Broad-flattened bandwidth wavelength conversion based on difference-frequency generation in aperiodically poled lithium niobate

The development of high-performance, room-temperature, continuous wave (CW), widely-tunable terahertz (THz) lasers has attracted considerable industrial interest and has shown great potential for applications in various fields ranging from THz spectroscopy and biomedical imaging to safety and security applications. Among the commonly used lasers, direct quantum cascade lasers are regarded as the most popular sources offering the highest-to-date wall-plug efficiency. However, such lasers generally require cryogenic cooling and suffer from the drawbacks in terms of the production complexity and the lack of broad tunability. On the other hand, high-performance laser sources based on semiconductor disk lasers (SDLs) and periodically-poled ferroelectric crystals can offer an access to new wavelengths not covered by available gain media, particularly in the THz spectral region. In this regard, vertical-external-cavity surface-emitting lasers (VECSELs), with remarkable output parameters, can allow the demonstration of highly-efficient THz emission in the 0.8 – 2 THz spectral window at room temperature based on difference frequency generation in advanced quasi-phase-matched crystals, such as periodically-poled lithium niobate (PPLN) and aperiodically-poled lithium niobate (aPPLN) crystals. In recent years, a few VECSEL-based THz sources, referred to as Terahertz-External-Cavity Surface-Emitting Lasers (TECSELs), have been demonstrated utilizing PPLN crystals [1–4], generating non-tunable multimode or, with certain efforts, even single-frequency THz signal.

We present a difference frequency generator (DFG) based on a 2-mm-long MgO-doped aperiodically-poled lithium-niobate (APPLN) crystal with a linearly-varying poling-period. The DFG was pumped by a femtosecond fiber-laser system with a repetition-rate of 53.4 MHz. It produced broadband mid-infrared light source having an instantaneous-bandwidth covering 1.9–4.6 μm, with an average power of 30 mW. Moreover, by numerical simulations, we revealed that due to the relatively high intensity of the incident signal wave, an APPLN crystal with a length of a few milli-meters was sufficient to produce efficient and broadband nonlinear frequency conversion in a high-repetition-rate, ultra-short-pulsed DFG system.

A hybrid genetic algorithm (GA) is proposed. Simulating two test functions shows that the proposed GA can effectively solve the multimodal optimization problems, and the three movies demonstrate the detailed procedure of each generation. The conversion efficiency and bandwidth, based on quasi-phase-matching (QPM) difference frequency generation (DFG), are optimized by the matrix operator and our GA. Optimized examples for five-, six- and seven-segment QPM gratings are given, respectively. The optimal results show that adding the segment number of QPM can obviously broaden the conversion bandwidth, which is sensitive to the fluctuation of bandwidth and the variation of QPM grating period.

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.

Apodized chirped aperiodically poled lithium niobate is designed to generate smooth second harmonic of a pump wave over a wide bandwidth. The effect of different apodization functions and the ratio of apodization have been examined for the reduction of ripples in the efficiency response of APPLN.

Quasi phase-matching (QPM) is an attractive technique for optical wavelength conversion applications instead of the conventional birefringent phase matching. An advantage of the QPM is that any interaction within the transparency range of the material can be noncritically phase matched at a specific temperature by using the largest nonlinear optical coefficient. The periodically poled lithium niobate (PPLN) has been extensively used for the quasi phase-matched optical parametric oscillator (QPM-OPO). The effective nonlinear coefficient of the PPLN is 21.6 pm/V and enables one to realize efficient wavelength conversion. Usually the wavelength tuning of the QPM-OPO is achieved by changing the crystal temperature. Although the wide wavelength tuning range is achieved with the temperature tuning, the response is not fast. In this paper we demonstrate the wavelength tuning of the QPM-OPO by the electro-optic effect for fast wavelength tuning.

