8.3 Exploring the Mechanism Behind Self-Mode-Locking in VECSELs
235
The aim to harness self-mode-locking is understandable, as saturable-absorber
mirrors
3 have to be individually designed and produced for the targeted operation
wavelength, even though for growth experts it appears to be a reasonably little additional effort to deliver such extra chip once high-quality gain mirror growth is accomplished in a certain spectral range. However, self-mode-locked SDLs are expected to
circumvent other restrictions—i.e. other than an additional design, optimisation and
growth process—naturally set by saturable-absorber based devices, such as thermal
management of SESAMs, degradation at high powers and thereby peak-power limitations, and additional costs and space for the implementation of SESAM chips in
VECSELs.
In fact, self-mode-locking is an appealing pathway towards ultrashort pulses, but
the mechanisms behind this phenomenon in VECSELs still remain unclear and are
not understood. Nevertheless, self-mode-locking has been successfully obtained for
different devices, which are based on quantum-well [13] or quantum-dot [12] gain
media, with advanced characterisation in support of a mode-locked operation claim
(for an overview on self-mode-locked VECSELs, see [18]). Owing to the fact that
SDL research in Marburg contributed to these achievements with key demonstrations
in 2014, the aim of the author’s first major VECSEL project has been to perform
important investigations regarding self-mode-locking within the scope of the first—
and in the meanwhile also second—funding period. Several experiments have been
envisioned and partly realised in order to uncover the mechanisms and nonlinear
effects responsible for mode-locking, and to boost the development of fs-pulsed
SDLs towards powerful, cost-efficient devices for a variety of applications.
In summary, within the author’s VECSEL project, pump–probe experiments on a
running SDL’s active region are to be performed in order to acquire a time-resolved
picture of the gain dynamics. This may reveal the influence of gain saturation on an
intensity-dependent refractive index, which causes self-phase modulation and selffocusing. Further experimental studies on the laser chip aim at providing a direct
measurement of a possible Kerr-lensing effect, which is assumed to play a major
role. Given the fact that the chip exhibits such an intensity-dependent lensing, and
shows little influence of the optical pumping situation, as demonstrated so far by the
author’s Z-scan experiments, one can already draw a conclusion on the effective nonlinear refractive index of a VECSEL chip (see preliminary measurements [19–21]
and advanced studies [22, 23]). However, only time-resolved measurements of the
nonlinearity may provide important insights into the origin of nonlinear lensing that
can be attributed to either an ultrafast bound-electronic Kerr effect (BEKE)—if taking place on the fs scale—or to free-carrier nonlinearities (FCN)—on the ps scale. In
fact, an interplay of both effects, which needs to be unravelled still, may be the reason
for stable self-mode-locking, acting as both an ultrafast and slow artificial saturable
absorber, respectively (see discussion in [22]). Additional measurements regarding
the phase information of the optical pulses may finally allow for a clear identification of the main mechanism behind mode-locking and a better understanding of the
properties of the pulsed light source.
3 SAMs, usually semiconductor SAMs, known as SESAMs.
235
The aim to harness self-mode-locking is understandable, as saturable-absorber
mirrors
3 have to be individually designed and produced for the targeted operation
wavelength, even though for growth experts it appears to be a reasonably little additional effort to deliver such extra chip once high-quality gain mirror growth is accomplished in a certain spectral range. However, self-mode-locked SDLs are expected to
circumvent other restrictions—i.e. other than an additional design, optimisation and
growth process—naturally set by saturable-absorber based devices, such as thermal
management of SESAMs, degradation at high powers and thereby peak-power limitations, and additional costs and space for the implementation of SESAM chips in
VECSELs.
In fact, self-mode-locking is an appealing pathway towards ultrashort pulses, but
the mechanisms behind this phenomenon in VECSELs still remain unclear and are
not understood. Nevertheless, self-mode-locking has been successfully obtained for
different devices, which are based on quantum-well [13] or quantum-dot [12] gain
media, with advanced characterisation in support of a mode-locked operation claim
(for an overview on self-mode-locked VECSELs, see [18]). Owing to the fact that
SDL research in Marburg contributed to these achievements with key demonstrations
in 2014, the aim of the author’s first major VECSEL project has been to perform
important investigations regarding self-mode-locking within the scope of the first—
and in the meanwhile also second—funding period. Several experiments have been
envisioned and partly realised in order to uncover the mechanisms and nonlinear
effects responsible for mode-locking, and to boost the development of fs-pulsed
SDLs towards powerful, cost-efficient devices for a variety of applications.
In summary, within the author’s VECSEL project, pump–probe experiments on a
running SDL’s active region are to be performed in order to acquire a time-resolved
picture of the gain dynamics. This may reveal the influence of gain saturation on an
intensity-dependent refractive index, which causes self-phase modulation and selffocusing. Further experimental studies on the laser chip aim at providing a direct
measurement of a possible Kerr-lensing effect, which is assumed to play a major
role. Given the fact that the chip exhibits such an intensity-dependent lensing, and
shows little influence of the optical pumping situation, as demonstrated so far by the
author’s Z-scan experiments, one can already draw a conclusion on the effective nonlinear refractive index of a VECSEL chip (see preliminary measurements [19–21]
and advanced studies [22, 23]). However, only time-resolved measurements of the
nonlinearity may provide important insights into the origin of nonlinear lensing that
can be attributed to either an ultrafast bound-electronic Kerr effect (BEKE)—if taking place on the fs scale—or to free-carrier nonlinearities (FCN)—on the ps scale. In
fact, an interplay of both effects, which needs to be unravelled still, may be the reason
for stable self-mode-locking, acting as both an ultrafast and slow artificial saturable
absorber, respectively (see discussion in [22]). Additional measurements regarding
the phase information of the optical pulses may finally allow for a clear identification of the main mechanism behind mode-locking and a better understanding of the
properties of the pulsed light source.
3 SAMs, usually semiconductor SAMs, known as SESAMs.