Μm Waveband Research Articles

An Ultra-Broadband Polarization Beam Splitter Based on the Digital Meta-Structure at the 2 µm Waveband

The 2 μm waveband is considered to have great potential in optical communications. Driven by the demands on high-performance functional devices in this spectral band, various integrated photonic components have been demonstrated. In this work, an analog and digital topology optimization method is proposed to design an ultra-broadband polarization beam splitter at the 2 μm waveband. Within an optical bandwidth of 213 nm, the excess losses of TE and TM modes are <0.53 dB and 0.3 dB, respectively. The corresponding polarization extinction ratios are >16.5 dB and 18.1 dB. The device has a very compact footprint of only 2.52 µm × 5.4 µm. According to our best knowledge, this is a benchmark demonstration of an ultra-broadband and ultra-compact polarization beam splitter enabled by the proposed optimization method.

Hierarchical-Morphology Metal/Polymer Heterostructure for Scalable Multimodal Thermal Management.

The cooling and heating energy consumption of buildings poses a serious threat to the energy supply and increases greenhouse gas emissions, thus adversely impacting global warming and the long-term climate change trends. Here, inspired by the structure of the louver, this work demonstrates a multimodal device that integrates radiative cooling, natural lighting, and solar heating to deal with the grand challenge of building energy consumption. The blades integrate a selective radiative cooling material with a solar heating material. The selective radiative cooling material (solar reflectance ∼97%, selective emittance ∼0.82 in the 8-13 μm waveband) combines a solar reflective melt-blown polypropylene film and a solar transparent mid-infrared emitter polyethylene/silicon dioxide film. In addition, the heating material (solar absorptance ∼91%, thermal emittance ∼0.04) is zinc (Zn) film deposited with copper (Cu) nanoparticles, based on the Cu-Zn galvanic-displacement reaction. Hence, by rotating the blades, the conversion of radiative cooling, solar heating, and natural lighting functions can be realized. In the daytime, the multimodal device displays a subambient temperature of 4 °C, a superambient temperature of 2 °C, and a superambient temperature of 5 °C for the cooling mode, transmitting mode, and solar heating mode, respectively. On the basis of the energy-savings simulation, integrating these modes and dynamic converting these modes in the corresponding climate could save ∼746 GJ in the contiguous United States for one year (38% of the baseline energy consumption), which is equivalent to ∼147 tons of carbon dioxide emission reduction. Because of its excellent multimodal thermal management performance, this multimodal device will push forward the transformative change of building thermal management toward decarbonization and sustainability and being more green.

Lossy mode resonance refractometer operating in the 1.55 µm waveband based on TiOxNy thin films deposited onto no-core multimode fiber by DC magnetron sputtering

We propose and demonstrate the use of titanium oxynitrides (TiOxNy) as flexible material for lossy-mode resonance (LMR) fiber sensing applications. The LMR characteristics of TiOxNy are investigated for a fiber refractometer consisting of a multimode fiber structure, which includes a no-core fiber coated with a thin film of TiOxNy, because of transitional phases between TiN and TiO2. The material coating was achieved through the N-O exchange during the film synthesis by reactive-magnetron DC sputtering. For a suitable sputtered coating with thickness of ∼270 nm, the 2nd LMR order is generated within the 1.55 µm wavelength region, compatible with fiber optic systems. The spectral response for the 2nd and the 3rd LMR orders of the candidate fiber structure was characterized for liquid refractive index (RI) variations from a set of isopropyl alcohol (IPA)/glycerin solutions. For glycerin concentrations ranged from 0% to 35% (RI from 1.3602 to 1.4091), it was found that the 2nd LMR order is significantly more sensitive than the 3rd LMR order, whose sensitivities are of 1238.445 nm/RIU and 111.39 nm/RIU, respectively.

Connector-style hollow-core fiber interconnections.

