Multiwavelength midinfrared difference frequency generation based on a uniform grating periodically poled lithium niobate

Phase matching must be performed to realize four-wave mixing (FWM) in a silicon waveguide, which is challenging when a signal wavelength is highly deviated from an idler wavelength. To solve this problem, quasi-phase-matching (QPM) based on grating-assisted directional coupling (GADC) in a hybrid structure is theoretically investigated. The proposed system consists of a silicon strip with periodic width modulation as the grating and a silicon nitride strip that is vertically aligned to the silicon strip. GADC occurs between the TE_S mode (mainly confined in the silicon strip) and TE_N mode (mainly confined in the silicon nitride strip) at an idler wavelength. This phenomenon compensates for the phase mismatch occurring in FWM among the TE_S modes at the pump, signal, and idler wavelengths. Results of analysis of the hybrid structure show that the TE_S and TE_N modes at an idler wavelength of 2.1177 μm are efficiently generated with the TE_S modes at a pump wavelength of 1.58 μm and signal wavelength of 1.2601 μm. Moreover, the signal TE_S mode can be efficiently implemented with the pump TE_S mode and idler TE_N mode. Owing to the GADC characteristics, the conversion bandwidth of the signal is 0.8 nm; however, the signal wavelength can be thermally tuned, with a temperature change of 50 °C corresponding to a signal wavelength change of 3.6 nm. The hybrid structure with the GADC-based QPM can be used to generate and detect mid-infrared light with well-developed O-band and L-band devices.

Forward optical parametric oscillators (OPO’s) based on quasi-phase matching (QPM) were implemented in LiNbO3 [1], However, a forward OPO requires a cavity to establish oscillation. Harris [2] introduced the concept of a backward OPO (BOPO) based on conventional phase matching: a cavity is not required to establish oscillation. However, in Ref. [2], only a threshold condition was obtained. Here, we present our results on BOPO’s [3] and transversely-pumped counter-propagating OPO’s (TPCOPO’s) [4]. A TPCOPO does not require a cavity to establish oscillation either. Second-order susceptibility of a nonlinear medium is spatially modulated with a period the pump wavelength in the medium to achieve QPM. A pump wave at the wavelength in vacuum λ3 propagates along a waveguide for a BOPO or onto the surface for a TPCOPO. Two counter-propagating waves at the wavelengths λ1 and λ2 can be generated in the nonlinear medium. To tune the output frequencies of the signal and idler, we can change the incident angle of the pump wave in the TPCOPO or BOPO. The gain for the signal or idler is effectively balanced by the loss of the signal or idler at the respective exit plane to reach a steady-state oscillation. Because a cavity is eliminated, a BOPO or TPCOPO is more stable while a forward OPO is sensitive to the slight mirror translation. For a TPCOPO [4], there is an optimal pump power ≈3.4Pth (where Pth is the threshold, pump power) at which η reaches the maximum value of 44%. If P3≫Pth, there is a huge build-up of the oscillating fields inside the medium. The efficient sum-frequency generation saturates the TPCOPO. Consider GaAs/Al0.8Ga0.2 As multilayers [5] with the optimized structure dimensions: if λ3≈0.49μm, Pth≈7.3kW and tuning range: 1.4-2.6 μm (or 3.1-5.8 μm if λ3≈2μm). Consider ZnSe/ZnS multilayers: if λ3 ≈ 0.49 μm, Pth≈0.92kW and the tuning range: 0.7-1.7 μm, Consider GaAs/AlAs asymmetric coupled quantum-well domain structure [6]: if λ3 = 10 μm, Pth ≈ 10W and the tuning range: 15-29 μm. Consider a nondegenerate BOPO: |k1 − k2| ≫ 1/L, where k1,2 are the corresponding wave vectors and L is the length of the medium. If P3≈1.1 Pth, the conversion efficiency for the BOPO is η ≈ 20%. When P3 ≈ 3.4Pth, η ≈ 44% for the TPCOPO and η ≈ 95% for the BOPO. Consider a degenerate BOPO: λ1=λ2. A mirror for the pump wave with the reflectivity R2ω is attached to the right facet to increase the conversion efficiencies, However, it is not required for the oscillation to occur. When the pump intensity is Ip≈4I′th≈Ith/4, where Ith and I′th are the thresholds for a nearly-degenerate and degenerate BOPO, η ≈ 99.7% if R2ω=99%. Therefore, compared with the nondegenerate BOPO, the degenerate BOPO offers higher conversion efficiencies. The decrease of the conversion efficiency as Ip (>4I′th) increases is due to generation of a backward wave at the pump wavelength, which propagates along the direction opposite to that of the pump wave. Consider a poled LiNbO3 [1], If the spatial period of the domains is Λ=4μm, λ3=1.1μm, λ1,2≈2.2μm, and L≈2.6cm, the threshold pump intensity for the degenerate BOPO is I′th=2.9×108 W/cm2. For a QPM KTP [7]: Λ=0.7μm, λ3=1.3μm, λ1,2≈2.6μm, and L≈3cm, I′th=2.8×106 W/cm2. In the presence of a cavity for the signal and idler, the thresholds can be reduced by several orders of magnitude. Both the BOPO and TPCOPO can be also implemented in nonlinear optical polymers. Our parametric processes can be used to achieve large amplifications and difference-frequency generation [8].

