Quantum-dot heterostructure lasers

Quantum-dot (QD) heterostructures are nanoscale coherent insertions of narrow-gap material in a single-crystalline matrix. These tiny structures provide unique opportunities to modify and extend all basic principles of heterostructure lasers and advance their applications. Despite early predictions, fabrication of QD heterostructure (QDHS) lasers appeared to be a much more challenging task, as compared to quantum well (QW) devices. The breakthrough occurred when techniques for self-organized growth of QD's allowed the fabrication of dense arrays of coherent islands, uniform in shape and size, and, simultaneously, free from undesirable defects. Recently, the figure of merit of QDHS lasers surpasses some of the key characteristics of QW devices in some of the most important applications     

InGaAs-GaAs quantum-dot lasers

Quantum-dot (QD) lasers provide superior lasing characteristics compared to quantum-well (QW) and QW wire lasers due to their delta like density of states. Record threshold current densities of 40 A·cm ^-2 at 77 K and of 62 A·cm^-2 at 300 K are obtained while a characteristic temperature of 385 K is maintained up to 300 K. The internal quantum efficiency approaches values of ~80 %. Currently, operating QD lasers show broad-gain spectra with full-width at half-maximum (FWHM) up to ~50 meV, ultrahigh material gain of ~10⁵ cm^-1, differential gain of ~10^-13 cm² and strong nonlinear gain effects with a gain compression coefficient of ~10^-16 cm³. The modulation bandwidth is limited by nonlinear gain effects but can be increased by careful choice of the energy difference between QD and barrier states. The linewidth enhancement factor is ~0.5. The InGaAs-GaAs QD emission can be tuned between 0.95 μm and 1.37 μm at 300 K     

Nd-YAG and Yb-YAG rotary disk lasers

The unconventional rotary disk laser holds promise in the development of new laser systems that have low gain and that are currently below threshold due to thermal effects at the high pump intensity required for lasing. It can be made to operate in almost any solid-state laser gain medium and can significantly increase the single-mode output power in CW or pulsed operation over the current state of the art. This article reports on the efficient single-mode operation of a Nd-YAG rotary disk laser and presents the first experimental data on its beam quality, pointing error, and power fluctuation. It also presents preliminary results from a single-mode Yb-YAG rotary disk laser.     

Ultrabroadband infrared solid-state lasers

Ultrabroadband infrared transition metal ion-doped solid-state lasers have come of age and are increasingly being used in trace gas monitoring, remote sensing, telecommunications, ophthalmology, and neurosurgery. Operating at room temperature, they are stable, versatile, and easy to handle successors to the color center lasers. They are becoming the critical components in optical frequency standards, space-based remote sensing systems, and may soon find application in femtochemistry and attosecond science. The article reviews the principles and basic physics of these types of lasers, which are distinguished by their ability to support the shortest pulses down to single optical cycle durations and the ultimately broad tuning ranges. The paper further reviews the state of the art in the existing diode-pumped sources of broadly tunable continuous wave, and ultrashort pulsed radiation in the infrared, and provides examples of their successful application to supercontinuum generation, trace gas measurements, and ultrasensitive intracavity spectroscopy. Developments in such lasers as Cr:YAG, Cr:ZnSe, Cr:ZnS, as well as the recently proposed mixed Cr:ZnS/sub x/Se/sub 1-x/ laser, are discussed in more detail. These lasers nearly continuously cover the infrared spectral region between 1.3 and 3.1 /spl mu/m. The gain spectra of these lasers perfectly match and extend toward the infrared spectra of such established ultrabroadband lasers, operating at shorter wavelengths between /spl sim/0.7-1.3 /spl mu/m, as Ti:sapphire, Cr:LiSAF/Cr:LiSGaF and Cr:forsterite. This opens up new opportunities for synthesis of single-cycle optical pulses and frequency combs in the infrared.     