To go beyond the fundamental limits imposed by latency, nonlinearity, and laser damage threshold in silica glass fibers, the hollow-core fiber (HCF) technique has been intensively investigated for decades. Recent breakthroughs in ultralow-loss HCF clearly imply that long-haul applications of HCF in communications and lasers are going to appear. Nevertheless, up to now, the HCF technique as a whole is still hampered by the limited length of a single span and the lack of HCF-based functional devices. To resolve these two issues, it is of importance to develop ultralow-loss and plug-and-play HCF interconnections. In this work, we report on HCF interconnections with the lowest-ever insertion losses (0.10 dB for HCF to standard single-mode fiber (SMF) and 0.13 dB for HCF to itself in the 1.5 µm waveband) and in a pluggable means. Two fiber mode-field adapters, one based on a graded-index multi-mode fiber (GIF) and the other utilizing a thermally expanded core (TEC) SMF, have been tested and compared. An extra insertion loss arising from imperfect refractive index distribution in a commercial GIF is observed. Our HCF interconnections also realize a back-reflection of <-35 dB over a 100 nm bandwidth as well as other critical metrics in favor of practical applications. Our technique is viable for any type of HCF.

Broadband and stable linearly polarized mode converter based on polarization-maintaining fiber long-period grating

We demonstrate a broadband and stable linearly polarized mode converter by inscribing long-period grating (LPG) in few-mode polarization-maintaining fibers (FPMFs). The operating wavelength of the mode converter at 1.0- and 1.5- μm waveband has been achieved by using PMF with different cut-off wavelengths. Theoretical and experimental demonstrated broadband linearly polarized mode conversion can be achieved with a maximum 10 dB bandwidth of 140- and 189- nm at 1.0- and 1.5- μm waveband respectively due to the turning point coupling mechanism. Then, we propose and demonstrate a controllable linearly polarized orbital angular momentum (LP-OAM) mode generator using a single LPG-based mode converter and a length of FPMF. The LP-OAM mode is produced by turning axial strain and can be smoothly switched between ± 1 order. Our all-FPMF based grating fabrication offers a new option towards a stable and broadband polarized mode converter and LP-OAM mode generator for application in a few-mode fiber laser and communication system.

Direct-binary-search algorithm for fabrication-tolerant photonic-crystal-like subwavelength structures and its application to a four-mode waveguide crossing in 2 μm waveband

A novel direct-binary-search algorithm for the design of fabrication-tolerant photonic-crystal-like subwavelength structures is proposed. In the proposed method, structures based on holes with different diameters are considered to enhance the device performance under diameter variation. The usefulness of the proposed method is demonstrated through the design of a fabrication-tolerant four-mode waveguide crossing in the 2 μm waveband. With the proposed method, the transmission of TE3 mode is increased by 4.9% (for diameter of 130 nm) and 9.5% (for diameter of 150 nm) on average of 15 samples, compared with the conventional direct-binary-search method. The designed device provides insertion losses of <0.87 dB and crosstalks of <−24 dB in spite of diameter variations of ±10 nm from the ideal diameter.

Passively Q-switched Tm:YAP laser with a tantalum aluminum carbide saturable absorber.

Tantalum aluminum carbide (Ta4AlC3) phase ceramic (MAX) material has attracted much attention because of its high conductivity, high strength, corrosion resistance, and good optical properties. However, there are too few reports on lasers with Ta4AlC3-based saturable absorbers (SAs). We prepared and characterized a Ta4AlC3-based SA whose nonlinear absorption performances were achieved at a 2 µm waveband range and which was used in a passively Q-switched (PQS) Tm:YAP laser. In the PQS mode, a maximum average output power of 0.78 W was achieved with the central output wavelength of 1991.86 nm from a PQS Tm:YAP laser, corresponding to a pulse duration of 926 ns at 143.8 kHz.

Silicon photonic arrayed waveguide grating with 64 channels for the 2 µm spectral range.

Driven by the demand to extend optical fiber communications wavelengths beyond the C + L band, the 2 µm wave band has proven to be a promising candidate. Extensive efforts have been directed into developing high-performance and functional photonic devices. Here we report an integrated silicon photonic arrayed waveguide grating (AWG) fabricated in a commercial foundry. The device has 64 channels with a spacing of approximately 50 GHz (0.7 nm), covering the bandwidth from 1967 nm to 2012 nm. The on-chip insertion loss of the AWG is measured to be approximately 5 dB. By implementing a TiN metal layer, the AWG spectrum can be thermally tuned with an efficiency of 0.27 GHz/mW. The device has a very compact configuration with a footprint of 2.3 mm × 2 mm. The demonstrated AWG can potentially be used for dense wavelength division multiplexing in the 2 µm spectral band.