Nonlinear Cherenkov radiations (NCR) have attracted increasing research interest in the past years. Similar to the traditional Cherenkov radiation effect in particle physics, NCR requires that the nonlinear polarization wave propagates faster than the harmonic waves. Cherenkov frequency up-conversions can be automatically realized in normal dispersion materials and extensive work on Cherenkov second-harmonic generation, sum-frequency generation and high-order harmonic generation have been reported in both bulk and waveguide systems. However, Cherenkov frequency down-conversion process is still a challenge for the restriction of phase velocity threshold. Recently, we have proposed two schemes to realize the degenerated Cherenkov difference-frequency generation (CDFG) by modulating the phase velocity of the nonlinear polarization wave using both quasi-phase-matching (QPM) and birefringence-phasematching (BPM) respectively. For the first scheme, we used the QPM method. The reciprocal vectors provided by the domain engineered structure can modulate the phase velocity of the nonlinear polarization wave, and particularly, the backward vectors can accelerate the nonlinear polarization wave. If the backward reciprocal vector is large enough, Cherenkov DFG can be realized. In experiment, a proton exchanged LiTaO 3 planar waveguide with a poling period of 4.5 µm was used and the wavelengths of pump and signal were 532 and 1064nm respectively. At a radiation angle close to the theoretical one, the infrared CDFG spot can be observed with a CCD. For the second scheme, the birefringence property of KTP was utilized. For birefringent crystal, light with different polarization has different refractive index. By proper choice of the polarization states of the interacting waves, the refractive index of the signal wave can be larger than that of the pump wave, thus the phase velocity of the nonlinear polarization wave of the DFG process will exceed that of the harmonic wave. In experiment, we set the polarization of 532-nm pump and the 1064-nm signal waves along x and z axis of KTP respectively. CDFG was realized and some cascaded Cherenkov processes were observed as well. The method based on BPM can reduce the requirements for fabricating short-period domain structures and can be extended to mode-matching in waveguides for realizing nonlinear Cherenkov radiations.

All-optical wavelength conversion between ps-pulses based on cascaded sum- and difference frequency generation (SFG+DFG) is proposed and experimentally demonstrated in periodically poled LiNbO3 (PPLN) waveguides. The signal pulse with 40-GHz repetition rate and 1.57- ps pulse width is adopted. The converted idler wavelength can be tuned from 1527.4 to 1540.5nm as the signal wavelength is varied from 1561.9 to 1548.4nm. No obvious changes of the pulse shape and width, also no chirp are observed in the converted idler pulse. The results imply that single-to-multiple channel wavelength conversions can be achieved by appropriately tuning the two pump wavelengths.

