Broadband Frequency Conversion Research Articles

Adiabatic second-harmonic generation.

Adiabatic three-wave mixing processes enable broadband, efficient, and robust frequency conversion by slowly varying the phase mismatch between the interacting waves along the interaction region. Up until now, this method was mainly used in the case in which one of the waves was undepleted. Here we experimentally study fully nonlinear adiabatic processes by implementation in type I and type II second-harmonic generation processes, where the undepleted pump approximation does not hold. Using quasi-phase-matched interaction in chirped gratings, we obtain conversion efficiency approaching 60% and 80%, with corresponding wide thermal acceptance bandwidths of >100°C and 30°C, respectively. The transition between the depleted and undepleted pump regimes is also studied by varying the input polarization angle in the type II process; thus we also test current theory with arbitrary initial conditions. The results are in excellent agreement with analytic predictions for the fully nonlinear adiabatic process.

Theory of second-harmonic generation in a chirped 2D nonlinear optical superlattice under nonlinear Raman-Nath diffraction

We analyze second-harmonic generation (SHG) in a two-dimensional nonlinear optical superlattice (NLOS) with its modulation period being chirped in the propagation direction and constant in the transverse direction. This results in efficient multiple SHG via nonlinear Raman–Nath diffraction. We obtain exact analytical expressions for a SH amplitude generated in chirped 2D NLOSs and for its quasi-phase-matching bandwidth. The results of analytical calculations are in excellent agreement with the numerical ones. We show that the process is robust to angular deviations of NLOS and it can be applied to enable tunable and broadband frequency conversion.

Infrared supercontinuum frequency doubling via nonlinear Raman-Nath and Cerenkov scattering processes

We consider experimentally the frequency doubling of a near infrared supercontinuum light in a periodically poled lithium niobate crystal. The supercontinuum light was generated by a photonic crystal fiber and then filtered by an infrared passband filter, in order to obtain a spectrum from 1100nm to 1500nm. Once that spectrum was injected in the crystal, we observed a broadband frequency conversion towards the visible domain between 560nm and 700nm. Our goal is to demonstrate the capability of the spatio-temporal quadratic unidirectional pulse propagation model, providing insight into the complex experimental observations. We enlighten the role of Cerenkov second-harmonic emission, nonlinear Raman-Nath and nonlinear Bragg scattering processes in the broadband frequency conversion from infrared to visible light.

Broadband frequency conversion of laser radiation in ZnGeP2 crystal

The phase matching conditions in the ZnGeP2 crystal are calculated and analyzed from the viewpoint of broadband frequency conversion of mid-IR radiation. The frequency conversion of Cr:ZnS laser radiation in the ZnGeP2 crystal is calculated. The results show the possibility of developing a broadband laser source whose spectrum spans a wavelength range of 3.2–9.5 µm.

Use of tunable second‐harmonic signal from KNbO3 nanoneedles to find optimal wavelength for deep‐tissue imaging

AbstractNonlinear optical (NLO) responses of perovskite‐type nanostructures have a variety of potential applications owing to the highly efficient frequency conversion guaranteed by both the material itself and the nanometer‐scale configuration. KNbO3 (KN) nanoneedles have been identified as a promising NLO material because of the superior broadband frequency conversion efficiency, and if incident light is propagating in a direction perpendicular to the axis of a nanoneedle, then the phase‐matching constraint can be relaxed. Here, the second‐harmonic generation (SHG) and third‐harmonic generation (THG) responses of both individual and clustered KN nanoneedles are reported. Based on these results, a novel method is proposed for determining the optimal excitation wavelength for NLO imaging of several biological samples, with KN nanoneedles acting as NLO agents. The method is shown to provide the optical features in the focal plane and a more reliable estimation of the optimal excitation wavelength for deep‐tissue imaging.

Robust and broadband frequency conversion in composite crystals with tailored segment widths and χ(2) nonlinearities of alternating signs.

We propose an efficient, robust, and broadband nonlinear optical frequency conversion technique, which uses segmented crystals constructed in analogy with the composite pulses in nuclear magnetic resonance and quantum optics. The composite crystals are made of several macroscopic segments of nonlinear susceptibilities of opposite signs and specific thicknesses, which are determined from the condition to maximize the conversion efficiency with respect to variations in the experimental parameters. These crystals deliver broadband operation for significantly lower pump intensities than single bulk crystals. We demonstrate this technique by numerical simulation of sum-frequency generation in MgO:LiNbO3 crystal.

