1464
OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
Actively mode-locked Tm3+-doped silica fiber laser with wavelength-tunable, high average output power Christian Kneis,1,2,* Brenda Donelan,1 Antoine Berrou,1 Inka Manek-Hönninger,2 Thierry Robin,3 Benoît Cadier,3 Marc Eichhorn,1 and Christelle Kieleck1 1
French-German Research Institute of Saint-Louis ISL, 5 rue du Général Cassagnou, 68301 Saint-Louis, France 2
Centre Lasers Intenses et Applications (CELIA), Université Bordeaux—CNRS—CEA—UMR5107 (formerly at LOMA—UMR5798), 351 cours de la Libération, 33405 Talence, France 3
iXFiber, Rue Paul Sabatier, 22300 Lannion, France *Corresponding author: [email protected]
Received December 17, 2014; revised March 2, 2015; accepted March 3, 2015; posted March 3, 2015 (Doc. ID 230829); published March 27, 2015 A diode-pumped, actively mode-locked high-power thulium (Tm3 )-doped double-clad silica fiber laser is demonstrated, providing an average output power in mode-locked (continuous wave) operation of 53 W (72 W) with a slope efficiency of 34% (38%). Mode-locking in the 6th-harmonic order was obtained by an acousto-optic modulator driven at 66 MHz without dispersion compensation. The shortest measured output pulse width was 200 ps. Owing to a diffraction grating as cavity end mirror, the central wavelength could be tuned from 1.95 to 2.13 μm. The measured beam quality in mode-locked and continuous wave operation has been close to the diffraction limit. © 2015 Optical Society of America OCIS codes: (140.4050) Mode-locked lasers; (140.3510) Lasers, fiber; (140.3600) Lasers, tunable. http://dx.doi.org/10.1364/OL.40.001464
Mode-locked Tm3 -doped fiber lasers emitting at eyesafe wavelengths around 2 μm are promising sources for applications that include plastic and glass processing [1], medicine [2,3], and optical sensing [4,5], or as a pump source for mid-infrared supercontinuum generation [6,7]. The broad amplification bandwidth of Tm3 -doped silica fiber lasers, ranging from ∼1.7 to ∼2.1 μm, makes them very suitable for short-pulse generation [8]. For many of those applications, crystal-based solidstate lasers are currently being replaced by fiber lasers due to the advantages of the fiber geometry compared to the commonly used bulk geometry: a higher gain factor, broader gain bandwidth, flexible and robust single- (transverse) mode operation and high efficiencies, to mention some of them [9]. This trend is also noticeable at wavelengths around 2 μm. Owing to the mentioned high-amplification bandwidth, mode-locked Tm3 -doped fiber lasers have been investigated very intensively in the past decade. Many different mode-locking techniques have been studied, whereby passive modulation methods have been mostly used. Saturable absorbers in the form of semiconductor saturable-absorber mirrors (SESAMs) [10] are the most popular passive elements, but topological insulators [11], carbon nanotubes [8], and graphene [12] have emerged as useful alternatives. For active modulation techniques, acousto-optic modulators (AOMs) have been analyzed in the majority of cases [13,14], but also approaches with electro-optic modulators have been reported [15]. It is generally claimed that passive mode-locking provides shorter pulses when compared to active techniques [16]. The output power and pulse energy in passive modulation techniques from single oscillators are limited though, up to a certain output power level for stable operation, or by the damage threshold of the available modulators [13]. If high peak and average output power are required, either actively mode-locked single-oscillator designs [6] or passively mode-locked seed lasers with 0146-9592/15/071464-04$15.00/0