We propose a general routine, Genetic-Simulated Annealing method (GSA) for the optimal design of quasi-phase matched (QPM) non-uniform grating. Here this method is applied to the design of the flat broadened wavelength conversion bandwidth of 40-mm aperiodically poled lithium niobate (APLN) based on difference-frequency generation (DFG). The flat conversion bandwidth of APLN is broadened from the original 65nm in a 40-mm periodically poled lithium niobate (PPLN) to the pre-designated 130 nm in a 40-mm APLN, which conversion efficiency is 6.7 dB higher than that of 12.4-mm PPLN under the same conversion bandwidth.

Here we present an analysis of a fully planar optical sensor based on ARROW waveguides. We consider a device geometry that consists of two intersecting ARROW waveguides, one with solid and one with liquid core. The pump wavelength is input from the solid core waveguide and penetrates through the wall into the liquid core of the other waveguide where the fluorescence is excited in the sample material. Then it is captured by the ARROW waveguide and guided to a detector at the end. Pump and signal wavelength can be guided with low loss through the solid and hollow core respectively. At the same time, high loss discrimination inside the core can be obtained by tailoring the thickness of ARROW layers, leading to efficient filtering. The pump wave can be transmitted efficiently in and out of the core, allowing for multiple intersections. In the following experimental part, we investigate the waveguide loss of the liquid core as a function of core width, we measure values as low as 1.7cm-1 that can be further reduced by improving the thickness control of ARROW layers in the fabrication process. Finally, we report fluorescence experiments on Alexa dye molecules. We demonstrate fluorescence detection at concentrations as low as 10-8 mol/l from a detection volume of 21pl. Photo-bleaching is observed and discussed as a function of input power intensity.

We propose the idea of using a grating structure for solving the problem of the incomplete quasi-phase matching (QPM) period and demonstrate the simulation results of phase compensation in achieving efficient round-trip optical parametric oscillation in periodically poled LiNbO/sub 3/ (PPLN). The layout of a monolithic optical parametric oscillator (OPO) with two surface gratings at both ends is shown. The two gratings are chirped to guarantee that their phase variations over the Bragg reflection windows are larger than 2/spl pi/. One of them has a length of 1 mm and the other has a length of 0.8 mm. The pump input is applied from the long grating side and the signal output is obtained from the short grating side. The whole PPLN length is 5 mm. For our numerical study, we consider the pump wavelength at 1064 nm and signal wavelength at 1550 nm. The idler wavelength is hence at 3.393 /spl mu/m. The corresponding QPM period of PPLN is 29.6 /spl mu/m. The central grating period is 361.83 nm, which corresponds to the Bragg wavelength 1550 nm. To obtain numerical results, we solved the coupled equations for the forward and backward pump complex amplitudes, the forward and backward signal complex amplitudes, and the forward and backward idler complex amplitudes.

In this paper, mid-IR light generation based on difference frequency generation (DFG) in a single piece of PPLN is proposed and studied. In the studies, a Yb doped fiber laser at 1.092 μm and a tunable laser around 1.55 μm were used. The output mid-IR laser with the wavelength tunable around 3.7 μm was generated. Since compact Yb doped fiber lasers at 1.092 μm and tunable semiconductor laser diodes around 1.55 μm are available on the market, the proposed mid-IR laser is potentially portable. Our simulations show that tunable mid-IR light as broad as 800 nm can be obtained from a single PPLN chip simply by tuning temperature (from 30 ^oC to 500 ^oC) and signal wavelength (from 1.48 μm to 1.62 μm).

The generation of THz pulses having tens of microjoules energy by tilted pulse-front excitation is reviewed. Possibilities of further up-scaling the THz energy as well as existing and future applications of these pulses are analyzed.

