Μm Waveband Research Articles

Tunable Broadband Mode Converter Based on Long-Period Fiber Gratings at 2-μm Waveband

We demonstrate the fabrication of a broadband mode converter (MC) operating at 2 μm waveband based on long-period fiber gratings (LPFGs) written in the two-mode fiber (TMF) by CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser. The proposed MC offers a 10-dB bandwidth of 58 nm, making it suitable for high-speed broad-band optical communication system at 2 μm waveband. The maximum conversion efficiency reaches 98.89% at wavelength of 2 μm. The torsion, strain, bending, and polarization characteristics of the broadband MC were measured experimentally. The torsion sensitivity of the LPFGs was measured to be 1.32 nm/(rad/m) at 2 μm waveband, which was almost three times higher than that of the grating with the same order mode coupling at 1.55 μm waveband. The proposed broadband MC can function as not only a promotion in fiber communication system but also a high sensitivity torsion sensor in fiber sensing system.

Compact dual-wavelength vertical-external-cavity surface-emitting laser with simple elements.

Dual-wavelength lasers with separation from several nanometers to tens of nanometers at 1 µm waveband are attractive light sources for terahertz generation. This work reports a compact dual-wavelength vertical-external-cavity surface-emitting laser with simple elements. The gain chip is regularly designed and epitaxially grown, and the laser cavity is the most common straight line cavity. By the use of a blade as the tuning element in cavity, the laser wavelength can be tuned continuously, and the stable dual-wavelength oscillation can be obtained when the blade is situated at a certain position. The total output power of 85 mW, which is limited by our pump source, is produced when the laser wavelengths are at 961 nm and 970 nm. We have also analyzed the evolution mechanism and the stability of this dual-wavelength laser.

Infrared radiation and thermal properties of Al-doped SrZrO3 perovskites for potential infrared stealth coating materials in the high-temperature environment

The Al-doped SrZrO3 perovskite powder with low infrared emissivity at high temperatures was prepared. The infrared radiation performance and thermophysical properties of the perovskites at high temperatures were discussed. As a result, the infrared emissivity of the Al-doped SrZrO3 perovskite powder is associated with Al3+-doping content, phase composition and particle morphology. The flaky particles SrZr0.85Al0.15O2.925 formed by heat treatment at 1000 °C for 6 h have the lowest infrared emissivity of 0.245 in 3–5 μm wavebands at 590 °C. The perovskite powder's infrared emissivity is positively correlated with its electrical resistivity and has no apparent change after heating over 800 °C for long-term. The SrZr0.85Al0.15O2.925 perovskite ceramic formed by pressureless sintering still maintains ideal heat insulation performance with the thermal conductivity from 1.17 to 2.21 W m−1 K−1 below 1400 °C. The Al-doped SrZrO3 perovskite tablet exhibits significant weak radiation intensity due to its characteristics of both low infrared emissivity and thermal conductivity at high temperatures.

Ultrafast thulium-doped fiber laser mode-locked by antimonides.

We report, for the first time, the nonlinear absorption at the 2 µm waveband of three Sb-related materials including two Sb compounds, GaSb and InSb, and one Sb alloy, Ge8Sb92. These saturable absorbers (SAs) were coated on tapered single mode fibers by the magnetron-sputtering deposition method. By incorporating these SAs into Tm-doped fiber lasers, ultrafast mode-locked solitons could be readily obtained. Stable pulse trains with 922 fs/753 fs/1005 fs pulse durations, 31.35 mW/37.70 mW/16.60 mW output powers, 93 dB/80 dB/92 dB signal-to-noise ratios were achieved with GaSb/InSb/Ge8Sb92, respectively. Our findings demonstrate that these materials can be widely used for photonic devices in the 2 µm waveband where ultrafast optical switching and modulating are desired.