amplifier stages [17,18] have to be used. Passively modelocked systems can be designed in an all-fiber format, which is not possible for active mode-locking due to the nonavailability of fiber-pigtailed modulators. However, master oscillator power-amplifier systems suffer from higher complexity and sensitivity to fiber nonlinearities compared to single-oscillator actively mode-locked lasers. In this Letter, we present results of a diode-pumped, actively mode-locked Tm3 -doped double-clad silica fiber laser. The laser is harmonically mode-locked by an AOM and can be wavelength-tuned by tilting the diffraction grating, which acts as cavity end mirror. The setup of the said fiber laser system is shown in Fig. 1. The Tm3 -doped silica fiber is symmetrically endpumped by two fiber-coupled pump diodes with a maximum output power of 300 W each. The diode modules emit at a wavelength of 798 nm and 800 nm, respectively. The radiation is delivered via multimode delivery fibers with a core diameter of 400 μm and a numerical aperture (NA) of 0.22. The pump light is collimated by AR-coated achromatic lenses with a focal length of 25 mm and combined with the laser signal using dichroic mirrors, which are transparent for the laser and reflective for the pump wavelength. Pump injection and laser beam collimation
Fig. 1. Setup of the Tm3 -doped fiber laser. © 2015 Optical Society of America
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
are provided by aspheric coupling lenses, AR-coated for pump and laser wavelength with a focal length of 15 mm. It is assumed that all pump light, despite the 4% Fresnel losses, can be coupled into the Tm3 -doped fiber. For that reason, all results in this Letter are presented with respect to incident pump power. The silica fiber is doped with 2.8 wt.% Tm3 and has a core diameter of 25 μm with a NA of 0.08. The hexagonal cladding has a flat-to-flat distance of 300 μm with a NA of 0.46. The absorption of Tm3 -doped silica at the pump wavelength of 800 nm is approximately 40% of the peak value, which results in an absorption of 2.2 dB∕m. For that reason, a fiber length of 8.8 m is used to guarantee adequate absorption of the pump light for efficient laser operation, and to avoid damage that occurs if transmitted pump light is coupled back into the delivery fiber of the other diode module. The active fiber is cleaved on one end under an angle of 0° to use the 4% Fresnel reflections as feedback for the laser resonator. The other end of the fiber, which faces the approximately 50-cm-long free-space cavity, is cleaved under an angle of 8° to prevent parasitic lasing owing to back reflections into the fiber. The fiber is actively water-cooled at 19°C during operation. In order to adjust the beam size in the external laser cavity, a lens has been inserted. The diffraction grating is blazed at 2.8 μm with 300 groves per millimeter and provides an average reflectivity around 70% at 2 μm. For active mode-locking, an AR-coated free-space AOM is deployed, which is placed directly in front of the diffraction grating. The AOM is driven by a tunable digital-frequency synthesizer. At every zero crossing of the standing acoustic wave inside the modulator, the intracavity pulse passes the modulator toward the grating and back to the fiber. There are two zero-crossings per period, which is why the frequency synthesizer has to be operated at half of the repetition rate of the mode-locked laser. Owing to the relative long active fiber, the fundamental repetition rate of the laser was 11 MHz, and the frequency synthesizer would have to be operated at 5.5 MHz for fundamental mode-locking. This frequency is not in the operating range of the synthesizer; that is why harmonic mode-locking had to be chosen for the available equipment. The dispersion of the investigated fiber laser is not compensated. The average output power with respect to incident pump power of the described fiber laser system has been measured for continuous wave and mode-locked operation. The results are plotted in Fig. 2. At an incident pump power of 186 W, 72 W of average output power in continuous wave operation with a slope efficiency of 38% has been achieved. In mode-locked operation, the laser system provided up to 53 W at an incident pump power of 158 W resulting in a slope efficiency of 34%. The threshold for both regimes has been approximately 6 W. The slope efficiency can be further increased by decreasing the losses of around 30% at the diffraction grating. The curve for mode-locking shows a bend at an incident pump power around 100 W. The reason has not yet been investigated and will be the scope of further investigations. The beam propagation factor M 2 has been measured in continuous wave and mode-locked operation at an
1465
Fig. 2. Output power over pump power for continuous wave (CW) and mode-locked (ML) operation.
