The current status as well as the history of reliability of the RF-excited CO2 waveguide lasers manufactured by Laakmann Electro-Optics, Inc. are outlined herein. Specific problems, such as gas leaks, outgassing, and optics damage, as well as our solutions, are discussed.
Two types of gas lasers, the helium-neon and the sealed-off, waveguide carbon dioxide, are discussed. Prior to hard seals in helium-neon lasers, failures due to contamination of the gas mixture had far outnumbered optical failures. Now that hard seals have been established, the failure rate constant associated with the lasing medium has been reduced below the failure rate constant associated with the optics. To accomplish the corresponding reduction in failures associated with the lasing medium of sealed-off, waveguide carbon dioxide lasers, it will be necessary not only to establish true hard seals but also to prevent a general dissociation of carbon dioxide by catalyzing the reverse reaction.
The real cost of He-Ne Lasers in systems is made up of many factors, only one of which is the actual purchase price of the laser itself. It behooves the system designer to thoroughly understand both the requirements of his system and the capabilities and limitations of the various lasers available in the market. Only in this way can the designer minimize the overall cost of his system and provide the best possible value to his customer.
The growing use of helium cadmium lasers in commercial applications is examined. Particular attention is paid to the reasons for selecting a low power cadmium laser over an argon ion laser for specific applications. Suitability of wavelength to the application, long term reliability and cost per milliwatt hour of use are considered. Life test statistics on the Liconix 10mwatt hecd laser tube are detailed.
From the perspective of the OEM user, the lifetime and reliability of lasers is discussed. Field failure symptoms are related to failure modes within laser systems. Hard and soft failure modes are discussed along with a description of normal failure probability statistics. Hard sealed integral mirror and brewster window plasma tube lifetime are compared.
Consistent performance and long lifetime in the space environment at high data rates are key issues in determining the applicability of semiconductor lasers for space communications. An understanding of the physical and optical characteristics of the laser diodes is required before failure criteria for this application can be stated. Many early life failures can be eliminated by a combination of quality screening and manufacturing process control. Essential device evaluation and failure analysis techniques are being developed to assure availability of long life diode lasers for this application.
We discuss the need for evaluating the performance of a semiconductor laser in terms of the requirements imposed by the system in which it is used. We find that the median life-time for 45 Mb/s optical transmitters, based on the criteria of average power drift and extinction ratio degradation, is on the order of 104 h. This result is well below the demonstrated cw median lifetime of over 105 h for the (AlGa)As multimoded lasers used in the transmitter. The processes leading to the shortened transmitter lifetime are traced to laser degradation. Circuit strategies which compensate for the effects of the aging of the laser are presented.
The verbatim transcripts of the Panel Discussion have been edited without altering the general conversational tone of the individual panelist's comments. The opinions expressed by individual panelists are solely their own and do not necessarily agree with, or represent, those of the organizations, government agencies, or companies with which they are affiliated.
A survey of the published literature on radiation effects in laser diodes and photodiodes as applied to space communications is presented. Laser diodes should be relatively hard to nuclear environments, especially if operated well above threshold, and should be quite hard to the natural environment. Photodiodes, on the other hand, may experience excess noise due to sustained ionization by Van Allen radiation, especially if this radia-tion is enhanced by a recent nuclear event. Avalanche photodiodes (APDs) are especially sensitive to sustained ionizing dose rate effects. Silicon p-i-n photodiodes are relatively insensitive to neutron damage in contrast to APDs which are considerably more vulnerable. Methods of improving the radiation tolerance of p-i-n photodiodes have been sucessfully implemented by employing double heterostructure junctions. Additional experimental work is needed for improved modeling of these devices.
High energy effects on fiber optic data systems generally fall into several categories, ranging from transient signal upset and burnout of its circuits to glass coloration and residual luminescence from the Cerenkov effect. These results occur from electron, proton, gamma, x-ray, and electromagnetic pulse (EMP) effects. These variations exist because of different operational system applications and nuclear weapon effects in variable, natural environments. Fiber optic systems have become very desirable for military application because of the increased volume of information to be transferred. The optical power source is an important component in fiber optic transmission system. The preferred type of transmitter is either a laser diode or a light emitting diode (LED) made up of gallium aluminum arsenide (GaAlAs/GaAs) materials. The selection of this source is based on its reliability, price, and compatibility with the rest of the components in the transmission system.
Exoatmospheric detonations of nuclear weapons produce a broad spectrum of effects which can prevent operational space missions from being successfully accomplished. The spacecraft may be exposed to the prompt radiation from the detonations which can cause upset or burnout of critical mission components through Transient Radiation Effects on Electronics (TREE) or System Generated Electromagnetic Pulse (SGEMP). Continual exposure to the trapped radiation environment may cause component failure due to total dose or Electron Caused EMP (ECEMP). Satellite links to ground and airborne terminals are subject to serious degradation due to signal absorption and scintillation. The ground data stations and lines of communications are subject to failure from the broad range effects of high-altitude EMP.
The effects of low energy proton irradiations (< 1.5 MeV) on GaAs CUT and pulsed laser diodes (RCA C30130 and Laser Diode Lab LD-60) operated at room temperature are reported in this study. A strong energy dependence is observed from 200 keV to 1 MeV which is the result of the nonuniform damage produced by protons. One potential complication is proton energy attenuation by surface materials (metallization, contacts, leads). A strong dependence on diode orientation is also observed---1.5 MeV protons incident parallel to the junc-tion plane and normal to the laser emission facet produce anywhere from a factor of ~ 5 to 50 times more damage than when incident normal to the broqd, currentinjecting face of the laser diode. Proton fluences as low as 10 to 1012 p/cm2 produced significant degradation in laser output power.
The effects of neutron irradiation on a variety of semiconductor laser diodes have been studied as a function of temperature between 200K and 320K. Total light output was measured as a function of current from well below threshold to the maximum operating point. The results show that the radiation sensitivity of all the laser types examined is less at lower temperatures. As long as the operating current is well above threshold following irradiation, the light output is relatively insensitive to neutron irradiation. In one of the laser types, neutron-enhanced operational degradation was also observed. Because of the complex structure of these laser diodes, it is difficult to associate particular device parameters with the degree of neutron sensitivity. However, in general the responses of the lasers to irradiation are similar. From the applied point of view, the results indicate that laser diodes with maximum operating currents well above their threshold currents are appropriate for radiation environment applications.*
Thin-film dielectric coatings for laser optics have been exposed to several types of radiation. The forms of radiation (UV, protons, and electrons) were chosen to deposit energy in the coatings and their substrates. Certain multilayer coatings for mirrors improved after irradiation in vacuum. Their absorptance near 2.7 micrometers in the infrared decreased after exposure to UV and visible light for as much as 1600 hours. The design (Al203/Si)2AgCrMo showed the most improvement, its absorption being roughly halved by this UV radiation exposure. Different changes were noted, however, in other samples of this design exposed to 25-keV protons and 40-keV electrons. Fluences were >1016 cm-2 and dosage >109 rad(Si). Similar results were obtained with the designs (SiOx/Si)nAgCrMo and (Al203/ZnS)4AgCrMo. This last design is not as stable as the other two, but was pursued as an improvement over its predecessors. The individual constituents of these coatings have also been irradiated in other experiments as single-layer films on CaF2 windows. Changes in absorptance and refractive index have been observed there. Attempts have been made to correlate the changes observed by laser absorption calorimetry, with results obtained in surface studies such as XPS. Compositional changes in some materials are implied.