High-speed silicon microring modulator at the 2 µm waveband with analysis and observation of optical bistability

Recently, significantly raised interests have emerged for the 2 µm waveband as an extended new window for fiber optic communication. Much research progress has been made on the photonic integrated circuits for the 2 µm waveband, especially on the CMOS-compatible silicon-on-insulator wafer. In this work, a silicon integrated microring modulator (MRM) with record high-speed performances at the 2 µm waveband was demonstrated. An L-shaped PN junction was specially designed for 2 µm to achieve a high modulation efficiency with V π L of 0.85 V · cm . The measured 3 dB bandwidth is 18 GHz, supporting up to 50 Gbps signaling at 2 µm. Additionally, optical bistability induced by the thermo-optical effect and nonlinear effects was analyzed theoretically and observed experimentally in the 2 µm MRM for the first time to our knowledge. Nonlinear coupled mode theory and the Runge–Kutta method were used to simulate the behaviors of bistability in the 2 µm MRM. The simulation and experimental results indicate that, when the MRM is launched by a high optical power, the distorted resonant spectrum under an optical bistable state deteriorates the modulation efficiency and signal performances. This work breaks the record of high-speed silicon MRM at 2 µm, drawing a promising prospect for the silicon photonic integration and high-speed interconnection at the 2 µm waveband, and it provides the referenceable analysis of optical bistability, which guides the design and experimental investigation of 2 µm MRM.

Spectral and Sensing Performance of Long-Period Fiber Gratings at 2 μm Waveband

In this paper, we demonstrate the transmission spectral and surrounding refractive index (SRI) sensing performance of long-period fiber gratings (LPFGs) at 2 &#x03BC;m waveband. The cladding modes operating at 2 &#x03BC;m waveband show higher SRI sensitivity than 1.55 &#x03BC;m waveband. The influence of a little off resonance region of grating period, refractive index modulation and duty cycle of gratings are analyzed theoretically. Additionally, the LPFGs of LP₀₈ mode near dispersion turning point operating at 2 &#x03BC;m waveband are investigated theoretically and experimentally. Mode splitting brings out with the increasing SRI. After the resonant dip splitting into two dips (dip A and dip A&#x2032;), the SRI sensitivity of dip A are measured to be as high as &#x2212;8495.2 nm&#x002F;RIU in the SRI region of 1.445&#x2013;1.457. The proposed 2-&#x03BC;m-waveband LPFGs makes it a potential in biochemical and environmental monitoring.

2 µm sub-GHz harmonic mode-locked soliton generation based on a Bi2S3 saturable absorber.

Saturable absorber (SA) based harmonic mode-locking (HML) techniques at 2 µm waveband are much less reported than those at 1.5 µm waveband, the maximum repetition rate of the harmonic pulse generated by such techniques at 2 µm waveband is also much lower than those generated at 1.5 µm waveband. In this paper, the 39th harmonic with the repetition rate of 908.6 MHz is realized in a Bi2S3-based thulium-doped fiber laser. The fundamental mode-locked pulse has a central wavelength of 1954.2 nm and a 3-dB bandwidth of 5.1 nm. The repetition rate is 23.27 MHz and the pulse width is 902 fs. The characteristics of the material and harmonic mode-locked pulse are investigated. To the best of our knowledge, this is the highest and the closest resonance frequency to GHz among the reported SA-based harmonic mode-locked fiber lasers operating at 2 µm waveband.

