0.7 MW output power from a two-arm coherently combined Q-switched photonic crystal fiber laser Boris Rosenstein,1,∗ Avry Shirakov,1,2 Daniel Belker,1 and Amiel A. Ishaaya1 1 Department
of Electrical and Computer Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel 2 Department of Physics, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel ∗ [email protected]
Abstract: We demonstrate a high peak power, Q-switched pulsed, intracavity coherently combined fiber laser system. The system is based on two Yb-doped, rod-type, photonic crystal fibers which are passively phase-locked and combined into the single output beam in a power oscillator configuration. Experimental evidence indicate that this oscillator system provides record high peak power of ∼ 0.7 MW with pulse duration of ∼ 10 ns at 1 kHz repetition rate. The measured beam quality shows near-diffraction-limited operation of the coherently combined system. © 2014 Optical Society of America OCIS codes: (140.3298) Laser beam combining; (140.3510) Lasers, fiber; (140.3540) Lasers, Q-switched.
References and links 1. D. Richardson, J. Nilsson, and W. Clarkson, “High power fiber lasers: current status and future perspectives [invited],” J. Opt. Soc. Am. B 27, B63–B92 (2010). 2. J. P. Koplow, D. A. Kliner, and L. Goldberg, “Single-mode operation of a coiled multimode fiber amplifier,” Opt. Lett. 25, 442–444 (2000). 3. W. Wadsworth, R. Percival, G. Bouwmans, J. Knight, and P. Russell, “High power air-clad photonic crystal fibre laser,” Opt. Express 11, 48–53 (2003). 4. I. Hu, X. Ma, A. Galvanauskas et al., “Single mode 9.1 mJ and 10ns pulses from 55μ m core yb-doped CCC fiber mopa,” in “CLEO: Science and Innovations,” (OSA, 2013), pp. CTu1K–5. 5. J. Limpert, F. Stutzki, F. Jansen, H.-J. Otto, T. Eidam, C. Jauregui, and A. T¨unnermann, “Yb-doped largepitch fibers: effective single-mode operation based on higher-order mode delocalization,” Light: Sci. App. 1, 1–5 (2012). 6. J. Bourderionnet, C. Bellanger, J. Primot, and A. Brignon, “Collective coherent phase combining of 64 fibers,” Opt. Express 19, 17053–17058 (2011). 7. J. R. Leger, G. J. Swanson, and W. B. Veldkamp, “Coherent laser addition using binary phase gratings,” Appl. Opt. 26, 4391–4399 (1987). 8. T. Fan, “Laser beam combining for high-power, high-radiance sources,” IEEE J. Sel. Top. Quantum Electron. 11, 567–577 (2005). 9. A. Ishaaya, N. Davidson, L. Shimshi, and A. Friesem, “Intracavity coherent addition of gaussian beam distributions using a planar interferometric coupler,” Appl. Phys. Lett. 85, 2187–2189 (2004). 10. A. A. Ishaaya, N. Davidson, and A. A. Friesem, “Passive laser beam combining with intracavity interferometric combiners,” IEEE J. Sel. Top. Quantum Electron. 15, 301–311 (2009). 11. D. Sabourdy, A. Desfarges-Berthelemot, V. Kermene, and A. Barth´el´emy, “Coherent combining of q-switched fibre lasers,” Electron. Lett. 40, 1254–1255 (2004). 12. B. Shulga and A. A. Ishaaya, “Imposing temporal and frequency characteristics in a system of coherently combined high peak power photonic crystal fiber lasers,” in “CLEO/EUROPE - IQEC,” (OSA, 2013).
