We know that since the 1990s, WDM wavelength division multiplexing technology has been used for hundreds or even thousands of kilometers of long-distance optical fiber links. For most countries, fiber optic infrastructure is their most expensive asset, while the cost of transceiver components is relatively low.
However, with the explosive growth of data transmission rates in networks such as 5G, WDM technology is also becoming increasingly important in short-distance links, which are deployed in much greater volumes and therefore have an impact on the cost of transceiver components. and size are also more sensitive.
Currently, these networks still rely on thousands of single-mode optical fibers for parallel transmission through spatial division multiplexing channels, and the data rate of each channel is relatively low, only a few hundred Gbit/s (800G) at most. T-level is possible There are few applications.
But in the foreseeable future, the concept of ordinary space parallelization will soon reach the limit of its scalability, and must be supplemented by spectral parallelization of the data flow in each fiber to sustain further increases in data rates. This may open up a whole new application space for wavelength division multiplexing technology, where maximum scalability of channel number and data rate is crucial.
In this context, the optical frequency comb generator (FCG) plays a key role as a compact, fixed multi-wavelength light source that can deliver a large number of well-defined optical carriers. In addition, a particularly important advantage of optical frequency combs is that the comb lines are inherently equidistant in frequency, thus relaxing the requirements for inter-channel guard bands and avoiding the need for traditional schemes using DFB laser arrays. Frequency control on a single line.
It is important to note that these advantages apply not only to the WDM transmitter, but also to its receiver, where an array of discrete local oscillators (LOs) can be replaced by a single comb generator. Digital signal processing of wavelength division multiplexed channels can be further facilitated using an LO comb generator, thereby reducing receiver complexity and improving phase noise margin.
In addition, using LO comb signals with phase locking function for parallel coherent reception can even reconstruct the time domain waveform of the entire wavelength division multiplexed signal, thereby compensating for damage caused by optical nonlinearity of the transmission fiber. In addition to these conceptual advantages based on comb signaling, smaller size and cost-effective mass production are also key for future wavelength division multiplexing transceivers.
Therefore, among various comb signal generator concepts, chip-scale devices are of particular interest. When combined with highly scalable photonic integrated circuits for data signal modulation, multiplexing, routing, and reception, such devices could become the key to compact, efficient wavelength-division multiplexing transceivers that can operate at low It is cost-effective to manufacture in large quantities, and the transmission capacity of each optical fiber can reach tens of Tbit/s.
The figure below depicts the schematic diagram of a wavelength division multiplexing transmitter using an optical frequency comb FCG as a multi-wavelength light source. The FCG comb signal is first separated in the demultiplexer (DEMUX) and then enters the EOM electro-optical modulator. Through, in order to obtain the best spectral efficiency (SE), the signal is subjected to advanced QAM quadrature amplitude modulation.
At the transmitter outlet, each channel is recombined in a multiplexer (MUX), and the wavelength division multiplexed signal is transmitted through single-mode optical fiber. At the receiving end, the wavelength division multiplexing receiver (WDM Rx) uses the LO local oscillator of the second FCG to perform multi-wavelength coherent detection. The channels of the input wavelength division multiplexed signal are separated by a demultiplexer and then fed into the coherent receiver array (Coh. Rx). Among them, the demultiplexing frequency of the local oscillator LO is used as the phase reference of each coherent receiver. The performance of such a wavelength division multiplexing link obviously depends heavily on the basic comb signal generator, specifically the light width and the optical power of each comb line.
Of course, optical frequency comb technology is still in the development stage, and its application scenarios and market size are relatively small. If it can overcome technical bottlenecks, reduce costs and improve reliability, it will be possible to achieve large-scale applications in optical transmission.
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