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Laser Machining of Metals
Published in V. K. Jain, Advanced Machining Science, 2023
The material capable of sustaining stimulated emission and amplifying it is called gain medium. For stimulated emission to occur, the gain medium must be supplied with external energy to pump atoms from the ground state to an excited state, also called pumping. Pumping is essential in creating a population inversion, wherein the number of atoms in the excited state is more than that in the ground state. Pumping is typically done using electrical or optical energy. The most common pumping sources are a flash lamp or another laser source. Population inversion is necessary for the gain medium to amplify light produced by stimulated emission. Light amplification is achieved by providing optical feedback by placing a pair of mirrors on either side of the gain medium, as shown in Figure 7.2. The mirrors could be flat or curved. The setup is known as an optical cavity or a laser cavity. Light confined in the optical cavity bounces back and forth upon being reflected by the mirrors, producing standing waves. Every time the light passes through the gain medium, it gets amplified. One of the mirrors is fully reflective, while the other is partially transparent (output coupler), thus allowing some light to escape the optical cavity, generating a laser beam. The laser wavelength depends on the type of gain medium used, which could be a gas, liquid, or solid. Examples of gain medium include ruby crystal, CO2 gas, helium-neon gas, Nd: YAG crystal, doped-fibers, etc.
Cavity–Matter Interaction in Weak- and Strong-Coupling Regime: From White OLEDs to Organic Polariton Lasers
Published in Marco Anni, Sandro Lattante, Organic Lasers, 2018
Marco Mazzeo, Fabrizio Mariano, Armando Genco, Claudia Triolo, Salvatore Patanè
According to this equation, the peak transmission frequency of a Fabry–Pérot cavity changes by one FSR if the cavity length is modified of half a wavelength. Another parameter that defines the working performances of an optical cavity is the quality factor (Q-factor). It parameterizes the frequency width of the resonant enhancement, and it is simply defined as the ratio of a resonant cavity frequency, νc $ \nu _\text{ c} $ , to the linewidth (FWHM) of the cavity mode, δνc $ \delta \nu _\text{ c} $ : Q=νcδνc $$ \begin{aligned} Q=\frac{\nu _{\text{ c}}}{\delta \nu _{\text{ c}}} \end{aligned} $$
The Basics of Lasers
Published in Helmut H. Telle, Ángel González Ureña, Laser Spectroscopy and Laser Imaging, 2018
Helmut H. Telle, Ángel González Ureña
In the simplest case, an optical cavity consists of two mirrors facing each other. The most fundamental and frequently utilized configuration is that of two plane mirrors, which result in a so-called plane-parallel or Fabry–Perot cavity. While fundamentally simple (from the point of view of calculation), flat mirrors are rarely used because they are difficult to align to the required precision to support a standing wave pattern, unless the cavity length is very short (as a rule of thumb, L < 1 cm). Such an exception is found in semiconductor diode lasers, which possess an intrinsic, perfectly aligned plane-parallel cavity (see Chapter 5.1). When curved mirrors (both concave and convex) are incorporated into the setup, different resonator types ensue. These are distinguished by the focal lengths of the mirror(s), with radii of curvature R1 and R2, and the distance between them, the cavity length L.
Cavity-enhanced double resonance spectroscopy of HD
Published in Molecular Physics, 2022
M.-Y. Yu, Q.-H. Liu, C.-F. Cheng, S.-M. Hu
A schematic configuration of the COCA-DR spectroscopy setup is shown in Figure 1(e). Two external cavity diode lasers (Toptica DL Pro) were locked to respective modes of a high-finesse (130000) optical cavity using the Pound-Drever-Hall (PDH) method. The optical cavity is composed of a pair of high-reflective (HR) mirrors (Layetec, radius of curvature ∼1 m) and the cavity length is about 62.4 cm, corresponding to a free spectral range (FSR) of about 240 MHz. One of the mirrors is mounted on a piezoelectric actuator (PZT), which allows for tuning the cavity length. The sample gas cell with open ends is made of oxygen-free high thermal conductivity copper and contacted to a two-stage cryogenic cooler. The whole cavity and the cold sample cell are enclosed in a stainless-steel vacuum chamber, which can be pumped to 10 Pa by a turbo pump. To insulate the optical cavity from the mechanical noise generated by the cooler and pump, we separate the ring-down cavity mirrors with the sample cell and damp the vibration with several-stage bellows. The detailed structure of the cavity and cryogenic chamber (not shown in Figure 1 for simplification) has been described in our previous work [29]. It allows us to measure molecular spectroscopy in the temperature range of 10–296 K with an accuracy better than 0.1 K.
Dual-cavity spectrometer for monitoring broadband light extinction by atmospheric aerosols
Published in Aerosol Science and Technology, 2020
Aiswarya Saseendran, Susan Mathai, Shreya Joshi, Anoop Pakkattil, Tyler Capek, Gregory Kinney, Claudio Mazzoleni, Ravi Varma
In an IBBCEAS setup, the light from an incoherent source is coupled to a stable optical cavity formed by two concave highly reflective mirrors. The transmitted intensities are measured using spectrometers (Varma et al. 2009), photodiodes (Venables 2016), or photo multiplier tubes (Chandran, Puthukkudy, and Varma 2017). Initially, the transmitted spectrum from either an evacuated cavity or a cavity filled with a reference gas (I0) is measured, followed by the measurement of the transmitted intensity in a sample atmosphere (I). The cavity is having an effective optical pathlength of L, which can be determined from the separation between the mirrors (d) and the calibrated reflectivity (R) (Fiedler, Hese, and Ruth 2003). For small extinction, the extinction coefficients [ε(λ)] of aerosols as well as single trace gases can be calculated (Fiedler, Hese, and Ruth 2003) as: