High color rendering and high-luminance laser lighting using all inorganic nitride phosphor films.

Despite the huge advances that have been made in the development of ultra-high luminance laser lighting, achieving high color rendering properties in such systems at the same time remains a challenge. Recent studies show that in most cases, the luminous efficacy (LE) of laser lighting is compromised to improve the color rendering index (CRI). In this study, a possible solution to this problem has been proposed by preparing phosphor-in-glass (PiG) films comprised of the yellow-emitting phosphor (LSN:Ce3+) and the red-emitting phosphor (CASN:Eu2+). The composite material synthesized in this study exhibited outstanding optical and thermal properties. A uniform white light with a high CRI of 80.0 and a high LE of 185.9 lm W-1 was achieved by optimizing the yellow/red ratio and the emission peak position of the blue laser. Furthermore, it was found that this design enabled the phosphor to restrict the light emission area effectively, thus attaining a high luminous exitance of 1302 lm mm-2. With their superior optical performance, the PiG films can be regarded as promising color converter candidates for future high-quality laser-based white light sources.

Influence of pump beam shaping and noise on performance of a direct diode-pumped ultrafast Ti:sapphire laser.

One of the major factors that limits the widespread use of ultrafast titanium sapphire (Ti:S) lasers in life science is its expensive and complex pump source. Broad area diode laser (BA-DL) based pump sources have high potential to solve this problem, since they are compact, inexpensive and very efficient. However, their non-diffraction limited beam profile makes it challenging to achieve high powers in Kerr-lens mode-locked (KLM) operation of Ti:S lasers. In this work, we show that the ideal way to beam shape two spectrally combined BA-DLs with different beam qualities is to aim for a compromise between matching to the cavity mode diameter and the cavity mode Rayleigh range. We furthermore conclude that the relative intensity noise (RIN) of the BA-DL pumped Ti:S laser, another important parameter for imaging applications, is sufficiently low for a wide range of life science applications. However, for applications that are highly sensitive to noise, new laser diode designs are likely necessary to reduce inherent noise originating within the laser diode.

Phosphor material dependent spot size limitations in laser lighting.

In laser lighting, a major benefit over other lighting techniques is the possibility to achieve very high luminous exitance. Focusing the exciting laser to a very small spot size on the phosphor, however, does not necessarily provide a very small emitting area for the white light. In this study we investigate experimentally and numerically the relationship between the white light spot size and the incident blue laser spot size. We show that the specific phosphor material properties have significant impact on this relationship and on the achievable minimum spot size. This constitutes a limitation on the minimum spot size achievable in laser lighting and has important implications in applications.

Investigation of Saturation Effects in Ceramic Phosphors for Laser Lighting.

We report observations of saturation effects in a Ce:LuAG and Eu-doped nitride ceramic phosphor for conversion of blue laser light for white light generation. The luminous flux from the phosphors material increases linearly with the input power until saturation effects limit the conversion. It is shown that the temperature of the phosphor layer influences the saturation power level and the conversion efficiency. It is also shown that the correlated color temperature (CCT), phosphor conversion efficiency and color rendering index (CRI) are dependent both on the incident power and spot size diameter of the illumination. A phosphor conversion efficiency up to 140.8 lm/W with CRI of 89.4 was achieved. The saturation in a ceramic phosphor, when illuminated by high intensity laser diodes, is estimated to play the main role in limiting the available luminance from laser-based lighting systems.

Green high-power tunable external-cavity GaN diode laser at 515 nm.

A 480 mW green tunable diode laser system is demonstrated for the first time to our knowledge. The laser system is based on a GaN broad-area diode laser and Littrow external-cavity feedback. The green laser system is operated in two modes by switching the polarization direction of the laser beam incident on the grating. When the laser beam is p-polarized, an output power of 50 mW with a tunable range of 9.2 nm is achieved. When the laser beam is s-polarized, an output power of 480 mW with a tunable range of 2.1 nm is obtained. This constitutes the highest output power from a tunable green diode laser system.

Tuning range and output power optimization of an external-cavity GaN diode laser at 455 nm.

In this paper we discuss how different feedback gratings affect the tuning range and the output power of external feedback diode laser systems. A tunable high-power narrow-spectrum external-cavity diode laser system around 455 nm is investigated. The laser system is based on a high-power GaN diode laser in a Littrow external-cavity. Both a holographic diffraction grating and a ruled diffraction grating are used as feedback elements in the external cavity. The output power, spectral bandwidth, and tunable range of the external cavity diode laser system are measured and compared with the two gratings at different injected currents. When the holographic grating is used, the laser system can be tuned over a range of 1.4 nm with an output power around 530 mW. When the ruled grating is used, the laser system can be tuned over a range of 6.0 nm with an output power around 80 mW. The results can be used as a guide for selecting gratings for external-cavity diode lasers for different requirements.

Generation of single-frequency tunable green light in a coupled ring tapered diode laser cavity.

