Wavelength Selective Element Research Articles

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.

A Tunable Hyperspectral Imager for Detection and Quantification of Marine Biofouling on Coated Surfaces

Fouling control coatings (FCCs) are used to prevent the accumulation of marine biofouling on, e.g., ship hulls, which causes increased fuel consumption and the global spread of non-indigenous species. The standards for performance evaluations of FCCs rely on visual inspections, which induce a degree of subjectivity. The use of RGB images for objective evaluations has already received interest from several authors, but the limited acquired information restricts detailed analyses class-wise. This study demonstrates that hyperspectral imaging (HSI) expands the specificity of biofouling assessments of FCCs by capturing distinguishing spectral features. We developed a staring-type hyperspectral imager using a liquid crystal tunable filter as the wavelength selective element. A novel light-emitting diode illumination system with high and uniform irradiance was designed to compensate for the low-filter transmittance. A spectral library was created from reflectance-calibrated optical signatures of representative biofouling species and coated panels. We trained a neural network on the annotated library to assign a class to each pixel. The model was evaluated on an artificially generated target, and global accuracy of 95% was estimated. The classifier was tested on coated panels (exposed at the CoaST Maritime Test Centre) with visible intergrown biofouling. The segmentation results were used to determine the coverage percentage per class. Although a detailed taxonomic description might be complex due to spectral similarities among groups, these results demonstrate the feasibility of HSI for repeatable and quantifiable biofouling detection on coated surfaces.

Thulium-doped fiber laser with bidirectional output in a ring laser cavity

A thulium-doped fiber laser (TDFL) with bidirectional output was proposed and demonstrated herein. Clockwise (CW) and counter-clockwise (CCW) lasing output with different lasing wavelengths was realized by two uniform fiber Bragg gratings (UFBGs) and two optical circulators. The UFBGs located at 1941.238 and 2049.325 nm were incorporated in the laser cavity to act as the wavelength-selective element. Switchable operation was realized through tuning the gain and loss of the circulating light by applying curvature on a section of the single-mode fiber. When the TDFL was operated in the CW direction, single-wavelength output with a center wavelength, optical signal-to-noise ratio (OSNR), and maximal output power of 2049.335 nm, 33.175 dB, and 280.6 mW was obtained, and the fluctuation of the center wavelength and the output power were less than 0.041 nm and 1.400 dB within an observation time of 60 min. When the TDFL was operated in the CCW direction, single-wavelength output with a center wavelength, OSNR, and maximal output power of 1941.312 nm, 37.108 dB, and 486 mW could be obtained, and the maximal fluctuation of the center wavelength and the output power were 0.128 nm and 0.832 dB within an observation period of 60 min. In addition, the proposed TDFL can operate in both CC and CCW directions simultaneously with different output wavelengths. The potential application of a TDFL strain sensor with sensitivities of 1.33 and 1.22 pm/με in the CW and CCW directions was also demonstrated and the proposed TDFL may also well suited for use in medical and free-space optical communication systems.

Wavelength-stabilized tunable mode-locked thulium-doped fiber laser beyond 2 µm.

We report the development of a widely tunable mode-locked thulium-doped fiber laser based on a robust chirped fiber Bragg grating (CFBG). By applying mechanical tension and compression to the CFBG, an overall tunability of 20.1 nm, spanning from 2022.1 nm to 2042.2 nm, was achieved. The observed mode-locked pulse train from this fiber laser has a repetition rate of 9.4 MHz with an average power of 12.6 dBm and a pulse duration between 9.0 ps and 12.8 ps, depending on the central wavelength. To the best of our knowledge, this is the first demonstration of a tunable mode-locked thulium-doped fiber laser operating beyond 2 µm using a CFBG as a wavelength-selective element.

