Ultra-high Numerical Aperture Research Articles

1.7-μm dissipative soliton Tm-doped fiber laser

We report on the dissipative soliton generation in a 1.7-μm net-normal dispersion Tm-doped fiber laser by nonlinear polarization rotation technique. An intra-cavity bandpass filter was employed to suppress the long-wavelength emission, while the cavity dispersion was compensated by a segment of ultrahigh numerical aperture (UHNA4) fiber. The dissipative soliton with a central wavelength of 1746 nm was obtained, covering a spectral range from 1737 nm to 1754 nm. The de-chirped duration and energy of the dissipative soliton were 370 fs and 0.2 nJ, respectively. In addition, the dynamics of multiple dissipative solitons were also investigated. Through optimization of the cavity dispersion, the 50 nm broadband dissipative soliton with de-chirped pulse duration of 230 fs could be achieved. The development of dissipative soliton seed laser represents the first step in achieving the chirped pulse amplification system at the 1.7-μm wave band, which would find potential applications in fields such as biomedical imaging and material processing.

Systematic exploration and characterization on the influence of dispersion to pulse characteristics in Tm-doped NPE mode-locked fiber oscillator

We present a systematic and experimental exploration on the influence of the second-order and third-order dispersion on pulse characteristics in an ultra-high numerical aperture (UHNA) fiber dispersion-managed, Tm-doped and nonlinear polarization evolution (NPE) mode-locked fiber laser cavity; and obtained the detailed regularity of the variation of the key parameters for mode-locked pulse, including pulse duration, pulse spectral profile, and pulse signal-to-noise ratio (SNR) with dispersion. In this process, we find that there are two interesting second-order dispersion points of −0.207 ps2 and 0.09 ps2, in which the pulse SNRs can be optimized to as high as 79 dB and 78 dB, respectively, considerably higher than that in other positions. Besides, the experimental results show that the value of third-order dispersion can not only impact the pulse spectrum but also monotonously affect the quality of pulse SNR. To the best of our knowledge, this is the first report on the detailed analysis of the effects of both the second and the third-order dispersion values on the pulse characteristics in 2-μm mode-locked fiber laser cavity and it will be an important reference to the realization of a Tm-doped mode-locked laser with high pulse contrast.

Ultrahigh numerical aperture meta-fibre for flexible optical trapping

Strong focusing on diffraction-limited spots is essential for many photonic applications and is particularly relevant for optical trapping; however, all currently used approaches fail to simultaneously provide flexible transportation of light, straightforward implementation, compatibility with waveguide circuitry, and strong focusing. Here, we demonstrate the design and 3D nanoprinting of an ultrahigh numerical aperture meta-fibre for highly flexible optical trapping. Taking into account the peculiarities of the fibre environment, we implemented an ultrathin meta-lens on the facet of a modified single-mode optical fibre via direct laser writing, leading to a diffraction-limited focal spot with a record-high numerical aperture of up to NA ≈ 0.9. The unique capabilities of this flexible, cost-effective, bio- and fibre-circuitry-compatible meta-fibre device were demonstrated by optically trapping microbeads and bacteria for the first time with only one single-mode fibre in combination with diffractive optics. Our study highlights the relevance of the unexplored but exciting field of meta-fibre optics to a multitude of fields, such as bioanalytics, quantum technology and life sciences.

Use of Dielectric Metasurfaces to Generate Deep‐Subwavelength Nondiffractive Bessel‐Like Beams with Arbitrary Trajectories and Ultralarge Deflection

AbstractFor decades, accelerating beams have attracted considerable interest in fundamental physics and in various emerging applications. However, not only are conventional accelerating beam generators (such as spatial light modulators) bulky and diffraction‐inefficient, they also have a poorly resolved ability of phase manipulation that limits the accelerating beam's minimum size and maximum degree of curvature. In this study, a dielectric metasurface is used to generate highly focused nondiffractive Bessel‐like accelerating beams with predefined arbitrary trajectories within a broadband spectral range of 550–710 nm. In particular, a similar metasurface with a combined phase profile allows the generation of a Bessel‐like vortex beam with an ultrahigh numerical aperture of 0.79, resulting in a subwavelength beamwidth of 234 nm (≈0.43λ) down to the diffraction limit. In addition, various accelerating Bessel‐like beams and beam arrays with different predefined trajectories and characteristic beam parameters that are not otherwise achievable with conventional spatial light modulators are demonstrated with synthetic‐phase metasurfaces. This study promotes the diversity of Bessel‐like accelerating beams for practical applications in fields such as optical manipulation, optical storage, biomedical imaging, and material processing.

