Bollig, C.

Collett, O. J. P.

Esser, Daniel

Jacobs, C.

Koen, W.

Preussler, D.

Strauss, H. J.

English

H. J. Strauss

D. Preussler

M. J. D. Esser

W. Koen

C. Jacobs

O. J. P. Collett

C. Bollig

Crossref

Optics Letters

330 mJ single-frequency Ho:YLF slab amplifier

We report on a double-pass Ho:YLF slab amplifier which delivered 350 ns long single-frequency pulses of up to 330 mJ at 2064 nm, with a maximum M-2 of 1.5 at 50 Hz. It was end pumped with a diode-pumped Tm:YLF slab laser and seeded with up to 50 mJ of single-frequency pulses. (C) 2013 Optical Society of America</p

Heriot Watt Pure

    Heriot-Watt University Research Gateway                                                          Heriot-Watt University330 mJ single-frequency Ho:YLF slab amplifierStrauss, H. J.; Preussler, D.; Koen, W.; Esser, Matthew Jan Daniel; Jacobs, C.; Collett, O. J.P.; Bollig, C.Published in:Optics LettersDOI:10.1364/OL.38.001022Publication date:2013Link to publication in Heriot-Watt Research GatewayCitation for published version (APA):Strauss, H. J., Preussler, D., Koen, W., Esser, D., Jacobs, C., Collett, O. J. P., & Bollig, C. (2013). 330 mJsingle-frequency Ho:YLF slab amplifier. Optics Letters, 38(7), 1022-1024. 10.1364/OL.38.001022General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.330 mJ single-frequency Ho:YLF slab amplifierH. J. Strauss,1,* D. Preussler,1 M. J. D. Esser,1 W. Koen,1 C. Jacobs,1 O. J. P. Collett,1 and C. Bollig1,21National Laser Centre, Council for Scientific and Industrial Research, P.O. Box 395, Pretoria 0001, South Africa2Carl von Ossietzky Universität Oldenburg, Institut für Physik, AG Meeresphysik, Oldenburg D-26111, Germany*Corresponding author: hstrauss@csir.co.zaReceived December 19, 2012; revised February 8, 2013; accepted February 16, 2013;posted February 20, 2013 (Doc. ID 182054); published March 20, 2013We report on a double-pass Ho:YLF slab amplifier which delivered 350 ns long single-frequency pulses of up to330 mJ at 2064 nm, with a maximum M2 of 1.5 at 50 Hz. It was end pumped with a diode-pumped Tm:YLF slab laserand seeded with up to 50 mJ of single-frequency pulses. © 2013 Optical Society of AmericaOCIS codes: 140.0140, 140.3070, 140.3280, 140.3570, 140.3580, 140.5680.High-energy 2 μm sources are sought after in a widevariety of applications, some of which are in defense,medicine, material processing, and remote sensing [1].Ho doped into YLiF4 (YLF) is an excellent gain mediumwith which to obtain high-energy Q-switched 2 μmpulses. This is because it has a high gain cross section,long upper level lifetime, and weak thermal lensing and isnaturally birefringent [2,3]. Recently, diffraction-limited2 μm pulses with energies up to ∼170 mJ at 100 Hz havebeen demonstrated from a Ho:YLF master oscillatorpower amplifier [4] which was pumped with three120 W Tm-fiber lasers. High average power 1.9 μmT mslab lasers can also be used as pump sources, especiallyfor amplifiers where the good beam quality of fiber lasersis not required for the pump beam.We recently demonstrated 210 mJ of single-frequency2 μm pulses using a Tm slab pumped, two crystal Ho:YLFand Ho:LuLF slab amplifier [5]. Subsequent simulations[6] indicated that using longer gain crystals and doublepassing the amplified beam would lead to a significantincrease in gain.Consequently we used two 0.5% doped, Ho:YLF slabsfrom VLOC in series. Both had dimensions 10 mm ×2 mm × 50 mm (c × a × a) and were a-cut with theirc-axis horizontal. The slabs were mounted with indiumfoil between custom-made, 22°C, water-cooled mounts.The experimental setup is illustrated in Fig. 1. The slabswere placed in series to provide a 200 mm gain lengthwhen double passed. As pump source, we used aTm:YLF slab oscillator [5] delivering 187 