Quasi-phase Matched Structure Research Articles

Studies on tunable simultaneous RGB generation in domain engineered magnesium oxide doped periodically poled lithium niobate structures

Simultaneous generation of red (R), green (G), and blue (B) wavelengths in different grating structures of 5% MgO doped domain engineered LiNbO3 is studied theoretically by employing the type-0 quasiphase-matched (QPM) technique. The spectral output from two cascaded, four segmented, and two-phase shifter (PS) QPM structures using second-harmonic generation, difference-frequency generation, sum-frequency generation, and third harmonic generation were analyzed and interpreted. We observed RGB in the range of 620 to 650 nm, 530 to 580 nm, and 430 to 468 nm for red, green, and blue, respectively, for various configurations of input wavelength, period, device length, segment length, location, and width of the PS domain. The influence of the input pump, operating temperature, pump detuning, PS width, and phase matching periods are analyzed for all the proposed schemes. Fluctuation in the output wavelengths of about 10 to 20 nm was observed due to the temperature variations from 25°C to 150°C in steps of 25°C. Further, the advantages of using a MgO-PS QPM structure over the other proposed schemes have also been addressed for their potential application in a laser-based projection display system.

Stimulated Raman Amplification in GaAs / AlAs Intermixed Superlattices

We show the effectiveness of a χ(3)-quasi-phase-matched structure made of a GaAs/AlAs inter-mixed superlattice in stimulated Raman amplification for an enhancement in the output efficiency of anti-Stokes waves, where the efficiency is originally extremely small due to a phase mismatch for the anti-Stokes waves (although the output efficiency of Stokes waves is sufficiently large because of no such phase mismatch). We demonstrate that our quasi-phase matched structure enhances the anti-Stokes output efficiency up to the order of 103 when compared with that without the quasi-phase matched structure, particularly at a small pump intensity. Here, the effects of four-wave mixing, self- and cross-phase modulations are included in addition to Raman amplification with linear optical losses. In this efficiency enhancement, we see that the output efficiency of the anti-Stokes waves is boosted via simultaneous four-wave mixing. Moreover, we show how much the effects of the self- and cross-phase-modulations degrade this efficiency enhancement. With all those effects taken into account, we show that there is an optimal superlattice length that provides the highest efficiency. In addition, we analyze the efficiency degradation due to two-photon absorption, and point out that in order to keep the highest efficiency at the optimal length, the Stokes, anti-Stokes, and pump wavelengths should be set larger than the half-bandgap wavelength of the intermixed superlattice.

Flattop Broadband Wavelength Conversion Based on Cascaded Second-Harmonic Generation and Difference Frequency Generation in Step-Segmented Quasi-Phase Matched Gratings

Bandwidth enhancement and response flattening of wavelength conversion based on single-pass and double-pass cascaded second harmonic generation and difference frequency generation in step-segmented quasi-phase matched (QPM) gratings are investigated. For the same waveguide length, high conversion efficiency, flat response and broad signal bandwidth can be obtained simultaneously by optimizing the poling period of the step-segmented QPM structure. The conversion bandwidths in a 3-cm-long five-step-segmented waveguide are 134 nm for single-pass scheme and 132 nm for the double-pass one, respectively, which are both over the whole conventional band and long-wavelength band. The ripple on the matching response can keeps less than 0.2 dB as well.

A simplified model for quadratic parametric processes considering pump depletion in a layered quasi-phase-matching structure

Quadratic parametric processes in a one-dimensional nonlinear structure were investigated by using a nonlinear transfer matrix and iterative algorithm. Our method can provide an accurate description in the regime of pump depletion. Detailed analysis shows iteration is important for achieving a precise result. In order to simplify the complex iterative derivation, we develop a simplified model for quasi-phase-matched (QPM) second harmonic generation. We extend the simplification strategy to treat other quadratic nonlinear interactions in a QPM structure. The validity of simplification is demonstrated by numerically comparing conversion efficiencies. Our method provides a simple and efficient way to investigate second-order parametric processes in QPM structures.

