Key CLARITY technologies II – Four-Wave Mixing wavelength conversion National and Kapodistrian University of Athens Department of Informatics and Telecommunications Photonics Technology Laboratory

Many optical systems may not be naturally compatible with one another and require a means of converting photons of different energies. Wavelength/frequency conversion is a technique used to alter the wavelength of an optical field. The new wavelength can be within the same waveband or in a totally different waveband. A variety of media can be used: - Passive (waveguides, optical fibers …) - Active (semiconductor lasers, amplifiers …) Introduction - Wavelength conversion Wavelengthconversiondevice λ λiλi λ λoλo

In an optical system non-linear response can occur when there is sufficiently intense illumination. The nonlinearity is exhibited in the polarization of the material (P) which is often represented by a power series expansion of the total applied optical field (E): Optical non-linearity usually occurs due to 2 nd and 3 rd susceptibility: χ (2), χ (3) Different non-linear processes which depend on the material can occur: - Cross gain saturation - Cross-phase modulation - Four-wave mixing Introduction - Non-linear processes 1

In most techniques more than one optical fields are required: - the field to be wavelength converted at λ 1 (signal) - an optical pumping field at λ 2 (pump) The signal photons are scattered to a new energy due to a non-linear process present in the medium. Four-Wave Mixing is a χ (3) process and can take place in many media Different non-linear physical mechanisms can contribute to the FWM process: - gain - Kerr effect - two-photon absorption - … Introduction - Non-linear processes 2 INPUTOUTPUT λ λ1λ1 λ2λ2 λ λ3λ3 λ1λ1 λ2λ2 Non-linear process Four-Wave Mixing (FWM) Cross-Phase Modulation (XPM) Cross-Gain Modulation (XGM)

In FWM process four optical fields are involved: - at the input: the signal and the pump - at the output: the conjugate or idler and the satellite Consider two input frequencies present, a strong pump field at ω p, and a signal field at ω s (Ω = ω p – ω s ). New components are generated at the output due to the non-linear polarization proportional to the third order susceptibility: - the idler at ω i, ω i = 2ω p - ω s = ω p + Ω - the satellite at ω s, ω s = 2ω s - ω p = ω s - Ω The idler is the phase conjugate of the signal and the satellite is the conjugate of the pump Four-Wave Mixing 1 INPUTOUTPUT ω ωsωs ωpωp ω ωiωi ωsωs ωpωp Four-Wave Mixing ωsωs Ω

The efficiency of the FWM process (strength of the new products) depends on the pump power. In order to obtain high efficiency, the FWM process the phase matching condition is required (β is the propagation constant): Conversion of a waveband is possible FWM is an efficient wavelength conversion tool for wavelength-division multiplexed (WDM) telecommunication networks But it plays a negative role in the propagation of multi-wavelength signals in optical fibers, as new undesired wavelengths are generated. Four-Wave Mixing 2

Conversion from mid-IR to near-IR using FWM - The concept Within CLARITY the FWM process will be used to convert optical signals from the mid- infrared (MIR) regime for detection to the near-IR (NIR) regime. 3 rd order non-linear materials will be used to realize broadband parametric amplification. For conversion of the signal which lies within the MIR regime (3 – 5 μm) to the NIR regime (1.4 – 1.7 μm), the pump should be around 2 μm.

Conversion from mid-IR to near-IR using FWM - Engineering issues Phase matching condition depends on: Input wavelengths Waveguide dispersion and non-linear properties Input pump power High conversion efficiency and broadband operation can be achieved following specific design rules: Engineering the waveguide geometry: - Small effective mode area at the pump wavelength regime is required in order to exploit the high power of the pump field (<1 μm 2 ) - Mode overlap close to 1 in order to maximize the non-linear interaction between the FWM fields Engineering the waveguide dispersion: zero dispersion at the pump wavenegth regime Proper selection of input wavelengths: pump tunability is required High pump power: ~W range