The internal losses of green and blue laser diodes are challenging to determine because of the narrow longitudinal mode spacing. Furthermore, the internal losses of state-of-the-art blue and green ...laser diodes are in the range of only a few inverse centimeter. Therefore the dynamical range given by the maxima and minima of the longitudinal mode spectrum is very large, even for moderate optical gain. Under these conditions, the usually employed, so-called Hakki-Paoli method to determine the optical gain becomes inaccurate. Now, we compare this with two other methods, the Cassidy method and an evaluation based on a Fourier transformation for a green laser diode. An error estimation as well as a correction of the systematic error caused by the spectral resolution of the setup were established. The overall highest gain was measured with the Cassidy method in the range of the lasing wavelength, as this method is least affected by the spectral resolution. In comparison of all methods, the highest gain for the wavelengths above the lasing wavelength is observed for one variation of the Fourier method, because background noise has the least influence on this method. For wavelengths below lasing wavelength we see similiar optical gain for all methods.
Introduction
Acute hyperammonemic encephalopathy is associated with distinct brain MRI findings, namely, hyperintensity in T2-weighted sequences as well as restricted diffusion in diffusion-weighted ...imaging with accentuation in the insular cortex and cingulate gyrus. The pathophysiology and the histopathological correlates of these characteristic MRI findings are largely unknown.
Case report
We present a 57-year-old male with a history of chronic alcohol abuse, liver cirrhosis and portal hypertension, and a clinical syndrome (variceal bleeding, depression of consciousness, seizures), elevated plasma ammonia levels, and characteristic brain MRI abnormalities suggestive of acute hyperammonemic encephalopathy. A postmortem histopathological examination revealed extensive hypoxic ischemic encephalopathy without evidence for metabolic encephalopathy. No episodes of prolonged cerebral hypoxemia were documented throughout the course of the disease. We conducted a review of the literature, which exhibited no reports of hyperammonemic encephalopathy in association with characteristic brain MRI findings and a consecutive histopathological examination.
Conclusion
This is the first report of a patient with acute hyperammonemic encephalopathy together with characteristic brain MRI findings and a histopathological correlation. Although characteristic MRI findings of acute hyperammonemic encephalopathy were present, a histopathological examination revealed only hypoxic pathology without signs of metabolic encephalopathy.
(Al,In)GaN laser diodes have various relevant applications, especially in projection systems for virtual and augmented reality devices and in optical communication, all requiring fast modulation. ...This corresponds to pulses in the nanosecond to microsecond range, where a rich longitudinal mode dynamics occurs. We investigate this spectral-temporal dynamics experimentally with a streak camera system and simulate it using a longitudinal multi-mode rate equation model. We observe an interplay of effects, which have been observed selectively, such as relaxation oscillations, mode competition and inhomogeneous pumping of multiple quantum wells. A mechanism is included in the simulations to model the red-shift of the gain spectrum due to the carrier density in the quantum wells exceeding threshold density, which is amplified by inhomogeneous pumping. Mode competition leads to spectral cycles of the active mode with a noticeable jitter, which is observed in single pulse measurements in comparison to multi pulse averaged measurements where blurring occurs. Here, some statistical behavior as well as repeating patterns are investigated at the same effect. Also thermal effects as laser diode self-heating are discussed and have been measured over six orders of magnitude in time.
In blue GaN‐based laser diodes with a 40 μm broad‐ridge waveguide, lateral modes of different order are selectively observed by high‐resolution spectroscopy combined with lateral near‐field scanning. ...Longitudinal mode clusters contribute to different lateral modes, close above the threshold current. Longitudinal mode competition is observed in each longitudinal mode cluster as intensity modulation and wavelength shift, repeating itself with a frequency of around 60–80 MHz. Despite the spectral separation of the different clusters and different lateral mode order, the mode competition processes share the same frequency and show a stable phase relation, especially at low currents. This indicates coupling of separate mode clusters via the charge carrier density. Due to the lateral modes occurring at separate wavelengths, lateral–longitudinal mode competition is found, which not only causes spectral effects but also time‐dependent periodic variations in the lateral near field.
