Small Mode Field Research Articles

Ultra-broadband and low-loss edge coupler for highly efficient second harmonic generation in thin-film lithium niobate

Thin-film lithium niobate is a promising material platform for integrated nonlinear photonics, due to its high refractive index contrast with the excellent optical properties. However, the high refractive index contrast and correspondingly small mode field diameter limit the attainable coupling between the waveguide and fiber. In second harmonic generation processes, lack of efficient fiber-chip coupling schemes covering both the fundamental and second harmonic wavelengths has greatly limited the overall efficiency. We design and fabricate an ultra-broadband tri-layer edge coupler with a high coupling efficiency. The coupler allows efficient coupling of 1 dB / facet at 1550 nm and 3 dB / facet at 775 nm. This enables us to achieve an ultrahigh overall second harmonic generation normalized efficiency (fiber-to-fiber) of 1027 % W − 1 cm − 2 (on-chip second harmonic efficiency ∼3256 % W − 1 cm − 2) in a 5-mm-long periodically-poled lithium niobate waveguide, which is two to three orders of magnitude higher than that in state-of-the-art devices.

Simultaneous modal phase and group velocity matching in microstructured optical fibers for second harmonic generation with ultrashort pulses.

Optical fibers provide a favorable medium for nonlinear optical processes owing to the small mode field size and concurrently high optical intensity combined with the extended interaction lengths. Second harmonic generation (SHG) is one of those processes that has been demonstrated in silica glass optical fibers. Since silica is centrosymmetric, generating SHG in an optical fiber requires poling of the glass. In addition and when one wants to use ultrashort pulses for SHG, achieving both phase and group velocity matching is crucial. Although fibers that feature either modal phase velocity or group velocity matching for SHG have been reported, the possibility of simultaneous modal phase and group velocity matching was never reported before. In this paper we address this challenge, and for the first time to our knowledge, we show that it is feasible to do so with silica microstructured optical fibers featuring at least one ring of air holes in the cladding and a heavily Germanium doped core (above 25 mol.%) by exploiting the LP01(ω) and LP02(2ω) modes at 1.06 µm pump and 0.53 µm second harmonic wavelengths. This finding can greatly impact applications requiring waveguide based SHG generation with ultrashort pulses, including microscopy, material characterization and nonlinear imaging.

Ion-Doped Photonic Crystal Fiber Lasers

Compared with conventional solid-state lasers, fiber lasers have the advantages of small size, simple cooling system, and good output beam quality, enabling them an extended service lifetime in industrialized environments. Periodically arranged photonic crystals have been the most important gain medium for high-power laser applications, which overcame the problems in fiber lasers such as small mode field, low degree of nonlinearity, and non-adjustable dispersion. In this mini-review, we summarize the recent advances of typical ion-doped photonic crystal fiber lasers doped, discuss the challenges, and provide an outlook on the future developments in ion-doped photonic crystal fiber lasers.

Strong convergent LP11 beam for nanoparticles trapping

As the size of research objects reaches the nanometer level, there is an increasing desire to trap and detect nanoparticles. However, the means to trap and study individual nanoparticles is not mature. Here, we propose and demonstrate a method for trapping a single nanoparticle stably and moving it in a three-dimensional space using a single-mode optical fiber probe based on a strong convergent LP11 mode beam. The small mode field diameter of the LP11 beam and the particular probe structure are combined to obtain a nanoscale convergence area to trap a 100 nm nanoparticle stably. The proposed optical fiber tweezers have the advantages of high spatial resolution, simple structure, and easy operation. They are expected to be developed to control organelles in single cells, screen particles of different sizes, and extract virus cells.

CW demonstration of SHG spectral narrowing in a PPLN waveguide generating 2.5 W at 780 nm.

