Superluminescent Diode Research Articles

Enhanced spectrum superluminescent diodes fabricated by infrared laser rapid thermal annealing

Abstract We report on the fabrication of superluminescent diodes (SLD) from a graded bandgap quantum well intermixed (QWI) material obtained by an infrared laser rapid thermal annealing (IR Laser-RTA) technique. The processed semiconductor wafer consisted of an InGaAs/InGaAsP/InP (001) QW laser heterostructure originally emitting at 1.55 μm. The combined beams of a 150 W laser diode operating at 980 nm and a 30 W Nd:YAG laser operating at 1064 nm are used to heat the sample. While the laser diode is used for back-side heating of the wafer, the Nd:YAG laser beam is swept along the sample surface, resulting in temperature gradient changing in the direction perpendicular to the scan. This contactless RTA approach, allowed to obtain a graded bandgap material that was employed for the fabrication of SLD devices with a broadened emission bandwidth. The lasing effect in a series of 3 mm long broad area injection diodes was suppressed by tilting their facets by 7.5° with respect to the [110] direction. The best SLD devices had their FWHM (full-width-at-half-maximum) emission increased by 33% in comparison to the FWHM of 36 nm observed for devices made from the as grown material at an equal output power of 0.8 mW.

Comparison of VEP with contrast sensitivity and other measurements of central visual function

In order to evaluate alternative visual acuity testing techniques, especially to discriminate between small changes and for high visual acuity, we conducted a study covering several state-of-the-art techniques. In this cross-sectional study, a homogeneous cohort of healthy and young patients (n = 33; 66 eyes) underwent ETDRS vision acuity (VA) testing, testing for contrast sensitivity (CS), VA determination with spatial frequency sweep visual evoked potentials (VEP) and a series of examinations of perifoveal retinal nerve fibre layer thickness (RNFLT) using Spectralis SD-OCT. To simulate the effect of artificial media opacity, CS, and VEP were repeated with Bangerter foils. We found that Bangerter foils can be used to reduce VA effectively measured by VA testing and VEP VA. CS correlated significantly with VA (correlation coefficients ranging from 0.54 to 0.77). VEP may be used to estimate VA; nevertheless, we found no significant correlation. RNFLT did not correlate significantly with VA. CS seems to correlate well with VA when used for high VA. All other used examinations seem to have difficulties distinguishing between small differences in VA or when the VA is high.

Performance of Random Number Generators Using Noise-Based Superluminescent Diode and Chaos-Based Semiconductor Lasers

We investigate two optical sources used for random number generation: superluminescent diode (SLD) and semiconductor lasers. Amplified spontaneous emission noise is generated in the SLD and chaotic intensity fluctuation is generated in a semiconductor laser. We investigate the performance of random number generation for both optical sources. For single-bit generation of random numbers, the maximum generation rate is 8.33 Gb/s for both the SLD and the laser with a similar bandwidth of ~15 GHz. For multibit generation schemes, we obtain the generation rate up to 400 Gb/s for both the SLD and the laser. The overall characteristics are similar between the SLD and the laser, since similar bandwidths of the RF spectra are used. The probability density function of the SLD is more symmetric than that of the chaotic laser. This fact results in slightly good performance of random number generation using the SLD for multibit generation.

Hybrid Quantum Well/Quantum Dot Structure for Broad Spectral Bandwidth Emitters

We report a hybrid quantum well (QW)/quantum dot active element for an application in broadband sources. These structures consist of an InGaAs QW and six InAs dot-in-well (DWELL) layers. The single QW is designed to emit at a wavelength coincident with the second excited state of the quantum dot. We compare two hybrid QW/quantum dot samples where the QW position is changed, and show that carrier transport effects make QW placement very important through current-voltage, capacitance-voltage, photocurrent, and temperature-dependent spontaneous emission measurements. Using the optimal structure, due to the combined effects of quantum dot ground states, first excited state, and QW emission, a positive modal gain spanning ~300 nm is achieved for the segmented contact device. The values for modal gain are further confirmed by simultaneous three-state lasing, which is studied spectroscopically. Finally, a hybrid QW/quantum dot superluminescent diode (SLD) is reported; the device exhibits a 3 dB emission spectrum of 213 nm, centered at 1230 nm with a corresponding output power of 1.1 mW. The hybrid SLD is then assessed for an application in an optical coherence tomography system; an axial resolution of ~4 μm is predicted.

