Intracavity semiconductor saturable absorber mirrors (SESAM's) 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 10's of picoseconds to sub-10 fs. This paper reviews the design requirements of SESAM's for stable pulse generation in both the mode-locked and Q-switched regime. The combination of device structure and material parameters for SESAM's 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/.

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

We report experiments and theory on the effects of electric fields on the optical absorption near the band edge in GaAs/AlGaAs quantum-well structures. We find distinct physical effects for fields parallel and perpendicular to the quantum-well layers. In both cases, we observe large changes in the absorption near the exciton peaks. In the parallel-field case, the excitons broaden with field, disappearing at fields \ensuremath{\sim}${10}^{4}$ V/cm; this behavior is in qualitative agreement with previous theory and in order-of-magnitude agreement with direct theoretical calculations of field ionization rates reported in this paper. This behavior is also qualitatively similar to that seen with three-dimensional semiconductors. For the perpendicular-field case, we see shifts of the exciton peaks to lower energies by up to 2.5 times the zero-field binding energy with the excitons remaining resolved at up to \ensuremath{\sim}${10}^{5}$ V/cm: This behavior is qualitatively different from that of bulk semiconductors and is explained through a mechanism previously briefly described by us [D. A. B. Miller et al., Phys. Rev. Lett. 53, 2173 (1984)] called the quantum-confined Stark effect. In this mechanism the quantum confinement of carriers inhibits the exciton field ionization. To support this mechanism we present detailed calculations of the shift of exciton peaks including (i) exact solutions for single particles in infinite wells, (ii) tunneling resonance calculations for finite wells, and (iii) variational calculations of exciton binding energy in a field. We also calculate the tunneling lifetimes of particles in the wells to check the inhibition of field ionization. The calculations are performed using both the 85:15 split of band-gap discontinuity between conduction and valence bands and the recently proposed 57:43 split. Although the detailed calculations differ in the two cases, the overall shift of the exciton peaks is not very sensitive to split ratio. We find excellent agreement with experiment with no fitted parameters.

Electric field dependence of optical absorption near the band gap of quantum-well structures.

The optical properties of glasses containing a small amount of thermally developed CdSexS1−x microcrystalline phase are studied with emphasis on quantum confinement effects exhibited at small crystallite size. Optical absorption, photoluminescence, x‐ray diffraction, and transmission electron microscopy are used to examine microcrystallites as a function of composition and development. Results are presented for a series of commercially available CorningR filter glasses with a selenium mole fraction in the range 0.28≤x≤0.74, as well as for several experimental glasses in which the average microcrystallite diameters range from 30 to 80 A. Optical effects observed in the experimental glasses that are due to electron and hole confinement are not present in the filter glasses considered; variations in optical properties of the filters are due to changes in stoichiometry of the CdSexS1−x mixed anion system. A brief discussion of other microcrystalline phases in glass is also presented. These microcrystallites show room‐temperature optical absorption structure analogous to bulk crystal excitons; the temperature dependence of this structure is contrasted with that resulting from quantum confinement in CdSexS1−x glasses.

Quantum confinement effects of semiconducting microcrystallites in glass

Apparatus and method for increasing the sensitivity in the detection of optical coherence tomography and low coherence interferometry (“LCI”) signals by detecting a parallel set of spectral bands, each band being a unique combination of optical frequencies. The LCI broad bandwidth source is split into N spectral bands. The N spectral bands are individually detected and processed to provide an increase in the signal-to-noise ratio by a factor of N. Each spectral band is detected by a separate photo detector and amplified. For each spectral band the signal is band pass filtered around the signal band by analog electronics and digitized, or, alternatively, the signal may be digitized and band pass filtered in software. As a consequence, the shot noise contribution to the signal is reduced by a factor equal to the number of spectral bands. The signal remains the same. The reduction of the shot noise increases the dynamic range and sensitivity of the system.

