December 15, 2013 / Vol. 38, No. 24 / OPTICS LETTERS
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Fiber-laser-based green-pumped picosecond MgO:sPPLT optical parametric oscillator S. Chaitanya Kumar1,* and M. Ebrahim-Zadeh1,2 1
2
ICFO-Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860 Castelldefels, Barcelona, Spain Institucio Catalana de Recerca i Estudis Avancats (ICREA), Passeig Lluis Companys 23, Barcelona 08010, Spain *Corresponding author:
[email protected] Received October 22, 2013; accepted November 5, 2013; posted November 11, 2013 (Doc. ID 199903); published December 9, 2013
We report a stable, high-power, picosecond optical parametric oscillator (OPO) at 160 MHz repetition rate synchronously pumped by a frequency-doubled mode-locked Yb-fiber laser at 532 nm and tunable in the near-infrared, across 874–1008 nm (signal) and 1126–1359 nm (idler). Using a 30-mm-long MgO:sPPLT crystal, the OPO provides average output power up to 780 mW in the signal at 918.58 nm and 600 mW in the idler at 1242 nm. The device operates stably over many days, even close to degeneracy, exhibiting passive long-term power stability better than 1.8% rms in the signal and 2.4% rms in the idler over 2.5 h at a temperature of 55°C. We investigate spectral and temporal characteristics of the signal pulses under different conditions and demonstrate cavity-length tuning enabled by the dispersion properties of MgO:sPPLT. The output signal pulses have a duration of 2.4 ps at 967 nm. © 2013 Optical Society of America OCIS codes: (190.7110) Ultrafast nonlinear optics; (190.4970) Parametric oscillators and amplifiers; (190.4400) Nonlinear optics, materials; (320.7160) Ultrafast technology. http://dx.doi.org/10.1364/OL.38.005349
Progress in synchronously pumped picosecond optical parametric oscillators (OPOs) operating in the nearinfrared (near-IR) has enabled the development of powerful techniques such as coherent anti-Stokes Raman scattering microscopy, paving the way for a variety of applications in biology and medicine [1–3]. Efficient and practical operation of such OPOs at high average power relies on stable pump sources in the green and exploitation of suitable nonlinear materials with desirable properties, including high damage threshold, sufficiently large nonlinearity, long interaction length, and noncritical phase-matching capability. Traditionally, green-pumped picosecond OPOs have been developed by exploiting birefringent nonlinear crystals such as LiB3 O6 (LBO) and KTiOPO4 (KTP). Using a 30-mm-long, Brewster-cut LBO crystal, a picosecond OPO tunable across 740–930 nm (signal) and 1240–1890 nm (idler) at a repetition rate of 80 MHz was demonstrated [2]. In an earlier work, a highrepetition-rate picosecond OPO based on a 5-mm-long KTP crystal, operating at 352 MHz, was reported [4]. Using 6-mm-long KTP crystals, a picosecond OPO was developed at 76 MHz repetition rate, providing signal tuning in the 1010–1100 nm spectral range [5]. The advent of quasi-phase-matched (QPM) nonlinear materials enabled further development of green-pumped picosecond OPOs based on periodically poled (PP) crystals. Using a 10.8-mm-long PPKTP crystal, picosecond pulses at 80 MHz and tunable in the 890–1325 nm wavelength range were generated from an OPO [3]. Recently, a picosecond OPO based on a 20-mm-long, MgO-doped PP LiNbO3 (MgO:PPLN) crystal, pumped by 20 ps green pulses at a repetition rate of 230 MHz, generating a total (signal plus idler) power of >120 mW for 0.5 W of average pump power was also demonstrated [6]. However, the MgO:PPLN crystal was physically damaged after 30 min of operation, while pumping at 2 W of green power. In practice, the development of green-pumped OPOs using the most well-established QPM nonlinear 0146-9592/13/245349-04$15.00/0
