Digitally tunable ring laser using ladder filter and ring resonator

We propose a digitally tunable ring laser incorporating a ladder filter and a ring resonator. The widely tunable ladder filter consists of two input-output waveguides and a waveguide array, and it selects one channel from the periodic outputs of the ring resonator. With this device, the passband of the ladder filter is important in terms of obtaining stable lasing operation and it becomes narrower with increasing diffraction order. However, the free-spectral range of the filter is reduced with increasing diffraction order and this induces lasing mode instability. We therefore optimize the diffraction order of the ladder filter. The device is monolithically integrated by using the InP-InGaAsP material system. We achieved 37 channel 1000-GHz spacing digitally tunable laser operation. A promising way of improving the device performance is to use a chirped ladder filter because this filter has one dominant passband.     

Fabrication of saturable absorbers in InGaAsP-InP bulksemiconductor laser diodes by heavy ion implantation

Several semiconductor Fabry-Perot laser diodes with InGaAsP-InP bulk active layers have been implanted with oxygen and phosphorus ions to form saturable absorbers. The characteristics of the lasing threshold current increase and the change in the optical spectrum have been investigated as a function of the ion fluence. Based on existing models for the formation of point defects in solids, a theory has been derived that effectively describes these laser parameters, as well as radiation-induced losses, as function of the ion fluence. The lasing threshold current of the laser diodes increased up to more than four times due to ion implantation, accompanied by a wavelength shift of more than 30 nm to the blue. Bistability for optical injection is observed     

GaInNAs: a novel material for long-wavelength semiconductor lasers

GaInNAs was proposed and created in 1995 by the authors. It can be grown pseudomorphically on a GaAs substrate and is a light-emitting material having a bandgap energy suitable for long-wavelength laser diodes (1.3-1.55 μm and longer wavelengths). By combining GaInNAs with GaAs or other wide-gap materials that can be grown on a GaAs substrate, a type-I band lineup is achieved and, thus, very deep quantum wells can be fabricated, especially in the conduction band. Since the electron overflow from the wells to the barrier layers at high temperatures can he suppressed, the novel material of GaInNAs is very attractive to overcome the poor temperature characteristics of conventional long-wavelength laser diodes used for optical fiber communication systems. GaInNAs with excellent crystallinity was grown by gas-source molecular beam epitaxy in which a nitrogen radical was used as the nitrogen source. GaInNAs was applied in both edge-emitting and vertical-cavity surface-emitting lasers (VCSELs) in the long-wavelength range. In edge-emitting laser diodes, operation under room temperature continuous-wave (CW) conditions with record high temperature performance (T₀=126 K) was achieved. The optical and physical parameters, such as quantum efficiency and gain constant, are also systematically investigated to confirm the applicability of GaInNAs to laser diodes for optical fiber communications. In a VCSEL, successful lasing action was obtained under room-temperature (RT) CW conditions by photopumping with a low threshold pump intensity and a lasing wavelength of 1.22 μm     

Longitudinal spatial inhomogeneities in high-power semiconductorlasers

We study the spatial distribution of the temperature, gain, and carrier density along the longitudinal direction of a semiconductor laser cavity. In high-power laser diodes, the use of asymmetrical facet reflectivities creates a spatially nonuniform photon intensity profile and results in inhomogeneous temperature and carrier distributions along the active stripe. These profiles are determined from direct measurements of blackbody radiation and the spontaneous emission from the laser cavity. The temperature of the active stripe is observed to be significantly higher than that of the heat sink during lasing, and the effect of temperature on the modal gain spectrum is analyzed. We demonstrate that the local carrier density and optical gain within a laser are not pinned beyond threshold. A spatially inhomogeneous gain profile is possible in laser cavities as long as the threshold condition that the averaged round-trip gain equals the total losses is maintained. A theoretical model is presented which explains the observed experimental data     

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.     

