Multimode Emission Research Articles

Distributed-feedback terahertz quantum-cascade lasers with laterally corrugated metal waveguides

We report the demonstration of distributed-feedback terahertz quantum-cascade lasers based on a first-order grating fabricated via a lateral corrugation in a double-sided metal ridge waveguide. The phase of the facet reflection was precisely set by lithographically defined facets by dry etching. Single-mode emission was observed at low to moderate injection currents, although multimode emission was observed far beyond threshold owing to spatial hole burning. Finite-element simulations were used to calculate the modal and threshold characteristics for these devices, with results in good agreement with experiments.

Photopumped multimode blue laser emission from cylindrical microcavities of conducting polymers with heteroatoms in main chains

We have studied optical and electric properties of conducting polymers with heteroatoms such as Si and Sn in main chains. Heteroatoms, which are the same group with carbon atom, in each monomer unit of polymer main chains, are expected to possess unique characteristics and functionalities. The strong photoluminescence (PL) with high quantum efficiency and electroluminescence have been clarified, and these polymers have been exhibited the excellent optical properties and spectral narrowing. We also demonstrated pulsed, photopumped multimode laser emission from cylindrical microcavities formed by the polymer thin films deposited around optical fibers and the polymer microdisks. The bright blue laser emission could be observed.

Dual ring laser emission of conducting polymers in microcapillary structures

We have demonstrated photopumped multimode laser emission from cylindrical microcavities of conducting polymer thin films, which were formed by deposition on the inside surface of the glass microcapillary with capillary action. The laser emission was characterized by narrow emission lines and a well-defined excitation threshold. We also fabricated dual ring cavities on inner and outer surfaces of microcapillaries, and laser emission from both sides of capillaries was observed by photopumping with one excitation source. Fabricating two cylindrical microcavities on inner and outer surfaces of the microcapillary by utilizing a different sort of conducting polymers, dual laser emissions, blue and red in color, were obtained concurrently.

Directional emission from asymmetric microlaser resonators of π-conjugated polymers

A π-conjugated polymer film was fabricated into an asymmetric microlaser resonator having a quadrapole deformation with irregular boundaries and a Q factor of 600. At high excitation intensities above the threshold for lasing, we observed multimode laser emission spectra and directional emission at four different angles. Chaotic ray dynamics explains the observed emission pattern.

Double-clad 10-W Yb3+-doped fiber master oscillator power fiber amplifier for He3+ optical pumping.

We describe an all-fiber ytterbium-doped laser followed by a double-stage ytterbium-doped double-clad fiber amplifier of 10-W output power for helium pumping. Different cavity designs are investigated with the goal of achieving high-power multimode emission at 1083 nm, wavelength tunability over the helium absorption bands, and linewidth envelope control over the range 1-3 GHz. We point out the domains with unstable output power and discuss their origin.

Optical properties and microring laser of conducting polymers with Sn atoms in main chains

Optical properties of conducting polymers with Sn atoms in main chains, such as optical absorption, photoluminescence (PL), and electroluminescence, have been studied. The electronic energy structures have been determined by optical and electrochemical measurements. Strong PL with high quantum efficiency has been clarified in the films, and photopumped multimode laser emission was demonstrated from cylindrical microcavities formed by these polymers coated around optical fibers.

High-temperature performance of GaAs-based bound-to-continuum quantum-cascade lasers

GaAs-based quantum-cascade lasers based on a bound-to-continuum transition have been realized and characterized. This band structure design combines the advantages of the well known three-well and superlattice active regions. We observed lasing of Fabry–Pérot lasers in pulsed mode up to a temperature of 100 °C. Multimode emission with a pulsed peak power of 340 mW is observed at room temperature and 42 mW at 80 °C. Further, from aging tests we expect a lifetime of over 60 years for these devices.

