Silicon-nitride microring resonators for nonlinear optical and biosensing applications.

We discuss the design, fabrication, and characterization of silicon-nitride microring resonators for nonlinear-photonic and biosensing device applications. The first part presents new theoretical and experimental results that overcome highly normal dispersion of silicon-nitride microresonators by adding a dispersive coupler. The latter parts review our work on highly efficient second-order nonlinear interaction in a hybrid silicon-nitride slot waveguide with nonlinear polymer cladding and silicon-nitride microring application as a biosensor for human stress indicator neuropeptide Y at the nanomolar level.

Measurement of effective nonlinear coefficients in few-mode fibers.

Because of random mode coupling, the nonlinear coefficient in few-mode fibers (FMFs) is averaged to an effective value, which can be theoretically modeled and calculated by using the multi-mode Manakov equations. In this Letter, we experimentally measure the effective nonlinear coefficients in a 530-m-long FMF supporting two mode groups, namely, the $ {\rm LP}_{01} $LP01 and $ {\rm LP}_{11} $LP11 mode groups, by exploiting the self-phase and cross-phase modulations of pulsed fields. By using the nonlinear coefficient of the $ {\rm LP}_{01} $LP01 mode as a reference and comparing the spectral broadening of the pulsed fields, we obtain the intra-modal effective nonlinear coefficient of the $ {\rm LP}_{11} $LP11 mode and the inter-modal effective nonlinear coefficient between the $ {\rm LP}_{01} $LP01 and $ {\rm LP}_{11} $LP11 modes. The experimental results are in good agreement with the theoretical predictions of the multi-mode Manakov equations.

All-optical regenerator of multi-channel signals.

One of the main reasons why nonlinear-optical signal processing (regeneration, logic, etc.) has not yet become a practical alternative to electronic processing is that the all-optical elements with nonlinear input-output relationship have remained inherently single-channel devices (just like their electronic counterparts) and, hence, cannot fully utilise the parallel processing potential of optical fibres and amplifiers. The nonlinear input-output transfer function requires strong optical nonlinearity, e.g. self-phase modulation, which, for fundamental reasons, is always accompanied by cross-phase modulation and four-wave mixing. In processing multiple wavelength-division-multiplexing channels, large cross-phase modulation and four-wave mixing crosstalks among the channels destroy signal quality. Here we describe a solution to this problem: an optical signal processor employing a group-delay-managed nonlinear medium where strong self-phase modulation is achieved without such nonlinear crosstalk. We demonstrate, for the first time to our knowledge, simultaneous all-optical regeneration of up to 16 wavelength-division-multiplexing channels by one device. This multi-channel concept can be extended to other nonlinear-optical processing schemes.Nonlinear optical processing devices are not yet fully practical as they are single channel. Here the authors demonstrate all-optical regeneration of up to 16 channels by one device, employing a group-delay-managed nonlinear medium where strong self-phase modulation is achieved without nonlinear inter-channel crosstalk.

Spatially-resolved self-heterodyne spectroscopy of lateral modes of broad-area laser diodes.

We investigate the fine structure of the optical spectrum of a broad-area laser diode with approximately 30-MHz resolution using spatially-resolved self-heterodyning technique. We show that this method is capable of measuring the relative powers and spacings of the individual lateral modes.

Fundamental eigenmode of traveling-wave phase-sensitive optical parametric amplifier: experimental generation and verification.

We investigate experimentally the eigenmodes of a Gaussian-beam-pumped traveling-wave phase-sensitive optical parametric amplifier (PSA). By varying the waist of an input LG(00) signal mode, we show that PSA performance improves with increasing spatial overlap between the input and the theoretically predicted fundamental eigenmode. For optimum waist, we report amplification and deamplification markedly higher than those observed for the traditional case of signal waist=√2× (pump waist). Lastly, we demonstrate the generation and verification of the PSA fundamental eigenmode.

Optimization of gain in traveling-wave optical parametric amplifiers by tuning the offset between pump- and signal-waist locations.

We present experimental demonstration and modeling of the optimization of a phase-sensitive optical parametric amplifier by tuning the relative position between the pump and signal beam waists along the propagation direction. At the optimum position, the pump beam focuses after the signal beam, and this departure from colocated waists increases with increasing pump power. Such optimization leads to more than 3 dB improvement in the measured deamplification response of the amplifier.

