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OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
Multi-gigahertz picosecond optical parametric oscillator pumped by 80-MHz Yb-fiber laser Ossi Kimmelma,1 S. Chaitanya Kumar,1,* Adolfo Esteban-Martin,1 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, 08010 Barcelona, Spain *Corresponding author: [email protected] Received July 12, 2013; revised September 24, 2013; accepted October 1, 2013; posted October 2, 2013 (Doc. ID 193843); published November 5, 2013
We report a multi-gigahertz (GHz) repetition-rate picosecond optical parametric oscillator (OPO) based on MgO: PPLN, synchronously pumped by a Yb-fiber laser operating at 80 MHz, where the multiplication of repetition frequency is achieved using fractional increment in the OPO cavity length. Using this simple technique, we achieve OPO operation up to the 88th harmonic of the pump laser frequency, corresponding to a repetition rate as high as 7 GHz. Deploying a 5% output coupler, we are able to extract up to 960 mW of average signal power at the fundamental with 600 mW at the 88th harmonic (7 GHz), using a pump power of 5.6 W. The measured relative standard deviations of the fundamental and fifth harmonic signal power are recorded to be 1.6% and 3.5%, respectively, while the fundamental signal pulse duration is measured to be 18.4 ps. © 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.38.004550
Techniques to generate laser pulses at high repetition rate (RR) have been studied widely because of the demand for such sources in applications such as optical clocking, high-speed and electro-optic sampling [1–3]. For high RR ultrashort pulse generation near the important telecommunication wavelength of 1.5 μm, a number of techniques have been exploited, including mode-locked Er, Yb:glass lasers [4], with some using rational-harmonic modelocking [5], where the RR of a laser source is multiplied by changing the ratio of the laser cavity round-trip time and the gain or loss modulation frequency. For tunable ultrashort pulse generation in the near- to mid-IR, synchronously pumped optical parametric oscillators (OPOs) represent the most viable route, providing picosecond and femtosecond pulses across the 1–5 μm spectral range. Using direct synchronous pumping, output pulses at the same RR as the mode-locked pump laser are obtained from the OPO, typically limited to 99%) for the signal over 1.3–2.2 μm and highly transmitting for the pump (T > 90%) at 1064 nm, while the OC has a transmission of 5% over the signal wavelength range of 1.43–1.63 μm. All mirrors, except the OC, are also highly transmitting (T > 87%) at idler wavelengths over 2.2–4 μm, ensuring singly resonant oscillation. The nonlinear crystal for the OPO is a 50-mm-long MgO:PPLN sample containing seven uniform poling periods from 28.5 to 31.5 μm, although we only use the 30.5 μm grating period in this work. The crystal is maintained at 100°C in an oven with a stability of 0.1°C, resulting in a signal wavelength of 1564 nm and a corresponding idler wavelength of 3.33 μm. However, with the available mirrors, the signal wavelength can be readily tuned over 1.43–1.63 μm by varying the crystal temperature and grating period [10]. The pump beam is focused to a waist radius of w0 ∼ 50 μm at the center of the crystal, which is positioned at the primary focus in between M1 and M2. The signal beam is collimated in the X-cavity arms to maintain a constant mode size while varying the arm length for high RR pulse generation. To achieve pulse RR multiplication from the input 80 MHz to several GHz at the output, the OPO cavity length was extended by translating the OC. Under synchronous pumping, the fundamental OPO cavity length matches the input pump pulse RR according to L0 c∕2RR 1.88 m. The increase, ΔL L0 ∕Q, results in OPO output pulses at the Qth harmonic of the input RR, as noted earlier, so at specific values of ΔL, new harmonics of the input RR should be obtained. Figure 2 shows an example of the generated OPO signal pulse trains at the fundamental and seventh harmonic of the input RR, measured using a 3.5 GHz oscilloscope with a (LeCroy WavePro 735Zi) and a fast photodiode with a bandwidth of 20 GHz (New Focus 1444). For the seventh harmonic, the OC mirror was displaced by 26.9 cm from the fundamental position, being 1/7th of the synchronized cavity length. To measure the output pulse RR at higher frequencies and for various values of ΔL, we used a radio frequency (RF) spectrum analyzer with maximum measurable RF frequency of 13.6 GHz. Examples of the generated RF spectra of the OPO output signal pulse train at 1.3 and 7 GHz are presented in Figs. 3(a) and 3(b), respectively. These frequencies correspond to the 16th and 88th harmonic of the input RR at 80 MHz, and were obtained for fractional OPO cavity length increments (ΔL LOPO − L0 ) of 118 and 21 mm, respectively. In the spectra, two periods can be observed: the fine structure formed by the spectral lines separated
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Fig. 2. Oscilloscope traces of output signal pulses at the fundamental (∼80 MHz) and seventh harmonic of the input RR (∼560 MHz).
by 80 MHz, corresponding to the fundamental pump laser RR, and the main peaks located at the selected harmonic of the pump RR and the subsequent harmonics of this frequency. The behavior of OPO signal average output power as a function of the measured RR is presented in Fig. 4, where it can be clearly seen that power is highest at the fundamental frequency and drops as the RR increases. To control the higher- and lower-harmonic spectral peaks and to ensure that the measured RF peak was the strongest at the intended frequency, we restricted the highest RR directly deduced from the RF spectrum analyzer to 7 GHz, with the results shown in the inset of Fig. 4. At higher RRs, due to the limitations of the RF spectrum analyzer, the frequencies were deduced by measuring the displacement of the OC mirror, ΔL, on the micrometer
Fig. 3. RF spectra of the pulse trains with RR at (a) 1.3 GHz (16th harmonic) and (b) 7.0 GHz (88th harmonic). Details of the main peak at (c) 1.3 GHz and (d) 7 GHz.
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OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
Fig. 4. OPO signal power as a function of estimated RR (lower abscissa) and the respective mirror displacement related to the fundamental mirror position (upper abscissa). Inset: OPO output power as a function of the measured RR.
scale and recording the corresponding power. As evident from Fig. 4, as the RR is increased, the average output power decreases up to a frequency of ∼20 GHz, beyond which the power begins to rise again. As the pulse RR increases, ΔL decreases, and the OPO cavity length, LOPO , approaches the fundamental cavity length, L0 . Ultimately, an increase in the average power is expected when the consecutive pulses begin to overlap, resulting in the seeding of the signal in the OPO. The pulse overlap becomes significant when the temporal separation between the pulses approaches twice the pulse duration (1∕e2 ). We measured the FWHM duration of the signal pulses from the OPO to be 18.4 ps, for which the corresponding RR, at which the pulse overlap occurs, is estimated to be ∼30 GHz, which corresponds to the 377th harmonic of the input RR. The increment of the power is evident from Fig. 4, where it can be clearly seen that the signal power starts to increase from 20 GHz toward 30 GHz and beyond. However, due to the limitation of the RF spectrum analyzer, it was not possible to determine the exact RR at the highest frequencies. We determined the duration of output signal pulses at an arbitrary RR of ∼400 MHz, corresponding to the fifth harmonic of the fundamental. The signal pulse duration was measured using a homemade interferometric autocorrelator based on two-photon absorption in a Si photodetector. The result is shown in Fig. 5. The autocorrelation measurement results in a pulse duration of 16.9 ps, assuming a Gaussian pulse shape. The corresponding measurements for the signal at the fundamental RR and for the pump resulted in duration of 18.4 and 19.5 ps, respectively. The signal spectrum is presented in inset (a) of Fig. 5, showing the main peak at the 1564 nm with a small peak close by. The power stability of the output signal at the fundamental RR and fifth harmonic recorded over 30 min are shown in the inset (b) of Fig. 5. The relative standard deviations for the fundamental and fifth harmonic are 1.6% and 3.5%, respectively. As expected, OPO operation with higher losses in the fifth
Fig. 5. Autocorrelation trace of signal pulses at the fifth harmonic of fundamental RR, resulting in FWHM pulse duration of 16.9 ps. Inset: (a) spectrum of the fifth harmonic. (b) Power stability over 30 min for fundamental (blue) and fifth harmonic (red). (c) Intensity profiles of the output signal beam in orthogonal directions at a RR of 12.8 GHz.
harmonic produced less stable output and lower power. The spatial profile of the output signal beam in the two orthogonal directions, measured at the distance of 44 cm from the output coupling mirror, is shown in inset (c) of Fig. 5, confirming that the beam profile is close to symmetric and Gaussian. In conclusion, we have demonstrated a fiber-pumped picosecond OPO at ∼7 GHz using the incremental cavity length technique, with the RR limited by the temporal overlap of ∼18 ps signal pulses. Using optimized output coupling, higher powers than 600 mW will be readily attainable, with potential for further increase in output power, given the power scaling capability of fiber lasers and the possibility to optimize of output coupling [11,12]. Extension of the technique to femtosecond OPOs using high-power fiber lasers should enable the generation of pulses at tens of GHz before the onset of temporal overlap. The GHz RR, wide tuning range, practical power, and compact fiber-pumped design make the OPO viable for a variety of applications in the near- to mid-IR. 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 FA8655-12-12128 and the Catalan Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) through grant SGR 20092013. O. Kimmelma acknowledges financial support from the Jenny and Antti Wihuri Foundation, Finland. References 1. K. J. Weingarten, M. J. W. Rodwell, and D. M. Bloom, IEEE J. Quantum Electron. 24, 198 (1988). 2. C. Janke, M. Först, M. Nagel, H. Kurz, and A. Bartels, Opt. Lett. 30, 1405 (2005). 3. A. Stolow and D. M. Jonas, Science 305, 1575 (2004). 4. A. E. H. Oehler, M. C. Stumpf, S. Pekarek, T. Sudmeyer, K. J. Weingarten, and U. Keller, Appl. Phys. B 99, 53 (2010). 5. E. Yoshida and M. Nakazawa, Electron. Lett. 32, 1370 (1996).
November 15, 2013 / Vol. 38, No. 22 / OPTICS LETTERS 6. R. Paschotta, L. Krainer, S. Lecomte, G. J. Spuhler, S. C. Zeller, A. Aschwanden, D. Lorenser, H. J. Unold, K. J. Weingarten, and U. Keller, New J. Phys. 6, 174 (2004). 7. S. Chaitanya Kumar, A. Agnesi, P. Dallocchio, F. Pirzio, G. Reali, K. T. Zawilski, P. G. Schunemann, and M. Ebrahim-Zadeh, Opt. Lett. 36, 3236 (2011). 8. P. J. Phillips, S. Das, and M. Ebrahimzadeh, Appl. Phys. Lett. 77, 469 (2000).
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9. A. Esteban-Martin, O. Kokabee, K. Moutzouris, and M. Ebrahim-Zadeh, Opt. Lett. 34, 428 (2009). 10. O. Kokabee, A. Esteban-Martin, and M. Ebrahim-Zadeh, Opt. Lett. 35, 3210 (2010). 11. S. Chaitanya Kumar, A. Esteban-Martin, and M. Ebrahim-Zadeh, Opt. Lett. 36, 1068 (2011). 12. A. Esteban-Martin, O. Kokabee, and M. Ebrahim-Zadeh, Opt. Lett. 35, 2786 (2010).
OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
Multi-gigahertz picosecond optical parametric oscillator pumped by 80-MHz Yb-fiber laser Ossi Kimmelma,1 S. Chaitanya Kumar,1,* Adolfo Esteban-Martin,1 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, 08010 Barcelona, Spain *Corresponding author: [email protected] Received July 12, 2013; revised September 24, 2013; accepted October 1, 2013; posted October 2, 2013 (Doc. ID 193843); published November 5, 2013
We report a multi-gigahertz (GHz) repetition-rate picosecond optical parametric oscillator (OPO) based on MgO: PPLN, synchronously pumped by a Yb-fiber laser operating at 80 MHz, where the multiplication of repetition frequency is achieved using fractional increment in the OPO cavity length. Using this simple technique, we achieve OPO operation up to the 88th harmonic of the pump laser frequency, corresponding to a repetition rate as high as 7 GHz. Deploying a 5% output coupler, we are able to extract up to 960 mW of average signal power at the fundamental with 600 mW at the 88th harmonic (7 GHz), using a pump power of 5.6 W. The measured relative standard deviations of the fundamental and fifth harmonic signal power are recorded to be 1.6% and 3.5%, respectively, while the fundamental signal pulse duration is measured to be 18.4 ps. © 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.38.004550
Techniques to generate laser pulses at high repetition rate (RR) have been studied widely because of the demand for such sources in applications such as optical clocking, high-speed and electro-optic sampling [1–3]. For high RR ultrashort pulse generation near the important telecommunication wavelength of 1.5 μm, a number of techniques have been exploited, including mode-locked Er, Yb:glass lasers [4], with some using rational-harmonic modelocking [5], where the RR of a laser source is multiplied by changing the ratio of the laser cavity round-trip time and the gain or loss modulation frequency. For tunable ultrashort pulse generation in the near- to mid-IR, synchronously pumped optical parametric oscillators (OPOs) represent the most viable route, providing picosecond and femtosecond pulses across the 1–5 μm spectral range. Using direct synchronous pumping, output pulses at the same RR as the mode-locked pump laser are obtained from the OPO, typically limited to 99%) for the signal over 1.3–2.2 μm and highly transmitting for the pump (T > 90%) at 1064 nm, while the OC has a transmission of 5% over the signal wavelength range of 1.43–1.63 μm. All mirrors, except the OC, are also highly transmitting (T > 87%) at idler wavelengths over 2.2–4 μm, ensuring singly resonant oscillation. The nonlinear crystal for the OPO is a 50-mm-long MgO:PPLN sample containing seven uniform poling periods from 28.5 to 31.5 μm, although we only use the 30.5 μm grating period in this work. The crystal is maintained at 100°C in an oven with a stability of 0.1°C, resulting in a signal wavelength of 1564 nm and a corresponding idler wavelength of 3.33 μm. However, with the available mirrors, the signal wavelength can be readily tuned over 1.43–1.63 μm by varying the crystal temperature and grating period [10]. The pump beam is focused to a waist radius of w0 ∼ 50 μm at the center of the crystal, which is positioned at the primary focus in between M1 and M2. The signal beam is collimated in the X-cavity arms to maintain a constant mode size while varying the arm length for high RR pulse generation. To achieve pulse RR multiplication from the input 80 MHz to several GHz at the output, the OPO cavity length was extended by translating the OC. Under synchronous pumping, the fundamental OPO cavity length matches the input pump pulse RR according to L0 c∕2RR 1.88 m. The increase, ΔL L0 ∕Q, results in OPO output pulses at the Qth harmonic of the input RR, as noted earlier, so at specific values of ΔL, new harmonics of the input RR should be obtained. Figure 2 shows an example of the generated OPO signal pulse trains at the fundamental and seventh harmonic of the input RR, measured using a 3.5 GHz oscilloscope with a (LeCroy WavePro 735Zi) and a fast photodiode with a bandwidth of 20 GHz (New Focus 1444). For the seventh harmonic, the OC mirror was displaced by 26.9 cm from the fundamental position, being 1/7th of the synchronized cavity length. To measure the output pulse RR at higher frequencies and for various values of ΔL, we used a radio frequency (RF) spectrum analyzer with maximum measurable RF frequency of 13.6 GHz. Examples of the generated RF spectra of the OPO output signal pulse train at 1.3 and 7 GHz are presented in Figs. 3(a) and 3(b), respectively. These frequencies correspond to the 16th and 88th harmonic of the input RR at 80 MHz, and were obtained for fractional OPO cavity length increments (ΔL LOPO − L0 ) of 118 and 21 mm, respectively. In the spectra, two periods can be observed: the fine structure formed by the spectral lines separated
4551
Fig. 2. Oscilloscope traces of output signal pulses at the fundamental (∼80 MHz) and seventh harmonic of the input RR (∼560 MHz).
by 80 MHz, corresponding to the fundamental pump laser RR, and the main peaks located at the selected harmonic of the pump RR and the subsequent harmonics of this frequency. The behavior of OPO signal average output power as a function of the measured RR is presented in Fig. 4, where it can be clearly seen that power is highest at the fundamental frequency and drops as the RR increases. To control the higher- and lower-harmonic spectral peaks and to ensure that the measured RF peak was the strongest at the intended frequency, we restricted the highest RR directly deduced from the RF spectrum analyzer to 7 GHz, with the results shown in the inset of Fig. 4. At higher RRs, due to the limitations of the RF spectrum analyzer, the frequencies were deduced by measuring the displacement of the OC mirror, ΔL, on the micrometer
Fig. 3. RF spectra of the pulse trains with RR at (a) 1.3 GHz (16th harmonic) and (b) 7.0 GHz (88th harmonic). Details of the main peak at (c) 1.3 GHz and (d) 7 GHz.
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OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
Fig. 4. OPO signal power as a function of estimated RR (lower abscissa) and the respective mirror displacement related to the fundamental mirror position (upper abscissa). Inset: OPO output power as a function of the measured RR.
scale and recording the corresponding power. As evident from Fig. 4, as the RR is increased, the average output power decreases up to a frequency of ∼20 GHz, beyond which the power begins to rise again. As the pulse RR increases, ΔL decreases, and the OPO cavity length, LOPO , approaches the fundamental cavity length, L0 . Ultimately, an increase in the average power is expected when the consecutive pulses begin to overlap, resulting in the seeding of the signal in the OPO. The pulse overlap becomes significant when the temporal separation between the pulses approaches twice the pulse duration (1∕e2 ). We measured the FWHM duration of the signal pulses from the OPO to be 18.4 ps, for which the corresponding RR, at which the pulse overlap occurs, is estimated to be ∼30 GHz, which corresponds to the 377th harmonic of the input RR. The increment of the power is evident from Fig. 4, where it can be clearly seen that the signal power starts to increase from 20 GHz toward 30 GHz and beyond. However, due to the limitation of the RF spectrum analyzer, it was not possible to determine the exact RR at the highest frequencies. We determined the duration of output signal pulses at an arbitrary RR of ∼400 MHz, corresponding to the fifth harmonic of the fundamental. The signal pulse duration was measured using a homemade interferometric autocorrelator based on two-photon absorption in a Si photodetector. The result is shown in Fig. 5. The autocorrelation measurement results in a pulse duration of 16.9 ps, assuming a Gaussian pulse shape. The corresponding measurements for the signal at the fundamental RR and for the pump resulted in duration of 18.4 and 19.5 ps, respectively. The signal spectrum is presented in inset (a) of Fig. 5, showing the main peak at the 1564 nm with a small peak close by. The power stability of the output signal at the fundamental RR and fifth harmonic recorded over 30 min are shown in the inset (b) of Fig. 5. The relative standard deviations for the fundamental and fifth harmonic are 1.6% and 3.5%, respectively. As expected, OPO operation with higher losses in the fifth
Fig. 5. Autocorrelation trace of signal pulses at the fifth harmonic of fundamental RR, resulting in FWHM pulse duration of 16.9 ps. Inset: (a) spectrum of the fifth harmonic. (b) Power stability over 30 min for fundamental (blue) and fifth harmonic (red). (c) Intensity profiles of the output signal beam in orthogonal directions at a RR of 12.8 GHz.
harmonic produced less stable output and lower power. The spatial profile of the output signal beam in the two orthogonal directions, measured at the distance of 44 cm from the output coupling mirror, is shown in inset (c) of Fig. 5, confirming that the beam profile is close to symmetric and Gaussian. In conclusion, we have demonstrated a fiber-pumped picosecond OPO at ∼7 GHz using the incremental cavity length technique, with the RR limited by the temporal overlap of ∼18 ps signal pulses. Using optimized output coupling, higher powers than 600 mW will be readily attainable, with potential for further increase in output power, given the power scaling capability of fiber lasers and the possibility to optimize of output coupling [11,12]. Extension of the technique to femtosecond OPOs using high-power fiber lasers should enable the generation of pulses at tens of GHz before the onset of temporal overlap. The GHz RR, wide tuning range, practical power, and compact fiber-pumped design make the OPO viable for a variety of applications in the near- to mid-IR. 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 FA8655-12-12128 and the Catalan Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) through grant SGR 20092013. O. Kimmelma acknowledges financial support from the Jenny and Antti Wihuri Foundation, Finland. References 1. K. J. Weingarten, M. J. W. Rodwell, and D. M. Bloom, IEEE J. Quantum Electron. 24, 198 (1988). 2. C. Janke, M. Först, M. Nagel, H. Kurz, and A. Bartels, Opt. Lett. 30, 1405 (2005). 3. A. Stolow and D. M. Jonas, Science 305, 1575 (2004). 4. A. E. H. Oehler, M. C. Stumpf, S. Pekarek, T. Sudmeyer, K. J. Weingarten, and U. Keller, Appl. Phys. B 99, 53 (2010). 5. E. Yoshida and M. Nakazawa, Electron. Lett. 32, 1370 (1996).
November 15, 2013 / Vol. 38, No. 22 / OPTICS LETTERS 6. R. Paschotta, L. Krainer, S. Lecomte, G. J. Spuhler, S. C. Zeller, A. Aschwanden, D. Lorenser, H. J. Unold, K. J. Weingarten, and U. Keller, New J. Phys. 6, 174 (2004). 7. S. Chaitanya Kumar, A. Agnesi, P. Dallocchio, F. Pirzio, G. Reali, K. T. Zawilski, P. G. Schunemann, and M. Ebrahim-Zadeh, Opt. Lett. 36, 3236 (2011). 8. P. J. Phillips, S. Das, and M. Ebrahimzadeh, Appl. Phys. Lett. 77, 469 (2000).
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9. A. Esteban-Martin, O. Kokabee, K. Moutzouris, and M. Ebrahim-Zadeh, Opt. Lett. 34, 428 (2009). 10. O. Kokabee, A. Esteban-Martin, and M. Ebrahim-Zadeh, Opt. Lett. 35, 3210 (2010). 11. S. Chaitanya Kumar, A. Esteban-Martin, and M. Ebrahim-Zadeh, Opt. Lett. 36, 1068 (2011). 12. A. Esteban-Martin, O. Kokabee, and M. Ebrahim-Zadeh, Opt. Lett. 35, 2786 (2010).