#204219 - $15.00 USD (C) 2014 OSA
Received 6 Jan 2014; revised 2 Mar 2014; accepted 3 Mar 2014; published 12 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006416 | OPTICS EXPRESS 6416
13. K. Duan, J. Wang, J. Li, Y. Wang, Z. Zhao, and W. Zhao, “Experimental study of phase-locking of two photonic crystal fiber lasers,” Opt. Commun. 281, 2557–2560 (2008). 14. D. Kouznetsov, J.-F. Bisson, A. Shirakawa, and K.-I. Ueda, “Limits of coherent addition of lasers: Simple estimate,” Opt. Rev. 12, 445–447 (2005). 15. J. Guillot, A. Desfarges-Berthelemot, V. Kerm`ene, and A. Barth´el´emy, “Experimental study of cophasing dynamics in passive coherent combining of fiber lasers,” Opt. Lett. 36, 2907–2909 (2011). 16. V. Kozlov, J. Hernandez-Cordero, and T. Morse, “All-fiber coherent beam combining of fiber lasers,” Opt. Lett. 24, 1814–1816 (1999). 17. J. Bromage, J. M. Fini, C. Dorrer, and J. D. Zuegel, “Characterization and optimization of yb-doped photoniccrystal fiber rod amplifiers using spatially resolved spectral interferometry,” Appl. Opt. 50, 2001–2007 (2011).
1.
Introduction
Over the past decade, fiber laser sources have become one of the most popular and fastdeveloping laser technologies. This is mainly because fiber lasers possess several key advantages over bulk solid-state lasers, including good heat dissipation handling, high gain, easy alignment (or even the lack thereof, in all-fiber configurations) and single-mode wave guiding that provides excellent beam quality [1]. However, as yet, fiber lasers cannot replace bulk solidstate lasers when high peak power is required. Specifically, three main factors limit the singlemode output power that can be extracted from a single fiber laser source: material damage, deleterious nonlinear effects, and modal instabilities with high average powers. These limiting factors directly result from the extremely high average and peak powers within the relatively small fiber core. Increasing the effective single-mode area of the core, or combining several fiber laser sources coherently, may potentially circumvent these limitations and increase the output peak power of the laser. To increase the effective area of the single-mode core compared with standard step-index single-mode fibers, large mode area step-index fibers can be used; however, these are not “truly” single-mode fibers [2]. Other alternatives include employing photonic crystal fibers (PCFs) [3] or the recently developed chirally coupled core fibers [4]. The largest single-mode fiber reported is a rod-type PCF with a core diameter of 135 μ m and a corresponding mode field diameter of ∼ 100 μ m [5]; however, also this fiber can withstand limited peak power. The second approach for increasing the output peak power is the coherent combining approach, wherein several “independent” lasers (or amplifiers) are combined into a single coherent output. This approach is advantageous over increasing the effective single-mode core area in three main aspects. First, it is more accessible as the laser system can be based only upon commercially available components. Second, larger cores are highly sensitive to bending (in terms of mode losses) and often require a ruggedized fiber design; in contrast, in coherent combining, the fiber as an active medium may be flexible, increasing the compactness and robustness of the system. Third, the output peak and average powers may be increased simultaneously in the coherent combining approach by increasing the number of combined channels. Such promising scalability is made possible through distribution of the energy between several “independent” channels with no performance degradation. In the core size increasing approach, by contrast, the thermal load increases with the increase in average output power, resulting in mode instability and, reciprocally, in performance degradation [5]. In light of these clear advantages, coherent combining of laser sources has attracted much interest in the past decade. To achieve coherent combining, the relative phases of the laser sources should be locked and properly adjusted, either actively or passively. In active phase-locking techniques, the phase of each “independent” laser source is actively and continuously adjusted by means of electronic feedback loops [6]. In passive phase-locking techniques, the “independent” laser sources are self-phase-locked by means of a passive coupling mechanism [7, 8, 9, 10, 11, 12, 13]. Although passive techniques are generally regarded as more limited in terms of the maximum number
#204219 - $15.00 USD (C) 2014 OSA
Received 6 Jan 2014; revised 2 Mar 2014; accepted 3 Mar 2014; published 12 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006416 | OPTICS EXPRESS 6417
of combined laser channels [14], they are simpler, more robust and require no phase-control feedback loops that must be “smart” and fast enough, especially in short pulsed lasing systems [15]. Intracavity coherently combined fiber lasers with high average and peak power in an oscillator configuration, and especially with PCFs, have not been reported previously. Here, we describe the experimental investigation of high peak power passive interferometric coherent combining of two rod-type, Yb-doped, PCF laser channels within a common Q-switched cavity. In this approach, the two identical single-mode laser channels are operated in a phase-locked manner, and the output beam is a coherent superposition of the individual beams that preserves an excellent beam quality. We have achieved, to the best of our knowledge, a new record with regard to the peak power obtained from a Q-switched fiber oscillator. 2.
