Efficient Er:YAG lasers at nm, resonantly pumped with narrow bandwidth diode laser modules at 1532 nm, for methane detection

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1 Efficient Er:YAG lasers at nm, resonantly pumped with narrow bandwidth diode laser modules at 1532 nm, for methane detection H. Fritsche* a, O. Lux a, C. Schuett a, S. Heinemann b, W. Gries b, H. J. Eichler a a Institute for Optics and Atomic Physics, Technische Universität von Berlin, Berlin ; b DirectPhotonics Industries GmbH, Max-Planck-Str. 3, Berlin Germany ABSTRACT Eye safe laser operation at nm (6077 cm -1 ) of resonantly pumped Er:YAG laser systems is demonstrated in cw and Q-switched operation. High brightness diode laser modules emitting at 1532 nm have been utilized as pump sources providing an absorption efficiency of up to 96%. This leads to an overall efficiency of the Er:YAG laser of 30%. For cw operation, 9 W output power is possible at pump power of 30 W while Q-switching results in generation of more than 7 mj pulses with duration of 60 ns and repetition rate of 500 Hz. The Er:YAG laser systems have been applied for methane detection measurements demonstrating their feasibility for CH 4 -DIAL applications Keywords: resonant pumping, Er:YAG, eye safe, narrow linewidth diode laser, high brightness pump source, methane detection 1. INTRODUCTION Resonant pumping of Er:YAG is rapidly emerging as one promising way to generate high power room temperature lasers operating around 1.6 µm. The resonant pumping process of Er:YAG involves pump sources emitting at 1455 nm, 1470 nm or 1532 nm wavelength. Apart from fiber lasers, high brightness diode lasers are suitable in order to realize Er:YAG lasers with high quantum efficiency. In addition, for long propagation in free space with potential for human exposure, both high eye safety and low absorption rates are crucial. For erbium-doped materials lasing at 2.9 µm, it is very difficult to achieve high pump efficiencies from the 4 I 15/2 4 I 11/2 transition due to competing up conversion processes. The resonant pumping process is characterized by high quantum efficiency, which can be achieved at pump wavelengths around 1.5 µm. A quasi-two level laser process involving the 4 I 13/2 4 I 15/2 levels lead to laser radiation at 1617 nm or 1645 nm..fovw áw Energy [cm-1] yntntppt VOppD00 0J aoñ tv0000 tv0 1532nm 1469nm nm 975nm 1645nm t 1617nm 0000go NNWWpp ÑVWiVWN Fig. 1 Laser levels of Er:YAG laser with lasing wavelength at 1.6 µm and 2.9 µm *fritsche@physik.tu-berlin.de; phone: ; fax: ; Solid State Lasers XXII: Technology and Devices, edited by W. Andrew Clarkson, Ramesh Shori, Proc. of SPIE Vol. 8599, 85990G 2013 SPIE CCC code: X/13/$18 doi: / Proc. of SPIE Vol G-1

2 These two transitions are pumped with the same wavelength and compete with each other. For wavelength selection an etalon or volume Bragg grating can be used, exploitation of the gain cross-section is another option. The gain-cross section progresses differently for both wavelengths whereby 1617 nm becomes the dominant wavelength at an inversion parameter of about 0.3[1]. By varying the losses within the resonator the lasing wavelength can be adjusted which allows detection of CO 2 at 1617 nm and methane (CH 4 ) at 1645 nm with the same setup within a lidar system. This laser process is only affected by up conversion and excited state absorption (ESA), which can be observed by green light emission of the crystal. Because of that, the erbium concentration has to be very low (below 1 at%). Another benefit of Er:YAG is the long lifetime of the upper level which is about 7 ms. Due to that long lifetime Er:YAG is suitable for cwpumping to achieve high energy in q-switched operation instead of using a pulsed pump source as it is common. Already in 1972, K.White and S.Schleusener suggested Er:YAG lasers at 1.6 µm for methane detection[2], however resonant pumping was not yet possible with GaAs-based laser. With the development of high power InP-based laser diodes around 1.5 µm, a new way for pumping erbium lasers has now become available. There are now several diode lasers available at wavelength from 1450 nm up to 1532 nm which can be used for resonant pumping whereas the pumping efficiency is increased with the use of longer pump wavelength, on the other hand, the pump levels become more narrow with longer wavelength. 2.1 Narrow bandwidth diodes lasers as pump source 2. EXPERIMENT For the resonant pumping process a very stable narrow bandwidth pump source is needed to achieve high pump efficiencies. Common laser diode spectra show a FWHM of more than 5 nm and a center wavelength shift, depending on pump current and diode temperature. For most classic solid state lasers this is good enough, but for the resonantly pumped Er:YAG lasers that means a loss of pump power due to the narrow pump levels of the Er:YAG. All the power, which is not deponated in the pump levels is lost through up-conversion, ESA and heat. In order to increase the pump efficiency it is also possible to pump with 1532 nm instead of 1455 nm because of the higher quantum efficiency. On the other hand the 1532 nm band is narrower and a pump source with a very stable center wavelength and narrow bandwidth is needed for effective pumping. In the ultra-high brightness diode laser modules from DirectPhotonics Industries GmbH (DPI) a new way of building up high brightness power diode lasers with narrow bandwidth has been realized and tested for resonantly pumped Er:YAG lasers. 12 single emitter broad strip laser diodes are stacked in a staircase arrangement. Each single emitter has a single fast axis collimation lens which is individually aligned for achieving best possible brightness. These single emitter laser diodes are then optically stacked via a slow axis collimator (SAC). This SAC consists of a monolithic copper block with 12 curved facets forming all laser beams. Polarization multiplexing is deployed prior to focusing into a 100 µm fiber. By using this free space collimation, the beam profile is only limited by the size of the emitting area of the diodes. Currently, the modules consist of 100 µm broad stripe diodes, so that coupling into a 100 µm fiber, 0.15 NA can be achieved, which results in a beam parameter product of 7.5 mm mrad.[3] 1,0 = 1532,33 nm 0,8 C 0,6 0,4 FWHM = 0.17 nm 0,2 0, wavelength / nm Fig. 2 Spectrum of a DirectPhotonics DirectPump module at Proc. of SPIE Vol G-2

