Unidirectional ring laser operation and tunable single-frequency emission using differential parametric gain

In this Letter, a novel approach for unidirectional operation of a 1064 nm solid-state ring laser is demonstrated based on difference frequency mixing. Unidirectional operation is achieved exploiting the directional parametric gain from a single-pass diode laser, facilitated through a periodically poled LiNbO3 crystal. In addition to achieving unidirectional operation, the nonlinear process further enables the generation of single-frequency mid-infrared light. Using a single-pass tapered diode laser, tunable in the range from 780 to 815 nm, the generated mid-infrared signal covers the 2.9 to 3.5 µm range while optimizing the phase-match condition of the difference frequency generation process.

Geological, multispectral, and meteorological imaging results from the Mars 2020 Perseverance rover in Jezero crater

Perseverance's Mastcam-Z instrument provides high-resolution stereo and multispectral images with a unique combination of spatial resolution, spatial coverage, and wavelength coverage along the rover's traverse in Jezero crater, Mars. Images reveal rocks consistent with an igneous (including volcanic and/or volcaniclastic) and/or impactite origin and limited aqueous alteration, including polygonally fractured rocks with weathered coatings; massive boulder-forming bedrock consisting of mafic silicates, ferric oxides, and/or iron-bearing alteration minerals; and coarsely layered outcrops dominated by olivine. Pyroxene dominates the iron-bearing mineralogy in the fine-grained regolith, while olivine dominates the coarse-grained regolith. Solar and atmospheric imaging observations show significant intra- and intersol variations in dust optical depth and water ice clouds, as well as unique examples of boundary layer vortex action from both natural (dust devil) and Ingenuity helicopter-induced dust lifting. High-resolution stereo imaging also provides geologic context for rover operations, other instrument observations, and sample selection, characterization, and confirmation.

Neuroscience and architecture: Modulating behavior through sensorimotor responses to the built environment

As we move through the world, natural and built environments implicitly guide behavior by appealing to certain sensory and motor dynamics. This process can be motivated by automatic attention to environmental features that resonate with specific sensorimotor responses. This review aims at providing a psychobiological framework describing how environmental features can lead to automated sensorimotor responses through defined neurophysiological mechanisms underlying attention. Through the use of automated processes in subsets of cortical structures, the goal of this framework is to describe on a neuronal level the functional link between the designed environment and sensorimotor responses. By distinguishing between environmental features and sensorimotor responses we elaborate on how automatic behavior employs the environment for sensorimotor adaptation. This is realized through a thalamo-cortical network integrating environmental features with motor aspects of behavior. We highlight the underlying transthalamic transmission from an Enactive and predictive perspective and review recent studies that effectively modulated behavior by systematically manipulating environmental features. We end by suggesting a promising combination of neuroimaging and computational analysis for future studies.

Speckle-displacement-based wavemeter for mode-hop and side-mode detection

A speckle-displacement-based wavemeter is combined with a spatial-fundamental-mode-pass filter to eliminate the influence of multimode operation on the directionality of the resulting output from a distributed Bragg reflector (DBR) tapered laser. The proposed setup is characterized theoretically and experimentally, and detections of mode hops and side-mode suppression ratios (SMSRs) in the optical output are demonstrated. The laser illuminates a rough surface at an oblique angle, and a camera observes the corresponding speckle pattern from an almost identical back-scattering direction. As the wavelength of the laser shifts, the speckle pattern responds with a corresponding displacement, which is approximately linear with respect to the shift within the detection area. The wavemeter tracks continuously the shifts of the speckles pattern by tracking the peak of the covariance function of sequentially acquired images. In this way, the speckle-displacement-based wavemeter achieves a spectral resolution of 10.4 MHz. Mode hops in the laser do not cause any impeding decorrelation of the speckle patterns. Interestingly, the actual SMSR is related to the peak height and width of the absolute covariance function. A wavemeter, which is capable of measuring wavelengths, mode hops, and SMSRs, is highly useful for spectroscopy, quantum optics, nonlinear frequency conversion, and other applications requiring stable single-frequency laser light, especially when using diode lasers.

Microsecond pulse-mode operation of a micro-integrated high-power external-cavity tapered diode laser at 808 nm

We investigate microsecond pulse-mode operation of a micro-integrated high-power diode laser based on volume Bragg grating external-cavity feedback around 808 nm. The laser system contains a tapered amplifier consisting of a ridge-waveguide section and a tapered section with separated electrical contacts. Thus, the diode laser system can be pulsed by modulating the injected current either to the ridge waveguide section (I_RW) or to the tapered amplifier section (I_TA). With a trigger signal of a 50 µs pulse width and a 10 kHz repetition rate, comparing the modulation depth, peak output power, beam propagation factor, and spectral bandwidth, we conclude that the pulse-mode operation achieved by modulating the I_TA gives better results than by modulating the I_RW due to the decreased thermal effect. At a constant I_RW of 0.2 A and a modulated I_TA of 6.0 A, 4.3 W of peak output power is obtained with an emission spectral bandwidth with an upper bound of 0.2 nm, and a beam propagation factor in the slow axis, Mslow2, of 2.6 (1/e²). The modulation depth is almost 100%. The results show that the tapered diode laser system may be a good candidate for microsecond pulse-mode solid-state lasers.

Micro-integrated high-power narrow-linewidth external-cavity tapered diode laser at 808 nm

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 5mm×13mm aluminum nitride micro-optical bench on a conduction cooled package mount with a footprint of 25mm×25mm. The diode laser system is characterized by measuring the output power and spectrum with the injection currents to the ridge-waveguide section (I_RW) and tapered amplifier section (I_TA) changed in steps of 25 and 50 mA, respectively. At I_RW=200mA and I_TA=6.0A, 3.5 W 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.

Coherent combining of high brightness tapered amplifiers for efficient non-linear conversion

We report on a coherent beam combination of three high-brightness tapered amplifiers, which are seeded by a single-frequency laser at λ = 976 nm in a simple architecture with efficiently cooled emitters. The maximal combined power of 12.9 W is achieved at a combining efficiency of > 65%, which is limited by the amplifiers' intrinsic beam quality. The coherent combination cleans up the spatial profile, as the central lobe's power content increases by up to 86%. This high-brightness infrared beam is converted into the visible by second harmonic generation. This results in a high non-linear conversion efficiency of 4.5%/W and a maximum power over 2 W at 488 nm, which is limited by thermal effects in the periodically poled lithium niobate (PPLN).

Second-harmonic-generation-based technique for examining laser diode wavelength dynamics in the μs to ms range

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.

Deep modulation of second-harmonic light by wavelength detuning of a laser diode

Power modulated visible lasers are interesting for a number of applications within areas such as laser displays and medical laser treatments. In this paper, we present a system for modulating the second-harmonic light generated by single-pass frequency doubling of a distributed feedback (DFB) master oscillator power amplifier (MOPA) laser diode with separate electrical contacts for the MO and the PA. A modulation depth in excess of 97% from 0.1 Hz to 10 kHz is demonstrated. This is done by wavelength tuning of the laser diode using only a 40 mA adjustment of the current through the MO. The bandwidth of the modulation is limited by the electronics. This method has the potential to decrease the size as well as cost of modulated visible lasers. The achievable optical powers will increase as DFB MOPAs are further developed.

Efficient generation of 1.9 W yellow light by cascaded frequency doubling of a distributed Bragg reflector tapered diode

Watt-level yellow emitting lasers are interesting for medical applications, due to their high hemoglobin absorption, and for efficient detection of certain fluorophores. In this paper, we demonstrate a compact and robust diode-based laser system in the yellow spectral range. The system generates 1.9 W of single-frequency light at 562.4 nm by cascaded single-pass frequency doubling of the 1124.8 nm emission from a distributed Bragg reflector (DBR) tapered laser diode. The absence of a free-space cavity makes the system stable over a base-plate temperature range of 30 K. At the same time, the use of a laser diode enables the modulation of the pump wavelength by controlling the drive current. This is utilized to achieve a power modulation depth above 90% for the second harmonic light, with a rise time below 40  μs.

Highly efficient single-pass sum frequency generation by cascaded nonlinear crystals

The cascading of nonlinear crystals has been established as a simple method to greatly increase the conversion efficiency of single-pass second-harmonic generation compared to a single-crystal scheme. Here, we show for the first time that the technique can be extended to sum frequency generation, despite differences in the phase relations of the involved fields. An unprecedented 5.5 W of continuous-wave diffraction-limited green light is generated from the single-pass sum frequency mixing of two diode lasers in two periodically poled nonlinear crystals (conversion efficiency 50%). The technique is generally applicable and can be applied to any combination of fundamental wavelengths and nonlinear crystals.

Concept for power scaling second harmonic generation using a cascade of nonlinear crystals

Within the field of high-power second harmonic generation (SHG), power scaling is often hindered by adverse crystal effects such as thermal dephasing arising from the second harmonic (SH) light, which imposes limits on the power that can be generated in many crystals. Here we demonstrate a concept for efficient power scaling of single-pass SHG beyond such limits using a cascade of nonlinear crystals, in which the first crystal is chosen for high nonlinear efficiency and the subsequent crystal(s) are chosen for power handling ability. Using this highly efficient single-pass concept, we generate 3.7 W of continuous-wave diffraction-limited (M(2)=1.25) light at 532 nm from 9.5 W of non-diffraction-limited (M(2)=7.7) light from a tapered laser diode, while avoiding significant thermal effects. Besides constituting the highest SH power yet achieved using a laser diode, this demonstrates that the concept successfully combines the high efficiency of the first stage with the good power handling properties of the subsequent stages. The concept is generally applicable and can be expanded with more stages to obtain even higher efficiency, and extends also to other combinations of nonlinear media suitable for other wavelengths.

Simultaneous dual wavelength eye-tracked ultrahigh resolution retinal and choroidal optical coherence tomography

We demonstrate an optical coherence tomography device that simultaneously combines different novel ultrabroad bandwidth light sources centered in the 800 and 1060 nm regions, operating at 66 kHz depth scan rate, and a confocal laser scanning ophthalmoscope-based eye tracker to permit motion-artifact-free, ultrahigh resolution and high contrast retinal and choroidal imaging. The two wavelengths of the device provide the complementary information needed for diagnosis of subtle retinal changes, while also increasing visibility of deeper-lying layers to image pathologies that include opaque media in the anterior eye segment or eyes with increased choroidal thickness.

Deep tissue optical imaging of upconverting nanoparticles enabled by exploiting higher intrinsic quantum yield through use of millisecond single pulse excitation with high peak power

We have accomplished deep tissue optical imaging of upconverting nanoparticles at 800 nm, using millisecond single pulse excitation with high peak power. This is achieved by carefully choosing the pulse parameters, derived from time-resolved rate-equation analysis, which result in higher intrinsic quantum yield that is utilized by upconverting nanoparticles for generating this near infrared upconversion emission. The pulsed excitation approach thus promises previously unreachable imaging depths and shorter data acquisition times compared with continuous wave excitation, while simultaneously keeping the possible thermal side-effects of the excitation light moderate. These key results facilitate means to break through the general shallow depth limit of upconverting-nanoparticle-based fluorescence techniques, necessary for a range of biomedical applications, including diffuse optical imaging, photodynamic therapy and remote activation of biomolecules in deep tissues.

Fourier domain mode-locked swept source at 1050 nm based on a tapered amplifier

While swept source optical coherence tomography (OCT) in the 1050 nm range is promising for retinal imaging, there are certain challenges. Conventional semiconductor gain media have limited output power, and the performance of high-speed Fourier domain mode-locked (FDML) lasers suffers from chromatic dispersion in standard optical fiber. We developed a novel light source with a tapered amplifier as gain medium, and investigated the FDML performance comparing two fiber delay lines with different dispersion properties. We introduced an additional gain element into the resonator, and thereby achieved stable FDML operation, exploiting the full bandwidth of the tapered amplifier despite high dispersion. The light source operates at a repetition rate of 116 kHz with an effective average output power in excess of 30 mW. With a total sweep range of 70 nm, we achieved an axial resolution of 15 microm in air (approximately 11 microm in tissue) in OCT measurements. As our work shows, tapered amplifiers are suitable gain media for swept sources at 1050 nm with increased output power, while high gain counteracts dispersion effects in an FDML laser.

980 nm high brightness external cavity broad area diode laser bar

We demonstrate off-axis spectral beam combining applied to a 980 nm high power broad area diode laser bar. The experiments yielded 9 W of optical power at 30 A of operating current and the measured M2 values of the combined beam from 12 emitters were 1.9 and 6.4 for the fast and the slow axis, respectively. The slow axis beam quality was 5-6 times better than the value obtained from a single emitter in free running mode. A high brightness of 79 MW/cm2-str was achieved using this configuration. To our knowledge, this is the highest brightness level ever achieved from a broad area diode laser bar.

Hypokalemic myopathy during treatment with diuretics

Two male patients with severe reversible muscle weakness and excessive potassium deficiency associated with alkalosis during treatment with diuretics are presented. The case reports are further illustrated by the morphologic changes as seen in light and electron microscopic examination of muscle biopsies. Hypokalemia and muscle dysfunction are discussed in relation to other investigations of altered potassium metabolism and myopathy during treatment with certain diuretics.