Intersubband quantum-box semiconductor lasers

It is shown that semiconductor lasers utilizing intersubband transitions in quantum boxes, so-called intersubband quantum-box (IQB) lasers, can have significantly lower threshold current densities and operating voltages than quantum cascade (QC) lasers. In order to achieve this result, an enhancement factor of about 20 in the LO-phonon-assisted electron relaxation time is necessary. The increased gain for the radiative stage in an IQB laser eliminates the need for a multiradiative-stage structure (typically 25 stages in QC lasers). In turn, the electron injector and Bragg mirror regions on either side of the active region can be separately optimized. Due to their inherently lower input power requirements, IQB lasers operating in the mid-IR wavelength range should be capable of higher average-output powers than QC lasers at all temperatures. Furthermore, continuous-wave (CW) operation at room temperature with high wallplug efficiency becomes possible     

Monolithic tunable diode lasers

After over two decades of exploration, tunable diode lasers are beginning to find significant applications, driven largely by the huge demand for bandwidth that is guiding many developments in the optical fiber communication business today. In the paper, some of the history and key developments that have led to the technologies available today are reviewed from the perspective of the author. After discussion of some of the early work, the focus shifts to widely tunable diode lasers, which would appear to be key enablers for future dense wavelength-division multiplexing and optical switching and networking systems. The distinguishing characteristics of the current technological alternatives are summarized     

Nonlinear optics and solid-state lasers: 2000

Progress in solid-state laser sources and the nonlinear frequency conversion of lasers has been impressive over the first forty years of their development. The paper reviews the progress with an emphasis on the interactions of the scientists and engineers involved in the work and the motivation for the research. This account, is highly personal and necessarily incomplete. The references cited point to the key results and to reviews of progress in nonlinear optics and solid-state laser sources and should assist those seeking to learn about the field as it developed     

Glass-fiber lasers in the ultraviolet and visible

The recent fabrication of rare earth-doped fluoro-zirconate (ZBLAN) glass fiber has spurred the development of a family of visible and ultraviolet fiber lasers pumped by upconversion. The performance of CW room temperature devices demonstrated to date is reviewed with emphasis on the recently reported Nd-doped ZBLAN fiber laser operating in the ultraviolet at 381 nm     

InGaN-GaN multiquantum-well blue and green light-emitting diodes

InGaN-GaN multiquantum-well (MQW) blue and green light-emitting diodes (LEDs) were prepared by organometallic vapor phase epitaxy, and the properties of these LEDs were evaluated by photoluminescence (PL), double crystal X-ray diffraction, and electroluminescence (EL) measurements. It was found that there were only small shifts observed in PL and EL peak positions of the blue MQW LEDs when the number of quantum well (QW) increased. However, significant shifts in PL and EL peak positions were observed in green MQW LEDs when the number of QW increased. It was also found that there was a large blue shift in EL peak position under high current injection in blue MQW LEDs. However, the blue shift in green MQW LEDs was negligibly small when the injection current was large. These observations could all be attributed to the rapid relaxation in green MQW LEDs since the In composition ratio in the InGaN well was high for the green MQW LEDs. The forward voltage V_f of green MQW LEDs was also found to be larger than that of blue MQW LEDs due to the same reason     

Progress in color center lasers

We report here on two areas of color center laser (CCL) development. The first involves an optimization of the commercial room temperature (RT) operable CCL, “MALSAN”, in which the active medium provides high generation efficiencies up to 40% for LiF:F₂ ^- (1.08-1.27 μm) and 25% for LiF:(F₂⇒F₂⁺) (0.82-1.1 μm), and line widths less than 0.3 cm^-1. The second area is a demonstration of two unique CCL optical configurations. One entails lasing across the entire emission region of the active medium (1.1-1.24 μm for the F₂^- CC and 0.85-1.09 μm for the F ₂⁺ CC with efficiencies of 15% and 10%, respectively), with simultaneous frequency doubling in the blue-red spectral region (0.43-0.62 μm). The other involves multifrequency lasing, using a special mask or image controller placed in the pump beam     

Resonantly pumped eyesafe erbium lasers

The viability of high-power and high-energy, direct eyesafe emission from bulk erbium lasers has recently been demonstrated. In this paper, we present a review of eyesafe erbium lasers that are resonantly pumped by both fiber and diode lasers. High brightness pumping with a 1.53-/spl mu/m erbium fiber laser has yielded 60 W of continuous wave (CW) output, 10 W of repetitively Q-switched output, and as much as 16 mJ of pulse energy. Diode laser pumping has yielded 38 W of quasi-CW output and >40 mJ of Q-switched output.     

