100 GHz Multiple Colliding Pulse Generation from Cleaved Facet-free Multi-section Semiconductor Laser Diode Student Paper Jessica Cesar Cuello 1 , Robinson Guzman 1 , Alberto Zarzuelo 1 , Mu Chieh Lo 1 , Muhsin Ali 1 , Guillermo Carpintero 1 1 University Carlos III of Madrid, UC3M, Electronics Technology Department e-mail: rcguzman@ing.uc3m.es ABSTRACT A monolithically integrated semiconductor laser with a cavity laser of a 25 GHz fundamental repetition rate has been designed to generate an optical signal of four-times of the fundamental repetition rate working on the fourth harmonic colliding pulse mode locking configuration. This device was developed and fabricated within a multi-project wafer run in an InP-based active-passive generic foundry. The Fabry-Pérot laser cavity structure of the semiconductor laser is formed with two on-chip reflector building blocks rather than cleaved facets of the chip. The Fabry-Perot laser cavity length is around 1.66 mm the cavity three absorber sections symmetrically divide the cavity into four gain segments. An electrical linewidth of 350 KHz and 150 KHz with a frequency spacing of 25 GHz y 100 GHz is generated by the laser in passive regimen condition, respectively. Keywords: semiconductor laser, electrical linewidth, multiple colliding pulse, mode locking laser. 1. INTRODUCTION Passive mode locking (PML) based on III-V semiconductor lasers in photonic integrated circuits (PIC) generating optical frequency combs (OFC) have been increasingly attracting attention in recent years, due to its chip-scaled compactness and simplicity of DC operation. Especially, Passive mode locking laser diodes at repetition rates in millimetre-wave (mmW, 30-300 GHz) and terahertz (THz, 0.1-10 THz) ranges have been considered promising in communications, spectroscopy, and sensing [1] – [2]. However, in most cases an extremely high repetition rate (>100 GHz) of a mode-locked lasers (MLL) is corresponding to only a few hundreds of µm cavity length (<400 µm). For this reason, harmonic mode locking (HML) schemes have been investigated to produce multiple pulses per round trip in a sufficiently long cavity, thus pushing the RR beyond the low-GHz fundamental cavity round trip frequency [3]. HML is achieved by means of colliding pulse ML [4], coupled cavity ML [5], and methods based on the wavelength selectivity of distributed Bragg reflector (DBR) grating [6]. In colliding pulse ML, one saturable absorber is placed at the midpoint of cavity, where two counter propagating pulses circulate and collide, producing a train of pulses at a repetition rates that are twice the fundamental round trip frequency. Evolving from colliding pulse ML, multiple colliding pulse ML features multiple saturable absorbers concatenated with gain sections, for repetition rates multiplication >2 as has been extensively investigated [7] –[8]. Recently, a new class of on-chip broadband reflector based on the multimode interference (MMI) principle has been proposed [9], and demonstrated its wide applicability [10] – [11]. Such multimode interference reflectors (MIRs) are so simple to create in lithography with greater fabrication tolerance to replace DBRs and cleaved facets. Furthermore, they can be placed anywhere on a chip, and the transmitted light is manipulable on chip to fulfill more functionalities [12]. 2. DEVICE DESCRIPTION Fig. 1 shows a photograph of the layout of the semiconductor laser with a multi-section structure. The Perot- Perot laser cavity is formed with a pair of MIRs, in which there are four gain sections separated by three saturable absorber sections, placed at every quarter of the cavity length. The on-chip reflectors MIRs, define a cavity length (L) of around 1.66 mm, corresponding to a cavity round trip frequency of 25 GHz. Figure 1. Photograph of the layout of the investigated semiconductor laser diode.