Conversion efficiency frequency doubling

The resonant frequency conversion with UV laser radiation (jOr - generated by the efficient frequency doubling in BBO - offers several advani ages. First, the frequency can be tuned to the two photon transition, which provides the largest resonant enhancement of Second, the spectral width of the UV light is determined by the line width of the dy< laser radiation and thus narrowband radiation is easily obtained (the spectral width of the radiation at is usually limited by... 

Lick Observatory. The success of the LLNL/AVLIS demonstration led to the deployment of a pulsed dye laser / AO system on the Lick Observatory 3-m telescope (Friedman et al., 1995). LGS system (Fig. 14). The dye cells are pumped by 4 70 W, frequency-doubled, flashlamp-pumped, solid-state Nd YAG lasers. Each laser dissipates 8 kW, which is removed by watercooling. The YAG lasers, oscillator, dye pumps and control system are located in a room in the Observatory basement to isolate heat production and vibrations from the telescope. A grazing incidence dye master oscillator (DMO) provides a single frequency 589.2 nm pulse, 100-150 ns in length at an 11 kHz repetition rate. The pulse width is a compromise between the requirements for Na excitation and the need for efficient conversion in the dye, for which shorter pulses are optimum. The laser utilizes a custom designed laser dye, R-2 perchlorate, that lasts for 1-2 years of use before replacement is required. 

The 6 Nd YAG lasers pump the DM0, preamplifier and power amplifier (Fig. 19, Friedman et al., 1998). The YAG lasers are built from commercially available flashlamp/laser rod assemblies, acousto-optic Q-switches and frequency doubling crystals (LBO and KTP). Most of the mirror mounts and crystal holders are commercial. Nd YAGs are frequency doubled to 532 nm using a nonlinear crystal. The Nd YAG rod and nonlinear crystal are both in the pump laser cavity to provide efficient frequency conversion. The 532 nm light is coupled out through a dichroic and fed to multimode fibers which transport the light to the DM0 and amplifier dye cells. 

Second harmonic generation (SHG) was first observed in single crystal quartz by Franken and coworkers (1) in 1961. These early workers frequency doubled the output of a ruby laser (694.3 nm) into the ultraviolet (347.15 nm) with a conversion efficiency of only about 10 % in their best experiments, but the ground had been broken. 

The titanium-sapphire wavelength itself is too short for vibrational excitation in most molecules, and too long for electronic excitation. However, these high peak powers permit efficient frequency conversion. For example, certain crystals can convert two photons with frequency co into a single photon with frequency 2co. In many ways this can be viewed as similar to a second-order chemical reaction, such as the dimerization of NO2 to form N2O4. The rate of that reaction is proportional to the square of the NO2 concentration the rate of this frequency doubling is proportional to the square of the photon concentration (the intensity), so high powers are very useful. It is also possible to combine two photons with different frequencies co and o>i in either sum-... 

Three features of this laser source merit further discussion. First, in a typical kinetic experiment, the 1/e chemical lifetime of the photolytically produced radicals varies between 0.2 and 25 ms, a representative mean being ti/e = 2 ms. For statistical reasons, one desires to collect a minimum of 20 concentration versus time data points per 1/e concentration decay period. For multichannel scaling detection, these typical kinetic conditions imply a maximum dwell period per channel of 100 ys. The ultraviolet laser source described above emits 2.5 x 10 pulses per 100 ys interval thus, relative to chemical decays, this rapidly pulsed source is viewed by the experiment as a cw excitation probe. Second, given that a pulsed initiation/cw detection kinetics configuration is desired, one may ask why a cw laser source is not used. The rationale here is that the visible-to-ultraviolet conversion efficiency is much higher when the quasi-cw source rather than a cw source is used. Frequency doubling efficiency varies in proportion to the fundamental peak power density present in the second harmonic generation crystal,... 

In the non-depleted wave approximation the efficiency t] for frequency-doubling by conversion of a guided fundamental mode into a guided second-harmonic mode using the nonlinear optical coefficient djj is given by... 

Table 3. Comparison of frequency-doubling conversion efficiencies in DCANP LB waveguides for several phase-matching configurations.

Jurdik et al. (2002)). It should be noted that, for a number of fixed-wavelength lasers (such as NdrYAG lasers), frequency doubling is implemented in intra-cavity configurations, which exhibit the benefit of automatic resonator-enhanced circulating power of the pump wave conversion efficiencies of more than 50 per cent are routinely achieved in commercial green CW Nd YAG laser systems, and values of close to 90 per cent are not uncommon (e.g. see Schneider et al. (1996)). 

In the two-color experiments the pulses of the Tiisapphire laser (1.47 eV) were frequency-doubled by a 1 mm BBO crystal with a conversion efficiency of 15%. A dichroic mirror separated the fundamental from the second harmonic (2.94 eV). For convenience, a positive delay here means the pump beam is the second harmonic at 2.94 eV and the probe beam is the fundamental at 1.47 eV. For negative delay times the energies of the pump and probe pulses are interchanged. 

The one-color experiment was performed with pump and probe pulses of the same photon energy (2.00eV). In this case a synchronously pumped femtosecond optical parametric oscillator was used (see Sect. 2.1.1). At A = 1.3 im the signal wave s maximum output reached more than 400 mW, corresponding to 20% conversion efficiency. The signal wave was frequency-doubled by a BBO crystal (60 mW). By measuring an interferometric autocorrelation... 

Figure 26.12 shows the chemical structure for an epoxy-based polymer that was used in prototype devices for frequency-doubling applications [46,110]. The absorption maximum of the NLO moiety in this polymer is at 380 nm. The polymer exhibited a ch , of 13 pm/V at 1.064 /Ltm. Cerenkov SHG of blue and green light at 450 and 532 nm has been demonstrated. Flowever, only a 0.03%/watt conversion efficiency in the waveguide was obtained. An attempt to reduce the cutoff wavelength to 340 nm by using 4-aminobenzonitrile chromophore in a similar polymer resulted in a rather small NLO coefficient (J33 = 4 pm/V at 1.064 /xm) [111]. 

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