Ultra-high Numerical Aperture Fiber Research Articles

The Impact of 1030 nm fs-Pulsed Laser on Enhanced Rayleigh Scattering in Optical Fibers.

This article presents a comprehensive study on the impact of irradiation optical fiber cores with a femtosecond-pulsed laser, operating at a wavelength of 1030 nm, on the signal amplitude in Rayleigh scattering-based optical frequency domain reflectometry (OFDR). The experimental study involves two fibers with significantly different levels of germanium doping: the standard single-mode fiber (SMF-28) and the ultra-high numerical aperture fiber (UHNA7). The research findings reveal distinct characteristics of reflected and scattered light amplitudes as a function of pulse energy. Although different amplitude changes are observed for the examined fibers, both can yield an enhancement of amplitude. The paper further investigates the effect of fiber Bragg grating inscription on the overall amplitude of reflected light. The insights gained from this study could be beneficial for controlling the enhancement of light scattering amplitude in fibers with low or high levels of germanium doping.

Wavelength tunable dissipative soliton resonant holmium-doped fiber laser beyond [formula omitted

We propose a wavelength tunable dissipative soliton resonant (DSR) mode-locked holmium-doped fiber laser operating at wavelength exceeding 2.1μm. Mode-locking is realized by employing nonlinear amplifying loop mirror technique. Using a piece of long gain fiber to offer enough gain above 2.1μm. A section of ultra-high numerical aperture fiber (UHNAF) with normal dispersion and high nonlinearity is used to reduce mode-locking threshold and increase the intracavity nonlinearity. At the optimized gain fiber length of 9.8 m and UHNAF length of 55 m, 2109 nm DSR pulse with 3-dB spectrum width of 0.46 nm is obtained. Thanks to the birefringence-induced spectral filter effect, the central wavelength of DSR can be tuned from 2102.7 to 2120.5 nm with wavelength range of 17.8 nm.

2.1 m, high-energy dissipative soliton resonance from a holmium-doped fiber laser system

Abstract We propose a 2.1 μm high-energy dissipative soliton resonant (DSR) fiber laser system based on a mode-locked seed laser and dual-stage amplifiers. In the seed laser, the nonlinear amplifying loop mirror technique is employed to realize mode-locking. The utilization of an in-band pump scheme and long gain fiber enables effectively exciting 2.1 μm pulses. A section of ultra-high numerical aperture fiber (UHNAF) with normal dispersion and high nonlinearity and an output coupler with a large coupling ratio are used to achieve a high-energy DSR system. By optimizing the UHNAF length to 55 m, a 2103.7 nm, 88.1 nJ DSR laser with a 3-dB spectral bandwidth of 0.48 nm and a pulse width of 17.1 ns is obtained under a proper intracavity polarization state and pump power. The output power and conversion efficiency are 0.233 W and 4.57%, respectively, both an order of magnitude higher than those of previously reported holmium-doped DSR seed lasers. Thanks to the high output power and nanosecond pulse width of the seed laser, the average power of the DSR laser is linearly scaled up to 50.4 W via a dual-stage master oscillator power amplifier system. The 3-dB spectral bandwidth broadens slightly to 0.52 nm, and no distortion occurs in the amplified pulse waveform. The corresponding pulse energy reaches 19.1 μJ, which is the highest pulse energy in a holmium-doped mode-locked fiber laser system to the best of our knowledge. Such a 2.1 μm, high-energy DSR laser with relatively wide pulse width has prospective applications in mid-infrared nonlinear frequency conversion.

