- Email: [email protected]
Co2+:MgAl2O4 microchip lasers
Optics Communications 356 (2015) 166–169
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Monolithic thermally bonded Er3 þ , Yb3 þ :glass/Co2 þ :MgAl2O4 microchip lasers Jaroslaw Mlynczak n, Nabil Belghachem Institute of Optoelectronics, Military University of Technology, Kaliskiego 2 Str., 00-908 Warsaw, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 17 April 2015 Received in revised form 3 July 2015 Accepted 21 July 2015
The highest ever reported 10 kW peak power in monolithic thermally bonded Er3 þ , Yb3 þ :glass/Co2 þ : MgAl2O4 microchip laser was achieved. To show the superiority of monolithic microchip lasers over those with external mirrors the laser generation characteristics of the same samples in both cases were compared. & 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Thermally bonded microchip laser Monolithic microchip laser Erbium pulse microchip laser
1. Introduction Microchip lasers generating at 1.5 mm wavelength find many applications in telecommunication as well as in the army [1,2]. The radiation at this wavelength is considered to be “eye-safe” and its transmission in the atmosphere and in optical fibres is very high. For this reason many different glass as well as crystalline active media doped with erbium and ytterbium ions have been developed for the last two decades [3–19]. Crystals are characterized by good mechanical and thermal properties but because of up-conversion processes their operation efficiency is limited. This limitation does not apply to glasses (to obtain efficient up-conversion luminescence the medium should possess relatively low phonon energy which is not characteristics of glasses [20]) and although they have worse thermal and mechanical parameters they became preferable active media for “eye-safe” microchip lasers [5,7,8,11]. From the point of view of most applications pulse generation is especially desirable and it can be easily achieved by the use of a semiconductor saturable absorber mirror (SESAM) [21] or a saturable absorber crystal [8]. Even though SESAMs enable to generate pulses with relatively high peak power (up to 10.6 kW) [22] they cannot withstand high energies because of their relatively low damage threshold (10–100 mJ/cm2) [22]. This situation effectively limits the pulse energy that can be generated by such lasers. In case of saturable absorber crystals the damage threshold is 2–3 orders of magnitude higher (several J/cm2) [23] which makes them preferable for high energy microchip laser applications. The n
Corresponding author. E-mail address: [email protected] (J. Mlynczak).
most efficient type of saturable absorber at 1.5 mm is Co2 þ : MgAl2O4 crystal (MALO) which can be rigidly joined [8] or thermally bonded [24] with the active media. The peak power generated in both cases using external mirrors reached 2.2 kW and 7.68 kW. However it can be further increase by depositing thin layers of input and output mirrors directly on the thermally bonded sample making one monolithic microchip laser. In this paper we present the highest ever reported peak power (over 10 kW) of monolithic thermally bonded Er3 þ , Yb3 þ :glass/ Co2 þ :MgAl2O4 microchip laser. The generation parameters of thermally bonded samples using external mirrors were compared with the generation parameters of monolithic microchip lasers made of the same samples.
2. Experimental setup and results Four samples of the phosphate glass doped with Er3 þ and Yb3 þ ions 1.9 mm long were thermally bonded with four samples of MALO saturable absorber 0.29 mm long with small signal transmission equal to 97.50%. The thermally bonded samples were characterized by 4 4 mm2 cross section and the length 2.19 mm. Both sides of them were polished to make them plane parallel. The active media and MALO crystals used to prepare the samples were described in more details in paper [24]. During the first experiment external plane mirrors were used. Input mirror was characterized by high transmission at 975 nm and high reflection at 1535 nm while output mirror was partially transmitting ( T ¼3.5%) at 1535 nm. The samples were in direct contact with the mirrors so the length of the resonator was equal
http://dx.doi.org/10.1016/j.optcom.2015.07.055 0030-4018/& 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
J. Mlynczak, N. Belghachem / Optics Communications 356 (2015) 166–169
Fig. 1. Experimental setup for laser generation investigation with external mirrors. FoP, fibre of the pumping laser diode; PB, pumping beam; Op, optics; AM, active medium; SA, saturable absorber; GB, generated beam.
