Multimode Diode Research Articles

Sub-30-fs Yb:CALGO laser oscillator based on cross-polarized multi-mode diode pumping.

Ytterbium (Yb)-doped materials are an excellent choice for efficient and powerful ultrafast lasers. They exhibit favorable emission properties, which include a low quantum defect and compatibility with cost-effective high-power pump diodes. While being strongly beneficial for efficiency, the low quantum defect is a challenge for operation in the few-cycle regime. In the standard collinear pumping scheme, the optical pump light passes a dichroic mirror, whose spectral transmission and dispersion properties can substantially perturbate the oscillating broad-band pulse. Recently, we introduced a broadband cross-polarization pumping scheme that avoids this issue and enables significant improvements in optical-to-optical efficiency and output average power of modelocked few-cycle Yb-bulk laser oscillators. However, our initial prototype relied on a high-power, diffraction-limited fiber laser for pumping. Here, we demonstrate that the broadband cross-polarization pumping scheme is also well-suited for cost-effective multimode diode pumping of Kerr-lens modelocked Yb-bulk lasers. In doing so, we developed a hard-aperture Kerr-lens modelocked Yb:CALGO bulk laser oscillator delivering ultrashort pulses in the sub-30 fs regime. We present two configurations, which both operate at 85 MHz repetition rate. The first delivers 22-fs pulses at 0.3 W of average output power with a 4% optical-to-optical efficiency, while the second produces 28-fs pulses at 0.5 W of average power with a 12% optical-to-optical efficiency. This study validates the high potential of cross-polarization pumping for cost-effective, high-efficiency Yb-lasers operating in the few-cycle regime, achieving shorter pulse durations and higher power levels than previously demonstrated for multimode diode pumping.

Super-resolved fluorescence imaging utilising accessible stochastic optical reconstruction microscopy (easySTORM) implemented on a low-cost, modular open-source (openFrame) microscope

Abstract Optical super-resolution microscope is a powerful tool for the life sciences, including cell biology and pathology yielding information on, e.g., biological structure and protein distribution in the nano-scale range. In recent years, there has been an exponential rise in the interest to apply super-resolution microscopes in diverse areas, but its wider utility is hindered by the cost of purchasing and maintaining super-resolved microscopes. In this paper we present the implementation of easySTORM, an accessible implementation of stochastic optical reconstruction microscopy (STORM) at Indian Institute of Technology Guwahati (IITG), India, in a flexible, user friendly and cost-effective manner using a state-of-the-art, modular, open-source, optical microscope platform called: "openFrame". Providing comparable imaging performance to commercial optical super-resolution microscopes, the openFrame-based implementation of easySTORM uses in-expensive multimode diode lasers and industry grade CMOS cameras, and the open-source and modular nature of the instrument makes it easy to maintain and to upgrade. To demonstrate its successful implementation at IITG, we image quantum dots and actin-tubulin structure in both normal and cancer cells, resolved features separated by a few tens of nanometers. This work demonstrates that openFrame-enabled easySTORM instrumentation can be widely accessible to provide affordable, research grade super-resolution microscopy capability for academic and medical research.

975 nm multimode semiconductor lasers with high-order Bragg diffraction gratings

The 975 nm multimode diode lasers with high-order surface Bragg diffraction gratings have been simulated and calculated using the 2D finite difference time domain (FDTD) algorithm and the scattering matrix method (SMM). The periods and etch depth of the grating parameters have been optimized. A board area laser diode (BA-LD) with high-order diffraction gratings has been designed and fabricated. At output powers up to 10.5 W, the measured spectral width of full width at half maximum (FWHM) is less than 0.5 nm. The results demonstrate that the designed high-order surface gratings can effectively narrow the spectral width of multimode semiconductor lasers at high output power.

Brightness enhancement on random-distributed-feedback Raman fiber lasers pumped by multimode diodes

Abstract The power scaling on short wavelength (SW) fiber lasers operating around 1 μm are in significant demand for applications in energy, environment and industry. The challenge for performance scalability of high-power SW lasers based on rare-earth-doped fiber primarily lies in the physical limitations, including reabsorption, amplified spontaneous emission and parasitic laser oscillation. Here, we demonstrate an all-fiberized, purely passive SW (1018 nm) random-distributed-feedback Raman fiber laser (RRFL) to validate the capability of achieving high-power output at SWs based on multimode laser diodes (LDs) direct pumping. Directly pumped by multimode LDs, the high-brightness RRFL delivers over 656 W, with an electro-optical efficiency of 20% relative to the power. The slope efficiency is 94%. The beam quality M2 factor is 2.9 (which is ~20 times that of the pump) at the maximum output signal power, achieving the highest brightness enhancement of 14.9 in RRFLs. To the best of our knowledge, this achievement also represents the highest power record of RRFLs utilizing multimode diodes for direct pumping. This work may not only provide a new insight into the realization of high-power, high-brightness RRFLs but also is a promising contender in the power scaling of SWs below 1 μm.

