NOPA - overview and principles

LMU München
Fakultät für Physik




(c) 2002 BMO


Design principles of a NOPA

A NOPA is an optical parametric converter designed to generate ultrashort tunable visible pulses from blue pump pulses. The tuning range is 470 to 1600 nm for pumping with a fre-quency doubled Ti:sapphire amplifier system. Most NOPAs are used at kHz repetition rate, but we have also built units for up to 200 kHz and for just a few 10 Hz. In the visible the spec-trum of the pulses can be made sufficiently wide to support sub-10 fs pulses. Sub-30 fs pulse lengths are readily available by compression in a simple prism compressor made of SF10 prisms. In the NIR sub-50 fs pulses are generated.

A single-stage NOPA consists of three main functional blocks:

  • Continuum generation in a sapphire disk pumped by a small fraction of the NIR pump light.
  • Parametric amplification of the seed light in a BBO crystal pumped by the frequency doubled pump light.
  • Compression of the broadband output pulses.

Pulse lengthening in collinearly phasematched OPA

In an optical parametric amplifier (OPA) photons contained in a short wavelength (blue) pump beam are split into one signal (visible) and one idler photon (NIR) each. The "active" medium is a nonlinear optical crystal, such as BBO. For efficient conversion the phase veloci-ties of pump, signal and idler are matched by proper rotation of the birefringent crystal. Un-fortunately, however, this does not simultaneously assure matching of the three group veloci-ties. Therefore the three pulses propagate at differing speeds in the crystal and as a result the amplified signal and idler pulses exiting the crystal are typically 100 fs long for collinear phase matching in a 1 or 2 mm BBO crystal.

The situation is illustrated in the Figure. A 100 to 150 fs long pump pulse (violet) propagates through the crystal and a very short visible seed (signal; green) pulse is injected in addition. This signal pulse is amplified by the parametric interaction and an infrared idler pulse (red) is generated at the difference frequency between pump and signal. This idler pulse propagates somewhat faster than the signal. Continuously signal and idler are amplified as they travel fur-ther through the crystal and the signal generates more idler just as the idler generates more signal (shown as hashed areas in the Figure). Due to the differing group velocity, the new sig-nal photons are added on the leading edge of the signal pulse and the new idler photons are added on the trailing edge of the idler pulse. As a result lengthened pulses cannot be avoided in collinearly phasematched visible OPAs.

Noncollinear phasematching for ultrashort tunable pulses

Work by G. M. Gale and coworkers (Opt. Lett. 20, 1562 (1995)) demonstrated an elegant solution to the problem of pulse lengthening in collinearly phase matched parametric interaction. They used a noncollinear arrangement in a quasi-cw pumped visible OPO that produced sub-20-fs pulses. This is made possible by the following situation. The chosen noncollinear ar-rangement of pump and seed (signal) beam - angle Ψ - results in an angle Ω between the idler and the signal that is approximately given by

The idler group velocity is somewhat larger than the signal one and therefore a suitable angle Ω can be found that the projection of the idler group velocity onto the signal k vector (propagation direction) will match the signal group velocity, i.e.

This situation is illustrated in the Figure. Now the extra signal photons generated by the amplification of the idler are produced at the same position as the ones due to amplification of the signal itself and no lengthening of the pulse results. The transversal displacement of the idler only leads to a slight spatial widening of the signal beam.

It can easily be shown that effective matching of the signal and idler group velocities is equivalent to very broadband phase matching in the parametric process (Appl. Phys. B 71, 457 (2000)). As a consequence a very wide spectral range out of the seed light can be amplified and the resulting output pulse can be compressed to pulse lengths well below 10 fs.

For calculations of the phase matching angles in the noncollinear parametric interaction a program SNLO written by Arlee Smith is most helpful. Any angles and geometries needed for the alignment of the NOPA can easily be calculated with the help of this program. In the original OPO reported by Gale the tuning range was limited by the resonator optics and the operation of the OPO depends critically on a very high average pump power. Wilhelm, Piel and Riedle were the first ones to show that a traveling wave noncollinearly phase matched OPA pumped by blue pulses at 1 kHz repetition rate can readily yield sub-20-fs pulses tunable throughout the visible (Opt. Lett. 22, 1494 (1997)).

