In order to provide higher bandwidth in the access network part, global operators have begun to gradually implement the “optical in copper retreat” plan, deploying passive optical networks represented by EPON and GPON. From a technical point of view, EPON and GPON both work in the time division multiplexing mode, which can be collectively referred to as TDM-PON.
TDM-PIN’s mechanism of allocating time slices to each user at a single wavelength not only limits the available bandwidth per user, but also greatly wastes the available bandwidth of the fiber itself. The introduction of wavelength division multiplexing technology into the PON system, that is, WDM-PON, will greatly increase the user access bandwidth and meet the ultimate needs of users, so WDM-PON is also considered to be the solution of the next generation access network.
Transmitter light source
ONU light source
The various ONU light source technologies in the WDM-PON system belong to the category of single-wavelength light sources. FP-LD and RSOA are the implementation technologies of colorless ONU mainly used in the current WDM-PON system. Among them, FP-LD has been widely used in today’s optical communication systems, although the FP-LD used in WDM-PON systems is slightly different (such as requiring low reflectivity of the front surface and higher rear surface), its cost is still relatively low, and the output is relatively large. In addition to its use as an amplifier, SOA can be used in various functions such as modulation, wavelength conversion, regeneration, and high-speed (especially 40Gb/s or more) optical exchange. Slightly modified to its structure results in a reflective RSOA, which is particularly useful in WDM-PON systems.
In general, although SOA/RSOA devices are diverse, mature processes, and can be optimized for different applications, they can be regarded as still in the laboratory application stage, the commercial market is in its infancy, and there is no driving force for the widespread application of SOA/RSOA devices. There are not too many suppliers of SOA/RSOA products in the world, including CIP in the United Kingdom, Kamelian in Scotland, etc., and ETRI in South Korea is also developing RSOA devices for WDM-PON systems and providing them to Corecess. However, the current RSOA devices for WDM-PON are still relatively expensive, and need to be scaled up to further reduce costs.
OLT light source
For OLTs, it is inconvenient to use this single-wavelength light source solution because different wavelengths are required to communicate with each ONU. OLT light sources can also use spectrum segmentation for wide-spectrum light sources, but because spectrum segmentation will introduce large losses (about 18dB), which will cause tight power budgets, multi-wavelength light sources are mainly used at present. A multi-wavelength light source is an integrated device that can produce multiple wavelengths of light at the same time, which is suitable for use as an OLT light source in WDM-PON systems. At present, there are mainly the following multi-wavelength light sources.
Multi-frequency laser (MFL): As shown in Figure 1, in a multifrequency laser, a 1×N array waveguide grating and N optical amplifiers are integrated, and an optical amplifier is integrated at each input of the array waveguide grating. An optical cavity is formed between the optical amplifier and the output of the array waveguide grating, and if the amplifier provides enough gain to overcome the losses in the cavity, there is a laser output, and the output wavelength is determined by the filtering characteristics of the array waveguide grating. By directly modulating the bias current of each amplifier, a multi-wavelength downstream signal can be generated.
Figure 1: Schematic diagram of the structure of a multi-frequency laser
The wavelength interval of MFL is determined by the waveguide length difference in the array waveguide grating, which can be accurately controlled, and each wavelength can be uniformly adjusted by controlling the same temperature, which is convenient for wavelength monitoring and is an ideal OLT light source. Direct modulation is also possible in multi-frequency lasers, but the modulation speed is also limited due to the relatively long laser cavity. 16 MFLs with a channel spacing of 200 GHz and 20 channels of 400 GHz have been introduced with a direct modulation rate of 622 Mbit/s.
Gain-coupled DFB laser array: DFB laser array is to fabricate multiple InGaAsP/InP multi-quantum well waveguide lasers with the same properties on the same substrate, which is an integrated multi-wavelength light source. The DFB laser array combines a gain coupling mechanism and tuning capability on a laser module, and wavelength tuning is achieved by controlling temperature. Thin-film resistors are integrated into the device, and controlling its temperature can change the wavelength, which results in near-continuous tuning. The advantages of this device are its compact size and high-speed modulation characteristics, but it also has the major problem of precisely controlling the wavelength of each laser in the array, because each laser wavelength is determined by an independent filter.