output power of 30 W and 25 W, respectively. The fiber laser provided an excellent beam quality in both operation regimes with a beam propagation factor of less than 1.1 in both directions. The results are displayed in Figs. 3 and 4 with insets showing the beam profiles of the collimated beams. The pulse duration of the mode-locked pulses has been monitored using an extended, biased Indium-GalliumArsenide (InGaAs) photo receiver (PR) with a bandwidth of 12.5 GHz, connected to an oscilloscope via a suitable high-bandwidth BNC cable. The oscilloscope had a bandwidth of 3 GHz and a sampling rate of 20 Gs/s. For the analysis of the mode-locked pulse duration, the measured pulse width Δtmeas has to be corrected if this value is close to the detection limit of the measurement system (Δtsys ). To calculate the real pulse , the following correlation has to be used: width Δtq real
Δtreal Δtmeas 2 − Δtsys 2 [19]. All time constants are the full-width at halfmaximum of the pulses in time domain. Hereby, the detection limit of the measurement system Δtsys r K Osc 2 K PR 2 can be calculated with Δtsys Δf Osc Δf PR ,
considering the oscilloscope and the PR of the measurement system and neglecting the influence of the BNC cable. Δf denotes the full width at half-maximum of
Fig. 3. M 2 measurement at an output power of 30 W in continuous wave operation; inset: profile of the collimated beam.
1466
OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
Fig. 4. M 2 measurement at an output power of 25 W in modelocked operation; inset: profile of the collimated beam.
Fig. 6. Tunability of the Tm3 -doped fiber laser; inset: laser output spectrum at one operation point of the laser system.
the oscilloscope and the PR in frequency domain. The constants K for both devices are around 0.4 and 0.31 for the oscilloscope and the PR, respectively [19]. With the above-mentioned equipment, a theoretical detection limit of 150 ps results. To verify this value, the response of the measurement system for an input pulse of a commercial mode-locked laser, delivering a known pulse width in the range of 20 ps, has been recorded. The output pulse duration has been around 220 ps, which can be taken as the time constant Δtsys of the used measurement system. During mode-locked operation, the shortest measured pulse duration has been 300 ps (Fig. 5), which resulted in a real pulse width around Δtreal 200 ps. At 53 W of output power, this corresponds to a pulse energy of 0.8 μJ and a pulse peak power of approximately 8 kW, assuming a Gaussian pulse shape. The calculated threshold power for Raman scattering is 9 kW for the this fiber laser [20]. To realize higher peak power operation, the threshold for nonlinearities has to be pushed, either by a shorter active fiber or different fiber design. Figure 6 shows the tunability of the laser. The output power as a function of the central laser wavelength, which corresponds to different angles of the diffraction grating, has been measured at a constant incident pump power of 30 W. The wavelength of the laser was tunable between 1.95 and 2.13 μm. The curve is not completely
flat, which is probably caused by the dependence of the reflectivity of the grating on the wavelength of the incident light. The result also shows a shift of the overall spectrum toward longer wavelengths, if it is compared to other tunable Tm3 -doped fiber laser systems from the literature [21]. The reason for this effect is the relatively long active fiber, which influences the lasing bandwidth of quasi-three-level fiber laser systems [22]. Mode-locked operation was possible over the whole tunable bandwidth of the fiber laser and for all different incident pump power levels. For stable and optimized operation, some physical effects during operation must be taken into consideration. The optical length of the cavity is influenced by the wavelength of the laser system owing to dispersion. For a Tm3 -doped silica fiber with a length of 8.8 m, the change of the optical length of the cavity per δl change in lasing wavelength δλopt is in the order of 0.1 mm/nm, which is immense regarding the broad tunability of the laser system. Also the temperature of the fiber and therefore pump power level influence the refractive index and hence the optical cavity length. This effect is in that case 0.07 mm/K. If the lasing wavelength of the system or the pump power level is changed significantly, either the repetition rate of the synthesizer or the cavity length has to be adapted for stable operation. Furthermore, the diffraction efficiency and hence the modulation depth of the AOM depend on the lasing wavelength, which is why the parameters of the mode-locked pulses also vary for different lasing wavelengths. A detailed characterization of the explained influences on laser performance has not been yet done and will be the scope of further investigations. The inset of Fig. 6 shows the laser output spectrum at set tuning angle of the grating: the linewidth is around 0.15 nm and nearly constant over the entire tunable wavelength range. Assuming a Gaussian pulse shape, the time-bandwidth product for the given linewidth results in a minimal achievable pulse duration of approximately 39 ps, if the pulse would be compressed externally and/or the dispersion of the fiber laser would be compensated. Dispersion-compensation or external compression has not been tried. The modulation characteristics of the AOM could also cause a difference between the theoretical limit and the achieved pulse duration.