W of outputpower. It was forced to operate on the pi-polarization byinserting a Brewster plate. Its wavelength was 1890 nm,which matched one of the strong absorption peaks ofHo:YLF. The beam had an M2x of 603 and an M2y of2.78. The polarization was kept horizontal so that it wasabsorbed by the stronger absorbing pi-polarization ofHo:YLF. Its output power was monitored by a Gentecpower meter (PS-310WB) placed behind a 99% mirror(M2). The pump beam was then steered into the Ho:YLFamplifier crystals using a high reflector (HR) at 1.9 μm.To shape the beam, it was first collimated with an f 100 mm spherical lens (L1) and focussed into theHo:YLF slabs with an f  60 mm spherical lens (L3).An f  250 mm horizontal cylindrical lens (L2) was usedto match the horizontal focus position after L3 to that ofthe vertical. The Ho:YLF crystals were placed so that thepump focus was positioned in the gap between them,accounting for the refractive index of YLF. The 1890 nmpump beam radii were measured with a pyroelectriccamera (PY-III-CA from Spiricon) as 3.5 mm × 0.4 mmon the inner facing surfaces of the slabs and 4.5 mm ×0.45 mm (wx × wy) on the outer surfaces using the sec-ond moment method and had the focus intensity profileshown in Fig. 1. The transmitted pump power was mea-sured with a power meter (LM 1000 HTD from Coherent)and the amplifier and pump laser were both enclosed in adry air box to avoid spiking in the Tm:YLF laser outputcaused by water absorption.The seed laser for the amplifier was a single-frequency,ring laser/amplifier system, pumped with a commercial80 W 1940 nm Tm:fiber laser [7,8]. It delivered diffraction-limited output with energies of up to 70 mJ per pulse at50 Hz, and had a pulse duration of 370 ns. The seed laserbeam power was monitored by power meter Pm2 (LM 10HTD from Coherent) placed behind a 95% partial reflec-tor (M4). The seed beam was then steered into theHo:YLF amplifier crystals using various HRs at 2 μm.In the vertical direction, two cylindrical lenses L4,(f y  774 mm) and L5,( f y  504 mm) were used to fo-cus the seed beam through the Ho:YLF crystals onto aFig. 1. (Color online) Experimental setup of the double-passHo:YLF amplifier.1022 OPTICS LETTERS / Vol. 38, No. 7 / April 1, 20130146-9592/13/071022-03$15.00/0 © 2013 Optical Society of Americanormal incidence, HR at 2 μm, high transmission (HT) at1890nm mirror (M3),which reflected it back for a secondpass at a slight horizontal angle to the incident beam di-rection. The vertical second moment beam radii of theseed beam in the crystals were 0.38 mm on the first crys-tal surface it encountered and 0.34 mm on the crystal sur-face closest to M3, with similar beam sizes on the returnpath. In the horizontal plane, a single cylindrical lens L6(f x  2133 mm) was used to collimate the seed beam toa radius of 3.3 mm. The second pass of the amplifiedbeam was almost spatially symmetrical to the incidentbeam and passed through cylindrical lenses L6 and L5before being separated by mirror M5 (0° HR at 2 μm)from the seed laser beam. The seed polarization was keptvertical (perpendicular to the crystalline c-axis) so that itwas amplified on the sigma-polarization of the Ho:YLFslabs, which has intrinsic weak thermal lensing [9], atthe cost of lower gain due to its lower emission crosssection comparedto the pi-polarization. Optical feedbackinto the master ring oscillator/amplifier was prevented byinserting an optical isolator (IO-4-2050-HP from OFR)after the ring oscillator. This reduced the available seedenergy incident on the amplifier gain crystals to ∼50 mJat 50 Hz.Various HRs and a partial reflector at 2 μm( M6)r e -flected the amplified beam onto power meter Pm4(LM-45 HTD from Coherent), which measured the aver-age amplified power. The beam