A novel 8-channel DWDM demultiplexer on silicon photonic crystal slab: Design and analysis

In this work an 8-channel DWDM demultiplexer is proposed with multiple designs and numerical analysis. Photonic crystal ring resonators of airholes on a 220 nm thick silicon slab are used as wavelength selective elements. Demultiplexers of three different designs are considered and their characteristics are analyzed. For all the three designs, the resolved wavelengths are in the range of 1530–1540 nm, where Erbium Doped Fiber Amplifiers are applicable. For the optimized device, the average coupling efficiency, cross talk are found to be 70.4% and − 16.76 dB, respectively. Maximum and minimum coupling efficiencies of 79% and 56% are achieved. Photonic bandgap computation is performed by using plane wave expansion method and spectral characteristics are obtained by applying 3D finite difference time domain method.

EDF laser temperature sensor using double-pass cascaded-chirped long-period fiber grating in sigma-cavity configuration

We propose a novel Erbium-doped fiber laser sensor in a sigma-cavity configuration (EDFσL) for temperature sensing. In this configuration, the sigma-branch of the EDFσL, or the double-pass cascaded-chirped long-period fiber grating (C-CLPG), consists of an optical circulator, a C-CLPG, and a Faraday rotator mirror (FRM), and works as a reflective sensor device as well as a wavelength selection element with a comb-like spectrum, namely a channeled spectrum. The double-pass C-CLPG has the advantage of not only a flexible sensor arrangement, especially for remote sensing, but also the compensation effect for the polarization fluctuations in the sigma-branch caused by the measurand. In the experiment, it is shown that the FRM reflection scheme can stabilize the output polarization state of the double-pass C-CLPG during the temperature change around the C-CLPG, and it is found that the EDFσL enables a more stable and precise measurement than the previous simple EDF ring laser using the C-CLPG. Furthermore, it is also demonstrated that the oscillation is stabilized by the polarization compensation effect of the double-pass C-CLPG even if the polarization state is intentionally changed by applying the mechanical deformation near the C-CLPG.

Heat-induced wavelength-switchable high-power CW orange Pr3+:YLF lasers

Pr3+:YLF crystal has been demonstrated as a successful laser gain medium in the visible region. However, the 604-nm laser based on this crystal is limited in power due to the intrinsic reabsorption in the orange region. We propose a laser diode (LD) pumped continuous-wave (CW) 604-nm laser based on a lowly-doped Pr3+:YLF crystal. A maximum output power of 3.28 W with a total slope efficiency of 34.2% is achieved at 604 nm. The slope efficiency increases to 43.0% under high-power pumping conditions. To the best of our knowledge, the output power of the 604-nm laser we achieved is the highest to date and no output power saturation is observed. Besides, dual-wavelength operations at 604 and 607 nm are achieved by tuning the thermal lensing effects of the crystal, without any additional wavelength-selection elements. The dual-wavelength laser at 604 and 607 nm with a maximum output power of 3.30 W (highest to date as well) is obtained by this approach. Since no extra intracavity losses are introduced by intracavity wavelength-selection elements, the method we proposed will be promising to obtain high-power wavelength-switchable lasers.

Distributed feedback lasers using surface gratings in Bragg waveguides.

This Letter presents, to the best of our knowledge, the first report of a narrow-linewidth ∼790-800nm edge-emitting semiconductor distributed feedback Bragg reflection waveguide diode laser (DFB2RL). The DFB2RLs were fabricated using a ridge waveguide structure with 5th order, surface-etched grating forming the wavelength selective element. Unbonded devices with a 500 µm cavity length exhibited continuous wave threshold currents in the region of 25 mA with an output power of 2.5 mW per (uncoated) facet at 100 mA drive current. The devices operated in a single longitudinal mode, with side-mode suppression ratio (SMSR) as high as 49 dB and linewidths as low as 207 kHz. Devices maintained single mode operation with high SMSR over a 9 nm wavelength range as the temperature was swept from 15°C to 50°C.

