Experimental results on parametric optical processes in saturable ytterbium-doped fibers (YDFs) at the wavelength of 1064 nm are reported. Significant photoinduced change of the refractive index under saturation of the fiber- optical absorption in the near-infrared range enables us to consider this medium as an effective optical Kerr ( chi 000 ) medium. For pumping, it needs continuous-wave Nd:YAG laser power of 10-mW scale, but the saturation process has the characteristic time <= 1 ms, governed by the metastable level lifetime in YDF. We investigate the nearly degenerate collinear configuration of the optical parametric amplification (OPA). Here we focus on unidirectional transformation of the initial amplitude modulation in the incident pump wave to the output phase modulation as a manifestation of OPA in a narrowband case. This effect results from the conventional coupled-wave equations for the signal and idler waves-if one considers them as the modulation sidebands. It can also be treated as a result of the self-phase modulation of the periodically modulated pump in the Kerr medium. OPA gain of about 0.64 with the net gain approximate to 0.29 was experimentally estimated for the utilized 2-m-long YDF in the frequency band approximate to 200 Hz. (c) 2024 Optica Publishing Group

We present experimental advances in comparative studies of optical parametric amplification (OPA) in microstructured fused silica solid-core fibers and hollow-core fibers filled with acetylene (C2H2). Both media exhibit third-order nonlinearity, enabling the OPA process in collinear configurations with a high spatial concentration of light power. In the former, non-resonant case, we investigated the parametric amplification via four-wave mixing (FWM) with a degenerate pump by picosecond laser pulses centered at a wavelength of 737 nm. This process ensured the generation of the correlated signal/idler photon pairs that could be parametrically amplified in a similar nonlinear micro-structured fiber. For the resonant acetylene-filled fibers, we present an experimental evaluation of the OPA gain in a degenerate collinear FWM at 1530 nm near the P9 acetylene absorption line. We specifically studied the transformation of amplitude modulation in the quasi-continuous W-scale input pump wave to output phase modulation and vice versa. Our research compares OPA efficiencies and the potential to generate squeezed and entangled light states in resonant and non- resonant fiber-based media.

Operation modes of the optoelectronic oscillator (OEO), based on a phase electrooptic modulator with an acetylene reference cell as a photonic filter, have been investigated. For the wideband phase-to-amplitude demodulation that was observed when the laser wavelength was tuned to one side of the acetylene absorption line, an additional tunable electronic yttrium iron garnet (YIG) filter was introduced to observe single-mode OEO generation. This configuration generated a stable monochromatic signal in the 4–12 GHz frequency range, with phase noise of −122 dBc/Hz at 10 kHz offset from the carrier frequency. In the narrowband demodulation mode (when one optical modulation sideband was tuned to the acetylene absorption line), the quasi-single-mode oscillation could be observed without additional electronic filtering. In this case, the generation frequency was controlled optically by tuning the laser wavelength.

We report results on characterization of the self-reference interferometric configuration based on quantum memory of the acetylene (12C2H2) molecules at the wavelength of 1530.37 nm of the vibrational–rotational P9 absorption line. The demodulator under consideration is robust, simple, operates at room temperature with the wavefronts of high etendue of the sub-mW power scale, and needs neither phase, no frequency locking loops. Proposed original balance detection schemes enable effective suppression of the semiconductor laser phase noise, and the shot-noise-limited demodulation resolution at frequencies higher than the high-pass configuration cut-off frequency ≈0.25 GHz.

We report polarization ellipse self-rotation and polarization hole-burning in the acetylene-filled hollow-core photonic crystal fiber in spectral range 1520–1540 nm. P5 oscillation rotation line demonstrated the strongest results that are in agreement with classical model.

We report experimental measurements of noises and resolution in a bulk self-referencing interferometric configuration based on quantum memory of acetylene (C2H2) molecules at the wavelength of 1530.37 nm of P9 absorption line.

