Transcript CLARITY Tutorial on Four Wave Mixing

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
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.
λi
λ
Wavelength
conversion
device
A variety of media can be used:
- Passive (waveguides, optical fibers …)
- Active (semiconductor lasers, amplifiers …)
λo
λ
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
-…
INPUT
OUTPUT
Non-linear process
λ
λ1
λ2
Four-Wave Mixing (FWM)
Cross-Phase Modulation (XPM)
Cross-Gain Modulation (XGM)
λ
λ1
λ2
λ3
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 a signal 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
INPUT
OUTPUT
Four-Wave Mixing
Ω
ω
ωs
ωp
ω
ωs
ωs
ωp
ωi
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 midinfrared (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 (<1 μm2)
- 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