Quantitative imaging of volcanic SO<sub>2</sub> plumes using Fabry–Pérot interferometer correlation spectroscopy

Abstract. We present first measurements with a novel imaging technique for atmospheric trace gases in the UV spectral range. Imaging Fabry–Pérot interferometer correlation spectroscopy (IFPICS) employs a Fabry–Pérot interferometer (FPI) as the wavelength-selective element. Matching the FPI's distinct, periodic transmission features to the characteristic differential absorption structures of the investigated trace gas allows us to measure differential atmospheric column density (CD) distributions of numerous trace gases with high spatial and temporal resolution. Here we demonstrate measurements of sulfur dioxide (SO2), while earlier model calculations show that bromine monoxide (BrO) and nitrogen dioxide (NO2) are also possible. The high specificity in the spectral detection of IFPICS minimises cross-interferences to other trace gases and aerosol extinction, allowing precise determination of gas fluxes. Furthermore, the instrument response can be modelled using absorption cross sections and a solar atlas spectrum from the literature, thereby avoiding additional calibration procedures, e.g. using gas cells. In a field campaign, we recorded the temporal CD evolution of SO2 in the volcanic plume of Mt. Etna, with an exposure time of 1 s and 400×400 pixel spatial resolution. The temporal resolution of the time series was limited by the available non-ideal prototype hardware to about 5.5 s. Nevertheless, a detection limit of 2.1×1017 molec cm−2 could be reached, which is comparable to traditional and much less selective volcanic SO2 imaging techniques.

Isolator-free unidirectional dual-wavelength thulium-doped fiber laser assisted by a two-mode fiber filter

A switchable, thulium-doped, isolator-free fiber laser is proposed and demonstrated. The isolator-free configuration, wavelength selective element and stabilization mechanism were achieved by using a theta cavity design, a two-mode fiber filter and nonlinear polarization rotation, respectively. By properly adjusting the polarization controllers to induce polarization-dependent loss, single- and dual-wavelength outputs can be obtained, and the stability of single- and dual-wavelength operations was measured for 30 min in each case. The single wavelength can be switched from 1959.17 to 1994.86 nm in a lasing range of ~35.7 nm with an optical signal-to-noise ratio (OSNR) of up to ~60 dB. When lasing dual-wavelength, the wavelength interval can be changed from ~4 to 27.36 nm with an OSNR higher than 42 dB, and the maximal lasing range of the dual-wavelength is 23.38 nm. In addition, the wavelength drift and power fluctuation of the proposed fiber laser are less than 0.14 nm and 1.976 dB, respectively, at room temperature, which demonstrates that the thulium-doped fiber laser has favorable wavelength and power stability.

Wide-Range Wavelength-Tunable Mode-Locked Fiber Laser Based on Fiber Bragg Grating

We demonstrated a wide-range wavelength-tunable passively mode-locked fiber laser based on the fiber Bragg grating (FBG) fabricated by femtosecond laser as the wavelength selection element. A ring fiber cavity with the nonlinear-optical loop mirror as its saturable absorber was employed for mode-locking operation. The peak wavelength of the fiber laser was 1547.9 nm with a spectral full-width-at-half-maximum of 0.90 nm at room temperature. The pulse duration of the fiber laser was about 2.94 ps closed to the Fourier-limited pulse duration of 2.80 ps by assuming sech <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> pulse shape. The repetition rate was 860.5 kHz and the pulse peak power was about 56.2 W. The wavelength tuning range of the fiber laser achieved 14.2 nm by varying the FBG temperature from 25 °C to 1000 °C.

A Wavelength-Selective Multiwavelength Ring-Assisted Mach–Zehnder Interferometer Switch

Wavelength-selective switches have been proposed for datacenter use to enhance the scalability of their networks to help in meeting ever-increasing traffic demands. We have previously demonstrated a 4 × 4 ring-based crossbar silicon photonic switch in which each cross-point contained three ring pairs to partition the free spectral range (FSR) into three equal regions to reduce wavelength tuning range per ring pair—thereby reducing both the tuning power consumption and stress on the rings—while maintaining full routing flexibility. However, the question of scalability remains for such a crossbar switch in which 96 signal pads—one routed to each ring—are required to fully control it. In this article we present a 4-port silicon photonic ring-assisted Mach–Zehnder interferometer (RAMZI) switch with multiple-sized rings per switching elements in a Benes network configuration to reduce the number of electrical pads required to 36 signal pads for the equivalent number of wavelength-selective switch elements per switch cell. The switch is 500 μm × 3 mm in size and is packaged on a custom PCB. Another advantage the RAMZI switch has over the crossbar switch is that the loss through the switch is not path-dependent due to its balanced path configuration. In the crossbar switch, the difference between the shortest and longest paths is 2(N − 1) switching elements. Finally, we present results from the fabricated switch and the outlook for further scaling of the switch architecture.