We report the realization of a tapered diode laser operated in a coupled ring cavity that significantly improves the coherence properties of the tapered laser and efficiently generates tunable light at the second harmonic frequency. The tapered diode laser is tunable with single-frequency output in the broad wavelength range from 1049 nm to 1093 nm and the beam propagation factor is improved from M(2) = 2.8 to below 1.1. The laser frequency is automatically locked to the cavity resonance frequency using optical feedback. Furthermore, we show that this adaptive external cavity approach leads to efficient frequency doubling. More than 500 mW green output power is obtained by placing a periodically poled LiNbO(3) crystal in the external cavity. The single frequency green output from the laser system is tunable in the 530 nm to 533 nm range limited by the LiNbO(3) crystal. The optical to optical conversion efficiency exceeds 30%.

The still unknown azobenzene - wavelength dependent photoanisotropy in amorphous azobenzene polymers.

We demonstrate a new type of anisotropy in thin films of amorphous azobenzene polymers induced between 570 and 633 nm, where the absorbance in the film is on the order of 0.05. The anisotropy has a pronounced radial contribution. This observation points to an additional mechanism for the alignment of azobenzene molecules.

Efficient concept for generation of diffraction-limited green light by sum-frequency generation of spectrally combined tapered diode lasers.

In order to increase the power of visible diode laser systems in an efficient manner, we propose spectral beam combining with subsequent sum-frequency generation. We show that this approach, in comparison with second harmonic generation of single emitters, can enhance the available power significantly. By combining two distributed Bragg reflector tapered diode lasers we achieve a 2.5-3.2 fold increase in power and a maximum of 3.9 W of diffraction-limited green light. At this power level, green diode laser systems have a high application potential, e.g., within the biomedical field. Our concept can be expanded combining multiple diode lasers to increase the power even further.

Frequency-doubled DBR-tapered diode laser for direct pumping of Ti:sapphire lasers generating sub-20 fs pulses.

For the first time a single-pass frequency doubled DBR-tapered diode laser suitable for pumping Ti:sapphire lasers generating ultrashort pulses is demonstrated. The maximum output powers achieved when pumping the Ti:sapphire laser are 110 mW (CW) and 82 mW (mode-locked) respectively at 1.2 W of pump power. This corresponds to a reduction in optical conversion efficiencies to 75% of the values achieved with a commercial diode pumped solid-state laser. However, the superior electro-optical efficiency of the diode laser improves the overall efficiency of the Ti:sapphire laser by a factor > 2. The optical spectrum emitted by the Ti:sapphire laser when pumped with our diode laser shows a spectral width of 112 nm (FWHM). Based on autocorrelation measurements, pulse widths of less than 20 fs can therefore be expected.

High-power dual-wavelength external-cavity diode laser based on tapered amplifier with tunable terahertz frequency difference.

Tunable dual-wavelength operation of a diode laser system based on a tapered diode amplifier with double-Littrow external-cavity feedback is demonstrated around 800 nm. The two wavelengths can be tuned individually, and the frequency difference of the two wavelengths is tunable from 0.5 to 5.0 THz. An output power of 1.54 W is achieved with a frequency difference of 0.86 THz, the output power is higher than 1.3 W in the 5.0 THz range of frequency difference, and the amplified spontaneous emission intensity is more than 20 dB suppressed in the range of frequency difference. To our knowledge, this is the highest output power from a dual-wavelength diode laser system operating with tunable terahertz frequency difference.

Experimental investigation of relative timing jitter in passively synchronized Q-switched lasers.

Relative timing jitter between synchronized Q-switched lasers, or lack thereof, is important for stable sum-frequency generation. Experimental investigation of two passively synchronized lasers shows that the jitter is minimized when the free-running repetition rates of the two lasers are close to, but not exactly, matching. When the free-running repetition rates are matched, the jitter is significantly large. At the best operating point, the pulse-to-pulse period was 200 µs, while the relative jitter between the two lasers was 9 ns. If the effect of the master laser's pulse-to-pulse jitter is removed, the residual timing jitter between the two lasers was 6 ns, which corresponds to the lower limit set by pump power fluctuations and noise from spontaneous emission.

Narrow line width operation of a 980 nm gain guided tapered diode laser bar.

We demonstrate two different schemes for the spectral narrowing of a 12 emitter 980 nm gain guided tapered diode laser bar. In the first scheme, a reflective grating has been used in a Littman Metcalf configuration and the wavelength of the laser emission could be narrowed down from more than 5.5 nm in the free running mode to 0.04 nm (FWHM) at an operating current of 30 A with an output power of 8 W. The spectrum was found to be tunable within a range of 16 nm. In the second scheme, a volume Bragg grating has been used to narrow the wavelength of the laser bar from over 5 nm to less than 0.2 nm with an output of 5 W at 20 A. To our knowledge, this is the first time spectral narrowing has been performed on a gain guided tapered diode laser bar. In the Littman Metcalf configuration, the spectral brightness has been increased by 86 times and in the volume Bragg grating cavity the spectral brightness has been improved over 18 times when compared to the free running operation. These schemes could be also extended for other wavelengths of interest in the future.

