Idler-resonant femtosecond tandem optical ... - OSA Publishing

Tillman et al.

Vol. 21, No. 8 / August 2004 / J. Opt. Soc. Am. B

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Idler-resonant femtosecond tandem optical parametric oscillator tuning from 2.1 ␮m to 4.2 ␮m Karl A. Tillman and Derryck T. Reid Ultrafast Optics Group, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, UK (www.phy.hw.ac.uk/resrev/ufast)

David Artigas ICFO-Institut de Cie`ncies Foto`niques, and Department of Signal Theory and Communications, Universitat Politecnica de Catalunya, 08034 Barcelona, Spain

T. Y. Jiang HC Photonics Corporation, Hsinchu 300, Taiwan Received October 21, 2003; revised manuscript received February 20, 2004; accepted March 16, 2004 A Ti:sapphire-pumped idler-resonant femtosecond tandem optical parametric oscillator is reported that is based on periodically poled lithium niobate and operates with wavelength tuning from 2.1 to 4.2 ␮m (idlers) and 1.25 to 1.40 ␮m (signal). The configuration uses two cascaded gratings arranged so that the nonresonant signal from the first grating acts as a pump for the second grating. Novel phase-matching behavior is described and explained, including full tandem operation, degenerate parametric downconversion of the signal pulses, and simultaneous tandem operation with seeded optical parametric amplification. Signal spectra show characteristic depletion profiles, implying substantial conversion from the signal to the tandem idler output. © 2004 Optical Society of America OCIS codes: 190.4410, 190.4970, 320.2250, 320.7080, 190.7110.

1. INTRODUCTION A. Concept of the Idler-Resonant Tandem Optical Parametric Oscillator Quasi-phase-matched crystals based on periodically poled lithium niobate (PPLN) can be readily fabricated to contain multiple gratings, and commonly this practice is applied to create parallel gratings on the same crystal so that, in an optical parametric oscillator (OPO), tuning can be achieved by moving the crystal transversely across the pump beam. A less common implementation involves cascading two or more gratings in series so that the light generated in the first grating undergoes further frequency conversion in the second grating, and the possibility of exploiting this approach to obtain efficient conversion from a near-infrared pump to a mid-infrared idler has been studied theoretically and experimentally for signal-resonant OPOs1–3 and theoretically for idlerresonant OPOs.4,5 The form of the secondary frequency-conversion step in these systems depends on the desired output, and several different processes have been considered both theoretically and experimentally, including sum-frequency generation6,7 and difference-frequency mixing.1,3 These early approaches used two separate crystals placed in series in an intracavity arrangement with each crystal phase matched for a different conversion process. However, owing to the improvements in modern fabrication 0740-3224/2004/081551-08$15.00

techniques, it is possible for both conversion processes to occur within a single crystal with an appropriate dualgrating design, and tandem conversion systems using a single crystal with a dual-grating design have been demonstrated for both sum-frequency-generation8 and difference-frequency-mixing9 processes. Tandem OPO devices,10 where a single grating crystal has been used to phase match both conversion processes simultaneously, have been developed both experimentally11 and theoretically.12 However, owing to requirements of the individual parametric conversion processes, it is not always possible to maximize the conversion efficiency of both of the processes. Each conversion stage converts a different pump wavelength, both of which have a different optimal interaction length, and therefore it is not possible to maximize both of these lengths in a single crystal. This means that simultaneously phase-matched tandem OPO devices based on single grating crystals, although simpler in design, may not necessarily achieve high depletion at both the pump wavelengths. The oscillator we report is based on a scheme in which femtosecond pump pulses at a frequency ␻ p are converted in the first grating to nonresonant signal pulses of frequency ␻ s and resonant idler pulses at a frequency ␻ i so that ␻ p ⫽ ␻ s ⫹ ␻ i . In the second grating, the nonresonant signal pulse acts as a second pump for a further parametric interaction according to ␻ s ⫽ ␻ i ⫹ ␻ i2 , in which the idler pulses remain resonant and a second, © 2004 Optical Society of America

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fore creates a second, tandem OPO that shares the same resonator with the first. As mentioned in Ref. 3, the behavior of an idler-resonant tandem OPO is similar to a cascaded-process OPO with the difference that simultaneous strong depletion of the pump and signal waves is not possible. In the femtosecond regime, different intracavity pulse synchronization constraints apply to the two cases because of group-velocity walk-away between the signal and idler pulses in the first grating. Tandem operation leads to the generation of a second, longer wavelength idler output, idler(2), whose presence provides a useful experimental signature of tandem operation. Although schematically the tandem OPO is represented as consisting of two separate crystals, in practice only a single monolithic crystal was used, and the conversion takes place in two differently quasi-phase-matched gratings.