Fig. 5.
Measured pulse trace of the mode-locked pulse.
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
In conclusion, an actively mode-locked Tm3 -doped silica fiber laser with wavelength-tunability and high average output power has been demonstrated. 72 W in continuous wave operation and 53 W in mode-locked operation with pulse energies up to 0.8 μJ and peak intensities of 8 kW have been achieved at 66 MHz repetition rate. The laser system was tunable from 1.95 to 2.13 μm. Further investigation will be performed to scale the output power and decrease the pulse duration by an appropriate laser design. References 1. K. Scholle, S. Lamrini, P. Koopmann, and P. Fuhrberg, 2 μm Laser Sources and Their Possible Applications, Frontiers in Guided Wave Optics and Optoelectronics (InTech, 2010). 2. N. M. Fried, Lasers Surg. Med. 36, 52 (2005). 3. K. D. Polder and S. Bruce, Dermatol. Surg. 38, 199 (2012). 4. M. Eichhorn and S. D. Jackson, Opt. Lett. 33, 1044 (2008). 5. S. W. Henderson, P. J. M. Suni, C. P. Hale, S. M. Hannon, J. R. Magee, D. L. Bruns, and E. H. Yuen, IEEE Trans. Photon. Technol. Lett. 31, 415 (1993). 6. M. Eckerle, C. Kieleck, J. Swiderski, S. D. Jackson, G. Mazé, and M. Eichhorn, Opt. Lett. 37, 512 (2012). 7. J. Swiderski, M. Michalska, C. Kieleck, M. Eichhorn, and G. Mazé, IEEE Photon. Technol. Lett. 26, 150 (2014).
1467
8. M. A. Solodyankin, E. D. Obraztsova, A. S. Lobach, A. I. Chernov, A. V. Tausenev, V. I. Konov, and E. M. Dianov, Opt. Lett. 33, 1336 (2008). 9. D. J. Richardson, J. Nilsson, and W. A. Clarkson, J. Opt. Soc. Am. B 27, B63 (2010). 10. R. C. Sharp, D. E. Spock, N. Pan, and J. Elliot, Opt. Lett. 21, 881 (1996). 11. M. Jung, J. Lee, J. Koo, J. Park, Y.-W. Song, K. Lee, S. Lee, and J. H. Lee, Opt. Express 22, 7865 (2014). 12. H. Ahmad, K. Thambiratnam, F. D. Muhammad, M. Z. Zulkifli, A. Z. Zulkifli, M. C. Paul, and S. W. Harun, IEEE J. Sel. Top. Quantum Electron. 20, 1100108 (2014). 13. P. Hübner, C. Kieleck, S. D. Jackson, and M. Eichhorn, Opt. Lett. 36, 2483 (2011). 14. J. Lee and J. H. Lee, J. Opt. Soc. Am. B 30, 1479 (2013). 15. K. Yin, B. Zhang, W. Yang, H. Chen, S. Chen, and J. Hou, Opt. Lett. 39, 4259 (2014). 16. U. Keller, Nature 424, 831 (2003). 17. J. Liu, J. Xu, K. Liu, F. Tan, and P. Wang, Opt. Lett. 38, 4150 (2013). 18. P. Wan, L.-M. Yang, and J. Liu, Opt. Express 21, 21374 (2013). ˇ 19. M. Jelínek, V. Kubeček, and M. Cech, Photodiodes— Communications, Bio-Sensings, Measurements and HighEnergy Physics (InTech, 2011). 20. G. P. Agrawal, Nonlinear Fiber Optics (Academic, 2001). 21. M. Eichhorn and S. D. Jackson, Appl. Phys. B 90, 35 (2008). 22. M. Eichhorn, Appl. Phys. B 93, 269 (2008).
OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
Actively mode-locked Tm3+-doped silica fiber laser with wavelength-tunable, high average output power Christian Kneis,1,2,* Brenda Donelan,1 Antoine Berrou,1 Inka Manek-Hönninger,2 Thierry Robin,3 Benoît Cadier,3 Marc Eichhorn,1 and Christelle Kieleck1 1
French-German Research Institute of Saint-Louis ISL, 5 rue du Général Cassagnou, 68301 Saint-Louis, France 2
Centre Lasers Intenses et Applications (CELIA), Université Bordeaux—CNRS—CEA—UMR5107 (formerly at LOMA—UMR5798), 351 cours de la Libération, 33405 Talence, France 3
iXFiber, Rue Paul Sabatier, 22300 Lannion, France *Corresponding author: [email protected]
Received December 17, 2014; revised March 2, 2015; accepted March 3, 2015; posted March 3, 2015 (Doc. ID 230829); published March 27, 2015 A diode-pumped, actively mode-locked high-power thulium (Tm3 )-doped double-clad silica fiber laser is demonstrated, providing an average output power in mode-locked (continuous wave) operation of 53 W (72 W) with a slope efficiency of 34% (38%). Mode-locking in the 6th-harmonic order was obtained by an acousto-optic modulator driven at 66 MHz without dispersion compensation. The shortest measured output pulse width was 200 ps. Owing to a diffraction grating as cavity end mirror, the central wavelength could be tuned from 1.95 to 2.13 μm. The measured beam quality in mode-locked and continuous wave operation has been close to the diffraction limit. © 2015 Optical Society of America OCIS codes: (140.4050) Mode-locked lasers; (140.3510) Lasers, fiber; (140.3600) Lasers, tunable. http://dx.doi.org/10.1364/OL.40.001464
Mode-locked Tm3 -doped fiber lasers emitting at eyesafe wavelengths around 2 μm are promising sources for applications that include plastic and glass processing [1], medicine [2,3], and optical sensing [4,5], or as a pump source for mid-infrared supercontinuum generation [6,7]. The broad amplification bandwidth of Tm3 -doped silica fiber lasers, ranging from ∼1.7 to ∼2.1 μm, makes them very suitable for short-pulse generation [8]. For many of those applications, crystal-based solidstate lasers are currently being replaced by fiber lasers due to the advantages of the fiber geometry compared to the commonly used bulk geometry: a higher gain factor, broader gain bandwidth, flexible and robust single- (transverse) mode operation and high efficiencies, to mention some of them [9]. This trend is also noticeable at wavelengths around 2 μm. Owing to the mentioned high-amplification bandwidth, mode-locked Tm3 -doped fiber lasers have been investigated very intensively in the past decade. Many different mode-locking techniques have been studied, whereby passive modulation methods have been mostly used. Saturable absorbers in the form of semiconductor saturable-absorber mirrors (SESAMs) [10] are the most popular passive elements, but topological insulators [11], carbon nanotubes [8], and graphene [12] have emerged as useful alternatives. For active modulation techniques, acousto-optic modulators (AOMs) have been analyzed in the majority of cases [13,14], but also approaches with electro-optic modulators have been reported [15]. It is generally claimed that passive mode-locking provides shorter pulses when compared to active techniques [16]. The output power and pulse energy in passive modulation techniques from single oscillators are limited though, up to a certain output power level for stable operation, or by the damage threshold of the available modulators [13]. If high peak and average output power are required, either actively mode-locked single-oscillator designs [6] or passively mode-locked seed lasers with 0146-9592/15/071464-04$15.00/0
