High second-order nonlinearities in poled silicate fibers

Linear Electro-optic Effect in Silicon Nitride Waveguides Enabled by Electric-Field Poling

Stoichiometric silicon nitride (Si3N4) is one of the most mature integrated photonic platforms for linear and nonlinear optical applications on-chip. However, because it is a centrosymmetric material, second-order nonlinear processes are inherently not available in Si3N4, limiting its use for multiple classical and quantum applications. In this work, we implement thermally assisted electric-field poling, which allows charge carrier separation in the waveguide core, leading to a depletion zone formation and the inscription of a strong electric field reaching 20 V/μm. The latter results in an effective second-order susceptibility (χ(2)) inside the Si3N4 waveguide, making linear electro-optic modulation accessible on the platform for the first time. We develop a numerical model for simulating the poling process inside the waveguide and use it to calculate the diffusion coefficient and the concentration of the charge carriers responsible for the field formation. The charge carrier concentration, as well as the waveguide core size, is found to play a significant role in determining the achievable effective nonlinearity experienced by the optical mode inside the waveguide. Current findings establish a strong groundwork for further advancement of χ(2)-based devices on Si3N4.

Optical poling by means of electrical corona discharge

Electrical corona discharge is employed in this work to deposit ions on the surface of an optical fiber, creating a strong electric field that is used for poling. Green laser light propagating in the core frees photocarriers that are displaced by the poling field. The technique presented can induce a higher optical nonlinearity than previously obtained in traditional optical poling with internal metal electrodes. To date, a maximum second order nonlinearity 0.13 pm/V has been achieved for a 15 kV corona discharge bias.

Thermal Poling of Optical Fibers: A Numerical History

This review gives a perspective of the thermal poling technique throughout its chronological evolution, starting in the early 1990s when the first observation of the permanent creation of a second order non-linearity inside a bulk piece of glass was reported. We then discuss a number of significant developments in this field, focusing particular attention on working principles, numerical analysis and theoretical advances in thermal poling of optical fibers, and conclude with the most recent studies and publications by the authors. Our latest works show how in principle, optical fibers of any geometry (conventional step-index, solid core microstructured, etc) and of any length can be poled, thus creating an advanced technological platform for the realization of all-fiber quadratic non-linear photonics.

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 2.5 m tellurite fiber with conversion efficiency up to ∼1.15% at 20 mW, 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.

Thermal poling of multi-wire array optical fiber

We demonstrate in this paper thermal poling of multi-wire array fibers, which extends poling of fibers with two anodes to ~50 and ~500 wire array anodes. The second harmonic microscopy observations show that second order nonlinearity (SON) layers are developed surrounding all the rings of wires in the ~50 anode array fiber with poling of 1.8kV, 250°C and 30min duration, and the outer rings of the ~500 anode array fiber at lower poling temperature. Our simulations based on a two-dimensional charge dynamics model confirm this can be explained by the self-adjustment mechanism, and show the SON layers are induced from the outer rings to the inner rings.

Instant and efficient second-harmonic generation and downconversion in unprepared graded-index multimode fibers

We show that germanium-doped graded-index multimode silica fibers can exhibit relatively high conversion efficiencies (∼6.5%) for second-harmonic generation when excited at 1064 nm. This frequency-doubling behavior is also found to be accompanied by an effective downconversion. As opposed to previous experiments carried out in single- and few-mode fibers where hours of preparation were required, in our system, these χ⁽²⁾ related processes occur almost instantaneously. The efficiencies observed in our experiments are, to the best of our knowledge, among the highest ever reported in unprepared fibers.

Second harmonic generation in multimode graded-index fibers: spatial beam cleaning and multiple harmonic sideband generation

We study experimentally and numerically the spectral and spatial dynamics of second harmonic generation in an all-optically poled multimode graded-index fiber. In contrast with poled single-mode fibers, in a multimode graded-index fiber a pump can generate a series of sharp sidebands around its second harmonic (SH) that originate from the sub-millimetric periodic evolution of the intensity at the fundamental frequency. The mutual interaction between the fundamental and its SH may also strongly affect the spatial distribution of guided light for both colors: when increasing the pump power, both fundamental and SH output beams evolve from disordered multimode speckles into two bell-shaped beams.