Fiber laser refractometer operating in the 2 μm spectral region based on the MMI effect

We experimentally demonstrate a fiber laser refractometer based on the use of a multimode interference (MMI) fiber structure. The MMI filter is constructed with an uncladded fiber segment which acts as sensing element to determine the refractive index (RI) of liquid solutions. The laser emission, generated at the 2 μm waveband, is wavelength shifted in linear proportion to the RI of the liquid. The proposed fiber laser refractometer exhibits high sensibility of 937.81 nm/RIU.

The effect of Al3+ doping on the infrared radiation and thermophysical properties of SrZrO3 perovskites as potential low thermal infrared material

A series of Al3+-doped SrZrO3 perovskite powders with low infrared emissivity were prepared. The doping-effect of Al3+ on infrared radiation ability and thermophysical properties was investigated in detail by theoretical and experimental methods. As a result, Al3+-doped SrZrO3 perovskite's infrared emissivity is related to its electronic structure and particle morphology. It can be adjusted by the Al3+-doped concentration. The SrZr0.86Al0.14O2.935 powder with stronger electronic conductivity and flake particles has the lowest infrared emissivity with 0.256 in 3-5 μm waveband at 590 °C and 0.383 in 8-14 μm waveband at 550 °C. Al3+-doped SrZrO3 perovskites still maintain ideal thermal stability and low thermal conductivity above 800 °C. Base on both low infrared emissivity and thermal conductivity, Al3+-doped SrZrO3 tablets exhibited weaker thermal radiation intensity in the high-temperature condition and might be the potential low thermal infrared materials servicing under extreme temperature in the future.

1.1 μm waveband tunable laser using emission-wavelength-controlled InAs quantum dots for swept-source optical coherence tomography applications

In this study, an optical gain chip using emission-wavelength-controlled self-assembled InAs quantum dots (QDs) was developed for swept-source optical coherence tomography (SS-OCT) applications. The optical characterizations indicated that the QDs emission wavelength and optical gain spectra were controlled in the 1.1μm waveband by optimizing the QDs growth conditions. This waveband is useful for obtaining a large imaging depth of OCT because of an optimal balance between absorption and scattering in biological samples. In addition, continuous tunable lasing in the waveband was achieved by introducing the QD-based gain chip into a grating-coupled external cavity. This tunable laser was introduced into an SS-OCT setup, and the point spread function (PSF) was evaluated. The PSF position was observed to vary according to the optical path length differences. These results demonstrate the feasibility of the application of emission-wavelength-controlled QDs for SS-OCT.

High power tunable multiwavelength random fiber laser at 1.3 μm waveband.

Multiwavelength fiber lasers, especially those operating at optical communication wavebands such as 1.3 μm and 1.5 μm wavebands, have huge demands in wavelength division multiplexing communications. In the past decade, multiwavelength fiber lasers operating at 1.5 μm waveband have been widely reported. Nevertheless, 1.3 μm waveband multiwavelength fiber laser is rarely studied due to the lack of proper gain mechanism. Random fiber laser (RFL), owing to its good temporal stability and flexible wavelength tunability, is a great candidate for multiwavelength generation. Here, we reported high power multiwavelength generation at 1.3 μm waveband in RFL for the first time. At first, we employed a section of 10 km G655C fiber to provide Raman gains, as a result of which, 1.07 W multiwavelength generation at 1.3 μm waveband with an optical to signal noise ratio of ∼33 dB is demonstrated. By tuning the pump wavelength from 1055 nm to 1070 nm, tunable multiwavelength output covering the range of 1300-1330 nm can be achieved. Furtherly, we realized 4.67 W multiwavelength generation at 1.3 μm waveband by shortening the fiber length to 4 km. To the best of our knowledge, this is the highest output power ever reported for multiwavelength fiber lasers.

