The use of Digital Light Processing (DLP)-based technologies has driven innovation in industries such as additive manufacturing, metrology, lithography and, increasingly, biomedical research and bioprinting. In addition to image quality parameters (magnification, line contrast, distortion), two key characteristics govern the manufacturing success: intensity on the image plane, and Full On/Full Off (FO:FO) contrast. Both need to be balanced carefully in the illumination design. We discuss detailed considerations for developing an UV DLP projector. Specifically, by choosing TIR prism design rationale, fine tuning the exact geometry, tailoring the other illumination optics and an improved coating design, we achieve an illumination that is both high-contrast and high-intensity.
Bragg-grating-based distributed-feedback waveguide resonators, with a discrete phase shift introduced inside the Bragg grating, exhibit within their grating reflection band a Lorentzian-shaped resonance line with an ultranarrow linewidth. If the phase shift is π/2, the resonance is located at the center of the reflection band, i.e., at the Bragg wavelength, where the grating reflectivity is maximum, hence the resonance linewidth is minimum. Alternatively, the required π/2 phase shift is often introduced by a distributed change in effective refractive index, e.g. by adiabatically widening the waveguide. Despite careful design and fabrication, the experimentally observed resonance wavelength deviates from the designed one. Besides deviations owing to fabrication errors, a fundamental, systematic shift towards shorter wavelengths occurs. We show theoretically and experimentally that the decay of light intensity during propagation from the phase-shift center into both sides of the Bragg grating due to (i) reflection by the periodic grating and (ii) the adiabatic refractive-index change causes an incomplete accumulation of designed phase shift by the oscillating light, thereby systematically shifting the resonance to a shorter wavelength. Calculations are performed based on the characteristic-matrix approach. Experimental studies are carried out in distributed-feedback channel-waveguide resonators in an amorphous aluminum oxide thin film on silicon with a distributed phase shift introduced by adiabatic widening of the waveguide according to a sin2 function. Calculations and experiments show good agreement. Considering in the design the overlap integral between distributed phase shift and light intensity provides a performance that is much closer to the desired value.
Distributed-feedback waveguide lasers based on Bragg-grating resonators generate ultranarrow-linewidth emission. Oscillation at the center of the reflection band ensures maximum reflectivity, hence minimum laser linewidth. The required μ/2 phase shift is often introduced by a distributed change in effective refractive index, e.g. by adiabatically widening the waveguide. Despite careful design and fabrication, the experimentally observed resonance wavelength deviates, thereby placing the resonance and laser emission at a position with lower reflectivity inside the reflection band. This effect is usually incorrectly attributed to fabrication errors. Here we show theoretically and experimentally that the decay of light intensity during propagation from the phase-shift center into both sides of the Bragg grating due to (i) reflection by the periodic grating and (ii) the adiabatic refractive-index change causes an incomplete accumulation of designed phase shift, thereby systematically shifting the resonance to a shorter wavelength. Calculations are performed based on the characteristic-matrix approach. Experimental studies are carried out in a distributed-feedback channel-waveguide resonator in amorphous Al2O3 on silicon with a distributed phase shift introduced by adiabatic widening of the waveguide according to a sin2 function. Calculations and experiments show good agreement. Considering in the design the overlap integral between distributed phase shift and light intensity provides the desired performance.
The spectral response of a distributed-feedback resonator with a thermal chirp is investigated. An Al2O3 channel waveguide with a surface Bragg grating inscribed into its SiO2 top cladding is studied. A linear temperature gradient along the resonator leads to a corresponding variation of the grating period. We characterize its spectral response with respect to wavelength and linewidth changes of the resonance peak. Simulated results show good agreement with the experimental data, indicating that the resonance wavelength is determined by the total accumulated phase shift. The calculated grating reflectivities at the resonance wavelength largely explain the observed changes of the resonance linewidth. This agreement demonstrates that the linewidth increase is caused by the increase of resonator outcoupling losses.
We thoroughly investigate the Fabry-Pérot resonator, avoid approximations, and derive its generic Airy distribution, equaling the internal resonance enhancement, and all related Airy distributions, such as the commonly known transmission. We verify that the sum of the mode profiles of all longitudinal modes is the fundamental physical function characterizing the Fabry-Pérot resonator and generating the Airy distributions. We investigate the influence of frequency-dependent mirror reflectivities on the mode profiles and the resulting Airy distributions. The mode profiles then deviate from simple Lorentzian lines and exhibit peaks that are not located at resonant frequencies. Our simple, yet accurate analysis greatly facilitates the characterization of Fabry-Pérot resonators with strongly frequency-dependent mirror reflectivities.
Distributed-feedback (DFB) laser resonators are widely recognized for their advantage of generating laser emission with extremely narrow linewidth. Our investigation concerns ytterbium-doped amorphous Al2O3 channel waveguides with a corrugated homogeneous Bragg grating inscribed into its SiO2 top cladding, in which a λ/4 phase-shift provides a resonance and allows for laser emission with a linewidth as narrow as a few kHz. Pump absorption imposes a thermal chirp of the grating period, which has implications for the spectral characteristics of the resonator. Thermal effects on the spectral response of a DFB passive resonator were investigated via simulations using Coupled Mode Theory by considering (i) a constant deviation of the grating period or (ii) a chirp with a linear profile. We report an increase of the resonance linewidth up to 15%. This result is due to two factors, namely changes of the grating reflectivity at the resonance frequency up to 2.4% and of the shift of resonance frequency up to 61 pm due to an accumulated phase shift imposed on the grating by the chirp profile. The linewidth decrease due to gain is on the order of 106, which is a much larger value. Nevertheless, according to the Schawlow-Townes equation the linewidth increase of the passive resonator due to a thermal chirp quadratically increases the laser linewidth.
In this work we demonstrate for the first time, to the best of our knowledge, quasi-continuous wave (qcw) laser operation of a diode-side-pumped Nd:YVO4 self-Raman laser operating at 1176 nm. The double beam mode controlling (DBMC) technique used in this work allows fundamental mode laser oscillation, resulting in a beam quality M2 of 2.42 and 2.18 in the horizontal and vertical directions, respectively. More than 3.5 W of peak output power at 1176 nm was achieved with TEM00 laser mode, corresponding to an optical conversion efficiency of 5.4%.With multimode operation, more than 8W of peak output power was achieved, corresponding to 11.7% optical conversion efficiency.
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