290
OPTICS LETTERS / Vol. 30, No. 3 / February 1, 2005
Optical parametric oscillator with a pulse repetition rate of 39 GHz and 2.1-W signal average output power in the spectral region near 1.5 mm Steve Lecomte and Rüdiger Paschotta Institute of Quantum Electronics, Department of Physics, Swiss Federal Institute of Technology (ETH), ETH Zürich Hönggerberg, Wolfgang-Pauli-Strasse 16, CH-8093 Zürich, Switzerland
Susanne Pawlik and Berthold Schmidt Bookham (Switzerland) AG, Binzstrasse 17, CH-8045 Zürich, Switzerland
Kentaro Furusawa, Andrew Malinowski, and David J. Richardson Optoelectronics Research Center, University of Southampton, Southampton SO17 1BJ, UK
Ursula Keller Institute of Quantum Electronics, Department of Physics, Swiss Federal Institute of Technology (ETH), ETH Zürich Hönggerberg, Wolfgang-Pauli-Strasse 16, CH-8093 Zürich, Switzerland Received July 19, 2004 We present a singly resonant, synchronously pumped optical parametric oscillator with a record-high repetition rate of 39 GHz. The transform-limited 2.2-ps pulses at 1570 nm have as much as 2.1-W average output power. The all-solid-state pump source is based on a diode-pumped passively mode-locked 39-GHz Nd:YVO4 laser and an efficient ytterbium-doped fiber amplifier. © 2005 Optical Society of America OCIS codes: 190.4970, 190.2620, 320.7110, 320.7090, 140.4050, 060.4510.
Sources of broadly wavelength-tunable ultrashort pulses in the spectral region near 1.5 mm with gigahertz repetition rates are of interest, e.g., for fiber-optic communication as well as for test and measurement purposes in various application areas. There are several approaches to building such sources: Harmonically and actively or passively mode-locked fiber ring lasers1 – 3 can be operated at repetition rates of 40 GHz and higher, and wavelength tuning covering the telecommunication C band has been demonstrated.3 Actively and hybrid mode-locked diode lasers can reach even higher repetition rates, up to 1.2 THz,4 but at the cost of limited average output power and tunability. We recently demonstrated compact diode-pumped Er:Yb:glass lasers with repetition rates up to 50 GHz (Ref. 5) that were passively mode locked with a semiconductor saturable absorber.6,7 This class of laser emits as many as several tens of milliwatts of average output power in good-quality picosecond pulses. Wavelength tuning of Er:Yb:glass oscillator pulse-generating lasers over the C band has already been demonstrated for devices with repetition rates of as much as 25 GHz.8 Optical parametric oscillators (OPOs) offer attractive properties that can outperform those of other types of source mentioned above in terms of average output power and wavelength tunability. Recently singly resonant synchronously pumped picosecond OPOs with repetition rates of 2.5 GHz (Ref. 9) and 10 GHz (Ref. 10) were demonstrated. Further device improvements by use of a nonmonolithic cavity led 0146-9592/05/030290-03$15.00/0
to 10-GHz repetition rate OPOs emitting as much as 353-mW signal average output power in 13.9-ps pulses.11 Very broad wavelength tuning of 154 nm covering the S, C, and L bands was possible with a single nonmonolithic device by varying the temperature and the poled grating period of the periodically poled LiNbO3 (PPLN) nonlinear crystal.11 Whereas 10 GHz was the highest repetition rate previously demonstrated for an OPO, in this Letter we present a singly resonant synchronously pumped OPO with a record-high repetition rate of 39 GHz and as much as 2.1 W of signal average output power in 2.2-ps transform-limited pulses. The system is based entirely on all-solid-state technology. Its high power goes beyond what is currently needed for telecommunications but may facilitate new applications, e.g., related to supercontinuum generation and other nonlinear processes. The main challenge for operating an OPO in the multigigahertz regime lies in generating sufficient peak power of the pump pulses to exceed the OPO threshold. The powers achievable from state-of-the-art diode-pumped passively mode-locked Nd:YVO4 lasers are suff icient to operate an optimized low-loss OPO at a repetition rate of 10 GHz.10,11 However, at 40 GHz, four times higher average power is required (assuming the same pulse duration, PPLN crystal length, and focusing parameter). Unfortunately, it is far more difficult to achieve high average powers at such high repetition rates. At present, the best 40-GHz lasers12 do not have suff icient © 2005 Optical Society of America
