Laser Threshold Condition Research Articles

Spectral dynamics of pseudo-distributed erbium gain in Raman random fiber laser

This work explores the spectral dynamics of pseudo-distributed erbium gain in a Raman-based open cavity hybrid random fiber laser (RFL) and compares it to two other schemes; without erbium-doped fiber (EDF) and with EDF integrated mid-cavity. The pseudo-distribution of erbium gain is simulated by splicing short segments of EDF in between single-mode fiber (SMF) spools. The three schemes have similar total SMF lengths of 78 km, and a total of 30 m EDF if any. It was established that the laser wavelength shifts significantly from 1555.936 nm to 1565.904 nm simply by the insertion of EDF mid-cavity, while the distribution of EDF segments along the cavity allowed the generation of dual-wavelength lasing at 1559.964 and 1565.236 nm. The distribution of EDF also generated a higher maximum output power of 485.60 mW compared to the schemes with lumped EDF and without EDF of 409.17 mW and 352.02 mW output powers, respectively. However, the increment came at the expense of a longer instability event from homogenous gain broadening of SBS induced Stokes and a delayed onset of laser threshold condition. We discuss the formations of these different laser schemes and its dynamics permitted solely due to the uniqueness of random fiber lasers. The scheme presents an interesting opportunity in the development of hybrid random fiber lasers, and the analysis of this structure offers insights for better designed dual-wavelength random fiber lasers and the role that rare-earth Erbium dopant plays in the formation of laser in a Raman-based fiber cavity.

Study on Dual-Wavelength Wide-Spectrum Erbium-Doped Fiber Ring Laser for Optical Fiber Interferometric Sensing System

In this paper, a novel dual-wavelength wide-spectrum erbium-doped fiber ring laser (EDFRL) for optical fiber sensing system is proposed and experimentally demonstrated. The output of EDFRL is theoretically simulated and classified depending on the theoretical analysis of laser threshold condition into “excited state” and “nonexcited state”. The influence of pump wavelength, fiber laser configuration, EDF length and output coupling ratio of EDFRL is discussed. The EDFRL pumped at 1480 nm wavelength and backward-pumped mode could achieve the stable maximum wide-spectrum output power. The output power fluctuation is lower than 0.06 dB and the center wavelengths shift is less than 0.01 nm. The practicability of EDFRL in a hybrid M-Z and Sagnac interferometer optical fiber sensing system is demonstrated through the comparison of SLD and EDFRL. Consequently, the EDFRL proposed in this study is expected to replace the traditional SLD in the optical fiber interferometric sensing system at a lower cost.

1.5-μm Indium Phosphide-Based Quantum Dot Lasers and Optical Amplifiers: The Impact of Atom-Like Optical Gain Material for Optoelectronics Devices

Using quantum dot (QD) structures as active material for optoelectronics was already in focus before the development of quantum well (QW) lasers and before semiconductor lasers occupied an important place in the market. The big step in reducing laser threshold conditions by substituting bulk gain materials with QWs should find a logical continuation by further reducing the dimensionality toward full 3D carrier confinement by QDs. This was predicted by theoretical considerations at the beginning of the 1980s [1], [2]. However, no practical technologies were available at that time. Top-down approaches, as originally addressed, to produce nanoscale species on very large surfaces and interface areas resulted in the accumulation of a large defect density within the active region and the domination of nonradiative carrier recombination mechanisms. Only during the early 1990s was a major breakthrough achieved with the introduction of the bottom-up approach of strain-driven, self-organized epitaxial QD growth. The first lasers were obtained in the gallium arsenide (GaAs)-based material system [3].

Spectral singularities and tunable slab lasers with 2D material coating

We investigate linear and nonlinear spectral singularities in the transverse electric and transverse magnetic modes of a slab laser consisting of an active planar slab sandwiched between a pair of graphene or Weyl semimetal thin sheets. The requirement of the presence of linear spectral singularities gives the laser threshold condition while the existence of nonlinear spectral singularities due to an induced weak Kerr nonlinearity allows for computing the laser output intensity in the vicinity of the threshold. The presence of the graphene and Weyl semimetal sheets introduces additional physical parameters that we can use to tune the output intensity of the laser. We provide a comprehensive study of this phenomenon and report peculiarities of lasing in the transverse magnetic (TM) modes of the slab with Weyl semimetal coatings. In particular, we reveal the existence of a critical angle such that no lasing seems possible for TM modes of the slab with the smaller emission angle. Our results suggest that for TM modes with an emission angle slightly exceeding the critical angle, the laser output intensity becomes highly sensitive to the physical parameters of the coating.

DC-Biased Optofluidic Biolaser for Uric Acid Detection

Optofluidic biolaser is an emerging technology for chemical and biomedical sensing. However, the sensitivity of optofluidic laser is still limited by the laser threshold condition. In this paper we report a DC-biased optofluidic biolaser for uric acid (UA) detection. Inside a Fabry-Perot laser cavity, a coupled enzyme reaction was employed to transform the detection of UA into the detection of analyte-converted hydrogen peroxide (H2O2). The fluorescent product of the enzyme-catalyzed reaction, resorufin, was employed as gain medium for optofluidic lasing. An optimized concentration of H2O2 was pre-added as a DC bias to counterbalance the requirement of high analyte concentration for lasing. UA detection with a limit of detection of 3.63 μM was achieved with a low sample volume as small as 10 μl. This work provides a sensitive technology using optofluidic biolaser for the detection of substance in human body fluids that can be converted into H2O2.

Solving scattering problems in the half-line using methods developed for scattering in the full line

We reduce the solution of the scattering problem defined on the half-line [0,∞) by a real or complex potential v(x) and a general homogeneous boundary condition at x=0 to that of the extension of v(x) to the full line that vanishes for x<0. We find an explicit expression for the reflection amplitude of the former problem in terms of the reflection and transmission amplitudes of the latter. We obtain a set of conditions on these amplitudes under which the potential in the half-line develops bound states, spectral singularities, and time-reversed spectral singularities where the potential acts as a perfect absorber. We examine the application of these results in the study of the scattering properties of a δ-function potential and a finite barrier potential defined in [0,∞), discuss optical systems modeled by these potentials, and explore the configurations in which these systems act as a laser or perfect absorber. In particular, we derive an explicit formula for the laser threshold condition for a slab laser with a single mirror and establish the surprising fact that a nearly perfect mirror gives rise to a lower threshold gain than a perfect mirror. We also offer a nonlinear extension of our approach which allows for utilizing a recently developed nonlinear transfer matrix method in the full line to deal with finite-range nonlinear scattering interactions defined in the half-line.

Spectral singularities, threshold gain, and output intensity for a slab laser with mirrors

We explore the consequences of the emergence of linear and nonlinear spectral singularities in TE modes of a homogeneous slab of active optical material that is placed between two mirrors. We use the results together with two basic postulates regarding the behavior of laser light emission to derive explicit expressions for the laser threshold condition and output intensity for these modes of the slab and discuss their physical implications. In particular, we reveal the details of the dependence of the threshold gain and output intensity on the position and properties of the mirrors and on the real part of the refractive index of the gain material.

Mode Volume, Energy Transfer, and Spaser Threshold in Plasmonic Systems with Gain

We present a unified approach to describe spasing in plasmonic systems modeled by quantum emitters interacting with resonant plasmon mode. We show that spaser threshold implies detailed energy transfer balance between the gain and plasmon mode and derive explicit spaser condition valid for arbitrary plasmonic systems. By defining carefully the plasmon mode volume relative to the gain region, we show that the spaser condition represents, in fact, the standard laser threshold condition extended to plasmonic systems with dispersive dielectric function. For extended gain region, the saturated mode volume depends solely on the system parameters that determine the lower bound of threshold population inversion.

Point interactions, metamaterials, and [formula omitted]-symmetry

We express the boundary conditions for TE and TM waves at the interfaces of an infinite planar slab of homogeneous metamaterial as certain point interactions and use them to compute the transfer matrix of the system. This allows us to demonstrate the omnidirectional reflectionlessness of Veselago’s slab for waves of arbitrary wavelength, reveal the translational and reflection symmetry of this slab, explore the laser threshold condition and coherent perfect absorption for active negative-index metamaterials, introduce a point interaction modeling phase-conjugation, determine the corresponding antilinear transfer matrix, and offer a simple proof of the equivalence of Veselago’s slab with a pair of parallel phase-conjugating plates. We also study the connection between certain optical setups involving metamaterials and a class of PT-symmetric quantum systems defined on wedge-shape contours in the complex plane. This provides a physical interpretation for the latter.

Towards loss compensated and lasing terahertz metamaterials based on optically pumped graphene

We evidence by numerical calculations that optically pumped graphene is suitable for compensating inherent loss in terahertz (THz) metamaterials. We calculate the complex conductivity of graphene under optical pumping and determine the proper conditions for terahertz amplification in single layer graphene. It is shown that amplification in graphene occurs up to room temperature for moderate pump intensities at telecommunication wavelength λ = 1.5 μm. Furthermore, we investigate the coupling between a plasmonic split ring resonator (SRR) metamaterial and optically pumped graphene at a temperature T = 77 K and a pump intensity I = 300 mW/mm(2). We find that the loss of a SRR metamaterial can be compensated by optically stimulated amplification in graphene. Moreover, we show that a hybrid material consisting of asymmetric split-ring resonators and optically pumped graphene can emit coherent THz radiation at minimum output power levels of 60 nW/mm(2).

First principles derivation of microcavity semiconductor laser threshold condition and its application to FDTD active cavity modeling

A laser threshold condition derived from the instantaneous form of Maxwell’s equations is presented. This derivation incorporates the passive quality factor of the resonator and is particularly amenable to semiconductor microcavity lasers. The optical confinement factor is derived and compared to previous reports. The threshold condition derived here is compared to the results of active cavity finite-difference time-domain calculations, and excellent agreement is found.

