Compact Diode Laser Research Articles

Combined frequency domain photoacoustic and ultrasound imaging for intravascular applications.

Intravascular photoacoustic (IVPA) imaging has the potential to characterize lipid-rich structures based on the optical absorption contrast of tissues. In this study, we explore frequency domain photoacoustics (FDPA) for intravascular applications. The system employed an intensity-modulated continuous wave (CW) laser diode, delivering 1W over an intensity modulated chirp frequency of 4-12MHz. We demonstrated the feasibility of this approach on an agar vessel phantom with graphite and lipid targets, imaged using a planar acoustic transducer co-aligned with an optical fibre, allowing for the co-registration of IVUS and FDPA images. A frequency domain correlation method was used for signal processing and image reconstruction. The graphite and lipid targets show an increase in FDPA signal as compared to the background of 21dB and 16dB, respectively. Use of compact CW laser diodes may provide a valuable alternative for the development of photoacoustic intravascular devices instead of pulsed laser systems.

Compact and inexpensive iodine-stabilized diode laser system with an output at 531 nm for gauge block interferometers

A compact and inexpensive iodine-stabilized diode laser system with an output at 531nm has been applied to long gauge block measurements. Although the optical frequency of the output beam was widely modulated (modulation width of ∼22MHz), the coherence length and interference phase stability are sufficiently long and high, respectively, for the interferometric measurement of long gauge blocks of up to 1000mm in length. The effective uncertainty of laser frequency in the interferometric measurement was theoretically and experimentally confirmed to be less than 10−9.

A compact and robust diode laser system for atom interferometry on a sounding rocket

We present a diode laser system optimized for laser cooling and atom interferometry with ultra-cold rubidium atoms aboard sounding rockets as an important milestone towards space-borne quantum sensors. Design, assembly and qualification of the system, combing micro-integrated distributed feedback (DFB) diode laser modules and free space optical bench technology is presented in the context of the MAIUS (Matter-wave Interferometry in Microgravity) mission. This laser system, with a volume of 21 liters and total mass of 27 kg, passed all qualification tests for operation on sounding rockets and is currently used in the integrated MAIUS flight system producing Bose-Einstein condensates and performing atom interferometry based on Bragg diffraction. The MAIUS payload is being prepared for launch in fall 2016. We further report on a reference laser system, comprising a rubidium stabilized DFB laser, which was operated successfully on the TEXUS 51 mission in April 2015. The system demonstrated a high level of technological maturity by remaining frequency stabilized throughout the mission including the rocket's boost phase.

Effects of polarization mode dispersion on polarization-entangled photons generated via broadband pumped spontaneous parametric down-conversion.

An inexpensive and compact frequency multi-mode diode laser enables a compact two-photon polarization entanglement source via the continuous wave broadband pumped spontaneous parametric down-conversion (SPDC) process. Entanglement degradation caused by polarization mode dispersion (PMD) is one of the critical issues in optical fiber-based polarization entanglement distribution. We theoretically and experimentally investigate how the initial entanglement is degraded when the two-photon polarization entangled state undergoes PMD. We report an effect of PMD unique to broadband pumped SPDC, equally applicable to pulsed pumping as well as cw broadband pumping, which is that the amount of the entanglement degradation is asymmetrical to the PMD introduced to each quantum channel. We believe that our results have important applications in long-distance distribution of polarization entanglement via optical fiber channels.

Fast 3D visualization of endogenous brain signals with high-sensitivity laser scanning photothermal microscopy.

A fast, high-sensitivity photothermal microscope was developed by implementing a spatially segmented balanced detection scheme into a laser scanning microscope. We confirmed a 4.9 times improvement in signal-to-noise ratio in the spatially segmented balanced detection compared with that of conventional detection. The system demonstrated simultaneous bi-modal photothermal and confocal fluorescence imaging of transgenic mouse brain tissue with a pixel dwell time of 20 μs. The fluorescence image visualized neurons expressing yellow fluorescence proteins, while the photothermal signal detected endogenous chromophores in the mouse brain, allowing 3D visualization of the distribution of various features such as blood cells and fine structures probably due to lipids. This imaging modality was constructed using compact and cost-effective laser diodes, and will thus be widely useful in the life and medical sciences.

