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Near-infrared (NIR) imaging, as a newly emerged technique, demonstrates immense potential in various imaging applications such as biological detection, night vision, and anti-counterfeiting. The imaging quality of the currently available NIR light sources is limited by their low radiant exitance and poor beam quality. Herein, a quasi-point NIR light source based on a laser-driven photoluminescence technique was successfully developed. A single blue laser diode (LD) with a power of ∼2250 mW and a minimum spot size of ≈ 0.13 mm-2 is employed as the pumping source. A Cr-doped MgO ceramic displaying strong luminescence in the NIR region is used as the emitter. Interestingly, the prepared MgO:Cr ceramic is able to withstand the blue laser irradiation density of > 5600 mW·mm-2, and therefore, the fabricated NIR light source demonstrates a high radiant flux of ∼234 mW with a high radiant exitance of ∼139 mW·mm-2. Furthermore, the emitting area is as small as ∼1.6 mm2, which is highly beneficial for optical design and device miniaturization. The overall performance of the quasi-point NIR light source in blood vessel imaging and night vision applications is evaluated.

Laser lighting is emerging as a viable replacement for other light sources in applications requiring high luminance not achievable with LEDs. Phosphor materials for laser lighting are often optimized in terms of luminous efficiency and/or colorimetric properties, while the light homogeneity is often neglected. We present a thorough investigation of the homogeneity of the chromaticity and the luminance profile for the most common types of phosphors used in laser lighting. We find that the achievable luminance and homogeneity of the light spot depends significantly on the phosphor used to convert the blue laser light to white light. The findings of these investigations will present guidelines for optimal phosphor material parameters to achieve high luminance combined with homogeneous chromaticity.

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.

Laser diodes, in general, are sensitive to optical feedback, especially with regard to maintaining single-frequency operation. Until now, however, the feedback sensitivity of high-power devices such as single-frequency distributed Bragg reflector (DBR) tapered laser diodes has not been investigated in quantitative detail. In this paper, we analyze the impact of very weak optical feedback between -105 dB and -40 dB on a high-power DBR tapered laser diode. The measurement setup is validated using a typical DFB laser diode. The results are in good agreement with the theory at low-feedback levels. (C) 2021 Optical Society of America

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.

A novel compact micro-integrated high-power narrow-linewidth external-cavity diode laser around 808 nm is demonstrated. The laser system contains a tapered amplifier consisting of a ridge-waveguide section and a tapered section with separated electrical contacts. Thus, the injection currents to both sections can be controlled independently. An external volume Bragg grating is utilized for spectral narrowing and stabilization. The diode laser system is integrated on a 5 mm x 13 mm aluminum nitride micro-optical bench on a conduction cooled package mount with a footprint of 25 mm x 25 mm. The diode laser system is characterized by measuring the output power and spectrum with the injection currents to the ridge-waveguide section (IRW) and tapered amplifier section (ITA) changed in steps of 25 and 50 mA, respectively. At IRW = 200 mA and ITA = 6.0 A, 3.5 watts of output power is obtained with an emission spectral linewidth with an upper bound of 6 pm, and a beam propagation factor in the slow axis, M2, of 2.6 (1/e2). The characterization of the temperature stabilization of the laser system shows an increase of the wavelength at a rate of 6.5 pm/K, typical for the applied volume Bragg grating. © 2019 Optical Society of America

Laser lighting is an emerging technology to generate high luminance lighting. To achieve high luminance or high luminous exitance, the light emitter must have high flux and small size simultaneously. When laser light is focused to a small spot size on the phosphor material, the two main limitations are saturation of the phosphor material and the spot size of the generated light. Here, we investigate experimentally and numerically the spot size of laser lighting dependent on the spot size of the incident laser light and the material properties of the wavelength converting phosphor material. We find numerically that the spot size of the generated white light is significantly influenced by the phosphor properties. The spot size of the white light determines the étendue and thereby the possibility to collect and shape the light. This has important implications in applications of 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.

A significant obstacle in the development of phosphor-converted white-laser diodes (pc-wLDs) is associated with the laser-induced luminescence saturation of phosphors. YAG:Ce is a canonical yellow-emitting phosphor. However, there has been few systematic research of measurement and understanding of the luminescence saturation in YAG:Ce yet. Here, we analyze the luminescence saturation of a single-crystal YAG:Ce with a unique phosphor geometry. A novel characterization method, which enables the optical signals to be fully collected and analyzed, is introduced. The effects of both laser power and laser power density on the luminescence properties and colorimetric properties of the YAG:Ce are studied systematically. The results show that a single-crystal YAG:Ce has a high saturation threshold (over 360 W/mm2) so that only a very slight saturation behavior can be observed at a high power density. Furthermore, a pc-wLDs light source based on the combination of a single laser diode and a YAG:Ce phosphor exhibits remarkable efficiency and stability. Under the excitation of a 3.38 W blue laser (~100 W/mm2), the white light emission with the luminous flux of 465 lm, the luminous efficacy (LE) of 145 lm/W, and the correlated color temperature (CCT) of 4980 K can be achieved.