Toward generation of mid-infrared orbital angular momentum beams by tailoring four-wave mixing in chalcogenide photonic crystal fiber

The nonlinear process of orbital angular momentum (OAM) beams in multimode fiber has attracted renewed interest due to its enhanced intermodal nonlinear interactions. Here, we designed and simulated a novel photonic crystal fiber (PCF) based on A s 2 S e 3 glass that supports stable OAM modes. By optimizing the structure parameters of the fiber, both the effective index separation and the phase-matching condition of OAM modes were satisfied in the fiber. Mid-infrared OAM light can be generated by intermodal four-wave mixing in the chalcogenide fiber. The idler conversion efficiency at the 5 µm waveband and the signal optical gain spectrum near 1.9 µm is calculated when the pump light is around 2.8 µm. Furthermore, the crosstalk of the FWM process from the other vector modes vanished due to the lower overlap factor compared with the interacting OAM modes. Our work demonstrates that the vortex beam at 5 µm could be generated, and its conversion efficiency can reach 21.96% with appropriate optimization.

Passive devices at 2 µm wavelength on 200 mm CMOS-compatible silicon photonics platform [Invited

As a promising spectral window for optical communication and sensing, it is of great significance to realize on-chip devices at the 2 µm waveband. The development of the 2 µm silicon photonic platform mainly depends on the performance of passive devices. In this work, the passive devices were fabricated in the silicon photonic multi-project wafer process. The designed micro-ring resonator with a 0.6 µm wide silicon ridge waveguide based on a 220 nm silicon-on-insulator platform achieves a high intrinsic quality factor of 3.0×105. The propagation loss is calculated as 1.62 dB/cm. In addition, the waveguide crossing, multimode interferometer, and Mach–Zehnder interferometer were demonstrated at 2 µm with good performances.

Design of airborne dual-band common aperture photoelectric aiming optical system

Aiming at the problem of central obstruction and assembly difficulties in the reflective imaging of the airborne dual-band common-aperture photoelectric sighting system, a common-aperture optical system with front-light path refraction imaging was designed. The initial structure of the optical system was calculated using the two-component zoom theory. Based on the Minimum Resolvable Temperature Difference (MRTD) model, the operating distance of infrared system was analyzed, and the compensation residual error of forward image motion was analyzed according to Rayleigh criterion. When the airborne dual-band common-aperture photoelectric aiming optical system worked in the waveband of 0.38-0.76 μm, it can achieve 5× continuous zoom from 36 to 180 mm. When it worked in the 3-5 μm waveband, it can realize three fields of view transformations, and the ratio of the three focal lengths of the three fields is 3. The working F number was 4. The design results show that, in the working environment of −40-60 ℃, the optical system undergoes optical passive athermalization treatment, which meets the requirements of system imaging quality.

Silicon subwavelength grating-assisted asymmetric directional coupler around 2 μm and its applications

We propose a silicon-based asymmetric directional coupler assisted by subwavelength grating anisotropic metamaterial operating at the emerging 2 μm waveband. The effective medium theory is utilized to explain the principle of the proposed structure. Three devices with mode-order conversions are presented. Firstly, TE0-to-TE1, TE0-to-TE2, and TE0-to-TE3 conversions with remarkably high transmittance (>−0.17 dB) and extremely high mode purity (>98.81%) are achieved at the central wavelength of 1985 nm. The fabrication tolerance analyses demonstrate reasonable robustness of the proposed structure. Secondly, such structure can realize a reciprocal optical diode with an exceedingly high contrast ratio (>34 dB) based on spatial symmetry breaking. Finally, we present a symmetric 3 dB mode converter and splitter just by adjusting the length of subwavelength grating and the gap of asymmetric directional coupler. The results reveal that the transmittances of both output ports approach −3 dB around 2 μm and the transmittance difference remains below −10 dB above 40 nm within the waveband from 1.95 μm to 2.05 μm.

Single-longitudinal-mode cascaded crystalline Raman laser at 1.7 µm.