Apparatus and method for ranging and noise reduction of low coherence interferometry LCI and optical coherence tomography OCT signals by parallel detection of spectral bands

An apparatus and method are provided. In particular, at least one first electro-magnetic radiation may be provided to a sample and at least one second electro-magnetic radiation can be provided to a non-reflective reference. A frequency of the first and/or second radiations varies over time. An interference is detected between at least one third radiation associated with the first radiation and at least one fourth radiation associated with the second radiation. Alternatively, the first electro-magnetic radiation and/or second electro-magnetic radiation have a spectrum which changes over time. The spectrum may contain multiple frequencies at a particular time. In addition, it is possible to detect the interference signal between the third radiation and the fourth radiation in a first polarization state. Further, it may be preferable to detect a further interference signal between the third and fourth radiations in a second polarization state which is different from the first polarization state. The first and/or second electro-magnetic radiations may have a spectrum whose mean frequency changes substantially continuously over time at a tuning speed that is greater than 100 Tera Hertz per millisecond.

Method and apparatus for performing optical imaging using frequency-domain interferometry

By using a GaAs/GaAIAs multiple-quantum-well sample as a saturable absorber in an external resonator, we have passively mode locked a GaAs laser to obtain pulses as short as 1.6 psec, the shortest pulses ever observed to our knowledge from a mode-locked diode laser in a regulator pulse train.

Passive mode locking of a semiconductor diode laser

Semiconductor microstructures whose dimensions are comparable to atomic dimensions and whose interfaces are atomically smooth exhibit novel optical properties not encountered in the parent compounds. Quantum well structures, which consist of ultrathin semiconductor layers alternately grown one on the other, possess remarkable optical nolinearities and electro-optical properties with potential applications in optoelectronics. In this paper we review our studies of GaAs/AIGaAs quantum well structures (QWS). We briefly discuss the physics of absorption in QWS at room temperature, and we describe the mechanisms of saturation of excitonic absorption and refraction and that of electroabsorption in QWS. Finally, we review the applications of these effects to optical processing and optoelectronic devices.

Nonlinear Optical Properties Of GaAs/GaAlAs Multiple Quantum Well Material: Phenomena And Applications

Two-dimensional all-optical devices include a planar single-mode waveguide formed on or in a major surface of a supporting substrate. The waveguide comprises a nonlinear medium having an intensity-dependent index of refraction. For input optical signals above a critical intensity, self-focusing occurs in the waveguide. Microminiature high-speed bistable and switching devices based on this phenomenon and compatible with integrated-optics technology are thereby provided.

Integrated all-optical switching devices

Publisher Summary This chapter discusses the nonlinear optical properties of multiple quantum well structures for optical signal processing. Two properties of quantum wells that are relevant for nonlinear optical devices are: first, quantum wells show optical absorption saturation associated with the remarkably distinct room-temperature excitonic resonance. Second, they show a large electricfield dependence of the absorption, both associated with the excitonic transition and also directly with the transition between two-dimensional subbands. All these effects are seen near the optical absorption edge. The work reviewed here is primarily, from recent experiments on GaAs/AlCaAs multiple quantum well structures (MQWS). The chapter discusses the linear absorption in MQWS with a particular emphasis on room-temperature effects. It presents the measurements of nonlinear optical effects in MQWS, including absorption saturation and four-wave mixing. Further, the experimental and theoretical studies of electroabsorption in MQWS are reviewed, and the example of a new category of devices that exploits both the absorption saturation and electroabsorption is discussed.

Chapter 5 Nonlinear Optical Properties of Multiple Quantum Well Structures for Optical Signal Processing

Passive mode locking of a semiconductor diode laser using GaAs/ GaAlAs multiple quantum well (MQW) material as a saturable absorber has been reported recently [1]. The strong room-temperature excitonic absorption line in this material [2] saturates at relatively low powers, and it can be matched to the GaAs laser wavelength. Such mode-locked lasers are potentially important sources for future ultra-fast photonic signal processing.

P. W. Smith

Papers

Passive mode locking of a semiconductor diode laser

Nonlinear Optical Properties Of GaAs/GaAlAs Multiple Quantum Well Material: Phenomena And Applications

Integrated all-optical switching devices

Chapter 5 Nonlinear Optical Properties of Multiple Quantum Well Structures for Optical Signal Processing

Fast Multiple Quantum Well Absorber for Mode Locking of Semiconductor Lasers