My research concentrates of the creation an application of short laser pulses, as well as high tech applications of computer vision and computational photography. Currently I am working on femtosecond transient imaging.
Femtosecond Transient Imaging: A camera that looks around corners.
A camera with a short flash and a very fast response time is aware of the time of flight of light through a scene. This time of flight information can be used to reveal information that is hidden to other imaging devices. We are biulding such an ultra-fast transient imaging camera and developing new algorithms to process and understand its images. With our combination of ultra-fast hardware and advanced signal processing we can use scattered light to reconstrict images of an object that we can not see directly, effectively "looking around the corner". We also have developed methods to obtain reflectance properties (BRDFs) of materials in a single measurement. This project has been featured in numerous media outlets including the BBC and the Economist. Please see here for details.
Light in Motion Visualizations
A streak camera is modified to provide 1 nanosecond long movies of repetitive events with 672 by 1000 pixel resolution. We use this camera to show the motion and scattering of a light pulse through a scene, to analyze scattering and to visualize and unterstand ultra-fast phenomena. Some smaple videos are shown in this talk. Please follow this link for more information.
BRDF Detection for Transient Imaging: Measuring reflectance properties from a single position.
This work introduces the concept of time-of-flight reflectance estimation, and
demonstrates a new technique that allows a camera to rapidly acquire
reflectance properties of objects from a single viewpoint, over relatively long
distances and without encircling equipment. We measure material properties by
indirectly illuminating an object by a laser source, and observing its
reflected light indirectly using a time-of-flight camera.
Synchronously pumped optical parametric oscillator as intracavity interferometer
The frequency comb of mode-locked pulses is fixed to their laser cavity. If two pulses are resonant in the same cavity and a phase shift is applied to one of them (e. g. with an electro-optic modulator) the pulses have different carrier frequencies. The difference in frequency, and thus the phase shift can be measured as a beat note between the two pulses. The approach is insensitive to intensity fluctuations and to mechanical noise. We have shown beat notes down to 100 Hz and beat note stabilities below 1 Hz corresponding to a phase shift of less than 10-7 radians or a length change of less than 10 femtometers - little more than one billionth of the wavelength of the light used. This phase measurement, just as in an interferometer, can be used to measure many different quantities. The OPO built by me is the first stable OPO device reaching (and exceeding) this accuracy. The OPO cavity in this setup is twice as long as the cavity of the pump laser, creating the required two pulses in the OPO. This device was used to measure the nonlinear index of refraction of LiNbO3 with an accuracy outperforming the established z-scan method. My work on the project includes the initial design idea for this specific device, theoretical modeling, as well as the major contribution to the construction of the device and the measurement and evaluation of the data.
Simulation of intracavity pumped optical parametric oscillators
In a synchronously intracavity pumped Optical Parametric Oscillator (OPO) the OPO gain crystal is placed inside the pump laser cavity. This setup promises very high efficiency and a low threshold, but also leads to amplitude instabilities such as self Q-switching. The purpose of this project was to better understand the complex dynamics of such a device in order to avoid instabilities. The simulation is written in FORTRAN. It employs a rate equation approach to simulate saturable gain and loss in the pump laser. The interaction in the OPO gain crystal is modeled in the frequency domain to include dispersive effects and is applicable to pulse durations down to a few optical cycles. The results from this simulation indicate that the instabilities are caused by the time delayed feedback of the OPO cavity to the pump. This feedback amplifies transient oscillations of the pump laser. Possible solutions are the use of semiconductor pump sources and stabilization by nonlinear loss in the OPO or pump laser. This project is entirely my own work.
Intracavity pumped Ti:Sapphire OPO
While an intracavity pumped Ti:Sapphire OPO has been realized before, it was highly unstable and is of limited use for measurement applications. My simulation of intracavity pumped OPOs provides strategies to stabilize the device. I present a stable intracavity pumped OPO using these strategies. It also employs a new way of avoiding back reflections from the facets of the OPO gain crystal. Since these reflections tend to destabilize the pump laser, previous devices used Brewster cut OPO gain crystals making the alignment very challenging. If however a long (~1 cm) anti reflection coated crystal is used, back reflections into the cavity mode are minimized. While an intracavity pumped OPO with equal length for pump and OPO cavity generally produces two OPO pulses, the intensity of those two pulses is not equal. This causes an intensity dependent bias beat note, which is very disadvantageous. This device therefore uses a length ratio of 2/3 between the pump and OPO cavities to create pairs of equal pulses in the OPO cavity. All work on this project, including the idea for the design novelties, the design and the construction was done by me.
