External cavities

In practical application, Raman sensors exclusively use frequency-stabilised laser sources to compensate for the low intensity of the Raman radiation. For Raman sensors, prevalently compact high-intensity external cavity laser diodes are used, operated in CW (continuous wave) mode. These diode lasers combine high intensity with the spectral stability required for Raman applications and are commercially available at various wavelengths. 

R. Maulini, M. Beck, J. Faist, E. Gini, Broadband tuning of external cavity bound-to-continuum quantum-cascade lasers. AppZ. Phys. Lett, 84 (10) 1659 (2004). 

For example, a diode-laser sensor was applied to measure CO and CO2 concentrations in combustion gases using fast extraction-sampling techniques [8]. The sensor was based on an external cavity diode laser (ECDL) operating over the spectral region 6321-6680 cm which includes the i -branch of the CO band, the i -branch of the 2i/i + 2v2 + v, CO2 band, and selected lines of the... 

Furthermore, design modifications to the STR were also considered equipping the STR with marbles to ensure a larger distribution of the gel beads inside the reactor while damping down the shearing effect of the Rushton impeller and equipping the STR with a perforated stainless still cylinder that confines the beads in the external cavity between the reactor walls and the impeller zone. A description of the explained configurations is shown in Fig. 1. 

The STR was equipped with temperature, pH, and Po2 control units. Only one Rushton impeller was used to homogenize the system. Almost 1L of the culture medium supplemented with the carbon sources was used during the test runs. In the configuration with marbles, the total volume occupied by marbles (average diameter of 16 mm) was roughly 350 mL. In the configuration with the metal cylinder around the stirrer axle, the volume available in the external cavity was 565 mL. 

Figure 7.7. External cavity diode laser, such as the Spectra Diode Labs 8530. All components are contained in a 3 x 4 x 10 in. case, and the output power is 300 mW.

ECDL External Cavity Diode Laser OI Optical Isolator L Lens AOS Acousto-Optic Switch PZT Piezo-Electric actuator Me Cavity mirror APD Avalanche Photodiode PC Pressure Controller P Pump... 

One can obtain sufficient conditions for system (6.14) to have only one external cavity mode, i.e. a unique solution of (6.15). For this, we write the equation for w as... 

The saddle-node bifurcation, which gives rise to additional external cavity modes, can be identified [25] as a double root of (6.16). Hence, differentiating it with respect to cu, we obtain 1 = TT] a sin(i - - cut) — cos(v + cut)). It is clear that the condition... 

T. Heil, I. Fischer, W. Elsafier, B. Krauskopf, K. Green, and A. Gavrielides. Delay dynamics of semiconductor lasers with short external cavities Bifurcation scenarios and mechanisms. Phys. Rev. E, 67 066214, 2003. 

E. M. Shahverdiev, S. Sivaprakasam, and K. A. Shore. Parameter mismatches and perfect anticipating synchronization in bidirectionally coupled external cavity laser diodes. Phys. Rev. E, 66 017206, 2002. 

In the adsorption step, every external cavity is checked one by one. The molecule enters into the lattice if the chosen element is empty and the random number generated by a uniform distribution is less than the predetermined adsorption probability. In the diffusion step, each molecule in the system is associated with a velocity vector. The molecular diffusion was determined by jumps between elements of the network. In our simulations each kind of molecule had a probability to diffuse in the pore structure, with the values fixed to 1.0, 0.5 and 0.5 respectively for reactant, product, and precursor. Molecules can pass through an occupied cavity with a probability of 0.2. The molecules directions in the lattice were chosen randomly. 

In the reaction section, the molecule entering a cavity can react with an adsorbed molecule in the same cavity with a reaction probability of 0.1. The reaction occurs when one molecule jump to a chosen cavity adsorbed by another molecule. As the adsorption, in the desorption step every external cavity was checked one by one, throwing to the gas phase reactant, product and precursor with the followings probabilities 0.8,0.6 and 0.4, respectively. 

As has been pointed out above, a laser basically consists of an active material and a resonator. The latter enables the build-up of certain resonant modes and essentially determines the lasing characteristics. In most conventional devices, the optical feedback is provided by an external cavity with two end mirrors forming the resonator. With the advent of polymers as active materials, various new feedback structures were invented. Initially, a microcavity resonator device of the type shown schematically in Fig. 6.13 a was employed [48]. 

Example 1.6 For a resonator length of r/ = 8 cm, which is typical of a diode laser with an external cavity, and X = 800 nm, the integer q becomes q = 2x 10. For a Doppler width of A = 1 GHz, a length change of Ad = 2 pm is sufficient to shift the laser frequency periodically over the absorption profile. 

External passive resonators may become advantageous when the absorption cell cannot be placed directly inside the active laser resonator. However, there also exist some drawbacks the cavity length has to be changed synchronously with the tunable-laser wavelength in order to keep the external cavity always in resonance. Furthermore, one has to take care to prevent optical feedback from the passive to... 

In order to achieve more accurate values for the ratio of electron mass to proton mass from which the fine-structure constant can be derived, a frequency comb in the infrared region was developed which can be used to measure vibrational-rotational transitions in HD+-ions. The frequency distance q between the modes of the comb, which equals the repetition rate of the femto-second pulses, is stabilized on the resonance frequency of a cryogenic ultra-stable sapphire resonator. For the measurement of the transitions in the HD+-ion diode lasers with external cavity and a grating for achieving single mode operation are used. Their frequency is stabilized onto the centre of the molecular transition and is then compared with the adjacent mode of the frequency comb. 

There are, firstly, the improvement of frequency-doubling techniques in external cavities, the realization of more reliable cw-parametric oscillators with large output power, and the development of tunable narrow-band UV sources, which have expanded the possible applications of coherent light sources in molecular spectroscopy. Furthermore, new sensitive detection techniques for the analysis of small molecular concentrations or for the measurement of weak transitions, such as overtone transitions in molecules, could be realized. Examples are Cavity Ringdown Spectroscopy, which allows the measurement of absolute absorption coefficients with great sensitivity or specific modulation techniques that push the minimum detectable absorption coefficient down to 10 " cm ... 

The input data are the temperature at the centre of the core, the total decay heat, and the dimensions of the core, the vessel and the external cavity. 

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