Henning Engelbrecht Larsen

Catastrophic optical mirror damage in diode lasers monitored during single-pulse operation

Catastrophic optical mirror damage (COMD) is analyzed for 808 nm emitting diode lasers in single-pulse operation in order to separate facet degradation from subsequent degradation processes. During each pulse, nearfield and thermal images are monitored. A temporal resolution better than 7 μs is achieved. The thermal runaway process is unambiguously related to the occurrence of a “thermal flash.” A one-by-one correlation between nearfield, thermal flash, thermal runaway, and structural damage is observed. The single-pulse excitation technique allows for controlling the propagation of the structural damage into the cavity. We propose this technique for the analysis of early stages of COMD.

Physical limits of semiconductor laser operation: A time-resolved analysis of catastrophic optical damage

The early stages of catastrophic optical damage (COD) in 808 nm emitting diode lasers are mapped by simultaneously monitoring the optical emission with a 1 ns time resolution and deriving the device temperature from thermal images. COD occurs in highly localized damage regions on a 30 to 400 ns time scale which is determined by the accumulation of excess energy absorbed from the optical output. We identify regimes in which COD is avoided by the proper choice of operation parameters.

Field programmable gate-array-based real-time optical Doppler tomography system for in vivo imaging of cardiac dynamics in the chick embryo

We demonstrate a field programmable gate- array-based real-time optical Doppler tomography system. A complex-valued bandpass filter is used for the first time in optical coherence tomography signal processing to create the analytic signal. This method simplifies the filter design, and allows efficient and compact implementation by combin- ing the conversion to an analytic signal with a pulse shaping function without the need for extra resources as compared to the Hilbert transform method. The conversion of the analytic signal to amplitude and phase is done by use of the coordi- nate rotation digital computer CORDIC algorithm, which is an efficient algorithm that maps well to the field program- mable gate array. Flow phantom experiments, and the use of this system for in vivo imaging of cardiac dynamics in the chick embryo, are presented. We demonstrate the visualiza- tion of blood flow in the early embryonic heart as well as in the aorta, small peripheric vitelline vessels, and coronary ar- teries of fully formed chick hearts.

Optical Doppler tomography based on a field programmable gate array

Abstract We report the design of and results obtained by using a field programmable gate array (FPGA) to digitally process optical Doppler tomography signals. The processor fits into the analog signal path in an existing optical coherence tomography setup. We demonstrate both Doppler frequency and envelope extraction using the Hilbert transform, all in a single FPGA. An FPGA implementation has certain advantages over general purpose digital signal processor (DSP) due to the fact that the processing elements operate in parallel as opposed to the DSP, which is primarily a sequential processor.

Optical Doppler coherence tomography based on a field-programmable gate array

We report the design of and results obtained by using a Field Programmable Gate Array (FPGA) to digitally process Optical Doppler Tomography signals. The processor fits into the analog signal path in an existing OCT setup. We demonstrate both Doppler frequency and envelope extraction using the Hilbert transform, all in a single FPGA. An FPGA implementation has certain advantages over a general purpose Digital Signal Processor (DSP) due to the fact that the processing elements operate in parallel as opposed to the DSP, which is primarily a sequential processor. In this paper, we demonstrate that the use of a FPGA enables sampling rates exceeding DSP-based solutions. In addition, this implementation has the important feature that calculation of the phase in addition to the amplitude of the interference fringe pattern only requires few additional resources. The proposed implementation of Doppler frequency extraction in a single FPGA is feasible for real-time Doppler OCT applications requiring high signal sampling rates.

Sensing roughness and polish direction

As a part of the work carried out on a project supported by the Danish council for technology and innovation, we have investigated the option of smoothing standard CNC machined surfaces. In the process of constructing optical prototypes, involving custom-designed optics, the development cost and time consumption can become relatively large numbers in a research budget. Machining the optical surfaces directly is expensive and time consuming. Alternatively, a more standardized and cheaper machining method can be used, but then the object needs to be manually polished. During the polishing process the operator needs information about the RMS-value of the surface roughness and the current direction of the scratches introduces by the polishing process. The RMS-value indicates to the operator how far he is from the final finish, and the scratch orientation is often specified by the customer in order to avoid complications during the casting process. In this work we present a method for measuring the RMS-values of the surface roughness while simultaneously determining the polishing direction. We are mainly interested in the RMS-values in the range from 0 – 100 nm, which corresponds to the finish categories of A1, A2 and A3. Based on simple intensity measurements we estimates the RMS-value of the surface roughness, and by using a sectioned annual photo-detector to collect the scattered light we can determine the direction of polishing and distinguish light scattered from random structures and light scattered from scratches.

Compact optical system for measuring linear and angular displacement of solid structures

We present a compact, low-cost optical method for detection of in-plane speckle translation, which e.g. could be a measure of in-plane translation or rotation of a solid structure. The speckles are produced by illuminating a non-specular target surface with coherent light. The scattered light propagates through free-space to the sensor inlet. The sensor is based on a lenticular array, which implements a narrow spatial band-pass filter, acting on the translating speckle patterns. The sensor detects speckle translation, which for the given configuration can be caused to detect both translation and rotation of the target. The presented free space propagation design can provide a sensor with no direct sensitivity on the working distance. The electrical signals from the sensor are processed with a digital algorithm, based on zero-crossings detection to provide real-time displacement measurements. The spatial filter of the sensor is characterized here, and the precision of the sensor, integrated with a processor, which applies zero-crossing detection to the signal, is considered.