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Abstract
A Q-switched polarization-maintaining (PM) Thulium (Tm3+):Holmium (Ho3+)-codoped triple-clad fiber (THTF) laser is under investigation with special focus on important optical parameters enabling efficient frequency conversion into mid-infrared (mid-IR). An active fiber of 5 m length is used in a single-oscillator free-space cavity arrangement and operated in continuous-wave and pulsed operation. In Q-switched regime at a repetition rate of 63 kHz, high pulse energies up to 760 μJ were obtained while reaching pulse widths as low as 45.6 ns, resulting in pulse peak powers of 15.7 kW with an average output power of 48 W. In addition, an $M^{2}_{\textrm{x,y}}<1.2$ and a FWHM of 290 pm for the emission spectrum was measured.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical parametric oscillators are promising sources to generate mid-infrared (mid-IR) radiation useful for medical applications, spectroscopy, and environmental sensing. During the last years, crystal manufacturers made large progress, developing and improving nonlinear crystals for efficient high-power optical parametric generation, like Orientation-Patterned Gallium Arsenide (OP-GaAs) and Zinc Germanium Phosphide (ZGP) [1]. This puts a special focus on high energy as well as high power 2 µm laser sources with outstanding optical performance driving an efficient frequency conversion using these nonlinear crystals. Efficient frequency conversion into the mid-IR (3-5 µm) has already been demonstrated with pulsed 2 µm sources pumping OP-GaAs [2,3] as well as ZGP [4,5]. Depending on the application, different pulsed 2 µm laser sources can be used for pumping optical parametric devices. 2 µm laser sources under investigation are for example crystal lasers [6] and master oscillator power amplifiers (MOPA) systems [7]. This paper is focusing on fiber based single-oscillators taking advantage of a simple cavity design.
The single-oscillator fiber laser needs to deliver high output powers and energies in order to surpass the nonlinear threshold, a good beam quality, a narrow linewidth, and a high polarization extinction ratio (PER), enabling an efficient frequency conversion. In addition the laser needs to emit at wavelengths well beyond 2 µm in order to minimize absorption in nonlinear crystals, especially for ZGP and to prevent from two-photon absorption in OP-GaAs.
Therefore, typically lasers based on holmium ions are used which provide longer emission wavelengths compared to lasers based on thulium ions [8,9]. Incorporated into silica-glass fibers, trivalent holmium ions show a broad emission spectrum from 2 µm to 2.1 µm [10]. For an efficient operation at a wavelength of 2.1 µm, holmium lasers are usually in-band pumped by thulium lasers [11,12]. However, in order to stay at a single-oscillator and reduce system complexity, Tm$^{3+}$:Ho$^{3+}$-codoped fiber lasers are preferred, as they can be directly diode-pumped at around 793 nm and exploit a direct ion-ion energy transfer between thulium and holmium ions [13,14].
Generating high output power in Tm$^{3+}$:Ho$^{3+}$-codoped fiber lasers is challenging as the heat load of the active fiber is significantly higher compared to singly Ho$^{3+}$-doped fibers. The polymer cladding, which is usually transparent for the pump light at a wavelength around 793 nm, may exhibit thermally induced darkening at increased fiber temperatures. As soon as polymer darkening occurs the pump light reflected at the cladding-to-polymer interface becomes partially absorbed in the darkened polymer, causing additional heat and thereby increased darkening, finally resulting in a destructive runaway effect. In addition, it is often unavoidable that 2 µm laser light couples into the first cladding of the fiber, which serves as the pump cladding, at fiber-to-fiber splices, fiber-to-endcap splices, or free-space laser light (re-)injection into the fiber in the case of external cavities. In standard double-clad fibers this 2 µm light interacts with the polymer cladding. The polymer absorbs some of the light and is damaged already at low power levels. To reduce the risks of 2 µm and pump light absorption in the polymer, triple-clad fibers can be used to shield the optical radiation from the polymer coating. Therefore, this concept has here been transferred to Tm$^{3+}$:Ho$^{3+}$-codoped fibers, which was so far not studied in literature.