Bloch wave formalism of photon lifetime in distributed feedback lasers

A rigorous analytic Bloch wave formalism is presented to calculate and analyze the photon lifetime of distributed feedback (DFB) lasers. By expressing the electromagnetic fields in the DFB structure as a superposition of two counterpropagating Bloch waves, the DFB can be considered as a Fabry-Pérot cavity for the Bloch waves. Analytic formulas for the laser threshold condition and the photon lifetime of DFB lasers are then derived, which have the same simple forms and physical insight as those for Fabry-Pérot lasers. Numerical results are presented and interpreted along with comparisons with those obtained from the coupled mode theory and the transfer matrix method.

Dispersive-grating distributed feedback lasers

This paper proposed a novel distributed feedback (DFB) laser by incorporating a dispersive grating, whose coupling strength is dependent on the operating wavelength. Analysis of the laser threshold conditions shows that the proposed structure guarantees single-mode operation due to the inherent threshold gain discrimination on the two otherwise degenerated lasing modes.

Ultralow threshold laser using a single quantum dot and a microsphere cavity

We propose a novel semiconductor microlaser, made by capturing the light emitted from a single InAs/GaAs quantum dot in the whispering-gallery mode of a glass microsphere. We demonstrate that such an arrangement allows the laser threshold condition to be satisfied. The corresponding threshold current should be several orders of magnitude lower than is currently possible in semiconductor lasers.

Recombination processes in type I and type II Sb-based laser structures

Radiative and non-radiative recombination processes in Sb-based laser structures are calculated and discussed with a focus on the differences between type I and type II laser structures. Auger recombination, optical gain, and intervalence band absorption are computed as a function of temperature for ideal laser threshold conditions. These quantities as well as laser threshold currents are discussed for each type of structure. The calculations show that the type I structure is dominated by radiative recombination at low temperatures and by electron Auger processes at high temperatures, and it has the lowest threshold current of the two laser structures studied. The type II structure has similar behavior except it is dominated by hole Auger processes at high temperatures. Finally, these recombination rates are correlated with qualitative features in the bandstructures.

Laser modes and threshold conditions in N-mirror resonators

Two formal methods for finding laser modes and threshold conditions in laser resonators containing as many as N mirrors are presented. The first method is based on an analysis determining the reflectivity and the transmittivity of an N-mirror system with gain. This is an extension of the classical 2 × 2 matrix method. The second method is based on self-consistency equations for the system and directly yields the circulating fields of the individual resonators. A set of rules has been proved to allow these fields to be calculated directly by means of inspection. The laser oscillation condition for an N-mirror system is found. Examples are given for systems with as many as five mirrors.

Feedback relationships and threshold current in semiconductor injection lasers: old problems revisited

Diagrammatic analysis applied to the stationary version of the equations describing electron and photon kinetics in semiconductor injection lasers reveals the existence of a feedback loop in the pattern of mutual relationships between parameters. Associated with this loop is the concept of causal feedback. The analysis affords simple approximate expressions for the photon density versus pumping rate characteristic. A fundamental system-theoretical meaning is found for the laser threshold condition as one for which the transmission function of the feedback loop equals-1.

Analysis of TE and TM modes in DFB lasers by two-dimensional theory

The two-dimensional theory of a distributed feedback (DFB) laser (which was previously presented and applied to the analysis of the laser threshold conditions for the transverse-electric (TE) mode in a simple three-layer waveguide structure) is developed to treat both TE and transverse-magnetic (TM) modes in a four-layer waveguide structure with a thin grating layer, which more closely reflects actual DFB laser structure. The differences between TE and TM modes for the dispersion relations and the laser threshold conditions are clarified. The effects of the waveguide structure (including grating layer thickness, refractive indexes of layers, coupling constant, and corrugation period) on the threshold gains and the gain differences between the two longitudinal modes on both sides of the Bragg frequencies are studied in detail for both TE and TM modes. >

Output characteristics of diode pumped fiber ASE sources

High power amplified spontaneous emission (ASE) from multicore Nd:fibers pumped by laser diode arrays is investigated experimentally and theoretically. Particular examination is made of threshold conditions at which lasing occurs. A model is presented which describes output power for arbitrary end reflectors. Experimental measurements of lasing threshold reflectivity versus pump power are made and are shown to deviate from the usual laser threshold condition at high pump powers. It is demonstrated that feedback from Rayleigh backscattering can be sufficient to drive the ASE source above lasing threshold in certain configurations. >

Analysis of distributed feedback semiconductor lasers by two-dimensional theory

The two-dimensional theory of distributed feedback (DFB) lasers to take account of the planar waveguide structure which was presented in a previous paper (see ibid., vol.26, no.3, p.467-72, 1990) is applied to the analysis of the DFB laser threshold conditions with respect to the effects of the waveguide structure and the facet reflection. The asymmetric properties of the transverse functions of the coupled modes with respect to the Bragg frequency in the dispersion relations are found to be enhanced by the asymmetric index waveguide structure and by the asymmetric facet reflectivity. Therefore, the resulting confinement factor differences in the grating layer between the two adjacent lasing modes on both sides of the Bragg frequency give large threshold gain differences.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>