Transverse Spatial Hole Burning Research Articles

Self-mixing interference in a thin-slice solid-state laser with few feedback photons per observation period

Formation of a peculiar lasing pattern leading to a TEM00-like mode accompanied by a nonzero bright outer-ring emission (namely, a modified TEM00 mode) was observed in a 300-\mu m-thick LiNdP4O12 (LNP) laser with reflective flat end mirrors that was pumped with a laser diode whose focused spot size was larger than the lasing beam spot size determined by the thermal lens effect. The photon lifetimes of the laser oscillations were found to be greatly shortened to \tau_{p} = 6.74 ps as compared with those in pure TEM00 mode operation, while the fluorescence lifetime was \tau = 130 \mu s. The formation of the peculiar transverse mode was theoretically treated in terms of the pump-intensity-dependent thermal lens effect, which yields the lasing beam spot sizes in the resultant optical cavity. In addition, transverse spatial hole burning of population inversions due to the preceding transverse eigenmode intensity and the associated anti-guidance of the pure TEM00 field leading to formation of the TEM00 with an emergent outer ring emission was shown to appear with increasing pump power through the electric lens (i.e., population lens) effect, in which almost all of the population inversion contributes to lasing with a pronounced effective modal gain. Self-mixing laser Doppler velocimetry (LDV) experiments and numerical simulations revealed that spontaneous emission factors in the peculiar mode oscillations were greatly decreased in comparison with those in pure TEM00 mode operation. The present thin-slice solid-state laser possessing a large fluorescence-to-photon lifetime and decreased spontaneous emission factor enabled us to detect LDV signals in the regime of few feedback photons per observation period.

Universal Output Characteristics of Single-Mode Operation in Low-Loss Large- V -Number Multimode Waveguide Lasers with Transverse Spatial Hole Burning

We study the performance limit of single-mode operation in low-loss large-V-number planar waveguide lasers, with transverse spatial hole burning as the dominating mechanism for transverse-mode competition. By introducing normalized variables, we develop a simple semianalytical model to describe the universal output characteristics of single-mode emission before the onset of high-order modes, which can be easily scaled to a wide range of laser configuration parameters. Our model is validated using exact numerical solutions, which show applicability beyond that of the low-loss approximation. Our analysis establishes a minimum criterion of the loss ratio between the fundamental and first higher-order mode for a robust single-mode operation. This criterion is much weaker than that of conventional wisdom based on the pure wave-propagation argument, which we attribute to a resonator enhancement effect. Our universal outcome can be denormalized to establish the relationship between single-mode extraction efficiency and optimum output coupling in a multimode laser with arbitrary modal loss ratios over a wide range of single-pass unsaturated gain and loss.

Theoretical study of mode evolution in active long tapered multimode fiber.

A concise and effective model based on coupled mode theory to describe mode evolution in long tapered active fiber is presented in this manuscript. The mode coupling due to variation of core radius and slight perturbation have been analyzed and local gain with transverse spatial hole burning (TSHB) effect, loss and curvature have been taken into consideration in our model. On the base of this model, the mode evolution behaviors under different factors have been numerically investigated. Our model and results can provide instructive suggestions when designing long tapered fiber based laser and amplifiers.

Impact of high-order-mode loss on high-power fiber amplifiers

In this paper, the impact of the loss of high-order mode (HOM) on the performances of high-power fiber amplifiers (HPFA) has been numerically investigated by using the model considering transverse spatial-hole burning. The HPFA is modeled with various parameters such as pump wavelength, pumping direction, and fiber length. The fact that the output power of HOM will decrease with increasing loss of HOM is explained by the numerical analysis for the first time to the best of our knowledge. Moreover, the results show that the extracted efficiency of the amplifier will decrease dramatically with increasing HOM loss when the loss is low. The efficiency will increase smoothly and finally converge, as the growth of the HOM loss in the regime where the efficiency crosses the minimum. The evolution of the output power of HOM and the efficiency can be attributed to the interaction between the loss and the nonuniformly distributed local gain, which will be clearly illustrated by simulations. We believe the results will be beneficial to understanding the impact of the loss of HOM on the amplifier performances and providing guidelines on designing large-mode-area fiber and optimizing the HPFA aiming at effective single-mode output based on differential loss between the HOMs and the fundamental mode.