Fig. 1. Schematic representations of idler-resonant OPO systems where each resonator is highly reflective (HR) at the idler wavelength: (a) One-stage process using a single crystal. (b) Two-stage cascaded conversion process with two cavities arranged in tandem. The signal output from the first cavity is the pump for the second cavity. (c) Alternative two-stage conversion process using an intracavity tandem configuration. A single cavity contains two crystals, each with a different grating, with the signal output from the first crystal pumping the second crystal.

nonresonant idler [referred to here as ‘‘idler(2)’’] is generated with a frequency ␻ i2 . In this way, the idler pulses ( ␻ i ) experience gain twice with the possibility (under strong pumping) to obtain more than 100% quantum efficiency in the conversion from the pump to the idler. Because the second grating also contributes gain to the resonant wave, the system can be properly considered to be a tandem OPO, although an equivalent description is as an OPO–difference-frequency-mixing device.3 Figure 1 illustrates schematically the difference between the conventional idler-resonant OPO [Fig. 1(a)], a cascaded OPO system with the idler resonant in both cavities [Fig. 1(b)], and the tandem idler-resonant OPO [Fig. 1(c)]. In all cases, the cavities have high reflectivity at the idler wavelength and high transmission for all other wavelengths. In the first case, as the pump light is depleted in the OPO crystal, the signal wave experiences gain and, in proportion to the exact ratios of the idler and signal photon energies, the majority of the pump energy is coupled from the cavity in the signal wave. In the cascaded OPO concept [Fig. 1(b)], the signal extracted from the first OPO is used as a pump for a second OPO, which is resonant at the same wavelength as the first. The gratings in the first and second OPOs are chosen to phase match the interactions ␻ p → ␻ s ⫹ ␻ i and ␻ s → ␻ i ⫹ ␻ i2 , respectively. The cascaded OPO system is conceptually very similar to the idler-resonant tandem configuration [Fig. 1(c)] in which the signal wave is again used as a pump for a second parametric downconversion stage that takes place in a second grating situated immediately after the first. In this second crystal, the signal light is depleted and supplies further parametric gain to the resonant idler, and the inclusion of this crystal there-

B. Femtosecond Tandem Optical Parametric Oscillator Design In previous theoretical work,5 we used a numerical model to investigate the efficiency of conversion in a tandem OPO from the pump to the idler in idler-resonant and signal-resonant configurations. Figure 2 illustrates one

Fig. 2. Contour plot showing the theoretical efficiency of the tandem conversion process as a function of the lengths of the two gratings. The contour lines represent lines of equal conversion efficiency (defined as the ratio of output idler power to input pump power), and the peak represents the ratio of grating lengths giving the best conversion efficiency. The dashed line corresponds to the optimal combination of grating lengths for pump pulse durations higher than 100 fs according to the length ratio grating 2/grating 1 ⫽ 1.6.

Fig. 3. Cavity design used in the tandem OPO illustrating the three-mirror V-cavity configuration used with the pump laser characteristics and the details of the seven double grating periods.

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Fig. 4. Quasi-phase-matching efficiency plots for four representative gratings showing how the conversion efficiency varies with wavelength and including the experimental data recorded. The gray-scale plots represent the efficiency of the conversion process based on the phase-matching function, sinc2 (⌬kl/2), where ⌬k ⫽ k s ⫺ k i ⫺ k i2 and the pump wavelength, ␭ p , is fixed at 845 nm. Darker shading represents higher efficiency, with the black indicating maximum efficiency. The horizontal dashed lines indicate the reflectivity bandwidth of the cavity mirrors (2.1 ␮m–2.5 ␮m), and the solid curves represent the expected wavelength tuning predicted by applying conservation of photon energy to the waves interacting in the primary grating and secondary gratings. (a) Tuning data near degeneracy for ⌳ 1 ⫽ 23.09 ␮m and ⌳ 2 ⫽ 34.68 ␮m, and (b) tandem operation and simultaneous parametric down-conversion of the signal for ⌳ 1 ⫽ 23.02 ␮m and ⌳ 2 ⫽ 34.83 ␮m; (c) tuning data for ⌳ 1 ⫽ 22.85 ␮m and ⌳ 2 ⫽ 34.13 ␮m and (d) for ⌳ 1 ⫽ 22.77 ␮m and ⌳ 2 ⫽ 32.78 ␮m.