In this paper, we present the current advances achieved in Q-switched Tm$^{3+}$:Ho$^{3+}$-codoped polarization-maintaining single-oscillator fiber lasers. Recent focus was put on a diffraction-limited laser output. In addition, we have demonstrated that this fiber type can withstand high output power operation [15]. In this paper we focus on Q-switched operation for moderate output power levels which will already be sufficient to generate high mid-IR output powers. High pulse energies of 760 $\mu$J for silica fiber single-oscillators and pulse widths of 45.6 ns were achieved, resulting in a pulse peak power of greater than 15 kW in Q-switched operation while providing excellent beam quality and narrow emission spectrum. In the first part, the fiber is characterized in continuous-wave (CW) operation. The degree of polarization, the beam quality, and the spectrum of the fiber laser are investigated. In the second part, the fiber laser is characterized in Q-switched operation: we investigate the degree of polarization, the beam quality, and the spectrum of the fiber laser. The performance of the pulsed laser is investigated for the same incident pump power as the CW laser. Therefore, the impact of Q-switching on the laser performance is deduced.
2. Experimental setup
Figure 1 displays the experimental setup of the high power single-oscillator fiber laser. The active fiber is a Tm$^{3+}$:Ho$^{3+}$-codoped panda type triple-clad silica fiber which is designed to be close to single mode. The fiber had a core diameter of 18 µm and a numerical aperture of 0.09 thanks to a pedestal surrounding the core. The thulium to holmium ratio was 10 and the aluminium to thulium ratio was > 8. The normalized frequency was calculated to be 2.483, at a wavelength of 2050 nm. The first cladding consisted of pure silica with a diameter of 270 µm (NA = 0.22). By design, the fiber had a clad absorption greater than 4 dB/km at the pump wavelength of 793 nm. The second cladding had a diameter of 300 µm and was made out of fluorine doped silica. The third cladding consisted of a low-index polymer coating with a diameter of 490 µm. Two glass stress rods were incorporated into the first cladding to obtain the necessary birefringence for polarization-maintaining operation.
Fig. 1. Schematic diagram of the experimental setup displaying a double-end pumped polarization-maintaining THTF in an external-cavity arrangement.
The fiber length was $\sim$5 m with a gain maximum at around 2050 nm. Antireflection (AR)-coated glass endcaps were fusion spliced onto both ends of the fiber using a CO2 laser splicer, preventing fiber end face damage caused by high intensities at the fiber surface. High splicing temperatures cause a strong diffusion process in the fiber core and pedestal region, leading to a blurred refractive index profile. As a consequence, the fundamental mode can couple into higher-order modes at splice interfaces [16] and can degrade the laser performance. Therefore, special care was taken in reducing diffusion of the heavily doped active fiber core while splicing endcaps. The THTF was coiled and placed inside of a container, filled with temperature controlled deionized water. The fiber was pumped from each side using fiber coupled laser diodes with up to 300 W of 793 nm light in a 200 ${\mathrm{\mu}}$m core diameter (NA=0.22) multimode fiber. The coupling between the multimode fibers and the THTF was performed using AR-coated achromatic lenses with an effective focal length of 15 mm. The lenses as well as the endcaps were AR coated at the pump wavelength of 793 nm and the fiber laser output wavelength of about 2-2.1 µm. The pump light was collimated using identical lenses and redirected by dichroic mirrors, which were highly reflective for 793 nm and highly transmissive for the spectral laser emission range. The laser cavity was formed by a single-side AR-coated plane-parallel glass substrate with a Fresnel reflectivity of 3 % acting as the output coupler (OC) and a reflective Volume Bragg Grating (VBG) centered at $\sim$2050 nm. The AR coating of the OC was also highly transmittive for the 793 nm radiation as well as the 2 ${\mathrm{\mu}}$m signal. A polarizer was inserted between the active fiber and the VBG to obtain a polarized output of the fiber laser. For analyzing the polarization of the laser, a second polarizer was placed outside the laser cavity behind the OC. To operate the laser in pulsed mode an acousto-optic modulator (AOM) was placed in between the VBG and the polarizer creating periodic losses for repetitive active Q-switching.
