Transverse Mode Selection Research Articles

Transverse mode selection in a vertical-cavity surface-emitting laser by using preferential alignment of optical feedback

Effect of the alignment of optical feedback on a multi-transverse-mode vertical-cavity surface-emitting laser is investigated experimentally. Enhancement of the fundamental mode or higher-order mode is achieved by adjusting the alignment of the feedback mirror. When the strength of feedback is strong enough, single-transverse-mode operation is obtained in a wide current range.

Microlensed microchip VECSEL

We report a 1.055-mum microchip VECSEL array which uses a microlens-patterned diamond both as a heatspreader and as an array of concave output mirrors. This configuration, which is suitable for laser array operation, is here exploited to perform a systematic study of a set of microchip lasers with the same semiconductor structure but different cavity properties. The transverse mode selection of individual VECSELs is found to depend on the mode-matching conditions and on the microlens aperture size. Mode-matched single-device emission in the fundamental mode (M2~1.1) with pump-limited output power of 70 mW is demonstrated.

Self-induced transverse mode selection in a photorefractive extended cavity laser diode

Previous works on photorefractive self-organizing laser cavities were about lasers that oscillate, prior the self-organization process occurs, on a set of axial modes sharing the same transverse structure. In a well-designed broad-area laser diode extended cavity, we theoretically and experimentally demonstrate that the insertion of a photorefractive crystal can also affect the transverse modal structure to force the laser, initially oscillating on several transverse modes, to oscillate on a single transverse and axial mode. This spatial self-organization process leads to an enhancement of the single mode operating range of the laser.

Analysis of transverse mode competition and selection in multicore fiber lasers

We present an analysis of the transverse supermode competition mechanism in multicore fiber lasers based on rate equations extended to fibers with multiple cores. The model is used to analyze 7-core and 19-core fiber lasers with three commonly used feedback mechanisms as boundary conditions: butt-contact mirror or fiber Bragg grating, Talbot cavity, and collimating lens plus mirror. For the two last-named cases, different feedback (reflection) factors for different modes result from the diffraction effect. The diffraction effect favors the in-phase supermode. Because of mode mixing, a pure in-phase supermode is not obtained. However, for the 7-core fiber laser the in-phase mode can dominate the output.

Multi-frequency transverse properties of a Michelson mirror

Usually, the Michelson interferometer used as a selective mirror in a coupled-cavity laser is treated in the framework of plane waves although the laser beam has a finite lateral extent. We show that in general this approximation is not valid since the reflectivity of the Michelson mirror depends on the transverse profile of the input beam. This could allow for transverse mode selection in a laser without resorting to intracavity apertures. Single and multi-frequency cases are compared when the input is a TEM00 and a TEM10 Laguerre–Gauss beams.

Fourier-optical transverse mode selection in external-cavity broad-area lasers: experimental and numerical results

Broad-area lasers (BALs) with external Fourier-optical cavities with spatial filter for transverse mode selection have been studied experimentally and theoretically. The transverse mode structure of BALs is modeled using a three mirror cavity approach. The model accounts for gain saturation, carrier diffusion, and anti-guiding effects. Near- and far-field distributions are calculated and compared to experimental results. Transverse mode selection is achieved for BALs with a residual front facet reflectivity of 10% at low pump currents. For BALs having a very low front facet reflectivity of 0.001%, transverse modes can be selectively excited up to pump currents more than 200% above laser threshold. Calculations show that BALs having a 0.001%-antireflection coating with an external Fourier-optical cavity high above threshold can operate in a self-Q-switched-like mode with pulse durations of 2-4 ns and repetition rates of 100-200 MHz. Experimental results obtained with a hybrid integrated-optical external cavity for transverse mode selection are also presented.

Efficient selection of high-order Laguerre-Gaussian modes in a Q-switched Nd:YAG laser

Intra-cavity binary phase elements are incorporated into a Q-switched Nd:YAG laser resonator to obtain efficient high-order transverse mode selection. The resonator configuration is analyzed using the propagation-matrix diagonalization method and the Fox-Li algorithm, and a simple model for predicting the relative output powers of the selected modes is developed. The predicted results are verified experimentally with binary phase elements for selecting the TEM/sub 01/, TEM/sub 02/ and TEM/sub 03/, degenerate Laguerre-Gaussian modes. The output energy per pulse was 15 mJ for the TEM/sub 01/, 16.5 mJ for TEM/sub 02/ and 18.3 mJ for TEM/sub 03/, all higher than the 10 mJ for the TEM/sub 00/. The performance in Q-switched operation was found to be similar to that in free-running operation. The numerical calculations, experimental procedure and experimental results are presented.