Temperature imaging in low-pressure flames using diode laser two-line atomic fluorescence employing a novel indium seeding technique

The use of diode lasers for spatially resolved temperature imaging is demonstrated in low-pressure premixed methane–air flames using two-line atomic fluorescence of seeded indium atoms. This work features the advantages of using compact diode lasers as the excitation sources with the benefits of two-dimensional planar imaging, which is normally only performed with high-power pulsed lasers. A versatile and reliable seeding technique with minimal impact on flame properties is used to introduce indium atoms into the combustion environment for a wide range of flame equivalence ratios. A spatial resolution of around 210 µm for this calibration-free thermometry technique is achieved for three equivalence ratios at a pressure of 50 mbar in a laminar flat flame.

Laser diode pumped, actively Q-switched, and mode-locked Nd:YAG/PbWO_4 Raman laser

A compact laser diode pumped, actively Q-switched, and mode-locked Nd:YAG/PbWO(4) Raman laser is demonstrated. With the incident pump power of 9.11 W and pulse repetition frequency of 15 kHz, a maximum average output power of 1.003 W at 1178 nm was obtained and the corresponding conversion efficiency was 11%. A perfect stable mode-locking operation with the modulation depth of 100% was obtained and the corresponding mode-locking pulse width was less than 207 ps. The actively Q-switched and mode-locked Raman laser was theoretically analyzed and calculations were conducted using the fluctuation rate equation model.

Compact silicon photonic wavelength-tunable laser diode with ultra-wide wavelength tuning range

We present a wavelength-tunable laser diode with a 99-nm-wide wavelength tuning range. It has a compact wavelength-tunable filter with high wavelength selectivity fabricated using silicon photonics technology. The silicon photonic wavelength-tunable filter with wide wavelength tuning range was realized using two ring resonators and an asymmetric Mach-Zehnder interferometer. The wavelength-tunable laser diode fabricated by butt-joining a silicon photonic filter and semiconductor optical amplifier shows stable single-mode operation over a wide wavelength range.

Simultaneous dual-wavelength imaging of nonfluorescent tissues with 3D subdiffraction photothermal microscopy.

Multi-wavelength microscopic imaging is essential to visualize a variety of nanoscale cellular components with high specificity and high spatial resolution. However, previous techniques are based on fluorescence, and thus cannot visualize nonfluorescent species, which are much less suffered from photodamage or photobleaching and hence are intrinsically useful in wider range of optical microscopy. Here, we show that simultaneous multi-wavelength imaging of nonfluorescent species can be achieved with the use of a photothermal microscope. Dual-wavelength subdiffraction imaging of biological tissues stained with hematoxylin and eosin is demonstrated. Three-dimensional label-free imaging of mouse melanoma tissue section is also presented to demonstrate the effectiveness of the enhanced spatial resolution. Our technique can be implemented using cost-effective and compact laser diodes and is applicable for various types of both fluorescent and nonfluorescent tissues.

High brightness diode-pumped organic solid-state laser

High-power, diffraction-limited organic solid-state laser operation has been achieved in a vertical external cavity surface-emitting organic laser (VECSOL), pumped by a low-cost compact blue laser diode. The diode-pumped VECSOLs were demonstrated with various dyes in a polymer matrix, leading to laser emissions from 540 nm to 660 nm. Optimization of both the pump pulse duration and output coupling leads to a pump slope efficiency of 11% for a DCM based VECSOLs. We report output pulse energy up to 280 nJ with 100 ns long pump pulses, leading to a peak power of 3.5 W in a circularly symmetric, diffraction-limited beam.

Compact high‐current diode laser nanosecond‐pulse source with high efficiency and 13 µJ output energy

A compact gain-switched nanosecond diode laser pulse source at 920 nm with an output energy of 13 µJ is presented. The developed nanosecond electronic driver is based on GaN transistors and controlled by transistor–transistor logic compatible input pulses. A peak current of more than 430 A for 20–50 ns-long pulses with a repetition rate up to 100 kHz and an electrical efficiency of more than 50% is reached. About 50 ns optical pulses with a peak power of 250 W (13 µJ) are generated with a tapered ridge waveguide bar with 29 emitters. The rise and fall time of the pulses is about 13 ns.

All-polymer photonic sensing platform based on whispering-gallery mode microgoblet lasers.

We present an all-polymer photonic sensing platform based on whispering-gallery mode microgoblet lasers integrated into a microfluidic chip. The chip is entirely made from polymers, enabling the use of the devices as low-cost disposables. The microgoblet cavities feature quality factors exceeding 10(5) and are fabricated from poly(methyl methacrylate) (PMMA) using spin-coating, mask-based optical lithography, wet chemical etching, and thermal reflow. In contrast to silica-based microtoroid resonators, this approach replaces technically demanding vacuum-based dry etching and serial laser-based reflow techniques by solution-based processing and parallel thermal reflow. This enables scaling to large-area substrates, and hence significantly reduces device costs. Moreover, the resonators can be fabricated on arbitrary substrate materials, e.g., on transparent and flexible polymer foils. Doping the microgoblets with the organic dye pyrromethene 597 transforms the passive resonators into lasers. Devices have lasing thresholds below 0.6 nJ per pulse and can be efficiently pumped via free-space optics using a compact and low-cost green laser diode. We demonstrate that arrays of microgoblet lasers can be readily integrated into a state-of-the-art microfluidic chip replicated via injection moulding. In a proof-of-principle experiment, we show the viability of the lab-on-a-chip via refractometric sensing, demonstrating a bulk refractive index sensitivity (BRIS) of 10.56 nm per refractive index unit.

Cavity-enhanced frequency doubling from 795nm to 397.5nm ultra-violet coherent radiation with PPKTP crystals in the low pump power regime.

We demonstrate a simple, compact and cost-efficient diode laser pumped frequency doubling system at 795 nm in the low power regime. In two configurations, a bow-tie four-mirror ring enhancement cavity with a PPKTP crystal inside and a semi-monolithic PPKTP enhancement cavity, we obtain 397.5nm ultra-violet coherent radiation of 35mW and 47mW respectively with a mode-matched fundamental power of about 110mW, corresponding to a conversion efficiency of 32% and 41%. The low loss semi-monolithic cavity leads to the better results. The constructed ultra-violet coherent radiation has good power stability and beam quality, and the system has huge potential in quantum optics and cold atom physics.

A new Micro-Stereolithography-System based on Diode Laser Curing (DLC)

In this study, a compact diode laser based Micro-Stereolithography (DLC — Diode Laser Curing) system with linear stages has been developed. A suspended cross table was used to position a diode laser with a focusing lens that cures liquid photosensitive resin. Therefore the apparatus does not need complex beam forming and beam guiding, e.g., galvanometer scanner, optic fibers or similar. Only one replaceable focusing lens is needed. A commercial photo-curable resin was adapted by UV-absorber for μ-SLA. To demonstrate the functionality Single-lines were cured with different parameters such as laser power, scan speed and absorber concentration. Afterwards, specimen with simple geometries like edges and circles were cured. Thereby the limitations of roundness and sharp geometries were determined. Finally, 3D-parts are built with optimized curing parameters. The results show that the new Micro-Stereolithography-System, based on diode laser and fast linear stages is suitable for an effective fabrication of 3D micro-parts.

Frequency domain approach for time-resolved pump-probe microscopy using intensity modulated laser diodes.

We present a scheme for time-resolved pump-probe microscopy using intensity modulated laser diodes. The modulation frequencies of the pump and probe beams are varied up to 500 MHz with fixed frequency detuning typically set at 15 kHz. The frequency response of the pump-probe signal is detected using a lock-in amplifier referenced at the beat frequency. This frequency domain method is capable of characterizing the nanosecond to picosecond relaxation dynamics of sample species without the use of a high speed detector or a high frequency lock-in amplifier. Furthermore, as the pump-probe signal is based on the nonlinear interaction between the two laser beams and the sample, our scheme provides better spatial resolution than the conventional diffraction-limited optical microscopes. Time-resolved pump-probe imaging of fluorescence beads and aggregates of quantum dots demonstrates that this method is useful for the microscopic analysis of optoelectronic devices. The system is implemented using compact and low-cost laser diodes, and thus has a broad range of applications in the fields of photochemistry, optical physics, and biological imaging.