High-efficient coupler for thin-film lithium niobate waveguide devices.

Lithium niobate (LN) devices have been widely used in optical communication and nonlinear optics due to its attractive optical properties. The emergence of the thin-film lithium niobate on insulator (LNOI) improves performances of LN-based devices greatly. However, a high-efficient fiber-chip optical coupler is still necessary for the LNOI-based devices for practical applications. In this paper, we demonstrate a highly efficient and polarization-independent edge coupler based on LNOI. The coupler, fabricated by a standard semiconductor process, shows a low fiber-chip coupling loss of 0.54 dB/0.59 dB per facet at 1550 nm for TE/TM light, respectively, when coupled with an ultra-high numerical aperture fiber (UHNAF) of which the mode field diameter is about 3.2 μm. The coupling loss is lower than 1dB/facet for both TE and TM light in the wavelength range of 1527 nm to 1630 nm. A relatively large tolerance for optical misalignment is also proved, due to the coupler's large mode spot size up to 3.2 μm. The coupler shows a promising stability in high optical power and temperature variation.

A highly birefringent bend-insensitive porous core PCF for endlessly single-mode operation in THz regime: an analysis with core porosity

We present an endlessly single-mode, bend-insensitive and highly birefringent photonic crystal fibre (PCF) for efficient wave transmission in the terahertz (THz) spectrum. To facilitate flat dispersion and high birefringence, a diamond-shaped porous core is introduced inside the Zeonex (background material)-based hexagonal porous cladding. Moreover, the dense geometrical arrangement of the regular hexagonal lattice in the cladding region holds maximum usable optical power inside the porous core. The major geometrical parameter of the proposed PCF such as core porosity has been optimized to improve the optical guiding parameters. The numerically investigated outcomes have shown an ultrahigh birefringence and numerical aperture of 0.0888 and 0.57, respectively. Apart from this, very low bending and material losses of $$3.01 \times 10^{ - 20}$$ cm−1 and 0.01451 cm−1 with flattened dispersion variation of ± 0.2463 ps/THz/cm have been achieved over the broad THz band (i.e. 0.8–1.3 THz). Additionally, our proposed PCF sustains endlessly single-mode operation with a very high core power fraction of 75.28%. With all these promising results, the proposed PCF would be a potential candidate for polarization preserving as well as for efficient broadband transmission applications in the THz regime.

A W-Type Double-Cladding IR Fiber With Ultra-High Numerical Aperture

We have prepared a novel fiber consisting of three different glasses with W-type double-cladding structure and constant core-to-cladding ratio by the extrusion method. An ultra-high numerical aperture (NA ≥ 2.17) indicates that this fiber has a good capability of collecting and confining light in a wide wavelength range. The lowest fiber loss is approximately 1.7 dB/m at 6.1 μm. Supercontinuum spectrum covering 1.6 μm to 10.85 μm at a bandwidth of -30 dB has been demonstrated in a 21 cm long fiber pumped by 5 μm laser. This chalcogenide fiber has high-NA and W-type double-cladding structure, and thus shows great potentials in the field of high power laser delivery, high efficiency imaging and high-capacity signal transmission in the mid-infrared.

Power threshold reduction and laser efficiency improvement of Brillouin fiber laser based on an As[formula omitted]S[formula omitted] Chalcogenide fiber via a mode field adaptor

A Brillouin fiber laser with low power threshold and high laser efficiency based on a 4.2-m-long As2S3 chalcogenide step-index fiber is presented in this work. Due to the free-space system between the As2S3 and single mode fiber (SMF), a high coupling efficiency is required to reduce the ring cavity loss and laser threshold, and hence improve the laser conversion efficiency. Compared with the lensed fiber and ultrahigh numerical aperture (UHNA) fiber which are commonly used as butt-coupling devices, the mode field adaptor (MFA) employed in this work can better match the mode field diameter and numerical aperture of the As2S3 and SMF, exhibiting a total coupling loss of only 1 dB. A low Brillouin laser threshold of 14 mW is achieved benefiting from the high cavity feedback of 16%. A Stokes power of ∼50 mW is generated for an injected pump power of 120 mW, showing a high laser efficiency of 42%. Furthermore, the Brillouin laser with the MFA butt-coupling exhibits lower relative intensity noise and phase noise and hence narrower laser linewidth than that with the lensed fiber and UHNA fiber butt-coupling. These results suggest the significance of the MFA which can reduce the cavity loss for a Brillouin laser with good performance based on As2S3 chalcogenide fiber.

