Interferometric noise reduction through intrabit frequency evolution of directly modulated DFB lasers

On the various time constants of wavelength changes of a DFB laser under direct modulation

“…This behavior has been previously demonstrated using high-frequency phase modulation of the laser output, or, equivalently, by adding a chirp-generating signal to the datacarrying injection current [5]. But in a recent publication [6] it was shown that the thermal-like chirp directly reduces the interferometric noise without the need for any additional modulation. However, for this effect to be significant, the change in frequency during each bit must be at least several hundreds of megahertz, requiring the relevant frequency chirp time constant to be on the order of the bit length [6].…”

“…But in a recent publication [6] it was shown that the thermal-like chirp directly reduces the interferometric noise without the need for any additional modulation. However, for this effect to be significant, the change in frequency during each bit must be at least several hundreds of megahertz, requiring the relevant frequency chirp time constant to be on the order of the bit length [6]. This effect was successfully demonstrated [3] for frequencies as high as 622 Mb/s, implying the existence of time constants much shorter than 100 ns.…”

“…While the adiabatic term is proportional to the temporal injected current, the thermal chirp follows the junction instantaneous temperature, which, in turn, could be given by a convolution of the modulating current sequence and the experimentally obtained impulse response. Therefore, an increase in the injection current will lead to a frequency change of the form (6) where both and are positive constants, describing, respectively, the coefficients of adiabatic and thermal chirps.…”

Analysis of Incoherent Homodyne Crosstalk in Optical Networks

“…In this analysis, we assume that the three fields are co-polarized, giving maximum crosstalk [5]; and that all crosstalk products fall within the receiver bandwidth [3]. The receiver photocurrent from these fields is given by [6-8, 18, 19]:…”

“…If the main and interfering fields are not phase-correlated, then they will beat at the receiver, causing fluctuations in the received power level and an increase in the bit-error rate (BER) of the received signal. This phenomenon is often referred to as incoherent crosstalk [2,3], although there is considerable variation in the nomenclature used in the literature (e. g. [4,5]). …”