An adaptive optical fiber sensor/demodulator of an optical phase modulation with a Sagnac interferometer configuration is reported. The dynamic population grating recorded in ytterbium-doped fiber (YDF) at a wavelength of 1064 nm enables adaptive properties of this configuration with a high-pass transfer function and with the cut-off frequency of about 260 Hz at ∼10mW cw recording power. A linear response with nearly 100% modulation depth is ensured by effective formation of the nonshifted phase dynamic grating with the amplitude one order of magnitude greater than can be expected from the saturation of the YDF absorption at the recording wavelength. This is associated with the photoinduced changes in the UV optical absorption of the YDF and enables minimal detected amplitude of the phase modulation ≈0.7∗10-7Hz in our experimental configuration. We believe that, in general, this mechanism of the phase grating formation can ensure the sensor resolution limited by the photonic noise of the utilized light power only.

We report on the first experimental observation of the polarization ellipse self-rotation (PSR) and the polarization hole burning (PHB) nonlinear optical effects for the acetylene ( C 2 H 2 ) vibrational–rotational transitions in the 1520–1530 nm spectral range. The experiments were performed using a 1-m-long acetylene-filled hollow-core photonic crystal fiber cell at room temperature and gas pressure of 0.4–4 Torr. The PHB effect was experimentally observed as a photo-induced input polarization ellipticity change in the output light. Among all the investigated acetylene transitions P9, P7, P5, and P3, the maximum ellipticity change of ∼ 35 % was observed at zero detuning from the center of the P5 absorption line with the initial optical absorbance about 6. The obtained results are in good agreement with the theoretical predictions of a classical analysis based on the model of the acetylene molecules as gyrators (circular oscillators) with random orientations. For the inhomogeneously broadened acetylene absorption P5 line ( ∼ 500 M H z ), the PSR effect observed for tuning at the sides of the P5 line, as expected, proved to be by factor Δ ω i h / Δ ω h (inhomogeneous-to-homogeneous broadening ratio) lower than the maximum PHB. The PSR constant for these conditions proved to be about 0.1 m − 1 , which is about 2 orders of magnitude lower than that reported earlier for the alkali metal (K, Rb) vapors.

The delayed optical nutation effect has been used to evaluate the longitudinal relaxation time(T 1 ) inside an acetylene-filled hollow-core photonic crystal fiber (HC-PCF) at room temperature. In this experiment, sequences of two short optical pulses of 30ns and 15ns in width, respectively, with the maximum peak power up to similar to 2.5W, were generated at a laser wavelength of 1530.37nm, which corresponds to the acetylene ro-vibrational P9 absorption line. The gas pressure inside the 1m-long HC-PCF varied in the range 0.1-1Torr. Numerical simulations based on the optical Bloch equations allowed us to evaluate the longitudinal relaxation time asT 1 similar to 9 mml:mspace width=".1em"mml:mspaceSns, proving to be fundamentally limited by the transit-time broadening. Also, the obtained results revealed the influence of slow and fast molecules in the form of a non-exponential decay of the delayed-nutation signals.

New applications of dynamic population Bragg gratings recorded in saturable ytterbium doped fibers (YDF) by 10mW-scale cw Nd:YAG laser power at 1064nm are considered. In particular, adaptive interferometric Sagnac configuration for detection of optical phase modulation with resolution close to that determined by photon noise is reported. Spectral and nonlinear properties of all-fiber resonance cavity filled with an artificial dispersive media – dynamic Bragg grating in YDF - are also investigated.

Experimental results on effective two-wave mixing (TWM) via dynamic population gratings in saturable erbium doped fibers (EDFs) at liquid nitrogen temperature (77 K) are reported. It is shown that formation of the spectral holes (SHs) by the recording coherent waves in the wavelength region 1500-1540 nm results in a complex dynamics of the transient TWM response. In addition to conventional slow, even-type response due to amplitude grating formation via spectrally uniform EDF absorption saturation we also observed a dominating contribution due to SH formation. In particular, at the recording wavelength 1527 nm the relaxation time of the SH contribution to the TWM process was significantly shorter (< 4.7 ms) and with the effective recording power similar to 4 times lower than that for conventional EDF population grating.