Senile retinoschisis versus retinal detachment, the additional value of peripheral retinal OCT scans (SL SCAN‐1, Topcon)

A peripheral senile retinoschisis is sometimes difficult to distinguish from a retinal detachment by biomicroscopy alone. This study evaluated spectral domain OCT (SD-OCT) scans of the far peripheral retina to improve the differentiation between these diseases. Patients were included in whom the distinction between retinal detachment and senile retinoschisis was not clear based on biomicroscopy alone, or who had a presumed clinical diagnosis of senile retinoschisis. OCT scans were made with the SLSCAN-1, through a hand-held lens and a 3-mirror contact lens, while the area of interest was simultaneously observed with the slit lamp. The SLSCAN-1 is a SD-OCT integrated into a slit lamp, superluminescent diode light source with a central wavelength of 830 nm, bandwidth 30 nm, scan resolution 8-9 μm in tissue, scan depth 2 mm, scan speed 5000 A-scans per second. In the 11 patients with an uncertain diagnosis, OCT scans showed a senile retinoschisis in four and a retinal detachment in seven patients. Fifteen of the 18 patients with a presumed diagnosis of retinoschisis showed a retinoschisis on OCT. In five of them, the OCT scans revealed a schisis detachment. Interestingly, three of the 18 patients had a retinal detachment. Senile retinoschisis may mimic a peripheral retinal detachment and vice versa. Differentiation is important with respect to therapeutic decisions. OCT scans of the far peripheral retina, which can be made with the SLSCAN-1 through a 3-mirror contact lens, can contribute significantly to this differentiation. The obtained scans of the far peripheral retina clearly visualize the anatomic differences between both diseases.

Blue Superluminescent Light-Emitting Diodes with Output Power above 100 mW for Picoprojection

We present a blue InGaN research and development superluminescent light-emitting diode (SLED) that is suitable for picoprojection. The SLED reaches an output power of >100 mW with a peak wavelength of 443 nm and a spectral bandwidth of >2.6 nm as well as a single-mode far-field driven in cw mode at 25 °C. In order to figure out an optimized waveguide design, which enables such a high output power at lowest operation current, we compare the performance of diodes with curved and tilted shaped ridges in detail, using the lasing threshold current as a criterion for lasing or superluminescence, respectively.

광간섭 단층 촬영 장치와 광음향 현미경의 결합을 통한 동시 이미지 획득 연구

In this study, we developed an integrated optical coherence tomography - photoacoustic microscopy (OCT- PAM) system to simultaneously provide optical absorption and scattering information. Two different laser sources, such as a pulsed laser for PAM and a superluminescent diode for OCT, were employed to implement the integrated OCT-PAM system. The performance of the OCT-PAM system was measured by imaging carbon fibers. We then imaged black and white hairs to demonstrate the simultaneous OCT-PAM imaging capabilities. As a result, OCT can produce 3-D images of both black and white hairs, whereas PAM is only able to image the black hair due to strong optical absorption of black hair.

Strain-imposed External Cavity Tunable Lasers Operating for NIR Wavelength

For demonstrating widely tunable external cavity lasers operating for near-infrared (NIR) wavelength, a flexible polymer waveguide with an imbedded Bragg grating is incorporated. Due to the superior flexibility of the polymer material, the reflection wavelength of the Bragg grating is widely tunable by imposing tensile and compressive strains on the flexible Bragg grating. A third-order Bragg grating is formed on the device for facilitating the fabrication method. With a superluminescent laser diode as a gain medium of ECL, the tunable laser exhibited output power of -3 dBm and a tuning range of 32 nm.

Tailored first- and second-order coherence properties of quantum dot superluminescent diodes via optical feedback

We demonstrate experimentally that the first- and second-order coherence properties of light emitted by a quantum dot superluminescent diode can be simultaneously tailored by well-controlled optical feedback. Depending on feedback intensity and feedback spectral range we achieve a spectral width Δλ between 120 and 0.26 nm, corresponding to a coherence length in first order in the range between 13 and 5820 μm, while the central second-order coherence degree g((2))(τ=0) is tuned gradually from a thermal value of g((2))(0)~1.8 down to the coherent laser limit of g((2))(0)=1.0. These results are complemented by comprehensive investigations of relative intensity noise, which are in excellent agreement with the observed intensity correlation behavior.