Simulating the space-charge field formation in thermally poled optical fibers: a new two-rate model for hydrogenated cations

In thermally poled optical fibers, the second-order nonlinearity has been observed to be distributed in a narrow layer more than 10 μm beneath the anode surface, a result that cannot be explained by current models. In this Letter, we propose a new model based on slow migration of H₃O⁺ and fast H⁺ migration related to the H₃O⁺ concentration. The numerical simulation results show a narrow layer-shaped distribution of the induced second-order nonlinearity, which retains its magnitude and distribution profile in the migration process, and is in good agreement with the experimental observations.

Pockels effect of silicate glass-ceramics: Observation of optical modulation in Mach-Zehnder system

Silicate glass has been used for long time because of its advantages from material’s viewpoint. In this paper, we report the observation of Pockels effect by Mach–Zehnder interferometer in polycrystalline ceramics made from a ternary silicate glass via crystallization due to heat-treatment, i.e., glass-ceramics. Since the silicate system is employed as the precursor, merits of glass material are fully utilized to fabricate the optical device component, in addition to that of functional crystalline material, leading us to provide an electro-optic device, which is introducible into glass-fiber network.

Optical creation and erasure of the linear electrooptical effect in silica fiber

We study the creation and erasure of the linear electrooptical effect in silicate fibers by optical poling. Carriers are released by exposure to green light and displaced with simultaneous application of an internal dc field. The second order nonlinear coefficient induced grows with poling bias. The field recorded (~10⁸ V/m) is comparable to that obtained through classical thermal poling of fibers. In the regime studied here, the second-order nonlinearity induced (~0.06 pm/V) is limited by the field applied during poling (1.2 × 10⁸ V/m). Optical erasure with high-power green light alone is very efficient. The dynamics of the writing and erasing process is discussed, and the two dimensional (2D) field distribution across the fiber is simulated.

Hindering effect of the core-cladding interface in thermally poled optical fibers

In this paper, the hindering effect of the core-cladding interface (CCI) in thermally poled optical fiber is investigated based on the two-dimensional charge dynamics model. The influence of the mobility of charge carriers at the CCI, and the mobility and concentration of charge carriers in the fiber core on the thermal poling process is presented. It is found that lower mobility of charge carriers at the CCI is responsible for the hindering effect of the CCI. The hindering effect of the CCI could either be enhanced by increasing the mobility and concentration of charge carriers in the fiber core or be overcome by decreasing the mobility and concentration of sodium ions in the fiber core, which could increase the χ⁽²⁾ in propagation mode. The results provide theoretical insight into the underlying mechanism of the hindering effect and may find applications in thermal poling optical waveguides and multilayered structures.

Study of thermally poled fibers with a two-dimensional model

A two-dimensional (2D) numerical model is implemented to describe the movement of ions under thermal poling for the specific case of optical fibers. Three types of cations are considered (representing Na(+), Li(+) and H3O(+)) of different mobility values. A cross-sectional map of the carrier concentration is obtained as a function of time. The role of the various cations is investigated. The assumptions of the model are validated by comparing the predictions to experimental data of the time evolution of the nonlinearity induced. A variational analysis of poling parameters including temperature, poling voltage, sign of the bias potential and initial ionic concentrations is performed for a particular fiber geometry. The analysis allows identifying the impact of these parameters on the induced second-order nonlinearity in poled fibers.

Pulse selection at 1 MHz with electrooptic fiber switch

Two 78-cm long electrooptic fibers with nonlinear coefficient χ((2)) ~0.26 pm/V are used in a Sagnac loop for pulse selection at up to 1 MHz repetition rate. Laser pulses of 1.5 µm wavelength arriving at every 140 ns are selected with an extinction ratio as high as -30 dB. The arrangement is entirely based on silica fiber.

Investigating the effectiveness of thermally poling optical fibers with various internal electrode configurations

Twin-hole fibers were thermally poled with different internal electrode configurations, including having only one anode wire in the hole, two anode wires in the two holes, one cathode wire, and two cathode wires in the holes, in comparison to the conventional one anode wire and one cathode wire combination. Second harmonic microscopy was utilized to visually reveal the spatial distribution and to measure the magnitude of the induced second-order optical nonlinearity within the poled fibers. It was found that both one- and two-anode configurations resulted in strong nonlinearity comparable with the conventional case but the two-anode configuration was more reproducible than the one-anode case; for the one-cathode-wire and two-cathode-wire configuration, strong nonlinearity in a ring shape concentric with the fiber outer surface was induced as if the cathode metal wire were in the center of the twin-hole fiber rather than substantially offset. These new results provide strong support for the proposed model of a "self-adjustment" mechanism and point the way to simplified and more repeatable experimental techniques.

