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Hybrid Neodymium-doped passively Q-switched waveguide laser
Materials Science and Engineering B 149 (2008) 181–184
Hybrid Neodymium-doped passively Q-switched waveguide laser Rafael Salas-Montiel, Lionel Bastard ∗ , Gr´egory Grosa, Jean-Emmanuel Broquin IMEP-LAHC, 3 Parvis Louis N´eel BP 257, 38016 Grenoble Cedex 1, France Received 31 October 2007; accepted 8 November 2007
Abstract In the mid 80s, the doping of optical fibers’ core with rare earth atoms has been a major breakthrough in the field of optical telecommunications since it allowed the realization of in line optical amplifiers. However, erbium-doped fiber amplifiers are a few meters long and a huge effort has been made in order to realize compact and efficient active devices based on rare-earth-doped waveguides. For this purpose the use of phosphate glasses instead of silicate ones has been investigated because they allow a better solubility of the inserted rare earths. In this paper we present the realization of a hybrid Neodymium-doped passively Q-switched waveguide laser made by an ion-exchange on a Schott IOG-1 phosphate laser glass combined with the deposition of a bis(4-dimethylaminodithiobenzil)nickel (BDN) saturable absorber diluted in a cellulose acetate polymer cladding. In a first step, we present the continuous wave (CW) operation of the laser with an undoped cladding. We show that for a 3.5 m wide, 1.5 cm long waveguide realized by a silver–sodium ion-exchange, a 6 mW output has been achieved by creating a Fabry–Perot cavity with dielectric multilayers mirrors sticked to the chip facets. Then, the characterizations performed on the BDN-doped layers are presented. It is shown that a proper selection of the hybrid guiding condition and saturable absorber concentration entail a non-saturated excess absorption of 3.4 dB/cm. Finally, we present the results we obtained on the Q-switched behaviour of the laser. Indeed a repetition rate of 330 kHz is achieved for a pulse energy of 10 nJ and a peak power of 1 W. © 2007 Elsevier B.V. All rights reserved. Keywords: Glass integrated optics; Ion-exchange; Q-switch laser
1. Introduction Ion-exchange on glass has been widely used for the realization of passive integrated optics devices since the demonstration of the first optical waveguide by Izawa and Nakagome [1]. However, the use of rare-earth-doped glasses has also allowed obtaining optical amplifiers [2,3] and lasers [4]. Based on the use of Er3+ or Nd3+ ions for emissions around λ = 1.5 m and λ = 1.06 m, these ion-exchanged glass integrated lasers have been demonstrated with the main types of feed-back which are the Fabry–Perot cavity [4], the distributed Bragg reflector (DBR) configuration [5] and the distributed feed-back (DFB) one [6]. Nonetheless, except one report by Sanford et al. [7] on a Ybranch laser and the use of a Neodymium-doped ion-exchanged waveguides as active medium into a bulk optic Q-switched laser [8], no integrated pulsed lasers have been investigated with the ion-exchange technology on glass. Integrated pulsed lasers could however present an interesting alternative to microchip lasers
∗
Corresponding author. Tel.: +33 4 56 52 95 30. E-mail address: [email protected] (L. Bastard).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.11.012
[9], which are now commonly used for drilling, marking or nonlinear optic applications. Indeed, microchip devices are usually realized by bonding to a rod of a rare-earth-doped crystal, a saturable absorber and two dielectric mirrors and their transverse single mode operation is only ensured by the thermal lens that is created when the laser is pumped with a high power laser diode. On the opposite, the transverse single mode operation of an ionexchanged integrated laser is imposed by the waveguide and therefore intrinsically stable. Moreover, the possibility of realizing DFB structure can guaranty a single frequency operation without mode-hopping. In this article, we report the realization of a passively Qswitched laser based on the hybridization of a polymer doped with a saturable absorber dye on an Nd3+ -doped ion-exchanged waveguide amplifier. Section 2 is devoted to the presentation of the hybrid laser principle of operation whereas Section 3 deals with the realization of the ion-exchanged waveguide amplifiers and their characterization and use as a continuous wave (CW) laser. Finally, Section 4 presents the hybridization of the CW laser with the saturable absorber-doped polymer cladding as well as the characterizations that have been carried-out on the soobtained passively Q-switch laser.