Periodically poled lithium niobate (PPLN) waveguides are a proven and popular means for efficient wavelength conversion. However, conventional PPLN waveguides typically have small mode field diameters (MFD) (≲6 µm) or significant insertion and/or propagation losses, limiting their ability to operate at multi-watt power levels. In this work we utilise zinc indiffused PPLN ridge waveguides that have a larger MFD, favourable pump/SHG modal overlap, and low insertion losses. Here for the first time, we have demonstrated continuous wave (CW) spectral narrowing from a PPLN waveguide, both with high efficiency and multi-watt second harmonic generation (SHG). 2.5 W of 780 nm has been produced by SHG of an amplified 1560 nm telecom laser with a device efficiency of 58% in a 4.0-cm long ridge waveguide. We have modelled conversion efficiency and applied experimentally measured waveguide parameters to show excellent agreement to the SHG spectra. Spectral narrowing of the full width half maximum (FWHM) of 35.7% has been measured as the nonlinear drive is increased. This work demonstrates that single-pass, multi-watt, CW SHG at 780 nm is feasible from our PPLN waveguide in the large conversion regime.

Multimode and single-mode fiber compatible graded-index multicore fiber for high density optical interconnect application

We designed and fabricated a graded-index (GI) multicore fiber (MCF) compatible with both standard multimode and single-mode fiber for high density optical interconnect application in large-scale data centers. The proposed fiber supports long-distance multimode transmission at 850 nm as well as quasi-single mode transmission at 1310 nm and 1550 nm. The parameters of the GI-MCF have been optimized to obtain both a small differential mode delay at 850 nm and a small mode field diameter mismatch of less than 0.5 μm with single-mode fiber at 1310 nm and 1550 nm with a negligible inter-core crosstalk. In experiment, we successfully realized the multimode operation over 1 km-long GI-MCF at 850 nm and the quasi-single mode operation over 12.4 km-long GI-MCF at 1310 nm and 1550 nm at a data rate of 7×10-Gb/s. The multi-wavelengths multicore transmission was demonstrated for the first time. The experiment results imply that the proposed GI-MCF satisfies various requirements in such as operating wavelength, accessible distance and interconnect density of large-scale data center, and can effectively reduce the fiber numbers and system complexity.

Generation of 6.8 W of CW output power at 1550 nm using small mode field diameter Er:Yb co-doped double clad fiber in laser oscillator configuration

We report studies on erbium:ytterbium doped fiber laser, that uses a commercially available off-the-shelf Er:Yb co-doped single mode fiber with small mode field diameter of 7 µm, which is compatible with single mode fibers used in most of the communication systems. In a FBG based laser oscillator configuration pumped at 976 nm, the single transverse mode fiber laser emitted more than 6.87 Watts of output power at 1550 nm with a conversion efficiency of ~32% over a narrow line width. The pump radiation is coupled into active fiber using free-space optics. To the best of our knowledge, this is the highest power reported in the 1.55 µm region by using commercially available off-the-shelf active fiber of small mode field diameter (~7 µm) in a simple laser oscillator configuration with 9×× nm pumping. We also observed the effect of weak secondary grating structure in FBGs on the spectral characteristics of EYDFLs and explained its significance for co-doped fiber lasers such as erbium–ytterbium fiber lasers.

Study of Optimized Coupling Based on Micro-lensed Fibers for Fibers and Photonic Integrated Circuits in the Framework of Telecommunications and Sensing Applications

We demonstrate the interest of expanded beam microlenses (around 55 µm of mode field diameter) to relax positioning tolerances and to decrease reflectance in single mode fiber to fiber interconnexions. We also point out the interest of micro-lenses of very small mode field diameter (around 2 µm) to improve coupling efficiency in specialty fibers and integrated waveguides for non linear effects based functions and for sensors applications at a wavelength of 1.55 µm.