Hyperspectral absorption of water around 2 μm based on a boradband tunable, narrow linewidth Tm-doped fiber laser

The 1.8–2.0 μm waveband contains abundant absorption lines of water, which are much stronger than those in the traditional 1.3–1.5 μm waveband, exhibiting huge potentials for absorption spectrum applications of water. In the hyperspectral absorption spectrum, physical parameters of the target molecule can be derived from lots of absorption lines within a broadband scanning range, achieving the results more robust, accurate and versatile than the results from the conventional tunable diode laser absorption spectrum in which only one or two absorption lines are used. The key to hyperspectral absorption is the development of broadband tunable, narrow linewidth laser sources emitting in the wavelength range of interest. With a tunable fiber FP filter and a fiber saturable absorber, a Tm-doped fiber laser is established, featuring broadband tenability and narrow linewidth. Taking advantage of the re-absorption characteristics of Tm-doped silica fibers, a wavelength tuning range covering 60 nm from 1910–1970 nm is obtained through the appropriately designing of the active fiber length. The measured laser linewidth at steady state is smaller than 0.1 nm, which is suitable for water absorption spectrum. Hyperspectral absorption measurements of water in air and alcohol flame are conducted. In room-temperature air, more than 40 absorption lines are recognized within a tuning range of 1910–1965 nm, while, in alcohol flame, the number of detected lines reaches about 50. Comparison with the HITRAN2016 database gives a laser linewidth of about 0.06 nm which is very close to the static linewidth measured by an OSA. The temperature of the air is derived to be 298 K with a water mole fraction of about 2%, which is consistent with the measurement of the hygrothermograph. And the calculation indicates an alcohol flame temperature of about 1220 K, which is very close to the measurement result of the thermocouple.

High Energy Pulsed Laser in 1.6 μm Waveband Based on Deuterium Gas Stimulated Raman Scattering

High Energy Pulsed Laser in 1.6 μm Waveband Based on Deuterium Gas Stimulated Raman Scattering

Research progress in 2 μm waveband on-chip photonic integrated devices (Invited)

Research progress in 2 μm waveband on-chip photonic integrated devices (Invited)

Compact, Hybrid III-V/Silicon Vernier Laser Diode Operating From 1955–1992 nm

The 2 µm waveband is capable of enabling pervasive applications. The demonstration of the hollow-core photonic bandgap fiber and the thulium-doped fiber amplifier has highlighted the fiber propagation and amplification aspects of fiber communications, indicating its potential as an adjunct to present communication infrastructure at the O/C bands. The above is especially imperative given the current concerns with regards to the upper bandwidth limit of the single-mode fiber. Furthermore, the waveband could facilitate many more applications such as LIDAR and free-space communication. However, water absorption (OH-) is high at most of the 2 μm waveband and this will impact the optical insertion loss of applications implemented in the wavelength region. The relative low water absorption region of the waveband falls within 1950 – 2000 nm. As such, the development of a hybrid/heterogeneous III-V/silicon laser source that operates within the region is important for 2 µm silicon photonics. In this work, we demonstrate a III-V/Si hybrid tunable laser operating from 1955 - 1992 nm for the first time. Room temperature continuous wave operation is achieved with a maximum laser output power of 8.1 mW. This wavelength-tunable laser operates specifically within the low water absorption window, indicating good wavelength suitability for applications at the 2 μm waveband.

Generation of stable mode-locked fiber laser based MWCNTs in 1.5-µm waveband

Pulses fiber laser had been successfully generated by using multi-wall carbon nanotubes thin film saturable absorber at room temperature. The saturable absorber is incorporated into a ring laser cavity. A stable Kelly sidebands mode-locked pulse spectrum with 1561.3 nm wavelength at the pump power of 86.8 mW. The repetition rate and pulse width of 12.3 MHz and 0.51 picosecond, respectively. A stable operation is observed for an hour at room temperature. These simple and reliable system features offer interesting research study especially in mode-locked pulse generation at 1.5 μm waveband.

Ultra-broadband 3 dB power splitter from 1.55 to 2 µm wave band.

