Seeding of picosecond and femtosecond optical parametric amplifiers by weak single mode continuous lasers

Optical parametric amplifiers are typically seeded with either parametric superfluorescence or broadband continuum pulses. We show both with picosecond and femtosecond pump pulses, that single longitudinal mode cw lasers with mW power can be well used to generate nearly Fourier-transform-limited output pulses. The 532 nm pumped picosecond system is seeded in the near infrared and fully tunable from 1260 to 1630 nm. The femtosecond system operates stable with just hundreds of seed photons. The output spectral width matches closely to the width of individual spectral features found in single shot spectra of parametric superfluorescence. Both systems provide interesting radiation sources for nonlinear optics experiments that need highly controlled and clean excitation. ©2013 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers; (230.4480) Optical amplifiers; (270.6630) Superradiance, superfluorescence; (320.7110) Ultrafast nonlinear optics. 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Introduction
Optical parametric amplifiers (OPAs) offer the unique possibility to amplify optical signals from the UV to the mid infrared.This is possible without the necessity for resonances in the amplifying medium like it is encountered in laser amplifiers.It was only through the use of laser dyes that similar tunability was achieved [1].But this comes at the cost of dye solutions that have to be replenished quite frequently.OPAs on the other hand are fully solid state devices and need practically no replacement of consumables.Even though OPA was demonstrated already very early, the process became technically most important with the advent of pico-and femtosecond lasers.This is due to the fact that the amplification scales exponentially with the square root of the pump intensity and for long pulses a high energy is needed.At the same time the damage threshold fluence scales as τ 0.5 [2], which leads to a damage intensity proportional to τ -0. 5 .
For pulsed dye lasers it is well recognized that even the use of elaborate resonator designs does not lead to Fourier-transform-limited (FL) pulses.However, injection seeding or even locking with single longitudinal mode cw lasers readily leads to FL pulses [3][4][5][6].The concept of cw seeding was subsequently transferred to nanosecond optical parametric oscillators and amplifiers and is used in a wide range of wavelengths [7][8][9][10].The typical seeding power of a few mW renders roughly 10 7 photons during the ns amplification window and therefore leads to stable operation.
If the pump pulse duration is decreased to a few picoseconds, the number of seed photons goes down to 10 4 .Still the feasibility of cw seeded OPA could be demonstrated and compared to the more often used case of optical parametric generation (OPG) [11].The seeding indeed led to nearly FL pulses and a lowered OPA threshold.Recently even first reports of cw seeded femtosecond OPAs became available, however at the expense of 500 mW seeding [12] and only slight tunability in the near infrared (NIR) [13].
In this work we demonstrate that mW levels suffice to cleanly seed a fs OPA in the visible and a widely tunable ps OPA in the NIR.Both systems operate in the µJ range and with kHz repetition rate and are aimed at experiments that need highly controlled and clean optical excitation.The fs setup explores the limits of cw seeding, the ps OPA renders an unprecedented source of close to FL pulses with about 5 ps duration.This translates to less than 1 nm or about 3 cm −1 bandwidth.Such a bandwidth is below the homogeneous broadening in the condensed phase and therefore allows the optimal resolution of any excitation dependent dynamics.FL tunable pulses in the ps regime are otherwise hard to generate with an OPA, as the continuum seeding used in many modern fs OPAs is not available due to the lack of a compressible ps continuum.On the other hand, OPG as a source needs a rather high degree of spectral filtering with the associated loss in effective seed power.Even the most advanced synchronously pumped optical parametric oscillators (OPOs) in the fs or ps domain can at best render output pulses in the 10 nJ regime.Still higher pulse energies come at the expense of largely increased complexity and limited beam quality as well as tunability [14].For many nonlinear applications pulse energies beyond these levels are needed.