Experimental configuration
The experimental setup is based on two identical PCF lasers that are passively combined in a coherent manner into a single output. The schematic configuration of the system is shown in Fig. 1. The resonator is formed by two active fibers, an acousto-optic Q-switch (AOQ), a highly reflective (HR) broadband rear mirror common for both channels, and a partially reflective output coupler (OC) with 4% reflectivity. To enable intracavity interferometric coherent combining between the channels, the resonator includes an un-polarized 50% beam splitter (BS), and the OC is common for both channels. Due to the intracavity mode competition, the relative phase between the channels is passively adjusted and locked. The system chooses the lowest loss state with zero output from the loss channel (Fig. 1). This results in a constructive interference at the BS’s output (directed towards the OC) which feeds back to the cavity, and a destructive interference at the BS’s loss output which does not contribute to the intracavity circulating energy [9, 16].
2x active PCFs
AOQ
pump pump
CCD near-field
HR mirror
loss channel OC 4%
CCD far-field photodiode
BS
far-field lens
output
Fig. 1. Schematic configuration of the Q-switched intracavity coherently combined fiber laser.
Each channel consists of a commercially available, 55 cm long, double-clad, Yb-doped, single-mode, rod-type PCF (NKT DC-200/70-PM-Yb-ROD), with a core diameter of 70 μ m and MFD of ∼ 55 μ m; a 30 W fiber coupled pump diode with central wavelength at 976 nm; a dichroic mirror to separate between the pump and lasing beams; and suitable coupling optics. In order to use the same Q-switch for both channels we used a right angle HR broadband-coated prism, which brings the beams from both channels close to each other without overlap. Both channels are therefore Q-switched simultaneously, and the phase relation is determined only by the interference conditions on the BS. One of the key parameters of the combined laser system is the combining efficiency. In our study, this parameter is defined as the ratio between the output power from the coherently combined system and the sum of the outputs from the two channels operating separately under #204219 - $15.00 USD (C) 2014 OSA
Received 6 Jan 2014; revised 2 Mar 2014; accepted 3 Mar 2014; published 12 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006416 | OPTICS EXPRESS 6418
the same conditions. To measure the output power from the two separate single PCF laser channels we used the setup described in Fig. 2, wherein the BS is removed and the OC is placed in each channel separately.
AOQ
active PCF CCD near-field
pump OC 4%
HR mirror CCD far-field photodiode
far-field lens
output
Fig. 2. Schematic configuration of the single fiber laser channel setup. The same setup is used for both channels.
3.