contained a continuouswave component which originated from the injectionseeding of the Ho:YLF ring laser which was also ampli-fied. This component had a maximum value of 340 mWand was characterized at various 1890 nm pump powers.These were then deducted from the total measuredaverage powers of the amplified pulses.The amplifier was first optimized at 350 Hz, where therisk of optical damage was the lowest, and the pulse rep-etition frequency (PRF) was then gradually decreased to40 Hz. The output results are given in Fig. 2.The limit where the average power does not increasewith PRF was found to be at PRFs above ∼300 Hz andwas ∼43 W. For lower PRFs, the average output powerdecreased to ∼12 W at 40 Hz. The pulse energy increasedfrom 122 mJ at 350 Hz to above 300 mJ at PRFs below50 Hz, without optical damage. The amplifier was thenoptimized at low PRFs with respect to seed and pumpalignment. At a minimum PRF of 30 Hz, maximum pulseenergy of 333 mJ was obtained. At the required PRF of50 Hz, 325 mJ was measured (at which subsequentmeasurements were performed).The optimized amplifier was subsequently character-ized with respect to 1890 nm Tm:YLF pump power andthe results are given in Fig. 3. It was estimated that upto 96% of the 1890 nm power was absorbed, droppingto 83% at maximum pump power. This indicated thatthe total effect of the Ho:YLF pump saturation was min-imal due to the long combined length of the slabs. Theoutput energy increased linearly from low levels andslightly rolled over to the maximum output energy of325 mJ. The slight roll over was attributed to a small in-crease in beam diameter of the 1890 nm pump beam withpower, which was caused by strong negative thermallensing in the Tm:YLF slab crystal.From the gain curve (amplified energy/seed energy), itcan be seen that the crystals only reached transparency(gain  1)a t∼41 W of pump power. At full pump power,the gain increased to ∼5.7.The amplifier performance as a function of the seedenergy at full pump power is shown in Fig 4. The seedenergy was varied by placing different partial reflectorsinto the incident seed beam. The gain at low seedenergies was ∼40.The amplified beam transmitted through M6 wasfocused by a f  827 mm lens combination L7 (f 522  f  −1415 mm) to create a waist which was thenused to spatially characterize the output using a pyroelec-tric camera (PYIII-CA from Spiricon). In Fig. 5, thehorizontal and vertical second moment beam radii(ISO 11146) are plotted as a functions of the distancefrom the f  827 mm lens combination. The beamquality factors in the two transverse directions weredetermined by fitting the beam radii to the Gaussianbeam propagation equation.The astigmatism was due to the fact that the amplifiedbeam was not shaped with L4 to be circular symmetric, aswell as the astigmatic thermal lens associated with theHo:YLF amplifier crystals [9]. Figure 6(a) shows the am-plified beam intensity profile at maximum output energy.It can be seen that there was a slight degradation inhorizontal beam quality. This was also reflected in an in-creased horizontal beam quality factor (M2x) of 1.5 andFig. 2. (Color online) Seed and amplified 2064 nm outputenergy (left axis) and the average power of the amplified pulses(right axis) as functions of PRF.Fig. 3. (Color online) Amplified output energy (right axis) andgain (left axis) as functions of incident 1890 nm Tm:YLF pumppower.April 1, 2013 / Vol. 38, No. 7 / OPTICS LETTERS 1023was possibly due to the variation in the horizontal inten-sity profile of the pump beam, which had a large M2 valuein this direction. In the vertical direction, the beamstayed diffraction limited and had a M2y of 1.01.It was also verified that the output was single fre-quency by only observing significant