We demonstrate a picosecond optical parametric oscillator (OPO) that is synchronously pumped by a fiber-amplified gain-switched laser diode. At 24W of pump power, up to 7.3W at 1.54µm and 3.1W at 3.4µm is obtained in separate output beams. The periodically poled MgO-doped LiNbO3 OPO operates with ~17ps pulses at a fundamental repetition rate of 114.8MHz but can be switched to higher repetition rates up to ~1GHz. Tunabilty between 1.4µm and 1.7µm (signal) and 2.9µm and 4.4µm (idler) is demonstrated by translating the nonlinear crystal to access different poling-period gratings and typical M2 values of 1.1 by 1.2 (signal) and 1.6 by 3.2 (idler) are measured at high power for the singly resonant oscillator.
©2010 Optical Society of America
Synchronously pumped optical parametric oscillators (SPOPOs) are of great interest as a source of broadly tunable picosecond and femtosecond pulses. Such systems are normally pumped by mode-locked bulk solid-state laser systems with typical fixed pulse repetition rates of ~100MHz. However, the emergence of new sources of ultrashort pulses has led to various demonstrations of SPOPOs with repetition rates up to 82GHz , and combined signal and idler average powers up to ~27W . The pump sources have also become more compact, allowing SPOPOs based on amplified mode-locked diode sources , fiber lasers , and passively mode-locked miniature bulk lasers [1,5]. In this work we demonstrate, for the first time, an SPOPO pumped by a simple gain-switched laser diode amplified by a chain of Yb-doped fiber amplifiers. This pump system benefits from a highly compact and simple design with a minimum of free-space components, a user-controlled repetition rate up to the GHz regime, and the potential for scaling to high average powers . High-repetition rate ultrashort pulse sources are of interest for a range of applications including telecommunications  and non-invasive nonlinear microscopy [7,8], and SPOPOs in particular are well-suited to CARS microscopy as they deliver synchronized pulses at two different wavelengths .
2. Experimental configuration and results
The pump laser used in these experiments has been described previously , and is shown in Fig. 1 . It consists of a gain-switched laser diode and a chain of diode-pumped Yb-doped fiber amplifiers with just one free-space coupling component, making it a highly stable and practical pump source. It delivers linearly polarized, 21ps pulses with average powers up to 100W at a center wavelength of 1.06µm. The repetition rate can be varied between 100MHz and 1GHz through the use of an electro-optic modulator pulse-picker and the output beam has a stable M2 of 1.02 due to the use of a tapered splice into the otherwise multi-mode final power amplifier. For high repetition rates there is no significant spectral broadening due to nonlinear effects in the fibers, while at low repetition rates, and hence higher peak powers, the spectral broadening is sufficient to allow pulse compression down to 1.2ps, although this is not exploited in this work. The polarization extinction ratio (PER) of the final power amplifier output is 19dB but this is degraded somewhat by the optical isolator. Since a linearly polarized pump beam is required for an efficient parametric interaction in the SPOPO, a λ/2 waveplate and a polarizing beamsplitter (PER>23dB) is used to restore the linear polarization. A two-lens telescope and a final focusing lens are used after the polarizing beamsplitter to obtain the required pump waist size.
The SPOPO used a periodically poled 5% MgO-doped congruent LiNbO3 (MgO:PPLN) crystal for the nonlinear gain medium. The MgO-doping helps to reduce photorefractive beam distortion and green-induced infrared absorption , as well as extending the mid-infrared transparency to ~5µm . The crystal, provided by Covesion Ltd., was 40mm long, 10mm wide and 0.5mm thick with eleven 0.5mm wide poled gratings with periods from 26.5µm to 31.5µm. It was held in an oven at 150°C to eliminate any residual photorefraction. A length of 4cm was chosen taking into account the size of the beams that can pass through the 0.5mm by 0.5mm aperture without clipping and the desire for high nonlinear conversion. The differences in the group velocities of the interacting waves for a 1.06µm pump, a 1.4 - 1.7µm signal, and 3 - 4µm idler are small enough to give a pump acceptance bandwidth that copes with the nonlinearly broadened pump spectrum, as well as minimal temporal walk-off.