3. Results in continuous wave operation
Figure 2 shows the output power of the single-oscillator fiber laser, using an active fiber length of 5 m. The fiber laser was operated within a moderate power regime up to $\sim$100 W. The slope efficiency after a thermal settling of the setup was 41.6 %. The high slope efficiency is indicating an efficient cross-relaxation in the fiber between the thulium ions as well as an efficient energy transfer between the thulium and holmium ions. The triple-clad fiber laser shows a good slope efficiency thanks to a well optimized thulium and holmium concentrations within the active fiber [17]. A thulium to holmium ratio of 10 is stated to enable an efficient energy transfer.
The optical laser output characteristics are investigated for an incident pump power of 180 W in order to compare it to the Q-switched results, later on. The emission spectrum for a laser output power of 53 W is shown in Fig. 3, which was recorded with an optical spectrum analyzer (Yokogawa). The fiber laser is featuring one strong line with two peaks at shorter wavelengths. Similar behavior has already been reported earlier [18]. The featured wavelengths are within the reflectance bandwidth of the VBG. One possible explanation could be a local heating of the VBG due to high power operation. A small deformation of the inscribed grating can lead to a disturbance of the plane phase front interaction with the reflected 2 ${\mathrm{\mu}}$m laser light. However, further investigations are needed to fully understand this effect and suppress it, if possible. The impact of additional side peaks on Q-switched operation and additional spectral broadening in view of a potential pump source for frequency conversion is discussed later on. The PER was 19.9 dB in CW operation.
Fig. 3. Continuous-wave operation: Spectral emission of the fiber laser with a fiber length of 5 m for an output power of 53 W.
Figure 4 displays the measurement of the laser beam caustic for an output power of 53 W, for determining the beam propagation factor M$^{2}$. A lens with a focal length of $\sim$40 cm was used to focus the collimated laser beam. Additional 2 µm signal, which came from a scattering at the fiber-to-endcap-splice interface was cut off by placing an iris in front of the focusing lens. Typically, the power of this scattered part was measured to be smaller than 1 % and therefore not relevant to the performance. However, due to the strong sensitivity of the M$^{2}$ measurement with an Ophir camera (spot size determination) on even slight background radiation, this light has to be cut off. The M$^{2}$ was 1.07 and 1.09 in x- and y-direction, respectively, which shows a significant improvement to previously reported values [15]. This value is in good agreement with the predicted single-mode behavior by a calculated V-number of 2.483. Despite directly splicing the active fiber on the endcaps, the M$^{2}$ does not show any degradation at high temperatures. The very good M$^{2}$ value obtained without significant power loss proves that the use of an iris for background elimination is acceptable. The single-oscillator fiber laser shows excellent optical characteristics as CW fiber source and lays a promising basis for Q-switched operation.
Fig. 4. Continuous-wave operation: Measurement of the caustic of the 2 ${\mathrm{\mu}}$m signal and determination of the M$^{2}$ value. Diffraction-limited output beam with an $M^{2}<1.1$.