Two-stage picosecond OPA
The setup used for the OPA with cw seed light is quite similar to the generic setup of continuum seeded OPAs [15,16].The laser output at 1064 nm is frequency doubled to obtain visible pump light.In our recent work we have discussed the advantages of visible or UV pumping in OPAs [17].The increased group velocity mismatch is of no concern for the modestly short pulses generated.
The setup for the ps OPA is shown in Fig. 1.A commercial Nd:YVO 4 regenerative amplifier system (picoREGEN IC-10000; High Q Laser) delivers 10.6 ps pulses of 360 µJ energy at 1064 nm with 5 kHz repetition rate.These are frequency doubled in a 1.5 mm BBO crystal cut at 23.5° to 7 ps green pulses.To ensure a high second harmonic generation (SHG) of 130 µJ, we focus the fundamental light with an f = 250 mm AR coated lens, place the BBO crystal sufficiently behind the focus to avoid saturation, and finally recollimate the SHG light with another identical lens.We checked that the SHG divergence does not change as the BBO crystal is moved along the axis.In this way the SHG output can be smoothly varied without affecting the OPA alignment.As seed system an external cavity diode laser (TSL-210F; santec Corporation) tunable without mode hops from 1260 to 1630 nm is used.The pump light is split into two beams that each pump one of two OPA stages.The pump beams are focused with R = −500 mm dielectrically coated mirrors into the amplifier crystals.The focusing mirrors are both placed below the seed and the pre-amplified beam path to introduce a noncollinearity in the plane containing the BBO crystal axis.The output of the fiber delivering the NIR seed light is collimated and simultaneously weakly focused by an aspherical lens (f = 11.0 mm, N.A. 0.25; Thorlabs).This leads to a very clean focus and an excellent spatial overlap with the pump beam.The polarization of the cw seed light is set externally to horizontal polarization with a fiber polarization controller.In the first amplifier stage 44 µJ pump energy are used and focused to a diameter of below 100 µm.This corresponds to an intensity of about 10 GW/cm 2 at the amplifier crystal, which is again placed behind the focus.As amplifier crystal we use an 8 mm thick BBO crystal cut at 26.5° for type I phase matching.In a preliminary experiment we tried shorter crystals as we were concerned about the limited beam overlap in long crystals.We only found weak amplification and rapid crystal damage.This is due to the fact that the same overall amplification has to be generated in a shorter crystal through a higher intensity.Already in early work on ps OPAs it has been measured that the damage threshold of BBO for ps pulses is in the order of 10 GW/cm 2 for 250 ps pulses [18].The above given scaling law thus predicts a damage threshold of 60 GW/cm 2 for our 7 ps pulses.Only through the use of the long crystal can we stay below the damage threshold and still get the desired singlestage amplification of 3 x 10 4 needed to amplify the mW seed to the µJ level.At the used pump intensity no damage was observed in many months of operation.
The second amplifier uses the same pump geometry at 84 µJ energy and an identical BBO crystal.We varied the noncollinearity angle in both amplifiers to find the maximum single pass amplification.The seeding in the idler branch does not allow the increase in amplification bandwidth found for a noncollinear OPA (NOPA) operated in the signal regime [16,19].However, the experiments showed that the best amplification is still found for an external angle of about 4.0° in both amplifiers and the crystals placed in walk-off compensating geometry.We believe that this geometry leads to the best compromise between signal-pump and idler-pump spatial walk-off and therefore the highest gain over the crystal length.
In between the two amplifiers we use an f = 250 mm fused silica lens to image all preamplified light into the pumped volume of the second amplifier.The spatial overlap between seed and first pump, and between pre-amplified beam and second pump is quite critical as the whole setup is just marginally operated in saturation with respect to the seed.
Due to the cw seed, the temporal overlap in the first stage is automatically ensured and the alignment consists simply of spatially overlapping seed and pump within the BBO crystal.After the initial alignment of the tilt angle of the amplifier crystal, the daily operation is quite easy despite the NIR operation.Even in the second amplifier the comparatively long pump pulse makes finding the temporal overlap quite simple.It can best be found with a high speed photodiode and an oscilloscope.Only for the final optimization monitoring of the increased output with a NIR detector or a sensitive powermeter is preferable.Quite helpful is the observation of the off-axis signal output.Its wavelength tunes from 920 to 790 nm.If the longest seed wavelengths are used, the signal can already be seen with the bare eye on a white card.This greatly facilitates the initial alignment.Once the temporal and spatial overlap is found, the seed can be tuned to the desired wavelength.Only a slight change of the internal phase matching angle by 0.6° is needed over the full tuning range.With the described system we obtain up to 4 µJ output.The average power is up to 20 mW and thereby much higher than the 1 mW seed level used.Still, if a photodiode is used to determine the amplification factor, one has to consider its temporal response function of 1 ns in our case, which decreases the contrast from the full 3 x 10 4 amplification to about 150.We find roughly the same amplification in both OPA stages.The contribution of parametric superfluorescence is determined to be below 5%.
Typical output spectra are shown in Fig. 2 as measured with a spectrum analyzer (86142B OSA; Agilent Technologies, Inc.).In Fig. 2(a) it is shown how the output can be tuned over the full range from 1260 to 1630 nm without any gaps or discontinuities beyond the 0.1 nm tuning steps of the seed.This is done mainly by computer controlled tuning of the seed and only a minor adjustment of the phase matching angles.Only the polarization of the cw seed laser has to be readjusted, because the seed laser changes its polarization when the wavelength is scanned.For a limited tuning range of about 20 nm, e.g., 1490 to 1510 nm as shown in Fig. 2(b), no mechanical adjustment at all is needed.The new ps source therefore affords the possibility to do precise frequency spectroscopy with simultaneous high peak power for nonlinear excitation and gating in the ps range.
Measurements at high resolution show a smooth spectral distribution of the OPA output with a typical width of 0.63 nm at 1300 nm (see Fig. 3(a)).This can be compared to the measured pulse length (Pulsecheck, APE GmbH) of 5.3 ps (Fig. 3(b)).The resulting timebandwidth-product is 0.59 and close to Fourier-limited.At the same time the spatial mode of the output is close to Gaussian (see Fig. 3(c)) and the energy fluctuations are only about 1% rms.

