Detecting weapons concealed underneath clothing, analyzing the contents of suspicious-looking envelopes, or even spotting the onset of cancer: these are just some of the exciting prospects that have been turning terahertz wave research into one of the most important topics in photonics. Most broadband pulsed THz sources are based on the excitation of different materials with ultrashort laser pulses. So far, generation of tunable narrow-band THz radiation has been demonstrated using ultrafast solid state lasers as a source of high-intensity optical pulses. The lack of a high-power, low-cost, portable room-temperature THz source is the most significant limitation of modern THz systems. Advances in fiber laser technology can be used to further the capabilities of the homeland security. Using semiconductor saturable absorber mirrors allows for reliable mode-locked operation with different values of cavity dispersion in a broad spectrum ranged from 900 to 1600 nm. Semiconductor saturable absorbers mirrors have been used successfully to initiate and to sustain mode-locking in a wide range of core-pumped fiber lasers. The main advantage of the semiconductor saturable absorber mirrors (SESAM) is the possibility to control important parameters such as absorption recovery time, saturation fluence and modulation depth through the device design, growth conditions and post-growth processing. The SESAM as a cavity mirror in the fiber laser results in compact size, environmentally stable and simple ultrashort pulse lasers that can cover wide wavelength range and generate optical pulses with durations from picoseconds to femtoseconds. Employing SESAM technology for mode-locking, the double-clad fiber laser promises superior pulse quality, high stability and pulse energy without need for power booster that eventually degrades the pulse quality due to nonlinear distortions in the amplifier fiber.
We give an overview of recent achievements in ultrafast fiber lasers; discuss basic properties, technical challenges and methods to achieve stable short pulse operation with high average and peak powers from all-fiber devices. The important aspects of the mode-locked fiber lasers relevant to practical security systems are presented. Particularly, the effect of the amplified spontaneous emission (ASE) on the performance of the SESAMs in mode-locked fiber lasers has been investigated. We show that high level of ASE intensity typical for fiber lasers can saturate the absorption and degrade significantly the nonlinear response of the SESAM. We studied the effect of the absorber recovery time and demonstrated that the ion-irradiated SESAMs with fast nonlinear response are less affected by the ASE radiation and, consequently, in the presence of the high-power ASE they exhibit better self-starting capability compared with slow absorbers. The promising method for noise suppression based on the cavity-enhanced optical limiting is another important issue described. Optical limiting and saturable absorption are studied by placing two-photon absorption material and InGaAs quantum wells in a microcavity. We show that field enhancement that occurs in a cavity affects strongly the limiting threshold and dynamic range of roll-over in the nonlinear response.
To achieve energy levels sufficient for different security systems, power scaling technique should be employed. We present a stretched-pulse double-clad ytterbium-doped fiber laser mode-locked with SESAM. High modulation depth in the nonlinear response of the SESAM allows for self-starting pulse operation without any dispersion compensation in the laser cavity. The chirp on the output pulses is highly linear and can be compensated for with dispersion in photonic bandgap fiber. The results is a fully self-starting source of 150-fs pulse with 63-nJ of energy at a 8-MHz repetition rate.