Superluminescent Diode Research Articles

Simultaneous tomography and topography of silicon integrated circuits using full-field swept-source optical coherence tomography

We demonstrate simultaneous tomography and topography of micro-electro-mechanical systems based on silicon integrated circuits using full-field swept-source optical coherence tomography. The optical set-up consists of a swept-source system, a compact Michelson interferometer, and an area detector. The swept-source system comprises a superluminescent diode as a broadband light source in conjunction with an acousto-optic tunable filter as a fast frequency tuning device. By means of sweeping the frequency of the light source, multiple interferograms were recorded and both the amplitude and the phase map of the interference fringe signal were reconstructed. Optically sectioned images of the silicon integrated circuits were obtained by selective Fourier filtering and the topography was retrieved from the phase map. The main advantages of the proposed system are completely non-mechanical scanning, ease of alignment, high stability because of its nearly common-path geometry, and compactness.

Longitudinal bandgap modulated broadband (>150 nm) InGaAs/GaAs MQWs superluminescent diodes by selective area MOVPE growth

The characteristics of broadband superluminescent diodes (SLDs) are presented. The longitudinal bandgap modulated InGaAs/GaAs multiple quantum wells of broadband SLDs are grown by selective area metal organic vapour phase epitaxy (MOVPE) growth. The centre wavelength was 1060 nm. The 3 dB bandwidth was 130 nm and the 10 dB bandwidth was 174 nm. The output power was 0.3 mW.

Modeling of Broadband Chirped Quantum-Dot Super-Luminescent Diodes

We propose a novel non-excitonic computationally efficient numerical model based on rate equations to describe the spectral characteristics of quantum-dot (QD) super-luminescent diodes (SLDs). It takes into account the inhomogeneous gain broadening due to dot-size dispersion, the effect of QD layers emitting at different wavelengths, and the nonuniform longitudinal distribution of gain and optical power along the waveguide. The model is applied to evaluate the effectiveness of chirping the QD layers in improving the SLD spectral bandwidth and output power. Simulated and experimental results are in good agreement in terms of output power and spectral characteristics versus current. Finally, an example of the possibility to apply the model to optimize SLD performances in terms of spectral bandwidth and power is shown.

Simultaneous measurement of substrate temperature and thin-film thickness on SiO2/Si wafer using optical-fiber-type low-coherence interferometry

This paper proposes a technique for simultaneously monitoring the thickness of a SiO2 thin film and the temperature of a Si substrate. This technique uses low-coherence interferometry and has the potential to be used for online monitoring of semiconductor manufacturing processes. In low-coherence interferometry, when the optical path length of a layer is shorter than the coherence length of the light source, the two interference at the top and bottom interfaces of the layer overlap each other. In this case the detected peak position of the interference is shifted from the actual interface, resulting in an error in the temperature measurement, since the temperature is derived from the optical path length of the layer. To improve the accuracy of the temperature measurement, the effect of the overlapping interference was compensated by measuring the SiO2 thickness. The thickness of the Si substrate was 750 μm and the thickness of the SiO2 film was varied between 0 and 2 μm. The SiO2 thickness, which is shorter than the coherence length of the light source, was measured from the ratio of interference intensities of two superluminescent diodes (wavelengths: 1.55 and 1.31 μm). The measured ratio corresponded well with the theoretical one for SiO2 film thicknesses between 0 and 2 μm, and the error was less than 25 nm. The Si temperature was measured from the optical path length. In order to compensate for the overlapping interference, the shift in the peak position of the interference at the SiO2/Si interface was estimated from the measurement results of the SiO2 thickness. This improved the accuracy of the temperature measurement from 5.3 to 3.5 °C.

Detection and monitoring of early airway injury effects of half-mustard (2-chloroethylethylsulfide) exposure using high-resolution optical coherence tomography

Optical coherence tomography (OCT) is a noninvasive, high-resolution imaging technology capable of delivering real-time, near-histologic images of tissues. Mustard gas is a vesicant-blistering agent that can cause severe and lethal damage to airway and lungs. The ability to detect and assess airway injury in the clinical setting of mustard exposure is currently limited. The purpose of this study is to assess the ability to detect and monitor progression of half-mustard [2-chloroethylethylsulfide (CEES)] airway injuries with OCT techniques. A ventilated rabbit mustard exposure airway injury model is developed. A flexible fiber optic OCT probe is introduced into the distal trachea to image airway epithelium and mucosa in vivo. Progression of airway injury is observed over eight hours with OCT using a prototype time-domain superluminescent diode OCT system. OCT tracheal images from CEES exposed animals are compared to control rabbits for airway mucosal thickening and other changes. OCT detects the early occurrence and progression of dramatic changes in the experimental group after exposure to CEES. Histology and immunofluorescence staining confirms this finding. OCT has the potential to be a high resolution imaging modality capable of detecting, assessing, and monitoring treatment for airway injury following mustard vesicant agent exposures.