material, MgO:PPLN, remains challenging due to various limitations including photorefractive damage. As such, it would be desirable to investigate alternative nonlinear materials for stable, high-power, and reliable long-term operation of green-pumped near-IR picosecond OPOs. One such material is the QPM nonlinear crystal, MgO-doped PP stoichiometric LiTaO3 (MgO:sPPLT), which has in recent years proved to be an attractive alternative for high-power frequency conversion from the visible to mid-infrared [7–10], offering superior thermal properties [7,8]. In spite of a lower effective nonlinear coefficient (deff ∼ 9 pm∕V) than PPLN (deff ∼ 16 pm∕V), increased resistance to photorefractive damage and green-induced infrared absorption have enabled highpower operation of green-pumped OPOs based on MgO:sPPLT in the continuous-wave (cw) regime to generate watt-level near-IR output power [10]. In this Letter, we report, for the first time to our knowledge, a stable, high-power, picosecond OPO for the nearIR based on MgO:sPPLT as the nonlinear material and pumped at 532 nm in the green. The OPO is synchronously pumped by the frequency-doubled output of a modelocked picosecond Yb-fiber laser, resulting in a compact and robust architecture, and provides tuning across 874–1008 nm in the signal and 1126–1359 nm in the idler. It delivers practical powers and operates with excellent long-term stability over many days. Using a 30-mm-long crystal, we generate a maximum signal power of 0.78 W at 918.58 nm and idler power of 0.6 W at 1242 nm. Even when operating near degeneracy, the OPO exhibits passive power stability better than 1.8% rms for the signal and 2.4% rms for the idler over 2.5 h, while the green pump exhibits power stability better than 0.4% rms. The output signal pulses have a duration of 2.4 ps at 967 nm, corresponding to a MgO:sPPLT crystal temperature of 100°C. We also demonstrate cavity-length tuning enabled by the dispersion properties of MgO: sPPLT and investigate spectral and temporal characteristics of the signal pulses under different conditions. © 2013 Optical Society of America
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The schematic of the experimental setup is shown in Fig. 1. The primary pump source is a mode-locked Ybfiber laser (Fianium, FP1060-20) delivering ∼20.8 ps pulses at 80 MHz repetition rate and operating at a center wavelength of 1064 nm. The laser has a double-peak spectrum with a bandwidth of 1.4 nm (FWHM). The picosecond green pump radiation for the OPO is obtained by single-pass second-harmonic generation of the Yb-fiber laser in bismuth triborate, BiB3 O6 (BIBO), providing up to 5.4 W of average green power at 532 nm with a FWHM spectral bandwidth of 0.57 nm [11]. The green pump exhibits passive power stability better than 0.24% rms over 15 h and has a TEM00 spatial mode with an elliptic beam profile due to spatial walk-off between the fundamental and the second-harmonic beam in the BIBO crystal. The beam is circularized using two cylindrical lenses (not shown in the Fig. 1) with focal lengths, f 75, 150 mm, separated by 245 mm, resulting in a circularity of ∼0.7 before pumping the OPO. The nonlinear crystal for the OPO is a 30-mm-long MgO:sPPLT sample with a single grating period (Λ 7.97 μm) and is housed in an oven with a stability of 0.1°C and adjustable from room temperature to 200°C. The green beam is focused to an elliptic waist radius of w0 ∼ 46 × 69 μm at the center of the nonlinear crystal. The OPO is configured in a standing-wave X cavity with two plano-concave mirrors, M 1 − M 2 (r 100 mm), and a plane mirror, M 3 . All mirrors are highly transmitting (T > 90%) for the pump at 532 nm, highly reflecting (R > 99%) for the signal over 840–1000 nm, with good transmission (T ∼ 85%–90%) for the idler over 1100–1500 nm, thus ensuring singly resonant oscillator (SRO) configuration. A plane output coupler (OC) is used to extract the signal power from the OPO, and a dichroic mirror, M 4 , separates the generated idler from the pump. The mirrors M 2 and M 3 are separated by ∼41 cm, while the total optical length of the OPO cavity is ∼1.875 m, corresponding to a repetition rate of 160 MHz, ensuring synchronization at the harmonic of the pump laser repetition rate. In order to characterize the OPO, we initially performed measurements of temperature tuning. By varying the temperature of the MgO:sPPLT crystal from 50°C to 200°C, we were able to tune the OPO across 874–1008 nm in the signal, together with idler tuning over 1126– 1359 nm, resulting in a total signal plus idler wavelength coverage of 367 nm. The simultaneously measured signal and idler power across the tuning range is shown in Fig. 2.