3 Wavelength stabilization is another key feature and is realized by one single volume Bragg grating (VBG) located behind the SAC which provides an external feedback to all single emitter lasers at the same time by coupling a small percentage back into the diodes. This way all emitters are locked to one specific wavelength (see Fig. 2). For the experiments we used a DPI DirectPump P LEF laser module. That laser emitted at a wavelength of and FWHM of 0.17 nm. Long term measurements showed a frequency stability of about 230 MHz over one hour (see Fig. 3 left). Because of the VBG wavelength stabilization it needs only to be passively water cooled, the wavelength is nearly independent on current changes and measured to only 0.01 nm / W (see Fig. 3 right) mean wavelength: nm frequency stability: 230 MHz linewidth: 0.17 nm shift of central wavelength = 0,01 nm / W E c wavelength E Ç rn ca1 t L J linewidth _ 200 > , m 3 _ -- f-g r I 1 I I I time / min output power / W Fig. 3 long term stability of DPI DirectPump was observed with 230 MHz (left side) and center wavelength shift by increasing output power (right side) with only 0.01 nm / W 2.2 Er:YAG lasers operating at 1645 nm For first experiments with the narrow bandwidth pumping we used a 12 W DPI laser module for pumping a 0.5 at% doped 30 mm long Er:YAG crystal in a short linear cavity with a curved in coupling mirror (radius of curvature R = mm). We observed an output power of 2.5 W at a pump power of 12.5 W while the laser threshold was observed to be 5.5 W. The slope efficiency was highest, η = 42%, with out coupling mirror reflectivity around 80% while the whole resonator has an overall length of only 10 cm. Another benefit of lower doped Er:YAG crystals is the absence of saturation effects within the possible pump power range. The output wavelength was measured to be 1646 nm, linewidth 0.02 nm (2.2 GHz) and the wavelength stability 113 MHz, respectively (see Fig. 4). 1646, ,040 - = linewidth < 20 pm 1646,035 - Su = 113 MHz 1646, , , , r time / s Fig. 4 wavelength stability measurement of the free running 0.5 at% Er:YAG laser By utilizing an out coupling mirror which was only one side coated it acts already as etalon and prevented lasing at 1617 nm even with resonator losses over 0.3. Independent of the out coupling reflectivity the output wavelength shifted stable to nm. For generating 1617 nm lasing output, different techniques of wavelength tuning are required. Proc. of SPIE Vol G-3

