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Abstract
A PPLN non-resonant optical parametric oscillator injection-seeded by narrowband sub-100-mW CW radiation at the signal wavelength produces > 3 W idler average power at 2376 nm for a 20-kHz repetition rate, with a sub-2-nm spectral linewidth. The maximum quantum efficiency reaches 39.5%, roughly 1.4 times higher compared to narrowband operation achieved with a volume Bragg grating at the same pump level. Seed levels as low as 40 mW are sufficient to produce the desired spectral narrowing effect.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Without wavelength selective elements, the output spectrum of an optical parametric oscillator (OPO) is usually much broader than the pump spectrum due to the large parametric spectral gain bandwidth of the nonlinear crystal. While the latter depends also on the parametric gain, the main factor remains the spectral acceptance as calculated for a difference frequency generation three-wave interaction process assuming a narrowband pump, which is then inversely proportional to the crystal length and the difference between the inverse group velocities of the signal and idler [1].
Having chosen the three wavelengths for the desired conversion process and the polarization scheme for maximum effective nonlinearity, the only parameter to be optimized remains the crystal length. Long crystals ensure higher amplification but existing crystals are limited to a few centimeters, corresponding to bandwidths supporting few–picosecond pulses in the time domain. Shorter pulses possess higher peak powers that provide higher parametric gain but typical pump pulse durations are in the nanosecond range to enable multiple cavity round trips for achieving reasonable conversion efficiency. Since the group velocity difference vanishes near degeneracy for type-I (ooe or eeo) and type-0 (eee) phase-matching when the signal and idler have the same polarization, the spectral acceptance can drastically increase in this limit, corresponding to the femtosecond scale in the time domain. Such broadband nanosecond OPO sources with a time-bandwidth product as high as 100000 (= 10 ns / 100 fs) will have rather limited application potential in spectroscopy because of the low spectral density and resolution, and the accompanying restricted tunability. Another adverse effect is related to the pump spectral acceptance (calculated assuming a narrowband resonated signal wave) if the OPO is to be used as a first stage in cascade configuration, to reach further into the mid-infrared (IR) pumping a non-oxide nonlinear crystal in a second OPO stage.
One of the most widely used nonlinear crystals in OPOs pumped by the powerful and mature laser sources near 1 µm (Nd- or Yb-lasers), is periodically-poled LiNbO3 (PPLN) based on type-0 quasi-phase matching (QPM) [2]. PPLN samples have limited thickness in the electric-field poling direction but a length of up to few centimeters. Since the surface damage limit is determined by the fluence, this means that PPLN is predestined for operation at high average powers with kilohertz repetition rates. For the same reasons, PPLN is perfect for pairing with orientation patterned GaAs (OP-GaAs) in a second stage, which is the best QPM material for the mid-IR with transparency extending up to 18 µm [1]. However, the pump spectral acceptance of OP-GaAs is so small that it will be detrimental for the conversion efficiency in such a cascade scheme if the first stage output is not spectrally narrowed [3,4].
While the methods for spectral narrowing the output of an OPO are well known [2–5], including dispersive elements, etalons, injection seeding, etc., we are interested here in a specific OPO configuration called the non-resonant OPO (NRO) [6,7]. In a NRO, in contrast to a conventional OPO, none of the waves is resonant and the signal and idler leave the cavity after just one round trip in opposite directions. This can alleviate the degradation of the beam quality and conversion efficiency caused by back conversion and the scheme is particularly attractive for high gain materials such as PPLN. One of the NRO cavity mirrors is highly reflective (HR) at the signal and highly transmissive (HT) at the idler while the other cavity mirror is the opposite (HT signal/HR idler). The pump wave propagates in both directions and so transfers energy to the signal and idler. A nanosecond NRO can be very efficient, e.g. the 30% pump depletion achieved in [8] was very close to the efficiency of a singly resonant OPO. Recently, we demonstrated the use of a volume Bragg grating (VBG) for spectrally narrowing the output of a PPLN NRO [9] with a conversion efficiency of 47.5%.
Injection seeding is another well-known method for spectral narrowing and NROs seem well suited to this approach given their cavity is open from one side for each of the output waves. This is clear considering that such a mode-less cavity can be equally regarded and employed as a multipass parametric amplifier [8,10]. Our initial studies of this approach with a Nd:YVO4 laser pumped 1-mm thick PPLN in a nanosecond NRO revealed, however, that at high parametric gain the spectral narrowing effect is overwhelmed by parametric generation [11]. Thus, at a repetition rate of 20 kHz, maximum average pump powers of only 3-4 W could be applied independent of the CW seed power level [11]. In the present work we investigate the power scaling capabilities of the injection seeded narrowband NRO utilizing the full power available from the pump laser, by employing a larger aperture PPLN crystal (3 × 3 mm2 vs 1 × 1 mm2 in [11]). Injection seeded NROs offer some potential advantages over VBG NRO operation, including the broader tuning capability (compared to chirped VBGs) and the straightforward reconfiguration to single frequency operation using low-power seed laser diodes.