Design and analysis of low bend losses of the air core optical fiber for wavelength selective devices

In this paper, the bending loss, modal field diameter and the modal field distribution of the air core optical fiber are investigated. The effect of optical and geometrical parameters on the bending loss, power confinement and modal field diameter are examined in this special fiber. Detailed design parameters and operation principles of the air core optical fiber are discussed. The structure is based on a unique three layered structure of air core optical fiber, having the central air core, germanium silicate ring core, and silica cladding. It has been demonstrated that air core optical fiber has an excellent mode transformation capability. The air core optical fiber is expected to have a versatile application in local area optical communication networks and tunable wavelength selective devices. The main advantages of air core optical fiber are low bend loss and small mode field diameter, which is a prime focus of this paper.

Design of a single-polarization single-mode photonic crystal fiber with a near-Gaussian mode field and wide bandwidth

Single-polarization single-mode (SPSM) fiber can efficiently eliminate polarization mode coupling, polarization mode dispersion, and polarization-dependent loss. Up to now, most single-polarization fibers have been designed based on form birefringence, which would result in a non-Gaussian field distribution and a small effective mode field area. In this paper, a novel structure of SPSM photonic crystal fibers based on the resonant coupling phenomena is proposed and analyzed by using a full-vector finite-element method with a second-order transparent boundary condition. From the numerical results it is confirmed that this fiber has a near-Gaussian mode field within the wavelength range from 1.46 to 2.2 μm, where only one polarized mode exists effectively, and the mode field area is about 79 μm(2) at the wavelength of 1.55 μm, matching that of the conventional single-mode fiber.

Low Loss Vertical Optical Path Conversion Using 45° Mirror for Coupling between Optical Waveguide Devices and Planar Devices

To realize low loss optical interconnects between optical waveguide devices and two dimensional array-type devices, we investigated a novel 45° mirror device buried by index matching epoxy resin and coated with an anti-reflection layer. A groove with an isosceles right triangle cross section was formed using a dicing saw so as to cut into the single-mode optical waveguide, and aluminum thin film was obliquely deposited on the 45° side wall of the groove to increase the reflectance. The groove was filled with a spin-coating of UV-curable epoxy resin to planarize the top surface, and low index polymer (Teflon) was formed on the top surface as the anti-reflection layer. The optimum depth of the groove for reducing shadowing of the deflected beam was analyzed. As a result the insertion loss of the 45° mirror was reduced 1.3 dB compared to the insertion loss of straight waveguide. In addition, a novel optical tap device is proposed and demonstrated using a similar fabrication process to that of the 45° mirror device. The isosceles right triangle-shaped groove was formed on the upper cladding layer so as to almost touch the core layer. After filling it in and finishing it with the epoxy resin and AR coating used in the 45° mirror device, the loss of the through port was reduced 0.5 dB compared to the insertion loss of straight waveguide and -14 dB (4.0%) of the optical power was transmitted to the tap port. Lastly as a model of the coupling between waveguide devices and two-dimensional arrayed devices such as vertical cavity surface emitting lasers (VCSEL's), the vertical port of the 45° mirror device was connected to a single mode fiber with a very small mode field diameter (3.6 µm). This configuration transmitted -11.8 dB (6.6%) of the input power from the input port to the vertical tap port and vice versa.

Power limitation induced by nonlinear effects in pulsed high-power fiber amplifiers

The combining of double-clad (DC) doped fibers and semiconductor pump laser diodes has allowed the development of high power optical sources in CW or pulsed regimes. Compared to bulk medium, the main limitations for achieving high peak powers inside optical fibers are due to small mode field size and large propagation length. Nonlinear effects such as the optical Kerr effect, Stimulated Raman Scattering (SRS) or Stimulated Brillouin Scattering (SBS) can be observed in high power amplifiers as short as a few meters of doped fiber length. A description of the different non-linear dynamics for optical pulse amplification in dependence with temporal pulse duration (ps, ns and μs regimes) will be presented. Theoretical modelling has also investigated to confirm the observed signal distortions in the case of a ∼6 μm core doped fiber amplifier. The power increase requires the design of new doped fibers with a large core to enhance simultaneously stored energy and nonlinear effects threshold. The peak power threshold in dependence with temporal pulse duration (ps, ns and μs regimes) will be given in relation with core diameters of double-clad fibers. To cite this article: Y. Jaouën et al., C. R. Physique 7 (2006).