Experimental results and discussion
The output power in CW mode, as measured from each channel separately and from the coherently combined system, is presented in Fig. 3(a). The measured values indicate a combining efficiency higher than 95% at any pumping condition, and a maximum output power of 22 W (limited by the available pump source). The combining efficiency is higher at low pump power levels and somewhat decreases at higher pump power levels. This dependence is probably due to the higher pump powers that lead to higher gain in the fibers, which in turn translates to beam wandering when aligning the two beams (angularly and laterally) on the BS. We believe that this beam wandering, which occurs only when attempting to combine the two channels, is due to the non-strict spatial single-mode operation of the particular rod-type PCFs used in the experiments [17]. Such wandering decreases the combining efficiency, which strongly depends on the spatial overlap of the beams on the combining element. We next measured the output characteristics of the coherently combined system in Qswitched pulsed operation at different repetition rates. The measured output power is shown in Fig. 3(b-d), with corresponding repetition rates of 100 kHz, 10 kHz, and 1 kHz, respectively. At low repetition rates (10 kHz and 1 kHz) the measured average output power was confirmed by measuring the pulse energy with a pyro-electric detector. This was used to verify that amplified spontaneous emission or parasitic CW lasing do not occur simultaneously with the pulsed operation. The data show that the combining efficiency is higher than 95% at any repetition rate. In the case of 100 kHz, the output power is very similar to that in CW mode [see Fig. 3(a)], with a maximum output power of 21 W . As expected, at lower repetition rates the average output power is lower compared with the CW mode, demonstrating a maximum of ∼ 17 W and ∼ 6 W for the 10 kHz and 1 kHz repetition rates, respectively. Although the measuring methodology described above is practical in indicating the net gain of using the coherent combining scheme, the combining efficiency can also be defined as the ratio between the power emitted from the OC and the sum of powers emitted from OC and the loss channel. This measurement, which takes into account possible interactions between the two channels, indicated higher values than those presented above (97 − 99%). Figure 4 shows the measured peak power (green dots) and pulse duration (blue squares) as function of the launched pump power at 1 kHz repetition rate. The pulse energy increases with the pump power to reach a maximal value of 7.2 mJ and the pulse duration becomes shorter at high pump powers, reaching the shortest value of ∼ 10 ns. The corresponding peak power for
#204219 - $15.00 USD (C) 2014 OSA
Received 6 Jan 2014; revised 2 Mar 2014; accepted 3 Mar 2014; published 12 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006416 | OPTICS EXPRESS 6419
Fig. 3. Laser output power as function of the single channel launched pump power, as measured for the upper and lower channels operated separately (blue triangles and green down triangles, respectively) and for the coherently combined system (black squares). Purple circles represent the combining efficiency as function of the launched pump power (values shown on the right y-axis). The dotted lines indicate a linear fit between the measured values. Panel (a) shows the CW mode; panels (b-d) show pulsed cases with repetition rates of 100 kHz (b), 10 kHz (c), and 1 kHz (d).
Fig. 4. Measured peak power (green dots) and pulse duration (blue squares) as function of the single channel launched pump power from the coherently combined laser system at 1 kHz repetition rate (lines represent connections between the points).
the maximal pulse energy and shortest pulse is ∼ 0.7 MW , corresponding with a peak intensity of ∼ 15 GW /cm2 in each fiber core.
#204219 - $15.00 USD (C) 2014 OSA
Received 6 Jan 2014; revised 2 Mar 2014; accepted 3 Mar 2014; published 12 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006416 | OPTICS EXPRESS 6420
We continued by measuring the laser output beam quality parameter (M2 ) by means of the 4σ method, in the case of the maximum peak power output (Fig. 5). The beam quality, which is calculated from the near- and far-field intensity distributions as shown in Fig. 5, reveals singlemode, near T EM00 operation of the system, with a measured beam quality factor of M 2 < 1.2. This indicates that the beam quality is preserved and is comparable to that of a single, separately operated laser channel.
(a)
(b)
(c)
(d)
3 2.5 2 1.5 1 0.5
Fig. 5. Measured intensity distributions of the output laser beam. (a, b) show the near- and far-field, respectively, of a single laser channel operated separately. (c, d) show the nearand far-field, respectively, of the coherently combined laser configuration.
4.
Conclusion
To the best of our knowledge, this is the first demonstration of passive intracavity coherent combining of PCF lasers in a power oscillator configuration with high peak power. Our experimental measurements indicate the highest pulse energy and peak power obtained from any fiber-based power oscillator, with a near-diffraction-limited beam quality. The maximum measured peak-power in our study was limited by the available pump power, and therefore future experiments should employ higher pump powers. In light of our results, we believe that coherently combined pulsed PCF lasers (either two or more) may yield even higher peak power fiber lasers with excellent beam quality. These, in turn, can be employed for numerous applications such as material processing, light detection and ranging, and medical procedures.