Soliton generation from an actively mode-locked fiber laser incorporating an electro-optic fiber modulator

This work demonstrates an actively mode-locked fiber laser operating in soliton regime and employing an all-fiber electro-optic modulator. Nonlinear polarization rotation is utilized for femtosecond pulse generation. Stable operation of the all-fiber ring laser is readily achieved at a fundamental repetition rate of 2.6 MHz and produces 460 fs pulses with a spectral bandwidth of 5.3 nm.

Integrated fiber Michelson interferometer based on poled hollow twin-core fiber

We propose an integrated fiber Michelson interferometer based on a poled hollow twin-core fiber. The Michelson interferometer can be used as an electro-optic modulator by thermal poling one core of the twin-core fiber and introducing second-order nonlinearity in the fiber. The proposed fiber Michelson interferometer is experimentally demonstrated under driving voltages at the frequency range of 149 to 1000 Hz. The half-wave voltage of the poled fiber is 135 V, and the effective second-order nonlinear coefficient χ² is 1.23 pm/V.

Multicolor upconversion emissions in Tm 3+/Er3+ codoped tellurite photonic microwire between silica fiber tapers

We report multicolor upconversion emissions including the blue-violet, green, and red lights in a Tm 3+/Er3+codoped tellurite glass photonic microwire between two silica fiber tapers. A silica fiber is tapered until its evanescent field is exposed and then angled-cleaved at the tapered center to divide the tapered fibers into two parts. A tellurite glass is melted by a gas flame to cluster into a sphere at the tip of one tapered fiber. The other angled-cleaved tapered fiber is blended into the melted tellurite glass. When the tellurite glass is melted, the two silica fiber tapers are simultaneously moving outwards to draw the tellurite glass into a microwire in between. The advantage of angled-cleaving on fiber tapers is to avoid cavity resonances in high index photonic microwire. Thus, the broadband white light can be transmitted between silica fibers and a special optical property like high intensity upconversion emission can be achieved. A cw 1064 nm Nd:YAG laser light is launched into the Tm 3+/Er3+ codoped tellurite microwire through a silica fiber taper to generate the multicolor upconversion emissions, including the blue-violet, green, and red lights, simultaneously.

Measurement of chi((2)) symmetry in a poled fiber

We measure the values of individual chi((2)) tensor components in a birefringent periodically poled silica fiber through spectrally separated type I and type II second-harmonic generation. We demonstrate that the chi((2)) tensor symmetry is consistent with that of chi((3)) in silica and thereby provide experimental evidence that this chi((2)) originates from a chi((3)) process.

Aperiodically poled silica fibers for bandwidth control of quasi-phase-matched second-harmonic generation

Precise control of the bandwidth of quasi-phase-matched second-harmonic generation in silica fibers is realized through chirped-period poling. The bandwidth is expanded by a factor of 33 over a uniform-period poled fiber of the same interaction length.

Unprecedented Raman cascading and four-wave mixing from second-harmonic generation in optical fiber

We experimentally demonstrate strong second-harmonic-generation from a self-induced all-optical poling in germanium-doped fiber with a subnanosecond laser pump at 1064 nm. The large second-harmonic conversion efficiency allows nonlinear spectral broadening at visible wavelengths so that up to nine distinct Raman sidebands have been obtained. In this work we emphasize how the Raman scattering, induced from the pump in the IR region, can drastically affect the optical poling effect, limiting in turn second-harmonic generation.

Who needs a cathode? Creating a second-order nonlinearity by charging glass fiber with two anodes

We report that it is possible to create a fiber electret by having both internal electrodes of a twin-hole fiber at the same anodic potential, i.e., without the use of a contacted cathode electrode. We find that a stronger and more temperature-stable charge distribution results when the fiber core is subjected to an external field near zero. Negative charges from the air surrounding the fiber are sufficient for the recording of an electric field across the core of the fiber that is twice stronger than when one anode and one cathode electrode are used. The enhancement in stability and in the strength of the effective chi((2)) induced are a significant step towards the wider use of fibers with a second order optical nonlinearity.

