To study the influence of the carrier density on the lifetime of broad area high power diodes three different epitaxial structures (double and single quantum wells, different confinement factors) were investigated. The fabricated lasers have similar emission wavelengths of 880 nm and 890 nm, the same lateral design and identical facet coatings. Because of the different vertical structures the lasers have different threshold current densities and therefore different carrier densities during operation. The experimental results show that the lifetimes depend strongly on the carrier densities. The measured catastrophical optical mirror damage (COMD) levels and the facet temperatures show the same dependence. The results achieved are explained by a theoretical model for additional heat generation at the facets in comparison to the bulk material. The calculations show a proportional relationship between the heat generation, leading to additional degradation mechanism, and the carrier density a few diffusion lengths away from the facet.
We report on the potential of the photocurrent technique as analytical tool for diode laser testing. The physics involved into the generation of photocurrents as well as experimental requirements for detecting them are highlighted. Based on a number of practical examples, we demonstrate how knowledge about the photoelectrical properties of diode lasers can help to learn about stress and defects within packaged devices or how non-perfect device fabrication may be discovered. These results are discussed in conjunction with device reliability issues.
Thermal effects are of fundamental importance in the cw operation of both gain- and index-guided VCSELs. At today, the actual temperature of operating devices is usually estimated from the spectral shift of the laser emission. This method only probes the temperature distribution averaged over the whole cavity volume and cannot provide spatially resolved information across the VCSEL cross- section. One single experiment has been performed to locally measure the temperature distribution in VCSEL by using a thermal scanning microscope. However, such technique required the cleaving and re-processing of the device. We present a new non-invasive technique to map the temperature of operating VCSELs that can be used to test devices at the wafer level. The method is based on the analysis of the spontaneous emission transmitted through the DBR mirrors. While the sample is temperature stabilized and held onto a xy piezo stage, it is scanned across with an optical microscope (achieving 1 micrometers spatial resolution). The signal is spectrally resolved and analysed by a CCD. By comparing the spectra taken under cw and pulsed current injection, the temperature contribution to the emission lineshape can be extracted straightforwardly. We demonstrate this technique by mapping the temperature rise of a broad area proton implanted VCSEL.
In this paper, we present the results of a preliminary investigation on the reliability of high power optical diodes. Commercially available 970 nm optical diodes were subjected to various levels of stress, including: operating current, optical power and operating temperature. Optical diodes that failed during testing were subsequently analyzed using a variety of techniques, including: optical microscopy, scanning electron microscopy, eletroluminescence, and near-field profiling. It has been observed that the major cause of optical failure can be attributed to damage on the emitting facet of the optical diodes. Preliminary evidence suggests that facet damage is a result of catastrophic optical damage.
High spatial resolution infrared (3-5 and 8-12 micrometers ) scanning microscopy study shows that light and heat signatures of conventional planar light emitting diodes for 3-5 micrometers spectral range. (InGaAs, InAs, InAsSb) are drastically affected by current crowding effect. As a result, Joule heating causes unavoidable heat traps in the vicinity of point contact (those are most pronounced in substrate down structures), whereas large uniform emitting areas are difficult to produce. Contrary to this, emitters based on magnetoconcentration effect (InSb) are free of current crowding and could be made of larger areas (of some mm2). For the diode stripe (width of 100 micrometers ) lasers (AlGaAs/GaAs, InGaAsP) we show that heat concentrates at lateral stripe sides that are difficult to penetrate. Some details of an infrared micromapping system characterized by 20 micrometers spatial resolution and 10 microsecond(s) time resolved interval are also given.
We report on specular reflectivity measurements at the position of the waveguide at front facets of commercial diode laser arrays. Since the waveguide thickness is such semiconductor structures amounts about 1 micrometers an even better spatial resolution of the probe light spot is required. For this purpose, a micro-reflectance setup was designed and implemented. For re-locating the optically active region, e.g. after stepped-up operation time, we employ the photosensitivity of the active region by using the photocurrent induced by the probe beam for auto- alignment of the setup. We show for coated InGaAlAs/GaAs- single chip devices that during long-term operation the diode laser front facet reflectivity at the position of an emitter is almost constant with a slight tendency (about 0.002 at 633 nm) to increase. The results are explained in the framework of defect-induced refractive index changes within the semiconductor material close to the interface between waveguide and facet coating.
Ag-Sb-Te alloy and films are developed as Optical recording material based on amorphous-crystalline phase transformation. The crystallization process of Ag-Sb-Te films is systematically studied through measurement of recording characteristics to solve the trade off problem between data stability and erasing sensitivity. Phase change optical recording disks have been found to demonstrate long thermal stability of the amorphous recording marks. In the present work, preparation and characterization of the chalcogenide allow Agx - Sb2(1-x) - Te3(1-x) with different composition (xequals0.16, 0.18 and 0.20) has been presented. Samples were prepared using melt quenching technique and the films were grown by thermal evaporation system. The crystallization process of Ag-Sb-Te material was studied using differential thermal analysis (DTA) and Optical analysis (Transmittance and reflectance) respectively. The films were studied for both cases: before and after annealing. The Differential thermal analysis curves were recorded for different compositions and Glass transition temperature (Tg), crystallization temperature (Tc) and melting temperature (Tm) have been obtained. It may also be concluded that Tg/Tm ratio is closer to required condition for the phase change optical data storage material. Thermal and optical analysis shows that the Ag-Sb-Te material is a potential candidate for phase change optical data storage. The optimized composition has also been obtained.
We describe a method for generating an amplitude modulated optical wave in the range of telecommunication frequencies. The principle is based on optical heterodying of two free DFB lasers. Frequency modulation of the optical wave resulting from beat is equal to the difference of the two laser wave frequencies. Control in temperature of laser sources ensure their wavelength stability. We theoretically study and also simulate the influence of the spectral width of the laser rays on spectral purity of the generated signal in a quadratic receiver. We then describe the experimental set up which permits to obtain a tunability of the source up to 275 GHz. We present the results obtained in reception by beat of two identical lasers in a 60 Ghz bandwidth photodiode. The signal generated in quadratic receiver is in terms of spectral width (FWHM) of about 37 MHz for a power level of - 35 dBm. Study of power and frequency stability of the generated signal in photodiode is carried out. This work permits to test large bandwidth photodiodes, it would also allow to characterize in bandwidth passive optical components.
Semiconductor lasers with high beam quality and high optical output power are very attractive for a variety of applications such as optical pumping of solid-state lasers, fiber amplifiers and medical treatment. When easy and low- cost fabrication is a further requirement, devices based on tapered gain sections are the most promising candidates. Low modal gain, single quantum well InGaAs/AlGaAs devices emitting at 940 nm were grown by molecular beam epitaxy. The lateral design consists of a tapered gain guided and a ridge-waveguide section having an overall length between 2 mm and 3 mm. Whereas the length of the tapered structure determines the high output power, the high brightness requires a ridge-waveguide structure with sufficient length. Here the length of the ridge section has been chosen to 500 micrometers . We achieved an optical output power of up to 5.3 W at room temperature in continuous wave mode. The threshold current density depends on the tapered length with values between 200 A/cm2 and 650 A/cm2. The slope efficiency is around 0.9 W/A for all devices. The wall plug efficiency reaches 44% at a current of 3 A. The beam quality factor remains nearly constant up to about 2.2 W having an M2-value of 1.3. At higher optical powers M2 increases fast. The lifetime of such devices has been extrapolated to more than 7500 h at room temperature.
High brightness is a major issue in most high-power diode laser applications. The brightness of a laser diode primarily depends on the optical power and the product of beam-quality factors in x- and y-direction, i.e. M2xx M2y. Even though there are many broad-area laser diodes which show an impressive brightness, all emitters lack a symmetric beam quality. In the direction perpendicular to the laser junction, the fast axis, the beam quality is perfect, showing an M2y-value of 1. Parallel to the junction, in the slow axis, the beam quality is much worse, usually having an M2x-value of 20- 100. For most pumping applications or fiber coupling, symmetrical beam parameters are required. Therefore usually the accessible brightness of the diodes is determined by the beam quality of the slow axis, wasting the perfect beam quality of the fast axis. We present a novel micro-optical beam shaping technique to symmetrize the beam quality of a broad area single emitter. This technique sacrifices the perfect beam quality in the fast axis while increasing the beam quality in the slow axis yielding (formula available in paper) This symmetrized beam can be coupled into a much smaller fiber/pump area than the direct beam from the laser diode. We report on some pumping applications using our beam- shaped, high-power diode-lasers as well as the benefits achieved by the symmetrized beam.
