Second-harmonic Generation In Optical Fibers Research Articles

Multi-wavelength second-harmonic generation in a double-clad high nonlinear fiber induced by multiple phase-matching.

In this study, multi-wavelength second-harmonic generation (SHG) based on self-phase modulation (SPM) broadband supercontinuum (SC) was observed by employing a double-clad high nonlinear optical fiber (HNLF) in conjunction with a femtosecond laser. At a wavelength of 1050 nm and an average pump power of 320 mW, multiple phase-matching conditions were achieved, and SH signals of central wavelengths ∼530.7 nm, ∼525.1 nm, ∼503.5 nm, and ∼478.7 nm were observed, with SHG efficiency reaching ∼1.34 × 10-4. The SHG in this experiment can be attributed to the utilization of a doped optical fiber, where dopants create defect states, facilitating optical-chemical transformation and enhancing second-order polarization susceptibility. Additionally, theoretical simulations were conducted, aligning closely with the experimental findings. To the best of our knowledge, this work marks the first demonstration of multi-wavelength SHG in optical fibers. It offers a distinctive avenue for customizing multi-wavelength ultrafast light sources, exhibiting great application potential in the fields of medical diagnostics and optical sensing.

Highly efficient second-harmonic generation in a tellurite optical fiber.

A step-index tellurite optical fiber with loss ∼0.02 dB/m at ∼1545 nm was fabricated based on TeO2-Bi2O3-ZnO-Na2O glass. With a nanosecond laser operated at ∼1545 nmas the pump source, second-harmonic generation (SHG) was observed in the 2.5m tellurite fiber with conversion efficiency up to ∼1.15% at 20mW, which, to the best of our knowledge, is the highest for non-silica optical fibers. The experimental phenomenon can be explained by the χ(2)-induced model of nonlinear polarization at the core-cladding interface. Furthermore, it can also be explained by a χ(3)-induced model via the four-wave-mixing effect, where a quasi-SH signal and a millimeter wave were generated simultaneously. This new model may help shed light on the physical mechanism of SHG in optical fibers.

Second-Harmonic Generation in Optical Fibres Induced by aCross-Phase Modulation Effect

When two optical pulses copropagate inside a single-mode fibre,intensity-dependent refractive index couples the pulses through across-phase modulation (XPM). We show that a second-harmonicgeneration (SHG) on a continuous-wave background is possible inthe optical fibre induced by the XPM effect. By means of amulti-scale method the nonlinearly coupled envelope equations forthe SHG are derived and their explicit solutions are provided anddiscussed.

Second-harmonic generation in optical fibers on a continuous-wave background

We predict that a different type of second-harmonic generation (SHG) in nonlinear optical fibers is possible for the waves excited from a continuous-wave background. We show that in a normal dispersion regime and near the zero-dispersion point of a single-mode optical fiber the phase-matching condition of the SHG can be satisfied by suitably choosing the wave vectors and frequencies of fundamental and second-harmonic waves. Using a multiscale method the nonlinearly coupled envelope equations for the SHG are derived and their explicit solutions are provided and checked by numerical simulation.

Evaluation of Saturation of Photoinduced Second-Harmonic Generation in Doped Glass Fibers

The saturation of self-organized second-harmonic generation in optical fibers, predicted by the directional photoionization model and the phenomenological cumulative local-response model, is confronted with data of some previous experiments. It was found that the experimentally determined value of ( I 2ω/ I 2 ω) sat ranges from 0.2 × 10- 16 to 2 × 10 -16 m 2/W, which is in reasonable agreement with the predictions of the directional photoionization model. The self-organized parametric down conversion is proposed for a very effective preparation of optical fibers.

Growth rate of second-harmonic generation in optical fibers

Reported are experimental results concerning harmonic generation efficiency and speed of fiber preparation in erbium 3+ -doped optical fibers of various ion concentrations. The maximum saturated value of the second-harmonic generation efficiency is found to be independent of pulse duration, pump peak intensity and doping of the fiber. The time-integrated input energy required to reach the saturated level is shown to vary significantly with pump peak power. Holographic grating build-up time is shown to be significantly reduced in Er-doped fibers. The results are compared to the spatially anisotropic ionisation model of Anderson et al.

