Effects of thermalization on Q-switched laser properties

The conventional rate equations for a Q-switched laser are augmented to explicitly include the effects of time- and level-dependent pumping, thermalization among the sublevels in the upper and lower multiplets, and multiplet relaxation in a homogeneously broadened four-level laser medium. To make the numerical computations more generally valid, we introduce a number of dimensionless variables. We show that the initial set of five coupled differential equations can be reduced to a simple set of two coupled equations for the inversion density and photon flux. Via numerical modeling, we have investigated the manner in which both thermalization and lower multiplet relaxation affect Nd:YAG laser characteristics such as output energy and temporal waveform. Our numerical results confirm earlier predictions that the Q-switched Nd:YAG laser output energy increases monotonically by a factor of 3.33 as one progresses from the assumption of slow to rapid thermalization and by an additional factor of 1.46 if one further assumes a terminal multiplet relaxation which is fast relative to the resonator photon decay time. We also find that the laser pulsewidth is substantially broadened when the resonator photon decay time is comparable to the thermalization, and to a lesser extent, the terminal multiplet relaxation times     

Einstein relations for gain-guided semiconductor lasers

The Einstein equations which relate the transition rates for stimulated emission, spontaneous emission and stimulated absorption in a laser active medium are re-examined for the special case of a semiconductor laser with complex wave-guiding. It is shown that, owing to the nonorthogonality of the gain-guided laser modes, the Einstein relations have to be modified. In particular, the spontaneous emission rate per laser mode no longer equals the stimulated emission rate per laser photon.     

Group theory and realization of a helical distributed feedback laser

This study is devoted to theory and first realization of a helical distributed feedback (DFB) laser. The theory emphasizes the group theoretic aspects of helical and linear periodic DFB lasers. It is demonstrated that the dispersion relations of the helical DFB modes are neither periodic nor necessarily symmetric with respect to the propagation constant. Nevertheless, DFB effects exist in helical as well as in linear periodic laser structures. Finally, the first experimental realization of a helical DFB laser is described.     

Phase Correlation and Linewidth Reduction of 40 GHz Self-Pulsation in Distributed Bragg Reflector Semiconductor Lasers

In this paper, self-pulsation (SP) in a distributed Bragg reflector (DBR) semiconductor laser without a saturable absorber is experimentally and theoretically investigated. Detailed experimental characterizations of the SP DBR laser are reported in the optical and radio-frequency domains. Phase correlation between the longitudinal modes selected by the DBR mirror has been experimentally demonstrated. A theoretical model based on coupled rate equations for three modes has been developed to study the time evolution of phases and amplitudes of the modes. The carrier density modulation, resulting from the beating between adjacent longitudinal modes generates four-wave mixing (FWM) and is responsible for mutual injection locking, leading to passive mode-locking. The calculated power spectral density of the frequency noise derived from the model is in agreement with experimental results and proves that the phases of the longitudinal modes are identically correlated through the FWM process in this type of SP lasers     

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     

Pump power dependence of ruby laser starting and stopping time

Experimental results from a number of small ruby lasers show a linear relationship between the pump power and the inverse of the time delay between pump flash and the onset of laser oscillation. These experimental results are quantitatively predicted by a simple rate-equation analysis, yielding a single theoretical curve with no adjustable parameters and in good agreement with the experiments. Additional evidence is also presented verifying the onset at higher pump powers of additional "bouncing-ball" or "light-pipe" modes of oscillation in ruby rods with polished side walls. These additional modes account, in particular, for the abrupt cessation of laser output after an anomalously short duration of laser action, as is typically observed in such rods. These anomalous modes, which are usually not well coupled to the detection circuitry, apparently suppress the normal mode of oscillation once they begin and limit the available output energy in the normal mode. The anomalous modes are eliminated by roughening the sides of the rod, after which normal duration of laser action is observed. The laser stopping time is also predicted with reasonable accuracy by the rate-equation analysis.     

Nonthermal Dynamics of Dielectric Functions in a Resonantly Bonded Photoexcited Material

A fundamental question of whether it is thermal or nonthermal has often been argued, when the system undergoes an ultrafast phase change by femtosecond laser photoexcitation. One of the difficulties therein is how to consider the thermalization of a photoexcited system, in which the “lattice” and “electron” systems are decoupled from an equilibrium state just after photoexcitation. This work shows a methodology to identify a thermalization‐time domain of a photoexcited semiconductor, where it can be detected by monitoring the time‐resolved photoabsorption broadband spectrum and comparing that to thermally induced spectrum changes, by which the electron–lattice thermalization time is evaluated to be 12 ps for PbTe. Further analyzing the dynamic dielectric functions substantiates that thermalization time is related to the recovery time of the resonant bonding. Such a crucial determination of thermalization time would eliminate an uncertainty of “thermal or nonthermal” in ultrafast phase change of the resonantly bonded materials.     