of the key results of this model in the form of a plot showing the conversion efficiency as a function of the lengths of grating 1 and grating 2. The results indicate that optimal conversion is achieved when the ratio of the lengths of gratings 1 and 2 is approximately 1.6. The absolute maximum in conversion efficiency is predicted when the first grating has a length of ⬃0.6 mm and the second a length of ⬃0.9 mm. These lengths originate in the model from an assumption that the interacting pulse durations are ⬃100 fs, which therefore limits the interaction distances within each crystal between the pump, signal, and idler pulses because of their different group velocities and pulse walk-off distances at the wavelengths involved.5 In practice, we chose to use a crystal with the optimum ratio of grating sizes but with lengths of 1 mm (grating 1) and 1.6 mm (grating 2) because our laser produced longer pulses than those assumed in the original numerical model (dashed line in Fig. 2). The experimental configuration of the tandem OPO is represented in Fig. 3. The pump source for the OPO was a self-mode-locked Ti:sapphire laser operating at 845 nm and which produced pulses with durations of 150 fs at a repetition frequency of 106 MHz and a typical average output power of 750 mW. The PPLN crystal used was held at approximately 150 °C in the focus of a synchronously pumped three-mirror cavity with high finesse at

2.3 ␮m and comprising two concave reflectors with radii of curvature R ⫽ ⫺100 mm or R ⫽ ⫺200 mm and a plane high reflector (reflectivity ⬎ 99.9%) or a plane output coupler (reflectivity ⫽ 98%). The crystal comprised seven separate double gratings (see Fig. 3), which allowed the OPO to be extensively tuned by adjusting the lateral crystal position. Depending on the orientation of the crystal, the OPO could be operated in a tandem configuration (shorter-period grating nearest the pump entrance face of the crystal) or in a nontandem configuration (longer-period grating nearest the pump entrance face of the crystal). In the following section, we present and contrast results obtained from both configurations.

2. SPECTRAL CHARACTERIZATIONS A. Tandem Optical Parametric Oscillator Phase Matching Phase matching for an idler-resonant OPO is readily represented by a plot of idler and idler(2) wavelengths against the signal wavelength, which effectively acts as the pump for grating 2. In a synchronously pumped femtosecond OPO, the oscillator responds to a small change in the cavity length by modifying the center wavelength of the resonant pulses so that the cavity period remains constant due to the wavelength dependence of the group de-

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lay in the crystal. For each grating pair, cavity-length tuning was used to modify the signal wavelength, and Fig. 4 depicts the experimental idler and idler(2) data obtained as the signal wavelength generated by grating 1 was varied in this way. The four plots shown correspond to operation on grating pairs G [Fig. 4(a)], F [Fig. 4(b)], D [Fig. 4(c)], and C [Fig. 4(d)]. In each case, the phasematching diagram is constrained to a finite range of signal and idler wavelengths and is shown as an efficiency map where the normalized efficiency is sinc2 (⌬kl/2) and ⌬k ⫽ k s ⫺ k i ⫺ k i2 ⫺ 2 ␲ /⌳ 2 , with ⌳ 2 as the period of grating 2 and k j as the wave vector of wave j. Darker regions of the phase-matching map imply higher conversion efficiency from the signal pulse to the idler and idler(2) pulses. Owing to the broad spectral bandwidths of the signal, idler, and idler(2) pulses, each data point plotted in Fig. 4 includes both vertical and horizontal error bars indicating the spectral bandwidth full width halfmaximum (FWHM) of either the signal pulses (horizontal bars) or the idler or idler(2) pulses (vertical bars). Tandem operation of an idler-resonant OPO is constrained by a number of experimental factors. First, the

Fig. 5. Normalized selected idler and idler(2) spectra indicating, first, nontandem operation (solid black curve) and progressing through the onset of tandem operation (solid gray curve and dotted black curve) into the approach to degenerate operation (dotted gray curve), where the two idler wavelengths are the same.