High-resolution fiber optic temperature sensors using nonlinear spectral curve fitting technique

A generic new data processing method is developed to accurately calculate the absolute optical path difference of a low-finesse Fabry-Perot cavity from its broadband interference fringes. The method combines Fast Fourier Transformation with nonlinear curve fitting of the entire spectrum. Modular functions of LabVIEW are employed for fast implementation of the data processing algorithm. The advantages of this technique are demonstrated through high performance fiber optic temperature sensors consisting of an infrared superluminescent diode and an infrared spectrometer. A high resolution of 0.01 °C is achieved over a large dynamic range from room temperature to 800 °C, limited only by the silica fiber used for the sensor.

A high-speed magnetic tweezer beyond 10,000 frames per second

The magnetic tweezer is a single-molecule instrument that can apply a constant force to a biomolecule over a range of extensions, and is therefore an ideal tool to study biomolecules and their interactions. However, the video-based tracking inherent to most magnetic single-molecule instruments has traditionally limited the instrumental resolution to a few nanometers, above the length scale of single DNA base-pairs. Here we have introduced superluminescent diode illumination and high-speed camera detection to the magnetic tweezer, with graphics processing unit-accelerated particle tracking for high-speed analysis of video files. We have demonstrated the ability of the high-speed magnetic tweezer to resolve particle position to within 1 Å at 100 Hz, and to measure the extension of a 1566 bp DNA with 1 nm precision at 100 Hz in the presence of thermal noise.

InAs/GaAs submonolayer quantum-dot superluminescent diodes with active multimode interferometer configuration

With a chirped InAs/GaAs SML-QD (quantum dot) structure serving as the active region, the superluminescent diodes emitting at wavelength of around 970 nm are fabricated. By using an active multimode interferometer configuration, these devices exhibit high continue-wave output powers from the narrow ridge waveguides. At continue-wave injection current of 800 mA, an output power of 18.5 mW, and the single Gaussian-like emission spectrum centered at 972 nm with a full width at half maximum of 18 nm are obtained.

Equivalence of displacement radiation damage in superluminescent diodes induced by protons and heavy ions

The degradation of optical power for superluminescent diodes is in situ measured under exposures of protons with various energies (170keV, 3MeV and 5MeV), and 25MeV carbon ions for several irradiation fluences. Experimental results show that the optical power of the SLDs decreases with increasing fluence. The protons with lower energies cause more degradation in the optical power of SLDs than those with higher energies at a given fluence. Compared to the proton irradiation with various energies, the 25MeV carbon ions induce more severe degradation to the optical power. To characterize the radiation damage of the SLDs, the displacement doses as a function of chip depth in the SLDs are calculated by SRIM code for the protons and carbon ions. Based on the irradiation testing and calculation results, an approach is given to normalize the equivalence of displacement damage induced by various charged particles in SLDs.

Interferometric homogeneity test using adaptive frequency comb illumination

The homogeneity test of glass plates in a Fizeau interferometer requires the measurement of the glass sample in reflected as well as in transmitted light. For the measurement in transmitted light, the sample has to be inserted into the ray path of a Fizeau or Twyman-Green interferometer, which leads to a nested cavity setup. To separate the interference signals from the different cavities, we illuminate a Fizeau interferometer with an adaptive frequency comb. In this way, rigid glass plates can be measured, and linear variations in the homogeneity can also be detected. The adaptive frequency comb is provided by a variable Fabry-Perot filter under broadband illumination from a superluminescence diode. Compared to approaches using a two-beam interferometer as a filter for the broadband light source, the visibility of the fringe system is considerably higher.

Reliability Improvement of superluminescent diodes emitting at 1.0 µm band using InGaAsP barrier structure

A report is presented on the reliability evaluation of broad-band InGaAs superluminescent diodes emitting at 1.0 µm band wavelength. The devices using the InGaAsP barrier layer showed better reliability characteristics owing to the low optical power density in the active region, compared to the devices with using the GaAs barrier layer. In the lifetime test under auto-power-control mode at the output power of 30 mW at 25 °C, a noticeable degradation was observed in the devices with the GaAs barrier, while the devices with the InGaAsP barrier have shown almost no degradation over 3000 h.

