Benjamin Döpke

Self-optimizing femtosecond semiconductor laser

A self-optimizing approach to intra-cavity spectral shaping of external cavity mode-locked semiconductor lasers using edge-emitting multi-section diodes is presented. An evolutionary algorithm generates spectrally resolved phase- and amplitude masks that lead to the utilization of a large part of the net gain spectrum for mode-locked operation. Using these masks as a spectral amplitude and phase filter, a bandwidth of the optical intensity spectrum of 3.7 THz is achieved and Fourier-limited pulses of 216 fs duration are generated after further external compression.

Mode-locked semiconductor laser system with intracavity spatial light modulator for linear and nonlinear dispersion management

We analyze the influence of second and third order intracavity dispersion on a passively mode-locked diode laser by introducing a spatial light modulator (SLM) into the external cavity. The dispersion is optimized for chirped pulses with highest possible spectral bandwidth that can be externally compressed to the sub picosecond range. We demonstrate that the highest spectral bandwidth is achieved for a combination of second and third order dispersion. With subsequent external compression pulses with a duration of 437 fs are generated.

Femtosecond semiconductor laser system with resonator-internal dispersion adaptation

We present a femtosecond laser diode system that is capable of autonomously adjusting itself to compensate for the external dispersion in an arbitrary application. The laser system contains a spatial light modulator inside the cavity which is controlled by an evolutionary algorithm in order to allow for phase and amplitude shaping of the laser emission. The cavity-internal dispersion control is shown to be much more efficient than an external control with a pulse shaper.

Passively Mode-Locked Diode Laser With Optimized Dispersion Management

We investigate passively mode-locked diode lasers with external cavity for ultrashort pulse generation. Our strategy to achieve ultrashort pulses is to generate strongly chirped pulses with a maximized bandwidth and to compress them externally. By managing intracavity dispersion with an evolutionary algorithm, we obtain pulse widths as short as 278 fs following this approach. We analyze the bandwidth of the optimized pulses in comparison to the available net gain bandwidth of the diode laser device to derive further strategies for achieving shorter pulses.

Intracavity loss and dispersion managed mode-locked diode laser

We present a self-optimizing diode laser, which generates 216 fs Fourier-limited pulses after external pulse compression. This is achieved by a closed-loop learning concept that optimizes the intracavity spectral phase and amplitude.

Femtosecond semiconductor laser system with arbitrary intracavity phase and amplitude manipulation

In this work we present a laser cavity with a spatial light modulator (SLM), which allows for arbitrary phase and amplitude manipulation. In comparison to previous setups, it allows the manipulation of spectral components inside the laser cavity without the introduction of spatial chirp. An electrically driven ultrafast semiconductor laser system is used for proper alignment of the laser cavity. We were able to demonstrate that the gain of the laser supports mode-locking operation over a spectral range greater than 12 nm with a central wavelength of 850 nm. This bandwidth has the potential to generate sub 150 fs pulses.

In-situ calibration of spatial light modulators in femtosecond pulse shapers

Femtosecond pulse shapers are an important tool for the manipulation of ultrashort pulses. Two-Liquid-Crystal (LC)- Panel Spatial Light Modulator (SLM) pulse shapers5 are especially useful, as they permit simultaneous control of phase and amplitude of spectral components, thereby allowing for a wide range of possible manipulations of the temporal shape of ultrashort pulses. In this work, a detailed description of the alignment of a pulse shaper optimized for modelocked semiconductor lasers is presented. Several methods to calibrate the phase-retardance-LC-voltagerelationship are reviewed and a calibration method is presented which is robust to non-idealities of the components of the setup.

Self-optimizing passively, actively and hybridly mode-locked diode lasers

Due to their large gain bandwidth, semiconductor lasers are an interesting alternative to conventional ultrafast laser systems. Their potential pulse width is predicted to be well below 60 fs [1]. In comparison to other sources, they are directly electrically pumped, are compact in size and their emission range can be designed by bandgap engineering. Yet semiconductor lasers for ultrashort pulse generation still face challenges.

Linking transducer transfer function with multi-pulse excitation photoacoustic response

It has been shown recently, that varying the excitation sequence could deliver additional benefits for photoacoustic imaging, for instance, bringing additional information on the sample under study, or reducing the total acquisition time. However, for the typically used solid state laser systems, such modification requires significant increase of the systems’ complexity. We are taking an advantage of high pulse repetition rates that semiconductor laser diodes could offer. That allows the usage of dense pulse bursts with varied number of pulses and inter-pulse delays in the range of the transducer waveform duration to study the effects of the overlay of the single pulse photoacoustic responses. In this study, we conduct a pump-probe experiment, using multi-pulse excitation sequences with varied inter-pulse delays while registering the acoustic response. We show that pulse burst excitation can be beneficial for increasing the registered amplitude and suitable inter-pulse delay values can be obtained from the transducer transfer function, either known or measured. Additionally, we examine the frequency content of the multi-pulse photoacoustic response and show that it is dominated by the pulse repetition rate used. We focus on low central frequency transducers as being widely used for clinical applications.

Dynamics of the photoacoustic response of single-element PZT transducers to pulse burst excitation

Achieving a good signal-to-noise ratio at increased depths remains a challenge, even for photoacoustic imaging, which stimulates the search for possible contrast improvements. Both double-pulse and pulse burst excitation are shown beneficial for increasing the signal-to-noise ratio or acquiring additional information about the sample. We use the advantage of semiconductor laser diodes offering great opportunities regarding both number of pulses in the burst and inter-pulse delay times to study the dynamics of the pulse burst excitation responses of the single-element PZT transducers, looking for possibilities towards contrast improvement. We concentrate on inter-pulse delay ranges of few hundred nanoseconds and low central frequency transducers as they are mainly used for clinical applications We show that using pulse burst excitation with up to five pulses per burst and transducer-matched inter-pulse delays can increase the registered maximum amplitude, leading to signal-to-noise ratio improvement. The multi-pulse response amplitude increase amounted to 20% of the amplitude of a single-pulse response in the performed measurement.