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Laser linewidth can have a significant impact on the transmission performance of signals within long spans of fibre with accumulated dispersion. The underlying mechanism is that a finite optical linewidth reflects random phase noise fluctuation of the optical field resulting from phase-amplitude coupling combined with the stochastic nature of the spontaneous emission processes within the active region. The transmission of a signal exhibiting phase noise results in phase modulation to amplitude modulation (PM-AM) conversion [1], thus contributing a penalty to the overall transmission performance. Furthermore, laser phase noise is particularly important in coherent communications where currently external cavity lasers are generally employed to fulfil the narrow linewidth requirements for both transmitter and local oscillator. In this paper we report a reduction of Lorentzian linewidth for a C-band monolithic tunable digital supermode (DS) distributed Bragg reflector (DBR) laser [2]. A prototype device containing a InGaAlAs/InP aluminium quaternary [Al(Q)] active region is compared alongside a device containing a traditionally used InGaAsP/InP phosphorus quaternary [P(Q)] active region. The DBR tuning regions and section dimensions for both devices are identical. Our previous publication [3] demonstrates elevated temperature operation for the prototype device, maintaining full tunability and single mode operation up to 55°C. Here, we have compared the results of Lorentzian linewidth across the C-band. The mean linewidth for the Al(Q) and P(Q) devices is 0.28 MHz and 0.61 MHz, respectively, and the maximum 0.59 MHz and 1.69 MHz, respectively. The theoretical expression for Lorentzian linewidth, δυ, is given by Henry[4], equation, where R is the cavity spontaneous emission rate (clamped above threshold), S is the cavity photon population, and α H is the linewidth enhancement parameter. For the DS-DBR laser studied here, full C-band tunability is achieved across seven supermodes; individual supermodes are selected using a front chirped grating. Tunability within each supermode is provided by a rear phase grating. With increasing grating current, free carrier scattering losses increase, decreasing the cavity photon population relative to the spontaneous emission rate and increasing the linewidth. The reduced linewidth of Al(Q) compared to P(Q) is primarily attributed to reduced α H [5], supported by our Hakki-Paoli measurements. A secondary effect is attributed to reduced photon losses within the Al(Q) active region. The reduced linewidths exhibited by the Al(Q) devices show promise for monolithically integrated transmitters and receivers for coherent systems. |
