Standard Laser Diode Research Articles

Absolute angle measurement using dual-wavelength laser speckle: Theory and method

This paper presents a method utilising the speckle pattern formed by dual-wavelength illumination for the measurement of the two out-of-plane surface angles with respect to the sensor frame. Theoretical expressions are derived relating the observed speckle shift between patterns formed by two wavelengths for tilted surfaces with both on-axis and off-axis detector positions. These expressions are verified experimentally, showing RMS errors of between 0.5–1.0 ▪. Finally, an on-axis implementation of the concept is presented using dual-wavelength illumination generated from two modes of a standard FP diode laser. Simplified expressions for the calculation of surface angles from measured speckle shift using this arrangement are presented, given in terms of three sensor constants; the responsivity or sensitivity of the sensor, C, and the zero surface tilt speckle shifts, Ax0 and Ay0. Experimental results using this sensor for a range of surface tilts between ▪ and ▪ showed an RMS error of ▪ in θx and ▪ in θy.

Elimination of catastrophic optical mirror damage in continuous-wave high-power laser diodes using multi-section waveguides.

One of the persistent obstacles for high-power laser diodes (LDs) has been the catastrophic optical mirror damage (COMD), which limits the operating power level and lifetime of commercial high-power LDs. The output facet of LD reaches a critical temperature resulting in COMD, which is an irreversible device failure. Here, we fabricate multi-section LDs by tailoring the waveguide structure along the cavity that separates the output facet from the heat-generating lasing region. In this method, the LD waveguide is divided into electrically isolated laser and window sections along the cavity. The laser section is pumped at a high current to achieve high output power, and the window is biased at a low current with negligible heat generation. This design restricts the thermal impact of the laser section on the facet, and the window section allows lossless transport of the laser to the output facet. The lasers were operated continuous-wave up to the maximum achievable power. While standard LDs show COMD failures, the multi-section waveguide LDs are COMD-free. Our technique and results provide a pathway for high-reliability LDs, which would find diverse applications in semiconductor lasers.

Nonlinearity Reduction in a Fiber Fabry-Perot Interferometer Interrogated by a Wavelength Scanning Optical Source

A novel method to reduce the nonlinearity effect of a wavelength scanning diode laser interrogating a low-reflectivity Fabry-Perot interferometer is presented. The interferometer is used for refractive index measurement. The proposed method allows improving the resolution of the system from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> ; and generates a temperature insensitive sensing system. Using the resolution improvement and the temperature insensitivity, the proposed system was used to measure the thermo-optic coefficient of two liquid samples with well-known refractive index. To demonstrate the reliability of the proposed system, experimental results of the long-term performance of a six-month interval, using the same samples but different sensing heads, are presented. The proposed method allows the implementation of a standard four-pin diode laser as an optical source and a standard PIN photodetector for signal detection; permitting the implementation of a high-performance fiber sensor system based on small, light, and standard optoelectronic components. These characteristics are important for different applications such as medical and aerospace, for example, to monitor strain, curvature, pressure, and refractive index using fiber interferometers.

A Stable and Efficient Red‐Emitting Color Converter Based on K2SiF6:Mn4+ Phosphor‐in‐Glass Film for Next‐Generation Laser‐Excited Lighting and Display

Laser diodes (LDs) are expected to be the next‐generation high luminance sources because of the unobvious “efficiency droop” phenomenon under high power. To achieve high‐quality white lighting, red emission is essential for white lighting devices. K2SiF6:Mn4+ (KSFM) is a highly suitable red phosphor for its strong broadband excitation in the UV and blue region and intense red emission at 631 nm. In addition, the ultranarrow emission makes it suitable for wide gamut displays. Herein, a stable and easily prepared red color converter, KSFM based phosphor‐in‐glass films on a coated sapphire substrate (CSA) is developed, which exhibits a high thermal conductivity of 32.02 W m−1 K−1. Compared with the KSFM phosphor, this KSFM–CSA composite shows an overall increase in thermal quenching and irradiation resistance abilities. A wide color gamut covering 128% of the National Television Standards Committee (NTSC) standard white LD (WLD) is fabricated by coupling a green‐emitting CsPbBr3 embedded glass plate and red‐emitting KSFM–CSA with blue LDs. Furthermore, a high‐quality WLD with a luminous flux (520 lm), luminous efficiency (93.0 lm W−1), and color rendering index (CRI, 81.4) is achieved. With its excellent properties, the KSFM–CSA has great potential in high‐power laser lighting and display.

Estimation of sheath electric field in inductively coupled hydrogen plasma on the basis of Doppler-broadened absorption spectrum of hydrogen Balmer-α line

We examined the applicability of the Doppler-broadened absorption spectrum of the hydrogen Balmer-α line to the estimation of the sheath electric field in plasma. The Stark splitting of the fine-structure components was calculated by solving the time-independent Schrödinger equation at various electric field strengths, and the theoretical absorption spectrum was obtained by the superposition of the fine-structure components with the same Doppler broadening widths. The spectrum of the Balmer-α line of atomic hydrogen, which was measured by standard diode laser absorption spectroscopy, was fitted with the theoretical spectrum. We succeeded in determining the translational temperature of atomic hydrogen, which was obtained from the Doppler broadening width, and the electric field strength by the spectral fitting. We have evaluated that the minimum electric field strength that can be detected by the present method is approximately 350 V cm−1 when the translational temperature of atomic hydrogen is 400–500 K.

Distributed-feedback blue laser diode utilizing a tunnel junction grown by plasma-assisted molecular beam epitaxy

In this paper, we demonstrate a novel approach utilizing tunnel junction (TJ) to realize GaN-based distributed feedback (DFB) laser diodes (LDs). Thanks to the use of the TJ the top metal contact is moved to the side of the ridge and the DFB grating is placed directly on top of the ridge. The high refractive index contrast between air and GaN, together with the high overlap of optical mode with the grating, provides a high coupling coefficient. The demonstrated DFB LD operates at λ=450.15 nm with a side mode suppression ratio higher than 35dB. The results are compared to a standard Fabry-Perot LD.

Resolution Improvement in a Multi-Point Fiber Refractometer Based on Coherent-OFDR

In this letter, we propose a signal processing method for improving the resolution of a multi-point fiber refractometer from 10 -4 to 10 -6 . The fiber refractometer uses the Fresnel reflection at the fiber tip to measure the refractive index (RI), based on the coherent optical frequency domain reflectometry (C-OFDR) technique; using a standard four-pin coaxial pigtailed DFB diode laser as the optical source. The proposed method is divided into two steps; first the raw signal is processed to reduce the nonlinearity generated by the optical frequency sweeping laser, and then a pattern that repeats itself in the sensing signal is identified, extracted and replicated several times to improve the sensing signal used to calculate the RI. In the experimental results, using 30 ms of effective acquisition time, and the proposed method, an improvement of two orders of magnitude in the resolution of the system was obtained.

Two-wavelength thermal–optical determination of light-absorbing carbon in atmospheric aerosols