Spatial dopant profiles for transverse-mode selection in multimode waveguides

We theoretically investigated the effect of the spatial distribution of the active-ion concentration in multimode step-index waveguides on transverse-mode selection for continuous-wave laser operation. We found that uniform doping of a central portion of as much as 60% of the full waveguide core width is highly effective for the selection of fundamental-mode operation, even under highly saturated, high-power conditions. Profiling the dopant distribution to match that of the particular mode desired was also found to be effective, especially if it is the saturated inversion profile that is matched to the shape of the mode.

Effects of carrier transport on the transverse-mode selection of index-guided vertical-cavity surface-emitting lasers

We study the transverse mode selection of a weakly index-guided vertical-cavity surface-emitting laser (VCSEL), based on an extension of the model of Valle et al. [Opt. Commun. 115 (1995) 297]. In the model we incorporate carrier capture and escape processes between the separate confinement heterostructure (SCH) layer and the quantum well (QW) active region of a VCSEL. These effects are characterized with phenomenological capture and escape times, τ cap, τ esc. We show analytically and by numerical simulations that when the carrier loss in the SCH region is taken into account, carrier capture and escape critically affects the transverse-mode selection of a VCSEL. A large ratio between τ cap and τ esc diminishes the number of photons in the QWs, leading to a decrease of the spatial-hole burning, which in turn benefits emission on the fundamental transverse mode. On the contrary, a small ratio between τ cap and τ esc increases the number of photons in the QWs, leading to a large spatial-hole burning which in turn favors higher-order transverse modes. If the carrier loss in the SCH region is neglected the total number of photons in the QW region is independent of τ cap and τ esc. In this case carrier capture and escape play no fundamental role in the transverse-mode selection of the VCSEL, but modify the transient dynamics of the relaxation oscillations.

Limitations of the transverse coherence in the self-amplified spontaneous emission FEL

In this paper we analyze the process of the formation of transverse coherence of radiation from a self-amplified spontaneous emission (SASE) FEL. It is shown that in the high-gain linear regime the degree of transverse coherence approaches unity asymptotically as z −1, but not exponentially, as one would expect from a simple physical assumption that the transverse coherence is established due to the transverse mode selection. It has been found that even after finishing the transverse mode selection process the degree of transverse coherence of the radiation from SASE FEL visibly differs from unity. This is a consequence of the interdependence of the longitudinal and transverse coherence. The SASE FEL has poor longitudinal coherence which develops slowly with the undulator length thus preventing a full transverse coherence.

Metallic waveguide mirrors in polymer film waveguides

A technology for the fabrication of metallic waveguide mirrors is developed. Plane and curved waveguide mirrors, the latter acting in the same way as cylindrical lenses, are realized in benzocyclobutene (BCB) film waveguides. The waveguide mirror structure is dry-etched into the BCB film waveguide. To enhance the reflectivity of the waveguide mirrors, the waveguide edge is metallized. The BCB film waveguide mirrors are characterized with respect to waveguide attenuation and mirror reflectivity. The waveguide attenuation of the processed BCB waveguide is 0.5 dB/cm. Ag-coated BCB waveguide mirrors show a reflectivity of 71%. The efficiency of total internal reflection (TIR, i.e. in the case without metallization) at the dry-etched waveguide edge is 74%. As an application of the BCB waveguide mirrors a hybrid integrated optical module for Fourier-optical transverse mode selection in broad area lasers (BAL) is proposed.

Electrically steerable lasers using wide-aperture VCSELs

A wide-aperture vertical-cavity surface-emitting laser has been fabricated so as to enhance the transverse mode selection. It was found that the far field emission can be made to exhibit two distinct steerable beams. The laser beams may be steered from 30/spl deg/ away from the normal right up to normal emission by varying the driving current. The steering effect is a thermal effect as confirmed by transient waveform analysis. A two-channel switch was built with switching time between channels of 10 /spl mu/s and crosstalk of greater than 18 dB.

New class of optical apodizing diaphragmson the basis of dyed polymers

A new class of apodizing diaphragms, obtained bygradient photobleaching of initially uniform dyed polyurethaneacrylate films by the adapted penumbra technique, isproposed. Two types of intracavity soft apertures are proposed: apodizing selectors that should be used with a separate Qswitch, and selectors combined with an optical switch (a singleelement contains a passive Q switch and an apodizing selectorof transverse modes of the cavity). Soft apertures of the firsttype, produced on the basis of polyurethaneacrylate dopedwith the dithiobenzyl nickel complex, were used for transverse-mode selection in a Nd3+:YAG laser.

Selection of transverse modes in laser cavitiescontaining waveguides and open parts

The transverse modes of a submillimetre lasercavity that contains waveguides and open parts were studiedtheoretically and experimentally with the purpose of findingmethods for mode selection. Two methods based on thefiltering of the Fourier spectra of the waveguide modes andthe use of their interference were substantiated numericallyand realised in experiment. Special attention was paid to themode selection in tunable lasers. Scaling laws allowing one touse the obtained results in a wide range of the cavity parameters and wavelengths are presented.

