Materials Science and Engineering B74 (2000) 80 – 83 Coupled cavity DQW semiconductor lasers A. Serpengu ¨ zel a, *, B.E. Sag ˘ol b , E.A. Avrutin c , R.M. De La Rue c , P.J.R. Laybourn c , C.R. Stanley c a Physics Department, Koc ¸ Uniersity, Istinye, 80860 Istanbul, Turkey b Max -Planck -Institut fu ¨r Festko ¨rperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany c Electronics and Electrical Engineering Department, Glasgow Uniersity, Glasgow G12 8LT, UK Abstract Coupled cavity effects has been observed in the electroluminescence spectra of monolithic GaAs/GaAlAs double quantum well graded index separate confinement heterostructure semiconductor diode lasers. A time domain analysis has been performed in order to simulate the experimentally observed results. The theoretically calculated spectra are in good agreement with the experimentally observed spectra. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Coupled cavity; Diode laser; Semiconductor www.elsevier.com/locate/mseb 1. Introduction The ability to generate high speed and multiple wave- length optical signals using semiconductor diode lasers is crucial for future time domain multiplexing (TDM) and wavelength domain multiplexing (WDM) optoelec- tronic systems, which will be used in telecommunication networks, ultrafast data processing, optical-clock distri- bution for computing, opto-microwave-electronic inter- facing, and optical time domain reflectometry. Because of their ability to generate optical signals at harmonics of the laser cavity round-trip frequency, coupled cavity semiconductor diode lasers are good light source candi- dates for future TDM and WDM systems [1]. Although the coupled cavity concept is usually applied to the Fabry – Perot geometry lasers, it has also been applied to vertical cavity surface emitting lasers for dual wave- length generation [2]. Recently, coupled cavity laser diodes have been realized by focused ion beam etching techniques [3], and passive mode locking of semicon- ductor diode lasers has been achieved using a coupled cavity geometry [4]. Here, we report the coupled cavity effects observed in the electroluminescence spectra of monolithic semiconductor diode lasers. In order to simulate our experimentally observed results, we have also performed a theoretical time domain analysis [5,6]. 2. Device fabrication The material used in the fabrication of the coupled cavity semiconductor diode lasers is a GaAs/GaAlAs double quantum well graded index separate confi- nement heterostructure grown by molecular beam epi- taxy. The details of the semiconductor diode laser structure is given elsewhere [7]. The semiconductor diode lasers have been fabricated using standard semi- conductor processing techniques. The scanning electron microgram in Fig. 1 shows the end mirror facet at the edge of the semiconductor diode laser. The mesa is defined by photolithography and a ridge 1 m high and 4 m wide is wet etched on the samples. Afterwards, a 100 nm thick layer of silicon oxide (SiO x ) was deposited by low-temperature plasma-enhanced chemical vapor deposition. The SiO x on the mesa was later lifted-off in order to deposit contact metal. After lift-off, p-contact metallization and annealing (of the metal for ohmic contact) were performed. The samples were later thinned by wet etch to reduce their serial resistance. After the thinning process, n-contact metallization and annealing (of the metal for ohmic contact) were per- formed. Afterwards, the samples were diced at different optical Fabry – Perot cavity lengths. From the current – voltage and current – optical power measurements, the threshold current density per quantum well of the semi- conducting laser diode material was found to be 200 A * Corresponding author. E-mail address: aserpenguzel@ku.edu.tr (A. Serpengu ¨ zel) 0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0921-5107(99)00539-5