On-chip integrated zero-RAM frequency modulator for noise reduction and linewidth narrowing in discrete-mode lasers

Abstract

Wavelength tuneable lasers for 40G and 100G coherent optical communications systems need to meet stringent requirements on narrow linewidth emission across the entire tuning range, with typical values of 300-500 kHz required in commercial systems. Higher capacities can be achieved in next generation systems by employing higher order modulation formats such as 16QAM or 64QAM. However, such systems have even more stringent linewidth requirements [1]. For example, square 64QAM transmission at data rate 40Gbit/s (the baud rate is 6.7G symbols / second) demands a laser with 1 kHz linewidth. Although linewidths as low as 70 kHz and even lower have been demonstrated in free-running Discrete Mode Laser Diodes (DMLD), further (active) linewidth reduction is required. This can be achieved by introducing a phase modulator on chip and applying a Pound-Drever-Hall (PDH) technique to stabilize on an optical reference cavity for instance. In the usual implementation of the PDH technique an external acousto-optic modulator (AOM) is used for active phase noise correction (Figure 1 (a), compare blue and red curves). This worsens significantly the intensity noise (Figure 1 (b)) rendering the laser unsuitable for QAM applications. In this talk we will report a DMLD laser with on-chip integrated frequency modulator based on Joule heating and showing zero residual amplitude modulation (RAM) (Figure 1 (a) and (b) green curves show preliminary stabilization results). DMLDs with integrated phase modulators are highly interesting for coherent optical communications and other applications demanding actively narrowed linewidth emission while offering an economic approach with a focus on high volume manufacturability of monolithic semiconductor lasers [2].