300-mW average power in 22-fs pulses from a multi-mode diode pumped Yb:CALGO oscillator

We present a multi-mode diode-pumped Yb:CALGO laser oscillator based on cross-polarization pumping. Using this method, we demonstrate 22-fs pulses at 0.3 W, which is the shortest duration for any Yb- based bulk laser oscillator utilizing multimode-diode pumping.

High-energy picosecond pulses with a single spatial mode from a passively mode-locked, broad-area semiconductor laser

We present a mode-locked semiconductor laser oscillator that emits few picosecond pulses (5-8ps at a repetition rate of 379MHz and wavelength of 1064nm) with record peak power (112W) and pulse energy (0.5nJ) directly out of the oscillator (with no amplifier). To achieve this high power performance we employ a high-current broad-area, spatially multi-mode diode amplifier (0.3×5mm), placed in an external cavity that enforces oscillation in a single spatial mode. Consequently, the brightness of the beam is near-ideal (M2 = 1.3). Mode locking is achieved by dividing the large diode chip (edge emitter) into two sections with independent electrical control: one large section for gain and another small section for a saturable absorber. Precise tuning of the reverse voltage on the absorber section allows to tune the saturation level and recovery time of the absorber, providing a convenient knob to optimize the mode-locking performance for various cavity conditions.

Brightness of AlGaInAs/InP Multimode Diode Lasers with Different Aperture Widths.

A set of semiconductor lasers with different stripe widths is fabricated based on the AlGaInAs/InP heterostructure with an ultra-narrow waveguide. The key characteristics of the lasers (light-current curves (L-I), current-voltage curves (I-V), and spectral and spatial characteristics) are measured, and their dependence on the stripe width is shown. The operating optical power increases from 1.4 W to 4.3 W; however, the lateral brightness decreases from 1.09 W/(mm*mrad) to 0.65 W/(mm*mrad) as the stripe width increases from 20 to 150 μm.

OpenFrame: A modular, sustainable, open microscopy platform with single-shot, dual-axis optical autofocus module providing high precision and long range of operation.

'openFrame' is a modular, low-cost, open-hardware microscopy platform that can be configured or adapted to most light microscopy techniques and is easily upgradeable or expandable to multiple modalities. The ability to freely mix and interchange both open-source and proprietary hardware components or software enables low-cost, yet research-grade instruments to be assembled and maintained. It also enables rapid prototyping of advanced or novel microscope systems. For long-term time-lapse image data acquisition, slide-scanning or high content analysis, we have developed a novel optical autofocus incorporating orthogonal cylindrical optics to provide robust single-shot closed-loop focus lock, which we have demonstrated to accommodate defocus up to ±37μm with <200nm accuracy, and a two-step autofocus mode which we have shown can operate with defocus up to ±68μm. We have used this to implement automated single molecule localisation microscopy (SMLM) in a relatively low-cost openFrame-based instrument using multimode diode lasers for excitation and cooled CMOS cameras.

Multi-frequency Doppler-free spectroscopy of cesium using an external cavity diode laser

We employed a different approach to develop multi-frequency saturated-absorption spectroscopy (SAS) involving both cesium hyperfine ground state levels using a multimode external cavity diode laser (ECDL), which could operate with neither another independent laser nor a modulator. The multi-frequency SAS is formed by atomic velocity groups on resonance with both of the laser modes from an ECDL in multimode operation, which are counterpropagated through the vapor cell as a quasicoherent pair of laser beams. A sign reversal of the sub-Doppler resonance under special pump–probe polarization with and without applied magnetic fields is observed. Simultaneously, the optical microwave generation of the multimode ECDL is also investigated experimentally. The free-running linewidth of the beat note spectra between two modes is about 475 Hz, which indicates a high coherence between them. This oscillator- and modulator-free approach provides a complementary scheme for existing optical microwave generation and has potential for improvements.

Improving performance prediction of diode end-pumped solid-state Nd:YAG rod amplifiers by incorporating pump mode evolution.