Tuning Superluminescent Diode Characteristics for Optical Coherence Tomography Systems by Utilizing a Multicontact Device Incorporating Wavelength-Modulated Quantum Dots

This paper details the development of broadband sources at 1050 nm for optical coherence tomography applications. A method for obtaining a broad and smooth emission and gain spectrum for 1050 nm quantum dot (QD) layers is presented. The design, fabrication, and operating characteristics of multicontact superluminescent diodes are then set out, and the operating characteristics of this device, incorporating the broadband QD material, are described. It is shown that this device allows the tuning of the emission spectrum peak position, peak shape, emission bandwidth, and power, which are advantageous for imaging applications. A simplified device, utilizing a single current source, is achieved by incorporating a resistor network. The operation of the multicontact device under various drive topologies is discussed.

Modified Fabry-Perot interferometric method for waveguide loss measurement

An extension to the Fabry-Perot interferometric method is demonstrated to calculate the optical loss and the reflectivity for optical waveguides simultaneously. The method uses an excitation of the waveguide with a broadband amplified spontaneous emission source (a superluminescent diode in our case) and curve fitting to account for the change of input power, thereby simplifying the measurement procedure. The use of a broadband source as opposed to tunable lasers allows for simultaneous measurements over multiple wavelengths and decreased sensitivity to reflections in the cavity. Further, waveguides of different lengths are measured to calculate the optical loss and the reflectivity simultaneously. It is shown that, if the value for reflectivity is assumed, there could be a large error in the measurement of loss especially for short waveguides. Optical loss for ridge waveguides is measured and compared by using a tunable laser as the input source. The method can be used for a generic case where it is suspected that the input power changes during the measurement.

Design Considerations for Asymmetric Multiple Quantum Well Broad Spectral Width Superluminescent Diodes

We investigated the design of broad spectral width super luminescent diodes (SLDs) with asymmetric multiple-quantum-well (AMQW) active regions. The design is based on theoretical modeling, which starts with calculation of the gain. Two InGaAsP/InP AMQW SLD structures centered at 1.3 mum were grown and processed. Both gave a broad spectral width of >80 nm when operated at the transition carrier density (TCD). Structure 1 was designed based on gain simulations only and was found to be able to provide broad spectral width only at low injection currents. Thus, Structure 1 gives low output power. Based on this experience, structure 2 was designed to achieve a broad gain at a higher TCD than Structure 1. Experimental results show that structure 2 provided a reasonable power and broad spectral width at the TCD. Thus, we conclude that, compared with normal design criterion which depends on the gain spectrum only, the design of an AMQW SLD has to include a relatively high TCD to obtain reasonable output power. The SLDs were ridge waveguide structures with seven degree tilted-stripes. Use of double section waveguides could further broaden the SLD spectrum at the red side by >10 nm.

In vivo imaging of dynamic biological specimen by real-time single-shot full-field optical coherence tomography

We demonstrate the feasibility of a compact single-shot full-field time domain optical coherence tomography (OCT) for imaging dynamic biological sample in real-time. The system is based on a Linnik type polarization Michelson interferometer and a four-quadrature phase-stepper optics, which can simultaneously capture four quadraturely phase-stepped interferograms on a single CCD. Using a superluminescent diode as light source with center wavelength of 842nm and spectral width of 16.2nm, the system yields an axial resolution of 19.8μm, and covers a field of view of 280×320μm2 (220×250 pixels) with a transverse resolution of 4.4μm by using a 10× microscope objective (0.3NA). Three-dimensional OCT images of biological samples such as an onion slice and a diaptomus were obtained without any image averaging or pixel binning. In addition, in vivo depth resolved dynamic imaging was demonstrated to show the beating internal structure of a diaptomus with a fame rate of 5fps.