Fabrication of scale gratings for surface encoders by using laser interference lithography with 405 nm laser diodes

This paper presents a cost-effective fabrication method of grating structures with a period of sub-μm for surface encoders. By employing an inexpensive and compact blue laser diode, which has a wavelength of 405 nm and is used in Blu-ray disc drives, a Lloyd’s mirror interferometer has been developed to carry out laser interference lithography for the fabrication of grating structures. Experimental results showed that the developed fabrication system is capable of fabricating grating structures with a pitch of 570 nm. The relative pitch deviation is confirmed to be approximately 1%, which meets the requirement for the scale grating used in a surface encoder. The influence of the limited coherence length of the laser diode on the fabrication area of the grating structures has also been investigated. To expand the fabrication area of the grating structure, a laser source, which has a wavelength of 405 nm and a longer coherence length with a mode selection cavity, is employed in the fabrication system. It has been verified by experiments that the fabrication area of the sub-μm periodic grating structures can be expanded to 300 mm2 with the laser source.

A continuous wave Yb:KGW laser with polarization-independent pump absorption

A polarization-independent pumping scheme for an end-pumped Yb:KGd(WO4)2 laser oscillator is proposed. It enables the use of compact, high power and high brightness fiber-coupled laser diode modules as pump sources and results in reduced thermal effects, good beam quality and improved output power stability. Based on this design, a continuous wave Yb:KGW laser with polarization-independent pump scheme is demonstrated. The laser delivered 2 W of average output power in TEM00 mode at 1040 nm. The induced thermal lens is reduced by a factor of 2 when compared to conventional diode pumping schemes, which clearly demonstrates the potential of the geometry of this crystal for power scaling.

Real-time vapor detection of nitroaromatic explosives by catalytic thermal dissociation blue diode laser cavity ring-down spectroscopy

A compact blue diode laser catalytic thermal dissociation cavity ring-down spectrometer (cTD–CRDS) to detect vapors of nitroaromatic explosives is described. The instrument uses heated platinum(IV) oxide catalyst to convert nitroaromatic compounds to NO2, which is detected at 405nm. Using the relatively volatile nitrobenzene as a test compound, we show by Fourier Transform Infrared Spectroscopy (FTIR) in off-line experiments that nitroaromatics can be quantitatively converted to NO2. The cTD–CRDS detection limit was 0.3parts-per-billion by volume (ppbv) and sufficiently low to allow the detection of a room temperature sample of 2,4,6-trinitrotoluene (TNT) without sample preconcentration.

Measurements of the Weak UV Absorptions of Isoprene and Acetone at 261–275 nm Using Cavity Ringdown Spectroscopy for Evaluation of a Potential Portable Ringdown Breath Analyzer

The weak absorption spectra of isoprene and acetone have been measured in the wavelength range of 261–275 nm using cavity ringdown spectroscopy. The measured absorption cross-sections of isoprene in the wavelength region of 261–266 nm range from 3.65 × 10−21 cm2·molecule−1 at 261 nm to 1.42 × 10−21 cm2·molecule−1 at 266 nm; these numbers are in good agreement with the values reported in the literature. In the longer wavelength range of 270–275 nm, however, where attractive applications using a single wavelength compact diode laser operating at 274 nm is located, isoprene has been reported in the literature to have no absorption (too weak to be detected). Small absorption cross-sections of isoprene in this longer wavelength region are measured using cavity ringdown spectroscopy for the first time in this work, i.e., 6.20 × 10−23 cm2·molecule−1 at 275 nm. With the same experimental system, wavelength-dependent absorption cross-sections of acetone have also been measured. Theoretical detection limits of isoprene and comparisons of absorbance of isoprene, acetone, and healthy breath gas in this wavelength region are also discussed.

Dual-wavelength monolithic Y-branch distributed Bragg reflection diode laser at 671 nm suitable for shifted excitation Raman difference spectroscopy

A dual-wavelength monolithic Y-branch distributed Bragg reflection (DBR) diode laser at 671 nm is presented. The device is realized with deeply etched surface DBR gratings by one-step epitaxy. A maximum optical output power of 110 mW is obtained in cw-operation for each laser cavity. The emission wavelengths of the device are 670.5 nm and 671.0 nm with a spectral width of 13 pm (0.3 cm−1) and a mean spectral distance of 0.46 nm (10.2 cm−1) over the whole operating range. Together with a free running power stability of ± 1.1% this most compact diode laser is ideally suited as an excitation light source for portable shifted excitation Raman difference spectroscopy (SERDS).