A single-longitudinal-mode crystalline Raman laser in the 1.7 µm wave band was reported for the first time, to the best of our knowledge. The YVO4 Raman laser, which was intracavity-pumped by an actively Q-switched 1314 nm Nd:YLF laser, demonstrated the cascaded Stokes oscillation at 1715 nm. By inserting an etalon in the fundamental resonator, linewidth narrowing and power scaling of the second-Stokes laser were realized based on the spatial-hole-burning-free Raman gain. With an optimal pulse repetition frequency of 4 kHz, the maximum single-longitudinal-mode average output power of 1.8 W was acquired with the spectrum linewidth of ∼340MHz. Further increasing the incident pump power, the second-Stokes laser transitioned to multimode regime, and the maximum average output power reached 2.7 W with the peak power as high as ∼380kW.

Adjustable color and emissivity based on amorphous arrays composed of SiO2@ZnO

SiO2@ZnO amorphous photonic crystals (APCs) were mainly manufactured by cladding and self-assembling. In this work, the SiO2 APCs framework are contributed to the desired structural color and the coated ZnO particles are served for reducing the infrared emissivity in 3–5 µm waveband. The effects of SiO2 particle size and ZnO adhesion amount on the visible light properties have been investigated as well as the infrared performance. The results show that with the diminution of SiO2 particles size, the packing quality of SiO2@ZnO is constantly improved since the smaller particle possesses higher electric density and surface energy. In the effects of nuclei size, SiO2@ZnO APCs with different size of building units display different color and low emissivity, and the sample with 250 nm core and 1:2 SiO2/ZnO mass ratio has blue structure and emissivity of 0.303. Meanwhile, the increasing ZnO adhesion amount can also alter the structural color due to the red shift of photonic pseudogap, and the minimum emissivity values keep dropping from 0.496 to 0.236. Finally, the SiO2@ZnO APCs with SiO2 core size of 250 nm and SiO2/ZnO mass ratio of 1:4 exhibit prominent performance with light green and low emissivity of 0.236 which contributes to the compatibility in green vegetation camouflage background and infrared stealth.

High-order mode conversion in a few-mode fiber via laser-inscribed long-period gratings at 1.55 µm and 2 µm wavebands.

We demonstrate high-order mode conversion in a few-mode fiber (FMF) via CO2 laser inscribed long-period fiber gratings (LPFGs) at both the 1.55 µm and 2 µm wavebands. At the 1.55 µm waveband, five high-order core modes (the LP11, LP21, LP02, LP31, and LP12 modes) can be coupled from the LP01 mode with high efficiency by the FMF-LPFGs. The orbital angular momentum beams with different topological charges (±1,±2,±3) are experimentally generated by adjusting the polarization controllers. At the 2 µm waveband, three high-order modes (the LP11, LP21, and LP02 mode) can be coupled by the FMF-LPFGs with different grating periods. The proposed LPFG-based mode converters could have a potential prospects in mode-division multiplexing and multiwindow broadband optical communication applications.

MXene Ti3C2Tx (T = F, O, or OH) saturable absorber for a 2 μm doubly Q-switched laser with AOM

MXene Ti3C2Tx (T = F, O, or OH) was fabricated by aqueous acid etching and liquid phase exfoliation method. Its appearance, the spectra and the saturated absorption characteristics were measured. By using MXene Ti3C2Tx as saturable absorber (SA) and an acousto-optic modulator (AOM), a doubly Q-switched Tm:LuAG laser at 2 μm waveband was realized. Under the incident pump power of 9.5 W at modulated repetition rate 1 kHz of AOM, the narrowest laser pulse width of 178 ns was obtained, corresponding to the highest pulse peak power of 1062 W. To our knowledge, this is the shortest pulse width and highest peak power obtained in the Q-switched lasers with MXene Ti3C2Tx as SA. In comparison to the singly Q-switched laser with only MXene Ti3C2Tx or AOM, the doubly Q-switched laser can generate shorter pulse width and higher peak power. The maximum compression ratio of pulse width 6.0 and the highest enhancement factor of peak power 205 were achieved. The experimental results show that MXene Ti3C2Tx is an excellent SA in 2 μm waveband laser system and the doubly Q-switched laser can compress pulse width and improve pulse peak power.