Fig. 1. Experimental configuration of picosecond greenpumped OPO based on MgO:sPPLT. FI, Faraday isolator; λ∕2, half-wave plate; L, lens; PBS, polarizing beam splitter; M, mirrors; OC, output coupler.
Fig. 2. Variation of the (a) signal and (b) idler power across the tuning range of green-pumped MgO:sPPLT picosecond OPO.
A partially reflecting OC with variable transmission over the signal wavelength range was used to extract a significant amount of intracavity power. The transmission of the OC varies from 5% at 874 nm to 86% at 1008 nm. Deploying such an output coupler enabled stable longterm operation of the OPO with reduced risk of damage to the nonlinear crystal. For a fixed green power of 2.8 W, the signal power extracted through the output coupler varied from 0.44 W at 874 nm to 0.23 W at 1008 nm, with a maximum of 0.78 W at 918.58 nm and >0.6 W over 58% of the signal tuning range. The corresponding idler power varied from 0.21 W at 1126 nm to 0.51 W at 1359 nm, with a maximum of 0.6 W at 1242 nm and >0.5 W over 70% of the idler tuning range, while a maximum pump depletion of 74% was recorded. It is to be noted that data presented here were not corrected for any residual coating losses on the crystal faces, mirrors, or transmission optics at the input and output of the OPO.
Fig. 3. Variation of the signal and idler power from the OPO as a function of the green power. Inset: signal spectra at low and high pumping levels.
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The power scaling results for the green-pumped MgO: sPPLT picosecond OPO are shown in Fig. 3. At a fixed temperature of 100°C and for a signal output coupling of ∼30%, we extracted as much as 0.7 W of signal power at 958 nm, together with 0.55 W of idler power at 1196 nm, for a pump power of 3.1 W. This corresponds to a total power of 1.25 W, representing an overall extraction efficiency of 40%. The threshold of the OPO is recorded to be 0.68 W, and the slope efficiencies of the extracted signal and idler power are 31% and 23%, respectively. Operating the OPO in pure SRO configuration, with no output coupling, resulted in the generation of as much as 1 W of idler power for a green pump power of 4.74 W, but the OPO operation was unstable. In order to avoid any damage to the MgO:sPPLT crystal, we limited the green pump power to 15 nm is recorded when pumping 4.1 times above threshold. This spectral broadening is signature of self phase-modulation of the intracavity signal pulses at high circulating intensities and is typically observed in synchronously pumped OPOs operating efficiently well above threshold. In addition to temperature tuning, we also investigated cavity-length tuning of the OPO for a fixed green power of 2.7 W, at a temperature of 50°C, close to degeneracy. The simultaneously recorded signal and idler output power as a function of cavity-length detuning is shown in Fig. 4. The signal output coupling in this wavelength range is ∼86%, leading to a higher OPO pump threshold of 1.4 W. Under perfect synchronization, while pumping ∼2 times above threshold, we extracted as much as 232 mW of signal and 213 mW of idler, corresponding to an overall extraction efficiency of 16.5%. As the cavity length is detuned from −72 μm, the signal power increases from 99 mW to a maximum of 232 mW at zero detuning, beyond which it decreases to 160 mW at 30 μm of detuning. It is to be noted that the signal and idler power follow each other, while the OPO output power varies asymmetrically with respect to the zero
Fig. 4. Variation of the signal and idler power from the MgO: sPPLT SPOPO as a function of the cavity detuning. Inset: signal spectra at negative and positive detuning positions.
