Quasi-phase Matched Waveguide Research Articles

Femtosecond laser direct writing quasi-phase matched type-II waveguide in lithium niobate

In this paper, we demonstrate a quasi-phase matched type-II waveguide fabricated by femtosecond laser direct writing in Z-cut MgO doped lithium niobate. Compared with the previous work that employs type-III configuration to realize the all-optical fabricated quasi-phase matched waveguide, our type-II waveguide has a simpler dual-line structure that can support TM-mode quasi-phase matched second harmonic generation which employs d33 naturally, thus the fabrication process can be significantly simplified. In experiment, an enhanced second-harmonic generation with a normalized conversion efficiency of 8.76 %W−1cm−2 is observed, corresponding to a nonlinear modulation depth of 0.89. The proposed design and fabrication method expand the application of femtosecond laser direct writing type-II waveguide in the field of monolithic integrated nonlinear devices.

Quasi-phase matching waveguides on lithium niobate and KTP for nonlinear frequency conversion: A comparison

Established waveguide fabrication technologies on lithium niobate (LN) and potassium titanyl phosphate (KTP) were revisited, and a comparative analysis of their performance for type-0 quasi-phase matched second-harmonic generation at 1.55 µm was provided based on literature data and our simulations. This analysis aided identifying gaps where the waveguide performance is below the theoretical predictions, and the possible reasons are discussed. It provides the reader with a roadmap for choosing the most appropriate waveguide type and material choice between LN and KTP for desired performance of targeted applications.

Efficient generation of ultra-broadband parametric fluorescence using chirped quasi-phase-matched waveguide devices.

We present a highly efficient photon pair source using chirped quasi-phase-matched (QPM) devices with a ridge waveguide structure. We developed QPM waveguide devices with chirp rates of 3% and 6.7%. Spectrum measurements reveal that the generated photons have bandwidths of 229 nm and 325 nm in full width at half maximum (FWHM), alternatively, 418 nm and 428 nm in base-to-base width for the 3% and 6.7% chirped devices, respectively, which are much broader than the bandwidth of 16 nm in FWHM observed with a non-chirp device. We also evaluate the generation efficiency of photon pairs from coincidence measurements using two superconducting single photon detectors (SSPDs). The estimated generation efficiencies of photon pairs were 2.7 × 106 pairs/s·µW and 1.2 × 106 pairs/s·µW for the 3% and 6.7% chirped devices, respectively, which are comparable to the generation efficiency for the non-chirp device of 2.7 × 106 pairs/s·µW. We also measured the frequency correlation of the photon pairs generated from the 6.7% chirped device. The experimental results clearly show the frequency correlation of the generated broadband photon pairs.

Thin film wavelength converters for photonic integrated circuits

Quasi-phase-matched (QPM) wavelength converters are highly desirable for emerging nonlinear optics applications in photonic integrated circuits, but available waveguide and quasi-phase-matching technologies have so far constrained their realization. In this work, we present a periodically poled lithium niobate (LN) waveguide on a silicon nitride–thin film LN platform. It contains a submicrometer waveguide core for enhancing nonlinear interactions that is more than one order of magnitude smaller than those of previous QPM waveguides. Periodic poling was applied directly to the thin film LN for quasi-phase-matching by a new surface poling technology. We demonstrated 160% W−1·cm−2 normalized efficiency for second harmonic generation at 1530 nm with ultralow propagation loss (0.3 dB/cm) in the telecom band. This highly efficient and compact wavelength converter has the potential for straightforward integration with various photonic platforms, e.g., on-chip microsystems such as optical communication networks, quantum storage, and optical frequency referencing.

Monolithic fabrication of quasi phase-matched waveguides by femtosecond laser structuring the χ(2) nonlinearity

We demonstrate second harmonic generation in quasi phase-matched waveguide structures fabricated by direct laser writing. Circular waveguides are inscribed in z-cut lithium niobate that provide well confined guiding of the fundamental and second harmonic wave. In contrast to classic schemes that employ periodically poled crystals, quasi phase-matching is realized by a laser-induced modulation of the nonlinearity inside the waveguide core. The proposed design allows monolithic integration of buried frequency conversion devices with tailored nonlinear response and excellent compatibility to on-chip optical elements. Second harmonic generation of 1064 nm radiation is demonstrated for different grating periods and associated matching temperatures. A maximum conversion efficiency of 5.72% is obtained for a 6 mm long, laser-induced quasi phase-matching grating.

Supercontinuum generation in quasi-phasematched waveguides

We numerically investigate supercontinuum generation in quasi-phase-matched waveguides using a single-envelope approach to capture second and third order nonlinear processes involved in the generation of octave-spanning spectra. Simulations are shown to agree with experimental results in reverse-proton-exchanged lithium-niobate waveguides. The competition between χ((2)) and χ((3)) self phase modulation effects is discussed. Chirped quasi-phasematched gratings and stimulated Raman scattering are shown to enhance spectral broadening, and the pulse dynamics involved in the broadening processes are explained.