Fig. 6. Logarithmic spectral intensity plot of idler and idler(2) outputs indicating low-efficiency, single-pass, optical parametric amplification seeded by the wings of the idler pulses and attributed to the high intracavity powers of the idler pulses.

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Table 1. Observable Phase-Matched Processes and Their Wavelength Coverage Output or Process Signal Idler Idler(2) Nonresonant optical parametric amplification Degenerate downconversion

Wavelength Coverage (␮m) 1.25–1.40 2.1–2.5 2.5–4.2 2.0 and 3.85 2.60–2.75

Table 2. Wavelengths Generated by Particular Conversion Processes Wavelength

Mechanism

844 nm 1.32 ␮m 2.34 ␮m 3.028 ␮m 1.170 ␮m 422 nm 512 nm 624 nm 662 nm

␻p ␻s ⫽ ␻p ⫺ ␻i ␻i ⫽ ␻p ⫺ ␻s ␻ i2 ⫽ ␻ s ⫺ ␻ i ␻i ⫹ ␻i ␻p ⫹ ␻p ␻p ⫹ ␻s ␻i ⫹ ␻i ⫹ ␻s , ␻p ⫹ ␻i ␻ s ⫹ ␻ s , ␻ p ⫹ ␻ i2

OPO can only operate at idler wavelengths lying within the mirror-reflectivity bandwidth, whose range is indicated by the parallel horizontal dashed lines in the lower part of each plot. A further constraint exists because, in common with all OPOs, the idler, idler(2), and signal wavelengths are related directly to the Ti:sapphire pump wavelength by photon energy conservation as shown by the gray curves. For tandem operation, the grating 1 energy conservation curves must intercept a region of high conversion efficiency in the phase-matching plots of grating 2. This principle was confirmed by experiment, which indicated tandem operation near degeneracy from 2.3 to 2.7 ␮m for grating G [Fig. 4(a)] and by other data for grating C [Fig. 4(d)], showing only a narrow band of tandem operation where the two phase-matching loci intersected. The experimental phase-matching data shown in Fig. 4 illustrate some novel and unexpected behavior. In Fig. 4(b), it is obvious that infrared outputs were generated in the 2.60–2.75-␮m wavelength region that could not be explained by tandem oscillation. A strong output was observed at wavelengths that departed strongly from the regions of the phase-matching diagram where tandem operation was expected. These data are plotted in the center of the darkest region of the phase-matching diagram [Fig. 4(b)], but because this region does not intercept the gray phase-matching curves corresponding to grating 1, the data are not caused by tandem operation. In Fig. 4(b) (grating F) the phase-matching locus has only a single maximum, and the significance of this is that the darkest region indicates efficient degenerate parametric downconversion of the signal wave according to ␻ s → ( ␻ s /2) ⫹ ( ␻ s /2). In addition to the downconversion process, the same grating exhibits true tandem operation near degeneracy, where the phase-matching loci of grat-

