Off-axis Feedback Research Articles

Beam quality improvement of 1.95-μm GaSb-based broad-area self-pulsed laser by off-axis feedback

It was proposed and demonstrated that a high beam quality self-pulse could be achieved by off-axis feedback for 1.95-μm GaSb-based broad-area laser. The comparative studies of on-axis feedback and off-axis feedback were conducted in an experiment. A highly reflecting (HR) mirror with sharp edge was used in the off-axis feedback system and the overlapping size was changed by moving the HR mirror. At the direct current of 1.8 A, 45-cm external cavity length, and 36% strength of feedback, the off-axis feedback with the overlapping size of 1.2 mm could accomplish the regular periodic pulse with the frequency of 153 MHz. The beam quality M2 was 12.75 in slow axis and improved nearly doubled compared with that in the on-axis feedback system.

Going beyond the beam quality limit of spectral beam combining of diode lasers in a V-shaped external cavity.

A novel spectral beam combining (SBC) approach based on off-axis feedback in a V-shaped external cavity (VSBC) was proposed and demonstrated. A highly reflecting mirror was used to supply the optical feedback by partial overlapping the beam. The advantages of simple setup, output coupler free, tunable beam quality and emission power over traditional SBC were presented. The beam quality exceeds single emitter with the similar energy conversion efficiency to the traditional SBC. The M2 factors of 2.31 × 3.76 in fast and slow axes and a brightness of 122 MWcm-2sr-1 were realized at 30 A based on a commercial broad-area diode laser array.

Off-axis spectral beam combining of Bragg reflection waveguide photonic crystal diode lasers

The spectral beam combining (SBC) of Bragg reflection waveguide photonic crystal (BRW-PC) diode lasers was studied for the first time. An off-axis feedback system was constructed using a stripe mirror and a spatial filter to control beam quality in the external cavity. It was found that the BRW-PC diode lasers with a low divergence and a circular beam provided a simplified and cost-effective SBC. The off-axis feedback broke the beam quality limit of a single element, and an M2 factor of 3.8 times lower than that of a single emitter in the slow axis was demonstrated.

Dynamical regimes of a multistripe laser array with external off-axis feedback

We theoretically study the dynamics of a multistripe laser array with an external cavity formed by either one or two off-axis feedback mirrors, which allow us to select a single lateral supermode with transversely modulated intensity distribution. We derive and analyze a reduced model of such an array based on a set of delay differential equations, taking into account transverse carrier grating in the semiconductor medium. With the help of the bifurcation analysis of the reduced model, we show the existence of single and multimode instabilities leading to periodic and irregular pulsations of the output intensity. In particular, we observe a multimode instability leading to a periodic regime with antiphase oscillating intensities of the two counter-propagating waves in the external cavity. This is in agreement with the result obtained earlier with the help of a 2+1 dimensional traveling wave model.

Theoretical Investigation of Striped and Non-Striped Broad Area Lasers With Off-Axis Feedback

We report a method to improve the beam quality of broad area lasers by using a V-shaped external cavity formed by two off-axis feedback mirrors that allow to select a single transverse mode with transversally modulated intensity distribution. In the case when one of the two feedback mirrors is absent a spontaneous formation of self-induced transverse population grating leading to a reduction of the lasing threshold is observed. Most favorable conditions for stabilization of a single transverse supermode and creation of a high power and high brightness plane wave traveling in the extended cavity are obtained for equal reflectivities of the two external reflectors.

Coherent beam combining of high power broad-area laser diode array with a closed-V-shape external Talbot cavity

We have coherently combined a high-power broad-area laser diode array by using a feedback loop closed off-axis external Talbot cavity. The off-axis feedback from two gratings provides transverse-mode control of broad-area lasers. The Talbot configuration of the external cavity implements diffractive coupling among laser diodes. Feedback from two gratings increases external cavity quality factor and spectrum selection capability. As a result, spatial coherence was improved and spectral linewidth was narrowed down. The high visibility of the far-field profile indicates that high spatial coherence was achieved. We also observed symmetric far-field profiles indicating that laser array was phase locked to in-phase and out-of-phase super-modes, respectively. Transition between these super-modes was observed by tuning one grating's tilted angle.

Lasers with nonlocal feedback, diffraction, and diffusion

An off-axis feedback loop originates a two-point nonlocality that changes the instabilities of lasers of classes A and B: after crossing a first threshold these devices amplify while they lase above a second one. We obtain the instability diagram in presence of off-axis feedback and show that transverse phase and group velocities can be tuned to have the same or opposite sign, depending on control parameters. Counterpropagating noise sustained patterns and localized perturbations are found for an out-of-phase feedback when the laser operates as a signal splitter. We predict these effects in a broad class of devices, with either positive or negative refractive index, diffusion, and in presence of noise.

Signal Amplification and Control in Optical Cavities with Off-Axis Feedback

We consider a large class of optical cavities and gain media with an off-axis external feedback which introduces a two-point nonlocality. This nonlocality moves the lasing threshold and opens large windows of control parameters where weak light spots can be strongly amplified while the background radiation remains very low. Furthermore, transverse phase and group velocities of a signal can be independently tuned and this enables us to steer it nonmechanically, to control its spatial chirping, and to split it into two counterpropagating ones.

Near-diffraction-limited segmented broad area diode laser based on off-axis spectral beam combining

The beam quality of a 500-μm-wide broad area diode laser with five active segments has been improved beyond the beam quality of the individual segments. The principle of this new laser system is based on off-axis feedback in combination with spectral beam combining. By using a double-feedback scheme we are able to improve the beam quality of the laser by a factor of 23 from M2=55 for the free-running diode laser to M2=2.4 for the laser with feedback at a drive current of 2.2 A. The improved M2 value is a factor of 3.4 below M2=8.2 for a single free-running segment. This is the first time that the beam quality of a segmented broad area diode laser has been improved beyond the beam quality of the individual segments.