amplifier stages [17,18] have to be used. Passively modelocked systems can be designed in an all-fiber format, which is not possible for active mode-locking due to the nonavailability of fiber-pigtailed modulators. However, master oscillator power-amplifier systems suffer from higher complexity and sensitivity to fiber nonlinearities compared to single-oscillator actively mode-locked lasers. In this Letter, we present results of a diode-pumped, actively mode-locked Tm3 -doped double-clad silica fiber laser. The laser is harmonically mode-locked by an AOM and can be wavelength-tuned by tilting the diffraction grating, which acts as cavity end mirror. The setup of the said fiber laser system is shown in Fig. 1. The Tm3 -doped silica fiber is symmetrically endpumped by two fiber-coupled pump diodes with a maximum output power of 300 W each. The diode modules emit at a wavelength of 798 nm and 800 nm, respectively. The radiation is delivered via multimode delivery fibers with a core diameter of 400 μm and a numerical aperture (NA) of 0.22. The pump light is collimated by AR-coated achromatic lenses with a focal length of 25 mm and combined with the laser signal using dichroic mirrors, which are transparent for the laser and reflective for the pump wavelength. Pump injection and laser beam collimation
Fig. 1. Setup of the Tm3 -doped fiber laser. © 2015 Optical Society of America
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
are provided by aspheric coupling lenses, AR-coated for pump and laser wavelength with a focal length of 15 mm. It is assumed that all pump light, despite the 4% Fresnel losses, can be coupled into the Tm3 -doped fiber. For that reason, all results in this Letter are presented with respect to incident pump power. The silica fiber is doped with 2.8 wt.% Tm3 and has a core diameter of 25 μm with a NA of 0.08. The hexagonal cladding has a flat-to-flat distance of 300 μm with a NA of 0.46. The absorption of Tm3 -doped silica at the pump wavelength of 800 nm is approximately 40% of the peak value, which results in an absorption of 2.2 dB∕m. For that reason, a fiber length of 8.8 m is used to guarantee adequate absorption of the pump light for efficient laser operation, and to avoid damage that occurs if transmitted pump light is coupled back into the delivery fiber of the other diode module. The active fiber is cleaved on one end under an angle of 0° to use the 4% Fresnel reflections as feedback for the laser resonator. The other end of the fiber, which faces the approximately 50-cm-long free-space cavity, is cleaved under an angle of 8° to prevent parasitic lasing owing to back reflections into the fiber. The fiber is actively water-cooled at 19°C during operation. In order to adjust the beam size in the external laser cavity, a lens has been inserted. The diffraction grating is blazed at 2.8 μm with 300 groves per millimeter and provides an average reflectivity around 70% at 2 μm. For active mode-locking, an AR-coated free-space AOM is deployed, which is placed directly in front of the diffraction grating. The AOM is driven by a tunable digital-frequency synthesizer. At every zero crossing of the standing acoustic wave inside the modulator, the intracavity pulse passes the modulator toward the grating and back to the fiber. There are two zero-crossings per period, which is why the frequency synthesizer has to be operated at half of the repetition rate of the mode-locked laser. Owing to the relative long active fiber, the fundamental repetition rate of the laser was 11 MHz, and the frequency synthesizer would have to be operated at 5.5 MHz for fundamental mode-locking. This frequency is not in the operating range of the synthesizer; that is why harmonic mode-locking had to be chosen for the available equipment. The dispersion of the investigated fiber laser is not compensated. The average output power with respect to incident pump power of the described fiber laser system has been measured for continuous wave and mode-locked operation. The results are plotted in Fig. 2. At an incident pump power of 186 W, 72 W of average output power in continuous wave operation with a slope efficiency of 38% has been achieved. In mode-locked operation, the laser system provided up to 53 W at an incident pump power of 158 W resulting in a slope efficiency of 34%. The threshold for both regimes has been approximately 6 W. The slope efficiency can be further increased by decreasing the losses of around 30% at the diffraction grating. The curve for mode-locking shows a bend at an incident pump power around 100 W. The reason has not yet been investigated and will be the scope of further investigations. The beam propagation factor M 2 has been measured in continuous wave and mode-locked operation at an
1465
Fig. 2. Output power over pump power for continuous wave (CW) and mode-locked (ML) operation.
output power of 30 W and 25 W, respectively. The fiber laser provided an excellent beam quality in both operation regimes with a beam propagation factor of less than 1.1 in both directions. The results are displayed in Figs. 3 and 4 with insets showing the beam profiles of the collimated beams. The pulse duration of the mode-locked pulses has been monitored using an extended, biased Indium-GalliumArsenide (InGaAs) photo receiver (PR) with a bandwidth of 12.5 GHz, connected to an oscilloscope via a suitable high-bandwidth BNC cable. The oscilloscope had a bandwidth of 3 GHz and a sampling rate of 20 Gs/s. For the analysis of the mode-locked pulse duration, the measured pulse width Δtmeas has to be corrected if this value is close to the detection limit of the measurement system (Δtsys ). To calculate the real pulse , the following correlation has to be used: width Δtq real
Δtreal Δtmeas 2 − Δtsys 2 [19]. All time constants are the full-width at halfmaximum of the pulses in time domain. Hereby, the detection limit of the measurement system Δtsys r K Osc 2 K PR 2 can be calculated with Δtsys Δf Osc Δf PR ,
considering the oscilloscope and the PR of the measurement system and neglecting the influence of the BNC cable. Δf denotes the full width at half-maximum of
Fig. 3. M 2 measurement at an output power of 30 W in continuous wave operation; inset: profile of the collimated beam.
1466
OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
Fig. 4. M 2 measurement at an output power of 25 W in modelocked operation; inset: profile of the collimated beam.
Fig. 6. Tunability of the Tm3 -doped fiber laser; inset: laser output spectrum at one operation point of the laser system.
the oscilloscope and the PR in frequency domain. The constants K for both devices are around 0.4 and 0.31 for the oscilloscope and the PR, respectively [19]. With the above-mentioned equipment, a theoretical detection limit of 150 ps results. To verify this value, the response of the measurement system for an input pulse of a commercial mode-locked laser, delivering a known pulse width in the range of 20 ps, has been recorded. The output pulse duration has been around 220 ps, which can be taken as the time constant Δtsys of the used measurement system. During mode-locked operation, the shortest measured pulse duration has been 300 ps (Fig. 5), which resulted in a real pulse width around Δtreal 200 ps. At 53 W of output power, this corresponds to a pulse energy of 0.8 μJ and a pulse peak power of approximately 8 kW, assuming a Gaussian pulse shape. The calculated threshold power for Raman scattering is 9 kW for the this fiber laser [20]. To realize higher peak power operation, the threshold for nonlinearities has to be pushed, either by a shorter active fiber or different fiber design. Figure 6 shows the tunability of the laser. The output power as a function of the central laser wavelength, which corresponds to different angles of the diffraction grating, has been measured at a constant incident pump power of 30 W. The wavelength of the laser was tunable between 1.95 and 2.13 μm. The curve is not completely
flat, which is probably caused by the dependence of the reflectivity of the grating on the wavelength of the incident light. The result also shows a shift of the overall spectrum toward longer wavelengths, if it is compared to other tunable Tm3 -doped fiber laser systems from the literature [21]. The reason for this effect is the relatively long active fiber, which influences the lasing bandwidth of quasi-three-level fiber laser systems [22]. Mode-locked operation was possible over the whole tunable bandwidth of the fiber laser and for all different incident pump power levels. For stable and optimized operation, some physical effects during operation must be taken into consideration. The optical length of the cavity is influenced by the wavelength of the laser system owing to dispersion. For a Tm3 -doped silica fiber with a length of 8.8 m, the change of the optical length of the cavity per δl change in lasing wavelength δλopt is in the order of 0.1 mm/nm, which is immense regarding the broad tunability of the laser system. Also the temperature of the fiber and therefore pump power level influence the refractive index and hence the optical cavity length. This effect is in that case 0.07 mm/K. If the lasing wavelength of the system or the pump power level is changed significantly, either the repetition rate of the synthesizer or the cavity length has to be adapted for stable operation. Furthermore, the diffraction efficiency and hence the modulation depth of the AOM depend on the lasing wavelength, which is why the parameters of the mode-locked pulses also vary for different lasing wavelengths. A detailed characterization of the explained influences on laser performance has not been yet done and will be the scope of further investigations. The inset of Fig. 6 shows the laser output spectrum at set tuning angle of the grating: the linewidth is around 0.15 nm and nearly constant over the entire tunable wavelength range. Assuming a Gaussian pulse shape, the time-bandwidth product for the given linewidth results in a minimal achievable pulse duration of approximately 39 ps, if the pulse would be compressed externally and/or the dispersion of the fiber laser would be compensated. Dispersion-compensation or external compression has not been tried. The modulation characteristics of the AOM could also cause a difference between the theoretical limit and the achieved pulse duration.
