There is a controversy, to which extend cochlear stimulation with near infrared laser pulses at a wavelength of 1860 nm is based on optoacoustic stimulation of intact hair cells or -in contrast- is based on direct stimulation of the nerve cells in absence of functional hair cells. Thermal and stress confinement conditions apply, because of the pulse duration range (5 ns, 10 μs-20 ms) of the two lasers used. The dependency of the signal characteristics on pulse peak power and pulse duration was investigated in this study. The compound action potential (CAP) was measured during stimulation of the cochlea of four anaesthetized guinea pigs, which were hearing at first and afterwards acutely deafened using intracochlear neomycin-rinsing. For comparison hydrophone measurements in a water tank were performed to investigate the optoacoustic signals at different laser interaction regimes. With rising pulse peak power CAPs of the hearing animals showed first a threshold, then a positively correlated and finally a saturating dependency. CAPs also showed distinct responses at laser onset and offset separated with the pulse duration. At pulse durations shorter than physiological response times the signals merged. Basically the same signal characteristics were observed in the optoacoustic hydrophone measurements, scaled with the sensitivity and response time of the hydrophone. Taking together the qualitative correspondence in the signal response and the absence of any CAPs in deafened animals our results speak in favor of an optoacoustic stimulation of intact hair cells rather than a direct stimulation of nerve cells.

The direct and therefore forgery-proof marking of singled horticultural products can be done by laser radiation. The potential codification and the harmlessness of markings at micro scale were investigated. The use of different laser wavelengths and parameter settings allows for superficial laser markings, which could be re-identified by image processing, and for sub-epidermal laser markings, which could be read by optical coherence tomography. Thus, parameter studies were performed using different laser systems (800 nm femtosecond-pulsed laser and a quasi-continuous wave 10,600 nm CO2 laser) on horticultural products (rhododendron cuttings and apples). An image processing algorithm was developed to evaluate the superficial laser markings. The sub-epidermal laser markings were re-identified using optical coherence tomography. Furthermore, the effect of the different laser irradiation (laser wavelength, applied laser energy, location of the marking) onto the product quality was determined concerning surface changes. Safe and re-identifiable laser markings could be labeled using wavelength-dependent energy doses. Sub-epidermal markings showed minor quality loss.

Laser surgery has gained clinical importance due to numerous advantages including contact-free processing, arbitrary cutting geometries, and high precision. However, online process control remains a challenge for widespread clinical use. Therefore, we established a combined setup of a pulsed Er:YAG laser (lambda = 2940 nm) and an optical coherence tomography (OCT) (lambda = 930 nm) for in situ monitoring of hard tissue ablation. The optical setup facilitates an interactive control of the laser ablation depth and remaining tissue strength through the depth resolution of OCT. The 3D OCT data-set, which is acquired after ablation, provides contours and layer thicknesses.

Den Vorteilen der Laserchirurgie (kontaktfreie Bearbeitung, beliebige Geometrien, hohe Genauigkeit) steht bislang die fehlende Tiefeninformation wahrend des Gewebeabtrags gegenuber. Ein optischer Aufbau mit vereintem Strahlengang eines Er:YAG-Lasers zur Gewebebearbeitung und einer optischen Koharenztomographie bietet die Moglichkeit zur In-situ-Abtragsuberwachung. Der prasentierte Losungsansatz beinhaltet eine automatische Oberflachensegmentierung sowie eine Auswertung der Ablationsgeometrie. Mit einem Vergleich zwischen Soll- und Ist-Tiefe lassen sich nachfolgende Bearbeitungsschritte prazise planen und zukunftig ein Regelkreis zur Abtragssteuerung schliesen. Schlusselworte: Laserosteotomie, Segmentierung, Ablationsuberwachung

Article Pulsed wavelength-dependent laser stimulation of the inner ear was published on September 6, 2012 in the journal Biomedical Engineering / Biomedizinische Technik (volume 57, issue SI-1-Track-P).

Optical stimulation of the inner ear, the cochlea, is discussed as a possible alternative to conventional cochlear implants with the hypothetical improvement of dynamic range and frequency resolution. In this study nanosecond-pulsed optical stimulation of the hearing and non-hearing inner ear is investigated in vivo over a wide range of optical wavelengths and at different beam delivery locations. Seven anaesthetized guinea pigs were optically stimulated before and after neomycin induced destruction of hair cells. An optical parametric oscillator was tuned to different wavelengths (420 nm–2150 nm, ultraviolet to near-infrared) and delivered 3–5 ns long pulses with 6 µJ pulse energy via a multimode optical fiber located either extracochlearly in front of the intact round window membrane or intracochlearly within the scala tympani. Cochlear responses were measured using registration of compound action potentials (CAPs). With intact hair cells CAP similar to acoustic stimulation were measured at both locations, while the neomycin treated cochleae did not show any response in any case. The CAP amplitudes of the functional cochleae showed a positive correlation to the absorption coefficient of hemoglobin and also to moderate water absorption. A negative correlation of CAP amplitude with a water absorption coefficient greater than 5.5 cm−1 indicates additional phenomena. We conclude that in our stimulation paradigm with ns-pulses the most dominant stimulation effect is of optoacoustic nature and relates to functional hair cells.

We report on an all-fiber femtosecond ytterbium laser without dispersion compensation consisting of all-normal dispersion fibers. Mode-locking was achieved by nonlinear polarization evolution in combination with additional amplitude modulation generated by a fiber-based spectral filter. The generated pulses were highly chirped and had a maximum pulse energy of 1.8 nJ. The output pulse duration was 7.6 ps and could be dechirped to 179 fs.

We present a mode-locked ytterbium fiber laser with a higher-order mode fiber compensating the group-velocity dispersion and partially the third-order dispersion of the single-mode fiber at a wavelength of 1 microm. The generated pulses had an energy of 0.5 nJ and could be dechirped externally to a pulse duration of less than 60 fs. The power spectrum shows a spectral full width at half-maximum of 57 nm.

We report on an all-fiber femtosecond Ytterbium laser without dispersion compensation operating in the similariton pulse regime. The oscillator was mode-locked with a saturable Bragg reflector along with nonlinear polarization evolution. A pulse energy of 0.8 nJ was achieved with a pulse duration of 10 ps. The pulses could be externally dechirped to 627 fs which was within 8.7% of the Fourier transform limited pulse duration. The ealized oscillator combines compact size, high stability and alignment-free operation.