Fig. 2. Characteristics of the average output power versus the pump power incident on the active medium.
to the length of the samples (2.19 mm). The basic mechanism to get a stable resonator in this case is thermal expansion of the active media which compensates the negative thermal lensing (dn/dT o0) of Er:Yb:glass. Fibre coupled laser diode with core diameter 100 mm operating at 976 nm at room temperature (25 °C) was used to longitudinally pump the samples. The focus size of the pump beam was limited by the fibre core diameter and was equal to 0.78 10 4 cm2. The damage threshold of the laser setup was mainly determined by the damage threshold of the glass samples which was assumed to be around 500 mW of the applied pump power [8]. So as to avoid discontinuation of the experiments caused by possible damage of the glass samples the pump power was limited to 375 nm and a quasi-cw pumping regime with a period 20 ms and a duty-cycle 50% was applied (pump pulse width equal to 10 ms). The experimental setup is shown in Fig. 1. The generation characteristics showing the average output power versus the average pump power incident on the active medium along with straight line approximation are presented in Fig. 2. The generation parameters such as peak power Pp , pulse width τp , pulse energy Ep , slope efficiency σ , threshold Pth and pulse repetition rate fr at Pp ¼375 mW, calculated on the basis of these characteristics and measured by the oscilloscope are shown in Table 1. One can see that there are some differences between generation parameters of the investigated samples. The second sample
167
seems to be the best one with the highest peak power reaching 7.61 kW and the highest slop efficiency equal to 7.92%. The pulse width for all samples was around 3.5 ns. The lower peak power relates to lower threshold and higher repetition rate. Such situation may suggest that the differences may be caused by small variances of the length of the saturable absorber. The change of the length of the saturable absorber by 10% results in the change of the small signal transmission by 0.25% (the absorption coefficient of MALO was 0.873 cm 1). Such difference is relatively big in case of microchip lasers and may result in big changes of the output peak power. The differences may also be caused by some inhomogeneities in the glass and in MALO or in the joints between them. Using laser beam analyser the M2 parameter for sample 2 was measured to be M2x ¼ 3.28 and M2y ¼3.03. During the second experiment the input and output mirrors were directly deposited on the samples making monolithic microchip lasers. The characteristics of these mirrors were the same as in case of the first experiment (high transmitting input mirror at 975 nm and high reflecting at 1535 nm and the output mirror partially transmitting ( T ¼ 3.5%) at 1535 nm). The length of the resonator was also equal to 2.19 mm. The stability of the resonator in this case is again achieved by thermal expansion of the glass sample. To pump the samples the same fibre coupled laser diode was used operating in the same quasi-cw regime. The setup used in this experiment is presented in Fig. 3. The generation characteristics showing the average output power in function of the average pump power incident on the active medium along with straight line approximation are presented in Fig. 4. The generation parameters are shown in Table 2. The monolithic microchip lasers show similar differences of generation parameters which can be explained in the same way as for the samples with separate mirrors. The best results were achieved for the second microchip reaching the output peak power of 10.18 kW. One can see that the results for monolithic microchip lasers are much better than for the same samples with separate mirrors. The peak power is almost two times higher while the threshold was significantly decreased. The main reason for better behaviour of the monolithic microchip lasers compared to that with external mirrors are lower losses in the cavity caused by reflections at the boundaries between different media (in case of monolithic samples there are no air gaps in the cavity). Moreover MALO works as an additional heat sink due to high thermal conductivity (13.8 J K 1 cm 1 s 1) [8]. Additionally, monolithic setup ensures stability with respect to mechanical vibration and thermal deformation of the resonator and the alignment of the laser is much easily achievable. Compering the results in Table 1 with that in Table 2 one can see that for monolithic microchip lasers the differences between the microchips (especially in case of the threshold and pulse repetition rate) are much lower. It shows that, from technological point of view, microchip lasers are more easily reproducible than lasers with separate mirrors. In Figs. 5, 6 and 7 the pulse shape, pulse train (for 675.68 Hz repetition rate) and spectrum generated by the second monolithic
Table 1 Generation parameters of the samples with external mirrors. Sample
Pp [kW]
τp [ns]
Ep [mJ]
σ [%]
Pth [mW]
fr [khz]
1 2 3 4
4.35 7.61 3.76 3.43
3.44 3.16 3.20 3.64
14.96 24.05 12.03 12.49
3.90 7.92 5.80 6.19
299 301 255 224
0.909 0.862 1.785 1.818
Fig. 3. Experimental setup for laser generation investigation of monolithic microchip lasers. FoP, fibre of the pumping laser diode; PB, pumping beam; Op, optics; AM, active medium; SA, saturable absorber; GB, generated beam.
