Paul Michael Petersen

Power Scaling of Nonlinear Frequency Converted Tapered Diode Lasers for Biophotonics

Diode lasers have proven to be versatile light sources for a wide range of applications. Nonlinear frequency conversion of high brightness diode lasers has recently resulted in visible light power levels in the watts range enabling an increasing number of applications within biophotonics. This review provides an overview of the developments within nonlinear frequency converted high power laser diodes in the visible spectral range. Single-pass nonlinear frequency doubling is presented as a nonsophisticated method to achieve watt-level output powers and possible routes to higher power and efficiency are included. Application examples within pumping of mode-locked Ti:sapphire lasers and implementation of such lasers in optical coherence tomography are presented showing the application potential of these lasers.

Widely Tunable High-Power Tapered Diode Laser at 1060 nm

We report a large tuning range from 1018 to 1093 nm from a InGaAs single quantum-well 1060-nm external cavity tapered diode laser. More than 2.5-W output power has been achieved. The tuning range is to our knowledge the widest obtained from a high-power InGaAs single quantum-well tapered laser operating around 1060 nm. The light emitted by the laser has a nearly diffraction limited beam quality and a narrow linewidth of less than 6 pm everywhere in the tuning range.

Nonlinear gain amplification due to two-wave mixing in a broad-area semiconductor amplifier with moving gratings

Two-wave mixing in a broad-area semiconductor amplifier with moving gratings is investigated. It is shown that depending on direction of the moving gratings and the anti-guiding parameter the optical gain may increase or decrease.

A novel diode laser system for photodynamic therapy

In this paper a novel diode laser system for photodynamic therapy is demonstrated. The system is based on linear spatial filtering and optical phase conjugate feedback from a photorefractive BaTiO3 crystal. The spatial coherence properties of the diode laser are significantly improved. The system provides an almost diffraction limited output which is efficiently coupled into a 50 micrometers core diameter fiber. The optical power transmitted through the fiber is increased by a factor of six when the feedback is applied to the diode laser. 85 percent of the power from the freely running laser diode is extracted in a high-quality beam and 80 percent of the output power is extracted through the fiber. The power transmitted through the fiber scales linearly with the power of the laser diode, which means that a laser diode emitting 1.7 W multi-mode radiation would provide 1 W of optical power through a 50 micrometers core diameter fiber. The system is compact, portable, stable, and easy to operate.

Local diffuse reflectance from three-layered skin tissue structures

We consider two different skin structure models. The first structure consists of epidermis, dermis/blood, and subcutaneous tissue. The second structure consists of epidermis/dermis, adipose tissue and muscle tissue. A new solution based on diffusion theory of the cw local diffuse reflectance from a three-layered skin tissue structure is presented. Comparisons with Monte Carlo simulations are carried out favorably. It is shown that the functional form of the radial dependence of the diffuse reflectance from multilayer and single layer models are identical. We use a modified expression originating from diffusion theory to fit the diffuse reflectance. We discuss the sensitivity of the local diffuse reflectance as a function of the optical properties of separate layers in both structures. Moreover, we investigate the influence on the local diffuse reflectance with changes in the optical properties corresponding to normal changes in tissue glucose concentration and blood volume. The necessity of multilayer models lies within their ability to provide a detailed description of the light-tissue interaction rate than their applicability to practical data analysis of the local diffuse reflectance measurements.

Nonlinear self-refraction of Gaussian laser beams in silica sono-gels doped with copper tetrasulfonated phthalocyanine

In this paper we present a new theory for nonlinear self- refraction of Gaussian laser beams in Kerr-like materials and its application to determine the third order nonlinear susceptibility in silica sono-gels doped with copper tetrasulfonated phthalocyanine. In the theory we assume that the incident Gaussian beam induces a phase shift that varies as a Gaussian function of the beam radius in the sample. The profile of the intensity in the sample is furthermore assumed to remain Gaussian. The beam propagation after the sample is determined by using the Huygens-Fresnel integral formula. By solving this complicated Huygens-Fresnel integral we obtain analytical expressions for the spatial beam profile in the near-field and in the far-field after the nonlinear sample.