Dynamics and coherence of a multimode semiconductor laser with optical feedback in an intermediate-length external-cavity regime

We use a traveling-wave numerical model of the temporal dynamics in semiconductor lasers to investigate the dynamics and coherence properties of a multimode semiconductor laser subject to optical feedback. Considering an external cavity of intermediate length, we have observed features characteristic of the so-called short-cavity regime in a region where the relaxation oscillations frequency is larger than the external-cavity frequency. The time scales of the dynamics are controlled in this case by the strength of the optical feedback. We have examined the coherence properties of the multimode emission and have studied how the coherence time is reduced as the laser becomes more multimode.

Observability of the Risken–Nummedal–Graham–Haken instability in Nd:YAG lasers

Multilongitudinal mode instability in ring Nd:YAG lasers is theoretically analyzed. After we review the way in which the standard two-level laser theory applies to this laser we extend the theoretical treatment to include transverse effects. We do this by taking into account the finite transverse section of the active medium and by assuming a Gaussian transverse distribution for the intracavity field. Finally we demonstrate that multimode emission develops whenever the intracavity field waist diameter is almost equal to the active rod diameter. We conclude that continuous-wave diode-pumped Nd:YAG lasers with low cavity losses are good candidates for the observation of the Risken–Nummedal–Graham–Haken instability.

Studies of π-conjugated polymer coupled microlasers

A resonance structure formed from two connected microdisks was fabricated on a film of dioctyloxy poly-phenlene vinylene (DOO-PPV). When optically excited above the laser threshold this structure showed multimode laser emission, which at high intensities form a whispering gallery (WG) mode that traverses both microdisks.

Photon statistics of the SASE FEL

The photon statistics of the free-electron laser radiation are examined. The photon number moments are calculated for the multimode self-amplified spontaneous emission including the effects of the Gaussian free-electron laser spectral distribution. It is shown that, in the parameter regime where the pulse duration is of the order of the coherence time, the expectation value of the photon number moments deviate from those predicted by a negative-binomial probability distribution.

Long-wavelength (λ≈16 μm), room-temperature, single-frequency quantum-cascade lasers based on a bound-to-continuum transition

Room-temperature operation of long-wavelength, Fabry–Perot and single-mode quantum-cascade lasers at λ≈16 μm is reported. Multimode emission with pulsed peak power up to 400 mW at −40 °C and 220 mW at 30 °C is demonstrated. Single-mode emission up to 60 mW peak power has been achieved at 30 °C.

Longitudinal pumping of polymer microring lasers

Pulsed, longitudinal photopumped multi-mode laser emission is demonstrated in the visible spectral range from cylindrical microcavities formed by luminescent polymer thin films coated around optical fibers. Thresholds for laser oscillation for picosecond excitation are 80 pJ/mm for longitudinal pumping as compared to 4 nJ/mm (5 μJ/cm 2 or 0.05 MW/cm 2) for transversal pumping. An important advantage of the longitudinal pumping configuration lies in a significantly lower excitation density at threshold in comparison with the transverse pumping configuration. This improvement is ascribed to a larger overlap factor between the optical pump and the gain medium in the longitudinal geometry. The emission spectra of plastic microrings under these different excitation configurations are reported and analyzed. We note that lowering the lasing threshold is an important prerequisite criterion towards the development of electrically pumped plastic lasers.

Multimode emission in inhomogeneously broadened ring lasers

The threshold for multilongitudinal-mode emission in inhomogeneously broadened ring lasers is analytically investigated. In the homogeneous limit the multimode instability corresponds to the classical Risken–Nummedal–Graham–Haken instability. It is found that by increasing the inhomogeneous linewidth, the instability threshold is decreased and the growth of high-frequency side modes is favored. In the limit where the population-inversion decay rate γ‖ is much smaller than the polarization decay rate γ⊥ (class B lasers), analytical expressions for the instability threshold are found, which are then generalized to three-level lasers for a comparison with experimental results obtained with erbium-doped fiber lasers. It is shown that even in class B lasers a full Maxwell–Bloch description (in opposition to a rate-equations approach) is necessary when the free spectral range of the cavity is less than ∼(γ‖/γ⊥)1/4γ⊥.