#204219 - $15.00 USD (C) 2014 OSA
Received 6 Jan 2014; revised 2 Mar 2014; accepted 3 Mar 2014; published 12 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006416 | OPTICS EXPRESS 6421
of Electrical and Computer Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel 2 Department of Physics, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel ∗ [email protected]
Abstract: We demonstrate a high peak power, Q-switched pulsed, intracavity coherently combined fiber laser system. The system is based on two Yb-doped, rod-type, photonic crystal fibers which are passively phase-locked and combined into the single output beam in a power oscillator configuration. Experimental evidence indicate that this oscillator system provides record high peak power of ∼ 0.7 MW with pulse duration of ∼ 10 ns at 1 kHz repetition rate. The measured beam quality shows near-diffraction-limited operation of the coherently combined system. © 2014 Optical Society of America OCIS codes: (140.3298) Laser beam combining; (140.3510) Lasers, fiber; (140.3540) Lasers, Q-switched.
References and links 1. D. Richardson, J. Nilsson, and W. Clarkson, “High power fiber lasers: current status and future perspectives [invited],” J. Opt. Soc. Am. B 27, B63–B92 (2010). 2. J. P. Koplow, D. A. Kliner, and L. Goldberg, “Single-mode operation of a coiled multimode fiber amplifier,” Opt. Lett. 25, 442–444 (2000). 3. W. Wadsworth, R. Percival, G. Bouwmans, J. Knight, and P. Russell, “High power air-clad photonic crystal fibre laser,” Opt. Express 11, 48–53 (2003). 4. I. Hu, X. Ma, A. Galvanauskas et al., “Single mode 9.1 mJ and 10ns pulses from 55μ m core yb-doped CCC fiber mopa,” in “CLEO: Science and Innovations,” (OSA, 2013), pp. CTu1K–5. 5. J. Limpert, F. Stutzki, F. Jansen, H.-J. Otto, T. Eidam, C. Jauregui, and A. T¨unnermann, “Yb-doped largepitch fibers: effective single-mode operation based on higher-order mode delocalization,” Light: Sci. App. 1, 1–5 (2012). 6. J. Bourderionnet, C. Bellanger, J. Primot, and A. Brignon, “Collective coherent phase combining of 64 fibers,” Opt. Express 19, 17053–17058 (2011). 7. J. R. Leger, G. J. Swanson, and W. B. Veldkamp, “Coherent laser addition using binary phase gratings,” Appl. Opt. 26, 4391–4399 (1987). 8. T. Fan, “Laser beam combining for high-power, high-radiance sources,” IEEE J. Sel. Top. Quantum Electron. 11, 567–577 (2005). 9. A. Ishaaya, N. Davidson, L. Shimshi, and A. Friesem, “Intracavity coherent addition of gaussian beam distributions using a planar interferometric coupler,” Appl. Phys. Lett. 85, 2187–2189 (2004). 10. A. A. Ishaaya, N. Davidson, and A. A. Friesem, “Passive laser beam combining with intracavity interferometric combiners,” IEEE J. Sel. Top. Quantum Electron. 15, 301–311 (2009). 11. D. Sabourdy, A. Desfarges-Berthelemot, V. Kermene, and A. Barth´el´emy, “Coherent combining of q-switched fibre lasers,” Electron. Lett. 40, 1254–1255 (2004). 12. B. Shulga and A. A. Ishaaya, “Imposing temporal and frequency characteristics in a system of coherently combined high peak power photonic crystal fiber lasers,” in “CLEO/EUROPE - IQEC,” (OSA, 2013).