Application of the picosecond OPA
The unique properties of the cw seeded ps OPA are advantageously used in experiments investigating nonlinear effects in lithium niobate waveguides and one-dimensional waveguide arrays (WGAs).In these systems, interactions like SHG, nonlinear phase shifts and soliton propagation take place.These are controlled by the input power, the utilized waveguide modes of fundamental wave (FW) and SH and the phase mismatch between the modes.A suitable laser source to explore these nonlinear effects has to fulfill a number of requirements.First, the available peak power must be high enough to trigger the nonlinear processes, usually in the range of 1-20 kW.Second, the laser needs to be easily tunable to allow for measurements of different effects associated with different SH modes or phase mismatches.Since the phase mismatch changes with the wavelength, the bandwidth of the laser spectrum is required to be rather small, ideally around 0.5 nm.Finally, the laser should provide a clean beam for efficient coupling to the waveguides.These requirements are best met by a pulsed source with a nearly Fourier-limited pulse length of around 5 ps.The peak power then translates to at least 100 nJ, a pulse energy still out of range for synchronously pumped OPOs.The requirements are, however, well met by the OPA described above, which was actually designed for the described experimental investigations.Figures 4(a) and 4(b) show the SH output power of a single waveguide in dependence on the input wavelength when only the seed beam of the OPA is coupled to the first order mode of the waveguide.We find several maxima of the SH power, corresponding to the phase matching wavelengths to different SH modes shown in the insets.At the wavelengths denoted by the dotted lines we show the qualitatively different propagation regimes in a WGA consisting of 101 waveguides.A wide elliptic beam is coupled to the WGA and excites several waveguides.At low peak power the beam simply diffracts (see blue lines in Figs.