Abstract. Thermal–optical analysis is widely adopted for the quantitative determination of total (TC), organic (OC), and elemental (EC) carbon in atmospheric aerosol sampled by suitable filters. Nevertheless, the methodology suffers from several uncertainties and artifacts such as the well-known issue of charring affecting the OC–EC separation. In the standard approach, the effect of the possible presence of brown carbon, BrC, in the sample is neglected. BrC is a fraction of OC, usually produced by biomass burning with a thermic behavior intermediate between OC and EC. BrC is optically active: it shows an increasing absorbance when the wavelength moves to the blue–UV region of the electromagnetic spectrum. Definitively, the thermal–optical characterization of carbonaceous aerosol should be reconsidered to address the possible BrC content in the sample under analysis. We introduce here a modified Sunset Lab Inc. EC–OC analyzer. Starting from a standard commercial instrument, the unit has been modified at the physics department of the University of Genoa (Italy), making possible the alternative use of the standard laser diode at λ=635 nm and of a new laser diode at λ=405 nm. In this way, the optical transmittance through the sample can be monitored at both wavelengths. Since at shorter wavelengths the BrC absorbance is higher, a better sensitivity to this species is gained. The modified instrument also gives the possibility to quantify the BrC concentration in the sample at both wavelengths. The new unit has been thoroughly tested, with both artificial and real-world aerosol samples: the first experiment, in conjunction with the multi-wavelength absorbance analyzer (MWAA; Massabò et al., 2013, 2015), resulted in the first direct determination of the BrC mass absorption coefficient (MAC) at λ=405 nm: MAC =23±1 m2 g−1.

True-blue laser diodes with tunnel junctions grown monolithically by plasma-assisted molecular beam epitaxy

We demonstrate true-blue 450 nm tunnel junction (TJ) laser diodes (LDs) grown by plasma-assisted molecular beam epitaxy (PAMBE). The absence of hydrogen during PAMBE growth allows us to achieve TJs with low resistance. We compare TJ LDs with LDs of standard construction with p-type metal contact. For both types of LD, the threshold current density is around 3 kA/cm2 and the slope efficiency is 0.5 W/A. We do not observe any significant changes in optical losses and differential gain in TJ LDs compared with standard LDs. The differential resistivity of the TJs for current densities higher than 2 kA/cm2 is below 10−4 Ω·cm2.

Micromechanical Resonator Driven by Radiation Pressure Force

Radiation pressure exerted by light on any surface is the pressure generated by the momentum of impinging photons. The associated force – fundamentally, a quantum mechanical aspect of light – is usually too small to be useful, except in large-scale problems in astronomy and astrodynamics. In atomic and molecular optics, radiation pressure can be used to trap or cool atoms and ions. Use of radiation pressure on larger objects such as micromechanical resonators has been so far limited to its coupling to an acoustic mode, sideband cooling, or levitation of microscopic objects. In this Letter, we demonstrate direct actuation of a radio-frequency micromechanical plate-type resonator by the radiation pressure force generated by a standard laser diode at room temperature. Using two independent methods, the magnitude of the resonator’s response to forcing by radiation pressure is found to be proportional to the intensity of the incident light.

Transected sciatic nerve repair by diode laser protein soldering

Background and ObjectiveDespite advances in microsurgical techniques, repair of peripheral nerve injuries (PNI) is still a major challenge in regenerative medicine. The standard treatment for PNI includes suturing and anasthomosis of the transected nerve. The objective of this study was to compare neurorraphy (nerve repair) using standard suturingto diode laser protein soldering on the functional recovery of transected sciatic nerves. Study Design/Materials and MethodsThirty adult male Fischer-344 Wistar rats were randomly assigned to 3 groups: 1. The control group, no repair, 2. the standard of care suture group, and 3. The laser/protein solder group. For all three groups, the sciatic nerve was transected and the repair was done immediately. For the suture repair group, 10.0 prolene suture was used and for the laser/protein solder group a diode laser (500mW output power) in combination with bovine serum albumen and indocyanine green dye was used. Behavioral assessment by sciatic functional index was done on all rats biweekly. At 12weeks post-surgery, EMG recordings were done on all the rats and the rats were euthanized for histological evaluation of the sciatic nerves. The one-way ANOVA test was used for statistical analysis. ResultsThe average time required to perform the surgery was significantly shorter for the laser-assisted nerve repair group compared to the suture group. The EMG evaluation revealed no difference between the two groups. Based on the sciatic function index the laser group was significantly better than the suture group after 12weeks (p<0.05). Histopathologic evaluation indicated that the epineurium recovery was better in the laser group (p<0.05). There was no difference in the inflammation between the suture and laser groups. ConclusionBased on this evidence, laser/protein nerve soldering is a more efficient and efficacious method for repair of nerve injury compared to neurorraphy using standard suturing methods.

