Mode coherence measurements across a 15 THz spectral bandwidth of a passively mode-locked quantum dash laser

Abstract

The mode coherence of adjacent and non-adjacent spectral modes of a passively mode locked quantum dash (QDash) semiconductor laser are deduced through radio frequency beat-tone linewidth measurements. A wavelength conversion scheme that uses degenerate four wave mixing in a semiconductor optical amplifier is proposed which considerably extends the mode spacing beyond the limit imposed by conventional fast-photodetection and electrical spectrum analysis of around 100 GHz. Using this scheme, the mode coherence of the QDash laser was measured out to the thirty-first harmonic, or a mode separation of 1.5 THz.

Terahertz-bandwidth coherence measurements of a quantum dash laser in passive and active mode-locking operation

“…4 as a function of the spectral mode separation. The red circles specify the measured RF beat-tone linewidths of the QDash laser for the passive case and follow a trend in agreement with measurements performed on a similar device [7]. A quadratic trend is fitted to measured data, indicated by the solid red line, following theory outlined in [11].…”

“…Quantum dot-based lasers can also achieve the terahertz bandwidths, but require either passive mode-locking with a saturable absorber, or active or hybrid mode-locking [8,9]. QDash lasers, on the other hand, have demonstrated passive modelocking without the use of a saturable absorber section [6,7]. In this Letter, terahertz beat-tone coherence measurements of a QDash laser when passively mode-locked and injection mode-locked using both electronic and optical master signals are demonstrated.…”

“…QDash semiconductor modelocked lasers are a promising technology fit owing to their low power requirements, millimeter-scale device footprint, and monolithic manufacturability [6]. With a spectral full width half-maximum of 1.6 THz, passively mode-locked QDash lasers have previously been shown to have an radio frequency (RF) beat tone of ∼10 MHz at a 1 THz spectral mode separation [7]. Quantum dot-based lasers can also achieve the terahertz bandwidths, but require either passive mode-locking with a saturable absorber, or active or hybrid mode-locking [8,9].…”

“…Subsequently, the RF beat-tone linewidth of the two modes can be measured using a 10 GHz photodiode, an RF amplifier, and a 10 GHz bandwidth real-time scope. This technique has an electrical resolution bandwidth less than 80 kHz, but has an instrument resolution bandwidth of ∼330 kHz imposed by the FWM-based wavelength converter, as described in detail in [7]. The experiment was carried out for the passively and actively mode-locked QDash laser setups shown in Fig.…”

Phase Noise Characterization of a Mode-Locked Quantum-Dot Coherent Optical Frequency Comb Source Laser

“…This is accomplished through an analysis of the RF beat tone linewidth across the spectrum of an optical frequency comb[49]. Other experiments have attempted to quantify coherence through an optical and RF linewidth ratio as a function of mode number[50]. In both cases, the results showed that the term used to quantify coherence decreased as the spacing between observed spectral modes increased, indicating that coherence decreases with an increase in modal separation.…”