Compensation-free broadband entangled photon pair sources

Quantum sources that provide broadband biphotons entangled in both polarization and time-energy degrees of freedom are a rich quantum resource that finds many applications in quantum communication, sensing, and metrology. Creating such a source while maintaining high entanglement quality over a broad spectral range is a challenge, which conventionally requires various compensation steps to erase temporal, spectral, or spatial distinguishabilities. Here, we point out that in fact compensation is not always necessary. The key to generate broadband polarization-entangled biphotons via type-II spontaneous parametric downcoversion (SPDC) without compensation is to use nonlinear materials with sufficiently low group birefringence that the biphoton bandwidth becomes dispersion-limited. Most nonlinear crystals or waveguides cannot meet this condition, but it is easily met in fiber-based systems. We reveal the interplay of group birefringence and dispersion on SPDC bandwidth and polarization entanglement quality. We show that periodically poled silica fiber (PPSF) is an ideal medium to generate high-concurrence (>0.977) polarization-entangled photons over a broad spectral range (>77nm), directly and without compensation. This is the highest polarization-entanglement concurrence reported that is maintained over a broad spectral range from a compensation-free source.

Thermal poling of silica optical fibers using liquid electrodes

Thermal poling is a well-known technique for inducing second-order nonlinearities in centrosymmetric silica optical fibers. However, some 25 years since its discovery, there still remain a number of issues that prevent the realization of very long length, highly efficient all-fiber nonlinear device applications that include frequency conversion or sources of polarization-entangled photon pairs. In this Letter, we report a thermal poling method that utilizes a novel range of liquid metal and aqueous electrodes embedded into the optical fibers. We demonstrate that it is possible to pole samples that are potentially meters in length, characterized by very low losses for efficient second-harmonic generation processes. The maximum estimated effective value of χ⁽²⁾ (0.12 pm/V) obtained using mercury electrodes is the highest reported in periodically poled silica fibers.

All-fiber fourth and fifth harmonic generation from a single source

All-fiber fourth and fifth harmonic generation from a single source is demonstrated experimentally and analyzed theoretically. Light from a fully fiberized high power master oscillator power amplifier is launched into a periodically poled silica fiber generating the second harmonic. The output is then sent through two optical microfibers that generate the third and fourth harmonic, respectively, via four wave mixing (FWM). For a large range of pump wavelengths in the silica optical transmission window, phase matched FWM can be achieved in the microfibers at two different diameters with relatively wide fabrication tolerances of up to +/-5 nm. Our simulations indicate that by optimizing the second harmonic generation efficiency and the diameters and lengths of the two microfibers, conversion efficiencies to the fourth harmonic in excess of 25% are theoretically achievable.

Optical fiber poling by induction: analysis by 2D numerical modeling

Since their first demonstration some 25 years ago, thermally poled silica fibers have been used to realize device functions such as electro-optic modulation, switching, polarization-entangled photons, and optical frequency conversion with a number of advantages over bulk free-space components. We have recently developed an innovative induction poling technique that could allow for the development of complex microstructured fiber geometries for highly efficient χ(2)-based device applications. To systematically implement these more advanced poled fiber designs, we report here the development of comprehensive numerical models of the induction poling mechanism itself via two-dimensional (2D) simulations of ion migration and space-charge region formation using finite element analysis.

Phase matched parametric amplification via four-wave mixing in optical microfibers

Four-wave mixing (FWM) based parametric amplification in optical microfibers (OMFs) is demonstrated over a wavelength range of over 1000 nm by exploiting their tailorable dispersion characteristics to achieve phase matching. Simulations indicate that for any set of wavelengths satisfying the FWM energy conservation condition there are two diameters at which phase matching in the fundamental mode can occur. Experiments with a high-power pulsed source working in conjunction with a periodically poled silica fiber (PPSF), producing both fundamental and second harmonic signals, are undertaken to investigate the possibility of FWM parametric amplification in OMFs. Large increases of idler output power at the third harmonic wavelength were recorded for diameters close to the two phase matching diameters. A total amplification of more than 25 dB from the initial signal was observed in a 6 mm long optical microfiber, after accounting for the thermal drift of the PPSF and other losses in the system.

Optical fiber poling by induction

Conventional thermal poling methods require direct physical contact to internal fiber electrodes. Here, we report an indirect electrostatic induction technique using electrically floating wires inside the fiber combined with external electric fields that can allow for facile poling of complex microstructured fibers (MOFs) of arbitrarily long lengths. In combination with our unique ability to use liquid gallium electrodes, inducing second-order nonlinearities inside otherwise difficult to access multi-core or multi-hole MOFs now becomes entirely feasible and practical. The formation of a permanent second-order nonlinearity is unequivocally demonstrated by realizing quasi-phase-matched frequency doublers using periodic UV erasure methods in the induction-poled fibers. The second-order susceptibility created inside the fiber is driven by the potential difference established between the floating electrodes, which we calculate via numerical simulations.