2. Experimental setup
The experimental setup of the PPLN-NRO is shown in Fig. 1. It is pumped by a multi-longitudinal mode (spectral linewidth ∼0.66 nm) Nd:YVO4 master oscillator power amplifier (MOPA) laser system (Canlas GmbH), delivering a maximum average power of 20 W for a pulse duration of ~8 ns at a repetition rate of 20 kHz, with a beam quality factor of M2∼1.3.
A half waveplate and a polarizer are used to adjust the pump power while keeping the pump laser characteristics constant at its maximum output level. A Faraday isolator (FI-1) is employed to prevent the optical feedback to the pump laser, and a second half waveplate after the FI rotates the polarization to vertical for type-0 (eee) phase-matching in the PPLN crystal. The pump beam is down-collimated by an achromatic beam expander (GBE02-C, Thorlabs) to a 1/e2 beam diameter of 1.8 mm. The 25-mm long, antireflection (AR)-coated 5 mol% MgO-doped PPLN (PPMgLN, HC Photonics) has an aperture of 3 × 3 mm2. Its QPM period of 32.25 µm provides a signal wavelength of 1927 nm and an idler wavelength of 2376 nm at an oven temperature of 30°C.
The narrowband NRO cavity consists of signal and idler output couplers (OC-1 and OC-2) separated by a physical length of 10 cm, as shown in Fig. 1. OC-1 is 95% reflective for the idler and 95% transmissive for the signal. OC-2 is 99% reflective for the signal and 90% transmissive for the idler. The dichroic mirrors DM-1/DM-2 couple the pump in and out of the NRO - they are highly transmissive for the signal (95%) and idler (91%) and highly reflective at the pump and its second harmonic. The dichroic mirror DM-3 is highly reflective at the pump and transmissive at 532 nm, enabling the obligatory double pass pumping of the NRO and outcoupling parasitic green second harmonic light. The continuous-wave (CW) seed source is a tunable (1908-1937 nm), narrow linewidth (<0.5 nm) 1-W Tm fiber laser (TLT-1-1930, IPG Photonics), collimated with a 5.95-mm aspheric lens (CO228TME-D, Thorlabs) to a diameter of 1.75 mm to match the pump. A second FI (FI-2) is used to prevent back reflections to the seed laser and a half waveplate to match the pump polarization in the PPMgLN. All idler powers were characterized using a long pass filter to eliminate residual pump and signal light.
3. Results and discussion
All pump powers quoted below were measured at the input of the NRO cavity (in front of DM-1) while all idler powers are already corrected for the transmission (96%) of the cut-on filter used in front of the power meter. The CW seed powers are those measured in front of OC-1. Figure 2 shows the average idler output power at 2376 nm versus pump power for a seed level of 95 mW at 1927 nm. The maximum idler average power exceeds 3 W and the quantum conversion efficiency (or pump depletion) exceeds 38% for 18 W pump power.
Fig. 2. Idler power (red squares) versus pump power at 20 kHz for a seed level of 95 mW with corresponding quantum conversion efficiency (blue circles). Lines to guide eye only.
Tuning the NRO idler output was possible by tuning the Tm fiber seed laser wavelength at a fixed PPMgLN temperature, see Fig. 3, with a power reduction across the tuning range not exceeding 20%. For a seed power of 95 mW, the idler output had a spectral bandwidth (FWHM) of 1.5 nm at 2376 nm at the maximum power level, see Fig. 4, measured with a spectral resolution of 0.5 nm. Without seeding, the idler spectral FWHM was roughly 20 nm, i.e., the spectral narrowing was more than an order of magnitude.
Fig. 3. Idler spectra at a pump level of 18 W and a seed level of 95 mW recorded at a fixed temperature (30°C) of the PPMgLN crystal by tuning the Tm fiber seed laser.
Fig. 4. Idler spectra for an average pump power of 18 W without seeding (black), at a CW seed level of 95 mW (red) and with the VBG for the signal instead of seeding (blue).