Extending the optical communication wavelengths to 2 µm can significantly increase data capacity. Silicon photonics, which is a proven device integration technology, has made rapid progress at 2 µm recently. As a fundamental functional element in the photonic design kit, the 3 dB power splitter has been extensively studied in both the 1.55 µm and 2 µm regime. While the device is highly desirable to operate over both wave bands, the large waveguide dispersion in silicon makes it challenging. In this work, we demonstrate an ultra-broadband power splitter on silicon, which has a 0.2 dB bandwidth exceeding 520 nm from 1500 to 2020 nm according to simulations. The beam splitter is realized by a triple tapered Y-junction, and its operational bandwidth is greatly increased by subwavelength grating structure. The device has an ultra-compact footprint of only 3µm×2µm. Due to the limitations on the setup and coupling technique, we measure the device bandwidth in 1.55 µm and 2 µm wave bands. The device insertion loss is measured to be below 0.4 dB from 1500 to 1620 nm and from 1960 to 2020 nm, respectively. According to these results, the proposed device is believed to be capable of operating over a broadband from 1.55 µm and 2 µm wavelengths.

A promising inorganic YFeO3 pigments with high near-infrared reflectance and infrared emission

The colored non-toxic near-infrared (NIR) reflective and infrared radiative pigments are desired in reducing the energy consumption of buildings. In this work, yttrium ferrite (YFeO3) pigments with controllable crystalline structure were successfully synthesized via the method of solid-state reaction at low-heating temperature. The effects of the crystalline structure and microstructure, especially charge state distribution, on the bandgap, chromatic properties, infrared reflection and radiation performance of YFeO3 pigments were investigated for the first time. With the calcining temperature increased, the structure of obtained pigments changed from hexagonal to orthorhombic, and the bandgap was obviously widened. The black hexagonal YFeO3 pigments possessed small particle size, and their intensive optical absorption in the NIR range was related to the noticeable multiple reflections induced by the grain boundary. While, the brown orthorhombic YFeO3 powdered pigments, as well as the corresponding pigmented coatings, possessed good NIR reflection characteristics, and the high solar reflectance values clearly highlight the potential of these pigments as cool pigments. In addition, the obtained YFeO3 pigmented coatings showed high emissivity in 8–14 μm waveband (atmospheric window), indicating their potential applications in energy saving of buildings by combining NIR shielding and space cooling. The larger amount of Fe2+-Fe3+ ions pairs in the orthorhombic pigments led to the enhancement of free carrier absorption, which enhanced infrared emission and contributed to the slightly decreased NIR reflectance.

Fabrication of an Anti-Reflective Microstructure on ZnS by Femtosecond Laser Bessel Beams.

As an important mid-infrared to far-infrared optical window, ZnS is extremely important to improve spectral transmission performance, especially in the military field. However, on account of the Fresnel reflection at the interface between the air and the high-strength substrate, surface optical loss occurs in the ZnS optical window. In this study, the concave antireflective sub-wavelength structures (ASS) on ZnS have been experimentally investigated to obtain high transmittance in the far-infrared spectral range from 6 μm to 10 μm. We proposed a simple method to fabricate microhole array ASS by femtosecond Bessel beam, which further increased the depth of the microholes and suppressed the thermal effects effectively, including the crack and recast layer of the microhole. The influence of different Gaussian and Bessel beam parameters on the microhole morphology were explored, and three ASS structures with different periods were prepared by the optimized Bessel parameters. Ultimately, the average transmittance of the sample with the ASS microhole array period of 2.6 μm increased by 4.1% in the 6 μm to 10 μm waveband, and the transmittance was increased by 5.7% at wavelength of 7.2 μm.

Efficient, compact, and robust GeOI-based photonic polarization rotator in the 2 [formula omitted]m waveband

Based on the germanium-on-insulator (GeOI) platform, we numerically demonstrate a photonic polarization rotator (PR) operating in the 2 μm waveband. Utilizing the partial-height etching to break the symmetry of germanium waveguide cross section, the proposed PR realizes a 90° polarization rotation between the fundamental transverse electric (TE) and magnetic (TM) modes. The device footprint occupies only 0.4 μm ×0.4μm ×3μm. This PR shows a low insertion loss of 0.096 dB, a remarkably high polarization conversion efficiency (PCE) of 98.9%, and an extremely high extinction ratio of 33.45 dB at the wavelength of 2005 nm The PCE keeps above 95.4% within the waveband between 1.9 μm and 2.1 μm. Furthermore, a reasonable robustness is demonstrated.