Medium-power CW Raman fiber lasers

A review of recent results on medium-power CW Raman fiber lasers pumped by laser diodes is presented. Most attention is given to Raman lasers based on phosphosilicate fibers, the latter providing a number of advantages compared to commonly used germanosilicate fibers     

InGaN-based blue laser diodes

The continuous-wave (CW) operation of InGaN multiquantum-well (MQW) structure laser diodes (LDs) was demonstrated at room temperature (RT) with a lifetime of 100 h. The threshold current and the voltage of the LDs were 50 mA and 5 V, respectively. The threshold current density was 8.8 kA/cm². The carrier lifetime and the threshold carrier density were estimated to be 3.5 ns and 1.8×10²⁰/cm³, respectively. The Stokes shift of the energy difference between the absorption and the emission energy of the InGaN MQW LD's were 140 meV. Both spontaneous and stimulated emission of the LD's originated from this deep localized energy state which is equivalent to a quantum dot-like state. From the measurements of gain spectra and an external differential quantum efficiency dependence on the cavity length, the differential gain coefficient, the transparent carrier density, threshold gain and internal loss were estimated to be 5.8×10^-17 cm², 9.3×10 ¹⁹ cm^-3, 5200 cm^-1, and 43 cm^-1 respectively     

Highly phase stable mode-locked lasers

The authors report on stabilizing the carrier-envelope phase of mode-locked Ti:sapphire lasers. Optimization of the construction of the lasers for ease of phase stabilization is discussed. Results demonstrating long-term phase coherence of the generated pulse train are presented, yielding a phase coherence time of at least 326 s, measurement time limited. The conversion of amplitude noise to phase noise in the microstructured fiber, which is used to obtain an octave spanning spectrum, is measured. The resulting phase noise is found to be sufficiently small so as to not corrupt the phase stabilization. Shift of carrier-envelope phase external to the laser cavity due to propagation through a dispersive material is measured.     

Diode-pumped grazing incidence slab lasers

Diode-pumped grazing incidence slab (GISL) lasers, and materials which may be used with this configuration, are discussed. Of the materials investigated so far, Nd:YVO₄ has been shown to yield high efficiency and high small-signal gain, and a compact, 1-kHz repetition rate laser, using a Nd:YVO₄ slab has generated 3-ns duration, millijoule pulses with high beam quality. A numerical model for GISL lasers has been developed and used to simulate a hybrid Nd:YVO₄/Nd:YAG oscillator. A preliminary, experimental investigation of such a hybrid system has yielded results that are consistent with the model predictions     

Strained-layer InGaAs quantum-well heterostructure lasers

The incorporation of intentional strain in heterostructure lasers was almost unheard of a decade ago or so and considered a problem to be avoided. Advances in both epitaxial crystal growth technology and the understanding of the physics and reliability of these materials have led to a remarkable increase in the commercial use of strained-layer lasers. The industry has benefited from an increase in the available range of emission wavelengths from quantum-well diode lasers and dramatic improvement in their time-zero performance. In the paper, we review the characteristics of strained-layer InGaAs quantum-well heterostructure lasers that have resulted in the emergence of this important technology     

Transient inversion soft X-ray lasers

The collisionally excited transient inversion scheme is shown to produce exceptionally high gain coefficients and gain-length products. Data are presented for the Ne-like titanium and germanium and Ni-like silver X-ray lasers (XRL's) pumped using a combination of nanosecond and picosecond duration laser pulses. This method leads to a dramatic reduction of the required pump energy and makes down-sizing of XRL's possible, an important prerequisite if they are to become commonly used tools in the long-term     

Random lasers with coherent feedback

We review our recent work on lasing in active random media. Light scattering, which had been regarded as detrimental to lasing action for a long time, actually provided coherent feedback for lasing. The fundamental difference and transition between a random laser with coherent feedback and a random laser with incoherent feedback were illustrated. We also trapped laser light in micrometer-sized random media. The trapping was caused by disorder-induced scattering and interference. This nontraditional way of light confinement has important applications to microlasers.     

Noise of mode-locked semiconductor lasers

Analytical expressions for the amplitude, frequency, timing, and carrier phase noise of mode-locked laser diodes (MLLDs) are derived. It is found both experimentally and theoretically that carrier dynamics contribute to the total noise of MLLDs. In addition, we demonstrate how to capture the high-frequency timing jitter with optical cross correlations     

Universal properties of random lasers

The design and fabrication of laser resonators is often difficult. However, random lasers occur in gain media with numerous scatterers and produce coherent laser emission without any predesigned cavity. The generation of coherent emission from multiple scattering is quite general and its basic principles are shown here using two model systems, namely /spl pi/-conjugated polymer films and rhodamine-TiO/sub 2/ suspensions. Above a threshold excitation intensity, both systems show narrow emission lines (<0.5 nm), coherence that is determined by photon statistics, and a fundamental cavity length in the disordered material that is revealed by averaging multiple power Fourier transform spectra.