Master Oscillator Power Amplifier (MOPA) systems find extensive use in laser development to increase the optical power of laser emissions from a Master Oscillator (MO). Commonly used are the cylindrical rod MOPAs that are optically excited using a multimode fiber-coupled (FC) diode laser emission in an end-pumped configuration. Current analytical 3D models that incorporate thermal effects, gain saturation, and iterative Fourier beam propagation methods, collectively, rely on static approximations of the evolution of the FC pump beam profile over the longitudinal volume of the amplifier crystal. Furthermore, in general, the spectral behavior of the FC diode emission is assumed to be static, and the thermal wavelength shift is not accounted for in the simulation. In this work, we demonstrate a novel approach for accurate modeling of the multimode FC pump beam emission as a complex field using a phase-only Gaussian to flat-top (FT) diffractive optical element, thus allowing for the inclusion of the pump beam into the iterative propagation method. Additionally, we present a method for precise calibration of the model using simple experimental measurements of the diode emission spectrum. The theoretical model is experimentally validated using an end-pumped Nd:YAG crystal rod to perform single-pass amplification of a Gaussian beam, showing excellent agreement with predicted output powers over the calibrated range of pump powers. Furthermore, we provide experimental data that exhibits a strong correlation between the Gaussian to FT phase-only transformation and the multimode FC diode evolution in free-space propagation.

Theoretical analysis of time delay echoes in multimode semiconductor diode laser with optical feedback

Theoretical analysis of time delay echoes in multimode semiconductor diode laser with optical feedback

Terahertz Quasi-Time Domain Spectroscopy using a 808nm multimode diode laser

We report on a terahertz quasi-time domain spectroscopy (QTDS) system based on a low-cost continuous wave multimode diode laser. Commercially available low-temperature grown gallium arsenide (LT-GaAs) based photoconductive antennas (PCAs) with spiral and dipole configurations were used as emitter and detector, respectively. Terahertz pulses spaced at approximately 55 ps with a bandwidth of 400 GHz were obtained. Parametric measurements of the terahertz peak-to-peak intensity were performed by varying the injection current and temperature while maintaining incident laser power. The highest peak-to-peak intensity was obtained at 170mA injection current and 20° C temperature settings. The change in the THz peak-to-peak intensity is attributed to the mode hopping characteristics of the device which in turn, is dependent on injection current and temperature.

Continuous-Wave Bismuth-Doped Fiber Lasers with Multimode Diode Pumping

Continuous-Wave Bismuth-Doped Fiber Lasers with Multimode Diode Pumping

Clinical evaluation and assessment of laser irradiation on the stability of orthodontic implant - A prospective experimental study

ABSTRACT Background: Currently, orthodontic implants have reached a peak where they are considered a dependable modality to provide temporary supplemental anchoring in orthodontic therapy. When absolute anchoring is a necessity or in cases of minimally cooperative patients, these devices can help manage skeletal anchorage. However, its failure is a serious multi-factorial issue that happens during orthodontic treatment. The stability of the mini-implant is crucial to the outcome of orthodontic intervention. Approaches to increasing the stability of the mini-implant were researched. Hence, this study was carried out to compare and contrast and clinically assess the integrity of orthodontic implants over time. Subjects and Methods: Split mouth technique of treatment was carried out on 16 patients, i.e., one side of the mandible was considered as the experimental group (implant site irradiated with laser after placement), and the other was considered as the control side (implant site not irradiated with laser). Titanium mini-implants of the dimensions 1.5 mm diameter and 6 mm length were employed in the present study. They were positioned in the inter radicular space between the first molar and second premolar in the mandibular posterior region, 7 mm apical to the alveolar crest. During the whole process, the laser utilized was a multimode GaAs diode laser with a wavelength of 980 nm. It had 0.5–10 W output power which was adjustable with the frequency of 1–20 kHz and its main body input voltage was DC12 to further analyze the stability of the implant which in turn would aid in success assessment, the resonance frequency concept was utilized. The readings were recorded (T0) after insertion, (T1) 24 h after insertion, (T2) 2 weeks after insertion, (T3) 4 weeks after insertion, (T4) 6 weeks after insertion, and (T5) 8 weeks after insertion. The higher the implant stability quotient values the greater the stability and hence the optimal loading time. Results: The test employed for statistical analysis was Mann–Whitney U, Kruskal Wallis, and analysis of variance test. After analysis of all the readings, it was found that low-level laser therapy has a significant role in the stability of orthodontic mini-implant. Conclusion: The findings from this study suggest that low-level laser irradiation at the time of implant placement controls the inflammatory reaction around the implant and improves its stability.

Ultrafast Yb:CALGO laser oscillator based on cross-polarization pumping with a cost-efficient multi-mode diode

We demonstrate a multimode diode-pumped Yb:CALGO laser oscillator based on bandwidth-optimized cross-polarization pumping targeting sub-30-fs operation. In our first proof of principle experiment we achieved mode-locked operation at 83 MHz repetition rate with 0.4 W of average power and a 33-nm-bandwidth optical spectrum supporting sub-40-fs pulses. This concept offers a simple and cost-efficient alternative to green-pumped Ti:sapphire lasers.

