Jan C. Balzer

Femtosecond passively modelocked diode laser with intracavity dispersion management

We report on the generation of ultrashort pulses by dispersion management of a passively modelocked external cavity diode laser. Pulse widths down to 200 fs are obtained at 830 nm emission wavelength. We use intracavity dispersion management to increase the spectral bandwidth and compress the strongly chirped pulses externally with a grating compressor.

3D printed dielectric rectangular waveguides, splitters and couplers for 120 GHz

We use a 3D printer to fabricate rectangular dielectric single mode waveguides for 120 GHz. The rectangular waveguides consisting of polystyrene showed an attenuation of 6.3 dB/m, which is low enough for short devices. We also characterize 3D printed Y-splitters and a 1x3-splitter based on multimode interference. Further, we construct and measure a variable planar waveguide coupler which can be used as a 3-dB coupler, a cross-coupler and no coupler at all.

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.

Quality Control of Sugar Beet Seeds With THz Time-Domain Spectroscopy

Sugar beet seeds have a comparably high rate of defective seeds of about 35%-40% which are discarded before dissemination in the field. The separation of proper and defective seeds is a complex and labor-intensive process. We show how THz time-domain spectroscopy can be used for the examination of the seeds including an algorithm for automated classification of the seeds.

All semiconductor high power fs laser system with variable repetition rate

Laser diodes offer an interesting alternative to commercially available light sources for the generation of ultrashort pulses. They have the unique feature that they can be directly electrically pumped and that the emission wavelength can be controlled over a huge spectral range by changing the composition of the laser material. Hence they have the potential of being a highly flexible, compact and cost effective light source. However there is a considerable chirp of the pulses generated by a diode laser as a consequence of the strong coupling of real and imaginary part of the susceptibility in the semiconductor. This problem is solved by using an external cavity with intracavity dispersion management. By applying this technique we are able to generate pulse durations with less then 200 fs if an additional external pulse compressor is used. By using such a cavity in a master oscillator power amplifier setup the peak power can be increased up to 6.5 kW. This enables a huge field of possible applications like time domain terahertz spectroscopy or material processing. Anyway for some applications like fluorescence lifetime imaging even the repetition rate of an external cavity laser is too high. To solve this problem an ultrafast semiconductor pulse picking element is implemented to reduce the repetition rate into the kHz region. In conclusion we will demonstrate a compact all semiconductor laser system which is capable to generate sub ps pulses with a high peak power and a variable repetition rate at central wavelength of approximately 840 nm.

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.

Terahertz quasi time-domain spectroscopy based on telecom technology for 1550 nm

We present a fiber-coupled terahertz quasi time-domain spectroscopy system driven by a laser with a central wavelength of 1550 nm. By using a commercially available multimode laser diode in combination with state-of-the-art continuous wave antennas, a bandwidth of more than 1.8 THz is achieved. The peak signal-to-noise ratio is around 60 dB. A simulation based on the optical spectrum of the laser diode and the transfer function of the THz path is in agreement with the experimental results. The system is used to extract the refractive index from two different samples and the results indicate that the performance is up to 1.8 THz comparable to a terahertz time-domain spectroscopy system.

Extending the Alvarez-Lens Concept to Arbitrary Optical Devices: Tunable Gratings, Lenses, and Spiral Phase Plates

The Alvarez lenses consist of two parts with surface geometry designed to produce a change in refractive power with transversal translation between the upper and lower parts of the device. In this study, we present a new and straightforward approach to calculating the geometry of such devices and show that an Alvarez configuration can be found not only for lenses, but also for virtually any refractive device. Surface profiles required to realize an optical wedge, a blazed grating, a lens, and a spiral phase plate are presented. The performance of the devices is verified by numerical simulations and experimentally.

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.