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ings 1 and 2 intercept, but also operates at locations in the phase-matching diagram corresponding to the firstorder sidebands of the sinc2 (⌬kl/2) function that describes the conversion efficiency. In Fig. 4(b), the sinc2 (⌬kl/2) sidebands appear as concentric ellipses surrounding the intense region of the phase-matching diagram. As the period of grating 2 was decreased, the character of the phase-matching diagram changed from a single maximum to a distributed phase-matching locus taking the form of a tilted ellipse [see Figs. 4(c) and 4(d)]. In these configurations strong tandem operation was observed, as expected, when the principal phase-matching loci of gratings 1 and 2 overlapped [Fig. 4(c)], but tandem behavior was again observed when the sidebands of the grating 2 phase-matching locus intercepted the gray phase-matching curves for grating 1 [Fig. 4(d)]. Although operation on the phase-matching sidebands was not as efficient as operation on the principal phasematching locus, we were able to observe easily measurable spectra under these conditions, and we attribute this to the high nonlinearity of PPLN and the high power of the intracavity idler pulses. B. Spectral Measurements During Tandem Operation When the crystal was oriented correctly for tandem operation, cavity-length tuning enabled either tandem or nontandem operation to be selected. By modifying the wavelength of the resonant idler pulses, cavity-length tuning also changed the signal wavelength, which in turn affected the viability of phase matching in grating 2. Examples of the tandem and nontandem idler spectra recorded from the OPO in this way are shown in Fig. 5, and the center wavelengths could be tuned smoothly toward or away from each other by adjusting the OPO cavity length. The spectrum of the resonant idler wave lacks the characteristic self-phase-modulated structure often observed in signal-resonant OPOs because the net cavity dispersion around 2.3 ␮m is negative, and therefore soliton-shaping effects are responsible for the pulse shape. Four different conditions are represented in the data shown in Fig. 5, all of which were recorded during operation in the lateral grating G. The shortest-wavelength idler spectrum (solid black curve) corresponds to nontandem operation in which the associated signal wavelength generated from grating 1 is not suitable for achieving phase matching in grating 2 and only a single peak is present. As the cavity length was varied, the signal wavelength became closer to that needed for tandem operation, and the next spectrum (solid gray curve) shows the onset of tandem operation at 2.7 ␮m where two peaks are present, the lower wavelength corresponding to the idler and the higher to idler(2). The third curve (dotted black curve) shows idler and idler(2) spectra at the location of maximum idler(2) generation, illustrating significant idler(2) generation. As cavity-length tuning was continued, the idler and idler(2) wavelengths converged toward the degeneracy point already discussed in Subsection 2.A shown by the fourth curve (dotted gray curve). A novel effect was observed at high intracavity powers (⬃2 W) in which the wings of the idler spectrum appeared to act as a seed for optical parametric amplification


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pumped by the signal pulses and present at the same time as normal tandem operation. The spectrum shown in Fig. 6 depicts on a logarithmic intensity scale the outputs that were recorded under this condition. Tandem operation is responsible for the two center spectra according to 1.32 ␮m (signal) → 2.38 ␮m (idler) ⫹ 2.94 ␮m [idler(2)], and optical parametric amplification gives rise to the remaining spectra according to 1.32 ␮m (signal) → 2.0 ␮m ⫹ 3.85 ␮m. Both wavelengths generated in the second case lay well outside the reflectivity band of the cavity mirrors, and therefore none of the interacting waves was resonant. The single-pass optical parametric amplification observed was partially phase-matched on a sideband of the grating 2 phase-matching diagram and would therefore be expected to proceed with very low ef-

Fig. 7. Tuning data for the reverse crystal orientation for selected gratings. The dashed lines represent the reflectivity bandwidths of the mirrors (2.1 ␮m–2.5 ␮m), and the solid curves represent the idler and idler(2) tuning expected from applying the conversation of photon energy. (a) Tuning data for grating G (⌳ 1 ⫽ 23.09 ␮m), indicating low-power tandem operation; (b) data from grating F (⌳ 1 ⫽ 23.02 ␮m), also indicating low-power tandem operation; (c) data from grating D (⌳ 1 ⫽ 22.85 ␮m); (d) data from grating C (⌳ 1 ⫽ 22.77 ␮m).

Fig. 8. Normalized idler spectrum recorded in the reverse crystal orientation indicating the difference in peak intensities between the idler output at around 2.4 ␮m with the idler(2) output at around 2.7 ␮m in the absence of phase-matched tandem operation. A comparison with Fig. 5 (black curve and symbols) shows that tandem operation is much less efficient in the reverse-crystal orientation.