168
J. Mlynczak, N. Belghachem / Optics Communications 356 (2015) 166–169
Fig. 4. Characteristics of the average output power versus the pump power incident on the monolithic microchip lasers.
Table 2 Generation parameters of the monolithic microchip lasers. Microchip
Pp [kW]
τp [ns]
Ep [mJ]
σ [%]
Pth [mW]
fr [kHz]
1 2 3 4
7.91 10.18 5.31 6.00
3.6 3.2 4.0 3.9
28.48 32.58 21.24 23.40
5.00 5.95 3.29 5.21
228 229 231 228
0.909 0.833 0.862 0.961
Fig. 6. Pulse train generated by the second monolithic microchip laser for 675.68 Hz repetition rate.
Fig. 7. Spectrum generated by the second monolithic microchip laser.
Fig. 5. Pulse shape generated by the second monolithic microchip laser.
microchip laser are presented, respectively. Using laser beam analyser the M2 parameter for this microchip was measured to be M2x ¼2.77 and M2y ¼2.27. The far-field distribution of the beam intensity is presented in Fig. 8. The blue square frame around the intensity distribution of the beam is caused by the aperture of the laser beam analyser and should not be taken into consideration.
3. Summary In conclusion we have demonstrated the monolithic thermally bonded Er3 þ , Yb3 þ :glass/Co2 þ :MgAl2O4 microchip lasers, that generated the highest 10.18 kW peak power ever reported for such type of lasers. The advantage of monolithic microchip lasers over those with external mirrors was shown.
Fig. 8. Far-field distribution of the beam intensity for the second monolithic microchip laser.
J. Mlynczak, N. Belghachem / Optics Communications 356 (2015) 166–169
Acknowledgements The authors would like to thank the Institute of Electronic Materials Technology for the development of the glass active medium and the saturable absorber MALO as well as the Institute of Applied Optics for the thermal bonding of the samples. The work was sponsored by the Polish National Centre for Research and Development, Project DOB-1-6/1/PS/2014.
[11]
[12]
[13] [14]
References [15] [1] J.J. Zayhowski, A.L. Wilson Jr, Miniature eye-safe laser system for high-resolution three-dimensional lidar, Appl. Opt. 46 (2007) 5951–5956. [2] J. Młyńczak, K. Kopczyński, Z. Mierczyk, M. Zygmunt, S. Natkański, M. Muzal, J. Wojtanowski, P. Kirwil, M. Jakubaszek, P. Knysak, W. Piotrowski, A. Zarzycka, A. Gawlikowski, Practical application of pulsed “eye-safe” microchip laser to laser rangefinders, Optoelectron. Rev. 21 (2013) 332–337. [3] N.V. Kuleshov, A.A. Lagatsky, V.G. Shcherbitsky, V.P. Mikhailov, E. Heumann, T. Jensen, A. Diening, G. Huber, CW laser performance of Yb and Er, Yb doped tungstates, Appl. Phys. B: Lasers Opt. 64 (1998) 409–413. [4] P. Laporta, S. Taccheo, S. Longhi, O. Svelto, C. Svelto, Erbium–ytterbium microlasers: optical properties and lasing characteristics, Opt. Mater. 11 (1999) 269–288. [5] G. Karlsson, V. Pasiskevicius, F. Laurell, J. Tellefsen, B. Denker, B. Galagan, V. Osiko, S. Sverchkov, Diode-pumped Er–Yb:glass laser passively Q switched by use of Co(2 þ ):MgAl(2)O(4) as a saturable absorber, Appl. Opt. 39 (2000) 6188–6192. [6] E. Georgiou, O. Musset, J. Boquillon, B. Denker, S. Sverchkov, 50 mJ/30 ns FTIR Q-switched diode-pumped Er:Yb:glass 1.54 μm laser, Opt. Commun. 198 (2001) 147–153. [7] B. Denker, B. Galagan, V. Osiko, S. Sverchkov, Materials and components for miniature diode-pumped 1.5 μm erbium glass lasers, Laser Phys. 12 (2002) 697–701. [8] G. Karlsson, F. Laurell, J. Tellefsen, B. Denker, B. Galagan, V. Osiko, S. Sverchkov, Development and characterization of Yb–Er laser glass for high average power laser diode pumping, Appl. Phys. B: Lasers Opt. 75 (2002) 41–46. [9] Z. Liu, C. Qi, S. Dai, Y. Jiang, L. Hu, Spectra and laser properties of Er3 þ , Yb3 þ : phosphate glasses, Opt. Mater. 21 (2003) 789–794. [10] P.A. Burns, J.M. Dawes, P. Dekker, J.A. Piper, H. Jiang, J. Wang, Optimization of