High power fiber lasers in the SWIR band using Raman lasers

Identifying the scope for high power laser sources around 1.6 μm band in LIDAR and free space optical communication applications, because of high atmospheric transparency in this band, we have built a high power laser that generates >25 W of output power at 1570 nm using sixth-order cascaded Raman amplification of a low power, Erbium–Ytterbium seed. A high power Ytterbium doped fiber laser operating at 1117 nm generating >100 W CW output power was built and used as the primary laser source for Raman laser experiments. We also demonstrated a novel, drive scheme for standard laser diode modules (without wavelength locking) used for pumping rare-earth doped lasers and amplifiers. This scheme improves the pumping efficiency and reliability of the laser without increasing nonlinearity in a cost effective manner.

Microchip laser converter based on InGaN laser diode and (Zn)CdSe quantum dot heterostructure

A high-efficiency microchip violet–green laser converter based on a molecular-beam epitaxy grown green-emitting (Zn)CdSe quantum dot (QD) II–VI laser heterostructure, optically pumped by emission of a cheap violet InGaN laser diode (LD) was demonstrated. The active region of the II–VI laser heterostructure consisted of three electronically coupled (Zn)CdSe QD sheets in a single ZnSe quantum well (QW) placed asymmetrically inside a graded-index ZnMgSSe/ZnSe superlattice optical waveguide. The cavity length of the cleaved-facet II–VI laser was 130 μm which is a nearly optimal value for minimizing the excitation power. InGaN LD radiation was focused into a narrow stripe on the surface of the II–VI laser by a microlens optical system. The converter showed laser emission at 541 nm with threshold excitation power of ∼0.5 W. The maximal output pulse power and conversion efficiency of 1.5 W and ∼15%, respectively, were achieved. The device was mounted in a standard TO-18 LD package. Photograph of the operating violet–green microchip laser converter.

Three stage cascade diode lasers generating 500 mW near 3.2 μm

GaSb-based type-I quantum well 3.1–3.2 μm diode lasers with two- and three-cascade heterostructures were designed, fabricated, and characterized. Devices with ∼100-μm-wide aperture, 3-mm-long cavity, and anti-/high-reflection coated mirrors demonstrated continuous wave output power of 500 mW, threshold current density of ∼200 A/cm2, and peak power conversion efficiency of ∼7% at 17 °C. This corresponds to more than twofold improvement in terms of output power and efficiency as compared to standard diode lasers operating in the same spectral region. The experiment showed that the increase in the number of cascades from two to three led to critical enhancement of the differential gain and reduction of the threshold current density. Light p-doping of the AlGaAsSb graded section did not introduce extra optical loss but aided hole transport as required for realization of the efficient multi-stage cascade pumping scheme.

Output characteristics of diode laser array by three different spectral beam combing structures

Based on spectral beam combining technology, with diffraction grating and external cavity feedback, each element of an array of diode laser is locked at different wavelengths in a narrow linewidth by inserting a beam shaping system. And the output beam is an approximately parallel light along the direction of combination. Beam quality improvement and linewidth narrowing of an array of diode laser are achieved. The standard diode laser array used in our experiment has 19 elements, with a width of 100 m and a period of 500 m for each element. Experiment is conducted on different spectral beam combing setups, including spectral beam combing after collimation in both fast and slow axes, spectral beam combing after beam shaping and beam combing in an external cavity with narrow-line width. The combining output beam has the same beam quality as that of a single emitter and the linewidth of such a beam is narrow. The experiment results are analyzed.