Generation of the chirped seed continuum

Hard focusing of less than 1 ÁJ of 100 fs 800 nm pulses into a sapphire disk leads to self focusing and generation of a single filament continuum. The visible part of the spectral distribution of such a continuum is shown in the Figure. In the range from about 460 to 700 nm a nearly flat distribution is found that allows the use of this continuum as seed light for the NOPA. The continuum pulse is strongly chirped due to the dispersion in the sapphire itself and the optics used to collimate the emerging beam. In addition the continuum propagates somewhat faster than the blue pump pulse in the amplifier crystal. This slippage and the continuum chirp can be matched to select both the desired center wavelength of the output pulses (choice of pump - seed delay) and the bandwidth of the pulses.

Need for pulse compression

The broad bandwidth output pulses of the NOPA are chirped due to the initial chirp of the seed continuum and additional chirp caused by the dispersive properties of the BBO crystal, the optics and the air. The chirped pulses can readily be compressed in a simple compressor made of two prisms that are double passed. With SF10 prisms a pulse length below 30 fs can be reached throughout the tuning range with a small spacing of the prisms. To get down to 10 fs fused silica prisms are needed with a spacing around 1 m.

Note: Even the compression of the pulses in the prism sequence only minimizes their length at one single point in space. Transversal of further optics - including plain laboratory air - will cause the pulses to lengthen again. This means that the pulse length has to be determined in the spot of the spectroscopic experiment or an equivalent position. The prism compressor can, however, be well used to precompensate the chirp encountered downstream.

Commercial availability

In a very fruitful collaboration with HORIBA Jobin Yvon (Dr. Hans-Erik Swoboda) and Clark-MXR, Inc. we have developed a series of boxed NOPAs that have been installed in leading laboratories around the world. The single-stage NOPA described so far is available as NO PA slim. Its fundamental tuning range of 470 to 700 nm can be extended by different means:

-Wavelengths between 870 and 1600 nm can be generated with high pulse energies and high efficiency in the 2-stage NO PA.
-To generate ultrashort pulses in the 700 to 870 nm range, suitable seed light has to be generated. The continuum generated from the 775 nm CPA output is strongly structured in this range and therefore not suited. In the 2-stage NOPA plus the first stage is operated in the NIR and the output is used to pump a second continuum. Selected portions are then amplified in the second stage.
-To access the UV and deep blue range (240 to 470 nm) the NO PA output can be fre-quency doubled or sum frequency mixed with the Ti:sapphire fundamental in a thin BBO crystal. Pulse lengths well below 30 fs are obtained with typically 10 % conversion efficiency.

For the analysis of the ultrashort pulses we developed a compact autocorrelator (NOPA pal) that can measure pulses as short as 10 fs in the range from 420 to 1600 nm.

Former co-workers: Thomas Wilhelm, Johannes Piel, Matthias Beutter, Kai Stock 
"Femtosecond two-photon photoemission at 150 kHz utilizing two noncollinear optical parametric amplifiers for measuring ultrafast electron dynamics"
L. Gundlach, R. Ernstorfer, E. Riedle, R. Eichberger, and F. Willig
Appl. Phys. B 80, 727 - 731 (2005)

"Generation of sub-20 fs tunable visible pulses from a 100 kHz NOPA for measuring ultrafast heterogeneous electron transfer"
R. Ernstorfer, L. Gundlach, C. Zimmermann, F. Willig, R. Eichberger, and E. Riedle
Ultrafast Optics IV: Selected Contributions to the 4th International Conference on Ultrafast Optics, F. Krausz, G. Korn, P. Corkum, and I. A. Walmsley, eds., Springer Series in Optical Sciences, vol. 95, 393 - 398 (2004)

"Real-time characterization and optimal phase control of tunable visible pulses with a flexible compressor"
P. Baum, S. Lochbrunner, L. Gallmann, G. Steinmeyer, U. Keller, E. Riedle
Appl. Phys. B 74 [Suppl.], S219 - S224 (2002)

"20 to 50 fs pulses tunable across the NIR from a blue pumped noncollinear parametric amplifier"
J. Piel, M. Beutter, E. Riedle
Opt. Lett. 25, 180 - 182 (2000)

"Generation of 10 to 50 fs pulses tunable through all of the visible and the NIR"
E. Riedle, M. Beutter, S. Lochbrunner, J. Piel, S. Schenkl, S. Spörlein, W. Zinth
Appl. Phys. B 71, 457 - 465 (2000)

"20 Femtosecond Visible Pulses Go Tunable by Noncollinear Parametric Amplification"
T. Wilhelm, E. Riedle
Opt. Phot. News 8, 50 (1997)

"Sub-20-fs pulses tunable across the visible from a blue pumped single pass noncollinear parametric converter"
T. Wilhelm, J. Piel, E. Riedle
Opt. Lett. 22, 1494 - 1496 (1997)