4 --Moc=93%;n:17% --Moc=85%; : 42% Alec =75%;n:42% Moc = 60 % ; n:18% Fig. 5 : setup of the linear resonator with an overall length of only 100 mm; right: resulting output power at 1645 nm In order to achieve pulsed mode operation we extended the resonator while preserving the pump beam volume inside the crystal. For that we built up a L-shaped resonator and extended the radius of curvature to R = mm. The slope efficiency is η = 38 % and the laser threshold was increased. The longer resonator design offered the possibility for implementing a q-switch and wavelength control elements by conserved performance. Fig. 6 : setup of the L-shaped resonator with an overall length of 200 mm; right: resulting output power at 1645 nm The long upper level life time of approx. 7 ms makes Er:YAG laser very suitable for q-switched mode operation with cw pump sources. We used an Alphalas LiNbO 3 pockels cell in Brewster cut for q-switching, placed in in front of the curved mirror in an L-shaped resonator (see Fig. 7 left). A maximum energy of 6.7 mj and a pulse width of 60 ns could be observed (see Fig. 7 right) at a pump power of 11.5 W. The repetition rate can be set up to 500 Hz without any changes in the resulting pulse width. Implementation of the pockels cell leads to additional losses and a wavelength shift to nm occurred nm R = mm Pump Laser Q-Switch f, f, Er:YAG I 1645 nm> Mc Moc 7 E_ arni a> m, t- o. f =50Hz T=60ns r /7/ i pump power / W Fig. 7 left: setup of the L-shaped resonator for cw and q-switched operation; right: resulting output energy in q-switched mode using a pockels cell. Proc. of SPIE Vol G-4

5 This setup is very close to the specifications needed for methane detection [4] and LIDAR applications. Methane shows strong absorption lines at cm -1 ( nm) and cm -1 ( nm)[4] (see Fig. 10). For tuning the wavelength to one of these lines an etalon was inserted in the cavity. By rotating the etalon we could tune the laser wavelength between cm -1 and cm -1, even cm -1 ( nm), which coincides with strong CO 2 absorption lines, could be realized. Using a etalon with 3 mm thickness increased the linewidth to 0.01 cm -1 (0.002 nm) and frequency stability to 39 MHz (see Fig. 8 left) Another small wavelength shift can be realized by temperature control of the Er:YAG crystal (see Fig 8 right). These two features together allow a very fast, stable and precise wavelength switching which is very use full for DIAL applications. Fig. 8 left: by utilizing an etalon the linewidth was increases to less than 2 pm while the long term wavelength stability increased to less than 40 MHz. right: by additional control of the of the Er:YAG crystal temperature the output wavelength can be adjusted in the pm region. 2.3 Application The actual approach for space born methane detection mission aims at the development of injection-seeded optical parametric oscillator/amplifier (OPO/OPA) systems which are pumped by high power Nd:YAG master oscillator power amplifier systems [5]. Due to the frequency conversion and amplification steps, these systems are rather complex and sensitive which complicates their application on satellites. Compared to that an Er:YAG laser is much more compact and easier to handle. The potential of the developed cw and pulsed Er:YAG laser sources in terms of methane LIDAR applications was evaluated by absorption measurements which were performed with the setup depicted in Fig. 9. this purpose, the collimated output radiation of the Er:YAG laser is separated into three portions by two beam splitters (BS).While the first part is guided to an InGaAs photodiode (PD1) which measures a reference intensity, the second part is coupled into a wavemeter to monitor the exact radiation frequency. The third beam portion is focused into a multi-pass absorption cell (New Focus, model 5612) by a biconvex lens with a focal length of 500 mm. A detailed description is given in [6]. wavemeter compact piezo gauge 0 Pump Laser 1532 nm vacuum system (p 2 10'6 mbar) Fig. 9 Schematic diagram of the experimental setup for methane absorption measurements Proc. of SPIE Vol G-5

6 The optical distance between the first and second photo diode was measured to be 170 ns, so this setup could be easily aligned with the 60 ns short pulses of the Er:YAG laser. After successful alignment also cw-measurments could be done. The multi-pass cell is equipped with two gas outlets. While one is connected to a compact piezo gauge (Balzers AG, model APR 260) to measure the internal pressure of the cell, the other one is attached to a CH 4 reservoir as well as a vacuum system to control an internal pressure in the low vacuum range (~10-1 mbar). In the wavelength region of 1.6 µm we can find several absorption lines we can reach with the tuned Er:YAG lasers (see fig. 10). Fig. 10 simulation of transmission spectra of methane around nm for room temperature, plotted for various pressures.... (grey curves) based,... on the HITRAN database[7]. For the experiment we evacuated the multi-pass cell in the first step and afterwards applied low methane pressures up to 300 mbar and measured the intensity of the transmitted signal while monitoring the wavelength using a wavemeter (HighFinesse-Ångstrom, model WS6-IR). Without setup we can use two different types of measurements, one is scanning the methane absorption line at constant pressure. For higher pressures we can observe, according to the theory, the reduction of the transmittance through the multi pass cell while tuning the wavelength of the Er:YAG laser over a methane absorption line. Here is the feasibility demonstrated at a constant pressure of about 30 mbar methane and a strong but small absorption line at nm (see fig. 11, compare fig. 10). That absorption line is very narrow but also the broad absorption lines at longer wavelength can already be observed. 1,00-0,95-0,90 - E. 0,85-0,80-0,75-0,70-0,65-0, ,1 6078,2 6078,3 6078,4 6078,5 6078,6 wavenumbers / cm Fig. 11 measurement of a fast scan over a methane absorption line, at a methane pressure of about 30 mbar Proc. of SPIE Vol G-6

