Spatial Hole Burning Effect Research Articles

Single-axial mode, intracavity doubled Nd:YAG laser

A diode-pumped, single-axial mode, intracavity doubled laser has been demonstrated using a cavity consisting of only two intracavity components, a Brewster angle cut piece of Nd:YAG and a KTP doubling crystal. Up to 4 mW single-ended output power has been obtained for 260 mW incident power on the Nd:YAG. Threshold power was 31 mW of diode power incident on the YAG; thus, single-mode operation was attained at up to approximately 8.5 times threshold. At the highest power the laser was susceptible to multimode operation due to spatial hole burning effects but below approximately 5 times threshold reliable single mode operation was achievable. >

Stabilization of the longitudinal mode against spatial hole burning in lambda /4-shifted DFB lasers by quantum size effect

A novel method for stabilizing the spectral properties of lambda /4-shifted distributed-feedback (DFB) lasers against the longitudinal spatial hole burning by using the quantum size effect in MQW structures is proposed and demonstrated experimentally and theoretically. The effect of the longitudinal spatial hole burning on the spectral behavior is shown to be determined by the alpha parameter, not by the differential index or by the optical confinement in the active layer. The reduction of the alpha parameter is found to be very effective in suppressing the spectral instabilities induced by the longitudinal spatial hole burning. >

Corrugation-pitch-modulated MQW-DFB laser with narrow spectral linewidth (170 kHz)

A spectral linewidth of 170 kHz was achieved at a 1.5- mu m wavelength in a 1200- mu m long corrugation-pitch-modulated (CPM) distributed feedback (DFB) laser having multiple quantum wells (MQW). This is the narrowest linewidth ever reported for InGaAsP DFB lasers. The CPM structure is effective in maintaining the single longitudinal mode at a high output power by suppressing the spatial hole burning effect. The linewidth-power product is 400 kHz-mW, and the extrapolated residual spectral linewidth is 150 kHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurements and modeling of the harmonic distortion in InGaAsP distributed feedback lasers

Measurements of the second harmonic distortion in 1.3 mu m InGaAsP distributed feedback lasers are reported. At low modulation frequencies ( approximately 50 MHz), the second harmonic distortion exhibits a minimum at a certain bias current. The minimum, in general, is not observed at high modulation frequencies ( approximately 1 GHz). A model of the harmonic distortion, based on the equivalent electrical circuit of the laser, is presented to explain the observed behavior. The spatial hole burning effect in the active region introduces additional nonlinearity, its effects can be significant compared to that of the leakage path. >

Spectral linewidth of DFB lasers including the effects of spatial holeburning and nonuniform current injection

An analytic expression for the linewidth of distributed-feedback (DFB) lasers is derived. Local fluctuations in carrier density are shown to provide a significant contribution to laser linewidth in the presence of spatial holeburning. In a single-electrode DFB laser the effect can be avoided by careful optimization of the kl-product, but for a multielectrode DFB laser the linewidth can also be reduced by adjusting the injected current profile to compensate for spatial holeburning. Numerical results for a three-electrode lambda /4-shifted DFB laser are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

FM operation of Nd: YAG lasers with standing wave and ring cavity configurations

We have investigated the performance of laser pumped Nd: YAG lasers in the FM mode of operation. We show that the spectrum of these lasers is very strongly influenced by the effects of spatial hole burning. We compare two standing-wave laser designs with a unidirectional ring and find that the ring is the only system that approaches ideal FM operation.

Spectral linewidth reduction by low spatial hole burning in 1.5 μm multi-quantum-well λ/4-shifted DFB lasers

The spectral linewidth of MQW λ/4-shifted DFB lasers has been reduced by increasing κL up to 3–4 without degradation of side-mode suppression by the spatial hole burning effect. A narrow sub-MHz (0.79 MHz) spectral linewidth is achieved for the first time in conventional 300–400 μm long cavity DFB lasers.