Compact representation of the spatial modes of a phase-sensitive image amplifier.

We compute the eigenmodes of a spatially-broadband optical parametric amplifier with elliptical Gaussian pump and show that the well-amplified eigenmodes can be compactly represented by a low-dimensional subspace of the first few Laguerre- or Hermite-Gaussian (LG or HG, respectively) modes of an appropriate waist size. We also show that the first few eigenmodes are well matched to single LG or HG modes. For sufficiently large pump waists, the optimum waist size of the compact basis is in the vicinity of the geometric average of the pump waist size and the inverse spatial bandwidth of the nonlinear crystal in the parametric amplifier. The use of such compact representation can greatly simplify numerical computation of the spatial eigenmodes of the amplifier and thus lead to improving the experiments on traveling-wave image amplification and spatially-broadband vacuum squeezing.

Frequency up-conversion of quantum images.

We develop the theory of frequency up-conversion of an arbitrary spatially-broadband quantum state (quantum image), deriving the analytical solutions for the cases of plane-wave pump and short crystal with arbitrary pump. By using an example of the quantum image imposed by the orbital angular momentum of the pump beam in spontaneous parametric down-conversion, we show that 99%-fidelity up-conversion of quantum images from an infrared wavelength to the visible wavelength can be obtained in periodically-poled lithium-niobate crystals at reasonable pump intensities well below the crystal damage threshold.

Spontaneous decay of CdSe/ZnS core-shell quantum dots at the air-dielectric interface.

We report measurements of the fluorescence decay times of CdSe/ZnS core-shell quantum dots at the air-dielectric interface for several dielectrics with different refractive indices. The results are in agreement with a simple theory that accounts for the impact of the refractive index on the density of states and magnitude of the vacuum field, as well as for the local-field correction inside the quantum dot. The results suggest that, by embedding the quantum dots into a high-index dielectric material, one can reduce the spontaneous decay time to sub-nanosecond scale while preserving high quantum efficiency.

NALM-based, phase-preserving 2R regenerator of high-duty-cycle pulses.

We explore the potential of the nonlinear amplifying loop mirror (NALM)-based phase-preserving 2R (reamplification and reshaping) regenerator for simultaneous regeneration of multiple wavelength-division-multiplexed (WDM) channels. While not considering nonlinear multi-channel propagation, we address two issues of the phase-preserving NALM that appear to us as the major obstacles in adopting it for realistic WDM applications: a high operating power and a detrimental effect of non-small (33% - 50%) pulse duty cycles. After thorough optimization, we find a new operating regime of this regenerator with the non-small duty-cycle capability and approximately an order of magnitude reduction of the required operating power. In addition, we show that the plateau in the input-output power transfer curve does not automatically lead to the reduction of the amplitude jitter, which is particularly noticeable for the non-small duty-cycle pulses.

Spatial modes of phase-sensitive parametric image amplifiers with circular and elliptical Gaussian pumps.

We develop a method for finding the number and shapes of the independently squeezed or amplified modes of a spatially-broadband, travelling-wave, frequency- and polarization-degenerate optical parametric amplifier in the general case of an elliptical Gaussian pump. The obtained results show that for tightly focused pump only one mode is squeezed, and this mode has a Gaussian TEM(00) shape. For larger pump spot sizes that support multiple modes, the shapes of the most-amplified modes are close to Hermite- or Laguerre-Gaussian profiles. These results can be used to generate matched local oscillators for detecting high amounts of squeezing and to design parametric image amplifiers that introduce minimal distortion.

Modeling and measurement of the noise figure of a cascaded non-degenerate phase-sensitive parametric amplifier.

Semi-classical noise characteristics are derived for the cascade of a non-degenerate phase-insensitive (PI) and a phase-sensitive (PS) fiber optical parametric amplifier (FOPA). The analysis is proved to be consistent with the quantum theory under the large-photon number assumption. Based on this, we show that the noise figure (NF) of the PS-FOPA at the second stage can be obtained via relative-intensity-noise (RIN) subtraction method after averaging the signal and idler NFs. Negative signal and idler NFs are measured, and <2 dB NF at >16 dB PS gain is estimated when considering the combined signal and idler input, which is believed to be the lowest measured NF of a non-degenerate PS amplifier to this date. The limitation of the RIN subtraction method attributed to pump transferred noise and Raman phonon induced noise is also discussed.