4. Results in pulsed operation
In order to investigate the Q-switched operation, the AOM is placed inside the cavity, between the active fiber and the VBG. The AOM is modulating the losses in the cavity, which leads to the repetitive generation of pulses. The fiber laser was operated at a constant incident pump power of 180 W. This results in an output power of around 50 W, which can currently be handled by commercially available isolators for future nonlinear experiments. Figure 5 displays the pulse energy and the average output power versus repetition rate. The repetition rate ranged from 63 kHz up to 150 kHz. For lower repetition rates the laser shows a strong increase in amplified spontaneous emission or/and parasitic lasing which can be measured as a decrease in degree of polarization and an increase in intra-cavity power on the polarization perpendicular to the laser polarization. Pulse energies from 350 ${\mathrm{\mu}}$J up to 760 ${\mathrm{\mu}}$J were generated with decreasing repetition rates. In addition, the average output power decreased from 52 W to 48 W. Figure 6 shows the pulse peak power as well as the pulse width versus repetition rate. The pulse width decreased from 115 ns down to 45.6 ns. Therefore it leads to a pulse peak power as high as 15.7 kW at a repetition rate of 63 kHz. The PER was larger or equal to 19 dB for all operation points. A typical output pulse of the Q-switched single-oscillator fiber laser is shown in the inset of Fig. 6. The measured output pulse had a FWHM of 45.6 ns and was recorded for a repetition rate of 63 kHz. A Gaussian fit to the data (red curve) shows a good agreement between the fit and the measured temporal profile of the pulse.
Fig. 6. Pulse peak power and pulse width as a function of the AOM’s repetition rate. The inset shows a Gaussian pulse with a FWHM of 45.6 ns.
The M$^{2}$ value was measured for an average output power of 49 W and a repetition rate of 70 kHz. Compared to CW operation, a minor degradation was observed. Figure 7 shows the laser caustic. The beam propagation was still close to diffraction-limited and the M$^{2}$ was 1.19 for x-direction and 1.14 for y-direction.
Fig. 7. Q-switched operation: Measurement of the caustic of the 2 µm signal and determination of the M$^{2}$ value. Diffraction-limited output beam with an $M^{2}<1.2$
Figure 8 shows the spectral emission for a repetition rate of 63 kHz. Compared to CW operation, we observe that the spectrum is broadened in Q-switched operation. There are two effects that are superimposed. First, the typical base broadening in Q-switched operation is observed. According to [19] this effect is based on several nonlinear effects. A major influence can be attributed to self-phase modulation. Due to an intensity-dependent refractive index, the pulse is imposing a phase modulation on itself which leads to a spectral pulse broadening. This effect scales with fiber length and pulse peak power. Second, as the pulse repetition frequency decreases we observe multiple side peaks appearing. For repetition rates close to the lower limit of 63 kHz, the side peaks are more and more covered by the overall nonlinearities. Despite the spectral broadening, the spectral acceptance bandwidth for ZGP ($\Delta \lambda \sim ~7\;nm$) and OP-GaAs ($\Delta \lambda \sim ~4\;nm$) are still much larger than the spectral output of the Q-switched fiber laser with an FWHM of 0.29 nm.
Fig. 8. Q-switched operation: Spectral emission of the fiber laser with a fiber length of 5 m for an output power of 49 W.
5. Discussion
In Q-switched regime as the repetition rate decreases, the Q-switched laser process is starting to compete with an amplified spontaneous emission which leads to a decrease in average output power. In addition, it was shown that the operation at the highest possible pulse energies causes a slight decrease in overall laser performance. The M$^{2}$ and the PER decreased from roughly 1.1 and 19.9 dB to 1.2 and 19 dB, respectively. The spectral bandwidth increased from 0.054 nm in CW operation to 0.29 nm for a repetition rate of 63 kHz in Q-switched operation.
We compare our laser performance to state-of-the-art Q-switched Tm$^{3+}$:Ho$^{3+}$-codoped single oscillators. Since the application is to pump an OPO, emphasis is put on obtaining the highest possible pulse energy at simultaneously high average powers as well as high spatial and spectral brightness of the laser. The Q-switched Tm$^{3+}$:Ho$^{3+}$-codoped fiber laser with so far the highest output power of 55 W was presented in [20] and has a partially similar setup to our laser. Dalloz et al. achieved energies of $\sim$ 720 ${\mathrm{\mu}}$J only at an average output power of $\sim$ 28.5 W for a repetition rate of 40 kHz. However, in a quasi-CW regime, they reported an average output power of 55 W with an M$^{2}<1.7$, a slope efficiency of 35 %, and a laser linewidth of 0.2 nm for a repetition rate of 200 kHz. The spectral emission presented a broad base distribution, which cannot be well described by a FWHM value. Thus, a significant spectral energy was outside of the 0.2 nm FWHM main peak. The fiber laser showed a comparable PER of 19 dB.