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2. Hybrid Q-switched laser principle of operation A Q-switch laser is composed of an amplifying medium providing the gain to the optical oscillator, an optical resonator to provide the feed-back and a saturable absorber which role is to periodically damp the quality factor of the cavity yielding hence to a pulsed operation of the laser. In our case, the amplifying medium is a IOG1 phosphate glass provided by Schott and doped with 1.5 × 1026 m−3 Nd3+ ions, in which an ion-exchange is carried-out in order to create a single mode Neodymiumdoped waveguide amplifier (NdDWA). The saturable absorber is a cellulose acetate polymer film which has been doped by a bis(4-dimethylaminodithiobenzil)nickel (BDN) dye. The polymer is deposited on the glass substrate entailing a distributed interaction of the guided mode with the BDN molecules within the laser cavity, which is constituted by two multilayers dielectric mirrors stuck to the waveguide facets. The overall structure of the hybrid laser is sketched in Fig. 1. The operation of a Q-switched pulsed laser is intrinsically governed by the competition between the intra-cavity losses created by the saturable absorber and the gain provided by the optical amplifier. In bulk optics or microchip Q-switched laser, gain and saturable loss media are cascaded in such a way that setting their respective length sets their respective influence. For hybrid laser, the polymer containing the absorbing dye has a refractive index of 1.475 at λ = 1.064 m, which is lower than the one of the IOG1 (1.515 at λ = 1.064 m). It behaves therefore as a cladding for the NdDWA and the guided mode that is amplified within the waveguide core and substrate also interacts evanescently with the BDN. Fig. 2 depicts a typical simulation of the guided mode of such a structure where its interaction with the polymer cladding can clearly be seen. Calculations have been carried-out with a scalar mode solver [10] coupled with an ion-diffusion solver. The interaction coefficient is defined as the normalized overlap of the guided mode with the cladding by the following relation: 2 cladding |E(x, y)| dx dy Γ = (1) |E(x, y)|2 dx dy where E(x, y) is the guided mode electric field. Γ can easily be set from 1% to 10% by varying the waveguide characteristics through the ion-exchange time and diffusion aperture.
Fig. 1. Schematic representation of the hybrid Q-switched integrated laser.
Fig. 2. Calculated mode intensity distribution of an hybrid waveguide at λ = 1.064 m.
3. NdDWA realization, characterization and CW laser operation The waveguide has been realized using the IOG1 phosphate glass following the standard process described in [6] to deposit and pattern an alumina mask on its surface before realizing a 4 min. Ion-exchange at 330 ◦ C in a 20%AgNO3 –80%NaNO3 salt melt through 1 m wide diffusion apertures. After dicing and polishing of the waveguide facets, passive and active characterizations have been performed. First the mode profiles of the waveguide have been measured at pump and signal wavelength, respectively λ = 0.8 m and λ = 1.054 m, by injecting light through a single mode optical fiber and imaging the waveguide output on an InGaAs CCD camera. From the images depicted in Fig. 3, mode dimensions of 8 ± 0.2 m by 5.1 ± 0.2 m and 5.5 ± 0.2 m by 4.5 ± 0.2 m have been measured at λ = 1.015 m and λ = 0.8 m, respectively. Replacing the camera by a calibrated detector, propagation losses of 1.2 ± 0.3 dB/cm have been determined. This relatively quite high value is mainly due to a deterioration of the waveguide surface quality during the mask patterning process. Although passive characterizations have not been carried-out at λ = 1.054 m, it can be considered that the results obtained at λ = 1.015 m describes well the behaviour of the waveguide at the signal wavelength since their difference is small and the mode propagation properties do not depend strongly on wavelength when far from cut-off.
Fig. 3. Near field mode intensity pictures: (a) at λ = 1.015 m and (b) at λ = 0.8 m.
R. Salas-Montiel et al. / Materials Science and Engineering B 149 (2008) 181–184
Fig. 4. Laser output power as a function of launched pump power for a cavity with 99% and 85% reflectivity mirrors. Inset presents the laser emission spectrum.
