Squeezing across two harmonic generations utilizing coupled nonlinear waveguide systems

In this paper we will describe the most recent advances in the development of Ti:Al2O3, unstable-resonator lasers. Results include demonstration of improved long-wavelength operation and efficient second, third and fourth harmonic generation. Second harmonic generation (SHG) results will be described for three nonlinear crystals: deuterated potassium di-hydrogen phosphate (D-KDP), beta barium borate (BBO) and lithium triborate (LBO). Third harmonic generation (THG) in BBO and D-KDP will be presented along with fourth harmonic generation (FHG) data obtained with BBO crystals.

The role of the ratio of the two design periods for phase reversed QPM structure is theoretically investigated. It is found that the efficiency of third and fourth harmonic generation strongly depends on this ratio. The results for third harmoic generation and fourth harmonic generation in LiTaO3, LiNbO3 and KTP are shown in graphical form.

We propose and report a 193 nm narrow-linewidth light generation by a frequency mixing of Yb and Er-fiber lasers as the seed for an ArF excimer laser. The Yb-fiber laser includes a pulsed distributed feedback (DFB) or external cavity diode laser (ECDL), acousto-optic modulator(AOM), fiber amplifiers and an Yb:YAG single crystal fiber (SCF) power amplifier with more than 7 W output. The Er-fiber laser consists of a continuous-wave (CW) DFB laser, a semiconductor optical amplifier (SOA), and fiber amplifiers. The second harmonic generation (SHG) and the fourth harmonic generation (FHG) of Yb laser at 515 nm and 258 nm reach 5 W and 1.5 W, respectively. Two stages of sum-frequency generation (SFG) produce the power of 100 mW for 193 nm laser by use of CLBO crystals.

In this paper, a frequency doubler is designed that converts from 1.15 GHz input signal to 2.3 GHz output signal using TSMC 0.18 CMOS technology. The proposed doubler consist of a fully adjustable Schmitt trigger (FAST), time-delay components, XOR gate and a voltage controlled delay line (VCDL). Time-delay components use RC integrator and comparator. Advantages of this topology include good fundamental frequency and harmonic frequency suppression, compact layout and low power consumption. The desired signal at 2.3 GHz is 0.871 dBm. The fundamental frequency at 1.15 GHz and third harmonic frequency at 3.45 GHz approximately 37 dBc below the 2.3 GHz signal. The fourth harmonic frequency at 4.6 GHz is approximately 24 dBc below the 2.3 GHz signal. The power consumption is 11.07 mW and the phase noise of the output signal is 100.5 dBc/Hz at 10 KHz offset.

The non-classical light resonance on the cesium D<sub>1</sub> (894.6 nm) line has important applications in solid-state quantum information networks due to its unique advantages. The cesium D<sub>1</sub> line has a simplified hyperfine structure and can be used to realize a light-atom interface. In our previous work, we demonstrated 2.8-dB quadrature squeezed vacuum light at cesium D<sub>1</sub> line in an optical parametric oscillator(OPO) with a periodically poled KTP(PPKTP) crystal. However, the squeezing level is relatively low, and the tunability that has practical significance for squeezed light has not been further investigated. Theoretically, the increase of the transmittance of output mirror and the decrease of the intra-cavity loss of the OPO can improve the squeezing level. Here, we use super-polished and optimal coating cavity mirrors to improve the nonlinear process in OPO. We prepare 447.3 nm blue light from 894.6 nm fundamental light by a second harmonic generation cavity (SHG). The SHG is a two-mirror standing-wave cavity with a PPKTP crystal as the nonlinear medium. The power of generated blue laser is 32 mW when the incident infrared power is 120 mW. Using the blue light to pump an OPO, we achieve quadrature squeezed vacuum light at cesium D<sub>1</sub> line. The OPO is a two-mirror standing-wave cavity with a PPKTP crystal. The threshold of OPO is reduced to 28 mW. The squeezing level of generated quadrature squeezed vacuum light is increased to 3.3 dB when the pump power is 15 mW. Taking into account the overall detection efficiency, the actual squeezing reaches 5.5 dB. We inject a weak signal beam into the OPO cavity to act as an optical parametric amplifier (OPA), and test the tunability of squeezzed light. The blue light and the squeezed light are tuned by using a low-frequency triangular wave signal to scan the Ti: sapphire laser. Gradually increasing the amplitude of the scanning triangle wave signal, the generated bright squeezed light can be continuously tuned over a range around 80 MHz without losing the stability of the whole system. The generated squeezed light offers the possibility for the efficient coupling between the non-classical source and solid medium in the process of quantum interface.

We demonstrate the generation of squeezed states of light due to the second harmonic generation and Kerr effect in an array of nonlinear waveguides mediated through a linear one. We characterized the electromagnetic field by a quantum mechanical Hamiltonian and the density operator time evolution is obtained from the Von-Neumann equation of motion. Using the quasiprobability positive P of phase space representation, the classical Fokker-Planck equation is obtained from the master equation and translated to its classical matching set of nonlinear differential equations. We showed that because of the new possibilities of correlation between the linear and nonlinear channel waveguides, highly nonclassical light may be produced.

We study optical harmonic generation on the 1S exciton-polariton in the semiconductor ZnSe. Intense and spectrally narrow exciton resonances are found in optical second (SHG), third (THG), and fourth (FHG) harmonic generation spectra. The resonances are shifted to higher energy by 3.2 meV from the exciton energy in the linear reflectivity spectrum. Additional resonances are observed in the THG and FHG spectra and assigned to combinations of incident and backscattered photons in the crystal. Rotational anisotropy diagrams are measured and further information on the origin of the optical harmonic generation and the involved exciton states is obtained by a symmetry analysis using group theory in combination with a microscopic consideration.