Recent Progress on Ultrathin Metalenses for Flat Optics.

SummaryAs technology advances, electrical devices such as smartphones have become more and more compact, leading to a demand for the continuous miniaturization of optical components. Metalenses, ultrathin flat optical elements composed of metasurfaces consisting of arrays of subwavelength optical antennas, provide a method of meeting those requirements. Moreover, metalenses have many other distinctive advantages including aberration correction, active tunability, and semi-transparency, compared to their conventional refractive and diffractive counterparts. Therefore, over the last decade, great effort has been focused on developing metalenses to investigate and broaden the capabilities of metalenses for integration into future applications. Here, we discuss recent progress on metalenses including their basic design principles and notable characteristics such as aberration correction, tunability, and multifunctionality.

On-chip metalenses based on one-dimensional gradient trench in the broadband visible.

Metasurfaces are composed of flat, ultrathin subwavelength nanoantennas with strong capability in manipulating light propagation by modulations on its phase, amplitude, and polarization. For instance, the invention of two-dimensional (2D) metalenses has enabled light focusing and imaging in three-dimensional (3D) free space with miniaturized thickness and device size at a planar surface. However, such inherent form of 2D arrays and focusing functionality at 3D optical free-space limits the degree of freedom for light propagation and manipulation along a 2D planar surface and eventually the possibility of on-chip photonic system integration. Here, we theoretically study and demonstrate a new type of planar on-chip metalens, which enables light focusing and strong localization at a 2D surface. The planar on-chip architecture design is based on the one-dimensional (1D) length or width gradient trench metalens (GTM), which could yield the elaborately engineered phase shift for propagating light within the on-chip waveguide at the visible wavelength of 500 nm. By generating 1D phase arrangement at the nanoscale, a miniature on-chip metalens with ∼3×0.5µm dimension could achieve light focusing on a 2D waveguide surface with the flexibility to design scalable focal lengths and ultra-high numerical aperture of up to ∼0.99. Additionally, GTM metalens designs could also exhibit overlapped high depth-of-focus, which consequently could behave as achromatic-like lensing at the selected focal plane. Furthermore, we manifest that the focusing functionality can also be subject to dynamically tuning and switching on-and-off with TE/TM polarization change or waveguide index alteration. We believe this new form of on-chip 1D metalens holds potential applications including on-chip light manipulation functionality of focusing and diverging, optical on-chip sensing, next-generation on-chip optical communication, signal processing as well as imaging devices, etc.

Design of Compact and Efficient Silicon Photonic Micro Antennas With Perfectly Vertical Emission

Compact and efficient optical antennas are fundamental components for many applications, including high-density fiber-chip coupling and optical phased arrays. Here we present the design of grating-based micro-antennas with perfectly vertical emission in the 300-nm silicon-on-insulator platform. We leverage a methodology combining adjoint optimization and machine learning dimensionality reduction to efficiently map the multiparameter design space of the antennas, analyse a large number of relevant performance metrics, carry out the required multi-objective optimization, and discover high performance designs. Using a one-step apodized grating we achieve a vertical upward diffraction efficiency of almost 92% with a 3.6 {\mu}m-long antenna. When coupled with an ultra-high numerical aperture fiber, the antenna exhibits a coupling efficiency of more than 81% (-0.9 dB) and a 1-dB bandwidth of almost 158 nm. The reflection generated by the perfectly vertical antenna is smaller than -20 dB on a 200-nm bandwidth centered at {\lambda} = 1550 nm.

