Hello, everyone!
Today, I would like to share with you an article about Chirped Management Laser (CML) as a way to expand the bandwidth of GI-POF.
The demand of bandwidth for Internet traffic and access networks in the premises are rapidly increasing, fueled by video and graphic-rich applications. Therefore, the data rate at 40-Gb/s per channel is expanding to next-generation optical access networks and very- short-reach (VSR) optical links. Unlike long-haul and metro networks, access and VSR networks require low hardware cost and low operation expenses to make the transmission technology attractive and practical.
There is a growing interest in utilising directly modulated lasers (DMLs) in cost-sensitive metro and access optical links because of their potentially low cost, compact sise, low power consumption, and high output power characteristics when compared with other transmitter sources using external modulation (EM) scheme such as electro-absorption modulator (EAM) or Mach-Zehnder modulator (MZM).
As it is well known, however, DMLs are the carrier density modulation via drive current, giving rise to inherent and highly component-specific frequency chirp, i.e., a residual phase modulation (PM) accompanying the desired intensity modulation (IM). This chirp results in broad spectrum that severely limits the maximum transmission distance within ~20 and ~2-km SSMF for 10 and 40 Gb/s without dispersion compensation because of its interaction with fiber dispersion along the transmission link.
One way to overcome this issue is to use the special fiber with a negative dispersion characteristic, which is a good choice to take advantage of the positive chirp characteristics of DMLs to increase the reach without dispersion compensation modules that can cost as much as the transmission fiber. However, it is only suitable to new deployment of optical transport system but not fit to upgrade and change of the installed base of metro fiber links.
Fig. a. Schematic of chirped-managed DML transmitter.
Chirp-managed laser (CML) can provide a good optical source for access systems. In order to support high dispersion tolerance, a DFB laser biased at high direct current (DC) far above the threshold is used, digital data directly modulate this DFB laser, and a suitable optical filter is used to control the phase between the adjacent bits.
Fig. b. Schematic of signal waveform.
The additional benefits of the higher bias are :-
High output power
Wide modulation bandwidth
Low timing jitter
Suppressed transient chirp.
CML technology simultaneously meets two market needs:
The data rate upgrade from 2.5 to 10 Gb/s, even to 40 Gb/s, in the emerging metro market.
The migration of small form factor pluggable optics from short reach to high- performance long reach and WDM links.
The directly modulated signals have low extinction ratio (ER) and an accompanying adiabatic chirp. An optical spectrum reshaping (OSR) filter is placed at the laser output to perform frequency modulation (FM) to amplitude modulation (AM) conversion to increase the ER and convert the slowly-varying adiabatically chirped pulses to flat-topped chirp pulses with abrupt phase transitions. The output of the CML has been shown to have tolerance to both negative and positive dispersion for 10-Gb/s optical links.
The CML technique has been applied in 10-Gb/s data links with 200-km transmission over SMF without dispersion compensation and 675-km transmission using a combination of electronic dispersion compensation (EDC) and tunable dispersion compensating modules at the receiver. We have developed a 40-Gb/s CML transmitter with high dispersion tolerance using a simple combination of a directly modulated DFB laser and the subsequent conventional optical filter.
You are welcome to leave a message and exchange in the comment area. Thank you!