In the work presented here, we demonstrate higher pulse energies of 760 ${\mathrm{\mu}}$J at almost twice the average output power, and a significant increase in spatial brightness was achieved due to a close to diffraction limited beam. Furthermore, a slope efficiency of 41.6 % compared to 35 % shows a good precondition for future power scaling of the Q-switched fiber laser. Using [20] as a benchmark, the Q-switched fiber laser presented in this paper is showing a significant overall improvement to the state-of-the-art in Q-switched single-oscillator Tm$^{3+}$:Ho$^{3+}$-codoped fiber lasers and especially superior performance concerning spatial and spectral brightness.
6. Conclusion
In conclusion we have demonstrated a close to diffraction-limited Q-switched Tm$^{3+}$:Ho$^{3+}$-codoped triple-clad polarization-maintaining silica fiber laser. A single-oscillator with a fiber length of 5 m was investigated. The fiber laser had a high slope efficiency of 41.6 % with respect to the incident pump power and a high PER $\geq 19.9$ dB in continuous-wave operation. The beam quality factor $M^{2}_{\textrm{x,y}}$ was smaller than 1.1 and the emission spectrum had a FWHM of 54 pm centered at 2050 nm. Operating in a pulsed regime, the THTF laser delivered pulses with a pulse width down to 45.6 ns and a pulse energy of 760 ${\mathrm{\mu}}$J. A constant average output power of 48 W was reached with pulse peak powers of 15.7 kW. Towards low repetition rates, i.e. maximum achievable pulse energies, the 2 ${\mathrm{\mu}}$m signal still showed an almost diffraction-limited behavior $M^{2}_{\textrm{x,y}}<1.2$ and a FWHM for the emission spectrum of 0.29 nm due to spectral pulse broadening.
Future work will be devoted to shift the emission of the Tm$^{3+}$:Ho$^{3+}$-codoped silica fiber from 2050 nm to longer wavelengths. In addition, the Q-switched fiber laser will be investigated for a higher incident pump power.
Funding
Bundesministerium der Verteidigung; Bundesamt für Ausrüstung, Informationstechnik und Nutzung der Bundeswehr.
Acknowledgment
We acknowledge the support of the mechanical workshop of the IOSB and Artur Schander, who designed and fabricated special optomechanical components for the experimental setup.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. P. G. Schunemann, K. T. Zawilski, L. A. Pomeranz, D. J. Creeden, and P. A. Budni, “Advances in nonlinear optical crystals for mid-infrared coherent sources,” J. Opt. Soc. Am. B 33(11), D36–D43 (2016). [CrossRef]
2. C. Kieleck, A. Hildenbrand, M. Eichhorn, D. Faye, E. Lallier, B. Gérard, and S. D. Jackson, “OP-GaAs OPO pumped by 2 μm Q-switched lasers: Tm;Ho:silica fiber laser and Ho:YAG laser,” Proc. SPIE 7836, 783607 (2010). [CrossRef]
3. C. Kieleck, M. Eichhorn, A. Hirth, D. Faye, and E. Lallier, “High-efficiency 20–50 kHz mid-infrared orientation-patterned GaAs optical parametric oscillator pumped by a 2 μm holmium laser,” Opt. Lett. 34(3), 262–264 (2009). [CrossRef]
4. C. Kieleck, A. Berrou, B. Donelan, B. Cadier, T. Robin, and M. Eichhorn, “6.5 W ZnGeP2 OPO directly pumped by a Q-switched Tm3+-doped single-oscillator fiber laser,” Opt. Lett. 40(6), 1101–1104 (2015). [CrossRef]
