Christian Homann

Approaching the full octave: Noncollinear optical parametric chirped pulse amplification with two-color pumping

We present a new method to broaden the amplification range in optical parametric amplification toward the bandwidth needed for single cycle femtosecond pulses. Two-color pumping of independent stages is used to sequentially amplify the long and short wavelength parts of the ultrabroadband seed pulses. The concept is tested in two related experiments. With multi-mJ pumping pulses with a nearly octave spanning spectrum and an uncompressed energy of 3 mJ are generated at low repetition rate. The spectral phase varies slowly and continuously in the overlap region as shown with 100 kHz repetition rate. This should allow the compression to the Fourier limit of below 5 fs in the high energy system.

Octave wide tunable UV-pumped NOPA: pulses down to 20 fs at 0.5 MHz repetition rate

Femtosecond laser pulses, which are tunable from 440 to 990 nm, are generated at MHz repetition rates by noncollinear parametric amplification (NOPA). The pulses have durations of 20 to 30 fs over the major part of the tuning range and a high energy stability of 1.3% (rms). The NOPA is pumped with ultraviolet pulses from the third harmonic of an ytterbium doped fiber laser system and seeded by a smooth continuum generated in bulk sapphire. The residual second harmonic is used to pump an additional NOPA, which is independently tunable from 620 to 990 nm. Interference experiments show that the two NOPA systems have a precisely locked relative phase, despite of being pumped by different harmonics with a random phase jitter. This demonstrates that the phase of pulses generated by optical parametric amplification does not depend on the pump phase.

Carrier-envelope phase stable sub-two-cycle pulses tunable around 1.8 μm at 100 kHz

We present a simple and efficient concept for the generation of ultrashort infrared pulses with passively stabilized carrier-envelope phase at 100 kHz repetition rate. The central wavelength is tunable between 1.6 and 2.0 µm with pulse durations between 8.2 and 12.8 fs, corresponding to a sub-two-cycle duration over the whole tuning range. Pulse energies of up to 145 nJ are achieved. As a first application we measure the high nonlinearity of multiphoton photoemission from a nanoscale metal tip.

Mid-IR femtosecond pulse generation on the microjoule level up to 5 μm at high repetition rates

We show efficient generation of mid-IR pulses tunable between 1 and 5 μm from 100 kHz class femtosecond systems. The concept can be applied to various sources, particularly based on Ti:sapphire and the newly evolving Yb+ lasers. The mid-IR pulses are generated as the idler of a collinear optical parametric amplifier pumped by the laser fundamental. The seed for this amplifier is the idler of a previous amplification stage pumped with the second harmonic and seeded with a visible continuum. This enhances the energy and allows us to influence the bandwidth of the final output. Pulses with microjoule energy and Fourier limits of 50 fs are achieved.

Non-invasive detection of iron deficiency by fluorescence measurement of erythrocyte zinc protoporphyrin in the lip

Worldwide, more individuals have iron deficiency than any other health problem. Most of those affected are unaware of their lack of iron, in part because detection of iron deficiency has required a blood sample. Here we report a non-invasive method to optically measure an established indicator of iron status, red blood cell zinc protoporphyrin, in the microcirculation of the lower lip. An optical fibre probe is used to illuminate the lip and acquire fluorescence emission spectra in ∼1 min. Dual-wavelength excitation with spectral fitting is used to distinguish the faint zinc protoporphyrin fluorescence from the much greater tissue background fluorescence, providing immediate results. In 56 women, 35 of whom were iron-deficient, the sensitivity and specificity of optical non-invasive detection of iron deficiency were 97% and 90%, respectively. This fluorescence method potentially provides a rapid, easy to use means for point-of-care screening for iron deficiency in resource-limited settings lacking laboratory infrastructure.