16 W output power by high-efficient spectral beam combining of DBR-tapered diode lasers.

Up to 16 W output power has been obtained using spectral beam combining of two 1063 nm DBR-tapered diode lasers. Using a reflecting volume Bragg grating, a combining efficiency as high as 93.7% is achieved, resulting in a single beam with high spatial coherence. The result represents the highest output power achieved by spectral beam combining of two single element tapered diode lasers. Since spectral beam combining does not affect beam propagation parameters, M2-values of 1.8 (fast axis) and 3.3 (slow axis) match the M2-values of the laser with lowest spatial coherence. The principle of spectral beam combining used in our experiments can be expanded to combine more than two tapered diode lasers and hence it is expected that the output power may be increased even further in the future.

Tunable high-power narrow-spectrum external-cavity diode laser at 675 nm as a pump source for UV generation.

High-power narrow-spectrum diode laser systems based on tapered gain media in an external cavity are demonstrated at 675 nm. Two 2 mm long amplifiers are used, one with a 500 µm long ridge-waveguide section (device A), the other with a 750 µm long ridge-waveguide section (device B). Laser system A based on device A is tunable from 663 to 684 nm with output power higher than 0.55 W in the tuning range; as high as 1.25 W output power is obtained at 675.34 nm. The emission spectral bandwidth is less than 0.05 nm throughout the tuning range, and the beam quality factor M(2) is 2.07 at an output power of 1.0 W. Laser system B based on device B is tunable from 666 to 685 nm. As high as 1.05 W output power is obtained around 675.67 nm. The emission spectral bandwidth is less than 0.07 nm throughout the tuning range, and the beam quality factor M(2) is 1.13 at an output power of 0.93 W. Laser system B is used as a pump source for the generation of 337.6 nm UV light by single-pass frequency doubling in a bismuth triborate (BIBO) crystal. An output power of 109 µW UV light, corresponding to a conversion efficiency of 0.026% W(-1), is attained.

Optical effects of exposing intact human lenses to ultraviolet radiation and visible light.

BACKGROUND: The human lens is continuously exposed to high levels of light. Ultraviolet radiation is believed to play a causative role in the development of cataract. In vivo, however, the lens is mainly exposed to visible light and the ageing lens absorbs a great part of the short wavelength region of incoming visible light. The aim of the present study was to examine the optical effects on human lenses of short wavelength visible light and ultraviolet radiation. METHODS: Naturally aged human donor lenses were irradiated with UVA (355 nm), violet (400 and 405 nm) and green (532 nm) lasers. The effect of irradiation was evaluated qualitatively by photography and quantitatively by measuring the direct transmission before and after irradiation. Furthermore, the effect of pulsed and continuous laser systems was compared as was the effect of short, intermediate and prolonged exposures. RESULTS: Irradiation with high intensity lasers caused scattering lesions in the human lenses. These effects were more likely to be seen when using pulsed lasers because of the high pulse intensity. Prolonged irradiation with UVA led to photodarkening whereas no detrimental effects were observed after irradiation with visible light. CONCLUSIONS: Irradiation with visible light does not seem to be harmful to the human lens except if the lens is exposed to laser irradiances that are high enough to warrant thermal protein denaturation that is more readily seen using pulsed laser systems.

All passive synchronized Q-switching of a quasi-three-level and a four-level Nd:YAG laser.

Using an all passive approach, synchronized Q-switching of two Nd:YAG lasers, at 946 nm and 1064 nm, is reported. Two laser crystals are used to avoid gain competition, and stable operation is reported for the first time. The pulse trains are synchronized over a wide range of pump powers and a relative timing jitter of 36 ns is achieved. A minimum delay of 64 ns is observed between the two laser pulses, and by making the 946 nm pulse relatively long, a 79% temporal overlap is obtained when compared to the zero-delay scenario.

Spectral beam combining of a 980 nm tapered diode laser bar.

We demonstrate spectral beam combining of a 980 nm tapered diode laser bar. The combined beam from 12 tapered emitters on the bar yielded an output power of 9.3 W at 30 A of operating current. An M2 value of 5.3 has been achieved along the slow axis. This value is close to that of a free running single tapered emitter on the bar at the same current level. The overall spectral beam combining efficiency was measured to be 63%.

1.5 W green light generation by single-pass second harmonic generation of a single-frequency tapered diode laser.

More than 1.5 W of green light at 531 nm is generated by single-pass second harmonic generation in periodically poled MgO:LiNbO3. The pump laser is a high power tapered laser with a distributed Bragg reflector etched in the ridge section of the laser to provide wavelength selectivity. The output power of the single-frequency tapered laser is 9.3 W in continuous wave operation. A conversion efficiency of 18.5 % was achieved in the experiments.

High power 404 nm source based on second harmonic generation in PPKTP of a tapered external feedback diode laser.

We present results on a 404 nm laser system based on second harmonic generation in a new compact external cavity configuration. We obtain a stable 318 mW cw diffraction limited output from the system with a mode-matched pump power of 630 mW. We observe up to 620 mW SHG, when the cavity is operating in scanning mode. The pump source is an external cavity grating feedback tapered diode laser operating at 808 nm. We find that thermal effects in the nonlinear crystal severely limit the efficiency of the setup with high input powers.