Browsing by Author "Ackemann, T."

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Analysis of polarization states of broad-area vertical-cavity surface-emitting lasers below and above threshold
(2009)
Schulz-Ruhtenberg, M.
;
Babushkin, I. V.
;
Loiko, N. A.
;
Huang, K. F.
;
Ackemann, T.
The polarization direction of broad-area vertical-cavity surface-emitting lasers is found to be parallel to the wavevector below threshold. The characteristics of the transition through threshold are studied.
Control of the spatial emission structure of broad-area vertical-cavity surface-emitting lasers by feedback
(2009-03-07)
Schulz-Ruhtenberg, M.
;
Tanguy, Y.
;
Huang, K. F.
;
Jaeger, R.
;
Ackemann, T.
The wave number of transverse spatial structures in broad-area vertical-cavity surface-emitting lasers (VCSELs) is controlled via frequency-selective feedback from an external self-imaging cavity in a broad range of wave numbers and emission frequencies. The selected states follow the dispersion curves of the free-running laser. A control range of about 2.5 mu m(-1) in spatial frequency space and 2.5 nm in emission wavelength was obtained for square VCSELs and of about 3 mu m(-1) and 8 nm for circular VCSELs having a different dispersion curve. By spatial filtering in Fourier space, the shape of the structures can also be controlled to some extent. It is argued that the feedback techniques are useful to 'probe' emission states of the free-running laser.
Coupling of polarization and spatial degrees of freedom of highly divergent emission in broad-area square vertical-cavity surface-emitting lasers
(2008-05-30)
Babushkin, I. V.
;
Schulz-Ruhtenberg, M.
;
Loiko, N. A.
;
Huang, K. F.
;
Ackemann, T.
The polarization of highly divergent modes of broad-area square vertical-cavity surface-emitting lasers is shown to be only marginally affected by material anisotropies but determined by an interplay of the polarization properties of the Bragg cavity mirrors and of the transverse boundary conditions. This leads to a locking of the polarization direction to the boundaries and its indeterminacy for wave vectors oriented along the diagonal. We point out a non-Poissonian character of nearest-neighbor frequency spacing distribution and the impossibility of single-wave number solutions.
Polarization properties in the transition from below to above lasing threshold in broad-area vertical-cavity surface-emitting lasers
(2010-02-01)
Schulz-Ruhtenberg, M.
;
Babushkin, I. V.
;
Loiko, N. A.
;
Huang, K. F.
;
Ackemann, T.
For highly divergent emission of broad-area vertical-cavity surface-emitting lasers, a rotation of the polarization direction by up to 90 degrees occurs when the pump rate approaches the lasing threshold. Well below threshold the polarization is parallel to the direction of the transverse wave vector and is determined by the transmissive properties of the Bragg reflectors that form the cavity mirrors. In contrast, near-threshold and above-threshold emission is more affected by the reflective properties of the reflectors and is predominantly perpendicular to the direction of transverse wave vectors. Two qualitatively different types of polarization transition are demonstrated: an abrupt transition, where the light polarization vanishes at the point of the transition, and a smooth transition, where it is significantly nonzero during the transition.