Cerenkov free-electron laser with a dielectric-lined double-slab waveguide

The dispersion relations of dielectric-lined double-slab waveguides used in Cerenkov free-electron lasers are derived and the existence of a group of odd modes, which can be simultaneously excited with the previously considered even modes, is reported. A discussion on the dependence of the radiation frequency and single-pass gain on the system parameters, the dielectric thickness, the beam energy and the beam channel width, is presented. The effects of the odd modes on the Cerenkov free-electron laser operation are investigated.     

A kinematic model for the oligo-mode action of a CW dye laser

Several stable oligo-mode spectra of a CW dye laser were recorded with the aid of an optical spectrum analyzer (scannable Fabry-Perot) giving Fourier transforms of the light field. Simultaneously the total intensity was measured with a fast photodiode and analyzed for beat signals with an RF spectrum analyzer. The spectra observed can be explained consistently in terms of a kinematic model postulating that competing modes cannot exist simultaneously and therefore show a "spiking" behavior. The repetition frequency of the spiking was found to be the frequency difference of the competing modes, a mechanism known from mode-locking. For the spectra observed experimentally, analytic expressions for the time development of the field amplitudes are given which are in good agreement with the experimental results.     

Performance improvement of a self-heterodyne detection BOTDR system employing broad-band laser

The self-heterodyne detection Brillouin optical time domain reflectometer (BOTDR) system using broad-band laser is proposed to reduce coherent Rayleigh noise and improve the system performance. Compared with the system with narrow-band laser, the stimulated Brillouin scattering (SBS) threshold can be improved by about 3 dB. The experimental results of the narrow-band laser measurements for three times independently and the broad-band laser measurement for one time are compared. The root-mean-square (RMS) errors of Brillouin linewidth for two systems with narrow-band laser and broad-band laser are 6.9 MHz and 2.7 MHz, respectively, and the RMS errors of temperature for the heated fiber are about 1.3 °C and 0.7 °C. With the broad-band laser, signal-to-noise ratio (SNR) of the unheated fiber is approximately equivalent to that of the integrated three independent Brillouin signals for the narrowband laser, and the results are believed to be beneficial for performance improvement and measurement time reduction.     

Influence of the lifetime and degeneracy of the4I11/2level on Nd-glass amplifiers

Gain saturation of an Nd-glass amplifier by 1.06-μm light pulses is determined by the lifetimes and degeneracies of the two laser states as well as by thermalization rates among the components of the two multiplets to which the laser levels belong. Reported are measurements showing that although small signal gain was the same for amplified microsecond and picosecond pulses, comparable energy gain saturation occurred when the energy density of the mode-locked pulses was 1.9 times smaller than the amount for the microsecond pulses indicating terminal level filling with the shorter pulses. Our measurements indicate the terminal laser level, which is one of two unresolved groups in the⁴I_{11/2}multiplet, has a lifetime greater than 50 ns and a degeneracy likely to be one or two. Pulse train distortion due to saturating amplification was measured to find whether the thermalization rates among the levels of the laser multiplets were fast or slow compared to the 10^-11-s pulses, but experimental uncertainties prohibit a determination.     

Trapped radiation in injection lasers

The effect of trapped radiation on the stimulated emission threshold and power in injection lasers is investigated theoretically and experimentally. The density of trapped radiation is determined by the relation between the gain and the loss coefficients for this radiation in the active region. The last value depends upon treatment of the cavity sidewalls and laser diode geometry. The absorption of spontaneous emission in the active diode region provides for an additional optical excitation of the crystal and can result in a decrease of threshold current. The GaAs laser diodes with Fabry-Perot cavities and four-ended resonators were studied. The lasing threshold for the axial modes is increased with the diode width and that for the nonaxial radiating modes is decreased with it. The luminescence spectra from the cavity ends and sidewalls are investigated as a function of diode length and current density. It is shown that the gained luminescence can be an essential source of energy losses in injection lasers and leads to limitations of dimensions of laser diodes with planar p-n-junctions. Various methods for suppression of harmful radiation and for increase of the stimulated emission power in injection lasers are discussed.     

