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Raman Microscopy
Published in John Girkin, A Practical Guide to Optical Microscopy, 2019
A typical CARS system is illustrated in Figure 10.6. In order to achieve CARS excitation one needs two laser sources tuned to the correct wavelength with sufficiently high power to achieve a detectable signal. Although in principle it is possible to achieve this using a continuous laser, all practical CARS microscopy has been undertaken with short-pulsed lasers. In the system illustrated the output from a mode-locked laser has been sent into an optical parametric oscillator (OPO). This OPO uses non-linear optical techniques to produce a tunable and intense light source. The second laser pulse is typically the initial laser used to seed the OPO. The OPO output is then tuned to ensure that the wavelength combination drives the required transition in the molecule of interest.
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Published in Philip A. Laplante, Comprehensive Dictionary of Electrical Engineering, 2018
where z j is the output of the jth neuron, w ji is the interconnection weight between the ith input neuron and the jth neuron, xi is the input coming from the ith input neuron, j is the bias in the jth neuron, and f is a nonlinear transfer function. Notice that z j and xi are binary. Nonlinear transfer function f could simply be a threshold function. The formation of interconnection weight matrix is called learning process. It appears that in recognizing process, the neural network has to perform a matrix-vector multiplication and a nonlinear operation. The matrix-vector multiplication can be easily performed using an optical system. However, the nonlinear operation can not be performed by optical means. It may be performed using electronic circuits. For two-dimensional neural processing, tensor-matrix multiplication is needed, which can also be realized by optical means. optical parametric oscillator a nonlinear optical device that can produce a frequency-tunable output when pumped by a fixed-frequency laser beam. The device consists of a second-order nonlinear optical crystal placed inside of an optical resonator as well as additional components for
Optical Parametric Devices
Published in Chunlei Guo, Subhash Chandra Singh, Handbook of Laser Technology and Applications, 2021
The potential of OPOs derives from their exceptional wavelength flexibility, which allows access to spectral regions unavailable to conventional lasers. The OPO can readily provide widely tunable radiation across substantial portions of the optical spectrum by a suitable choice of non-linear material and laser pump source. Figure 23.12 shows the wavelength tuning range of a number of OPO devices developed to date. The spectral range available to several conventional tunable lasers (as in Figure 23.1) is also shown for comparison. It is seen that many wavelength regions unavailable to lasers are readily accessible to OPOs. Moreover, spectral regions far more extensive than any tunable laser can be accessed by a single device using one non-linear crystal. In addition to its spectral versatility, the OPO can be configured as a highly compact device, has a simple tuning mechanism and offers high efficiencies in converting the input pump energy into useful output. It also has a practical solid-state design unlike, for example, tunable dye lasers employing liquids as the gain medium. Another important characteristic of OPOs is their temporal flexibility, which allows these devices to operate across all temporal regimes. This property is a consequence of the instantaneous nature of electronic polarization, the origin of non-linear gain. In contrast to conventional lasers, where the generation of the shortest optical pulses is limited by the upper-state lifetime of the laser transition, OPOs can provide output in all temporal regimes, from the cw to ultrafast femtosecond timescales, by the suitable choice of laser pump source.
The effect of elliptical focusing on the threshold of a nanosecond BBO crystal optical parametric oscillator
Published in Journal of Modern Optics, 2019
C. S. Rao, G. Kondayya, S. Kundu, Alok K. Ray
It is clearly understood from our numerical model results, as discussed in the earlier section, that the elliptical focused spot size of the pump beam must be much greater than the optimum size (15.5 µm) to avoid the optical damage at the crystal surface. Hence, we need to determine the maximum elliptical size of the pump beam for which the threshold pulse energy requirement of the pump laser reduces to sub mJ. To calculate the threshold of the BBO OPO using the numerical model, we assumed that a Type-I phase matched BBO crystal of length 15 mm is placed inside a cavity formed by the two plane mirrors M1 and M2 as shown in Figure 3. The OPO is pumped with a third harmonic (355 nm) of an Nd;YAG laser. The input cavity mirror (M1) possesses high reflectivity >99% and the output coupler (M2) is coated for 70% reflectivity for the signal wavelength (420–700 nm) and both mirrors have a high transmission at pump and idler wavelengths to prevent doubly resonant operation. The length of the BBO OPO cavity is chosen as 25 mm. Figure 4 shows the variation of the threshold pulse energy of the BBO OPO with the ellipticity factor (ϑellipse) of the input pump beam for different confocal focusing parameters (ξp). As shown in Figure 4, the threshold pulse energy of the BBO OPO for a confocal parameter of ξp= 0.03 corresponding to a pump beam waist size of 130 µm reduces to 0.74 mJ at a beam ellipticity factor (ϑellipse) of 15. The spot size is calculated to be the maximum when compared to other confocal parameters with a ϑellipse factor for which the threshold of the OPO reduces to 0.74 mJ. As a result, the peak intensity with this focused spot size reduces by a factor of ∼46 when compared to the earlier optimum focusing. Hence, using this improved optical focusing geometry of the pump beam, a high repetition rate DPSSL system, which can deliver pulse energies of 2–3 mJ can be used for reliable and sustainable pumping operation of the OPO without damaging its intracavity BBO crystal.
Precise measurement of the D2 S1(0) vibrational transition frequency
Published in Molecular Physics, 2022
The experimental setup used for these measurements consists of a precision laser system that produces a narrow-linewidth, continuous-wave beam near 3158 nm with a well-defined and tunable absolute frequency and a molecular beam apparatus for producing a cold collimated beam of isolated deuterium molecules. The laser system has been described in a previous work [10]. In short: a 1064-nm reference laser is stabilised at an optical frequency of by frequency doubling the laser to using second-harmonic generation and interrogating the component of the R(56) 32–0 transition of molecular iodine. The two degrees of freedom of a Ti:Sapphire-based optical frequency comb are stabilised so that mode number is 100 MHz lower in frequency than and mode number is 200 MHz lower in frequency than . This scheme results in a carrier-envelope offset frequency of and a repetition rate of . The exact value of during the measurement is recorded using a radio-frequency counter referenced to a rubidium oscillator which is disciplined by a global navigation satellite system (GNSS) receiver. Part of the 1064-nm beam is amplified to ∼10 W and used to pump a continuous-wave optical parametric oscillator (OPO), which down converts the pump photons into signal and idler photons at 1606 nm and 3158 nm, respectively. The signal beam is frequency doubled to 803 nm, and the frequency of the doubled beam is measured relative to mode of the optical frequency comb using a beat note detector. A phase-locked loop acting on a piezo mirror in the OPO cavity maintains the beatnote at a frequency , which is defined using a tunable radio-frequency reference. The ∼2-W idler beam, with an absolute frequency given by is used to excite the D S(0) vibrational transition.