Degradation processes in high-power diode lasers under external optical feedback

The effect of moderate external feedback on the gradual degradation of 808 nm emitting AlGaAs-based high-power broad-area diode lasers is analyzed. Eventually the quantum well that actually experiences the highest total optical load remains unaffected by the aging, while severe impact to the waveguide by point defects is observed.

Red Tunable High-Power Narrow-Spectrum External-Cavity Diode Laser Based on Tapered Amplifier

Diffraction-limited high-power narrow-spectrum red diode lasers are attractive for many applications, such as photodynamic therapy, laser display, and as a pump source to generate UV light by second harmonic generation (SHG). High-power, diffraction-limited diode lasers can be realized by the technology of lasers with a tapered gain-region (Kintzer et al., 1993; Donnelly et al., 1998; Wenzel et al., 2003; Paschke et al., 2005; Sumpf et al., 2009; Fiebig et al., 2010). The tapered laser devices can be used in applications where narrowspectrum is not needed such as photodynamic therapy, but for other applications such as a pump source for UV light generation, the spectral quality of these devices has to be improved. In order to improve the spectral quality of a tapered laser, different techniques are applied, such as a monolithically integrated master oscillator power amplifier by forming Bragg gratings in the semiconductor material (O’Brien et al., 1993, 1997a), injection locking to an external single-mode laser (Goldberg et al., 1993; Mehuys et al., 1993b; O’Brien et al., 1997b; Wilson et al., 1998; Ferrari et al., 1999; Spieβberger et al., 2011), and different external-cavity feedback techniques (Jones et al., 1995; Cornwell & Thomas, 1997; Morgott et al., 1998; Goyal et al., 1998; Pedersen & Hansen, 2005; Chi et al., 2005; Lucas-Leclin et al., 2008; Tien et al., 2008; Sakai et al., 2009). Up to 1 W output power at 668 nm from a Fabry-Perot tapered diode laser was obtained with a beam quality factor of 1.7, and the spectral width was smaller than 0.2 nm (Sumpf et al., 2007). Around 670 nm, tunable narrow-linewidth diffraction-limited output was also achieved from an injection-locking tapered diode laser system seeded with a single-mode external-cavity diode laser (Haring et al., 2007); the output power was up to 970 mW. A 670 nm micro-external-cavity tapered diode laser system was demonstrated with a reflecting volume Bragg grating as a feedback element; in continuous wave (CW) mode, more than 0.5 W output power was obtained, and in pulse mode, 5 W peak power was obtained with a beam quality factor of 10 and a spectral width below 150 pm (Tien et al., 2008). Up to 1.2 W output power at 675 nm from a tapered laser

Phase locking of laser diode arrays using a photorefractive Rh:BaTiO-3 crystal

We have developed an extremely stable phase conjugate single-mode laser diode array with unique spatial and temporal coherence properties. The laser is based on a novel concept of frequency selective phase conjugate feedback. A laser diode array is coupled to the phase conjugate feedback system that comprise a rhodium doped BaTiO3 crystal, a Fabry-Perot etalon, and a spatial filter. This feedback system forces the laser diode array to operate in a single spatial and a single longitudinal mode. In comparison with the free running laser diode array, the coherence length is increased significantly and the output is close to the diffraction limit. More than 80% of the total energy provided by the free running laser diode array can be extracted from this single-mode laser system.

Multiple grating interactions in photorefractive optical interconnects

0 gating pump and probe was 49 cm- at 9 = 159 where 0 is the angle between the grating vector and the c axis. Using the BaTi03 crystal, the phase-conjugate rise time of 35 ms and the steady-state reflectivity of 50% were obtained for 10 mW input power. Finally, the phase-conjugate mirror with BaTiO, was used to compensate for the fluctuations of laser beams caused by air turbulence. A 400-m folded optical path was constructed using the phaseconjugate mirror. The positional fluctuations of laser beams were reduced by phase conjugation as compared to the case of a conventional corner-cube reflector for frequencies up to 10 Hz. We believe the reduction of the fluctuations of laser beams will be useful for metrology in the use of long optical paths of hundreds of meters.