February 1, 2005 / Vol. 30, No. 3 / OPTICS LETTERS
output power to reach the OPO threshold. Therefore we are now using an ytterbium-doped fiber amplif ier to boost the available pump power. Because of the large amplif ier gain 共艐30 dB兲, the Nd:YVO4 seed laser can now be optimized for the shortest pump pulse duration rather than for the highest output power. The shortest pulse duration of 2.7 ps from a Nd:YVO4 -based laser was obtained with a quasimonolithic cavity design.13 The miniature quasimonolithic laser presented in Ref. 13 was pumped with a Ti:sapphire laser. Here, owing to progress in 808-nm diode manufacturing, we could replace the bulky Ti:sapphire pump laser with an AlGaAs兾GaAs-based single-mode pump diode provided by Bookham (Switzerland) AG. The 39-GHz passively mode-locked laser is described in detail in Ref. 12. Here is a summary of its main characteristics: 50-mW average output power; 3-ps pulse duration in transform-limited pulses; beam quality factor, M 2 , 1.15; and pulse repetition rate, 38.99 GHz. To boost the OPO pump power we used an Yb-doped f iber amplif ier based on a 4.5-m-long Yb-doped double-clad large mode area fiber. The fiber had a step-index refractive-index prof ile design with a core N.A. of 0.06 and a core diameter of 30 mm. The core was doped with 8000 parts in 106 of Yb31 ions. The f iber had a 300-mm diameter D-shaped inner cladding and was end pumped with a 24-W 915-nm f iber-coupled laser diode. Fiber of the same design was recently demonstrated as an amplifier for high-energy femtosecond pulses from a f iber oscillator14 and in a Q-switched fiber laser configuration.15 The duration and spectral width of the pulses were unchanged during the amplif ication, whereas the average power was boosted to as much as 9.8 W in a diffraction-limited beam 共M 2 , 1.1兲. For 24-W pump power at 915 nm, the amplif ier was saturated with a few milliwatts of coupled average seed power. Because the Yb-doped f iber did not maintain the polarization state, we introduced a polarization controller made from l兾4, l兾2, and l兾4 wave plates before the amplifier to generate a linear polarization state as required for pumping the OPO. The complete experimental setup is shown schematically in Fig. 1. The OPO is based on a 21-mm-long PPLN crystal from Crystal Technology, Inc., which is antiref lection coated for the pump, signal, and idler wavelengths. It has a single poled grating with a period of 29.6 mm and is kept at a temperature of 180 ±C in a homemade temperature-stabilized oven. To keep the parasitic intracavity signal losses as low as possible and to minimize the idler feedback, we used a ring cavity.16 The two curved cavity mirrors have a radius of curvature of 75 mm and are 96 mm apart. The two other cavity mirrors are f lat. All the cavity mirrors have BK7 glass substrates, absorbing the idler wave, such that no idler beam can be extracted from the cavity. The free spectral range of the cavity is 628 MHz. For pumping with a repetition rate of 39 GHz, 62 pulses circulate in the OPO cavity. Although this situation is reminiscent of harmonic mode locking, it does not introduce problems with timing jitter, as the timing of all circulating pulses is determined by
291
the timing of the pump pulses. The advantage of the relatively long cavity is that it allows us to use a longer nonlinear crystal with correspondingly higher gain and lower threshold. The mirror coatings have a high transmission for the pump and idler wavelengths and a higher ref lection for the signal wavelength, except for the second curved mirror, which has 1.7% transmission at the signal wavelength. The signal waist’s radius in the PPLN crystal was 49.6 mm, resulting in a focusing parameter js (ratio of the crystal length to the confocal parameter in the crystal) of 0.99. The pump beam was focused to a radius of 43 mm, corresponding to a focusing parameter
Fig. 1. Experimental setup: 39-GHz diode-pumped passively mode-locked Nd:YVO4 laser, Yb-doped fiber amplifier, and optical parametric oscillator. HT, high transmission; HR, high ref lection; SESAM, semiconductor saturable-absorber mirror.
