Difference Frequency Generation (DFG)
| Experimental Apparatus
Mid-infrared laser sources come in many different varieties shown in Fig. For their application to spectroscopic measurement, and specifically for quantitative measurements, the ideal source would have the following properties: (1) sufficient optical power to overcome inherent electronic detection noise and ensure high laser signal-tonoise ratios, (2) narrow linewidth to obtain high selectivity and sensitivity, (3) single longitudinal mode operation.
A number of IR DFG sources have been constructed and used for spectroscopy. In the case of our DFG laser shown in Fig. 3.2, Ti:sapphire (“pump”) and Nd:YAG (“signal”) laser at different frequencies combined in a nonlinear material with suitable dispersion characteristics generate a beam at the difference-frequency (“idler”). The narrow emission spectra of the “pump” (highest frequency) and “signal” (middle frequency) translate into a similarly narrow spectrum of the idler wave. In general, idler wavelength tuning is accomplished by tuning the pump laser, or signal laser, or both. In order to building up the idler wave continually while the beams pass collinearly through the nonlinear material, the three waves must stay in proper phase.
The experimental arrangement for 4.3 μm 12CO2 Doppler-free saturation spectroscopy: DM = dichroic mirror; PBS = polarizing beam splitter; HWP = half wave plate; QWP = Quarter wave plate; DBS = dichroic beam splitter; MMO = mode matching optics; PG = pressure gauge |
| Frequency Stabilizatio
Schematic diagram of frequency locking of a Nd:YAG laser to the Dopplerfree transition of iodine. Visible radiation(532 nm) is produced by double-frequency generation in a MgO:PPLN crystal. The produced green light is retroreflected through the iodine cell to observe the Doppler-free spectrum: OI = optical isolator; HWP = half-wave plate; MgO:PPLN = periodically poled MgO doped lithium niobate crystal; DBS = dichroic beamsplitter; PBS = polarizing beamsplitter; QWP = quarter-wave plate; PR = partial reflector; PD = photodetector. |
| Experimental Results
P(20) Doppler profile using directly pump-prob saturation spectroscopy @ 2.7 μm The first-derivative Lamb-dip spectrum of [1001,0201]I 0000 band of P(20) line for the following experimental condition: Pressure P = 20 mTorr, modulation amplitude Am = 1.5 MHz, modulation frequency m = 400 Hz, lock-in time constant lock-in = 300 ms, frequency span span = 500 MHz, scan time Tscan = 500 s.
The second-derivative Lamb-dip spectrum of [1001,0201]I 0000 band of P(20) line for the following experimental condition: Pressure P = 20 mTorr, modulation amplitude Am = 1.9 MHz, modulation frequency m = 400 Hz, lock-in time constant lock-in = 300 ms, frequency span span = 10 MHz, scan time Tscan = 500 s |