mode-beating whenthe ring laser was unlocked [10]. The amplified pulselength at full pump power was measured to be ∼350 ns.This can be seen from the amplified pulse shape that isgiven juxtaposed with that of the seed in Fig. 6(b). Thepulse to pulse energy variation was measured to be lessthan 5%. The amplified beam was subsequently used topump an HBr gas laser/amplifier system [11]. The2064 nm amplified beam was 50% absorbed over anHBr cell with a length of 510 mm (at a pressure of40 Torr). This implied that its linewidth was narrowerthan the 400 MHz linewidth of HBr [12].The output energy can still be significantly scaled. Themaximum fluence in the system was calculated to be10.9 J∕cm2, which was still below the optical damagethreshold of good quality coatings (20–45 J∕cm2)[ 2,13].The same architecture can therefore be used to scalethe energy at least 3 times higher by simply adding moreamplifier modules. However, from calculations [6]w ebelieve that slight modifications to the system in its cur-rent form can also lead to improvement. Higher gain isexpected if the 28 W of unabsorbed 1890 nm pump poweris reflected back into the amplifier crystals. Amplificationon the stronger gain, pi-polarization would also bepossible if thicker slab crystals were to be used. Itwas not possible to implement it in this setup becauseof clipping of the amplified beam due to the relativelystrong thermal lensing associated with the pi-polarizationof YLF [9].In conclusion, we have demonstrated a 2064 nm,single-frequency, Ho:YLF slab amplifier, emitting a goodquality beam with energies up to 333 mJ. The work showsthat Ho:YLF amplifiers, pumped with Tm:YLF slabs,provide an effective way to obtain high energies at2 μm, which could be scaled even higher using the samearchitecture.References1. K. Scholle, S. Lamrini, P. Koopmann, and P. Fuhrberg, inFrontiers in Guided Wave Optics and Optoelectronics,B. Pal, ed. (InTech, 2010).2. M. Eichhorn, Appl. Phys. B 93, 269 (2008).3. B. M. Walsh, N. P. Barnes, and B. Di Bartolo, J. Appl. Phys.83, 2772 (1998).4. A. Dergachev, D. Armstrong, A. Smith, T. Drake, and M.Dubois, Proc. SPIE 6875, 687507 (2008).5. H. J. Strauss, W. Koen, C. Bollig, M. J. D. Esser, C. Jacobs,O. J. P. Collett, and D. R. Preussler, Opt. Express 19, 13974(2011).6. O. J. P. Collett, C. Bollig, and M. J. D. Esser, in CLEOEUROPE and EQEC Conference Digest, OSA TechnicalDigest (CD) (Optical Society of America, 2011),paper CA_P24.7. C. Bollig, M. J. D. Esser, C. Jacobs, W. Koen, D. Preussler,K. Nyangaza, and M. Schellhorn, in Mid-InfraredCoherent Sources (European Physical Society, 2009),invited talk Mo3.8. L. R. Botha, C. Bollig, M. J. Esser, R. N. Campbell, C. Jacobs,and D. R. Preussler, Opt. Express 17, 20615 (2009).9. V. V. Zelenogorskii and E. A. Khazanov, Quantum Electron.40, 40 (2010).10. A. Dergachev, Opt. Express 19, 6797 (2011).11. W. Koen, C. Jacobs, C. Bollig, H. J. Strauss, L. Botha, andM. J. D. Esser, Proc. SPIE 8543, 85430E (2012).12. J. Nicholson, D. Neumann, and W. Rudolph, Air ForceResearch Laboratory report AFRL-DE 2001–1015 (AirForce Research Laboratory, 2001).13. M. Schellhorn and M. Eichhorn, Appl. Phys. B 109, 351(2012).Fig. 4. (Color online) Amplified output energy (right axis) andgain (left axis) as functions of seed energy at a PRF of 50 Hz.Fig. 5. (Color online) Horizontal and vertical second momentbeam radii as functions of distance from an f  827 mm lenscombination at maximum output at 50 Hz. The lines are fitsof the beam propagation equation to the measured data andthe crosses are the fitted beam waists.Fig. 6. (Color online) Amplified beam profile in the far field (a)and (b) the seed and amplified pulses at maximum output and aPRF of 50 Hz.1024 OPTICS LETTERS / Vol. 38, No. 7 / April 1, 2013