4(c)-4(e)).
Between the phase matching wavelengths to different modes, the phase shifts induced by the two SH interactions on the FW compete [20] and nonlinear effects are inhibited [21] (see red line in Fig. 4(c) at peak power of 1.5 kW) and the profile matches that at low power.For wavelengths above the two SH resonances, the two phase shifts act cumulative and quadratic spatial solitons with two SH components exist, seen as narrow FW output (red line in Fig. 4(d)) [22].Spatial solitons with only one SH component [23] are shown in Fig. 4(e).
All the experiments described above were performed with the ps OPA presented in this paper.Only minor adjustments of the BBO crystals in the OPA were necessary to switch between the wavelength ranges indicated in Figs.4(a) and 4(b), whereas wavelength tuning within the wavelength ranges in either Fig. 4(a) or Fig. 4(b) could be conducted without adjustments by simply changing the wavelength of the seed diode laser.Thus, the presented OPA concept proves to be an invaluable instrument for conducting experiments in quadratically nonlinear systems.

Single-stage femtosecond OPA
In the ps OPA described so far, about 2 x 10 4 photons from the seed laser are amplified.They are found to be sufficient to render output pulses with fluctuations just given by the pump laser fluctuations.If the pump pulse length is decreased to the typical duration of amplified Ti:sapphire lasers of about 100 fs, the number of photons delivered in a 1 mW cw seed beam during the amplification window is just a few hundred.It is therefore not a priori clear whether such a low seed level is sufficient.In addition, it has to be seen whether the cw seed can compete with the parametric superfluorescence always present in OPAs [24].
The setup for amplification of mW level cw seed light with fs pump pulses is shown in Fig. 5.As pump laser we use a commercial Ti:sapphire amplifier system (CPA2001; Clark MXR), which delivers 190 fs pulses around 775 nm with a repetition rate of 1 kHz.Pulses with an energy of 220 µJ are frequency doubled in a 0.7 mm thick BBO crystal, cut at 30° for type I phase matching to generate the pump pulses for the NOPA.A motorized half-wave plate in front of the BBO crystal is used to continuously adjust the energy of the frequencydoubled pulses.Their pulse duration is measured with an autocorrelator based on two-photon absorption [25] in a 100 µm thick gadolinium gallium garnet crystal to 150 fs.
As seed we use a single longitudinal mode cw laser (Torus; LaserQuantum) which delivers up to 100 mW power at 532 nm.A combination of a motorized half-wave plate and a polarizing beam splitter cube is used to adjust the energy sent to the NOPA.The seed laser has a sideband suppression of about 60 dB.This is quite important for the measurements, as already a −20 dB second longitudinal mode (1% power level) will lead to a 20% mode beating and variation of the instantaneous photon number delivered to the amplifier.
The pump pulses are focused with a spherical mirror (R = -1000 mm) towards the amplifier BBO crystal (2 mm, type I, cut at 32.5°), the cw seed light with an achromatic lens (f = 500 mm), whereby both beams have their focus in front of the crystal [16].At the position of the crystal the pump beam has a diameter of 300 µm (FWHM), the optimal seed diameter for largest output was found to be 150 µm.
To achieve optimal amplification of the seed, the correct phase matching angle of the crystal, the correct noncollinearity angle and good spatial overlap between pump and seed have to be ensured.As an initial alignment help we additionally generated a supercontinuum by tightly focusing a small fraction of the Ti:sapphire fundamental pulses into a sapphire crystal, as is commonly done in NOPA applications [15,19].We overlapped the cw light and the continuum seed with a metallic beam splitter, and first looked for amplification of the continuum seed with blocked cw light.