Mode-locked 790 nm VECSEL for intracavity pumping of an OPO
A Semiconductor Vertical Cavity Surface Emitting Laser (VECSEL) is designed to intracavity pump an OPO. Our simulations indicate that the short gain lifetime of the VECSEL will make stable intracavity operation possible. Accessing the high intracavity intensity will enable an OPO dramatically more efficient and less expensive that what is available today. The VECSELs under construction for this purpose include a conventional design with a distributed Bragg reflector (DBR) mirror as part of the VECSEL structure, as well as a new approach using the VECSEL active region in transmission bonded to a heat spreader. My work includes contributions to the design of the structures, the cutting, etching and bonding and the selection of the heat spreader.
Tapered semiconductor amplifier ring laser
Construction of a mode locked ring laser using a tapered semiconductor amplifier as gain medium. This device generates sub picosecond pulses with a pulse energy of up to 0.5 nJ which is unusually high for a semiconductor laser. It is tunable over 20 nm and provides a good beam quality. Since it is very insensitive to losses it is well suited for intracavity experiments such as the investigantion of autostabilization with rubidium vapor. My work includes contributions to alignment and characterization of the device as well as optimization of the mode-locking process.
Simulation of dark line resonances in rubidium vapor for laser autostabilization
We model dark line resonances in rubidium using a 4-level density matrix. The purpose is to study the behavior of such a system inside a laser cavity and to evaluate its potential for passive repetition rate stabilization. The simulation code is written in FORTRAN and intended to be used with the laser simulation program written for the intracavity pumped OPO (see above). My work includes adapting the existing rubidium simulation code to work with my laser simulation. I also present a greatly simplified model to simulate the effect of the rubidium. This model has shown repetition rate stabilization.
The PUlsed Laser Simulation Tool for Education and Research provides ways to model, simulate and analyze mode-locked laser cavities. Pulster is a multi-threaded application written in C++ and FORTRAN and provides a graphical user interface using the Qt and Qwt libraries. It contains over 16000 lines of my own code. The simulation routines are the ones developed in the projects mentioned above. The program can load user written binary plug in files to simulate additional cavity elements. It includes an algebra interpreter that allows to formulate conditions and evaluate data during the simulation. It also allows to observe and adjust all parameters and variables while the simulation is in progress, enabling the user to ''experiment'' with the setup while it is running. Data from the simulation can also be analyzed right inside the application with octave (using Matlab syntax and providing most of Matlabs functionality). It is intended to be used as a design tool as well as an educational program. Pulster is developed by me under Linux and is in an advanced beta status. Because of license issues the Linux binaries cannot be made publicly available at this point. Demonstrations, as well as single personalized copies are however possible.
A numerical tool for ray tracing and propagation of Gaussian beams inside and outside of optical cavities. It can be used to design laser cavities as well as simple imaging systems. The program is written in C++ using the wxWidgets library for the graphical interface. It provides a fast and intuitive way to design a laser cavity and plot the beam profile. Any cavity element that can be simulated by an ABCD matrix can easily be included in the functionality of the program. Some investigations are also possible for dynamic and nonlinear effects like Kerr lensing and thermal lensing. A beta version with limited functionality is available for Linux and Windows. The project is entirely my own.
Cr:LiSAF Laser for intracavity pumping of OPOs
Cr:LiSAF lasers have been of large interest because of their ability to produce femtosecond pulses from a diode pumped setup. A diode pumped Cr:LiSAF laser is designed to replace the Ti:Sapphire laser as an OPO pump. Once computer simulations pointed out the inherent problems with crystalline host lasers as OPO pump lasers (see above) the project was abandoned in favor of semiconductor lasers. My work includes contributions to alignment and mode-locking of the laser.