The power of laser diodes has increased in a spectacular fashion during recent years. The focus of this brief report will be to consider some of the problems associated with using high power diodes to pump fiber lasers and to discuss some of the schemes that have been proposed to overcome these problems. This is of particular importance at the present time because of recent interest in what might be termed extremely high power output. Over 100 W CW have already been obtained from a single Yb double clad fiber1, and elsewhere research is in progress to scale this up several orders of magnitude. At the present time, there are several sources of single mode fiber out put at 10-20 W, and one source is offering a single mode fiber laser of 100 W (IPG Polus)14. The Department of Defense has recently requested the submission of proposals for work that would lead to fiber lasers from 1 kW to 100 kW. This latter will clearly be concerned with the combination of multiple fiber lasers, and the aspect of coherent combination will not be considered in this presentation. It will be clear, however, that there is a need for design of fiber lasers that can efficiently utilize such very high diode pumping power.
The performance of interferometric fiber optical gyroscopes (IFOGs) has continued to advance resulting in expanded applications for both commercial and military inertial sensors. The primary advantages of the IFOG technology for inertial systems are the high reliability and lower cost. Most IFOG designs incorporate a fiber light source, a fiber sensing coil with discrete components connected, typically, with optical fiber pigtails. Fiber light sources require several optical components and are expensive to produce as well as bulky to package. The use of superluminescent diodes (SLDs) as a light source provides a much smaller, less expensive alternative and provides more flexibility in the integration architecture. The challenge for SLD development is the achievement of high power while maintaining the spectral quality and long lifetime. Presented here are the source requirements and the performance achieved for SLD's designed for these applications.
We investigate the effect of light source coherence length on longitudinal and en-face OCT images of the retina. The sources used in this comparative study are a superluminescent diode (SLD), a superliminescent diode fitted with an interference filter at its output and a tunable coherence length three-electrode laser device (3EL). We show that the use of sources of shortest coherence length is ideal for longitudinal OCT imaging. However, there are reasons for using adjustable coherence length sources for en-face OCT imaging. The effect of adjustable coherence length (and implicitly spectrum FWHM) on the achievable signal to noise ratio in the Oct is also presented. An increase in the coherence length enhances the excess photon noise but, at the same time, increases the signal collected from scattering tissue due to a larger thickness of the coherence gated backscattering layer in the target tissue. This suggests that the signal to noise ratio should not change with the light source coherence length. Nevertheless, the effect of light source coherence length change on the signal to noise ratio is more complex due to other noise sources in the system.
Superluminescent diodes (SLD) are high gain optical amplifiers whose output is the amplified spontaneous emission (ASE) of a current-pumped semiconductor laser gain medium. Because of the high gain (>40 dB), it is very important to reduce facet reflections to 10-6 or lower in order to maintain spectral modulation due to feedback from facet reflection below a few percents. Tilting the SLD waveguide at an angle with respect to the facets is an effective way to reduce facet reflection. However, the angle cannot be chosen arbitrarily, because the facet reflection does not decrease monotonically with angle. This is due to the geometry of the mode profile, which is a trigonometric function inside the waveguide and a decaying exponential in the outside of the region. It also depends on the stripe width and lateral index contrast of the structure. This paper will discuss the design criteria for achieving low spectral modulation in ordinary index-guided structures and will describe several applications of SLDs. It will also discuss the design of SLDs with output power of several hundred milliwatts using the so-called diamond or inverse bow-tie waveguide structure.
We summarize the results of development of superluminescent diodes (SLEDs) for OCT applications. It is shown that powerful, wide spectrum and low rippled SLEDs may be realized at different spectral bands from 800 nm to 1600 nm on the base of relatively simple bent-angled SLED structure with double-layer protective/antireflective (AR) coatings on both facets of SLED crystal. Up to 100 mW free space and 30 mW SM fiber couple outputs are realized at 820 and 980 nm bands, and 15-35 mW free space (5-15 mW SM fiber) at 1550 nm and 1300 nm bands. Various methods to widen SLED spectrum are presented. 50 nm of gaussian-like spectrum is obtained from 1040 nm SLD. 70 nm FWHM and 20 mW free space output power is realized at 940 nm by u sing two-electrode pumping of strained QW SLED. 70 nm FWHM is obtained from 1550 nm band MQW SLED with 5 mW output power. All SLDs are characterized by very low parasitic Fabry-Perot modulation. Example of SLED use in commercial OCT setup is presented.