Fabrication and characterization of Teflon-bonded periodic GaAs structures for THz generation

A novel technique for low-temperature bonding of GaAs wafers using an interboundary Teflon film is developed. A fabricated stack of ten 25x25 mm2 diced wafers demonstrated 75% transmission of a 10 microm CO2 laser beam. Modeling of these Teflon-bonded (TB) structures as sequences of Fabry-Perot etalons gives a good agreement with transmission measurements. A 20x20 mm2 quasi-phase matched structure of five wafers pumped by CO2 laser lines generated the narrow-band THz radiation at a wavelength of 343 microm via a difference frequency mixing process.

Influence of Cascaded Nonlinear Phase Shifts on Second-Harmonic Generation in High-Intensity Pumped QPM Structures

Cascaded nonlinear phase shifts may be imposed on the interacting waves during second-harmonic generation (SHG) in a quasi-phase-matched (QPM) structure, which are severe in high-intensity regime and may result in lower conversion efficiency. We propose a configuration of QPM structure with reduced domain-length, which may depress the nonlinear phase shifts to some extent and lead to an improvement on the conversion efficiency. The numerical analyses on the conversion efficiency as well as the relative phase angle are discussed in detail for better understanding of the SHG process.

Gouy phase compensation in quasi-phase matching

In any focussed nonlinear interaction the focus induced phase shift, known as the Gouy phase shift, provides an imperfection in phase matching for any linearly invariant material. However, using an appropriately designed quasi-phase matched structure it is theoretically possible to compensate for the deleterious effects of the Gouy phase shift, allowing a symmetric frequency response and tighter optimal focussing than in a uniform material.

Generation of tunable blue-green light using ZnO periodically poled lithium niobate crystal fiber by self-cascaded second-order nonlinearity

Using a novel self-cascaded first-order second-harmonic generation (SHG) and third-order sum-frequency generation (SFG) in a ZnO periodically poled lithium niobate crystal fiber, tunable blue-green light was demonstrated. At a domain pitch of 15.45 μm, the SHG signal and its fundamental signal at 1423.9 nm can satisfy the third-order SFG quasi-phase-matched (QPM) condition. The measured SHG power at 714.2 nm was 12.25 mW under 100 mW input power, and the estimated nonlinear coefficient (d33) achieved was 25.3 pm/V. The self-cascaded SHG+SFG power measured at 477.1 nm was ∼700 μW under 350 mW input power. The maximum internal efficiency of the SHG is 14.84%. The tuning range of the self-cascaded SHG and SFG generated tunable blue-green light was more than 40 nm, from 471.3 to 515 nm. The maximum simulated 3 dB bandwidth achieved using a gradient-period QPM structure is 196 nm, which is from 1476 to 1672 nm. The gain-bandwidth product of the self-cascaded SHG and SFG processes decreases drastically as the bandwidth is broadened.

Multiple scale analysis of quasi-phase-matched quadratic dielectrics for second harmonic generation

Light propagation in quasi-phase-matched (QPM) media is analysed by the multiple scale technique. By means of this approach we model the non homogeneous QPM structure with an equivalent homogeneous medium; we then analyse the induced Kerr effects to discuss how they depend on the ratio of the structure's wave vector to the absolute phase detuning. The validity of the proposed model is tested by exhaustive numerical simulations: in particular we focus our attention to the regions close to perfect quasi-phase-matching where other models fail in the description of both wave's amplitude and phase.

Two-color generation in a rare-earth-doped quasiphase-matched structure

A numerical study of two-color operation of quasiphase-matched structure in the lasing regime is presented for the case when a 980 nm pump produces the output both at 1550 and 775 nm. Successful operation of the two-color device relies on the use of a novel gain medium which consists of diode-pumped high-density rare-earth ions and on the utilization of a saturable amplification and absorption in the pumped and unpumped section of the device, correspondingly.

Quasi-phase-matched second harmonic generation in a polymer waveguide with a periodic poled structure

Quasi-phase-matched (QPM) second harmonic generation in a poled polymer waveguide has been realized. A waveguide with a periodic structure was fabricated by conventional photolithography and reactive ion etching. The waveguide with a QPM structure was composed of poled polymer and UV cured epoxy resin. Phase-matched second harmonic generation was observed, using laser light with a fundamental wavelength 1.48–1.65 μm. This phase matched condition was dependent on the periodic length. The waveguide had a typical second harmonic generation efficiency of about 4×10−1% W−1 cm−2 around a wavelength of 1.55 μm.