Analysis of influence of the Maxwell distribution of the transverse thermal velocities and of the flight-time-determined characteristic relaxation rates (i.e. the inverse relaxation times T1,2) of the acetylene (C2H2) molecules in the hollow-core photonic crystal fiber on nonlinear optical effects are presented. The theoretical predictions are compared with the experimental data obtained in the ~0.4Torr acetylene-filled fiber cell at the wavelength 1530.37nm of the most effective P9 vibrational-rotational transition of 12C2H2. At room temperature and the fiber mode field diameter of 7.5 μm, the average transverse thermal velocity of ~390m/s ensured relaxation times T1,2 ~8-10ns. These are in good agreement with the corresponding values experimentally measured using delayed optical nutation and two-photon echo techniques. The experimentally observed nonlinear effect of the polarization ellipse self-rotation proves to be at least two orders of magnitude less efficient comparing with that reported earlier for the alkali metals vapors.

Application of the phase memory of acetylene (C2H2) vibrational-rotational transitions in the 1520-1540 nm wavelength range for a self-referencing homodyne detection of a sub-nanosecond optical phase modulation is reported. In the proposed configuration the collinearly propagating coherent dipole radiation of the excited molecule acts like a phase-locked reference wave (local oscillator) that enables transformation of the initial phase modulation into the intensity one. This technique does not need high light intensity and can operate in a linear range of the medium optical absorption. The linear optical phase demodulation (i.e., transformation of the phase-to-amplitude light modulation) is interpreted as an introduction of an additional phase shift to the carrier frequency component of the modulated signal in the maximum of the dispersion curve of a narrow optical absorption peak. It has been experimentally demonstrated with the bulk 10 cm long cuvette filled with low pressure (similar to 2 Torr) acetylene at room temperature. Effective demodulation of the milliwatt-scale incident laser wave of a single-and multi-mode structure is shown. As expected, the response to the fast (< 1 ns) phase modulation was quadratic when the acetylene inhomogeneous Doppler-broadened (similar to 500 MHz) absorption line is excited in its center and was linearized by tuning at one of the absorption line sides. It is of a differentiating (high-pass) type with the cutoff frequency determined by the total spectral width of the utilized absorption line. Expected detection resolution is determined by the photon noise of the incident light intensity. (C) 2019 Optical Society of America

Detection (or demodulation) of the phase modulation is a critical operation in different photonic applications, in particular, in interferometric fiber sensors, material evaluation via laser induced ultrasound, coherent communications etc. Different techniques were proposed to overcome the main problem of conventional interferometers: necessity of stabilization of the quadrature operation point and compensation of the complex spatial structure of the detected wave. Here one can mention, active operation point stabilization via external optoelectronic loop [1], adaptive dynamic gratings based systems [2], self-reference (i.e. without external reference wave) confocal Fabry-Perot cavities [3], and configuration based on quantum phase memory of an ensemble of two-level atoms/molecules [4]. All these seemingly different approaches have the same common feature - they are based on utilization of a narrow absorption/gain spectral profile, artificial, as in the first two above-mentioned techniques, or natural, as in the two last cases. Here we analyse a general case of passing of the phase modulated wave through a narrow absorption/gain spectral line as a technique for its demodulation (i.e. its partial transformation into the intensity modulation wave) with the following detection by conventional photodetector.

We propose to use phase memory of an ensemble of two-level centers, irradiated by the resonantly tuned detected signal wave, for homodyne interferometric detection of fast optical phase modulation. Collinearly propagating dipole radiation of the excited two-level centers acts as a coherent reference wave (local oscillator) necessary to transform the phase modulation into the amplitude modulation behind the resonant medium. The phase of this radiation follows the average phase of the incident detected wave with the characteristic transverse relaxation time T-2 that ensures adaptive properties of this interferometric detection configuration to slowly varying environmental conditions. We present experimental demonstration with the acetylene-filled hollow-core microstructured optical fibers at a wavelength of 1530.37 nm of the acetylene P9 absorption line, which is inside the telecommunications wavelengths range. It is shown that the response is quadratic when the acetylene similar to 500 MHz inhomogeneously broadened absorption line is excited in its center, but can be essentially linearized for excitation at one of the line sides.