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detuning, which corresponds to the maximum output power. Moreover, the output power from the OPO is more sensitive to positive detuning. This asymmetry in output power with cavity length detuning in synchronously pumped OPOs is mainly attributed to the delay in the temporal overlap of the signal and pump pulses in the crystal [12], which is essentially determined by the group velocities of the signal and pump. At a temperature of 50°C, the group velocities of the pump and signal in MgO:sPPLT are calculated to be vgp ∼ c∕2.38 and vgs ∼ c∕2.18, respectively, where c is the velocity of light. This difference in the group velocities results in a time delay of ∼20 ps between the signal and the pump pulse, while traveling through the 30-mm-long MgO:sPPLT crystal. Also shown in Fig. 4 is the variation of the signal wavelength as a function of cavity detuning, with the corresponding spectra at the extremes. As the cavity is detuned from −76 μm to 30 μm, the signal wavelength varies from 987 to 1013 nm. Further, we characterized the OPO with regard to output stability by performing measurements of long-term average power fluctuation. For a fixed pump power of 2.8 W, the simultaneously recorded power stability of the pump, signal, and idler at full output power is shown in Fig. 5. It can be seen that the pump exhibits passive power stability better than 0.4% rms, while the signal and idler power stability is recorded to be better than 1.8% rms and 2.4% rms, respectively, over a period of 2.5 h under free-running conditions. The measurements were performed at a temperature of 55°C, corresponding to signal (1002 nm) and idler (1134 nm) wavelengths close to degeneracy. We also operated the OPO continuously over more than 24 h without any damage to the MgO:sPPLT crystal. Finally, we performed spectral and temporal characterization of the signal pulses from the green-pumped OPO at different temperatures of the MgO:sPPT crystal using a homemade interferometric autocorrelator based on two-photon absorption in a Si photodetector. The signal pulse duration measured at a crystal temperature of 100°C resulted in a temporal width of Δτ ∼ 2.4 ps (assuming Gaussian pulse shape), as shown in Fig. 6(a).
Fig. 5. Log-term power stability of the (a) green, (b) signal, and (c) idler from the MgO:sPPLT picosecond OPO over 2.5 h.
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sPPLT synchronously pumped in the green. The compact Yb-fiber-laser-based OPO provides near-IR signal and idler pulses at 160 MHz with tuning coverage across 874– 1008 nm and 1126–1359 nm, respectively, limited by the available mirrors. It delivers an average signal plus idler power of >1.2 W with excellent passive long-term stability over many days and continuous operation over >24 h without damage to the MgO:sPPLT crystal. Further improvements in the overall extraction efficiency can be achieved by optimizing the signal output coupling [15]. These results indicate that MgO:sPPLT is a promising nonlinear material for green pumping in the picosecond time-scale, making the described OPO a viable and reliable source of tunable high-repetition-rate picosecond pulses in the near-IR for a variety of applications in spectroscopy and microscopy. This research was supported by the Ministry of Science and Innovation, Spain, through project OPTEX (TEC2012-37853) and the Consolider program SAUUL (CSD2007-00013). We also acknowledge partial support by the European Office of Aerospace Research and Development (EOARD) through grant FA8655-12-1-2128 and the Catalan Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) through grant SGR 2009-2013.
Fig. 6. Autocorrelation trace of the extracted signal pulse at MgO:sPPLT crystal temperature of (a) T 100°C and (b) T 50°C. Inset: corresponding signal spectra.
It is to be noted that this measurement was performed toward the positive cavity detuning range of the OPO. The corresponding signal spectrum is recorded to have a FWHM spectral width of 0.7 nm, centered at 967.3 nm. These measurements correspond to a time-bandwidth product of ΔυΔτ ∼ 0.53, close to the transform limit. Similar measurements close to the zero detuning position, at a temperature of 50°C, resulted in chirped pulses with a duration of Δτ ∼ 5.5 ps and an FWHM spectral width of 10.4 nm at 1008 nm, corresponding to a time-bandwidth product of ΔυΔτ ∼ 16.8. Using intracavity dispersion compensation for temporal and spectral control, as well as further optimization of output coupling, significant improvements in the timebandwidth product are expected. We also investigated the pulse duration as a function of cavity detuning, where we obtained variations from 7.1 to 3.3 ps at a crystal temperature of 50°C, as also observed in previous reports [13,14]. In conclusion, we have demonstrated, for the first time to our knowledge, a picosecond OPO based on MgO:
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