[16]
[17]
[18]
[19]
[20] [21]
[22] [23]
[24]
169
Er, Yb:YCOB for CW laser operation, IEEE J. Quantum Electron. 40 (2004) 1575–1582. B. Denker, B. Galagan, L. Ivleva, V. Osiko, S. Sverchkov, I. Voronina, J. E. Hellstrom, F. Laurell, Luminescent and laser properties of Yb–Er:GdCa4O (BO3)3: a new crystal for eye-safe 1.5-μm lasers, Appl. Phys. B: Lasers Opt. 79 (2004) 577–581. N.A. Tolstik, S.V. Kurilchik, V.E. Kisel, N.V. Kuleshov, V.V. Maltsev, O. V. Pilipenko, E.V. Koporulina, N.I. Leonyuk, Efficient 1 W continuous-wave diode-pumped Er,Yb:Yal3(BO3)4 laser, Opt. Lett. 32 (2007) 3233–3235. J. Sulc, H. Jelinkova, W. Ryba-Romanowski, T. Lukasiewicz, 1.6 μm microchip laser, Laser Phys. Lett. 6 (2009) 207–211. J. Młyńczak, K. Kopczynski, Z. Mierczyk, M. Malinowska, P. Osiwiański, Comparison of cw laser generation in Er3 þ , Yb3 þ :glass microchip lasers with different types of glasses, Optoelectron. Rev. 19 (2011) 491–495. V.E. Kisel., K.N. Gorbachenya., A.S. Yasukevich., A.M. Ivashko., N.V. Kuleshov., V. V. Maltsev., N.I. Leonyuk, Passively Q-switched microchip Er, Yb:YAl3(BO3)4 diode-pumped laser, Opt. Lett. 37 (2012) 2745–2747. Y.J. Chen, Y.F. Lin, J.H. Huang, X.H. Gong, Z.D. Luo, Y.D. Huang, Diode-pumped monolithic Er3 þ :Yb3 þ :YAl3(BO3)4 micro-laser at 1.6 μm, Opt. Commun. 285 (2012) 751–754. Y. Chen, Y. Lin, Y. Zou, J. Huang, X. Gong, Z. Luo, Y. Huang, Diode-pumped 1.5– 1.6 μm laser operation in Er3 þ doped YbAl3(BO3)4 microchip, Opt. Express 22 (2014) 13969–13974. J.M. Serres, X. Mateos, P. Loiko, K. Yumashev, N. Kuleshov, V. Petrov, U. Griebner, M. Aguiló, F. Díaz, Diode-pumped microchip Tm:KLu(WO4)2 laser with more than 3 W of output power, Opt. Lett. 39 (2014) 4247–4250. Y. Chen, J. Huang, Y. Zou, Y. Lin, X. Gong, Z. Luo, Y. Huang, Diode-pumped passively Q-switched Er3 þ :Yb3 þ :Sr3Lu2(BO3)4 laser at 1534 nm, Opt. Express 22 (2014) 8333–8338. C. Ming, F. Song, H. Liu, Intense blue up-conversion emission in Tm3 þ /Yb3 þ co-doped phosphate glass, J. Non-Crystal. Solids 360 (2013) 1–3. R. Fluck, R. Haring, R. Paschotta, E. Gini, H. Melchior, U. Keller, Eyesafe pulsed microchip laser using semiconductor saturable absorber mirrors, Appl. Phys. Lett. 72 (1998) 3273–3275. R. Haring, R. Paschotta, R. Fluck, E. Gini, H. Melchior, U. Keller, Passively Q-switched microchip laser at 1.5 μm, J. Opt. Soc. Am. B 18 (2001) 1805–1812. V.G. Shcherbitsky, S. Girard, M. Fromager, R. Moncorge, N.V. Kuleshov, V. I. Levchenko, V.N. Yakimovich, B. Ferrand, Accurate method for the measurement of absorption cross sections of solid-state saturable absorbers, Appl. Phys. B 74 (2002) 367–374. J. Mlynczak, N. Belghachem, High peak power generation in thermally bonded Er3 þ , Yb3 þ :glass/Co2 þ :MgAl2O3 microchip laser for telemetry application, Laser Phys. Lett. 12 (2015) 045803.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Monolithic thermally bonded Er3 þ , Yb3 þ :glass/Co2 þ :MgAl2O4 microchip lasers Jaroslaw Mlynczak n, Nabil Belghachem Institute of Optoelectronics, Military University of Technology, Kaliskiego 2 Str., 00-908 Warsaw, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 17 April 2015 Received in revised form 3 July 2015 Accepted 21 July 2015
The highest ever reported 10 kW peak power in monolithic thermally bonded Er3 þ , Yb3 þ :glass/Co2 þ : MgAl2O4 microchip laser was achieved. To show the superiority of monolithic microchip lasers over those with external mirrors the laser generation characteristics of the same samples in both cases were compared. & 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Thermally bonded microchip laser Monolithic microchip laser Erbium pulse microchip laser
