I.D. Jung

Semiconductor saturable absorber mirrors (SESAM's) for femtosecond to nanosecond pulse generation in solid-state lasers

Intracavity semiconductor saturable absorber mirrors (SESAMs) offer unique and exciting possibilities for passively pulsed solid-state laser systems, extending from Q-switched pulses in the nanosecond and picosecond regime to mode-locked pulses from 10s of picoseconds to sub-10 fs. This paper reviews the design requirements of SESAMs for stable pulse generation in both the mode-locked and Q-switched regime. The combination of device structure and material parameters for SESAMs provide sufficient design freedom to choose key parameters such as recovery time, saturation intensity, and saturation fluence, in a compact structure with low insertion loss. We have been able to demonstrate, for example, passive modelocking (with no Q-switching) using an intracavity saturable absorber in solid-state lasers with long upper state lifetimes (e.g., 1-/spl mu/m neodymium transitions), Kerr lens modelocking assisted with pulsewidths as short as 6.5 fs from a Ti:sapphire laser-the shortest pulses ever produced directly out of a laser without any external pulse compression, and passive Q-switching with pulses as short as 56 ps-the shortest pulses ever produced directly from a Q-switched solid-state laser. Diode-pumping of such lasers is leading to practical, real-world ultrafast sources, and we will review results on diode-pumped Cr:LiSAF, Nd:glass, Yb:YAG, Nd:YAG, Nd:YLF, Nd:LSB, and Nd:YVO/sub 4/.

Soliton mode-locking with saturable absorbers

We investigate ultrashort pulse generation based on the fundamental soliton generation that is stabilized by a saturable absorber. The case of an absorber with a recovery time much longer than the pulsewidth of the generated soliton is investigated in detail. Based on soliton perturbation theory we derive equations for the soliton variables and the continuum generated in a mode-locked laser. Analytic criteria for the transition from stable to unstable soliton generation are derived. The results demonstrate the possibility of ultrashort pulse generation by a slow saturable absorber only. The theoretical results are compared with experiments. We generate pulses as short as 13 fs using only semiconductor saturable absorbers.

Self-starting 6.5-fs pulses from a Ti:sapphire laser

We demonstrate self-starting 6.5-fs pulses from a Kerr-lens mode-locked Ti:sapphire laser with 200-mW average output power at a pulse repetition rate of ~86 M Hz. This is to our knowledge the shortest pulse ever generated directly from a laser. For dispersion compensation we used a prism pair in combination with double-chirped mirrors, which balances the higher-order dispersion of the prism pair and therefore flattens the average total group-delay dispersion in the laser cavity. For self-starting mode locking we used a broadband semiconductor saturable-absorber mirror.

Experimental verification of soliton mode locking using only a slow saturable absorber

We demonstrate experimentally that solid-state lasers with strong solitonlike pulse shaping can be mode locked by a slow saturable absorber only, i.e., the response time is much slower than the width of the soliton. A Ti:sapphire laser mode locked by a low-temperature-grown GaAs absorber with 10-ps recovery time generates pulses as short as 300 fs without the need for Kerr-lens mode locking and critical cavity alignment. An extrapolation of this result would predict that an asymptotically equal to 100-fs recovery time of a semiconductor absorber could support pulses into the 10-fs regime.

Characterization of short-pulse oscillators by means of a high-dynamic-range autocorrelation measurement

A high-dynamic-range autocorrelation technique was used to characterize the temporal pulse shape of ultrashort laser pulses produced from four separate oscillators. These lasers included two Kerr-lens mode-locked Ti:sapphire oscillators as well as a Nd:glass and a Ti:sapphire oscillator, each passively mode locked by an antiresonant Fabry-Perot semiconductor saturable absorber. It was shown that the Nd:glass oscillator supported a pulse that was temporally clean over 8 orders of magnitude.

Self-starting 6.5-fs pulses from a Ti:sapphire laser using a semiconductor saturable absorber and double chirped mirrors

We demonstrate self-starting 6.5-fs pulses from a Kerr-lens-mode-locked Ti:sapphire laser with an average output power of 200 mW at a pulse repetition rate of 86 MHz. We have achieved a mode-locking buildup time of only 60 /spl mu/s, using a broad-band semiconductor saturable absorber mirror to initiate the pulse formation. The dispersion has been compensated with a prism pair in combination with improved double-chirped mirrors. The prism pair allows for the flexible adjustment of both the duration and the center wavelength of the pulse. The double-chirped mirrors show a high reflectivity better than 99.8% over the full bandwidth of 300 nm and a controlled group delay over more than 250 nm. The choice of a proper output coupler turns out to be critical for ultrashort pulse generation directly from the laser.

Scaling of the antiresonant Fabry–Perot saturable absorber design toward a thin saturable absorber

We demonstrate and discuss the scaling of the antiresonant Fabry-Perot saturable absorber toward a novel antiref lection-coated thin saturable absorber. With a Ti:sapphire laser we obtained self-starting Kerr-lens mode-locked pulses as short as 19 fs. With the higher modulation depth of the thin saturable absorber we obtained soliton mode-locked self-starting 34-fs pulses over the full cavity stability regime with a significantly shorter mode-locking buildup time. The pulse duration was limited only by the semiconductor Bragg mirror.

Wide bandwidth (100) GaAs/fluorides quarter‐wavelength Bragg reflectors grown by molecular beam epitaxy

Broadband quarter‐wavelength Bragg reflectors that consist of periodic stacks of fluorides (CaF2–BaF2–CaF2) and GaAs, centered at 1.4 μm, were grown by molecular beam epitaxy. Despite a total fluoride thickness as high as 720 nm, crack‐free surface morphology was obtained. In this letter, we report a crack‐free standard quarter‐wavelength III–V semiconductor‐fluoride Bragg reflector. With only three stacks, the bandwidth with reflectance above 95% is about 650 nm (1.15–1.80 μm), while, near the center wavelength, the reflectivity is as high as 99%. Both important wavelengths of 1.3 and 1.55 μm for optical communication are included in the very wide high reflectance plateau. These mirrors are expected to have wide applications for optical and optoelectronic devices.

Highly efficient 980 nm single mode modules with over 0.5 Watt pump power

We present narrow beam divergence InGaAs/AlGaAs 980 nm ridge waveguide laser diodes with over 1 W CW-rollover power. Together with superior power conversion efficiency a maximum fiber-coupled light output power of over 0.5 W has been reached.

Self-starting 6.S-fs pulses from a KLM Ti:sapphire laser

We demonstrate self-starting 6.5-fs pulses, which are to our knowledge the shortest pulses directly out of a laser, with 200-mW average output power by using prism pairs in combination with a chirped mirror.