Laser lighting is rapidly becoming a viable alternative to LEDs in many applications. However, the high power density from the lasers put strict requirements on the phosphor materials used for downconverting the blue laser light. Here we demonstrate high efficiency white light generation using single-crystal Ce:YAG phosphors. More than 4700 lm is achieved using 30 W of incident blue laser light.

Unwanted optical feedback is a common problem in many optical setups of laser systems. To quantify the effect, the influence of a controlled external feedback on the emission properties of a low power distributed feedback ridge waveguide and a high power distributed Bragg reflector tapered diode laser are analyzed. The measured influence of the phase dependent feedback over several orders of magnitude in feedback power attenuation on emission wavelength is discussed and compared to theory.

Modulated yellow lasers may enable more efficient photocoagulation for treatments of ophthalmic diseases. In this regard, we present a laser system emitting 2.7 W of true-yellow light at 576 nm by frequency doubling the emission of a tapered diode laser emitting 7 W at 1153 nm. The frequency doubling is based on a single-pass configuration using a cascade of two 40-mm periodically poled lithium niobate (PPLN) crystals. The stabilization of the yellow power over 10 h showed a standard deviation of 0.10% and a relative intensity noise of 0.032% rms. Moreover, we demonstrate the generation of yellow pulses with 900 mW of amplitude and a high extinction ratio in the microsecond and millisecond regimes, as required for photocoagulation.

In this chapter, both blue and green high-power tunable diode laser systems based on GaN broad-area diode laser (BAL) in Littrow external cavity are demonstrated. For blue diode laser system, for high-power application, an output power around 530 mW over a 1.4 nm tunable range is obtained; for wide tunable range application, an output power around 80 mW over a 6.0 nm tunable range is obtained. For the green diode laser system, for high-power application, an output power around 480 mW with a tunable range of 2.1 nm is achieved; for wide tunable range application, an output power of 50 mW with a tunable range of 9.2 nm is achieved. The tuning range and output power optimization of an external-cavity diode laser system is investigated based on the experimental results obtained in the blue and green external-cavity GaN diode laser systems. The obtained results can be used as a guide for selecting gratings for external-cavity diode lasers for different requirements. The temporal dynamics of the green diode laser system is studied experimentally, and pulse package oscillation is observed, for the first time to our knowledge, in a BAL with an external-cavity grating feedback.

Although external-cavity feedback with a grating is widely used to achieve a tunable high-power narrow-linewidth broad-area diode laser (BAL) system, the dynamics of such a system is seldom studied. In this paper, the temporal dynamics of a tunable high-power green external-cavity diode laser system based on a GaN BAL and Littrow external-cavity is investigated experimentally. The regular pulse package oscillation (PPO) is observed just above the threshold. The oscillating period of the pulse package decreases with the increasing injected current. As the current increases further, the pulse package oscillates irregularly, and finally changes to a chaotic state. The PPO is observed, for the first time to our knowledge, in a BAL with an external-cavity grating feedback.

Wavelength information is essential for any researcher in optics and photonics, and for this reason, a wide range of devices is available for measuring it. However, the techniques available today are limited either to a resolution of nanometers or a measurement rate of kHz. In this paper, we present a simple but highly versatile technique based on second-harmonic generation to measure fast wavelength dynamics of laser diodes. We demonstrate a resolution of 0.7 pm and a measurement rate in the MHz range. The measurement rate is limited only by the photodetector, and the wavelength resolution is limited mainly by the length of the nonlinear crystal and the noise of the detectors. The technique can, e.g., be used to investigate the mode-hop behavior of laser diodes during pulsed operation. To demonstrate this, we show the wavelength changes of a laser diode during a single pulse. (c) 2018 Optical Society of America

In this chapter, both blue and green high-power tunable diode laser systems based on GaN broad-area diode laser (BAL) in Littrow external cavity are demonstrated. For blue diode laser system, for high-power application, an output power around 530 mW over a 1.4 nm tunable range is obtained; for wide tunable range application, an output power around 80 mW over a 6.0 nm tunable range is obtained. For the green diode laser system, for high-power application, an output power around 480 mW with a tunable range of 2.1 nm is achieved; for wide tunable range application, an output power of 50 mW with a tunable range of 9.2 nm is achieved. The tuning range and output power optimization of an external-cavity diode laser system is investigated based on the experimental results obtained in the blue and green external-cavity GaN diode laser systems. The obtained results can be used as a guide for selecting gratings for external-cavity diode lasers for different requirements. The temporal dynamics of the green diode laser system is studied experimentally, and pulse package oscillation is observed, for the first time to our knowledge, in a BAL with an external-cavity grating feedback.

