Polarisation-sensitive four-wave mixing and soliton self-frequency shift effect in the highly birefringent photonic crystal fibre

By adjusting the polarisation state of the pump at 805 nm parallel to slow （x） and fast （y） axes of the highly birefringent photonic crystal fibre with zero dispersion wavelengths 790 nm and 750 nm, this paper demonstrates the efficient polarisation-sensitive four wave mixing involved in pump, anti-Stokes and Stokes signals and soliton self- frequency shift effects induced by the phase-matching between red-shifted solitons and blue-shifted dispersive waves. If the reduction of coupling efficiency to the circular pump laser mode or other circular fibres due to asymmetry of the core is neglected, more than 98% of the total input power is kept in a single linear polarisation. Controlled dispersion characteristic of the doublet of fundamental guided-modes results in achieving light field strongly confined in principal axes of photonic crystal fibre, and enhancing the corresponding nonlinear-optical process through the remarkable nonlinear birefringence.

Observation of four-wave mixing in caesium atoms using a noncycling transition

In this paper the generation of four-wave mixing （FWM） signal using a noncycling transition of caesium atoms is investigated when the pumping laser is locked to the transition 6S1/2F= 4→6P3/2^F1 = 4, and meanwhile the probe frequency is scanned across the 6S1/2F = 4→6P3/2 transition. The efficiency of the four-wave mixing signal as a function of the intensity of the pumping beams and the detuning of the pumping beams is also studied. In order to increase the detection efficiency, a repumping laser which is resonant with 6S1/2F = 3→6P3/2F′= 4 transition is used. A theoretical model is also introduced, and the theoretical results are in qualitative agreement with experimental ones.

Performance analysis of a hybrid optical CDMA/DWDM system against inter-symbol interference and four wave mixing

This paper reports the implementation and theoretical model for analyzing an optical CDMA/DWDM hybrid system to reduce two major problems,the Inter-Symbol Interference(ISI)and the Four-Wave Mixing(FWM)effects and improve the performance of optical subscriber access networks by using Zero Cross-Correlation(ZCC)optical codes,which helps to reduce the effect of the Multiple Access Interference(MAI).A squeezing method is used in the proposed hybrid system to completely suppress the ISI.In this method,the sequence interval of the signature code is squeezed into a duration of less than one-bit.The hybrid system is capable of accommodating 120 optical CDMA users carried by only 10 DWDM wavelengths spaced by 0.2 nm with 60 Gb/s/wavelength transmitted over 105.075 km of optical fiber.The result shows that the optimum interval of the code sequence is a quarter(i.e.,25%)of the bit duration.Moreover,the results reveal that the CDMA technology based on the spread spectrum is capable of increasing the nonlinear tolerance of the proposed hybrid system as the energy of bits is distributed over the chip sequence code.Also,the number of ones/weight and the positions between them have a significant effect on the performance of the proposed hybrid system.

Four-wave mixing based 10-Gb/s tunable wavelength conversion in dispersion-flattened microstructure fibers

All-optical wavelength conversion of 10-Gb/s signal based on four-wave mixing is experimentally demonstrated in a 30-m-long dispersion-flattened microstructure fiber with small positive dispersion. For an average pump power of 26 dBm, the conversion efficiency was around -19.5 dB with the fluctuation less than ±1.4 dB, covering a conversion bandwidth of 20 nm. The eye diagram of the converted signal shows good eye opening.

Terahertz-wave generation by surface-emitted four-wave mixing in optical fiber

We propose a novel terahertz-wave source through the four-wave mixing effect in a conventional single-mode optical fiber pumped by a dual-wavelength laser whose difference frequency lies in the terahertz range. Surface-emitted geometry is employed to decrease absorption loss. A detailed derivation of the terahertz-wave power expression is presented using the coupled-wave theory. This is a promising way for realizing a reasonable narrow-band terahertz-wave source.

Four-wave mixing model solutions for polarization control of terahertz pulse generated by a two-color laser field in air

A four-wave mixing （FWM） model is used to analyze the polarization control of terahertz （THz） pulse generated by a two-color laser field in air. The analytic formula for the THz intensity varying with the THz polarizer angle, and the relative phase between the two pulses, are obtained. The corresponding numerical results agree well with both numerical result obtained from a quantum model and measured data reported. Moreover, possible phenomena are predicted for variables not found in other experiments. Compared with the quantum model, the FWM model gives analytic formulas and clear physical pictures, and has the advantage of efficient computing time.