1. Introduction Microchip lasers generating at 1.5 mm wavelength find many applications in telecommunication as well as in the army [1,2]. The radiation at this wavelength is considered to be “eye-safe” and its transmission in the atmosphere and in optical fibres is very high. For this reason many different glass as well as crystalline active media doped with erbium and ytterbium ions have been developed for the last two decades [3–19]. Crystals are characterized by good mechanical and thermal properties but because of up-conversion processes their operation efficiency is limited. This limitation does not apply to glasses (to obtain efficient up-conversion luminescence the medium should possess relatively low phonon energy which is not characteristics of glasses [20]) and although they have worse thermal and mechanical parameters they became preferable active media for “eye-safe” microchip lasers [5,7,8,11]. From the point of view of most applications pulse generation is especially desirable and it can be easily achieved by the use of a semiconductor saturable absorber mirror (SESAM) [21] or a saturable absorber crystal [8]. Even though SESAMs enable to generate pulses with relatively high peak power (up to 10.6 kW) [22] they cannot withstand high energies because of their relatively low damage threshold (10–100 mJ/cm2) [22]. This situation effectively limits the pulse energy that can be generated by such lasers. In case of saturable absorber crystals the damage threshold is 2–3 orders of magnitude higher (several J/cm2) [23] which makes them preferable for high energy microchip laser applications. The n
Corresponding author. E-mail address: [email protected] (J. Mlynczak).
most efficient type of saturable absorber at 1.5 mm is Co2 þ : MgAl2O4 crystal (MALO) which can be rigidly joined [8] or thermally bonded [24] with the active media. The peak power generated in both cases using external mirrors reached 2.2 kW and 7.68 kW. However it can be further increase by depositing thin layers of input and output mirrors directly on the thermally bonded sample making one monolithic microchip laser. In this paper we present the highest ever reported peak power (over 10 kW) of monolithic thermally bonded Er3 þ , Yb3 þ :glass/ Co2 þ :MgAl2O4 microchip laser. The generation parameters of thermally bonded samples using external mirrors were compared with the generation parameters of monolithic microchip lasers made of the same samples.
2. Experimental setup and results Four samples of the phosphate glass doped with Er3 þ and Yb3 þ ions 1.9 mm long were thermally bonded with four samples of MALO saturable absorber 0.29 mm long with small signal transmission equal to 97.50%. The thermally bonded samples were characterized by 4 4 mm2 cross section and the length 2.19 mm. Both sides of them were polished to make them plane parallel. The active media and MALO crystals used to prepare the samples were described in more details in paper [24]. During the first experiment external plane mirrors were used. Input mirror was characterized by high transmission at 975 nm and high reflection at 1535 nm while output mirror was partially transmitting ( T ¼3.5%) at 1535 nm. The samples were in direct contact with the mirrors so the length of the resonator was equal
http://dx.doi.org/10.1016/j.optcom.2015.07.055 0030-4018/& 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
J. Mlynczak, N. Belghachem / Optics Communications 356 (2015) 166–169
Fig. 1. Experimental setup for laser generation investigation with external mirrors. FoP, fibre of the pumping laser diode; PB, pumping beam; Op, optics; AM, active medium; SA, saturable absorber; GB, generated beam.