Information on the wavelength is essential for most laser applications and a wide range of devices are available for measuring it. Commercially available wavemeters can provide femtometer resolution in a wide wavelength range but their refresh rate rarely goes into the kHz range. Streak cameras, on the other hand, provide extremely fast measurements with a wide spectrum. However, the spectral resolution is severely limited due to the use of a grating as the wavelength separating element. Here we present a wavemeter that combines a megahertz measurement rate and sub-picometer wavelength resolution. The technique uses the steep wavelength acceptance curve of a thick non-linear crystal to calculate the wavelength from just two power measurements. The bandwidth is limited only by the speed of a photodiode while the resolution and wavelength range can be engineered by choosing a suitable crystal type and geometry. We use the wavemeter to examine how the longitudinal mode evolves during a single pulse from a tapered diode laser. High resolution, high speed measurements of the wavelength can give new information about laser diodes, which is valuable for applications requiring short but wavelength stable pulses, such as pulsing of the second harmonic light.

A wide range of laser medical treatments are based on coagulation of blood by absorption of the laser radiation. It has, therefore, always been a goal of these treatments to maximize the ratio of absorption in the blood to that in the surrounding tissue. For this purpose lasers at 577 nm are ideal since this wavelength is at the peak of the absorption in oxygenated hemoglobin. Furthermore, 577 nm has a lower absorption in melanin when compared to green wavelengths (515 − 532 nm), giving it an advantage when treating at greater penetration depth. Here we present a laser system based on frequency doubling of an 1154 nm Distributed Bragg Reflector (DBR) tapered diode laser, emitting 1.1 W of single frequency and diffraction limited yellow light at 577 nm, corresponding to a conversion efficiency of 30.5%. The frequency doubling is performed in a single pass configuration using a cascade of two bulk non-linear crystals. The system is power stabilized over 10 hours with a standard deviation of 0.13% and the relative intensity noise is measured to be 0.064 % rms.

In this paper, we investigate the noise performance and transfer in a titanium sapphire (Ti:S) laser system. This system consists of a DBR tapered diode laser, which is frequency doubled in two cascaded nonlinear crystals and used to pump the Ti:S laser oscillator. This investigation includes electrical noise characterizations of the utilized power supplies, the optical noise of the fundamental light, the second harmonic light, and finally the optical noise of the femtosecond pulses emitted by the Ti:S laser. Noise features originating from the electric power supply are evident throughout the whole transfer chain. It is demonstrated that improving the electrical noise provides an easy method for reducing the relative intensity noise (RIN) in all stages. The frequency doubled light is shown to have a higher RIN than the fundamental light. In particular, the cascaded system is seen to exhibit higher RIN than a setup with only a single nonlinear crystal. The Ti:S is shown to have a cut-off frequency around 500 kHz, which means that noise structures of the pump laser above this frequency are strongly suppressed. Finally, the majority of the Ti:S noise seems to originate from the laser itself, which partly can be explained by the effect of relaxation oscillation frequency.

The results of photoluminescence (PL) imaging performed with a laser line scan in a prototype setup are presented in this work. Here we show the PL images with the photovoltaic cell positioned at different distances from the laser line source, therefore with the beam at different intensities and line characteristics. This study is motivated by the time demanding process of acquiring luminescence images for accurate PV inspections in the field, usually made with electroluminescence imaging. The PL imaging induced by a laser line has the potential to be compact enough to be portable and carried out by drones for field inspections.

More than 3.5 W of green light at 515 nm is generated by frequency doubling a single-frequency high power DBR tapered diode laser. The frequency doubling is performed in a cascade of PPMgLN and PPMgSLT crystals in order to reach high power and avoid thermal effects present in PPMgLN at high power. The green light is diffraction limited (M2 <1.1) and single-frequency operation is demonstrated with a linewidth less than 2 pm.

Semiconductor lasers are ideal sources for efficient electrical-to-optical power conversion and for many applications where their small size and potential for low cost are required to meet market demands. Yellow lasers find use in a variety of bio-related applications, such as photocoagulation, imaging, flow cytometry, and cancer treatment. However, direct generation of yellow light from semiconductors with sufficient beam quality and power has so far eluded researchers. Meanwhile, tapered semiconductor lasers at near-infrared wavelengths have recently become able to provide neardiffraction- limited, single frequency operation with output powers up to 8 W near 1120 nm. We present a 1.9 W single frequency laser system at 562 nm, based on single pass cascaded frequency doubling of such a tapered laser diode. The laser diode is a monolithic device consisting of two sections: a ridge waveguide with a distributed Bragg reflector, and a tapered amplifier. Using single-pass cascaded frequency doubling in two periodically poled lithium niobate crystals, 1.93 W of diffraction-limited light at 562 nm is generated from 5.8 W continuous-wave infrared light. When turned on from cold, the laser system reaches full power in just 60 seconds. An advantage of using a single pass configuration, rather than an external cavity configuration, is increased stability towards external perturbations. For example, stability to fluctuating case temperature over a 30 K temperature span has been demonstrated. The combination of high stability, compactness and watt-level power range means this technology is of great interest for a wide range of biological and biomedical applications. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE