Optical parametric oscillator with a pulse repetitionrate 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
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 repetitionrate of 39 GHz. The transform-limited 2.2-ps pulses at 1570 nm have as much as 2.1-W average outputpower. The all-solid-state pump source is based on a diode-pumped passively mode-locked 39-GHz Nd:YVO4
Sources of broadly wavelength-tunable ultrashortpulses in the spectral region near 1.5 mm withgigahertz repetition rates are of interest, e.g., forfiber-optic communication as well as for test andmeasurement purposes in various application ar-eas. There are several approaches to building suchsources: Harmonically and actively or passivelymode-locked fiber ring lasers1 – 3 can be operatedat repetition rates of 40 GHz and higher, andwavelength tuning covering the telecommunicationC band has been demonstrated.3 Actively and hybridmode-locked diode lasers can reach even higher repe-tition rates, up to 1.2 THz,4 but at the cost of limitedaverage output power and tunability. We recentlydemonstrated compact diode-pumped Er:Yb:glasslasers with repetition rates up to 50 GHz (Ref. 5)that were passively mode locked with a semiconductorsaturable absorber.6,7 This class of laser emits asmany as several tens of milliwatts of average outputpower in good-quality picosecond pulses. Wavelengthtuning of Er:Yb:glass oscillator pulse-generating lasersover the C band has already been demonstrated fordevices with repetition rates of as much as 25 GHz.8
Optical parametric oscillators (OPOs) offer attrac-tive properties that can outperform those of othertypes of source mentioned above in terms of averageoutput power and wavelength tunability. Recentlysingly resonant synchronously pumped picosecondOPOs with repetition rates of 2.5 GHz (Ref. 9) and10 GHz (Ref. 10) were demonstrated. Further deviceimprovements by use of a nonmonolithic cavity led
to 10-GHz repetition rate OPOs emitting as muchas 353-mW signal average output power in 13.9-pspulses.11 Very broad wavelength tuning of 154 nmcovering the S, C, and L bands was possible with asingle nonmonolithic device by varying the tempera-ture and the poled grating period of the periodicallypoled LiNbO3 (PPLN) nonlinear crystal.11 Whereas10 GHz was the highest repetition rate previouslydemonstrated for an OPO, in this Letter we presenta singly resonant synchronously pumped OPO witha record-high repetition rate of 39 GHz and as muchas 2.1 W of signal average output power in 2.2-pstransform-limited pulses. The system is based en-tirely on all-solid-state technology. Its high powergoes beyond what is currently needed for telecom-munications but may facilitate new applications,e.g., related to supercontinuum generation and othernonlinear processes.
The main challenge for operating an OPO inthe multigigahertz regime lies in generating suf-ficient peak power of the pump pulses to exceedthe OPO threshold. The powers achievable fromstate-of-the-art diode-pumped passively mode-lockedNd:YVO4 lasers are suff icient to operate an op-timized low-loss OPO at a repetition rate of10 GHz.10,11 However, at 40 GHz, four times higheraverage power is required (assuming the same pulseduration, PPLN crystal length, and focusing parame-ter). Unfortunately, it is far more difficult to achievehigh average powers at such high repetition rates. Atpresent, the best 40-GHz lasers12 do not have suff icient
output power to reach the OPO threshold. Thereforewe are now using an ytterbium-doped fiber amplif ierto boost the available pump power. Because of thelarge amplif ier gain ��30 dB�, the Nd:YVO4 seed lasercan now be optimized for the shortest pump pulseduration rather than for the highest output power.
The shortest pulse duration of 2.7 ps from aNd:YVO4-based laser was obtained with a quasi-monolithic cavity design.13 The miniature quasi-monolithic laser presented in Ref. 13 was pumpedwith a Ti:sapphire laser. Here, owing to progressin 808-nm diode manufacturing, we could re-place the bulky Ti:sapphire pump laser with anAlGaAs�GaAs-based single-mode pump diode pro-vided by Bookham (Switzerland) AG. The 39-GHzpassively mode-locked laser is described in detailin Ref. 12. Here is a summary of its main char-acteristics: 50-mW average output power; 3-pspulse duration in transform-limited pulses; beamquality factor, M2 , 1.15; and pulse repetition rate,38.99 GHz. To boost the OPO pump power we usedan Yb-doped fiber amplif ier based on a 4.5-m-longYb-doped double-clad large mode area fiber. Thefiber had a step-index refractive-index profile de-sign with a core N.A. of 0.06 and a core diameterof 30 mm. The core was doped with 8000 parts in106 of Yb31 ions. The fiber had a 300-mm diameterD-shaped inner cladding and was end pumped witha 24-W 915-nm fiber-coupled laser diode. Fiber ofthe same design was recently demonstrated as anamplifier for high-energy femtosecond pulses froma fiber oscillator14 and in a Q-switched fiber laserconfiguration.15 The duration and spectral width ofthe pulses were unchanged during the amplif ication,whereas the average power was boosted to as much as9.8 W in a diffraction-limited beam �M2 , 1.1�. For24-W pump power at 915 nm, the amplif ier was satu-rated with a few milliwatts of coupled average seedpower. Because the Yb-doped fiber did not maintainthe polarization state, we introduced a polarizationcontroller made from l�4, l�2, and l�4 wave platesbefore the amplifier to generate a linear polarizationstate as required for pumping the OPO. The completeexperimental setup is shown schematically in Fig. 1.
The OPO is based on a 21-mm-long PPLN crystalfrom Crystal Technology, Inc., which is antiref lectioncoated for the pump, signal, and idler wavelengths.It has a single poled grating with a period of 29.6 mmand is kept at a temperature of 180 ±C in a homemadetemperature-stabilized oven. To keep the parasiticintracavity signal losses as low as possible and to mini-mize the idler feedback, we used a ring cavity.16 Thetwo curved cavity mirrors have a radius of curvatureof 75 mm and are 96 mm apart. The two othercavity mirrors are f lat. All the cavity mirrors haveBK7 glass substrates, absorbing the idler wave, suchthat 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, 62pulses circulate in the OPO cavity. Although thissituation is reminiscent of harmonic mode locking,it does not introduce problems with timing jitter, asthe timing of all circulating pulses is determined by
the timing of the pump pulses. The advantage ofthe relatively long cavity is that it allows us to use alonger nonlinear crystal with correspondingly highergain and lower threshold. The mirror coatings have ahigh transmission for the pump and idler wavelengthsand a higher ref lection for the signal wavelength,except for the second curved mirror, which has 1.7%transmission at the signal wavelength. The signalwaist’s radius in the PPLN crystal was 49.6 mm,resulting in a focusing parameter js (ratio of thecrystal length to the confocal parameter in the crystal)of 0.99. The pump beam was focused to a radiusof 43 mm, corresponding to a focusing parameter
Fig. 2. (a) Measured autocorrelation (lighter, broadercurve) 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 pulsetrain with 2.1-W average power taken with 0.08-nmresolution. The longitudinal modes with 39-GHz spacingare resolved.
jp of 0.90. We conservatively chose the signal andpump beam radii to prevent damage to the PPLNcrystal. The OPO threshold was reached with 2.2 Wof average pump power incident upon the PPLNcrystal. With 7.8 W of average pump power the OPOgenerated as much as 2.1 W of average signal power.Figure 2 shows the autocorrelation and the opticalspectrum of the 2.2-ps pulses with a central wave-length of 1570 nm. The pulses were transformlimited. The pump depletion reached 71% at fullpower. By using the Manely–Rowe relation andpump depletion we calculated the round-trip loss tobe 1.2%, which was somewhat higher than expected.With 2.1 W of signal average output power, theprocess of parasitic sum-frequency generation of thepump and signal waves generated more than 10-mWaverage power at 634 nm. However, the signal lossthat resulted from this effect is rather small.
For the full pump power of 7.8 W, the range of cavitylengths at which the OPO oscillates is 25 mm wide.By varying the PPLN crystal temperature from 120to 220 ±C we could tune the signal wavelength from
1540.8 to 1592.8 nm. The idler wavelength variedfrom 3438.4 to 3204.9 nm but was not measured. Fora much wider tuning range we could use a multiperiodcrystal as described in Ref. 11.
In conclusion, we have demonstrated a singly reso-nant synchronously pumped OPO with a record-highrepetition rate of 39 GHz and a very high signal aver-age output power of 2.1 W in transform-limited 2.2-pspulses in the spectral region near 1.5-mm. We believethat this approach will also be suitable for repetitionrates of 80 GHz and higher.
This research was supported by the Swiss inno-vation promotion agency KTI/CTI (Commission forTechnology and Innovation) and the Hasler Stiftung.We thank Gigatera as an industrial partner andL. Krainer and G. J. Spühler for fruitful discussions.S. Lecomte’s e-mail address is [email protected].
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