In order to determine the gain of the 1.4 cm long NdDWA, it has been used to realize CW lasers with different sets of dielectric mirrors stuck to its facets in order to create a cavity. Knowing the mirror transmission and the propagation losses, the measurement of the laser threshold allows hence determining the waveguide small signal gain since at this point it matches the laser cavity losses. CW laser operation has been achieved with sets of mirrors having reflectivities of 99%, 85% and 4%. The maximum gain measured was 7 ± 0.3 dB for a 290 mW launched pump power, which corresponds to a laser cavity composed of a 4% (i.e. Fresnel reflection on the waveguide facet) and a 99% reflectivity mirrors. Fig. 4 presents the evolution of the power emitted by the laser that was realized with a 99% reflectivity mirror on one facet and a 85% reflectivity mirror on the other one. The pump threshold is 60 ± 5 mW, the slope efficiency is 4.5% and a maximum output power of more than 6 mW has been obtained for a launched pump power of 200 mW. These results prove that the NdDWA that we have realized allows obtaining laser emission and they can serve as a starting point to realize passively Q-switched lasers.
183
increase the cavity losses, it has been decided to take some margin and thus reduce the BDN concentration to 3 × 1016 cm−3 . Although it might limit the laser pulse energy, a reduction of the saturable absorber excess loss ensures indeed that the 7 dB maximum gain of the NdDWA is high enough to compensate both the cavity losses, including the ones due to potential mirrors misalignments, and the BDN absorption. After having implemented the new concentration polymer cladding, the sticking of two mirrors with reflectivity of 99% and 85%, respectively completed the realization process of the hybrid Q-switch laser. The device has been pumped with a Ti–W laser emitting at λ = 808 nm while its output was monitored by a 25 GHz bandwidth photodetector coupled to a 6 GHz digital oscilloscope. Fig. 5 displays the trace of two pulses that have been measured for a launched pump power of 160 mW. It can be seen that the repetition rate of the laser is 330 kHz. This value can be changed by modifying the pump power since the more gain is created in the waveguide, the less time is needed to saturate the BDN molecules. With this sample, repetition rates ranging from 65 kHz to 350 kHz have been observed. The temporal shape of laser pulses has also been measured and is presented in Fig. 6. The pulse width is 10 ± 1 ns and does not depend on the pump power, which is in agreement the theoretical behaviour of passively Q-switched lasers [12]. Measuring the average power emitted by the laser to be 0.5 mW for a launched pump power of 160 mW a pulse energy of 10 nJ has been derived corresponding to a pulse peak power of 1 W. Although all this results can be dramatically improved by reducing the waveguide propagation losses, optimizing the BDN concentration or integrating the mirrors on the chip, it must be noticed that the values we have obtained confirms the interest of the integrated optic approach since they are similar or better than the ones published by Aust et al. [8] with the same active medium and saturable absorber elements but using a nonintegrated extended cavity scheme.
4. Hybrid Q-switch laser realization and characterization In order to implement the saturable absorber on the NdDWA, BDN molecules have been first dissolved into acetone then incorporated into a cellulose acetate–acetone solution. After partial evaporation of the solvent, the obtained BDN-doped cellulose acetate has been deposited as a thick film on the waveguide surface where it has finished drying creating a solid thick film of roughly 100 m with a BDN concentration of 1.2 × 1017 cm−3 . The hybrid waveguide transmission has been measured and the excess loss due to the saturable absorber have been determined to be 3.4 ± 0.2 dB/cm which gives an interaction coefficient of Γ = 5% assuming that the absorption cross-section of BDN at λ = 1.054 m is 1.3 × 10−16 cm2 [11] and remains unchanged when incorporated in the polymer matrix. Nonetheless, because the impact of the polymer cladding presence on the mirror positioning accuracy was a major unknown that could dramatically
Fig. 5. Laser pulse train measured on a digital oscilloscope for a launched pump power of 240 mW.
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R. Salas-Montiel et al. / Materials Science and Engineering B 149 (2008) 181–184
cellulose acetate as a host for BDN saturable absorber molecules has allowed realizing an integrated optic Q-switched laser with a repetition rate ranging from 65 kHz to 350 kHz, a pulse width of 10 ± 1 ns, a pulse energy of 10 nJ with a 1 W peak power. This first demonstration can be dramatically improved by reducing the waveguide propagation losses, optimizing the BDN concentration and integrating the cavity mirrors on the chip. However, it must be noticed that the characteristics of our integrated Q-switch laser made by ion-exchange on glass are at least equivalent to the ones of free space devices using the same active material and saturable absorber. References
Fig. 6. Emitted laser pulse for a launched pump power of 160 mW.
