Laser Sources
Generation of coherent light is essential to high-resolution laser spectroscopy. In our high-resolution lab, the spectral regions of mid-IR, near IR, visible, and UV are covered without gaps.
1. Continuous-Wave Ring Lasers:
Ring lasers are continuous-wave (CW), single-frequency, narrow-linewidth, tunable lasers with high stability that are built to match the requirements of high-resolution high-precision spectroscopy. In our high-resolution lab, there are:
1(a). One dye ring laser (Coherent 899-29), pumped by a Ar+ laser (Coherent, Innova Sabre), and
1(b). One Ti: Sapphire ring laser (Coherent 899-29), pumped by a diode-pumped solid-state laser (Coherent: Verdi-V10).
1(c). One MSquared Ti:Sapphire ring laser (SolsTiS) with frequency-doubling unit (ECD).
2. Nd; YAG Lasers:
Q-switch Nd: YAG lasers provide high pulse energy and peak power needed in molecular spectroscopy. Our high-resolution lab is equipped with six Nd: YAG lasers (Spectra-Physics, DCR-2A, GCR-4, Quanta-Ray Pro250, Quanta-Ray Pro270 x2; Continuum, Powerlite Precision II 8000). They have several different uses:
(i) The fundamental output (1064 nm) is used for laser ablation/vaporization to initiate chemical reactions in a free jet expansion.
(ii) The second and third harmonics (532 and 355 nm) are used to pump pulsed dye lasers or OPOs (355 nm only, see below);
(ii) The third and fourth harmonics (355 and 266 nm) are used as the photolysis laser in the production of free radicals, either in a flow cell or in a jet expansion.
3. Excimer Lasers:
Excimer lasers are still indispensable light source in the UV region. We have three Excimer lasers (Lambda Physics, Complex 110, EMG103-MSC, LPX-120-1) in our high-resolution lab.
4. Pulsed Dye Laser:
Pulsed dye lasers provide wide tunability in the visible region and the narrow bandwidth that is needed in molecular spectroscopy. Our high-resolution lab currently has two pulsed dye lasers:
4(a). Sirah, Precision-Scan
4(b). Sirah, Cobra-Stretch
5. Optical Parametric Amplifiers (OPO):
OPOs can cover both the visible and the near-IR region. We have one OPO (Continuum, Panther EX).
6. Laser Amplifiers
When both narrow linewidth and high pulse energy are needed, laser amplifiers are the choice of molecular spectroscopists. Laser amplifiers are usually seeded by a single-mode CW laser and pumped by a high-energy pulsed laser (Nd: YAG or excimer). In our lab, we use three amplification stages of the pulsed dye laser as a dye amplifier. We also have a home-made Ti: Sapphire amplifier system to generate laser pulses in the near-IR region with Fourier-transform-limited linewidth.
7. Frequency conversion
Direct generation of tunable pulsed lasers in the UV and IR regions is still relatively difficult. Coherent radiation in these regions is usually produced by frequency up-conversion (to UV) or down-conversion (to IR) using a visible source such as pulsed dye lasers and OPOs. Two techniques are used in our lab:
7(a). Frequency doubling
Frequency doubling of the tunable pulsed dye lasers is used to cover the UV region.
7(b). Raman shifting.
The Raman shift cell can convert visible pulses generated by a pulsed dye laser to the near IR region (900 .. 1500 nm). Wavelength conversion is achieved by the Raman effect, i. e. Stokes-shift, in a gas cell filled with, e.g., hydrogen under high pressure. Conversion efficiency to the first order Stokes is typically above 10% while that to the second order Stokes is above 1%. In our high-resolution lab, we have both single-pass cells and a home-made multi-pass cell for Raman shifting.
7(c). Difference-frequency mixing (DFM).
A frequency doubled, injection seeded Nd: YAG laser is used to pump the pulsed dye laser, operated in the red spectral region. The dye laser output beam and the residual 1064 nm pulse from the Nd: YAG is mixed in a temperature-stabilized LiNbO3 crystal. DFM using a pulsed dye laser can cover the mid-IR region (1.5-5.0 um).