Ralf Menzel

Advances in broad bandwidth light sources for ultrahigh resolution optical coherence tomography

Novel ultra-broad bandwidth light sources enabling unprecedented sub-2 microm axial resolution over the 400 nm-1700 nm wavelength range have been developed and evaluated with respect to their feasibility for clinical ultrahigh resolution optical coherence tomography (UHR OCT) applications. The state-of-the-art light sources described here include a compact Kerr lens mode locked Ti:sapphire laser (lambdaC = 785 nm, delta lambda = 260 nm, P(out) = 50 mW) and different nonlinear fibre-based light sources with spectral bandwidths (at full width at half maximum) up to 350 nm at lambdaC = 1130 nm and 470 nm at lambdaC = 1375 nm. In vitro UHR OCT imaging is demonstrated at multiple wavelengths in human cancer cells, animal ganglion cells as well as in neuropathologic and ophthalmic biopsies in order to compare and optimize UHR OCT image contrast, resolution and penetration depth.

External resonator design for high-power laser diodes that yields 400 mW of TEM 00 power

Normal diode lasers with average output powers of 1 W or more exhibit bad beam quality and therefore cannot be applied for high-precision applications or nonlinear optics. Therefore an external output coupling mirror was used in our experiments. Diffraction-limited operation was achieved, which yielded 400 mW of power and a factor-of-12 improvement in brightness. With this resonator type 1.1 W of average output power was also obtained, with a beam propagation factor of 2.6 in the slow axis; fast axis emission is always diffraction limited.

Diode-pumped Nd:YAG master oscillator power amplifier with high pulse energy, excellent beam quality, and frequency-stabilized master oscillator as a basis for a next-generation lidar system

A pulsed, diode-laser-pumped Nd:YAG master oscillator power amplifier (MOPA) in rod geometry, frequency stabilized with a modified Pound-Drever-Hall scheme is presented. The apparatus delivers 33-ns pulses with a maximum pulse energy of 0.5 J at 1064 nm. The system was set up in two different configurations for repetition rates of 100 or 250 Hz. The beam quality was measured to be 1.5 times the diffraction limit at a pulse energy of 405 mJ and a repetition rate of 100 Hz. At 250 Hz with the same pulse energy, the M2 was better than 2.1. The radiation is frequency converted with an efficiency of 50% to 532 nm. This MOPA system will be the pump laser of transmitters for a variety of high-end, scanning lidar systems.

Highly efficient single-pass frequency doubling of a continuous-wave distributed feedback laser diode using a PPLN waveguide crystal at 488 nm

A continuous-wave distributed feedback diode laser emitting at 976 nm was frequency doubled by the use of a periodically poled lithium niobate waveguide crystal with a channel size of 3 microm x 5 microm and an interaction length of 10 mm. A laser to waveguide coupling efficiency of 75% could be achieved resulting in 304 mW of incident infrared light inside the waveguide. Blue laser light emission of 159 mW at 488 nm has been generated, which equals to a conversion efficiency of 52%. The resulting wall plug efficiency was 7.4%.

Quasi-continuous-wave birefringence-compensated single- and double-rod Nd:YAG lasers

Compensation of thermally induced birefringence directed toward compensation of depolarization and bifocusing in laser rods is treated with simple beam transfer matrices. When we apply a 90-deg polarization-rotating element to a resonator, the radial and the tangential eigensolutions of the resonator change significantly. The effect of this alteration on the resonators stability is investigated in detail. The outcome is used to design a single- and double-rod resonator resulting in 53 W with an M2 approximately 1.5 and 182 W of output power with an M2 approximately 1.2, respectively.

Hindrance of the rotational relaxation in the excited singlet state of biphenyl and para-terphenyl in cooled solutions by methyl substituents

Abstract The measured fluorescence spectra of biphenyl (BP) and para-terphenyl (PTP) in solution reveal with decreasing temperature first an increasing structure and a slight red shift and below the glass transition a loss in structure and a strong blue shift. The effect is discussed using a torsional potential scheme in agreement with QCFF/PI calculations. Comparison of the experimental blue shifts with the results of CNDOS calculations suggest an incomplete hindrance of the relaxation process of the unsubstituted molecules. A nearly perfect hindrance could be detected for the tetra-methyl substituted BP and PTP molecules.

Tuning high-power laser diodes with as much as 0.38 W of power and M 2 = 1.2 over a range of 32 nm with 3-GHz bandwidth

Gain-guided diode lasers usually have emission wavelengths determined by the manufacturing process, with typically 0.5-1-nm bandwidth. Furthermore, their beam quality is rather poor. We show that external cavities allow for tunable narrow-bandwidth operation of gain-guided diode lasers. At the same time the beam quality is drastically improved; almost diffraction-limited light of more than 200 mW has been achieved over the whole tuning range from 910 to 942 nm with narrow bandwidth.

Stripe-array diode-laser in an off-axis external cavity: theory and experiment.

Stripe-array diode lasers naturally operate in an anti-phase supermode. This produces a sharp double lobe far field at angles +/-alpha depending on the period of the array. In this paper a 40 emitter gain guided stripe-array laterally coupled by off-axis filtered feedback is investigated experimentally and numerically. We predict theoretically and confirm experimentally that at doubled feedback angle 2alpha a stable higher order supermode exists with twice the number of emitters per array period. The theoretical model is based on time domain traveling wave equations for optical fields coupled to the carrier density equation taking into account diffusion of carriers. Feedback from the external reflector is modeled using Fresnel integration.

Compact white-light source with an average output power of 2.4 W and 900 nm spectral bandwidth

Abstract The appropriate combination of a passively mode-locked Nd:YVO 4 pump laser with 10 ps pump pulse duration at 1064 nm and a dispersion-adapted air–silica microstructured fiber generates efficiently supercontinuum radiation with an average power of 2.4 W in a spectral range from 700 to 1600 nm. The presented setup allows a very compact design of the white-light source. By means of experimental results will be shown, that newly generated frequency components mainly originate from self-phase modulation, parametric four-wave mixing as well as stimulated Raman amplification.

100-watt average output power 1.2 diffraction limited beam from pulsed neodymium single-rod amplifier with SBS phase conjugation

We describe a solid-state master oscillator-amplifier-laser (MOPA) with a minimum number of elements for the double-pass single-rod amplifier. It produces 100-W average output power of pulsed light with an almost diffraction limited beam. The Q-switched system works with an average repetition rate of about 3 kHz and the pulse width was 70 ns. As active material we used Nd:YALO (Nd:YAP) which has negligible depolarization. The strong thermal lens of the material with focal lengths of less than 15 cm at pump powers of 8 kW was compensated by a phase conjugating mirror based on stimulated Brillouin scattering (SBS). Therefore, the output power could be varied from 2 W up to 100 W without changing the transversal beam profile. The total efficiency of the amplifier including the phase conjugator is 1.25%. >