#204219 - $15.00 USD (C) 2014 OSA
Received 6 Jan 2014; revised 2 Mar 2014; accepted 3 Mar 2014; published 12 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006416 | OPTICS EXPRESS 6416
13. K. Duan, J. Wang, J. Li, Y. Wang, Z. Zhao, and W. Zhao, “Experimental study of phase-locking of two photonic crystal fiber lasers,” Opt. Commun. 281, 2557–2560 (2008). 14. D. Kouznetsov, J.-F. Bisson, A. Shirakawa, and K.-I. Ueda, “Limits of coherent addition of lasers: Simple estimate,” Opt. Rev. 12, 445–447 (2005). 15. J. Guillot, A. Desfarges-Berthelemot, V. Kerm`ene, and A. Barth´el´emy, “Experimental study of cophasing dynamics in passive coherent combining of fiber lasers,” Opt. Lett. 36, 2907–2909 (2011). 16. V. Kozlov, J. Hernandez-Cordero, and T. Morse, “All-fiber coherent beam combining of fiber lasers,” Opt. Lett. 24, 1814–1816 (1999). 17. J. Bromage, J. M. Fini, C. Dorrer, and J. D. Zuegel, “Characterization and optimization of yb-doped photoniccrystal fiber rod amplifiers using spatially resolved spectral interferometry,” Appl. Opt. 50, 2001–2007 (2011).
1.
Introduction
Over the past decade, fiber laser sources have become one of the most popular and fastdeveloping laser technologies. This is mainly because fiber lasers possess several key advantages over bulk solid-state lasers, including good heat dissipation handling, high gain, easy alignment (or even the lack thereof, in all-fiber configurations) and single-mode wave guiding that provides excellent beam quality [1]. However, as yet, fiber lasers cannot replace bulk solidstate lasers when high peak power is required. Specifically, three main factors limit the singlemode output power that can be extracted from a single fiber laser source: material damage, deleterious nonlinear effects, and modal instabilities with high average powers. These limiting factors directly result from the extremely high average and peak powers within the relatively small fiber core. Increasing the effective single-mode area of the core, or combining several fiber laser sources coherently, may potentially circumvent these limitations and increase the output peak power of the laser. To increase the effective area of the single-mode core compared with standard step-index single-mode fibers, large mode area step-index fibers can be used; however, these are not “truly” single-mode fibers [2]. Other alternatives include employing photonic crystal fibers (PCFs) [3] or the recently developed chirally coupled core fibers [4]. The largest single-mode fiber reported is a rod-type PCF with a core diameter of 135 μ m and a corresponding mode field diameter of ∼ 100 μ m [5]; however, also this fiber can withstand limited peak power. The second approach for increasing the output peak power is the coherent combining approach, wherein several “independent” lasers (or amplifiers) are combined into a single coherent output. This approach is advantageous over increasing the effective single-mode core area in three main aspects. First, it is more accessible as the laser system can be based only upon commercially available components. Second, larger cores are highly sensitive to bending (in terms of mode losses) and often require a ruggedized fiber design; in contrast, in coherent combining, the fiber as an active medium may be flexible, increasing the compactness and robustness of the system. Third, the output peak and average powers may be increased simultaneously in the coherent combining approach by increasing the number of combined channels. Such promising scalability is made possible through distribution of the energy between several “independent” channels with no performance degradation. In the core size increasing approach, by contrast, the thermal load increases with the increase in average output power, resulting in mode instability and, reciprocally, in performance degradation [5]. In light of these clear advantages, coherent combining of laser sources has attracted much interest in the past decade. To achieve coherent combining, the relative phases of the laser sources should be locked and properly adjusted, either actively or passively. In active phase-locking techniques, the phase of each “independent” laser source is actively and continuously adjusted by means of electronic feedback loops [6]. In passive phase-locking techniques, the “independent” laser sources are self-phase-locked by means of a passive coupling mechanism [7, 8, 9, 10, 11, 12, 13]. Although passive techniques are generally regarded as more limited in terms of the maximum number
#204219 - $15.00 USD (C) 2014 OSA
Received 6 Jan 2014; revised 2 Mar 2014; accepted 3 Mar 2014; published 12 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006416 | OPTICS EXPRESS 6417
of combined laser channels [14], they are simpler, more robust and require no phase-control feedback loops that must be “smart” and fast enough, especially in short pulsed lasing systems [15]. Intracavity coherently combined fiber lasers with high average and peak power in an oscillator configuration, and especially with PCFs, have not been reported previously. Here, we describe the experimental investigation of high peak power passive interferometric coherent combining of two rod-type, Yb-doped, PCF laser channels within a common Q-switched cavity. In this approach, the two identical single-mode laser channels are operated in a phase-locked manner, and the output beam is a coherent superposition of the individual beams that preserves an excellent beam quality. We have achieved, to the best of our knowledge, a new record with regard to the peak power obtained from a Q-switched fiber oscillator. 2.
Experimental configuration
The experimental setup is based on two identical PCF lasers that are passively combined in a coherent manner into a single output. The schematic configuration of the system is shown in Fig. 1. The resonator is formed by two active fibers, an acousto-optic Q-switch (AOQ), a highly reflective (HR) broadband rear mirror common for both channels, and a partially reflective output coupler (OC) with 4% reflectivity. To enable intracavity interferometric coherent combining between the channels, the resonator includes an un-polarized 50% beam splitter (BS), and the OC is common for both channels. Due to the intracavity mode competition, the relative phase between the channels is passively adjusted and locked. The system chooses the lowest loss state with zero output from the loss channel (Fig. 1). This results in a constructive interference at the BS’s output (directed towards the OC) which feeds back to the cavity, and a destructive interference at the BS’s loss output which does not contribute to the intracavity circulating energy [9, 16].
2x active PCFs
AOQ
pump pump
CCD near-field
HR mirror
loss channel OC 4%
CCD far-field photodiode
BS
far-field lens
output
Fig. 1. Schematic configuration of the Q-switched intracavity coherently combined fiber laser.
Each channel consists of a commercially available, 55 cm long, double-clad, Yb-doped, single-mode, rod-type PCF (NKT DC-200/70-PM-Yb-ROD), with a core diameter of 70 μ m and MFD of ∼ 55 μ m; a 30 W fiber coupled pump diode with central wavelength at 976 nm; a dichroic mirror to separate between the pump and lasing beams; and suitable coupling optics. In order to use the same Q-switch for both channels we used a right angle HR broadband-coated prism, which brings the beams from both channels close to each other without overlap. Both channels are therefore Q-switched simultaneously, and the phase relation is determined only by the interference conditions on the BS. One of the key parameters of the combined laser system is the combining efficiency. In our study, this parameter is defined as the ratio between the output power from the coherently combined system and the sum of the outputs from the two channels operating separately under #204219 - $15.00 USD (C) 2014 OSA
Received 6 Jan 2014; revised 2 Mar 2014; accepted 3 Mar 2014; published 12 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006416 | OPTICS EXPRESS 6418
the same conditions. To measure the output power from the two separate single PCF laser channels we used the setup described in Fig. 2, wherein the BS is removed and the OC is placed in each channel separately.
AOQ
active PCF CCD near-field
pump OC 4%
HR mirror CCD far-field photodiode
far-field lens
output
Fig. 2. Schematic configuration of the single fiber laser channel setup. The same setup is used for both channels.
3.