Phase-sensitive image amplification with elliptical Gaussian pump.

We numerically analyze phase-sensitive parametric amplification and de-amplification of a multimode field representing a multi-pixel text image. We optimize pumping configuration and demonstrate that approximately 10-dB gain is achievable with relatively moderate approximately 10- kW total pump peak power available from compact pump sources.

A comparative study of noisy signal evolution in 2R all-optical regenerators with normal and anomalous average dispersions using an accelerated Multicanonical Monte Carlo method.

In [Opt. Express 15, 10061 (2007)] we proposed a new regime of multichannel all-optical regeneration that required anomalous average dispersion. This regime is superior to the previously studied normal-dispersion regime when signal distortions are deterministic in their temporal shape. However, there was a concern that the regenerator with anomalous average dispersion may be prone to noise amplification via modulational instability. Here, we show that this, in general, is not the case. Moreover, in the range of input powers that is of interest for multichannel regeneration, the device with anomalous average dispersion may even provide less noise amplification than the one with normal dispersion. These results are obtained with an improved version of the parallelized modification of the Multicanonical Monte Carlo method proposed in [IEEE J. Sel. Topics Quantum Electron. 14, 599 (2008)].

In-line phase-sensitive amplification of multi-channel CW signals based on frequency nondegenerate four-wave-mixing in fiber.

We demonstrate phase-sensitive amplification of multiple wavelength-division-multiplexed continuous-wave (CW) signals by frequency nondegenerate four-wave-mixing process in optical fiber. By fine-tuning the optical wavelengths of the CW signals, simultaneous phase-sensitive in-line amplification of three signal channels is realized. This indicates the possibility of amplifying multiple data channels by an in-line phase-sensitive fiber parametric amplifier. We also discuss a potential system architecture employing such amplifiers.

A new robust regime for a dispersion-managed multichannel 2R regenerator.

We study the performance of a multichannel version [M. Vasilyev and T.I. Lakoba, Opt. Lett. 30, 1458 (2005)] of the all-optical Mamyshev regenerator in a practically important situation where one of its key components - a periodic-group-delay device - has a realistic amplitude characteristic of a bandpass filter. We show that in this case, the regenerator can no longer operate in the regime reported in our original paper. Instead, we have found a new regime in which the regenerator's performance is robust not only to such filtering, but also to considerable variations of regenerator parameters. In this regime, the average dispersion of the regenerator must be (relatively) large and anomalous, in constrast to what was considered in all earlier studies of such (single-channel) regenerators based on spectral broadening followed by off-center filtering. In addition, hardware implementation of a regenerator in the new regime is somewhat simpler than that in the original regime.

All-optical multichannel 2R regeneration in a fiber-based device.

We propose the design of an all-optical 2R regenerator capable of handling multiple wavelength-division-multiplexed channels simultaneously. It extends the known concept of off-center filtering of self-phase-modulation-broadened signal spectra. The novel feature of the proposed device is a dispersion map that strongly suppresses interchannel impairments. The map employs several sections of nonlinear fiber with high normal dispersion, separated by dispersion compensators with spectrally periodic group delay. The results of our numerical simulations indicate the feasibility of such a multichannel regenerator.

Distributed phase-sensitive amplification.

We propose a phase-sensitive amplifier scheme that balances fiber loss and parametric gain everywhere in a fiber span. We show that, for long links, such a distributed phase-sensitive amplifier has a 3-dB lower noise figure than an ideal distributed phase-insensitive amplifier (e.g. Raman), even if simple direct detection is employed. This sets the ultimate limit for the optimum noise-nonlinearity trade-off in transmission systems.

Gain characteristics of a frequency nondegenerate phase-sensitive fiber-optic parametric amplifier with phase self-stabilized input.

We experimentally demonstrate, for the first time to our knowledge, a phase-sensitive amplifier based on frequency nondegenerate parametric amplification in optical fiber, where the input signal-idler pair is prepared all-optically. Using two fiber-optic parametric amplifier sections separated by a fiber-based wavelength-dependent phase shifter, we observe and investigate phase-sensitive gain profile in the 1550 nm region both experimentally and theoretically. The realized scheme automatically generates gain-defining phase that is environmentally stable, making it advantageous for building phase-sensitive transmission links.