We experimentally demonstrate quasi-phase-matched (QPM) four-wave-mixing (FWM) in silicon (Si) nanowire waveguides with sinusoidally modulated width. We perform discrete wavelength conversion over 250 nm, and observe 12 dB conversion efficiency (CE) enhancement for targeted wavelengths more than 100 nm away from the edge of the 3-dB conversion bandwidth. The QPM process in Si nanowires is rigorously modeled, with results explaining experimental observations. The model is further used to investigate the dependence of the CE on key device parameters, and to introduce devices that facilitate wavelength conversion between the C-band and mid-IR. Devices based on a superposition of sinusoidal gratings are investigated theoretically, and are shown to provide CE enhancement over the entire C-band. Width-modulation is further shown to be compatible with zero-dispersion-wavelength pumping for broadband wavelength conversion. The results indicate that QPM via width-modulation is an effective technique for extending the spectral domain of efficient FWM in Si waveguides.

A multiwavelength midinfrared quasi-phase-matched (QPM) difference frequency generation (DFG) scheme is proposed based on a segmented temperature controlling technique for the uniform grating periodically poled lithium niobate (PPLN). The QPM DFG output performances have been investigated under the fixed pump and signal wavelength conditions, respectively. Our theoretical results show that when the pump wavelength is fixed at 1.08 μm, three idler QPM bands, located at 3.46, 3.50, and 3.57 μm, have been obtained, where the temperatures of the three PPLN segments with the same length are set at 20°C, 70°C, and 100°C. However, when the signal wavelength is fixed at 1.58 μm, six idler QPM bands, located at 2.95, 3.06, 3.14, 3.63, 3.72, and 3.83 μm, have been achieved with the same crystal temperature distribution of 20°C, 70°C, and 100°C.

Quasi-phase matching (QPM) technique have realized an efficient and various types of nonlinear wavelength conversion by using a desired nonlinear coefficient in arbitrary wavelength by specially designed crystal structure. Ferroelectric crystals such as LiNbO3 (LN) and LiTaO3 (LT) have been reported for major materials of the QPM device from visible to mid infrared (MIR) wavelength region. In last several years, we have reported a large-aperture QPM device using a Mg-doped congruent LN (MgLN) for high-power/energy operation, and demonstrated a highly efficient and high-energy optical parametric oscillation with > 0.5 J output energy by a 10-mm-thick periodically poled MgLN (PPMgLN) [1]. As increasing of both conversion efficiency and handling power/energy in PPMgLN device, crystal damage have become severe problem, which needed to find another choice for the QPM material. Compared to LN-type crystals, LT-type crystals have relatively wide transparent range, small absorption, and high thermal conductivity, although their nonlinear coefficients are low, which are suitable for high-power/energy QPM devices. LT crystal with Mg-doped stoichiometric composition, grown by double crucible Czochralski or vapor phase epitaxy method, have been already reported [2]. In this study, we focus on the Mg-doped congruent LT (MgLT) grown by conventional Czochralski method. An increase of Mg doping can basically improve various properties for the QPM device, though high-quality crystal growth become difficult. Here we focus on the characterization of 8 mol% MgLT (Mg8LT), and compare with low Mg-doped MgLTs [3] and 5 mol% MgLN.

Hybrid multiplexing technology has been considered as an effective method to increase the optical communication capacity. An inverse design method is proposed for thin-film aperiodically poled lithium niobate (TFAPLN) waveguides to support all-optical wavelength conversion of wavelength-mode hybrid multiplexed signals. The TFAPLN waveguide structure is optimized using a simulated annealing algorithm with the objective function of maximum difference frequency generation (DFG) efficiency and minimum uniformity. By adjusting the spacing between adjacent target wavelength channels, a 44-nm 1-dB conversion bandwidth is obtained for three modes (TE0, TE1, and TE2) with an average conversion efficiency of -6.2 dB under a 100-mW pump. The conversion uniformity is less than 0.0243.