Investigations of optical feedback used to enhance stimulatedscattering

The technique of using optical feedback to enhance the phase-conjugation process of stimulated Brillouin scattering is experimentally and theoretically investigated. Several feedback arrangements are experimentally studied and demonstrate the ability to reduce significantly the threshold power requirement, enhance the efficiency, and improve the phase-conjugate fidelity of the Brillouin process. Steady-state analytical solutions for the interactions, including the effects of laser pump depletion, are derived. Numerical modeling is used to introduce the more complete transient effects. Good agreement between the experimental results using a pulsed Nd:YAG laser system and the numerical simulations is achieved. The results are applicable, with modification, to other forms of stimulated scattering and also to self-pumped photoreflection     

Symmetry relations of two-dimensional photonic crystal cavity modes

Degeneracy of resonant modes in two-dimensional (2-D) photonic crystal cavities are investigated using the symmetry relations. The 2-D photonic crystal cavity tends to have either a pair of doubly degenerate modes or nondegenerate modes. We derive simple relations between degenerate modes without using a rigorous group theory. These relations are useful for classifying the resonant modes into degenerate pairs and nondegenerate ones.     

Near-field emission from GaAs double-heterostructure laser mirrors

The near fields of GaAs double-heterostructure (DH) laser mirrors are studied both in photoluminescent (PL) excitation and in lasing emission. The various liquid-phase-epitaxial (LPE) layers of the laser diode are optically delineated, withmp0.1-mum resolution, in a wavelength-selective PL detection system. The near fields of the transverse lasing modes are correlated with the LPE layers that constitute the optical waveguide. With special emphasis on the large-cavity fundamental-mode Ppn'N laser, it is found that small changes in refractive index within the waveguide have pronounced effects on the distribution of stimulated power within the LPE layers. The fundamental mode is found to be contained within the slightly higher refractive-index gain region. This explains the previously observed localization of catastrophic mirror damage and the anomalously large angles of beam divergence. High-order modes are also excited in the gain region when its thickness and refractive-index step within the waveguide exceed some prescribed limits.     

Frequency pulling in a Brillouin fiber ring laser

The lasing frequencies of a Brillouin fiber ring laser are shifted slightly from the resonant frequencies of the cold fiber ring resonator due to the optical Kerr effect and the nonlinear phase shift associated with stimulated Brillouin scattering (SBS) used as the gain mechanism in this type of laser. These frequency pulling effects are investigated in detail both theoretically and experimentally. The results of this study are useful in many Brillouin fiber ring laser applications such as Brillouin fiber optic gyroscopes, microwave generators, and frequency shifters. To the authors' knowledge this is the first time the frequency pulling effect caused by the nonlinear phase shift associated with SBS is reported     

激光辐照光伏型锑化铟探测器的损伤机制研究

本文研究激光辐照光伏型梯化铟探测器的损伤机制，指出激光的损伤使局部ｐｎ结退化为电阻，对器件性能的影响等效于并联一个电阻，用来解释实验中的各种现象，理论与实验曲线符合得很好。该模型也可以解释闪光效应，即光照对此种器件的修复作用。     

Theoretical and experimental study of optical gain and linewidth enhancement factor of type-I quantum-cascade lasers

A theoretical and experimental study of the optical gain and the linewidth enhancement factor (LEF) of a type-I quantum-cascade (QC) laser is reported. QC lasers have a symmetrical gain spectrum because the optical transition occurs between conduction subbands. According to the Kramers-Kronig relation, a zero LEF is predicted at the gain peak, but there has been no experimental observation of a zero LEF. There are other mechanisms that affect the LEF such as device self-heating, and the refractive index change due to other transition states not involved in lasing action. In this paper, the effects of these mechanisms on the LEF of a type-I QC laser are investigated theoretically and experimentally. The optical gain spectrum and the LEF are measured using the Hakki-Paoli method. Device self-heating on the wavelength shift in the Fabry-Perot modes is isolated by measuring the shift of the lasing wavelength above the threshold current. The band structure of a QC laser is calculated by solving the Schro/spl uml/dinger-Poisson equation self-consistently. We use the Gaussian lineshape function for gain change and the confluent hypergeometric function of the first kind for refractive index change, which satisfies the Kramers-Kronig relation. The refractive index change caused by various transition states is calculated by the theoretical model of a type-I QC laser. The calculated LEF shows good agreement with the experimental measurement.