5. Conclusion In this article, the realization and characterization of an integrated passively Q-switch laser made by ion-exchange on glass and hybridization of a doped polymer has been presented. After an explanation of the device principle of operation, the realization of Neodymium-doped optical amplifiers made by a silver–sodium ion-exchange on glass has been presented. The 1.4 cm long NdDWA has a maximum gain of 7 ± 0.3 dB for a launched pump power of 290 mW and it has been successfully used to realize a CW laser emitting more than 6 mW of output power for a launched pump power of 200 mW. The use of
[1] T. Izawa, H. Nakagome, Appl. Phys. Lett. 21 (1972) 584. [2] J.M.P. Delavaux, S. Granlund, O. Mizuhara, L.D. Tzeng, D. Barbier, M. Rattay, F. Saint-Andr´e, A. Kevorkian, IEEE Photon. Technol. Lett. 9 (2) (1997) 247–249. [3] F. Gardillou, L. Bastard, J.-E. Broquin, Appl. Phys. Lett. 85 (2004) 5176. [4] A. Sanford, K.J. Malone, D.R. Larson, Opt. Lett. 15 (1990) 366. [5] D.L. Vasey, D.S. Funk, N.A. Sanford, Appl. Phys. Lett. 74 (6) (1999) 789–791. [6] L. Bastard, S. Blaize, J.-E. Broquin, Opt. Eng. 42 (10) (2003) 2800–2804. [7] N.A. Sanford, K.J. Malone, D.R. Larson, R.K. Hickernell, Opt. Lett. 16 (1991) 1168. [8] J.A. Aust, K.J. Malone, D.L. Veasey, N.A. Sanford, A. Roshko, Opt. Lett. 19 (1994) (1849). [9] J.J. Zayhowski, Opt. Mater. 11 (1999) 255–267. [10] D. Heathley, G. Vitrant, A. Kevorkian, Opt. Quantum Elect. 26 (1994). [11] K.H. Drexhage, G.A. Reynolds, Opt. Commun. 10 (1) (1974) 18. [12] A.E. Siegman, Lasers (1990).
Hybrid Neodymium-doped passively Q-switched waveguide laser Rafael Salas-Montiel, Lionel Bastard ∗ , Gr´egory Grosa, Jean-Emmanuel Broquin IMEP-LAHC, 3 Parvis Louis N´eel BP 257, 38016 Grenoble Cedex 1, France Received 31 October 2007; accepted 8 November 2007
Abstract In the mid 80s, the doping of optical fibers’ core with rare earth atoms has been a major breakthrough in the field of optical telecommunications since it allowed the realization of in line optical amplifiers. However, erbium-doped fiber amplifiers are a few meters long and a huge effort has been made in order to realize compact and efficient active devices based on rare-earth-doped waveguides. For this purpose the use of phosphate glasses instead of silicate ones has been investigated because they allow a better solubility of the inserted rare earths. In this paper we present the realization of a hybrid Neodymium-doped passively Q-switched waveguide laser made by an ion-exchange on a Schott IOG-1 phosphate laser glass combined with the deposition of a bis(4-dimethylaminodithiobenzil)nickel (BDN) saturable absorber diluted in a cellulose acetate polymer cladding. In a first step, we present the continuous wave (CW) operation of the laser with an undoped cladding. We show that for a 3.5 m wide, 1.5 cm long waveguide realized by a silver–sodium ion-exchange, a 6 mW output has been achieved by creating a Fabry–Perot cavity with dielectric multilayers mirrors sticked to the chip facets. Then, the characterizations performed on the BDN-doped layers are presented. It is shown that a proper selection of the hybrid guiding condition and saturable absorber concentration entail a non-saturated excess absorption of 3.4 dB/cm. Finally, we present the results we obtained on the Q-switched behaviour of the laser. Indeed a repetition rate of 330 kHz is achieved for a pulse energy of 10 nJ and a peak power of 1 W. © 2007 Elsevier B.V. All rights reserved. Keywords: Glass integrated optics; Ion-exchange; Q-switch laser
1. Introduction Ion-exchange on glass has been widely used for the realization of passive integrated optics devices since the demonstration of the first optical waveguide by Izawa and Nakagome [1]. However, the use of rare-earth-doped glasses has also allowed obtaining optical amplifiers [2,3] and lasers [4]. Based on the use of Er3+ or Nd3+ ions for emissions around λ = 1.5 m and λ = 1.06 m, these ion-exchanged glass integrated lasers have been demonstrated with the main types of feed-back which are the Fabry–Perot cavity [4], the distributed Bragg reflector (DBR) configuration [5] and the distributed feed-back (DFB) one [6]. Nonetheless, except one report by Sanford et al. [7] on a Ybranch laser and the use of a Neodymium-doped ion-exchanged waveguides as active medium into a bulk optic Q-switched laser [8], no integrated pulsed lasers have been investigated with the ion-exchange technology on glass. Integrated pulsed lasers could however present an interesting alternative to microchip lasers