Diffraction effects in the self-amplified spontaneous emission FEL

In this paper we present a systematic approach for analytical description of SASE FEL (SASE: self-amplified spontaneous emission) in the linear mode. We calculate the average radiation power, radiation spectrum envelope, angular distribution of the radiation intensity in far zone, longitudinal and transverse correlation functions, degree of transverse coherence etc. Using the results of analytical calculations presented in reduced form, we analyze various features of the SASE FEL in the linear mode. The general result is applied to the special case of an electron beam having Gaussian profile and Gaussian energy distribution. These analytical results can serve as a primary standard for testing the codes. In this paper we present numerical study of the process of amplification in the SASE FEL using three-dimension time-dependent code FAST. Comparison with analytical results shows that in the high-gain linear limit there is good agreement between the numerical and analytical results. It has been found that even after finishing the transverse mode selection process the degree of transverse coherence of the radiation from SASE FEL visibly differs from unity. This is consequence of the interdependence of the longitudinal and transverse coherence. The SASE FEL has poor longitudinal coherence which develops slowly with the undulator length thus preventing a full transverse coherence.

Transverse mode selection in large-area oxide-confined vertical-cavity surface-emitting lasers using a shallow surface relief

Transverse mode selection has been introduced in a large area oxide-confined vertical-cavity surface-emitting laser (VCSEL) by etching a shallow (only 40-nm deep) surface relief. The circular relief pattern, intended for fundamental mode selection, selects low order modes, resulting in significantly reduced beam divergence (from 48/spl deg/ to 13/spl deg/ and less over the entire drive current range) and improved spectral purity (width of emission spectrum reduced from 5 to less than 0.3 nm) compared to VCSEL's without surface relief. A maximum output power of 10 mW was measured.

Transverse-mode selection and noise properties of external-cavity vertical-cavity surface-emitting lasers including multiple-reflection effects

A theoretical analysis has been undertaken of steady-state transverse-mode selection in vertical-cavity surface-emitting lasers (VCSEL’s) that are subject to optical feedback in an external-cavity configuration. Multiple reflections in the external cavity are taken into account. The influence of thermal effects on mode selection is also treated, and noise properties of the configuration are discussed. In the analysis specific attention is paid to the competition between the two lowest-order modes (LP01 and LP11 degenerate modes) that are appropriate to the chosen circularly symmetric vertical-cavity surface-emitting laser structure. It is shown that preferential excitation of either mode can be effected by the appropriate choice of optical feedback delay. It is also shown that multiple reflections in the external cavity can affect the mode selection, depending on the optical feedback delay. Thermal effects are shown to be detrimental for transverse-mode selection. Strategies for the minimization of thermal effects on mode selection are also discussed. Finally, the relative intensity noise spectrum for each transverse mode is shown to maintain its double-peaked structure and low mode-partition noise in the presence of optical feedback. A change from a twin-peaked to a single-peaked structure of this spectrum is also observed when the optical feedback delay is changed.

Regenerative ring-laser design by use of an intracavity diffractive mode-selecting element

We report what we believe to be the first application of diffractive phase elements for transverse mode selection in laser ring resonators. We show that this resonator type offers several advantages over Fabry-Pérot resonators with diffractive mirrors. The design for a regenerative ring resonator that produces an eighth-order super-Gaussian intensity profile beam is presented. Numerical simulations, including modeling of the gain saturation and experimental tests, have been carried out to demonstrate the performance of this approach for cw and pulsed operations.

Fourier-optical selection of higher order transverse modes in broad area lasers

A partially and a highly antire ection coated broad area laser are operated in an external cavity Fourier-optical 4f set-up to experimentally investigate transverse mode selection. The external cavity consists of a lens and a spatial frequency filter. Running freely the lasers show non-stationary filamentation. Placing the spatial filter unit directly onto the optical axis gives cw fundamental mode operation and a transverse shift of the spatial filter in the plane of the active region allows for selective excitation of higher order modes.

Increasing the power of broad-waveguide lasers by additional selection of transverse modes

A new method is proposed for transverse mode selection in high-power, broad-waveguide, separate-confinement, heterostructure quantum-well lasers by orienting the front mirror at an angle other than normal to the direction of radiation propagation. The dependence of the mode reflection coefficients on the laser mirror parameters is investigated. Optimal values are found for the mirror inclination angles and the thicknesses and refractive indices of the dielectric coating at which the fundamental guide mode dominates. The possibility of a 1.2-fold increase in the output power of a broad-waveguide, separate-confinement, heterostructure quantum-well diode laser is demonstrated.