Growth and fabrication of quantum dots superluminescent diodes using the indium-flush technique: A new approach in controlling the bandwidth

Broadband superluminescent diodes incorporating multiple layers of InAs quantum dots (QDs), where the dots height was deliberately varied from one layer to another have been grown and characterized. We used the indium-flush process to accurately control the emission energy of each layer of dots, enabling us to reliability and predictably engineer the bandwidth of the overlapped layers. Photoluminescence spectrum of four combined layers of QDs with full-width at half-maximum of 125 nm at peak wavelength energy of 1.06 μm was obtained. A 3 dB bandwidth emission spectrum of 80 nm and output power of 1 mW was obtained under CW operation mode at room temperature.

Numerical investigation of power tunability in two-section QD superluminescent diodes

We present the results of a numerical model for the simulation of quantum dot (QD) multi-section superluminescent diodes (SLD). Model parameters are obtained by fitting experimental PI characteristics of a single section SLD. We propose a method to tune the output power in a 2-section SLD by maintaining equal ground state and excited state contributions and therefore ensuring wide emission spectra.

Computations for the design of an integrated surface acoustic wave controlled semiconductor optical source

A superluminescent diode monolithically integrated with a surface acoustic wave Bragg-cell to operate as a beam deflector is proposed. The analysis procedures for determining the acoustic and optical modes, and modal diffraction of the optical beam are employed to develop a practical model for the integrated device. Results from this model are used to assess the operational characteristics of the proposed integrated device and to provide design guidelines.

High-speed, high-resolution Fourier-domain optical coherence tomography system for retinal imaging in the 1060 nm wavelength region

A high-speed (47,000 A-scans/s), ultrahigh axial resolution Fourier domain optical coherence tomography (OCT) system for retinal imaging at approximately 1060 nm, based on a 1024 pixel linear array, 47 kHz readout rate InGaAs camera is presented. When interfaced with a custom superluminescent diode (lambda(c) = 1020 nm, Deltalambda = 108 nm, Pout = 9 mW), the system provides 3.3 microm axial OCT resolution at the surface of biological tissue, approximately 4.5 microm in vivo in rat retina, approximately 5.7 microm in vivo in human retina, and 110 dB sensitivity for 870 microW incident power and 21 mus integration time. Retinal tomograms acquired in vivo from a human volunteer and a rat animal model show clear visualization of all intraretinal layer and increased penetration into the choroid.

High performance 1.3 μm InGaAsN superluminescent diodes

High performance 1.3 μm InGaAsN superluminescent diodes (SLDs) were fabricated with Schottky contact. The structure was grown by metal organic chemical vapor deposition (MOCVD). Output power of 3 mW was obtained in continuous wave (CW) mode at room temperature. The full width at half maximum (FWHM) of the emission spectrum was 30 nm. The devices operated up to 100°C.

Transverse-junction superluminescent diodes at the 1.1μm wavelength regime

We demonstrate, for the first time to our knowledge, GaAs-based transverse-junction (TJ) superluminescent diodes (SLDs) that operate at a wavelength of 1.1 microm. Due to lateral current injection by use of TJ, specified as transverse carrier flow spread in each quantum well horizontally instead of vertical well-by-well injection, nonuniform carrier distribution can be minimized among different multiple quantum wells (MQWs), which is a problem in vertical-junction (VJ) SLDs whose electroluminescent (EL) spectrum is governed by the center wavelength of QWs near the p side. In contrast with a VJ SLD, the EL spectrum of our device is determined by QWs that have a larger differential gain than the positions of QWs neighbored with a p side layer.

Wideband Infrared Emission from Yb3+- and Nd3+-Doped Bi2O3–B2O3 Glass Phosphor for an Optical Coherence Tomography Light Source

We have investigated the spectroscopic properties of Bi2O3–B2O3 glasses doped with Nd3+ and Yb3+ separately, and doubly doped with the same ions, in order to search for appropriate phosphors for an optical coherence tomography (OCT) light source. The effects of Yb3+ and Nd3+ co-doping and varying Yb3+ concentration on the spectrum shape were studied to obtain an optimum line shape for OCT. We successfully obtained a Gaussian-like shape spectrum from 5.0Yb2O3–1.0Nd2O3–46.0Bi2O3–46.0B2O3 (in nominal molar composition) with a full-width-at-half-maximum (FWHM) of 101 nm in the wavelength region around 1000 nm. This FWHM corresponds to a depth resolution of 4.5 µm, which is about twice that of conventional light emitting diodes (LEDs) and super luminescent diodes (SLDs). The Gaussian-like line shape is very important to suppress ghost images in OCT. These results indicate that a Yb3+, Nd3+ co-doped Bi2O3–B2O3 glass phosphor can indeed be applied as a new type of OCT light source.

