In this paper we present the first observation of passive mode-locking in a quantum dot (QD) ring laser operating at wavelengths around 1.5 µm. The device consists of an 18-mm long (electrically pumped) ring cavity, corresponding to a 5-GHz roundtrip frequency. The waveguide width is 2 µm. A saturable absorber (SA) section with a length of 300 µm is located in the cavity, opposite to a directional coupler which couples part of the light to the output waveguides. The fabrication technology and the quantum dot gain material are described in . Peaks of over 50 dB are observed in the RF spectrum from the 50GHz photodiode signal of the laser output, for injection currents of 720 mA to 890 mA and an SA bias voltage of -1 V. The width of the (fundamental) RF-peak at -20 dB is between 300 kHz and 400 kHz in this range, which is almost half the width as observed with Fabry-Pérot-type lasers of the same QD material . A detailed view of the spectrum is shown in Fig. 1. Mode-locking is further confirmed by an autocorrelator measurement, showing a 55-ps duration second harmonic generated signal at FWHM. The optical spectra of both laser outputs, i.e. from clockwise (CW) and counterclockwise (CCW) propagation, are given in Fig. 2. As can be seen the CW-spectrum has three groups of laser modes for injection currents of 720 mA to 890 mA, located around 1490 nm, 1505 nm and 1520 nm. The width of these individual groups is approximately 3 nm FWHM. We ascribe these spectral features to lasing transitions from the electron ground and excited dot states. When the CW and CCW spectra are compared it can be seen that they are complementary, i.e. the CCW spectrum fills in the holes of the CW spectrum (Fig. 2). This means that the CW and CCW signals lase at different wavelengths and consequently that they make use of different sets of quantum dots, with different sizes.
|Titel||Proceedings of the European Semiconductor Laser Workshop 2008 (ESLW2008), 19 - 20 September 2008, Eindhoven, Netherlands|
|Plaats van productie||Eindhoven, Netherlands|
|Uitgeverij||Technische Universiteit Eindhoven|
|Status||Gepubliceerd - 2008|
|Evenement||2008 European Semiconductor Laser Workshop (ESLW 2008) - Eindhoven, Nederland|
Duur: 19 sep 2008 → 20 sep 2008
|Workshop||2008 European Semiconductor Laser Workshop (ESLW 2008)|
|Verkorte titel||ESLW 2008|
|Periode||19/09/08 → 20/09/08|
|Ander||European Semiconductor Laser Workshop 208|
Heck, M. J. R., Renault, A., Bente, E. A. J. M., Oei, Y. S., Smit, M. K., Eikema, K. S. E., ... Nötzel, R. (2008). 5-GHz passively mode-locked quantum dot ring laser diode at 1.5 μm. In Proceedings of the European Semiconductor Laser Workshop 2008 (ESLW2008), 19 - 20 September 2008, Eindhoven, Netherlands (blz. 7-7). Eindhoven, Netherlands: Technische Universiteit Eindhoven.