Supercontinuum laser light source: A femtosecond pulse is generated using a femtosecond laser, which is transmitted through a nonlinear medium and causes pulse expansion and linear frequency chirping due to self-phase modulation effects. On the broadened spectrum, the wavelength increases linearly with time, so different wavelengths occupy different time slots, and the downstream data is modulated on each channel by TDM. The broadened spectrum can be amplified and split to support multiple PONs and be shared by a large number of users.
WDM multiplexer
In WDM-PIN, the wavelength division multiplexer is often called a wavelength router, which demultiplexes the downstream signal and assigns it to a designated ONU, while multiplexing the upstream signal into a fiber and transmitting it to the OLT. Its main indicators are insertion loss, crosstalk, channel spacing, polarization dependence and temperature sensitivity.
At present, there are a variety of structural devices, such as thin-film interference filters, acousto-optic filters, fiber gratings, AWGs, etc. In the case of a small number of channels, thin-film interference filters and fiber gratings are a better choice; For WDM systems with more than 16 channels, AWG is mostly used for multiplexed/demultiplexed devices, mainly because AWG is a loss independent of the number of paths. In recent years, the array waveguide grating developed in recent years has the advantages of small size, easy integration, narrow channel spacing and stable performance, which has promoted the development of WDM-PIN. Although AWG has been widely used in DWDM systems, when applied to PON networks, it is difficult to use active temperature control devices and will face wavelength drift problems caused by temperature changes, so thermally insensitive AWG is essential for WDM-PON systems. At present, the thermal insensitive AWG technology is relatively mature, but the price is slightly more expensive than ordinary AWG, if it can be mass-produced and widely used, the cost of thermal insensitive AWG will be basically the same as the cost of ordinary AWG.
WDM receiver
The receiver in the WDM-PON system includes a photodetector and a companion circuit (digital optical receiver) for signal recovery. Commonly used photodetectors are PIN photodiodes and avalanche photodiodes, which have different applications depending on the required sensitivity. Digital optical receivers typically consist of a preamplifier, a master amplifier, and a clock data recovery circuit (CDR). The receiver in WDM-PON consists of a demultiplexer and a receiver array. In WDM receivers, the linear crosstalk at the demultiplexer needs to be considered, and the linear crosstalk can cause a rapid increase in power loss. The methods of controlling crosstalk include equalizing the power from each ONU and double filtering the received signal.
Wavelength monitoring
Since multiple wavelengths are employed in WDM-PON, and because AWG is generally placed in the open air and there is no temperature control, the effect of temperature on AWG passband changes is very important. Generally speaking, the temperature difference range of AWG is -40~85°C, and the passband offset rate is 0.011nm/°C. Therefore, at such a temperature difference, there will be a shift of 1.4 nm in the wavelength. Such an offset will be separated by the wavelength of DWDM on the same order of magnitude (100~200GHz), which will seriously affect the work of WDM-PIN. Therefore, wavelength detection and tuning work is required in the OLT. Wavelength monitoring uses a differential algorithm, comparing the transmitted power of a channel with the power through the wavelength router, to obtain a difference signal, if less than the difference signal at the previous moment, the temperature changes ΔT in the current direction, on the contrary, it indicates that the channel mismatch increases, and the temperature changes ΔT in the opposite direction. This method should appropriately select the rate and step ΔT of temperature regulation.
Wavelength monitoring can be achieved by monitoring the downlink channel power and monitoring the uplink channel power. For composite PONS that use WDM only downstream, only downstream channel power can be monitored, which requires additional loopback fiber, or a monitoring channel and fiber grating. For the upstream using spectrum splitting WDM-PON, the upstream signal power before and after demultiplexing can be compared at the OLT, and wavelength monitoring only requires the addition of couplers and no additional channels.