Broadband Emitting Superluminescent Diodes With InAs Quantum Dots in AlGaAs Matrix

Superluminescent diodes were fabricated by using InAs-AlGaAs self-assembled quantum dots (QDs) as the active region. The ultrawide emitting spectrum of 142 nm was achieved. The short migration length of indium adatoms on AlGaAs surface increases the size dispersion of InAs QDs, resulting in the broadening of optical gain spectrum.

Validation of Spectral Domain Optical Coherence Tomographic Doppler Shifts Using an In Vitro Flow Model

To validate velocity measurements produced by spectral domain optical coherence tomography (SD-OCT) in an in vitro laminar flow model. A 30-mL syringe filled with skim milk was inserted into a syringe pump. Intravenous (i.v.) tubing connected the syringe within the pump to a glass capillary tube (internal diameter, 0.579 mm) shallowly embedded in agarose gel, then to a collection reservoir. SD-OCT imaging was performed with an anterior segment eye scanner and optics engine coupled with a 100-nm bandwidth broadband superluminescent diode. Scan density of 128 x 128 A-scans was spread over a 4 x 4 mm area, and each A-scan was 2 mm in length. Fifteen sequential stationary A-scans were obtained at each 128 x 128 position, and Doppler shifts were calculated from temporal changes in phase. The beam-to-flow vector Doppler angle was determined from three-dimensional scans. In all reflectance and Doppler images, a clear laminar flow pattern was observed, with v(max) appearing in the center of the flow column. Phase wrapping was observed at all measured flow velocities, and fringe washout progressively shattered reflectance and phase signals beyond the Nyquist limit. The observed percentages of the velocity profile at or below Nyquist frequency was highly correlated with the predicted percentages (R(2)=0.934; P=0.007). SD-OCT provides objective Doppler measurements of laminar fluid flow in an in vitro flow system in a range up to the Nyquist limit.

Cone mosaic imaging using an adaptive optics flood illumination camera system

Abstract Purpose The objective of this study was to develop an adaptive optics (AO) retinal imaging system and conduct in vivo pilot tests in a group of healthy eyes and in several pathological eyes. The experiments aimed at determining the device ability to image cone photoreceptors in presence of a variety of refractive errors. Methods We examined 20 healthy eyes and 3 eyes with inherited maculopathy using the instrument. The AO system was based on a 52‐actuator electromagnetic deformable mirror and a 1024‐lenslet Shack‐Hartmann wavefront sensor (respectively mirao52e and haso32e, both Imagine Eyes, France). The device was completed with a modified Badal arrangement in order to compensate for possibly large defocus errors. A super luminescent diode operating at 750nm was focused at the retina and the wavefront sensor. Additionally the retina was illuminated by a 3°x3°produced by an electroluminescent diode at 850nm. The center of this field was located at 3° off the fovea. The images were detected using a low‐noise CCD camera (Ropper Scientific, USA). Results Cone photoreceptor mosaics were imaged in the eyes with spherical refraction ranging between –7D and +4D and astigmatism up to 4.5D. The AO correction residual error ranged between 0.08 and 0.2µm in all tested eyes. The imaging resolution was estimated to be better than 5µm in all cases. The mosaic was visible over the entire 3°x3° field in several eyes. Conclusion It appeared that the retinal isoaplanetic field width was at least 3° experimentally in several tested eyes. This study demonstrated the feasibility of cellular retinal imaging in eyes with relatively large refractive errors using a compact AO instrument. Commercial interest

Broadband near-IR double quantum-well heterostructure superluminescent diodes

Superluminescent diodes (SLDs) based on double quantum-well (AlGa)As/GaAs heterostructures emitting between 800 and 900 nm are studied experimentally. These SLDs provide a high enough output power in the emission bandwidth more than 55 nm at much shorter active channel lengths than SLDs based on similar single quantum-well heterostructures. Despite the high injection current density, the service life of the SLDs exceeds 10000 hours.