Graphics processing unit aided highly stable real-time spectral-domain optical coherence tomography at 1375 nm based on dual-coupled-line subtraction

We have proposed and demonstrated a highly stable spectral-domain optical coherence tomography (SD-OCT) system based on dual-coupled-line subtraction. The proposed system achieved an ultrahigh axial resolution of 5 μm by combining four kinds of spectrally shifted superluminescent diodes at 1375 nm. Using the dual-coupled-line subtraction method, we made the system insensitive to fluctuations of the optical intensity that can possibly arise in various clinical and experimental conditions. The imaging stability was verified by perturbing the intensity by bending an optical fiber, our system being the only one to reduce the noise among the conventional systems. Also, the proposed method required less computational complexity than conventional mean- and median-line subtraction. The real-time SD-OCT scheme was implemented by graphics processing unit aided signal processing. This is the first reported reduction method for A-line-wise fixed-pattern noise in a single-shot image without estimating the DC component.

Optical Monitoring of Tumors in BALB/c Nude Mice Using Optical Coherence Tomography

We report a method for optical monitoring of tumors in an animal model using optical coherence tomography (OCT). In a spectral domain OCT system, a superluminescent diode light source with a full width of 66 nm at half maximum and peak wavelength of 950 nm was used to take images having an axial resolution of 6.8 μ m. Cancer cells of PC-3 were cultured and inoculated into the hypodermis of auricle tissues in BALB/c nude mice. We observed tumor formation and growth at the injection region of cancer cells in vivo and obtained the images of tumor mass center and sparse circumferences. On the 5 th day from an inoculation of cancer cells, histological images of the tumor region using cross-sectional slicing and dye staining of specimens were taken in order to confirm the correlation with the high resolution OCT images. The OCT image of tumor mass compared with normal tissues was analyzed using its A-scan data so as to obtain a tissue attenuation rate which increases according to tumor growth.

Growth and spectral analysis of stacked quantum dots for broadband superluminescent diodes

Stacks of different quantum dot (QD) structures were adopted and characterized for broadband superluminescent diodes (SLDs). Each QD structure in the stacked QD structures exhibited a unique peak in the photoluminescence (PL) spectrum, showing ground-state peaks of ∼1272, ∼1185 and ∼1090 nm for InGaAs-capped InAs QDs, InAs QDs, and InGaAs QDs, respectively. Two SLD structures, one with the stack of InGaAs-capped InAs QDs, InAs QDs, and InGaAs QDs and the other with the stack of InGaAs-capped InAs QDs and InAs QDs, were grown and fabricated into devices. The SLDs with a stack of three different QD structures and with a stack of two different QD structures showed spectral bandwidths of 173.6 nm and 188.2 nm, respectively, in the electroluminescence (EL) measurements. The larger spectral bandwidth of the SLD with three different QD structures is thought to be due to the addition of the short-wavelength-emitting InGaAs QD structure. We analyzed the sub-peaks in the EL spectra of the SLD structures and suggested the stacking of QD structures to get broadband SLDs.

Temperature Measurement of Si Substrate Using Optical-Fiber-Type Low-Coherence Interferometry Employing Supercontinuum Light

We have measured the temperature of a Si substrate using an optical low-coherence interferometer employing supercontinuum light (SC). The accuracy of temperature measurement and the minimum measurable thickness of a layer are determined by the maximum resolving power of the optical path length of the medium in low-coherence interferometry, which depends on the coherent length defined by the spectrum profile and the wavelength of the light source. Low-noise, ultraflat, and highly coherent SC, generated using ultrashort laser pulses and optical fibers, was used as a light source. The wavelength dispersion of SC on the Si substrate was compensated by using a silicon mirror as a reference mirror, resulting in shaper interference waveforms of SC at the front and back surfaces of Si substrate than those of the superluminescent diode (SLD) light used as a conventional low-coherence light source. The measurement accuracy of the temperature using SC was improved to be ±0.4 °C from ±1.0 °C for the case of using the SLD. The temperatures of the Si substrate and SiO2 thin film were simultaneously measured using SC on an 800-µm-thick Si substrate with an 8.55-µm-thick SiO2 film. The temperature of the thin film, the thickness of which is several micrometers, was measured using SC and a compensation technique of wavelength dispersion using the silicon reference mirror.

Segmented superluminescent diode with linear power–current characteristics and adjustable radiation pattern

This study presents the modelling and experimental characterisation of 980 nm segmented-contact superluminescent diodes designed to produce linear power–current characteristics for applications requiring high output powers and readily reproducible operational characteristics, with adjustable beam width, radiation pattern and spectral width. The maximum measured output power was 160 mW at 4 A pulsed driving current, resulting in 2% wall-plug efficiency per pulse, with ripple-free broad-band output. Coupling into multi- and single-mode optical fibres was achieved with efficiencies of, respectively, 60 and 17%.