Fig. 2. Characteristics of the average output power versus the pump power incident on the active medium.
to the length of the samples (2.19 mm). The basic mechanism to get a stable resonator in this case is thermal expansion of the active media which compensates the negative thermal lensing (dn/dT o0) of Er:Yb:glass. Fibre coupled laser diode with core diameter 100 mm operating at 976 nm at room temperature (25 °C) was used to longitudinally pump the samples. The focus size of the pump beam was limited by the fibre core diameter and was equal to 0.78 10 4 cm2. The damage threshold of the laser setup was mainly determined by the damage threshold of the glass samples which was assumed to be around 500 mW of the applied pump power [8]. So as to avoid discontinuation of the experiments caused by possible damage of the glass samples the pump power was limited to 375 nm and a quasi-cw pumping regime with a period 20 ms and a duty-cycle 50% was applied (pump pulse width equal to 10 ms). The experimental setup is shown in Fig. 1. The generation characteristics showing the average output power versus the average pump power incident on the active medium along with straight line approximation are presented in Fig. 2. The generation parameters such as peak power Pp , pulse width τp , pulse energy Ep , slope efficiency σ , threshold Pth and pulse repetition rate fr at Pp ¼375 mW, calculated on the basis of these characteristics and measured by the oscilloscope are shown in Table 1. One can see that there are some differences between generation parameters of the investigated samples. The second sample
167
seems to be the best one with the highest peak power reaching 7.61 kW and the highest slop efficiency equal to 7.92%. The pulse width for all samples was around 3.5 ns. The lower peak power relates to lower threshold and higher repetition rate. Such situation may suggest that the differences may be caused by small variances of the length of the saturable absorber. The change of the length of the saturable absorber by 10% results in the change of the small signal transmission by 0.25% (the absorption coefficient of MALO was 0.873 cm 1). Such difference is relatively big in case of microchip lasers and may result in big changes of the output peak power. The differences may also be caused by some inhomogeneities in the glass and in MALO or in the joints between them. Using laser beam analyser the M2 parameter for sample 2 was measured to be M2x ¼ 3.28 and M2y ¼3.03. During the second experiment the input and output mirrors were directly deposited on the samples making monolithic microchip lasers. The characteristics of these mirrors were the same as in case of the first experiment (high transmitting input mirror at 975 nm and high reflecting at 1535 nm and the output mirror partially transmitting ( T ¼ 3.5%) at 1535 nm). The length of the resonator was also equal to 2.19 mm. The stability of the resonator in this case is again achieved by thermal expansion of the glass sample. To pump the samples the same fibre coupled laser diode was used operating in the same quasi-cw regime. The setup used in this experiment is presented in Fig. 3. The generation characteristics showing the average output power in function of the average pump power incident on the active medium along with straight line approximation are presented in Fig. 4. The generation parameters are shown in Table 2. The monolithic microchip lasers show similar differences of generation parameters which can be explained in the same way as for the samples with separate mirrors. The best results were achieved for the second microchip reaching the output peak power of 10.18 kW. One can see that the results for monolithic microchip lasers are much better than for the same samples with separate mirrors. The peak power is almost two times higher while the threshold was significantly decreased. The main reason for better behaviour of the monolithic microchip lasers compared to that with external mirrors are lower losses in the cavity caused by reflections at the boundaries between different media (in case of monolithic samples there are no air gaps in the cavity). Moreover MALO works as an additional heat sink due to high thermal conductivity (13.8 J K 1 cm 1 s 1) [8]. Additionally, monolithic setup ensures stability with respect to mechanical vibration and thermal deformation of the resonator and the alignment of the laser is much easily achievable. Compering the results in Table 1 with that in Table 2 one can see that for monolithic microchip lasers the differences between the microchips (especially in case of the threshold and pulse repetition rate) are much lower. It shows that, from technological point of view, microchip lasers are more easily reproducible than lasers with separate mirrors. In Figs. 5, 6 and 7 the pulse shape, pulse train (for 675.68 Hz repetition rate) and spectrum generated by the second monolithic
Table 1 Generation parameters of the samples with external mirrors. Sample
Pp [kW]
τp [ns]
Ep [mJ]
σ [%]
Pth [mW]
fr [khz]
1 2 3 4
4.35 7.61 3.76 3.43
3.44 3.16 3.20 3.64
14.96 24.05 12.03 12.49
3.90 7.92 5.80 6.19
299 301 255 224
0.909 0.862 1.785 1.818
Fig. 3. Experimental setup for laser generation investigation of monolithic microchip lasers. FoP, fibre of the pumping laser diode; PB, pumping beam; Op, optics; AM, active medium; SA, saturable absorber; GB, generated beam.