Intensity-dependent shift of phasematched wavelength in second-harmonic generation in optical fibres

The dependence of the pump intensity on the phase-matching conditions in second-harmonic generation in optical fibres has been studied. It has been shown experimentally that there is a power-dependent shift of the phase-matched fundamental wavelength.

Seeded second-harmonic generation in optical fibers: the effect of phase fluctuations

Phase fluctuations between the infrared and green beams during seeding of second-harmonic generation (SHG) in optical fibers lead to a drastic reduction of the initial SHG intensity and to a strong time dependence of the SHG after seeding. Experiments using mode-locked lasers with a stable phase relation between the infrared and green writing beams lead to reliable, reproducible results. Under these conditions we do not observe a significant difference in SH efficiency between fibers with Ge concentrations of 3 and 14 mol. %.

Photoinduced second-harmonic generation in gamma-ray-irradiated optical fibers

The effect of gamma-ray irradiation on photoinduced second-harmonic generation in optical fibers is investigated for the first time to our knowledge. A significant increase (by a factor of 20) in the conversion efficiency of gamma-ray-irradiated optical fibers is found. An explanation of the experimental results is suggested, which is based on the photovoltaic mechanism of the photoinduced second-harmonic generation.

On the Phenomenology of Self-organized Quasi-phase-matched Second-harmonic Generation in Optical Fibres

Abstract A simple phenomenological theory of second-harmonic generation in centrosymmetric media is presented in this analytical paper. It is shown that in the medium without memory the energy transfer between interacting waves is negligibly small. For the medium with an optical memory (like phosphorus-germanium-doped glass fibres), the process of formation of χ (2) grating, owing to the third-order optical rectification and the electric-field-induced deprivation of centrosymmetric structure, is described from the phenomenological point of view. Second harmonic generation at periodical χ (2) grating is studied. In the case of resonant interaction, second-harmonic radiation can be systematically amplified. For the non-monochromatic and pulse-form radiations the effective length of the medium is limited largely due to the Kerr nonlinearities. The estimated effective fibre length is in good agreement with previous experiments.

Polarization-dependent and intensity-dependent phase mismatch of second-harmonic generation in optical fibers

The second-harmonic dependence on the input fundamental light has been measured in optical fibers having a chi((2)) induced by seeding. The results are explained with a model that includes self-phase and cross-phase modulation in the equations governing second-harmonic generation and seeding. This leads to an intensity-dependent phase mismatch whose strength also depends on the relative polarizations of the fundamental and second-harmonic lights. During seeding, the same effects result in a chi((2)) grating detuned from exact phase matching.

Enhancement of second-harmonic generation in optical fibers by a hydrogen and heat treatment

Optical fiber exposed to high-pressure hydrogen gas for 3–4 days, and subsequently heated in air at temperatures in the range 250–500 °C for 8–24 h, shows an enhanced second-harmonic conversion efficiency after seeding with 1.06 μm and 532 nm light, when compared with untreated fiber seeded under similar conditions. Up to now, an enhancement by as much as a factor of 4.4 has been observed. Some possible correlations between this enhancement and the change in the defect content of the fiber due to the H2 -heat treatment are pointed out.

Phase-matched periodic electric-field-induced second-harmonic generation in optical fibers

Phase-matched electric-field-induced second-harmonic generation is demonstrated in single-mode germania-doped silica fibers. A periodic second-order nonlinearity is induced by a simple interdigitated electrode structure, which can be rotated to permit phase matching between all propagating modes. The most efficient mode interaction between HE11ω and HE112ω is achieved at 1.064 μm by using a Q-switched Nd+3:YAG laser. In principle, phase matching at any propagating wavelength is possible. This technique could be applied to planar as well as cylindrical waveguides and can be used with many non-χ(2) materials. The asymmetry in the applied electric field enhances the optical-field overlaps between modes of dissimilar orders, and this is also demonstrated. A conversion efficiency of 4.0 × 10−4% has been obtained in unoptimized devices. Device optimization is also discussed.

Investigations of the preparation process for efficient second-harmonic generation in optical fibers

Experiments were carried out to elucidate aspects of the preparation process, which enables glass fibers to efficiently frequency-double infrared laser light. A picture is described which is consistent with most of the observations.