BIOP Research Program
New laser systems for diagnostic and therapeutic applications
Development of state-of-the-art laser systems is of utmost importance for deriving new clinical procedures, diagnostics and therapy, and for biochemical quantification. The overall goal of this focus area is therefore to research and develop new lasers and systems with improved characteristics in terms of beam quality, wavelength, tunability of laser wavelength, output power, noise and coherence properties.
The figure below shows a novel laser system, invented at Risų National Laboratory, with high output power and excellent temporal and spatial coherence properties. The system consists of a broad area semiconductor diode laser. The entire broad area laser is phase-locked using a novel external feedback loop. By using this feedback system, the entire broad area gain medium is brought to oscillate in the same mode converting the multi-mode output into single-mode output with conversion efficiencies exceeding 80% of the multi-mode power. The output is readily coupled through single-mode fibers. Such high-power lasers have numerous applications in laser therapy.
Optimized broad area laser diode system.
Today, treatment of various skin diseases, such as photodynamic therapy (PDT) for treating skin cancer, is mainly carried out by the use of lasers. For the PDT procedure, a contrast agent is needed, which accumulates in the malignant tissue, i.e. the tumors. The molecules of the contrast agent are excited to a higher energy level using lasers, and this process leads to the formation of singlet oxygen. In turn, this leads to a highly selective destruction (necrosis) of the tumor cells. The method is thus highly selective, since the contrast agent is not accumulated in healthy cells outside the tumor(s). The lasers for excitation are high-power lasers, several Watts (cw), in the 630-750 nm wavelength range.
In BIOP, the above-mentioned system is currently being optimized for use in photodynamic therapy (PDT). The aim is to develop a high-power laser diode array emitting at 635 nm having single-mode output.
At the Technical University of Denmark, a new kind of light guiding has been investigated and it has resulted in the development of photonic bandgap fibers. These fibers do not allow transmission of light in certain frequency bands. Furthermore, they possess the intriguing property that the main part of the light propagating in the fiber is confined in the airholes. It is therefore expected that this new type of light guiding may prove superior compared to conventional fibers for optical sensor applications. The unique properties of these photonic bandgap fibers may also be used to form wave guides with ultra small effective cross sections, which facilitates fibers with extremely high optical nonlinearities and novel dispersion characteristics compared to conventional fibers. The latter may be used to broaden the optical spectra through self-phase modulation and Raman effects, which is readily applicable for light sources in, for example, optical coherence tomography systems.
Cross-sectional image of the intensity pattern of a photonic bandgap fiber (simulated data).
The selection of laser frequencies from natural or synthetic crystals is limited, and only a few laser materials perform reliably and efficiently enough to form the basis for commercial systems. Optical parametric oscillation is a method based on nonlinear optics that allow the generation of arbitrary combinations of frequencies in the same manner as electronic frequency synthesizers can create free selection of electronic frequencies.
In the figure below, we show one of our on-going project projects at Department of Physics, DTU; a tunable source of high quality coherent light in the mid-infrared. The nonlinear process is the so-called optical parametric oscillation (OPO). The nonlinear crystal, a periodically poled lithium niobate or KTP-crystal, is placed inside a laser cavity. The laser acts as a pump of high quality coherent light at 1064 nm, which interacts with a resonant optical field in the near infrared (1.2 - 5 micrometers). The exact oscillation frequency is determined by the crystal temperature and orientation, and the OPO output frequency can be tuned by small corrections to these crystal operating parameters.
The quality of the output light and the efficiency of the frequency conversion are determined by the quality of the circulating laser beam. In our case we use a so-called diode-pumped, unidirectional ring laser, which operates at a single frequency with a circulating cw-power of about 100 W. The OPO-resonator may be either a linear cavity or another ring cavity coupled to the laser through dielectric beam splitters.
In other experiments we pump the OPO-resonator with frequency doubled light at 532 nm to obtained tunable light in the visible, and in future projects we would like to go even higher into the near UV-frequency range.
Bow-tie laser coupled to resonant SHG.
Our current activities are concentrated on:
- Novel high-power broad area diode laser systems with improved coherence properties, especially optimized for PDT treatment,
- Photonic bandgap fibers,
- Optical parametric oscillators for generation of new, high quality laser frequencies.