Continuous-wave operation

All chemical laser systems discussed so far were operated in a pulsed mode and they need either flashlamp excitation or a gas discharge to initiate the chemical reaction by producing free radicals. Recently these limitations have been overcome and pure chemical lasers with continuous-wave operation have been developed. 

Continuous wave operation of COIL is facilitated by the hyperfine structure of the atom. Iodine has a nuclear spin of, so the P /2 and Pz/2 levels are split by hyperfine interactions. Figure 8 shows the allowed transitions between the hyperfine sublevels and a high resolution emission spectrum. The F = 3 — F" = 4 transition is most intense, and this is the laser line under normal conditions. Collisional relaxation between the hyperfine sub-levels of Pz 2 maintains the population inversion, while transfer between the Fi/2 levels extracts energy stored in the F = 2 level. Hence, if it is not sufficiently rapid, hyperfine relaxation can limit power extraction. 

K. Takaichi, H. Yagi, J. Lu, J.F. Bisson, A. Shirakawa, K. Ueda, et al. Highly efficient continuous-wave operation at 1030 and 1075 nm wavelengths of LD-pumped Yb Y203 ceramic lasers. Applied Physics Letters, 84, 317-9 (2004). 

Highly efficient continuous-wave operation at 1030 and 1075 nm wavelengths of LD-pumped Yb Y203 ceramic Appl Phys. Lett. 84, 317-319 (2004). 

Zhukov AE et al (1999) Continuous-wave operation of long-wavelength quantum-dot diode laser on a GaAs substrate. Photon IEEE Tech Lett 11 1345-1347... 

Bai Y, Slivken S., Darvish S. R., and Razeghi M., Room temperature continuous wave operation of quantum cascade lasers with 12.5% wall plug efficiency, RID C-6716-2011, RID B-7273-2009, RID B-7265-2009, App/. Phys. Lett, 93, 021103-021103 (2008). 

To operate an atom laser in a continuous-wave mode was not an easy problem, for this task could only be coped with by decreasing the losses and replenishing the condensate. This goal was achieved by Ghikkatur et al. (2002), who produced condensates in two different vacuum chambers. The condensate periodically produced in one of the chambers was fed to the other chamber to merge with the condensate contained therein. This procedure provided a permanent reservoir of condensed atoms. The continuous release of the condensate from the former chamber thus provided for the continuous-wave operation of the atom laser. 

Fig. 1. High-resolution spectrum of ATP. (a) 1959 Single-scan, 24.3-MHz, 5-mm tube, continuous-wave operation of a Varian 4302 dual-purpose NMR spectrometer. [ATP] = 500 mM. (b) 1976 10 scans, 72.9 MHz, 20-mm tube, in the Fourier-transform mode of operation, 20-sec repetition rate, and proton decoupling on a Bruker WH 180 NMR spectrometer. [ATP] = 1 mM. From Cohn (1979).

Population inversion is difficult not only to achieve but also to maintain. Indeed, for many laser systems there is no method of pumping which will maintain a population inversion continuously. For such systems inversion can be brought about only by means of a pumping source which delivers short, high-energy pulses. The result is a pulsed laser as opposed to a continuous wave, or CW, laser which operates continuously. 

New impetus was given to photomedicine by development of lasers that are compatible with the clinical environment. These include HeNe, Ar ion, mby, and tunable dye lasers operating in the continuous wave (cw) mode. Prior to the advent of lasers in medicine, only the treatment of newborn jaundice, and the appHcation of long wavelength uv irradiation in conjunction with adininistration (or topical appHcation) of psoralen class sensitizers to treatment of skin diseases (86), principally psoriasis, were clinically important phototherapies. 

The common liquid lasers utilize a flowing dye as the active medium and are pumped by a flash lamp or another laser. These are typically more complex systems requiring more maintenance. They can he operated as either CW (continuous wave) or pulsed. One advantage liquid lasers have is they can be tuned for different wavelengths over a 100-nm range. 

W.m (see Ch.l4). To get enough return flux at the minimum laser power, one needs to optimize the laser specifications (continuous wave or pulsed, pulse width, pulse repetition rate, (average) power, spectral profile) taking into both saturation, technological, budget and operation constraints. This is the challenge described in detail in the above mentioned chapter. 

Depending on how the secondary magnetic field is applied, there are two fundamentally different types of spectrometers, namely, continuous wave (CW) and pulse Fourier transform (PFT) spectrometers. The older continuous wave NMR spectrometers (the equivalent of dispersive spectrometry) were operated in one of two modes (i) fixed magnetic field strength and frequency (vi) sweeping of Bi irradiation or (ii) fixed irradiation frequency and variable field strength. In this way, when the resonance condition is reached for a particular type of nuclei (vi = vo), the energy is absorbed and... 

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. 

Some lasers produce a continuous-wave (CW) beam, where the timescale of the output cycle is of the same order as the time taken to remove photons from the system. CW lasers can be modified to produce a pulsed output, whereas other lasers are inherently pulsed due to the relative rates of the pumping and emission processes. For example, if the rate of decay from the upper laser level is greater than the rate of pumping then a population inversion cannot be maintained and pulsed operation occurs. 

Additional laser diode technologies recently reported include a continuous-wave rhodamine 700 dye laser with a maximum wavelength output at 758 nm, powered by two laser diodes each operated with two standard AA batteries (RDT E division of the US Naval Command, Control, and Ocean Surveillance Center, San Diego, California), and a tunable laser diode with output laser wavelengths of 650, 780, 850, and 1320 nm (New Focus, Mountain View, California). 

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