Lifetime analysis of laser induced fluorescence by means of Time-Correlated Single Photon Counting (TCSPC) provides a powerful discrimination method to distinguish molecules of interest from background and other species. This has made the technique extremely valuable for sensitive analysis down to the single molecule level. We have developed the first complete range of compact picosecond to nanosecond excitation sources for fluorescence lifetime measurements based on laser diodes and LEDs. Using a common driver with interchangeable LED and laser heads the system is adaptable to almost all of the needs for sensitive chemical and biochemical analysis. The sources provide pulse durations under one nanosecond and repetition rates up to 80 MHz. These features qualify them for use in fast TCSPC applications, in particular where short data acquisition time is crucial. The sources can be used in combination with common inexpensive single photon detectors such as Photomultiplier Tubes and Single Photon Avalanche Photodiodes. Compact, low cost and easy to use fluorescence lifetime spectrometers can be built from these sources together with integrated TCSPC electronics. We will demonstrate the performance of the sources and complete systems in terms of power, repetition rate, stability, IRF and fluorescence decay fit quality in various setups and with different fluorescent materials.
For automobile interior, surface-mounted (SMT) LED's have replaced incandescent bulbs and through-hole LED's to backlight the dashboard, radio, displays, and switches for many years. For automobile exterior, high-flux surface- mounted LED's have started replacing the incandescent bulbs in signal lamps, such as center high mounted stop lamp, rear combination lamp and side markers. For these applications, surface-mounted LED's have certain advantages over through- hold LED's and other types of high-flux LED's. Surface- mounted LED's are assembled to the printed circuit board through high-speed automatic pick-and-place standard re-flow soldering processes, so its assembly is more reliable and more cost-efficient than that of through-hole and other types of LED's. Surface-mounted LED's are much smaller than through-hold and other types of LED's, therefore the dashboard or the signal lamps with surface-mounted LED's can be much thinner than with other types of LED's; the liquid crystal displays can be backlit uniformly by coupling the surface-mounted LED's light into a thin light pipe. Surface- mounted LED's can be assembled on the flexible printed circuit substrate, which can be bent like a plastic paper. This provides a solution of stylish 3-dimensional signal lamps, which is more flexible, and more cost-effective than through-hole and other types of LED's can do.
A new advanced infrared-based analyzer was developed to perform fast and reliable analysis of ultra trace gas impurities. The complete analytical system, the MTO-1000, is capable of measuring moisture to less than 200 parts-per- trillion (PPT) and perform single species detection of other ultra trace gas impurities such as NH3, CO, Hcl, HF and CH4 to parts-per-billion (PPB) and sub-PPB concentrations. Trace moisture calibration data will be presented to demonstrate the speed of response, sensitivity and accuracy of infrared Cavity Ring-Down Spectroscopy (CRDS). Emerging broadband capability to measure multiple trace gas species by CRDS technology will also be discussed.
Raman spectroscopy using a simple external Power Build-up Cavity (PBC) pumped with a diode laser was demonstrated. The PBC is very simple configuration consisted of anti- reflection coated low power (10mW, (lambda) equals670nm) laser diode, Graded index (GRIN) lens, and extremely high finesse external cavity, where the beam intensity can reaches up to 100W intracavity beam. Such a high finesse external cavity could optimized parameter attribute to high spectral brightness mostly suitable for a compact Raman light source without any sophisticated temperature and/or current injection control. PBC pumped Raman spectrum was measured by an optical fiber coupled optical multi-channel analyzer (OMA). The Raman signal from intracavity beam was imaged on the optical fiber by two focusing lenses (fequals38mm, 50mm) located in perpendicular to the optical axis. Stokes Raman spectra of N2 and O2 of gas mixture were simultaneously measured with real time operation, approximately.