Continuous wavelength tuning of nondegenerate femtosecond doubly resonant optical parametric oscillators

We report an approach for continuous tuning of the central wavelengths of a femtosecond nondegenerate doubly resonant optical parametric oscillator (DRO). Key in this scheme is the insertion of a wavelength-selective element into the DRO cavity-length-locking system, which allows selected narrow-band emission from a DRO to be detected and enables the DRO cavity length to be locked away from the peak of its cavity resonances. In a preliminary experiment, we demonstrated a femtosecond nondegenerate DRO with its central-wavelength continuously-tunable over each of its cavity resonances and achieved a wavelength-tuning range of 1910-2070 nm for the signal wave and 2140-2340 nm for the idler wave. This is, to the best of our knowledge, the first demonstration of the continuous-wavelength-tuning capability of DRO systems.

EDF ring laser using cascaded-chirped long period fiber grating for temperature measurement.

A novel tunable erbium-doped fiber ring laser (EDFRL) using a cascaded-chirped long period fiber grating (C-CLPG) as a wavelength selection element is proposed. The oscillation wavelength is determined by the one of the spectral peaks of the C-CLPG used, and the oscillation output provides a high signal to noise (S/N) ratio detection and a highly sensitive measurement of the temperature due to its high power and narrow spectral output. In the experiment, it is confirmed that the wavelength of the output shifts in accordance with the temperature-induced spectral shift of the C-CLPG transmittance spectrum and the temperature sensitivity is obtained to be ∼-0.2 nm/°C within the wavelength range of 1567.30 ∼ 1575.78 nm. The oscillation wavelength range is to be limited depending on the fringe spacing of the channeled spectrum of C-CLPG, which limits the temperature measurement range, but a data processing approach to solve this problem is additionally proposed and its availability is also presented.

Widely tunable erbium doped fiber ring laser based on loop and double-pass EDFA design

This study presents two novel tunable erbium doped fiber ring laser (EDFRL) configurations based on loop EDFA design and double-pass EDFA design, respectively to achieve an ultra-wideband tuning range and stable single-longitudinal-mode lasing operation. A micro-electro-mechanical system (MEMS) based optical tunable band pass filter was utilized as the wavelength selection element. Output spectral and power characteristics of these EDFRLs were experimentally analyzed and discussed. The EDFRLs had moderate and stable output power level (~-3 dBm) over 70 nm tuning range that covers C+L band. Moreover, the proposed EDFRLs could also suppress the undesired multi-longitudinal-modes successfully and have a high (~60 dB) OSNR level in whole tuning range.

Spectroscopic gas detection using a Bragg grating - stabilized external cavity laser, custom written in planar integrated silica-on-silicon.

We present the development of an external cavity Bragg grating stabilized laser for tunable diode laser spectroscopy (TDLS). Our design uses a planar integrated silica-on-silicon platform incorporating a custom written Bragg grating as the wavelength-selective element of the laser cavity. We have developed a prototype singlemode laser at 1651 nm and performed a detailed characterization of its performance for the purpose of spectroscopic measurement of methane at this wavelength using a 25 cm path-length single-pass cell. Mode hop-free tuning of 0.13 nm has been demonstrated at frequencies of up to 10 kHz. A single-point limit of detection for TDLS of ΔI/I0 = 8.3 × 10-5 AU was achieved, which is consistent with the performance of standard distributed feedback lasers. The new device exhibits a side-mode suppression ratio of -40 dB and a low RIN of <-150 dB/Hz, and thus avoids the high levels of noise or instability normally associated with larger, mechanically driven external cavity lasers. The silica-on-silicon platform has the potential for low-volume manufacturing of special lasers at the custom wavelengths required for gas detection, without the need for investment in foundry solutions.

