We report the generation of 200-nm-bandwidth mid-infrared pulses at 3.5-µm from an optical parametric oscillator incorporating a 25-mm MgO:PPLN crystal and synchronously-pumped by chirped pulses from a fiber-amplified Yb:KYW laser. A long nonlinear crystal permits efficient transfer of the pump bandwidth into the idler pulses, achieves exceptional passive stability and enables pumping using chirped pulses directly from a fiber-amplifier, avoiding the need to use lossy pulse-compression optics.
© 2011 OSA
Femtosecond lasers based on Yb-doped gain media are becoming established as serious competitors to Ti:sapphire in many applications because of their compatibility with high-gain Yb-fiber amplifiers, which enable their average powers to be scaled to several Watts. Most practical ultrafast fiber-amplifiers adopt a chirped-pulse amplification (CPA) approach  in which the seed pulses are stretched, amplified, then re-compressed after amplification. The final compression stage is normally unavoidably lossy – requiring a double-pass through a diffraction-grating pair – and even the best transmission gratings introduce 20 - 30% loss in this configuration. In this paper we present an optical parametric oscillator (OPO) which is directly pumped by the chirped 3-ps output of an Yb-fiber amplifier, seeded by a Yb:KYW laser, eliminating the need for post-amplification pulse compression. Our scheme exploits an inflexion point of the phase-matching curve in a long MgO:PPLN crystal, which allows efficient parametric transfer of the pump bandwidth into the idler pulses for a narrowband resonant signal pulse. Broadband idler pulses of the kind available from the OPO have immediate applications in femtosecond Fourier-transform infrared spectroscopy [2,3].
2. Broadband Phasematching using a Long MgO:PPLN Crystal
It is well known that certain crystals possess unique phase-matching properties permitting efficient coupling between broadband and narrowband waves. Examples include Type-I BBO, used in broadband OPOs , and Type-II KDP, with applications in optical pulse characterization [5,6]. Quasi-phase-matched MgO:PPLN exhibits phasematching properties which permit broadband pump pulses to efficiently amplify narrowband signal pulses, even for long crystals. Figure 1(a) illustrates this scenario by showing the phase-matching map for a 25-mm-long MgO:PPLN crystal with a grating period of 30 µm and a temperature of 40°C.
Conventionally, achieving an acceptance bandwidth sufficient to allow femtosecond pumping requires a short (few-mm) nonlinear crystal, but this requirement is relaxed when inflexions in the phasematching curve exist. Despite using a long crystal, the above configuration has an acceptance bandwidth of ~20 nm at 1050 nm, permitting pumping with pulses as short at 60 fs (sech2(t) intensity profile assumed) and arising from the group-velocity matching of the pump and idler pulses at the inflexion point, illustrated by Fig. 1(b). The corresponding signal acceptance bandwidth is only 2.5 nm because of the large signal-idler temporal walkoff , and this narrowband signal enables efficient transfer of the pump bandwidth into the idler . The pump-signal temporal walkoff of 2.4 ps allows efficient pumping by chirped pulses with comparable durations, making a long MgO:PPLN crystal suitable for pumping with the uncompressed output pulses from an Yb:fiber amplifier. The next section presents experimental data validating this approach.
3.1 Pump Source
The pump was a master-oscillator-power-amplifier (MOPA) and is shown in Fig. 2 . The seed laser was a single-mode-diode-pumped, semiconductor-saturable-absorber-mirror modelocked Yb:KYW laser, which produced pulses with FWHM (full-width at half-maximum) durations of 250 fs, and spectra centred at 1045 nm with FWHM bandwidths of 5 nm. The average output power was 150 mW and the repetition rate was 94 MHz. The seed was amplified by a cladding-pumped Yb-doped fiber amplifier, pumped by a 6-W, 915-nm multimode fiber-coupled diode laser. The amplifier fiber was a 6-m long, double-clad fiber with a single-mode and polarization-maintaining (PM) core. With 6 W pump power we obtained 2.5 W output power in a spectrum (Fig. 3(a) ) broadened to a FWHM bandwidth of 18 nm (4.82 THz) due to self-phase modulation effects in the amplifier, while the central wavelength of the output pulses was red-shifted to 1058 nm because of the large difference between the central wavelengths of the amplifier gain and seed laser. The output pulses were strongly chirped due to the normal dispersion of the amplifier fiber. With an interferometric autocorrelation (IAC) technique based on two-photon absorption (TPA) [10,11], the durations of the amplified pulses were determined to be ~3.0 ps, corresponding to a duration-bandwidth-product (DBP) of 14.3. The IAC trace for the amplifier output pulses at the maximum pump power is shown in Fig. 3(b). By adjusting the half-wave plate so that the seed polarization was launched parallel to the slow axis of the PM amplifier fiber, a linearly polarized output was obtained.