5. L. G. Holmen and H. Fonnum, “Holmium-doped fiber amplifier for pumping a ZnGeP2 optical parametric oscillator,” Opt. Express 29(6), 8477–8489 (2021). [CrossRef]
6. E. Lippert, S. Nicolas, G. Arisholm, K. Stenersen, and G. Rustad, “Midinfrared laser source with high power and beam quality,” Appl. Opt. 45(16), 3839–3845 (2006). [CrossRef]
7. D. Creeden, P. A. Ketteridge, P. A. Budni, S. D. Setzler, Y. E. Young, J. C. McCarthy, K. Zawilski, P. G. Schunemann, T. M. Pollak, E. P. Chicklis, and M. Jiang, “Mid-infrared ZnGeP2 parametric oscillator directly pumped by a pulsed 2 μm Tm-doped fiber laser,” Opt. Lett. 33(4), 315–317 (2008). [CrossRef]
8. S. D. Jackson, “Midinfrared holmium fiber lasers,” IEEE J. Quantum Electron. 42(2), 187–191 (2006). [CrossRef]
9. Stuart D Jackson, “Towards high-power mid-infrared emission from a fibre laser,” Nat. Photonics 6(7), 423–431 (2012). [CrossRef]
10. A. Hemming, N. Simakov, J. Haub, and A. Carter, “A review of recent progress in holmium-doped silica fibre sources,” Opt. Fiber Technol. 20(6), 621–630 (2014). [CrossRef]
11. A. Hemming, S. Bennetts, N. Simakov, A. Davidson, J. Haub, and A. Carter, “High power operation of cladding pumped holmium-doped silica fibre lasers,” Opt. Express 21(4), 4560–4566 (2013). [CrossRef]
12. N. Simakov, A. Hemming, W. A. Clarkson, J. Haub, and A. Carter, “A cladding-pumped, tunable holmium doped fiber laser,” Opt. Express 21(23), 28415–28422 (2013). [CrossRef]
13. S. D. Jackson, A. Sabella, A. Hemming, S. Bennetts, and D. G. Lancaster, “High-power 83 W holmium-doped silica fiber laser operating with high beam quality,” Opt. Lett. 32(3), 241–243 (2007). [CrossRef]
14. M. Eichhorn and S. D. Jackson, “High-pulse-energy, actively Q-switched Tm3+, Ho3+-codoped silica 2 μm fiber laser,” Opt. Lett. 33(10), 1044–1046 (2008). [CrossRef]
15. P. Forster, C. Romano, M. Eichhorn, and C. Kieleck, “Recent advances in high-power 2 μm fiber lasers for frequency conversion into the mid-IR,” Proc. SPIE 11264, 73–82 (2020). [CrossRef]
16. T. Walbaum, M. Heinzig, T. Schreiber, R. Eberhardt, and A. Tünnermann, “Monolithic thulium fiber laser with 567 W output power at 1970 nm,” Opt. Lett. 41(11), 2632–2635 (2016). [CrossRef]
17. N. J. Ramírez-Martínez, M. Núñez-Velázquez, and J. K. Sahu, “Study on the dopant concentration ratio in thulium-holmium doped silica fibers for lasing at 2.1 μm,” Opt. Express 28(17), 24961–24967 (2020). [CrossRef]
18. C. Kieleck, M. Eichhorn, A. Hirth, D. Faye, E. Lallier, and S. Jackson, “OP-GaAs OPO Pumped by a Q-switched Tm, Ho:silica Fiber Laser,” in Conference on Lasers and Electro-Optics/International Quantum Electronics Conference, (2009), p. CWJ2.
19. Y. Wang and C.-Q. Xu, “Actively Q-switched fiber lasers: Switching dynamics and nonlinear processes,” Prog. Quantum Electron. 31(3-5), 131–216 (2007). [CrossRef]
20. N. Dalloz, T. Robin, B. Cadier, C. Kieleck, M. Eichhorn, and A. Hildenbrand-Dhollande, “55 W actively Q-switched single oscillator Tm3+, Ho3+-codoped silica polarization maintaining 2.09 μm fiber laser,” Opt. Express 27(6), 8387–8394 (2019). [CrossRef]