Experimental results and discussion
The output power in CW mode, as measured from each channel separately and from the coherently combined system, is presented in Fig. 3(a). The measured values indicate a combining efficiency higher than 95% at any pumping condition, and a maximum output power of 22 W (limited by the available pump source). The combining efficiency is higher at low pump power levels and somewhat decreases at higher pump power levels. This dependence is probably due to the higher pump powers that lead to higher gain in the fibers, which in turn translates to beam wandering when aligning the two beams (angularly and laterally) on the BS. We believe that this beam wandering, which occurs only when attempting to combine the two channels, is due to the non-strict spatial single-mode operation of the particular rod-type PCFs used in the experiments [17]. Such wandering decreases the combining efficiency, which strongly depends on the spatial overlap of the beams on the combining element. We next measured the output characteristics of the coherently combined system in Qswitched pulsed operation at different repetition rates. The measured output power is shown in Fig. 3(b-d), with corresponding repetition rates of 100 kHz, 10 kHz, and 1 kHz, respectively. At low repetition rates (10 kHz and 1 kHz) the measured average output power was confirmed by measuring the pulse energy with a pyro-electric detector. This was used to verify that amplified spontaneous emission or parasitic CW lasing do not occur simultaneously with the pulsed operation. The data show that the combining efficiency is higher than 95% at any repetition rate. In the case of 100 kHz, the output power is very similar to that in CW mode [see Fig. 3(a)], with a maximum output power of 21 W . As expected, at lower repetition rates the average output power is lower compared with the CW mode, demonstrating a maximum of ∼ 17 W and ∼ 6 W for the 10 kHz and 1 kHz repetition rates, respectively. Although the measuring methodology described above is practical in indicating the net gain of using the coherent combining scheme, the combining efficiency can also be defined as the ratio between the power emitted from the OC and the sum of powers emitted from OC and the loss channel. This measurement, which takes into account possible interactions between the two channels, indicated higher values than those presented above (97 − 99%). Figure 4 shows the measured peak power (green dots) and pulse duration (blue squares) as function of the launched pump power at 1 kHz repetition rate. The pulse energy increases with the pump power to reach a maximal value of 7.2 mJ and the pulse duration becomes shorter at high pump powers, reaching the shortest value of ∼ 10 ns. The corresponding peak power for
#204219 - $15.00 USD (C) 2014 OSA
Received 6 Jan 2014; revised 2 Mar 2014; accepted 3 Mar 2014; published 12 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006416 | OPTICS EXPRESS 6419
Fig. 3. Laser output power as function of the single channel launched pump power, as measured for the upper and lower channels operated separately (blue triangles and green down triangles, respectively) and for the coherently combined system (black squares). Purple circles represent the combining efficiency as function of the launched pump power (values shown on the right y-axis). The dotted lines indicate a linear fit between the measured values. Panel (a) shows the CW mode; panels (b-d) show pulsed cases with repetition rates of 100 kHz (b), 10 kHz (c), and 1 kHz (d).
Fig. 4. Measured peak power (green dots) and pulse duration (blue squares) as function of the single channel launched pump power from the coherently combined laser system at 1 kHz repetition rate (lines represent connections between the points).
the maximal pulse energy and shortest pulse is ∼ 0.7 MW , corresponding with a peak intensity of ∼ 15 GW /cm2 in each fiber core.
#204219 - $15.00 USD (C) 2014 OSA
Received 6 Jan 2014; revised 2 Mar 2014; accepted 3 Mar 2014; published 12 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006416 | OPTICS EXPRESS 6420
We continued by measuring the laser output beam quality parameter (M2 ) by means of the 4σ method, in the case of the maximum peak power output (Fig. 5). The beam quality, which is calculated from the near- and far-field intensity distributions as shown in Fig. 5, reveals singlemode, near T EM00 operation of the system, with a measured beam quality factor of M 2 < 1.2. This indicates that the beam quality is preserved and is comparable to that of a single, separately operated laser channel.
(a)
(b)
(c)
(d)
3 2.5 2 1.5 1 0.5
Fig. 5. Measured intensity distributions of the output laser beam. (a, b) show the near- and far-field, respectively, of a single laser channel operated separately. (c, d) show the nearand far-field, respectively, of the coherently combined laser configuration.
4.
Conclusion
To the best of our knowledge, this is the first demonstration of passive intracavity coherent combining of PCF lasers in a power oscillator configuration with high peak power. Our experimental measurements indicate the highest pulse energy and peak power obtained from any fiber-based power oscillator, with a near-diffraction-limited beam quality. The maximum measured peak-power in our study was limited by the available pump power, and therefore future experiments should employ higher pump powers. In light of our results, we believe that coherently combined pulsed PCF lasers (either two or more) may yield even higher peak power fiber lasers with excellent beam quality. These, in turn, can be employed for numerous applications such as material processing, light detection and ranging, and medical procedures.
#204219 - $15.00 USD (C) 2014 OSA
Received 6 Jan 2014; revised 2 Mar 2014; accepted 3 Mar 2014; published 12 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006416 | OPTICS EXPRESS 6421