Since the number of photons in the window of the 150 fs pump pulses is much higher for the continuum seed than for the cw laser (~10 9 compared to 4 x 10 4 for 100 mW seeding power), amplification is found here more easily even for non-optimal settings of the critical parameters.An additional advantage is that amplification is seen much more easily by eye, as the amplified output changes its color drastically from the white looking continuum.When some amplification is found, the NOPA can be optimized for best performance around the cw seed wavelength of 532 nm.Once this is achieved, the whitelight seed is blocked and the cw The amplified output is monitored with an AC coupled photodiode [26].Although insensitive to cw light of small powers, the diode circuitry is saturated by the cw seed light for powers of more than about 30 mW.Since here we are only interested in the amplification behavior for small seed levels, this was of no great concern.We achieve amplification factors of up to 10 8 for a pump energy of 35 µJ with seed powers in the range from 0.1 mW to 0.5 mW, where the output energy scales linearly with the seed power.For a typical seed power of 1 mW and an amplification of 10 7 , the output pulse energy is in the range of 1 nJ.When we block the seed beam, the remaining output caused by parametric superfluorescence is in the range of 200 pJ, i.e. 20% of the seeded output.We find that the parametric superfluorescence is extremely broadband and therefore an even better discrimination can be obtained with a bandpass filter.Due to the low number of seed photons we find a fluctuation of the output energy of a few percent.
The temporal autocorrelation of the seeded output is measured for pump energies in the range from 24 µJ to 27 µJ and for seed powers between 0.3 mW and 8 mW.In these ranges the measured output pulse is Gaussian with a duration of almost constant 71 fs (see Fig. 6(a)).
To measure the output spectrum, the NOPA has to be slightly misaligned in noncollinearity and phase matching angle, so that the cw seed and the amplified output propagate in slightly different directions.If both beams are perfectly overlapped, our spectrometer, which integrates over a minimum of 1 ms, mainly measures the cw seed.Only when seed and amplified beam are spatially separated can we measure a clean amplified spectrum (see Fig. 6(b)).The measured spectral width of 6.4 nm corresponds to a FL of 65 fs, which means that we obtain nearly Fourier-limited pulses with a time bandwidth product of 0.48 even without a compressor.Figure 6(c) shows a typical spectrum of a single pulse when seeding the same amplifier with the parametric superfluorescence generated by focusing a part of the frequency doubled pump pulses into an additional identical BBO crystal for optical parametric generation (OPG).The measured spectra vary strongly in spectral distribution from shot to shot.However, they all show peaked structures that have nearly Gaussian shape and a spectral width of 6.5 ± 1.4 nm.This is very similar to the spectral width of the amplified output of 6.4 nm when using the cw seed.We therefore suggest that the peaks originate from single "monochromatic" seed photons present in the front part of the OPG crystal that get amplified.The different heights could correspond to different first interaction points of these seed photons with the pump pulse in the OPG crystal.Therefore for some of the spontaneous seed photons only part of the OPG crystal is available for pre-amplification.This concept is investigated in detail in a series of measurements described in a forthcoming publication.As a result we are able to measure the number of relevant modes in the vacuum fluctuations of the optical electromagnetic field.
Another noteworthy observation is that the center of the amplified spectrum is redshifted with respect to the cw seed by about 2 nm (see Fig. 6(b)).A similar observation was already reported in [11], where the amplified output of an OPA pumped by 3.5 ps pulses at 767 nm and seeded by a cw laser at 1310 nm was redshifted by about 1 nm.Although various parameters were varied, this observation could not be explained satisfactorily.In our own experiments with the ps pump pulses we did not see such a large offset, but only a maximum shift of about 0.1 nm.We performed a number of tests to clarify the origin of the shift in the fs setup.We think that wavelength pulling caused by the operation off the amplification maximum and group velocity mismatch and spatial walk-off effects contribute jointly to the situation.A full explanation is beyond the scope of the current work.