330 mJ single-frequency Ho:YLF slab amplifier

330 mJ single-frequency Ho:YLF slab amplifierH. J. Strauss,1,* D. Preussler,1 M. J. D. Esser,1 W. Koen,1 C. Jacobs,1 O. J. P. Collett,1 and C. Bollig1,21National Laser Centre, Council for Scientific and Industrial Research, P.O. Box 395, Pretoria 0001, South Africa2Carl von Ossietzky Universität Oldenburg, Institut für Physik, AG Meeresphysik, Oldenburg D-26111, Germany*Corresponding author: hstrauss@csir.co.zaReceived December 19, 2012; revised February 8, 2013; accepted February 16, 2013;posted February 20, 2013 (Doc. ID 182054); published March 20, 2013We report on a double-pass Ho:YLF slab amplifier which delivered 350 ns long single-frequency pulses of up to330 mJ at 2064 nm, with a maximumM2 of 1.5 at 50 Hz. It was end pumped with a diode-pumped Tm:YLF slab laserand seeded with up to 50 mJ of single-frequency pulses. © 2013 Optical Society of AmericaOCIS codes: 140.0140, 140.3070, 140.3280, 140.3570, 140.3580, 140.5680.High-energy 2 μm sources are sought after in a widevariety of applications, some of which are in defense,medicine, material processing, and remote sensing [1].Ho doped into YLiF4 (YLF) is an excellent gain mediumwith which to obtain high-energy Q-switched 2 μmpulses. This is because it has a high gain cross section,long upper level lifetime, and weak thermal lensing and isnaturally birefringent [2,3]. Recently, diffraction-limited2 μm pulses with energies up to ∼170 mJ at 100 Hz havebeen demonstrated from a Ho:YLF master oscillatorpower amplifier [4] which was pumped with three120 W Tm-fiber lasers. High average power 1.9 μm Tmslab lasers can also be used as pump sources, especiallyfor amplifiers where the good beam quality of fiber lasersis not required for the pump beam.We recently demonstrated 210 mJ of single-frequency2 μm pulses using a Tm slab pumped, two crystal Ho:YLFand Ho:LuLF slab amplifier [5]. Subsequent simulations[6] indicated that using longer gain crystals and doublepassing the amplified beam would lead to a significantincrease in gain.Consequently we used two 0.5% doped, Ho:YLF slabsfrom VLOC in series. Both had dimensions 10 mm ×2 mm × 50 mm (c × a × a) and were a-cut with theirc-axis horizontal. The slabs were mounted with indiumfoil between custom-made, 22°C, water-cooled mounts.The experimental setup is illustrated in Fig. 1. The slabswere placed in series to provide a 200 mm gain lengthwhen double passed. As pump source, we used aTm:YLF slab oscillator [5] delivering 187 W of outputpower. It was forced to operate on the pi-polarization byinserting a Brewster plate. Its wavelength was 1890 nm,which matched one of the strong absorption peaks ofHo:YLF. The beam had an M2x of 603 and an M2y of2.78. The polarization was kept horizontal so that it wasabsorbed by the stronger absorbing pi-polarization ofHo:YLF. Its output power was monitored by a Gentecpower meter (PS-310WB) placed behind a 99% mirror(M2). The pump beam was then steered into the Ho:YLFamplifier crystals using a high reflector (HR) at 1.9 μm.To shape the beam, it was first collimated with an f 100 mm spherical lens (L1) and focussed into theHo:YLF slabs with an f  60 mm spherical lens (L3).An f  250 mm horizontal cylindrical lens (L2) was usedto match the horizontal focus position after L3 to that ofthe vertical. The Ho:YLF crystals were placed so that thepump focus was positioned in the gap between them,accounting for the refractive index of YLF. The 1890 nmpump beam radii were measured with a pyroelectriccamera (PY-III-CA from Spiricon) as 3.5 mm × 0.4 mmon the inner facing surfaces of the slabs and 4.5 mm ×0.45 mm (wx ×wy) on the outer surfaces using the sec-ond moment method and had the focus intensity profileshown in Fig. 1. The transmitted pump power was mea-sured with a power meter (LM 1000 HTD from Coherent)and the amplifier and pump laser were both enclosed in adry air box to avoid spiking in the Tm:YLF laser outputcaused by water absorption.The seed laser for the amplifier was a single-frequency,ring laser/amplifier system, pumped with a commercial80 W 1940 nm Tm:fiber laser [7,8]. It delivered diffraction-limited output with energies of up to 70 mJ per pulse at50 Hz, and had a pulse duration of 370 ns. The seed laserbeam power was monitored by power meter Pm2 (LM 10HTD from Coherent) placed behind a 95% partial reflec-tor (M4). The seed beam was then steered into theHo:YLF amplifier crystals using various HRs at 2 μm.In the vertical direction, two cylindrical lenses L4,(f y  774 mm) and L5, (f y  504 mm) were used to fo-cus the seed beam through the Ho:YLF crystals onto aFig. 1. (Color online) Experimental