We present in this work the design of second harmonic generation process in χ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> nonlinear waveguides by a new time domain simulator. The presented Hertzian potential formulation (HPF) is applied to quasi-phase-matched GaAs/AlGaAs nonlinear waveguide with dielectric discontinuities. With the introduction of the presented rigorous time domain method it is possible to represent the physical phenomena such as light propagation and second harmonic generation (SHG) process inside a nonlinear optical device with a good convergent solution and low computational cost. We first check the convergence of the HPF approach by comparing the numerical results with experimental ones for a GaAs/AlGaAs nonlinear waveguide with a fundamental mode at λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FU</inf> =1.955 μm coupled to a coprapagating second-harmonic SH mode (λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SH</inf> =9.775 μm) through an appropriate nonlinear susceptibility coefficient. Then we apply the modelling to a GaAs/AlGaAs ridge waveguide with λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FU</inf> =1.55 μm and λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SH</inf> =0.775 μm by evaluating the second harmonic conversion efficiency versus the waveguide length. The model is generic and can be applied also to periodic microwave waveguides.

We have developed a widely tunable deep-ultraviolet (DUV) laser in the wavelength range from 175 to 210 nm by the fourth harmonic generation of Ti:Sapphire laser. The fourth harmonic generation is performed by direct second-harmonic generation (SHG) of a frequency doubled Ti:Sapphire laser with KBBF crystal. The highest output power is 2.23 mW at 193 nm, and the power of the DUV laser is more than 1 mW from 182 nm to 210 nm. To our knowledge, it is the first demonstration of milliwatt-level widely tunable DUV all-solid-state laser below 200 nm by direct SHG technique.

Experimental researches on non-critical phase-matching fourth harmonic generation with ADP and DKDP crystals are reported, in which high efficiency (>84%) has been demonstrated with small-scale beam, although both crystals suffer from spatial non-uniformity problems.

We report a comprehensive numerical study of a type of quasi-phase-matched Cherenkov third harmonic generation in a nonlinear waveguide, and this Cherenkov third harmonic generation consists of a guided-to-guided second harmonic generation cascaded with a guided-to-radiated sum-frequency generation. Following the coupled-mode theory, the temperature-detuning characteristics of third harmonic (TH) radiation under different pumping-power densities were studied. TH power dependences on interaction length have been discussed as well, which includes three situations: without reciprocal vector participating, and forward and backward reciprocal vector participating. In addition, we demonstrate that the Cherenkov angle of each TH radiation is not sensitive to the temperature variation.

Nonlinear optical (NLO) properties of CsLiB6O10 (CLBO) have been investigated. Frequency quadrupling of the 1.064 µ m Nd:YAG laser output for type-I phase matching was observed by using CLBO. The walk-off angle and the effective NLO coefficient deff were estimated to be 1.83° and 1.01 pm/V for fourth harmonics generation of Nd:YAG laser output, respectively. Phase matching for sum frequency generation of 4ω (fourth harmonics of Nd:YAG laser)+ω (fundamental of Nd:YAG laser), resulting in fifth harmonics of Nd:YAG laser output, has been observed.

We have developed an angle-resolved photoemission spectrometer with tunable vacuum ultraviolet laser as a photon source. The photon source is based on the fourth harmonic generation of a near IR beam from a Ti:sapphire laser pumped by a CW green laser and tunable between 5.3 eV and 7 eV. The most important part of the set-up is a compact, vacuum enclosed fourth harmonic generator based on potassium beryllium fluoroborate crystals, grown hydrothermally in the US. This source can deliver a photon flux of over 10(14) photon/s. We demonstrate that this energy range is sufficient to measure the k(z) dispersion in an iron arsenic high temperature superconductor, which was previously only possible at synchrotron facilities.

The photoinjector laser system of Shenzhen Superconducting Soft X-Ray Free Electron Laser (S3FEL) is reported in this paper. This laser system operates at up to 1 MHz and produces more than 50 μJ infrared (IR) laser pulses. With a customized fourth harmonic generation (FHG) module, more than 2 μJ ultraviolet (UV) laser pulses were obtained. The power standard deviations of the IR laser and the UV laser are 0.093% and 0.395% respectively. While the pulse energy standard deviations are 1.087% and 1.746% correspondingly. We implemented the pulse stacking scheme to generate flat-top pulses. With four birefringent uniaxial crystals, the Gaussian pulses were converted to flat-top shape, featuring 10 ps pulse width and 0.5 ps rising and falling edges. A cut-Gaussian transverse profile with very sharp rising and falling edges can be produced after the spatial pulse shaper.

Generation of an ultrabroadband optical pulse with a fluent frequency-dependency of the phase is important, for example, for shaping multi-wavelength optical pulses and monocyclization. When two optical pulses, one fundamental, the other generated by second harmonic generation, are co-propagated in a nonlinear waveguide, induced-phase modulation (IPM) between them, as well as self-phase modulation (SPM), can be used to broaden the spectrum of each pulse. Since the second-harmonic wave as well as the fundamental wave are generated from one common femtosecond pulse, the carrier-phase difference between their waves is constant and these can be synthesized constructively after propagation. For a case where a capillary fiber filled with noble gas is used as an almost-dispersionless, nonlinear waveguide, we analysed the optimum delay between two pulses and powers required for covering spectrum between two pulses and it was predicted that the generation of the optical pulses whose spectrum covers from near-ultraviolet to near-infrared was possible. To verify these predictions, experimental studies have been performed.