Laser diodes show dynamic mode competition that causes cyclic variations in the laser spectrum in the frequency range of 50–100 MHz. In broad‐ridge laser diodes, lateral modes of different order occupy separate spectral clusters and take part in mode competition. Coupling between lateral modes is investigated in terms of frequency and phase difference of mode competition effects.
A diode‐based external cavity diode laser (ECDL) is built in Littrow configuration and its modal behavior as well as its current and temperature‐induced wavelength shift is investigated. The ECDL is ...based on a laser diode (LD) in the blue spectral range with a straight ridge and low‐reflectivity front facet. A high‐resolution grating spectrometer (SPEX 1404 0.85 m double spectrometer) as well as a scanning Fabry–Pérot Interferometer (FPI, Burleigh RC‐110 with RG‐93 ramp generator) are used for spectral characterization. With the high resolution of the FPI, an upper bound of the ECDL linewidth of about 50 fm is found, which is 75 MHz in terms of frequency. The spectral shifts with temperature and with driving current are determined. The shift of 1.15 pm mA−1 of the individual mode of the external cavity is caused by a change of the group refractive index and corresponding change of the effective cavity length. The shift of 2.7 pm mA−1 of the spectral center of gravity can be explained by the faster shift of the internal modes. The single‐mode shift with temperature of 16.71 pm K−1 is consistently measured both by the spectrometer and the FPI.
Narrow‐linewidth lasers are used for many applications for example quantum computing. A self‐built external cavity diode laser in single‐mode operation is shown and investigated with a Fabry–Pérot interferometer. With increasing current the wavelength shifts to longer wavelengths and two different slopes are visible. Both shifts correspond to a change in the refractive index and are differentiated with the FPI.
We investigate the lateral-longitudinal mode dynamics of blue broad-area laser diodes in short pulses. With a streak camera and a lateral scanning mechanism, the spectral-spatial-temporal resolved ...dynamics can be observed. A high-resolution spectrometer allows measuring multiple longitudinal mode combs and the corresponding lateral modes.
The number of elderly patients with head and neck cancer is increasing. However, there are few valid data on postoperative course after head and neck cancer surgery in elderly patients. The aim of ...this study was to evaluate the oncological outcome of elderly patients after surgical treatment for oro- and hypopharyngeal cancer.
The clinical data of 81 patients, separated into two age groups (62 < 65 years vs. 19 ≥ 65 years), were retrospectively analysed. The cohort comprised T1 and T2 oro- and hypopharyngeal cancer patients, who had undergone primary treatment with transoral laser surgery and neck dissection. Overall and disease-free survival times of the patients were analysed. Additionally, comorbidities and perioperative complications were compared between the two age groups. Median follow-up time was 5.9 years.
Comparison of different clinical and histopathological data revealed no significant differences between the age groups. The Kaplan-Meier method revealed no significant difference in disease-free survival between the age groups (p = 0.52). Age had no effect on disease-free survival in uni- or multivariate analysis (p = 0.53 vs. 0.94). Surgery-related complications were observed in 13 patients (16 %), 11 cases of which concerned the younger group of patients.
Transoral laser surgery and neck dissection can lead to satisfactory oncological and surgical outcomes in elderly patients with oro- and hypopharyngeal cancer.
In this work, it is aimed to study the interplay of different longitudinal mode combs corresponding to higher‐order lateral modes of broad‐area Fabry–Pérot‐type laser diodes. Coherent coupling of ...longitudinal modes of different mode combs influences the near‐ and far‐field pattern of the diode. Herein, commercial state‐of‐the‐art high‐power laser diodes based on the (Al,In)GaN material system in the blue spectral range with a 40 μm broad ridge are investigated. To spectrally resolve the different lateral modes, a sequential combination of a high‐resolution double grating spectrometer and a Fabry–Pérot interferometer (FPI) is used. The grating spectrometer acts as bandpass with a spectral width smaller than the free spectral range (FSR) of the FPI. The FPI with a finesse of approximately boosts the resolution to 0.08 pm. A spectral clustering of lateral modes with increasing current and also a variation of the number of distinguishable lateral modes between 7 and 11 are observed. Within the current range from 1.25 I th to 2.25 I th , there is no clear evidence for coherent coupling. It is concluded that mode‐pulling is not strong enough to coherently couple modes on the investigated time and wavelength range.
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