial confinement fusion motivation. The challenges and prospects for RFLs are also discussed.

Switchable and tunable dual-wavelength thulium-doped fiber laser based on the parallel filter of the all-fiber Sagnac interferometer embedded with an FBG and the Fabry-Perot filter

A switchable and tunable dual-wavelength thulium-doped fiber laser (ST-DW-TDFL) based on the parallel filter is proposed and experimentally demonstrated in this study. In the proposed ST-DW-TDFL, there are two independent branches to form the parallel filter. One branch uses an all-fiber Sagnac interferometer embedded with a fiber Bragg grating (FBG-FSI) as the wavelength selection element, and the other branch uses a Fabry-Perot (FP) filter as the wavelength selection element. By adjusting two optical attenuators to change the light power of two branches, single and dual-wavelength output can be achieved and switched. On the basis of achieving dual-wavelength output, independent and continuous tuning of two wavelengths can be achieved by heating the FBG and adjusting the FP filter. The tuning range of one wavelength is up to 5.84 nm, while the tuning range of the other wavelength is up to 99.86 nm. The maximum wavelength variation and power fluctuation are less than 0.03 nm and 0.5 dB at room temperature. And the experimental results show that the proposed ST-DW-TDFL has flexible wavelength switching and tuning capabilities and good wavelength and power stabilities.

Broadband tunable Raman fiber laser with monochromatic pump.

Raman fiber laser (RFL) has been widely adopted in astronomy, optical sensing, imaging, and communication due to its unique advantages of flexible wavelength and broadband gain spectrum. Conventional RFLs are generally based on silica fiber. Here, we demonstrate that the phosphosilicate fiber has a broader Raman gain spectrum as compared to the common silica fiber, making it a better choice for broadband Raman conversion. By using the phosphosilicate fiber as gain medium, we propose and build a tunable RFL, and compare its operation bandwidth with a silica fiber-based RFL. The silica fiber-based RFL can operate within the Raman shift range of 4.9 THz (9.8-14.7 THz), whereas in the phosphosilicate fiber-based RFL, efficient lasing is achieved over the Raman shift range of 13.7 THz (3.5-17.2 THz). The operation bandwidths of the two RFLs are also calculated theoretically. The simulation results agree well with experimental data, where the operation bandwidth of the phosphosilicate fiber-based RFL is more than twice of that of the silica fiber-based RFL. This work reveals the phosphosilicate fiber's unique advantage in broadband Raman conversion, which has great potential in increasing the reach and capacity of optical communication systems.

Compact diode-pumped tunable single- and dual-wavelength single-frequency semiconductor disk lasers

Optically pumped external cavity semiconductor disk laser (SDL) combines the characteristics of semiconductor laser and solid-state laser, and has important research value. In this paper, an InGaAs SDL with different emission peak from others was designed. Under free-running operation and a cooling temperature of 12 °C, a few-longitudinal-mode continuous wave laser at 1037 nm with a maximum output power of 2.53 W and a slope efficiency of 41.3% with respect to an absorbed power of about 8.0 W was achieved. Based on this experiment, stable single-wavelength single frequency SDL with maximum output power of 1.79 W was obtained by inserting an etalon into the laser resonator. The single frequency laser was measured to have near-diffraction-limited beam quality. Flexible and continuous wavelength tuning from 1034.77 nm to 1041.10 nm of the single-wavelength single frequency SDL was also realized by introducing another etalon. Under this situation, we also obtained tunable dual-wavelength single frequency SDLs, for the first time to our knowledge. Compared to complex methods such as particular bandgap engineering design or multi-SDL resonator for generating dual-wavelength lasers, the method proposed in this work by using etalons is a simple, feasible, and low-cost solution.

High Energy Singular Beams Generation From a Dissipative Soliton Resonance Raman Fiber Laser

We demonstrated the high energy 1.65 μm singular beam generation from a dissipative soliton resonance (DSR) all-fiber Raman laser experimentally. By utilizing advantages of the high nonlinearity and home-made mode-selective coupler (MSC), different DSR pulsed singular beams, including cylindrical vector beam (CVB) and vortex beams (VB), have been generated experimentally. The maximum pulse energy can be enhanced up to 698.19 nJ with high signal to noise ratio of 80 dB by optimizing the intra-cavity nonlinearity. Our research may provide a new method to generate high energy pulsed singular beams with a flexible operation wavelength.

Broadband synchronization of ultrafast pulse generation with double-walled carbon nanotubes.

Double-walled carbon nanotubes have shown competitive properties in broadband optical pulse generation owning to the intrinsic electronic properties. Synchronization of ultrafast optical pulses in multiple wavelengths is a key technique for numerous applications, such as nonlinear frequency conversion, ultrafast pump-probe, coherent Raman scattering spectroscopy, coherent optical synthesis, etc. In this work, we demonstrate the mode-locking and synchronization of 1.55 µm pulses with 1 µm and 1.9 µm pulses via a single saturable absorber based on double-walled carbon nanotubes. The large optical nonlinearity and broadband optical absorption in the double-walled carbon nanotubes enable independent and synchronized mode-locking in >900 nm bandwidth. In addition, we present a creative concept to realize multi-wavelength synchronization from a single laser system. Our results demonstrate a straightforward and feasible approach towards pulse synchronization over ultra-broad bandwidth with flexible wavelength selection in the near-infrared region.

