Multi-transverse Mode Operation Research Articles

Controlling the beam quality in DPALs by changing the resonator parameters

The beam quality of diode-pumped alkali lasers (DPALs) is of major concern in the studies of these lasers. We report on experimental studies and modeling of the beam propagation factor M2 in flowing-gas Cs DPALs with stable optical resonators and of its dependence on the resonator geometry. The measured values of M2 are in agreement with those calculated using the optical model of multi-transverse mode operation (Auslender et al. in Opt Express 25:19767, 2017). Conditions for substantial improvement of the output laser beam quality, reducing M2 to close to unity, are found. Changing the length of the resonator, and/or the radius of curvature of the high reflection mirror, leaving all other parameters of the laser unchanged, makes it possible to control the beam quality. Techniques for controlling M2 in DPALs with wide-aperture beams have not been studied elsewhere and are very important for the applications of this laser. In addition to the optical model, a simple analytical method based on the model suggested by Siegman (IEEE J Quantum Electron 29:1212, 1993) is applied to the experimental results and predicts a simple scaling law for M2 as a function of the size of the resonator fundamental mode.

Single transverse mode operation of GaN-based vertical-cavity surface-emitting laser with monolithically incorporated curved mirror

We report single transverse mode operation of a blue GaN-based vertical-cavity surface-emitting laser (GaN-VCSEL) with a monolithically incorporated curved mirror. For a device with a 4 μm current aperture diameter and a curved mirror with a radius of curvature (ROC) of 51 μm, single transverse mode operation was confirmed up to an output power of 3.2 mW under continuous wave operation at 20 °C. For a device with a smaller ROC of 31 μm, multi transverse mode operation was confirmed, indicating that the transverse mode can be controlled by the cavity design of such GaN-VCSELs.

General model of DPAL output power and beam quality dependence on pump beam parameters: experimental and theoretical studies

A general model that reproduces the output parameters of diode-pumped alkali lasers with different pump beam apertures and shapes is reported. Comprehensive experimental and theoretical parametric studies of static diode-pumped Cs lasers are presented, including the dependence of the output laser power and the beam quality factor M2 on the power of the pump beam for different shapes of the pump beam, pump-to-laser beam overlaps, and laser cell lengths. Two different pump lasers with circular and rectangular beam shapes and pump powers of 65 W and 7 W, respectively, were used in two sets of experiments. An optical model of multi-transverse mode operation of alkali vapor lasers [Opt. Express25, 19767 (2017)OPEXFF1094-408710.1364/OE.25.019767] was modified and applied to the experimental results. The values of the laser power and M2 predicted by the model are in good agreement with the measured values for a wide range of the laser parameters.

Multi-transverse mode operation of alkali vapor lasers: modeling and comparison with experiments

In high-power diode pumped alkali lasers with stable resonators the radius of the pump beam is usually larger than that of the fundamental laser mode and thus several high order transverse modes of the resonator can participate in the lasing. A simple optical model of multi-transverse mode operation of alkali vapor lasers is reported. The model is based on calculations of the pump and laser beam intensities in the gain medium, where the laser beam intensity is a linear combination of the azimuthally-symmetric Laguerre-Gaussian modes. It was applied to Ti:Sapphire and diode pumped cesium vapor lasers. The model predicts that for low pump power only the fundamental lasing mode oscillates. However, for higher pump powers several transverse modes participate in oscillation. The number and intensities of the oscillating modes as a function of the pump beam power and radius were found. The model predicts linear dependence of the laser power on the pump power, the values of the former being in agreement with the experimental results obtained for diode pumped cesium laser [Electron. Lett.44, 582 (2008)]. The mode-matching efficiency for the multi-transverse mode lasing is ~0.8 - 0.85 which means that in this case almost complete overlap of the laser and pump beams takes place. The laser beam quality factorM2increases with increasing pump power from 1 at the threshold power to 5-6 at maximum values of the pump power resulting in lower beam quality at high powers.

High-frequency microwave signal generation using multi-transverse mode VCSELs subject to two-frequency optical injection

In this paper we report a new method of photonic generation of microwave signals using a multi-transverse mode VCSEL subject to two-frequency optical injection. Numerical simulations show that double injection locking involving two transverse modes can be obtained in these systems. We show that the higher-order transverse mode is excited with a much larger amplitude than that of the fundamental transverse mode. The comparison with the case of a single-transverse mode VCSEL subject to similar two-frequency optical injection shows that multi-transverse mode operation of the VCSEL enhances the performance of the photonic microwave generation system. Broad tuning ranges, beyond the THz region, and narrow linewidths are demonstrated in our system. The maximum frequency of the generated microwave signals can be substantially increased if multimode VCSELs are used instead of single-mode VCSELs.

Power-Scaling of Diamond Microlensed Microchip Semiconductor Disk Lasers

Power-scaling of an optically pumped 1040-nm microlensed microchip semiconductor disk laser is reported, where the monolithic cavity and lens structure is embodied in single-crystal diamond. Low threshold and circularly symmetric single-transverse-mode emission (M2<1.2) at output power up to 180 mW is obtained for a mode-matched device having an integrated output coupler of 9.4-mm radius of curvature and a 300-mum aperture. Multitransverse mode operation above 1 W is also presented.