Summary and discussion
Seeding of ultrafast optical parametric amplifiers is shown to be possible by mW levels of a cw source.By utilizing a commercial Nd:YVO 4 regenerative amplifier at 5 kHz repetition rate a unique source of 5 ps and 4 µJ pulses becomes available.It is fully tunable from 1260 to 1630 nm, presently limited by the commercial seed source.The pulses are both close to a Fourier-limited time-bandwidth product and nearly Gaussian in the far field.The successful use for an extended study of multi-waveguide second harmonic coupling is demonstrated.
Compared to the classical approach for ps OPA systems, i.e. the seeding by OPG, there are a number of silent advantages in the new system.The separation of the desired NIR output from the green pump and the red signal is readily achieved spatially.This is due to the fact that the highest small signal gain and output power is found for a noncollinear geometry.Since no lossy spectral filtering is needed for the nearly Fourier-limited operation, the low seed level is still sufficient to obtain a good contrast of the output to the amplified parametric superfluorescence.
The system is believed to be of great interest for CARS [27], wavelength critical machining [28] and laser ablation mass spectroscopy utilizing narrow spectral resonances [29].With further amplification stages also a highly versatile source for molecular gas phase dynamics will result.The present spectral resolution of 3 cm −1 can be further tailored by varying the length of the of pump pulses.
With an amplified femtosecond Ti:sapphire laser as pump system, we amplified as few as 400 photons in a noncollinear blue pumped OPA.As a result nearly Fourier-limited output pulses at 532 nm with a duration of 71 fs result.Other seed wavelengths should work as well and therefore cw seeding is found to provide extremely controlled ultrafast pulses in the visible.Compared to previous work the presented system operates with much less cw power [12] and at much shorter pulse lengths [13].The amplification factor of ~10 7 -10 8 in the single stage renders output pulse energy of about 1 nJ.Further amplification in a second NOPA stage already allowed us to generate well above 1 µJ of output (setup and data not shown).
The fs amplified cw seed system is a unique source for many quantum optical experiments.We have already analyzed the absolute strength of OPG and use the cw seed laser as an internal reference.Most interesting is the influence of seed statistics on the output fluctuations.For these measurements even seed levels below 100 photons are utilized that have an inherent Poissonian fluctuation already well above the technical noise of the pump laser.In this way we will be able to determine how the transition from the photon picture appropriate for weak cw sources evolves into the classical wave picture that is typically used in the description of OPAs.
Both the fs and the ps OPA seeded by a single frequency continuous laser are sources of pulses with highest control of pulse parameters available at present in the µJ regime.Such pulse energies are needed for many nonlinear excitation schemes.The concept of cw seeding in combination with the appropriate pump laser gives the experimentalist new possibilities for dedicated investigations.

Fig. 1 .
Fig. 1.Schematic of the two-stage noncollinear optical parametric amplifier pumped by a ps laser.BS: beam splitter; FPC: fiber polarization controller.

Fig. 2 .
Fig. 2. Sample spectra of the ps-pumped NOPA output showing (a) the large tuning range from 1260 nm to 1630 nm, (b) the fine tuning capability exemplarily from 1490 nm to 1510 nm (the spikes are due to an imperfect subtraction of the cw background).

Fig. 3 .
Fig. 3. Spectrum (a), autocorrelation trace (b), and far field beam profile (c) of the ps-pumped NOPA output at 1300 nm.The violet and red line in (a) and (b) are Gaussian fits to the data.

Fig. 4 .
Fig. 4. a,b) Normalized SH output power in dependence on the FW input wavelength.The insets show measured mode profiles of the excited SH modes at the wavelengths of maximum conversion efficiency.(c,d,e) Normalized FW output intensity profiles of a WGA for linear propagation (blue line) and nonlinear propagation (red line) at the wavelengths marked by the dotted lines in (a,b).We find nonlinear competition and soliton formation (c) with higher order (d) and the first order SH component (e).

Fig. 6 .
Fig. 6.Autocorrelation (a) and spectrum (b) of the fs-pumped cw seeded NOPA.The red line in (a) shows a Gaussian fit to the data.The red line in (b) indicates the spectral position of the cw seed and the resolution of the spectrometer.(c) Single shot output spectrum of an OPG seeded NOPA.The green lines show Gaussian fits to the individual peaks.