External Modulation

For characterization purposes the most useful form of external modulation is electromodulation, because it provides the sharpest structure (third derivative of R in bulk or thin films) and is sensitive to surface or interface electric fields. The most widely used contacdess mode of electromodulation is termed Photoreflectance (PR) 5.7.8... 

User Authentication. User authentication is a vital required function, particularly for fully electronic systems, but one whose implementation varies widely from one company to another. Different companies may use different authentication mechanisms and technologies to validate their users, for example, Windows username, Oracle usernames, smartcards (such as SAFE-compliant cards provided by a trust authority), and biometrics. Typical enterprise ELN systems will delegate this authentication process to an external module that is customized to match the corporate IT infrastructure. 

Closing and Reopening of Experiments. Software interfaces can also be used to detect when an experiment is about to close and, before closing it, consult an external module that scans the experiment and other related information to make a decision on whether to accept and resume the closing operation or cancel out the closure. 

Figure 12 Schematic representation for reversibly changing drug permeation through membranes by external modulation. M, indicates the diffused amount of drug at time t. 

Frequently a modulation of light is introduced to the system in order to increase the signal to noise ratio. Flash lamps by their construction give pulses of light with repetition, which can be controlled by the user. Other lamps cannot be modulated through their driving current because the emitted radiation would be unstable over time. In this case, the application of an external modulator, e.g. a mechanical chopper, is the only solution. In both cases, the frequency of modulation is rather low - up to kilohertz. 

Monochromatic light can also be obtained from other types of lasers solid state, gas, ion, dye. Among them argon ion laser with its many lines is an especially valuable light source used in many sensors. However, these types of lasers are expensive, the modulation of the light cannot be done internally and external modulators (e.g. choppers) should be used. Wavelengths emitted by some exemplary lasers are presented in Table 1. 

The smaller the duty cycle the larger the number of harmonics and the less power available per modulation frequency of the excitation. 30 In this case care must be taken to not place to much power in the unused high-frequency region by decreasing the duty cycle of the excitation. A good rule of thumb for externally modulated sources is the following ... 

The excitation pulse tram is used to trigger of the excitation. Mode-locked lasers and cavity-dumped lasers are versatile excitation sources however, they are not ideal to implement portable instruments. Externally modulated solid state microlasers are a... 

Figure 9.12. Block diagrams of fiberoptic sensing instrumentation Top externally modulated laser with an acousto-optic modulator. (From Ref. 20 with permission.) Bottom internally modulated laser, (From Ref. 21 with permission.)...

With the advance of pharmaceutical science, it has been recognized that constant release is not the only way to maximize drug effectiveness and minimize side effects and that the assumptions used for constant release rate sometimes fail due to physiological conditions. From this perspective, zero-order dmg release is not acceptable in all cases and externally modulated or self-regulating dmg delivery systems have been developed as novel approaches to deliver dmgs as required. To realize such dmg delivery systems, it is important to constmct a system where the dmg itself senses environmental stimuli and responds appropriately to control the dmg release. For this purpose, the phase transition polymers have been intensively exploited as a candidate material during last decade [21]. 

Modulation can be accomplished in a variety of ways. For example, the output of a laser can be modulated directly or external modulation can be applied using electro-optic or electro-absorptive modulators. Currently, commercial electrooptic modulators are fabricated from lithium niobate. The performance characteristics of one family of lithium niobate modulators, produced by Lucent Technologies (Breinigsville, PA), are given in Table 1. 

The consequence of the long lifetime for a potential laser is that population inversion is easily achieved, but the gain will suffer. Furthermore, external modulation will probably be necessary for high frequency operation. 

The principal part of the pilot plant comprises a biological reactor (bioreactor) in whose interior the biomass is placed, and two external modules of ultrafiltration membranes placed in series. Figure 41.2 depicts a flow diagram of this pilot plant, which is described in more detail below. 

The top plate is connected to a laterally driven spring, of spring constant Xj, and to a spring X that is used to control the motion in the normal direction. f/(X, Z) is the effective potential experienced by the plate due to the presence of the embedded system, b is its periodicity, and ct characterizes the corrugation of the potential in the lateral direction. The parameters ri and are responsible for the dissipation of the plate kinetic energy due to the motion in the lateral and normal directions. In contrast to the traditional Prandtl Tomlinson model, here the dependence of U and ri on the distance Z between plates is taken into account. The detailed distance dependence is determined by the nature of the interaction between the plate and embedded system. As an example, we assume an exponential decrease of U and ri with a rate A as Z increases. The possibility of an external modulation of the normal load L (t) = X [Zp(f) — Z] is taken into account by introducing a time dependence into the position of the normal stage, Zp(f). 

FigureSvSS. fteqaeiKyreq>onseoffilBpHpfobeSNAFLr2iDe sured witii a Uue LED ( ) or widi an externally modulated argoiHion laser at 488 nm (+). From Ref. 1 IS. Rqmnled with permissiofi from the American Instiaite

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