Spatial dynamics, thermalization, and gain clamping in a photon condensate

We study theoretically the effects of pump-spot size and location on photon condensates. By exploring the inhomogeneous molecular excitation fraction, we make clear the relation between spatial equilibration, gain clamping, and thermalization in a photon condensate. This provides a simple understanding of several recent experimental results. We find that as thermalization breaks down, gain clamping is imperfect, leading to ``transverse spatial hole burning'' and multimode condensation. This opens the possibility of engineering the gain profile to control the condensate structure.

Transverse mode competition in index-antiguided waveguide lasers

Transverse mode competition in large-mode-area index-antiguided planar waveguide lasers is investigated. We show that, with very large core width and/or index difference between the core and cladding, high-order modes can oscillate and contribute to output lasing power. We have developed a theoretical model for transverse mode competition that takes into account transverse spatial hole burning. Experiments conducted on index-antiguided waveguide lasers with core width of 220 μm and 400 μm are compared to theory with good agreement.

Impact of transverse spatial-hole burning on beam quality in large-mode-area Yb-doped fibers

The beam-quality factor of an amplified spontaneous emission source based on a Yb-doped, large-mode-area (LMA), multimode fiber was found to be optimized when the gain became saturated. A model using spatially resolved gain and transverse-mode decomposition of the optical field showed that transverse spatial-hole burning (TSHB) was responsible for the observed behavior. A simplified model without TSHB failed to predict the observed behavior of beam quality. A comparison of both models shows TSHB is also critical for properly modeling beam quality in LMA fiber amplifiers.

Frequency and intensity modulation characteristics of GaAs lasers in an external cavity

Frequency and intensity modulation characteristics were measured for external cavity GaAs diode lasers as a function of modulation frequency. The data, displayed as a chirp-to-power ratio (CPR), showed at low modulation frequencies a flat response and a 0 degrees or 180 degrees relative phase, depending on laser structure. A model incorporating a carrier-density-dependent imaginary part of the differential gain (Henry alpha factor) is developed to explain the data. The model yields simple scaling of the CPR with injection current and photon lifetime. The agreement between the model and data including scaling is excellent. These results provide strong evidence for transverse spatial hole burning in these lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Influence of quadratic pump power distribution on hole burning in neodymium lasers

The influence of quadratic pump power distribution on transverse spatial hole burning in homogeneously broadened lasers is analyzed. The optimum relationship between pump size and mode size is given. Theoretical and experimental results are compared for neodymium lasers with helical xenon lamp pumping. From the excellent agreement between theory and experiment we can conclude that the pump intensity radial distribution near axis is close to the quadratic law. The experimental quadratic term is measured. Laser performance as a function of the optical cavity parameters is analyzed.

Laser power calculations: sources of error

The physical phenomena that dominate the power characteristics of a laser depend on the detailed nature of the amplifying medium and the resonator structure. In predicting the power characteristics, numerous approximations are always required. The most important approximations are considered here in detail, and error estimates are presented so that a designer can select the appropriate model for a particular application. Emphasis is placed on analytic solutions and specific phenomena considered include longitudinal and transverse spatial hole burning, large single-pass gain, and mixed line broadening.

Saturation behavior of the spontaneous emission from double-heterostructure junction lasers operating high above threshold

Saturation of the spontaneous emission intensity from stripe-geometry double-heterostructure junction lasers has been observed for currents from threshold up to three times threshold. The present observation of saturated luminescence depends upon the spatial uniformity of the saturated gain obtained in a laser with a thin active region ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\sim0.2-0.3 \mu</tex> m) and weak optical confinement. In lasers with thicker active regions ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\sim1 \mu</tex> m) and stronger optical confinement, the spontaneous emission does not saturate at threshold but continues to increase with current at a reduced rate. The increasing emission is shown to originate in spatial regions where the field intensity is low and consequently the gain is not saturated. At pumping levels high above threshold, the nonuniform saturation of the gain (transverse spatial hole burning) leads to increased gain and finally oscillation for other transverse modes.