Low Noise Ultra-Flexible SiP Switching Platform for mmWave OCDM & Multi-Band OFDM ARoF Fronthaul

A highly flexible wavelength and space switched analog radio-over-fiber (ARoF) fronthaul transmission of a millimeter-wave (mmWave) emerging 6G waveform over a centralized/ cloud radio access network (C-RAN) is experimentally demonstrated in this work. A spread spectrum multiplexing technique — orthogonal chirp division multiplexing (OCDM) — which is highly resilient to inter-channel interference and enables enhanced channel estimation is utilized in the fronthaul transmission demonstration. The flexible properties of a low noise silicon photonic (SiP) microring resonator (MRR) based tunable laser and a low cross-talk 4×4 SiP optical wavelength/space switch are combined to form a reconfigurable 10 km fronthaul system enabling 64-QAM OCDM transmission at 24 GHz with consistent performances ~5% EVM across all test wavelengths/ports. A signal constituting Wi-Fi and 5G NR standard compatible 64-QAM orthogonal frequency division multiplexing (OFDM) bands, at 10 GHz and 24 GHz respectively, are also transmitted and evaluated in the proposed system with EVM performances below 64-QAM EVM limit (8%) achieved, thus demonstrating the system’s potential in a future converged multi-service environment.

Wideband SOA fiber-to-fiber gain and saturation output power in the C-band: impact of characteristic parameters

Semiconductor Optical Amplifiers (SOAs) have numerous attractive characteristics; for example: minimum power usage, broadband, flexible wavelength, wide dynamic range and significant nonlinearities. In this study, we present a comprehensive model of output saturation power and fiber-to-fiber gain of wideband SOAs. Furthermore, we investigate the impact of optical confinement factor, bias current, input signal power, temperature, and amplifier length on the fiber-to-fiber gain spectrum in the C-band. The obtained numerical results are approved by comparison with simulation results showing a fair agreement. The results reveal that output saturation power is improved by decreasing the optical confinement factor. Additionally, when input signal power rises, the maximum fiber-to-fiber gain value declines. Furthermore, as the temperature rises, the fiber-to-fiber gain decreases. Moreover, the maximum fiber-to-fiber gain is obtained at shorter amplifier lengths. Additionally, the effect of carrier density on the spectrum of coefficient of material gain is investigated, showing an increase in its value when increasing the carrier density, with a peak near 1550 nm.

Generation of 1.3 µm femtosecond pulses by cascaded nonlinear optical gain modulation in phosphosilicate fiber.

Nonlinear optical gain modulation (NOGM) is a simple and effective approach to generate highly coherent ultrafast pulses with a flexible wavelength. In this work, we demonstrate 34 nJ, 170 fs pulse generation at 1319 nm through a piece of phosphorus-doped fiber by two-stage cascaded NOGM with a 1064 nm pulsed pump. Beyond the experiment, numerical results show that 668 nJ, 391 fs pulses at 1.3 µm can be produced with up to 67% conversion efficiency by increasing the pump pulse energy and optimizing the pump pulse duration. This would offer an efficient method to obtain high-energy sub-picosecond laser sources for applications such as multiphoton microscopy.

Mid-infrared cross-comb spectroscopy

Dual-comb spectroscopy has been proven beneficial in molecular characterization but remains challenging in the mid-infrared region due to difficulties in sources and efficient photodetection. Here we introduce cross-comb spectroscopy, in which a mid-infrared comb is upconverted via sum-frequency generation with a near-infrared comb of a shifted repetition rate and then interfered with a spectral extension of the near-infrared comb. We measure CO2 absorption around 4.25 µm with a 1-µm photodetector, exhibiting a 233-cm−1 instantaneous bandwidth, 28000 comb lines, a single-shot signal-to-noise ratio of 167 and a figure of merit of 2.4 × 106 Hz1/2. We show that cross-comb spectroscopy can have superior signal-to-noise ratio, sensitivity, dynamic range, and detection efficiency compared to other dual-comb-based methods and mitigate the limits of the excitation background and detector saturation. This approach offers an adaptable and powerful spectroscopic method outside the well-developed near-IR region and opens new avenues to high-performance frequency-comb-based sensing with wavelength flexibility.

Unidirectional ring vortex laser using a wedge-plate shearing interferometer.

In response to growing demand from optical vortex (OV) beam applications, numerous generation techniques have been developed competing in power scalability, purity, and wavelength flexibility. Direct vortex emission from lasers typically grants access to efficient, high power, and pure mode generation. In this work we demonstrate a compact, unidirectional Nd:YVO4 ring laser with an intracavity wedge-plate shearing interferometer (WPSI) as an output coupler, which converted the internal Gaussian mode to LG01 (Laguerre-Gaussian OV) output. It directly generated a watt-level LG01 OV with high mode purity (98%) in a single longitudinal mode. The monolithic WPSI has advantages in stability and simplicity compared to other designs. The system is compact and cheap, using off-the-shelf components, and can be readily adapted to any gain media, widening the scope for OV generation at wavelengths currently unobtainable using competing methods.