7 Another measurement is to maintain a stable Er:YAG wavelength and observe the absorption change by varying the methane pressure inside the cell. In our setup pressures from 1 mbar (full transmission) to 350 mbar (zero transmission at an absorption line) was successfully realized. These types of measurements do not only show the presence of methane but also concentrations at different distances can be monitored. 3. CONCULUSION AND OUTLOOK. We developed cw as well as Q-switched Er:YAG lasers pumped by fiber-coupled, wavelength-stabilized diode laser modules provided by DirectPhotonics. The high power pump modules feature narrowband emission at 1532 nm with a linewidth of only 0.17 nm. At a pump power of 12.5 W we measured an output power of the Er:YAG System of 2.5 W, with the new 30 W pump module with increased wavelength stability we can measure the absorption efficiency to be 96% which allow a cw output power of the Er:YAG laser for more than 9 W, corresponding to an overall efficiency of 30%. Utilization of an electro-optic modulator enables pulsed operation at a repetition rate of 500 Hz with pulse energy of about 6.7 mj and pulse duration of 60 ns. Due to the high wavelength stability of the pump lasers, the free-running Er:YAG laser shows stable operation at nm with long-term fluctuations of less than 113 MHz while wavelength tuning can be obtained by employing intra-cavity etalons which also increases wavelength fluctuations to less than 39 MHz and linewidth of only 2 pm. The developed Er:YAG laser were applied for methane absorption measurements using a multi-pass absorption cell. Good agreement of the experimental results with theoretical simulations of the absorption characteristics demonstrated the potential of the developed Er:YAG lasers in terms of methane detection at low and high gas pressures, in the next step atomospherical DIAL experiments will follow. REFERENCES [1] Kudryashow, I., Ter-Gabrielyan, N. and Bubinsjii, M., Resonantly diode-pumped Er:YAG laser: 1470-nm vs nm CW pumping case, Proc. SPIE 7325, (2009) [2] White, K.O. and Schleusener, S.A., Coincidence of Er : YAG laser emission with methane absorption at nm, Appl. Phys. Lett. 21, 419 (1972) [3] Heinemann, S., Lewis, B., Regard, B. and Schmidt, T., Single emitter based diode lasers with high brightness high power and narrow linewidth, Proc. SPIE 7918, 79180M (February 21, 2011) [4] Kiemle, C., Quatrevalet, M., Ehret, G., Amediek, A., Fix, A. and Wirth, M., Sensitivity studies for a space-based methane lidar mission, Atmos. Meas. Tech. 4, (2011) [5] Fix, a., Büdenbender, C., Wirth, M., Quatrevalet, M., Amediek, A., Kiemle, C. and Ehret, G., Optical parametric oscillators and amplifiers for airborne and spaceborne active remote sensing for CO2 and CH4, Proc. SPIE 8182, (2011) [6] McManus, J., Kebabian, P. and Zahniser, M., Astigmatic mirror multipass absorption cells for longpass-length spectroscopy, Appl. Opt. 34, (1995) [7] Rothman, L., Gordon, I., Barbe, A., Benner, D. C., Bernath, P., Birk, M., Boudon, V., Brown, L., Campargue, A., Champion, J.-P., Chance, K., Coudert, L., Dana, V., Devi, V. M., Fally, S., Flaud, J.-M., Gamache, R., Goldman, A., Jacquemart, D., Kleiner, I., Lacome, N., Lafferty, W., Mandin, J.-Y., Massie, S., Mikhailenko, S., Miller, C., Moazzen-Ahmadi, N., Naumenko, O., Nikitin, A., Orphal, J., Perevalov, V., Perrin, A., Predoi- Cross, A., Rinsland, C., Rotger, M., Šimečková, M., Smith, M., Sung, K., Tashkun, S., Tennyson, J., Toth, R., Vandaele, A. and Auwera, J. V. et- al., The HITRAN 2008 molecular spectroscopic database, Journal of Quantitative Spectroscopy & Radiative Transfer 110 (9-10), (2009). Proc. of SPIE Vol G-7