2.1 Standing-wave cavity optical parametric oscillator
Initial trials of the SPOPO performance were carried out with a bow-tie configuration standing-wave resonator, as shown in Fig. 2 . The MgO:PPLN was orientated with its optic axis aligned in the plane of the resonator and thus the pump polarization was also set to this axis.
If we assume diffraction-limited beams and set the resonated signal and the pump waist spot sizes (radii), ws and wp, at the center of the MgO:PPLN crystal to be roughly equal then, given the relationship between these spot sizes and the idler polarization spot size , i.e. wi = (ws−2 + wp−2)-1/2, we are limited to a range of signal spot sizes between ~90µm and ~160µm to avoid clipping of any of the beams by the 0.5mm by 0.5mm poling aperture (here we define clipping as occurring when a beam spot size becomes greater than 1/3 of the aperture at any point in the crystal). In practice we used 250mm radius of curvature mirrors for M1 and M2 (see Fig. 2) and with the cavity length set to match the lowest repetition rate of the pump source (114.8MHz) the calculated signal waist was ~98µm with insignificant astigmatism due to the θ = 12° folding angle (see Fig. 2). With the focusing lenses available we set the pump beam waist to be 125µm and hence obtained a calculated idler polarization spot size of 77µm. Thus the pump focusing is considerably weaker than confocal, (a 56µm spot size would correspond to confocal focusing over the 4cm long MgO:PPLN crystal).
All mirrors M1-M4 were highly reflective for the signal wavelength, apart from the output coupler M3, and highly transmissive for the idler in the mid-infrared (CaF2 substrates are used). The pump input coupler M1 transmitted 92% of the pump light and the idler output coupler M2 transmitted 88% of the generated idler light. A modified Findlay-Clay analysis of the internal losses of the cavity , indicated a roundtrip signal loss of 9.5%. The MgO:PPLN, with MgF2 single-layer anti-reflection (AR) coatings on its end-faces, had a measured single-pass transmission of 95% at 1.5µm. As the round trip involved two passes through the crystal it appears that the internal losses at the signal wavelength were dominated by the AR coatings.
Figure 3 shows the initial low-power characterization of the standing-wave SPOPO using an R = 95% reflectivity signal output coupler (M3) and a pulse repetition rate of 918.4MHz (8 pulses circulating in the cavity). Pump depletion of up to 50% was observed with maximum output signal and idler powers of 1.2W and 1.3W for a pump power incident on M1 of 11.4W. The threshold of just over 2W indicates that even with these relatively large internal losses, the pump source, which is at present scalable to 100W average power, could provide sufficient peak power to allow SPOPO operation three times above threshold at repetition rates of more than 10GHz. However, when using a larger output coupling (R = 65%) and increased pump power the roll-over of the SPOPO output, which can already be seen in Fig. 3, became much stronger, with oscillation ceasing at ~21W of pump power. That thermal effects were, at least in part, responsible for this effect was confirmed by observing that when a 50% duty cycle chopper was placed in the pumping beam to reduce the average power, oscillation was recovered at the high pump power level.
This result is at variance with the simplistic notion that the OPO should not exhibit any significant thermal input. Also, previous reports of high-power operation of periodically-poled LiTaO3 , and LiNbO3 , did not show such effects. No improvement was seen in this behavior by tuning further away from the mid-infrared transparency boundary to idler wavelengths of less than 3µm and there was no increase in the pump bandwidth at higher powers in this high repetition rate regime, which might have led to poorer conversion efficiency. As the MgO:PPLN was in contact with a temperature controlled oven (fixed to 0.1°C), we would also expect that any physical clipping of the idler beam due to non-diffraction-limited performance would not cause significant variations in temperature. However, further indication of a thermal input to the nonlinear crystal was given by an observed linear tuning of the output signal wavelength by 0.155nm/W, as the pump power is increased. For a pump power increase from 2.6W to 23.8W, the signal wavelength increases by ~3.3nm, which, if due only to a change in temperature , would correspond to a ~6°C rise in temperature within the crystal at the location of the interacting beams.