Focus on lasers, imaging, microscopy, and nanoscience

Focus on lasers, imaging, microscopy, and nanoscience

Cylindrical vector beam generation from a passively mode-locked Raman fiber laser

We experimentally investigate cylindrical vector beam (CVB) generation from a passively mode-locked Raman fiber laser. The mode-locked Raman fiber lasers based on nonlinear polarization rotation (NPR), utilize the Raman gain resulted from the stimulated Raman scattering (SRS) effect in passive fibers, instead of stimulated emission amplification of active fibers in conventional mode-locked fiber lasers. With a mode-selective coupler (MSC) incorporated into the mode-locked Raman fiber laser, the generation of CVBs has been realized. In experiment, using a Raman pump source of 1454 nm, mode-locked CVB output with central wavelength of 1562 nm was obtained. The output polarization state can be switched between the radial and azimuthal polarization states, and the beam purity is above 90%. Due to the broadband Raman gain in optical fiber, the CVB output with the desired wavelength is available in principle by selecting a suitable pump source. This kind of flexible wavelength CVB Raman fiber laser may have potential applications in optical communications and fiber optics.

Spectrally programmable Raman fiber laser with adaptive wavefront shaping

Raman fiber lasers (RFLs) have broadband tunability due to cascaded stimulated Raman scattering, providing extensive degrees of freedom for spectral manipulation. However, the spectral diversity of RFLs depends mainly on the wavelength flexibility of the pump, which limits the application of RFLs. Here, a spectrally programmable RFL is developed based on two-dimensional spatial-to-spectral mapping of light in multimode fibers (MMFs). Using an intracavity wavefront shaping method combined with genetic algorithm optimization, we launch light with a selected wavelength(s) at MMF output into the active part of the laser for amplification. In contrast, the light of undesired wavelengths is blocked. We demonstrate spectral shaping of the high-order RFL, including a continuously tunable single wavelength and multiple wavelengths with a designed spectral shape. Due to the simultaneous control of different wavelength regions, each order of Raman Stokes light allows flexible and independent spectral manipulation. Our research exploits light manipulation in a fiber platform with multi-eigenmodes and nonlinear gain, mapping spatial control to the spectral domain and extending linear light control in MMFs to active light emission, which is of great significance for applications of RFLs in optical imaging, sensing, and spectroscopy.

Multi-color switchable visible light source generated via nonlinear frequency conversion of a random fiber laser.

Nonlinear frequency conversion of random fiber lasers could provide new possibilities to realize visible and mid-infrared light with flexible wavelength and low temporal/spatial coherence. Frequency doubling of random fiber laser is reported to generate visible light with single-color output. Here, we propose a new way to generate multi-color switchable visible light source from a dual-wavelength switchable 1st-order random Raman fiber laser (RRFL) with phosphosilicate fiber. Taking advantage of the existence of the two Raman gain peaks with significant different Raman gain bandwidth at the frequency shifts of 13.2 THz (silica-related one with broad Raman gain bandwidth) and 39.9 THz (phosphorus-related one with narrow Raman gain bandwidth) in phosphosilicate fiber, a dual-wavelength switchable RRFL is developed which can emit 1120 and 1238 nm random Raman lasing individually or simultaneously with 3-watt level output power and sub-1 nm bandwidth by precisely tuning the pump wavelength to manipulate the Raman gain at two fixed Raman Stokes wavelengths. It is expected that the output power can be further increased with a shorter fiber length and more powerful pump, and the spectral bandwidth can be much narrower by adopting a narrowband point reflector in 1st-order RRFL. Based on the dual-wavelength RRFL with a flexible power ratio and a periodically poled lithium niobate (PPLN) crystal array containing three separate poled gratings with different periods, the second-harmonic generation of 1120 nm or 1238 nm random lasing and sum-frequency generation of 1120 nm and 1238 nm random lasing can be performed. As a result, the switchable output of green light at 560 nm, yellow light at 588 nm and red light at 619 nm can be realized with optical power of 22.2 mW, 16.9 mW and 18.5 mW, respectively. Our work demonstrates dual-wavelength RRFL could act as a new platform for generating visible light source with flexible color output which has potential applications in imaging, sensing and visible temporal ghost imaging.

Flexible wavelength-, pulse-controlled mode-locked all-fiber laser based on a fiber Lyot filter.

In this paper, we report a flexible wavelength-, pulse-controlled mode-locked all-fiber laser based on a novel fiber optic Lyot filter. The wavelength, pulse duration and spectral bandwidth of passive mode-locked lasers can be tuned by controlling the polarization controller. The proposed Lyot filter was constructed by a single-mode fiber insertion between two polarization-maintaining fibers. The filter bandwidth and laser output tunability were based on the birefringence characteristics of the polarization-maintaining fibers. This all-fiber laser is simple and stable and can be used for various applications where width-tunable or wavelength-tunable pulses are necessary.