Adiabatic processes in frequency conversion

Adiabatic evolution, an important dynamical process in a variety of classical and quantum systems providing a robust way of steering a system into a desired state [1-5], was introduced only recently to frequency conversion [6-9]. Adiabatic frequency conversion allowed the achievement of efficient scalable broadband frequency conversion [8, 9] and was applied successfully to the conversion of ultrashort pulses, demonstrating near-100% efficiency for ultrabroadband spectrum [10-16]. The underlying analogy between undepleted pump nonlinear processes and coherently excited quantum systems was extended in the past few years to multi-level quantum systems, demonstrating new concepts in frequency conversion, such as complete frequency conversion through an absorption band [17-20]. Additionally, the undepleted pump restriction was removed, enabling the exploration of adiabatic processes in the fully nonlinear dynamics regime of nonlinear optics [21-26]. In this article, the basic concept of adiabatic frequency conversion is introduced, and recent advances in ultrashort physics, multi-process systems, and the fully nonlinear dynamics regime are reviewed.

Efficient, broadband, and robust frequency conversion by fully nonlinear adiabatic three-wave mixing

A comprehensive physical model of adiabatic three-wave mixing is developed for the fully nonlinear regime, i.e., without making the undepleted pump approximation. The conditions for adiabatic evolution are rigorously derived, together with an estimate of the bandwidth of the process. Furthermore, these processes are shown to be robust and efficient. Finally, numerical simulations demonstrate adiabatic frequency conversion in a wide variety of physically attainable configurations.

Efficient broadband frequency conversion via simultaneous adiabatic three wave mixing processes

Two frequency-cascaded and spatially simultaneous three-wave-mixing processes, each pumped by a different frequency, are studied in a quasi-phase-matching crystal with chirped poling. Theory, numerical simulation, and experiment all show that efficient two-process adiabatic frequency conversion takes place over a broad range of input wavelengths. Experimentally, up to five process cascades are obtained. Furthermore, it is demonstrated that reversing the chirp sign results in a different conversion for the same input wavelength. This method can be applied to generation of an all-optically tunable frequency comb.

Broadband frequency conversion and shaping of single photons emitted from a nonlinear cavity

Much recent effort has focused on coupling individual quantum emitters to optical microcavities in order to produce single photons on demand, enable single-photon optical switching, and implement functional nodes of a quantum network. Techniques to control the bandwidth and frequency of the outgoing single photons are of practical importance, allowing direct emission into telecommunications wavelengths and "hybrid" quantum networks incorporating different emitters. Here, we describe an integrated approach involving a quantum emitter coupled to a nonlinear optical resonator, in which the emission wavelength and pulse shape are controlled using the intra-cavity nonlinearity. Our scheme is general in nature, and demonstrates how the photonic environment of a quantum emitter can be tailored to determine the emission properties. As specific examples, we discuss a high Q-factor, TE-TM double-mode photonic crystal cavity design that allows for direct generation of single photons at telecom wavelengths (1425 nm) starting from an InAs/GaAs quantum dot with a 950 nm transition wavelength, and a scheme for direct optical coupling between such a quantum dot and a diamond nitrogen-vacancy center at 637 nm.

Ultra-broadband optical parametric amplification by tailoring the group-velocity dispersion of Bragg reflection waveguides

The possibility of extending the signal-acceptance bandwidth for a non-degenerate optical parametric amplification process using a non-linear Bragg reflection waveguide (BRW) is explored. The strongly dispersive features of BRWs made up of semiconductors, such as GaN/AlxGa1−x N, are employed to maintain the phase-matching condition over a broad range of signal wavelengths. It is found that the signal-acceptance bandwidth could extend over 20 THz in the optical communication band (∼193 THz) which could facilitate simultaneous parametric amplification and wavelength conversion across the entire S–C–L band. Moreover, this idea could also find important applications in optical communication schemes involving ultra-short pulses. The broad bandwidth is essentially a direct consequence of appropriate phase- and group-velocity dispersion compensation by the idler mode in order to preserve the phase-matching condition over a broad spectral range. Since the idea exploits the dispersive features of Bragg reflection based geometries, the scheme could be implemented to realize broadband frequency conversion in any desired spectral region within the constraints imposed by the transparency of non-linear materials and the possibility of periodic poling.

Broad-band continuous-wave parametric wavelength conversion in silicon nanowaveguides

We demonstrate highly broad-band frequency conversion via four-wave mixing in silicon nanowaveguides. Through appropriate engineering of the waveguide dimensions, conversion bandwidths greater than 150 nm are achieved and peak conversion efficiencies of -9.6 dB are demonstrated. Furthermore, utilizing fourth-order dispersion, wave-length conversion across four telecommunication bands from 1477 nm (S-band) to 1672 nm (U-band) is demonstrated with an efficiency of -12 dB.

