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National and Kapodistrian University of Athens Department of Informatics and Telecommunications Photonics Technology Laboratory. Key CLARITY technologies II – Four-Wave Mixing wavelength conversion. Introduction - Wavelength conversion. Wavelength conversion device. λ i. λ o. λ. λ.

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key clarity technologies ii four wave mixing wavelength conversion

National and Kapodistrian University of Athens

Department of Informatics and Telecommunications

Photonics Technology Laboratory

Key CLARITY technologiesII – Four-Wave Mixingwavelength conversion


Introduction - Wavelength conversion








  • 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 - Non-linear processes 1

  • 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 2nd and 3rd 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 2








  • In most techniques more than one optical fields are required:
  • - the field to be wavelength converted at λ1(“signal”)
  • - an optical pumping field at λ2 (“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- …



Non-linear process

Four-Wave Mixing (FWM)Cross-Phase Modulation (XPM)Cross-Gain Modulation (XGM)


Four-Wave Mixing 1









  • 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 ωp, and asignal field at ωs (Ω = ωp – ωs).
  • New components are generated at the output due to the non-linear polarization proportional to the third order susceptibility:
  • - the idler at ωi, ωi = 2ωp - ωs = ωp + Ω
  • - the satellite at ωs, ωs= 2ωs - ωp = ωs - Ω
  • The idler is the phase conjugate of the signal and the satellite is the conjugate of the pump



Four-Wave Mixing



Four-Wave Mixing 2

  • 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.

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.
  • 3rd 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 (<1μm2)

- 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