setup of the double-passHo:YLF amplifier.1022 OPTICS LETTERS / Vol. 38, No. 7 / April 1, 20130146-9592/13/071022-03$15.00/0 © 2013 Optical Society of Americanormal incidence, HR at 2 μm, high transmission (HT) at1890 nmmirror (M3), which reflected it back for a secondpass at a slight horizontal angle to the incident beam di-rection. The vertical second moment beam radii of theseed beam in the crystals were 0.38 mm on the first crys-tal surface it encountered and 0.34 mm on the crystal sur-face closest to M3, with similar beam sizes on the returnpath. In the horizontal plane, a single cylindrical lens L6(f x  2133 mm) was used to collimate the seed beam toa radius of 3.3 mm. The second pass of the amplifiedbeam was almost spatially symmetrical to the incidentbeam and passed through cylindrical lenses L6 and L5before being separated by mirror M5 (0° HR at 2 μm)from the seed laser beam. The seed polarization was keptvertical (perpendicular to the crystalline c-axis) so that itwas amplified on the sigma-polarization of the Ho:YLFslabs, which has intrinsic weak thermal lensing [9], atthe cost of lower gain due to its lower emission crosssection compared to the pi-polarization. Optical feedbackinto the master ring oscillator/amplifier was prevented byinserting an optical isolator (IO-4-2050-HP from OFR)after the ring oscillator. This reduced the available seedenergy incident on the amplifier gain crystals to ∼50 mJat 50 Hz.Various HRs and a partial reflector at 2 μm (M6) re-flected the amplified beam onto power meter Pm4(LM-45 HTD from Coherent), which measured the aver-age amplified power. The beam contained a continuouswave component which originated from the injectionseeding of the Ho:YLF ring laser which was also ampli-fied. This component had a maximum value of 340 mWand was characterized at various 1890 nm pump powers.These were then deducted from the total measuredaverage powers of the amplified pulses.The amplifier was first optimized at 350 Hz, where therisk of optical damage was the lowest, and the pulse rep-etition frequency (PRF) was then gradually decreased to40 Hz. The output results are given in Fig. 2.The limit where the average power does not increasewith PRF was found to be at PRFs above ∼300 Hz andwas ∼43 W. For lower PRFs, the average output powerdecreased to ∼12 W at 40 Hz. The pulse energy increasedfrom 122 mJ at 350 Hz to above 300 mJ at PRFs below50 Hz, without optical damage. The amplifier was thenoptimized at low PRFs with respect to seed and pumpalignment. At a minimum PRF of 30 Hz, maximum pulseenergy of 333 mJ was obtained. At the required PRF of50 Hz, 325 mJ was measured (at which subsequentmeasurements were performed).The optimized amplifier was subsequently character-ized with respect to 1890 nm Tm:YLF pump power andthe results are given in Fig. 3. It was estimated that upto 96% of the 1890 nm power was absorbed, droppingto 83% at maximum pump power. This indicated thatthe total effect of the Ho:YLF pump saturation was min-imal due to the long combined length of the slabs. Theoutput energy increased linearly from low levels andslightly rolled over to the maximum output energy of325 mJ. The slight roll over was attributed to a small in-crease in beam diameter of the 1890 nm pump beam withpower, which was caused by strong negative thermallensing in the Tm:YLF slab crystal.From the gain curve (amplified energy/seed energy), itcan be seen that the crystals only reached transparency(gain  1) at ∼41 W of pump power. At full pump power,the gain increased to ∼5.7.The amplifier performance as a function of the seedenergy at full pump power is shown in Fig 4. The seedenergy was varied by placing different partial reflectorsinto the incident seed beam. The gain at low seedenergies was ∼40.The amplified beam transmitted through M6 wasfocused by a f  827 mm lens combination L7 (f 522 f  −1415 mm) to create a waist which was thenused to spatially characterize the output using a pyroelec-tric camera (PYIII-CA from Spiricon). In Fig. 5, thehorizontal and vertical second moment beam radii(ISO 11146) are plotted as a functions of the distancefrom the f  827 mm lens combination. The beamquality factors in the two transverse directions weredetermined by fitting the beam radii to the Gaussianbeam propagation equation.The astigmatism was due to the fact that the amplifiedbeamwas not shaped with L4 to be circular symmetric, aswell as the astigmatic thermal lens associated with theHo:YLF amplifier crystals [9]. Figure 6(a) shows the am-plified beam intensity profile at maximum output energy.It can be seen that there was a slight degradation inhorizontal beam quality. This was also reflected in an in-creased horizontal beam quality factor (M2x) of 1.5 andFig. 