High-average-power second-harmonic generation from periodically poled silica fibers

The generation of 236 mW of second-harmonic power in a 32-cm-long periodically poled silica fiber, corresponding to an average conversion efficiency of 15.2+/-0.5%, is reported. This represents the highest normalized second-harmonic conversion and the highest average second-harmonic power ever reported for a periodically poled silica fiber, to our knowledge. The enhancement is attributed to an improved design of the specialty twin-hole fiber and the extension of the nonlinear interaction length.

Space-selectively crystallized fiber with second-order optical nonlinearity for variable optical attenuation

We have fabricated BaO-TiO2-GeO2-SiO2-based glass fibers with the oriented space-selectively crystallized structure by laser irradiation and also demonstrated variable optical attenuation induced by electro-optical birefringence change based on second-order optical nonlinearity. The transmittance of a polarized signal is controlled by an electric field applied to the fiber, and the electro-optic fiber devices are operated with extremely low nanowatt electric power dissipation.

Broadly tunable second-harmonic generation in periodically poled silica fibers

Broadband tunability of the second-harmonic wavelength is achieved in periodically poled silica fibers. A wavelength tuning range of almost 45 nm of the fundamental wave is demonstrated through mechanical compression tuning of the quasi-phase-matched periodic structure. The uniform strain applied along the entire periodic structure enables the spectral profile and the conversion efficiency of the generated second harmonic to be preserved for the full tuning range. To our knowledge, the achieved tuning range realized through this technique is far greater than that possible with uniform periodically poled crystals.

Electro-optical fiber modulation in a Sagnac interferometer

The linear electro-optical effect was induced in optical fibers by thermal poling. A Sagnac all-spliced fiber loop was constructed incorporating an electro-optical fiber used as a phase modulator at 1.5 microm and stable switching was obtained. A rise time of approximately 40 ns was obtained, and signals <2 V generated a measurable optical response.

Characterization of a second-order nonlinear layer profile in thermally poled optical fibers with second-harmonic microscopy

Second-harmonic microscopy has been successfully applied to characterize the second-order nonlinear layer in optical fibers thermally poled at 280 degrees C and 3.5 kV for 30 min. The nonlinear layer was found to be approximately 5.1 microm deep under the anode and was not always centered about the closest point between the electrodes.

Integrated fiber Mach-Zehnder interferometer for electro-optic switching

Molten alloys under high pressure were used to obtain fibers with long internal electrodes that are solid at room temperature. An integrated Mach-Zehnder interferometer was constructed from a twin-core twin-hole fiber that permitted application of an electric field preferentially to one of the cores. Good stability and a switching voltage of 1.4kV were measured with a 1-m-long fiber device with a quadratic voltage dependence.

Effect of poling conditions on second-harmonic generation in fused silica

A systematic study of the effects of poling time and applied voltage on second-harmonic (SH) generation in thermally poled silica glass reveals that the SH signal is proportional to the square of the applied voltage and that the speed of the poling process increases with the applied voltage. Prior treatment of the samples is found to affect the poling process, and the optimum poling conditions are observed to depend on the poling atmosphere. The mechanism of thermal poling is discussed in the light of these new results.

Correlation of Ge E' defect sites with second-harmonic generation in poled high-water fused silica

Second-harmonic generation (SHG) in poled high-water fused silica was found to be linearly dependent on the relative concentration of Ge E' defect sites (<10(-15) defect sites/cm(3)) for the highly pure UV-grade material. For commercial-grade material characterized by higher concentrations of Ge E' defects sites, the SHG appeared to saturate at approximately 0.5 pm/V. No dependence of the SHG was found on the relative concentration of Ge E' defects sites in poled commercial-grade low-water fused silica.

Blue-light generation by quasi-phase-matched frequency doubling in thermally poled optical fibers

Periodically patterned second-order nonlinearities are created in optical fibers by thermal poling in vacuo. Cw quasi-phase-matched frequency conversion to the blue is demonstrated. The measured bandwidth, 0.78 nm cm, is close to the theoretical prediction.