Dual-wavelength excitation to reduce background fluorescence for fluorescence spectroscopic quantitation of erythrocyte zinc protoporphyrin-IX and protoporphyrin-IX from whole blood and oral mucosa

Erythrocyte zinc protoporphyrin-IX (ZnPP) and protoporphyrin-IX (PPIX) accumulate in a variety of disorders that restrict or disrupt the biosynthesis of heme, including iron deficiency and various porphyrias. We describe a reagent-free spectroscopic method based on dual-wavelength excitation that can measure simultaneously both ZnPP and PPIX fluorescence from unwashed whole blood while virtually eliminating background fluorescence. We further aim to quantify ZnPP and PPIX non-invasively from the intact oral mucosa using dual-wavelength excitation to reduce the strong tissue background fluorescence while retaining the faint porphyrin fluorescence signal originating from erythrocytes. Fluorescence spectroscopic measurements were made on 35 diluted EDTA blood samples using a custom front-face fluorometer. The difference spectrum between fluorescence at 425 nm and 407 nm excitation effectively eliminated background autofluorescence while retaining the characteristic porphyrin peaks. These peaks were evaluated quantitatively and the results compared to a reference HPLC-kit method. A modified instrument using a single 1000 μm fiber for light delivery and detection was used to record fluorescence spectra from oral mucosa. For blood measurements, the ZnPP and PPIX fluorescence intensities from the difference spectra correlated well with the reference method (ZnPP: Spearman’s rho rs = 0.943, p < 0.0001; PPIX: rs = 0.959, p < 0.0001). In difference spectra from oral mucosa, background fluorescence was reduced significantly, while porphyrin signals remained observable. The dual-wavelength excitation method evaluates quantitatively the ZnPP/heme and PPIX/heme ratios from unwashed whole blood, simplifying clinical laboratory measurements. The difference technique reduces the background fluorescence from measurements on oral mucosa, allowing for future non-invasive quantitation of erythrocyte ZnPP and PPIX.

Seeding of picosecond and femtosecond optical parametric amplifiers by weak single mode continuous lasers

Optical parametric amplifiers are typically seeded with either parametric superfluorescence or broadband continuum pulses. We show both with picosecond and femtosecond pump pulses, that single longitudinal mode cw lasers with mW power can be well used to generate nearly Fourier-transform-limited output pulses. The 532 nm pumped picosecond system is seeded in the near infrared and fully tunable from 1260 to 1630 nm. The femtosecond system operates stable with just hundreds of seed photons. The output spectral width matches closely to the width of individual spectral features found in single shot spectra of parametric superfluorescence. Both systems provide interesting radiation sources for nonlinear optics experiments that need highly controlled and clean excitation.

Intraoperative assessment of laryngeal pathologies with optical coherence tomography (oct) integrated into a surgical microscope

Endoscopic examination followed by tissue biopsy is the gold standard in the evaluation of lesions of the upper aerodigestive tract. However, it can be difficult to distinguish between healthy mucosa, dysplasia, and invasive carcinoma. Optical coherence tomography (OCT) is a non‐invasive technique which acquires high‐resolution, cross‐sectional images of tissue in vivo. Integrated into a surgical microscope, it allows the intraoperative evaluation of lesions simultaneously with microscopic visualization.

Detection of cervical intraepithelial neoplasia by using optical coherence tomography in combination with microscopy

Abstract. Optical coherence tomography (OCT) is a noninvasive high-resolution imaging technique that permits the detection of cancerous and precancerous lesions of the uterine cervix. The purpose of this study was to evaluate a new system that integrates an OCT device into a microscope. OCT images were taken from loop electrosurgical excision procedure (LEEP) specimens under microscopic guidance. The images were blinded with respect to their origin within the microscopic image and analyzed independently by two investigators using initially defined criteria and later compared to the corresponding histology. Sensitivity and specificity were calculated with respect to the correct identification of high-grade squamous intraepithelial lesions (HSIL). The interinvestigator agreement was assessed by using Cohen’s kappa statistics. About 160 OCT images were obtained from 20 LEEP specimens. Sixty randomly chosen images were used to define reproducible criteria for evaluation. The assessment of the remaining 100 images showed a sensitivity of 88% (second investigator 84%) and a specificity of 69% (65%) in detecting HSIL. Surgical microscopy-guided OCT appears to be a promising technique for immediate assessment of microanatomical changes. In the gynecological setting, the combination of OCT with a colposcope may improve the detection of high-grade squamous intraepithelial lesions.

Two-photon absorption as convenient method for 20 fs pulse length measurement in the deep UV

For many applications in ultrafast spectroscopy, ultrashort ultraviolet (UV) laser pulses are needed. When generating UV pulses one has to be able to optimize and characterize the pulse compression. This can be done by e.g. fully characterizing the pulse with methods like self-diffraction FROG or ZAP-SPIDER [1]. However, in cases, where it is sufficient to measure and optimize the pulse duration, simpler methods are more desirable.