Continuous wave second-harmonic generation using domain-disordered quasi-phase matching waveguides

Second-harmonic generation in domain-disordered quasi-phase-matched GaAs/AlGaAs superlattice-core waveguides was demonstrated using a continuous wave fundamental source. Output second-harmonic powers of up to 1.6 μW were measured when on a Fabry-Pérot resonance peak. Temperature-related bistable behavior was observed in both the fundamental and second-harmonic output when tuning either the input power or input wavelength.

Modeling and Optimization of Quasi-Phase Matching Via Domain-Disordering

Second-harmonic generation in a quasi-phase matched waveguide produced using a domain-disordered GaAs-AlAs superlattice is modeled including the effects of group velocity mismatch, nonlinear refraction, two-photon absorption, and linear loss. The model predicts our experimentally observed second-harmonic powers within an order of magnitude. Self-phase modulation and two-photon absorption led to reduced conversion efficiencies of up to 33% at input peak powers >50 W. Group velocity mismatch results in a reduction of 23% in conversion efficiency using estimated group velocities calculated from the measured effective refractive index. The modeling also shows that the conversion efficiency peaks at propagation lengths longer than the pulse walk off length and that duty cycle variations induced shifts in the tuning curves. Group velocity mismatch also increased the conversion bandwidth by ~ 30%. Incomplete modulation of chi <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> in disordered regions reduced the output conversion efficiency by up to two orders of magnitude. Grating-assisted phase matching led to a 7% efficiency drop for a Deltan of 0.045 at the second-harmonic and 0.01 at the fundamental. This model serves as a valuable tool to provide insight into the optimization of these devices.

High-confined second harmonic generation in nano-scale slot waveguides

We theoretically analyse quasi-phase matched second harmonic generation in a quadratic slot waveguide. Our results show that in this system both fundamental frequency and second harmonic (SH) modes can be confined in a nano-sized low-index region with the mode overlap integral intensity two orders larger than that achieved in the form birefringence phase matching slot waveguide and the conventional quasi-phase matched waveguide. For the unity length of 1.0 mm, the SH in the novel waveguide can be one order larger than those which have been investigated.

High resolution patterning of quadratic non-linear optical properties in doped photopolymer thin films

We demonstrate the patterning of the quadratic non-linear optical (QNLO) properties in a photopolymerizable resin doped with push–pull chromophores. Advantage is taken of the crosslinking process to perform the patterning directly. QNLO gratings with a period of 8.5 μm have been achieved over areas of a few cm 2. This spacing is comparable to the typical visible and near infrared coherence lengths in this material and is therefore suitable for the elaboration of quasi-phase matched waveguides. An accurate characterization of the QNLO gratings has been performed using a high spatial resolution second harmonic generation microscope with a femtosecond laser source.

Retiming of picosecond pulses by a cascaded second-order nonlinear process in quasi-phase-matched LiNbO_3 waveguides

A new technique for reducing timing jitter by using a cascaded second-order nonlinear process in quasi-phase-matched (QPM) LiNbO3 waveguides is proposed. It was found that the timing jitter of a signal at QPM wavelength is not transferred to the converted pulses during cascading in a 30-mm-long LiNbO3 QPM waveguide. Timing-jitter transfer characteristics in QPM LiNbO3 waveguides are evaluated by calculation of pulse evolution, and retiming of 1.5-ps pulses in 10- and 30-mm-long waveguides is demonstrated experimentally.

Second-harmonic generation in periodically poled nonlinear polymer waveguides.

We demonstrate quasi-phase-matched (QPM) second-harmonic generation (SHG) at the optical communication wavelengths with side-chain polymer waveguides. A ridge waveguide structure is designed to support fundamental mode guiding at both the pump and the second harmonics, leading to a high field overlap integral of the guided modes. The nonlinearity contrast in the +/0 type QPM waveguide is maximized under a QPM poling electrode width of nearly half the coherence length. Using these configurations, we record a normalized SHG efficiency of 2.2% W(-1) cm(-2) in the polymer waveguide.

Waveguide Nonlinear-Optic Devices for Optical Communications

Waveguide-type quasi-phase matched (QPM) nonlinear-optic devices offer numerous applications in optoelectronics area. Wavelength converters based on QPM difference frequency generation (DFG) have advantages of wide wavelength coverage, wide bandwidth, high efficiency, high transparency for various data format, low noise and integration compatibility. Various ultrafast optical signal processing such as optical sampling and optical gate switching can also be performed. Recent progress in this subarea is reviewed with emphasis on research work of the author’s group using LiNbO3 QPM waveguides.