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ficiency. For this reason, the process is unlikely to occur without seeding, and we conclude that the intense wings of the resonant idler wave contributed sufficient power to seed the optical parametric amplification process. The tuning and wavelength coverage of the phasematched or partially phase-matched processes observed in the tandem OPO are summarized in Table 1. In addition to the phase-matched processes referred to above, various non-phase-matched processes were also observed, often at significant powers. These commonly generated light at visible wavelengths through sum-frequency mixing processes, but non-phase-matched second-harmonic generation was also responsible for the creation of new wavelengths, and, in particular, second-harmonic generation of the resonant idler pulses led to a significant amount of infrared light at 1.2 ␮m. The wavelengths generated in this way are summarized in Table 2 along with the processes responsible. C. Tuning Behavior for Nontandem Operation As the crystal was made from a single monolithic crystal and contained two grating structures in series, it was not possible to study the nontandem behavior of the primary grating (responsible for converting the pump) in the absence of the secondary grating (responsible for converting the signal). In order to study the nontandem spectral behavior of the primary grating without the tandem secondary signal conversion taking place, it was necessary to reverse the orientation of the crystal. In this way, the generated signal could be removed from the cavity before it encountered the second grating and was recycled as a pump for the tandem process. The spectral data taken in the reverse configuration are plotted in Fig. 7, and the gratings used correspond directly with those used in the forward-alignment phasematching plots given in Fig. 4. All of the lateral gratings produced the expected standard nontandem OPO operation, but in two of the plots, 7(a) and 7(b), some of the data points apparently indicated that tandem operation still occurred even with the crystal in the reverse alignment. An explanation for this unexpected tandem operation is that both of the conversion processes could take place inside the primary grating because, at the wavelengths involved, the signal and idler group velocities are the same, and the necessary temporal overlap between the pulses is present as they are generated. In this situation, the process is similar to a standard OPO where the role of the pump, signal, and idler is now performed by the signal, idler, and idler(2), respectively. The result is that a small amount of the generated signal pulses is converted immediately into idler and idler(2) outputs, but as this now occurs within the primary grating and the process is not phase matched, the conversion is significantly less efficient. Numerical calculations of the wave-vector mismatch, ⌬k, suggest that for the given data points, the efficiency of the secondary conversion of the signal pulses in the 1-mm grating is much lower than in the 1.6-mm grating (forward direction) but still large enough to see some conversion. This point is better illustrated by comparing the relative intensities of the idler and idler(2) outputs in the forward-orientation arrangement with those seen in the reverse orientation of the tandem system. Figure 8

Fig. 9. (a) Interferometric autocorrelation and (b) intensity autocorrelation of the idler pulses with (c) a corresponding spectrum. Assuming a Gaussian intensity spectral profile, the autocorrelation measurements correspond to a pulse duration of 420 fs, and the shape of the interferometric autocorrelation indicates the presence of substantial frequency chirp in the idler pulse.

shows a selected spectrum taken from the data points presented in Fig. 7(a) and corresponds to the maximum idler(2) power generated with the crystal in the reverse orientation. By comparing the relative peak intensities of the idler and idler(2) outputs in this plot to the relative idler and idler(2) intensities in Fig. 5, the difference in the generated idler(2) is obvious, indicating a reduction in idler(2) power by a factor of approximately 6. This dif-

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ference is consistent with calculations of the efficiency factor, sinc2 (⌬kl/2), for the same processes in the first and second gratings. When tandem conversion occurs in grating 2, the normalized efficiency factor is of the order of 1, but for the conversion in grating 1 (whose period is not designed to phase match the ␻ s → ␻ i ⫹ ␻ i2 process), this falls by an order of magnitude to ⬃0.1.

3. TIME-DOMAIN CHARACTERIZATION With the OPO operating using a 2.5% plane output coupler, an autocorrelation measurement was recorded of the idler pulses leaving the cavity. In the configuration that was used, the average power of the idler output was 30 mW. The autocorrelation measurement was implemented using two-photon absorption in an InGaAs photodiode (cutoff wavelength ⬃1.7 ␮m), which gave a strong nonlinear response at the idler wavelength of 2.38 ␮m. Figure 9 shows the interferometric and intensity autocorrelations measured [Figs. 9(a) and 9(b)], and the corresponding spectrum is shown in Fig. 9(c). The intensity autocorrelation duration was 650 fs, and, assuming a Gaussian intensity profile, this corresponded to idler pulse durations of 420 fs. It can be seen from the interferometric autocorrelation [Fig. 9(a)] that the pulses contained substantial frequency chirp, and this was confirmed by the duration–bandwidth product of 1.24 that implied the pulses were approximately a factor of 3 longer than the transform limit.