Effect of external cavity length on the coherence properties in a multimode semiconductor laser

Investigation of the coherence properties of a multimode semiconductor diode laser subjected to an external optical feedback is reported. Suitable multimode Lang-Kobayashi rate equations are solved numerically for this investigation. Suppression of threshold reduction has been found to occur for relatively long cavity delay times and for relatively high modal operation. This investigation provides numerical evidence of the role of multimode operation on the laser coherence without having to use any explicit decoherence element in the external cavity.

Fiber-coupled light-emitting diodes (LEDs) as safe and convenient light sources for the characterization of optoelectronic devices.

Optoelectronic device characterization requires to probe the electrical transport changes upon illumination with light of different incident powers, wavelengths, and modulation frequencies. This task is typically performed using laser-based or lamp + monochromator-based light sources, that result complex to use and costly to implement. Here, we describe the use of multimode fiber-coupled light-emitting diodes (LEDs) as a simple, low-cost alternative to more conventional light sources, and demonstrate their capabilities by extracting the main figures of merit of optoelectronic devices based on monolayer MoS 2, i.e. optical absorption edge, photoresponsivity, response time and detectivity. The described light sources represent an excellent alternative for performing optoelectronic characterization experiments on a limited budget.

Broadly tunable (993-1110 nm) Yb:YLF laser.

We have investigated room-temperature continuous-wave (cw) lasing performance of Yb:YLF oscillators in detail using rod-type crystals with low Yb-doping (2%). The laser is pumped by a low-cost, high brightness, 10 W, 960 nm single-emitter multimode diode. Laser performance is acquired in both E//a and E//c configurations, using 12 different output couplers with transmission ranging from 0.015% to 70%. We have estimated the passive loss of the Yb:YLF crystal as 0.06% per cm, corresponding to an impressive crystal figure of merit above 4000. The low-doping level not only reduces the system losses but also minimizes the thermal load as the low doped crystals enable distribution of heat load in a greater volume. Using the advantages of lower loss and improved thermal behavior, we have achieved cw output power above 4 W, cw slope efficiencies up to 78%, and a record cw tuning range covering the 993-1110 nm region (117 nm). The output power performance achieved in this initial work is limited by the available pump power, and future room-temperature Yb:YLF systems have the potential to produce higher output power levels.

Semiconductor saturable absorber mirror mode-locked Yb:YLF laser with pulses of 40 fs.

We have generated pulses as short as 40 fs with an average power of 265 mW from a semiconductor saturable absorber mirror (SESAM) mode-locked Yb:YLF oscillator employing a 1% transmitting output coupler (OC). The room-temperature laser is pumped by a low-cost 960 nm single-emitter multimode diode and dispersion compensation is provided via double chirped mirrors (DCMs). The 40-fs pulses are centered around 1050 nm with a width of 34 nm at a repetition rate of 87.3 MHz. By increasing the output coupling to 5% and by using Gires-Tournois interferometer (GTI) mirrors for dispersion compensation, we have also demonstrated 380-fs pulses with 1.85 W of average power around 1025 nm at a repetition rate of 190.4 MHz. Using an intracavity off-surface optic axis birefringent filter, the central wavelength of the pulses could be tuned in the 1020-1025 nm and 1019-1047 nm ranges for the 5% and 1% transmitting OCs, respectively. To the best of our knowledge, these are the shortest pulses and highest average and peak powers generated from room-temperature Yb:YLF lasers to date.

Amplitude-Modulated Cavity-Enhanced Absorption Spectroscopy with Phase-Sensitive Detection: A New Approach Applied to the Fast and Sensitive Detection of NO2.

Accurate and sensitive measurements of NO2 play an extremely important role in atmospheric studies. Increasingly, studies require NO2 measurements with parts per trillion by volume (pptv-level) detection limits. Other desirable instrument attributes include ease of use, long-term stability, and low maintenance. In this work, we report the development of an amplitude-modulated multimode-diode-laser-based cavity-enhanced absorption spectroscopy (AM-CEAS) system operating at 406 nm that uses phase-sensitive detection for extremely sensitive NO2 detection. The laser was TTL-modulated at 35 kHz. The mirror reflectivity was determined to be 99.985% based on the ring-down time measurement. The cavity base length was 47.5 cm, giving an effective absorption pathlength of ∼3.26 km. AM-CEAS achieved a 1σ detection precision of 35 pptv in a 1 s data acquisition time (4.98 × 10-10 cm-1), over 4 times lower than that attained using a ring-down approach and the same optical system. The AM-CEAS precision improved to 8 pptv over a data acquisition time of 30 s (1.14 × 10-10 cm-1). The AM-CEAS method with the multimode diode laser integrates the advantages of high light injection efficiency like on-axis alignment cavity ring-down spectroscopy, low cavity-mode noise like off-axis alignment CEAS, and narrow-bandwidth high-sensitivity weak signal detection of modulation spectroscopy, providing a powerful, straightforward, and general method for ultrasensitive absorption and extinction measurements.