Tunable wavelength conversion of picosecond (ps)-pulses based on cascaded sum- and difference frequency generation is proposed and demonstrated by use of a fiber ring laser incorporating a periodically poled lithium niobate waveguide and two parallel-arranged tunable filters (TFs). The pulsed signal with 40-GHz repetition rate and 1.57-ps pulsewidth is employed. No external pump and control sources are required in the wavelength converter. By properly adjusting two TFs to change the lasing pump and control wavelengths, flexible wavelength conversion from a changeable input signal wavelength to a variable output idler wavelength is achieved

Coherent beam combining (CBC) by active phase control could be useful for power scaling fiber-laser-pumped optical frequency converters like optical parametric oscillators (OPOs). However, a phase modulator operating at the frequency-converted wavelength would be needed, which is a non-standard component. Fortunately, nonlinear conversion processes rely on a phase-matching condition, correlating not only the wave-vectors of the coupled waves, but also their phases. It is therefore possible to control the phase indirectly, using more standard phase modulators. Feasibility of this technique was previously demonstrated for second harmonic generators (SHG). Controlling the phase of the fundamental wave, excellent harmonic wave combining efficiency was achieved in both cases of phase matching and quasi phase matching, with lower than λ/30 residual phase error. In this paper, coherent combining of difference frequency generators (DFG) is experimentally tested. Even if DFG is more challenging than SHG as it implies handling three waves instead of two, phase control of the sole 1-μm pump waves is sufficient to combine the 3.4-μm waves generated. The mid-infrared DFG crystals are pumped and signal-seeded with standard all-fiber sources at 1 μm and 1.5 μm respectively. Phase control is performed with an electro-optic phase modulator which is a standard all-fiber component operating at 1 μm. CBC of mid-infrared DFG modules is a first step towards combining continuous wave OPOs.

We propose AND, NOR, and XNOR logic gates realized simultaneously for 40-Gb/s networks, in which the realization of NOR and XNOR logic gates using only MgO-doped periodically poled lithium niobate (MgO: PPLN) is reported.In our configuration, we exploit broadband quasi-phase matching (QPM) cascaded second harmonic and difference-frequency generation (cSHG/DFG), cascaded sum-frequency and differencefrequency generation (cSFG/DFG) in one MgO:PPLN, and the narrow band QPM sum-frequency generation (SFG) in another MgO:PPLN.The performance, including the quality-factor (Q-factor) and extinction ratio (ER), of the proposed multifunctional logic device is also simulated.

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.

A compact and stable terahertz (THz) source is demonstrated based on difference frequency generation (DFG) pumped by an efficient dual-wavelength acousto-optic (AO) Q-switched solid-state Nd:YLF laser with composite gain media (a-cut and c-cut) in the coaxial pumping configuration. Optimal power ratio and pulse synchronization of the orthogonal polarized 1047/1053 nm dual-wavelength laser could be realized by varying the pump focusing depth and/or pump wavelength. The total power of 2.92 W was obtained at 5 kHz pumped by 10-W laser-diode power at 803 nm. Such an efficient dual-wavelength laser demonstrated good stability and inconspicuous timing jitter benefiting from the suppressed gain competition between two resonating wavelengths. An 8-mm-long GaSe crystal was employed to generate THz waves at 1.64 THz by DFG and the maximum THz average output power was about 0.93 μW. This compact coaxial pumping method can be extended to all kinds of neodymium (Nd) doped laser crystals to produce different dual-wavelength lasers for various THz wavelength generation, which have good prospects for portable and costless applications like imaging, non-destructive inspection, etc.

In this paper, noise characteristics of second-harmonic generation (SHG) in periodically poled lithium niobate (PPLN) using the quasi-phase-matching (QPM) technique are studied experimentally. In the experiment, a 0.78-mm second-harmonic (SH) wave was generated when a 1.55-mm fundamental wave passed through a PPLN crystal (bulk or annealed proton-exchanged waveguide). The fundamental and SH waves were then separated through a beam splitter and sent to two photodetectors, respectively. The time-domain and frequency-domain characteristics of the fundamental and SH waves were analyzed. By using the pump-probe method, the noise characteristics were studied further when 532-nm irradiation light co-propagated with the 1.55-mm fundamental light in the PPLN crystals. It is found that for the bulk and waveguides of PPLN crystals, the SH wave has a higher relative noise level than the corresponding fundamental wave. For the same fundamental wave, the SH wave has lower noise in a bulk crystal than in a waveguide, and in MgO-doped PPLN than in undoped PPLN. In addition, the photorefractive effect incurred by the irradiation light can influence the SHG noise.