2. (Color online) Seed and amplified 2064 nm outputenergy (left axis) and the average power of the amplified pulses(right axis) as functions of PRF.Fig. 3. (Color online) Amplified output energy (right axis) andgain (left axis) as functions of incident 1890 nm Tm:YLF pumppower.April 1, 2013 / Vol. 38, No. 7 / OPTICS LETTERS 1023was possibly due to the variation in the horizontal inten-sity profile of the pump beam, which had a largeM2 valuein this direction. In the vertical direction, the beamstayed diffraction limited and had a M2y of 1.01.It was also verified that the output was single fre-quency by only observing significant mode-beating whenthe ring laser was unlocked [10]. The amplified pulselength at full pump power was measured to be ∼350 ns.This can be seen from the amplified pulse shape that isgiven juxtaposed with that of the seed in Fig. 6(b). Thepulse to pulse energy variation was measured to be lessthan 5%. The amplified beam was subsequently used topump an HBr gas laser/amplifier system [11]. The2064 nm amplified beam was 50% absorbed over anHBr cell with a length of 510 mm (at a pressure of40 Torr). This implied that its linewidth was narrowerthan the 400 MHz linewidth of HBr [12].The output energy can still be significantly scaled. Themaximum fluence in the system was calculated to be10.9 J∕cm2, which was still below the optical damagethreshold of good quality coatings (20–45 J∕cm2) [2,13].The same architecture can therefore be used to scalethe energy at least 3 times higher by simply adding moreamplifier modules. However, from calculations [6] webelieve that slight modifications to the system in its cur-rent form can also lead to improvement. Higher gain isexpected if the 28 W of unabsorbed 1890 nm pump poweris reflected back into the amplifier crystals. Amplificationon the stronger gain, pi-polarization would also bepossible if thicker slab crystals were to be used. Itwas not possible to implement it in this setup becauseof clipping of the amplified beam due to the relativelystrong thermal lensing associated with the pi-polarizationof YLF [9].In conclusion, we have demonstrated a 2064 nm,single-frequency, Ho:YLF slab amplifier, emitting a goodquality beamwith energies up to 333 mJ. The work showsthat Ho:YLF amplifiers, pumped with Tm:YLF slabs,provide an effective way to obtain high energies at2 μm, which could be scaled even higher using the samearchitecture.References1. K. Scholle, S. Lamrini, P. Koopmann, and P. Fuhrberg, inFrontiers in Guided Wave Optics and Optoelectronics,B. Pal, ed. (InTech, 2010).2. M. Eichhorn, Appl. Phys. B 93, 269 (2008).3. B. M. Walsh, N. P. Barnes, and B. Di Bartolo, J. Appl. Phys.83, 2772 (1998).4. A. Dergachev, D. Armstrong, A. Smith, T. Drake, and M.Dubois, Proc. SPIE 6875, 687507 (2008).5. H. J. Strauss, W. Koen, C. Bollig, M. J. D. Esser, C. Jacobs,O. J. P. Collett, and D. R. Preussler, Opt. Express 19, 13974(2011).6. O. J. P. Collett, C. Bollig, and M. J. D. Esser, in CLEOEUROPE and EQEC Conference Digest, OSA TechnicalDigest (CD) (Optical Society of America, 2011),paper CA_P24.7. C. Bollig, M. J. D. Esser, C. Jacobs, W. Koen, D. Preussler,K. Nyangaza, and M. Schellhorn, in Mid-InfraredCoherent Sources (European Physical Society, 2009),invited talk Mo3.8. L. R. Botha, C. Bollig, M. J. Esser, R. N. Campbell, C. Jacobs,and D. R. Preussler, Opt. Express 17, 20615 (2009).9. V. V. Zelenogorskii and E. A. Khazanov, Quantum Electron.40, 40 (2010).10. A. Dergachev, Opt. Express 19, 6797 (2011).11. W. Koen, C. Jacobs, C. Bollig, H. J. Strauss, L. Botha, andM. J. D. Esser, Proc. SPIE 8543, 85430E (2012).12. J. Nicholson, D. Neumann, and W. Rudolph, Air ForceResearch Laboratory report AFRL-DE 2001–1015 (AirForce Research Laboratory, 2001).13. M. Schellhorn and M. Eichhorn, Appl. Phys. B 109, 351(2012).Fig. 4. (Color online) Amplified output energy (right axis) andgain (left axis) as functions of seed energy at a PRF of 50 Hz.Fig. 5. (Color online) Horizontal and vertical second momentbeam radii as functions of distance from an f  827 mm lenscombination at maximum output at 50 Hz. The lines are fitsof the beam propagation equation to the measured data andthe crosses are the fitted beam waists.Fig. 6. (Color online) Amplified beam profile in the far field (a)and (b) the seed and amplified pulses at maximum output and aPRF of 50 Hz.1024 OPTICS LETTERS / Vol. 38, No. 7 / April 1, 2013

330 mJ single-frequency Ho:YLF slab amplifier

Abstract

Similar works

Full text

Available Versions

Crossref

Heriot Watt Pure

Heriot Watt Pure