∗
Corresponding author. Tel.: +33 4 56 52 95 30. E-mail address: [email protected] (L. Bastard).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.11.012
[9], which are now commonly used for drilling, marking or nonlinear optic applications. Indeed, microchip devices are usually realized by bonding to a rod of a rare-earth-doped crystal, a saturable absorber and two dielectric mirrors and their transverse single mode operation is only ensured by the thermal lens that is created when the laser is pumped with a high power laser diode. On the opposite, the transverse single mode operation of an ionexchanged integrated laser is imposed by the waveguide and therefore intrinsically stable. Moreover, the possibility of realizing DFB structure can guaranty a single frequency operation without mode-hopping. In this article, we report the realization of a passively Qswitched laser based on the hybridization of a polymer doped with a saturable absorber dye on an Nd3+ -doped ion-exchanged waveguide amplifier. Section 2 is devoted to the presentation of the hybrid laser principle of operation whereas Section 3 deals with the realization of the ion-exchanged waveguide amplifiers and their characterization and use as a continuous wave (CW) laser. Finally, Section 4 presents the hybridization of the CW laser with the saturable absorber-doped polymer cladding as well as the characterizations that have been carried-out on the soobtained passively Q-switch laser.
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2. Hybrid Q-switched laser principle of operation A Q-switch laser is composed of an amplifying medium providing the gain to the optical oscillator, an optical resonator to provide the feed-back and a saturable absorber which role is to periodically damp the quality factor of the cavity yielding hence to a pulsed operation of the laser. In our case, the amplifying medium is a IOG1 phosphate glass provided by Schott and doped with 1.5 × 1026 m−3 Nd3+ ions, in which an ion-exchange is carried-out in order to create a single mode Neodymiumdoped waveguide amplifier (NdDWA). The saturable absorber is a cellulose acetate polymer film which has been doped by a bis(4-dimethylaminodithiobenzil)nickel (BDN) dye. The polymer is deposited on the glass substrate entailing a distributed interaction of the guided mode with the BDN molecules within the laser cavity, which is constituted by two multilayers dielectric mirrors stuck to the waveguide facets. The overall structure of the hybrid laser is sketched in Fig. 1. The operation of a Q-switched pulsed laser is intrinsically governed by the competition between the intra-cavity losses created by the saturable absorber and the gain provided by the optical amplifier. In bulk optics or microchip Q-switched laser, gain and saturable loss media are cascaded in such a way that setting their respective length sets their respective influence. For hybrid laser, the polymer containing the absorbing dye has a refractive index of 1.475 at λ = 1.064 m, which is lower than the one of the IOG1 (1.515 at λ = 1.064 m). It behaves therefore as a cladding for the NdDWA and the guided mode that is amplified within the waveguide core and substrate also interacts evanescently with the BDN. Fig. 2 depicts a typical simulation of the guided mode of such a structure where its interaction with the polymer cladding can clearly be seen. Calculations have been carried-out with a scalar mode solver [10] coupled with an ion-diffusion solver. The interaction coefficient is defined as the normalized overlap of the guided mode with the cladding by the following relation: 2 cladding |E(x, y)| dx dy Γ = (1) |E(x, y)|2 dx dy where E(x, y) is the guided mode electric field. Γ can easily be set from 1% to 10% by varying the waveguide characteristics through the ion-exchange time and diffusion aperture.
Fig. 1. Schematic representation of the hybrid Q-switched integrated laser.