The MgO:LiNbO3 crystal was in thermal contact with a gold-plated temperature-controlled copper block on one of its 40mm by 10mm faces with the other being in contact with an indium-tin-oxide-coated piece of glass (for management of electrostatic effects) such that heat was effectively dissipated from just one side of the crystal. While a full thermal model of this experimental arrangement has not yet been undertaken we note that MgO-doped, or undoped, LiNbO3 has a large value for (dne/dT)/Kc, where dne/dT is the rate of change of the extraordinary refractive index with temperature , and Kc is the thermal conductivity. In comparison, the value of (dn/dT)/Kc in a well-known high-power laser material such as YAG is ~20 times smaller. Using these material parameters in a simple cylindrical rod model , leads to predictions that if less than 1% of the pump were converted to heat within the volume occupied by the beam then a thermal lens of focal length ~1cm, sufficient to cause the cavity to become unstable, would be present.
2.2 Ring cavity optical parametric oscillator
In an attempt to reduce the cavity losses and the effect of any thermal aberrations by only having one pass per round trip through the MgO:PPLN, the cavity shown in Fig. 2 was modified to a ring resonator. In order to continue to match the fundamental repetition rate of 114.8MHz the overall cavity length was increased by a factor of 2 compared to the standing-wave oscillator and this led to a smaller and more asymmetric calculated signal waist size in the MgO:PPLN of 47µm (in the plane of the cavity) by 59µm. To maintain good spatial overlap, the pump waist size was adjusted to 50µm, leading to an expected idler polarization waist size of ~35µm and, consequently, an expectation of significant idler clipping due to the increased beam divergence, even if the beam was diffraction-limited. A similar Findlay-Clay analysis to that used for the standing-wave cavity gave a higher value for the round trip losses of 15.7%, despite the single pass through the MgO:PPLN crystal. However, additional losses for the idler in this configuration due to increased diffraction losses and clipping would reduce the parametric gain [18,19], and hence increase the effective loss as determined by the Findlay-Clay measurement.
Figure 4 shows the output power characteristics and pump depletion for 1, 4, and 8 pulses circulating in the cavity for pump average powers up to 24W, currently limited by the thermal effects in the isolator used between the pump source and the SPOPO. The first observation was that the roll-over effect was not present for the lower repetition rates and while it was still present for the 918.4MHz results, oscillation was maintained up to the full 24W of pump power. For 114.8MHz operation we obtained 7.3W of signal at 1.54µm and 3.1W of idler at 3.4µm, with a pump depletion that saturated at ~70%. An R = 65% output coupler was used for all the results in Fig. 4 and it is likely that a larger output coupler could have provided greater signal output power for the low repetition rate case, as it operated at 33 times above threshold at maximum power.
The greatly improved performance with the ring resonator suggests that it benefited from only having one pass of the signal through the nonlinear crystal per round trip. Modeling of the ring and standing-wave cavities suggest that they would require thermal lenses of similar strength to drive them into instability and so we speculate that the reduced aberrations imposed on the signal beam by only passing once through the crystal was the more important effect. It may also be the case that the single pass of the signal through the crystal led to less heat being deposited, although the mechanism by which any such absorption occurs is not clear. It is interesting to note that the performance has improved despite the expectation that the idler would not be able to pass through the nonlinear crystal in this configuration, without a significant degree of interception by the surfaces of the MgO:PPLN slab.
The presence of the power roll-over in the high repetition rate results is intriguing as there should be no difference in the thermal load for the different repetition rates. The only change is that the gain is lower due to the reduced peak intensity of the pump pulse, which is reflected in the higher average power threshold.
In order to confirm the potential for tunable operation we translated the nonlinear crystal to access gratings with different poling periods. The experimental results, taken at a repetition rate of 918.4MHz, are shown in Fig. 4(d) together with the theoretical curve derived from the temperature-dependent Sellmeier equation for MgO-doped congruent LiNbO3 . The temperature of the crystal was set to 150°C and the pump power was adjusted to be approximately 15% higher than the threshold level at each signal wavelength. Tuning was demonstrated between 1.40 and 1.68µm for the signal and 2.87µm and 4.36µm for the idler.