We present a new concept of the homodyne interferometric adaptive detection of optical phase modulation. To ensure adaptivity, i.e. stabilization of the interferometer operation point, we utilize the phase memory of a two-level quantum system, resonantly illuminated with the information bearing signal wave. Phase modulation of the transmitted signal wave transforms into the intensity modulation via interference with the collinearly propagating dipole radiation of the excited two-level system. The latter acts like a reference wave since it has a phase corresponding to that of the signal wave but averaged over the transverse relaxation time T-2 of the quantum system. Experimental demonstration with the acetylene-filled hollow-core micro-structured optical fiber at the communication wavelength of 1530nm of the acetylene P9 absorption line is presented. It is shown that the response to the introduced phase modulation is quadratic when the acetylene inhomogeneously broadened absorption line is excited in its center and is a linear one if it is excited at one of the absorption line sides.

We report detailed experimental results on the optical nutation effect in acetylene-filled hollow-core photonic crystal fibers (HC-PCFs). This coherent quantum optical effect is manifested by the appearance of attenuated oscillations in the profiles of transmitted step-like coherent optical pulses. The experiments used 15 ns optical pulses with peak powers up to ∼4 W and centered at 1530.37 nm, which corresponds to the acetylene (12C2H2) vibration-rotation resonance absorption line P9. The all-fiber cell made from a HC-PCF with internal diameter ∼10.3 μm contained a gas pressure of about 0.12 Torr. The Maxwell-Bloch equations are used to analyze the influence of the homogeneous relaxation, the inhomogeneous Doppler broadening of the absorption line, and the random orientation of the acetylene molecules. By comparing the experimental results with numerical simulations, we obtain the longitudinal and the transverse relaxation times to be close to 10 ns and find that the transition dipole moment of the acetylene P9 absorption line is ∼1.36×10−32 Cm.

Experimental results on the transient optical nutation effect inside an acetylene-filled hollow-core photonic crystal fiber (HC-PCF) are reported. The experiments used 15 ns optical pulses with peak powers up to 5 W. The light wavelength was centered at 1530.37 nm, which corresponds to the P9 acetylene (12C2H2) vibrational-rotational absorption line. The gas pressure inside the PCF, with hollow core diameter of ∼10.3 μm, was kept around 0.12 Torr. Comparison of the experimental data with numerical simulations using the Maxwell-Bloch equations allowed us to evaluate the characteristic longitudinal and transverse relaxation times around 10 ns, as well as the transition dipole moment (1.36 × 10−32 Cm).

Two-pulse photon echo in acetylene-filled hollow-core photonic crystal fiber at 1530.37 nm is reported. Initial transverse relaxation time T2≈11 ns, determined by the molecule flight-time across fiber mode diameter, decreased to 7 ns at acetylene pressure 0.6 Torr.

Summary form only given. We propose a method that combines a low loss of the Ti-indiffused technology and passive permanent trimming of DLW. Two techniques of trimming, i.e., (i) micro-inscription of refractive index change and (ii) micro-machining of loading metal films, have been investigated. We demonstrate that a local micro-scale change of refractive index due to LN modification can be efficiently used for precise adjustment of the splitting ratio of waveguide optical splitters and couplers and does not increase the optical loss. The other trimming method is loading of a metal film on top of a waveguide and its treatment with a laser beam, as shown in fig. la. It is based on the dependence of the plazmon polariton excitation efficiency on the metal film thickness. It has been experimentally found that the insertion loss of a single-mode waveguide change from 0.1 dB/mm for 400 nm aluminium loading film to more than 1 6 dB/mm for a substantially thinner one. The laser beam of a Ti: Sapphire femtosecond laser (Avesta Project, λ = 800 nm) with a pulse duration of 100 fs and repetition rate of 80 MHz the average power of which was controlled in the range 20+100 mW was focused by an Olympus objective (40x, NA = 0.75). To position the sample with respect to the laser beam, a precise nanopositioning system was used. The zigzag track on the 100-nm aluminium loading film placed on the single mode Ti-indiffused channel waveguide allowed a precise control of trimming.