168
J. Mlynczak, N. Belghachem / Optics Communications 356 (2015) 166–169
Fig. 4. Characteristics of the average output power versus the pump power incident on the monolithic microchip lasers.
Table 2 Generation parameters of the monolithic microchip lasers. Microchip
Pp [kW]
τp [ns]
Ep [mJ]
σ [%]
Pth [mW]
fr [kHz]
1 2 3 4
7.91 10.18 5.31 6.00
3.6 3.2 4.0 3.9
28.48 32.58 21.24 23.40
5.00 5.95 3.29 5.21
228 229 231 228
0.909 0.833 0.862 0.961
Fig. 6. Pulse train generated by the second monolithic microchip laser for 675.68 Hz repetition rate.
Fig. 7. Spectrum generated by the second monolithic microchip laser.
Fig. 5. Pulse shape generated by the second monolithic microchip laser.
microchip laser are presented, respectively. Using laser beam analyser the M2 parameter for this microchip was measured to be M2x ¼2.77 and M2y ¼2.27. The far-field distribution of the beam intensity is presented in Fig. 8. The blue square frame around the intensity distribution of the beam is caused by the aperture of the laser beam analyser and should not be taken into consideration.
3. Summary In conclusion we have demonstrated the monolithic thermally bonded Er3 þ , Yb3 þ :glass/Co2 þ :MgAl2O4 microchip lasers, that generated the highest 10.18 kW peak power ever reported for such type of lasers. The advantage of monolithic microchip lasers over those with external mirrors was shown.
Fig. 8. Far-field distribution of the beam intensity for the second monolithic microchip laser.
J. Mlynczak, N. Belghachem / Optics Communications 356 (2015) 166–169
Acknowledgements The authors would like to thank the Institute of Electronic Materials Technology for the development of the glass active medium and the saturable absorber MALO as well as the Institute of Applied Optics for the thermal bonding of the samples. The work was sponsored by the Polish National Centre for Research and Development, Project DOB-1-6/1/PS/2014.
[11]
[12]
[13] [14]
References [15] [1] J.J. Zayhowski, A.L. Wilson Jr, Miniature eye-safe laser system for high-resolution three-dimensional lidar, Appl. Opt. 46 (2007) 5951–5956. [2] J. Młyńczak, K. Kopczyński, Z. Mierczyk, M. Zygmunt, S. Natkański, M. Muzal, J. Wojtanowski, P. Kirwil, M. Jakubaszek, P. Knysak, W. Piotrowski, A. Zarzycka, A. Gawlikowski, Practical application of pulsed “eye-safe” microchip laser to laser rangefinders, Optoelectron. Rev. 21 (2013) 332–337. [3] N.V. Kuleshov, A.A. Lagatsky, V.G. Shcherbitsky, V.P. Mikhailov, E. Heumann, T. Jensen, A. Diening, G. Huber, CW laser performance of Yb and Er, Yb doped tungstates, Appl. Phys. B: Lasers Opt. 64 (1998) 409–413. [4] P. Laporta, S. Taccheo, S. Longhi, O. Svelto, C. Svelto, Erbium–ytterbium microlasers: optical properties and lasing characteristics, Opt. Mater. 11 (1999) 269–288. [5] G. Karlsson, V. Pasiskevicius, F. Laurell, J. Tellefsen, B. Denker, B. Galagan, V. Osiko, S. Sverchkov, Diode-pumped Er–Yb:glass laser passively Q switched by use of Co(2 þ ):MgAl(2)O(4) as a saturable absorber, Appl. Opt. 39 (2000) 6188–6192. [6] E. Georgiou, O. Musset, J. Boquillon, B. Denker, S. Sverchkov, 50 mJ/30 ns FTIR Q-switched diode-pumped Er:Yb:glass 1.54 μm laser, Opt. Commun. 