Wavelength conversion is an important function in WDM (Wavelength Division Multiplexing) network, as it enables better utilization of bandwidth and reduces blocking probabilities, which is caused by the factors of insufficient network resources like wavelength or bandwidth, lack of wavelength converters in network, routing and wavelength assignment decision made on outdated network state information. The wavelength continuity constraint increases the blocking probability. Wavelength continuity constraint can be relaxed by using wavelength converters with the development of traffic grooming technique. A wavelength converter is a single input/output device that converts the wavelength of an optical signal arriving at its input port to the different wavelength as the signal departs from its output port, but otherwise leaves the optical signal unchanged. Different levels of wavelength conversion shown in Figure 1.
Figure1: Different levels of wavelength conversion
Depending on the mapping functions and the form of control signals, wavelenth converters can be classified into three types: optoelectronic (OEO), optical gating, and wave-mixing. Figure 2 (b)-(d) shows functional block diagrams for the three types of wavelength converters. (a) is a general wavelength converter’s functional block diagrams.
Figure2: Functional Block Diagrams for Different Wavelength Converters
Optoelectronic (O/E-E/O) Wavelength Conversion
In this method, the incident signal at the input wavelength λ1 to be converted into an electrical bit pattern, then amplification and reshaping and then convert to optical signal at the desired wavelength λ2 (shown in Figure 3). Such a way is relatively easy to implement as it uses standard components. Its other advantages include an insensitivity to input polarization and the possibility of net ampliﬁcation. Among its disadvantages are limited transparency to bit rate and data format, speed limited by electronics, and a relatively high cost, all of which stem from the optoelectronic nature of wavelength conversion.
Figure3: O/E-E/O Wavelength Conversion
Optical Gating and Wave-mixing Wavelength Conversion
These two methods for wavelength conversion belong to All-Optical Wavelength Conversion. In these all-optical methods, the optical signal is allowed to remain in the optical domain throughout the conversion process. Note that, in this context, all-optical, refers to the fact that there is no O/E conversion involved. What are optical gating and Wave-mixing? Do not worry, you will have a clear understanding after reading the following content.
Optical Gating is a series of techniques using cross-modulation to achieve wavelength conversion. These techniques utilize active semiconductor optical devices such as semiconductor optical amplifier (SOA) and lasers. Cross modulation methods can be further divided into cross-gain modulation (XGM) and cross-phase modulation (XPM) Mode. To date, the most promising method for wavelength conversion has been cross-modulation in an SLA (Semiconductor laser amplifiers) in which either the gain or the phase can be modulated. A basic XGM converter is shown in Figure 4 (a). The idea behind XGM is to mix the input signal with a cw (continuous wave) beam at the new desired wavelength in the SLA. Due to gain saturation, the cw beam will be intensity modulated so that after the SLA it carries the same information as the input signal. We can see that a filter is placed after the SLA, which is used to to terminate the original wavelength. The signal and the cw beam can be either co- or counterpropagating. A counterpropagation approach has the advantage of not requiring the filter as well as making it possible for no wavelength conversion to take place. A typical XGM SLA converter is polarization independent but suffers from an inverted output signal and low extinction ratio. Figure 4 (b) shows Cross-phase modulation using an SLA for wavelength conversion which makes it possible to generate a noninverted output signal with improved extinction ratio. The XPM relies on the fact that the refractive index in the active region of an SLA depends on the carrier density. Therefore, when an intensity-modulated signal propagates through the active region of an SLA it depletes the carrier density, thereby modulating the refractive index, which results in phase modulation of a CW beam propagating through the SLA simultaneously.
Figure4: Use of an SLA for wavelength conversion. (a) Cross-gain modulation. (b) Cross-phase modulation.
Wave-mixing (Figure 5) arises from a nonlinear optical response of a medium when more than one wave is present. It results in the generation of another wave whose intensity is proportional to the product of the interacting wave intensities. Wave-mixing preserves both phase and amplitude information, offering strict transparency. It also allows simultaneous conversion of a set of multiple input wavelengths to another set of multiple output wavelengths and could potentially accommodate signals with bit rates exceeding 100 Gb/s. There are two types of Wave-mixing: FWM (Four-wave mixing) and DFG (Difference frequency generation). FWM is an intermodulation phenomenon in non-linear optics, whereby interactions between two wavelengths produce two extra wavelengths in the signal. It is similar to the third-order intercept point in electrical systems. Four-wave mixing can be compared to the intermodulation distortion in standard electrical systems. DFG is a consequence of a second-order nonlinear interaction of a medium with two optical waves: a pump wave and a signal wave. This technique offers a full range of
transparency without adding excess noise to the signal, and spectrum inversion capabilities, but it suffers from low efficiency. The main difficulties in implementing this technique lie in the phase matching of interacting waves and in fabricating a low-loss waveguide for high conversion efficiency.
Figure5: A wavelength converter based on nonlinear wave-mixing effects. the value n=3 corresponds to FWM and n=2 corresponds to DFG
This article mentions the various techniques used in the design of a wavelength converter. They have their own advantages and disadvantages. The actual choice of the technology to be employed in wavelength conversion in a network depends on the requirements of the particular system.