Autocorrelation measurements, taken at 918.4MHz, gave full-width half-maximum pulse durations of ~17ps (assuming a Gaussian pulse shape) for both signal and idler. Figure 5 shows an interferometric autocorrelation of the 3.4µm idler pulse taken using two-photon absorption in an InGaAs photodiode with an extended sensitivity up to 2.1µm. The fully filled-out fringes suggest a nearly bandwidth-limited pulse, but we did not have a suitable spectrometer available to measure the idler spectrum.
The beam quality, M2, for both the signal and idler, again taken at 918.4MHz, was measured at both medium and high powers. For the signal M2 measurements, a 100mm focal length lens was used to produce a beam waist through which a beam profiler was translated, recording the 1/e2-beam radius at points along the beam. The M2 value was then fitted to this data. The beam profiler was not sensitive in the idler spectral region and so a knife-edge method was used to determine the 15% and 85% power transmission points, allowing an estimation to be made of the 1/e2-beam radius. This was repeated for several positions along the idler beam spanning the waist region and again an M2 value was fitted to the data. The results presented in Table 1 show that the resonated signal was near diffraction-limited with just a small degradation at higher pump powers. The M2 of the idler shows a significant departure from diffraction-limited performance, which we expect as the non-resonated beam was composed of an addition of the idler radiation generated from all points along the 4cm length of nonlinear crystal, which is much greater than the confocal parameter corresponding to the idler polarization waist size. We also note that the M2 measured in the plane of the resonator (denoted as the x-axis) was consistently worse than in the y-axis. There were various factors that distinguish the two axes and which may be responsible for this difference. These include the asymmetry in the resonated signal spot size due to the angle θ at the curved mirrors, stronger thermal lensing in the x-plane as this corresponds to the heat removal axis due to the slab geometry of the crystal, and the presence of a hard aperture in the x-axis due to the oven.
We also made spectral measurements of the depleted and undepleted pump at high average powers in the two repetition rate extremes, where nonlinear broadening is either absent or at its strongest. The results are shown in Fig. 6(a) and 6(b), where we have set the areas under the curves to represent the measured average powers of the input pump and the residual pump after depletion and thus give directly comparable spectra. It appears that depletion occurred across the entire spectrum confirming no limitation due to the phase-matching bandwidth.
Measurements of the signal bandwidth at the two repetition rate extremes and at 24W incident pump power are shown in Fig. 6 (c) and (d). Both spectra have similar full-width half maximum values and show some sign of structure although it is much more pronounced in the lower repetition rate case.
We have demonstrated a picosecond optical parametric oscillator pumped by a fiber-amplified 1.06µm gain-switched laser diode. The pump source is highly compact and has a minimum number of free-space components. The SPOPO can deliver pulses at 114.8MHz, 229.6MHz, 459.2MHz, or 918.4MHz by simply controlling the source repetition rate via a pulse picker and having 1, 2, 4, or 8 pulses circulating in the SPOPO cavity without any need to adjust the cavity length. Output powers as high as 7.3W at 1.54µm and 3.1W at 3.4µm were observed with 24W of pump power, and tunability between 1.4µm and 1.7µm (signal) and 2.9µm and 4.4µm (idler) was demonstrated. The pulse duration for both signal and idler was ~17ps.
The pump source allows great potential for future scalability in average power, reduced pulse duration, and a greater range of repetition rates. However, we have observed under some experimental conditions that a power roll-over effect and even complete cessation of oscillation can occur as the pump power is increased. We speculate that this may be due to an unexpected thermal input into the nonlinear crystal which, in combination with adverse material thermal coefficients, leads to strong thermal lensing and aberrations. The use of a ring cavity as opposed to a standing-wave cavity appears to reduce the effect, perhaps due primarily to having only a single pass per round trip of the signal beam through the aberrated thermal lens. Future work will be directed towards investigating these limiting effects more fully in order to reduce their impact so that the full power scaling potential offered by this pump source can be realized and consequently to achieve significant power and repetition rate scaling beyond the results presented here.
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