Passive harmonic mode locking by mode selection in Fabry–Perot diode lasers with patterned effective index

Abstract

We demonstrate passive harmonic mode-locking of a quantum well laser diode designed to support a discrete comb of Fabry-Perot modes. Spectral filtering of the mode spectrum was achieved using a non-periodic patterning of the cavity effective index. By selecting six modes spaced at twice the fundamental mode spacing, near-transform limited pulsed output with 2 ps pulse duration was obtained at a repetition rate of 100 GHz.The generation of high power, ultrashort optical pulses using mode-locked diode lasers is of considerable importance across a range of applications in optical communication systems and data processing [1,2]. In recent years, the wide gain bandwidth of certain self-assembled gain materials has enabled sub-picosecond and femtosecond pulsewidths to be obtained [3][4][5]. In parallel, various techniques such as colliding pulse mode-locking (CPML) and harmonic mode-locking have been adapted for diode lasers that allow higher repetition rates to be achieved while avoiding the lower average power associated with short device lengths [1].CPML places the saturable absorber section at the center of the cavity. Two counterpropagating pulses are formed and collide at the center. In this way the saturation of the absorption is enhanced and the repetition rate is simultaneously doubled from the fundamental [6]. Harmonic mode-locking of diode lasers can be achieved by forcing the device to lock on a set of equally spaced but non-adjacent cavity modes. With this approach, terahertz (THz) repetition rates have been achieved in devices that incorporate distributed Bragg reflector mirrors [7] and intra-cavity reflectors [8]. A limitation of these approaches to harmonic mode-locking however is the fact that only the harmonic frequency rather than the precise form of the mode-locked spectrum itself can be specified.In this letter we demonstrate an approach to passive mode-locking of diode lasers based on the selection of individual modes from the spectrum of Fabry-Perot resonances. Mode selection is achieved by a distributed reflection mechanism that is designed as a perturbation of the Fabry-Perot mode spectrum. Such an approach has some features in common with that described in [8], although, in our case the effective index profile along the device is derived directly from an inverse problem solution. This approach has allowed us to design singlemode [9], and two-mode Fabry-Perot diode lasers [10] with high spectral purity and can in principle provide a much greater degree of control of the mode-locked spectrum of the laser. To demonstrate passive mode-locking of a discrete comb of Fabry-Perot modes, a saturable absorber section is placed adjacent to one of the cavity mirrors. In the device we consider here, six primary modes are chosen with a spacing of two fundamental modes leading to mode-locking at the first harmonic of the cavity.We have shown that a set of self-consistent equations for the lasing modes can be found by making an expansion about the cavity resonance condition in a FabryPerot laser [11]. The ef...

Design Optimization of Passively Mode-Locked Semiconductor Lasers With Intracavity Grating Spectral Filters

“…Experimental data from the second device of length 875 µm are presented in Fig. 3 [13]. In this example the filter was designed to select a comb of six primary modes with 100 GHz spacing at a wavelength of 1545 nm.…”

“…In recent work we have demonstrated a number of twosection Fabry-Perot lasers with engineered spectra defined by an intracavity grating spectral filter [12,13]. The grating filter in these devices is designed to select a finite number of predetermined lasing modes so that precise tailoring of the comb line spectrum is possible.…”

Dispersion-induced dynamics of coupled modes in a semiconductor laser with saturable absorption

“…Examples include stable terahertz frequency generation and tailored comb-line emission demonstrated recently by our group [17][18][19]. These Fabry-Perot (FP) lasers included a spectral filter to limit the number of active modes, and here it may be appropriate to formulate the problem of describing the dynamics in the frequency domain.…”

Mode-Locking of Semiconductor Lasers with Intracavity Grating Spectral Filters

“…In recent work we have demonstrated a number of Fabry-Perot [FP] ML devices with engineered spectra defined by an intracavity grating spectral filter [2,3]. These designs can also be adapted for integrated lightwave circuits based on open grating resonators [4].…”