In this paper, the comparison of theoretical analysis and numerical simulation in conversion efficiency, bandwidth, frequency difference and multichannel propagation are expatiated distinctly according to the conversion principle and structure model of SP-cSHG+DFG-DC-AOWC (Single Pass All Optical Wavelength Converter Based on Cascaded Second Harmonic Generation and Difference Frequency Generation with Double Conversion Configuration) and SP-cSFG+DFG-DC-AOWC (Single Pass All Optical Wavelength Converter Based on Cascaded Sum Frequency Generation and Difference Frequency Generation with Double Conversion Configuration). Furthermore, it is pointed out that the structure model of SP-cSFG+DFG-DC-AOWC will be an ideal one due to its characteristics and advantages in conversion efficiency, bandwidth, frequency difference and multichannel propagation.

We present the numerical results for the optimization of the pump-to-idler conversion efficiencies of nanosecond idler wavelength tunable cascaded optical parametric oscillators (OPO) in different wavelength tuning ranges, where the primary signals from the OPO process are recycled to enhance the pump-to-idler conversion efficiencies via the simultaneous difference frequency generation (DFG) process by monolithic aperiodically poled, magnesium oxide doped lithium niobate (APMgLN) crystals. The APMgLN crystals are designed with different chirp parameters for the DFG process to broaden their thermal acceptance bandwidths to different extents. The idler wavelength tuning of the cascaded OPO is realized by changing the temperature of the designed APMgLN crystal and the cascaded oscillation is achieved in a single pump pass singly resonant linear cavity. The pump-to-idler conversion efficiencies with respect to the pump pulse duration and ratio of OPO coefficient to DFG coefficient are calculated by numerically solving the coupled wave equations. The optimal working conditions of the tunable cascaded OPOs pumped by pulses with energies of 350 μJ and 700 μJ are compared to obtain the general rules of optimization. It is concluded that the optimization becomes the interplay between the ratio of OPO coefficient to DFG coefficient and the pump pulse duration when the idler wavelength tuning range and the pump pulse energy are fixed. Besides, higher pump pulse energy is beneficial for reaching higher optimal pump-to-idler conversion efficiency as long as the APMgLN crystal is optimized according to this pump condition. To the best of our knowledge, this is the first numerical analysis of idler wavelength tunable cascaded OPOs based on chirp-assisted APMgLN crystals.

We report on the fabrication of ridge waveguides formed in congruent periodically poled lithium niobate (PPLN). The waveguides were fabricated by periodic poling, proton exchange, subsequent annealing and reverse proton exchange followed by diamond blade dicing. Up to 1 W of single-pass second-harmonic generation at 775 nm has been realized in 50 mm long ridge waveguides with internal conversion efficiency of 70%. Furthermore, difference frequency generation at 3.3 μm of two pump waves at 1.05 μm and 1.55 μm has been obtained in similarly fabricated PPLN ridge waveguides with an appropriate poling period.

Exploiting the cascaded sum- and difference-frequency generation in a periodically poled lithium niobate (PPLN) waveguide, a novel scheme of tunable all-optical wavelength conversion is proposed and experimentally demonstrated by use of a PPLN-based double-ring cavity configuration. Wavelength conversion of picosecond pulses from the signal wavelength to the idler wavelength is realized. A pulsed signal source with a repetition rate of 40 GHz and a pulse width of 1.57ps is adopted. The pump and control waves are generated from the double-ring fiber laser. Therefore, no external continuous-wave (CW) light is required.

Periodically poled lithium niobate (PPLN) is a relatively new non-linear optical material which can be used for such processes as second harmonic generation, sum and difference frequency generation and optical parametric oscillation. The use of periodically-poled lithium niobate in spectroscopy and chemical sensing offers many potential advantages over systems employing more traditional laser sources. When pumped by the fundamental of a Nd:YAG laser PPLN offers the promise of high efficiency, high power, broad tunability (1.5–3.5 µm) and compact size. The broad tunability over the C-H stretch region is an important advantage for many chemical sensing applications. At Sandia, we are developing IR sources based on PPLN for both remote and in-situ chemical sensing.