Room-temperature lasing operation of GaInNAs-GaAssingle-quantum-well laser diodes

We have succeeded in demonstrating continuous-wave (CW) operation of GaInNAs-GaAs single-quantum-well (SQW) laser diodes at room temperature (RT). The threshold current density was about 1.4 kA/cm², and the operating wavelength was approximately 1.18 μm for a broad-stripe geometry. Evenly spaced multiple longitudinal modes were clearly observed in the lasing spectrum. The full-angle-half-power far-field beam divergence measured parallel and perpendicular to the junction plane was 4.5° and 45°, respectively. A high characteristic temperature (T₀) of 126 K under CW operation and a small wavelength shift per ambient temperature change of 0.48 nm/°C under pulsed operation were obtained. These experimental results indicate the applicability of GaInNAs to long-wavelength laser diodes with excellent high-temperature performance     

Miniature erbium:ytterbium fiber Fabry-Perot multiwavelength lasers

We demonstrate stable simultaneous lasing of up to 29 wavelengths in miniature 1- and 2-mm-long Er³⁺:Yb³⁺ fiber Fabry-Perot lasers. The wavelengths are separated by 0.8 (100 GHz) and 0.4 nm (50 GHz), respectively, corresponding to the free spectral range of the laser cavity. The number of lasing wavelengths and the power stability of the individual modes are greatly enhanced by cooling the laser in liquid nitrogen (77 K). The polarization modes and linewidth of each wavelength are measured with high resolution by heterodyning with a local oscillator. The homogeneous linewidth of the Er³⁺:Yb ³⁺ fiber at 77 K is determined to be ~0.5 nm, from spectral-hole-burning measurements, which accounts for the generation of a stable multiwavelength lasing comb with wavelength separations of 0.4 nm     

Bistable laser diodes and their applications: state of the art

Recent progresses in research on bistable laser diodes and their applications in optical communications and photonic switching are reviewed. In addition to the conventional absorptive and dispersive bistable laser diodes, bistability in two-mode lasers via gain saturation has recently attracted attention, because of its ultra high speed. On the other hand, bistable laser diodes with saturable absorbers are mainly used in the system applications because of their stable operations at present. This paper presents the theoretical analysis of the two-mode bistable laser diodes, the stripe lasers and the vertical-cavity surface-emitting lasers (VCSELs) as the two major representatives of bistable lasers, and the profound discussion of their possible applications     

Lasing mechanism of InGaN-GaN-AlGaN MQW laser diode grown on SiC bylow-pressure metal-organic vapor phase epitaxy

We studied the lasing mechanism of an InGaN-GaN-AlGaN multiquantum-well (MQW) laser diode by making various optical characterizations on the diode. Excitation power dependence of photoluminescence (PL) intensity was obtained to investigate the carrier recombination process of the laser. Surface emission and edge emission were compared by optical pumping to clarify where the lasing lines were located in relation to the absorption continuum. From the results, we demonstrate that lasing phenomena in our laser are dominated by free carriers. PL mapping was also taken on the same laser chip to examine the in-cavity bandgap inhomogeneity. We found a very large bandgap scattering of 100 meV. We also found that the wavelength distribution has a periodic modulation. We clarified that the various stimulated emission lines observed in our lasers are caused by the in-cavity spatial bandgap inhomogeneity of the InGaN MQW     

Influence of strain on lasing performances of Al-freestrained-layer Ga(In)As(P)-GaInAsP-GaInP quantum-well lasers emitting at0.78<λ<1.1 μm

The influence of strain on lasing performances of Al-free strained-layer Ga(In)As(P)-GaInAsP-GaInP quantum-well lasers is investigated for the first time over a large emission range of 0.78<λ<1.1 μm. GaAsP and InGaAs are used for tensile and compressive-strained quantum-well layers, respectively, while GaAs and GaInAsP lattice-matched to GaAs are applied for unstrained quantum wells. The laser structures were prepared by using gas-source molecular beam epitaxy, and broad-area and ridge waveguide Fabry-Perot laser diodes were fabricated. This study shows that applying both tensile and compressive strains in the quantum well reduces threshold current density for the Al-free strained-layer quantum-well lasers. However, it was found that the lattice relaxation set a limitation of maximum compressive strain (i.e., maximum lasing wavelength) for the compressive strained InGaAs lasers while the carrier confinement determined the acceptable maximum tensile strain (i.e., minimum lasing wavelength) and lasing performances for the tensile strained GaAsP lasers. Threshold current density as low as 164 A/cm² has been obtained for 1.4% compressive-strained InGaAs-GaInAsP-GaInP lasers having a 12-nm thick quantum well. However, excellent characteristics, such as low threshold current, high efficiency low internal loss, and high output power, have been achieved for the Al-free strained-layer quantum-well lasers     