292
OPTICS LETTERS / Vol. 30, No. 3 / February 1, 2005
1540.8 to 1592.8 nm. The idler wavelength varied from 3438.4 to 3204.9 nm but was not measured. For a much wider tuning range we could use a multiperiod crystal as described in Ref. 11. In conclusion, we have demonstrated a singly resonant synchronously pumped OPO with a record-high repetition rate of 39 GHz and a very high signal average output power of 2.1 W in transform-limited 2.2-ps pulses in the spectral region near 1.5-mm. We believe that this approach will also be suitable for repetition rates of 80 GHz and higher. This research was supported by the Swiss innovation promotion agency KTI/CTI (Commission for Technology and Innovation) and the Hasler Stiftung. We thank Gigatera as an industrial partner and L. Krainer and G. J. Spühler for fruitful discussions. S. Lecomte’s e-mail address is
[email protected]. References
Fig. 2. (a) Measured autocorrelation (lighter, broader curve) of the signal pulses with 2.1-W average power. The pulse length is 2.2 ps, assuming a sech2 shape (fit, darker curve). (b) Optical spectrum of the 39-GHz pulse train with 2.1-W average power taken with 0.08-nm resolution. The longitudinal modes with 39-GHz spacing are resolved.
jp of 0.90. We conservatively chose the signal and pump beam radii to prevent damage to the PPLN crystal. The OPO threshold was reached with 2.2 W of average pump power incident upon the PPLN crystal. With 7.8 W of average pump power the OPO generated as much as 2.1 W of average signal power. Figure 2 shows the autocorrelation and the optical spectrum of the 2.2-ps pulses with a central wavelength of 1570 nm. The pulses were transform limited. The pump depletion reached 71% at full power. By using the Manely– Rowe relation and pump depletion we calculated the round-trip loss to be 1.2%, which was somewhat higher than expected. With 2.1 W of signal average output power, the process of parasitic sum-frequency generation of the pump and signal waves generated more than 10-mW average power at 634 nm. However, the signal loss that resulted from this effect is rather small. For the full pump power of 7.8 W, the range of cavity lengths at which the OPO oscillates is 25 mm wide. By varying the PPLN crystal temperature from 120 to 220 ±C we could tune the signal wavelength from
1. L. Schares, L. Occhi, and G. Guekos, IEEE Photon. Technol. Lett. 15, 1348 (2003). 2. E. Yoshida and M. Nakazawa, Electron. Lett. 32, 1370 (1996). 3. B. Bakhshi and P. A. Andrekson, Electron. Lett. 36, 411 (2000). 4. S. Arahira, Y. Matsui, and Y. Ogawa, IEEE J. Quantum Electron. 32, 1211 (1996). 5. S. C. Zeller, L. Krainer, G. J. Spühler, R. Paschotta, M. Golling, D. Ebling, K. J. Weingarten, and U. Keller, Electron. Lett. 40, 875 (2004). 6. U. Keller, K. J. Weingarten, F. X. Kärtner, D. Kopf, B. Braun, I. D. Jung, R. Fluck, C. Hönninger, N. Matuschek, and J. Aus der Au, IEEE J. Sel. Top. Quantum Electron. 2, 435 (1996). 7. U. Keller, Nature 424, 831 (2003). 8. G. J. Spühler, P. S. Golding, L. Krainer, I. J. Kilburn, P. A. Crosby, M. Brownell, K. J. Weingarten, R. Paschotta, M. Haiml, R. Grange, and U. Keller, Electron. Lett. 39, 778 (2003). 9. A. Robertson, M. E. Klein, M. A. Tremont, K.-J. Boller, and R. Wallenstein, Opt. Lett. 25, 657 (2000). 10. S. Lecomte, L. Krainer, R. Paschotta, M. J. P. Dymott, K. J. Weingarten, and U. Keller, Opt. Lett. 27, 1714 (2002). 11. S. Lecomte, R. Paschotta, M. Golling, D. Ebling, and U. Keller, J. Opt. Soc. Am. B 21, 844 (2004). 12. S. Lecomte, M. Kalisch, L. Krainer, G. J. Spühler, R. Paschotta, L. Krainer, M. Golling, D. Ebling, T. Ohgoh, T. Hayakawa, S. Pawlik, B. Schmidt, and U. Keller, “Diode-pumped passively mode-locked Nd:YVO4 lasers with 40-GHz repetition rate,” IEEE J. Quantum Electron. (to be published). 13. L. Krainer, R. Paschotta, S. Lecomte, M. Moser, K. J. Weingarten, and U. Keller, IEEE J. Quantum Electron. 38, 1331 (2002). 14. A. Malinowski, A. Piper, J. H. V. Price, K. Furusawa, Y. Jeong, J. Nilsson, and D. J. Richardson, Opt. Lett. 29, 2073 (2004). 15. A. Piper, A. Malinowski, K. Furusawa, and D. J. Richardson, Electron. Lett. 40, 928 (2004). 16. W. R. Bosenberg, A. Drobshoff, J. I. Alexander, L. E. Myers, and R. L. Byer, Opt. Lett. 21, 1336 (1996).
