Continuously tunable, narrow linewidth mm-wave generation from a monolithically integrated triple DFB laser chip

Abstract

Summary form only given. Generation of stable and tunable mm-wave and THz signals is extremely attractive for applications including tomography, gas sensing and imaging for security systems. Currently photonic methods for generating signals at these frequencies show either very limited tunability, as in the case of passively mode-locked lasers and THz quantum cascade lasers , or poor spectral purity, evinced in the photomixing of two uncorrelated laser sources. In addition, current systems using the combination of a number of discrete optical elements require the use of bulky alignment optics increasing packaging costs and sensitivity to environmental noise sources. In this work a relatively simple arrangement of three DFB laser sources on a single chip is proposed and demonstrated, a schematic of the scheme is shown in Fig. 1(a). Two lasers DFB1 and DFB2 inject into DFB3 whose free running frequency is designed to emit between those of DFB. The result is that Four Wave Mixing (FWM) products of the pairs of lasers DFB and DFB are produced within DFB. These FWM signals then produce feedback signals that coincide in frequency with the complimentary laser source, i.e. the laser pair DFB produce a FWM product equal in frequency to DFB, thus locking their phases and stabilising the optical beating signal. Furthermore, by simply tuning the frequency spacing of DFB and DFB, ensuring that DFB tracks their centre point, the generated beating signal can be continuously tuned over a wide frequency range. This scheme was fabricated on a multi-quantum well AlGaInAs\InP material system and the DFB lasers were defined using upper-cladding sidewall modulation gratings in order to generate precise spacing of the three laser wavelengths [3]. Tuning of the laser wavelengths was achieved by direct current injection variation, with integrated SOA sections ensuring the optimal injection amplitude range was maintained. Measurements of the RF signal generated by the beating signal from the output at DFB show a linewidth reduction of an order of magnitude (to 2.5MHz) when the system is in the phase locked condition. Locking range measurements show a tolerance of ~2GHz on the position of the wavelength of DFB. Furthermore, the locked frequency can be tuned continuously from a few GHz to 40GHz (the maximum measureable frequency of our RF spectrum analyser) as shown in Fig.1 (b).