Passively Mode-Locked 4.6 and 10.5 GHz Quantum Dot Laser Diodes Around 1.55 μm With Large Operating Regime

Passive mode-locking in two-section InAs/InP quantum dot laser diodes operating at wavelengths around 1.55 $mu$m is reported. For a 4.6-GHz laser, a large operating regime of stable mode-locking, with RF-peak heights of over 40 dB, is found for injection currents of 750 mA up to 1.0 A and for values of the absorber bias voltage of 0 V down to −3 V. Optical output spectra are broad, with a bandwidth of 6–7 nm. However, power exchange between different spectral components of the laser output leads to a relatively large phase jitter, resulting in a total timing jitter of around 35 ps. In a 4-mm-long, 10.5-GHz laser, it is shown that the operating regime of stable mode-locking is limited by the appearance of quantum dot excited state lasing, since higher injection current densities are necessary for these shorter lasers. The output pulses are stretched in time and heavily up-chirped with a value of 16–20 ps/nm. This mode of operation can be compared to Fourier domain mode-locking. The lasers have been realized using a fabrication technology that is compatible with further photonic integration. This makes such lasers promising candidates for, e.g., a coherent multiwavelength source in a complex photonic chip.      

Room temperature operation of InAs quantum-dot laser utilizing GaAs photonic-crystal-slab-based line-defect waveguide with optical pump

We have successfully fabricated InAs quantum dots (QDs) embedded in a line-defect waveguide in an air-bridge type GaAs photonic-crystal slab (PCS) and observed laser action from optical-pumping. This lasing is found to occur without any optical cavity, such as a set of Fabry-Perot mirrors. Comparison of the observed transmittance spectrum with the calculated band dispersion of the triple-lines defect mode enables us to specify the lasing wavelength as that at the band edge. From this fact, it follows that the distributed feedback mechanism at the band edge with an infinitely small group velocity is responsible for the present lasing.     

Optical phenotyping of human mitochondria in a biocavity laser

We report a new nanolaser technique for measuring characteristics of human mitochondria. Because mitochondria are so small, it has been difficult to study large populations using standard light microscope or flow cytometry techniques. We recently discovered a nanooptical transduction method for high-speed analysis of submicrometer organelles that is well suited to mitochondrial studies. This ultrasensitive detection technique uses nanosqueezing of light into photon modes imposed by the ultrasmall organelle dimensions in a semiconductor biocavity laser. In this paper, we use the method to study the lasing spectra of normal and diseased mitochondria. We find that the diseased mitochondria exhibit larger physical diameter and standard deviation. These morphological differences are also revealed in the lasing spectra. The diseased specimens have a larger spectral linewidth than normal, and have more variability in their statistical distributions.     

Near-field optical studies of semiconductor heterostructures andlaser diodes

Near-field optical microscopy and spectroscopy is emerging as a powerful tool for the investigation of semiconductor structures. Tunable excitation combined with sub-wavelength resolution is providing an unprecedented level of detail on the local optical properties of semiconductor structures. Recent near-field optical studies have addressed issues of laser diode mode profiling, minority carrier transport, near-field photocurrent response of quantum-well structures and laser diodes, imaging of local waveguide properties, and location and studies of dislocations in semiconductor thin films. We present results on the intrinsic resolution limitations of near-field photoconductivity in quantum-well heterostructures and demonstrate that the resolution depends strongly on the amount of evanescent and propagating field components in the semiconductor. Spectroscopic mode-profiling of high-power laser diode emission details the spatial dependence of multiple spectral modes. This paper presents an overview of NSOM techniques for semiconductor systems, its limitations, and present status     

Technologies and applications of Al‐free high‐power laser diodes

We report high‐power technologies in 0.8‐µm Al‐free InGaAsP/InGaP laser diodes. To realize the high‐power operation, the improvement of catastrophic optical mirror damage (COMD) power density level is required. In addition to the use of low surface recombination velocity of Al‐free materials, optimization of waveguide thickness in broad waveguide structure with tensile‐strained barriers and current blocking structure near facets has led to high COMD power density level. Highly stable operation of Al‐free laser diodes with these structures has been obtained over 2500 hours at 2 W from a stripe width of µm. Applications of high‐power laser diodes are also described. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 158(1): 53–59, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20286     