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nontandem configuration (represented in Fig. 11 by the crosses) was 101 mW. In the forward configuration, the maximum idler power recorded from the OPO was 68 mW and was measured at 2.3 ␮m for a pump power of 750 mW. At close to a wavelength of 1.3 ␮m, the signal spectrum measured during tandem operation had a profile very similar to the characteristic shape associated with pump depletion and that was recorded at the Ti:sapphire pump laser wavelength. Directly confirming that this shape was a result of signal depletion caused by tandem operation was difficult because there was no way of interrupting the operation of grating 2 while still maintaining oscillation on grating 1 to generate an undepleted signal pulse to use as a comparison. Nevertheless, one indirect measurement exists that corroborates the assumption that the spectral shape is a result of signal depletion due to tandem operation. Inspection of the wavelength of maximum depletion in the signal spectrum [Fig. 12(a)]

4. POWER CONVERSION PERFORMANCE The conversion performance of the OPO was studied by investigating a variety of measurements including the pump depletion, the oscillation threshold, the output power, and the slope efficiency. By measuring the fractional pump depletion, it was possible to determine the overall efficiency of the oscillator, and this was recorded for a pump wavelength of 840 nm to be 78% when the OPO resonator was configured using two R ⫽ ⫺100 mm high-reflector mirrors and one plane high-reflector mirror. The pump depletion was measured by recording the pump spectrum immediately after the end R ⫽ ⫺100 mm highreflector mirror when the OPO cavity was running and when it was blocked internally. Figure 10 shows the depleted and undepleted pump spectra from which the pump depletion was inferred. The pump-depletion value includes all parasitic losses present in the cavity and was substantially higher than the idler extraction efficiency. With the 2.5% output coupler installed, we measured the idler slope efficiency in the forward, tandem configuration across a range of pump powers from 0 to 450 mW and obtained a value of 11%, while in the reverse, nontandem configuration, using a range of pump powers from 0 to 600 mW, a slope-efficiency value of 6.5% was recorded. The slope-efficiency measurements carried out in both the tandem and nontandem configurations are displayed in Fig. 11, and the threshold values for each configuration were also measured and were similar in both cases. For the tandem configuration (represented in Fig. 11 by the circles), the threshold when the resonator entirely comprised high-reflector mirrors was 94 mW and for the

Fig. 10. Spectra indicating a 78% depletion of the Ti:sapphire pump wave (␭ ⫽ 840 nm) when the OPO was operating in the tandem alignment and producing an average idler output power of 68 mW.

Fig. 11. Idler slope-efficiency data taken from the system in both of the two possible crystal configurations. Data taken with the crystal in the forward direction implied a slope efficiency of 11% and are shown as circles, while data taken with the crystal in the reverse direction implied a value of 6.5% and are shown as crosses.

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5. CONCLUSIONS We have described the configuration and performance of an idler-resonant femtosecond tandem optical parametric oscillator that operated with wavelength tuning from the near- to mid-infrared with tuning ranges of 1.25–1.40 ␮m (signal), 2.1–2.5 ␮m (idler), and 2.5–4.2 ␮m [idler(2)]. The OPO has revealed effects previously unreported in an ultrafast system including novel phase-matching effects like seeded optical parametric generation and degenerate downconversion of the nonresonant signal wave. The OPO itself represents a source of high-quality midinfrared pulses because the idler-resonant cavity potentially ensures that the beam quality of the midinfrared output is diffraction-limited and therefore suitable for demanding experiments in nonlinear imaging and other fields.

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2. 3. 4. 5.

Fig. 12. (a) Signal spectrum and (b) idler spectra recorded during tandem operation. The central wavelength of the signal spectrum corresponds with the maxima of the generated idler spectra, suggesting that the signal spectrum indicates depletion due to tandem conversion.

shows that it corresponds, through the relation 1/␭ s ⫽ 1/␭ i ⫹ 1/␭ i2 , to the signal wavelength inferred from the peak idler and idler(2) wavelengths shown in Fig. 12(b). Although this evidence is not a conclusive indicator of signal depletion (other effects such as group-velocity walk-off between the pump, signal, and idler pulses can cause similar spectral shapes), if correct, it indicates that there is substantial conversion from the signal to the idler and idler(2) pulses and that the tandem configuration is successful in increasing the amount of idler generated. Absolute measurements of the average idler(2) power generated were not possible because of the poor transmission of the idler(2) wavelength through the cavity mirror coatings and substrates and the difficulty of separating the idler and idler(2) wavelengths in order to make a power measurement of only the idler(2) output.

6. 7.

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