Consideration for Broadband Frequency Conversion Based on Ba0.5Sr0.5TiO3 Films

A frequency converter based on a BaxSr1-xTiO3 (x=0.5) (BST) film was successfully realized using the nonlinearity of the film. Frequency conversion occurred due to the distortion of a signal across the BST film, because the relation between electric charge and external bias is nonlinear in the film. Frequency conversion at low frequency from 2 to 6 MHz and 4 to 12 MHz was realized using a parallel-plate BST capacitor. Frequency conversion at high frequency from 500 MHz to 1 GHz was demonstrated, yielding a conversion loss of 30 dB at an input power of 19 dBm. The conversion loss of the frequency converter can be improved by increasing the input power on a BST film.

SIGNAL PROCESSING IN PHOTONIC CRYSTALS AND NANOSTRUCTURES

Optical devices employing photonic crystals and novel nanostructure materials may exhibit useful properties for applications to all-optical signal processing. In this work we analyze as a first example four-wave mixing of polarized beams in photonic crystal fibers. We show that by properly tuning the pump wavelength and the linear dispersion properties of the fiber one may obtain broadband parametric amplification and frequency conversion. Next we consider the in-line periodic amplification of short optical pulses by means of quantum-dot semiconductor optical amplifiers. We show by numerical simulations that pattern-free amplification of a 40 Gbit/s soliton signal at 1300 nm is possible without any inter-symbol interference or nonlinear pulse distortion caused by the fast gain dynamics.

TDFA/FOPA hybrid for broadband amplification and frequency conversion in optical communications

A serial combination of a Tm3+-doped fibre and a fibre-optic parametric amplifier is explored for broadband signal amplification and frequency conversion. Amplification of the entire S-band and frequency conversion into the C- and L-band are achieved. Performance limitations are discussed and improved configurations proposed.

Analysis of Possible Group-Velocity-Matched Broadband Second-Harmonic Generation in Various Periodically Poled Nonlinear Crystals

For the efficient second-harmonic generation of ultrashort pulses in wide spectral ranges, we theoretically studied all possible group velocity matched broadband frequency conversion processes in different periodically poled materials, using quasi-phase matching. Important parameters such as the poling period, type of interaction, spectral bandwidth and conversion efficiency are estimated in detail. The results show that some materials can be efficiently applied to the generation of sub-100 fs pulses near the wavelengths of a widely used Ti:sapphire laser.

Miro´: Complete modeling and software for pulse amplification and propagation in high-power laser systems

This paper presents some of the most important features of the MirO code, which is a laser propagation program for high-peak-power laser setups. The main propagation models are discussed: the paraxial nonlinear Schrodinger approximation, broadband computation including dispersion effects, and spatial and temporal Talanov transforms are described, as well as the main optical components: a four-level multipass amplifier including both saturation and narrowing effects; broadband and noncollinear frequency conversion; the grating, including high-order dispersion effects; and laser damage effects. Some elementary examples of MirO computation are also provided.

Raman-assisted three-wave mixing of non-phase-matched waves in optical fibres: application to wide-range frequency conversion

We analyse theoretically and experimentally the Raman-assisted parametric coupling between non-phase-matched waves propagating in normally dispersive single-mode fibres. We perform a careful analysis of the wave-coupling behaviour, which shows that scalar and vector three-wave mixing (TWM) interactions induce a relatively small periodic power flow between a central-frequency pump at frequency ω 0 and a pair of up-shifted (anti-Stokes) and down-shifted (Stokes) sidebands at frequencies ω 0+Ω and ω 0−Ω , respectively. For sufficiently high pump powers, the stimulated Raman scattering enters into play, causing a unilateral transfer of energy from higher to lower frequency waves. This energy transfer destroys the spatial periodicity of the parametric energy-exchange process. As a result, parametric seeding and subsequent Raman amplification of a Stokes idler wave is achieved by mixing a strong pump with a weak anti-Stokes signal. This Raman-induced Stokes power-gain enhancement leads to efficient anti-Stokes → Stokes frequency conversion, with frequency detunings which can be relatively large (typically, from 7 to 30 THz), even for very short parametric coherence. Raman-assisted TWM thus overcomes the strict spectral limitation usually imposed by the phase-matching condition, leading to broadband frequency conversion processes that are inaccessible with a pure parametric interaction.

A new technique for the generation of television test signals in all bands

The paper deals with a new technique for the generation of r.f. signals for television testing. A signal generator tunable continuously through all the television bands is described. The operating principle is based on the generation of sound- and vision-modulated test signals at the standard intermediate frequency followed by broadband frequency conversion to the range 25 to 1000 MHz. The method is justified in a description of the technique of up-conversion to a high intermediate frequency with subsequent broadband down-conversion. This is followed by a presentation of the specification necessary for the local conversion oscillator, together with its operating principle and electrical characteristics. The method of frequency stabilization, indication and tuning is looked at in greater depth. In conclusion, the characteristics of the overall system are discussed, including a description of the modulator section, and the wide range of applications indicated.