L-Band Tunable and Dual-Wavelength Mode-Locked Fiber Laser With NCF-GIMF-Based SA

A hybrid structure of no-core fiber and graded-index multimode fiber (NCF-GIMF) is demonstrated, which can be used as both the saturable absorber (SA) and the wavelength selective element. By employing such a device, an L-band wavelength tunable Er-doped mode-locked laser is realized and stable single solitons of ~1.7 ps, with continuously tunable wavelengths from 1574 to 1601.1 nm, are generated. Moreover, dual-wavelength mode-locked laser operation at 1575.4 and 1603 nm is achieved, with a pulse duration of 1.2 and 1.7 ps, respectively. The proposed device is simple and compact and represents an effective approach for tunable and dual-wavelength mode-locked fiber laser operation.

Tunable high-average-power optical parametric oscillators near 2 μm

We report on the development of high-average-power nanosecond and picosecond laser sources tunable near 2 μm based on optical parametric oscillators (OPOs) pumped by solid-state Nd:YAG and Yb-fiber lasers at 1.064 μm. By exploiting 50-mm-long MgO-doped lithium niobate (MgO:PPLN) as the nonlinear crystal and operating the OPO in a near-degenerate doubly resonant configuration with intracavity wavelength selection elements, we have generated tunable high-average-power radiation across 1880–2451 nm in high spectral and spatial beam quality with excellent output stability. In nanosecond operation, pumping with a Q-switched Nd:YAG laser and using an intracavity prism for spectral control, we have generated more than 2 W of average power in pulses of 10 ns duration at 80 kHz repetition rate with narrow linewidth (<3 nm), with M2<2.8, and a passive power stability better than 2.2% rms over 1 h. In picosecond operation, pumping with a mode-locked Yb-fiber laser and using a diffraction grating as the wavelength selection element, we have generated more than 5 W of average power in pulses of 20 ps at 80 MHz repetition rate with narrow bandwidth (∼2.5 nm), with M2<1.8 and a passive power stability better than 1.3% rms over 2 h. The demonstrated sources represent viable alternatives to Tm3+/Ho3+-doped solid-state and fiber lasers for the generation of high-power radiation in the ∼2 μm spectral range.

Wavelength stability in a hybrid photonic crystal laser through controlled nonlinear absorptive heating in the reflector

The need for miniaturized, fully integrated semiconductor lasers has stimulated significant research efforts into realizing unconventional configurations that can meet the performance requirements of a large spectrum of applications, ranging from communication systems to sensing. We demonstrate a hybrid, silicon photonics-compatible photonic crystal (PhC) laser architecture that can be used to implement cost-effective, high-capacity light sources, with high side-mode suppression ratio and milliwatt output output powers. The emitted wavelength is set and controlled by a silicon PhC cavity-based reflective filter with the gain provided by a III–V-based reflective semiconductor optical amplifier (RSOA). The high power density in the laser cavity results in a significant enhancement of the nonlinear absorption in silicon in the high Q-factor PhC resonator. The heat generated in this manner creates a tuning effect in the wavelength-selective element, which can be used to offset external temperature fluctuations without the use of active cooling. Our approach is fully compatible with existing fabrication and integration technologies, providing a practical route to integrated lasing in wavelength-sensitive schemes.

Ring Fiber Lasers Based on EIT-Like Fano Resonances as a Wavelength-Selective Element

Based on strong over-coupling between a thin fiber taper and a microcylinder, electromagnetically induced transparency (EIT)-like Fano resonances resulting from a destructive interference between the microcylinder resonances and the background continuum transmission in the fiber are featured as a narrowband optical filter. The quality factor ( $Q$ factor) and the extinction ratio can be up to $2\times 10^{5}$ and 10 dB, respectively. When such an optical filter is applied in an erbium (Er)-doped fiber laser, a stable continuous-wave laser output can be generated with a 27.9-mW pump threshold and a ~20-MHz linewidth. In addition, by adjusting the polarization of the laser cavity, we demonstrate the tuning of the output wavelength of the ring laser.