Efficient and scalable edge coupler based on silica planar lightwave circuits and lithium niobate thin films

Recently, lithium niobate on insulator (LNOI) has emerged as a promising photonic integration platform for optical communication due to its outstanding material properties. However, an optical coupler that features high-efficiency is essential to improve the practicability of LN devices. In this work, we proposed a high coupling efficiency and scalable edge coupler, which consists of silica-based planar lightwave circuit (PLC) and LN bilayer taper. The silica-based PLC is positioned on the bottom of LN bilayer taper, which serves as the optical interface connecting the LNOI waveguide to the fiber. The silica-based PLC is interfaced to optical fibers through butt-coupling. Then, LN bilayer taper transfers the light form silica PLC to LN ridge waveguide through adiabatic coupling and converts it gradually into the fundamental mode. When coupled to an ultra-high numerical aperture fiber (UHNA), the simulation results show a low coupling loss of 0.11 dB and 0.128 dB per facet for TE and TM mode, respectively. The scalability of the coupler for large numbers of channels has been proven, which meets the performance requirements of LN devices employed in high-bandwidth, multi-channel systems. These results open a path for lithium niobate photonic integration circuit (PIC) and low-cost optical packaging.

Grazing-Angle Fiber-to-Waveguide Coupler

The silicon photonics market has grown rapidly over recent decades due to the demand for high bandwidth and high data-transfer capabilities. Silicon photonics leverage well-developed semiconductor fabrication technologies to combine various photonic functionalities on the same chip. Complicated silicon photonic integrated circuits require a mass-producible packaging strategy with broadband, high coupling efficiency, and fiber-array fiber-to-chip couplers, which is a big challenge. In this paper, we propose a new approach to fiber-array fiber-to-chip couplers which have a complementary metal-oxide semiconductor-compatible silicon structure. An ultra-high numerical aperture fiber is polished at a grazing angle and positioned on a taper-in silicon waveguide. Our simulation results demonstrate a coupling efficiency of more than 90% over hundreds of nanometers and broad alignment tolerance ranges, supporting the use of a fiber array for the packaging. We anticipate that the proposed approach will be able to be used in commercialized systems and other photonic integrated circuit platforms, including those made from lithium niobate and silicon nitride.

Wavelength switchable and tunable noise-like pulses from a 2 μm figure-eight all-fiber laser

We propose the generation of wavelength switchable and tunable noise-like pulses (NLPs) in 2 μm all-fiber laser mode-locking with nonlinear amplifier loop mirror technique. An properly segment of ultra-high numerical aperture fiber is inserted in the cavity to reduce the pump power required for achieving mode-locking at different wavelengths, and a suitable length of single mode fiber is employed to induce more intracavity birefringence. By increasing the pump power and adjusting the intracavity polarization state, the fiber laser can deliver NLPs with switchable wavelengths between 1979.4 nm and 2003.8 nm. Based on the spectral filter effect induced by fiber birefringence, continuous wavelength tunable NLP operation at these two wavebands can be achieved by adjusting polarization controllers. The center wavelength of NLP can be continuously tuned from 1952.1 nm to 1999.0 nm and from 2002.6 nm to 2014.4 nm, with corresponding spectral tuning ranges of 46.9 nm and 11.8 nm, respectively. This wavelength switchable and tunable NLP fiber laser has great potential in fiber sensing and industrial 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.

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.

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.

Manufacturing a Long-Period Grating with Periodic Thermal Diffusion Technology on High-NA Fiber and Its Application as a High-Temperature Sensor.

We demonstrated a kind of long-period fiber grating (LPFG), which is manufactured with a thermal diffusion treatment. The LPFG was inscribed on an ultrahigh-numerical-aperture (UHNA) fiber, highly doped with Ge and P, which was able to easily diffuse at high temperatures within a few seconds. We analyzed how the elements diffused at a high temperature over 1300 °C in the UHNA fiber. Then we developed a periodically heated technology with a CO2 laser, which was able to cause the diffusion of the elements to constitute the modulations of an LPFG. With this technology, there is little damage to the outer structure of the fiber, which is different from the traditional LPFG, as it is periodically tapered. Since the LPFG itself was manufactured under high temperature, it can withstand higher temperatures than traditional LPFGs. Furthermore, the LPFG presents a higher sensitivity to high temperature due to the large amount of Ge doping, which is approximately 100 pm/°C. In addition, the LPFG shows insensitivity to the changing of the environment’s refractive index and strain.

Multistate passively mode-locked thulium-doped fiber laser with nonlinear amplifying loop mirror.