1.3 μm passively Q-switched mode-locked laser with Fe3O4 nanoparticle saturable absorber

Fe3O4 nanoparticles (FONP) are made into saturable absorbers and assembled into the resonant cavity, for the first time, and passively Q-switched mode-locked (QML) pulse output is realized in a 1.3 μm all-solid-state laser. When the pump power increases to 8 W, the repetition frequency of the Q-switched pulse increases from 47 kHz to 116 kHz, the pulse width changes from 1.137 μs to 767 ns, and the corresponding single pulse energy is 1.2 μJ. At the same time, the passively QML operation was observed on the oscilloscope, with a pulse repetition frequency of 154.5 MHz and a pulse width of 660 ps. Our experiments demonstrate that FONP exhibits good nonlinear optical characteristics in the 1.3 μm waveband, and has the potential to realize giant ultrashort and multiwavelength tunable pulse output in all-solid-state pulsed lasers.

Room temperature watt-level 3.87 µm MgO:PPLN optical parametric oscillator under pumping with a Tm:YAP laser.

A room-temperature highly efficient Tm:YAP laser pumped MgO:PPLN optical parametric oscillator operating at 3.87 µm near degeneracy is demonstrated. The pump source is an acousto-optical (AO) Q-switched Tm:YAP laser, which delivers a maximum output power of 6.17 W with a pulse duration of 45 ns and a repetition rate of 6 kHz. The temperature dependent wavelength tuning characteristics of the PPLN-OPO is investigated, and a maximum OPO output power of 1.2 W at around 3.87 µm is achieved at 35°C, corresponding to an optical-optical conversion efficiency of 19.4%. To the best of our knowledge, this is the maximum output power ever reported from 2 µm waveband laser pumped 3-5 µm MgO:PPLN OPOs.

Bandwidth optimization of cascaded long-period gratings for broadband mode conversion over 1.0-2.2 µm waveband.

We investigated theoretically and experimentally the cascaded long-period fiber gratings (c-LPFGs) in a few-mode fiber (FMF) for the generation of LP11 core mode in a broad wavelength range. The dependence of the transmission spectra of the c-LPFGs on the spacing between the gratings, and grating periods are studied in detail. The c-LPFGs experimentally generate LP11 core mode in a 10-dB bandwidth of 193.6 nm in 1.55 µm waveband and 447.5 nm in 2 µm waveband, respectively. The first-order orbital angular momentum mode can be converted by the c-LPFGs with the same broadband wavelength range. The 10-dB bandwidth and corresponding wavelength range for mode conversion can be adjusted by changing the grating spacing and grating periods.

The fabrication technology of VCSELs emitting in the 1.55 μm waveband

The paper presents the results on fabrication technique of a 1,55 μm wafer-fused vertical-cavity surface-emitting lasers (VCSELs) based on the InAlGaAsP/InP optical cavity and AlGaAs/GaAs distributed Bragg reflectors (DBRs) grown by solid-source molecular beam epitaxy. The optical cavity InAlGaAsP/InP containes an active region based on multiple InGaAs quantum wells, n++/p++-In (Al)GaAs buried tunnel junction (BTJ), InGaAsP intracavity contact layers and n-InP spreading layers. The top and bottom AlGaAs/GaAs DBRs were grown on GaAs substrate, while the optical cavity InAlGaAsP/InP was grown on InP substrate. The main fabrication proceseses of 1,55 μm VCSELs such as: BTJ fabrication, dry etching of first mesa on top DBR, wet etching of second VCSEL mesa, forming of the ohmic contacts and passivation of the VCSEL structure were described in details.