All-fiber frequency-doubled visible laser

All-fiber ns-pulsed visible laser at λ=521  nm is realized by frequency doubling an Yb-doped fiber laser with a periodically poled silica fiber. A 50-mW second-harmonic (SH) output power is produced that is over 6-orders of magnitude greater than previous results obtained with poled fibers in the visible spectral range. The normalized conversion efficiency of 0.3%/W is to date the largest demonstrated with poled fiber technology. Furthermore, 21% conversion efficiency is achieved for the doubling of 8-ps pulses from a neodymium-doped yttrium vanadate solid-state laser. The advances are made possible by the precision and flexibility offered by using the continuous periodic UV erasure, as opposite to photolithographic methods, for the fabrication of over 20-cm-long χ(2)-gratings for quasi-phase matched SH generation.

Poled-fiber source of broadband polarization-entangled photon pairs

We demonstrate broadband polarization-entangled photon pair generation in a poled fiber phase matched for Type II downconversion in the 1.5 μm telecom band. Even with signal-idler separation greater than 100 nm, we observe fringe visibilities greater than 97% and tangle greater than 0.8. A Hong-Ou-Mandel interference experiment is also used to experimentally confirm the broadband nature of the entanglement.

Femtosecond versus picosecond laser machining of nano-gratings and micro-channels in silica glass

The ability of 8 picosecond pulse lasers for three dimensional direct-writing in the bulk of transparent dielectrics is assessed through a comparative study with a femtosecond laser delivering 600 fs pulses. The comparison addresses two main applications: the fabrication of birefringent optical elements and two-step machining by laser exposure and post-processing by chemical etching. Formation of self-organized nano-gratings in glass by ps-pulses is demonstrated. Differential etching between ps-laser exposed regions and unexposed silica is observed. Despite attaining values of retardance (>100 nm) and etching rate (2 μm/min) similar to fs pulses, ps pulses are found unsuitable for bulk machining in silica glass primarily due to the build-up of a stress field causing scattering, cracks and non-homogeneous etching. Additionally, we show that the so-called "quill-effect", that is the dependence of the laser damage from the direction of writing, occurs also for ps-pulse laser machining. Finally, an opposite dependence of the retardance from the intra-pulse distance is observed for fs- and ps-laser direct writing.

Tunable, continuous-wave Ti:sapphire channel waveguide lasers written by femtosecond and picosecond laser pulses

Fabrication and cw lasing at 798.25 nm is reported for femtosecond (fs) and picosecond (ps) laser-inscribed channel waveguides in Ti:sapphire crystals. Lasing in channels written by fs (ps) pulses was obtained above a threshold of 84 mW (189 mW) with a maximum output power and a slope efficiency of 143 mW (45 mW) and 23.5% (7.1%), respectively. The emission wavelength was tuned over a 170 nm range by using a birefringent filter in an external cavity.

Direct generation of polarization-entangled photon pairs in a poled fiber

We experimentally demonstrate the direct generation of polarization-entangled photon pairs in an optical fiber at room temperature by exploiting type-II phase-matched spontaneous parametric down-conversion. A second-order nonlinearity is artificially induced in the 17-cm-long weakly birefringent step-index fiber through the process of thermal poling, and quasi-phase-matching allows for the generation of entangled photons in the 1.5-micron telecom band when the fiber is pumped at 775 nm. A greater-than 80:1 coincidence-to-accidental ratio is achieved, limited mainly by multiphoton pair generation. Without the need to subtract accidentals or to compensate for walk-off, the raw two-photon interference visibility is found to be better than 95%, and violation of Bell's inequality is observed by more than 18 standard deviations. This makes for a truly alignment-free, plug-and-play source of polarization-entangled photon pairs.

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.

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.

Low loss photonic components in high index bismuth borate glass by femtosecond laser direct writing

Single mode, low loss waveguides were fabricated in high index bismuth borate glass by femtosecond laser direct writing. A specific set of writing parameters leading to waveguides perfectly mode matched to standard single-mode fibers at 1.55 microm with an overall insertion loss of approximately 1 dB and with propagation loss below 0.2 dB/cm was identified. Photonic components such as Y-splitters and directional couplers were also demonstrated. A close agreement between their performances and theoretical predictions based upon the characterization of the waveguide properties is shown. Finally, the nonlinear refractive index of the waveguides has been measured to be 6.6 x 10(-15) cm(2)/W by analyzing self-phase modulation of the propagating femtosecond laser pulse at the wavelength of 1.46 microm. Broadening of the transmitted light source as large as 500 nm was demonstrated through a waveguide with the length of 1.8 cm.

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.

Photon pair source based on parametric fluorescence in periodically poled twin-hole silica fiber

We present observations of quasi-phase matched parametric fluorescence in a periodically poled twin-hole silica fiber. The phase matching condition in the fiber enables the generation of a degenerate signal field in the fiber-optic communication band centered on 1556 nm. We performed coincidence measurements and a Hong-Ou-Mandel experiment to validate that the signal arises from photon pairs. A coincidence peak with a signal to noise ratio (SNR) of 4 using 43 mW of pump power and a Hong-Ou-Mandel dip showing 40% net visibility were measured. Moreover, the experiments were performed with standard single mode fibers spliced at both ends of the poled section, which makes this source easy to integrate in fiber-optic quantum communication applications.