Fig. 2. Calculated mode intensity distribution of an hybrid waveguide at λ = 1.064 m.
3. NdDWA realization, characterization and CW laser operation The waveguide has been realized using the IOG1 phosphate glass following the standard process described in [6] to deposit and pattern an alumina mask on its surface before realizing a 4 min. Ion-exchange at 330 ◦ C in a 20%AgNO3 –80%NaNO3 salt melt through 1 m wide diffusion apertures. After dicing and polishing of the waveguide facets, passive and active characterizations have been performed. First the mode profiles of the waveguide have been measured at pump and signal wavelength, respectively λ = 0.8 m and λ = 1.054 m, by injecting light through a single mode optical fiber and imaging the waveguide output on an InGaAs CCD camera. From the images depicted in Fig. 3, mode dimensions of 8 ± 0.2 m by 5.1 ± 0.2 m and 5.5 ± 0.2 m by 4.5 ± 0.2 m have been measured at λ = 1.015 m and λ = 0.8 m, respectively. Replacing the camera by a calibrated detector, propagation losses of 1.2 ± 0.3 dB/cm have been determined. This relatively quite high value is mainly due to a deterioration of the waveguide surface quality during the mask patterning process. Although passive characterizations have not been carried-out at λ = 1.054 m, it can be considered that the results obtained at λ = 1.015 m describes well the behaviour of the waveguide at the signal wavelength since their difference is small and the mode propagation properties do not depend strongly on wavelength when far from cut-off.
Fig. 3. Near field mode intensity pictures: (a) at λ = 1.015 m and (b) at λ = 0.8 m.
R. Salas-Montiel et al. / Materials Science and Engineering B 149 (2008) 181–184
Fig. 4. Laser output power as a function of launched pump power for a cavity with 99% and 85% reflectivity mirrors. Inset presents the laser emission spectrum.
In order to determine the gain of the 1.4 cm long NdDWA, it has been used to realize CW lasers with different sets of dielectric mirrors stuck to its facets in order to create a cavity. Knowing the mirror transmission and the propagation losses, the measurement of the laser threshold allows hence determining the waveguide small signal gain since at this point it matches the laser cavity losses. CW laser operation has been achieved with sets of mirrors having reflectivities of 99%, 85% and 4%. The maximum gain measured was 7 ± 0.3 dB for a 290 mW launched pump power, which corresponds to a laser cavity composed of a 4% (i.e. Fresnel reflection on the waveguide facet) and a 99% reflectivity mirrors. Fig. 4 presents the evolution of the power emitted by the laser that was realized with a 99% reflectivity mirror on one facet and a 85% reflectivity mirror on the other one. The pump threshold is 60 ± 5 mW, the slope efficiency is 4.5% and a maximum output power of more than 6 mW has been obtained for a launched pump power of 200 mW. These results prove that the NdDWA that we have realized allows obtaining laser emission and they can serve as a starting point to realize passively Q-switched lasers.
183
increase the cavity losses, it has been decided to take some margin and thus reduce the BDN concentration to 3 × 1016 cm−3 . Although it might limit the laser pulse energy, a reduction of the saturable absorber excess loss ensures indeed that the 7 dB maximum gain of the NdDWA is high enough to compensate both the cavity losses, including the ones due to potential mirrors misalignments, and the BDN absorption. After having implemented the new concentration polymer cladding, the sticking of two mirrors with reflectivity of 99% and 85%, respectively completed the realization process of the hybrid Q-switch laser. The device has been pumped with a Ti–W laser emitting at λ = 808 nm while its output was monitored by a 25 GHz bandwidth photodetector coupled to a 6 GHz digital oscilloscope. Fig. 5 displays the trace of two pulses that have been measured for a launched pump power of 160 mW. It can be seen that the repetition rate of the laser is 330 kHz. This value can be changed by modifying the pump power since the more gain is created in the waveguide, the less time is needed to saturate the BDN molecules. With this sample, repetition rates ranging from 65 kHz to 350 kHz have been observed. The temporal shape of laser pulses has also been measured and is presented in Fig. 6. The pulse width is 10 ± 1 ns and does not depend on the pump power, which is in agreement the theoretical behaviour of passively Q-switched lasers [12]. Measuring the average power emitted by the laser to be 0.5 mW for a launched pump power of 160 mW a pulse energy of 10 nJ has been derived corresponding to a pulse peak power of 1 W. Although all this results can be dramatically improved by reducing the waveguide propagation losses, optimizing the BDN concentration or integrating the mirrors on the chip, it must be noticed that the values we have obtained confirms the interest of the integrated optic approach since they are similar or better than the ones published by Aust et al. [8] with the same active medium and saturable absorber elements but using a nonintegrated extended cavity scheme.