Fig. 5.
Measured pulse trace of the mode-locked pulse.
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
In conclusion, an actively mode-locked Tm3 -doped silica fiber laser with wavelength-tunability and high average output power has been demonstrated. 72 W in continuous wave operation and 53 W in mode-locked operation with pulse energies up to 0.8 μJ and peak intensities of 8 kW have been achieved at 66 MHz repetition rate. The laser system was tunable from 1.95 to 2.13 μm. Further investigation will be performed to scale the output power and decrease the pulse duration by an appropriate laser design. References 1. K. Scholle, S. Lamrini, P. Koopmann, and P. Fuhrberg, 2 μm Laser Sources and Their Possible Applications, Frontiers in Guided Wave Optics and Optoelectronics (InTech, 2010). 2. N. M. Fried, Lasers Surg. Med. 36, 52 (2005). 3. K. D. Polder and S. Bruce, Dermatol. Surg. 38, 199 (2012). 4. M. Eichhorn and S. D. Jackson, Opt. Lett. 33, 1044 (2008). 5. S. W. Henderson, P. J. M. Suni, C. P. Hale, S. M. Hannon, J. R. Magee, D. L. Bruns, and E. H. Yuen, IEEE Trans. Photon. Technol. Lett. 31, 415 (1993). 6. M. Eckerle, C. Kieleck, J. Swiderski, S. D. Jackson, G. Mazé, and M. Eichhorn, Opt. Lett. 37, 512 (2012). 7. J. Swiderski, M. Michalska, C. Kieleck, M. Eichhorn, and G. Mazé, IEEE Photon. Technol. Lett. 26, 150 (2014).
1467
8. M. A. Solodyankin, E. D. Obraztsova, A. S. Lobach, A. I. Chernov, A. V. Tausenev, V. I. Konov, and E. M. Dianov, Opt. Lett. 33, 1336 (2008). 9. D. J. Richardson, J. Nilsson, and W. A. Clarkson, J. Opt. Soc. Am. B 27, B63 (2010). 10. R. C. Sharp, D. E. Spock, N. Pan, and J. Elliot, Opt. Lett. 21, 881 (1996). 11. M. Jung, J. Lee, J. Koo, J. Park, Y.-W. Song, K. Lee, S. Lee, and J. H. Lee, Opt. Express 22, 7865 (2014). 12. H. Ahmad, K. Thambiratnam, F. D. Muhammad, M. Z. Zulkifli, A. Z. Zulkifli, M. C. Paul, and S. W. Harun, IEEE J. Sel. Top. Quantum Electron. 20, 1100108 (2014). 13. P. Hübner, C. Kieleck, S. D. Jackson, and M. Eichhorn, Opt. Lett. 36, 2483 (2011). 14. J. Lee and J. H. Lee, J. Opt. Soc. Am. B 30, 1479 (2013). 15. K. Yin, B. Zhang, W. Yang, H. Chen, S. Chen, and J. Hou, Opt. Lett. 39, 4259 (2014). 16. U. Keller, Nature 424, 831 (2003). 17. J. Liu, J. Xu, K. Liu, F. Tan, and P. Wang, Opt. Lett. 38, 4150 (2013). 18. P. Wan, L.-M. Yang, and J. Liu, Opt. Express 21, 21374 (2013). ˇ 19. M. Jelínek, V. Kubeček, and M. Cech, Photodiodes— Communications, Bio-Sensings, Measurements and HighEnergy Physics (InTech, 2011). 20. G. P. Agrawal, Nonlinear Fiber Optics (Academic, 2001). 21. M. Eichhorn and S. D. Jackson, Appl. Phys. B 90, 35 (2008). 22. M. Eichhorn, Appl. Phys. B 93, 269 (2008).