3.2 OPO Cavity Configuration
The OPO crystal was 5-mol.% MgO:PPLN (HC Photonics) and was 25-mm long and 1-mm thick, with a grating period of 30 µm, and was housed in an aluminum heat-sink held at 30 °C. The cavity is shown in Fig. 4 , in which focusing mirrors M1 and M2 (radius of curvature 150 mm) and plane mirror M3 were coated on YAG substrates for high transmission at the pump and idler wavelengths (T >90% at 1020 – 1110 nm and 2.3 – 3.7 µm), and high reflectivity at the signal wavelength (R >99.8% at 1.4 – 1.8 µm). The OPO was singly resonant for the signal, which was extracted via a 2 – 20% output coupler (OC). The cavity length was adjusted for synchronism with the pump laser. The idler and depleted pump passed through M2 and were collimated by an anti-reflection (AR) coated CaF2 lens, following which an AR-coated Ge window was used to isolate the idler pulses prior to characterization.
3.3 Results and Discussion
With the 2% OC, the threshold pump power was 200 mW, and Fig. 5(a) shows the measured signal / idler powers, and the idler power inferred from the signal power by using the Manley-Rowe relationship (assuming signal and idler wavelengths of 1503 nm and 3575 nm respectively). The discrepancy between the measured and derived idler power arises largely from the non-optimized anti-reflection coatings of the crystal at the idler wavelength, estimated to have R ≤ 40%. Figure 5(b) shows the residual pump power coupled from M2 versus the incident pump power. At the maximum incident pump power of 2.2 W, 603 mW signal and 144 mW idler were measured, while 252 mW idler power was inferred. The quantum conversion efficiencies for the signal and idler outputs were 39% and 22% respectively, and the pump conversion was 75%. When a 10% OC was used, the threshold and maximum signal power were 400 mW and 910 mW respectively. The signal quantum conversion efficiency was 59%. With a 20% OC, the threshold power and maximum signal output power were 800 mW and 510 mW respectively, with a signal quantum conversion efficiency of 33%. A 2% OC was used in all of the other results presented here. Figure 5(c) shows the spectra of the undepleted and depleted pump pulses at 2.2-W pump power. The intensity is normalized to the measured depletion ratio of 75% and the data show that the pump was depleted uniformly across most of its bandwidth, with negligible back-conversion.
The signal spectrum of the OPO, pumped at full power, is shown in Fig. 6(a) . Its central wavelength is 1503 nm and its FWHM was measured to be 2.4 nm (0.32 THz). This bandwidth is much narrower than the pump bandwidth (4.8 THz), implying that a broadband idler can be obtained, because the pump spectrum is the convolution of the signal and idler spectra, and almost the full width of the pump spectrum was depleted in the OPO. Such a narrow signal bandwidth is mainly due to the relatively long MgO:PPLN grating. The origin of the satellite peak at 1527 nm is unclear, however this wavelength is phasematched to the edges of the pump spectrum by a +/− 50-nm domain error, which is within the tolerance of the crystal photomask design. The signal pulses at full pump power were measured using the IAC technique based on TPA by using a Si photodiode , and the result shown in Fig. 6(b) implied a FWHM duration of ~1.4 ps. The DBP was calculated to be 0.45, showing that the signal pulses were moderately chirped.
Figure 6(c) presents the idler spectrum obtained at full pump power, measured with a scanning monochromator. Its central wavelength is 3573 nm and its FWHM bandwidth is 200 nm (4.70 THz), closely following the pump bandwidth of 4.82 THz and confirming effective parametric transfer, as expected from theory. The IAC of the idler pulse at full pump power was measured using TPA in an extended-InGaAs photodiode  (see Fig. 6(d)) and implied a FWHM duration of ~3.0 ps and DBP of 14.1, nearly identical to that of the pump pulses.
The output power stability of the signal at 1503 nm was detected with an InGaAs photodiode and recorded in the time domain over a period of 16 seconds with a sampling rate of 256 kHz. The corresponding relative intensity noise (RIN) and cumulative power error were calculated and shown in Fig. 7 . For comparison, the pump power stability was also recorded and shown in Fig. 7. The cumulative power fluctuation of the OPO signal output was around 0.1%. This excellent power-stability is attributed to the use of the long nonlinear crystal, which introduces large dispersion in the OPO cavity and therefore reduces the sensitivity of the signal central wavelength to fluctuations in the OPO cavity length arising from mechanical vibrations or temperature variations. To demonstrate this, we recorded the signal central wavelength of the OPO as a function of the cavity length detuning. The OPO oscillated over a cavity-length detuning range of 0.12 mm, and over this tuning range, the variation of the signal wavelength was less than 3 nm.
4. Summary and Conclusions
We have presented a mid-infrared MgO:PPLN OPO synchronously-pumped by chirped pulses from a Yb:fiber amplifier and exhibiting a quantum conversion efficiency of up to 59%. With 2.2 W incident pump power, 144 mW average power of idler was directly measured at a central wavelength of 3573 nm and with a 200-nm FWHM bandwidth.
We attribute the broadband idler spectrum to the use of a long MgO:PPLN crystal, whose phasematching provides a narrow signal acceptance bandwidth when pumped near 1050 nm, facilitating accurate parametric transfer from the broadband pump to the idler. The principal advantages of this scheme are its compatibility with using chirped pump pulses, which eliminates the complexity and loss associated with compressing the pulses leaving the fiber amplifier, and low back-conversion because of the broad acceptance bandwidth at the pump wavelength.
The authors gratefully acknowledge financial support from the UK Engineering and Physical Sciences Research Council under grants EP/H018190/1 and EP/H000011/1, and from the United Kingdom's National Physical Laboratory under its Strategic Research Programme.
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