The effects of spatial hole burning and energy diffusion on the single-mode operation of standing-wave lasers

Spatial hole burning in the absence of energy diffusion is examined. A formula is derived which predicts how far above threshold a homogeneously broadened standing-wave laser operates in a single-longitudinal mode. This is a conservative formula, since energy diffusion decreases the effects of spatial hole burning and increases the power level at which single-mode operation can be maintained. The role of energy diffusion is also examined. A formula is derived for how far above threshold a standing-wave laser oscillates in a single-longitudinal mode when energy diffusion is accounted for. Examples of the use of these formulas are given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

CLADISS-a longitudinal multimode model for the analysis of the static, dynamic, and stochastic behavior of diode lasers with distributed feedback

A computer model called CLADISS is presented for the analysis of multisection diode lasers. The model allows for the analysis of a wide variety of multisection devices with discrete or distributed internal reflections. The simulator can carry out a threshold, DC, AC, and a noise analysis. The threshold analysis determines the threshold of the various longitudinal modes of the laser. The power versus current and the wavelength versus current characteristics are found with the self-consistent DC analysis. CLADISS includes all of the longitudinal variations by dividing each laser section in many short segments. Both the optical field and carrier density are discretized according to this segmentation. To demonstrate the capabilities of CLADISS some nonlinear effects in DFB lasers are treated. Instabilities induced in the side-mode suppression ratio by spatial hole burning are considered. The effects of spatial hole burning and side modes on the FM response on the linewidth are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Influence of nonlinear gain on DFB laser linewidth

A linewidth theory for DFB lasers which includes the combined effect of spatial holeburning and nonlinear gain is presented. It is shown that the conventional form of nonlinear gain leads to a small reduction of the effective linewidth enhancement factor and cannot explain the linewidth floor or rebroadening observed for DFB lasers at high output power.

Static frequency chirping in lambda /4-phase-shifted distributed-feedback semiconductor lasers: influence of carrier-density nonuniformity due to spatial hole burning

An experimental and theoretical study of the effect of carrier-density nonuniformity on frequency modulation (FM) characteristics of 1.3- μm λ/4-phase-shifted distributed-feedback (DFB) lasers is discussed. The static frequency chirping is measured using a recently developed method that is free from dynamical chirping as well as thermal frequency shift. The amount of chirping shows a strong dependence on the bias current level. This is explained theoretically by considering the nonuniform carrier-density distribution due to the spatial hole burning effect.

Large-signal harmonic and intermodulation distortions in wide-bandwidth GaInAsP semiconductor lasers

Harmonic and intermodulation distortion are two crucial performance parameters of laser diodes in determining their utility in wide-bandwidth analog links. The multimode rate equations are solved numerically to give information on the bandwidth phase, harmonic, and intermodulation distortion of GaInAsP laser diodes at both small- and large-signal levels. The results of the theoretical analysis agree well with experimental results, and show that the effects of both spatial and spectral hole burning are necessary to create an accurate theoretical model that correctly predicts the modulation damping and distortion levels for a range of lateral cavity structures. Anomalous variations in the low-frequency harmonic and intermodulation curves at high modulation levels are explained with this model. The model can be used for any arbitrary input current waveform, and provides information on the output optical spectra which can be used to analyze the propagation of the waveform along an optical fiber. >

Suppression of longitudinal spatial hole-burning effect in lambda /4-shifted DFB lasers by nonuniform current distribution

The mode and the output properties of an asymmetric lambda /4-shifted DFB (distributed-feedback) laser with a distributed injection current density along the cavity are investigated theoretically and experimentally. Coupled wave equations that describe the longitudinal spatial hole-burning phenomenon due to both distributions of a stimulated recombination carrier density and an injection current density are developed. The calculations show that an appropriate distribution of the injection current similar to the intensity profile can be effectively suppress the longitudinal spatial hole-burning effect from the aspect of threshold gai difference Delta alpha /sub th/. A 1.55- mu m InGaAsP/InP asymmetric lambda /4-shifted DFB laser was fabricated with three divided electrodes. Improvement of the linearity of the output characteristics, decrease of threshold current, stability of the single-mode property, and narrowing of the spectral linewidth by injection current and distribution along the cavity were observed. >