Recent progress in high-power blue-violet lasers

The property of GaInN-AlGaN heterostructures and GaInN multiple quantum well (MQW) gain GaInN laser diodes with low internal loss are described. GaInN blue-violet laser diodes have been developed as a light source for optical disk recording. However, the threshold current density of these diodes has been difficult to reduce and remains high at around 3-4 kA/cm/sup 2/. This is thought to be due to the large transparency current density Jt and the large optical internal loss /spl alpha//sub i/. Recently, the internal loss was successfully reduced to 13.6 cm/sup -1/ by optimizing the design of the near active region and achieved stable continuous operation under 50-mW continuous wave at 70/spl deg/C. Other laser characteristics such as far-field patterns and laser noise have also been improved for optical disk use.     

Densely arrayed eight-wavelength semiconductor lasers fabricated bymicroarray selective epitaxy

This paper describes a novel epitaxial growth technique, called microarray selective epitaxy (MASE), for fabricating extremely small integrated photonic devices. The MASE technique makes it possible to form densely arrayed (pitch <10 μm) multiple-quantum-well (MQW) waveguides without semiconductor etching as well as to control the bandgap energy of each waveguide. The technique is demonstrated for fabricating an eight-channel 10-μm-spacing microarray MQW structure, and the bandgap wavelength of each channel is successfully controlled by changing the SiO₂ mask pattern over a range of 90 nm. The technique is also applied to the fabrication of densely arrayed, eight-wavelength, Fabry-Perot laser diodes. The laser section is only 70 pm wide and 400 μm long. Eight different lasing wavelengths (each over 80 nm), a uniform threshold current of less than 9 mA, and an output power of over 10 mW are obtained     

Dynamic characteristics of bistable laser diodes

A stability analysis of bistable laser diodes, which gives a first analytical explanation of the recently observed high-speed bistable switching and self-pulsation, is presented by introducing two key parameters. It is shown that switching and self-pulsation are associated with two time scales. Turn-on and turn-off dynamics can be characterized by two different sets of eigenvalues which are used to analytically explain the switch-off and switch-on transients observed in experiments. It is also shown that the gain function profile can significantly affect the performance of self-pulsation operation of bistable laser diodes. The analysis also proves that the existence of two critical points has an important effect on the switching time if switching occurs around one of them. The results of the present analysis agree well with those of an exact numerical simulation. In addition, the analysis can be applied to study dynamic characteristics of a bistable laser diode (BLD) with other more complicated structures such as distributed feedback (DFB) laser diodes, multiquantum-well (MQW) laser diodes, etc     

Power scalable TEM/sub 00/ CW Nd:YAG laser with thermal lens compensation

We present finite-element analysis and experimental results to validate our approach for building high-power single-mode Nd:YAG lasers. We show that the thermooptical and thermomechanical properties of a slab laser can be controlled. This is essential for the use of the proposed unstable resonator, We include demonstration of an efficient subscale laser operating at 20 W TEM/sub 00/.     

Miniaturization and Power Scaling of Fundamental Mode Optically Pumped Semiconductor Lasers

We demonstrate that TEM₀₀ mode optically pumped semiconductor lasers (OPSLs) may be scaled to tens of watts in the visible wavelength range using laser cavities an order of magnitude smaller than those of conventional solid-state lasers. In particular, we show that the output power may be scaled linearly by increasing the number of optically pumped semiconductor (OPS) devices and derive a unique solution for a dynamically stable resonator that is independent of the physical cavity length and internal design. This enables miniaturization of high-power OPS lasers to ~1 cm footprints without compromising many resonator performance metrics. The results are applied to demonstrate a 15-mm footprint cavity producing 7.3-W output at 486 nm, and a cavity with two OPS chips with 24-W output at 561 nm. In addition, we show that efficient TEM₀₀ mode performance may be realized using free-space-coupled, high-power laser diode bars. Single-frequency operation is also demonstrated, and an rms noise level less than 0.01% is achieved.