In this paper, a multistate passively mode-locked thulium-doped fiber laser with nonlinear amplifying loop mirror is reported. An ultra-high numerical aperture fiber is employed to compensate the cavity abnormal dispersion, and the net cavity dispersion value is fixed at -0.046 ps2. By changing the pump power, three different mode-locked regions, namely Q-switched mode-locking operation, mode-locking operation, and dual-wavelength mode-locking operation, are observed sequentially. When the pump power is raised to the threshold of 1.60W, the transition state of the Q-switched mode locking begins to be observed. When the pump power increases from 2.88W to 8.75W, stable single-wavelength mode-locking operation is obtained. And the center wavelength is 1988.73nm corresponding to the 3dB spectral bandwidth of 11.21nm. As the pump power is raised beyond 8.75W, dual-wavelength mode-locking operation is developed, where the dual wavelengths are 1969.70nm and 1984.12nm with the 3dB spectral bandwidth of ∼4.11 nm and ∼6.14 nm, respectively.

Cavity-birefringence-dependent h-shaped pulse generation in a thulium-holmium-doped fiber laser.

We report on a type of 2μm h-shaped pulse generation in a thulium-holmium-doped fiber laser and experimentally investigate its cavity birefringence and pump power dependences. An asymmetric nonlinear optical loop mirror is employed as an artificial saturable absorber, which incorporates ∼52.7 m dispersion-shifted fiber and ∼3.8 m ultra-high numerical aperture fiber to enhance the nonlinearity. The h-shaped pulse shows both a polarization state (PS) and pump power related evolutions, even when randomly weak birefringence fibers are employed. By further incorporating different lengths of high birefringence polarization-maintaining fiber (PMF), i.e., introducing different amounts of linear cavity birefringence, much larger pulse tuning ranges can be realized. In particular, when the PMF is lengthened to ∼2.3 m through manipulating the PS, the achieved longest pulse duration of ∼318.14 ns can almost cover the whole repetition period of ∼323.96 ns, corresponding to a pulse duty circle of ∼98.2%, the largest ever reported from a fiber laser, to the best of our knowledge. We demonstrate the related characteristics in detail.

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source.

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Spectral Compression of a Dispersion-Managed Mode-Locked Tm:Fiber Laser at $1.9~\mu \text{m}$

We demonstrate self-phase modulation (SPM)-based spectral compression of the pulse from an all-fiber dispersion-managed Thulium (Tm)-doped fiber laser at 1.9 $\mu \text{m}$ . The laser consists of ultrahigh numerical aperture fiber for the dispersion management to avoid soliton mode-locking and is mode-locked by nonlinear amplified loop mirror. Spectrum of the pulse is compressed from 7 to 3 nm by SPM in an anomalous dispersion core-pumped Tm-doped fiber, together with a passive fiber. Numerical simulations agree well with the experimental results. Limitation of high-efficiency spectral compression due to soliton evolution in the anomalous dispersion fibers is also discussed.

Dissipative Soliton Resonance in a Wavelength-Tunable Thulium-Doped Fiber Laser With Net-Normal Dispersion

We demonstrate an all-fiber figure-eight mode-locked thulium-doped fiber laser with a wide tunable range in both pulsewidth and wavelength. A 45-m-long ultrahigh numerical aperture fiber is used to manage the cavity dispersion, and the net cavity dispersion is calculated to be 0.8585 ps 2 at 1900 nm. With net-normal dispersion, the experimental laser, operating in a dissipative soliton resonance region, generates stable rectangular pulses. The pulsewidth varies from 480 ps to 6.19 ns with the increasing pump power, and its center wavelength has a 28.95-nm tunable range (from 1940.22 to 1969.17 nm) by properly adjusting the polarization controllers. The maximum of average output power and pulse energy is 60.73 mW and 19.51 nJ, respectively. The rectangular pulses have a clamped peak power of about 3.16 W. The wavelength-tunable fiber laser with high-energy output operating at 2 μm has great potential in various application fields.