4. Hybrid Q-switch laser realization and characterization In order to implement the saturable absorber on the NdDWA, BDN molecules have been first dissolved into acetone then incorporated into a cellulose acetate–acetone solution. After partial evaporation of the solvent, the obtained BDN-doped cellulose acetate has been deposited as a thick film on the waveguide surface where it has finished drying creating a solid thick film of roughly 100 m with a BDN concentration of 1.2 × 1017 cm−3 . The hybrid waveguide transmission has been measured and the excess loss due to the saturable absorber have been determined to be 3.4 ± 0.2 dB/cm which gives an interaction coefficient of Γ = 5% assuming that the absorption cross-section of BDN at λ = 1.054 m is 1.3 × 10−16 cm2 [11] and remains unchanged when incorporated in the polymer matrix. Nonetheless, because the impact of the polymer cladding presence on the mirror positioning accuracy was a major unknown that could dramatically
Fig. 5. Laser pulse train measured on a digital oscilloscope for a launched pump power of 240 mW.
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R. Salas-Montiel et al. / Materials Science and Engineering B 149 (2008) 181–184
cellulose acetate as a host for BDN saturable absorber molecules has allowed realizing an integrated optic Q-switched laser with a repetition rate ranging from 65 kHz to 350 kHz, a pulse width of 10 ± 1 ns, a pulse energy of 10 nJ with a 1 W peak power. This first demonstration can be dramatically improved by reducing the waveguide propagation losses, optimizing the BDN concentration and integrating the cavity mirrors on the chip. However, it must be noticed that the characteristics of our integrated Q-switch laser made by ion-exchange on glass are at least equivalent to the ones of free space devices using the same active material and saturable absorber. References
Fig. 6. Emitted laser pulse for a launched pump power of 160 mW.
5. Conclusion In this article, the realization and characterization of an integrated passively Q-switch laser made by ion-exchange on glass and hybridization of a doped polymer has been presented. After an explanation of the device principle of operation, the realization of Neodymium-doped optical amplifiers made by a silver–sodium ion-exchange on glass has been presented. The 1.4 cm long NdDWA has a maximum gain of 7 ± 0.3 dB for a launched pump power of 290 mW and it has been successfully used to realize a CW laser emitting more than 6 mW of output power for a launched pump power of 200 mW. The use of
[1] T. Izawa, H. Nakagome, Appl. Phys. Lett. 21 (1972) 584. [2] J.M.P. Delavaux, S. Granlund, O. Mizuhara, L.D. Tzeng, D. Barbier, M. Rattay, F. Saint-Andr´e, A. Kevorkian, IEEE Photon. Technol. Lett. 9 (2) (1997) 247–249. [3] F. Gardillou, L. Bastard, J.-E. Broquin, Appl. Phys. Lett. 85 (2004) 5176. [4] A. Sanford, K.J. Malone, D.R. Larson, Opt. Lett. 15 (1990) 366. [5] D.L. Vasey, D.S. Funk, N.A. Sanford, Appl. Phys. Lett. 74 (6) (1999) 789–791. [6] L. Bastard, S. Blaize, J.-E. Broquin, Opt. Eng. 42 (10) (2003) 2800–2804. [7] N.A. Sanford, K.J. Malone, D.R. Larson, R.K. Hickernell, Opt. Lett. 16 (1991) 1168. [8] J.A. Aust, K.J. Malone, D.L. Veasey, N.A. Sanford, A. Roshko, Opt. Lett. 19 (1994) (1849). [9] J.J. Zayhowski, Opt. Mater. 11 (1999) 255–267. [10] D. Heathley, G. Vitrant, A. Kevorkian, Opt. Quantum Elect. 26 (1994). [11] K.H. Drexhage, G.A. Reynolds, Opt. Commun. 10 (1) (1974) 18. [12] A.E. Siegman, Lasers (1990).