198 (2001) 147–153. [7] B. Denker, B. Galagan, V. Osiko, S. Sverchkov, Materials and components for miniature diode-pumped 1.5 μm erbium glass lasers, Laser Phys. 12 (2002) 697–701. [8] G. Karlsson, F. Laurell, J. Tellefsen, B. Denker, B. Galagan, V. Osiko, S. Sverchkov, Development and characterization of Yb–Er laser glass for high average power laser diode pumping, Appl. Phys. B: Lasers Opt. 75 (2002) 41–46. [9] Z. Liu, C. Qi, S. Dai, Y. Jiang, L. Hu, Spectra and laser properties of Er3 þ , Yb3 þ : phosphate glasses, Opt. Mater. 21 (2003) 789–794. [10] P.A. Burns, J.M. Dawes, P. Dekker, J.A. Piper, H. Jiang, J. Wang, Optimization of
[16]
[17]
[18]
[19]
[20] [21]
[22] [23]
[24]
169
Er, Yb:YCOB for CW laser operation, IEEE J. Quantum Electron. 40 (2004) 1575–1582. B. Denker, B. Galagan, L. Ivleva, V. Osiko, S. Sverchkov, I. Voronina, J. E. Hellstrom, F. Laurell, Luminescent and laser properties of Yb–Er:GdCa4O (BO3)3: a new crystal for eye-safe 1.5-μm lasers, Appl. Phys. B: Lasers Opt. 79 (2004) 577–581. N.A. Tolstik, S.V. Kurilchik, V.E. Kisel, N.V. Kuleshov, V.V. Maltsev, O. V. Pilipenko, E.V. Koporulina, N.I. Leonyuk, Efficient 1 W continuous-wave diode-pumped Er,Yb:Yal3(BO3)4 laser, Opt. Lett. 32 (2007) 3233–3235. J. Sulc, H. Jelinkova, W. Ryba-Romanowski, T. Lukasiewicz, 1.6 μm microchip laser, Laser Phys. Lett. 6 (2009) 207–211. J. Młyńczak, K. Kopczynski, Z. Mierczyk, M. Malinowska, P. Osiwiański, Comparison of cw laser generation in Er3 þ , Yb3 þ :glass microchip lasers with different types of glasses, Optoelectron. Rev. 19 (2011) 491–495. V.E. Kisel., K.N. Gorbachenya., A.S. Yasukevich., A.M. Ivashko., N.V. Kuleshov., V. V. Maltsev., N.I. Leonyuk, Passively Q-switched microchip Er, Yb:YAl3(BO3)4 diode-pumped laser, Opt. Lett. 37 (2012) 2745–2747. Y.J. Chen, Y.F. Lin, J.H. Huang, X.H. Gong, Z.D. Luo, Y.D. Huang, Diode-pumped monolithic Er3 þ :Yb3 þ :YAl3(BO3)4 micro-laser at 1.6 μm, Opt. Commun. 285 (2012) 751–754. Y. Chen, Y. Lin, Y. Zou, J. Huang, X. Gong, Z. Luo, Y. Huang, Diode-pumped 1.5– 1.6 μm laser operation in Er3 þ doped YbAl3(BO3)4 microchip, Opt. Express 22 (2014) 13969–13974. J.M. Serres, X. Mateos, P. Loiko, K. Yumashev, N. Kuleshov, V. Petrov, U. Griebner, M. Aguiló, F. Díaz, Diode-pumped microchip Tm:KLu(WO4)2 laser with more than 3 W of output power, Opt. Lett. 39 (2014) 4247–4250. Y. Chen, J. Huang, Y. Zou, Y. Lin, X. Gong, Z. Luo, Y. Huang, Diode-pumped passively Q-switched Er3 þ :Yb3 þ :Sr3Lu2(BO3)4 laser at 1534 nm, Opt. Express 22 (2014) 8333–8338. C. Ming, F. Song, H. Liu, Intense blue up-conversion emission in Tm3 þ /Yb3 þ co-doped phosphate glass, J. Non-Crystal. Solids 360 (2013) 1–3. R. Fluck, R. Haring, R. Paschotta, E. Gini, H. Melchior, U. Keller, Eyesafe pulsed microchip laser using semiconductor saturable absorber mirrors, Appl. Phys. Lett. 72 (1998) 3273–3275. R. Haring, R. Paschotta, R. Fluck, E. Gini, H. Melchior, U. Keller, Passively Q-switched microchip laser at 1.5 μm, J. Opt. Soc. Am. B 18 (2001) 1805–1812. V.G. Shcherbitsky, S. Girard, M. Fromager, R. Moncorge, N.V. Kuleshov, V. I. Levchenko, V.N. Yakimovich, B. Ferrand, Accurate method for the measurement of absorption cross sections of solid-state saturable absorbers, Appl. Phys. B 74 (2002) 367–374. J. Mlynczak, N. Belghachem, High peak power generation in thermally bonded Er3 þ , Yb3 þ :glass/Co2 þ :MgAl2O3 microchip laser for telemetry application, Laser Phys. Lett. 12 (2015) 045803.