Shyam K. Mandre

Control of the spatiotemporal emission of a broad-area semiconductor laser by spatially filtered feedback

We demonstrate the control of the spatiotemporal emission dynamics of a broad-area semiconductor laser in an external optical feedback configuration formed by a spatially filtering mirror. The emission properties are studied with a single-shot streak camera with temporal resolution of approximately 7 ps and spatial resolution of approximately 5 microm. Our results show a significant reduction of the spatial filamentation and, furthermore, suppression of the spatiotemporal instabilities, which are both intrinsic emission properties of standard high-power broad-area lasers. Associated with the control of the emission dynamics, strong improvement of the beam quality, which is essential for numerous high-power applications, is possible.

Spatial decoherence of pulsed broad-area vertical-cavity surface-emitting lasers

We report a strong reduction of spatial coherence of the emission of large aperture vertical-cavity surface-emitting lasers when they are driven by microsecond electrical pulses. We give evidence that this is due to a breakdown of the modal emission of these lasers. The spatial decoherence manifests itself in the formation of a Gaussian far field intensity distribution. The coherence radius we extract is 1.4 micrometer under these operating conditions, irrespective of the Fresnel number of the vertical-cavity surface-emitting laser. Finally, the spatial coherence properties can be varied by changing the pulse duration or pulse amplitude.

Evolution from modal to spatially incoherent emission of a broad-area VCSEL

Broad-area vertical-cavity surface-emitting lasers (BA-VCSELs) can exhibit a state of spatially incoherent emission, as we recently reported in [M. Peeters et al., Opt. Express, 13, 9337 (2005)]. Here, we experimentally study the evolution of a BA-VCSEL under pulsed operation from well-defined modal emission with a multitude of transverse cavity modes to such spatially incoherent emission. The transition is studied using a high-speed intensified CCD camera and differential image analysis with which single-shot measurements of the imaged nearfield, farfield, spatial coherence, and spectral emission properties are acquired. This combination of experimental characterization tools allows for a detailed description of the BA-VCSELs emission behavior, which is necessary for an in-depth understanding of the processes involved. We find the interplay between the thermal chirp and the build-up of a spatially distributed thermal lens to be decisive for the break-up of the global cavity modes.

Spatially Resolved Characterization of the Coherence Area in the Incoherent Emission Regime of a Broad-Area Vertical-Cavity Surface-Emitting Laser

We present direct measurements of the spatial coherence area of a pulsed broad-area vertical-cavity surface-emitting laser using a reversing wavefront interferometer. With this technique, we can assess the size and uniformity of the coherence area across the laser aperture, being of importance for projection applications. We show that the output beam can be considered quasi-homogeneous and that the measured coherence area corresponds well with the coherence area deduced from the far-field emission profile. We demonstrate that the coherence area is limited in size by the radial temperature gradient in the device and discuss the origin of coherence variations.

Thermally Controlled Onset of Spatially Incoherent Emission in a Broad-Area Vertical-Cavity Surface-Emitting Laser

We present an experimental study of the physical process that leads to spatially incoherent, nonmodal emission in broad-area vertical-cavity surface-emitting lasers. We show that this special emission regime that occurs in pulsed operation of these lasers is due to a combination of a spatially distributed thermal or refractive index gradient (thermal lens) and thermal expansion of the cavity during the pulse (thermal chirp). Our measurements are based on preinstalling a thermal lens through a current bias, and subsequently, modulating a pulse onto the bias. This approach allows us to independently investigate the role of both thermal effects in the onset of nonmodal emission.

Determining the temporally and radially resolved temperature distribution inside a pulsed broad-area vertical-cavity surface-emitting laser cavity

The authors experimentally determine temporally and radially resolved profiles of the temperature distribution within the cavity of a broad-area vertical-cavity surface-emitting laser in pulsed operation. For this, the recently discovered state of spatially incoherent emission [M. Peeters et al., Opt. Express 13, 9337 (2005)] is harnessed. Using single-shot measurements of spectrally resolved near field profiles acquired by a fast-gated charge coupled device camera, the wavelength shift due to heating of the device is temporally and radially resolved. From the wavelength shift the temperature shift is extracted, which finally allows for determination of absolute temperature profiles.

Nonmodal emission characteristics of broad-area vertical-cavity surface-emitting lasers

An overview of the idiosyncratic emission characteristics of pulsed broad-area VCSELs is presented, together with a statistical model which describes these devices as quasi-homogeneous sources. The predicted properties of such sources, which include two reciprocity relations between near and far field aspects of the cross-spectral density as well as its propagation, are verified experimentally. We finish by showing how and which thermal effects are at the origin of this nonmodal emission.

Direct Generation of 50 ps-Pulses from an External Cavity Diode Laser Yielding 200 mW Average Output Power

We present a broad area diode laser with a very simple external resonator which shows self-induced modelocking of about 10 longitudinal modes at a repetition rate of 1-3 GHz, resulting in short pulses.

ps-Pulse generation by self-induced modelocking of a high power broad area diode laser in an external resonator

Up to 250 mW of ps-pulses with a pulsewidth of 50 ps and a repetition rate of 1-3 GHz are generated by self-induced modelocking of a high-power diode-laser working in an external resonator.

Role of thermal effects on the onset of spatially incoherent emission in a Broad-Area Vertical-Cavity Surface-Emitting Laser

We present an experimental study of the spatial coherence properties of Broad-Area Vertical-Cavity Surface-Emitting Lasers (BA-VCSELs). In particular we demonstrate that the spatially incoherent emission regime [1] that can be invoked in pulsed operation stems from two different thermal effects: thermal lens and thermal chirp. We have separately measured the influence of these effects on the appearance of the spatially incoherent emission by pre-installing a thermal lens through a current bias and subsequently modulating a pulse onto the bias. By means of a differential method we are able to analyze the emission character during the additional current pulse alone.