Micro-lens on polarization maintaining fibre for coupling with 1.55 μm quantum dot devices

We propose a simple micro-lens scheme on polarization maintaining single mode fibre for characterization of 1.55μm wavelength quantum dot devices such as lasers or amplifiers. The proposed micro-lens offers quite long working distance (45μm), very small mode field diameter (<2μm), low back reflections (around 45dB) and easy fabrication process for high coupling efficiency. The coupling scheme principle and fabrication process are described. Characterization results of mode field diameter, working distance and polarization extinction ratio (around 22dB) of this micro-lens are presented. The use of this micro-lens for characterizing polarization sensitivity of InAs/InP on 311B quantum dot wave-guides is also discussed.

Hybrid integrated external cavity laser using 1.5% PLC

An integrated external cavity laser (ECL) using planar lightwave circuits (PLCs) with a higher than usual refractive index contrast has been successfully demonstrated. The small modefield of the PLCs enables use of a laser diode without a spot-size converter and eliminates the need for a Si terrace structure. The oscillation characteristics of the ECL are as good as those of a conventional ECL with a lower contrast.

Low pump power photonic crystal fibre amplifiers

Designs of low pump power optical amplifiers, based on photonic crystal fibres are presented. The potential of these fibre amplifiers is investigated, and it is demonstrated that such amplifiers may deliver gains of more than 15 dB at 1550 nm with less than 1 mW of optical pump power. We focus on the design of erbium-doped photonic crystal fibre amplifiers operated at very low pump powers. We achieve this by using PCFs with large air holes and thereby large index contrasts. With such high index contrast fibres, we may obtain very small mode field diameters.

Characteristics of Wavelength Tunable Femtosecond Soliton Pulse Generation Using Femtosecond Pump Laser and Polarization Maintaining Fiber

We have investigated both experimentally and numerically the characteristics of wavelength tunable femtosecond soliton pulse generation using a pulse width variable fiber laser and two different types of polarization maintaining fibers. The generated soliton shows the feature of the pulsewidth becoming almost constant at - 250 fs under any conditions of the pump pulse for 220m fiber. High conversion efficiency from pump pulse to a generated soliton pulse accounting for as much as 73% can be obtained. This efficiency decreases with increase in the input power or wavelength shift. A superior conversion efficiency and broad wavelength shift can be obtained by using a more shortened pump pulse. In the numerical calculations, it is predicted that under a condition of constant power of pump pulse, the maximum wavelength shift is achieved when the soliton number N is -1.4. The difference of mode field diameter and the group velocity dispersion (GVD) coefficient β2 affect the wavelength shift and conversion efficiency. Using the fiber with small mode field diameter and small absolute value of GVD coefficient β2, a high conversion efficiency and large wavelength shift can be obtained.

A Tunable 1.6 μm Band Optical Time-domain Reflectometer

A tunable 1.6 μm band optical time-domain reflectometer (OTDR) is reported which uses a tunable Raman fiber laser (RFL). The RFL optical peak power of 7 W at 1652 nm was obtained by using a tunable Q-switched Er 3+ -doped fiber ring laser (QERL) as a pump light source, and a Raman fiber with a small mode field diameter. By using this RFL, an OTDR dynamic range over 22.5 dB was achieved in the wavelength region from 1650 nm to 1680 nm with a spatial resolution of 100 m.

Bend loss measurements for small mode field diameter fibres

We measured the loss of bend resistant fibres at small bend radii of 1 to 5 mm. A spectral interpolation technique was used to measure the bend loss over 1300–1600 nm range. Fibres with 7.3μm mode field diameter at 1550 nm show &lt;0.4dB/cm at 2 mm radius.