The average idler output power as a function of seed power is shown in Fig. 5. It increases significantly faster in the 75-95 mW range of seed power than at higher and lower seeds. This steeper dependence was observed only at the maximum pump level of 18 W but not at lower pump powers, e.g. at 16 W. It is attributed to local (in space and time) back conversion and the resulting distortion of the pump spatial and temporal profile in the presence of the CW seed. Such back conversion should occur at high integral (energy) conversion efficiency (see Fig. 2) considering the roughly four times higher peak on-axis value.
Fig. 5. Average idler output power at an average pump power of 18 W versus CW seed power at the signal wavelength.
At the highest seed level applied (165 mW) an average idler power of 3.18 W was achieved (compared to 3.08 W at 95 mW seed), see Fig. 5, an improvement by roughly an order of magnitude compared to [11]. Thus, the 95 mW seed level was chosen for detailed characterization because of the marginal improvement above it. At the maximum pump level of 18 W, spectrally clean, i.e. free of satellites, and narrow (< 2 nm) idler spectra were observed down to a CW seed power of ~40 mW (well suited for external cavity single frequency laser diodes), with the output idler power decreasing by only 10%.
The above observations were very similar when the pump level was decreased to 16 and 14 W. The same spectral narrowing effect for the idler output was achieved at 95 mW of CW seed power and this narrowing was effective starting again from a seed level around 40 mW. The main difference was in the average idler output power generated, see Fig. 2.
The measured idler pulse duration at maximum power (FWHM = 7.6 ns) was slightly shorter compared to the pump pulse (8 ns), see Fig. 6. The beam profiles were measured by a Pyrocam PY-III-C-B camera (Ophir-Spiricon) using a 300-mm CaF2 focusing lens. The output idler beam had a circular shape with an M2 factor of 3.24 in the vertical and 2.75 in the horizontal direction, see Fig. 7. These increased M2 values can be attributed to the previously mentioned spatially local back conversion effects.
Fig. 6. Pump and idler temporal profiles at an average pump power of 18 W and CW seed power of 95 mW measured with an InGaAs photodiode with 70 ps response time.
Fig. 7. M2 measurement of the idler output beam of the narrowband NRO in the two planes at an average pump power of 16 W for a 95 mW CW seed level. The inset represents a beam profile recorded at 80 cm from OC-2.
We did not characterize the spectral properties of the signal because narrowband idler radiation is only possible if injection seeding narrows the signal spectrum. In [11] we also showed how the NRO signal output can be separated from the seed beam using an additional FI.
Finally, we substituted OC-2 by a VBG for 1922 nm (AR-coated for signal and idler, and HT for idler) and compared the operation of the NRO without seeding for the maximum pump level of 18 W at 20 kHz and a PPMgLN temperature of 29.3°C (adjusted to the slightly different signal wavelength). As can be seen by the blue curve in Fig. 4, similar spectral bandwidths were recorded for the idler, however, the maximum average power was 26% lower (2.28 W compared to 3.08 W at 95 mW CW seed level).
4. Conclusion
In conclusion, injection seeding with a narrowband CW source of a NRO at the shorter (signal) wavelength was capable of narrowing the output at the longer (idler) wavelength by more than an order of magnitude, simultaneously increasing the output power compared to the unseeded case. A maximum average output idler power of 3.18 W at 2376 nm was achieved for a repetition rate of 20 kHz (single pulse energy of 159 µJ). The maximum quantum efficiency reached 39.5% in this case corresponding to a seed level of 165 mW. The spectral narrowing was effective from seed powers as low as 40 mW.
It shall be outlined that the optimization of the beam sizes was essential for achieving the present results and this depends on the repetition rate for a given maximum average power of the pump laser. Thus at lower repetition rates it might be necessary to increase the beam size, the seed power and even the crystal aperture [12]. On the opposite, for pump sources operating at higher repetition rates, e.g. 100 kHz, lower seed levels will be sufficient and a standard PPLN thickness of 1 mm will work.
The seeded PPLN NRO will be employed for pumping OP-GaAs in an external OPO or DFG [13] cascade configuration. In general, seeded operation seems feasible also for spectral narrowing of NROs based on very thick QPM or bulk crystals [14] which can also be paired with bulk non-oxide crystals in tandem schemes [15].
Funding
Imperial College European Partners Fund; Institute of Security Science and Technology Champions Fund.
Acknowledgments
We gratefully acknowledge support from the Imperial College European Partners Fund and the Institute of Security Science and Technology Champions Fund. We thank I. B. Divliansky (CREOL, University of Central Florida, Orlando, USA) for providing the VBG used here for comparison.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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