High Modulation Bandwidth Implant-Confined Photonic Crystal Vertical-Cavity Surface-Emitting Lasers

High-speed implant-confined photonic crystal vertical-cavity surface-emitting laser (VCSEL) diodes are fabricated and characterized. The maximum achievable 3-dB bandwidth is a function of both the index confinement provided by the etched pattern (which serves to reduce the diffraction loss and provide for stabilized fundamental mode operation) and the increased parasitic resistance caused by the removal of doped material. In optimized designs, the reduction in net loss provided by the etched pattern can more than offset the increased parasitics, resulting in a higher 3-dB bandwidth than possible in an unetched conventional implant VCSELs for both single- and multi transverse mode operation. A maximum 3-dB small-signal modulation bandwidth of 15 and 18 GHz is reported for an optimized single transverse-mode and multimode VCSEL, respectively.

Multi-transverse Mode Operation in Vertical-cavity Surface-emitting Lasers: Effects of Wideband Modelling on Spectral Characteristics and Noise

A dynamic model for the vertical-cavity surface-emitting laser aimed to study the precise oscillation frequencies of the transverse modes is presented. The model performs the computation of the longitudinal distribution of the field inside the laser cavity, to take account of distributed boundary conditions at the Bragg reflectors. The latter are approximated by means of autoregressive moving-average (ARMA) digital filters, in order to reduce the computational load without loss of precision in wideband scenarios. With the aid of the developed model, a detailed analysis of the frequency and relative intensity noise spectra is carried out, thus allowing to evaluate the spectral quality of the laser emission. Particularly, the results reveal that each of the transverse modes oscillates with a frequency detuning from the central design wavelength, which is in good agreement with experimental observation.

Polarization and Transverse Mode Behaviour of VCSELs under Optical Injection

A theoretical analysis of the polarization and transverse modes of Vertical-cavity Surface-emitting lasers with external optical injection is performed. We show that, for single-mode operation, weak injection of linearly polarized light can lead to stable output power in the injected polarization. We also show that, for multitransverse mode operation, weak injection of linearly-polarized light can lead to stable emission of the fundamental transverse mode in the injected polarization over wide current ranges. The previous selection can occur when the transverse modes have parallel or orthogonal polarizations.

High-Power Optically Pumped 1550-nm VECSEL With a Bonded Silicon Heat Spreader

We present an optically pumped 1550-nm vertical-external-cavity surface-emitting laser with improved output power and operating temperature using a bonded heat spreader. The laser comprises an InGaAsP-based gain element, with a resonant periodic gain structure on top of a distributed Bragg reflector, and a high reflectivity spherical mirror as the external reflector. Heat transport is enhanced by a Si heat spreader bonded to the InGaAsP surface by liquid capillary bonding. Optical pumping is achieved using a 1250-nm fiber Raman laser. A maximum continuous output power of 250 mW was obtained under multitransverse mode operation at 240 K. Under operation in the fundamental near-Gaussian mode, we obtained a maximum power of 230 mW with a beam quality factor (M/sup 2/) of 1.22. At room temperature, the output power was limited to 12 mW.

InP-Based Optically Pumped VECSEL Operating CW at 1550 nm

We present a vertical external-cavity surface-emitting laser operating at 1550 nm. The laser comprises an InGaAsP-based gain element, with a resonant periodic gain structure on top of a distributed Bragg reflector, and a high reflectivity spherical mirror as the external reflector. Optical pumping is achieved using a 1250-nm fiber Raman laser. A maximum continuous output power of 70 mW was obtained under multitransverse mode operation at 233 K. Under single-mode operation, we obtained a maximum power of 60 mW with a beam quality factor M/sup 2/ less than 1.1.

Optical characteristics of multitransverse-mode two-dimensional vertical-cavity top surface-emitting laser arrays

We observe for the first time up to nine transverse modes lasing in the individual elements of a two-dimensional vertical-cavity top surface-emitting laser array under pulsed operation. These modes consist of the TEM/sub 0,0/, TEM/sub 0,1/, TEM/sub 1,0/, TEM/sub 1,1/, TEM/sub 0,2/, TEM/sub 2,0/, TEM/sub 1,2/, TEM/sub 2,1/, and TEM/sub 2,2/ modes, Each of these modes couples coherently across the array to form very distinct supermodes. The transverse modes as well as the resulting supermodes are identified by examining the near-field and far-field profiles as well as the corresponding spectra. We also obtain a record 1.4 W (pulsed) of coherent output power from the array during multitransverse-mode operation. In addition, the polarization properties of the array are investigated, It is found that the TEM/sub 0,1/ mode is horizontally polarized along the [011] crystalline axis while all the other modes are dominantly vertically polarized along the [011~] axis.

Large area multitransverse-mode VCSELs for modal noise reduction in multimode fibre systems

The first operation is reported of large area multitransversemode vertical cavity surface emitting lasers (VCSELs) to reduce the effect of modal noise in multimode fibre systems. A BER &lt; 10−13 is measured with a 25 μm VCSEL operated at 1 Gbit/s in a multimode optical link with 10 dB of mode selective loss at 16 m. The reduction in modal noise is from a decrease in the coherence of the large VCSELs with multitransverse-mode operation.

Chaos in a CO2 waveguide laser due to transverse-mode competition.

We report on the spontaneous onset of temporal chaos in a waveguide ${\mathrm{CO}}_{2}$ laser due to the interaction between two transverse modes. A nonstandard configuration of the resonator has been used in order to obtain multitransverse mode operation. Starting from a mirror setting where two modes have a stable coexistence, further tiny variations of the cavity length induce a period-doubling cascade to chaos.