Laser-Like Emission in the Blue-Green Spectral Range from InGaN/GaN/AlGaN Structures under Optical Pumping

The optical properties of InGaN/GaN/AlGaN structures emitting in the blue-green spectral range were studied by conventional photoluminescence and optical pumping studies in stripe geometry. Multi-mode laser-like emission with clearly resolved threshold was observed with longest achieved lasing wavelength of 496 nm. The threshold excitation densities were 7 and 17 kW/cm 2 at 77 and 293 K, respectively.

Laser-Like Emission in the Blue-Green Spectral Range from InGaN/GaN/AlGaN Structures under Optical Pumping

The optical properties of InGaN/GaN/AlGaN structures emitting in the blue-green spectral range were studied by conventional photoluminescence and optical pumping studies in stripe geometry. Multi-mode laser-like emission with clearly resolved threshold was observed with longest achieved lasing wavelength of 496 nm. The threshold excitation densities were 7 and 17 kW/cm 2 at 77 and 293 K, respectively.

Multimode instability in inhomogeneously broadened class-Bring lasers: Beyond the uniform-field limit

The multimode emission threshold of class-$B$ ring lasers is analytically investigated taking into account the localized nature of the cavity losses and the inhomogeneous broadening of the amplifying medium. This analysis finds a relevant application to erbium-doped fiber lasers (EDFL's). The main conclusion is that the predictions of the simplest models are deeply modified both quantitatively and qualitatively. Thus, any attempt to interpret multimode emission in EDFL's must incorporate the two considered factors. Two main results are: (i) While in homogeneously broadened lasers instabilities are inhibited for values of the mirrors' reflectivity $\mathcal{R}<0.54ca,$ this limitation disappears as soon as the inhomogeneous width exceeds $0.9$ times the homogeneous width; and (ii) the band of unstable modes is reduced as cavity losses increase.

Defect recognition via longitudinal mode analysis of high power fundamental mode and broad area edge emitting laser diodes

A nondestructive method is presented which allows a precise detection of defects and their positions inside the cavity of semiconductor lasers. The defect recognition is based on the measurement of the longitudinal mode spectrum below threshold and the inspection of its Fourier transformation. Using a theoretical model, it is shown that a small distortion inside the cavity leads to a peak in the Fourier transformed spectrum from which the position of the distortion relative to the facets can be determined. For a ridge waveguide laser we find a direct correlation between defects identified by the analysis of the longitudinal mode spectrum and cathodoluminescence imaging. The applicability of this method for nondestructive defect recognition will also be demonstrated for broad area laser diodes with lateral multimode emission. The investigations reveal that the presented method can be used to assess the crystal quality of manufactured laser diodes.

L-I characteristics of fiber Bragg grating stabilized 980-nm pump lasers

Light versus current (L-I) characteristics, of fiber Bragg grating stabilized 980-nm pump lasers, is experimentally studied and theoretically modeled. It is shown that a conventional design of such laser modules can result in sudden transitions between a coherence-collapse multimode emission, and a coherent single-mode state of operation.

Experimental studies of the multimode spectral emission in quantum dot lasers

We investigate the electroluminescence spectra of edge-emitting lasers having self-assembled quantum dots as the active medium. A broad laser emission is observed with a modulation of intensity corresponding to single or bunches (supermodes) of Fabry–Pérot modes. The variation of the laser spectra with magnetic field shows that the supermodes originate from laser cavity effects and are not related directly to the electronic properties of the quantum dots. Measurements taken on devices of different cavity height, length, and lateral width indicate that the important parameter controlling the laser multimode emission is the cavity height, effectively the substrate thickness. In particular, the period of the supermodes is inversely proportional to this thickness, indicating that the modulation of the laser emission intensity is due to the leakage of modes into the transparent substrate.