Radio frequency and terahertz signals generated by passively mode-locked semiconductor lasers

There are several different approaches to generating periodic signals using semiconductor lasers, for example: Q-switching, gain switching or mode-locking schemes. In general the active or passive mode-locking techniques require the use of a modulator or a saturable absorber in order to achieve the phase synchronisation. The laser diodes studied in this thesis, are demonstrated to operate in the mode-locked regime, while not requiring any direct or external modulation, nor the saturable absorbtion element in order to achieve the phase synchronisation. It has been demonstrated previously, that in a multimode semiconductor laser, the third order nonlinearities of a gain medium resulting in the four-wave-mixing effects, are responsible for the phase synchronisation and lead to phase locking. The repetition rate of the generated signal is fixed by the free-spectral range of the longitudinal spectrum. Therefore, with a passively mode-locked laser (PMLL) it is possible to cover a wide range of frequencies from the Radio-Frequency (RF) to the TeraHertz (THz) domain. Radio frequency signals generated by semiconductor lasers have many applications in optical communications, such as radio-over-fibre, or all-optical clock extraction. Terahertz signals are the focus of many research bodies nowadays, due to their interaction with matter. They have potential applications in areas like: industry, pharmacy, security (military), telecommunication and medicine. With continuous improvement of materials processing and technology, new ways of generation and detection of such types of signals have appeared. The key advantage of the optical RF or THz generation is that this type of device is direct current biased and operates at room temperature. In this thesis, a comprehensive study of various PMLLs, from distributed Bragg reflector bulk laser to quantum dashed Fabry-Perot lasers is given, demonstrating the origin of the phase synchronisation in these structures and some applications for these lasers such as all-optical clock recovery or THz signal generation

Passively mode-locked semiconductor lasers and their applications

In this paper we present some characterizations of passively mode-locked semiconductor lasers. These lasers are multimode and they exhibit a modulation of their output power even though they are DC-biased. The modulation frequency corresponds to their free-spectral range. We demonstrate their potential: for generation of ultra-fast modulation for THz wave generation at room temperature without any direct modulations applied, in the radio frequency range they synchronise their modulation to the bit-rate of an incoming signal used for clock extraction. Therefore these devices can be used for all-optical clock recovery

Terahertz wave generation from a dc-biased multimode laser

We present results achieved in the generation of terahertz wave by a semiconductor laser. It is a Fabry–Pérot based device with shallow grooves implemented on its p-side to engineer the longitudinal mode spectrum. The laser is dc-biased and temperature controlled at 298 K. The main two modes are separated by 3 nm at 1550 nm with a side-mode-suppression ratio of 25 dB. Using a frequency resolved optical gating, evidence of mode beating at 373 GHz is observed. With a bolometer interfaced to a Fourier transform interferometer, the second harmonic signal is measured at 690 GHz

RF or THz signals generated from DC biased multimode lasers

Although self pulsating (SP) lasers are DC biased, they feature a modulation of the output power. For the results presented in this paper, the SP frequency corresponds to the frequency spacing between longitudinal modes or its second harmonic. The performances of both a 40 GHz self pulsating distributed Bragg reflector laser and of a 660 GHz slotted laser are presented. For the first laser, the radio frequency (RF) signal was analysed on electrical spectrum analyser and its linewidth was smaller that the sum of the main optical modes, proving a passive modelocking of the mode phases. For the slotted laser, a bolometer interfaced to a FT IR spectrometer is used for the terahertz (THz) detection. A signal 10 times larger than the noise level was measured with this set up. Both lasers have demonstrated to be an easy solution to produce RF or THz signal generator

Wave-mixing analysis for THz-signals generation in dc-biased semiconductor optical devices at room temperature

Wave-mixing at 370-GHz in a SOA and FP semiconductor laser is investigated by using a FROG-system. A comparison of the optical time-fluctuations measured at their output stresses the importance of a resonant cavity in THz-signals generation from semiconductor optical devices

Investigation on the origin of terahertz waves generated by dc-biased multimode semiconductor lasers at room temperature

A technique to measure a terahertz wave generated by spectrum tailored Fabry–Pérot lasers (FP) is assessed. A dc-biased and 25 °C temperature controlled FP is probed by a continuous wave signal, tuned at 20 nm away from its lasing modes. With a 0.02 nm resolution optical spectrum analyzer (OSA), the terahertz generated signal frequency is measured from the interval between the probe and its side-band modulations. The terahertz waves emitted by these FPs are measured at 370±5 GHz and at 1.157±0.005 THz, respectively, within a precision set by our OSA. The origin of the terahertz wave is due to passive mode-locked through intracavity four-wave-mixing processes

Semiconductor optical amplifier-based heterodyning detection for resolving optical terahertz beat-tone signals from passively mode-locked semiconductor lasers

An all-optical heterodyne approach based on a room-temperature controlled semiconductor optical amplifier (SOA) for measuring the frequency and linewidth of the terahertz beat-tone signal from a passively mode-locked laser is proposed. Under the injection of two external cavity lasers, the SOA acts as a local oscillator at their detuning frequency and also as an optical frequency mixer whose inputs are the self-modulated spectrum of the device under test and the two laser beams. Frequency and linewidth of the intermediate frequency signal and therefore, the beat-tone signal are resolved by using a photodiode and an electrical spectrum analyze

Multi data-rate synchronization of 40 GHz mode-locked quantum-dash lasers diodes

Beat-tones at 40-GHz from a QDash-ML laser under 10 to 160 Gb/s optical injection are analyzed. A FWHM linewidth less than 8 Hz is obtained regardless of the rate of injected data strea

Analysis of optical THz-signals from mode-locked semiconductor laser by using a semiconductor optical amplifier-based detection scheme

An all-optical approach based on nonlinear interactions inside a semiconductor optical amplifier for measuring the THz beat-tone of a passively mode-locked laser is proposed. This novel approach can be applied to beat-tones in the range from GHz to few TH

Short pulse generation with 40 GHz passively-mode locked Q-dashed Fabry-Perot laser

Generation of sub-picosecond pulses by a dc-biased passively mode-locked Fabry-Pérot laser is demonstrated. By using a tunable band-pass filter, it is observed that the width of the generated pulses decreases in direct proportion to the optical modes emitted by the laser. Furthermore, performing a passive compression by a piece of single mode fiber, we demonstrate the generation of 720 fs pulses by this type of mode-locked Fabry-Pérot laser