Beam shaping of laser diode stacks for compact and efficient illumination devices at the French-German Research Institute of Saint-Louis

Abstract Laser diode stacks are applied for the realization of compact and efficient laser illumination devices for active imaging (e.g., laser gated viewing). An efficient use of illumination power and sensor arrays has to adapt the laser illumination to the sensor’s field of view (FOV) with a perfectly matched and homogeneous illumination field. In the past decades, ISL has studied different methods for beam shaping and homogenization of laser diode stacks. In this publication, the authors give a review of the development of different beam-shaping techniques for auto-stack, minibar stack in a specific configuration and standard laser diode stacks. The presented methods are based on the application of wedge-type waveguides, fast axis and slow axis collimation, and the rearrangement of the laser beams by polarization overlapping and virtual restacking. All presented methods have very high transmission efficiencies (&gt;80%) and lead to a homogeneous illumination matched to the sensor’s field of view.

The laser-diode-excited 5d-4f luminescence of Ce3+ and Pr3+ ions embedded into a BaR2F8 matrix

We show the possibility of obtaining UV luminescence from 5d-4f transitions of rare-earth ions in the BaY2F8: (Yb3+, Pr3+, Ce3+) crystal under upconversion excitation by standard laser diodes with lasing wavelengths of 960, 808, and 840 nm. Various upconversion mechanisms of pumping for populating the higher-lying energy levels of the active ions, as well as methods of adaptation of the active medium BaY2F8: (Yb3+, Pr3+, Ce3+) to these mechanisms, are considered.

Frequency-domain optical probing of coherent spins in nanocrystal quantum dots

Spin-photon interactions such as the Faraday effect provide techniques for measuring coherent spin dynamics in semiconductors. In contrast to typical ultrafast pulsed laser techniques, which measure spin dynamics in the time domain with an intense, spectrally broad probe pulse, we demonstrate a frequency-domain spin-photon resonance effect using modulated continuous-wave lasers which enables measurement of GHz-scale coherent spin dynamics in semiconductors with minimal spectral linewidth. This technique permits high-resolution spectroscopic measurements not possible with ultrafast methods. We have employed this effect to observe coherent spin dynamics in CdSe nanocrystals using standard diode lasers. By fitting the results to the expected model, we extract electron g-factors, and spin coherence and dephasing times in agreement with time-domain measurements.

Alternative configuration for an optical ring resonator angular rate sensor by using a standard laser diode

In this work, we present an alternative approach to angular velocity optical sensing based on two-ring resonators. This configuration admits the use of a standard laser diode source (0.1 nm, 10,000 MHz, FWHM) reaching higher sensitivities when narrow spectral laser sources (1 MHz, FWHM) are used. We compare this configuration with the standard single-ring resonator angular rate sensor (SRARS), which must use a narrow laser at input. Finally, we conclude that the sensitivity of this new approach can also be enhanced by coupling high-power broadband laser sources in a large range (from 1°/h to 10,000°/h), reaching performance similar to that of a standard SRARS configuration.

Nonlinear Identification of Laser Welding Process

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> It has been well recognized that weld pool geometry plays a critical role in fusion welding process such as laser welding. In this study, the authors establish a standard diode laser welding system and perform a series of experiments to investigate correlations between welding parameters and the weld pool geometry. Custom digital camera for image acquisition and software for image processing are implemented in the system to obtain the surface width of the weld pool. Experimental data has demonstrated significant nonlinearity in the diode laser welding process. The authors thus propose a continuous Hammerstein identification methodology to approximate this process. A single-input–single-output (SISO) nonlinear continuous model is then identified and validated for the diode laser welding process using the experimental data. Because by nature laser welding is a heat transferring process in which the heat applied to a unit length of the work-piece along the weld seam is inversely proportional to the travelling speed, the model takes the reciprocal of welding speed as the input and the top surface width as the output. For this methodology, all experiments need to be dynamic and be conducted using either Step or PRTS (pseudo-random ternary signal) inputs. In a revised and simplified version, the linear dynamics is identified first from a single dynamic experiment and then used in the identification of the nonlinearity with other static response experiments. The validation proves both identification methods are capable of predicting the surface width of the weld pool. </para>