Optical Wavelength Converters: Types and Methods

As the name suggests, a wavelength converter converts an optical signal from one wavelength to another. These converters are widely used in WDM (Wavelength Division Multiplexing) networks. Optical wavelength converters can be categorized into four types based on their input and output wavelength handling capabilities:

• Fixed Input, Fixed Output
• Variable Input, Fixed Output
• Fixed Input, Variable Output
• Variable Input, Variable Output

A Fixed Input, Fixed Output converter accepts a signal with a fixed wavelength as input and converts it to a fixed wavelength output signal. A Variable Input, Fixed Output converter accepts an arbitrary wavelength as input and provides a fixed wavelength as output. A Fixed Input, Variable Output converter takes a fixed wavelength signal as input and converts it to a variety of wavelengths as the output signal. Finally, a Variable Input, Variable Output converter converts a variable wavelength input signal to a variable wavelength output signal.

Wavelength Conversion Methods

Several methods are used for wavelength conversion, including optoelectronic, optical gating, interferometric, and wave mixing techniques.

Optoelectronic Conversion

This type of optical wavelength converter is typically a variable input, fixed output type. It converts the input optical signal to an electrical signal, regenerates it, and then transmits it using a laser. An optical receiver takes either 1310 nm or 1550 nm wavelength as input. A tunable laser device is used to generate the desired output wavelength.

Optical Gating

This method utilizes cross-gain modulation and exploits the nonlinear effects of a Semiconductor Optical Amplifier (SOA). It typically takes a variable input and provides either a fixed or variable optical signal as output. This optical gating method uses an optical device whose characteristics vary based on the intensity of the input optical signal to achieve wavelength conversion.

Interferometric Conversion

This method also leverages phase distortion from cross-gain modulation to achieve wavelength signal conversion. It’s often referred to as cross-phase modulation. The carrier density varies based on the input signal, which in turn changes the refractive index and modulates the phase of a probe signal.

Wave Mixing

This method utilizes the nonlinear phenomena of the transmission medium to achieve wavelength conversion. Four-wave mixing produces frequencies $f_1$, $f_2$, $f_3$, and $f_1 + f_2 - f_3$. This is exploited to obtain the desired conversion with the help of an optical filter. If we consider $f_s$ as the signal frequency and $f_p$ as the probe frequency, we will have $2f_s - f_p$ and $2f_p - f_s$ as the resulting frequencies.

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