A novel method to characterize photorefractive damage in quasiphase-matched wavelength converters

A novel two-beam method is proposed and applied for the first time, to characterize photorefractive damage (PRD) in a LiNbO3 quasiphase-matched (QPM) wavelength converter. In the proposed method, irradiation light from a Ti sapphire laser and a broadband probe beam from an erbium-doped fiber amplifier are coupled into a LiNbO3 QPM waveguide. The PRD effect caused by the irradiation is studied by monitoring the generated second-harmonic light spectrum of the probe light. It is shown that PRD in the LiNbO3 QPM waveguide can be qualitatively characterized by the proposed method, and relevant information relating to the QPM wavelength conversion can be extracted directly.

Waveguide second-harmonic generation device with broadened flat quasi-phase-matching response by use of a grating structure with located phase shifts

We report on a theoretical analysis and experiments for bandwidth broadening in quasi-phase-matched (QPM) second-harmonic generation (SHG).We used phase-shifted segments of a periodic grating to obtain a spectrally broadened, nearly flat response simultaneously with high conversion efficiency. We used an x-cut MgO:LiNbO(3) QPM waveguide in our analysis and experiments. The spectral range of the 850-nm fundamental for which SHG conversion exceeded 0.95 of the maximum value broadened from 0.02 to 0.12 nm when a 1-cm-long grating was divided into three segments with optimum phase shift. SHG conversion efficiency was 300%/W for this waveguide. The SHG efficiency and phase-matching characteristics showed good agreement with theoretical results.

Surface emitted second-harmonic generation from a quasi-phase matched waveguide in an Al_xGa_1-xAs/Al_2O_3 microcavity

A nonlinear Al(x)Ga(1-x)As waveguide consisting of a quasi-phase matched heterostructure embedded in a microcavity has been designed and fabricated. The microcavity resonator is formed by Al(2)O(3)/Al(0.32)Ga(0.68)As multilayer mirrors located above and below the waveguide core. The cavity resonantly enhances the surface emitting second-harmonic generation. The SH conversion efficiency has been measured for wavelengths between l = 1525 and 1575 nm. A simple waveguide loss measurement technique based on the SH autocorrelation of short optical pulses in a III-V waveguide is also demonstrated.

AlGaAs Semiconductor Quasiphase-Matched Wavelength Converters

Wavelength conversion based on sum frequency generation (SFG) using a semiconductor quasiphase-matched (QPM) waveguide with a periodically crystal domain inverted structure has been studied both theoretically and experimentally. Mathematical expressions with clear physical insight into QPM SFG are derived, taking into account waveguide propagation loss. SFG from AlGaAs QPM waveguides fabricated by a direct wafer bonding technique have been characterized. Nearly polarization insensitive wavelength conversion is demonstrated, and a normalized conversion efficiency of 810%/(W cm2) is obtained for a 0.5-mm-long waveguide with a propagation loss of over -100 dB/cm. The dependence of conversion efficiency and polarization sensitivity on device length is also discussed, and it is shown that the experimental results agree well with the theoretical simulations.

Visible laser sources based on frequency doubling in nonlinear waveguides

This paper reviews nonlinear quasi-phase-matching (QPM) waveguides and laser diodes with application to conversion of infrared laser diode wavelengths to the visible. The discussion of nonlinear QPM waveguides includes Ti-diffusion poled and E-field poled LiNbO/sub 3/ and KTP waveguides. Up to 25 mW of blue output power has been demonstrated for 120-mW infrared power injected into a nonlinear waveguide. Semiconductor laser sources suitable for frequency doubling are discussed, including distributed Bragg reflection (DBR) lasers operating at up to 200 mW output power, master oscillator power amplifiers with over 1 W output power, and wavelength-tunable flared semiconductor lasers with over 0.5 W output. A compact blue laser with 1-4 mW output power at 425-nm wavelength has been demonstrated based on a DBR laser frequency doubled in a nonlinear waveguide.

Single-step fabrication of strongly confining quasiphase-matching waveguides in LiTaO3

Strongly confining waveguides were fabricated in LiTaO3 at high temperature (540–610 °C) by short-time (≤6 s) rapid thermal annealing. Single-step photolithography is able to accomplish the simultaneous fabrication of a waveguide and a periodically domain-inverted structure for second-order quasiphase-matching blue-light generation. A measured normalized conversion efficiency of 7%/W cm2 for a physical length of 0.55 mm in a planar quasiphase-matching waveguide showed that this single-step process can lead to high conversion efficiency in channel waveguides.

Drift of phasematching wavelength forquasi-phasematching LiTaO 3 waveguides

A drift in the refractive index in annealed proton-exchanged LiTaO/sub 3/ waveguides leads to a changing phase-matching wavelength in quasi-phasematching waveguides for blue-light generation.