Linearity-Enhanced Refractometric Sensor Utilizing Ultra-High Numerical Aperture Fiber Combined With a Plastic Prism

Compact optical sensors are deemed to be of significant importance in industrial applications owing to their high performance and cost-effectiveness. In this study, a linearity-enhanced refractometric sensor was developed by employing an ultra-high numerical aperture (UHNA) fiber tethered to a plastic prism. The proposed refractometer can appropriately alter the intensity profile of incident light over the sensing area associated with the prism by adopting UHNA fibers with different mode profiles as the lead-in fiber. This provides an adjustable sensing regime that yields a linear response with a coefficient of determination (R 2 ) over 0.99. The fabricated sensor achieved a sensitivity of 13.6 per refractive index unit (RIU) and a high resolution of 5.80 × 10 -5 RIU for refractive indices ranging from 1.3166 to 1.3826 at λ = 1550 nm. Further, the proposed sensor was successfully applied to inspect the refractive index of a sample of coolant used in metalworking equipment, indicating that the proposed refractometric sensor can facilitate real-time monitoring of industrial production processes.

Switchable generation of a sub-200 fs dissipative soliton and a noise-like pulse in a normal-dispersion Tm-doped mode-locked fiber laser.

We report on the switchable generation of a dissipative soliton (DS) pulse and a noise-like pulse (NLP) in an all-fiberized Tm-doped fiber laser in the normal-dispersion region. Mode-locking operation is achieved through a nonlinear polarization rotation component, and the cavity dispersion is compensated using ultra-high numerical aperture (UHNA4) fiber that is easy to integrate and low in cost. At a pump threshold of 510 mW, DS operation can first be achieved without additional filter. The 3 dB spectrum bandwidth of the DS pulse is greater than 50 nm, and the duration of the de-chirped pulse is 193 fs. By increasing the pump power to 880 mW, the mode-locking state can evolve into NLP operation with proper cavity polarization state. The 3 dB spectrum bandwidth and duration of de-chirped coherence spike are 105.6 nm and 121 fs, respectively. Meanwhile, ultra-broadband NLP (over 150 nm considering 3 dB spectrum width) can also be observed with the appropriate cavity parameters. All the proposed pulse patterns present good capacity for achieving narrow pulse width and withstanding high pulse energy.

Polymer Microlens on Pillar Grown From Single-Mode Fiber Core for Silicon Photonics

A novel coupling device composed of a combination of microlens and pillar having an approximately $8~\mu \text{m}$ diameter and $20~\mu \text{m}$ height and grown from single-mode fiber (SMF) core end is proposed. This device was successfully fabricated by using a self-written waveguide and dipping method with UV-curable resin. Theoretical and experimental studies were done. A high coupling efficiency of more than −0.97dB was obtained for coupling to an ultrahigh numerical aperture (0.41) SMF with a spot diameter of $3.7~\mu \text{m}$ at a $1.55~\mu \text{m}$ wavelength in place of a silicon chip with an integrated spot-size converter. In contrast, it is only −4.0dB by a conventional SMF.

Tapered ultra-high numerical aperture optical fiber tip for nitrogen vacancy ensembles based endoscope in a fluidic environment

Fixing a diamond containing a high density of Nitrogen-Vacancy (NV) center ensembles on the apex of a multimode optical fiber (MMF) extends the applications of NV-based endoscope sensors. Replacing the normal MMF with a tapered MMF (MMF-taper) has enhanced the fluorescence (FL) collection efficiency from the diamond and achieved a high spatial resolution NV-based endoscope. The MMF-taper's high FL collection efficiency is the direct result of multiple internal reflections in the tapered region caused by silica, which has a higher refractive index (RI) than the surrounding air. However, for applications involving fluidic environments whose RI is close to or higher than that of the silica, the MMF-taper loses its FL collection significantly. Here, to overcome this challenge and achieve a high spatial resolution NV-based endoscope in a fluidic environment, we conceptually proposed a tapered ultra-high numerical aperture microstructured optical fiber (MOF) whose air capillaries at the tapered end are sealed. Since the end-sealed air capillaries along the tapered MOF (MOF-taper) have isolated the MOF core from the surrounding medium, the core retains its high FL collection and NV excitation efficiency in liquids regardless of their RI values. Replacing the MMF-taper with the MOF-taper will achieve a versatile NV-based endoscope that could potentially find widespread applications in fluidic environments where many biological processes and chemical reactions occur.

Narrow bandwidth optimization using a polymer microring resonator in a thulium–holmium fiber laser cavity

A thulium–holmium fiber (THDF) laser cavity has been demonstrated with a SU-8 polymer microring resonator (PMRR) in the cavity. The PMRR has a 500μm radius and fabricated using the lithography method, with the SU-8 polymer acting as a host material. The butt coupling method was used for the horizontal coupling of light from a ultra-high numerical aperture (UHNA) fiber to the polymer waveguide. Lasing in the cavity without the PMRR is obtained at a center wavelength of 1.910μm, 1.869μm when the PMMR is inserted into the cavity. A maximum power of -5 dBm was extracted from the laser oscillator, and the laser linewidth was measured to be ∼26.6 kHz by radio-frequency spectrum analyzer analysis. The PMRR was able to generate an output with a free spectral range of 0.79μm at a frequency of 59.25 GHz.

Dissipative soliton resonance in a simple linear cavity Tm3+-doped double clad fiber laser with dispersion management

We report on the investigation of large energy dissipative soliton resonance (DSR) pulse generation in a simple linear cavity Tm3+-doped fiber laser (TDFL), by using semiconductor saturable absorption mirror as saturable absorber. This DSR TDFL is implemented by utilizing ultra-high numerical aperture 4 (UHNA4) fiber for dispersion compensation. With further dispersion management, the DSR TDFL can generate an average output power of 242 mW with corresponding pulse energy of ~61 nJ. Moreover, the soliton molecules mode-locked operation has also been demonstrated in this TDFL. Our results can provide an easily accessible way to develop large energy TDFLs.

Observation and optimization of 2 μm mode-locked pulses in all-fiber net anomalous dispersion laser cavity

We integrally demonstrate 2 μm mode-locked pulses performances in all-fiber net anomalous dispersion cavity. Stable mode-locking operations with the center wavelength around 1950-1980 nm can be achieved by using the nonlinear polarization rotation structure and properly designing the dispersion management component. Conventional soliton is firstly obtained with a total anomalous dispersion cavity. Due to the contribution of commercial ultra-high numerical aperture fibers, net dispersion is reduced to -0.077 ps². So that stretched pulse with 19.4 nm optical bandwidth is obtained and the de-chirped pulse-width can reach 312 fs using extra-cavity compression. Under pump power greater than 890 mW, stretched pulse can evolve into noise-like pulse with 41.3 nm bandwidth. The envelope and peak of such broadband pulse can be compressed with up to 2.2 ps and 145 fs, respectively. The single pulse energy of largely chirped stretched and noise-like pulse can reach 1.785 nJ and 1.53 nJ, respectively. Furthermore, extra-cavity compression can also contribute to a significant increase of peak power.

Multi-wavelength, nano-second actively mode-locked Yb-fiber oscillator with 100 nm wide Raman broadened spectrum

An all-normal dispersion, actively mode-locked Yb-fiber oscillator has been demonstrated which delivers multi-pulsing output with Raman broadened spectrum spanning more than 100 nm. Pulses of nano-second duration at 953.81 kHz repetition rate were obtained due to an elongated cavity consisting of 200 m long single mode fiber introducing high nonlinearity. The individual pulses of the multi-pulsing output corresponded to different wavelengths in the output spectrum and concurrently generated multiple first order Raman Stokes sidebands at close wavelength intervals. This resulted in a smoother broadband spectrum compared to usually obtained Raman broadened spectrum in normal dispersion regime which contains multiple higher order Stokes lines at large wavelength intervals when pumped by a single nano-second pulse. Analysis of the spectral power content revealed that nearly 50% of the output power contained in the Raman band around the 1120 nm region. By amplification of the generated pulses and further non-linear broadening through an ultra-high numerical aperture (UHNA) fiber, a broad spectrum having a wavelength span of nearly 260 nm with 1.5 Watt average power has been demonstrated showing application capability of the designed source.

Fabrication and characterization of multimaterial Ge25Se10Te65/As2S3 chalcogenide fiber with a high value of the numerical aperture

The design and fabrication of a novel multimaterial Ge25Se10Te65/As2S3 chalcogenide glass fiber with ultra-high numerical aperture (NA) by using the rod-in-tube method was reported. The NA of fiber of approximately 2.2 is the maximum NA of step-index chalcogenide fibers obtained experimentally. The Ge25Se10Te65 glass was selected as the core material, and its mechanical and thermal properties were investigated. The optical properties and bending strength were measured to investigate fiber performance. The low bending loss of fiber was demonstrated through a simulation and an experiment. The fundamental dispersion characteristic, effective-mode-area, and nonlinear coefficient of fiber were simulated numerically.