Coupled phase-shift distributed-feedback semiconductor lasers for narrow linewidth operation

Mode properties of a coupled phase-shift distributed-feedback (DFB) structure are analyzed and the feasibility of narrow-linewidth emission is shown theoretically. The structure consists of a multiple number of phase-shift DFB laser units which are arranged in tandem and coupled with each other through additional phase shift of corrugation. Optimum structure parameters were determined for two coupled laser units so that the frequency detuning is zero and the mode pattern is the flattest. The mode analysis shows that the intensity distribution is flatter and the normalized coupling constant can be larger than those for the conventional DFB laser with a single phase shift. This indicates that the cavity length can be extended with less influence from the longitudinal spatial hole burning effect. >

Above-threshold mode discrimination in external reflector and phase-shifted DFB lasers

The longitudinal mode discrimination above threshold in both distributed feedback lasers with one external reflector and those with phase-shifted grating structures is studied. By numerically solving a pair of coupled mode equations, it is shown that when these types of lasers have nonsaturable index coupling, they lose their mode discrimination at powers only a few times the saturation power of the medium due to the effects of spatial hole burning. >

Spatial hole burning effects in distributed feedback lasers

The effects of spatial hole burning in a steady-state distributed feedback (DFB) laser are examined by numerically solving the coupled mode equations that describe the system. An approximate solution for the gain above threshold is derived and compared to the exact solution. It is shown that the self-induced grating that arises due to spatial hole burning significantly reduces the mode discrimination of index-coupled DFB lasers. This makes it difficult for these lasers to maintain single-longitudinal-mode behavior above threshold. However, it is found in addition that bulk-modulated (gain-coupled) DFB lasers do not lose their mode selectivity above threshold, indicating that these lasers may be better choices for narrow-linewidth operation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of linewidth of separated-electrode DFB laser diode

We present a new linewidth formula for DFB lasers, which includes the spatial hole burning effect. The linewidth of DFB lasers with three separated electrodes is calculated in various bias conditions. The optimum bias condition is discussed to obtain a narrow linewidth.

Distributed feedback lasers with multiple phase-shift regions

Phase-shifted distributed-feedback (PSDFB) lasers with multiple phase-shift regions are proposed to reduce the effect of spatial hole burning in conventional PSDFB lasers where a single phase shift leads to a highly nonuniform axial distribution of the mode intensity. We analyze multiple phase-shift PSDFB lasers by solving numerically the coupled-wave equations. Our results show that the use of even two phase-shift regions can substantially reduce the axial inhomogeneity of the mode intensity. Although the gain margin is generally reduced by the use of multiple phase shifts, it is still large enough for most practical purposes if the phase shifts are suitably optimized.

Transient chirping in distributed-feedback (DFB) lasers: effect of spatial hole-burning along the laser axis

An analysis of the chirping behavior of directly modulated distributed-feedback (DFB) lasers which takes into consideration the effects of spatial hole-burning along the laser axis is discussed. Rate equations including these effects are derived. To calculate the rate equations, the threshold condition of phase-shifted DFB lasers with a nonuniform refractive index distribution is analyzed. The calculations are carried out on phase-shifted DFB structures under the condition of random nonreturn to zero modulation at 1.8 Gb/s. The results show that the spatial hole-burning along the axis significantly affects the chirping behavior, adding to the effects induced by the spectral hole-burning alone.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Semiconductor laser arrays with enhanced mode stability

An analysis is presented which shows how a semiconductor laser array with a highly uniform intensity distribution for the fundamental mode can be constructed. The high uniformity is desirable for improved modal stability at high-power levels because the effect of spatial hole burning is strongly reduced.