Terahertz generation based on four-wave mixing difference frequency by resonance-enhanced third-order nonlinear of media

Using nanosecond pulse near-infrared and mid-infrared laser pulses as the pump source, we obtain terahertz wave sources via four-wave difference frequency mixing. From the coupled wave theory, we analyze the four-wave mixing process of GaSe crystal and alkali metal vapor in detail, get the analytical expression of terahertz wave output power, and discuss the conditions for achieving phase matching. By adjusting the pump frequency, the third-order nonlinear polarization of alkali metal vapor is resonance-enhanced. This program offers a new type of high-power terahertz radiation source.

Type-Ⅱmicro-comb generation in a filter-driven four wave mixing laser[Invited]

We experimentally demonstrate the generation of highly coherent Type-II micro-combs based on a microresonator nested in a fiber cavity loop, known as the filter-driven four wave mixing （FD-FWM） laser scheme. In this system, the frequency spacing of the comb can be adjusted to integer multiples of the free-spectral range （FSR） of the nested mlcro-resonator by properly tuning the fiber cavity length. Sub-comb lines with single FSR spacing around the primary comb lines can be generated. Such a spectral emission is known as a ＂Type-II comb＂. Our system achieves a fully coherent output. This behavior is verified by numerical simulations. This study represents an important step forward in controlling and manipulating the dynamics of an FD-FWM laser.

Multiwavelength Raman fiber ring laser with the spectrum profile broadened by parametric four wave mixing in highly nonlinear dispersion-shifted fibers

A broadband multiwavelength Raman fiber ring laser （RFRL） covering the whole C-band at room temperature are presented. The effect of the intracavity highly nonlinear dispersion-shifted fiber on broadening and flattening the output spectrum envelope is discussed and experimentally demonstrated. More than 45-dB extinction-ratio multiwavelength output from 1527.76 to 1566.86 nm with 100-GHz channel spacing and 2.1-dB power ripple has been achieved by carefully controlling the individual powers of three pump lasers.

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing （FWM）-based wavelength conversion in a 26.5-km dispersion shifted fiber （DSF） is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave （CW） light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.

Quantum optical devices based on four-wave mixing in hot rubidium vapor

In this paper,we briefly review the recent experimental progresses in quantum optics based on four-wave mixing(FWM) processes in hot rubidium vapor,particularly our two recent experiments in quantum information.We have experimentally produced strong quantum correlations between three bright beams generated by two cascaded FWM processes.The intensity difference squeezing with the cascaded system is enhanced to(-7.0±0.1)d B from the(-5.5±0.1)d B/(4.5±0.1)d B with only one FWM process.Also,this system can be easily extended to multiple modes using multiple FWM processes.Besides,we have also successfully realized a cascade all-optical transistor(AOT),which is driven by a very weak light beam about 800 photons in total.The required probe power for achieving a switching efficiency of 50% can be as low as 180 p W,and it can manipulate a light beam with power of 5.0×106 times more,which proves the cascade of the AOT.Both experiments may find wide applications in quantum information and optical communication.

Highly nonlinear enhanced-core photonic crystal fiber with low dispersion for wavelength conversion based on four-wave mixing

In this paper, a new structure of highly nonlinear low dispersion photonic crystal fiber （HN- PCF） by elliptical concentration of GeO2 in the PCF core has been proposed. Using finite difference time domain （FDTD） method, we have analyzed the dispersion proper- ties and effective mode area in the HN-PCF. Simulative results show that the dispersion variation is within 4-0.65 ps/（nm.km） in C-band, especially 0.24ps/（nm.km） in 1.55 ~tm wavelength. Effective area and nonlinear coeffi- cient are 1.764pm2 and 72.6Wl＂km-1 respectively at 1.55 ~tm wavelength. The proposed PCF demonstrates high nonlinear coefficient, ultra small effective mode area and nearly-zero flattened dispersion characteristics over C- band, which can have important application in all-optical wavelength conversion based on four wave mixing （FWM）.

A Refractive Index Sensor Based on Four-Wave Mixing in D-Shaped Tellurite Photonic Crystal Fiber

In this study,we design a refractive index(RI)sensor using a novel cadmium telluride photonic crystal fiber(TPCF).Based on four-wave mixing(FWM),the changes in RI can be accurately detected,and RI sensing in the mid-infrared region(MIR)can be achieved by detecting wavelength shifts in the Stokes and anti-Stokes spectra caused by the changes in RI of the liquid to be measured.When the pump wavelength of FWM lies in the normal and abnormal dispersion regions of the TPCF,the RI response of the idler frequency wave and the signal wave are analyzed by numerical simulation methods.The simulation results show that the RI sensitivity of the sensor can be as high as 7692 nm/RIU with a linearity is up to 99.9%at the pump wavelength of 3380 nm.To our knowledge,the RI sensing sensitivity of the MIR is presented for the first time in this study by using FWM in the non-silicon PCF.

Cascaded four-wave mixing generation with pumping in the normal dispersion regime

we report on the generation of cascaded four-wave mixing （CFWM） in a photonic crystal fiber with a high peak power picosecond pulse pumping in the normal dispersion region and a weak continuous wave seeded. We also experimentally investigate the dependence of CFWM bandwidth on the seeding wavelengths. Experimental results demonstrate the existence of optimum seeding wavelengths for broadband CFWM even though the pulse wavelength is 50 nm shorter than the zero dispersion wavelength. And CFWM products spanning more than 300 nm are experimentally obtained.

Quantum Effects of Bose-Einstein Condensates

In this paper,we study quadrature squeezings of two Bose-Einstein condensates with collision and non- classical properties of pair entanglement in four wave mixing in Bose-Einstein condensates.With the aid of a numerical method,we find that the two modes(pair entanglement modes)α_1 and α_2 may exhibit quadrature squeezing,in which they are affected by the initial phase.It is shown that the two pump modes exhibit the same super-Poissonian distri- bution.The analysis for the mode-mode correlation shows that there always exists a violation of the Cauchy-Schwartz inequality,which means that correlation between the two pump modes is nonclassical.

Observation of two multiwave mixing processes via dual optical pumping channels

Based on double optical pumping channels, we experimentally study the competition between two coexistent six-wave mixing （SWM） processes falling into two electromagnetically induced transparency windows by scanning the frequency of the probe field in two similar five-level atomic systems of S5Rb. By blocking one optical pumping channel unrelated to the four-wave mixing （FWM） process, one SWM process, together with the FWM process, generated by a conjugated small-angle static grating could be observed in the spectrum. Moreover, the other SWM process obtained by blocking the first SWM channel is also observed together with the FWM process in a lower N-type four-level subsystem. These experimental results agree well with theoretical predictions.

Saturation of biphoton generation near atomic resonance

We have numerically investigated the biphoton generation rate as a function of several parameters in the sponta- neous four-wave mixing in cold atoms. It has been found that the biphoton generation rate can easily reach saturation with the intensity of the coupling laser increasing. The saturation intensity is mainly dependent on the dephasing rate of the ground states, unrelated to the pumping laser. It implies that though the biphoton waveform can be manipulated by the coupling laser, the generation rate of the biphoton cannot increase markedly after the saturation. The saturation effect also suggests that there is an optimal coupling laser for obtaining the largest biphoton generation rate with a sufficiently long coherence time.

Experiment study of wavelength conversion in a dispersion-flattened photonic crystal fiber

Wavelength conversion based on four-wave mixing （FWM） has been demonstrated using a 40-m dispersion flattened highly nonlinear photonic crystal fiber （HNL-PCF）. A conversion efficiency of -26 dB for a pump power of 19.5 dBm and a conversion bandwidth of 28 nm have been obtained, which are limited by the continuous wave （CW） laser wavelength range and tunability of optical band pass filters （OBPFs）.

Stable multi-wavelength erbium-doped fiber laser based on dispersion-shifted fiber and Sagnac loop filter

A multi-wavelength erbium-doped fiber laser （MEDFL） with simple line structure is experimentally demonstrated by using a Sagnac interferometer as a comb filter. It is shown that the multi-wavelength lasing is quite stable at room temperature due to the four-wave mixing （FWM） effect among different laser channels in the dispersion-shifted fiber cooperated in the laser cavity.

Combined nonlinear effects for UV to visible wavelength generation in a photonic crystal fiber

We experimentally study the combined nonlinear effects, including four-wave mixing, stimulated Raman scat- tering, soliton dynamics, and cross-phase modulation by coupling femtoseeond pulses around 850 nm into the normal dispersion region near the zero-dispersion wavelength in the fundamental mode of a homemade silica photonic crystal fiber. The nonlinear optical dynamics at different stages are demonstrated, and the discrete ultraviolet （UV） to visible wavelengths widely separated from the pump wave are generated by the interaction of several nonlinear effects involved. The UV to visible wavelengths can be used as short pulse sources for multi- photon ionization, fluorescence spectroscopy, and biochemical imaging.